wdiff rfc7296.original rfc7296.txt

Network Working Group
Internet Engineering Task Force (IETF) C. Kaufman
Internet-Draft
Request for Comments: 7296 Microsoft
Obsoletes: 5996 (if approved)
STD: 79 P. Hoffman
Intended status: Standards Track
Obsoletes: 5996 VPN Consortium
Expires: December 8, 2014
Category: Standards Track Y. Nir
ISSN: 2070-1721 Check Point
P. Eronen
Independent
T. Kivinen
INSIDE Secure
June 6,
October 2014

Internet Key Exchange Protocol Version 2 (IKEv2)
draft-kivinen-ipsecme-ikev2-rfc5996bis-04.txt

Abstract

This document describes version 2 of the Internet Key Exchange (IKE)
protocol. IKE is a component of IPsec used for performing mutual
authentication and establishing and maintaining Security Associations
(SAs). This document obsoletes RFC 5996, and includes all of the
errata for it. It advances IKEv2 to be an Internet Standard.

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
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Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 5741.

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This Internet-Draft will expire on December 8, 2014.
http://www.rfc-editor.org/info/rfc7296.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5 ....................................................5
1.1. Usage Scenarios . . . . . . . . . . . . . . . . . . . . . 6 ............................................7
1.1.1. Security Gateway to Security Gateway in
Tunnel Mode . 7 .........................................7
1.1.2. Endpoint-to-Endpoint Transport Mode . . . . . . . . . 7 .................8
1.1.3. Endpoint to Security Gateway in Tunnel Mode . . . . . 8 .........8
1.1.4. Other Scenarios . . . . . . . . . . . . . . . . . . . 9 .....................................9
1.2. The Initial Exchanges . . . . . . . . . . . . . . . . . . 9 ......................................9
1.3. The CREATE_CHILD_SA Exchange . . . . . . . . . . . . . . 13 ..............................13
1.3.1. Creating New Child SAs with the
CREATE_CHILD_SA Exchange . . . . . . . . . . . . . . . . . . . . . . 14 ...........................14
1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA
Exchange . 15 ...........................................16
1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA
Exchange . . . . . . . . . . . . . . . . . . . . . . 16 ...........................................16
1.4. The INFORMATIONAL Exchange . . . . . . . . . . . . . . . 17 ................................17
1.4.1. Deleting an SA with INFORMATIONAL Exchanges . . . . . 17 ........18
1.5. Informational Messages outside of an IKE SA . . . . . . . 18 ...............19
1.6. Requirements Terminology . . . . . . . . . . . . . . . . 19 ..................................20
1.7. Significant Differences between RFC 4306 and RFC5996 . . 19 RFC 5996 .....20
1.8. Differences between RFC 5996 and This Document . . . . . 22 ............23
2. IKE Protocol Details and Variations . . . . . . . . . . . . . 23 ............................23
2.1. Use of Retransmission Timers . . . . . . . . . . . . . . 23 ..............................24
2.2. Use of Sequence Numbers for Message ID . . . . . . . . . 25 ....................25
2.3. Window Size for Overlapping Requests . . . . . . . . . . 26 ......................26
2.4. State Synchronization and Connection Timeouts . . . . . . 27 .............28
2.5. Version Numbers and Forward Compatibility . . . . . . . . 29 .................30
2.6. IKE SA SPIs and Cookies . . . . . . . . . . . . . . . . . 30 ...................................32
2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD . . . . 33 .......35
2.7. Cryptographic Algorithm Negotiation . . . . . . . . . . . 34 .......................35
2.8. Rekeying . . . . . . . . . . . . . . . . . . . . . . . . 35 ..................................................36
2.8.1. Simultaneous Child SA Rekeying . . . . . . . . . . . 37 .....................38
2.8.2. Simultaneous IKE SA Rekeying . . . . . . . . . . . . 39 .......................40
2.8.3. Rekeying the IKE SA versus Reauthentication . . . . . 40 ........42
2.9. Traffic Selector Negotiation . . . . . . . . . . . . . . 41 ..............................42
2.9.1. Traffic Selectors Violating Own Policy . . . . . . . 44 .............45
2.9.2. Traffic Selectors in Rekeying ......................46
2.10. Nonces . . . . . . . . . . . . . . . . . . . . . . . . . 44 ...................................................46
2.11. Address and Port Agility . . . . . . . . . . . . . . . . 45 .................................47
2.12. Reuse of Diffie-Hellman Exponentials . . . . . . . . . . 45 .....................47
2.13. Generating Keying Material . . . . . . . . . . . . . . . 46 ...............................48
2.14. Generating Keying Material for the IKE SA . . . . . . . . 47 ................49
2.15. Authentication of the IKE SA . . . . . . . . . . . . . . 48 .............................50
2.16. Extensible Authentication Protocol Methods . . . . . . . 50 ...............52
2.17. Generating Keying Material for Child SAs . . . . . . . . 52 .................54
2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange . . . . 53 ........55
2.19. Requesting an Internal Address on a Remote Network . . . 54 .......56
2.20. Requesting the Peer's Version . . . . . . . . . . . . . . 55 ............................58
2.21. Error Handling . . . . . . . . . . . . . . . . . . . . . 56 ...........................................58
2.21.1. Error Handling in IKE_SA_INIT . . . . . . . . . . . . 56 .....................59
2.21.2. Error Handling in IKE_AUTH . . . . . . . . . . . . . 57 ........................59
2.21.3. Error Handling after IKE SA is Authenticated . . . . 58 ......60
2.21.4. Error Handling Outside IKE SA . . . . . . . . . . . . 58 .....................60
2.22. IPComp . . . . . . . . . . . . . . . . . . . . . . . . . 59 ...................................................61
2.23. NAT Traversal . . . . . . . . . . . . . . . . . . . . . . 60 ............................................62
2.23.1. Transport Mode NAT Traversal . . . . . . . . . . . . 64 ......................66
2.24. Explicit Congestion Notification (ECN) . . . . . . . . . 68 ...................70
2.25. Exchange Collisions . . . . . . . . . . . . . . . . . . . 68 ......................................70
2.25.1. Collisions while Rekeying or Closing Child SAs . . . 69 ....71
2.25.2. Collisions while Rekeying or Closing IKE SAs . . . . 70 ......71
3. Header and Payload Formats . . . . . . . . . . . . . . . . . 70 .....................................72
3.1. The IKE Header . . . . . . . . . . . . . . . . . . . . . 70 ............................................72
3.2. Generic Payload Header . . . . . . . . . . . . . . . . . 73 ....................................75
3.3. Security Association Payload . . . . . . . . . . . . . . 75 ..............................77
3.3.1. Proposal Substructure . . . . . . . . . . . . . . . . 79 ..............................80
3.3.2. Transform Substructure . . . . . . . . . . . . . . . 80 .............................81
3.3.3. Valid Transform Types by Protocol . . . . . . . . . . 84 ..................85
3.3.4. Mandatory Transform IDs . . . . . . . . . . . . . . . 84 ............................85
3.3.5. Transform Attributes . . . . . . . . . . . . . . . . 85 ...............................86
3.3.6. Attribute Negotiation . . . . . . . . . . . . . . . . 87 ..............................88
3.4. Key Exchange Payload . . . . . . . . . . . . . . . . . . 88 ......................................89
3.5. Identification Payloads . . . . . . . . . . . . . . . . . 89 ...................................90
3.6. Certificate Payload . . . . . . . . . . . . . . . . . . . 91 .......................................92
3.7. Certificate Request Payload . . . . . . . . . . . . . . . 94 ...............................95
3.8. Authentication Payload . . . . . . . . . . . . . . . . . 96 ....................................97
3.9. Nonce Payload . . . . . . . . . . . . . . . . . . . . . . 97 .............................................98
3.10. Notify Payload . . . . . . . . . . . . . . . . . . . . . 98 ...........................................99
3.10.1. Notify Message Types . . . . . . . . . . . . . . . . 99 .............................101
3.11. Delete Payload . . . . . . . . . . . . . . . . . . . . . 102 ..........................................104
3.12. Vendor ID Payload . . . . . . . . . . . . . . . . . . . . 103 .......................................105
3.13. Traffic Selector Payload . . . . . . . . . . . . . . . . 105 ................................106
3.13.1. Traffic Selector . . . . . . . . . . . . . . . . . . 106 .................................108
3.14. Encrypted Payload . . . . . . . . . . . . . . . . . . . . 108 .......................................110
3.15. Configuration Payload . . . . . . . . . . . . . . . . . . 110 ...................................112
3.15.1. Configuration Attributes . . . . . . . . . . . . . . 111 .........................113
3.15.2. Meaning of INTERNAL_IP4_SUBNET and
INTERNAL_IP6_SUBNET . . . . . . . . . . . . . . . . . 114 ..............................116
3.15.3. Configuration Payloads for IPv6 . . . . . . . . . . . 116 ..................118
3.15.4. Address Assignment Failures . . . . . . . . . . . . . 117 ......................119
3.16. Extensible Authentication Protocol (EAP) Payload . . . . 118 ........120
4. Conformance Requirements . . . . . . . . . . . . . . . . . . 119 ......................................122
5. Security Considerations . . . . . . . . . . . . . . . . . . . 121 .......................................124
5.1. Traffic Selector Authorization . . . . . . . . . . . . . 124 ...........................127
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 125 ...........................................128
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 125
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 127
8.1. ....................................................128
7.1. Normative References . . . . . . . . . . . . . . . . . . 127
8.2. .....................................128
7.2. Informative References . . . . . . . . . . . . . . . . . 128 ...................................130
Appendix A. Summary of Changes from IKEv1 . . . . . . . . . . . 132 ........................136
Appendix B. Diffie-Hellman Groups . . . . . . . . . . . . . . . 133 ................................137
B.1. Group 1 - 768-bit MODP . . . . . . . . . . . . . . . . . 134 ....................................137
B.2. Group 2 - 1024-bit MODP . . . . . . . . . . . . . . . . . 134 ...................................137
Appendix C. Exchanges and Payloads . . . . . . . . . . . . . . . 134 ...............................138
C.1. IKE_SA_INIT Exchange . . . . . . . . . . . . . . . . . . 135 ......................................138
C.2. IKE_AUTH Exchange without EAP . . . . . . . . . . . . . . 136 .............................138
C.3. IKE_AUTH Exchange with EAP . . . . . . . . . . . . . . . 137 ................................139
C.4. CREATE_CHILD_SA Exchange for Creating or Rekeying
Child SAs . . . . . . . . . . . . . . . . . . . . . . . . 138 .................................................140
C.5. CREATE_CHILD_SA Exchange for Rekeying the IKE SA . . . . 138 ..........140
C.6. INFORMATIONAL Exchange . . . . . . . . . . . . . . . . . 138 ....................................141
Acknowledgements .................................................141
Authors' Addresses ...............................................142

1. Introduction

IP Security (IPsec) provides confidentiality, data integrity, access
control, and data source authentication to IP datagrams. These
services are provided by maintaining shared state between the source
and the sink of an IP datagram. This state defines, among other
things, the specific services provided to the datagram, which
cryptographic algorithms will be used to provide the services, and
the keys used as input to the cryptographic algorithms.

Establishing this shared state in a manual fashion does not scale
well. Therefore, a protocol to establish this state dynamically is
needed. This document describes such a protocol -- the Internet Key
Exchange (IKE). Version 1 of IKE was defined in RFCs 2407 [DOI],
2408 [ISAKMP], and 2409 [IKEV1]. IKEv2 replaced all of those RFCs.
IKEv2 was defined in [IKEV2] (RFC 4306) and was clarified in [Clarif]
(RFC 4718). The [RFC5996] replaced and updated RFC RFCs 4306 and RFC
4718, and this 4718. This
document replaces the RFC 5996 and the intended status
for this document will be Internet Standard. 5996. IKEv2 as stated in RFC 4306 was a change
to the IKE protocol that was not backward compatible. RFC 5996
revised RFC 4306 to provide a clarification of IKEv2, making minimum minimal
changes to the IKEv2 protocol. The current This document slightly revises replaces RFC 5996 5996,
slightly revising it to make it suitable for progression to Internet
Standard. A list of the significant differences between RFC RFCs 4306
and RFC 5996 is given in Section 1.7 1.7, and differences between RFC 5996
and this document is are given in Section 1.8.

IKE performs mutual authentication between two parties and
establishes an IKE security association Security Association (SA) that includes shared
secret information that can be used to efficiently establish SAs for
Encapsulating Security Payload (ESP) [ESP] or Authentication Header
(AH) [AH] and a set of cryptographic algorithms to be used by the SAs
to protect the traffic that they carry. In this document, the term
"suite" or "cryptographic suite" refers to a complete set of
algorithms used to protect an SA. An initiator proposes one or more
suites by listing supported algorithms that can be combined into
suites in a mix-and-match fashion. IKE can also negotiate use of IP
Compression (IPComp) [IP-COMP] in connection with an ESP or AH SA.
The SAs for ESP or AH that get set up through that IKE SA we call
"Child SAs".

All IKE communications consist of pairs of messages: a request and a
response. The pair is called an "exchange", and is sometimes called
a "request/response pair". The first exchange two exchanges of messages
establishing an IKE SA are called the IKE_SA_INIT exchange and the
IKE_AUTH
exchanges; exchange; subsequent IKE exchanges are called the either
CREATE_CHILD_SA exchanges or INFORMATIONAL exchanges. In the common
case, there is a single IKE_SA_INIT exchange and a single IKE_AUTH
exchange (a total of four messages) to establish the IKE SA and the
first Child SA. In exceptional cases, there may be more than one of
each of these exchanges. In all cases, all IKE_SA_INIT exchanges
MUST complete before any other exchange type, then all IKE_AUTH
exchanges MUST complete, and following that, any number of
CREATE_CHILD_SA and INFORMATIONAL exchanges may occur in any order.
In some scenarios, only a single Child SA is needed between the IPsec
endpoints, and therefore there would be no additional exchanges.
Subsequent exchanges MAY be used to establish additional Child SAs
between the same authenticated pair of endpoints and to perform
housekeeping functions.

An IKE message flow always consists of a request followed by a
response. It is the responsibility of the requester to ensure
reliability. If the response is not received within a timeout
interval, the requester needs to retransmit the request (or abandon
the connection).

The first exchange of an IKE session, IKE_SA_INIT, negotiates
security parameters for the IKE SA, sends nonces, and sends Diffie-
Hellman
Diffie-Hellman values.

The second exchange, IKE_AUTH, transmits identities, proves knowledge
of the secrets corresponding to the two identities, and sets up an SA
for the first (and often only) AH or ESP Child SA (unless there is
failure setting up the AH or ESP Child SA, in which case the IKE SA
is still established without the Child SA).

The types of subsequent exchanges are CREATE_CHILD_SA (which creates
a Child SA) and INFORMATIONAL (which deletes an SA, reports error
conditions, or does other housekeeping). Every request requires a
response. An INFORMATIONAL request with no payloads (other than the
empty Encrypted payload required by the syntax) is commonly used as a
check for liveness. These subsequent exchanges cannot be used until
the initial exchanges have completed.

In the description that follows, we assume that no errors occur.
Modifications to the flow when errors occur are described in
Section 2.21.

1.1. Usage Scenarios

IKE is used to negotiate ESP or AH SAs in a number of different
scenarios, each with its own special requirements.

1.1.1. Security Gateway to Security Gateway in Tunnel Mode

Figure 1: Security Gateway to Security Gateway Tunnel

In this scenario, neither endpoint of the IP connection implements
IPsec, but network nodes between them protect traffic for part of the
way. Protection is transparent to the endpoints, and depends on
ordinary routing to send packets through the tunnel endpoints for
processing. Each endpoint would announce the set of addresses
"behind" it, and packets would be sent in tunnel mode where the inner
IP header would contain the IP addresses of the actual endpoints.

1.1.2. Endpoint-to-Endpoint Transport Mode

Figure 2: Endpoint to Endpoint

In this scenario, both endpoints of the IP connection implement
IPsec, as required of hosts in [IPSECARCH]. Transport mode will
commonly be used with no inner IP header. A single pair of addresses
will be negotiated for packets to be protected by this SA. These
endpoints MAY implement application-layer access controls based on
the IPsec authenticated identities of the participants. This
scenario enables the end-to-end security that has been a guiding
principle for the Internet since [ARCHPRINC], [TRANSPARENCY], and a
method of limiting the inherent problems with complexity in networks
noted by [ARCHGUIDEPHIL]. Although this scenario may not be fully
applicable to the IPv4 Internet, it has been deployed successfully in
specific scenarios within intranets using IKEv1. It should be more
broadly enabled during the transition to IPv6 and with the adoption
of IKEv2.

It is possible in this scenario that one or both of the protected
endpoints will be behind a network address translation (NAT) node, in
which case the tunneled packets will have to be UDP encapsulated so
that port numbers in the UDP headers can be used to identify
individual endpoints "behind" the NAT (see Section 2.23).

1.1.3. Endpoint to Security Gateway in Tunnel Mode

Figure 3: Endpoint to Security Gateway Tunnel

In this scenario, a protected endpoint (typically a portable roaming
computer) connects back to its corporate network through an IPsec-
protected tunnel. It might use this tunnel only to access
information on the corporate network, or it might tunnel all of its
traffic back through the corporate network in order to take advantage
of protection provided by a corporate firewall against Internet-based
attacks. In either case, the protected endpoint will want an IP
address associated with the security gateway so that packets returned
to it will go to the security gateway and be tunneled back. This IP
address may be static or may be dynamically allocated by the security
gateway. In support of the latter case, IKEv2 includes a mechanism
(namely, configuration payloads) for the initiator to request an IP
address owned by the security gateway for use for the duration of
its SA.

In this scenario, packets will use tunnel mode. On each packet from
the protected endpoint, the outer IP header will contain the source
IP address associated with its current location (i.e., the address
that will get traffic routed to the endpoint directly), while the
inner IP header will contain the source IP address assigned by the
security gateway (i.e., the address that will get traffic routed to
the security gateway for forwarding to the endpoint). The outer
destination address will always be that of the security gateway,
while the inner destination address will be the ultimate destination
for the packet.

In this scenario, it is possible that the protected endpoint will be
behind a NAT. In that case, the IP address as seen by the security
gateway will not be the same as the IP address sent by the protected
endpoint, and packets will have to be UDP encapsulated in order to be
routed properly. Interaction with NATs is covered in detail in
Section 2.23.

1.1.4. Other Scenarios

Other scenarios are possible, as are nested combinations of the
above. One notable example combines aspects of Sections 1.1.1 and
1.1.3. A subnet may make all external accesses through a remote
security gateway using an IPsec tunnel, where the addresses on the
subnet are routed to the security gateway by the rest of the
Internet. An example would be someone's home network being virtually
on the Internet with static IP addresses even though connectivity is
provided by an ISP that assigns a single dynamically assigned IP
address to the user's security gateway (where the static IP addresses
and an IPsec relay are provided by a third party located elsewhere).

1.2. The Initial Exchanges

Communication using IKE always begins with IKE_SA_INIT and IKE_AUTH
exchanges (known in IKEv1 as Phase 1). These initial exchanges
normally consist of four messages, though in some scenarios that
number can grow. All communications using IKE consist of request/
response pairs. We'll describe the base exchange first, followed by
variations. The first pair of messages (IKE_SA_INIT) negotiate
cryptographic algorithms, exchange nonces, and do a Diffie-Hellman
exchange [DH].

The second pair of messages (IKE_AUTH) authenticate the previous
messages, exchange identities and certificates, and establish the
first Child SA. Parts of these messages are encrypted and integrity
protected with keys established through the IKE_SA_INIT exchange, so
the identities are hidden from eavesdroppers and all fields in all
the messages are authenticated. See Section 2.14 for information on
how the encryption keys are generated. (A man-in-the-middle attacker
who cannot complete the IKE_AUTH exchange can nonetheless see the
identity of the initiator.)

All messages following the initial exchange are cryptographically
protected using the cryptographic algorithms and keys negotiated in
the IKE_SA_INIT exchange. These subsequent messages use the syntax
of the Encrypted payload described in Section 3.14, encrypted with
keys that are derived as described in Section 2.14. All subsequent
messages include an Encrypted payload, even if they are referred to
in the text as "empty". For the CREATE_CHILD_SA, IKE_AUTH, or
INFORMATIONAL exchanges, the message following the header is
encrypted and the message including the header is integrity protected
using the cryptographic algorithms negotiated for the IKE SA.

Every IKE message contains a Message ID as part of its fixed header.
This Message ID is used to match up requests and responses, and to
identify retransmissions of messages.

In the following descriptions, the payloads contained in the message
are indicated by names as listed below.

Notation Payload
-----------------------------------------
AUTH Authentication
CERT Certificate
CERTREQ Certificate Request
CP Configuration
D Delete
EAP Extensible Authentication
HDR IKE header (not a payload)
IDi Identification - Initiator
IDr Identification - Responder
KE Key Exchange
Ni, Nr Nonce
N Notify
SA Security Association
SK Encrypted and Authenticated
TSi Traffic Selector - Initiator
TSr Traffic Selector - Responder
V Vendor ID

The details of the contents of each payload are described in section
Section 3. Payloads that may optionally appear will be shown in
brackets, such as [CERTREQ]; this indicates that a Certificate
Request payload can optionally be included.

The initial exchanges are as follows:

Initiator Responder
-------------------------------------------------------------------
HDR, SAi1, KEi, Ni -->

HDR contains the Security Parameter Indexes (SPIs), version numbers,
Exchange Type, Message ID, and flags of various sorts. The SAi1
payload states the cryptographic algorithms the initiator supports
for the IKE SA. The KE payload sends the initiator's Diffie-Hellman
value. Ni is the initiator's nonce.

<-- HDR, SAr1, KEr, Nr, [CERTREQ]

The responder chooses a cryptographic suite from the initiator's
offered choices and expresses that choice in the SAr1 payload,
completes the Diffie-Hellman exchange with the KEr payload, and sends
its nonce in the Nr payload.

At this point in the negotiation, each party can generate a quantity
called SKEYSEED (see Section 2.14), from which all keys are derived
for that IKE SA. The messages that follow are encrypted and
integrity protected in their entirety, with the exception of the
message headers. The keys used for the encryption and integrity
protection are derived from SKEYSEED and are known as SK_e
(encryption) and SK_a (authentication, a.k.a. integrity protection);
see Sections 2.13 and 2.14 for details on the key derivation. A
separate SK_e and SK_a is computed for each direction. In addition
to the keys SK_e and SK_a derived from the Diffie-Hellman value for
protection of the IKE SA, another quantity SK_d is derived and used
for derivation of further keying material for Child SAs. The
notation SK { ... } indicates that these payloads are encrypted and
integrity protected using that direction's SK_e and SK_a.

HDR, SK {IDi, [CERT,] [CERTREQ,]
[IDr,] AUTH, SAi2,
TSi, TSr} -->
The initiator asserts its identity with the IDi payload, proves
knowledge of the secret corresponding to IDi and integrity protects
the contents of the first message using the AUTH payload (see
Section 2.15). It might also send its certificate(s) in CERT
payload(s) and a list of its trust anchors in CERTREQ payload(s). If
any CERT payloads are included, the first certificate provided MUST
contain the public key used to verify the AUTH field.

The optional payload IDr enables the initiator to specify to which of
the responder's identities it wants to talk. This is useful when the
machine on which the responder is running is hosting multiple
identities at the same IP address. If the IDr proposed by the
initiator is not acceptable to the responder, the responder might use
some other IDr to finish the exchange. If the initiator then does
not accept the fact that responder used an IDr different than the one
that was requested, the initiator can close the SA after noticing the
fact.

The Traffic Selectors (TSi and TSr) are discussed in Section 2.9.

The initiator begins negotiation of a Child SA using the SAi2
payload. The final fields (starting with SAi2) are described in the
description of the CREATE_CHILD_SA exchange.

<-- HDR, SK {IDr, [CERT,] AUTH,
SAr2, TSi, TSr}

The responder asserts its identity with the IDr payload, optionally
sends one or more certificates (again with the certificate containing
the public key used to verify AUTH listed first), authenticates its
identity and protects the integrity of the second message with the
AUTH payload, and completes negotiation of a Child SA with the
additional fields described below in the CREATE_CHILD_SA exchange.
Both parties in the IKE_AUTH exchange MUST verify that all signatures
and Message Authentication Codes (MACs) are computed correctly. If
either side uses a shared secret for authentication, the names in the
ID payload MUST correspond to the key used to generate the AUTH
payload.

Because the initiator sends its Diffie-Hellman value in the
IKE_SA_INIT, it must guess the Diffie-Hellman group that the
responder will select from its list of supported groups. If the
initiator guesses wrong, the responder will respond with a Notify
payload of type INVALID_KE_PAYLOAD indicating the selected group. In
this case, the initiator MUST retry the IKE_SA_INIT with the
corrected Diffie-Hellman group. The initiator MUST again propose its
full set of acceptable cryptographic suites because the rejection
message was unauthenticated and otherwise an active attacker could
trick the endpoints into negotiating a weaker suite than a stronger
one that they both prefer.

If creating the Child SA during the IKE_AUTH exchange fails for some
reason, the IKE SA is still created as usual. The list of Notify
message types in the IKE_AUTH exchange that do not prevent an IKE SA
from being set up include at least the following: NO_PROPOSAL_CHOSEN,
TS_UNACCEPTABLE, SINGLE_PAIR_REQUIRED, INTERNAL_ADDRESS_FAILURE, and
FAILED_CP_REQUIRED.

If the failure is related to creating the IKE SA (for example, an
AUTHENTICATION_FAILED Notify error message is returned), the IKE SA
is not created. Note that although the IKE_AUTH messages are
encrypted and integrity protected, if the peer receiving this Notify
error message has not yet authenticated the other end (or if the peer
fails to authenticate the other end for some reason), the information
needs to be treated with caution. More precisely, assuming that the
MAC verifies correctly, the sender of the error Notify message is
known to be the responder of the IKE_SA_INIT exchange, but the
sender's identity cannot be assured.

Note that IKE_AUTH messages do not contain KEi/KEr or Ni/Nr payloads.
Thus, the SA payloads in the IKE_AUTH exchange cannot contain
Transform Type 4 (Diffie-Hellman group) with any value other than
NONE. Implementations SHOULD omit the whole transform substructure
instead of sending value NONE.

1.3. The CREATE_CHILD_SA Exchange

The CREATE_CHILD_SA exchange is used to create new Child SAs and to
rekey both IKE SAs and Child SAs. This exchange consists of a single
request/response pair, and some of its function was referred to as a
Phase 2 exchange in IKEv1. It MAY be initiated by either end of the
IKE SA after the initial exchanges are completed.

An SA is rekeyed by creating a new SA and then deleting the old one.
This section describes the first part of rekeying, the creation of
new SAs; Section 2.8 covers the mechanics of rekeying, including
moving traffic from old to new SAs and the deletion of the old SAs.
The two sections must be read together to understand the entire
process of rekeying.

Either endpoint may initiate a CREATE_CHILD_SA exchange, so in this
section the term initiator refers to the endpoint initiating this
exchange. An implementation MAY refuse all CREATE_CHILD_SA requests
within an IKE SA.

The CREATE_CHILD_SA request MAY optionally contain a KE payload for
an additional Diffie-Hellman exchange to enable stronger guarantees
of forward secrecy for the Child SA. The keying material for the
Child SA is a function of SK_d established during the establishment
of the IKE SA, the nonces exchanged during the CREATE_CHILD_SA
exchange, and the Diffie-Hellman value (if KE payloads are included
in the CREATE_CHILD_SA exchange).

If a CREATE_CHILD_SA exchange includes a KEi payload, at least one of
the SA offers MUST include the Diffie-Hellman group of the KEi. The
Diffie-Hellman group of the KEi MUST be an element of the group the
initiator expects the responder to accept (additional Diffie-Hellman
groups can be proposed). If the responder selects a proposal using a
different Diffie-Hellman group (other than NONE), the responder MUST
reject the request and indicate its preferred Diffie-Hellman group in
the INVALID_KE_PAYLOAD Notify payload. There are two octets of data
associated with this notification: the accepted Diffie-Hellman group
number in big endian order. In the case of such a rejection, the
CREATE_CHILD_SA exchange fails, and the initiator will probably retry
the exchange with a Diffie-Hellman proposal and KEi in the group that
the responder gave in the INVALID_KE_PAYLOAD Notify payload.

The responder sends a NO_ADDITIONAL_SAS notification to indicate that
a CREATE_CHILD_SA request is unacceptable because the responder is
unwilling to accept any more Child SAs on this IKE SA. This
notification can also be used to reject IKE SA rekey. Some minimal
implementations may only accept a single Child SA setup in the
context of an initial IKE exchange and reject any subsequent attempts
to add more.

1.3.1. Creating New Child SAs with the CREATE_CHILD_SA Exchange

A Child SA may be created by sending a CREATE_CHILD_SA request. The
CREATE_CHILD_SA request for creating a new Child SA is:

Initiator Responder
-------------------------------------------------------------------
HDR, SK {SA, Ni, [KEi], [KEi,]
TSi, TSr} -->

The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
payload, optionally a Diffie-Hellman value in the KEi payload, and
the proposed Traffic Selectors for the proposed Child SA in the TSi
and TSr payloads.

The CREATE_CHILD_SA response for creating a new Child SA is:

<-- HDR, SK {SA, Nr, [KEr], [KEr,]
TSi, TSr}

The responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, a nonce in the Nr payload, and a
Diffie-Hellman value in the KEr payload if KEi was included in the
request and the selected cryptographic suite includes that group.

The Traffic Selectors for traffic to be sent on that SA are specified
in the TS payloads in the response, which may be a subset of what the
initiator of the Child SA proposed.

The USE_TRANSPORT_MODE notification MAY be included in a request
message that also includes an SA payload requesting a Child SA. It
requests that the Child SA use transport mode rather than tunnel mode
for the SA created. If the request is accepted, the response MUST
also include a notification of type USE_TRANSPORT_MODE. If the
responder declines the request, the Child SA will be established in
tunnel mode. If this is unacceptable to the initiator, the initiator
MUST delete the SA. Note: Except when using this option to negotiate
transport mode, all Child SAs will use tunnel mode.

The ESP_TFC_PADDING_NOT_SUPPORTED notification asserts that the
sending endpoint will not accept packets that contain Traffic Flow
Confidentiality (TFC) padding over the Child SA being negotiated. If
neither endpoint accepts TFC padding, this notification is included
in both the request and the response. If this notification is
included in only one of the messages, TFC padding can still be sent
in the other direction.

The NON_FIRST_FRAGMENTS_ALSO notification is used for fragmentation
control. See [IPSECARCH] for a fuller explanation. Both parties
need to agree to sending non-first fragments before either party does
so. It is enabled only if NON_FIRST_FRAGMENTS_ALSO notification is
included in both the request proposing an SA and the response
accepting it. If the responder does not want to send or receive non-
first
non-first fragments, it only omits NON_FIRST_FRAGMENTS_ALSO
notification from its response, but does not reject the whole Child
SA creation.

An IPCOMP_SUPPORTED notification, covered in Section 2.22, can also
be included in the exchange.

A failed attempt to create a Child SA SHOULD NOT tear down the IKE
SA: there is no reason to lose the work done to set up the IKE SA.
See Section 2.21 for a list of error messages that might occur if
creating a Child SA fails.

1.3.2. Rekeying IKE SAs with the CREATE_CHILD_SA Exchange

The CREATE_CHILD_SA request for rekeying an IKE SA is:

Initiator Responder
-------------------------------------------------------------------
HDR, SK {SA, Ni, KEi} -->

The initiator sends SA offer(s) in the SA payload, a nonce in the Ni
payload, and a Diffie-Hellman value in the KEi payload. The KEi
payload MUST be included. A new initiator SPI is supplied in the SPI
field of the SA payload. Once a peer receives a request to rekey an
IKE SA or sends a request to rekey an IKE SA, it SHOULD NOT start any
new CREATE_CHILD_SA exchanges on the IKE SA that is being rekeyed.

The CREATE_CHILD_SA response for rekeying an IKE SA is:

<-- HDR, SK {SA, Nr, KEr}

The responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, a nonce in the Nr payload, and a
Diffie-Hellman value in the KEr payload if the selected cryptographic
suite includes that group. A new responder SPI is supplied in the
SPI field of the SA payload.

The new IKE SA has its message counters set to 0, regardless of what
they were in the earlier IKE SA. The first IKE requests from both
sides on the new IKE SA will have Message ID 0. The old IKE SA
retains its numbering, so any further requests (for example, to
delete the IKE SA) will have consecutive numbering. The new IKE SA
also has its window size reset to 1, and the initiator in this rekey
exchange is the new "original initiator" of the new IKE SA.

Section 2.18 also covers IKE SA rekeying in detail.

1.3.3. Rekeying Child SAs with the CREATE_CHILD_SA Exchange

The CREATE_CHILD_SA request for rekeying a Child SA is:

Initiator Responder
-------------------------------------------------------------------
HDR, SK {N(REKEY_SA), SA, Ni, [KEi], [KEi,]
TSi, TSr} -->

The notifications described in Section 1.3.1 may also be sent in a
rekeying exchange. Usually, these will be the same notifications
that were used in the original exchange; for example, when rekeying a
transport mode SA, the USE_TRANSPORT_MODE notification will be used.

The REKEY_SA notification MUST be included in a CREATE_CHILD_SA
exchange if the purpose of the exchange is to replace an existing ESP
or AH SA. The SA being rekeyed is identified by the SPI field in the
Notify payload; this is the SPI the exchange initiator would expect
in inbound ESP or AH packets. There is no data associated with this
Notify message type. The Protocol ID field of the REKEY_SA
notification is set to match the protocol of the SA we are rekeying,
for example, 3 for ESP and 2 for AH.

The CREATE_CHILD_SA response for rekeying a Child SA is:

<-- HDR, SK {SA, Nr, [KEr], [KEr,]
TSi, TSr}

The responder replies (using the same Message ID to respond) with the
accepted offer in an SA payload, a nonce in the Nr, Nr payload, and a Diffie-
Hellman
Diffie-Hellman value in the KEr payload if KEi was included in the
request and the selected cryptographic suite includes that group.

The Traffic Selectors for traffic to be sent on that SA are specified
in the TS payloads in the response, which may be a subset of what the
initiator of the Child SA proposed.

1.4. The INFORMATIONAL Exchange

At various points during the operation of an IKE SA, peers may desire
to convey control messages to each other regarding errors or
notifications of certain events. To accomplish this, IKE defines an
INFORMATIONAL exchange. INFORMATIONAL exchanges MUST ONLY occur
after the initial exchanges and are cryptographically protected with
the negotiated keys. Note that some informational messages, not
exchanges, can be sent outside the context of an IKE SA.
Section 2.21 also covers error messages in great detail.

Control messages that pertain to an IKE SA MUST be sent under that
IKE SA. Control messages that pertain to Child SAs MUST be sent
under the protection of the IKE SA that generated them (or its
successor if the IKE SA was rekeyed).

Messages in an INFORMATIONAL exchange contain zero or more
Notification, Delete, and Configuration payloads. The recipient of
an INFORMATIONAL exchange request MUST send some response; otherwise,
the sender will assume the message was lost in the network and will
retransmit it. That response MAY be an empty message. The request
message in an INFORMATIONAL exchange MAY also contain no payloads.
This is the expected way an endpoint can ask the other endpoint to
verify that it is alive.

The INFORMATIONAL exchange is defined as:

Initiator Responder
-------------------------------------------------------------------
HDR, SK {[N,] [D,]
[CP,] ...} -->
<-- HDR, SK {[N,] [D,]
[CP],
[CP,] ...}

The processing of an INFORMATIONAL exchange is determined by its
component payloads.

1.4.1. Deleting an SA with INFORMATIONAL Exchanges

ESP and AH SAs always exist in pairs, with one SA in each direction.
When an SA is closed, both members of the pair MUST be closed (that
is, deleted). Each endpoint MUST close its incoming SAs and allow
the other endpoint to close the other SA in each pair. To delete an
SA, an INFORMATIONAL exchange with one or more Delete payloads is
sent listing the SPIs (as they would be expected in the headers of
inbound packets) of the SAs to be deleted. The recipient MUST close
the designated SAs. Note that one never sends Delete payloads for
the two sides of an SA in a single message. If there are many SAs to
delete at the same time, one includes Delete payloads for the inbound
half of each SA pair in the INFORMATIONAL exchange.

Normally, the response in the INFORMATIONAL exchange will contain
Delete payloads for the paired SAs going in the other direction.
There is one exception. If, by chance, both ends of a set of SAs
independently decide to close them, each may send a Delete payload
and the two requests may cross in the network. If a node receives a
delete request for SAs for which it has already issued a delete
request, it MUST delete the outgoing SAs while processing the request
and the incoming SAs while processing the response. In that case,
the responses MUST NOT include Delete payloads for the deleted SAs,
since that would result in duplicate deletion and could in theory
delete the wrong SA.

Similar to ESP and AH SAs, IKE SAs are also deleted by sending an
Informational
INFORMATIONAL exchange. Deleting an IKE SA implicitly closes any
remaining Child SAs negotiated under it. The response to a request
that deletes the IKE SA is an empty INFORMATIONAL response.

Half-closed ESP or AH connections are anomalous, and a node with
auditing capability should probably audit their existence if they
persist. Note that this specification does not specify time periods,
so it is up to individual endpoints to decide how long to wait. A
node MAY refuse to accept incoming data on half-closed connections
but MUST NOT unilaterally close them and reuse the SPIs. If
connection state becomes sufficiently messed up, a node MAY close the
IKE SA, as described above. It can then rebuild the SAs it needs on
a clean base under a new IKE SA.

1.5. Informational Messages outside of an IKE SA

There are some cases in which a node receives a packet that it cannot
process, but it may want to notify the sender about this situation.

o If an ESP or AH packet arrives with an unrecognized SPI. This
might be due to the receiving node having recently crashed and
lost state, or because of some other system malfunction or attack.

o If an encrypted IKE request packet arrives on port 500 or 4500
with an unrecognized IKE SPI. This might be due to the receiving
node having recently crashed and lost state, or because of some
other system malfunction or attack.

o If an IKE request packet arrives with a higher major version
number than the implementation supports.

In the first case, if the receiving node has an active IKE SA to the
IP address from whence the packet came, it MAY send an INVALID_SPI
notification of the wayward packet over that IKE SA in an
INFORMATIONAL exchange. The Notification Data contains the SPI of
the invalid packet. The recipient of this notification cannot tell
whether the SPI is for AH or ESP, but this is not important because
in many cases the SPIs will be different for the two. If no suitable
IKE SA exists, the node MAY send an informational message without
cryptographic protection to the source IP address, using the source
UDP port as the destination port if the packet was UDP (UDP-
encapsulated ESP or AH). In this case, it should only be used by the
recipient as a hint that something might be wrong (because it could
easily be forged). This message is not part of an INFORMATIONAL
exchange, and the receiving node MUST NOT respond to it because doing
so could cause a message loop. The message is constructed as
follows: there are no IKE SPI values that would be meaningful to the
recipient of such a notification; using zero values or random values
are both acceptable, this being the exception to the rule in
Section 3.1 that prohibits zero IKE Initiator SPIs. The Initiator
flag is set to 1, the Response flag is set to 0, and the version
flags are set in the normal fashion; these flags are described in
Section 3.1.

In the second and third cases, the message is always sent without
cryptographic protection (outside of an IKE SA), and includes either
an INVALID_IKE_SPI or an INVALID_MAJOR_VERSION notification (with no
notification data). The message is a response message, and thus it
is sent to the IP address and port from whence it came with the same
IKE SPIs and the Message ID and Exchange Type are copied from the
request. The Response flag is set to 1, and the version flags are
set in the normal fashion.

1.6. Requirements Terminology

Definitions of the primitive terms in this document (such as Security
Association or SA) can be found in [IPSECARCH]. It should be noted
that parts of IKEv2 rely on some of the processing rules in
[IPSECARCH], as described in various sections of this document.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [MUSTSHOULD].

1.7. Significant Differences between RFC 4306 and RFC5996 RFC 5996

This document contains clarifications and amplifications to IKEv2
[IKEV2]. Many of the clarifications are based on [Clarif]. The
changes listed in that document were discussed in the IPsec Working
Group and, after the Working Group was disbanded, on the IPsec
mailing list. That document contains detailed explanations of areas
that were unclear in IKEv2, and is thus useful to implementers of
IKEv2.

The protocol described in this document retains the same major
version number (2) and minor version number (0) as was used in
RFC 4306. That is, the version number is *not* changed from
RFC 4306. The small number of technical changes listed here are not
expected to affect RFC 4306 implementations that have already been
deployed at the time of publication of this document.

This document makes the figures and references a bit more consistent
than they were in [IKEV2].

IKEv2 developers have noted that the SHOULD-level requirements in
RFC 4306 are often unclear in that they don't say when it is OK to
not obey the requirements. They also have noted that there are MUST-
level requirements that are not related to interoperability. This
document has more explanation of some of these requirements. All
non-capitalized uses of the words SHOULD and MUST now mean their
normal English sense, not the interoperability sense of [MUSTSHOULD].

IKEv2 (and IKEv1) developers have noted that there is a great deal of
material in the tables of codes in Section 3.10.1 in RFC 4306. This
leads to implementers not having all the needed information in the
main body of the document. Much of the material from those tables
has been moved into the associated parts of the main body of the
document.

This document removes discussion of nesting AH and ESP. This was a
mistake in RFC 4306 caused by the lag between finishing RFC 4306 and
RFC 4301. Basically, IKEv2 is based on RFC 4301, which does not
include "SA bundles" that were part of RFC 2401. While a single
packet can go through IPsec processing multiple times, each of these
passes uses a separate SA, and the passes are coordinated by the
forwarding tables. In IKEv2, each of these SAs has to be created
using a separate CREATE_CHILD_SA exchange.

This document removes discussion of the INTERNAL_ADDRESS_EXPIRY
configuration attribute because its implementation was very
problematic. Implementations that conform to this document MUST
ignore proposals that have configuration attribute type 5, the old
value for INTERNAL_ADDRESS_EXPIRY. This document also removed
INTERNAL_IP6_NBNS as a configuration attribute.

This document removes the allowance for rejecting messages in which
the payloads were not in the "right" order; now implementations
MUST NOT reject them. This is due to the lack of clarity where the
orders for the payloads are described.

The lists of items from RFC 4306 that ended up in the IANA registry
were trimmed to only include items that were actually defined in
RFC 4306. Also, many of those lists are now preceded with the very
important instruction to developers that they really should look at
the IANA registry at the time of development because new items have
been added since RFC 4306.

This document adds clarification on when notifications are and are
not sent encrypted, depending on the state of the negotiation at the
time.

This document discusses more about how to negotiate combined-mode
ciphers.

In Section 1.3.2, "The KEi payload SHOULD be included" was changed to
be "The KEi payload MUST be included". This also led to changes in
Section 2.18.

In Section 2.1, there is new material covering how the initiator's
SPI and/or IP is used to differentiate if this is a "half-open" IKE
SA or a new request.

This document clarifies the use of the critical flag in Section 2.5.

In Section 2.8, "Note that, when rekeying, the new Child SA MAY have
different Traffic Selectors and algorithms than the old one" was
changed to "Note that, when rekeying, the new Child SA SHOULD NOT
have different Traffic Selectors and algorithms than the old one".

The new Section 2.8.2 covers simultaneous IKE SA rekeying.

The new Section 2.9.2 covers Traffic Selectors in rekeying.

This document adds the restriction in Section 2.13 that all
pseudorandom functions (PRFs) used with IKEv2 MUST take variable-
sized keys. This should not affect any implementations because there
were no standardized PRFs that have fixed-size keys.

Section 2.18 requires doing a Diffie-Hellman exchange when rekeying
the IKE_SA. In theory, RFC 4306 allowed a policy where the Diffie-
Hellman exchange was optional, but this was not useful (or
appropriate) when rekeying the IKE_SA.

Section 2.21 has been greatly expanded to cover the different cases
where error responses are needed and the appropriate responses to
them.

Section 2.23 clarified that, in NAT traversal, now both UDP-
encapsulated IPsec packets and non-UDP-encapsulated IPsec packets
need to be understood when receiving.

Added Section 2.23.1 to describe NAT traversal when transport mode is
requested.

Added Section 2.25 to explain how to act when there are timing
collisions when deleting and/or rekeying SAs, and two new error
notifications (TEMPORARY_FAILURE and CHILD_SA_NOT_FOUND) were
defined.

In Section 3.6, "Implementations MUST support the HTTP method "http:" scheme for
hash-and-URL lookup. The behavior of other URL methods schemes is not
currently specified, and such methods schemes SHOULD NOT be used in the
absence of a document specifying them" was added.

In Section 3.15.3, a pointer to a new document that is related to
configuration of IPv6 addresses was added.

Appendix C was expanded and clarified.

1.8. Differences between RFC 5996 and This Document

Fixed section 3.6

Clarified in the Abstract and 3.10 as specified the Introduction section that the
status of this document is Internet Standard.

The new Section 2.9.2 covers Traffic Selectors in rekeying.

Added reference to RFC 6989 when reusing Diffie-Hellman exponentials
(Section 2.12).

Added name "Last Substruc" for the RFC5996 errata 2707 Proposal Substructure and
Transform Substructure header (Sections 3.3.1 and 3036. 3.3.2) for the 0
(last) or 2/3 (more) field.

Added reference to RFC 6989 when using groups that are not
Sophie Germain Modular Exponentiation (MODP) groups (Section 3.3.2).

Added reference to RFC 4945 in the Identification Payloads section
(Section 3.5).

Deprecated Raw RSA Public keys. public keys in Section 3.6. There is new work ongoing to replace
that with in
progress adding a more generic format for generic raw public keys.

Added reference to

Fixed Sections 3.6 and 3.10 as specified in the RFC6989 when using non Sophie-Germain Diffie-
Hellman groups, or when reusing Diffie-Hellman Exponentials. errata for RFC 5996
(RFC Errata IDs 2707 and 3036).

Added reference to the RFC4945 a note in the Identification Payloads
section.

Added IANA Considerations section note (Section 6) about
deprecating the Raw RSA Key, and removed the old contents which (which was
already done during
RFC5996 processing. RFC 5996 processing). Added a note that IANA
should update IKEv2
registry all references to RFC 5996 to point to this document instead of RFC5996.

Clarified that the intended status of this document is Internet
Standard both in abstract and Introduction section.

Added name Last Substruc for the Proposal and Transform Substructure
header for the 0 (last) or 2/3 (more) field. document.

2. IKE Protocol Details and Variations

IKE normally listens and sends on UDP port 500, though IKE messages
may also be received on UDP port 4500 with a slightly different
format (see Section 2.23). Since UDP is a datagram (unreliable)
protocol, IKE includes in its definition recovery from transmission
errors, including packet loss, packet replay, and packet forgery.
IKE is designed to function so long as (1) at least one of a series
of retransmitted packets reaches its destination before timing out;
and (2) the channel is not so full of forged and replayed packets so
as to exhaust the network or CPU capacities of either endpoint. Even
in the absence of those minimum performance requirements, IKE is
designed to fail cleanly (as though the network were broken).

Although IKEv2 messages are intended to be short, they contain
structures with no hard upper bound on size (in particular, digital
certificates), and IKEv2 itself does not have a mechanism for
fragmenting large messages. IP defines a mechanism for fragmentation
of oversized UDP messages, but implementations vary in the maximum
message size supported. Furthermore, use of IP fragmentation opens
an implementation to denial-of-service (DoS) attacks [DOSUDPPROT].
Finally, some NAT and/or firewall implementations may block IP
fragments.

All IKEv2 implementations MUST be able to send, receive, and process
IKE messages that are up to 1280 octets long, and they SHOULD be able
to send, receive, and process messages that are up to 3000 octets
long. IKEv2 implementations need to be aware of the maximum UDP
message size supported and MAY shorten messages by leaving out some
certificates or cryptographic suite proposals if that will keep
messages below the maximum. Use of the "Hash and URL" formats rather
than including certificates in exchanges where possible can avoid
most problems. Implementations and configuration need to keep in
mind, however, that if the URL lookups are possible only after the
Child SA is established, recursion issues could prevent this
technique from working.

The UDP payload of all packets containing IKE messages sent on
port 4500 MUST begin with the prefix of four zeros; otherwise, the
receiver won't know how to handle them.

2.1. Use of Retransmission Timers

All messages in IKE exist in pairs: a request and a response. The
setup of an IKE SA normally consists of two exchanges. Once the IKE
SA is set up, either end of the Security Association may initiate
requests at any time, and there can be many requests and responses
"in flight" at any given moment. But each message is labeled as
either a request or a response, and for each exchange, one end of the
Security Association is the initiator and the other is the responder.

For every pair of IKE messages, the initiator is responsible for
retransmission in the event of a timeout. The responder MUST never
retransmit a response unless it receives a retransmission of the
request. In that event, the responder MUST ignore the retransmitted
request except insofar as it causes a retransmission of the response.
The initiator MUST remember each request until it receives the
corresponding response. The responder MUST remember each response
until it receives a request whose sequence number is larger than or
equal to the sequence number in the response plus its window size
(see Section 2.3). In order to allow saving memory, responders are
allowed to forget the response after a timeout of several minutes.
If the responder receives a retransmitted request for which it has
already forgotten the response, it MUST ignore the request (and not,
for example, attempt constructing a new response).

IKE is a reliable protocol: the initiator MUST retransmit a request
until it either receives a corresponding response or deems the IKE SA
to have failed. In the latter case, the initiator discards all state
associated with the IKE SA and any Child SAs that were negotiated
using that IKE SA. A retransmission from the initiator MUST be
bitwise identical to the original request. That is, everything
starting from the IKE header (the IKE SA initiator's SPI onwards)
must be bitwise identical; items before it (such as the IP and UDP
headers) do not have to be identical.

Retransmissions of the IKE_SA_INIT request require some special
handling. When a responder receives an IKE_SA_INIT request, it has
to determine whether the packet is a retransmission belonging to an
existing "half-open" IKE SA (in which case the responder retransmits
the same response), or a new request (in which case the responder
creates a new IKE SA and sends a fresh response), or it belongs to an
existing IKE SA where the IKE_AUTH request has been already received
(in which case the responder ignores it).

It is not sufficient to use the initiator's SPI and/or IP address to
differentiate between these three cases because two different peers
behind a single NAT could choose the same initiator SPI. Instead, a
robust responder will do the IKE SA lookup using the whole packet,
its hash, or the Ni payload.

The retransmission policy for one-way messages is somewhat different
from that for regular messages. Because no acknowledgement is ever
sent, there is no reason to gratuitously retransmit one-way messages.
Given that all these messages are errors, it makes sense to send them
only once per "offending" packet, and only retransmit if further
offending packets are received. Still, it also makes sense to limit
retransmissions of such error messages.

2.2. Use of Sequence Numbers for Message ID

Every IKE message contains a Message ID as part of its fixed header.
This Message ID is used to match up requests and responses and to
identify retransmissions of messages. Retransmission of a message
MUST use the same Message ID as the original message.

The Message ID is a 32-bit quantity, which is zero for the
IKE_SA_INIT messages (including retries of the message due to
responses such as COOKIE and INVALID_KE_PAYLOAD), and incremented for
each subsequent exchange. Thus, the first pair of IKE_AUTH messages
will have an ID of 1, the second (when EAP is used) will be 2, and so
on. The Message ID is reset to zero in the new IKE SA after the IKE
SA is rekeyed.

Each endpoint in the IKE Security Association maintains two "current"
Message IDs: the next one to be used for a request it initiates and
the next one it expects to see in a request from the other end.
These counters increment as requests are generated and received.
Responses always contain the same Message ID as the corresponding
request. That means that after the initial exchange, each integer n
may appear as the Message ID in four distinct messages: the nth
request from the original IKE initiator, the corresponding response,
the nth request from the original IKE responder, and the
corresponding response. If the two ends make a very different number
of requests, the Message IDs in the two directions can be very
different. There is no ambiguity in the messages, however, because
the Initiator and Response flags in the message header specify which
of the four messages a particular one is.

Throughout this document, "initiator" refers to the party who
initiated the exchange being described. The "original initiator"
always refers to the party who initiated the exchange that resulted
in the current IKE SA. In other words, if the "original responder"
starts rekeying the IKE SA, that party becomes the "original
initiator" of the new IKE SA.

Note that Message IDs are cryptographically protected and provide
protection against message replays. In the unlikely event that
Message IDs grow too large to fit in 32 bits, the IKE SA MUST be
closed or rekeyed.

2.3. Window Size for Overlapping Requests

The SET_WINDOW_SIZE notification asserts that the sending endpoint is
capable of keeping state for multiple outstanding exchanges,
permitting the recipient to send multiple requests before getting a
response to the first. The data associated with a SET_WINDOW_SIZE
notification MUST be 4 octets long and contain the big endian
representation of the number of messages the sender promises to keep.
The window size is always one until the initial exchanges complete.

An IKE endpoint MUST wait for a response to each of its messages
before sending a subsequent message unless it has received a
SET_WINDOW_SIZE Notify message from its peer informing it that the
peer is prepared to maintain state for multiple outstanding messages
in order to allow greater throughput.

After an IKE SA is set up, in order to maximize IKE throughput, an
IKE endpoint MAY issue multiple requests before getting a response to
any of them, up to the limit set by its peer's SET_WINDOW_SIZE.
These requests may pass one another over the network. An IKE
endpoint MUST be prepared to accept and process a request while it
has a request outstanding in order to avoid a deadlock in this
situation. An IKE endpoint may also accept and process multiple
requests while it has a request outstanding.

An IKE endpoint MUST NOT exceed the peer's stated window size for
transmitted IKE requests. In other words, if the responder stated
its window size is N, then when the initiator needs to make a request
X, it MUST wait until it has received responses to all requests up
through request X-N. An IKE endpoint MUST keep a copy of (or be able
to regenerate exactly) each request it has sent until it receives the
corresponding response. An IKE endpoint MUST keep a copy of (or be
able to regenerate exactly) the number of previous responses equal to
its declared window size in case its response was lost and the
initiator requests its retransmission by retransmitting the request.

An IKE endpoint supporting a window size greater than one ought to be
capable of processing incoming requests out of order to maximize
performance in the event of network failures or packet reordering.

The window size is normally a (possibly configurable) property of a
particular implementation, and is not related to congestion control
(unlike the window size in TCP, for example). In particular, what
the responder should do when it receives a SET_WINDOW_SIZE
notification containing a smaller value than is currently in effect
is not defined. Thus, there is currently no way to reduce the window
size of an existing IKE SA; you can only increase it. When rekeying
an IKE SA, the new IKE SA starts with window size 1 until it is
explicitly increased by sending a new SET_WINDOW_SIZE notification.

The INVALID_MESSAGE_ID notification is sent when an IKE Message ID
outside the supported window is received. This Notify message
MUST NOT be sent in a response; the invalid request MUST NOT be
acknowledged. Instead, inform the other side by initiating an
INFORMATIONAL exchange with Notification data Data containing the four-
octet
four-octet invalid Message ID. Sending this notification is
OPTIONAL, and notifications of this type MUST be rate limited.

2.4. State Synchronization and Connection Timeouts

An IKE endpoint is allowed to forget all of its state associated with
an IKE SA and the collection of corresponding Child SAs at any time.
This is the anticipated behavior in the event of an endpoint crash
and restart. It is important when an endpoint either fails or
reinitializes its state that the other endpoint detect those
conditions and not continue to waste network bandwidth by sending
packets over discarded SAs and having them fall into a black hole.

The INITIAL_CONTACT notification asserts that this IKE SA is the only
IKE SA currently active between the authenticated identities. It MAY
be sent when an IKE SA is established after a crash, and the
recipient MAY use this information to delete any other IKE SAs it has
to the same authenticated identity without waiting for a timeout.
This notification MUST NOT be sent by an entity that may be
replicated (e.g., a roaming user's credentials where the user is
allowed to connect to the corporate firewall from two remote systems
at the same time). The INITIAL_CONTACT notification, if sent, MUST
be in the first IKE_AUTH request or response, not as a separate
exchange afterwards; receiving parties MAY ignore it in other
messages.

Since IKE is designed to operate in spite of DoS attacks from the
network, an endpoint MUST NOT conclude that the other endpoint has
failed based on any routing information (e.g., ICMP messages) or IKE
messages that arrive without cryptographic protection (e.g., Notify
messages complaining about unknown SPIs). An endpoint MUST conclude
that the other endpoint has failed only when repeated attempts to
contact it have gone unanswered for a timeout period or when a
cryptographically protected INITIAL_CONTACT notification is received
on a different IKE SA to the same authenticated identity. An
endpoint should suspect that the other endpoint has failed based on
routing information and initiate a request to see whether the other
endpoint is alive. To check whether the other side is alive, IKE
specifies an empty INFORMATIONAL message request that (like all IKE requests)
requires an acknowledgement (note that within the context of an IKE
SA, an "empty" message consists of an IKE header followed by an
Encrypted payload that contains no payloads). If a cryptographically
protected (fresh, i.e., not retransmitted) message has been received
from the other side recently, unprotected Notify messages MAY be
ignored. Implementations MUST limit the rate at which they take
actions based on unprotected messages.

The number of retries and length of timeouts are not covered in this
specification because they do not affect interoperability. It is
suggested that messages be retransmitted at least a dozen times over
a period of at least several minutes before giving up on an SA, but
different environments may require different rules. To be a good
network citizen, retransmission times MUST increase exponentially to
avoid flooding the network and making an existing congestion
situation worse. If there has only been outgoing traffic on all of
the SAs associated with an IKE SA, it is essential to confirm
liveness of the other endpoint to avoid black holes. If no
cryptographically protected messages have been received on an IKE SA
or any of its Child SAs recently, the system needs to perform a
liveness check in order to prevent sending messages to a dead peer.
(This is sometimes called "dead peer detection" or "DPD", although it
is really detecting live peers, not dead ones.) Receipt of a fresh
cryptographically protected message on an IKE SA or any of its Child
SAs ensures liveness of the IKE SA and all of its Child SAs. Note
that this places requirements on the failure modes of an IKE
endpoint. An implementation needs to stop sending over any SA if
some failure prevents it from receiving on all of the associated SAs.
If a system creates Child SAs that can fail independently from one
another without the associated IKE SA being able to send a delete
message, then the system MUST negotiate such Child SAs using separate
IKE SAs.

There is a

One type of DoS attack on the initiator of an IKE SA that can be avoided
if the initiator takes the proper care. Since care: since the first two messages of
an SA setup are not cryptographically protected, an attacker could
respond to the initiator's message before the genuine responder and
poison the connection setup attempt. To prevent this, the initiator
MAY be willing to accept multiple responses to its first message,
treat each response as potentially legitimate, respond to it, each one,
and then discard all the invalid half-open connections when it
receives a valid cryptographically protected response to any one of
its requests. Once a cryptographically valid response is received,
all subsequent responses should be ignored whether or not they are
cryptographically valid.

Note that with these rules, there is no reason to negotiate and agree
upon an SA lifetime. If IKE presumes the partner is dead, based on
repeated lack of acknowledgement to an IKE message, then the IKE SA
and all Child SAs set up through that IKE SA are deleted.

An IKE endpoint may at any time delete inactive Child SAs to recover
resources used to hold their state. If an IKE endpoint chooses to
delete Child SAs, it MUST send Delete payloads to the other end
notifying it of the deletion. It MAY similarly time out the IKE SA.
Closing the IKE SA implicitly closes all associated Child SAs. In
this case, an IKE endpoint SHOULD send a Delete payload indicating
that it has closed the IKE SA unless the other endpoint is no longer
responding.

2.5. Version Numbers and Forward Compatibility

This document describes version 2.0 of IKE, meaning the major version
number is 2 and the minor version number is 0. This document is a
replacement for [IKEV2]. It is likely that some implementations will
want to support version 1.0 and version 2.0, and in the future, other
versions.

The major version number should be incremented only if the packet
formats or required actions have changed so dramatically that an
older version node would not be able to interoperate with a newer
version node if it simply ignored the fields it did not understand
and took the actions specified in the older specification. The minor
version number indicates new capabilities, and MUST be ignored by a
node with a smaller minor version number, but used for informational
purposes by the node with the larger minor version number. For
example, it might indicate the ability to process a newly defined
Notify message type. The node with the larger minor version number
would simply note that its correspondent would not be able to
understand that message and therefore would not send it.

If an endpoint receives a message with a higher major version number,
it MUST drop the message and SHOULD send an unauthenticated Notify
message of type INVALID_MAJOR_VERSION containing the highest
(closest) version number it supports. If an endpoint supports major
version n, and major version m, it MUST support all versions between
n and m. If it receives a message with a major version that it
supports, it MUST respond with that version number. In order to
prevent two nodes from being tricked into corresponding with a lower
major version number than the maximum that they both support, IKE has
a flag that indicates that the node is capable of speaking a higher
major version number.

Thus, the major version number in the IKE header indicates the
version number of the message, not the highest version number that
the transmitter supports. If the initiator is capable of speaking
versions n, n+1, and n+2, and the responder is capable of speaking
versions n and n+1, then they will negotiate speaking n+1, where the
initiator will set a flag indicating its ability to speak a higher
version. If they mistakenly (perhaps through an active attacker
sending error messages) negotiate to version n, then both will notice
that the other side can support a higher version number, and they
MUST break the connection and reconnect using version n+1.

Note that IKEv1 does not follow these rules, because there is no way
in v1 of noting that you are capable of speaking a higher version
number. So an active attacker can trick two v2-capable nodes into
speaking v1. When a v2-capable node negotiates down to v1, it should
note that fact in its logs.

Also, for forward compatibility, all fields marked RESERVED MUST be
set to zero by an implementation running version 2.0, and their
content MUST be ignored by an implementation running version 2.0 ("Be
conservative in what you send and liberal in what you receive" [IP]).
In this way, future versions of the protocol can use those fields in
a way that is guaranteed to be ignored by implementations that do not
understand them. Similarly, payload types that are not defined are
reserved for future use; implementations of a version where they are
undefined MUST skip over those payloads and ignore their contents.

IKEv2 adds a "critical" flag to each payload header for further
flexibility for forward compatibility. If the critical flag is set
and the payload type is unrecognized, the message MUST be rejected
and the response to the IKE request containing that payload MUST
include a Notify payload UNSUPPORTED_CRITICAL_PAYLOAD, indicating an
unsupported critical payload was included. In that Notify payload,
the notification data Notification Data contains the one-octet payload type. If the
critical flag is not set and the payload type is unsupported, that
payload MUST be ignored. Payloads sent in IKE response messages
MUST NOT have the critical flag set. Note that the critical flag
applies only to the payload type, not the contents. If the payload
type is recognized, but the payload contains something that is not
(such as an unknown transform inside an SA payload, or an unknown
Notify Message Type inside a Notify payload), the critical flag is
ignored.

Although new payload types may be added in the future and may appear
interleaved with the fields defined in this specification,
implementations SHOULD send the payloads defined in this
specification in the order shown in the figures in Sections 1 and 2;
implementations MUST NOT reject as invalid a message with those
payloads in any other order.

2.6. IKE SA SPIs and Cookies

The initial two eight-octet fields in the header, called the "IKE
SPIs", are used as a connection identifier at the beginning of IKE
packets. Each endpoint chooses one of the two SPIs and MUST choose
them so as to be unique identifiers of an IKE SA. An SPI value of
zero is special: it indicates that the remote SPI value is not yet
known by the sender.

Incoming IKE packets are mapped to an IKE SA only using the packet's
SPI, not using (for example) the source IP address of the packet.

Unlike ESP and AH where only the recipient's SPI appears in the
header of a message, in IKE the sender's SPI is also sent in every
message. Since the SPI chosen by the original initiator of the IKE
SA is always sent first, an endpoint with multiple IKE SAs open that
wants to find the appropriate IKE SA using the SPI it assigned must
look at the Initiator flag in the header to determine whether it
assigned the first or the second eight octets.

In the first message of an initial IKE exchange, the initiator will
not know the responder's SPI value and will therefore set that field
to zero. When the IKE_SA_INIT exchange does not result in the
creation of an IKE SA due to INVALID_KE_PAYLOAD, NO_PROPOSAL_CHOSEN,
or COOKIE, the responder's SPI will be zero also in the response
message. However, if the responder sends a non-zero responder SPI,
the initiator should not reject the response for only that reason.

Two expected attacks against IKE are state and CPU exhaustion, where
the target is flooded with session initiation requests from forged IP
addresses. These attacks can be made less effective if a responder
uses minimal CPU and commits no state to an SA until it knows the
initiator can receive packets at the address from which it claims to
be sending them.

When a responder detects a large number of half-open IKE SAs, it
SHOULD reply to IKE_SA_INIT requests with a response containing the
COOKIE notification. The data associated with this notification MUST
be between 1 and 64 octets in length (inclusive), and its generation
is described later in this section. If the IKE_SA_INIT response
includes the COOKIE notification, the initiator MUST then retry the
IKE_SA_INIT request, and include the COOKIE notification containing
the received data as the first payload, and all other payloads
unchanged. The initial exchange will then be as follows:

Initiator Responder
-------------------------------------------------------------------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE)
HDR(A,0), N(COOKIE), SAi1,
KEi, Ni -->
<-- HDR(A,B), SAr1, KEr,
Nr, [CERTREQ]
HDR(A,B), SK {IDi, [CERT,]
[CERTREQ,] [IDr,] AUTH,
SAi2, TSi, TSr} -->
<-- HDR(A,B), SK {IDr, [CERT,]
AUTH, SAr2, TSi, TSr}

The first two messages do not affect any initiator or responder state
except for communicating the cookie. In particular, the message
sequence numbers in the first four messages will all be zero and the
message sequence numbers in the last two messages will be one. 'A'
is the SPI assigned by the initiator, while 'B' is the SPI assigned
by the responder.

An IKE implementation can implement its responder cookie generation
in such a way as to not require any saved state to recognize its
valid cookie when the second IKE_SA_INIT message arrives. The exact
algorithms and syntax used to generate cookies do not affect
interoperability and hence are not specified here. The following is
an example of how an endpoint could use cookies to implement limited
DoS protection.

A good way to do this is to set the responder cookie to be:

Cookie = <VersionIDofSecret> | Hash(Ni | IPi | SPIi | <secret>)

where <secret> is a randomly generated secret known only to the
responder and periodically changed and | indicates concatenation.
<VersionIDofSecret> should be changed whenever <secret> is
regenerated. The cookie can be recomputed when the IKE_SA_INIT
arrives the second time and compared to the cookie in the received
message. If it matches, the responder knows that the cookie was
generated since the last change to <secret> and that IPi must be the
same as the source address it saw the first time. Incorporating SPIi
into the calculation ensures that if multiple IKE SAs are being set
up in parallel they will all get different cookies (assuming the
initiator chooses unique SPIi's). Incorporating Ni in the hash
ensures that an attacker who sees only message 2 can't successfully
forge a message 3. Also, incorporating SPIi in the hash prevents an
attacker from fetching one cookie from the other end, and then
initiating many IKE_SA_INIT exchanges all with different initiator
SPIs (and perhaps port numbers) so that the responder thinks that
there are a lot of machines behind one NAT box that are all trying to
connect.

If a new value for <secret> is chosen while there are connections in
the process of being initialized, an IKE_SA_INIT might be returned
with other than the current <VersionIDofSecret>. The responder in
that case MAY reject the message by sending another response with a
new cookie or it MAY keep the old value of <secret> around for a
short time and accept cookies computed from either one. The
responder should not accept cookies indefinitely after <secret> is
changed, since that would defeat part of the DoS protection. The
responder should change the value of <secret> frequently, especially
if under attack.

When one party receives an IKE_SA_INIT request containing a cookie
whose contents do not match the value expected, that party MUST
ignore the cookie and process the message as if no cookie had been
included; usually this means sending a response containing a new
cookie. The initiator should limit the number of cookie exchanges it
tries before giving up, possibly using exponential back-off. An
attacker can forge multiple cookie responses to the initiator's
IKE_SA_INIT message, and each of those forged cookie replies will
cause two packets to be sent: one packet from the initiator to the
responder (which will reject those cookies), and one response from
responder to initiator that includes the correct cookie.

A note on terminology: the term "cookies" originates with Karn and
Simpson [PHOTURIS] in Photuris, an early proposal for key management
with IPsec, and it has persisted. The Internet Security Association
and Key Management Protocol (ISAKMP) [ISAKMP] fixed message header
includes two eight-octet fields called "cookies", and that syntax is
used by both IKEv1 and IKEv2, although in IKEv2 they are referred to
as the "IKE SPI" and there is a new separate field in a Notify
payload holding the cookie.

2.6.1. Interaction of COOKIE and INVALID_KE_PAYLOAD

There are two common reasons why the initiator may have to retry the
IKE_SA_INIT exchange: the responder requests a cookie or wants a
different Diffie-Hellman group than was included in the KEi payload.
If the initiator receives a cookie from the responder, the initiator
needs to decide whether or not to include the cookie in only the next
retry of the IKE_SA_INIT request, or in all subsequent retries as
well.

If the initiator includes the cookie only in the next retry, one
additional round trip may be needed in some cases. An additional
round trip is needed also if the initiator includes the cookie in all
retries, but the responder does not support this. For instance, if
the responder includes the KEi payloads in cookie calculation, it
will reject the request by sending a new cookie.

If both peers support including the cookie in all retries, a slightly
shorter exchange can happen.

Initiator Responder
-----------------------------------------------------------
HDR(A,0), SAi1, KEi, Ni -->
<-- HDR(A,0), N(COOKIE)
HDR(A,0), N(COOKIE), SAi1, KEi, Ni -->
<-- HDR(A,0), N(INVALID_KE_PAYLOAD)
HDR(A,0), N(COOKIE), SAi1, KEi', Ni -->
<-- HDR(A,B), SAr1, KEr, Nr

Implementations SHOULD support this shorter exchange, but MUST NOT
fail if other implementations do not support this shorter exchange.

2.7. Cryptographic Algorithm Negotiation

The payload type known as "SA" indicates a proposal for a set of
choices of IPsec protocols (IKE, ESP, or AH) for the SA as well as
cryptographic algorithms associated with each protocol.

An SA payload consists of one or more proposals. Each proposal
includes one protocol. Each protocol contains one or more transforms
-- each specifying a cryptographic algorithm. Each transform
contains zero or more attributes (attributes are needed only if the
Transform ID does not completely specify the cryptographic
algorithm).

This hierarchical structure was designed to efficiently encode
proposals for cryptographic suites when the number of supported
suites is large because multiple values are acceptable for multiple
transforms. The responder MUST choose a single suite, which may be
any subset of the SA proposal following the rules below.

Each proposal contains one protocol. If a proposal is accepted, the
SA response MUST contain the same protocol. The responder MUST
accept a single proposal or reject them all and return an error. The
error is given in a notification of type NO_PROPOSAL_CHOSEN.

Each IPsec protocol proposal contains one or more transforms. Each
transform contains a Transform Type. The accepted cryptographic
suite MUST contain exactly one transform of each type included in the
proposal. For example: if an ESP proposal includes transforms
ENCR_3DES, ENCR_AES w/keysize 128, ENCR_AES w/keysize 256,
AUTH_HMAC_MD5, and AUTH_HMAC_SHA, the accepted suite MUST contain one
of the ENCR_ transforms and one of the AUTH_ transforms. Thus, six
combinations are acceptable.

If an initiator proposes both normal ciphers with integrity
protection as well as combined-mode ciphers, then two proposals are
needed. One of the proposals includes the normal ciphers with the
integrity algorithms for them, and the other proposal includes all
the combined-mode ciphers without the integrity algorithms (because
combined-mode ciphers are not allowed to have any integrity algorithm
other than "none"). "NONE").

2.8. Rekeying

IKE, ESP, and AH Security Associations use secret keys that should be
used only for a limited amount of time and to protect a limited
amount of data. This limits the lifetime of the entire Security
Association. When the lifetime of a Security Association expires,
the Security Association MUST NOT be used. If there is demand, new
Security Associations MAY be established. Reestablishment of
Security Associations to take the place of ones that expire is
referred to as "rekeying".

To allow for minimal IPsec implementations, the ability to rekey SAs
without restarting the entire IKE SA is optional. An implementation
MAY refuse all CREATE_CHILD_SA requests within an IKE SA. If an SA
has expired or is about to expire and rekeying attempts using the
mechanisms described here fail, an implementation MUST close the IKE
SA and any associated Child SAs and then MAY start new ones.
Implementations may wish to support in-place rekeying of SAs, since
doing so offers better performance and is likely to reduce the number
of packets lost during the transition.

To rekey a Child SA within an existing IKE SA, create a new,
equivalent SA (see Section 2.17 below), and when the new one is
established, delete the old one. Note that, when rekeying, the new
Child SA SHOULD NOT have different Traffic Selectors and algorithms
than the old one.

To rekey an IKE SA, establish a new equivalent IKE SA (see
Section 2.18 below) with the peer to whom the old IKE SA is shared
using a CREATE_CHILD_SA within the existing IKE SA. An IKE SA so
created inherits all of the original IKE SA's Child SAs, and the new
IKE SA is used for all control messages needed to maintain those
Child SAs. After the new equivalent IKE SA is created, the initiator
deletes the old IKE SA, and the Delete payload to delete itself MUST
be the last request sent over the old IKE SA.

SAs should be rekeyed proactively, i.e., the new SA should be
established before the old one expires and becomes unusable. Enough
time should elapse between the time the new SA is established and the
old one becomes unusable so that traffic can be switched over to the
new SA.

A difference between IKEv1 and IKEv2 is that in IKEv1 SA lifetimes
were negotiated. In IKEv2, each end of the SA is responsible for
enforcing its own lifetime policy on the SA and rekeying the SA when
necessary. If the two ends have different lifetime policies, the end
with the shorter lifetime will end up always being the one to request
the rekeying. If an SA has been inactive for a long time and if an
endpoint would not initiate the SA in the absence of traffic, the
endpoint MAY choose to close the SA instead of rekeying it when its
lifetime expires. It can also do so if there has been no traffic
since the last time the SA was rekeyed.

Note that IKEv2 deliberately allows parallel SAs with the same
Traffic Selectors between common endpoints. One of the purposes of
this is to support traffic quality of service (QoS) differences among
the SAs (see [DIFFSERVFIELD], [DIFFSERVARCH], and Section 4.1 of
[DIFFTUNNEL]). Hence unlike IKEv1, the combination of the endpoints
and the Traffic Selectors may not uniquely identify an SA between
those endpoints, so the IKEv1 rekeying heuristic of deleting SAs on
the basis of duplicate Traffic Selectors SHOULD NOT be used.

There are timing windows -- particularly in the presence of lost
packets -- where endpoints may not agree on the state of an SA. The
responder to a CREATE_CHILD_SA MUST be prepared to accept messages on
an SA before sending its response to the creation request, so there
is no ambiguity for the initiator. The initiator MAY begin sending
on an SA as soon as it processes the response. The initiator,
however, cannot receive on a newly created SA until it receives and
processes the response to its CREATE_CHILD_SA request. How, then, is
the responder to know when it is OK to send on the newly created SA?

From a technical correctness and interoperability perspective, the
responder MAY begin sending on an SA as soon as it sends its response
to the CREATE_CHILD_SA request. In some situations, however, this
could result in packets unnecessarily being dropped, so an
implementation MAY defer such sending.

The responder can be assured that the initiator is prepared to
receive messages on an SA if either (1) it has received a
cryptographically valid message on the other half of the SA pair, or
(2) the new SA rekeys an existing SA and it receives an IKE request
to close the replaced SA. When rekeying an SA, the responder
continues to send traffic on the old SA until one of those events
occurs. When establishing a new SA, the responder MAY defer sending
messages on a new SA until either it receives one or a timeout has
occurred. If an initiator receives a message on an SA for which it
has not received a response to its CREATE_CHILD_SA request, it
interprets that as a likely packet loss and retransmits the
CREATE_CHILD_SA request. An initiator MAY send a dummy ESP message
on a newly created ESP SA if it has no messages queued in order to
assure the responder that the initiator is ready to receive messages.

2.8.1. Simultaneous Child SA Rekeying

If the two ends have the same lifetime policies, it is possible that
both will initiate a rekeying at the same time (which will result in
redundant SAs). To reduce the probability of this happening, the
timing of rekeying requests SHOULD be jittered (delayed by a random
amount of time after the need for rekeying is noticed).

This form of rekeying may temporarily result in multiple similar SAs
between the same pairs of nodes. When there are two SAs eligible to
receive packets, a node MUST accept incoming packets through either
SA. If redundant SAs are created though through such a collision, the SA
created with the lowest of the four nonces used in the two exchanges
SHOULD be closed by the endpoint that created it. "Lowest" means an
octet-by-octet comparison (instead of, for instance, comparing the
nonces as large integers). In other words, start by comparing the
first octet; if they're equal, move to the next octet, and so on. If
you reach the end of one nonce, that nonce is the lower one. The
node that initiated the surviving rekeyed SA should delete the
replaced SA after the new one is established.

The following is an explanation on the impact this has on
implementations. Assume that hosts A and B have an existing Child SA
pair with SPIs (SPIa1,SPIb1), and both start rekeying it at the same
time:

Host A Host B
-------------------------------------------------------------------
send req1: N(REKEY_SA,SPIa1),
SA(..,SPIa2,..),Ni1,.. -->
<-- send req2: N(REKEY_SA,SPIb1),
SA(..,SPIb2,..),Ni2
recv req2 <--

At this point, A knows there is a simultaneous rekeying happening.
However, it cannot yet know which of the exchanges will have the
lowest nonce, so it will just note the situation and respond as
usual.

send resp2: SA(..,SPIa3,..),
Nr1,.. -->
--> recv req1

Now B also knows that simultaneous rekeying is going on. It responds
as usual.

<-- send resp1: SA(..,SPIb3,..),
Nr2,..
recv resp1 <--
--> recv resp2

At this point, there are three Child SA pairs between A and B (the
old one and two new ones). A and B can now compare the nonces.
Suppose that the lowest nonce was Nr1 in message resp2; in this case,
B (the sender of req2) deletes the redundant new SA, and A (the node
that initiated the surviving rekeyed SA), deletes the old one.

send req3: D(SPIa1) -->
<-- send req4: D(SPIb2)
--> recv req3
<-- send resp3: D(SPIb1)
recv req4 <--
send resp4: D(SPIa3) -->

The rekeying is now finished.

However, there is a second possible sequence of events that can
happen if some packets are lost in the network, resulting in
retransmissions. The rekeying begins as usual, but A's first packet
(req1) is lost.

Host A Host B
-------------------------------------------------------------------
send req1: N(REKEY_SA,SPIa1),
SA(..,SPIa2,..),
Ni1,.. --> (lost)
<-- send req2: N(REKEY_SA,SPIb1),
SA(..,SPIb2,..),Ni2
recv req2 <--
send resp2: SA(..,SPIa3,..),
Nr1,.. -->
--> recv resp2
<-- send req3: D(SPIb1)
recv req3 <--
send resp3: D(SPIa1) -->
--> recv resp3

From B's point of view, the rekeying is now completed, and since it
has not yet received A's req1, it does not even know that there was
simultaneous rekeying. However, A will continue retransmitting the
message, and eventually it will reach B.

resend req1 -->
--> recv req1

To B, it looks like A is trying to rekey an SA that no longer exists;
thus, B responds to the request with something non-fatal such as
CHILD_SA_NOT_FOUND.

<-- send resp1: N(CHILD_SA_NOT_FOUND)
recv resp1 <--

When A receives this error, it already knows there was simultaneous
rekeying, so it can ignore the error message.

2.8.2. Simultaneous IKE SA Rekeying

Probably the most complex case occurs when both peers try to rekey
the IKE_SA at the same time. Basically, the text in Section 2.8
applies to this case as well; however, it is important to ensure that
the Child SAs are inherited by the correct IKE_SA.

The case where both endpoints notice the simultaneous rekeying works
the same way as with Child SAs. After the CREATE_CHILD_SA exchanges,
three IKE SAs exist between A and B: the old IKE SA and two new IKE
SAs. The new IKE SA containing the lowest nonce SHOULD be deleted by
the node that created it, and the other surviving new IKE SA MUST
inherit all the Child SAs.

In addition to normal simultaneous rekeying cases, there is a special
case where one peer finishes its rekey before it even notices that
other peer is doing a rekey. If only one peer detects a simultaneous
rekey, redundant SAs are not created. In this case, when the peer
that did not notice the simultaneous rekey gets the request to rekey
the IKE SA that it has already successfully rekeyed, it SHOULD return
TEMPORARY_FAILURE because it is an IKE SA that it is currently trying
to close (whether or not it has already sent the delete notification
for the SA). If the peer that did notice the simultaneous rekey gets
the delete request from the other peer for the old IKE SA, it knows
that the other peer did not detect the simultaneous rekey, and the
first peer can forget its own rekey attempt.

Host A Host B
-------------------------------------------------------------------
send req1:
SA(..,SPIa1,..),Ni1,.. -->
<-- send req2: SA(..,SPIb1,..),Ni2,..
--> recv req1
<-- send resp1: SA(..,SPIb2,..),Nr2,..
recv resp1 <--
send req3: D() -->
--> recv req3

At this point, host B sees a request to close the IKE_SA. There's
not much more to do than to reply as usual. However, at this point
host B should stop retransmitting req2, since once host A receives
resp3, it will delete all the state associated with the old IKE_SA
and will not be able to reply to it.

<-- send resp3: ()

The TEMPORARY_FAILURE notification was not included in RFC 4306, and
support of the TEMPORARY_FAILURE notification is not negotiated.
Thus, older peers that implement RFC 4306 but not this document may
receive these notifications. In that case, they will treat it the
same as any other unknown error notification, and will stop the
exchange. Because the other peer has already rekeyed the exchange,
doing so does not have any ill effects.

2.8.3. Rekeying the IKE SA versus Reauthentication

Rekeying the IKE SA and reauthentication are different concepts in
IKEv2. Rekeying the IKE SA establishes new keys for the IKE SA and
resets the Message ID counters, but it does not authenticate the
parties again (no AUTH or EAP payloads are involved).

Although rekeying the IKE SA may be important in some environments,
reauthentication (the verification that the parties still have access
to the long-term credentials) is often more important.

IKEv2 does not have any special support for reauthentication.
Reauthentication is done by creating a new IKE SA from scratch (using
IKE_SA_INIT/IKE_AUTH exchanges, without any REKEY_SA Notify
payloads), creating new Child SAs within the new IKE SA (without
REKEY_SA Notify payloads), and finally deleting the old IKE SA (which
deletes the old Child SAs as well).

This means that reauthentication also establishes new keys for the
IKE SA and Child SAs. Therefore, while rekeying can be performed
more often than reauthentication, the situation where "authentication
lifetime" is shorter than "key lifetime" does not make sense.

While creation of a new IKE SA can be initiated by either party
(initiator or responder in the original IKE SA), the use of EAP
and/or and/
or Configuration payloads means in practice that reauthentication has
to be initiated by the same party as the original IKE SA. IKEv2 does
not currently allow the responder to request reauthentication in this
case; however, there are extensions that add this functionality such
as [REAUTH].

2.9. Traffic Selector Negotiation

When an RFC4301-compliant IPsec subsystem receives an IP packet that
matches a "protect" selector in its Security Policy Database (SPD),
the subsystem protects that packet with IPsec. When no SA exists
yet, it is the task of IKE to create it. Maintenance of a system's
SPD is outside the scope of IKE, although some implementations might
update their SPD in connection with the running of IKE (for an
example scenario, see Section 1.1.3).

Traffic Selector (TS) payloads allow endpoints to communicate some of
the information from their SPD to their peers. These must be
communicated to IKE from the SPD (for example, the PF_KEY API [PFKEY]
uses the SADB_ACQUIRE message). TS payloads specify the selection
criteria for packets that will be forwarded over the newly set up SA.

This can serve as a consistency check in some scenarios to assure
that the SPDs are consistent. In others, it guides the dynamic
update of the SPD.

Two TS payloads appear in each of the messages in the exchange that
creates a Child SA pair. Each TS payload contains one or more
Traffic Selectors. Each Traffic Selector consists of an address
range (IPv4 or IPv6), a port range, and an IP protocol ID.

The first of the two TS payloads is known as TSi (Traffic Selector-
initiator). The second is known as TSr (Traffic Selector-responder).
TSi specifies the source address of traffic forwarded from (or the
destination address of traffic forwarded to) the initiator of the
Child SA pair. TSr specifies the destination address of the traffic
forwarded to (or the source address of the traffic forwarded from)
the responder of the Child SA pair. For example, if the original
initiator requests the creation of a Child SA pair, and wishes to
tunnel all traffic from subnet 198.51.100.* on the initiator's side
to subnet 192.0.2.* on the responder's side, the initiator would
include a single Traffic Selector in each TS payload. TSi would
specify the address range (198.51.100.0 - 198.51.100.255) and TSr
would specify the address range (192.0.2.0 - 192.0.2.255). Assuming
that proposal was acceptable to the responder, it would send
identical TS payloads back.

IKEv2 allows the responder to choose a subset of the traffic proposed
by the initiator. This could happen when the configurations of the
two endpoints are being updated but only one end has received the new
information. Since the two endpoints may be configured by different
people, the incompatibility may persist for an extended period even
in the absence of errors. It also allows for intentionally different
configurations, as when one end is configured to tunnel all addresses
and depends on the other end to have the up-to-date list.

When the responder chooses a subset of the traffic proposed by the
initiator, it narrows the Traffic Selectors to some subset of the
initiator's proposal (provided the set does not become the null set).
If the type of Traffic Selector proposed is unknown, the responder
ignores that Traffic Selector, so that the unknown type is not
returned in the narrowed set.

To enable the responder to choose the appropriate range in this case,
if the initiator has requested the SA due to a data packet, the
initiator SHOULD include as the first Traffic Selector in each of TSi
and TSr a very specific Traffic Selector including the addresses in
the packet triggering the request. In the example, the initiator
would include in TSi two Traffic Selectors: the first containing the
address range (198.51.100.43 - 198.51.100.43) and the source port and
IP protocol from the packet and the second containing (198.51.100.0 -
198.51.100.255) with all ports and IP protocols. The initiator would
similarly include two Traffic Selectors in TSr. If the initiator
creates the Child SA pair not in response to an arriving packet, but
rather, say, upon startup, then there may be no specific addresses
the initiator prefers for the initial tunnel over any other. In that
case, the first values in TSi and TSr can be ranges rather than
specific values.

The responder performs the narrowing as follows:

o If the responder's policy does not allow it to accept any part of
the proposed Traffic Selectors, it responds with a TS_UNACCEPTABLE
Notify message.

o If the responder's policy allows the entire set of traffic covered
by TSi and TSr, no narrowing is necessary, and the responder can
return the same TSi and TSr values.

o If the responder's policy allows it to accept the first selector
of TSi and TSr, then the responder MUST narrow the Traffic
Selectors to a subset that includes the initiator's first choices.
In this example above, the responder might respond with TSi being
(198.51.100.43 - 198.51.100.43) with all ports and IP protocols.

o If the responder's policy does not allow it to accept the first
selector of TSi and TSr, the responder narrows to an acceptable
subset of TSi and TSr.

When narrowing is done, there may be several subsets that are
acceptable but their union is not. In this case, the responder
arbitrarily chooses one of them, and MAY include an
ADDITIONAL_TS_POSSIBLE notification in the response. The
ADDITIONAL_TS_POSSIBLE notification asserts that the responder
narrowed the proposed Traffic Selectors but that other Traffic
Selectors would also have been acceptable, though only in a separate
SA. There is no data associated with this Notify type. This case
will occur only when the initiator and responder are configured
differently from one another. If the initiator and responder agree
on the granularity of tunnels, the initiator will never request a
tunnel wider than the responder will accept.

It is possible for the responder's policy to contain multiple smaller
ranges, all encompassed by the initiator's Traffic Selector, and with
the responder's policy being that each of those ranges should be sent
over a different SA. Continuing the example above, the responder
might have a policy of being willing to tunnel those addresses to and
from the initiator, but might require that each address pair be on a
separately negotiated Child SA. If the initiator didn't generate its
request based on the packet, but (for example) upon startup, there
would not be the very specific first Traffic Selectors helping the
responder to select the correct range. There would be no way for the
responder to determine which pair of addresses should be included in
this tunnel, and it would have to make a guess or reject the request
with a SINGLE_PAIR_REQUIRED Notify message.

The SINGLE_PAIR_REQUIRED error indicates that a CREATE_CHILD_SA
request is unacceptable because its sender is only willing to accept
Traffic Selectors specifying a single pair of addresses. The
requestor is expected to respond by requesting an SA for only the
specific traffic it is trying to forward.

Few implementations will have policies that require separate SAs for
each address pair. Because of this, if only some parts of the TSi
and TSr proposed by the initiator are acceptable to the responder,
responders SHOULD narrow the selectors to an acceptable subset rather
than use SINGLE_PAIR_REQUIRED.

2.9.1. Traffic Selectors Violating Own Policy

When creating a new SA, the initiator needs to avoid proposing
Traffic Selectors that violate its own policy. If this rule is not
followed, valid traffic may be dropped. If you use decorrelated
policies from [IPSECARCH], this kind of policy violations cannot
happen.

This is best illustrated by an example. Suppose that host A has a
policy whose effect is that traffic to 198.51.100.66 is sent via
host B encrypted using AES, and traffic to all other hosts in
198.51.100.0/24 is also sent via B, but must use 3DES. Suppose also
that host B accepts any combination of AES and 3DES.

If host A now proposes an SA that uses 3DES, and includes TSr
containing (198.51.100.0-198.51.100.255), (198.51.100.0 - 198.51.100.255), this will be accepted by
host B. Now, host B can also use this SA to send traffic from
198.51.100.66, but those packets will be dropped by A since it
requires the use of AES for this traffic. Even if host A creates a
new SA only for 198.51.100.66 that uses AES, host B may freely
continue to use the first SA for the traffic. In this situation,
when proposing the SA, host A should have followed its own policy,
and included a TSr containing ((198.51.100.0-
198.51.100.65),(198.51.100.67-198.51.100.255)) ((198.51.100.0 - 198.51.100.65),
(198.51.100.67 - 198.51.100.255)) instead.

In general, if (1) the initiator makes a proposal "for traffic X
(TSi/TSr), do SA", and (2) for some subset X' of X, the initiator
does not actually accept traffic X' with SA, and (3) the initiator
would be willing to accept traffic X' with some SA' (!=SA), valid
traffic can be unnecessarily dropped since the responder can apply
either SA or SA' to traffic X'.

2.9.2. Traffic Selectors in Rekeying

Rekeying is used to replace an existing Child SA with another. If
the new SA would be allowed to have a narrower set of selectors than
the original, traffic that was allowed on the old SA would be dropped
in the new SA, thus violating the idea of "replacing". Thus, the new
SA MUST NOT have narrower selectors than the original. If the
rekeyed SA would ever need to have a narrower scope than the
currently used SA, that would mean that the policy was changed in a
way such that the currently used SA is against the policy. In that
case, the SA should have been already deleted after the policy change
took effect.

When the initiator attempts to rekey the Child SA, the proposed
Traffic Selectors SHOULD be either the same as, or a superset of, the
Traffic Selectors used in the old Child SA. That is, they would be
the same as, or a superset of, the currently active (decorrelated)
policy. The responder MUST NOT narrow down the Traffic Selectors
narrower than the scope currently in use.

Because a rekeyed SA can never have a narrower scope than the one
currently in use, there is no need for the selectors from the packet,
so those selectors SHOULD NOT be sent.

2.10. Nonces

The IKE_SA_INIT messages each contain a nonce. These nonces are used
as inputs to cryptographic functions. The CREATE_CHILD_SA request
and the CREATE_CHILD_SA response also contain nonces. These nonces
are used to add freshness to the key derivation technique used to
obtain keys for Child SA, and to ensure creation of strong
pseudorandom bits from the Diffie-Hellman key. Nonces used in IKEv2
MUST be randomly chosen, MUST be at least 128 bits in size, and MUST
be at least half the key size of the negotiated pseudorandom function
(PRF). However, the initiator chooses the nonce before the outcome
of the negotiation is known. Because of that, the nonce has to be
long enough for all the PRFs being proposed. If the same random
number source is used for both keys and nonces, care must be taken to
ensure that the latter use does not compromise the former.

2.11. Address and Port Agility

IKE runs over UDP ports 500 and 4500, and implicitly sets up ESP and
AH associations for the same IP addresses over which it runs. The IP
addresses and ports in the outer header are, however, not themselves
cryptographically protected, and IKE is designed to work even through
Network Address Translation (NAT) boxes. An implementation MUST
accept incoming requests even if the source port is not 500 or 4500,
and MUST respond to the address and port from which the request was
received. It MUST specify the address and port at which the request
was received as the source address and port in the response. IKE
functions identically over IPv4 or IPv6.

2.12. Reuse of Diffie-Hellman Exponentials

IKE generates keying material using an ephemeral Diffie-Hellman
exchange in order to gain the property of "perfect forward secrecy".
This means that once a connection is closed and its corresponding
keys are forgotten, even someone who has recorded all of the data
from the connection and gets access to all of the long-term keys of
the two endpoints cannot reconstruct the keys used to protect the
conversation without doing a brute force search of the session key
space.

Achieving perfect forward secrecy requires that when a connection is
closed, each endpoint MUST forget not only the keys used by the
connection but also any information that could be used to recompute
those keys.

Because computing Diffie-Hellman exponentials is computationally
expensive, an endpoint may find it advantageous to reuse those
exponentials for multiple connection setups. There are several
reasonable strategies for doing this. An endpoint could choose a new
exponential only periodically though this could result in less-than-
perfect forward secrecy if some connection lasts for less than the
lifetime of the exponential. Or it could keep track of which
exponential was used for each connection and delete the information
associated with the exponential only when some corresponding
connection was closed. This would allow the exponential to be reused
without losing perfect forward secrecy at the cost of maintaining
more state.

Whether and when to reuse Diffie-Hellman exponentials are private
decisions in the sense that they will not affect interoperability.
An implementation that reuses exponentials MAY choose to remember the
exponential used by the other endpoint on past exchanges and if one
is reused to avoid the second half of the calculation. See [REUSE]
and [RFC6989] for a security analysis of this practice and for
additional security considerations when reusing ephemeral Diffie-
Hellman
Diffie-Hellman keys.

2.13. Generating Keying Material

In the context of the IKE SA, four cryptographic algorithms are
negotiated: an encryption algorithm, an integrity protection
algorithm, a Diffie-Hellman group, and a pseudorandom function (PRF).
The PRF is used for the construction of keying material for all of
the cryptographic algorithms used in both the IKE SA and the
Child SAs.

We assume that each encryption algorithm and integrity protection
algorithm uses a fixed-size key and that any randomly chosen value of
that fixed size can serve as an appropriate key. For algorithms that
accept a variable-length key, a fixed key size MUST be specified as
part of the cryptographic transform negotiated (see Section 3.3.5 for
the definition of the Key Length transform attribute). For
algorithms for which not all values are valid keys (such as DES or
3DES with key parity), the algorithm by which keys are derived from
arbitrary values MUST be specified by the cryptographic transform.
For integrity protection functions based on Hashed Message
Authentication Code (HMAC), the fixed key size is the size of the
output of the underlying hash function.

It is assumed that PRFs accept keys of any length, but have a
preferred key size. The preferred key size MUST be used as the
length of SK_d, SK_pi, and SK_pr (see Section 2.14). For PRFs based
on the HMAC construction, the preferred key size is equal to the
length of the output of the underlying hash function. Other types of
PRFs MUST specify their preferred key size.

Keying material will always be derived as the output of the
negotiated PRF algorithm. Since the amount of keying material needed
may be greater than the size of the output of the PRF, the PRF is
used iteratively. The term "prf+" describes a function that outputs
a pseudorandom stream based on the inputs to a pseudorandom function
called "prf".

In the following, | indicates concatenation. prf+ is defined as:

prf+ (K,S) = T1 | T2 | T3 | T4 | ...

where:
T1 = prf (K, S | 0x01)
T2 = prf (K, T1 | S | 0x02)
T3 = prf (K, T2 | S | 0x03)
T4 = prf (K, T3 | S | 0x04)
...

This continues until all the material needed to compute all required
keys has been output from prf+. The keys are taken from the output
string without regard to boundaries (e.g., if the required keys are a
256-bit Advanced Encryption Standard (AES) key and a 160-bit HMAC
key, and the prf function generates 160 bits, the AES key will come
from T1 and the beginning of T2, while the HMAC key will come from
the rest of T2 and the beginning of T3).

The constant concatenated to the end of each prf function is a single
octet. The prf+ function is not defined beyond 255 times the size of
the prf function output.

2.14. Generating Keying Material for the IKE SA

The shared keys are computed as follows. A quantity called SKEYSEED
is calculated from the nonces exchanged during the IKE_SA_INIT
exchange and the Diffie-Hellman shared secret established during that
exchange. SKEYSEED is used to calculate seven other secrets: SK_d
used for deriving new keys for the Child SAs established with this
IKE SA; SK_ai and SK_ar used as a key to the integrity protection
algorithm for authenticating the component messages of subsequent
exchanges; SK_ei and SK_er used for encrypting (and of course
decrypting) all subsequent exchanges; and SK_pi and SK_pr, which are
used when generating an AUTH payload. The lengths of SK_d, SK_pi,
and SK_pr MUST be the preferred key length of the PRF agreed upon.

SKEYSEED and its derivatives are computed as follows:

SKEYSEED = prf(Ni | Nr, g^ir)

{SK_d | SK_ai | SK_ar | SK_ei | SK_er | SK_pi | SK_pr } SK_pr}
= prf+ (SKEYSEED, Ni | Nr | SPIi | SPIr ) SPIr)

(indicating that the quantities SK_d, SK_ai, SK_ar, SK_ei, SK_er,
SK_pi, and SK_pr are taken in order from the generated bits of the
prf+). g^ir is the shared secret from the ephemeral Diffie-Hellman
exchange. g^ir is represented as a string of octets in big endian
order padded with zeros if necessary to make it the length of the
modulus. Ni and Nr are the nonces, stripped of any headers. For
historical backward-compatibility reasons, there are two PRFs that
are treated specially in this calculation. If the negotiated PRF is
AES-XCBC-PRF-128 [AESXCBCPRF128] or AES-CMAC-PRF-128 [AESCMACPRF128],
only the first 64 bits of Ni and the first 64 bits of Nr are used in
calculating SKEYSEED, but all the bits are used for input to the prf+
function.

The two directions of traffic flow use different keys. The keys used
to protect messages from the original initiator are SK_ai and SK_ei.
The keys used to protect messages in the other direction are SK_ar
and SK_er.

2.15. Authentication of the IKE SA

When not using extensible authentication (see Section 2.16), the
peers are authenticated by having each sign (or MAC using a padded
shared secret as the key, as described later in this section) a block
of data. In these calculations, IDi' and IDr' are the entire ID
payloads excluding the fixed header. For the responder, the octets
to be signed start with the first octet of the first SPI in the
header of the second message (IKE_SA_INIT response) and end with the
last octet of the last payload in the second message. Appended to
this (for the purposes of computing the signature) are the
initiator's nonce Ni (just the value, not the payload containing it),
and the value prf(SK_pr, IDr'). Note that neither the nonce Ni nor
the value prf(SK_pr, IDr') are transmitted. Similarly, the initiator
signs the first message (IKE_SA_INIT request), starting with the
first octet of the first SPI in the header and ending with the last
octet of the last payload. Appended to this (for purposes of
computing the signature) are the responder's nonce Nr, and the value
prf(SK_pi, IDi'). It is critical to the security of the exchange
that each side sign the other side's nonce.

The initiator's signed octets can be described as:

Note that all of the payloads are included under the signature,
including any payload types not defined in this document. If the
first message of the exchange is sent multiple times (such as with a
responder cookie and/or a different Diffie-Hellman group), it is the
latest version of the message that is signed.

Optionally, messages 3 and 4 MAY include a certificate, or
certificate chain providing evidence that the key used to compute a
digital signature belongs to the name in the ID payload. The
signature or MAC will be computed using algorithms dictated by the
type of key used by the signer, and specified by the Auth Method
field in the Authentication payload. There is no requirement that
the initiator and responder sign with the same cryptographic
algorithms. The choice of cryptographic algorithms depends on the
type of key each has. In particular, the initiator may be using a
shared key while the responder may have a public signature key and
certificate. It will commonly be the case (but it is not required)
that, if a shared secret is used for authentication, the same key is
used in both directions.

Note that it is a common but typically insecure practice to have a
shared key derived solely from a user-chosen password without
incorporating another source of randomness. This is typically
insecure because user-chosen passwords are unlikely to have
sufficient unpredictability to resist dictionary attacks and these
attacks are not prevented in this authentication method.
(Applications using password-based authentication for bootstrapping
and IKE SA should use the authentication method in Section 2.16,
which is designed to prevent off-line dictionary attacks.) The pre-
shared
pre-shared key needs to contain as much unpredictability as the
strongest key being negotiated. In the case of a pre-shared key, the
AUTH value is computed as:

For the initiator:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<InitiatorSignedOctets>)
For the responder:
AUTH = prf( prf(Shared Secret, "Key Pad for IKEv2"),
<ResponderSignedOctets>)

where the string "Key Pad for IKEv2" is 17 ASCII characters without
null termination. The shared secret can be variable length. The pad
string is added so that if the shared secret is derived from a
password, the IKE implementation need not store the password in
cleartext, but rather can store the value prf(Shared Secret,"Key Pad
for IKEv2"), which could not be used as a password equivalent for
protocols other than IKEv2. As noted above, deriving the shared
secret from a password is not secure. This construction is used
because it is anticipated that people will do it anyway. The
management interface by which the shared secret is provided MUST
accept ASCII strings of at least 64 octets and MUST NOT add a null
terminator before using them as shared secrets. It MUST also accept
a hex encoding of the shared secret. The management interface MAY
accept other encodings if the algorithm for translating the encoding
to a binary string is specified.

There are two types of EAP authentication (described in
Section 2.16), and each type uses different values in the AUTH
computations shown above. If the EAP method is key-generating,
substitute master session key (MSK) for the shared secret in the
computation. For non-key-generating methods, substitute SK_pi and
SK_pr, respectively, for the shared secret in the two AUTH
computations.

2.16. Extensible Authentication Protocol Methods

In addition to authentication using public key signatures and shared
secrets, IKE supports authentication using methods defined in
RFC 3748 [EAP]. Typically, these methods are asymmetric (designed
for a user authenticating to a server), and they may not be mutual.
For this reason, these protocols are typically used to authenticate
the initiator to the responder and MUST be used in conjunction with a
public-key-signature-based authentication of the responder to the
initiator. These methods are often associated with mechanisms
referred to as "Legacy Authentication" mechanisms.

While this document references [EAP] with the intent that new methods
can be added in the future without updating this specification, some
simpler variations are documented here. [EAP] defines an
authentication protocol requiring a variable number of messages.
Extensible Authentication authentication is implemented in IKE as additional
IKE_AUTH exchanges that MUST be completed in order to initialize the
IKE SA.

An initiator indicates a desire to use EAP by leaving out the AUTH
payload from the first message in the IKE_AUTH exchange. (Note that
the AUTH payload is required for non-EAP authentication, and is thus
not marked as optional in the rest of this document.) By including
an IDi payload but not an AUTH payload, the initiator has declared an
identity but has not proven it. If the responder is willing to use
an EAP method, it will place an Extensible Authentication Protocol
(EAP) payload in the response of the IKE_AUTH exchange and defer
sending SAr2, TSi, and TSr until initiator authentication is complete
in a subsequent IKE_AUTH exchange. In the case of a minimal EAP
method, the initial SA establishment will appear as follows:

Initiator Responder
-------------------------------------------------------------------
HDR, SAi1, KEi, Ni -->
<-- HDR, SAr1, KEr, Nr, [CERTREQ]
HDR, SK {IDi, [CERTREQ,]
[IDr,] SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
EAP }
EAP}
HDR, SK {EAP} -->
<-- HDR, SK {EAP (success)}
HDR, SK {AUTH} -->
<-- HDR, SK {AUTH, SAr2, TSi, TSr } TSr}

As described in Section 2.2, when EAP is used, each pair of IKE SA
initial setup messages will have their message numbers incremented;
the first pair of IKE_AUTH messages will have an ID of 1, the second
will be 2, and so on.

For EAP methods that create a shared key as a side effect of
authentication, that shared key MUST be used by both the initiator
and responder to generate AUTH payloads in messages 7 and 8 using the
syntax for shared secrets specified in Section 2.15. The shared key
from EAP is the field from the EAP specification named MSK. This
shared key generated during an IKE exchange MUST NOT be used for any
other purpose.

EAP methods that do not establish a shared key SHOULD NOT be used, as
they are subject to a number of man-in-the-middle attacks [EAPMITM]
if these EAP methods are used in other protocols that do not use a
server-authenticated tunnel. Please see the Security Considerations
section for more details. If EAP methods that do not generate a
shared key are used, the AUTH payloads in messages 7 and 8 MUST be
generated using SK_pi and SK_pr, respectively.

The initiator of an IKE SA using EAP needs to be capable of extending
the initial protocol exchange to at least ten IKE_AUTH exchanges in
the event the responder sends notification messages and/or retries
the authentication prompt. Once the protocol exchange defined by the
chosen EAP authentication method has successfully terminated, the
responder MUST send an EAP payload containing the Success message.
Similarly, if the authentication method has failed, the responder
MUST send an EAP payload containing the Failure message. The
responder MAY at any time terminate the IKE exchange by sending an
EAP payload containing the Failure message.

Following such an extended exchange, the EAP AUTH payloads MUST be
included in the two messages following the one containing the EAP
Success message.

When the initiator authentication uses EAP, it is possible that the
contents of the IDi payload is used only for Authentication,
Authorization, and Accounting (AAA) routing purposes and selecting
which EAP method to use. This value may be different from the
identity authenticated by the EAP method. It is important that
policy lookups and access control decisions use the actual
authenticated identity. Often the EAP server is implemented in a
separate AAA server that communicates with the IKEv2 responder. In
this case, the authenticated identity, if different from that in the
IDi payload, has to be sent from the AAA server to the IKEv2
responder.

2.17. Generating Keying Material for Child SAs

A single Child SA is created by the IKE_AUTH exchange, and additional
Child SAs can optionally be created in CREATE_CHILD_SA exchanges.
Keying material for them is generated as follows:

KEYMAT = prf+(SK_d, Ni | Nr)

Where Ni and Nr are the nonces from the IKE_SA_INIT exchange if this
request is the first Child SA created or the fresh Ni and Nr from the
CREATE_CHILD_SA exchange if this is a subsequent creation.

For CREATE_CHILD_SA exchanges including an optional Diffie-Hellman
exchange, the keying material is defined as:

KEYMAT = prf+(SK_d, g^ir (new) | Ni | Nr ) Nr)

where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros in the high-order
bits if necessary to make it the length of the modulus).

A single CREATE_CHILD_SA negotiation may result in multiple Security
Associations. ESP and AH SAs exist in pairs (one in each direction),
so two SAs are created in a single Child SA negotiation for them.
Furthermore, Child SA negotiation may include some future IPsec
protocol(s) in addition to, or instead of, ESP or AH (for example,
ROHC_INTEG as described in [ROHCV2]). In any case, keying material
for each Child SA MUST be taken from the expanded KEYMAT using the
following rules:

o All keys for SAs carrying data from the initiator to the responder
are taken before SAs going from the responder to the initiator.

o If multiple IPsec protocols are negotiated, keying material for
each Child SA is taken in the order in which the protocol headers
will appear in the encapsulated packet.

o If an IPsec protocol requires multiple keys, the order in which
they are taken from the SA's keying material needs to be described
in the protocol's specification. For ESP and AH, [IPSECARCH]
defines the order, namely: the encryption key (if any) MUST be
taken from the first bits and the integrity key (if any) MUST be
taken from the remaining bits.

Each cryptographic algorithm takes a fixed number of bits of keying
material specified as part of the algorithm, or negotiated in SA
payloads (see Section 2.13 for description of key lengths, and
Section 3.3.5 for the definition of the Key Length transform
attribute).

2.18. Rekeying IKE SAs Using a CREATE_CHILD_SA Exchange

The CREATE_CHILD_SA exchange can be used to rekey an existing IKE SA
(see Sections 1.3.2 and 2.8). New initiator and responder SPIs are
supplied in the SPI fields in the Proposal structures inside the
Security Association (SA) payloads (not the SPI fields in the IKE
header). The TS payloads are omitted when rekeying an IKE SA.
SKEYSEED for the new IKE SA is computed using SK_d from the existing
IKE SA as follows:

SKEYSEED = prf(SK_d (old), g^ir (new) | Ni | Nr)

where g^ir (new) is the shared secret from the ephemeral Diffie-
Hellman exchange of this CREATE_CHILD_SA exchange (represented as an
octet string in big endian order padded with zeros if necessary to
make it the length of the modulus) and Ni and Nr are the two nonces
stripped of any headers.

The old and new IKE SA may have selected a different PRF. Because
the rekeying exchange belongs to the old IKE SA, it is the old IKE
SA's PRF that is used to generate SKEYSEED.

The main reason for rekeying the IKE SA is to ensure that the
compromise of old keying material does not provide information about
the current keys, or vice versa. Therefore, implementations MUST
perform a new Diffie-Hellman exchange when rekeying the IKE SA. In
other words, an initiator MUST NOT propose the value "NONE" for the
Diffie-Hellman transform, and a responder MUST NOT accept such a
proposal. This means that a successful exchange rekeying the IKE SA
always includes the KEi/KEr payloads.

The new IKE SA MUST reset its message counters to 0.

SK_d, SK_ai, SK_ar, SK_ei, and SK_er are computed from SKEYSEED as
specified in Section 2.14, using SPIi, SPIr, Ni, and Nr from the new
exchange, and using the new IKE SA's PRF.

2.19. Requesting an Internal Address on a Remote Network

Most commonly occurring in the endpoint-to-security-gateway scenario,
an endpoint may need an IP address in the network protected by the
security gateway and may need to have that address dynamically
assigned. A request for such a temporary address can be included in
any request to create a Child SA (including the implicit request in
message 3) by including a CP payload. Note, however, it is usual to
only assign one IP address during the IKE_AUTH exchange. That
address persists at least until the deletion of the IKE SA.

This function provides address allocation to an IPsec Remote Access
Client (IRAC) trying to tunnel into a network protected by an IPsec
Remote Access Server (IRAS). Since the IKE_AUTH exchange creates an
IKE SA and a Child SA, the IRAC MUST request the IRAS-controlled
address (and optionally other information concerning the protected
network) in the IKE_AUTH exchange. The IRAS may procure an address
for the IRAC from any number of sources such as a DHCP/BOOTP
(Bootstrap Protocol) server or its own address pool.

Initiator Responder
-------------------------------------------------------------------
HDR, SK {IDi, [CERT,]
[CERTREQ,] [IDr,] AUTH,
CP(CFG_REQUEST), SAi2,
TSi, TSr} -->
<-- HDR, SK {IDr, [CERT,] AUTH,
CP(CFG_REPLY), SAr2,
TSi, TSr}
In all cases, the CP payload MUST be inserted before the SA payload.
In variations of the protocol where there are multiple IKE_AUTH
exchanges, the CP payloads MUST be inserted in the messages
containing the SA payloads.

CP(CFG_REQUEST) MUST contain at least an INTERNAL_ADDRESS attribute
(either IPv4 or IPv6) but MAY contain any number of additional
attributes the initiator wants returned in the response.

For example, message from initiator to responder:

CP(CFG_REQUEST)=
INTERNAL_ADDRESS()
TSi = (0, 0-65535,0.0.0.0-255.255.255.255) 0-65535, 0.0.0.0-255.255.255.255)
TSr = (0, 0-65535,0.0.0.0-255.255.255.255) 0-65535, 0.0.0.0-255.255.255.255)

NOTE: Traffic Selectors contain (protocol, port range, address
range).

Message from responder to initiator:

CP(CFG_REPLY)=
INTERNAL_ADDRESS(192.0.2.202)
INTERNAL_NETMASK(255.255.255.0)
INTERNAL_SUBNET(192.0.2.0/255.255.255.0)
TSi = (0, 0-65535,192.0.2.202-192.0.2.202) 0-65535, 192.0.2.202-192.0.2.202)
TSr = (0, 0-65535,192.0.2.0-192.0.2.255) 0-65535, 192.0.2.0-192.0.2.255)

All returned values will be implementation dependent. As can be seen
in the above example, the IRAS MAY also send other attributes that
were not included in CP(CFG_REQUEST) and MAY ignore the non-
mandatory non-mandatory
attributes that it does not support.

The responder MUST NOT send a CFG_REPLY without having first received
a CP(CFG_REQUEST) from the initiator, because we do not want the IRAS
to perform an unnecessary configuration lookup if the IRAC cannot
process the REPLY.

In the case where the IRAS's configuration requires that CP be used
for a given identity IDi, but IRAC has failed to send a
CP(CFG_REQUEST), IRAS MUST fail the request, and terminate the Child
SA creation with a FAILED_CP_REQUIRED error. The FAILED_CP_REQUIRED
is not fatal to the IKE SA; it simply causes the Child SA creation to
fail. The initiator can fix this by later starting a new
Configuration payload request. There is no associated data in the
FAILED_CP_REQUIRED error.

2.20. Requesting the Peer's Version

An IKE peer wishing to inquire about the other peer's IKE software
version information MAY use the method below. This is an example of
a configuration request within an INFORMATIONAL exchange, after the
IKE SA and first Child SA have been created.

An IKE implementation MAY decline to give out version information
prior to authentication or even after authentication in case some
implementation is known to have some security weakness. In that
case, it MUST either return an empty string or no CP payload if CP is
not supported.

Initiator Responder
-------------------------------------------------------------------
HDR, SK{CP(CFG_REQUEST)} SK {CP(CFG_REQUEST)} -->
<-- HDR, SK{CP(CFG_REPLY)} SK {CP(CFG_REPLY)}

CP(CFG_REQUEST)=
APPLICATION_VERSION("")

CP(CFG_REPLY) APPLICATION_VERSION("foobar v1.3beta, (c) Foo Bar
Inc.")

2.21. Error Handling

There are many kinds of errors that can occur during IKE processing.
The general rule is that if a request is received that is badly
formatted, or unacceptable for reasons of policy (such as no matching
cryptographic algorithms), the response contains a Notify payload
indicating the error. The decision whether or not to send such a
response depends whether or not there is an authenticated IKE SA.

If there is an error parsing or processing a response packet, the
general rule is to not send back any error message because responses
should not generate new requests (and a new request would be the only
way to send back an error message). Such errors in parsing or
processing response packets should still cause the recipient to clean
up the IKE state (for example, by sending a Delete for a bad SA).

Only authentication failures (AUTHENTICATION_FAILED and EAP failure)
and malformed messages (INVALID_SYNTAX) lead to a deletion of the IKE
SA without requiring an explicit INFORMATIONAL exchange carrying a
Delete payload. Other error conditions MAY require such an exchange
if policy dictates that this is needed. If the exchange is
terminated with EAP Failure, an AUTHENTICATION_FAILED notification is
not sent.

2.21.1. Error Handling in IKE_SA_INIT

Errors that occur before a cryptographically protected IKE SA is
established need to be handled very carefully. There is a trade-off
between wanting to help the peer to diagnose a problem and thus
responding to the error and wanting to avoid being part of a DoS
attack based on forged messages.

In an IKE_SA_INIT exchange, any error notification causes the
exchange to fail. Note that some error notifications such as COOKIE,
INVALID_KE_PAYLOAD or INVALID_MAJOR_VERSION may lead to a subsequent
successful exchange. Because all error notifications are completely
unauthenticated, the recipient should continue trying for some time
before giving up. The recipient should not immediately act based on
the error notification unless corrective actions are defined in this
specification, such as for COOKIE, INVALID_KE_PAYLOAD, and
INVALID_MAJOR_VERSION.

2.21.2. Error Handling in IKE_AUTH

All errors that occur in an IKE_AUTH exchange, causing the
authentication to fail for whatever reason (invalid shared secret,
invalid ID, untrusted certificate issuer, revoked or expired
certificate, etc.) SHOULD result in an AUTHENTICATION_FAILED
notification. If the error occurred on the responder, the
notification is returned in the protected response, and is usually
the only payload in that response. Although the IKE_AUTH messages
are encrypted and integrity protected, if the peer receiving this
notification has not authenticated the other end yet, that peer needs
to treat the information with caution.

If the error occurs on the initiator, the notification MAY be
returned in a separate INFORMATIONAL exchange, usually with no other
payloads. This is an exception for the general rule of not starting
new exchanges based on errors in responses.

Note, however, that request messages that contain an unsupported
critical payload, or where the whole message is malformed (rather
than just bad payload contents), MUST be rejected in their entirety,
and MUST only lead to an UNSUPPORTED_CRITICAL_PAYLOAD or
INVALID_SYNTAX Notification sent as a response. The receiver should
not verify the payloads related to authentication in this case.

If authentication has succeeded in the IKE_AUTH exchange, the IKE SA
is established; however, establishing the Child SA or requesting
configuration information may still fail. This failure does not
automatically cause the IKE SA to be deleted. Specifically, a
responder may include all the payloads associated with authentication
(IDr, CERT, and AUTH) while sending error notifications for the
piggybacked exchanges (FAILED_CP_REQUIRED, NO_PROPOSAL_CHOSEN, and so
on), and the initiator MUST NOT fail the authentication because of
this. The initiator MAY, of course, for reasons of policy later
delete such an IKE SA.

In an IKE_AUTH exchange, or in the INFORMATIONAL exchange immediately
following it (in case an error happened when processing a response to
IKE_AUTH), the UNSUPPORTED_CRITICAL_PAYLOAD, INVALID_SYNTAX, and
AUTHENTICATION_FAILED notifications are the only ones to cause the
IKE SA to be deleted or not created, without a Delete payload.
Extension documents may define new error notifications with these
semantics, but MUST NOT use them unless the peer has been shown to
understand them, such as by using the Vendor ID payload.

2.21.3. Error Handling after IKE SA is Authenticated

After the IKE SA is authenticated, all requests having errors MUST
result in a response notifying about the other end of the error.

In normal situations, there should not be cases where a valid
response from one peer results in an error situation in the other
peer, so there should not be any reason for a peer to send error
messages to the other end except as a response. Because sending such
error messages as an INFORMATIONAL exchange might lead to further
errors that could cause loops, such errors SHOULD NOT be sent. If
errors are seen that indicate that the peers do not have the same
state, it might be good to delete the IKE SA to clean up state and
start over.

If a peer parsing a request notices that it is badly formatted (after
it has passed the message authentication code checks and window
checks) and it returns an INVALID_SYNTAX notification, then this
error notification is considered fatal in both peers, meaning that
the IKE SA is deleted without needing an explicit Delete payload.

2.21.4. Error Handling Outside IKE SA

A node needs to limit the rate at which it will send messages in
response to unprotected messages.

If a node receives a message on UDP port 500 or 4500 outside the
context of an IKE SA known to it (and the message is not a request to
start an IKE SA), this may be the result of a recent crash of the
node. If the message is marked as a response, the node can audit the
suspicious event but MUST NOT respond. If the message is marked as a
request, the node can audit the suspicious event and MAY send a
response. If a response is sent, the response MUST be sent to the IP
address and port from where it came with the same IKE SPIs and the
Message ID copied. The response MUST NOT be cryptographically
protected and MUST contain an INVALID_IKE_SPI Notify payload. The
INVALID_IKE_SPI notification indicates an IKE message was received
with an unrecognized destination SPI; this usually indicates that the
recipient has rebooted and forgotten the existence of an IKE SA.

A peer receiving such an unprotected Notify payload MUST NOT respond
and MUST NOT change the state of any existing SAs. The message might
be a forgery or might be a response that a genuine correspondent was
tricked into sending. A node should treat such a message (and also a
network message like ICMP destination unreachable) as a hint that
there might be problems with SAs to that IP address and should
initiate a liveness check for any such IKE SA. An implementation
SHOULD limit the frequency of such tests to avoid being tricked into
participating in a DoS attack.

If an error occurs outside the context of an IKE request (e.g., the
node is getting ESP messages on a nonexistent SPI), the node SHOULD
initiate an INFORMATIONAL exchange with a Notify payload describing
the problem.

A node receiving a suspicious message from an IP address (and port,
if NAT traversal is used) with which it has an IKE SA SHOULD send an
IKE Notify payload in an IKE INFORMATIONAL exchange over that SA.
The recipient MUST NOT change the state of any SAs as a result, but
may wish to audit the event to aid in diagnosing malfunctions.

2.22. IPComp

Use of IP Compression [IP-COMP] can be negotiated as part of the
setup of a Child SA. While IP Compression involves an extra header
in each packet and a compression parameter index (CPI), the virtual
"compression association" has no life outside the ESP or AH SA that
contains it. Compression associations disappear when the
corresponding ESP or AH SA goes away. It is not explicitly mentioned
in any Delete payload.

Negotiation of IP Compression is separate from the negotiation of
cryptographic parameters associated with a Child SA. A node
requesting a Child SA MAY advertise its support for one or more
compression algorithms through one or more Notify payloads of type
IPCOMP_SUPPORTED. This Notify message may be included only in a
message containing an SA payload negotiating a Child SA and indicates
a willingness by its sender to use IPComp on this SA. The response
MAY indicate acceptance of a single compression algorithm with a
Notify payload of type IPCOMP_SUPPORTED. These payloads MUST NOT
occur in messages that do not contain SA payloads.

The data associated with this Notify message includes a two-octet
IPComp CPI followed by a one-octet Transform ID optionally followed
by attributes whose length and format are defined by that Transform
ID. A message proposing an SA may contain multiple IPCOMP_SUPPORTED
notifications to indicate multiple supported algorithms. A message
accepting an SA may contain at most one.

The Transform IDs are listed here. The values in the following table
are only current as of the publication date of RFC 4306. Other
values may have been added since then or will be added after the
publication of this document. Readers should refer to [IKEV2IANA]
for the latest values.

Name Number Defined In
-------------------------------------
----------------------------------------
IPCOMP_OUI 1 (UNSPECIFIED)
IPCOMP_DEFLATE 2 RFC 2394
IPCOMP_LZS 3 RFC 2395
IPCOMP_LZJH 4 RFC 3051

Although there has been discussion of allowing multiple compression
algorithms to be accepted and to have different compression
algorithms available for the two directions of a Child SA,
implementations of this specification MUST NOT accept an IPComp
algorithm that was not proposed, MUST NOT accept more than one, and
MUST NOT compress using an algorithm other than one proposed and
accepted in the setup of the Child SA.

A side effect of separating the negotiation of IPComp from
cryptographic parameters is that it is not possible to propose
multiple cryptographic suites and propose IP Compression with some of
them but not others.

In some cases, Robust Header Compression (ROHC) may be more
appropriate than IP Compression. [ROHCV2] defines the use of ROHC
with IKEv2 and IPsec.

2.23. NAT Traversal

Network Address Translation (NAT) gateways are a controversial
subject. This section briefly describes what they are and how they
are likely to act on IKE traffic. Many people believe that NATs are
evil and that we should not design our protocols so as to make them
work better. IKEv2 does indeed specify some unintuitive processing
rules in
order so that NATs are more likely to work.

NATs exist primarily because of the shortage of IPv4 addresses,
though there are other rationales. IP nodes that are "behind" a NAT
have IP addresses that are not globally unique, but rather are
assigned from some space that is unique within the network behind the
NAT but that are likely to be reused by nodes behind other NATs.
Generally, nodes behind NATs can communicate with other nodes behind
the same NAT and with nodes with globally unique addresses, but not
with nodes behind other NATs. There are exceptions to that rule.
When those nodes make connections to nodes on the real Internet, the
NAT gateway "translates" the IP source address to an address that
will be routed back to the gateway. Messages to the gateway from the
Internet have their destination addresses "translated" to the
internal address that will route the packet to the correct endnode.

NATs are designed to be "transparent" to endnodes. Neither software
on the node behind the NAT nor the node on the Internet requires
modification to communicate through the NAT. Achieving this
transparency is more difficult with some protocols than with others.
Protocols that include IP addresses of the endpoints within the
payloads of the packet will fail unless the NAT gateway understands
the protocol and modifies the internal references as well as those in
the headers. Such knowledge is inherently unreliable, is a network
layer violation, and often results in subtle problems.

Opening an IPsec connection through a NAT introduces special
problems. If the connection runs in transport mode, changing the IP
addresses on packets will cause the checksums to fail and the NAT
cannot correct the checksums because they are cryptographically
protected. Even in tunnel mode, there are routing problems because
transparently translating the addresses of AH and ESP packets
requires special logic in the NAT and that logic is heuristic and
unreliable in nature. For that reason, IKEv2 will use UDP
encapsulation of IKE and ESP packets. This encoding is slightly less
efficient but is easier for NATs to process. In addition, firewalls
may be configured to pass UDP-encapsulated IPsec traffic but not
plain, unencapsulated ESP/AH or vice versa.

It is a common practice of NATs to translate TCP and UDP port numbers
as well as addresses and use the port numbers of inbound packets to
decide which internal node should get a given packet. For this
reason, even though IKE packets MUST be sent to and from UDP port 500
or 4500, they MUST be accepted coming from any port and responses
MUST be sent to the port from whence they came. This is because the
ports may be modified as the packets pass through NATs. Similarly,
IP addresses of the IKE endpoints are generally not included in the
IKE payloads because the payloads are cryptographically protected and
could not be transparently modified by NATs.

Port 4500 is reserved for UDP-encapsulated ESP and IKE. An IPsec
endpoint that discovers a NAT between it and its correspondent (as
described below) MUST send all subsequent traffic from port 4500,
which NATs should not treat specially (as they might with port 500).

An initiator can use port 4500 for both IKE and ESP, regardless of
whether or not there is a NAT, even at the beginning of IKE. When
either side is using port 4500, sending ESP with UDP encapsulation is
not required, but understanding received UDP-encapsulated ESP packets
is required. UDP encapsulation MUST NOT be done on port 500. If
Network Address Translation Traversal (NAT-T) is supported (that is,
if NAT_DETECTION_*_IP payloads were exchanged during IKE_SA_INIT),
all devices MUST be able to receive and process both UDP-encapsulated
ESP and non-UDP-encapsulated ESP packets at any time. Either side
can decide whether or not to use UDP encapsulation for ESP
irrespective of the choice made by the other side. However, if a NAT
is detected, both devices MUST use UDP encapsulation for ESP.

The specific requirements for supporting NAT traversal [NATREQ] are
listed below. Support for NAT traversal is optional. In this
section only, requirements listed as MUST apply only to
implementations supporting NAT traversal.

o Both the IKE initiator and responder MUST include in their
IKE_SA_INIT packets Notify payloads of type
NAT_DETECTION_SOURCE_IP and NAT_DETECTION_DESTINATION_IP. Those
payloads can be used to detect if there is NAT between the hosts,
and which end is behind the NAT. The location of the payloads in
the IKE_SA_INIT packets is just after the Ni and Nr payloads
(before the optional CERTREQ payload).

o The data associated with the NAT_DETECTION_SOURCE_IP notification
is a SHA-1 digest of the SPIs (in the order they appear in the
header), IP address, and port from which this packet was sent.

There MAY be multiple NAT_DETECTION_SOURCE_IP payloads in a
message if the sender does not know which of several network
attachments will be used to send the packet.

o The data associated with the NAT_DETECTION_DESTINATION_IP
notification is a SHA-1 digest of the SPIs (in the order they
appear in the header), IP address, and port to which this packet
was sent.

o The recipient of either the NAT_DETECTION_SOURCE_IP or
NAT_DETECTION_DESTINATION_IP notification MAY compare the supplied
value to a SHA-1 hash of the SPIs, source or recipient IP address
(respectively), address,
and port, port (respectively), and if they don't match, it SHOULD enable
NAT traversal. In the case there is a mismatch of the
NAT_DETECTION_SOURCE_IP hash with all of the
NAT_DETECTION_SOURCE_IP payloads received, the recipient MAY
reject the connection attempt if NAT traversal is not supported.
In the case of a mismatching NAT_DETECTION_DESTINATION_IP hash, it
means that the system receiving the NAT_DETECTION_DESTINATION_IP
payload is behind a NAT and that system SHOULD start sending
keepalive packets as defined in [UDPENCAPS]; alternately, it MAY
reject the connection attempt if NAT traversal is not supported.

o If none of the NAT_DETECTION_SOURCE_IP payload(s) received matches
the expected value of the source IP and port found from the IP
header of the packet containing the payload, it means that the
system sending those payloads is behind a NAT (i.e., someone along
the route changed the source address of the original packet to
match the address of the NAT box). In this case, the system
receiving the payloads should allow dynamic updates of the other
systems'
system's IP address, as described later.

o The IKE initiator MUST check the NAT_DETECTION_SOURCE_IP or
NAT_DETECTION_DESTINATION_IP payloads if present, and if they do
not match the addresses in the outer packet, MUST tunnel all
future IKE and ESP packets associated with this IKE SA over UDP
port 4500.

o To tunnel IKE packets over UDP port 4500, the IKE header has
four octets of zero zeros prepended and the result immediately follows
the UDP header. To tunnel ESP packets over UDP port 4500, the ESP
header immediately follows the UDP header. Since the first
four octets of the ESP header contain the SPI, and the SPI cannot
validly be zero, it is always possible to distinguish ESP and IKE
messages.

o Implementations MUST process received UDP-encapsulated ESP packets
even when no NAT was detected.

o The original source and destination IP address required for the
transport mode TCP and UDP packet checksum fixup (see [UDPENCAPS])
are obtained from the Traffic Selectors associated with the
exchange. In the case of transport mode NAT traversal, the
Traffic Selectors MUST contain exactly one IP address, which is
then used as the original IP address. This is covered in greater
detail in Section 2.23.1.

o There are cases where a NAT box decides to remove mappings that
are still alive (for example, the keepalive interval is too long,
or the NAT box is rebooted). This will be apparent to a host if
it receives a packet whose integrity protection validates, but has
a different port, address, or both from the one that was
associated with the SA in the validated packet. When such a
validated packet is found, a host that does not support other
methods of recovery such as IKEv2 Mobility and Multihoming
(MOBIKE) [MOBIKE], and that is not behind a NAT, SHOULD send all
packets (including retransmission packets) to the IP address and
port in the validated packet, and SHOULD store this as the new
address and port combination for the SA (that is, they SHOULD
dynamically update the address). A host behind a NAT SHOULD NOT
do this type of dynamic address update if a validated packet has
different port and/or address values because it opens a possible
DoS attack (such as allowing an attacker to break the connection
with a single packet). Also, dynamic address update should only
be done in response to a new packet; otherwise, an attacker can
revert the addresses with old replayed packets. Because of this,
dynamic updates can only be done safely if replay protection is
enabled. When IKEv2 is used with MOBIKE, dynamically updating the
addresses described above interferes with MOBIKE's way of
recovering from the same situation. See Section 3.8 of [MOBIKE]
for more information.

2.23.1. Transport Mode NAT Traversal

Transport mode used with NAT Traversal traversal requires special handling of
the Traffic Selectors used in the IKEv2. The complete scenario looks
like:

+------+ +------+ +------+ +------+
|Client| IP1 | NAT | IPN1 IPN2 | NAT | IP2 |Server|
|node |<------>| A |<---------->| B |<------->| |
+------+ +------+ +------+ +------+

(Other scenarios are simplifications of this complex case, so this
discussion uses the complete scenario.)

In this scenario, there are two address translating NATs: NAT A and
NAT B. NAT A is a dynamic NAT that maps the client's source address
IP1 to IPN1. NAT B is a static NAT configured so that connections
coming to IPN2 address are mapped to the gateway's address IP2, that
is, IPN2 destination address is mapped to IP2. This allows the
client to connect to a server by connecting to the IPN2. NAT B does
not necessarily need to be a static NAT, but the client needs to know
how to connect to the server, and it can only do that if it somehow
knows the outer address of the NAT B, that is, the IPN2 address. If
NAT B is a static NAT, then its address can be configured to the
client's configuration. Another option would be to find it using
some other protocol (like DNS), but that is outside of scope of
IKEv2.

In this scenario, both the client and server are configured to use
transport mode for the traffic originating from the client node and
destined to the server.

When the client starts creating the IKEv2 SA and Child SA for sending
traffic to the server, it may have a triggering packet with source IP
address of IP1, and a destination IP address of IPN2. Its Peer
Authorization Database (PAD) and SPD needs to have a configuration
matching those addresses (or wildcard entries covering them).
Because this is transport mode, it uses exactly same addresses as the
Traffic Selectors and outer IP address of the IKE packets. For
transport mode, it MUST use exactly one IP address in the TSi and TSr
payloads. It can have multiple Traffic Selectors if it has, for
example, multiple port ranges that it wants to negotiate, but all TSi
entries must use the IP1-IP1 range as the IP addresses, and all TSr
entries must have the IPN2-IPN2 range as IP addresses. The first
Traffic Selector of TSi and TSr SHOULD have very specific Traffic
Selectors including protocol and port numbers, such as from the
packet triggering the request.

NAT A will then replace the source address of the IKE packet from IP1
to IPN1, and NAT B will replace the destination address of the IKE
packet from IPN2 to IP2, so when the packet arrives to the server it
will still have the exactly same Traffic Selectors that were sent by
the client, but the IP address of the IKE packet has been replaced by
IPN1 and IP2.

When the server receives this packet, it normally looks in the Peer
Authorization Database (PAD) described in RFC 4301 [IPSECARCH] based
on the ID and then searches the SPD based on the Traffic Selectors.
Because IP1 does not really mean anything to the server (it is the
address client has behind the NAT), it is useless to do a lookup
based on that if transport mode is used. On the other hand, the
server cannot know whether transport mode is allowed by its policy
before it finds the matching SPD entry.

In this case, the server should first check that the initiator
requested transport mode, and then do address substitution on the
Traffic Selectors. It needs to first store the old Traffic Selector
IP addresses to be used later for the incremental checksum fixup (the
IP address in the TSi can be stored as the original source address
and the IP address in the TSr can be stored as the original
destination address). After that, if the other end was detected as
being behind a NAT, the server replaces the IP address in TSi
payloads with the IP address obtained from the source address of the
IKE packet received (that is, it replaces IP1 in TSi with IPN1). If
the server's end was detected to be behind NAT, it replaces the IP
address in the TSr payloads with the IP address obtained from the
destination address of the IKE packet received (that is, it replaces
IPN2 in TSr with IP2).

After this address substitution, both the Traffic Selectors and the
IKE UDP source/destination addresses look the same, and the server
does SPD lookup based on those new Traffic Selectors. If an entry is
found and it allows transport mode, then that entry is used. If an
entry is found but it does not allow transport mode, then the server
MAY undo the address substitution and redo the SPD lookup using the
original Traffic Selectors. If the second lookup succeeds, the
server will create an SA in tunnel mode using real Traffic Selectors
sent by the other end.

This address substitution in transport mode is needed because the SPD
is looked up using the addresses that will be seen by the local host.
This also will make sure also ensure that the Security Association Database (SAD)
entries for the tunnel exit checks and return packets is are added using
the addresses as seen by the local operating system stack.

The most common case is that the server's SPD will contain wildcard
entries matching any addresses, but this also allows making different
SPD entries, for example, for different known NATs' outer addresses.

After the SPD lookup, the server will do Traffic Selector narrowing
based on the SPD entry it found. It will again use the already
substituted Traffic Selectors, and it will thus send back Traffic
Selectors having IPN1 and IP2 as their IP addresses; it can still
narrow down the protocol number or port ranges used by the Traffic
Selectors. The SAD entry created for the Child SA will have the
addresses as seen by the server, namely IPN1 and IP2.

When the client receives the server's response to the Child SA, it
will do similar processing. If the transport mode SA was created,
the client can store the original returned Traffic Selectors as
original source and destination addresses. It will replace the IP
addresses in the Traffic Selectors with the ones from the IP header
of the IKE packet: it will replace IPN1 with IP1 and IP2 with IPN2.
Then, it will use those Traffic Selectors when verifying the SA
against sent Traffic Selectors, and when installing the SAD entry.

A summary of the rules for NAT traversal in transport mode is:

For the client proposing transport mode:

- The TSi entries MUST have exactly one IP address, and that MUST
match the source address of the IKE SA.

- The TSr entries MUST have exactly one IP address, and that MUST
match the destination address of the IKE SA.

- The first TSi and TSr Traffic Selectors SHOULD have very specific
Traffic Selectors including protocol and port numbers, such as
from the packet triggering the request.

- There MAY be multiple TSi and TSr entries.

- If transport mode for the SA was selected (that is, if the server
included USE_TRANSPORT_MODE notification in its response):

- Store the original Traffic Selectors as the received source and
destination address.

- If the server is behind a NAT, substitute the IP address in the
TSr entries with the remote address of the IKE SA.

- If the client is behind a NAT, substitute the IP address in the
TSi entries with the local address of the IKE SA.

- Do address substitution before using those Traffic Selectors
for anything other than storing original content of them.
This includes verification that Traffic Selectors were narrowed
correctly by the other end, creation of the SAD entry, and so on.

For the responder, when transport mode is proposed by client:

- Store the original Traffic Selector IP addresses as received source
and destination address, in case undo address substitution is
needed, to use as the "real source and destination address"
specified by [UDPENCAPS], and for TCP/UDP checksum fixup.

- If the client is behind a NAT, substitute the IP address in the
TSi entries with the remote address of the IKE SA.

- If the server is behind a NAT, substitute the IP address in the
TSr entries with the local address of the IKE SA.

- Do PAD and SPD lookup using the ID and substituted Traffic
Selectors.

- If no SPD entry was found, or if found (if found) the SPD entry does not
allow transport mode, undo the Traffic Selector substitutions.
Do PAD and SPD lookup again using the ID and original Traffic
Selectors, but also searching for tunnel mode SPD entry (that
is, fall back to tunnel mode).

- However, if a transport mode SPD entry was found, do normal
traffic selection narrowing based on the substituted Traffic
Selectors and SPD entry. Use the resulting Traffic Selectors when
creating SAD entries, and when sending Traffic Selectors back to
the client.

2.24. Explicit Congestion Notification (ECN)

When IPsec tunnels behave as originally specified in [IPSECARCH-OLD],
ECN usage is not appropriate for the outer IP headers because tunnel
decapsulation processing discards ECN congestion indications to the
detriment of the network. ECN support for IPsec tunnels for IKEv1-
based
IKEv1-based IPsec requires multiple operating modes and negotiation
(see [ECN]). IKEv2 simplifies this situation by requiring that ECN
be usable in the outer IP headers of all tunnel mode Child SAs
created by IKEv2. Specifically, tunnel encapsulators and
decapsulators for all tunnel mode SAs created by IKEv2 MUST support
the ECN full-
functionality full-functionality option for tunnels specified in [ECN] and
MUST implement the tunnel encapsulation and decapsulation processing
specified in [IPSECARCH] to prevent discarding of ECN congestion
indications.

2.25. Exchange Collisions

Because IKEv2 exchanges can be initiated by either peer, it is
possible that two exchanges affecting the same SA partly overlap.
This can lead to a situation where the SA state information is
temporarily not synchronized, and a peer can receive a request that
it cannot process in a normal fashion.

Obviously, using a window size greater than 1 leads to more complex
situations, especially if requests are processed out of order. This
section concentrates on problems that can arise even with a window
size of 1, and recommends solutions.

A TEMPORARY_FAILURE notification SHOULD be sent when a peer receives
a request that cannot be completed due to a temporary condition such
as a rekeying operation. When a peer receives a TEMPORARY_FAILURE
notification, it MUST NOT immediately retry the operation; it MUST
wait so that the sender may complete whatever operation caused the
temporary condition. The recipient MAY retry the request one or more
times over a period of several minutes. If a peer continues to
receive TEMPORARY_FAILURE on the same IKE SA after several minutes,
it SHOULD conclude that the state information is out of sync and
close the IKE SA.

A CHILD_SA_NOT_FOUND notification SHOULD be sent when a peer receives
a request to rekey a Child SA that does not exist. The SA that the
initiator attempted to rekey is indicated by the SPI field in the
Notify payload, which is copied from the SPI field in the REKEY_SA
notification. A peer that receives a CHILD_SA_NOT_FOUND notification
SHOULD silently delete the Child SA (if it still exists) and send a
request to create a new Child SA from scratch (if the Child SA does
not yet exist).

2.25.1. Collisions while Rekeying or Closing Child SAs

If a peer receives a request to rekey a Child SA that it is currently
trying to close, it SHOULD reply with TEMPORARY_FAILURE. If a peer
receives a request to rekey a Child SA that it is currently rekeying,
it SHOULD reply as usual, and SHOULD prepare to close redundant SAs
later based on the nonces (see Section 2.8.1). If a peer receives a
request to rekey a Child SA that does not exist, it SHOULD reply with
CHILD_SA_NOT_FOUND.

If a peer receives a request to close a Child SA that it is currently
trying to close, it SHOULD reply without a Delete payload (see
Section 1.4.1). If a peer receives a request to close a Child SA
that it is currently rekeying, it SHOULD reply as usual, with a
Delete payload. If a peer receives a request to close a Child SA
that does not exist, it SHOULD reply without a Delete payload.

If a peer receives a request to rekey the IKE SA, and it is currently
creating, rekeying, or closing a Child SA of that IKE SA, it SHOULD
reply with TEMPORARY_FAILURE.

2.25.2. Collisions while Rekeying or Closing IKE SAs

If a peer receives a request to rekey an IKE SA that it is currently
rekeying, it SHOULD reply as usual, and SHOULD prepare to close
redundant SAs and move inherited Child SAs later based on the nonces
(see Section 2.8.2). If a peer receives a request to rekey an IKE SA
that it is currently trying to close, it SHOULD reply with
TEMPORARY_FAILURE.

If a peer receives a request to close an IKE SA that it is currently
rekeying, it SHOULD reply as usual, and forget about its own rekeying
request. If a peer receives a request to close an IKE SA that it is
currently trying to close, it SHOULD reply as usual, and forget about
its own close request.

If a peer receives a request to create or rekey a Child SA when it is
currently rekeying the IKE SA, it SHOULD reply with
TEMPORARY_FAILURE. If a peer receives a request to delete a Child SA
when it is currently rekeying the IKE SA, it SHOULD reply as usual,
with a Delete payload.

3. Header and Payload Formats

In the tables in this section, some cryptographic primitives and
configuration attributes are marked as "UNSPECIFIED". These are
items for which there are no known specifications and therefore
interoperability is currently impossible. A future specification may
describe their use, but until such specification is made,
implementations SHOULD NOT attempt to use items marked as
"UNSPECIFIED" in implementations that are meant to be interoperable.

3.1. The IKE Header

IKE messages use UDP ports 500 and/or 4500, with one IKE message per
UDP datagram. Information from the beginning of the packet through
the UDP header is largely ignored except that the IP addresses and
UDP ports from the headers are reversed and used for return packets.
When sent on UDP port 500, IKE messages begin immediately following
the UDP header. When sent on UDP port 4500, IKE messages have
prepended four octets of zero. zeros. These four octets of zeros are not
part of the IKE message and are not included in any of the length
fields or checksums defined by IKE. Each IKE message begins with the
IKE header, denoted HDR in this document. Following the header are
one or more IKE payloads each identified by a "Next Payload" Next Payload field in
the preceding payload. Payloads are identified in the order in which
they appear in an IKE message by looking in the "Next Payload" Next Payload field in
the IKE header, and subsequently according to the "Next Payload" Next Payload field
in the IKE payload itself until a "Next Payload" Next Payload field of zero
indicates that no payloads follow. If a payload of type "Encrypted"
is found, that payload is decrypted and its contents parsed as
additional payloads. An Encrypted payload MUST be the last payload
in a packet and an Encrypted payload MUST NOT contain another
Encrypted payload.

The responder's SPI in the header identifies an instance of an IKE
Security Association. It is therefore possible for a single instance
of IKE to multiplex distinct sessions with multiple peers, including
multiple sessions per peer.

All multi-octet fields representing integers are laid out in big
endian order (also known as "most significant byte first", or
"network byte order").

The format of the IKE header is shown in Figure 4.

Figure 4: IKE Header Format

o Initiator's SPI (8 octets) - A value chosen by the initiator to
identify a unique IKE Security Association. This value MUST NOT
be zero.

o Responder's SPI (8 octets) - A value chosen by the responder to
identify a unique IKE Security Association. This value MUST be
zero in the first message of an IKE initial exchange (including
repeats of that message including a cookie).

o Next Payload (1 octet) - Indicates the type of payload that
immediately follows the header. The format and value of each
payload are defined below.

o Major Version (4 bits) - Indicates the major version of the IKE
protocol in use. Implementations based on this version of IKE
MUST set the major version to 2. Implementations based on
previous versions of IKE and ISAKMP MUST set the major version
to 1. Implementations based on this document's version
(version 2) of IKE MUST reject or ignore messages containing a
version number greater than 2 with an INVALID_MAJOR_VERSION
notification message as described in Section 2.5.

o Minor Version (4 bits) - Indicates the minor version of the IKE
protocol in use. Implementations based on this version of IKE
MUST set the minor version to 0. They MUST ignore the minor
version number of received messages.

o Exchange Type (1 octet) - Indicates the type of exchange being
used. This constrains the payloads sent in each message in an
exchange. The values in the following table are only current as
of the publication date of RFC 4306. Other values may have been
added since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest
values.

Exchange Type Value
----------------------------------
IKE_SA_INIT 34
IKE_AUTH 35
CREATE_CHILD_SA 36
INFORMATIONAL 37

o Flags (1 octet) - Indicates specific options that are set for the
message. Presence of options is indicated by the appropriate bit
in the flags field being set. The bits are as follows:

+-+-+-+-+-+-+-+-+
|X|X|R|V|I|X|X|X|
+-+-+-+-+-+-+-+-+

In the description below, a bit being 'set' means its value is
'1', while 'cleared' means its value is '0'. 'X' bits MUST be
cleared when sending and MUST be ignored on receipt.

* R (Response) - This bit indicates that this message is a
response to a message containing the same Message ID. This bit
MUST be cleared in all request messages and MUST be set in all
responses. An IKE endpoint MUST NOT generate a response to a
message that is marked as being a response (with one exception;
see Section 2.21.2).

* V (Version) - This bit indicates that the transmitter is
capable of speaking a higher major version number of the
protocol than the one indicated in the major version number
field. Implementations of IKEv2 MUST clear this bit when
sending and MUST ignore it in incoming messages.

* I (Initiator) - This bit MUST be set in messages sent by the
original initiator of the IKE SA and MUST be cleared in
messages sent by the original responder. It is used by the
recipient to determine which eight octets of the SPI were
generated by the recipient. This bit changes to reflect who
initiated the last rekey of the IKE SA.

o Message ID (4 octets, unsigned integer) - Message identifier used
to control retransmission of lost packets and matching of requests
and responses. It is essential to the security of the protocol
because it is used to prevent message replay attacks. See
Sections 2.1 and 2.2.

o Length (4 octets, unsigned integer) - Length of the total message
(header + payloads) in octets.

3.2. Generic Payload Header

Each IKE payload defined in Sections 3.3 through 3.16 begins with a
generic payload header, shown in Figure 5. Figures for each payload
below will include the generic payload header, but for brevity, the
description of each field will be omitted.

Figure 5: Generic Payload Header

The Generic Payload Header fields are defined as follows:

o Next Payload (1 octet) - Identifier for the payload type of the
next payload in the message. If the current payload is the last
in the message, then this field will be 0. This field provides a
"chaining" capability whereby additional payloads can be added to
a message by appending each one to the end of the message and
setting the "Next Payload" Next Payload field of the preceding payload to
indicate the new payload's type. An Encrypted payload, which must
always be the last payload of a message, is an exception. It
contains data structures in the format of additional payloads. In
the header of an Encrypted payload, the Next Payload field is set
to the payload type of the first contained payload (instead of 0);
conversely, the Next Payload field of the last contained payload
is set to zero. The payload type values are listed here. The
values in the following table are only current as of the
publication date of RFC 4306. Other values may have been added
since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest
values.

Next Payload Type Notation Value
--------------------------------------------------
No Next Payload 0
Security Association SA 33
Key Exchange KE 34
Identification - Initiator IDi 35
Identification - Responder IDr 36
Certificate CERT 37
Certificate Request CERTREQ 38
Authentication AUTH 39
Nonce Ni, Nr 40
Notify N 41
Delete D 42
Vendor ID V 43
Traffic Selector - Initiator TSi 44
Traffic Selector - Responder TSr 45
Encrypted and Authenticated SK 46
Configuration CP 47
Extensible Authentication EAP 48

(Payload type values 1-32 should not be assigned in the
future so that there is no overlap with the code assignments
for IKEv1.)

o Critical (1 bit) - MUST be set to zero if the sender wants the
recipient to skip this payload if it does not understand the
payload type code in the Next Payload field of the previous
payload. MUST be set to one if the sender wants the recipient to
reject this entire message if it does not understand the payload
type. MUST be ignored by the recipient if the recipient
understands the payload type code. MUST be set to zero for
payload types defined in this document. Note that the critical
bit applies to the current payload rather than the "next" payload
whose type code appears in the first octet. The reasoning behind
not setting the critical bit for payloads defined in this document
is that all implementations MUST understand all payload types
defined in this document and therefore must ignore the critical
bit's value. Skipped payloads are expected to have valid Next
Payload and Payload Length fields. See Section 2.5 for more
information on this bit.

o RESERVED (7 bits) - MUST be sent as zero; MUST be ignored on
receipt.

o Payload Length (2 octets, unsigned integer) - Length in octets of
the current payload, including the generic payload header.

Many payloads contain fields marked as "RESERVED". Some payloads in
IKEv2 (and historically in IKEv1) are not aligned to 4-octet
boundaries.

3.3. Security Association Payload

The Security Association payload, denoted SA in this document, is
used to negotiate attributes of a Security Association. Assembly of
Security Association payloads requires great peace of mind. An SA
payload MAY contain multiple proposals. If there is more than one,
they MUST be ordered from most preferred to least preferred. Each
proposal contains a single IPsec protocol (where a protocol is IKE,
ESP, or AH), each protocol MAY contain multiple transforms, and each
transform MAY contain multiple attributes. When parsing an SA, an
implementation MUST check that the total Payload Length is consistent
with the payload's internal lengths and counts. Proposals,
Transforms, and Attributes each have their own variable-length
encodings. They are nested such that the Payload Length of an SA
includes the combined contents of the SA, Proposal, Transform, and
Attribute information. The length of a Proposal includes the lengths
of all Transforms and Attributes it contains. The length of a
Transform includes the lengths of all Attributes it contains.

The syntax of Security Associations, Proposals, Transforms, and
Attributes is based on ISAKMP; however, the semantics are somewhat
different. The reason for the complexity and the hierarchy is to
allow for multiple possible combinations of algorithms to be encoded
in a single SA. Sometimes there is a choice of multiple algorithms,
whereas other times there is a combination of algorithms. For
example, an initiator might want to propose using ESP with either
(3DES and HMAC_MD5) or (AES and HMAC_SHA1).

One of the reasons the semantics of the SA payload have changed from
ISAKMP and IKEv1 is to make the encodings more compact in common
cases.

The Proposal structure contains within it a Proposal Num and an IPsec
protocol ID. Each structure MUST have a proposal number one (1)
greater than the previous structure. The first Proposal in the
initiator's SA payload MUST have a Proposal Num of one (1). One
reason to use multiple proposals is to propose both standard crypto
ciphers and combined-mode ciphers. Combined-mode ciphers include
both integrity and encryption in a single encryption algorithm, and
MUST either offer no integrity algorithm or a single integrity
algorithm of "none", "NONE", with no integrity algorithm being the
RECOMMENDED method. If an initiator wants to propose both combined-
mode ciphers and normal ciphers, it must include two proposals: one
will have all the combined-mode ciphers, and the other will have all
the normal ciphers with the integrity algorithms. For example, one
such proposal would have two proposal structures. Proposal 1 is ESP
with AES-128, AES-192, and AES-256 bits in Cipher Block Chaining
(CBC) mode, with either HMAC-SHA1-96 or XCBC-96 as the integrity
algorithm; Proposal 2 is AES-128 or AES-256 in GCM mode with an
8-octet Integrity Check Value (ICV). Both proposals allow but do not
require the use of ESNs (Extended Sequence Numbers). This can be
illustrated as:

Each Proposal/Protocol structure is followed by one or more transform
structures. The number of different transforms is generally
determined by the Protocol. AH generally has two transforms:
Extended Sequence Numbers (ESNs) and an integrity check algorithm.
ESP generally has three: ESN, an encryption algorithm, and an
integrity check algorithm. IKE generally has four transforms: a
Diffie-Hellman group, an integrity check algorithm, a PRF algorithm,
and an encryption algorithm. For each Protocol, the set of
permissible transforms is assigned Transform ID numbers, which appear
in the header of each transform.

If there are multiple transforms with the same Transform Type, the
proposal is an OR of those transforms. If there are multiple
transforms with different Transform Types, the proposal is an AND of
the different groups. For example, to propose ESP with (3DES or AES-
CBC)
AES-CBC) and (HMAC_MD5 or HMAC_SHA), the ESP proposal would contain
two Transform Type 1 candidates (one for 3DES and one for AEC-CBC)
and two Transform Type 3 candidates (one for HMAC_MD5 and one for
HMAC_SHA). This effectively proposes four combinations of
algorithms. If the initiator wanted to propose only a subset of
those, for example (3DES and HMAC_MD5) or (IDEA and HMAC_SHA), there
is no way to encode that as multiple transforms within a single
Proposal. Instead, the initiator would have to construct two
different Proposals, each with two transforms.

A given transform MAY have one or more Attributes. Attributes are
necessary when the transform can be used in more than one way, as
when an encryption algorithm has a variable key size. The transform
would specify the algorithm and the attribute would specify the key
size. Most transforms do not have attributes. A transform MUST NOT
have multiple attributes of the same type. To propose alternate
values for an attribute (for example, multiple key sizes for the AES
encryption algorithm), an implementation MUST include multiple
transforms with the same Transform Type each with a single Attribute.

Note that the semantics of Transforms and Attributes are quite
different from those in IKEv1. In IKEv1, a single Transform carried
multiple algorithms for a protocol with one carried in the Transform
and the others carried in the Attributes.

Figure 6: Security Association Payload

o Proposals (variable) - One or more proposal substructures.

The payload type for the Security Association payload is
thirty-three (33).

3.3.1. Proposal Substructure

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Last Substruc | RESERVED | Proposal Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Proposal Num | Protocol ID | SPI Size |Num Transforms|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ SPI (variable) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ <Transforms> ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Proposal Substructure

o Last Substruc (1 octet) - Specifies whether or not this is the
last Proposal Substructure in the SA. This field has a value of 0
if this was the last Proposal Substructure, and a value of 2 if
there are more Proposal Substructures. This syntax is inherited
from ISAKMP, but is unnecessary because the last Proposal could be
identified from the length of the SA. The value (2) corresponds
to a payload type of Proposal in IKEv1, and the first four octets
of the Proposal structure are designed to look somewhat like the
header of a payload.

o RESERVED (1 octet) - MUST be sent as zero; MUST be ignored on
receipt.

o Proposal Length (2 octets, unsigned integer) - Length of this
proposal, including all transforms and attributes that follow.

o Proposal Num (1 octet) - When a proposal is made, the first
proposal in an SA payload MUST be 1, and subsequent proposals MUST
be one more than the previous proposal (indicating an OR of the
two proposals). When a proposal is accepted, the proposal number
in the SA payload MUST match the number on the proposal sent that
was accepted.

o Protocol ID (1 octet) - Specifies the IPsec protocol identifier
for the current negotiation. The values in the following table
are only current as of the publication date of RFC 4306. Other
values may have been added since then or will be added after the
publication of this document. Readers should refer to [IKEV2IANA]
for the latest values.

Protocol Protocol ID
-----------------------------------
IKE 1
AH 2
ESP 3

o SPI Size (1 octet) - For an initial IKE SA negotiation, this field
MUST be zero; the SPI is obtained from the outer header. During
subsequent negotiations, it is equal to the size, in octets, of
the SPI of the corresponding protocol (8 for IKE, 4 for ESP
and AH).

o Num Transforms (1 octet) - Specifies the number of transforms in
this proposal.

o SPI (variable) - The sending entity's SPI. Even if the SPI Size
is not a multiple of 4 octets, there is no padding applied to the
payload. When the SPI Size field is zero, this field is not
present in the Security Association payload.

o Transforms (variable) - One or more transform substructures.

3.3.2. Transform Substructure

Figure 8: Transform Substructure

o Last Substruc (1 octet) - Specifies whether or not this is the
last Transform Substructure in the Proposal. This field has a
value of 0 if this was the last Transform Substructure, and a
value of 3 if there are more Transform Substructures. This syntax
is inherited from ISAKMP, but is unnecessary because the last
transform could be identified from the length of the proposal.
The value (3) corresponds to a payload type of Transform in IKEv1,
and the first four octets of the Transform structure are designed
to look somewhat like the header of a payload.

o RESERVED - MUST be sent as zero; MUST be ignored on receipt.

o Transform Length - The length (in octets) of the Transform
Substructure including Header and Attributes.

o Transform Type (1 octet) - The type of transform being specified
in this transform. Different protocols support different
Transform Types. For some protocols, some of the transforms may
be optional. If a transform is optional and the initiator wishes
to propose that the transform be omitted, no transform of the
given type is included in the proposal. If the initiator wishes
to make use of the transform optional to the responder, it
includes a transform substructure with Transform ID = 0 as one of
the options.

o Transform ID (2 octets) - The specific instance of the Transform
Type being proposed.

The Transform Type values are listed below. The values in the
following table are only current as of the publication date of
RFC 4306. Other values may have been added since then or will be
added after the publication of this document. Readers should refer
to [IKEV2IANA] for the latest values.

Description Trans. Used In
Type
------------------------------------------------------------------
Encryption Algorithm (ENCR) 1 IKE and ESP
Pseudorandom Function (PRF) 2 IKE
Integrity Algorithm (INTEG) 3 IKE*, AH, optional in ESP
Diffie-Hellman group Group (D-H) 4 IKE, optional in AH & ESP
Extended Sequence Numbers (ESN) 5 AH and ESP

(*) Negotiating an integrity algorithm is mandatory for the
Encrypted payload format specified in this document. For example,
[AEAD] specifies additional formats based on authenticated
encryption, in which a separate integrity algorithm is not
negotiated.

For Transform Type 1 (Encryption Algorithm), the Transform IDs are
listed below. The values in the following table are only current as
of the publication date of RFC 4306. Other values may have been
added since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest values.

Name Number Defined In
---------------------------------------------------
ENCR_DES_IV64 1 (UNSPECIFIED)
ENCR_DES 2 (RFC2405), [RFC2405], [DES]
ENCR_3DES 3 (RFC2451) [RFC2451]
ENCR_RC5 4 (RFC2451) [RFC2451]
ENCR_IDEA 5 (RFC2451), [RFC2451], [IDEA]
ENCR_CAST 6 (RFC2451) [RFC2451]
ENCR_BLOWFISH 7 (RFC2451) [RFC2451]
ENCR_3IDEA 8 (UNSPECIFIED)
ENCR_DES_IV32 9 (UNSPECIFIED)
ENCR_NULL 11 (RFC2410) [RFC2410]
ENCR_AES_CBC 12 (RFC3602) [RFC3602]
ENCR_AES_CTR 13 (RFC3686) [RFC3686]

For Transform Type 2 (Pseudorandom Function), the Transform IDs are
listed below. The values in the following table are only current as
of the publication date of RFC 4306. Other values may have been
added since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest values.

Name Number Defined In
------------------------------------------------------
------------------------------------------------------------------
PRF_HMAC_MD5 1 (RFC2104), [RFC2104], [MD5]
PRF_HMAC_SHA1 2 (RFC2104), [SHA] [RFC2104], [FIPS.180-4.2012]
PRF_HMAC_TIGER 3 (UNSPECIFIED)

For Transform Type 3 (Integrity Algorithm), defined Transform IDs are
listed below. The values in the following table are only current as
of the publication date of RFC 4306. Other values may have been
added since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest values.

Name Number Defined In
----------------------------------------
NONE 0
AUTH_HMAC_MD5_96 1 (RFC2403) [RFC2403]
AUTH_HMAC_SHA1_96 2 (RFC2404) [RFC2404]
AUTH_DES_MAC 3 (UNSPECIFIED)
AUTH_KPDK_MD5 4 (UNSPECIFIED)
AUTH_AES_XCBC_96 5 (RFC3566) [RFC3566]
For Transform Type 4 (Diffie-Hellman group), defined Transform IDs
are listed below. The values in the following table are only current
as of the publication date of RFC 4306. Other values may have been
added since then or will be added after the publication of this
document. Readers should refer to [IKEV2IANA] for the latest values.

Name Number Defined In
----------------------------------------
------------------------------------------
NONE 0
768-bit MODP Group 1 Appendix B
1024-bit MODP Group 2 Appendix B
1536-bit MODP Group 5 [ADDGROUP]
2048-bit MODP Group 14 [ADDGROUP]
3072-bit MODP Group 15 [ADDGROUP]
4096-bit MODP Group 16 [ADDGROUP]
6144-bit MODP Group 17 [ADDGROUP]
8192-bit MODP Group 18 [ADDGROUP]

Although ESP and AH do not directly include a Diffie-Hellman
exchange, a Diffie-Hellman group MAY be negotiated for the Child SA.
This allows the peers to employ Diffie-Hellman in the CREATE_CHILD_SA
exchange, providing perfect forward secrecy for the generated Child
SA keys.

Note,

Note that the MODP Diffie-Hellman groups listed above does do not need any
special validity tests to be performed, but other types of groups
(ECP
(elliptic curve groups, and MODP groups with small subgroups) need to
have some additional tests to be performed on them to use them securely.
See "Additional Diffie-Hellman Tests for IKEv2" ([RFC6989]) for more
information.

For Transform Type 5 (Extended Sequence Numbers), defined Transform
IDs are listed below. The values in the following table are only
current as of the publication date of RFC 4306. Other values may
have been added since then or will be added after the publication of
this document. Readers should refer to [IKEV2IANA] for the latest
values.

Name Number
--------------------------------------------
No Extended Sequence Numbers 0
Extended Sequence Numbers 1

Note that an initiator who supports ESNs will usually include two ESN
transforms, with values "0" and "1", in its proposals. A proposal
containing a single ESN transform with value "1" means that using
normal (non-extended) sequence numbers is not acceptable.

Numerous additional Transform Types have been defined since the
publication of RFC 4306. Please refer to the IANA IKEv2 "Internet Key
Exchange Version 2 (IKEv2) Parameters" registry for details.

3.3.3. Valid Transform Types by Protocol

The number and type of transforms that accompany an SA payload are
dependent on the protocol in the SA itself. An SA payload proposing
the establishment of an SA has the following mandatory and optional
Transform Types. A compliant implementation MUST understand all
mandatory and optional types for each protocol it supports (though it
need not accept proposals with unacceptable suites). A proposal MAY
omit the optional types if the only value for them it will accept is
NONE.

Protocol Mandatory Types Optional Types
---------------------------------------------------
IKE ENCR, PRF, INTEG*, D-H
ESP ENCR, ESN INTEG, D-H
AH INTEG, ESN D-H

3.3.4. Mandatory Transform IDs

The specification of suites that MUST and SHOULD be supported for
interoperability has been removed from this document because they are
likely to change more rapidly than this document evolves. At the
time of publication of this document, [RFC4307] specifies these
suites, but note that it might be updated in the future, and other
RFCs might specify different sets of suites.

An important lesson learned from IKEv1 is that no system should only
implement the mandatory algorithms and expect them to be the best
choice for all customers.

It is likely that IANA will add additional transforms in the future,
and some users may want to use private suites, especially for IKE
where implementations should be capable of supporting different
parameters, up to certain size limits. In support of this goal, all
implementations of IKEv2 SHOULD include a management facility that
allows specification (by a user or system administrator) of Diffie-
Hellman parameters (the generator, modulus, and exponent lengths and
values) for new Diffie-Hellman groups. Implementations SHOULD
provide a management interface through which these parameters and the
associated Transform IDs may be entered (by a user or system
administrator), to enable negotiating such groups.

All implementations of IKEv2 MUST include a management facility that
enables a user or system administrator to specify the suites that are
acceptable for use with IKE. Upon receipt of a payload with a set of
Transform IDs, the implementation MUST compare the transmitted
Transform IDs against those locally configured via the management
controls, to verify that the proposed suite is acceptable based on
local policy. The implementation MUST reject SA proposals that are
not authorized by these IKE suite controls. Note that cryptographic
suites that MUST be implemented need not be configured as acceptable
to local policy.

3.3.5. Transform Attributes

Each transform in a Security Association payload may include
attributes that modify or complete the specification of the
transform. The set of valid attributes depends on the transform.
Currently, only a single attribute type is defined: the Key Length
attribute is used by certain encryption transforms with variable-
length keys (see below for details).

The attributes are type/value pairs and are defined below.
Attributes can have a value with a fixed two-octet length or a
variable-length value. For the latter, the attribute is encoded as
type/length/value.

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A| Attribute Type | AF=0 Attribute Length |
|F| | AF=1 Attribute Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AF=0 Attribute Value |
| AF=1 Not Transmitted |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: Data Attributes

o Attribute Format (AF) (1 bit) - Indicates whether the data
attribute follows the Type/Length/Value (TLV) format or a
shortened Type/Value (TV) format. If the AF bit is zero (0), then
the attribute uses TLV format; if the AF bit is one (1), the TV
format (with two-byte value) is used.

o Attribute Type (15 bits) - Unique identifier for each type of
attribute (see below).

o Attribute Value (variable length) - Value of the attribute
associated with the attribute type. If the AF bit is a zero (0),
this field has a variable length defined by the Attribute Length
field. If the AF bit is a one (1), the Attribute Value has a
length of 2 octets.

The only currently defined attribute type (Key Length) is fixed
length; the variable-length encoding specification is included only
for future extensions. Attributes described as fixed length MUST NOT
be encoded using the variable-length encoding unless that length
exceeds two bytes. Variable-length attributes MUST NOT be encoded as
fixed-length even if their value can fit into two octets. Note: This
is a change from IKEv1, where increased flexibility may have
simplified the composer of messages but certainly complicated the
parser.

The values in the following table are only current as of the
publication date of RFC 4306. Other values may have been added since
then or will be added after the publication of this document.
Readers should refer to [IKEV2IANA] for the latest values.

Attribute Type Value Attribute Format
------------------------------------------------------------
Key Length (in bits) 14 TV

Values 0-13 and 15-17 were used in a similar context in IKEv1, and
should not be assigned except to matching values.

The Key Length attribute specifies the key length in bits (MUST use
network byte order) for certain transforms as follows:

o The Key Length attribute MUST NOT be used with transforms that use
a fixed-length key. For example, this includes ENCR_DES,
ENCR_IDEA, and all the Type 2 (Pseudorandom function) Function) and Type 3
(Integrity Algorithm) transforms specified in this document. It
is recommended that future Type 2 or 3 transforms do not use this
attribute.

o Some transforms specify that the Key Length attribute MUST be
always included (omitting the attribute is not allowed, and
proposals not containing it MUST be rejected). For example, this
includes ENCR_AES_CBC and ENCR_AES_CTR.

o Some transforms allow variable-length keys, but also specify a
default key length if the attribute is not included. For example,
these transforms include ENCR_RC5 and ENCR_BLOWFISH.

Implementation note: To further interoperability and to support
upgrading endpoints independently, implementers of this protocol
SHOULD accept values that they deem to supply greater security. For
instance, if a peer is configured to accept a variable-length cipher
with a key length of X bits and is offered that cipher with a larger
key length, the implementation SHOULD accept the offer if it supports
use of the longer key.

Support for this capability allows a responder to express a concept
of "at least" a certain level of security -- "a key length of _at
least_ X bits for cipher Y". However, as the attribute is always
returned unchanged (see the next section), an initiator willing to
accept multiple key lengths has to include multiple transforms with
the same Transform Type, each with a different Key Length attribute.

3.3.6. Attribute Negotiation

During Security Association negotiation initiators present offers to
responders. Responders MUST select a single complete set of
parameters from the offers (or reject all offers if none are
acceptable). If there are multiple proposals, the responder MUST
choose a single proposal. If the selected proposal has multiple
transforms with the same type, the responder MUST choose a single
one. Any attributes of a selected transform MUST be returned
unmodified. The initiator of an exchange MUST check that the
accepted offer is consistent with one of its proposals, and if not
MUST terminate the exchange.

If the responder receives a proposal that contains a Transform Type
it does not understand, or a proposal that is missing a mandatory
Transform Type, it MUST consider this proposal unacceptable; however,
other proposals in the same SA payload are processed as usual.
Similarly, if the responder receives a transform that it does not
understand, or one that contains a Transform Attribute it does not
understand, it MUST consider this transform unacceptable; other
transforms with the same Transform Type are processed as usual. This
allows new Transform Types and Transform Attributes to be defined in
the future.

Negotiating Diffie-Hellman groups presents some special challenges.
SA offers include proposed attributes and a Diffie-Hellman public
number (KE) in the same message. If in the initial exchange the
initiator offers to use one of several Diffie-Hellman groups, it
SHOULD pick the one the responder is most likely to accept and
include a KE corresponding to that group. If the responder selects a
proposal using a different Diffie-Hellman group (other than NONE),
the responder will indicate the correct group in the response and the
initiator SHOULD pick an element of that group for its KE value when
retrying the first message. It SHOULD, however, continue to propose
its full supported set of groups in order to prevent a man-in-the-
middle
man-in-the-middle downgrade attack. If one of the proposals offered
is for the Diffie-Hellman group of NONE, and the responder selects
that Diffie-
Hellman Diffie-Hellman group, then it MUST ignore the initiator's KE
payload and omit the KE payload from the response.

3.4. Key Exchange Payload

The Key Exchange payload, denoted KE in this document, is used to
exchange Diffie-Hellman public numbers as part of a Diffie-Hellman
key exchange. The Key Exchange payload consists of the IKE generic
payload header followed by the Diffie-Hellman public value itself.

Figure 10: Key Exchange Payload Format

A Key Exchange payload is constructed by copying one's Diffie-Hellman
public value into the "Key Exchange Data" portion of the payload.
The length of the Diffie-Hellman public value for modular
exponentiation group (MODP) MODP groups MUST be
equal to the length of the prime modulus over which the
exponentiation was performed, prepending zero bits to the value if
necessary.

The Diffie-Hellman Group Num identifies the Diffie-Hellman group in
which the Key Exchange Data was computed (see Section 3.3.2). This
Diffie-Hellman Group Num MUST match a Diffie-Hellman group specified
in a proposal in the SA payload that is sent in the same message, and
SHOULD match the Diffie-Hellman group in the first group in the first
proposal, if such exists. If none of the proposals in that SA
payload specifies a Diffie-Hellman group, the KE payload MUST NOT be
present. If the selected proposal uses a different Diffie-Hellman
group (other than NONE), the message MUST be rejected with a Notify
payload of type INVALID_KE_PAYLOAD. See also Sections 1.2 and 2.7.

The payload type for the Key Exchange payload is thirty-four (34).

3.5. Identification Payloads

The Identification payloads, denoted IDi and IDr in this document,
allow peers to assert an identity to one another. This identity may
be used for policy lookup, but does not necessarily have to match
anything in the CERT payload; both fields may be used by an
implementation to perform access control decisions. When using the
ID_IPV4_ADDR/ID_IPV6_ADDR identity types in IDi/IDr payloads, IKEv2
does not require this address to match the address in the IP header
of IKEv2 packets, or anything in the TSi/TSr payloads. The contents
of IDi/IDr are used purely to fetch the policy and authentication
data related to the other party.

NOTE: In IKEv1, two ID payloads were used in each direction to hold
Traffic Selector (TS) information for data passing over the SA. In
IKEv2, this information is carried in TS payloads (see Section 3.13).

The Peer Authorization Database (PAD) as described in RFC 4301
[IPSECARCH] describes the use of the ID payload in IKEv2 and provides
a formal model for the binding of identity to policy in addition to
providing services that deal more specifically with the details of
policy enforcement. The PAD is intended to provide a link between
the SPD and the IKE Security Association management. See
Section 4.4.3 of RFC 4301 for more details.

The Identification payload consists of the IKE generic payload header
followed by identification fields as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID Type | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Identification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 11: Identification Payload Format
o ID Type (1 octet) - Specifies the type of Identification being
used.

o RESERVED - MUST be sent as zero; MUST be ignored on receipt.

o Identification Data (variable length) - Value, as indicated by the
Identification Type. The length of the Identification Data is
computed from the size in the ID payload header.

The payload types for the Identification payload are thirty-five (35)
for IDi and thirty-six (36) for IDr.

The following table lists the assigned semantics for the
Identification Type field. The values in the following table are
only current as of the publication date of RFC 4306. Other values
may have been added since then or will be added after the publication
of this document. Readers should refer to [IKEV2IANA] for the latest
values.

ID Type Value
-------------------------------------------------------------------
ID_IPV4_ADDR 1
A single four (4) octet IPv4 address.

ID_FQDN 2
A fully-qualified domain name string. An example of an ID_FQDN
is "example.com". The string MUST NOT contain any terminators
(e.g., NULL, CR, etc.). All characters in the ID_FQDN are ASCII;
for an "internationalized domain name", the syntax is as defined
in [IDNA], for example "xn--tmonesimerkki-bfbb.example.net".

ID_RFC822_ADDR 3
A fully-qualified RFC 822 email address string. An example of a
ID_RFC822_ADDR is "jsmith@example.com". The string MUST NOT
contain any terminators. Because of [EAI], implementations would
be wise to treat this field as UTF-8 encoded text, not as
pure ASCII.

ID_IPV6_ADDR 5
A single sixteen (16) octet IPv6 address.

ID_DER_ASN1_DN 9
The binary Distinguished Encoding Rules (DER) encoding of an
ASN.1 X.500 Distinguished Name [PKIX].

ID_DER_ASN1_GN 10
The binary DER encoding of an ASN.1 X.509 GeneralName [PKIX].

ID_KEY_ID 11
An opaque octet stream that may be used to pass vendor-
specific information necessary to do certain proprietary
types of identification.

Two implementations will interoperate only if each can generate a
type of ID acceptable to the other. To assure maximum
interoperability, implementations MUST be configurable to send at
least one of ID_IPV4_ADDR, ID_FQDN, ID_RFC822_ADDR, or ID_KEY_ID, and
MUST be configurable to accept all of these four types.
Implementations SHOULD be capable of generating and accepting all of
these types. IPv6-capable implementations MUST additionally be
configurable to accept ID_IPV6_ADDR. IPv6-only implementations MAY
be configurable to send only ID_IPV6_ADDR instead of ID_IPV4_ADDR for
IP addresses.

EAP [EAP] does not mandate the use of any particular type of
identifier, but often EAP is used with Network Access Identifiers
(NAIs) defined in [NAI]. Although NAIs look a bit like email
addresses (e.g., "joe@example.com"), the syntax is not exactly the
same as the syntax of email address in [MAILFORMAT]. For those NAIs
that include the realm component, the ID_RFC822_ADDR identification
type SHOULD be used. Responder implementations should not attempt to
verify that the contents actually conform to the exact syntax given
in [MAILFORMAT], but instead should accept any reasonable-looking
NAI. For NAIs that do not include the realm component, the ID_KEY_ID
identification type SHOULD be used.

See "IPsec "The Internet IP Security PKI Profile of IKEv1, IKEv2 IKEv1/ISAKMP, IKEv2, and
PKIX" ([RFC4945]) for more information about matching Identification
payloads and the contents of the PKIX Certificates.

3.6. Certificate Payload

The Certificate payload, denoted CERT in this document, provides a
means to transport certificates or other authentication-related
information via IKE. Certificate payloads SHOULD be included in an
exchange if certificates are available to the sender. The Hash and
URL formats of the Certificate payloads should be used in case the
peer has indicated an ability to retrieve this information from
elsewhere using an HTTP_CERT_LOOKUP_SUPPORTED Notify payload. Note
that the term "Certificate payload" is somewhat misleading, because
not all authentication mechanisms use certificates and data other
than certificates may be passed in this payload.

The Certificate payload is defined as follows:

Figure 12: Certificate Payload Format

o Certificate Encoding (1 octet) - This field indicates the type of
certificate or certificate-related information contained in the
Certificate Data field. The values in the following table are
only current as of the publication date of RFC 4306. Other values
may have been added since then or will be added after the
publication of this document. Readers should refer to [IKEV2IANA]
for the latest values.

Certificate Encoding Value
----------------------------------------------------
PKCS #7 wrapped X.509 certificate 1 UNSPECIFIED
PGP Certificate 2 UNSPECIFIED
DNS Signed Key 3 UNSPECIFIED
X.509 Certificate - Signature 4
Kerberos Token 6 UNSPECIFIED
Certificate Revocation List (CRL) 7
Authority Revocation List (ARL) 8 UNSPECIFIED
SPKI Certificate 9 UNSPECIFIED
X.509 Certificate - Attribute 10 UNSPECIFIED
Deprecated (Was (was Raw RSA Key) 11 DEPRECATED
Hash and URL of X.509 certificate 12
Hash and URL of X.509 bundle 13

o Certificate Data (variable length) - Actual encoding of
certificate data. The type of certificate is indicated by the
Certificate Encoding field.

The payload type for the Certificate payload is thirty-seven (37).

Specific syntax for some of the certificate type codes above is not
defined in this document. The types whose syntax is defined in this
document are:

o "X.509 Certificate - Signature" contains a DER-encoded X.509
certificate whose public key is used to validate the sender's AUTH
payload. Note that with this encoding, if a chain of certificates
needs to be sent, multiple CERT payloads are used, only the first
of which holds the public key used to validate the sender's AUTH
payload.

o "Certificate Revocation List" contains a DER-encoded X.509
certificate revocation list.

o Hash and URL encodings allow IKE messages to remain short by
replacing long data structures with a 20-octet SHA-1 hash (see
[SHA])
[FIPS.180-4.2012]) of the replaced value followed by a variable-length variable-
length URL that resolves to the DER-encoded data structure itself.
This improves efficiency when the endpoints have certificate data
cached and makes IKE less subject to DoS attacks that become
easier to mount when IKE messages are large enough to require IP
fragmentation [DOSUDPPROT].

The "Hash and URL of a bundle" type uses the following ASN.1
definition for the X.509 bundle:

CertBundle
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-cert-bundle(34) }

DEFINITIONS EXPLICIT TAGS ::=
BEGIN

IMPORTS
Certificate, CertificateList
FROM PKIX1Explicit88
{ iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-pkix1-explicit(18) } ;

CertificateOrCRL ::= CHOICE {
cert [0] Certificate,
crl [1] CertificateList }

CertificateBundle ::= SEQUENCE OF CertificateOrCRL

END
Implementations MUST be capable of being configured to send and
accept up to four X.509 certificates in support of authentication,
and also MUST be capable of being configured to send and accept the
two Hash and URL formats (with HTTP URLs). If multiple certificates
are sent, the first certificate MUST contain the public key
associated with the private key used to sign the AUTH payload. The
other certificates may be sent in any order.

Implementations MUST support the HTTP [HTTP] method "http:" scheme for hash-and-URL
lookup. The behavior of other URL methods schemes [URLS] is not currently
specified, and such methods schemes SHOULD NOT be used in the absence of a
document specifying them.

3.7. Certificate Request Payload

The Certificate Request payload, denoted CERTREQ in this document,
provides a means to request preferred certificates via IKE and can
appear in the IKE_INIT_SA response and/or the IKE_AUTH request.
Certificate Request payloads MAY be included in an exchange when the
sender needs to get the certificate of the receiver.

The Certificate Request payload is defined as follows:

Figure 13: Certificate Request Payload Format

o Certificate Encoding (1 octet) - Contains an encoding of the type
or format of certificate requested. Values are listed in
Section 3.6.

o Certification Authority (variable length) - Contains an encoding
of an acceptable certification authority for the type of
certificate requested.

The payload type for the Certificate Request payload is
thirty-eight (38).

The Certificate Encoding field has the same values as those defined
in Section 3.6. The Certification Authority field contains an
indicator of trusted authorities for this certificate type. The
Certification Authority value is a concatenated list of SHA-1 hashes
of the public keys of trusted Certification Authorities (CAs). Each
is encoded as the SHA-1 hash of the Subject Public Key Info element
(see section Section 4.1.2.7 of [PKIX]) from each Trust Anchor certificate.
The 20-octet hashes are concatenated and included with no other
formatting.

The contents of the "Certification Authority" Certification Authority field are defined only
for X.509 certificates, which are types 4, 12, and 13. Other values
SHOULD NOT be used until Standards-Track specifications that specify
their use are published.

Note that the term "Certificate Request" is somewhat misleading, in
that values other than certificates are defined in a "Certificate"
payload and requests for those values can be present in a Certificate
Request payload. The syntax of the Certificate Request payload in
such cases is not defined in this document.

The Certificate Request payload is processed by inspecting the "Cert
Encoding"
Cert Encoding field to determine whether the processor has any
certificates of this type. If so, the "Certification Authority" Certification Authority field
is inspected to determine if the processor has any certificates that
can be validated up to one of the specified certification
authorities. This can be a chain of certificates.

If an end-entity certificate exists that satisfies the criteria
specified in the CERTREQ, a certificate or certificate chain SHOULD
be sent back to the certificate requestor if the recipient of the
CERTREQ:

o is configured to use certificate authentication,

o is allowed to send a CERT payload,

o has matching CA trust policy governing the current negotiation,
and

o has at least one time-wise and usage-appropriate end-entity
certificate chaining to a CA provided in the CERTREQ.

Certificate revocation checking must be considered during the
chaining process used to select a certificate. Note that even if two
peers are configured to use two different CAs, cross-certification
relationships should be supported by appropriate selection logic.

The intent is not to prevent communication through the strict
adherence of selection of a certificate based on CERTREQ, when an
alternate certificate could be selected by the sender that would
still enable the recipient to successfully validate and trust it
through trust conveyed by cross-certification, CRLs, or other out-of-
band
out-of-band configured means. Thus, the processing of a CERTREQ
should be seen as a suggestion for a certificate to select, not a
mandated one. If no certificates exist, then the CERTREQ is ignored.
This is not an error condition of the protocol. There may be cases
where there is a preferred CA sent in the CERTREQ, but an alternate
might be acceptable (perhaps after prompting a human operator).

The HTTP_CERT_LOOKUP_SUPPORTED notification MAY be included in any
message that can include a CERTREQ payload and indicates that the
sender is capable of looking up certificates based on an HTTP-based
URL (and hence presumably would prefer to receive certificate
specifications in that format).

3.8. Authentication Payload

The Authentication payload, denoted AUTH in this document, contains
data used for authentication purposes. The syntax of the
Authentication data Data varies according to the Auth Method as specified
below.

The Authentication payload is defined as follows:

Figure 14: Authentication Payload Format

o Auth Method (1 octet) - Specifies the method of authentication
used. The types of signatures are listed here. The values in the
following table are only current as of the publication date of
RFC 4306. Other values may have been added since then or will be
added after the publication of this document. Readers should
refer to [IKEV2IANA] for the latest values.

Mechanism Value
-----------------------------------------------------------------
RSA Digital Signature 1
Computed as specified in Section 2.15 using an RSA private key
with RSASSA-PKCS1-v1_5 signature scheme specified in [PKCS1]
(implementers should note that IKEv1 used a different method for
RSA signatures). To promote interoperability, implementations
that support this type SHOULD support signatures that use SHA-1
as the hash function and SHOULD use SHA-1 as the default hash
function when generating signatures. Implementations can use the
certificates received from a given peer as a hint for selecting a
mutually understood hash function for the AUTH payload signature.
Note, however, that the hash algorithm used in the AUTH payload
signature doesn't have to be the same as any hash algorithm(s)
used in the certificate(s).

Shared Key Message Integrity Code 2
Computed as specified in Section 2.15 using the shared key
associated with the identity in the ID payload and the
negotiated PRF.

DSS Digital Signature 3
Computed as specified in Section 2.15 using a DSS private key
(see [DSS]) over a SHA-1 hash.

o RESERVED - MUST be sent as zero; MUST be ignored on receipt.

o Authentication Data (variable length) - see Section 2.15.

The payload type for the Authentication payload is thirty-nine (39).

3.9. Nonce Payload

The Nonce payload, denoted as Ni and Nr in this document for the
initiator's and responder's nonce, respectively, contains random data
used to guarantee liveness during an exchange and protect against
replay attacks.

The Nonce payload is defined as follows:

Figure 15: Nonce Payload Format

o Nonce Data (variable length) - Contains the random data generated
by the transmitting entity.

The payload type for the Nonce payload is forty (40).

The size of the Nonce Data MUST be between 16 and 256 octets,
inclusive. Nonce values MUST NOT be reused.

3.10. Notify Payload

The Notify payload, denoted N in this document, is used to transmit
informational data, such as error conditions and state transitions,
to an IKE peer. A Notify payload may appear in a response message
(usually specifying why a request was rejected), in an INFORMATIONAL
Exchange
exchange (to report an error not in an IKE request), or in any other
message to indicate sender capabilities or to modify the meaning of
the request.

The Notify payload is defined as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol ID | SPI Size | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Parameter Index (SPI) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Notification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 16: Notify Payload Format

o Protocol ID (1 octet) - If this notification concerns an existing
SA whose SPI is given in the SPI field, this field indicates the
type of that SA. For notifications concerning Child SAs, this
field MUST contain either (2) to indicate AH or (3) to indicate
ESP. Of the notifications defined in this document, the SPI is
included only with INVALID_SELECTORS, REKEY_SA REKEY_SA, and
CHILD_SA_NOT_FOUND. If the SPI field is empty, this field MUST be
sent as zero and MUST be ignored on receipt.

o SPI Size (1 octet) - Length in octets of the SPI as defined by the
IPsec protocol ID or zero if no SPI is applicable. For a
notification concerning the IKE SA, the SPI Size MUST be zero and
the field must be empty.

o Notify Message Type (2 octets) - Specifies the type of
notification message.

o SPI (variable length) - Security Parameter Index.

o Notification Data (variable length) - Status or error data
transmitted in addition to the Notify Message Type. Values for
this field are type specific (see below).

The payload type for the Notify payload is forty-one (41).

3.10.1. Notify Message Types

Notification information can be error messages specifying why an SA
could not be established. It can also be status data that a process
managing an SA database wishes to communicate with a peer process.

The table below lists the Notification notification messages and their
corresponding values. The number of different error statuses was
greatly reduced from IKEv1 both for simplification and to avoid
giving configuration information to probers.

Types in the range 0 - 16383 are intended for reporting errors. An
implementation receiving a Notify payload with one of these types
that it does not recognize in a response MUST assume that the
corresponding request has failed entirely. Unrecognized error types
in a request and status types in a request or response MUST be
ignored, and they should be logged.

Notify payloads with status types MAY be added to any message and
MUST be ignored if not recognized. They are intended to indicate
capabilities, and as part of SA negotiation, are used to negotiate
non-cryptographic parameters.

More information on error handling can be found in Section 2.21.

The values in the following table are only current as of the
publication date of RFC 4306, plus two error types added in this
document. Other values may have been added since then or will be
added after the publication of this document. Readers should refer
to [IKEV2IANA] for the latest values.

NOTIFY messages: error types Value
-------------------------------------------------------------------
UNSUPPORTED_CRITICAL_PAYLOAD 1
See Section 2.5.

INVALID_IKE_SPI 4
See Section 2.21.

INVALID_MAJOR_VERSION 5
See Section 2.5.

INVALID_SYNTAX 7
Indicates the IKE message that was received was invalid because
some type, length, or value was out of range or because the
request was rejected for policy reasons. To avoid a DoS
attack using forged messages, this status may only be
returned for and in an encrypted packet if the Message ID and
cryptographic checksum were valid. To avoid leaking information
to someone probing a node, this status MUST be sent in response
to any error not covered by one of the other status types.
To aid debugging, more detailed error information should be
written to a console or log.

INVALID_MESSAGE_ID 9
See Section 2.3.

INVALID_SPI 11
See Section 1.5.

NO_PROPOSAL_CHOSEN 14
None of the proposed crypto suites was acceptable. This can be
sent in any case where the offered proposals (including but not
limited to SA payload values, USE_TRANSPORT_MODE notify,
IPCOMP_SUPPORTED notify) are not acceptable for the responder.
This can also be used as "generic" Child SA error when Child SA
cannot be created for some other reason. See also Section 2.7.

INVALID_KE_PAYLOAD 17
See Sections 1.2 and 1.3.

AUTHENTICATION_FAILED 24
Sent in the response to an IKE_AUTH message when, for some
reason, the authentication failed. There is no associated
data. See also Section 2.21.2.

SINGLE_PAIR_REQUIRED 34
See Section 2.9.

NO_ADDITIONAL_SAS 35
See Section 1.3.

INTERNAL_ADDRESS_FAILURE 36
See Section 3.15.4.

FAILED_CP_REQUIRED 37
See Section 2.19.

TS_UNACCEPTABLE 38
See Section 2.9.

INVALID_SELECTORS 39
MAY be sent in an IKE INFORMATIONAL exchange when a node receives
an ESP or AH packet whose selectors do not match those of the SA
on which it was delivered (and that caused the packet to be
dropped). The Notification Data contains the start of the
offending packet (as in ICMP messages) and the SPI field of the
notification is set to match the SPI of the Child SA.

TEMPORARY_FAILURE 43
See section Section 2.25.

CHILD_SA_NOT_FOUND 44
See section Section 2.25.

NOTIFY messages: status types Value
-------------------------------------------------------------------
INITIAL_CONTACT 16384
See Section 2.4.

SET_WINDOW_SIZE 16385
See Section 2.3.

ADDITIONAL_TS_POSSIBLE 16386
See Section 2.9.

IPCOMP_SUPPORTED 16387
See Section 2.22.

NAT_DETECTION_SOURCE_IP 16388
See Section 2.23.

NAT_DETECTION_DESTINATION_IP 16389
See Section 2.23.

COOKIE 16390
See Section 2.6.

USE_TRANSPORT_MODE 16391
See Section 1.3.1.

HTTP_CERT_LOOKUP_SUPPORTED 16392
See Section 3.6.

REKEY_SA 16393
See Section 1.3.3.

ESP_TFC_PADDING_NOT_SUPPORTED 16394
See Section 1.3.1.

NON_FIRST_FRAGMENTS_ALSO 16395
See Section 1.3.1.

3.11. Delete Payload

The Delete payload, denoted D in this document, contains a protocol-
specific
protocol-specific Security Association identifier that the sender has
removed from its Security Association database and is, therefore, no
longer valid. Figure 17 shows the format of the Delete payload. It
is possible to send multiple SPIs in a Delete payload; however, each
SPI MUST be for the same protocol. Mixing of protocol identifiers
MUST NOT be performed in the Delete payload. It is permitted,
however, to include multiple Delete payloads in a single
INFORMATIONAL exchange where each Delete payload lists SPIs for a
different protocol.

Deletion of the IKE SA is indicated by a protocol ID of 1 (IKE) but
no SPIs. Deletion of a Child SA, such as ESP or AH, will contain the
IPsec protocol ID of that protocol (2 for AH, 3 for ESP), and the SPI
is the SPI the sending endpoint would expect in inbound ESP or AH
packets.

The Delete payload is defined as follows:

Figure 17: Delete Payload Format

o Protocol ID (1 octet) - Must be 1 for an IKE SA, 2 for AH, or 3
for ESP.

o SPI Size (1 octet) - Length in octets of the SPI as defined by the
protocol ID. It MUST be zero for IKE (SPI is in message header)
or four for AH and ESP.

o Num of SPIs (2 octets, unsigned integer) - The number of SPIs
contained in the Delete payload. The size of each SPI is defined
by the SPI Size field.

o Security Parameter Index(es) (variable length) - Identifies the
specific Security Association(s) to delete. The length of this
field is determined by the SPI Size and Num of SPIs fields.

The payload type for the Delete payload is forty-two (42).

3.12. Vendor ID Payload

The Vendor ID payload, denoted V in this document, contains a vendor-
defined constant. The constant is used by vendors to identify and
recognize remote instances of their implementations. This mechanism
allows a vendor to experiment with new features while maintaining
backward compatibility.

A Vendor ID payload MAY announce that the sender is capable of
accepting certain extensions to the protocol, or it MAY simply
identify the implementation as an aid in debugging. A Vendor ID
payload MUST NOT change the interpretation of any information defined
in this specification (i.e., the critical bit MUST be set to 0).
Multiple Vendor ID payloads MAY be sent. An implementation is not
required to send any Vendor ID payload at all.

A Vendor ID payload may be sent as part of any message. Reception of
a familiar Vendor ID payload allows an implementation to make use of
private use numbers described throughout this document, such as
private payloads, private exchanges, private notifications, etc.
Unfamiliar Vendor IDs MUST be ignored.

Writers of documents who wish to extend this protocol MUST define a
Vendor ID payload to announce the ability to implement the extension
in the document. It is expected that documents that gain acceptance
and are standardized will be given "magic numbers" out of the Future
Use range by IANA, and the requirement to use a Vendor ID will go
away.

The Vendor ID payload fields are defined as follows:

Figure 18: Vendor ID Payload Format

o Vendor ID (variable length) - It is the responsibility of the
person choosing the Vendor ID to assure its uniqueness in spite of
the absence of any central registry for IDs. Good practice is to
include a company name, a person name, or some such information.
If you want to show off, you might include the latitude and
longitude and time where you were when you chose the ID and some
random input. A message digest of a long unique string is
preferable to the long unique string itself.

The payload type for the Vendor ID payload is forty-three (43).

3.13. Traffic Selector Payload

The Traffic Selector payload, denoted TS in this document, allows
peers to identify packet flows for processing by IPsec security
services. The Traffic Selector payload consists of the IKE generic
payload header followed by individual Traffic Selectors as follows:

Figure 19: Traffic Selectors Payload Format

o Number of TSs (1 octet) - Number of Traffic Selectors being
provided.

o RESERVED - This field MUST be sent as zero and MUST be ignored on
receipt.

o Traffic Selectors (variable length) - One or more individual
Traffic Selectors.

The length of the Traffic Selector payload includes the TS header and
all the Traffic Selectors.

The payload type for the Traffic Selector payload is forty-four (44)
for addresses at the initiator's end of the SA and forty-five (45)
for addresses at the responder's end.

There is no requirement that TSi and TSr contain the same number of
individual Traffic Selectors. Thus, they are interpreted as follows:
a packet matches a given TSi/TSr if it matches at least one of the
individual selectors in TSi, and at least one of the individual
selectors in TSr.

For instance, the following Traffic Selectors:

TSi = ((17, 100, 198.51.100.66-198.51.100.66),
(17, 200, 198.51.100.66-198.51.100.66))
TSr = ((17, 300, 0.0.0.0-255.255.255.255),
(17, 400, 0.0.0.0-255.255.255.255))

would match UDP packets from 198.51.100.66 to anywhere, with any of
the four combinations of source/destination ports (100,300),
(100,400), (200,300), and (200, 400).

Thus, some types of policies may require several Child SA pairs. For
instance, a policy matching only source/destination ports (100,300)
and (200,400), but not the other two combinations, cannot be
negotiated as a single Child SA pair.

3.13.1. Traffic Selector

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS Type |IP Protocol ID*| Selector Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Start Port* | End Port* |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Starting Address* ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Ending Address* ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 20: Traffic Selector

*Note: All fields other than TS Type and Selector Length depend on
the TS Type. The fields shown are for TS Types 7 and 8, the only two
values currently defined.

o TS Type (one octet) - Specifies the type of Traffic Selector.

o IP protocol ID (1 octet) - Value specifying an associated IP
protocol ID (such as UDP, TCP, and ICMP). A value of zero means
that the protocol ID is not relevant to this Traffic Selector --
the SA can carry all protocols.

o Selector Length (2 octets, unsigned integer) - Specifies the
length of this Traffic Selector substructure including the header.

o Start Port (2 octets, unsigned integer) - Value specifying the
smallest port number allowed by this Traffic Selector. For
protocols for which port is undefined (including protocol 0), or
if all ports are allowed, this field MUST be zero. ICMP and
ICMPv6 Type and Code values, as well as Mobile IP version 6
(MIPv6) mobility header (MH) Type values, are represented in this
field as specified in Section 4.4.1.1 of [IPSECARCH]. ICMP Type
and Code values are treated as a single 16-bit integer port
number, with Type in the most significant eight bits and Code in
the least significant eight bits. MIPv6 MH Type values are
treated as a single 16-bit integer port number, with Type in the
most significant eight bits and the least significant eight bits
set to zero.

o End Port (2 octets, unsigned integer) - Value specifying the
largest port number allowed by this Traffic Selector. For
protocols for which port is undefined (including protocol 0), or
if all ports are allowed, this field MUST be 65535. ICMP and
ICMPv6 Type and Code values, as well as MIPv6 MH Type values, are
represented in this field as specified in Section 4.4.1.1 of
[IPSECARCH]. ICMP Type and Code values are treated as a single
16-bit integer port number, with Type in the most significant
eight bits and Code in the least significant eight bits. MIPv6 MH
Type values are treated as a single 16-bit integer port number,
with Type in the most significant eight bits and the least
significant eight bits set to zero.

o Starting Address - The smallest address included in this Traffic
Selector (length determined by TS Type).

o Ending Address - The largest address included in this Traffic
Selector (length determined by TS Type).

Systems that are complying with [IPSECARCH] that wish to indicate
"ANY" ports MUST set the start port to 0 and the end port to 65535;
note that according to [IPSECARCH], "ANY" includes "OPAQUE". Systems
working with [IPSECARCH] that wish to indicate "OPAQUE" ports, but
not "ANY" ports, MUST set the start port to 65535 and the end port
to 0.

The Traffic Selector types 7 and 8 can also refer to ICMP or ICMPv6
type and code fields, as well as MH Type fields for the IPv6 mobility
header [MIPV6]. Note, however, that neither ICMP nor MIPv6 packets
have separate source and destination fields. The method for
specifying the Traffic Selectors for ICMP and MIPv6 is shown by
example in Section 4.4.1.3 of [IPSECARCH].

The following table lists values for the Traffic Selector Type field
and the corresponding Address Selector Data. The values in the
following table are only current as of the publication date of
RFC 4306. Other values may have been added since then or will be
added after the publication of this document. Readers should refer
to [IKEV2IANA] for the latest values.

TS Type Value
-------------------------------------------------------------------
TS_IPV4_ADDR_RANGE 7

A range of IPv4 addresses, represented by two four-octet
values. The first value is the beginning IPv4 address
(inclusive) and the second value is the ending IPv4 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.

TS_IPV6_ADDR_RANGE 8

A range of IPv6 addresses, represented by two sixteen-octet
values. The first value is the beginning IPv6 address
(inclusive) and the second value is the ending IPv6 address
(inclusive). All addresses falling between the two specified
addresses are considered to be within the list.

3.14. Encrypted Payload

The Encrypted payload, denoted SK{...} SK {...} in this document, contains
other payloads in encrypted form. The Encrypted payload, if present
in a message, MUST be the last payload in the message. Often, it is
the only payload in the message. This payload is also called the
"Encrypted and Authenticated" payload.

The algorithms for encryption and integrity protection are negotiated
during IKE SA setup, and the keys are computed as specified in
Sections 2.14 and 2.18.

This document specifies the cryptographic processing of Encrypted
payloads using a block cipher in CBC mode and an integrity check
algorithm that computes a fixed-length checksum over a variable size
message. The design is modeled after the ESP algorithms described in
RFCs 2104 [HMAC], 4303 [ESP], and 2451 [ESPCBC]. This document
completely specifies the cryptographic processing of IKE data, but
those documents should be consulted for design rationale. Future
documents may specify the processing of Encrypted payloads for other
types of transforms, such as counter mode encryption and
authenticated encryption algorithms. Peers MUST NOT negotiate
transforms for which no such specification exists.

When an authenticated encryption algorithm is used to protect the IKE
SA, the construction of the Encrypted payload is different than what
is described here. See [AEAD] for more information on authenticated
encryption algorithms and their use in IKEv2.

The payload type for an Encrypted payload is forty-six (46). The
Encrypted payload consists of the IKE generic payload header followed
by individual fields as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (length is block size for encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Encrypted IKE Payloads ~
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| | Pad Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Integrity Checksum Data ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 21: Encrypted Payload Format

o Next Payload - The payload type of the first embedded payload.
Note that this is an exception in the standard header format,
since the Encrypted payload is the last payload in the message and
therefore the Next Payload field would normally be zero. But
because the content of this payload is embedded payloads and there
was no natural place to put the type of the first one, that type
is placed here.

o Payload Length - Includes the lengths of the header,
initialization vector (IV), Encrypted IKE payloads, Padding, Pad
Length, and Integrity Checksum Data.

o Initialization Vector - For CBC mode ciphers, the length of the
initialization vector (IV) is equal to the block length of the
underlying encryption algorithm. Senders MUST select a new
unpredictable IV for every message; recipients MUST accept any
value. The reader is encouraged to consult [MODES] for advice on
IV generation. In particular, using the final ciphertext block of
the previous message is not considered unpredictable. For modes
other than CBC, the IV format and processing is specified in the
document specifying the encryption algorithm and mode.

o IKE payloads are as specified earlier in this section. This field
is encrypted with the negotiated cipher.

o Padding MAY contain any value chosen by the sender, and MUST have
a length that makes the combination of the payloads, the Padding,
and the Pad Length to be a multiple of the encryption block size.
This field is encrypted with the negotiated cipher.

o Pad Length is the length of the Padding field. The sender SHOULD
set the Pad Length to the minimum value that makes the combination
of the payloads, the Padding, and the Pad Length a multiple of the
block size, but the recipient MUST accept any length that results
in proper alignment. This field is encrypted with the negotiated
cipher.

o Integrity Checksum Data is the cryptographic checksum of the
entire message starting with the Fixed IKE header through the Pad
Length. The checksum MUST be computed over the encrypted message.
Its length is determined by the integrity algorithm negotiated.

3.15. Configuration Payload

The Configuration payload, denoted CP in this document, is used to
exchange configuration information between IKE peers. The exchange
is for an IRAC to request an internal IP address from an IRAS and to
exchange other information of the sort that one would acquire with
Dynamic Host Configuration Protocol (DHCP) if the IRAC were directly
connected to a LAN.

The Configuration payload is defined as follows:

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| CFG Type | RESERVED |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Configuration Attributes ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 22: Configuration Payload Format
The payload type for the Configuration payload is forty-seven (47).

o CFG Type (1 octet) - The type of exchange represented by the
Configuration Attributes. The values in the following table are
only current as of the publication date of RFC 4306. Other values
may have been added since then or will be added after the
publication of this document. Readers should refer to [IKEV2IANA]
for the latest values.

CFG Type Value
--------------------------
CFG_REQUEST 1
CFG_REPLY 2
CFG_SET 3
CFG_ACK 4

o RESERVED (3 octets) - MUST be sent as zero; MUST be ignored on
receipt.

o Configuration Attributes (variable length) - These are type length
value (TLV) structures specific to the Configuration payload and
are defined below. There may be zero or more Configuration
Attributes in this payload.

3.15.1. Configuration Attributes

1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Attribute Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Value ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 23: Configuration Attribute Format

o Reserved (1 bit) - This bit MUST be set to zero and MUST be
ignored on receipt.

o Attribute Type (15 bits) - A unique identifier for each of the
Configuration Attribute Types.

o Length (2 octets, unsigned integer) - Length in octets of value.

o Value (0 or more octets) - The variable-length value of this
Configuration Attribute. The following lists the attribute types.

The values in the following table are only current as of the
publication date of RFC 4306 (except INTERNAL_ADDRESS_EXPIRY and
INTERNAL_IP6_NBNS
INTERNAL_IP6_NBNS, which were removed by this document). RFC 5996). Other values may
have been added since then or will be added after the publication of
this document. Readers should refer to [IKEV2IANA] for the latest
values.

Attribute Type Value Multi-Valued Length
------------------------------------------------------------
INTERNAL_IP4_ADDRESS 1 YES* 0 or 4 octets
INTERNAL_IP4_NETMASK 2 NO 0 or 4 octets
INTERNAL_IP4_DNS 3 YES 0 or 4 octets
INTERNAL_IP4_NBNS 4 YES 0 or 4 octets
INTERNAL_IP4_DHCP 6 YES 0 or 4 octets
APPLICATION_VERSION 7 NO 0 or more
INTERNAL_IP6_ADDRESS 8 YES* 0 or 17 octets
INTERNAL_IP6_DNS 10 YES 0 or 16 octets
INTERNAL_IP6_DHCP 12 YES 0 or 16 octets
INTERNAL_IP4_SUBNET 13 YES 0 or 8 octets
SUPPORTED_ATTRIBUTES 14 NO Multiple of 2
INTERNAL_IP6_SUBNET 15 YES 17 octets

* These attributes may be multi-valued on return only if
multiple values were requested.

o INTERNAL_IP4_ADDRESS, INTERNAL_IP6_ADDRESS - An address on the
internal network, sometimes called a red node address or private
address, and it MAY be a private address on the Internet. In a
request message, the address specified is a requested address (or
a zero-length address if no specific address is requested). If a
specific address is requested, it likely indicates that a previous
connection existed with this address and the requestor would like
to reuse that address. With IPv6, a requestor MAY supply the low-
order address octets it wants to use. Multiple internal addresses
MAY be requested by requesting multiple internal address
attributes. The responder MAY only send up to the number of
addresses requested. The INTERNAL_IP6_ADDRESS is made up of two
fields: the first is a 16-octet IPv6 address, and the second is a
one-octet prefix-length as defined in [ADDRIPV6]. The requested
address is valid as long as this IKE SA (or its rekeyed
successors) requesting the address is valid. This is described in
more detail in Section 3.15.3.

o INTERNAL_IP4_NETMASK - The internal network's netmask. Only one
netmask is allowed in the request and response messages (e.g.,
255.255.255.0), and it MUST be used only with an
INTERNAL_IP4_ADDRESS attribute. INTERNAL_IP4_NETMASK in a
CFG_REPLY means roughly the same thing as INTERNAL_IP4_SUBNET
containing the same information ("send traffic to these addresses
through me"), but also implies a link boundary. For instance, the
client could use its own address and the netmask to calculate the
broadcast address of the link. An empty INTERNAL_IP4_NETMASK
attribute can be included in a CFG_REQUEST to request this
information (although the gateway can send the information even
when not requested). Non-empty values for this attribute in a
CFG_REQUEST do not make sense and thus MUST NOT be included.

o INTERNAL_IP4_DNS, INTERNAL_IP6_DNS - Specifies an address of a DNS
server within the network. Multiple DNS servers MAY be requested.
The responder MAY respond with zero or more DNS server attributes.

o INTERNAL_IP4_NBNS - Specifies an address of a NetBios Name Server
(WINS) within the network. Multiple NBNS servers MAY be
requested. The responder MAY respond with zero or more NBNS
server attributes.

o INTERNAL_IP4_DHCP, INTERNAL_IP6_DHCP - Instructs the host to send
any internal DHCP requests to the address contained within the
attribute. Multiple DHCP servers MAY be requested. The responder
MAY respond with zero or more DHCP server attributes.

o APPLICATION_VERSION - The version or application information of
the IPsec host. This is a string of printable ASCII characters
that is NOT null terminated.

o INTERNAL_IP4_SUBNET - The protected sub-networks that this edge-
device protects. This attribute is made up of two fields: the
first being an IP address and the second being a netmask.
Multiple sub-networks MAY be requested. The responder MAY respond
with zero or more sub-network attributes. This is discussed in
more detail in Section 3.15.2.

o SUPPORTED_ATTRIBUTES - When used within a Request, this attribute
MUST be zero-length and specifies a query to the responder to
reply back with all of the attributes that it supports. The
response contains an attribute that contains a set of attribute
identifiers each in 2 octets. The length divided by 2 (octets)
would state the number of supported attributes contained in the
response.

o INTERNAL_IP6_SUBNET - The protected sub-networks that this edge-
device
edge-device protects. This attribute is made up of two fields:
the first is a 16-octet IPv6 address, and the second is a
one-octet prefix-length as defined in [ADDRIPV6]. Multiple
sub-networks MAY be requested. The responder MAY respond with
zero or more sub-
network sub-network attributes. This is discussed in more
detail in Section 3.15.2.

Note that no recommendations are made in this document as to how an
implementation actually figures out what information to send in a
response. That is, we do not recommend any specific method of an
IRAS determining which DNS server should be returned to a requesting
IRAC.

The CFG_REQUEST and CFG_REPLY pair allows an IKE endpoint to request
information from its peer. If an attribute in the CFG_REQUEST
Configuration payload is not zero-length, it is taken as a suggestion
for that attribute. The CFG_REPLY Configuration payload MAY return
that value, or a new one. It MAY also add new attributes and not
include some requested ones. Unrecognized or unsupported attributes
MUST be ignored in both requests and responses.

The CFG_SET and CFG_ACK pair allows an IKE endpoint to push
configuration data to its peer. In this case, the CFG_SET
Configuration payload contains attributes the initiator wants its
peer to alter. The responder MUST return a Configuration payload if
it accepted any of the configuration data data, and it the Configuration
payload MUST contain the attributes that the responder accepted with
zero-length data. Those attributes that it did not accept MUST NOT
be in the CFG_ACK Configuration payload. If no attributes were
accepted, the responder MUST return either an empty CFG_ACK payload
or a response message without a CFG_ACK payload. There are currently
no defined uses for the CFG_SET/CFG_ACK exchange, though they may be
used in connection with extensions based on Vendor IDs. An
implementation of this specification MAY ignore CFG_SET payloads.

3.15.2. Meaning of INTERNAL_IP4_SUBNET and INTERNAL_IP6_SUBNET

INTERNAL_IP4/6_SUBNET attributes can indicate additional subnets,
ones that need one or more separate SAs, that can be reached through
the gateway that announces the attributes. INTERNAL_IP4/6_SUBNET
attributes may also express the gateway's policy about what traffic
should be sent through the gateway; the client can choose whether
other traffic (covered by TSr, but not in INTERNAL_IP4/6_SUBNET) is
sent through the gateway or directly to the destination. Thus,
traffic to the addresses listed in the INTERNAL_IP4/6_SUBNET
attributes should be sent through the gateway that announces the
attributes. If there are no existing Child SAs whose Traffic
Selectors cover the address in question, new SAs need to be created.

For instance, if there are two subnets, 198.51.100.0/26 and
192.0.2.0/24, and the client's request contains the following:

CP(CFG_REQUEST) =
INTERNAL_IP4_ADDRESS()
TSi = (0, 0-65535, 0.0.0.0-255.255.255.255)
TSr = (0, 0-65535, 0.0.0.0-255.255.255.255)

then a valid response could be the following (in which TSr and
INTERNAL_IP4_SUBNET contain the same information):

CP(CFG_REPLY) =
INTERNAL_IP4_ADDRESS(198.51.100.234)
INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
TSr = ((0, 0-65535, 198.51.100.0-198.51.100.63),
(0, 0-65535, 192.0.2.0-192.0.2.255))

In these cases, the INTERNAL_IP4_SUBNET does not really carry any
useful information.

A different possible response would have been this:

That response would mean that the client can send all its traffic
through the gateway, but the gateway does not mind if the client
sends traffic not included by INTERNAL_IP4_SUBNET directly to the
destination (without going through the gateway).

A different situation arises if the gateway has a policy that
requires the traffic for the two subnets to be carried in separate
SAs. Then a response like this would indicate to the client that
if it wants access to the second subnet, it needs to create a
separate SA:

CP(CFG_REPLY) =
INTERNAL_IP4_ADDRESS(198.51.100.234)
INTERNAL_IP4_SUBNET(198.51.100.0/255.255.255.192)
INTERNAL_IP4_SUBNET(192.0.2.0/255.255.255.0)
TSi = (0, 0-65535, 198.51.100.234-198.51.100.234)
TSr = (0, 0-65535, 198.51.100.0-198.51.100.63)
INTERNAL_IP4_SUBNET can also be useful if the client's TSr included
only part of the address space. For instance, if the client requests
the following:

CP(CFG_REQUEST) =
INTERNAL_IP4_ADDRESS()
TSi = (0, 0-65535, 0.0.0.0-255.255.255.255)
TSr = (0, 0-65535, 192.0.2.155-192.0.2.155)

then the gateway's response might be:

Because the meaning of INTERNAL_IP4_SUBNET/INTERNAL_IP6_SUBNET in
CFG_REQUESTs is unclear, they cannot be used reliably in
CFG_REQUESTs.

3.15.3. Configuration Payloads for IPv6

The Configuration payloads for IPv6 are based on the corresponding
IPv4 payloads, and do not fully follow the "normal IPv6 way of doing
things". In particular, IPv6 stateless autoconfiguration or router
advertisement messages are not used, neither is neighbor discovery.
Note that there is an additional document that discusses IPv6
configuration in IKEv2, [IPV6CONFIG]. At the present time, it is an
experimental document, but there is a hope that with more
implementation experience, it will gain the same standards treatment
as this document.

A client can be assigned an IPv6 address using the
INTERNAL_IP6_ADDRESS Configuration payload. A minimal exchange might
look like this:

CP(CFG_REQUEST) =
INTERNAL_IP6_ADDRESS()
INTERNAL_IP6_DNS()
TSi = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)
TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

CP(CFG_REPLY) =
INTERNAL_IP6_ADDRESS(2001:DB8:0:1:2:3:4:5/64)
INTERNAL_IP6_DNS(2001:DB8:99:88:77:66:55:44)
TSi = (0, 0-65535, 2001:DB8:0:1:2:3:4:5 - 2001:DB8:0:1:2:3:4:5)
TSr = (0, 0-65535, :: - FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF:FFFF)

The client MAY send a non-empty INTERNAL_IP6_ADDRESS attribute in the
CFG_REQUEST to request a specific address or interface identifier.
The gateway first checks if the specified address is acceptable, and
if it is, returns that one. If the address was not acceptable, the
gateway attempts to use the interface identifier with some other
prefix; if even that fails, the gateway selects another interface
identifier.

The INTERNAL_IP6_ADDRESS attribute also contains a prefix length
field. When used in a CFG_REPLY, this corresponds to the
INTERNAL_IP4_NETMASK attribute in the IPv4 case.

Although this approach to configuring IPv6 addresses is reasonably
simple, it has some limitations. IPsec tunnels configured using
IKEv2 are not fully featured "interfaces" in the IPv6 addressing
architecture sense [ADDRIPV6]. In particular, they do not
necessarily have link-local addresses, and this may complicate the
use of protocols that assume them, such as [MLDV2].

3.15.4. Address Assignment Failures

If the responder encounters an error while attempting to assign an IP
address to the initiator during the processing of a Configuration
payload, it responds with an INTERNAL_ADDRESS_FAILURE notification.
The IKE SA is still created even if the initial Child SA cannot be
created because of this failure. If this error is generated within
an IKE_AUTH exchange, no Child SA will be created. However, there
are some more complex error cases.

If the responder does not support Configuration payloads at all, it
can simply ignore all Configuration payloads. This type of
implementation never sends INTERNAL_ADDRESS_FAILURE notifications.

If the initiator requires the assignment of an IP address, it will
treat a response without CFG_REPLY as an error.

The initiator may request a particular type of address (IPv4 or IPv6)
that the responder does not support, even though the responder
supports Configuration payloads. In this case, the responder simply
ignores the type of address it does not support and processes the
rest of the request as usual.

If the initiator requests multiple addresses of a type that the
responder supports, and some (but not all) of the requests fail, the
responder replies with the successful addresses only. The responder
sends INTERNAL_ADDRESS_FAILURE only if no addresses can be assigned.

If the initiator does not receive the IP address(es) required by its
policy, it MAY keep the IKE SA up and retry the Configuration payload
as separate INFORMATIONAL exchange after suitable timeout, or it MAY
tear down the IKE SA by sending a Delete payload inside a separate
INFORMATIONAL exchange and later retry IKE SA from the beginning
after some timeout. Such a timeout should not be too short
(especially if the IKE SA is started from the beginning) because
these error situations may not be able to be fixed quickly; the
timeout should likely be several minutes. For example, an address
shortage problem on the responder will probably only be fixed when
more entries are returned to the address pool when other clients
disconnect or when responder is reconfigured with larger address
pool.

3.16. Extensible Authentication Protocol (EAP) Payload

The Extensible Authentication Protocol payload, denoted EAP in this
document, allows IKE SAs to be authenticated using the protocol
defined in RFC 3748 [EAP] and subsequent extensions to that protocol.
When using EAP, an appropriate EAP method needs to be selected. Many
of these methods have been defined, specifying the protocol's use
with various authentication mechanisms. EAP method types are listed
in [EAP-IANA]. A short summary of the EAP format is included here
for clarity.

Figure 24: EAP Payload Format

The payload type for an EAP payload is forty-eight (48).

Figure 25: EAP Message Format

o Code (1 octet) indicates - Indicates whether this message is a Request (1),
Response (2), Success (3), or Failure (4).

o Identifier (1 octet) is used - Used in PPP to distinguish replayed
messages from repeated ones. Since in IKE, EAP runs over a
reliable protocol, it the Identifier serves no function here. In a
response message, this octet MUST be set to match the identifier
in the corresponding request.

o Length (2 octets, unsigned integer) is the - The length of the EAP
message and
message. MUST be four less than the Payload Length of the
encapsulating payload.

o Type (1 octet) is present - Present only if the Code field is Request (1) or
Response (2). For other codes, the EAP message length MUST be
four octets and the Type and Type_Data fields MUST NOT be present.
In a Request (1) message, Type indicates the data being requested.
In a Response (2) message, Type MUST either be Nak or match the
type of the data requested. Note that since IKE passes an
indication of initiator identity in the first message in the
IKE_AUTH exchange, the responder SHOULD NOT send EAP Identity
requests (type 1). The initiator MAY, however, respond to such
requests if it receives them.

o Type_Data (Variable Length) varies (variable length) - Varies with the Type of Request and
the associated Response. For the documentation of the EAP
methods, see [EAP].

Note that since IKE passes an indication of initiator identity in the
first message in the IKE_AUTH exchange, the responder should not SHOULD NOT send
EAP Identity requests. The initiator may, MAY, however, respond to such
requests if it receives them.

4. Conformance Requirements

In order to assure that all implementations of IKEv2 can
interoperate, there are "MUST support" requirements in addition to
those listed elsewhere. Of course, IKEv2 is a security protocol, and
one of its major functions is to allow only authorized parties to
successfully complete establishment of SAs. So a particular
implementation may be configured with any of a number of restrictions
concerning algorithms and trusted authorities that will prevent
universal interoperability.

IKEv2 is designed to permit minimal implementations that can
interoperate with all compliant implementations. The following are
features that can be omitted in a minimal implementation:

o Ability to negotiate SAs through a NAT and tunnel the resulting
ESP SA over UDP.

o Ability to request (and respond to a request for) a temporary IP
address on the remote end of a tunnel.

o Ability to support EAP-based authentication.

o Ability to support window sizes greater than one.

o Ability to establish multiple ESP or AH SAs within a single
IKE SA.

o Ability to rekey SAs.

To assure interoperability, all implementations MUST be capable of
parsing all payload types (if only to skip over them) and to ignore
payload types that it does not support unless the critical bit is set
in the payload header. If the critical bit is set in an unsupported
payload header, all implementations MUST reject the messages
containing those payloads.

Every implementation MUST be capable of doing four-message
IKE_SA_INIT and IKE_AUTH exchanges establishing two SAs (one for IKE,
one for ESP or AH). Implementations MAY be initiate-only or respond-
only if appropriate for their platform. Every implementation MUST be
capable of responding to an INFORMATIONAL exchange, but a minimal
implementation MAY respond to any request in the INFORMATIONAL
exchange with an empty response (note that within the context of an
IKE SA, an "empty" message consists of an IKE header followed by an
Encrypted payload with no payloads contained in it). A minimal
implementation MAY support the CREATE_CHILD_SA exchange only in so
far as to recognize requests and reject them with a Notify payload of
type NO_ADDITIONAL_SAS. A minimal implementation need not be able to
initiate CREATE_CHILD_SA or INFORMATIONAL exchanges. When an SA
expires (based on locally configured values of either lifetime or
octets passed), and an implementation MAY either try to renew it with a
CREATE_CHILD_SA exchange or it MAY delete (close) the old SA and
create a new one. If the responder rejects the CREATE_CHILD_SA
request with a NO_ADDITIONAL_SAS notification, the implementation
MUST be capable of instead deleting the old SA and creating a
new one.

Implementations are not required to support requesting temporary IP
addresses or responding to such requests. If an implementation does
support issuing such requests and its policy requires using temporary
IP addresses, it MUST include a CP payload in the first message in
the IKE_AUTH exchange containing at least a field of type
INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS. All other fields are
optional. If an implementation supports responding to such requests,
it MUST parse the CP payload of type CFG_REQUEST in the first message
in the IKE_AUTH exchange and recognize a field of type
INTERNAL_IP4_ADDRESS or INTERNAL_IP6_ADDRESS. If it supports leasing
an address of the appropriate type, it MUST return a CP payload of
type CFG_REPLY containing an address of the requested type. The
responder may include any other related attributes.

For an implementation to be called conforming to this specification,
it MUST be possible to configure it to accept the following:

o Public Key Infrastructure using X.509 (PKIX) Certificates
containing and signed by RSA keys of size 1024 or 2048 bits, where
the ID passed is any of ID_KEY_ID, ID_FQDN, ID_RFC822_ADDR, or
ID_DER_ASN1_DN.

o Shared key authentication where the ID passed is any of ID_KEY_ID,
ID_FQDN, or ID_RFC822_ADDR.

o Authentication where the responder is authenticated using PKIX
Certificates and the initiator is authenticated using shared key
authentication.

5. Security Considerations

While this protocol is designed to minimize disclosure of
configuration information to unauthenticated peers, some such
disclosure is unavoidable. One peer or the other must identify
itself first and prove its identity first. To avoid probing, the
initiator of an exchange is required to identify itself first, and
usually is required to authenticate itself first. The initiator can,
however, learn that the responder supports IKE and what cryptographic
protocols it supports. The responder (or someone impersonating the
responder) not only can probe the initiator not only for its identity, identity but may,
by using CERTREQ payloads may payloads, be able to determine what certificates the
initiator is willing to use.

Use of EAP authentication changes the probing possibilities somewhat.
When EAP authentication is used, the responder proves its identity
before the initiator does, so an initiator that knew the name of a
valid initiator could probe the responder for both its name and
certificates.

Repeated rekeying using CREATE_CHILD_SA without additional Diffie-
Hellman exchanges leaves all SAs vulnerable to cryptanalysis of a
single key. Implementers should take note of this fact and set a
limit on CREATE_CHILD_SA exchanges between exponentiations. This
document does not prescribe such a limit.

The strength of a key derived from a Diffie-Hellman exchange using
any of the groups defined here depends on the inherent strength of
the group, the size of the exponent used, and the entropy provided by
the random number generator used. Due to these inputs, it is
difficult to determine the strength of a key for any of the defined
groups. Diffie-Hellman group number two, when used with a strong
random number generator and an exponent no less than 200 bits, is
common for use with 3DES. Group five provides greater security than
group two. Group one is for historic purposes only and does not
provide sufficient strength except for use with DES, which is also
for historic use only. Implementations should make note of these
estimates when establishing policy and negotiating security
parameters.

Note that these limitations are on the Diffie-Hellman groups
themselves. There is nothing in IKE that prohibits using stronger
groups nor is there anything that will dilute the strength obtained
from stronger groups (limited by the strength of the other algorithms
negotiated including the PRF). In fact, the extensible framework of
IKE encourages the definition of more groups; use of elliptic curve
groups may greatly increase strength using much smaller numbers.

It is assumed that all Diffie-Hellman exponents are erased from
memory after use.

The IKE_SA_INIT and IKE_AUTH exchanges happen before the initiator
has been authenticated. As a result, an implementation of this
protocol needs to be completely robust when deployed on any insecure
network. Implementation vulnerabilities, particularly DoS attacks,
can be exploited by unauthenticated peers. This issue is
particularly worrisome because of the unlimited number of messages in
EAP-based authentication.

The strength of all keys is limited by the size of the output of the
negotiated PRF. For this reason, a PRF whose output is less than
128 bits (e.g., 3DES-CBC) MUST NOT be used with this protocol.

The security of this protocol is critically dependent on the
randomness of the randomly chosen parameters. These should be
generated by a strong random or properly seeded pseudorandom source
(see [RANDOMNESS]). Implementers should take care to ensure that use
of random numbers for both keys and nonces is engineered in a fashion
that does not undermine the security of the keys.

For information on the rationale of many of the cryptographic design
choices in this protocol, see [SIGMA] and [SKEME]. Though the
security of negotiated Child SAs does not depend on the strength of
the encryption and integrity protection negotiated in the IKE SA,
implementations MUST NOT negotiate NONE as the IKE integrity
protection algorithm or ENCR_NULL as the IKE encryption algorithm.

When using pre-shared keys, a critical consideration is how to assure
the randomness of these secrets. The strongest practice is to ensure
that any pre-shared key contain as much randomness as the strongest
key being negotiated. Deriving a shared secret from a password,
name, or other low-entropy source is not secure. These sources are
subject to dictionary and social-engineering attacks, among others.

The NAT_DETECTION_*_IP notifications contain a hash of the addresses
and ports in an attempt to hide internal IP addresses behind a NAT.
Since the IPv4 address space is only 32 bits, and it is usually very
sparse, it would be possible for an attacker to find out the internal
address used behind the NAT box by trying all possible IP addresses
and trying to find the matching hash. The port numbers are normally
fixed to 500, and the SPIs can be extracted from the packet. This
reduces the number of hash calculations to 2^32. With an educated
guess of the use of private address space, the number of hash
calculations is much smaller. Designers should therefore not assume
that use of IKE will not leak internal address information.

When using an EAP authentication method that does not generate a
shared key for protecting a subsequent AUTH payload, certain man-in-
the-middle and server-impersonation attacks are possible [EAPMITM].
These vulnerabilities occur when EAP is also used in protocols that
are not protected with a secure tunnel. Since EAP is a general-
purpose authentication protocol, which is often used to provide
single-signon facilities, a deployed IPsec solution that relies on an
EAP authentication method that does not generate a shared key (also
known as a non-key-generating EAP method) can become compromised due
to the deployment of an entirely unrelated application that also
happens to use the same non-key-generating EAP method, but in an
unprotected fashion. Note that this vulnerability is not limited to
just EAP, but can occur in other scenarios where an authentication
infrastructure is reused. For example, if the EAP mechanism used by
IKEv2 utilizes a token authenticator, a man-in-the-middle attacker
could impersonate the web server, intercept the token authentication
exchange, and use it to initiate an IKEv2 connection. For this
reason, use of non-key-generating EAP methods SHOULD be avoided where
possible. Where they are used, it is extremely important that all
usages of these EAP methods SHOULD utilize a protected tunnel, where
the initiator validates the responder's certificate before initiating
the EAP authentication. Implementers should describe the
vulnerabilities of using non-key-generating EAP methods in the
documentation of their implementations so that the administrators
deploying IPsec solutions are aware of these dangers.

An implementation using EAP MUST also use a public-key-based
authentication of the server to the client before the EAP
authentication begins, even if the EAP method offers mutual
authentication. This avoids having additional IKEv2 protocol
variations and protects the EAP data from active attackers.

If the messages of IKEv2 are long enough that IP-level fragmentation
is necessary, it is possible that attackers could prevent the
exchange from completing by exhausting the reassembly buffers. The
chances of this can be minimized by using the Hash and URL encodings
instead of sending certificates (see Section 3.6). Additional
mitigations are discussed in [DOSUDPPROT].

Admission control is critical to the security of the protocol. For
example, trust anchors used for identifying IKE peers should probably
be different than those used for other forms of trust, such as those
used to identify public web servers. Moreover, although IKE provides
a great deal of leeway in defining the security policy for a trusted
peer's identity, credentials, and the correlation between them,
having such security policy defined explicitly is essential to a
secure implementation.

5.1. Traffic Selector Authorization

IKEv2 relies on information in the Peer Authorization Database (PAD)
when determining what kind of Child SAs a peer is allowed to create.
This process is described in Section 4.4.3 of [IPSECARCH]. When a
peer requests the creation of an a Child SA with some Traffic Selectors,
the PAD must contain "Child SA Authorization Data" linking the
identity authenticated by IKEv2 and the addresses permitted for
Traffic Selectors.

For example, the PAD might be configured so that authenticated
identity "sgw23.example.com" is allowed to create Child SAs for
192.0.2.0/24, meaning this security gateway is a valid
"representative" for these addresses. Host-to-host IPsec requires
similar entries, linking, for example, "fooserver4.example.com" with
198.51.100.66/32, meaning this identity is a valid "owner" or
"representative" of the address in question.

As noted in [IPSECARCH], "It is necessary to impose these constraints
on creation of child SAs to prevent an authenticated peer from
spoofing IDs associated with other, legitimate peers". In the
example given above, a correct configuration of the PAD prevents
sgw23 from creating Child SAs with address 198.51.100.66, and
prevents fooserver4 from creating Child SAs with addresses from
192.0.2.0/24.

It is important to note that simply sending IKEv2 packets using some
particular address does not imply a permission to create Child SAs
with that address in the Traffic Selectors. For example, even if
sgw23 would be able to spoof its IP address as 198.51.100.66, it
could not create Child SAs matching fooserver4's traffic.

The IKEv2 specification does not specify how exactly IP address
assignment using Configuration payloads interacts with the PAD. Our
interpretation is that when a security gateway assigns an address
using Configuration payloads, it also creates a temporary PAD entry
linking the authenticated peer identity and the newly allocated inner
address.

It has been recognized that configuring the PAD correctly may be
difficult in some environments. For instance, if IPsec is used
between a pair of hosts whose addresses are allocated dynamically
using DHCP, it is extremely difficult to ensure that the PAD
specifies the correct "owner" for each IP address. This would
require a mechanism to securely convey address assignments from the
DHCP server, and link them to identities authenticated using IKEv2.

Due to this limitation, some vendors have been known to configure
their PADs to allow an authenticated peer to create Child SAs with
Traffic Selectors containing the same address that was used for the
IKEv2 packets. In environments where IP spoofing is possible (i.e.,
almost everywhere) this essentially allows any peer to create Child
SAs with any Traffic Selectors. This is not an appropriate or secure
configuration in most circumstances. See [H2HIPSEC] for an extensive
discussion about this issue, and the limitations of host-to-host
IPsec in general.

6. IANA Considerations

[IKEV2] defined many field types and values. IANA has already
registered those types and values in [IKEV2IANA], so they are not
listed here again.

One item has been deprecated from the IKEv2 "IKEv2 Certificate Encodings
table: Encodings"
registry: "Raw RSA Key".

IANA has updated all references to RFC 5996 to point to this
document.

7. Acknowledgements

Many individuals in the IPsecME Working Group were very helpful in
contributing ideas and text for this document, as well as in
reviewing the clarifications suggested by others.

The acknowledgements from the IKEv2 document were:

This document is a collaborative effort of the entire IPsec WG. If
there were no limit to the number of authors that could appear on an
RFC, the following, in alphabetical order, would have been listed:
Bill Aiello, Stephane Beaulieu, Steve Bellovin, Sara Bitan, Matt
Blaze, Ran Canetti, Darren Dukes, Dan Harkins, Paul Hoffman, John
Ioannidis, Charlie Kaufman, Steve Kent, Angelos Keromytis, Tero
Kivinen, Hugo Krawczyk, Andrew Krywaniuk, Radia Perlman, Omer
Reingold, and Michael Richardson. Many other people contributed to
the design. It is an evolution of IKEv1, ISAKMP, and the IPsec DOI,
each of which has its own list of authors. Hugh Daniel suggested the
feature of having the initiator, in message 3, specify a name for the
responder, and gave the feature the cute name "You Tarzan, Me Jane".
David Faucher and Valery Smyslov helped refine the design of the
Traffic Selector negotiation.

8. References

8.1.

7.1. Normative References

[ADDGROUP]
Kivinen, T. and M. Kojo, "More Modular Exponential (MODP)
Diffie-Hellman groups for Internet Key Exchange (IKE)",
RFC 3526, May 2003. 2003,
<http://www.rfc-editor.org/info/rfc3526>.

[ADDRIPV6]
Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006. 2006,
<http://www.rfc-editor.org/info/rfc4291>.

[AEAD] Black, D. and D. McGrew, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282, August 2008.
2008, <http://www.rfc-editor.org/info/rfc5282>.

[AESCMACPRF128]
Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
Advanced Encryption Standard-Cipher-based Message
Authentication Code-Pseudo-Random Function-128 (AES-CMAC-
PRF-128) Algorithm for the Internet Key Exchange Protocol
(IKE)", RFC 4615, August 2006. 2006,
<http://www.rfc-editor.org/info/rfc4615>.

[AESXCBCPRF128]
Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the
Internet Key Exchange Protocol (IKE)", RFC 4434, February 2006.
2006, <http://www.rfc-editor.org/info/rfc4434>.

[EAP] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004. 2004, <http://www.rfc-editor.org/info/rfc3748>.

[ECN] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
of Explicit Congestion Notification (ECN) to IP", RFC
3168, September 2001. 2001,
<http://www.rfc-editor.org/info/rfc3168>.

[ESPCBC] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
Algorithms", RFC 2451, November 1998.

[HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. 1998,
<http://www.rfc-editor.org/info/rfc2451>.

[IKEV2IANA]
"Internet Key Exchange Version 2 (IKEv2) Parameters", <htt
p://www.iana.org/assignments/ikev2-parameters/
ikev2-parameters.xhtml>.
<http://www.iana.org/assignments/ikev2-parameters/>.

[IPSECARCH]
Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005. 2005,
<http://www.rfc-editor.org/info/rfc4301>.

[MUSTSHOULD]
Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography
Standards (PKCS) #1: RSA Cryptography Specifications
Version 2.1", RFC 3447, February 2003. 2003,
<http://www.rfc-editor.org/info/rfc3447>.

[PKIX] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, May 2008. 2008,
<http://www.rfc-editor.org/info/rfc5280>.

[RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the
Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
December 2005. 2005, <http://www.rfc-editor.org/info/rfc4307>.

[UDPENCAPS]
Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets", RFC
3948, January 2005. 2005,
<http://www.rfc-editor.org/info/rfc3948>.

[URLS] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.

8.2. 2005,
<http://www.rfc-editor.org/info/rfc3986>.

7.2. Informative References

[AH] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
2005, <http://www.rfc-editor.org/info/rfc4302>.

[ARCHGUIDEPHIL]
Bush, R. and D. Meyer, "Some Internet Architectural
Guidelines and Philosophy", RFC 3439, December 2002. 2002,
<http://www.rfc-editor.org/info/rfc3439>.

[ARCHPRINC]
Carpenter, B., "Architectural Principles of the Internet",
RFC 1958, June 1996. 1996,
<http://www.rfc-editor.org/info/rfc1958>.

[Clarif] Eronen, P. and P. Hoffman, "IKEv2 Clarifications and
Implementation Guidelines", RFC 4718, October 2006. 2006,
<http://www.rfc-editor.org/info/rfc4718>.

[DES] American National Standards Institute, "American National
Standard for Information Systems-Data Link Encryption",
ANSI X3.106, 1983.

[DH] Diffie, W. and M. Hellman, "New Directions in
Cryptography", IEEE Transactions on Information Theory,
V.IT-22 n. 6, June 1977.

[DIFFSERVARCH]
Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998. 1998,
<http://www.rfc-editor.org/info/rfc2475>.

[DIFFSERVFIELD]
Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998.
1998, <http://www.rfc-editor.org/info/rfc2474>.

[DIFFTUNNEL]
Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000. 2000,
<http://www.rfc-editor.org/info/rfc2983>.

[DOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998. 1998,
<http://www.rfc-editor.org/info/rfc2407>.

[DOSUDPPROT]
C.
Kaufman, R. C., Perlman, R., and B. Sommerfeld, "DoS
protection for UDP-based protocols", ACM Conference on
Computer and Communications Security, October 2003.

[DSS] National Institute of Standards and Technology, U.S.
Department of Commerce, "Digital Signature Standard",
Draft Standard
(DSS)", FIPS 186-3, June 2008. 186-4, July 2013,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.186-4.pdf>.

[EAI] Yang, A., Steele, S., and N. Freed, "Internationalized
Email Headers", RFC 6532, February 2012. 2012,
<http://www.rfc-editor.org/info/rfc6532>.

[EAP-IANA]
"Extensible Authentication Protocol (EAP) Registry: Method
Types", <http://www.iana.org>. <http://http://www.iana.org/assignments/eap-eke/>.

[EAPMITM] N. Asokan, V. Nierni, N., Niemi, V., and K. Nyberg, "Man-in-the-Middle
in Tunneled Authentication Protocols", November 2002,
<http://eprint.iacr.org/2002/163>.

[ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005. 2005,
<http://www.rfc-editor.org/info/rfc4303>.

[EXCHANGEANALYSIS]
Perlman, R. Perlman and C. Kaufman, "Analysis of the IPsec key
exchange Standard", WET-ICE Security Conference, MIT,
2001,
<http://sec.femto.org/wetice-2001/papers/radia-paper.pdf>. <http://www.computer.org/csdl/proceedings/
wetice/2001/1269/00/12690150.pdf>.

[FIPS.180-4.2012]
National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard (SHS)", FIPS
180-4, March 2012,
<http://csrc.nist.gov/publications/fips/fips180-4/
fips-180-4.pdf>.

[H2HIPSEC]
Aura, T., Roe, M., and A. Mohammed, "Experiences with
Host-to-Host IPsec", 13th International Workshop on
Security Protocols, Cambridge, UK, April 2005.

[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997.
1997, <http://www.rfc-editor.org/info/rfc2104>.

[IDEA] X. Lai, X., "On the Design and Security of Block Ciphers",
ETH Series in Information Processing, v. 1, Konstanz: Hartung-
Gorre
Hartung-Gorre Verlag, 1992.

[IDNA] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010. 2010,
<http://www.rfc-editor.org/info/rfc5890>.

[IKEV1] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998. 1998,
<http://www.rfc-editor.org/info/rfc2409>.

[IKEV2] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005. 2005,
<http://www.rfc-editor.org/info/rfc4306>.

[IP] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
1981, <http://www.rfc-editor.org/info/rfc791>.

[IP-COMP] Shacham, A., Monsour, B., Pereira, R., and M. Thomas, "IP
Payload Compression Protocol (IPComp)", RFC 3173,
September 2001. 2001, <http://www.rfc-editor.org/info/rfc3173>.

[IPSECARCH-OLD]
Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998. 1998,
<http://www.rfc-editor.org/info/rfc2401>.

[IPV6CONFIG]
Eronen, P., Laganier, J., and C. Madson, "IPv6
Configuration in Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 5739, February 2010. 2010,
<http://www.rfc-editor.org/info/rfc5739>.

[ISAKMP] Maughan, D., Schneider, M., and M. Schertler, "Internet
Security Association and Key Management Protocol
(ISAKMP)", RFC 2408, November 1998. 1998,
<http://www.rfc-editor.org/info/rfc2408>.

[MAILFORMAT]
Resnick, P., Ed., "Internet Message Format", RFC 5322,
October 2008. 2008, <http://www.rfc-editor.org/info/rfc5322>.

[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992. 1992, <http://www.rfc-editor.org/info/rfc1321>.

[MIPV6] Perkins, C., Johnson, D., and J. Arkko, "Mobility Support
in IPv6", RFC 6275, July 2011. 2011,
<http://www.rfc-editor.org/info/rfc6275>.

[MLDV2] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004. 2004,
<http://www.rfc-editor.org/info/rfc3810>.

[MOBIKE] Eronen, P., "IKEv2 Mobility and Multihoming Protocol
(MOBIKE)", RFC 4555, June 2006. 2006,
<http://www.rfc-editor.org/info/rfc4555>.

[MODES] National Institute of Standards and Technology, U.S.
Department of Commerce, Dworkin, M., "Recommendation for Block Cipher Modes of
Operation", SP 800-38A, National Institute of Standards and
Technology, NIST Special Publication 800-38A 2001 Edition,
December 2001.

[NAI] Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
Network Access Identifier", RFC 4282, December 2005. 2005,
<http://www.rfc-editor.org/info/rfc4282>.

[NATREQ] Aboba, B. and W. Dixon, "IPsec-Network Address Translation
(NAT) Compatibility Requirements", RFC 3715, March 2004. 2004,
<http://www.rfc-editor.org/info/rfc3715>.

[OAKLEY] Orman, H., "The OAKLEY Key Determination Protocol", RFC
2412, November 1998. 1998,
<http://www.rfc-editor.org/info/rfc2412>.

[PFKEY] McDonald, D., Metz, C., and B. Phan, "PF_KEY Key
Management API, Version 2", RFC 2367, July 1998. 1998,
<http://www.rfc-editor.org/info/rfc2367>.

[PHOTURIS]
Karn, P. and W. Simpson, "Photuris: Session-Key Management
Protocol", RFC 2522, March 1999. 1999,
<http://www.rfc-editor.org/info/rfc2522>.

[RANDOMNESS]
Eastlake,
Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005. 2005, <http://www.rfc-editor.org/info/rfc4086>.

[REAUTH] Nir, Y., "Repeated Authentication in Internet Key Exchange
(IKEv2) Protocol", RFC 4478, April 2006. 2006,
<http://www.rfc-editor.org/info/rfc4478>.

[REUSE] Menezes, A. and B. Ustaoglu, "On Reusing Ephemeral Keys In
Diffie-Hellman Key Agreement Protocols", December 2008, <
http://www.cacr.math.uwaterloo.ca/techreports/2008/
<http://www.cacr.math.uwaterloo.ca/techreports/2008/
cacr2008-24.pdf>.

[RFC4945] Korver, B., "The Internet IP Security PKI Profile of
IKEv1/ISAKMP, IKEv2, and PKIX", RFC 4945, August 2007. 2007,
<http://www.rfc-editor.org/info/rfc4945>.

[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
5996, September 2010. 2010,
<http://www.rfc-editor.org/info/rfc5996>.

[RFC6989] Sheffer, Y. and S. Fluhrer, "Additional Diffie-Hellman
Tests for the Internet Key Exchange Protocol Version 2
(IKEv2)", RFC 6989, July 2013. 2013,
<http://www.rfc-editor.org/info/rfc6989>.

[ROHCV2] Ertekin, E., Christou, C., Jasani, R., Kivinen, T., and C.
Bormann, "IKEv2 Extensions to Support Robust Header
Compression over IPsec", RFC 5857, May 2010.

[SHA] National Institute of Standards and Technology, U.S.
Department of Commerce, "Secure Hash Standard",
FIPS 180-3, October 2008. 2010,
<http://www.rfc-editor.org/info/rfc5857>.

[SIGMA] H. Krawczyk, H., "SIGMA: the `SIGn-and-MAc' 'SIGn-and-MAc' Approach to
Authenticated Diffie-Hellman and its Use in the IKE
Protocols", Advances in Cryptography - CRYPTO 2003
Proceedings LNCS 2729, 2003, <http://
www.informatik.uni-trier.de/~ley/db/conf/crypto/
<http://www.informatik.uni-trier.de/~ley/db/conf/crypto/
crypto2003.html>.

[SKEME] H. Krawczyk, H., "SKEME: A Versatile Secure Key Exchange
Mechanism for Internet", IEEE Proceedings of the 1996
Symposium on Network and Distributed Systems Security , Security,
1996.

[TRANSPARENCY]
Carpenter, B., "Internet Transparency", RFC 2775, February 2000.
2000, <http://www.rfc-editor.org/info/rfc2775>.

Appendix A. Summary of Changes from IKEv1

The goals of this revision to IKE are:

1. To define the entire IKE protocol in a single document,
replacing RFCs 2407, 2408, and 2409 and incorporating subsequent
changes to support NAT Traversal, traversal, Extensible Authentication, and
Remote Address acquisition;

2. To simplify IKE by replacing the eight different initial
exchanges with a single four-message exchange (with changes in
authentication mechanisms affecting only a single AUTH payload
rather than restructuring the entire exchange) see
[EXCHANGEANALYSIS];

3. To remove the Domain of Interpretation (DOI), Situation (SIT),
and Labeled Domain Identifier fields, and the Commit and
Authentication only bits;

4. To decrease IKE's latency in the common case by making the
initial exchange be 2 round trips (4 messages), and allowing the
ability to piggyback setup of a Child SA on that exchange;

5. To replace the cryptographic syntax for protecting the IKE
messages themselves with one based closely on ESP to simplify
implementation and security analysis;

6. To reduce the number of possible error states by making the
protocol reliable (all messages are acknowledged) and sequenced.
This allows shortening CREATE_CHILD_SA exchanges from 3 messages
to 2;

7. To increase robustness by allowing the responder to not do
significant processing until it receives a message proving that
the initiator can receive messages at its claimed IP address;

8. To fix cryptographic weaknesses such as the problem with
symmetries in hashes used for authentication (documented by Tero
Kivinen);

9. To specify Traffic Selectors in their own payloads type rather
than overloading ID payloads, and making more flexible the
Traffic Selectors that may be specified;

10. To specify required behavior under certain error conditions or
when data that is not understood is received in order to make it
easier to make future revisions in a way that does not break
backward compatibility;
11. To simplify and clarify how shared state is maintained in the
presence of network failures and DoS attacks; and

12. To maintain existing syntax and magic numbers to the extent
possible to make it likely that implementations of IKEv1 can be
enhanced to support IKEv2 with minimum effort.

Appendix B. Diffie-Hellman Groups

There are two Diffie-Hellman groups defined here for use in IKE.
These groups were generated by Richard Schroeppel at the University
of Arizona. Properties of these primes are described in [OAKLEY].

The strength supplied by group 1 may not be sufficient for typical
uses and is here for historic reasons.

Additional Diffie-Hellman groups have been defined in [ADDGROUP].

B.1. Group 1 - 768-bit MODP

This group is assigned ID 1 (one).

The prime is: 2^768 - 2 ^704 - 1 + 2^64 * { [2^638 pi] + 149686 }
Its hexadecimal value is:

FFFFFFFF FFFFFFFF C90FDAA2 2168C234 C4C6628B 80DC1CD1
29024E08 8A67CC74 020BBEA6 3B139B22 514A0879 8E3404DD
EF9519B3 CD3A431B 302B0A6D F25F1437 4FE1356D 6D51C245
E485B576 625E7EC6 F44C42E9 A63A3620 FFFFFFFF FFFFFFFF

The generator is 2.

B.2. Group 2 - 1024-bit MODP

This group is assigned ID 2 (two).

The prime is 2^1024 - 2^960 - 1 + 2^64 * { [2^894 pi] + 129093 }.
Its hexadecimal value is:

The generator is 2.

Appendix C. Exchanges and Payloads

This appendix contains a short summary of the IKEv2 exchanges, and
what payloads can appear in which message. This appendix is purely
informative; if it disagrees with the body of this document, the
other text is considered correct.

Vendor ID (V) payloads may be included in any place in any message.
This sequence here shows what are the most logical places for them.

C.1. IKE_SA_INIT Exchange

request --> [N(COOKIE)], [N(COOKIE),]
SA, KE, Ni,
[N(NAT_DETECTION_SOURCE_IP)+,
N(NAT_DETECTION_DESTINATION_IP)],
N(NAT_DETECTION_DESTINATION_IP),]
[V+][N+]

normal response <-- SA, KE, Nr,
(no cookie) [N(NAT_DETECTION_SOURCE_IP),
N(NAT_DETECTION_DESTINATION_IP)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
N(NAT_DETECTION_DESTINATION_IP),]
[[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
[V+][N+]

cookie response <-- N(COOKIE),
[V+][N+]

different Diffie- <-- N(INVALID_KE_PAYLOAD),
Hellman group [V+][N+]
wanted

C.2. IKE_AUTH Exchange without EAP

request --> IDi, [CERT+],
[N(INITIAL_CONTACT)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
[IDr], [CERT+,]
[N(INITIAL_CONTACT),]
[[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
[IDr,]
AUTH,
[CP(CFG_REQUEST)],
[N(IPCOMP_SUPPORTED)+],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
[CP(CFG_REQUEST),]
[N(IPCOMP_SUPPORTED)+,]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, TSi, TSr,
[V+][N+]
response <-- IDr, [CERT+], [CERT+,]
AUTH,
[CP(CFG_REPLY)],
[N(IPCOMP_SUPPORTED)],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
[CP(CFG_REPLY),]
[N(IPCOMP_SUPPORTED),]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, TSi, TSr,
[N(ADDITIONAL_TS_POSSIBLE)],
[N(ADDITIONAL_TS_POSSIBLE),]
[V+][N+]

error in Child SA <-- IDr, [CERT+], [CERT+,]
creation AUTH,
N(error),
[V+][N+]

C.3. IKE_AUTH Exchange with EAP

first request --> IDi,
[N(INITIAL_CONTACT)],
[[N(HTTP_CERT_LOOKUP_SUPPORTED)], CERTREQ+],
[IDr],
[CP(CFG_REQUEST)],
[N(IPCOMP_SUPPORTED)+],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
[N(INITIAL_CONTACT),]
[[N(HTTP_CERT_LOOKUP_SUPPORTED),] CERTREQ+,]
[IDr,]
[CP(CFG_REQUEST),]
[N(IPCOMP_SUPPORTED)+,]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, TSi, TSr,
[V+][N+]

first response <-- IDr, [CERT+], [CERT+,] AUTH,
EAP,
[V+][N+]

/ --> EAP
repeat 1..N times |
\ <-- EAP
last request --> AUTH

last response <-- AUTH,
[CP(CFG_REPLY)],
[N(IPCOMP_SUPPORTED)],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)],
[CP(CFG_REPLY),]
[N(IPCOMP_SUPPORTED),]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, TSi, TSr,
[N(ADDITIONAL_TS_POSSIBLE)],
[N(ADDITIONAL_TS_POSSIBLE),]
[V+][N+]

C.4. CREATE_CHILD_SA Exchange for Creating or Rekeying Child SAs

request --> [N(REKEY_SA)],
[CP(CFG_REQUEST)],
[N(IPCOMP_SUPPORTED)+],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)], [N(REKEY_SA),]
[CP(CFG_REQUEST),]
[N(IPCOMP_SUPPORTED)+,]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, Ni, [KEi], [KEi,] TSi, TSr TSr,
[V+][N+]

normal <-- [CP(CFG_REPLY)], [CP(CFG_REPLY),]
response [N(IPCOMP_SUPPORTED)],
[N(USE_TRANSPORT_MODE)],
[N(ESP_TFC_PADDING_NOT_SUPPORTED)],
[N(NON_FIRST_FRAGMENTS_ALSO)], [N(IPCOMP_SUPPORTED),]
[N(USE_TRANSPORT_MODE),]
[N(ESP_TFC_PADDING_NOT_SUPPORTED),]
[N(NON_FIRST_FRAGMENTS_ALSO),]
SA, Nr, [KEr], [KEr,] TSi, TSr,
[N(ADDITIONAL_TS_POSSIBLE)]
[N(ADDITIONAL_TS_POSSIBLE),]
[V+][N+]

error case <-- N(error)

different Diffie- <-- N(INVALID_KE_PAYLOAD),
Hellman group [V+][N+]
wanted

C.5. CREATE_CHILD_SA Exchange for Rekeying the IKE SA

request --> SA, Ni, KEi KEi,
[V+][N+]

response <-- SA, Nr, KEr KEr,
[V+][N+]

C.6. INFORMATIONAL Exchange

request --> [N+],
[D+], [N+,]
[D+,]
[CP(CFG_REQUEST)]

response <-- [N+],
[D+], [N+,]
[D+,]
[CP(CFG_REPLY)]

Acknowledgements

Many individuals in the IPsecME Working Group were very helpful in
contributing ideas and text for this document, as well as in
reviewing the clarifications suggested by others.

The acknowledgements from the IKEv2 document were:

Authors' Addresses

Charlie Kaufman
Microsoft
1 Microsoft Way
Redmond, WA 98052
US

Phone: 1-425-707-3335
United States

EMail: charliek@microsoft.com charliekaufman@outlook.com

Paul Hoffman
VPN Consortium
127 Segre Place
Santa Cruz, CA 95060
US
United States

Phone: 1-831-426-9827
EMail: paul.hoffman@vpnc.org

Yoav Nir
Check Point Software Technologies Ltd.
5 Hasolelim St.
Tel Aviv 6789735
Israel

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Pasi Eronen
Independent

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Tero Kivinen
INSIDE Secure
Eerikinkatu 28
HELSINKI FI-00180
FI
Finland

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