wdiff rfc9063.original rfc9063.txt

Network Working Group

Internet Engineering Task Force (IETF) R. Moskowitz, Ed.
Internet-Draft
Request for Comments: 9063 HTT Consulting
Obsoletes: 4423 (if approved) M. Komu
Intended status:
Category: Informational Ericsson
Expires: August 18, 2019 February 14, 2019
ISSN: 2070-1721 July 2021

Host Identity Protocol Architecture
draft-ietf-hip-rfc4423-bis-20

Abstract

This memo describes the Host Identity (HI) namespace, that which provides
a cryptographic namespace to applications, and the associated
protocol layer, the Host Identity Protocol, located between the
internetworking and transport layers, that supports end-host
mobility, multihoming multihoming, and NAT traversal. Herein are presented the
basics of the current namespaces, their strengths and weaknesses, and
how a HI namespace will add completeness to them. The roles of the
HI namespace in the protocols are defined.

This document obsoletes RFC 4423 and addresses the concerns raised by
the IESG, particularly that of crypto agility. The Security
Considerations section on
security considerations describe also describes measures against flooding
attacks, usage of identities in access control lists, weaker types of
identifiers
identifiers, and trust on first use. This document incorporates
lessons learned from the implementations of RFC 5201 7401 and goes further
to explain how HIP works as a secure signaling channel.

Status of This Memo

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

Internet-Drafts are working documents not an Internet Standards Track specification; it is
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approved by the IESG are candidates for a maximum any level of Internet
Standard; see Section 2 of RFC 7841.

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This Internet-Draft will expire on August 18, 2019.
https://www.rfc-editor.org/info/rfc9063.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Terms common Common to other documents . . . . . . . . . . . . . . 4 Other Documents
2.2. Terms specific Specific to this This and other Other HIP documents . . . . . . 5 Documents
3. Background . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. A desire Desire for a namespace Namespace for computing platforms . . . . . 8 Computing Platforms
4. Host Identity namespace . . . . . . . . . . . . . . . . . . . 9 Namespace
4.1. Host Identifiers . . . . . . . . . . . . . . . . . . . . . 10
4.2. Host Identity Hash (HIH) . . . . . . . . . . . . . . . . . 11
4.3. Host Identity Tag (HIT) . . . . . . . . . . . . . . . . . . 11
4.4. Local Scope Identifier (LSI) . . . . . . . . . . . . . . . 12
4.5. Storing Host Identifiers in directories . . . . . . . . . . 13 Directories
5. New stack architecture . . . . . . . . . . . . . . . . . . . 14 Stack Architecture
5.1. On the multiplicity Multiplicity of identities . . . . . . . . . . . . . 15 Identities
6. Control plane . . . . . . . . . . . . . . . . . . . . . . . . 16 Plane
6.1. Base exchange . . . . . . . . . . . . . . . . . . . . . . . 16 Exchange
6.2. End-host mobility End-Host Mobility and multi-homing . . . . . . . . . . . . 17 Multihoming
6.3. Rendezvous mechanism . . . . . . . . . . . . . . . . . . . 18 Mechanism
6.4. Relay mechanism . . . . . . . . . . . . . . . . . . . . . . 18 Mechanism
6.5. Termination of the control plane . . . . . . . . . . . . . 18 Control Plane
7. Data plane . . . . . . . . . . . . . . . . . . . . . . . . . 18 Plane
8. HIP and NATs . . . . . . . . . . . . . . . . . . . . . . . . 19
8.1. HIP and Upper-layer checksums . . . . . . . . . . . . . . . 20 Upper-Layer Checksums
9. Multicast . . . . . . . . . . . . . . . . . . . . . . . . . . 20
10. HIP policies . . . . . . . . . . . . . . . . . . . . . . . . 21 Policies
11. Security considerations . . . . . . . . . . . . . . . . . . . 21 Considerations
11.1. MiTM MitM Attacks . . . . . . . . . . . . . . . . . . . . . . . 22
11.2. Protection against flooding attacks . . . . . . . . . . . 23 Flooding Attacks
11.3. HITs used Used in ACLs . . . . . . . . . . . . . . . . . . . . 24
11.4. Alternative HI considerations . . . . . . . . . . . . . . 25 Considerations
11.5. Trust On on First Use . . . . . . . . . . . . . . . . . . . . 25
12. IANA considerations . . . . . . . . . . . . . . . . . . . . . 28 Considerations
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
14. Changes from RFC 4423 . . . . . . . . . . . . . . . . . . . . 29
15.
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 29
15.1.
14.1. Normative References . . . . . . . . . . . . . . . . . . . 29
15.2.
14.2. Informative references . . . . . . . . . . . . . . . . . . 31 References
Appendix A. Design considerations . . . . . . . . . . . . . . . 38 Considerations
A.1. Benefits of HIP . . . . . . . . . . . . . . . . . . . . . . 38
A.2. Drawbacks of HIP . . . . . . . . . . . . . . . . . . . . . 41
A.3. Deployment and adoption considerations . . . . . . . . . . 43 Adoption Considerations
A.3.1. Deployment analysis . . . . . . . . . . . . . . . . . . . 43 Analysis
A.3.2. HIP in 802.15.4 networks . . . . . . . . . . . . . . . . 44 Networks
A.3.3. HIP and Internet of Things . . . . . . . . . . . . . . . 44
A.3.4. Infrastructure Applications . . . . . . . . . . . . . . . 46
A.3.5. Management of Identities in a Commercial Product . . . . 47
A.4. Answers to NSRG questions . . . . . . . . . . . . . . . . . 48 Questions
Acknowledgments
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50

1. Introduction

The Internet has two important global namespaces: Internet Protocol
(IP) addresses and Domain Name Service (DNS) names. These two
namespaces have a set of features and abstractions that have powered
the Internet to what it is today. They also have a number of
weaknesses. Basically, since they are all we have, we try to do too
much with them. Semantic overloading and functionality extensions
have greatly complicated these namespaces.

The proposed Host Identity namespace is also a global namespace, and
it fills an important gap between the IP and DNS namespaces. A Host
Identity conceptually refers to a computing platform, and there may
be multiple such Host Identities per computing platform (because the
platform may wish to present a different identity to different
communicating peers). The Host Identity namespace consists of Host
Identifiers (HI). There is exactly one Host Identifier for each Host
Identity (although there may be transient periods of time such as key
replacement when more than one identifier may be active). While this
text later talks about non-cryptographic Host Identifiers, the
architecture focuses on the case in which Host Identifiers are
cryptographic in nature. Specifically, the Host Identifier is the
public key of an asymmetric key-pair. key pair. Each Host Identity uniquely
identifies a single host, i.e., no two hosts have the same Host
Identity. If two or more computing platforms have the same Host
Identifier, then they are instantiating a distributed host. The Host
Identifier can either be public (e.g., published in the DNS), DNS) or
unpublished. Client systems will tend to have both public and
unpublished Host Identifiers.

There is a subtle but important difference between Host Identities
and Host Identifiers. An Identity refers to the abstract entity that
is identified. An Identifier, on the other hand, refers to the
concrete bit pattern that is used in the identification process.

Although the Host Identifiers could be used in many authentication
systems, such as IKEv2 [RFC4306], [RFC7296], the presented architecture
introduces a new protocol, called the Host Identity Protocol (HIP),
and a cryptographic exchange, called the HIP base exchange; see also
Section 6. HIP provides for limited forms of trust between systems,
enhances mobility, multi-homing multihoming, and dynamic IP renumbering, aids in
protocol translation / and transition, and reduces certain types of
denial-of-service (DoS) attacks.

When HIP is used, the actual payload traffic between two HIP hosts is
typically, but not necessarily, protected with ESP Encapsulating Security
Payload (ESP) [RFC7402]. The Host Identities are used to create the
needed ESP Security Associations (SAs) and to authenticate the hosts.
When ESP is used, the actual payload IP packets do not differ in any
way from standard
ESP protected ESP-protected IP packets.

Much has been learned about HIP [RFC6538] since [RFC4423] was
published. This document expands Host Identities beyond their
original use to enable IP connectivity and security to enable general
interhost secure
signalling signaling at any protocol layer. The signal may
establish a security association between the hosts, hosts or simply pass
information within the channel.

2. Terminology

2.1. Terms common Common to other documents
+---------------+---------------------------------------------------+ Other Documents

+==========+===================================================+
| Term | Explanation |
+---------------+---------------------------------------------------+
+==========+===================================================+
| Public key | The public key of an asymmetric cryptographic key |
| key | pair. Used as a publicly known identifier for |
| | cryptographic identity authentication. Public is |
| | a relative term here, ranging from "known to |
| | peers only" to "known to the world." world". |
+----------+---------------------------------------------------+
| Private key | The private or secret key of an asymmetric |
| key | cryptographic key pair. Assumed to be known only |
| | to the party identified by the corresponding |
| | public key. Used by the identified party to |
| | authenticate its identity to other parties. |
+----------+---------------------------------------------------+
| Public key | An asymmetric cryptographic key pair consisting |
| key pair | of public and private keys. For example, Rivest- |
| | Shamir-Adleman (RSA), Digital Signature Algorithm |
| | (DSA) and Elliptic Curve DSA (ECDSA) key pairs |
| | are such key pairs. |
+----------+---------------------------------------------------+
| End-point Endpoint | A communicating entity. For historical reasons, |
| | the term 'computing platform' is used in this |
| | document as a (rough) synonym for end-point. endpoint. |
+---------------+---------------------------------------------------+
+----------+---------------------------------------------------+

Table 1

2.2. Terms specific Specific to this This and other Other HIP documents Documents

It should be noted that many of the terms defined herein are
tautologous, self-referential self-referential, or defined through circular reference
to other terms. This is due to the succinct nature of the
definitions. See the text elsewhere in this document and the base
specification [RFC7401] for more elaborate explanations.

+---------------+---------------------------------------------------+

Table 2

3. Background

The Internet is built from three principal components: computing
platforms (end-points), (endpoints), packet transport (i.e., internetworking)
infrastructure, and services (applications). The Internet exists to
service two principal components: people and robotic services
(silicon-based people, if you will). All these components need to be
named in order to interact in a scalable manner. Here we concentrate
on naming computing platforms and packet transport elements.

There are two principal namespaces in use in the Internet for these
components: IP addresses, and Domain Names. Domain Names provide
hierarchically assigned names for some computing platforms and some
services. Each hierarchy is delegated from the level above; there is
no anonymity in Domain Names. Email, HTTP, and SIP addresses all
reference Domain Names.

The IP addressing namespace has been overloaded to name both
interfaces (at layer-3) Layer 3) and endpoints (for the endpoint-specific part
of layer-3, Layer 3 and for layer-4). Layer 4). In their role as interface names, IP
addresses are sometimes called "locators" and serve as an endpoint
within a routing topology.

IP addresses are numbers that name networking interfaces, and
typically only when the interface is connected to the network.
Originally, IP addresses had long-term significance. Today, the vast
number of interfaces use ephemeral and/or non-unique IP addresses.
That is, every time an interface is connected to the network, it is
assigned an IP address.

In the current Internet, the transport layers are coupled to the IP
addresses. Neither can evolve separately from the other. IPng
deliberations were strongly shaped by the decision that a
corresponding TCPng would not be created.

There are three critical deficiencies with the current namespaces.
Firstly,
First, the establishing of initial contact and the sustaining of data
flows between two hosts can be challenging due to private address
realms and the ephemeral nature of addresses. Secondly, Second,
confidentiality is not provided in a consistent, trustable manner.
Finally, authentication for systems and datagrams is not provided.
All of these deficiencies arise because computing platforms are not
well named with the current namespaces.

3.1. A desire Desire for a namespace Namespace for computing platforms Computing Platforms

An independent namespace for computing platforms could be used in
end-to-end operations independent of the evolution of the
internetworking layer and across the many internetworking layers.
This could support rapid readdressing of the internetworking layer
because of mobility, rehoming, or renumbering.

If the namespace for computing platforms is based on public-key
cryptography, it can also provide authentication services. If this
namespace is locally created without requiring registration, it can
provide anonymity.

Such a namespace (for computing platforms) and the names in it should
have the following characteristics:

* The namespace should be applied to the IP 'kernel' or stack. The
IP stack is the 'component' between applications and the packet
transport infrastructure.

* The namespace should fully decouple the internetworking layer from
the higher layers. The names should replace all occurrences of IP
addresses within applications (like in the Transport Control
Block, TCB). This replacement can be handled transparently for
legacy applications as the LSIs Local Scope Identifiers (LSIs) and HITs
are compatible with IPv4 and IPv6 addresses [RFC5338]. However,
HIP-aware applications require some modifications from the
developers, who may employ networking API extensions for HIP
[RFC6317].

* The introduction of the namespace should not mandate any
administrative infrastructure. Deployment must come from the
bottom up, in a pairwise deployment.

* The names should have a fixed-length representation, for easy
inclusion in datagram headers and existing programming interfaces
(e.g
(e.g., the TCB).

* Using the namespace should be affordable when used in protocols.
This is primarily a packet size issue. There is also a
computational concern in affordability.

* Name collisions should be avoided as much as possible. The
mathematics of the birthday paradox can be used to estimate the
chance of a collision in a given population and hash space. In
general, for a random hash space of size n bits, we would expect
to obtain a collision after approximately 1.2*sqrt(2**n) 1.2*sqrt(2^n) hashes
were obtained. For 64 bits, this number is roughly 4 billion. A
hash size of 64 bits may be too small to avoid collisions in a
large population; for example, there is a 1% chance of collision
in a population of 640M. For 100 bits (or more), we would not
expect a collision until approximately 2**50 2^50 (1 quadrillion) hashes
were generated. With the currently used hash size of 96 bits
[RFC7343], the figure is 2**48 2^48 (281 trillions).

* The names should have a localized abstraction so that they can be
used in existing protocols and APIs.

* It must be possible to create names locally. When such names are
not published, this can provide anonymity at the cost of making
resolvability very difficult.

* The namespace should provide authentication services.

* The names should be long-lived, but replaceable at any time. This
impacts access control lists; short lifetimes will tend to result
in tedious list maintenance or require a namespace infrastructure
for central control of access lists.

In this document, the namespace approaching these ideas is called the
Host Identity namespace. Using Host Identities requires its own
protocol layer, the Host Identity Protocol, between the
internetworking and transport layers. The names are based on public-
key cryptography to supply authentication services. Properly
designed, it can deliver all of the above-stated requirements.

4. Host Identity namespace Namespace

A name in the Host Identity namespace, a Host Identifier (HI),
represents a statistically globally unique name for naming any system
with an IP stack. This identity is normally associated with, but not
limited to, an IP stack. A system can have multiple identities, some
'well known', some unpublished or 'anonymous'. A system may self-
assert its own identity, or may use a third-party authenticator like
DNSSEC [RFC2535], PGP, [RFC4033], Pretty Good Privacy (PGP), or X.509 to 'notarize'
the identity assertion to another namespace.

In theory, any name that can claim to be 'statistically globally
unique' may serve as a Host Identifier. In the HIP architecture, the
public key of a private-public key pair has been chosen as the Host
Identifier because it can be self-managed and it is computationally
difficult to forge. As specified in the Host Identity Protocol
[RFC7401] specification,
specification [RFC7401], a public-key-based HI can authenticate the
HIP packets and protect them from man-in-the-middle (MitM) attacks.
Since authenticated datagrams are mandatory to provide much of HIP's
denial-of-service protection, the Diffie-Hellman exchange in HIP base
exchange has to be authenticated. Thus, only public-key HI and
authenticated HIP messages are supported in practice.

In this document, some non-cryptographic forms of HI and HIP are
referenced, but cryptographic forms SHOULD should be preferred because they
are more secure than their non-cryptographic counterparts. There has
been past research in challenge puzzles to use using non-cryptographic HI, HI
for Radio Frequency IDentification (RFID), in an HIP exchange
tailored to the workings of such challenges (as described further in
[urien-rfid] and [urien-rfid-draft]).

4.1. Host Identifiers

Host Identity adds two main features to Internet protocols. The
first is a decoupling of the internetworking and transport layers;
see Section 5. This decoupling will allow for independent evolution
of the two layers. Additionally, it can provide end-to-end services
over multiple internetworking realms. The second feature is host
authentication. Because the Host Identifier is a public key, this
key can be used for authentication in security protocols like ESP.

An identity is based on public-private key cryptography in HIP. The
Host Identity is referred to by its public component, the public key.
Thus, the name representing a Host Identity in the Host Identity
namespace, i.e., the Host Identifier, is the public key. In a way,
the possession of the private key defines the Identity itself. If
the private key is possessed by more than one node, the Identity can
be considered to be a distributed one.

Architecturally, any other Internet naming convention might form a
usable base for Host Identifiers. However, non-cryptographic names
should only be used in situations of high trust - and/or low risk.
That is any place where host authentication is not needed (no risk of
host spoofing) and no use of ESP. However, at least for
interconnected networks spanning several operational domains, the set
of environments where the risk of host spoofing allowed by non-
cryptographic Host Identifiers is acceptable is the null set. Hence,
the current HIP documents do not specify how to use any other types
of Host Identifiers but public keys. For instance, Back-to-My-Mac the Back to My
Mac service [RFC6281] from Apple comes pretty close to the
functionality of HIP, but unlike HIP, it is based on non-cryptographic non-
cryptographic identifiers.

The actual Host Identifiers are never directly used at the transport
or network layers. The corresponding Host Identifiers (public keys)
may be stored in various DNS or other directories as identified
elsewhere in this document, and they are passed in the HIP base
exchange. A Host Identity Tag (HIT) is used in other protocols to
represent the Host Identity. Another representation of the Host
Identities, the Local Scope Identifier (LSI), can also be used in
protocols and APIs.

4.2. Host Identity Hash (HIH)

The Host Identity Hash (HIH) is the cryptographic hash algorithm used
in producing the HIT from the HI. It is also the hash used
throughout the HIP protocol for consistency and simplicity. It is possible for
the two hosts in the HIP exchange to use different hash algorithms.

Multiple HIHs within HIP are needed to address the moving target of
creation and eventual compromise of cryptographic hashes. This
significantly complicates HIP and offers an attacker an additional
downgrade attack that is mitigated in the HIP protocol [RFC7401].

4.3. Host Identity Tag (HIT)

A Host Identity Tag (HIT) is a 128-bit representation for a Host
Identity. Due to its size, it is suitable to be used for use in the existing
sockets API in the place of IPv6 addresses (e.g., in sockaddr_in6
structure, sin6_addr member) without modifying applications. It is
created from an HIH, an IPv6 prefix [RFC7343] [RFC7343], and a hash identifier.
There are two advantages of using the HIT over using the Host
Identifier in protocols. Firstly, First, its fixed length makes for easier
protocol coding and also better manages the packet size cost of this
technology. Secondly, Second, it presents the identity in a consistent format
to the protocol independent of the cryptographic algorithms used.

In essence, the HIT is a hash over the public key. As such, two
algorithms affect the generation of a HIT: the public-key algorithm
of the HI and the used HIH. The two algorithms are encoded in the
bit presentation of the HIT. As the two communicating parties may
support different algorithms, [RFC7401] defines the minimum set for
interoperability. For further interoperability, the responder Responder may
store its keys in DNS records, and thus the initiator Initiator may have to
couple destination HITs with appropriate source HITs according to
matching HIH.

In the HIP packets, the HITs identify the sender and recipient of a
packet. Consequently, a HIT should be unique in the whole IP
universe as long as it is being used. In the extremely rare case of
a single HIT mapping to more than one Host Identity, the Host
Identifiers (public keys) will make the final difference. If there
is more than one public key for a given node, the HIT acts as a hint
for the correct public key to use.

Although it may be rare for an accidental collision to cause a single
HIT mapping to more than one Host Identity, it may be the case that
an attacker succeeds to find, by brute force or algorithmic weakness,
a second Host Identity hashing to the same HIT. This type of attack
is known as a preimage attack, and the resistance to finding a second
Host Identifier (public key) that hashes to the same HIT is called
second preimage resistance. Second preimage resistance in HIP is
based on the hash algorithm strength and the length of the hash
output used. Through HIPv2 [RFC7401], this resistance is 96 bits
(less than the 128 bit 128-bit width of an IPv6 address field due to the
presence of the ORCHID Overlay Routable Cryptographic Hash Identifiers
(ORCHID) prefix [RFC7343]). 96 bits of resistance was considered
acceptable strength during the design of HIP, HIP but may eventually be
considered insufficient for the threat model of an envisioned
deployment. One possible mitigation would be to augment the use of
HITs in the deployment with the HIs themselves (and mechanisms to
securely bind the HIs to the HITs), so that the HI becomes the final
authority. It also may be possible to increase the difficulty of a
brute force attack by making the generation of the HI more
computationally difficult, such as the hash extension approach of SEND CGAs
Secure Neighbor Discovery Cryptographically Generated Addresses
(CGAs) [RFC3972], although the HIP specifications through HIPv2 do
not provide such a mechanism. Finally, deployments that do not use
ORCHIDs (such as certain types of overlay networks) might also use
the full 128-bit width of an IPv6 address field for the HIT.

4.4. Local Scope Identifier (LSI)

An LSI is a 32-bit localized representation for a Host Identity. Due
to its size, it is suitable to be used for use in the existing sockets API in
the place of IPv4 addresses (e.g., in sockaddr_in structure, sin_addr
member) without modifying applications. The purpose of an LSI is to
facilitate using Host Identities in existing APIs for IPv4-based
applications. LSIs are never transmitted on the wire; when an
application sends data using a pair of LSIs, the HIP layer (or
sockets handler) translates the LSIs to the corresponding HITs, and
vice versa for the receiving of data. Besides facilitating HIP-based
connectivity for legacy IPv4 applications, the LSIs are beneficial in
two other scenarios [RFC6538].

In the first scenario, two IPv4-only applications are residing reside on two
separate hosts connected by IPv6-only network. With HIP-based
connectivity, the two applications are able to communicate despite of
the mismatch in the protocol families of the applications and the
underlying network. The reason is that the HIP layer translates the
LSIs originating from the upper layers into routable IPv6 locators
before delivering the packets on the wire.

The second scenario is the same as the first one, but with the
difference that one of the applications supports only IPv6. Now two
obstacles hinder the communication between the application: applications: the
addressing families of the two applications differ, and the
application residing at the IPv4-only side is again unable to
communicate because of the mismatch between addressing families of
the application (IPv4) and network (IPv6). With HIP-based
connectivity for applications, this scenario works; the HIP layer can
choose whether to translate the locator of an incoming packet into an
LSI or HIT.

Effectively, LSIs improve IPv6 interoperability at the network layer
as described in the first scenario and at the application layer as
depicted in the second example. The interoperability mechanism
should not be used to avoid transition to IPv6; the authors firmly
believe in IPv6 adoption and encourage developers to port existing
IPv4-only applications to use IPv6. However, some proprietary,
closed-source, IPv4-only applications may never see the daylight of
IPv6, and the LSI mechanism is suitable for extending the lifetime of
such applications even in IPv6-only networks.

The main disadvantage of an LSI is its local scope. Applications may
violate layering principles and pass LSIs to each other in
application-layer protocols. As the LSIs are valid only in the
context of the local host, they may represent an entirely different
host when passed to another host. However, it should be emphasized
here that the LSI concept is effectively a host-based NAT and does
not introduce any more issues than the prevalent middlebox based middlebox-based NATs
for IPv4. In other words, the applications violating layering
principles are already broken by the NAT boxes that are ubiquitously
deployed.

4.5. Storing Host Identifiers in directories Directories

The public Host Identifiers should be stored in DNS; the unpublished
Host Identifiers should not be stored anywhere (besides the
communicating hosts themselves). The (public) HI along with the
supported HIHs are stored in a new RR Resource Record (RR) type. This
RR type is defined in the HIP DNS Extension extension [RFC8005].

Alternatively, or in addition to storing Host Identifiers in the DNS,
they may be stored in various other directories. For instance, a
directory based on the Lightweight Directory Access Protocol (LDAP)
or a Public Key Infrastructure (PKI) [RFC8002] may be used.
Alternatively, Distributed Hash Tables (DHTs) [RFC6537] have
successfully been utilized [RFC6538]. Such a practice may allow them
to be used for purposes other than pure host identification.

Some types of applications may cache and use Host Identifiers
directly, while others may indirectly discover them through a
symbolic host name (such as FQDN) a Fully Qualified Domain Name (FQDN))
look up from a directory. Even though Host Identities can have a
substantially longer lifetime associated with them than routable IP
addresses, directories may be a better approach to manage the
lifespan of Host Identities. For example, an LDAP-
based LDAP-based directory or
DHT can be used for locally published identities whereas DNS can be
more suitable for public advertisement.

5. New stack architecture Stack Architecture

One way to characterize Host Identity is to compare the proposed HI-
based architecture with the current one. Using the terminology from
the IRTF Name Space Research Group Report [nsrg-report] and, e.g.,
the unpublished Internet-Draft Endpoints document on "Endpoints and Endpoint Names Names" [chiappa-endpoints],
the IP addresses currently embody the dual role of locators and end-point
endpoint identifiers. That is, each IP address names a topological
location in the Internet, thereby acting as a routing direction
vector, or locator. At the same time, the IP address names the
physical network interface currently located at the
point-of-attachment, point-of-
attachment, thereby acting as a end-point an endpoint name.

In the HIP architecture, the end-point endpoint names and locators are
separated from each other. IP addresses continue to act as locators.
The Host Identifiers take the role of end-point endpoint identifiers. It is
important to understand that the end-point endpoint names based on Host
Identities are slightly different from interface names; a Host
Identity can be simultaneously reachable through several interfaces.

The difference between the bindings of the logical entities are
illustrated in Figure 1. The left side illustrates the current TCP/
IP architecture and the right side the HIP-based architecture.

Figure 1

Architecturally, HIP provides for a different binding of transport-
layer protocols. That is, the transport-layer associations, i.e.,
TCP connections and UDP associations, are no longer bound to IP
addresses but rather to Host Identities. In practice, the Host
Identities are exposed as LSIs and HITs for legacy applications and
the transport layer to facilitate backward compatibility with
existing networking APIs and stacks.

The HIP layer is logically located at layer Layer 3.5, between the
transport and network layers, in the networking stack. It acts as
shim layer for transport data utilizing LSIs or HITs, HITs but leaves other
data intact. The HIP layer translates between the two forms of HIP
identifiers originating from the transport layer into routable
IPv4/IPv6 IPv4/
IPv6 addresses for the network layer, layer and vice versa for the reverse
direction.

5.1. On the multiplicity Multiplicity of identities Identities

A host may have multiple identities both at the client and server
side. This raises some additional concerns that are addressed in
this section.

For security reasons, it may be a bad idea to duplicate the same Host
Identity on multiple hosts because the compromise of a single host
taints the identities of the other hosts. Management of machines
with identical Host Identities may also present other challenges and,
therefore, it is advisable to have a unique identity for each host.

At the server side, utilizing DNS is a better alternative than a
shared Host Identity to implement load balancing. A single FQDN
entry can be configured to refer to multiple Host Identities. Each
of the FQDN entries can be associated with the related locators, locators or
with a single shared locator in the case the servers are using the
same HIP rendezvous server Section 6.3 (Section 6.3) or HIP relay server Section 6.4.
(Section 6.4).

Instead of duplicating identities, HIP opportunistic mode can be
employed, where the initiator Initiator leaves out the identifier of the
responder
Responder when initiating the key exchange and learns it upon the
completion of the exchange. The tradeoffs trade-offs are related to lowered
security guarantees, but a benefit of the approach is to avoid the
publishing of Host Identifiers in any directories [komu-leap]. Since
many public servers already employ DNS as their directory,
opportunistic mode may be more suitable for, e.g, e.g., peer-to-peer
connectivity. It is also worth noting that opportunistic mode is
also required in practice when anycast IP addresses would be utilized
as locators.

HIP opportunistic mode could be utilized in association with HIP
rendezvous servers or HIP relay servers [komu-diss]. In such a
scenario, the Initiator sends an I1 message with a wildcard
destination HIT to the locator of a HIP rendezvous/relay server.
When the receiving rendezvous/relay server is serving multiple
registered Responders, the server can choose the ultimate destination
HIT, thus acting as a HIP based HIP-based load balancer. However, this
approach is still experimental and requires further investigation.

At the client side, a host may have multiple Host Identities, for
instance, for privacy purposes. Another reason can be that the
person utilizing the host employs different identities for different
administrative domains as an extra security measure. If a HIP-aware
middlebox, such as a HIP-based firewall, is on the path between the
client and server, the user or the underlying system should carefully
choose the correct identity to avoid the firewall to unnecessarily
drop
dropping HIP-based connectivity [komu-diss].

Similarly, a server may have multiple Host Identities. For instance,
a single web server may serve multiple different administrative
domains. Typically, the distinction is accomplished based on the DNS
name, but also the Host Identity could be used for this purpose.
However, a more compelling reason to employ multiple identities are is
the HIP-aware firewalls firewall that are is unable to see the HTTP traffic inside
the encrypted IPsec tunnel. In such a case, each service could be
configured with a separate identity, thus allowing the firewall to
segregate the different services of the single web server from each
other [lindqvist-enterprise].

6. Control plane Plane

HIP decouples the control and data plane planes from each other. Two end-hosts end-
hosts initialize the control plane using a key exchange procedure
called the base exchange. The procedure can be assisted by HIP HIP-
specific infrastructural intermediaries called rendezvous or relay
servers. In the event of IP address changes, the end-hosts sustain
control plane connectivity with mobility and multihoming extensions.
Eventually, the end-hosts terminate the control plane and remove the
associated state.

6.1. Base exchange Exchange

The base exchange is a key exchange procedure that authenticates the
initiator
Initiator and responder Responder to each other using their public keys.
Typically, the initiator Initiator is the client-side host and the responder Responder is
the server-side host. The roles are used by the state machine of a
HIP implementation, implementation but then discarded upon successful completion.

The exchange consists of four messages during which the hosts also
create symmetric keys to protect the control plane with Hash-based
message authentication codes
Message Authentication Codes (HMACs). The keys can be also used to
protect the data plane, and IPsec ESP [RFC7402] is typically used as
the data-plane data plane protocol, albeit HIP can also accommodate others.
Both the control and data plane planes are terminated using a closing
procedure consisting of two messages.

In addition, the base exchange also includes a computational puzzle
[RFC7401] that the initiator Initiator must solve. The responder Responder chooses the
difficulty of the puzzle puzzle, which permits the responder Responder to delay new
incoming initiators Initiators according to local policies, for instance, when
the responder Responder is under heavy load. The puzzle can offer some
resiliency against DoS attacks because the design of the puzzle
mechanism allows the responder Responder to remain stateless until the very end
of the base exchange [aura-dos]. HIP puzzles have also been studied
under steady-state DDoS attacks [beal-dos], on multiple adversary
models with varying puzzle difficulties [tritilanunt-dos] [tritilanunt-dos], and with
ephemeral Host Identities [komu-mitigation].

6.2. End-host mobility End-Host Mobility and multi-homing Multihoming

HIP decouples the transport from the internetworking layer, layer and binds
the transport associations to the Host Identities (actually through
either the HIT or LSI). After the initial key exchange, the HIP
layer maintains transport-layer connectivity and data flows using its
extensions for mobility [RFC8046] and multihoming [RFC8047] extensions. [RFC8047].
Consequently, HIP can provide for a degree of internetworking
mobility and multi-homing multihoming at a low infrastructure cost. HIP mobility
includes IP address changes (via any method) to either party. Thus,
a system is considered mobile if its IP address can change
dynamically for any reason like PPP, DHCP, IPv6 prefix reassignments,
or a NAT device remapping its translation. Likewise, a system is
considered multi-homed multihomed if it has more than one globally routable IP
address at the same time. HIP links IP addresses together, together when
multiple IP addresses correspond to the same Host Identity. If one
address becomes unusable, or a more preferred address becomes
available, existing transport associations can easily be moved to
another address.

When a mobile node moves while communication is already on-going, ongoing, address
changes are rather straightforward. The mobile node sends a HIP
UPDATE packet to inform the peer of the new address(es), and the peer
then verifies that the mobile node is reachable through these
addresses. This way, the peer can avoid flooding attacks as further
discussed in Section 11.2.

6.3. Rendezvous mechanism Mechanism

Establishing a contact to a mobile, moving node is slightly more
involved. In order to start the HIP exchange, the initiator Initiator node has
to know how to reach the mobile node. For instance, the mobile node
can employ Dynamic DNS [RFC2136] to update its reachability
information in the DNS. To avoid the dependency to DNS, HIP provides
its own HIP-specific alternative: the HIP rendezvous mechanism as
defined in the HIP Rendezvous specifications rendezvous specification [RFC8004].

Using the HIP rendezvous extensions, the mobile node keeps the
rendezvous infrastructure continuously updated with its current IP
address(es). The mobile nodes trusts the rendezvous mechanism in
order to properly maintain their HIT and IP address mappings.

The rendezvous mechanism is especially useful in scenarios where both
of the nodes are expected to change their address at the same time.
In such a case, the HIP UPDATE packets will cross each other in the
network and never reach the peer node.

6.4. Relay mechanism Mechanism

The HIP relay mechanism [I-D.ietf-hip-native-nat-traversal] [RFC9028] is an alternative to the HIP
rendezvous mechanism. The HIP relay mechanism is more suitable for
IPv4 networks with NATs because a HIP relay can forward all control
and data plane communications in order to guarantee successful NAT
traversal.

6.5. Termination of the control plane Control Plane

The control plane between two hosts is terminated using a secure two-
message exchange as specified in base exchange specification
[RFC7401]. The related state (i.e. (i.e., host associations) should be
removed upon successful termination.

7. Data plane Plane

The encapsulation format for the data plane used for carrying the
application-layer traffic can be dynamically negotiated during the
key exchange. For instance, HICCUPS extensions [RFC6078] define one
way to transport application-layer datagrams directly over the HIP
control plane, protected by asymmetric key cryptography. Also, SRTP
Secure Real-time Transport Protocol (SRTP) has been considered as the
data encapsulation protocol [hip-srtp]. However, the most widely
implemented method is the Encapsulated Security Payload (ESP)
[RFC7402] that is protected by symmetric keys derived during the key
exchange. ESP Security Associations (SAs) offer both confidentiality
and integrity protection, of which the former can be disabled during
the key exchange. In the future, other ways of transporting
application-layer data may be defined.

The ESP SAs are established and terminated between the initiator Initiator and
the responder Responder hosts. Usually, the hosts create at least two SAs, one
in each direction (initiator-to-responder (Initiator-to-Responder SA and responder-to-
initiator Responder-to-
Initiator SA). If the IP addresses of either host changes, the HIP
mobility extensions can be used to re-negotiate renegotiate the corresponding SAs.

On the wire, the difference in the use of identifiers between the HIP
control and data plane planes is that the HITs are included in all control
packets, but not in the data plane when ESP is employed. Instead,
the ESP employs SPI Security Parameter Index (SPI) numbers that act as
compressed HITs. Any HIP-
aware HIP-aware middlebox (for instance, a HIP-aware
firewall) interested in the ESP based ESP-based data plane should keep track
between the control and data plane identifiers in order to associate
them with each other.

Since HIP does not negotiate any SA lifetimes, all lifetimes are
subject to local policy. The only lifetimes a HIP implementation
must support are sequence number rollover (for replay protection), protection) and
SA timeout. An SA times out if no packets are received using that
SA. Implementations may support lifetimes for the various ESP
transforms and other data-plane data plane protocols.

8. HIP and NATs

Passing packets between different IP addressing realms requires
changing IP addresses in the packet header. This may occur, for
example, when a packet is passed between the public Internet and a
private address space, or between IPv4 and IPv6 networks. The
address translation is usually implemented as Network Address
Translation (NAT) [RFC3022] or the historic NAT Protocol translation Translation
(NAT-PT) [RFC2766].

In a network environment where identification is based on the IP
addresses, identifying the communicating nodes is difficult when NATs
are employed because private address spaces are overlapping. In
other words, two hosts cannot be distinguished from each other solely
based on their IP address. addresses. With HIP, the transport-layer end-points
(i.e. endpoints
(i.e., applications) are bound to unique Host Identities rather than
overlapping private addresses. This allows two end-points endpoints to
distinguish one other even when they are located in different private
address realms. Thus, the IP addresses are used only for routing
purposes and can be changed freely by NATs when a packet between two
HIP capable
HIP-capable hosts traverses through multiple private address realms.

NAT traversal extensions for HIP [I-D.ietf-hip-native-nat-traversal] [RFC9028] can be used to realize the
actual end-to-end connectivity through NAT devices. To support basic
backward compatibility with legacy NATs, the extensions encapsulate
both HIP control and data plane planes in UDP. The extensions define
mechanisms for forwarding the two planes through an intermediary host
called HIP relay and procedures to establish direct end-to-end
connectivity by penetrating NATs. Besides this "native" NAT
traversal mode for HIP, other NAT traversal mechanisms have been
successfully utilized, such as Teredo [RFC4380] (as described in
further detail in [varjonen-split]).

Besides legacy NATs, a HIP-aware NAT has been designed and
implemented [ylitalo-spinat]. For a HIP-based flow, a HIP-aware NAT
or HIP-aware historic NAT-PT system tracks the mapping of HITs, and
the corresponding ESP SPIs, to an IP address. The NAT system has to
learn mappings both from HITs and from SPIs to IP addresses. Many
HITs (and SPIs) can map to a single IP address on a NAT, simplifying
connections on address-poor NAT interfaces. The NAT can gain much of
its knowledge from the HIP packets themselves; however, some NAT
configuration may be necessary.

8.1. HIP and Upper-layer checksums Upper-Layer Checksums

There is no way for a host to know if any of the IP addresses in an
IP header are the addresses used to calculate the TCP checksum. That
is, it is not feasible to calculate the TCP checksum using the actual
IP addresses in the pseudo header; the addresses received in the
incoming packet are not necessarily the same as they were on the
sending host. Furthermore, it is not possible to recompute the
upper-layer checksums in the NAT/NAT-PT system, since the traffic is
ESP protected. Consequently, the TCP and UDP checksums are
calculated using the HITs in the place of the IP addresses in the
pseudo header. Furthermore, only the IPv6 pseudo header format is
used. This provides for IPv4 / IPv6 protocol translation.

9. Multicast

A number of studies investigating HIP-based multicast have been
published (including [shields-hip], [xueyong-hip], [xueyong-hip], [zhu-hip], [amir-hip], [kovacshazi-host]
[kovacshazi-host], and [xueyong-secure]). [zhu-secure]). In particular, so-called Bloom
filters, that which allow compressing the compression of multiple labels into small
data structures, may be a promising way forward [sarela-bloom].
However, the different schemes have not been adopted by the HIP
working group (nor the HIP research group in the IRTF), so the
details are not further elaborated here.

10. HIP policies Policies

There are a number of variables that influence the HIP exchange that
each host must support. All HIP implementations should support at
least 2 two HIs, one to publish in DNS or a similar directory service
and an unpublished one for anonymous usage (that should expect to be
rotated frequently in order to disrupt linkability/trackability). linkability and/or
trackability). Although unpublished HIs will be rarely be used as responder
Responder HIs, they are likely to be common for initiators. Initiators. As
stated in [RFC7401], "all HIP implementations MUST support more than
one simultaneous HI, at least one of which SHOULD be reserved for
anonymous usage", and "support for more than two HIs is RECOMMENDED".
This provides new challenges for systems or users to decide which
type of HI to expose when they start a new session.

Opportunistic mode (where the initiator Initiator starts a HIP exchange without
prior knowledge of the responder's Responder's HI) presents a security tradeoff. trade-off.
At the expense of being subject to MITM MitM attacks, the opportunistic
mode allows the initiator Initiator to learn the identity of the responder Responder
during communication rather than from an external directory.
Opportunistic mode can be used for registration to HIP-based services
[RFC8003] (i.e. (i.e., utilized by HIP for its own internal purposes) or by
the application layer [komu-leap]. For security reasons, especially
the latter requires some involvement from the user to accept the
identity of the responder Responder similar to how SSH the Secure Shell (SSH)
protocol prompts the user when connecting to a server for the first
time [pham-leap]. In practice, this can be realized in end-host end-host-
based firewalls in the case of legacy applications [karvonen-usable]
or with native APIs for HIP APIs [RFC6317] in the case of HIP-aware
applications.

As stated in [RFC7401], "Initiators [RFC7401]:

| Initiators MAY use a different HI for different Responders to
| provide basic privacy. Whether such private HIs are used
| repeatedly with the same Responder, and how long these HIs are
| used, are decided by local policy and depend on the privacy
| requirements of the Initiator". Initiator.

According to [RFC7401], "Responders [RFC7401]:

| Responders that only respond to selected Initiators require an
| Access Control List (ACL), representing for which hosts they
| accept HIP base exchanges, and the preferred transport format and
| local lifetimes. Wildcarding SHOULD be supported for such ACLs,
| and also for Responders that offer public or anonymous services". services.

11. Security considerations Considerations

This section includes discussion on some issues and solutions related
to security in the HIP architecture.

11.1. MiTM MitM Attacks

HIP takes advantage of the Host Identity paradigm to provide secure
authentication of hosts and to provide a fast key exchange for ESP.
HIP also attempts to limit the exposure of the host to various
denial-of-service (DoS) and man-in-the-middle (MitM) attacks. In so
doing, HIP itself is subject to its own DoS and MitM attacks that
potentially could be more damaging to a host's ability to conduct
business as usual.

Resource exhausting denial-of-service DoS attacks take advantage of the cost of setting
up a state for a protocol on the responder Responder compared to the
'cheapness' on the initiator. Initiator. HIP allows a responder Responder to increase the
cost of the start of state on the initiator Initiator and makes an effort to
reduce the cost to the responder. Responder. This is done by having the responder
Responder start the authenticated Diffie-Hellman exchange instead of
the initiator, Initiator, making the HIP base exchange 4 four packets long. The
first packet sent by the responder Responder can be prebuilt to further
mitigate the costs. This packet also includes a computational puzzle
that can optionally be used to further delay the initiator, Initiator, for
instance, when the responder Responder is overloaded. The details are
explained in the base exchange specification [RFC7401].

Man-in-the-middle (MitM)

MitM attacks are difficult to defend against, against without third-party
authentication. A skillful MitM could easily handle all parts of the
HIP base exchange, but HIP indirectly provides the following
protection from a MitM attack. If the
responder's Responder's HI is retrieved
from a signed DNS zone or securely obtained by some other means, the initiator
Initiator can use this to authenticate the signed HIP packets.
Likewise, if the initiator's Initiator's HI is in a secure DNS zone, the responder
Responder can retrieve it and validate the signed HIP packets.
However, since an initiator Initiator may choose to use an unpublished HI, it
knowingly risks a MitM attack. The
responder Responder may choose not to
accept a HIP exchange with an initiator Initiator using an unknown HI.

Other types of MitM attacks against HIP can be mounted using ICMP
messages that can be used to signal about problems. As an overall
guideline, the ICMP messages should be considered as unreliable
"hints" and should be acted upon only after timeouts. The exact
attack scenarios and countermeasures are described in full detail in
the base exchange specification [RFC7401].

A MitM attacker could try to replay older I1 or R1 messages using
weaker cryptographic algorithms as described in section Section 4.1.4 in of
[RFC7401]. The base exchange has been augmented to deal with such an
attack by restarting on detecting the detection of the attack. At worst worst, this
would only lead to a situation in which the base exchange would never
finish (or would be aborted after some retries). As a drawback, this
leads to a 6-way six-way base exchange exchange, which may seem bad at first.
However, since this only occurs in an attack scenario and since the
attack can be handled (so it is not interesting to mount anymore), we
assume the subsequent messages do not represent a security threat.
Since the MitM cannot be successful with a downgrade attack, these
sorts of attacks will only occur as 'nuisance' attacks. So, the base
exchange would still be usually just four packets even though
implementations must be prepared to protect themselves against the
downgrade attack.

In HIP, the Security Association for ESP is indexed by the SPI; the
source address is always ignored, and the destination address may be
ignored as well. Therefore, HIP-enabled Encapsulated Security
Payload (ESP) ESP is IP address
independent. This might seem to make attacking easier, but ESP with
replay protection is already as well protected as possible, and the
removal of the IP address as a check should not increase the exposure
of ESP to DoS attacks.

11.2. Protection against flooding attacks Flooding Attacks

Although the idea of informing about address changes by simply
sending packets with a new source address appears appealing, it is
not secure enough. That is, even if HIP does not rely on the source
address for anything (once the base exchange has been completed), it
appears to be necessary to check a mobile node's reachability at the
new address before actually sending any larger amounts of traffic to
the new address.

Blindly accepting new addresses would potentially lead to flooding
Denial-of-Service
DoS attacks against third parties [RFC4225]. In a distributed
flooding attack attack, an attacker opens high volume high-volume HIP connections with a
large number of hosts (using unpublished HIs), HIs) and then claims to all
of these hosts that it has moved to a target node's IP address. If
the peer hosts were to simply accept the move, the result would be a
packet flood to the target node's address. To prevent this type of
attack, HIP mobility extensions include a return routability check
procedure where the reachability of a node is separately checked at
each address before using the address for larger amounts of traffic.

A credit-based authorization approach for host mobility "Host Mobility with the
Host Identity Protocol Protocol" [RFC8046] can be used between hosts for
sending data prior to completing the address tests. Otherwise, if
HIP is used between two hosts that fully trust each other, the hosts
may optionally decide to skip the address tests. However, such
performance optimization must be restricted to peers that are known
to be trustworthy and capable of protecting themselves from malicious
software.

11.3. HITs used Used in ACLs

At end-hosts, HITs can be used in IP-based access control lists at
the application and network layers. At middleboxes, HIP-aware
firewalls [lindqvist-enterprise] can use HITs or public keys to
control both ingress and egress access to networks or individual
hosts, even in the presence of mobile devices because the HITs and
public keys are topology independent. As discussed earlier in
Section 7, once a HIP session has been established, the SPI value in
an ESP packet may be used as an index, indicating the HITs. In
practice, firewalls can inspect HIP packets to learn of the bindings
between HITs, SPI values, and IP addresses. They can even explicitly
control ESP usage, dynamically opening ESP only for specific SPI
values and IP addresses. The signatures in HIP packets allow a
capable firewall to ensure that the HIP exchange is indeed occurring
between two known hosts. This may increase firewall security.

A potential drawback of HITs in ACLs is their 'flatness' 'flatness', which means
they cannot be aggregated, and this could potentially result in
larger table searches in HIP-aware firewalls. A way to optimize this
could be to utilize Bloom filters for grouping of HITs [sarela-bloom].
However, it should be noted that it is also easier to exclude
individual, misbehaving hosts out when the firewall rules concern
individual HITs rather than groups.

There has been considerable bad experience with distributed ACLs that
contain public key material related material, to public keys, for example, with SSH. If
the owner of a key needs to revoke it for any reason, the task of
finding all locations where the key is held in an ACL may be
impossible. If the reason for the revocation is due to private key
theft, this could be a serious issue.

A host can keep track of all of its partners that might use its HIT
in an ACL by logging all remote HITs. It should only be necessary to
log responder Responder hosts. With this information, the host can notify the
various hosts about the change to the HIT. There have been attempts
to develop a secure method to issue the HIT revocation notice
[zhang-revocation].

Some of the HIP-aware middleboxes, such as firewalls
[lindqvist-enterprise] or NATs [ylitalo-spinat], may observe the on-
path traffic passively. Such middleboxes are transparent by their
nature and may not get a notification when a host moves to a
different network. Thus, such middleboxes should maintain soft state
and timeout time out when the control and data plane planes between two HIP end-hosts
has end-
hosts have been idle too long. Correspondingly, the two end-hosts
may send periodically keepalives, such as UPDATE packets or ICMP
messages inside the ESP tunnel, to sustain state at the on-path
middleboxes.

One general limitation related to end-to-end encryption is that
middleboxes may not be able to participate to in the protection of data
flows. While the issue may affect also affect other protocols, Heer at al et al.
[heer-end-host] have analyzed the problem in the context of HIP.
More specifically, when ESP is used as the data-plane data plane protocol for
HIP, the association between the control and data plane planes is weak and
can be exploited under certain assumptions. In the scenario, the
attacker has already gained access to the target network protected by
a HIP-aware firewall, but wants to circumvent the HIP-based firewall.
To achieve this, the attacker passively observes a base exchange
between two HIP hosts and later replays it. This way, the attacker
manages to penetrate the firewall and can use a fake ESP tunnel to
transport its own data. This is possible because the firewall cannot
distinguish when the ESP tunnel is valid. As a solution, HIP-aware
middleboxes may participate to in the control plane interaction by
adding random nonce parameters to the control traffic, which the end-
hosts have to sign to guarantee the freshness of the control traffic
[heer-midauth]. As an alternative, extensions for transporting the
data plane directly over the control plane can be used [RFC6078].

11.4. Alternative HI considerations Considerations

The definition of the Host Identifier states that the HI need not be
a public key. It implies that the HI could be any value; value, for example
example, a FQDN. This document does not describe how to support such
a non-
cryptographic non-cryptographic HI, but examples of such protocol variants do
exist ([urien-rfid], [urien-rfid-draft]). A non-cryptographic HI
would still offer the services of the HIT or LSI for NAT traversal.
It would be possible to carry HITs in HIP packets that had neither
privacy nor authentication. Such schemes may be employed for
resource constrained
resource-constrained devices, such as small sensors operating on
battery power, but are not further analyzed here.

If it is desirable to use HIP in a low security low-security situation where
public key computations are considered expensive, HIP can be used
with very short Diffie-Hellman and Host Identity keys. Such use
makes the participating hosts vulnerable to MitM and connection
hijacking attacks. However, it does not cause flooding dangers,
since the address check mechanism relies on the routing system and
not on cryptographic strength.

11.5. Trust On on First Use

[RFC7435] highlights four design principles for Leap of Faith, or
Trust On First Use (TOFU), protocols that apply also to opportunistic
HIP:

1. Coexist with explicit policy

2. Prioritize communication

3. Maximize security peer by peer

4. No misrepresentation of security

According to the first TOFU design principle, "opportunistic "Opportunistic security
never displaces or preempts explicit policy". Some application data
may be too sensitive, so the related policy could require
authentication (i.e, (i.e., the public key or certificate) in such a case
instead of the unauthenticated opportunistic mode. In practice, this
has been realized in HIP implementations as follows [RFC6538].

The OpenHIP implementation allowed an Initiator to use opportunistic
mode only with an explicitly configured Responder IP address, when
the Responder's HIT is unknown. At the Responder, OpenHIP had an
option to allow opportunistic mode with any Initiator -- trust any
Initiator.

HIP for Linux (HIPL) developers experimented with more fine-grained
policies operating at the application level. The HIPL implementation
utilized so called so-called "LD_PRELOAD" hooking at the application layer that
allowed a dynamically linked library to intercept socket-related
calls without rebuilding the related application binaries. The
library acted as a shim layer between the application and transport
layers. The shim layer translated the non-HIP based non-HIP-based socket calls
from the application into HIP-based socket calls. While the shim
library involved some level of complexity as described in more detail
in [komu-leap], it achieved the goal of applying opportunistic mode
at the granularity of individual applications.

The second TOFU principle essentially states that communication
should be first class citizen instead of prioritized over security. So opportunistic mode should be,
in general, allowed even if no authentication is present, and even
possibly a fallback to non-encrypted unencrypted communications could be allowed
(if policy permits) instead of blocking communications. In practice,
this can be realized in three steps. In the first step, a HIP
Initiator can look up the HI of a Responder from a directory such as
DNS. When the Initiator discovers a HI, it can use the HI for
authentication and skip the rest of the following steps. In the
second step, the Initiator can, upon failing to find a HI, try
opportunistic mode with the Responder. In the third step, the
Initiator can fall back to non-HIP based non-HIP-based communications upon failing
with opportunistic mode if the policy allows it. This three
step three-step
model has been implemented successfully and described in more detail
in [komu-leap].

The third TOFU principle suggests that security should be maximized,
so that at least opportunistic security would be employed. The three
step
three-step model described earlier prefers authentication when it is
available, e.g., via DNS records (and possibly even via DNSSEC when
available) and falls back to opportunistic mode when no out-of-band
credentials are available. As the last resort, fallback to non-HIP non-HIP-
based communications can be used if the policy allows it. Also,
since perfect forward security secrecy (PFS) is explicitly mentioned in the
third design principle, it is worth mentioning that HIP supports it.

The fourth TOFU principle states that users and non-interactive noninteractive
applications should be properly informed about the level of security
being applied. In practice, non-HIP aware non-HIP-aware applications would assume
that no extra security is being applied, so misleading at least a non-
interactive
noninteractive application should not be possible. In the case of
interactive desktop applications, system-level prompts have been
utilized in earlier HIP experiments [karvonen-usable], [karvonen-usable] [RFC6538] to
guide the user about the underlying HIP-based security. In general,
users in those experiments perceived when HIP-based security was
being used versus not used. However, the users failed to notice the
difference between opportunistic opportunistic, non-authenticated HIP and non-opportunistic non-
opportunistic, authenticated HIP. The reason for this was that the
opportunistic HIP (i.e. (i.e., lowered level of security) was not clearly
indicated in the prompt. This provided a valuable lesson to further
improve the user interface.

In the case of HIP-aware applications, native sockets APIs for HIP as
specified in [RFC6317] can be used to develop application-specific
logic instead of using generic system-level prompting. In such a
case, the application itself can directly prompt the user or
otherwise manage the situation in other ways. In this case, also non-
interactive
noninteractive applications also can properly log the level of
security being employed because the developer can now explicitly
program the use of authenticated HIP, opportunistic HIP HIP, and plain-text plain-
text communication.

It is worth mentioning a few additional items discussed in [RFC7435].
Related to active attacks, HIP has built-in protection against
cipher-suite down-grade
ciphersuite downgrade attacks as described in detail in [RFC7401].
In addition, pre-deployed certificates could be used to mitigate
against active attacks in the case of opportunistic mode as mentioned
in [RFC6538].

Detection of peer capabilities is also mentioned in the TOFU context.
As discussed in this section, the three-step model can be used to
detect peer capabilities. A host can achieve the first step of
authentication, i.e., discovery of a public key, via DNS, for
instance. If the host found finds no keys, the host can then try
opportunistic mode as the second step. Upon a timeout, the host can
then proceed to the third step by falling back to non-HIP based non-HIP-based
communications if the policy permits. This last step is based on an
implicit timeout rather an explicit (negative) acknowledgment like in
the case of DNS, so the user may conclude prematurely that the
connectivity has failed. To speed up the detection phase by
explicitly detecting if the peer supports opportunistic HIP,
researchers have proposed TCP specific TCP-specific extensions [RFC6538], [RFC6538]
[komu-leap]. In a nutshell, an Initiator sends simultaneously both
an opportunistic I1 packet and the related TCP SYN datagram equipped
with a special TCP option to a peer. If the peer supports HIP, it
drops the SYN packet and responds with an R1. If the peer is HIP
incapable, it drops the HIP packet (and the unknown TCP option) and
responds with a TCP SYN-ACK. The benefit of the proposed scheme is a
faster, one round-trip fallback to non-HIP based non-HIP-based communications. The
drawback is that the approach is tied to TCP (IP-options (IP options were also
considered, but do not work well with firewalls and NATs).
Naturally, the approach does not work against an active attacker, but
opportunistic mode is not anyway supposed to protect against such an
adversary.
adversary anyway.

It is worth noting that while the use of opportunistic mode has some
benefits related to incremental deployment, it does not achieve all
the benefits of authenticated HIP [komu-diss]. Namely, authenticated
HIP supports persistent identifiers in the sense that hosts are
identified with the same HI independently independent of their movement.
Opportunistic HIP meets this goal only partially: after the first
contact between two hosts, HIP can successfully sustain connectivity
with its mobility management extensions, but problems emerge when the
hosts close the HIP association and try to re-establish reestablish connectivity.
As hosts can change their location, it is no longer guaranteed that
the same IP address belongs to the same host. The same address can
be temporally assigned to different hosts, e.g., due to the reuse of
IP addresses (e.g., by a DHCP service), the overlapping of private
address realms (see also the discussion on Internet transparency in
Appendix A.1) A.1), or due to an attempted attack.

12. IANA considerations Considerations

This document has no actions for IANA. IANA actions.

13. Acknowledgments

For the people historically involved in the early stages of HIP, see
the Acknowledgments section in the Host Identity Protocol
specification.

During the later stages of this document, when the editing baton was
transferred to Pekka Nikander, the comments from the early
implementers and others, including Jari Arkko, Jeff AhrenHolz, Tom
Henderson, Petri Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan
Melen, Tim Shepard, Jukka Ylitalo, Sasu Tarkoma, and Jorma Wall, were
invaluable. Also, the comments from Lars Eggert, Spencer Dawkins,
Dave Crocker and Erik Giesa were also useful.

The authors want to express their special thanks to Tom Henderson,
who took the burden of editing the document in response to IESG
comments at the time when both of the authors were busy doing other
things. Without his perseverance original document might have never
made it as RFC4423.

This main effort to update and move HIP forward within the IETF
process owes its impetuous to a number of HIP development teams. The
authors are grateful for Boeing, Helsinki Institute for Information
Technology (HIIT), NomadicLab of Ericsson, and the three
universities: RWTH Aachen, Aalto and University of Helsinki, for
their efforts. Without their collective efforts HIP would have
withered as on the IETF vine as a nice concept.

Thanks also for Suvi Koskinen for her help with proofreading and with
the reference jungle.

14. Changes from RFC 4423

In a nutshell, the changes from RFC 4423 [RFC4423] are mostly
editorial, including clarifications on topics described in a
difficult way and omitting some of the non-architectural
(implementation) details that are already described in other
documents. A number of missing references to the literature were
also added. New topics include the drawbacks of HIP, a discussion on
802.15.4 and MAC security, HIP for IoT scenarios, deployment
considerations
considerations, and a description of the base exchange.

15.

14. References

15.1.

14.1. Normative References

[I-D.ietf-hip-dex]
Moskowitz, R. and R. Hummen, "HIP Diet EXchange (DEX)",
draft-ietf-hip-dex-06 (work in progress), December 2017.

[I-D.ietf-hip-native-nat-traversal]
Keranen, A., Melen, J., and M. Komu, "Native NAT Traversal
Mode for the Host Identity Protocol", draft-ietf-hip-
native-nat-traversal-28 (work in progress), March 2018.

[RFC5482] Eggert, L. and F. Gont, "TCP User Timeout Option",
RFC 5482, DOI 10.17487/RFC5482, March 2009,
<https://www.rfc-editor.org/info/rfc5482>.

[RFC6079] Camarillo, G., Nikander, P., Hautakorpi, J., Keranen, A.,
and A. Johnston, "HIP BONE: Host Identity Protocol (HIP)
Based Overlay Networking Environment (BONE)", RFC 6079,
DOI 10.17487/RFC6079, January 2011, <https://www.rfc-
editor.org/info/rfc6079>.
<https://www.rfc-editor.org/info/rfc6079>.

[RFC7086] Keranen, A., Camarillo, G., and J. Maenpaa, "Host Identity
Protocol-Based Overlay Networking Environment (HIP BONE)
Instance Specification for REsource LOcation And Discovery
(RELOAD)", RFC 7086, DOI 10.17487/RFC7086, January 2014,
<https://www.rfc-editor.org/info/rfc7086>.

[RFC7343] Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
Routable Cryptographic Hash Identifiers Version 2
(ORCHIDv2)", RFC 7343, DOI 10.17487/RFC7343, September
2014, <https://www.rfc-editor.org/info/rfc7343>.

[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, DOI 10.17487/RFC7401, April 2015,
<https://www.rfc-editor.org/info/rfc7401>.

[RFC7402] Jokela, P., Moskowitz, R., and J. Melen, "Using the
Encapsulating Security Payload (ESP) Transport Format with
the Host Identity Protocol (HIP)", RFC 7402,
DOI 10.17487/RFC7402, April 2015, <https://www.rfc-
editor.org/info/rfc7402>.
<https://www.rfc-editor.org/info/rfc7402>.

[RFC8002] Heer, T. and S. Varjonen, "Host Identity Protocol
Certificates", RFC 8002, DOI 10.17487/RFC8002, October
2016, <https://www.rfc-editor.org/info/rfc8002>.

[RFC8003] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Registration Extension", RFC 8003, DOI 10.17487/RFC8003,
October 2016, <https://www.rfc-editor.org/info/rfc8003>.

[RFC8004] Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
October 2016, <https://www.rfc-editor.org/info/rfc8004>.

[RFC8005] Laganier, J., "Host Identity Protocol (HIP) Domain Name
System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
October 2016, <https://www.rfc-editor.org/info/rfc8005>.

[RFC8046] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host Mobility
with the Host Identity Protocol", RFC 8046,
DOI 10.17487/RFC8046, February 2017, <https://www.rfc-
editor.org/info/rfc8046>.
<https://www.rfc-editor.org/info/rfc8046>.

[RFC8047] Henderson, T., Ed., Vogt, C., and J. Arkko, "Host
Multihoming with the Host Identity Protocol", RFC 8047,
DOI 10.17487/RFC8047, February 2017, <https://www.rfc-
editor.org/info/rfc8047>.

15.2.
<https://www.rfc-editor.org/info/rfc8047>.

[RFC9028] Keränen, A., Melén, J., and M. Komu, Ed., "Native NAT
Traversal Mode for the Host Identity Protocol", RFC 9028,
DOI 10.17487/RFC9028, July 2021,
<https://www.rfc-editor.org/info/rfc9028>.

14.2. Informative references References

[amir-hip] Amir, K., Forsgren, H., Grahn, K., Karvi, T., and G.
Pulkkis, "Security and Trust of Public Key Cryptography
for HIP and HIP Multicast", International Journal of
Dependable and Trustworthy Information Systems (IJDTIS),
2(3),
Vol. 2, Issue 3, pp. 17-35, DOI: DOI 10.4018/jdtis.2011070102, 2013.
2013, <https://doi.org/10.4018/jdtis.2011070102>.

[aura-dos] Aura, T., Nikander, P., and J. Leiwo, "DOS-resistant "DOS-Resistant
Authentication with Client Puzzles", 8th International
Workshop on Security Protocols, pages 170-177. Security Protocols 2000,
Lecture Notes in Computer Science, Vol. 2133, pp. 170-177,
Springer, ,
April 2001. DOI 10.1007/3-540-44810-1_22, September 2001,
<https://doi.org/10.1007/3-540-44810-1_22>.

[beal-dos] Beal, J. and T. Shephard, Shepard, "Deamplification of DoS Attacks
via Puzzles", , October 2004.

[camarillo-p2psip]
Camarillo, G., Maeenpaeae, Mäenpää, J., Keraenen, Keränen, A., and V. Anderson,
"Reducing delays related to NAT traversal in P2PSIP
session establishments", IEEE Consumer Communications and
Networking Conference (CCNC), pp.
549-553 DOI: 549-553,
DOI 10.1109/CCNC.2011.5766540, 2011. 2011,
<https://doi.org/10.1109/CCNC.2011.5766540>.

[chiappa-endpoints]
Chiappa, J., "Endpoints and Endpoint Names: A Proposed
Enhancement to the Internet Architecture",
URL http://www.chiappa.net/~jnc/tech/endpoints.txt, 1999. 1999,
<http://mercury.lcs.mit.edu/~jnc/tech/endpoints.txt>.

[heer-end-host]
Heer, T., Hummen, R., Komu, M., Goetz, Gotz, S., and K. Wehre,
"End-host Wehrle,
"End-Host Authentication and Authorization for Middleboxes
based
Based on a Cryptographic Namespace", ICC2009 Communication
and Information Systems Security Symposium, , 2009. 2009 IEEE
International Conference on Communications,
DOI 10.1109/ICC.2009.5198984, 2009,
<https://doi.org/10.1109/ICC.2009.5198984>.

[heer-midauth]
Heer, T. T., Ed., Hummen, R., Wehrle, K., and M. Komu, "End-Host "End-
Host Authentication for HIP Middleboxes", Working draft draft-heer-hip-middle-auth-02,
September 2009. Work in
Progress, Internet-Draft, draft-heer-hip-middle-auth-04,
31 October 2011, <https://datatracker.ietf.org/doc/html/
draft-heer-hip-middle-auth-04>.

[henderson-vpls]
Henderson, T. R., Venema, S. C., and D. Mattes, "HIP-based
Virtual Private LAN Service (HIPLS)", Working draft draft-henderson-hip-
vpls-07, Dec 2013. Work in Progress,
Internet-Draft, draft-henderson-hip-vpls-11, 3 August
2016, <https://datatracker.ietf.org/doc/html/draft-
henderson-hip-vpls-11>.

[hip-dex] Moskowitz, R., Ed., Hummen, R., and M. Komu, "HIP Diet
EXchange (DEX)", Work in Progress, Internet-Draft, draft-
ietf-hip-dex-24, 19 January 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-hip-dex-
24>.

[hip-lte] Liyanage, M., Kumar, P., Ylianttila, M., and A. Gurtov,
"Novel secure VPN architectures for LTE backhaul
networks", Security and Communication Networks Networks, Vol. 9,
pp. 1198-1215, DOI 10.1002/sec.1411, November 2015. January 2016,
<https://doi.org/10.1002/sec.1411>.

[hip-srtp] Tschofenig, H., Muenz, F., and M. Shanmugam, M., and F. Muenz, "Using SRTP
transport format with HIP", Working draft draft-
tschofenig-hiprg-hip-srtp-01, Work in Progress, Internet-
Draft, draft-tschofenig-hiprg-hip-srtp-02, 25 October 2005.
2006, <https://datatracker.ietf.org/doc/html/draft-
tschofenig-hiprg-hip-srtp-02>.

[hummen] Hummen, R., Hiller, J., Henze, M., and K. Wehrle, "Slimfit
- A HIP DEX Compression Layer compression layer for the IP-based Internet of
Things", 2013 IEEE 9th International Conference on
Wireless and Mobile Computing, Networking and
Communications (WiMob), 2013 IEEE 9th International
Conference on , page 259-266. DOI: pp. 259-266,
DOI 10.1109/WiMOB.2013.6673370, October 2013.

[IEEE.802-15-4.2011]
"Information technology - Telecommunications and
information exchange between systems - Local and
metropolitan area networks - Specific requirements - Part
15.4: Wireless Medium Access Control (MAC) and Physical
Layer (PHY) Specifications 2013,
<https://doi.org/10.1109/WiMOB.2013.6673370>.

[IEEE.802.15.4]
IEEE, "IEEE Standard for Low-Rate Wireless Personal
Area Networks (WPANs)", Networks",
IEEE Standard 802.15.4, September
2011, <http://standards.ieee.org/getieee802/
download/802.15.4-2011.pdf>.

[IEEE.802-15-9] DOI 10.1109/IEEESTD.2020.9144691,
July 2020, <https://ieeexplore.ieee.org/document/9144691>.

[IEEE.802.15.9]
IEEE, "IEEE Draft Recommended Practice for Transort Transport of
Key Management Protocol (KMP) Datagrams",
IEEE P802.15.9/D04, May 2015.

[karvonen-usable]
Karvonen, K., Komu, M., and A. Gurtov, "Usable Security
Management security
management with Host Identity Protocol", 7th ACS/IEEE host identity protocol", 2009 IEEE/ACS
International Conference on Computer Systems and
Applications, (AICCSA-2009), 2009. pp. 279-286,
DOI 10.1109/AICCSA.2009.5069337, 2009,
<https://doi.org/10.1109/AICCSA.2009.5069337>.

[komu-cloud]
Komu, M., Sethi, M., Mallavarapu, R., Oirola, H., Khan,
R., and S. Tarkoma, "Secure Networking for Virtual
Machines in the Cloud", 2012 IEEE International Workshop Conference
on Power
and QoS Aware Cluster Computing (PQoSCom2012), IEEE, ISBN:
978-1-4244-8567-3, September 2012. Workshops, pp. 88-96,
DOI 10.1109/ClusterW.2012.29, 2012,
<https://doi.org/10.1109/ClusterW.2012.29>.

[komu-diss]
Komu, M., "A Consolidated Namespace for Network
Applications, Developers, Administrators and Users",
Dissertation, Aalto University, Espoo, Finland ISBN: Finland,
ISBN 978-952-60-4904-5 (printed), ISBN: ISBN 978-952-60-4905-2
(electronic). ,
(electronic), December 2012.

[komu-leap]
Komu, M. and J. Lindqvist, "Leap-of-Faith Security is
Enough for IP Mobility", 2009 6th Annual IEEE Consumer
Communications and Networking Conference IEEE CCNC 2009, Conference, Las Vegas, Nevada, , NV,
USA, pp. 1-5, DOI 10.1109/CCNC.2009.4784729, January 2009. 2009,
<https://doi.org/10.1109/CCNC.2009.4784729>.

[komu-mitigation]
Komu, M., Tarkoma, S., and A. Lukyanenko, "Mitigation of
Unsolicited Traffic Across Domains with Host Identities
and Puzzles", 15th Nordic Conference on Secure IT Systems
(NordSec 2010), Springer Systems,
NordSec 2010, Lecture Notes in Computer Science, Volume Vol.
7127, pp. 33-48, ISBN: Springer, ISBN 978-3-642-27936-2,
DOI 10.1007/978-3-642-27937-9_3, October 2010. 2010,
<https://doi.org/10.1007/978-3-642-27937-9_3>.

[kovacshazi-host]
Kovacshazi, Z. and R. Vida, "Host Identity Specific
Multicast", International conference Conference on Networking and
Services (ICNS'06), IEEE Computer Society, Los Alamitos,
CA, USA, http://doi.ieeecomputersociety.org/10.1109/
ICNS.2007.66, 2007.

[leva-barriers]
Levae, A.,
Services (ICNS '07), Athens, Greece, pp. 1-1,
DOI 10.1109/ICNS.2007.66, 2007,
<https://doi.org/10.1109/ICNS.2007.66>.

[levae-barriers]
Levä, T., Komu, M., and S. Luukkainen, "Adoption Barriers barriers
of Network-layer Protocols: network layer protocols: the Case of Host Identity
Protocol", The International Journal case of host identity
protocol", Computer and
Telecommunications Networking, ISSN: Networks, Vol. 57, Issue 10, pp.
2218-2232, ISSN 1389-1286,
DOI 10.1016/j.comnet.2012.11.024, March
2013. 2013,
<https://doi.org/10.1016/j.comnet.2012.11.024>.

[lindqvist-enterprise]
Lindqvist, J., Vehmersalo, E., Manner, J., and M. Komu, M., and J. Manner,
"Enterprise Network Packet Filtering for Mobile
Cryptographic Identities", International Journal of
Handheld Computing Research, 1 (1), Research (IJHCR), Vol. 1, Issue 1, pp.
79-94, , January-March
2010. DOI 10.4018/jhcr.2010090905, 2010,
<https://doi.org/10.4018/jhcr.2010090905>.

[Nik2001] Nikander, P., "Denial-of-Service, Address Ownership, and
Early Authentication in the IPv6 World", in Proceesings
of Security Protocols, 9th International Workshop,
Cambridge, UK, April 25-27
Workshop on Security Protocols, Security Protocols 2001, LNCS
Lecture Notes in Computer Science, Vol. 2467, pp. 12-26, 12-21,
Springer, 2002. DOI 10.1007/3-540-45807-7_3, 2002,
<https://doi.org/10.1007/3-540-45807-7_3>.

[nsrg-report]
Lear, E. and R. Droms, "What's In A Name:Thoughts Name: Thoughts from
the NSRG", draft-irtf-nsrg-report-10 (work Work in progress), Progress, Internet-Draft, draft-irtf-
nsrg-report-10, 22 September 2003. 2003,
<https://datatracker.ietf.org/doc/html/draft-irtf-nsrg-
report-10>.

[paine-hip]
Paine, R., R. H., "Beyond HIP: The End to Hacking As We Know
It", BookSurge Publishing, ISBN: ISBN-10 1439256047, 9781439256046,
ISBN-13 978-1439256046, 2009.

[pham-leap]
Pham, V. and T. Aura, "Security Analysis of Leap-of-Faith
Protocols", Seventh ICST 7th International Conference on ICST Conference, Security
and Privacy for Communication Networks, ,
September 2011. SecureComm 2011,
Lecture Notes of the Institute for Computer Sciences,
Social Informatics and Telecommunications Engineering,
Vol. 96, DOI 10.1007/978-3-642-31909-9_19, 2012,
<https://doi.org/10.1007/978-3-642-31909-9_19>.

[ranjbar-synaptic]
Ranjbar, A., Komu, M., Salmela, P., and T. Aura,
"SynAPTIC: Secure and Persistent Connectivity for
Containers", 2017 17th IEEE/ACM International Symposium on
Cluster, Cloud and Grid Computing (CCGRID), Madrid, 2017,
pp. 262-267 doi: 262-267, DOI 10.1109/CCGRID.2017.62, 2017. 2017,
<https://doi.org/10.1109/CCGRID.2017.62>.

[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.

[RFC2535] Eastlake 3rd, D., "Domain Name System Security
Extensions", RFC 2535, DOI 10.17487/RFC2535, March 1999,
<https://www.rfc-editor.org/info/rfc2535>.

[RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address
Translation - Protocol Translation (NAT-PT)", RFC 2766,
DOI 10.17487/RFC2766, February 2000, <https://www.rfc-
editor.org/info/rfc2766>.
<https://www.rfc-editor.org/info/rfc2766>.

[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
DOI 10.17487/RFC3022, January 2001, <https://www.rfc-
editor.org/info/rfc3022>.
<https://www.rfc-editor.org/info/rfc3022>.

[RFC3102] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102,
DOI 10.17487/RFC3102, October 2001, <https://www.rfc-
editor.org/info/rfc3102>.
<https://www.rfc-editor.org/info/rfc3102>.

[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.

[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<https://www.rfc-editor.org/info/rfc3972>.

[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, DOI 10.17487/RFC4033, March 2005,
<https://www.rfc-editor.org/info/rfc4033>.

[RFC4225] Nikander, P., Arkko, J., Aura, T., Montenegro, G., and E.
Nordmark, "Mobile IP Version 6 Route Optimization Security
Design Background", RFC 4225, DOI 10.17487/RFC4225,
December 2005, <https://www.rfc-editor.org/info/rfc4225>.

[RFC4306] Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
Protocol", RFC 4306, DOI 10.17487/RFC4306, December 2005,
<https://www.rfc-editor.org/info/rfc4306>.

[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
DOI 10.17487/RFC4380, February 2006, <https://www.rfc-
editor.org/info/rfc4380>.
<https://www.rfc-editor.org/info/rfc4380>.

[RFC4423] Moskowitz, R. and P. Nikander, "Host Identity Protocol
(HIP) Architecture", RFC 4423, DOI 10.17487/RFC4423, May
2006, <https://www.rfc-editor.org/info/rfc4423>.

[RFC5218] Thaler, D. and B. Aboba, "What Makes for a Successful
Protocol?", RFC 5218, DOI 10.17487/RFC5218, July 2008,
<https://www.rfc-editor.org/info/rfc5218>.

[RFC5338] Henderson, T., Nikander, P., and M. Komu, "Using the Host
Identity Protocol with Legacy Applications", RFC 5338,
DOI 10.17487/RFC5338, September 2008, <https://www.rfc-
editor.org/info/rfc5338>.
<https://www.rfc-editor.org/info/rfc5338>.

[RFC5887] Carpenter, B., Atkinson, R., and H. Flinck, "Renumbering
Still Needs Work", RFC 5887, DOI 10.17487/RFC5887, May
2010, <https://www.rfc-editor.org/info/rfc5887>.

[RFC6078] Camarillo, G. and J. Melen, "Host Identity Protocol (HIP)
Immediate Carriage and Conveyance of Upper-Layer Protocol
Signaling (HICCUPS)", RFC 6078, DOI 10.17487/RFC6078,
January 2011, <https://www.rfc-editor.org/info/rfc6078>.

[RFC6250] Thaler, D., "Evolution of the IP Model", RFC 6250,
DOI 10.17487/RFC6250, May 2011, <https://www.rfc-
editor.org/info/rfc6250>.
<https://www.rfc-editor.org/info/rfc6250>.

[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service",
RFC 6281, DOI 10.17487/RFC6281, June 2011,
<https://www.rfc-editor.org/info/rfc6281>.

[RFC6317] Komu, M. and T. Henderson, "Basic Socket Interface
Extensions for the Host Identity Protocol (HIP)",
RFC 6317, DOI 10.17487/RFC6317, July 2011,
<https://www.rfc-editor.org/info/rfc6317>.

[RFC6537] Ahrenholz, J., "Host Identity Protocol Distributed Hash
Table Interface", RFC 6537, DOI 10.17487/RFC6537, February
2012, <https://www.rfc-editor.org/info/rfc6537>.

[RFC6538] Henderson, T. and A. Gurtov, "The Host Identity Protocol
(HIP) Experiment Report", RFC 6538, DOI 10.17487/RFC6538,
March 2012, <https://www.rfc-editor.org/info/rfc6538>.

[RFC7296] Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
Kivinen, "Internet Key Exchange Protocol Version 2
(IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
2014, <https://www.rfc-editor.org/info/rfc7296>.

[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <https://www.rfc-editor.org/info/rfc7435>.

[sarela-bloom]
Saerelae,
Särelä, M., Esteve Rothenberg, C., Zahemszky, A.,
Nikander, P., and J. Ott, "BloomCasting: Security in Bloom
filter based multicast", ,
Filter Based Multicast", Information Security Technology
for Applications, NordSec 2010, Lecture Notes in Computer
Science 2012, ,
Science, Vol. 7127, pages 1-16, Springer Berlin Heidelberg,
2012. Springer,
DOI 10.1007/978-3-642-27937-9_1, 2012,
<https://doi.org/10.1007/978-3-642-27937-9_1>.

[schuetz-intermittent]
Schuetz,
Schütz, S., Eggert, L., Schmid, S., and M. Brunner,
"Protocol enhancements for intermittently connected
hosts", ACM SIGCOMM Comput. Commun. Rev., 35(3):5-18, , Computer Communication Review, Vol.
35, Issue 3, pp. 5-18, DOI 10.1145/1070873.1070875, July
2005.
2005, <https://doi.org/10.1145/1070873.1070875>.

[shields-hip]
Shields, C. and J. J. Garcia-Luna-Aceves, "The HIP
protocol for hierarchical multicast routing", Proceedings
of the seventeenth annual ACM symposium on Principles of
distributed computing, pages 257-266. ACM, New York, NY,
USA, ISBN: pp. 257-266, ISBN 0-89791-977-7, DOI:
DOI 10.1145/277697.277744,
1998. 1998,
<https://doi.org/10.1145/277697.277744>.

[tempered-networks]
Tempered Networks, "Identity-Defined Network (IDN)
Architecture: Unified, Secure Networking Made Simple",
White Paper , Paper, 2016.

[tritilanunt-dos]
Tritilanunt, S., Boyd, C., Foo, E., and J. J.M.G. Nieto,
"Examining the DoS Resistance of HIP", On the Move to
Meaningful Internet Systems 2006: OTM Workshops (1),
volume 4277 of 2006 Workshops,
Lecture Notes in Computer Science, pages
616-625,Springer , 2006. Vol. 4277, pp. 616-625,
Springer, DOI 10.1007/11915034_85, 2006,
<https://doi.org/10.1007/11915034_85>.

[urien-rfid]
Urien, P., Chabanne, H., Pepin, C., Orga, S., Bouet, M.,
de Cunha, D., D.O., Guyot, V., Pujolle, G., Paradinas, P.,
Gressier, E., and J. J.-F. Susini, "HIP-based RFID Networking
Architecture", 2007 IFIP International Conference on
Wireless and Optical Communications Networks, DOI: pp. 1-5,
DOI 10.1109/WOCN.2007.4284140,
July 2007. 2007,
<https://doi.org/10.1109/WOCN.2007.4284140>.

[urien-rfid-draft]
Urien, P., Lee, G., G. M., and G. Pujolle, "HIP support for
RFIDs", IRTF Working draft draft-irtf-hiprg-rfid-07, Work in Progress, Internet-Draft, draft-irtf-
hiprg-rfid-07, 23 April
2013. 2013,
<https://datatracker.ietf.org/doc/html/draft-irtf-hiprg-
rfid-07>.

[varjonen-split]
Varjonen, S., Komu, M., and A. Gurtov, "Secure and
Efficient IPv4/IPv6 Handovers Using Host-Based Identifier-
Location Split", Journal of Communications Software and
Systems, 6(1), 2010, ISSN: Vol. 6, Issue 1, ISSN 18456421, 2010.
DOI 10.24138/jcomss.v6i1.193, 2010,
<https://doi.org/10.24138/jcomss.v6i1.193>.

[xin-hip-lib]
Xin, G., "Host Identity Protocol Version 2.5", Master's
Thesis, Aalto University, Espoo, Finland, , June 2012.

[xueyong-hip]
Xueyong, Z., Zhiguo, D., and W. Xinling, "A Multicast
Routing Algorithm Applied to HIP-Multicast Model",
Proceedings of the 2011 International Conference on
Network Computing and Information Security - Volume 01
(NCIS '11), Vol. 1. IEEE Computer Society, Washington, DC,
USA, pages 169-174, DOI: 10.1109/NCIS.2011.42, 2011.

[xueyong-secure]
Xueyong, Z. and J. Atwood, "A Secure Multicast Model for
Peer-to-Peer and Access Networks Using the Host Identity
Protocol", Consumer Communications and Networking
Conference. CCNC 2007. 4th IEEE, pages 1098,1102, DOI:
10.1109/CCNC.2007.221, January 2007. 2012.

[ylitalo-diss]
Ylitalo, J., "Secure Mobility at Multiple Granularity
Levels over Heterogeneous Datacom Networks", Dissertation,
Helsinki University of Technology, Espoo, Finland Finland,
ISBN 978-951-22-9531-9, 2008.

[ylitalo-spinat]
Ylitalo, J., Salmela, P., and H. Tschofenig, "SPINAT:
Integrating IPsec into overlay routing", Proceedings of
the Overlay Routing", First
International Conference on Security and Privacy for
Emerging Areas in Communication Networks (SecureComm
2005). Networks, SECURECOMM'05,
Athens, Greece. IEEE Computer Society, pages Greece, pp. 315-326, ISBN: ISBN 0-7695-2369-2, September 2005.
DOI 10.1109/SECURECOMM.2005.53, 2005,
<https://doi.org/10.1109/SECURECOMM.2005.53>.

[zhang-revocation]
Zhang, D., Kuptsov, D., and S. Shen, "Host Identifier
Revocation in HIP", IRTF Working draft draft-irtf-hiprg-
revocation-05, Mar 2012. Work in Progress, Internet-Draft,
draft-irtf-hiprg-revocation-05, 9 March 2012,
<https://datatracker.ietf.org/doc/html/draft-irtf-hiprg-
revocation-05>.

[zhu-hip] Zhu, X., Ding, Z., and X. Wang, "A Multicast Routing
Algorithm Applied to HIP-Multicast Model", 2011
International Conference on Network Computing and
Information Security, Guilin, China, pp. 169-174,
DOI 10.1109/NCIS.2011.42, 2011,
<https://doi.org/10.1109/NCIS.2011.42>.

[zhu-secure]
Zhu, X. and J. W. Atwood, "A Secure Multicast Model for
Peer-to-Peer and Access Networks Using the Host Identity
Protocol", 2007 4th IEEE Consumer Communications and
Networking Conference, Las Vegas, NV, USA, pages
1098-1102, DOI 10.1109/CCNC.2007.221, 2007,
<https://doi.org/10.1109/CCNC.2007.221>.

Appendix A. Design considerations Considerations

A.1. Benefits of HIP

In the beginning, the network layer protocol (i.e., IP) had the
following four "classic" invariants:

1. Non-mutable: The address sent is the address received.

2. Non-mobile: The address doesn't change during the course of an
"association".

3. Reversible: A return header can always be formed by reversing the
source and destination addresses.

4. Omniscient: Each host knows what address a partner host can use
to send packets to it.

Actually, the fourth can be inferred from 1 and 3, but it is worth
mentioning explicitly for reasons that will be obvious soon if not
already.

In the current "post-classic" world, we are intentionally trying to
get rid of the second invariant (both for mobility and for multi-
homing),
multihoming), and we have been forced to give up the first and the
fourth. Realm Specific IP [RFC3102] is an attempt to reinstate the
fourth invariant without the first invariant. IPv6 is attempts to
reinstate the first invariant.

Few client-side systems on the Internet have DNS names that are
meaningful. That is, if they have a Fully Qualified Domain Name
(FQDN), that name typically belongs to a NAT device or a dial-up
server, and does not really identify the system itself but its
current connectivity. FQDNs (and their extensions as email names)
are application-layer names; more frequently naming services than
particular systems. This is why many systems on the Internet are not
registered in the DNS; they do not have services of interest to other
Internet hosts.

DNS names are references to IP addresses. This only demonstrates the
interrelationship of the networking and application layers. DNS, as
the Internet's only deployed and distributed database, is also the
repository of other namespaces, due in part to DNSSEC and application
specific
application-specific key records. Although each namespace can be
stretched (IP with v6, DNS with KEY records), neither can adequately
provide for host authentication or act as a separation between
internetworking and transport layers.

The Host Identity (HI) namespace fills an important gap between the
IP and DNS namespaces. An interesting thing about the HI is that it
actually allows a host to give up all but the 3rd network-layer
invariant. That is to say, as long as the source and destination
addresses in the network-layer protocol are reversible, HIP takes
care of host identification, and reversibility allows a local host to
receive a packet back from a remote host. The address changes
occurring during NAT transit (non-mutable) or host movement (non-
omniscient or non-mobile) can be managed by the HIP layer.

With the exception of High-Performance Computing high-performance computing applications, the
Sockets
sockets API is the most common way to develop network applications.
Applications use the Sockets sockets API either directly or indirectly
through some libraries or frameworks. However, the Sockets sockets API is
based on the assumption of static IP addresses, and DNS with its
lifetime values was invented at later stages during the evolution of
the Internet. Hence, the Sockets sockets API does not deal with the lifetime
of addresses [RFC6250]. As the majority of the end-user equipment is
mobile today, their addresses are effectively ephemeral, but the
Sockets
sockets API still gives a fallacious illusion of persistent IP
addresses to the unwary developer. HIP can be used to solidify this
illusion because HIP provides persistent persistent, surrogate addresses to the
application layer in the form of LSIs and HITs.

The persistent identifiers as provided by HIP are useful in multiple
scenarios (see, e.g., [ylitalo-diss] or [komu-diss], [komu-diss] for a more
elaborate discussion):

* When a mobile host moves physically between two different WLAN
networks and obtains a new address, an application using the
identifiers remains isolated regardless of the topology changes
while the underlying HIP layer re-establishes reestablishes connectivity (i.e. (i.e., a
horizontal handoff).

* Similarly, the application utilizing the identifiers remains again
unaware of the topological changes when the underlying host
equipped with WLAN and cellular network interfaces switches
between the two different access technologies (i.e. (i.e., a vertical
handoff).

* Even when hosts are located in private address realms,
applications can uniquely distinguish different hosts from each
other based on their identifiers. In other words, it can be
stated that HIP improves Internet transparency for the application
layer [komu-diss].

* Site renumbering events for services can occur due to corporate
mergers or acquisitions, or by changes in Internet Service
Provider. service
provider. They can involve changing the entire network prefix of
an organization, which is problematic due to hard-coded addresses
in service configuration files or cached IP addresses at the
client side [RFC5887]. Considering such human errors, a site
employing location-independent identifiers as promoted by HIP may
experience fewer problems while renumbering their network.

* More agile IPv6 interoperability can be achieved, as discussed in
Section 4.4. IPv6-based applications can communicate using HITs
with IPv4-based applications that are using LSIs. Additionally,
the underlying network type (IPv4 or IPv6) becomes independent of
the addressing family of the application.

* HITs (or LSIs) can be used in IP-based access control lists as a
more secure replacement for IPv6 addresses. Besides security, HIT
based
HIT-based access control has two other benefits. First, the use
of HITs can potentially halve the size of access control lists
because separate rules for IPv4 are not needed [komu-diss].
Second, HIT-based configuration rules in HIP-aware middleboxes
remain static and independent of topology changes, thus
simplifying administrative efforts particularly for mobile
environments. For instance, the benefits of HIT-based access
control have been harnessed in the case of HIP-aware firewalls,
but can be utilized directly at the end-hosts as well [RFC6538].

While some of these benefits could be and have been redundantly
implemented by individual applications, providing such generic
functionality at the lower layers is useful because it reduces
software development effort and networking software bugs (as the
layer is tested with multiple applications). It also allows the
developer to focus on building the application itself rather than
delving into the intricacies of mobile networking, thus facilitating
separation of concerns.

HIP could also be realized by combining a number of different
protocols, but the complexity of the resulting software may become
substantially larger, and the interaction between multiple multiple, possibly
layered protocols may have adverse effects on latency and throughput.
It is also worth noting that virtually nothing prevents realizing the
HIP architecture, for instance, as an application-layer library,
which has been actually implemented in the past [xin-hip-lib].
However, the tradeoff trade-off in moving the HIP layer to the application
layer is that legacy applications may not be supported.

A.2. Drawbacks of HIP

In computer science, many problems can be solved with an extra layer
of indirection. However, the indirection always involves some costs
as there is no such a thing as a "free lunch". In the case of HIP,
the main costs could be stated as follows:

* In general, an additional layer and a namespace always involve
some initial effort in terms of implementation, deployment deployment, and
maintenance. Some education of developers and administrators may
also be needed. However, the HIP community at the IETF has spent
years in experimenting, exploring, testing, documenting documenting, and
implementing HIP to ease the adoption costs.

* HIP introduces a need to manage HIs and requires a centralized
approach to manage HIP-aware endpoints at scale. What were
formerly IP address-based ACLs are now trusted HITs, and the HIT
to IP HIT-
to-IP address mappings as well as access policies must be managed.
HIP-aware endpoints must also be able to operate autonomously to
ensure mobility and availability (an endpoint must be able to run
without having to have a persistent management connection). The
users who want this better security and mobility of HIs instead of
IP address based address-based ACLs have to then manage this additional
'identity layer' in a non-persistent nonpersistent fashion. As exemplified in
Appendix A.3.5, these challenges have been already solved in an
infrastructure setting to distribute policy and manage the
mappings and trust relationships between HIP-aware endpoints.

* HIP decouples identifier and locator roles of IP addresses.
Consequently, a mapping mechanism is needed to associate them
together. A failure to map a HIT to its corresponding locator may
result in failed connectivity because a HIT is "flat" by its
nature and cannot be looked up from the hierarchically organized
DNS. HITs are flat by design due to a security tradeoff. trade-off. The
more bits that are allocated for the hash in the HIT, the less
likely there will be (malicious) collisions.

* From performance viewpoint, HIP control and data plane processing
introduces some overhead in terms of throughput and latency as
elaborated below.

Related to deployment drawbacks, firewalls are commonly used to
control access to various services and devices in the current
Internet. Since HIP introduces an additional namespace, it is
expected that also the HIP namespace would be filtered for unwanted
connectivity.
connectivity also. While this can be achieved with existing tools
directly in the end-hosts, filtering at the middleboxes requires
modifications to existing firewall software or additional middleboxes
[RFC6538].

The key exchange introduces some extra latency (two round trips) in
the initial transport-layer connection establishment between two
hosts. With TCP, additional delay occurs if the underlying network
stack implementation drops the triggering SYN packet during the key
exchange. The same cost may also occur during HIP handoff
procedures. However, subsequent TCP sessions using the same HIP
association will not bear this cost (within the key lifetime). Both
the key exchange and handoff penalties can be minimized by caching
TCP packets. The latter case can further be optimized with TCP user
timeout extensions [RFC5482] as described in further detail by
Schuetz Schütz
et al al. [schuetz-intermittent].

The most CPU-intensive operations involve the use of the asymmetric
keys and Diffie-Hellman key derivation at the control plane, but this
occurs only during the key exchange, its maintenance (handoffs, (handoffs and
refreshing of key material) material), and tear-down teardown procedures of HIP
associations. The data plane is typically implemented with ESP
because it has a smaller overhead due to symmetric key encryption.
Naturally, even ESP involves some overhead in terms of latency
(processing costs) and throughput (tunneling) (see, e.g.,
[ylitalo-diss] for a performance evaluation).

A.3. Deployment and adoption considerations Adoption Considerations

This section describes some deployment and adoption considerations
related to HIP from a technical perspective.

A.3.1. Deployment analysis Analysis

HIP has been adapted and deployed in an industrial control network in
a production factory, in which HIP's strong network layer network-layer identity
supports the secure coexistence of the control network with many
untrusted network devices operated by third-party vendors
[paine-hip]. Similarly, HIP has also been included in a security
product to support layer-two Virtual Private Networks Layer 2 VPNs [henderson-vpls] to enable security
zones in a supervisory control and data acquisition (SCADA) network.
However, HIP has not been a "wild success" [RFC5218] in the Internet
as argued by Levae Levä et al
[leva-barriers]. al. [levae-barriers]. Here, we briefly
highlight some of their findings based on interviews with 19 experts
from the industry and academia.

From a marketing perspective, the demand for HIP has been low and
substitute technologies have been favored. Another identified reason
has been that some technical misconceptions related to the early
stages of HIP specifications still persist. Two identified
misconceptions are that HIP does not support NAT traversal, traversal and that
HIP must be implemented in the OS kernel. Both of these claims are
untrue; HIP does have NAT traversal extensions
[I-D.ietf-hip-native-nat-traversal], [RFC9028], and kernel
modifications can be avoided with modern operating systems by
diverting packets for userspace processing.

The analysis by Levae Levä et al al. clarifies infrastructural requirements
for HIP. In a minimal set up, setup, a client and server machine have to run
HIP software. However, to avoid manual configurations, usually DNS
records for HIP are set up. For instance, the popular DNS server
software Bind9 does not require any changes to accommodate DNS
records for HIP because they can be supported in binary format in its
configuration files [RFC6538]. HIP rendezvous servers and firewalls
are optional. No changes are required to network address points,
NATs, edge routers routers, or core networks. HIP may require holes in
legacy firewalls.

The analysis also clarifies the requirements for the host components
that consist of three parts. First, a HIP control plane component is
required, typically implemented as a userspace daemon. Second, a
data plane component is needed. Most HIP implementations utilize the
so called BEET
so-called Bound End-to-End Tunnel (BEET) mode of ESP that has been
available since Linux kernel 2.6.27, but the BEET mode is also
included as a userspace component in a few of the implementations.
Third, HIP systems usually provide a DNS proxy for the local host
that translates HIP DNS records to LSIs and HITs, and communicates
the corresponding locators to the HIP userspace daemon. While the
third component is not mandatory, it is very useful for avoiding
manual configurations. The three components are further described in
the HIP experiment report [RFC6538].

Based on the interviews, Levae Levä et al al. suggest further directions to
facilitate HIP deployment. Transitioning a number of HIP
specifications to the standards track Standards Track in the IETF has already taken
place, but the authors suggest other additional measures based on the
interviews. As a more radical measure, the authors suggest to
implement HIP as a purely application-layer library [xin-hip-lib] or
other kind of middleware. On the other hand, more conservative
measures include focusing on private deployments controlled by a
single stakeholder. As a more concrete example of such a scenario,
HIP could be used by a single service provider to facilitate secure
connectivity between its servers [komu-cloud].

A.3.2. HIP in 802.15.4 networks Networks

The IEEE 802 standards have been defining MAC layered MAC-layer security. Many
of these standards use EAP Extensible Authentication Protocol (EAP)
[RFC3748] as a Key Management System (KMS) transport, but some like
IEEE 802.15.4 [IEEE.802-15-4.2011] [IEEE.802.15.4] leave the KMS and its transport as "Out "out
of Scope". scope".

HIP is well suited as a KMS in these environments:

* HIP is independent of IP addressing and can be directly
transported over any network protocol.

* Master Keys keys in 802 protocols are commonly pair-based with group
keys transported from the group controller using pair-wise pairwise keys.

o AdHoc

* Ad hoc 802 networks can be better served by a peer-to-peer KMS
than the EAP client/server model.

* Some devices are very memory constrained constrained, and a common KMS for
both MAC and IP security represents a considerable code savings.

A.3.3. HIP and Internet of Things

HIP requires certain amount computational resources from a device due
to cryptographic processing. HIP scales down to phones and small
system-on-chip devices (such as Raspberry Pis, Intel Edison), but
small sensors operating with small batteries have remained
problematic. Different extensions to the HIP have been developed to
scale HIP down to smaller devices, typically with different security
tradeoffs.
trade-offs. For example, the non-cryptographic identifiers have been
proposed in RFID scenarios. The slimfit Slimfit approach [hummen] proposes a
compression layer for HIP to make it more suitable for constrained
networks. The approach is applied to a light-weight lightweight version of HIP
(i.e.
(i.e., "Diet HIP") in order to scale down to small sensors.

The HIP Diet Exchange [I-D.ietf-hip-dex] EXchange (DEX) [hip-dex] design aims at reducing to reduce the
overhead of the employed cryptographic primitives by omitting public-
key signatures and hash functions. In doing so, the main goal is to
still deliver similar security properties similar to the Base Exchange (BEX).

DEX is primarily designed for computation computation- or memory- constrained memory-constrained
sensor/actuator devices. Like BEX, it is expected to be used
together with a suitable security protocol such as the Encapsulated
Security Payload (ESP) ESP for the
protection of upper layer upper-layer protocol data. In addition, DEX can also
be used as a keying mechanism for security primitives at the MAC
layer, e.g., for IEEE 802.15.9 networks ([IEEE.802-15-9]. [IEEE.802.15.9].

The main differences between HIP BEX and DEX are:

1. Minimum collection of cryptographic primitives to reduce the
protocol overhead.

* Static Elliptic Curve Diffie-Hellman (ECDH) key pairs for peer
authentication and encryption of the session key.

* AES-CTR for symmetric encryption and AES-CMAC for MACing
function.

* A simple fold function for HIT generation.

2. Forfeit of Perfect Forward Secrecy perfect forward secrecy with the dropping of an
ephemeral Diffie-Hellman key agreement.

3. Forfeit of digital signatures with the removal of a hash
function. Reliance on ECDH derived the ECDH-derived key used in HIP_MAC to
prove ownership of the private key.

4. Diffie-Hellman derived key ONLY used to protect the HIP packets.
A separate secret exchange within the HIP packets creates the
session key(s).

5. Optional retransmission strategy tailored to handle the
potentially extensive processing time of the employed
cryptographic operations on computationally constrained devices.

A.3.4. Infrastructure Applications

The HIP experimentation report [RFC6538] enumerates a number of
client and server applications that have been trialed with HIP.
Based on the report, this section highlights and complements some
potential ways how HIP could be exploited in existing infrastructure
such as routers, gateways gateways, and proxies.

HIP has been successfully used with forward web proxies (i.e.,
client-side proxies). HIP was used between a client host (web
browser) and a forward proxy (Apache server) that terminated the HIP/
ESP-tunnel.
ESP tunnel. The forward web proxy translated HIP-based traffic
originating from the client into non-HIP traffic towards any web
server in the Internet. Consequently, the HIP-capable client could
communicate with HIP-incapable web servers. This way, the client
could utilize mobility support as provided by HIP while using the
fixed IP address of the web proxy, for instance, to access services
that were allowed only from the IP address range of the proxy.

HIP has also been experimented with reverse web proxies (i.e. server-
side (i.e., server-side proxies) has also
been investigated, as described in more detail in [komu-cloud]. In
this scenario, a HIP-incapable client accessed a HIP-capable web
service via an intermediary load balancer (that was a web based (a web-based load balancer
implementation called HAProxy). The load balancer translated non-HIP
traffic originating from the client into HIP-based traffic for the
web service (consisting of front-end and back-end servers). Both the
load balancer and the web service were located in a datacenter. data center. One
of the key benefits for encrypting the web traffic with HIP in this
scenario was to support supporting a private-public cloud scenario (i.e. (i.e., hybrid
cloud) where the load balancer, front-end servers servers, and back-end
servers can be were located in different datacenters and, thus, data centers, and thus the traffic needs
needed to be protected when it passes passed through potentially insecure
networks between the borders of the private and public clouds.

While HIP could be used to secure access to intermediary devices
(e.g., access to switches with legacy telnet), it has also been used
to secure intermittent connectivity between middlebox infrastructure.
For instance, earlier research [komu-mitigation] utilized HIP between
Secure
Simple Mail Transport Protocol (SMTP) servers in order to exploit the
computational puzzles of HIP as a spam mitigation mechanism. A
rather obvious practical challenge in this approach was the lack of
HIP adoption on existing SMTP servers.

To avoid deployment hurdles with existing infrastructure, HIP could
be applied in the context of new protocols with little deployment.
Namely, HIP has been experimented studied in the context of a new protocol,
peer-to-peer peer-
to-peer SIP [camarillo-p2psip]. The work has resulted in a number of
related RFCs [RFC6078], [RFC6079], and [RFC7086]. The key idea in
the research work was to avoid redundant, time consuming time-consuming ICE
procedures by grouping different connections (i.e. (i.e., SIP and media
streams) together using the low-layer HIP HIP, which executes NAT
traversal procedures only once per host. An interesting aspect in
the approach was the use of P2P-SIP infrastructure as rendezvous
servers for the HIP control plane instead of utilizing the
traditional HIP rendezvous services [RFC8004].

Researchers have proposed to use using HIP in cellular networks as a
mobility, multihoming multihoming, and security solution. [hip-lte] provides a
security analysis and simulation measurements of using HIP in Long
Term Evolution (LTE) backhaul networks.

HIP has been experimented with studied for securing cloud internal connectivity. First
with virtual machines [komu-cloud] and then later also between Linux containers
[ranjbar-synaptic]. In both cases, HIP was suggested as a solution
to NAT traversal that could be utilized both internally by a cloud
network and between multi-cloud deployments. Specifically in the
former case, HIP was beneficial sustaining connectivity with a
virtual machine while it migrates migrated to a new location. In the latter
case, a Software-Defined Networking (SDN) controller acted as a
rendezvous server for HIP-capable containers. The controller
enforced strong replay protection by adding middlebox nonces
[heer-end-host] to the passing HIP base exchange and UPDATE messages.

A.3.5. Management of Identities in a Commercial Product

Tempered Networks provides HIP-based products. They refer to their
platform as Identity-Defined Networking (IDN) [tempered-networks]
because of HIP's identity-first networking architecture. Their
objective has been to make it simple and non-disruptive nondisruptive to deploy HIP HIP-
enabled services widely in production environments with the purpose
of enabling transparent device authentication and authorization,
cloaking, segmentation, and end-to-end networking. The goal is to
eliminate much of the circular dependencies, exploits, and layered
complexity of traditional "address-defined networking" that prevents
mobility and verifiable device access control. The products in the
portfolio of Tempered Networks utilize HIP are as follows:

HIP Switches / Gateways - these
These are physical or virtual appliances that serve as the HIP
gateway and policy enforcement point for non
HIP-aware non-HIP-aware
applications and devices located behind it. No IP or
infrastructure changes are required in order to connect, cloak,
and protect the non-HIP aware non-HIP-aware devices. Currently known supported
platforms for HIP gateways are: are x86 and ARM chipsets, ESXi, Hyper-
V, KVM, AWS, Azure, and Google clouds.

HIP Relays / Rendezvous -
These are physical or virtual appliances that serve as identity identity-
based routers authorizing and bridging HIP endpoints without
decrypting the HIP session. A HIP Relay relay can be deployed as a
standalone appliance or in a cluster for horizontal scaling. All HIP aware
HIP-aware endpoints and the devices they're connecting and
protecting can remain privately addressed, addressed. The appliances
eliminate IP conflicts, tunnel through NAT and CGNAT, carrier-grade NAT,
and require no changes to the underlay underlying infrastructure. The only
requirement is that a HIP endpoint should have outbound access to
the Internet and that a HIP Relay should have a public address.

HIP-Aware Clients and Servers -
This is software that installs is installed in the host's network stack and
enforces policy for that host. HIP clients support split
tunneling. Both the HIP client and HIP server can interface with
the local host firewall firewall, and the HIP Server server can be locked down to
listen only on the port used for HIP, making the server invisible
from unauthorized devices. Currently known supported platforms
are Windows, OSX, OS X, iOS, Android, Ubuntu, CentOS CentOS, and other Linux
derivatives.

Policy Orchestration Managers - a
These physical or virtual appliance
that serves appliances serve as the engine to define
and distribute network and security policy (HI and IP mappings,
overlay networks networks, and whitelist policies policies, etc.) to HIP-aware
endpoints. Orchestration does not need to persist to the HIP
endpoints and vice versa versa, allowing for autonomous host networking
and security.

A.4. Answers to NSRG questions Questions

The IRTF Name Space Research Group has posed a number of evaluating
questions in their report [nsrg-report]. In this section, we provide
answers to these questions.

1. How would a stack name improve the overall functionality of the
Internet?

HIP decouples the internetworking layer from the transport layer,
allowing each to evolve separately. The decoupling makes end-host end-
host mobility and multi-homing multihoming easier, also across IPv4 and IPv6
networks. HIs make network renumbering easier, and they also
make process migration and clustered servers easier to implement.
Furthermore, being cryptographic in nature, they provide the
basis for solving the security problems related to end-host
mobility and multi-homing. multihoming.

2. What does a stack name look like?

A HI is a cryptographic public key. However, instead of using
the keys directly, most protocols use a fixed-size hash of the
public key.

3. What is its lifetime?

HIP provides both stable and temporary Host Identifiers. Stable
HIs are typically long-lived, with a lifetime of years or more.
The lifetime of temporary HIs depends on how long the upper-layer
connections and applications need them, and can range from a few
seconds to years.

4. Where does it live in the stack?

The HIs live between the transport and internetworking layers.

5. How is it used on the end points? endpoints?

The Host Identifiers may be used directly or indirectly (in the
form of HITs or LSIs) by applications when they access network
services. Additionally, the Host Identifiers, as public keys,
are used in the built-in key agreement protocol, called the HIP
base exchange, to authenticate the hosts to each other.

6. What administrative infrastructure is needed to support it?

In some environments, it is possible to use HIP
opportunistically, without any infrastructure. However, to gain
full benefit from HIP, the HIs must be stored in the DNS or a
PKI, and the rendezvous mechanism is needed [RFC8005].

7. If we add an additional layer layer, would it make the address list in
SCTP unnecessary?

Yes

8. What additional security benefits would a new naming scheme
offer?

HIP reduces dependency on IP addresses, making the so-called
address ownership [Nik2001] problems easier to solve. In
practice, HIP provides security for end-host mobility and
multi-homing.
multihoming. Furthermore, since HIP Host Identifiers are public
keys, standard public key certificate infrastructures can be
applied on the top of HIP.

9. What would the resolution mechanisms be, or what characteristics
of a resolution mechanisms would be required?

For most purposes, an approach where DNS names are resolved
simultaneously to HIs and IP addresses is sufficient. However,
if it becomes necessary to resolve HIs into IP addresses or back
to DNS names, a flat resolution infrastructure is needed. Such
an infrastructure could be based on the ideas of Distributed Hash
Tables, but would require significant new development and
deployment.

Acknowledgments

For the people historically involved in the early stages of HIP, see
the Acknowledgments section in the Host Identity Protocol
specification.

During the later stages of this document, when the editing baton was
transferred to Pekka Nikander, the comments from the early
implementers and others, including Jari Arkko, Jeff Ahrenholz, Tom
Henderson, Petri Jokela, Miika Komu, Mika Kousa, Andrew McGregor, Jan
Melen, Tim Shepard, Jukka Ylitalo, Sasu Tarkoma, and Jorma Wall, were
invaluable. Also, the comments from Lars Eggert, Spencer Dawkins,
Dave Crocker, and Erik Giesa were also useful.

The authors want to express their special thanks to Tom Henderson,
who took the burden of editing the document in response to IESG
comments at the time when both of the authors were busy doing other
things. Without his perseverance, the original document might have
never made it as RFC 4423.

This main effort to update and move HIP forward within the IETF
process owes its impetus to a number of HIP development teams. The
authors are grateful for Boeing, Helsinki Institute for Information
Technology (HIIT), NomadicLab of Ericsson, and the three
universities: RWTH Aachen, Aalto, and University of Helsinki for
their efforts. Without their collective efforts, HIP would have
withered as on the IETF vine as a nice concept.

Thanks also to Suvi Koskinen for her help with proofreading and with
the reference jungle.

Authors' Addresses

Robert Moskowitz (editor)
HTT Consulting
Oak Park Park, Michigan
USA
United States of America

Email: rgm@labs.htt-consult.com

Miika Komu
Ericsson
Hirsalantie 11
02420
FI-02420 Jorvas
Finland

Email: miika.komu@ericsson.com