wdiff rfc8489v1.txt rfc8489.txt

Internet Engineering Task Force (IETF) M. Petit-Huguenin
Request for Comments: 8489 Impedance Mismatch
Obsoletes: 5389 G. Salgueiro
Category: Standards Track Cisco
ISSN: 2070-1721 J. Rosenberg
Five9
D. Wing
Citrix
R. Mahy
Unaffiliated
P. Matthews
Nokia
July 2019
February 2020

Session Traversal Utilities for NAT (STUN)

Abstract

Session Traversal Utilities for NAT (STUN) is a protocol that serves
as a tool for other protocols in dealing with NAT traversal. It can
be used by an endpoint to determine the IP address and port allocated
to it by a NAT. It can also be used to check connectivity between
two endpoints and as a keep-alive protocol to maintain NAT bindings.
STUN works with many existing NATs and does not require any special
behavior from them.

STUN is not a NAT traversal solution by itself. Rather, it is a tool
to be used in the context of a NAT traversal solution.

This document obsoletes RFC 5389.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8489.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. STUN Message Structure . . . . . . . . . . . . . . . . . . . 9
6. Base Protocol Procedures . . . . . . . . . . . . . . . . . . 11
6.1. Forming a Request or an Indication . . . . . . . . . . . 11
6.2. Sending the Request or Indication . . . . . . . . . . . . 12
6.2.1. Sending over UDP or DTLS-over-UDP . . . . . . . . . . 13
6.2.2. Sending over TCP or TLS-over-TCP . . . . . . . . . . 14
6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP . . . . . 15
6.3. Receiving a STUN Message . . . . . . . . . . . . . . . . 16
6.3.1. Processing a Request . . . . . . . . . . . . . . . . 17
6.3.1.1. Forming a Success or Error Response . . . . . . . 17
6.3.1.2. Sending the Success or Error Response . . . . . . 18
6.3.2. Processing an Indication . . . . . . . . . . . . . . 18
6.3.3. Processing a Success Response . . . . . . . . . . . . 19
6.3.4. Processing an Error Response . . . . . . . . . . . . 19
7. FINGERPRINT Mechanism . . . . . . . . . . . . . . . . . . . . 20
8. DNS Discovery of a Server . . . . . . . . . . . . . . . . . . 20
8.1. STUN URI Scheme Semantics . . . . . . . . . . . . . . . . 21
9. Authentication and Message-Integrity Mechanisms . . . . . . . 22
9.1. Short-Term Credential Mechanism . . . . . . . . . . . . . 22 23
9.1.1. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 23
9.1.2. Forming a Request or Indication . . . . . . . . . . . 23
9.1.3. Receiving a Request or Indication . . . . . . . . . . 23
9.1.4. Receiving a Response . . . . . . . . . . . . . . . . 24 25
9.1.5. Sending Subsequent Requests . . . . . . . . . . . . . 25
9.2. Long-Term Credential Mechanism . . . . . . . . . . . . . 25
9.2.1. Bid-Down Attack Prevention . . . . . . . . . . . . . 27
9.2.2. HMAC Key . . . . . . . . . . . . . . . . . . . . . . 27
9.2.3. Forming a Request . . . . . . . . . . . . . . . . . . 28
9.2.3.1. First Request . . . . . . . . . . . . . . . . . . 28
9.2.3.2. Subsequent Requests . . . . . . . . . . . . . . . 28
9.2.4. Receiving a Request . . . . . . . . . . . . . . . . . 29
9.2.5. Receiving a Response . . . . . . . . . . . . . . . . 31
10. ALTERNATE-SERVER Mechanism . . . . . . . . . . . . . . . . . 32
11. Backwards Compatibility with RFC 3489 . . . . . . . . . . . . 33
12. Basic Server Behavior . . . . . . . . . . . . . . . . . . . . 34 33
13. STUN Usages . . . . . . . . . . . . . . . . . . . . . . . . . 35 34
14. STUN Attributes . . . . . . . . . . . . . . . . . . . . . . . 36
14.1. MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . . . 37
14.2. XOR-MAPPED-ADDRESS . . . . . . . . . . . . . . . . . . . 38 37
14.3. USERNAME . . . . . . . . . . . . . . . . . . . . . . . . 39 38
14.4. USERHASH . . . . . . . . . . . . . . . . . . . . . . . . 39
14.5. MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . 39 40
14.6. MESSAGE-INTEGRITY-SHA256 . . . . . . . . . . . . . . . . 40
14.7. FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . 41
14.8. ERROR-CODE . . . . . . . . . . . . . . . . . . . . . . . 41 42
14.9. REALM . . . . . . . . . . . . . . . . . . . . . . . . . 43
14.10. NONCE . . . . . . . . . . . . . . . . . . . . . . . . . 43 44
14.11. PASSWORD-ALGORITHMS . . . . . . . . . . . . . . . . . . 44
14.12. PASSWORD-ALGORITHM . . . . . . . . . . . . . . . . . . . 44 45
14.13. UNKNOWN-ATTRIBUTES . . . . . . . . . . . . . . . . . . . 45
14.14. SOFTWARE . . . . . . . . . . . . . . . . . . . . . . . . 45 46
14.15. ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . . 46
14.16. ALTERNATE-DOMAIN . . . . . . . . . . . . . . . . . . . . 46
15. Operational Considerations . . . . . . . . . . . . . . . . . 46 47
16. Security Considerations . . . . . . . . . . . . . . . . . . . 46 47
16.1. Attacks against the Protocol . . . . . . . . . . . . . . 46 47
16.1.1. Outside Attacks . . . . . . . . . . . . . . . . . . 46 47
16.1.2. Inside Attacks . . . . . . . . . . . . . . . . . . . 47 48
16.1.3. Bid-Down Attacks . . . . . . . . . . . . . . . . . . 48
16.2. Attacks Affecting the Usage . . . . . . . . . . . . . . 50
16.2.1. Attack I: Distributed DoS (DDoS) against a Target . 50 51
16.2.2. Attack II: Silencing a Client . . . . . . . . . . . 51
16.2.3. Attack III: Assuming the Identity of a Client . . . 51 52
16.2.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . 51 52
16.3. Hash Agility Plan . . . . . . . . . . . . . . . . . . . 52
17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 52 53
18. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
18.1. STUN Security Features Registry . . . . . . . . . . . . 53
18.2. STUN Methods Registry . . . . . . . . . . . . . . . . . 53 54
18.3. STUN Attributes Registry . . . . . . . . . . . . . . . . 53 54
18.3.1. Updated Attributes . . . . . . . . . . . . . . . . . 53 55
18.3.2. New Attributes . . . . . . . . . . . . . . . . . . . 54 55
18.4. STUN Error Codes Registry . . . . . . . . . . . . . . . 54 56
18.5. STUN Password Algorithm Algorithms Registry . . . . . . . . . . . . 54 56
18.5.1. Password Algorithms . . . . . . . . . . . . . . . . 55 56
18.5.1.1. MD5 . . . . . . . . . . . . . . . . . . . . . . 55 56
18.5.1.2. SHA-256 . . . . . . . . . . . . . . . . . . . . 55 57
18.6. STUN UDP and TCP Port Numbers . . . . . . . . . . . . . 55 57
19. Changes since RFC 5389 . . . . . . . . . . . . . . . . . . . 56 57
20. References . . . . . . . . . . . . . . . . . . . . . . . . . 56 58
20.1. Normative References . . . . . . . . . . . . . . . . . . 56 58
20.2. Informative References . . . . . . . . . . . . . . . . . 59 60
Appendix A. C Snippet to Determine STUN Message Types . . . . . 61 63
Appendix B. Test Vectors . . . . . . . . . . . . . . . . . . . . 62 64
B.1. Sample Request with Long-Term Authentication with
MESSAGE-INTEGRITY-SHA256 and USERHASH . . . . . . . . . . 62 64
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 63 65
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 64 66
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 64 66

1. Introduction

The protocol defined in this specification, Session Traversal
Utilities for NAT (STUN), provides a tool for dealing with NATs. Network
Address Translators (NATs). It provides a means for an endpoint to
determine the IP address and port allocated by a NAT that corresponds
to its private IP address and port. It also provides a way for an
endpoint to keep a NAT binding alive. With some extensions, the
protocol can be used to do connectivity checks between two endpoints
[RFC8445] or to relay packets between two endpoints [RFC5766].

In keeping with its tool nature, this specification defines an
extensible packet format, defines operation over several transport
protocols, and provides for two forms of authentication.

STUN is intended to be used in the context of one or more NAT
traversal solutions. These solutions are known as "STUN Usages".
Each usage describes how STUN is utilized to achieve the NAT
traversal solution. Typically, a usage indicates when STUN messages
get sent, which optional attributes to include, what server is used,
and what authentication mechanism is to be used. Interactive
Connectivity Establishment (ICE) [RFC8445] is one usage of STUN. SIP
Outbound [RFC5626] is another usage of STUN. In some cases, a usage
will require extensions to STUN. A STUN extension can be in the form
of new methods, attributes, or error response codes. More
information on STUN Usages can be found in Section 13.

2. Overview of Operation

This section is descriptive only.

/-----\
// STUN \\
| Server |
\\ //
\-----/

+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+

+--------------+ Private Network 2
................| NAT 1 |.......................
+--------------+

/-----\
// STUN \\
| Client |
\\ // Private Network 1
\-----/

Figure 1: One Possible STUN Configuration

One possible STUN configuration is shown in Figure 1. In this
configuration, there are two entities (called STUN agents) that
implement the STUN protocol. The lower agent in the figure is the
client, which is connected to private network 1. This network
connects to private network 2 through NAT 1. Private network 2
connects to the public Internet through NAT 2. The upper agent in
the figure is the server, which resides on the public Internet.

STUN is a client-server protocol. It supports two types of
transactions. One is a request/response transaction in which a
client sends a request to a server, and the server returns a
response. The second is an indication transaction in which either
agent -- client or server -- sends an indication that generates no
response. Both types of transactions include a transaction ID, which
is a randomly selected 96-bit number. For request/response
transactions, this transaction ID allows the client to associate the
response with the request that generated it; for indications, the
transaction ID serves as a debugging aid.

All STUN messages start with a fixed header that includes a method, a
class, and the transaction ID. The method indicates which of the
various requests or indications this is; this specification defines
just one method, Binding, but other methods are expected to be
defined in other documents. The class indicates whether this is a
request, a success response, an error response, or an indication.
Following the fixed header comes zero or more attributes, which are
Type-Length-Value extensions that convey additional information for
the specific message.

This document defines a single method called "Binding". The Binding
method can be used either in request/response transactions or in
indication transactions. When used in request/response transactions,
the Binding method can be used to determine the particular binding a
NAT has allocated to a STUN client. When used in either request/
response or in indication transactions, the Binding method can also
be used to keep these bindings alive.

In the Binding request/response transaction, a Binding request is
sent from a STUN client to a STUN server. When the Binding request
arrives at the STUN server, it may have passed through one or more
NATs between the STUN client and the STUN server (in Figure 1, there
are two such NATs). As the Binding request message passes through a
NAT, the NAT will modify the source transport address (that is, the
source IP address and the source port) of the packet. As a result,
the source transport address of the request received by the server
will be the public IP address and port created by the NAT closest to
the server. This is called a "reflexive transport address". The
STUN server copies that source transport address into an XOR-MAPPED-
ADDRESS attribute in the STUN Binding response and sends the Binding
response back to the STUN client. As this packet passes back through
a NAT, the NAT will modify the destination transport address in the
IP header, but the transport address in the XOR-MAPPED-ADDRESS
attribute within the body of the STUN response will remain untouched.
In this way, the client can learn its reflexive transport address
allocated by the outermost NAT with respect to the STUN server.

In some usages, STUN must be multiplexed with other protocols (e.g.,
[RFC8445] and [RFC5626]). In these usages, there must be a way to
inspect a packet and determine if it is a STUN packet or not. STUN
provides three fields in the STUN header with fixed values that can
be used for this purpose. If this is not sufficient, then STUN
packets can also contain a FINGERPRINT value, which can further be
used to distinguish the packets.

STUN defines a set of optional procedures that a usage can decide to
use, called "mechanisms". These mechanisms include DNS discovery, a
redirection technique to an alternate server, a fingerprint attribute
for demultiplexing, and two authentication and message-integrity
exchanges. The authentication mechanisms revolve around the use of a
username, password, and message-integrity value. Two authentication
mechanisms, the long-term credential mechanism and the short-term
credential mechanism, are defined in this specification. Each usage
specifies the mechanisms allowed with that usage.

In the long-term credential mechanism, the client and server share a
pre-provisioned username and password and perform a digest challenge/
response exchange inspired by the one defined for HTTP [RFC7616] but
differing in details. In the short-term credential mechanism, the
client and the server exchange a username and password through some
out-of-band method prior to the STUN exchange. For example, in the
ICE usage [RFC8445], the two endpoints use out-of-band signaling to
exchange a username and password. These are used to integrity
protect and authenticate the request and response. There is no
challenge or nonce used.

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.

4. Definitions

STUN Agent: A STUN agent is an entity that implements the STUN
protocol. The entity can be either a STUN client or a STUN
server.

STUN Client: A STUN client is an entity that sends STUN requests and
receives STUN responses and STUN indications. A STUN client can
also send indications. In this specification, the terms "STUN
client" and "client" are synonymous.

STUN Server: A STUN server is an entity that receives STUN requests
and STUN indications and that sends STUN responses. A STUN server
can also send indications. In this specification, the terms "STUN
server" and "server" are synonymous.

Transport Address: The combination of an IP address and port number
(such as a UDP or TCP port number).

Reflexive Transport Address: A transport address learned by a client
that identifies that client as seen by another host on an IP
network, typically a STUN server. When there is an intervening
NAT between the client and the other host, the reflexive transport
address represents the mapped address allocated to the client on
the public side of the NAT. Reflexive transport addresses are
learned from the mapped address attribute (MAPPED-ADDRESS or XOR-
MAPPED-ADDRESS) in STUN responses.

Mapped Address: Same meaning as reflexive address. This term is
retained only for historic reasons and due to the naming of the
MAPPED-ADDRESS and XOR-MAPPED-ADDRESS attributes.

Long-Term Credential: A username and associated password that
represent a shared secret between client and server. Long-term
credentials are generally granted to the client when a subscriber
enrolls in a service and persist until the subscriber leaves the
service or explicitly changes the credential.

Long-Term Password: The password from a long-term credential.

Short-Term Credential: A temporary username and associated password
that represent a shared secret between client and server. Short-
term credentials are obtained through some kind of protocol
mechanism between the client and server, preceding the STUN
exchange. A short-term credential has an explicit temporal scope,
which may be based on a specific amount of time (such as 5
minutes) or on an event (such as termination of a Session
Initiation Protocol (SIP) [RFC3261] dialog). The specific scope
of a short-term credential is defined by the application usage.

Short-Term Password: The password component of a short-term
credential.

STUN Indication: A STUN message that does not receive a response.

Attribute: The STUN term for a Type-Length-Value (TLV) object that
can be added to a STUN message. Attributes are divided into two
types: comprehension-required and comprehension-optional. STUN
agents can safely ignore comprehension-optional attributes they
don't understand but cannot successfully process a message if it
contains comprehension-required attributes that are not
understood.

RTO: Retransmission TimeOut, which defines the initial period of
time between transmission of a request and the first retransmit of
that request.

5. STUN Message Structure

STUN messages are encoded in binary using network-oriented format
(most significant byte or octet first, also commonly known as big-
endian). The transmission order is described in detail in Appendix B
of [RFC0791]. Unless otherwise noted, numeric constants are in
decimal (base 10).

All STUN messages comprise a 20-byte header followed by zero or more
attributes. The STUN header contains a STUN message type, message
length, magic cookie, and transaction ID.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0| STUN Message Type | Message Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Magic Cookie |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Transaction ID (96 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Format of STUN Message Header

The most significant 2 bits of every STUN message MUST be zeroes.
This can be used to differentiate STUN packets from other protocols
when STUN is multiplexed with other protocols on the same port.

The message type defines the message class (request, success
response, error response, or indication) and the message method (the
primary function) of the STUN message. Although there are four
message classes, there are only two types of transactions in STUN:
request/response transactions (which consist of a request message and
a response message) and indication transactions (which consist of a
single indication message). Response classes are split into error
and success responses to aid in quickly processing the STUN message.

The STUN Message Type field is decomposed further into the following
structure:

0 1
2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+
|M |M |M|M|M|C|M|M|M|C|M|M|M|M|
|11|10|9|8|7|1|6|5|4|0|3|2|1|0|
+--+--+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: Format of STUN Message Type Field

Here the bits in the STUN Message Type field are shown as most
significant (M11) through least significant (M0). M11 through M0
represent a 12-bit encoding of the method. C1 and C0 represent a
2-bit encoding of the class. A class of 0b00 is a request, a class
of 0b01 is an indication, a class of 0b10 is a success response, and
a class of 0b11 is an error response. This specification defines a
single method, Binding. The method and class are orthogonal, so that
for each method, a request, success response, error response, and
indication are possible for that method. Extensions defining new
methods MUST indicate which classes are permitted for that method.

For example, a Binding request has class=0b00 (request) and
method=0b000000000001 (Binding) and is encoded into the first 16 bits
as 0x0001. A Binding response has class=0b10 (success response) and
method=0b000000000001 and is encoded into the first 16 bits as
0x0101.

Note: This unfortunate encoding is due to assignment of values in
[RFC3489] that did not consider encoding indication messages,
success responses, and errors responses using bit fields.

The Magic Cookie field MUST contain the fixed value 0x2112A442 in
network byte order. In [RFC3489], this the 32 bits comprising the Magic
Cookie field was were part of the transaction ID; placing the magic
cookie in this location allows a server to detect if the client will
understand certain attributes that were added to STUN by [RFC5389].
In addition, it aids in distinguishing STUN packets from packets of
other protocols when STUN is multiplexed with those other protocols
on the same port.

The transaction ID is a 96-bit identifier, used to uniquely identify
STUN transactions. For request/response transactions, the
transaction ID is chosen by the STUN client for the request and
echoed by the server in the response. For indications, it is chosen
by the agent sending the indication. It primarily serves to
correlate requests with responses, though it also plays a small role
in helping to prevent certain types of attacks. The server also uses
the transaction ID as a key to identify each transaction uniquely
across all clients. As such, the transaction ID MUST be uniformly
and randomly chosen from the interval 0 .. 2**96-1 and MUST be
cryptographically random. Resends of the same request reuse the same
transaction ID, but the client MUST choose a new transaction ID for
new transactions unless the new request is bit-wise identical to the
previous request and sent from the same transport address to the same
IP address. Success and error responses MUST carry the same
transaction ID as their corresponding request. When an agent is
acting as a STUN server and STUN client on the same port, the
transaction IDs in requests sent by the agent have no relationship to
the transaction IDs in requests received by the agent.

The message length MUST contain the size of the message in bytes, not
including the 20-byte STUN header. Since all STUN attributes are
padded to a multiple of 4 bytes, the last 2 bits of this field are
always zero. This provides another way to distinguish STUN packets
from packets of other protocols.

Following the STUN fixed portion of the header are zero or more
attributes. Each attribute is TLV (Type-Length-Value) encoded.
Details of the encoding and the attributes themselves are given in
Section 14.

6. Base Protocol Procedures

This section defines the base procedures of the STUN protocol. It
describes how messages are formed, how they are sent, and how they
are processed when they are received. It also defines the detailed
processing of the Binding method. Other sections in this document
describe optional procedures that a usage may elect to use in certain
situations. Other documents may define other extensions to STUN, by
adding new methods, new attributes, or new error response codes.

6.1. Forming a Request or an Indication

When formulating a request or indication message, the agent MUST
follow the rules in Section 5 when creating the header. In addition,
the message class MUST be either "Request" or "Indication" (as
appropriate), and the method must be either Binding or some method
defined in another document.

The agent then adds any attributes specified by the method or the
usage. For example, some usages may specify that the agent use an
authentication method (Section 9) or the FINGERPRINT attribute
(Section 7).

If the agent is sending a request, it SHOULD add a SOFTWARE attribute
to the request. Agents MAY include a SOFTWARE attribute in
indications, depending on the method. Extensions to STUN should
discuss whether SOFTWARE is useful in new indications. Note that the
inclusion of a SOFTWARE attribute may have security implications; see
Section 16.1.2 for details.

For the Binding method with no authentication, no attributes are
required unless the usage specifies otherwise.

All STUN messages sent over UDP or DTLS-over-UDP [RFC6347] SHOULD be
less than the path MTU, if known.

If the path MTU is unknown for UDP, messages SHOULD be the smaller of
576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
IPv6 [RFC8200]. This value corresponds to the overall size of the IP
packet. Consequently, for IPv4, the actual STUN message would need
to be less than 548 bytes (576 minus 20-byte IP header, minus 8-byte
UDP header, assuming no IP options are used).

If the path MTU is unknown for DTLS-over-UDP, the rules described in
the previous paragraph need to be adjusted to take into account the
size of the (13-byte) DTLS Record header, the Message Authentication
Code (MAC) size, and the padding size.

STUN provides no ability to handle the case where the request is
smaller than the MTU but the response is larger than the MTU. It is
not envisioned that this limitation will be an issue for STUN. The
MTU limitation is a SHOULD, not a MUST, to account for cases where
STUN itself is being used to probe for MTU characteristics [RFC5780].
See also [STUN-PMTUD] for a framework that uses STUN to add Path MTU
Discovery to protocols that lack such a mechanism. Outside of this
or similar applications, the MTU constraint MUST be followed.

6.2. Sending the Request or Indication

The agent then sends the request or indication. This document
specifies how to send STUN messages over UDP, TCP, TLS-over-TCP, or
DTLS-over-UDP; other transport protocols may be added in the future.
The STUN Usage must specify which transport protocol is used and how
the agent determines the IP address and port of the recipient.
Section 8 describes a DNS-based method of determining the IP address
and port of a server that a usage may elect to use.

At any time, a client MAY have multiple outstanding STUN requests
with the same STUN server (that is, multiple transactions in
progress, with different transaction IDs). Absent other limits to
the rate of new transactions (such as those specified by ICE for
connectivity checks or when STUN is run over TCP), a client SHOULD
limit itself to ten outstanding transactions to the same server.

6.2.1. Sending over UDP or DTLS-over-UDP

When running STUN over UDP or STUN over DTLS-over-UDP [RFC7350], it
is possible that the STUN message might be dropped by the network.
Reliability of STUN request/response transactions is accomplished
through retransmissions of the request message by the client
application itself. STUN indications are not retransmitted; thus,
indication transactions over UDP or DTLS-over-UDP are not reliable.

A client SHOULD retransmit a STUN request message starting with an
interval of RTO ("Retransmission TimeOut"), doubling after each
retransmission. The RTO is an estimate of the round-trip time (RTT)
and is computed as described in [RFC6298], with two exceptions.
First, the initial value for RTO SHOULD be greater than or equal to
500 ms. The exception cases for this "SHOULD" are when other
mechanisms are used to derive congestion thresholds (such as the ones
defined in ICE for fixed-rate streams) or when STUN is used in non-
Internet environments with known network capacities. In fixed-line
access links, a value of 500 ms is RECOMMENDED. Second, the value of
RTO SHOULD NOT be rounded up to the nearest second. Rather, a 1 ms
accuracy SHOULD be maintained. As with TCP, the usage of Karn's
algorithm is RECOMMENDED [KARN87]. When applied to STUN, it means
that RTT estimates SHOULD NOT be computed from STUN transactions that
result in the retransmission of a request.

The value for RTO SHOULD be cached by a client after the completion
of the transaction and used as the starting value for RTO for the
next transaction to the same server (based on equality of IP
address). The value SHOULD be considered stale and discarded if no
transactions have occurred to the same server in the last 10 minutes.

Retransmissions continue until a response is received or until a
total of Rc requests have been sent. Rc SHOULD be configurable and
SHOULD have a default of 7. If, after the last request, a duration
equal to Rm times the RTO has passed without a response (providing
ample time to get a response if only this final request actually
succeeds), the client SHOULD consider the transaction to have failed.
Rm SHOULD be configurable and SHOULD have a default of 16. A STUN
transaction over UDP or DTLS-over-UDP is also considered failed if
there has been a hard ICMP error [RFC1122]. For example, assuming an
RTO of 500 ms, requests would be sent at times 0 ms, 500 ms, 1500 ms,
3500 ms, 7500 ms, 15500 ms, and 31500 ms. If the client has not
received a response after 39500 ms, the client will consider the
transaction to have timed out.

6.2.2. Sending over TCP or TLS-over-TCP

For TCP and TLS-over-TCP [RFC5246], [RFC8446], the client opens a TCP connection
to the server.

In some usages of STUN, STUN is sent as the only protocol over the TCP
connection. In this case, it can be sent without the aid of any
additional framing or demultiplexing. In other usages, or with other
extensions, it may be multiplexed with other data over a TCP
connection. In that case, STUN MUST be run on top of some kind of
framing protocol, specified by the usage or extension, which allows
for the agent to extract complete STUN messages and complete
application-layer messages. The STUN service running on the well-
known port or ports discovered through the DNS procedures in
Section 8 is for STUN alone, and not for STUN multiplexed with other
data. Consequently, no framing protocols are used in connections to
those servers. When additional framing is utilized, the usage will
specify how the client knows to apply it and what port to connect to.
For example, in the case of ICE connectivity checks, this information
is learned through out-of-band negotiation between client and server.

Reliability of STUN over TCP and TLS-over-TCP is handled by TCP
itself, and there are no retransmissions at the STUN protocol level.
However, for a request/response transaction, if the client has not
received a response by Ti seconds after it sent the request message,
it considers the transaction to have timed out. Ti SHOULD be
configurable and SHOULD have a default of 39.5 s. This value has
been chosen to equalize the TCP and UDP timeouts for the default
initial RTO.

In addition, if the client is unable to establish the TCP connection,
or the TCP connection is reset or fails before a response is
received, any request/response transaction in progress is considered
to have failed.

The client MAY send multiple transactions over a single TCP (or TLS-
over-TCP) connection, and it MAY send another request before
receiving a response to the previous request. The client SHOULD keep
the connection open until it:

o has no further STUN requests or indications to send over that
connection,

o has no plans to use any resources (such as a mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[RFC5766]) that were learned though STUN requests sent over that
connection,
o if multiplexing other application protocols over that port, has
finished using those other protocols,

o if using that learned port with a remote peer, has established
communications with that remote peer, as is required by some TCP
NAT traversal techniques (e.g., [RFC6544]).

The details of an eventual keep-alive mechanism are left to each STUN
Usage. In any case, if a transaction fails because an idle TCP
connection doesn't work anymore, the client SHOULD send a RST and try
to open a new TCP connection.

At the server end, the server SHOULD keep the connection open and let
the client close it, unless the server has determined that the
connection has timed out (for example, due to the client
disconnecting from the network). Bindings learned by the client will
remain valid in intervening NATs only while the connection remains
open. Only the client knows how long it needs the binding. The
server SHOULD NOT close a connection if a request was received over
that connection for which a response was not sent. A server MUST NOT
ever open a connection back towards the client in order to send a
response. Servers SHOULD follow best practices regarding connection
management in cases of overload.

6.2.3. Sending over TLS-over-TCP or DTLS-over-UDP

When STUN is run by itself over TLS-over-TCP or DTLS-over-UDP, the
TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 ciphersuites MUST be
implemented, and other
implemented (for compatibility with older versions of this protocol),
except if deprecated by rules of a specific STUN usage. Other
ciphersuites MAY be implemented. Note that STUN clients and servers
that implement TLS version 1.3 [RFC8446] or subsequent versions are
also required to implement mandatory ciphersuites from those
specifications and SHOULD disable usage of deprecated ciphersuites
when they detect support for those specifications. Perfect Forward
Secrecy (PFS) ciphersuites MUST be preferred over non-PFS
ciphersuites. Ciphersuites with known weaknesses, such as those
based on (single) DES and RC4, MUST NOT be used. Implementations
MUST disable TLS-level compression.

These recommendations are just a part of the recommendations in
[BCP195] that implementations and deployments of a STUN Usage using
TLS or DTLS MUST follow.

When it receives the TLS Certificate message, the client MUST verify
the certificate and inspect the site identified by the certificate.
If the certificate is invalid or revoked, or if it does not identify
the appropriate party, the client MUST NOT send the STUN message or
otherwise proceed with the STUN transaction. The client MUST verify
the identity of the server. To do that, it follows the
identification procedures defined in [RFC6125], with a certificate
containing an identifier of type DNS-ID or CN-ID, optionally with a
wildcard character as the leftmost label, but not of type SRV-ID or
URI-ID.

When STUN is run multiplexed with other protocols over a TLS-over-TCP
connection or a DTLS-over-UDP association, the mandatory ciphersuites
and TLS handling procedures operate as defined by those protocols.

6.3. Receiving a STUN Message

This section specifies the processing of a STUN message. The
processing specified here is for STUN messages as defined in this
specification; additional rules for backwards compatibility are
defined in Section 11. Those additional procedures are optional, and
usages can elect to utilize them. First, a set of processing
operations is applied that is independent of the class. This is
followed by class-specific processing, described in the subsections
that follow.

When a STUN agent receives a STUN message, it first checks that the
message obeys the rules of Section 5. It checks that the first two
bits are 0, that the Magic Cookie field has the correct value, that
the message length is sensible, and that the method value is a
supported method. It checks that the message class is allowed for
the particular method. If the message class is "Success Response" or
"Error Response", the agent checks that the transaction ID matches a
transaction that is still in progress. If the FINGERPRINT extension
is being used, the agent checks that the FINGERPRINT attribute is
present and contains the correct value. If any errors are detected,
the message is silently discarded. In the case when STUN is being
multiplexed with another protocol, an error may indicate that this is
not really a STUN message; in this case, the agent should try to
parse the message as a different protocol.

The STUN agent then does any checks that are required by a
authentication mechanism that the usage has specified (see
Section 9).

Once the authentication checks are done, the STUN agent checks for
unknown attributes and known-but-unexpected attributes in the
message. Unknown comprehension-optional attributes MUST be ignored
by the agent. Known-but-unexpected attributes SHOULD be ignored by
the agent. Unknown comprehension-required attributes cause
processing that depends on the message class and is described below.

At this point, further processing depends on the message class of the
request.

6.3.1. Processing a Request

If the request contains one or more unknown comprehension-required
attributes, the server replies with an error response with an error
code of 420 (Unknown Attribute) and includes an UNKNOWN-ATTRIBUTES
attribute in the response that lists the unknown comprehension-
required attributes.

Otherwise, the server then does any additional checking that the
method or the specific usage requires. If all the checks succeed,
the server formulates a success response as described below.

When run over UDP or DTLS-over-UDP, a request received by the server
could be the first request of a transaction or could be a
retransmission. The server MUST respond to retransmissions such that
the following property is preserved: if the client receives the
response to the retransmission and not the response that was sent to
the original request, the overall state on the client and server is
identical to the case where only the response to the original
retransmission is received or where both responses are received (in
which case the client will use the first). The easiest way to meet
this requirement is for the server to remember all transaction IDs
received over UDP or DTLS-over-UDP and their corresponding responses
in the last 40 seconds. However, this requires the server to hold
state and is inappropriate for any requests that are not
authenticated. Another way is to reprocess the request and recompute
the response. The latter technique MUST only be applied to requests
that are idempotent (a request is considered idempotent when the same
request can be safely repeated without impacting the overall state of
the system) and result in the same success response for the same
request. The Binding method is considered to be idempotent. Note
that there are certain rare network events that could cause the
reflexive transport address value to change, resulting in a different
mapped address in different success responses. Extensions to STUN
MUST discuss the implications of request retransmissions on servers
that do not store transaction state.

6.3.1.1. Forming a Success or Error Response

When forming the response (success or error), the server follows the
rules of Section 6. The method of the response is the same as that
of the request, and the message class is either "Success Response" or
"Error Response".

For an error response, the server MUST add an ERROR-CODE attribute
containing the error code specified in the processing above. The
reason phrase is not fixed but SHOULD be something suitable for the
error code. For certain errors, additional attributes are added to
the message. These attributes are spelled out in the description
where the error code is specified. For example, for an error code of
420 (Unknown Attribute), the server MUST include an UNKNOWN-
ATTRIBUTES attribute. Certain authentication errors also cause
attributes to be added (see Section 9). Extensions may define other
errors and/or additional attributes to add in error cases.

If the server authenticated the request using an authentication
mechanism, then the server SHOULD add the appropriate authentication
attributes to the response (see Section 9).

The server also adds any attributes required by the specific method
or usage. In addition, the server SHOULD add a SOFTWARE attribute to
the message.

For the Binding method, no additional checking is required unless the
usage specifies otherwise. When forming the success response, the
server adds an XOR-MAPPED-ADDRESS attribute to the response; this
attribute contains the source transport address of the request
message. For UDP or DTLS-over-UDP, this is the source IP address and
source UDP port of the request message. For TCP and TLS-over-TCP,
this is the source IP address and source TCP port of the TCP
connection as seen by the server.

6.3.1.2. Sending the Success or Error Response

The response (success or error) is sent over the same transport as
the request was received on. If the request was received over UDP or
DTLS-over-UDP, the destination IP address and port of the response
are the source IP address and port of the received request message,
and the source IP address and port of the response are equal to the
destination IP address and port of the received request message. If
the request was received over TCP or TLS-over-TCP, the response is
sent back on the same TCP connection as the request was received on.

The server is allowed to send responses in a different order than it
received the requests.

6.3.2. Processing an Indication

If the indication contains unknown comprehension-required attributes,
the indication is discarded and processing ceases.

Otherwise, the agent then does any additional checking that the
method or the specific usage requires. If all the checks succeed,
the agent then processes the indication. No response is generated
for an indication.

For the Binding method, no additional checking or processing is
required, unless the usage specifies otherwise. The mere receipt of
the message by the agent has refreshed the bindings in the
intervening NATs.

Since indications are not re-transmitted over UDP or DTLS-over-UDP
(unlike requests), there is no need to handle re-transmissions of
indications at the sending agent.

6.3.3. Processing a Success Response

If the success response contains unknown comprehension-required
attributes, the response is discarded and the transaction is
considered to have failed.

Otherwise, the client then does any additional checking that the
method or the specific usage requires. If all the checks succeed,
the client then processes the success response.

For the Binding method, the client checks that the XOR-MAPPED-ADDRESS
attribute is present in the response. The client checks the address
family specified. If it is an unsupported address family, the
attribute SHOULD be ignored. If it is an unexpected but supported
address family (for example, the Binding transaction was sent over
IPv4, but the address family specified is IPv6), then the client MAY
accept and use the value.

6.3.4. Processing an Error Response

If the error response contains unknown comprehension-required
attributes, or if the error response does not contain an ERROR-CODE
attribute, then the transaction is simply considered to have failed.

Otherwise, the client then does any processing specified by the
authentication mechanism (see Section 9). This may result in a new
transaction attempt.

The processing at this point depends on the error code, the method,
and the usage; the following are the default rules:

o If the error code is 300 through 399, the client SHOULD consider
the transaction as failed unless the ALTERNATE-SERVER extension
(Section 10) is being used.

o If the error code is 400 through 499, the client declares the
transaction failed; in the case of 420 (Unknown Attribute), the
response should contain a UNKNOWN-ATTRIBUTES attribute that gives
additional information.

o If the error code is 500 through 599, the client MAY resend the
request; clients that do so MUST limit the number of times they do
this. Unless a specific error code specifies a different value,
the number of retransmissions SHOULD be limited to 4.

Any other error code causes the client to consider the transaction
failed.

7. FINGERPRINT Mechanism

This section describes an optional mechanism for STUN that aids in
distinguishing STUN messages from packets of other protocols when the
two are multiplexed on the same transport address. This mechanism is
optional, and a STUN Usage must describe if and when it is used. The
FINGERPRINT mechanism is not backwards compatible with RFC 3489 and
cannot be used in environments where such compatibility is required.

In some usages, STUN messages are multiplexed on the same transport
address as other protocols, such as the Real-Time Transport Protocol
(RTP). In order to apply the processing described in Section 6, STUN
messages must first be separated from the application packets.

Section 5 describes three fixed fields in the STUN header that can be
used for this purpose. However, in some cases, these three fixed
fields may not be sufficient.

When the FINGERPRINT extension is used, an agent includes the
FINGERPRINT attribute in messages it sends to another agent.
Section 14.7 describes the placement and value of this attribute.

When the agent receives what it believes is a STUN message, then, in
addition to other basic checks, the agent also checks that the
message contains a FINGERPRINT attribute and that the attribute
contains the correct value. Section 6.3 describes when in the
overall processing of a STUN message the FINGERPRINT check is
performed. This additional check helps the agent detect messages of
other protocols that might otherwise seem to be STUN messages.

8. DNS Discovery of a Server

This section describes an optional procedure for STUN that allows a
client to use DNS to determine the IP address and port of a server.
A STUN Usage must describe if and when this extension is used. To
use this procedure, the client must know a STUN URI [RFC7064]; the
usage must also describe how the client obtains this URI. Hard-
coding a STUN URI into software is NOT RECOMMENDED in case the domain
name is lost or needs to change for legal or other reasons.

When a client wishes to locate a STUN server on the public Internet
that accepts Binding request/response transactions, the STUN URI
scheme is "stun". When it wishes to locate a STUN server that
accepts Binding request/response transactions over a TLS or DTLS
session, the URI scheme is "stuns".

The syntax of the "stun" and "stuns" URIs is defined in Section 3.1
of [RFC7064]. STUN Usages MAY define additional URI schemes.

8.1. STUN URI Scheme Semantics

If the <host> part of a "stun" URI contains an IP address, then this
IP address is used directly to contact the server. A "stuns" URI
containing an IP address MUST be rejected. A future STUN extension
or usage may relax this requirement, provided it demonstrates how to
authenticate the STUN server and prevent man-in-the-middle attacks.

If the URI does not contain an IP address, the domain name contained
in the <host> part is resolved to a transport address using the SRV
procedures specified in [RFC2782]. The DNS SRV service name is the
content of the <scheme> part. The protocol in the SRV lookup is the
transport protocol the client will run STUN over: "udp" for UDP and
"tcp" for TCP.

The procedures of RFC 2782 are followed to determine the server to
contact. RFC 2782 spells out the details of how a set of SRV records
is sorted and then tried. However, RFC 2782 only states that the
client should "try to connect to the (protocol, address, service)"
without giving any details on what happens in the event of failure.
When following these procedures, if the STUN transaction times out
without receipt of a response, the client SHOULD retry the request to
the next server in the ordered order defined by RFC 2782. Such a retry is
only possible for request/response transmissions, since indication
transactions generate no response or timeout.

In addition, instead of querying either the A or the AAAA resource
records for a domain name, a dual-stack IPv4/IPv6 client MUST query
both and try the requests with all the IP addresses received, as
specified in [RFC8305].

The default port for STUN requests is 3478, for both TCP and UDP.
The default port for STUN over TLS and STUN over DTLS requests is
5349. Servers can run STUN over DTLS on the same port as STUN over
UDP if the server software supports determining whether the initial
message is a DTLS or STUN message. Servers can run STUN over TLS on
the same port as STUN over TCP if the server software supports
determining whether the initial message is a TLS or STUN message.

Administrators of STUN servers SHOULD use these ports in their SRV
records for UDP and TCP. In all cases, the port in DNS MUST reflect
the one on which the server is listening.

If no SRV records are found, the client performs both an A and AAAA
record lookup of the domain name, as described in [RFC8305]. The
result will be a list of IP addresses, each of which can be
simultaneously contacted at the default port using UDP or TCP,
independent of the STUN Usage. For usages that require TLS, the
client connects to the IP addresses using the default STUN over TLS
port. For usages that require DTLS, the client connects to the IP
addresses using the default STUN over DTLS port.

9. Authentication and Message-Integrity Mechanisms

This section defines two mechanisms for STUN that a client and server
can use to provide authentication and message integrity; these two
mechanisms are known as the short-term credential mechanism and the
long-term credential mechanism. These two mechanisms are optional,
and each usage must specify if and when these mechanisms are used.
Consequently, both clients and servers will know which mechanism (if
any) to follow based on knowledge of which usage applies. For
example, a STUN server on the public Internet supporting ICE would
have no authentication, whereas the STUN server functionality in an
agent supporting connectivity checks would utilize short-term
credentials. An overview of these two mechanisms is given in
Section 2.

Each mechanism specifies the additional processing required to use
that mechanism, extending the processing specified in Section 6. The
additional processing occurs in three different places: when forming
a message, when receiving a message immediately after the basic
checks have been performed, and when doing the detailed processing of
error responses.

Note that agents MUST ignore all attributes that follow MESSAGE-
INTEGRITY, with the exception of the MESSAGE-INTEGRITY-SHA256 and
FINGERPRINT attributes. Similarly, agents MUST ignore all attributes
that follow the MESSAGE-INTEGRITY-SHA256 attribute if the MESSAGE-
INTEGRITY attribute is not present, with the exception of the
FINGERPRINT attribute.

9.1. Short-Term Credential Mechanism

The short-term credential mechanism assumes that, prior to the STUN
transaction, the client and server have used some other protocol to
exchange a credential in the form of a username and password. This
credential is time-limited. The time limit is defined by the usage.
As an example, in the ICE usage [RFC8445], the two endpoints use out-
of-band signaling to agree on a username and password, and this
username and password are applicable for the duration of the media
session.

This credential is used to form a message-integrity check in each
request and in many responses. There is no challenge and response as
in the long-term mechanism; consequently, replay is limited by virtue
of the time-limited nature of the credential.

9.1.1. HMAC Key

For short-term credentials, the Hash-Based Message Authentication
Code (HMAC) key is defined as follow:

key = OpaqueString(password)

where the OpaqueString profile is defined in [RFC8265]. The encoding
used is UTF-8 [RFC3629].

9.1.2. Forming a Request or Indication

For a request or indication message, the agent MUST include the
USERNAME, MESSAGE-INTEGRITY-SHA256, and MESSAGE-INTEGRITY attributes
in the message unless the agent knows from an external indication mechanism
which message integrity algorithm is supported by both agents. In
this case, either MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 MUST
be included in addition to USERNAME. The HMAC for the MESSAGE-
INTEGRITY attribute is computed as described in Section 14.5, and the
HMAC for the MESSAGE-INTEGRITY-SHA256 attributes is computed as
described in Section 14.6. Note that the password is never included
in the request or indication.

9.1.3. Receiving a Request or Indication

After the agent has done the basic processing of a message, the agent
performs the checks listed below in the order specified:

o If the message does not contain 1) a MESSAGE-INTEGRITY or a
MESSAGE-INTEGRITY-SHA256 attribute and 2) a USERNAME attribute:

* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 400 (Bad Request).

* If the message is an indication, the agent MUST silently
discard the indication.

o If the USERNAME does not contain a username value currently valid
within the server:

* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401 (Unauthenticated).

* If the message is an indication, the agent MUST silently
discard the indication.

o If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
value for the message integrity as described in Section 14.6,
using the password associated with the username. If the MESSAGE-
INTEGRITY-SHA256 attribute is not present, then use the same
password to compute the value for the message integrity as
described in Section 14.5. If the resulting value does not match
the contents of the corresponding attribute (MESSAGE-INTEGRITY-
SHA256 or MESSAGE-INTEGRITY):

* If the message is a request, the server MUST reject the request
with an error response. This response MUST use an error code
of 401 (Unauthenticated).

* If the message is an indication, the agent MUST silently
discard the indication.

If these checks pass, the agent continues to process the request or
indication. Any response generated by a server to a request that
contains a MESSAGE-INTEGRITY-SHA256 attribute MUST include the
MESSAGE-INTEGRITY-SHA256 attribute, computed using the password
utilized to authenticate the request. Any response generated by a
server to a request that contains only a MESSAGE-INTEGRITY attribute
MUST include the MESSAGE-INTEGRITY attribute, computed using the
password utilized to authenticate the request. This means that only
one of these attributes can appear in a response. The response MUST
NOT contain the USERNAME attribute.

If any of the checks fail, a server MUST NOT include a MESSAGE-
INTEGRITY-SHA256, MESSAGE-INTEGRITY, or USERNAME attribute in the
error response. This is because, in these failure cases, the server
cannot determine the shared secret necessary to compute the MESSAGE-
INTEGRITY-SHA256 or MESSAGE-INTEGRITY attributes.

9.1.4. Receiving a Response

The client looks for the MESSAGE-INTEGRITY or the MESSAGE-INTEGRITY-
SHA256 attribute in the response. If present and if the client only
sent one of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attributes in the request (because of the external indication in
Section 9.1.2 or because this is a subsequent request as defined in
Section 9.1.5), the algorithm in the response has to match;
otherwise, the response MUST be discarded.

The client then computes the message integrity over the response as
defined in Section 14.5 for the MESSAGE-INTEGRITY attribute or
Section 14.6 for the MESSAGE-INTEGRITY-SHA256 attribute, using the
same password it utilized for the request. If the resulting value
matches the contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, respectively, the response is considered
authenticated. If the value does not match, or if both MESSAGE-
INTEGRITY and MESSAGE-INTEGRITY-SHA256 are absent, the processing
depends on whether the request was sent over a reliable or an
unreliable transport.

If the request was sent over an unreliable transport, the response
MUST be discarded, as if it had never been received. This means that
retransmits, if applicable, will continue. If all the responses
received are discarded, then instead of signaling a timeout after
ending the transaction, the layer MUST signal that the integrity
protection was violated.

If the request was sent over a reliable transport, the response MUST
be discarded, and the layer MUST immediately end the transaction and
signal that the integrity protection was violated.

9.1.5. Sending Subsequent Requests

A client sending subsequent requests to the same server MUST send
only the MESSAGE-INTEGRITY-SHA256 or the MESSAGE-INTEGRITY attribute
that matches the attribute that was received in the response to the
initial request. Here, "same server" means same IP address and port
number, not just the same URI or SRV lookup result.

9.2. Long-Term Credential Mechanism

The long-term credential mechanism relies on a long-term credential,
in the form of a username and password that are shared between client
and server. The credential is considered long-term since it is
assumed that it is provisioned for a user and remains in effect until
the user is no longer a subscriber of the system or until it is
changed. This is basically a traditional "log-in" username and
password given to users.

Because these usernames and passwords are expected to be valid for
extended periods of time, replay prevention is provided in the form
of a digest challenge. In this mechanism, the client initially sends
a request, without offering any credentials or any integrity checks.
The server rejects this request, providing the user a realm (used to
guide the user or agent in selection of a username and password) and
a nonce. The nonce provides a limited replay protection. It is a
cookie, selected by the server and encoded in such a way as to
indicate a duration of validity or client identity from which it is
valid. Only the server needs to know about the internal structure of
the cookie. The client retries the request, this time including its
username and the realm and echoing the nonce provided by the server.
The client also includes one of the message-integrity attributes
defined in this document, which provides an HMAC over the entire
request, including the nonce. The server validates the nonce and
checks the message integrity. If they match, the request is
authenticated. If the nonce is no longer valid, it is considered
"stale", and the server rejects the request, providing a new nonce.

In subsequent requests to the same server, the client reuses the
nonce, username, realm, and password it used previously. In this
way, subsequent requests are not rejected until the nonce becomes
invalid by the server, in which case the rejection provides a new
nonce to the client.

Note that the long-term credential mechanism cannot be used to
protect indications, since indications cannot be challenged. Usages
utilizing indications must either use a short-term credential or omit
authentication and message integrity for them.

To indicate that it supports this specification, a server MUST
prepend the NONCE attribute value with the character string composed
of "obMatJos2" concatenated with the (4-character) base64 [RFC4648]
encoding of the 24-bit STUN Security Features as defined in
Section 18.1. The 24-bit Security Feature set is encoded as 3 bytes,
with bit 0 as the most significant bit of the first byte and bit 23
as the least significant bit of the third byte. If no security
features are used, then a byte array with all 24 bits set to zero
MUST be encoded instead. For the remainder of this document, the
term "nonce cookie" will refer to the complete 13-character string
prepended to the NONCE attribute value.

Since the long-term credential mechanism is susceptible to offline
dictionary attacks, deployments SHOULD utilize passwords that are
difficult to guess. In cases where the credentials are not entered
by the user, but are rather placed on a client device during device
provisioning, the password SHOULD have at least 128 bits of
randomness. In cases where the credentials are entered by the user,
they should follow best current practices around password structure.

9.2.1. Bid-Down Attack Prevention

This document introduces two new security features that provide the
ability to choose the algorithm used for password protection as well
as the ability to use an anonymous username. Both of these
capabilities are optional in order to remain backwards compatible
with previous versions of the STUN protocol.

These new capabilities are subject to bid-down attacks whereby an
attacker in the message path can remove these capabilities and force
weaker security properties. To prevent these kinds of attacks from
going undetected, the nonce is enhanced with additional information.

The value of the "nonce cookie" will vary based on the specific STUN
Security Feature bits selected. When this document makes reference
to the "nonce cookie" in a section discussing a specific STUN
Security Feature it is understood that the corresponding STUN
Security Feature bit in the "nonce cookie" is set to 1.

For example, in Section 9.2.4, which discusses when the PASSWORD-
ALGORITHMS PASSWORD-ALGORITHMS security feature, it feature (defined
in Section 9.2.4) is implied that used, the corresponding "Password algorithms" bit, as defined
bit (defined in Section 18.1, 18.1) is set to 1 in the "nonce cookie".

9.2.2. HMAC Key

For long-term credentials that do not use a different algorithm, as
specified by the PASSWORD-ALGORITHM attribute, the key is 16 bytes:

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

Where MD5 is defined in [RFC1321] and [RFC6151], and the OpaqueString
profile is defined in [RFC8265]. The encoding used is UTF-8
[RFC3629].

The 16-byte key is formed by taking the MD5 hash of the result of
concatenating the following five fields: (1) the username, with any
quotes and trailing nulls removed, as taken from the USERNAME
attribute (in which case OpaqueString has already been applied); (2)
a single colon; (3) the realm, with any quotes and trailing nulls
removed and after processing using OpaqueString; (4) a single colon;
and (5) the password, with any trailing nulls removed and after
processing using OpaqueString. For example, if the username is
'user', the realm is 'realm', and the password is 'pass', then the
16-byte HMAC key would be the result of performing an MD5 hash on the
string 'user:realm:pass', the resulting hash being
0x8493fbc53ba582fb4c044c456bdc40eb.

The structure of the key when used with long-term credentials
facilitates deployment in systems that also utilize SIP [RFC3261].
Typically, SIP systems utilizing SIP's digest authentication
mechanism do not actually store the password in the database.
Rather, they store a value called "H(A1)", which is equal to the key
defined above. For example, this mechanism can be used with the
authentication extensions defined in [RFC5090].

When a PASSWORD-ALGORITHM is used, the key length and algorithm to
use are described in Section 18.5.1.

9.2.3. Forming a Request

There are two cases when forming a request. In the first case, this
is the

The first request from the client to the server (as identified by
hostname if the DNS procedures of Section 8 are used and by IP
address if not). In not) is handled according to the second case, rules in Section 9.2.3.1.
When the client is submitting initiates a subsequent request once a previous
request/response transaction has completed successfully. successfully, it follows
the rules in Section Section 9.2.3.2. Forming a request as a
consequence of a 401 (Unauthenticated) or 438 (Stale Nonce) error
response is covered in Section 9.2.5 and is not considered a
"subsequent request" and thus does not utilize the rules described in
Section 9.2.3.2.

The difference between a first request and a subsequent request is
the presence or absence Each of some attributes, so omitting or including
them is these types of requests have a MUST. different
mandatory attributes.

9.2.3.1. First Request

If the client has not completed a successful request/response
transaction with the server, it MUST omit the USERNAME, USERHASH,
MESSAGE-INTEGRITY, MESSAGE-INTEGRITY-SHA256, REALM, NONCE, PASSWORD-
ALGORITHMS, and PASSWORD-ALGORITHM attributes. In other words, the
first request is sent as if there were no authentication or message
integrity applied.

9.2.3.2. Subsequent Requests

Once a request/response transaction has completed, the client will
have been presented a realm and nonce by the server and selected a
username and password with which it authenticated. The client SHOULD
cache the username, password, realm, and nonce for subsequent
communications with the server. When the client sends a subsequent
request, it MUST include either the USERNAME or USERHASH, REALM,
NONCE, and PASSWORD-ALGORITHM attributes with these cached values.
It MUST include a MESSAGE-INTEGRITY attribute or a MESSAGE-INTEGRITY-
SHA256 attribute, computed as described in Sections 14.5 and 14.6
using the cached password. The choice between the two attributes
depends on the attribute received in the response to the first
request.

9.2.4. Receiving a Request

After the server has done the basic processing of a request, it
performs the checks listed below in the order specified. Note that
it is RECOMMENDED that the REALM value be the domain name of the
provider of the STUN server:

o If the message does not contain a MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute, the server MUST generate an error
response with an error code of 401 (Unauthenticated). This
response MUST include a REALM value. The response MUST include a
NONCE, selected by the server. The server MUST NOT choose the
same NONCE for two requests unless they have the same source IP
address and port. The server MAY support alternate password
algorithms, in which case it can list them in preferential order
in a PASSWORD-ALGORITHMS attribute. If the server adds a
PASSWORD-ALGORITHMS attribute, it MUST set the STUN Security
Feature "Password algorithms" bit set to 1. The server MAY support
anonymous username, in which case it MUST set the STUN Security
Feature "Username anonymity" bit set to 1. The response SHOULD
NOT contain a USERNAME, USERHASH, MESSAGE-INTEGRITY, or
MESSAGE-INTEGRITY-SHA256 MESSAGE-
INTEGRITY-SHA256 attribute.

Note: Reusing a NONCE for different source IP addresses or ports
was not explicitly forbidden in [RFC5389].

o If the message contains a MESSAGE-INTEGRITY or a MESSAGE-
INTEGRITY-SHA256 attribute, but is missing either the USERNAME or
USERHASH, REALM, or NONCE attribute, the server MUST generate an
error response with an error code of 400 (Bad Request). This
response SHOULD NOT include a USERNAME, USERHASH, NONCE, or REALM
attribute. The response cannot contain a MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, as the attributes required to
generate them are missing.

o If the NONCE attribute starts with the "nonce cookie" with the
STUN Security Feature "Password algorithms" bit set to 1, the
server performs these checks in the order specified:

* If the request contains neither the PASSWORD-ALGORITHMS nor
PASSWORD-ALGORITHM, the
PASSWORD-ALGORITHM algorithm, then the request is processed as
though PASSWORD-ALGORITHM were MD5. (Note that if the PASSWORD-
ALGORITHMS attribute is present but does not contain MD5, this
will result in an error code of 400 (Bad Request) in a later
step.)
* Otherwise, unless (1) PASSWORD-ALGORITHM and

* Otherwise, unless (1) PASSWORD-ALGORITHM and PASSWORD-
ALGORITHMS are both present, (2) PASSWORD-ALGORITHMS matches
the value sent in the response that sent this NONCE, and (3)
PASSWORD-ALGORITHM matches one of the entries in PASSWORD-
ALGORITHMS, the server MUST generate an error response with an
error code of 400 (Bad Request).

o If the NONCE is no longer valid and the MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute is invalid, the server MUST
generate an error response with an error code of 401
(Unauthenticated). This response MUST include NONCE, REALM, and
PASSWORD-ALGORITHMS attributes and SHOULD NOT include the USERNAME
or USERHASH attribute. The NONCE attribute value MUST be valid.
The response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, using the previous NONCE to calculate it.

o If the NONCE is no longer valid, the server MUST generate an error
response with an error code of 438 (Stale Nonce). This response
MUST include NONCE, REALM, and PASSWORD-ALGORITHMS attributes and
SHOULD NOT include the USERNAME and USERHASH attributes. The
NONCE attribute value MUST be valid. The response MAY include a
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, using the
previous NONCE to calculate it. Servers can revoke nonces in
order to provide additional security. See Section 5.4 of
[RFC7616] for guidelines.

o If the value of the USERNAME or USERHASH attribute is not valid,
the server MUST generate an error response with an error code of
401 (Unauthenticated). This response MUST include a REALM value.
The response MUST include a NONCE, selected by the server. The
response MUST include a PASSWORD-ALGORITHMS attribute. The
response SHOULD NOT contain a USERNAME or USERHASH attribute. The
response MAY include a MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute, using the previous key to calculate it.

o If the MESSAGE-INTEGRITY-SHA256 attribute is present, compute the
value for the message integrity as described in Section 14.6,
using the password associated with the username. Else, Otherwise, using
the same password, compute the value for the message integrity MESSAGE-INTEGRITY
attribute as described in Section 14.5. If the resulting value
does not match the contents of the MESSAGE-INTEGRITY attribute or
the MESSAGE-
INTEGRITY-SHA256 MESSAGE-INTEGRITY-SHA256 attribute, the server MUST reject the
request with an error response. This response MUST use an error
code of 401 (Unauthenticated). It MUST include the REALM and
NONCE attributes and SHOULD NOT include the USERNAME, USERHASH, MESSAGE-
INTEGRITY,
MESSAGE-INTEGRITY, or MESSAGE-INTEGRITY-SHA256 attribute.

If these checks pass, the server continues to process the request.
Any response generated by the server MUST include the MESSAGE-
INTEGRITY-SHA256 attribute, computed using the username and password
utilized to authenticate the request, unless the request was
processed as though PASSWORD-ALGORITHM was MD5 (because the request
contained neither PASSWORD-ALGORITHMS nor PASSWORD-ALGORITHM). In
that case, the MESSAGE-INTEGRITY attribute MUST be used instead of
the MESSAGE-INTEGRITY-SHA256 attribute. The attribute, and the REALM, NONCE,
USERNAME, and USERHASH attributes SHOULD NOT be included.

9.2.5. Receiving a Response

If the response is an error response with an error code of 401
(Unauthenticated) or 438 (Stale Nonce), the client MUST test if the
NONCE attribute value starts with the "nonce cookie". If the test
succeeds so and the
"nonce cookie" has the STUN Security Feature "Password algorithms"
bit set to 1 but no PASSWORD-ALGORITHMS attribute is present, then
the client MUST NOT retry the request with a new transaction.

If the response is an error response with an error code of 401
(Unauthenticated), the client SHOULD retry the request with a new
transaction. This request MUST contain a USERNAME or a USERHASH,
determined by the client as the appropriate username for the REALM
from the error response. If the "nonce cookie" is present and has
the STUN Security Feature "Username anonymity" bit set to 1, then the
USERHASH attribute MUST be used; else, the USERNAME attribute MUST be
used. The request MUST contain the REALM, copied from the error
response. The request MUST contain the NONCE, copied from the error
response. If the response contains a PASSWORD-ALGORITHMS attribute,
the request MUST contain the PASSWORD-ALGORITHMS attribute with the
same content. If the response contains a PASSWORD-ALGORITHMS
attribute, and this attribute contains at least one algorithm that is
supported by the client, then the request MUST contain a PASSWORD-
ALGORITHM attribute with the first algorithm supported on the list.
If the response contains a PASSWORD-ALGORITHMS attribute, and this
attribute does not contain any algorithm that is supported by the
client, then the client MUST NOT retry the request with a new
transaction. The client MUST NOT perform this retry if it is not
changing the USERNAME, USERHASH, REALM, or its associated password
from the previous attempt.

If the response is an error response with an error code of 438 (Stale
Nonce), the client MUST retry the request, using the new NONCE
attribute supplied in the 438 (Stale Nonce) response. This retry
MUST also include either the USERNAME or USERHASH, the REALM, and
either the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute.

For all other responses, if the NONCE attribute starts with the
"nonce cookie" with the STUN Security Feature "Password algorithms"
bit set to 1 but PASSWORD-ALGORITHMS is not present, the response
MUST be ignored.

If the response is an error response with an error code of 400 (Bad
Request) and does not contain either the MESSAGE-INTEGRITY or
MESSAGE-INTEGRITY-SHA256 attribute, then the response MUST be
discarded, as if it were never received. This means that
retransmits, if applicable, will continue.

Note: In this case, the 400 response will never reach the
application, resulting in a timeout.

The client looks for the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-
SHA256 attribute in the response (either success or failure). If
present, the client computes the message integrity over the response
as defined in Sections 14.5 or 14.6, using the same password it
utilized for the request. If the resulting value matches the
contents of the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256
attribute, the response is considered authenticated. If the value
does not match, or if both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-
SHA256 are absent, the processing depends on the request being sent
over a reliable or an unreliable transport.

If the request was sent over a reliable transport, the response MUST
be discarded, and the layer MUST immediately end the transaction and
signal that the integrity protection was violated.

If the response contains a PASSWORD-ALGORITHMS attribute, all the
subsequent requests MUST be authenticated using MESSAGE-INTEGRITY-
SHA256 only.

10. ALTERNATE-SERVER Mechanism

This section describes a mechanism in STUN that allows a server to
redirect a client to another server. This extension is optional, and
a usage must define if and when this extension is used. The
ALTERNATE-SERVER attribute carries an IP address.

A server using this extension redirects a client to another server by
replying to a request message with an error response message with an
error code of 300 (Try Alternate). The server MUST include at least
one ALTERNATE-SERVER attribute in the error response, which MUST
contain an IP address of the same address family as the source IP
address of the request message. The server SHOULD include an
additional ALTERNATE-SERVER attribute, after the mandatory one, that
contains an IP address of the address family other than the source IP
address of the request message. The error response message MAY be
authenticated; however, there are use cases for ALTERNATE-SERVER
where authentication of the response is not possible or practical.
If the transaction uses TLS or DTLS, if the transaction is
authenticated by a MESSAGE-INTEGRITY-SHA256 attribute, and if the
server wants to redirect to a server that uses a different
certificate, then it MUST include an ALTERNATE-DOMAIN attribute
containing the name inside the subjectAltName of that certificate.
This series of conditions on the MESSAGE-INTEGRITY-SHA256 attribute
indicates that the transaction is authenticated and that the client
implements this specification and therefore can process the
ALTERNATE-DOMAIN attribute.

A client using this extension handles a 300 (Try Alternate) error
code as follows. The client looks for an ALTERNATE-SERVER attribute
in the error response. If one is found, then the client considers
the current transaction as failed and reattempts the request with the
server specified in the attribute, using the same transport protocol
used for the previous request. That request, if authenticated, MUST
utilize the same credentials that the client would have used in the
request to the server that performed the redirection. If the
transport protocol uses TLS or DTLS, then the client looks for an
ALTERNATE-DOMAIN attribute. If the attribute is found, the domain
MUST be used to validate the certificate using the recommendations in
[RFC6125]. The certificate MUST contain an identifier of type DNS-ID
or CN-ID (eventually with wildcards) but not of type SRV-ID or URI-
ID. If the attribute is not found, the same domain that was used for
the original request MUST be used to validate the certificate. If
the client has been redirected to a server to which it has already
sent this request within the last five minutes, it MUST ignore the
redirection and consider the transaction to have failed. This
prevents infinite ping-ponging between servers in case of redirection
loops.

11. Backwards Compatibility with RFC 3489

In addition to the backward compatibility already described in
Section 12 of [RFC5389], DTLS MUST NOT be used with [RFC3489]
(referred to as "classic STUN"). Any STUN request or indication
without the magic cookie (see Section 6 of [RFC5389]) over DTLS MUST
be considered invalid: all requests MUST generate a 500 (Server
Error) error response, and indications MUST be ignored.

12. Basic Server Behavior

This section defines the behavior of a basic, stand-alone STUN
server.

Historically, "classic STUN" [RFC3489] only defined the behavior of a
server that was providing clients with server reflexive transport
addresses by receiving and replying to STUN Binding requests.
[RFC5389] redefined the protocol as an extensible framework, and the
server functionality became the sole STUN Usage defined in that
document. This STUN Usage is also known as "Basic STUN Server".

The STUN server MUST support the Binding method. It SHOULD NOT
utilize the short-term or long-term credential mechanism. This is
because the work involved in authenticating the request is more than
the work in simply processing it. It SHOULD NOT utilize the
ALTERNATE-SERVER mechanism for the same reason. It MUST support UDP
and TCP. It MAY support STUN over TCP/TLS or STUN over UDP/DTLS;
however, DTLS and TLS provide minimal security benefits in this basic
mode of operation. It does not require a keep-alive mechanism
because a TCP or TLS-over-TCP connection is closed after the end of
the Binding transaction. It MAY utilize the FINGERPRINT mechanism
but MUST NOT require it. Since the stand-alone server only runs
STUN, FINGERPRINT provides no benefit. Requiring it would break
compatibility with RFC 3489, and such compatibility is desirable in a
stand-alone server. Stand-alone STUN servers SHOULD support
backwards compatibility with clients using [RFC3489], as described in
Section 11.

It is RECOMMENDED that administrators of STUN servers provide DNS
entries for those servers as described in Section 8. If both A and
AAAA resource records are returned, then the client can
simultaneously send STUN Binding requests to the IPv4 and IPv6
addresses (as specified in [RFC8305]), as the Binding request is
idempotent. Note that the MAPPED-ADDRESS or XOR-MAPPED-ADDRESS
attributes that are returned will not necessarily match the address
family of the server address used.

A basic STUN server is not a solution for NAT traversal by itself.
However, it can be utilized as part of a solution through STUN
Usages. This is discussed further in Section 13.

13. STUN Usages

STUN by itself is not a solution to the NAT traversal problem.
Rather, STUN defines a tool that can be used inside a larger
solution. The term "STUN Usage" is used for any solution that uses
STUN as a component.

A STUN Usage defines how STUN is actually utilized -- when to send
requests, what to do with the responses, and which optional
procedures defined here (or in an extension to STUN) are to be used.
A usage also defines:

o Which STUN methods are used.

o What transports are used. If DTLS-over-UDP is used, then
implementing the denial-of-service countermeasure described in
Section 4.2.1 of [RFC6347] is mandatory.

o What authentication and message-integrity mechanisms are used.

o The considerations around manual vs. automatic key derivation for
the integrity mechanism, as discussed in [RFC4107].

o What mechanisms are used to distinguish STUN messages from other
messages. When STUN is run over TCP or TLS-over-TCP, a framing
mechanism may be required.

o How a STUN client determines the IP address and port of the STUN
server.

o How simultaneous use of IPv4 and IPv6 addresses (Happy Eyeballs
[RFC8305]) works with non-idempotent transactions when both
address families are found for the STUN server.

o Whether backwards compatibility to RFC 3489 is required.

o What optional attributes defined here (such as FINGERPRINT and
ALTERNATE-SERVER) or in other extensions are required.

o If MESSAGE-INTEGRITY-SHA256 truncation is permitted, and the
limits permitted for truncation.

o The keep-alive mechanism if STUN is run over TCP or TLS-over-TCP.

o If anycast addresses can be used for the server in case 1) TCP or
TLS-over-TCP or 2) authentication is used.

In addition, any STUN Usage must consider the security implications
of using STUN in that usage. A number of attacks against STUN are
known (see the Security Considerations section in this document), and
any usage must consider how these attacks can be thwarted or
mitigated.

Finally, a usage must consider whether its usage of STUN is an
example of the Unilateral Self-Address Fixing approach to NAT
traversal and, if so, address the questions raised in RFC 3424
[RFC3424].

14. STUN Attributes

After the STUN header are zero or more attributes. Each attribute
MUST be TLV encoded, with a 16-bit type, 16-bit length, and value.
Each STUN attribute MUST end on a 32-bit boundary. As mentioned
above, all fields in an attribute are transmitted most significant
bit first.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value (variable) ....
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: Format of STUN Attributes

The value in the Length field MUST contain the length of the Value
part of the attribute, prior to padding, measured in bytes. Since
STUN aligns attributes on 32-bit boundaries, attributes whose content
is not a multiple of 4 bytes are padded with 1, 2, or 3 bytes of
padding so that its value contains a multiple of 4 bytes. The
padding bits MUST be set to zero on sending and MUST be ignored by
the receiver.

Any attribute type MAY appear more than once in a STUN message.
Unless specified otherwise, the order of appearance is significant:
only the first occurrence needs to be processed by a receiver, and
any duplicates MAY be ignored by a receiver.

To allow future revisions of this specification to add new attributes
if needed, the attribute space is divided into two ranges.
Attributes with type values between 0x0000 and 0x7FFF are
comprehension-required attributes, which means that the STUN agent
cannot successfully process the message unless it understands the
attribute. Attributes with type values between 0x8000 and 0xFFFF are
comprehension-optional attributes, which means that those attributes
can be ignored by the STUN agent if it does not understand them.

The set of STUN attribute types is maintained by IANA. The initial
set defined by this specification is found in Section 18.3.

The rest of this section describes the format of the various
attributes defined in this specification.

14.1. MAPPED-ADDRESS

The MAPPED-ADDRESS attribute indicates a reflexive transport address
of the client. It consists of an 8-bit address family and a 16-bit
port, followed by a fixed-length value representing the IP address.
If the address family is IPv4, the address MUST be 32 bits. If the
address family is IPv6, the address MUST be 128 bits. All fields
must be in network byte order.

The format of the MAPPED-ADDRESS attribute is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Family | Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Address (32 bits or 128 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 5: Format of MAPPED-ADDRESS Attribute

The address family can take on the following values:

0x01:IPv4
0x02:IPv6

The first 8 bits of the MAPPED-ADDRESS MUST be set to 0 and MUST be
ignored by receivers. These bits are present for aligning parameters
on natural 32-bit boundaries.

This attribute is used only by servers for achieving backwards
compatibility with [RFC3489] clients.

14.2. XOR-MAPPED-ADDRESS

The XOR-MAPPED-ADDRESS attribute is identical to the MAPPED-ADDRESS
attribute, except that the reflexive transport address is obfuscated
through the XOR function.

The format of the XOR-MAPPED-ADDRESS is:

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0| Family | X-Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| X-Address (Variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 6: Format of XOR-MAPPED-ADDRESS Attribute

The Family field represents the IP address family and is encoded
identically to the Family field in MAPPED-ADDRESS.

X-Port is computed by XOR'ing the mapped port with the most
significant 16 bits of the magic cookie. If the IP address family is
IPv4, X-Address is computed by XOR'ing the mapped IP address with the
magic cookie. If the IP address family is IPv6, X-Address is
computed by XOR'ing the mapped IP address with the concatenation of
the magic cookie and the 96-bit transaction ID. In all cases, the
XOR operation works on its inputs in network byte order (that is, the
order they will be encoded in the message).

The rules for encoding and processing the first 8 bits of the
attribute's value, the rules for handling multiple occurrences of the
attribute, and the rules for processing address families are the same
as for MAPPED-ADDRESS.

Note: XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their
encoding of the transport address. The former encodes the transport
address by XOR'ing it with the magic cookie. The latter encodes it
directly in binary. RFC 3489 originally specified only MAPPED-
ADDRESS. However, deployment experience found that some NATs rewrite
the 32-bit binary payloads containing the NAT's public IP address,
such as STUN's MAPPED-ADDRESS attribute, in the well-meaning but
misguided attempt to provide a generic Application Layer Gateway
(ALG) function. Such behavior interferes with the operation of STUN
and also causes failure of STUN's message-integrity checking.

14.3. USERNAME

The USERNAME attribute is used for message integrity. It identifies
the username and password combination used in the message-integrity
check.

The value of USERNAME is a variable-length value containing the
authentication username. It MUST contain a UTF-8-encoded [RFC3629]
sequence of less fewer than 509 bytes and MUST have been processed using
the UsernameCasePreserved OpaqueString profile [RFC8265]. A compliant implementation MUST
be able to parse a UTF-8-encoded sequence of 763 or less bytes fewer octets to
be compatible with [RFC5389], which [RFC5389].

Note: [RFC5389] mistakenly referenced the definition of UTF-8 in
[RFC2279]. [RFC2279] assumed up to 6 bytes octets per characters
encoded. [RFC2279] was replaced by [RFC3629], which allows only 4
octets per character encoded, consistent with changes made in
Unicode 2.0 and ISO/IEC 10646.

Note: This specification uses the OpaqueString profile instead of
the UsernameCasePreserved profile for username string processing
in order to improve compatibility with deployed password stores.
Many password databases used for HTTP and SIP Digest
authentication store the MD5 hash of username:realm:password
instead of storing a plain text password. In [RFC3489], STUN
authentication was designed to be compatible with these existing
databases to the extent possible, which like SIP and HTTP
performed no pre-processing of usernames and passwords other than
prohibiting non-space ASCII control characters. The next revision
of the STUN specification, [RFC5389], used the SASLprep [RFC4013]
stringprep [RFC3454] profile to pre-process usernames and
passwords. SASLprep uses Unicode Normalization Form KC
(Compatibility Decomposition, followed by Canonical Composition)
[UAX15] and prohibits various control, space, and non-text,
deprecated, or inappropriate codepoints. The PRECIS framework
[RFC8264] obsoletes stringprep. PRECIS handling of usernames and
passwords [RFC8265] uses Unicode Normalization Form C (Canonical
Decomposition, followed by Canonical Composition). While there
are specific cases where different username strings under HTTP
Digest could be mapped to a single STUN username processed with
OpaqueString, these cases are extremely unlikely and easy to
detect and correct. With a UsernameCasePreserved profile, it
would be more likely that valid usernames under HTTP Digest would
not match their processed forms (specifically usernames containing
bidirectional text and compatibility forms). Operators are free
to further restrict the allowed codepoints in usernames to avoid
problematic characters.

14.4. USERHASH

The USERHASH attribute is used as a replacement for the USERNAME
attribute when username anonymity is supported.

The value of USERHASH has a fixed length of 32 bytes. The username
MUST have been processed using the UsernameCasePreserved OpaqueString profile [RFC8265],
and the realm MUST have been processed using the OpaqueString profile
[RFC8265] before hashing.

The following is the operation that the client will perform to hash
the username:

userhash = SHA-256(UsernameCasePreserved(username) SHA-256(OpaqueString(username) ":" OpaqueString(realm))

14.5. MESSAGE-INTEGRITY

The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [RFC2104] of
the STUN message. The MESSAGE-INTEGRITY attribute can be present in
any STUN message type. Since it uses the SHA-1 hash, the HMAC will
be 20 bytes.

The key for the HMAC depends on which credential mechanism is in use.
Section 9.1.1 defines the key for the short-term credential
mechanism, and Section 9.2.2 defines the key for the long-term
credential mechanism. Other credential mechanisms MUST define the
key that is used for the HMAC.

The text used as input to HMAC is the STUN message, up to and
including the attribute preceding the MESSAGE-INTEGRITY attribute.
The Length field of the STUN message header is adjusted to point to
the end of the MESSAGE-INTEGRITY attribute. The value of the
MESSAGE-INTEGRITY attribute is set to a dummy value.

Once the computation is performed, the value of the MESSAGE-INTEGRITY
attribute is filled in, and the value of the length in the STUN
header is set to its correct value -- the length of the entire
message. Similarly, when validating the MESSAGE-INTEGRITY, the
Length field in the STUN header must be adjusted to point to the end
of the MESSAGE-INTEGRITY attribute prior to calculating the HMAC over
the STUN message, up to and including the attribute preceding the
MESSAGE-INTEGRITY attribute. Such adjustment is necessary when
attributes, such as FINGERPRINT and MESSAGE-INTEGRITY-SHA256, appear
after MESSAGE-INTEGRITY. See also [RFC5769] for examples of such
calculations.

14.6. MESSAGE-INTEGRITY-SHA256

The MESSAGE-INTEGRITY-SHA256 attribute contains an HMAC-SHA256
[RFC2104] of the STUN message. The MESSAGE-INTEGRITY-SHA256
attribute can be present in any STUN message type. The MESSAGE-
INTEGRITY-SHA256 attribute contains an initial portion of the HMAC-
SHA-256 [RFC2104] of the STUN message. The value will be at most 32
bytes, but it MUST be at least 16 bytes and MUST be a multiple of 4
bytes. The value must be the full 32 bytes unless the STUN Usage
explicitly specifies that truncation is allowed. STUN Usages may
specify a minimum length longer than 16 bytes.

The text used as input to HMAC is the STUN message, up to and
including the attribute preceding the MESSAGE-INTEGRITY-SHA256
attribute. The Length field of the STUN message header is adjusted
to point to the end of the MESSAGE-INTEGRITY-SHA256 attribute. The
value of the MESSAGE-INTEGRITY-SHA256 attribute is set to a dummy
value.

Once the computation is performed, the value of the MESSAGE-
INTEGRITY-SHA256 attribute is filled in, and the value of the length
in the STUN header is set to its correct value -- the length of the
entire message. Similarly, when validating the MESSAGE-INTEGRITY-
SHA256, the Length field in the STUN header must be adjusted to point
to the end of the MESSAGE-INTEGRITY-SHA256 attribute prior to
calculating the HMAC over the STUN message, up to and including the
attribute preceding the MESSAGE-INTEGRITY-SHA256 attribute. Such
adjustment is necessary when attributes, such as FINGERPRINT, appear
after MESSAGE-INTEGRITY-SHA256. See also Appendix B.1 for examples
of such calculations.

14.7. FINGERPRINT

The FINGERPRINT attribute MAY be present in all STUN messages.

The value of the attribute is computed as the CRC-32 of the STUN
message up to (but excluding) the FINGERPRINT attribute itself,
XOR'ed with the 32-bit value 0x5354554e. (The XOR operation ensures
that the FINGERPRINT test will not report a false positive on a
packet containing a CRC-32 generated by an application protocol.)
The 32-bit CRC is the one defined in ITU V.42 [ITU.V42.2002], which
has a generator polynomial of x^32 + x^26 + x^23 + x^22 + x^16 + x^12
+ x^11 + x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1. See the sample
code for the CRC-32 in Section 8 of [RFC1952].

When present, the FINGERPRINT attribute MUST be the last attribute in
the message and thus will appear after MESSAGE-INTEGRITY and MESSAGE-
INTEGRITY-SHA256.

The FINGERPRINT attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 7.

As with MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256, the CRC used
in the FINGERPRINT attribute covers the Length field from the STUN
message header. Therefore, prior to computation of the CRC, this
value must be correct and include the CRC attribute as part of the
message length. When using the FINGERPRINT attribute in a message,
the attribute is first placed into the message with a dummy value;
then, the CRC is computed, and the value of the attribute is updated.
If the MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute is
also present, then it must be present with the correct message-
integrity value before the CRC is computed, since the CRC is done
over the value of the MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256
attributes as well.

14.8. ERROR-CODE

The ERROR-CODE attribute is used in error response messages. It
contains a numeric error code value in the range of 300 to 699 plus a
textual reason phrase encoded in UTF-8 [RFC3629]; it is also
consistent in its code assignments and semantics with SIP [RFC3261]
and HTTP [RFC7231]. The reason phrase is meant for diagnostic
purposes and can be anything appropriate for the error code.
Recommended reason phrases for the defined error codes are included
in the IANA registry for error codes. The reason phrase MUST be a
UTF-8-encoded [RFC3629] sequence of less fewer than 128 characters (which
can be as long as 509 bytes when encoding them or 763 bytes when
decoding them).

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved, should be 0 |Class| Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reason Phrase (variable) ..
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 7: Format of ERROR-CODE Attribute

To facilitate processing, the class of the error code (the hundreds
digit) is encoded separately from the rest of the code, as shown in
Figure 7.

The Reserved bits SHOULD be 0 and are for alignment on 32-bit
boundaries. Receivers MUST ignore these bits. The Class represents
the hundreds digit of the error code. The value MUST be between 3
and 6. The Number represents the binary encoding of the error code
modulo 100, and its value MUST be between 0 and 99.

The following error codes, along with their recommended reason
phrases, are defined:

300 Try Alternate: The client should contact an alternate server for
this request. This error response MUST only be sent if the
request included either a USERNAME or USERHASH attribute and a
valid MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute;
otherwise, it MUST NOT be sent and error code 400 (Bad Request)
is suggested. This error response MUST be protected with the
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 attribute, and
receivers MUST validate the MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 of this response before redirecting themselves
to an alternate server.

Note: Failure to generate and validate message integrity for a
300 response allows an on-path attacker to falsify a 300
response thus causing subsequent STUN messages to be sent to a
victim.

400 Bad Request: The request was malformed. The client SHOULD NOT
retry the request without modification from the previous
attempt. The server may not be able to generate a valid
MESSAGE-INTEGRITY or MESSAGE-INTEGRITY-SHA256 for this error, so
the client MUST NOT expect a valid MESSAGE-INTEGRITY or MESSAGE-
INTEGRITY-SHA256 attribute on this response.

401 Unauthenticated: The request did not contain the correct
credentials to proceed. The client should retry the request
with proper credentials.

420 Unknown Attribute: The server received a STUN packet containing
a comprehension-required attribute that it did not understand.
The server MUST put this unknown attribute in the UNKNOWN-
ATTRIBUTE attribute of its error response.

438 Stale Nonce: The NONCE used by the client was no longer valid.
The client should retry, using the NONCE provided in the
response.

500 Server Error: The server has suffered a temporary error. The
client should try again.

14.9. REALM

The REALM attribute may be present in requests and responses. It
contains text that meets the grammar for "realm-value" as described
in [RFC3261] but without the double quotes and their surrounding
whitespace. That is, it is an unquoted realm-value (and is therefore
a sequence of qdtext or quoted-pair). It MUST be a UTF-8-encoded
[RFC3629] sequence of less fewer than 128 characters (which can be as long
as 509 bytes when encoding them and as long as 763 bytes when
decoding them) and MUST have been processed using the OpaqueString
profile [RFC8265].

Presence of the REALM attribute in a request indicates that long-term
credentials are being used for authentication. Presence in certain
error responses indicates that the server wishes the client to use a
long-term credential in that realm for authentication.

14.10. NONCE

The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
[RFC3261]. Note that this means that the NONCE attribute will not
contain the actual surrounding quote characters. The NONCE attribute
MUST be less fewer than 128 characters (which can be as long as 509 bytes
when encoding them and a long as 763 bytes when decoding them). See
Section 5.4 of [RFC7616] for guidance on selection of nonce values in
a server.

14.11. PASSWORD-ALGORITHMS

The PASSWORD-ALGORITHMS attribute may be present in requests and
responses. It contains the list of algorithms that the server can
use to derive the long-term password.

The set of known algorithms is maintained by IANA. The initial set
defined by this specification is found in Section 18.5.

The attribute contains a list of algorithm numbers and variable
length parameters. The algorithm number is a 16-bit value as defined
in Section 18.5. The parameters start with the length (prior to
padding) of the parameters as a 16-bit value, followed by the
parameters that are specific to each algorithm. The parameters are
padded to a 32-bit boundary, in the same manner as an attribute.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 | Algorithm 1 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 1 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 | Algorithm 2 Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm 2 Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ...

Figure 8: Format of PASSWORD-ALGORITHMS Attribute

14.12. PASSWORD-ALGORITHM

The PASSWORD-ALGORITHM attribute is present only in requests. It
contains the algorithms algorithm that the server must use to derive a key from
the long-term password.

The set of known algorithms is maintained by IANA. The initial set
defined by this specification is found in Section 18.5.

The attribute contains an algorithm number and variable length
parameters. The algorithm number is a 16-bit value as defined in
Section 18.5. The parameters starts with the length (prior to
padding) of the parameters as a 16-bit value, followed by the
parameters that are specific to the algorithm. The parameters are
padded to a 32-bit boundary, in the same manner as an attribute.
Similarly, the padding bits MUST be set to zero on sending and MUST
be ignored by the receiver.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm | Algorithm Parameters Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Algorithm Parameters (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 9: Format of PASSWORD-ALGORITHM Attribute

14.13. UNKNOWN-ATTRIBUTES

The UNKNOWN-ATTRIBUTES attribute is present only in an error response
when the response code in the ERROR-CODE attribute is 420 (Unknown
Attribute).

The attribute contains a list of 16-bit values, each of which
represents an attribute type that was not understood by the server.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 1 Type | Attribute 2 Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute 3 Type | Attribute 4 Type ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 10: Format of UNKNOWN-ATTRIBUTES Attribute

Note: In [RFC3489], this field was padded to 32 by duplicating the
last attribute. In this version of the specification, the normal
padding rules for attributes are used instead.

14.14. SOFTWARE

The SOFTWARE attribute contains a textual description of the software
being used by the agent sending the message. It is used by clients
and servers. Its value SHOULD include manufacturer and version
number. The attribute has no impact on operation of the protocol and
serves only as a tool for diagnostic and debugging purposes. The
value of SOFTWARE is variable length. It MUST be a UTF-8-encoded
[RFC3629] sequence of less fewer than 128 characters (which can be as long
as 509 when encoding them and as long as 763 bytes when decoding
them).

14.15. ALTERNATE-SERVER

The alternate server represents an alternate transport address
identifying a different STUN server that the STUN client should try.

It is encoded in the same way as MAPPED-ADDRESS and thus refers to a
single server by IP address.

14.16. ALTERNATE-DOMAIN

The alternate domain represents the domain name that is used to
verify the IP address in the ALTERNATE-SERVER attribute when the
transport protocol uses TLS or DTLS.

The value of ALTERNATE-DOMAIN is variable length. It MUST be a valid
DNS name [RFC1123] (including A-labels [RFC5890]) of 255 or less fewer
ASCII characters.

15. Operational Considerations

STUN MAY be used with anycast addresses, but only with UDP and in
STUN Usages where authentication is not used.

16. Security Considerations

Implementations and deployments of a STUN Usage using TLS or DTLS
MUST follow the recommendations in [BCP195].

Implementations and deployments of a STUN Usage using the long-term
credential mechanism (Section 9.2) MUST follow the recommendations in
Section 5 of [RFC7616].

16.1. Attacks against the Protocol

16.1.1. Outside Attacks

An attacker can try to modify STUN messages in transit, in order to
cause a failure in STUN operation. These attacks are detected for
both requests and responses through the message-integrity mechanism,
using either a short-term or long-term credential. Of course, once
detected, the manipulated packets will be dropped, causing the STUN
transaction to effectively fail. This attack is possible only by an
on-path attacker.

An attacker that can observe, but not modify, STUN messages in-
transit (for example, an attacker present on a shared access medium,
such as Wi-Fi) can see a STUN request and then immediately send a
STUN response, typically an error response, in order to disrupt STUN
processing. This attack is also prevented for messages that utilize
MESSAGE-INTEGRITY. However, some error responses, those related to
authentication in particular, cannot be protected by MESSAGE-
INTEGRITY. When STUN itself is run over a secure transport protocol
(e.g., TLS), these attacks are completely mitigated.

Depending on the STUN Usage, these attacks may be of minimal
consequence and thus do not require message integrity to mitigate.
For example, when STUN is used to a basic STUN server to discover a
server reflexive candidate for usage with ICE, authentication and
message integrity are not required since these attacks are detected
during the connectivity check phase. The connectivity checks
themselves, however, require protection for proper operation of ICE
overall. As described in Section 13, STUN Usages describe when
authentication and message integrity are needed.

Since STUN uses the HMAC of a shared secret for authentication and
integrity protection, it is subject to offline dictionary attacks.

When authentication is utilized, it SHOULD be with a strong password
that is not readily subject to offline dictionary attacks.
Protection of the channel itself, using TLS or DTLS, mitigates these
attacks.

STUN supports both MESSAGE-INTEGRITY and MESSAGE-INTEGRITY-SHA256,
which makes STUN subject to bid-down attacks by an on-path attacker.
An attacker could strip the MESSAGE-INTEGRITY-SHA256 attribute,
leaving only the MESSAGE-INTEGRITY attribute and thus exploiting a
potential vulnerability. Protection of the channel itself, using TLS
or DTLS, mitigates these attacks. Timely removal of the support of
MESSAGE-INTEGRITY in a future version of STUN is necessary.

Note: The use of SHA-256 for password hashing does not meet modern
standards, which are aimed at slowing down exhaustive password
searches by providing a relatively slow minimum time to compute the
hash. Although better algorithms such as Argon2 [Argon2] are
available, SHA-256 was chosen for consistency with [RFC7616].

16.1.2. Inside Attacks

A rogue client may try to launch a DoS attack against a server by
sending it a large number of STUN requests. Fortunately, STUN
requests can be processed statelessly by a server, making such
attacks hard to launch effectively.

A rogue client may use a STUN server as a reflector, sending it
requests with a falsified source IP address and port. In such a
case, the response would be delivered to that source IP and port.
There is no amplification of the number of packets with this attack
(the STUN server sends one packet for each packet sent by the
client), though there is a small increase in the amount of data,
since STUN responses are typically larger than requests. This attack
is mitigated by ingress source address filtering.

Revealing the specific software version of the agent through the
SOFTWARE attribute might allow them to become more vulnerable to
attacks against software that is known to contain security holes.
Implementers SHOULD make usage of the SOFTWARE attribute a
configurable option.

16.1.3. Bid-Down Attacks

This document adds the possibility of selecting different algorithms
to protect the confidentiality of the passwords stored on the server
side when using the long-term credential mechanism while still
ensuring compatibility with MD5, which was the algorithm used in a
previous version of this protocol.
[RFC5389]. This selection works by having the server send to the
client the list of algorithms supported in a PASSWORD-ALGORITHMS
attribute and having the client send back a PASSWORD-ALGORITHM
attribute containing the algorithm selected.

Because the PASSWORD-ALGORITHMS attribute has to be sent in an
unauthenticated response, an on-path attacker wanting to exploit an
eventual vulnerability in MD5 can just strip the PASSWORD-ALGORITHMS
attribute from the unprotected response, thus making the server
subsequently act as if the client was implementing a previous the version of
this protocol. protocol defined in [RFC5389].

To protect against this attack and other similar bid-down attacks,
the nonce is enriched with a set of security bits that indicates
which security features are in use. In the case of the selection of
the password algorithm, the matching bit is set in the nonce returned
by the server in the same response that contains the PASSWORD-
ALGORITHMS attribute. Because the nonce used in subsequent
authenticated transactions is verified by the server to be identical
to what was originally sent, it cannot be modified by an on-path
attacker. Additionally, the client is mandated to copy the received
PASSWORD-ALGORITHMS attribute in the next authenticated transaction
to that server.

An on-path attack that removes the PASSWORD-ALGORITHMS will be
detected because the client will not be able to send it back to the
server in the next authenticated transaction. The client will detect
that attack because the security bit is set but the matching
attribute is missing; this will end the session. A client using an
older version of this protocol will not send the PASSWORD-ALGORITHMS
back but can only use MD5 anyway, so the attack is inconsequential.

The on-path attack may also try to remove the security bit together
with the PASSWORD-ALGORITHMS attribute, but the server will discover
that when the next authenticated transaction contains an invalid
nonce.

An on-path attack that removes some algorithms from the PASSWORD-
ALGORITHMS attribute will be equally defeated because that attribute
will be different from the original one when the server verifies it
in the subsequent authenticated transaction.

Note that the bid-down protection mechanism introduced in this
document is inherently limited by the fact that it is not possible to
detect an attack until the server receives the second request after
the 401 (Unauthenticated) response.

SHA-256 was chosen as the new default for password hashing for its
compatibility with [RFC7616], but because SHA-256 (like MD5) is a
comparatively fast algorithm, it does little to deter brute-force
attacks. Specifically, this means that if the user has a weak
password:

o An
password, an attacker who that captures a single exchange can use a
brute-force attack to learn the user's password and then potentially
impersonate the user to the server and to other servers where the
same password was used. Note that such an attacker can impersonate
the user to the server itself without any brute-force attack.

o An attacker that captures a single exchange can brute force the
user's password and then potentially impersonate the user to the
server and to other servers where the same password was used.

A stronger (which is to say, slower) algorithm, like Argon2 [Argon2],
would help both of these cases; however, in the first case, it would
only help after until the database entry for this user is updated to
exclusively use that stronger mechanism.

The bid-down defenses in this protocol prevent an attacker from
forcing the client and server to complete a handshake using weaker
algorithms than they jointly support, but only if the weakest joint
algorithm is strong enough that it cannot be compromised by a brute-
force attack. However, this does not defend against many attacks on
those algorithms; specifically, an on-path attacker might perform a
bid-down attack on a client that supports both Argon2 [Argon2] and
SHA-256 for password hashing and use that to collect a MESSAGE-
INTEGRITY-SHA256 value that it can then use for an offline brute-
force attack. This would be detected when the server receives the
second request, but that does not prevent the attacker from obtaining
the MESSAGE-INTEGRITY-SHA256 value.

Similarly, an attack against the USERHASH mechanism will not succeed
in establishing a session as the server will detect that the feature
was discarded on path, but the client would still have been convinced
to send its username in the clear in the USERNAME attribute, thus
disclosing it to the attacker.

Finally, when the bid-down protection mechanism is employed for a
future upgrade of the HMAC algorithm used to protect messages, it
will offer only a limited protection if the current HMAC algorithm is
already compromised.

16.2. Attacks Affecting the Usage

This section lists attacks that might be launched against a usage of
STUN. Each STUN Usage must consider whether these attacks are
applicable to it and, if so, discuss countermeasures.

Most of the attacks in this section revolve around an attacker
modifying the reflexive address learned by a STUN client through a
Binding request/response transaction. Since the usage of the
reflexive address is a function of the usage, the applicability and
remediation of these attacks are usage-specific. In common
situations, modification of the reflexive address by an on-path
attacker is easy to do. Consider, for example, the common situation
where STUN is run directly over UDP. In this case, an on-path
attacker can modify the source IP address of the Binding request
before it arrives at the STUN server. The STUN server will then
return this IP address in the XOR-MAPPED-ADDRESS attribute to the
client and send the response back to that (falsified) IP address and
port. If the attacker can also intercept this response, it can
direct it back towards the client. Protecting against this attack by
using a message-integrity check is impossible, since a message-
integrity value cannot cover the source IP address and the
intervening NAT must be able to modify this value. Instead, one
solution to prevent the attacks listed below is for the client to
verify the reflexive address learned, as is done in ICE [RFC8445].

Other usages may use other means to prevent these attacks.

16.2.1. Attack I: Distributed DoS (DDoS) against a Target

In this attack, the attacker provides one or more clients with the
same faked reflexive address that points to the intended target.
This will trick the STUN clients into thinking that their reflexive
addresses are equal to that of the target. If the clients hand out
that reflexive address in order to receive traffic on it (for
example, in SIP messages), the traffic will instead be sent to the
target. This attack can provide substantial amplification,
especially when used with clients that are using STUN to enable
multimedia applications. However, it can only be launched against
targets for which packets from the STUN server to the target pass
through the attacker, limiting the cases in which it is possible.

16.2.2. Attack II: Silencing a Client

In this attack, the attacker provides a STUN client with a faked
reflexive address. The reflexive address it provides is a transport
address that routes to nowhere. As a result, the client won't
receive any of the packets it expects to receive when it hands out
the reflexive address. This exploitation is not very interesting for
the attacker. It impacts a single client, which is frequently not
the desired target. Moreover, any attacker that can mount the attack
could also deny service to the client by other means, such as
preventing the client from receiving any response from the STUN
server, or even a DHCP server. As with the attack described in
Section 16.2.1, this attack is only possible when the attacker is on
path for packets sent from the STUN server towards this unused IP
address.

16.2.3. Attack III: Assuming the Identity of a Client

This attack is similar to attack II. However, the faked reflexive
address points to the attacker itself. This allows the attacker to
receive traffic that was destined for the client.

16.2.4. Attack IV: Eavesdropping

In this attack, the attacker forces the client to use a reflexive
address that routes to itself. It then forwards any packets it
receives to the client. This attack allows the attacker to observe
all packets sent to the client. However, in order to launch the
attack, the attacker must have already been able to observe packets
from the client to the STUN server. In most cases (such as when the
attack is launched from an access network), this means that the
attacker could already observe packets sent to the client. This
attack is, as a result, only useful for observing traffic by
attackers on the path from the client to the STUN server, but not
generally on the path of packets being routed towards the client.

Note that this attack can be trivially launched by the STUN server
itself, so users of STUN servers should have the same level of trust
in the users of STUN servers as any other node that can insert itself
into the communication flow.

16.3. Hash Agility Plan

This specification uses HMAC-SHA256 for computation of the message
integrity, sometimes in combination with HMAC-SHA1. If, at a later
time, HMAC-SHA256 is found to be compromised, the following remedy
should be applied:

o Both a new message-integrity attribute and a new STUN Security
Feature bit will be allocated in a Standards Track document. The
new message-integrity attribute will have its value computed using
a new hash. The STUN Security Feature bit will be used to
simultaneously 1) signal to a STUN client using the long-term
credential mechanism that this server supports this new hash
algorithm and 2) prevent bid-down attacks on the new message-
integrity attribute.

o STUN clients and servers using the short-term credential mechanism
will need to update the external mechanism that they use to signal
what message-integrity attributes are in use.

The bid-down protection mechanism described in this document is new
and thus cannot currently protect against a bid-down attack that
lowers the strength of the hash algorithm to HMAC-SHA1. This is why,
after a transition period, a new document updating this one will
assign a new STUN Security Feature bit for deprecating HMAC-SHA1.
When used, this bit will signal that HMAC-SHA1 is deprecated and
should no longer be used.

Similarly, if HMAC-SHA256 is found to be compromised, a new userhash
attribute and a new STUN Security Feature bit will be allocated in a
Standards Track document. The new userhash attribute will have its
value computed using a new hash. The STUN Security Feature bit will
be used to simultaneously 1) signal to a STUN client using the long-
term credential mechanism that this server supports this new hash
algorithm for the userhash attribute and 2) prevent bid-down attacks
on the new userhash attribute.

17. IAB Considerations

The IAB has studied the problem of Unilateral Self-Address Fixing
(UNSAF), which is the general process by which a client attempts to
determine its address in another realm on the other side of a NAT
through a collaborative protocol reflection mechanism [RFC3424].
STUN can be used to perform this function using a Binding request/
response transaction if one agent is behind a NAT and the other is on
the public side of the NAT.

The IAB has suggested that protocols developed for this purpose
document a specific set of considerations. Because some STUN Usages
provide UNSAF functions (such as ICE [RFC8445]) and others do not
(such as SIP Outbound [RFC5626]), answers to these considerations
need to be addressed by the usages themselves.

18. IANA Considerations

18.1. STUN Security Features Registry

A STUN Security Feature set defines 24 bits as flags.

IANA has created a new registry containing the STUN Security Features
that are protected by the bid-down attack prevention mechanism
described in Section 9.2.1.

The initial STUN Security Features are:

Bit 0: Password algorithms
Bit 1: Username anonymity
Bit 2-23: Unassigned

Bits are assigned starting from the most significant side of the bit
set, so Bit 0 is the leftmost bit and Bit 23 is the rightmost bit.

New Security Features are assigned by Standards Action [RFC8126].

18.2. STUN Methods Registry

A STUN method is a hex number in the range 0x000-0x0FF. The encoding
of a STUN method into a STUN message is described in Section 5.

STUN methods in the range 0x000-0x07F are assigned by IETF Review
[RFC8126]. STUN methods in the range 0x080-0x0FF are assigned by
Expert Review [RFC8126]. The responsibility of the expert is to
verify that the selected codepoint(s) is not in use and that the
request is not for an abnormally large number of codepoints.
Technical review of the extension itself is outside the scope of the
designated expert responsibility.

IANA has updated the name for method 0x002 as described below as well
as updated the reference from RFC 5389 to RFC 8489 for the following
STUN methods:

0x000: Reserved
0x001: Binding
0x002: Reserved; was SharedSecret prior to [RFC5389]

18.3. STUN Attributes Registry

A STUN attribute type is a hex number in the range 0x0000-0xFFFF.
STUN attribute types in the range 0x0000-0x7FFF are considered
comprehension-required; STUN attribute types in the range
0x8000-0xFFFF are considered comprehension-optional. A STUN agent
handles unknown comprehension-required and comprehension-optional
attributes differently.

STUN attribute types in the first half of the comprehension-required
range (0x0000-0x3FFF) and in the first half of the comprehension-
optional range (0x8000-0xBFFF) are assigned by IETF Review [RFC8126].
STUN attribute types in the second half of the comprehension-required
range (0x4000-0x7FFF) and in the second half of the comprehension-
optional range (0xC000-0xFFFF) are assigned by Expert Review
[RFC8126]. The responsibility of the expert is to verify that the
selected codepoint(s) are not in use and that the request is not for
an abnormally large number of codepoints. Technical review of the
extension itself is outside the scope of the designated expert
responsibility.

18.3.1. Updated Attributes

IANA has updated the names for attributes 0x0002, 0x0004, 0x0005,
0x0007, and 0x000B as well as updated the reference from RFC 5389 to
RFC 8489 for each the following STUN methods: methods.

In addition, [RFC5389] introduced a mistake in the name of attribute
0x0003; [RFC5389] called it CHANGE-ADDRESS when it was actually
previously called CHANGE-REQUEST. Thus, IANA has updated the
description for 0x0003 to read "Reserved; was CHANGE-REQUEST prior to
[RFC5389]".

Comprehension-required range (0x0000-0x7FFF):
0x0000: Reserved
0x0001: MAPPED-ADDRESS
0x0002: Reserved; was RESPONSE-ADDRESS prior to [RFC5389]
0x0003: Reserved; was CHANGE-REQUEST prior to [RFC5389]
0x0004: Reserved; was SOURCE-ADDRESS prior to [RFC5389]
0x0005: Reserved; was CHANGED-ADDRESS prior to [RFC5389]
0x0006: USERNAME
0x0007: Reserved; was PASSWORD prior to [RFC5389]
0x0008: MESSAGE-INTEGRITY
0x0009: ERROR-CODE
0x000A: UNKNOWN-ATTRIBUTES
0x000B: Reserved; was REFLECTED-FROM prior to [RFC5389]
0x0014: REALM
0x0015: NONCE
0x0020: XOR-MAPPED-ADDRESS

Comprehension-optional range (0x8000-0xFFFF)
0x8022: SOFTWARE
0x8023: ALTERNATE-SERVER
0x8028: FINGERPRINT

18.3.2. New Attributes

IANA has added the following attribute to the "STUN Attributes"
registry:

Comprehension-required range (0x0000-0x7FFF):
0x001C: MESSAGE-INTEGRITY-SHA256
0x001D: PASSWORD-ALGORITHM
0x001E: USERHASH

Comprehension-optional range (0x8000-0xFFFF)
0x8002: PASSWORD-ALGORITHMS
0x8003: ALTERNATE-DOMAIN

18.4. STUN Error Codes Registry

A STUN error code is a number in the range 0-699. STUN error codes
are accompanied by a textual reason phrase in UTF-8 [RFC3629] that is
intended only for human consumption and can be anything appropriate;
this document proposes only suggested values.

STUN error codes are consistent in codepoint assignments and
semantics with SIP [RFC3261] and HTTP [RFC7231].

New STUN error codes are assigned based on IETF Review [RFC8126].
The specification must carefully consider how clients that do not
understand this error code will process it before granting the
request. See the rules in Section 6.3.4.

IANA has updated the reference from RFC 5389 to RFC 8489 for the
error codes defined in Section 14.8.

IANA has changed the name of the 401 error code from "Unauthorized"
to "Unauthenticated".

18.5. STUN Password Algorithm Algorithms Registry

IANA has created a new registry titled "STUN Password Algorithm". Algorithms".

A password algorithm is a hex number in the range 0x0000-0xFFFF.

The initial contents of the "Password Algorithm" registry are as
follows:

0x0000: Reserved
0x0001: MD5
0x0002: SHA-256
0x0003-0xFFFF: Unassigned

Password algorithms in the first half of the range (0x0000-0x7FFF)
are assigned by IETF Review [RFC8126]. Password algorithms in the
second half of the range (0x8000-0xFFFF) are assigned by Designated Expert
Review [RFC8126].

18.5.1. Password Algorithms

18.5.1.1. MD5

This password algorithm is taken from [RFC1321].

The key length is 16 bytes, and the parameters value is empty.

Note: This algorithm MUST only be used for compatibility with
legacy systems.

key = MD5(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.5.1.2. SHA-256

This password algorithm is taken from [RFC7616].

The key length is 32 bytes, and the parameters value is empty.

key = SHA-256(username ":" OpaqueString(realm)
":" OpaqueString(password))

18.6. STUN UDP and TCP Port Numbers

IANA has updated the reference from RFC 5389 to RFC 8489 for the
following ports in the "Service Name and Transport Protocol Port
Number Registry".

stun 3478/tcp Session Traversal Utilities for NAT (STUN) port
stun 3478/udp Session Traversal Utilities for NAT (STUN) port
stuns 5349/tcp Session Traversal Utilities for NAT (STUN) port

19. Changes since RFC 5389

This specification obsoletes [RFC5389]. This specification differs
from RFC 5389 in the following ways:

o Added support for DTLS-over-UDP [RFC6347].

o Made clear that the RTO is considered stale if there are no
transactions with the server.

o Aligned the RTO calculation with [RFC6298].

o Updated the ciphersuites for TLS.

o Added support for STUN URI [RFC7064].

o Added support for SHA256 message integrity.

o Updated the Preparation, Enforcement, and Comparison of
Internationalized Strings (PRECIS) support to [RFC8265].

o Added protocol and registry to choose the password encryption
algorithm.

o Added support for anonymous username.

o Added protocol and registry for preventing bid-down attacks.

o Specified that sharing a NONCE is no longer permitted.

o Added the possibility of using a domain name in the alternate
server mechanism.

o Added more C snippets.

o Added test vector.

20. References

20.1. Normative References

[ITU.V42.2002]
International Telecommunication Union, "Error-correcting
procedures for DCEs using asynchronous-to-synchronous
conversion", ITU-T Recommendation V.42, March 2002.

[KARN87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", SIGCOMM '87,
Proceedings of the ACM workshop on Frontiers in computer
communications technology, Pages 2-7,
DOI 10.1145/55483.55484, August 1987.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.

[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.

[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<https://www.rfc-editor.org/info/rfc1321>.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<https://www.rfc-editor.org/info/rfc2104>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<https://www.rfc-editor.org/info/rfc4648>.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.

[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<https://www.rfc-editor.org/info/rfc5890>.

[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <https://www.rfc-editor.org/info/rfc6125>.

[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, DOI 10.17487/RFC6151, March 2011,
<https://www.rfc-editor.org/info/rfc6151>.

[RFC6298] Paxson, V., Allman, M., Chu, J., and M. Sargent,
"Computing TCP's Retransmission Timer", RFC 6298,
DOI 10.17487/RFC6298, June 2011,
<https://www.rfc-editor.org/info/rfc6298>.

[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.

[RFC7064] Nandakumar, S., Salgueiro, G., Jones, P., and M. Petit-
Huguenin, "URI Scheme for the Session Traversal Utilities
for NAT (STUN) Protocol", RFC 7064, DOI 10.17487/RFC7064,
November 2013, <https://www.rfc-editor.org/info/rfc7064>.

[RFC7350] Petit-Huguenin, M. and G. Salgueiro, "Datagram Transport
Layer Security (DTLS) as Transport for Session Traversal
Utilities for NAT (STUN)", RFC 7350, DOI 10.17487/RFC7350,
August 2014, <https://www.rfc-editor.org/info/rfc7350>.

[RFC7616] Shekh-Yusef, R., Ed., Ahrens, D., and S. Bremer, "HTTP
Digest Access Authentication", RFC 7616,
DOI 10.17487/RFC7616, September 2015,
<https://www.rfc-editor.org/info/rfc7616>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.

[RFC8265] Saint-Andre, P. and A. Melnikov, "Preparation,
Enforcement, and Comparison of Internationalized Strings
Representing Usernames and Passwords", RFC 8265,
DOI 10.17487/RFC8265, October 2017,
<https://www.rfc-editor.org/info/rfc8265>.

[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.

20.2. Informative References

[Argon2] Biryukov, A., Dinu, D., Khovratovich, D., and S.
Josefsson, "The memory-hard Argon2 password hash and
proof-of-work function", Work in Progress, draft-irtf-
cfrg-argon2-04,
cfrg-argon2-09, November 2018. 2019.

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<https://www.rfc-editor.org/info/bcp195>.

[RFC1952] Deutsch, P., "GZIP file format specification version 4.3",
RFC 1952, DOI 10.17487/RFC1952, May 1996,
<https://www.rfc-editor.org/info/rfc1952>.

[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, DOI 10.17487/RFC2279, January 1998,
<https://www.rfc-editor.org/info/rfc2279>.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
DOI 10.17487/RFC3261, June 2002,
<https://www.rfc-editor.org/info/rfc3261>.

[RFC3424] Daigle, L., Ed. and IAB, "IAB Considerations for
UNilateral Self-Address Fixing (UNSAF) Across Network
Address Translation", RFC 3424, DOI 10.17487/RFC3424,
November 2002, <https://www.rfc-editor.org/info/rfc3424>.

[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
DOI 10.17487/RFC3454, December 2002,
<https://www.rfc-editor.org/info/rfc3454>.

[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
DOI 10.17487/RFC3489, March 2003,
<https://www.rfc-editor.org/info/rfc3489>.

[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User Names
and Passwords", RFC 4013, DOI 10.17487/RFC4013, February
2005, <https://www.rfc-editor.org/info/rfc4013>.

[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, DOI 10.17487/RFC4107,
June 2005, <https://www.rfc-editor.org/info/rfc4107>.

[RFC5090] Sterman, B., Sadolevsky, D., Schwartz, D., Williams, D.,
and W. Beck, "RADIUS Extension for Digest Authentication",
RFC 5090, DOI 10.17487/RFC5090, February 2008,
<https://www.rfc-editor.org/info/rfc5090>.

[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
DOI 10.17487/RFC5389, October 2008,
<https://www.rfc-editor.org/info/rfc5389>.

[RFC5626] Jennings, C., Ed., Mahy, R., Ed., and F. Audet, Ed.,
"Managing Client-Initiated Connections in the Session
Initiation Protocol (SIP)", RFC 5626,
DOI 10.17487/RFC5626, October 2009,
<https://www.rfc-editor.org/info/rfc5626>.

[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766,
DOI 10.17487/RFC5766, April 2010,
<https://www.rfc-editor.org/info/rfc5766>.

[RFC5769] Denis-Courmont, R., "Test Vectors for Session Traversal
Utilities for NAT (STUN)", RFC 5769, DOI 10.17487/RFC5769,
April 2010, <https://www.rfc-editor.org/info/rfc5769>.

[RFC5780] MacDonald, D. and B. Lowekamp, "NAT Behavior Discovery
Using Session Traversal Utilities for NAT (STUN)",
RFC 5780, DOI 10.17487/RFC5780, May 2010,
<https://www.rfc-editor.org/info/rfc5780>.

[RFC6544] Rosenberg, J., Keranen, A., Lowekamp, B., and A. Roach,
"TCP Candidates with Interactive Connectivity
Establishment (ICE)", RFC 6544, DOI 10.17487/RFC6544,
March 2012, <https://www.rfc-editor.org/info/rfc6544>.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.

[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC8264] Saint-Andre, P. and M. Blanchet, "PRECIS Framework:
Preparation, Enforcement, and Comparison of
Internationalized Strings in Application Protocols",
RFC 8264, DOI 10.17487/RFC8264, October 2017,
<https://www.rfc-editor.org/info/rfc8264>.

[RFC8445] Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive
Connectivity Establishment (ICE): A Protocol for Network
Address Translator (NAT) Traversal", RFC 8445,
DOI 10.17487/RFC8445, July 2018,
<https://www.rfc-editor.org/info/rfc8445>.

[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.

[STUN-PMTUD]
Petit-Huguenin, M. and G. M., Salgueiro, G., and F. Garrido, "Path
MTU Discovery Using Session Traversal Utilities for NAT
(STUN)", Work in Progress, draft-ietf-tram-stun-pmtud-10, September 2018. draft-ietf-tram-stun-pmtud-14,
November 2019.

[UAX15] Unicode Standard Annex #15, "Unicode Normalization Forms",
edited by Mark Davis and Ken Whistler. An integral part
of The Unicode Standard,
<http://unicode.org/reports/tr15/>.

Appendix A. C Snippet to Determine STUN Message Types

Given a 16-bit STUN message type value in host byte order in msg_type
parameter, below are C macros to determine the STUN message types:

<CODE BEGINS>
#define IS_REQUEST(msg_type) (((msg_type) & 0x0110) == 0x0000)
#define IS_INDICATION(msg_type) (((msg_type) & 0x0110) == 0x0010)
#define IS_SUCCESS_RESP(msg_type) (((msg_type) & 0x0110) == 0x0100)
#define IS_ERR_RESP(msg_type) (((msg_type) & 0x0110) == 0x0110)
<CODE ENDS>

A function to convert method and class into a message type:

<CODE BEGINS>
int type(int method, int cls) {
return (method & 0x1F80) << 2 | (method & 0x0070) << 1
| (method & 0x000F) | (cls & 0x0002) << 7
| (cls & 0x0001) << 4;
}
<CODE ENDS>

A function to extract the method from the message type:

<CODE BEGINS>
int method(int type) {
return (type & 0x3E00) >> 2 | (type & 0x00E0) >> 1
| (type & 0x000F);
}
<CODE ENDS>

A function to extract the class from the message type:

<CODE BEGINS>
int cls(int type) {
return (type & 0x0100) >> 7 | (type & 0x0010) >> 4;
}
<CODE ENDS>

Appendix B. Test Vectors

This section augments the list of test vectors defined in [RFC5769]
with MESSAGE-INTEGRITY-SHA256. All the formats and definitions
listed in Section 2 of [RFC5769] apply here.

B.1. Sample Request with Long-Term Authentication with MESSAGE-
INTEGRITY-SHA256 and USERHASH

This request uses the following parameters:

Username: "<U+30DE><U+30C8><U+30EA><U+30C3><U+30AF><U+30B9>" (without
quotes) unaffected by OpaqueString [RFC8265] processing

Password: "The<U+00AD>M<U+00AA>tr<U+2168>" and "TheMatrIX" (without
quotes) respectively before and after OpaqueString [RFC8265]
processing

Nonce: "obMatJos2QAAAf//499k954d6OL34oL9FSTvy64sA" "obMatJos2AAACf//499k954d6OL34oL9FSTvy64sA" (without quotes)

Realm: "example.org" (without quotes)
00 01 00 9c Request type and message length
21 12 a4 42 Magic cookie
78 ad 34 33 }
c6 ad 72 c0 } Transaction ID
29 da 41 2e }
XX XX
00 1e 00 20 USERHASH attribute header
4a 3c f3 8f }
ef 69 92 bd }
a9 52 c6 78 }
04 17 da 0f } Userhash value (32 bytes)
24 81 94 15 }
56 9e 60 b2 }
05 c4 6e 41 }
40 7f 17 04 }
00 15 00 29 NONCE attribute header
6f 62 4d 61 }
74 4a 6f 73 }
32 41 41 41 }
43 66 2f 2f }
34 39 39 6b } Nonce value and padding (3 bytes)
39 35 34 64 }
36 4f 4c 33 }
34 6f 4c 39 }
46 53 54 76 }
79 36 34 73 }
41 00 00 00 }
00 14 00 0b REALM attribute header
65 78 61 6d }
70 6c 65 2e } Realm value (11 bytes) and padding (1 byte)
6f 72 67 00 }
XX XX
00 1c 00 20 MESSAGE-INTEGRITY-SHA256 attribute header
c4 ec a2 b6
e4 68 6c 8f }
24 6f 26 be
0e de b5 90 }
bc 2f 77 49
13 e0 70 90 }
07 c2 00 a3
01 0a 93 ef } HMAC-SHA256 value
76 c7 c2 8e
cc bc cc 54 }
b4 d1 26 60
4c 0a 45 d9 }
bb fe 9f 28
f8 30 aa 6d }
0e 85 71 f2
6f 73 5a 01 }

Acknowledgements

Thanks to Michael Tuexen, Tirumaleswar Reddy, Oleg Moskalenko, Simon
Perreault, Benjamin Schwartz, Rifaat Shekh-Yusef, Alan Johnston,
Jonathan Lennox, Brandon Williams, Olle Johansson, Martin Thomson,
Mihaly Meszaros, Tolga Asveren, Noriyuki Torii, Spencer Dawkins, Dale
Worley, Matthew Miller, Peter Saint-Andre, Julien Elie, Mirja
Kuehlewind, Eric Rescorla, Ben Campbell, Adam Roach, Alexey Melnikov,
and Benjamin Kaduk for the comments, suggestions, and questions that
helped improve this document.

The Acknowledgements section of RFC 5389 appeared as follows:

The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
Jennings, Bob Penfield, Xavier Marjou, Magnus Westerlund, Miguel
Garcia, Bruce Lowekamp, and Chris Sullivan for their comments, and
Baruch Sterman and Alan Hawrylyshen for initial implementations.
Thanks for Leslie Daigle, Allison Mankin, Eric Rescorla, and Henning
Schulzrinne for IESG and IAB input on this work.

Contributors

Christian Huitema and Joel Weinberger were original coauthors of
RFC 3489.

Authors' Addresses

Marc Petit-Huguenin
Impedance Mismatch

Email: marc@petit-huguenin.org

Gonzalo Salgueiro
Cisco
7200-12 Kit Creek Road
Research Triangle Park, NC 27709
United States of America

Email: gsalguei@cisco.com

Jonathan Rosenberg
Five9
Edison, NJ
United States of America

Email: jdrosen@jdrosen.net
URI: http://www.jdrosen.net

Dan Wing
Citrix Systems, Inc.
United States of America

Email: dwing-ietf@fuggles.com
Rohan Mahy
Unaffiliated

Email: rohan.ietf@gmail.com

Philip Matthews
Nokia
600 March Road
Ottawa, Ontario K2K 2T6
Canada

Phone: 613-784-3139
Email: philip_matthews@magma.ca