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Network Working Group J. Rosenberg
Request for Comments: 5389 Cisco
Obsoletes: 3489 R. Mahy
Category: Standards Track P. Matthews
Unaffiliated

D. Wing
Cisco
October 2008

Session Traversal Utilities for NAT (STUN)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

Session Traversal Utilities for NAT (STUN) is a protocol that serves
as a tool for other protocols in dealing with Network Address

Translator (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 is an
important change from the previous version of this specification (RFC
3489), which presented STUN as a complete solution.

This document obsoletes RFC 3489.

Table of Contents

1. Introduction .............................................................................4
2. Evolution from RFC 3489 ....................................................4
3. Overview of Operation .........................................................5
4. Terminology .............................................................................8
5. Definitions .................................................................................8
6. STUN Message Structure .......................................................10
7. Base Protocol Procedures .....................................................12
7.1. Forming a Request or an Indication ........................12
7.2. Sending the Request or Indication .........................13
7.2.1. Sending over UDP .................................................13
7.2.2. Sending over TCP or TLS-over-TCP ...................14
7.3. Receiving a STUN Message ..................................16
7.3.1. Processing a Request ...............................17
7.3.1.1. Forming a Success or Error Response .......18
7.3.1.2. Sending the Success or Error Response .....19
7.3.2. Processing an Indication .........................................19
7.3.3. Processing a Success Response ......................19
7.3.4. Processing an Error Response .......................20
8. FINGERPRINT Mechanism ........................................................20
9. DNS Discovery of a Server ....................................................21
10. Authentication and Message-Integrity Mechanisms ...............22
10.1. Short-Term Credential Mechanism ..........................22
10.1.1. Forming a Request or Indication ...................23
10.1.2. Receiving a Request or Indication .................23
10.1.3. Receiving a Response ..............................24
10.2. Long-Term Credential Mechanism ...........................24
10.2.1. Forming a Request .................................25
10.2.1.1. First Request ............................25
10.2.1.2. Subsequent Requests ......................26
10.2.2. Receiving a Request ...............................26
10.2.3. Receiving a Response ..............................27
11. ALTERNATE-SERVER Mechanism ....................................28
12. Backwards Compatibility with RFC 3489 .........................28
12.1. Changes to Client Processing .............................29
12.2. Changes to Server Processing .............................29
13. Basic Server Behavior .......................................................30
14. STUN Usages .................................................................30
15. STUN Attributes .............................................................31
15.1. MAPPED-ADDRESS ...........................................32
15.2. XOR-MAPPED-ADDRESS .......................................33
15.3. USERNAME ...............................................................34
15.4. MESSAGE-INTEGRITY ........................................34
15.5. FINGERPRINT ............................................................36
15.6. ERROR-CODE .............................................................36
15.7. REALM ..................................................................38
15.8. NONCE ..................................................................38
15.9. UNKNOWN-ATTRIBUTES .......................................38
15.10. SOFTWARE ..............................................................39
15.11. ALTERNATE-SERVER ........................................39
16. Security Considerations .....................................................39
16.1. Attacks against the Protocol .............................39
16.1.1. Outside Attacks .................................................39
16.1.2. Inside Attacks ..................................................40
16.2. Attacks Affecting the Usage ..............................40
16.2.1. Attack I: Distributed DoS (DDoS) against a
Target ........................................................................41
16.2.2. Attack II: Silencing a Client .....................41
16.2.3. Attack III: Assuming the Identity of a Client .....42
16.2.4. Attack IV: Eavesdropping ..........................42
16.3. Hash Agility Plan ......................................................42
17. IAB Considerations ..........................................................42
18. IANA Considerations .........................................................43
18.1. STUN Methods Registry ....................................43
18.2. STUN Attribute Registry ..................................43
18.3. STUN Error Code Registry .................................44
18.4. STUN UDP and TCP Port Numbers ............................45
19. Changes since RFC 3489 ......................................................45
20. Contributors ..............................................................................47
21. Acknowledgements ............................................................47
22. References ................................................................................47
22.1. Normative References ...................................................47
22.2. Informative References .................................................48
Appendix A. C Snippet to Determine STUN Message Types .............50

1. Introduction

The protocol defined in this specification, Session Traversal
Utilities for NAT, provides a tool for dealing with 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 [MMUSIC-ICE], or to relay
packets between two endpoints [BEHAVE-TURN].

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 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) [MMUSIC-ICE] is one usage of STUN. SIP Outbound
[SIP-OUTBOUND] 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 14.

2. Evolution from RFC 3489

STUN was originally defined in RFC 3489 [RFC3489]. That
specification, sometimes referred to as "classic STUN", represented
itself as a complete solution to the NAT traversal problem. In that
solution, a client would discover whether it was behind a NAT,
determine its NAT type, discover its IP address and port on the
public side of the outermost NAT, and then utilize that IP address
and port within the body of protocols, such as the Session Initiation
Protocol (SIP) [RFC3261]. However, experience since the publication
of RFC 3489 has found that classic STUN simply does not work
sufficiently well to be a deployable solution. The address and port
learned through classic STUN are sometimes usable for communications
with a peer, and sometimes not. Classic STUN provided no way to
discover whether it would, in fact, work or not, and it provided no
remedy in cases where it did not. Furthermore, classic STUN's
algorithm for classification of NAT types was found to be faulty, as
many NATs did not fit cleanly into the types defined there.

Classic STUN also had a security vulnerability -- attackers could
provide the client with incorrect mapped addresses under certain
topologies and constraints, and this was fundamentally not solvable
through any cryptographic means. Though this problem remains with
this specification, those attacks are now mitigated through the use
of more complete solutions that make use of STUN.

For these reasons, this specification obsoletes RFC 3489, and instead
describes STUN as a tool that is utilized as part of a complete NAT
traversal solution. ICE [MMUSIC-ICE] is a complete NAT traversal
solution for protocols based on the offer/answer [RFC3264]
methodology, such as SIP. SIP Outbound [SIP-OUTBOUND] is a complete
solution for traversal of SIP signaling, and it uses STUN in a very
different way. Though it is possible that a protocol may be able to
use STUN by itself (classic STUN) as a traversal solution, such usage
is not described here and is strongly discouraged for the reasons
described above.

The on-the-wire protocol described here is changed only slightly from
classic STUN. The protocol now runs over TCP in addition to UDP.
Extensibility was added to the protocol in a more structured way. A
magic cookie mechanism for demultiplexing STUN with application
protocols was added by stealing 32 bits from the 128-bit transaction
ID defined in RFC 3489, allowing the change to be backwards
compatible. Mapped addresses are encoded using a new exclusive-or
format. There are other, more minor changes. See Section 19 for a
more complete listing.

Due to the change in scope, STUN has also been renamed from "Simple
Traversal of UDP through NAT" to "Session Traversal Utilities for
NAT". The acronym remains STUN, which is all anyone ever remembers
anyway.

3. Overview of Operation

This section is descriptive only.

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, and 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, and 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
were 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.,
[MMUSIC-ICE], [SIP-OUTBOUND]). 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 (but differing in details) to the one
defined for HTTP [RFC2617]. 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 [MMUSIC-ICE] 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.

4. Terminology

In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119
[RFC2119] and indicate requirement levels for compliant STUN
implementations.

5. 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. 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 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 SIP 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.

6. 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 RFC 791 [RFC0791]. Unless otherwise noted, numeric constants are
in decimal (base 10).

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

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, failure 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 message type field is decomposed further into the following
structure:

Figure 3: Format of STUN Message Type Field

Here the bits in the 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 Indications, Success, and
Errors using bit fields.

The magic cookie field MUST contain the fixed value 0x2112A442 in
network byte order. In RFC 3489 [RFC3489], this field was 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 in this revised specification. 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 SHOULD 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, in bytes, of the message
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. The
details of the encoding, and of the attributes themselves are given
in Section 15.

7. 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.

7.1. Forming a Request or an Indication

When formulating a request or indication message, the agent MUST
follow the rules in Section 6 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 10) or the FINGERPRINT attribute
(Section 8).

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.

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

All STUN messages sent over UDP SHOULD be less than the path MTU, if
known. If the path MTU is unknown, messages SHOULD be the smaller of
576 bytes and the first-hop MTU for IPv4 [RFC1122] and 1280 bytes for
IPv6 [RFC2460]. 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). STUN provides no
ability to handle the case where the request is under the MTU but the
response would be larger than the MTU. It is not envisioned that
this limitation will be an issue for STUN. The MTU limitation is a
SHOULD, and not a MUST, to account for cases where STUN itself is
being used to probe for MTU characteristics [BEHAVE-NAT]. Outside of
this or similar applications, the MTU constraint MUST be followed.

7.2. Sending the Request or Indication

The agent then sends the request or indication. This document
specifies how to send STUN messages over UDP, TCP, or TLS-over-TCP;
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 9
describes a DNS-based method of determining the IP address and port
of a server that a usage may elect to use. STUN may be used with
anycast addresses, but only with UDP and in usages where
authentication is not used.

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
space new transactions to a server by RTO and SHOULD limit itself to
ten outstanding transactions to the same server.

7.2.1. Sending over UDP

When running STUN over UDP, 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 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 RFC 2988 [RFC2988], with two
exceptions. First, the initial value for RTO SHOULD be configurable
(rather than the 3 s recommended in RFC 2988) and SHOULD be greater
than 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 after
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 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.

7.2.2. Sending over TCP or TLS-over-TCP

For TCP and TLS-over-TCP, 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 9 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.

When STUN is run by itself over TLS-over-TCP, the
TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be implemented at a
minimum. Implementations MAY also support any other ciphersuite.
When it receives the TLS Certificate message, the client SHOULD
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 Section 3.1 of RFC 2818
[RFC2818]. Those procedures assume the client is dereferencing a
URI. For purposes of usage with this specification, the client
treats the domain name or IP address used in Section 8.1 as the host
portion of the URI that has been dereferenced. Alternatively, a
client MAY be configured with a set of domains or IP addresses that
are trusted; if a certificate is received that identifies one of
those domains or IP addresses, the client considers the identity of
the server to be verified.

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

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 SYN to establish
the connection, it considers the transaction to have timed out. Ti
SHOULD be configurable and SHOULD have a default of 39.5s. 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. The client SHOULD keep the
connection open until it:

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

o has no plans to use any resources (such as a mapped address
(MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) or relayed address
[BEHAVE-TURN]) that were learned though STUN requests sent over
that connection, and

o if multiplexing other application protocols over that port, has
finished using that other application, and

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., [MMUSIC-ICE-TCP]).

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.

7.3. 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 12. 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 6. 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 10).

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.

7.3.1. 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.

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, a request received by the server could be the
first request of a transaction, or 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 and their
corresponding responses in the last 40 seconds. However, this
requires the server to hold state, and will be inappropriate for any
requests which 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.

7.3.1.1. 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 10). 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 10).

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 a XOR-MAPPED-ADDRESS attribute to the response, where the
contents of the attribute are the source transport address of the

request message. For 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.

7.3.1.2. 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,
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.

7.3.2. Processing an Indication

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

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 (unlike requests),
there is no need to handle re-transmissions of indications at the
sending agent.

7.3.3. 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.

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.

7.3.4. 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.

The client then does any processing specified by the authentication
mechanism (see Section 10). 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 is
being used. See Section 11.

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.

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

8. 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 7, STUN
messages must first be separated from the application packets.

Section 6 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 15.5 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 7.3 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.

9. 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 server's domain name and a
service name; the usage must also describe how the client obtains
these. Hard-coding the domain name of the server 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 in the public Internet
that accepts Binding request/response transactions, the SRV service
name is "stun". When it wishes to locate a STUN server that accepts
Binding request/response transactions over a TLS session, the SRV
service name is "stuns". STUN usages MAY define additional DNS SRV
service names.

The domain name is resolved to a transport address using the SRV
procedures specified in [RFC2782]. The DNS SRV service name is the
service name provided as input to this procedure. The protocol in
the SRV lookup is the transport protocol the client will run STUN
over: "udp" for UDP and "tcp" for TCP. Note that only "tcp" is
defined with "stuns" at this time.

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 defined by RFC 2782. Such a retry is
only possible for request/response transmissions, since indication
transactions generate no response or timeout.

The default port for STUN requests is 3478, for both TCP and UDP.

Administrators of STUN servers SHOULD use this port 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. The default port for STUN
over TLS is 5349. 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.

If no SRV records were found, the client performs an A or AAAA record
lookup of the domain name. The result will be a list of IP
addresses, each of which can be contacted at the default port using
UDP or TCP, independent of the STUN usage. For usages that require
TLS, the client connects to one of the IP addresses using the default
STUN over TLS port.

10. 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 3.

Each mechanism specifies the additional processing required to use
that mechanism, extending the processing specified in Section 7. 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.

10.1. 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 [MMUSIC-ICE], 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 prevented by
virtue of the time-limited nature of the credential.

10.1.1. Forming a Request or Indication

For a request or indication message, the agent MUST include the
USERNAME and MESSAGE-INTEGRITY attributes in the message. The HMAC
for the MESSAGE-INTEGRITY attribute is computed as described in
Section 15.4. Note that the password is never included in the
request or indication.

10.1.2. Receiving a Request or Indication

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

o If the message does not contain both a MESSAGE-INTEGRITY and 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 (Unauthorized).

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

o Using the password associated with the username, compute the value
for the message integrity as described in Section 15.4. If the
resulting value does not match the contents of the MESSAGE-
INTEGRITY 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 401 (Unauthorized).

* 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 MUST include the
MESSAGE-INTEGRITY attribute, computed using the password utilized to
authenticate the request. The response MUST NOT contain the USERNAME
attribute.

If any of the checks fail, a server MUST NOT include a 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 MESSAGE-INTEGRITY.

10.1.3. Receiving a Response

The client looks for the MESSAGE-INTEGRITY attribute in the response.
If present, the client computes the message integrity over the
response as defined in Section 15.4, using the same password it
utilized for the request. If the resulting value matches the
contents of the MESSAGE-INTEGRITY attribute, the response is
considered authenticated. If the value does not match, or if
MESSAGE-INTEGRITY was absent, the response MUST be discarded, as if
it was never received. This means that retransmits, if applicable,
will continue.

10.2. 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 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 the 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. 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 a message-integrity, 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.

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.

10.2.1. Forming a Request

There are two cases when forming a request. In the first case, this
is the first request from the client to the server (as identified by
its IP address and port). In the second case, the client is
submitting a subsequent request once a previous request/response
transaction has completed successfully. Forming a request as a
consequence of a 401 or 438 error response is covered in
Section 10.2.3 and is not considered a "subsequent request" and thus
does not utilize the rules described in Section 10.2.1.2.

10.2.1.1. First Request

If the client has not completed a successful request/response
transaction with the server (as identified by hostname, if the DNS
procedures of Section 9 are used, else IP address if not), it SHOULD
omit the USERNAME, MESSAGE-INTEGRITY, REALM, and NONCE attributes.
In other words, the very first request is sent as if there were no
authentication or message integrity applied.

10.2.1.2. Subsequent Requests

Once a request/response transaction has completed successfully, 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 SHOULD include the USERNAME, REALM, and NONCE
attributes with these cached values. It SHOULD include a MESSAGE-
INTEGRITY attribute, computed as described in Section 15.4 using the
cached password.

10.2.2. Receiving a Request

After the server has done the basic processing of a request, it
performs the checks listed below in the order specified:

o If the message does not contain a MESSAGE-INTEGRITY attribute, the
server MUST generate an error response with an error code of 401
(Unauthorized). This response MUST include a REALM value. It is
RECOMMENDED that the REALM value be the domain name of the
provider of the STUN server. The response MUST include a NONCE,
selected by the server. The response SHOULD NOT contain a
USERNAME or MESSAGE-INTEGRITY attribute.

o If the message contains a MESSAGE-INTEGRITY attribute, but is
missing the USERNAME, 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, NONCE,
REALM, or MESSAGE-INTEGRITY attribute.

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 and REALM attributes and SHOULD NOT include the
USERNAME or MESSAGE-INTEGRITY attribute. Servers can invalidate
nonces in order to provide additional security. See Section 4.3
of [RFC2617] for guidelines.

o If the username in the USERNAME attribute is not valid, the server
MUST generate an error response with an error code of 401
(Unauthorized). This response MUST include a REALM value. It is
RECOMMENDED that the REALM value be the domain name of the
provider of the STUN server. The response MUST include a NONCE,
selected by the server. The response SHOULD NOT contain a
USERNAME or MESSAGE-INTEGRITY attribute.

o Using the password associated with the username in the USERNAME
attribute, compute the value for the message integrity as
described in Section 15.4. If the resulting value does not match
the contents of the MESSAGE-INTEGRITY attribute, the server MUST
reject the request with an error response. This response MUST use
an error code of 401 (Unauthorized). It MUST include REALM and
NONCE attributes and SHOULD NOT include the USERNAME or MESSAGE-
INTEGRITY attribute.

If these checks pass, the server continues to process the request.
Any response generated by the server (excepting the cases described
above) MUST include the MESSAGE-INTEGRITY attribute, computed using
the username and password utilized to authenticate the request. The
REALM, NONCE, and USERNAME attributes SHOULD NOT be included.

10.2.3. Receiving a Response

If the response is an error response with an error code of 401
(Unauthorized), the client SHOULD retry the request with a new
transaction. This request MUST contain a USERNAME, determined by the
client as the appropriate username for the REALM from the error
response. The request MUST contain the REALM, copied from the error
response. The request MUST contain the NONCE, copied from the error
response. The request MUST contain the MESSAGE-INTEGRITY attribute,
computed using the password associated with the username in the
USERNAME attribute. The client MUST NOT perform this retry if it is
not changing the USERNAME or 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
supplied in the 438 (Stale Nonce) response. This retry MUST also
include the USERNAME, REALM, and MESSAGE-INTEGRITY.

The client looks for the MESSAGE-INTEGRITY attribute in the response
(either success or failure). If present, the client computes the
message integrity over the response as defined in Section 15.4, using
the same password it utilized for the request. If the resulting
value matches the contents of the MESSAGE-INTEGRITY attribute, the
response is considered authenticated. If the value does not match,
or if MESSAGE-INTEGRITY was absent, the response MUST be discarded,
as if it was never received. This means that retransmits, if
applicable, will continue.

11. 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.

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 an
ALTERNATE-SERVER attribute in the error response. The error response
message MAY be authenticated; however, there are uses cases for
ALTERNATE-SERVER where authentication of the response is not possible
or practical.

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 client has been redirected to a server on which
it has already tried 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.

12. Backwards Compatibility with RFC 3489

This section defines procedures that allow a degree of backwards
compatibility with the original protocol defined in RFC 3489
[RFC3489]. This mechanism is optional, meant to be utilized only in
cases where a new client can connect to an old server, or vice versa.
A usage must define if and when this procedure is used.

Section 19 lists all the changes between this specification and RFC
3489 [RFC3489]. However, not all of these differences are important,
because "classic STUN" was only used in a few specific ways. For the
purposes of this extension, the important changes are the following.
In RFC 3489:

o UDP was the only supported transport.

o The field that is now the magic cookie field was a part of the
transaction ID field, and transaction IDs were 128 bits long.

o The XOR-MAPPED-ADDRESS attribute did not exist, and the Binding
method used the MAPPED-ADDRESS attribute instead.

o There were three comprehension-required attributes, RESPONSE-
ADDRESS, CHANGE-REQUEST, and CHANGED-ADDRESS, that have been
removed from this specification.

* CHANGE-REQUEST and CHANGED-ADDRESS are now part of the NAT
Behavior Discovery usage [BEHAVE-NAT], and the other is
deprecated.

12.1. Changes to Client Processing

A client that wants to interoperate with an [RFC3489] server SHOULD
send a request message that uses the Binding method, contains no
attributes, and uses UDP as the transport protocol to the server. If
successful, the success response received from the server will
contain a MAPPED-ADDRESS attribute rather than an XOR-MAPPED-ADDRESS
attribute. A client seeking to interoperate with an older server
MUST be prepared to receive either. Furthermore, the client MUST
ignore any Reserved comprehension-required attributes that might
appear in the response. Of the Reserved attributes in Section 18.2,
0x0002, 0x0004, 0x0005, and 0x000B may appear in Binding responses
from a server compliant to RFC 3489. Other than this change, the
processing of the response is identical to the procedures described
above.

12.2. Changes to Server Processing

A STUN server can detect when a given Binding request message was
sent from an RFC 3489 [RFC3489] client by the absence of the correct
value in the magic cookie field. When the server detects an RFC 3489
client, it SHOULD copy the value seen in the magic cookie field in
the Binding request to the magic cookie field in the Binding response
message, and insert a MAPPED-ADDRESS attribute instead of an XOR-
MAPPED-ADDRESS attribute.

The client might, in rare situations, include either the RESPONSE-
ADDRESS or CHANGE-REQUEST attributes. In these situations, the
server will view these as unknown comprehension-required attributes
and reply with an error response. Since the mechanisms utilizing
those attributes are no longer supported, this behavior is
acceptable.

The RFC 3489 version of STUN lacks both the magic cookie and the
FINGERPRINT attribute that allows for a very high probability of
correctly identifying STUN messages when multiplexed with other
protocols. Therefore, STUN implementations that are backwards

compatible with RFC 3489 SHOULD NOT be used in cases where STUN will
be multiplexed with another protocol. However, that should not be an
issue as such multiplexing was not available in RFC 3489.

13. Basic Server Behavior

This section defines the behavior of a basic, stand-alone STUN
server. A basic STUN server provides clients with server reflexive
transport addresses by receiving and replying to STUN Binding
requests.

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; however, TLS provides
minimal security benefits in this basic mode of operation. 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 [RFC3489]
clients, as described in Section 12.

It is RECOMMENDED that administrators of STUN servers provide DNS
entries for those servers as described in Section 9.

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 14.

14. 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.

At the time of writing, three STUN usages are defined: Interactive
Connectivity Establishment (ICE) [MMUSIC-ICE], Client-initiated
connections for SIP [SIP-OUTBOUND], and NAT Behavior Discovery
[BEHAVE-NAT]. Other STUN usages may be defined in the future.

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 would also define:

o Which STUN methods are used.

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, a framing mechanism may be
required.

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

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].

15. 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.

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 are ignored, and may be any value.

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 18.2.

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

15.1. 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:

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 RFC 3489 [RFC3489] clients.

15.2. 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:

Figure 6: Format of XOR-MAPPED-ADDRESS Attribute

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

X-Port is computed by taking the mapped port in host byte order,
XOR'ing it with the most significant 16 bits of the magic cookie, and
then the converting the result to network byte order. If the IP
address family is IPv4, X-Address is computed by taking the mapped IP
address in host byte order, XOR'ing it with the magic cookie, and
converting the result to network byte order. If the IP address
family is IPv6, X-Address is computed by taking the mapped IP address
in host byte order, XOR'ing it with the concatenation of the magic
cookie and the 96-bit transaction ID, and converting the result to
network byte order.

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 exclusive-or'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 at providing a generic ALG function. Such
behavior interferes with the operation of STUN and also causes
failure of STUN's message-integrity checking.

15.3. 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. It MUST contain a
UTF-8 [RFC3629] encoded sequence of less than 513 bytes, and MUST
have been processed using SASLprep [RFC4013].

15.4. 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 SHA1 hash, the HMAC will be
20 bytes. The text used as input to HMAC is the STUN message,
including the header, up to and including the attribute preceding the
MESSAGE-INTEGRITY attribute. With the exception of the FINGERPRINT
attribute, which appears after MESSAGE-INTEGRITY, agents MUST ignore
all other attributes that follow MESSAGE-INTEGRITY.

The key for the HMAC depends on whether long-term or short-term
credentials are in use. For long-term credentials, the key is 16
bytes:

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

That is, 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 SASLprep has already been applied);
(2) a single colon; (3) the realm, with any quotes and trailing nulls
removed; (4) a single colon; and (5) the password, with any trailing
nulls removed and after processing using SASLprep. For example, if
the username was 'user', the realm was 'realm', and the password was
'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.

For short-term credentials:

key = SASLprep(password)

where MD5 is defined in RFC 1321 [RFC1321] and SASLprep() is defined
in RFC 4013 [RFC4013].

The structure of the key when used with long-term credentials
facilitates deployment in systems that also utilize SIP. 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.

Based on the rules above, the hash used to construct MESSAGE-
INTEGRITY includes the length field from the STUN message header.
Prior to performing the hash, the MESSAGE-INTEGRITY attribute MUST be
inserted into the message (with dummy content). The length MUST then
be set to point to the length of the message up to, and including,
the MESSAGE-INTEGRITY attribute itself, but excluding any attributes
after it. Once the computation is performed, the value of the
MESSAGE-INTEGRITY attribute can be filled in, and the value of the
length in the STUN header can be set to its correct value -- the
length of the entire message. Similarly, when validating the
MESSAGE-INTEGRITY, the length field should be adjusted to point to
the end of the MESSAGE-INTEGRITY attribute prior to calculating the
HMAC. Such adjustment is necessary when attributes, such as
FINGERPRINT, appear after MESSAGE-INTEGRITY.

15.5. 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 helps in cases where an
application packet is also using CRC-32 in it). The 32-bit CRC is
the one defined in ITU V.42 [ITU.V42.2002], which has a generator
polynomial of x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x+1.
When present, the FINGERPRINT attribute MUST be the last attribute in
the message, and thus will appear after MESSAGE-INTEGRITY.

The FINGERPRINT attribute can aid in distinguishing STUN packets from
packets of other protocols. See Section 8.

As with MESSAGE-INTEGRITY, the CRC used in the FINGERPRINT attribute
covers the length field from the STUN message header. Therefore,
this value must be correct and include the CRC attribute as part of
the message length, prior to computation of the CRC. 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
then the value of the attribute is updated. If the MESSAGE-INTEGRITY
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 attribute as well.

15.6. 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], and is consistent
in its code assignments and semantics with SIP [RFC3261] and HTTP
[RFC2616]. The reason phrase is meant for user consumption, 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 [RFC3629] encoded
sequence of less than 128 characters (which can be as long as 763
bytes).

Figure 7: 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 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 a USERNAME attribute and a valid MESSAGE-
INTEGRITY 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 attribute, and receivers
MUST validate the MESSAGE-INTEGRITY 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 for this error, so the client MUST NOT expect
a valid MESSAGE-INTEGRITY attribute on this response.

401 Unauthorized: 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.

15.7. REALM

The REALM attribute may be present in requests and responses. It
contains text that meets the grammar for "realm-value" as described
in RFC 3261 [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 [RFC3629] encoded sequence of less than 128 characters (which
can be as long as 763 bytes), and MUST have been processed using
SASLprep [RFC4013].

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 for authentication.

15.8. NONCE

The NONCE attribute may be present in requests and responses. It
contains a sequence of qdtext or quoted-pair, which are defined in
RFC 3261 [RFC3261]. Note that this means that the NONCE attribute
will not contain actual quote characters. See RFC 2617 [RFC2617],
Section 4.3, for guidance on selection of nonce values in a server.

It MUST be less than 128 characters (which can be as long as 763
bytes).

15.9. UNKNOWN-ATTRIBUTES

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

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

Figure 8: 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.

15.10. 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 [RFC3629]
encoded sequence of less than 128 characters (which can be as long as
763 bytes).

15.11. 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. The IP address family MUST be identical
to that of the source IP address of the request.

16. Security Considerations

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 14, 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, mitigates these attacks.
However, STUN is most often run over UDP and in those cases, strong
passwords are the only way to protect against these attacks.

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.

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.

16.2. 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 counter-measures.

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, since the
intervening NAT must be able to modify this value. Instead, one
solution to preventing the attacks listed below is for the client to
verify the reflexive address learned, as is done in ICE [MMUSIC-ICE].
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 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 would allow 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.

16.3. Hash Agility Plan

This specification uses HMAC-SHA-1 for computation of the message
integrity. If, at a later time, HMAC-SHA-1 is found to be
compromised, the following is the remedy that will be applied.

We will define a STUN extension that introduces a new message-
integrity attribute, computed using a new hash. Clients would be
required to include both the new and old message-integrity attributes
in their requests or indications. A new server will utilize the new
message-integrity attribute, and an old one, the old. After a
transition period where mixed implementations are in deployment, the
old message-integrity attribute will be deprecated by another
specification, and clients will cease including it in requests.

It is also important to note that the HMAC is done using a key that
is itself computed using an MD5 of the user's password. The choice
of the MD5 hash was made because of the existence of legacy databases
that store passwords in that form. If future work finds that an HMAC
of an MD5 input is not secure, and a different hash is needed, it can
also be changed using this plan. However, this would require
administrators to repopulate their databases.

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
[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 mandated that protocols developed for this purpose
document a specific set of considerations. Because some STUN usages
provide UNSAF functions (such as ICE [MMUSIC-ICE] ), and others do
not (such as SIP Outbound [SIP-OUTBOUND]), answers to these
considerations need to be addressed by the usages themselves.

18. IANA Considerations

IANA has created three new registries: a "STUN Methods Registry", a
"STUN Attributes Registry", and a "STUN Error Codes Registry". IANA
has also changed the name of the assigned IANA port for STUN from
"nat-stun-port" to "stun".

18.1. STUN Methods Registry

A STUN method is a hex number in the range 0x000 - 0xFFF. The
encoding of STUN method into a STUN message is described in
Section 6.

The initial STUN methods are:

0x000: (Reserved)
0x001: Binding
0x002: (Reserved; was SharedSecret)

STUN methods in the range 0x000 - 0x7FF are assigned by IETF Review
[RFC5226]. STUN methods in the range 0x800 - 0xFFF are assigned by
Designated Expert [RFC5226]. 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.

18.2. STUN Attribute 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.

The initial STUN Attributes types are:

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

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

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
[RFC5226]. 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
Designated Expert [RFC5226]. 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. STUN Error Code 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 [RFC2616].

The initial values in this registry are given in Section 15.6.

New STUN error codes are assigned based on IETF Review [RFC5226].
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 7.3.4.

18.4. STUN UDP and TCP Port Numbers

IANA has previously assigned port 3478 for STUN. This port appears
in the IANA registry under the moniker "nat-stun-port". In order to
align the DNS SRV procedures with the registered protocol service,
IANA is requested to change the name of protocol assigned to port
3478 from "nat-stun-port" to "stun", and the textual name from
"Simple Traversal of UDP Through NAT (STUN)" to "Session Traversal
Utilities for NAT", so that the IANA port registry would read:

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

In addition, IANA has assigned port number 5349 for the "stuns"
service, defined over TCP and UDP. The UDP port is not currently
defined; however, it is reserved for future use.

19. Changes since RFC 3489

This specification obsoletes RFC 3489 [RFC3489]. This specification
differs from RFC 3489 in the following ways:

o Removed the notion that STUN is a complete NAT traversal solution.
STUN is now a tool that can be used to produce a NAT traversal
solution. As a consequence, changed the name of the protocol to
Session Traversal Utilities for NAT.

o Introduced the concept of STUN usages, and described what a usage
of STUN must document.

o Removed the usage of STUN for NAT type detection and binding
lifetime discovery. These techniques have proven overly brittle
due to wider variations in the types of NAT devices than described
in this document. Removed the RESPONSE-ADDRESS, CHANGED-ADDRESS,
CHANGE-REQUEST, SOURCE-ADDRESS, and REFLECTED-FROM attributes.

o Added a fixed 32-bit magic cookie and reduced length of
transaction ID by 32 bits. The magic cookie begins at the same
offset as the original transaction ID.

o Added the XOR-MAPPED-ADDRESS attribute, which is included in
Binding responses if the magic cookie is present in the request.
Otherwise, the RFC 3489 behavior is retained (that is, Binding
response includes MAPPED-ADDRESS). See discussion in XOR-MAPPED-
ADDRESS regarding this change.

o Introduced formal structure into the message type header field,
with an explicit pair of bits for indication of request, response,
error response, or indication. Consequently, the message type
field is split into the class (one of the previous four) and
method.

o Explicitly point out that the most significant 2 bits of STUN are
0b00, allowing easy differentiation with RTP packets when used
with ICE.

o Added the FINGERPRINT attribute to provide a method of definitely
detecting the difference between STUN and another protocol when
the two protocols are multiplexed together.

o Added support for IPv6. Made it clear that an IPv4 client could
get a v6 mapped address, and vice versa.

o Added long-term-credential-based authentication.

o Added the SOFTWARE, REALM, NONCE, and ALTERNATE-SERVER attributes.

o Removed the SharedSecret method, and thus the PASSWORD attribute.
This method was almost never implemented and is not needed with
current usages.

o Removed recommendation to continue listening for STUN responses
for 10 seconds in an attempt to recognize an attack.

o Changed transaction timers to be more TCP friendly.

o Removed the STUN example that centered around the separation of
the control and media planes. Instead, provided more information
on using STUN with protocols.

o Defined a generic padding mechanism that changes the
interpretation of the length attribute. This would, in theory,
break backwards compatibility. However, the mechanism in RFC 3489
never worked for the few attributes that weren't aligned naturally
on 32-bit boundaries.

o REALM, SERVER, reason phrases, and NONCE limited to 127
characters. USERNAME to 513 bytes.

o Changed the DNS SRV procedures for TCP and TLS. UDP remains the
same as before.

20. Contributors

Christian Huitema and Joel Weinberger were original co-authors of RFC
3489.

21. Acknowledgements

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.

22. References

22.1. Normative References

[ITU.V42.2002] International Telecommunications Union, "Error-
correcting Procedures for DCEs Using Asynchronous-
to-Synchronous Conversion", ITU-T Recommendation
V.42, March 2002.

[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
October 1989.

[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",
RFC 1321, April 1992.

[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication",
RFC 2104, February 1997.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[RFC2460] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.

[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J.,
Lawrence, S., Leach, P., Luotonen, A., and L.
Stewart, "HTTP Authentication: Basic and Digest
Access Authentication", RFC 2617, June 1999.

[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS
RR for specifying the location of services (DNS
SRV)", RFC 2782, February 2000.

[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's
Retransmission Timer", RFC 2988, November 2000.

[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.

[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for
User Names and Passwords", RFC 4013, February 2005.

22.2. Informative References

[BEHAVE-NAT] MacDonald, D. and B. Lowekamp, "NAT Behavior
Discovery Using STUN", Work in Progress, July 2008.

[BEHAVE-TURN] Rosenberg, J., Mahy, R., and P. Matthews,
"Traversal Using Relays around NAT (TURN): Relay
Extensions to Session Traversal Utilities for NAT
(STUN)", Work in Progress, July 2008.

[KARN87] Karn, P. and C. Partridge, "Improving Round-Trip
Time Estimates in Reliable Transport Protocols",
SIGCOMM 1987, August 1987.

[MMUSIC-ICE] Rosenberg, J., "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal for Offer/Answer
Protocols", Work in Progress, October 2007.

[MMUSIC-ICE-TCP] Rosenberg, J., "TCP Candidates with Interactive
Connectivity Establishment (ICE)", Work
in Progress, July 2008.

[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee,
"Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2616, June 1999.

[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley,
M., and E. Schooler, "SIP: Session Initiation
Protocol", RFC 3261, June 2002.

[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with Session Description Protocol (SDP)",
RFC 3264, June 2002.

[RFC3424] Daigle, L. and IAB, "IAB Considerations for
UNilateral Self-Address Fixing (UNSAF) Across
Network Address Translation", RFC 3424,
November 2002.

[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, March 2003.

[RFC4107] Bellovin, S. and R. Housley, "Guidelines for
Cryptographic Key Management", BCP 107, RFC 4107,
June 2005.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for
Writing an IANA Considerations Section in RFCs",
BCP 26, RFC 5226, May 2008.

[SIP-OUTBOUND] Jennings, C. and R. Mahy, "Managing Client
Initiated Connections in the Session Initiation
Protocol (SIP)", Work in Progress, June 2008.

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:

#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)

Authors' Addresses

Jonathan Rosenberg
Cisco
Edison, NJ
US

EMail: [email protected]
URI: http://www.jdrosen.net

Rohan Mahy
Unaffiliated

EMail: [email protected]

Philip Matthews
Unaffiliated

EMail: [email protected]

Dan Wing
Cisco
771 Alder Drive
San Jose, CA 95035
US

EMail: [email protected]

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Rosenberg, et al. Standards Track [Page 51]