rfc5389.txt

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



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        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



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        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



































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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.






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   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.















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




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



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




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


                 Figure 1: One Possible STUN Configuration

   One possible STUN configuration is shown in Figure 1.  In this
   configuration, there are two entities (called STUN agents) that
   implement the STUN protocol.  The lower agent in the figure is the
   client, 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





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   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.



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   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).




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   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.




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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.

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

                  Figure 2: Format of STUN Message Header

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

   The message type defines the message class (request, success
   response, 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.













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   The message type field is decomposed further into the following
   structure:

                        0                 1
                        2  3  4 5 6 7 8 9 0 1 2 3 4 5

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

                Figure 3: Format of STUN Message Type Field

   Here the bits in the 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



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   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).




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   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




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   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.






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   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.




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   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.




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   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



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   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



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   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



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   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.




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   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




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   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.



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   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:




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      *  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



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   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.




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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.






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   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.








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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.





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   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



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   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:



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   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.

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

                    Figure 4: Format of STUN Attributes




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   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.
















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   The format of the MAPPED-ADDRESS attribute is:

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

               Figure 5: Format of MAPPED-ADDRESS Attribute

   The address family can take on the following values:

   0x01:IPv4
   0x02:IPv6

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

   This attribute is used only by servers for achieving backwards
   compatibility with 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:

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

             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.





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   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.




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   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.






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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).

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

                      Figure 7: ERROR-CODE Attribute



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   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.



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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.

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


             Figure 8: Format of UNKNOWN-ATTRIBUTES Attribute






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      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.




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   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



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   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.




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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



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   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:



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   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.






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   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.







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   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.



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   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.





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   [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.




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   [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.





















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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: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Rohan Mahy
   Unaffiliated

   EMail: rohan@ekabal.com


   Philip Matthews
   Unaffiliated

   EMail: philip_matthews@magma.ca


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

   EMail: dwing@cisco.com










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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
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   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at
   ietf-ipr@ietf.org.












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