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Versions: (RFC 3489) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 RFC 5389

BEHAVE                                                      J. Rosenberg
Internet-Draft                                             Cisco Systems
Expires: January 12, 2007                                     C. Huitema
                                                               Microsoft
                                                                 R. Mahy
                                                             Plantronics
                                                                 D. Wing
                                                           Cisco Systems
                                                           July 11, 2006


  Simple Traversal Underneath Network Address Translators (NAT) (STUN)
                    draft-ietf-behave-rfc3489bis-04

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on January 12, 2007.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Simple Traversal Underneath NATs (STUN) is a lightweight protocol
   that serves as a tool for application protocols in dealing with NAT
   traversal.  It allows a client to determine the IP address and port



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   allocated to them by a NAT and to keep NAT bindings open.  It can
   also serve as a check for connectivity between a client and a server
   in the presence of NAT, and for the client to detect failure of the
   server.  STUN works with many existing NATs, and does not require any
   special behavior from them.  As a result, it allows a wide variety of
   applications to work through existing NAT infrastructure.


Table of Contents

   1.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  5
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   5.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  7
   6.  STUN Message Structure . . . . . . . . . . . . . . . . . . . . 11
   7.  STUN Transactions  . . . . . . . . . . . . . . . . . . . . . . 14
     7.1.   Request/Response Transactions . . . . . . . . . . . . . . 14
     7.2.   Indications . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1.   Discovery . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.2.   Obtaining a Shared Secret . . . . . . . . . . . . . . . . 16
     8.3.   Request/Response Transactions . . . . . . . . . . . . . . 17
       8.3.1.  Formulating the Request Message  . . . . . . . . . . . 17
       8.3.2.  Processing Responses . . . . . . . . . . . . . . . . . 18
     8.4.   Indication Transactions . . . . . . . . . . . . . . . . . 21
   9.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 22
     9.1.   Request/Response Transactions . . . . . . . . . . . . . . 22
       9.1.1.  Receive Request Message  . . . . . . . . . . . . . . . 23
       9.1.2.  Constructing the Response  . . . . . . . . . . . . . . 25
       9.1.3.  Sending the Response . . . . . . . . . . . . . . . . . 26
     9.2.   Indication Transactions . . . . . . . . . . . . . . . . . 26
   10. Demultiplexing of STUN and Application Traffic . . . . . . . . 27
   11. STUN Attributes  . . . . . . . . . . . . . . . . . . . . . . . 28
     11.1.  MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . . . 28
     11.2.  USERNAME  . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.3.  PASSWORD  . . . . . . . . . . . . . . . . . . . . . . . . 29
     11.4.  MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 30
     11.5.  FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 30
     11.6.  ERROR-CODE  . . . . . . . . . . . . . . . . . . . . . . . 30
     11.7.  REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.8.  NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.9.  UNKNOWN-ATTRIBUTES  . . . . . . . . . . . . . . . . . . . 32
     11.10. XOR-MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . 33
     11.11. SERVER  . . . . . . . . . . . . . . . . . . . . . . . . . 34
     11.12. ALTERNATE-SERVER  . . . . . . . . . . . . . . . . . . . . 34
     11.13. REFRESH-INTERVAL  . . . . . . . . . . . . . . . . . . . . 34
   12. STUN Usages  . . . . . . . . . . . . . . . . . . . . . . . . . 34



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     12.1.  Binding Discovery . . . . . . . . . . . . . . . . . . . . 35
       12.1.1. Applicability  . . . . . . . . . . . . . . . . . . . . 35
       12.1.2. Client Discovery of Server . . . . . . . . . . . . . . 36
       12.1.3. Server Determination of Usage  . . . . . . . . . . . . 36
       12.1.4. New Requests or Indications  . . . . . . . . . . . . . 37
       12.1.5. New Attributes . . . . . . . . . . . . . . . . . . . . 37
       12.1.6. New Error Response Codes . . . . . . . . . . . . . . . 37
       12.1.7. Client Procedures  . . . . . . . . . . . . . . . . . . 37
       12.1.8. Server Procedures  . . . . . . . . . . . . . . . . . . 37
       12.1.9. Security Considerations for Binding Discovery  . . . . 37
     12.2.  Connectivity Check  . . . . . . . . . . . . . . . . . . . 37
       12.2.1. Applicability  . . . . . . . . . . . . . . . . . . . . 37
       12.2.2. Client Discovery of Server . . . . . . . . . . . . . . 38
       12.2.3. Server Determination of Usage  . . . . . . . . . . . . 38
       12.2.4. New Requests or Indications  . . . . . . . . . . . . . 38
       12.2.5. New Attributes . . . . . . . . . . . . . . . . . . . . 39
       12.2.6. New Error Response Codes . . . . . . . . . . . . . . . 39
       12.2.7. Client Procedures  . . . . . . . . . . . . . . . . . . 39
       12.2.8. Server Procedures  . . . . . . . . . . . . . . . . . . 39
       12.2.9. Security Considerations for Connectivity Check . . . . 39
     12.3.  NAT Keepalives  . . . . . . . . . . . . . . . . . . . . . 39
       12.3.1. Applicability  . . . . . . . . . . . . . . . . . . . . 39
       12.3.2. Client Discovery of Server . . . . . . . . . . . . . . 40
       12.3.3. Server Determination of Usage  . . . . . . . . . . . . 40
       12.3.4. New Requests or Indications  . . . . . . . . . . . . . 40
       12.3.5. New Attributes . . . . . . . . . . . . . . . . . . . . 40
       12.3.6. New Error Response Codes . . . . . . . . . . . . . . . 40
       12.3.7. Client Procedures  . . . . . . . . . . . . . . . . . . 41
       12.3.8. Server Procedures  . . . . . . . . . . . . . . . . . . 41
       12.3.9. Security Considerations for NAT Keepalives . . . . . . 41
     12.4.  Short-Term Password . . . . . . . . . . . . . . . . . . . 41
       12.4.1. Applicability  . . . . . . . . . . . . . . . . . . . . 42
       12.4.2. Client Discovery of Server . . . . . . . . . . . . . . 42
       12.4.3. Server Determination of Usage  . . . . . . . . . . . . 42
       12.4.4. New Requests or Indications  . . . . . . . . . . . . . 42
       12.4.5. New Attributes . . . . . . . . . . . . . . . . . . . . 43
       12.4.6. New Error Response Codes . . . . . . . . . . . . . . . 43
       12.4.7. Client Procedures  . . . . . . . . . . . . . . . . . . 43
       12.4.8. Server Procedures  . . . . . . . . . . . . . . . . . . 44
       12.4.9. Security Considerations for Short-Term Password  . . . 44
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 46
     13.1.  Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 46
       13.1.1. Attack I: DDoS Against a Target  . . . . . . . . . . . 46
       13.1.2. Attack II: Silencing a Client  . . . . . . . . . . . . 47
       13.1.3. Attack III: Assuming the Identity of a Client  . . . . 47
       13.1.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 47
     13.2.  Launching the Attacks . . . . . . . . . . . . . . . . . . 47
       13.2.1. Approach I: Compromise a Legitimate STUN Server  . . . 48



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       13.2.2. Approach II: DNS Attacks . . . . . . . . . . . . . . . 48
       13.2.3. Approach III: Rogue Router or NAT  . . . . . . . . . . 48
       13.2.4. Approach IV: Man in the Middle . . . . . . . . . . . . 49
       13.2.5. Approach V: Response Injection Plus DoS  . . . . . . . 49
       13.2.6. Approach VI: Duplication . . . . . . . . . . . . . . . 50
     13.3.  Countermeasures . . . . . . . . . . . . . . . . . . . . . 50
     13.4.  Residual Threats  . . . . . . . . . . . . . . . . . . . . 52
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 52
     14.1.  Problem Definition  . . . . . . . . . . . . . . . . . . . 52
     14.2.  Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 52
     14.3.  Brittleness Introduced by STUN  . . . . . . . . . . . . . 53
     14.4.  Requirements for a Long Term Solution . . . . . . . . . . 54
     14.5.  Issues with Existing NAPT Boxes . . . . . . . . . . . . . 55
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 56
     15.1.  STUN Message Type Registry  . . . . . . . . . . . . . . . 56
     15.2.  STUN Attribute Registry . . . . . . . . . . . . . . . . . 56
   16. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 57
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 58
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 58
     18.1.  Normative References  . . . . . . . . . . . . . . . . . . 58
     18.2.  Informational References  . . . . . . . . . . . . . . . . 59
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 61
   Intellectual Property and Copyright Statements . . . . . . . . . . 62




























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1.  Applicability Statement

   This protocol is not a cure-all for the problems associated with NAT.
   It is a tool that is typically used in conjunction with other
   protocols, such as Interactive Connectivity Establishment (ICE) [11]
   for a more complete solution.  The binding discovery usage, defined
   by this specification, can be used by itself with numerous
   application protocols as a solution for NAT traversal.  However, when
   used in that way, STUN will not work with applications that require
   incoming TCP connections through NAT.  It will allow incoming UDP
   packets through NAT, but only through a subset of existing NAT types.
   In particular, the STUN binding usage by itself does not enable
   incoming UDP packets through NATs whose mapping property is address
   dependent or address and port dependent [12].  Furthermore, the
   binding usage, when used by itself, does not work when a client is
   communicating with a peer which happens to be behind the same NAT.
   Nor will it work when the STUN server is not in a common shared
   address realm.

   The STUN relay usage, defined in [14], allows a client to obtain an
   IP address and port that actually reside on the STUN server.  The
   STUN relay usage, when used by itself, eliminates all of the
   limitations of using the binding usage by itself, as described above.
   However, it requires a server to act as a relay for application
   traffic, which can be expensive to provide, operate and manage.

   For multimedia protocols based on the offer/answer model [20],
   including the Session Initiation Protocol (SIP) [9], Interactive
   Connectivity Establishment (ICE) uses both the binding usage and
   relay usage, and furthermore makes use of the connectivity check
   usage defined here to help decide which of those two mechanisms ought
   to be used.

   Implementors should be aware of the specific deployment scenarios
   that are of interest, and of the specific protocol (whether its SIP
   or something else) in order to determine whether STUN is suitable as
   a tool to facilitate NAT traversal, and which usage should be used.


2.  Introduction

   Network Address Translators (NATs), while providing many benefits,
   also come with many drawbacks.  The most troublesome of those
   drawbacks is the fact that they break many existing IP applications,
   and make it difficult to deploy new ones.  Guidelines have been
   developed [18] that describe how to build "NAT friendly" protocols,
   but many protocols simply cannot be constructed according to those
   guidelines.  Examples of such protocols include almost all peer-to-



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   peer protocols, such as multimedia communications, file sharing and
   games.

   To combat this problem, Application Layer Gateways (ALGs) have been
   embedded in NATs.  ALGs perform the application layer functions
   required for a particular protocol to traverse a NAT.  Typically,
   this involves rewriting application layer messages to contain
   translated addresses, rather than the ones inserted by the sender of
   the message.  ALGs have serious limitations, including scalability,
   reliability, and speed of deploying new applications.

   Many existing proprietary protocols, such as those for online games
   (such as the games described in RFC3027 [19]) and Voice over IP, have
   developed tricks that allow them to operate through NATs without
   changing those NATs and without relying on ALG behavior in the NATs.
   This document takes some of those ideas and codifies them into an
   interoperable protocol that can meet the needs of many applications.

   The protocol described here, Simple Traversal Underneath NAT (STUN),
   provides a toolkit of functions.  These functions allow entities
   behind a NAT to learn the address bindings allocated by the NAT, to
   keep those bindings open, and communicate with other STUN-aware
   entities to validate connectivity and liveness.  STUN requires no
   changes to NATs, and works with an arbitrary number of NATs in tandem
   between the application entity and the public Internet.


3.  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
   [1] and indicate requirement levels for compliant STUN
   implementations.


4.  Definitions

   STUN Client: A STUN client (also just referred to as a client) is an
      entity that generates STUN requests and receives STUN responses.
      Clients can also generate STUN indications.

   STUN Server: A STUN Server (also just referred to as a server) is an
      entity that receives STUN requests, and sends STUN responses.
      Servers also send STUN indications.






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

   Reflexive Transport Address: A transport address learned by a client
      which 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 binding 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: The source IP address and port of the STUN Binding
      Request packet received by the STUN server and inserted into the
      mapped address attribute (MAPPED-ADDRESS or XOR-MAPPED-ADDRESS) of
      the Binding Response message.

   Long Term Credential: A username and associated password which
      represent a shared secret between client and server.  Long term
      credentials are generally granted to the client when a subscriber
      enrolles in a service and persists 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
      which represent a shared secret between client and server.  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.  A
      short term credential can be obtained from a Shared Secret
      request, though other mechanisms are possible.

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


5.  Overview of Operation

   This section is descriptive only.  Normative behavior is described in
   Section 8 and Section 9









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


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


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

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


   Figure 1: Typical STUN Server Configuration

   The typical STUN configuration is shown in Figure 1.  A STUN client
   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 STUN server resides on the public
   Internet.

   STUN is a simple client-server protocol.  It supports two types of
   transactions.  One is a request/response transaction in which client
   sends a request to a server, and the server returns a response.  The
   second are indications which are initiated by the server or the
   client and do not elicit a response.  There are two types of requests
   defined in this specification - Binding Requests and Shared Secret
   Requests.  There are no indications defined by this specification.

   Binding Requests are sent from the client towards the 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 a result, the source
   transport address of the request received by the server will be the
   mapped address created by the NAT closest to the server.  The STUN
   server copies that source IP address and port into a STUN Binding
   Response, and sends it back to the source IP address and port of the
   STUN request.  Every type of NAT will route that response so that it



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   arrives at the STUN client.  From this response, the client knows its
   IP address and port allocated by the outermost NAT towards the STUN
   server.

   STUN provides several mechanisms for authentication and message
   integrity.  The client and server can share a pre-provisioned shared
   secret, which is used for a digest challenge/response authentication
   operation.  This is known as a long-term credential or long-term
   shared secret.

   Alternatively, if the shared secret is obtained by some out-of-bands
   means and has a lifetime that is temporally scoped, a simple HMAC is
   provided, without a challenge operation.  This is known as a short
   term credential or short term password.  Short-term passwords are
   useful when there is no long-term relationship with a STUN server and
   thus no long-term password is shared between the STUN client and STUN
   server.  Even if there is a long-term password, the issuance of a
   short-term password is useful to prevent dictionary attacks.

   STUN itself provides a mechanism for obtaining such short term
   credentials, using the Shared Secret Request.  Shared Secret requests
   are sent over TLS [5] over TCP.  Shared Secret Requests ask the
   server to return a temporary username and password that can be used
   in subsequent STUN requests.

   There are many ways in which these basic mechanisms can be used to
   accomplish a specific task.  As a result, STUN has the notion of a
   usage.  A usage is a specific use case for the STUN protocol.  The
   usage will define what it is that the client does with the mapped
   address it receives, defines when the client would send Binding
   requests and why, and would constrain the set of authentication
   mechanisms or attributes that get used in that usage.  STUN usages
   can also define new attributes and message types, if needed.  This
   specification defines four STUN usages - binding discovery,
   connectivity check, NAT keepalives, and short-term password.

   The binding discovery usage is sometimes referred to as 'classic
   STUN', since it is the usage originally envisioned in RFC 3489 [13],
   the predecessor to this specification.  The purpose of the binding
   discovery usage is for the client to obtain an IP address and port at
   which it is reachable, that it can include in application layer
   signaling messages, such as the Session Description Protocol (SDP)
   [17] body of a SIP message, utilized to receive Real Time Transport
   Protocol (RTP [15]) traffic.  In this usage, the STUN server is
   typically located on the public Internet and run by the service
   provider offering the application service (such as a SIP provider),
   though this need not be the case.  The client would utilize the STUN
   request just prior to sending a protocol message (such as a SIP



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   INVITE request or 200 OK response) which requires the client to embed
   its IP address in it.

   In the connectivity check usage, two hosts on the Internet have used
   a protocol such as SIP to rendezvous, and have used it to exchange IP
   addresses and ports at which they might be reachable.  However, each
   host does not know whether it can actually connect to the other host
   using that IP address and port, and whether that remote host can
   reach it.  To figure this out, each host will send a STUN Binding
   Request to the other host, and if a reply is received, it knows that
   the remote host was reachable.  Furthermore, the mapped address
   returned in the response tells the host the address and port at which
   it can be reached by the remote host.  The connectivity check usage
   is used by ICE [11], for example.

   In the binding keepalive usage, a client sends an application
   protocol message (such as a SIP REGISTER message) to a server.  The
   server notes the source IP address and port of the request, and
   remembers it.  Later on, if it needs to reach the client, it sends a
   message to that IP address and port.  However, this message will only
   be received by the client if the binding in the NAT is still alive.
   Since bindings allocated by NAT expire unless refreshed, the client
   must generate keepalive messages towards the server to refresh the
   binding.  Rather than use expensive application layer messages, a
   STUN binding request is sent by the client to the server, and is sent
   to the exact same IP address and port used by the server for the
   application protocol.  In the case of SIP, this would typically mean
   port 5060 or 5061.  This has the effect of keeping the bindings in
   the NAT alive.  The STUN binding responses also inform the client
   that the server is still responsive, and also inform the client if
   its IP address and port towards the server have changed, in which
   case it may need application layer protocol messaging to update its
   IP address and port as seen by the server.  The binding keepalive
   usage is used by the SIP outbound mechanism, for example [16].

   These three usages all utilize the same Binding Request message, and
   all require the same basic processing on the server.  They differ
   only in where the server is (embedded in a peer host, a standalone
   server in the network, or embedded in an application layer server),
   when the Binding Request is used, and what the client does with the
   mapped address that is returned.

   The short-term password usage makes use of the Shared Secret request
   and response, and allows a client to obtain a temporary set of
   credentials to authenticate itself with the STUN server.  The
   credentials obtained from this usage can be used in requests for any
   other usage.




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   Many of the usages (such as the binding keepalive and connectivity
   check usages) require STUN messages to be sent on the same IP address
   and port as some application protocol, such as RTP or SIP.  To
   facilitate the demultiplexing of the two, STUN defines a special
   field in the message called the magic cookie, which is a fixed 32 bit
   value that identifies STUN traffic.  STUN requests also contain a
   fingerprint, which is a cryptographic hash of the message, that allow
   for validation that the message was a STUN request and not a data
   packet that happened to have the same 32 bit value in the right place
   in the message.

   STUN servers can be discovered through DNS, though this is not
   necessary in all usages.  For those usages where it is needed, STUN
   makes use of SRV records [3] to facilitate discovery.  This discovery
   allows for different IP addresses and ports to be found for different
   usages.


6.  STUN Message Structure

   STUN messages are TLV (type-length-value) encoded using big endian
   (network ordered) binary.  STUN messages are encoded using binary
   fields.  All integer fields are carried in network byte order, that
   is, most significant byte (octet) first.  This byte order is commonly
   known as big-endian.  The transmission order is described in detail
   in Appendix B of RFC791 [2].  Unless otherwise noted, numeric
   constants are in decimal (base 10).  All STUN messages start with a
   single STUN header followed by a STUN payload.  The payload is a
   series of STUN attributes, the set of which depends on the message
   type.  The STUN header contains a STUN message type, transaction ID,
   and length.  The length indicates the total length of the STUN
   payload, not including the 20-byte header.

   There are two types of transactions in STUN - request/response
   transactions, which utilize a request message and a response message,
   and an indication transaction, which utilizes a single indication
   message.  Furthermore, responses are broken into two types - success
   responses and error responses.  The specific message (for example,
   that it is a Binding Response or a Shared Secret Request) is encoded
   into the message type field of the STUN header.  For a particular
   request message, its success response has a type that is always 0x100
   higher than its own type, and its error response has a type that is
   0x110 higher than its own type.  Extensions defining new requests,
   responses and error responses MUST use message type values 0x100 and
   0x110 higher for their success and error responses, respectively.

   STUN Requests are sent reliably.  STUN can run over UDP or TCP.  When
   run over UDP, STUN requests are retransmitted in order to achieve



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   reliability.  The transaction ID is used to correlate requests and
   responses.

   An indication message can be sent from the client to the server, or
   from the server to the client.  Indication messages can be sent over
   TCP or UDP.  STUN itself does not provide reliability for these
   messages, though they will be delivered reliably when sent over TCP.
   Indication messages do not have an associated success response
   message type or associated error response message type.  Indication
   messages can be sent from the server to the client or client to
   server.  The transaction ID is used to distinguish indication
   messages.

   All STUN messages consist of a 20 byte header:

         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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                                                        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 2: Format of STUN Message Header

   The most significant two bits of every STUN message are both zeroes.
   This, combined with the magic cookie and the fingerprint attribute,
   aid in differentiating STUN packets from other protocols when STUN is
   multiplexed with other protocols on the same port.

   The STUN message types Binding Request, Response, and Error Response
   are defined in Section 8 and Section 9.1.  The Shared Secret Request,
   Response, and Error Response are described in Section 12.4.  Their
   values are enumerated in Section 15.

   The message length is the size, in bytes, of the message not
   including the 20 byte STUN header.

   The magic cookie is a fixed value, 0x2112A442.  In the previous
   version of this specification [13] this field was part of the
   transaction ID.  This fixed value is used as part of the
   identification of a STUN message when STUN is multiplexed with other



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   protocols on the same port, as is done for example in [11] and [16].
   The magic cookie additionally indicates the STUN client is compliant
   with this specification.  The magic cookie is present in all STUN
   messages -- requests, success responses, error responses and
   indications.

   The transaction ID is a 96 bit identifier.  STUN transactions are
   identified by their unique 96-bit transaction ID.  For request/
   response transactions, the transaction ID is chosen by the STUN
   client and MUST be unique for each new STUN transaction generated by
   that STUN client.  Any two requests that are not bit-wise identical,
   and not sent to the same server from the same IP address and port,
   MUST have a different transaction ID.  The transaction ID MUST be
   uniformly and randomly distributed between 0 and 2**96 - 1.  The
   large range is needed because the transaction ID serves as a form of
   randomization, helping to prevent replays of previously signed
   responses from the server.  A reponse to the STUN request, whether it
   be a success or error response, carries the same transaction ID as
   the request.  Indications are also identified by their transaction
   ID.  The value is chosen by the server an MUST be unique for each
   unique indication generated by the server.  Any two requests that are
   not bit-wise identical, and not sent to the same server from the same
   IP address and port, MUST have a different transaction ID.  The
   transaction ID MUST be uniformly and randomly distributed between 0
   and 2**96 - 1.

   After the STUN header are zero or more attributes.  Each attribute is
   TLV encoded, with a 16 bit type, 16 bit length, and variable value.
   Each STUN attribute ends on a 32 bit boundary:

         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                 ....        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3: Format of STUN Attributes

   The Length refers to the length of the actual useful content of the
   Value portion of the attribute, 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 they are a multiple of 4 btyes.  Such padding is only needed
   with attributes that take freeform strings, such as USERNAME and
   PASSWORD.  For attributes that contain more structured data, the
   attributes are constructed to align on 32 bit boundaries.  The value



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   in the Length field refers to the length of the Value part of the
   attribute prior to padding - i.e., the useful content.  Consequently,
   when parsing messages, implementations will need to round up the
   Length field to the nearest multiple of four in order to find the
   start of the next attribute.

   The attribute types defined in this specification are in Section 11 .


7.  STUN Transactions

   STUN defines two types of transactions - request/response
   transactions and indication transactions.

7.1.  Request/Response Transactions

   STUN clients are allowed to pipeline STUN requests.  That is, a STUN
   client MAY have multiple outstanding STUN requests with different
   transaction IDs and not wait for completion of a STUN request/
   response exchange before sending another STUN request.

   When running STUN over UDP it is possible that the STUN request or
   its response might be dropped by the network.  Reliability of STUN
   request message types is is accomplished through client
   retransmissions.  Clients SHOULD retransmit the request starting with
   an interval of 100ms, doubling every retransmit until the interval
   reaches 1.6 seconds.  Retransmissions continue with intervals of 1.6
   seconds until a response is received, or a total of 9 requests have
   been sent.  If no response is received by 1.6 seconds after the last
   request has been sent, the client SHOULD consider the transaction to
   have failed.  In other words, requests would be sent at times 0ms,
   100ms, 300ms, 700ms, 1500ms, 3100ms, 4700ms, 6300ms, and 7900ms.  At
   9500ms, the client considers the transaction to have failed if no
   response has been received.  A STUN transaction over UDP is also
   considered failed if there has been a transport failure of some sort,
   such as a fatal ICMP error.

   When running STUN over TCP, TCP is responsible for ensuring delivery.
   The STUN application SHOULD NOT retransmit STUN requests when running
   over TCP.  If the client has not received a response after 9500ms, it
   considers the transaction to have failed.

   Regardless of whether TCP or UDP was used for the transaction, if a
   failure occurs and the client has other servers it can reach (as a
   consequence of an SRV query which provides a multiplicity of STUN
   servers Section 8.1, for example), the client SHOULD create a new
   request, which is identical to the previous, but has a different
   transaction ID (and consequently a different MESSAGE INTEGRITY and



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   FINGERPRINT attribute).

7.2.  Indications

   Indications are sent from the client to the server, or from the
   server to the client.  Though no indications are used by this
   specification, they are used by the STUN relay usage [14].  When sent
   over UDP, there are no retransmissions, and reliability is not
   provided.  When sent over TCP, reliability is provided by TCP.

   If a data indication solicits a fatal ICMP error, or causes a TCP
   error, the transaction is considered to have failed.  In such a case,
   the client SHOULD create a new transaction, which is identical to the
   previous, but has a different transaction ID (and consequently a
   different MESSAGE INTEGRITY and FINGERPRINT attribute).  That request
   is sent to the next element in the list as specified by RFC2782.


8.  Client Behavior

   Client behavior can be broken down into several steps.  The first is
   discovery of the STUN server.  The next is obtaining a shared secret.
   For request/response transactions, the next steps are formulating the
   request and processing the response.  For indication transactions,
   the next step is formulating the indication.

8.1.  Discovery

   Unless stated otherwise by a STUN usage, DNS is used to discover the
   STUN server following these procedures.

   The client will be configured with a domain name of the provider of
   the STUN servers.  This domain name is resolved to an IP address and
   port using the SRV procedures specified in RFC2782 [3].  The
   mechanism for configuring the STUN client with the domain name to
   look up is not in scope of this document.

   The DNS SRV service name depends on the application usage.  For the
   binding usage, the service name is "stun".  The protocol can be "udp"
   for UDP, "tcp" for TCP and "tls" for TLS over TCP.  For the short
   term password application usage, the service name is "stun-pass".
   The protocol is always "tls" for TLS over TCP.  The binding keepalive
   and connectivity check usages always start with an IP address, so no
   DNS SRV service names are defined for them.  New STUN usages MAY
   define additional DNS SRV service names.  These SHOULD start with
   "stun".

   The procedures of RFC 2782 are followed to determine the server to



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   contact.  RFC 2782 spells out the details of how a set of SRV records
   are sorted and then tried.  However, RFC2782 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;
   those details for STUN are described in Section 8.3.1.

   A STUN server supporting multiple usages (such as the short term
   password and binding discovery usage) MAY use the same ports for
   different usages, as long as ports are not needed to differentiate
   the usages.  Different ports are not needed to differentiate the
   usages defined in this specification.  Different ports SHOULD be used
   for TCP and TCP/TLS, so that the server can determine whether the
   first message it will receive after the TCP connection is set up is a
   STUN message or a TLS message.

   The default port for STUN requests is 3478, for both TCP and UDP.
   There is no default port for STUN over TLS.  Administrators SHOULD
   use this port in their SRV records for UDP and TCP, but MAY use
   others.  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, such as the short term password usage, lack of SRV
   records is equivalent to a failure of the transaction, since the
   request or indication MUST NOT be sent unless SRV records provided an
   address and port specifically for TLS.

8.2.  Obtaining a Shared Secret

   As discussed in Section 13, there are several attacks possible on
   STUN systems.  Many of these attacks are prevented through integrity
   protection of requests and responses.  To provide that integrity,
   STUN makes use of a shared secret between client and server which is
   used as the keying material for the MESSAGE-INTEGRITY attribute in
   STUN messages.  STUN allows for the shared secret to be obtained in
   any way.  The application usage defines the mechanism and required
   implementation strength for shared secrets.

   Some usages, such as the connectivity check usage, assume that out of
   band protocols, such as ICE, are used to obtain the necessary
   credentials.  Other usages, such as binding keepalives, don't use
   authentication, as it is not required.  Others, such as the binding
   discovery, allows for authentication using either a long term shared
   secret or a short term shared secret.  The latter can be obtained by
   using the short term password usage to obtain a short term shared
   secret.

   Consequently, the STUN usages define rules for obtaining shared



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   secrets prior to sending a request.

8.3.  Request/Response Transactions

8.3.1.  Formulating the Request Message

   The client follows the syntax rules defined in Section 6 and the
   transmission rules of Section 7.  The message type of the MUST be a
   request type; "Binding Request" or "Shared Secret Request" are the
   two defined by this document.

   The client creates a STUN message following the STUN message
   structure described in Section 6.  The client SHOULD add a MESSAGE-
   INTEGRITY and USERNAME attribute to the Request message if the usage
   employs authentication.  The specific credentials to use are
   described by the STUN usage, which can specify no credentials, a
   short term credential, or a long term credential.  The procedures for
   each are:

   1.  If the STUN usage specifies that no credentials are used, the
       message is sent without MESSAGE-INTEGRITY

   2.  If a short term credential is to be used, the STUN Request or
       STUN Indication would contain the USERNAME and MESSAGE-INTEGRITY
       attributes.  The message would not contain the NONCE attribute.
       The key for MESSAGE-INTEGRITY is the password.

   3.  If a long term credential is to be used, the STUN request
       contains the USERNAME, REALM, and MESSAGE-INTEGRITY attributes.
       The 16-byte key for MESSAGE-INTEGRITY HMAC 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, (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.  For example, if the USERNAME
       field were 'user', the REALM field were '"realm"', and the
       PASSWORD field were 'pass', then the 16-byte HMAC key would be
       the result of performing an MD5 hash on the string 'user:realm:
       pass', or 0x8493fbc53ba582fb4c044c456bdc40eb.

      This format for the key was chosen so as to enable a common
      authentication database for SIP, which uses digest authentication
      as defined in RFC 2617 [7] and STUN, as it is expected that
      credentials are usually stored in their hashed forms.

   The NONCE is included in the request only if a short or long term
   credential is being used, and only if the STUN request is a retry as
   a consequence of a previous error response which provided the client



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   with a NONCE.

   For TCP and TLS-over-TCP, the client opens a TCP connection to the
   server.  The TLS_RSA_WITH_AES_128_CBC_SHA ciphersuite MUST be
   supported at a minimum by implementers when TLS is used with STUN.
   Implementers 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, 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 [4].
   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 8.1as the host portion of
   the URI that has been dereferenced.  If DNS was not used, the client
   MUST be configured with a set of authorized domains whose
   certificates will be accepted.

   The client MUST include a FINGERPRINT attribute as the last
   attribute.

   Next, the client sends the request.  For UDP-based requests,
   reliability is accomplished through client retransmissions, following
   the procedure in Section 7.1.  For TCP (including TLS over TCP),
   there are no retransmissions.

   For TCP and TLS over TCP, the client MAY send multiple requests on
   the connection, and it MAY pipeline requests (that is, it can have
   multiple requests outstanding at the same time).  When using TCP or
   TLS over TCP, the client SHOULD close the connection as soon as it
   has received the STUN Response, if it has no plans to send further
   requests.

8.3.2.  Processing Responses

   All responses, whether success responses or error responses, MUST
   first be authenticated by the client.  Authentication is performed by
   first comparing the Transaction ID of the response to an oustanding
   request.  If there is no match, the client MUST discard the response.
   Then the client SHOULD check the response for a MESSAGE-INTEGRITY
   attribute.  If not present, and the client placed a MESSAGE-INTEGRITY
   attribute into the associated request, it MUST discard the response.
   If MESSAGE-INTEGRITY is present, the client computes the HMAC over
   the response as described in Section 11.4.  The key that is used MUST
   be same as used to compute the MESSAGE-INTEGRITY attribute in the
   request.



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   If the computed HMAC matches the one from the response, processing
   continues.

   If the response is an Error Response, the client checks the response
   code from the ERROR-CODE attribute of the response.  For a 400
   response code, the client SHOULD display the reason phrase to the
   user.  For a 420 response code, the client SHOULD retry the request,
   this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
   attribute of the response.

   If the client receives a 401 response and had not included a MESSAGE-
   INTEGRITY attribute in the request, it is an indication from the
   server that credentials are required.  If the REALM attribute was
   present in the response, it is a signal to the client to use a long
   term shared secret and retry the request.  The client SHOULD retry
   the request, using the username and password associated with the
   REALM.  If the server had provided a nonce in the 401 response, the
   client SHOULD include the same nonce in the retry.  If the REALM
   attribute was absent in the resposne, it is a signal to the client to
   use a short term shared secret and retry the request.  If the client
   doesn't have a short term shared secret, it SHOULD use the Shared
   Secret request to obtain one, and then retry the request with the
   username and password obtained as a result.  If the 401 response had
   contained a NONCE attribute, that same nonce is included in the
   retry.

   If the client receives a 401 response but had included a MESSAGE-
   INTEGRITY attribute in the request, there has been an unrecoverable
   error.  This shouldn't ever happen, but if it does, the client SHOULD
   NOT retry the request.

   If the client receives a 432 response, and the client had omitted the
   USERNAME from the request but included a MESSAGE-INTEGRITY, the
   client SHOULD retry the request and include both MESSAGE-INTEGRITY
   and USERNAME.  If the 432 response had contained a NONCE attribute,
   that same nonce is included in the retry.  If the client receives a
   432 but had included both MESSAGE-INTEGRITY and USERNAME in the
   request, there has been an unrecoverable error.  This shouldn't ever
   happen, but if it does, the client SHOULD NOT retry the request.

   If the client receives a 435 response, but had included a NONCE in
   the request, an unrecoverable error has occurred and the client
   SHOULD NOT retry.  However, if it had omitted the NONCE in the
   request and received a 435, or it had included the NONCE but received
   a 438, it is a request from the server to retry using the same
   credential, but with a different nonce.  The client SHOULD retry the
   request, using the nonce provided in the NONCE attribute of the 435
   or 438 response.



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   If the client receives a 430 response, it means that the client used
   a short term credential which has expired.  If the client had
   submitted the request using a short term credential obtained from a
   Shared Secret request, the client SHOULD generate a new Shared Secret
   request to obtain a new short term credential, and then retry the
   request with that credential.  Note that the Shared Secret request
   may or may not go to the same server which generated the 430
   response; the server that receives the Shared Secret request is
   determined by the DNS procedures defined above.  If a 430 response
   was received and the client had used a short term credential provided
   through some other means, the client SHOULD obtain a new short term
   credential using that mechanism.  If the client had not used a short
   term credential in the request, the 430 error is unrecoverable and
   the request SHOULD NOT be retried.  If the 430 response had contained
   a NONCE attribute, that same nonce is included in the retry.

   For a 431 response code, the client SHOULD alert the user, and if a
   short term credential obtained from a Shared Secret request had been
   used previously, the client MAY try the request again after obtaining
   a new short term username and password.  If the 431 response had
   contained a NONCE attribute, that same nonce is included in the
   retry.

   If the client receives a 433 response, and the request was a Shared
   Secret request which was not sent over TLS, the client SHOULD retry
   the request, and MUST send it using TLS.  If this response is
   received to any other request except for a Shared Secret request, or
   if the client had sent the Shared Secret request over TLS, it is an
   unrecoverable error and the client SHOULD NOT retry.  As with other
   error responses, the 433 can contain a NONCE, and if present, that
   nonce is used in the request retry.

   If the client receives a 434 response, and had omitted the REALM in
   the request, but had included MESSAGE-INTEGRITY, it is an indication
   that, though a short-term credential was used for the request, the
   server desires the client to use a long term credential.  The client
   SHOULD retry the request using the username and password associated
   with the REALM.  As with other error responses, the 434 can contain a
   NONCE, and if present, that nonce is used in the request retry.  If
   the 434 was received but the request had contained a REALM, and the
   REALM in the response differs from the REALM in the request, the
   client SHOULD retry using the username and password associated with
   the REALM in the response.  If the REALMS were equal, this is an
   unrecoverable error and the client SHOULD NOT retry.

   It the client receives a 436 response, it means that the username it
   provided in the request is unknown.  For usages where the username
   was collectedd from the user, the client SHOULD alert the user.  The



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   client SHOULD NOT retry with the same username.

   For error responses which can contain a NONCE, the client includes
   the NONCE in a subsequent retry as discussed above.  Furthermore, the
   client SHOULD cache the nonce, and continue using it in subsequent
   requests sent to the same server, identified by IP address and port.

   For a 300 response code, the client SHOULD attempt a new transaction
   to the server indicated in the ALTERNATE-SERVER attribute.  This is
   useful for load balancing requests across a STUN server cluster, when
   those requests require some amount of resources to process.  Though
   this specification allows the 300 response to be applied to Binding
   Requests, it is generally not useful to do so, since the work of
   redirecting a Binding Request is equal to, if not more than, the work
   of just processing the Binding Request.  Consequently, the 300
   response code is targeted for other usages of STUN, such as the relay
   usage [14].

   For a 500 response code, the client MAY wait several seconds and then
   retry the request on the same server.  Or, if the server was learned
   through DNS SRV records, the client MAY try the request on a
   different server.  The same username and password MAY be used.  For a
   600 response code, client MUST NOT retry the request on this server,
   or if the server was learned through DNS, any other server found
   through the DNS resolution procedures.  Unknown response codes
   between 400 and 499 are treated like a 400, unknown response codes
   between 500 and 599 are treated like a 500, and unknown response
   codes between 600 and 699 are treated like a 600.  Any response
   between 100 and 299 MUST result in the cessation of request
   retransmissions, but otherwise is discarded.

   Responses containing unknown optional attributes (greater than
   0x7FFF) MUST be ignored by the STUN client.  Responses containing
   unknown mandatory attributions (less than or equal to 0x7FFF) MUST be
   discarded and considered immediately as a failed transaction.

   It is also possible for an IPv4 host to receive a XOR-MAPPED-ADDRESS
   or MAPPED-ADDRESS containing an IPv6 address, or for an IPv6 host to
   receive a XOR-MAPPED-ADDRESS or MAPPED-ADDRESS containing an IPv4
   address.  Clients MUST be prepared for this case.

   The processing of other attributes in the response, such as the
   mapped address (present in the XOR-MAPPED-ADDRESS attribute or
   MAPPED-ADDRESS attribute) depends on the STUN usage.

8.4.  Indication Transactions

   This section applies to client and server behavior for sending an



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   Indication message.

   The client or server follows the syntax rules defined in Section 6
   and the transmission rules of Section 7.  The message type MUST be
   one of the Indication message types; none are defined by this
   document.

   Indication messages cannot be challenged or rejected.  Consequently,
   they cannot be authenticated using long term credentials.  If a STUN
   usage specifies that authentication is needed for an indication
   message, it can only be done using a short term credential.  In that
   case, the client or server MUST add a MESSAGE-INTEGRITY and USERNAME
   attribute to the Request message.  The key for MESSAGE-INTEGRITY is
   the password.

   The client or server MUST include a FINGERPRINT attribute as the last
   attribute of any Indication message.

   Typically, indication messages are sent to the same IP address and
   port, or over the same TCP connections as a previous request message.
   However, a usage can specify that indication messages are sent based
   on a DNS query, in which case the discovery procedures in Section 8.1
   are followed, along with the TCP/TLS connection establishment
   procedures defined in Section 8.3.1.

   Indication message types are not sent reliably, do not elicit a
   response from the server, and are not retransmitted.

   For TCP and TLS over TCP, the client or server MAY send multiple
   indications on the connection.  By definition, since indications to
   not generate a response, they can be pipelined on the connection.
   For clients, the connection is closed once it determines it has no
   further messages to send to the server.  Servers do not normally
   close TCP connections.


9.  Server Behavior

   As with clients, server behavior depends on whether it is a request/
   response transaction or indication.

9.1.  Request/Response Transactions

   The server behavior for receiving request message types is described
   in this section.






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9.1.1.  Receive Request Message

   A STUN server MUST be prepared to receive request messages on the IP
   address and UDP or TCP port that will be discovered by the STUN
   client when the STUN client follows its discovery procedures
   described in Section 8.1.  Depending on the usage, the STUN server
   will listen for incoming UDP STUN messages, incoming TCP STUN
   messages, or incoming TLS exchanges followed by TCP STUN messages.

   When a STUN request is received, the server determines the usage.
   The usages describe how the STUN server makes this determination.

   Based on the usage, the server determines whether the request
   requires any authentication and message integrity checks.  It can
   require none, short-term credential based authentication, or long-
   term credential authentication.

   If authentication is required, the server checks for the presence of
   the MESSAGE-INTEGRITY attribute.  If not present, the server
   generates an error response with an ERROR-CODE attribute and a
   response code of 401.  If the server wishes the client to use a short
   term credential, the REALM is omitted from the response.  If the
   server wishes the client to use a long term credential, the REALM is
   included in the response containing a realm from which the username
   and password are scoped [7].  When the REALM is present in the
   response, the server MUST include a NONCE attribute in the response.
   The nonce includes a random value that the server wishes the client
   to reflect back in a subsequent request (and therefore include in the
   message integrity computation).  When the REALM is absent in the
   response, the server MAY include a NONCE in the response if it wishes
   to use nonces along with short-term shared secrets.  However, there
   is little reason to do so, since the short-term password is, by
   definition, short-term, and thus additional temporal scoping through
   the nonce is not needed.

   If the MESSAGE-INTEGRITY attribute was present, the server checks for
   the existence of the USERNAME attribute.  If it was not present, the
   server MUST generate an error response.  The error response MUST
   include an ERROR-CODE attribute with a response code of 432.  If the
   server is using a long term credential for authentication, the
   response MUST include a REALM and a NONCE.  If the server is using a
   short-term credential, it MUST NOT include a REALM in the response
   and MAY include a NONCE.

   If the server is using long term credentials for authentication, and
   the request contained the MESSAGE-INTEGRITY and USERNAME attributes,
   the server checks for the existence of the REALM attribute.  If the
   attribute is not present, the server MUST generate an error response.



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   That error response MUST include an ERROR-CODE attribute with
   response code of 434.  That error response MUST also include a NONCE
   and a REALM attribute.

   If the REALM attribute was present and the server is using a long
   term credential for authentication, the server checks for the
   existence of the NONCE attribute.  If the NONCE attribute is not
   present, the server MUST generate an error response.  That error
   response MUST include an ERROR-CODE attribute with a response code of
   435.  That error response MUST include a NONCE attribute and a REALM
   attribute.  If the NONCE was absent and the server is authenticating
   with short term credentials, the server MAY generate an error
   response with an ERROR-CODE attribute with a response code of 435.
   This response MUST included a NONCE.  If the NONCE was present in the
   request, but the server has determined it is stale, the server MUST
   generate an error response with an ERROR-CODE attribute with a
   response code of 438.

   Next, if the server is authenticating the request, it checks for the
   presence of the USERNAME attribute.  If absent, the server generates
   an error response with an ERROR-CDE attribute with a response code of
   432.  If the server is authenticating using long term credentials, it
   MUST include a REALM and NONCE in the response.  If the server is
   authenticating with short term credentials, it MUST NOT include a
   REALM and MAY include a NONCE.

   If the server is authenticating the request with a short term
   credential, it checks the value of the USERNAME field.  If the
   USERNAME was previously valid but has expired, the server generates
   an error response with an ERROR-CODE attribute with a response code
   of 430.  This error response MAY include a NONCE.  If the server is
   authenticating with either short or long term credentials, it
   determines whether the USERNAME contains a known entity, and in the
   case of a long-term credential, known within the realm of the REALM
   attribute of the request.  If the USERNAME is unknown, the server
   generates an error response with an ERROR-CODE attribute with a
   response code of 436.  For authentication using long-term
   credentials, that error response MUST include a NONCE attribute and a
   REALM attribute.  For authentication using short-term credentials, it
   MAY include a NONCE but MUST NOT include a REALM.

   At this point, if the server is doing authentication, the request
   contains everything needed for that purpose.  The server computes the
   HMAC over the request as described in Section 11.4.  The key depends
   on the credential.  For short-term credentials, it equals the
   password associated with the username.  For long term credentials, it
   is computed as described in Section 8.3.1.




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   If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
   attribute in the request, the server MUST generate an error response
   with an ERROR-CODE attribute with a response code of 431.  If long
   term credentials are being used for authentication, this response
   MUST include a NONCE attribute and a REALM attribute.  If short term
   credentials are being used, it MAY include a NONCE and MUST NOT
   include a REALM.

   At this point, the request has been authentication checked and
   integrity verified.

   Next, the server MUST check for any mantadory attributes in the
   request (values less than or equal to 0x7fff) which it does not
   understand.  If it encounters any, the server MUST generate an error
   response, and it MUST include an ERROR-CODE attribute with a 420
   response code.  Any attributes that are known, but are not supposed
   to be present in a message (MAPPED-ADDRESS in a request, for example)
   MUST be ignored.

9.1.2.  Constructing the Response

   To construct the STUN Response the STUN server follows the message
   structure described in Section 6.  The server then copies the
   Transaction ID from the Request to the Response.  If the STUN
   response is a success response, the STUN server adds 0x100 to the
   Message Type of the request.  If the STUN response is a success
   response, the STUN server adds 0x110 to the Message Type of the
   request.

   The attributes that get added to the response depend on the type of
   response.  See Figure 4 for a summary.

   If the response is a type which can carry either MAPPED-ADDRESS or
   XOR-MAPPED-ADDRESS (the Binding Response as defined in this
   specification meets that criteria), the server examines the magic
   cookie in the STUN header.  If it has the value 0x2112A442, it
   indicates that the client supports this version of the specification.
   The server MUST insert a XOR-MAPPED-ADDRESS into the response,
   carrying the exclusive-or of the source IP address and port from the
   request.  If the magic cookie did not have this value, it indicates
   that the client supports the previous version of this specification.
   The server MUST insert a MAPPED-ADDRESS attribute into the response,
   carrying the souce IP address and port from the request.  Insertion
   of either XOR-MAPPED-ADDRESS or MAPPED-ADDRESS happens regardless of
   the transport protocol used for the request.

   XOR-MAPPED-ADDRESS and MAPPED-ADDRESS differ only in their encoding
   of the IP address and port.  The former, as implied by the name,



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   encodes the IP address and port by exclusive-or'ing them with the
   magic cookie.  The latter encodes them 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.

   The server SHOULD include a SERVER attribute in all responses,
   indicating the identity of the server generating the response.  This
   is useful for diagnostic purposes.

   The server MUST include a FINGERPRINT attribute as the last attribute
   of any Indication message.

   In cases where the server cannot handle the request, due to
   exhaustion of resources, the server MAY generate a 300 response with
   an ALTERNATE-SERVER attribute.  This attribute identifies an
   alternate server which can service the requests.  It is not expected
   that 300 responses or this attribute would be used by the requests
   defined in this specification.

9.1.3.  Sending the Response

   All UDP response messages are sent to the IP address and port the
   associated Binding Request came from, and sent from the IP address
   and port the Binding Request was sent to.  All TCP or TLS over TCP
   responses messages are sent on the TCP connections that the request
   arrived on.

9.2.  Indication Transactions

   Indication messages cause the server to change its state.  Indication
   message types to not cause the server to send a response message.

   A STUN server MUST be prepared to receive indication messages on the
   IP address and UDP or TCP port that will be discovered by the STUN
   client when the STUN client follows its discovery procedures
   described in Section 8.1.  Depending on the usage, the STUN server
   will listen for incoming UDP STUN messages, incoming TCP STUN
   messages, or incoming TLS exchanges followed by TCP STUN messages.

   When a STUN indication is received, the server determines the usage.
   The usages describe how the STUN server makes this determination.

   Based on the usage, the server determines whether the indication
   requires any authentication and message integrity checks.  It can



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   require none or short-term credential based authentication.  If
   short-term credentials are utilized, the server follows the same
   procedures as defined in Section 9.1.1, but if those procedures
   require transmission of an error response, the server MUST instead
   silently discard the indication.

   Once authenticated (if authentication was in use), the processing of
   the indication message depends on the message type.  This
   specification doesn't define any indication messages.


10.  Demultiplexing of STUN and Application Traffic

   In both the connectivity check and binding refresh usages, STUN
   traffic is multiplexed on the same IP address and port as application
   traffic, such as RTP.  In order to apply the processing described in
   this specification, STUN messages must first be separated from the
   application packets.  This disambiguation is done identically for
   requests and responses of all types.

   First, all STUN messages start with two bits equal to zero.  If STUN
   is being multiplexed with application traffic where it is known that
   the topmost two bits are never zeroes, the presence of these two
   zeroes signals STUN traffic.

   If this mechanism doesn't suffice, the magic cookie can be used.  All
   STUN messages have the value 0x2112A442 as the second 32 bit word.
   If the application traffic can not have this value as the second 32
   bit word, then any packets with this value are STUN packets.  Even if
   the application packet can have this value (for example, in cases
   where the application packets contain random binary data), there is
   only a one in 2^32 chance that an application packet will have a
   value of 0x2112A442 in its second 32 bit word.  If this probability
   is deemed sufficiently small for the application at hand (for
   example, it is considered adequate for Voice over IP applications),
   then any packet with this value in its second 32 bit word is
   processed as a STUN packet.

   However, a mis-classification of 1 in 2^32 may still be too high for
   some usages of STUN.  Consequently, all STUN messages contain a
   FINGERPRINT attribute.  This is a cryptographic hash over the
   message, covering everything prior to the attribute.  This attribute
   is different from MESSAGE-INTEGRITY.  The latter uses a keyed HMAC,
   and thus requires a shared secret.  FINGERPRINT does not use a
   password, and can be computed just by examining the STUN message.
   Thus, if a packet appears to be a STUN message because it has a value
   of 0x2112A442 in its second 32 bit word, a client or server then
   assumes the message is a STUN message, and computes the value for the



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   fingerprint.  It then looks for the FINGERPRINT attribute in the
   message, and if the value equals the computed value, the message is
   considered to be a STUN message.  If not, it is considered to be an
   application packet


11.  STUN Attributes

   To allow future revisions of this specification to add new attributes
   if needed, the attribute space is divided into optional and mandatory
   ones.  Attributes with values greater than 0x7fff are optional, which
   means that the message can be processed by the client or server even
   though the attribute is not understood.  Attributes with values less
   than or equal to 0x7fff are mandatory to understand, which means that
   the client or server cannot successfully process the message unless
   it understands the attribute.

   The values of the message attributes are enumerated in Section 15.

   The following figure indicates which attributes are present in which
   messages.  An M indicates that inclusion of the attribute in the
   message is mandatory, O means its optional, C means it's conditional
   based on some other aspect of the message, and - means that the
   attribute is not applicable to that message type.

                                                   Error
              Attribute         Request  Response Response Indication
              _______________________________________________________
              MAPPED-ADDRESS       -        C         -       -
              USERNAME             O        -         -       O
              PASSWORD             -        C         -       -
              MESSAGE-INTEGRITY    O        O         O       O
              ERROR-CODE           -        -         M       -
              ALTERNATE-SERVER     -        -         C       -
              REALM                C        -         C       -
              NONCE                C        -         C       -
              UNKNOWN-ATTRIBUTES   -        -         C       -
              XOR-MAPPED-ADDRESS   -        C         -       -
              SERVER               -        O         O       O
              REFRESH-INTERVAL     -        O         -       -
              FINGERPRINT          M        M         M       M

   Figure 4: Mandatory Attributes and Message Types

11.1.  MAPPED-ADDRESS

   The MAPPED-ADDRESS attribute indicates the mapped IP address and
   port.  It consists of an eight bit address family, and a sixteen bit



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   port, followed by a fixed length value representing the IP address.
   If the address family is IPv4, the address is 32 bits.  If the
   address family is IPv6, the address is 128 bits.

   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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |x x x x x x x x|    Family     |           Port                |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                   Address  (variable)
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 5: Format of MAPPED-ADDRESS attribute

   The address family can take on the following values:

   0x01:IPv4
   0x02:IPv6

   The port is a network byte ordered representation of the port the
   request arrived from.

   The first 8 bits of the MAPPED-ADDRESS are ignored for the purposes
   of aligning parameters on natural 32 bit boundaries.

11.2.  USERNAME

   The USERNAME attribute is used for message integrity.  It identifies
   the shared secret used in the message integrity check.  Consequently,
   the USERNAME MUST be included in any request that contains the
   MESSAGE-INTEGRITY attribute.

   The USERNAME is also always present in a Shared Secret Response,
   along with the PASSWORD, which informs a client of a short term
   password.

   The value of USERNAME is a variable length opaque value.  Note that,
   as described above, if the USERNAME is not a multiple of four bytes
   it is padded for encoding into the STUN message, in which case the
   attribute length represents the length of the USERNAME prior to
   padding.

11.3.  PASSWORD

   If the message type is Shared Secret Response it MUST include the
   PASSWORD attribute.



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   The value of PASSWORD is a variable length opaque value.  The
   password returned in the Shared Secret Response is used as the HMAC
   in the MESSAGE-INTEGRITY attribute of a subsequent STUN transaction.
   Note that, as described above, if the USERNAME is not a multiple of
   four bytes it is padded for encoding into the STUN message, in which
   case the attribute length represents the length of the USERNAME prior
   to padding.

11.4.  MESSAGE-INTEGRITY

   The MESSAGE-INTEGRITY attribute contains an HMAC-SHA1 [8] 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.  That text is then padded with zeroes so as to
   be a multiple of 64 bytes.  As a result, the MESSAGE-INTEGRITY
   attribute is generally the next to last attribute in any STUN
   message.  With the exception of the FINGERPRINT attribute, which
   appears after MESSAGE-INTEGRITY, elements MUST ignore all other
   attributes which follow MESSAGE-INTEGRITY.

   The key used as input to HMAC depends on the STUN usage and the
   shared secret mechanism.

11.5.  FINGERPRINT

   The FINGERPRINT attribute is present in all STUN messages.  It is
   computed as a SHA1 hash of the STUN message up to (but excluding) the
   FINGERPRINT attribute itself.  The value of the attribute is the
   actual binary output of the SHA-1 function.  The FINGERPRINT
   attribute MUST be the last attribute in the message.

11.6.  ERROR-CODE

   The ERROR-CODE attribute is present in the Binding Error Response and
   Shared Secret Error Response.  It is a numeric value in the range of
   100 to 699 plus a textual reason phrase encoded in UTF-8, and is
   consistent in its code assignments and semantics with SIP [9] and
   HTTP [10].  The reason phrase is meant for user consumption, and can
   be anything appropriate for the response code.  Recommended reason
   phrases for the defined response codes are presented below.









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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.

         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                     |Class|     Number    |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Reason Phrase (variable)                                ..
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The class represents the hundreds digit of the response code.  The
   value MUST be between 1 and 6.  The number represents the response
   code modulo 100, and its value MUST be between 0 and 99.

   If the reason phrase has a length that is not a multiple of four
   bytes, it is padded for encoding into the STUN message, in which case
   the attribute length represents the length of the entire ERROR-CODE
   attribute (including the reason phrase) prior to padding.

   The following response codes, along with their recommended reason
   phrases (in brackets) are defined at this time:

   300  (Try Alternate): The client should contact an alternate server
        for this request.

   400  (Bad Request): The request was malformed.  The client should not
        retry the request without modification from the previous
        attempt.

   401  (Unauthorized): The request did not contain a MESSAGE-INTEGRITY
        attribute.

   420  (Unknown Attribute): The server did not understand a mandatory
        attribute in the request.

   430  (Stale Credentials): The request did contain a MESSAGE-INTEGRITY
        attribute, but it used a shared secret that has expired.  The
        client should obtain a new shared secret and try again.

   431  (Integrity Check Failure): The request contained a MESSAGE-
        INTEGRITY attribute, but the HMAC failed verification.  This
        could be a sign of a potential attack, or client implementation
        error.







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   432  (Missing Username): The request contained a MESSAGE-INTEGRITY
        attribute, but not a USERNAME attribute.  Both USERNAME and
        MESSAGE-INTEGRITY must be present for integrity checks.

   433  (Use TLS): The Shared Secret request has to be sent over TLS,
        but was not received over TLS.

   434  (Missing Realm): The REALM attribute was not present in the
        request.

   435  (Missing Nonce): The NONCE attribute was not present in the
        request.

   436  (Unknown Username): The USERNAME supplied in the request is not
        known or is not known to the server.

   438  (Stale Nonce): The NONCE attribute was present in the request
        but wasn't valid.

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

   600  (Global Failure): The server is refusing to fulfill the request.
        The client should not retry.

11.7.  REALM

   The REALM attribute is present in requests and responses.  It
   contains text which meets the grammar for "realm" as described in RFC
   3261 [9], and will thus contain a quoted string (including the
   quotes).

   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.

11.8.  NONCE

   The NONCE attribute is present in requests and in error responses.
   It contains a sequence of qdtext or quoted-pair, which are defined in
   RFC 3261 [9].  See RFC 2617 [7] for guidance on selection of nonce
   values in a server.

11.9.  UNKNOWN-ATTRIBUTES

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



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   The attribute contains a list of 16 bit values, each of which
   represents an attribute type that was not understood by the server.
   If the number of unknown attributes is an odd number, one of the
   attributes MUST be repeated in the list, so that the total length of
   the list is a multiple of 4 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
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Attribute 1 Type           |     Attribute 2 Type        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |      Attribute 3 Type           |     Attribute 4 Type    ...
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Figure 8: Format of UNKNOWN-ATTRIBUTES attribute

11.10.  XOR-MAPPED-ADDRESS

   The XOR-MAPPED-ADDRESS attribute is present in responses.  It
   provides the same information that would present in the MAPPED-
   ADDRESS attribute but because the NAT's public IP address is
   obfuscated through the XOR function, STUN messages are able to pass
   through NATs which would otherwise interfere with STUN.

   This attribute MUST always be present in a Binding Response and may
   be used in other responses as well.  Usages defining new requests and
   responses should specify if XOR-MAPPED-ADDRESS is applicable to their
   responses.

   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 9: Format of XOR-MAPPED-ADDRESS Attribute

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

   X-Port is the mapped port, exclusive or'd with most significant 16
   bits of the magic cookie.  If the IP address family is IPv4,
   X-Address is the mapped IP address exclusive or'd with the magic



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   cookie.  If the IP address family is IPv6, the X-Address is the
   mapped IP address exclusively or'ed with the magic cookie and the 96-
   bit transaction ID.

   For example, using the "^" character to indicate exclusive or, if the
   IP address is 192.168.1.1 (0xc0a80101) and the port is 5555 (0x15B3),
   the X-Port would be 0x15B3 ^ 0x2112 = 0x34A1, and the X-Address would
   be 0xc0a80101 ^ 0x2112A442 = 0xe1baa543.

11.11.  SERVER

   The server attribute contains a textual description of the software
   being used by the server, including 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
   SERVER is variable length.  If the value of SERVER is not a multiple
   of four bytes, it is padded for encoding into the STUN message, in
   which case the attribute length represents the length of the USERNAME
   prior to padding.

11.12.  ALTERNATE-SERVER

   The alternate server represents an alternate IP address and port for
   a different STUN server to try.  It is encoded in the same way as
   MAPPED-ADDRESS.

   This attribute is MUST only appear in an error response.

11.13.  REFRESH-INTERVAL

   The REFRESH-INTERVAL indicates the number of milliseconds that the
   server suggests the client should use between refreshes of the NAT
   bindings between the client and server.  Even though the server may
   not know the binding lifetimes in intervening NATs, this attribute
   serves as a useful configuration mechanism for suggesting a value for
   use by the client.  Furthermore, when the NAT Keepalive usage is
   being used, the server may become overloaded with Binding Requests
   that are being used for keepalives.  The REFRESH-INTERVAL provies a
   mechanism for the server to gradually reduce the load on itself by
   pushing back on the client.

   REFRESH-INTERVAL is specified as an unsigned 32 bit integer, and
   represents an interval measured in millseconds.  It can be present in
   Binding Responses.


12.  STUN Usages




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   STUN is a simple request/response protocol that provides a useful
   capability in several situations.  In this section, different usages
   of STUN are described.  Each usage may differ in how STUN servers are
   discovered, when the STUN requests are sent, what message types are
   used, what message attributes are used, and how authentication is
   performed.

   This specification defines the STUN usages for binding discovery
   (Section 12.1), connectivity check (Section 12.2), NAT keepalives
   (Section 12.3) and short-term password (Section 12.4).

   New STUN usages may be defined by other standards-track documents.
   New STUN usages MUST describe their applicability, client discovery
   of the STUN server, how the server determines the usage, new message
   types (requests or indications), new message attributes, new error
   response codes, and new client and server procedures.

12.1.  Binding Discovery

   The previous version of this specification, RFC3489 [13], described
   only this binding discovery usage.

12.1.1.  Applicability

   Binding discovery is used to learn reflexive addresses from servers
   on the network, generally the public Internet.  That is, it is used
   by a client to determine its dynamically-bound 'public' IP address
   and UDP port that is assigned by a NAT between a STUN client and a
   STUN server.  This address and port will be present in the mapped
   address of the STUN Binding Response.

   The mapped address present in the binding response can be used by
   clients to facilitate traversal of NATs for many applications.  NAT
   traversal is problematic for applications which require a client to
   insert an IP address and port into a message, to which subsequent
   messages will be delivered by other entities in a network.  Normally,
   the client would insert the IP address and port from a local
   interface into the message.  However, if the client is behind a NAT,
   this local interface will be a private address.  Clients within other
   address realms will not be able to send messages to that address.

   An example of a such an application is SIP, which requires a client
   to include IP address and port information in several places,
   including the Session Description Protocol (SDP [17]) body carried by
   SIP.  The IP address and port present in the SDP is used for receipt
   of media.

   To use STUN as a technique for traversal of SIP and other protocols,



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   when the client wishes to send a protocol message, it figures out the
   places in the protocol data unit where it is supposed to insert its
   own IP address along with a port.  Instead of directly using a port
   allocated from a local interface, the client allocates a port from
   the local interface, and from that port, generates a STUN Binding
   Request.  The mapped address in the Binding Response (XOR-MAPPED-
   ADDRESS or MAPPED-ADDRESS) provides the client with an alternative IP
   address and port which it can then include in the protocol payload.
   This IP address and port may be within a different address family
   than the local interfaces used by the client.  This is not an error
   condition.  In such a case, the client would use the learned IP
   address and port as if the client was a host with an interface within
   that address family.

   In the case of SIP, to populate the SDP appropriately, a client would
   generate two STUN Binding Request messages at the time a call is
   initiated or answered.  One is used to obtain the IP address and port
   for RTP, and the other, for the Real Time Control Protocol
   (RTCP)[15].  The client might also need to use STUN to obtain IP
   addresses and ports for usage in other parts of the SIP message.  The
   detailed usage of STUN to facilitate SIP NAT traversal is outside the
   scope of this specification.

   As discussed above, the addresses learned by STUN may not be usable
   with all entities with whom a client might wish to communicate.  The
   way in which this problem is handled depends on the application
   protocol.  The ideal solution is for a protocol to allow a client to
   include a multiplicity of addresses and ports in the PDU.  One of
   those can be the address and port determined from STUN, and the
   others can include addresses and ports learned from other techniques.
   The application protocol would then provide a means for dynamically
   detecting which one works.  An example of such an an approach is
   Interactive Connectivity Establishment (ICE [11]).

12.1.2.  Client Discovery of Server

   Clients SHOULD be configured with a domain name for a STUN server to
   use.  In cases where the client has no explicit configuration
   mechanism for STUN, but knows the domain of its service provider, the
   client SHOULD use that domain (in the case of SIP, this would be the
   domain from their Address-of-Record).  The discovery mechanisms
   defined in Section 8.1 are then applied to that domain name.

12.1.3.  Server Determination of Usage

   It is anticipated that servers would advertise a specific port in the
   DNS for the Binding Discovery usage.  Thus, when a request arrives at
   that particular port, the server knows that the binding usage is in



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   use.  This fact is only needed for purposes of determining the
   authentication and message integrity mechanism to apply.

12.1.4.  New Requests or Indications

   This usage does not define any new message types.

12.1.5.  New Attributes

   This usage does not define any new message attributes.

12.1.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.1.7.  Client Procedures

   The binding discovery is utilized by a client just prior to
   generating an application PDU that requires the client to include its
   IP address and port.  The client MAY first obtain a short term
   credential using the short term password STUN usage.  The credential
   that is obtained is then using in Binding Request messages.  A
   Binding Request message is generated for each distinct IP address and
   port that the client requires to formulate the application PDU.

12.1.8.  Server Procedures

   It is RECOMMENDED that servers utilize short term credentials,
   obtained by the client from a Shared Secret request, for
   authentication and message integrity.  Consequently, if a Binding
   Request is generated without a short term credential, the server
   SHOULD challenge for one.

12.1.9.  Security Considerations for Binding Discovery

   There are no security considerations for this usage beyond those
   described in Section 13.

12.2.  Connectivity Check

12.2.1.  Applicability

   This STUN usage primarily provides a connectivity check to a peer
   discovered through rendezvous protocols.  The usage presumes that
   some other mechanism, such as ICE [11] has been used allow two peer
   agents to exchange IP addresses and ports.  The agents would like to
   initiate direct communications with each other, using those IP
   addresses and ports.  However, it is not known which IP addresses and



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   ports will actually work for direct exchange of communications, due
   to NAT, firewall and other network connectivity issues.
   Consequently, each agent uses a STUN Binding Request from each of
   their transport addresses to each of the transport addresses of their
   peers.  This check serves numerous purposes.  Firstly, if a response
   is received to a Binding Request, an agent knows that that particular
   5-tuple (the transport address that the Binding Request was sent to,
   along with the one it was sent from) are viable for direct
   communciations.  Secondly, the mapped address from the Binding
   Response tells the agent its reflexive address towards its peer,
   which may be another candidate for receipt of communications.
   Finally, the connectivity checks can keep NAT bindings in intervening
   NATs active.

   It is fundamental to this STUN usage that the addresses and ports
   used for direct communications are the same ones used for the Binding
   Requests and responses.  Consequently, it will be necessary to
   demultiplex STUN traffic from whatever the application data traffic
   is.  This demultiplexing is done using the magic cookie along with
   the FINGERPRINT attribute.

   Because the connectivity check usage is always used in conjunction
   with some kind of rendezvous protocol, it is assumed that the
   rendezvous protocol provides the exchange of a short term credential.
   This credential is then used for authentication and message integrity
   of the STUN Binding Requests and Responses.

   The username and password exchanged in the rendezvous protocol is
   valid for the duration of the connection being checked.

12.2.2.  Client Discovery of Server

   The client does not follow the general procedure in Section 8.1.
   Instead, the client discovers the STUN server's IP address and port
   through a rendezvous protocol.  Note that the STUN server is a
   logical entity in this usage, and it will be running on the exact
   same IP address and port as is used for actual communications.

12.2.3.  Server Determination of Usage

   The server is aware of this usage because it signalsed this port
   through the rendezvous protocol.  Any STUN packets received on this
   port will be for the connectivity check usage.

12.2.4.  New Requests or Indications

   This usage does not define any new message types.




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12.2.5.  New Attributes

   This usage does not define any new message attributes.

12.2.6.  New Error Response Codes

   This usage does not define any new error response codes.

12.2.7.  Client Procedures

   A client will generate a connectivity check based on the procedures
   defined in the rendezvous protocol that uses this usage.  Generally
   this will be done after an exchange of IP addresses and ports has
   occurred between the two clients, at which point connectivity checks
   will begin.

   Clients MUST include the FINGERPRINT attribute, to aid in
   disambiguation of STUN and application traffic.  Clients MUST used a
   short term credential obtained through the rendezvous protocol.  The
   specific structure and format of the username and password are
   defined by the rendezvous protocol.  Receipt of any non-recoverable
   STUN error is an indication that there is no connectivity to that IP
   address and port.

12.2.8.  Server Procedures

   In this usage, the short-term password is valid as long as the UDP
   port is listening for STUN packets.  For example when used with ICE,
   the short-term password would be valid as long as the RTP session
   (which multiplexes STUN and RTP) is active.

12.2.9.  Security Considerations for Connectivity Check

   The username and password, which are used for STUN's message
   integrity, are exchanged in the rendezvous protocol.  Failure to
   encrypt and integrity protect the rendezvous protocol is equivalent
   in risk to using STUN without message integrity.

12.3.  NAT Keepalives

12.3.1.  Applicability

   In this STUN usage, a client is connected to a server for a
   particular application protocol (for example, a SIP proxy server).
   The connection is long-lived, allowing for asynchronous messaging
   from the server to the client.  The client is connected to the server
   either using TCP, in which case there is a long-lived TCP connection
   from the client to the server, or using UDP, in which case the server



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   stores the source IP address and port of a message from a client
   (such as SIP REGISTER), and sends messages to the client using that
   IP address and port.

   Since the connection between the client and server is very-long
   lived, the bindings established by that connection need to be
   maintained in any intervening NATs.  Rather than implement expensive
   application-layer keepalives, the keepalives can be accomplished
   using STUN Binding Requests.  The client will periodically sending a
   Binding Request to the server, using the same IP address and ports
   used for the application protocol.  These Binding Requests are
   demultiplexed at the server using the magic cookie and possibly
   FINGERPRINT.  The response from the server informs the client that
   the server is still alive.  The STUN message also keeps the binding
   active in intervening NATs.  The client can also examine the mapped
   address in the Binding Response.  If it has changed, the client can
   re-initiate application layer procedures to inform the server of its
   new IP address and port.

12.3.2.  Client Discovery of Server

   In this usage, the STUN server and the application protocol are using
   the same fixed port.  While the multiplexing of two applications on
   the same port is similar to the connectivity check (Section 12.2)
   usage, this usage is differs as the server's port is fixed and the
   server's port isn't communicated using a rendezvous protocol.

12.3.3.  Server Determination of Usage

   The server multiplexes both STUN and its application protocol on the
   same port.  The server knows it is has this usage because the URI
   that gets resolved to this port indicates the server supports this
   multiplexing.

12.3.4.  New Requests or Indications

   This usage does not define any new message types.

12.3.5.  New Attributes

   This usage does not define any new message attributes.

12.3.6.  New Error Response Codes

   This usage does not define any new error response codes.






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12.3.7.  Client Procedures

   If the STUN Response indicates the client's mapped address has
   changed from the client's expected mapped address, the client SHOULD
   inform other applications of its new mapped address.  For example, a
   SIP client should send a new registration message indicating the new
   mapped address.

   The client SHOULD NOT include a MESSAGE-INTEGRITY attribute, since
   authentication is not used with this STUN usage.

12.3.8.  Server Procedures

   The server MUST NOT authenticate the client or look for a MESSAGE-
   INTEGRITY attribute.  Authentication is not used with this STUN
   usage.

12.3.9.  Security Considerations for NAT Keepalives

   This STUN usage does not employ message integrity or authentication
   of any sort.  This is because the client never actually uses the
   mapped address from the STUN response.  It merely treats a change in
   that address as a hint that the client should re-apply application
   layer procedures for connection establishment and registration.

   An attacker could attempt to inject faked responses, or modify
   responses in transit.  Such an attack would require the attacker to
   be on-path in order to determine the transaction ID.  In the worse
   case, the attack would cause the client to see a change in IP address
   or port, and then perform an application layer re-registration.  Such
   a re-registration would not use the IP address and port obtained from
   the Binding Response.  Thus, the worst that the attacker can do is
   cause the client to re-register every half minute or so, when it
   otherwise wouldn't need to.  Given the difficulty in launching this
   attack (it requires the attacker to be on-path and to disrupt the
   actual response from the server) compared to the benefit, there is
   little motivation for authentication or integrity mechanisms.

12.4.  Short-Term Password

   In order to ensure interoperability, this usage describes a TLS-based
   mechanism to obtain a short-term credential.  The usage makes use of
   the Shared Secret Request and Response messages.  It is defined as a
   separate usage in order to allow it to run on a separate port, and to
   allow it to be more easily separated from the different STUN usages,
   only some of which require this mechanism.





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12.4.1.  Applicability

   To thwart some on-path attacks described in Section 13, it is
   necessary for the STUN client and STUN server to integrity protect
   the information they exchange over UDP.  In the absence of a long-
   term secret (password) that is shared between them, a short-term
   password can be obtained using the usage described in this section.

   The username and password returned in the STUN Shared Secret Response
   are valid for use in subsequent STUN transactions for nine (9)
   minutes with any hosts that have the same SRV Priority value as
   discovered via Section 12.4.2.  The username and password obtained
   with this usage are used as the USERNAME and in the HMAC for the
   MESSAGE-INTEGRITY in a subsequent STUN message, respectively.

12.4.2.  Client Discovery of Server

   The client follows the procedures in Section 8.1.  The SRV protocol
   is "tls" and the service name "stun-pass".

   For example a client would look up "_stun-pass._tls.example.com" in
   DNS.

12.4.3.  Server Determination of Usage

   The server advertises this port in the DNS as capable of receiving
   TLS over TCP connections, along with the Shared Secret messages that
   run over it.  The server MAY also advertise this same port in DNS for
   other TLS over TCP usages if the server is capable of multiplexing
   those different usages.  For example, the server could advertise the
   short-term password and binding discovery usages on the same TLS/TCP
   port.

12.4.4.  New Requests or Indications

   The message type Shared Secret Request and its associated Shared
   Secret Response and Shared Secret Error Response are defined in this
   section.  Their values are enumerated in Section 15.













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   The following figure indicates which attributes are present in the
   Shared Secret Request, Response, and Error Response.  An M indicates
   that inclusion of the attribute in the message is mandatory, O means
   its optional, C means it's conditional based on some other aspect of
   the message, and - means that the attribute is not applicable to that
   message type.  Attributes not listed are not applicable to Shared
   Secret Request, Response, or Error Response.

                          Shared   Shared    Shared
                          Secret   Secret    Secret
       Attribute          Request  Response  Error
                                             Response
       ____________________________________________________________________
       USERNAME             O         M         -
       PASSWORD             -         M         -
       MESSAGE-INTEGRITY    O         O         O
       ERROR-CODE           -         -         M
       ALTERNATE-SERVER     -         -         C
       UNKNOWN-ATTRIBUTES   -         -         C
       SERVER               -         O         O
       REALM                C         -         C
       NONCE                C         -         C

   The Shared Secret requests, like other STUN requests, can be
   authenticated.  However, since its purpose is to obtain a short-term
   credential, the Shared Secret request itself cannot be authenticated
   with a short-term credential.  However, it can be authenticated with
   a long-term credential.

12.4.5.  New Attributes

   No new attributes are defined by this usage.

12.4.6.  New Error Response Codes

   This usage defines the 433 error response.  Only the MESSAGE-
   INTEGRITY, ERROR-CODE and SERVER attributes are applicable to this
   response.

12.4.7.  Client Procedures

   Shared Secret requests are formed like other STUN requests, with the
   following additions.  Clients MUST NOT use a short-term credential
   with a Shared Secret request.  They SHOULD send the request with no
   credentials (omitting MESSAGE-INTEGRITY and USERNAME).

   Processing of the Shared Secret response follows that of any other
   STUN response.  Note that clients MUST be prepared to be challenged



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   for a long-term credential.

   If the response was a Shared Secret Response, the it will contain a
   short lived username and password, encoded in the USERNAME and
   PASSWORD attributes, respectively.  A client SHOULD use these
   credentials whenever short term credentials are needed for any server
   discovered using the same domain name as was used to discover the one
   which returned those credentials.  For example, if a client used a
   domain name of example.com, it would have looked up _stun-
   pass._tls.example.com in DNS, found a server, and sent a Shared
   Secret request that provided a credential to the client.  The client
   would use this credential with a server discovered by looking up
   _stun._udp.example.com in the DNS.

   If the response was a Shared Secret Error Response, and ERROR-CODE
   attribute was present with a response code of 433, and the client had
   not sent the request over TLS, the client SHOULD establish a TLS
   connection to the server and retry the request over that connection.
   If the client had used TLS, this error response is unrecoverable and
   the client SHOULD NOT retry.

12.4.8.  Server Procedures

   The procedures for general processing of STUN requests apply to
   Shared Secret requests.  Servers MAY challenge the client for a long-
   term credential if one was not provided in a request.  However, they
   MUST NOT challenge the request for a short-term credential.

   If the Shared Secret Request did not arrive over a TLS connection,
   the server MUST generate a Shared Secret Error response with an
   ERROR-CODE attribute that has a response code of 433.

   If the request is valid and authenticated (assuming the server is
   performing authentication), the server MUST create a short term
   credential for the user.  This credential consists of a username and
   password.  The credentials MUST be valid for a duration of at least
   nine minutes, and SHOULD NOT be valid for a duration of longer than
   thirty minutes.  The username MUST be distinct, with extremely high
   probabilities, from all usernames that have been handed out across
   all servers that are returned from DNS SRV queries for the same
   domain name.  Extremely high probability means that the likelihood of
   collision SHOULD be better than 1 in 2**64.  The password for each
   username MUST be cryptographically random with at least 128 bits of
   entropy.

12.4.9.  Security Considerations for Short-Term Password

   The security considerations in Section 13 do not apply to the Shared



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   Secret request and response, since these messages do not make use of
   mapped addresses, which is the primary source of security
   consideration discussed there.  Rather, shared secret requests are
   used to obtain short term credentials that are used in the
   authentication of other messages.

   Because the Shared Secret response itself carries a credential, in
   the form of a username and password, it must be sent encrypted.  For
   this reason, STUN servers MUST reject any Shared Secret request that
   has not arrived over a TLS connection.

   Malicious clients could generate a multiplicity of Shared Secret
   requests, each of which causes the server to allocate shared secrets,
   each of which might consume memory and processing resources.  If
   shared secret requests are not being authenticated, this leads to a
   possible denial-of-service attack.  Indeed, even if the requestor is
   authenticated, attacks are still possible.

   To prevent being swamped with traffic, a STUN server SHOULD limit the
   number of simultaneous TLS connections it will hold open by dropping
   an existing connection when a new connection request arrives (based
   on an Least Recently Used (LRU) policy, for example).

   Similarly, servers SHOULD allocate only a small number of shared
   secrets to a host with a particular source IP address and port.
   Requests from the same IP address and port which exceed this limit
   SHOULD be rejected with a 600 response.  Servers SHOULD also limit
   the total number of shared secrets they will provide at a time across
   all clients, based on the number of users and expected loads during
   normal peak usage.  If a Shared Secret request arrives and the server
   has exceeded its limit, it SHOULD reject the request with a 500
   response.

   Furthermore, for servers which are not authenticating shared secret
   requests, it is RECOMMENDED that short-term credentials be
   constructed in a way such that they do not require memory or disk to
   store.

   This can be done by intelligently computing the username and
   password.  One approach is to construct the USERNAME as:

         USERNAME = <prefix,rounded-time,clientIP,hmac>

   Where prefix is some random text string (different for each shared
   secret request), rounded-time is the current time modulo 20 minutes,
   clientIP is the source IP address where the Shared Secret Request
   came from, and hmac is an HMAC [13] over the prefix, rounded-time,
   and client IP, using a server private key.



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   The password is then computed as:

         password = <hmac(USERNAME,anotherprivatekey)>

   With this structure, the username itself, which will be present in
   the Binding Request, contains the source IP address where the Shared
   Secret Request came from.  That allows the server to meet the
   requirements specified in Section 8.1 for constructing the REFLECTED-
   FROM attribute.  The server can verify that the username was not
   tampered with, using the hmac present in the username.


13.  Security Considerations

   Attacks on STUN systems vary depending on the usage.  The short term
   password usage is quite different from the other usages defined here,
   and its security considerations are unique to it and discussed as
   part of the usage definition.  However, all of the other usages are
   very similar, and share a similar set of security considerations as a
   consequence of their usage of the mapped address from STUN Binding
   Responses.  Consequently, these security considerations apply to
   usage of the mapped address.

13.1.  Attacks on STUN

   Generally speaking, attacks on STUN can be classified into denial of
   service attacks and eavesdropping attacks.  Denial of service attacks
   can be launched against a STUN server itself, or against other
   elements using the STUN protocol.  The attacks of greater interest
   are those in which the STUN server and client are used to launch
   denial of service (DoS) attacks against other entities, including the
   client itself.  Many of the attacks require the attacker to generate
   a response to a legitimate STUN request, in order to provide the
   client with a faked mapped address.  The attacks that can be launched
   using such a technique include:

13.1.1.  Attack I: DDoS Against a Target

   In this case, the attacker provides a large number of clients with
   the same faked mapped address that points to the intended target.
   This will trick all the STUN clients into thinking that their
   addresses are equal to that of the target.  The clients then hand out
   that address in order to receive traffic on it (for example, in SIP
   or H.323 messages).  However, all of that traffic becomes focused at
   the intended target.  The attack can provide substantial
   amplification, especially when used with clients that are using STUN
   to enable multimedia applications.




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13.1.2.  Attack II: Silencing a Client

   In this attack, the attacker seeks to deny a client access to
   services enabled by STUN (for example, a client using STUN to enable
   SIP-based multimedia traffic).  To do that, the attacker provides
   that client with a faked mapped address.  The mapped address it
   provides is an IP 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 mapped 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.

13.1.3.  Attack III: Assuming the Identity of a Client

   This attack is similar to attack II.  However, the faked mapped
   address points to the attacker themself.  This allows the attacker to
   receive traffic which was destined for the client.

13.1.4.  Attack IV: Eavesdropping

   In this attack, the attacker forces the client to use a mapped
   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.

13.2.  Launching the Attacks

   It is important to note that attacks of this nature (injecting
   responses with fake mapped addresses) require that the attacker be
   capable of eavesdropping requests sent from the client to the server
   (or to act as a man in the middle for such attacks).  This is because
   STUN requests contain a transaction identifier, selected by the
   client, which is random with 96 bits of entropy.  The server echoes
   this value in the response, and the client ignores any responses that
   don't have a matching transaction ID.  Therefore, in order for an
   attacker to provide a faked response that is accepted by the client,
   the attacker needs to know the transaction ID of the request.  The
   large amount of randomness, combined with the need to know when the



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   client sends a request and the IP address and UDP ports used for that
   request, precludes attacks that involve guessing the transaction ID.

   Since all of the above attacks rely on this one primitive - injecting
   a response with a faked mapped address - preventing the attacks is
   accomplished by preventing this one operation.  To prevent it, we
   need to consider the various ways in which it can be accomplished.
   There are several:

13.2.1.  Approach I: Compromise a Legitimate STUN Server

   In this attack, the attacker compromises a legitimate STUN server
   through a virus or Trojan horse.  Presumably, this would allow the
   attacker to take over the STUN server, and control the types of
   responses it generates.  Compromise of a STUN server can also lead to
   discovery of open ports.  Knowledge of an open port creates an
   opportunity for DoS attacks on those ports (or DDoS attacks if the
   traversed NAT is a full cone NAT).  Discovering open ports is already
   fairly trivial using port probing, so this does not represent a major
   threat.

13.2.2.  Approach II: DNS Attacks

   STUN servers are discovered using DNS SRV records.  If an attacker
   can compromise the DNS, it can inject fake records which map a domain
   name to the IP address of a STUN server run by the attacker.  This
   will allow it to inject fake responses to launch any of the attacks
   above.  Clearly, this attack is only applicable for usages which
   discover servers through DNS.

13.2.3.  Approach III: Rogue Router or NAT

   Rather than compromise the STUN server, an attacker can cause a STUN
   server to generate responses with the wrong mapped address by
   compromising a router or NAT on the path from the client to the STUN
   server.  When the STUN request passes through the rogue router or
   NAT, it rewrites the source address of the packet to be that of the
   desired mapped address.  This address cannot be arbitrary.  If the
   attacker is on the public Internet (that is, there are no NATs
   between it and the STUN server), and the attacker doesn't modify the
   STUN request, the address has to have the property that packets sent
   from the STUN server to that address would route through the
   compromised router.  This is because the STUN server will send the
   responses back to the source address of the request.  With a modified
   source address, the only way they can reach the client is if the
   compromised router directs them there.

   If the attacker is on a private network (that is, there are NATs



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   between it and the STUN server), the attacker will not be able to
   force the server to generate arbitrary mapped addresses in responses.
   They will only be able force the STUN server to generate mapped
   addresses which route to the private network.  This is because the
   NAT between the attacker and the STUN server will rewrite the source
   address of the STUN request, mapping it to a public address that
   routes to the private network.  Because of this, the attacker can
   only force the server to generate faked mapped addresses that route
   to the private network.  Unfortunately, it is possible that a low
   quality NAT would be willing to map an allocated public address to
   another public address (as opposed to an internal private address),
   in which case the attacker could forge the source address in a STUN
   request to be an arbitrary public address.  This kind of behavior
   from NATs does appear to be rare.

13.2.4.  Approach IV: Man in the Middle

   As an alternative to approach III (Section 13.2.3), if the attacker
   can place an element on the path from the client to the server, the
   element can act as a man-in-the-middle.  In that case, it can
   intercept a STUN request, and generate a STUN response directly with
   any desired value of the mapped address field.  Alternatively, it can
   forward the STUN request to the server (after potential
   modification), receive the response, and forward it to the client.
   When forwarding the request and response, this attack is subject to
   the same limitations on the mapped address described in Approach III
   (Section 13.2.3).

13.2.5.  Approach V: Response Injection Plus DoS

   In this approach, the attacker does not need to be a MitM (as in
   approaches III and IV).  Rather, it only needs to be able to
   eavesdrop onto a network segment that carries STUN requests.  This is
   easily done in multiple access networks such as ethernet or
   unprotected 802.11.  To inject the fake response, the attacker
   listens on the network for a STUN request.  When it sees one, it
   simultaneously launches a DoS attack on the STUN server, and
   generates its own STUN response with the desired mapped address
   value.  The STUN response generated by the attacker will reach the
   client, and the DoS attack against the server is aimed at preventing
   the legitimate response from the server from reaching the client.
   Arguably, the attacker can do without the DoS attack on the server,
   so long as the faked response beats the real response back to the
   client, and the client uses the first response, and ignores the
   second (even though it's different).






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13.2.6.  Approach VI: Duplication

   This approach is similar to approach V (Section 13.2.5).  The
   attacker listens on the network for a STUN request.  When it sees it,
   it generates its own STUN request towards the server.  This STUN
   request is identical to the one it saw, but with a spoofed source IP
   address.  The spoofed address is equal to the one that the attacker
   desires to have placed in the mapped address of the STUN response.
   In fact, the attacker generates a flood of such packets.  The STUN
   server will receive the one original request, plus a flood of
   duplicate fake ones.  It generates responses to all of them.  If the
   flood is sufficiently large for the responses to congest routers or
   some other equipment, there is a reasonable probability that the one
   real response is lost (along with many of the faked ones), but the
   net result is that only the faked responses are received by the STUN
   client.  These responses are all identical and all contain the mapped
   address that the attacker wanted the client to use.

   The flood of duplicate packets is not needed (that is, only one faked
   request is sent), so long as the faked response beats the real
   response back to the client, and the client uses the first response,
   and ignores the second (even though it's different).

   Note that, in this approach, launching a DoS attack against the STUN
   server or the IP network, to prevent the valid response from being
   sent or received, is problematic.  The attacker needs the STUN server
   to be available to handle its own request.  Due to the periodic
   retransmissions of the request from the client, this leaves a very
   tiny window of opportunity.  The attacker must start the DoS attack
   immediately after the actual request from the client, causing the
   correct response to be discarded, and then cease the DoS attack in
   order to send its own request, all before the next retransmission
   from the client.  Due to the close spacing of the retransmits (100ms
   to a few seconds), this is very difficult to do.

   Besides DoS attacks, there may be other ways to prevent the actual
   request from the client from reaching the server.  Layer 2
   manipulations, for example, might be able to accomplish it.

   Fortunately, this approach is subject to the same limitations
   documented in Approach III (Section 13.2.3), which limit the range of
   mapped addresses the attacker can cause the STUN server to generate.

13.3.  Countermeasures

   STUN provides mechanisms to counter the approaches described above,
   and additional, non-STUN techniques can be used as well.




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   First off, it is RECOMMENDED that networks with STUN clients
   implement ingress source filtering [6].  This is particularly
   important for the NATs themselves.  As Section 13.2.3 explains, NATs
   which do not perform this check can be used as "reflectors" in DDoS
   attacks.  Most NATs do perform this check as a default mode of
   operation.  We strongly advise people that purchase NATs to ensure
   that this capability is present and enabled.

   Secondly, for usages where the STUN server is not co-located with
   some kind of application (such as the binding discovery usage), it is
   RECOMMENDED that STUN servers be run on hosts dedicated to STUN, with
   all UDP and TCP ports disabled except for the STUN ports.  This is to
   prevent viruses and Trojan horses from infecting STUN servers, in
   order to prevent their compromise.  This helps mitigate Approach I
   (Section 13.2.1).

   Thirdly, to prevent the DNS attack of Section 13.2.2, Section 8.2
   recommends that the client verify the credentials provided by the
   server with the name used in the DNS lookup.

   Finally, all of the attacks above rely on the client taking the
   mapped address it learned from STUN, and using it in application
   layer protocols.  If encryption and message integrity are provided
   within those protocols, the eavesdropping and identity assumption
   attacks can be prevented.  As such, applications that make use of
   STUN addresses in application protocols SHOULD use integrity and
   encryption, even if a SHOULD level strength is not specified for that
   protocol.  For example, multimedia applications using STUN addresses
   to receive RTP traffic would use secure RTP [21].

   The above three techniques are non-STUN mechanisms.  STUN itself
   provides several countermeasures.

   Approaches IV (Section 13.2.4), when generating the response locally,
   and V (Section 13.2.5) require an attacker to generate a faked
   response.  A faked response must match the 96-bit transaction ID of
   the request.  The attack further prevented by using the message
   integrity mechanism provided in STUN, described in Section 11.4.

   Approaches III (Section 13.2.3), IV (Section 13.2.4), when using the
   relaying technique, and VI (Section 13.2.6), however, are not
   preventable through server signatures.  These three approaches are
   functional when the attacker modifies nothing but the source address
   of the STUN request.  Sadly, this is the one thing that cannot be
   protected through cryptographic means, as this is the change that
   STUN itself is seeking to detect and report.  It is therefore an
   inherent weakness in NAT, and not fixable in STUN.




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13.4.  Residual Threats

   None of the countermeasures listed above can prevent the attacks
   described in Section 13.2.3 if the attacker is in the appropriate
   network paths.  Specifically, consider the case in which the attacker
   wishes to convince client C that it has address V. The attacker needs
   to have a network element on the path between A and the server (in
   order to modify the request) and on the path between the server and V
   so that it can forward the response to C. Furthermore, if there is a
   NAT between the attacker and the server, V must also be behind the
   same NAT.  In such a situation, the attacker can either gain access
   to all the application-layer traffic or mount the DDOS attack
   described in Section 13.1.1.  Note that any host which exists in the
   correct topological relationship can be DDOSed.  It need not be using
   STUN.


14.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing"
   (UNSAF), which is the general process by which a client attempts to
   determine its address in another realm on the other side of a NAT
   through a collaborative protocol reflection mechanism (RFC3424 [22]).
   STUN is an example of a protocol that performs this type of function
   for the binding discovery and connectivity check usages.  The IAB has
   mandated that any protocols developed for this purpose document a
   specific set of considerations.  This section meets those
   requirements for those two usages.

14.1.  Problem Definition

   From RFC3424 [22], any UNSAF proposal must provide:

      Precise definition of a specific, limited-scope problem that is to
      be solved with the UNSAF proposal.  A short term fix should not be
      generalized to solve other problems; this is why "short term fixes
      usually aren't".

   The specific problem being solved by STUN is to provide a means for a
   client to obtain a mapped address which can be used for the receipt
   of incoming application packets.

14.2.  Exit Strategy

   From RFC3424 [22], any UNSAF proposal must provide:

      Description of an exit strategy/transition plan.  The better short
      term fixes are the ones that will naturally see less and less use



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      as the appropriate technology is deployed.

   STUN by itself does not provide an exit strategy.  This is provided
   by techniques, such as Interactive Connectivity Establishment (ICE
   [11]), which allow a client to determine whether addresses learned
   from STUN are needed, or whether other addresses, such as the one on
   the local interface, will work when communicating with another host.
   With such a detection technique, as a client finds that the addresses
   provided by STUN are never used, STUN queries can cease to be made,
   thus allowing them to phase out.

14.3.  Brittleness Introduced by STUN

   From RFC3424 [22], any UNSAF proposal must provide:

      Discussion of specific issues that may render systems more
      "brittle".  For example, approaches that involve using data at
      multiple network layers create more dependencies, increase
      debugging challenges, and make it harder to transition.

   STUN introduces brittleness into the system in several ways:

   o  Transport addresses discovered by STUN in the Binding Discovery
      usage will only be useful for receiving packets from a peer if the
      NAT does not have address or address and port dependent mapping
      properties.  When this usage is used in isolation, this makes STUN
      brittle, since its effectiveness depends on the type of NAT.  This
      brittleness is eliminated when the Binding Discovery usage is used
      in concert with mechanisms which can verify the transport address
      and use others if it doesn't work.  ICE is an example of such a
      mechanism.

   o  Transport addresses discovered by STUN in the Binding Discovery
      usage will only be useful for receive packets from a peer if the
      STUN server subtends the address realm of the peer.  For example,
      consider client A and B, both of which have residential NAT
      devices.  Both devices connect them to their cable operators, but
      both clients have different providers.  Each provider has a NAT in
      front of their entire network, connecting it to the public
      Internet.  If the STUN server used by A is in A's cable operator's
      network, an address obtained by it will not be usable by B. The
      STUN server must be in the network which is a common ancestor to
      both - in this case, the public Internet.  When this usage is used
      in isolation, this makes STUN brittle, since its effectiveness
      depends on the topological placement of the STUN server.  This
      brittleness is eliminated when the Binding Discovery usage is used
      in concert with mechanisms which can verify the transport address
      and use others if it doesn't work.  ICE is an example of such a



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      mechanism.

   o  The bindings allocated from the NAT need to be continuously
      refreshed.  Since the timeouts for these bindings is very
      implementation specific, the refresh interval cannot easily be
      determined.  When the binding is not being actively used to
      receive traffic, but to wait for an incoming message, the binding
      refresh will needlessly consume network bandwidth.

   o  The use of the STUN server in the Binding Discovery usage as an
      additional network element introduces another point of potential
      security attack.  These attacks are largely prevented by the
      security measures provided by STUN, but not entirely.

   o  The use of the STUN server as an additional network element
      introduces another point of failure.  If the client cannot locate
      a STUN server, or if the server should be unavailable due to
      failure, the application cannot function.

   o  The use of STUN to discover address bindings may result in an
      increase in latency for applications.

   o  Transport addresses discovered by STUN in the Binding Discovery
      usage will only be useful for receive packets from a peer behind
      the same NAT if the STUN server supports hairpinning [12].  When
      this usage is used in isolation, this makes STUN brittle, since
      its effectiveness depends on the topological placement of the STUN
      server.  This brittleness is eliminated when the Binding Discovery
      usage is used in concert with mechanisms which can verify the
      transport address and use others if it doesn't work.  ICE is an
      example of such a mechanism.

   o  Most significantly, STUN introduces potential security threats
      which cannot be eliminated through cryptographic means.  These
      security problems are described fully in Section 13.

14.4.  Requirements for a Long Term Solution

   From RFC3424 [22], any UNSAF proposal must provide:

      Identify requirements for longer term, sound technical solutions
      -- contribute to the process of finding the right longer term
      solution.

   Our experience with STUN has led to the following requirements for a
   long term solution to the NAT problem:





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   o  Requests for bindings and control of other resources in a NAT need
      to be explicit.  Much of the brittleness in STUN derives from its
      guessing at the parameters of the NAT, rather than telling the NAT
      what parameters to use, or knowing what parameters the NAT will
      use.

   o  Control needs to be in-band.  There are far too many scenarios in
      which the client will not know about the location of middleboxes
      ahead of time.  Instead, control of such boxes needs to occur in-
      band, traveling along the same path as the data will itself
      travel.  This guarantees that the right set of middleboxes are
      controlled.

   o  Control needs to be limited.  Users will need to communicate
      through NATs which are outside of their administrative control.
      In order for providers to be willing to deploy NATs which can be
      controlled by users in different domains, the scope of such
      controls needs to be extremely limited - typically, allocating a
      binding to reach the address where the control packets are coming
      from.

   o  Simplicity is Paramount.  The control protocol will need to be
      implement in very simple clients.  The servers will need to
      support extremely high loads.  The protocol will need to be
      extremely robust, being the precursor to a host of application
      protocols.  As such, simplicity is key.

14.5.  Issues with Existing NAPT Boxes

   From RFC3424 [22], any UNSAF proposal must provide:

      Discussion of the impact of the noted practical issues with
      existing, deployed NA[P]Ts and experience reports.

   Originally, RFC 3489 was developed as a standalone solution for NAT
   traversal for several types of applications, including VoIP.
   However, practical experience found that the limitations of its usage
   in isolation made it impractical as a complete solution.  There were
   too many NATs which didn't support hairpinning or which had address
   and port dependent mapping properties.

   Consequently, STUN was revised to produce this specification, which
   turns STUN into a tool that is used as part of a broader solution.
   For multimedia communciations protocols, this broader solution is
   ICE.  ICE uses the binding discovery and connectivity check usages
   together.  When done this way, ICE eliminates almost all of the
   brittleness and issues found with RFC 3489 alone.




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15.  IANA Considerations

   IANA is hereby requsted to create two new registries STUN Message
   Types and STUN Attributes.  IANA must assign the following values to
   both registeries before publication of this document as an RFC.  New
   values for both STUN Message Type and STUN Attributes are assigned
   through the IETF consensus process via RFCs approved by the IESG
   [23].

15.1.  STUN Message Type Registry

   For STUN Message Types that are request message types, they must be
   registered including associated Response message types and Error
   Response message types, and those responses must have values that are
   0x100 and 0x110 higher than their respective Request values.

   For STUN Message Types that are Indication message types, no
   associated restriction applies.  As the message type field is only 14
   bits the range of valid values is 0x001 through 0x3FFF.

   The initial STUN Message Types are:

    0x0001:Binding Request
    0x0101:Binding Response
    0x0111:Binding Error Response
    0x0002:Shared Secret Request
    0x0102:Shared Secret Response
    0x0112:Shared Secret Error Response

15.2.  STUN Attribute Registry

   STUN attributes values above 0x7FFF are considered optional
   attributes; attributes equal to 0x7FFF or below are considered
   mandatory attributes.  The STUN client and STUN server process
   optional and mandatory attributes differently.  IANA should assign
   values based on the RFC consensus process.















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   The initial STUN Attributes are:

    0x0001: MAPPED-ADDRESS
    0x0006: USERNAME
    0x0007: PASSWORD
    0x0008: MESSAGE-INTEGRITY
    0x0009: ERROR-CODE
    0x000A: UNKNOWN-ATTRIBUTES
    0x0014: REALM
    0x0015: NONCE
    0x0020: XOR-MAPPED-ADDRESS
    0x0023: FINGERPRINT
    0x8022: SERVER
    0x8023: ALTERNATE-SERVER
    0x8024: REFRESH-INTERVAL



16.  Changes Since RFC 3489

   This specification updates RFC3489 [13].  This specification differs
   from RFC3489 in the following ways:

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

   o  Added the XOR-MAPPED-ADDRESS attribute, which is included in
      Binding Responses if the magic cookie is present in the request.
      Otherwise the RFC3489 behavior is retained (that is, Binding
      Response includes MAPPED-ADDRESS).  See discussion in XOR-MAPPED-
      ADDRESS regarding this change.

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





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

   o  Added long-term credential-based authentication.

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

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

   o  Introduced the concept of STUN usages and defined four usages -
      Binding Discovery, Connectivity Check, NAT Keepalive, and Short
      term password.


17.  Acknowledgements

   The authors would like to thank Cedric Aoun, Pete Cordell, Cullen
   Jennings, Bob Penfield 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.


18.  References

18.1.  Normative References

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

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

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

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

   [5]  Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
        RFC 2246, January 1999.

   [6]  Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating
        Denial of Service Attacks which employ IP Source Address
        Spoofing", BCP 38, RFC 2827, May 2000.

   [7]  Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
        Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:



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        Basic and Digest Access Authentication", RFC 2617, June 1999.

18.2.  Informational References

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

   [9]   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.

   [10]  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.

   [11]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-08 (work in
         progress), March 2006.

   [12]  Audet, F. and C. Jennings, "NAT Behavioral Requirements for
         Unicast UDP", draft-ietf-behave-nat-udp-07 (work in progress),
         June 2006.

   [13]  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.

   [14]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         of UDP Through NAT (STUN)", draft-ietf-behave-turn-00 (work in
         progress), March 2006.

   [15]  Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
         "RTP: A Transport Protocol for Real-Time Applications",
         RFC 3550, July 2003.

   [16]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-03 (work in progress), March 2006.

   [17]  Handley, M., "SDP: Session Description Protocol",
         draft-ietf-mmusic-sdp-new-26 (work in progress), January 2006.

   [18]  Senie, D., "Network Address Translator (NAT)-Friendly
         Application Design Guidelines", RFC 3235, January 2002.

   [19]  Holdrege, M. and P. Srisuresh, "Protocol Complications with the
         IP Network Address Translator", RFC 3027, January 2001.



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

   [21]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
         Norrman, "The Secure Real-time Transport Protocol (SRTP)",
         RFC 3711, March 2004.

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

   [23]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
         Considerations Section in RFCs", BCP 26, RFC 2434,
         October 1998.





































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Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems
   600 Lanidex Plaza
   Parsippany, NJ  07054
   US

   Phone: +1 973 952-5000
   Email: jdrosen@cisco.com
   URI:   http://www.jdrosen.net


   Christian Huitema
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: huitema@microsoft.com


   Rohan Mahy
   Plantronics
   345 Encinal Street
   Santa Cruz, CA  95060
   US

   Email: rohan@ekabal.com


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

   Email: dwing@cisco.com













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