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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
Obsoletes: 3489 (if approved)                                 C. Huitema
Intended status: Standards Track                               Microsoft
Expires: September 6, 2007                                       R. Mahy
                                                             Plantronics
                                                                 D. Wing
                                                           Cisco Systems
                                                           March 5, 2007


              Session Traversal Utilities for (NAT) (STUN)
                    draft-ietf-behave-rfc3489bis-06

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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   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on September 6, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   Session Traversal Utilities for NAT (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 . . . . . . . . . . . . . . . . . 19
       8.3.3.  Timeouts . . . . . . . . . . . . . . . . . . . . . . . 22
     8.4.   Indication Transactions . . . . . . . . . . . . . . . . . 22
   9.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . . . 23
     9.1.   Request/Response Transactions . . . . . . . . . . . . . . 23
       9.1.1.  Receive Request Message  . . . . . . . . . . . . . . . 23
       9.1.2.  Constructing the Response  . . . . . . . . . . . . . . 26
       9.1.3.  Sending the Response . . . . . . . . . . . . . . . . . 27
     9.2.   Indication Transactions . . . . . . . . . . . . . . . . . 27
   10. Demultiplexing of STUN and Application Traffic . . . . . . . . 28
   11. STUN Attributes  . . . . . . . . . . . . . . . . . . . . . . . 29
     11.1.  MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . . . 29
     11.2.  USERNAME  . . . . . . . . . . . . . . . . . . . . . . . . 30
     11.3.  PASSWORD  . . . . . . . . . . . . . . . . . . . . . . . . 31
     11.4.  MESSAGE-INTEGRITY . . . . . . . . . . . . . . . . . . . . 31
     11.5.  FINGERPRINT . . . . . . . . . . . . . . . . . . . . . . . 31
     11.6.  ERROR-CODE  . . . . . . . . . . . . . . . . . . . . . . . 31
     11.7.  REALM . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     11.8.  NONCE . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     11.9.  UNKNOWN-ATTRIBUTES  . . . . . . . . . . . . . . . . . . . 33
     11.10. XOR-MAPPED-ADDRESS  . . . . . . . . . . . . . . . . . . . 34
     11.11. SERVER  . . . . . . . . . . . . . . . . . . . . . . . . . 35
     11.12. ALTERNATE-SERVER  . . . . . . . . . . . . . . . . . . . . 35
     11.13. REFRESH-INTERVAL  . . . . . . . . . . . . . . . . . . . . 35



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   12. STUN Usages  . . . . . . . . . . . . . . . . . . . . . . . . . 36
     12.1.  Binding Discovery . . . . . . . . . . . . . . . . . . . . 36
       12.1.1. Applicability  . . . . . . . . . . . . . . . . . . . . 36
       12.1.2. Client Discovery of Server . . . . . . . . . . . . . . 37
       12.1.3. Server Determination of Usage  . . . . . . . . . . . . 38
       12.1.4. New Requests or Indications  . . . . . . . . . . . . . 38
       12.1.5. New Attributes . . . . . . . . . . . . . . . . . . . . 38
       12.1.6. New Error Response Codes . . . . . . . . . . . . . . . 38
       12.1.7. Client Procedures  . . . . . . . . . . . . . . . . . . 38
       12.1.8. Server Procedures  . . . . . . . . . . . . . . . . . . 38
       12.1.9. Security Considerations for Binding Discovery  . . . . 38
     12.2.  NAT Keepalives  . . . . . . . . . . . . . . . . . . . . . 39
       12.2.1. Applicability  . . . . . . . . . . . . . . . . . . . . 39
       12.2.2. Client Discovery of Server . . . . . . . . . . . . . . 39
       12.2.3. Server Determination of Usage  . . . . . . . . . . . . 39
       12.2.4. New Requests or Indications  . . . . . . . . . . . . . 39
       12.2.5. New Attributes . . . . . . . . . . . . . . . . . . . . 40
       12.2.6. New Error Response Codes . . . . . . . . . . . . . . . 40
       12.2.7. Client Procedures  . . . . . . . . . . . . . . . . . . 40
       12.2.8. Server Procedures  . . . . . . . . . . . . . . . . . . 40
       12.2.9. Security Considerations for NAT Keepalives . . . . . . 40
     12.3.  Short-Term Password . . . . . . . . . . . . . . . . . . . 41
       12.3.1. Applicability  . . . . . . . . . . . . . . . . . . . . 41
       12.3.2. Client Discovery of Server . . . . . . . . . . . . . . 41
       12.3.3. Server Determination of Usage  . . . . . . . . . . . . 42
       12.3.4. New Requests or Indications  . . . . . . . . . . . . . 42
       12.3.5. New Attributes . . . . . . . . . . . . . . . . . . . . 43
       12.3.6. New Error Response Codes . . . . . . . . . . . . . . . 43
       12.3.7. Client Procedures  . . . . . . . . . . . . . . . . . . 43
       12.3.8. Server Procedures  . . . . . . . . . . . . . . . . . . 43
       12.3.9. Security Considerations for Short-Term Password  . . . 44
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 45
     13.1.  Attacks on STUN . . . . . . . . . . . . . . . . . . . . . 45
       13.1.1. Attack I: DDoS Against a Target  . . . . . . . . . . . 46
       13.1.2. Attack II: Silencing a Client  . . . . . . . . . . . . 46
       13.1.3. Attack III: Assuming the Identity of a Client  . . . . 46
       13.1.4. Attack IV: Eavesdropping . . . . . . . . . . . . . . . 46
     13.2.  Launching the Attacks . . . . . . . . . . . . . . . . . . 47
       13.2.1. Approach I: Compromise a Legitimate STUN Server  . . . 47
       13.2.2. Approach II: DNS Attacks . . . . . . . . . . . . . . . 47
       13.2.3. Approach III: Rogue Router or NAT  . . . . . . . . . . 48
       13.2.4. Approach IV: Man in the Middle . . . . . . . . . . . . 48
       13.2.5. Approach V: Response Injection Plus DoS  . . . . . . . 49
       13.2.6. Approach VI: Duplication . . . . . . . . . . . . . . . 49
     13.3.  Countermeasures . . . . . . . . . . . . . . . . . . . . . 50
     13.4.  Residual Threats  . . . . . . . . . . . . . . . . . . . . 51
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 51
     14.1.  Problem Definition  . . . . . . . . . . . . . . . . . . . 52



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     14.2.  Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 52
     14.3.  Brittleness Introduced by STUN  . . . . . . . . . . . . . 52
     14.4.  Requirements for a Long Term Solution . . . . . . . . . . 54
     14.5.  Issues with Existing NAPT Boxes . . . . . . . . . . . . . 55
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 55
     15.1.  STUN Methods Registry . . . . . . . . . . . . . . . . . . 55
     15.2.  STUN Attribute Registry . . . . . . . . . . . . . . . . . 55
   16. Changes Since RFC 3489 . . . . . . . . . . . . . . . . . . . . 56
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 57
   18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57
     18.1.  Normative References  . . . . . . . . . . . . . . . . . . 57
     18.2.  Informational References  . . . . . . . . . . . . . . . . 58
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 59
   Intellectual Property and Copyright Statements . . . . . . . . . . 61





































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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) [13]
   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 [14].  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 [16], 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 [22],
   including the Session Initiation Protocol (SIP) [11], Interactive
   Connectivity Establishment (ICE) uses both the binding usage and
   relay usage, and furthermore defines a connectivity check usage to
   help determine which transport address to use.

   Implementers should be aware of the specific deployment scenarios and
   the specific protocol (SIP, etc) being used to determine whether NAT
   traversal can be facilitated by STUN and which STUN usages are
   required.


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 [20] 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-
   peer protocols such as multimedia communications, file sharing and



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   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 [21]) 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, Session Traversal Utilities for NAT
   (STUN), provides a toolkit of functions.  These functions allow
   entities behind a NAT to learn the address bindings allocated by the
   NAT and to keep those bindings open.  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.

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





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   Reflexive Transport Address:  A transport address learned by a client
      that identifies that client as seen by another host on an IP
      network, typically a STUN server.  When there is an intervening
      NAT between the client and the other host, the reflexive transport
      address represents the 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 that
      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 persist until the subscriber leaves the
      service or explicitly changes the credential.

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

   Short Term Credential:  A temporary username and associated password
      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 that 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 transport address into a STUN Binding
   Response and sends it back to the source transport address 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
   transport address 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 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 three STUN usages - binding discovery, 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 [15],
   the predecessor to this specification.  The purpose of the binding
   discovery usage is for the client to obtain a transport address at
   which it is reachable.  The client can include these transport
   addresses in application layer signaling messages such as the Session
   Description Protocol (SDP) [19] (present in the body of SIP
   messages), where it indicates where the client wants to receive Real
   Time Transport Protocol (RTP [17]) 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



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   utilize the STUN request just prior to sending a protocol message
   (such as a SIP INVITE request or 200 OK response) that requires the
   client to embed its transport address.

   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 transport address of the request, and
   remembers it.  Later on, if it needs to reach the client, it sends a
   message to that transport address.  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 toward 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 transport address 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 transport address towards the server have changed (its reflexive
   transport address), in which case it may need application layer
   protocol messaging to update its transport address as seen by the
   server.  The binding keepalive usage is used by the SIP outbound
   mechanism, for example [18].

   These two 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 (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.

   Some usages (such as the binding keepalive) require STUN messages to
   be sent on the same transport address 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.




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   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 transport addresses 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, magic cookie,
   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 indication transactions, which utilizes a single indication
   message.  Furthermore, responses are broken into two types - success
   responses and error responses.  Two bits in the message type field of
   the STUN header indicate the class of the message - whether the
   message is a request, a success response, an indication, or a failure
   response.  An additional 12 bits in the message type indicate the
   method, which is the primary function of the message.  This
   specification defines two methods, Binding and Shared Secret.

   STUN Requests are sent reliably.  STUN can run over UDP, TCP or TCP/
   TLS.  When run over UDP, STUN requests are retransmitted in order to
   achieve 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.
   The transaction ID is used to distinguish indication messages.








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   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 message type field is decomposed further into the following
   structure:

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

                Figure 3: Format of STUN Message Type Field

   M11 through M0 represent a 12-bit encoding of the method.  C1 through
   C0 represent a 2 bit encoding of the class.  A class of 0 is a
   Request, a class of 1 is an indication, a class of 2 is a success
   response, and a class of 3 is an error response.  This specification
   defines two methods, Binding and Shared Secret.  Their method 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 [15] 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 [13] and [18].
   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.  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
   transaction ID there MUST also be uniformly and randomly distributed
   between 0 and 2**96 - 1.As with requests, the value is chosen by the
   server and MUST be unique for each unique indication generated by the
   server.  Unless a request or indication is bit-wise identical to a
   previous request, and was sent to the same server from the same
   transport address, a client MUST choose a new transaction ID for it.

   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 4: 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 bytes.  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
   in the Length field refers to the length of the Value part of the
   attribute prior to padding - i.e., the useful content.  Consequently,



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   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 accomplished through client retransmissions.
   Clients SHOULD retransmit the request starting with an interval of
   RTO, doubling after each retransmission.  RTO is an estimate of the
   round-trip-time, and is computed as described in RFC 2988 [8], with
   two exceptions.  First, the initial value for RTO SHOULD be
   configurable (rather than the 3s recommended in RFC 2988).  In fixed-
   line access links, a value of 100ms is RECOMMENDED.  Secondly, the
   value of RTO MUST NOT be rounded up to the nearest second.  Rather, a
   1ms accuracy MUST be maintained.  As with TCP, the usage of Karn's
   algorithm is RECOMMENDED.  When applied to STUN, it means that RTT
   estimates SHOULD NOT be computed from STUN transactions which result
   in the retransmission of a request.

   The value for RTO SHOULD be cached by an agent after the completion
   of the transaction, and used as the starting value for RTO for the
   next transaction to the same host (based on equality of IP address).
   The value SHOULD be considered stale and discarded after 10 minutes.

   Retransmissions continue until a response is received, or a total of
   7 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.  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.  For example, assuming an RTO of 100ms,
   requests would be sent at times 0ms, 100ms, 300ms, 700ms, 1500ms,
   3100ms, and 6300ms.  At 7900ms, the agent would consider the
   transaction to have timed out if no response has been received.



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   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 7900ms, it
   considers the transaction to have timed out.

   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/or
   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 [16].  When sent
   over UDP, there are no retransmissions, and reliability is not
   provided.  When sent over TCP, reliability is provided by TCP.

   Regardless of whether TCP or UDP was used for the indication, if a
   failure occurs (due to a fatal ICMP error or TCP error), 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 indication, which is
   identical to the previous, but has a different transaction ID (and
   consequently a different MESSAGE INTEGRITY and/or FINGERPRINT
   attribute).


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 a transport
   address using the SRV procedures specified in RFC2782 [3].  The
   mechanism for configuring the STUN client with the domain name to



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   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
   usage always starts with a transport address, so no DNS SRV service
   names are defined for it.  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
   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 8.3.3.

   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 a
   transport address 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



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   implementation strength for shared secrets.

   Some usages assume that out of band protocols 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
   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 class MUST be a
   request.

   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 MUST NOT contain the REALM 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:



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       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
   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 8.1 as 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.

   When STUN is being multiplexed on the same transport address as
   application data, and there are valid application layer data packets
   which could be confused with STUN packets (because, for example, bits
   32 through 63 can contain an arbitrary binary value which might be
   equal to 0x2112A442), the FINGERPRINT attribute MUST be present.
   Otherwise, its inclusion is RECOMMENDED.

   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.  When using TCP or TLS over TCP, the client SHOULD
   keep the connection open until it has no further requests to send,
   and has no plans to use any resources (such as a mapped address or
   relayed address [16]) learned though STUN requests sent over that
   connection.



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   Regardless of the transport protocol, a client MAY pipeline requests
   (that is, it can have multiple requests outstanding at the same
   time).

8.3.2.  Processing Responses

   Once the client has received a response to its request that it did
   not discard, it MUST discard any further responses for the same
   request.

   All responses that were not discarded, 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.  If the
   client did not place a MESSAGE-INTEGRITY attribute into the request,
   it MUST ignore the MESSAGE-INTEGRITY attribute in the response and
   continue processing the response.

   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 (Bad
   Request) response code, the client SHOULD display the reason phrase
   to the user.  For a 420 (Unknown Attribute) 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 (Unauthorized) 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 (this username and password are assumed to
   be pre-provisioned into the client through some other means).  If the
   REALM attribute was absent in the response, 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.



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   If the client receives a 401 (Unauthorized) 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 (Missing Username) 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 client receives a 432
   (Missing Username) 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 (Missing Nonce) 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.

   If the client receives a 430 (Stale Credentials) response, it means
   that the client used a short term credential that 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 (Stale Credentials) response; the server that receives the
   Shared Secret request is determined by the DNS procedures defined
   above.  If a 430 (Stale Credentials) 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 (Stale Credentials) error is unrecoverable
   and the request SHOULD NOT be retried.

   For a 431 (Integrity Check Failure) 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 client receives a 433 (Use TLS) 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



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   TLS, it is an unrecoverable error and the client SHOULD NOT retry.

   If the client receives a 434 (Missing Realm) 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.  If the 434 (Missing Realm)
   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 (Unknown Username) response, it means
   that the username it provided in the request is unknown.  For usages
   where the username was collected from the user, the client SHOULD
   alert the user.  The client SHOULD NOT retry with the same username.
   If the username was obtained using the Shared Secret request, the
   client SHOULD obtain a new credential and retry.  However, if the
   retries are repeatedly rejected with a 436 (Unknown Username), it
   SHOULD cease retrying.

   For error responses which can contain a NONCE, if the error response
   results in a retry, the client MUST include the NONCE in a subsequent
   retry.  Furthermore, the client SHOULD cache the nonce, and continue
   using it in subsequent requests sent to the same server, identified
   by transport address.

   For a 300 (Try Alternate) response code, the client SHOULD attempt a
   new transaction to the server indicated in the ALTERNATE-SERVER
   attribute.  The client SHOULD reuse its credentials (username and
   password) when retrying.  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
   (Try Alternate) 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 (Try
   Alternate) response code is targeted for other usages of STUN, such
   as the relay usage [16].

   For a 500 (Server Error) 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 the next server in the list.  The same username and
   password MAY be used.  For a 600 (Global Failure) response code,
   client MUST NOT retry the request on this server, or if the server



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   was learned through DNS, any other server found through the DNS
   resolution procedures.

   Unknown response codes between 300 and 399 are treated like a 300.
   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.

   Unknown optional attributes in a response (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.

   For a success response, the client SHOULD cache any nonce present in
   the response, and continue using it in subsequent requests sent to
   the same server, identified by transport address.

8.3.3.  Timeouts

   If the STUN transaction times out without receipt of a response, the
   client SHOULD consider it a failure and retry the request to the next
   server in the list of servers from the DNS SRV response, as specified
   in RFC 2782.

8.4.  Indication Transactions

   This section applies to client and server behavior for sending an
   Indication message.

   The client or server follows the syntax rules defined in Section 6
   and the transmission rules of Section 7.  The message class MUST be
   an indication.

   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.

   When STUN is being multiplexed on the same transport address as
   application data, and there are valid application layer data packets
   which could be confused with STUN packets (because, for example, bits
   32 through 63 can contain an arbitrary binary value which might be
   equal to 0x2112A442), the FINGERPRINT attribute MUST be present.



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   Otherwise, its inclusion is RECOMMENDED.

   Typically, indication messages are sent to the same transport
   address, 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.  When using TCP or TLS over TCP, the
   client SHOULD close the connection as soon as it determines it has no
   further messages to send to the server.

   By definition, since indications do not generate a response, they can
   be pipelined, regardless of the transport protocol.


9.  Server Behavior

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

9.1.  Request/Response Transactions

9.1.1.  Receive Request Message

   A STUN server MUST be prepared to receive request messages on the
   transport address 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.

   If the request is a retransmission of a request for which the server
   has already generated a response within the last 10 seconds, the
   server MUST retransmit the response.  A server can do this either by
   remembering the response it transmitted, or by re-processing the
   request and computing the response.  The latter technique can only be
   applied to requests which are idempotent and would result in the same
   response for the same request.  This is the case for the Binding
   Request, but not for the Shared Secret Request.  Extensions to STUN
   SHOULD state whether their request types have this property or not.

   When a STUN request is received, the server determines the usage.



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   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 (Unauthorized).  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].

   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 (Missing
   Username).  If the server is using a long term credential for
   authentication, the response MUST include a REALM.  If the server is
   using a short-term credential, it MUST NOT include a REALM in the
   response.

   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.
   That error response MUST include an ERROR-CODE attribute with
   response code of 434 (Missing Realm).  That error response MUST also
   include 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 (Missing Nonce).  That error response MUST include 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
   (Missing Nonce).  This response MUST include 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 (Stale Nonce).

   If the server is authenticating the request with a short term



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   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 (Stale Credentials).  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
   (Unknown Username).  For authentication using long-term credentials,
   that error response MUST include a REALM attribute.  For
   authentication using short-term credentials, it 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.

   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 (Integrity
   Check Failure).  If long term credentials are being used for
   authentication, this response MUST include a REALM attribute.  If
   short term credentials are being used, it MUST NOT include a REALM.

   When an error response is to be generated by the server as a
   consequence of authentication problems (error codes 401, 432, 434,
   435, 430 and 436, and the REALM is present in the response
   (signifying the usage of a long term credential), 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 (with the exception of 435, where
   NONCE is mandatory even for short term credentials).  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.

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

   If the method of the request is unknown to the server, it MUST
   generate an error response which includes an ERROR-CORE attribute



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   with a 400 response code.

   Next, the server MUST check for any mandatory 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 message type MUST indicate
   either a success response or error response class and MUST indicate
   the same method as the request.  The server MUST copy the transaction
   ID from the request to the response.

   The attributes that get added to the response depend on the type of
   response.  See Figure 5 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 transport address and magic
   cookie.  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 transport address 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 transport address.  The former, as implied by the name,
   encodes the transport address 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.

   If the request contained the MESSAGE-INTEGRITY attribute, the server
   MUST include a MESSAGE-INTEGRITY attribute in a successful response.



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   The MESSAGE-INTEGRITY attribute MUST use the same username and
   password used to authenticate the request.  If long term credentials
   were used, the response MUST include a NONCE.  For short term
   credentials, a NONCE MAY be included.

   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.

   When STUN is being multiplexed on the same transport address as
   application data, and there are valid application layer data packets
   which could be confused with STUN packets (because, for example, bits
   32 through 63 can contain an arbitrary binary value which might be
   equal to 0x2112A442), the FINGERPRINT attribute MUST be present in
   the response.  Otherwise, its inclusion is RECOMMENDED.

   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 methods
   defined in this specification.

9.1.3.  Sending the Response

   All UDP response messages are sent to the transport address the
   associated Binding Request came from, and sent from the transport
   address 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 do not cause the server to send a response message.

   A STUN server MUST be prepared to receive indication messages on the
   transport address 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 method.  This specification
   doesn't define any indication messages.


10.  Demultiplexing of STUN and Application Traffic

   In the binding refresh usage, STUN traffic is multiplexed on the same
   transport address 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 all message 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, STUN messages can 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
   fingerprint.  It then looks for the FINGERPRINT attribute in the



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   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             C        -         -       O
              PASSWORD             -        C         -       -
              MESSAGE-INTEGRITY    O        C         C       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          O        O         O       O

             Figure 5: Mandatory Attributes and Message Types

11.1.  MAPPED-ADDRESS

   The MAPPED-ADDRESS attribute indicates the mapped transport address.
   It consists of an eight bit address family, and a sixteen bit port,
   followed by a fixed length value representing the IP address.  If the



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   address family is IPv4, the address is 32 bits, in network byte
   order.  If the address family is IPv6, the address is 128 bits in
   network byte order.

   The format of the MAPPED-ADDRESS attribute is:

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

               Figure 6: 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.

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

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.



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11.3.  PASSWORD

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

   The value of PASSWORD is a variable length opaque value.  The
   password returned in the Shared Secret Response is used as the HMAC
   key 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 [10] 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 either the last attribute, or the next to last attribute
   in any STUN message (depending on whether FINGERPRINT is present).
   With the exception of the FINGERPRINT attribute, which appears after
   MESSAGE-INTEGRITY, elements MUST ignore all other attributes that
   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 can be present in all STUN messages.  It is
   computed as the CRC-32 of the STUN message up to (but excluding) the
   FINGERPRINT attribute itself, xor-d with the 32 bit value 0x5354554e
   (the XOR helps in cases where an application packet is also using
   CRC-32 in it).  The 32 bit CRC is the one defined in ITU V.42 [9],
   which has a generator polynomial of x32+x26+x23+x22+x16+x12+x11+x10+
   x8+x7+x5+x4+x2+x+1.  When present, the FINGERPRINT attribute MUST be
   the last attribute in the message.

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 [11] and



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   HTTP [12].  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.

   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 [11], 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 [11].  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 9: 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 10: 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.

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

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 transport address for a
   different STUN server to try.  It is encoded in the same way as
   MAPPED-ADDRESS.

   This attribute 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



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   Binding Responses.


12.  STUN Usages

   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), NAT keepalives (Section 12.2) and short-term password
   (Section 12.3).

   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 [15], 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' UDP transport
   address that is assigned by a NAT between a STUN client and a STUN
   server.  This transport address 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 that require a client to
   insert a transport address into a message, to which subsequent
   messages will be delivered by other entities in a network.  Normally,
   the client would insert the transport address 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 transport address information in several places, including



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   the Session Description Protocol (SDP [19]) body carried by SIP.  The
   transport address present in the SDP is used for receipt of media.

   To use STUN as a technique for traversal of SIP and other protocols,
   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 transport address.  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 transport address
   that it can then include in the protocol payload.  This transport
   address 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 transport address
   for RTP, and the other, for the Real Time Control Protocol
   (RTCP)[17].  The client might also need to use STUN to obtain
   transport addresses 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 transport 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 transport addresses in the PDU.
   One of those can be the transport address determined from STUN, and
   the others can include transport addresses 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 [13]).

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.





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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
   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
   transport address.  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 transport
   address that the client requires to formulate the application PDU.

   A successful response message will carry either an XOR-MAPPED-ADDRESS
   or MAPPED-ADDRESS attribute, depending on the version of the server.
   A client SHOULD use the XOR-MAPPED-ADDRESS if present.  If not, it
   uses the MAPPED-ADDRESS.

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.




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12.2.  NAT Keepalives

12.2.1.  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
   stores the source transport address of a message from a client (such
   as SIP REGISTER), and sends messages to the client using that
   transport address.

   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 send a
   Binding Request to the server, using the same transport addresses
   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 transport address.

12.2.2.  Client Discovery of Server

   In this usage, the STUN server and the application protocol are using
   the same fixed port.

12.2.3.  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.

12.2.4.  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

   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 could use the binding discovery usage to obtain a new
   mapped address, and then register it using SIP registration
   procedures.

   The client SHOULD NOT include a MESSAGE-INTEGRITY attribute unless
   prompted for one by the server, since authentication is not generally
   used with this STUN usage.

12.2.8.  Server Procedures

   The server SHOULD NOT authenticate the client or look for a MESSAGE-
   INTEGRITY attribute.  Since the keepalives come with some regularity,
   and will come for each client that is connected to the server, the
   processing cost associated with authenticating each request is very
   high.  Consequently, authentication should only be used by small
   servers, for whom the processing cost is not an issue, or when used
   with application protocols where the consequences of a fake response
   are very significant.

12.2.9.  Security Considerations for NAT Keepalives

   This STUN usage does not recommend the usage of message integrity or
   authentication.  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 worst
   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 transport address 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



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   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.

   When used with application protocols where the cost of "re-
   registration" is in fact high, the keepalive usage can still be used
   without authentication.  However, the usage would serve ONLY to keep
   NAT bindings alive; it would not be useful for detecting failures of
   the server or of intervening NAT.  In such a case, the client would
   not perform any application layer processing based on the STUN
   response, even if it indicated a change in transport address.

12.3.  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.

12.3.1.  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 applicable hosts as described in Section 12.3.2.
   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.

12.3.2.  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.







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12.3.3.  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.

12.3.4.  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.

   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.







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12.3.5.  New Attributes

   No new attributes are defined by this usage.

12.3.6.  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.

12.3.7.  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
   for a long-term credential.

   If the response was a Shared Secret Response, 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.

12.3.8.  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,



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   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.

12.3.9.  Security Considerations for Short-Term Password

   The security considerations in Section 13 do not apply to the Shared
   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 transport address.
   Requests from the same transport address 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



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   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 that 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,hmac>

   Where prefix is some random text string (different for each shared
   secret request), rounded-time is the current time modulo 20 minutes,
   and hmac is an HMAC [13] over the prefix and rounded-time, using a
   server private key.

   The password is then computed as:

         password = <hmac(USERNAME,anotherprivatekey)>

   With this structure 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



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   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.

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 a transport address that routes to nowhere.  As a result,
   the client won't receive any of the packets it expects to receive
   when it hands out the 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



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   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
   client sends a request and the transport addresses 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.







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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 transport 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 transport address of the request.
   With a modified source transport 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
   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
   transport 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).






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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).

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
   one, 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



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   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.

   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 who 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 [23].




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   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 is 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.

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 [24]).
   STUN is an example of a protocol that performs this type of function
   for the binding discovery usage.  The IAB has mandated that any
   protocols developed for this purpose document a specific set of
   considerations.  This section meets those requirements for the
   binding discovery usage.





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14.1.  Problem Definition

   From RFC3424 [24], 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 the
   functionality necessary to describe how to connect two endpoints
   regardless of the location of type of NATs in the topology.

14.2.  Exit Strategy

   From RFC3424 [24], 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
      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
   [13]), that 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 [24], 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



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      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 receiving 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
      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 receiving packets from a peer behind
      the same NAT if the STUN server supports hairpinning [14].  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



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      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 [24], 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:

   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
      implemented 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.







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14.5.  Issues with Existing NAPT Boxes

   From RFC3424 [24], 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 communications protocols, this broader solution is
   ICE.  ICE uses the binding discovery usage and defines its own
   connectivity check usage, and then utilizes them together.  When done
   this way, ICE eliminates almost all of the brittleness and issues
   found with RFC 3489 alone.


15.  IANA Considerations

   IANA is hereby requested to create two new registries - STUN methods
   and STUN Attributes.  IANA must assign the following values to both
   registries before publication of this document as an RFC.  New values
   for both STUN methods and STUN attributes are assigned through the
   IETF consensus process via RFCs approved by the IESG [25].

15.1.  STUN Methods Registry

   The initial STUN methods are:

    0x001:Binding
    0x002:Shared Secret

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
    0x8023: FINGERPRINT
    0x8022: SERVER
    0x8023: ALTERNATE-SERVER
    0x8024: REFRESH-INTERVAL



16.  Changes Since RFC 3489

   This specification updates RFC3489 [15].  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  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  Introduced formal structure into the Message Type header field,
      with an explicit pair of bits for indication of request, response,
      error response or indication.  Consequently, the message type
      field is split into the class (one of the previous four) and
      method.

   o  Explicitly point out that the most significant 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 three usages -
      Binding Discovery, NAT Keepalive, and Short term password.

   o  Changed transaction timers to be more TCP friendly.

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


17.  Acknowledgements

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


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.



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

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

   [9]   International Telecommunications Union, "Error-correcting
         Procedures for DCEs Using Asynchronous-to-Synchronous
         Conversion", ITU-T Recommendation V.42, 1994.

18.2.  Informational References

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

   [11]  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.

   [12]  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.

   [13]  Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
         Methodology for Network  Address Translator (NAT) Traversal for
         Offer/Answer Protocols", draft-ietf-mmusic-ice-13 (work in
         progress), January 2007.

   [14]  Audet, F. and C. Jennings, "NAT Behavioral Requirements for
         Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress),
         October 2006.

   [15]  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.

   [16]  Rosenberg, J., "Obtaining Relay Addresses from Simple Traversal
         Underneath NAT (STUN)", draft-ietf-behave-turn-02 (work in
         progress), October 2006.

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



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   [18]  Jennings, C. and R. Mahy, "Managing Client Initiated
         Connections in the Session Initiation Protocol  (SIP)",
         draft-ietf-sip-outbound-07 (work in progress), January 2007.

   [19]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
         Description Protocol", RFC 4566, July 2006.

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

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

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

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

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

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


Authors' Addresses

   Jonathan Rosenberg
   Cisco
   Edison, NJ
   US

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


   Christian Huitema
   Microsoft
   One Microsoft Way
   Redmond, WA  98052
   US

   Email: huitema@microsoft.com




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   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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Full Copyright Statement

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