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Versions: (draft-rosenberg-midcom-turn) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 5766

Behave                                                      J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                                 R. Mahy
Expires: May 18, 2008                                        Plantronics
                                                             P. Matthews
                                                                   Avaya
                                                                 D. Wing
                                                                   Cisco
                                                       November 15, 2007


 Traversal Using Relays around NAT (TURN): Relay Extensions to Session
                   Traversal Utilities for NAT (STUN)
                       draft-ietf-behave-turn-05

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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   This Internet-Draft will expire on May 18, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This specification defines an extension of the Session Traversal
   Utilities for NAT (STUN) Protocol for asking the STUN server to relay



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   packets towards a client.  This extension, called Traversal Using
   Relays around NAT (TURN), is useful for hosts behind address
   dependent NATs.  The extension purposefully restricts the ways in
   which the relayed address can be used.  In particular, it prevents
   users from running general purpose servers on ports obtained from the
   TURN server.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Transports . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  About Tuples . . . . . . . . . . . . . . . . . . . . . . .  9
     3.3.  Keepalives . . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  TURN Framing Mechanism . . . . . . . . . . . . . . . . . . . . 11
   5.  General Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Managing Allocations . . . . . . . . . . . . . . . . . . . . . 13
     6.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . 13
       6.1.1.  Initial Allocate Requests  . . . . . . . . . . . . . . 13
       6.1.2.  Refresh Requests . . . . . . . . . . . . . . . . . . . 14
     6.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . 15
       6.2.1.  Initial Allocate         Requests  . . . . . . . . . . 15
       6.2.2.  Refresh Requests . . . . . . . . . . . . . . . . . . . 18
   7.  Sending and Receiving Data . . . . . . . . . . . . . . . . . . 19
     7.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . 19
       7.1.1.  Sending  . . . . . . . . . . . . . . . . . . . . . . . 19
       7.1.2.  Receiving  . . . . . . . . . . . . . . . . . . . . . . 20
     7.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . 20
       7.2.1.  Receiving Data from the Client . . . . . . . . . . . . 20
       7.2.2.  Receiving Data from Peers  . . . . . . . . . . . . . . 22
       7.2.3.  Allocation Activity Timer and Permission Timeout . . . 23
   8.  New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 23
     8.1.  CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 23
     8.2.  LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.3.  BANDWIDTH  . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.4.  PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 24
     8.5.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.6.  RELAY-ADDRESS  . . . . . . . . . . . . . . . . . . . . . . 24
     8.7.  REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 24
     8.8.  REQUESTED-TRANSPORT  . . . . . . . . . . . . . . . . . . . 25
     8.9.  REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 26
   9.  New Error Response Codes . . . . . . . . . . . . . . . . . . . 26
   10. Client Discovery of TURN Servers . . . . . . . . . . . . . . . 27
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 27
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
     12.1. New STUN Methods . . . . . . . . . . . . . . . . . . . . . 29



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     12.2. New STUN Attributes  . . . . . . . . . . . . . . . . . . . 30
     12.3. New STUN Response Codes  . . . . . . . . . . . . . . . . . 30
   13. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 30
   14. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
   15. Changes since version -04  . . . . . . . . . . . . . . . . . . 35
   16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 36
     17.2. Informative References . . . . . . . . . . . . . . . . . . 36
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37
   Intellectual Property and Copyright Statements . . . . . . . . . . 39








































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1.  Introduction

   Session Traversal Utilities for NAT (STUN)
   [I-D.ietf-behave-rfc3489bis] provides a suite of tools for
   facilitating the traversal of NAT.  Specifically, it defines the
   Binding method, which is used by a client to determine its reflexive
   transport address towards the STUN server.  The reflexive transport
   address can be used by the client for receiving packets from peers,
   but only when the client is behind "good" NATs.  In particular, if a
   client is behind a NAT whose mapping behavior [RFC4787] is address or
   address and port dependent (sometimes called "bad" NATs), the
   reflexive transport address will not be usable for communicating with
   a peer.

   The only way to obtain a UDP transport address that can be used for
   corresponding with a peer through such a NAT is to make use of a
   relay.  The relay sits on the public side of the NAT, and allocates
   transport addresses to clients reaching it from behind the private
   side of the NAT.  These allocated transport addresses are from IP
   addresses belonging to the relay.  When the relay receives a packet
   on one of these allocated addresses, the relay forwards it toward the
   client.

   This specification defines an extension to STUN, called TURN, that
   allows a client to request an address on the TURN server, so that the
   TURN server acts as a relay.  This extension defines a handful of new
   STUN methods.  The Allocate method is the most fundamental component
   of this set of extensions.  It is used to provide the client with a
   transport address that is relayed through the TURN server.  A
   transport address which relays through an intermediary is called a
   relayed transport address.

   Though a relayed transport address is highly likely to work when
   corresponding with a peer, it comes at high cost to the provider of
   the relay service.  As a consequence, relayed transport addresses
   should only be used as a last resort.  Protocols using relayed
   transport addresses should make use of mechanisms to dynamically
   determine whether such an address is actually needed.  One such
   mechanism, defined for multimedia session establishment protocols
   based on the offer/answer protocol in RFC 3264 [RFC3264], is
   Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice].

   Though originally invented for Voice over IP applications, TURN is
   designed to be a general-purpose relay mechanism for NAT traversal.







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2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Relayed Transport Address:  A transport address that terminates on a
      server, and is forwarded towards the client.  The TURN Allocate
      request can be used to obtain a relayed transport address, for
      example.

   TURN client:  A STUN client that implements this specification.  It
      obtains a relayed transport address that it provides to a small
      number of peers (usually one).

   TURN server:  A STUN server that implements this specification.  It
      relays data between a TURN client and its peer(s).

   Peer:  A node with which the TURN client wishes to communicate.  The
      TURN server relays traffic between the TURN client and its
      peer(s).

   Allocation:  The IP address and port granted to a client through an
      Allocate request, along with related state, such as permissions
      and expiration timers.

   5-tuple:  A combination of the source IP address and port,
      destination IP address and port, and transport protocol (UDP, or
      TCP).  It uniquely identifies a TCP connection or bi-directional
      flow of UDP datagrams.

   Permission:  A record of an IP address and transport of a peer that
      is permitted to send traffic to the TURN client.  The TURN server
      will only forward traffic to its client from remote peers that
      match an existing permission.


3.  Overview of Operation

   In a typical configuration, a TURN client is connected to a private
   network and through one or more NATs to the public Internet.  On the
   public Internet is a TURN server.  Elsewhere in the Internet are one
   or more peers that the TURN client wishes to communicate with.  This
   specification defines a framing mechanism and several new STUN
   methods.  Together, these add the ability for a STUN server to act as
   a packet relay.

   The framing mechanism serves two purposes.  First, it contains a



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   length field that allow TURN nodes to find the boundaries between
   chunks of application data when the communication with the TURN
   server is over a stream-based transport such as TCP.  Second, it
   carries a channel number.  Channel zero is used for TURN control
   messages, while the other channel numbers are used for application
   data traveling to or from various peers.  The channel number allows
   the client to know which peer sent data to it, and to specify which
   peer is to be the recipient of data.  Application data flowing on any
   non-zero channel is unencapsulated, meaning that the application data
   starts immediately after the framing header.  The framing header is
   just four bytes.  This allows TURN to operate with minimal overhead,
   which is important for the real-time protocols it is designed to
   support.  Application data can also flow in encapsulated format,
   meaning that it is carried in certain TURN messages on channel 0.
   Channel numbers are independent in each direction: for example,
   channel 5 might indicate one peer in the client to server direction,
   but a different peer in the server to client direction.

   When the client wants to obtain a relayed transport address, the
   client first sends an Allocate request to the server, which the
   server authenticates.  The server generates an Allocate response with
   the allocated address, port, and target transport.  All other STUN
   messages defined by this specification happen in the context of an
   allocation.

   A successful Allocate transaction just reserves a transport address
   on the TURN server.  Data does not flow through an allocated
   transport address until the TURN client asks the TURN server to open
   a permission, which is done with a Send Indication.  While the client
   can request more than one permission per allocation, it needs to
   request each permission explicitly and one at a time.  This insures
   that a client can't use a TURN server to run a traditional server,
   and partially protects the client from DoS attacks.

   Once a permission is open, the client can then receive data flowing
   back from its peer.  Initially this data is encapsulated in a Data
   Indication.  Since multiple permissions can be open simultaneously,
   the Data Indication contains the PEER-ADDRESS attribute so the TURN
   client knows which peer sent the data, and a CHANNEL-NUMBER attribute
   so the client knows how the server will refer to traffic from this
   peer when sent unencapsulated.  Likewise when the client initially
   sends to a new peer, it uses a Send Indication with the peer address
   in the PEER-ADDRESS attribute, along with a channel number so the
   server knows how the client will refer to unencapsulated data to this
   peer.






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   TURN                   TURN          peer
   client                 server
     |--- Allocate Req  -->|             |
     |<-- Allocate Resp ---|             |
     |                     |             |
     |--- Send (chan 2) -->|    data     |
     |                     |============>|
     |<-- ChannelConfirm --|             |
     |                     |    data     |
     |                     |<============|
     |<-- Data (chan 5) ---|             |
     |--- ChannelConfirm ->|             |
     |                     |             |
     |--- [2] + data ----->|    data     |
     |                     |============>|
     |                     |    data     |
     |                     |<============|
     |<-- [5] + data ------|             |

                    Figure 1: Example Usage of Channels

   When the client and server communicate over UDP, data and control
   messages can arrive out of order.  For this reason, the client needs
   to verify the server knows the client channel mapping before the
   client sends unencapsulated, and the server needs to verify the
   client knows the server channel mapping before the server sends
   unencapsulated.  When the client and server communicate over UDP, a
   Channel Confirmation indication is sent after the Send (or Data)
   indication so the client (or server) knows that it can send
   unencapsulated.

   Figure 1 demonstrates how this works.  The client performs an
   Allocate Request, and gets a response.  It decides to send data to a
   specific peer.  Initially, it sends data to that peer using a TURN
   Send indication on channel 0.  That Send Indication tells the TURN
   server that, once confirmed, the client will send data unencapsulated
   to that peer on channel 2.  Whenever the TURN server receives a Send
   indication, it stores the mapping from channel number to peer, and
   sends a ChannelConfirm indication (on channel 0).  Once the
   confirmation has been received by the client, the client can send
   data to the peer on channel 2.  Prior to receipt of the
   ChannelConfirm, any other data the client wishes to send to the peer
   is sent using Send indications, all of which indicate that channel 2
   is to be used for unencapsulated data.  The same procedure happens
   from server to client; the TURN server initially sends data using a
   Data indication on channel 0, and once confirmed with a
   ChannelConfirm, it can send it unencapsulated on its selected channel
   (channel 5 in the example).



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   Over a reliable transport, such as TCP, the confirmation step is not
   needed so the Channel Confirmation indication is not used.  Clients
   can immediately send the next piece of data to the peer on the
   requested channel.

   Allocations can also request specific attributes such as the desired
   Lifetime of the allocation and the maximum Bandwidth.  Clients can
   also request specific port assignment behavior, for example, a
   specific port number, odd or even port numbers, or pairs of
   sequential port numbers.

3.1.  Transports

   TURN clients can communicate with a TURN server using UDP, TCP, or
   TLS over TCP.  A TURN server can then relay traffic between a
   reliable transport used between the client and server (TCP or TLS
   over TCP), and UDP used from server to peer.  When relaying data sent
   from a stream-based protocol to a UDP peer, the TURN server emits
   datagrams which are the same length as the length field in the TURN
   framing or the length of the DATA attribute in a Send Indication.
   Likewise, when a UDP datagram is received by the TURN server and
   relayed to the client over a stream-based transport, the length of
   the datagram is the length of the TURN framing or Data Indication's
   DATA attribute.

   The following table shows the possible combinations of transport
   protocols from client to server and from server to peer:

              +-----------------------+---------------------+
              | client to TURN server | TURN server to peer |
              +-----------------------+---------------------+
              | UDP                   | UDP                 |
              | TCP                   | UDP                 |
              | TLS                   | UDP                 |
              +-----------------------+---------------------+

   For TURN clients, using TLS over TCP provides two benefits.  When
   using TLS, the client can be assured that the address of the client's
   peers are not visible to an attacker except by traffic analysis
   downstream of the TURN server.  Second, the client may be able to
   communicate with TURN servers using TLS when it would not be able to
   communicate with the same server using TCP or UDP, due to the
   configuration of a firewall between the TURN client and its server.
   TLS between the client and TURN server in this case just facilitates
   traversal.

   In addition, an extension to TURN is planned to add support for TCP
   allocations [I-D.ietf-behave-turn-tcp].



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3.2.  About Tuples

   To relay data to and from the correct location, the TURN server
   maintains an association between the 5-tuple used to communicate with
   the client and the 5-tuple used to communicate with each of the
   client's peers.  The 5-tuple on the client side will consist of the
   client's reflexive address -- the apparent source address and port of
   the client (typically as rewritten by the last NAT)--and the
   destination address and port used by the TURN server.  The figure
   below (Figure 2) shows a typical topology.  In this diagram, the
   client 5-tuple is for a UDP flow between 192.0.2.1:7000 and
   192.0.2.15:3490.  The 5-tuple between the TURN server and Peer B is
   for a UDP flow between 192.0.2.15:9000 (the TURN allocated address)
   and 192.0.2.210:18200.

      While the terminology used in this document refers to 5-tuples,
      the TURN server can store whatever identifier it likes that yields
      identical results.  Specifically, many implementations may use a
      file-descriptor in place of a 5-tuple to represent a TCP
      connection.































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                                                             +---------+
                                                             |         |
                                                             |         |
                                                           / |  Peer A |
       Client's              TURN                        //  |         |
       Host                  Server                     /    |         |
       Address               Address                  //     +---------+
    10.1.1.2:17240       192.0.2.15:3490             / 192.0.2.180:16400
          |                    |                   //
          |        +-+         |                  /
          |        | |         |                 /
          v        | |         |               //      192.0.2.210:18200
    +---------+    | |         |+---------+   /              +---------+
    |         |    |N|         ||         | //               |         |
    | TURN    |    | |         v|  TURN   |/                 |         |
    | Client  |----|A|----------|  Server |------------------|  Peer B |
    |         |    | |^         |         |^                ^|         |
    |         |    |T||         |         ||                ||         |
    +---------+    | ||         +---------+|                |+---------+
                   | ||                    |                |
                   | ||                    |                |
                   +-+|                    |                |
                      |                    |                |
                      |
                    Client's           TURN              Peer B
                    Reflexive          Allocated         Transport
                    Address            Address           Address
                 192.0.2.1:7000    192.0.2.15:9000     192.0.2.210:18200

                                 Figure 2

3.3.  Keepalives

   Since the main purpose of STUN and TURN is to traverse NATs, it is
   natural to consider which elements are responsible for generating
   sufficient periodic traffic to insure that NAT bindings stay alive.
   TURN clients need to send data frequently enough to keep both NAT
   bindings and the TURN server permissions fresh.  Like NAT bindings,
   the TURN server permissions are refreshed by ordinary data traffic
   relayed from the client to the peer.  Unlike permissions, allocations
   on the TURN server have an explicit expiration time and need to be
   refreshed explicitly by the client with a TURN Refresh request.  When
   an allocation expires, all permissions associated with that
   allocation are automatically deleted.







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4.  TURN Framing Mechanism

   All TURN control messages and all application data sent between the
   client and the server MUST start with the TURN framing header.  This
   header is used for two purposes: indicating the channel number, and
   for framing.

   TURN uses a channel number to distinguish control traffic from data,
   and to distinguish among multiple peers using the same allocation.
   Channel number zero is reserved for TURN control messages.  All TURN
   requests, responses and indications between the client and server
   MUST be sent on channel 0, and MUST NOT be sent on any other channel.
   Channel 0xFFFF is reserved for future use and MUST NOT be used by
   clients or servers compliant to this specification.  Other channel
   numbers are assigned and communicated as described in Section 7.
   Because the framing is always used, TURN needs to run on a separate
   port number from unframed STUN requests.

   Over stream-based transports, the TURN client and server also need to
   include an explicit length so that the TURN server can perform
   conversion from streams to datagrams and vice versa.  TURN framing
   has a 2 octet channel number and a 2 octet length field.  Over
   stream-based transports, the length field counts the number of octets
   immediately after the length field itself.  Over UDP the length is
   always set to zero.

    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Channel Number        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Channel numbers are always defined within a particular allocation.
   If a client has multiple allocations on a TURN server, there is no
   relationship whatsoever between the channel numbers in each
   allocation.  Once created, a channel number persists for the lifetime
   of the allocation.  There is no way to explicitly remove a channel.
   Consequently, a client which obtains an allocation with the intent of
   holding it for extremely long periods, possibly for communication
   with many different peers over time, may eventually exhaust the set
   of channels.  In that case, the client will need to obtain a new
   allocation.


5.  General Behavior

   After the initial Allocate transaction, all subsequent TURN
   transactions need to be sent in the context of a valid allocation.



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   The source and destination IP address and ports for these TURN
   messages MUST match the internal 5-tuple of an existing allocation.
   These are processed using the general server procedures in
   [I-D.ietf-behave-rfc3489bis] with a few important additions.  For
   requests (in this specification, the only subsequent request possible
   is a Refresh request), if there is no matching allocation, the server
   MUST generate a 437 (Allocation Mismatch) error response.  For
   indications, if there is no matching allocation, the indication is
   silently discarded.  An Allocate request MUST NOT be sent by a client
   within the context of an existing allocation.  Such a request MUST be
   rejected by the server with a 437 (Allocation Mismatch) error
   response.

   A subsequent request MUST be authenticated using the same username
   and realm as the one used in the Allocate request that created the
   allocation.  If the request was authenticated but not with the
   matching credential, the server MUST reject the request with a 401
   (Unauthorized) error response.

   When a server returns an error response, it MAY include an ALTERNATE-
   SERVER attribute if it has positive knowledge that the problem
   reported in the error response will not be a problem on the alternate
   server.  For example, a 443 response (Invalid IP Address) with an
   ALTERNATE-SERVER means that the other server is responsible for that
   IP address.  A 442 (Unsupported Transport Protocol) with this
   attribute means that the other server is known to support that
   transport protocol.  A 507 (Insufficient Capacity) means that the
   other server is known to have sufficient capacity.  Using the
   ALTERNATE-SERVER mechanism in the 507 (Insufficient Capacity)
   response can only be done if the rejecting server has definitive
   knowledge of available capacity on the target.  This will require
   some kind of state sharing mechanism between TURN servers, which is
   beyond the scope of this specification.  If a TURN server attempts to
   redirect to another server without knowledge of available capacity,
   it is possible that all servers are in a congested state, resulting
   in series of rejections that only serve to further increase the load
   on the system.  This can cause congestion collapse.

   If a client sends a request to a server and gets a 500 class error
   response without an ALTERNATE-SERVER, or the transaction times out
   without a response, and the client was utilizing the SRV procedures
   of [I-D.ietf-behave-rfc3489bis] to contact the server, the client
   SHOULD try another server based on those procedures.  However, the
   client SHOULD cache the fact that the request to this server failed,
   and not retry that server again for a configurable period of time.
   Five minutes is RECOMMENDED.

   TURN clients and servers MUST NOT include the FINGERPRINT attribute



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   in any of the methods defined in this document.


6.  Managing Allocations

   Communications between a TURN client and a TURN server on a new flow
   begin with an Allocate transaction.  All subsequent transactions
   happen in the context of that allocation.  The client refreshes
   allocations and deallocates them using a Refresh transaction.

6.1.  Client Behavior

6.1.1.  Initial Allocate Requests

   When a client wishes to obtain a transport address, it sends an
   Allocate request to the server.  This request is constructed and sent
   using the general procedures defined in [I-D.ietf-behave-rfc3489bis].
   Clients MUST implement the long term credential mechanism defined in
   [I-D.ietf-behave-rfc3489bis], and be prepared for the server to use
   it.

   The client SHOULD include a BANDWIDTH attribute, which indicates the
   maximum bandwidth that will be used with this binding.  If the
   maximum is unknown, the attribute is not included in the request.

      OPEN ISSUE: Bandwidth is very much underspecified.  Is anyone
      actually using it for capacity planning?  If not we should remove.

   The client MAY request a particular lifetime for the allocation by
   including it in the LIFETIME attribute in the request.

   The client MUST include a REQUESTED-TRANSPORT attribute.  In this
   specification, the REQUESTED-TRANSPORT will always be UDP.  This
   attribute is included to allow for future extensions to TURN.

   The client MAY include a REQUESTED-PORT-PROPS or REQUESTED-IP
   attribute in the request to obtain specific types of transport
   addresses.  Whether these are needed depends on the application using
   the TURN server.  As an example, the Real Time Transport Protocol
   (RTP) [RFC3550] requires that RTP and RTCP ports be an adjacent pair,
   even and odd respectively, for compatibility with a previous version
   of that specification.  The REQUESTED-PORT-PROPS attribute allows the
   client to ask the relay for those properties.

   Processing of the response follows the general procedures of
   [I-D.ietf-behave-rfc3489bis].  A successful response will include
   both a RELAY-ADDRESS and an XOR-MAPPED-ADDRESS attribute, providing
   both a relayed transport address and a reflexive transport address,



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   respectively, to the client.  The value of the LIFETIME attribute in
   the response indicates the amount of time after which the server will
   expire the allocation, if not refreshed with a Refresh request.  The
   server will allow the user to send and receive at least the amount of
   data indicated in the BANDWIDTH attribute per allocation.  (At its
   discretion the server can optionally discard UDP data above this
   threshold.)

   If the response is an error response and contains a 442, 443 or 444
   error code, the client knows that its requested properties could not
   be met.  The client MAY retry with different properties, with the
   same properties (in a hope that something has changed on the server),
   or give up, depending on the needs of the application.  However, if
   the client retries, it SHOULD wait 500ms, and if the request fails
   again, wait 1 second, then 2 seconds, and so on, exponentially
   backing off.

6.1.2.  Refresh Requests

   Before 3/4 of the lifetime of the allocation has passed (the lifetime
   of the allocation is conveyed in the LIFETIME attribute of the
   Allocate Response), the client SHOULD refresh the allocation with a
   Refresh transaction if it wishes to keep the allocation.

   To perform a refresh, the client generates a Refresh Request.  The
   client MUST use the same username, realm and password for the Refresh
   request as it used in its initial Allocate Request.  The Refresh
   request MAY contain a proposed LIFETIME attribute.  The client MAY
   include a BANDWIDTH attribute if it wishes to request more or less
   bandwidth than in the original request.  If absent, it indicates no
   change in the requested bandwidth from the Allocate request.  The
   client MUST NOT include a REQUESTED-IP, REQUESTED-TRANSPORT, or
   REQUESTED-PORT-PROPS attribute in the Refresh request.

   In a successful response, the LIFETIME attribute indicates the amount
   of additional time (the number of seconds after the response is
   received) that the allocation will live without being refreshed.  A
   successful response will also contain a BANDWIDTH attribute,
   indicating the bandwidth the server is allowing for this allocation.
   Note that an error response does not imply that the allocation has
   expired, just that the refresh has failed.

   If a client no longer needs an allocation, it SHOULD perform an
   explicit deallocation.  If the client wishes to explicitly remove the
   allocation because it no longer needs it, it sends a Refresh request,
   but sets the LIFETIME attribute to zero.  This will cause the server
   to remove the allocation, and all associated permissions and channel
   numbers.  For connection-oriented transports such as TCP, the client



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   can also remove the allocation (and all associated bindings) by
   closing the relevant connection with the TURN server.

6.2.  Server Behavior

   The server first processes the request according to the base protocol
   procedures in [I-D.ietf-behave-rfc3489bis], extended with the
   procedures for the long-term credential mechanism.

6.2.1.  Initial Allocate         Requests

   When the server receives an Allocate request, the server attempts to
   allocate a relayed transport address.  It first looks for the
   BANDWIDTH attribute in the request.  If present, the server
   determines whether or not it has sufficient capacity to handle a
   binding that will generate the requested bandwidth.

   If it does, the server attempts to allocate a transport address for
   the client.  The Allocate Request can contain several additional
   attributes that allow the client to request specific characteristics
   of the transport address.

6.2.1.1.  REQUESTED-TRANSPORT

   First, the server checks for the REQUESTED-TRANSPORT attribute.  This
   indicates the transport protocol requested by the client.  This
   specification defines a value for UDP only, but support for TCP
   allocations is planned in [I-D.ietf-behave-turn-tcp].

      As a consequence of the REQUESTED-TRANSPORT attribute, it is
      possible for a client to connect to the server over TCP or TLS
      over TCP and request a UDP transport address.  In this case, the
      server will relay data between the transports.

   If the requested transport is supported, the server allocates a port
   using the requested transport protocol.  If the REQUESTED-TRANSPORT
   attribute contains a value of the transport protocol unknown to the
   server, or known to the server but not supported by the server in the
   context of this request, the server MUST reject the request and
   include a 442 (Unsupported Transport Protocol) in the response.  If
   the request did not contain a REQUESTED-TRANSPORT attribute, the
   server MUST use the same transport protocol as the request arrived
   on.

6.2.1.2.  REQUESTED-IP

   Next, the server checks for the REQUESTED-IP attribute.  If present,
   it indicates a specific IP address from which the client would like



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   its transport address allocated.  (The client could do this if it
   requesting the second address in a specific port pair).  If this IP
   address is not a valid one for allocations on the server, the server
   MUST reject the request and include a 443 (Invalid IP Address) error
   code in the response, or else redirect the request to a server that
   is known to support this IP address.  If the IP address is one that
   is valid for allocations (presumably, the server is configured to
   know the set of IP addresses from which it performs allocations), the
   server MUST provide an allocation from that IP address.  If the
   attribute is not present, the selection of an IP address is at the
   discretion of the server.

6.2.1.3.  REQUESTED-PORT-PROPS

   Finally, the server checks for the REQUESTED-PORT-PROPS attribute.
   If present, it indicates specific port properties desired by the
   client.  This attribute is split into two portions: one portion for
   port behavior and the other for requested port alignment (whether the
   allocated port is odd, even, reserved as a pair, or at the discretion
   of the server).

   If the port behavior requested is for a Specific Port, the server
   MUST attempt to allocate that specific port for the client.  If the
   specific port is not available (in use or reserved), the server MUST
   reject the request with a 444 (Invalid Port) response.  For example,
   the STUN server could reject a request for a Specific Port because
   the port is temporarily reserved as part of an adjacent pair of
   ports, or because the requested port is a well-known port (1-1023).

   If the client requests "even" port alignment, the server MUST attempt
   to allocate an even port for the client.  If an even port cannot be
   obtained, the server MUST reject the request with a 444 (Invalid
   Port) response or redirect to an alternate server.  If the client
   requests odd port alignment, the server MUST attempt to allocate an
   odd port for the client.  If an odd port cannot be obtained, the
   server MUST reject the request with a 444 (Invalid Port) response or
   redirect to an alternate server.  Finally, the "Even port with hold
   of the next higher port" alignment is similar to requesting an even
   port.  It is a request for an even port, and MUST be rejected by the
   server if an even port cannot be provided, or redirected to an
   alternate server.  However, it is also a hint from the client that
   the client will request the next higher port with a separate Allocate
   request.  As such, it is a request for the server to allocate an even
   port whose next higher port is also available, and furthermore, a
   request for the server to not allocate that one higher port to any
   other request except for one that asks for that port explicitly.  The
   server can honor this request for adjacency at its discretion.  The
   only constraint is that the allocated port has to be even.



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      Port alignment requests exist for compatibility with
      implementations of RTP which predate RFC 3550.  These
      implementations use the port numbering conventions in (now
      obsolete) RFC 1889.

6.2.1.4.  Creating the Allocation

   If any of the requested or desired constraints cannot be met, whether
   it be bandwidth, transport protocol, IP address or port, instead of
   rejecting the request, the server can alternately redirect the client
   to a different server that may be able to fulfill the request.  This
   is accomplished using the 300 error response and ALTERNATE-SERVER
   attribute.  If the server does not redirect and cannot service the
   request because the server has reached capacity, it sends a 507
   (Insufficient Capacity) response.  The server can also reject the
   request with a 486 (Allocation Quota Reached) if the user or client
   is not authorized to request additional allocations.

   The server SHOULD only allocate ports in the range 1024-65535.  This
   is one of several ways to prohibit relayed transport addresses from
   being used to attempt to run standard services.

   Once a port is allocated, the server associates the allocation with
   the 5-tuple used to communicate between the client and the server.
   For TCP, this amounts to associating the TCP connection from the TURN
   client with the allocated transport address.

   The new allocation MUST also be associated with the username,
   password and realm used to authenticate the request.  These
   credentials are used in all subsequent requests to ensure that only
   the same client can use or modify the allocation it was given.

   In addition, the allocation created by the server is associated with
   a set of permissions.  Each permission is a specific IP address
   identifying an external client.  Initially, this list is null.

   If the LIFETIME attribute was present in the request, and the value
   is larger than the maximum duration the server is willing to use for
   the lifetime of the allocation, the server MAY lower it to that
   maximum.  However, the server MUST NOT increase the duration
   requested in the LIFETIME attribute.  If there was no LIFETIME
   attribute, the server may choose a duration at its discretion.  Ten
   minutes is RECOMMENDED.  In either case, the resulting duration is
   added to the current time, and a timer, called the allocation
   expiration timer, is set to fire at or after that time.
   Section 7.2.3 discusses behavior when the timer fires.  Note that the
   LIFETIME attribute an Allocate request can be zero, though this is
   effectively a no-op, since it will create and destroy the allocation



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   in one transaction.

6.2.1.5.  Sending the Allocate Response

   Once the port has been obtained and the allocation expiration timer
   has been started, the server generates an Allocate Response using the
   general procedures defined in [I-D.ietf-behave-rfc3489bis], including
   the ones for long term authentication.  The transport address
   allocated to the client MUST be included in the RELAY-ADDRESS
   attribute in the response.  In addition, this response MUST contain
   the XOR-MAPPED-ADDRESS attribute.  This allows the client to
   determine its reflexive transport address in addition to a relayed
   transport address, from the same Allocate request.

   The server MUST add a LIFETIME attribute to the Allocate Response.
   This attribute contains the duration, in seconds, of the allocation
   expiration timer associated with this allocation.

   The server MUST add a BANDWIDTH attribute to the Allocate Response.
   This MUST be equal to the attribute from the request, if one was
   present.  Otherwise, it indicates a per-allocation limit that the
   server is placing on the bandwidth usage on each binding.  Such
   limits are needed to prevent against denial-of-service attacks (See
   Section 11).

6.2.2.  Refresh Requests

   A Refresh request is processed using the general server and long term
   authentication procedures in [I-D.ietf-behave-rfc3489bis].  It is
   used to refresh and extend an allocation, or to cause an immediate
   deallocation.  It is processed as follows.

   First, the request MUST be authenticated using the same shared secret
   as the one associated with the allocation.  If the request was
   authenticated but not with such a matching credential, the server
   MUST generate a Refresh Error Response with a 401 response.

   If the Refresh request contains a BANDWIDTH attribute, the server
   checks that it can relay the requested volume of traffic.

   Finally, a Refresh Request will set a new allocation expiration timer
   for the allocation, effectively canceling the previous allocation
   expiration timer.  As with an Allocate request, the server can offer
   a shorter allocation lifetime, but never a longer one.

   A success Refresh response MUST contain a LIFETIME attribute and a
   BANDWIDTH attribute.




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7.  Sending and Receiving Data

   As described in Section 4, TURN allows a client to send and receive
   data without utilizing TURN Send and Data indications, by sending and
   receiving them on channels.  Before sending client-to-peer or peer-
   to-client data for a new peer, a TURN client or server needs to
   assign a channel number that corresponds to that remote peer.  Once a
   channel number is assigned, it remains assigned through the duration
   of the allocation.  It cannot be unassigned or reassigned to a
   different peer.

7.1.  Client Behavior

7.1.1.  Sending

   When the client wants to forward data to a peer, it checks if it has
   assigned a channel number for communications with this peer (as
   identified by its IP address and port) over this allocation:

   o  If one has not been assigned, the client assigns one of its own
      choosing.  This channel number MUST be one that is currently
      unassigned by the client for this allocation.  It MUST be between
      1 and 65534.  It is RECOMMENDED that the client choose one of the
      unassigned numbers randomly, rather than sequentially.  The state
      of the channel is set to unconfirmed.

   o  If one has been assigned, that channel MUST be selected.

   Next, the client checks if the channel number has been confirmed by
   the server.  If the channel number has been confirmed, the client
   simply sends the data to the TURN server with the appropriate channel
   number in the TURN framing.

   If the channel number has not been confirmed, the client creates a
   Send indication.  It places the selected channel number in a CHANNEL-
   NUMBER attribute, the peer IP address and port in a PEER-ADDRESS
   attribute, and puts the data to be sent in a DATA attribute.  (If the
   client just wishes to create a permission, it can omit the DATA
   attribute.)  If the Send indication is sent over a reliable transport
   (ex: TCP), the client marks that the channel number as confirmed.
   When the client receives a ChannelConfirmation Indication, and the
   channel number, IP address and port match the channel number assigned
   to that peer, the client marks that the channel number is confirmed.

   Since Send is an Indication, it generates no response.  The client
   must rely on application layer mechanisms to determine if the data
   was received by the peer.  A ChannelConfirmation Indication just
   means that some Send indication was received by the TURN server.  It



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   does not mean that a specific Send indication was received by the
   peer.

      Note that Send Indications are not authenticated and do not
      contain a MESSAGE-INTEGRITY attribute.  Just like non-relayed data
      sent over UDP or TCP, the authenticity and integrity of this data
      can only be assured using security mechanisms at higher layers.

7.1.2.  Receiving

   When the client receives a Data indication, it:

   o  records the channel number used by the server (from the CHANNEL-
      NUMBER attribute) and associates it with the IP address and port
      in the PEER-ADDRESS attribute, which identify the peer that sent
      the data.  The resulting mapping from channel number to transport
      address MUST be stored by the client for the duration of the
      allocation.

   o  delivers the contents of the DATA attribute to the client
      application as if it was received from the peer's IP address and
      port.

   o  If the Data indication was received over UDP, the client MUST
      confirm the channel used by the server, by sending a
      ChannelConfirmation Indication to the server.  This indication
      MUST contain the same PEER-ADDRESS and CHANNEL-NUMBER attributes
      included in the Data indication.  This indication is sent to the
      server on channel 0 using the 5-tuple associated with this
      allocation.  Note that, due to round trip delays, a client may
      receive several Data indications with the same channel number for
      the same remote peer.  It MUST process each as defined here,
      resulting in several ChannelConfirmation indications.

   When the client receives unencapsulated data, it checks the received
   channel number.  If the client has a mapping associated with the
   server channel number it delivers the data to the client application
   as if it was received directly from that peer.  Otherwise, it
   silently discards the data.

7.2.  Server Behavior

7.2.1.  Receiving Data from the Client

   When the server receives a Data indication from the client, it:

   o  records the channel number used by the client (from the CHANNEL-
      NUMBER attribute) and associates it with the IP address and port



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      in the PEER-ADDRESS attribute, which identify the peer to which
      the data is to be sent.  The resulting mapping from channel number
      to peer transport address MUST be stored by the server for the
      duration of the allocation.

   o  sends the contents of the DATA attribute in a UDP datagram,
      sending it to the PEER-ADDRESS and sending from the allocated
      transport address.

   o  if one doesn't exist, creates a permission for the IP address from
      the PEER-ADDRESS (the port is ignored), and attaches the
      permission to the allocation

   o  checks if a timer has been set for this permission.  If none has
      been started, the server starts one.  It is RECOMMENDED that it
      have a value of sixty seconds.  If the timer is already running,
      it MUST be reset.

   o  If the Send indication was received over UDP, the server MUST
      confirm the channel used by the client, by sending a
      ChannelConfirmation Indication to the client.  This indication
      MUST contain the same PEER-ADDRESS and CHANNEL-NUMBER attributes
      included in the Send indication.  This indication is sent to the
      client on channel 0 using the 5-tuple associated with this
      allocation.  Note that, due to round trip delays, a server may
      receive several Send indications with the same channel number for
      the same remote peer.  It MUST process each as defined here,
      resulting in several ChannelConfirmation indications.

   When the server receives unencapsulated data, it checks the received
   channel number:

   o  If the server has a mapping associated with the client channel
      number it:

      *  sends a UDP datagram to the peer using the transport address
         from the mapping, and sends from the allocated transport
         address.

      *  checks if a permission activity timer is running for the
         destination IP address of the peer.  If one is not running, the
         server starts one.  It is RECOMMENDED that it have a value of
         sixty seconds.  If the timer is already running, it MUST be
         reset.

   o  If the server has no mapping, it silently discards the data.





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7.2.2.  Receiving Data from Peers

   If a server receives a UDP packet on an allocated UDP transport
   address, it checks the permissions associated with that allocation.
   If the source IP address of the UDP packet matches one of the
   permissions (the source port is not used), the UDP packet is
   accepted.  Otherwise, it is discarded.  If the packet is accepted, it
   is forwarded to the client as described below.

   The server checks if it has assigned a channel number for
   communications from this peer (as identified by its IP address and
   port) over this allocation:

   o  If one has not been assigned, the client assigns one of its own
      choosing.  This channel number MUST be one that is currently
      unassigned by the server for this allocation.  It MUST be between
      1 and 65534.  It is RECOMMENDED that the server choose one of the
      unassigned numbers randomly, rather than sequentially.  The state
      of the channel is set to unconfirmed.

   o  If one has been assigned, that channel MUST be selected.

   Note that data from peers does not reset the permission activity
   timer.

   Next, the server checks if the channel number has been confirmed by
   the client.  If the channel number has been confirmed, the server
   simply sends the data to the client with the appropriate channel
   number in the TURN framing.

   If the channel number has not been confirmed, the server creates a
   Data indication.  It places the selected channel number in a CHANNEL-
   NUMBER attribute, the peer IP address and port in a PEER-ADDRESS
   attribute, and puts the data to be sent in a DATA attribute.  If the
   Data indication is sent over a reliable transport (ex: TCP), the
   server marks that the channel number as confirmed.  When the server
   receives a ChannelConfirmation Indication, and the channel number, IP
   address and port match the channel number assigned to that peer, the
   server marks that the channel number is confirmed.

   Since Data is an Indication, it generates no response.  The server
   does not provide reliability for the data.  When sending over a
   reliable transport to the client, if the server is unable to send the
   data received from the peer (for example, because the TCP connection
   cannot accept any more messages right now), it can silently discards
   UDP data received from the peer.





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      Note that Send Indications are not authenticated and do not
      contain a MESSAGE-INTEGRITY attribute.  Just like non-relayed data
      sent over UDP or TCP, the authenticity and integrity of this data
      can only be assured using security mechanisms at higher layers.

7.2.3.  Allocation Activity Timer and Permission Timeout

   When the allocation activity timer expires, the server MUST destroy
   the allocation.  This involves freeing the allocated transport
   address, deleting permissions and channel numbers, and removing other
   state associated with the allocation.

   When a permission times out, the TURN server MUST NOT forward a
   packet from that TURN peer to the TURN client.


8.  New Attributes

   This STUN extension defines the following new attributes:


   0x000C: CHANNEL-NUMBER
   0x000D: LIFETIME
   0x0010: BANDWIDTH
   0x0012: PEER-ADDRESS
   0x0013: DATA
   0x0016: RELAY-ADDRESS
   0x0018: REQUESTED-PORT-PROPS
   0x0019: REQUESTED-TRANSPORT
   0x0022: REQUESTED-IP

8.1.  CHANNEL-NUMBER

   The channel number attribute represents the channel number assigned
   by the sender, that corresponds with the peer specified in the PEER-
   ADDRESS attribute.  It is a 16-bit unsigned integer, plus two octets
   of padding which MUST be set to zero.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Channel Number         |         Reserved = 0          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+








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8.2.  LIFETIME

   The lifetime attribute represents the duration for which the server
   will maintain an allocation in the absence of a refresh.  It is a 32
   bit unsigned integral value representing the number of seconds
   remaining until expiration.

8.3.  BANDWIDTH

   The bandwidth attribute represents the peak bandwidth, measured in
   kilobits per second, that the client expects to use on the allocation
   in each direction.

8.4.  PEER-ADDRESS

   The PEER-ADDRESS specifies the address and port of the peer as seen
   from the TURN server.  It is encoded in the same way as XOR-MAPPED-
   ADDRESS.

8.5.  DATA

   The DATA attribute is present in most Send Indications and Data
   Indications.  It contains raw payload data that is to be sent (in the
   case of a Send Request) or was received (in the case of a Data
   Indication).

8.6.  RELAY-ADDRESS

   The RELAY-ADDRESS is present in Allocate responses.  It specifies the
   address and port that the server allocated to the client.  It is
   encoded in the same way as XOR-MAPPED-ADDRESS.

8.7.  REQUESTED-PORT-PROPS

   This attribute allows the client to request certain properties for
   the port that is allocated by the server.  The attribute can be used
   with any transport protocol that has the notion of a 16 bit port
   space (including TCP and UDP).  The attribute is 32 bits long.  Its
   format 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Reserved = 0        | A |    Specific Port Number       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The two bits labeled A in the diagram above are for requested port



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   alignment and have the following meaning:

   00 no specific port alignment
   01 odd port number
   10 even port number
   11 even port number; reserve next higher port

   If the value of the A field is 00 (no specific port alignment), then
   the Specific Port Number field can either be 0 or some non-zero port
   number.  If the Specific Port Number field is 0, then the client is
   not putting any restrictions on the port number it would like
   allocated.  If the Specific Port Number is some non-zero port number,
   then the client is requesting that the server allocate the specified
   port.

   If the value of the A field is 01 (odd port number), then the
   Specific Port Number field must be zero, and the client is requesting
   the server allocate an odd-numbered port.

   If the value of the A field is 10 (even port number), then the
   Specific Port number field must be zero, and the client is requesting
   the server allocate an even-numbered port.

   If the value of the A field is 11 (even port number; reserve next
   higher port), then the Specific Port Number field must be zero, and
   the client is requesting the server allocate an even-numbered port.
   In addition, the client is requesting the server reserve the next
   higher port (i.e., N+1 if the server allocates port N), and should
   only allocate the N+1 port number if it is explicit requested (with a
   subsequent request specifying that exact port number)

   In all cases, if a port with the requested properties cannot be
   allocated, the server responds with a error response with an error
   code of 444 (Invalid Port).

8.8.  REQUESTED-TRANSPORT

   This attribute is used by the client to request a specific transport
   protocol for the allocated transport address.  It is a 32 bit
   unsigned integer.  Its values are:


   0x0000 0000: UDP
   0x0000 0001: Reserved for TCP

   If an Allocate request is sent over TCP and requests a UDP
   allocation, or an Allocate request is sent over TLS over TCP and
   requests a UDP allocation, the server will relay data between the two



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   transports.

   Extensions to TURN can define additional transport protocols in an
   IETF-consensus RFC.

8.9.  REQUESTED-IP

   The REQUESTED-IP attribute is used by the client to request that a
   specific IP address be allocated to it.  This attribute is needed
   since it is anticipated that TURN servers will be multi-homed so as
   to be able to allocate more than 64k transport addresses.  As a
   consequence, a client needing a second transport address on the same
   interface as a previous one can make that request.

   The format of this attribute is identical to XOR-MAPPED-ADDRESS.
   However, the port component of the attribute is ignored by the
   server.  If a client wishes to request a specific IP address and
   port, it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS
   attributes.


9.  New Error Response Codes

   This document defines the following new Error response codes:

   437  (Allocation Mismatch): A request was received by the server that
      requires an allocation to be in place, but there is none, or a
      request was received which requires no allocation, but there is
      one.

   442  (Unsupported Transport Protocol): The Allocate request asked for
      a transport protocol to be allocated that is not supported by the
      server.  If the server is aware of another server that supports
      the requested protocol, it SHOULD include the other server's
      address in an ALTERNATE-SERVER attribute in the error response.

   443  (Invalid IP Address): The Allocate request asked for a transport
      address to be allocated from a specific IP address that is not
      valid on the server.

   444  (Invalid Port): The Allocate request asked for a port to be
      allocated that is not available on the server.

   486  (Allocation Quota Reached): The user or client is not authorized
      to request additional allocations.






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   507  (Insufficient Capacity): The server cannot allocate a new port
      for this client as it has exhausted its relay capacity.


10.  Client Discovery of TURN Servers

   The STUN extensions introduced by TURN differ from the binding
   requests defined in [I-D.ietf-behave-rfc3489bis] in that they are
   sent with additional framing and demand substantial resources from
   the TURN server.  In addition, it seems likely that administrators
   might want to block connections from clients to the TURN server for
   relaying separately from connections for the purposes of binding
   discovery.  As a consequence, TURN runs on a separate port from STUN.
   The client discovers the address and port of the TURN server using
   the same DNS procedures defined in [I-D.ietf-behave-rfc3489bis], but
   using an SRV service name of "turn" (or "turns" for TURN over TLS)
   instead of just "stun".

   For example, to find TURN servers in the example.com domain, the TURN
   client performs a lookup for '_turn._udp.example.com',
   '_turn._tcp.example.com', and '_turns._tcp.example.com' if the STUN
   client wants to communicate with the TURN server using UDP, TCP, or
   TLS over TCP, respectively.


11.  Security Considerations

   TURN servers allocate bandwidth and port resources to clients, in
   contrast to the Binding method defined in
   [I-D.ietf-behave-rfc3489bis].  Therefore, a TURN server requires
   authentication and authorization of STUN requests.  This
   authentication is provided by mechanisms defined in the STUN
   specification itself, in particular digest authentication.

   Because TURN servers allocate resources, they can be susceptible to
   denial-of-service attacks.  All Allocate transactions are
   authenticated, so that an unknown attacker cannot launch an attack.
   An authenticated attacker can generate multiple Allocate Requests,
   however.  To prevent a single malicious user from allocating all of
   the resources on the server, it is RECOMMENDED that a server
   implement a modest per user limit on the amount of bandwidth that can
   be allocated.  Such a mechanism does not prevent a large number of
   malicious users from each requesting a small number of allocations.
   Attacks such as these are possible using botnets, and are difficult
   to detect and prevent.  Implementors of TURN should keep up with best
   practices around detection of anomalous botnet attacks.

   A client will use the transport address learned from the RELAY-



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   ADDRESS attribute of the Allocate Response to tell other users how to
   reach them.  Therefore, a client needs to be certain that this
   address is valid, and will actually route to them.  Such validation
   occurs through the message integrity checks provided in the Allocate
   response.  They can guarantee the authenticity and integrity of the
   allocated addresses.  Note that TURN is not susceptible to the
   attacks described in Section 12.2.3, 12.2.4, 12.2.5 or 12.2.6 of
   [I-D.ietf-behave-rfc3489bis] [[TODO: Update section number references
   to 3489bis]].  These attacks are based on the fact that a STUN server
   mirrors the source IP address, which cannot be authenticated.  STUN
   does not use the source address of the Allocate Request in providing
   the RELAY-ADDRESS, and therefore, those attacks do not apply.

   TURN cannot be used by clients for subverting firewall policies.
   TURN has fairly limited applicability, requiring a user to explicitly
   authorize permission to receive data from a peer, one IP address at a
   time.  Thus, it does not provide a general technique for
   externalizing sockets.  Rather, it has similar security properties to
   the placement of an address-restricted NAT in the network, allowing
   messaging in from a peer only if the internal client has sent a
   packet out towards the IP address of that peer.  This limitation
   means that TURN cannot be used to run web servers, email servers, SIP
   servers, or other network servers that service a large number of
   clients.  Rather, it facilitates rendezvous of NATted clients that
   use some other protocol, such as SIP, to communicate IP addresses and
   ports for communications.

   Confidentiality of the transport addresses learned through Allocate
   transactions does not appear to be that important.  If required, it
   can be provided by running TURN over TLS.

   TURN does not and cannot guarantee that UDP data is delivered in
   sequence or to the correct address.  As most TURN clients will only
   communicate with a single peer, the use of a single channel number
   will be very common.  Consider an enterprise where Alice and Bob are
   involved in separate calls through the enterprise NAT to their
   corporate TURN server.  If the corporate NAT reboots, it is possible
   that Bob will obtain the exact NAT binding originally used by Alice.
   If Alice and Bob were using identical channel numbers, Bob will
   receive unencapsulated data intended for Alice and will send data
   accidentally to Alice's peer.  This is not a problem with TURN.  This
   is precisely what would happen if there was no TURN server and Bob
   and Alice instead provided a (STUN) reflexive transport address to
   their peers.  If detecting this misdelivery is a problem, the client
   and its peer need to use message integrity on their data.

   One TURN-specific DoS attack bears extra discussion.  An attacker who
   can corrupt, drop, or cause the loss of a Send or Data indication



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   sent over UDP, and then forge a Channel Confirmation indication for
   the corresponding channel number, can cause a TURN client (server) to
   start sending unencapsulated data that the server (client) will
   discard.  Since indications are not integrity protected, this attack
   is not prevented by cryptographic means.  However, any attacker who
   can generate this level of network disruption could simply prevent a
   large fraction of the data from arriving at its destination, and
   therefore protecting against this attack does not seem important.
   The ChannelConfirmation forging attack is not possible when the
   client to server communication is over TCP or TLS over TCP.

   Relay servers are useful even for users not behind a NAT.  They can
   provide a way for truly anonymous communications.  A user can cause a
   call to have its media routed through a TURN server, so that the
   user's IP addresses are never revealed.

   Any relay addresses learned through an Allocate request will not
   operate properly with IPSec Authentication Header (AH) [RFC4302] in
   transport or tunnel mode.  However, tunnel-mode IPSec ESP [RFC4303]
   should still operate.


12.  IANA Considerations

   This specification defines several new STUN methods, STUN attributes,
   and STUN response codes.  This section directs IANA to add these new
   protocol elements to the IANA registry of STUN protocol elements.

12.1.  New STUN Methods


   Request/Response Transactions
   0x003  :  Allocate
   0x004  :  Refresh

   Indications
   0x006  :  Send
   0x007  :  Data
   0x009  :  Channel Confirmation












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12.2.  New STUN Attributes


   0x000C: CHANNEL-NUMBER
   0x000D: LIFETIME
   0x0010: BANDWIDTH
   0x0012: PEER-ADDRESS
   0x0013: DATA
   0x0016: RELAY-ADDRESS
   0x0018: REQUESTED-PORT-PROPS
   0x0019: REQUESTED-TRANSPORT
   0x0022: REQUESTED-IP

12.3.  New STUN Response Codes

   437    Allocation Mismatch
   442    Unsupported Transport Protocol
   443    Invalid IP Address
   444    Invalid Port
   486    Allocation Quota Reached
   507    Insufficient Capacity


13.  IAB Considerations

   The IAB has studied the problem of "Unilateral Self Address Fixing",
   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 RFC 3424 [RFC3424].  The
   TURN extension is an example of a protocol that performs this type of
   function.  The IAB has mandated that any protocols developed for this
   purpose document a specific set of considerations.

   TURN is an extension of the STUN protocol.  As such, the specific
   usages of STUN that use the TURN extensions need to specifically
   address these considerations.  Currently the only STUN usage that
   uses TURN is ICE [I-D.ietf-mmusic-ice].


14.  Example

   In this example, a TURN client is behind a NAT.  This TURN client is
   running SIP.  The client has a private address of 10.0.1.1.  The TURN
   server is on the public side of the NAT, and is listening for TURN
   requests on 192.0.2.3:8776.  The public side of the NAT has an IP
   address of 192.0.2.1.  The client is attempting to send a SIP INVITE
   to a peer, and wishes to allocate an IP address and port for
   inclusion in the SDP of the INVITE.  Normally, TURN would be used in



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   conjunction with ICE when applied to SIP.  However, to keep the
   example simple, TURN is shown without ICE.

   The client communicates with a SIP user agent on the public network.
   This user agent uses a 192.0.2.17:12734 for receipt of its RTP
   packets.

   10.0.1.1            192.0.2.1           192.0.2.3         192.0.2.17
    Client                NAT             TURN Server            Peer
       |                   |                   |                   |
       |(1) Allocate       |(2) Allocate       |                   |
       |S=10.0.1.1:4334    |S=192.0.2.1:63346  |                   |
       |D=192.0.2.3:8776   |D=192.0.2.3:8776   |                   |
       |------------------>|------------------>|                   |
       |                   |                   |                   |
       |(4) Error          |(3) Error          |                   |
       |S=192.0.2.3:8776   |S=192.0.2.3:8776   |                   |
       |D=10.0.1.1:4334    |D=192.0.2.1:63346  |                   |
       |<------------------|<------------------|                   |
       |                   |                   |                   |
       |(5) Allocate       |(6) Allocate       |                   |
       |S=10.0.1.1:4334    |S=192.0.2.1:63346  |                   |
       |D=192.0.2.3:8776   |D=192.0.2.3:8776   |                   |
       |------------------>|------------------>|                   |
       |                   |                   |                   |
       |                   |          (allocates port 32766)       |
       |                   |                   |                   |
       |                   |                   |                   |
       |(8) Response       |(7) Response       |                   |
       |RA=192.0.2.3:32766 |RA=192.0.2.3:32766 |                   |
       |MA=192.0.2.1:63346 |MA=192.0.2.1:63346 |                   |
       |S=192.0.2.3:8776   |S=192.0.2.3:8776   |                   |
       |D=10.0.1.1:4334    |D=192.0.2.1:63346  |                   |
       |<------------------|<------------------|                   |
       |                   |                   |                   |
       |(9) SIP INVITE     |                   |                   |
       |SDP=192.0.2.3:32766|                   |                   |
       |---------------------------------------------------------->|
       |                   |                   |                   |
       |(10) SIP 200 OK    |                   |                   |
       |SDP=192.0.2.17:12734                   |                   |
       |<----------------------------------------------------------|
       |                   |                   |                   |
       |                   |                   |(11) RTP           |
       |                   |                   |S=192.0.2.17:12734 |
       |                   |                   |D=192.0.2.3:32766  |
       |                   |                   |<------------------|
       |                   |                   |                   |



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       |                   |    (no permission; packet dropped)    |
       |                   |                   |                   |
       |(12) SIP ACK       |                   |                   |
       |---------------------------------------------------------->|
       |                   |                   |                   |
       |(13) Send Indic.   |(14) Send Indic.   |                   |
       |TURN Channel=0     |TURN Channel=0     |                   |
       |STUN DATA=RTP      |STUN DATA=RTP      |                   |
       |CHANNEL-NUMER=77   |CHANNEL-NUMBER=77  |                   |
       |PA=192.0.2.17:12734|PA=192.0.2.17:12734|                   |
       |S=10.0.1.1:4334    |S=192.0.2.1:63346  |                   |
       |D=192.0.2.3:8776   |D=192.0.2.3:8776   |                   |
       |------------------>|------------------>|                   |
       |                   |                   |                   |
       |                   |            permission created         |
       |                   |                   |                   |
       |                   |                   |(15) RTP           |
       |                   |                   |S=192.0.2.3:32766  |
       |                   |                   |D=192.0.2.17:12734 |
       |                   |                   |------------------>|
       |                   |                   |                   |
       |(17) ChannelConf   |(16) ChannelConf   |                   |
       |TURN Channel=0     |TURN Channel=0     |                   |
       |CHANNEL-NUMBER=77  |CHANNEL-NUMBER=77  |                   |
       |PA=192.0.2.17:12734|PA=192.0.2.17:12734|                   |
       |S=192.0.2.3:8776   |S=192.0.2.3:8776   |                   |
       |D=10.0.1.1:4334    |D=192.0.2.1:63346  |                   |
       |<------------------|<------------------|                   |
       |                   |                   |                   |
       |(18) TURN Framed   |(19) TURN Framed   |                   |
       |TURN Channel=77    |TURN Channel=77    |(20) RTP           |
       |S=10.0.1.1:4334    |S=192.0.2.1:63346  |S=192.0.2.3:32766  |
       |D=192.0.2.3:8776   |D=192.0.2.3:8776   |D=192.0.2.17:12734 |
       |------------------>|------------------>|------------------>|
       |                   |                   |                   |
       |(23) Data Indic.   |(22) Data Indic.   |                   |
       |TURN Channel=0     |TURN Channel=0     |                   |
       |CHANNEL-NUMBER=33  |CHANNEL-NUMBER=33  |(21) RTP           |
       |S=192.0.2.3:8776   |S=192.0.2.3:8776   |S=192.0.2.17:12734 |
       |D=10.0.1.1:4334    |D=192.0.2.1:63346  |D=192.0.2.3:32766  |
       |<------------------|<------------------|<------------------|
       |                   |                   |                   |
       |(24) ChannelConf   |(25) ChannelConf   |                   |
       |TURN Channel=0     |TURN Channel=0     |                   |
       |CHANNEL-NUMBER=33  |CHANNEL-NUMBER=33  |                   |
       |S=10.0.0.1:4334    |S=192.0.2.3:8776   |                   |
       |D=192.0.2.3:8776   |D=192.0.2.3:8776   |                   |
       |------------------>|------------------>|                   |



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       |                   |                   |                   |
       |(28) TURN Framed   |(27) TURN Framed   |                   |
       |TURN Channel=33    |TURN Channel=33    |(26) RTP           |
       |S=192.0.2.3:8776   |S=192.0.2.3:8776   |S=192.0.2.17:12734 |
       |D=10.0.1.1:4334    |D=192.0.2.1:63346  |D=192.0.2.3:32766  |
       |<------------------|<------------------|<------------------|
       |                   |                   |                   |


                                 Figure 12

   The message flow is shown in Figure 12.  In step 1-2, the client
   allocates a UDP port from the local operating system on its private
   interface, obtaining 4334.  It then attempts to obtain a port for RTP
   traffic.  RTCP processing is not shown in the example.

   In step 1, the client sends an Allocate Request (1) with a source
   address (denoted by S) of 10.0.1.1:4334 and a destination (denoted by
   D) of 192.0.2.3:8776.  This passes through the NAT (2), which
   allocates a new UDP port (63346) on the NAT's public interface
   (192.0.2.1), and creates an internal mapping between the internal
   address 10.0.1.1:4334 and that external address 192.0.2.1:63346.  The
   NAT sends this request to the TURN server (3).  The TURN server
   challenges the request, requesting credentials by sending a STUN
   error and including the NONCE and REALM attributes.  Message 3 is
   relayed, by the NAT, to the TURN client (4).  The client sends a new
   request (from the same UDP port), including its credentials (5, 6).
   The TURN server authenticates the request.  The TURN server allocates
   a new UDP port on one of its interfaces, 192.0.2.3:32766.  The TURN
   server puts 192.0.2.3:32766 into the RELAY-ADDRESS (denoted by RA)
   attribute of the response, and puts the source IP address and UDP
   port of the request (as seen by the TURN server) into the XOR-MAPPED-
   ADDRESS attribute (denoted by MA).  In step 7, this message is sent
   back to the TURN client and relayed by the NAT in step 8.

   The client now proceeds to perform a basic SIP call setup.  In
   message 9, the TURN client includes the TURN server's address (which
   it learned in message 8) in the SDP of its INVITE (e.g., using syntax
   described in[I-D.ietf-mmusic-ice]).  The called party responds with
   its SDP in a provisional response (18x) or a final response (200 Ok).
   The called party's SDP includes its IP address and UDP port,
   192.0.2.17:12734.  Immediately after sending its 200 Ok, the called
   party sends an RTP packet to the TURN server's IP address (11).  This
   RTP packet is dropped by the TURN server, because the TURN server has
   not been given permission to relay that data.  Incoming packets are
   dropped until a permission is created.  The SIP exchange completes
   with an SIP 200 Ok message (12).




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   Steps 13-20 show the client performing a channel allocation.  The
   TURN client needs to send an RTP packet.  Since no channels and no
   permissions have been created, the TURN client sends the RTP packet
   inside of a Send Indication, using channel number 0, with the
   CHANNEL-NUMBER attribute set to the channel number the TURN client
   wants to use for subsequent communication with this TURN peer (77 is
   shown in the example).  The TURN peer's IP address and UDP port
   (which were learned from the SDP answer received in step 10) are
   placed in the PEER-ADDRESS attribute (denoted by PA).  In message 13,
   the TURN client sends this Send Indication, and it is relayed by the
   NAT to the TURN server (14).  Upon receipt of that message, the TURN
   server creates a permission, which allows subsequent traffic from
   that same peer address to be relayed to that TURN client's IP address
   and UDP port.  The TURN server sends the contents of the Send
   Indication's DATA attribute towards the PEER-ADDRESS (15); this will
   typically be an RTP packet.  Note that the source address and port of
   message 15 is the TURN server's address, 192.0.2.3:32766, which is
   the allocated transport address communicated to the TURN client in
   messages 7 and 8.

   In step 16, the TURN server sends a channel confirmation message to
   the TURN client.  Once the TURN client receives this message, it can
   forgo using the Send Indication for that channel.  Instead, it can
   utilize the channel number in the TURN framing header.  Steps 18 and
   19 show the TURN client sending a message to TURN server using the
   TURN framing header, with channel=1.  Step 20 shows the TURN server
   removing the TURN framing and sending the RTP packet to the TURN
   peer.

   Steps 21-28 show an RTP packet from the TURN peer, which causes a
   channel allocation by the TURN server.  In packet 21, an RTP packet
   is sent by the TURN peer to the TURN server.  There is an existing
   permission (created in step 14), so the TURN server accepts this
   incoming RTP packet.  The TURN server knows the TURN client to send
   this packet to, but does not yet have a channel assigned for traffic
   in this direction.  The TURN server chooses a channel number (33 in
   the example), and sends a Data Indication to the TURN client (message
   22).  The NAT relays this to the TURN client (message 23).  The TURN
   client sends an Channel Confirmation message (24) which is relayed by
   the NAT (25).  When the TURN server receives the Channel
   Confirmation, it no longer needs to use a Send Indication for traffic
   from that remote peer; instead, it can use TURN framing with its
   chosen channel number (33).  The next RTP packet that arrives from
   that peer (26) is sent by the TURN server using TURN framing
   indicating the channel number (message 27) and relayed by the NAT to
   the TURN client (28).





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15.  Changes since version -04

   This section lists the major changes between thiis document and
   draft-ietf-behave-turn-04:

   o  Removed the ability to allocate addresses for TCP relaying.  This
      is now covered in a separate document.  However, communication
      between the client and the server can still run over TCP or TLS/
      TCP.  This resulted in the removal of the Connect method and the
      TIMER-VAL and CONNECT-STAT attributes.

   o  Added the concept of channels.  All communication between the
      client and the server flows on a channel.  Channels are numbered
      0..65535.  Channel 0 is used for TURN messages, while the
      remaining channels are used for sending unencapsulated data to/
      from a remote peer.  This concept adds a new Channel Confirmation
      method and a new CHANNEL-NUMBER attribute.  The new attribute is
      also used in the Send and Data methods.

   o  The framing mechanism formally used just for stream-oriented
      transports is now also used for UDP, and the former Type and
      Reserved fields in the header have been replaced by a Channel
      Number field.  The length field is zero when running over UDP.

   o  TURN now runs on its own port, rather than using the STUN port.
      The use of channels requires this.

   o  Removed the SetActiveDestination concept.  This has been replaced
      by the concept of channels.

   o  Changed the allocation refresh mechanism.  The new mechanism uses
      a new Refresh method, rather than repeating the Allocation
      transaction.

   o  Changed the syntax of SRV requests for secure transport.  The new
      syntax is "_turns._tcp" rather than the old "_turn._tls".  This
      change mirrors the corresponding change in STUN SRV syntax.

   o  Renamed the old REMOTE-ADDRESS attribute to PEER-ADDRESS, and
      changed it to use the XOR-MAPPED-ADDRESS format.

   o  Changed the RELAY-ADDRESS attribute to use the XOR-MAPPED-ADDRESS
      format (instead of the MAPPED-ADDRESS format)).

   o  Renamed the 437 error code from "No Binding" to "Allocation
      Mismatch".





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   o  Added a discussion of what happens if a client's public binding on
      its outermost NAT changes.

   o  The document now consistently uses the term "peer" as the name of
      a remote endpoint with which the client wishes to communicate.

   o  Rewrote much of the document to describe the new concepts.  At the
      same time, tried to make the presentation clearer and less
      repetitive.


16.  Acknowledgements

   The authors would like to thank Marc Petit-Huguenin for his comments
   and suggestions.


17.  References

17.1.  Normative References

   [I-D.ietf-behave-rfc3489bis]
              Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for (NAT) (STUN)",
              draft-ietf-behave-rfc3489bis-12 (work in progress),
              November 2007.

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

17.2.  Informative References

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

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



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              Translation", RFC 3424, November 2002.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-19 (work in progress), October 2007.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

   [I-D.ietf-behave-turn-tcp]
              Rosenberg, J. and R. Mahy, "Traversal Using Relays around
              NAT (TURN) Extensions for TCP Allocations",
              draft-ietf-behave-turn-tcp-00 (work in progress),
              November 2007.


Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems, Inc.
   Edison, NJ
   US

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


   Rohan Mahy
   Plantronics, Inc.

   Email: rohan@ekabal.com


   Philip Matthews
   Avaya, Inc.
   1135 Innovation Drive
   Ottawa, Ontario  K2K 3G7
   Canada

   Phone: +1 613 592-4343 x223
   Fax:
   Email: philip_matthews@magma.ca
   URI:





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Internet-Draft                    TURN                     November 2007


   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Phone:
   Fax:
   Email: dwing@cisco.com
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Internet-Draft                    TURN                     November 2007


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