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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: July 25, 2008                                       Plantronics
                                                             P. Matthews
                                                                   Avaya
                                                        January 22, 2008


 Traversal Using Relays around NAT (TURN): Relay Extensions to Session
                   Traversal Utilities for NAT (STUN)
                       draft-ietf-behave-turn-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
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   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on July 25, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2008).

Abstract

   If a host is located behind a NAT, then in certain situations it can
   be impossible for that host to communicate directly with other hosts
   (peers) located behind other NATs.  In these situations, it is
   necessary for the host to use the services of an intermediate node



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   that acts as a communication relay.  This specification defines a
   protocol, called TURN (Traversal Using Relays around NAT), that
   allows the host to control the operation of the relay and to exchange
   packets with its peers using the relay.

   The TURN protocol can be used in isolation, but is more properly used
   as part of the ICE (Interactive Connectivity Establishment) approach
   to NAT traversal.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of Operation  . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Transports . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Allocations  . . . . . . . . . . . . . . . . . . . . . . .  8
     2.3.  Exchanging Data with Peers . . . . . . . . . . . . . . . .  9
     2.4.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . 10
     2.5.  Channels . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  General Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Managing Allocations . . . . . . . . . . . . . . . . . . . . . 14
     5.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . . . 14
       5.1.1.  Initial Allocate Requests  . . . . . . . . . . . . . . 14
       5.1.2.  Refresh Requests . . . . . . . . . . . . . . . . . . . 15
     5.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . . . 16
       5.2.1.  Receiving an Allocate Request  . . . . . . . . . . . . 16
       5.2.2.  Refresh Requests . . . . . . . . . . . . . . . . . . . 20
   6.  Send and Data Indications  . . . . . . . . . . . . . . . . . . 21
     6.1.  Forming and Sending an Indication  . . . . . . . . . . . . 21
     6.2.  Receiving an Indication  . . . . . . . . . . . . . . . . . 22
     6.3.  Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Channel Mechanism  . . . . . . . . . . . . . . . . . . . . . . 23
     7.1.  Forming and Sending a ChannelBind Request  . . . . . . . . 23
     7.2.  Receiving a ChannelBind Request and Sending a Response . . 24
     7.3.  Receiving a ChannelBind Response . . . . . . . . . . . . . 25
     7.4.  The ChannelData Message  . . . . . . . . . . . . . . . . . 25
     7.5.  Forming and Sending a ChannelData Message  . . . . . . . . 25
     7.6.  Receiving a ChannelData Message  . . . . . . . . . . . . . 26
     7.7.  Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 26
   8.  New STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 27
   9.  New STUN Attributes  . . . . . . . . . . . . . . . . . . . . . 27
     9.1.  CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 28
     9.2.  LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.3.  BANDWIDTH  . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.4.  PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 28
     9.5.  DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.6.  RELAY-ADDRESS  . . . . . . . . . . . . . . . . . . . . . . 28



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     9.7.  REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 28
     9.8.  REQUESTED-TRANSPORT  . . . . . . . . . . . . . . . . . . . 30
     9.9.  REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 30
   10. New STUN Error Response Codes  . . . . . . . . . . . . . . . . 30
   11. Client Discovery of TURN Servers . . . . . . . . . . . . . . . 31
   12. Security Considerations  . . . . . . . . . . . . . . . . . . . 32
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 34
   14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 34
   15. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
   16. Changes from Previous Versions . . . . . . . . . . . . . . . . 35
     16.1. Changes from -05 to -06  . . . . . . . . . . . . . . . . . 35
     16.2. Changes from -04 to -05  . . . . . . . . . . . . . . . . . 35
   17. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     17.1. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . 37
     17.2. Closed Issues  . . . . . . . . . . . . . . . . . . . . . . 39
   18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     19.2. Informative References . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
   Intellectual Property and Copyright Statements . . . . . . . . . . 43






























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1.  Introduction

   NOTE TO THE READER: This document is a work-in-progress.  Please see
   the list of open and closed issues in Section 17.  With only a few
   exceptions, if there is an open issue the text has NOT been updated
   in this area pending resolution of this issue - keep this in mind
   when reading the text.  In addition, in the interest of getting the
   document out quickly in order to make progress on open issues, the
   authors have elected to release the document is a bit more "raw"
   state than they would prefer, resulting in some rough spots in the
   presentation.




   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



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   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.


2.  Overview of Operation

   This section gives an overview of the operation of TURN.  It is non-
   normative.

   In a typical configuration, a TURN client is connected to a private
   network [RFC1918] 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.  These peers may or may not be behind one or more
   NATs.





























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                                                          +---------+
                                                          |         |
                                                          |         |
                                                        / |  Peer A |
    Client's              TURN                        //  |         |
    Host Transport        Server                     /    |         |
    Address               Address              +-+ //     +---------+
 10.1.1.2:17240       192.0.2.15:3478          |N|/  192.168.100.2:16400
       |                    |                  |A|
       |        +-+         |                 /|T|
       |        | |         |                / +-+
       v        | |         |               /       192.0.2.210:18200
 +---------+    | |         |+---------+   /              +---------+
 |         |    |N|         ||         | //               |         |
 | TURN    |    | |         v|  TURN   |/                 |         |
 | Client  |----|A|----------|  Server |------------------|  Peer B |
 |         |    | |^         |         |^                ^|         |
 |         |    |T||         |         ||                ||         |
 +---------+    | ||         +---------+|                |+---------+
                | ||                    |                |
                | ||                    |                |
                +-+|                    |                |
                   |                    |                |
                   |                    |                |
             Client's                   |            Peer B
             Server-Reflexive    Relayed             Transport
             Transport Address   Transport Address   Address
             192.0.2.1:7000      192.0.2.15:9000     192.0.2.210:18200

                                 Figure 1

   Figure 1 shows a typical deployment.  In this figure, the TURN client
   and the TURN server are separated by a NAT, with the client on the
   private side and the server on the public side of the NAT.  This NAT
   is assumed to be a "bad" NAT; for example, it might have a mapping
   property of address-and-port-dependent mapping (see [RFC4787]) for a
   description of what this means).

   The client has allocated a local port on one of its addresses for use
   in communicating with the server.  The combination of an IP address
   and a port is called a TRANSPORT ADDRESS and since this (IP address,
   port) combination is located on the client and not on the NAT, it is
   called the client's HOST transport address.

   The client sends TURN messages from its host transport address to a
   transport address on the TURN server which is known as the TURN
   SERVER ADDRESS.  The client learns the server's address through some
   unspecified means (e.g., configuration), and this address is



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   typically used by many clients simultaneously.  The TURN server
   address is used by the client to send both commands and data to the
   server; the commands are processed by the TURN server, while the data
   is relayed on to the peers.

   Since the client is behind a NAT, the server sees these packets as
   coming from a transport address on the NAT itself.  This address is
   known as the client's SERVER-REFLEXIVE transport address; packets
   sent by the server to the client's server-reflexive transport address
   will be forwarded by the NAT to the client's host transport address.

   The client uses TURN commands to allocate a RELAYED transport
   address, which is an transport address located on the server.  The
   server ensures that there is a one-to-one relationship between the
   client's server-reflexive transport address and the relayed transport
   address; thus a packet received at the relayed transport address can
   be unambiguously relayed by the server to the client.

   The client will typically communicate this relayed transport address
   to one or more peers through some mechanism not specified here (e.g.,
   an ICE offer or answer [I-D.ietf-mmusic-ice]).  Once this is done,
   peers can send data packets to the relayed transport address and the
   server will forward them to the client.  In the reverse direction,
   the client can send data packets to the server (at its TURN server
   address) and these will be forwarded by the server to the appropriate
   peer, and the peer will see them as coming from the relayed transport
   address; in this direction, the client must specify the appropriate
   peer.

2.1.  Transports

   TURN as defined in this specification only allows the use of UDP
   between the server and the peer.  However, this specification allows
   the use of any one of UDP, TCP, or TLS over TCP to carry the TURN
   messages between the client and the server.

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

   For TURN clients, using TLS over TCP to communicate with the TURN
   server provides two benefits.  First, the client can be assured that
   the addresses of its peers are not visible to any attackers between
   it and the server.  Second, the client may be able to communicate



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   with TURN servers using TLS when it would not be able to communicate
   with the same server using TCP or UDP, due to the policy of a
   firewall between the TURN client and its server.  In this second
   case, TLS between the client and TURN server facilitates traversal.

   There is a planned extension to TURN to add support for TCP between
   the server and the peers [I-D.ietf-behave-turn-tcp].  For this
   reason, allocations that use UDP between the server and the peers are
   known as UDP allocations, while allocations that use TCP between the
   server and the peers are known as TCP allocations.  This
   specification describes only UDP allocations.

2.2.  Allocations

   To allocate a relayed transport address, the client uses an Allocate
   transaction.  The client sends a Allocate Request to the server, and
   the server replies with an Allocate Response containing the allocated
   relayed transport address.  The client can include attributes in the
   Allocate Request that describe the type of allocation it desires
   (e.g., the lifetime of the allocation).  And since relaying data can
   require lots of bandwidth, the server may require that the client
   authenticate itself using STUN's long-term credential mechanism, to
   show that it is authorized to use the server.

   Once a relayed transport address is allocated, a client must keep the
   allocation alive.  This is done by the client periodically doing a
   Refresh transaction with the server, where the client includes the
   allocated relayed transport address in the Refresh Request.  TURN
   deliberately uses a different method (Refresh rather than Allocate)
   for refreshes to ensure that the client is informed if the allocation
   vanishes for some reason.

   The frequency of the Refresh transaction is determined by the
   lifetime of the allocation.  The client can request a lifetime in the
   Allocate Request and may modify its request in a Refresh Request, and
   the server always indicates the actual lifetime in the response.  The
   client must issue a new Refresh transaction within 'lifetime' seconds
   of the previous Allocate or Refresh transaction.  If a client no
   longer wishes to use an Allocation, it should do a Refresh
   transaction with a requested lifetime of 0.

   Note that sending or receiving data from a peer DOES NOT refresh the
   allocation.

   The server remembers the 5-tuple used in the Allocate Request.
   Subsequent transactions between the client and the server use this
   same 5-tuple.  In this way, the server knows which client owns the
   allocated relayed transport address.  If the client wishes to



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   allocate a second relayed transport address, it must use a different
   5-tuple for this allocation (e.g., by using a different client host
   address).

      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 use a file-
      descriptor in place of a 5-tuple to represent a TCP connection.

2.3.  Exchanging Data with Peers

   The client can use the relayed transport address to exchange data
   with its peers by using Send and Data indications.  A Send Indication
   is sent from a client to the TURN server and contains, in attributes
   inside the message, the transport address of the peer and the data to
   be sent to that peer.  When the TURN server receives the Send
   Indication, it extracts the data from the Send Indication and sends
   it in a UDP datagram to the peer, using the allocated relay address
   as the source address.  In the reverse direction, UDP datagrams
   arriving at the relay address on the TURN server are converted into
   Data Indications and sent to the client, with the transport address
   of the peer included in an attribute in the Data Indication.

   Note that a client can use a single relayed transport address to
   exchange data with multiple peers at the same time.
   TURN                   TURN          Peer          Peer
   client                 server         A             B
     |--- Allocate Req  -->|             |             |
     |<-- Allocate Resp ---|             |             |
     |                     |             |             |
     |--- Send (Peer A)--->|             |             |
     |                     |=== data ===>|             |
     |                     |             |             |
     |                     |<== data ====|             |
     |<-- Data (Peer A)----|             |             |
     |                     |             |             |
     |--- Send (Peer B)--->|             |             |
     |                     |=== data =================>|
     |                     |             |             |
     |                     |<== data ==================|
     |<-- Data (Peer B)----|             |             |

                                 Figure 2

   In the figure above, the client first allocates a relayed transport
   address.  It then sends data to Peer A using a Send Indication; at
   the server, the data is extracted and forwarded in a UDP datagram to
   Peer A, using the relayed transport address as the source transport



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   address.  When a UDP datagram from Peer A is received at the relayed
   transport address, the contents are placed into a Data Indication and
   forwarded to the client.  A similar exchange happens with Peer B.

2.4.  Permissions

   To ease concerns amongst enterprise IT administrators that TURN could
   be used to bypass corporate firewall security, TURN includes the
   notion of permissions.  TURN permissions mimic the address-restricted
   filtering mechanism of NATs that comply with [RFC4787].

   A TURN server will drop a UDP datagram arriving at a relayed
   transport address from a peer unless the client has recently sent
   data to a peer with the same IP address (the port numbers can
   differ).  See the normative description for the precise definition of
   "recently".

   A permission will timeout if not refreshed periodically.  The client
   refreshes a permission by sending data to the corresponding peer.
   Data received from the peer DOES NOT refresh the permission.

2.5.  Channels

   In some applications, the overhead of using Send and Data indications
   can be substantial.  For example, for applications like VoIP which
   utilize small packets, Send and Data Indications, with 36 bytes of
   overhead, can have a substantial impact on overall bandwidth usage.
   To remedy this, TURN clients can assign a CHANNEL to a peer.  Data to
   and from such a peer can then be sent using an alternate packet
   format that adds only 4 bytes per packet of overhead.

   The alternate packet format is known as the ChannelData message.  The
   ChannelData message does not use the STUN header used by other TURN
   messages, but instead has a 4-byte header that includes a number
   known as a channel number.

   To create a channel, the client sends a ChannelBind request to the
   server, and includes an unallocated channel number and the transport
   address of the peer.  Once the client receives the response to the
   ChannelBind request, it can send data to that peer using a
   ChannelData message.  Similarly, once the server has received the
   request, it can relay data from that peer towards the client using a
   ChannelData message.  There is no way to modify channel bindings, so
   once a channel is bound to a peer, it remains bound for the lifetime
   of the allocation.

   When the server receives a ChannelData message from the client, it
   uses the channel number to determine the destination peer and then



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   forwards the data inside a UDP datagram to the peer.  In the reverse
   direction, when a UDP datagram arives at the relayed transport
   address from that peer, the server inserts it into a ChannelData
   message containing the channel number bound to that peer; in this way
   the client can determine the peer that send the UDP datagram.
   TURN                   TURN          Peer          Peer
   client                 server         A             B
     |--- Allocate Req  -->|             |             |
     |<-- Allocate Resp ---|             |             |
     |                     |             |             |
     |--- Send (Peer A)--->|             |             |
     |                     |=== data ===>|             |
     |                     |             |             |
     |                     |<== data ====|             |
     |<-- Data (Peer A)----|             |             |
     |                     |             |             |
     |- ChannelBind Req -->|             |             |
     | (Peer A to 0x4001)  |             |             |
     |                     |             |             |
     |<- ChannelBind Resp -|             |             |
     |                     |             |             |
     |-- [0x4001] data --->|             |             |
     |                     |=== data ===>|             |
     |                     |             |             |
     |                     |<== data ====|             |
     |<- [0x4001] data --->|             |             |
     |                     |             |             |
     |--- Send (Peer B)--->|             |             |
     |                     |=== data =================>|
     |                     |             |             |
     |                     |<== data ==================|
     |<-- Data (Peer B)----|             |             |

                                 Figure 3

   The figure above shows the channel mechanism in use.  The client
   begins by allocating a relayed transport address, and then uses that
   address to exchange data with Peer A. After a bit, the client decides
   to bind a channel to Peer A. To do this, it sends a ChannelBind
   Request to the server, specifying the transport address of Peer A and
   a channel number (0x4001).  After that, the client can send
   application data encapsulated inside ChannelData messages to Peer A:
   this is shown as "[0x4001] data" where 0x4001 is the channel number.

   Note that ChannelData messages can only be used for peers to which
   the client has bound a channel.  In the example above, Peer A has
   been bound to a channel, but Peer B has not, so application data to
   and from Peer B uses Send and Data indications.



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   Channel bindings are always initiated by the client.


3.  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].

   Readers are expected to be familar with [I-D.ietf-behave-rfc3489bis]
   and the terms defined there.

   The following terms are used in this document:

   TURN:  A protocol spoken between a TURN client and a TURN server.  It
      is an extension to the STUN protocol [I-D.ietf-behave-rfc3489bis].
      The protocol allows a client to allocate and use a relayed
      transport address.

   TURN client:  A STUN client that implements this specification.

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

   Peer:  A host with which the TURN client wishes to communicate.  The
      TURN server relays traffic between the TURN client and its
      peer(s).  The peer does not interact with the TURN server using
      the protocol defined in this document; rather, the peer receives
      data sent by the TURN server and the peer sends data towards the
      TURN server.

   Host Transport Address:  A transport address allocated on a host.

   Server-Reflexive Transport Address:  A transport address on the
      "public side" of a NAT.  This address is allocated by the NAT to
      correspond to a specific host transport address.

   Relayed Transport Address:  A transport address that exists on a TURN
      server.  If a permission exists, packets that arrive at this
      address are relayed towards the TURN client.

   Allocation:  The transport address granted to a client through an
      Allocate request, along with related state, such as permissions
      and expiration timers.  See also Relayed Transport Address.







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

   Permission:  The IP address and transport protocol (but not the port)
      of a peer that is permitted to send traffic to the TURN server and
      have that traffic relayed to the TURN client.  The TURN server
      will only forward traffic to its client from peers that match an
      existing permission.


4.  General Behavior

   After the initial Allocate transaction, all subsequent TURN
   transactions need to be sent in the context of a valid allocation.
   The source and destination IP address and ports for these TURN
   messages MUST match the those used in the initial Allocate Request.
   These are processed using the general server procedures in
   [I-D.ietf-behave-rfc3489bis] with a few important additions.  For
   requests, 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,
   password 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



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   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 STUN 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
   in any of the methods defined in this document.


5.  Managing Allocations

   Communications between a TURN client and a TURN server begin with an
   Allocate transaction.  All subsequent transactions happen in the
   context of that allocation, and happen on the same 5-tuple.  The
   client refreshes allocations and deallocates them using a Refresh
   transaction.

5.1.  Client Behavior

5.1.1.  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
   demand credentials for requests.

   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.

   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 MUST always be UDP.  This
   attribute is included to allow for future extensions to TURN (e.g.,
   [I-D.ietf-behave-turn-tcp])



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   The client MAY include a REQUESTED-PORT-PROPS or REQUESTED-IP
   attribute in the request to obtain specific types of transport
   addresses, if desired.

   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,
   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.

5.1.2.  Refresh Requests

   TURN permissions are kept alive by traffic flowing through them, and
   persist for the lifetime of the allocation.  However, The allocations
   themselves have to be kept alive through 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 (this might also be the first
   time the TURN client indicates bandwidth to the TURN server).  If the
   BANDWIDTH attribute is 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.




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

5.2.  Server Behavior

5.2.1.  Receiving an Allocate Request

   When the server receives an Allocate request, the server attempts to
   allocate a relayed transport address.

   When the server receives the Allocate Request, it begins by
   processing it according to the base protocol procedures described in
   [I-D.ietf-behave-rfc3489bis], plus the Long-Term Credential Mechanism
   procedures if the server is using this mechanism.

   It then checks if the 5-tuple used for the Allocate Request matches
   the 5-tuple used for an existing allocation.  If there is a match,
   then:

   o  If the transport protocol is UDP, and the transaction id in the
      request message matches the transaction id used for the original
      allocation, then the server treats this as a retransmission of the
      original request, and replies with the same response as it did to
      the original request.  The server may do this by either storing
      its original response and resending it, or by rebuilding its
      original response from other state data.

   o  If the transport protocol is not UDP, or if the transaction id in
      the request message does not match the transaction id used for the
      original allocation, then the server replies with an error
      response containing the error code 437 Allocation Mismatch.

   If the 5-tuple does not match an existing allocation, then processing
   continues as described below.



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5.2.1.1.  BANDWIDTH

   The server checks 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.  If it doesn't, it sends an error response.

5.2.1.2.  REQUESTED-TRANSPORT

   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.

5.2.1.3.  REQUESTED-IP

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



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   discretion of the server.

5.2.1.4.  REQUESTED-PORT-PROPS

   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 number MUST be even.

      Port alignment requests exist for compatibility with
      implementations of RTP which predate [RFC3550].  These
      implementations use the port numbering conventions in (now
      obsolete) [RFC1889].








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5.2.1.5.  Lifetime

   The server checks for a LIFETIME attribute.  If present, it indicates
   the lifetime the client would like the server to assign to the
   allocation.

   If the LIFETIME attribute is malformed, or if the requested lifetime
   value is less than 32 seconds, the server replies with an error
   response with an error code of XXX Lifetime Malformed or Invalid.

5.2.1.6.  Creating the Allocation

   If any of the requested or desired constraints cannot be met, whether
   it be bandwidth, transport protocol, IP address or port, the server
   can 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 from the range 49152 - 65535
   (the Dynamic and/or Private Port range [Port-Numbers]), unless the
   TURN server application knows, through some means not specified here,
   that other applications running on the same host as the TURN server
   application will not be impacted by allocating ports outside this
   range.  This condition can often be satisfied by running the TURN
   server application on a dedicated machine and/or by arranging that
   any other applications on the machine allocate ports before the TURN
   server application starts.  In any case, the TURN server SHOULD NOT
   allocate ports in the range 0 - 1023 (the Well-Known Port range) to
   discourage clients from using TURN 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 and a set of channel bindings.  Each set is
   initially empty.



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   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 expire at or after that time.  Note that
   the LIFETIME attribute in an Allocate request can be zero, though
   this is effectively a no-op, since it will create and destroy the
   allocation in one transaction.

5.2.1.7.  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 12).

5.2.2.  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.




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   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 MAY
   utilize a shorter allocation lifetime, but MUST NOT utilize a longer
   lifetime.

   A success Refresh response MUST contain a LIFETIME attribute.  If its
   associated Allocate request contained the BANDWIDTH attribute, or
   this Refresh request contained a new BANDWIDTH attribute, the
   response MUST also contain the BANDWIDTH attribute.


6.  Send and Data Indications

   TURN supports two ways to send and receive data from peers.  This
   section describes the use of Send and Data indications, while
   Section 7 describes the use of the Channel Mechanism.

6.1.  Forming and Sending an Indication

   When the client has data to send to a peer, it uses a Send Indication
   to pass the data to the server.  When the server has data to send to
   the client, it uses a Data Indication to pass the data to the client.
   A client can also use a Send Indication without a DATA attribute to
   install or refresh a permission for the specified IP address.  Both
   indications are formed following the general rules described in [ref
   3489bis] with the extra considerations described below.

   A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
   contain a DATA attribute, while a Data Indication MUST contain both
   attributes.  The PEER-ADDRESS attribute contains the transport
   address of the peer to which the data is to be sent (in the case of a
   Send Indication) or from which the data was received (in the case of
   a Data Indication).  This peer address is the transport address of
   the peer as seen by the server, which may not be the same as the host
   transport address of the peer.  The DATA attribute contains the
   actual application data.  Note that the application data may need to
   be padded to ensure the DATA attribute length is a multiple of 4.

   No other attributes are included.  For example, neither the
   FINGERPRINT attribute nor any authentication attributes are included.
   The latter holds even if the server is using the Long-Term Credential
   Mechanism, since indications cannot be authenticated using this
   mechanism.




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   Both the Send and Data indications MUST be sent using the 5-tuple of
   the original allocation.  Thus, in the case of the Send Indication,
   the source transport address is the client's host transport address,
   the destination transport address is the TURN server address, and the
   transport protocol is the same as was used for the Allocate request.
   For the Data Indication, the source and destination transport
   addresses are the reverse.

6.2.  Receiving an Indication

   When a Send Indication is received at the server, or a Data
   Indication is received at the client, the receiver first does the
   basic indication processing described in [3489bis].  Once this is
   done, it does the processing specific to the Send and Data methods
   described below.

   A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
   contain a DATA attribute, while a Data Indication MUST contain both
   attributes.  Any other attributes appearing in the message are
   treated as unexpected.

      TODO: Add check that Send or Data indication arrives with
      appropriate 5-tuple.  Since this check applies to all STUN
      messages, not just Send and Data indications, perhaps this goes
      under the general processing section.

6.3.  Relaying

   When the server receives a valid Send Indication contains a DATA
   attribute, it forms a UDP datagram as follows:

   o  the source transport address is the relayed transport address of
      the allocation, where the allocation is determined by the 5-tuple
      on which the Send Indication arrived;

   o  the destination transport address is taken from the PEER-ADDRESS
      attribute;

   o  the data following the UDP header is the contents of the value
      field of the DATA attribute;

   o  the Length field in the UDP header is set to the Length field of
      the DATA attribute;

   o  the Checksum field in the UDP header is computed as described in
      [RFC 768].

   The resulting UDP datagram is then sent to the peer.



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   When the server receives a valid Send Indication (with or without a
   DATA attribute), it also updates the permission associated with the
   IP address contained in the PEER-ADDRESS attribute.  For a certain
   interval after the permission is updated, UDP datagrams received from
   peers with source IP address equal to the IP address contained in the
   PEER-ADDRESS attribute can be forwarded to the client.  Note that
   only the IP addresses are considered and the port numbers are
   irrelevent.  This permission is specific to the allocation and has no
   affect on any other allocation.  The recommended length of time is 60
   seconds from when the Send Indication is received.

   When the server receives a UDP datagram with a destination transport
   address corresponding to an active (i.e., still alive) allocation,
   then it first checks to see if it is permitted to relay the datagram.
   If it is not permitted, the UDP datagram MUST be discarded.

   If relaying is permitted, the server forms and send a Data Indication
   as described in Section 6.1, using the data following the UDP header
   as the application data.


7.  Channel Mechanism

   As described in the overview, channel mechanism provides a way for a
   client and server to send application data using ChannelData
   messages, which have less overhead than Send and Data indications.

   Channel bindings are always initiated by the client.  The client can
   bind a channel to a peer at any time during the lifetime of the
   allocation.  The client may bind a channel to a peer before
   exchanging data with it, or after exchanging data with it (using Send
   and Data indications) for some time, or may choose never to bind a
   channel it.  The client can also bind channels to some peers while
   not binding channels to other peers.

   Once a channel is bound to a peer, the channel binding cannot be
   changed.  There is no way to unbind a channel or bind it to a
   different peer.

   Channel bindings are specific to an allocation, so that a binding in
   one allocation has no relationship to a binding in any other
   allocation.  If an allocation expires, all its channel bindings
   expire with it.

7.1.  Forming and Sending a ChannelBind Request

   When a client wishes to bind a channel to a peer in an allocation, it
   forms a ChannelBind Request.  The Request formed following the



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   general rules described in [I-D.ietf-behave-rfc3489bis] with the
   extra considerations described below.

   A ChannelBind Request MUST contain both a CHANNEL-NUMBER attribute
   and a PEER-ADDRESS attribute.  The CHANNEL-NUMBER attribute specifies
   the number of the channel that the client wishes to bind to the peer.
   The channel number MUST be in the range 0x4000 to 0xFFFE (inclusive)
   and the channel MUST NOT be already bound to a different peer.  It is
   acceptable to rebind a channel to the peer it is already bound to.
   The PEER-ADDRESS attribute specifies the peer address to bind the
   channel to.

   Once formed, the ChannelBind Request is sent using the 5-tuple for
   the allocation.

   The client SHOULD be prepared to receive ChannelData messages on the
   channel as soon as it has sent the ChannelBind Request.  Over UDP, it
   is possible for the client to receive these before it receives a
   ChannelBind Success Response.

   Over UDP, the client SHOULD NOT send ChannelData messages on the
   channel until it has received a ChannelBind Success Response for the
   binding attempt.  Sending them before the success response is
   received risks having them dropped by the server if he ChannelBind
   Request was lost.

7.2.  Receiving a ChannelBind Request and Sending a Response

   When the server receives a ChannelBind Request, it first does the
   basic request processing described in [I-D.ietf-behave-rfc3489bis].
   Once this is done, it does the processing specific to the ChannelBind
   method described below.

   The server checks that the ChannelBind Request contains both a
   CHANNEL-NUMBER attribute and a PEER-ADDRESS attribute.  If the PEER-
   ADDRESS attribute is missing or malformed, then the server rejects
   the request with an Error Response containing the error code XXX
   "Peer address missing or invalid".  If the CHANNEL-NUMBER attribute
   is missing or malformed, or the channel number is not in the range
   0x4000 to 0xFFFE (inclusive), or the channel is already bound to
   another peer (already bound to the same peer is OK) the server
   rejects the request with an Error Response containing the error code
   XXX "Channel number missing or invalid".  Otherwise, if no errors are
   detected, the server replies with a ChannelBind Success Response.







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7.3.  Receiving a ChannelBind Response

   When the client receives a ChannelBind response (either success or
   error), it processes it as specified in [3489bis].  Any additional
   processing is implementation specific.

7.4.  The ChannelData Message

   The ChannelData message is used to carry application data between the
   client and the server.  It has the following format:

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   /                       Application Data                        /
   /                                                               /
   |                                                               |
   |                               +-------------------------------+
   |                               |
   +-------------------------------+

   The Channel Number field specifies the number of the channel on which
   the data is traveling, and thus the address of the peer that is
   sending or is to receive the data.  The channel number MUST be in the
   range 0x4000 - 0xFFFF, with channel number 0xFFFF being reserved for
   possible future extensions.

   Channel numbers 0x0000 - 0x3FFF cannot be used because bits 0 and 1
   are used to distinguish ChannelData messages from STUN-formatted
   messages (i.e., Allocate, Send, Data, ChannelBind, etc).  STUN-
   formatted messages always have bits 0 and 1 as "00", while
   ChannelData messages use combinations "01", "10", and "11".

   The Length field specifies the length in bytes of the application
   data field (i.e., it does not include the size of the ChannelData
   header).  Note that 0 is a valid length.

   The Application Data field carries the data the client is trying to
   send to the peer, or that the peer is sending to the client.

7.5.  Forming and Sending a ChannelData Message

   Once a client has bound a channel to a peer, then when the client has
   data to send to that peer it may use either a ChannelData message or
   a Send Indication; that is, the client is not obligated to use the



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   channel when it exists and may freely intermix the two message types
   when sending data to the peer.  The server, on the other hand, SHOULD
   use the ChannelData message if a channel has been bound to the peer.

   The fields of the ChannelData message are filled in as described in
   Section 7.4.

   Over stream transports, the ChannelData message MUST be padded to a
   multiple of four bytes in order to ensure the alignment of subsequent
   messages.  The padding is not reflected in the length field of the
   ChannelData message, so the actual size of a ChannelData message
   (including padding) is (4 + Length) rounded up to the nearest
   multiple of 4.  Over UDP, the padding is not required but MAY be
   included.

   The ChannelData message is then sent on the 5-tuple associated with
   the allocation.

7.6.  Receiving a ChannelData Message

   The receiver of the ChannelData message uses bits 0 and 1 to
   distinguish it from STUN-formatted messages, as described in
   Section 7.4.

   If the ChannelData message is received in a UDP datagram, and if the
   UDP datagram is too short to contain the claimed length of the
   ChannelData message (i.e., the UDP header length field value is less
   than the ChannelData header length field value + 4 + 8), then the
   message is silently discarded.

   If the ChannelData message is received over TCP or over TLS over TCP,
   then the actual length of the ChannelData message is as described in
   Section 7.5.

   If the ChannelData message is received on a channel which is not
   bound to any peer, then the message is silently discarded.

7.7.  Relaying

   When the server receives a valid ChannelData message, it forms a UDP
   datagram as follows: the source transport address is the relayed
   transport address of the allocation, where the allocation is
   determined by the 5-tuple on which the ChannelData message arrived;
   the destination transport address is the peer address to which the
   channel is bound; the data following the UDP header is the contents
   of the data field of the ChannelData message; the Length field in the
   UDP header is set to the Length field of the ChannelData message + 8;
   and the Checksum field in the UDP header is computed as described in



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   [RFC 768].  The resulting UDP datagram is then sent to the peer.

   The server also updates the permission associated with the IP address
   part of the peer address to which the UDP datagram is sent.

   When the server receives a UDP datagram with a destination transport
   address corresponding to an active (i.e., still alive) allocation,
   then it first checks to see if it is permitted to relay the datagram.
   If the allocation contains an active permission for the source IP
   address (from the IP header) of the received UDP datagram, then the
   UDP datagram is permitted.  Otherwise, the UDP datagram MUST be
   discarded.

   To relay the UDP datagram, the server forms and send a ChannelData
   message as described in Section 7.5


8.  New STUN Methods

   This section lists the codepoints for the new STUN methods defined in
   this specification.  See elsewhere in this document for the semantics
   of these new methods.

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

     Indications
       0x006  :  Send
       0x007  :  Data


9.  New STUN 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






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9.1.  CHANNEL-NUMBER

   The CHANNEL-NUMBER attribute contains the number of the channel.  It
   is a 16-bit unsigned integer, followed by a two-octet RFFU field
   which MUST be set to 0 on transmission and ignored on reception.

      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         |         RFFU                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

9.2.  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.

9.3.  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.

9.4.  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.

9.5.  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).

9.6.  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.

9.7.  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



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   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
   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 and the server MUST provide that 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.  The server MUST provide an
   odd port number.

   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.  The server MUST provide
   an even port number.

   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.
   The server MUST return an even port number.  In addition, the client
   is requesting the server reserve the next higher port (i.e., N+1 if
   the server allocates port N).  The server SHOULD only allocate the
   N+1 port number if it is explicitly requested (with a subsequent
   request specifying that exact port number by the same TURN client,
   over a different alllocation).

   In all cases, if a port with the requested properties cannot be
   allocated, the server MUST respond with a error response with an



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   error code of 444 (Invalid Port).

9.8.  REQUESTED-TRANSPORT

   This attribute is used by the client to request a specific transport
   protocol for the allocated transport address.  It has the following
   format:
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Protocol   |                    RFFU                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Protocol field specifies the desired protocol.  The codepoints
   used in this field are taken from those allowed in the Protocol field
   in the IPv4 header and the NextHeader field in the IPv6 header
   [Protocol-Numbers].  This specification only allows the use of
   codepoint 17 (User Datagram Protocol).

   The RFFU field is set to zero on transmission and ignored on
   receiption.  It is reserved for future uses.

9.9.  REQUESTED-IP

   The REQUESTED-IP attribute is used by the client to request that a
   specific IP address be allocated by the TURN server.  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 use this attribute to request a
   remote address from the same TURN server interface as the TURN
   client's previous remote address.

   The format of this attribute is identical to XOR-MAPPED-ADDRESS.
   However, the port component of the attribute MUST be 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.


10.  New STUN Error Response Codes

   This document defines the following new error response codes:








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   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.

   (tbd)  (Channel Number Missing or Invalid): The request requires a
      channel number, but the CHANNEL-NUMBER attribute is missing, or
      the specified channel number is invalid in some way.

   (tbd)  (Peer Address Missing or Invalid): The request requires a peer
      transport address, but the PEER-ADDRESS attribute is missing, or
      the specified peer transport address is invalid in some way.

   (tbd)  (Lifetime Malformed or Invalid): The LIFETIME attribute is
      malformed or the specified lifetime is invalid in some way.

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


11.  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)



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   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.


12.  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-
   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



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   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
   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.




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   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.


13.  IANA Considerations

   Since TURN is an extension to STUN [I-D.ietf-behave-rfc3489bis], the
   methods, attributes and error codes defined in this specification are
   new method, attributes, and error codes for STUN.  This section
   directs IANA to add these new protocol elements to the IANA registry
   of STUN protocol elements.

   The codepoints for the new STUN methods defined in this specification
   are listed in Section 8.

   The codepoints for the new STUN attributes defined in this
   specification are listed in Section 9.

   The codepoints for the new STUN error codes defined in this
   specification are listed in Section 10.

   Extensions to TURN can be made through IETF consensus.


14.  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 [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].


15.  Example

   TBD






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16.  Changes from Previous Versions

   Note to RFC Editor: Please remove this section prior to publication
   of this document as an RFC.

   This section lists the changes between the various versions of this
   specification.

16.1.  Changes from -05 to -06

   o  Changed the mechanism for allocating channels to the one proposed
      by Eric Rescorla at the Dec 2007 IETF meeting.

   o  Removed the framing mechanism (which was used to frame all
      messages) and replaced it with the ChannelData message.  As part
      of this change, noted that the demux of ChannelData messages from
      TURN messages can be done using the first two bits of the message.

   o  Rewrote the sections on transmitted and receiving data as a result
      of the above to changes, splitting it into a section on Send and
      Data Indications and a separate section on channels.

   o  Clarified the handling of Allocate Request messages.  In
      particular, subsequent Allocate Request messages over UDP with the
      same transaction id are not an error but a retransmission.

   o  Restricted the range of ports available for allocation to the
      Dynamic and/or Private Port range, and noted when ports outside
      this range can be used.

   o  Changed the format of the REQUESTED-TRANSPORT attribute.  The
      previous version used 00 for UDP and 01 for TCP; the new version
      uses protocol numbers from the IANA protocol number registry.  The
      format of the attribute also changed.

   o  Made a large number of changes to the non-normative portion of the
      document to reflect technical changes and improve the
      presentation.

   o  Added the Issues section.

16.2.  Changes from -04 to -05

   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.



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   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".

   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.






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17.  Issues

   NOTE to RFC Editor: Please remove this section prior to publication
   of this document as an RFC.

   This section lists the open and now closed issues in this document.
   The descriptions here are brief, and the reader should consult the
   corresponding thread on the mailing list for a more in-depth
   description of the issue and the resolutions being considered.

17.1.  Open Issues

   1.   Bandwidth: What should we do with the BANDWIDTH attribute, which
        is currently ill-specified?  Should we remove it?  Or should we
        try to come up with a good specification, perhaps using ideas
        from RSVP?

   2.   Permission Policy: What should the permission policy be?
        Address-restricted, as is currently specified in the document?
        Or address-and-port-restricted, as many firewalls implement
        today?  Or should we leave this open to the implementor, under
        the assumption that the IT administrator will only allow clients
        to contact those servers that implement whatever permission
        policy the IT administrator can accept?

   3.   Port Adjacency: The spec currently allows a client to request
        that the server allocate a port and also reserve the next higher
        port number for a possible future allocation (on a different
        5-tuple).  However, the exact behavior of the server in this
        case is ill-specified.  For example, must the next-higher-port
        be available for the allocation of the lower port number to
        succeed?  How long is the next-higher-port reserved? 30 seconds?
        For the lifetime of the lower-numbered-port's allocation?  Or
        should we just ditch this feature, since it is difficult to
        implement, it is at odds with port randomization, and paired
        port numbers applications don't work well with NATs anyway?

   4.   Demuxing ChannelData messages: How does a client or server demux
        STUN-formatted messages from ChannelData messages?  Does it use
        the first two bits (as currently specified) or just one bit?
        And how many channels do we need anyway?  Some people are
        questioning the need for any more than 200 channels.  If we
        don't need many channels, then the demux algorithm might become
        simpler.

   5.   Deallocating Channels: Do we need a mechanism for deallocating
        channels?  Some have argued for this feature, because a TURN
        server administrator will want a way to recover resources for



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        channels no longer in active use.  If yes, then what is the
        mechanism?  For example, should a channel binding expire when
        the corresponding permission expires?

   6.   Permissions and Channel Allocations: Should allocating a channel
        for a peer automatically install a permission for that peer's IP
        address?

   7.   Permission and Allocation Lifetimes: What should the default
        permission lifetime be?  Should there be a minumum value?
        Should there be a way for the client to modify the permission
        lifetime?  Should there be a way for the client to learn the
        current permission lifetime?  And what is the relationship of
        the permission lifetime to the allocation lifetime?  Does it
        make sense for the allocation lifetime to be less than the
        permission lifetime?

   8.   Preserving bits in the IP header: What bits (if any) should be
        preserved in the IP header when a packet is relayed by the
        server?  The bits under consideration are currently the Don't
        Fragment (DF) bit, the Explicit Congestion Notification (ECN)
        bits, and the DiffServ (DS) bits.

   9.   Exceeding the Path MTU Size: TURN adds an overhead of 4 bytes
        (ChannelData msg) or 36 bytes (Send or Data Indication), thus
        potentially exceeding the path MTU between the client and
        server.  This could either cause IP fragmentation, or cause the
        packet to be dropped if the DF bit is set.  Who handles this
        problem?  Does TURN need to handle this, or is this left up to
        the application to handle?

   10.  Allowed PEER-ADDRESS values: Should there be any restrictions on
        the IP address the client can specify in the PEER-ADDRESS
        attribute?  Are multicast addresses allowed?  What about
        0.0.0.0?  Any other restrictions?

   11.  Discarding UDP datagrams: If the server discards a received UDP
        datagram on the relayed transport address (because there is no
        corresponding permission), then does the server send an ICMP
        response?  If so, what error code does it use?  (What does RFC
        4787 say about the corresponding situation in NATs?  I believe
        many NATs silently discard these packets by default, or have a
        "stealth mode" that enables this behavior.)

   12.  Authentication: Is the use of STUN's Long-Term Authentication
        Mechanism by a TURN server mandatory?  The document currently
        implicitly assumes "yes", but what about someone who wants to
        operate a public TURN server?



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   13.  Re-using the 5-tuple: If an allocation expires, is there any
        reason a client should not be able to immediately create a new
        allocation using the same 5-tuple?

   14.  Password change: Is it possible to change the password for the
        Long-Term Authentication mechanism during the lifetime of an
        allocation?  If so, how is it done?

   15.  IPv6: TURN probably works fine in an all IPv6 environment, but
        there are a number of mixed IPv4/IPv6 cases that are ill-
        specified.  As an example, the server needs to check that the
        PEER-ADDRESS in a Send Indication is of the same address family
        as the relayed transport address.  Should we carefully work
        through all these cases and make sure we have caught them all,
        or should we just state that this document covers the IPv4 case
        only, and punt the specification of IPv6 and mixed IPv4/IPv6
        operation to draft-ietf-behave-turn-ipv6?  Does the current
        interest in resurecting IPv4-to-IPv6 NATs have any impact on
        TURN?

17.2.  Closed Issues

   1.  Channel Allocation: Should TURN use the mechanism proposed by EKR
       to allocate channels?  RESOLUTION: Yes. Document now reflects
       this.

   2.  Stateful Allocations: Does a TURN server need to distinguish
       between the case where the client retransmits the initial
       Allocate Request because the Allocate Response was lost and the
       case where the client sends an Allocate Request because it thinks
       the allocation does not exist?  RESOLUTION: Yes. Document now
       reflects this.

   3.  Port Range: From what range of port numbers should a TURN server
       allocate ports?  RESOLUTION: The server SHOULD allocate from the
       Dynamic and/or Private Port range unless it is sure it will not
       interfere with other apps on the same machine.  Document now
       reflects this.

   4.  Framing Header for STUN-formatted messages: Should TURN use the
       framing mechanism for STUN-formatted messages?  RESOLUTION: NO.
       Document now reflects this.  However, see related issues.

   5.  Length field in ChannelData header: Over UDP, the length of the
       application data field in the ChannelData message can be
       determined from the length field in the UDP header.  So should
       the length field in the ChannelData header be set to zero in this
       case?  RESOLUTION: No, the ChannelData length field should have



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       the same semantics over both TCP and UDP.  Document now reflects
       this.


18.  Acknowledgements

   The authors would like to thank the various participants in the
   BEHAVE working group for their many comments on this draft.  Marc
   Petit-Huguenin, Remi Denis-Courmont, Cullen Jennings, Lars Eggert,
   Magnus Westerlund, and Eric Rescorla have been particularly helpful,
   with Eric also suggesting the channel allocation mechanism.
   Christian Huitema was an early contributor to this document and was a
   co-author on the first few drafts.  Finally, the authors would like
   to thank Dan Wing for his huge help in restarting progress on this
   draft after work had stalled.


19.  References

19.1.  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-13 (work in progress),
              November 2007.

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

19.2.  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.

   [RFC1889]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 1889, January 1996.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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




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   [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
              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.

   [Port-Numbers]
              "IANA Port Numbers Registry",
              <http://www.iana.org/assignments/port-numbers>.

   [Protocol-Numbers]
              "IANA Protocol Numbers Registry", 2005,
              <http://www.iana.org/assignments/protocol-numbers>.


Authors' Addresses

   Jonathan Rosenberg
   Cisco Systems, Inc.
   Edison, NJ
   USA

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







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   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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