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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 WG                                                   J. Rosenberg
Internet-Draft                                                     Cisco
Intended status: Standards Track                                 R. Mahy
Expires: December 26, 2008                                   Plantronics
                                                             P. Matthews
                                                          (Unaffiliated)
                                                           June 24, 2008


 Traversal Using Relays around NAT (TURN): Relay Extensions to Session
                   Traversal Utilities for NAT (STUN)
                       draft-ietf-behave-turn-08

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   This Internet-Draft will expire on December 26, 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
   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.



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   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  . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Transports . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.2.  Allocations  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Exchanging Data with Peers . . . . . . . . . . . . . . . .  8
     2.4.  Channels . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.5.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . 11
     2.6.  Preserving vs. Non-Preserving Allocations  . . . . . . . . 11
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  General Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Allocations  . . . . . . . . . . . . . . . . . . . . . . . . . 14
   6.  Creating an Allocation . . . . . . . . . . . . . . . . . . . . 16
     6.1.  Sending an Allocate Request  . . . . . . . . . . . . . . . 16
     6.2.  Receiving an Allocate Request  . . . . . . . . . . . . . . 17
     6.3.  Receiving an Allocate Response . . . . . . . . . . . . . . 21
   7.  Refreshing an Allocation . . . . . . . . . . . . . . . . . . . 23
     7.1.  Sending a Refresh Request  . . . . . . . . . . . . . . . . 23
     7.2.  Receiving a Refresh Request  . . . . . . . . . . . . . . . 23
     7.3.  Receiving a Refresh Response . . . . . . . . . . . . . . . 24
   8.  Permissions  . . . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  Send and Data Indications  . . . . . . . . . . . . . . . . . . 25
     9.1.  Sending a Send Indication  . . . . . . . . . . . . . . . . 25
     9.2.  Receiving a Send Indication  . . . . . . . . . . . . . . . 26
     9.3.  Receiving a UDP Datagram . . . . . . . . . . . . . . . . . 26
     9.4.  Receiving a Data Indication  . . . . . . . . . . . . . . . 27
   10. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     10.1. Sending a ChannelBind Request  . . . . . . . . . . . . . . 28
     10.2. Receiving a ChannelBind Request  . . . . . . . . . . . . . 28
     10.3. Receiving a ChannelBind Response . . . . . . . . . . . . . 29
     10.4. The ChannelData Message  . . . . . . . . . . . . . . . . . 29
     10.5. Sending a ChannelData Message  . . . . . . . . . . . . . . 30
     10.6. Receiving a ChannelData Message  . . . . . . . . . . . . . 30
     10.7. Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 31
   11. IP and ICMP  . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.1. IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
     11.2. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
   12. New STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 36
   13. New STUN Attributes  . . . . . . . . . . . . . . . . . . . . . 37
     13.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 37
     13.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 37
     13.3. PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 37



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     13.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
     13.5. RELAY-ADDRESS  . . . . . . . . . . . . . . . . . . . . . . 38
     13.6. REQUESTED-PROPS  . . . . . . . . . . . . . . . . . . . . . 38
     13.7. REQUESTED-TRANSPORT  . . . . . . . . . . . . . . . . . . . 38
     13.8. RESERVATION-TOKEN  . . . . . . . . . . . . . . . . . . . . 39
   14. New STUN Error Response Codes  . . . . . . . . . . . . . . . . 39
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 40
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 41
   17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42
   18. Example  . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
   19. Changes from Previous Versions . . . . . . . . . . . . . . . . 42
     19.1. Changed from -07 to -08  . . . . . . . . . . . . . . . . . 42
     19.2. Changes from -06 to -07  . . . . . . . . . . . . . . . . . 43
     19.3. Changes from -05 to -06  . . . . . . . . . . . . . . . . . 45
     19.4. Changes from -04 to -05  . . . . . . . . . . . . . . . . . 46
   20. Open Issues  . . . . . . . . . . . . . . . . . . . . . . . . . 47
   21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
   22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     22.1. Normative References . . . . . . . . . . . . . . . . . . . 48
     22.2. Informative References . . . . . . . . . . . . . . . . . . 48
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
   Intellectual Property and Copyright Statements . . . . . . . . . . 51





























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1.  Introduction

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

   The only reliable 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, called
   relayed transport address, are IP addresses and ports on 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 a relayed transport address on a TURN
   server.

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

   TURN was originally invented to support multimedia sessions signaled
   using SIP.  Since SIP supports forking, TURN supports multiple peers
   per client; a feature not supported by other approaches (e.g., SOCKS
   [RFC1928]).  However, care has been taken in the later stages of its
   development to make sure that TURN is suitable for other types of
   applications.


2.  Overview of Operation

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



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   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.

                                                          +---------+
                                                          |         |
                                                          |         |
                                                        / |  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



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   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
   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 TURN 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,
   the client can send data to the server to relay towards its peers.
   In the reverse direction, peers can send data to the the relayed
   transport address of the client.  The server will relay this data to
   the client as long as the client explicitly created a permission (see
   Section 2.5) for the IP address of the 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.









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

   If TCP or TLS over TCP is used between the client and the server,
   then the server will convert between stream transport and UDP
   transport when relaying data.  TURN allows both TCP and TLS over TCP
   as transports in part because many firewalls are configured to not
   pass any UDP traffic.

   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
   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 may
   require lots of bandwidth, the server typically requires 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.  To do this, the client periodically sends a
   Refresh Request to the server with the allocated related transport
   address.  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.



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   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 keeps track of the client reflexive transport address and
   port, the server transport address and port, and the protocol used by
   the client to communicate with the server.  (Together known as a
   5-tuple.  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 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

   There are two ways for the client and peers to exchange data using
   the TURN server.  The first way uses Send and Data indications, the
   second way uses channels.  Common to both ways is the ability of the
   client to communicate with multiple peers using a single allocated
   relayed transport address; thus both ways include a means for the
   client to indicate to the server which peer to forward the data to,
   and for the server to indicate which peer sent the data.

   When using the first way, the client sends a Send indication to the
   TURN server containing, in attributes inside the indication, 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.



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   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
   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.  Channels

   For some applications (e.g.  Voice over IP), the 36 bytes of overhead
   that a Send or Data indication adds to the application data can
   substantially increase the bandwidth required between the client and
   the server.  To remedy this, TURN offers a second way for the client
   and server to associate data with a specific peer.

   This second way uses an alternate packet format 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.  Each channel
   number in use is bound to a specific peer and thus serves as a
   shorthand for the peer's address.

   To bind a channel to a peer, the client sends a ChannelBind request
   to the server, and includes an unbound channel number and the
   transport address of the peer.  Once the channel is bound, the client
   can use a ChannelData message to send the server data destined for
   the peer.  Similarly, the server can relay data from that peer
   towards the client using a ChannelData message.



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   Channel bindings last for 10 minutes unless refreshed.  Channel
   bindings are refreshed by sending ChannelData messages from the
   client to the server, or by rebinding the channel to the peer.

   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.

2.5.  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].

   The client can install a permission by sending data to a peer (or by
   doing certain other things).  Once a permission is installed, any
   peer with the same IP address (the ports numbers can differ) is
   permitted to send data to the client.  After 5 minutes, the
   permission times out and the server drops any UDP datagrams arriving
   at the relayed transport from that IP address.  Note that permissions
   are within the context of an allocation, so adding or expiring a
   permission in one allocation does not affect other allocations.

   Data received from the peer DOES NOT refresh the permission.

2.6.  Preserving vs. Non-Preserving Allocations

   Some applications that use TURN are quite tolerant of the different
   possible ways a TURN server could set the Diff-Serv, ECN, TTL / Hop
   Limit, and Flow Label fields in the IP header of the outgoing packet.
   Other applications require that the TURN server set these fields in a
   specific way, and also require that the TURN server relay ICMP error
   packets.  Applications in the second class typically wish to do Path
   MTU Discovery or end-to-end QOS.

   Unfortunately, reading and manipulating fields in the IP header and
   relaying ICMP messages usually requires the server to have special
   permissions (e.g., access to RAW sockets or be loaded into the
   kernel), something that the person setting up the server may be
   unwilling or unable to grant.  This is especially true when the
   server is part of a larger application, for example a peer-to-peer
   application.  It is also significantly more difficult to implement
   this type of server than just relaying at the UDP layer.

   To allow TURN to cater to both usage scenarios, TURN defines the
   concept of Preserving vs. Non-Preserving allocations.  A Preserving
   allocation sets the fields in outgoing IP header correctly, and also
   relays ICMP messages, while a Non-Preserving allocation may not relay
   correctly in every case.  The relaying rules for a Preserving are
   designed to guarantee the following:

   o  Path MTU Discovery works end-to-end (i.e. client-to-peer), using
      either the old algorithm ([RFC1191] and [RFC1981]) or the new one



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      ([RFC4821]);

   o  ECN and Diff-Serv works end-to-end;

   o  Loops are prevented by copying and decrementing the TTL/Hop Count
      field.

   If the client knows its application or usage scenario requires a
   Preserving allocation, then it can request one in its Allocate
   request.  If the server is unable to grant this request, then it
   rejects the Allocate request.

   Note that a Preserving allocation only makes sense when the transport
   protocol to the client is UDP; when the transport is TCP or TLS, the
   allocation is always Non-Preserving.


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.







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   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 relayed transport address granted to a client
      through an Allocate request, along with related state, such as
      permissions and expiration timers.

   5-tuple:  The combination (client IP address and port, server IP
      address and port, and transport protocol (UDP or TCP)) used to
      communicate between the client and the server .  The 5-tuple
      uniquely identifies this communication stream.  The 5-tuple also
      uniquely identifies the Allocation on the server.

   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.

   Preserving Allocation  An allocation that sets the the fields in the
      IP header in a specific manner when relaying application data, and
      which also relays ICMP messages.  An allocation that may not do
      this in some cases is called a Non-Preserving allocation.


4.  General Behavior

   This section contains general TURN processing rules that apply to all
   TURN messages.

   TURN is an extension to STUN.  All TURN messages, with the exception
   of the ChannelData message, are STUN-formatted messages.  All the
   base processing rules described in [I-D.ietf-behave-rfc3489bis] apply
   to STUN-formatted messages.  This means that all the message-forming
   and -processing descriptions in this document are implicitly prefixed
   with the rules of [I-D.ietf-behave-rfc3489bis].

   In addition, the server SHOULD require that all TURN requests use the
   Long-Term Credential mechanism described in
   [I-D.ietf-behave-rfc3489bis], and the client MUST be prepared to
   authenticate requests if required.  The server's administrator MUST



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   choose a realm value that will uniquely identify the username and
   password combination that the client must use, even if the client
   uses multiple servers under different administrations.  The server's
   administrator MAY choose to allocate a unique username to each
   client, or MAY choose to allocate the same username to more than one
   client (for example, to all clients from the same department or
   company).

   The client and/or the server MAY include the FINGERPRINT attribute in
   any of the methods defined in this document.  However, TURN does not
   use the backwards-compatibility mechanism described in
   [I-D.ietf-behave-rfc3489bis].

   By default, TURN runs on the same port as STUN.  However, either the
   SRV procedures or the ALTERNATE-SERVER procedures described in
   Section 6 may be used to run TURN on a different port.


5.  Allocations

   All TURN operations revolve around allocations, and all TURN messages
   are associated with an allocation.  An allocation conceptually
   consists of the following state data:

   o  the relayed transport address

   o  The 5-tuple: client IP address, client port, server IP address,
      server port, transport protocol

   o  the username

   o  the transaction ID of the Allocate request

   o  the time-to-expiry

   o  A list of permissions

   o  A list of channel to peer bindings

   o  A flag indicating whether or not the allocation is Preserving

   The relayed transport address is the transport address allocated by
   the server for communicating with peers, while the 5-tuple describes
   the communication path between the client and the server.  Both of
   these MUST be unique across all allocations, so either one can be
   used to uniquely identify the allocation.

   When a TURN message arrives at the server from the client, the server



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   uses the 5-tuple in the message to identify the associated
   allocation.  For all TURN messages (including ChannelData) EXCEPT an
   Allocate request, if the 5-tuple does not identify an existing
   allocation, then the message MUST either be rejected with a 437
   Allocation Mismatch error (if it is a request), or silently ignored
   (if it is an indication or a ChannelData message).  A client
   receiving a 437 error response to a request other than Allocate MUST
   assume the allocation no longer exists.

   The username and password of the allocation is the username and
   password of the authenticated Allocate request that creates the
   allocation.  Subsequent requests on an allocation use the same
   username and password as those used to create the allocation, to
   prevent attackers from hijacking the client's allocation.
   Specifically, if the server requires the use of the Long-Term
   Credential mechanism, and if a non-Allocate request passes
   authentication under this mechanism, and if the 5-tuple identifies an
   existing allocation, but the request does not use the same username
   as used to create the allocation, then the request MUST be rejected
   with a 438 (Wrong Credentials) error.

   The transaction ID of the allocation is the transaction ID used in
   the Allocate request.  This is used to detect retransmissions of the
   Allocate request over UDP (see Section 6.2 for details).

   The time-to-expiry is the time in seconds left until the allocation
   expires.  Each Allocate or Refresh transaction sets this timer, which
   then ticks down towards 0.  By default, each Allocate or Refresh
   transaction resets this timer to 600 seconds (10 minutes), but the
   client can request a different value in the Allocate and Refresh
   request.  Allocations can only be refreshed using the Refresh
   request; sending data to a peer does not refresh an allocation.  When
   an allocation expires, the state data associated with the allocation
   is freed.  However the server MUST ensure that neither the relayed
   transport address nor the client reflexive transport address from the
   5-tuple are re-used in other allocations until 2 minutes after the
   allocation expires; this ensures that any messages that are in
   transit when the allocation expires are gone before either of these
   transport addresses are re-used.

   The list of permissions is described in Section 8 and the list of
   channels is described in Section 10.

   The differences between a Preserving and a Non-Preserving allocation
   are described in Section 11.






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6.  Creating an Allocation

   An allocation on the server is created using an Allocate transaction.

6.1.  Sending an Allocate Request

   The client forms an Allocate request as follows.

   The client first needs to pick a host transport address that the
   server does not think is currently in use, or was recently in use.
   The client SHOULD pick a currently-unused transport address on the
   client's host (typically by allowing its OS to pick a currently-
   unused port for a new socket).

   The client needs to pick a transport protocol to use between the
   client and the server.  The transport protocol MUST be one of UDP,
   TCP, or TLS over TCP.  Since this specification only allows UDP
   between the server and the peers, it is RECOMMENDED that the client
   pick UDP unless it has a reason to use a different transport.  One
   reason to pick a different transport would be that the client
   believes, either through configuration or by experiment, that it is
   unable to contact any TURN server using UDP.  See Section 2.1 for
   more discussion.

   The client must also pick a server transport address.  Typically,
   this is done by the client learning (perhaps through configuration)
   one or more domain names for TURN servers.  In this case, the client
   uses the DNS procedures described in [I-D.ietf-behave-rfc3489bis],
   but using an SRV service name of "turn" (or "turns" for TURN over
   TLS) instead of "stun" (or "stuns").  For example, to find servers in
   the example.com domain, the client performs a lookup for
   '_turn._udp.example.com', '_turn._tcp.example.com', and
   '_turns._tcp.example.com' if the client wants to communicate with the
   server using UDP, TCP, or TLS over TCP, respectively.

   The client MUST include a REQUESTED-TRANSPORT attribute in the
   request.  This attribute specifies the transport protocol between the
   server and the peers (note that this is NOT the transport protocol
   that appears in the 5-tuple).  In this specification, the REQUESTED-
   TRANSPORT type is always UDP.  This attribute is included to allow
   future extensions specify other protocols (e.g.,
   [I-D.ietf-behave-turn-tcp]).

   If the client wishes the server to initialize the time-to-expire
   field of the allocation to some value other the default lifetime,
   then it MAY include a LIFETIME attribute specifying its desired
   value.  This is just a request, and the server may elect to use a
   different value.  Note that the server will ignore requests to



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   initialize the field to less tha the default value.

   If the client required the allocation to satisfy certain properties,
   then the client includes the REQUESTED-PROPS attribute.  This
   attribute is optional, and can be omitted if no special properties
   are required.

   Using the E and R bits in the REQUESTED-PROPS attribute, the client
   can request:

   o  (E=1, R=0 ) That the server allocate a relayed transport address
      with an even port number; OR

   o  (E=1, R=1) That the server reserve a pair of relayed transport
      addresses with adjacent port numbers N and N+1, where N is even
      and N+1 is odd, and then use port N for the current allocation.
      In this case, the server returns a RESERVATION-TOKEN attribute in
      the response which the client can then include in a subsequent
      Allocate request to create an allocation with port number N+1.

   Note that the client cannot request a pair of adjacent ports unless
   it also requests that the lower numbered port be even.  Thus the
   combination (E=0, R=1) is not allowed.

   Similarly, by setting the P bit to 1 in the REQUESTED-PROPS
   attribute, the client can request that the server allocate a
   Preserving allocation.

   For all the various REQUESTED-PROPS flags, if the server cannot
   satisfy the request, the Allocate request is rejected.

   The client MAY also include a RESERVATION-TOKEN attribute in the
   request to ask the server to use a previously reserved port for the
   allocation.  If the RESERVATION-TOKEN attribute is included, then the
   client MUST either omit the REQUESTED-PROPS attribute or set E=0 and
   R=0, since doing otherwise would make no sense.

   Once constructed, the client sends the Allocate request on the
   5-tuple.

6.2.  Receiving an Allocate Request

   When the server receives an Allocate request, it performs the
   following checks:

   1.  The server checks the credentials of the request, as per the
       Long-Term Credential mechanism of [I-D.ietf-behave-rfc3489bis].




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   2.  The server checks if the 5-tuple is currently in use by an
       existing allocation, or was it in use by another allocation
       within the last 2 minutes.  If yes, then there are two sub-cases:

       *  If the transport protocol in the 5-tuple is UDP, and if the
          5-tuple is currently in use by an existing allocation, and if
          the transaction id of the request matches the transaction id
          stored with the allocation, then the request is a
          retransmission of the original request.  The server replies
          either with a stored copy of the original response, or with a
          response rebuilt from the stored state data.  If the server
          chooses to rebuild the response, then (a) it need not parse
          the request further, but can immediately start building a
          success response, (b) the value of the LIFETIME attribute can
          be set to the current value of the time-to-expire timer, and
          (c) the server may need to include an extra field in the
          allocation to store the token returned in a RESERVATION-TOKEN
          attribute.

       *  Otherwise, the server rejects the request with a 437
          (Allocation Mismatch) error.

       NOTE: If the request includes credentials that are acceptable to
       server, but the 5-tuple is already in use, then it is important
       that the server reject the request with a 437 (Allocation
       Mismatch) error rather than a 401 (Unauthorized) error.  This
       ensures that the client knows that the problem is with the
       5-tuple, rather than (wrongly) believing that the problem lies
       with its credentials.

   3.  The server checks if the request contain a REQUESTED-TRANPORT
       attribute.  If the REQUESTED-TRANSPORT attribute is not included
       or is malformed, the server rejects the request with a 400 (Bad
       Request) error.  Otherwise, if the attribute is included but
       specifies a protocol other that UDP, the server rejects the
       request with a 422 (Unsupported Transport Protocol) error.

   4.  The server checks if the request contains a REQUESTED-PROPS
       attribute.  If yes, then the server checks that it understands
       and can satisfy all the flags that are set to 1.  If a flag is
       not understood, or if the server cannot satisfy the request, then
       the server rejects the request with a 508 (Insufficient Port
       Capacity) error.  Note that the combination (E=0, R=1) MUST be
       treated as unsupported.

   5.  The server checks if the request contains a RESERVATION-TOKEN
       attribute.  If yes, and the request also contains a REQUESTED-
       PROPS attribute, then the server rejectes the request with a 400



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       (Bad Request) error.  Otherwise it checks to see if the token is
       valid (i.e., the token is in range and has not expired, and the
       corresponding relayed transport address is still available).  If
       the token is not valid for some reason, the server rejects the
       request with a 508 (Insufficient Port Capacity) error.

   6.  At any point, the server MAY also choose to reject the request
       with a 486 (Allocation Quota Reached) error if it feels the
       client is trying to exceed some locally-defined allocation quota.
       The server is free to define this allocation quota any way it
       wishes, but SHOULD define it based on the username used to
       authenticate the request, and not on the client's transport
       address.

   If the server rejects the request with one of the error codes 422
   (Unsupported Transport Protocol), 486 (Allocation Quota Reached) or
   508 (Insufficient Port Capacity), it MAY include an ALTERNATE-SERVER
   attribute in the error response redirecting the client to another
   server that it believes will accept the request.  If the attribute is
   included, the address MUST be from the same address family as the
   server's transport address.  Note that, if the attribute is included,
   the client will try this alternate server before trying the other
   servers given by the SRV procedures.

   If all the checks pass, the server creates the allocation.  The
   5-tuple is set to the 5-tuple from the Allocate request, while the
   list of permissions and the list of channels are initially empty.

   When allocating a relayed transport address for the allocation, the
   server MUST allocate an IP address from the same family (e.g, IPv4
   vs. IPv6) as that on which the request was received (i.e., the
   server's IP address in the 5-tuple for the allocation).

      NOTE: An extension to TURN to allow an address from a different
      address family is currently in progress
      [I-D.ietf-behave-turn-ipv6].

   In addition, 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



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   standard services.

   If the request contains a REQUESTED-PROPS attribute with the R flag
   set, then the server looks for a pair of port numbers N and N+1 on
   the same IP address, where N is even.  Port N is used in the current
   allocation, while the relayed transport address with port N+1 is
   assigned a token and reserved for a future allocation.  The server
   MUST hold this reservation for at least 30 seconds, and MAY choose to
   hold longer (e.g. until the allocation with port N expires).  The
   server then includes the token in a RESERVATION-TOKEN attribute in
   the success response.

   If the request contains a RESERVATION-TOKEN, the server uses the
   previously-reserved transport address corresponding to the included
   token (if it is still available).

   The server determines the initial value of the time-to-expire field
   as follows.  If the request contains a LIFETIME attribute, and the
   proposed lifetime value is greater than the default lifetime, and the
   proposed lifetime value is otherwise acceptable to the server, then
   the server uses that value.  Otherwise, the server uses the default
   value.  It is RECOMMENDED that the server impose a maximum lifetime
   of no more than 3600 seconds (1 hour).

      NOTE: The time-to-expire is recomputed with each successful
      Refresh request.  Thus the value computed here applies only until
      the first refresh.

   Once the allocation is created, the server replies with a success
   response.  The success response contains:

   o  A RELAYED-ADDRESS attribute containing the relayed transport
      address;

   o  A LIFETIME attribute containing the current value of the time-to-
      expire timer;

   o  A RESERVATION-TOKEN attribute (if a second relayed transport
      address was reserved).

   o  An XOR-MAPPED-ADDRESS attribute containing the client's IP address
      and port (from the 5-tuple);

      NOTE: The XOR-MAPPED-ADDRESS attribute is included in the response
      as a convenience to the client.  TURN itself does not make use of
      this value, but clients running ICE can often need this value and
      can thus avoid having to do an extra Binding transaction with some
      STUN server to learn it.



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   The response (either success or error) is sent back to the client on
   the 5-tuple.

6.3.  Receiving an Allocate Response

   If the client receives a success response, then it MUST check that
   the relayed transport address is in an address family that the client
   understands and is prepared to deal with.  This specification only
   covers the case where the relayed transport address is of the same
   address family as the client's transport address.  If the relayed
   transport address is not in an address family that the client is
   prepared to deal with, then the client MUST delete the allocation
   (Section 7) and MUST NOT attempt to create another allocation on that
   server until it believes the mismatch has been fixed.

      The IETF is currently considering mechanisms for transitioning
      between IPv4 and IPv6 that could result in a client originating an
      Allocate request over IPv4, but the request would arrive at the
      server over IPv6, or vica-versa.  Hence the importance of this
      check.

   Otherwise, the client creates its own copy of the allocation data
   structure to track what is happening on the server.  In particular,
   the client needs to remember the actual lifetime received back from
   the server, rather than the value sent to the server in the request.
   The client must also remember the 5-tuple used for the request and
   the username and password it used to authenticate the request to
   ensure that it reuses them for subsequent messages.  The client also
   needs to track the channels and permissions it establishes on the
   server.

   The client will probably wish to send the relayed transport address
   to peers (using some method not specified here) so the peers can
   communicate with it.  The client may also wish to use the server-
   reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in
   its ICE processing.

   If the client receives an error response, then the processing depends
   on the actual error code returned:

   o  (Request timed out): There is either a problem with the server, or
      a problem reaching the server with the chosen transport.  The
      client MAY choose to try again using a different transport (e.g.,
      TCP instead of UDP), or the client MAY try a different server.

   o  400 (Bad Request): The server believes the client's request is
      malformed for some reason.  The client MAY notify the user or
      operator and SHOULD NOT retry the same request with this server



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      until it believes the problem has been fixed.  The client MAY try
      a different server.

   o  401 (Unauthorized): If the client has followed the procedures of
      the Long-Term Credential mechanism and still gets this error, then
      the server is not accepting the client's credentials.  The client
      SHOULD notify the user or operator and SHOULD NOT send any further
      requests to this server until it believes the problem has been
      fixed.  The client MAY try a different server.

   o  437 (Allocation Mismatch): This indicates that the client has
      picked a 5-tuple which the server sees as already in use or which
      was recently in use.  One way this could happen is if an
      intervening NAT assigned a mapped transport address that was
      recently used by another allocation.  The client SHOULD pick
      another client transport address and retry the Allocate request
      (using a different transaction id).  The client SHOULD try three
      different client transport addresses before giving up on this
      server.  Once the client gives up on the server, it SHOULD NOT try
      to create another allocation on the server for 2 minutes.

   o  438 (Wrong Credentials): The client should not receive this error
      in response to a Allocate request.  The client MAY notify the user
      or operator and SHOULD NOT retry the same request with this server
      until it believes the problem has been fixed.  The client MAY try
      a different server.

   o  442 (Unsupported Transport Address): The client should not receive
      this error in response to a request for a UDP allocation.  The
      client MAY notify the user or operator and SHOULD NOT retry the
      same request with this server until it believes the problem has
      been fixed.  The client MAY try a different server.

   o  486 (Allocation Quota Reached): The server is currently unable to
      create any more allocations with this username.  The client SHOULD
      wait at least 1 minute before trying to create any more
      allocations on the server.  The client MAY try a different server.

   o  508 (Insufficient Port Capacity): The server has no more relayed
      transport addresses avaiable, or has none with the requested
      properties, or the one that was reserved is no longer available.
      If the client is using either the REQUESTED-PROPS or the
      RESERVATION-TOKEN attribute, then the client MAY choose to remove
      this attribute and try again immediately.  Otherwise, the client
      SHOULD wait at least 1 minute before trying to create any more
      allocations on this server.  The client MAY try a different
      server.




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   If the error response contains an ALTERNATE-SERVER attribute, and the
   client elects to try a different server, the the client SHOULD try
   the alternate server specified in that attribute (while obeying the
   rules in [I-D.ietf-behave-rfc3489bis] for avoiding redirection loops)
   before trying any other servers found using the SRV procedures of
   [I-D.ietf-behave-rfc3489bis].


7.  Refreshing an Allocation

   A Refresh transaction can be used to either (a) refresh an existing
   allocation and update its time-to-expire, or (b) delete an existing
   allocation.

   If a client wishes to continue using an allocation, then the client
   MUST refresh it before it expires.  It is suggested that the client
   refresh the allocation roughly 1 minute before it expires.  If a
   client no longer wishes to use an allocation, then it SHOULD
   explicitly delete the allocation.  A client MAY also change the time-
   to-expire of an allocation at any time for other reasons.

7.1.  Sending a Refresh Request

   If the client wishes to immediately delete an existing allocation, it
   includes a LIFETIME attribute with a value of 0.  All other forms of
   the request refresh the allocation.

   The Refresh transaction updates the time-to-expire timer of an
   allocation.  If the client wishes the server to set the time-to-
   expire timer to something other than the default lifetime, it
   includes a LIFETIME attribute with the requested value.  The server
   then computes a new time-to-expire value in the same way as it does
   for an Allocate transaction, with the exception that a requested
   lifetime of 0 causes the server to immediately delete the allocation.

   The Refresh transaction is sent on the 5-tuple for the allocation.

7.2.  Receiving a Refresh Request

   When the server receives a Refresh request, it processes it as
   follows.  If, during processing, an error in the request is detected
   (for example, a syntax error in the request which causes a 400
   error), then the request is rejected with an error response but the
   allocation is NOT deleted (but note that a 437 error will indicate
   that the allocation was not found).

   The server determines the new value for the time-to-expire field as
   follows.  If the request contains a LIFETIME attribute, and the



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   attribute value is 0, then the server uses a value of 0, which causes
   the allocation to expire.  Otherwise, if the request contains a
   LIFETIME attribute and the attribute value is greater than the
   default lifetime, and the attribute value is otherwise acceptable to
   the server, then the server uses the attribute value.  Otherwise, the
   server uses the default value.  It is RECOMMENDED that the server
   impose a maximum lifetime of no more than 3600 seconds (1 hour).

   The server then constructs a success response containing:

   o  A LIFETIME attribute containing the current value of the time-to-
      expire timer.

   The response is then sent on the 5-tuple.

7.3.  Receiving a Refresh Response

   If the client receives a success response to its Refresh request, it
   updates its copy of the allocation data structure with the time-to-
   expire value contained in the response.

   If the client receives an 437 (Allocation Mismatch) error response to
   its Refresh request, then it must consider the allocation as having
   expired, as described in Section 4.  All other errors indicate a
   software error on the part of either the client or the server.


8.  Permissions

   For each allocation, the server keeps a list of zero or more
   permissions.  Each permission consists an IP address which uniquely
   identifies the permission, and an associated time-to-expiry.  The IP
   address describes a peer that is allowed to send data to the client,
   and the time-to-expiry is the number of seconds until the permission
   expires.

   Various events, as described in subsequent sections, can cause a
   permission for a given IP address to be installed or refreshed.  This
   causes one of two things to happen:

   o  If no permission for that IP address exists, then a permission is
      created with the given IP address and a time-to-expiry equal to
      the default permission lifetime.

   o  If a permission for that IP address already exists, then the
      lifetime for that permission is reset to the default permission
      lifetime.




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   The default permission lifetime MUST be 300 seconds (= 5 minutes).

   Each permission's time-to-expire decreases down once per second until
   it reaches 0, at which point the permission expires and is deleted.

   When a UDP datagram arrives at the relayed transport address for the
   allocation, the server checks the list of permissions for that
   allocation.  If there is a permission with an IP address that is
   equal to the source IP address of the UDP datagram, then the UDP
   datagram can be relayed to the client.  Otherwise, the UDP datagram
   is silently discarded.  Note that only IP addresses are compared;
   port numbers are irrelevant.

   The permissions for one allocation are totally unrelated to the
   permissions for a different allocation.  If an allocation expires,
   all its permissions expire with it.

      NOTE: Though TURN permissions expire after 5 minutes, many NATs
      deployed at the time of publication expire their UDP bindings
      considerably faster.  Thus an application using TURN will probably
      wish to send some sort of keep-alive traffic at a much faster
      rate.  Applications using ICE should follow the keep-alive
      guidelines of ICE [I-D.ietf-mmusic-ice], and applications not
      using ICE are advised to do something similar.


9.  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 10 describes the use of the Channel Mechanism.

9.1.  Sending a Send Indication

   A client can use a Send Indication to pass data to the server for
   relaying to a peer.  A client can also use a Send Indication without
   a DATA attribute to install or refresh a permission for the specified
   IP address.  A client may use a Send indication to send data to a
   peer even if a channel is bound to that peer.

   When forming a Send Indication, the client MUST include a PEER-
   ADDRESS attribute and MAY include a DATA attribute.  If the DATA
   attribute is included, then the DATA attribute contains the actual
   application data to be sent to the peer, and the PEER-ADDRESS
   attribute contains the transport address of the peer to which the
   data is to be sent.  If the DATA attribute is not present, then the
   PEER-ADDRESS attribute contains the IP address for which a permission
   is to be installed or refreshed; in this case the port specified in



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   the attribute is ignored.

   Note that no authentication attributes are included, since
   indications cannot be authenticated using the Long-Term Credential
   mechanism.

   The Send Indication MUST be sent using the same 5-tuple used for the
   original allocation.

9.2.  Receiving a Send Indication

   When the server receives a Send indication, it processes it as
   follows.

   If the received Send indication contains a DATA attribute, then 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.

   The resulting UDP datagram is then sent to the peer.  If any errors
   are detected during this process (e.g., the Send indication does not
   contain a PEER-ADDRESS attribute), the received indication is
   silently discarded and no UDP datagram is sent.

   When the server receives a valid Send Indication, either with or
   without a DATA attribute, it also installs or refreshes a permission
   for the IP address contained in the PEER-ADDRESS attribute (see
   Section 8).

9.3.  Receiving a UDP Datagram

   When the server receives a UDP datagram at a currently allocated
   relayed transport address, the server looks up the allocation
   associated with the relayed transport address.  It then checks to see
   if relaying is permitted, as described in section Section 8).

   If relaying is permitted, and there is no channel bound to the peer
   that sent the UDP datagram (see ISection 10), then the server forms
   and sends a Data indication.  The Data indication MUST contain both a
   PEER-ADDRESS and a DATA attribute.  The DATA attribute is set to the



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   value of the 'data octets' field from the datagram, and the PEER-
   ADDRESS attribute is set to the source transport address of the
   received UDP datagram.  The Data indication is then sent on the
   5-tuple associated with the allocation.

9.4.  Receiving a Data Indication

   When the client receives a Data indication, it checks that the Data
   indication contains both a PEER-ADDRESS and a DATA attribute.  It
   then delivers the data octets inside the DATA attribute to the
   application, along with an indication that they were received from
   the peer whose transport address is given by the PEER-ADDRESS
   attribute.


10.  Channels

   Channels provide a way for the 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.

   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.

   A channel binding consists of:

   o  A channel number;

   o  A transport address (of the peer);

   o  A time-to-expiry timer.

   Within the context of an allocation, a channel binding is uniquely
   identified either by the channel number or by the transport address.
   Thus the same channel cannot be bound to two different transport
   addresses, nor can the same transport address be bound to two
   different channels.




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   A channel binding last for 10 minutes unless refreshed.  Refreshing
   the binding (by the server receiving either a ChannelBind request
   rebinding the channel to the same peer, or by the server receiving a
   ChannelData message on that channel) resets the time-to-expire timer
   back to 10 minutes.  When the channel binding expires, the channel
   becomes unbound and available for binding to a different transport
   address.

   When binding a channel to a peer, the client SHOULD be prepared to
   receive ChannelData messages on the channel from the server as soon
   as it has sent the ChannelBind request.  Over UDP, it is possible for
   the client to receive ChannelData messages from the server before it
   receives a ChannelBind success response.

   In the other direction, the client MAY elect to send ChannelData
   messages before receiving the ChannelBind success response.  Doing
   so, however, runs the risk of having the ChannelData messages dropped
   by the server if the ChannelBind request does not succeed for some
   reason (e.g., packet lost if the request is sent over UDP, or the
   server being unable to fulfill the request).  A client that wishes to
   be safe should either queue the data, or use Send indications until
   the channel binding is confirmed.

10.1.  Sending a ChannelBind Request

   A channel binding is created using a ChannelBind transaction.  A
   channel binding can also be refreshed using a ChannelBind
   transaction.

   To initiate the ChannelBind transaction, the client forms a
   ChannelBind request.  The channel to be bound is specified in a
   CHANNEL-NUMBER attribute, and the peer's transport address is
   specified in a PEER-ADDRESS attribute.  Section 10.2 describes the
   restrictions on these attributes.

   Note that rebinding a channel to the same transport address that it
   is already bound to provides a way to refresh a channel binding
   without sending data to the peer.

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

10.2.  Receiving a ChannelBind Request

   When the server receives a ChannelBind request, it checks the
   following:





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   o  The request contains both a CHANNEL-NUMBER and a PEER-ADDRESS
      attribute;

   o  The channel number is in the range 0x4000 to 0xFFFE (inclusive);

   o  The channel number is not currently bound to a different transport
      address (same transport address is OK);

   o  The transport address is not currently bound to a different
      channel number.

   If any of these tests fail, the server replies with an error response
   with error code 400 "Bad Request".  Otherwise, the ChannelBind
   request is valid and the server replies with a ChannelBind success
   response.

   If ChannelBind request is valid, then the server creates or refreshes
   the channel binding using the channel number in the CHANNEL-ADDRESS
   attribute and the transport address in the PEER-ADDRESS attribute.
   The server also installs or refreshes a permission for the IP address
   in the PEER-ADDRESS attribute.

10.3.  Receiving a ChannelBind Response

   When the client receives a successful ChannelBind response, it
   updates its data structures to record that the channel binding is now
   active.

10.4.  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



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   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.

10.5.  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
   channel when it exists and may freely intermix the two message types
   when sending data to the peer.  The server, on the other hand, MUST
   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 10.4.

   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.

10.6.  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 10.4.

   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



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   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 10.5.

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

10.7.  Relaying

   When a server receives a ChannelData message, it first processes it
   as described in the previous section.  If no errors are detected, it
   relays the application data to the peer by forming 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 ChannelData message arrived;

   o  the destination transport address is the transport address to
      which the channel is bound;

   o  the data following the UDP header is the contents of the data
      field of the ChannelData message.

   The resulting UDP datagram is then sent to the peer.

   If the ChannelData message is valid, then the server refreshes the
   channel binding, and also installs or refreshes a permission for the
   IP address part of the transport address to which the UDP datagram is
   sent (see Section 8).

   In the other direction, when the server receives a UDP datagram on
   the relayed transport address associated with an allocation, then it
   first checks to see if it is permitted to relay the datagram.  This
   check is done as described in Section 8.  If relaying is permitted,
   then the server checks to see if there is a channel bound to the peer
   that sent the UDP datagram.  If there is, then it SHOULD form and
   send a ChannelData message as described in Section 10.5.  If no
   channel is bound to the peer, then it MUST form and send a Data
   indication as described in Section 9.3.







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11.  IP and ICMP

   This section describes how the server sets various fields in the IP
   header when relaying between the client and the peer or vica-versa.
   It also describes how the server relays ICMP messages.  The
   descriptions in this section apply: (a) when the server receives a
   Send indication or ChannelData message from the client and sends a
   UDP datagram to the peer, (b) when the server receives a UDP datagram
   on the relayed-transport address and sends a Data indication or
   ChannelData message to the client, or (c) when the server receives an
   ICMP message.  This section does not apply when the server sends TURN
   control messages.

   The descriptions below have two parts: a preferred behavior and an
   alternate behavior.  A Preserving allocation MUST implement the
   preferred behavior.  A non-preserving allocation with UDP transport
   to the client SHOULD implement the preferred behavior, but if that is
   not possible for a particular field, then it SHOULD implement the
   alternative behavior.  A non-preserving allocation with TCP or TLS
   transport to client SHOULD implement the alternate behavior, except
   where this conflicts with standard TCP or TLS behavior.

11.1.  IP

   This section describes the preferred and alternate behavior for
   various fields in the IP header.

   Time to Live (IPv4) or Hop Count (IPv6)

      Preferred Behavior: If the incoming value is 0, then send an ICMP
      Time Exceeded message back to the sender.  Otherwise set the
      outgoing Time to Live/Hop Count to one less than the incoming
      value.

      Alternate Behavior: Set the outgoing value to the default for
      outgoing packets.


   Diff-Serv Code Point

      Preferred Behavior: Set the outgoing value to the incoming value,
      unless the server, though configuration or other means, believes
      that a different setting is more appropriate.

      Alternate Behavior: Set the outgoing value to Best Effort, unless
      the server, through configuration or other means, believes a
      different setting is more appropriate.




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   ECN

      Preferred Behavior: Set the outgoing value to the incoming value,
      UNLESS the server is doing Active Queue Management, the incoming
      ECN field is 01 or 10, and the server wishes to indicate that
      congestion has been experienced, in which case set the outgoing
      value to 11.

      Alternate Behavior: Set the outgoing value to 00 (ECN not
      supported)


   Flow Label

      Preferred Behavior: Set the outgoing flow label to 0.

      Alternate Behavior: Same as the Preferred behavior.


   IPv4 Fragmentation

      Preferred Behavior:

         If the outgoing packet size does not exceed the outgoing link's
         MTU, then send the outgoing packet unfragmented.  Set the DF
         bit in the outgoing packet to the value of the DF bit in the
         incoming packet, and set the other fragmentation fields
         (Identification, MF, Fragment Offset) as appropriate for a
         packet originating from the server.

         Otherwise, if the outgoing link's MTU is exceeded and the
         incoming DF bit is 0, then fragment the packet before sending.
         Set the outgoing DF to 0, and set the other fragmentation
         fields as appropriate for fragments originated from the server.

         Otherwise [link MTU exceeded and incoming DF set], drop the
         outgoing packet and send an ICMP message of type 3 code 4
         ("fragmentation needed and DF set") to the sender of the
         incoming packet.

      Alternate Behavior: As described in the Preferred Behavior, except
      always assume the incoming DF bit is 0.


   IPv6 Fragmentation

      Preferred Behavior:




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         If the incoming packet did not include a Fragmentation header
         and the outgoing packet size does not exceed the outgoing
         link's MTU, then send the outgoing packet without a
         Fragmentation header.

         If the incoming packet included a Fragment header and if the
         outgoing packet size (with a Fragmentation header included)
         does not exceed the outgoing link's MTU, then send the outgoing
         packet with a Fragmentation header.  Set the fields of the
         Fragmentation header as appropriate for a packet originating
         from the server.

         If the incoming packet did not include a Fragmentation header
         and the outgoing packet size exceeds the outgoing link's MTU ,
         then drop the outgoing packet and send an ICMP message of type
         2 code 0 ("Packet too big") to the sender of the incoming
         packet.  If the packet is being sent to the peer, then reduce
         the MTU reported in the ICMP message by 48 bytes to allow room
         for the overhead of a Data indication.

         Otherwise, if the link's MTU is exceeded and the incoming
         packet contained a Fragmentation header, then fragment the
         outgoing packet into fragments of no more than 1280 bytes.  Set
         the fields of the Fragmentation header as appropriate for a
         packet originating from the server.

      Alternate Behavior: As described in the Preferred Behavior, except
      always assume incoming packet has a Fragmentation header.


   IPv4 Options

      Preferred Behavior: The outgoing packet is sent without any IPv4
      options.

      Alternate Behavior: Same as preferred.


   IPv6 Extention Headers

      Preferred Behavior: The outgoing packet is sent without any IPv6
      extension headers, with the exception of the Fragmentation header
      as described above

      Alternate Behavior: Same as preferred.






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11.2.  ICMP

   This sub-section describes the preferred behavior of ICMP relaying.
   The corresponding alternate behavior is to not relay ICMP messages.

   When an ICMP message arrives at the server, the copy of the original
   IP packet present inside the ICMP message is examined.  The server
   first checks that the original IP packet header is immediately
   followed by a UDP protocol header, such that the original source
   transport address was X and the original destination transport
   address was Y. The server also checks that the type and code values
   in the ICMP header are one of those relayed (see below).  Other ICMP
   messages are either ignored, or used by the server internally in an
   unspecified manner.

   The server then checks if one of the following two cases applies:

   Case 1: X is a relayed-transport-address currently assigned to an
   active allocation on the server, and there exists a permission for
   the IP address of Y in the allocation.

   In this case, the original IP packet was traveling from the server to
   a peer, so the the server relays the ICMP message back to the client.
   The server creates a Data indication where the PEER-ADDRESS attribute
   contains Y, and the ICMP attribute contains the type and code from
   the incoming ICMP message, and the DATA attribute contains
   application data from the original IP packet starting AFTER the UDP
   header.  The server SHOULD include as much application data as
   possible consistent with not exceeding a total IP packet size of
   either 576 bytes (for IPv4) or 1280 bytes (for IPv6).

      Note that there is no point in including the original IP or UDP
      header in the DATA attribute because those headers were generated
      by the server, not the client.

   Case 2: There is an active allocation where X is the server transport
   address, Y is the client transport address, and UDP is used as
   transport between the client and the server.  Furthermore, the packet
   after the UDP header is either (a) a ChannelData header which
   contains an active channel number in the allocation, or (b) a Data
   indication whose PEER-ADDRESS attribute contains an IP address for
   which there exists a permission in the allocation.

   In this case, the original IP packet was traveling from the server to
   the client, so the server creates and sends an ICMP message to the
   peer.  The outgoing ICMP message contains the type and code fields
   from the incoming ICMP message and then contains an approximation to
   the original IP packet sent from the peer to the server (the one the



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   server was trying to relay to the client inside the ChannelData or
   Data indication).  This approximation contains a synthesized IP
   header, a synthesized UDP header, and some application data.  The
   synthesis is done as follows:

   o  The destination transport address is the relayed-transport-address
      of the allocation;

   o  The source transport address is the peer's transport address
      determined from either (a) the channel number or (b) the PEER-
      ADDRESS attribute;

   o  The application data is taken from either (a) the ChannelData
      message or (b) the DATA attribute.  The server SHOULD include as
      much application data as possible consistent with not exceeding
      either 576 bytes (for IPv4) or 1280 bytes (for IPv6).

   The remaining fields in the IP and UDP headers are simply set to
   sensible values, since for most of them there is no way to
   reconstruct the original values.

   The server SHOULD relay as all ICMP type/code combinations and MUST
   relay at at least the following combinations.  For IPv4:

      Type 3, code 4: Fragmentation needed and DF set

   For IPv6:

      Type 2, code <any>: Packet too big


12.  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
       0x009  :  ChannelBind

     Indications
       0x006  :  Send
       0x007  :  Data






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13.  New STUN Attributes

   This STUN extension defines the following new attributes:

     0x000C: CHANNEL-NUMBER
     0x000D: LIFETIME
     0x0010: Reserved (was BANDWIDTH)
     0x0012: PEER-ADDRESS
     0x0013: DATA
     0x0016: RELAY-ADDRESS
     0x0018: REQUESTED-PROPS
     0x0019: REQUESTED-TRANSPORT
     0x0022: RESERVATION-TOKEN


13.1.  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 (Reserved
   For Future Use) field which MUST be set to 0 on transmission and MUST
   be 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 = 0              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

13.2.  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.

13.3.  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.

13.4.  DATA

   The DATA attribute is present in all Data Indications and most Send
   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).




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13.5.  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.

13.6.  REQUESTED-PROPS

   This attribute allows the client to request that the allocation have
   certain properties.  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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |E|R|P|                      MUST be 0                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first part of the attribute value contains a number of one-bit
   flags.  These are:

   E: If 1, the port number for the relayed-transport-address must be
      even.  If 0, the port number can be even or odd.

   R: If 1, the server must reserve the next highest port for a
      subsequent allocation.  If 0, no such reservation is requested.
      If the client sets the R bit to 1, it MUST also set the E bit to 1
      (however, the E bit may be 1 when the R bit is 0).

   P: If 1, the allocation must be a Preserving allocation.  If 0, the
      allocation can be either Preserving or Non-Preserving.

   All these flags have the property that if the bit is 1, and the
   server cannot create an allocation that satisfies the request, then
   the Allocate request is rejected.  To allow future TURN extensions to
   define new flags that also have this property, the client MUST set
   the rest of the attribute to zero, and the server MUST fail the
   Allocate request if any bits which the server does not support are
   set to 1.  By doing this, any new flags that are not recognized by
   the server will cause the Allocate request to fail.

13.7.  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:






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      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 MUST be set to zero on transmission and MUST be
   ignored on receiption.  It is reserved for future uses.

13.8.  RESERVATION-TOKEN

   The RESERVATION-TOKEN attribute contains a token that uniquely
   identifies a relayed transport address being held in reserve by the
   server.  The server includes this attribute in a success response to
   tell the client about the token, and the client includes this
   attribute in a subsequent Allocate request to request the server use
   that relayed transport address for the allocation.

   The attribute value is a 64-bit-long field containing the token
   value.


14.  New STUN Error Response Codes

   This document defines the following new error response codes:

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

   438  (Wrong Credentials): The credentials in the (non-Allocate)
      request, though otherwise acceptable to the server, do not match
      those used to create the allocation.

   442  (Unsupported Transport Protocol): The Allocate request asked the
      server to use a transport protocol between the server and the peer
      that the server does not support.  NOTE: This does NOT refer to
      the transport protocol used in the 5-tuple.






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   486  (Allocation Quota Reached): No more allocations using this
      username can be created at the present time.

   508  (Insufficient Port Capacity): The server has no more relayed
      transport addresses available right now, or has none with the
      requested properties, or the one that corresponds to the specified
      token is not available.


15.  Security Considerations

   TURN servers allocate resources to clients, in contrast to the
   Binding method defined in [I-D.ietf-behave-rfc3489bis].  Therefore, a
   TURN server may require the 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 per user limit on the number of allocations that can
   active at one time.  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 attempts to adhere as closely as possible to common firewall
   policies, consistent with allowing data to flow.  TURN has fairly
   limited applicability, requiring a user to explicitly authorize



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   permission to receive data from a peer, one IP address at a time.
   Thus, it does not provide a general technique for externalizing
   sockets.  Rather, it has similar security properties to the placement
   of an address-restricted NAT in the network, allowing messaging in
   from a peer only if the internal client has sent a packet out towards
   the IP address of that peer.  This limitation means that TURN cannot
   be used to run, for example, SIP servers, NTP servers, FTP 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.

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

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


16.  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



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   of STUN protocol elements.

   The codepoints for the new STUN methods defined in this specification
   are listed in Section 12.

   The codepoints for the new STUN attributes defined in this
   specification are listed in Section 13.

   The codepoints for the new STUN error codes defined in this
   specification are listed in Section 14.

   Extensions to TURN can be made through IETF consensus.


17.  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].


18.  Example

   TBD


19.  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.

19.1.  Changed from -07 to -08

   o  Removed the BANDWIDTH attribute and all associated text (including
      error code 507 "Insufficient Bandwidth Capacity"), as the
      requirements for this feature were not clear and it was felt the



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      feature could be easily added later.

   o  Changed the format of the REQUESTED-PROPS attribute from a one-
      byte field to a set of bit flags.  Changed the semantics of the
      unused portion of the value from RFFU to "MUST be 0" to give a
      more desirable behavior when new flags are defined.

   o  Introduced the concept of Preserving vs. Non-Preserving
      allocations.  As a result, completely revamped the rules for how
      to set the fields in the IP header, and added rules for relaying
      ICMP messages when the allocation is Preserving.

19.2.  Changes from -06 to -07

   o  Rewrote the General Behavior section, making various changes in
      the process.

   o  Changed the usage of authentication from MUST to SHOULD.

   o  Changed the requirement that subsequent requests use the same
      username and password from MUST to SHOULD to allow for the
      possibility of changing the credentials using some unspecified
      mechanism.

   o  Introduced a 438 (Wrong Credentials) error which is used when a
      non-Allocate request authenticates but does not use the same
      username and password as the Allocate request.  Having a separate
      error code for this case avoids the client being confused over
      what the error actually is.

   o  The server must now prevent the relayed transport address and the
      5-tuple from being reused in different allocations for 2 minutes
      after the allocation expires.

   o  Changed the usage of FINGERPRINT from MUST NOT to MAY, to allow
      for the possible multiplexing of TURN with some other protocol.

   o  Rewrote much of the section on Allocations, splitting it into
      three new sections (one on allocations in general, one on creating
      an allocation, and one on refreshing an allocation).

   o  Replaced the mechanism for requesting relayed transport addresses
      with specific properties.  The new mechanism is less powerful: a
      client can request an even port, or a pair of ports, but cannot
      request a single odd port or a specific port as was possible under
      the old mechanism.  Nor can the client request a specific IP
      address.




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   o  Changed the rules for handling ALTERNATE-SERVER, removing the
      requirement that the referring server have "positive knowledge"
      about the state of the alternate server.  The new rules instead
      rely on text in STUN to prevent referral loops.

   o  Changed the rules for allocation lifetimes.  Allocations lifetimes
      are now a minimum of 10 minutes; the client can ask for longer
      values, but requests for shorter values are ignored.  The text now
      recommends that the client refresh an allocation one minute before
      it expires.

   o  Put in temporary procedures for handling the BANDWIDTH attribute,
      modelled on the LIFETIME attribute.  These procedures are mostly
      placeholders and likely to change in the next revision.

   o  Added a detailed description of how a client reacts to the various
      errors it can receive in reply to an Allocate request.  This
      replaces the various descriptions that were previously scattered
      throughout the document, which were inconsistent and sometimes
      contradictory.

   o  Added a new section that gives the normative rules for
      permissions.

   o  Changed the rules around permission lifetimes.  The text used to
      recommend a value of one minute; it MUST now be 5 minutes.

   o  Removed the errors "Channel Missing or Invalid", "Peer Address
      Missing or Invalid" and "Lifetime Malformed or Invalid" and used
      400 "Bad Request" instead.

   o  Rewrote portions of the section on Send and Data indications and
      the section on Channels to try to make the client vs. server
      behavior clearer.

   o  Channel bindings now expire after 10 minutes, and must be
      refreshed to keep them alive.

   o  Binding a channel now installs or refreshes a permission for the
      IP address of corresponding peer.

   o  Changed the wording describing the situation when the client sends
      a ChannelData message before receiving the ChannelBind success
      response. -06 said that client SHOULD NOT do this; -07 now says
      that a client MAY, but describes the consequences of doing it.

   o  Added a section discussing the setting of fields in the IP header.




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   o  Replaced the REQUESTED-PORT-PROPS attribute with the REQUESTED-
      PROPS attribute that has a different format and semantics, but
      reuses the same code point.

   o  Replaced the REQUESTED-IP attribute with the RESERVATION-TOKEN
      attribute, which has a different format and semantics, but reuses
      the same code point.

   o  Removed error codes 443 and 444, and replaced them with 508
      (Insufficient Port Capacity).  Also changed the error text for
      code 507 from "Insufficient Capacity" to "Insufficient Bandwidth
      Capacity".

19.3.  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.






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19.4.  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.

   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.





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   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.


20.  Open Issues

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

   Bandwidth: How should bandwidth be specified?  What are the right
   rules around bandwidth?

   Alternate Server: Do we still want this mechanism?  Is the current
   proposal acceptable?  Note that the usage of the ALTERNATE-SERVER
   attribute in this document is inconsistent with its usage in STUN.
   In STUN, if the ALTERNATE-SERVER attribute is used, then the error
   that the server would otherwise generate is replaced by a 300 (Try
   Alternate) code.  In this document, the 300 error code is not used,
   and the server returns an appropriate error code and then includes
   the ALTERNATE-SERVER attribute in the response.  In this way, the
   client can see the actual error code, rather than always seeing error
   code 300, and can thus make a more intelligent decision on whether it
   wishes to try the alternate server.

   Public TURN servers: The text currently says that a server "SHOULD"
   use the Long-Term Credential mechanism, with the unstated idea that a
   public TURN server would not use it.  But this really weakens the
   security of TURN.  Is there a better way to allow public servers?  Or
   should we just drop the notion of a public server entirely?


21.  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, Derek MacDonald, Cullen
   Jennings, Lars Eggert, Magnus Westerlund, and Eric Rescorla have been
   particularly helpful, with Eric also suggesting the channel
   allocation mechanism, and Cullen suggesting the REQUESTED-PROPS
   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 both his contributions to
   the text and his huge help in restarting progress on this draft after
   work had stalled.



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22.  References

22.1.  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-15 (work in progress),
              February 2008.

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

   [RFC3697]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
              "IPv6 Flow Label Specification", RFC 3697, March 2004.

22.2.  Informative References

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

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

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

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

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



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   [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.

   [I-D.ietf-behave-turn-ipv6]
              Camarillo, G. and O. Novo, "Traversal Using Relays around
              NAT (TURN) Extension for IPv4/IPv6  Transition",
              draft-ietf-behave-turn-ipv6-04 (work in progress),
              January 2008.

   [I-D.ietf-tsvwg-udp-guidelines]
              Eggert, L. and G. Fairhurst, "Guidelines for Application
              Designers on Using Unicast UDP",
              draft-ietf-tsvwg-udp-guidelines-08 (work in progress),
              June 2008.

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

   [RFC1928]  Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
              L. Jones, "SOCKS Protocol Version 5", RFC 1928,
              March 1996.

   [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
   (Unaffiliated)

   Fax:
   Email: philip_matthews@magma.ca
   URI:







































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Full Copyright Statement

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