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Versions: (draft-rosenberg-midcom-turn) 00 01
02 03 04 05 06 07 08 09 10 11 12 13
14 15 16 RFC 5766
Behave J. Rosenberg
Internet-Draft Cisco
Intended status: Standards Track R. Mahy
Expires: July 25, 2008 Plantronics
P. Matthews
Avaya
January 22, 2008
Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-behave-turn-06
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Copyright Notice
Copyright (C) The IETF Trust (2008).
Abstract
If a host is located behind a NAT, then in certain situations it can
be impossible for that host to communicate directly with other hosts
(peers) located behind other NATs. In these situations, it is
necessary for the host to use the services of an intermediate node
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that acts as a communication relay. This specification defines a
protocol, called TURN (Traversal Using Relays around NAT), that
allows the host to control the operation of the relay and to exchange
packets with its peers using the relay.
The TURN protocol can be used in isolation, but is more properly used
as part of the ICE (Interactive Connectivity Establishment) approach
to NAT traversal.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
2.1. Transports . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2. Allocations . . . . . . . . . . . . . . . . . . . . . . . 8
2.3. Exchanging Data with Peers . . . . . . . . . . . . . . . . 9
2.4. Permissions . . . . . . . . . . . . . . . . . . . . . . . 10
2.5. Channels . . . . . . . . . . . . . . . . . . . . . . . . . 10
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. General Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
5. Managing Allocations . . . . . . . . . . . . . . . . . . . . . 14
5.1. Client Behavior . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Initial Allocate Requests . . . . . . . . . . . . . . 14
5.1.2. Refresh Requests . . . . . . . . . . . . . . . . . . . 15
5.2. Server Behavior . . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Receiving an Allocate Request . . . . . . . . . . . . 16
5.2.2. Refresh Requests . . . . . . . . . . . . . . . . . . . 20
6. Send and Data Indications . . . . . . . . . . . . . . . . . . 21
6.1. Forming and Sending an Indication . . . . . . . . . . . . 21
6.2. Receiving an Indication . . . . . . . . . . . . . . . . . 22
6.3. Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Channel Mechanism . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Forming and Sending a ChannelBind Request . . . . . . . . 23
7.2. Receiving a ChannelBind Request and Sending a Response . . 24
7.3. Receiving a ChannelBind Response . . . . . . . . . . . . . 25
7.4. The ChannelData Message . . . . . . . . . . . . . . . . . 25
7.5. Forming and Sending a ChannelData Message . . . . . . . . 25
7.6. Receiving a ChannelData Message . . . . . . . . . . . . . 26
7.7. Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 26
8. New STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 27
9. New STUN Attributes . . . . . . . . . . . . . . . . . . . . . 27
9.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 28
9.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.3. BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . 28
9.4. PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 28
9.5. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
9.6. RELAY-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 28
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9.7. REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 28
9.8. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . . 30
9.9. REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 30
10. New STUN Error Response Codes . . . . . . . . . . . . . . . . 30
11. Client Discovery of TURN Servers . . . . . . . . . . . . . . . 31
12. Security Considerations . . . . . . . . . . . . . . . . . . . 32
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 34
15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
16. Changes from Previous Versions . . . . . . . . . . . . . . . . 35
16.1. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 35
16.2. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 35
17. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
17.1. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 37
17.2. Closed Issues . . . . . . . . . . . . . . . . . . . . . . 39
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 40
19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
19.1. Normative References . . . . . . . . . . . . . . . . . . . 40
19.2. Informative References . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41
Intellectual Property and Copyright Statements . . . . . . . . . . 43
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1. Introduction
NOTE TO THE READER: This document is a work-in-progress. Please see
the list of open and closed issues in Section 17. With only a few
exceptions, if there is an open issue the text has NOT been updated
in this area pending resolution of this issue - keep this in mind
when reading the text. In addition, in the interest of getting the
document out quickly in order to make progress on open issues, the
authors have elected to release the document is a bit more "raw"
state than they would prefer, resulting in some rough spots in the
presentation.
Session Traversal Utilities for NAT (STUN)
[I-D.ietf-behave-rfc3489bis] provides a suite of tools for
facilitating the traversal of NAT. Specifically, it defines the
Binding method, which is used by a client to determine its reflexive
transport address towards the STUN server. The reflexive transport
address can be used by the client for receiving packets from peers,
but only when the client is behind "good" NATs. In particular, if a
client is behind a NAT whose mapping behavior [RFC4787] is address or
address and port dependent (sometimes called "bad" NATs), the
reflexive transport address will not be usable for communicating with
a peer.
The only way to obtain a UDP transport address that can be used for
corresponding with a peer through such a NAT is to make use of a
relay. The relay sits on the public side of the NAT, and allocates
transport addresses to clients reaching it from behind the private
side of the NAT. These allocated transport addresses are from IP
addresses belonging to the relay. When the relay receives a packet
on one of these allocated addresses, the relay forwards it toward the
client.
This specification defines an extension to STUN, called TURN, that
allows a client to request an address on the TURN server, so that the
TURN server acts as a relay. This extension defines a handful of new
STUN methods. The Allocate method is the most fundamental component
of this set of extensions. It is used to provide the client with a
transport address that is relayed through the TURN server. A
transport address which relays through an intermediary is called a
relayed transport address.
Though a relayed transport address is highly likely to work when
corresponding with a peer, it comes at high cost to the provider of
the relay service. As a consequence, relayed transport addresses
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should only be used as a last resort. Protocols using relayed
transport addresses should make use of mechanisms to dynamically
determine whether such an address is actually needed. One such
mechanism, defined for multimedia session establishment protocols
based on the offer/answer protocol in RFC 3264 [RFC3264], is
Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice].
Though originally invented for Voice over IP applications, TURN is
designed to be a general-purpose relay mechanism for NAT traversal.
2. Overview of Operation
This section gives an overview of the operation of TURN. It is non-
normative.
In a typical configuration, a TURN client is connected to a private
network [RFC1918] and through one or more NATs to the public
Internet. On the public Internet is a TURN server. Elsewhere in the
Internet are one or more peers that the TURN client wishes to
communicate with. These peers may or may not be behind one or more
NATs.
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+---------+
| |
| |
/ | Peer A |
Client's TURN // | |
Host Transport Server / | |
Address Address +-+ // +---------+
10.1.1.2:17240 192.0.2.15:3478 |N|/ 192.168.100.2:16400
| | |A|
| +-+ | /|T|
| | | | / +-+
v | | | / 192.0.2.210:18200
+---------+ | | |+---------+ / +---------+
| | |N| || | // | |
| TURN | | | v| TURN |/ | |
| Client |----|A|----------| Server |------------------| Peer B |
| | | |^ | |^ ^| |
| | |T|| | || || |
+---------+ | || +---------+| |+---------+
| || | |
| || | |
+-+| | |
| | |
| | |
Client's | Peer B
Server-Reflexive Relayed Transport
Transport Address Transport Address Address
192.0.2.1:7000 192.0.2.15:9000 192.0.2.210:18200
Figure 1
Figure 1 shows a typical deployment. In this figure, the TURN client
and the TURN server are separated by a NAT, with the client on the
private side and the server on the public side of the NAT. This NAT
is assumed to be a "bad" NAT; for example, it might have a mapping
property of address-and-port-dependent mapping (see [RFC4787]) for a
description of what this means).
The client has allocated a local port on one of its addresses for use
in communicating with the server. The combination of an IP address
and a port is called a TRANSPORT ADDRESS and since this (IP address,
port) combination is located on the client and not on the NAT, it is
called the client's HOST transport address.
The client sends TURN messages from its host transport address to a
transport address on the TURN server which is known as the TURN
SERVER ADDRESS. The client learns the server's address through some
unspecified means (e.g., configuration), and this address is
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typically used by many clients simultaneously. The TURN server
address is used by the client to send both commands and data to the
server; the commands are processed by the TURN server, while the data
is relayed on to the peers.
Since the client is behind a NAT, the server sees these packets as
coming from a transport address on the NAT itself. This address is
known as the client's SERVER-REFLEXIVE transport address; packets
sent by the server to the client's server-reflexive transport address
will be forwarded by the NAT to the client's host transport address.
The client uses TURN commands to allocate a RELAYED transport
address, which is an transport address located on the server. The
server ensures that there is a one-to-one relationship between the
client's server-reflexive transport address and the relayed transport
address; thus a packet received at the relayed transport address can
be unambiguously relayed by the server to the client.
The client will typically communicate this relayed transport address
to one or more peers through some mechanism not specified here (e.g.,
an ICE offer or answer [I-D.ietf-mmusic-ice]). Once this is done,
peers can send data packets to the relayed transport address and the
server will forward them to the client. In the reverse direction,
the client can send data packets to the server (at its TURN server
address) and these will be forwarded by the server to the appropriate
peer, and the peer will see them as coming from the relayed transport
address; in this direction, the client must specify the appropriate
peer.
2.1. Transports
TURN as defined in this specification only allows the use of UDP
between the server and the peer. However, this specification allows
the use of any one of UDP, TCP, or TLS over TCP to carry the TURN
messages between the client and the server.
+----------------------------+---------------------+
| TURN client to TURN server | TURN server to peer |
+----------------------------+---------------------+
| UDP | UDP |
| TCP | UDP |
| TLS over TCP | UDP |
+----------------------------+---------------------+
For TURN clients, using TLS over TCP to communicate with the TURN
server provides two benefits. First, the client can be assured that
the addresses of its peers are not visible to any attackers between
it and the server. Second, the client may be able to communicate
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with TURN servers using TLS when it would not be able to communicate
with the same server using TCP or UDP, due to the policy of a
firewall between the TURN client and its server. In this second
case, TLS between the client and TURN server facilitates traversal.
There is a planned extension to TURN to add support for TCP between
the server and the peers [I-D.ietf-behave-turn-tcp]. For this
reason, allocations that use UDP between the server and the peers are
known as UDP allocations, while allocations that use TCP between the
server and the peers are known as TCP allocations. This
specification describes only UDP allocations.
2.2. Allocations
To allocate a relayed transport address, the client uses an Allocate
transaction. The client sends a Allocate Request to the server, and
the server replies with an Allocate Response containing the allocated
relayed transport address. The client can include attributes in the
Allocate Request that describe the type of allocation it desires
(e.g., the lifetime of the allocation). And since relaying data can
require lots of bandwidth, the server may require that the client
authenticate itself using STUN's long-term credential mechanism, to
show that it is authorized to use the server.
Once a relayed transport address is allocated, a client must keep the
allocation alive. This is done by the client periodically doing a
Refresh transaction with the server, where the client includes the
allocated relayed transport address in the Refresh Request. TURN
deliberately uses a different method (Refresh rather than Allocate)
for refreshes to ensure that the client is informed if the allocation
vanishes for some reason.
The frequency of the Refresh transaction is determined by the
lifetime of the allocation. The client can request a lifetime in the
Allocate Request and may modify its request in a Refresh Request, and
the server always indicates the actual lifetime in the response. The
client must issue a new Refresh transaction within 'lifetime' seconds
of the previous Allocate or Refresh transaction. If a client no
longer wishes to use an Allocation, it should do a Refresh
transaction with a requested lifetime of 0.
Note that sending or receiving data from a peer DOES NOT refresh the
allocation.
The server remembers the 5-tuple used in the Allocate Request.
Subsequent transactions between the client and the server use this
same 5-tuple. In this way, the server knows which client owns the
allocated relayed transport address. If the client wishes to
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allocate a second relayed transport address, it must use a different
5-tuple for this allocation (e.g., by using a different client host
address).
While the terminology used in this document refers to 5-tuples,
the TURN server can store whatever identifier it likes that yields
identical results. Specifically, many implementations use a file-
descriptor in place of a 5-tuple to represent a TCP connection.
2.3. Exchanging Data with Peers
The client can use the relayed transport address to exchange data
with its peers by using Send and Data indications. A Send Indication
is sent from a client to the TURN server and contains, in attributes
inside the message, the transport address of the peer and the data to
be sent to that peer. When the TURN server receives the Send
Indication, it extracts the data from the Send Indication and sends
it in a UDP datagram to the peer, using the allocated relay address
as the source address. In the reverse direction, UDP datagrams
arriving at the relay address on the TURN server are converted into
Data Indications and sent to the client, with the transport address
of the peer included in an attribute in the Data Indication.
Note that a client can use a single relayed transport address to
exchange data with multiple peers at the same time.
TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
Figure 2
In the figure above, the client first allocates a relayed transport
address. It then sends data to Peer A using a Send Indication; at
the server, the data is extracted and forwarded in a UDP datagram to
Peer A, using the relayed transport address as the source transport
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address. When a UDP datagram from Peer A is received at the relayed
transport address, the contents are placed into a Data Indication and
forwarded to the client. A similar exchange happens with Peer B.
2.4. Permissions
To ease concerns amongst enterprise IT administrators that TURN could
be used to bypass corporate firewall security, TURN includes the
notion of permissions. TURN permissions mimic the address-restricted
filtering mechanism of NATs that comply with [RFC4787].
A TURN server will drop a UDP datagram arriving at a relayed
transport address from a peer unless the client has recently sent
data to a peer with the same IP address (the port numbers can
differ). See the normative description for the precise definition of
"recently".
A permission will timeout if not refreshed periodically. The client
refreshes a permission by sending data to the corresponding peer.
Data received from the peer DOES NOT refresh the permission.
2.5. Channels
In some applications, the overhead of using Send and Data indications
can be substantial. For example, for applications like VoIP which
utilize small packets, Send and Data Indications, with 36 bytes of
overhead, can have a substantial impact on overall bandwidth usage.
To remedy this, TURN clients can assign a CHANNEL to a peer. Data to
and from such a peer can then be sent using an alternate packet
format that adds only 4 bytes per packet of overhead.
The alternate packet format is known as the ChannelData message. The
ChannelData message does not use the STUN header used by other TURN
messages, but instead has a 4-byte header that includes a number
known as a channel number.
To create a channel, the client sends a ChannelBind request to the
server, and includes an unallocated channel number and the transport
address of the peer. Once the client receives the response to the
ChannelBind request, it can send data to that peer using a
ChannelData message. Similarly, once the server has received the
request, it can relay data from that peer towards the client using a
ChannelData message. There is no way to modify channel bindings, so
once a channel is bound to a peer, it remains bound for the lifetime
of the allocation.
When the server receives a ChannelData message from the client, it
uses the channel number to determine the destination peer and then
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forwards the data inside a UDP datagram to the peer. In the reverse
direction, when a UDP datagram arives at the relayed transport
address from that peer, the server inserts it into a ChannelData
message containing the channel number bound to that peer; in this way
the client can determine the peer that send the UDP datagram.
TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|- ChannelBind Req -->| | |
| (Peer A to 0x4001) | | |
| | | |
|<- ChannelBind Resp -| | |
| | | |
|-- [0x4001] data --->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<- [0x4001] data --->| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
Figure 3
The figure above shows the channel mechanism in use. The client
begins by allocating a relayed transport address, and then uses that
address to exchange data with Peer A. After a bit, the client decides
to bind a channel to Peer A. To do this, it sends a ChannelBind
Request to the server, specifying the transport address of Peer A and
a channel number (0x4001). After that, the client can send
application data encapsulated inside ChannelData messages to Peer A:
this is shown as "[0x4001] data" where 0x4001 is the channel number.
Note that ChannelData messages can only be used for peers to which
the client has bound a channel. In the example above, Peer A has
been bound to a channel, but Peer B has not, so application data to
and from Peer B uses Send and Data indications.
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Channel bindings are always initiated by the client.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Readers are expected to be familar with [I-D.ietf-behave-rfc3489bis]
and the terms defined there.
The following terms are used in this document:
TURN: A protocol spoken between a TURN client and a TURN server. It
is an extension to the STUN protocol [I-D.ietf-behave-rfc3489bis].
The protocol allows a client to allocate and use a relayed
transport address.
TURN client: A STUN client that implements this specification.
TURN server: A STUN server that implements this specification. It
relays data between a TURN client and its peer(s).
Peer: A host with which the TURN client wishes to communicate. The
TURN server relays traffic between the TURN client and its
peer(s). The peer does not interact with the TURN server using
the protocol defined in this document; rather, the peer receives
data sent by the TURN server and the peer sends data towards the
TURN server.
Host Transport Address: A transport address allocated on a host.
Server-Reflexive Transport Address: A transport address on the
"public side" of a NAT. This address is allocated by the NAT to
correspond to a specific host transport address.
Relayed Transport Address: A transport address that exists on a TURN
server. If a permission exists, packets that arrive at this
address are relayed towards the TURN client.
Allocation: The transport address granted to a client through an
Allocate request, along with related state, such as permissions
and expiration timers. See also Relayed Transport Address.
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5-tuple: A combination of the source IP address and port,
destination IP address and port, and transport protocol (UDP or
TCP). A 5-tuple uniquely identifies a TCP connection or the bi-
directional flow of UDP datagrams.
Permission: The IP address and transport protocol (but not the port)
of a peer that is permitted to send traffic to the TURN server and
have that traffic relayed to the TURN client. The TURN server
will only forward traffic to its client from peers that match an
existing permission.
4. General Behavior
After the initial Allocate transaction, all subsequent TURN
transactions need to be sent in the context of a valid allocation.
The source and destination IP address and ports for these TURN
messages MUST match the those used in the initial Allocate Request.
These are processed using the general server procedures in
[I-D.ietf-behave-rfc3489bis] with a few important additions. For
requests, if there is no matching allocation, the server MUST
generate a 437 (Allocation Mismatch) error response. For
indications, if there is no matching allocation, the indication is
silently discarded. An Allocate request MUST NOT be sent by a client
within the context of an existing allocation. Such a request MUST be
rejected by the server with a 437 (Allocation Mismatch) error
response.
A subsequent request MUST be authenticated using the same username,
password and realm as the one used in the Allocate request that
created the allocation. If the request was authenticated but not
with the matching credential, the server MUST reject the request with
a 401 (Unauthorized) error response.
When a server returns an error response, it MAY include an ALTERNATE-
SERVER attribute if it has positive knowledge that the problem
reported in the error response will not be a problem on the alternate
server. For example, a 443 response (Invalid IP Address) with an
ALTERNATE-SERVER means that the other server is responsible for that
IP address. A 442 (Unsupported Transport Protocol) with this
attribute means that the other server is known to support that
transport protocol. A 507 (Insufficient Capacity) means that the
other server is known to have sufficient capacity. Using the
ALTERNATE-SERVER mechanism in the 507 (Insufficient Capacity)
response can only be done if the rejecting server has definitive
knowledge of available capacity on the target. This will require
some kind of state sharing mechanism between TURN servers, which is
beyond the scope of this specification. If a TURN server attempts to
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redirect to another server without knowledge of available capacity,
it is possible that all servers are in a congested state, resulting
in series of rejections that only serve to further increase the load
on the system. This can cause congestion collapse.
If a client sends a request to a server and gets a 500 class error
response without an ALTERNATE-SERVER, or the STUN transaction times
out without a response, and the client was utilizing the SRV
procedures of [I-D.ietf-behave-rfc3489bis] to contact the server, the
client SHOULD try another server based on those procedures. However,
the client SHOULD cache the fact that the request to this server
failed, and not retry that server again for a configurable period of
time. Five minutes is RECOMMENDED.
TURN clients and servers MUST NOT include the FINGERPRINT attribute
in any of the methods defined in this document.
5. Managing Allocations
Communications between a TURN client and a TURN server begin with an
Allocate transaction. All subsequent transactions happen in the
context of that allocation, and happen on the same 5-tuple. The
client refreshes allocations and deallocates them using a Refresh
transaction.
5.1. Client Behavior
5.1.1. Initial Allocate Requests
When a client wishes to obtain a transport address, it sends an
Allocate request to the server. This request is constructed and sent
using the general procedures defined in [I-D.ietf-behave-rfc3489bis].
Clients MUST implement the long term credential mechanism defined in
[I-D.ietf-behave-rfc3489bis], and be prepared for the server to
demand credentials for requests.
The client SHOULD include a BANDWIDTH attribute, which indicates the
maximum bandwidth that will be used with this binding. If the
maximum is unknown, the attribute is not included in the request.
The client MAY request a particular lifetime for the allocation by
including it in the LIFETIME attribute in the request.
The client MUST include a REQUESTED-TRANSPORT attribute. In this
specification, the REQUESTED-TRANSPORT MUST always be UDP. This
attribute is included to allow for future extensions to TURN (e.g.,
[I-D.ietf-behave-turn-tcp])
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The client MAY include a REQUESTED-PORT-PROPS or REQUESTED-IP
attribute in the request to obtain specific types of transport
addresses, if desired.
Processing of the response follows the general procedures of
[I-D.ietf-behave-rfc3489bis]. A successful response will include
both a RELAY-ADDRESS and an XOR-MAPPED-ADDRESS attribute, providing
both a relayed transport address and a reflexive transport address,
respectively, to the client. The value of the LIFETIME attribute in
the response indicates the amount of time after which the server will
expire the allocation, if not refreshed with a Refresh request. The
server will allow the user to send and receive at least the amount of
data indicated in the BANDWIDTH attribute per allocation. (At its
discretion the server can optionally discard UDP data above this
threshold.)
If the response is an error response and contains a 442, 443 or 444
error code, the client knows that its requested properties could not
be met. The client MAY retry with different properties, with the
same properties (in a hope that something has changed on the server),
or give up, depending on the needs of the application. However, if
the client retries, it SHOULD wait 500ms, and if the request fails
again, wait 1 second, then 2 seconds, and so on, exponentially
backing off.
5.1.2. Refresh Requests
TURN permissions are kept alive by traffic flowing through them, and
persist for the lifetime of the allocation. However, The allocations
themselves have to be kept alive through Refresh Requests.
Before 3/4 of the lifetime of the allocation has passed (the lifetime
of the allocation is conveyed in the LIFETIME attribute of the
Allocate Response), the client SHOULD refresh the allocation with a
Refresh transaction if it wishes to keep the allocation.
To perform a refresh, the client generates a Refresh Request. The
client MUST use the same username, realm and password for the Refresh
request as it used in its initial Allocate Request. The Refresh
request MAY contain a proposed LIFETIME attribute. The client MAY
include a BANDWIDTH attribute if it wishes to request more or less
bandwidth than in the original request (this might also be the first
time the TURN client indicates bandwidth to the TURN server). If the
BANDWIDTH attribute is absent, it indicates no change in the
requested bandwidth from the Allocate request. The client MUST NOT
include a REQUESTED-IP, REQUESTED-TRANSPORT, or REQUESTED-PORT-PROPS
attribute in the Refresh request.
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In a successful response, the LIFETIME attribute indicates the amount
of additional time (the number of seconds after the response is
received) that the allocation will live without being refreshed. A
successful response will also contain a BANDWIDTH attribute,
indicating the bandwidth the server is allowing for this allocation.
Note that an error response does not imply that the allocation has
expired, just that the refresh has failed.
If a client no longer needs an allocation, it SHOULD perform an
explicit deallocation. If the client wishes to explicitly remove the
allocation because it no longer needs it, it sends a Refresh request,
but sets the LIFETIME attribute to zero. This will cause the server
to remove the allocation, and all associated permissions and channel
numbers. For connection-oriented transports such as TCP, the client
can also remove the allocation (and all associated bindings) by
closing the relevant connection with the TURN server.
5.2. Server Behavior
5.2.1. Receiving an Allocate Request
When the server receives an Allocate request, the server attempts to
allocate a relayed transport address.
When the server receives the Allocate Request, it begins by
processing it according to the base protocol procedures described in
[I-D.ietf-behave-rfc3489bis], plus the Long-Term Credential Mechanism
procedures if the server is using this mechanism.
It then checks if the 5-tuple used for the Allocate Request matches
the 5-tuple used for an existing allocation. If there is a match,
then:
o If the transport protocol is UDP, and the transaction id in the
request message matches the transaction id used for the original
allocation, then the server treats this as a retransmission of the
original request, and replies with the same response as it did to
the original request. The server may do this by either storing
its original response and resending it, or by rebuilding its
original response from other state data.
o If the transport protocol is not UDP, or if the transaction id in
the request message does not match the transaction id used for the
original allocation, then the server replies with an error
response containing the error code 437 Allocation Mismatch.
If the 5-tuple does not match an existing allocation, then processing
continues as described below.
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5.2.1.1. BANDWIDTH
The server checks for the BANDWIDTH attribute in the request. If
present, the server determines whether or not it has sufficient
capacity to handle a binding that will generate the requested
bandwidth.
If it does, the server attempts to allocate a transport address for
the client. The Allocate Request can contain several additional
attributes that allow the client to request specific characteristics
of the transport address. If it doesn't, it sends an error response.
5.2.1.2. REQUESTED-TRANSPORT
The server checks for the REQUESTED-TRANSPORT attribute. This
indicates the transport protocol requested by the client. This
specification defines a value for UDP only, but support for TCP
allocations is planned in [I-D.ietf-behave-turn-tcp].
As a consequence of the REQUESTED-TRANSPORT attribute, it is
possible for a client to connect to the server over TCP or TLS
over TCP and request a UDP transport address. In this case, the
server will relay data between the transports.
If the requested transport is supported, the server allocates a port
using the requested transport protocol. If the REQUESTED-TRANSPORT
attribute contains a value of the transport protocol unknown to the
server, or known to the server but not supported by the server in the
context of this request, the server MUST reject the request and
include a 442 (Unsupported Transport Protocol) in the response. If
the request did not contain a REQUESTED-TRANSPORT attribute, the
server MUST use the same transport protocol as the request arrived
on.
5.2.1.3. REQUESTED-IP
The server checks for the REQUESTED-IP attribute. If present, it
indicates a specific IP address from which the client would like its
transport address allocated. (The client could do this if it
requesting the second address in a specific port pair). If this IP
address is not a valid one for allocations on the server, the server
MUST reject the request and include a 443 (Invalid IP Address) error
code in the response, or else redirect the request to a server that
is known to support this IP address. If the IP address is one that
is valid for allocations (presumably, the server is configured to
know the set of IP addresses from which it performs allocations), the
server MUST provide an allocation from that IP address. If the
attribute is not present, the selection of an IP address is at the
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discretion of the server.
5.2.1.4. REQUESTED-PORT-PROPS
The server checks for the REQUESTED-PORT-PROPS attribute. If
present, it indicates specific port properties desired by the client.
This attribute is split into two portions: one portion for port
behavior and the other for requested port alignment (whether the
allocated port is odd, even, reserved as a pair, or at the discretion
of the server).
If the port behavior requested is for a Specific Port, the server
MUST attempt to allocate that specific port for the client. If the
specific port is not available (in use or reserved), the server MUST
reject the request with a 444 (Invalid Port) response. For example,
the STUN server could reject a request for a Specific Port because
the port is temporarily reserved as part of an adjacent pair of
ports, or because the requested port is a well-known port (1-1023).
If the client requests "even" port alignment, the server MUST attempt
to allocate an even port for the client. If an even port cannot be
obtained, the server MUST reject the request with a 444 (Invalid
Port) response or redirect to an alternate server. If the client
requests odd port alignment, the server MUST attempt to allocate an
odd port for the client. If an odd port cannot be obtained, the
server MUST reject the request with a 444 (Invalid Port) response or
redirect to an alternate server. Finally, the "Even port with hold
of the next higher port" alignment is similar to requesting an even
port. It is a request for an even port, and MUST be rejected by the
server if an even port cannot be provided, or redirected to an
alternate server. However, it is also a hint from the client that
the client will request the next higher port with a separate Allocate
request. As such, it is a request for the server to allocate an even
port whose next higher port is also available, and furthermore, a
request for the server to not allocate that one higher port to any
other request except for one that asks for that port explicitly. The
server can honor this request for adjacency at its discretion. The
only constraint is that the allocated port number MUST be even.
Port alignment requests exist for compatibility with
implementations of RTP which predate [RFC3550]. These
implementations use the port numbering conventions in (now
obsolete) [RFC1889].
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5.2.1.5. Lifetime
The server checks for a LIFETIME attribute. If present, it indicates
the lifetime the client would like the server to assign to the
allocation.
If the LIFETIME attribute is malformed, or if the requested lifetime
value is less than 32 seconds, the server replies with an error
response with an error code of XXX Lifetime Malformed or Invalid.
5.2.1.6. Creating the Allocation
If any of the requested or desired constraints cannot be met, whether
it be bandwidth, transport protocol, IP address or port, the server
can redirect the client to a different server that may be able to
fulfill the request. This is accomplished using the 300 error
response and ALTERNATE-SERVER attribute. If the server does not
redirect and cannot service the request because the server has
reached capacity, it sends a 507 (Insufficient Capacity) response.
The server can also reject the request with a 486 (Allocation Quota
Reached) if the user or client is not authorized to request
additional allocations.
The server SHOULD only allocate ports from the range 49152 - 65535
(the Dynamic and/or Private Port range [Port-Numbers]), unless the
TURN server application knows, through some means not specified here,
that other applications running on the same host as the TURN server
application will not be impacted by allocating ports outside this
range. This condition can often be satisfied by running the TURN
server application on a dedicated machine and/or by arranging that
any other applications on the machine allocate ports before the TURN
server application starts. In any case, the TURN server SHOULD NOT
allocate ports in the range 0 - 1023 (the Well-Known Port range) to
discourage clients from using TURN to run standard services.
Once a port is allocated, the server associates the allocation with
the 5-tuple used to communicate between the client and the server.
For TCP, this amounts to associating the TCP connection from the TURN
client with the allocated transport address.
The new allocation MUST also be associated with the username,
password and realm used to authenticate the request. These
credentials are used in all subsequent requests to ensure that only
the same client can use or modify the allocation it was given.
In addition, the allocation created by the server is associated with
a set of permissions and a set of channel bindings. Each set is
initially empty.
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If the LIFETIME attribute was present in the request, and the value
is larger than the maximum duration the server is willing to use for
the lifetime of the allocation, the server MAY lower it to that
maximum. However, the server MUST NOT increase the duration
requested in the LIFETIME attribute. If there was no LIFETIME
attribute, the server may choose a duration at its discretion. Ten
minutes is RECOMMENDED. In either case, the resulting duration is
added to the current time, and a timer, called the allocation
expiration timer, is set to expire at or after that time. Note that
the LIFETIME attribute in an Allocate request can be zero, though
this is effectively a no-op, since it will create and destroy the
allocation in one transaction.
5.2.1.7. Sending the Allocate Response
Once the port has been obtained and the allocation expiration timer
has been started, the server generates an Allocate Response using the
general procedures defined in [I-D.ietf-behave-rfc3489bis], including
the ones for long term authentication. The transport address
allocated to the client MUST be included in the RELAY-ADDRESS
attribute in the response. In addition, this response MUST contain
the XOR-MAPPED-ADDRESS attribute. This allows the client to
determine its reflexive transport address in addition to a relayed
transport address, from the same Allocate request.
The server MUST add a LIFETIME attribute to the Allocate Response.
This attribute contains the duration, in seconds, of the allocation
expiration timer associated with this allocation.
The server MUST add a BANDWIDTH attribute to the Allocate Response.
This MUST be equal to the attribute from the request, if one was
present. Otherwise, it indicates a per-allocation limit that the
server is placing on the bandwidth usage on each binding. Such
limits are needed to prevent against denial-of-service attacks (see
Section 12).
5.2.2. Refresh Requests
A Refresh request is processed using the general server and long term
authentication procedures in [I-D.ietf-behave-rfc3489bis]. It is
used to refresh and extend an allocation, or to cause an immediate
deallocation. It is processed as follows.
First, the request MUST be authenticated using the same shared secret
as the one associated with the allocation. If the request was
authenticated but not with such a matching credential, the server
MUST generate a Refresh Error Response with a 401 response.
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If the Refresh request contains a BANDWIDTH attribute, the server
checks that it can relay the requested volume of traffic.
Finally, a Refresh Request will set a new allocation expiration timer
for the allocation, effectively canceling the previous allocation
expiration timer. As with an Allocate request, the server MAY
utilize a shorter allocation lifetime, but MUST NOT utilize a longer
lifetime.
A success Refresh response MUST contain a LIFETIME attribute. If its
associated Allocate request contained the BANDWIDTH attribute, or
this Refresh request contained a new BANDWIDTH attribute, the
response MUST also contain the BANDWIDTH attribute.
6. Send and Data Indications
TURN supports two ways to send and receive data from peers. This
section describes the use of Send and Data indications, while
Section 7 describes the use of the Channel Mechanism.
6.1. Forming and Sending an Indication
When the client has data to send to a peer, it uses a Send Indication
to pass the data to the server. When the server has data to send to
the client, it uses a Data Indication to pass the data to the client.
A client can also use a Send Indication without a DATA attribute to
install or refresh a permission for the specified IP address. Both
indications are formed following the general rules described in [ref
3489bis] with the extra considerations described below.
A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
contain a DATA attribute, while a Data Indication MUST contain both
attributes. The PEER-ADDRESS attribute contains the transport
address of the peer to which the data is to be sent (in the case of a
Send Indication) or from which the data was received (in the case of
a Data Indication). This peer address is the transport address of
the peer as seen by the server, which may not be the same as the host
transport address of the peer. The DATA attribute contains the
actual application data. Note that the application data may need to
be padded to ensure the DATA attribute length is a multiple of 4.
No other attributes are included. For example, neither the
FINGERPRINT attribute nor any authentication attributes are included.
The latter holds even if the server is using the Long-Term Credential
Mechanism, since indications cannot be authenticated using this
mechanism.
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Both the Send and Data indications MUST be sent using the 5-tuple of
the original allocation. Thus, in the case of the Send Indication,
the source transport address is the client's host transport address,
the destination transport address is the TURN server address, and the
transport protocol is the same as was used for the Allocate request.
For the Data Indication, the source and destination transport
addresses are the reverse.
6.2. Receiving an Indication
When a Send Indication is received at the server, or a Data
Indication is received at the client, the receiver first does the
basic indication processing described in [3489bis]. Once this is
done, it does the processing specific to the Send and Data methods
described below.
A Send Indication MUST contain a PEER-ADDRESS attribute and MAY
contain a DATA attribute, while a Data Indication MUST contain both
attributes. Any other attributes appearing in the message are
treated as unexpected.
TODO: Add check that Send or Data indication arrives with
appropriate 5-tuple. Since this check applies to all STUN
messages, not just Send and Data indications, perhaps this goes
under the general processing section.
6.3. Relaying
When the server receives a valid Send Indication contains a DATA
attribute, it forms a UDP datagram as follows:
o the source transport address is the relayed transport address of
the allocation, where the allocation is determined by the 5-tuple
on which the Send Indication arrived;
o the destination transport address is taken from the PEER-ADDRESS
attribute;
o the data following the UDP header is the contents of the value
field of the DATA attribute;
o the Length field in the UDP header is set to the Length field of
the DATA attribute;
o the Checksum field in the UDP header is computed as described in
[RFC 768].
The resulting UDP datagram is then sent to the peer.
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When the server receives a valid Send Indication (with or without a
DATA attribute), it also updates the permission associated with the
IP address contained in the PEER-ADDRESS attribute. For a certain
interval after the permission is updated, UDP datagrams received from
peers with source IP address equal to the IP address contained in the
PEER-ADDRESS attribute can be forwarded to the client. Note that
only the IP addresses are considered and the port numbers are
irrelevent. This permission is specific to the allocation and has no
affect on any other allocation. The recommended length of time is 60
seconds from when the Send Indication is received.
When the server receives a UDP datagram with a destination transport
address corresponding to an active (i.e., still alive) allocation,
then it first checks to see if it is permitted to relay the datagram.
If it is not permitted, the UDP datagram MUST be discarded.
If relaying is permitted, the server forms and send a Data Indication
as described in Section 6.1, using the data following the UDP header
as the application data.
7. Channel Mechanism
As described in the overview, channel mechanism provides a way for a
client and server to send application data using ChannelData
messages, which have less overhead than Send and Data indications.
Channel bindings are always initiated by the client. The client can
bind a channel to a peer at any time during the lifetime of the
allocation. The client may bind a channel to a peer before
exchanging data with it, or after exchanging data with it (using Send
and Data indications) for some time, or may choose never to bind a
channel it. The client can also bind channels to some peers while
not binding channels to other peers.
Once a channel is bound to a peer, the channel binding cannot be
changed. There is no way to unbind a channel or bind it to a
different peer.
Channel bindings are specific to an allocation, so that a binding in
one allocation has no relationship to a binding in any other
allocation. If an allocation expires, all its channel bindings
expire with it.
7.1. Forming and Sending a ChannelBind Request
When a client wishes to bind a channel to a peer in an allocation, it
forms a ChannelBind Request. The Request formed following the
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general rules described in [I-D.ietf-behave-rfc3489bis] with the
extra considerations described below.
A ChannelBind Request MUST contain both a CHANNEL-NUMBER attribute
and a PEER-ADDRESS attribute. The CHANNEL-NUMBER attribute specifies
the number of the channel that the client wishes to bind to the peer.
The channel number MUST be in the range 0x4000 to 0xFFFE (inclusive)
and the channel MUST NOT be already bound to a different peer. It is
acceptable to rebind a channel to the peer it is already bound to.
The PEER-ADDRESS attribute specifies the peer address to bind the
channel to.
Once formed, the ChannelBind Request is sent using the 5-tuple for
the allocation.
The client SHOULD be prepared to receive ChannelData messages on the
channel as soon as it has sent the ChannelBind Request. Over UDP, it
is possible for the client to receive these before it receives a
ChannelBind Success Response.
Over UDP, the client SHOULD NOT send ChannelData messages on the
channel until it has received a ChannelBind Success Response for the
binding attempt. Sending them before the success response is
received risks having them dropped by the server if he ChannelBind
Request was lost.
7.2. Receiving a ChannelBind Request and Sending a Response
When the server receives a ChannelBind Request, it first does the
basic request processing described in [I-D.ietf-behave-rfc3489bis].
Once this is done, it does the processing specific to the ChannelBind
method described below.
The server checks that the ChannelBind Request contains both a
CHANNEL-NUMBER attribute and a PEER-ADDRESS attribute. If the PEER-
ADDRESS attribute is missing or malformed, then the server rejects
the request with an Error Response containing the error code XXX
"Peer address missing or invalid". If the CHANNEL-NUMBER attribute
is missing or malformed, or the channel number is not in the range
0x4000 to 0xFFFE (inclusive), or the channel is already bound to
another peer (already bound to the same peer is OK) the server
rejects the request with an Error Response containing the error code
XXX "Channel number missing or invalid". Otherwise, if no errors are
detected, the server replies with a ChannelBind Success Response.
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7.3. Receiving a ChannelBind Response
When the client receives a ChannelBind response (either success or
error), it processes it as specified in [3489bis]. Any additional
processing is implementation specific.
7.4. The ChannelData Message
The ChannelData message is used to carry application data between the
client and the server. It has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Application Data /
/ /
| |
| +-------------------------------+
| |
+-------------------------------+
The Channel Number field specifies the number of the channel on which
the data is traveling, and thus the address of the peer that is
sending or is to receive the data. The channel number MUST be in the
range 0x4000 - 0xFFFF, with channel number 0xFFFF being reserved for
possible future extensions.
Channel numbers 0x0000 - 0x3FFF cannot be used because bits 0 and 1
are used to distinguish ChannelData messages from STUN-formatted
messages (i.e., Allocate, Send, Data, ChannelBind, etc). STUN-
formatted messages always have bits 0 and 1 as "00", while
ChannelData messages use combinations "01", "10", and "11".
The Length field specifies the length in bytes of the application
data field (i.e., it does not include the size of the ChannelData
header). Note that 0 is a valid length.
The Application Data field carries the data the client is trying to
send to the peer, or that the peer is sending to the client.
7.5. Forming and Sending a ChannelData Message
Once a client has bound a channel to a peer, then when the client has
data to send to that peer it may use either a ChannelData message or
a Send Indication; that is, the client is not obligated to use the
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channel when it exists and may freely intermix the two message types
when sending data to the peer. The server, on the other hand, SHOULD
use the ChannelData message if a channel has been bound to the peer.
The fields of the ChannelData message are filled in as described in
Section 7.4.
Over stream transports, the ChannelData message MUST be padded to a
multiple of four bytes in order to ensure the alignment of subsequent
messages. The padding is not reflected in the length field of the
ChannelData message, so the actual size of a ChannelData message
(including padding) is (4 + Length) rounded up to the nearest
multiple of 4. Over UDP, the padding is not required but MAY be
included.
The ChannelData message is then sent on the 5-tuple associated with
the allocation.
7.6. Receiving a ChannelData Message
The receiver of the ChannelData message uses bits 0 and 1 to
distinguish it from STUN-formatted messages, as described in
Section 7.4.
If the ChannelData message is received in a UDP datagram, and if the
UDP datagram is too short to contain the claimed length of the
ChannelData message (i.e., the UDP header length field value is less
than the ChannelData header length field value + 4 + 8), then the
message is silently discarded.
If the ChannelData message is received over TCP or over TLS over TCP,
then the actual length of the ChannelData message is as described in
Section 7.5.
If the ChannelData message is received on a channel which is not
bound to any peer, then the message is silently discarded.
7.7. Relaying
When the server receives a valid ChannelData message, it forms a UDP
datagram as follows: the source transport address is the relayed
transport address of the allocation, where the allocation is
determined by the 5-tuple on which the ChannelData message arrived;
the destination transport address is the peer address to which the
channel is bound; the data following the UDP header is the contents
of the data field of the ChannelData message; the Length field in the
UDP header is set to the Length field of the ChannelData message + 8;
and the Checksum field in the UDP header is computed as described in
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[RFC 768]. The resulting UDP datagram is then sent to the peer.
The server also updates the permission associated with the IP address
part of the peer address to which the UDP datagram is sent.
When the server receives a UDP datagram with a destination transport
address corresponding to an active (i.e., still alive) allocation,
then it first checks to see if it is permitted to relay the datagram.
If the allocation contains an active permission for the source IP
address (from the IP header) of the received UDP datagram, then the
UDP datagram is permitted. Otherwise, the UDP datagram MUST be
discarded.
To relay the UDP datagram, the server forms and send a ChannelData
message as described in Section 7.5
8. New STUN Methods
This section lists the codepoints for the new STUN methods defined in
this specification. See elsewhere in this document for the semantics
of these new methods.
Request/Response Transactions
0x003 : Allocate
0x004 : Refresh
Indications
0x006 : Send
0x007 : Data
9. New STUN Attributes
This STUN extension defines the following new attributes:
0x000C: CHANNEL-NUMBER
0x000D: LIFETIME
0x0010: BANDWIDTH
0x0012: PEER-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
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9.1. CHANNEL-NUMBER
The CHANNEL-NUMBER attribute contains the number of the channel. It
is a 16-bit unsigned integer, followed by a two-octet RFFU field
which MUST be set to 0 on transmission and ignored on reception.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2. LIFETIME
The lifetime attribute represents the duration for which the server
will maintain an allocation in the absence of a refresh. It is a 32
bit unsigned integral value representing the number of seconds
remaining until expiration.
9.3. BANDWIDTH
The bandwidth attribute represents the peak bandwidth, measured in
kilobits per second, that the client expects to use on the allocation
in each direction.
9.4. PEER-ADDRESS
The PEER-ADDRESS specifies the address and port of the peer as seen
from the TURN server. It is encoded in the same way as XOR-MAPPED-
ADDRESS.
9.5. DATA
The DATA attribute is present in most Send Indications and Data
Indications. It contains raw payload data that is to be sent (in the
case of a Send Request) or was received (in the case of a Data
Indication).
9.6. RELAY-ADDRESS
The RELAY-ADDRESS is present in Allocate responses. It specifies the
address and port that the server allocated to the client. It is
encoded in the same way as XOR-MAPPED-ADDRESS.
9.7. REQUESTED-PORT-PROPS
This attribute allows the client to request certain properties for
the port that is allocated by the server. The attribute can be used
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with any transport protocol that has the notion of a 16 bit port
space (including TCP and UDP). The attribute is 32 bits long. Its
format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved = 0 | A | Specific Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The two bits labeled A in the diagram above are for requested port
alignment and have the following meaning:
00 no specific port alignment
01 odd port number
10 even port number
11 even port number; reserve next higher port
If the value of the A field is 00 (no specific port alignment), then
the Specific Port Number field can either be 0 or some non-zero port
number. If the Specific Port Number field is 0, then the client is
not putting any restrictions on the port number it would like
allocated. If the Specific Port Number is some non-zero port number,
then the client is requesting that the server allocate the specified
port and the server MUST provide that port.
If the value of the A field is 01 (odd port number), then the
Specific Port Number field MUST be zero, and the client is requesting
the server allocate an odd-numbered port. The server MUST provide an
odd port number.
If the value of the A field is 10 (even port number), then the
Specific Port number field MUST be zero, and the client is requesting
the server allocate an even-numbered port. The server MUST provide
an even port number.
If the value of the A field is 11 (even port number; reserve next
higher port), then the Specific Port Number field MUST be zero, and
the client is requesting the server allocate an even-numbered port.
The server MUST return an even port number. In addition, the client
is requesting the server reserve the next higher port (i.e., N+1 if
the server allocates port N). The server SHOULD only allocate the
N+1 port number if it is explicitly requested (with a subsequent
request specifying that exact port number by the same TURN client,
over a different alllocation).
In all cases, if a port with the requested properties cannot be
allocated, the server MUST respond with a error response with an
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error code of 444 (Invalid Port).
9.8. REQUESTED-TRANSPORT
This attribute is used by the client to request a specific transport
protocol for the allocated transport address. It has the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol field specifies the desired protocol. The codepoints
used in this field are taken from those allowed in the Protocol field
in the IPv4 header and the NextHeader field in the IPv6 header
[Protocol-Numbers]. This specification only allows the use of
codepoint 17 (User Datagram Protocol).
The RFFU field is set to zero on transmission and ignored on
receiption. It is reserved for future uses.
9.9. REQUESTED-IP
The REQUESTED-IP attribute is used by the client to request that a
specific IP address be allocated by the TURN server. This attribute
is needed since it is anticipated that TURN servers will be multi-
homed so as to be able to allocate more than 64k transport addresses.
As a consequence, a client needing a second transport address on the
same interface as a previous one can use this attribute to request a
remote address from the same TURN server interface as the TURN
client's previous remote address.
The format of this attribute is identical to XOR-MAPPED-ADDRESS.
However, the port component of the attribute MUST be ignored by the
server. If a client wishes to request a specific IP address and
port, it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS
attributes.
10. New STUN Error Response Codes
This document defines the following new error response codes:
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437 (Allocation Mismatch): A request was received by the server that
requires an allocation to be in place, but there is none, or a
request was received which requires no allocation, but there is
one.
442 (Unsupported Transport Protocol): The Allocate request asked for
a transport protocol to be allocated that is not supported by the
server. If the server is aware of another server that supports
the requested protocol, it SHOULD include the other server's
address in an ALTERNATE-SERVER attribute in the error response.
443 (Invalid IP Address): The Allocate request asked for a transport
address to be allocated from a specific IP address that is not
valid on the server.
444 (Invalid Port): The Allocate request asked for a port to be
allocated that is not available on the server.
486 (Allocation Quota Reached): The user or client is not authorized
to request additional allocations.
(tbd) (Channel Number Missing or Invalid): The request requires a
channel number, but the CHANNEL-NUMBER attribute is missing, or
the specified channel number is invalid in some way.
(tbd) (Peer Address Missing or Invalid): The request requires a peer
transport address, but the PEER-ADDRESS attribute is missing, or
the specified peer transport address is invalid in some way.
(tbd) (Lifetime Malformed or Invalid): The LIFETIME attribute is
malformed or the specified lifetime is invalid in some way.
507 (Insufficient Capacity): The server cannot allocate a new port
for this client as it has exhausted its relay capacity.
11. Client Discovery of TURN Servers
The STUN extensions introduced by TURN differ from the binding
requests defined in [I-D.ietf-behave-rfc3489bis] in that they are
sent with additional framing and demand substantial resources from
the TURN server. In addition, it seems likely that administrators
might want to block connections from clients to the TURN server for
relaying separately from connections for the purposes of binding
discovery. As a consequence, TURN runs on a separate port from STUN.
The client discovers the address and port of the TURN server using
the same DNS procedures defined in [I-D.ietf-behave-rfc3489bis], but
using an SRV service name of "turn" (or "turns" for TURN over TLS)
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instead of just "stun".
For example, to find TURN servers in the example.com domain, the TURN
client performs a lookup for '_turn._udp.example.com',
'_turn._tcp.example.com', and '_turns._tcp.example.com' if the STUN
client wants to communicate with the TURN server using UDP, TCP, or
TLS over TCP, respectively.
12. Security Considerations
TURN servers allocate bandwidth and port resources to clients, in
contrast to the Binding method defined in
[I-D.ietf-behave-rfc3489bis]. Therefore, a TURN server requires
authentication and authorization of STUN requests. This
authentication is provided by mechanisms defined in the STUN
specification itself, in particular digest authentication.
Because TURN servers allocate resources, they can be susceptible to
denial-of-service attacks. All Allocate transactions are
authenticated, so that an unknown attacker cannot launch an attack.
An authenticated attacker can generate multiple Allocate Requests,
however. To prevent a single malicious user from allocating all of
the resources on the server, it is RECOMMENDED that a server
implement a modest per user limit on the amount of bandwidth that can
be allocated. Such a mechanism does not prevent a large number of
malicious users from each requesting a small number of allocations.
Attacks such as these are possible using botnets, and are difficult
to detect and prevent. Implementors of TURN should keep up with best
practices around detection of anomalous botnet attacks.
A client will use the transport address learned from the RELAY-
ADDRESS attribute of the Allocate Response to tell other users how to
reach them. Therefore, a client needs to be certain that this
address is valid, and will actually route to them. Such validation
occurs through the message integrity checks provided in the Allocate
response. They can guarantee the authenticity and integrity of the
allocated addresses. Note that TURN is not susceptible to the
attacks described in Section 12.2.3, 12.2.4, 12.2.5 or 12.2.6 of
[I-D.ietf-behave-rfc3489bis] [[TODO: Update section number references
to 3489bis]]. These attacks are based on the fact that a STUN server
mirrors the source IP address, which cannot be authenticated. STUN
does not use the source address of the Allocate Request in providing
the RELAY-ADDRESS, and therefore, those attacks do not apply.
TURN cannot be used by clients for subverting firewall policies.
TURN has fairly limited applicability, requiring a user to explicitly
authorize permission to receive data from a peer, one IP address at a
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time. Thus, it does not provide a general technique for
externalizing sockets. Rather, it has similar security properties to
the placement of an address-restricted NAT in the network, allowing
messaging in from a peer only if the internal client has sent a
packet out towards the IP address of that peer. This limitation
means that TURN cannot be used to run web servers, email servers, SIP
servers, or other network servers that service a large number of
clients. Rather, it facilitates rendezvous of NATted clients that
use some other protocol, such as SIP, to communicate IP addresses and
ports for communications.
Confidentiality of the transport addresses learned through Allocate
transactions does not appear to be that important. If required, it
can be provided by running TURN over TLS.
TURN does not and cannot guarantee that UDP data is delivered in
sequence or to the correct address. As most TURN clients will only
communicate with a single peer, the use of a single channel number
will be very common. Consider an enterprise where Alice and Bob are
involved in separate calls through the enterprise NAT to their
corporate TURN server. If the corporate NAT reboots, it is possible
that Bob will obtain the exact NAT binding originally used by Alice.
If Alice and Bob were using identical channel numbers, Bob will
receive unencapsulated data intended for Alice and will send data
accidentally to Alice's peer. This is not a problem with TURN. This
is precisely what would happen if there was no TURN server and Bob
and Alice instead provided a (STUN) reflexive transport address to
their peers. If detecting this misdelivery is a problem, the client
and its peer need to use message integrity on their data.
One TURN-specific DoS attack bears extra discussion. An attacker who
can corrupt, drop, or cause the loss of a Send or Data indication
sent over UDP, and then forge a Channel Confirmation indication for
the corresponding channel number, can cause a TURN client (server) to
start sending unencapsulated data that the server (client) will
discard. Since indications are not integrity protected, this attack
is not prevented by cryptographic means. However, any attacker who
can generate this level of network disruption could simply prevent a
large fraction of the data from arriving at its destination, and
therefore protecting against this attack does not seem important.
The ChannelConfirmation forging attack is not possible when the
client to server communication is over TCP or TLS over TCP.
Relay servers are useful even for users not behind a NAT. They can
provide a way for truly anonymous communications. A user can cause a
call to have its media routed through a TURN server, so that the
user's IP addresses are never revealed.
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Any relay addresses learned through an Allocate request will not
operate properly with IPSec Authentication Header (AH) [RFC4302] in
transport or tunnel mode. However, tunnel-mode IPSec ESP [RFC4303]
should still operate.
13. IANA Considerations
Since TURN is an extension to STUN [I-D.ietf-behave-rfc3489bis], the
methods, attributes and error codes defined in this specification are
new method, attributes, and error codes for STUN. This section
directs IANA to add these new protocol elements to the IANA registry
of STUN protocol elements.
The codepoints for the new STUN methods defined in this specification
are listed in Section 8.
The codepoints for the new STUN attributes defined in this
specification are listed in Section 9.
The codepoints for the new STUN error codes defined in this
specification are listed in Section 10.
Extensions to TURN can be made through IETF consensus.
14. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [RFC3424]. The TURN
extension is an example of a protocol that performs this type of
function. The IAB has mandated that any protocols developed for this
purpose document a specific set of considerations.
TURN is an extension of the STUN protocol. As such, the specific
usages of STUN that use the TURN extensions need to specifically
address these considerations. Currently the only STUN usage that
uses TURN is ICE [I-D.ietf-mmusic-ice].
15. Example
TBD
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16. Changes from Previous Versions
Note to RFC Editor: Please remove this section prior to publication
of this document as an RFC.
This section lists the changes between the various versions of this
specification.
16.1. Changes from -05 to -06
o Changed the mechanism for allocating channels to the one proposed
by Eric Rescorla at the Dec 2007 IETF meeting.
o Removed the framing mechanism (which was used to frame all
messages) and replaced it with the ChannelData message. As part
of this change, noted that the demux of ChannelData messages from
TURN messages can be done using the first two bits of the message.
o Rewrote the sections on transmitted and receiving data as a result
of the above to changes, splitting it into a section on Send and
Data Indications and a separate section on channels.
o Clarified the handling of Allocate Request messages. In
particular, subsequent Allocate Request messages over UDP with the
same transaction id are not an error but a retransmission.
o Restricted the range of ports available for allocation to the
Dynamic and/or Private Port range, and noted when ports outside
this range can be used.
o Changed the format of the REQUESTED-TRANSPORT attribute. The
previous version used 00 for UDP and 01 for TCP; the new version
uses protocol numbers from the IANA protocol number registry. The
format of the attribute also changed.
o Made a large number of changes to the non-normative portion of the
document to reflect technical changes and improve the
presentation.
o Added the Issues section.
16.2. Changes from -04 to -05
o Removed the ability to allocate addresses for TCP relaying. This
is now covered in a separate document. However, communication
between the client and the server can still run over TCP or TLS/
TCP. This resulted in the removal of the Connect method and the
TIMER-VAL and CONNECT-STAT attributes.
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o Added the concept of channels. All communication between the
client and the server flows on a channel. Channels are numbered
0..65535. Channel 0 is used for TURN messages, while the
remaining channels are used for sending unencapsulated data to/
from a remote peer. This concept adds a new Channel Confirmation
method and a new CHANNEL-NUMBER attribute. The new attribute is
also used in the Send and Data methods.
o The framing mechanism formally used just for stream-oriented
transports is now also used for UDP, and the former Type and
Reserved fields in the header have been replaced by a Channel
Number field. The length field is zero when running over UDP.
o TURN now runs on its own port, rather than using the STUN port.
The use of channels requires this.
o Removed the SetActiveDestination concept. This has been replaced
by the concept of channels.
o Changed the allocation refresh mechanism. The new mechanism uses
a new Refresh method, rather than repeating the Allocation
transaction.
o Changed the syntax of SRV requests for secure transport. The new
syntax is "_turns._tcp" rather than the old "_turn._tls". This
change mirrors the corresponding change in STUN SRV syntax.
o Renamed the old REMOTE-ADDRESS attribute to PEER-ADDRESS, and
changed it to use the XOR-MAPPED-ADDRESS format.
o Changed the RELAY-ADDRESS attribute to use the XOR-MAPPED-ADDRESS
format (instead of the MAPPED-ADDRESS format)).
o Renamed the 437 error code from "No Binding" to "Allocation
Mismatch".
o Added a discussion of what happens if a client's public binding on
its outermost NAT changes.
o The document now consistently uses the term "peer" as the name of
a remote endpoint with which the client wishes to communicate.
o Rewrote much of the document to describe the new concepts. At the
same time, tried to make the presentation clearer and less
repetitive.
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17. Issues
NOTE to RFC Editor: Please remove this section prior to publication
of this document as an RFC.
This section lists the open and now closed issues in this document.
The descriptions here are brief, and the reader should consult the
corresponding thread on the mailing list for a more in-depth
description of the issue and the resolutions being considered.
17.1. Open Issues
1. Bandwidth: What should we do with the BANDWIDTH attribute, which
is currently ill-specified? Should we remove it? Or should we
try to come up with a good specification, perhaps using ideas
from RSVP?
2. Permission Policy: What should the permission policy be?
Address-restricted, as is currently specified in the document?
Or address-and-port-restricted, as many firewalls implement
today? Or should we leave this open to the implementor, under
the assumption that the IT administrator will only allow clients
to contact those servers that implement whatever permission
policy the IT administrator can accept?
3. Port Adjacency: The spec currently allows a client to request
that the server allocate a port and also reserve the next higher
port number for a possible future allocation (on a different
5-tuple). However, the exact behavior of the server in this
case is ill-specified. For example, must the next-higher-port
be available for the allocation of the lower port number to
succeed? How long is the next-higher-port reserved? 30 seconds?
For the lifetime of the lower-numbered-port's allocation? Or
should we just ditch this feature, since it is difficult to
implement, it is at odds with port randomization, and paired
port numbers applications don't work well with NATs anyway?
4. Demuxing ChannelData messages: How does a client or server demux
STUN-formatted messages from ChannelData messages? Does it use
the first two bits (as currently specified) or just one bit?
And how many channels do we need anyway? Some people are
questioning the need for any more than 200 channels. If we
don't need many channels, then the demux algorithm might become
simpler.
5. Deallocating Channels: Do we need a mechanism for deallocating
channels? Some have argued for this feature, because a TURN
server administrator will want a way to recover resources for
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channels no longer in active use. If yes, then what is the
mechanism? For example, should a channel binding expire when
the corresponding permission expires?
6. Permissions and Channel Allocations: Should allocating a channel
for a peer automatically install a permission for that peer's IP
address?
7. Permission and Allocation Lifetimes: What should the default
permission lifetime be? Should there be a minumum value?
Should there be a way for the client to modify the permission
lifetime? Should there be a way for the client to learn the
current permission lifetime? And what is the relationship of
the permission lifetime to the allocation lifetime? Does it
make sense for the allocation lifetime to be less than the
permission lifetime?
8. Preserving bits in the IP header: What bits (if any) should be
preserved in the IP header when a packet is relayed by the
server? The bits under consideration are currently the Don't
Fragment (DF) bit, the Explicit Congestion Notification (ECN)
bits, and the DiffServ (DS) bits.
9. Exceeding the Path MTU Size: TURN adds an overhead of 4 bytes
(ChannelData msg) or 36 bytes (Send or Data Indication), thus
potentially exceeding the path MTU between the client and
server. This could either cause IP fragmentation, or cause the
packet to be dropped if the DF bit is set. Who handles this
problem? Does TURN need to handle this, or is this left up to
the application to handle?
10. Allowed PEER-ADDRESS values: Should there be any restrictions on
the IP address the client can specify in the PEER-ADDRESS
attribute? Are multicast addresses allowed? What about
0.0.0.0? Any other restrictions?
11. Discarding UDP datagrams: If the server discards a received UDP
datagram on the relayed transport address (because there is no
corresponding permission), then does the server send an ICMP
response? If so, what error code does it use? (What does RFC
4787 say about the corresponding situation in NATs? I believe
many NATs silently discard these packets by default, or have a
"stealth mode" that enables this behavior.)
12. Authentication: Is the use of STUN's Long-Term Authentication
Mechanism by a TURN server mandatory? The document currently
implicitly assumes "yes", but what about someone who wants to
operate a public TURN server?
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13. Re-using the 5-tuple: If an allocation expires, is there any
reason a client should not be able to immediately create a new
allocation using the same 5-tuple?
14. Password change: Is it possible to change the password for the
Long-Term Authentication mechanism during the lifetime of an
allocation? If so, how is it done?
15. IPv6: TURN probably works fine in an all IPv6 environment, but
there are a number of mixed IPv4/IPv6 cases that are ill-
specified. As an example, the server needs to check that the
PEER-ADDRESS in a Send Indication is of the same address family
as the relayed transport address. Should we carefully work
through all these cases and make sure we have caught them all,
or should we just state that this document covers the IPv4 case
only, and punt the specification of IPv6 and mixed IPv4/IPv6
operation to draft-ietf-behave-turn-ipv6? Does the current
interest in resurecting IPv4-to-IPv6 NATs have any impact on
TURN?
17.2. Closed Issues
1. Channel Allocation: Should TURN use the mechanism proposed by EKR
to allocate channels? RESOLUTION: Yes. Document now reflects
this.
2. Stateful Allocations: Does a TURN server need to distinguish
between the case where the client retransmits the initial
Allocate Request because the Allocate Response was lost and the
case where the client sends an Allocate Request because it thinks
the allocation does not exist? RESOLUTION: Yes. Document now
reflects this.
3. Port Range: From what range of port numbers should a TURN server
allocate ports? RESOLUTION: The server SHOULD allocate from the
Dynamic and/or Private Port range unless it is sure it will not
interfere with other apps on the same machine. Document now
reflects this.
4. Framing Header for STUN-formatted messages: Should TURN use the
framing mechanism for STUN-formatted messages? RESOLUTION: NO.
Document now reflects this. However, see related issues.
5. Length field in ChannelData header: Over UDP, the length of the
application data field in the ChannelData message can be
determined from the length field in the UDP header. So should
the length field in the ChannelData header be set to zero in this
case? RESOLUTION: No, the ChannelData length field should have
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the same semantics over both TCP and UDP. Document now reflects
this.
18. Acknowledgements
The authors would like to thank the various participants in the
BEHAVE working group for their many comments on this draft. Marc
Petit-Huguenin, Remi Denis-Courmont, Cullen Jennings, Lars Eggert,
Magnus Westerlund, and Eric Rescorla have been particularly helpful,
with Eric also suggesting the channel allocation mechanism.
Christian Huitema was an early contributor to this document and was a
co-author on the first few drafts. Finally, the authors would like
to thank Dan Wing for his huge help in restarting progress on this
draft after work had stalled.
19. References
19.1. Normative References
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-13 (work in progress),
November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
19.2. Informative References
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 1889, January 1996.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
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[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[I-D.ietf-behave-turn-tcp]
Rosenberg, J. and R. Mahy, "Traversal Using Relays around
NAT (TURN) Extensions for TCP Allocations",
draft-ietf-behave-turn-tcp-00 (work in progress),
November 2007.
[Port-Numbers]
"IANA Port Numbers Registry",
<http://www.iana.org/assignments/port-numbers>.
[Protocol-Numbers]
"IANA Protocol Numbers Registry", 2005,
<http://www.iana.org/assignments/protocol-numbers>.
Authors' Addresses
Jonathan Rosenberg
Cisco Systems, Inc.
Edison, NJ
USA
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Rohan Mahy
Plantronics, Inc.
Email: rohan@ekabal.com
Philip Matthews
Avaya, Inc.
1135 Innovation Drive
Ottawa, Ontario K2K 3G7
Canada
Phone: +1 613 592-4343 x223
Fax:
Email: philip_matthews@magma.ca
URI:
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