wdiff draft-ietf-behave-turn-05.txt draft-ietf-behave-turn-06.txt

Behave J. Rosenberg
Internet-Draft Cisco
Intended status: Standards Track R. Mahy
Expires: May 18, July 25, 2008 Plantronics
P. Matthews
Avaya
D. Wing
Cisco
November 15, 2007
January 22, 2008

Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-behave-turn-05
draft-ietf-behave-turn-06

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Abstract

This specification defines an extension of the Session Traversal
Utilities

If a host is located behind a NAT, then in certain situations it can
be impossible for NAT (STUN) Protocol that host to communicate directly with other hosts
(peers) located behind other NATs. In these situations, it is
necessary for asking the STUN server host to relay
packets towards use the services of an intermediate node
that acts as a client. communication relay. This extension, specification defines a
protocol, called Traversal TURN (Traversal Using Relays around NAT (TURN), is useful for hosts behind address
dependent NATs. The extension purposefully restricts NAT), that
allows the ways in
which host to control the relayed address operation of the relay and to exchange
packets with its peers using the relay.

The TURN protocol can be used. In particular, it prevents
users from running general purpose servers on ports obtained from used in isolation, but is more properly used
as part of the
TURN server. ICE (Interactive Connectivity Establishment) approach
to NAT traversal.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
2.1. Transports . . . . . 5
3. Overview of Operation . . . . . . . . . . . . . . . . . . . 7
2.2. Allocations . 5
3.1. Transports . . . . . . . . . . . . . . . . . . . . . . 8
2.3. Exchanging Data with Peers . . . 8
3.2. About Tuples . . . . . . . . . . . . . 9
2.4. Permissions . . . . . . . . . . 9
3.3. Keepalives . . . . . . . . . . . . . 10
2.5. Channels . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. TURN Framing Mechanism
3. Terminology . . . . . . . . . . . . . . . . . . . . 11
5. . . . . . 12
4. General Behavior . . . . . . . . . . . . . . . . . . . . . . . 11
6. 13
5. Managing Allocations . . . . . . . . . . . . . . . . . . . . . 13
6.1. 14
5.1. Client Behavior . . . . . . . . . . . . . . . . . . . . . 13
6.1.1. 14
5.1.1. Initial Allocate Requests . . . . . . . . . . . . . . 13
6.1.2. 14
5.1.2. Refresh Requests . . . . . . . . . . . . . . . . . . . 14
6.2. 15
5.2. Server Behavior . . . . . . . . . . . . . . . . . . . . . 15
6.2.1. Initial 16
5.2.1. Receiving an Allocate Requests Request . . . . . . . . . . 15
6.2.2. . . 16
5.2.2. Refresh Requests . . . . . . . . . . . . . . . . . . . 18
7. Sending 20
6. Send and Receiving Data Indications . . . . . . . . . . . . . . . . . . 19
7.1. Client Behavior 21
6.1. Forming and Sending an Indication . . . . . . . . . . . . 21
6.2. Receiving an Indication . . . . . . . . . . . 19
7.1.1. Sending . . . . . . 22
6.3. Relaying . . . . . . . . . . . . . . . . . 19
7.1.2. Receiving . . . . . . . . 22
7. Channel Mechanism . . . . . . . . . . . . . . 20 . . . . . . . . 23
7.1. Forming and Sending a ChannelBind Request . . . . . . . . 23
7.2. Server Behavior Receiving a ChannelBind Request and Sending a Response . . 24
7.3. Receiving a ChannelBind Response . . . . . . . . . . . . . 25
7.4. The ChannelData Message . . . . . . 20
7.2.1. Receiving Data from the Client . . . . . . . . . . . 25
7.5. Forming and Sending a ChannelData Message . 20
7.2.2. . . . . . . . 25
7.6. Receiving Data from Peers a ChannelData Message . . . . . . . . . . . . . 26
7.7. Relaying . 22
7.2.3. Allocation Activity Timer and Permission Timeout . . . 23 . . . . . . . . . . . . . . . . . . . . . 26
8. New Attributes STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 27
9. New STUN Attributes . . . . . 23
8.1. . . . . . . . . . . . . . . . . 27
9.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 23
8.2. 28
9.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.3. 28
9.3. BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . 24
8.4. 28
9.4. PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 24
8.5. 28
9.5. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
8.6. 28
9.6. RELAY-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 24
8.7. 28
9.7. REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 24
8.8. 28
9.8. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . . 25
8.9. 30
9.9. REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 26
9. 30
10. New STUN Error Response Codes . . . . . . . . . . . . . . . . . . . 26
10. 30
11. Client Discovery of TURN Servers . . . . . . . . . . . . . . . 27
11. 31
12. Security Considerations . . . . . . . . . . . . . . . . . . . 27
12. 32
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
12.1. New STUN Methods 34
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 29
12.2. New STUN Attributes . 34
15. Example . . . . . . . . . . . . . . . . . . 30
12.3. New STUN Response Codes . . . . . . . . . 34
16. Changes from Previous Versions . . . . . . . . 30
13. IAB Considerations . . . . . . . . 35
16.1. Changes from -05 to -06 . . . . . . . . . . . . . . 30
14. Example . . . 35
16.2. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 35
17. Issues . . . . . . . . . . 30
15. Changes since version -04 . . . . . . . . . . . . . . . . . . 35
16. 37
17.1. Open Issues . . . . . . . . . . . . . . . . . . . . . . . 37
17.2. Closed Issues . . . . . . . . . . . . . . . . . . . . . . 39
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 36
17. 40
19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
17.1. 40
19.1. Normative References . . . . . . . . . . . . . . . . . . . 36
17.2. 40
19.2. Informative References . . . . . . . . . . . . . . . . . . 36 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 37 41
Intellectual Property and Copyright Statements . . . . . . . . . . 39 43

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
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. Terminology

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

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

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

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

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

Allocation: The IP address and port granted to a client through Operation

This section gives an
Allocate request, along with related state, such as permissions
and expiration timers.

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

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

3. Overview of Operation 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. This
specification defines a framing mechanism and several new STUN
methods. Together, these add the ability for a STUN server to act as
a packet relay.

The framing mechanism serves two purposes. First, it contains a
length field that allow TURN nodes to find the boundaries between
chunks of application data when the communication with the TURN
server is over a stream-based transport such as TCP. Second, it
carries a channel number. Channel zero is used for TURN control
messages, while the other channel numbers are used for application
data traveling to These peers may or from various peers. The channel number allows
the client to know which peer sent data to it, and to specify which
peer is to may not be the recipient of data. Application data flowing on any
non-zero channel is unencapsulated, meaning that the application data
starts immediately after the framing header. The framing header is
just four bytes. This allows TURN to operate with minimal overhead,
which is important for the real-time protocols it is designed to
support. Application data can also flow in encapsulated format,
meaning that it is carried in certain TURN messages on channel 0.
Channel numbers are independent in each direction: for example,
channel 5 might indicate behind one peer in the client to server direction,
but a different peer in the server to client direction.

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

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

Once a permission is open, the client can then receive data flowing
back from its peer. Initially this data is encapsulated in a Data
Indication. Since multiple permissions can be open simultaneously,
the Data Indication contains the PEER-ADDRESS attribute so the
NATs.

+---------+
| |
| |
/ | Peer A |
Client's TURN
client knows which peer sent the data, and a CHANNEL-NUMBER attribute
so the client knows how the server will refer to traffic from this
peer when sent unencapsulated. Likewise when the client initially
sends to a new peer, it uses a Send Indication with the peer address
in the PEER-ADDRESS attribute, along with a channel number so the
server knows how the client will refer to unencapsulated data to this
peer. // | |
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 peer
client server
|--- Allocate Req -->| |/ |
|<-- Allocate Resp ---| |
| Client |----|A|----------| Server |------------------| Peer B |
|
|--- Send (chan 2) -->| data | | |============>|
|<-- ChannelConfirm --| |^ | |^ ^| |
| data | |T|| | |<============|
|<-- Data (chan 5) ---| || || |
|--- ChannelConfirm ->|
+---------+ | || +---------+| |+---------+
| || | |
|--- [2] + data ----->| data
| || | |
+-+| | | |============>|
| | data |
| |<============|
|<-- [5] + data ------| | |
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: Example Usage of Channels

When the client and server communicate over UDP, data and control
messages can arrive out of order. For 1

Figure 1 shows a typical deployment. In this reason, figure, the TURN client needs
to verify
and the TURN server knows are separated by a NAT, with the client channel mapping before on the
client sends unencapsulated,
private side and the server needs to verify the
client knows the server channel mapping before on the server sends
unencapsulated. When public side of the client and server communicate over UDP, a
Channel Confirmation indication NAT. This NAT
is sent after the Send (or Data)
indication so the client (or server) knows that assumed to be a "bad" NAT; for example, it can send
unencapsulated.

Figure 1 demonstrates how might have a mapping
property of address-and-port-dependent mapping (see [RFC4787]) for a
description of what this works. means).

The client performs has allocated a local port on one of its addresses for use
in communicating with the server. The combination of an
Allocate Request, IP address
and gets a response. It decides to send data to port is called a
specific peer. Initially, 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 data TURN messages from its host transport address to that peer using a TURN
Send indication
transport address on channel 0. That Send Indication tells the TURN server that, once confirmed, which is known as the TURN
SERVER ADDRESS. The client will send data unencapsulated
to that peer on channel 2. Whenever 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 receives a Send
indication, it stores
address is used by the mapping from channel number client to peer, send both commands and
sends a ChannelConfirm indication (on channel 0). Once data to the
confirmation has been received
server; the commands are processed by the client, TURN server, while the client can send data to the peer
is relayed on channel 2. Prior to receipt of the
ChannelConfirm, any other data peers.

Since the client wishes to send to is behind a NAT, the peer 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 using Send indications, all of which indicate that channel 2
is by the server to the client's server-reflexive transport address
will be used for unencapsulated data. The same procedure happens
from server forwarded by the NAT to client; the client's host transport address.

The client uses TURN server initially sends data using commands to allocate a
Data indication RELAYED transport
address, which is an transport address located on channel 0, the server. The
server ensures that there is a one-to-one relationship between the
client's server-reflexive transport address and once confirmed with the relayed transport
address; thus a
ChannelConfirm, it packet received at the relayed transport address can send it unencapsulated on its selected channel
(channel 5 in
be unambiguously relayed by the example).

Over a reliable transport, such as TCP, server to the confirmation step is client.

The client will typically communicate this relayed transport address
to one or more peers through some mechanism not
needed so the Channel Confirmation indication specified here (e.g.,
an ICE offer or answer [I-D.ietf-mmusic-ice]). Once this is not used. Clients done,
peers can immediately send the next piece of data packets to the peer on relayed transport address and the
requested channel.

Allocations can also request specific attributes such as
server will forward them to the desired
Lifetime of client. In the allocation and reverse direction,
the maximum Bandwidth. Clients can
also request specific port assignment behavior, for example, a
specific port number, odd or even port numbers, or pairs of
sequential port numbers.

3.1. Transports

TURN clients client can communicate with a TURN send data packets to the server using UDP, TCP, or
TLS over TCP. A (at its TURN server can then relay traffic between a
reliable transport used between the client and server (TCP or TLS
over TCP),
address) and UDP used from these will be forwarded by the server to peer. When relaying data sent
from a stream-based protocol to a UDP peer, the TURN server emits
datagrams which are appropriate
peer, and the same length peer will see them as coming from the length field relayed transport
address; in this direction, the client must specify the appropriate
peer.

2.1. Transports

TURN
framing or as defined in this specification only allows the length use of the DATA attribute in a Send Indication.
Likewise, when a UDP datagram is received by
between the TURN server and
relayed to the client over a stream-based transport, peer. However, this specification allows
the length use of
the datagram is the length any one of UDP, TCP, or TLS over TCP to carry the TURN framing or Data Indication's
DATA attribute.

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

+-----------------------+---------------------+ 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. When
using TLS, First, the client can be assured that
the address addresses of the client's its peers are not visible to an attacker except by traffic analysis
downstream of any attackers between
it and the TURN server. Second, the client may be able to communicate
with TURN servers using TLS when it would not be able to communicate
with the same server using TCP or UDP, due to the
configuration policy of a
firewall between the TURN client and its server. In this second
case, TLS between the client and TURN server in this case just facilitates traversal.

In addition, an

There is a planned extension to TURN is planned to add support for TCP
allocations [I-D.ietf-behave-turn-tcp].

3.2. About Tuples

To relay data to between
the server and from the correct location, peers [I-D.ietf-behave-turn-tcp]. For this
reason, allocations that use UDP between the TURN server
maintains an association and the peers are
known as UDP allocations, while allocations that use TCP between the 5-tuple used to communicate with
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 5-tuple used to communicate server replies with each of an Allocate Response containing the
client's peers. allocated
relayed transport address. The 5-tuple on the client side will consist can include attributes in the
Allocate Request that describe the type of allocation it desires
(e.g., the
client's reflexive address -- lifetime of the apparent source address and port allocation). And since relaying data can
require lots of bandwidth, the server may require that the client (typically as rewritten
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 last NAT)--and client periodically doing a
Refresh transaction with the
destination server, where the client includes the
allocated relayed transport address and port used by in the Refresh Request. TURN server. The figure
below (Figure 2) shows
deliberately uses a typical topology. In this diagram, different method (Refresh rather than Allocate)
for refreshes to ensure that the client 5-tuple 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 UDP flow between 192.0.2.1:7000 lifetime in the
Allocate Request and may modify its request in a Refresh Request, and
192.0.2.15:3490.
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 TURN server client and Peer B is 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 UDP flow between 192.0.2.15:9000 (the TURN allocated address)
and 192.0.2.210:18200. 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 may use a
file-descriptor file-
descriptor in place of a 5-tuple to represent a TCP connection.

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

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 |
Client's Send Indication
is sent from a client to the TURN // | |
Host Server / | |
Address Address // +---------+
10.1.1.2:17240 192.0.2.15:3490 / 192.0.2.180:16400
| | //
| +-+ | /
| | | | /
v | | | // 192.0.2.210:18200
+---------+ | | |+---------+ / +---------+
| 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 -->| | |N| || | //
|<-- Allocate Resp ---| | |
| TURN | | | v| TURN |/
|--- Send (Peer A)--->| | |
| Client |----|A|----------| Server |------------------| Peer B |=== data ===>| |
| | | |^ | |^ ^|
| |<== data ====| |
|<-- Data (Peer A)----| | |T|| | || ||
|
+---------+ | || +---------+| |+---------+ | || |
|--- Send (Peer B)--->| | | ||
| |=== data =================>|
|
+-+| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
Client's TURN Peer B
Reflexive Allocated Transport
Address Address Address
192.0.2.1:7000 192.0.2.15:9000 192.0.2.210:18200

Figure 2

3.3. Keepalives

Since

In the main purpose of STUN and TURN is figure above, the client first allocates a relayed transport
address. It then sends data to traverse NATs, it Peer A using a Send Indication; at
the server, the data is
natural extracted and forwarded in a UDP datagram to consider which elements
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 responsible for generating
sufficient periodic traffic placed into a Data Indication and
forwarded to insure the client. A similar exchange happens with Peer B.

2.4. Permissions

To ease concerns amongst enterprise IT administrators that NAT bindings stay alive. TURN clients need to send data frequently enough could
be used to keep both NAT
bindings and bypass corporate firewall security, TURN includes the
notion of permissions. TURN server permissions fresh. Like NAT bindings, mimic the address-restricted
filtering mechanism of NATs that comply with [RFC4787].

A TURN server permissions are refreshed by ordinary data traffic 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 peer. Unlike permissions, allocations
on same IP address (the port numbers can
differ). See the TURN server have an explicit expiration time and need to be normative description for the precise definition of
"recently".

A permission will timeout if not refreshed explicitly periodically. The client
refreshes a permission by sending data to the client 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 Refresh request. When clients can assign a CHANNEL to a peer. Data to
and from such a peer can then be sent using an allocation expires, all permissions associated with alternate packet
format that
allocation are automatically deleted.

4. TURN Framing Mechanism

All 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 control messages and all application data sent between
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 MUST start with has received the TURN framing header. This
header is used for two purposes: indicating
request, it can relay data from that peer towards the client using a
ChannelData message. There is no way to modify channel number, and bindings, so
once a channel is bound to a peer, it remains bound for framing.

TURN uses the lifetime
of the allocation.

When the server receives a ChannelData message from the client, it
uses the channel number to distinguish control traffic from data, determine the destination peer and then
forwards the data inside a UDP datagram to distinguish among multiple peers using the same allocation.
Channel 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 zero is reserved for bound to that peer; in this way
the client can determine the peer that send the UDP datagram.
TURN control messages. All TURN
requests, responses and indications between 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 server
MUST be sent on then uses that
address to exchange data with Peer A. After a bit, the client decides
to bind a channel 0, to Peer A. To do this, it sends a ChannelBind
Request to the server, specifying the transport address of Peer A and MUST NOT be sent on any other channel.
Channel 0xFFFF
a channel number (0x4001). After that, the client can send
application data encapsulated inside ChannelData messages to Peer A:
this is reserved for future use and MUST NOT shown as "[0x4001] data" where 0x4001 is the channel number.

Note that ChannelData messages can only be used by
clients or servers compliant for peers to this specification. Other channel
numbers 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.

Channel bindings are assigned always initiated by the client.

3. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and communicated "OPTIONAL" in this
document are to be interpreted as described in Section 7.
Because RFC 2119 [RFC2119].

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

The following terms are used in this document:

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

TURN client: A STUN requests.

Over stream-based transports, 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 also need to
include an explicit length so that using
the protocol defined in this document; rather, the peer receives
data sent by the TURN server can perform
conversion from streams to datagrams and vice versa. the peer sends data towards the
TURN framing
has a 2 octet channel number and server.

Host Transport Address: A transport address allocated on a 2 octet length field. Over
stream-based transports, the length field counts host.

Server-Reflexive Transport Address: A transport address on the number
"public side" of octets
immediately after the length field itself. Over UDP the length a NAT. This address is
always set allocated by the NAT to
correspond to zero.

Channel numbers are always defined within a particular allocation.
If a client has multiple allocations specific host transport address.

Relayed Transport Address: A transport address that exists on a TURN server, there is no
relationship whatsoever between the channel numbers in each
allocation. Once created,
server. If a channel number persists for the lifetime
of permission exists, packets that arrive at this
address are relayed towards the allocation. There is no way TURN client.

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

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 intent bi-
directional flow of
holding it for extremely long periods, possibly for communication
with many different peers over time, may eventually exhaust UDP datagrams.

Permission: The IP address and transport protocol (but not the set port)
of channels. In a peer that case, is permitted to send traffic to the client TURN server and
have that traffic relayed to the TURN client. The TURN server
will need only forward traffic to obtain a new
allocation.

5. 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 internal 5-tuple of an existing allocation. 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 (in this specification, the only subsequent request possible
is a Refresh request),
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 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
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 on a new flow begin with an
Allocate transaction. All subsequent transactions happen in the
context of that allocation. allocation, and happen on the same 5-tuple. The
client refreshes allocations and deallocates them using a Refresh
transaction.

6.1.

5.1. Client Behavior

6.1.1.

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 use
it.
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.

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

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

The client MUST include a REQUESTED-TRANSPORT attribute. In this
specification, the REQUESTED-TRANSPORT will MUST always be UDP. This
attribute is included to allow for future extensions to TURN. TURN (e.g.,
[I-D.ietf-behave-turn-tcp])
The client MAY include a REQUESTED-PORT-PROPS or REQUESTED-IP
attribute in the request to obtain specific types of transport
addresses. Whether these are needed depends on the application using
the TURN server. As an example, the Real Time Transport Protocol
(RTP) [RFC3550] requires that RTP and RTCP ports be an adjacent pair,
even and odd respectively, for compatibility with a previous version
of that specification. The REQUESTED-PORT-PROPS attribute allows the
client to ask the relay for those properties.
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.

6.1.2.

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. 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.

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.

6.2.

5.2. Server Behavior

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

6.2.1. Initial

5.2.1. Receiving an Allocate Requests Request

When the server receives an Allocate request, the server attempts to
allocate a relayed transport address. It first looks for the
BANDWIDTH attribute in the request. If present,

When the server
determines whether or not receives the Allocate Request, it has sufficient 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.

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.

6.2.1.1. If it doesn't, it sends an error response.

5.2.1.2. REQUESTED-TRANSPORT

First, the

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

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

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

6.2.1.2.

5.2.1.3. REQUESTED-IP

Next, the

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
discretion of the server.

6.2.1.3.

5.2.1.4. REQUESTED-PORT-PROPS

Finally, the

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

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

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

Port alignment requests exist for compatibility with
implementations of RTP which predate RFC 3550. [RFC3550]. These
implementations use the port numbering conventions in (now
obsolete) RFC 1889.

6.2.1.4. [RFC1889].

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

The server SHOULD only allocate ports in from the range 1024-65535. 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
is one of several ways 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 prohibit relayed transport addresses
discourage clients from
being used to attempt 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. Each permission is permissions and a specific IP address
identifying an external client. Initially, this list set of channel bindings. Each set is null.
initially empty.

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

6.2.1.5.

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 (see
Section 11).

6.2.2. 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.

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

Finally, a Refresh Request will set a new allocation expiration timer
for the allocation, effectively canceling the previous allocation
expiration timer. As with an Allocate request, the server can offer MAY
utilize a shorter allocation lifetime, but never MUST NOT utilize a longer one.
lifetime.

A success Refresh response MUST contain a LIFETIME attribute and 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.

7. Sending

6. Send and Receiving Data

As described in Section 4, Indications

TURN allows a client supports two ways to send and receive data without utilizing TURN from peers. This
section describes the use of Send and Data indications, by sending and
receiving them on channels. Before sending client-to-peer or peer-
to-client data for a new peer, a TURN client or server needs to
assign a channel number that corresponds to that remote peer. Once a
channel number is assigned, it remains assigned through while
Section 7 describes the duration use of the allocation. It cannot be unassigned or reassigned to a
different peer.

7.1. Client Behavior

7.1.1. Channel Mechanism.

6.1. Forming and Sending an Indication

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

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

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

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

If client, it uses a Data Indication to pass the channel number has not been confirmed, data to the client.
A client creates can also use a Send indication. It places the selected channel number in Indication without a CHANNEL-
NUMBER attribute, DATA attribute to
install or refresh a permission for the peer specified IP address and port 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, attribute and puts the data to be sent in MAY
contain a DATA attribute. (If the
client just wishes to create attribute, while a permission, it can omit the DATA
attribute.) If Data Indication MUST contain both
attributes. The PEER-ADDRESS attribute contains the Send indication is sent over a reliable transport
(ex: TCP), the client marks that the channel number as confirmed.
When the client receives a ChannelConfirmation Indication, and the
channel number, IP
address and port match of the channel number assigned peer to that peer, the client marks that which the channel number is confirmed.

Since Send data is an Indication, it generates no response. The client
must rely on application layer mechanisms to determine if be sent (in the case of a
Send Indication) or from which the data was received by (in the peer. A ChannelConfirmation Indication just
means that some Send indication was received case of
a Data Indication). This peer address is the transport address of
the peer as seen by the TURN server. It
does server, which may not mean that a specific Send indication was received by be the same as the host
transport address of the peer. The DATA attribute contains the
actual application data. Note that Send Indications are not authenticated and do not
contain a MESSAGE-INTEGRITY attribute. Just like non-relayed data
sent over UDP or TCP, the authenticity and integrity of this application data
can only may need to
be assured using security mechanisms at higher layers.

7.1.2. Receiving

When padded to ensure the client receives DATA attribute length is a Data indication, it:

o records multiple of 4.

No other attributes are included. For example, neither the channel number used by
FINGERPRINT attribute nor any authentication attributes are included.
The latter holds even if the server (from is using the CHANNEL-
NUMBER attribute) and associates it with Long-Term Credential
Mechanism, since indications cannot be authenticated using this
mechanism.

Both the IP address Send and port Data indications MUST be sent using the 5-tuple of
the original allocation. Thus, in the PEER-ADDRESS attribute, which identify case of the peer that sent Send Indication,
the data. The resulting mapping from channel number to source transport address MUST be stored by the client for is the duration of client's host transport address,
the
allocation.

o delivers destination transport address is the contents of TURN server address, and the DATA attribute to
transport protocol is the client
application same as if it was received from used for the peer's IP address and
port.

o If Allocate request.
For the Data indication was received over UDP, Indication, the client MUST
confirm source and destination transport
addresses are the channel used by reverse.

6.2. Receiving an Indication

When a Send Indication is received at the server, by sending or a
ChannelConfirmation Data
Indication to is received at the server. This 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 the same a PEER-ADDRESS attribute and CHANNEL-NUMBER MAY
contain a DATA attribute, while a Data Indication MUST contain both
attributes. Any other attributes
included appearing in the message are
treated as unexpected.

TODO: Add check that Send or Data indication. This indication is sent to the
server on channel 0 using the 5-tuple associated arrives with
appropriate 5-tuple. Since this
allocation. Note that, due check applies to round trip delays, a client may
receive several all STUN
messages, not just Send and Data indications with the same channel number for indications, perhaps this goes
under the same remote peer. It MUST process each as defined here,
resulting in several ChannelConfirmation indications. general processing section.

6.3. Relaying

When the client server receives unencapsulated data, it checks the received
channel number. If the client has a mapping associated with the
server channel number valid Send Indication contains a DATA
attribute, it delivers the data to the client application forms a UDP datagram as if it was received directly from that peer. Otherwise, it
silently discards the data.

7.2. Server Behavior

7.2.1. Receiving Data from follows:

o the Client

When source transport address is the server receives a Data indication from relayed transport address of
the client, it:

o records allocation, where the channel number used allocation is determined by the client (from 5-tuple
on which the CHANNEL-
NUMBER attribute) and associates it with Send Indication arrived;

o the IP destination transport address and port
in is taken from the PEER-ADDRESS attribute, which identify the peer to which
attribute;

o the data following the UDP header is to be sent. The resulting mapping from channel number
to peer transport address MUST be stored by the server for contents of the
duration value
field of the allocation. DATA attribute;

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

o the Checksum field in a the UDP datagram,
sending it header is computed as described in
[RFC 768].

The resulting UDP datagram is then sent to the PEER-ADDRESS and sending from peer.

When the allocated
transport address.

o if one doesn't exist, creates server receives a valid Send Indication (with or without a
DATA attribute), it also updates the permission for associated with the
IP address from contained in the PEER-ADDRESS (the port attribute. For a certain
interval after the permission is ignored), 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 attaches the port numbers are
irrelevent. This permission is specific to the allocation

o checks if a timer has been set for this permission. If none and has
been started, no
affect on any other allocation. The recommended length of time is 60
seconds from when the server starts one. It Send Indication is RECOMMENDED that it
have a value of sixty seconds. If received.

When the timer 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 already running, permitted to relay the datagram.
If it is not permitted, the UDP datagram MUST be reset.

o discarded.

If the Send indication was received over UDP, relaying is permitted, the server MUST
confirm the channel used by the client, by sending forms and send a
ChannelConfirmation Data Indication to
as described in Section 6.1, using the client. This indication
MUST contain data following the same PEER-ADDRESS and CHANNEL-NUMBER attributes
included UDP header
as the application data.

7. Channel Mechanism

As described in the Send indication. This indication is sent 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 on can
bind a channel 0 using to a peer at any time during the lifetime of the 5-tuple associated with this
allocation. Note that, due The client may bind a channel to round trip delays, a server may
receive several Send indications peer before
exchanging data with the same channel number 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 same remote channel binding cannot be
changed. There is no way to unbind a channel or bind it to a
different peer. It MUST process each as defined here,
resulting

Channel bindings are specific to an allocation, so that a binding in several ChannelConfirmation indications.
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 the server receives unencapsulated data, a client wishes to bind a channel to a peer in an allocation, it checks
forms a ChannelBind Request. The Request formed following the received
channel number:

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

A ChannelBind Request MUST contain both a mapping associated with 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 it:

* sends MUST be in the range 0x4000 to 0xFFFE (inclusive)
and the channel MUST NOT be already bound to a UDP datagram different peer. It is
acceptable to rebind a channel to the peer using it is already bound to.
The PEER-ADDRESS attribute specifies the transport peer address
from to bind the mapping, and sends from
channel to.

Once formed, the allocated transport
address.

* checks if a permission activity timer ChannelBind Request is running sent using the 5-tuple for
the
destination IP address of allocation.

The client SHOULD be prepared to receive ChannelData messages on the peer. If one is not running,
channel as soon as it has sent the
server starts one. It ChannelBind Request. Over UDP, it
is RECOMMENDED that possible for the client to receive these before it have receives a value of
sixty seconds. If
ChannelBind Success Response.

Over UDP, the timer is already running, it MUST be
reset.

o If client SHOULD NOT send ChannelData messages on the server has no mapping,
channel until it silently discards has received a ChannelBind Success Response for the data.

7.2.2.
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 Data from Peers

If a ChannelBind Request and Sending a Response

When the server receives a UDP packet on an allocated UDP transport
address, ChannelBind Request, it checks first does the permissions associated with
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 allocation. the ChannelBind Request contains both a
CHANNEL-NUMBER attribute and a PEER-ADDRESS attribute. If the source IP PEER-
ADDRESS attribute is missing or malformed, then the server rejects
the request with an Error Response containing the error code XXX
"Peer address of missing or invalid". If the UDP packet matches one of CHANNEL-NUMBER attribute
is missing or malformed, or the
permissions (the source port channel number is not used), in the UDP packet is
accepted. Otherwise, it range
0x4000 to 0xFFFE (inclusive), or the channel is discarded. If already bound to
another peer (already bound to the packet same peer is accepted, 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.

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 forwarded implementation specific.

7.4. The ChannelData Message

The ChannelData message is used to carry application data between the
client as described below.

The server checks if it and the server. It has assigned a channel 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 for
communications from this peer (as identified by its IP address of the channel on which
the data is traveling, and
port) over this allocation:

o If one has not been assigned, thus the client assigns one address of its own
choosing. This the peer that is
sending or is to receive the data. The channel number MUST be one that is currently
unassigned by in the server
range 0x4000 - 0xFFFF, with channel number 0xFFFF being reserved for this allocation. It MUST
possible future extensions.

Channel numbers 0x0000 - 0x3FFF cannot be between 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 65534. It is RECOMMENDED that 1 as "00", while
ChannelData messages use combinations "01", "10", and "11".

The Length field specifies the server choose one length in bytes of the
unassigned numbers randomly, rather than sequentially. The state application
data field (i.e., it does not include the size of the channel is set to unconfirmed.

o If one has been assigned, that channel MUST be selected. ChannelData
header). Note that 0 is a valid length.

The Application Data field carries the data from peers does not reset the permission activity
timer.

Next, client is trying to
send to the server checks if peer, or that the channel number has been confirmed by peer is sending to the client. If the channel number

7.5. Forming and Sending a ChannelData Message

Once a client has been confirmed, the server
simply sends 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 with is not obligated to use the appropriate
channel
number in when it exists and may freely intermix the TURN framing.

If two message types
when sending data to the peer. The server, on the other hand, SHOULD
use the ChannelData message if a channel number has not been confirmed, bound to the server creates a
Data indication. It places peer.

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

Over stream transports, the ChannelData message MUST be padded to a CHANNEL-
NUMBER attribute,
multiple of four bytes in order to ensure the peer IP address and port alignment of subsequent
messages. The padding is not reflected in a PEER-ADDRESS
attribute, and puts the data 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 DATA attribute. 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
Data indication ChannelData message is sent received over a reliable transport (ex: TCP), TCP or over TLS over TCP,
then the
server marks that actual length of the channel number ChannelData message is as confirmed. 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 ChannelConfirmation Indication, and valid ChannelData message, it forms a UDP
datagram as follows: the channel number, IP source transport address and port match is the channel number assigned to that peer, relayed
transport address of the
server marks that allocation, where the channel number is confirmed.

Since Data allocation is an Indication, it generates no response. The server
does not provide reliability for
determined by the data. When sending over a
reliable transport to 5-tuple on which the client, if ChannelData message arrived;
the server destination transport address is unable the peer address to send which the
data received from
channel is bound; the peer (for example, because data following the TCP connection
cannot accept any more messages right now), it can silently discards UDP data received from header is the contents
of the peer.

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

7.2.3. Allocation Activity Timer the ChannelData message + 8;
and Permission Timeout

When the allocation activity timer expires, Checksum field in the server MUST destroy UDP header is computed as described in

[RFC 768]. The resulting UDP datagram is then sent to the allocation. This involves freeing peer.

The server also updates the allocated transport
address, deleting permissions and channel numbers, and removing other
state permission associated with the allocation. IP address
part of the peer address to which the UDP datagram is sent.

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

8.1.

9.1. CHANNEL-NUMBER

The channel number CHANNEL-NUMBER attribute represents contains the channel number assigned
by the sender, that corresponds with the peer specified in of the PEER-
ADDRESS attribute. channel. It
is a 16-bit unsigned integer, plus two octets
of padding followed by a two-octet RFFU field
which MUST be set to zero. 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 | Reserved = 0 RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

8.2.

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.

8.3.

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.

8.4.

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.

8.5.

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).

8.6.

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.

8.7.

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
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 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 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 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), and should N). The server SHOULD only allocate the
N+1 port number if it is explicit explicitly requested (with a subsequent
request specifying that exact port number) 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 responds MUST respond with a error response with an
error code of 444 (Invalid Port).

8.8.

9.8. REQUESTED-TRANSPORT

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

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

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

Extensions IPv6 header
[Protocol-Numbers]. This specification only allows the use of
codepoint 17 (User Datagram Protocol).

The RFFU field is set to TURN can define additional transport protocols in an
IETF-consensus RFC.

8.9. 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 to it. by the TURN server. This attribute
is needed since it is anticipated that TURN servers will be multi-homed multi-
homed so as to be able to allocate more than 64k transport addresses.
As a consequence, a client needing a second transport address on the
same interface as a previous one can make that request. 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 is 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 error response codes:

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

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

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

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

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

(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.

10.

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)
instead of just "stun".

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

11.

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
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.

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

12.

13. IANA Considerations

This specification defines several new

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

12.1. New

The codepoints for the new STUN Methods

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

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

12.2. New methods defined in this specification
are listed in Section 8.

The codepoints for the new STUN Attributes

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

12.3. New attributes defined in this
specification are listed in Section 9.

The codepoints for the new STUN Response Codes

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

13. 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 RFC 3424 [RFC3424]. The TURN
extension is an example of a protocol that performs this type of
function. The IAB has mandated that any protocols developed for this
purpose document a specific set of considerations.

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

14.

15. Example

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

TBD

16. Changes from Previous Versions

Note to SIP. However, RFC Editor: Please remove this section prior to keep the
example simple, TURN is shown without ICE.

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

10.0.1.1 192.0.2.1 192.0.2.3 192.0.2.17
Client NAT TURN Server Peer
| | | |
|(1) Allocate |(2) Allocate | |
|S=10.0.1.1:4334 |S=192.0.2.1:63346 | |
|D=192.0.2.3:8776 |D=192.0.2.3:8776 | |
|------------------>|------------------>| |
| | | |
|(4) Error |(3) Error | |
|S=192.0.2.3:8776 |S=192.0.2.3:8776 | |
|D=10.0.1.1:4334 |D=192.0.2.1:63346 | |
|<------------------|<------------------| |
| | | |
|(5) Allocate |(6) Allocate | |
|S=10.0.1.1:4334 |S=192.0.2.1:63346 | |
|D=192.0.2.3:8776 |D=192.0.2.3:8776 | |
|------------------>|------------------>| |
| | | |
| | (allocates port 32766) |
| | | |
| | | |
|(8) Response |(7) Response | |
|RA=192.0.2.3:32766 |RA=192.0.2.3:32766 | |
|MA=192.0.2.1:63346 |MA=192.0.2.1:63346 | |
|S=192.0.2.3:8776 |S=192.0.2.3:8776 | |
|D=10.0.1.1:4334 |D=192.0.2.1:63346 | |
|<------------------|<------------------| |
| | | |
|(9) SIP INVITE | | |
|SDP=192.0.2.3:32766| | |
|---------------------------------------------------------->|
| | | |
|(10) SIP 200 OK | | |
|SDP=192.0.2.17:12734 | |
|<----------------------------------------------------------|
| | | |
| | |(11) RTP |
| | |S=192.0.2.17:12734 |
| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| | (no permission; packet dropped) |
| | | |
|(12) SIP ACK | | |
|---------------------------------------------------------->|
| | | |
|(13) Send Indic. |(14) Send Indic. | |
|TURN Channel=0 |TURN Channel=0 | |
|STUN DATA=RTP |STUN DATA=RTP | |
|CHANNEL-NUMER=77 |CHANNEL-NUMBER=77 | |
|PA=192.0.2.17:12734|PA=192.0.2.17:12734| |
|S=10.0.1.1:4334 |S=192.0.2.1:63346 | |
|D=192.0.2.3:8776 |D=192.0.2.3:8776 | |
|------------------>|------------------>| |
| | | |
| | permission created |
| | | |
| | |(15) RTP |
| | |S=192.0.2.3:32766 |
| | |D=192.0.2.17:12734 |
| | |------------------>|
| | | |
|(17) ChannelConf |(16) ChannelConf | |
|TURN Channel=0 |TURN Channel=0 | |
|CHANNEL-NUMBER=77 |CHANNEL-NUMBER=77 | |
|PA=192.0.2.17:12734|PA=192.0.2.17:12734| |
|S=192.0.2.3:8776 |S=192.0.2.3:8776 | |
|D=10.0.1.1:4334 |D=192.0.2.1:63346 | |
|<------------------|<------------------| |
| | | |
|(18) TURN Framed |(19) TURN Framed | |
|TURN Channel=77 |TURN Channel=77 |(20) RTP |
|S=10.0.1.1:4334 |S=192.0.2.1:63346 |S=192.0.2.3:32766 |
|D=192.0.2.3:8776 |D=192.0.2.3:8776 |D=192.0.2.17:12734 |
|------------------>|------------------>|------------------>|
| | | |
|(23) Data Indic. |(22) Data Indic. | |
|TURN Channel=0 |TURN Channel=0 | |
|CHANNEL-NUMBER=33 |CHANNEL-NUMBER=33 |(21) RTP |
|S=192.0.2.3:8776 |S=192.0.2.3:8776 |S=192.0.2.17:12734 |
|D=10.0.1.1:4334 |D=192.0.2.1:63346 |D=192.0.2.3:32766 |
|<------------------|<------------------|<------------------|
| | | |
|(24) ChannelConf |(25) ChannelConf | |
|TURN Channel=0 |TURN Channel=0 | |
|CHANNEL-NUMBER=33 |CHANNEL-NUMBER=33 | |
|S=10.0.0.1:4334 |S=192.0.2.3:8776 | |
|D=192.0.2.3:8776 |D=192.0.2.3:8776 | |
|------------------>|------------------>| |
| | | |
|(28) TURN Framed |(27) TURN Framed | |
|TURN Channel=33 |TURN Channel=33 |(26) RTP |
|S=192.0.2.3:8776 |S=192.0.2.3:8776 |S=192.0.2.17:12734 |
|D=10.0.1.1:4334 |D=192.0.2.1:63346 |D=192.0.2.3:32766 |
|<------------------|<------------------|<------------------|
| | | |

Figure 12

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

In step 1, the client sends this document as an Allocate Request (1) with a source
address (denoted by S) of 10.0.1.1:4334 and a destination (denoted by
D) of 192.0.2.3:8776. RFC.

This passes through the NAT (2), which
allocates a new UDP port (63346) on section lists the NAT's public interface
(192.0.2.1), and creates an internal mapping changes between the internal
address 10.0.1.1:4334 and that external address 192.0.2.1:63346. The
NAT sends various versions of this request
specification.

16.1. Changes from -05 to -06

o Changed the TURN server (3). The TURN server
challenges mechanism for allocating channels to the request, requesting credentials one proposed
by sending a STUN
error and including Eric Rescorla at the NONCE and REALM attributes. Message 3 is
relayed, by Dec 2007 IETF meeting.

o Removed the NAT, framing mechanism (which was used to frame all
messages) and replaced it with the TURN client (4). The client sends a new
request (from the same UDP port), including its credentials (5, 6).
The TURN server authenticates ChannelData message. As part
of this change, noted that the request. The TURN server allocates
a new UDP port on one demux of its interfaces, 192.0.2.3:32766. The ChannelData messages from
TURN
server puts 192.0.2.3:32766 into messages can be done using the RELAY-ADDRESS (denoted by RA)
attribute first two bits of the response, and puts message.

o Rewrote the source IP address sections on transmitted and UDP
port receiving data as a result
of the request (as seen by the TURN server) into the XOR-MAPPED-
ADDRESS attribute (denoted by MA). In step 7, this message is sent
back above to the TURN client changes, splitting it into a section on Send and
Data Indications and relayed by the NAT in step 8.

The client now proceeds to perform a basic SIP call setup. In
message 9, the TURN client includes the TURN server's address (which
it learned in message 8) in separate section on channels.

o Clarified the SDP handling of its INVITE (e.g., using syntax
described in[I-D.ietf-mmusic-ice]). The called party responds with
its SDP in a provisional response (18x) or a final response (200 Ok).
The called party's SDP includes its IP address and Allocate Request messages. In
particular, subsequent Allocate Request messages over UDP port,
192.0.2.17:12734. Immediately after sending its 200 Ok, the called
party sends an RTP packet to the TURN server's IP address (11). This
RTP packet is dropped by the TURN server, because with the TURN server has
not been given permission to relay that data. Incoming packets
same transaction id are
dropped until a permission is created. The SIP exchange completes
with not an SIP 200 Ok message (12).

Steps 13-20 show the client performing error but a channel allocation. The
TURN client needs to send an RTP packet. Since no channels and no
permissions have been created, the TURN client sends retransmission.

o Restricted the RTP packet
inside range of a Send Indication, using channel number 0, with the
CHANNEL-NUMBER attribute set to the channel number the TURN client
wants to use ports available for subsequent communication with this TURN peer (77 is
shown in the example). The TURN peer's IP address and UDP port
(which were learned from the SDP answer received in step 10) are
placed in the PEER-ADDRESS attribute (denoted by PA). In message 13,
the TURN client sends this Send Indication, and it is relayed by the
NAT allocation to the TURN server (14). Upon receipt of that message, the TURN
server creates a permission, which allows subsequent traffic from
that same peer address to be relayed to that TURN client's IP address
Dynamic and/or Private Port range, and UDP port. The TURN server sends the contents of the Send
Indication's DATA attribute towards the PEER-ADDRESS (15); noted when ports outside
this will
typically range can be an RTP packet. Note that used.

o Changed the source address and port format of
message 15 is the TURN server's address, 192.0.2.3:32766, which is the allocated transport address communicated to the TURN client in
messages 7 and 8.

In step 16, the TURN server sends a channel confirmation message to
the TURN client. Once the TURN client receives this message, it can
forgo using the Send Indication REQUESTED-TRANSPORT attribute. The
previous version used 00 for that channel. Instead, it can
utilize the channel number in the TURN framing header. Steps 18 and
19 show the TURN client sending a message to TURN server using the
TURN framing header, with channel=1. Step 20 shows the TURN server
removing the TURN framing UDP and sending the RTP packet to 01 for TCP; the TURN
peer.

Steps 21-28 show an RTP packet new version
uses protocol numbers from the TURN peer, which causes a
channel allocation by the TURN server. In packet 21, an RTP packet
is sent by the TURN peer to the TURN server. There is an existing
permission (created in step 14), so the TURN server accepts this
incoming RTP packet. IANA protocol number registry. The TURN server knows
format of the TURN client to send
this packet to, but does not yet have a channel assigned for traffic
in this direction. The TURN server chooses attribute also changed.

o Made a channel large number (33 in
the example), and sends a Data Indication to the TURN client (message
22). The NAT relays this of changes to the TURN client (message 23). The TURN
client sends an Channel Confirmation message (24) which is relayed by
the NAT (25). When the TURN server receives non-normative portion of the Channel
Confirmation, it no longer needs
document to use a Send Indication for traffic
from that remote peer; instead, it can use TURN framing with its
chosen channel number (33). The next RTP packet that arrives from
that peer (26) is sent by the TURN server using TURN framing
indicating the channel number (message 27) reflect technical changes and relayed by improve the NAT to
presentation.

o Added the TURN client (28).

15. Issues section.

16.2. Changes since version from -04

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

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

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

16.

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
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?

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

18. Acknowledgements

The authors would like to thank Marc Petit-Huguenin the various participants in the
BEHAVE working group for his their many comments on this draft. Marc
Petit-Huguenin, Remi Denis-Courmont, Cullen Jennings, Lars Eggert,
Magnus Westerlund, and suggestions.

17. 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

17.1.

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-12
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.

17.2.

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.

[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
US
USA

Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Rohan Mahy
Plantronics, Inc.

Email: rohan@ekabal.com

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

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

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

Phone:
Fax:
Email: dwing@cisco.com
URI:

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