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Versions: (draft-rosenberg-midcom-turn) 00 01
02 03 04 05 06 07 08 09 10 11 12 13
14 15 16 RFC 5766
BEHAVE WG J. Rosenberg
Internet-Draft Cisco
Intended status: Standards Track R. Mahy
Expires: December 26, 2008 Plantronics
P. Matthews
(Unaffiliated)
June 24, 2008
Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-behave-turn-08
Status of this Memo
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Abstract
If a host is located behind a NAT, then in certain situations it can
be impossible for that host to communicate directly with other hosts
(peers) located behind other NATs. In these situations, it is
necessary for the host to use the services of an intermediate node
that acts as a communication relay. This specification defines a
protocol, called TURN (Traversal Using Relays around NAT), that
allows the host to control the operation of the relay and to exchange
packets with its peers using the relay.
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The TURN protocol can be used in isolation, but is more properly used
as part of the ICE (Interactive Connectivity Establishment) approach
to NAT traversal.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
2.1. Transports . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Allocations . . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Exchanging Data with Peers . . . . . . . . . . . . . . . . 8
2.4. Channels . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.5. Permissions . . . . . . . . . . . . . . . . . . . . . . . 11
2.6. Preserving vs. Non-Preserving Allocations . . . . . . . . 11
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. General Behavior . . . . . . . . . . . . . . . . . . . . . . . 13
5. Allocations . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Creating an Allocation . . . . . . . . . . . . . . . . . . . . 16
6.1. Sending an Allocate Request . . . . . . . . . . . . . . . 16
6.2. Receiving an Allocate Request . . . . . . . . . . . . . . 17
6.3. Receiving an Allocate Response . . . . . . . . . . . . . . 21
7. Refreshing an Allocation . . . . . . . . . . . . . . . . . . . 23
7.1. Sending a Refresh Request . . . . . . . . . . . . . . . . 23
7.2. Receiving a Refresh Request . . . . . . . . . . . . . . . 23
7.3. Receiving a Refresh Response . . . . . . . . . . . . . . . 24
8. Permissions . . . . . . . . . . . . . . . . . . . . . . . . . 24
9. Send and Data Indications . . . . . . . . . . . . . . . . . . 25
9.1. Sending a Send Indication . . . . . . . . . . . . . . . . 25
9.2. Receiving a Send Indication . . . . . . . . . . . . . . . 26
9.3. Receiving a UDP Datagram . . . . . . . . . . . . . . . . . 26
9.4. Receiving a Data Indication . . . . . . . . . . . . . . . 27
10. Channels . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
10.1. Sending a ChannelBind Request . . . . . . . . . . . . . . 28
10.2. Receiving a ChannelBind Request . . . . . . . . . . . . . 28
10.3. Receiving a ChannelBind Response . . . . . . . . . . . . . 29
10.4. The ChannelData Message . . . . . . . . . . . . . . . . . 29
10.5. Sending a ChannelData Message . . . . . . . . . . . . . . 30
10.6. Receiving a ChannelData Message . . . . . . . . . . . . . 30
10.7. Relaying . . . . . . . . . . . . . . . . . . . . . . . . . 31
11. IP and ICMP . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.1. IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
11.2. ICMP . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
12. New STUN Methods . . . . . . . . . . . . . . . . . . . . . . . 36
13. New STUN Attributes . . . . . . . . . . . . . . . . . . . . . 37
13.1. CHANNEL-NUMBER . . . . . . . . . . . . . . . . . . . . . . 37
13.2. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.3. PEER-ADDRESS . . . . . . . . . . . . . . . . . . . . . . . 37
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13.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13.5. RELAY-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 38
13.6. REQUESTED-PROPS . . . . . . . . . . . . . . . . . . . . . 38
13.7. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . . 38
13.8. RESERVATION-TOKEN . . . . . . . . . . . . . . . . . . . . 39
14. New STUN Error Response Codes . . . . . . . . . . . . . . . . 39
15. Security Considerations . . . . . . . . . . . . . . . . . . . 40
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 41
17. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 42
18. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
19. Changes from Previous Versions . . . . . . . . . . . . . . . . 42
19.1. Changed from -07 to -08 . . . . . . . . . . . . . . . . . 42
19.2. Changes from -06 to -07 . . . . . . . . . . . . . . . . . 43
19.3. Changes from -05 to -06 . . . . . . . . . . . . . . . . . 45
19.4. Changes from -04 to -05 . . . . . . . . . . . . . . . . . 46
20. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . . 47
21. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
22. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
22.1. Normative References . . . . . . . . . . . . . . . . . . . 48
22.2. Informative References . . . . . . . . . . . . . . . . . . 48
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
Intellectual Property and Copyright Statements . . . . . . . . . . 51
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1. Introduction
Session Traversal Utilities for NAT (STUN)
[I-D.ietf-behave-rfc3489bis] provides a suite of tools for
facilitating the traversal of NAT. Specifically, it defines the
Binding method, which is used by a client to determine its reflexive
transport address towards the STUN server. The reflexive transport
address can be used by the client for receiving packets from peers,
but only when the client is behind "good" NATs. In particular, if a
client is behind a NAT whose mapping behavior [RFC4787] is address or
address and port dependent (sometimes called "bad" NATs), the
reflexive transport address will not be usable for communicating with
a peer.
The only reliable way to obtain a UDP transport address that can be
used for corresponding with a peer through such a NAT is to make use
of a relay. The relay sits on the public side of the NAT, and
allocates transport addresses to clients reaching it from behind the
private side of the NAT. These allocated transport addresses, called
relayed transport address, are IP addresses and ports on the relay.
When the relay receives a packet on one of these allocated addresses,
the relay forwards it toward the client.
This specification defines an extension to STUN, called TURN, that
allows a client to request a relayed transport address on a TURN
server.
Though a relayed transport address is highly likely to work when
corresponding with a peer, it comes at high cost to the provider of
the relay service. As a consequence, relayed transport addresses
should only be used as a last resort. Protocols using relayed
transport addresses should make use of mechanisms to dynamically
determine whether such an address is actually needed. One such
mechanism, defined for multimedia session establishment protocols
based on the offer/answer protocol in RFC 3264 [RFC3264], is
Interactive Connectivity Establishment (ICE) [I-D.ietf-mmusic-ice].
TURN was originally invented to support multimedia sessions signaled
using SIP. Since SIP supports forking, TURN supports multiple peers
per client; a feature not supported by other approaches (e.g., SOCKS
[RFC1928]). However, care has been taken in the later stages of its
development to make sure that TURN is suitable for other types of
applications.
2. Overview of Operation
This section gives an overview of the operation of TURN. It is non-
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normative.
In a typical configuration, a TURN client is connected to a private
network [RFC1918] and through one or more NATs to the public
Internet. On the public Internet is a TURN server. Elsewhere in the
Internet are one or more peers that the TURN client wishes to
communicate with. These peers may or may not be behind one or more
NATs.
+---------+
| |
| |
/ | Peer A |
Client's TURN // | |
Host Transport Server / | |
Address Address +-+ // +---------+
10.1.1.2:17240 192.0.2.15:3478 |N|/ 192.168.100.2:16400
| | |A|
| +-+ | /|T|
| | | | / +-+
v | | | / 192.0.2.210:18200
+---------+ | | |+---------+ / +---------+
| | |N| || | // | |
| TURN | | | v| TURN |/ | |
| Client |----|A|----------| Server |------------------| Peer B |
| | | |^ | |^ ^| |
| | |T|| | || || |
+---------+ | || +---------+| |+---------+
| || | |
| || | |
+-+| | |
| | |
| | |
Client's | Peer B
Server-Reflexive Relayed Transport
Transport Address Transport Address Address
192.0.2.1:7000 192.0.2.15:9000 192.0.2.210:18200
Figure 1
Figure 1 shows a typical deployment. In this figure, the TURN client
and the TURN server are separated by a NAT, with the client on the
private side and the server on the public side of the NAT. This NAT
is assumed to be a "bad" NAT; for example, it might have a mapping
property of address-and-port-dependent mapping (see [RFC4787]) for a
description of what this means).
The client has allocated a local port on one of its addresses for use
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in communicating with the server. The combination of an IP address
and a port is called a TRANSPORT ADDRESS and since this (IP address,
port) combination is located on the client and not on the NAT, it is
called the client's HOST transport address.
The client sends TURN messages from its host transport address to a
transport address on the TURN server which is known as the TURN
SERVER ADDRESS. The client learns the server's address through some
unspecified means (e.g., configuration), and this address is
typically used by many clients simultaneously. The TURN server
address is used by the client to send both commands and data to the
server; the commands are processed by the TURN server, while the data
is relayed on to the peers.
Since the client is behind a NAT, the server sees these packets as
coming from a transport address on the NAT itself. This address is
known as the client's SERVER-REFLEXIVE transport address; packets
sent by the server to the client's server-reflexive transport address
will be forwarded by the NAT to the client's host transport address.
The client uses TURN commands to allocate a RELAYED TRANSPORT
ADDRESS, which is an transport address located on the TURN server.
The server ensures that there is a one-to-one relationship between
the client's server-reflexive transport address and the relayed
transport address; thus a packet received at the relayed transport
address can be unambiguously relayed by the server to the client.
The client will typically communicate this relayed transport address
to one or more peers through some mechanism not specified here (e.g.,
an ICE offer or answer [I-D.ietf-mmusic-ice]). Once this is done,
the client can send data to the server to relay towards its peers.
In the reverse direction, peers can send data to the the relayed
transport address of the client. The server will relay this data to
the client as long as the client explicitly created a permission (see
Section 2.5) for the IP address of the peer.
2.1. Transports
TURN as defined in this specification only allows the use of UDP
between the server and the peer. However, this specification allows
the use of any one of UDP, TCP, or TLS over TCP to carry the TURN
messages between the client and the server.
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+----------------------------+---------------------+
| TURN client to TURN server | TURN server to peer |
+----------------------------+---------------------+
| UDP | UDP |
| TCP | UDP |
| TLS over TCP | UDP |
+----------------------------+---------------------+
If TCP or TLS over TCP is used between the client and the server,
then the server will convert between stream transport and UDP
transport when relaying data. TURN allows both TCP and TLS over TCP
as transports in part because many firewalls are configured to not
pass any UDP traffic.
For TURN clients, using TLS over TCP to communicate with the TURN
server provides two benefits. First, the client can be assured that
the addresses of its peers are not visible to any attackers between
it and the server. Second, the client may be able to communicate
with TURN servers using TLS when it would not be able to communicate
with the same server using TCP or UDP, due to the policy of a
firewall between the TURN client and its server. In this second
case, TLS between the client and TURN server facilitates traversal.
There is a planned extension to TURN to add support for TCP between
the server and the peers [I-D.ietf-behave-turn-tcp]. For this
reason, allocations that use UDP between the server and the peers are
known as UDP allocations, while allocations that use TCP between the
server and the peers are known as TCP allocations. This
specification describes only UDP allocations.
2.2. Allocations
To allocate a relayed transport address, the client uses an Allocate
transaction. The client sends a Allocate Request to the server, and
the server replies with an Allocate Response containing the allocated
relayed transport address. The client can include attributes in the
Allocate Request that describe the type of allocation it desires
(e.g., the lifetime of the allocation). And since relaying data may
require lots of bandwidth, the server typically requires that the
client authenticate itself using STUN's long-term credential
mechanism, to show that it is authorized to use the server.
Once a relayed transport address is allocated, a client must keep the
allocation alive. To do this, the client periodically sends a
Refresh Request to the server with the allocated related transport
address. TURN deliberately uses a different method (Refresh rather
than Allocate) for refreshes to ensure that the client is informed if
the allocation vanishes for some reason.
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The frequency of the Refresh transaction is determined by the
lifetime of the allocation. The client can request a lifetime in the
Allocate Request and may modify its request in a Refresh Request, and
the server always indicates the actual lifetime in the response. The
client must issue a new Refresh transaction within 'lifetime' seconds
of the previous Allocate or Refresh transaction. If a client no
longer wishes to use an Allocation, it should do a Refresh
transaction with a requested lifetime of 0.
Note that sending or receiving data from a peer DOES NOT refresh the
allocation.
The server keeps track of the client reflexive transport address and
port, the server transport address and port, and the protocol used by
the client to communicate with the server. (Together known as a
5-tuple. The server remembers the 5-tuple used in the Allocate
Request. Subsequent transactions between the client and the server
use this same 5-tuple. In this way, the server knows which client
owns the allocated relayed transport address. If the client wishes
to allocate a second relayed transport address, it must use a
different 5-tuple for this allocation (e.g., by using a different
client host address).,
While the terminology used in this document refers to 5-tuples,
the TURN server can store whatever identifier it likes that yields
identical results. Specifically, many implementations use a file-
descriptor in place of a 5-tuple to represent a TCP connection.
2.3. Exchanging Data with Peers
There are two ways for the client and peers to exchange data using
the TURN server. The first way uses Send and Data indications, the
second way uses channels. Common to both ways is the ability of the
client to communicate with multiple peers using a single allocated
relayed transport address; thus both ways include a means for the
client to indicate to the server which peer to forward the data to,
and for the server to indicate which peer sent the data.
When using the first way, the client sends a Send indication to the
TURN server containing, in attributes inside the indication, the
transport address of the peer and the data to be sent to that peer.
When the TURN server receives the Send Indication, it extracts the
data from the Send Indication and sends it in a UDP datagram to the
peer, using the allocated relay address as the source address. In
the reverse direction, UDP datagrams arriving at the relay address on
the TURN server are converted into Data Indications and sent to the
client, with the transport address of the peer included in an
attribute in the Data Indication.
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TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
Figure 2
In the figure above, the client first allocates a relayed transport
address. It then sends data to Peer A using a Send Indication; at
the server, the data is extracted and forwarded in a UDP datagram to
Peer A, using the relayed transport address as the source transport
address. When a UDP datagram from Peer A is received at the relayed
transport address, the contents are placed into a Data Indication and
forwarded to the client. A similar exchange happens with Peer B.
2.4. Channels
For some applications (e.g. Voice over IP), the 36 bytes of overhead
that a Send or Data indication adds to the application data can
substantially increase the bandwidth required between the client and
the server. To remedy this, TURN offers a second way for the client
and server to associate data with a specific peer.
This second way uses an alternate packet format known as the
ChannelData message. The ChannelData message does not use the STUN
header used by other TURN messages, but instead has a 4-byte header
that includes a number known as a channel number. Each channel
number in use is bound to a specific peer and thus serves as a
shorthand for the peer's address.
To bind a channel to a peer, the client sends a ChannelBind request
to the server, and includes an unbound channel number and the
transport address of the peer. Once the channel is bound, the client
can use a ChannelData message to send the server data destined for
the peer. Similarly, the server can relay data from that peer
towards the client using a ChannelData message.
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Channel bindings last for 10 minutes unless refreshed. Channel
bindings are refreshed by sending ChannelData messages from the
client to the server, or by rebinding the channel to the peer.
TURN TURN Peer Peer
client server A B
|--- Allocate Req -->| | |
|<-- Allocate Resp ---| | |
| | | |
|--- Send (Peer A)--->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<-- Data (Peer A)----| | |
| | | |
|- ChannelBind Req -->| | |
| (Peer A to 0x4001) | | |
| | | |
|<- ChannelBind Resp -| | |
| | | |
|-- [0x4001] data --->| | |
| |=== data ===>| |
| | | |
| |<== data ====| |
|<- [0x4001] data --->| | |
| | | |
|--- Send (Peer B)--->| | |
| |=== data =================>|
| | | |
| |<== data ==================|
|<-- Data (Peer B)----| | |
Figure 3
The figure above shows the channel mechanism in use. The client
begins by allocating a relayed transport address, and then uses that
address to exchange data with Peer A. After a bit, the client decides
to bind a channel to Peer A. To do this, it sends a ChannelBind
request to the server, specifying the transport address of Peer A and
a channel number (0x4001). After that, the client can send
application data encapsulated inside ChannelData messages to Peer A:
this is shown as "[0x4001] data" where 0x4001 is the channel number.
Note that ChannelData messages can only be used for peers to which
the client has bound a channel. In the example above, Peer A has
been bound to a channel, but Peer B has not, so application data to
and from Peer B uses Send and Data indications.
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Channel bindings are always initiated by the client.
2.5. Permissions
To ease concerns amongst enterprise IT administrators that TURN could
be used to bypass corporate firewall security, TURN includes the
notion of permissions. TURN permissions mimic the address-restricted
filtering mechanism of NATs that comply with [RFC4787].
The client can install a permission by sending data to a peer (or by
doing certain other things). Once a permission is installed, any
peer with the same IP address (the ports numbers can differ) is
permitted to send data to the client. After 5 minutes, the
permission times out and the server drops any UDP datagrams arriving
at the relayed transport from that IP address. Note that permissions
are within the context of an allocation, so adding or expiring a
permission in one allocation does not affect other allocations.
Data received from the peer DOES NOT refresh the permission.
2.6. Preserving vs. Non-Preserving Allocations
Some applications that use TURN are quite tolerant of the different
possible ways a TURN server could set the Diff-Serv, ECN, TTL / Hop
Limit, and Flow Label fields in the IP header of the outgoing packet.
Other applications require that the TURN server set these fields in a
specific way, and also require that the TURN server relay ICMP error
packets. Applications in the second class typically wish to do Path
MTU Discovery or end-to-end QOS.
Unfortunately, reading and manipulating fields in the IP header and
relaying ICMP messages usually requires the server to have special
permissions (e.g., access to RAW sockets or be loaded into the
kernel), something that the person setting up the server may be
unwilling or unable to grant. This is especially true when the
server is part of a larger application, for example a peer-to-peer
application. It is also significantly more difficult to implement
this type of server than just relaying at the UDP layer.
To allow TURN to cater to both usage scenarios, TURN defines the
concept of Preserving vs. Non-Preserving allocations. A Preserving
allocation sets the fields in outgoing IP header correctly, and also
relays ICMP messages, while a Non-Preserving allocation may not relay
correctly in every case. The relaying rules for a Preserving are
designed to guarantee the following:
o Path MTU Discovery works end-to-end (i.e. client-to-peer), using
either the old algorithm ([RFC1191] and [RFC1981]) or the new one
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([RFC4821]);
o ECN and Diff-Serv works end-to-end;
o Loops are prevented by copying and decrementing the TTL/Hop Count
field.
If the client knows its application or usage scenario requires a
Preserving allocation, then it can request one in its Allocate
request. If the server is unable to grant this request, then it
rejects the Allocate request.
Note that a Preserving allocation only makes sense when the transport
protocol to the client is UDP; when the transport is TCP or TLS, the
allocation is always Non-Preserving.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Readers are expected to be familar with [I-D.ietf-behave-rfc3489bis]
and the terms defined there.
The following terms are used in this document:
TURN: A protocol spoken between a TURN client and a TURN server. It
is an extension to the STUN protocol [I-D.ietf-behave-rfc3489bis].
The protocol allows a client to allocate and use a relayed
transport address.
TURN client: A STUN client that implements this specification.
TURN server: A STUN server that implements this specification. It
relays data between a TURN client and its peer(s).
Peer: A host with which the TURN client wishes to communicate. The
TURN server relays traffic between the TURN client and its
peer(s). The peer does not interact with the TURN server using
the protocol defined in this document; rather, the peer receives
data sent by the TURN server and the peer sends data towards the
TURN server.
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Host Transport Address: A transport address allocated on a host.
Server-Reflexive Transport Address: A transport address on the
"public side" of a NAT. This address is allocated by the NAT to
correspond to a specific host transport address.
Relayed Transport Address: A transport address that exists on a TURN
server. If a permission exists, packets that arrive at this
address are relayed towards the TURN client.
Allocation: The relayed transport address granted to a client
through an Allocate request, along with related state, such as
permissions and expiration timers.
5-tuple: The combination (client IP address and port, server IP
address and port, and transport protocol (UDP or TCP)) used to
communicate between the client and the server . The 5-tuple
uniquely identifies this communication stream. The 5-tuple also
uniquely identifies the Allocation on the server.
Permission: The IP address and transport protocol (but not the port)
of a peer that is permitted to send traffic to the TURN server and
have that traffic relayed to the TURN client. The TURN server
will only forward traffic to its client from peers that match an
existing permission.
Preserving Allocation An allocation that sets the the fields in the
IP header in a specific manner when relaying application data, and
which also relays ICMP messages. An allocation that may not do
this in some cases is called a Non-Preserving allocation.
4. General Behavior
This section contains general TURN processing rules that apply to all
TURN messages.
TURN is an extension to STUN. All TURN messages, with the exception
of the ChannelData message, are STUN-formatted messages. All the
base processing rules described in [I-D.ietf-behave-rfc3489bis] apply
to STUN-formatted messages. This means that all the message-forming
and -processing descriptions in this document are implicitly prefixed
with the rules of [I-D.ietf-behave-rfc3489bis].
In addition, the server SHOULD require that all TURN requests use the
Long-Term Credential mechanism described in
[I-D.ietf-behave-rfc3489bis], and the client MUST be prepared to
authenticate requests if required. The server's administrator MUST
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choose a realm value that will uniquely identify the username and
password combination that the client must use, even if the client
uses multiple servers under different administrations. The server's
administrator MAY choose to allocate a unique username to each
client, or MAY choose to allocate the same username to more than one
client (for example, to all clients from the same department or
company).
The client and/or the server MAY include the FINGERPRINT attribute in
any of the methods defined in this document. However, TURN does not
use the backwards-compatibility mechanism described in
[I-D.ietf-behave-rfc3489bis].
By default, TURN runs on the same port as STUN. However, either the
SRV procedures or the ALTERNATE-SERVER procedures described in
Section 6 may be used to run TURN on a different port.
5. Allocations
All TURN operations revolve around allocations, and all TURN messages
are associated with an allocation. An allocation conceptually
consists of the following state data:
o the relayed transport address
o The 5-tuple: client IP address, client port, server IP address,
server port, transport protocol
o the username
o the transaction ID of the Allocate request
o the time-to-expiry
o A list of permissions
o A list of channel to peer bindings
o A flag indicating whether or not the allocation is Preserving
The relayed transport address is the transport address allocated by
the server for communicating with peers, while the 5-tuple describes
the communication path between the client and the server. Both of
these MUST be unique across all allocations, so either one can be
used to uniquely identify the allocation.
When a TURN message arrives at the server from the client, the server
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uses the 5-tuple in the message to identify the associated
allocation. For all TURN messages (including ChannelData) EXCEPT an
Allocate request, if the 5-tuple does not identify an existing
allocation, then the message MUST either be rejected with a 437
Allocation Mismatch error (if it is a request), or silently ignored
(if it is an indication or a ChannelData message). A client
receiving a 437 error response to a request other than Allocate MUST
assume the allocation no longer exists.
The username and password of the allocation is the username and
password of the authenticated Allocate request that creates the
allocation. Subsequent requests on an allocation use the same
username and password as those used to create the allocation, to
prevent attackers from hijacking the client's allocation.
Specifically, if the server requires the use of the Long-Term
Credential mechanism, and if a non-Allocate request passes
authentication under this mechanism, and if the 5-tuple identifies an
existing allocation, but the request does not use the same username
as used to create the allocation, then the request MUST be rejected
with a 438 (Wrong Credentials) error.
The transaction ID of the allocation is the transaction ID used in
the Allocate request. This is used to detect retransmissions of the
Allocate request over UDP (see Section 6.2 for details).
The time-to-expiry is the time in seconds left until the allocation
expires. Each Allocate or Refresh transaction sets this timer, which
then ticks down towards 0. By default, each Allocate or Refresh
transaction resets this timer to 600 seconds (10 minutes), but the
client can request a different value in the Allocate and Refresh
request. Allocations can only be refreshed using the Refresh
request; sending data to a peer does not refresh an allocation. When
an allocation expires, the state data associated with the allocation
is freed. However the server MUST ensure that neither the relayed
transport address nor the client reflexive transport address from the
5-tuple are re-used in other allocations until 2 minutes after the
allocation expires; this ensures that any messages that are in
transit when the allocation expires are gone before either of these
transport addresses are re-used.
The list of permissions is described in Section 8 and the list of
channels is described in Section 10.
The differences between a Preserving and a Non-Preserving allocation
are described in Section 11.
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6. Creating an Allocation
An allocation on the server is created using an Allocate transaction.
6.1. Sending an Allocate Request
The client forms an Allocate request as follows.
The client first needs to pick a host transport address that the
server does not think is currently in use, or was recently in use.
The client SHOULD pick a currently-unused transport address on the
client's host (typically by allowing its OS to pick a currently-
unused port for a new socket).
The client needs to pick a transport protocol to use between the
client and the server. The transport protocol MUST be one of UDP,
TCP, or TLS over TCP. Since this specification only allows UDP
between the server and the peers, it is RECOMMENDED that the client
pick UDP unless it has a reason to use a different transport. One
reason to pick a different transport would be that the client
believes, either through configuration or by experiment, that it is
unable to contact any TURN server using UDP. See Section 2.1 for
more discussion.
The client must also pick a server transport address. Typically,
this is done by the client learning (perhaps through configuration)
one or more domain names for TURN servers. In this case, the client
uses the DNS procedures described in [I-D.ietf-behave-rfc3489bis],
but using an SRV service name of "turn" (or "turns" for TURN over
TLS) instead of "stun" (or "stuns"). For example, to find servers in
the example.com domain, the client performs a lookup for
'_turn._udp.example.com', '_turn._tcp.example.com', and
'_turns._tcp.example.com' if the client wants to communicate with the
server using UDP, TCP, or TLS over TCP, respectively.
The client MUST include a REQUESTED-TRANSPORT attribute in the
request. This attribute specifies the transport protocol between the
server and the peers (note that this is NOT the transport protocol
that appears in the 5-tuple). In this specification, the REQUESTED-
TRANSPORT type is always UDP. This attribute is included to allow
future extensions specify other protocols (e.g.,
[I-D.ietf-behave-turn-tcp]).
If the client wishes the server to initialize the time-to-expire
field of the allocation to some value other the default lifetime,
then it MAY include a LIFETIME attribute specifying its desired
value. This is just a request, and the server may elect to use a
different value. Note that the server will ignore requests to
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initialize the field to less tha the default value.
If the client required the allocation to satisfy certain properties,
then the client includes the REQUESTED-PROPS attribute. This
attribute is optional, and can be omitted if no special properties
are required.
Using the E and R bits in the REQUESTED-PROPS attribute, the client
can request:
o (E=1, R=0 ) That the server allocate a relayed transport address
with an even port number; OR
o (E=1, R=1) That the server reserve a pair of relayed transport
addresses with adjacent port numbers N and N+1, where N is even
and N+1 is odd, and then use port N for the current allocation.
In this case, the server returns a RESERVATION-TOKEN attribute in
the response which the client can then include in a subsequent
Allocate request to create an allocation with port number N+1.
Note that the client cannot request a pair of adjacent ports unless
it also requests that the lower numbered port be even. Thus the
combination (E=0, R=1) is not allowed.
Similarly, by setting the P bit to 1 in the REQUESTED-PROPS
attribute, the client can request that the server allocate a
Preserving allocation.
For all the various REQUESTED-PROPS flags, if the server cannot
satisfy the request, the Allocate request is rejected.
The client MAY also include a RESERVATION-TOKEN attribute in the
request to ask the server to use a previously reserved port for the
allocation. If the RESERVATION-TOKEN attribute is included, then the
client MUST either omit the REQUESTED-PROPS attribute or set E=0 and
R=0, since doing otherwise would make no sense.
Once constructed, the client sends the Allocate request on the
5-tuple.
6.2. Receiving an Allocate Request
When the server receives an Allocate request, it performs the
following checks:
1. The server checks the credentials of the request, as per the
Long-Term Credential mechanism of [I-D.ietf-behave-rfc3489bis].
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2. The server checks if the 5-tuple is currently in use by an
existing allocation, or was it in use by another allocation
within the last 2 minutes. If yes, then there are two sub-cases:
* If the transport protocol in the 5-tuple is UDP, and if the
5-tuple is currently in use by an existing allocation, and if
the transaction id of the request matches the transaction id
stored with the allocation, then the request is a
retransmission of the original request. The server replies
either with a stored copy of the original response, or with a
response rebuilt from the stored state data. If the server
chooses to rebuild the response, then (a) it need not parse
the request further, but can immediately start building a
success response, (b) the value of the LIFETIME attribute can
be set to the current value of the time-to-expire timer, and
(c) the server may need to include an extra field in the
allocation to store the token returned in a RESERVATION-TOKEN
attribute.
* Otherwise, the server rejects the request with a 437
(Allocation Mismatch) error.
NOTE: If the request includes credentials that are acceptable to
server, but the 5-tuple is already in use, then it is important
that the server reject the request with a 437 (Allocation
Mismatch) error rather than a 401 (Unauthorized) error. This
ensures that the client knows that the problem is with the
5-tuple, rather than (wrongly) believing that the problem lies
with its credentials.
3. The server checks if the request contain a REQUESTED-TRANPORT
attribute. If the REQUESTED-TRANSPORT attribute is not included
or is malformed, the server rejects the request with a 400 (Bad
Request) error. Otherwise, if the attribute is included but
specifies a protocol other that UDP, the server rejects the
request with a 422 (Unsupported Transport Protocol) error.
4. The server checks if the request contains a REQUESTED-PROPS
attribute. If yes, then the server checks that it understands
and can satisfy all the flags that are set to 1. If a flag is
not understood, or if the server cannot satisfy the request, then
the server rejects the request with a 508 (Insufficient Port
Capacity) error. Note that the combination (E=0, R=1) MUST be
treated as unsupported.
5. The server checks if the request contains a RESERVATION-TOKEN
attribute. If yes, and the request also contains a REQUESTED-
PROPS attribute, then the server rejectes the request with a 400
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(Bad Request) error. Otherwise it checks to see if the token is
valid (i.e., the token is in range and has not expired, and the
corresponding relayed transport address is still available). If
the token is not valid for some reason, the server rejects the
request with a 508 (Insufficient Port Capacity) error.
6. At any point, the server MAY also choose to reject the request
with a 486 (Allocation Quota Reached) error if it feels the
client is trying to exceed some locally-defined allocation quota.
The server is free to define this allocation quota any way it
wishes, but SHOULD define it based on the username used to
authenticate the request, and not on the client's transport
address.
If the server rejects the request with one of the error codes 422
(Unsupported Transport Protocol), 486 (Allocation Quota Reached) or
508 (Insufficient Port Capacity), it MAY include an ALTERNATE-SERVER
attribute in the error response redirecting the client to another
server that it believes will accept the request. If the attribute is
included, the address MUST be from the same address family as the
server's transport address. Note that, if the attribute is included,
the client will try this alternate server before trying the other
servers given by the SRV procedures.
If all the checks pass, the server creates the allocation. The
5-tuple is set to the 5-tuple from the Allocate request, while the
list of permissions and the list of channels are initially empty.
When allocating a relayed transport address for the allocation, the
server MUST allocate an IP address from the same family (e.g, IPv4
vs. IPv6) as that on which the request was received (i.e., the
server's IP address in the 5-tuple for the allocation).
NOTE: An extension to TURN to allow an address from a different
address family is currently in progress
[I-D.ietf-behave-turn-ipv6].
In addition, the server SHOULD only allocate ports from the range
49152 - 65535 (the Dynamic and/or Private Port range [Port-Numbers]),
unless the TURN server application knows, through some means not
specified here, that other applications running on the same host as
the TURN server application will not be impacted by allocating ports
outside this range. This condition can often be satisfied by running
the TURN server application on a dedicated machine and/or by
arranging that any other applications on the machine allocate ports
before the TURN server application starts. In any case, the TURN
server SHOULD NOT allocate ports in the range 0 - 1023 (the Well-
Known Port range) to discourage clients from using TURN to run
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standard services.
If the request contains a REQUESTED-PROPS attribute with the R flag
set, then the server looks for a pair of port numbers N and N+1 on
the same IP address, where N is even. Port N is used in the current
allocation, while the relayed transport address with port N+1 is
assigned a token and reserved for a future allocation. The server
MUST hold this reservation for at least 30 seconds, and MAY choose to
hold longer (e.g. until the allocation with port N expires). The
server then includes the token in a RESERVATION-TOKEN attribute in
the success response.
If the request contains a RESERVATION-TOKEN, the server uses the
previously-reserved transport address corresponding to the included
token (if it is still available).
The server determines the initial value of the time-to-expire field
as follows. If the request contains a LIFETIME attribute, and the
proposed lifetime value is greater than the default lifetime, and the
proposed lifetime value is otherwise acceptable to the server, then
the server uses that value. Otherwise, the server uses the default
value. It is RECOMMENDED that the server impose a maximum lifetime
of no more than 3600 seconds (1 hour).
NOTE: The time-to-expire is recomputed with each successful
Refresh request. Thus the value computed here applies only until
the first refresh.
Once the allocation is created, the server replies with a success
response. The success response contains:
o A RELAYED-ADDRESS attribute containing the relayed transport
address;
o A LIFETIME attribute containing the current value of the time-to-
expire timer;
o A RESERVATION-TOKEN attribute (if a second relayed transport
address was reserved).
o An XOR-MAPPED-ADDRESS attribute containing the client's IP address
and port (from the 5-tuple);
NOTE: The XOR-MAPPED-ADDRESS attribute is included in the response
as a convenience to the client. TURN itself does not make use of
this value, but clients running ICE can often need this value and
can thus avoid having to do an extra Binding transaction with some
STUN server to learn it.
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The response (either success or error) is sent back to the client on
the 5-tuple.
6.3. Receiving an Allocate Response
If the client receives a success response, then it MUST check that
the relayed transport address is in an address family that the client
understands and is prepared to deal with. This specification only
covers the case where the relayed transport address is of the same
address family as the client's transport address. If the relayed
transport address is not in an address family that the client is
prepared to deal with, then the client MUST delete the allocation
(Section 7) and MUST NOT attempt to create another allocation on that
server until it believes the mismatch has been fixed.
The IETF is currently considering mechanisms for transitioning
between IPv4 and IPv6 that could result in a client originating an
Allocate request over IPv4, but the request would arrive at the
server over IPv6, or vica-versa. Hence the importance of this
check.
Otherwise, the client creates its own copy of the allocation data
structure to track what is happening on the server. In particular,
the client needs to remember the actual lifetime received back from
the server, rather than the value sent to the server in the request.
The client must also remember the 5-tuple used for the request and
the username and password it used to authenticate the request to
ensure that it reuses them for subsequent messages. The client also
needs to track the channels and permissions it establishes on the
server.
The client will probably wish to send the relayed transport address
to peers (using some method not specified here) so the peers can
communicate with it. The client may also wish to use the server-
reflexive address it receives in the XOR-MAPPED-ADDRESS attribute in
its ICE processing.
If the client receives an error response, then the processing depends
on the actual error code returned:
o (Request timed out): There is either a problem with the server, or
a problem reaching the server with the chosen transport. The
client MAY choose to try again using a different transport (e.g.,
TCP instead of UDP), or the client MAY try a different server.
o 400 (Bad Request): The server believes the client's request is
malformed for some reason. The client MAY notify the user or
operator and SHOULD NOT retry the same request with this server
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until it believes the problem has been fixed. The client MAY try
a different server.
o 401 (Unauthorized): If the client has followed the procedures of
the Long-Term Credential mechanism and still gets this error, then
the server is not accepting the client's credentials. The client
SHOULD notify the user or operator and SHOULD NOT send any further
requests to this server until it believes the problem has been
fixed. The client MAY try a different server.
o 437 (Allocation Mismatch): This indicates that the client has
picked a 5-tuple which the server sees as already in use or which
was recently in use. One way this could happen is if an
intervening NAT assigned a mapped transport address that was
recently used by another allocation. The client SHOULD pick
another client transport address and retry the Allocate request
(using a different transaction id). The client SHOULD try three
different client transport addresses before giving up on this
server. Once the client gives up on the server, it SHOULD NOT try
to create another allocation on the server for 2 minutes.
o 438 (Wrong Credentials): The client should not receive this error
in response to a Allocate request. The client MAY notify the user
or operator and SHOULD NOT retry the same request with this server
until it believes the problem has been fixed. The client MAY try
a different server.
o 442 (Unsupported Transport Address): The client should not receive
this error in response to a request for a UDP allocation. The
client MAY notify the user or operator and SHOULD NOT retry the
same request with this server until it believes the problem has
been fixed. The client MAY try a different server.
o 486 (Allocation Quota Reached): The server is currently unable to
create any more allocations with this username. The client SHOULD
wait at least 1 minute before trying to create any more
allocations on the server. The client MAY try a different server.
o 508 (Insufficient Port Capacity): The server has no more relayed
transport addresses avaiable, or has none with the requested
properties, or the one that was reserved is no longer available.
If the client is using either the REQUESTED-PROPS or the
RESERVATION-TOKEN attribute, then the client MAY choose to remove
this attribute and try again immediately. Otherwise, the client
SHOULD wait at least 1 minute before trying to create any more
allocations on this server. The client MAY try a different
server.
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If the error response contains an ALTERNATE-SERVER attribute, and the
client elects to try a different server, the the client SHOULD try
the alternate server specified in that attribute (while obeying the
rules in [I-D.ietf-behave-rfc3489bis] for avoiding redirection loops)
before trying any other servers found using the SRV procedures of
[I-D.ietf-behave-rfc3489bis].
7. Refreshing an Allocation
A Refresh transaction can be used to either (a) refresh an existing
allocation and update its time-to-expire, or (b) delete an existing
allocation.
If a client wishes to continue using an allocation, then the client
MUST refresh it before it expires. It is suggested that the client
refresh the allocation roughly 1 minute before it expires. If a
client no longer wishes to use an allocation, then it SHOULD
explicitly delete the allocation. A client MAY also change the time-
to-expire of an allocation at any time for other reasons.
7.1. Sending a Refresh Request
If the client wishes to immediately delete an existing allocation, it
includes a LIFETIME attribute with a value of 0. All other forms of
the request refresh the allocation.
The Refresh transaction updates the time-to-expire timer of an
allocation. If the client wishes the server to set the time-to-
expire timer to something other than the default lifetime, it
includes a LIFETIME attribute with the requested value. The server
then computes a new time-to-expire value in the same way as it does
for an Allocate transaction, with the exception that a requested
lifetime of 0 causes the server to immediately delete the allocation.
The Refresh transaction is sent on the 5-tuple for the allocation.
7.2. Receiving a Refresh Request
When the server receives a Refresh request, it processes it as
follows. If, during processing, an error in the request is detected
(for example, a syntax error in the request which causes a 400
error), then the request is rejected with an error response but the
allocation is NOT deleted (but note that a 437 error will indicate
that the allocation was not found).
The server determines the new value for the time-to-expire field as
follows. If the request contains a LIFETIME attribute, and the
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attribute value is 0, then the server uses a value of 0, which causes
the allocation to expire. Otherwise, if the request contains a
LIFETIME attribute and the attribute value is greater than the
default lifetime, and the attribute value is otherwise acceptable to
the server, then the server uses the attribute value. Otherwise, the
server uses the default value. It is RECOMMENDED that the server
impose a maximum lifetime of no more than 3600 seconds (1 hour).
The server then constructs a success response containing:
o A LIFETIME attribute containing the current value of the time-to-
expire timer.
The response is then sent on the 5-tuple.
7.3. Receiving a Refresh Response
If the client receives a success response to its Refresh request, it
updates its copy of the allocation data structure with the time-to-
expire value contained in the response.
If the client receives an 437 (Allocation Mismatch) error response to
its Refresh request, then it must consider the allocation as having
expired, as described in Section 4. All other errors indicate a
software error on the part of either the client or the server.
8. Permissions
For each allocation, the server keeps a list of zero or more
permissions. Each permission consists an IP address which uniquely
identifies the permission, and an associated time-to-expiry. The IP
address describes a peer that is allowed to send data to the client,
and the time-to-expiry is the number of seconds until the permission
expires.
Various events, as described in subsequent sections, can cause a
permission for a given IP address to be installed or refreshed. This
causes one of two things to happen:
o If no permission for that IP address exists, then a permission is
created with the given IP address and a time-to-expiry equal to
the default permission lifetime.
o If a permission for that IP address already exists, then the
lifetime for that permission is reset to the default permission
lifetime.
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The default permission lifetime MUST be 300 seconds (= 5 minutes).
Each permission's time-to-expire decreases down once per second until
it reaches 0, at which point the permission expires and is deleted.
When a UDP datagram arrives at the relayed transport address for the
allocation, the server checks the list of permissions for that
allocation. If there is a permission with an IP address that is
equal to the source IP address of the UDP datagram, then the UDP
datagram can be relayed to the client. Otherwise, the UDP datagram
is silently discarded. Note that only IP addresses are compared;
port numbers are irrelevant.
The permissions for one allocation are totally unrelated to the
permissions for a different allocation. If an allocation expires,
all its permissions expire with it.
NOTE: Though TURN permissions expire after 5 minutes, many NATs
deployed at the time of publication expire their UDP bindings
considerably faster. Thus an application using TURN will probably
wish to send some sort of keep-alive traffic at a much faster
rate. Applications using ICE should follow the keep-alive
guidelines of ICE [I-D.ietf-mmusic-ice], and applications not
using ICE are advised to do something similar.
9. Send and Data Indications
TURN supports two ways to send and receive data from peers. This
section describes the use of Send and Data indications, while
Section 10 describes the use of the Channel Mechanism.
9.1. Sending a Send Indication
A client can use a Send Indication to pass data to the server for
relaying to a peer. A client can also use a Send Indication without
a DATA attribute to install or refresh a permission for the specified
IP address. A client may use a Send indication to send data to a
peer even if a channel is bound to that peer.
When forming a Send Indication, the client MUST include a PEER-
ADDRESS attribute and MAY include a DATA attribute. If the DATA
attribute is included, then the DATA attribute contains the actual
application data to be sent to the peer, and the PEER-ADDRESS
attribute contains the transport address of the peer to which the
data is to be sent. If the DATA attribute is not present, then the
PEER-ADDRESS attribute contains the IP address for which a permission
is to be installed or refreshed; in this case the port specified in
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the attribute is ignored.
Note that no authentication attributes are included, since
indications cannot be authenticated using the Long-Term Credential
mechanism.
The Send Indication MUST be sent using the same 5-tuple used for the
original allocation.
9.2. Receiving a Send Indication
When the server receives a Send indication, it processes it as
follows.
If the received Send indication contains a DATA attribute, then it
forms a UDP datagram as follows:
o the source transport address is the relayed transport address of
the allocation, where the allocation is determined by the 5-tuple
on which the Send Indication arrived;
o the destination transport address is taken from the PEER-ADDRESS
attribute;
o the data following the UDP header is the contents of the value
field of the DATA attribute.
The resulting UDP datagram is then sent to the peer. If any errors
are detected during this process (e.g., the Send indication does not
contain a PEER-ADDRESS attribute), the received indication is
silently discarded and no UDP datagram is sent.
When the server receives a valid Send Indication, either with or
without a DATA attribute, it also installs or refreshes a permission
for the IP address contained in the PEER-ADDRESS attribute (see
Section 8).
9.3. Receiving a UDP Datagram
When the server receives a UDP datagram at a currently allocated
relayed transport address, the server looks up the allocation
associated with the relayed transport address. It then checks to see
if relaying is permitted, as described in section Section 8).
If relaying is permitted, and there is no channel bound to the peer
that sent the UDP datagram (see ISection 10), then the server forms
and sends a Data indication. The Data indication MUST contain both a
PEER-ADDRESS and a DATA attribute. The DATA attribute is set to the
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value of the 'data octets' field from the datagram, and the PEER-
ADDRESS attribute is set to the source transport address of the
received UDP datagram. The Data indication is then sent on the
5-tuple associated with the allocation.
9.4. Receiving a Data Indication
When the client receives a Data indication, it checks that the Data
indication contains both a PEER-ADDRESS and a DATA attribute. It
then delivers the data octets inside the DATA attribute to the
application, along with an indication that they were received from
the peer whose transport address is given by the PEER-ADDRESS
attribute.
10. Channels
Channels provide a way for the client and server to send application
data using ChannelData messages, which have less overhead than Send
and Data indications.
Channel bindings are always initiated by the client. The client can
bind a channel to a peer at any time during the lifetime of the
allocation. The client may bind a channel to a peer before
exchanging data with it, or after exchanging data with it (using Send
and Data indications) for some time, or may choose never to bind a
channel it. The client can also bind channels to some peers while
not binding channels to other peers.
Channel bindings are specific to an allocation, so that a binding in
one allocation has no relationship to a binding in any other
allocation. If an allocation expires, all its channel bindings
expire with it.
A channel binding consists of:
o A channel number;
o A transport address (of the peer);
o A time-to-expiry timer.
Within the context of an allocation, a channel binding is uniquely
identified either by the channel number or by the transport address.
Thus the same channel cannot be bound to two different transport
addresses, nor can the same transport address be bound to two
different channels.
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A channel binding last for 10 minutes unless refreshed. Refreshing
the binding (by the server receiving either a ChannelBind request
rebinding the channel to the same peer, or by the server receiving a
ChannelData message on that channel) resets the time-to-expire timer
back to 10 minutes. When the channel binding expires, the channel
becomes unbound and available for binding to a different transport
address.
When binding a channel to a peer, the client SHOULD be prepared to
receive ChannelData messages on the channel from the server as soon
as it has sent the ChannelBind request. Over UDP, it is possible for
the client to receive ChannelData messages from the server before it
receives a ChannelBind success response.
In the other direction, the client MAY elect to send ChannelData
messages before receiving the ChannelBind success response. Doing
so, however, runs the risk of having the ChannelData messages dropped
by the server if the ChannelBind request does not succeed for some
reason (e.g., packet lost if the request is sent over UDP, or the
server being unable to fulfill the request). A client that wishes to
be safe should either queue the data, or use Send indications until
the channel binding is confirmed.
10.1. Sending a ChannelBind Request
A channel binding is created using a ChannelBind transaction. A
channel binding can also be refreshed using a ChannelBind
transaction.
To initiate the ChannelBind transaction, the client forms a
ChannelBind request. The channel to be bound is specified in a
CHANNEL-NUMBER attribute, and the peer's transport address is
specified in a PEER-ADDRESS attribute. Section 10.2 describes the
restrictions on these attributes.
Note that rebinding a channel to the same transport address that it
is already bound to provides a way to refresh a channel binding
without sending data to the peer.
Once formed, the ChannelBind Request is sent using the 5-tuple for
the allocation.
10.2. Receiving a ChannelBind Request
When the server receives a ChannelBind request, it checks the
following:
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o The request contains both a CHANNEL-NUMBER and a PEER-ADDRESS
attribute;
o The channel number is in the range 0x4000 to 0xFFFE (inclusive);
o The channel number is not currently bound to a different transport
address (same transport address is OK);
o The transport address is not currently bound to a different
channel number.
If any of these tests fail, the server replies with an error response
with error code 400 "Bad Request". Otherwise, the ChannelBind
request is valid and the server replies with a ChannelBind success
response.
If ChannelBind request is valid, then the server creates or refreshes
the channel binding using the channel number in the CHANNEL-ADDRESS
attribute and the transport address in the PEER-ADDRESS attribute.
The server also installs or refreshes a permission for the IP address
in the PEER-ADDRESS attribute.
10.3. Receiving a ChannelBind Response
When the client receives a successful ChannelBind response, it
updates its data structures to record that the channel binding is now
active.
10.4. The ChannelData Message
The ChannelData message is used to carry application data between the
client and the server. It has the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
/ Application Data /
/ /
| |
| +-------------------------------+
| |
+-------------------------------+
The Channel Number field specifies the number of the channel on which
the data is traveling, and thus the address of the peer that is
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sending or is to receive the data. The channel number MUST be in the
range 0x4000 - 0xFFFF, with channel number 0xFFFF being reserved for
possible future extensions.
Channel numbers 0x0000 - 0x3FFF cannot be used because bits 0 and 1
are used to distinguish ChannelData messages from STUN-formatted
messages (i.e., Allocate, Send, Data, ChannelBind, etc). STUN-
formatted messages always have bits 0 and 1 as "00", while
ChannelData messages use combinations "01", "10", and "11".
The Length field specifies the length in bytes of the application
data field (i.e., it does not include the size of the ChannelData
header). Note that 0 is a valid length.
The Application Data field carries the data the client is trying to
send to the peer, or that the peer is sending to the client.
10.5. Sending a ChannelData Message
Once a client has bound a channel to a peer, then when the client has
data to send to that peer it may use either a ChannelData message or
a Send Indication; that is, the client is not obligated to use the
channel when it exists and may freely intermix the two message types
when sending data to the peer. The server, on the other hand, MUST
use the ChannelData message if a channel has been bound to the peer.
The fields of the ChannelData message are filled in as described in
Section 10.4.
Over stream transports, the ChannelData message MUST be padded to a
multiple of four bytes in order to ensure the alignment of subsequent
messages. The padding is not reflected in the length field of the
ChannelData message, so the actual size of a ChannelData message
(including padding) is (4 + Length) rounded up to the nearest
multiple of 4. Over UDP, the padding is not required but MAY be
included.
The ChannelData message is then sent on the 5-tuple associated with
the allocation.
10.6. Receiving a ChannelData Message
The receiver of the ChannelData message uses bits 0 and 1 to
distinguish it from STUN-formatted messages, as described in
Section 10.4.
If the ChannelData message is received in a UDP datagram, and if the
UDP datagram is too short to contain the claimed length of the
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ChannelData message (i.e., the UDP header length field value is less
than the ChannelData header length field value + 4 + 8), then the
message is silently discarded.
If the ChannelData message is received over TCP or over TLS over TCP,
then the actual length of the ChannelData message is as described in
Section 10.5.
If the ChannelData message is received on a channel which is not
bound to any peer, then the message is silently discarded.
10.7. Relaying
When a server receives a ChannelData message, it first processes it
as described in the previous section. If no errors are detected, it
relays the application data to the peer by forming a UDP datagram as
follows:
o the source transport address is the relayed transport address of
the allocation, where the allocation is determined by the 5-tuple
on which the ChannelData message arrived;
o the destination transport address is the transport address to
which the channel is bound;
o the data following the UDP header is the contents of the data
field of the ChannelData message.
The resulting UDP datagram is then sent to the peer.
If the ChannelData message is valid, then the server refreshes the
channel binding, and also installs or refreshes a permission for the
IP address part of the transport address to which the UDP datagram is
sent (see Section 8).
In the other direction, when the server receives a UDP datagram on
the relayed transport address associated with an allocation, then it
first checks to see if it is permitted to relay the datagram. This
check is done as described in Section 8. If relaying is permitted,
then the server checks to see if there is a channel bound to the peer
that sent the UDP datagram. If there is, then it SHOULD form and
send a ChannelData message as described in Section 10.5. If no
channel is bound to the peer, then it MUST form and send a Data
indication as described in Section 9.3.
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11. IP and ICMP
This section describes how the server sets various fields in the IP
header when relaying between the client and the peer or vica-versa.
It also describes how the server relays ICMP messages. The
descriptions in this section apply: (a) when the server receives a
Send indication or ChannelData message from the client and sends a
UDP datagram to the peer, (b) when the server receives a UDP datagram
on the relayed-transport address and sends a Data indication or
ChannelData message to the client, or (c) when the server receives an
ICMP message. This section does not apply when the server sends TURN
control messages.
The descriptions below have two parts: a preferred behavior and an
alternate behavior. A Preserving allocation MUST implement the
preferred behavior. A non-preserving allocation with UDP transport
to the client SHOULD implement the preferred behavior, but if that is
not possible for a particular field, then it SHOULD implement the
alternative behavior. A non-preserving allocation with TCP or TLS
transport to client SHOULD implement the alternate behavior, except
where this conflicts with standard TCP or TLS behavior.
11.1. IP
This section describes the preferred and alternate behavior for
various fields in the IP header.
Time to Live (IPv4) or Hop Count (IPv6)
Preferred Behavior: If the incoming value is 0, then send an ICMP
Time Exceeded message back to the sender. Otherwise set the
outgoing Time to Live/Hop Count to one less than the incoming
value.
Alternate Behavior: Set the outgoing value to the default for
outgoing packets.
Diff-Serv Code Point
Preferred Behavior: Set the outgoing value to the incoming value,
unless the server, though configuration or other means, believes
that a different setting is more appropriate.
Alternate Behavior: Set the outgoing value to Best Effort, unless
the server, through configuration or other means, believes a
different setting is more appropriate.
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ECN
Preferred Behavior: Set the outgoing value to the incoming value,
UNLESS the server is doing Active Queue Management, the incoming
ECN field is 01 or 10, and the server wishes to indicate that
congestion has been experienced, in which case set the outgoing
value to 11.
Alternate Behavior: Set the outgoing value to 00 (ECN not
supported)
Flow Label
Preferred Behavior: Set the outgoing flow label to 0.
Alternate Behavior: Same as the Preferred behavior.
IPv4 Fragmentation
Preferred Behavior:
If the outgoing packet size does not exceed the outgoing link's
MTU, then send the outgoing packet unfragmented. Set the DF
bit in the outgoing packet to the value of the DF bit in the
incoming packet, and set the other fragmentation fields
(Identification, MF, Fragment Offset) as appropriate for a
packet originating from the server.
Otherwise, if the outgoing link's MTU is exceeded and the
incoming DF bit is 0, then fragment the packet before sending.
Set the outgoing DF to 0, and set the other fragmentation
fields as appropriate for fragments originated from the server.
Otherwise [link MTU exceeded and incoming DF set], drop the
outgoing packet and send an ICMP message of type 3 code 4
("fragmentation needed and DF set") to the sender of the
incoming packet.
Alternate Behavior: As described in the Preferred Behavior, except
always assume the incoming DF bit is 0.
IPv6 Fragmentation
Preferred Behavior:
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If the incoming packet did not include a Fragmentation header
and the outgoing packet size does not exceed the outgoing
link's MTU, then send the outgoing packet without a
Fragmentation header.
If the incoming packet included a Fragment header and if the
outgoing packet size (with a Fragmentation header included)
does not exceed the outgoing link's MTU, then send the outgoing
packet with a Fragmentation header. Set the fields of the
Fragmentation header as appropriate for a packet originating
from the server.
If the incoming packet did not include a Fragmentation header
and the outgoing packet size exceeds the outgoing link's MTU ,
then drop the outgoing packet and send an ICMP message of type
2 code 0 ("Packet too big") to the sender of the incoming
packet. If the packet is being sent to the peer, then reduce
the MTU reported in the ICMP message by 48 bytes to allow room
for the overhead of a Data indication.
Otherwise, if the link's MTU is exceeded and the incoming
packet contained a Fragmentation header, then fragment the
outgoing packet into fragments of no more than 1280 bytes. Set
the fields of the Fragmentation header as appropriate for a
packet originating from the server.
Alternate Behavior: As described in the Preferred Behavior, except
always assume incoming packet has a Fragmentation header.
IPv4 Options
Preferred Behavior: The outgoing packet is sent without any IPv4
options.
Alternate Behavior: Same as preferred.
IPv6 Extention Headers
Preferred Behavior: The outgoing packet is sent without any IPv6
extension headers, with the exception of the Fragmentation header
as described above
Alternate Behavior: Same as preferred.
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11.2. ICMP
This sub-section describes the preferred behavior of ICMP relaying.
The corresponding alternate behavior is to not relay ICMP messages.
When an ICMP message arrives at the server, the copy of the original
IP packet present inside the ICMP message is examined. The server
first checks that the original IP packet header is immediately
followed by a UDP protocol header, such that the original source
transport address was X and the original destination transport
address was Y. The server also checks that the type and code values
in the ICMP header are one of those relayed (see below). Other ICMP
messages are either ignored, or used by the server internally in an
unspecified manner.
The server then checks if one of the following two cases applies:
Case 1: X is a relayed-transport-address currently assigned to an
active allocation on the server, and there exists a permission for
the IP address of Y in the allocation.
In this case, the original IP packet was traveling from the server to
a peer, so the the server relays the ICMP message back to the client.
The server creates a Data indication where the PEER-ADDRESS attribute
contains Y, and the ICMP attribute contains the type and code from
the incoming ICMP message, and the DATA attribute contains
application data from the original IP packet starting AFTER the UDP
header. The server SHOULD include as much application data as
possible consistent with not exceeding a total IP packet size of
either 576 bytes (for IPv4) or 1280 bytes (for IPv6).
Note that there is no point in including the original IP or UDP
header in the DATA attribute because those headers were generated
by the server, not the client.
Case 2: There is an active allocation where X is the server transport
address, Y is the client transport address, and UDP is used as
transport between the client and the server. Furthermore, the packet
after the UDP header is either (a) a ChannelData header which
contains an active channel number in the allocation, or (b) a Data
indication whose PEER-ADDRESS attribute contains an IP address for
which there exists a permission in the allocation.
In this case, the original IP packet was traveling from the server to
the client, so the server creates and sends an ICMP message to the
peer. The outgoing ICMP message contains the type and code fields
from the incoming ICMP message and then contains an approximation to
the original IP packet sent from the peer to the server (the one the
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server was trying to relay to the client inside the ChannelData or
Data indication). This approximation contains a synthesized IP
header, a synthesized UDP header, and some application data. The
synthesis is done as follows:
o The destination transport address is the relayed-transport-address
of the allocation;
o The source transport address is the peer's transport address
determined from either (a) the channel number or (b) the PEER-
ADDRESS attribute;
o The application data is taken from either (a) the ChannelData
message or (b) the DATA attribute. The server SHOULD include as
much application data as possible consistent with not exceeding
either 576 bytes (for IPv4) or 1280 bytes (for IPv6).
The remaining fields in the IP and UDP headers are simply set to
sensible values, since for most of them there is no way to
reconstruct the original values.
The server SHOULD relay as all ICMP type/code combinations and MUST
relay at at least the following combinations. For IPv4:
Type 3, code 4: Fragmentation needed and DF set
For IPv6:
Type 2, code <any>: Packet too big
12. New STUN Methods
This section lists the codepoints for the new STUN methods defined in
this specification. See elsewhere in this document for the semantics
of these new methods.
Request/Response Transactions
0x003 : Allocate
0x004 : Refresh
0x009 : ChannelBind
Indications
0x006 : Send
0x007 : Data
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13. New STUN Attributes
This STUN extension defines the following new attributes:
0x000C: CHANNEL-NUMBER
0x000D: LIFETIME
0x0010: Reserved (was BANDWIDTH)
0x0012: PEER-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: RESERVATION-TOKEN
13.1. CHANNEL-NUMBER
The CHANNEL-NUMBER attribute contains the number of the channel. It
is a 16-bit unsigned integer, followed by a two-octet RFFU (Reserved
For Future Use) field which MUST be set to 0 on transmission and MUST
be ignored on reception.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Channel Number | RFFU = 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
13.2. LIFETIME
The lifetime attribute represents the duration for which the server
will maintain an allocation in the absence of a refresh. It is a 32-
bit unsigned integral value representing the number of seconds
remaining until expiration.
13.3. PEER-ADDRESS
The PEER-ADDRESS specifies the address and port of the peer as seen
from the TURN server. It is encoded in the same way as XOR-MAPPED-
ADDRESS.
13.4. DATA
The DATA attribute is present in all Data Indications and most Send
Indications. It contains raw payload data that is to be sent (in the
case of a Send Request) or was received (in the case of a Data
Indication).
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13.5. RELAY-ADDRESS
The RELAY-ADDRESS is present in Allocate responses. It specifies the
address and port that the server allocated to the client. It is
encoded in the same way as XOR-MAPPED-ADDRESS.
13.6. REQUESTED-PROPS
This attribute allows the client to request that the allocation have
certain properties. The attribute is 32 bits long. Its format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E|R|P| MUST be 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The first part of the attribute value contains a number of one-bit
flags. These are:
E: If 1, the port number for the relayed-transport-address must be
even. If 0, the port number can be even or odd.
R: If 1, the server must reserve the next highest port for a
subsequent allocation. If 0, no such reservation is requested.
If the client sets the R bit to 1, it MUST also set the E bit to 1
(however, the E bit may be 1 when the R bit is 0).
P: If 1, the allocation must be a Preserving allocation. If 0, the
allocation can be either Preserving or Non-Preserving.
All these flags have the property that if the bit is 1, and the
server cannot create an allocation that satisfies the request, then
the Allocate request is rejected. To allow future TURN extensions to
define new flags that also have this property, the client MUST set
the rest of the attribute to zero, and the server MUST fail the
Allocate request if any bits which the server does not support are
set to 1. By doing this, any new flags that are not recognized by
the server will cause the Allocate request to fail.
13.7. REQUESTED-TRANSPORT
This attribute is used by the client to request a specific transport
protocol for the allocated transport address. It has the following
format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | RFFU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The Protocol field specifies the desired protocol. The codepoints
used in this field are taken from those allowed in the Protocol field
in the IPv4 header and the NextHeader field in the IPv6 header
[Protocol-Numbers]. This specification only allows the use of
codepoint 17 (User Datagram Protocol).
The RFFU field MUST be set to zero on transmission and MUST be
ignored on receiption. It is reserved for future uses.
13.8. RESERVATION-TOKEN
The RESERVATION-TOKEN attribute contains a token that uniquely
identifies a relayed transport address being held in reserve by the
server. The server includes this attribute in a success response to
tell the client about the token, and the client includes this
attribute in a subsequent Allocate request to request the server use
that relayed transport address for the allocation.
The attribute value is a 64-bit-long field containing the token
value.
14. New STUN Error Response Codes
This document defines the following new error response codes:
437 (Allocation Mismatch): A request was received by the server that
requires an allocation to be in place, but there is none, or a
request was received which requires no allocation, but there is
one.
438 (Wrong Credentials): The credentials in the (non-Allocate)
request, though otherwise acceptable to the server, do not match
those used to create the allocation.
442 (Unsupported Transport Protocol): The Allocate request asked the
server to use a transport protocol between the server and the peer
that the server does not support. NOTE: This does NOT refer to
the transport protocol used in the 5-tuple.
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486 (Allocation Quota Reached): No more allocations using this
username can be created at the present time.
508 (Insufficient Port Capacity): The server has no more relayed
transport addresses available right now, or has none with the
requested properties, or the one that corresponds to the specified
token is not available.
15. Security Considerations
TURN servers allocate resources to clients, in contrast to the
Binding method defined in [I-D.ietf-behave-rfc3489bis]. Therefore, a
TURN server may require the authentication and authorization of STUN
requests. This authentication is provided by mechanisms defined in
the STUN specification itself, in particular digest authentication.
Because TURN servers allocate resources, they can be susceptible to
denial-of-service attacks. All Allocate transactions are
authenticated, so that an unknown attacker cannot launch an attack.
An authenticated attacker can generate multiple Allocate Requests,
however. To prevent a single malicious user from allocating all of
the resources on the server, it is RECOMMENDED that a server
implement a per user limit on the number of allocations that can
active at one time. Such a mechanism does not prevent a large number
of malicious users from each requesting a small number of
allocations. Attacks such as these are possible using botnets, and
are difficult to detect and prevent. Implementors of TURN should
keep up with best practices around detection of anomalous botnet
attacks.
A client will use the transport address learned from the RELAY-
ADDRESS attribute of the Allocate Response to tell other users how to
reach them. Therefore, a client needs to be certain that this
address is valid, and will actually route to them. Such validation
occurs through the message integrity checks provided in the Allocate
response. They can guarantee the authenticity and integrity of the
allocated addresses. Note that TURN is not susceptible to the
attacks described in Section 12.2.3, 12.2.4, 12.2.5 or 12.2.6 of
[I-D.ietf-behave-rfc3489bis] [[TODO: Update section number references
to 3489bis]]. These attacks are based on the fact that a STUN server
mirrors the source IP address, which cannot be authenticated. STUN
does not use the source address of the Allocate Request in providing
the RELAY-ADDRESS, and therefore, those attacks do not apply.
TURN attempts to adhere as closely as possible to common firewall
policies, consistent with allowing data to flow. TURN has fairly
limited applicability, requiring a user to explicitly authorize
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permission to receive data from a peer, one IP address at a time.
Thus, it does not provide a general technique for externalizing
sockets. Rather, it has similar security properties to the placement
of an address-restricted NAT in the network, allowing messaging in
from a peer only if the internal client has sent a packet out towards
the IP address of that peer. This limitation means that TURN cannot
be used to run, for example, SIP servers, NTP servers, FTP servers or
other network servers that service a large number of clients.
Rather, it facilitates rendezvous of NATted clients that use some
other protocol, such as SIP, to communicate IP addresses and ports
for communications.
Confidentiality of the transport addresses learned through Allocate
transactions does not appear to be that important. If required, it
can be provided by running TURN over TLS.
TURN does not and cannot guarantee that UDP data is delivered in
sequence or to the correct address. As most TURN clients will only
communicate with a single peer, the use of a single channel number
will be very common. Consider an enterprise where Alice and Bob are
involved in separate calls through the enterprise NAT to their
corporate TURN server. If the corporate NAT reboots, it is possible
that Bob will obtain the exact NAT binding originally used by Alice.
If Alice and Bob were using identical channel numbers, Bob will
receive unencapsulated data intended for Alice and will send data
accidentally to Alice's peer. This is not a problem with TURN. This
is precisely what would happen if there was no TURN server and Bob
and Alice instead provided a (STUN) reflexive transport address to
their peers. If detecting this misdelivery is a problem, the client
and its peer need to use message integrity on their data.
Relay servers are useful even for users not behind a NAT. They can
provide a way for truly anonymous communications. A user can cause a
call to have its media routed through a TURN server, so that the
user's IP addresses are never revealed.
Any relay addresses learned through an Allocate request will not
operate properly with IPSec Authentication Header (AH) [RFC4302] in
transport or tunnel mode. However, tunnel-mode IPSec ESP [RFC4303]
should still operate.
16. IANA Considerations
Since TURN is an extension to STUN [I-D.ietf-behave-rfc3489bis], the
methods, attributes and error codes defined in this specification are
new method, attributes, and error codes for STUN. This section
directs IANA to add these new protocol elements to the IANA registry
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of STUN protocol elements.
The codepoints for the new STUN methods defined in this specification
are listed in Section 12.
The codepoints for the new STUN attributes defined in this
specification are listed in Section 13.
The codepoints for the new STUN error codes defined in this
specification are listed in Section 14.
Extensions to TURN can be made through IETF consensus.
17. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [RFC3424]. The TURN
extension is an example of a protocol that performs this type of
function. The IAB has mandated that any protocols developed for this
purpose document a specific set of considerations.
TURN is an extension of the STUN protocol. As such, the specific
usages of STUN that use the TURN extensions need to specifically
address these considerations. Currently the only STUN usage that
uses TURN is ICE [I-D.ietf-mmusic-ice].
18. Example
TBD
19. Changes from Previous Versions
Note to RFC Editor: Please remove this section prior to publication
of this document as an RFC.
This section lists the changes between the various versions of this
specification.
19.1. Changed from -07 to -08
o Removed the BANDWIDTH attribute and all associated text (including
error code 507 "Insufficient Bandwidth Capacity"), as the
requirements for this feature were not clear and it was felt the
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feature could be easily added later.
o Changed the format of the REQUESTED-PROPS attribute from a one-
byte field to a set of bit flags. Changed the semantics of the
unused portion of the value from RFFU to "MUST be 0" to give a
more desirable behavior when new flags are defined.
o Introduced the concept of Preserving vs. Non-Preserving
allocations. As a result, completely revamped the rules for how
to set the fields in the IP header, and added rules for relaying
ICMP messages when the allocation is Preserving.
19.2. Changes from -06 to -07
o Rewrote the General Behavior section, making various changes in
the process.
o Changed the usage of authentication from MUST to SHOULD.
o Changed the requirement that subsequent requests use the same
username and password from MUST to SHOULD to allow for the
possibility of changing the credentials using some unspecified
mechanism.
o Introduced a 438 (Wrong Credentials) error which is used when a
non-Allocate request authenticates but does not use the same
username and password as the Allocate request. Having a separate
error code for this case avoids the client being confused over
what the error actually is.
o The server must now prevent the relayed transport address and the
5-tuple from being reused in different allocations for 2 minutes
after the allocation expires.
o Changed the usage of FINGERPRINT from MUST NOT to MAY, to allow
for the possible multiplexing of TURN with some other protocol.
o Rewrote much of the section on Allocations, splitting it into
three new sections (one on allocations in general, one on creating
an allocation, and one on refreshing an allocation).
o Replaced the mechanism for requesting relayed transport addresses
with specific properties. The new mechanism is less powerful: a
client can request an even port, or a pair of ports, but cannot
request a single odd port or a specific port as was possible under
the old mechanism. Nor can the client request a specific IP
address.
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o Changed the rules for handling ALTERNATE-SERVER, removing the
requirement that the referring server have "positive knowledge"
about the state of the alternate server. The new rules instead
rely on text in STUN to prevent referral loops.
o Changed the rules for allocation lifetimes. Allocations lifetimes
are now a minimum of 10 minutes; the client can ask for longer
values, but requests for shorter values are ignored. The text now
recommends that the client refresh an allocation one minute before
it expires.
o Put in temporary procedures for handling the BANDWIDTH attribute,
modelled on the LIFETIME attribute. These procedures are mostly
placeholders and likely to change in the next revision.
o Added a detailed description of how a client reacts to the various
errors it can receive in reply to an Allocate request. This
replaces the various descriptions that were previously scattered
throughout the document, which were inconsistent and sometimes
contradictory.
o Added a new section that gives the normative rules for
permissions.
o Changed the rules around permission lifetimes. The text used to
recommend a value of one minute; it MUST now be 5 minutes.
o Removed the errors "Channel Missing or Invalid", "Peer Address
Missing or Invalid" and "Lifetime Malformed or Invalid" and used
400 "Bad Request" instead.
o Rewrote portions of the section on Send and Data indications and
the section on Channels to try to make the client vs. server
behavior clearer.
o Channel bindings now expire after 10 minutes, and must be
refreshed to keep them alive.
o Binding a channel now installs or refreshes a permission for the
IP address of corresponding peer.
o Changed the wording describing the situation when the client sends
a ChannelData message before receiving the ChannelBind success
response. -06 said that client SHOULD NOT do this; -07 now says
that a client MAY, but describes the consequences of doing it.
o Added a section discussing the setting of fields in the IP header.
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o Replaced the REQUESTED-PORT-PROPS attribute with the REQUESTED-
PROPS attribute that has a different format and semantics, but
reuses the same code point.
o Replaced the REQUESTED-IP attribute with the RESERVATION-TOKEN
attribute, which has a different format and semantics, but reuses
the same code point.
o Removed error codes 443 and 444, and replaced them with 508
(Insufficient Port Capacity). Also changed the error text for
code 507 from "Insufficient Capacity" to "Insufficient Bandwidth
Capacity".
19.3. Changes from -05 to -06
o Changed the mechanism for allocating channels to the one proposed
by Eric Rescorla at the Dec 2007 IETF meeting.
o Removed the framing mechanism (which was used to frame all
messages) and replaced it with the ChannelData message. As part
of this change, noted that the demux of ChannelData messages from
TURN messages can be done using the first two bits of the message.
o Rewrote the sections on transmitted and receiving data as a result
of the above to changes, splitting it into a section on Send and
Data Indications and a separate section on channels.
o Clarified the handling of Allocate Request messages. In
particular, subsequent Allocate Request messages over UDP with the
same transaction id are not an error but a retransmission.
o Restricted the range of ports available for allocation to the
Dynamic and/or Private Port range, and noted when ports outside
this range can be used.
o Changed the format of the REQUESTED-TRANSPORT attribute. The
previous version used 00 for UDP and 01 for TCP; the new version
uses protocol numbers from the IANA protocol number registry. The
format of the attribute also changed.
o Made a large number of changes to the non-normative portion of the
document to reflect technical changes and improve the
presentation.
o Added the Issues section.
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19.4. Changes from -04 to -05
o Removed the ability to allocate addresses for TCP relaying. This
is now covered in a separate document. However, communication
between the client and the server can still run over TCP or TLS/
TCP. This resulted in the removal of the Connect method and the
TIMER-VAL and CONNECT-STAT attributes.
o Added the concept of channels. All communication between the
client and the server flows on a channel. Channels are numbered
0..65535. Channel 0 is used for TURN messages, while the
remaining channels are used for sending unencapsulated data to/
from a remote peer. This concept adds a new Channel Confirmation
method and a new CHANNEL-NUMBER attribute. The new attribute is
also used in the Send and Data methods.
o The framing mechanism formally used just for stream-oriented
transports is now also used for UDP, and the former Type and
Reserved fields in the header have been replaced by a Channel
Number field. The length field is zero when running over UDP.
o TURN now runs on its own port, rather than using the STUN port.
The use of channels requires this.
o Removed the SetActiveDestination concept. This has been replaced
by the concept of channels.
o Changed the allocation refresh mechanism. The new mechanism uses
a new Refresh method, rather than repeating the Allocation
transaction.
o Changed the syntax of SRV requests for secure transport. The new
syntax is "_turns._tcp" rather than the old "_turn._tls". This
change mirrors the corresponding change in STUN SRV syntax.
o Renamed the old REMOTE-ADDRESS attribute to PEER-ADDRESS, and
changed it to use the XOR-MAPPED-ADDRESS format.
o Changed the RELAY-ADDRESS attribute to use the XOR-MAPPED-ADDRESS
format (instead of the MAPPED-ADDRESS format)).
o Renamed the 437 error code from "No Binding" to "Allocation
Mismatch".
o Added a discussion of what happens if a client's public binding on
its outermost NAT changes.
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o The document now consistently uses the term "peer" as the name of
a remote endpoint with which the client wishes to communicate.
o Rewrote much of the document to describe the new concepts. At the
same time, tried to make the presentation clearer and less
repetitive.
20. Open Issues
NOTE to RFC Editor: Please remove this section prior to publication
of this document as an RFC.
Bandwidth: How should bandwidth be specified? What are the right
rules around bandwidth?
Alternate Server: Do we still want this mechanism? Is the current
proposal acceptable? Note that the usage of the ALTERNATE-SERVER
attribute in this document is inconsistent with its usage in STUN.
In STUN, if the ALTERNATE-SERVER attribute is used, then the error
that the server would otherwise generate is replaced by a 300 (Try
Alternate) code. In this document, the 300 error code is not used,
and the server returns an appropriate error code and then includes
the ALTERNATE-SERVER attribute in the response. In this way, the
client can see the actual error code, rather than always seeing error
code 300, and can thus make a more intelligent decision on whether it
wishes to try the alternate server.
Public TURN servers: The text currently says that a server "SHOULD"
use the Long-Term Credential mechanism, with the unstated idea that a
public TURN server would not use it. But this really weakens the
security of TURN. Is there a better way to allow public servers? Or
should we just drop the notion of a public server entirely?
21. Acknowledgements
The authors would like to thank the various participants in the
BEHAVE working group for their many comments on this draft. Marc
Petit-Huguenin, Remi Denis-Courmont, Derek MacDonald, Cullen
Jennings, Lars Eggert, Magnus Westerlund, and Eric Rescorla have been
particularly helpful, with Eric also suggesting the channel
allocation mechanism, and Cullen suggesting the REQUESTED-PROPS
mechanism. Christian Huitema was an early contributor to this
document and was a co-author on the first few drafts. Finally, the
authors would like to thank Dan Wing for both his contributions to
the text and his huge help in restarting progress on this draft after
work had stalled.
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22. References
22.1. Normative References
[I-D.ietf-behave-rfc3489bis]
Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-15 (work in progress),
February 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
22.2. Informative References
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[I-D.ietf-mmusic-ice]
Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols",
draft-ietf-mmusic-ice-19 (work in progress), October 2007.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
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[I-D.ietf-behave-turn-tcp]
Rosenberg, J. and R. Mahy, "Traversal Using Relays around
NAT (TURN) Extensions for TCP Allocations",
draft-ietf-behave-turn-tcp-00 (work in progress),
November 2007.
[I-D.ietf-behave-turn-ipv6]
Camarillo, G. and O. Novo, "Traversal Using Relays around
NAT (TURN) Extension for IPv4/IPv6 Transition",
draft-ietf-behave-turn-ipv6-04 (work in progress),
January 2008.
[I-D.ietf-tsvwg-udp-guidelines]
Eggert, L. and G. Fairhurst, "Guidelines for Application
Designers on Using Unicast UDP",
draft-ietf-tsvwg-udp-guidelines-08 (work in progress),
June 2008.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC1928] Leech, M., Ganis, M., Lee, Y., Kuris, R., Koblas, D., and
L. Jones, "SOCKS Protocol Version 5", RFC 1928,
March 1996.
[Port-Numbers]
"IANA Port Numbers Registry",
<http://www.iana.org/assignments/port-numbers>.
[Protocol-Numbers]
"IANA Protocol Numbers Registry", 2005,
<http://www.iana.org/assignments/protocol-numbers>.
Authors' Addresses
Jonathan Rosenberg
Cisco Systems, Inc.
Edison, NJ
USA
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
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Rohan Mahy
Plantronics, Inc.
Email: rohan@ekabal.com
Philip Matthews
(Unaffiliated)
Fax:
Email: philip_matthews@magma.ca
URI:
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