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14 15 16 RFC 5766
Behave J. Rosenberg
Internet-Draft Cisco Systems
Intended status: Standards Track R. Mahy
Expires: January 9, 2008 Plantronics
C. Huitema
Microsoft
July 8, 2007
Traversal Using Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)
draft-ietf-behave-turn-04.txt
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This specification defines an extension of the Session Traversal
Utilities for NAT (STUN) Protocol for asking the STUN server to relay
packets towards a client. This extension, called Traversal Using
Relays around NAT (TURN), is useful for elements behind NATs whose
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mapping behavior is address and port dependent. The extension
purposefully restricts the ways in which the relayed address can be
used. In particular, it prevents users from running general purpose
servers from ports obtained from the STUN server.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Overview of Operation . . . . . . . . . . . . . . . . . . . . 5
4.1. Transports . . . . . . . . . . . . . . . . . . . . . . . . 7
4.2. Tuple Terminology . . . . . . . . . . . . . . . . . . . . 8
4.3. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . 9
5. New Framing Mechanism for Stream-Oriented Transports . . . . . 10
6. New STUN Requests and Indications . . . . . . . . . . . . . . 10
6.1. Allocate Request . . . . . . . . . . . . . . . . . . . . . 11
6.1.1. Client Behavior . . . . . . . . . . . . . . . . . . . 11
6.1.2. Server Behavior . . . . . . . . . . . . . . . . . . . 13
6.2. Procedures for all other Requests and Indications . . . . 17
6.3. Set Active Destination Request . . . . . . . . . . . . . . 18
6.3.1. Client Behavior . . . . . . . . . . . . . . . . . . . 18
6.3.2. Server Behavior . . . . . . . . . . . . . . . . . . . 19
6.4. Connect Request . . . . . . . . . . . . . . . . . . . . . 19
6.4.1. Server Behavior . . . . . . . . . . . . . . . . . . . 20
6.5. Connection Status Indication . . . . . . . . . . . . . . . 20
6.6. Send Indication . . . . . . . . . . . . . . . . . . . . . 20
6.6.1. Client Behavior . . . . . . . . . . . . . . . . . . . 21
6.6.2. Server Behavior . . . . . . . . . . . . . . . . . . . 21
6.7. Data Indication . . . . . . . . . . . . . . . . . . . . . 22
6.7.1. Client Behavior . . . . . . . . . . . . . . . . . . . 22
6.7.2. Server Behavior . . . . . . . . . . . . . . . . . . . 22
7. New Attributes . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. LIFETIME . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.2. BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . . . 23
7.3. REMOTE-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 23
7.4. DATA . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.5. RELAY-ADDRESS . . . . . . . . . . . . . . . . . . . . . . 24
7.6. REQUESTED-PORT-PROPS . . . . . . . . . . . . . . . . . . . 24
7.7. REQUESTED-TRANSPORT . . . . . . . . . . . . . . . . . . . 25
7.8. REQUESTED-IP . . . . . . . . . . . . . . . . . . . . . . . 25
7.9. CONNECT_STAT . . . . . . . . . . . . . . . . . . . . . . . 25
8. New Error Response Codes . . . . . . . . . . . . . . . . . . . 26
9. Client Procedures . . . . . . . . . . . . . . . . . . . . . . 26
9.1. Receiving and Sending Unencapsulated Data . . . . . . . . 26
9.1.1. Datagram Protocols . . . . . . . . . . . . . . . . . . 27
9.1.2. Stream Transport Protocols . . . . . . . . . . . . . . 27
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10. Server Procedures . . . . . . . . . . . . . . . . . . . . . . 27
10.1. Receiving Data on Allocated Transport Addresses . . . . . 27
10.1.1. TCP Processing . . . . . . . . . . . . . . . . . . . . 27
10.1.2. UDP Processing . . . . . . . . . . . . . . . . . . . . 28
10.2. Receiving Data on Internal Local Transport Addresses . . . 29
10.3. Lifetime Expiration . . . . . . . . . . . . . . . . . . . 29
11. Client Discovery of TURN Servers . . . . . . . . . . . . . . . 30
12. Security Considerations . . . . . . . . . . . . . . . . . . . 30
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
13.1. New STUN Requests, Responses, and Indications . . . . . . 32
13.2. New STUN Attributes . . . . . . . . . . . . . . . . . . . 33
13.3. New STUN response codes . . . . . . . . . . . . . . . . . 33
14. IAB Considerations . . . . . . . . . . . . . . . . . . . . . . 33
15. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 38
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
17.1. Normative References . . . . . . . . . . . . . . . . . . . 38
17.2. Informative References . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 39
Intellectual Property and Copyright Statements . . . . . . . . . . 40
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1. Introduction
Session Traversal Utilities for NAT (STUN) [1] provides a suite of
tools for facilitating the traversal of NAT. Specifically, it
defines the Binding Request, 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 [9]
is address or address and port dependent (sometimes called "bad"
NATs), the reflexive transport address will not be usable for
communicating with a peer.
The only way to obtain a 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 addresses are from interfaces 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 of STUN, called TURN, that
allows a client to request an address on the STUN server itself, so
that the STUN server acts as a relay. To accomplish that, this
extension defines a handful of new STUN requests and indications.
The Allocate request is the most fundamental component of this set of
extensions. It is used to provide the client with a transport
address that is relayed through the STUN server. A transport address
which relays through an intermediary is called a relayed transport
address.
Though a relayed 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 [4], is Interactive Connectivity
Establishment (ICE) [8].
The mechanism defined here was previously a standalone protocol
called Traversal Using Relay NAT (TURN), and is now defined as an
extension of STUN. A STUN server that supports these extensions can
be called a 'STUN relay' or more simply a 'TURN server'.
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [2].
3. Definitions
Relayed Transport Address: A transport address that terminates on a
server, and is forwarded towards the client. The STUN Allocate
Request can be used to obtain a relayed transport address, for
example.
TURN client: A STUN client that implements this specification. It
obtains a relayed transport address that it provides to a small
number of peers (usually one).
TURN server: A STUN server that implements this specification. It
relays data between a TURN client and its peer.
5-tuple: A combination of the source IP address and port,
destination IP address and port, and transport protocol (UDP, TCP,
or TLS over TCP). It uniquely identifies a TCP connection, TLS
channel, or bi-directional flow of UDP datagrams.
Permission: A record of an IP address and transport of a peer that
is permitted to send traffic to the TURN client. The TURN server
will only forward traffic to its client from peers that match an
existing permission.
4. Overview of Operation
In a typical configuration, a TURN client is connected to a private
network and through one or more NATs to the public Internet. On the
public Internet is a TURN server. This specification defines several
new messages and a new framing mechanism that add the ability for a
STUN server to act as a packet relay. The text in this section
explains the typical usage of this relay extension.
First the client sends an Allocate request to the server, which the
server authenticates. The server generates an Allocate response with
the allocated address, port, and target transport. All other STUN
messages defined by this specification happen in the context of an
allocation.
A successful Allocate Request just reserves an address on the TURN
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server. Data does not flow through an allocated port until the TURN
client asks the TURN server to open a permission. It can do this by
sending data to the far end with a Send Indication for UDP
allocations, by sending a ConnectRequest for TCP allocations, or by
setting the default destination for either transport. While the
client can request more than one permission per allocation, it needs
to request each permission explicitly and one at a time. This
insures that a client can't use a TURN server to run a traditional
server, and partially protects the client from DoS attacks.
Once a permission is open, the client can then receive data flowing
back from its peer. Initially this data is wrapped in a STUN Data
Indication. Since multiple permissions can be open simultaneously,
the Data Indication contains the Remote Address attribute so the TURN
client knows which peer sent the data. The client can send data to
any of its peers with the Send Indication.
Once the client wants to primarily receive from one peer, it can send
a SetActiveDestination request. All subsequent data received from
the active peer is forwarded directly to the client and vice versa,
except that it is wrapped or framed according to the protocol used
between the TURN client and TURN server. The client can send
subsequent SetActiveDestination requests to change or remove the
active destination.
When the TURN client to server communication is over a datagram
protocol (UDP), any datagram received from the active peer that has
the STUN magic cookie is wrapped in a Data Indication. Likewise any
datagram sent by the client that has the STUN magic cookie and is
intended for the active peer is wrapped in a Send Indication. This
wrapping prevents the STUN relay server from inappropriately
interpreting end-to-end data.
Over stream-based transports (TCP and TLS over TCP), the TURN client
and server always use some additional framing (defined in Section 5)
so that end-to-end data is distinguishable from STUN control
messages. This additional framing just has a type and a length
field. The value of the type field was chosen so it is always
distinguishable from an unframed STUN request or response.
The SetActiveDestination Request does not close other bindings. Data
to and from other peers is still wrapped in Send and Data indications
respectively.
Allocations can also request specific attributes such as the desired
Lifetime of the allocation, and the maximum Bandwidth. Clients can
also request specific port assignment behavior, for example, a
specific port number, odd or even port numbers, or pairs of
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sequential port numbers.
4.1. Transports
TURN clients can communicate with a TURN server using UDP, TCP, or
TLS over TCP. A TURN can even relay traffic between two different
transports with certain restrictions. A TURN can never relay from an
unreliable transport (client to server) to a reliable transport to
the peer. Note that a TURN server never has a TLS relationship with
a client's peer, since the TURN server does not interpret data above
the TCP layer. When relaying data sent from a stream-based protocol
to a UDP peer, the TURN server emits datagrams which are the same
length as the length field in the STUN TCP framing or the length
field in a Send Indication. Likewise, when a UDP datagram is relayed
from a peer over a stream-based transport, the length of the datagram
is the length of the TCP framing or Data Indication.
+----------------+--------------+
| client to TURN | TURN to peer |
+----------------+--------------+
| UDP | UDP |
| TCP | TCP |
| TCP | UDP |
| TLS | TCP |
| TLS | UDP |
+----------------+--------------+
For TURN clients, using TLS over TCP provides two benefits. When
using TLS, the client can be assured that the address of the client's
peers are not visible to an attacker except by traffic analysis
downstream of the TURN server. Second, the client may be able to
communicate with TURN servers using TLS that it would not be able to
communicate with using TCP or UDP due to the configuration of a
firewall between the TURN client and its server. TLS between the
client and TURN server in this case just facilitates traversal.
For TCP connections, the Connection Request allows the client to ask
the server to open a connection to the peer. This also adds a
permission to accept an incoming TCP connection from the remote
address of the peer. When the server and the peer try to open a TCP
connection at the same time, this is called TCP simultaneous open.
When the TURN-to-peer leg is TCP, the TURN client needs to be aware
of the status of these TCP connections. The TURN extension defines
application states for a TCP connection as follows: LISTEN,
ESTABLISHED, and CLOSED. Consequently, the TURN server sends a
ConnectionState Indication for a binding whenever the relay
connection status changes for one of the client's bindings, except
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when the status changes due to a TURN client request (ex: an explicit
binding deallocation).
4.2. Tuple Terminology
To relay data to and from the correct location, the TURN server
maintains an association between an internal address (called a
5-tuple) and one or more external 5-tuples, as shown in Figure 1.
The internal 5-tuple identifies the path between the TURN client and
the TURN server. It consists of the protocol (UDP, TCP, or TLS over
TCP), the internal local IP address and port number and the source IP
address and port number of the STUN client, as seen by the relay
server. For example, for UDP, the internal 5-tuple is the
combination of the IP address and port from which the STUN client
sent its Allocate Request, with the IP address and port from which
the corresponding Allocate Response was sent.
The external local transport address is the IP address and port
allocated to the TURN client (the allocated transport address). The
external 5-tuple is the combination of the external local transport
address and the IP address and port of an external client that the
STUN client is communicating with through the STUN server.
Initially, there aren't any external 5-tuples, since the STUN client
hasn't communicated with any other hosts yet. As packets are
received on or sent from the allocated transport address, external
5-tuples are created.
While the terminology used in this document refers to 5-tuples,
the TURN server can store whatever identifier it likes that yields
identical results. Specifically, many implementations may use a
file-descriptor in place of a 5-tuple to represent a TCP
connection.
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+---------+
| |
| External|
/ | Client |
// | |
/ | |
// +---------+
/
//
+-+ /
| | /
| | //
+---------+ | | +---------+ / +---------+
| | |N| | | // | |
| STUN | | | | |/ | External|
| Client |----|A|----------| STUN |------------------| Client |
| | | |^ ^| Server |^ ^| |
| | |T|| || || || |
+---------+ | || |+---------+| |+---------+
^ | || | | |
| | || | | |
| +-+| | | |
| | | | |
|
Internal Internal External External
Client Remote Local Local Remote
Performing Transport Transport Transport Transport
Allocations Address Address Address Address
| | | |
+-----+----+ +--------+-------+
| |
| |
Internal External
5-Tuple 5-tuple
Figure 1
4.3. Keepalives
Since the main purpose of STUN and the relay extension are to
traverse NATs, it is natural to consider which elements are
responsible for generating sufficient periodic traffic to insure that
NAT bindings stay alive. Relay clients need to send data frequently
enough to keep both NAT bindings and the TURN server internal
permissions fresh. Like NAT bindings, the TURN server bindings are
refreshed by ordinary data traffic relayed to and from the peer.
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Unlike permissions, allocations on the TURN server have an explicit
expiration time and need to be refreshed explicitly by the client.
When an allocation expires, all permissions associated with that
allocation are automatically deleted.
5. New Framing Mechanism for Stream-Oriented Transports
Over stream-based transports, the TURN client and server need to use
additional framing so that end-to-end data is distinguishable from
STUN control messages, and so that the TURN server can perform
conversion from streams to datagrams and vice versa. This additional
framing has a one octet type, one reserved octet, and a 2 octet
length field. The first octet of this framing is 0x02 to indicate
STUN messages or 0x03 to indicate end-to-end data to or from the
active destination. Note that the first octet is always
distinguishable from an unframed STUN request or response (which is
always 0x00 or 0x01). The second octet is reserved and MUST be set
to zero. The length field counts the number of octets immediately
after the length field itself.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved = 0 | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Use of this framing mechanism is discussed in Section 9 and
Section 10.
6. New STUN Requests and Indications
This document defines three new requests (along with their success
and error responses) and three indications. It also defines
processing rules for the STUN server and client on receipt of non-
STUN messages. See Section 9 and Section 10
The new messages are:
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Request/Response Transactions
0x003 : Allocate
0x004 : Set Active Destination
0x005 : Connect
Indications
0x006 : Send
0x007 : Data
0x008 : Connect Status
In addition to STUN Messages (Requests, Responses, and Indications),
TURN clients and servers send and receive non-STUN packets on the
same ports used for STUN messages. How these entities distinguish
STUN and non-STUN traffic is discussed in Section 9 and Section 10.
6.1. Allocate Request
6.1.1. Client Behavior
Client behavior for Allocate requests depends on whether the request
is an initial one, for the purposes of obtaining a new relayed
transport address, or a subsequent one, used for refreshing an
existing allocation.
6.1.1.1. Initial Requests
When a client wishes to obtain a transport address, it sends an
Allocate Request to the server. This request is constructed and sent
using the general procedures defined in [1]. The server will
challenge the request for credentials. The client MAY either provide
its credentials to the server directly, or it MAY obtain a short-term
set of credentials using the Shared Secret request and then use those
as the credentials in the Allocate request.
The client SHOULD include a BANDWIDTH attribute, which indicates the
maximum bandwidth that will be used with this binding. If the
maximum is unknown, the attribute is not included in the request.
The client MAY request a particular lifetime for the allocation by
including it in the LIFETIME attribute in the request. The default
lifetime is 10 minutes.
The client MAY include a REQUESTED-PORT-PROPS, REQUESTED-TRANSPORT,
or REQUESTED-IP attribute in the request to obtain specific types of
transport addresses. Whether these are needed depends on the
application using the TURN server. As an example, the Real Time
Transport Protocol (RTP) [3] requires that RTP and RTCP ports be an
adajacent pair, even and odd respectively, for compatibility with a
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previous version of that specification. The REQUESTED-PORT-PROPS
attribute allows the client to ask the relay for those properties.
The client MUST NOT request the TCP transport in an Allocate request
sent to the TURN server over UDP.
Processing of the response follows the general procedures of [1]. A
successful response will include both a RELAY-ADDRESS and an XOR-
MAPPED-ADDRESS attribute, providing both a relayed transport address
and a reflexive transport address, respectively, to the client. The
server will expire the allocation after LIFETIME seconds have passed
if not refreshed by another Allocate request. The server will allow
the user to send and receive at least the amount of data indicated in
the BANDWIDTH attribute per allocation. (At its discretion the
server can optionally discard data above this threshold.)
If the response is an error response and contains a 442, 443 or 444
error code, the client knows that its requested properties could not
be met. The client MAY retry with different properties, with the
same properties (in a hope that something has changed on the server),
or give up, depending on the needs of the application. However, if
the client retries, it SHOULD wait 500ms, and if the request fails
again, wait 1 second, then 2 seconds, and so on, exponentially
backing off.
6.1.1.2. Subsequent Requests
Before 3/4 of the lifetime of the allocation has passed (the lifetime
of the allocation is conveyed in the LIFETIME attribute of the
Allocate Response), the client SHOULD refresh the allocation with
another Allocate Request if it wishes to keep the allocation.
To perform a refresh, the client generates an Allocate Request as
described in Section 6.1.1.1. If the initial request was
authenticated with a shared secret P that the client holds with the
server, or using a short term password derived from P through a
Shared Secret request, the client MUST use shared secret P, or a
short-term password derived from it, in the subsequent request.
In a successful response, the RELAY-ADDRESS contains the same
transport address as previously obtained, indicating that the binding
has been refreshed. The LIFETIME attribute indicates the amount of
additional time the binding will live without being refreshed. Note
that an error response does not imply that the binding has been
expired, just that the refresh has failed.
If a client no longer needs an allocation, it SHOULD perform an
explict deallocation. If the client wishes to explicitly remove the
allocation because it no longer needs it, it generates a subsequent
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Allocate request, but sets the LIFETIME attribute to zero. This will
cause the server to remove the allocation, and all associated
bindings. For connection-oriented transports such as TCP, the client
can also remove the allocation (and all associated bindings) by
closing the relevant connection with the TURN server.
6.1.2. Server Behavior
The server first processes the request according to the general
request processing rules in [1]. This includes performing
authentication, and checking for mandatory unknown attributes. Due
to the fact that the STUN server is allocating resources for
processing the request, Allocate requests MUST be authenticated, and
furthermore, MUST be authenticated using either a shared secret known
between the client and server, or a short term password derived from
it.
Note that Allocate requests, like most other STUN requests, can be
sent to the TURN server over UDP, TCP, or TCP/TLS.
The behavior of the server when receiving an Allocate Request depends
on whether the request is an initial one, or a subsequent one. An
initial request is one whose source and destination transport address
do not match the internal remote and local transport addresses of an
existing internal 5-tuple. A subsequent request is one whose source
and destination transport address matches the internal remote and
local transport address of an existing internal 5-tuple.
6.1.2.1. Initial Requests
The server attempts to allocate transport addresses. It first looks
for the BANDWIDTH attribute for the request. If present, the server
determines whether or not it has sufficient capacity to handle a
binding that will generate the requested bandwidth.
If it does, the server attempts to allocate a transport address for
the client. The Allocate request can contain several additional
attributes that allow the client to request specific characteristics
of the transport address. First, the server checks for the
REQUESTED-TRANSPORT attribute. This indicates the transport protocol
requested by the client. This specification defines values for UDP
and TCP.
As a consequence of the REQUESTED-TRANSPORT attribute, it is
possible for a client to connect to the server over TCP or TLS
over TCP and request a UDP transport address. In this case, the
server will relay data between the transports.
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If the requested transport is supported, the server allocates a port
using the requested transport protocol. If the REQUESTED-TRANSPORT
attribute contains a value of the transport protocol unknown to the
server, or known to the server but not supported by the server in the
context of this request, the server MUST reject the request and
include a 442 (Unsupported Transport Protocol) in the response, or
redirect the request. If the request did not contain a REQUESTED-
TRANSPORT attribute, the server MUST use the same transport protocol
as the request arrived on.
Next, the server checks for the REQUESTED-IP attribute. If present,
it indicates a specific interface from which the client would like
its transport address allocated. If this interface is not a valid
one for allocations on the server, the server MUST reject the request
and include a 443 (Invalid IP Address) error code in the response, or
else redirect the request to a server that is known to support this
IP address. If the IP address is one that is valid for allocations
(presumably, the server is configured to know the set of IP addresses
from which it performs allocations), the server MUST provide an
allocation from that IP address. If the attribute is not present,
the selection of an IP address is at the discretion of the server.
Finally, the server checks for the REQUESTED-PORT-PROPS attribute.
If present, it indicates specific port properties desired by the
client. This attribute is split into two portions: one portion for
port behavior and the other for requested port alignment (whether the
allocated port is odd, even, reserved as a pair, or at the discretion
of the server).
If the port behavior requested is for a Specific Port, the server
MUST attempt to allocate that specific port for the client. If the
port is allocated to a different internal 5-tuple associated with the
same STUN long-term credentials, the client is requesting a move.
The server SHOULD replace the old internal 5-tuple with the new tuple
over which this Allocate request arrived. The server MUST reject the
move request if any of the attributes other than LIFETIME have
changed (BANDWIDTH, REQUESTED-TRANSPORT, etc.).
If the specific port is not available (in use or reserved), the
server MUST reject the request with a 444 (Invalid Port) response or
redirect to an alternate server. For example, the STUN server could
reject a request for a Specific Port because the port is temporarily
reserved as part of an adjacent pair of ports, or because the
requested port is a well-known port (1-1023).
If the client requests "even" port alignment, the server MUST attempt
to allocate an even port for the client. If an even port cannot be
obtained, the server MUST reject the request with a 444 (Invalid
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Port) response or redirect to an alternate server. If the client
requests odd port alignment, the server MUST attempt to allocate an
odd port for the client. If an odd port cannot be obtained, the
server MUST reject the request with a 444 (Invalid Port) response or
redirect to an alternate server. Finally, the "Even port with hold
of the next higher port" alignment is similar to requesting an even
port. It is a request for an even port, and MUST be rejected by the
server if an even port cannot be provided, or redirected to an
alternate server. However, it is also a hint from the client that
the client will request the next higher port with a separate Allocate
request. As such, it is a request for the server to allocate an even
port whose next higher port is also available, and furthermore, a
request for the server to not allocate that one higher port to any
other request except for one that asks for that port explicitly. The
server can honor this request for adjacency at its discretion. The
only constraint is that the allocated port has to be even.
Port alignment requests exist for compatibility with
implementations of RTP which pre-date RFC 3550. These
implementations use the port numbering conventions in (now
obsolete) RFC 1889.
If any of the requested or desired constraints cannot be met, whether
it be bandwidth, transport protocol, IP address or port, instead of
rejecting the request, the server can alternately redirect the client
to a different server that may be able to fulfill the request. This
is accomplished using the 300 error response and ALTERNATE-SERVER
attribute. If the server does not redirect and cannot service the
request because the server has reached capacity, it sends a 507
(Insufficient Capacity) response. The server can also reject the
request with a 486 (Allocation Quota Reached) if the user or client
is not authorized to request additional allocations.
The server SHOULD only allocate ports in the range 1024-65535. This
is one of several ways to prohibit relayed transport addresses from
being used to attempt to run standard services. These guidelines are
meant to be consistent with [9], since the relay is effectively a
NAT.
Once the port is allocated, the server associates it with the
internal 5-tuple and fills in that 5-tuple. The internal remote
transport address of the internal 5-tuple is set to the source
transport address of the Allocate Request. The internal local
transport address of the internal 5-tuple is set to the destination
transport address of the Allocate Request. For TCP, this amounts to
associating the TCP connection from the TURN client with the
allocated transport address.
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If the Allocate request was authenticated using a shared secret
between the client and server, this credential MUST be associated
with the allocation. If the request was authenticated using a short
term password derived from a shared secret, that shared secret MUST
be associated with the allocation. This is used in all subsequent
requests and indications to ensure that only the same client can use
or modify the allocation it was given.
The allocation created by the Allocate request is also associated
with a transport address, called the active destination. This
transport address is used for forwarding data through the TURN
server, and is described in more detail later. It is initially set
to null when the allocation is created. In addition, the allocation
created by the server is associated with a set of permissions. Each
permission is a specific IP address identifying an external client.
Initially, this list is null.
If the LIFETIME attribute was present in the request, and the value
is larger than the maximum duration the server is willing to use for
the lifetime of the allocation, the server MAY lower it to that
maximum. However, the server MUST NOT increase the duration
requested in the LIFETIME attribute. If there was no LIFETIME
attribute, the server may choose a default duration at its
discretion. In either case, the resulting duration is added to the
current time, and a timer, called the allocation expiration timer, is
set to fire at or after that time. Section 10.3 discusses behavior
when the timer fires. Note that the LIFETIME attribute in the
request can be zero. This typically happens for subsequent
Allocations, and provides a mechanism to delete the allocation. It
will force the immediate deletion of the allocation.
Once the port has been obtained and the activity timer started for
the port binding, the server generates an Allocate Response using the
general procedures defined in [1]. The transport address allocated
to the client MUST be included in the RELAY-ADDRESS attribute in the
response. In addition, this response MUST contain the XOR-MAPPED-
ADDRESS attribute. This allows the client to determine its reflexive
transport address in addition to a relayed transport address, from
the same Allocate request.
The server MUST add a LIFETIME attribute to the Allocate Response.
This attribute contains the duration, in seconds, of the allocation
expiration timer associated with this allocation.
The server MUST add a BANDWIDTH attribute to the Allocate Response.
This MUST be equal to the attribute from the request, if one was
present. Otherwise, it indicates a per-binding cap that the server
is placing on the bandwidth usage on each binding. Such caps are
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needed to prevent against denial-of-service attacks (See Section 12).
The server MUST add, as the final attribute of the request, a
MESSAGE-INTEGRITY attribute. The key used in the HMAC MUST be the
same as that used to validate the request.
6.1.2.2. Subsequent Requests
A subsequent Allocate request is one received whose source and
destination IP address and ports match the internal 5-tuple of an
existing allocation. The request is processed using the general
server procedures in [1] and is processed identically to
Section 6.1.2.1, with a few important exceptions.
First, the request MUST be authenticated using the same shared secret
as the one associated with the allocation, or be authenticated using
a short term password derived from that shared secret. If the
request was authenticated but not with such a matching credential,
the server MUST generate an Allocate Error Response with an
appropriate error response code.
Secondly, if the allocated transport address given out previously to
the client still matches the constraints in the request (in terms of
request ports, IP addresses and transport protocols), the same
allocation granted previously MUST be returned. However, if one of
the constraints is not met any longer, because the client changed
some aspect of the request, the server MUST free the previous
allocation and allocate a new request to the client.
Finally, a subsequent Allocate request will set a new allocation
expiration timer for the allocation, effectively canceling the
previous lifetime expiration timer.
6.2. Procedures for all other Requests and Indications
Other than initial Allocate Requests, all requests and indications
defined in this document need to be sent in the context of a valid
allocation. The source and destination IP address and ports for
these STUN messages MUST match the internal 5-tuple of an existing
allocation. These processed using the general server procedures in
[1] with a few important additions. For requests, if there is no
matching allocation, the server MUST generate a 437 (No Binding) Send
Error Response. For indications, if there is no matching allocation,
the indication is silently discarded.
All requests and indications MUST be authenticated using the same
shared secret as the one associated with the allocation, or be
authenticated using a short term password derived from that shared
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secret. If the request was authenticated but not with such a
matching credential, the server MUST generate an Allocate Error
Response with an appropriate error response code, such as a 431
(Integrity Failure) or 436 (Unknown User).
6.3. Set Active Destination Request
6.3.1. Client Behavior
The Set Active Destination request allows the client to create an
optimized relay function between the client and the server. When the
server receives packets from a particular preferred external peer,
the server will forward those packets towards the client without
encapsulating them in a Data Indication. Similarly, the client can
send non-STUN packets to the server without encapsulation in a Send
Indication, and these packets are forwarded to the external peer.
Sending and receiving data in unencapsulated form is critical for
efficiency purposes. One of the primary use cases for the STUN relay
extensions is in support of Voice over IP (VoIP), which uses very
small UDP packets to begin with. The extra overhead of an additional
layer of encapsulation is considered unacceptable.
The Set Active Destination request is used by the client to provide
the identity of this preferred external peer. The Set Active
Destination address MAY contain a REMOTE-ADDRESS attribute. This
attribute, when present, provides the address of the preferred
external peer to the server. When absent, it clears the value of the
preferred external peer. As a convenience, if the client sets the
REMOTE-ADDRESS attribute to a peer without a permission, the server
will add the corresponding permission.
The client MUST NOT send a Set Active Destination request with a
REMOTE-ADDRESS attribute over an unreliable link (ex: UDP) if an
active destination is already set for that allocation. If the client
wishes to set a new active destination, it MUST wait until a
successful response is received to a Set Destination Request removing
the active destination. The client SHOULD then continue to wait for
an additional period of up to 5 seconds until it is extremely
unlikely that any data from the previous active destination might
still arrive. Failure to wait could cause the client to receive and
attribute late data forwarded by the TURN server to the wrong peer.
The client MAY wait a shorter period of time if the application has
built-in addressing (such as the RTP [3] Sender Source) that makes it
unlikely the client would incorrectly attribute late data. [OPEN
ISSUE: is this OK with the WG? ]
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Consider the case where the active destination is set, and the
server is relaying packets towards the client. The client knows
the IP address and port where the packets came from - the current
value of the active destination. The client issues a Set Active
Destination Request to change the active destination, and receives
a response. A moment later, a data packet is received, not
encapsulated in a STUN Data Indication. What is the source if
this packet? Is it the active destination that existed prior to
the Set Active Destination request, or the one after? If the
transport between the client and the STUN server is not reliable,
there is no way to know.
6.3.2. Server Behavior
The Set Active Destination Request is used by a client to set the
forwarding destination of all data that is not encapsulated in STUN
Send Indications. In addition, when a matching permission is
present, all data received from that external peer will be forwarded
to the STUN client without being encapsulated in a Data Indication.
If the Set Active Destination request does not contain a REMOTE-
ADDRESS attribute, the value of the active destination is cleared.
If the Set Active Destination request contains a REMOTE-ADDRESS
attribute, and the active destination is not set, the active
destination is set to that IP address and port. If an active
destination is already set, and the request was received over a
reliable transport, the active destination is changed to the new
value. If the active destination is already set and the request was
received over UDP, the Set Active Destination request is rejected
with a 439 Active Destination Already Set error response. This
prevents the race condition described in the previous section.
If the server sets the active destination and there is no permission
associated with the REMOTE-ADDRESS, the server adds the corresponding
permission. Note that if the permission associated with the active
destination becomes invalid, the server does not reset the active
destination. The client is expected to do this explicitly.
6.4. Connect Request
The Connect Request is used by a client when it has obtained an
allocated transport address that is TCP. The client can use the
Connect Request to ask the server to open a TCP connection to a
specified destination address included in the request.
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6.4.1. Server Behavior
If the allocation is for a UDP port, the server MUST reject the
request with a 445 (Operation for TCP Only) response. If the request
does not contain a REMOTE-ADDRESS attribute, the server sends a 400
(Bad Request) Connect error response,.
If the request contains a REMOTE-ADDRESS attribute, the IP address
contained within it is added to the permissions for this allocation,
if it was not already present. This happens regardless of whether
the subsequent TCP connection attempt succeeds or not.
If a connection already exists for this address and port, the server
returns a 446 (Connection Already Exists) Connect error response.
Otherwise the server tries to establish the corresponding TCP
connection and returns a Connect Success Response. This just
indicates that the server added the permission and is attempting to
establish a TCP connection. The server does not wait for the
connection attempt to succeed or fail. The status of the connection
attempt is returned via the Connect Status Indication.
Note that the server needs to use the same source connection
address for all connections/permissions associated with an
allocation. For servers written using Berkeley sockets, the
SO_REUSEADDR flag is typically used to use the same local address
with multiple sockets.
6.5. Connection Status Indication
When the TURN to peer leg is TCP, the TURN client needs to be aware
of the status of these TCP connections. The TURN extension defines
application states for a TCP connection as follows: LISTEN,
ESTABLISHED, CLOSED. Consequently, the TURN server sends a
Connection State Indication for a TCP permission whenever the relay
connection status changes for one of the client's permissions except
when the status changes due to a TURN client request (ex: an explicit
binding close or deallocation).
A TURN can only relay to a peer over TCP if the client
communicates with the server over TCP or TLS over TCP. Because of
this, the server can be assured that Connection Status Indications
are received reliably.
6.6. Send Indication
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6.6.1. Client Behavior
The Send Indication is used to ask the relay to forward data to a
peer. It is typically used to send to a peer other than the active
destination. For TCP allocated transport addresses, the client needs
to wait for the peer to open a connection to the TURN server before
it can send data. Data sent with a Send request prior to the opening
of a TCP connection is discarded silently by the server.
The Send Indication MUST contain a REMOTE-ADDRESS attribute, which
contains the IP address and port that the data is being sent to. The
DATA attribute MAY be present, and contains the data that is to be
sent towards REMOTE-ADDRESS. If absent, the server will send an
empty UDP packet in the case of UDP. In the case of TCP, the server
will do nothing.
Since Send is an Indication, it generates no response. The client
must rely on application layer mechanisms to determine if the data
was received by the peer.
Note that Send Indications are not authenticated and do not
contain a MESSAGE-INTEGRITY attribute. Just like non-relayed data
sent over UDP or TCP, the authenticity and integrity of this data
can only be assured using security mechanisms at higher layers.
6.6.2. Server Behavior
A Send Indication is sent by a client after it has completed its
Allocate transaction, in order to create permissions in the server
and send data to an external client.
If a Send Indication contains no REMOTE-ADDRESS, the indication is
discarded. If there is no DATA attribute, and the corresponding
allocation is for TCP, the indication is discarded.
If the allocation is a UDP allocation, the server creates a UDP
packet whose payload equals that content. The server sets the source
IP address of the packet equal to the allocated transport address.
The destination transport address is set to the contents of the
REMOTE-ADDRESS attribute. If a permission does not exist for this
destination the server creates one for this allocation. The server
then sends the UDP packet. Note that any retransmissions of this
packet which might be needed are not handled by the server. It is
the responsibility of the client to generate another Send indication
if needed.
If the allocation is a TCP allocation, the server checks if it has an
existing TCP connection open from the allocated transport address to
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the address in the REMOTE-ADDRESS attribute. If so, the server
extracts the content of the DATA attribute and sends it over the
matching TCP connection. If the server doesn't have an existing TCP
connection to the destination, it adds the REMOTE-ADDRESS to the
permission list and discards the data.
6.7. Data Indication
6.7.1. Client Behavior
Once a client has obtained an allocation and created permissions for
a particular external client, the server can begin to relay packets
from that external client towards the client. If the external client
is not the active destination, this data is relayed towards the
client in encapsulated form using the Data Indication.
The Data Indication contains two attributes - DATA and REMOTE-
ADDRESS. The REMOTE-ADDRESS attribute indicates the source transport
address that the request came from, and it will equal the external
remote transport address of the external peer. When processing this
data, a client MUST treat the data as if it came from this address,
rather than the stun server itself. The DATA attribute contains the
data from the UDP packet or TCP segment that was received. Note that
the TURN server will not retransmit this indication over UDP.
Note that Data Indications are not authenticated and do not
contain a MESSAGE-INTEGRITY attribute. Just like non-relayed data
sent over UDP or TCP, the authenticity and integrity of this data
can only be assured using security mechanisms at higher layers.
6.7.2. Server Behavior
A server MUST send data packets towards the client using a Data
Indication under the conditions described in Section 10.1. Data
Indications MUST contain a DATA attribute containing the data to
send, and MUST contain a REMOTE-ADDRESS attribute indicating where
the data came from.
7. New Attributes
This STUN extension defines the following new attributes:
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0x000D: LIFETIME
0x0010: BANDWIDTH
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
0x0021: TIMER-VAL
0x0023: CONNECT_STAT
7.1. LIFETIME
The lifetime attribute represents the duration for which the server
will maintain an allocation in the absence of data traffic either
from or to the client. It is a 32 bit value representing the number
of seconds remaining until expiration.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.2. BANDWIDTH
The bandwidth attribute represents the peak bandwidth, measured in
kbits per second, that the client expects to use on the binding in
each direction.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
7.3. REMOTE-ADDRESS
The REMOTE-ADDRESS specifies the address and port of the peer as seen
from the TURN server. It is encoded in the same way as MAPPED-
ADDRESS.
7.4. DATA
The DATA attribute is present in Send Indications and Data
Indications. It contains raw payload data that is to be sent (in the
case of a Send Request) or was received (in the case of a Data
Indication). It is padded with zeros if its length is not divisible
evenly by 4 octets
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7.5. 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 MAPPED-ADDRESS.
7.6. REQUESTED-PORT-PROPS
This attribute allows the client to request certain properties for
the port that is allocated by the server. The attribute can be used
with any transport protocol that has the notion of a 16 bit port
space (including TCP and UDP). The attribute is 32 bits long. Its
format is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved = 0 | A | Specific Port Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The two bits labeled A in the diagram above are for requested port
alignment and have the following meaning:
00 no specific port alignment
01 odd port number
10 even port number
11 even port number; reserve next higher port
If the Specific Port Number field is zero, this means that no
specific port is requested. If a specific port number is requested
the value will be in the two low order octets. All other bits in
this attribute are reserved and MUST be set to zero.
Even Port is a request to the server to allocate a port with even
parity. The port filter is not used with this property. Odd Port is
a request to the server to allocate a port with odd parity. The port
filter is not used with this property. Even port, with a hold on the
next higher port, is a request to the server to allocate an even
port. Furthermore, the client indicates that it will want the next
higher port as well. As such, the client requests that the server,
if it can, not allocate the next higher port to anyone unless that
port is explicitly requested, which the client will itself do. The
port filter is not used with this property. Finally, the Specific
Port property is a request for a specific port. The port that is
requested is contained in the Port filter.
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7.7. REQUESTED-TRANSPORT
This attribute is used by the client to request a specific transport
protocol for the allocated transport address. It is a 32 bit
unsigned integer. Its values are:
0x0000 0000: UDP
0x0000 0001: TCP
If an Allocate request is sent over TCP and requests a UDP
allocation, or an Allocate request is sent over TLS over TCP and
requests a UDP or TCP allocation, the server will relay data between
the two transports.
Extensions to TURN can define additional transport protocols in an
IETF-consensus RFC.
7.8. REQUESTED-IP
The REQUESTED-IP attribute is used by the client to request that a
specific IP address be allocated to it. This attribute is needed
since it is anticipated that TURN servers will be multi-homed so as
to be able to allocate more than 64k transport addresses. As a
consequence, a client needing a second transport address on the same
interface as a previous one can make that request.
The format of this attribute is identical to MAPPED-ADDRESS.
However, the port component of the attribute is ignored by the
server. If a client wishes to request a specific IP address and
port, it uses both the REQUESTED-IP and REQUESTED-PORT-PROPS
attributes.
7.9. CONNECT_STAT
This attribute is used by the server to convey the status of server-
to-peer connections. It is a 32 bit unsigned integer. Its values
are:
0x0000 0000: LISTEN
0x0000 0001: ESTABLISHED
0x0000 0002: CLOSED
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8. New Error Response Codes
This document defines the following new Error response codes:
437 (No Binding): A request was received by the server that
requires an allocation to be in place. However, there is none yet
in place.
439 (Active Destination Already Set): A Set Active Destination
request was received by the server over UDP. However, the active
destination is already set to another value. The client should
reset the active destination, wait for the hold-down period, and
set the active destination to the new value.
442 (Unsupported Transport Protocol): The Allocate request asked
for a transport protocol to be allocated that is not supported by
the server.
443 (Invalid IP Address): The Allocate request asked for a
transport address to be allocated from a specific IP address that
is not valid on the server.
444 (Invalid Port): The Allocate request asked for a port to be
allocated that is not available on the server.
445 (Operation for TCP Only): The client tried to send a request
to perform a TCP-only operation on an allocation, and allocation
is UDP.
446 (Connection Already Exists): The client tried to open a
connection to a peer, but a connection to that peer already
exists.
486 (Allocation Quota Reached): The user or client is not
authorized to request additional allocations.
507 (Insufficient Capacity): The server cannot allocate a new port
for this client as it has exhausted its relay capacity.
9. Client Procedures
9.1. Receiving and Sending Unencapsulated Data
Once the active destination has been set, a client will receive both
STUN and non-STUN data on the socket on which the Allocate Request
was sent. The encapsulation behavior depends on the transport
protocol used between the STUN client and the TURN server.
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9.1.1. Datagram Protocols
If the allocation was over UDP, datagrams which contain the STUN
magic cookie are treated as STUN requests. All other data is non-
STUN data, which MUST be processed as if it had a source IP address
and port equal to the value of the active destination.
If the client wants to send data to the peer which contains the magic
cookie in the same location as a STUN request, it MUST send that data
encapsulated in a Send Indication, even if the active destination is
set.
In addition, once the active destination has been set, the client can
send data to the active destination by sending the data
unencapsulated on that same socket. Unencapsulated data MUST NOT be
sent if no active destination is set. Of course, even if the active
destination is set, the client can send data to that destination at
any time by using the Send Indication.
9.1.2. Stream Transport Protocols
If the allocation was over TCP or TLS over TCP, the client will
receive data framed as described in Section 5. The client MUST treat
data encapsulated as data with this framing as if it originated from
the active destination.
For the any of the methods defined in this document, the client
always sends data encapsulated using this framing scheme. It SHOULD
frame data to the active destination as data or it MAY place the data
inside a Send Indications and frame this as STUN traffic.
10. Server Procedures
Besides the processing of the request and indications described
above, this specification defines rules for processing of data
packets received by the STUN server. There are two cases - receipt
of any packets on an allocated address, and receipt of non-STUN data
on its internal local transport address.
10.1. Receiving Data on Allocated Transport Addresses
10.1.1. TCP Processing
If a server receives a TCP connection request on an allocated TCP
transport address, it checks the permissions associated with that
allocation. If the source IP address of the TCP SYN packet matches
one of the permissions (the source port does not need to match), the
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TCP connection is accepted. Otherwise, it is rejected. When a TCP
connection is accepted, the server sends the corresponding client a
Connect Status Indication with the CONNECT_STAT attribute set to
ESTABLISHED. No information is passed to the client if the server
rejects the connection because there is no corresponding permission.
If a server receives data on a TCP connection that terminates on the
allocated TCP transport address, the server checks the value of the
active destination. If it equals the source IP address and port of
the data packet (in other words, if the active destination identifies
the other side of the TCP connection), the data is taken from the TCP
connection and sent towards the client in unencapsulated form.
Otherwise, the data is sent towards the client in a Data Indication,
also known as encapsulated form. In this form, the server MUST add a
REMOTE-ADDRESS which corresponds to the external remote transport
address of the TCP connection, and MUST add a DATA attribute
containing the data received on the TCP connection.
Note that, because data is forwarded blindly across TCP bindings,
TLS will successfully operate over a TURN allocated TCP port if
the linkage to the client is also TCP.
10.1.2. UDP Processing
If a server receives a UDP packet on an allocated UDP transport
address, it checks the permissions associated with that allocation.
If the source IP address of the UDP packet matches one of the
permissions (the source port does not need to match), the UDP packet
is accepted. Otherwise, it is discarded. If the packet is accepted,
it is forwarded to the client. It will be forwarded in either
encapsulated or unencapsulated form.
If the client to server communication is via UDP, the server looks
for the existence of the STUN magic cookie in the data received from
the peer. If the data contains the magic cookie, the server
encapsulates the data in a Data Indication, sets the REMOTE_ADDRESS
attribute, and forwards the indication to the client. If the magic
cookie is not present, the server checks if the peer is the active
destination. If so the data is forwarded unencapsulated, directly to
the client. Otherwise the server encapsulates the data in a Data
Indication, sets the REMOTE_ADDRESS and forwards to the client.
If the client to server communication is via TCP or TLS, the server
checks if the peer is the active destination. If so, the data from
the peer is framed as Data and sent to the client over the client to
server connection. Otherwise, the server encapsulates the data in a
Data Indication, sets the REMOTE_ADDRESS attribute, frames the
indication as STUN traffic, and sends the indication over the
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connection to the client. If the TCP connection generates an error
(because, for example, the incoming UDP packet rate exceeds the
throughput of the TCP connection), the data is discarded silently by
the server.
10.2. Receiving Data on Internal Local Transport Addresses
If a server receives non-STUN UDP data from the client on its
internal local transport address, and it is coming from an internal
remote transport address associated with an existing allocation, it
represents UDP data that the client wishes to forward. If there is
no allocation associated with the source IP address and port number,
or if there is an associated allocation but the active destination is
not set, the server MUST discard the packet. If the active
destination is set, the server places the data from the client in a
UDP payload, and sets the destination address and port to the active
destination. The UDP packet is then sent with a source IP address
and port equal to the allocated transport address. Note that the
server will not retransmit the UDP datagram.
If a server receives framed data on a TCP connection from a client,
the server retrieves the allocation bound to that connection. If the
active destination for the allocation is not set, the server MUST
discard the data and close the connection. If the active destination
is set, and the allocated transport protocol is TCP, the server
forwards the data over the connection to the active destination. The
data is then sent over that connection. If the connection is not
established or if the transmission fails due to a TCP error, the data
is discarded silently by the server. If the active destination is
set, and the allocated transport protocol is UDP, the server places
the data from the client in a UDP payload, and sets the destination
address and port to the active destination. The UDP packet is then
sent with a source IP address and port equal to the allocated
transport address. Note that the server will not retransmit the UDP
datagram.
If a TCP connection from a client is closed, the associated
allocation is destroyed. This involves terminating any TCP
connections from the allocated transport address to external peer
(applicable only when the allocated transport address was TCP), and
then freeing the allocated transport address (and all associated
state maintained by the server) for use by other clients.
10.3. Lifetime Expiration
When the allocation expiration timer for a binding fires, the server
MUST destroy the allocation. This involves terminating any TCP
connections from the allocated transport address to external peers
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(applicable only when the allocated transport address was TCP), and
then freeing the allocated transport address (and all associated
state maintained by the server) for use by other clients. A
suggested value for the allocation expiration timer is 10 minutes.
The server is also expected to run a permission inactivity timer for
each permission associated with an Allocation. If no traffic from
the client is received, the permission inactivity timer will
eventually expire and the server MUST delete the permission. A
suggested value for the permission inactivity timer for UDP
allocations is 60 seconds.
11. Client Discovery of TURN Servers
The STUN relay extensions differ from the binding requests defined in
[1] in that they demands substantial resources from the STUN server.
In addition, it seems likely that administrators might want to block
connections from clients to the STUN server for relaying separately
from connections for the purposes of binding discovery. As a
consequence, TURN is expected to typically run on a separate port
from basic STUN. The client discovers the address and port of the
TURN server using the same DNS procedures defined in [1], but using
an SRV service name of "stun-relay" instead of just "stun".
For example, to find TURN servers in the example.com domain, the TURN
client performs a lookup for '_stun-relay._udp.example.com', '_stun-
relay._tcp.example.com', and '_stun-relay._tls.example.com' if the
STUN client wants to communicate with the TURN server using UDP, TCP,
or TLS over TCP, respectively. The client assumes that all
permissable transport protocols are supported from the TURN server to
the peer for the client to server protocol selected.
The STUN server is designed so the relay usage can run on a
separate source port from non-relay usages. Since the client
looks up the port number for the relay usage separately, servers
can be configured to rely on this property. The STUN server MAY
accept both relay and non-relay usages on the same port number, in
which case it uses framing hints and choice of STUN messages to
detect the STUN usage in use by a specific client.
12. Security Considerations
STUN servers implementing the TURN extensions allocate bandwidth and
port resources to clients, in contrast to the Binding method defined
in [1]. Therefore, a STUN server providing the relay usage requires
authentication and authorization of STUN requests. This
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authentication is provided by mechanisms defined in the STUN
specification itself. In particular, digest authentication and the
usage of short-term passwords, obtained through a digest exchange
over TLS, are available. The usage of short-term passwords ensures
that the Allocate Requests, which often do not run over TLS, are not
susceptible to offline dictionary attacks that can be used to guess
the long lived shared secret between the client and the server.
Because TURN servers allocate resources, they can be susceptible to
denial-of-service attacks. All Allocate Requests are authenticated,
so that an unknown attacker cannot launch an attack. An
authenticated attacker can generate multiple Allocate Requests,
however. To prevent a single malicious user from allocating all of
the resources on the server, it is RECOMMENDED that a server
implement a modest per user cap on the amount of bandwidth that can
be allocated. Such a mechanism does not prevent a large number of
malicious users from each requesting a small number of allocations.
Attacks 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 RFC
3489 [[TODO: Update references once 3489bis is more stable]]. These
attacks are based on the fact that a STUN server mirrors the source
IP address, which cannot be authenticated. STUN does not use the
source address of the Allocate Request in providing the RELAY-
ADDRESS, and therefore, those attacks do not apply.
TURN cannot be used by clients for subverting firewall policies.
TURN has fairly limited applicability, requiring a user to send a
packet to a peer before being able to receive a packet from that
peer. This applies to both TCP and UDP. Thus, it does not provide a
general technique for externalizing TCP and UDP sockets. Rather, it
has similar security properties to the placement of an address-
restricted NAT in the network, allowing messaging in from a peer only
if the internal client has sent a packet out towards the IP address
of that peer. This limitation means that TURN cannot be used to run
web servers, email servers, SIP servers, or other network servers
that service a large number of clients. Rather, it facilitates
rendezvous of NATted clients that use some other protocol, such as
SIP, to communicate IP addresses and ports for communications.
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Confidentiality of the transport addresses learned through Allocate
requests does not appear to be that important, and therefore, this
capability is not provided.
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 STUN server, so that the
user's IP addresses are never revealed.
TCP transport addresses allocated by Allocate requests will properly
work with TLS and SSL. However, any relay addresses learned through
an Allcoate will not operate properly with IPSec Authentication
Header (AH) [5] in transport mode. IPSec ESP [6] and any tunnel-mode
ESP or AH should still operate.
13. IANA Considerations
This specification defines several new STUN messages, STUN
attributes, and STUN response codes. This section directs IANA to
add these new protocol elements to the IANA registry of STUN protocol
elements.
13.1. New STUN Requests, Responses, and Indications
Request/Response Transactions
0x003 : Allocate
0x004 : Set Active Destination
0x005 : Connect
Indications
0x006 : Send
0x007 : Data
0x008 : Connect Status
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13.2. New STUN Attributes
0x000D: LIFETIME
0x0010: BANDWIDTH
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0016: RELAY-ADDRESS
0x0018: REQUESTED-PORT-PROPS
0x0019: REQUESTED-TRANSPORT
0x0022: REQUESTED-IP
0x0021: TIMER-VAL
0x0023: CONNECT_STAT
13.3. New STUN response codes
437 No Binding
439 Active Destination Already Set
442 Unsupported Transport Protocol
443 Invalid IP Address
444 Invalid Port
445 Operation for TCP Only
446 Connection Already Exists
486 Allocation Quota Reached
507 Insufficient Capacity
14. IAB Considerations
The IAB has studied the problem of "Unilateral Self Address Fixing",
which is the general process by which a client attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism RFC 3424 [7]. 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 [8].
15. Example
In this example, a client is behind a NAT. The client has a private
address of 10.0.1.1. The STUN server is on the public side of the
NAT, and is listening for TURN requests on 192.0.2.3:8776. The
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public side of the NAT has an IP address of 192.0.2.1. The client is
attempting to send a SIP INVITE to a peer, and wishes to allocate an
IP address and port for inclusion in the SDP of the INVITE.
Normally, TURNs would be used in conjunction with ICE when applied to
SIP. For simplicities sake, TURN is showed without ICE.
The client communicates with a SIP user agent on the public network.
This user agent uses a 192.0.2.17:12734 for receipt of its RTP
packets.
Client NAT STUN Server Peer
| | | |
|(1) Allocate | | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(2) Allocate | |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| |(3) Error | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(4) Error | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
|(5) Allocate | | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(6) Allocate | |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| |(7) Response | |
| |RA=192.0.2.3:32766 | |
| |MA=192.0.2.1:63346 | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
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| |<------------------| |
|(8) Response | | |
|RA=192.0.2.3:32766 | | |
|MA=192.0.2.1:63346 | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
|(9) INVITE | | |
|SDP=192.0.2.3:32766| | |
|---------------------------------------------------------->|
| | | |
| | | |
|(10) 200 OK | | |
|SDP=192.0.2.17:12734 | |
|<----------------------------------------------------------|
| | | |
| | | |
| | | |
|(11) ACK | | |
|---------------------------------------------------------->|
| | | |
|(12) Send | | |
|DATA=RTP | | |
|DA=192.0.2.17:12734| | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(13) Send | |
| |DATA=RTP | |
| |DA=192.0.2.17:12734| |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| | |(14) RTP |
| | |S=192.0.2.3:32766 |
| | |D=192.0.2.17:12734 |
| | |------------------>|
| | | |
| | |Permission |
| | |Created |
| | |192.0.2.17 |
| | | |
| | |(15) RTP |
| | |S=192.0.2.17:12734 |
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| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| |(16) DataInd | |
| |DATA=RTP | |
| |RA=192.0.2.17:12734| |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
|(17) DataInd | | |
|DATA=RTP | | |
|RA=192.0.2.17:12734| | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
|(18) SetAct | | |
|DA=192.0.2.17:12734| | |
|S=10.0.1.1:4334 | | |
|D=192.0.2.3:8776 | | |
|------------------>| | |
| | | |
| |(19) SetAct | |
| |DA=192.0.2.17:12734| |
| |S=192.0.2.1:63346 | |
| |D=192.0.2.3:8776 | |
| |------------------>| |
| | | |
| |(20) Response | |
| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(21) Response | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
| | | after 3s|
| | | |
| | | |
| | |(22) RTP |
| | |S=192.0.2.17:12734 |
| | |D=192.0.2.3:32766 |
| | |<------------------|
| | | |
| |(23) RTP | |
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| |S=192.0.2.3:8776 | |
| |D=192.0.2.1:63346 | |
| |<------------------| |
| | | |
|(24) RTP | | |
|S=192.0.2.3:8776 | | |
|D=10.0.1.1:4334 | | |
|<------------------| | |
| | | |
| | | |
Figure 14
The call flow is shown in Figure 14. The client allocates a port
from the local operating system on its private interface, obtaining
4334. It then attempts to secure a port for RTP traffic. RTCP
processing is not shown. The client sends an Allocate request (1)
with a source address (denoted by S) of 10.0.1.1:4334 and a
destination (denoted by D) of 192.0.2.3:8776. This passes through
the NAT (2), which creates a mapping from the 192.0.2.1:63346 to the
source IP address and port of the request, 10.0.1.1:4334. This
request is received at the STUN server, which challenges it (3),
requesting credentials. This response is passed to the client (4).
The client retries the request, this time with credentials (5). This
arrives at the server (6). The request is now authenticated. The
server provides a UDP allocation, 192.0.2.3:32766, and places it into
the RELAY-ADDRESS (denoted by RA) in the response (7). It also
reflects the source IP address and port of the request into the
MAPPED-ADDRESS (denoted by MA) in the response. This passes through
the NAT to the client (8). The client now proceeds to perform a
basic SIP call setup. In message 9, it includes the relay address
into the SDP of its INVITE. The called party responds with a 200 OK,
and includes its IP address - 192.0.2.17:12734. The exchange
completes with an ACK (11).
Next, user A sends an RTP packet. Since the active destination has
not been set, the client decides to use the Send indication. It does
so, including the RTP packet as the contents of the DATA attribute.
The REMOTE-ADDRESS attribute (denoted by DA) is set to 192.0.2.17:
12734, learned from the 200 OK. This is sent through the NAT
(message 12) and arrives at the STUN server (message 13). The server
extracts the data contents, and sends the packet towards REMOTE-
ADDRESS (message 14). Note how the source address and port in this
packet is 192.0.2.3:32766, the allocated transport address given to
the client. The act of sending the packet with Send causes the STUN
server to install a permission for 192.0.2.17.
Indeed, the called party now sends an RTP packet toward the client
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(message 15). This arrives at the STUN server. Since a permission
has been set for the IP address in the source of this packet, it is
accepted. As no active destination is set, the STUN server
encapsulates the contents of the packet in a Data Indication (message
16), and sends it towards the client. The REMOTE-ADDRESS attribute
(denoted by RA) indicates the source of the packet - 192.0.2.17:
12734. This is forwarded through the NAT to the client (message 17).
The client decides to optimize the path for packets to and from
192.0.2.17:12734. So, it issues a Set Active Destination request
(message 18) with a REMOTE-ADDRESS of 192.0.2.17:12734. This passes
through the NAT and arrives at the STUN server (message 19). This
generates a successful response (message 20) which is passed to the
client (message 21). At this point, the server and client are in the
transitioning state. A little over 3 seconds later (by default), the
state machines transition back to "Set". Until this point, packets
from the called party would have been relayed back to the client in
Data Indications. Now, the next RTP packet shows up at the STUN
server (message 22). Since the source IP address and port match the
active destination, the RTP packet is relayed towards the client
without encapsulation (message 23 and 24).
16. Acknowledgements
The authors would like to thank Marc Petit-Huguenin for his comments
and suggestions.
17. References
17.1. Normative References
[1] Rosenberg, J., "Session Traversal Utilities for (NAT) (STUN)",
draft-ietf-behave-rfc3489bis-06 (work in progress), March 2007.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
17.2. Informative References
[3] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[4] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
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[5] Kent, S., "IP Authentication Header", RFC 4302, December 2005.
[6] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.
[7] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[8] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols", draft-ietf-mmusic-ice-16 (work in
progress), June 2007.
[9] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-08 (work in progress),
October 2006.
Authors' Addresses
Jonathan Rosenberg
Cisco Systems
600 Lanidex Plaza
Parsippany, NJ 07054
US
Phone: +1 973 952-5000
Email: jdrosen@cisco.com
URI: http://www.jdrosen.net
Rohan Mahy
Plantronics
Email: rohan@ekabal.com
Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
US
Email: huitema@microsoft.com
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