Internet DRAFT - draft-rosenberg-midcom-turn
draft-rosenberg-midcom-turn
MIDCOM J. Rosenberg
Internet-Draft Cisco Systems
Expires: March 13, 2006 R. Mahy
Airspace
C. Huitema
Microsoft
September 9, 2005
Traversal Using Relay NAT (TURN)
draft-rosenberg-midcom-turn-08
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Traversal Using Relay NAT (TURN) is a protocol that allows for an
element behind a NAT or firewall to receive incoming data over TCP or
UDP connections. It is most useful for elements behind symmetric
NATs or firewalls that wish to be on the receiving end of a
connection to a single peer. TURN does not allow for users to run
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servers on well known ports if they are behind a nat; it supports the
connection of a user behind a nat to only a single peer. In that
regard, its role is to provide the same security functions provided
by symmetric NATs and firewalls, but to "turn" them into port-
restricted NATs.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Definitions . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Applicability Statement . . . . . . . . . . . . . . . . . . 5
5. Overview of Operation . . . . . . . . . . . . . . . . . . . 5
6. Message Overview . . . . . . . . . . . . . . . . . . . . . . 9
7. Server Behavior . . . . . . . . . . . . . . . . . . . . . . 9
7.1 Shared Secret Request . . . . . . . . . . . . . . . . . . 9
7.2 Allocate Request . . . . . . . . . . . . . . . . . . . . . 12
7.2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . 12
7.2.2 Initial Requests . . . . . . . . . . . . . . . . . . . 12
7.2.3 Subsequent Requests . . . . . . . . . . . . . . . . . 15
7.3 Send Request . . . . . . . . . . . . . . . . . . . . . . . 16
7.4 Receiving Packets and Connections on the Allocated
Transport Address . . . . . . . . . . . . . . . . . . . . 18
7.5 Receiving a Set Active Destination Request . . . . . . . . 19
7.6 Receiving Data from the TURN Client . . . . . . . . . . . 21
7.7 Lifetime Expiration . . . . . . . . . . . . . . . . . . . 21
8. Client Behavior . . . . . . . . . . . . . . . . . . . . . . 22
8.1 Discovery . . . . . . . . . . . . . . . . . . . . . . . . 22
8.2 Obtaining a One Time Password . . . . . . . . . . . . . . 22
8.3 Allocating a Binding . . . . . . . . . . . . . . . . . . . 24
8.4 Processing Allocate Responses . . . . . . . . . . . . . . 24
8.5 Refreshing a Binding . . . . . . . . . . . . . . . . . . . 25
8.6 Sending Encapsulated Data . . . . . . . . . . . . . . . . 26
8.7 Receiving a Data Indication . . . . . . . . . . . . . . . 27
8.8 Setting the Active Destination . . . . . . . . . . . . . . 28
8.9 Tearing Down a Binding . . . . . . . . . . . . . . . . . . 29
8.10 Receiving and Sending Unencapsulated Data . . . . . . . 29
9. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 29
9.1 Message Types . . . . . . . . . . . . . . . . . . . . . . 30
9.2 Message Attributes . . . . . . . . . . . . . . . . . . . . 30
9.2.1 LIFETIME . . . . . . . . . . . . . . . . . . . . . . . 30
9.2.2 ALTERNATE-SERVER . . . . . . . . . . . . . . . . . . . 31
9.2.3 MAGIC-COOKIE . . . . . . . . . . . . . . . . . . . . . 31
9.2.4 BANDWIDTH . . . . . . . . . . . . . . . . . . . . . . 31
9.2.5 DESTINATION-ADDRESS . . . . . . . . . . . . . . . . . 31
9.2.6 REMOTE-ADDRESS . . . . . . . . . . . . . . . . . . . . 31
9.2.7 DATA . . . . . . . . . . . . . . . . . . . . . . . . . 31
9.2.8 NONCE . . . . . . . . . . . . . . . . . . . . . . . . 32
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9.2.9 REALM . . . . . . . . . . . . . . . . . . . . . . . . 32
9.2.10 Response Codes . . . . . . . . . . . . . . . . . . . 32
10. Security Considerations . . . . . . . . . . . . . . . . . . 33
11. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 34
11.1 Problem Definition . . . . . . . . . . . . . . . . . . . 34
11.2 Exit Strategy . . . . . . . . . . . . . . . . . . . . . 35
11.3 Brittleness Introduced by TURN . . . . . . . . . . . . . 35
11.4 Requirements for a Long Term Solution . . . . . . . . . 36
11.5 Issues with Existing NAPT Boxes . . . . . . . . . . . . 36
12. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 37
13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 37
14.1 Normative References . . . . . . . . . . . . . . . . . . 37
14.2 Informative References . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 38
Intellectual Property and Copyright Statements . . . . . . . 40
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1. Introduction
Network Address Translators (NATs), while providing many benefits,
also come with many drawbacks. The most troublesome of those
drawbacks is the fact that they break many existing IP applications,
and make it difficult to deploy new ones. Guidelines [9] have been
developed that describe how to build "NAT friendly" protocols, but
many protocols simply cannot be constructed according to those
guidelines. Examples of such protocols include multimedia
applications and file sharing.
Simple Traversal of UDP Through NAT (STUN) [1] provides one means for
an application to traverse a NAT. STUN allows a client to obtain a
transport address (and IP address and port) which may be useful for
receiving packets from a peer. However, addresses obtained by STUN
may not be usable by all peers. Those addresses work depending on
the topological conditions of the network. Therefore, STUN by itself
cannot provide a complete solution for NAT traversal.
A complete solution requires a means by which a client can obtain a
transport address from which it can receive media from any peer which
can send packets to the public Internet. This can only be
accomplished by relaying data though a server that resides on the
public Internet. This specification describes Traversal Using Relay
NAT (TURN), a protocol that allows a client to obtain IP addresses
and ports from such a relay.
Although TURN will almost always provide connectivity to a client, it
comes at high cost to the provider of the TURN server. It is
therefore desirable to use TURN as a last resort only, preferring
other mechanisms (such as STUN or direct connectivity) when possible.
To accomplish that, the Interactive Connectivity Establishment (ICE)
[13] methodology can be used to discover the optimal means of
connectivity.
2. Terminology
In this document, the key words MUST, MUST NOT, REQUIRED, SHALL,
SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL are to
be interpreted as described in RFC 2119 [2] and indicate requirement
levels for compliant TURN implementations.
3. Definitions
TURN Client: A TURN client (also just referred to as a client) is
an entity that generates TURN requests. A TURN client can be an
end system, such as a Session Initiation Protocol (SIP) [6] User
Agent, or can be a network element, such as a Back-to-Back User
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Agent (B2BUA) SIP server. The TURN protocol will provide the TURN
client with IP addresses that route to it from the public
Internet.
TURN Server: A TURN Server (also just referred to as a server) is
an entity that receives TURN requests, and sends TURN responses.
The server is capable of acting as a data relay, receiving data on
the address it provides to clients, and forwarding them to the
clients.
Transport Address: An IP address and port.
4. Applicability Statement
TURN is useful for applications that require a client to place a
transport address into a protocol message, with the expectation that
the client will be able to receive packets from a single host that
will send to this address. Examples of such protocols include SIP,
which makes use of the Session Description Protocol (SDP) [7]. SDP
carries and IP address on which the client will receive media packets
from its peer. Another example of a protocol meeting this criteria
is the Real Time Streaming Protocol (RTSP) [8].
When a client is behind a NAT, transport addresses obtained from the
local operating system will not be publically routable, and
therefore, not useful in these protocols. TURN allows a client to
obtain a transport address, from a server on the public Internet,
which can be used in protocols meeting the above criteria. However,
the transport addresses obtained from TURN servers are not generally
useful for receiving data from anywhere. They are only useful for
communicating with a single peer. This is accomplished by having the
TURN server emulate the behavior of an address-restricted NAT. In
particular, the TURN server will only relay packets from an external
IP address towards the client if the client had previously sent a
packet through the TURN server towards that IP address. As a result
of this, when a TURN server is placed in front of a symmetric NAT,
the resulting combined system has identical security properties to a
system that just had an address restricted NAT. Since clients behind
such devices cannot run public servers, they cannot run them behind
TURN servers either.
5. Overview of Operation
The typical TURN configuration is shown in Figure 1. A TURN client
is connected to private network 1. This network connects to private
network 2 through NAT 1. Private network 2 connects to the public
Internet through NAT 2. On the public Internet is a TURN server.
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/-----\
// TURN \\
| Server |
\\ //
\-----/
+--------------+ Public Internet
................| NAT 2 |.......................
+--------------+
+--------------+ Private NET 2
................| NAT 1 |.......................
+--------------+
/-----\
// TURN \\
| Client |
\\ // Private NET 1
\-----/
Figure 1
TURN is a simple client-server protocol. It is identical in syntax
and general operation to STUN, in order to facilitate a joint
implementation of both. TURN defines a request message, called
Allocate, which asks the TURN server to allocate a public IP address
and port. TURN can run over UDP and TCP, as it allows for a client
to request address/port pairs for receiving both UDP and TCP.
A TURN client first discovers the address of a TURN server. This can
be preconfigured, or it can be discovered using SRV records [3] This
will allow for different TURN servers for UDP and TCP. Once a TURN
server is discovered, the client sends a TURN Allocate request to the
TURN server. TURN provides a mechanism for mutual authentication and
integrity checks for both requests and responses, based on a shared
secret. Assuming the request is authenticated and has not been
tampered with, the TURN server allocates a transport address to the
TURN client, called the allocated transport address, and returns it
in the response to the Allocate Request. Normally, the allocated
transport address will be on one of the interfaces on the TURN server
itself. However, it is also allowed for the TURN server to be behind
a NAT, in which case the allocated transport address may correspond
to the NAT, which is then mapped to the private address of the TURN
server. Proper operation of the TURN server will require it to have
many bindings established in the NAT ahead of time; the means for
doing so are outside the scope of this specification.
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However, the TURN server will not relay any packets from PA to SA
until the client sends a packet through the TURN server towards a
correspondent. To do that, a client sends a TURN Send command, which
includes a data packet and a destination IP address and port. The
TURN server, upon receipt of this command, will forward the packet to
that IP address and port, add a "permission" for that IP address, so
that inbound packets from that address and port are permitted. In
the case of TCP, the Send Request will cause the TURN server to open
a TCP connection towards the target if one is not already up.
Packets received from the TURN server via UDP or via the TCP
connections opened by a Send Request are then forwarded towards the
client, encapsulated in Data Indication messages. The usage of Send
Requests and Data Indication messages is inefficient. As a result,
once the client has concluded on a specific external client with
which it wishes to communicate, it can issue a Set Active Destination
Request. This request informs the server of a destination for which
unencapsulated packets are to be forwarded. As such, if a client
sends a packet to the TURN server which is not a TURN packet, it gets
sent to this destination. Similarly, packets from the external
client to the TURN server are forwarded to the client without
encapsulation in a Data Indication message.
Once an active destination is set, it cannot be changed for TCP.
Effectively, the TCP connection from the client to the TURN server
"switches" to the application's ownership once the Set Active
Destination Request has been received.
To do all of this, the TURN server will maintain a binding between an
internal 5-tuple and 1 or more external 5-tuples, as shown in
Figure 2. The internal 5-tuple represents the "connection" between
the TURN server and the TURN client. It is the actual connection in
the case of TCP, and in the case of UDP, it is the combination of the
IP address and port from which the TURN client sent its Allocate
Request, with the IP address and port to which that Allocate Request
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 TURN client is communicating with through the TURN server.
Initially, there aren't any external 5-tuples, since the TURN 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.
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+---------+
| |
| External|
/ | Client |
// | |
/ | |
// +---------+
/
//
+-+ /
| | /
| | //
+---------+ | | +---------+ / +---------+
| | |N| | | // | |
| TURN | | | | |/ | External|
| Client |----|A|----------| TURN |------------------| 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 2
For TCP, the TURN server does not need to examine the data received;
it merely forwards all data between the socket pairs it has
associated together. In the case of UDP, the TURN server looks for a
magic cookie in the first 128 bytes of each UDP packet. If present,
it indicates that the packet is a TURN control packet, used for
keepalives and teardown of the binding. In the case of TCP, if
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either side closes a connection, the TURN server closes the other
connection. For both UDP and TCP, the TURN server can also time out
a connection in the event data is not received after some configured
time out period. This period is sent to the client in the TURN
response to the Allocate request.
A TURN server will accept UDP packets and TCP connections from
external clients only when a permission for accepting it has been
created at the TURN server. The client creates this permission by
sending a packet to a specific transport address through the TURN
server.
6. Message Overview
TURN messages are identical to STUN messages in their syntax. TURN
defines several new messages - the Allocate Request, the Allocate
Response, the Allocate Error Response, the Send Request, the Send
Response, the Send Error Response, the Set Active Destination
Request, Set Active Destination Response, Set Actice Destination
Error Response and the Data Indication. TURN also uses the Shared
Secret Request, Shared Secret Response, and Shared Secret Error
Response defined by STUN. TURN makes use of some of the STUN
attributes (MAPPED-ADDRESS, USERNAME, MESSAGE-INTEGRITY, ERROR-CODE,
and UNKNOWN-ATTRIBUTES) and also defines several of its own.
Specifically, TURN adds the LIFETIME attribute, which allows the TURN
server to tell the client when the binding will be released. It
defines the ALTERNATE-SERVER attribute, which allows the server to
redirect the TURN client to connect to an alternate server. It
defines the MAGIC-COOKIE attribute, which allows the TURN client to
find TURN messages in a stream of UDP packets. It defines the
BANDWIDTH attribute, which allows a client to inform the server of
the expected bandwidth usage on the connection. It defines the
DESTINATION-ADDRESS attribute, which is used in the Send Request to
identify where the data should be sent to. It defines the REMOTE-
ADDRESS, which appears in a Data Indication, and tells the client
where the data came from. It defines the DATA attribute, which
contains the content in a Data Indication. Finally, it defines the
NONCE and REALM attributes, used for authentication.
7. Server Behavior
The server behavior depends on whether the request is a Shared Secret
Request, an Allocate Request or a Set Active Destination Request.
7.1 Shared Secret Request
Unlike a STUN server, a TURN server provides resources to clients
that connect to it. Therefore, only authorized clients can gain
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access to a TURN server. This requires that TURN requests be
authenticated. TURN assumes the existence of a long-lived shared
secret between the client and the TURN server in order to achieve
this authentication. The client uses this long-lived shared secret
to authenticate itself in a Shared Secret Request, sent over TLS.
The Shared Secret Response provides the client with a one-time
username and password. This one-time credential is then used by the
server to authenticate an Allocate Request. The usage of a separate
long lived and one-time credentials prevents dictionary attacks,
whereby an observer of a message and its HMAC could guess the
password by an offline dictionary search.
When a TURN server receives a Shared Secret Request, it first
executes the processing described in the first three paragraphs of
Section 8.2 of STUN. This processing will ensure that the Shared
Secret Request is received over TLS.
Assuming it was, the server checks the Shared Secret Request for a
MESSAGE-INTEGRITY attribute. If not present, the server generates a
Shared Secret Error Response with an ERROR-CODE attribute with
response code 401. That response MUST include a NONCE attribute,
containing a nonce that the server wishes the client to reflect back
in a subsequent Shared Secret Request (and therefore include the
message integrity computation). The response MUST include a REALM
attribute, containing a realm from which the username and password
are scoped [4].
If the MESSAGE-INTEGRITY attribute was present, the server checks for
the existence of the REALM attribute. If the attribute is not
present, the server MUST generate a Shared Secret Error Response.
That response MUST include an ERROR-CODE attribute with response code
434. That response MUST include a NONCE and a REALM attribute.
If the REALM attribute was present, the server checks for the
existence of the NONCE attribute. If the NONCE attribute is not
present, the server MUST generate a Shared Secret Error Response.
That response MUST include an ERROR-CODE attribute with response code
435. That response MUST include a NONCE attribute and a REALM
attribute.
If the NONCE attribute was present, the server checks for the
existence of the USERNAME attribute. If it was not present, the
server MUST generate a Shared Secret Error Response. The Shared
Secret Error Response MUST include an ERROR-CODE attribute with
response code 432. It MUST include a NONCE attribute and a REALM
attribute.
If the USERNAME is present, the server computes the HMAC over the
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request as described in Section 11.2.8 of STUN. The key is computed
as MD5(unq(USERNAME-value) ":" unq(REALM-value) ":" passwd), where
the password is the password associated with the username and realm
provided in the request. If the server does not have a record for
that username within that realm, the server generates a Shared Secret
Error Response. That response MUST include an ERROR-CODE attribute
with response code 436. That response MUST include a NONCE attribute
and a REALM attribute.
This format for the key was chosen so as to enable a common
authentication database for SIP and for TURN, as it is expected
that credentials are usually stored in their hashed forms.
If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
attribute in the request, the server MUST generate a Shared Secret
Error Response with an ERROR-CODE attribute with response code 431.
This response MUST include a NONCE attribute and a REALM attribute.
If the computed HMAC doesn't differ from the one in the request, but
the nonce is stale, the server MUST generate a Shared Secret Error
Response. That response MUST include an ERROR-CODE attribute with
response code 430. That response MUST include a NONCE attribute and
a REALM attribute.
In all cases, the Shared Secret Error Response is sent over the TLS
connection on which the Shared Secret Request was received.
The server proceeds to authorize the client. The means for
authorization are outside the scope of this specification. It is
anticipated that TURN servers will be run by providers that also
provide an application service, such as SIP or RTSP. In that case, a
user would be authorized to use TURN if they are authorized to use
the application service.
The server then generates a Shared Secret Response as in Section 8.2
of STUN. This response will contain a USERNAME and PASSWORD, which
are used by the client as a short-term shared secret in subsequent
Allocate requests. Note that STUN specifies that the server has to
invalidate this username and password after 30 minutes. This is not
the case in TURN. In TURN, the server MUST store the allocated
username and password for a duration of at least 30 minutes. Once an
Allocate request has been authenticated using that username and
password, if the result was an Allocate Error Response, the username
and password are discarded. If the result was an Allocate Response,
resulting in the creation of a new binding, the username and password
become associated with that binding. They can only be used to
authenticate Allocate requests sent from the same source transport
address in order to refresh or de-allocate that binding. Once the
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binding is deleted, the username and password are discarded.
This policy avoids replay attacks, whereby a recorded Allocate
request is replayed in order to obtain a binding without proper
authentication. It also ensures that existing bindings can be
refreshed without needed to continuously obtain one-time passwords
from the TURN server.
7.2 Allocate Request
7.2.1 Overview
Allocate requests are used to obtain an IP address and port that the
client can use to receive UDP and TCP packets from any host on the
network, even when the client is behind a symmetric NAT. To do this,
a TURN server allocates a local transport address, and passes it to
the client in an Allocate Response. The TURN server has a configured
policy that defines whether or not a packet received from an external
client will be passed to the TURN client. This is a set of IP
addresses and optionally ports that identify the permitted external
clients. This set of addresses is built up as a consequence of Send
requests from the TURN client.
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
matches the internal remote and local transport addresses of an
existing internal 5-tuple. A subsequent request is one whose source
and destination transport address do not match the internal remote
and local transport address of an existing internal 5-tuple.
7.2.2 Initial Requests
A TURN server MUST be prepared to receive Allocate Requests over TCP
and UDP. The port on which to listen is based on the DNS SRV entries
provided by the server. Typically, this will be XXXX, the default
TURN port.
The server MUST check the Allocate Request for a MESSAGE-INTEGRITY
attribute. If not present, the server generates a Allocate Error
Response with an ERROR-CODE attribute with response code 401.
If the MESSAGE-INTEGRITY attribute was present, the server checks for
the existence of the USERNAME attribute. If it was not present, the
server MUST generate a Allocate Error Response. The Allocate Error
Response MUST include an ERROR-CODE attribute with response code 432.
If the USERNAME is present, the server computes the HMAC over the
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request as described in Section 11.2.8 of STUN. The key is equal to
the password associated with the username in the request, where that
username is a short term username allocated by the TURN server. The
username MUST be one which has been allocated by the server in a
Shared Secret Response, but has not yet been used to authenticate an
Allocate request. If that username is not known by the server, or
has already been used, the server generates an Allocate Error
Response. That response MUST include an ERROR-CODE attribute with
response code 430.
If the computed HMAC differs from the one from the MESSAGE-INTEGRITY
attribute in the request, the server MUST generate a Allocate Error
Response with an ERROR-CODE attribute with response code 431.
Assuming the message integrity check passed, processing continues.
The server MUST check for any attributes in the request with values
less than or equal to 0x7fff which it does not understand. If it
encounters any, the server MUST generate an Allocate Error Response,
and it MUST include an ERROR-CODE attribute with a 420 response code.
That response MUST contain an UNKNOWN-ATTRIBUTES attribute listing
the attributes with values less than or equal to 0x7fff which were
not understood.
If the Allocate request arrived over TCP, the Allocate Error Response
is sent on the connection from which the request arrived. If the
Allocate request arrived over UDP, the Allocate Error Response is
sent to the transport address from which the request was received
(i.e., the source IP address and port), and sent from the transport
address on which the request was received (i.e., the destination IP
address and port).
Assuming the Allocate request was authenticated and was well-formed,
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 port for the client. If the clients
source port was in the range 1024-65535, it is RECOMMENDED that the
server allocate a port in that range. If the clients source port was
in the range of 1-1024, port selection is at the discrtion of the
administrator. It is RECOMMENDED that a port in the range of 1024-
65535 be allocated. This is one of several ways to prohibit TURN
from being used to attempt to run standard services. These
guidelines are meant to be consistent with [14], since the TURN relay
is effectively a NAT.
If a port meeting the bandwidth constraints cannot be allocated, the
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server MUST generate a Allocate Error Response that includes an
ERROR-CODE attribute with a response code of 300. That response MAY
include an ALTERNATE-SERVER attribute pointing to an alternate server
which can be used by the client.
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.
The server MUST remember the one-time username and password used to
obtain the allocated transport address, and also associate it with
the internal 5-tuple.
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 binding, 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 is set
to fire at or after that time. Section 7.7 discusses behavior when
the timer fires.
Once the port has been obtained from the operating system and the
activity timer started for the port binding, the server generates an
Allocate Response. The Allocate Response MUST contain the same
transaction ID contained in the Allocate Request. The length in the
message header MUST contain the total length of the message in bytes,
excluding the header. The Allocate Response MUST have a message type
of "Allocate Response".
The response MUST contain a MAGIC-COOKIE as the first attribute (this
is done so that endpoints can consistently use the presence of MAGIC-
COOKIE to discern TURN packets). The server MUST add a MAPPED-
ADDRESS attribute to the Allocate Response, and set it to the
allocated transport address.
The server MUST add a LIFETIME attribute to the Allocate Response.
This attribute contains the duration, in seconds, of the activity
timer associated with this binding.
The server MUST add a BANDWIDTH attribute to the Allocate Response.
This MUST be equal to the attribute from the request, if one was
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present. Otherwise, it indicates a per-binding cap that the server
is placing on the bandwidth usage on each binding. Such caps are
needed to prevent against denial-of-service attacks (See Section 10.
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.
The TURN server then sends the response. If the Allocate request was
received over TCP, the response is sent over that TCP connection. If
the Allocate request was received over UDP, the response is sent to
the transport address from which the request was received (i.e., the
source IP address and port), and sent from the transport address on
which the request was received (i.e., the destination IP address and
port).
If the allocated port for TCP, the server MUST be prepared to receive
a TCP connection request on that port.
7.2.3 Subsequent Requests
Once a binding has been created for UDP and permissions installed,
the client can send subsequent Allocate requests to the TURN server.
To determine which packets are for the TURN server, and which need to
be relayed, the server looks at the packet. If the packet is shorter
than 28 bytes, it is not a TURN request. If it is longer than 28
bytes, the server checks bytes 25-28. If these bytes are equal to
the MAGIC-COOKIE, the request is a TURN request. Otherwise, it is a
data packet, and is to be relayed.
The server first authenticates the request. This is done as in
Section 7.2.2. The request MUST be authenticated using the same one-
time username and password used previously. That is, the source and
destination transport address of the Allocate Request are compared,
respectively, with the internal remote and local transport addresses
associated with existing allocations. If there is a match, the same
username and password used to obtain that allocation must match the
ones used in the request. If there is not a match, the server MUST
generate an Allocate Error Response with a 441 response code.
The server looks for the LIFETIME attribute in the Allocate Request.
If not found, it determines the default refresh duration, in seconds,
for this binding. If the LIFETIME attribute was present in the
request, and the value is larger than the maximum duration the server
is willing to extend the lifetime of the binding, the server MAY
lower it to that maximum. However, the server MUST NOT increase the
duration requested in the LIFETIME attribute. The resulting duration
is added to the current time, and the activity timer for this binding
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is reset to fire at or after that time. Section 7.7 discusses
behavior when the timer fires.
Once the timer is set, the server MUST generate an Allocate Response.
The Allocate Response MUST contain the same transaction ID contained
in the Allocate Request. The length in the message header MUST
contain the total length of the message in bytes, excluding the
header. The Allocate Response MUST have a message type of "Allocate
Response". The response MUST contain a MAGIC-COOKIE as the first
attribute. It MUST contain a MAPPED-ADDRESS which contains the
allocated transport address. It MUST contain a LIFETIME attribute
which contains the time from now until the point at which the binding
will be deleted. The final attribute MUST be a MESSAGE-INTEGRITY
attribute, which MUST use the same one-time username and password
used to authenticate the request.
The TURN server then sends the response. The response is sent to the
transport address from which the request was received (i.e., the
source IP address and port), and sent from the transport address on
which the request was received (i.e., the destination IP address and
port).
Subsequent allocation requests cannot be sent for TCP, and as such,
the server should never receive them. Indeed, a server MUST NOT look
for them in the TCP data stream.
7.3 Send Request
A Send request 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. Send requests are used with
both UDP and TCP. A server can disambiguate a UDP Send Request from
a data packet by looking for the MAGIC-COOKIE attribute, as described
in Section 7.2.3. Such disambiguation is not needed for TCP, since
the client cannot send this request after an external 5-tuple has
been activated.
Once the server has identified a request as a Send request, the
server verifies that it has arrived with a source and destination
transport address that matches the internal remote and local
transport address of an internal 5-tuple associated with an existing
allocation. If there is no matching allocation, the server MUST
generate a 437 (No Binding) Send Error Response.
Next, the server authenticates the request. This is done as in
Section 7.2.2. The request MUST be authenticated using the same one-
time username and password used previously. That is, the source and
destination transport address of the Allocate Request are compared,
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respectively, with the internal remote and local transport addresses
associated with existing allocations. If there is a match, the same
username and password used to obtain that allocation must match the
ones used in the request. If there is not a match, the server MUST
generate a Send Error Response with a 441 response code.
Once the request has been authenticated, the server validates it.
The request should contain a DESTINATION-ADDRESS attribute and a DATA
attribute. If it doesn't, the server MUST reject the request with a
400 (Bad Request) Send Error Response.
Assuming the Send Request has been validated, the server then takes
the contents of the DATA attribute. In the case of UDP, it creates a
UDP packet whose payload equals that content. The server sets the
source IP address equal to the allocated transport address. The
destination transport address is set to the contents of the
DESTINATION-ADDRESS attribute. 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 clients responsibility to
generate another Send Request if needed. If the TURN client hasn't
previously sent to this destination IP address and port, an external
5-tuple is instantiated in the TURN server. Its local and remote
transport addresses, respectively, are set to the source and
destination transport addresses of the UDP packet.
In the case of TCP, the server checks if it has an existing TCP
connection open from the allocated transport address to the address
in the DESTINATION-ADDRESS attribute. If so, the server extracts the
content of the DATA attribute and sends it on matching TCP
connection. If the server doesn't have an existing TCP connection to
the destination, it MUST open one from an epheral port on the same
interface as the allocated transport address, and do so to the
transport address in the DESTINATION-ADDRESS attribute. Once the
connection is established, the server MUST send the contents of the
DATA attribute onto that connection. If the connection could not be
opened, or if the transmission of the data resulted in an error, the
TURN server MUST generate a Send Error Response with a 438 (Send
Failed) response code.
If the UDP packet or TCP data was sent without errors, the server
generates a Send Response. The Send Response MUST have a message
type of "Send Response". The response MUST contain a MAGIC-COOKIE as
the first attribute and a MESSAGE-INTEGRITY attribute as the last.
If the server needs to generate a Send Error Response, that message
MUST contain a message type of "Send Error Response", and MUST
contain a MAGIC-COOKIE as the first attribute. It MUST contain an
ERROR-CODE with the appropriate response code. For UDP, both the
Send Response and Send Error Response are sent back to the source IP
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and port where the request came from, and sent from the same address
and port where the request was sent to. For TCP, the response is
sent on the TCP connection that the server received the request on.
The server then adds the IP address of the DESTINATION-ADDRESS
attribute to the permission list for this allocation. This happens
regardless of whether, in the case of TCP, the data was sent
successfully.
7.4 Receiving Packets and Connections on the Allocated Transport
Address
If a TURN server receives a TCP connection request on an allocated
transport address, it checks the permissions associated with that
allocation. If the source IP address of the TCP SYN packet match one
of the permissions, the TCP connection is accepted. Otherwise, it is
rejected.
If a TURN server receives a UDP packet on an allocated 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 UDP packet is accepted. Otherwise, it is discarded.
This emulates the address-restricted behavior of a NAT, as opposed
to the stricter port and address restricted behavior. This allows
for interoperation of TURN with clients that don't perform
symmetric RTP, and is needed to avoid double relays for sessions
between clients that are both behind symmetric NAT.
If the source and destination transport address of the UDP packet is
equal, respectively, to the remote and local transport addresses of
the active 5-tuple, the UDP packet is forwarded to the client and not
encapsulated in a TURN packet. To forward, the packet is sent with a
source IP address and port equal to the internal local transport
address, and with a destination address and port equal to the
internal remote transport address.
Similarly, if data is received on a TCP connection, and that
connection is the active connection, the data on that connection is
copied onto the connection to the TURN client.
If data is received from an external client on a TCP connection, and
that connection is not the active connection, the data are sent to
the client in a Data Indication message. The Data Indication message
MUST contain a MAGIC-COOKIE attribute as the first attribute. The
Data Indication message MUST contain a DATA attribute whose contents
are equal to the data just received on the TCP connection from the
external client. The message MUST contain a REMOTE-ADDRESS attribute
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whose content is equal to the external remote transport address.
This packet is sent to the TURN client over the TCP connection to the
TURN client.
If a UDP packet is received from an external client, and the external
5-tuple don't match the active 5-tuple, the data is sent to the
client in a Data Indication message. This message is not
retransmitted by the server, and which does not generate a response.
As a result, like data packets which are forwarded, there is no
reliability guarantee provided by the TURN server for this
indication. The Data Indication message MUST contain a MAGIC-COOKIE
attribute as the first attribute. It MUST contain a DATA attribute
whose contents are equal to the payload of the UDP packet. The
message MUST contain a REMOTE-ADDRESS attribute whose content is
equal to the source IP address and port of the UDP packet received by
the TURN server. This packet is sent to the internal remote
transport address, and sent from the internal local transport
address.
Note that, because data is forwarded blindly across TCP bindings, TLS
will successfully operate over a TURN allocated TCP port.
7.5 Receiving a Set Active Destination Request
The Set Active Destination Request is used by a client to determine
an external 5-tuple that will be used as the forwarding destination
of all non-TURN data from the TURN client. Furthermore, all data
from that external client will be forwarded to the TURN client
without encapsulation in a Data Indication.
A server can disambiguate a UDP Set Active Destination Request from a
data packet by looking for the MAGIC-COOKIE attribute, as described
in Section 7.2.3. Such disambiguation is not needed for TCP, since
the client cannot send this request more than once.
The active destination is initially null. It is always explicitly
set by the Set Active Destination Request.
Once the server has identified a request as a Set Active Destination
request, the server verifies that it has arrived with a source and
destination transport address that matches the internal remote and
local transport address of an internal 5-tuple associated with an
existing allocation. If there is no matching allocation, the server
MUST generate a 437 (No Binding) Send Error Response.
Next, the server authenticates the request. This is done as in
Section 7.2.2. The request MUST be authenticated using the same one-
time username and password used previously. That is, the source and
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destination transport address of the Allocate Request are compared,
respectively, with the internal remote and local transport addresses
associated with existing allocations. If there is a match, the same
username and password used to obtain that allocation must match the
ones used in the request. If there is not a match, the server MUST
generate a Set Active Destination Error Response with a 441 response
code.
Once the request has been authenticated, the server validates it.
The request should contain a DESTINATION-ADDRESS attribute. If it
doesn't, the server MUST reject the request with a 400 (Bad Request)
Set Active Destination Error Response.
If there is already an active 5-tuple, and the external remote
transport address of that 5-tuple matches the DESTINATION-ADDRESS,
the request is basically a no-op. The server MUST generate a Set
Active Destination Response. This response contains no attributes.
If there is an active 5-tuple but its external remote transport
address doesn't match, the request is asking for a change in the
destination. The server checks its existing external 5-tuples for
one whose external remote transport address matches the DESTINATION-
ADDRESS. If none is found, the request is rejected with a 440 (No
Destination) response. If one is found, the currently active one is
deactivated. A timer, Ta, is set to fire in 3 seconds. The server
sets its state to "transitioning". When Ta fires, the server returns
to normal operations, and then sets the external 5-tuple that matched
the DESTINATION-ADDRESS to the active one. While in the
transitioning state, the server behaves as described in this
specification, except for the processing of the Set Active
Destination Request as described below.
If there is no active 5-tuple, but the server is in the transitioning
state, it MUST reject the request with a Set Active Destination Error
Response that includes a 439 (Transitioning) response code.
If there is no active 5-tuple, and the server is not in the
transitioning state, the server checks its existing external 5-tuples
for one whose external remote transport address matches the
DESTINATION-ADDRESS. If one is found, that external 5-tuple is made
the active one, and a Set Active Destination Response is sent. If
none is found, the request is rejected with a 440 (No Destination)
response.
If the server needs to send a Set Active Destination Response, that
message MUST contain a message type of "Set Active Destination
Response", and MUST contain a MAGIC-COOKIE as the first attribute and
a MESSAGE-INTEGRITY as the last. If the server needs to send a Set
Active Destination Error Response, that message MUST contain a
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message type of "Set Active Destination Error Response", and MUST
contain a MAGIC-COOKIE as the first attribute. It MUST contain an
ERROR-CODE with the appropriate response code. For UDP, both the
responses are sent back to the source IP and port where the request
came from, and sent from the same address and port where the request
was sent to. For TCP, the response is sent on the TCP connection
that the server received the request on.
7.6 Receiving Data from the TURN Client
If a TURN server receives a UDP packet from the TURN client on its
internal local transport address, and it is coming from an internal
remote transport address associated with an existing allocation, and
the UDP packet is not a TURN packet (known by the absence of the
MAGIC-COOKIE attribute), it represents UDP data that the client
wishes to forward. If there is an active 5-tuple, the TURN server
MUST forward the UDP packet on that 5-tuple. The destination address
and port of the UDP packet is set to the external remote transport
address of the active 5-tuple. The source IP address and port of the
UDP packet is set to the external local transport address of the
active 5-tuple. The TURN server SHOULD NOT retransmit the packet
once it has forwarded it. Such retransmissions are the
responsibility of the client.
If there is no active 5-tuple, the UDP packet is discarded.
If data is received on a TCP connection from the TURN client, and the
previous TURN message from the client was a Set Active Destination
Request, the data is forwarded to the active connection. Note that,
once a connection has been activated with a Set Active Destination
TURN Request, TURN messaging from the client over its TCP connection
to the server is not allowed. Thus, all subsequent data will be non-
TURN data by definition.
If a TCP connection associated with an allocated transport address is
closed, any connections to external clients MUST be closed. At that
point, the binding is destroyed. Similarly, if a connection from the
TURN server to an external client is closed, and that connection was
the active connection, the corresponding connection to the TURN
client MUST be closed, and the binding is destroyed.
7.7 Lifetime Expiration
When the activity timer for a binding fires, the server checks to see
if there has been any activity on the binding since its creation, or
since the last firing of the timer, whichever is more recent.
Activity is defined as connection establishment, or packet
transmission in either direction. If there has been activity, the
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timer is set to fire once again in M seconds, where M is the value of
the LIFETIME attribute returned in the most recent Allocate Response
for this binding. Note that, with TCP connections, a client cannot
send TURN requests once a connection has been set to active. As
such, the value of M that is used will be the one from the first
Allocate Request; refreshes will not take place with TURN messages.
If there has been no activity, the server MUST destroy the binding,
along with its associated one-time password. If the binding was over
TCP, the server MUST close any connections it is holding to the
client and to the remote client.
8. Client Behavior
Client behavior is broken into several separate steps. First, the
client obtains a one-time username and password. Secondly, it
generates initial Allocate Requests, and processes the responses. It
manages those addresses (refreshing and tearing them down), issues
Send Requests and Set Active Destination Requests, and processes TURN
indications and data received on those addresses.
8.1 Discovery
Generally, the client will be configured with a domain name of the
provider of the TURN servers. This domain name is resolved to an IP
address and port of using the SRV procedures [3]. When sending a
Shared Secret request, the service name is "turn" and the protocol is
"tcp". RFC 2782 spells out the details of how a set of SRV records
are sorted and then tried. However, it only states that the client
should "try to connect to the (protocol, address, service)" without
giving any details on what happens in the event of failure. Those
details are described here for TURN.
For TURN requests, failure occurs if there is a transport failure of
some sort (generally, due to fatal ICMP errors in UDP or connection
failures in TCP). Failure also occurs if the request does not
solicit a response after 9.5 seconds. If a failure occurs, the
client SHOULD create a new request, which is identical to the
previous, but has a different transaction ID and MESSAGE-INTEGRITY
attribute. That request is sent to the next element in the list as
specified by RFC~2782.
8.2 Obtaining a One Time Password
In order to allocate addresses, a client must obtain a one-time
username and password from the TURN server. A unique username and
password are required for each distinct address allocated from the
server.
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To obtain a one-time username and password, the client generates and
sends a Shared Secret Request. This is done as described in Section
9.2 of STUN. This request will have no attributes, and therefore,
based on the processing in Section 7.1, the server will reject it
with a Shared Secret Error Response with a 401 response code. That
response will contain a NONCE and a REALM. The client SHOULD
generate a new Shared Secret Request (with a new transaction ID),
which contains the NONCE and REALM attributes copied from the 401
response. The request MUST include the USERNAME attribute, which
contains a username supplied by the user for the specified realm.
The request MUST include a MESSAGE-INTEGRITY attribute as the last
attribute. The key for the HMAC is computed as described in
Section 7.1.
If the response (either to the initial request or to the second
attempt with the credentials) is a Shared Secret Error Response, the
processing depends on the value of the response code in the ERROR-
CODE attribute. If the response code was a 430, the client SHOULD
generate a new Shared Secret Request, using the username and password
provided by the user, and the REALM and NONCE provided in the 430
response. For a 431 or 436 response code, the client SHOULD alert
the user. For a 432, 434 and 435 response codes, if the client had
omitted the USERNAME, REALM or NONCE attributes, respectively, from
the previous request, it SHOULD retry, this time including the
USERNAME, NONCE, REALM, and MESSAGE-INTEGRITY attributes. For a 500
response code, the client MAY wait several seconds and then retry the
request. For a 600 response code, the client MUST NOT retry the
request, and SHOULD display the reason phrase to the user. Unknown
attributes between 400 and 499 are treated like a 400, unknown
attributes between 500 and 599 are treated like a 500, and unknown
attributes between 600 and 699 are treated like a 600. Any response
between 100 and 399 MUST result in the cessation of request
retransmissions, but otherwise is discarded.
If a client receives a Shared Secret Response with an attribute whose
type is unknown and greater than 0x7fff, the attribute MUST be
ignored. If the client receives a Shared Secret Response with an
attribute whose type is unknown and less than or equal to 0x7fff, the
response is ignored.
If the response is a Shared Secret Response, it will contain the
USERNAME and PASSWORD attributes. The client can use these to
authenticate an Allocate Request, as described below.
A client MAY send multiple Shared Secret Requests over the same TLS
connection, and MAY do so without waiting for responses to previous
requests. The client SHOULD close its connection when it has
completed allocating usernames and passwords.
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8.3 Allocating a Binding
When a client wishes to obtain a transport address, it sends an
Allocate Request to the TURN server. Requests for TCP transport
addresses MUST be sent over a TCP connection, and requests for UDP
transport addresses MUST be sent over UDP.
First, the client obtains a one-time username and password, using the
mechanisms described in Section 8.2. The client then formulates an
Allocate Request. The request MUST contain a transaction ID, unique
for each request, and uniformly and randomly distributed between 0
and 2**128 - 1. The message type of the request MUST be "Allocate
Request". The length is set as described in Section 11.1 of STUN.
The Allocate request MUST contain the MAGIC-COOKIE attribute as the
first attribute.
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 binding by
including it in the LIFETIME attribute in the request. If the no
data is sent or received on the binding before expiration of the
lifetime, the binding will be deleted by the client.
The client MUST include a USERNAME attribute, containing a username
obtained from a previous Shared Secret Response. The request MUST
include a MESSAGE-INTEGRITY attribute as the last attribute. The key
is equal to the password obtained from the PASSWORD attribute of the
Shared Secret Response. The Allocate Request MUST be sent to the
same IP address and port as the Shared Secret Request, but from a
different local ephemeral port (in other words, the TCP/TLS
connection used to obtain the shared secret is not reused for
allocations). This is because one time passwords are expected to be
host-specific. Rules for retransmissions for Allocate Requests sent
over UDP are identical to those for STUN Binding Requests. Allocate
Requests sent over TCP are not retransmitted. Transaction timeouts
are identical to those for STUN Binding Requests, independent of the
transport protocol.
8.4 Processing Allocate Responses
If the response is an Allocate Error Response, the client checks the
response code from the ERROR-CODE attribute of the response. For a
400 response code, the client SHOULD display the reason phrase to the
user. For a 420 response code, the client SHOULD retry the request,
this time omitting any attributes listed in the UNKNOWN-ATTRIBUTES
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attribute of the response. For a 430 response code, the client
SHOULD obtain a new one-time username and password, and retry the
Allocate Request with a new transaction. For 401 and 432 response
codes, if the client had omitted the USERNAME or MESSAGE-INTEGRITY
attribute as indicated by the error, it SHOULD try again with those
attributes. A new one-time username and password is needed in that
case. For a 431 response code, the client SHOULD alert the user, and
MAY try the request again after obtaining a new username and
password. For a 300 response code, the client SHOULD attempt a new
TURN transaction to the server indicated in the ALTERNATE-SERVER
attribute. For a 500 response code, the client MAY wait several
seconds and then retry the request with a new username and password.
For a 600 response code, the client MUST NOT retry the request, and
SHOULD display the reason phrase to the user. Unknown attributes
between 400 and 499 are treated like a 400, unknown attributes
between 500 and 599 are treated like a 500, and unknown attributes
between 600 and 699 are treated like a 600. Unknown attributes
between 300 and 399 are treated like 300. Any response between 100
and 299 MUST result in the cessation of any request retransmissions,
but otherwise is discarded.
If a client receives a response with an attribute whose type is
unknown and greater than 0x7fff, the attribute MUST be ignored. If
the client receives a response with an attribute whose type is
unknown and less than or equal to 0x7fff, any request retransmissions
MUST cease, but the entire response is otherwise ignored.
If the response is an Allocate Response, the client MUST check the
response for a MESSAGE-INTEGRITY attribute. If not present, the
client MUST discard the response. If present, the client computes
the HMAC over the response. The key MUST be same as used to compute
the MESSAGE-INTEGRITY attribute in the request. If the computed HMAC
differs from the one in the response, the client MUST discard the
response, and SHOULD alert the user about a possible attack. If the
computed HMAC matches the one from the response, processing
continues.
The MAPPED-ADDRESS in the Allocate Response can be used by the client
for receiving packets. The server will expire the binding after
LIFETIME seconds have passed with no activity. The server will allow
the user to send and receive no more than the amount of data
indicated in the BANDWIDTH attribute.
8.5 Refreshing a Binding
If there has been no activity on a UDP binding for a period of time
equalling 3/4 of the lifetime of the binding (as conveyed in the
LIFETIME attribute of the Allocate Response), the client SHOULD
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refresh the binding with another Allocate Request if it wishes to
keep it. Note that only UDP bindings can be refreshed. For TCP,
application-specific keepalives are needed.
To perform a refresh, the client generates an Allocate Request as
described in Section 8.3. However, the one-time username and
password used MUST be the same as those used in the successful
Allocate Request for that binding. The client will need to look for
the TURN response amongst the data packets using the MAGIC-COOKIE, as
described in Section 7.2.3. Processing of that response is as
defined in Section 8.4. If the response was an Allocate Response,
and the MAPPED-ADDRESS contains the same transport address as
previously obtained, the binding has been refreshed. The LIFETIME
attribute indicates the amount of additional time the binding will
live without activity. If, however, the response was an Allocate
Error Response with an ERROR-CODE indicating a 430 response, it means
that the binding has expired at the server. The client MAY use the
procedures in Section 8.3 to obtain a new binding (this will require
a new one-time username and password. Other response codes do not
imply that the binding has been expired, just that the refresh has
failed.
8.6 Sending Encapsulated Data
Before receiving any UDP or TCP data, a client has to send first. To
do that, it uses the Send Request. For UDP, a client MAY send this
request at any time. For TCP, it MUST NOT send it after it has
transmitted a Set Active Destination Request that yielded a
successful result.
The Send request MUST contain a transaction ID, unique for each
request, and uniformly and randomly distributed between 0 and 2**128
- 1. The message type of the request MUST be "Send Request". The
length is set as described in Section 11.1 of STUN.
The Send request MUST contain the MAGIC-COOKIE attribute as the first
attribute. The client MUST include a USERNAME attribute, containing
the same username used in the Allocate request for this binding. The
request MUST include a MESSAGE-INTEGRITY attribute as the last
attribute. The key is equal to the password used for the Allocate
request for this binding. For UDP, the Send Request MUST be sent to
the same IP address and port as the Allocate Request, and MUST be
sent from the same source IP and port used to send the Allocate
request for the binding. For TCP, it MUST be sent over the same
connection used for the initial allocation. Rules for
retransmissions for Send Requests sent over UDP are identical to
those for STUN Binding Requests. Transaction timeouts are identical
to those for STUN Binding Requests, independent of the transport
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protocol.
The Send Request MUST contain a DESTINATION-ADDRESS attribute, which
contains the IP address and port that the data is being sent to.
If the server successfully sends the data, the client will receive a
Send Response. Note that, as with responses to Allocate refreshes,
the client will need to pick a UDP Send Response (or Send Error
Response) out of the packet stream by searching for the MAGIC-COOKIE
in each received UDP packet.
If the response is an Send Response, the client MUST check the
response for a MESSAGE-INTEGRITY attribute. If not present, the
client MUST discard the response. If present, the client computes
the HMAC over the response. The key MUST be same as used to compute
the MESSAGE-INTEGRITY attribute in the request. If the computed HMAC
differs from the one in the response, the client MUST discard the
response, and SHOULD alert the user about a possible attack. If the
computed HMAC matches the one from the response, processing
continues.
If the response is a Send Error Response, it is processed as
described in the first two paragraphs of Section 8.4. TCP Send
Responses are readily identifiable since application data and TURN
cannot be used simultaneously on a connection. Thus, until the Set
Active Destination Request is sent, the connection is used
exclusively for TURN.
8.7 Receiving a Data Indication
Once a client has allocated a binding and used Send to send data on
it, the TURN server will start to accept UDP data and incoming TCP
connections from the IP addresses that the client sent or connected
to. Prior to the establishment of an active destination, all such
data received by the server is forwarded to the client using a DATA-
INDICATION message. This message generates no response. It contains
two attributes - DATA and REMOTE-ADDRESS. The REMOTE-ADDRESS
attribute indicates the source transport address that the request
came from. 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.
After an active destination is established, for TCP, no more DATA-
INDICATION messages will arrive. For UDP, a DATA-INDICATION message
will arrive when data comes from an external client that is not equal
to the active destination.
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8.8 Setting the Active Destination
Once the client has sent data on a binding, it can activate the
5-tuple between the TURN server and a particular external client. By
activating it, it means that non-TURN data sent by the client are
forwarded on this 5-tuple, and data from the external client are
forwarded to the TURN client without encapsulation in a DATA-
INDICATION. For TCP, once a destination is activated, TURN signaling
over the TCP connection is no longer possible.
To activate a destination, the client constructs a Set Active
Destination Request. This request MUST contain a transaction ID,
unique for each request, and uniformly and randomly distributed
between 0 and 2**128 - 1. The message type of the request MUST be
"Set Active Destination Request". The length is set as described in
Section 11.1 of STUN.
The Set Active Destination request MUST contain the MAGIC-COOKIE
attribute as the first attribute. The client MUST include a USERNAME
attribute, containing the same username used in the Allocate request
for this binding. The request MUST include a MESSAGE-INTEGRITY
attribute as the last attribute. The key is equal to the password
used for the Allocate request for this binding. For UDP, the Set
Active Destination Request MUST be sent to the same IP address and
port as the Allocate Request, and MUST be sent from the same source
IP and port used to send the Allocate request for the binding. For
TCP, it MUST be sent over the same connection used for the initial
allocation. Rules for retransmissions for Set Active Destination
Requests sent over UDP are identical to those for STUN Binding
Requests. Transaction timeouts are identical to those for STUN
Binding Requests, independent of the transport protocol.
The Set Active Destination Request MUST contain a DESTINATION-ADDRESS
attribute, which contains the IP address and port of the external
client corresponding to the active 5-tuple.
If the response is an Set Active Destination Response, the client
MUST check the response for a MESSAGE-INTEGRITY attribute. If not
present, the client MUST discard the response. If present, the
client computes the HMAC over the response. The key MUST be same as
used to compute the MESSAGE-INTEGRITY attribute in the request. If
the computed HMAC differs from the one in the response, the client
MUST discard the response, and SHOULD alert the user about a possible
attack. If the computed HMAC matches the one from the response,
processing continues.
If the response is a Set Active Destination Response, the TURN client
MUST start a timer, Ta, and set it to 3 seconds. It MUST set its
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state to "transitioning". When the timer fires, the client MUST
return to normal state. While transitioning, the client MUST NOT
send another Set Active Destination request. Furthermore, the client
MUST NOT send data without TURN encapsulation (i.e., using a Send
Request) while transitioning.
If the response is a Set Active Destination Error Response, and the
ERROR-CODE attribute in the response had a value of 439, it means
that the client tried to set the active destination while the server
was transitioning. The client SHOULD set a timer for 2 seconds, and
when the timer fires, retry. If the client received a 440, it is
because the client tried to set an active destination to an unknown
external client. The TURN client MAY retry with a different
destination.
8.9 Tearing Down a Binding
If a client no longer needs a binding, it SHOULD tear it down. For
TCP, this is done by closing the connection. For UDP, this is done
by performing a refresh, as described in Section 8.5, but with a
LIFETIME attribute indicating a time of 0.
8.10 Receiving and Sending Unencapsulated Data
Once a client has set the active destination, it MUST be prepared to
receive data from the socket on which the Allocate Request was sent.
For UDP, the client MUST be prepared to disambiguate TURN messages
from data for the lifetime of the binding. This disambiguation is
done using the MAGIC-COOKIE, as described in Section 7.2.3. For TCP,
all subsequent data from the server will be application data, and not
TURN data.
Once a destination has been activated, the client MAY send data to
its peer by sending data on that same socket. Any UDP packets
received by the server are forwarded to the default destination
address until that address is changed by a subsequent Set Active
Destination command. Similarly, any data sent over the TCP
connection are forwarded to the TCP connection to the external
client.
9. Protocol Details
This section presents the detailed encoding of the message types,
attributes, and response codes which are new to TURN. The general
message structure of TURN is identical to STUN [1].
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9.1 Message Types
TURN defines ten new Message Types:
0x0003 : Allocate Request
0x0103 : Allocate Response
0x0113 : Allocate Error Response
0x0004 : Send Request
0x0104 : Send Response
0x0114 : Send Error Response
0x0115 : Data Indication
0x0006 : Set Active Destination Request
0x0106 : Set Active Destination Response
0x0116 : Set Active Destination Error Response
9.2 Message Attributes
TURN defines the following message attributes:
0x000d: LIFETIME
0x000e: ALTERNATE-SERVER
0x000f: MAGIC-COOKIE
0x0010: BANDWIDTH
0x0011: DESTINATION-ADDRESS
0x0012: REMOTE-ADDRESS
0x0013: DATA
0x0014: NONCE
0x0015: REALM
9.2.1 LIFETIME
The lifetime attribute represents the duration for which the server
will maintain a binding 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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9.2.2 ALTERNATE-SERVER
The alternate server represents an alternate IP address and port for
a different TURN server to try. It is encoded in the same way as
MAPPED-ADDRESS.
9.2.3 MAGIC-COOKIE
The MAGIC-COOKIE is used by TURN clients and servers to disambiguate
TURN traffic from data traffic. Its value ix 0x72c64bc6.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|1|1|1|0|0|1|0|1|1|0|0|0|1|1|0|0|1|0|0|1|0|1|1|1|1|0|0|0|1|1|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2.4 BANDWIDTH
The bandwidth attribute represents the peak bandwidth, measured in
kbits per second, that the client expects to use on the binding. The
value represents the sum in the receive and send directions.
[[Editors note: Need to define leaky bucket parameters for this.]]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bandwidth |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
9.2.5 DESTINATION-ADDRESS
The DESTINATION-ADDRESS is present in Send Requests and Set Active
Destination Requests. It specifies the address and port where the
data is to be sent. It is encoded in the same way as MAPPED-ADDRESS.
9.2.6 REMOTE-ADDRESS
The REMOTE-ADDRESS is present in Data Indications. It specifies the
address and port from which a packet was received. It is encoded in
the same way as MAPPED-ADDRESS.
9.2.7 DATA
The DATA attribute is present in Send Requests 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).
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9.2.8 NONCE
The NONCE attribute is present in Shared Secret Requests and Shared
Secret Error responses. It contains a sequence of qdtext or quoted-
pair, which are defined in [6].
9.2.9 REALM
The REALM attribute is present in Shared Secret Requests and Shared
Secret Responses. It contains text which meets the grammar for
"realm" as described in RFC 3261, and will thus contain a quoted
string (including the quotes).
9.2.10 Response Codes
TURN defines the following new response codes:
300 (Try Alternate): The client should contact an alternate server
for this request.
434 (Missing Realm): The REALM attribute was not present in the
request.
435 (Missing Nonce): The NONCE attribute was not present in the
request.
436 (Unknown Username): The USERNAME supplied in the Shared Secret
Request is not known in the given REALM.
437 (No Binding): A Send Request was received by the server, but
there is no binding in place for the source 5-tuple.
438 (Send Failed): A Send Request was received by the server over
TCP, but the server wasn't able to transmit the data to the
requested destination.
439 (Transitioning): A Set Active Destination request was received
by the server. However, a previous request was sent within the
last three seconds, and the server is still transitioning to that
active destination. Please repeat the request once three seconds
have elapsed.
440 (No Destination): A Set Active Destination request was
received by the server. However, the requested destination has
not been one corresponding to the destination of a Send Request,
and has not been one for which packets or a connection attempt
have been received.
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441 (Wrong Username): A TURN request was received for an allocated
binding, but it did not use the same username and password that
were used in the allocation. The client must supply the proper
credentials, and if it cannot, it should teardown its binding,
allocate a new one time password, and try again.
10. Security Considerations
TURN servers allocate bandwidth and port resources to clients.
Therefore, a TURN server requires authentication and authorization of
TURN requests. This authentication is provided by a client digest
over TLS, which results in the generation of a one-time password that
is used in a single subsequent Allocate Request. This mechanism
protects against eavesdropping attacks and man-in-the-middle attacks.
The usage of one-time passwords ensures that the Allocate Requests,
which 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, but
each requires a new one-time username and password. It is
RECOMMENDED that servers implement a cap on the number of one-time
passwords that are allocated to any specific user at a time (around 5
or 10 should be sufficient). This will prevent floods of Allocate
requests from a single user, in an attempt to use up the resources of
the system. A single malicious user could generate a single Allocate
Request, obtain a binding, and then flood the server with data over
this binding, in an attempt to deny others service. However, this
attack requires the attacker themselves to receive the data being
sent at the server. To ameliorate these kinds of attacks, servers
SHOULD implement a bandwidth cap on each binding (conveyed to the
client in the BANDWIDTH attribute of the Allocate Response), and
discard packets beyond the threshold.
A client will use the transport address learned from the MAPPED-
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 TLS and HMAC-based authentication and integrity
checks provided in TURN. They can guarantee the authenticity and
integrity of the mapped addressses. 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 STUN. These attacks are based on the fact that a
STUN server mirrors the source IP address, which cannot be
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authenticated. TURN does not use the source address of the Allocate
Request, 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 TURN cannot be used to run web
servers, email servers, SIP servers, or other network servers that
service a large number of clients. Rather, it facilitates rendezvous
of NATted clients that use some other protocol, such as SIP, to
communicate IP addresses and ports for communications.
Confidentiality of the transport addresses learned through TURN does
not appear to be that important, and therefore, this capability is
not provided.
TURN 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.
TCP transport addresses allocated by TURN will properly work with TLS
and SSL. However, any addresses allocated by TURN will not operate
properly with IPSec Authentication Header (AH) [10] in transport
mode. IPSec ESP [11] and any tunnel-mode ESP or AH should still
operate.
11. 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 [12].
TURN 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. This section meets those
requirements.
11.1 Problem Definition
From RFC 3424 [12], any UNSAF proposal must provide:
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Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short term fix should not be
generalized to solve other problems; this is why "short term
fixes usually aren't".
The specific problem being solved by TURN is for a client, which may
be located behind a NAT of any type, to obtain an IP address and port
on the public Internet, useful for applications that require a client
to place a transport address into a protocol message, with the
expectation that the client will be able to receive packets from a
single host that will send to this address. Both UDP and TCP are
addressed. It is also possible to send packets so that the recipient
sees a source address equal to the allocated address. TURN, by
design, does not allow a client to run a server (such as a web or
SMTP server) using a TURN address. TURN is useful even when NAT is
not present, to provide anonymity services.
11.2 Exit Strategy
From [12], any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better short
term fixes are the ones that will naturally see less and less use
as the appropriate technology is deployed.
It is expected that TURN will be useful indefinitely, to provide
anonymity services. When used to facilitate NAT traversal, TURN does
not iself provide an exit strategy. That is provided by the
Interactive Connectivity Establishment (ICE) [13] mechanism. ICE
allows two cooperating clients to interactively determine the best
addresses to use when communicating. ICE uses TURN-allocated
addresses as a last resort, only when no other means of connectivity
exists. As a result, as NATs phase out, and as IPv6 is deployed, ICE
will increasingly use other addresses (host local addresses).
Therefore, clients will allocate TURN addresses, but not use them,
and therefore, de-allocate them. Servers will see a decrease in
usage. Once a provider sees that its TURN servers are not being used
at all (that is, no media flows through them), they can simply remove
them. ICE will operate without TURN-allocated addresses.
11.3 Brittleness Introduced by TURN
From [12], any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
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TURN introduces brittleness in a few ways. First, it adds another
server element to any system, which adds another point of failure.
TURN requires clients to demultiplex TURN packets and data based on
hunting for a MAGIC-COOKIE in the TURN messages. It is possible
(with extremely small probabilities) that this cookie could appear
within a data stream, resulting in mis-classification. That might
introduce errors into the data stream (they would appear as lost
packets), and also result in loss of a binding. TURN relies on any
NAT bindings existing for the duration of the bindings held by the
TURN server. Neither the client nor the TURN server have a way of
reliably determining this lifetime (STUN can provide a means, but it
is heuristic in nature and not reliable). Therefore, if there is no
activity on an address learned from TURN for some period, the address
might become useless spontaneously.
TURN will result in potentially significant increases in packet
latencies, and also increases in packet loss probabilities. That is
because it introduces an intermediary on the path of a packet from
point A to B, whose location is determined by application-layer
processing, not underlying routing topologies. Therefore, a packet
sent from one user on a LAN to another on the same LAN may do a trip
around the world before arriving. When combined with ICE, some of
the most problematic cases are avoided (such as this example) by
avoiding the usage of TURN addresses. However, when used, this
problem will exist.
Note that TURN does not suffer from many of the points of brittleness
introduced by STUN. TURN will work with all existing NAT types known
at the time of writing, and for the forseeable future. TURN does not
introduce any topological constraints. TURN does not rely on any
heuristics for NAT type classification.
11.4 Requirements for a Long Term Solution
From [12]}, any UNSAF proposal must provide:
Identify requirements for longer term, sound technical solutions
-- contribute to the process of finding the right longer term
solution.
Our experience with TURN continues to validate our belief in the
requirements outlined in Section 14.4 of STUN.
11.5 Issues with Existing NAPT Boxes
From [12], any UNSAF proposal must provide:
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Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market which
try and provide "generic" ALG functionality. These generic ALGs hunt
for IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This will interfere with
proper operation of any UNSAF mechanism, including TURN. However, if
a NAT tries to modify a MAPPED-ADDRESS in a TURN Allocate Response,
this will be detected by the client as an attack.
12. Examples
TODO.
13. Acknowledgements
The authors would like to thank Marc Petit-Huguenin for his comments
and suggestions.
14. References
14.1 Normative References
[1] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[3] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[4] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
14.2 Informative References
[5] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications",
RFC 3550, July 2003.
[6] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
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[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC 2326, April 1998.
[9] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[10] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
November 1998.
[11] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC 2406, November 1998.
[12] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation",
RFC 3424, November 2002.
[13] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols",
draft-ietf-mmusic-ice-04 (work in progress), February 2005.
[14] Audet, F. and C. Jennings, "NAT Behavioral Requirements for
Unicast UDP", draft-ietf-behave-nat-udp-02 (work in progress),
June 2005.
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
Airspace
Email: rohan@ekabal.com
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Christian Huitema
Microsoft
One Microsoft Way
Redmond, WA 98052-6399
US
Email: huitema@microsoft.com
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