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IETF RFC 6062
Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations
Last modified on Tuesday, November 30th, 2010
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Internet Engineering Task Force (IETF) S. Perreault, Ed.
Request for Comments: 6062 Viagenie
Category: Standards Track J. Rosenberg
ISSN: 2070-1721 jdrosen.net
November 2010
Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations
 Abstract
This specification defines an extension of Traversal Using Relays
around NAT (TURN), a relay protocol for Network Address Translator
(NAT) traversal. This extension allows a TURN client to request TCP
allocations, and defines new requests and indications for the TURN
server to open and accept TCP connections with the client's peers.
TURN and this extension both purposefully restrict 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
TURN server.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/RFC 6062.
Perreault & Rosenberg Standards Track PAGE 1 
RFC 6062 TURN TCP November 2010
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview of Operation . . . . . . . . . . . . . . . . . . . . 4
4. Client Processing . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Creating an Allocation . . . . . . . . . . . . . . . . . . 6
4.2. Refreshing an Allocation . . . . . . . . . . . . . . . . . 7
4.3. Initiating a Connection . . . . . . . . . . . . . . . . . 7
4.4. Receiving a Connection . . . . . . . . . . . . . . . . . . 7
4.5. Sending and Receiving Data . . . . . . . . . . . . . . . . 8
4.6. Data Connection Maintenance . . . . . . . . . . . . . . . 8
5. TURN Server Behavior . . . . . . . . . . . . . . . . . . . . . 8
5.1. Receiving a TCP Allocate Request . . . . . . . . . . . . . 8
5.2. Receiving a Connect Request . . . . . . . . . . . . . . . 9
5.3. Receiving a TCP Connection on a Relayed Transport
Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4. Receiving a ConnectionBind Request . . . . . . . . . . . . 11
5.5. Data Connection Maintenance . . . . . . . . . . . . . . . 11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6.1. New STUN Methods . . . . . . . . . . . . . . . . . . . . . 11
6.2. New STUN Attributes . . . . . . . . . . . . . . . . . . . 12
6.2.1. CONNECTION-ID . . . . . . . . . . . . . . . . . . . . 12
6.3. New STUN Error Codes . . . . . . . . . . . . . . . . . . . 12
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . . 13
Perreault & Rosenberg Standards Track PAGE 2
RFC 6062 TURN TCP November 2010
1. Introduction
Traversal Using Relays around NAT (TURN) [RFC 5766] is an extension to
the Session Traversal Utilities for NAT [RFC 5389] protocol. TURN
allows for clients to communicate with a TURN server and ask it to
allocate ports on one of its host interfaces, and then relay traffic
between that port and the client itself. TURN, when used in concert
with STUN and Interactive Connectivity Establishment (ICE) [RFC 5245],
forms a solution for NAT traversal for UDP-based media sessions.
However, TURN itself does not provide a way for a client to allocate
a TCP-based port on a TURN server. Such an allocation is needed for
cases where a TCP-based session is desired with a peer, and NATs
prevent a direct TCP connection. Examples include application
sharing between desktop softphones, or transmission of pictures
during a voice communications session.
This document defines an extension to TURN that allows a client to
obtain a TCP allocation. It also allows the client to initiate
outgoing TCP connections from that allocation to peers and to accept
incoming TCP connection requests from peers made towards that
allocation.
The term "TCP allocation" means a TURN allocation where TCP is used
as the transport protocol instead of UDP. Such an allocation is
uniquely identified by its relayed transport address, which consists
of an IP address and TCP port (defined in [RFC 5766]).
2. Conventions
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].
Perreault & Rosenberg Standards Track PAGE 3
RFC 6062 TURN TCP November 2010
3. Overview of Operation
+--------+
| |
| Peer1 |
/ | |
/ | |
/ +--------+
/
/
/ Peer Data 1
/
+--------+ Control +--------+ /
| | -------------- | | /
| Client | Client Data 1 | TURN |
| | -------------- | Server | \
| | -------------- | | \
+--------+ Client Data 2 +--------+ \
\
\
\ +--------+
\ | |
Peer Data 2 \ | Peer2 |
\ | |
| |
+--------+
Figure 1: TURN TCP Model
The overall model for TURN-TCP is shown in Figure 1. The client will
have two different types of connections to its TURN server. For each
allocated relayed transport address, it will have a single control
connection. Control connections are used to obtain allocations and
open up new connections. Furthermore, for each connection to a peer,
the client will have a single connection to its TURN server. These
connections are called data connections. Consequently, there is a
data connection from the client to its TURN server (the client data
connection) and one from the TURN server to a peer (the peer data
connection). Actual application data is sent on these connections.
Indeed, after an initial TURN message that binds the client data
connection to a peer data connection, only application data can be
sent -- no TURN messaging. This is in contrast to the control
connection, which only allows TURN messages and not application data.
To obtain a TCP-based allocation, a client first opens a TCP or TLS
connection to its TURN server. The client then sends an Allocate
request over that control connection. That request contains a
REQUESTED-TRANSPORT attribute, which indicates a TCP-based allocation
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RFC 6062 TURN TCP November 2010
is desired. A server that supports this extension will allocate a
TCP relayed transport address and begin listening for connection
requests on it. It then returns the allocated relayed transport
address to the client in the response to the Allocate request. The
connection on which the Allocate request was sent is the control
connection.
If a client wishes to establish a TCP connection to a peer from that
relayed transport address, it issues a Connect request to the TURN
server over the control connection. That request contains an XOR-
PEER-ADDRESS attribute identifying the peer IP address and port
(i.e., its "transport address") to which a connection is to be made.
The TURN server attempts to open the TCP connection, and assuming it
succeeds, then responds to the Connect request with a success
response. The server also creates a connection identifier associated
with this connection and passes that connection identifier back to
the client in the success response. Note that a maximum of one
connection to a given peer transport address can be established per
allocation.
Note: Establishing a relayed connection from the client to a peer
is done in two steps. First, the allocation is created, and
second, the connection is established. Combining the two is not
desirable for NAT traversal. It is expected that, between the
first and second steps, the client will communicate off-band with
the peer (e.g., using ICE [RFC 5245]) and tell it the relayed
transport address that the TURN server allocated and from which it
is about to initiate a connection. The peer can then "get ready":
open holes in its firewall, try to poke holes in a NAT, attempt a
TCP simultaneous open, etc.
In order to actually send data on the new connection or otherwise
utilize it in any way, the client establishes a new TCP connection to
its TURN server. Once established, it issues a ConnectionBind
request to the server over this new connection. That request echoes
back the connection identifier to the TURN server. The TURN server
uses it to correlate the two connections. As a consequence, the TCP
connection to the peer is associated with a TCP connection to the
client one-to-one. The two connections are now data connections. At
this point, if the server receives data from the peer, it forwards
that data towards the client, without any kind of encapsulation. Any
data received by the TURN server from the client over the client data
connection is forwarded to the peer, again without encapsulation or
framing of any kind. Once a connection has been bound using the
ConnectionBind request, TURN messaging is no longer permitted on the
connection.
Perreault & Rosenberg Standards Track PAGE 5
RFC 6062 TURN TCP November 2010
In a similar way, when a peer opens a TCP connection towards the
relayed transport address, the server checks if there is a permission
in place for that peer. If there is none, the connection is closed.
Permissions are created with the CreatePermission request sent over
the control connection, just as for UDP TURN. If there is a
permission in place, the TURN server sends to the client a
ConnectionAttempt Indication over the control connection. That
indication contains a connection identifier. Once again, the client
initiates a separate TCP connection to its TURN server, and over that
connection, issues a ConnectionBind request. Once received, the TURN
server will begin relaying data back and forth. The server closes
the peer data connection if no ConnectionBind request is received
after a timeout.
If the client closes a client data connection, the corresponding peer
data connection is closed. If the peer closes a peer data
connection, the corresponding client data connection is closed. In
this way, the status of the connection is directly known to the
client.
The TURN server will relay the data between the client and peer data
connections. End-to-end flow control is maintained by the relay
process: if the relay process is no longer able to write data to the
destination of the relayed data, the relay process stops reading data
from the source.
4. Client Processing
4.1. Creating an Allocation
To create a TCP allocation, a client MUST initiate a new TCP or TLS
connection to its TURN server, identical to the TCP or TLS procedures
defined in [RFC 5766]. TCP allocations cannot be obtained using a UDP
association between client and server.
Once set up, a client MUST send a TURN Allocate request. That
request MUST contain a REQUESTED-TRANSPORT attribute whose value is
6, corresponding to TCP.
The request MUST NOT include a DONT-FRAGMENT, RESERVATION-TOKEN, or
EVEN-PORT attribute. The corresponding features are specific to UDP-
based capabilities and are not utilized by TURN-TCP. However, a
LIFETIME attribute MAY be included, with semantics identical to the
UDP case.
The procedures for authentication of the Allocate request and
processing of success and failure responses are identical to those
for UDP.
Perreault & Rosenberg Standards Track PAGE 6
RFC 6062 TURN TCP November 2010
Once a success response is received, the TCP connection to the TURN
server is called the control connection for that allocation.
4.2. Refreshing an Allocation
The procedures for refreshing an allocation are identical to those
for UDP. Note that the Refresh MUST be sent on the control
connection.
4.3. Initiating a Connection
To initiate a TCP connection to a peer, a client MUST send a Connect
request over the control connection for the desired allocation. The
Connect request MUST include an XOR-PEER-ADDRESS attribute containing
the transport address of the peer to which a connection is desired.
If the connection is successfully established, the client will
receive a success response. That response will contain a
CONNECTION-ID attribute. The client MUST initiate a new TCP
connection to the server, utilizing the same destination transport
address to which the control connection was established. This
connection MUST be made using a different local transport address.
Authentication of the client by the server MUST use the same method
and credentials as for the control connection. Once established, the
client MUST send a ConnectionBind request over the new connection.
That request MUST include the CONNECTION-ID attribute, echoed from
the Connect Success response. When a response to the ConnectionBind
request is received, if it is a success, the TCP connection on which
it was sent is called the client data connection corresponding to the
peer.
If the result of the Connect request was an Error Response, and the
response code was 447 (Connection Timeout or Failure), it means that
the TURN server was unable to connect to the peer. The client MAY
retry with the same XOR-PEER-ADDRESS attribute, but MUST wait at
least 10 seconds.
As with any other request, multiple Connect requests MAY be sent
simultaneously. However, Connect requests with the same XOR-PEER-
ADDRESS parameter MUST NOT be sent simultaneously.
4.4. Receiving a Connection
After an Allocate request is successfully processed by the server,
the client will start receiving a ConnectionAttempt indication each
time a peer for which a permission has been installed attempts a new
connection to the relayed transport address. This indication will
contain CONNECTION-ID and XOR-PEER-ADDRESS attributes. If the client
Perreault & Rosenberg Standards Track PAGE 7
RFC 6062 TURN TCP November 2010
wishes to accept this connection, it MUST initiate a new TCP
connection to the server, utilizing the same destination transport
address to which the control connection was established. This
connection MUST be made using a different local transport address.
Authentication of the client by the server MUST use the same method
and credentials as for the control connection. Once established, the
client MUST send a ConnectionBind request over the new connection.
That request MUST include the CONNECTION-ID attribute, echoed from
the ConnectionAttempt indication. When a response to the
ConnectionBind request is received, if it is a success, the TCP
connection on which it was sent is called the client data connection
corresponding to the peer.
4.5. Sending and Receiving Data
Once a client data connection is established, data sent on it by the
client will be relayed as-is to the peer by the server. Similarly,
data sent by the peer to the server will be relayed as-is to the
client over the data connection.
4.6. Data Connection Maintenance
The client MUST refresh the allocation (corresponding to a data
connection) using the Refresh request as defined in [RFC 5766] for as
long as it wants to keep the data connection alive.
When the client wishes to terminate its relayed connection to the
peer, it closes the data connection to the server.
Note: No mechanism for keeping alive the NAT bindings (potentially
on the client data connection as well as on the peer data
connection) is included. This service is not provided by TURN-
TCP. If such a feature is deemed necessary, it can be implemented
higher up the stack, in the application protocol being tunneled
inside TURN-TCP. Also, TCP keep-alives MAY be used to keep the
NAT bindings on the client data connection alive.
5. TURN Server Behavior
5.1. Receiving a TCP Allocate Request
The process is similar to that defined in [RFC 5766], Section 6.2,
with the following exceptions:
1. If the REQUESTED-TRANSPORT attribute is included and specifies a
protocol other than UDP or TCP, the server MUST reject the
request with a 442 (Unsupported Transport Protocol) error. If
the value is UDP, and if UDP transport is allowed by local
Perreault & Rosenberg Standards Track PAGE 8
RFC 6062 TURN TCP November 2010
policy, the server MUST continue with the procedures of [RFC 5766]
instead of this document. If the value is UDP, and if UDP
transport is forbidden by local policy, the server MUST reject
the request with a 403 (Forbidden) error.
2. If the client connection transport is not TCP or TLS, the server
MUST reject the request with a 400 (Bad Request) error.
3. If the request contains the DONT-FRAGMENT, EVEN-PORT, or
RESERVATION-TOKEN attribute, the server MUST reject the request
with a 400 (Bad Request) error.
4. A TCP relayed transport address MUST be allocated instead of a
UDP one.
5. The RESERVATION-TOKEN attribute MUST NOT be present in the
success response.
If all checks pass, the server MUST start accepting incoming TCP
connections on the relayed transport address. Refer to Section 5.3
for details.
5.2. Receiving a Connect Request
When the server receives a Connect request, it processes the request
as follows.
If the request is received on a TCP connection for which no
allocation exists, the server MUST return a 437 (Allocation Mismatch)
error.
If the server is currently processing a Connect request for this
allocation with the same XOR-PEER-ADDRESS, it MUST return a 446
(Connection Already Exists) error.
If the server has already successfully processed a Connect request
for this allocation with the same XOR-PEER-ADDRESS, and the resulting
client and peer data connections are either pending or active, it
MUST return a 446 (Connection Already Exists) error.
If the request does not contain an XOR-PEER-ADDRESS attribute, or if
such attribute is invalid, the server MUST return a 400 (Bad Request)
error.
If the new connection is forbidden by local policy, the server MUST
reject the request with a 403 (Forbidden) error.
Perreault & Rosenberg Standards Track PAGE 9
RFC 6062 TURN TCP November 2010
Otherwise, the server MUST initiate an outgoing TCP connection. The
local endpoint is the relayed transport address associated with the
allocation. The remote endpoint is the one indicated by the XOR-
PEER-ADDRESS attribute. If the connection attempt fails or times
out, the server MUST return a 447 (Connection Timeout or Failure)
error. The timeout value MUST be at least 30 seconds.
If the connection is successful, it is now called a peer data
connection. The server MUST buffer any data received from the
client. The server adjusts its advertised TCP receive window to
reflect the amount of empty buffer space.
The server MUST include the CONNECTION-ID attribute in the Connect
success response. The attribute's value MUST uniquely identify the
peer data connection.
If no ConnectionBind request associated with this peer data
connection is received after 30 seconds, the peer data connection
MUST be closed.
5.3. Receiving a TCP Connection on a Relayed Transport Address
When a server receives an incoming TCP connection on a relayed
transport address, it processes the request as follows.
The server MUST accept the connection. If it is not successful,
nothing is sent to the client over the control connection.
If the connection is successfully accepted, it is now called a peer
data connection. The server MUST buffer any data received from the
peer. The server adjusts its advertised TCP receive window to
reflect the amount of empty buffer space.
If no permission for this peer has been installed for this
allocation, the server MUST close the connection with the peer
immediately after it has been accepted.
Otherwise, the server sends a ConnectionAttempt indication to the
client over the control connection. The indication MUST include an
XOR-PEER-ADDRESS attribute containing the peer's transport address,
as well as a CONNECTION-ID attribute uniquely identifying the peer
data connection.
If no ConnectionBind request associated with this peer data
connection is received after 30 seconds, the peer data connection
MUST be closed.
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RFC 6062 TURN TCP November 2010
5.4. Receiving a ConnectionBind Request
When a server receives a ConnectionBind request, it processes the
request as follows.
If the client connection transport is not TCP or TLS, the server MUST
return a 400 (Bad Request) error.
If the request does not contain the CONNECTION-ID attribute, or if
this attribute does not refer to an existing pending connection, the
server MUST return a 400 (Bad Request) error.
Otherwise, the client connection is now called a client data
connection. Data received on it MUST be sent as-is to the associated
peer data connection.
Data received on the associated peer data connection MUST be sent
as-is on this client data connection. This includes data that was
received after the associated Connect or request was successfully
processed and before this ConnectionBind request was received.
5.5. Data Connection Maintenance
If the allocation associated with a data connection expires, the data
connection MUST be closed.
When a client data connection is closed, the server MUST close the
corresponding peer data connection.
When a peer data connection is closed, the server MUST close the
corresponding client data connection.
6. IANA Considerations
This specification defines several new STUN methods, STUN attributes,
and STUN error codes. IANA added these new protocol elements to the
Session Traversal Utilities for NAT (STUN) Parameters registry.
6.1. New STUN Methods
This section lists the codepoints for the new STUN methods defined in
this specification. See Sections 4 and 5 for the semantics of these
new methods.
0x000a : Connect
0x000b : ConnectionBind
0x000c : ConnectionAttempt
Perreault & Rosenberg Standards Track PAGE 11
RFC 6062 TURN TCP November 2010
6.2. New STUN Attributes
This STUN extension defines the following new attributes:
0x002a : CONNECTION-ID
6.2.1. CONNECTION-ID
The CONNECTION-ID attribute uniquely identifies a peer data
connection. It is a 32-bit unsigned integral value.
6.3. New STUN Error Codes
446 Connection Already Exists
447 Connection Timeout or Failure
7. Security Considerations
After a TCP connection is established between the server and a peer,
and before a ConnectionBind request is received from the client, the
server buffers all data received from the peer. This protocol
specification lets the server drop the connection if the buffer size
is about to exceed a limit defined by local policy. This policy
should ensure that memory resources are not exceeded. See also
[RFC 4732], Section 2.1.3.
All the security considerations applicable to STUN [RFC 5389] and TURN
[RFC 5766] are applicable to this document as well.
8. Acknowledgements
Thanks to Rohan Mahy and Philip Matthews for their initial work on
getting this document started.
The authors would also like to thank Alfred E. Heggestad, Ari
Keranen, Marc Petit-Huguenin, Dave Thaler, and Dan Wing for their
comments and suggestions.
9. References
9.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
Perreault & Rosenberg Standards Track PAGE 12
RFC 6062 TURN TCP November 2010
[RFC 5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
9.2. Informative References
[RFC 4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[RFC 5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
Authors' Addresses
Simon Perreault (editor)
Viagenie
2875 boul. Laurier, suite D2-630
Quebec, QC G1V 2M2
Canada
Phone: +1 418 656 9254
EMail: simon.perreault@viagenie.ca
URI: http://www.viagenie.ca
Jonathan Rosenberg
jdrosen.net
Monmouth, NJ
US
EMail: jdrosen@jdrosen.net
URI: http://www.jdrosen.net
Perreault & Rosenberg Standards Track PAGE 13
Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations
RFC TOTAL SIZE: 28978 bytes
PUBLICATION DATE: Tuesday, November 30th, 2010
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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