Traversal Using Relays around NAT (TURN) Extensions for TCP Allocations Viagénie2600 boul. Laurier, suite 625QuébecQCG1V 4W1Canada+1 418 656 9254simon.perreault@viagenie.cahttp://www.viagenie.caCisco Systems600 Lanidex PlazaParsippanyNJ07054US+1 973 952-5000jdrosen@cisco.comhttp://www.jdrosen.net
Transport
BehaveNATTURNSTUNThis specification defines an extension of Traversal
Using Relays around NAT (TURN), a relay protocol for NAT traversal, to
allow 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.
Traversal Using Relays around NAT (TURN)
is an extension to the Session
Traversal Utilities for NAT 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)
form 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 which allows a client to
obtain a TCP allocation. It also allows the client to initiate outgoing TCP
connections from that allocation to peers, and accept incoming TCP connection
requests from peers made towards that allocation.
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 .
The overall model for TURN-TCP is shown in
. The client will have two different
types of connections to its TURN server. For each allocated port, 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
which 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 must have a TCP or TLS
connection to its TURN server. Using that connection, it sends an
Allocate request. That request contains a REQUESTED-TRANSPORT
attribute, which indicates a TCP-based allocation is desired. A server
which supports this extension will allocate a TCP port and begin
listening for connection requests on that port. It then returns the
allocated port 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
allocated address, it issues a Connect request to the TURN server over
the control connection. That request contains a XOR-PEER-ADDRESS
attribute identifying the peer IP address and port 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 (address and port combination) can be established
per allocation.
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. 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 1-to-1. 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 are 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.
In a similar way, when a peer opens a TCP connection towards the allocated port,
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, utilizing an internal buffer. However, back pressure is
used in order to achieve end-to-end flow control. If the buffer from
client to peer fills up, the TURN server ceases to read off the client
data connection, which causes TCP backpressure through the OS towards
the client.
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 . 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.
Once a success response is received, the TCP connection to the TURN
server is called the control connection for that allocation.
The procedures for refreshing an allocation are identical to those for
UDP. Note that the Refresh MUST be sent on the control connection.
To initiate a TCP connection to a peer, a client MUST send a Connect
request over the control channel for the desired allocation.
The Connect request MUST include a
XOR-PEER-ADDRESS attribute containing the IP address and port 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 IP address and port to which the control
connection was established. This connection MUST be made using a
different local IP address and/or port. 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. 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 a Error Response, and the
response code was 447, 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.
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 allocated address. This indication will contain a CONNECTION-ID and a
XOR-PEER-ADDRESS attributes. If the client wishes to accept this connection,
it MUST initiate a new TCP connection to the server, utilizing the same
destination IP address and port to which the control connection was
established. This connection MUST be made using a different local IP
address and/or port. 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. 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.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.The client MUST refresh the allocation corresponding to a data connection,
using the Refresh request as defined in , 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.The process is similar to that defined in , Section 6.2, with the following
exceptions: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, the
server MUST continue with the procedures of instead of this document.)If the client connection transport is not TCP or TLS, the server MUST
reject the request with a 400 (Bad Request) error.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.A TCP relayed transport address MUST be allocated instead of a UDP
one.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 for details.When the server receives a Connect request, it processes 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 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 a XOR-PEER-ADDRESS attribute, or if such
attribute is invalid, the server MUST return a 400 (Bad Request) error.Otherwise, and if the new connection is permitted by local policy, 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.If the connection is successful, it is now called a peer data connection.
The server MUST buffer any data received from the peer. Data MUST NOT be
lost unless the buffer is about to exceed a limit defined by local policy,
in which case the data connection MUST be closed. 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.When a server receives an incoming TCP connection on a relayed transport,
it processes 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. Data
MUST NOT be lost unless the buffer is about to exceed a limit defined by
local policy, in which case the data connection MUST be closed. The server
adjusts its advertised TCP receive window to reflect the amount of empty
buffer space.The server then sends a ConnectionAttempt indication to the client over the
control connection. The indication MUST include a XOR-PEER-ADDRESS attribute
containing the peer's 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.When a server receives a ConnectionBind request, it processes 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.If the allocation associated with a data connection expires, the data
connection MUST be closed.When a client data connection is closed or times out, the
server MUST close the corresponding peer data connection.When a peer data connection is closed or times out, the
server MUST close the corresponding client data connection.
This specification defines several new STUN methods, STUN attributes,
and STUN error codes. This section directs IANA to add these new
protocol elements to the IANA registry of STUN protocol elements.
The CONNECTION-ID attributes uniquely identifies a peer data connection. It
is a 32-bit unsigned integral value.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 , Section
2.1.3.All the security considerations applicable to STUN
and TURN are applicable to this
document as well.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.