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This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.
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This specification defines an extension of Traversal Using Relays around NAT (TURN), a relay protocol for NAT traversal, to 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 STUN server.
1.
Introduction
2.
Conventions
3.
Overview of Operation
4.
Client Processing
4.1.
Creating an Allocation
4.2.
Refreshing an Allocation
4.3.
Initiating a Connection
4.4.
Receiving a Connection
4.5.
Sending and Receiving Data
4.6.
Data Connection Maintenance
5.
TURN Server Behavior
5.1.
Receiving a TCP Allocate Request
5.2.
Receiving a Connect Request
5.3.
Receiving a TCP Connection on an Allocated Port
5.4.
Receiving a ConnectionBind Request
5.5.
Data Connection Maintenance
6.
IANA Considerations
6.1.
New STUN Methods
6.2.
New STUN Attributes
6.2.1.
CONNECTION-ID
6.2.2.
New STUN response codes
6.3.
Security Considerations
6.4.
Acknowledgements
7.
References
7.1.
Normative References
7.2.
Informative References
§
Authors' Addresses
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Traversal Using Relays around NAT (TURN) [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) is an extension to the Session Traversal Utilities for NAT [RFC5389] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for NAT (STUN),” October 2008.) 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) [I‑D.ietf‑mmusic‑ice] (Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” October 2007.) 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 connections from that allocation to peers, and accept connection requests from peers made towards that allocation.
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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 [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).
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+--------+
| |
| 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 (TURN TCP Model). 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 resposne 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.
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 processing 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.
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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 [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.). 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 TCP.
Once a success response is received, the TCP connection to the TURN server is called the control connection for that allocation.
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The procedures for refreshing an allocation are identical to those for UDP. Note that the Refresh MUST be sent on the control connection.
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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. Once established, the client MUST send a ConnectionBind request. That request MUST include the CONNECTION-ID attribute, mirrored from the Connect Success response. When a response to the ConnectionBind request is recevied, 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 XXX, it means that the TURN server was unable to connect to the peer. The client MAY retry, but MUST wait at least 10 seconds.
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After an Allocate request is successfully processed by the server, the client will start receiving a ConnectionAttempt indication each time a peer attemps 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. Once established, the client MUST send a ConnectionBind request. That request MUST include the CONNECTION-ID attribute, mirrorred 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.
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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. Data on a data connection MUST NOT be interpreted as STUN messages.
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The client MUST refresh the allocation corresponding to a data connection, using the Refresh request as defined in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.), 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.
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The process is similar to that defined in [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.), Section 6.2, with the following exceptions:
If all checks pass, the server MUST start accepting incoming TCP connections on the relayed transport address. Refer to Section 5.3 (Receiving a TCP Connection on an Allocated Port) for details.
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When the server receives a Connect request, it processes as follows.
If the request is received on a control 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, 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 XXX (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. It is up to the server to adjust its advertised TCP receive window should the buffer size become a limiting factor.
The server MUST include the CONNECTION-ID attribute in the Connect success response. The attribute's value MUST uniquely identify the peer data connection.
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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. It is up to the server to adjust its advertised TCP receive window should the buffer size become a limiting factor.
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.
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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 Listen request was successfully processed and before this ConnectionBind request was received.
Data received on a client or peer data connection MUST NOT be interpreted as a STUN message.
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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.
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This specification defines several new STUN methods, STUN attributes, and STUN response codes. This section directs IANA to add these new protocol elements to the IANA registry of STUN protocol elements.
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This section lists the codepoints for the new STUN methods defined in this specification. See elsewhere in this document for the semantics of these new methods.
0x0007 : Connect (only request/response semantics defined) 0x0008 : ConnectionBind (only request/response semantics defined) 0x0009 : ConnectionAttempt (only indication semantics defined)
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This STUN extension defines the following new attributes:
0x002A : CONNECTION-ID
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The CONNECTION-ID attributes uniquely identifies a peer data connection. It is a 32-bit unsigned integral value.
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446 Connection Already Exists 447 Connection Timeout or Failure
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The methods, attribute, and error response codes defined in this document do not have any special security considerations beyond those for other attributes and Error response codes. All the security considerations applicable to STUN [RFC5389] (Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for NAT (STUN),” October 2008.) and TURN [I‑D.ietf‑behave‑turn] (Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” April 2009.) are applicable to this document as well.
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Thanks to Rohan Mahy and Philip Matthews for their initial work on getting this document started.
The authors would also like to thank Marc Petit-Huguenin for his comments and suggestions.
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| [RFC5389] | Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, “Session Traversal Utilities for NAT (STUN),” RFC 5389, October 2008 (TXT). |
| [I-D.ietf-behave-turn] | Rosenberg, J., Mahy, R., and P. Matthews, “Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN),” draft-ietf-behave-turn-14 (work in progress), April 2009 (TXT). |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
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| [I-D.ietf-mmusic-ice] | Rosenberg, J., “Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols,” draft-ietf-mmusic-ice-19 (work in progress), October 2007 (TXT). |
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| Simon Perreault (editor) | |
| Viagénie | |
| 2600 boul. Laurier, suite 625 | |
| Québec, QC G1V 4W1 | |
| Canada | |
| Phone: | +1 418 656 9254 |
| Email: | simon.perreault@viagenie.ca |
| URI: | http://www.viagenie.ca |
| 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 |