Network Working Group C. Huitema Request for Comments: 3605 Microsoft Category: Standards Track October 2003 Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP) Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2003). All Rights Reserved.
AbstractThe Session Description Protocol (SDP) is used to describe the parameters of media streams used in multimedia sessions. When a session requires multiple ports, SDP assumes that these ports have consecutive numbers. However, when the session crosses a network address translation device that also uses port mapping, the ordering of ports can be destroyed by the translation. To handle this, we propose an extension attribute to SDP. RFC3261]) is often used to establish multi-media sessions on the Internet. There are often cases today in which one or both ends of the connection are hidden behind a network address translation device [RFC2766]. In this case, the SDP text must document the IP addresses and UDP ports as they appear on the "public Internet" side of the NAT. In this memo, we will suppose that the host located behind a NAT has a way to obtain these numbers. A possible way to learn these numbers is briefly outlined in section 3, however, just learning the numbers is not enough. The SIP messages use the encoding defined in SDP [RFC2327] to describe the IP addresses and TCP or UDP ports used by the various media. Audio and video are typically sent using RTP [RFC3550], which requires two UDP ports, one for the media and one for the control protocol (RTCP). SDP carries only one port number per media, and
states that "other ports used by the media application (such as the RTCP port) should be derived algorithmically from the base media port." RTCP port numbers were necessarily derived from the base media port in older versions of RTP (such as [RFC1889]), but now that this restriction has been lifted, there is a need to specify RTCP ports explicitly in SDP. Note, however, that implementations of RTP adhering to the earlier [RFC1889] specification may not be able to make use of the SDP attributes specified in this document. When the NAT device performs port mapping, there is no guarantee that the mappings of two separate ports reflects the sequencing and the parity of the original port numbers; in fact, when the NAT manages a pool of IP addresses, it is even possible that the RTP and the RTCP ports may be mapped to different addresses. In order to successfully establish connections despite the misordering of the port numbers and the possible parity switches caused by the NAT, we propose to use a specific SDP attribute to document the RTCP port and optionally the RTCP address. 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]. RFC2327]: "a=<attribute>:<value>". For the RTCP attribute: * the name is the ascii string "rtcp" (lower case), * the value is the RTCP port number and optional address. The formal description of the attribute is defined by the following ABNF [RFC2234] syntax: rtcp-attribute = "a=rtcp:" port [nettype space addrtype space connection-address] CRLF
In this description, the "port", "nettype", "addrtype" and "connection-address" tokens are defined as specified in "Appendix A: SDP Grammar" of [RFC2327]. Example encodings could be: m=audio 49170 RTP/AVP 0 a=rtcp:53020 m=audio 49170 RTP/AVP 0 a=rtcp:53020 IN IP4 188.8.131.52 m=audio 49170 RTP/AVP 0 a=rtcp:53020 IN IP6 2001:2345:6789:ABCD:EF01:2345:6789:ABCD The RTCP attribute MAY be used as a media level attribute; it MUST NOT be used as a session level attribute. Though the examples below relate to a method that will return only unicast addresses, both unicast and multicast values are valid. RFC3489]. We thus obtain a four step process: 1 - The host allocates two UDP ports numbers for an RTP/RTCP pair, 2 - The host sends a UDP message from each port to the STUN server, 3 - The STUN server reads the source address and port of the packet, and copies them in the text of a reply,
4 - The host parses the reply according to the STUN protocol and learns the external address and port corresponding to each of the two UDP ports. This algorithm supposes that the NAT will use the same translation for packets sent to the third party and to the "SDP peer" with which the host wants to establish a connection. There is no guarantee that all NAT boxes deployed on the Internet have this characteristic. Implementers are referred to the STUN specification [RFC3489] for an extensive discussion of the various types of NAT.
understand the "a=rtcp" attribute will simply ignore it; they will fail to send RTCP packets to the specified address, but they will at least be able to receive the media in the RTP packets. RFC3489]. STUN is a short term fix to the NAT traversal problem, which requires thus consideration of the general issues linked to "Unilateral self- address fixing" [RFC3424]. The RTCP attribute addresses a very specific problem, the documentation of port numbers as they appear after address translation by a port-mapping NAT. The RTCP attribute SHOULD NOT be used for other applications. We expect that, with time, one of two exit strategies can be developed. The IETF may develop an explicit "middlebox control" protocol that will enable applications to obtain a pair of port numbers appropriate for RTP and RTCP. Another possibility is the deployment of IPv6, which will enable use of "end to end" addressing and guarantee that the two hosts will be able to use appropriate ports. In both cases, there will be no need for documenting a "non standard" RTCP port with the RTCP attribute. RFC3369] is the technical method to be implemented and applied. This is compatible with SIP [RFC3261]. RFC2327] has been registered by IANA. This attribute field is designed for use at media level only.
[RFC1889] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson. "RTP: A Transport Protocol for Real-Time Applications", RFC 1889, January 1996. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 2234, November 1997. [RFC2327] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.
[RFC3489] 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. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson. "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003. [RFC2766] Tsirtsis, G. and P. Srisuresh. "Network Address Translation - Protocol Translation (NAT-PT)", RFC 2766, February 2000. [RFC3261] 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. [RFC3369] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369, August 2002. [RFC3424] Daigle, L., "IAB considerations for UNilateral Self- Address Fixing (UNSAF) across network address translation", RFC 3424, November 2002.
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