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Network Working Group                                         C. HuitemaRequest for Comments: 3605                                     MicrosoftCategory: Standards Track                                   October 2003Real Time Control Protocol (RTCP) attribute inSession 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.Abstract   The 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.1.  Introduction   The session invitation protocol (SIP, [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 insection 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, andHuitema                     Standards Track                     [Page 1]

RFC 3605                 RTCP attribute in SDP              October 2003   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].2.  Description of the Solution   The main part of our solution is the declaration of an SDP attribute   for documenting the port used by RTCP.2.1.  The RTCP Attribute   The RTCP attribute is used to document the RTCP port used for media   stream, when that port is not the next higher (odd) port number   following the RTP port described in the media line.  The RTCP   attribute is a "value" attribute, and follows the general syntax   specified page 18 of [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] CRLFHuitema                     Standards Track                     [Page 2]

RFC 3605                 RTCP attribute in SDP              October 2003   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 126.16.64.4    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.3.  Discussion of the Solution   The implementation of the solution is fairly straightforward.  The   questions that have been most often asked regarding this solution are   whether this is useful, i.e., whether a host can actually discover   port numbers in an unmodified NAT, whether it is sufficient, i.e.,   whether or not there is a need to document more than one ancillary   port per media type, and whether why should not change the media   definition instead of adding a new attribute.3.1.  How do we Discover Port Numbers?   The proposed solution is only useful if the host can discover the   "translated port numbers", i.e., the value of the ports as they   appear on the "external side" of the NAT.  One possibility is to ask   the cooperation of a well connected third party that will act as a   server according to STUN [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,Huitema                     Standards Track                     [Page 3]

RFC 3605                 RTCP attribute in SDP              October 2003   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.3.2.  Do we need to Support Multiple Ports?   Most media streams are transmitted using a single pair of RTP and   RTCP ports.  It is possible, however, to transmit a single media over   several RTP flows, for example using hierarchical encoding.  In this   case, SDP will encode the port number used by RTP on the first flow,   and the number of flows, as in:      m=video 49170/2 RTP/AVP 31   In this example, the media is sent over 2 consecutive pairs of ports,   corresponding respectively to RTP for the first flow (even number,   49170), RTCP for the first flow (odd number, 49171), RTP for the   second flow (even number, 49172), and RTCP for the second flow (odd   number, 49173).   In theory, it would be possible to modify SDP and document the many   ports corresponding to the separate encoding layers.  However,   layered encoding is not much used in practice, and when used is   mostly used in conjunction with multicast transmission.  The   translation issues documented in this memo apply uniquely to unicast   transmission, and thus there is no short term need for the support of   multiple port descriptions.  It is more convenient and more robust to   focus on the simple case in which a media is sent over exactly one   RTP/RTCP stream.3.3.  Why not Expand the Media Definition?   The RTP ports are documented in the media description line, and it   would seem convenient to document the RTCP port at the same place,   rather than create an RTCP attribute.  We considered this design   alternative and rejected it for two reasons: adding an extra port   number and an option address in the media description would be   awkward, and more importantly it would create problems with existing   applications, which would have to reject the entire media description   if they did not understand the extension.  On the contrary, adding an   attribute has a well defined failure mode: implementations that don'tHuitema                     Standards Track                     [Page 4]

RFC 3605                 RTCP attribute in SDP              October 2003   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.4.  UNSAF Considerations   The RTCP attribute in SDP is used to enable establishment of RTP/RTCP   flows through NAT.  This mechanism can be used in conjunction with an   address discovery mechanism such as STUN [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.5.  Security Considerations   This SDP extension is not believed to introduce any significant   security risk to multi-media applications.  One could conceive that a   malevolent third party would use the extension to redirect the RTCP   fraction of an RTP exchange, but this requires intercepting and   rewriting the signaling packet carrying the SDP text; if an   interceptor can do that, many more attacks are available, including a   wholesale change of the addresses and port numbers at which the media   will be sent.   In order to avoid attacks of this sort, when SDP is used in a   signaling packet where it is of the form application/sdp, end-to-end   integrity using S/MIME [RFC3369] is the technical method to be   implemented and applied.  This is compatible with SIP [RFC3261].6.  IANA Considerations   This document defines a new SDP parameter, the attribute field   "rtcp", which per [RFC2327] has been registered by IANA.  This   attribute field is designed for use at media level only.Huitema                     Standards Track                     [Page 5]

RFC 3605                 RTCP attribute in SDP              October 20037.  Intellectual Property Statement   The IETF takes no position regarding the validity or scope of any   intellectual property or other rights that might be claimed to   pertain to the implementation or use other technology described in   this document or the extent to which any license under such rights   might or might not be available; neither does it represent that it   has made any effort to identify any such rights.  Information on the   IETF's procedures with respect to rights in standards-track and   standards-related documentation can be found inBCP-11.  Copies of   claims of rights made available for publication and any assurances of   licenses to be made available, or the result of an attempt made to   obtain a general license or permission for the use of such   proprietary rights by implementers or users of this specification can   be obtained from the IETF Secretariat.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights which may cover technology that may be required to practice   this standard.  Please address the information to the IETF Executive   Director.8.  Acknowledgements   The original idea for using the "rtcp" attribute was developed by Ann   Demirtjis.  The document was reviewed by the MMUSIC and AVT working   groups of the IETF.9.  References9.1.  Normative References   [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.Huitema                     Standards Track                     [Page 6]

RFC 3605                 RTCP attribute in SDP              October 2003   [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 2003.9.2.  Informative References   [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)",RFC3369, August 2002.   [RFC3424]  Daigle, L., "IAB considerations for UNilateral Self-              Address Fixing (UNSAF) across network address              translation",RFC 3424, November 2002.10.  Author's Address   Christian Huitema   Microsoft Corporation   One Microsoft Way   Redmond, WA 98052-6399   EMail: huitema@microsoft.comHuitema                     Standards Track                     [Page 7]

RFC 3605                 RTCP attribute in SDP              October 200311.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assignees.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Huitema                     Standards Track                     [Page 8]