MMUSIC D. Wing Internet-Draft Cisco Systems Expires: March 24, 2005 September 23, 2004 Symmetric RTP and RTCP Considered Helpful draft-wing-mmusic-symmetric-rtprtcp-00 Status of this Memo This document is an Internet-Draft and is subject to all provisions of section 3 of RFC 3667. By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she become aware will be disclosed, in accordance with RFC 3668. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 24, 2005. Copyright Notice Copyright (C) The Internet Society (2004). Abstract This document defines symmetric RTP and symmetric RTCP and recommends their use. Requirements Language 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 [1]. Wing Expires March 24, 2005 [Page 1] Internet-Draft Symmetric RTP and RTCP September 2004 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Symmetric RTCP . . . . . . . . . . . . . . . . . . . . . . 3 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 6.1 Normative References . . . . . . . . . . . . . . . . . . . . 5 6.2 Informational References . . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . 7 Wing Expires March 24, 2005 [Page 2] Internet-Draft Symmetric RTP and RTCP September 2004 1. Introduction Because RTP and RTCP are not inheriently a bi-directional protocols, the usefulness of symmetry has been generally ignored. Many firewalls, NATs [6], and RTP implementations expect "Symmetric RTP", and do not work in the presense of non-symmetric RTP. However, this term has never been defined. This document defines Symmetric RTP and Symmetric RTCP. TCP [3], which is inheriently bidirectional, uses symmetric ports. That is, when a TCP connection is established from host A and its source TCP port "a" to a remote host, the remote host sends packets back to host A's source TCP port "a". UDP isn't inheriently bidirectional and UDP itself doesn't require similar port symmetry. Rather, some UDP applications (DNS [11]) have symmetry, some UDP applications (TFTP [12]) don't have symmetry, and other UDP applications (RTP [5]) don't mention symmetry. 2. Definitions 2.1 Symmetric RTP The advertisement of the UDP port number for RTP media is usually via SDP [7], and the SDP is usually carried by a signaling protocol such as SIP [8], SAP [9], or MGCP [10]. When advertising that RTP media should be received on a certain UDP port A, a device that supports Symmetric RTP will also transmit RTP media from that same UDP port A. A device which doesn't support Symmetric RTP would transmit RTP from a different port than the one used to receive RTP. 2.2 Symmetric RTCP The advertisement of the RTCP port number can be implicit (RTP port + 1, as described in RFC3550 [5] section 11) or explicit (as described in Alternative Network Address Types [4]). When advertising that RTCP should be received on a certain UDP port B, a device that supports Symmetric RTCP will also transmit RTCP from that same UDP port B. A device which doesn't support Symmetric RTCP would transmit RTCP from a different port than the one used to receive RTCP. Wing Expires March 24, 2005 [Page 3] Internet-Draft Symmetric RTP and RTCP September 2004 3. Requirements There are two specific instances where symmetric RTP and symmetric RTCP are required. The first instance is symmetric NATs, which require the endpoint use UDP port symmetry to establish bi-directional traffic. Symmetric NATs are defined in RFC3489 [2] as: A symmetric NAT is one where all requests from the same internal IP address and port, to a specific destination IP address and port, are mapped to the same external IP address and port. If the same host sends a packet with the same source address and port, but to a different destination, a different mapping is used. Furthermore, only the external host that receives a packet can send a UDP packet back to the internal host. The second instance is Session Border Controllers (SBCs), which relay RTP media and RTCP packets. SBCs are useful in conjunction with symmetric NATs to allow bi-directional media traffic across such NATs. However, if the RTP endpoint does not do symmetric RTP and symmetric RTCP, an SBC still cannot provide traversal of RTP media across a symmetric NAT. There are other instances where symmetric RTP and symmetric RTCP are helpful, but not required. For example, if a firewall can expect symmetric RTP and symmetric RTCP then the firewall's dynamic per-call port filter list can be more restrictive compared to non-symmetric RTP and non-symmetric RTCP. Thus, this document specifies two requirements to permit traversal of symmetric NATs and operation with SBCs, and to allow firewalls to employ more restrictive filters: 1. If, for a session, an RTP sender is also an RTP receiver, the RTP sender MUST send RTP using the same UDP port as it receives RTP. 2. If, for a session, an RTCP sender is also an RTCP receiver, the RTCP sender MUST send RTCP using the same UDP port as it receives RTCP. 4. Security Considerations There is no additional security exposure if a host complies with this specification. 5. IANA Considerations This document doesn't require any IANA registrations. Wing Expires March 24, 2005 [Page 4] Internet-Draft Symmetric RTP and RTCP September 2004 6. References 6.1 Normative References [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 6.2 Informational References [2] 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. [3] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. [4] Camarillo, G., "The Alternative Network Address Types Semantics for the Session Description Protocol Grouping Framework", draft-ietf-mmusic-anat-01 (work in progress), June 2004. [5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. [6] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [7] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [8] 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. [9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000. [10] Andreasen, F. and B. Foster, "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 3435, January 2003. [11] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [12] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350, July 1992. Wing Expires March 24, 2005 [Page 5] Internet-Draft Symmetric RTP and RTCP September 2004 Author's Address Dan Wing Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA EMail: dwing@cisco.com Wing Expires March 24, 2005 [Page 6] Internet-Draft Symmetric RTP and RTCP September 2004 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat 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 on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. The IETF has been notified of intellectual property rights claimed in regard to some or all of the specification contained in this document. For more information consult the online list of claimed rights. Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM 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. Copyright Statement Copyright (C) The Internet Society (2004). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. Wing Expires March 24, 2005 [Page 7] Internet-Draft Symmetric RTP and RTCP September 2004 Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Wing Expires March 24, 2005 [Page 8]