BEHAVE D. Wing Internet-Draft Cisco Systems Expires: November 22, 2005 May 21, 2005 A Definition of Symmetric RTP and Symmetric RTCP draft-wing-behave-symmetric-rtprtcp-00 Status of this Memo 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 becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 November 22, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document defines "symmetric RTP" and "symmetric RTCP" and explains the value of their use. RFC Category The author intends this Internet Draft to be published as an Informational RFC. Wing Expires November 22, 2005 [Page 1] Internet-Draft Symmetric RTP and RTCP May 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1 Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Symmetric RTCP . . . . . . . . . . . . . . . . . . . . . . 3 3. Recommended Usage . . . . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 4 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5 7.1 Normative References . . . . . . . . . . . . . . . . . . . 5 7.2 Informational References . . . . . . . . . . . . . . . . . 5 Author's Address . . . . . . . . . . . . . . . . . . . . . . . 6 Intellectual Property and Copyright Statements . . . . . . . . 7 Wing Expires November 22, 2005 [Page 2] Internet-Draft Symmetric RTP and RTCP May 2005 1. Introduction Because RTP and RTCP are not inheriently bi-directional protocols, the usefulness of symmetry has been generally ignored. Many firewalls, NATs [4], 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 with source TCP port "a" to a remote host, the remote host sends packets back to host A's source TCP port "a". However, UDP is not inheriently bidirectional and UDP does not require similar port symmetry. Rather, some UDP applications have symmetry (DNS [9]), some applications do not have symmetry (TFTP [10]), and other applications do not discuss symmetry (RTP [1]). The benefits of UDP port symmetry for RTP and RTCP are discussed below in Section 3. 2. Definitions 2.1 Symmetric RTP The UDP port number for RTP media stream is usually communicated using SDP [5]. The SDP is usually carried by a signaling protocol such as SIP [6], SAP [7], or MGCP [8]. A device supports simmetric RTP if, when receiving a bi-directional RTP media stream on UDP port A and IP address "a", it also transmits RTP media for that stream from the same source UDP port A and IP address "a". A device which doesn't support symmetric RTP would transmit RTP from a different port, or from a different IP address, than the port and IP address used to receive RTP. 2.2 Symmetric RTCP The advertisement of the UDP port number for RTCP is usually communicated using SDP, and the port number is either implicit (RTP port + 1, as described in RFC3550 [1] section 11) or explicit (as described in Alternative Network Address Types [11]). The SDP is usually carried by a signaling protocol such as SIP, SAP, or MGCP. A device supports symmetric RCTP if, when receiving RTCP for a media Wing Expires November 22, 2005 [Page 3] Internet-Draft Symmetric RTP and RTCP May 2005 stream on port B and IP address "b", it also transmits its RTCP messages for that stream from the same source UDP port B and IP address "b". A device which doesn't support symmetric RTCP would transmit RTCP from a different port, or from a different IP address, than the port and IP address used to receive RTCP. 3. Recommended Usage There are two specific instances where symmetric RTP and symmetric RTCP are required. The first instance is NATs that lack integrated Application Layer Gateway (ALG) functionality. Such NATs require that endpoints use UDP port symmetry to establish bi-directional traffic. This requirement exists for all four types of NATs described in section 5 of STUN [2]. ALGs are defined in section 4.4 of RFC3022 [4]. The second instance is Session Border Controllers (SBCs) and other forms of RTP and RTCP relays (TURN [12]. Media relays are necessary to establish bi-directional UDP communication across a "symmetric NAT". However, even with a media relay, UDP port symmetry is still required by such a NAT. "Symmetric NAT" is defined in section 5 of STUN [2]. 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. There are no cases where symmetric RTP or symmetric RTCP are harmful. 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. 6. Acknowledgments The author thanks Sunil Bhargo and Cullen Jennings for their assistance with this document. Wing Expires November 22, 2005 [Page 4] Internet-Draft Symmetric RTP and RTCP May 2005 7. References 7.1 Normative References [1] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003. 7.2 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] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [5] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998. [6] 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. [7] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000. [8] Andreasen, F. and B. Foster, "Media Gateway Control Protocol (MGCP) Version 1.0", RFC 3435, January 2003. [9] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, November 1987. [10] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC 1350, July 1992. [11] Huitema, C., "Real Time Control Protocol (RTCP) attribute in Session Description Protocol (SDP)", RFC 3605, October 2003. [12] Rosenberg, J., "Traversal Using Relay NAT (TURN)", draft-rosenberg-midcom-turn-07 (work in progress), February 2005. Wing Expires November 22, 2005 [Page 5] Internet-Draft Symmetric RTP and RTCP May 2005 Author's Address Dan Wing Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Email: dwing@cisco.com Wing Expires November 22, 2005 [Page 6] Internet-Draft Symmetric RTP and RTCP May 2005 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. 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 (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Wing Expires November 22, 2005 [Page 7]