BEHAVE D. Wing Internet-Draft Cisco Systems Expires: December 5, 2005 June 3, 2005 Common Local Transmit and Receive Ports (Symmetric RTP) draft-wing-behave-symmetric-rtprtcp-01 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 December 5, 2005. Copyright Notice Copyright (C) The Internet Society (2005). Abstract This document describes common local transmit and receive ports, commonly called "symmetric RTP" and "symmetric RTCP", and explains the advantages of using common local transmit and receive ports. RFC Category The author intends this Internet Draft to be published as an Informational RFC. Wing Expires December 5, 2005 [Page 1] Internet-Draft Symmetric RTP and RTCP June 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definition of Symmetric RTP and Symmetric RTCP . . . . . . . . 3 3. Recommended Usage . . . . . . . . . . . . . . . . . . . . . . 4 4. Security Considerations . . . . . . . . . . . . . . . . . . . 4 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 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 December 5, 2005 [Page 2] Internet-Draft Symmetric RTP and RTCP June 2005 1. Introduction TCP [3], which is inheriently bidirectional, uses common local transmit and receive 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 common local transmit and receive ports. Rather, some UDP applications use common local transmit and receive ports (DNS [9]), some applications use different local transmit and receive ports with explicit signaling (TFTP [10]), and other applications don't specify the behavior for local transmit and receive ports (RTP [1]). Because RTP and RTCP are not inheriently bi-directional protocols, and UDP isn't a bi-directional protocol, the usefulness of common local transmit and receive ports has been generally ignored for RTP and RTCP. 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". The UDP port number to receive media, and the UDP port to transmit media are both selected by the device that receives that media and transmits that media. For unicast flows, the receive port is usually communicated to the remote peer using SDP [5]. The SDP is usually carried by a signaling protocol such as SIP [6], SAP [7], or MGCP [8]. For multicast flows, the transmit port is communicated to the remote peer using similar signaling protocols. There is no correspondence between the common local port and the common remote port. That is, device "A" might choose its common local transmit and receive port 1234 and its RTP peer, device "B", might choose a common local transmit and receive port 5678. Such a correspondence is impossible because device "B" might already be using port 1234 for another application. The benefits of common local transmit and receive ports is described below in Section 3. 2. Definition of Symmetric RTP and Symmetric RTCP A device supports symmetric RTP if it selects, communicates, and uses IP addresses and port numbers such that, 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". That is, it uses a common local transmit and receive port for RTP. Wing Expires December 5, 2005 [Page 3] Internet-Draft Symmetric RTP and RTCP June 2005 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 for that bi-directional media steam. A device supports symmetric RTCP if it selects, communicates, and uses IP addresses and port numbers such that, when receiving RTCP packets for a media stream on UDP port "B" and IP address "b", it also transmits RTCP packets for that stream from the same source UDP port "B" and IP address "b". That is, it uses a common local transmit and receive port for RTCP. 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. Symmetric RTP and symmetric RTCP can also ease debugging and troubleshooting. Other UDP-based protocols can also benefit from common local transmit and receive ports. 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 Wing Expires December 5, 2005 [Page 4] Internet-Draft Symmetric RTP and RTCP June 2005 specification. 5. IANA Considerations This document doesn't require any IANA registrations. 6. Acknowledgments The author thanks Sunil Bhargo, Francois Le Faucheur, Cullen Jennings, and Joe Stone for their assistance with this document. 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, Wing Expires December 5, 2005 [Page 5] Internet-Draft Symmetric RTP and RTCP June 2005 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. [13] International Telecommunications Union, "PACKET-BASED MULTIMEDIA COMMUNICATIONS SYSTEM", ITU Recommendation H.323, 1998. [14] International Telecommunications Union, "CONTROL PROTOCOL FOR MULTIMEDIA COMMUNICATION", ITU Recommendation H.245, 1998. Author's Address Dan Wing Cisco Systems 170 West Tasman Drive San Jose, CA 95134 USA Email: dwing@cisco.com Wing Expires December 5, 2005 [Page 6] Internet-Draft Symmetric RTP and RTCP June 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. 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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 December 5, 2005 [Page 7]