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Network Working Group                                            D. WingRequest for Comments:  4961                                Cisco SystemsBCP:  131                                                      July 2007Category:  Best Current Practice              Symmetric RTP / RTP Control Protocol (RTCP)Status of This Memo   This document specifies an Internet Best Current Practices for the   Internet Community, and requests discussion and suggestions for   improvements.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The IETF Trust (2007).Abstract   This document recommends using one UDP port pair for both   communication directions of bidirectional RTP and RTP Control   Protocol (RTCP) sessions, commonly called "symmetric RTP" and   "symmetric RTCP".Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . .22.  Conventions Used in this Document . . . . . . . . . . . . . . .23.  Definition of Symmetric RTP and Symmetric RTCP  . . . . . . . .34.  Recommended Usage . . . . . . . . . . . . . . . . . . . . . . .35.  Security Considerations . . . . . . . . . . . . . . . . . . . .46.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . .47.  References  . . . . . . . . . . . . . . . . . . . . . . . . . .47.1.  Normative References  . . . . . . . . . . . . . . . . . . .47.2.  Informative References  . . . . . . . . . . . . . . . . . .4Wing                     Best Current Practice                  [Page 1]

RFC 4961                 Symmetric RTP and RTCP                July 20071.  Introduction   TCP [RFC0793], which is inherently bidirectional, transmits and   receives data using the same local port.  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 inherently bidirectional and UDP does not require   using the same port for sending and receiving bidirectional traffic.   Rather, some UDP applications use a single UDP port to transmit and   receive (e.g., DNS [RFC1035]), some applications use different UDP   ports to transmit and receive with explicit signaling (e.g., Trivial   File Transfer Protocol (TFTP) [RFC1350]), and other applications   don't specify the choice of transmit and receive ports (RTP   [RFC3550]).   Because RTP and RTCP are not inherently bidirectional protocols, and   UDP is not a bidirectional protocol, the usefulness of using the same   UDP port for transmitting and receiving has been generally ignored   for RTP and RTCP.  Many firewalls, Network Address Translators (NATs)   [RFC3022], and RTP implementations expect symmetric RTP, and do not   work in the presence of asymmetric 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   communicated to the remote peer (e.g., Session Description Protocol   (SDP) [RFC4566] carried in SIP [RFC3261], Session Announcement   Protocol (SAP) [RFC2974], or Megaco/H.248 [RFC3525]).   There is no correspondence between the local RTP (or RTCP) port and   the remote RTP (or RTCP) port.  That is, device "A" might choose its   local transmit and receive port to be 1234.  Its peer, device "B", is   not constrained to also use port 1234 for its port.  In fact, such a   constraint is impossible to meet because device "B" might already be   using that port for another application.   The benefits of using one UDP port pair is described below inSection 4.2.  Conventions Used in this Document   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].Wing                     Best Current Practice                  [Page 2]

RFC 4961                 Symmetric RTP and RTCP                July 20073.  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   bidirectional 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 the same UDP port to   transmit and receive one RTP stream.   A device that 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 bidirectional 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 the same UDP port to   transmit and receive one RTCP stream.   A device that 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.4.  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   symmetric UDP ports to establish bidirectional traffic.  This   requirement exists for all types of NATs described inSection 4 of   [RFC4787].  ALGs are defined inSection 4.4 of [RFC3022].   The second instance is Session Border Controllers (SBCs) and other   forms of RTP and RTCP relays (e.g., [TURN]).  Media relays are   necessary to establish bidirectional UDP communication across a NAT   that is 'Address-Dependent' or 'Address and Port-Dependent'   [RFC4787].  However, even with a media relay, symmetric UDP ports are   still required to traverse such a 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 asymmetric   RTP and asymmetric RTCP.  Symmetric RTP and symmetric RTCP can also   ease debugging and troubleshooting.Wing                     Best Current Practice                  [Page 3]

RFC 4961                 Symmetric RTP and RTCP                July 2007   Other UDP-based protocols can also benefit from common local transmit   and receive ports.   There are no known cases where symmetric RTP or symmetric RTCP are   harmful.   For these reasons, it is RECOMMENDED that symmetric RTP and symmetric   RTCP always be used for bidirectional RTP media streams.5.  Security Considerations   If an attacker learns the source and destination UDP ports of a   symmetric RTP or symmetric RTCP flow, the attacker can send RTP or   RTCP packets to that host.  This differs from asymmetric RTP and   asymmetric RTCP, where an attacker has to learn the UDP source and   destination ports used for the reverse traffic, before it can send   packets to that host.  Thus, if a host uses symmetric RTP or   symmetric RTCP, an attacker need only see one RTP or RTCP packet in   order to attack either RTP endpoint.  Note that this attack is   similar to that of other UDP-based protocols that use one UDP port   pair (e.g., DNS [RFC1035]).6.  Acknowledgments   The author thanks Francois Audet, Sunil Bhargo, Lars Eggert, Francois   Le Faucheur, Cullen Jennings, Benny Rodrig, Robert Sparks, and Joe   Stone for their assistance with this document.7.  References7.1.  Normative References   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels",BCP 14,RFC 2119, March 1997.7.2.  Informative References   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.              Jacobson, "RTP: A Transport Protocol for Real-Time              Applications", STD 64,RFC 3550, July 2003.   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation              (NAT) Behavioral Requirements for Unicast UDP",BCP 127,RFC 4787, January 2007.   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,RFC 793, September 1981.Wing                     Best Current Practice                  [Page 4]

RFC 4961                 Symmetric RTP and RTCP                July 2007   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network              Address Translator (Traditional NAT)",RFC 3022,              January 2001.   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session              Description Protocol",RFC 4566, July 2006.   [RFC1035]  Mockapetris, P., "Domain names - implementation and              specification", STD 13,RFC 1035, November 1987.   [RFC1350]  Sollins, K., "The TFTP Protocol (Revision 2)", STD 33,RFC 1350, July 1992.   [TURN]     Rosenberg, J., "Obtaining Relay Addresses from Simple              Traversal Underneath NAT (STUN)", Work in Progress,              July 2007.   [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.   [RFC2974]  Handley, M., Perkins, C., and E. Whelan, "Session              Announcement Protocol",RFC 2974, October 2000.   [RFC3525]  Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,              "Gateway Control Protocol Version 1",RFC 3525, June 2003.Author's Address   Dan Wing   Cisco Systems   170 West Tasman Drive   San Jose, CA  95134   USA   EMail:  dwing@cisco.comWing                     Best Current Practice                  [Page 5]

RFC 4961                 Symmetric RTP and RTCP                July 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   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, THE IETF TRUST 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Wing                     Best Current Practice                  [Page 6]