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rfc:rfc4961

Network Working Group D. Wing Request for Comments: 4961 Cisco Systems BCP: 131 July 2007 Category: 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 Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Conventions Used in this Document . . . . . . . . . . . . . . . 2
 3.  Definition of Symmetric RTP and Symmetric RTCP  . . . . . . . . 3
 4.  Recommended Usage . . . . . . . . . . . . . . . . . . . . . . . 3
 5.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
 6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 4
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 4
   7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 4
   7.2.  Informative References  . . . . . . . . . . . . . . . . . . 4

Wing Best Current Practice [Page 1] RFC 4961 Symmetric RTP and RTCP July 2007

1. 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 in
 Section 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 2007

3. 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 in Section 4 of
 [RFC4787].  ALGs are defined in Section 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. References

7.1. 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.com

Wing Best Current Practice [Page 5] RFC 4961 Symmetric RTP and RTCP July 2007

Full Copyright Statement

 Copyright (C) The IETF Trust (2007).
 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.
 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.

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 Intellectual Property Rights or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
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 might or might not be available; nor does it represent that it has
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 this standard.  Please address the information to the IETF at
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Wing Best Current Practice [Page 6]

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