Internet DRAFT - draft-wing-mmusic-symmetric-rtprtcp

draft-wing-mmusic-symmetric-rtprtcp





MMUSIC                                                           D. Wing
Internet-Draft                                             Cisco Systems
Expires: April 7, 2005                                   October 7, 2004


               Symmetric RTP and RTCP Considered Helpful
                 draft-wing-mmusic-symmetric-rtprtcp-01

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
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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].



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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.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   6.1   Normative References . . . . . . . . . . . . . . . . . . . .  4
   6.2   Informational References . . . . . . . . . . . . . . . . . .  4
       Author's Address . . . . . . . . . . . . . . . . . . . . . . .  5
       Intellectual Property and Copyright Statements . . . . . . . .  6





































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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 UDP port number for RTP media stream is usually communicated
   using SDP [7].  The SDP is usually carried by a signaling protocol
   such as SIP [8], SAP [9], or MGCP [10].

   A device supports Symmetric 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 [5] section 11) or explicit (as
   described in Alternative Network Address Types [4]).  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
   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".



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   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 the RTP endpoint use
   UDP port symmetry to establish bi-directional traffic.  ALGs are
   defined in section 4.4 of RFC3022 [6].

   The second instance is Session Border Controllers (SBCs) and TURN
   [13] servers, which relay RTP media and RTCP packets.  Media relays
   are useful in conjunction with symmetric NATs to allow bi-directional
   UDP traffic across such NATs.  However, if the RTP endpoint does not
   do symmetric RTP and symmetric RTCP, the media relay is unable to
   perform its function if there is a symmetric NAT in the path.
   "Symmetric NAT" is defined in section 5 of RFC3489 [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.

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.  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



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         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.

   [13]  Rosenberg, J., "Traversal Using Relay NAT (TURN)",
         draft-rosenberg-midcom-turn-05 (work in progress), July 2004.


Author's Address

   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   EMail: dwing@cisco.com



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Acknowledgment

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















































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