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
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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.
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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. 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
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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
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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.
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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.
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Author's Address
Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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