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
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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.
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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
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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.
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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
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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,
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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
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