BEHAVE D. Wing
Internet-Draft Cisco Systems
Intended status: Best Current February 26, 2007
Practice
Expires: August 30, 2007
Common Local Transmit and Receive Ports (Symmetric RTP)
draft-wing-behave-symmetric-rtprtcp-02
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Copyright (C) The IETF Trust (2007).
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in this Document . . . . . . . . . . . . . . . 3
3. Definition of Symmetric RTP and Symmetric RTCP . . . . . . . . 4
4. Recommended Usage . . . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . . 5
8.2. Informational References . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 6
Intellectual Property and Copyright Statements . . . . . . . . . . 7
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1. Introduction
TCP [RFC0793], 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
[RFC1035]), some applications use different local transmit and
receive ports with explicit signaling (TFTP [RFC1350]), and other
applications don't specify the behavior for local transmit and
receive ports (RTP [RFC3550]).
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 [RFC3022], 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
communicated to the remote peer (e.g., SDP [RFC4566] carried in SIP
[RFC3261], SAP [RFC2974], or MGCP [RFC3435]).
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 to be 1234. Its peer, device "B", is
not constrained to also use port 1234 for its common remote port. In
fact, such a constraint might be impossible to meet because device
"B" might already be using that port for another application.
The benefits of common local transmit and receive ports 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].
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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 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.
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.
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
UDP port symmetry to establish bi-directional 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., [I-D.ietf-behave-turn]). Media
relays are necessary to establish bi-directional UDP communication
across a NAT that is 'Address-Dependent' or 'Address and Port-
Dependent' [RFC4787]. However, even with a media relay, UDP port
symmetry is 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 non-symmetric
RTP and non-symmetric RTCP. Symmetric RTP and symmetric RTCP can
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also ease debugging and troubleshooting.
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 bi-directional RTP media streams.
5. Security Considerations
There is no additional security exposure if a host uses symmetric RTP
or symmetric RTCP.
6. IANA Considerations
This document doesn't require any IANA registrations.
7. Acknowledgments
The author thanks Francois Audet, Sunil Bhargo, Francois Le Faucheur,
Cullen Jennings, Benny Rodrig, Robert Sparks, and Joe Stone for their
assistance with this document.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informational 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,
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RFC 793, September 1981.
[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.
[I-D.ietf-behave-turn]
Rosenberg, J., "Obtaining Relay Addresses from Simple
Traversal Underneath NAT (STUN)",
draft-ietf-behave-turn-02 (work in progress),
October 2006.
[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.
[RFC3435] Andreasen, F. and B. Foster, "Media Gateway Control
Protocol (MGCP) Version 1.0", RFC 3435, January 2003.
Author's Address
Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
Email: dwing@cisco.com
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