MMUSIC D. Wing
Internet-Draft Cisco Systems
Expires: March 24, 2005 September 23, 2004
Symmetric RTP and RTCP Considered Helpful
draft-wing-mmusic-symmetric-rtprtcp-00
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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. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 5
6.2 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 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 advertisement of the UDP port number for RTP media is usually via
SDP [7], and the SDP is usually carried by a signaling protocol such
as SIP [8], SAP [9], or MGCP [10].
When advertising that RTP media should be received on a certain UDP
port A, a device that supports Symmetric RTP will also transmit RTP
media from that same UDP port A.
A device which doesn't support Symmetric RTP would transmit RTP from
a different port than the one used to receive RTP.
2.2 Symmetric RTCP
The advertisement of the RTCP port number can be implicit (RTP port +
1, as described in RFC3550 [5] section 11) or explicit (as described
in Alternative Network Address Types [4]).
When advertising that RTCP should be received on a certain UDP port
B, a device that supports Symmetric RTCP will also transmit RTCP from
that same UDP port B.
A device which doesn't support Symmetric RTCP would transmit RTCP
from a different port than the one used to receive RTCP.
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3. Requirements
There are two specific instances where symmetric RTP and symmetric
RTCP are required.
The first instance is symmetric NATs, which require the endpoint use
UDP port symmetry to establish bi-directional traffic. Symmetric
NATs are defined in RFC3489 [2] as:
A symmetric NAT is one where all requests from the same internal
IP address and port, to a specific destination IP address and
port, are mapped to the same external IP address and port. If the
same host sends a packet with the same source address and port,
but to a different destination, a different mapping is used.
Furthermore, only the external host that receives a packet can
send a UDP packet back to the internal host.
The second instance is Session Border Controllers (SBCs), which relay
RTP media and RTCP packets. SBCs are useful in conjunction with
symmetric NATs to allow bi-directional media traffic across such
NATs. However, if the RTP endpoint does not do symmetric RTP and
symmetric RTCP, an SBC still cannot provide traversal of RTP media
across a symmetric 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.
Thus, this document specifies two requirements to permit traversal of
symmetric NATs and operation with SBCs, and to allow firewalls to
employ more restrictive filters:
1. If, for a session, an RTP sender is also an RTP receiver, the RTP
sender MUST send RTP using the same UDP port as it receives RTP.
2. If, for a session, an RTCP sender is also an RTCP receiver, the
RTCP sender MUST send RTCP using the same UDP port as it receives
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.
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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
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.
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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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Acknowledgment
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Internet Society.
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