INTERNET DRAFT                                              C. Huitema
<draft-ietf-mmusic-sdp4nat-00.txt>                           Microsoft
Expires February 22, 2002                              August 22, 2001

                        RTCP attribute in SDP

Status of this memo

This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.

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The session description protocol (SDP) is used to describe the
parameters of media streams used in multimedia sessions. When a
session requires multiple ports, SDP assumes that these port have
consecutive numbers. However, when the session crosses a network
address translation device that also uses port mapping, the ordering
of ports can be destroyed by the translation. To handle this, we
propose an extension attribute to SDP.

1       Introduction

The session invitation protocol (SIP, [RFC2543]) is often used to
establish multi-media sessions on the Internet. There are often
cases today in which one or both end of the connection is hidden
behind a network address translation device [RFC2766]. In this
cases, the SDP text must document the IP addresses and UDP ports as
they appear on the "public Internet" side of the NAT; in this memo,
we will suppose that the host located behind a NAT has a way to
obtain these numbers; a possible way to learn these numbers is
briefly outlined in section 3. However, just learning the numbers is
not enough.

The SIP messages use the encoding defined in SDP [RFC2237] to
describe the IP addresses and TCP or UDP ports used my the various
media. Audio and video are typically sent using RTP [RFC1889], which
requires two UDP port, one for the media and one for the control

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protocol (RTCP). SDP carries only one port number per media, and
states that "other ports used by the media application (such as the
RTCP port) should be derived algorithmically from the base media
port." When the media is transmitted using RTP [RFC1889], the choice
of the port number is very specific: "for UDP and similar protocols,
RTP uses an even port number and the corresponding RTCP stream uses
the next higher (odd) port number; if an application is supplied
with an odd number for use as the RTP port, it should replace this
number with the next lower (even) number."

When the NAT device performs port mapping, there is no guarantee
that the mappings of two separate ports reflects the sequencing and
the parity of the original port numbers; in fact, when the NAT
manages a pool of IP addresses, it is even possible that the RTP and
the RTCP ports may be mapped to different addresses. In order to
successfully establish connections despites the misordering of the
port numbers and the possible parity switches caused by the NAT, we
propose to use a specific SDP attribute to document the RTCP port
and optionally the RTCP address, and we also propose to make the
behavior of RTP implementations more conforming to the robustness

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [RFC2119].

2       Description of the solution

The main part of our solution is the declaration of an SDP attribute
for documenting the port used by RTCP. In order for the solution to
be useful, the RTP implementation must be made more tolerant than
specified in [RFC1889].

2.1     The RTCP attribute

The RTCP attribute is used to document the RTCP port used for media
stream, when that port is not the next higher (odd) port number
following the RTP port described in the media line. The RTCP
attribute is a "value" attribute, and follows the general syntax
specified page 18 of [RFC2327]: "a=<attribute>:<value>". For the
RTCP attribute:

* the name is the ascii string "rtcp" (lower case),

* the value is the RTCP port number and optional address.

The formal description of the attribute is defined by the following
ABNF syntax:

rtcp-attribute =  "a=rtcp:" port  [nettype space addrtype space
                         connection-address] CRLF

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In this description, the "port", "nettype", "addrtype" and
"connection-address" tokens are defined as specified in "Appendix A:
SDP Grammar" of [RFC2327].

Example encodings could be:

    m=audio 49170 RTP/AVP 0

    m=audio 49170 RTP/AVP 0
    a=rtcp:53020 IN IP4

    m=audio 49170 RTP/AVP 0
    a=rtcp:53020 IN IP6 2001:2345:6789:ABCD:EF01:2345:6789:ABCD

The RTCP attribute MAY be used as a media level attribute; it MUST
NOT be used as a session level attribute.

2.2     Oddity tolerant RTP

In order to successfully exchange RTP packets with a host located
behind a NAT, a corresponding RTP implementation should be more
tolerant than specified in [RFC1889]. If it receives an SDP text
specifying the use of a specific port number for RTP, and another
specific port number for RTCP, the implementation SHOULD send
packets to exactly these port numbers, regardless of whether the
numbers are odd or even, in sequence or separate.

For compatibility with existing implementations, the modified RTP
behavior MUST NOT be used if the RTCP port is not explicitly
specified. An implementation that wishes to receive RTP packets on
an odd port number MUST document both the RTP and the RTCP ports in
the SDP description, even if the RTCP port is immediately
consecutive to the RTP port.

3       Discussion of the solution

The implementation of the solution is fairly straightforward. The
three questions that have been most often asked regarding this
solution are whether this is useful, i.e. whether a host can
actually discover port numbers in an unmodified NAT, whether it is
sufficient, i.e. whether or not there is a need to document more
than one ancillary port per media type, and whether relaxing the RTP
requirements is legitimate.

3.1     How do we discover port numbers ?

The proposed solution assumes that we can discover the "translated
port numbers", i.e. the value of the port as they appear on the
"external side" of the NAT. There are multiple ways to achieve this
result. One possibility is to ask the cooperation of a well
connected third party, using a three step process:

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1)      The host allocates two UDP port numbers for an RTP/RTCP pair,

2)      The host sends a UDP message from each port to the third party,

3)      The third party reads the source address and port of the packet,
and copy them in the text of a reply,

4)      The host parses the reply and learns the external address and
port corresponding to each of the two UDP port.

This algorithm supposes that the NAT will use the same translation
for packets sent to the third party and to the "SDP peer" with which
the host wants to establish a connection. The experience shows that
this is the case for a large fraction of NATs.

3.2     Do we need to support multiple ports ?

Most media streams are transmitted using a single pair of RTP and
RTCP ports. It is possible however to transmit a single media over
several RTP flows, for example using hierarchical encoding. In this
case, SDP will encode the port number used by RTP on the first flow,
and the number of flows, as in:

       m=video 49170/2 RTP/AVP 31

In this example, the media is sent over 2 consecutive pairs of
ports, corresponding respectively to RTP for the first flow (even
number, 49170), RTCP for the first flow (odd number, 49171), RTP for
the second flow (even number, 49172), and RTCP for the second flow
(odd number, 49173).

In theory, it would be possible to modify SDP and document the many
ports corresponding to the separate encoding layers. However,
layered encoding is not much used in practice, and when used is
mostly used in conjunction with multicast transmission. The
translation issues documented in this memo apply uniquely to unicast
transmission, and thus there is no short term need for the support
of multiple port descriptions. It is more convenient and more robust
to focus on the simple case in which a media is sent over exactly
one RTP/RTCP stream.

3.3     Why not expand the media definition?

The RTP ports are documented in the media description line, and it
would seem convenient to document the RTCP port at the same place,
rather than create an RTCP attribute. We considered this design
alternative and rejected it for two reasons: adding an extra port
number and an option address in the media description would be
awkward, and more importantly it would create problems with existing
applications, which would have to reject the entire media
description if they did not understand the extension. On the

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contrary, adding an attribute has a well defined failure mode:
implementations that don't understand the "a=rtcp" attribute will
simply ignore it; they will fail to send RTCP packets to the
specified address, but they will at least be able to receive the
media in the RTP packets.

3.4     Is a tolerant RTP legitimate?

Our solution explicitly asks implementers to disregard a part of the
RTP specification that mandates use of even port numbers for RTP and
the consecutive odd port number for RTCP. We believe that this is
very much in the spirit of the robustness principle attributed to
Jon Postel, i.e. "Be conservative in what you do, be liberal in what
you accept from others."

4       Security Considerations

This SDP extension is not believed to introduce any significant
security risk to multi-media applications. One could conceive that a
malevolent third party would use the extension to redirect the RTCP
fraction of an RTP exchange, but this require intercepting and
rewriting the signaling packet carrying the SDP text; if an
interceptor can do that, many more attacks are available, including
a wholesale change of the addresses and port numbers at which the
media will be sent.

5       IANA Considerations

This document does not call for an IANA action, unless the IANA is
registering attributes types for SDP - and there is no documentation
to that effect on the IANA web site.

6       Copyright

The following copyright notice is copied from RFC 2026 [Bradner,
1996], Section 10.4, and describes the applicable copyright for this

Copyright (C) The Internet Society March 21, 2001. All Rights

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works.  However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be

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followed, or as required to translate it into languages other than

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assignees.

This document and the information contained herein is provided on an

7       Intellectual Property

The following notice is copied from RFC 2026 [Bradner, 1996],
Section 10.4, and describes the position of the IETF concerning
intellectual property claims made against this document.

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use other technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights.  Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11.  Copies of
claims of rights made available for publication and any assurances
of licenses to be made available, or the result of an attempt made
to obtain a general license or permission for the use of such
proprietary rights by implementers or users of this specification
can be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any
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this standard.  Please address the information to the IETF Executive

8       Acknowledgements

The original idea for using the "rtcp" attribute was developed by
Ann Demirtjis.

9       References

[RFC2543] M. Handley, H. Schulzrinne, E. Schooler, J. Rosenberg "SIP:
Session Initiation Protocol", RFC 2543, March 1999.

[RFC2237] M. Handley, V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.

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[RFC1889] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson. "RTP:
A Transport Protocol for Real-Time Applications", RFC 1889, January

[RFC2766] G. Tsirtsis, P. Srisuresh. "Network Address Translation -
Protocol Translation (NAT-PT)", RFC 2766, February 2000.

[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.

[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax
Specifications:  ABNF", RFC 2234, November 1997.

10      Author's Addresses

Christian Huitema
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052-6399


Huitema                                                      [Page 7]