One document matched: draft-ietf-mmusic-sdp4nat-04.txt

Differences from draft-ietf-mmusic-sdp4nat-03.txt

INTERNET-DRAFT          RTCP attribute in SDP            May 12, 2003
                                                      
INTERNET DRAFT		                                   C. Huitema
<draft-ietf-mmusic-sdp4nat-04.txt>                          Microsoft
Expires November 12, 2003                                May 12, 2003

                        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.

This document is an Internet-Draft. Internet-Drafts are working 
documents of the Internet Engineering Task Force (IETF), its areas, 
and its working groups.  Note that other groups may also distribute 
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six 
months and may be updated, replaced, or obsoleted by other documents 
at any time.  It is inappropriate to use Internet- Drafts as 
reference material or to cite them other than as "work in progress."

The list of current Internet-Drafts can be accessed at 
http://www.ietf.org/ietf/1id-abstracts.txt.

The list of Internet-Draft Shadow Directories can be accessed at 
http://www.ietf.org/shadow.html.

Abstract

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, [RFC3261]) 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 are hidden 
behind a network address translation device [RFC2766]. In this case, 
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 [RFC2327] to 
describe the IP addresses and TCP or UDP ports used my the various 
media. Audio and video are typically sent using RTP [RTP-NEW], which 
requires two UDP ports, one for the media and one for the control 
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.' RTCP port numbers were necessarily derived from the base 
media port in older versions of RTP (such as [RFC1889]), but now 
that this restriction has been lifted, there is a need to specify 
RTCP ports explicitly in SDP. Note, however, that implementations of 
RTP adhering to the earlier [RFC1889] specification may not be able 
to make use of the SDP attributes specified in this document.

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 despite 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 
principle.

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]. 

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. 

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 ôvalueö 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 =  ôa=rtcp:ö port  [nettype space addrtype space
                         connection-address] CRLF

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

Example encodings could be:

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

    m=audio 49170 RTP/AVP 0
    a=rtcp:53020 IN IP4 126.16.64.4

    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. 

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 is only useful if the host can discover the 
ôtranslated port numbersö, i.e. the value of the ports as they 
appear on the 'external side' of the NAT. One possibility is to ask 
the cooperation of a well connected third party that will act as a 
server according to STUN [RFC3489]. We thus obtain a four step 
process:

1- The host allocate two UDP ports numbers for an RTP/RTCP pair,

2- The host sends a UDP message from each port to the STUN server,

3- The STUN server reads the source address and port of the packet, 
and copies them in the text of a reply, 

4- The host parses the reply according to the STUN protocol 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 ôSDP peerö with which 
the host wants to establish a connection. There is no guarantee that 
all NAT boxes deployed on the Internet have this characteristic. 
Implementers are referred to the STUN specification [RFC3489] for an 
extensive discussion of the various types of NAT.

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 
contrary, adding an attribute has a well defined failure mode: 
implementations that donÆt 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.

4.	UNSAF considerations

The RTCP attribute in SDP is used to enable establishment of 
RTP/RTCP flows through NAT. This mechanism can be used in 
conjunction with an address discovery mechanism such as STUN 
[RFC3489]. STUN is a short term fix to the NAT traversal problem, 
which requires thus consideration of the general issues linked to 
'Unilateral self-address fixing' [RFC3424].

The RTCP attribute addresses a very specific problem, the 
documentation of port numbers as they appear after address 
translation by a port-mapping NAT. The RTCP attribute SHOULD NOT be 
used for other applications.

We expect that, with time, one of two exit strategies can be 
developed. The IETF may develop an explicit 'middlebox control' 
protocol that will enable applications to obtain a pair of port 
numbers appropriate for RTP and RTCP. Another possibility is the 
deployment of IPv6, which will enable use of 'end to end' addressing 
and guarantee that the two hosts will be able to use appropriate 
ports. In both cases, there will be no need for documenting a ônon 
standardö RTCP port with the RTCP attribute.

5.	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.

In order to avoid attacks of this sort, when SDP is used in a 
signaling packet where it is of the form application/sdp, end-to-end 
integrity using S/MIME [RFC3369] is the technical method to be 
implemented and applied.  This is compatible with SIP [RFC3261].

6.	IANA Considerations

This document defines a new SDP parameter, the attribute field 
'rtcp', which per [RFC2327] should be registered by IANA. This 
attribute field is designed for use at media level only.

7.	Copyright

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

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

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

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 
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING 
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING 
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION 
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF 
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

8.	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 
copyrights, patents or patent applications, or other proprietary 
rights which may cover technology that may be required to practice 
this standard.  Please address the information to the IETF Executive 
Director.

9.	Acknowledgements

The original idea for using the 'rtcp' attribute was developed by 
Ann Demirtjis. The draft was reviewed by the MMUSIC and AVT working 
groups of the IETF.

10.	References

Normative references

[RFC2327] M. Handley, V. Jacobson, 'SDP: Session Description 
Protocol', RFC 2327, April 1998.

[RTP-NEW] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson. "RTP: 
A Transport Protocol for Real-Time Applications", Work in progress, 
March 2003.

[RFC1889] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson. "RTP: 
A Transport Protocol for Real-Time Applications", RFC 1889, January 
1996.

[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.

Informative references

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

[RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, 
J. Peterson, R. Sparks, M. Handley, E. Schooler. SIP: Session 
Initiation Protocol. RFC 3261, June 2002. 

[RFC3369] R. Housley. Cryptographic Message Syntax (CMS). RFC 3369, 
August 2002.

[RFC3424] L. Daigle, "IAB considerations for UNilateral self-address 
fixing (UNSAF) across network address translation," RFC 3424, 
November 2002.

[RFC3489] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy. ôSTUN - 
Simple Traversal of User Datagram Protocol (UDP) Through Network 
Address Translators (NATs)ö. RFC 3489, March 2003

11.	Author's Address

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

Email: huitema@microsoft.com