draft-ietf-mmusic-sdp4nat-04.txt   draft-ietf-mmusic-sdp4nat-05.txt 
INTERNET DRAFT C. Huitema INTERNET DRAFT C. Huitema
<draft-ietf-mmusic-sdp4nat-04.txt> Microsoft <draft-ietf-mmusic-sdp4nat-05.txt> Microsoft
Expires November 12, 2003 May 12, 2003 Expires November 30, 2003 May 30, 2003
RTCP attribute in SDP RTCP attribute in SDP
Status of this memo Status of this memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
skipping to change at line 46 skipping to change at page 1, line 46
of ports can be destroyed by the translation. To handle this, we of ports can be destroyed by the translation. To handle this, we
propose an extension attribute to SDP. propose an extension attribute to SDP.
1. Introduction 1. Introduction
The session invitation protocol (SIP, [RFC3261]) is often used to The session invitation protocol (SIP, [RFC3261]) is often used to
establish multi-media sessions on the Internet. There are often establish multi-media sessions on the Internet. There are often
cases today in which one or both end of the connection are hidden cases today in which one or both end of the connection are hidden
behind a network address translation device [RFC2766]. In this case, behind a network address translation device [RFC2766]. In this case,
the SDP text must document the IP addresses and UDP ports as they 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 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 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 these numbers; a possible way to learn these numbers is briefly
outlined in section 3. However, just learning the numbers is not outlined in section 3. However, just learning the numbers is not
enough. enough.
The SIP messages use the encoding defined in SDP [RFC2327] to The SIP messages use the encoding defined in SDP [RFC2327] to
describe the IP addresses and TCP or UDP ports used my the various describe the IP addresses and TCP or UDP ports used by the various
media. Audio and video are typically sent using RTP [RTP-NEW], which 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 requires two UDP ports, one for the media and one for the control
protocol (RTCP). SDP carries only one port number per media, and protocol (RTCP). SDP carries only one port number per media, and
states that 'other ports used by the media application (such as the states that "other ports used by the media application (such as the
RTCP port) should be derived algorithmically from the base media RTCP port) should be derived algorithmically from the base media
port.' RTCP port numbers were necessarily derived from the base port." RTCP port numbers were necessarily derived from the base
media port in older versions of RTP (such as [RFC1889]), but now media port in older versions of RTP (such as [RFC1889]), but now
that this restriction has been lifted, there is a need to specify that this restriction has been lifted, there is a need to specify
RTCP ports explicitly in SDP. Note, however, that implementations of RTCP ports explicitly in SDP. Note, however, that implementations of
RTP adhering to the earlier [RFC1889] specification may not be able RTP adhering to the earlier [RFC1889] specification may not be able
to make use of the SDP attributes specified in this document. to make use of the SDP attributes specified in this document.
When the NAT device performs port mapping, there is no guarantee When the NAT device performs port mapping, there is no guarantee
that the mappings of two separate ports reflects the sequencing and that the mappings of two separate ports reflects the sequencing and
the parity of the original port numbers; in fact, when the NAT 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 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 the RTCP ports may be mapped to different addresses. In order to
successfully establish connections despite the misordering of the successfully establish connections despite the misordering of the
port numbers and the possible parity switches caused by the NAT, we port numbers and the possible parity switches caused by the NAT, we
propose to use a specific SDP attribute to document the RTCP port propose to use a specific SDP attribute to document the RTCP port
and optionally the RTCP address, and we also propose to make the and optionally the RTCP address.
behavior of RTP implementations more conforming to the robustness
principle.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2. Description of the solution 2. Description of the solution
The main part of our solution is the declaration of an SDP attribute The main part of our solution is the declaration of an SDP attribute
for documenting the port used by RTCP. for documenting the port used by RTCP.
2.1. The RTCP attribute 2.1. The RTCP attribute
The RTCP attribute is used to document the RTCP port used for media 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 stream, when that port is not the next higher (odd) port number
following the RTP port described in the media line. The RTCP following the RTP port described in the media line. The RTCP
attribute is a ˘value÷ attribute, and follows the general syntax attribute is a "value" attribute, and follows the general syntax
specified page 18 of [RFC2327]: "a=<attribute>:<value>". For the specified page 18 of [RFC2327]: "a=<attribute>:<value>". For the
RTCP attribute: RTCP attribute:
* the name is the ascii string 'rtcp' (lower case), * the name is the ascii string "rtcp" (lower case),
* the value is the RTCP port number and optional address. * the value is the RTCP port number and optional address.
The formal description of the attribute is defined by the following The formal description of the attribute is defined by the following
ABNF syntax: ABNF syntax:
rtcp-attribute = ˘a=rtcp:÷ port [nettype space addrtype space rtcp-attribute = "a=rtcp:" port [nettype space addrtype space
connection-address] CRLF connection-address] CRLF
In this description, the 'port', 'nettype', 'addrtype' and In this description, the "port", "nettype", "addrtype" and
˘connection-address÷ tokens are defined as specified in 'Appendix A: "connection-address" tokens are defined as specified in "Appendix A:
SDP Grammar' of [RFC2327]. SDP Grammar" of [RFC2327].
Example encodings could be: Example encodings could be:
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtcp:53020 a=rtcp:53020
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtcp:53020 IN IP4 126.16.64.4 a=rtcp:53020 IN IP4 126.16.64.4
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=rtcp:53020 IN IP6 2001:2345:6789:ABCD:EF01:2345:6789:ABCD 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 The RTCP attribute MAY be used as a media level attribute; it MUST
NOT be used as a session level attribute. NOT be used as a session level attribute.
3. Discussion of the solution 3. Discussion of the solution
The implementation of the solution is fairly straightforward. The The implementation of the solution is fairly straightforward. The
three questions that have been most often asked regarding this questions that have been most often asked regarding this solution
solution are whether this is useful, i.e. whether a host can are whether this is useful, i.e. whether a host can actually
actually discover port numbers in an unmodified NAT, whether it is discover port numbers in an unmodified NAT, whether it is
sufficient, i.e. whether or not there is a need to document more 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 than one ancillary port per media type, and whether why should not
requirements is legitimate. change the media definition instead of adding a new attribute.
3.1. How do we discover port numbers? 3.1. How do we discover port numbers?
The proposed solution is only useful if the host can discover the The proposed solution is only useful if the host can discover the
˘translated port numbers÷, i.e. the value of the ports as they "translated port numbers", i.e. the value of the ports as they
appear on the 'external side' of the NAT. One possibility is to ask 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 the cooperation of a well connected third party that will act as a
server according to STUN [RFC3489]. We thus obtain a four step server according to STUN [RFC3489]. We thus obtain a four step
process: process:
1- The host allocate two UDP ports numbers for an RTP/RTCP pair, 1- The host allocates two UDP ports numbers for an RTP/RTCP pair,
2- The host sends a UDP message from each port to the STUN server, 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, 3- The STUN server reads the source address and port of the packet,
and copies them in the text of a reply, and copies them in the text of a reply,
4- The host parses the reply according to the STUN protocol and 4- The host parses the reply according to the STUN protocol and
learns the external address and port corresponding to each of the learns the external address and port corresponding to each of the
two UDP port. two UDP port.
This algorithm supposes that the NAT will use the same translation 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 for packets sent to the third party and to the "SDP peer" with which
the host wants to establish a connection. There is no guarantee that the host wants to establish a connection. There is no guarantee that
all NAT boxes deployed on the Internet have this characteristic. all NAT boxes deployed on the Internet have this characteristic.
Implementers are referred to the STUN specification [RFC3489] for an Implementers are referred to the STUN specification [RFC3489] for an
extensive discussion of the various types of NAT. extensive discussion of the various types of NAT.
3.2. Do we need to support multiple ports? 3.2. Do we need to support multiple ports?
Most media streams are transmitted using a single pair of RTP and Most media streams are transmitted using a single pair of RTP and
RTCP ports. It is possible, however, to transmit a single media over RTCP ports. It is possible, however, to transmit a single media over
several RTP flows, for example using hierarchical encoding. In this several RTP flows, for example using hierarchical encoding. In this
skipping to change at line 198 skipping to change at page 4, line 42
The RTP ports are documented in the media description line, and it The RTP ports are documented in the media description line, and it
would seem convenient to document the RTCP port at the same place, would seem convenient to document the RTCP port at the same place,
rather than create an RTCP attribute. We considered this design rather than create an RTCP attribute. We considered this design
alternative and rejected it for two reasons: adding an extra port alternative and rejected it for two reasons: adding an extra port
number and an option address in the media description would be number and an option address in the media description would be
awkward, and more importantly it would create problems with existing awkward, and more importantly it would create problems with existing
applications, which would have to reject the entire media applications, which would have to reject the entire media
description if they did not understand the extension. On the description if they did not understand the extension. On the
contrary, adding an attribute has a well defined failure mode: contrary, adding an attribute has a well defined failure mode:
implementations that donĂt understand the 'a=rtcp' attribute will implementations that don't understand the "a=rtcp" attribute will
simply ignore it; they will fail to send RTCP packets to the simply ignore it; they will fail to send RTCP packets to the
specified address, but they will at least be able to receive the specified address, but they will at least be able to receive the
media in the RTP packets. media in the RTP packets.
4. UNSAF considerations 4. UNSAF considerations
The RTCP attribute in SDP is used to enable establishment of The RTCP attribute in SDP is used to enable establishment of
RTP/RTCP flows through NAT. This mechanism can be used in RTP/RTCP flows through NAT. This mechanism can be used in
conjunction with an address discovery mechanism such as STUN conjunction with an address discovery mechanism such as STUN
[RFC3489]. STUN is a short term fix to the NAT traversal problem, [RFC3489]. STUN is a short term fix to the NAT traversal problem,
which requires thus consideration of the general issues linked to which requires thus consideration of the general issues linked to
'Unilateral self-address fixing' [RFC3424]. "Unilateral self-address fixing" [RFC3424].
The RTCP attribute addresses a very specific problem, the The RTCP attribute addresses a very specific problem, the
documentation of port numbers as they appear after address documentation of port numbers as they appear after address
translation by a port-mapping NAT. The RTCP attribute SHOULD NOT be translation by a port-mapping NAT. The RTCP attribute SHOULD NOT be
used for other applications. used for other applications.
We expect that, with time, one of two exit strategies can be We expect that, with time, one of two exit strategies can be
developed. The IETF may develop an explicit 'middlebox control' developed. The IETF may develop an explicit "middlebox control"
protocol that will enable applications to obtain a pair of port protocol that will enable applications to obtain a pair of port
numbers appropriate for RTP and RTCP. Another possibility is the numbers appropriate for RTP and RTCP. Another possibility is the
deployment of IPv6, which will enable use of 'end to end' addressing deployment of IPv6, which will enable use of "end to end" addressing
and guarantee that the two hosts will be able to use appropriate and guarantee that the two hosts will be able to use appropriate
ports. In both cases, there will be no need for documenting a ˘non ports. In both cases, there will be no need for documenting a "non
standard÷ RTCP port with the RTCP attribute. standard" RTCP port with the RTCP attribute.
5. Security Considerations 5. Security Considerations
This SDP extension is not believed to introduce any significant This SDP extension is not believed to introduce any significant
security risk to multi-media applications. One could conceive that a security risk to multi-media applications. One could conceive that a
malevolent third party would use the extension to redirect the RTCP malevolent third party would use the extension to redirect the RTCP
fraction of an RTP exchange, but this require intercepting and fraction of an RTP exchange, but this requires intercepting and
rewriting the signaling packet carrying the SDP text; if an rewriting the signaling packet carrying the SDP text; if an
interceptor can do that, many more attacks are available, including interceptor can do that, many more attacks are available, including
a wholesale change of the addresses and port numbers at which the a wholesale change of the addresses and port numbers at which the
media will be sent. media will be sent.
In order to avoid attacks of this sort, when SDP is used in a 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 signaling packet where it is of the form application/sdp, end-to-end
integrity using S/MIME [RFC3369] is the technical method to be integrity using S/MIME [RFC3369] is the technical method to be
implemented and applied. This is compatible with SIP [RFC3261]. implemented and applied. This is compatible with SIP [RFC3261].
6. IANA Considerations 6. IANA Considerations
This document defines a new SDP parameter, the attribute field This document defines a new SDP parameter, the attribute field
'rtcp', which per [RFC2327] should be registered by IANA. This "rtcp", which per [RFC2327] should be registered by IANA. This
attribute field is designed for use at media level only. attribute field is designed for use at media level only.
7. Copyright 7. Copyright
The following copyright notice is copied from RFC 2026 [Bradner, The following copyright notice is copied from RFC 2026 [Bradner,
1996], Section 10.4, and describes the applicable copyright for this 1996], Section 10.4, and describes the applicable copyright for this
document. document.
Copyright (C) The Internet Society March 21, 2001. All Rights Copyright (C) The Internet Society March 21, 2001. All Rights
Reserved. Reserved.
skipping to change at line 309 skipping to change at page 6, line 49
can be obtained from the IETF Secretariat. can be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
9. Acknowledgements 9. Acknowledgements
The original idea for using the 'rtcp' attribute was developed by The original idea for using the "rtcp" attribute was developed by
Ann Demirtjis. The draft was reviewed by the MMUSIC and AVT working Ann Demirtjis. The draft was reviewed by the MMUSIC and AVT working
groups of the IETF. groups of the IETF.
10. References 10. References
Normative references Normative references
[RFC2327] Handley, M., and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[RFC2327] M. Handley, V. Jacobson, 'SDP: Session Description [RTP-NEW] Schulzrinne, H., Casner, S., Frederick, R., and V.
Protocol', RFC 2327, April 1998. Jacobson. "RTP: A Transport Protocol for Real-Time Applications",
Work in progress, March 2003.
[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: [RFC1889] Schulzrinne, H., Casner, S., Frederick, R., and V.
A Transport Protocol for Real-Time Applications", RFC 1889, January Jacobson. "RTP: A Transport Protocol for Real-Time Applications",
1996. RFC 1889, January 1996.
[RFC2119] S. Bradner, 'Key words for use in RFCs to Indicate [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels', RFC 2119, March 1997. Requirement Levels", RFC 2119, March 1997.
[RFC2234] D. Crocker, P. Overell, "Augmented BNF for Syntax [RFC2234] Crocker, D., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997. Specifications: ABNF", RFC 2234, November 1997.
[RFC3489] 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
Informative references Informative references
[RFC2766] G. Tsirtsis, P. Srisuresh. 'Network Address Translation - [RFC2766] Tsirtsis, G., and P. Srisuresh. "Network Address
Protocol Translation (NAT-PT)', RFC 2766, February 2000. Translation - Protocol Translation (NAT-PT)", RFC 2766, February
2000.
[RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
J. Peterson, R. Sparks, M. Handley, E. Schooler. SIP: Session A., Peterson, J., Sparks, R., Handley, M., and E. Schooler. SIP:
Initiation Protocol. RFC 3261, June 2002. Session Initiation Protocol. RFC 3261, June 2002.
[RFC3369] R. Housley. Cryptographic Message Syntax (CMS). RFC 3369, [RFC3369] R. Housley. Cryptographic Message Syntax (CMS). RFC 3369,
August 2002. August 2002.
[RFC3424] L. Daigle, "IAB considerations for UNilateral self-address [RFC3424] L. Daigle, "IAB considerations for UNilateral Self-Address
fixing (UNSAF) across network address translation," RFC 3424, Fixing (UNSAF) across network address translation," RFC 3424,
November 2002. November 2002.
[RFC3489] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy. ˘STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)÷. RFC 3489, March 2003
11. Author's Address 11. Author's Address
Christian Huitema Christian Huitema
Microsoft Corporation Microsoft Corporation
One Microsoft Way One Microsoft Way
Redmond, WA 98052-6399 Redmond, WA 98052-6399
Email: huitema@microsoft.com Email: huitema@microsoft.com
 End of changes. 

This html diff was produced by rfcdiff 1.23, available from http://www.levkowetz.com/ietf/tools/rfcdiff/