Updates to RFC 5761
draft-holmberg-avtcore-5761-update-01.txt

Abstract

This document updates RFC 5761 by clarifying the SDP offer/answer negotiation of RTP and RTCP multiplexing. It makes it clear that an answerer can only include an "a=rtcp-mux" attribute in an SDP answer if the associated SDP offer contained the attribute.

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Table of Contents

• 1. Introduction
• 2. Conventions
• 3. Update to RFC 5761
• 3.1. Update to the sixth paragraph of section 5
• 4. Security Considerations
• 5. IANA Considerations
• 6. Acknowledgements
• 7. Change Log
• 8. Normative References
• Author's Address

1. Introduction

RFC 5761 [RFC5761] specifies how to multiplex RTP data packets and RTP Control Protocol (RTCP) packets on a single UDP port, and how to negotiate usage of such multiplexing using the SDP offer/answer mechanism [RFC3264], using an "a=rtcp-mux" attribute. However, the text is unclear on whether an answerer is allowed to include the attribute in an answer even if the associated offer did not contain an attribute.

This document updates RFC 5761 [RFC5761] by clarifying that an answerer can only include an "a=rtcp-mux" attribute in an answer if the associated offer contained the attribute. It also clarifies that the negotiation of RTP and RTCP multiplexing is for usage in both directions.

2. Conventions

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].

3. Update to RFC 5761

This section updates section 5.1.1 of RFC 5761.

3.1. Update to the sixth paragraph of section 5

OLD TEXT:

 When the Session Description Protocol (SDP) [8] is used to negotiate
 RTP sessions following the offer/answer model [9], the "a=rtcp-mux"
 attribute (see Section 8) indicates the desire to multiplex RTP and
 RTCP onto a single port.  The initial SDP offer MUST include this
 attribute at the media level to request multiplexing of RTP and RTCP
 on a single port.  For example:

     v=0
     o=csp 1153134164 1153134164 IN IP6 2001:DB8::211:24ff:fea3:7a2e
     s=-
     c=IN IP6 2001:DB8::211:24ff:fea3:7a2e
     t=1153134164 1153137764
     m=audio 49170 RTP/AVP 97
     a=rtpmap:97 iLBC/8000
     a=rtcp-mux

 This offer denotes a unicast voice-over-IP session using the RTP/AVP
 profile with iLBC coding.  The answerer is requested to send both RTP
 and RTCP to port 49170 on IPv6 address 2001:DB8::211:24ff:fea3:7a2e.

 If the answerer wishes to multiplex RTP and RTCP onto a single port,
 it MUST include a media-level "a=rtcp-mux" attribute in the answer.
 The RTP payload types used in the answer MUST conform to the rules in
 Section 4.

 If the answer does not contain an "a=rtcp-mux" attribute, the offerer
 MUST NOT multiplex RTP and RTCP packets on a single port.  Instead,
 it should send and receive RTCP on a port allocated according to the
 usual port-selection rules (either the port pair, or a signalled port
 if the "a=rtcp:" attribute [10] is also included).  This will occur
 when talking to a peer that does not understand the "a=rtcp-mux"
 attribute.

 When SDP is used in a declarative manner, the presence of an "a=rtcp-
 mux" attribute signals that the sender will multiplex RTP and RTCP on
 the same port.  The receiver MUST be prepared to receive RTCP packets
 on the RTP port, and any resource reservation needs to be made
 including the RTCP bandwidth.


NEW TEXT:

 When the Session Description Protocol (SDP) [8] is used to negotiate
 RTP sessions following the offer/answer model [9], the "a=rtcp-mux"
 attribute (see Section 8) indicates the desire to multiplex RTP and
 RTCP onto a single port, and the usage is always negotiated for both
 directions.

 If the offerer wishes to multiplex RTP and RTCP onto a single port,
 the initial SDP offer MUST include the attribute at the media level to
 request multiplexing of RTP and RTCP on a single port. For example:

     v=0
     o=csp 1153134164 1153134164 IN IP6 2001:DB8::211:24ff:fea3:7a2e
     s=-
     c=IN IP6 2001:DB8::211:24ff:fea3:7a2e
     t=1153134164 1153137764
     m=audio 49170 RTP/AVP 97
     a=rtpmap:97 iLBC/8000
     a=rtcp-mux

 This offer denotes a unicast voice-over-IP session using the RTP/AVP
 profile with iLBC coding.  The answerer is requested to send both RTP
 and RTCP to port 49170 on IPv6 address 2001:DB8::211:24ff:fea3:7a2e.

 If the offer contains the "a=rtcp-mux" attribute, and if the answerer
 wishes to multiplex RTP and RTCP onto a single port, it MUST include a
 media-level "a=rtcp-mux" attribute in the answer. The RTP payload
 types used in the answer MUST conform to the rules in Section 4. If
 the offer does not contain the "a=rtcp-mux" attribute the answerer
 MUST NOT include an attribute in the answer, and the answerer MUST NOT
 multiplex RTP and RTCP packets on a single port.

 If the answerer includes an "a=rtcp-mux" attribute in the answer, the
 offerer and answerer MUST multiplex RTP and RTCP packets on a single
 port.

 If the answer does not contain an "a=rtcp-mux" attribute, the offerer
 and answerer MUST NOT multiplex RTP and RTCP packets on a single port.
 Instead, they should send and receive RTCP on a port allocated
 according to the usual port-selection rules (either the port pair, or
 a signalled port if the "a=rtcp:" attribute [10] is also included).
 This will occur when talking to a peer that does not understand the
 "a=rtcp-mux" attribute.

 When SDP is used in a declarative manner, the presence of an "a=rtcp-
 mux" attribute signals that the sender will multiplex RTP and RTCP on
 the same port.  The receiver MUST be prepared to receive RTCP packets
 on the RTP port, and any resource reservation needs to be made
 including the RTCP bandwidth.

                

4. Security Considerations

The security considerations for RTP and RTCP multiplexing are described in RFC 5761. This specification does not impact those security considerations.

5. IANA Considerations

This specifcation makes no requests from IANA.

6. Acknowledgements

TBD

7. Change Log

[RFC EDITOR NOTE: Please remove this section when publishing]

Changes from foo

• Add text

8. Normative References

Author's Address