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rfc:rfc8035

Internet Engineering Task Force (IETF) C. Holmberg Request for Comments: 8035 Ericsson Updates: 5761 November 2016 Category: Standards Track ISSN: 2070-1721

   Session Description Protocol (SDP) Offer/Answer Clarifications
                     for RTP/RTCP Multiplexing

Abstract

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

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8035.

Holmberg Standards Track [Page 1] RFC 8035 RTP/RTCP Mux Clarifications November 2016

Copyright Notice

 Copyright (c) 2016 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Holmberg Standards Track [Page 2] RFC 8035 RTP/RTCP Mux Clarifications November 2016

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   3
 3.  Update to RFC 5761  . . . . . . . . . . . . . . . . . . . . .   3
   3.1.  Update to Section 5.1.1 . . . . . . . . . . . . . . . . .   4
 4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
 6.  Normative References  . . . . . . . . . . . . . . . . . . . .   6
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   6
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

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] with 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 by clarifying that an
 answerer can only include an "a=rtcp-mux" attribute in an answer if
 the associated offer contained the attribute, and by clarifying that
 the negotiation of RTP and RTCP multiplexing is for usage in both
 directions.

Holmberg Standards Track [Page 3] RFC 8035 RTP/RTCP Mux Clarifications November 2016

3.1. Update to Section 5.1.1

 In this section, any references to Sections 4 and 8 are to those
 sections in [RFC5761].
 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.

Holmberg Standards Track [Page 4] RFC 8035 RTP/RTCP Mux Clarifications November 2016

 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 Internet Low Bit Rate Codec (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 "a=rtcp-mux" attribute in the answer, and the
 answerer MUST NOT multiplex RTP and RTCP packets on a single port.
 If the answer contains an "a=rtcp-mux" attribute, 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.

Holmberg Standards Track [Page 5] RFC 8035 RTP/RTCP Mux Clarifications November 2016

 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

 IANA has added a reference to this document for the att-field (media
 level only) registration "rtcp-mux" in the "Session Description
 Protocol (SDP) Parameters" registry.

6. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
            with Session Description Protocol (SDP)", RFC 3264,
            DOI 10.17487/RFC3264, June 2002,
            <http://www.rfc-editor.org/info/rfc3264>.
 [RFC5761]  Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
            Control Packets on a Single Port", RFC 5761,
            DOI 10.17487/RFC5761, April 2010,
            <http://www.rfc-editor.org/info/rfc5761>.

Acknowledgements

 Thanks to Colin Perkins, Magnus Westerlund, Paul Kyzivat, and Roni
 Even for providing comments on the document.  Thomas Belling provided
 useful input in the discussions that took place in 3GPP and resulted
 in the submission of the document.  Elwyn Davies performed the
 Gen-ART review.  Rick Casarez performed the Ops-Dir review.  Alissa
 Cooper and Spencer Dawkins provided IESG review comments.

Holmberg Standards Track [Page 6] RFC 8035 RTP/RTCP Mux Clarifications November 2016

Author's Address

 Christer Holmberg
 Ericsson
 Hirsalantie 11
 Jorvas  02420
 Finland
 Email: christer.holmberg@ericsson.com

Holmberg Standards Track [Page 7]

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