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.




















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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
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  publication of this document.  Please review these documents
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  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.

























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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.










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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.






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  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.






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  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.








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RFC 8035               RTP/RTCP Mux Clarifications         November 2016


Author's Address

  Christer Holmberg
  Ericsson
  Hirsalantie 11
  Jorvas  02420
  Finland

  Email: [email protected]










































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