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Internet Engineering Task Force (IETF)                       C. HolmbergRequest for Comments: 8035                                      EricssonUpdates:5761                                              November 2016Category: Standards TrackISSN: 2070-1721Session Description Protocol (SDP) Offer/Answer Clarificationsfor RTP/RTCP MultiplexingAbstract   This document updatesRFC 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 inSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc8035.Holmberg                     Standards Track                    [Page 1]

RFC 8035               RTP/RTCP Mux Clarifications         November 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 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 2016Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .32.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .33.  Update toRFC 5761  . . . . . . . . . . . . . . . . . . . . .33.1.  Update toSection 5.1.1 . . . . . . . . . . . . . . . . .44.  Security Considerations . . . . . . . . . . . . . . . . . . .65.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .66.  Normative References  . . . . . . . . . . . . . . . . . . . .6   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .6   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .71.  IntroductionRFC 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 updatesRFC 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 toRFC 5761   This section updatesSection 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 20163.1.  Update toSection 5.1.1   In this section, any references to Sections4 and8 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 (seeSection 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 inSection 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 (seeSection 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 inSection 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 inRFC 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 2016Author's Address   Christer Holmberg   Ericsson   Hirsalantie 11   Jorvas  02420   Finland   Email: christer.holmberg@ericsson.comHolmberg                     Standards Track                    [Page 7]