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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RFC 8035 RTP/RTCP Mux Clarifications November 2016
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RFC 8035 RTP/RTCP Mux Clarifications November 2016
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.
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].
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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RFC 8035 RTP/RTCP Mux Clarifications November 2016
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:
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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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:
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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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.
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.
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