Session Description Protocol (SDP) Offer/Answer Clarifications for RTP/RTCP Multiplexing
A one-rule fix: an answer may only ask for RTP/RTCP multiplexing if the offer did.
Holmberg, C., "Session Description Protocol (SDP) Offer/Answer Clarifications for RTP/RTCP Multiplexing", RFC 8035, DOI 10.17487/RFC8035, November 2016, <https://www.rfc-editor.org/info/rfc8035>.
@misc{rfc8035,
series = {Request for Comments},
number = 8035,
howpublished = {RFC 8035},
publisher = {RFC Editor},
doi = {10.17487/RFC8035},
url = {https://www.rfc-editor.org/info/rfc8035},
author = {C. Holmberg},
title = {{Session Description Protocol (SDP) Offer/Answer Clarifications for RTP/RTCP Multiplexing}},
pagetotal = 7,
year = 2016,
month = nov,
} RFC text
Contents
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.
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.
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.
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.
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.
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.
Author's Address
Christer Holmberg
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: christer.holmberg@ericsson.com
AI ELI5
On this page
§1The Ambiguity Being Fixed
RFC 5761 describes how to multiplex RTP data packets and RTP Control Protocol (RTCP) packets on a single UDP port. It also describes how to negotiate that multiplexing with the SDP offer/answer mechanism from RFC 3264, using an a=rtcp-mux attribute.
The gap was narrow. The text was unclear on whether an answerer is allowed to include the attribute in an answer even if the offer did not contain it. This document closes that gap and also clarifies that the negotiation applies in both directions.
Two people are deciding how to share one phone line. Nobody had written down whether the second person could suggest sharing first. Now it is written down.
§2The Core Rule
The update covers two points. First, an answerer can include an a=rtcp-mux attribute in an answer only if the associated offer contained the attribute. Second, the negotiation of RTP and RTCP multiplexing is for usage in both directions.
The change is made as an OLD TEXT / NEW TEXT replacement of Section 5.1.1 of RFC 5761. Nothing else in RFC 5761 is changed by this document.
You can only say "yes, let us share" if the other person asked to share. And when you agree, both of you share, not just one.
§3How the Offer and Answer Behave
If the offerer wants to multiplex, the initial offer MUST include the attribute at the media level. If the offer contains a=rtcp-mux and the answerer wants to multiplex too, the answer MUST include a media-level a=rtcp-mux, and the RTP payload types in the answer MUST conform to the rules in Section 4 of RFC 5761.
If the offer does not contain the attribute, the answerer MUST NOT include it in the answer, and MUST NOT multiplex RTP and RTCP on a single port.
- If the answer contains
a=rtcp-mux, the offerer and the answerer MUST multiplex RTP and RTCP on a single port. - If the answer does not contain it, they MUST NOT multiplex. They should send and receive RTCP on a port chosen by the usual port-selection rules: either the port pair, or a signalled port if the
a=rtcp:attribute is also included. This happens when talking to a peer that does not understanda=rtcp-mux.
If you were not asked to share, you cannot start sharing. If you were asked and you say yes, you both share. If you say nothing, you both keep separate lines.
- Offer with a=rtcp-muxRequests RTP and RTCP on a single port.
- Answer with a=rtcp-muxIncluded only because the offer had it; both sides now multiplex.
§4The Example Offer
The RFC keeps the example offer from RFC 5761. It carries one audio stream, m=audio 49170 RTP/AVP 97, with a=rtpmap:97 iLBC/8000 and a=rtcp-mux, sent from IPv6 address 2001:DB8::211:24ff:fea3:7a2e.
The text describes it as 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 that address.
The new text also covers declarative use of SDP. There, the presence of a=rtcp-mux signals that the sender will multiplex RTP and RTCP on the same port. The receiver MUST be prepared to receive RTCP on the RTP port, and any resource reservation needs to include the RTCP bandwidth.
The example is like a note saying "call me on line 49170, and I will use that one line for talking and for notes."
§5Security and IANA
The security considerations for RTP and RTCP multiplexing are described in RFC 5761, and this specification does not impact them.
For IANA, a reference to this document has been added to the att-field (media level only) registration rtcp-mux in the "Session Description Protocol (SDP) Parameters" registry.
Because only a rule about who may say yes was clarified, nothing new is risky. The official list just got a note pointing to this new rule.