SDP Offer/Answer for RTP over QUIC (RoQ)

1.1. Notes for Readers

(Note to RFC Editor - if this document ever reaches you, please remove this section)¶

This document has not yet been adopted by any IETF working group, so does not carry any special status within the IETF.¶

3.1. The QUIC proto

The 'QUIC' protocol identifier is similar to the 'UDP' and 'TCP' protocol identifiers in that it only describes the transport protocol, and not the upper-layer protocol.¶

An 'm' line that specifies 'QUIC' MUST further qualify the application-layer protocol using an fmt identifier, such as "QUIC/RTP/AVPF".¶

Media described using an 'm' line containing the 'QUIC' protocol identifier are carried using QUIC streams, as defined in [RFC9000], or in QUIC DATAGRAMs, as defined in [RFC9221].¶

The following is an update to the ABNF for an 'm' line, as specified by [RFC8866], that defines a new value for the QUIC protocol.¶

   media-field =         %s"m" "=" media SP port \["/" integer\]
                             SP proto 1*(SP fmt) CRLF

   m= line parameter        parameter value(s)
   ------------------------------------------------------------------
   <media>:                 (unchanged from RFC8866)
   <proto>:                 'QUIC'
   <port>:                  UDP port number
   <fmt>:                   (unchanged from RFC8866)

3.2. RoQ RTP Protos

As much as possible, attributes used in this section are reused from other specifications, with references to the original definitions.¶

   media-field =         %s"m" "=" media SP port \["/" integer\]
                             SP proto 1*(SP fmt) CRLF

   m= line parameter        parameter value(s)
   ------------------------------------------------------------------
   <media>:                 (unchanged from RFC8866)
   <proto>:                 'QUIC/RTP/AVP'
   <port>:                  UDP port number
   <fmt>:                   (unchanged from RFC8866)

   media-field =         %s"m" "=" media SP port \["/" integer\]
                             SP proto 1*(SP fmt) CRLF

   m= line parameter        parameter value(s)
   ------------------------------------------------------------------
   <media>:                 (unchanged from RFC8866)
   <proto>:                 'QUIC/RTP/AVPF'
   <port>:                  UDP port number
   <fmt>:                   (unchanged from RFC8866)

   media-field =         %s"m" "=" media SP port \["/" integer\]
                             SP proto 1*(SP fmt) CRLF

   m= line parameter        parameter value(s)
   ------------------------------------------------------------------
   <media>:                 (unchanged from RFC8866)
   <proto>:                 'QUIC/RTP/SAVP'
   <port>:                  UDP port number
   <fmt>:                   (unchanged from RFC8866)

   media-field =         %s"m" "=" media SP port \["/" integer\]
                             SP proto 1*(SP fmt) CRLF

   m= line parameter        parameter value(s)
   ------------------------------------------------------------------
   <media>:                 (unchanged from RFC8866)
   <proto>:                 'QUIC/RTP/SAVPF'
   <port>:                  UDP port number
   <fmt>:                   (unchanged from RFC8866)

4.1. RoQ QUIC-DATAGRAMs Attribute

As noted in [I-D.ietf-avtcore-rtp-over-quic], the RoQ specification only assumes a baseline QUIC implementation as defined in [RFC8999], [RFC9000], [RFC9001], and [RFC9002], and this baseline does not provide unreliable datagrams, which are defined in [RFC9221].¶

It is very likely that RoQ implementers will wish to use QUIC DATAGRAMs, for a variety of reasons too large to list in this specification.¶

In order to support this capability, this section defines a new SDP media-level attribute, "quic-datagrams". The attribute can be associated with an SDP media description ("m=" line) with any of the QUIC proto values defined in Section 3.1.¶

Actual support for QUIC DATAGRAMs is negotiated between two QUIC endpoints, as described in Section 3 of [RFC9221], and nothing specified in SDP will cause a QUIC endpoint that does not advertise support for QUIC DATAGRAMs to suddenly begin to support them. However, it may be useful to tell a RoQ receiver that the RoQ sender plans to send QUIC DATAGRAMs, and to allow a RoQ receiver to tell the SDP sender that the RoQ receiver does not plan to support receiving QUIC DATAGRAMs for that media flow.¶

If the quic-datagrams attribute is present, the RoQ sender indicates its intention to use QUIC DATAGRAMs for the associated media flow, and the RoQ receiver indicates its willingness to accept QUIC DATAGRAMs for that media flow.¶

The quic-datagrams attribute is OPTIONAL for RoQ applications, even when the sender intends to use QUIC DATAGRAMs. Omitting the quic-datagrams attribute merely complicates the sender's decision whether to send specific media using QUIC DATAGRAMs.¶

If the attribute is not present in SDP, the sender sends its QUIC Initial packet with a non-zero max_datagram_frame_size QUIC transport parameter, and the receiver with a non-zero max_datagram_frame_size QUIC transport parameter, all will proceed normally. If the sender attempts to send DATAGRAMs before it receives a non-zero name=max_datagram_frame_size QUIC transport parameter in the initial handshake, this is a QUIC PROTOCOL_VIOLATION, as described in Section 3 of [RFC9221].¶

The definition of the SDP "quic-datagrams" attribute is:¶

Attribute name: quic-datagrams¶

Type of attribute: media¶

Mux category: IDENTICAL¶

• NOTE: This specification sets the mux category (as discussed in Section 4 of [RFC8859]) as IDENTICAL, as an RTP mixer which is multiplexing several incoming streams onto one connection needs to provide the same quidance to a RoQ receiver for all multiplexed media flows.¶

Subject to charset: No¶

Purpose: This attribute provides a hint as to whether the media associated with the SDP media description is likely to arrive via QUIC DATAGRAMs. It is a property attribute, which does not take a value.¶

Contact name: Spencer Dawkins¶

Contact e-mail: spencerdawkins.ietf@gmail.com¶

Reference: [I-D.dawkins-avtcore-sdp-roq] (This document)¶

Syntax:¶

    quic-datagrams

4.2. RoQ Flow Identifiers

Section 5.1 of [I-D.ietf-avtcore-rtp-over-quic] introduces a multiplexing identifier for RTP flows carried over a QUIC connection called "Flow Identifiers". This section defines a new SDP media-level attribute, "roq-flow-id". The attribute can be associated with an SDP media description ("m=" line) with any of the QUIC proto values defined in Section 3.1. In that case, the "m=" line port value indicates the port of the underlying QUIC transport UDP port, and the "roq-flow-id" value indicates the RoQ Flow Identifier.¶

No default value is defined for the SDP "roq-flow-id" attribute. Therefore, if the attribute is not present, the associated "m=" line MUST be considered invalid.¶

The definition of the SDP "roq-flow-id" attribute is:¶

Attribute name: roq-flow-id¶

Type of attribute: media¶

Mux category: CAUTION¶

• NOTE: This specification sets the mux category (as discussed in Section 4 of [RFC8859]) as CAUTION, as an RTP mixer which is multiplexing several incoming streams onto one connection needs to ensure that RoQ Flow Identifiers do not overlap, and might need to rewrite the Flow Identifiers in received streams when further multiplexing them.¶

Subject to charset: No¶

Purpose: This attribute indicates the RoQ Flow Idenfitier associated with the SDP media description.¶

Contact name: Spencer Dawkins¶

Contact e-mail: spencerdawkins.ietf@gmail.com¶

Reference: [I-D.dawkins-avtcore-sdp-roq] (This document)¶

Syntax:¶

    roq-flow-id = 1*19(DIGIT) ; DIGIT defined in RFC 4566

The RoQ flow identifier range is between 0 and 4611686018427387903 (2^62 - 1) (both included). Leading zeroes MUST NOT be used.¶

5.1. The SDP "setup" Attribute

The SDP "setup" attribute, defined for media over TCP in [RFC4145], is reused to indicate which endpoint initiates a QUIC connection (whether the endpoint actively opens a QUIC connection, or accepts an incoming QUIC connection. This attribute MUST be present in SDP offers and answers for RoQ.¶

5.2. The SDP "connection" Attribute

The SDP "connection" attribute, defined for TCP in [RFC4145], is reused to indicate whether the endpoint will open a new QUIC connection, or reuse an existing QUIC connection. This attribute MUST be present in SDP offers and answers for RoQ.¶

5.3. The SDP "fingerprint" Attribute

Because QUIC itself uses the TLS handshake as described in [RFC9001], the parties to a RoQ session MUST also provide authentication certificates as part of the TLS handshake procedure, as described in Section 5 of [RFC8122]. When self-signed certificates are used, certificate fingerprint is represented in SDP using the fingerprint SDP attribute, as illustrated in Section 3.4 of [RFC8122], in order to provide assurance that two endpoints with no prior relationship are not being subjected to a man-in-the-middle attack.¶

5.4. The SDP "rtcp-mux" Attribute

[I-D.ietf-avtcore-rtp-over-quic] defines how RTP and RTCP can be multiplexed onto a single QUIC connection. An application that will perform this multiplexing MUST include the "rtcp-mux" attribute defined in [RFC5761] in its SDP signaling.¶

7.1. Bundling Considerations

[RFC8843] describes a Session Description Protocol (SDP) Grouping Framework extension called 'BUNDLE'. The extension can be used with the SDP offer/answer mechanism to negotiate the usage of a single transport (5-tuple) for sending and receiving media described by multiple SDP media descriptions ("m=" sections).¶

The authors believe that no special considerations apply when using BUNDLE with a single QUIC connection carrying RoQ.¶

If an application uses multiple 5-tuples in order to allow QUIC Connection Migration as described in Section 9 of [RFC9000], it is assumed that only one QUIC path will be active at any given time.¶

If an application uses multiple 5-tuples in order to make use of the Multipath Extension for QUIC as described in [I-D.draft-ietf-quic-multipath], this would allow multiple QUIC paths to be active simultaneously, and this assumption will need revisiting when [I-D.draft-ietf-quic-multipath] is approved.¶

7.2. Implications of Replacing RTCP Feedback with QUIC Feedback

Section 10.4 of [I-D.ietf-avtcore-rtp-over-quic] describes how some RTCP feedback can be replaced by equivalent statistics that are already collected by QUIC. The exact RTCP feedback that can be replaced depends on the QUIC statistics exposed by the underlying QUIC implementation, and these QUIC statistics might depend in turn on QUIC extensions supported in the underlying QUIC implementation. The set of possible relevant QUIC extensions is not fixed, but some discussion appears in Section 11 of [I-D.ietf-avtcore-rtp-over-quic]. For these reasons, decisions about what RTCP feedback can be replaced will always be media-dependent and implementation-dependent.¶

It is assumed that an implementer will review the application requirements, the RTP proto in use, the available RTCP feedback for the media types being transferred, and available QUIC statistics, and will do the right thing.¶

More information about what RTCP feedback might be replaced by QUIC statistics, and what is possible, appears in Appendix B of [I-D.ietf-avtcore-rtp-over-quic].¶

Editors' Question: The API between QUIC and RoQ is, of course, a private matter, but in at least some cases, it might be useful to specify specific QUIC feedback substitutions so that the other RoQ endpoint does not provide RTCP feedback that this RoQ endpoint does not need and does not plan to use. We almost certainly need implementation and deployment experience before we can do more than offer a strawman proposal. Spencer suggests that we include any IETF-specified QUIC feedback substitutions in separate documents, as we do with RTCP extensions today, or even include them in the GitHub repository Wiki for this document.¶

7.3. Implications of Congestion Control

A significant distinction between QUIC transport and UDP transport is that QUIC transport is always congestion-controlled at the QUIC layer. For RTP media, this ought to be a difference without a difference. Any RTP application ought to perform flow control and congestion control using a control mechanism that is appropriate for the media being transferred, and this applies to RoQ applications as well.¶

Having said this, it is worth saying that RoQ applications can use any RTCP mechanisms such as Codec Control Messages [RFC5104] that can affect variables such as the Maximum Media Stream Bit Rate, as long as the RTP application respects the relevant congestion control considerations (in the case of Codec Control Messages, these considerations appear in Section 5 of [RFC5104]).¶

RoQ applications can also use RTP Control Protocol (RTCP) Feedback for Congestion Control, as described in [RFC8888].¶

Because RoQ applications are always congestion controlled at the QUIC connection level, QUIC congestion control also acts as an RTP Circuit Breaker [RFC8083], with no special considerations for RoQ.¶

7.4. Implications of using ICE with RoQ

Because a peer address is validated during QUIC connection establishment as described in Section 8.1 of [RFC9000], when a RoQ endpoint uses ICE [RFC8445] to communicate with another RoQ endpoint, an ICE agent will have already performed ICE candidate pair connectivity checking before a QUIC connection can be opened for use with RoQ.¶

An implementer should be aware that it is possible for a RoQ connection to be subject to "ping"/liveness checks at several different levels:¶

• QUIC PING frames, as described in Section 10.1.2 of [RFC9000]¶

• ICE keepalives, as described in Section 10 of [RFC5245] and in [RFC6263]¶

• ICE consent freshness, as described in [RFC7675]¶

• RTCP packets, as described in Section 6.2 of [RFC3550]¶

The following considerations are worth reviewing for implementers.¶

• QUIC PING frames are entirely under the control of an implementation. If a QUIC connection carries RTP/RTCP traffic, the RTCP transmission interval is likely to suffice for RTP liveness detection, but a wise implementer will look at this in their environment and proceed accordingly.¶

• ICE consent freshness, as described in Section 4 of [RFC7675], also serves the ICE keepalive function, so ICE keepalives are no longer necessary.¶

• At least some RTCP feedback might be unnecessary, as described in Section 7.2, so a wise implementer will look at what RTCP feedback can be replaced with QUIC feedback.¶

8.1. AV Profile-related Security Considerations

This document currently defines the QUIC/RTP/SAVP and QUIC/RTP/SAVPF secure profiles, although this might seem unnecessary, because RoQ already uses QUIC security mechanisms. That choice is made for two reasons:¶

• If an implementer wishes to use an existing RTP application over RoQ, continuing to support existing legacy secure profiles minimizes the changes required to the implementations at each end.¶

• If a RoQ RTP endpoint is communicating with a non-RoQ RTP endpoint using an Top-PtP-Translator RTP middlebox (as described in Section 3.2.1 of [RFC7667], the RoQ endpoint might reasonably use a secure AVP profile over QUIC when sending to the middlebox, because the sending endpoint doesn't have any way to control or even discover whether the RTP middlebox used a secure profile when forwarding RTP to a non-RoQ endpoint.¶

If this is not a good plan, there are two alternatives.¶

• We can REQUIRE conformant RoQ middleboxes to bridge between AVP and AVPF profiles carried over RoQ and SAVP and SAVPF profiles carried using other transports, so that insecure media flows are not relayed over insecure transport protocols.¶

• Alternatively, an implementation can use SFRAME as described in [RFC9605] to achieve end-to-end media security, at the price of disallowing some types of translating middleboxes (for example, Topo-Media-Translator middleboxes, as described in Section 3.2.1.2 of [RFC7667].¶

Editors' Question: We will need discussion within the working group to determine whether we do need to include QUIC/RTP/SAVP and QUIC/RTP/SAVPF in this specification.¶

9.1. quic-datagrams

This document defines a new SDP media-level attribute, "quic-datagrams". The details of the attribute are defined in Section 4.1.¶

9.2. roq-flow-id

This document defines a new SDP media-level attribute, "roq-flow-id". The details of the attribute are defined in Section 4.2.¶

10.1. Normative References

[I-D.dawkins-avtcore-sdp-roq]

"*** BROKEN REFERENCE ***".

[I-D.ietf-avtcore-rtp-over-quic]

Engelbart, M., Ott, J., and S. Dawkins, "RTP over QUIC (RoQ)", Work in Progress, Internet-Draft, draft-ietf-avtcore-rtp-over-quic-12, 21 October 2024, <https://datatracker.ietf.org/doc/html/draft-ietf-avtcore-rtp-over-quic-12>.

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC3264]

Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, DOI 10.17487/RFC3264, June 2002, <https://www.rfc-editor.org/rfc/rfc3264>.

[RFC3550]

Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550, July 2003, <https://www.rfc-editor.org/rfc/rfc3550>.

[RFC3551]

Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, DOI 10.17487/RFC3551, July 2003, <https://www.rfc-editor.org/rfc/rfc3551>.

[RFC3711]

Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, DOI 10.17487/RFC3711, March 2004, <https://www.rfc-editor.org/rfc/rfc3711>.

[RFC4145]

Yon, D. and G. Camarillo, "TCP-Based Media Transport in the Session Description Protocol (SDP)", RFC 4145, DOI 10.17487/RFC4145, September 2005, <https://www.rfc-editor.org/rfc/rfc4145>.

[RFC4585]

Ott, J., Wenger, S., Sato, N., Burmeister, C., and J. Rey, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, DOI 10.17487/RFC4585, July 2006, <https://www.rfc-editor.org/rfc/rfc4585>.

[RFC5124]

Ott, J. and E. Carrara, "Extended Secure RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/SAVPF)", RFC 5124, DOI 10.17487/RFC5124, February 2008, <https://www.rfc-editor.org/rfc/rfc5124>.

[RFC5761]

Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, DOI 10.17487/RFC5761, April 2010, <https://www.rfc-editor.org/rfc/rfc5761>.

[RFC7667]

Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667, DOI 10.17487/RFC7667, November 2015, <https://www.rfc-editor.org/rfc/rfc7667>.

[RFC8122]

Lennox, J. and C. Holmberg, "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)", RFC 8122, DOI 10.17487/RFC8122, March 2017, <https://www.rfc-editor.org/rfc/rfc8122>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

[RFC8445]

Keranen, A., Holmberg, C., and J. Rosenberg, "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal", RFC 8445, DOI 10.17487/RFC8445, July 2018, <https://www.rfc-editor.org/rfc/rfc8445>.

[RFC8866]

Begen, A., Kyzivat, P., Perkins, C., and M. Handley, "SDP: Session Description Protocol", RFC 8866, DOI 10.17487/RFC8866, January 2021, <https://www.rfc-editor.org/rfc/rfc8866>.

[RFC8999]

Thomson, M., "Version-Independent Properties of QUIC", RFC 8999, DOI 10.17487/RFC8999, May 2021, <https://www.rfc-editor.org/rfc/rfc8999>.

[RFC9000]

Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000, May 2021, <https://www.rfc-editor.org/rfc/rfc9000>.

[RFC9001]

Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021, <https://www.rfc-editor.org/rfc/rfc9001>.

[RFC9002]

Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection and Congestion Control", RFC 9002, DOI 10.17487/RFC9002, May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

[RFC9221]

Pauly, T., Kinnear, E., and D. Schinazi, "An Unreliable Datagram Extension to QUIC", RFC 9221, DOI 10.17487/RFC9221, March 2022, <https://www.rfc-editor.org/rfc/rfc9221>.

[RFC9605]

Omara, E., Uberti, J., Murillo, S. G., Barnes, R., Ed., and Y. Fablet, "Secure Frame (SFrame): Lightweight Authenticated Encryption for Real-Time Media", RFC 9605, DOI 10.17487/RFC9605, August 2024, <https://www.rfc-editor.org/rfc/rfc9605>.

[SDP-attribute-name]

"SDP Parameters - attribute-name", September 2021, <https://www.iana.org/assignments/sdp-parameters/sdp-parameters.xhtml#sdp-att-field>.

[SDP-protos]

"SDP Parameters - Proto", September 2021, <https://www.iana.org/assignments/sdp-parameters/sdp-parameters.xhtml#sdp-parameters-2>.

10.2. Informative References

[I-D.draft-ietf-quic-multipath]

Liu, Y., Ma, Y., De Coninck, Q., Bonaventure, O., Huitema, C., and M. Kühlewind, "Multipath Extension for QUIC", Work in Progress, Internet-Draft, draft-ietf-quic-multipath-12, 22 January 2025, <https://datatracker.ietf.org/doc/html/draft-ietf-quic-multipath-12>.

[I-D.ietf-avtcore-rtp-vvc]

Zhao, S., Wenger, S., Sanchez, Y., Wang, Y., and M. M. Hannuksela, "RTP Payload Format for Versatile Video Coding (VVC)", Work in Progress, Internet-Draft, draft-ietf-avtcore-rtp-vvc-18, 4 August 2022, <https://datatracker.ietf.org/doc/html/draft-ietf-avtcore-rtp-vvc-18>.

[RFC3261]

Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, DOI 10.17487/RFC3261, June 2002, <https://www.rfc-editor.org/rfc/rfc3261>.

[RFC5104]

Wenger, S., Chandra, U., Westerlund, M., and B. Burman, "Codec Control Messages in the RTP Audio-Visual Profile with Feedback (AVPF)", RFC 5104, DOI 10.17487/RFC5104, February 2008, <https://www.rfc-editor.org/rfc/rfc5104>.

[RFC5245]

Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, DOI 10.17487/RFC5245, April 2010, <https://www.rfc-editor.org/rfc/rfc5245>.

[RFC6263]

Marjou, X. and A. Sollaud, "Application Mechanism for Keeping Alive the NAT Mappings Associated with RTP / RTP Control Protocol (RTCP) Flows", RFC 6263, DOI 10.17487/RFC6263, June 2011, <https://www.rfc-editor.org/rfc/rfc6263>.

[RFC7675]

Perumal, M., Wing, D., Ravindranath, R., Reddy, T., and M. Thomson, "Session Traversal Utilities for NAT (STUN) Usage for Consent Freshness", RFC 7675, DOI 10.17487/RFC7675, October 2015, <https://www.rfc-editor.org/rfc/rfc7675>.

[RFC8083]

Perkins, C. and V. Singh, "Multimedia Congestion Control: Circuit Breakers for Unicast RTP Sessions", RFC 8083, DOI 10.17487/RFC8083, March 2017, <https://www.rfc-editor.org/rfc/rfc8083>.

[RFC8825]

Alvestrand, H., "Overview: Real-Time Protocols for Browser-Based Applications", RFC 8825, DOI 10.17487/RFC8825, January 2021, <https://www.rfc-editor.org/rfc/rfc8825>.

[RFC8842]

Holmberg, C. and R. Shpount, "Session Description Protocol (SDP) Offer/Answer Considerations for Datagram Transport Layer Security (DTLS) and Transport Layer Security (TLS)", RFC 8842, DOI 10.17487/RFC8842, January 2021, <https://www.rfc-editor.org/rfc/rfc8842>.

[RFC8843]

Holmberg, C., Alvestrand, H., and C. Jennings, "Negotiating Media Multiplexing Using the Session Description Protocol (SDP)", RFC 8843, DOI 10.17487/RFC8843, January 2021, <https://www.rfc-editor.org/rfc/rfc8843>.

[RFC8859]

Nandakumar, S., "A Framework for Session Description Protocol (SDP) Attributes When Multiplexing", RFC 8859, DOI 10.17487/RFC8859, January 2021, <https://www.rfc-editor.org/rfc/rfc8859>.

[RFC8888]

Sarker, Z., Perkins, C., Singh, V., and M. Ramalho, "RTP Control Protocol (RTCP) Feedback for Congestion Control", RFC 8888, DOI 10.17487/RFC8888, January 2021, <https://www.rfc-editor.org/rfc/rfc8888>.