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RTP over QUIC
draft-engelbart-rtp-over-quic-04

Document Type Active Internet-Draft (avtcore WG)
Authors Joerg Ott , Mathis Engelbart
Last updated 2022-07-20 (Latest revision 2022-06-24)
Stream Internet Engineering Task Force (IETF)
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draft-engelbart-rtp-over-quic-04
AVTCORE                                                           J. Ott
Internet-Draft                                              M. Engelbart
Intended status: Standards Track             Technical University Munich
Expires: 26 December 2022                                   24 June 2022

                             RTP over QUIC
                    draft-engelbart-rtp-over-quic-04

Abstract

   This document specifies a minimal mapping for encapsulating RTP and
   RTCP packets within QUIC.  It also discusses how to leverage state
   from the QUIC implementation in the endpoints to reduce the exchange
   of RTCP packets and how to implement congestion control.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/mengelbart/rtp-over-quic-draft.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 December 2022.

Copyright Notice

   Copyright (c) 2022 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 (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.

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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Notation  . . . . . . . . . . . . . . . . . .   3
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Connection Establishment and ALPN . . . . . . . . . . . . . .   5
     5.1.  Draft version identification  . . . . . . . . . . . . . .   6
   6.  Encapsulation . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  QUIC Streams  . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  QUIC Datagrams  . . . . . . . . . . . . . . . . . . . . .   8
   7.  RTCP  . . . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     7.1.  Transport Layer Feedback  . . . . . . . . . . . . . . . .   9
     7.2.  Application Layer Repair and other Control Messages . . .  11
   8.  Congestion Control  . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Congestion Control at the QUIC layer  . . . . . . . . . .  12
     8.2.  Congestion Control at the Application Layer . . . . . . .  13
     8.3.  Shared QUIC connections . . . . . . . . . . . . . . . . .  14
   9.  API Considerations  . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  Information to be exported from QUIC  . . . . . . . . . .  14
     9.2.  Functions to be exposed by QUIC . . . . . . . . . . . . .  15
   10. Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Flow Identifier  . . . . . . . . . . . . . . . . . . . .  16
     10.2.  Impact of Connection Migration . . . . . . . . . . . . .  16
     10.3.  0-RTT considerations . . . . . . . . . . . . . . . . . .  16
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Registration of a RTP over QUIC Identification String  .  17
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     13.2.  Informative References . . . . . . . . . . . . . . . . .  20
   Appendix A.  Experimental Results . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  21

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1.  Introduction

   The Real-time Transport Protocol (RTP) [RFC3550] is generally used to
   carry real-time media for conversational media sessions, such as
   video conferences, across the Internet.  Since RTP requires real-time
   delivery and is tolerant to packet losses, the default underlying
   transport protocol has been UDP, recently with DTLS on top to secure
   the media exchange and occasionally TCP (and possibly TLS) as a
   fallback.  With the advent of QUIC [RFC9000] and, most notably, its
   unreliable DATAGRAM extension [RFC9221], another secure transport
   protocol becomes available.  QUIC and its DATAGRAMs combine desirable
   properties for real-time traffic (e.g., no unnecessary
   retransmissions, avoiding head-of-line blocking) with a secure end-
   to-end transport that is also expected to work well through NATs and
   firewalls.

   Moreover, with QUIC's multiplexing capabilities, reliable and
   unreliable transport connections as, e.g., needed for WebRTC, can be
   established with only a single port used at either end of the
   connection.  This document defines a mapping of how to carry RTP over
   QUIC.  The focus is on RTP and RTCP packet mapping and on reducing
   the amount of RTCP traffic by leveraging state information readily
   available within a QUIC endpoint.  This document also describes
   different options for implementing congestion control for RTP over
   QUIC.

   The scope of this document is limited to unicast RTP/RTCP.

   This document does not cover signaling for session setup.  Signaling
   for RTP over QUIC can be defined in separate documents such as
   [I-D.draft-dawkins-avtcore-sdp-rtp-quic] does for SDP.

   Note that this draft is similar in spirit to but differs in numerous
   ways from [I-D.draft-hurst-quic-rtp-tunnelling].

2.  Terminology and Notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   The following terms are used:

   Datagram:  Datagrams exist in UDP as well as in QUICs unreliable

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      datagram extension.  If not explicitly noted differently, the term
      datagram in this document refers to a QUIC Datagram as defined in
      [RFC9221].

   Endpoint:  A QUIC server or client that participates in an RTP over
      QUIC session.

   Frame:  A QUIC frame as defined in [RFC9000].

   Media Encoder:  An entity that is used by an application to produce a
      stream of encoded media, which can be packetized in RTP packets to
      be transmitted over QUIC.

   Receiver:  An endpoint that receives media in RTP packets and may
      send or receive RTCP packets.

   Sender:  An endpoint that sends media in RTP packets and may send or
      receive RTCP packets.

   Packet diagrams in this document use the format defined in
   Section 1.3 of [RFC9000] to illustrate the order and size of fields.

3.  Scope

   RTP over QUIC mostly defines an application usage of QUIC
   [I-D.draft-ietf-quic-applicability].  As a baseline, the
   specification does not expect more than a standard QUIC
   implementation as defined in [RFC8999], [RFC9000], [RFC9001], and
   [RFC9002].  Nevertheless, the specification can benefit from QUIC
   extesions such as QUIC datagrams [RFC9221] as described below.
   Moreover, this document describes how a QUIC implementation and its
   API can be extended to improve efficiency of the protocol operation.

   On top of QUIC, this document defines an encapsulation of RTP and
   RTCP packets.

   The scope of this document is limited to carrying RTP over QUIC.  It
   does not attempt to enhance QUIC for real-time media or define a
   replacement or evolution of RTP.  Such new media transport protocols
   may be covered elsewhere, e.g., in the MOQ WG.

   Protocols for negotiating connection setup and the associated
   parameters are defined separately, e.g., in
   [I-D.draft-dawkins-avtcore-sdp-rtp-quic].

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4.  Protocol Overview

   This document introduces a mapping of the Real-time Transport
   Protocol (RTP) to the QUIC transport protocol.  RTP over QUIC allows
   the use of QUIC streams and unreliable QUIC datagrams to transport
   real-time data, and thus, the QUIC implementation MUST support QUICs
   unreliable datagram extension, if RTP packets should be sent over
   QUIC datagrams.  Since datagram frames cannot be fragmented, the QUIC
   implementation MUST also provide a way to query the maximum datagram
   size so that an application can create RTP packets that always fit
   into a QUIC datagram frame.

   [RFC3550] specifies that RTP sessions need to be transmitted on
   different transport addresses to allow multiplexing between them.
   RTP over QUIC uses a different approach to leverage the advantages of
   QUIC connections without managing a separate QUIC connection per RTP
   session.  QUIC does not provide demultiplexing between different
   flows on datagrams but suggests that an application implement a
   demultiplexing mechanism if required.  An example of such a mechanism
   are flow identifiers prepended to each datagram frame as described in
   Section 2.1 of [I-D.draft-ietf-masque-h3-datagram].  RTP over QUIC
   uses a flow identifier to replace the network address and port number
   to multiplex many RTP sessions over the same QUIC connection.

   A congestion controller can be plugged in to adapt the media bitrate
   to the available bandwidth.  This document does not mandate any
   congestion control algorithm.  Some examples include Network-Assisted
   Dynamic Adaptation (NADA) [RFC8698] and Self-Clocked Rate Adaptation
   for Multimedia (SCReAM) [RFC8298].  These congestion control
   algorithms require some feedback about the network's performance to
   calculate target bitrates.  Traditionally this feedback is generated
   at the receiver and sent back to the sender via RTCP.  Since QUIC
   also collects some metrics about the network's performance, these
   metrics can be used to generate the required feedback at the sender-
   side and provide it to the congestion controller to avoid the
   additional overhead of the RTCP stream.

5.  Connection Establishment and ALPN

   QUIC requires the use of ALPN [RFC7301] tokens during connection
   setup.  RTP over QUIC uses "rtp-mux-quic" as ALPN token in the TLS
   handshake (see also Section 12.

   Note that the use of a given RTP profile is not reflected in the ALPN
   token even though it could be considered part of the application
   usage.  This is simply because different RTP sessions, which may use
   different RTP profiles, may be carried within the same QUIC
   connection.

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      *Editor's note:* "rtp-mux-quic" indicates that RTP and other
      protocols may be multiplexed on the same QUIC connection using a
      flow identifier as described in Section 6.  Applications are
      responsible for negotiation of protocols in use by appropriate use
      of a signaling protocol such as SDP.

      *Editor's note:* This implies that applications cannot use RTP
      over QUIC as specified in this document over WebTransport.

5.1.  Draft version identification

      *RFC Editor's note:* Please remove this section prior to
      publication of a final version of this document.

   RTP over QUIC uses the token "rtp-mux-quic" to identify itself in
   ALPN.

   Only implementations of the final, published RFC can identify
   themselves as "rtp-mux-quic".  Until such an RFC exists,
   implementations MUST NOT identify themselves using this string.

   Implementations of draft versions of the protocol MUST add the string
   "-" and the corresponding draft number to the identifier.  For
   example, draft-engelbart-rtp-over-quic-04 is identified using the
   string "rtp-mux-quic-04".

   Non-compatible experiments that are based on these draft versions
   MUST append the string "-" and an experiment name to the identifier.

6.  Encapsulation

   QUIC supports two transport methods: reliable streams [RFC9000] and
   unreliable datagrams [RFC9221].  This document specifies a mapping of
   RTP to both of the transport modes.  The encapsulation format for RTP
   over QUIC is described in Figure 1.

   Section 6.1 and Section 6.2 explain the specifics of mapping of RTP
   to QUIC streams and QUIC datagrams respectively.

   Payload {
     Flow Identifier (i),
     RTP/RTCP Packet (..)
   }

                   Figure 1: RTP over QUIC Payload Format

   Flow Identifier:  Flow identifier to demultiplex different data flows
      on the same QUIC connection.

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   RTP/RTCP Packet:  The RTP/RTCP packet to transmit.

   For multiplexing different RTP and other data streams on the same
   QUIC connection, each RTP/RTCP packet is prefixed with a flow
   identifier.  A flow identifier is a QUIC variable-length integer
   which must be unique per stream.

   RTP and RTCP packets of a single RTP session MAY be sent using the
   same flow identifier (following the procedures defined in [RFC5761],
   or they MAY be sent using different flow identifiers.  The respective
   mode of operation MUST be indicated using the appropriate signaling.

   RTP and RTCP packets of different RTP sessions MUST be sent using
   different flow identifiers.

   Differentiating RTP/RTCP packets of different RTP sessions from non-
   RTP/RTCP datagrams is the responsibility of the application by means
   of appropriate use of flow identifiers and the corresponding
   signaling.

   This specification defines two ways of carrying RTP packets in QUIC:
   1) using reliable QUIC streams and 2) using unreliable QUIC
   DATAGRAMs.  Every RTP session MUST choose exactly one way of carrying
   RTP and RTCP packets, different RTP sessions MAY choose different
   ways.

6.1.  QUIC Streams

   An application MUST open a new QUIC stream for each Application Data
   Unit (ADU).  Each ADU MUST be encapsulated in a single RTP packet and
   the application MUST not send more than one RTP packet per stream.
   Opening a new stream for each packet adds implicit framing to RTP
   packets, allows to receive packets without strict ordering and gives
   an application the possibility to cancel certain packets.

   Large RTP packets sent on a stream will be fragmented in smaller QUIC
   frames, that are transmitted reliably and in order, such that a
   receiving application can read a complete packet from the stream.  No
   retransmission has to be implemented by the application, since QUIC
   frames that are lost in transit are retransmitted by the QUIC
   connection.  If it is known to either the sender or the receiver,
   that a packet, which was not yet successfully and completely
   transmitted, is no longer needed, either side can close the stream.

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      *Editor's Note:* We considered adding a framing like the one
      described in [RFC4571] to send multiple RTP packets on one stream,
      but we don't think it is worth the additional overhead only to
      reduce the number of streams.  Moreover, putting multiple ADUs
      into a single stream would also require defining policies when to
      use the same (and which) stream and when to open a new one.

      *Editor's Note:* Note, however, that using a single frame per
      stream in a single RTP packet may cause interworking issues when a
      translator wants to forward packets received via RTP-over-QUIC to
      an endpoint as UDP packets because the received ADUs may exceed
      the MTU size or even maximum UDP packet size.

6.2.  QUIC Datagrams

   RTP packets can be sent in QUIC datagrams.  QUIC datagrams are an
   extension to QUIC described in [RFC9221].  QUIC datagrams preserve
   frame boundaries, thus a single RTP packet can be mapped to a single
   QUIC datagram, without the need for an additional framing.  Senders
   SHOULD consider the header overhead associated with QUIC datagrams
   and ensure that the RTP/RTCP packets, including their payloads, QUIC,
   and IP headers, will fit into path MTU.

   If an application wishes to retransmit lost RTP packets, the
   retransmission has to be implemented by the application by sending a
   new datagram for the RTP packet, because QUIC datagrams are not
   retransmitted on loss (see also Section 7.1 for loss signaling).

7.  RTCP

   The RTP Control Protocol (RTCP) allows RTP senders and receivers to
   exchange control information to monitor connection statistics and to
   identify and synchronize streams.  Many of the statistics contained
   in RTCP packets overlap with the connection statistics collected by a
   QUIC connection.  To avoid using up bandwidth for duplicated control
   information, the information SHOULD only be sent at one protocol
   layer.  QUIC relies on certain control frames to be sent.

   In general, applications MAY send RTCP without any restrictions.
   This document specifies a baseline for replacing some of the RTCP
   packet types by mapping the contents to QUIC connection statistics.
   Future documents can extend this mapping for other RTCP format types.
   It is RECOMMENDED to expose relevant information from the QUIC layer
   to the application instead of exchanging addtional RTCP packets,
   where applicable.

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   This section discusses what information can be exposed from the QUIC
   connection layer to reduce the RTCP overhead and which type of RTCP
   messages cannot be replaced by similar feedback from the transport
   layer.  The list of RTCP packets in this section is not exhaustive
   and similar considerations SHOULD be taken into account before
   exchanging any other type of RTCP control packets.

7.1.  Transport Layer Feedback

   This section explains how some of the RTCP packet types which are
   used to signal reception statistics can be replaced by equivalent
   statistics that are already collected by QUIC.  The following list
   explains how this mapping can be achieved for the individual fields
   of different RTCP packet types.

   QUIC Datagrams are ack-eliciting packets, which means, that an
   acknowledgment is triggered when a datagram frame is received.  Thus,
   a sender can assume that an RTP packet arrived at the receiver or was
   lost in transit, using the QUIC acknowledgments of QUIC Datagram
   frames.  In the following, an RTP packet is regarded as acknowledged,
   when the QUIC Datagram frame that carried the RTP packet, was
   acknowledged.  For RTP packets that are sent over QUIC streams, an
   RTP packet can be considered acknowledged, when all frames which
   carried fragments of the RTP packet were acknowledged.

   When QUIC Streams are used, the application should be aware that the
   direct mapping proposed below may not reflect the real
   characteristics of the network.  RTP packet loss can seem lower than
   actual packet loss due to QUIC's automatic retransmissions.
   Similarly, timing information might be incorrect due to
   retransmissions.

   Some of the transport layer feedback that can be implemented in RTCP
   contains information that is not included in QUIC by default, but can
   be added via QUIC extensions.  One important example are arrival
   timestamps, which are not part of QUIC's default acknowledgment
   frames, but can be added using [I-D.draft-smith-quic-receive-ts] or
   [I-D.draft-huitema-quic-ts].  Another extension, that can improve the
   precision of the feedback from QUIC is
   [I-D.draft-ietf-quic-ack-frequency], which allows a sender to control
   the delay of acknowledgments sent by the receiver.

   *  _Receiver Reports_ (PT=201, Name=RR, [RFC3550])

      -  _Fraction lost_: The fraction of lost packets can be directly
         infered from QUIC's acknowledgments.  The calculation SHOULD
         include all packets up to the acknowledged RTP packet with the
         highest RTP sequence number.  Later packets SHOULD be ignored,

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         since they may still be in flight, unless other QUIC packets
         that were sent after the datagram frame, were already
         acknowledged.

      -  _Cumulative lost_: Similar to the fraction of lost packets, the
         cumulative loss can be infered from QUIC's acknowledgments
         including all packets up to the latest acknowledged packet.

      -  _Highest Sequence Number received_: The highest sequence number
         received is the sequence number of all RTP packets that were
         acknowledged.

      -  Interarrival jitter: If QUIC acknowledgments carry timestamps
         as described in one of the extensions referenced above, senders
         can infer from QUIC acks the interarrival jitter from the
         arrival timestamps.

      -  Last SR: Similar to RTP arrival times, the arrival time of RTCP
         Sender Reports can be inferred from QUIC acknowledgments, if
         they include timestamps.

      -  Delay since last SR: This field is not required when the
         receiver reports are entirely replaced by QUIC feedback.

   *  _Negative Acknowledgments_ (PT=205, FMT=1, Name=Generic NACK,
      [RFC4585])

      -  The generic negative acknowledgment packet contains information
         about packets which the receiver considered lost.
         Section 6.2.1. of [RFC4585] recommends to use this feature
         only, if the underlying protocol cannot provide similar
         feedback.  QUIC does not provide negative acknowledgments, but
         can detect lost packets through acknowledgments.

   *  _ECN Feedback_ (PT=205, FMT=8, Name=RTCP-ECN-FB, [RFC6679])

      -  ECN feedback packets report the count of observed ECN-CE marks.
         [RFC6679] defines two RTCP reports, one packet type (with
         PT=205 and FMT=8) and a new report block for the extended
         reports which are listed below.  QUIC supports ECN reporting
         through acknowledgments.  If the connection supports ECN, the
         reporting of ECN counts SHOULD be done using QUIC
         acknowledgments.

   *  _Congestion Control Feedback_ (PT=205, FMT=11, Name=CCFB,
      [RFC8888])

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      -  RTP Congestion Control Feedback contains acknowledgments,
         arrival timestamps and ECN notifications for each received
         packet.  Acknowledgments and ECNs can be infered from QUIC as
         described above.  Arrival timestamps can be added through
         extended acknowledgment frames as described in
         [I-D.draft-smith-quic-receive-ts] or
         [I-D.draft-huitema-quic-ts].

   *  _Extended Reports_ (PT=207, Name=XR, [RFC3611])

      -  Extended Reports offer an extensible framework for a variety of
         different control messages.  Some of the standard report blocks
         which can be implemented in extended reports such as loss RLE
         or ECNs can be implemented in QUIC, too.  For other report
         blocks, it SHOULD be evaluated individually, if the contained
         information can be transmitted using QUIC instead.

7.2.  Application Layer Repair and other Control Messages

   While the previous section presented some RTCP packet that can be
   replaced by QUIC features, QUIC cannot replace all of the available
   RTCP packet types.  This mostly affects RTCP packet types which carry
   control information that is to be interpreted by the application
   layer instead of the transport itself.

   _Sender Reports_ (PT=200, Name=SR, [RFC3550]) are similar to
   _Receiver Reports_. They are sent by media senders and additionally
   contain a NTP and a RTP timestamp and the number of packets and
   octets transmitted by the sender.  The timestamps can be used by a
   receiver to synchronize streams.  QUIC cannot provide a similar
   control information, since it does not know about RTP timestamps.  A
   QUIC receiver can also not calculate the packet or octet counts,
   since it does not know about lost datagrams.  Thus, sender reports
   are required in RTP over QUIC to synchronize streams at the receiver.
   The sender reports SHOULD not contain any receiver report blocks, as
   the information can be infered from the QUIC transport as explained
   in the previous section.

   Next to carrying transmission statistics, RTCP packets can contain
   application layer control information, that cannot directly be mapped
   to QUIC.  This includes for example the _Source Description_ (PT=202,
   Name=SDES), _Bye_ (PT=203, Name=BYE) and _Application_ (PT=204,
   Name=APP) packet types from [RFC3550] or many of the payload specific
   feedback messages (PT=206) defined in [RFC4585], which can for
   example be used to control the codec behavior of the sender.  Since
   QUIC does not provide any kind of application layer control
   messaging, these RTCP packet types SHOULD be used in the same way as
   they would be used over any other transport protocol.

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8.  Congestion Control

   Like any other application on the internet, RTP over QUIC needs to
   perform congestion control to avoid overloading the network.

   QUIC is a congestion controlled transport protocol.  Senders are
   required to employ some form of congestion control.  The default
   congestion control specified for QUIC is an alogrithm similar to TCP
   NewReno, but senders are free to choose any congestion control
   algorithm as long as they follow the guidelines specified in
   Section 3 of [RFC8085].

   RTP does not specify a congestion controller, but provides feedback
   formats for congestion control (e.g.  [RFC8888]) as well as different
   congestion control algorithms in separate RFCs (e.g.  SCReAM
   [RFC8298] and NADA [RFC8698]).  The congestion control algorithms for
   RTP are specifically tailored for real-time transmissions at low
   latencies.  The available congestion control algorithms for RTP
   expose a target_bitrate that can be used to dynamically reconfigure
   media codecs to produce media at a rate that can be sent in real-time
   under the observed network conditions.

   This section defines two architectures for congestion control and
   bandwidth estimation for RTP over QUIC, but it does not mandate any
   specific congestion control algorithm to use.  The section also
   discusses congestion control implications of using shared or multiple
   separate QUIC connections to send and receive multiple independent
   data streams.

   It is assumed that the congestion controller in use provides a pacing
   mechanism to determine when a packet can be sent to avoid bursts.
   The currently proposed congestion control algorithms for real-time
   communications provide such pacing mechanisms.  The use of congestion
   controllers which don't provide a pacing mechanism is out of scope of
   this document.

      *TODO:* Add considerations for bandwidth shares when a QUIC
      connection is shared between RTP and non-RTP streams?

8.1.  Congestion Control at the QUIC layer

   If congestion control is to be applied at the transport layer, it is
   RECOMMENDED to configure the QUIC Implementation to use a delay-based
   real-time congestion control algorithm instead of a loss-based
   algorithm.  The currently available delay-based congestion control
   algorithms depend on detailed arrival time feedback to estimate the
   current one-way delay between sender and receiver.  Since QUIC does
   not provide arrival timestamps in its acknowledgments, the QUIC

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   implementations of the sender and receiver MUST use an extension to
   add this information to QUICs acknowledgment frames, e.g.
   [I-D.draft-smith-quic-receive-ts].

   If congestion control is done by the QUIC implementation, the
   application needs a mechanism to query the currently available
   bandwidth to adapt media codec configurations.  The employed
   congestion controller of the QUIC connection SHOULD expose such an
   API to the application.  If a current bandwidth estimation is not
   available from the QUIC congestion controller, the sender can either
   implement an alternative bandwidth estimation at the application
   layer as described in Section 8.2 or a receiver can feedback the
   observed bandwidth through RTCP, e.g., using
   [I-D.draft-alvestrand-rmcat-remb].

      *Editor's note:* An alternative to the hard requirement to use a
      timestamp extension could be to use RTCP, but that would mean,
      that an application has to negotiate RTCP congestion control
      feedback which would then have to be passed to the QUIC congestion
      controller.

      *Editor's note:* How can a QUIC connection be shared with non-RTP
      streams, when SCReAM/NADA/GCC is used as congestion controller?
      Can these algorithms be adapted to allow different streams
      including non-real-time streams?  Do they even have to be adapted
      or _should_ this just work?

8.2.  Congestion Control at the Application Layer

   If an application cannot access a bandwidth estimation from the QUIC
   layer, or the QUIC implementation does not support a delay-based,
   low-latency congestion control algorithm, it can alternatively
   implement a bandwidth estimation algorithm at the application layer.
   Calculating a bandwidth estimation at the application layer can be
   done using the same bandwidth estimation algorithms as described in
   Section 8 (NADA, SCReAM).  The bandwidth estimation algorithm
   typically needs some feedback on the transmission performance.  This
   feedback can be collected following the guidelines in Section 7.

   If the application implements full congestion control rather than
   just a bandwidth estimation at the application layer using a
   congestion controller that satisfies the requirements of Section 7 of
   [RFC9002], and the connection is only used to send real-time media
   which is subject to the application layer congestion control, it is
   RECOMMENDED to disable any other congestion control that is possibly
   running at the QUIC layer.  Disabling the additional congestion
   controllers helps to avoid any interference between the different
   congestion controllers.

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8.3.  Shared QUIC connections

   Two endpoints may want to establish channels to exchange more than
   one type of data simultaneously.  The channels can be intended to
   carry real-time RTP data or other non-real-time data.  This can be
   realized in different ways.  A straightforward solution is to
   establish multiple QUIC connections, one for each channel.  Or all
   real-time channels are mapped to one QUIC connection, while a
   separate QUIC connection is created for the non-real-time channels.
   In both cases, the congestion controllers can be chosen to match the
   demands of the respective channels and the different QUIC connections
   will compete for the same resources in the network.  No local
   prioritization of data across the different (types of) channels would
   be necessary.

   Alternatively, (all or a subset of) real-time and non-real-time
   channels are multiplexed onto a single, shared QUIC connection, which
   can be done by using the flow identifier described in Section 6.
   Applications multiplexing multiple streams in one connection SHOULD
   implement some form of stream prioritization or bandwidth allocation.

9.  API Considerations

   The mapping described in the previous sections poses some interface
   requirements on the QUIC implementation.  Although a basic mapping
   should work without any of these requirements most of the
   optimizations regarding congestion control and RTCP mapping require
   certain functionalities to be exposed to the application.  The
   following to sections contain a list of information that is required
   by an application to implement different optimizations (Section 9.1)
   and functions that a QUIC implementation SHOULD expose to an
   application (Section 9.2).

   Each item in the following list can be considered individually.  Any
   exposed information or function can be used by RTP over QUIC
   regardless of whether the other items are available.  Thus, RTP over
   QUIC does not depend on the availability of all of the listed
   features but can apply different optimizations depending on the
   functionality exposed by the QUIC implementation.

9.1.  Information to be exported from QUIC

   This section provides a list of items that an application might want
   to export from an underlying QUIC implementation.  It is thus
   RECOMMENDED that a QUIC implementation exports the listed items to
   the application.

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   *  _Maximum Datagram Size_: The maximum datagram size that the QUIC
      connection can transmit.

   *  _Datagram Acknowledgment and Loss_: Section 5.2 of [RFC9221]
      allows QUIC implementations to notify the application that a QUIC
      Datagram was acknowledged or that it believes a datagram was lost.
      The exposed information SHOULD include enough information to allow
      the application to maintain a mapping between the datagram that
      was acknowledged/lost and the RTP packet that was sent in that
      datagram.

   *  _Stream States_: The QUIC implementation SHOULD expose to a
      sender, how much of the data that was sent on a stream was
      successfully delivered and how much data is still outstanding to
      be sent or retransmitted.

   *  _Arrival timestamps_: If the QUIC connection uses a timestamp
      extension like [I-D.draft-smith-quic-receive-ts] or
      [I-D.draft-huitema-quic-ts], the arrival timestamps or one-way
      delays SHOULD be exposed to the application.

   *  _ECN_: If ECN marks are available, they SHOULD be exposed to the
      application.

9.2.  Functions to be exposed by QUIC

   This sections lists functions that a QUIC implementation SHOULD
   expose to an application to implement different features of the
   mapping described in the previous sections of this document.

   *  _Cancel Streams_: To allow an application to cancel
      (re)transmission of packets that are no longer needed, the QUIC
      implementation MUST expose a way to cancel the corresponding QUIC
      streams.

   *  _Select Congestion Controller_: If congestion control is to be
      implemented at the QUIC connection layer as described in
      Section 8.1, the application must be able to choose an appropriate
      congestion control algorithm.

   *  _Disable Congestion Controller_: If congestion control is to be
      implemented at the application layer as described in Section 8.2,
      and the application layer is trusted to apply adequate congestion
      control, it is RECOMMENDEDto allow the application to disable QUIC
      layer congestion control entirely.

10.  Discussion

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10.1.  Flow Identifier

   [RFC9221] suggests to use flow identifiers to multiplex different
   streams on QUIC Datagrams, which is implemented in Section 6, but it
   is unclear how applications can combine RTP over QUIC with other data
   streams using the same QUIC connections.  If the non-RTP data streams
   use the same flow identifies, too and the application can make sure,
   that flow identifiers are unique, there should be no problem.  Flow
   identifiers could be problematic, if different specifications for RTP
   and non-RTP data streams over QUIC mandate different incompatible
   flow identifiers.

10.2.  Impact of Connection Migration

   RTP sessions are characterized by a continuous flow of packets into
   either or both directions.  A connection migration may lead to
   pausing media transmission until reachability of the peer under the
   new address is validated.  This may lead to short breaks in media
   delivery in the order of RTT and, if RTCP is used for RTT
   measurements, may cause spikes in observed delays.  Application layer
   congestion control mechanisms (and also packet repair schemes such as
   retransmissions) need to be prepared to cope with such spikes.

   If a QUIC connection is established via a signaling channel, this
   signaling may have involved Interactive Connectivity Establishment
   (ICE) exchanges to determine and choose suitable (IP address, port
   number) pairs for the QUIC connection.  Subsequent address change
   events may be noticed by QUIC via its connection migration handling
   and/or at the ICE or other application layer, e.g., by noticing
   changing IP addresses at the network interface.  This may imply that
   the two signaling and data "layers" get (temporarily) out of sync.

      *Editor's Note:* It may be desirable that the API provides an
      indication of connection migration event for either case.

10.3.  0-RTT considerations

   For repeated connections between peers, the initiator of a QUIC
   connection can use 0-RTT data for both QUIC streams and datagrams.
   As such packets are subject to replay attacks, applications shall
   carefully specify which data types and operations are allowed.  0-RTT
   data may be beneficial for use with RTP over QUIC to reduce the risk
   of media clipping, e.g., at the beginning of a conversation.

   This specification defines carrying RTP on top of QUIC and thus does
   not finally define what the actual application data are going to be.
   RTP typically carries ephemeral media contents that is rendered and
   possibly recorded but otherwise causes no side effects.  Moreover,

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   the amount of data that can be carried as 0-RTT data is rather
   limited.  But it is the responsibility of the respective application
   to determine if 0-RTT data is permissible.

      *Editor's Note:* Since the QUIC connection will often be created
      in the context of an existing signaling relationship (e.g., using
      WebRTC or SIP), specific 0-RTT keying material could be exchanged
      to prevent replays across sessions.  Within the same connection,
      replayed media packets would be discarded as duplicates by the
      receiver.

11.  Security Considerations

   RTP over QUIC is subject to the security considerations of RTP
   described in Section 9 of [RFC3550] and the security considerations
   of any RTP profile in use.

   The security considerations for the QUIC protocol and datagram
   extension described in Section 21 of [RFC9000], Section 9 of
   [RFC9001], Section 8 of [RFC9002] and Section 6 of [RFC9221] also
   apply to RTP over QUIC.

12.  IANA Considerations

12.1.  Registration of a RTP over QUIC Identification String

   This document creates a new registration for the identification of
   RTP over QUIC in the "TLS Application-Layer Protocol Negotiation
   (ALPN) Protocol IDs" registry [RFC7301].

   The "rtp-mux-quic" string identifies RTP over QUIC:

   Protocol:  RTP over QUIC

   Identification Sequence:  0x72 0x74 0x70 0x2D 0x6F 0x76 0x65 0x72
      0x2D 0x71 0x75 0x69 0x63 ("rtp-mux-quic")

   Specification:  This document

13.  References

13.1.  Normative References

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   [I-D.draft-huitema-quic-ts]
              Huitema, C., "Quic Timestamps For Measuring One-Way
              Delays", Work in Progress, Internet-Draft, draft-huitema-
              quic-ts-07, 6 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-huitema-quic-
              ts-07>.

   [I-D.draft-ietf-quic-ack-frequency]
              Iyengar, J. and I. Swett, "QUIC Acknowledgement
              Frequency", Work in Progress, Internet-Draft, draft-ietf-
              quic-ack-frequency-01, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              ack-frequency-01>.

   [I-D.draft-smith-quic-receive-ts]
              Smith, C. and I. Swett, "QUIC Extension for Reporting
              Packet Receive Timestamps", Work in Progress, Internet-
              Draft, draft-smith-quic-receive-ts-00, 25 October 2021,
              <https://datatracker.ietf.org/doc/html/draft-smith-quic-
              receive-ts-00>.

   [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>.

   [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>.

   [RFC3611]  Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
              "RTP Control Protocol Extended Reports (RTCP XR)",
              RFC 3611, DOI 10.17487/RFC3611, November 2003,
              <https://www.rfc-editor.org/rfc/rfc3611>.

   [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>.

   [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>.

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   [RFC6679]  Westerlund, M., Johansson, I., Perkins, C., O'Hanlon, P.,
              and K. Carlberg, "Explicit Congestion Notification (ECN)
              for RTP over UDP", RFC 6679, DOI 10.17487/RFC6679, August
              2012, <https://www.rfc-editor.org/rfc/rfc6679>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.

   [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>.

   [RFC8298]  Johansson, I. and Z. Sarker, "Self-Clocked Rate Adaptation
              for Multimedia", RFC 8298, DOI 10.17487/RFC8298, December
              2017, <https://www.rfc-editor.org/rfc/rfc8298>.

   [RFC8698]  Zhu, X., Pan, R., Ramalho, M., and S. Mena, "Network-
              Assisted Dynamic Adaptation (NADA): A Unified Congestion
              Control Scheme for Real-Time Media", RFC 8698,
              DOI 10.17487/RFC8698, February 2020,
              <https://www.rfc-editor.org/rfc/rfc8698>.

   [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>.

   [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>.

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   [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>.

13.2.  Informative References

   [I-D.draft-alvestrand-rmcat-remb]
              Alvestrand, H., "RTCP message for Receiver Estimated
              Maximum Bitrate", Work in Progress, Internet-Draft, draft-
              alvestrand-rmcat-remb-03, 21 October 2013,
              <https://datatracker.ietf.org/doc/html/draft-alvestrand-
              rmcat-remb-03>.

   [I-D.draft-dawkins-avtcore-sdp-rtp-quic]
              Dawkins, S., "SDP Offer/Answer for RTP using QUIC as
              Transport", Work in Progress, Internet-Draft, draft-
              dawkins-avtcore-sdp-rtp-quic-00, 28 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-dawkins-
              avtcore-sdp-rtp-quic-00>.

   [I-D.draft-hurst-quic-rtp-tunnelling]
              Hurst, S., "QRT: QUIC RTP Tunnelling", Work in Progress,
              Internet-Draft, draft-hurst-quic-rtp-tunnelling-01, 28
              January 2021, <https://datatracker.ietf.org/doc/html/
              draft-hurst-quic-rtp-tunnelling-01>.

   [I-D.draft-ietf-masque-h3-datagram]
              Schinazi, D. and L. Pardue, "HTTP Datagrams and the
              Capsule Protocol", Work in Progress, Internet-Draft,
              draft-ietf-masque-h3-datagram-11, 17 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-masque-
              h3-datagram-11>.

   [I-D.draft-ietf-quic-applicability]
              Kuehlewind, M. and B. Trammell, "Applicability of the QUIC
              Transport Protocol", Work in Progress, Internet-Draft,
              draft-ietf-quic-applicability-16, 6 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-quic-
              applicability-16>.

   [RFC4571]  Lazzaro, J., "Framing Real-time Transport Protocol (RTP)
              and RTP Control Protocol (RTCP) Packets over Connection-
              Oriented Transport", RFC 4571, DOI 10.17487/RFC4571, July
              2006, <https://www.rfc-editor.org/rfc/rfc4571>.

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   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/rfc/rfc8085>.

Appendix A.  Experimental Results

   An experimental implementation of the mapping described in this
   document can be found on Github (https://github.com/mengelbart/rtp-
   over-quic).  The application implements the RTP over QUIC Datagrams
   mapping and implements SCReAM congestion control at the application
   layer.  It can optionally disable the builtin QUIC congestion control
   (NewReno).  The endpoints only use RTCP for congestion control
   feedback, which can optionally be disabled and replaced by the QUIC
   connection statistics as described in Section 7.1.

   Experimental results of the implementation can be found on Github
   (https://github.com/mengelbart/rtp-over-quic-mininet), too.

Acknowledgments

   The authors would like to thank Spencer Dawkins, Lucas Pardue and
   David Schinazi for their valuable comments and suggestions
   contributing to this document.

Authors' Addresses

   Jörg Ott
   Technical University Munich
   Email: ott@in.tum.de

   Mathis Engelbart
   Technical University Munich
   Email: mathis.engelbart@gmail.com

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