| Internet-Draft | RTP over QUIC | March 2022 |
| Ott & Engelbart | Expires 8 September 2022 | [Page] |
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.¶
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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 and, most notably, its unreliable DATAGRAM extension, 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 briefly touches upon how to signal media over QUIC using the Session Description Protocol (SDP) [RFC8866].¶
The scope of this document is limited to unicast RTP/RTCP.¶
Note that this draft is similar in spirit to but differs in numerous ways from [draft-hurst-quic-rtp-tunnelling-01].¶
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:¶
QUIC specifies a congestion controller in Section 7 of [RFC9002], but the specific requirements for interactive real-time media lead to the development of dedicated congestion control algorithms. In this document, the term congestion controller refers to these algorithms dedicated to real-time applications.¶
Datagrams exist in UDP as well as in QUICs unreliable datagram extension. If not explicitly noted differently, the term datagram in this document refers to a QUIC Datagram as defined in [draft-ietf-quic-datagram-10].¶
A QUIC server or client that participates in an RTP over QUIC session.¶
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.¶
An endpoint that receives media in RTP packets and may send or receive RTCP packets.¶
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.¶
This document introduces a mapping of the Real-time Transport Protocol (RTP) to the QUIC transport protocol. QUIC supports two transport methods: reliable streams and unreliable datagrams [RFC9000], [draft-ietf-quic-datagram-10]. RTP over QUIC uses unreliable QUIC datagrams to transport real-time data, and thus, the QUIC implementation MUST support QUICs unreliable datagram extension. 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 [draft-schinazi-quic-h3-datagram-05]. 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.¶
All RTP and RTCP packets MUST be sent in QUIC datagram frames with the following format:¶
Datagram Payload {
Flow Identifier (i),
RTP/RTCP Packet (..)
}
Flow identifier to demultiplex different data flows on the same QUIC connection.¶
The RTP/RTCP packet to transmit.¶
For multiplexing RTP sessions on the same QUIC connection, each RTP/RTCP packet is prefixed with a flow identifier. This flow identifier serves as a replacement for using different transport addresses per session. 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, e.g., when using SDP as discussed in Section 6.¶
RTP and RTCP packets of different RTP sessions MUST be sent using different flow identifiers.¶
Differentiating RTP/RTCP datagrams 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.¶
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.¶
RTP over QUIC needs to employ congestion control to avoid overloading the
network. RTP and QUIC both offer different congestion control mechanisms. QUIC
specifies a congestion control algorithm similar to TCP NewReno, but allows
senders to choose a different algorithm, as long as the algorithm conforms to
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. [RFC8298] and [RFC8698]). The congestion control
algorithms for RTP are specifically tailored for real-time transmissions at low
latencies. RTP congestion control mostly works delay-based, using the growing
one-way delay as a congestion signal. The available congestion control
algorithms for RTP also expose a target_bitrate that can be used to
dynamically reconfigure media encoders to produce media at a rate that can be
sent in real-time under the given network conditions.¶
This section defines two options for congestion control for RTP over QUIC, but
it does not mandate which congestion control algorithms to use. The congestion
control algorithm MUST expose a target_bitrate to which the encoder should be
configured to fully utilize the available bandwidth. Furthermore, it is assumed
that the congestion controller 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 a pacing mechanism. The use
of congestion controllers which don't provide a pacing mechanism is out of scope
of this document.¶
Additionally, the section defines how the connection statistics obtained from QUIC can be used to reduce RTCP feedback overhead.¶
Since QUIC provides generic congestion signals which allow the implementation of different congestion control algorithms, senders are not dependent on RTCP feedback for congestion control. However, there are some restrictions, and the QUIC implementation MUST fulfill some requirements to use these signals for congestion control instead of RTCP feedback.¶
To estimate the currently available bandwidth, real-time congestion control algorithms keep track of the sent packets and typically require a list of successfully delivered packets together with the timestamps at which they were received by a receiver. The bandwidth estimation can then be used to decide whether the media encoder can be configured to produce output at a higher or lower rate.¶
A congestion controller used for RTP over QUIC should be able to compute an adequate bandwidth estimation using the following inputs:¶
t_current: A current timestamp¶
pkt_departure: The departure time for each RTP packet sent to the receiver.¶
pkt_arrival: The arrival time for each RTP packet that was successfully
delivered to the receiver.¶
The RTT estimations calculated by QUIC as described in Section 5 of [RFC9002]:¶
ecn: Optionally ECN marks may be used, if supported by the network and
exposed by the QUIC implementation.¶
The only value of these inputs not currently available in QUIC is the
pkt_arrival. The exact arrival times of QUIC Datagrams can be obtained by
using the QUIC extension described in [draft-smith-quic-receive-ts-00] or
[draft-huitema-quic-ts-05].¶
QUIC allows acknowledgments to be sent with some delay, which could cause problems for delay-based congestion control algorithms. Sender and receiver can use [draft-ietf-quic-ack-frequency-01] to avoid feedback inaccuracies caused by delayed acknowledgments.¶
If the QUIC extensions described in [draft-smith-quic-receive-ts-00]/[draft-huitema-quic-ts-05] and [draft-ietf-quic-ack-frequency-01] are not supported by sender and receiver, it is RECOMMENDED to use RTCP feedback reports instead of thee QUIC connection statistics for congestion control.¶
The first option implements congestion control at the QUIC layer by replacing the standard QUIC congestion control with one of the congestion control algorithms for RTP.¶
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?¶
Editor's note: If this option is chosen, but the required QUIC statistics/extensions are not available and the sender has to use RTCP feedback for congestion control, the feedback needs to be fed back to the QUIC implementation.¶
The second option implements real-time congestion control at the application layer. This gives an application more control over the congestion controller and the congestion control feedback to use. It is RECOMMENDED to disable QUIC's congestion control when this option is used to avoid interferences between the congestion controllers at different layers.¶
Editor's note: Maybe this option should be removed, as it has some issues: 1. It cannot be used in situations where the application is untrusted, such as in Webtransport, where the browser implements QUIC but cannot trust a JS application using it to do the right thing. 2. It is unclear how non-real-time data sharing the same connection can be congestion controlled. 3. If QUIC connection statistics should be used instead of RTCP, these have to be exposed to the application.¶
When the QUIC connection is shared between multiple data streams, a share of the available bandwidth should be allocated to each stream. An implementation MUST ensure that a real-time flow is always allowed to send data unless it has exhausted its allocated bandwidth share. This is especially important when the connection is shared with non-real-time flows.¶
Editor's note: This section may need to explain the problem that occurs when non-real-time data fills up the congestion window when a real-time flow does not fully use its assigned bandwidth share.¶
Independent from which option is chosen to implement congestion control, the
sender likely needs to reconfigure the media encoder in reaction to changing
network conditions. Common real-time congestion control algorithms expose a
target_bitrate for this purpose. An RTP over QUIC implementation can either
expose the most recent target_bitrate produced by the congestion controller to
the application or accept a callback from the application, which updates the
encoder bitrate whenever the congestion controller updates the target_bitrate.¶
Editor's note: See also [draft-dawkins-avtcore-sdp-rtp-quic].¶
QUIC is a connection-based protocol that supports connectionless transmissions of DATAGRAM frames within an established connection. As noted above, demultiplexing DATAGRAMS intended for different purposes is up to the application using QUIC.¶
There are several necessary steps to carry out jointly between the communicating peers to enable RTP over QUIC:¶
The peers must provide a means for identifying RTP sessions carried in QUIC DATAGRAMS. To enable using a common transport connection for one, two, or more media sessions in the first place, the BUNDLE grouping framework MUST be used [RFC8843]. All media sections belonging to a bundle group, except the first one, MUST set the port in the m= line to zero and MUST include the a=bundle-only attribute.¶
For disambiguating different RTP session, a reference needs to be provided for each m= line to allow associating this specific media session with a flow identifier. This could be achieved following different approaches:¶
Editor's note: It is likely preferable to use multiplexing using QUIC DATAGRAM flow identifiers because this multiplexing mechanisms will also work across RTP and non-RTP media streams.¶
In either case, the corresponding identifiers MUST be treated independently for each direction of transmission, so that an endpoint MAY choose its own identifies and only uses SDP to inform its peer which RTP sessions use which identifiers.¶
To this end, SDP MUST be used to indicate the respective flow identifiers for RTP and RTCP of the different RTP sessions (for which we borrow inspiration from [RFC3605]).¶
A sample session setup offer (liberally borrowed and extended from [RFC8843] and [RFC8122] could look as follows:¶
v=0 o=alice 2890844526 2890844526 IN IP6 2001:db8::3 s= c=IN IP6 2001:db8::3 t=0 0 a=group:BUNDLE abc xyz m=audio 10000 QUIC/RTP/AVP 0 8 97 a=setup:actpass a=connection:new a=fingerprint:SHA-256 \ 12:DF:3E:5D:49:6B:19:E5:7C:AB:4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF: \ 3E:5D:49:6B:19:E5:7C:AB:4A:AD b=AS:200 a=mid:abc a=rtcp-mux a=rtpmap:0 PCMU/8000 a=rtpmap:8 PCMA/8000 a=rtpmap:97 iLBC/8000 a=extmap:1 urn:ietf:params:<tbd> m=video 0 QUIC/RTP/AVP 31 32 b=AS:1000 a=bundle-only a=mid:bar a=rtcp-mux a=rtpmap:31 H261/90000 a=rtpmap:32 MPV/90000 a=extmap:2 urn:ietf:params:<tbd>
Signaling details to be worked out.¶
Any RTP packet can be sent over QUIC and no RTCP packets are used by default. Since QUIC already includes some features which are usually implemented by certain RTCP messages, RTP over QUIC implementations should not need to implement the following RTCP messages:¶
This document has no IANA actions.¶
TODO acknowledge.¶