| Internet-Draft | MoQ Use Cases and Requirements | July 2022 |
| Gruessing & Dawkins | Expires 12 January 2023 | [Page] |
This document describes use cases that have been discussed in the IETF community under the banner of "Media Over QUIC", provides analysis about those use cases, recommends a subset of use cases that cover live media ingest, syndication, and streaming for further exploration, and describes requirements that should guide the design of protocols to satisfy these use cases.¶
RFC Editor: please remove this section before publication¶
Source code and issues for this draft can be found at https://github.com/fiestajetsam/draft-gruessing-moq-requirements.¶
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This document describes use cases that have been discussed in the IETF community under the banner of "Media Over QUIC", provides analysis about those use cases, recommends a subset of use cases that cover live media ingest, syndication, and streaming for further exploration, and describes requirements that should guide the design of protocols to satisfy these use cases.¶
Most of the rest of this document provides background for these sections.¶
It is not the purpose of this document to argue against proposals for work on media applications that do not involve QUIC. Such proposals are simply out of scope for this document.¶
When work on the QUIC protocol ([RFC9000]) was chartered ([QUIC-goals]), the key goals for QUIC were:¶
These goals were chosen with HTTP ([I-D.draft-ietf-quic-http]) in mind.¶
While work on "QUIC version 1" (version codepoint 0x00000001) was underway, protocol designers considered potential advantages of the QUIC protocol for other applications. In addition to the key goals for HTTP applications, these advantages were immediately apparent for at least some media applications:¶
The specific advantages of interest may vary from use case to use case, but these advantages justify further investigation of "Media Over QUIC".¶
Protocol developers have been considering the implications of the QUIC protocol ([RFC9000]) for media transport for several years, resulting in a large number of possible meanings of the term "Media Over QUIC", or "MOQ". As of this writing, "Media Over QUIC" has had at least these meanings:¶
There may be IETF participants using other meanings as well.¶
As of this writing, the second bullet ("any kind of media carried indirectly over the QUIC protocol, as an RTP payload"), seems to be in scope for the IETF AVTCORE working group, and was discussed at some length at the February 2022 AVTCORE working group meeting [AVTCORE-2022-02], although no drafts in this space have yet been adopted by the AVTCORE working group.¶
}The "Operational Considerations for Streaming Media" document ([I-D.draft-ietf-mops-streaming-opcons]) described a range of latencies of interest to streaming media providers, as¶
Because the IETF Media Over QUIC community now expresses interest in interactive media Section 4.1 and live media Section 4.3} use cases will have requirements that are significantly less than the "streaming media"-defined "ultra-low latency".¶
Within this document, we are using¶
Perhaps obviously, these last two latency bands are the shortened form of "ultra-low latency - 50 ms" and "ultra-low-latency - 200 ms".¶
Perhaps less obviously, bikeshedding on better names and more useful values is welcomed.¶
Several draft specifications have been proposed which either encapsulate existing Media Transport Protocols in QUIC ([I-D.draft-sharabayko-srt-over-quic]), make use of RTP, RTCP, and SDP ([I-D.draft-engelbart-rtp-over-quic]) or define their own new Media Transport Protocol on top of QUIC. Some have already seen deployment into the wild (e.g. [I-D.draft-kpugin-rush], [I-D.draft-lcurley-warp]) where as others are unconfirmed. Whilst most just focus on defining wire format, [I-D.draft-jennings-moq-quicr-arch] defines an architecture using a pub/sub model for both producers and consumers.¶
Our goal in this section is to understand the range of use cases that have been proposed for "Media Over QUIC".¶
Although some of the use cases described in this section came out of "RTP over QUIC" proposals, they are worth considering in the broader "Media Over QUIC" context, and may be especially relevant to MOQ, depending on whether "RTP over QUIC" requires major changes to RTP and RTCP, in order to meet the requirements arising out of the corresponding use cases.¶
An early draft in the "media over QUIC" space, [I-D.draft-rtpfolks-quic-rtp-over-quic], defined several key use cases. Some of the following use cases have been inspired by that document, and others have come from discussions with the wider MOQ community (among other places, a side meeting at IETF 112).¶
For each use case in this section, we also define¶
It is likely that we should add other characteristics, as we come to understand them.¶
The use cases described in this section have one particular attribute in common - the target latency for these cases are on the order of one or two RTTs. In order to meet those targets, it is not possible to rely on protocol mechanisms that require multiple RTTs to function effectively. For example,¶
This may help to explain why these use cases often rely on protocols such as RTP [RFC3550], which provide low-level control of packetization and transmission.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to One |
| Bi-directional | Yes |
| Latency | Ull-50 |
Where media is received, and user inputs are sent by the client. This may also include the client receiving other types of signalling, such as triggers for haptic feedback. This may also carry media from the client such as microphone audio for in-game chat with other players.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to One |
| Bi-directional | Yes |
| Latency | Ull-50 |
Where media is received, and user inputs are sent by the client. Latency requirements with this usecase are marginally different than the gaming use case. This may also include signalling and/or transmitting of files or devices connected to the user's computer.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | Many to Many |
| Bi-directional | Yes |
| Latency | Ull-50 to Ull-200 |
Where media is both sent and received; This may include audio from both microphone(s) or other inputs, or may include "screen sharing" or inclusion of other content such as slide, document, or video presentation. This may be done as client/server, or peer to peer with a many to many relationship of both senders and receivers. The target for latency may be as large as Ull-200 for some media types such as audio, but other media types in this use case have much more stringent latency targets.¶
For the video conferencing/telephony use case, there can be additional scenarios where the audience greatly outnumbers the concurrent active participants, but any member of the audience could participate. As this has a much larger total number of participants - as many as Live Media Streaming Section 4.3.3, but with the bi-directionality of confercing, this should be considered a "hybrid".¶
The use cases in this section, unlike the use cases described in Section 4.1, still have "humans in the loop", but these humans expect media to be "responsive", where the responsiveness is more on the order of 5 to 10 RTTs. This allows the use of protocol mechanisms that require more than one or two RTTs - as noted in Section 4.1, end-to-end recovery from packet loss and congestion avoidance are two such protocol mechanisms that can be used with Live Media.¶
To illustrate the difference, the responsiveness expected with videoconferencing is much greater than watching a video, even if the video is being produced "live" and sent to a platform for syndication and distribution.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to One |
| Bi-directional | No |
| Latency | Ull-200 to Ultra-Low |
Where media is received from a source for onwards handling into a distribution platform. The media may comprise of multiple audio and/or video sources. Bitrates may either be static or set dynamically by signalling of connection inforation (bandwidth, latency) based on data sent by the receiver.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to One |
| Bi-directional | No |
| Latency | Ull-200 to Ultra-Low |
Where media is sent onwards to another platform for further distribution. The media may be compressed down to a bitrate lower than source, but larger than final distribution output. Streams may be redundant with failover mechanisms in place.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to Many |
| Bi-directional | No |
| Latency | Ull-200 to Ultra-Low |
Where media is received from a live broadcast or stream. This may comprise of multiple audio or video outputs with different codecs or bitrates. This may also include other types of media essence such as subtitles or timing signalling information (e.g. markers to indicate change of behaviour in client such as advertisement breaks). The use of "live rewind" where a window of media behind the live edge can be made available for clients to playback, either because the local player falls behind edge or because the viewer wishes to play back from a point in the past.¶
Finally, the "On-Demand" use cases described in this section do not have a tight linkage between ingest and streaming, allowing significant transcoding, processing, insertion of video clips in a news article, etc. The latency constraints for the use cases in this section may be dominated by the time required for whatever actions are required before media are available for streaming.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to Many |
| Bi-directional | No |
| Latency | On Demand |
Where media is ingested and processed for a system to later serve it to clients as on-demand media. This media provided from a pre-recorded source, or captured from live output, but in either case, this media is not immediately passed to viewers, but is stored for "on-demand" retrieval, and may be transcoded upon ingest.¶
| Attribute | Value |
|---|---|
| Senders/Receivers | One to Many |
| Bi-directional | No |
| Latency | On Demand |
Where media is received from a non-live, typically pre-recorded source. This may feature additional outputs, bitrates, codecs, and media types described in the live media streaming use case.¶
Our proposal is that "Media Over QUIC" discussions focus first on the use cases described in Section 4.3, which are Live Media Ingest (Section 4.3.1), Syndication (Section 4.3.2), and Streaming (Section 4.3.3). Our reasoning for this suggestion follows.¶
Each of the above use cases in Section 4 fit into one of three classifications of solutions.¶
The first group, Interactive Media, as described in Section 4.1, and covering gaming (Section 4.1.1), screen sharing (Section 4.1.2), and general video conferencing (Section 4.1.3), are largely covered by RTP, often in conjunction with WebRTC [WebRTC], and related protocols today.¶
Whilst there may be benefit in these use cases having a QUIC based protocol it may be more appropriate given the size of existing deployments to extend the RTP protocols and specifications.¶
The second group of classifications, in Section 4.3, covering Live Media Ingest (Section 4.3.1), Live Media Syndication (Section 4.3.2), and Live Media Streaming (Section 4.3.3) are likely the use cases that will benefit most from this work.¶
Existing ingest and streaming protocols such as HLS [RFC8216] and DASH [DASH] are reaching limits towards how low they can reduce latency in live streaming and for scenarios where low-bitrate audio streams are used, these protocols add a significant amount of overhead compared to the media bitstream itself.¶
For this reason, we suggest that work on "Media Over QUIC" protocols target these use cases at this time.¶
The third group, Section 4.4, covering On-Demand Media Ingest (Section 4.4.1) and On-Demand Media streaming (Section 4.4.2) is unlikely to benefit from work in this space. Without the same "Live Media" latency requirements that would motivate deployment of new protocols, existing protocols such as HLS and DASH are probably "good enough" to meet the needs of these use cases.¶
This does not mean that existing protocols in this space are perfect. Segmented protocols such as HLS and DASH were developed to overcome the deficiencies of TCP, as used in HTTP/1.1 [RFC7230] and HTTP/2 [RFC7540], and do not make full use of the possible congestion window along the path from sender to receiver. Other protocols in this space have their own deficiencies. For example, RTSP [RFC7826] does not have easy ways to add support for new media codecs.¶
Our expectation is that these use cases will not drive work in the "Media Over QUIC" space, but as new protocols come into being, they may very well be taken up for these use cases as well.¶
TODO: Quite a lot, really ...¶
This document makes no requests of IANA.¶
As this document is intended to guide discussion and consensus, it introduces no security considerations of its own.¶
The authors would like to thank the many authors of the specifications referenced in Section 3 for their work. The authors would also like to thank Alan Frindell, Luke Curley, and Maxim Sharabayko for text contributions to this draft.¶
James Gruessing would also like to thank Francesco Illy and Nicholas Book for their part in providing the needed motivation.¶