Media over QUIC - Transfork

Internet-Draft	moqtf	September 2024
Curley	Expires 27 March 2025	[Page]

Abstract

MoqTransfork is designed to serve a broadcast to an unbounded number of viewers with different latency and quality targets: the entire spectrum between real-time and VOD. MoqTransfork itself is a media agnostic transport, allowing relays and CDNs to forward the most important content under degraded networks without knowledge of codecs, containers, or even if the content is fully encrypted. Higher level protocols specify how to use MoqTransfork to encode and deliver video, audio, messages, or any form of live content.¶

3. Concepts

MoqTransfork consists of:¶

Session: An established connection between a client and server used to transmit any number of Broadcasts.¶
Broadcast: A collection of Tracks from a single producer (client). This is primarily used for routing, but Tracks within a Broadcast may be correlated.¶
Track: An append-only series of Groups, each of which can be delivered and decoded independently.¶
Group: An append-only series of Frames, each of which are delivered and decoded in order¶
Frame: A sized payload of bytes, intended to represent a single moment in time.¶

The application determines how to split data into broadcasts, tracks, groups, and frames. MoqTransfork only is responsible for the networking and deduplication by utilizing rules encoded in headers. This provides robust and generic one-to-many transmission, even for latency sensitive applications.¶

3.1. Session

A Session consists of a connection between a QUIC client and server.¶

A session is established after the necessary QUIC, WebTransport, and MoqTransfork handshakes. The MoqTransfork handshake consists of version information and an optional ROLE extension, signaling if the endpoint supports only supports publishing or subscribing.¶

The intent is that sessions are chained together via relays. A broadcaster could establish a session with an CDN ingest edge while the viewers establish separate sessions to CDN distribution edges. A MoqTransfork session is hop-by-hop, but the application should be designed end-to-end.¶

3.2. Broadcast

A Broadcast is a collection of tracks from a single producer, identified by a unique name within the session. A MoqTransfork session may be used to publish and subscribe to multiple, potentially unrelated, broadcasts.¶

The application determines if tracks within a broadcast are correlated. For example, a "video" track and "audio" track could share timestamp domains. This is possible as a broadcast is created by a single publisher, avoiding the need for clock synchronization.¶

A publisher can advertise available broadcasts via an ANNOUNCE message. This allows a subscriber to dynamically discover available broadcasts. Alternatively, the application can discover broadcasts via an out-of-band mechanism.¶

3.3. Track

A Track is a series of Groups within a Broadcast, identified by a unique name within the Broadcast.¶

Each subscription is scoped to a single Track. A subscription will always start at a Group boundary, either the latest group or a specified sequence number. A subscriber chooses the ordering and priority of each subscription, hinting to the publisher which Track should arrive first during congestion. This is critical for a decent user experience during network degradation and the core reason why QUIC can offer real-time latency.¶

There is currently no way to discover tracks within a broadcast; it must be negotiated out-of-band. This is often done with a named "catalog" track that lists all available tracks and their properties. An application may choose to use static and/or dynamic track names.¶

3.4. Group

A Group is an ordered stream of Frames within a Track.¶

A group is served by a dedicated QUIC stream which may be closed on completion, reset by the publisher, or cancelled by the subscriber. An active subscription involves delivering multiple potential concurrent groups. The Frames within a Group will arrive reliably and in order thanks to the QUIC stream. However, Groups within a Track can arrive in any order or not at all, and which the application should be prepared to handle.¶

A subscriber may FETCH a specific group starting at a given byte offset. This is similar to a HTTP request and may be used to recover from partial failures among other things.¶

3.5. Frame

A Frame is a payload of bytes within a Group.¶

A frame is used to represent a chunk of data with a known size. A frame should represent a single moment in time and avoid any buffering that would increase latency.¶

3.6. Liveliness

A media protocol can only be considered "live" if it can handle degraded network congestion. MoqTransfork handles this by prioritizing the most important media while the remainder is starved.¶

The importance of each broadcast/track/group/frame is signaled by the subscriber and the publisher will attempt to obey it. This is done via the Track Priority and the Group Order. Any data that is excessively starved may be dropped (by either endpoint) rather than block the live stream.¶

A publisher that serves multiple sessions, commonly a relay, should prioritize on a per-session basis. Alice may want real-time latency with a preference for audio, while Bob may want reliable playback while audio is muted. A relay MAY forward subscriber preferences upstream, but when there is a conflict (like the above example), the publisher's preference should be used as a tiebreaker.¶

4. Workflow

This section outlines the flow of messages within a MoqTransfork session. See the section for Messages section for the specific encoding.¶

4.1. Connection

MoqTransfork runs on top of WebTransport. WebTransport is a layer on top of QUIC and HTTP/3, required for web support. The API is nearly identical to QUIC, however notably lacks stream IDs and has fewer available error codes.¶

How the WebTransport connection is established is out-of-scope for this draft. For example, a service MAY use the WebTransport handshake to perform authentication via the URL.¶

4.2. Termination

QUIC bidirectional streams have an independent send and receive direction. Rather than deal with half-open states, MoqTransfork combines both sides. If an endpoint closes the send direction of a stream, the peer MUST also close the send direction.¶

MoqTransfork contains many long-lived transactions, such as subscriptions and announcements. These are terminated when the underlying QUIC stream is terminated.¶

To terminate a stream, an endpoint may: - close the send direction (STREAM with FIN) to gracefully terminate (all messages are flushed). - reset the send direction (RESET_STREAM) to immediately terminate.¶

After resetting the send direction, an endpoint MAY close the recv direction (STOP_SENDING). However, it is ultimately the other peer's responsibility to close their send direction.¶

4.3. Handshake

After a connection is established, the client opens a Session Stream and sends a SESSION_CLIENT message, to which the server replies with a SESSION_SERVER message. The session is active until either endpoint closes or resets the Session Stream.¶

This session handshake is used to negotiate the MoqTransfork version and any extensions. See the Extension section for more information.¶

4.4. Bidirectional Streams

Bidirectional streams are primarily used for control streams.¶

The first byte of each stream indicates the Stream Type. A Stream Type is created by an endpoint with the associated Role and MUST be enforced when the ROLE extension is negotiated.¶

Table 1
ID	Type	Role
0x0	Session	Client
0x1	Announce	Publisher
0x2	Subscribe	Subscriber
0x3	Fetch	Subscriber
0x4	Info	Subscriber

4.4.1. Session

The Session stream contains all messages that are session level.¶

The client MUST open a single Session Stream immediately after establishing the QUIC/WebTransport session. The client sends a SESSION_CLIENT message and the server replies with a SESSION_SERVER message.¶

Afterwards, both endpoints SHOULD send SESSION_UPDATE messages, such as after a significant change in the session bitrate.¶

The session remains active until the Session Stream is closed or reset by either endpoint.¶

4.4.2. Announce

A publisher can open an Announce Stream to advertise a broadcast. This is OPTIONAL and the application can determine the broadcast name out-of-band.¶

The publisher MUST start the stream with an ANNOUNCE message. The subscriber MUST reply with an ANNOUNCE_OK message or reset the stream. The announcement is active until the stream is closed or reset by either endpoint.¶

There is currently no expectation that a relay will forward an ANNOUNCE message downstream. A future draft may introduce a mechanism to discover broadcasts matching a prefix.¶

4.4.4. Fetch

A subscriber can open a Fetch Stream to receive a single Group at a specified offset. This is primarily used to recover from an abrupt stream termination, causing the truncation of a Group.¶

The subscriber MUST start a Fetch Stream with a FETCH message followed by any number of FETCH_UPDATE messages. The publisher MUST reply with the contents of a Group Stream, except starting at the specified offset after the GROUP message. Note that this includes any FRAME messages.¶

The fetch is active until both endpoints close the stream, or either endpoint resets the stream.¶

4.4.5. Info

A subscriber can open an Info Stream to request information about a track. This is not often necessary as SUBSCRIBE will trigger an INFO reply.¶

The subscriber MUST start the stream with a INFO_REQUEST message. The publisher MUST reply with an INFO message or reset the stream. Both endpoints MUST close the stream afterwards.¶

4.5. Unidirectional

Unidirectional streams are used for subscription data.¶

Table 2
ID	Stream	Role
0x0	Group	Publisher

4.5.1. Group

A publisher creates Group Streams in response to a Subscribe Stream.¶

A Group Stream MUST start with a GROUP message and MAY be followed by any number of FRAME messages. An application MAY use an empty GROUP and/or FRAME to signal gaps.¶

Both the publisher and subscriber MAY reset the stream at any time. When a Group stream is reset, the publisher MUST send a GROUP_DROP message on the corresponding Subscribe stream. A future version of this draft may utilize reliable reset instead.¶

5. Encoding

This section covers the encoding of each message.¶

Note that these message do not contain a type identifier. The message type is determined by the stream type and the current state.¶

5.1. SESSION_CLIENT

The client advertises supported versions and any extensions.¶

SESSION_CLIENT Message {
  Supported Versions Count (i)
  Supported Version (i)
  Extension Count (i)
  [
    Extension ID (i)
    Extension Payload (b)
  ]...
}

5.2. SESSION_SERVER

The server responds with the selected version and any extensions.¶

SESSION_SERVER Message {
  Selected Version (i)
  Extension Count (i)
  [
    Extension ID (i)
    Extension Payload (b)
  ]...
}

5.3. SESSION_UPDATE

SESSION_UPDATE Message {
  Session Bitrate (i)
}

Session Bitrate: The estimated bitrate of the QUIC connection in bits per second. This SHOULD be sourced directly from the QUIC congestion controller. A value of 0 indicates that this information is not available.¶

5.4. ANNOUNCE

A publisher sends an ANNOUNCE message to advertise a broadcast.¶

ANNOUNCE Message {
  Broadcast Name (b),
}

Broadcast Name: The name of the broadcast. A zero-sized name is valid but not recommended.¶

5.5. ANNOUNCE_OK

A subscriber replies to an ANNOUNCE with an ANNOUNCE_OK message.¶

ANNOUNCE_OK Message {
  Cool = 0x1
}

This message is a single byte long (😎).¶

5.7. SUBSCRIBE_UPDATE

A subscriber can modify a subscription with a SUBSCRIBE_UPDATE message.¶

SUBSCRIBE_UPDATE Message {
  Track Priority (i)
  Group Order (i)
  Group Expires (i)
  Group Min (i)
  Group Max (i)
}

Track Priority: The new track priority; see SUBSCRIBE. The publisher SHOULD use the new priority for any blocked streams.¶

Group Order: The new group order; see SUBSCRIBE. The publisher SHOULD use the new order for any blocked streams.¶

Group Min: The new minimum group sequence, or 0 if there is no update. This value MUST NOT be smaller than prior SUBSCRIBE and SUBSCRIBE_UPDATE messages.¶

Group Max: The new maximum group sequence, or 0 if there is no update. This value MUST NOT be larger than prior SUBSCRIBE or SUBSCRIBE_UPDATE messages.¶

If the Min and Max are updated, the publisher SHOULD reset any blocked streams that are outside the new range.¶

5.8. INFO

The INFO message contains the current information about a track.¶

INFO Message {
  Track Priority (i)
  Group Latest (i)
  Group Order (i)
  Group Expires (i)
}

Track Priority: The priority of this track within the broadcast. Note that this is slightly different than SUBSCRIBE, which is scoped to a session not broadcast. The publisher SHOULD transmit subscriptions with higher values first during congestion.¶

Group Latest: The latest group as currently known by the publisher. A relay without an active subscription SHOULD forward this request upstream¶

Group Order: The publisher's intended order of the groups within the subscription: none (0), ascending (1), or descending (2).¶

Group Expires: A duration in milliseconds. The group SHOULD be dropped if this duration has elapsed after group has finished, including any time spent cached. The Subscriber's Group Expires value SHOULD be used instead when smaller.¶

5.9. INFO_REQUEST

The INFO_REQUEST message is used to request an INFO response.¶

INFO_REQUEST Message {
  Broadcast Name (b)
  Track Name (b)
}

5.10. FETCH

A subscriber can request a byte offset within a Group with a FETCH message.¶

FETCH Message {
  Broadcast Name (b)
  Track Name (b)
  Track Priority (i)
  Group Sequence (i)
  Group Offset (i)
}

Track Priority: The priority of the group relative to all other FETCH and SUBSCRIBE requests within the session. The publisher should transmit higher values first during congestion.¶

Group Offset: The requested offset in bytes after the GROUP message.¶

5.11. FETCH_UPDATE

A subscriber can modify a FETCH request with a FETCH_UPDATE message.¶

FETCH_UPDATE Message {
  Track Priority (i)
}

Track Priority: The priority of the group relative to all other FETCH and SUBSCRIBE requests within the session. The publisher should transmit higher values first during congestion.¶

5.12. GROUP

The GROUP message contains information about a Group, as well as a reference to the subscription being served.¶

GROUP Message {
  Subscribe ID (i)
  Group Sequence (i)
}

Subscribe ID: The corresponding Subscribe ID. This ID is used to distinguish between multiple subscriptions for the same track.¶

Group Sequence: The sequence number of the group.¶

5.13. GROUP_DROP

A publisher transmits a GROUP_DROP message when it is unable to serve a group for a SUBSCRIBE.¶

GROUP_DROP {
  Group Start (i)
  Group Count (i)
  Group Error Code (i)
}

Group Start: The sequence number for the first group within the dropped range.¶

Group Count: The number of additional groups after the first. This value is 0 when only one group is dropped.¶

Error Code: An error code indicated by the application.¶

5.14. FRAME

The FRAME message consists of a length followed by that many bytes.¶

FRAME Message {
  Payload (b)
}

Payload: An application specific payload. A generic library or relay MUST NOT inspect or modify the contents unless otherwise negotiated.¶

Table 3
ID	Extension
0x0	Role

Table 4
ID	Role
0x0	Publisher
0x1	Subscriber
0x2	Both
0x3	Any

8. Appendix: Media Use-Cases

These are some recommended ways to use MoqTransfork for media delivery.¶

8.1. Video

Video encoding involves complex dependencies between frames/slices. The terminology in this section stems from H.264 but is applicable to most modern codecs.¶

Each frame of video is encoded as one or more slices but to simplify the discussion, we'll refer to a slice as a frame. There are three types of frames:¶

I-Frame: A frame that can be decoded independently.¶
P-Frame: A frame that depends on previous frames.¶
B-Frame: A frame that depends on previous or future frames.¶

8.1.1. Group of Pictures

A simple application can ignore the complexity of P/B frames and focus on I-Frames. This is the optimal approach for many encoding configurations.¶

Each I-Frame begins a Group of Pictures (GoP). A GoP is a set of frames that MAY depend on each other and MUST NOT depend on other GoPs. Each frame has a decode order (DTS) and a frame MUST NOT depend on frames with a higher DTS.¶

This perfectly maps to a QUIC stream, as they too are independent and ordered. The easiest way to use MoqTransfork is to send each GoP as a GROUP with each frame as a FRAME, hence the names.¶

Each SUBSCRIBE starts at a Group to ensure that it starts with an I-Frame. Each Group is delivered in decode order ensuring that all frames are decodable (no artifacts).¶

A subscriber can choose the Group Order based on the desired user experience:¶

SUBSCRIBE order=DESC: Transmits new Groups first to allow skipping, intended for low-latency live streams.¶
SUBSCRIBE order=ASC: Transmits old Groups first to avoid skipping, intended for VOD and reliable live streams.¶

A publisher or subscriber can skip the remainder of a Group by resetting a Group Stream or by issuing a SUBSCRIBE_UPDATE. A FETCH can be used to recover any partial groups.¶

8.1.2. Layers

An advanced application can subdivide a GoP into layers.¶

The most comprehensive way to do this is with Scalable Video Coding (SVC). There is a base layer and one or more enhancement layers that depend on lower layers. For example, a 4K stream could be broken into 4K, 1080p, and 360p (base) layers. However, SVC has limited support and is complex to encode.¶

Another approach is to use temporal scalability via something like B-pyramids. The frames within a GoP are sub-divided based on their dependencies, intentionally creating a hierarchy. For example, even frames could be prevented from referencing odd frames, creating a base 30fps layer and an enhancement 60fps layer. This is effectively a custom SVC scheme however it's limited to time and doesn't require special decoder support.¶

The purpose of these layers is to support degrading the quality of the broadcast. A subscriber could limit bandwidth usage by choose to only receive the base layer or a subset of the enhancements layers. During congestion, the base layer can be prioritized while the enhancement layers can be deprioritized or dropped. However, the cost is a small increase in bitrate (10%) as limiting potential references can only hurt the compression ratio.¶

When using MoqTransfork, each layer is delivered as a separate Track. This allows the subscriber to choose which layers to receive and how to prioritize them in SUBSCRIBE. It also enables layers to be prioritized within the layer application, for example Alice's base layer is more important than Bob's enhancement layer.¶

The application is responsible for determining the relationship between layers, since they're unrelated tracks as MoqTransport is concerned. The application could use a catalog to advertise the layers and how to synchronize them, for example based on the Group Sequence.¶

8.1.3. Non-Reference Frames

While not explicitly stated, I believe the complexity in MoqTransport stems almost entirely from a single use-case: the ability to drop individual non-reference frames in the middle of a group.¶

In theory, transmitting enhancement layers as tracks like mentioned above could introduce head-of-line blocking depending on the encoding. This would occur when enhancement layers are not self-referential, a rare configuration which also hurts the compression ratio. And in practice, there's no discernable user impact given the disproportionate size difference between base and enhancement layers.¶

The ability to drop individual non-reference frames in the middle of a group is an explicit non-goal for MoqTransfork.¶

8.2. Audio

Unlike video, audio is simple and yet has perhaps more potential for optimization.¶

8.2.1. Frames

Audio samples are very small and for the sake of compression, are grouped into a frame. This depends on the codec and the sample rate but each frame is typically 10-50ms of audio.¶

Audio frames are independent, which means they map nicely to MoqTransfork Groups. Each audio frame can be transmitted as a GROUP with a single FRAME.¶

8.2.2. Groups

Audio FRAMEs can also be combined into periodic GROUPs to reduce overhead at the cost of some introducing head-of-line blocking. This won't increase latency except under significant congestion as each FRAME is still streamed.¶

For example, an application could then subscribe to video and audio starting at group X for both tracks, instead of trying to maintain a mapping between the two based on timestamp. This is quite common in HLS/DASH as there's no reason to subdivide audio segments at frame boundaries.¶

8.2.3. FEC

Real-time audio applications often use Forward Error Correction (FEC) to conceal packet loss. Audio frames are a good candidate for FEC given that they are small and independent.¶

In an ideal world, FEC would be performed by QUIC based on the properties of the hop. However this is not currently not supported and FEC is left to the application.¶

In MoqTransfork, each FEC packet is transmitted as a separate GROUP with a single FRAME. A real-time subscriber issues a SUBSCRIBE with an aggressive Group Expires value in the milliseconds range. The publisher will drop any Groups that have not been transmitted or acknowledged within this time frame, potentially causing them to be lost.¶

Normally, FEC is performed by transmitting individual packets once as datagrams. However, QUIC streams are useful as they allow retransmissions when Group Expires is smaller than the RTT. If the RTT is too high, then the RESET_STREAM frame adds some overhead but it's inconsequential (~10 more bytes). This enables retransmitting lost packets on short hops and otherwise relying on FEC for long hops.¶

8.3. Metadata

There's a number of non-media use cases that can be served by MoqTransfork.¶

8.3.1. Catalog

Originally part of the transport itself, the catalog is a list of all tracks within a broadcast. It's since been delegated to the application and is now just another track with a well-known name.¶

The proposed MoQ catalog format supports live updates. It does this by encoding a base JSON blob and then applying JSON patches over time. If the number of deltas becomes too large, the producer can start over with a new base JSON blob.¶

In MoqTransfork, the base and all deltas are a single GROUP. The base is the first FRAME and all deltas are subsequent FRAMEs. The producer can create a new GROUP to start over, repeating the process.¶

8.3.2. Timeline

Another track that is commonly pitched is a timeline track. This records the presentation timestamp of each Group, giving a VOD viewer to seek to a specific time.¶

The timeline track is a single Group containing a Frame for each timestamp. The live nature of the timeline track is great for DVR applications while being concise enough for VOD. Timed metadata would use a similar approach or perhaps leverage this track.¶

8.3.3. Interaction

Another common use-case is to transmit user interactions, such as controller inputs or chat messages. It's up to the application to determine the format and encoding of these messages.¶

Let's take controller input as an example. The application needs to determine its loss vs latency tolerance, as reordering or dropping inputs will lead to a poor user experience.¶

If you don't want loss, then use a single GROUP with a FRAME per input.¶
If you don't want latency, then use a GROUP per input with a single FRAME.¶
If you want a hybrid, then use form of clustering inputs into GROUPs and FRAMEs based on time.¶

A publisher could monitor the session RTT or stream acknowledgements to get a sense of the latency and create Groups accordingly. However, this only applies to the first hop and won't be applicable when relays are involved.¶

8.4. Latency

One explicit goal of MoqTransfork is to support multiple latency targets.¶

This is accomplished by using the same Tracks and Group for all viewers, but slightly changing the behavior based on the subscription. This is driven by the subscriber, allowing them to choose the trade-off between latency and reliability. This may be done on the fly via SUBSCRIBE_UPDATE, for example if a high-latency viewer wishes to join the stage and suddenly needs real-time latency.¶

The below examples assume one audio and one video track. See the next section for more complicated broadcasts.¶

8.4.1. Real-Time

Real-time latency is accomplished by prioritizing the most important media during congestion and skipping the rest.¶

This is slightly different from other media protocols which instead opt to drop packets. The end result is similar, but prioritization means utilizing all available bandwidth as determined by the congestion controller. A subscriber or publisher can reset groups to avoid wasting bandwidth on old data.¶

A real-time viewer could issue:¶

SUBSCRIBE track=audio priority=1 order=DESC group_expires=100ms
SUBSCRIBE track=video priority=0 order=DESC group_expires=100ms

In this example, audio is higher priority than video, and newer groups are higher priority than older groups. Suppose a viewer fell behind after a burst of congestion and has to decide which groups to deliver next. This configuration would result in the transmission order:¶

GROUP track=audio sequence=102
GROUP track=audio sequence=101
GROUP track=audio sequence=100
GROUP track=video sequence=5
GROUP track=video sequence=4

The user experience depends on the amount of congestion:¶

If there's no congestion, all audio and video is delivered.¶
If there's moderate congestion, the tail of the old video group is dropped.¶
If there's severe congestion, all video will be late/dropped and some audio groups/frames will be dropped.¶

The value of group_expires is optional. In this example it means that the publisher automatically resets each group 100ms after they are no longer the latest. It's recommended to use the maximum jitter buffer size.¶

8.4.2. Unreliable Live

Unreliable live is a term I made up. Basically we want low latency, but we don't need it at all costs and we're willing to skip some video to achieve it. This is useful for broadcasts where latency is important but so is picture quality.¶

An unreliable live viewer could issue:¶

SUBSCRIBE track=audio priority=1 order=ASC
SUBSCRIBE track=video priority=0 order=DESC group_expires=3s

This example is different from the real-time one in that audio is fully reliable and delivered in order. Of course this is optional and up to the application, as it will result in buffering during significant congestion. If the viewer goes through a tunnel and then comes back online, they won't miss any audio.¶

A key difference is that our jitter buffer is much larger for video, 3s in this example. The player will tolerate up to 3s of latency before it starts skipping past video frames. Note that the group_expires value can be increased during buffering by issuing a SUBSCRIBE_UPDATE.¶

8.4.3. Reliable Live

Reliable live is another term I made up. This is when we have a live stream but primarily care about picture quality. A good example is a sports game where you want to see every frame.¶

A reliable live viewer could issue:¶

SUBSCRIBE track=audio priority=0 order=ASC
SUBSCRIBE track=video priority=0 order=ASC

This will deliver both audio and video in order, and with the same priority. The viewer won't miss any content unless the publisher resets a group. However, this can result in buffering during congestion and provides a similar user experience to HLS/DASH.¶

8.4.4. VOD / DVR

Video on Demand (VOD) and Digital Video Recorder (DVR) both involve seeking backwards in a live stream. MoqTransfork can serve this use-case too, don't worry.¶

A VOD viewer could issue:¶

SUBSCRIBE track=audio priority=0 order=ASC start=345 end=396
SUBSCRIBE track=video priority=0 order=ASC start=123 end=134

The application is responsible for determining the group sequence numbers based on the desired timestamp. This could be done via a timeline track or out-of-band.¶

A subscriber will need a specific end or else it will download too much data at once, as old media is transmitted at network speed and not encode speed. It will need to issue an updated SUBSCRIBE to expand the range as playback continues and the buffer depletes. A subscriber could use SUBSCRIBE_UPDATE, however there are race conditions involved.¶

A DVR player does the same thing but can automatically support joining the live stream. It's perfectly valid to specify a end in the future and it will behave like reliable live viewer once it reaches the live playhead.¶

Alternatively, a DVR player could prefetch the live playhead by issuing a parallel SUBSCRIBE at a lower priority. This would allow playback to immediately continue after clicking the "Go Live" button, canceling or deprioritizing the VOD subscription.¶

SUBSCRIBE track=video priority=1 order=ASC start=123 end=134
SUBSCRIBE track=video priority=0 order=DESC

8.4.5. Upstream

All of these separate viewers could be watching the same broadcast. How is a relay supposed to fetch the content from upstream?¶

MoqTransfork addresses this by providing the publisher's Track Priority and Group Order in the INFO message. This is the intended behavior for the first hop and dictates which viewers are preferred.¶

For example, suppose the producer chooses:¶

INFO track=audio priority=1 order=DESC
INFO track=video priority=0 order=DESC

If Alice is watching a VOD and issues:¶

SUBSCRIBE track=audio priority=0 order=ASC
SUBSCRIBE track=video priority=0 order=ASC

If Bob is watching real-time and issues:¶

SUBSCRIBE track=audio priority=1 order=DESC
SUBSCRIBE track=video priority=0 order=DESC

For any congestion on the first mile, then the relay will improve Bob's experience by following the producer's preference. However any congestion on the last mile will always use the viewer's preference.¶

A relay should use the publisher's priority/order only when there's a conflict. If viewers have the same priority/order, then the relay should use the viewer's preference and it can always issue a SUBSCRIBE_UPDATE when this changes.¶

8.5. Broadcast

A broadcast is a collection of tracks from a single producer. This usually includes an audio track and/or a video track, but there are reasons to have more than that.¶

8.5.1. ABR

Virtually all mass fan-out use-cases rely on Adaptive Bitrate (ABR) streaming. The idea is to encode the same content at multiple bitrates and resolutions, allowing the viewer to choose based on their unique situation.¶

MoqTransfork unsurprisingly supports this via multiple Tracks, but relies on the application to determine the relationship between them. This is often done via a catalog track that details each track's name, bitrate, resolution, and codec. This includes how a group in one track corresponds to a group in another track. A common approach is to use the same Group Sequence number for all tracks, or perhaps utilize a timeline track to map between Group Sequences and presentation timestamps.¶

The viewer may limit the available tracks based on capabilities or preferences. For example, the device may not support the 4K track since it uses AV1, or the screen size may be too small to justify the bandwidth. This is easy enough to support; just ignore these tracks in the catalog.¶

The primary reason to use ABR is to adapt to changing network conditions. The viewer learns about the estimated bandwidth via the SESSION_UPDATE message, or by measuring network traffic and can then choose the appropriate track based on bitrate.¶

Transitioning between tracks can be done seamlessly by utilizing prioritization. For example, suppose a viewer is watching the 360p track and wants to switch to 1080p at group 69.¶

A real-time or unreliable live viewer could issue:¶

SUBSCRIBE_UPDATE track=360p  priority=0 order=DESC end=69
SUBSCRIBE        track=1080p priority=1 order=DESC start=69

A reliable live or VOD viewer could issue:¶

SUBSCRIBE_UPDATE track=360p  priority=1 order=ASC end=69
SUBSCRIBE        track=1080p priority=0 order=ASC start=69

The difference between them is whether to prioritize the old track or the new track. In both scenarios, the subscription will seamlessly switch at group 69 even if it's seconds in the future. The same behavior can be used to switch down.¶

8.5.2. SVC

We touched on SVC before, but it's worth mentioning as an alternative to ABR. I want to see it used more often but I doubt it will be.¶

Instead of choosing the track based on the bitrate, the viewer subscribes to them all:¶

SUBSCRIBE track=360p  priority=2 order=DESC
SUBSCRIBE track=1080p priority=1 order=DESC
SUBSCRIBE track=4k    priority=0 order=DESC

During congestion, the 4k enhancement layer will be deprioritized followed by the 1080p enhancement layer. This is a more efficient use of bandwidth than ABR, but it requires more complex encoding.¶

8.6. Conferences

Some applications involve multiple producers, such as a conference calls or a live events. Even though these are separate broadcasts from potentially separate origins, MoqTransfork can still serve them over the same session.¶

8.6.1. Discovery

The first step to joining a conference is to discover the available broadcasts.¶

There is currently no discovery mechanism in MoqTransfork. However, an application can build one on top of a MoqTransfork track (of course!).¶

For example, suppose we have a conference room called room.12345. An index service could produce a room.12345 track that lists all broadcasts within the room. When Alice joins and ANNOUNCES room.12345.alice, the index service could update the room.12345 track to add a new FRAME +alice. The same can be done to remove her when she leaves.¶

8.6.2. Participants

Extending the idea that audio is more important than video, we can prioritize tracks regardless of the source. This works because SUBSCRIBE priority is scoped to the session and not the broadcast.¶

SUBSCRIBE broadcast=alice track=audio priority=3
SUBSCRIBE broadcast=frank track=audio priority=3
SUBSCRIBE broadcast=alice track=video priority=1
SUBSCRIBE broadcast=frank track=video priority=1

When Alice starts talking or is focused, we can actually issue a SUBSCRIBE_UPDATE to increase her priority:¶

SUBSCRIBE_UPDATE broadcast=alice track=audio priority=2
SUBSCRIBE_UPDATE broadcast=frank track=video priority=0

Note that audio is still more important than video, but Alice is now more important than Frank. (poor Frank)¶

This concept can further be extended to work with SVC or ABR:¶

SUBSCRIBE broadcast=alice track=360p priority=4
SUBSCRIBE broadcast=frank track=360p priority=3
SUBSCRIBE broadcast=alice track=720p priority=2
SUBSCRIBE broadcast=frank track=720p priority=1