Independent Submission S. Nandakumar
Internet-Draft Cisco
Intended status: Informational C. Huitema
Expires: 14 September 2023 Private Octopus Inc.
W. Law
Akamai
13 March 2023
MoQ Transport (moqt) - Unified Media Delivery Protocol over QUIC
draft-nandakumar-moq-transport-00
Abstract
This specification defined MoqTransport (moqt), an unified media
delivery protocol over QUIC. It aims at supporting multiple
application classes with varying latency requirements including ultra
low latency applications such as interactive communication and
gaming. It is based on a publish/subscribe metaphor where entities
publish and subscribe to data that is sent through, and received
from, relays in the cloud. The data is delivered in the strict
priority order. The information subscribed to is named such that
this forms an overlay information centric network. The relays allow
for efficient large scale deployments.
Status of This Memo
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This Internet-Draft will expire on 14 September 2023.
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Table of Contents
1. Introduction
1.1. Terms and definitions
2. Object Model
2.1. Tracks
2.2. Objects
2.3. Object Groups
3. Concepts
3.1. Emission
3.2. Catalog
3.3. MoQ Session
4. Protocol Design
4.1. Control Stream and Messages
4.1.1. Subscribe Message
4.1.2. SUBSCRIBE_REPLY Message
4.1.3. PUBLISH REQUEST Message
4.1.4. PUBLISH_REPLY Message.
4.1.5. RELAY_REDIRECT MESSAGE
4.1.6. Catalog Message
4.2. Stream Considerations
4.2.1. Group Header
4.2.2. Object header
4.3. Datagram considerations
4.3.1. Fragment Message
5. Drop Priority
5.1. Applying drop-priorities through the QUIC stack
5.2. Applying drop-priorities through active scheduling
5.3. Tracking drops
5.4. Marking objects with priorities
6. Relays
6.1. Relay - Subscriber Interactions
6.2. Relay - Publisher Interactions
6.3. Relay Discovery and Failover
6.4. Restoring connections through relays
6.5. Examples
7. Transport Usages
7.1. MoQ over QUIC
7.2. MoQ over WebTransport
7.2.1. Catalog Retrieval
7.2.2. Subscribing to Media
7.2.3. Publishing Media
8. Normative References
Appendix A. TODO
Appendix B. Security Considerations
Appendix C. IANA Considerations
Appendix D. References
D.1. Normative References
D.2. Informative references
Appendix E. Acknowledgments
Authors' Addresses
1. Introduction
Recently new use cases have emerged requiring higher scalability of
delivery for interactive realtime applications and much lower latency
for streaming applications and a combination thereof. On one side
are use cases such as normal web conferences wanting to distribute
out to millions of viewers and allow viewers to instantly move to
being a presenter. On the other side are uses cases such as
streaming a soccer game to millions of people including people in the
stadium watching the game live. Viewers watching an e-sports event
want to be able to comment with minimal latency to ensure the
interactivity aspects between what different viewers are seeing is
preserved. All of these uses cases push towards latencies that are
in the order of 100ms over the natural latency the network causes.
Interactive realtime applications, such as web conferencing systems,
require ultra low latency (< 150ms) delivery. Such applications
create their own application specific delivery network over which
latency requirements can be met. Realtime transport protocols such
as RTP over UDP provide the basic elements needed for realtime
communication, both contribution and distribution, while leaving
aspects such as resiliency and congestion control to be provided by
each application. On the other hand, media streaming applications
are much more tolerant to latency and require highly scalable media
distribution. Such applications leverage existing CDN networks, used
for optimizing web delivery, to distribute media. Streaming
protocols such as HLS and MPEG-DASH operates on top of HTTP and gets
transport-level resiliency and congestion control provided by TCP.
This specification defines MOQTransport, a publish and subscribe
based media delivery protocol over QUIC, where the principal idea is
entities publish unique named objects that are end-to-end encrypted
and consume data by subscribing to the named objects. The names used
are scoped and authorized to the domain operating the application
server (referred to as Origin/Provider in this specification).
The published data carry metadata identifying relative priority,
time-to-live and other useful metadata that's authenticated for
components implementing Relay functions to make drop/forwarding
decisions. MoQTransport is designed to make it easy to implement
relays so that fail over could happen between relays with minimal
impact to the clients and relays can redirect a client to a different
relay.
1.1. Terms and definitions
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.
Commonly used terms in this document are described below:
* Provider/Origin: Entity capable of hosting media application
session based on the MoQTransport. It is responsible for
authorizing the publishers/subscribers, managing the names used
for Tracks and is scoped under domain for a specific application.
In certain deployments, a provider is also responsible for
establishing trust between clients and relays for delivering
media.
* Emitter: Authorized entities that participate in a MoQTransport
Session under an Provider. Emitters are trusted with E2E
encryption keys. They operate on one or more uncompressed media
inputs, compress and possible encrypt it and send over Data
Streams. Each such encoded and/or encrpyted stream corresponds to
a Track within the MoQTransport.
* Catalog Maker: Entities performing Catalog Maker role compose or
aggregate tracks from multiple emissions to form a new emission.
Akin to the role of entities with the Relay role, Catalog Maker
role entities are not trusted with the E2E keys and they perform
publisher and subscriber roles. Catalog Makers are allowed to
publish tracks with a new name without changing the media content
of the received tracks.
* Control Stream: QUIC Stream to exchange control message to setup
appropriate context for media delivery and is scoped to a given
QUIC Connection. Functionally, Control Messages enable
authorization of names for tracks, setting up media properties and
starting/terminating media sessions.
* Data Stream: QUIC Stream or QUIC Datagram based transport for
delivering end to end encrypted application media objects. Such
objects shall carry metadata (unencrypted) for Relays to make
store/forwarding decisions along with the application payload.
2. Object Model
This section define various concepts that make up the object model
enabling media delivery over QUIC.
2.1. Tracks
Tracks form the central concept within the MoQ Transport protocol for
delivering media. A Track identifies the namespace and the
authorization scope under which MoQTransport objects Section 2.2 are
delivered.
A track is a transform of a uncompresss media using a specific
encoding process, a set of parameters for that encoding, and possibly
an encryption process.
The MoQTransport is designed to transport tracks.
Tracks have the following properties:
* Tracks MUST be owned by a single authorized MoQ Entity, such as an
Emitter or a Catalog Maker, under a single provider domain.
* Tracks MUST have a single encoding configuration.
* Tracks MUST have a single security configuration.
Tracks are identified by a globally unique identifier, called "Track
ID". Track ID MUST identify its owning provider by a standardized
identifier, such as domain name or equivalent, then followed by the
application context specific "Track Name", encoded a opaque string.
2.2. Objects
The binary content of a track is composed of a sequence of objects.
An Object is the smallest unit that makes sense to decode and may not
be independently decodable. An Object MUST belong to a group
Section 2.3
Few examples include, for video media an object could be an H.264 P
frame or could be just a single slice from inside the P Frame. For
audio media, it could be a single audio frame. Or a catalog payload.
Objects are not partially decodable. The end to end encryption and
authentication operations are performed across the whole object, thus
rendering partial objects unusable.
Objects MUST be uniquely identifiable within the MoQ delivery system.
Objects carry associated header/metadata containining priority, time
to live, and other information aiding the caching/forwarding decision
at the Relays. Objects MAY be optionally cached at Relays. The
content of the Objects are opaque to Relays and delivered on the
strict priority order Section 5
2.3. Object Groups
An Object MUST belong to a group. Groups are composition of objects
and the objects within the group carry the necessary dependecy
information needed to process the objects in the group. Objects that
carry information required to resolve dependencies are marked
appropriately in their headers. In cases where such information MAY
NOT be available, the first object in the group MUST have all the
dependency information needed to processs therest of the objects.
A group shall provide following utilities
* A way for subscribers to specifiy the appropriate consumption
point for enabling joins, rewinds and replay the objects, for
certain media usecases.
* A way to specify refresh points within a group, serving as decode
points, points of switching between qualties for audio/video media
* Serve as checkpoint for relays to implement appropriate congestion
responses.
3. Concepts
3.1. Emission
An Emission represents a collection of tracks sourced by an emission
source (Emitter or Catalog Maker) and owned by an application
Provider. An Emitter MUST be authorized to publish objects of the
tracks in a Emission. An Emitter can have one or more emissions.
Few example of Emissions include,
* Collection of audio and video tracks that makes up a broadcast for
a live stream by OBS client, the Emitter, to the provider, say
Twitch.
* Tracks from different participants (emitters) in a interactive
video conference
3.2. Catalog
Catalog is a MOQ Object scoped to a MoQ Session Section 3.3 that
provides information about tracks from one of more Emissions and is
used by the subscribers for consuming tracks and for publishers to
advertise the tracks. The content of "Catalog" is opaque to the
Relays and may be end to end encrypted in certain scenarios.
3.3. MoQ Session
A MoQ Session is a top level container under an application Provider
that represents one or more emissions, optionally a set of
participating relays, and set of publisher publishing content and
subscribers that are interested in consuming the content being
published.
4. Protocol Design
Media delivery is started by the publisher/subscriber setting up a
"Control Stream" for one or more Tracks. The control stream, which
is based on QUIC stream, is used to configure and setup properties
for the "Data Stream". Track media objects is delivered over one or
more "Data Streams" which can be unidirectional QUIC streams or over
QUIC Datagrams. The Control Channel can also be used to configure
in-session parameters.
4.1. Control Stream and Messages
The client starts by opening a new bilateral stream, acting as the
"control stream" for the exchange of data, carrying a series of
control messages in both directions.
The control stream is created for one or more tracks to be published
or subscribed to and will remain open as long as the peers are still
sending or receiving the media. If either peer closes the control
stream, the other peer will close its end of the stream and discard
the state associated with the media transfer.
Streams are "one way". If a peer both sends and receive media, there
will be different control streams for sending and receiving.
The control channel carry series of messages, encoded as a length
followed by a message value:
message {
length(16),
value(...)
}
The length is encoded as a 16 bit number in big endian network order.
Below sub-sections define various control messages defined in this
specification.
4.1.1. Subscribe Message
Entities that intend to receive media will do so via subscriptions to
one or more Tracks.
enum subscribe_intent
{
immediate(0),
catch_up(1),
wait_up(2),
}
track_info {
track_id_length(i),
track_id(...)...,
subscribe_intent intent
}
subscribe_message {
message_type(i),
tracks_length(i),
track_info tracks (...),
}
The message type will be set to SUBSCRIBE (1). tracks identifies the
list of tracks as defined by the track_info type.
The track_id captures the Track ID and the intent field specifies the
intended consumption point.
Following options are defined for the intent
* immediate: Deliver any new objects it receives for a given track.
* catch_up: Deliver any new objects it receives and in addition send
any previous objects it has received, beginning from the most
recent group and matching the given track.
* wait_up: Wait until next group before delivering the objects.
Subscriptions are typically long-lived transactions and they stay
active until one of the following happens
* a client local policy dictates expiration of a subscription.
* optionally, a server policy dictates subscription expiration.
* the underlying transport is disconnected.
The subscribe message is sent over the associated control stream and
the same is closed when the subscriptions for the tracks included are
no longer required. This implies the termination of all associated
data streams.
4.1.1.1. Aggregating Subscriptions
Subscriptions are aggregated at entities that perform Relay Function.
Aggregating subscriptions helps reduce the number of subscriptions
for a given track in transit and also enables efficient distribution
of published media with minimal copies between the client and other
relays, as well as reduce the latencies when there are multiple
subscribers for a given track behind a Relay or the provider.
4.1.2. SUBSCRIBE_REPLY Message
A subscribe_reply provides results of the subscription and is sent on
the control stream over which the subscribe control message was
received.
enum response
{
ok(0),
expired(1),
fail(2)
}
track_response {
Response response,
track_id_length(i),
track_id(...)...,
[ Reason Phrase Length (i) ],
[ Reason Phrase (...) ],
[ media_id(i) ]
}
subscribe_reply
{
message_type(i),
track_response tracks(...)
}
The message type will be set to SUBSCRIBE_REPLY (2). tracks capture
the result of subscription per track included in the subscribe
message.
For each track, the track_response provides result of subscription in
the response field, where a response of ok indicates successful
subscription. For failed or expired responses, the "Reason Phrase"
shall be populated with appropriate reason code.
The media_id for a given track is populated for a successful
subscription and represents an handle to the subscription to be
provided by the peer over the data streams(s). Given that the media
corresponding to a track can potentially arrive over multiple data
streams, the media_id provides the necessary mapping between the
control stream and the corresponding data streams. It also serves as
compression identifier for containing the size of object headers
instead of carrying complete track identifier information in every
object message.
While the subscription is active for a given name, the Relay(s) must
send objects for tracks it receives to all the matching subscribers.
Optionally, a client can refresh its subscriptions at any point by
sending a new subscribe_message.
4.1.3. PUBLISH REQUEST Message
The publish_request message provides one or more tracks that the
publisher intends to publish data.
track_info {
track_id_length(i),
track_id(...)...,
media_id(i),
}
publish_request {
message_type(i),
track_info tracks(...),
}
The message type will be set to PUBLISH_REQUEST (3). tracks
identifies the list of tracks. The media_id represents an handle to
the track to be used over the data streams(s). Given that media
corresponding to the track can potentially be sent over multiple data
streams, the media_id provides the necessary mapping between the
control stream and the associated data streams. media_id also serves
as compression identifier for containing the size of object headers
instead of carrying full formed Track Id in every object.
The publish_request message is sent on its own control stream and
akin to subscribes, the control stream's lifecycle bounds the media
transfer state. Terminating the control stream implies closing of
all the associated data streams for the tracks included in the
request.
4.1.4. PUBLISH_REPLY Message.
publish_reply provides the result of request to publish on the
track(s) in the publish_request. The publish_reply control message
is sent over the same control stream the request was received on.
publish_reply {
message_type(i),
track_response tracks(...),
}
The message_type is set to PUBLISH_REPLY (4).
tracks capture the result of publish request per track included in
the publish_request message. The semantics of track_response is same
as defined in Section 4.1.2 except the media_id is optionally
populated in the case where the media_id in the request cannot be
used.
4.1.5. RELAY_REDIRECT MESSAGE
relay_redirect control message provides an explicit signal to
indicate relay failover scenarios. This message is sent on all the
control streams that would be impacted by reduced operations of the
Relay.
relay_redirect
{
message_type(i),
relay_address_length(i),
relay_address(...)...
}
The message_type is set to RELAY_REDIRECT (5). relay_address
identifies the address of the relay to setup the new subscriptions or
publishes to.
4.1.6. Catalog Message
Catalog message provides information on tracks for a given MoQ
Session.
catalog {
message_type(i),
catalog_length(i),
data(...)...
}
The message_type is set to CATALOG (6). data is container specific
encoding of catalog information.
4.2. Stream Considerations
Certain applications can choose to send each group in their own
unidirectional QUIC stream. In such cases, stream will start with a
"group header" message specifying the media ID and the group ID,
followed for each object in the group by an "object header"
specifying the object ID and the object length and then the content
of the objects (as depicted below)
+--------+------------+-------+------------+-------+------
| Group | Object | Bytes | Object | Bytes |
| header | header (0) | (0) | header (1) | (1) | ...
+--------+------------+-------+------------+-------+------
The first object in the stream is object number 0, followed by 1,
etc. Arrival of objects out of order will be treated as a protocol
error.
TODO: this strict "in order" arrival is not verified if there is one
stream per drop-priority level. Add text to enable that.
Alternatively, certain applications can choose to send each object in
its own unidirectional QUIC stream. In such cases, each stream will
start with a "group header" message specifying the media ID and the
group ID, followed by a single "object header" and then the content
of the objects (as depicted below).
+--------+------------+-------+
| Group | Object | Bytes |
| header | header (n) | (n) |
+--------+------------+-------+
The MOQTransport doesn't enforce a rule to follow for the
applications, but instead aims to provide tools for the applications
to make the choices appropriate for their use-cases.
4.2.1. Group Header
The group header message is encoded as:
group_header {
message_type(i),
media_id(i),
group_id(i)
}
The message type is set to GROUP_HEADER, 11. media_id MUST correspond
to the one that was setup as part of publish_request control message
exchange Section 4.1.3. group_id always starts at 0 and increases
sequentially at the original media publisher.
4.2.2. Object header
Each object in the stream is encoded as an Object header, followed by
the content of the object. The Object header is encoded as:
object_header {
message_type(i),
object_id(i),
[nb_objects_previous_group(i),]
flags[8],
object_length(i)
}
The message type is set to OBJECT_HEADER, 12. object_id is identified
by a sequentially increasing integer, starting at 0.
The nb_objects_previous_group is present if and only if this is the
first fragment of the first object in a group, i.e., object_id and
offset are both zero. The number indicates how many objects were
sent in the previous group. It enables the receiver to check whether
all these objects have been received.
The flags field is used to maintain low latency by selectively
dropping objects in case of congestion. The flags field is encoded
as:
{
maybe_dropped(1),
drop_priority(7)
}
4.3. Datagram considerations
MoQ objects can be transmitted as QUIC datagrams, if the datagram
transmission option has been validated during the subscribe or
publish transaction. Such a option is chosen for non-relaible media
delivery scenarios.
When sent as datagrams, the object is split into a set of fragments.
Each fragment is sent as a separate datagram. The fragment header
contains enough information to enable reassembly. If the complete
set of fragments is not received in a reasonable time, the whole
object shall be considered lost.
4.3.1. Fragment Message
In the datagram variants, instead of sending a series of whole
objects on a stream, objects are sent as series of fragments, using
the Fragment message:
fragment {
message_type(i),
[ media_id(i) ],
[ group_id(i) ],
[ object_id(i) ],
fragment_offset(i),
object_length(i),
fragment_length(i),
data(...)
}
The message type will be set to FRAGMENT (13). The optional fields
media_id, group_id and object_id are provided in the cases where they
cannot be obtained from the context where the fragment message is
published. For typical cases, the group_header and the object_header
messages preceed the series of fragment messages and thus provide the
necessary context to tie the data to the object.
The fragment_offset value indicates where the fragment data starts in
the object designated by group_id and object_id. Successive messages
are sent in order, which means one of the following three conditions
must be verified:
* The group id and object id match the group id and object id of the
previous fragment, the previous fragment is not a last fragment,
and the offset matches the previous offset plus the previous
length.
* The group id matches the group id of the previous message, the
object id is equal to the object id of the previous fragment plus
1, the offset is 0, and the previous message is a last fragment.
* The group id matches the group id of the previous message plus 1,
the object id is 0, the offset is 0, and the previous message is a
last fragment.
The nb_objects_previous_group is present if and only if this is the
first fragment of the first object in a group, i.e., object_id and
offset are both zero. The number indicates how many objects were
sent in the previous groups. It enables the receiver to check
whether all these objects have been received.
The flags field is used to maintain low latency by selectively
dropping objects in case of congestion. The value must be the same
for all fragments belonging to the same object.
The flags field is encoded as:
{
maybe_dropped(1),
drop_priority(7)
}
The high order bit maybe_dropped indicates whether the object can be
dropped. The drop_priority allows nodes to selectively drop objects.
Objects with the highest priority as dropped first.
When an object is dropped, the relays will send a placeholder, i.e.,
a single fragment message in which:
* offset_and_fin indicates offset=0 and fin=1
* the length is set to zero
* the flags field is set to the all-one version 0xff.
Sending a placeholder allows node to differentiate between a
temporary packet loss, which will be soon corrected, and a deliberate
object drop.
5. Drop Priority
In case of congestion, the MoQ nodes may have to drop some traffic in
order to avoid building large queues. The drop algorithm must
respect the relative importance of objects within a track, as well as
the relative importance of tracks within an MoQ connection. Relays
base their decisions on two properties of objects:
* a "droppable" flag, which indicates whether the application would
rather see the object queued (droppable=False) or dropped
(droppable=True) in case of congestion.
* a "drop-priority" value, which indicates the relative priority of
this object versus other objects in the track or other tracks in
the connection.
Higher values of the drop-priority field indicate higher drop
priorities: objects mark with priority 0 would be the last to be
dropped, objects marked with priority 3 would be dropped before
dropping objects with priority 2, etc. Nodes support up to 8 drop-
priority levels, numbered 0 to 7.
Nodes may use drop-priorities in two ways: either by delegating to
the QUIC stack, or by monitoring the state of congestion and
performing their own scheduling.
5.1. Applying drop-priorities through the QUIC stack
Many QUIC stack allow application to associate a priority with a
stream. The MoQ transports can use that feature to delegate priority
enforcement to the QUIC stack. The actual delegation depends on the
transport choice.
If the MoQ transport uses the strategy where each object is
transmitted on a separate unidirectional QUIC stream, then that
stream should be marked with the object's priority. The QUIC API
should be set to request FIFO ordering of streams within a priority
layer.
If all the objects of a given group, say GOP, within a track are sent
in a single unidirectional QUIC stream. This strategy can be
modified to be priority aware. In a priority aware strategy, there
will be one unidirectional stream per group and per priority level,
and the priority of the unidirectional stream will match that level.
In both cases, if congestion happens, objects marked as "droppable"
will have be dropped by resetting the corresponding unidirectional
streams. This decision will happen separately for each track,
typically at the end of a group. At that point, the decision depends
on whether the content of the unidirectional streams have been sent
or not:
* if all objects have been sent, the stream can be closed normally.
* if some objects have not been sent, or not acknowledged, the
stream shall be reset, causing the corresponding objects to be
dropped.
These policies will normally ensure that for any congestion state,
only the most urgent objects are sent.
5.2. Applying drop-priorities through active scheduling
Some transport strategies prevent delegation of priority enforcement
to the QUIC stack. For example, if the policy is to use a single
QUIC stream or a single stream carrying objects of different
priorities. In such cases, nodes react to congestion by scheduling
some objects for transmission and explicitly dropping other objects.
Node should schedule objects as follow:
* if congestion is noticed, the node will delay or drop first the
numerically higher priority level. The node will drop all objects
marked at that priority, from the first dropped object to the end
of the group.
* if congestion persists despite dropping or delaying the "bottom"
level, the node will start dropping the next level, and continue
doing so until the end of the group.
* if congestion eases, the node will increase the delay or drop
level.
While the "drop level" is computed per connection, specific actions
will have to be performed at the "track" level:
* for a given track, the node remembers the highest priority level
for which objects were dropped in the current group. That level
will be maintained for that track until the end of the group.
* at the beginning of a group, the priority level is set to the
currently computed value for the connection.
5.3. Tracking drops
For management purposes, it is important to indicate which objects
have been dropped, as in "there was supposed to be here an object
number X or priority P but it has been dropped." In the scheduling
approach, this can be achieved by inserting a small placeholder for
the missing object. In the delegating approach, we need another
solution. One possibility would be to send a "previous group
summary" at the beginning of each group, stating the expected content
of the previous group.
5.4. Marking objects with priorities
The publishers mark objects with sequence numbers within groups and
with drop and priority values according to the need of the
application. This marks must be consistent with the encoding
requirements, making sure that:
* objects can only have encoding dependencies on other objects in
the same group,
* objects can only have encoding dependencies on other objects with
equal or or numerically lower priority levels.
With these constraints, applications have broad latitude to pick
priorities in order to match the desired user experience. When using
scalable video codecs, this could mean for example chosing between
"frame rate first" or "definition first" priorities, or some
compromise.
6. Relays
The Relays play an important role for enabling low latency media
delivery within the MoQ architecture. This specification allows for
a delivery protocol based on a publish/subscribe metaphor where some
endpoints, called publishers, publish media objects and some
endpoints, called subscribers, consume those media objects. Some
relays can leverage this publish/subscribe metaphor to form an
overlay delivery network similar/in-parallel to what CDN provides
today. While this type of overlay is expected to be a major
application of relays, other types of relays can also be defined to
offer various types of services.
Objects are received by "subscribing" to it. Objects are identified
such that it is unique for the relay/delivery network.
Relays provide several benefits including
* Scalability - Relays provide the fan-out necessary to scale up
streams to production levels (millions) of concurrent subscribers.
* Reliability - relays can improve the overall reliability of the
delivery system by providing alternate paths for routing content.
* Performance - Relays are usually positioned as close to the edge
of a network as possible and are well-connected to each other and
to the Origin via high capacity managed networks. This topography
minimizes the RTT over the unmanaged last mile to the end-user,
improving the latency and throughput compared to the client
connecting directly to the origin.'
* Security - Relays act to shield the origin from DDOS and other
malicious attacks.
6.1. Relay - Subscriber Interactions
Subscribers interact with the "Relays" by sending a "subscribe"
Section 4.1.1 command for the tracks of interest.
Relays MUST be willing to act on behalf of the subscriptions before
they can forward the media, which implies that the subscriptions MUST
to be authorized and it is done as follows:
1. Provider serving the tracks MUST be authorized. Track IDs
provide the necessary information to identify the Origin/
Provider.
2. Subscriptions MUST be authorized. This is typically done by
either subscriptions carrying enough authorization information or
subscriptions being forwarded to the Origin for obtaining
authorization. The mechanics of either of these approaches are
out of scope for this specification.
In all the scenarios, the end-point client making the subscribe
request is notified of the result of the subscription.
6.2. Relay - Publisher Interactions
Publishers MAY be configured to publish the objects to a relays based
on the application configuration and topology. Publishing set of
tracks through the relay starts with a "publish_request" transaction
that describes the track identifiers. That transaction will have to
be authorized by the Origin, using mechanisms similar to authorizing
subscriptions.
As specified with subscriber interactions, Relays MUST be authorized
to serve the provider and the publish_request MUST be authorized
before the Relays are willing to forward the published data for the
tracks.
Relays makes use of priority order and other metadata properties from
the published objects to make forward or drop decisions when reacting
to congestion as indicated by the underlying QUIC stack. The same
can be used to make caching decisions.
6.3. Relay Discovery and Failover
Relays are discovered via application defined ways that are out of
scope of this document. A Relay that wants to shutdown can send a
message to the client with the address of new relay. Client moves to
the new relay with all of its Subscriptions and then Client
unsubscribes from old relay and closes connection to it.
6.4. Restoring connections through relays
The transmission of a track can be interrupted by various events,
such as loss of connectivity between subscriber and relay. Once
connectivity is restored, the subscriber will want to resume
reception, ideally with as few visible gaps in the transmission as
possible, and certainly without having to "replay" media that was
already presented.
There is no guarantee that the restored connectivity will have the
same characteristics as the previous instance. The throughput might
be lower, forcing the subscriber to select a media track with lower
definition. The network addresses might be different, with the
subscriber connecting to a different relay.
6.5. Examples
Let’s consider the example as show in the picture below, where a
large number of subscribers are interested in media streams from the
publisher Alice. In this scenario, the publisher Alice has a live
broadcast on channel8 with video streams at 3 different quality (HD,
4K and SD)
More specifically,
1. Subscriber - S1 is interested in the just the low quality version
of the media, and asks for the all the media groups/objects under
the specific representation for "sd" quality.
2. Subscriber - S2 is fine with receiving highest quality video
streams published by Alice, hence asks for the all the media
objects under these representations 4k.
3. Rest of the Subscribers (say Sn,...) are fine with getting just
the Hi-def and low quality streams of the video from Alice, and
asks for the representations "sd" and "hd" qualities.
The Relay must forward all these subscription requests to the ingest
server in order to receive the content.
Note: The notation for identifying the resources for subscription are
for illustration purposes only.
sub: acme.tv/brodcasts/channel8/alice/sd
.─────.
┌──────( S1 )
│ `─────'
│
sub: acme.tv/broadcasts/channel8/alice/4 |
sub: acme.tv/brodcasts/channel8/alice/sd |
sub: acme.tv/brodcasts/channel8/alice/hd |
│ | sub: acme.tv/
┌──────────────┐ ┌──────────────┐ │ broadcasts/
│ │ │ │ | channel8/
| | | | | alice/4k
│ Ingest │ ◀────────| Relay-Edge │◀─┘ .─────.
│ │ │ │◀────( S2 )
└──────▲─────── └────────────── ◀──┐ `─────'
│ │ ◉
│ │ ◉
.─────. │ ◉
( Alice ) │
`─────' │ .─────.
pub: acme.tv/broadcasts/channel8/alice/hd └─( SN )
pub: acme.tv/broadcasts/channel8/alice/sd `─────'
pub: acme.tv/broadcasts/channel8/alice/4k
sub: acme.tv/brodcasts/channel8/alice/sd
sub: acme.tv/brodcasts/channel8/alice/hd
The relay does not intercept and parse the CATALOG messages,
therefore it does not know the entireity of the content being
produced by Alice. It simply aggregates and forwards all
subscription requests that it receives.
Similarly, below example shows an Interactive media session
pub: acme.com/meetings/m123/bob/video
pub: acme.com/meetings/m123/bob/audio
.─────. sub:acme.com/meetings/m123/alice/audio
( Bob )
`─────' sub:acme.com/meetings/m123/alice/video
│
│ sub:acme.com/meetings/m123/bob/audio
│
│ sub:acme.com/meetings/m123/bob/video
│
┌──────▼───────┐ ┌──────────────┐
│ │ │ │
│ Relay │ ◀─────────┤ Relay │◀───┐
│ │ │ │ |
└──────▲───────┘ └──────────────┘ |
│ │
│ │
│ │
│ │
.─────. │
( Alice ) │
`─────' │ .─────.
└────( S1 )
pub: acme.com/meetings/m123/alice/video `─────'
pub: acme.com/meetings/m123/alice/audio
sub:acme.com/meetings/m123/alice/audio
sub:acme.com/meetings/m123/alice/video
sub:acme.com/meetings/m123/bob/audio
sub:acme.com/meetings/m123/bob/video
The above picture shows as sample media delivery, where a tree
topography is formed with multiple relays in the network. The
example has 4 participants with Alice and Bob being the publishers
and S1 being the subscribers. Both Alice and Bob are capable of
publishing audio and video identified by their appropriate names. S1
subscribes to all the streams being published. The edge Relay
forwards the unique subscriptions to the downstream Relays as needed,
to setup the delivery network.
7. Transport Usages
Following subsections define usages of the MoQTransport over
WebTransport and over raw QUIC.
7.1. MoQ over QUIC
MoQ can run directly over QUIC. In that case, the following apply:
* Connection setup corresponds to the establishment of a QUIC
connection, in which the ALPN value indicates use of MoQ. For
versions implementing this draft, the ALPN value is set to "moq-
n00".
* Bilateral and unilateral streams are mapped directly to equivalent
QUIC streams
* Datagrams, when used, are mapped directly to QUIC datagram frames.
7.2. MoQ over WebTransport
MoQ can benefit from an infrastructure designed for HTTP3 by running
over WebTransport.
WebTransport provides protocol framework that enables clients
constrained by the Web security model to communicate with a remote
server using a secure multiplexed transport. WebTransport protocol
also provides support for unidirectional streams, bidirectional
streams and datagrams, all multiplexed within the same HTTP/3
connection.
Clients (publishers and subscribers) setup WebTransport Session via
HTTP CONNECT request for the application provided MoQSession and
provide the necessary authentication information (in the form of
authentication token). The ":protocol" value indicates use of MoQ.
For versions implementing this draft, the :protocol value is set to
"moq-n00".
In case of any errors, the session is terminated and reported to the
application.
Bilateral and unilateral streams are opened and used through the
WebTransport APIs.
7.2.1. Catalog Retrieval
On a successful connection setup, subscribers proceed by retrieving
the catalog (if not already retrieved), subscribing to the tracks of
their interest and consuming the data published as detailed below.
Catalog provides the details of tracks such as Track IDs and
corresponding configuration details (audio/video codec detail,
gamestate encoding details, for example).
Catalogs are identified as a special track, with its track name as
"catalog". Catalog objects are retrieved by subscribing to its
TrackID over its own control channel and the TrackID is formed as
shown below
Catalog TrackID :=<provider-domain>/<moq-session-id>/catalog
Ex: streaming.com/emission123/catalog
A successfull subscription will lead to one or more catalog objects
being published on a single unidirectional data stream. Successfull
subscriptions implies authorizaiton for subscribing to the tracks in
the catalog.
Unsuccessful subscriptions MUST result in closure of the WebTransport
session, followed by reporting the error obtained to the application.
Catalog Objects obtained MUST parse successfully, otherwise MUST be
treated as error, thus resulting the closure of the WebTransport
session.
7.2.2. Subscribing to Media
Once a catalog is successfully parsed, subscribers proceed to
subscribe to the tracks listed in the catalog. Applications can
choose to use the same WebTransport session or multiple of them to
perform the track subscriptions based on the application
requirements.
Also, It is typical for certain applications to group set of tracks
in to a single prioritization relationship and transmit them over a
single WebTransport Session.
Tracks subscription is done by sending subscribe message as definedin
Section 4.1.1
On successful subscription, subscribers should be ready to consume
media on one or more Data Streams as identified by their media_ids.
Failure to subscribe MUST result on closure of the control stream
associated with the track whose subscription failed and the error
MUST be reported to the application.
7.2.3. Publishing Media
On successful setup of the WebTransport session, publishers send
publish_request message listing the tracks they intend to publish
data on. Publisher MUST be authorized to publish on the tracks and
Relays MUST be willing to participate in the media delivery.
A sucessfull publish_reply allows publishers to publish on the tracks
advertised.
Publishing objects on the tracks follow the procedures defined in
Section 4.2 and Section 4.3.
8. Normative References
[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>.
[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>.
Appendix A. TODO
1. Add authorization details for the protocol messages.
Appendix B. Security Considerations
This section needs more work
Appendix C. IANA Considerations
TODO: fill this section. Register ALPN. Register WebTransport
protocol. Open new registry for MoQ message types. Possibly, open
registry for MoQ errors.
Appendix D. References
D.1. Normative References
[RFC XXX] Nandakumar, S "MoQ Base Protocol" Work in progress
D.2. Informative references
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126,
DOI 10.17487/RFC8126, June 2017, %gt;https://www.rfc-editor.org/info/
rfc8126>.
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000, DOI 10.17487/RFC9000,
May 2021, %gt;https://www.rfc-editor.org/rfc/rfc9000>.
[WebTransport] Frindell, A., Kinnear, E., and V. Vasiliev,
"WebTransport over HTTP/3", Work in Progress, Internet-Draft, draft-
ietf-webtrans-http3-04, 24 January 2023,
>https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3-04>.
Appendix E. Acknowledgments
Cullen Jennings, the IETF MoQ mailing lists and discussion groups.
Authors' Addresses
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
Christian Huitema
Private Octopus Inc.
Email: huitema@huitema.net
Will Law
Akamai
Email: wilaw@akamai.com