Internet DRAFT - draft-lcurley-moq-transport
draft-lcurley-moq-transport
Independent Submission L. Curley
Internet-Draft Twitch
Intended status: Informational K. Pugin
Expires: 27 November 2023 Meta
S. Nandakumar
Cisco
V. Vasiliev
Google
26 May 2023
Media over QUIC Transport
draft-lcurley-moq-transport-00
Abstract
This document defines the core behavior for Media over QUIC Transport
(MOQT), a media transport protocol over QUIC. MOQT allows a producer
of media to publish data and have it consumed via subscription by a
multiplicity of endpoints. It supports intermediate content
distribution networks and is designed for high scale and low latency
distribution.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 27 November 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1. Latency . . . . . . . . . . . . . . . . . . . . . . . 4
1.1.2. Leveraging QUIC . . . . . . . . . . . . . . . . . . . 4
1.1.3. Universal . . . . . . . . . . . . . . . . . . . . . . 4
1.1.4. Relays . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terms and Definitions . . . . . . . . . . . . . . . . . . 5
1.3. Notational Conventions . . . . . . . . . . . . . . . . . 6
2. Object Model . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Objects . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Groups . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Track . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Track Naming and Scopes . . . . . . . . . . . . . . . 7
2.3.2. Connection URL . . . . . . . . . . . . . . . . . . . 8
3. Sessions . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Session establishment . . . . . . . . . . . . . . . . . . 8
3.1.1. WebTransport . . . . . . . . . . . . . . . . . . . . 8
3.1.2. QUIC . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2. Session initialization . . . . . . . . . . . . . . . . . 9
3.3. Cancellation . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Termination . . . . . . . . . . . . . . . . . . . . . . . 10
4. Prioritization and Congestion Response . . . . . . . . . . . 10
4.1. Order Priorities and Options . . . . . . . . . . . . . . 11
4.1.1. Proposal - Send Order . . . . . . . . . . . . . . . . 11
4.1.2. Proposal - Ordering by Priorities . . . . . . . . . . 12
5. Relays . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.1. Subscriber Interactions . . . . . . . . . . . . . . . . . 13
5.2. Publisher Interactions . . . . . . . . . . . . . . . . . 13
5.3. Relay Discovery and Failover . . . . . . . . . . . . . . 14
5.4. Restoring connections through relays . . . . . . . . . . 14
5.5. Congestion Response at Relays . . . . . . . . . . . . . . 14
5.6. Relay Object Handling . . . . . . . . . . . . . . . . . . 14
6. Messages . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. SETUP . . . . . . . . . . . . . . . . . . . . . . . . . . 16
6.1.1. SETUP Parameters . . . . . . . . . . . . . . . . . . 16
6.2. OBJECT . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.3. SUBSCRIBE REQUEST . . . . . . . . . . . . . . . . . . . . 18
6.4. SUBSCRIBE OK . . . . . . . . . . . . . . . . . . . . . . 19
6.5. SUBSCRIBE ERROR . . . . . . . . . . . . . . . . . . . . . 19
6.6. ANNOUNCE . . . . . . . . . . . . . . . . . . . . . . . . 20
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6.6.1. Track Request Parameters . . . . . . . . . . . . . . 21
6.7. ANNOUNCE OK . . . . . . . . . . . . . . . . . . . . . . . 21
6.8. ANNOUNCE ERROR . . . . . . . . . . . . . . . . . . . . . 22
6.9. GOAWAY . . . . . . . . . . . . . . . . . . . . . . . . . 22
7. Security Considerations . . . . . . . . . . . . . . . . . . . 23
7.1. Resource Exhaustion . . . . . . . . . . . . . . . . . . . 23
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 24
References . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Normative References . . . . . . . . . . . . . . . . . . . . . 24
Informative References . . . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
Media Over QUIC Transport (MOQT) is a transport protocol that
utilizes the QUIC network protocol [QUIC], either directly or via
WebTransport [WebTransport], for the dissemination of media. MOQT
utilizes a publish/subscribe workflow in which producers of media
publish data in response to subscription requests from a multiplicity
of endpoints. MOQT supports wide range of use-cases with different
resiliency and latency (live, interactive) needs without compromising
the scalability and cost effectiveness associated with content
delivery networks.
MOQT is a generic protocol is designed to work in concert with
multiple MoQ Streaming Formats. These MoQ Streaming Formats define
how content is encoded, packaged, and mapped to MOQT objects, along
with policies for discovery and subscription.
* Section 2 describes the object model employed by MOQT.
* Section 3 covers aspects of setting up a MOQT session.
* Section 4 covers protocol considerations on prioritization schemes
and congestion response overall.
* Section 5 covers behavior at the relay entities.
* Section 6 covers how messages are encoded on the wire.
1.1. Motivation
The development of MOQT is driven by goals in a number of areas -
specifically latency, the robustness of QUIC, workflow efficiency and
relay support.
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1.1.1. Latency
HTTP Adaptive Streaming (HAS) has been successful at achieving scale
although often at the cost of latency. Latency is necessary to
correct for variable network throughput. Ideally live content is
consumed at the same bitrate it is produced. End-to-end latency
would be fixed and only subject to encoding and transmission delays.
Unfortunately, networks have variable throughput, primarily due to
congestion. Attempting to deliver content encoded at a higher
bitrate than the network can support causes queuing along the path
from producer to consumer. The speed at which a protocol can detect
and respond to queuing determines the overall latency. TCP-based
protocols are simple but are slow to detect congestion and suffer
from head-of-line blocking. UDP-based protocols can avoid queuing,
but the application is now responsible for the complexity of
fragmentation, congestion control, retransmissions, receiver
feedback, reassembly, and more. One goal of MOQT is to achieve the
best of both these worlds: leverage the features of QUIC to create a
simple yet flexible low latency protocol that can rapidly detect and
respond to congestion.
1.1.2. Leveraging QUIC
The parallel nature of QUIC streams can provide improvements in the
face of loss. A goal of MOQT is to design a streaming protocol to
leverage the transmission benefits afforded by parallel QUIC streams
as well exercising options for flexible loss recovery. Applying
[QUIC] to HAS via HTTP/3 has not yet yielded generalized improvements
in throughput. One reason for this is that sending segments down a
single QUIC stream still allows head-of-line blocking to occur.
1.1.3. Universal
Internet delivered media today has protocols optimized for ingest and
separate protocols optimized for distribution. This protocol switch
in the distribution chain necessitates intermediary origins which re-
package the media content. While specialization can have its
benefits, there are gains in efficiency to be had in not having to
re-package content. A goal of MOQT is to develop a single protocol
which can be used for transmission from contribution to distribution.
A related goal is the ability to support existing encoding and
packaging schemas, both for backwards compatibility and for
interoperability with the established content preparation ecosystem.
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1.1.4. Relays
An integral feature of a protocol being successful is its ability to
deliver media at scale. Greatest scale is achieved when third-party
networks, independent of both the publisher and subscriber, can be
leveraged to relay the content. These relays must cache content for
distribution efficiency while simultaneously routing content and
deterministically responding to congestion in a multi-tenant network.
A goal of MOQT is to treat relays as first-class citizens of the
protocol and ensure that objects are structured such that information
necessary for distribution is available to relays while the media
content itself remains opaque and private.
1.2. 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.
TODO: The terms defined here doesn't capture the ongoing discussions
within the Working Group (either as part of requirements or
architecture documents). This section will be updated to reflect the
discussions.
Commonly used terms in this document are described below.
Client: The party initiating a transport session.
Congestion: Packet loss and queuing caused by degraded or overloaded
networks.
Consumer: A QUIC endpoint receiving media over the network.
Endpoint: A QUIC Client or a QUIC Server.
Group: A temporal sequence of objects. A group represents a join
point in a track. See (Section 2.2).
Object: An object is an addressable unit whose payload is a sequence
of bytes. Objects form the base element in the MOQT model. See
(Section 2.1).
Producer: A QUIC endpoint sending media over the network.
Server: The party accepting an incoming transport session.
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Track: An encoded bitstream. Tracks contain a sequential series of
one or more groups and are the subscribable entity with MOQT.
Transport session: A raw QUIC connection or a WebTransport session.
1.3. Notational Conventions
This document uses the conventions detailed in Section 1.3 of [QUIC]
when describing the binary encoding.
This document also defines an additional field type for binary data:
x (b): Indicates that x consists of a variable length integer,
followed by that many bytes of binary data.
2. Object Model
MOQT has a hierarchical object model for data, comprised of objects,
groups and tracks.
2.1. Objects
The basic data element of MOQT is an object. An object is an
addressable unit whose payload is a sequence of bytes. All objects
belong to a group, indicating ordering and potential dependencies.
Section 2.2 Objects are comprised of two parts: metadata and a
payload. The metadata is never encrypted and is always visible to
relays. The payload portion may be encrypted, in which case it is
only visible to the producer and consumer. The application is solely
responsible for the content of the object payload. This includes the
underlying encoding, compression, any end-to-end encryption, or
authentication. A relay MUST NOT combine, split, or otherwise modify
object payloads.
2.2. Groups
A group is a collection of objects and is a sub-unit of a track
(Section 2.3). Objects within a group SHOULD NOT depend on objects
in other groups. A group behaves as a join point for subscriptions.
A new subscriber might not want to receive the entire track, and may
instead opt to receive only the latest group(s). The sender then
selectively transmits objects based on their group membership.
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2.3. Track
A track is a sequence of groups (Section 2.2). It is the entity
against which a consumer issues a subscription request. A subscriber
can request to receive individual tracks starting at a group
boundary, including any new objects pushed by the producer while the
track is active.
2.3.1. Track Naming and Scopes
In MOQT, every track has a track name and a track namespace
associated with it. A track name identifies an individual track
within the namespace.
A tuple of a track name and a track namespace together is known as a
full track name:
Full Track Name = Track Namespace Track Name
A MOQT scope is a set of servers (as identified by their connection
URIs) for which full track names are guaranteed to be unique. This
implies that within a single MOQT scope, subscribing to the same full
track name would result in the subscriber receiving the data for the
same track. It is up to the application using MOQT to define how
broad or narrow the scope has to be. An application that deals with
connections between devices on a local network may limit the scope to
a single connection; by contrast, an application that uses multiple
CDNs to serve media may require the scope to include all of those
CDNs.
The full track name is the only piece of information that is used to
identify the track within a given MOQT scope and is used as cache
key. MOQT does not provide any in-band content negotiation methods
similar to the ones defined by HTTP ([RFC9110], Section 10); if, at a
given moment in time, two tracks within the same scope contain
different data, they have to have different full track names.
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Example: 1
Track Namespace = live.example.com/meeting/123/member/alice/
Track Name = audio
Full Track Name = live.example.com/meeting/123/member/alice/audio
Example: 2
Track Namespace = live.example.com/
Track Name = uaCafDkl123/audio
Full Track Name = live.example.com/uaCafDkl123/audio
Example: 3
Track Namespace = security-camera.example.com/camera1/
Track Name = hd-video
Full Track Name = security-camera.example.com/camera1/hd-video
2.3.2. Connection URL
Each track MAY have one or more associated connection URLs specifying
network hosts through which a track may be accessed. The syntax of
the Connection URL and the associated connection setup procedures are
specific to the underlying transport protocol usage Section 3.
3. Sessions
3.1. Session establishment
This document defines a protocol that can be used interchangeably
both over a QUIC connection directly [QUIC], and over WebTransport
[WebTransport]. Both provide streams and datagrams with similar
semantics (see [I-D.ietf-webtrans-overview], Section 4); thus, the
main difference lies in how the servers are identified and how the
connection is established.
3.1.1. WebTransport
A MOQT server that is accessible via WebTransport can be identified
using an HTTPS URI ([RFC9110], Section 4.2.2). A MOQT session can be
established by sending an extended CONNECT request to the host and
the path indicated by the URI, as described in [WebTransport],
Section 3.
3.1.2. QUIC
A MOQT server that is accessible via native QUIC can be identified by
a URI with a "moq" scheme. The "moq" URI scheme is defined as
follows, using definitions from [RFC3986]:
moq-URI = "moq" "://" authority path-abempty [ "?" query ]
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The authority portion MUST NOT contain a non-empty host portion. The
moq URI scheme supports the /.well-known/ path prefix defined in
[RFC8615].
This protocol does not specify any semantics on the path-abempty and
query portions of the URI. The contents of those are left up to the
application.
The client can establish a connection to a MoQ server identified by a
given URI by setting up a QUIC connection to the host and port
identified by the authority section of the URI. The path-abempty and
query portions of the URI are communicated to the server using the
PATH parameter (Section 6.1.1.2) which is sent in the SETUP message
at the start of the session. The ALPN value [RFC7301] used by the
protocol is moq-00.
3.2. Session initialization
The first stream opened is a client-initiated bidirectional stream
where the peers exchange SETUP messages (Section 6.1). The
subsequent streams MAY be either unidirectional or bidirectional.
For exchanging content, an application would typically send a
unidirectional stream containing a single OBJECT message
(Section 6.2), as putting more than one object into one stream may
create head-of-line blocking delays. However, if one object has a
hard dependency on another object, putting them on the same stream
could be a valid choice.
3.3. Cancellation
A QUIC stream MAY be canceled at any point with an error code. The
producer does this via a RESET_STREAM frame while the consumer
requests cancellation with a STOP_SENDING frame.
When using order, lower priority streams will be starved during
congestion, perhaps indefinitely. These streams will consume
resources and flow control until they are canceled. When nearing
resource limits, an endpoint SHOULD cancel the lowest priority stream
with error code 0.
The sender MAY cancel streams in response to congestion. This can be
useful when the sender does not support stream prioritization.
TODO: this section actually describes stream cancellation, not
session cancellation. Is this section required, or can it be
deleted, or added to a new "workflow" section.
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3.4. Termination
The transport session can be terminated at any point. When native
QUIC is used, the session is closed using the CONNECTION_CLOSE frame
([QUIC], Section 19.19). When WebTransport is used, the session is
closed using the CLOSE_WEBTRANSPORT_SESSION capsule ([WebTransport],
Section 5).
The application MAY use any error message and SHOULD use a relevant
code, as defined below:
+======+====================+
| Code | Reason |
+======+====================+
| 0x0 | Session Terminated |
+------+--------------------+
| 0x1 | Generic Error |
+------+--------------------+
| 0x2 | Unauthorized |
+------+--------------------+
| 0x10 | GOAWAY |
+------+--------------------+
Table 1
* Session Terminated: No error occurred; however the endpoint wishes
to terminate the session.
* Generic Error: An unclassified error occurred.
* Unauthorized: The endpoint breached an agreement, which MAY have
been pre-negotiated by the application.
* GOAWAY: The endpoint successfully drained the session after a
GOAWAY was initiated (Section 6.9).
4. Prioritization and Congestion Response
TODO: This is a placeholder section to capture details on how the
MOQT protocol deals with prioritization and congestion overall.
This section is expected to cover details on:
* Prioritization Schemes.
* Congestion Algorithms and impacts.
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* Mapping considerations for one object per stream vs multiple
objects per stream.
* Considerations for merging multiple streams across domains onto
single connection and interactions with specific prioritization
schemes.
4.1. Order Priorities and Options
At the point of this writing, the working group has not reached
consensus on several important goals, such as:
* Ensuring that objects are delivered in the order intended by the
emitter
* Allowing nodes and relays to skip or delay some objects to deal
with congestion
* Ensuring that emitters can accurately predict the behavior of
relays
* Ensuring that when relays have to skip and delay objects belonging
to different tracks that they do it in a predictable way if tracks
are explicitly coordinated and in a fair way if they are not.
The working group has been considering two alternatives: marking
objects belonging to a track with an explicit "send order"; and,
defining algorithms combining tracks, priorities and object order
within a group. The two proposals are listed in Section 4.1.1 and
Section 4.1.2. We expect further work before a consensus is reached.
4.1.1. Proposal - Send Order
Media is produced with an intended order, both in terms of when media
should be presented (PTS) and when media should be decoded (DTS). As
stated in the introduction, the network is unable to maintain this
ordering during congestion without increasing latency.
The encoder determines how to behave during congestion by assigning
each object a numeric send order. The send order SHOULD be followed
when possible, to ensure that the most important media is delivered
when throughput is limited. Note that the contents within each
object are still delivered in order; this send order only applies to
the ordering between objects.
A sender MUST send each object over a dedicated QUIC stream. The
QUIC library should support prioritization (Section 4) such that
streams are transmitted in send order.
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A receiver MUST NOT assume that objects will be received in send
order, for the following reasons:
* Newly encoded objects can have a smaller send order than
outstanding objects.
* Packet loss or flow control can delay the send of individual
streams.
* The sender might not support QUIC stream prioritization.
TODO: Refer to Congestion Response and Prioritization Section for
further details on various proposals.
4.1.2. Proposal - Ordering by Priorities
Media is produced as a set of layers, such as for example low
definition and high definition, or low frame rate and high frame
rate. Each object belonging to a track and a group has two
attributes: the object-id, and the priority (or layer).
When nodes or relays have to choose which object to send next, they
apply the following rules:
* within the same group, objects with a lower priority number (e.g.
P1) are always sent before objects with a numerically greater
priority number (e.g., P2)
* within the same group, and the same priority level, objects with a
lower object-id are always sent before objects with a higher
object-id.
* objects from later groups are normally always sent before objects
of previous groups.
The latter rule is generally agreed as a way to ensure freshness, and
to recover quickly if queues and delays accumulate during a
congestion period. However, there may be cases when finishing the
transmission of an ongoing group results in better user experience
than strict adherence to the freshness rule. We expect that that the
working group will eventually reach consensus and define meta data
that controls this behavior.
There have been proposals to allow emitters to coordinate the
allocation of layer priorities across multiple coordinated tracks.
At this point, these proposals have not reached consensus.
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5. Relays
Relays are leveraged to enable distribution scale in the MoQ
architecture. Relays can be used to form an overlay delivery
network, similar in functionality to Content Delivery Networks
(CDNs). Additionally, relays serve as policy enforcement points by
validating subscribe and publish requests at the edge of a network.
5.1. Subscriber Interactions
Subscribers interact with the Relays by sending a "SUBSCRIBE REQUEST"
(Section 6.3) control message for the tracks of interest. Relays
MUST ensure subscribers are authorized to access the content
associated with the Full Track Name. The authorization information
can be part of subscription request itself or part of the
encompassing session. The specifics of how a relay authorizes a user
are outside the scope of this specification.
The subscriber making the subscribe request is notified of the result
of the subscription, via "SUBSCRIBE OK" (Section 6.4) or the
"SUBSCRIBE ERROR" Section 6.5 control message.
For successful subscriptions, the publisher maintains a list of
subscribers for each full track name. Each new OBJECT belonging to
the track is forwarded to each active subscriber, dependent on the
congestion response. A subscription remains active until it expires,
until the publisher of the track stops producing objects or there is
a subscription error (see Section 6.5).
Relays MAY aggregate authorized subscriptions for a given track when
multiple subscribers request the same track. Subscription
aggregation allows relays to make only a single forward subscription
for the track. The published content received from the forward
subscription request is cached and shared among the pending
subscribers.
5.2. Publisher Interactions
Publishing through the relay starts with publisher sending "ANNOUNCE"
control message with a Track Namespace (Section 2.3).
Relays MUST ensure that publishers are authorized by:
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* Verifying that the publisher is authorized to publish the content
associated with the set of tracks whose Track Namespace matches
the announced namespace. Specifics of where the authorization
happens, either at the relays or forwarded for further processing,
depends on the way the relay is managed and is application
specific (typically based on prior business agreement).
Relays respond with "ANNOUNCE OK" and/or "ANNOUNCE ERROR" control
messages providing the results of announcement.
OBJECT message header carry short hop-by-hop Track Id that maps to
the Full Track Name (see Section 6.4). Relays use the Track ID of an
incoming OBJECT message to identify its track and find the active
subscribers for that track. Relays MUST NOT depend on OBJECT payload
content for making forwarding decisions and MUST only depend on the
fields, such as priority order and other metadata properties in the
OBJECT message header. Unless determined by congestion response,
Relays MUST forward the OBJECT message to the matching subscribers.
5.3. Relay Discovery and Failover
TODO: This section shall cover aspects of relay failover and protocol
interactions.
5.4. Restoring connections through relays
TODO: This section shall cover reconnect considerations for clients
when moving between the Relays.
5.5. Congestion Response at Relays
TODO: Refer to Section 4. Add details to describe relay behavior
when merging or splitting streams and interactions with congestion
response.
5.6. Relay Object Handling
MOQT encodes the delivery information for a stream via OBJECT headers
(Section 6.2).
A relay MUST treat the object payload as opaque. A relay MUST NOT
combine, split, or otherwise modify object payloads. A relay SHOULD
prioritize streams (Section 4) based on the send order/priority.
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A relay that reads from a stream and writes to stream in order will
introduce head-of-line blocking. Packet loss will cause stream data
to be buffered in the QUIC library, awaiting in order delivery, which
will increase latency over additional hops. To mitigate this, a
relay SHOULD read and write QUIC stream data out of order subject to
flow control limits. See section 2.2 in [QUIC].
6. Messages
Both unidirectional and bidirectional QUIC streams contain sequences
of length-delimited messages.
MOQT Message {
Message Type (i),
Message Length (i),
Message Payload (..),
}
Figure 1: MOQT Message
The Message Length field contains the length of the Message Payload
field in bytes. A length of 0 indicates the message is unbounded and
continues until the end of the stream.
+======+=================================+
| ID | Messages |
+======+=================================+
| 0x0 | OBJECT (Section 6.2) |
+------+---------------------------------+
| 0x1 | SETUP (Section 6.1) |
+------+---------------------------------+
| 0x3 | SUBSCRIBE REQUEST (Section 6.3) |
+------+---------------------------------+
| 0x4 | SUBSCRIBE OK (Section 6.4) |
+------+---------------------------------+
| 0x5 | SUBSCRIBE ERROR (Section 6.5) |
+------+---------------------------------+
| 0x6 | ANNOUNCE (Section 6.6) |
+------+---------------------------------+
| 0x7 | ANNOUNCE OK (Section 6.7) |
+------+---------------------------------+
| 0x8 | ANNOUNCE ERROR (Section 6.8) |
+------+---------------------------------+
| 0x10 | GOAWAY (Section 6.9) |
+------+---------------------------------+
Table 2
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6.1. SETUP
The SETUP message is the first message that is exchanged by the
client and the server; it allows the peers to establish the mutually
supported version and agree on the initial configuration before any
objects are exchanged. It is a sequence of key-value pairs called
SETUP parameters; the semantics and format of which can vary based on
whether the client or server is sending. To ensure future
extensibility of MOQT, the peers MUST ignore unknown setup
parameters. TODO: describe GREASE for those.
The wire format of the SETUP message is as follows:
SETUP Parameter {
Parameter Key (i),
Parameter Value Length (i),
Parameter Value (..),
}
Client SETUP Message Payload {
Number of Supported Versions (i),
Supported Version (i) ...,
SETUP Parameters (..) ...,
}
Server SETUP Message Payload {
Selected Version (i),
SETUP Parameters (..) ...,
}
Figure 2: MOQT SETUP Message
The Parameter Value Length field indicates the length of the
Parameter Value.
The client offers the list of the protocol versions it supports; the
server MUST reply with one of the versions offered by the client. If
the server does not support any of the versions offered by the
client, or the client receives a server version that it did not
offer, the corresponding peer MUST close the connection.
The SETUP parameters are described in the Section 6.1.1 section.
6.1.1. SETUP Parameters
Every parameter MUST appear at most once within the SETUP message.
The peers SHOULD verify that and close the connection if a parameter
appears more than once.
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The ROLE parameter is mandatory for the client. All of the other
parameters are optional.
6.1.1.1. ROLE parameter
The ROLE parameter (key 0x00) allows the client to specify what roles
it expects the parties to have in the MOQT connection. It has three
possible values:
0x01: Only the client is expected to send objects on the connection.
This is commonly referred to as the ingestion case.
0x02: Only the server is expected to send objects on the connection.
This is commonly referred to as the delivery case.
0x03: Both the client and the server are expected to send objects.
The client MUST send a ROLE parameter with one of the three values
specified above. The server MUST close the connection if the ROLE
parameter is missing, is not one of the three above-specified values,
or it is different from what the server expects based on the
application.
6.1.1.2. PATH parameter
The PATH parameter (key 0x01) allows the client to specify the path
of the MoQ URI when using native QUIC ([QUIC]). It MUST NOT be used
by the server, or when WebTransport is used. If the peer receives a
PATH parameter from the server, or when WebTransport is used, it MUST
close the connection.
When connecting to a server using a URI with the "moq" scheme, the
client MUST set the PATH parameter to the path-abempty portion of the
URI; if query is present, the client MUST concatenate ?, followed by
the query portion of the URI to the parameter.
6.2. OBJECT
A OBJECT message contains a range of contiguous bytes from from the
specified track, as well as associated metadata required to deliver,
cache, and forward it.
The format of the OBJECT message is as follows:
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OBJECT Message {
Track ID (i),
Group Sequence (i),
Object Sequence (i),
Object Send Order (i),
Object Payload (b),
}
Figure 3: MOQT OBJECT Message
* Track ID: The track identifier obtained as part of subscription
and/or publish control message exchanges.
* Group Sequence : The object is a member of the indicated group
Section 2.2 within the track.
* Object Sequence: The order of the object within the group. The
sequence starts at 0, increasing sequentially for each object
within the group.
* Object Send Order: An integer indicating the object send order
Section 4.1.1 or priority Section 4.1.2 value.
* Object Payload: An opaque payload intended for the consumer and
SHOULD NOT be processed by a relay.
6.3. SUBSCRIBE REQUEST
A receiver issues a SUBSCRIBE REQUEST to a publisher to request a
track.
The format of SUBSCRIBE REQUEST is as follows:
Track Request Parameter {
Track Request Parameter Key (i),
Track Request Parameter Length (i),
Track Request Parameter Value (..),
}
SUBSCRIBE REQUEST Message {
Full Track Name Length (i),
Full Track Name (...),
Track Request Parameters (..) ...
}
Figure 4: MOQT SUBSCRIBE REQUEST Message
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* Full Track Name: Identifies the track as defined in
(Section 2.3.1).
* Track Request Parameters: As defined in Section 6.6.1.
On successful subscription, the publisher SHOULD start delivering
objects from the group sequence and object sequence as defined in the
Track Request Parameters.
6.4. SUBSCRIBE OK
A SUBSCRIBE OK control message is sent for successful subscriptions.
SUBSCRIBE OK
{
Full Track Name Length(i),
Full Track Name(...),
Track ID(i),
Expires (i)
}
Figure 5: MOQT SUBSCRIBE OK Message
* Full Track Name: Identifies the track for which this response is
provided.
* Track ID: Session specific identifier that is used as an alias for
the Full Track Name in the Track ID field of the OBJECT
(Section 6.2) message headers of the requested track. Track IDs
are generally shorter than Full Track Names and thus reduce the
overhead in OBJECT messages.
* Expires: Time in milliseconds after which the subscription is no
longer valid. A value of 0 indicates that the subscription stays
active until it is explicitly unsubscribed.
6.5. SUBSCRIBE ERROR
A publisher sends a SUBSCRIBE ERROR control message in response to a
failed SUBSCRIBE REQUEST.
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SUBSCRIBE ERROR
{
Full Track Name Length(i),
Full Track Name(...),
Error Code (i),
Reason Phrase Length (i),
Reason Phrase (...),
}
Figure 6: MOQT SUBSCRIBE ERROR Message
* Full Track Name: Identifies the track in the request message for
which this response is provided.
* Error Code: Identifies an integer error code for subscription
failure.
* Reason Phrase Length: The length in bytes of the reason phrase.
* Reason Phrase: Provides the reason for subscription error and
Reason Phrase Length field carries its length.
6.6. ANNOUNCE
The publisher sends the ANNOUNCE control message to advertise where
the receiver can route SUBSCRIBE REQUESTs for tracks within the
announced Track Namespace. The receiver verifies the publisher is
authorized to publish tracks under this namespace.
ANNOUNCE Message {
Track Namespace Length(i),
Track Namespace,
Track Request Parameters (..) ...,
}
Figure 7: MOQT ANNOUNCE Message
* Track Namespace: Identifies a track's namespace as defined in
(Section 2.3.1)
* Track Request Parameters: The parameters are defined in
Section 6.6.1.
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6.6.1. Track Request Parameters
The Track Request Parameters identify properties of the track
requested in either the ANNOUNCE or SUSBCRIBE REQUEST control
messages. The peers MUST close the connection if there are
duplicates. The Parameter Value Length field indicates the length of
the Parameter Value.
6.6.1.1. GROUP SEQUENCE Parameter
The GROUP SEQUENCE parameter (key 0x00) identifies the group within
the track to start delivering objects. The publisher MUST start
delivering the objects from the most recent group, when this
parameter is omitted. This parameter is applicable in SUBSCRIBE
REQUEST message.
6.6.1.2. OBJECT SEQUENCE Parameter
The OBJECT SEQUENCE parameter (key 0x01) identifies the object with
the track to start delivering objects. The GROUP SEQUENCE parameter
MUST be set to identify the group under which to start delivery. The
publisher MUST start delivering from the beginning of the selected
group when this parameter is omitted. This parameter is applicable
in SUBSCRIBE REQUEST message.
6.6.1.3. AUTHORIZATION INFO Parameter
AUTHORIZATION INFO parameter (key 0x02) identifies track's
authorization information. This parameter is populated for cases
where the authorization is required at the track level. This
parameter is applicable in SUBSCRIBE REQUEST and ANNOUNCE messages.
6.7. ANNOUNCE OK
The receiver sends an ANNOUNCE OK control message to acknowledge the
successful authorization and acceptance of an ANNOUNCE message.
ANNOUNCE OK
{
Track Namespace
}
Figure 8: MOQT ANNOUNCE OK Message
* Track Namespace: Identifies the track namespace in the ANNOUNCE
message for which this response is provided.
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6.8. ANNOUNCE ERROR
The receiver sends an ANNOUNCE ERROR control message for tracks that
failed authorization.
ANNOUNCE ERROR
{
Track Namespace Length(i),
Track Namespace(...),
Error Code (i),
Reason Phrase Length (i),
Reason Phrase (...),
}
Figure 9: MOQT ANNOUNCE ERROR Message
* Track Namespace: Identifies the track namespace in the ANNOUNCE
message for which this response is provided.
* Error Code: Identifies an integer error code for announcement
failure.
* Reason Phrase: Provides the reason for announcement error and
Reason Phrase Length field carries its length.
6.9. GOAWAY
The server sends a GOAWAY message to force the client to reconnect.
This is useful for server maintenance or reassignments without
severing the QUIC connection. The server can be a producer or a
consumer.
The server:
* MAY initiate a graceful shutdown by sending a GOAWAY message.
* MUST close the QUIC connection after a timeout with the GOAWAY
error code (Section 3.4).
* MAY close the QUIC connection with a different error code if there
is a fatal error before shutdown.
* SHOULD wait until the GOAWAY message and any pending streams have
been fully acknowledged, plus an extra delay to ensure they have
been processed.
The client:
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* MUST establish a new transport session upon receipt of a GOAWAY
message, assuming it wants to continue operation.
* SHOULD establish the new transport session using a different QUIC
connection to that on which it received the GOAWAY message.
* SHOULD remain connected on both connections for a short period,
processing objects from both in parallel.
7. Security Considerations
TODO: Expand this section.
7.1. Resource Exhaustion
Live content requires significant bandwidth and resources. Failure
to set limits will quickly cause resource exhaustion.
MOQT uses QUIC flow control to impose resource limits at the network
layer. Endpoints SHOULD set flow control limits based on the
anticipated bitrate.
Endpoints MAY impose a MAX STREAM count limit which would restrict
the number of concurrent streams which a MOQT Streaming Format could
have in flight.
The producer prioritizes and transmits streams out of order. Streams
might be starved indefinitely during congestion. The producer and
consumer MUST cancel a stream, preferably the lowest priority, after
reaching a resource limit.
8. IANA Considerations
TODO: fill out currently missing registries: * MOQT version numbers *
SETUP parameters * Track Request parameters * Subscribe Error codes *
Announce Error codes * Track format numbers * Message types * Object
headers
TODO: register the URI scheme and the ALPN
TODO: the MOQT spec should establish the IANA registration table for
MoQ Streaming Formats. Each MoQ streaming format can then register
its type in that table. The MoQ Streaming Format type MUST be
carried as the leading varint in catalog track objects.
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Contributors
* Alan Frindell
* Ali Begen
* Charles Krasic
* Christian Huitema
* Cullen Jennings
* James Hurley
* Jordi Cenzano
* Mike English
* Mo Zanaty
* Will Law
References
Normative References
[QUIC] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/rfc/rfc9000>.
[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>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/rfc/rfc7301>.
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[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>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/rfc/rfc8615>.
[RFC9110] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP Semantics", STD 97, RFC 9110,
DOI 10.17487/RFC9110, June 2022,
<https://www.rfc-editor.org/rfc/rfc9110>.
[WebTransport]
Frindell, A., Kinnear, E., and V. Vasiliev, "WebTransport
over HTTP/3", Work in Progress, Internet-Draft, draft-
ietf-webtrans-http3-06, 25 May 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-http3-06>.
Informative References
[I-D.ietf-webtrans-overview]
Vasiliev, V., "The WebTransport Protocol Framework", Work
in Progress, Internet-Draft, draft-ietf-webtrans-overview-
05, 24 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-
webtrans-overview-05>.
Authors' Addresses
Luke Curley
Twitch
Email: kixelated@gmail.com
Kirill Pugin
Meta
Email: ikir@meta.com
Suhas Nandakumar
Cisco
Email: snandaku@cisco.com
Victor Vasiliev
Google
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Email: vasilvv@google.com
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