Media over QUIC Transport

Introduction Media Over QUIC Transport (MOQT) is a protocol that is optimized for the QUIC protocol , either directly or via 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.

describes the object model employed by MOQT.
covers aspects of setting up a MOQT session.
covers mechanisms for prioritizing subscriptions.
covers behavior at the relay entities.
covers how control messages are encoded on the wire.
} covers how data messages are encoded on the wire.

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.

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. Protocols utilizing UDP directly can avoid queuing, but the application is then 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.

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 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.

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.

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.

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 when, and only when, they appear in all capitals, as shown here.

Client:: The party initiating a Transport Session.
Server:: The party accepting an incoming Transport Session.
Endpoint:: A Client or Server.
Publisher:: An endpoint that handles subscriptions by sending requested Objects from the requested track.
Subscriber:: An endpoint that subscribes to and receives tracks.
Original Publisher:: The initial publisher of a given track.
End Subscriber:: A subscriber that initiates a subscription and does not send the data on to other subscribers.
Relay:: An entity that is both a Publisher and a Subscriber, but not the Original Publisher or End Subscriber.
Upstream:: In the direction of the Original Publisher
Downstream:: In the direction of the End Subscriber(s)
Transport Session:: A raw QUIC connection or a WebTransport session.
Congestion:: Packet loss and queuing caused by degraded or overloaded networks.
Group:: A temporal sequence of objects. A group represents a join point in a track. See ().
Object:: An object is an addressable unit whose payload is a sequence of bytes. Objects form the base element in the MOQT data model. See ().
Track:: An encoded bitstream. Tracks contain a sequential series of one or more groups and are the subscribable entity with MOQT. See ().

Notational Conventions This document uses the conventions detailed in () when describing the binary encoding. As a quick reference, the following list provides a non normative summary of the parts of RFC9000 field syntax that are used in this specification.

x (L):: Indicates that x is L bits long
x (i):: Indicates that x holds an integer value using the variable-length encoding as described in ()
x (..):: Indicates that x can be any length including zero bits long. Values in this format always end on a byte boundary.
[x (L)]:: Indicates that x is optional and has a length of L
x (L) ...:: Indicates that x is repeated zero or more times and that each instance has a length of L

This document extends the RFC9000 syntax and with the additional field types:

x (b):: Indicates that x consists of a variable length integer encoding as described in (), followed by that many bytes of binary data
x (tuple):: Indicates that x is a tuple, consisting of a variable length integer encoded as described in (), followed by that many variable length tuple fields, each of which are encoded as (b) above.

To reduce unnecessary use of bandwidth, variable length integers SHOULD be encoded using the least number of bytes possible to represent the required value.

Object Data Model MOQT has a hierarchical data model, comprised of tracks which contain groups, and groups that contain objects. Inside of a group, the objects can be organized into subgroups. To give an example of how an application might use this data model, consider an application sending high and low resolution video using a codec with temporal scalability. Each resolution is sent as a separate track to allow the subscriber to pick the appropriate resolution given the display environment and available bandwidth. Each "group of pictures" in a video is sent as a group because the first frame is needed to decode later frames. This allows the client to join at the logical points where they can get the information to start decoding the stream. The temporal layers are sent as separate sub groups to allow the priority mechanism to favour the base layer when there is not enough bandwidth to send both the base and enhancement layers. Each frame of video on a given layer is sent as a single object.

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. An object is uniquely identified by its track namespace, track name, group ID, and object ID, and must be an identical sequence of bytes regardless of how or where it is retrieved. An Object can become unavailable, but its contents MUST NOT change over time. Objects are comprised of two parts: metadata and a payload. The metadata is never encrypted and is always visible to relays (see ). The payload portion may be encrypted, in which case it is only visible to the Original Publisher and End Subscribers. 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.

Subgroups A subgroup is a sequence of one or more objects from the same group () in ascending order by Object ID. Objects in a subgroup have a dependency and priority relationship consistent with sharing a QUIC stream. In some cases, a Group will be most effectively delivered using more than one QUIC stream. When a Track's forwarding preference (see ) is "Track" or "Datagram", Objects are not sent in Subgroups, no Subgroup IDs are assigned, and the description in the remainder of this section does not apply. QUIC streams offer in-order reliable delivery and the ability to cancel sending and retransmission of data. Furthermore, many implementations offer the ability to control the relative priority of streams, which allows control over the scheduling of sending data on active streams. Every object within a Group belongs to exactly one Subgroup. Objects from two subgroups cannot be sent on the same QUIC stream. Objects from the same Subgroup MUST NOT be sent on different QUIC streams, unless one of the streams was reset prematurely, or upstream conditions have forced objects from a Subgroup to be sent out of Object ID order. Original publishers assign each Subgroup a Subgroup ID, and do so as they see fit. The scope of a Subgroup ID is a Group, so Subgroups from different Groups MAY share a Subgroup ID without implying any relationship between them. In general, publishers assign objects to subgroups in order to leverage the features of QUIC streams as described above. An example strategy for using QUIC stream properties follows. If object B is dependent on object A, then delivery of B can follow A, i.e. A and B can be usefully delivered over a single QUIC stream. Furthermore, in this example:

If an object is dependent on all previous objects in a Subgroup, it is added to that Subgroup.
If an object is not dependent on all of the objects in a Subgroup, it goes in a different Subgroup.
There are often many ways to compose Subgroups that meet these criteria. Where possible, choose the composition that results in the fewest Subgroups in a group to minimize the number of QUIC streams used.

Groups A group is a collection of objects and is a sub-unit of a track (). Groups SHOULD be indendepently useful, so objects within a group SHOULD NOT depend on objects in other groups. A group provides a join point for subscriptions, so a subscriber that does not want to receive the entire track can opt to receive only the latest group(s). The publisher then selectively transmits objects based on their group membership.

Track A track is a sequence of groups (). It is the entity against which a subscriber issues a subscription request. A subscriber can request to receive individual tracks starting at a group boundary, including any new objects pushed by the publisher while the track is active.

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. Track namespace is an ordered N-tuple of bytes where N can be between 1 and 32. The structured nature of Track Namespace allows relays and applications to manipulate prefixes of a namespace. Track name is a sequence of bytes. In this specification, both the Track Namespace tuple fields and the Track Name are not constrained to a specific encoding. They carry a sequence of bytes and comparison between two Track Namespace tuple fields or Track Names is done by exact comparison of the bytes. Specifications that use MoQ Transport may constrain the information in these fields, for example by restricting them to UTF-8. Any specification that does needs to specify the canonicalization into the bytes in the Track Namespace or Track Name such that exact comparison works.

Scope A MOQT scope is a set of servers (as identified by their connection URIs) for which the tuple of Track Name and Track Namespace are guaranteed to be unique and identify a specific track. It is up to the application using MOQT to define how broad or narrow the scope is. 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. Because the tuple of Track Namespace and Track Name are unique within an MOQT scope, they can be used as a cache key. MOQT does not provide any in-band content negotiation methods similar to the ones defined by HTTP (); if, at a given moment in time, two tracks within the same scope contain different data, they have to have different names and/or namespaces.

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 .

Sessions

Session establishment This document defines a protocol that can be used interchangeably both over a QUIC connection directly , and over WebTransport . Both provide streams and datagrams with similar semantics (see ); thus, the main difference lies in how the servers are identified and how the connection is established. When using QUIC, datagrams MUST be supported via the [QUIC-DATAGRAM] extension, which is already a requirement for WebTransport over HTTP/3. There is no definition of the protocol over other transports, such as TCP, and applications using MoQ might need to fallback to another protocol when QUIC or WebTransport aren't available.

WebTransport A MOQT server that is accessible via WebTransport can be identified using an HTTPS URI (). 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 .

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 : The authority portion MUST NOT contain a non-empty host portion. The moq URI scheme supports the /.well-known/ path prefix defined in . 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 () which is sent in the CLIENT_SETUP message at the start of the session. The ALPN value used by the protocol is moq-00.

Version and Extension Negotiation Endpoints use the exchange of Setup messages to negotiate the MOQT version and any extensions to use. The client indicates the MOQT versions it supports in the CLIENT_SETUP message (see ). It also includes the union of all Setup Parameters required for a handshake by any of those versions. Within any MOQT version, clients request the use of extensions by adding Setup parameters corresponding to that extension. No extensions are defined in this document. The server replies with a SERVER_SETUP message that indicates the chosen version, includes all parameters required for a handshake in that version, and parameters for every extension requested by the client that it supports. New versions of MOQT MUST specify which existing extensions can be used with that version. New extensions MUST specify the existing versions with which they can be used. If a given parameter carries the same information in multiple versions, but might have different optimal values in those versions, there SHOULD be separate Setup parameters for that information in each version.

Session initialization The first stream opened is a client-initiated bidirectional control stream where the peers exchange Setup messages (). All messages defined in this draft except OBJECT and OBJECT_WITH_LENGTH are sent on the control stream after the Setup message. Control messages MUST NOT be sent on any other stream, and a peer receiving a control message on a different stream closes the session as a 'Protocol Violation'. Objects MUST NOT be sent on the control stream, and a peer receiving an Object on the control stream closes the session as a 'Protocol Violation'. This draft only specifies a single use of bidirectional streams. Objects are sent on unidirectional streams. Because there are no other uses of bidirectional streams, a peer MAY currently close the session as a 'Protocol Violation' if it receives a second bidirectional stream. The control stream MUST NOT be closed at the underlying transport layer while the session is active. Doing so results in the session being closed as a 'Protocol Violation'.

Stream Cancellation Streams aside from the control stream MAY be canceled due to congestion or other reasons by either the publisher or subscriber. Early termination of a stream does not affect the MoQ application state, and therefore has no effect on outstanding subscriptions.

Termination The Transport Session can be terminated at any point. When native QUIC is used, the session is closed using the CONNECTION_CLOSE frame (). When WebTransport is used, the session is closed using the CLOSE_WEBTRANSPORT_SESSION capsule (). The application MAY use any error message and SHOULD use a relevant code, as defined below:

Code	Reason
0x0	No Error
0x1	Internal Error
0x2	Unauthorized
0x3	Protocol Violation
0x4	Duplicate Track Alias
0x5	Parameter Length Mismatch
0x6	Too Many Subscribes
0x10	GOAWAY Timeout

No Error: The session is being terminated without an error.
Internal Error: An implementation specific error occurred.
Unauthorized: The endpoint breached an agreement, which MAY have been pre-negotiated by the application.
Protocol Violation: The remote endpoint performed an action that was disallowed by the specification.
Duplicate Track Alias: The endpoint attempted to use a Track Alias that was already in use.
Too Many Subscribes: The session was closed because the subscriber used a Subscribe ID equal or larger than the current Maximum Subscribe ID.
GOAWAY Timeout: The session was closed because the client took too long to close the session in response to a GOAWAY () message. See session migration ().

Migration MoqTransport requires a long-lived and stateful session. However, a service provider needs the ability to shutdown/restart a server without waiting for all sessions to drain naturally, as that can take days for long-form media. MoqTransport avoids this via the GOAWAY message (). The server sends a GOAWAY message, signaling that the client should establish a new session and migrate any active subscriptions. The GOAWAY message may contain a new URI for the new session, otherwise the current URI is reused. The server SHOULD terminate the session with 'GOAWAY Timeout' after a sufficient timeout if there are still open subscriptions on a connection. The GOAWAY message does not immediately impact subscription state. A subscriber SHOULD individually UNSUBSCRIBE for each existing subscription, while a publisher MAY reject new SUBSCRIBEs while in the draining state. When the server is a subscriber, it SHOULD send a GOAWAY message to downstream subscribers prior to any UNSUBSCRIBE messages to upstream publishers. After the client receives a GOAWAY, it's RECOMMENDED that the client waits until there are no more active subscriptions before closing the session with NO_ERROR. Ideally this is transparent to the application using MOQT, which involves establishing a new session in the background and migrating active subscriptions and announcements. The client can choose to delay closing the session if it expects more OBJECTs to be delivered. The server closes the session with a 'GOAWAY Timeout' if the client doesn't close the session quickly enough.

Priorities MoQ priorities allow a subscriber and original publisher to influence the transmission order of Objects within a session in the presence of congestion. Given the critical nature of control messages and their relatively small size, the control stream SHOULD be prioritized higher than all subscribed Objects. Both SUBSCRIBE and FETCH messages indicate a subscriber priority and group order, so the following text applies equally to both types of subscriptions. The subscriber indicates the priority of a subscription via the Subscriber Priority field and the original publisher indicates priority in every stream or datagram header. As such, the subscriber's priority is a property of the subscription and the original publisher's priority is a property of the Track and the Objects it contains. In both cases, a lower value indicates a higher priority, with 0 being the highest priority. When Objects are contained in Subgroups, all Objects in the Subgroup have the same priority. The Subscriber Priority is considered first when selecting a subscription to send data on within a given session. When two or more subscriptions have equal subscriber priority, the original publisher priority is considered next and can change within the track, so subscriptions are prioritized based on the highest priority data available to send. For example, if the subscription had data at priority 6 and priority 10 to send, the subscription priority would be 6. When both the subscriber and original publisher priorities for a subscription are equal, how much data to send from each subscription is implementation-dependent, but the expectation is that all subscriptions will be able to send some data. The subscriber's priority can be changed via a SUBSCRIBE_UPDATE message. This updates the priority of all unsent data within the subscription, though the details of the reprioritization are implementation-specific. Subscriptions have a Group Order of either 'Ascending' or 'Descending', which indicates whether the lowest or highest Group Id SHOULD be sent first when multiple Groups are available to send. A subscriber can specify either 'Ascending' or 'Descending' in the SUBSCRIBE message or they can specify they want to use the Original Publisher's Group Order, which is indicated in the corresponding SUBSCRIBE_OK. 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, regardless of the specified Group Order. If the group contains more than one Subgroup and the priority varies between these Subgroups, higher priority Subgroups are sent before lower priority Subgroups. If the specified priority of two Subgroups in a Group are equal, the lower Subgroup ID has priority. Within a Subgroup, Objects MUST be sent in increasing Object ID order. The Group Order cannot be changed via a SUBSCRIBE_UPDATE message, and instead an UNSUBSCRIBE and SUBSCRIBE can be used. Relays SHOULD respect the subscriber and original publisher's priorities. Relays SHOULD NOT directly use Subscriber Priority or Group Order from incoming subscriptions for upstream subscriptions. Relays use of Subscriber Priority for upstream subscriptions can be based on factors specific to it, such as the popularity of the content or policy, or relays can specify the same value for all upstream subscriptions. MoQ Sessions can span multiple namespaces, and priorities might not be coordinated across namespaces. The subscriber's priority is considered first, so there is a mechanism for a subscriber to fix incompatibilities between different namespaces prioritization schemes. Additionally, it is anticipated that when multiple namespaces are present within a session, the namespaces could be coordinating, possibly part of the same application. In cases when pooling among namespaces is expected to cause issues, multiple MoQ sessions, either within a single connection or on multiple connections can be used.

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. Relays are endpoints, which means they terminate Transport Sessions in order to have visibility of MoQ Object metadata. Relays MAY cache Objects, but are not required to.

Subscriber Interactions Subscribers interact with the Relays by sending a SUBSCRIBE () control message for the tracks of interest. Relays MUST ensure subscribers are authorized to access the content associated with the track. 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 is notified of the result of the subscription via a SUBSCRIBE_OK () or SUBSCRIBE_ERROR control message. The entity receiving the SUBSCRIBE MUST send only a single response to a given SUBSCRIBE of either SUBSCRIBE_OK or SUBSCRIBE_ERROR. If a relay does not already have a subscription for the track, or if the subscription does not cover all the requested Objects, it will need to make an upstream subscription. The relay SHOULD NOT return a SUBCRIBE_OK until at least one SUBSCRIBE_OK has been received for the track, to ensure the Group Order is correct. For successful subscriptions, the publisher maintains a list of subscribers for each track. Each new OBJECT belonging to the track within the subscription range is forwarded to each active subscriber, dependent on the congestion response. A subscription remains active until the publisher of the track terminates the subscription with a SUBSCRIBE_DONE (see ). A caching relay saves Objects to its cache identified by the Object's Full Track Name, Group ID and Object ID. Relays MUST be able to process objects for the same Full Track Name from multiple publishers and forward objects to active matching subscriptions. If multiple objects are received with the same Full Track Name, Group ID and Object ID, Relays MAY ignore subsequently received Objects or MAY use them to update the cache. Implementations that update the cache need to be protect against cache poisoning. Objects MUST NOT be sent for unsuccessful subscriptions, and if a subscriber receives a SUBSCRIBE_ERROR after receiving objects, it MUST close the session with a 'Protocol Violation'. A relay MUST NOT reorder or drop objects received on a multi-object stream when forwarding to subscribers, unless it has application specific information. 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 upstream subscription for the track. The published content received from the upstream subscription request is cached and shared among the pending subscribers. The application SHOULD use a relevant error code in SUBSCRIBE_ERROR, as defined below:

Code	Reason
0x0	Internal Error
0x1	Invalid Range
0x2	Retry Track Alias
0x3	Track Does Not Exist
0x4	Unauthorized
0x5	Timeout

The application SHOULD use a relevant status code in SUBSCRIBE_DONE, as defined below:

Code	Reason
0x0	Unsubscribed
0x1	Internal Error
0x2	Unauthorized
0x3	Track Ended
0x4	Subscription Ended
0x5	Going Away
0x6	Expired

Graceful Publisher Relay Switchover This section describes behavior a subscriber MAY implement to allow for a better user experience when a relay sends a GOAWAY. When a subscriber receives the GOAWAY message, it starts the process of connecting to a new relay and sending the SUBSCRIBE requests for all active subscriptions to the new relay. The new relay will send a response to the subscribes and if they are successful, the subscriptions to the old relay can be stopped with an UNSUBSCRIBE.

Publisher Interactions Publishing through the relay starts with publisher sending ANNOUNCE control message with a Track Namespace (). The announce enables the relay to know which publisher to forward a SUBSCRIBE to. Relays MUST ensure that publishers are authorized by:

Verifying that the publisher is authorized to publish the content associated with the set of tracks whose Track Namespace matches the announced namespace. Where the authorization and identification of the publisher occurs depends on the way the relay is managed and is application specific.

Relays respond with an ANNOUNCE_OK or ANNOUNCE_ERROR control message providing the result of announcement. The entity receiving the ANNOUNCE MUST send only a single response to a given ANNOUNCE of either ANNOUNCE_OK or ANNOUNCE_ERROR. A Relay can receive announcements from multiple publishers for the same Track Namespace and it SHOULD respond with the same response to each of the publishers, as though it was responding to an ANNOUNCE from a single publisher for a given tracknamespace. When a publisher wants to stop new subscriptions for an announced namespace it sends an UNANNOUNCE. A subscriber indicates it will no longer route subscriptions for a namespace it previously responded ANNOUNCE_OK to by sending an ANNOUNCE_CANCEL. A relay manages sessions from multiple publishers and subscribers, connecting them based on the track namespace. This MUST use an exact match on track namespace unless otherwise negotiated by the application. For example, a SUBSCRIBE namespace=foobar message will be forwarded to the session that sent ANNOUNCE namespace=foobar. When a relay receives an incoming SUBSCRIBE request that triggers an upstream subscription, it SHOULD send a SUBSCRIBE request to each publisher that has announced the subscription's namespace, unless it already has an active subscription for the Objects requested by the incoming SUBSCRIBE request from all available publishers. When a relay receives an incoming ANNOUCE for a given namespace, for each active upstream subscription that matches that namespace, it SHOULD send a SUBSCRIBE to that publisher that send the ANNOUNCE. OBJECT message headers carry a short hop-by-hop Track Alias that maps to the Full Track Name (see ). Relays use the Track Alias of an incoming OBJECT message to identify its track and find the active subscribers for that track. Relays MUST forward OBJECT messages to matching subscribers in accordance to each subscription's priority, group order, and delivery timeout.

Graceful Publisher Network Switchover This section describes behavior that a publisher MAY choose to implement to allow for a better users experience when switching between networks, such as WiFi to Cellular or vice versa. If the original publisher detects it is likely to need to switch networks, for example because the WiFi signal is getting weaker, and it does not have QUIC connection migration available, it establishes a new session over the new interface and sends an ANNOUCE. The relay will forward matching subscribes and the publisher publishes objects on both sessions. Once the subscriptions have migrated over to session on the new network, the publisher can stop publishing objects on the old network. The relay will drop duplicate objects received on both subscriptions. Ideally, the subscriptions downstream from the relay do no observe this change, and keep receiving the objects on the same subscription.

Graceful Publisher Relay Switchover This section describes behavior that a publisher MAY choose to implement to allow for a better user experience when a relay sends them a GOAWAY. When a publisher receives a GOAWAY, it starts the process of connecting to a new relay and sends announces, but it does not immediately stop publishing objects to the old relay. The new relay will send subscribes and the publisher can start sending new objects to the new relay instead of the old relay. Once objects are going to the new relay, the announcement and subscription to the old relay can be stopped.

Relay Object Handling MOQT encodes the delivery information for a stream via OBJECT headers (). A relay MUST NOT modify Object properties when forwarding. A relay MUST treat the object payload as opaque. A relay MUST NOT combine, split, or otherwise modify object payloads. A relay SHOULD prioritize sending Objects based on . A publisher SHOULD begin sending incomplete objects when available to avoid incurring additional latency. 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 library, awaiting in order delivery, which will increase latency over additional hops. To mitigate this, a relay SHOULD read and write stream data out of order subject to flow control limits. See section 2.2 in .

Control Messages MOQT uses a single bidirectional stream to exchange control messages, as defined in . Every single message on the control stream is formatted as follows:

MOQT Message

ID	Messages
0x2	SUBSCRIBE_UPDATE ()
0x3	SUBSCRIBE ()
0x4	SUBSCRIBE_OK ()
0x5	SUBSCRIBE_ERROR ()
0x6	ANNOUNCE ()
0x7	ANNOUNCE_OK ()
0x8	ANNOUNCE_ERROR ()
0x9	UNANNOUNCE ()
0xA	UNSUBSCRIBE ()
0xB	SUBSCRIBE_DONE ()
0xC	ANNOUNCE_CANCEL ()
0xD	TRACK_STATUS_REQUEST ()
0xE	TRACK_STATUS ()
0x10	GOAWAY ()
0x11	SUBSCRIBE_ANNOUNCES ()
0x12	SUBSCRIBE_ANNOUNCES_OK ()
0x13	SUBSCRIBE_ANNOUNCES_ERROR (
0x14	UNSUBSCRIBE_ANNOUNCES ()
0x15	MAX_SUBSCRIBE_ID ()
0x16	FETCH ()
0x17	FETCH_CANCEL ()
0x18	FETCH_OK ()
0x19	FETCH_ERROR ()
0x40	CLIENT_SETUP ()
0x41	SERVER_SETUP ()

An endpoint that receives an unknown message type MUST close the session. Control messages have a length to make parsing easier, but no control messages are intended to be ignored. If the length does not match the length of the message content, the receiver MUST close the session.

Parameters Some messages include a Parameters field that encode optional message elements. They contain a type, length, and value. Senders MUST NOT repeat the same parameter type in a message. Receivers SHOULD check that there are no duplicate parameters and close the session as a 'Protocol Violation' if found. Receivers ignore unrecognized parameters. The format of Parameters is as follows:

MOQT Parameter Parameter Type is an integer that indicates the semantic meaning of the parameter. Setup message parameters use a namespace that is constant across all MoQ Transport versions. All other messages use a version-specific namespace. For example, the integer '1' can refer to different parameters for Setup messages and for all other message types. SETUP message parameter types are defined in . Version- specific parameter types are defined in . The Parameter Length field of the String Parameter encodes the length of the Parameter Value field in bytes. Each parameter description will indicate the data type in the Parameter Value field. If a receiver understands a parameter type, and the parameter length implied by that type does not match the Parameter Length field, the receiver MUST terminate the session with error code 'Parameter Length Mismatch'.

Version Specific Parameters Each version-specific parameter definition indicates the message types in which it can appear. If it appears in some other type of message, it MUST be ignored. Note that since Setup parameters use a separate namespace, it is impossible for these parameters to appear in Setup messages.

AUTHORIZATION INFO AUTHORIZATION INFO parameter (key 0x02) identifies a track's authorization information in a SUBSCRIBE, SUBSCRIBE_ANNOUNCES or ANNOUNCE message. This parameter is populated for cases where the authorization is required at the track level. The value is an ASCII string.

DELIVERY TIMEOUT Parameter The DELIVERY TIMEOUT parameter (key 0x03) MAY appear in a SUBSCRIBE, SUBSCRIBE_OK, or a SUBSCRIBE_UDPATE message. It is the duration in milliseconds the relay SHOULD continue to attempt forwarding Objects after they have been received. The start time for the timeout is based on when the beginning of the Object is received, and does not depend upon the forwarding preference. There is no explicit signal that an Object was not sent because the delivery timeout was exceeded. If both the subscriber and publisher specify the parameter, they use the min of the two values for the subscription. The publisher SHOULD always specify the value received from an upstream subscription when there is one, and nothing otherwise. If an earlier Object arrives later than subsequent Objects, relays can consider the receipt time as that of the next later Object, with the assumption that the Object's data was reordered. If neither the subscriber or publisher specify DELIVERY TIMEOUT, Objects are delivered as indicated by their Group Order and Priority. When sent by a subscriber, this parameter is intended to be specific to a subscription, so it SHOULD NOT be forwarded upstream by a relay that intends to serve multiple subscriptions for the same track. Publishers SHOULD consider whether the entire Object is likely to be delivered before sending any data for that Object, taking into account priorities, congestion control, and any other relevant information.

MAX CACHE DURATION Parameter MAX_CACHE_DURATION (key 0x04): An integer expressing a number of milliseconds. If present, the relay MUST NOT start forwarding any individual Object received through this subscription after the specified number of milliseconds has elapsed since the beginning of the Object was received. This means Objects earlier in a multi-object stream will expire earlier than Objects later in the stream. Once Objects have expired, their state becomes unknown, and a relay that handles a subscription that includes those Objects re-requests them.

CLIENT_SETUP and SERVER_SETUP The CLIENT_SETUP and SERVER_SETUP messages are the first messages exchanged by the client and the server; they 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 messages are as follows:

MOQT Setup Messages The available versions and Setup parameters are detailed in the next sections.

Versions MoQ Transport versions are a 32-bit unsigned integer, encoded as a varint. This version of the specification is identified by the number 0x00000001. Versions with the most significant 16 bits of the version number cleared are reserved for use in future IETF consensus documents. 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 session. [[RFC editor: please remove the remainder of this section before publication.]] The version number for the final version of this specification (0x00000001), is reserved for the version of the protocol that is published as an RFC. Version numbers used to identify IETF drafts are created by adding the draft number to 0xff000000. For example, draft-ietf-moq-transport-13 would be identified as 0xff00000D.

Setup Parameters

ROLE The ROLE parameter (key 0x00) allows each endpoint to independently specify what functionality they support for the session. It has three possible values, which are of type varint:

0x01: Publisher: The endpoint can process subscriptions and send objects, but not subscribe. The endpoint MUST NOT send a SUBSCRIBE message and an ANNOUNCE MUST NOT be sent to it.
0x02: Subscriber: The endpoint can send subscriptions and receive objects, but not publish. The endpoint MUST NOT send an ANNOUNCE message and a SUBSCRIBE MUST NOT be sent to it.
0x03: PubSub: The endpoint can act as a publisher or subscriber, and can send or process any message type.

Both endpoints MUST send a ROLE parameter with one of the three values specified above. Both endpoints MUST close the session if the ROLE parameter is missing or is not one of the three above-specified values.

PATH The PATH parameter (key 0x01) allows the client to specify the path of the MoQ URI when using native 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. It follows the URI formatting rules . 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.

MAX_SUBSCRIBE_ID The MAX_SUBSCRIBE_ID parameter (key 0x02) communicates an initial value for the Maximum Subscribe ID to the receiving subscriber. The default value is 0, so if not specified, the peer MUST NOT create subscriptions.

GOAWAY The server sends a GOAWAY message to initiate session migration () with an optional URI. The server MUST terminate the session with a Protocol Violation () if it receives a GOAWAY message. The client MUST terminate the session with a Protocol Violation () if it receives multiple GOAWAY messages.

MOQT GOAWAY Message

New Session URI: The client MUST use this URI for the new session if provided. If the URI is zero bytes long, the current URI is reused instead. The new session URI SHOULD use the same scheme as the current URL to ensure compatibility.

SUBSCRIBE A subscription causes the publisher to send newly published objects for a track. A subscriber MUST NOT make multiple active subscriptions for a track within a single session and publishers SHOULD treat this as a protocol violation. The only objects prior to the current object that can be requested are those in the current group. Filter Types The subscriber specifies a filter on the subscription to allow the publisher to identify which objects need to be delivered. There are 4 types of filters: Latest Group (0x1) : Specifies an open-ended subscription with objects from the beginning of the current group. If no content has been delivered yet, the subscription starts with the first published or received group. Latest Object (0x2): Specifies an open-ended subscription beginning from the current object of the current group. If no content has been delivered yet, the subscription starts with the first published or received group. AbsoluteStart (0x3): Specifies an open-ended subscription beginning from the object identified in the StartGroup and StartObject fields. If the StartGroup is prior to the current group, the publisher MUST reply with a SUBSCRIBE_ERROR with code 'Invalid Range'. AbsoluteRange (0x4): Specifies a closed subscription starting at StartObject in StartGroup and ending at EndObject in EndGroup. The start and end of the range are inclusive. EndGroup and EndObject MUST specify the same or a later object than StartGroup and StartObject. If the StartGroup is prior to the current group, the publisher MUST reply with a SUBSCRIBE_ERROR with code 'Invalid Range'. A filter type other than the above MUST be treated as error. If a subscriber wants to subscribe to Objects both before and after the Latest Object, it can send a SUBSCRIBE for the Latest Object followed by a FETCH. Depending upon the application, one might want to send both messages at the same time or wait for the first to return before sending the second. The format of SUBSCRIBE is as follows:

MOQT SUBSCRIBE Message

Subscribe ID: The subscriber specified identifier used to manage a subscription. Subscribe ID is a variable length integer that MUST be unique and monotonically increasing within a session and MUST be less than the session's Maximum Subscribe ID.
Track Alias: A session specific identifier for the track. Messages that reference a track, such as OBJECT (), reference this Track Alias instead of the Track Name and Track Namespace to reduce overhead. If the Track Alias is already being used for a different track, the publisher MUST close the session with a Duplicate Track Alias error ().
Track Namespace: Identifies the namespace of the track as defined in ().
Track Name: Identifies the track name as defined in ().
Subscriber Priority: Specifies the priority of a subscription relative to other subscriptions in the same session. Lower numbers get higher priority. See .
Group Order: Allows the subscriber to request Objects be delivered in Ascending (0x1) or Descending (0x2) order by group. See . A value of 0x0 indicates the original publisher's Group Order SHOULD be used. Values larger than 0x2 are a protocol error.
Filter Type: Identifies the type of filter, which also indicates whether the StartGroup/StartObject and EndGroup/EndObject fields will be present.
StartGroup: The start Group ID. Only present for "AbsoluteStart" and "AbsoluteRange" filter types.
StartObject: The start Object ID. Only present for "AbsoluteStart" and "AbsoluteRange" filter types.
EndGroup: The end Group ID. Only present for the "AbsoluteRange" filter type.
EndObject: The end Object ID, plus 1. A value of 0 means the entire group is requested. Only present for the "AbsoluteRange" filter type.
Subscribe Parameters: The parameters are defined in .

On successful subscription, the publisher MUST reply with a SUBSCRIBE_OK, allowing the subscriber to determine the start group/object when not explicitly specified and the publisher SHOULD start delivering objects. If a publisher cannot satisfy the requested start or end or if the end has already been published it SHOULD send a SUBSCRIBE_ERROR with code 'Invalid Range'. A publisher MUST NOT send objects from outside the requested start and end.

SUBSCRIBE_UPDATE A subscriber issues a SUBSCRIBE_UPDATE to a publisher to request a change to a prior subscription. Subscriptions can only become more narrower, not wider, because an attempt to widen a subscription could fail. If Objects before the start or after the end of the current subscription are needed, a separate subscription can be made. The start Object MUST NOT decrease and when it increases, there is no guarantee that a publisher will not have already sent Objects before the new start Object. The end Object MUST NOT increase and when it decreases, there is no guarantee that a publisher will not have already sent Objects after the new end Object. A publisher SHOULD close the Session as a 'Protocol Violation' if the SUBSCRIBE_UPDATE violates either rule or if the subscriber specifies a Subscribe ID that does not exist within the Session. Unlike a new subscription, SUBSCRIBE_UPDATE can not cause an Object to be delivered multiple times. Like SUBSCRIBE, EndGroup and EndObject MUST specify the same or a later object than StartGroup and StartObject. The format of SUBSCRIBE_UPDATE is as follows:

MOQT SUBSCRIBE_UPDATE Message

Subscribe ID: The subscription identifier that is unique within the session. This MUST match an existing Subscribe ID.
StartGroup: The start Group ID.
StartObject: The start Object ID.
EndGroup: The end Group ID, plus 1. A value of 0 means the subscription is open-ended.
EndObject: The end Object ID, plus 1. A value of 0 means the entire group is requested.
Subscriber Priority: Specifies the priority of a subscription relative to other subscriptions in the same session. Lower numbers get higher priority. See .
Subscribe Parameters: The parameters are defined in .

UNSUBSCRIBE A subscriber issues a UNSUBSCRIBE message to a publisher indicating it is no longer interested in receiving media for the specified track and Objects should stop being sent as soon as possible. The publisher sends a SUBSCRIBE_DONE to acknowledge the unsubscribe was successful and indicate the final Object. The format of UNSUBSCRIBE is as follows:

MOQT UNSUBSCRIBE Message

Subscribe ID: Subscription Identifier as defined in .

FETCH A subscriber issues a FETCH to a publisher to request a range of already published objects within a track. The publisher responding to a FETCH is responsible for retrieving all available Objects. If there are gaps between Objects, the publisher omits them from the fetch response. All omitted objects have status Object Not Available. A publisher responds to a FETCH request with either a FETCH_OK or a FETCH_ERROR message. If it responds with FETCH_OK, the publisher creates a new unidirectional stream that is used to send the Objects. A relay MAY start sending objects immediately in response to a FETCH, even if sending the FETCH_OK takes longer because it requires going upstream to populate the latest object. The Object Forwarding Preference does not apply to fetches. Fetch specifies an inclusive range of Objects starting at StartObject in StartGroup and ending at EndObject in EndGroup. EndGroup and EndObject MUST specify the same or a later object than StartGroup and StartObject. The format of FETCH is as follows:

MOQT FETCH Message

Subscribe ID: The Subscribe ID identifies a given fetch request. Subscribe ID is a variable length integer that MUST be unique and monotonically increasing within a session.
Track Namespace: Identifies the namespace of the track as defined in ().
Track Name: Identifies the track name as defined in ().
Subscriber Priority: Specifies the priority of a fetch request relative to other subscriptions or fetches in the same session. Lower numbers get higher priority. See .
Group Order: Allows the subscriber to request Objects be delivered in Ascending (0x1) or Descending (0x2) order by group. See . A value of 0x0 indicates the original publisher's Group Order SHOULD be used. Values larger than 0x2 are a protocol error.
StartGroup: The start Group ID.
StartObject: The start Object ID.
EndGroup: The end Group ID.
EndObject: The end Object ID, plus 1. A value of 0 means the entire group is requested.
Parameters: The parameters are defined in .

Objects that are not yet published will not be retrieved by a FETCH. The latest available Object is indicated in the FETCH_OK, and is the last Object a fetch will return if the EndGroup and EndObject have not yet been published. A publisher MUST send fetched groups in the determined group order, either ascending or descending. Within each group, objects are sent in Object ID order; subgroup ID is not used for ordering. If StartGroup/StartObject is greater than the latest published Object group, the publisher MUST return FETCH_ERROR with error code 'No Objects'. A publisher MUST send fetched groups in group order, either ascending or descending. Within each group, objects are sent in Object ID order; subgroup ID is not used for ordering.

FETCH_CANCEL A subscriber issues a FETCH_CANCEL message to a publisher indicating it is no longer interested in receiving Objects for the fetch specified by 'Subscribe ID'. The publisher SHOULD close the unidirectional stream as soon as possible. The format of FETCH_CANCEL is as follows:

MOQT FETCH_CANCEL Message

Subscribe ID: Subscription Identifier as defined in .

ANNOUNCE_OK The subscriber sends an ANNOUNCE_OK control message to acknowledge the successful authorization and acceptance of an ANNOUNCE message.

MOQT ANNOUNCE_OK Message

Track Namespace: Identifies the track namespace in the ANNOUNCE message for which this response is provided.

ANNOUNCE_ERROR The subscriber sends an ANNOUNCE_ERROR control message for tracks that failed authorization.

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.

ANNOUNCE_CANCEL The subscriber sends an ANNOUNCE_CANCEL control message to indicate it will stop sending new subscriptions for tracks within the provided Track Namespace. If a publisher receives new subscriptions for that namespace after receiving an ANNOUNCE_CANCEL, it SHOULD close the session as a 'Protocol Violation'.

MOQT ANNOUNCE_CANCEL Message

Track Namespace: Identifies a track's namespace as defined in ().
Error Code: Identifies an integer error code for canceling the announcement.
Reason Phrase: Provides the reason for announcement cancelation.

TRACK_STATUS_REQUEST A potential subscriber sends a 'TRACK_STATUS_REQUEST' message on the control stream to obtain information about the current status of a given track. A TRACK_STATUS message MUST be sent in response to each TRACK_STATUS_REQUEST.

MOQT TRACK_STATUS_REQUEST Message

SUBSCRIBE_ANNOUNCES The subscriber sends the SUBSCRIBE_ANNOUNCES control message to a publisher to request the current set of matching announcements, as well as future updates to the set.

MOQT SUBSCRIBE_ANNOUNCES Message

Track Namespace Prefix: An ordered N-Tuple of byte fields which are matched against track namespaces known to the publisher. For example, if the publisher is a relay that has received ANNOUNCE messages for namespaces ("example.com", "meeting=123", "participant=100") and ("example.com", "meeting=123", "participant=200"), a SUBSCRIBE_ANNOUNCES for ("example.com", "meeting=123") would match both.
Parameters: The parameters are defined in .

The publisher will respond with SUBSCRIBE_ANNOUNCES_OK or SUBSCRIBE_ANNOUNCES_ERROR. If the SUBSCRIBE_ANNOUNCES is successful, the publisher will forward any matching ANNOUNCE messages to the subscriber that it has not yet sent. If the set of matching ANNOUNCE messages changes, the publisher sends the corresponding ANNOUNCE or UNANNOUNCE message. A subscriber cannot make overlapping namespace subscriptions on a single session. Within a session, if a publisher receives a SUBSCRIBE_ANNOUNCES with a Track Namespace Prefix that is a prefix of an earlier SUBSCRIBE_ANNOUNCES or vice versa, it MUST respond with SUBSCRIBE_ANNOUNCES_ERROR, with error code SUBSCRIBE_ANNOUNCES_OVERLAP. The publisher MUST ensure the subscriber is authorized to perform this namespace subscription. SUBSCRIBE_ANNOUNCES is not required for a publisher to send ANNOUNCE and UNANNOUNCE messages to a subscriber. It is useful in applications or relays where subscribers are only interested in or authorized to access a subset of available announcements.

UNSUBSCRIBE_ANNOUNCES A subscriber issues a UNSUBSCRIBE_ANNOUNCES message to a publisher indicating it is no longer interested in ANNOUNCE and UNANNOUNCE messages for the specified track namespace prefix. The format of UNSUBSCRIBE_ANNOUNCES is as follows:

MOQT UNSUBSCRIBE Message

Track Namespace Prefix: As defined in .

SUBSCRIBE_OK A publisher sends a SUBSCRIBE_OK control message for successful subscriptions.

MOQT SUBSCRIBE_OK Message

Subscribe ID: Subscription Identifier as defined in .
Expires: Time in milliseconds after which the subscription is no longer valid. A value of 0 indicates that the subscription does not expire or expires at an unknown time. Expires is advisory and a subscription can end prior to the expiry time or last longer.
Group Order: Indicates the subscription will be delivered in Ascending (0x1) or Descending (0x2) order by group. See . Values of 0x0 and those larger than 0x2 are a protocol error.
ContentExists: 1 if an object has been published on this track, 0 if not. If 0, then the Largest Group ID and Largest Object ID fields will not be present. Any other value is a protocol error and MUST terminate the session with a Protocol Violation ().
Largest Group ID: The largest Group ID available for this track. This field is only present if ContentExists has a value of 1.
Largest Object ID: The largest Object ID available within the largest Group ID for this track. This field is only present if ContentExists has a value of 1.
Subscribe Parameters: The parameters are defined in .

SUBSCRIBE_ERROR A publisher sends a SUBSCRIBE_ERROR control message in response to a failed SUBSCRIBE.

MOQT SUBSCRIBE_ERROR Message

Subscribe ID: Subscription Identifier as defined in .
Error Code: Identifies an integer error code for subscription failure.
Reason Phrase: Provides the reason for subscription error.
Track Alias: When Error Code is 'Retry Track Alias', the subscriber SHOULD re-issue the SUBSCRIBE with this Track Alias instead. If this Track Alias is already in use, the subscriber MUST close the connection with a Duplicate Track Alias error ().

FETCH_OK A publisher sends a FETCH_OK control message in response to successful fetches. A publisher MAY send Objects in response to a FETCH before the FETCH_OK message is sent, but the FETCH_OK MUST NOT be sent until the latest group and object are known.

MOQT FETCH_OK Message

Subscribe ID: Fetch Identifier as defined in .
Group Order: Indicates the fetch will be delivered in Ascending (0x1) or Descending (0x2) order by group. See . Values of 0x0 and those larger than 0x2 are a protocol error.
End Of Track: 1 if all objects have been published on this track, so the Largest Group ID and Object Id indicate the last Object in the track, 0 if not.
Largest Group ID: The largest Group ID available for this track. This field is only present if ContentExists has a value of 1.
Largest Object ID: The largest Object ID available within the largest Group ID for this track. This field is only present if ContentExists has a value of 1.
Subscribe Parameters: The parameters are defined in .

FETCH_ERROR A publisher sends a FETCH_ERROR control message in response to a failed FETCH.

MOQT FETCH_ERROR Message

Subscribe ID: Subscription Identifier as defined in .
Error Code: Identifies an integer error code for fetch failure.
Reason Phrase: Provides the reason for fetch error.

SUBSCRIBE_DONE A publisher sends a SUBSCRIBE_DONE message to indicate it is done publishing Objects for that subscription. The Status Code indicates why the subscription ended, and whether it was an error. The format of SUBSCRIBE_DONE is as follows:

MOQT SUBSCRIBE_DONE Message

Subscribe ID: Subscription identifier as defined in .
Status Code: An integer status code indicating why the subscription ended.
Reason Phrase: Provides the reason for subscription error.
ContentExists: 1 if an object has been published for this subscription, 0 if not. If 0, then the Final Group and Final Object fields will not be present. Any other value is a protocol error and MUST terminate the session with a Protocol Violation ().
Final Group: The largest Group ID sent by the publisher in an OBJECT message in this track.
Final Object: The largest Object ID sent by the publisher in an OBJECT message in the Final Group for this track.

MAX_SUBSCRIBE_ID A publisher sends a MAX_SUBSCRIBE_ID message to increase the number of subscriptions a subscriber can create within a session. The Maximum Subscribe Id MUST only increase within a session, and receipt of a MAX_SUBSCRIBE_ID message with an equal or smaller Subscribe ID value is a 'Protocol Violation'.

MOQT MAX_SUBSCRIBE_ID Message

Subscribe ID: The new Maximum Subscribe ID for the session. If a Subscribe ID equal or larger than this is received in any message, including SUBSCRIBE, the publisher MUST close the session with an error of 'Too Many Subscribes'. More on Subscribe ID in .

ANNOUNCE The publisher sends the ANNOUNCE control message to advertise where the receiver can route SUBSCRIBEs for tracks within the announced Track Namespace. The receiver verifies the publisher is authorized to publish tracks under this namespace.

MOQT ANNOUNCE Message

Track Namespace: Identifies a track's namespace as defined in ()
Parameters: The parameters are defined in .

UNANNOUNCE The publisher sends the UNANNOUNCE control message to indicate its intent to stop serving new subscriptions for tracks within the provided Track Namespace.

MOQT UNANNOUNCE Message

Track Namespace: Identifies a track's namespace as defined in ().

TRACK_STATUS A publisher sends a 'TRACK_STATUS' message on the control stream in response to a TRACK_STATUS_REQUEST message.

MOQT TRACK_STATUS Message The 'Status Code' field provides additional information about the status of the track. It MUST hold one of the following values. Any other value is a malformed message. 0x00: The track is in progress, and subsequent fields contain the highest group and object ID for that track. 0x01: The track does not exist. Subsequent fields MUST be zero, and any other value is a malformed message. 0x02: The track has not yet begun. Subsequent fields MUST be zero. Any other value is a malformed message. 0x03: The track has finished, so there is no "live edge." Subsequent fields contain the highest Group and object ID known. 0x04: The publisher is a relay that cannot obtain the current track status from upstream. Subsequent fields contain the largest group and object ID known. Any other value in the Status Code field is a malformed message. When a relay is subscribed to a track, it can simply return the highest group and object ID it has observed, whether or not that object was cached or completely delivered. If not subscribed, a relay SHOULD send a TRACK_STATUS_REQUEST upstream to obtain updated information. Alternatively, the relay MAY subscribe to the track to obtain the same information. If a relay cannot or will not do either, it should return its best available information with status code 0x04. The receiver of multiple TRACK_STATUS messages for a track uses the information from the latest arriving message, as they are delivered in order on a single stream.

SUBSCRIBE_ANNOUNCES_OK A publisher sends a SUBSCRIBE_ANNOUNCES_OK control message for successful namespace subscriptions.

MOQT SUBSCRIBE_ANNOUNCES_OK Message

Track Namespace Prefix: As defined in .

SUBSCRIBE_ANNOUNCES_ERROR A publisher sends a SUBSCRIBE_ANNOUNCES_ERROR control message in response to a failed SUBSCRIBE_ANNOUNCES.

MOQT SUBSCRIBE_ANNOUNCES_ERROR Message

Track Namespace Prefix: As defined in .
Error Code: Identifies an integer error code for the namespace subscription failure.
Reason Phrase: Provides the reason for the namespace subscription error.

Data Streams A publisher sends Objects matching a subscription on Data Streams. All unidirectional MOQT streams, as well as all datagrams, start with a variable-length integer indicating the type of the stream in question.

ID	Stream Type
0x1	OBJECT_DATAGRAM ()
0x4	STREAM_HEADER_SUBGROUP ()
0x5	FETCH_HEADER ()

An endpoint that receives an unknown stream type MUST close the session. Every Track has a single 'Object Forwarding Preference' and the Original Publisher MUST NOT mix different forwarding preferences within a single track. If a subscriber receives different forwarding preferences for a track, it SHOULD close the session with an error of 'Protocol Violation'.

Object Headers An 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. Objects are sent by publishers.

Canonical Object Fields A canonical MoQ Object has the following information:

Track Namespace and Track Name: The track this object belongs to.
Group ID: The object is a member of the indicated group ID within the track.
Object ID: The order of the object within the group. The IDs starts at 0, increasing sequentially for each object within the group.
Publisher Priority: An 8 bit integer indicating the publisher's priority for the Object .
Object Forwarding Preference: An enumeration indicating how a publisher sends an object. The preferences are Track, Subgroup, and Datagram. An Object MUST be sent according to its Object Forwarding Preference, described below.
Subgroup ID: The object is a member of the indicated subgroup ID () within the group. This field is omitted if the Object Forwarding Preference is Track or Datagram.
Object Status: As enumeration used to indicate missing objects or mark the end of a group or track. See below.
Object Payload: An opaque payload intended for an End Subscriber and SHOULD NOT be processed by a relay. Only present when 'Object Status' is Normal (0x0).

Object Status The Object Status informs subscribers what objects will not be received because they were never produced, are no longer available, or because they are beyond the end of a group or track. Status can have following values:

0x0 := Normal object. The payload is array of bytes and can be empty.
0x1 := Indicates Object does not exist. Indicates that this object does not exist at any publisher and it will not be published in the future. This SHOULD be cached.
0x3 := Indicates end of Group. ObjectId is one greater that the largest object produced in the group identified by the GroupID. This is sent right after the last object in the group. If the ObjectID is 0, it indicates there are no Objects in this Group. This SHOULD be cached. A publisher MAY use an end of Group object to signal the end of all open Subgroups in a Group.
0x4 := Indicates end of Track and Group. GroupID is one greater than the largest group produced in this track and the ObjectId is one greater than the largest object produced in that group. This is sent right after the last object in the track. This SHOULD be cached.
0x5 := Indicates end of Subgroup. Object ID is one greater than the largest normal object ID in the Subgroup.

Any other value SHOULD be treated as a protocol error and terminate the session with a Protocol Violation (). Any object with a status code other than zero MUST have an empty payload. Though some status information could be inferred from QUIC stream state, that information is not reliable and cacheable.

Object Datagram Message An OBJECT_DATAGRAM message carries a single object in a datagram. An Object received in an OBJECT_DATAGRAM message has an Object Forwarding Preference = Datagram. To send an Object with Object Forwarding Preference = Datagram, determine the length of the header and payload and send the Object as datagram. In certain scenarios where the object size can be larger than maximum datagram size for the session, the Object will be dropped.

MOQT OBJECT_DATAGRAM Message

Streams When objects are sent on streams, the stream begins with a stream header message and is followed by one or more sets of serialized object fields. If a stream ends gracefully in the middle of a serialized Object, terminate the session with a Protocol Violation. A publisher SHOULD NOT open more than one stream at a time with the same stream header message type and fields. TODO: figure out how a relay closes these streams

Stream Header Subgroup When a stream begins with STREAM_HEADER_SUBGROUP, all objects on the stream belong to the track requested in the Subscribe message identified by Subscribe ID and the subgroup indicated by 'Group ID' and Subgroup ID.

MOQT STREAM_HEADER_SUBGROUP Message All Objects received on a stream opened with STREAM_HEADER_SUBGROUP have an Object Forwarding Preference = Subgroup. To send an Object with Object Forwarding Preference = Subgroup, find the open stream that is associated with the subscription, Group ID and Subgroup ID, or open a new one and send the STREAM_HEADER_SUBGROUP. Then serialize the following fields. The Object Status field is only sent if the Object Payload Length is zero.

MOQT Group Stream Object Fields A publisher MUST NOT send an Object on a stream if its Object ID is less than a previously sent Object ID within a given group in that stream.

Fetch Header When a stream begins with FETCH_HEADER, all objects on the stream belong to the track requested in the Fetch message identified by Subscribe ID.

MOQT FETCH_HEADER Message Each object sent on a fetch stream after the FETCH_HEADER has the following format:

MOQT Fetch Object Fields The Object Status field is only sent if the Object Payload Length is zero.

Examples Sending a track on one stream: Sending a subgroup on one stream:

Security Considerations TODO: Expand this section, including subscriptions.

Resource Exhaustion Live content requires significant bandwidth and resources. Failure to set limits will quickly cause resource exhaustion. MOQT uses stream limits and 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 publisher prioritizes and transmits streams out of order. Streams might be starved indefinitely during congestion. The publisher and subscriber MUST cancel a stream, preferably the lowest priority, after reaching a resource limit.

IANA Considerations TODO: fill out currently missing registries:

MOQT version numbers
Setup parameters
Subscribe parameters
Subscribe Error codes
Subscribe Namespace Error codes
Announce Error codes
Announce Cancel Reason codes
Message types

TODO: register the URI scheme and the ALPN

Contributors

Alan Frindell
Ali Begen
Charles Krasic
Christian Huitema
Cullen Jennings
James Hurley
Jordi Cenzano
Mike English
Mo Zanaty
Will Law