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Web and Internet Transport Media Over QUIC media over quic subscription atomic switch This document defines a Media over QUIC Transport (MOQT) extension for atomically changing the Forward State of a set of established subscriptions. It allows a subscriber to replace one set of Tracks with another without an intermediate partially switched state at its peer. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Media Over QUIC Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction Applications often consume Tracks as a unit. A sports presentation can contain video, commentary, and captions, while a monitoring view can contain several camera Tracks. Switching each subscription independently can briefly activate only part of the destination set or forward both the old and new sets. This document defines BUNDLE_SWITCH, a one-shot operation over established subscriptions on one MOQT Session. The subscriber normally prepares the destination subscriptions with Forward State 0, then atomically deactivates the old set and activates the new set. The subscriptions remain independent after the switch; this extension creates no persistent bundle object. Atomicity applies to Forward State at the immediate peer. It does not guarantee simultaneous Object arrival or presentation, and data already in flight can still arrive after a switch.

Conventions 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. This document uses the terms Object, Track, Subscription, Forward State, Publisher, Subscriber, Relay, and Request ID as defined in .

Activation Set:

Established subscriptions whose Forward State changes from 0 to 1.

Deactivation Set:

Established subscriptions whose Forward State changes from 1 to 0.

Commit:

The single logical transition at which every requested Forward State change takes effect.

Negotiation Support is negotiated with the ATOMIC_SUBSCRIPTION_BUNDLES Setup Option. Its even-numbered Option Type is TBD1 and its value is a variable-length integer capability bit mask. Bit 0x01 enables BUNDLE_SWITCH; unknown bits are ignored. The capability is negotiated when both endpoints advertise bit 0x01. An endpoint MUST NOT send BUNDLE_SWITCH otherwise. The 0-RTT requirements of apply; a client MUST NOT use the extension in 0-RTT unless it has remembered peer support.

Preparing a Bundle Every bundle member is an Established subscription on the same Session. The subscriber SHOULD establish each destination subscription with Forward State 0 and wait for SUBSCRIBE_OK before sending BUNDLE_SWITCH. Filters, priorities, and other subscription parameters are configured on the subscription's own request stream and are not changed by BUNDLE_SWITCH. Preparing subscriptions in Forward State 0 allows failures to be detected while the current bundle continues forwarding. It can consume state at the Publisher or its upstream peers, but does not send Object payloads to the subscriber.

BUNDLE_SWITCH BUNDLE_SWITCH is a request sent as the first message on a new bidirectional stream. It consumes a Request ID under the rules of . The subscription Request IDs identify existing SUBSCRIBE requests, not Track Aliases. Each ID MUST occur exactly once, the two sets MUST be disjoint, and the combined number of IDs MUST be non-zero. All referenced subscriptions MUST have been initiated by the sender on this Session. Each deactivating subscription MUST be Established with Forward State 1, and each activating subscription MUST be Established with Forward State 0. The Publisher validates every member before changing any Forward State. If a member is invalid, unauthorized, terminated, or cannot be activated, the Publisher MUST send REQUEST_ERROR and leave all referenced subscriptions unchanged. INVALID_BUNDLE (TBD3) is used for an invalid member list or state; other errors such as UNAUTHORIZED, TIMEOUT, and EXCESSIVE_LOAD retain their MOQT meanings. Once all members are ready, the Publisher commits the switch. No Object from an activating subscription can be scheduled as a result of the switch until the Publisher has stopped scheduling Objects from every deactivating subscription. The normal MOQT rules for Forward State changes and outstanding Subgroup streams apply. At Commit, the Publisher MUST save the current Largest Location of each activating subscription as its Joining Location when Objects have been published on that Track. The Publisher then sends REQUEST_OK on the BUNDLE_SWITCH stream; this response is called BUNDLE_SWITCH_OK. Its Parameters and Track Properties MUST be empty. The deactivated subscriptions remain Established with Forward State 0. They can be reactivated by a later BUNDLE_SWITCH or terminated using normal MOQT procedures. A failure after Commit affects subscriptions independently and does not roll the switch back.

Concurrent Operations QUIC does not order operations sent on different streams. The Publisher MUST serialize BUNDLE_SWITCH with every operation that updates or terminates a referenced subscription, including REQUEST_UPDATE, subscriber cancellation using STOP_SENDING, PUBLISH_DONE, expiration, and another overlapping BUNDLE_SWITCH. Commit is the ordering point for BUNDLE_SWITCH. If another operation is processed first and leaves a member ineligible, the Publisher MUST reject BUNDLE_SWITCH with INVALID_BUNDLE and apply none of its requested Forward State changes. The earlier operation remains effective. If BUNDLE_SWITCH commits first, it succeeds and the later operation is processed normally without rolling back the switch. For example, cancellation of a deactivating subscription before Commit causes the bundle switch to fail; cancellation after Commit terminates the already-deactivated subscription. A subscriber that requires a particular order SHOULD wait for BUNDLE_SWITCH_OK before sending a conflicting operation on another stream.

Atomicity and Media Alignment Commit is an atomic control-plane transition at one Publisher or Relay. QUIC streams are independent, so Objects sent before Commit can arrive after Objects from the Activation Set. The extension does not align Groups, timestamps, or media presentation points across Tracks. Applications that require a seamless presentation switch need compatible media boundaries or application metadata. The operation is scoped to one MOQT Session. It cannot atomically switch subscriptions held on different peers or Sessions.

Relay Processing Request IDs are Session-local, so a Relay MUST NOT forward BUNDLE_SWITCH unchanged. It performs the downstream Commit locally and MAY prepare or update upstream subscriptions as needed. If it cannot make every activating subscription ready, it fails the downstream operation without changing the referenced downstream Forward States. Other downstream subscribers and upstream Forward States are outside the atomicity guarantee.

Security Considerations The security considerations of apply. BUNDLE_SWITCH does not bypass authorization on any member subscription. A Publisher MUST reject a Request ID that does not identify an eligible subscription initiated by the sender. Large bundles and prepared subscriptions can consume state. Implementations SHOULD limit bundle size and pending duration and MAY reject an operation with EXCESSIVE_LOAD. A malicious peer cannot use BUNDLE_SWITCH to alter another Session's subscriptions because Request IDs are Session-local.

IANA Considerations This document requests the following provisional registrations:

Registry	Value	Name	Specification
MOQT Setup Options	TBD1 (even)	ATOMIC_SUBSCRIPTION_BUNDLES
MOQT Message Types	TBD2	BUNDLE_SWITCH
MOQT REQUEST_ERROR Codes	TBD3	INVALID_BUNDLE

Normative References Cisco Google Google Meta This document defines Media over QUIC Transport (MOQT), a publish/ subscribe protocol that runs over QUIC and WebTransport. MOQT leverages the features of these transports, such as streams, datagrams, priorities, and partial reliability. MOQT operates both point-to-point and through intermediate relays, enabling scalable low-latency delivery. Despite its name, MOQT is media agnostic and can be used for a wide range of use cases. In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.

Acknowledgments This work was motivated by discussion of atomic multi-Track switching in the MOQT working group.

Change Log

draft-sharma-moq-atomic-subscription-bundles-00

• Initial version.

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