MoQ Multimodal Feedback

1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

The following terms are used throughout this document:¶

• MOQT: Media over QUIC Transport, following [MoQTransport].¶

• Object: The smallest semantic unit of data delivery in MOQT.¶

• Feedback Track: A MoQ Track that carries MMF feedback.¶

• MMF (MoQ Multimodal Feedback): A receiver report at the MoQ layer containing per-Object delivery status.¶

• Cross-Layer Control Interface: A mechanism for the MoQ layer to issue control commands to CC algorithms; the specific form is implementation- defined.¶

3.1. Three-Layer Architecture: Application / MoQ / Transport

This mechanism adopts a three-layer architecture: the application layer serves as the media source, the transport layer implements CC algorithms, and the MoQ layer resides in between, responsible for semantic understanding and signal distribution.¶

3.2. Dual-Layer Feedback Model: QUIC receive-ts and MMF

CC algorithms rely on per-packet delay signals for bandwidth estimation and congestion detection; this signal is provided by QUIC receive-ts ([quic-receive-ts]). MMF operates at the MoQ layer, forming a dual-layer feedback which could cooperate with QUIC receive-ts. Both mechanisms can be enabled simultaneously in implementations.¶

The two layers cover different granularities:¶

QUIC receive-ts provides per-packet inter-arrival delta, enabling CC algorithms to perform delay-based congestion detection and bandwidth estimation.¶

MMF supplements additional signals:¶

• Per-Object Signals: QUIC transport layer cannot determine whether an Object is complete or arrived within deadline. MMF provides per-Object status (RECEIVED_LATE, NOT_RECEIVED, etc.).¶

• Application-Layer Metrics: Playout headroom (PLAYOUT_AHEAD), receiver-side bandwidth estimation, etc.¶

3.3. Cross-Layer Control Interface

MoQ implementations could provide a cross-layer control interface that enables the MoQ layer to issue control commands to CC algorithms. The specific form of the interface is implementation-defined.¶

Distinct from approaches that pass raw data to CC algorithms for independent judgment, this mechanism could help the MoQ layer to achieve better performance. The MoQ layer possesses three kinds of information resource: MMF feedback, local CC output (BWE), and frame-level statistics (Section 8.1), achieving higher information completeness than CC algorithms alone.¶

CC algorithms could still run their own congestion detection logic, or adapted to accept control commands issued by the MoQ layer.¶

Control Commands:¶

MoQ Layer Decision Inputs:¶

The MoQ layer synthesizes the following signals for control decisions:¶

• MMF Signals (from peer): Object loss rate, expiration rate, Avg Inter-Arrival Delta, PLAYOUT_AHEAD_MS¶

• CC Output (from local): BWE, RTT, loss rate¶

• Frame-Level Statistics (from local, Section 8.1.3): per-Object transmission/loss/delivery duration¶

CC Algorithm Behavior:¶

CC algorithms SHOULD execute control commands after receiving them:¶

• Upon pacing_gain: Update sending rate as pacing_rate = BWE x pacing_gain¶

• Upon pacing_rate: Use this value directly as sending rate¶

• Upon target_bitrate: Record current application-layer sending budget for internal decision reference¶

When no control commands are issued by the MoQ layer, CC algorithms operate normally according to their own logic.¶

The cross-layer control interface is optional. CC algorithms that do not support this interface can still operate independently but cannot benefit from MMF-driven frame-level control capabilities.¶

3.4. Bidirectional MMF: Output Feedback and Input Feedback

The MMF report direction described in the preceding sections is from client to server, reporting delivery quality of downstream media (audio_response, text_response). In streaming input scenarios, the server simultaneously subscribes to upstream media (audio_input, video_input) and MAY establish a reverse feedback Track to report upstream media delivery quality to the client.¶

Client Server | | |----- audio_input ------------->| Server subscribes |<---- input_feedback (MMF) -----| Server reports input delivery quality | | |<---- audio_response -----------| Client subscribes |-- multimodal_feedback (MMF) -->| Client reports output delivery quality | | ¶

Output MMF (client-->server) and Input MMF (server-->client) use the same report format (Section 5). Their purposes differ:¶

• Output MMF: Server adjusts encoding bitrate, inference parameters, and CC based on it.¶

• Input MMF: Client adjusts upstream behavior based on it:¶

  • Audio Input: Adjust chunk size, encoding bitrate, transmission frequency¶

  • Video Input: Adjust frame rate, resolution, pause/resume¶

Input MMF is optional. Support of the input MMF is declared via bit-2 of the Setup Parameter bitmap.¶

4.1. Track Definition

The Feedback Track is a normal MoQ Track where the Payload of each Object is an MMF. Each MMF is published as an independent Object with monotonically increasing Object ID within a Group.¶

The Group division strategy for Feedback Track is implementation-defined. It is RECOMMENDED to use a single Group (Group ID = 0) with continuously incrementing Object IDs to simplify implementation.¶

Each Feedback Track MUST be associated 1:1 with a media Track. The association is established during the PUBLISH phase (Section 4.3) and is not repeated in MMF reports.¶

When feedback is needed for multiple media Tracks, independent Feedback Tracks MUST be established separately.¶

4.2. Track Naming

Feedback Track naming SHOULD follow these conventions:¶

• Namespace: Same Namespace as the media Track being fed back.¶

• Track Name: multimodal-feedback/<media_track_name>.¶

Examples: multimodal-feedback/audio_response, multimodal-feedback/video_response.¶

Media Track Names MUST NOT contain the / character to avoid parsing ambiguity in Feedback Track Names.¶

When reverse feedback exists in a session (see Section 3.4), the Input Feedback Track Name is input-feedback/<media_track_name>.¶

When a sender receives a PUBLISH request for a Feedback Track, it MUST identify the associated media Track via the <media_track_name> portion of the Track Name. If no matching established media Track is found, it SHOULD respond with REQUEST_ERROR.¶

4.3. Track Establishment and Lifecycle

The Feedback Track is established by the feedback generator (typically the media receiver) as Publisher through MOQT's PUBLISH / PUBLISH_OK negotiation.¶

Establishment Flow Example:¶

Media Receiver Media Sender (Media Subscriber / (Media Publisher / Feedback Publisher) Feedback Subscriber) | | | SUBSCRIBE(track=audio_response) | (1) Subscribe media Track |----------------------------------->| | SUBSCRIBE_OK | |<-----------------------------------| | | | PUBLISH(track=multimodal-feedback/ | (2) Publish Feedback Track | audio_response, | (Role reversal: receiver | namespace=same_as_media) | becomes feedback Publisher) |----------------------------------->| | PUBLISH_OK | |<-----------------------------------| | | | [Object: MMF seq=0] | (3) Send MMF |----------------------------------->| | [Object: MMF seq=1] | |----------------------------------->| | ... | ¶

Lifecycle Rules:¶

The Feedback Track lifecycle SHOULD align with the subscribed media Track it covers. After the media Track publisher sends PUBLISH_DONE for the media Track, the Feedback Track publisher (i.e., the media receiver) SHOULD send PUBLISH_DONE for the Feedback Track after transmitting the final MMF and stop publishing Objects.¶

When re-establishing a Feedback Track after connection interruption, the Report Sequence SHOULD restart from 0.¶

4.4. Transport and Priority

The Feedback Track SHOULD be carried over QUIC Stream (consistent with ordinary MoQ Objects).¶

The Subscriber Priority for Feedback Track SHOULD be set lower than media Tracks ([MoQTransport], Section 7). Example priority assignment:¶

When bandwidth contention occurs, media data SHOULD take precedence over feedback data transmission.¶

5.1. Report Structure

MoQ Multimodal Feedback { Report Timestamp (i), Report Sequence (i), Object Entry Count (i), Object Entry (..) ..., Summary Stats (..), Optional Metric Count (i), Optional Metric (..) ..., } ¶

The MMF format version is negotiated via Setup Parameter (Section 6.1) and is not repeated in each report. This document defines version 0.¶

Each MMF reports Object delivery status for a single media Track associated with its Feedback Track (1:1 association, see Section 4.1).¶

Report Timestamp (varint): The moment when the report is generated, using receiver's local monotonic clock value in microseconds. This value is only used for Receive Timestamp Delta chain anchoring and report ordering, not for cross-end time alignment. Monotonic clock MUST be used.¶

Report Sequence (varint): Monotonically increasing from 0. Senders detect feedback loss via sequence gaps (Section 10.1).¶

Object Entry Count (varint): Number of Object Entries that follow. When the value is 0, the report contains only Summary Stats (for heartbeat purposes).¶

5.2. Object Entry

Object Entry { Object ID (i), Status (i), [Receive Timestamp Delta (i)], } ¶

Object Entries within the same MMF MUST be sorted in ascending order by Object ID.¶

5.3. Delivery Status Codes

5.4. Summary Stats Block

Summary Stats { Report Interval (i), Total Objects Evaluated (i), Objects Received (i), Objects Received Late (i), Objects Lost (i), Avg Inter-Arrival Delta (i), } ¶

Summary Stats MUST always be included in every MMF, not controlled by negotiation bitmap. It provides windowed aggregate information, enabling lightweight CC consumers that do not parse Object Entry to obtain effective signals. The MoQ layer can compute control decisions based on this block and issue them to CC algorithms via the cross-layer control interface (Section 3.3).¶

The Report Interval window of Summary Stats is independent from the coverage of Object Entries. Object Entries MAY include Objects outside the Report Interval window (e.g., continuously reported NOT_RECEIVED entries, see Section 7.4).¶

5.5. Optional Media Metrics

Optional Media Metrics immediately follow Summary Stats. Optional Metric Count (varint) specifies the number of subsequent Optional Metrics; a value of 0 indicates no optional metrics are included.¶

Each metric uses Key-Value-Pair encoding (draft-ietf-moq-transport-17, Section 1.4.2):¶

Optional Metric { Metric Type (i), Metric Value (i), } ¶

Defined metric types are divided into two categories: Application-Layer Metrics and QUIC Layer Summary Metrics.¶

Application-Layer Metrics:¶

QUIC Layer Summary Metrics:¶

The following metrics expose the receiver's local QUIC connection transport state to the sender for CC algorithm cross-validation.¶

MMF's core CC signals rely on Receive Timestamp Delta in Object Entry (Section 5.2.3, referencing QUIC receive-ts / WebRTC TWCC per-packet delta encoding approach) and Summary Stats (Section 5.4), not Optional Metrics. Optional Metrics serve only as supplements.¶

Type values 0x00-0x1f are reserved for this specification. Values 0x20 and above are available for application-layer custom use.¶

Receiver MUST ignore unrecognized Metric Types.¶

Optional Media Metrics MAY be included in MMF only when both parties have declared bit1=1 in Setup negotiation (Section 6). When not negotiated or bit1=0, Optional Metric Count MUST be 0.¶

5.6. Report Size Control

A single MMF is RECOMMENDED not to exceed 1200 bytes to avoid QUIC packet fragmentation.¶

When the number of Objects to report exceeds the capacity of a single MMF, receiver SHOULD:¶

• Prioritize including recent Object Entries (largest Object IDs).¶

• Trim the oldest Object Entries.¶

• Ensure Summary Stats covers the complete Report Interval window (Summary Stats is not affected by Object Entry trimming).¶

6.1. Setup Parameter

During MOQT Setup phase, both parties declare Multimodal Feedback capability via Setup Parameter.¶

MOQT_MULTIMODAL_FEEDBACK Setup Parameter { Type = TBD1 (i), Length (i), Value (i), } ¶

The Value field is a capability bitmap (varint) with the following bit definitions:¶

Negotiation Example:¶

Client Server | | | CLIENT_SETUP( | | version=16, | | params=[ | | {type=0x00, value=0x02}, | (ROLE=Publisher) | {type=TBD1, value=0x03} | (MULTIMODAL_FEEDBACK: bit0|1=1) | ]) | |----------------------------------->| | | | SERVER_SETUP( | | version=16, | | params=[ | | {type=0x00, value=0x03}, | (ROLE=PubSub) | {type=TBD1, value=0x01} | (MULTIMODAL_FEEDBACK: bit0=1) | ]) | |<-----------------------------------| | | | Negotiation Result: | | bit0=1 (Output Feedback) | | bit1=0 (Client declared but | | Server did not, | | Optional Metrics disabled) | ¶

6.2. Capability Negotiation Rules

Feature enable conditions (both parties MUST declare corresponding bit as 1):¶

When a feature is not enabled:¶

• Receiver MUST NOT publish Feedback Track in corresponding direction.¶

• When sender receives PUBLISH request for Feedback Track in un-negotiated direction, it SHOULD respond with REQUEST_ERROR (draft-ietf-moq-transport-17, Section 9.8).¶

• MMF MUST NOT include un-negotiated optional fields (e.g., Optional Metrics).¶

6.3. Behavior When Parameter Not Declared

When peer's Setup does not include MOQT_MULTIMODAL_FEEDBACK Parameter, it is equivalent to Value=0 (no Multimodal Feedback capability supported).¶

This end MUST NOT proactively establish Feedback Track.¶

6.4. Runtime Capability Change

This version does not support runtime changes to Multimodal Feedback capability. If change is needed, MoQ session MUST be re-established.¶

7.1. Arrival Time Recording

Receiver MUST record the arrival time of each Object. Arrival time is defined as the moment when the last byte of the Object arrives at the receiver's MoQ layer. Implementation MUST use monotonic clock, unaffected by system time adjustments. Time precision SHOULD be no less than 1 millisecond, RECOMMENDED to be microsecond-level.¶

7.2. Delivery Status Determination

Receiver MUST maintain delivery status for each known Object:¶

• Last byte of Object arrives and within deadline: RECEIVED (0x00).¶

• Last byte of Object arrives but exceeds deadline: RECEIVED_LATE (0x01).¶

• Object has not arrived but is reasonably considered lost (see Section 5.3): NOT_RECEIVED (0x02).¶

• Object partially arrived and carrying Stream is closed: PARTIALLY_RECEIVED (0x03).¶

Object status MAY be updated in subsequent MMF. For example, from NOT_RECEIVED to RECEIVED (when a delayed Object is eventually received). Therefore, counts such as Objects Lost in Summary Stats reflect an observation snapshot at report generation time and are not guaranteed to be consistent with final statistics. Sender SHOULD use them as immediate signals rather than precise statistics.¶

7.3. MMF Generation Frequency

Receiver SHOULD generate MMF at the following recommended frequencies:¶

Generation frequency SHOULD NOT exceed once per 50ms (to avoid feedback itself consuming excessive bandwidth). Generation frequency SHOULD NOT be lower than once per 2s (to ensure sender receives timely feedback).¶

When receiver detects rapid deterioration in delivery quality (e.g., consecutive Object losses), it MAY immediately generate an additional MMF (without waiting for the scheduled cycle) to accelerate sender response.¶

7.4. Object Entry Selection

When generating MMF, receiver SHOULD follow these Object Entry selection strategies:¶

• Prioritize covering recent Objects (most recent in time).¶

• Total number of Object Entries per MMF is RECOMMENDED not to exceed 50.¶

• For Objects already reported in previous MMF with unchanged status, they MAY be omitted.¶

• Objects with status NOT_RECEIVED SHOULD be continuously reported (for at least 3 MMF cycles) until status changes or exceeding the report window.¶

7.5. Exception Handling

• Object Out-of-Order Arrival: Receiver MUST record by actual arrival time without reordering. Arrival time reflects actual network behavior; out-of-order itself is a valid network signal.¶

• Duplicate Objects: Receiver SHOULD ignore duplicate arrivals, retaining the first arrival time.¶

• Feedback Track Publish Failure: Receiver SHOULD retry establishing Feedback Track with backoff strategy. Retry interval is RECOMMENDED to use exponential backoff (initial 1s, maximum 30s).¶

8.1. Object to QUIC Packet Mapping

If a sender needs to correlate MMF feedback with local transmission events, it MUST maintain the mapping relationship between Objects and QUIC packets. Without this mapping, the sender cannot determine the number of packets, packet loss count, and delivery duration for a single video frame, and the per-Object status reported by MMF cannot be aligned with sender-side statistics.¶

8.2. Application-Layer Consumption

After the sender's MoQ layer receives MMF, it exposes the following information to the inference scheduler/ABR through application-layer callbacks:¶

• Delivery quality (loss rate, expiration rate) of the associated media Track¶

• Per-Object status and timestamps (can be used for detailed analysis)¶

• Optional media metrics (playout headroom, bandwidth estimation, etc.)¶

The inference scheduler makes decisions based on the above information (see Section 9 for details).¶

8.3. Transport-Layer Consumption (Cross-Layer Control)

The MoQ layer synthesizes MMF signals and local CC output, computes control decisions, and issues them to CC algorithms via the cross-layer control interface (Section 3.3). CC algorithms do not directly parse MMF, but instead execute control commands from the MoQ layer. Decision inputs, issuable commands, and CC behavior are described in Section 3.3.¶

8.4. Example: MoQ Layer Controlling BBR

The core formula of BBR is pacing_rate = bandwidth x pacing_gain.¶

BBR itself estimates bandwidth through ACK sampling and controls pacing_gain via its state machine. In cross-layer control mode, the MoQ layer takes over control of pacing_gain. BBR remains responsible for bandwidth estimation, while pacing_gain is determined by the MoQ layer based on MMF feedback:¶

``` MoQ Layer: 1. Read BBR's BWE 2. Read MMF: Objects Lost, Late, PLAYOUT_AHEAD_MS, Delta 3. Compute pacing_gain (comprehensive judgment) 4. Issue pacing_gain --> CC¶

BBR: 1. Receive pacing_gain 2. pacing_rate = BWE x pacing_gain 3. Send at pacing_rate ```¶

Example logic for MoQ layer computing pacing_gain:¶

1. Objects Lost > 0 and BBR local loss = 0 --> Reduce pacing_gain to 1.0 (CC local ACK is normal, but peer is actually losing frames; stop probing upward)¶

2. PLAYOUT_AHEAD_MS < 100ms --> Reduce pacing_gain to 1.0 (Insufficient playout headroom; avoid aggressive probing)¶

3. Avg Inter-Arrival Delta consistently positive --> Reduce pacing_gain to 0.9 (Receiver-side queuing is worsening; proactively reduce speed)¶

4. High proportion of Objects Received Late --> Reduce target_bitrate (Transmission has no packet loss but delay exceeds deadline; reduce per-frame data volume at the source)¶

5. None of the above conditions met --> Do not issue commands; BBR runs according to its own state machine¶

When the MoQ layer does not issue commands, BBR operates normally according to its own state machine. The MoQ layer overrides pacing_gain only when MMF signals indicate intervention is needed; otherwise, BBR operates fully autonomously.¶

9.1. MMF-Driven Rate-Quality Adaptation

The following adaptation rules apply to all MoQ streaming media scenarios:¶

These adaptations can be executed at the MoQ publishing layer with per-Object granularity and latency < 1 frame cycle, without requiring application-layer media source cooperation. When the application layer is an AI inference pipeline, MMF can also adjust inference pipeline parameters (real-time flush, dynamic chunk_size).¶

Unidirectional and bidirectional adaptation modes are described in Section 3.4 (Bidirectional MMF).¶

9.2. Typical Use Cases

The following use cases assume BBR as the CC algorithm (integration method described in Section 8.4) and illustrate the effects of MMF in different scenarios. All cases follow the strategy of prioritizing audio output and progressively reducing quality.¶