Internet-Draft moq-sub-flow-control October 2026
Frindell & Swett Expires 10 April 2027 [Page]
Workgroup:
Media Over QUIC
Internet-Draft:
draft-frindell-moq-subscription-flow-control-latest
Published:
Intended Status:
Standards Track
Expires:
Authors:
A. Frindell, Ed.
Meta
I. Swett, Ed.
Google

Subscription Flow Control Extension for Media over QUIC Transport

Abstract

This document defines an extension to Media over QUIC Transport (MOQT) that lets a subscriber limit the number of subgroup streams and the total bytes a publisher may send for an individual subscription. It defines a Setup Option to negotiate the extension and set initial limits, message parameters for advertising these limits, messages for granting credit and signaling flow control state, and a session error code.

About This Document

This note is to be removed before publishing as an RFC.

The latest revision of this draft can be found at https://afrind.github.io/draft-frindell-moq-subscription-flow-control/draft-frindell-moq-subscription-flow-control.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-frindell-moq-subscription-flow-control/.

Discussion of this document takes place on the Media Over QUIC Working Group mailing list (mailto:moq@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/moq/. Subscribe at https://www.ietf.org/mailman/listinfo/moq/.

Source for this draft and an issue tracker can be found at https://github.com/afrind/draft-frindell-moq-subscription-flow-control.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on 10 April 2027.

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Table of Contents

1. Introduction

Media over QUIC Transport (MOQT) [MOQT] delivers a subscription's Objects across one or more subgroup streams, but provides no way for a subscriber to bound how many streams or bytes a publisher sends for that subscription. Transport-layer flow control operates per stream and per session, so it cannot express a limit spanning a subscription's streams.

This extension lets a subscriber limit the total subgroup streams (MAX_SUB_STREAMS) and total bytes (MAX_SUB_BYTES) for a subscription, and grant further credit with SUB_FLOW_CONTROL_UPDATE. Publishers signal that they are blocked with SUB_STREAMS_BLOCKED and SUB_BYTES_BLOCKED, and report the final size of reset subgroup streams with SUBGROUP_RESET. Violations terminate the session with FLOW_CONTROL_EXCEEDED.

Support for this extension is negotiated during session establishment using the SUBSCRIPTION_FLOW_CONTROL Setup Option (Section 3).

2. 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.

This document uses the terminology and wire format notation of [MOQT]. The subscriber sets the limits and the publisher honors them, regardless of which endpoint is the client or server.

3. Extension Negotiation

3.1. SUBSCRIPTION_FLOW_CONTROL Setup Option

An endpoint indicates support for this extension by including the SUBSCRIPTION_FLOW_CONTROL Setup Option in its SETUP message. The option also sets the initial limits for subscriptions in which the endpoint is the subscriber (Section 4).

SUBSCRIPTION_FLOW_CONTROL Setup Option {
  Option Type (vi64) = 0x0B,
  Length (vi64),
  Initial Max Sub Streams (vi64),
  Initial Max Sub Bytes (vi64),
}
Figure 1: SUBSCRIPTION_FLOW_CONTROL Setup Option

  • Initial Max Sub Streams: The initial MAX_SUB_STREAMS limit.

  • Initial Max Sub Bytes: The initial MAX_SUB_BYTES limit.

When an endpoint receives a SUBSCRIPTION_FLOW_CONTROL Setup Option whose value does not contain exactly two varints, it MUST close the session with a KEY_VALUE_FORMATTING_ERROR.

The extension is negotiated when an endpoint has both sent and received this option, per the extension negotiation described in [MOQT]. It applies to both directions of the session.

An endpoint that offers this extension MUST support RESET_STREAM_AT ([RELIABLE-RESET]). When an endpoint negotiates this extension with a peer that does not support RESET_STREAM_AT, it MUST close the session with a PROTOCOL_VIOLATION.

4. Flow Control Model

Every subscription has a stream limit and a byte limit, initialized from the subscriber's SUBSCRIPTION_FLOW_CONTROL Setup Option (Section 3.1), which SUBSCRIBE or SUBSCRIBE_TRACKS can override (Section 5). The subscriber grants additional credit with SUB_FLOW_CONTROL_UPDATE (Section 6.1). Credit is not granted with REQUEST_UPDATE because it solicits a response, which is unnecessary.

The limits apply only to subgroup streams; Objects sent as datagrams are not counted. Limits and the messages defined here are scoped to a single session and are not forwarded. A relay honors its downstream subscriber's limits and independently sets its own limits toward its upstream publisher.

Limits are per subscription and cumulative over its lifetime: the stream count is the total number of subgroup streams opened, and the byte count is the total bytes sent across them (Section 4.3). A publisher MUST NOT exceed a limit in effect for a subscription. When a subscriber detects a violation, it MUST close the session with a FLOW_CONTROL_EXCEEDED (Section 7).

The subscriber attributes each subgroup stream to a subscription by its Track Alias, so when the extension is negotiated, a publisher MUST NOT assign the same Track Alias to more than one subscription in the session, even if the subscriptions are to the same Track or are not concurrent. When a subscriber detects a SUBSCRIBE_OK or PUBLISH with a Track Alias previously assigned to another subscription, it MUST close the session with a DUPLICATE_TRACK_ALIAS.

4.1. Stream Sequence

When the extension is negotiated, every SUBGROUP_HEADER includes a Stream Sequence field:

SUBGROUP_HEADER {
  Type Flags (vi64),
  Track Alias (vi64),
  Group ID (vi64),
  [Subgroup ID (vi64),]
  [Publisher Priority (8),]
  Stream Sequence (vi64),
}
Figure 2: SUBGROUP_HEADER with Stream Sequence

Stream Sequence uniquely identifies a subgroup stream within its subscription. The publisher sets it to 0 on the first subgroup stream it opens for a subscription and increments it by 1 for each subsequent stream.

When a subscriber receives a SUBGROUP_HEADER with a Stream Sequence greater than or equal to the stream limit, it MUST close the session with a FLOW_CONTROL_EXCEEDED. When a subscriber detects a Stream Sequence it has already received for the subscription, it MUST close the session with a PROTOCOL_VIOLATION.

4.2. Publisher Behavior When Blocked

When sending would exceed a limit, the publisher MUST NOT open a subgroup stream beyond the stream limit and MUST NOT send bytes beyond the byte limit, even if this means stopping in the middle of a stream. It retains the blocked Objects until it receives additional credit, and SHOULD send SUB_STREAMS_BLOCKED or SUB_BYTES_BLOCKED, as applicable.

Being blocked does not extend an Object's lifetime; the delivery timeouts of [MOQT] (SUBGROUP_DELIVERY_TIMEOUT and OBJECT_DELIVERY_TIMEOUT) continue to apply. A subgroup stream reset because of an expired timeout is accounted for as described in Section 4.3.

If retaining blocked Objects would exceed its resource limits, a publisher MAY terminate the subscription with PUBLISH_DONE and TOO_FAR_BEHIND ([MOQT]).

4.3. Byte Accounting

The byte count includes all bytes serialized on the subscription's subgroup streams: the SUBGROUP_HEADER (including the stream type) and each Object's header fields and payload. It excludes QUIC and WebTransport framing.

Accounting is at byte granularity. A publisher MAY send part of an Object and stop at the byte limit, leaving the stream open and resuming when credit arrives, subject to the delivery timeout and ordering rules of [MOQT].

Each subgroup stream consumes byte credit exactly once. For a stream closed with a FIN, the bytes received consume credit.

When a publisher resets a stream, it reports the bytes sent on it in the Final Size field of SUBGROUP_RESET (Section 6.2), and the stream consumes that much credit.

A publisher MUST reset subgroup streams using RESET_STREAM_AT with a reliable_size that includes the SUBGROUP_HEADER, so the subscriber always learns the stream's Stream Sequence (Section 4.1) and can match it to the corresponding SUBGROUP_RESET.

On native QUIC, this Final Size equals that of RESET_STREAM_AT ([RELIABLE-RESET]). WebTransport ([WebTransport]) implementations do not necessarily expose the transport Final Size. When a subscriber receives a SUBGROUP_RESET whose Final Size does not match the one reported by the transport, it MUST close the session with a PROTOCOL_VIOLATION.

For example, with 100 bytes of credit and a 200-byte Object:

1. Publisher sends 100 bytes (SUBGROUP_HEADER, Object header,
   partial payload), reaching the limit, and SHOULD send
   SUB_BYTES_BLOCKED.
2. The Object's delivery timeout expires.
3. Publisher resets the stream and sends SUBGROUP_RESET with
   Final Size = 100.
4. The stream consumes 100 bytes of credit: limit reached, not exceeded.

5. Message Parameters

This extension defines two Message Parameters ([MOQT]). If set in SUBSCRIBE or SUBSCRIBE_TRACKS, each parameter overrides the initial limit (Section 3.1), even if the value is smaller. In PUBLISH, it echoes the value from the corresponding SUBSCRIBE_TRACKS. When sent in SUB_FLOW_CONTROL_UPDATE, the value is added to the current limit.

A SUB_FLOW_CONTROL_UPDATE MUST NOT raise a limit above 2^64-1. When a publisher receives a SUB_FLOW_CONTROL_UPDATE that would raise a limit above 2^64-1, it MUST close the session with a PROTOCOL_VIOLATION.

5.1. MAX_SUB_STREAMS Parameter

MAX_SUB_STREAMS (Parameter Type 0x33) is a varint limiting the total number of subgroup streams the publisher can open for the subscription.

5.2. MAX_SUB_BYTES Parameter

MAX_SUB_BYTES (Parameter Type 0x36) is a varint limiting the total bytes the publisher can send across the subscription's subgroup streams (Section 4.3).

6. Messages

Each message below is sent on the subscription's request stream using MOQT control message framing with a 16-bit Length ([MOQT]). None consumes a Request ID or solicits a response.

6.1. SUB_FLOW_CONTROL_UPDATE

A subscriber sends SUB_FLOW_CONTROL_UPDATE to grant additional credit. It MAY be sent any time after the subscription is established, whether or not the publisher is blocked, so a subscriber that intends to grant credit MUST keep the send direction of the request stream open. It has no effect if received after the subscription ends.

Each parameter value is a delta to the current limit, so limits never decrease. When a publisher receives a SUB_FLOW_CONTROL_UPDATE with a delta of 0, it MUST close the session with a PROTOCOL_VIOLATION.

SUB_FLOW_CONTROL_UPDATE Message {
  Type (vi64) = 0x14,
  Length (16),
  Number of Parameters (vi64),
  Parameters (..) ...,
}
Figure 3: MOQT SUB_FLOW_CONTROL_UPDATE Message

  • Parameters: MAX_SUB_STREAMS and/or MAX_SUB_BYTES, each granting additional credit. When a publisher receives a SUB_FLOW_CONTROL_UPDATE with neither parameter, it MUST close the session with a PROTOCOL_VIOLATION.

6.2. SUBGROUP_RESET

A publisher sends SUBGROUP_RESET to report the final byte count of a reset subgroup stream (Section 4.3).

SUBGROUP_RESET Message {
  Type (vi64) = 0x1F,
  Length (16),
  Stream Sequence (vi64),
  Final Size (vi64),
}
Figure 4: MOQT SUBGROUP_RESET Message

  • Stream Sequence: The Stream Sequence of the reset subgroup stream (Section 4.1).

  • Final Size: The bytes sent on the stream before it was reset.

6.3. SUB_STREAMS_BLOCKED

A publisher sends SUB_STREAMS_BLOCKED when it wants to open a subgroup stream but MAX_SUB_STREAMS prevents it.

SUB_STREAMS_BLOCKED Message {
  Type (vi64) = 0x12,
  Length (16),
  Maximum Streams (vi64),
}
Figure 5: MOQT SUB_STREAMS_BLOCKED Message

  • Maximum Streams: The stream limit that was reached.

6.4. SUB_BYTES_BLOCKED

A publisher sends SUB_BYTES_BLOCKED when it has data to send but MAX_SUB_BYTES prevents it.

SUB_BYTES_BLOCKED Message {
  Type (vi64) = 0x13,
  Length (16),
  Maximum Bytes (vi64),
}
Figure 6: MOQT SUB_BYTES_BLOCKED Message

  • Maximum Bytes: The byte limit that was reached.

7. Error Handling

FLOW_CONTROL_EXCEEDED (0x1C):

A session termination error code indicating that the peer violated a subscription flow control limit (MAX_SUB_STREAMS or MAX_SUB_BYTES).

8. Security Considerations

These limits let a subscriber bound the resources a publisher consumes per subscription, complementing transport flow control. A subscriber SHOULD set limits consistent with the resources it will devote to a subscription.

SUB_STREAMS_BLOCKED and SUB_BYTES_BLOCKED are advisory. A subscriber MUST NOT rely on receiving them and MUST enforce limits regardless. A subscriber that grants credit only in response to them could stall a publisher that does not send them; granting credit proactively avoids this.

A misbehaving publisher could under-report Final Size in SUBGROUP_RESET to evade MAX_SUB_BYTES; Section 4.3 describes how a subscriber detects this when the transport reports the stream's Final Size.

9. IANA Considerations

This document registers entries in registries established by [MOQT]. All codepoints are provisional pending working group adoption and can be reassigned by IANA to avoid collisions.

9.1. Setup Option

IANA is requested to add the following entry to the "MOQT Setup Options" registry:

Table 1
Type Name Specification
0x0B SUBSCRIPTION_FLOW_CONTROL Section 3.1

9.2. Message Parameters

IANA is requested to add the following entries to the "MOQT Message Parameters" registry:

Table 2
Parameter Type Parameter Name Specification
0x33 MAX_SUB_STREAMS Section 5.1
0x36 MAX_SUB_BYTES Section 5.2

9.3. Message Types

IANA is requested to add the following entries to the "MOQT Message Types" registry. None is the first message on a stream.

Table 3
ID Messages Stream
0x12 SUB_STREAMS_BLOCKED (Section 6.3) Request
0x13 SUB_BYTES_BLOCKED (Section 6.4) Request
0x14 SUB_FLOW_CONTROL_UPDATE (Section 6.1) Request
0x1F SUBGROUP_RESET (Section 6.2) Request

9.4. Session Termination Error Code

IANA is requested to add the following entry to the "MOQT Session Termination Error Codes" registry:

Table 4
Name Code Specification
FLOW_CONTROL_EXCEEDED 0x1C Section 7

10. References

10.1. Normative References

[MOQT]
Nandakumar, S., Vasiliev, V., Swett, I., and A. Frindell, "Media over QUIC Transport", Work in Progress, Internet-Draft, draft-ietf-moq-transport-22, , <https://datatracker.ietf.org/doc/html/draft-ietf-moq-transport-22>.
[RELIABLE-RESET]
Seemann, M. and K. Oku, "QUIC Stream Resets with Partial Delivery", Work in Progress, Internet-Draft, draft-ietf-quic-reliable-stream-reset-11, , <https://datatracker.ietf.org/doc/html/draft-ietf-quic-reliable-stream-reset-11>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.

10.2. Informative References

[WebTransport]
Frindell, A., Kinnear, E., and V. Vasiliev, "WebTransport over HTTP/3", Work in Progress, Internet-Draft, draft-ietf-webtrans-http3-16, , <https://datatracker.ietf.org/doc/html/draft-ietf-webtrans-http3-16>.

Acknowledgments

This extension is derived from a proposal to add subscription flow control to the base Media over QUIC Transport protocol. The initial conversion of that proposal into this extension draft was drafted with the assistance of Claude Code.

Authors' Addresses

Alan Frindell (editor)
Meta
Ian Swett (editor)
Google