Add a telescoping SUBSCRIBE_DEMAND to moq-lite and a moq-transport extension

[kixelated]

Summary

Reports the downstream demand for a subscription back up the relay fan-out tree, in two places:

1. moq-lite (draft-lcurley-moq-lite.md) — a new SUBSCRIBE_DEMAND message on the Subscribe Stream.
2. A new moq-transport extension (draft-lcurley-moq-demand.md) — the same functionality as a standalone extension I-D against draft-ietf-moq-transport.

A relay aggregates many downstream subscriptions for a Track into one upstream subscription, so an origin can't see its true audience or what those subscribers need. SUBSCRIBE_DEMAND fixes that with values chosen to aggregate cheaply up the tree.

What it carries

• Audience size — Subscriptions Created and Subscriptions Closed, cumulative counts whose difference is the current subscriber count. A relay reports the sum of each across its downstreams, so demand telescopes for free: the origin reads its total audience across any number of hops from one upstream subscription. Reporting two counts (rather than one gauge) also lets a publisher distinguish churn from a new arrival — a rising Created is an implicit new-group signal.
• Group Request — the minimum group the subscriber wants produced (encoded like Group Start; 0 = no request). Unlike a PLI-style "new group now" trigger, this is a level: already satisfied if a group at or beyond it exists. That makes it idempotent and lets it aggregate as the maximum of downstream requests (less any a relay can serve from cache), so duplicate requests never produce duplicate groups. This is the deduplication win over draft-ietf-moq-transport-18, which gives 0 the special meaning "generate now".

moq-lite change

• New SUBSCRIBE_DEMAND message (Subscriptions Created, Subscriptions Closed, Group Request).
• Introduces a Type tag on the subscriber's post-SUBSCRIBE messages (0x0 SUBSCRIBE_UPDATE, 0x1 SUBSCRIBE_DEMAND), mirroring the publisher's already-typed responses.
• Relays SHOULD rate-limit demand updates to absorb churn; a Group Request increase is latency-sensitive and forwarded promptly.
• Changelog entry under moq-lite-05.

moq-transport extension (new draft)

• Fire-and-forget SUBSCRIBE_DEMAND control message on the subscription's request stream: consumes no Request ID and elicits no response (a poor fit for REQUEST_UPDATE, whose job is to modify the subscription).
• Negotiated per hop via a SUBSCRIBE_DEMAND Setup Option (0xC0117).
• Real Security/Privacy Considerations: per-hop opt-out for sensitive audience sizes; values are subscriber-supplied and untrusted (MUST NOT drive billing/admission/rate-limiting without verification); churn amplification bounded by the rate-limit and by the idempotent group request.
• IANA registrations for the Setup Option and the control message type.

Notes for reviewers

• Rebuilt cleanly on top of current main (drops the earlier stale history that predated Remove Frame/Object Duration from moq-lite and moq-timestamp #32/moq-lite-05: encode Hop ID as fixed-width 64-bit integer #33). Both drafts build OK (kramdown-rfc + xml2rfc-txt).
• Supersedes the earlier subscribe-stats framing: renamed to demand, demand split into created/closed counters, and the Group Request added.

🤖 Generated with Claude Code

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ℹ️ Review info

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Configuration used: Organization UI

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Reviewing files that changed from the base of the PR and between 80308a3 and b40032c.

📒 Files selected for processing (2)

• draft-lcurley-moq-lite.md
• draft-lcurley-moq-subscribe-stats.md

✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch claude/serene-golick-e3445c

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