Module draft21 

MoQT wire codec for draft-21.

Draft-21 is draft-20 restructured, and nothing a decoder can observe changed. Every message keeps its code point, every field its width and its place, every registry its rows. The one wire-visible consequence is the ALPN, which the draft derives from its own number: moqt-21.

What did change is where the rules are written down. Two hundred numbered sections became two hundred and fourteen and only eleven keep both their title and their number, so every section, table and figure reference in this module is draft-21’s own. A draft-20 number carried over would mostly still resolve — to a section that exists and states something else.

Two normative sentences were reworded on the way, so a restructure moves text without touching it is very nearly true and not quite. The AUTHORIZATION TOKEN repetition rule takes a different subject here than it took in draft-20, and LOCATION_FILTER’s definition lost the opening sentence that named its encoding — with the Length field still in the structure, so the wire is as it was. Neither changes what either requires, and both are quoted at the sites that read them.

The rest of this list is what a reader coming from draft-19 needs; draft-20 made these changes and draft-21 keeps every one of them.

• FETCH (0x16) is rewritten. The Fetch Type field, the Standalone Fetch and Joining Fetch structures and the Fetch Type registry are all gone. Track Namespace and Track Name are inline fields of FETCH itself, in the positions they occupied inside the old Standalone Fetch, and the range travels in the LOCATION_FILTER message parameter. The codepoint did not change, so a draft-19 decoder reads the Number of Track Namespace Fields count as a Fetch Type and mis-parses in silence; there is no in-band version signal to catch it (Section 9.11)
• PUBLISH_STATE_NOTIFY (0x22) is new. A publisher-only, unilateral report that a subscription’s state changed for a reason other than a subscriber-sent REQUEST_UPDATE. It has no Request ID field — the message is identified by the bidirectional request stream it arrives on, like SUBSCRIBE_OK, PUBLISH_DONE and FETCH_OK (Section 9.10)
• LOCATION_FILTER (0x21) is restructured. The Filter Type enum (draft-19’s 0x1 Next Group Start / 0x2 Largest Object / 0x3 AbsoluteStart / 0x4 AbsoluteRange) is gone; the filter’s shape now comes from how many vi64 fields its value holds (Section 9.20.10)
• Ranges are inclusive at both ends, on subscriptions and on fetches. Draft-19’s fetch end was “the last Object, plus 1; or 0 to indicate the entire Group”; both conventions are deleted. This change is absent from the draft’s own change log, and a draft-19 encoder ported forward with its end-location arithmetic intact fetches one object too many (Sections 3.3.1, 9.11, 9.12)
• FILL_PARAMETERS (0x23) is new: a length-prefixed parameter whose value is a nested parameter block, and whose mere presence on a SUBSCRIBE or a REQUEST_UPDATE asks the publisher to open a fill fetch stream (Section 9.20.16)
• INCLUDE_PROPERTIES (0x35) is new: a uint8 opt-out from Track Properties on the responding OK, default 1 (Section 9.20.22)
• Three code points are removed: SUBSCRIPTION_ENDED (PUBLISH_DONE 0x3), VERSION_NEGOTIATION_FAILED (session 0x15) and INVALID_JOINING_REQUEST_ID (REQUEST_ERROR 0x32). Two whole enumerations go with them: the Fetch Type registry and the Location Filter Type enum
• PUBLISH_DONE.Stream Count’s “unknown” sentinel moves from 2^62 - 1 to 2^64 - 1, and the count now includes fill fetch streams (Section 9.9)
• Subscription parameters moved off PUBLISH_OK. Six parameter definitions dropped it from their “MAY appear in” list and several gained PUBLISH; EXPIRES (0x08) is the only one that still names it (Section 9.20)
• The data-plane headers rename their leading field Type to Type Flags and move the validity rules out of the figure into prose. The field list, order, widths and the set of valid values are all unchanged; what changed is the receive path. Sections 11.2.1 and 11.3.1 give different rule sets — subgroup’s reserved bit 4 must be 1, datagram’s must be 0, and subgroup has no “unspecified bit” clause because bits 0-6 are all specified for it
• A fetch stream gains a third End of Range marker, End of Timed-Out Range (Serialization Flags 0x20C), for the Objects a relay abandoned when its FILL_TIMEOUT budget ran out. Draft-19 reported those as Unknown gaps (Section 11.4.1, Table 7)
• Draft-20 renumbered too, inserting PUBLISH_STATE_NOTIFY and shifting everything after it; draft-21 renumbered again and far more widely. Both are behind us: the numbers below are draft-21’s

Where this codec had to choose

Draft-21 leaves several wire questions open. Each decision is recorded at the encode or decode site that makes it, and each says that the draft does not state it. They are, with the site that carries the comment:

• FILL_PARAMETERS’s value begins with a Number of Parameters count — message::decode_fill_parameters
• the Type Delta chain restarts inside FILL_PARAMETERS and the outer chain is unaffected — message::decode_fill_parameters
• a LOCATION_FILTER’s field count comes from parsing, never from the byte length — message::decode_location_filter
• a non-minimally encoded Type Flags is accepted on receive and never emitted — data_stream::SubgroupHeader::decode and data_stream::DatagramHeader::decode
• PUBLISH_DONE.Stream Count’s 2^64 - 1 sentinel is indistinguishable from a well-formed exact count — message::publish_done_codes::STREAM_COUNT_UNKNOWN
• PUBLISH_STATE_NOTIFY’s parameter allow-list is treated as closed — message::parameter_in_scope
• Object Payload Length is present on an End-of-Range marker record, encoded as 0, and the ordinary delta arithmetic applies to the marker’s two fields — data_stream::FetchObjectHeader

Modules

data_stream

Draft-21 data stream header encoding and decoding.

error_codes

Error, status and stream-reset code registries defined by draft-ietf-moq-transport-21.

fields

This draft’s field names for a decoded control message.

message

Draft-21 control message encoding and decoding.

types

Draft-21 object status values.