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HTTP priority — RFC 9218

RFC 9218 defines a small, cross-version priority scheme that supersedes the HTTP/2 stream-dependency tree. The same scheme applies to HTTP/2 (RFC 9113 §5.3.2) and HTTP/3 (this module).

Typed priority​

public type H3Priority is {
    urgency:     UInt8,         ← 0..=7; 0 = highest priority
    incremental: Bool,          ← deliver progressively or contiguous
};

Defaults (§4.1):

  • urgency = 3 (DEFAULT_URGENCY)
  • incremental = false
  • urgency is clamped on construction to [0, MAX_URGENCY=7].

Priority header (§4)​

Wire form is a Structured Field dictionary emitted in the request or response headers:

priority: u=3, i
priority: u=0               ← highest urgency, non-incremental
priority: u=6, i=?0         ← explicit "not incremental"

Parser:

mount core.net.h3.priority.{H3Priority, parse_header, encode_header};

let p = parse_header(f"u=0, i").unwrap();
assert(p.urgency == 0);
assert(p.incremental == true);

let hdr_value = encode_header(&p);       // produces the Structured-Header text

Silent-tolerance rules (§4.2):

  • Unknown dictionary keys are ignored.
  • Out-of-range urgency values clamp to the nearest valid value.
  • Missing keys fall back to defaults.

See priority_rfc9218.

PRIORITY_UPDATE frame (§7)​

Clients and servers can update priority of an outstanding stream or push at any time via a control-stream frame:

PRIORITY_UPDATE (request streams) — frame type 0xF0700
PRIORITY_UPDATE (push IDs)        — frame type 0xF0701

Frame Payload {
    Prioritized Element ID (i),     ← stream_id or push_id
    Priority Field Value (..)       ← Structured-Header bytes
}

The frame types are greased-size varints (≥ 4 bytes on the wire) signalling "extension, may be absent" .8.

ADT:

public type PriorityUpdate is {
    target: PriorityUpdateTarget,   ← Request(stream_id) | Push(push_id)
    priority_field: List<Byte>,
};

public const PRIORITY_UPDATE_REQUEST: UInt64 = 0xF0700;
public const PRIORITY_UPDATE_PUSH:    UInt64 = 0xF0701;

SETTINGS interaction​

Servers advertise SETTINGS_NO_RFC7540_PRIORITIES = 0x09 (value 1) to indicate they've disabled the legacy HTTP/2 stream-dependency tree and rely solely on RFC 9218. Clients MAY skip emitting HEADERS.stream_dependency when this setting is set.

Scheduler integration​

The priority layer is parsing only — it does NOT implement send-ordering. The actual scheduler lives in core.net.h3.server / client. This module hands typed H3Priority values to the send-queue, which consumes them via a comparator:

1. Lower urgency sends first.
2. Within same urgency, non-incremental sends before incremental.
3. Within same urgency + incremental, round-robin.

References​

  • core/net/h3/priority.vr
  • vcs/specs/L2-standard/net/h3/priority_rfc9218.vr
  • vcs/specs/L2-standard/net/h3/priority_typecheck.vr