Sender-side track switching (SSTS) decision algorithms, ported from moqtail's moqtail-ssts crate (ee9753c): which member of each switching set a relay forwards for one media group. More...

#include "common/platform/sys/syscall.h"

Data Structures
struct	moqssts_member
	One encoding of a switching set. More...
struct	moqssts_set
	One switching set. More...
struct	moqssts_bp
	Per-connection backpressure state; reset it with moqssts_bp_init. More...
struct	moqssts_obs
	One group's congestion evidence. More...

Macros
#define	MOQSSTS_MAX_MEMBERS 4
	Members per switching set.
#define	MOQSSTS_MAX_SETS 32
	Switching sets per connection.
#define	MOQSSTS_ALG_DEFAULT 0x0ULL
	Algorithm id of the mandatory default split (SETUP option 0x09, SWITCHING_SET_ASSIGNMENT).
#define	MOQSSTS_ALG_BACKPRESSURE 0xff01ULL
	Algorithm id of moqtail's backpressure selector: the first id of the private range.
#define	MOQSSTS_NONE (-1)
	Decision entry for a set that forwards nothing this group.
#define	MOQSSTS_UNLIMITED_KBPS (~0ULL / 4)
	Budget ceiling for the default split (moqtail UNLIMITED_KBPS).
#define	MOQSSTS_DEPTH_TARGET 1ULL
	Backpressure: depth at or below which the connection is uncongested.
#define	MOQSSTS_DOWNSHIFT_DEPTH 2ULL
	Backpressure: depth at or above which the tier drops immediately.
#define	MOQSSTS_UPSHIFT_GOP_STREAK 5ULL
	Backpressure: consecutive uncongested groups before the tier rises.
#define	MOQSSTS_COOLDOWN_HIGH_STREAK 2ULL
	Backpressure: consecutive deeper cooldown groups before a further drop.
#define	MOQSSTS_EV_HELD 0
	Observation event: the tier stayed where it was.
#define	MOQSSTS_EV_COOLDOWN_EXIT 1
	Observation event: depth returned to the target, cooldown ended.
#define	MOQSSTS_EV_UPSHIFT 2
	Observation event: the tier rose after a clear streak.
#define	MOQSSTS_EV_DOWNSHIFT 3
	Observation event: the tier fell.

Functions
int	moqssts_alg_supported (u64 id)
	1 for an algorithm id this module implements (registry.rs), else 0.
int	moqssts_set_active (const moqssts_set *s)
	1 when activate > 0 and the set has at least activate members.
u64	moqssts_budget_kbps (u64 est_kbps, u64 cap_kbps)
	The stricter of estimate and cap; 0 means "unknown" / "uncapped", and neither signal yields ~0 (unconstrained).
u64	moqssts_active_depth (const moqssts_set sets, usz n, const u64 open)
	Largest open[i] over the ACTIVE sets[i] (open is parallel to sets): the deepest backed-up track, not the sum over tracks.
void	moqssts_default_decide (u64 budget_kbps, const moqssts_set sets, usz n, int out)
	Algorithm 0: stateless weighted strict-rank split of budget_kbps (clamped to MOQSSTS_UNLIMITED_KBPS).
void	moqssts_bp_init (moqssts_bp *st)
	Fresh backpressure state: lowest tier, no streaks, not in cooldown.
moqssts_obs	moqssts_obs_make (u64 depth, u64 timeouts)
	Observation from open-stream depth and timeouts since the last decision.
int	moqssts_bp_observe (moqssts_bp *st, moqssts_obs ob)
	Advance the tier state machine by one group; returns MOQSSTS_EV_*.
int	moqssts_bp_decide (moqssts_bp st, const moqssts_set sets, usz n, const u64 open, u64 timeouts, int out)
	Algorithm 0xff01: observe, clamp the tier to the longest ladder - 1, and write out[i] = min(tier, n_members-1) for active sets, else MOQSSTS_NONE.

Detailed Description

Sender-side track switching (SSTS) decision algorithms, ported from moqtail's moqtail-ssts crate (ee9753c): which member of each switching set a relay forwards for one media group.

Pure computation – no I/O, no hub types; the hub copies its sets in and applies the returned indices.

Every decision writes one entry per set: the chosen member index (into moqssts_set.m) or MOQSSTS_NONE when the set forwards nothing this group. The decide functions ignore moqssts_set.algorithm_id: the caller passes only the sets assigned to that algorithm (moqtail groups sets the same way).

Capacities are fixed (no allocator). Counts above a cap are clamped: extra members are ignored, and sets at index >= MOQSSTS_MAX_SETS are answered MOQSSTS_NONE and left out of the budget split and the stream depth.

Macro Definition Documentation

◆ MOQSSTS_MAX_MEMBERS

#define MOQSSTS_MAX_MEMBERS 4

Members per switching set.

A moqt_chat ladder is hi/lo, so 4 leaves headroom for a third rung.

◆ MOQSSTS_MAX_SETS

#define MOQSSTS_MAX_SETS 32

Switching sets per connection.

A subscriber holds one set per remote sharer, so this follows the hub's subscriber bound (WIRED_MOQTRUN_MAX_SUBS = 31). At most 32: the allocator keeps one bit per set in a u32 mask.

Function Documentation

◆ moqssts_bp_decide()

int moqssts_bp_decide	(	moqssts_bp *	st,
		const moqssts_set *	sets,
		usz	n,
		const u64 *	open,
		u64	timeouts,
		int *	out )

Algorithm 0xff01: observe, clamp the tier to the longest ladder - 1, and write out[i] = min(tier, n_members-1) for active sets, else MOQSSTS_NONE.

open is parallel to sets. Returns MOQSSTS_EV_* of the observation; an MOQSSTS_EV_UPSHIFT at the top rung is then cancelled by the clamp. Depth is moqssts_active_depth (deepest set), so several sharers with one stream each are uncongested.

◆ moqssts_default_decide()

void moqssts_default_decide	(	u64	budget_kbps,
		const moqssts_set *	sets,
		usz	n,
		int *	out )

Algorithm 0: stateless weighted strict-rank split of budget_kbps (clamped to MOQSSTS_UNLIMITED_KBPS).

Writes out[0..n).

Data Structures

Macros