open-standards/wai/extensions/co-watch.md

WAI Extension: Co-watch

Status: Draft. Registers the record wai.live.cowatch: people in different places watching one programme together in a room, with shared control and with their sync measured. The room authority signs a co-watch policy, WXP1, naming the source the room watches (a WAI package, a sequence of on-demand objects, or another room's recording) and the bounds every rule below holds to. A principal whose grant allows it sends controls (play, pause, seek and rate), each effective at a stated instant of the room clock a little ahead of when it is sent, so that every viewer applies it at that same instant whatever its own latency; controls for one instant resolve by a pinned order, and the controls together define the controlled position of the source at every instant. Each viewer's player signs, periodically, a presentation statement, WXS1: the source position it was presenting at a room-clock instant, under the player's key. From those statements, the room clock signs a skew statement, WXK1, per window of the room clock: how far each viewer was from the controlled position, and each pair of viewers from one another, as percentiles, with a verdict against the policy's bound, which anyone holding the room recomputes; a viewer whose statements are missing is unknown, never in sync. A viewer that drifts past the bound corrects by a pinned catch-up rule: a rate nudge within limits, or a seek past a threshold. Controls and statements ride in the room's events, as meta payloads that their authors sign. No new transport. The reference implementation is the co_watch module of wai-rs (feature co_watch); the corpus is live-conformance/cowatch/, and its second implementation is live-conformance/co_watch_verify.py. Keywords MUST, MUST NOT, SHOULD, MAY are RFC 2119/8174.

Texts read for this revision: the live family this extension builds on, as merged (live-events, live-rooms, live-recording, live-metering and ad-presentation), synthesis §10.3 and delivery-telemetry; RFC 8032 and FIPS 180-4. No outside text is pinned: the rules of this extension are WAI's own.

1. Scope

In scope: the co-watch policy, its signature, the policy in effect at each instant and the source it names (§3); how this extension's statements ride in a room's events, who may sign each, and the grant attribute it reads (§4); controls, who may send them, their rules and the order in which they apply (§5); the controlled position (§6); presentation statements (§7); windows, samples, errors, percentiles, verdicts and the skew statement (§8); the catch-up rule (§9); where each rule is enforced (§10); and verification (§11).

Out of scope: a transport; how a player fetches and decodes the source; how a player estimates the room clock (§7.3 states what it SHOULD do, and §14 what a wrong estimate does); how a sink renders a control, an indicator or a verdict; and the room's media and chat, which live-rooms and live-events already carry, so that a co-watch room is also a room whose participants may see and hear one another.

Relationship to the core spec: wai.live.cowatch is a record (SPEC §5): signed records a WAI tool emits and verifies. It names no payload format and is not dispatchable.

Relationship to live-events: this extension changes nothing in it. Its statements are payloads of the meta kind (§4.2 there), which live-events leaves to applications, so every statement is signed by its author, chained, stamped with the room clock and rate-bounded by its author's policy (live-events §5, §6). A control is not a control event (live-events §4.9): that kind's commands are fixed, and a record of another command is refused by every verifier of the current revision.

Relationship to live-rooms: the descriptor's and the grant's layouts are unchanged. A room states its co-watch policy in a record of its own, WXP1 (§3), chained like live-moderation's review policy. Who may control is the direct power (live-rooms §3.3, bit 1) or a grant attribute (§4.4), which live-rooms §4.4 signs with the grant and leaves to the extensions that read it: a new power bit would be refused by every room verifier of the current revision (live-rooms §3.4: no power above bit 4).

Relationship to live-recording: a room's recording is a source (§3.5). A viewer that watches it through a relay fetches its tracks below their live edge, from the relay's DVR window or its store, which live-rooms §10.6 grants by the seek power: a co-watch room watching a recording gives its viewers that power.

Relationship to ad-presentation and synthesis §10.3: a presentation statement is a player's signed claim about what it presented, as a presentation claim and a player's log are. It is signed under the player's key as a presentation claim is (ad-presentation §5.3), and chained, as a player's log is, by the event chain it rides in.

Relationship to delivery-telemetry: a player MAY also report its present and stall-start observations there. Those are unsigned and advisory (§1.1 there); a presentation statement is signed, and skew is computed from statements alone.

2. Terms

• Room, room clock, principal, event, chain, grant, grant in effect, room key, merged order: as in live-events §2 and live-rooms §2 and §9.2. A room key is the room authority's key or the clock's.
• Statement: the payload of a meta event whose first four bytes are one of the magics of §4.1. It is named by the digest of its event (live-events §3.2).
• Source: what the room watches (§3.5). A source position is an instant of the source's own media time, in microseconds from its start.
• Controlled position: the source position the controls define for an instant of the room clock (§6).
• Player: the software that presents the source to a viewer and states what it presented (§7). Its key is the key of the principal it presents for.
• Viewer: a principal, not a room key, present in the room under a grant that may subscribe (live-rooms §3.3).
• Error of a presentation statement: its position less the controlled position at its instant (§8.3). Skew of two viewers: the difference of their errors.
• µs is microseconds of room clock; a rate is in millionths, so 1 000 000 is normal speed.

Integers are little-endian; a signed integer is two's complement. A digest is SHA-256 (FIPS 180-4), 32 bytes. Ed25519 is RFC 8032's, verified strictly (SPEC §7 Signatures): a key that fails step 1 or 2 is bad_key, and a signature that fails a later step is bad_signature.

3. The co-watch policy

3.1 Layout

field	size	meaning
magic	4	WXP1 (ASCII)
version	1	1 in this revision
room	32	the room id (live-rooms §3.2)
prev	32	the digest of the policy this one replaces (§3.4); 32 zero bytes for the first
effective	8	u64: the room-clock instant from which this policy applies; 0 for the first
source_type	1	1 a WAI package, 2 an object sequence, 3 a recording of a room (§3.5)
source	32	the source's digest (§3.5)
duration_us	8	u64: the source's duration, above 0
lead_ms	4	u32: the least time from a control's sending to its instant (§5.3), above 0
horizon_ms	4	u32: the most time from a control's sending to its instant, at least lead_ms
min_rate, max_rate	4 each	u32: the rates a control and the catch-up rule may set, min_rate above 0 and at most 1 000 000, max_rate at least 1 000 000
period_ms	4	u32: the slot length: a player states its presentation at least once in each (§7.3, §8.1), above 0
window_ms	4	u32: the length of a skew window (§8.1), a multiple of period_ms of 1 to 1000 slots
settle_ms	4	u32: how long after a viewer's join, a control's instant or a source's start a sample is settling (§8.3)
bound_us	4	u32: the sync bound (§8.5), above 0
quantile	1	the percentile a verdict is taken at (§8.5), 1 to 100
tolerance_us	4	u32: the error the catch-up rule leaves alone (§9), at most bound_us
nudge	4	u32: the rate the catch-up rule adds or takes away (§9), above 0
seek_us	4	u32: the error past which the catch-up rule seeks (§9), above bound_us
sig	64	§3.2

3.2 Digest and signature

The digest of a policy is SHA-256 over its record from magic through seek_us. The signature is Ed25519 by the room descriptor's authority over the 20 bytes wai:co-watch-policy (ASCII) and 0x01, followed by the bytes the digest covers.

3.3 Reading and rules

A reader reads exactly the bytes it is given as one policy, in the order of §3.1, and refuses at the first of these that holds:

1. Fewer than 4 bytes: truncated. A magic other than WXP1: malformed. No byte after the magic: truncated. A version other than 1: unknown_version.
2. The bytes end before a field the layout requires: truncated.
3. Bytes after sig: malformed.

It then holds the policy to the rules of §3.1's table, and refuses one that breaks any of them as cw_policy_malformed: source_type is 1, 2 or 3; duration_us, lead_ms, period_ms, bound_us and nudge are above 0; horizon_ms is at least lead_ms; min_rate is above 0 and at most 1 000 000, and max_rate at least 1 000 000; window_ms is period_ms times a whole number from 1 to 1000; quantile is 1 to 100; tolerance_us is at most bound_us; and seek_us is above bound_us. Then its room is the room id (wrong_room), its signature verifies (bad_key, bad_signature), and its link holds (§3.4, cw_policy_unlinked).

3.4 The policy in effect

A verifier is given a room's policies in a list. The first has prev zero and effective 0. Each later one has as prev the digest of the one before it in the list, an effective above that one's, and, when it names the source of the one before it (the same source_type and source), that one's duration_us. A policy that breaks this is refused cw_policy_unlinked. The policy in effect at room-clock instant t is the last of the list whose effective is at or below t.

A policy is replaced, never amended. A policy that names another source than the one before it starts that source at its effective: the controlled position begins again (§6.4). One that names the same source changes the bounds alone, and the controlled position runs on.

3.5 The source

• A WAI package (1): source is the receipt_hash of the package's receipt (packaging §9). A source position is the package's presentation time.
• An object sequence (2): on-demand media objects presented in order, each its content hash (SHA-256 of the object's bytes as served) and its presentation duration in milliseconds (a u32, at least 1). source is SHA-256 over the 20 bytes wai:co-watch-source (ASCII) and 0x01, the objects' count (u32), and for each object in order its content hash and its duration; duration_us is 1000 times the sum of the durations. The object at index j holds the source positions from 1000 times the sum of the durations before it, for 1000 times its own. This is the form of ad-presentation §4's creative digest, under a label of its own.
• A recording of a room (3): source is the digest of the recording's manifest (live-recording §4.1). A source position is the recorded room's clock less the recording's first instant.

A verifier given an object sequence's objects for a policy checks them against it: the policy names an object sequence, its source is their digest, and its duration_us is 1000 times the sum of their durations. Otherwise cw_source_mismatch. A player checks the objects it fetches by their content hashes, so it presents what the policy names.

4. Statements

4.1 Carriage

magic	statement	section	who signs it
WXC1	control	§5.1	a room key, or a principal whose grant in effect allows the action (§5.2)
WXS1	presentation statement	§7.1	a viewer's player, with the viewer's key: any principal that is not a room key
WXK1	skew statement	§8.4	a room key

Each statement is the payload of a meta event: its magic, a version byte of 1, and the fields its section lists. A meta payload whose first four bytes are none of these magics is not this extension's, and a sink that does not implement it shows none of them: they are timed metadata for its application (live-events §4.2). Every statement is bounded by its author's policy (live-events §5, max_payload), 1024 bytes for a room key, which bounds the counts §8.4 allows.

4.2 Reading

A statement that ends before a field, that holds bytes after its last field, whose version is not 1, or whose field holds a value its section does not allow, is cw_statement_malformed.

4.3 Who signs

A presentation statement by a room key, and a skew statement by any other principal, are cw_not_permitted. A control's author is checked by §5.2. A grant's powers and attributes are those of the author's grant in effect at its event's t (live-rooms §9.2 step 4); a room key holds every power. Since a statement is a meta event, the room verifier has already held it to its author's grant: a principal whose policy does not permit meta cannot send one (live-rooms §9.2 step 4, kind_not_permitted).

4.4 The control attribute

This extension reads one grant attribute (live-rooms §4.4):

key	value	meaning
watch_control	a decimal integer from 1 to 15	the actions the principal may control: bit 0 play, bit 1 pause, bit 2 seek, bit 3 rate

A decimal integer here is 1 or 2 of the digits 0–9, the first not 0. A grant whose watch_control is not of that form, or is above 15, is refused cw_attr_malformed. A grant without it allows no action, unless its powers hold the direct power (§5.2). The authority gives a room's hosts every action and its guests some, or every viewer every action, as the room calls for.

4.5 The policy a statement is read under

A statement is read under the policy in effect at its event's t. A statement at an instant with no policy in effect is refused cw_no_policy.

5. Controls

5.1 Layout

field	size	meaning
magic, version	5	WXC1, 1
action	1	1 play, 2 pause, 3 seek, 4 rate
at	8	u64: the room-clock instant it takes effect
arg	8	u64: for seek, the source position to go to; for rate, the rate, 1 to 2^32 − 1; for play and pause, 0

A control whose action is outside 1 to 4, or whose arg is not as the table allows for its action, is cw_statement_malformed.

5.2 Who may control

A control's author is a room key, or a principal whose grant in effect at the control's t holds the direct power (live-rooms §3.3, bit 1), or carries watch_control with the action's bit set (§4.4). Otherwise the control is refused control_not_permitted.

5.3 Rules

A control is sent ahead of its instant, so that it reaches every viewer before the instant comes, and not far ahead. With t its event's t, under the policy in effect at t:

1. at is at least t + lead_ms × 1000: otherwise control_not_ahead. A control whose at is at or before its own t, in the past when it is sent, is so refused.
2. at is at most t + horizon_ms × 1000: otherwise control_beyond_horizon.

Then, under the policy in effect at at, the instant the control is for:

3. A seek's arg is at most duration_us, and a rate's is from min_rate to max_rate: otherwise control_out_of_range.

A relay or a room service that carries the room's events MUST NOT carry a control these rules or §5.2 refuse, and a player MUST NOT apply one.

5.4 The order of application

The accepted controls apply in ascending order of at; of two with one at, the one whose event has the lower t first; of two with one at and one t, the one whose event digest is the lower first (the digests compared as bytes). So of two controls for one instant, the one sent later prevails, and between two sent at one instant the order is still fixed: every party holding the controls applies them in one order, whatever order it received them in.

Of the controls at one instant, a later one overrides an earlier one that sets the same thing: play and pause set whether the source plays, seek sets the position, and rate sets the rate. A control that a later one at its instant overrides is stated overridden, and every other control applied. Controls at one instant that set different things all apply: a seek and a play at one instant go to the position and play from it.

6. The controlled position

6.1 The state

The controlled position is a function of the room clock, kept as a state: whether the source plays, its rate, and an anchor: an instant and the source position at it.

6.2 The position at an instant

At instant x at or after the anchor's instant a, with anchor position p, rate r and duration D (the duration_us of the policy in effect at x), the controlled position is:

• while the source plays: p + ⌊(x − a) × r / 1 000 000⌋, or D if that is above D;
• while it does not: p.

The product is computed without overflow (at least 128 bits).

6.3 Applying a control

A control at instant at first sets the anchor to (at, the controlled position at at), then:

• play: the source plays; pause: it does not;
• seek: the anchor's position becomes arg;
• rate: the rate becomes arg.

A source that reaches its duration while it plays stays at D: it has ended, and a seek before D sets it going again from there.

6.4 Where a source starts

A source starts at the first policy's effective, 0, and at the effective of each later policy that names another source (§3.4). At a start s, the state is: the source does not play, its rate is 1 000 000, and its anchor is (s, 0). A control applies to the source in effect at its at; a control whose at is a start applies after it.

The controlled position at x is then the state reached from the last start at or before x by applying, in the order of §5.4, every accepted control whose at is from that start to x, and §6.2 at x.

7. Presentation statements

7.1 Layout

field	size	meaning
magic, version	5	WXS1, 1
room	32	the room id
player	32	the key that signs it: the viewer's
source	32	the source it presents (§3.5)
at	8	u64: the room-clock instant it describes
position	8	u64: the source position it was presenting at at
state	1	1 playing, 2 paused, 3 stalled (waiting for data), 4 ended
rate	4	u32: the rate it was presenting at, above 0: the controlled rate, or a catch-up rate (§9)
sig	64	Ed25519 by player over the 26 bytes wai:co-watch-presentation (ASCII) and 0x01, followed by the record from magic through rate

A statement whose state is outside 1 to 4 or whose rate is 0 is cw_statement_malformed. Its digest, where a party names it apart from its event, is SHA-256 over the record from magic through rate.

The statement carries its own signature, beside its event's, so that it stands on its own: a viewer that shows a third party how closely it watched presents its statements and its grant, with no other event of the room.

7.2 Rules

A presentation statement is checked, in this order:

1. its room is the room id: otherwise wrong_room;
2. its player is its event's principal: otherwise presentation_wrong_key. A statement lifted from another viewer's chain, or made by a key the viewer does not hold, is so refused;
3. its signature verifies under player (§7.1): bad_key, bad_signature;
4. its at is at most its event's t: a statement describes what was presented, never what will be. Otherwise presentation_ahead;
5. its source is the source of the policy in effect at at: otherwise presentation_wrong_source.

7.3 The player

A player in a co-watch room MUST:

• apply each control at its at on its estimate of the room clock, and one that reaches it after its at at once, and present the controlled position by the catch-up rule (§9);
• state its presentation at least once in every slot (§8.1) while it is present, and SHOULD state it twice in each, so that timer jitter leaves no slot empty;
• state as position the position of the frame or sample it presented at at, as its media pipeline reports it, and as at the instant on its estimate of the room clock;
• state stalled while it waits for data, and ended once it has presented the source's end.

A player SHOULD estimate the room clock against a room key's clock, by a request whose answer states the instant it was made at, taking the answer of least round trip of several and the midpoint of that round trip, and SHOULD measure again at least each minute. Its statements state its estimate: a player whose clock is off states errors that are off by as much (§14).

8. Skew

8.1 Windows and slots

A window is a stretch of the room clock, from from up to, but not including, to, over which one policy is in effect and whose length is a whole number of that policy's slots, from 1 to 1000. Its slots are from + j × P to from + (j + 1) × P, j from 0, with P the policy's period_ms × 1000. A room clock that states windows regularly states them window_ms long, starting at multiples of window_ms × 1000; §8.4 checks any window a statement names.

8.2 Viewers and due slots

A principal is present from the t of a join of it to the t of the following leave of it, if any (live-rooms §7.2). A slot is due for a principal that is not a room key when it lies wholly within a stretch of its presence (the join at or before the slot's start, and any leave at or after its end), and the grant in effect for it at the slot's start may subscribe (live-rooms §3.3). The viewers of a window are the principals with a slot of the window due.

8.3 Samples, errors and settling

A skew statement names, for each viewer it lists, held: how many of the viewer's events the room clock held when it made the statement (its next seq). A viewer's sample in a slot is, of its accepted presentation statements whose event's seq is below held and whose at is within the slot, the first in the merged order. A statement the clock had not received when it made its statement, a later seq, is not a sample of that statement, whenever it arrived.

A sample's error is its position less the controlled position at its at (§6), a signed number of microseconds. A sample is settling when its at is within settle_ms × 1000 µs (of the policy in effect at its at) after a join of the viewer, the at of an accepted control, or a source's start: at or after that instant and before it plus settle_ms × 1000. A settling sample fills its slot, and is not counted: a viewer seeking to the room on its join, or every viewer applying a control, takes a moment that the bound is not about.

8.4 The skew statement

field	size	meaning
magic, version	5	WXK1, 1
from, to	8 each	u64: the window (§8.1)
bound_us	4	u32: the bound it judges by, the policy's
quantile	1	the percentile it judges at, the policy's
viewers	1 + 59 × v	a u8 count (1 to 8), then each viewer: principal (32), held (u64), due, samples, counted (u16 each), p50, pq, max (u32 each), verdict (u8)
pairs	1 + 17 × v(v − 1)/2	a u8 count, then each pair: a, b (u8 each, indexes into viewers), n (u16), p50, pq, max (u32 each), verdict (u8)

For a viewer: due is its due slots, samples the due slots it has a sample in, and counted those whose sample is counted; p50, pq and max are the 50th and the quantile-th percentile and the greatest of the absolute errors of its counted samples, and all three 0 when it has none; verdict is §8.5's. For a pair: n is the due slots in which both have a counted sample, and p50, pq and max are over the absolute values of the skew in those slots, the first viewer's error less the second's; verdict is §8.5's. A value above 2^32 − 1 is written 2^32 − 1.

A skew statement is cw_statement_malformed when: its viewers are not in strictly ascending order of principal; a verdict is outside 1 to 3; its pairs are not every pair of its viewers exactly once, in ascending order of (a, b) with a below b; from is not below to; to is above its event's t; the policy in effect at from is not in effect up to to, or to − from is not a whole number of its slots from 1 to 1000. Then, under the policy in effect at from:

1. bound_us and quantile are the policy's: otherwise skew_mismatch;
2. each viewer it lists is a viewer of the window, its held is at most the number of that viewer's events the room holds, and its entry is the one recomputed from the room: otherwise skew_mismatch;
3. each pair's entry is the one recomputed: otherwise skew_mismatch.

A statement may list a subset of a window's viewers, since one holds at most eight; it states the pairs among those it lists. §8.6 requires every viewer of the window to be listed by one.

8.5 Percentiles and verdicts

The q-th percentile of n values, n above 0, is the value at index ⌈q × n / 100⌉ − 1 of the values in ascending order: the nearest rank. A verdict is 1 in sync, 2 out of sync or 3 unknown:

• a viewer is unknown when it has a due slot with no sample, or no counted sample; it is in sync when its pq is at most bound_us, and out of sync otherwise;
• a pair is unknown when either of its viewers is unknown or its n is 0; it is in sync when its pq is at most bound_us, and out of sync otherwise.

"In sync" is so a claim with a stated bound, checked against the statements. A viewer whose statements are missing for a slot it was present for is unknown, never in sync: a player that stops stating is not counted as keeping up.

8.6 Completeness

A room clock that states a window states all of it. Once every record is read, for each window that an accepted skew statement names, every viewer of the window is listed by a skew statement of that window: otherwise skew_incomplete, at the window's first skew statement in the merged order.

9. Catch-up

A player that is off the controlled position corrects by this rule, so that two players given the same facts make the same correction, and a test reproduces it. At instant x, under the policy in effect at x, with c the controlled position, r the controlled rate and e the player's position less c:

• When the source does not play, or c is the duration (it has ended): seek to c when |e| is above tolerance_us, and hold otherwise.
• When it plays:
  • |e| at most tolerance_us: rate r;
  • |e| above tolerance_us and at most seek_us: rate r − nudge when e is above 0 (ahead: slower), and r + nudge when it is below (behind: faster), held within min_rate to max_rate;
  • |e| above seek_us: seek to c.

A player plays while the source plays and pauses while it does not; the rule sets its rate and its position. It evaluates the rule each time it states its presentation (§7.3), and keeps the outcome until the next. A rate held at a limit corrects nothing, and the error then grows until the rule seeks. A seek lands where the room is when it is made; what it took to land is left to the next evaluations' rates.

10. Where each rule is enforced

• A room service or relay that carries the room's events carries a control only when §5.2 and §5.3 accept it, and a presentation statement only when §7.2 does (§5.3 asks it of controls). It carries each viewer's events in its chain order. A skew statement it signs names, as held, what it held when it made it, so a statement that arrives later changes nothing that statement says.
• The room clock signs the lifecycle (live-rooms §7.2), and so when a viewer is present, and signs skew statements (§8.4): what the room states about how closely it watched together. The service that serves the source states the policy (§3) and serves the objects the policy names.
• The room authority signs the policy and gives the control attribute (§4.4).
• Players apply controls at their instants (§7.3), correct by the catch-up rule (§9) and state their presentation (§7). Sinks show the verdicts with the bound they are against.

After the fact, a verifier (§11) checks every one of these against the room: a control a service carried that §5 refuses, a statement signed by a key that is not its viewer's, and a skew statement that does not recompute or leaves a viewer out are each found out.

11. Verification

11.1 Inputs

A co-watch verifier is given what a room verifier is given (live-rooms §9.1: a descriptor, grants, a key-log head or none, and the chains), and:

• policies: the room's co-watch policies, in a list (§3.4);
• sources: for any policy, the objects of the object sequence it names (§3.5), or none.

11.2 Steps

It runs these steps and stops at the first refusal, which it reports with the reason and where:

1. The room (live-rooms §9): a refusal, at room with the room verifier's own place. A gap is not a refusal; the steps go on, and the checks of a skew statement that recompute from the room (§8.4 steps 2 and 3, and §8.6) are not made: each depends on events the verifier may not hold.
2. The policies, in the list's order (§3.3, §3.4), then the sources given (§3.5, cw_source_mismatch): at the policy's index.
3. The grants' attributes (§4.4), in the grants' order: at the grant's index.
4. The statements, in the merged order: each meta event the room accepted whose payload begins with a magic of §4.1: §4.2, then §4.5 (cw_no_policy), then §4.3, then the rules of its section, in the order each section gives. A skew statement's checks that recompute (§8.4 steps 2 and 3) are made after every statement has been read, in the merged order, since its samples may follow it there. At the record.
5. Completeness (§8.6): at the record.
6. The outcome (§11.3).

11.3 Outcomes and what a verification states

• refuse: the reason and where (§11.2).
• gap: no refusal, and the room has a gap: the gaps, under the code delivery_gap, and what accept states, computed from what the verifier holds, with each skew statement stated unchecked.
• accept: no refusal and no gap.

With accept or gap, the verifier states:

• the policies' digests, and each start of a source: its instant and the policy's index;
• each control, in the merged order: its digest, author, action, at, arg, and applied or overridden;
• the timeline: each instant at which a control or a start changed the state, in order, with the state after it (whether the source plays, the rate, and the anchor's position);
• each presentation statement, in the merged order: its digest, player, at, position, state, the controlled position at at, its error, and whether it is settling;
• each skew statement, in the merged order: its digest, its window, and for each viewer and each pair the entry recomputed, or unchecked.

12. Reasons

The codes this extension adds, each with where it is raised:

reason	raised at
cw_policy_malformed	§3.3
cw_policy_unlinked	§3.4
cw_source_mismatch	§3.5, §11.2 step 2
cw_attr_malformed	§4.4, §11.2 step 3
cw_statement_malformed	§4.2, §5.1, §7.1, §8.4
cw_no_policy	§4.5
cw_not_permitted	§4.3
control_not_permitted	§5.2
control_not_ahead	§5.3 step 1
control_beyond_horizon	§5.3 step 2
control_out_of_range	§5.3 step 3
presentation_wrong_key	§7.2 step 2
presentation_ahead	§7.2 step 4
presentation_wrong_source	§7.2 step 5
skew_mismatch	§8.4
skew_incomplete	§8.6

A co-watch verifier also raises every code a room verifier raises (live-rooms §11 and the live-events codes it lists), at the room; truncated, malformed and unknown_version for a policy that does not read; wrong_room, bad_key and bad_signature for a policy of another room or whose signature fails, and for a presentation statement (§7.2); and delivery_gap for a gap, which is not a refusal.

13. Conformance

live-conformance/cowatch/ holds the corpus, in one file, vectors.json, written from fixed seeds by wai_co_watch_vectors gen. It has rooms, each a room (descriptor, grants, key-log head or none, chains), its policies and sources, and the outcome §11.3 gives them with everything it states; and catch-up cases.

• Accepted: a watch party of three viewers, a host who may control every action, a guest who may play and pause, and a viewer who joins late and may control nothing: the host plays, the guest pauses and plays, the host seeks and sets a rate and sets it back, two controls for one instant resolve by the order of §5.4, the late joiner's first samples settle, and the clock's skew statements state every viewer in sync; a room whose second policy starts a new source; and a room in which one viewer drifts out of sync, one misses a slot and is unknown, one leaves partway, and a window is stated in two statements.
• Gap: the watch party with one viewer's record not delivered: its skew statements unchecked.
• Refused: each reason of §12 at least once, each vector a control of one defect: among them a control from a party without the control power, a control effective in the past, a presentation statement signed by another key than its viewer's and one lifted from another viewer's chain, a skew statement that does not recompute, and a viewer whose statements are missing reported in sync; and policies that do not read or are of another room.
• Catch-up cases: a policy, the controlled state at an instant and a player's position, and the correction §9 gives, for every branch of the rule and both limits of the rate.

A conforming verifier gives every room the outcome, the reason, the place, and everything else the corpus states, and every catch-up case its correction. wai_co_watch_vectors verify and the test tests/co_watch.rs check that the file regenerates byte for byte, that wai-rs gives every room its outcome, that every reason of §12 appears, and that §12 lists exactly the reference's codes. live-conformance/co_watch_verify.py is the second implementation, written from this text: it shares no code with wai-rs; it verifies each room with the live-rooms corpus's second implementation (live_rooms_verify.py) and every signature with WAI's strict rule (claim-conformance/_ed25519.py); and it mutates copies of the corpus and requires every copy to fail. The browser client's co-watch module (web/room/cowatch.js of joule-moq-web) gives every catch-up case its correction and writes the corpus's controls and presentation statements byte for byte.

14. Security and privacy considerations

• Every control and every statement is signed by its author and rides in its author's chain, and every policy is signed by the room authority. A service can withhold an event but alter none, and a withheld event is a gap in its author's chain. A withheld control is the room's to notice: its author sees its control missing from what the room carries.
• A control's instant is ahead of its sending by at least the policy's lead, so every viewer holds it before it applies, as long as it reaches them within the lead. One that reaches a viewer late is applied at once, and the viewer catches up by §9; the skew it causes settles within the policy's settle time, and a lead too short for the room's delivery shows as viewers out of sync.
• A presentation statement is a player's claim: it proves that the holder of the viewer's key stated it, not that it is true. A modified player can state any position, as a player of ad-presentation §13 can. Skew measures how well the room's players say they kept together; a relay's per-hop records (live-metering §3) of when it delivered the source's objects to each session are independent evidence against a statement of a position the session had not received.
• A player's at is on its own estimate of the room clock (§7.3). An estimate that is off shifts its errors by as much, in both directions alike, so a viewer that keeps its own clock wrong is measured as out of sync, never as in sync on another's behalf.
• The skew statement is the room clock's, and is recomputed by anyone holding the room: a clock that states a viewer in sync without the samples, or leaves an out-of-sync viewer out of a window, is refused (§8.4, §8.6). What remains to the clock is to omit a viewer's events from the room, which leaves a gap in the viewer's chain (live-events §6.2), and every skew statement of a room with a gap is unchecked.
• A viewer's statements say what it watched and when, which is personal data. A viewer that needs its watching unlinkable across rooms uses a key per room, as live-events §11 has every principal do. Taking part in a room under a grant is taking part in its mechanics, and this extension adds no consent step to them. Where a regime asks the operator for a lawful basis for processing what a room's players state, the operator records it as a lawful-basis claim (jwp-receipts §2.4.6).