sync_replay_project/docs/PROTOCOL.md

Protocol (proto = 11)

Wire format only. Behavior (gate, clock, timeouts) is in ARCHITECTURE.md. Source of truth: packages/core/lib/src/protocol.dart and packages/server/lib/src/server_hub.dart.

Any wire change bumps RelayConstants.protoVersion and edits this file in the same commit. Changes are additive where possible; a client and server with different proto refuse each other at hello (proto mismatch).

Transport

• WebSocket at /sync, JSON text frames, each an object with a t field.
• The first client frame must be hello within 10 s. Server pings every 5 s.
• Match incoming frames by t, never by order: welcome can race state.
• Failures: server sends {"t":"err","message":...} and closes. Messages: bad hello: ..., proto mismatch: got X, want Y, wrong secret, rejected: <reason> (e.g. avatar too large), bad message: ... (a frame with a non-finite number, e.g. NaN position), bad swap media, swap needs a url or the hosted file (empty swap).
• GET /health returns JSON (ok, name, proto, viewers, bytesServed, uptimeSecs; unauthenticated). GET /m/<sid>/<token>/<name> serves the host's file (Range table in ARCHITECTURE.md). GET /j/<sid>/<token>/<itemId> relays the current Jellyfin media (see "Jellyfin media" below). /r/<sid>/<token>/... is the relay of a member's media (see "Relay" below). GET /a/<sid>/<token>/<hash> serves a member's picture (see "Avatars" below), GET /i/<sid>/<token>/<hash> the current media's art (see "Media art" below).

Client to server

{"t":"hello","proto":11,"secret":"","user":{"name":"","avatar":""},
 "media":{"name":"","size":0,"duration":0,"url":null},"hostKey":"...",
 "resume":"<token>"}
{"t":"ready","ready":true,"source":"manual"}      source: manual | auto
{"t":"force","target":"all"|"<memberId>","ready":true}
{"t":"seek","position":123.4,"seq":3}
{"t":"swap","media":{"name":"","size":0,"duration":0,"url":"..."}}
{"t":"swap","media":{"name":"","size":0,"duration":0,"jellyfin":{<ref>}}}
{"t":"swap","media":{"name":"","size":123,"duration":0,"url":"...","relay":{}}}
{"t":"report","position":123.1,"buffered":6.2,"eventId":41,
 "duration":1440.5,"mediaId":2,"jf":{"<serverId>":true}}
{"t":"ping","c":1767225600000}
{"t":"chat","text":"..."}
{"t":"out","target":"<memberId>","out":true}
{"t":"nudge","target":"<memberId>"}
{"t":"profile","name":"...","avatar":"<base64>"}
{"t":"art","mediaId":2,"poster":"<base64>","banner":"<base64>"}

Field notes:

• hello.secret: the server password; a server with an empty secret is open. hello.media is carried but not used by the server today.
• hello.hostKey (optional, omitted when empty): the hosting app generates a random key per hosting, gives it to its server and sends it from its own client. The member whose key equals the server's is the host (members[].host). A server without a key (the dedicated server) has no host. Where the connection comes from does not matter.
• hello.resume (optional, omitted when empty): the token from this client's previous welcome, sent on reconnect. If the server still has that member, the connection takes it over: welcome returns the same token and memberId, the member keeps its flags, and there is no join/leave feed. Otherwise it is ignored and a new member joins.
• seek.seq (optional, default 0): increasing per client; echoed in state.seekSeq so the sender recognises its own seek.
• report.duration / report.mediaId (optional, omitted when 0): the open media's length in seconds and the mediaId it belongs to. The server takes the first one whose mediaId is current while its media duration is 0.
• report.jf (optional): Jellyfin access per ServerId, true = this client plays the item with its own account, false = through the relay. Sticky: a report without jf keeps the last one. At most 8 entries, keys up to 64 characters; others are dropped.
• ping.c: the client's clock in epoch ms; answered with pong.
• user.avatar: base64 PNG, at most 256 KiB of characters (else rejected: ...). name is trimmed, empty becomes ?, cut to 32 characters.
• profile: changes this member's name and/or picture mid-session. A field that is absent (or null) stays as it is; avatar: "" removes the picture. Same rules as hello.user; a too large avatar gets err rejected: avatar too large (the socket stays open, nothing changes).
• art: pictures of the current media (see "Media art").
• chat.text is cut to 500 characters.
• out: out:true sits the member target out (anyone, the host included); out:false brings the sender back and only works with its own member id as target. An out member is never in the set and never ready: ready from it and force on it are ignored, a swap leaves it out, the buffered of its reports counts as 0 (it keeps sending report: that keeps it fresh). Coming back puts it in the set like a join (unless playing), unready; it sends ready itself. Unknown targets and no-op changes are ignored. Feed: out (target = the name of who was sat out, absent when the actor sat itself out) and back.
• nudge: the server sends {"t":"nudge","from":<memberId>,"name":<name>} to target only (no feed). At most 3 per second and 20 per minute per sender; the rest, nudges to oneself and to unknown targets are dropped silently.
• swap.media.url is the link; name, size, duration are informational. swap.media.jellyfin makes it Jellyfin media (see below); then url is dropped. swap.media.relay makes it relayed media (see "Relay"). A swap keeps every member's ready and puts every fresh member in the set (inSet); clients that want to be unready for the new media send ready themselves (gate: ARCHITECTURE.md, "Ready gate").
• Frames before welcome (other than hello) are ignored; unparsable or unknown frames are ignored silently.

Server to client

{"t":"welcome","token":"...","memberId":"m1","mediaUrl":"...","relayUrl":"...",
 "state":{<state>}}
{"t":"state","eventId":7,"transport":"playing"|"held"|"finished","position":12.3,
 "serverTime":"<ISO-8601 UTC>","mediaId":1,
 "media":{name,size,duration,url?,jellyfin?,relay?,swapBy?,art?}|absent,
 "members":[{"id","name","avatarHash"?,"ready","inSet","buffered","stale","jf"?,
   "host"?,"out"?}],
 "seekBy":"m1","seekSeq":3,"relay":true}
{"t":"pong","c":1767225600000,"s":1767225600012}
{"t":"feed","actor":"","action":"","target":"","reason":"","at":"<ISO-8601>"}
{"t":"chat","text":"...","from":"<name>"}
{"t":"nudge","from":"m2","name":"Alice"}
{"t":"err","message":""}

• welcome.token is private to its owner and appears only in that member's media URL. mediaUrl is omitted when there is no media; it is a built /m/... URL for a host file (host part = what the client dialled), otherwise the shared link. welcome.state has the same shape as state. relayUrl is present only when state.relay is: this member's /j/... URL for the current Jellyfin media (private like token).
• state is sent every second and on every change. position is the value at serverTime (server clock); clients extrapolate while transport is playing, on the server's clock (see pong). finished: the media reached its end (position = duration); it stays until a seek or swap. eventId increases on seek, swap and transport changes. mediaId increases on swap. media.relay is present for relayed media (see "Relay"). media.duration is 0 until a client reports it. media.swapBy: member id of who swapped it in (omitted once that member left, and for the media the server started with). media.art: see "Media art". seekBy (member id) and seekSeq are present only while the current eventId came from a seek. relay (omitted when false): the server can relay the current Jellyfin media. members[].jf is the member's last report.jf, omitted when it never sent one. members[].avatarHash (omitted when the member has no picture) names the picture's bytes (see "Avatars"). members[].host (omitted when false): the member sent the server's hostKey (see hello.hostKey). members[].out (omitted when false): the member sits out (see out).
• pong.c echoes the ping's c; pong.s is the server's clock in epoch ms (the clock serverTime uses). With t3 the local receive time, the offset is s - (c + t3) / 2, accurate to half the round trip.
• feed.action values: ready, unready, force-ready, force-unready, force-play, force-hold, out, back, join, leave, stale, swap, seek, pause, resume, finished, chat, rename, avatar. target and reason are optional; reason carries auto/manual for ready and silent too long or no signal for leave/stale. rename: actor is the new name, target the old one. avatar: the actor changed only the picture.
• Chat is delivered twice: a chat frame with the text and a textless feed with action:"chat". Render the chat frame only; the reference client drops the textless feed entry.

Avatars

• The picture is sent as base64 in hello.user.avatar or profile.avatar; state carries only its hash: the first 16 hex characters of the sha256 of the decoded bytes. An avatar that is not valid base64, or whose bytes are not a PNG, JPEG, WebP or GIF (by magic bytes), counts as none.
• GET /a/<sid>/<token>/<hash>: the bytes with their image content type, cache-control: private, max-age=31536000, immutable (a hash names fixed bytes; clients fetch each hash once, from the host they dialled). Errors: 401 bad token, 404 no current member has that picture, 410 unknown session. Pictures go with their member (leave, change, removal).

Media art

client  {"t":"art","mediaId":2,"poster":"<base64>","banner":"<base64>"}
state   media.art = {"poster":"<hash>","banner":"<hash>"}

• A portrait poster and a wide banner for the current media, so every member (guests without Jellyfin included) can show them. Sent by the member who made the current swap (media.swapBy) or the host, for the current mediaId; from anyone else, or for another mediaId, it is ignored. Each field is optional; one that is not a picture (same check as avatars) or larger than its cap (poster 256 KiB, banner 768 KiB of decoded bytes) counts as none. A later art replaces the earlier one whole; any swap clears it.
• media.art carries hashes like avatarHash; a key is omitted when that picture is absent, art when both are.
• GET /i/<sid>/<token>/<hash>: as /a/ (see "Avatars"); 404 when the current media has no such art.

Jellyfin media

<ref> = {"serverId":"<Jellyfin ServerId>","itemId":"<32 hex>",
         "mediaSourceId":"...","title":"Pilot","subtitle":"Show - S01E01",
         "runtimeSecs":1440.5}

• serverId, itemId required; mediaSourceId, subtitle, runtimeSecs optional (omitted when absent); title always sent (may be empty). runtimeSecs is informational: the sync clock still uses the duration a client reports.
• The server keeps only the ref: for Jellyfin media no URL or token is stored or broadcast (a url sent along is dropped). serverId, itemId and mediaSourceId must match [A-Za-z0-9-]{1,64}, otherwise bad swap media. media.name is the swap's name, else the ref's title; names are sanitized like link names.
• Each client resolves the ref with its own account on that server. A client without one plays the relay URL when state.relay is true: welcome.relayUrl, or after a swap built like the /m one from the host it dialled: http://<host>/j/default/<token>/<itemId>.
• GET /j/<sid>/<token>/<itemId>: the server fetches /Videos/<itemId>/stream?static=true&mediaSourceId=<id> (the original file) with its own credential and streams it through. Range is forwarded as sent; status, Content-Range, Content-Length, Content-Type and Accept-Ranges come back as Jellyfin sent them. Errors: 401 bad token, 403 not the current item, 404 no relay for the current media, 410 unknown session, 502 Jellyfin unreachable or rejecting the server's credential. Bytes count toward bytesServed.
• Pasted Jellyfin links sent as plain url stay plain links.

Relay

Media whose bytes go through the server's buffer: a file on a member's computer, a link passed on by a member, a link the server fetches itself, or a Jellyfin item for viewers without an account. Behavior (buffer, flow control, cases) is in ARCHITECTURE.md, "Relay".

swap  media.relay = {}               the swapper's machine serves the bytes
      media.relay = {"tracks":[{"kind":"video"},{"kind":"audio"}]}
                                     the same, as two tracks played together
      media.relay = {"fetch":true}   the server reads media.url itself
state media.relay = {"source":"m2","live":true}   or {"fetch":true,"live":true}
      media.relay = {"fetch":true,"live":true,"tracks":[{"kind":"video"},{"kind":"audio"}]}

Tracks: a relayed media is one file, or several byte tracks (a page's separate video and audio files) that viewers play as one. tracks lists them in order, 2 to 4 entries of kind video or audio; absent (or anything else) means one file. Every track is a Range-readable byte stream; other kinds of track (e.g. segmented playlists) would add a field to the entry.

• swap: relay with jellyfin keeps the ref (the swapper serves it to viewers without an account; fetch is ignored there). relay without jellyfin: no URL is stored or broadcast; fetch needs an http(s) url (else relay fetch needs an http(s) url), which the server keeps to itself, and a server with a password (else server fetch needs a server password: an open server would fetch any address it can reach for anyone). A web page url is read through the server's yt-dlp; when the server cannot read the link, the swapper later gets The server can't fetch this link: <reason>. Any later swap ends the relay. tracks on a member's swap names the tracks its source will serve; the server ignores it with fetch and with jellyfin.
• state.media.relay.source: member id of the source (absent with fetch). live (omitted when false): bytes can flow (the source's channel is connected, or the server could read the link). tracks: as the swap named them, or, with fetch, set once the server has read the link (together with live).
• welcome.mediaUrl for relayed media without jellyfin: this member's /r/ URL; absent for the source itself (it plays its own copy) and for a fetched link the server has not read yet (clients open it on live). Clients build it after a swap like the /m one: http://<host>/r/default/<token>/<mediaId>/<name> (name with / and \ made _, then URL-encoded; the server ignores it). With tracks, it is an mpv EDL of every track's URL http://<host>/r/default/<token>/<mediaId>/<i>/<name>, each framed by its byte length: edl://!no_clip;%<len>%<track 0 url>;!new_stream;!no_clip;%<len>%<track 1 url>.
• GET /r/<sid>/<token>/<mediaId>/<name>: the media (one file), Range table as for /m. GET /r/<sid>/<token>/<mediaId>/<i>/<name>: track i (from 0) of a media with tracks (track 0 of a one-file media too). Errors: 401 bad token, 404 not the current relayed media (a player still on an older mediaId gets 404, never the new bytes) or no such track (the one-file URL of a media with tracks), 410 unknown session, 503 + retry-after the source never connected or the server could not read the link (60 s). A request whose bytes do not arrive within 60 s ends early. Bytes count toward bytesServed.
• GET /r/<sid>/<token>/source (WebSocket): the source's relay channel, dialled by the source (works behind NAT). Only the current source's token is accepted; anyone else, or the source after the next swap, is closed with code 4001 (the source stops instead of reconnecting). Frames:

source -> server  {"t":"info","size":<bytes>}            first frame, one file
                  {"t":"info","tracks":[{"kind":"video","size":<bytes>},
                                        {"kind":"audio","size":<bytes>}]}
                                                         first frame, tracks
                  binary: [u32 big-endian pull id][bytes]  in order per pull
                  {"t":"end","id":1}  {"t":"fail","id":1,"reason":"..."}
server -> source  {"t":"pull","id":1,"track":0,"start":0,"end":1048575}
                                                         inclusive; no track: 0
                  {"t":"ack","id":1,"n":65536}             bytes arrived
                  {"t":"cancel","id":1}                    stop that pull

Pull ids are unique across the tracks of a connection; track is sent only for a media with tracks. An info whose track count differs from the swap's tracks closes the channel with 4001. A track size different from the one before (a reconnect) drops that track's buffer.

Join and reconnect

hello then welcome, then the app opens mediaUrl, seeks to the server position and buffers. A joining member is ready=false; it is inSet at once unless the server is playing, then from its first ready (members present at a swap are put in the set by it). A socket the server sees close loses its token; reconnect is a fresh hello (with resume = the old token), after which the client re-sends its prior ready flag. If the old member was still there (unclean drop), resume takes it over and the token and media URL stay the same.

Reserved, not implemented

Server-held queue (queue-add, queue-skip) would auto-advance like a swap.