Galène's protocol

A group is a set of clients. It is identified by a human-readable name that must not start or end with a slash “/”, must not start with a period “.”, and must not contain the substrings “/../” or “/./”.

Client

A client is a peer that may originate offers and chat messages. It is identified by an id, an opaque string that is assumed to be unique. Peers that do not originate messages (servers) do not need to be assigned an id.

Stream

A stream is a set of related tracks. It is identified by an id, an opaque string. Streams in Galène are unidirectional. A stream is carried by exactly one peer connection (PC) (multiple streams in a single PC are not allowed). The offerer is also the RTP sender (i.e. all tracks sent by the offerer are of type sendonly).

Galène uses a symmetric, asynchronous protocol. In client-server usage, some messages are only sent in the client to server or in the server to client direction.

Before connecting

The client needs to know the location of the group, the (user-visible) URL at which the group is found. This may be obtained either by explicit configuration by the user, or by parsing the /public-groups.json file which may contain an array of group statuses (see below).

A client then performs an HTTP GET request on the file .status.json at the group’s location. This yields a single JSON object, which contains the following fields:

Connecting

The client connects to the websocket at the URL obtained at the previous step. Galene uses a symmetric, asynchronous protocol: there are no requests and responses, and most messages may be sent by either peer.

Message syntax

All messages are sent as JSON objects. All fields except type are optional; however, there are some fields that are common across multiple message types:

There are two kinds of errors. Unsolicited errors are sent using messages of type usermessage of kind error or warning. Errors sent in reply to a message use the same type as the usual reply, but with a specific kind (such as fail). In either case, the field value contains a human-readable error message, while the field error, if present, contains a stable, program-readable identifier for the error.

Establishing and maintaining a connection

The peer establishing the connection (the WebSocket client) sends a handshake message. The server replies with another handshake message. The client may wait for the server’s handshake, or it may immediately start pipelining messages to the server.

{
    type: 'handshake',
    version: ["2"],
    id: id
}

The version field contains an array of supported protocol versions, in decreasing preference order; the client may announce multiple versions, but the server will always reply with a single version. If the field id is absent, then the peer doesn’t originate streams.

{
    type: 'ping'
}

{
    type: 'pong'
}

Joining and leaving

{
    type: 'join',
    kind: 'join' or 'leave',
    group: group,
    username: username,
    password: password,
    data: data
}

If token-based authorisation is beling used, then the username and password fields are omitted, and a token field is included instead.

When the sender has effectively joined the group, the peer will send a ‘joined’ message of kind ‘join’; it may then send a ‘joined’ message of kind ‘change’ at any time, in order to inform the client of a change in its permissions or in the recommended RTC configuration.

{
    type: 'joined',
    kind: 'join' or 'fail' or 'change' or 'leave',
    error: may be set if kind is 'fail',
    group: group,
    username: username,
    permissions: permissions,
    status: status,
    data: data,
    rtcConfiguration: RTCConfiguration
}

The username field is the username that the server assigned to this user. The permissions field is an array of strings that may contain the values present, op and record. The status field is a dictionary that contains status information about the group, and updates the data obtained from the .status.json URL described above.

Maintaining group membership

Whenever a user joins or leaves a group, the server will send all other users a user message:

{
    type: 'user',
    kind: 'add' or 'change' or 'delete',
    id: id,
    username: username,
    permissions: permissions,
    status: status
}

Requesting streams

{
    type: 'request',
    request: requested
}

The field request is a dictionary that maps the labels of requested streams to a list containing either ‘audio’, or one of ‘video’ or ‘video-low’. The empty key '' serves as default. For example:

{
    type: 'request',
    request: {
        camera: ['audio', 'video-low'],
        '': ['audio', 'video']
    }
}

Pushing streams

{
    type: 'offer',
    id: id,
    label: label,
    replace: id,
    source: source-id,
    username: username,
    sdp: sdp
}

If a stream with the same id exists, then this is a renegotiation; otherwise this message creates a new stream. If the field replace is not empty, then this request additionally requests that an existing stream with the given id should be closed, and the new stream should replace it; this is used most notably when changing the simulcast envelope.

The field label is one of camera, screenshare or video, and will be matched against the keys sent by the receiver in their request message.

The field sdp contains the raw SDP string (i.e. the sdp field of a JSEP session description). Galène will interpret the nack, nack pli, ccm fir and goog-remb RTCP feedback types, and act accordingly.

The sender may either send a single stream per media section in the SDP, or use rid-based simulcasting with the streams ordered in decreasing order of throughput. In that case, it should send two video streams, the first one with high throughput, and the second one with throughput limited to roughly 100kbit/s. If more than two streams are sent, then only the first and the last one will be considered.

The receiver may either abort the stream immediately (see below), or send an answer.

{
    type: 'answer',
    id: id,
    sdp: SDP
}

{
    type: 'ice',
    id: id,
    candidate: candidate
}

The answerer may request a new offer of kind renegotiate and an ICE restart by sending a renegotiate message:

{
    type: 'renegotiate',
    id: id
}

At any time after answering, the client may change the set of streams being offered by sending a ‘requestStream’ request:

{
    type: 'requestStream'
    id: id,
    request: [audio, video]
}

Closing streams

{
    type: 'close',
    id: id
}

The answerer may request that the offerer close a stream by sending an abort message.

{
    type: 'abort',
    id: id
}

The stream will not be effectively closed until the offerer sends a matching close.

Sending messages

{
    type: 'chat',
    kind: '' or 'me',
    source: source-id,
    username: username,
    dest: dest-id,
    privileged: boolean,
    time: time,
    noecho: false,
    value: message
}

If dest is empty, the message is a broadcast message, destined to all of the clients in the group. If source is empty, then the message was originated by the server. The message is forwarded by the server without interpretation, the server only validates that the source and username fields are authentic. The field privileged is set to true by the server if the message was originated by a client with the op permission. The field time is the timestamp of the message, coded as a number in version 1 of the protocol, and as a string in ISO 8601 format in later versions. The field noecho is set by the client if it doesn’t wish to receive a copy of its own message.

The chathistory message is similar to the chat message, but carries a message taken from the chat history. Most clients should treat chathistory similarly to chat.

A user message is similar to a chat message, but is not conserved in the chat history, and is not expected to contain user-visible content.

{
    type: 'usermessage',
    kind: kind,
    source: source-id,
    username: username,
    dest: dest-id,
    privileged: boolean,
    value: value
}

Currently defined kinds include error, warning, info, kicked, clearchat (not to be confused with the clearchat group action), and mute.

{
    type: 'useraction',
    kind: kind,
    source: source-id,
    username: username,
    dest: dest-id,
    value: value
}

Currently defined kinds include op, unop, present, unpresent, kick and setdata.

{
    type: 'groupaction',
    kind: kind,
    source: source-id,
    username: username,
    value: value
}

Currently defined kinds include clearchat (not to be confused with the clearchat user message), lock, unlock, record, unrecord, subgroups and setdata.

Authorisation protocol

In addition to username/password authentication, Galene supports authentication using cryptographic tokens. Two flows are supported: using an authentication server, where Galene’s client requests a token from a third-party server, and using an authentication portal, where a third-party login portal redirects the user to Galene. Authentication servers are somewhat simpler to implement, but authentication portals are more flexible and avoid communicating the user’s password to Galene’s Javascript code.

Authentication server

If a group’s status dictionary has a non-empty authServer field, then the group uses an authentication server. Before joining, the client sends a POST request to the authorisation server URL containing in its body a JSON dictionary of the following form:

{
    "location": "https://galene.example.org/group/groupname/",
    "username": username,
    "password": password
}

If the user is not allowed to join the group, then the authorisation server replies with a code of 403 (“not authorised”), and Galene will reject the user. If the authentication server has no opinion about whether the user is allowed to join, it replies with a code of 204 (“no content”), and Galene will proceed with ordinary password authorisation.

If the user is allowed to join, then the authorisation server replies with a signed JWT (a “JWS”) the body of which has the following form:

{
    "sub": username,
    "aud": "https://galene.example.org/group/groupname/",
    "permissions": ["present"],
    "iat": now,
    "exp": now + 30s,
    "iss": authorisation server URL
}

The permissions field contains the permissions granted to the client, in the same format as in the joined message. Since the client will only use the token once, at the very beginning of the session, the tokens issued may have a short lifetime (on the order of 30s).

Authentication portal

If a group’s status dictionary has a non-empty authPortal field, Galene redirects the user agent to the URL indicated by authPortal. The authentication portal performs authorisation, generates a token as above, then redirects back to the group’s URL with the token stores in a URL query parameter named token: