This document describes the Socket.IO protocol. For a reference JavaScript implementation, take a look at socket.io-parser, socket.io-client and socket.io.

• Protocol version
• Packet format
  • Packet types
    • CONNECT
    • DISCONNECT
    • EVENT
    • ACK
    • ERROR
    • BINARY_EVENT
• Packet encoding
  • Encoding format
  • Examples
• Exchange protocol
  • Connection to the default namespace
  • Connection to a non-default namespace
  • Disconnection from a non-default namespace
  • Acknowledgement
• History
  • Difference between v3 and v2
  • Difference between v2 and v1
  • Initial revision

This is the revision 3 of the Socket.IO protocol, included in ̀socket.io@1.0.0...1.0.2.

The 4th revision (included in ̀socket.io@1.0.3...latest) can be found here: https://github.com/socketio/socket.io-protocol/tree/master

Both the 1st and the 2nd revisions were part of the work towards Socket.IO 1.0 but were never included in a Socket.IO release.

It is built on top of the 3rd revision of the Engine.IO protocol.

While the Engine.IO protocol describes the low-level plumbing with WebSocket and HTTP long-polling, the Socket.IO protocol adds another layer above in order to provide the following features:

• multiplexing (what we call Namespace)

Example of the Javascript API:

// server-side
const nsp = io.of("/admin");
nsp.on("connect", socket => {});

// client-side
const socket1 = io(); // default namespace
const socket2 = io("/admin");
socket2.on("connect", () => {});

• acknowledgement of packets

Example of the Javascript API:

// on one side
socket.emit("hello", 1, () => { console.log("received"); });
// on the other side
socket.on("hello", (a, cb) => { cb(); });

A packet contains the following fields:

• a type (integer, see below)
• a namespace (string)
• optionally, a payload (string | Array)
• optionally, an acknowledgment id (integer)

This event is sent:

• by the client when requesting access to a namespace
• by the server when accepting the connection to a namespace

It does not contain any payload nor acknowledgement id.

Example:

{
  "type": 0,
  "nsp": "/admin"
}

The client may include additional information (i.e. for authentication purpose) in the namespace field. Example:

{
  "type": 0,
  "nsp": "/admin?token=1234&uid=abcd"
}

This event is used when one side wants to disconnect from a namespace.

It does not contain any payload nor acknowledgement id.

Example:

{
  "type": 1,
  "nsp": "/admin"
}

This event is used when one side wants to transmit some data (without binary) to the other side.

It does contain a payload, and an optional acknowledgement id.

Example:

{
  "type": 2,
  "nsp": "/",
  "data": ["hello", 1]
}

With an acknowledgment id:

{
  "type": 2,
  "nsp": "/admin",
  "data": ["project:delete", 123],
  "id": 456
}

This event is used when one side has received an EVENT or a BINARY_EVENT with an acknowledgement id.

It contains the acknowledgement id received in the previous packet, and may contain a payload (without binary).

{
  "type": 3,
  "nsp": "/admin",
  "data": [],
  "id": 456
}

This event is sent by the server when the connection to a namespace is refused.

It may contain a payload indicating the reason of the refusal.

Example:

{
  "type": 4,
  "nsp": "/admin",
  "data": "Not authorized"
}

This event is used when one side wants to transmit some data (including binary) to the other side.

It does contain a payload, and an optional acknowledgement id.

Example:

{
  "type": 5,
  "nsp": "/",
  "data": ["hello", <Buffer 01 02 03>]
}

With an acknowledgment id:

{
  "type": 5,
  "nsp": "/admin",
  "data": ["project:delete", <Buffer 01 02 03>],
  "id": 456
}

This section details the encoding used by the default parser which is included in Socket.IO server and client, and whose source can be found here.

The JS server and client implementations also supports custom parsers, which have different tradeoffs and may benefit to certain kind of applications. Please see socket.io-json-parser or socket.io-msgpack-parser for example.

Please also note that each Socket.IO packet is sent as a Engine.IO message packet (more information here), so the encoded result will be prefixed by 4 when sent over the wire (in the request/response body with HTTP long-polling, or in the WebSocket frame).

<packet type>[<# of binary attachments>-][<namespace>,][<acknowledgment id>][JSON-stringified payload without binary]

+ binary attachments extracted

Note:

• the namespace is only included if it is different from the default namespace (/)

• CONNECT packet for the default namespace

{
  "type": 0,
  "nsp": "/"
}

is encoded to 0

• CONNECT packet for the /admin namespace

{
  "type": 0,
  "nsp": "/admin"
}

is encoded to 0/admin

• DISCONNECT packet for the /admin namespace

{
  "type": 1,
  "nsp": "/admin"
}

is encoded to 1/admin

• EVENT packet

{
  "type": 2,
  "nsp": "/",
  "data": ["hello", 1]
}

is encoded to 2["hello",1]

• EVENT packet with an acknowledgement id

{
  "type": 2,
  "nsp": "/admin",
  "data": ["project:delete", 123],
  "id": 456
}

is encoded to 2/admin,456["project:delete",123]

• ACK packet

{
  "type": 3,
  "nsp": "/admin",
  "data": [],
  "id": 456
}

is encoded to 3/admin,456[]

• ERROR packet

{
  "type": 4,
  "nsp": "/admin",
  "data": "Not authorized"
}

is encoded to 4/admin,"Not authorized"

• BINARY_EVENT packet

{
  "type": 5,
  "nsp": "/",
  "data": ["hello", <Buffer 01 02 03>]
}

is encoded to 51-["hello",{"_placeholder":true,"num":0}] + <Buffer 01 02 03>

• BINARY_EVENT packet with an acknowledgement id

{
  "type": 5,
  "nsp": "/admin",
  "data": ["project:delete", <Buffer 01 02 03>],
  "id": 456
}

is encoded to 51-/admin,456["project:delete",{"_placeholder":true,"num":0}] + <Buffer 01 02 03>

The server always send a CONNECT packet for the default namespace (/) when the connection is established.

That is, even if the client requests access to a non-default namespace, it will receive a CONNECT packet for the default namespace first.

Server > { type: CONNECT, nsp: "/" }

No response is expected from the client.

Client > { type: CONNECT, nsp: "/admin" }
Server > { type: CONNECT, nsp: "/admin" } (if the connection is successful)
or
Server > { type: ERROR, nsp: "/admin", data: "Not authorized" }

Client > { type: DISCONNECT, nsp: "/admin" }

And vice versa. No response is expected from the other-side.

Client > { type: EVENT, nsp: "/admin", data: ["hello"], id: 456 }
Server > { type: ACK, nsp: "/admin", data: [], id: 456 }

And vice versa.

• remove the usage of msgpack to encode packets containing binary objects (see also 299849b)

• add a BINARY_EVENT packet type

This was added during the work towards Socket.IO 1.0, in order to add support for binary objects. The BINARY_EVENT packets were encoded with msgpack.

This first revision was the result of the split between the Engine.IO protocol (low-level plumbing with WebSocket / HTTP long-polling, heartbeat) and the Socket.IO protocol. It was never included in a Socket.IO release, but paved the way for the next iterations.

MIT