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socket.io-protocol

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.

Table of Contents

Protocol version

This is the revision 4 of the Socket.IO protocol, included in socket.io@1.0.3...latest.

The 3rd revision (included in socket.io@1.0.0...1.0.2) can be found here: https://github.com/socketio/socket.io-protocol/tree/v3

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:

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(); });

Packet format

A packet contains the following fields:

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

Packet types

0 - CONNECT

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"
}

1 - DISCONNECT

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"
}

2 - EVENT

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
}

3 - ACK

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
}

4 - ERROR

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"
}

5 - BINARY_EVENT

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
}

6 - BINARY_ACK

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 contain a payload including binary.

Example:

{
  "type": 6,
  "nsp": "/admin",
  "data": [<Buffer 03 02 01>],
  "id": 456
}

Packet encoding

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).

Encoding format

<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 (/)

Examples

  • 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>

  • BINARY_ACK packet
{
  "type": 6,
  "nsp": "/admin",
  "data": [<Buffer 03 02 01>],
  "id": 456
}

is encoded to 61-/admin,456[{"_placeholder":true,"num":0}] + <Buffer 03 02 01>

Exchange protocol

Connection to the default namespace

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.

Connection to a non-default namespace

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

Disconnection from a non-default namespace

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

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

Acknowledgement

Client > { type: EVENT, nsp: "/admin", data: ["hello"], id: 456 }
Server > { type: ACK, nsp: "/admin", data: [], id: 456 }
or
Server > { type: BINARY_ACK, nsp: "/admin", data: [ <Buffer 01 02 03> ], id: 456 }

And vice versa.

Sample session

Here is an example of what is sent over the wire when combining both the Engine.IO and the Socket.IO protocols.

  • Request n°1 (open packet)
GET /socket.io/?EIO=3&transport=polling&t=N8hyd6w
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
96:0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}2:40

Details:

96          => number of characters (not bytes) of the first message
:           => separator
0           => Engine.IO "open" packet type
{"sid":...  => the Engine.IO handshake data
2           => number of characters of the 2nd message
:           => separator
4           => Engine.IO "message" packet type
0           => Socket.IO "CONNECT" packet type

Note: the t query param is used to ensure that the request is not cached by the browser.

  • Request n°2 (message in):

socket.emit('hey', 'Jude') is executed on the server:

GET /socket.io/?EIO=3&transport=polling&t=N8hyd7H&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
16:42["hey","Jude"]

Details:

16          => number of characters
:           => separator
4           => Engine.IO "message" packet type
2           => Socket.IO "EVENT" packet type
[...]       => content
  • Request n°3 (message out)

socket.emit('hello'); socket.emit('world'); is executed on the client:

POST /socket.io/?EIO=3&transport=polling&t=N8hzxke&sid=lv_VI97HAXpY6yYWAAAC
> Content-Type: text/plain; charset=UTF-8
11:42["hello"]11:42["world"]
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
ok

Details:

11          => number of characters of the 1st packet
:           => separator
4           => Engine.IO "message" packet type
2           => Socket.IO "EVENT" packet type
["hello"]   => the 1st content
11          => number of characters of the 2nd packet
:           => separator
4           => Engine.IO "message" packet type
2           => Socket.IO "EVENT" packet type
["world"]   => the 2nd content
  • Request n°4 (WebSocket upgrade)
GET /socket.io/?EIO=3&transport=websocket&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 101 Switching Protocols

WebSocket frames:

< 2probe                                        => Engine.IO probe request
> 3probe                                        => Engine.IO probe response
> 5                                             => Engine.IO "upgrade" packet type
> 42["hello"]
> 42["world"]
> 40/admin,                                     => request access to the admin namespace (Socket.IO "CONNECT" packet)
< 40/admin,                                     => grant access to the admin namespace
> 42/admin,1["tellme"]                          => Socket.IO "EVENT" packet with acknowledgement
< 461-/admin,1[{"_placeholder":true,"num":0}]   => Socket.IO "BINARY_ACK" packet with a placeholder
< <binary>                                      => the binary attachment (sent in the following frame)
... after a while without message
> 2                                             => Engine.IO "ping" packet type
< 3                                             => Engine.IO "pong" packet type
> 1                                             => Engine.IO "close" packet type

History

Difference between v4 and v3

  • add a BINARY_ACK packet type

Previously, an ACK packet was always treated as if it may contain binary objects, with recursive search for such objects, which could hurt performance.

Difference between v3 and v2

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

Difference between v2 and v1

  • 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.

Initial revision

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.

License

MIT

About

Socket.IO Protocol specification

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