socket.io/README.md

Engine.IO Protocol

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

Table of Contents:

• Revision
• Anatomy of an Engine.IO session
• URLs
• Encoding
  • Packet
    • 0 open
    • 1 close
    • 2 ping
    • 3 pong
    • 4 message
  • Payload
• Transports
  • Polling
    • XHR
    • JSONP
  • Server-sent events
  • WebSocket
• Transport fallback
• Timeouts
• Difference between v3 and v4
• Difference between v2 and v3

Revision

This is revision 4 of the Engine.IO protocol.

The revision 2 can be found here: https://github.com/socketio/engine.io-protocol/tree/v2

The revision 3 can be found here: https://github.com/socketio/engine.io-protocol/tree/v3

Anatomy of an Engine.IO session

1. Transport establishes a connection to the Engine.IO URL .
2. Server responds with an open packet with JSON-encoded handshake data:

• sid session id (String)
• pingTimeout server configured ping timeout, used for the client to detect that the server is unresponsive (Number)
• pingInterval server configured ping interval, used for the client to detect that the server is unresponsive (Number)

3. Client must respond to periodic ping packets sent by the server with pong packets.
4. Client and server can exchange message packets at will.
5. Polling transports can send a close packet to close the socket, since they're expected to be "opening" and "closing" all the time.

Example with WebSocket connection:

GET /engine.io/?EIO=4&transport=websocket&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 101 Switching Protocols

WebSocket frames:

< 0{"sid":"lv_VI97HAXpY6yYWAAAC","pingInterval":25000,"pingTimeout":5000} => handshake
> 4hello    => message (not concatenated)
> 4world
< 2         => "ping" packet type
> 3         => "pong" packet type
> 1         => "close" packet type

Example with polling:

• Request n°1 (open packet)

GET /engine.io/?EIO=4&transport=polling&t=N8hyd6w
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
0{"sid":"lv_VI97HAXpY6yYWAAAC","pingInterval":25000,"pingTimeout":5000}

Details:

0           => "open" packet type
{"sid":...  => the handshake data

• Request n°2 (message in):

GET /engine.io/?EIO=4&transport=polling&t=N8hyd7H&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
4hey

Details:

4           => "message" packet type
hey         => the actual message

• Request n°3 (message out)

POST /engine.io/?EIO=4&transport=polling&t=N8hzxke&sid=lv_VI97HAXpY6yYWAAAC
> Content-Type: text/plain; charset=UTF-8
4hello\x1e4world
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
ok

Details:

4           => "message" packet type
hello       => the 1st message
\x1e        => separator
4           => "message" message type
world       => the 2nd message

URLs

An Engine.IO url is composed as follows:

/engine.io/[?<query string>]

• The engine.io pathname should only be changed by higher-level frameworks whose protocol sits on top of engine's.

• The query string is optional and has six reserved keys:

  • transport: indicates the transport name. Supported ones by default are polling, websocket.
  • j: if the transport is polling but a JSONP response is required, j must be set with the JSONP response index.
  • sid: if the client has been given a session id, it must be included in the querystring.
  • EIO: the version of the protocol
  • t: a hashed-timestamp used for cache-busting

FAQ: Is the /engine.io portion modifiable?

Provided the server is customized to intercept requests under a different path segment, yes.

FAQ: What determines whether an option is going to be part of the path versus being encoded as part of the query string? In other words, why is the transport not part of the URL?

It's convention that the path segments remain only that which allows to disambiguate whether a request should be handled by a given Engine.IO server instance or not. As it stands, it's only the Engine.IO prefix (/engine.io) and the resource (default by default).

Encoding

There's two distinct types of encodings

• packet
• payload

Packet

An encoded packet can be UTF-8 string or binary data. The packet encoding format for a string is as follows

<packet type id>[<data>]

example:

4hello

For binary data the packet type is not included, since only "message" packet type can include binary.

0 open

Sent from the server when a new transport is opened (recheck)

1 close

Request the close of this transport but does not shutdown the connection itself.

2 ping

Sent by the server. Client should answer with a pong packet.

example

1. server sends: 2
2. client sends: 3

3 pong

Sent by the client to respond to ping packets.

4 message

actual message, client and server should call their callbacks with the data.

example 1

1. server sends: 4HelloWorld
2. client receives and calls callback socket.on('message', function (data) { console.log(data); });

example 2

1. client sends: 4HelloWorld
2. server receives and calls callback socket.on('message', function (data) { console.log(data); });

Payload

A payload is a series of encoded packets tied together. The payload encoding format is as follows when only strings are sent and XHR2 is not supported:

<packet1>\x1e<packet2>\x1e<packet3>

The packets are separated by the record separator ('\x1e'). More info here: https://en.wikipedia.org/wiki/C0_and_C1_control_codes#Field_separators

When binary data is included in the payload, it is sent as base64 encoded strings. For the purposes of decoding, an identifier b is put before a packet encoding that contains binary data. A combination of any number of strings and base64 encoded strings can be sent. Here is an example of base 64 encoded messages:

<packet1>\x1eb<packet2 data in b64>[...]

The payload is used for transports which do not support framing, as the polling protocol for example.

• Example without binary:

[
  {
    "type": "message",
    "data": "hello"
  },
  {
    "type": "message",
    "data": "€"
  }
]

is encoded to:

4hello\x1e4€

• Example with binary:

[
  {
    "type": "message",
    "data": "€"
  },
  {
    "type": "message",
    "data": buffer <01 02 03 04>
  }
]

is encoded to:

4€\x1eb4AQIDBA==

with

4           => "message" packet type
€
\x1e        => record separator
b           => indicates a base64 packet
AQIDBA==    => buffer content encoded in base64

Transports

An engine.io server must support three transports:

• websocket
• server-sent events (SSE)
• polling
  • jsonp
  • xhr

Polling

The polling transport consists of recurring GET requests by the client to the server to get data, and POST requests with payloads from the client to the server to send data.

XHR

The server must support CORS responses.

JSONP

The server implementation must respond with valid JavaScript. The URL contains a query string parameter j that must be used in the response. j is an integer.

The format of a JSONP packet.

`___eio[` <j> `]("` <encoded payload> `");`

To ensure that the payload gets processed correctly, it must be escaped in such a way that the response is still valid JavaScript. Passing the encoded payload through a JSON encoder is a good way to escape it.

Example JSONP frame returned by the server:

___eio[4]("packet data");

Posting data

The client posts data through a hidden iframe. The data gets to the server in the URI encoded format as follows:

d=<escaped packet payload>

In addition to the regular qs escaping, in order to prevent inconsistencies with \n handling by browsers, \n gets escaped as \\n prior to being POSTd.

Server-sent events

The client uses an EventSource object for receiving data, and an XMLHttpRequest object for sending data.

WebSocket

Encoding payloads should not be used for WebSocket, as the protocol already has a lightweight framing mechanism.

In order to send a payload of messages, encode packets individually and send() them in succession.

Transport fallback

A connection always starts with WebSocket (if supported by the client).

If the connection cannot be established, the client will try to establish a SSE stream.

If the connection still fails, the client will use polling as a fallback (either XHR or JSONP).

Timeouts

The client must use the pingTimeout and the pingInterval sent as part of the handshake (with the open packet) to determine whether the server is unresponsive.

The server sends a ping packet. If no packet type is received within pingTimeout, the server considers the socket disconnected. If a pong packet is actually received, the server will wait pingInterval before sending a ping packet again.

Since the two values are shared between the server and the client, the client will also be able to detect whether the server becomes unresponsive when it does not receive any data within pingTimeout + pingInterval.

Difference between v3 and v4

• reverse ping/pong mechanism

The ping packets will now be sent by the server, because the timers set in the browsers are not reliable enough. We suspect that a lot of timeout problems came from timers being delayed on the client-side.

• fallback to polling instead of upgrading to WebSocket

As of today, most browsers now support WebSocket: https://caniuse.com/#search=websocket

Please note that this does not remove the need to use sticky-session for the polling transport.

• always use base64 when encoding a payload with binary data

This change allows to treat all payloads (with or without binary) the same way, without having to take in account whether the client or the current transport supports binary data or not.

• use a record separator (\x1e) instead of counting of characters

Counting characters prevented (or at least makes harder) to implement the protocol in other languages, which may not use the UTF-16 encoding.

For example, € was encoded to 2:4€, though Buffer.byteLength('€') === 3.

Note: this assumes the record separator is not used in the data.

The revision 4 of the protocol will be included in Socket.IO v3.

Difference between v2 and v3

• add support for binary data

v2 is included in Socket.IO v0.9, while v3 is included in Socket.IO v1/v2.