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Engine.IO Protocol

This document describes the Engine.IO protocol. For a reference JavaScript implementation, take a look at, and

Table of Contents:


This is revision 4 of the Engine.IO protocol.

The revision 2 can be found here:

The revision 3 can be found here:

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)
  • upgrades possible transport upgrades (Array of 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)
  1. Client must respond to periodic ping packets sent by the server with pong packets.
  2. Client and server can exchange message packets at will.
  3. Polling transports can send a close packet to close the socket, since they're expected to be "opening" and "closing" all the time.

Sample session

  • Request n°1 (open packet)
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8


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

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

  • Request n°2 (message in):

socket.send('hey') is executed on the server:

< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8


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

Note: the sid query param contains the sid sent in the handshake.

  • Request n°3 (message out)

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

> Content-Type: text/plain; charset=UTF-8
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8


4           => "message" packet type
hello       => the 1st message
\x1e        => separator
4           => "message" message type
world       => the 2nd message
  • Request n°4 (WebSocket upgrade)
< HTTP/1.1 101 Switching Protocols

WebSocket frames:

< 2probe    => probe request
> 3probe    => probe response
> 5         => "upgrade" packet type
> 4hello    => message (not concatenated)
> 4world
> 2         => "ping" packet type
< 3         => "pong" packet type
> 1         => "close" packet type

Sample session with WebSocket only

In that case, the client only enables WebSocket (without HTTP polling).

< HTTP/1.1 101 Switching Protocols

WebSocket frames:

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


An Engine.IO url is composed as follows:

/[?<query string>]
  • The 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 / 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 (/ and the resource (default by default).


There's two distinct types of encodings

  • packet
  • payload


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>]



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.


  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); });

5 upgrade

Before switches a transport, it tests, if server and client can communicate over this transport. If this test succeed, the client sends an upgrade packets which requests the server to flush its cache on the old transport and switch to the new transport.

6 noop

A noop packet. Used primarily to force a poll cycle when an incoming websocket connection is received.

  1. client connects through new transport
  2. client sends 2probe
  3. server receives and sends 3probe
  4. client receives and sends 5
  5. server flushes and closes old transport and switches to new.


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:


The packets are separated by the record separator ('\x1e'). More info here:

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:

  • Example with binary:
    "type": "message",
    "data": "€"
    "type": "message",
    "data": buffer <01 02 03 04>

is encoded to:



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


An server must support three transports:

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


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.


The server must support CORS responses.


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.


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 upgrading

A connection always starts with polling (either XHR or JSONP). WebSocket gets tested on the side by sending a probe. If the probe is responded from the server, an upgrade packet is sent.

To ensure no messages are lost, the upgrade packet will only be sent once all the buffers of the existing transport are flushed and the transport is considered paused.

When the server receives the upgrade packet, it must assume this is the new transport channel and send all existing buffers (if any) to it.

The probe sent by the client is a ping packet with probe sent as data. The probe sent by the server is a pong packet with probe sent as data.

Moving forward, upgrades other than just polling -> x are being considered.


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.

  • 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.

Please note that this only applies to HTTP long-polling. Binary data is sent in WebSocket frames with no additional transformation.

  • 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.


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