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Engine.IO Protocol

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


This is revision 3 of the Engine.IO protocol.

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)
  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.


An Engine.IO url is composed as follows:

/ [ ? ]

  • 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 four reserved keys:

    • transport: indicates the transport name. Supported ones by default are polling, flashsocket, 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.
    • b64: if the client doesn't support XHR2, b64=1 is sent in the query string to signal the server that all binary data should be sent base64 encoded.

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 encoding is identical. When sending binary data, the packet type id is sent in the first byte of the binary contents, followed by the actual packet data. Example:


In the above example each byte is separated by a pipe character and shown as an integer. So the above packet is of type message (see below), and contains binary data that corresponds to an array of integers with values 0, 1, 2, 3, 4 and 5.

The packet type id is an integer. The following are the accepted packet types.

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 client. Server should answer with a pong packet containing the same data

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

3 pong

Sent by the server 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:

  • length: length of the packet in characters
  • packet: actual packets as descriped above

When XHR2 is not supported, the same encoding principle is used also when binary data is sent, but 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:

<length of base64 representation of the data + 1 (for packet type)>:b<packet1 type><packet1 data in b64>[...]

When XHR2 is supported, a similar principle is used, but everything is encoded directly into binary, so that it can be sent as binary over XHR. The format is the following:

<0 for string data, 1 for binary data><Any number of numbers between 0 and 9><The number 255><packet1 (first type,
then data)>[...]

If a combination of UTF-8 strings and binary data is sent, the string values are represented so that each character is written as a character code into a byte.

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


An server must support three transports:

  • websocket
  • flashsocket
  • 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.


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 sent as part of the handshake (with the open packet) to determine whether the server is unresponsive.

If no packet type is received withing pingTimeout, the client considers the socket disconnected.