A further breakdown of the topics covered here can be found in the Socket.IO Session Lifecycle and Socket.IO Packet Encoding pages.
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=4&transport=polling&t=N8hyd6w
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000,"maxPayload":1000000}
Details:
0 => Engine.IO "open" packet type
{"sid":... => the Engine.IO handshake data
Note: the t query param is used to ensure that the request is not cached by the browser.
- Request n°2 (namespace connection request):
POST /socket.io/?EIO=4&transport=polling&t=N8hyd7H&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
40
Details:
4 => Engine.IO "message" packet type
0 => Socket.IO "CONNECT" packet type
- Request n°3 (namespace connection approval)
GET /socket.io/?EIO=4&transport=polling&t=N8hyd7H&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
40{"sid":"wZX3oN0bSVIhsaknAAAI"}
- Request n°4
socket.emit('hey', 'Jude') is executed on the server:
GET /socket.io/?EIO=4&transport=polling&t=N8hyd7H&sid=lv_VI97HAXpY6yYWAAAC
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
42["hey","Jude"]
Details:
4 => Engine.IO "message" packet type
2 => Socket.IO "EVENT" packet type
[...] => content
- Request n°5 (message out)
socket.emit('hello'); socket.emit('world'); is executed on the client:
POST /socket.io/?EIO=4&transport=polling&t=N8hzxke&sid=lv_VI97HAXpY6yYWAAAC
> Content-Type: text/plain; charset=UTF-8
42["hello"]\x1e42["world"]
< HTTP/1.1 200 OK
< Content-Type: text/plain; charset=UTF-8
ok
Details:
4 => Engine.IO "message" packet type
2 => Socket.IO "EVENT" packet type
["hello"] => the 1st content
\x1e => separator
4 => Engine.IO "message" packet type
2 => Socket.IO "EVENT" packet type
["world"] => the 2nd content
- Request n°6 (WebSocket upgrade)
GET /socket.io/?EIO=4&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,{"sid":"-G5j-67EZFp-q59rADQM"} => 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