SocketIO websocket handshake is different from what is described by RFC 6455

Question

I am attempting to learn more about the websockets protocol. According to RFC 6455, the step of this protocol is the handshake, which begins with the HTTP Upgrade request:

 GET /chat HTTP/1.1
    Host: server.example.com
    Upgrade: websocket
    Connection: Upgrade
    Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
    Origin: http://example.com
    Sec-WebSocket-Protocol: chat, superchat
    Sec-WebSocket-Version: 13 

to which the server responds with a HTTP Switching Protocols message:

HTTP/1.1 101 Switching Protocols
    Upgrade: websocket
    Connection: Upgrade
    Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
    Sec-WebSocket-Protocol: chat

I attempted to observe this using a simple program based on socketIO (nodeJS websockets library. After capturing the traffic, I noticed that the first message the client sends the server is:

GET /socket.io/?EIO=3&transport=polling&t=1443149751115-0 HTTP/1.1
Host: localhost
Connection: keep-alive
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36            (KHTML, like Gecko) Chrome/45.0.2454.85 Safari/537.36
Origin: null
Accept: */*
Accept-Encoding: gzip, deflate, sdch
Accept-Language: en-US,en;q=0.8

to which the server responds with:

HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 101
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: null
Set-Cookie: io=3Z_TCqv9LKKXcWCjAAAD
Date: Fri, 25 Sep 2015 02:55:51 GMT
Connection: keep-alive

....0{"sid":"3Z_TCqv9LKKXcWCjAAAD","upgrades":    ["websocket"],"pingInterval":25000,"pingTimeout":60000}

is SocketIO not following the RFC for websockets or am I missing something?

Answer 1

socket.io has it's own connection management stuff BEFORE it connects on a webSocket that allows it to negotiate which type of connection it's actually going to make to the server and allows it to send some configuration options down to the client. This will confuse you if you're trying to study a plain webSocket.

socket.io is an additonal protocol on top of webSocket. It will eventually use a standard webSocket underneath, but it will have additional stuff around that. If you continued to follow the socket.io connection, you would have eventually seen a standard webSocket connection.

I would suggest you first just make a plain webSocket connection from your client (no socket.io) and study that network trace.