Trying to understand the wire protocol

[jcelerier]

Hello!

I'm trying to implement a client for a socket.io 5 server. I'm hitting a very likely trivial stumbling block but I cannot understand from the docs what's wrong:

I send:

GET /socket.io/?EIO=4&transport=polling

I receive:

120:0{"sid":"ewlhe0MUPiSeUC6JAAAS","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":20000,"maxPayload":500000000}

Nowhere do I see in the docs here (https://socket.io/docs/v4/socket-io-protocol/#sample-session) what the "120:" means or how I'm supposed to parse it. The page does not even contain the word "colon" when I ctrl-f.

Also, it is not clear what should the message format be if I directly initiate a ws connection, bypassing http long polling: does socket.io expect:

• /socket.io/?EIO=5&transport=polling to be the target of the WS handshake ?
• A "normal" WS handshake to be done to "/" followed by a text message /socket.io/?EIO=4&transport=polling ?
• A "normal" WS handshake to be done to "/" followed by a text message GET /socket.io/?EIO=4&transport=polling ?
• A "normal" WS handshake to be done to "/" followed by a whole HTTP request for GET /socket.io/?EIO=4&transport=polling but sent over WS ?
• Or is the only way to go first through HTTP connection and upgrade ?
  Thanks !

[jcelerier]

okay, issue was that I was calling with EIO=5 which downgraded to EIO=3