Question: is there any way to detect the used engine.io version?

[Benedicht]

I have a c# implementation that should work with both v3 and v4. Because v4 has a few breaking changes, i can't implement them in a backward compatible way and different code must be executed based on the version of the server.

[Benedicht]

A future proof solution would be if the handshake data would contain an additional version field. If it's missing than it's most probably a server using v3 otherwise its value is the version that i (and maybe other too) could use.

[darrachequesne]

You should be able to detect the version of the Engine.IO server based on the format of the handshake packet:

// v3
96:0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}
// v4
0{"sid":"lv_VI97HAXpY6yYWAAAC","upgrades":["websocket"],"pingInterval":25000,"pingTimeout":5000}

Details:

• https://github.com/socketio/engine.io-protocol#payload
• https://github.com/socketio/engine.io-protocol/tree/v3#payload

[Benedicht]

Thanks, that could work. Any advice for a websocket only session?

[darrachequesne]

The first WebSocket frame sent by the server will contain the handshake packet, so you should be able to make the same distinction as above.

[Benedicht]

AFAIK it's the same for v3 and v4.

These are received from different versions:

0{"sid":"iEDbJrPFWsgeDYk-AAAB","upgrades":[],"pingInterval":25000,"pingTimeout":5000}
0{"sid":"nR4WsvYPR1J8XrSvAAAA","upgrades":[],"pingInterval":25000,"pingTimeout":5000}

The documentation also looks like the same (i couldn't find any differences at least) for v3 and v4:

• https://github.com/socketio/engine.io-protocol#sample-session-with-websocket-only
• https://github.com/socketio/engine.io-protocol/tree/v3#sample-session-with-websocket-only

[darrachequesne]

Arf, you are right.

For a WebSocket-only session, I think the only difference is the encoding of a binary packet. The packet is prefixed with "4" in v3, whereas it is sent as is in v4.

[Benedicht]

Yep, but that's hard to use to decide whether it's v3 or v4, because "4" can be part of the binary data too.

[Benedicht]

@darrachequesne Any chance to include something in the handshake payload that would indicate the version of the protocol?

[darrachequesne]

We could check the value of the EIO query parameter on the server-side, and return a HTTP 400 if the query param is provided but does not match the value of the server. What do you think?

[Benedicht]

Hey, sorry for the late reply!

My only real objection is that it would add additional round-trips, but otherwise it seems good.

[darrachequesne]

The server now supports v3 and v4 clients: socketio/engine.io@663d326 (included in engine.io@4.1.0 & socket.io@3.1.0).