Comment by toast0

The EU marketplace disclosures on protocol seem to be pretty close to what I remember of the non-public protocols.

Chat is basically binary encoded XMPP, with essentially a compression dictionary, so per iq overhead is minimal. Especially for the start of connection stuff (login, offline message delivery), we counted bytes and made accomidations for typical network issues we would see. Not acking a big chunk of offline messages after a few tries? Let's send one at a time and see if that works, etc.

Our socket timeouts were rather long as well. Before the move into Facebook infra, servers were in the US only, and rural India is a long ways from the US; and last mile contention on 2G gets real rough out there too... I want to say timeouts were on the order of 30 seconds?

Multimedia (attachments) was https, with resumption. I don't remember the full history, originally I don't think we had resumption on uploads, there's some coordination required for that, which IIRC started as more or less send an IQ that you want to upload a file with a hash of the file, and get a response of either what the download url is if the file was complete, or where to upload and what byte to start with if not. I think it's likely different now, but probably still https based. I wanted to move it so multimedia would be either multiplexed on the chat channel or using a similar protocol to the chat channel, but I didn't have the pull, and I got redirected into pushing TLS 1.3 into our Android client's mms upload/download instead; I didn't do the code there, just prototyping to show it could be possible, and then was more of a facilitator than a contributor. I'm not sure I got all the benefits I was looking for, but there were some, and it kept me busy while I was wrapping up our pre-FB hosting and my time at WA.