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Comment by ndr | Hacker News Reader

"IMPP is a binary protocol that works by establishing a TCP connection to an IMPP server, authenticating with that server, and then exchanging messages with that server in order to reach other clients. Over the years, best efforts have been made to keep the protocol flow reasonably compatible with XMPP with the long term idea of enabling federation between IMPP and XMPP servers. As such, the general flow for establishing an IMPP session somewhat mirrors that of XMPP: [...]"

Source: https://www.trillian.im/impp/

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That explains what IMPP is, but it doesn't answer the question of how/why it's different from XMPP.

And the "keep the protocol flow reasonably compatible with XMPP" part only makes it more curious. The more similar your protocol is to XMPP, the stranger it seems that you don't just use XMPP.

The answer may be just inertia; the spec says they started working on IMPP in 2001, back when XMPP was still very young. It could be that they've just built a ton of internal infrastructure on top of this thing and can't afford to move it all to pure XMPP. But that doesn't really explain why anyone else would want to use it in 2013.

Yeah, inertia. What we've tried to do with IMPP is to roll all of the features that IM users actually want into the core protocol versus just saying "it's open!" or "it can be extended!" and hoping someone else does the dirty work. We build the server side and we build the client side, which is important; it means we're not off dreaming up pie-in-the-sky features that would be absolutely horrible for a client developer to implemement.

We have some experience reverse engineering the rest of the IM protocols out there as well, and they all have their own issues which we kept in mind when building ours. At the end of the day, our actual motivations were simple: publishing the protocol is the right thing to do and so we did it. It's been on our TODO for 6+ years. :)

Some advantages: binary instead of XML, compression, shared state between devices.

Since Google has dropped XMPP, at this point it's possible that IMPP already rivals it's usage numbers worldwide (no word on users, but Trillian has >40m downloads).

If anything it makes the original question even more pertinent. It's the "what" but not the "why".

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