XMPP problem is that in classical way a client needs to resume a stream. But to work well on iOS, an app must synchronize a state. WhatsApp is a heavily modifi

XMPP problem is that in classical way a client needs to resume a stream. But to work well on iOS, an app must synchronize a state. WhatsApp is a heavily modified xmpp, and does just that. That's what it works so well. Such approach is way easier in a federated environment.