Can you provide the capture file? What SRV record did it look up to connect to? The convention is to use the _xmpp-client._tcp.<domain> SRV record to publish servers clients should connect to. For example:

$ dig +short _xmpp-client._tcp.google.com SRV
20 0 5222 talk3.l.google.com.
20 0 5222 talk4.l.google.com.
20 0 5222 talk1.l.google.com.
5 0 5222 talk.l.google.com.
20 0 5222 talk2.l.google.com.

What "domain" does it use for Jabber IDs (e.g., if you have a WhatsApp account, what Jabber ID would you use in a standard Jabber client to access the account)?

How does the application register accounts? By the Jabber registration feature?

Understanding how the client works does not necessarily help much in "porting" it over, which traditionally means, making changes to the existing source code to work on a different platform.

It may help in writing a telepathy plugin to make the protocol accessible to the conversations app on N900 (and other apps on other platforms). But, if the protocol is actually XMPP, then there's no need for a separate plugin, if we have the details I asked for above.

Of course, it seems WhatsApp isn't interested in XMPP inter-operation, or they would have made these details available on their website.