How does Google Talk work?

How does Google T开发者_如何学编程alk work? When I am connected to my friend for a talk, what acts as an intermediate one to transfer my voice? Is that a Google server behind it (which act as an intermediate? Or does it provide a point-to-point protocol?

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They use XMPP ( jabber )

Extensible Messaging and Presence Protocol (XMPP)

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Try to look at these articles:

• GoogleTalk Architecture
• Architecture lessons from Google Talk