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UDP sockets, not TCP ones. And don't forget about the bunch of MIME and Internet Message format that precedes the XML. (-:

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SIP works over a variety of transports, both TCP and UDP are used. You can tell UDP is certainly more common but any RFC-compliant stack is supposed to support both because large UDP packets (> MTU) are supposed to be sent using TCP.

The bulk of the protocol doesn't require XML at all actually, it's the SIP SIMPLE and presence extensions that do. But for the core SIP spec and your run of the mill VoIP call setup, no XML is needed.

Apparently I was being too subtle with the gentle reminder not to forget the MIME and Internet Message stuff.

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