1.The host address where the request came from. Responses are sent back to the same host address, regardless of what the From header indicates. Note that different requests for the same call can come from different hosts.

2.The From address contains the logical source of the request. It remains unmodified as a SIP request traverses proxies, for example. The From address may not be the same as the host address that generated the SIP request, although that's the typical case.

3.The session description (e.g., SDP) contains one or more addresses where the caller expects media data (audio, video) to be sent. For some services, this address may not be the same as the From address.