Via header

[Sambhrama Mundkur]

Hi,
I had a question about the Via header.
When a UAS receives a request, is the response always sent back to the
host mentioned in the Via header? And if the port is not mentioned in the
via header, to the default port of 5060 ( even though the request might
have come though a proxy running on a different port?) . And what if there
is no via header?
Is From never considered while sending responses?

Thanx
Sambhrama