Better understanding WMP11 and UPNP relationships

Hi!
I am trying to better understand the implementation of UNPNP and SSDP
protocol in WMP11 within Vista.My basic understanding is that when using
"Media Sharing" option, the sharing is done via the SSDP protocol, which is
part of the UPNP standard.

I have installed two Vista machines, Enabled Network discovery and Media
sharing in both. in the "Media server" machine, I have added the desired MP3
files, and shared them to "Everyone" with Read permissions.
When I went to the "Media client" machine, and browse, I was able to play
the songs from the server. However, in the network capture of the whole
process, I could only see M-SEARCH packets went from one host to another, and
could not see any answers ("http OK 200")to the search packets or NOTIFY
packets from any host.

Does this implementation trial that I described used some protocol other
then UPNP ? Is there any missing step that is needed in order that the full
Media flow will use UPNP and SSDP protocols ?

I would appreciate any elaboration on the subject.