how to learn the peerID that has published a PipeAdvertisement?

"John D. Blair" <[email protected]> Tue, 6 Sep 2005 14:24:01 -0700
Newsgroups gmane.comp.java.jxta.user
Message-ID <[email protected]>
How can I determine the peerID of the peer that has published a 
PipeAdvertisement?  Further, can this information be learned for any 
advertisement?

thanks!
  -john.