use of id feild in ipsec.conf

hi
(pls have some patience and read thru........ ;)  )

iam trying to use freeswan on a mobile node and facing some problems
regarding automatic keying , where in SA negotiaiton has to be done for a
home address from the foreign network.i.e the mobile node is the initiator.

this is what my ipsec.conf file looks like
conn sample
      leftid=3ffe::1:207:95ff:fee1:1512
      left=3ffe::1:207:95ff:fee1:1512
      rightid=3ffe::1:207:95ff:fee1:ad50
      right=3ffe::1:207:95ff:fee1:ad50
      ...

now my questions are-
1. when this m/c moves to a new network and starts renogotiation from a new
address (though the id in the id feild remains the same).
how is the id feild used in this case. does it update the existing
connection information ?

but this gives me a problem -

2. how to negotiate from a new network for an address of home networK (
statically configured on eth0 and ipsec0).
is this possible as per the IKE and ISAKMP RFC's ? if yes then does
freeswan allow this.?

3. what is needed if such a negotiaton has to be tried. can  using
certificate based authentication help,
how will the subject altname feild be used at the responder.?

4 can RSA based public key authentiation method be used, in such a case,

thanks and regards