The peers <b>alice</b> and <b>bob</b> are hidden behind the NAT routers <b>moon</b> and <b>sun</b>,
respectively. Due to this double-NAT situation they cannot set up an IPsec tunnel directly 
but need the assistance from the mediation server <b>carol</b> with which they register under
the pseudonyms <b>6cu1UTVw@medsrv.org</b> and <b>av9oEPMz@medsrv.org</b>, respectively,
authenticating by means of individual pre-shared keys. Using the set of endpoints relayed 
to them by the mediation server, the peers are able to set up a direct IPsec connection..
In order to test the double NAT-ed IPsec tunnel both peers ping each other.