For the internal communication you should have a look to:
http://community.jboss.org/wiki/Shunning
http://community.jboss.org/wiki/FDVersusFDSOCK
http://community.jboss.org/wiki/JGroupsPbcastGMS
http://community.jboss.org/wiki/JGroupsFD
You will find a lot of information about it works inside.
With HTTP session-replication I do not work this time.
I know that the most common way is a buddy-replication, only two nodes keep the state of the session.
If the one where the session is connected fail an other server will process the next call. If this is not the 'buddy' the session must be copied to the current instance and this will take it's time depend to the size of the session data.