[SR-Users] question with "keep alive"

hi,all. I want to accomplish something like that: A kamailio server, and two clients (user agents) login to this server ( by registrar METHOD), and if possible, they tell each other the some params (for example, they connect to the server with 3G, wifi,etc.). Then, each of them can be awareness of the status of the opposite side.( online/offline, connected by 3G or Wifi) , even in case that one side was happened with network bankrupt. I faild after tried with module registrar,presence. Problems I faces were: 1, How to add one or more extra params to REGISTRAR call? or from the other words, Is it possible to tell the opposite end some customised information without a huge SIP-extended work ? 2, In case that one side was abort for some reason, how to notify the other side as quickly as possible? maybe the client can wait the notification after a presentity expired, but it bother the client to publish its status periodically.and also, in tcp module, I had set the params like tcp keep alive, but the client can not receive the notification even the physical connection was closed. Could some buddies kindly advice me something or hint some clue? All will be very much appreciated. B.R. Rixin.liu