It's interesting to look how Exchange ActiveSync handles this. They do a hanging request to the server, and the client keeps track of when the connection closes

It's interesting to look how Exchange ActiveSync handles this. They do a hanging request to the server, and the client keeps track of when the connection closes, and negotiates with the server to push the keepalive ping time up and up, until the device hits the carrier TCP timeout. The client then knows how long it can stay asleep before having to wake up to "PING" to keep the connection open.