Propagating State

  Slave nodes:
     Handle simple PPPIP packets immediately
     Unicast anything else to master
     Multicast “ping”s to cluster:
         version, undef, basetime
     Send session byte counters to master

  Master node:
     Handles all packets forwarded from slaves, or sent via L2TP
     Multicast “heartbeat”s to cluster:
         Header: version, sequence, basetime, serial
         Data: RLE compressed session and tunnel entries (changed/walk)
     Heartbeats are cached in a ring-buffer for possible re-send
     Heart-rate increased when slaves nodes join