rfc 4271...

this is an important distinction... although it's called keepalive, that's not its purpose...

http://www6.ietf.org/rfc/rfc4271

4.4.  KEEPALIVE Message Format

   BGP does not use any TCP-based, keep-alive mechanism to determine if
   peers are reachable.  Instead, KEEPALIVE messages are exchanged
   between peers often enough not to cause the Hold Timer to expire.  A
   reasonable maximum time between KEEPALIVE messages would be one third
   of the Hold Time interval.  KEEPALIVE messages MUST NOT be sent more
   frequently than one per second.  An implementation MAY adjust the
   rate at which it sends KEEPALIVE messages as a function of the Hold
   Time interval.