Hi Deep,
That will be difficult, all BGP traffic goes through the same TCP session (TCP port 179). If you want to make sure BGP remains up during congestion, I would configure your router to give it some preference. By default, all BGP traffic is marked with CS6.
If you really want to do this, I guess you could go crazy by doing some inspection, looking into the BGP packets only give preference to the “KEEPALIVE” packets.
Rene