The retransmit values are not fast enough for modern networks. Instead of 20 or 40 seconds, it should be somewhere between 1-4 seconds (to be discussed)
These should be configurable on a per-connection basis
These should be in milliseconds to allow greater precission.
Refined retransmission might would to improve battery life on hand held devices.
Experimentation is needed.
RFC 2988: "Computing TCP's Retransmission Timer" http://www.hjp.at/doc/rfc/rfc2988.txt