TCP / IP and HIPPI Performance in the CASA

We investigate the packet delay and loss characteristics of the wide-area HIPPI-based CASA gigabit testbed. Developed for high{speed local area device interconnects, HIPPI is a point{to{point, connection{oriented protocol. We show HIPPI blocking can degrade performance by increasing delay and/or packet loss. In the CASA network under conditions of blocking, a tradeoo exists between packet loss and delay variance. The tradeoo point is determined by a combination of factors: source packet rate, mean blocking rate, and a conngurable connection establishment timeout threshold. We show the delay/loss tradeoo manifests itself in TCP as inducing either the slow-start algorithm or requiring TCP to adjust retransmission timeout values due to increased delay variance. In the former case, we have measured TCP throughput to drop by as much as 97% of its unblocked maximum, as compared with 75% for the latter case.