This paper describes the design, implementation, and
performance evaluation of ST-TCP (Server fault-Tolerant
TCP), which is an extension of TCP to tolerate TCP server
failures. This is done by using an active backup server that
keeps track of the state of the TCP connection and takes
over the TCP connection whenever the primary fails. This
migration of the TCP connection to the backup is com-
pletely transparent to the client. Because no changes are
required on the client machine, any TCP client can access a
ST-TCP server. The performance overhead of ST-TCP over
standard TCP is minimal, and during normal operation its
behavior is the same as that of a regular TCP. In addition,
ST-TCP provides a fast and seamless failover whenever the
primary server fails. This is verified by a prototype imple-
mentation of ST-TCP in the Linux operating system, and
experiments with a number of simulated applications which
have different communication characteristics.