An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 63
mechanisms are still widely in use today in the all TCP implementations.
TCP Tahoe assumes that congestion signals are represented by lost segments
and that losses due to packet corruption are much less probable than
losses due to buffer overflows on the network.
As there is no explicit notification of packet loss through the ack mechanism,
TCP Tahoe introduces the notion whereby packet loss can occur if:
A TCP soft time-out occurs such that no acks have been received in a
period RT O since the last ‘normal’ ack was received. Therefore, the ack
clock has stopped. TCP Tahoe implements a more accurate calculation
of RT O.
Receipt of 3 dupacks The TCP connection is still active and there is still
ack clocking. A packet may have been lost but all successive packets
were delivered successfully.
The refinement of an accurate round-trip time estimator was also introduced
with TCP Tahoe (See Section 4.4).
The occurrence of an RT O timeout is taken as a signal of severe network
congestion and therefore the rate at which the TCP should send more segments
should be decreased to prevent network collapse. As such, the TCP
flow reinitiates the TCP connection in slow start so that the network conditions
can once again be probed to re-initialise the ack clock. By setting
cwnd to 1 packet, TCP Tahoe starts sending from a very low rate and only
if the network is capable of the extra traffic will the ack clocking reinitiate.
Fast Retransmits [Jac88] were proposed to reduce the long idle periods of
time during which the TCP on the sending host waits for a timeout to occur.
Fast Retransmit is a mechanism that sometimes triggers the retransmission