An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 68
until packet loss is detected at the 9 th RTT, where Fast Retransmission and
Fast Recovery is again initiated. At the 19 th RTT, a timeout occurs and
again ssthresh is recalculated, but cwnd is set to 1 packet whereby slow
start is initiated until cwnd ≥ ssthresh.
4.5.3 Motivation for Improving Loss Detection
A fundamental problem is that TCP acks are cumulative; an ack confirms
reception of all data up to a given sequence number, but provides no information
whether any bytes beyond this number were received. Therefore
upon loss, the Fast Retransmit and Fast Recovery algorithms assume that
only the segment at snd_una is lost per window. This can result in the loss
of ack clocking and timeouts if more than one segment is lost.
Due to the prevalence of FIFO queues in the internet, losses often occur
in bursts [JS00]. As link speeds increase and the internet becomes more
geographically distributed, with more hops, TCP cwnd sizes are increased
to make use of available capacity. The result of this amalgamation is that
single packet loss within a window is rare and multiple losses within the same
window are more likely to be the observed effect.
When Tahoe and Reno experience multiple packet losses in a window of
data (usually in the order of half a window under heavily congested links),
Fast Retransmissions and Fast Recovery are invoked several times in succession
leading to multiplicative decreases of cwnd and ssthresh.
As this happens several times in succession, the left edge of the sending
window advances only after each successive Fast Retransmit and the amount
of data in-flight (sent but not yet acked) eventually becomes more than the
cwnd (halved by the latest invocation of Fast Retransmit). As there are