An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 64
20
15
cwnd
ssthresh
Detected Packet Loss
Timeout
cwnd (Packets)
10
5
0
0 5 10 15 20 25 30
Duration (RTT)
Figure 4.4: Time Evolution of Example TCP Tahoe Trace.
of a dropped packet sooner than the regular timeout mechanism and hence
improves packet loss detection. It does not replace regular timeouts; it just
enhances that facility and keeps the ack clock from failing.
Fast Retransmits are implemented via the detection of dupacks at the
sender.
It suggests that either a packet has been lost, or received out of
order 4 . Upon the occurrence of three consecutive dupacks, TCP Tahoe decreases
the sending rate and the inferred lost packet is resent.
The main
difference between TCP Tahoe and variants such as TCP Reno (see Section
4.5.2) is that upon loss detection through dupacks, cwnd is set to one
packet rather than half of the previous value.
As the mechanism to set
ssthresh still holds, the TCP connection always restarts in slow start (after
loss detection) until cwnd reaches ssthresh and then the linear additive increase
of the congestion avoidance mechanism occurs until the whole process
is repeated or the connection is closed.
Figure 4.4 shows a typical time based evolution of TCP Tahoe. The TCP
4 Later implementations such as TCP SACK can calculate the exact missing segment
based on extra information provided by the receiver to prevent the need to retransmit
already received data.