An Investigation into Transport Protocols and Data Transport ...

4.5. TCP Variants 67
20
15
cwnd
ssthresh
Detected Packet Loss
Timeout
cwnd (Packets)
10
5
0
0 5 10 15 20 25 30
Duration (RTT)
Figure 4.5: Time Evolution of Example TCP Reno Trace.
cwnd upon each dupack received as each represents another data segment
which has left the network. Therefore, upon the receipt of each dupack, the
cwnd is inflated by one segment as each dupack signals the fact that another
packet has left the network.
When all the dupacks have been received (cwnd/2 packets corresponding
to the number of of packets that were in flight prior to the loss detection), the
next ack should be that caused by the retransmission and therefore should
acknowledge all packets. Upon the receipt of this normal ack, the sender
exits Fast Recovery and set its cwnd to ssthresh to maintain sending at a
rate which does not destroy the ack clock.
Therefore, under Reno, slow start is only invoked at the start of a TCP
connection and when a packet is timed out by the RT O, i.e. only when the
ack clock of the TCP flow has been destroyed.
Figure 4.5 shows a typical time evolution of TCP Reno. Slow start is
initiated whereby at the 4 th RTT packet loss is detected and ssthresh is
recalculated. Unlike TCP Tahoe, TCP Reno effectively sets cwnd to the value
of ssthresh after Fast Recovery. After this, congestion avoidance is initiated