An Investigation into Transport Protocols and Data Transport ...

5.4. Analysis of AIMD Congestion Control 103
slower rate of throughput increase.
This is especially evident after congestion as TCP halves cwnd. Therefore,
when a TCP connection runs over a low loss, un-congested link it
is unable to achieve high throughput in high-speed, wide area networks
due to the long recovery times and large value of optimal cwnd required
after congestion detection when cwnd is halved.
Therefore, as the geographical distance of the TCP connection increases,
the time required to reach congestion (assuming static networks
conditions) is proportionally increased and its average utilisation
is decreased.
To give context to these problems, a standard TCP connection with 1500B
packets and a 100ms round-trip time (typical trans-Atlantic connection),
would require an average congestion window of 83,333 segments and a packet
drop rate of at most one congestion event every 5 billion packets to achieve a
steady-state throughput of 10Gbit/sec. This requires at most one congestion
event every 1 hour and 40 minutes. This loss rate (which is assumed to be
related directly to the Bit Error Rate) is well below what is possible today
with the present optical fibre and router technology.
5.4.1 Throughput Evolution
A model of the algorithmic dynamics of TCP can be gathered through simple
geometry when considering the steady state behaviour of TCP as a cyclic
iteration over k congestion epochs.
Figure 5.9 shows the evolution of cwnd over time. Assuming that the
time for recovery and the time required to signal the loss is negligible, the