An Investigation into Transport Protocols and Data Transport ...

5.4. Analysis of AIMD Congestion Control 107
where the throughput B is related to the RTT, T , the timeout, T 0 , under
loss rates, p using packets of size s. It therefore demonstrates that the
throughput is increased for low latency flows experiencing low loss conditions.
Considering the case of large file transport where the TCP flow will spend
most of the time in congestion avoidance, Equation 5.7 can be approximated
to,
cwnd =
√ 3
2p
packets (5.8)
where the Bandwith Delay Product, cwnd = B × T (Equation 4.1) is
used to relate the throughput and the cwnd by the RTT. This is equivalent
to the models presented in [Flo91, LM97, MSMO97].
The importance of this model is that the throughput is inversely related
to the loss rate experienced by the TCP flow.
Therefore, a fundamental
limit on throughput of a TCP flow exists even without congestion due to
the physical existence of bit error rates on the Internet. Given a typical link
loss rate of 10 −7 , and a typical long distance latency of 100ms, TCP is only
capable of achieving approximately 450Mbit/sec.
5.4.3 TCP Generalisation
[JDXW03] considers the evolution of cwnd as a stability problem whereby
cwnd oscillates around an equilibrium position. They specify that the increase
of 1 packet per RTT results and the decrease of half results on a flow
level description of the cwnd (w) as:
dw i (t)
dt
= 1
T i (t) − w i(t)
T i (t) p i(t) 1 2
4
3 w i(t) (5.9)
Where the first term comes from the linear increase of w and the second