An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 86
Initial Window Size
For short TCP connections that only transfer a small amount of data, the
rate of transfer is limited primarily by the amount of time spent in slow
start. TCP is currently defined to start a connection with an initial cwnd of
one. Therefore, by increasing this value to three segments rather than one,
the amount of data transferred at the start of the connection is improved
by a factor of three and the connection can therefore terminate earlier for
the same amount of data sent. However, due to the larger initial burst, a
connection across links with very small router buffers may experience loss
sooner and therefore resort back into congestion avoidance.
[AFP98] defines an experimental extension that allows an increase of the
initial window to three or four segments. However, the number of segments
sent after an RT O is still fixed at one segment.
However, for large file transfers that take a long time, the time saved by
having a larger initial window is irrelevant as TCP will most likely spend
most of its time in congestion avoidance.
5.2.2 Host Queues
The txqueuelen is a Linux sysctl variable that determines the maximum
number of packets that can be buffered on the egress queue of the kernel
before it enters the NIC’s device driver [ABL + 03].
Higher queues sizes means that more packets can be buffered and hence
not lost. This is especially important for TCP in the Linux kernel as the
overflow of this queue will cause immediate congestion control to be instantiated,
even though the packet is lost locally rather than on the network. The
Linux Web100 patch [MHR03] determines such action as SendStalls and