An Investigation into Transport Protocols and Data Transport ...

5.2. TCP Tuning & Performance Improvements 95
the BDP of the network at just under 600 packets, which is the maximal rate
at which the receiver is able to receive packets as the sender is transferring
data at line rate.
It is clearly visible that the oscillatory behaviour of TCP is minimised
such that the sustained throughput is higher. However, it should also be
noted that due to the drop in cwnd upon loss, the utilisation of a DRS’d flow
is not maximal as with the case of normal TCP. The advantage of imposing
such an algorithm is two-fold: with appropriate measures to minimise the
socket-buffer size to that actually used by the flow (i.e. approximately the
value of cwnd) memory is not wasted; and induced loss through sending too
many packets into the network is also minimised, and therefore should the
buffer provision be small along the path, a DRS’d flow should be able to
obtain higher utilisation than a normal flow.
5.2.6 Socket Buffer Auto-Tuning & Caching
[SMM98] describes modifications to a NetBSD kernel that allow the kernel
to automatically resize the sender’s buffers. It enables a host that serves
many clients (such as a web server) to fairly share the available kernel buffer
memory and to provide better throughput than manually configured TCP
socket buffers. This is called socket buffer autotuning and is important as the
assignment of large default socket buffer sizes for all TCP connections would
result in the reduction of physical memory.
For applications that do not explicitly set the TCP send buffer size via
the setsockopt() options, socket buffer autotuning allows the sender socket
buffer to grow with cwnd and utilise the available bandwidth of the link
(up to the receiver’s advertised window). As the number of flows increases,