An Investigation into Transport Protocols and Data Transport ...

5.3. Network Aid in Congestion Detection 96
consuming physical memory, the kernel also facilitates the reduction of the
sender’s socket buffer window sizes to reduce the burden of memory swapping.
The modification of socket buffer auto-tuning algorithm were ported to
the Linux 2.4 kernel in 2001 and are controlled by new kernel variables
net.ipv4.tcp_rmem and net.ipv4.tcp_wmem defining the minimum, default
and maximum socket buffer memory allocated to each TCP connection for
the read and write buffers respectively 3 .
Independent of auto-tuning, the Linux 2.4 and 2.6 kernels implement a
system where connection performance histories of TCP transfers are cached
for up-to 10 minutes. This ‘retentive’ TCP stores details of the measured
/ experienced smoothed RTT and RTT variance, ssthresh and the path
reordering information for previously connected pairs of hosts. These values
are stored for up-to 10 minutes and will prevent a consecutive TCP flow for
that pair to reach congestion avoidance quickly without the excessive packet
losses of a typical slow start. It will also aid in preventing early spurious
timeouts. The cache can be prematurely cleared through the use of the
net.ipv4.route.flush sysctl.
5.3 Network Aid in Congestion Detection
Due to the transient nature of network utilisation, TCP gently probes the
network with increasing number of segments to determine whether it can
achieve more throughput. Therefore, in a sense, TCP has to cause congestion
in order to determine the capabilities of the network path.
Most current routers in TCP/IP networks do not detect incipient conges-
3 Note, however, that the maximal tunable socket buffer memory is still limited
by the maximum allocated socket buffer size in the kernel under net.ipv4.tcp mem,
net.core.rmem max and net.core.wmem max.