An Investigation into Transport Protocols and Data Transport ...

5.1. TCP Hardware Requirements 79
Goodput (mbit/sec)
1000
100
Socket Buffers From CERN (Switzerland) to RAL (England)
Mean
Median
Minimum
Maximum
Theory
10
10 100 1000 10000
Socket Buffer Size (KBytes)
Figure 5.1: Effect of limiting the socket buffer size on TCP goodput.
The typical socket buffer memory size for Linux and BSD systems is set
to 64KB. Depending on the latency and stable throughput of the TCP path,
one can see that TCP would perform abysmally over the example link and
that the throughput would be capped so that the TCP window fits within
the 64KB.
To address this problem, grid and network researchers continue to manually
optimise socket buffer sizes to keep the network pipe full, and thus
achieve increases of transport throughput by many orders of magnitude.
The amount of memory allocated to each TCP connection on Unix systems
can be changed using the standard Unix setsockopt() [Ste98] call.
However, under Linux systems, the kernel also imposes a limit on the maximum
size of the socket buffer memory that can be allocated for each connection.
As such system administrator privileges are often required to alter
this limit such that userspace applications such as FTP can utilise large(r)
socket buffer sizes.
This is achieved using the standard sysctl variables
net.core.rmem_max and net.core.wmem_max for the read and write socket
buffers respectively. Similarly, the default allocated socket buffer memory