http://www.performancewiki.com/linux-tuning.html

To view current TCP settings, run command:
root# sysctl net.ipv4.tcp_keepalive_time

net.ipv4.tcp_keepalive_time = 7200 // 2 hours
where net.ipv4.tcp_keepalive_time is a TCP tuning parameter.

To set a TCP parameter to a value, run command:
root# sysctl -w net.ipv4.tcp_keepalive_time=1800

A list of recommended TCP parameters, values, and their meanings:

    Parameter                   Value
    -------------------------------------------
    net.ipv4.tcp_tw_reuse 
    net.ipv4.tcp_tw_recycle     1
            Reuse sockets in the time-wait state 

    net.core.wmem_max           8388608
            Increase the maximum write buffer queue size 

    net.core.rmem_max           8388608
            Increase the maximum read buffer queue size 

    net.ipv4.tcp_rmem           4096 87380 8388608
            Set the minimum, initial, and maximum sizes for the read buffer.
            Note that this maximum should be less than or equal to the
            value set in net.core.rmem_max.

    net.ipv4.tcp_wmem           4096 87380 8388608
            Set the minimum, initial, and maximum sizes for the write buffer.
            Note that this maximum should be less than or equal to the
            value set in net.core.wmem_max.

    timeout_timewait            echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout
            Determines the time that must elapse before TCP/IP can release
            a closed connection and reuse its resources.  This interval
            between closure and release is known as the TIME_WAIT state or
            twice the maximum segment lifetime (2MSL) state.  During this
            time, reopening the connection to the client and server cost
            less than establishing a new connection.  By reducing the value
            of this entry, TCP/IP can release closed connections faster,
            providing more resources for new connections.  Adjust this
            parameter if the running application requires rapid release,
            the creation of new connections, and a low throughput due to
            many connections sitting in the TIME_WAIT state.

===========================================================================
http://www.softpanorama.org/Commercial_linuxes/performance_tuning.shtml
These reduce the amount of work the TCP stack has to do:

    echo 0 > /proc/sys/net/ipv4/tcp_sack
    echo 0 > /proc/sys/net/ipv4/tcp_timestamps 

    [ check security ]

===========================================================================
http://fasterdata.es.net/TCP-tuning/linux.html
TCP Tuning Guide
Version 1.0
Fri, 2 Apr 2010

Linux TCP Tuning

There are a lot of differences between Linux version 2.4 and 2.6, so first
we'll cover the tuning issues that are the same in both 2.4 and 2.6.  To change
TCP settings in, you add the entries below to the file /etc/sysctl.conf,
and then run "sysctl -p".

Like all operating systems, the default maximum Linux TCP buffer sizes are
way too small.  I suggest changing them to the following settings:

  # increase TCP max buffer size setable using setsockopt()
  net.core.rmem_max = 16777216
  net.core.wmem_max = 16777216

  # increase Linux autotuning TCP buffer limits
  # min, default, and max number of bytes to use
  # set max to at least 4MB, or higher if you use very high BDP paths
  net.ipv4.tcp_rmem = 4096 87380 16777216 
  net.ipv4.tcp_wmem = 4096 65536 16777216

You should also verify that these are all set to the default value of 1:

  sysctl net.ipv4.tcp_window_scaling 
  sysctl net.ipv4.tcp_timestamps 
  sysctl net.ipv4.tcp_sack

Note: you should leave tcp_mem alone, the defaults are fine.