Network Tuning Guidelines (AIX)
This article is a stub. Please expand it if you have more information.
Most of this page refers directly to AIX, however, the concepts apply equally well to all systems.
MTU
MTU (Maximum Transmission Unit) is the largest packet that will be sent. For standard Ethernet, this is 1500 bytes. "Jumbo frames", introduced with gigabit Ethernet, expands the MTU to 9000 bytes. Conversely, there are various elements that be used in the network fabric to reduce the MTU, commonly IPsec, stf, gif, and many other forms of tunnels and encapsulation.
RemMTU
Under AIX, each interface has a remmtu parameter, which may be displayed via
lsattr -El en0 -a remmtu
By default, this is 576 bytes, meaning that the packet size is reduced to this value for "remote" networks. Given that the default value for the no parameter subnetsarelocal is "1", AIX treats networks of a different class to be remote. In a large network, where different class networks are in use, this may unnecessarily decrease the MTU, increasing packet counts and overhead, and decreasing performance. Depending on network design, it should generally be possible to increase remmtu to 1500 bytes.
MTU Discovery
TCP Window Size
Calculating
Configuring
Selective Acknowledgements
Note: For AIX 5.3, do not enable SACKs unless APAR IY78947 is installed.
Normally, when running with large window sizes, a lost packet will result in the re-transmission of the entire window from the lost packet onwards. TCP Selective Acknowledgements (SACKs, RFC 2018) allows the receiving system to request the resending of just the lost data. To do this, a small length increase in the standard TCP headers is required, but the increased performance when running on LFNs (Long Fat Networks) with high Bandwidth Delay Product (BDP) and some packet loss, is worth the overhead.
no -p -o sack=1
