TCP checksum offload

The TCP checksum offload option enables the network adapter to compute the TCP checksum on transmit and receive, which saves the AIX® host CPU from having to compute the checksum.

The savings vary by packet size. Small packets have little or no savings with this option, while large packets have larger savings. On the PCI-X GigE adapters, the savings for MTU 1500 are typically about 5% reduction in CPU utilization, and for MTU 9000 (Jumbo Frames) the savings is approximately a 15% reduction in CPU utilization.

TCP streaming throughput with MTU 1500 is slower on machines that have processors faster than 400 MHz if the TCP checksum offload option is enabled because the host system can run the checksum faster than the Gigabit Ethernet PCI adapters, FC2969 and FC 2975. Therefore, by default, this option is off on these adapters. When these adapters use jumbo frames, it can run at wire speed even when it has to compute the checksum.

The PCI-X Gigabit Ethernet adapters can run at wire speeds with the TCP checksum offload option enabled and it reduces host CPU processing so it is enabled by default.