TCP segmentation offload

You can use TCP segmentation offload support to have the TCP/IP stack offload most IPv4 and IPv6 outbound TCP segmentation processing in theOSA feature. You can configure this function by specifying the SEGMENTATIONOFFLOAD parameter on the IPCONFIG or IPCONFIG6 profile statement. When using segmentation offload checksum processing will also be offloaded to the OSA feature. The TCP/IP stack performs TCP segmentation processing in the following cases in which segmentation cannot be offloaded:

  • Packets that go directly to another stack that shares the same OSA-Express port. Network Express supports segmentation for shared OSA.
  • IPSec-encapsulated packets
  • When multipath is in effect (unless all interfaces in the multipath group support segmentation offload)
  • When the SEGMENTATIONOFFLOAD parameter is not specified on the IPCONFIG or IPCONFIG6 statement
Tip: Applications that use large TCP send buffers will obtain the most benefit from TCP segmentation offload. The size of the TCP receive buffer on the other side of the TCP connection also affects the negotiated buffer size. You can control the size of these buffers using the following mechanisms:
  • The TCPSENDBFRSIZE parameter on the TCPCONFIG statement sets the default TCP send buffer size for all applications.
  • An application can use the SO_SNDBUF socket option to override the default TCP send buffer size.
  • The TCPRCVBUFRSIZE parameter on the TCPCONFIG statement sets the default TCP receive buffer size for all applications.
  • An application can use the SO_RCVBUF socket option to override the default TCP receive buffer size.