TCP large send offload

Edit online

The TCP large send offload option allows the AIX® TCP layer to build a TCP message up to 64 KB long. The adapter sends the message in one call down the stack through IP and the Ethernet device driver.

The adapter then breaks the message into multiple TCP frames to transmit data on the cable. The TCP packets sent on the cable are either 1500 byte frames for an Media Transmission Unit (MTU) of 1500 or up to 9000 byte frames for an MTU of 9000 (jumbo frames).

Without the TCP large send offload option, for the TCP option to send 64 KB of data it takes 44 calls down the stack by using 1500 byte packets. With the TCP large send option, the TCP option can send up to 64K bytes of data with one call down the stack, which reduces host processing and results in lower processor utilization on the host processor. The Ethernet adapter then does the TCP segmentation offload to segment the data into the MTU sized packets (typically 1500 bytes). The savings varies depending on the average TCP large send size. For example, a reduction of host processor CPU by 60 to 75% can be obtained with the PCI-eXtended (PCI-X) Gigabit Ethernet adapters with an MTU size of 1500. For jumbo frames (MTU 9000), the savings are less because the system already sends larger frames. For example, a 40% reduction of host processor CPU is typical with jumbo frames.

The large send offload option is enabled by default on Ethernet adapters that support the option when you are working in dedicated mode. This option improves the performance on 10 Gigabit Ethernet and faster adapters for workloads that manage data streaming (such as file transfer protocol (FTP), RCP, tape backup, and similar bulk data movement applications). The virtual Ethernet adapter and shared Ethernet adapter (SEA) devices are exceptions, where the large send offload option is disabled by default due to inter operability problems with the Linux® or IBM i operating system. Enabling Large Send and other performance features can be done in AIX and virtual Ethernet adapter or SEA environments.

The large send option is a device attribute that is specified as large_send. You can see the large send offload device attribute by using the following command where X is the device number :

lsattr -E -l entX