82598EB Datasheet, PDF (265/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Transmit Functionality
Unlike the TCP checksum, the UDP checksum is optional (for IPv4). Software must set the TXSM bit in
the TCP/IP Context Transmit Descriptor to indicate that a UDP checksum should be inserted. Hardware
does not overwrite the UDP checksum unless the TXSM bit is set.
3.5.3.4.6 Transmit Checksum Offloading with TCP Segmentation
The 82598 supports checksum off-loading as a component of the TCP segmentation offload feature and
as a standalone capability. Section 3.5.3.4.8 describes the interface for controlling the checksum off-
loading feature. This section describes the feature as it relates to TCP segmentation.
The 82598 supports IP and TCP/UDP header options in the checksum computation for packets that are
derived from the TCP segmentation feature.
Note:
The 82598 is capable of computing one level of IP header checksum and one TCP/UDP header
and payload checksum. In case of multiple IP headers, the software device driver has to
compute all but one IP header checksum. The 82598 calculates checksums on the fly on a
frame-by-frame basis and inserts the result in the IP/TCP/UDP headers of each frame. The
TCP and UDP checksums are a result of performing the checksum on all bytes of the payload
and the pseudo header.
Three specific types of checksum are supported by the hardware in the context of the TCP
segmentation offload feature:
• IPv4 checksum
• TCP checksum
• UDP checksum
Each packet that is sent via the TCP segmentation offload feature optionally includes the IPv4
checksum and the TCP or UDP checksum.
All checksum calculations use a 16-bit wide one's complement checksum. The checksum word is
calculated on the outgoing data. The checksum field is written with the 16-bit one's complement of the
one's complement sum of all 16-bit words in the range of CSS to CSE, including the checksum field
itself.
CFI
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