Intel PCI Transmit Checksum Offloading with TCP Segmentation, UDP Pseudo Header Diagram for IPv4

Receive and Transmit Description

UDP pseudo header has the same format as the TCP pseudo header. The IPv4 pseudo header conceptually prefixed to the UDP header contains the IPv4 source address, the IPv4 destination address, the IPv4 protocol field, and the UDP length (same as the TCP Length discussed above). The IPv6 pseudo header for UDP is the same as the IPv6 pseudo header for TCP. This checksum procedure is the same as is used in TCP.

IP Source Address

IP Destination Address

Zero

Layer 4

Protocol ID

UDP Length

Figure 3-17. UDP Pseudo Header Diagram for IPv4

	IP Source Address
	IP Destination Address
	Upper Layer Packet Length
Zero		Next Header

Figure 3-18. UDP PseudoHeader Diagram for IPv6

Note: The IP Destination Address is the final destination of the packet. Therefore, if a routing header is used, the last address in the route list is used in this calculation. The upper-layer packet length is the length of the UDP header and UDP payload.

Unlike the TCP checksum, the UDP checksum is optional. 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.

3.5.8Transmit Checksum Offloading with TCP Segmentation

The Ethernet controller supports checksum off-loading as a component of the TCP Segmentation offload feature and as a standalone capability. Section 3.5.8 describes the interface for controlling the checksum off-loading feature. This section describes the feature as it relates to TCP Segmentation.

The Ethernet controller supports IP and TCP/UDP header options in the checksum computation for packets that are derived from the TCP Segmentation feature. The Ethernet controller 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 driver has to compute all but one IP header checksum. The Ethernet controller calculates checksums on the fly on a frame by frame basis and inserts the result in the IP/TCP/UDP headers of each frame. TCP and UDP checksum are a result of performing the checksum on all bytes of the payload and the pseudo header.

64 Software Developer’s Manual