Intel PCI-X manual Transmit Descriptor Tdesc Layout, Transmit Description Descriptor Offload

Receive and Transmit Description

Table 3-14. Transmit Descriptor (TDESC) Layout

Transmit	Description
Descriptor Offload
	TCP/UDP Checksum Ending
TUCSE	Defines the ending byte for the TCP/UDP checksum offload feature.
	Setting TUCSE field to 0b indicates that the checksum covers from TUCCS to the
	end of the packet.
	TCP/UDP Checksum Offset
	Defines the offset where to insert the TCP/UDP checksum field in the packet data
TUCSO	buffer. This is used in situations where the software needs to calculate partial
	checksums (TCP pseudo-header, for example) to include bytes which are not
	contained within the range of start and end.
	If no partial checksum is required, software must write a value of 0b.
	TCP/UDP Checksum Start
TUCSS	Defines the starting byte for the TCP/UDP checksum offload feature.
	It must be defined even if checksum insertion is not desired for some reason.
	When setting the TCP segmentation context, TUCSS is used to indicate the start
	of the TCP header.
	IP Checksum Ending
	Defines the ending byte for the IP checksum offload feature.
IPCSE	It specifies where the checksum should stop. A 16-bit value supports checksum
	offloading of packets as large as 64KB.
	Setting IPCSE field to 0b indicates that the checksum covers from IPCCS to the
	end of the packet. In this way, the length of the packet does not need to be
	calculated.
	IP Checksum Offset
IPCSO	The IPCSO field specifies where the resulting IP checksum should be placed. It is
	limited to the first 256 bytes of the packet and must be less than or equal to the
	total length of a given packet. If this is not the case, the checksum is not inserted.
	IP Checksum Start
	IPCSS specifies the byte offset from the start of the transferred data to the first
	byte in be included in the checksum. Setting this value to 0b means the first byte of
	the data would be included in the checksum.
	Note that the maximum value for this field is 255. This is adequate for typical
IPCSS	applications.
	The IPCSS value needs to be less than the total transferred length of the packet. If
	this is not the case, the results are unpredictable.
	IPCSS must be defined even if checksum insertion is not desired for some reason.
	When setting the TCP segmentation context, IPCSS is used to indicate the start of
	the IP header.
	Maximum Segment Size
	Controls the Maximum Segment Size. This specifies the maximum TCP or UDP
MSS	payload “segment” sent per frame, not including any header. The total length of
	each frame (or “section”) sent by the TCP Segmentation mechanism (excluding
	802.3ac tagging and Ethernet CRC) is MSS bytes + HRDLEN. The one exception
	is the last packet of a TCP segmentation context which is (typically) shorter than
	“MSS+HDRLEN”. This field is ignored if TDESC.TSE is not set.
	Header Length
	Specifies the length (in bytes) of the header to be used for each frame (or
	“section”) of a TCP Segmentation operation. The first HDRLEN bytes fetched from
HDRLEN	data descriptor(s) are stored internally and used as a prototype header for each
	section, and are pre-pended to each payload segment to form individual frames.
	For UDP packets this is normally equal to “UDP checksum offset + 2”. For TCP
	packets it is normally equal to “TCP checksum offset + 4 + TCP header option
	bytes”. This field is ignored if TDESC.TSE is not set.

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