82598EB Datasheet, PDF (247/596 Pages) Intel Corporation – Intel® 82598EB 10 Gigabit Ethernet Controller Datasheet

Intel^® 82598EB 10 GbE Controller - Transmit Functionality
For an 802.1Q header, the offset values depend on the VLAN insertion enable bit (VLE). If they are not
set (VLAN tagging included in the packet buffers), the offset values should include the VLAN tagging. If
these bits are set (VLAN tagging is taken from the packet descriptor), the offset values should exclude
the VLAN tagging.
Hardware does not add the 802.1q Ether Type or the VLAN field following the 802.1q Ether Type to the
checksum. So for VLAN packets, software can compute the values to back out only on the encapsulated
packet rather than on the added fields.
Note:
UDP checksum calculation is not supported by the legacy descriptor as the legacy descriptor
does not support the translation of a checksum result of 0x0000 to 0xFFFF needed to
differentiate between a UDP packet with a checksum of zero and an UDP packet without
checksum.
As the CSO field is eight bits wide, it puts a limit on the location of the checksum to 255 bytes from the
beginning of the packet.
Note: CSO must be larger than CSS.
Software must compute an offsetting entry-to back out the bytes of the header that should not be
included in the TCP checksum-and store it in the position where the hardware computed checksum is to
be inserted.
Command Byte – CMD (8)
The CMD byte stores the applicable command and has the fields listed in Table 3-64.
Table 3-64. Transmit Command (TDESC.CMD) Layout
7
6
RSV
VLE
5
DEXT
4
3
2
RSV
RS
IC
1
IFCS
0
EOP
• RSV (bit 7) – Reserved
• VLE (bit 6) – VLAN Packet enable
• DEXT (bit 5) – Descriptor extension (0 for legacy mode)
• Reserved (bit 4) – Reserved
• RS (bit 3) – Report status
• IC (bit 2) – Insert checksum
• IFCS (bit 1) – Insert FCS
• EOP (bit 0) – End of packet
When EOP is set, it indicates the last descriptor making up the packet. One or many descriptors can be
used to form a packet. Hardware inserts a checksum at the offset indicated by the CSO field if the
Insert Checksum bit (IC) is set. Checksum calculations are for the entire packet starting at the byte
indicated by the CSS field. A value of 0b corresponds to the first byte in the packet. CSS must be set in
the first descriptor for a packet. In addition, IC is ignored if CSO or CSS are out of range. This occurs if
(CSS >/= length) OR (CSO >/= length = one).
RS signals the hardware to report the status information. This is used by software that does in-memory
checks of the transmit descriptors to determine which ones are done. For example, if software queues
up 10 packets to transmit, it can set the RS bit in the last descriptor of the last packet. If software
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