Intel PCI User Manual

21

layers. The packet checksum is always reported in the first descriptor (even in the case of multi-

descriptor packets).

Upon receipt of a packet for Ethernet controllers, hardware stores the packet data into the indicated 
buffer and writes the length, Packet Checksum, status, errors, and status fields. Length covers the 

data written to a receive buffer including CRC bytes (if any). Software must read multiple 

descriptors to determine the complete length for packets that span multiple receive buffers.

For standard 802.3 packets (non-VLAN) the Packet Checksum is by default computed over the 
entire packet from the first byte of the DA through the last byte of the CRC, including the Ethernet 

and IP headers. Software may modify the starting offset for the packet checksum calculation by 

means of the Receive Control Register. This register is described in 

. To verify the 

TCP checksum using the Packet Checksum, software must adjust the Packet Checksum value to 

back out the bytes that are not part of the true TCP Checksum. 

Status information indicates whether the descriptor has been used and whether the referenced 

buffer is the last one for the packet. Refer to 

 for the layout of the status field. Error status 

information is shown in 

For multi-descriptor packets, packet status is provided in the final descriptor of the packet (EOP 

set). If EOP is not set for a descriptor, only the Address, Length, and DD bits are valid.

PIF

IPCS

TCPCS

RSV

VP

IXSM

EOP

DD

PIF (bit 7)

Passed in-exact filter
Hardware supplies the PIF field to expedite software processing of packets. 
Software must examine any packet with PIF set to determine whether to accept 
the packet. If PIF is clear, then the packet is known to be for this station, so 
software need not look at the packet contents. Packets passing only the 
Multicast Vector has PIF set.

IPCS (bit 6)

IP Checksum Calculated on Packet
When Ignore Checksum Indication is deasserted (IXSM = 0b), IPCS bit indicates 
whether the hardware performed the IP checksum on the received packet.
0b = Do not perform IP checksum
1b = Perform IP checksum
Pass/Fail information regarding the checksum is indicated in the error bit (IPE) of 
the descriptor receive errors (RDESC.ERRORS)
IPv6 packets do not have the IPCS bit set.
Reads as 0b.