Intel 82558 Manuale Utente

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way, it may read an extra TBD (or 8 bytes) after the last valid TBD. The driver is responsible for 
allocating enough memory for the TBD array. Otherwise, it needs to ensure that an extra 8 byte 

read access from the PCI will not affect anything.

For better performance, the maximum TCP payload should be a multiple of 4. It is recommended 

to use the largest possible value for this parameter.

Large Send and checksum are orthogonal.

Large Send cannot be used with software parsing.

All frames of one Large Send instance are subject to the same scheduling policy. Specifically, if the 

prototype has a time value for transmission, all subsequent frames are considered ready for 
transmission after that time and may be transmitted back-to-back.

The 82550 and 82551 compute and verify the IP header checksum and the TCP/UDP checksum on 

frames successfully parsed. It indicates a checksum match or mismatch in the RFD. An incoming 

UDP frame with a checksum field of 0 is treated as a checksum match. If the 82550 or 82551 
cannot decrypt a frame, the checksum can only be verified to the first IP header only (relevant to 

types IV, V, and VI).

In checksum modes, the device checks incoming frames for format correctness. If the check fails, a 

receive parser failure occurs and the frame is passed as is. The incoming frame is subject to all 
MAC layer checks (for example, CRC, minimum length, address filtering). If the value of the word 

following the DA and SA fields is less than 5DCh (1500), the controller assumes it is an Ethernet 

v2 frame. If the word value is greater than or equal to 5DCh (1500), the 82550 and 82551 compare 
in the incoming frame format to SNAP.

In the 82559 compatibility mode, incoming frames are not verified for their format. The checksum 

is calculated on all non-MAC frame bytes. Supported frame formats are Ethernet v2 and SNAP.