Справочник Пользователя для Intel 82550

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Ethernet v2. If the Ethernet v2 type field equals 0800h, the first byte of IP header is expected 

right after the optional VLAN field.

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SNAP. The 82550 and 82551 skip the DSAP and SSAP fields. The Control field is expected to 
have a value of 03h. If a match occurs, the first byte of IP header is expected right after the 

optional VLAN field.

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IPv4 header checks. For IPv4 frames, no fragmentation occurs. The protocol field should 

equal:

— 06 for TCP

— 17 for UDP 

— 04 for IP

— 50 for ESP

— 51 for AH

Other protocols will not have a checksum.

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TCP header checks. The packet length should be greater than or equal to the TCP header.

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UDP header checks. The packet length should be greater than or equal to the UDP header.

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Checksum. The receive checksum machine computes the IP header checksum, the tunneled IP 

header checksum (if one is present), the TCP/UDP pseudo header checksum, and the TCP/
UDP checksum. If a mismatch occurs, the frame is transferred to memory with an error 

indication according to the configuration.

In 82559 compatibility mode, the 82550 and 82551 compute the checksum of the whole incoming 

packet excluding the MAC header and CRC. The checksum word is appended at the end of the 
packet in the receive buffer in host memory. If the CRC is posted to host memory, the checksum 

word follows the CRC. The byte count field in the receive memory structures includes the 

checksum word.

When the 82550 or 82551 receives a frame with a VLAN tag, it sets bit T in the RFD status word 
(bytes 1h:0h) and copies the tag (the two bytes after the constant 8100h) to the VLAN field in the 

RFD (bytes 11h:10h). If VLAN stripping is enabled, the device strips the four VLAN bytes, and 

the received packet is copied to host memory without these four bytes.