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10/100 Mbps Ethernet Controller Family Open Source Software Developer Manual 167
82550 and 82551QM Specific Information
B.4.1.1 Frame Types
•
Ethernet v2. If the Ethernet v2 type field equals 0800h, the first byte of IP header is expected
right after the optional VLAN field.
• 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.
B.4.1.2 Verification Types
• 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.
• TCP header checks. The packet length should be greater than or equal to the TCP header.
• UDP header checks. The packet length should be greater than or equal to the UDP header.
• 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.
B.4.2 82559 Compatibility
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.
B.5 VLAN Tagging
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.