The value of this field determines if it is interpreted as a length or a type field, as defined by IEEE Std 802.3-2008. A value of 1,536 decimals or greater is interpreted by the Ethernet MAC as a type field.
When used as a length field, the value in this field represents the number of bytes in the following data field. This value does not include any bytes that can be inserted in the pad field following the data field.
A length/type field value of
0x8100 indicates that the frame is a VLAN frame and a value of
0x8808 indicates a PAUSE MAC control frame.
For the transmission, the Ethernet MAC does not perform any processing of the length/type field.
For the reception,
- If this field is a length field, the Ethernet MAC receive engine interprets this value and removes any padding in the pad field when necessary.
- If the field is a length field and length/type checking is enabled, the Ethernet MAC compares the length against the actual data field length and flags an error if a mismatch occurs.
- If the field is a type field, the Ethernet MAC ignores the value and passes it along with the packet data with no further processing.
The length/type field is always retained in the receive packet data.