The base relative to which 'offset' is measured. Possible values are:
If specified, apply this mask (bitwise AND) to the field to ignore bits before matching. Encoded as a hexadecimal number (starting with "0x"). The last byte of the field (in network byte order) corresponds to the least significant byte of the mask.
Name of the user-defined field, as given in the definition.
Offset of the first byte of the field (in network byte order) relative to 'base'.
Size of the field in bytes. Valid values: 1-4.