Byte 15 Lower Nibble = IHL - Nibble with number of 32-bit words in IPV4 header (5 to 15 supported).
Bytes 20-21 = fragment[15-0] – Bit 13 is the MF bit and bits [12-0] are the Fragment offset. A packet is a fragment if the MF bit is set or if the fragment offset is non-zero. The first packet fragment has MF=1 with a zero offset. Middle fragments have MF=1 with a nonzero offset. The last packet fragment has MF=0 with a nonzero offset. Non-fragmented packets have MF=0 and a zero offset. A count is output for packet fragments but no errors are reported. First fragments have the UDP header included in the count. Middle and last fragments have only data included in the count (there is no UDP header).
Byte 24 = 0x11 for UDP protocol.
Received packet UDP checksum of zero means that there is no IPV4 checksum sent with the packet so no error will be issued.
Received packet UDP checksum of 0xFFFF means that the checksum was calculated to be 0xFFFF or 0x0000 but was sent in the transmitted packet as 0xFFFF by the sending originating entity.