Lectures notes for 2010 - KTH

UDP Checksum and Pseudo-Header
• UDP checksum covers more info than is present in the UDP datagram
alone: pseudo-header and pad byte (0) {to even number of 16 bit words}.
• Propose: to verify the UDP datagram reached its correct destination:
right port number at the right IP address.
• Pseudo-header and pad byte are not transmitted with the UDP
datagram, only used for checksum computation.
0 15 16 31
32 bit IP source address
32 bit IP destination address
UDP Pseudo
header
zero 8 bit protocol (UDP = 17) 16 bit UDP length
16 bit source port number 16 bit destination port number UDP header
16 bit UDP length 16 bit UDP checksum
data (if any)
data
Pad byte (0)
Maguire UDP Checksum and Pseudo-Header 4: 7 of 74
maguire@kth.se 2010.03.21 Internetworking/Internetteknik