c++ - How to calculate checksum of UDP packet embedded inside IP packet -


i have udp packet embedded inside ip packet , not able calculate checksum of udp can correctly find checksum of ip. can how udp checksum found.

[45 00 00 53 00 80 00 00 40 11 66 16 0a 00 00 03 0a 00 00 02] ca b1 ca b1 00 3f df a5

the bits enclosed in bracket ip packet , checksum given in bold.

**udp packet** ca b1 source port ca b1 destination port 00 3f length df a5 checksum

here how checksum "df a5" came. did 16 bit addition , took 1s complement still not getting value. whether need consider ip header calculate checksum of udp


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