Andrew S. Tanenbaum Edition 5th Exercise 5 Question 38 (Page No. 494)

Question

n IP, the checksum covers only the header and not the data. Why do you suppose this
design was chosen?

Answer 1

at each router the field in the header change such as TTL etc . so the router have to calculate the header checksum again . and the router has less CPU capacity so adding the header on the data increase burden on the router.

means to reduce the burden on the router we exclude the data while calculating header checksum