title: IPV4 header checksum verification
body: <p>When a packet is sent from source to destination,it has to pass by through several routers and each router decreases the value of TTL of packet by 1. So header checksum is to be recomputed at each router since one of the header field i.e. TTL surely changes. Then how does the destination verifies the presence of error by calculating the checksum ,though the checksum it got has changed than that of checksum of source side? </p>