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IP checksum has to definitely checked at the destination. But is the checksum verified by each and every router it passes through??
And does the router in the middle of the path drop the packed if it finds the checksum is incorrect?
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The Internet checksum is mandatory to detect errors in IPV6 UDP packets (including data payload). The Internet checksum is used to detect errors in ICMP packets (including data payload).
A checksum is a simple error-detection scheme in which each transmitted message that results in a numerical value based on the value of the bytes in a message. The sender places the calculated value in the message (usually in the message header) and sends the value with the message.
Checksum is a simple error detection mechanism to determine the integrity of the data transmitted over a network. Communication protocols like TCP/IP/UDP implement this scheme in order to determine whether the received data is corrupted along the network.
It's a matter of speed: the Internet backbone routers direct millions of packets per second and calculating a checksum over the entire contents would drastically slow down the packet processing. The IPv6 specification removes even the header checksum to allow routers to route packets even faster.
The RFC 791 says:
If the header checksum fails, the internet datagram is discarded at once by the entity which detects the error.
and later:
Header Checksum: 16 bits
A checksum on the header only. Since some header fields change
(e.g., time to live), this is recomputed and verified at each point
that the internet header is processed.
So, it is a must to check at any entity of network layer (layer 3) the IP header checksum before taking any action on a received packet.
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