Which of the following fields of an IP header is/are always modified by any router before it forward

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GATE-CS is a competitive examination conducted in India. This question is from the 2024 GATE-CS Computer Networks paper.

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Question

Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?(A) Source IP Address(B) Protocol(C) Time to Live (TTL)(D) Header Checksum

Accepted Answer

Correct Answer: (C) and (D)Concept -- IP Header Processing at Routers:(A) FALSE: Routers do not modify the Source IP Address during normal forwarding. The source IP remains the IP of the originating host throughout the packet's journey. (NAT is a special case not applicable here.)(B) FALSE: The Protocol field indicates the upper-layer protocol (TCP, UDP, ICMP, etc.) and is never modified by routers during forwarding. It is set by the source and read by the destination.(C) TRUE: Every router decrements the TTL by 1 before forwarding the packet. TTL prevents packets from circulating forever. If TTL reaches 0, the router discards the packet and sends an ICMP Time Exceeded message.(D) TRUE: Since the TTL field is always modified, the IP header checksum must be recomputed after the TTL decrement. The checksum covers the entire IP header, so any change to any header field requires recalculation. Routers always update the header checksum after modifying TTL.

Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?(A) Source IP Address(B) Protocol(C) Time to Live (TTL)(D) Header Checksum

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Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?(A) Source IP Address(B) Protocol(C) Time to Live (TTL)(D) Header Checksum

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