GATE CS 2024 Set 2 — Question 32

MSQ+1 / -0MediumIPv4 Packet & FragmentationNetwork Layer: AddressingComputer Networks

Computer Networks → Network Layer: Addressing → IPv4 Packet & Fragmentation

Last updated 20 August 2026

Question

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

Source IP Address

Protocol

Time to Live (TTL)

Header Checksum

Correct answer

Solution

When a router forwards an IPv4 packet, it performs the following steps regarding the header:

1.Time to Live (TTL): The router decrements the TTL field by at least one. This is done to prevent packets from circulating indefinitely in the network. Thus, the TTL is always modified.

2.Header Checksum: Since the TTL field is part of the IP header and its value changes, the checksum (which verifies the integrity of the header) becomes invalid. The router must recompute the checksum with the new TTL value and update the Header Checksum field. Thus, the Header Checksum is always modified.

The Source IP Address and Protocol fields are generally preserved by standard routers (unless Network Address Translation (NAT) is being performed, which is a specific function and not the general definition of a router's forwarding behavior).

Therefore, both Time to Live (TTL) and Header Checksum are always modified.

GATE CS 2024 Set 2 — Question 32

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Correct answer

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