Question
Which of the following fields of an IP header is/are always modified by any router before it forwards the IP packet?
Official answer and solution
- Official answer
- C and D
- From
- Official GATE 2024 answer key
Solution
Identify the IPv4 header fields updated at every hop (TTL and Header Checksum).
When an IPv4 router forwards an IP packet:
- Time to Live (TTL) (C) is always decremented by at every router hop to prevent packets from circulating indefinitely in routing loops.
- Header Checksum (D) covers the IPv4 header (including the TTL field), so because TTL is modified, the router must always recompute and update the Header Checksum.
- Source IP Address (A) (unless NAT is configured) and Protocol (B) are not modified by normal IP forwarding.
Final answer
Options (C), (D)
Common trap
Selecting only TTL (C) and forgetting that changing TTL requires updating the IPv4 Header Checksum (D).
The answer is the one in the official GATE answer key. The solution is this site's own explanation.
Source
- Paper
- GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
- Question
- 32 of 65
- Section
- Computer Science and Information Technology
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