GATE CS 2024 Set 2 Q22 — Fragmentation and IP Addressing, IPv4, CIDR

When an IP packet passes through a router, the router performs several operations on the IP header before forwarding it:

• Time to Live (TTL) (Option C): To prevent packets from circulating indefinitely in routing loops, every router decrements the TTL field by at least $1$. Thus, this field is always modified.
• Header Checksum (Option D): The Header Checksum covers the IP header fields, including the TTL field. Since the TTL field is modified at each hop, the checksum becomes invalid and must be recomputed and updated by every router. Thus, this field is always modified.

Let us look at the other options:

• Source IP Address (Option A): This field is generally preserved to identify the original sender. It is only modified if the router is performing Network Address Translation (NAT), which is not a standard routing requirement for every router.
• Protocol (Option B): This field indicates the upper-layer protocol (like TCP or UDP) that should receive the payload. It is never modified by routers during transit.

Therefore, the correct options are C and D.