GATE CS 2006 Q5 — Fragmentation and IP Addressing, IPv4, CIDR

The Time-to-Live (TTL) field in the IP header is an 8-bit field used to prevent packets from circulating endlessly in the network due to routing loops.

Here is how it works:

• The sender sets an initial TTL value (e.g., 64 or 128).
• Each router that forwards the IP packet decrements the TTL value by $1$.
• If the TTL value reaches $0$ before the packet reaches its destination, the packet is discarded, and an ICMP "Time Exceeded" message is typically sent back to the sender.

This mechanism ensures that packets caught in routing loops do not consume network bandwidth indefinitely.

Analysis of Options:

• Option A & B: Although TTL stands for "Time-to-Live", it is decremented per hop (router) rather than measuring actual physical time in seconds. Thus, it does not guarantee or measure arrival within a specific real-world time limit.
• Option C: Correct. It prevents packets from looping indefinitely.
• Option D: TTL is not used to limit queuing time in routers.