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In IP networking, the Time To Live (TTL) field prevents routing loops. Which description best explains how TTL is used?

Difficulty: Easy

Correct Answer: A technique used in best-effort delivery systems to avoid endlessly looping packets.

Explanation:


Introduction / Context:
TTL is a critical IPv4 header field (Hop Limit in IPv6) used to limit the lifetime of a packet in the network and thus prevent infinite circulation during routing anomalies.


Given Data / Assumptions:

  • Routers decrement TTL by 1 per hop.
  • Packet is discarded when TTL reaches 0.
  • ICMP Time Exceeded can be generated for diagnostics (e.g., traceroute).


Concept / Approach:
TTL bounds the maximum hop count a packet may traverse. This avoids persistent loops and aids troubleshooting by exposing path length characteristics.


Step-by-Step Solution:

1) Sender sets initial TTL (e.g., 64 or 128).2) Each router decrements TTL by 1.3) If TTL becomes 0, router drops packet and may send ICMP Time Exceeded.4) Tools like traceroute vary TTL to map the path.


Verification / Alternative check:
Observe traceroute hop counts; increasing TTL reveals successive routers.


Why Other Options Are Wrong:

Encapsulation (option B) describes layering, not TTL.Fragmentation (option C) concerns MTU and datagram pieces, not loop prevention.“All of the above” is wrong because only option A describes TTL correctly.


Common Pitfalls:
Confusing TTL with time duration (it is hops), mixing fragmentation with loop control, and assuming TTL prevents all routing issues.


Final Answer:
A technique used to avoid endlessly looping packets.

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