UGC NET CSE | August 2024 | Part 2 | Question: 65

Question

Host $A$ (on $\mathrm{TCP/IPv}4$ Networks) send an IP Datagram $D$ to host $B$ (also on $\mathrm{TCP/IPv}4$ network). Assume that no error occured during transmission of $D$ , when $D$ reaches $B$. Which of following header field may differ from that of original datagram $D$?

1. TTL
2. Checksum
3. Fragment offset
4. Source IP
5. Destination IP

Choose the correct answer from the options given below:

• A. $\text{(I)}$ and $\text{(II)}$ Only
• B. $\text{(I), (II)}$ and $\text{(III)}$ Only
• C. $\text{(I), (II), (III)}$ and $\text{(IV)}$ Only
• D. $\text{(I), (II), (III)}$ and $\text{(V)}$ Only

Answer 1

When an IP datagram is sent across networks, some header fields change while others must remain unchanged.

Let's analyze each field:

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✅ (A) TTL (Time To Live)

TTL decreases by 1 at every router hop, so it will be different.
✔ Will differ

✅ (B) Checksum

The IP header checksum is recalculated every time the packet is modified (such as TTL change).
✔ Will differ

❌ (C) Fragment offset

Fragment offset changes only if fragmentation occurs.
But the question states "Assume that no error occurred during transmission", not that fragmentation didn’t occur.
However, fragmentation does not depend on errors. It depends on MTU mismatch.

Since the question says host A and host B are on the same network, fragmentation will not occur.

So fragment offset will NOT change.
✘ Will NOT differ

❌ (D) Source IP

Source IP never changes during transmission.
✘ Will NOT differ

❌ (E) Destination IP

Destination IP never changes.
✘ Will NOT differ

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✅ Correct Answer: (A) and (B) Only

Comments

nothing about fragmentation is given in the question right?