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GO Classes | IIITH PGEE 2026 Mock Test 12 | Question: 67

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Consider a three-link configuration $\text{A-X-Y-B}$, where host $A$ is connected to router $X$, router $X$ is connected to router $Y$, and router $Y$ is connected to host $B$. All nodes support IPv4.

  •   The link $A-X$ has MTU $=1500$ bytes
  •   The link $X-Y$ has MTU $=500$ bytes
  •   The link $Y-B$ has MTU $=492$ bytes

Suppose host $A$ is sending a $1000$-byte packet to host $B$. Assume there is no packet loss due to bit errors or buffer overflow.

Also, assume that the IP header size $=20$ bytes.

How many packets will flow over the $X-Y$ link? And how many will arrive at host $B?$

  1. $3$ packets over $\mathrm{X}-\mathrm{Y}, 5$ packets arrive at host $B.$
  2. $2$ packets over $\mathrm{X}-\mathrm{Y}, 4$ packets arrive at host $B.$
  3. $5$ packets over $\mathrm{X}-\mathrm{Y}, 5$ packets arrive at host $B.$
  4. $3$ packets over $X-Y, 4$ packets arrive at host $B$.

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3 and 5. The 1000 byte packet from A can go to $X$ without problems, as the MTU on that link is larger than 1000. X has to fragment the packet into fragments of at most 500 bytes each. Since each fragment gets its own 20-byte IP header, the 1000 bytes from A do not fit in two 500-byte fragments, but need three, of sizes 500,500 , and 40 bytes. Each of these fragments arrive at $Y$, which cannot send the 500-byte fragments over the 492 byte MTU of the next link. So those fragments are fragmented further, leading to a total of five fragments arriving at Y.

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