Suppose a message of size 15000 bytes is transmitted from a source to a destination using IPv4 protocol via two routers as shown in the figure. Each router has a defined maximum transmission unit (MTU) as shown in the figure, including IP header. The number of fragments that will be delivered to the destination is ___________. (Answer in integer)
Show Hint
When calculating the number of fragments, always consider both the payload size and the MTU, as the fragmentation process depends on the smallest MTU along the transmission path.
We are given the following information:
- Message size: 15000 bytes.
- Router-1 MTU: 5000 bytes (including IP header of 20 bytes).
- Router-2 MTU: 3000 bytes (including IP header of 20 bytes).
Step 1: Fragmentation at Router-1
MTU at Router-1: 5000 bytes.
Payload size per fragment at Router-1:
\[
\text{Payload size} = 5000 - 20 = 4980 \text{ bytes}.
\]
The message size is 15000 bytes. To calculate how many fragments are required at Router-1:
\[
\text{Number of fragments at Router-1} = \frac{15000}{4980} \approx 3.01.
\]
Since a fragment cannot exceed the MTU, we round up, so 4 fragments are created at Router-1.
Step 2: Fragmentation at Router-2
MTU at Router-2: 3000 bytes.
Payload size per fragment at Router-2:
\[
\text{Payload size} = 3000 - 20 = 2980 \text{ bytes}.
\]
Now, each of the 4 fragments from Router-1 will be transmitted through Router-2. Each fragment needs to be fragmented again at Router-2:
\[
\text{Number of fragments at Router-2 per fragment} = \frac{4980}{2980} \approx 1.67.
\]
Since each fragment needs to be split further, the total number of fragments at Router-2 becomes 7.
Final Answer:
Thus, the total number of fragments delivered to the destination is \( \boxed{7} \).
