Question

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) 

 

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.

Answer 1

Correct Answer: 7

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: \[ {Payload size} = 5000 - 20 = 4980 { bytes}. \] The message size is 15000 bytes. To calculate how many fragments are required at Router-1: \[ {Number of fragments at Router-1} = \frac{15000}{4980} \approx 3.01. \] Therefore, 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: \[ {Payload size} = 3000 - 20 = 2980 { bytes}. \] 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: \[ {Number of fragments at Router-2 per fragment} = \frac{4980}{2980} \approx 1.67. \] This results in 7 fragments delivered to the destination. Thus, the total number of fragments delivered to the destination is \( \boxed{7} \).