Problem #4 (3 points): Consider sending a 1500-byte IP datagram (inclusive of a

Question

Problem #4 (3 points): Consider sending a 1500-byte IP datagram (inclusive of a minimum size header of 20 bytes) from host A to host B along the path shown in the following figure. Assume that the MTU of each of the shown links is inclusive of a minimum size header of 20 bytes. R1 R2 R3 MTU- 1,500 Bytes MTU- 600 Bytes MTU- 1,500 Bytes MTU- 1,500 Bytes 1. (1 point) Find the total number of fragments generated by router R1. 2. (2 points) Specify the header's "offset" field value of each fragment that is generated by router R1.

Explanation / Answer

Header + Data Payload   <=   600

because in this case maximum effective transmission unit is 600 byte

and also Data Payload must be multiple of 8 byte

1. So in order to send a 1500-byte datagram over a channel of MTU = 600 byte minimum 3 fragments needed

2. first fragment = data + header = 576 + 20 => offset field value = 0

second fragment = data + header = 576 + 20 => offset field value = 72

third fragment = data + header = 348 + 20 => offset field value = 143

note in last fragment some padding bits also added but that is not relevent for this question

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