An IPv4 datagram (identification #450) with 1600 bytes of application data encou

Question

An IPv4 datagram (identification #450) with 1600 bytes of application data encounters a router which has a 420-byte MTU. Assume the minimum IP and TCP header sizes, i.e., the IP header is 20 bytes and the TCP header is 20 bytes.

1) How many fragments are generated?

A. 3

B. 4

C. 5

D. 6

E. none of these

2) What is the identification number on the last fragment?

A. 451

B. 452

C. 453

D. 454

E. none of these

3) How many bytes of application data are carried in the second fragment?

A. 360

B. 380

C. 400

D. 420

E. none of these

Explanation / Answer

1) The IP hrader is 20 bytes, TCP header is 20 bytes, Appliaction data is 1600 bytes
    MTU is 420.
    Removing the header parts the MTU will have appliaction data as 420 - 40 = 380

    So number of fragements will be 1600/380 = 4.21 So we will be needing 5 fragements

2) Identification numbers on all the fragements will be 450. So the answer is non of these.

3) The application data bytes on the second fragment will be 380.

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