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Previous Year Question (PYQs)

Consider two hosts P and Q are connected through a router R. The maximum transfer unit $MTU$ value of the link between P and R is $1500$ bytes and between R and Q is $820$ bytes. A TCP segment of size of $1400$ bytes is transferred from P to Q through R with IP identification value of $0x1234$. Assume that IP header size is $20$ bytes. Further, the packet is allowed to be fragmented that is Don't Fragment $DF$ flag in the IP Header is not set by P. Which of the following statements is/are true?

I. Two fragments are created at R and IP datagram size carrying the second fragment is $620$ bytes
II. If the second fragment is lost then R resends the fragment with IP Identification value of $0x1234$
III. If the second fragment is lost then P is required to resend the entire TCP segment
IV. TCP destination port can be determined by analyzing the second fragment only

Choose the correct answer from the options given below:





Solution

TCP segment size:

$1400$ bytes

IP header size:

$20$ bytes

So, original IP datagram size is:

$1400+20=1420$ bytes

Since $1420 < 1500$, no fragmentation is required between P and R.

Now, between R and Q:

$MTU=820$ bytes

Each fragment will have an IP header of $20$ bytes.

So, maximum data in each fragment is:

$820-20=800$ bytes

Original data size is:

$1400$ bytes

So, fragments will be:

First fragment data:

$800$ bytes

First fragment total size:

$800+20=820$ bytes

Second fragment data:

$1400-800=600$ bytes

Second fragment total size:

$600+20=620$ bytes

So, statement I is true.

Now check statement II:

If the second fragment is lost, router R does not retransmit the fragment. IP is unreliable and does not handle retransmission.

So, statement II is false.

Now check statement III:

If the fragment is lost, TCP at source P will retransmit the TCP segment.

So, statement III is true.

Now check statement IV:

TCP destination port is present in the TCP header.

The TCP header is present only in the first fragment, not in the second fragment.

So, TCP destination port cannot be determined by analyzing only the second fragment.

So, statement IV is false.

Correct statements are:

I and III only


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