GATE CS 2021 Set 1 Q.55 (CS Q.45): IPv4/IPv6 Addressing, Subnetting, CIDR, ARP, ICMP and NAT

Consider two hosts $P$ and $Q$ 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 1400 bytes was transferred from $P$ to $Q$ through $R$ , with IP identification value as 0x1234. Assume that the IP header size is 20 bytes. Further, the packet is allowed to be fragmented, i.e., Don't Fragment (DF) flag in the IP header is not set by $P$ .

Which of the following statements is/are correct?

Two fragments are created at $R$ and the IP datagram size carrying the second fragment is 620 bytes.

If the second fragment is lost, $R$ will resend the fragment with the IP identification value 0x1234.

If the second fragment is lost, $P$ is required to resend the whole TCP segment.

TCP destination port can be determined by analysing only the second fragment.

Official answer and solution

Official answer: A and C
From: Official GATE 2021 answer key

Solution

Calculate IP fragmentation parameters at router R based on outgoing link MTU and analyze fragment retransmission requirements.

Parameters:

Original TCP segment size $= 1400$ bytes.
IP header size $= 20$ bytes.
Total incoming IP datagram size from $P = 1400 + 20 = 1420$ bytes.

Fragmentation at router $R$ (link $R-Q$ has $\text{MTU} = 820$ bytes):

Maximum payload per fragment must be a multiple of 8 and fit within $\text{MTU} - 20 = 820 - 20 = 800$ bytes.
Since 800 is divisible by 8 ( $800/8 = 100$ ), fragment 1 carries 800 bytes of data (offset 0, $\text{MF} = 1$ ). Total size $= 800 + 20 = 820$ bytes.
Fragment 2 carries the remaining $1400 - 800 = 600$ bytes of data (offset $800/8 = 100$ , $\text{MF} = 0$ ). Total size $= 600 + 20 = 620$ bytes.
Thus, statement A is correct.

Statements B and C:

IP is an unreliable, connectionless protocol that does not perform fragment-level retransmissions or buffer fragments at intermediate routers. Hence, $R$ will not resend any fragment (Statement B is false).
If any fragment is lost, reassembly at destination $Q$ fails and the entire IP datagram is discarded. TCP at sender $P$ will eventually time out and retransmit the entire 1400-byte segment (Statement C is correct).

Statement D:
The TCP header resides in the first 20 bytes of the TCP segment, which is placed exclusively inside the first fragment. The second fragment contains only data payload with no TCP header; therefore, the TCP destination port cannot be determined from the second fragment alone (Statement D is false).

Hence, statements A and C are correct.

Final answer

A, C

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology (CS, Set-1)
Question: 55 of 65, printed as CS Q.45
Section: Computer Science and Information Technology

Report an error in this question

GATE questions are the work of the organising institutes. This site is independent and is not affiliated with them.

Question

Solution

Source