GATE CS 2021 Set 1 Q45 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

Let's analyze each statement step-by-step:

• Statement A (Correct): The original IP datagram sent from $P$ has a total size of $1400 \text{ (TCP payload)} + 20 \text{ (IP header)} = 1420$ bytes. At router $R$, the outgoing link MTU is $820$ bytes. The maximum IP payload size per fragment is $\text{MTU} - \text{IP header} = 820 - 20 = 800$ bytes (which is a multiple of $8$).
  • Fragment 1: Payload = $800$ bytes, Total size = $820$ bytes.
  • Fragment 2: Remaining payload = $1400 - 800 = 600$ bytes, Total size = $600 + 20 = 620$ bytes.
  Thus, two fragments are created, and the second fragment's size is indeed $620$ bytes.
• Statement B (Incorrect): Routers operate at the Network Layer and do not perform retransmissions of lost fragments. Retransmission is handled end-to-end by the transport layer (TCP) at the source host $P$.
• Statement C (Correct): IP is an unreliable protocol. If any fragment of an IP datagram is lost, the destination host $Q$ cannot reassemble the datagram and discards all other fragments. Since TCP is unaware of IP-level fragmentation, the source $P$ must retransmit the entire TCP segment.
• Statement D (Incorrect): The TCP header (which contains the destination port) is located at the very beginning of the TCP segment. Therefore, it is entirely contained within the first fragment. The second fragment contains only the remaining segment payload and cannot be used to determine the destination port.