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

1. AI, II, and III only
2. BI and III only
3. CIII and IV only
4. DII and IV only

Pick an option to check yourself, or open the answer below.

Show answer & explanation

Correct answer: B. I and III only

Explanation

R creates two fragments, of 820 and 620 bytes (I true). IP does not retransmit, so TCP at P resends the whole segment (III true). Routers never resend (II false), and only the first fragment carries the TCP ports (IV false).