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 1400 bytes is transferred from P to Q through R with IP identification value of 0×1234. Assume that IP header size is 20 bytes. Further, the packet is allowed to be fragmented, meaning the Don't Fragment (DF) flag in the IP Header is not set by P. Which of the following statements is/are not true?

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

B. If the second fragment is lost then (R) resend the fragment with IP Identification value of 0×1234

C. If the second fragment is lost then P is required to resend the entire TCP segment

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

Choose the correct answer from the options given below:

1. AA and B Only
2. BA and C Only
3. CC and D Only
4. DB and D Only

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

Show answer & explanation

Correct answer: D. B and D Only

Explanation

Two fragments of 820 and 620 bytes are created (A true), and TCP at P must resend the whole segment if one is lost (C true). Routers never resend fragments (B false), and the TCP ports are only in the first fragment (D false). The statements that are not true are B and D.

Note on the official key: the official key says option B. Official/PW key is B (A and C), which are the true statements; the question asks for the statements that are not true. (The same question in June 2023 asks for the true ones, with key B.)