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?

Let us analyze the fragmentation process:

1. Packet creation at host $P$:

   • TCP segment size (Data payload for IP) $= 1400\text{ bytes}$.

   • IP header size $= 20\text{ bytes}$.

   • Total IP datagram size $= 1400 + 20 = 1420\text{ bytes}$.

   • Since MTU of link $P-R$ is $1500\text{ bytes} \ge 1420\text{ bytes}$, no fragmentation occurs between $P$ and $R$.

2. Fragmentation at Router $R$ (Link $R-Q$ has $\text{MTU} = 820\text{ bytes}$):

   • Maximum IP payload size per fragment $= 820 - 20 = 800\text{ bytes}$.

   • Since $800$ is a multiple of $8$, the maximum allowable payload per fragment is $800\text{ bytes}$.

   • Fragment 1:

     • IP Payload $= 800\text{ bytes}$

     • Total IP Datagram size $= 800 + 20 = 820\text{ bytes}$

     • Offset $= 0$, More Fragments $(\text{MF}) = 1$

   • Fragment 2:

     • Remaining IP Payload $= 1400 - 800 = 600\text{ bytes}$

     • Total IP Datagram size $= 600 + 20 = 620\text{ bytes}$

     • Offset $= 800 / 8 = 100$, More Fragments $(\text{MF}) = 0$

   • Thus, exactly two fragments are created, and the second fragment carries an IP datagram of size $620\text{ bytes}$. (A is correct).

3. Reliability and Retransmission:

   • Routers operate at the Network Layer (IP) which is connectionless and unreliable; routers do not buffer or retransmit lost fragments. Hence, (B is incorrect).

   • Fragment reassembly happens only at the destination host $Q$. If any fragment is lost, the entire IP datagram is discarded by $Q$. Therefore, the source transport layer (TCP at host $P$) will time out and retransmit the entire original TCP segment. (C is correct).

4. TCP Header inspection:

   • The TCP header (which contains the source and destination port numbers) is located at the very beginning of the TCP segment (bytes $0$ to $19$). It resides entirely in the first fragment. The second fragment only contains the subsequent payload data and cannot reveal the destination TCP port. Hence, (D is incorrect).

Therefore, the correct options are A and C.