Computer Networks Interview Questions — Medium

The TCP 3-way handshake establishes a reliable connection over an unreliable IP network by synchronizing sequence numbers:

1. SYN: The client sends a segment with a random initial sequence number ($ISN_C$) and the SYN bit set to 1. State: SYN-SENT.
2. SYN-ACK: The server responds with its own random initial sequence number ($ISN_S$), sets the SYN bit to 1, and acknowledges the client's packet by setting ACK to 1 and the acknowledgment number to $ISN_C + 1$. State: SYN-RECEIVED.
3. ACK: The client acknowledges the server's segment by setting the ACK bit to 1 and the acknowledgment number to $ISN_S + 1$. State: ESTABLISHED on both sides.

TCP connections are full-duplex, meaning each direction must be terminated independently:

1. FIN (Client to Server): Client sends a segment with the FIN bit set, indicating it has no more data to send. Client state: FIN-WAIT-1.
2. ACK (Server to Client): Server acknowledges the FIN with an ACK. Server state: CLOSE-WAIT; Client state: FIN-WAIT-2. The server can still send data.
3. FIN (Server to Client): Once the server finishes sending data, it sends its own FIN. Server state: LAST-ACK.
4. ACK (Client to Server): Client acknowledges the server's FIN with an ACK and enters the TIME-WAIT state, waiting for $2 \times MSL$ (Maximum Segment Lifetime) before transitioning to CLOSED. Server enters CLOSED immediately upon receiving the ACK.

The TIME-WAIT state (usually lasting 2 to 4 minutes) serves two critical purposes:

• Guarantees final ACK delivery: If the final ACK is lost, the server remains in LAST-ACK and retransmits its FIN. The client must remain active to re-send the ACK.
• Drains delayed packets: It allows older, delayed packets belonging to the connection to expire in the network, preventing them from corrupting subsequent connections using the same socket pairs.

• Flow Control: End-to-end mechanism that prevents a fast sender from overwhelming a slow receiver. It uses a Receiver Window (rwnd) advertised in the TCP header to limit unacknowledged data.
• Congestion Control: Global network mechanism that prevents sender nodes from overwhelming the intermediate routers and links. It uses a calculated Congestion Window (cwnd) in the OS kernel.
• Effective Window limit: The sender cannot transmit more than $\min(rwnd, cwnd)$ bytes of unacknowledged data.

• HTTP/1.1 HoL Blocking: Occurs because browsers can only send requests sequentially over a single TCP connection. If a resource takes long to generate, subsequent requests on that pipe are blocked.
• HTTP/2 Multiplexing: Resolves this by breaking HTTP transactions into binary frames. These frames are interleaved over a single TCP connection within independent concurrent streams.
• Remaining Limitation: HTTP/2 is still susceptible to TCP-level HoL blocking (if one packet is lost, TCP stalls all streams until that packet is retransmitted).