What this topic covers
10 subtopics- Congestion control13
- TCP vs UDP7
- Connection management5
- Flow control4
- Reliability4
- Ports3
- TCP header3
- Sequence numbers2
- Sockets2
- Timeouts2
- Beginner 15
- Intermediate 15
- Advanced 15
About this topic
The transport layer connects processes: UDP just delivers datagrams, while TCP adds connections, reliable in-order delivery, flow control and congestion control. GATE regularly asks for congestion-window values after slow start, timeouts and duplicate ACKs; interviews ask about the handshake and TCP vs UDP. Window traces here were computed with the standard rules (slow start doubles below ssthresh, then +1 per RTT; a timeout drops cwnd to 1; three duplicate ACKs in Reno halve it), and the socket programs really ran over localhost.
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Sample questions
Try each one first, then open the answer for the explanation and why the other options are wrong.
Which statement correctly contrasts TCP and UDP?
Show answer and explanation
Explanation
UDP suits short queries (DNS) and real-time media where a late packet is useless; TCP suits files, web pages and anything that must arrive intact.
Why the other options are wrong
- AAt least 20 bytes.
- BUDP does not.
- CReversed.
A TCP connection starts with cwnd = 1 MSS and ssthresh = 8 MSS, and no losses occur. What is cwnd (in MSS) at the start of transmission round 6?
The answer is a number – work it out, then open the solution.
Show answer and explanation
Explanation
Rounds 1–4: 1, 2, 4, 8 (slow start to ssthresh). Then congestion avoidance adds 1 per round: 9, 10. So round 6 starts with 10.
cwnd starts at 1 MSS with ssthresh 16 MSS. A timeout occurs at the end of round 6. What are the cwnd values at the start of rounds 1–10?
Show answer and explanation
Explanation
Slow start to 16, then 17 in congestion avoidance. The timeout at cwnd 17 sets ssthresh = 8 and cwnd = 1, and slow start resumes: 1, 2, 4, 8 (switching to +1 after reaching 8).
Why the other options are wrong
- AThat is Reno's reaction to duplicate ACKs, not a timeout.
- CAbove ssthresh growth is linear.
- DSlow start continues up to the new ssthresh of 8.