Consider the following statements regarding the slow start phase of the TCP…

2018

Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that cwnd stands for the TCP congestion window and MSS denotes the Maximum Segment Size.

(i) The cwnd increases by 2 MSS on every successful acknowledgment.

(ii) The cwnd approximately doubles on every successful acknowledgement.

(iii) The cwnd increases by 1 MSS every round trip time.

(iv) The cwnd approximately doubles every round trip time.

Which one of the following is correct?

  1. A.

    Only (ii) and (iii) are true

  2. B.

    Only (i) and (iii) are true

  3. C.

    Only (iv) is true

  4. D.

    Only (i) and (iv) are true

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Correct answer: C

Short answer: Only the statement that the congestion window approximately doubles every round trip time is correct.

Reasoning:

  • Statement (i) — "cwnd increases by 2 MSS on every successful acknowledgment": False. The usual slow start rule is to increase cwnd by about 1 MSS for each ACK, not 2 MSS.

  • Statement (ii) — "cwnd approximately doubles on every successful acknowledgment": False. Doubling does not occur per ACK; per-ACK increases accumulate over the RTT to produce the doubling effect.

  • Statement (iii) — "cwnd increases by 1 MSS every round trip time": False. Slow start increases cwnd by about 1 MSS per ACK, so the total increase per RTT is larger (roughly equal to the current cwnd), not a single MSS.

  • Statement (iv) — "cwnd approximately doubles every round trip time": True. If cwnd = W bytes, the sender transmits about W/MSS segments in one RTT and receives roughly W/MSS ACKs; each ACK increases cwnd by ~1 MSS, so total increase ≈ (W/MSS)×MSS = W, making cwnd grow from W to about 2W in one RTT (assumes one ACK per segment and no losses or delayed-ACK effects).

Note: Delayed ACKs (e.g., one ACK per two segments) or other protocol behaviors can slow the growth, so the doubling per RTT is an approximate result under typical assumptions.

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