Computer Networks · TEE · papers

Q1(d) · TCP congestion control

2023-01-27 · AI-only Sem V

Exact historical variant. Current-syllabus questions solved; excluded topics and source limitations stated.

Q1(d). What is congestion control in TCP and how is it implemented? [5 marks]

Answer

A TCP sender is limited by receiver capacity and network capacity. The receive window rwnd comes from the receiver; the congestion window cwnd is maintained by the sender. Roughly, allowed outstanding data is min(rwnd,cwnd), with the existing bytes in flight subtracted before sending new bytes.

\mathrm{new\ allowance}=\max(0,\min(rwnd,cwnd)-\mathrm{bytes\ in\ flight})LaTeX: \mathrm{new\ allowance}=\max(0,\min(rwnd,cwnd)-\mathrm{bytes\ in\ flight})
  1. In classic slow start, cwnd grows rapidly as ACKs arrive; under a simplified round-based model it approximately doubles each RTT.
  2. At the slow-start threshold, congestion avoidance grows more cautiously; classic additive increase is approximately one MSS per RTT.
  3. Loss/timeout or an explicit congestion signal causes the sender to reduce its sending allowance. Exact recovery behaviour depends on the TCP congestion-control algorithm.
  4. Fast retransmit can infer a missing segment from duplicate ACK evidence. It is distinct from waiting for a retransmission timeout.
  5. The simplified Reno-style outline is supporting detail for the syllabus’s congestion-control topic, not a claim that all modern TCP stacks use an identical curve.