Computer Networks · TEE · papers

Q6C · Open-loop and closed-loop control

Final · 5 December 2025

TEE · Actual TEE paper · all questions · current syllabus 702AI0C023

Actual PYQ · Q6C. Explain open-loop and closed-loop congestion control policy in transmission control protocol. Describe any two policies from each type. [10 marks]

Answer

Congestion occurs when offered traffic overloads network resources such as link capacity, router buffers or processing. Queueing delay grows, packets may be dropped, and excessive retransmission can make the overload worse. Flow control protects a receiver; congestion control protects the network.

Swipe across the diagram to read its labels ↔
Two congestion-control approaches
Diagram: Two congestion-control approaches. Labels: Two congestion-control approaches | Two congestion-control approaches | Open loop: prevent overload | Admission rules and retransmission/window/ACK policies reduce avoidable traffic before congestion | feedback. | Closed loop: observe and signal | Detect queues, loss or explicit congestion marks; return feedback toward the sender. | Change behaviour | Reduce offered load or change resource use. Receiver flow control addresses a different bottleneck.
Open-loop prevention and closed-loop feedback.
ApproachPolicy / mechanismHow it helps
Open loop: prevent congestion without runtime congestion feedbackRetransmission policyCarefully chosen timers/selective retransmission avoid needless extra traffic
Open loopWindow policyLimit outstanding data and avoid excessive retransmission after loss
Open loopAcknowledgement policyAvoid unnecessarily frequent ACK traffic while retaining correct recovery
Open loopAdmission policyRefuse new load when reserved capacity is insufficient
Closed loop: measure and react to congestionBackpressureA congested node tells upstream nodes to reduce/stop sending, possibly propagating hop by hop
Closed loopChoke/control signallingA congested node sends explicit feedback to the source
Closed loopImplicit signallingA sender infers congestion from loss/delay and changes its sending behaviour
Closed loopExplicit markingNetwork marks congestion (for example ECN); endpoints react to returned information

For an answer asking for two of each: explain retransmission and admission policies under open loop, then backpressure and explicit signalling under closed loop, with the mechanism and its effect. These are general congestion-control classifications used in the course; they are not all mandatory algorithms implemented inside TCP. TCP commonly uses end-to-end feedback such as loss or ECN.

Source: Final · 5 December 2025, original question paper page 2. Original marks retained; solution independently authored.