# Q6C · Open-loop and closed-loop control

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/tee-paper-final/q6c

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/tee-paper-final--q6c/

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](https://deep-learning-study-room.vercel.app/cn/fig/tee-congestion.c27c2303bb.svg)

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.

| Approach | Policy / mechanism | How it helps |
| --- | --- | --- |
| Open loop: prevent congestion without runtime congestion feedback | Retransmission policy | Carefully chosen timers/selective retransmission avoid needless extra traffic |
| Open loop | Window policy | Limit outstanding data and avoid excessive retransmission after loss |
| Open loop | Acknowledgement policy | Avoid unnecessarily frequent ACK traffic while retaining correct recovery |
| Open loop | Admission policy | Refuse new load when reserved capacity is insufficient |
| Closed loop: measure and react to congestion | Backpressure | A congested node tells upstream nodes to reduce/stop sending, possibly propagating hop by hop |
| Closed loop | Choke/control signalling | A congested node sends explicit feedback to the source |
| Closed loop | Implicit signalling | A sender infers congestion from loss/delay and changes its sending behaviour |
| Closed loop | Explicit marking | Network 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.

#### Practical example

A router’s output link can be overloaded even while the destination application reads quickly. Increasing receiver memory alone will not remove that network bottleneck.

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