Implementing a Simple Congestion Control Algorithm in Python

Congestion control is a fundamental mechanism in computer networks that prevents a network from becoming overwhelmed with data. When the network is congested, packet loss and increased latency occur. Your task is to implement a simplified congestion control algorithm that dynamically adjusts the rate at which data is sent to avoid overwhelming the network. This is crucial for maintaining network stability and ensuring efficient data transfer.

Problem Description

You need to create a Python class that simulates a sender's congestion control mechanism. This mechanism will be responsible for deciding how much data to send at any given time based on simulated network feedback.

Key Requirements:

1. Congestion Window (cwnd): Maintain a cwnd variable, representing the maximum amount of unacknowledged data that can be in transit. This window will grow and shrink.
2. Slow Start: Initially, the cwnd should increase exponentially (doubling) for each successful acknowledgment received until it reaches a defined slow_start_threshold.
3. Congestion Avoidance: Once cwnd reaches or exceeds slow_start_threshold, it should increase linearly (additively) by a small amount (e.g., 1 segment) for each acknowledgment received.
4. Congestion Detection (Packet Loss): When a simulated packet loss is detected (indicated by a timeout event or duplicate acknowledgments, though for simplicity, we'll focus on a timeout), the slow_start_threshold should be set to half of the current cwnd, and the cwnd should be reset to a small initial value (e.g., 1 segment).
5. Timeouts: The system should have a mechanism to detect timeouts. For this challenge, a timeout can be triggered after a certain number of unacknowledged packets have been sent or after a certain duration (which we'll simplify by using a packet count for this exercise).
6. Acknowledgments (ACKs): The algorithm should react to received ACKs. Each ACK signifies that a segment of data has been successfully delivered.

Expected Behavior:

• The cwnd will start small and grow rapidly during slow start.
• The cwnd will grow more slowly during congestion avoidance.
• Upon detecting congestion (timeout), the cwnd will be drastically reduced, and the slow_start_threshold will be updated.
• The algorithm should be able to handle multiple rounds of slow start, congestion avoidance, and congestion detection.

Important Edge Cases:

• Initial state: cwnd and slow_start_threshold must be initialized correctly.
• Handling timeouts when cwnd is very small.

Examples

Example 1: Basic Growth and a Single Timeout

Let's assume:

• initial_cwnd = 1
• ssthresh = 16
• packet_size = 1 (for simplicity, each unit in cwnd represents one packet)
• timeout_threshold = 10 (simulating a timeout after 10 unacknowledged packets are "in flight" which is not a true timeout but a simplification for this exercise. A real timeout would be based on RTT estimation).

Initial State: cwnd = 1 ssthresh = 16 packets_in_flight = 0

Simulated Events:

1. Send 1 packet: packets_in_flight becomes 1.
2. Receive ACK: cwnd increases (Slow Start: cwnd *= 2 because cwnd < ssthresh). packets_in_flight becomes 0.
   • cwnd = 2
   • packets_in_flight = 0
3. Send 2 packets: packets_in_flight becomes 2.
4. Receive ACK: cwnd increases. packets_in_flight becomes 1.
   • cwnd = 4
   • packets_in_flight = 1
5. Receive ACK: cwnd increases. packets_in_flight becomes 0.
   • cwnd = 8
   • packets_in_flight = 0
6. Send 8 packets: packets_in_flight becomes 8.
7. Receive ACK: cwnd increases. packets_in_flight becomes 7.
   • cwnd = 16
   • packets_in_flight = 7
8. Receive ACK: cwnd increases. packets_in_flight becomes 6.
   • cwnd = 17 (Now in Congestion Avoidance as cwnd >= ssthresh)
   • packets_in_flight = 6
9. Receive ACK: cwnd increases (Congestion Avoidance: cwnd += 1). packets_in_flight becomes 5.
   • cwnd = 18
   • packets_in_flight = 5
10. Simulated Timeout: timeout_threshold is reached with 5 packets in flight.
    • ssthresh is updated: ssthresh = floor(cwnd / 2) = floor(18 / 2) = 9
    • cwnd is reset: cwnd = 1
    • packets_in_flight is reset: packets_in_flight = 0

Output for Example 1 (State after each relevant event):

• Initial: cwnd=1, ssthresh=16
• After 1st ACK: cwnd=2, ssthresh=16
• After 2nd ACK: cwnd=4, ssthresh=16
• After 3rd ACK: cwnd=8, ssthresh=16
• After 4th ACK: cwnd=16, ssthresh=16
• After 5th ACK: cwnd=17, ssthresh=16
• After 6th ACK: cwnd=18, ssthresh=16
• After Timeout: cwnd=1, ssthresh=9

Example 2: Multiple Cycles

Assume same parameters as Example 1.

Initial State: cwnd = 1 ssthresh = 16 packets_in_flight = 0

Simulated Events:

1. ... (Events as in Example 1, leading to cwnd=18, ssthresh=9 after timeout)
2. Send 1 packet: packets_in_flight becomes 1.
3. Receive ACK: cwnd increases (Slow Start: cwnd *= 2 because cwnd < ssthresh). packets_in_flight becomes 0.
   • cwnd = 2
   • packets_in_flight = 0
4. Send 2 packets: packets_in_flight becomes 2.
5. Receive ACK: cwnd increases. packets_in_flight becomes 1.
   • cwnd = 4
   • packets_in_flight = 1
6. Receive ACK: cwnd increases. packets_in_flight becomes 0.
   • cwnd = 8
   • packets_in_flight = 0
7. Send 8 packets: packets_in_flight becomes 8.
8. Receive ACK: cwnd increases. packets_in_flight becomes 7.
   • cwnd = 9 (Now in Congestion Avoidance as cwnd >= ssthresh)
   • packets_in_flight = 7
9. Receive ACK: cwnd increases (Congestion Avoidance: cwnd += 1). packets_in_flight becomes 6.
   • cwnd = 10
   • packets_in_flight = 6
10. Receive ACK: cwnd increases. packets_in_flight becomes 5.
    • cwnd = 11
    • packets_in_flight = 5
11. Receive ACK: cwnd increases. packets_in_flight becomes 4.
    • cwnd = 12
    • packets_in_flight = 4
12. Receive ACK: cwnd increases. packets_in_flight becomes 3.
    • cwnd = 13
    • packets_in_flight = 3
13. Receive ACK: cwnd increases. becomes 2.

Output for Example 2 (Selected states):

• After 1st timeout: cwnd=1, ssthresh=9
• After 1st ACK of 2nd cycle: cwnd=2, ssthresh=9
• After 2nd ACK of 2nd cycle: cwnd=4, ssthresh=9
• After 3rd ACK of 2nd cycle: cwnd=8, ssthresh=9
• After 4th ACK of 2nd cycle: cwnd=9, ssthresh=9
• After 15th ACK of 2nd cycle: cwnd=16, ssthresh=9
• After 2nd timeout: cwnd=1, ssthresh=8

Constraints

• The congestion window (cwnd) and slow start threshold (ssthresh) should be integers.
• Packet size is considered to be 1 unit for simplicity.
• The simulation of a "timeout" will be simplified: a timeout occurs when packets_in_flight reaches a predefined timeout_threshold before any ACKs are received for those packets. In a real system, this would be based on Round-Trip Time (RTT) estimation.
• The slow_start_threshold must be an integer (use math.floor if division results in a float).
• Initial cwnd will always be 1.
• ssthresh will be provided as an initial parameter.
• The maximum value for cwnd is not explicitly limited, but it should reflect the algorithm's behavior.

Notes

• You will need to implement a class, let's call it CongestionController, with methods to handle incoming events like sending data and receiving acknowledgments.
• Consider what state variables your class needs to maintain (cwnd, ssthresh, packets_in_flight, etc.).
• Think about the sequence of operations when an ACK is received.
• Think about the conditions and actions for a timeout.
• For this challenge, we are not simulating actual network latency or RTT. The timeout_threshold is a proxy for a timeout event.
• The goal is to correctly implement the logic for Slow Start, Congestion Avoidance, and the reaction to a simulated timeout.
• A useful helper function might be one that simulates sending a certain number of packets and checks if a timeout has occurred before ACKs are received.