D-M Chiu, R. Jain, “Analysis of the Increase and Decrease Algorithms for Congestion Avoidance in Computer Networks,” *Computer Networks and ISDN Systems*, 17 (1989), pp 1-14. [PDF]

## Summary

“Low delay” and “high throughput”, two main operating objectives of a computer network, are frequently hampered by congestion of packets. Once there is a congestion, it takes considerable amount of time and network resource to get back to equilibrium (in this case, to *not-congested* state is probably more proper). Congestion avoidance is a proactive measure that strives for avoiding a congestion *just *before it occurs, thus improving network utilization and performance. This paper presents mathematical analyses of several existing increase/decrease *linear *congestion avoidance algorithms and compare them using multiple performance metrics in a simplified network model.

## Model

The authors consider variations of *decentralized* increase/decrease congestion avoidance algorithms each using “binary feedback scheme”. In a binary feedback control mechanism, network resources send back a *one* for overloaded state and a *zero* otherwise to the controllers. Basic assumptions of their model are:

- Users sharing the same bottleneck will receive the same feedback,
- Feedback and control loop for all users are synchronous (i.e., there take actions in lockstep), and
- There is only one bottleneck resource in the network.

Once users/end hosts receive feedback from the congested network resource, they take measures based on the following criteria:

*Efficiency*: Measured in terms of the utilization of resource; both overload and underload are undesirable.*Fairness*: Each user should take equal (proportional!) share of the bottleneck. Properties of a suitable fairness function can be found in the paper.*Distributedness*: Control requires minimum amount of feedback from the resource under consideration. No central processing is required.*Convergence*: The control scheme should converge within a reasonable level of tolerance.

## Results

Through mathematical reasoning and geometrical representation, the authors compare AIAD, AIMD, MIAD, and MIMD (A: additive, D: decrease, I: increase, M: multiplicative) control schemes and prove that only AIMD can optimally converge to both efficiency and fairness in a distributed manner with minimal feedback. They also find out the operating region and control parameters for the control scheme.

Non-linear controls are not considered in this paper citing the sake of practicality and modeling issues. While the authors do agree that non-linear control gives more flexibility, they argue that too much sensitivity to changing conditions in non-linear schemes can result in instability.

## Critique

At the end of the paper, the authors themselves raise several questions on different possible scenario that can affect AIMD in multiple ways; e.g., *how does delayed feedback affect the control*, *what is the marginal utility of increased bits of feedback*, *is it worthwhile to guess the current number of users*, and *what is the impact of asynchronous operation*. However, they left all of them as future work! I wonder what are the correct (if any) answers to these questions.

The paper itself is a wonderful read with fairly easy, understandable math with explanatory geometric representations. However, it is not clear how the authors came up with that massive logarithmic expression while calculating the conditions for optimal convergence to efficiency.

As far as I understand, CUBIC TCP for high-speed network uses non-linear model (cubic) function for congestion avoidance mechanism. Discussion/info on other variations of non-linear models is very much welcome.