Analysis of the Dynamics of TCP Congestion Avoidance:

John Baras

University of Maryland College Park


We consider the behavior of generalized TCP congestion avoidance when subject to randomized congestion feedback, such as RED and ECN. The window distribution of individual flows under a variable packet loss/marking probability is established and studied to demonstrate the desirability of specifying a less drastic reduction in the window size in response to ECN-basedcongestion feedback. A fixed-point based analysis is presented to derive the mean TCP window sizes and the mean queue occupancy when multiple such generalized TCP flows interact with a single bottleneck queue performing randomized congestion feedback. Recommendations on the use of memory (use of weighted averages of the past queue occupancy) and on the use of "drop-biasing" (minimum separation betwen consecutive drops) are provided to reduce the variability of the queue occupancy. Finally, the interaction of TCP congestion avoidance with randomized feedback is related to a framework for global optimization of network costs. Such a relation is used to provide the theory behind the shape of the marking (dropping) functions used in a randomized feedback scheme.

(* Joint work with Archan Misra and Teun Ott of Telcordia Research)

[John Baras]
[University of Maryland College Park]