# Analysis of the Dynamics of TCP Congestion Avoidance:

##
John Baras

### University of Maryland College Park

### Résumé:

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)