One desirable characteristics of any resource allocation (or differentiation) mechanism is that if many aggregates, each with its own reservation, are merged with other aggregates of the same class, each aggregate should get its reserved bandwidth and a fair share of the excess bandwidth. In particular, the performance of an aggregate should not be adversely affected by other aggregates and their congestion sensitivity. TCP flows are congestion sensitive while UDP flows are congestion insensitive in the sense that TCP flows reduce their traffic if any packets are lost. The goal of this study is to see if TCP flow aggregates will be punished for their good behavior in the presence of competing UDP flow aggregates in the same assured forwarding class. We identify several factors that affect the performance in the mixed environments and quantify their effects using a full factorial design of experiment methodology.

The factors that we studied are number of drop precendences required (one, two, or three), percentage of reserved (highest drop precedence) traffic, buffer management (Tail drop or Random Early Drop with different parameters), traffic types (TCP aggregates, UDP aggregates).

This study has four key results.

First, three drop precedences (green, yellow, and red) help clearly distinguish between congestion sensitive and insensitive flows.

Second, the reserved bandwidth should not be overbooked, that is, the sum should be less than the bottleneck link capacity. If the network operates close to its capacity, three levels of drop precedence are redundant as there is not much excess bandwidth to be shared.

Third, the excess congestion sensitive (TCP) packets should be marked as yellow while the excess congenstion insensitive (UDP) packets should be marked as red.

Fourth, the RED parameters have significant effect on the performance. The optimal setting of RED parameters is an area for further research.

This is a revised and comprehensive version of our previous study presented at the March and July 1999 IETF Meetings. The ps and pdf versions of this document with all the figures are available at: http://www.cse.wustl.edu/~jain/ietf/afstdy.htm

Complete contribution with figures in Adobe Acrobat 3.0 Format (683,184 bytes), Plain text W/O figures (38,059 bytes)

For a previous version of this document and the corresponding IETF presentation, please see