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.cis.ohio-state.edu/~jain/ietf/afstdy.htm
Complete contribution with figures in PostScript (759,637 bytes), Adobe Acrobat 3.0 Format (676,493 bytes), Plain text W/O figures (38,059 bytes)
For a previous version of this document and the corresponding IETF presentation, please see http://www.cis.ohio-state.edu/~durresi/ietf/dpstd2.htm.