[e2e] Question on traffic patterns for AQM design

[Zhang Miao]

Hi, Lynne
   Some more for comment.
   My question comes from the comparison between RED (proposed by Sally Floyd)
and PI controller(proposed by C.V.Hollet, etal.). One shortcoming of RED is that
as the number of flows increases, the average queue length is also increase.
With introduction of integral factor, PI controller eliminates the "steady-state
error" in RED. Unfortunately, there is no free meal. PI controller responses much
slower than RED. This tradeoff has been analyzed in books on control theory(such as
"Feedback Control of Dynamic Systems" by Gene F. Franklin, etal.)
   Whatever RED or PI, some time is needed for the end system to response. If we
don't consider the time for the controller to adapt to the changes in traffic,
the shortest response time is the RTT. Howerer, if the Internet traffic changes
very dramatically in very short time (approximate RTT or even shorter), it is 
nearly impossible for end system and for controller at the gateway to response.
   Here I have another question on the understanding of queue length. Many papers
use average queue length as a metric. When traffic changes frequently, the queue
length oscillates. Is it meaningful to have a little average queue length while the 
queue length ocillates between zero and maximum queue length(ie., buffer size)?
This will introduce delay jitter which is harmful for some applications.
   Anyway, using AQM is better than only using droptail. With more information on
the traffic patterns, we can make better tradeoff on AQM design.

>The upshot - Dr. Cerf himself has commented that unless one reduces retransmission, 
>congestion events will overwhelm. Timescales for active queue management must be fine 
>grained, not coarse grained. TCP at the fine timescale integrated throughout the 
>Internet is the only solution.

Using fine grained timescales in TCP can't solve the problem.
It is difficult for TCP to estimate the RTT.
Fine grained timer also adds burden to the end systems.

*****************************************************************
*    Zhang Miao                                                 *
*    Ph.D candidate,Department of Computer Science & Technology * 
*    Tsinghua University,Beijing,China(100084)                  *
*    Tel: (8610)-62785822                                       *
*    Email: zm at csnet1.cs.tsinghua.edu.cn                        *
*    Web: http://netlab.cs.tsinghua.edu.cn/~zm (domestic only ) *
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