[e2e] Question on traffic patterns for AQM design

[Zhang Miao]

Hi, Wu Wei
  In you mail, the key point is "Persistent Congestion" vs. "Transient Congestion".
When does congestion happen? Congestion happens when the requirement from the users
is beyond the ability of the network. How much will the users require? With the 
best-effort model, the users will require as much as the network can provide (if we
don't consider the limitation of host). The task of congestion control is to 
distribute resource between multiple users. 
  When the number of users changes, the resource should be re-distributed. If the
number of users increases, before the system can react, the old users still use 
their quota as before, now congestion happens. So the congestion is not because
of too many users, but because of the increase of number of users. Congestion 
happens at the moment of transfering from less users to more users.
  Consider the situation of Internet. Can we assume that the number of the users 
(ie., flows) is relative constant(for example, in 5 seconds)? If the number of users
always changes, there will be always congestion. Can we say such congestion is 
Transient Congestion? Then what is Persistent Congestion? The number of users
keeps increasing? 

>Hi,Zhang Miao
>
>Before we design AQM algorithms, we should make clean that what our goals are. 
>Network congestion can be divided into 2 classes according to their time scale:
>persistent congestion, which is typically longer than 1RTT and transient congestion, 
>which is less than 1RTT. AQM targets on detecting and avoiding persistent one, while
>accommodating transient ones. So just think network traffic go through a low-pass 
>filter (critical frenquency is less than 1/RTT), the output of the filter is what 
>we want to control. If AQM controller can adapt itself to track the output 
>traffic, AQM can response well enough then.
>
>Yes, the parameter settings of PI controller cannot work well for all network 
>dynamical conditions, especially when the buffer size is low or the network load 
>is heavy. The problems are the static parameter settings. We believe it is hard to 
>design an AQM algorithm, which can suit for all network conditions with only one 
>set of parameter setting. So the algorithms should be adaptive, adaptive to 
>network dynamics. Adaptive RED validates this point. We are working on an adaptive 
>PI controller recently. Preliminary results show that it can speed up the response
>of PI controller much.   
>
>Best Regards
>Wei Wu
>wuwei at sdp.ee.tsinghua.edu.cn
>|=====================================================================|
>| Wei Wu, Complexity Engineering System Labs (CESL)                   |
>| Electronics Engineering Department, Tsinghua University             |
>| Beijing, P.R.China                                                  | 
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*****************************************************************
*    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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