[e2e] How to create simulation scenario

[Zhang Miao]

Hi, 
I have some questions on how to create simulation scenario.
Nowadays, many papers use NS to validate the new designed algorithms for end2end
congestion control. After reading some papers, I see there are only 2 typical
topologies.
One is:(Topo1)
   s1                  k1
      \              /
       \            /
   s2--- r1 ---- r2 ---k2
    .  /            \   .
    . /              \  .
   sn                  kn

The other is:(Topo2)
 
   s1                    k1
     \                  /
      \                /
       r1 --- r2 ----r3
      /       | |      \
     /        | |       \
   s2        k2 s3       k3

My question are as follows:
1. How many pairs should be used in topo1?
   Ususally, there are only tens of flows used in simulation. Is it enough to 
simulate the realistic scenario? If only a few flows are used, the behavior of
individual flow will have much impact on the result.
2. How to set the bandwidth between r1 and r2 in topo1?
   Some paper used 1.5Mb. I doubt where this value came from. The bandwidth and the 
number of flows used in simulation determine the max cwnd for each TCP flows. Max 
cwnd will have impact on the duration time of slow start process. For example, if the 
bandwidth is relatively high, the TCP flow with finite data(say, 40KB) may terminate 
before congestion avoidance is started.
3. How to set the upper user scenario?
   Probably it is the most difficult in create a simulation. Usually, a mixture 
of permanent flows and transient flows is used. But how to design the exact scenario?
What is the relation between "elephants" and "mice"(as Steven Low mentioned early)? 
If there are too many mice, the traffic will tend to be more bursty.
4. How to evalute the interaction between multiple routers and bottleneck links?
   In topo1, only one bottleneck link is used. In topo2, 2 bottleneck links are used,
while only a few flows are introduced. Till now, I see little analysis on the 
interference between several routers. Could this impact be neglect?

Thanks.



*****************************************************************
*    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                        *
*****************************************************************