[ns] Increased loss with # of flows?

[Xidong Deng <xdeng@cse.psu.edu>]

 hi, all:
   I am simulating diffserv using the nortel patch. I use priority
 scheduling over three queues at the core router. The first one is a
 drop-tail queue  with a small buffer, but the highest priority, while the
 other two use rio_c with buffer size of 50. The link speed is 6MB.
   The main observation is on the first queue, I just use the other two
 to generate congestion. (All traffic are UDP CBR) . I found out that the
 loss rate of the first queue will increase dramatically when the # of
 flows in that queue increases, even if the total bandwidth keeps unchanged.
 For example: if I have 1 flow with 1.28Mbps, no loss will occur at the core
 router,but if instead I have 20 flows each with 64Kbps, then there will be a
 lot of packets dropped at the core router.
   If this happened at the edge router, I would understand because multiple
 flows can send their traffic at the same time so that the buffer may overflow.
 But in my case, I have a single queue for that edge to core, and with
 sufficient bandwidth and buffer space, I don't have any packet loss on
 that queue. Since every packet must follow that single queue to reach
 the core, they can't get there at the same time, right? Then why is there
 any difference between one 1.28M flow and 20 64k flows?
   I guessed that it might due to the burst of traffic introduced by the edge
 router, So I modified the script to use multiple queues with WRR on the
 edge-core route, by placing different weights on the queues, I expect an
 interleaving of the packets to reduce the possible burst, but still I got
 many loss at the core router.
   Now I am really very confused. Anybody have an idea?
   Thank you very much and have a good day!

 xdeng@cse.psu.edu