dummynet dropping TCP packets

[junfeng at ece.gatech.edu]

Hi! all,

I am using dummynet to simulate a network-connection with 65ms propagation
delay. Dummynet is installed on a workstation with two network-cards, and
is configured as a bridge (no IP for these interfaces). One network-card
is connected to a Video Server (IP:192.168.200.100), which sends out 2Mbps
video stream. The other network card is connected to a video client
(IP:192.168.200.50), which receives video packets and does ARQ if there
are packet loss. The feedback message is TCP.

We simulate random loss of video packet at server end. And use dummynet to
control the propagation delay between server and client. Here are the
commands I used to control dummynet


HZ=1000

ipfw flush
ipfw add pipe 100 ip from any to any
ipfw pipe 100 config delay 65ms

And I observed that some TCP packets are dropped when they are sent from
client to server. And then the operating system needs to retransmit these
lost TCP packets after TCP time out, which causes extra delay in ARQ.


I ever tried to seperate TCP from UDP packets and I got same result. ipfw
add pipe 100 TCP from any to any
ipfw add pipe 200 not TCP from any to any
ipfw pipe 100 config delay 65ms
ipfw pipe 200 config delay 65ms

Also, I tried to use the bi-direction pipes, but seems that all traffic
going through  dummynet is in-bound traffic, so there is no packet goes
through second pipe (pipe 200).
ipfw add pipe 100 ip from any to any in
ipfw add pipe 200  ip from any to any out
ipfw pipe 100 config delay 65ms
ipfw pipe 200 config delay 65ms

Is there any error in my configuration?

Thanks!

Junfeng