Packet drops and queue length upon bandwidth limiting in PF

[Ashish Awasthi]

Hi friends,

I am a relative newbie, so please don't flame me if my question doesn't make
sense.

In a network experiment to determine appropriate length of router buffers, I
am using pfctl on FreeBSD 5.3 to limit the bandwidth to 100 Mbps on a 1 Gig
link and limit the queue to 240 packets, and I use iperf for sending out
data. Connection is maintained between two routers running FreeBSD 5.3,
connected by a 1 Gig link. I monitor on sender the pfctl and iperf
statisitcs.

As I see the iperf throughput go down from 94 Mbps to 50 Mbps and then rise
again in accordance with the classic sawtooth curve of TCP, it is clear that
there must have been a packet drop, but "pfctl -s -queue -v -v" at the
sender shows 0 losses and 0 drops. Moreover, the queue length as reported
never overflows. Even netstat shows 0 retransmissions!

I tried this with queue lengths of 50, 100, 240, 10 and 5. Only when queue
length is on the order of 5 or 10 do I see packet drops in pfctl report (and
also retransmissions in the netstat report); however, since I have limited
the bandwidth and the outgoing traffic is shaped by this limitation, it is
clear that there must be some packet losses in other cases as well.

So, I tend to think that some other queueing is occuring apart from the
ALTQ, and drops are occuring there. If so, how can I obtain those
statistics?

Thanks a lot for your help!

Ashish