[afnog] Bandwidth measurement

[Brian Candler]

On Wed, Nov 24, 2004 at 07:31:29PM +0200, Antonio Godinho wrote:
> Does anyone in this list have a huge bandwidth pipe to the net? I would like 
> to run some bandwidth tests on my link but I need a server on the other side 
> connected to the Internet backbone (US, Europe).

If all you're going to do is ping that device, then often a good choice is
the router at the far end of the link, or another router within your
upstream network's ISP (unless they have configured ICMP rate limiting)

If the bandwidth of the pipe you're testing is not huge, then most any
remote server will do.

I find this simple test very useful:

# ping -c100 -s1472 x.x.x.x

That sends packets of size 1500 bytes (20 bytes IP header, 8 bytes ICMP
header, 1472 bytes payload).

Now, ping the far end of your link, and take the *lowest* round-trip time
you get. You can use this to estimate the bandwidth of the link between here
and there. You assume the packet is transmitted down the link at X bits per
second, when the whole packet has been received the reply is immediately
sent back down the link at X bits per second.

Example from my home DSL line:

--- 212.74.111.194 ping statistics ---
100 packets transmitted, 100 packets received, 0% packet loss
round-trip min/avg/max/stddev = 75.200/79.688/91.675/3.521 ms

Now, 1500 bytes = 12000 bits. The time is for a round-trip.

One-way time = 75.200 / 2 = 37.6 milliseconds = 0.0376 seconds

Bits per second = 12000 / 0.0376 = 319148

Actually, the link is asymmetrical, and the ISP claims it is 512Kbps down
and 224Kbps up.

So, the round-trip time I would expect to see is
12000/(512*1024)+12000/(224*1024)
= .0752040317 seconds

which matches, very closely, the time observed. So I believe my ISP is
telling the truth.

The variation in the round-trip time gives you an indication of congestion
on the link. An uncongested link will have min.time = avg.time = max.time

And, of course, you will expect to see 0% packet loss. Anything higher could
indicate bit errors, duplex mismatches, or (with big variance in RTT) severe
congestion.

Regards,

Brian.