latency Q

[alexs]

I have noticed a wierd (?!?) aspect of the network, weather it swiftel or Telsta (my bet is on the later).

I have a box on the swiftel network and another box on the Telstra Cable network. I do a lot of remote access between these two.

So I keep a mrtg graph of the ping times. I have noticed very good ping times consistanty ( ~30-40ms). But this did not correlate with my experience so I set up a graph of a ping with a load of 1350...... And now I get ping times ~250 - 300ms

This last figure correlates with my experience.

Is there a problem ?
Is this the norm ?

My guess is there is some shaping going on!

Alex

[Berkeley]

I'd have a look at MTU settings. . . what OS are you on?

[alexs]

Originally posted by Berkeley
I'd have a look at MTU settings. . . what OS are you on?

linux -> linux
over a ipsec tunel
mtu is 1460

same results with ping -s 1300 and similiar (but smaller with 1200)

Alex

[DSL_Tech]

Originally posted by alexs
ISo I keep a mrtg graph of the ping times. I have noticed very good ping times consistanty ( ~30-40ms). But this did not correlate with my experience so I set up a graph of a ping with a load of 1350...... And now I get ping times ~250 - 300ms


It would indicate that your line serialisation rate, at some point, is 128kbps.

[alexs]

Originally posted by DSL_Tech
It would indicate that your line serialisation rate, at some point, is 128kbps.

Okay I can accept that seeing as it is a 512/128 adsl connection.

But explain to me why 128Kbit slows down a single ping packet ?

usually I send a ping -c 1 -s 1300 ipaddres

or does it work somthing like 1300*8/128000 sec ?

[DSL_Tech]

Each time a packet arrives at an interface, it must be 'read in' or 'read out' bit by bit. A 128kbps interface can read a 1300 byte packet in (1300*8)/128000 = 81.25 milliseconds. Add protocol overheads etc to get roughly 100ms.

Since a ping packet goes out and comes back the same size, the packet must be read in and out of your modem - x2 = 200ms.

If you subtract the small packet return time of ~30-40ms, then the value for the large packet return matches the calculated value.

[alexs]

QED