Dummynet low bandwidth simulation

Ivan Voras ivoras at fer.hr
Mon Apr 26 17:25:38 PDT 2004

Luigi Rizzo wrote:
> are you sure you aren't running out of mbufs ?
> netstat -m should tell you.

No, its not mbufs (such messages are shown on the console I believe and 
I haven't seen them; also, running netstat -m states there's plenty of 
buffers remaining).

> Additionally, note that the 250ms of delay are probably way too much
> for your config (you are adding 250ms each way, which makes a 500ms RTT,
> not accounting for transmission times -- i think normal values here
> are more like 150-200ms rtt), and possibly irrelevant given how
> large your queues are -- a full-size packet is 12000 bits or 200ms,
> you can have up to 20 queued...

Oh sorry, I pasted from a wrong shell script. I've been experimenting 
with different values and am considering settling at 75ms, queue of 5. 
I'm still not sure: how does the number of buckets influence the operation?

I don't think it's clear to me how would such large queues produce my 
errors (connection reset by peer & broken pipe). Is this reasoning 
correct: in the above case, 200ms*20 = 4s, so in the worst case the 
packet from the end of a queue will travel 4s until it reaches its 
destination. Is 4s enough for a timeout of some sort?

I've tried running on a different machine, running 4.9-release, and 
there I also get this error (in large numbers):
Error: socket: address is unavailable.: Can't assign requested address

C isn't that hard: void (*(*f[])())() defines f as an array of
unspecified size, of pointers to functions that return pointers to
functions that return void.

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