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Date:      Fri, 30 Mar 2007 02:14:42 -0700
From:      "'Luigi Rizzo'" <rizzo@icir.org>
To:        Dave Raven <dave@raven.za.net>
Cc:        freebsd-ipfw@freebsd.org
Subject:   Re: Using "delay" to emulate a satellite link
Message-ID:  <20070330021442.A77652@xorpc.icir.org>
In-Reply-To: <01a501c772ab$1dcbd460$59637d20$@za.net>; from dave@raven.za.net on Fri, Mar 30, 2007 at 11:09:07AM %2B0200
References:  <015d01c77297$953ba250$bfb2e6f0$@za.net> <20070330005916.A76128@xorpc.icir.org> <01a501c772ab$1dcbd460$59637d20$@za.net>

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On Fri, Mar 30, 2007 at 11:09:07AM +0200, Dave Raven wrote:
> Hi Luigi,
> 	Thanks for the reply -- these are my related send/recv space
> settings on all of the boxes involved --
> 
> net.local.stream.sendspace: 65535
> net.local.stream.recvspace: 65535
> net.local.dgram.recvspace: 4096
> net.inet.tcp.sendspace: 65535
> net.inet.tcp.recvspace: 65535
> net.inet.udp.recvspace: 65535
> net.inet.raw.recvspace: 8192
> 
> What I have done is put 3 freebsd units plugged into eath other with
> crossover cables, like so --
> 
> BOX 1 ----- BOX 2 ----- BOX 3
> 
> Box 3 has a 500meg /dev/urandom text file on a webserver. Box 1 wget's that
> file. Box 2 has the ipfw delay setup. If I ping from BOX 1 to BOX 3 I get
> +/-1000ms of delay. When I download I get less than half of the link speed
> (1meg limited by box 2).

which is fine - you get 65kbytes/1000ms = 512kbit/s

> The latency directly affects the throughput - but my question is this -- If
> I download something from a web server on the internet that I have 500ms of
> delay to, I can get 1meg (on a 1meg link). When I emulate that delay with
> dummynet I can't -- is there a difference in the type of delay experienced?

well, it's twice as much in your case - your ping time is 1000ms instead
of 500ms

cheers
luigi

> Really anyone can - most people have delay of at least 300-500ms to remote
> webservers (e.g. from me in Africa to America); but it doesn't hamper
> download speed?
> 
> Thanks again
> Dave 
> 
> 
> 
> -----Original Message-----
> From: Luigi Rizzo [mailto:rizzo@icir.org] 
> Sent: Friday, March 30, 2007 9:59 AM
> To: Dave Raven
> Cc: freebsd-ipfw@freebsd.org
> Subject: Re: Using "delay" to emulate a satellite link
> 
> On Fri, Mar 30, 2007 at 08:49:19AM +0200, Dave Raven wrote:
> > Hi all,
> > 	I've been looking at the ipfw (dummynet) ability to do delay and
> > have a few questions - I hope this is the right list. I want to simulate a
> > 1000ms RTT on a satellite link. To do that I've created an inbound and
> > outbound pipe and given each 1mb and 500ms of delay.
> > 
> > However, I'm unable to get anywhere near 1mb of throughput on it until I
> > drop the delay. I believe I understand the slowdown due to the latency,
> but
> > my question is this - an http download through a 500ms "emulated" link
> > that's running 1 mb can't get 1mb, yet if I download over the internet on
> a
> > site that's pinging 500ms, it goes 1mb 
> > 
> > Whats the difference between dummynet delay and real life distance delay?
> 
> first make sure you are not comparing apples and oranges.
> what sender and receiver are you using to get 1Mbit/s on a 500ms link ?
> and, are you sure that if you ping from the source to the destination
> you are using for your tests with dummynet you get the delay
> you are expecting (1000ms as you configured it, and not 2000 ?)
> 
> 500ms of delay on each pipe give at least 1000ms total
> delay (assuming you have not misconfigured your dummynet box).
> In order to fill the pipe you need at least 1Mbit worth of data
> in the socket buffer/tcp window - the default on FreeBSD is
> 32kbytes sending, 64Kbytes receiving, so you won't be
> able to achieve that unless you increase these two sysctls:
> 
> 	net.inet.tcp.sendspace: 32768
> 	net.inet.tcp.recvspace: 65536
> 
> if you draw the bw vs delay that you achieve on your connection
> you will likely find that either the limit is your socket
> buffers or a misconfigured ipfw which results in twice the delay
> 
> 	cheers
> 	luigi



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