LARTC - Jul 2004 - Using Token Bucket Filter to simulate a low bandwidth radio link

Hello, 

I am attempting to use the LARTC traffic control to simulate a radio link that 
has variable bandwidth and availability. The basic bandwidth could be as low 
as 500 bits/sec but will generally be about 4000 bits/sec. If the simulated 
radio link is unavailable (zero bandwidth) then packets should be queued 
until a link is re-established. 

i.e. Initial bandwidth is 5000bits/sec
then to 0 bits/sec
and back to 5000bits/sec

However, I am having several different problems

Problem 1:
Setting a very low bandwidth tbf - if the rate is less than my ethernet MTU 
size then packets appear to just get thrown away

Problem 2:
Preventing packets being sent instantaneously then arrive at the tbf (because 
sufficient tokens are available in the bucket?) I have tried to set the burst 
and minburst parameters but packets still appear to be transmitted too soon 
(e.g. a 5000bit packet should be delayed for 1 second on a 5000bit link)

Problem 3:
Is it possible to stop the flow of data when the simulated link is unavailable 
- whilst still keeping a queue of packets


The tc command line I am using is
tc qdisc add dev eth0 root tbf rate 5kbit burst 0.01kbit limit 50kbit peakrate 
5kbit minburst 500

and to simulate a temporary link loss I use
tc qdisc change dev eth0 root tbf rate 0.001kbit burst 0.01kbit limit 50kbit 
peakrate 5kbit minburst 500 


Any help would be much appreciated.  
-- 
Nick Howden -  Senior IT Analyst
QinetiQ Trusted Information Management
Woodward Building, Room B009
Malvern Technology Park, WR14 3PS
Telephone 01684 895566, Fax 896660

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Nick Howden wrote:
>Hello, 
>
>I am attempting to use the LARTC traffic control to simulate a radio link
that
>has variable bandwidth and availability. The basic bandwidth could be as low
>as 500 bits/sec but will generally be about 4000 bits/sec. If the simulated 
>radio link is unavailable (zero bandwidth) then packets should be queued 
>until a link is re-established. 
>  
>
Yeah, I think you are going to find it hard to simulate equivalent 
latency as well?

I seem to remember a kernel module for slowing down a link for testing?  
Can''t remember the details, but I saw it in my 2.6 kernel somewhere.  
Perhaps check the docs on that.

For my own custom apps I simulated a satellite link simply by writing a 
small proxy program for my particular protocol (in perl and again in 
VB).  It queued packets and released them after a set time period at a 
set rate.  If you are only testing a simple app then this is another way 
to go

Ed W
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> I seem to remember a kernel module for slowing down a link for testing? 
> Can''t remember the details, but I saw it in my 2.6 kernel
somewhere.  Perhaps
> check the docs on that.
Yes, it''s Stephen''s sch_delay. Both 2.4 and 2.6 versions
available.
Just run the 2.4.26 or 2.6.7 and last version of iproute2.
Then: tc qdisc add dev eth0 root delay limit 100000 latency 3000ms
> For my own custom apps I simulated a satellite link simply by writing a
small
> proxy program for my particular protocol (in perl and again in VB).  It 
> queued packets and released them after a set time period at a set rate.  If
> you are only testing a simple app then this is another way to go
>
> Ed W
> _______________________________________________
> LARTC mailing list / LARTC@mailman.ds9a.nl
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>
---
Catalin(ux aka Dino) BOIE
catab at deuroconsult.ro
http://kernel.umbrella.ro/
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<P><BR></P><P><FONT
FACE="Monospace,Courier">!!Hello,<BR></FONT><BR><FONT
FACE="Monospace,Courier">!!I am attempting to use the LARTC traffic
control to simulate a radio link that<BR>!!has variable bandwidth and
availability. The basic bandwidth could be as low<BR>!!as 500 bits/sec but
will generally be about 4000 bits/sec. If the simulated<BR>!!radio link is
unavailable (zero bandwidth) then packets should be queued<BR>!!until a
link is re-established.<BR></FONT><BR><FONT
FACE="Monospace,Courier">!!i.e. Initial bandwidth is
5000bits/sec<BR>!!then to 0 bits/sec<BR>!!and back to
5000bits/sec<BR></FONT><BR><FONT
FACE="Monospace,Courier">!!However, I am having several different
problems<BR></FONT><BR><FONT
FACE="Monospace,Courier">!!Problem 1:<BR>!!Setting a very low
bandwidth tbf - if the rate is less than my ethernet MTU<BR>!!size then
packets appear to just get thrown
away<BR></FONT></P><P>Hi ,</P><P>the module
for simulating delay is sch_delay .</P><P>tc qdisc add dev eth0 root
delay latency 300ms rate
100mbit</P><P>regards,</P><P>U.SivaKumar,</P><P>HCL
Infosystems
Limited.</P><P>&nbsp;</P><P>&nbsp;</P><P>&nbsp;</P><P></P>_______________________________________________
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