[LARTC] Using Token Bucket Filter to simulate a low bandwidth radio link

[Nick Howden]

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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