[LARTC] Shaping traffic on heavily oversubscribed links?

[Chris Bennett]

Quick answer is: you can't.  You need to know the bandwidth so that you can 
control the queue.

You bring up some interesting theoretical ideas about monitoring the overall 
congestion level and forcing back-offs when you sense that you're reaching 
the current level, though.  That would sort of involve dynamically adjusting 
the shaping rules.  As far as I know there is nothing in existance that does 
this, but its an interesting idea to think about.  If you were going to try 
something like this I think you'd need some sort of reliable indicator of 
what the current congestion is like... perhaps some steady ping to use as 
your "canary in the coal mine".  Then set up several shaping scripts that 
assume different levels of bandwidth, and depending on the current "canary 
ping", either upgrade or downgrade your assumed bandwidth by calling the 
appropriate script.  So I guess if you wanted to try this, maybe you could 
set up three scripts, one set to 80% of your bandwidth, one set to 50%, and 
one set to 20%.  Then set up a cron job that checks the "ping" for your high 
priority traffic, and calls the appropriate script to adjust.  I doubt it'll 
work, but it could be an interesting experiment.

----- Original Message ----- 
From: "Justin Schoeman" <justin@expertron.co.za>
To: <lartc@mailman.ds9a.nl>
Sent: Thursday, November 25, 2004 3:17 AM
Subject: [LARTC] Shaping traffic on heavily oversubscribed links?


> Hi all,
>
> I am having some fun with traffic shaping, and have run into an 
> interesting situation.  Here is South Africa, most internet links are 
> heavily oversubscribed, which means that in most cases the local link is 
> _not_ the bottleneck, and shaping on the local link does not help that 
> much...
>
> Does anybody have some tips on shaping such links?  How can you get 
> interractive traffic if you don't know how much bandwidth to reserve for 
> it? How can you give fair access to a link if you don't know what the link 
> capacity is?
>
> Are there perhaps some tools to monitor retransmissions to try and 
> determine congestion levels, and from that adjust shaped bandwidth?
>
> Am I perhaps missing something simple in this scenario?
>
> Thanks!
> -justin
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