[altq 199] Re: Overcommitting Classes...

[Karl Pielorz <kpielorz@tdx.co.uk>]

Kenjiro Cho wrote:

> Your requirement does make sense but overcommitting bandwidth doesn't
> sound like a right solution.
> 
> AF (Assured Forwarding) of diffserv provides a much simpler mechanism
> to achieve similar effects.
> You can use trTCM (2-rate 3-color marker) for traffic conditioning and
> configure it in such a way that it marks green to packets under
> 64Kbps, yellow to packets between 64Kbps and 128Kbps, and discards
> packets more than 128Kbps.
> At the egress interface, the aggregated AF flow is classified to a
> class with 512Kbps limit.  When congestion occurs, RIO will drop only
> yellow packets.
> 
> But I have to admit that we do not have much experience with AF in a
> real ISP setting :)

Stefan Stefanov just commented:

"How about a Maximum_burst_bandwidth or something like that. A parameter
that will tell the maximum bandwidth a class can get when in burst. This
would limit a class not to burst to all available parent's bandwidth.
This could be a very useful option."

This would be great, and would have been one of my questions - had I not have
found the 'overcomit' behaviour first!

I like the sound of the diffserv solution above, It does sound a little
complex - I've read a little about diffserv, but not much...

Is the above doable with AltQ at present? - I don't think I could get it
passed as our primary link feed (a little too experimental) - but I might be
able to setup a secondary feed with it...

Or, alternatively being able to do:

class cbq fxp0 HTTP_agdc default_class pbandwidth 25 red borrow 30

Meaning, 25% bandwidth of default_class - but able to borrow up to 30% more
(if available) would be good addition, though this is realling moving away
from overcomit to 'new ideas'...

I guess overcomit is overcomit - the age old computer problem (e.g. with
Swap), though overcommit does happen - infact, swapfiles are one good example
of where it's a valid feature / point / need?

Maybe having a flag in AltQ to allow it wouldn't be such a bad idea? ;-)

-Karl