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Re: Congestion control/QoS

  • From: Jon 'tex' Boone
  • Date: Fri May 23 14:52:52 1997

Dirk,

Dirk Harms-Merbitz wrote:
> 
> Seems to me that a good way of limiting damage from runaway
> networks/hosts would be a modifaction of RED (Random Early
> Drop).

  RED (in the usage of Floyd and Jacobson) stands for Random Early
Detection.  There is a similar acronym (ERD) that refers to Early Random
Drop.  The two algorithms are very different - in RED, you attempt to
control the average queue size, but mark (usually drop) new arrivals
when necessary.  In ERD, you deal explicitly with the instantaneous
queue size and you drop all new arrivals until you get your
instantaneous queue size below the drop level.  Which were you intending
to refer to?

> Controlled Early Drop (CED? I just made this up, suggestions?)
> would allow a router to give a preference to connected
> networks/hosts. Packets from certain networks would have a higher
> chance of being dropped from a router queue then packets from
> other networks.

  This sounds like weighted RED - normally the probability of being
dropped in RED is dependent on the EWMA (exponentially weighted moving
average).  I think that in weighted RED, the "weight" is used to
influence the results of the normal RED algorithm.


> CED can be used to price connections!  You want 10%, you pay
> x. You want 50%, you pay y. Everybody gets full speed when the
> network is empty. When networks get congested everybody gets
> what they paid for.

  These aren't absolute guarantees, however.  Remember that in RED there
is still the possibility that all new arrivals will be dropped - if the
EWMA exceeds the maximum threshold and we are in serious congestion.

> Seems like a better way of pricing Internet connections. Right
> now way too many people are stuck behind small pipes. Most T1
> customers use only a small fraction of their circuit. How do
> you sell Ethernet connections? 100MB Ethernet connections?
> while still protecting your network. CED looks like a solution
> on this early Friday morning.

  A combination of weighted RED and RSVP might do a mighty fine job. 
Unless you are using Ethernet for the 10 Mb/s access speed with only one
customer per Ethernet, this doesn't really help.  CSMA/CD will still
take precedence in the face of a really congested link.  :-)

  You can check out a paper that I did with a partner on Floyd and
Jacobsen's RED paper.  It's at
http://pobox.upenn.edu/~tex/red_paper.html.


-- 

------------------------------------------------------------------------
Jon 'tex' Boone                                         Network Engineer
<[email protected]>                                       ISC Networking
(215) 898-2477                                University of Pennsylvania
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