[Codel] why RED is not considered as a solution to bufferbloat.

[Richard Scheffenegger]

Sahil.,

Codel tries to address the problems that RED couldn't; First, the input signal into the algorithm (sojourn time vs. average queue depth) is of a different quality; Second, Codel (in it's plain form) does drop/mark on dequeue, while RED drops/marks on enqueue. This means, that [TCP] congestion control loop is much quicker with Codel over RED; thus the reaction by the sender will probably be timely and relevant for that congestion epoch.  With RED; the congestion signal (that lost packet) has to traverse the filled-up buffer first, thus the control loop time is much larger (includes the instantaneous queue length of the buffer) - and is further delayed by the averaging going on.

Codel, by design, doesn't need to be tuned specifically for one particular drain rate (bandwidht) of the queue - unlike RED; So it adjusts much better to variable bandwidth MACs (Wifi, DOCSIS).

I've been told, that RED is easier to implement in HW due to that action being all done on enqueue. With PIE, there exists another AQM that tries to re-use the hw engines that exist for RED, but the control algorithms try to use a different input signal - making the best of that.


If you follow the AQM work in IETF, there is strong consensus steer to these more modern AQMs.

Best regards,
  Richard


  ----- Original Message ----- 
  From: sahil grover 
  To: Jonathan Morton 
  Cc: codel at lists.bufferbloat.net 
  Sent: Tuesday, February 24, 2015 5:20 PM
  Subject: Re: [Codel] why RED is not considered as a solution to bufferbloat.


  So we can say Codel is better than other AQM???


  On Tue, Feb 24, 2015 at 9:24 PM, Jonathan Morton <chromatix99 at gmail.com> wrote:

    Simply put, RED is a very old algorithm, one of the first viable AQM algorithms. However, it proved to be so difficult to configure properly that almost nobody uses it, even though many carrier grade routers implement it.

    Codel not only performs better than an ideally configured RED, but is far easier to configure. This makes it much more deployable.

    - Jonathan Morton






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