[Codel] SfqCodel

sahil grover sahilgrover013 at gmail.com
Sat Apr 11 04:11:41 EDT 2015

thanks for your help as always.

On Sat, Apr 11, 2015 at 2:25 AM, Jonathan Morton <chromatix99 at gmail.com>

> SFQ is quite simple in essence. It performs flow isolation, allowing
> packets from different flows to bypass each other.
> A flow is defined as packets possessing a common 5-tuple of source
> address, destination address, protocol (TCP, UDP, ICMP etc), source port
> and destination port.
> Ideally, Fair Queuing would perfectly separate all flows into queues, then
> service each of them equally and fairly in turn, for some measure of
> equality and fairness. This would provide ideal flow isolation.
> SFQ is not quite this ideal model. The 5-tuple is converted into a hash
> value which is used directly to index into a fixed list of queues; thus
> hash collisions can occur which mix traffic from multiple flows in the same
> queue. It also services queues in a strict round-robin, delivering one
> packet per cycle, regardless of the relative sizes of these packets. These
> are compromises intended to minimise CPU overhead and implementation
> complexity. (They made sense at the time when SFQ was cutting edge, which
> was a long time ago.)
> Fq_codel as implemented in Linux is not the same as sfq_codel as
> implemented in ns2. Instead of using SFQ to provide flow isolation, it uses
> DRR++, which provides bytewise fairness and a priority boost to sparse
> flows. Recent work has also found a way to significantly reduce the hash
> collision problem without imposing much additional overhead, and this has
> been incorporated into cake.
> - Jonathan Morton
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