On Thu, Dec 5, 2013 at 1:13 PM, Bob Briscoe <[email protected]> wrote:
> Fred, Gorry, all,
>
> I promised to suggest text for draft-ietf-aqm-recommendation about
> allowing the AQM's behaviour to be independent for ECN and non-ECN packets.
> In the process, I realised we can't talk about independent AQMs for ECN
> without also including Diffserv.
>
> This gets messy, because I believe a good AQM for BE traffic with and
> without ECN, should remove much if not all the need for Diffserv. But we
> can't ignore Diffserv.
>
I agree in principle with what Bob is trying to say here (and is very much
what I've been saying in my blog entry of last summer).
Once you have things under control, the need for Diffserv diminishes
dramatically (but does not go away).
But as Bob notes, there is still a good use for Diffserv: suitably marked
traffic may want to contend for access to the channel differently: your
marked VOIP packets may want to change the priority with which you request
channel access, so that you get more timely access to the medium. This
conserves transmit opportunities, which is often the scarcest resource in
many systems (e.g. 802.11, DOCSIS, etc.). This can be the difference
between your VOIP working well, and not working well, on a busy 802.11
network as well as using the channel as efficiently as possible.
Similarly, if you have packets you know are background, it's helpful to
know that to ensure that they never contend for access to the medium but
will always defer to other traffic, and just scavenge available space in
other transmit opportunities where possible.
I'm a bit loathe though to tie the behavior to queues, however; in
particular, best effort traffic may want to be sent in the same aggregate
as higher (or lower) priority traffic, if there is remaining space in the
aggregate.
In short, the mental model we've had that there is a one-to-one model of
hardware and software queues (not to mention flows in a given software
queue) is often incorrect (or at least seriously sub-optimal) in today's
systems (even if the hardware queues "work" properly, which it appears they
do not in 802.11).
So I'm not sure Bob's new section 3 here is how to best to state this (or
to deal with the terminology problem). Certainly, it may be the same
instance of an AQM algorithm, rather than different instances, for example.
And "
It SHOULD be possible" is more a pious wish than anything else. But I
agree in spirit with what Bob's trying to say.
- Jim
> ____________________________________________________________
> _____________________________
> {In Section 4: add another bullet between recommendations 2 & 3:}
>
> 3{New}. It SHOULD be possible to make different instances of an AQM
> algorithm apply to different subsets of packets that share the same queue.
>
>
> It SHOULD be possible to classify packets into these subsets at least by
> ECN codepoint [RFC3168] and Diffserv codepoint [RFC2474] (or the equivalent
> of these fields at lower layers).
>
> {Then a new section to expand on this before the current Section 4.3.}
> 4.3{New}. Independent AQM Instances for ECN and Diffserv
>
> The recommendation to provide a separate instance of the AQM for ECN
> packets goes beyond the assumptions of RFC 3168, which assumed that only
> one instance of an AQM will handle both ECN-capable and non-ECN-capable
> packets.
>
>
>
> Bob
>
>
>
> ________________________________________________________________
> Bob Briscoe, BT
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