On Tue, Dec 10, 2013 at 10:04 PM, Bob Briscoe <[email protected]> wrote:
> Jim, > > I'm just checking we're not talking past each other. I'll repeat two > quotes from each of us, then comment. > > On Thu, Dec 5, 2013 at 1:13 PM, Bob Briscoe <[email protected]> wrote: > > 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), > > > At 19:50 05/12/2013, Jim Gettys wrote: > > "Certainly, it may be the same instance of an AQM algorithm, rather than > different instances, for example." > > > That's true of course, but the case with one AQM handling all packets > within a queue is the norm. I want to check you're happy with the converse: > 1) A set-up more like WRED which was based on Dave Clark's RIO (RED with > in and out of contract). So we can have WPIE, WCoDel etc where the > differentiation between aggregates is provided by different AQM instances > in the same queue, not by different queues with different scheduling > priorities. > 2) Extending this so that AQM differentiation can be between ECN-capable > and Not-ECN-capable aggregates, not just between Diffserv classes (an > example being CoDel with a lower 'interval' for ECN-capable packets). > > I presented the evaluations of this last idea in tsvwg on the final Friday > of the Vancouver IETF - I don't think you were there. > <http://www.ietf.org/proceedings/88/slides/slides-88-tsvwg-20.pdf> > Yes, unfortunately I had to leave before the Friday session. > This is my primary motivation for this wordsmithing - I'm trying allow us > to move towards zero signalling delays in CoDel, PIE and RED (currently > defaults of 200ms, 100ms and 512packets respectively, which are not good > for dynamics). > Certainly signalling delays are very important: this is why I'm favorably inclined to "head mark/drop", as it signals TCP as quickly as possible, keeping the response of the TCP feedback loop as tight as possible (and part of why I like CoDel so much for the highly variable bandwidth problem we face at the edge of the net). It's *really* important than when the bandwidth drops suddenly that everyone gets told to slow down quickly (exactly how quickly probably depends on the propagation change characteristics of the medium), or packets can pile up in a big way. How quickly the mark/drop algorithm can figure out that signalling is appropriate is the *other* piece of getting good dynamics. Here I don't doubt that something may be discovered that is better than CoDel in the slightest. > > > Bob > My basic issue is one of terminology: people have talked about "best effort" queues. In reality, this is a "class" of service, rather than a single queue, and when you get into the mental model of BE being a single queue, (rather than a set of queues) it can lead one astray quickly and easily. It's really easy to fall into the idea of a single software queue mapping to some single hardware supported queue, and that's a cognitive mistake, as aggregating MACs are showing us; transmit ops are often the scarcest resource... Diffserv marking has the potential to give a "hint" to distinguish how particular flows should be handled (scheduled) in a service class, and as my previous example shows, that hint may be very useful in channel access decisions (e.g. voip on 802.11). But fq_codel teaches the lesson that packet scheduling combined with keeping TCP sane is a key improvement over handling either problem apart... In particular, the first packets of new flows/reappearing flows are vastly more "important" than other packets in terms of the latency cost to users of that service. Each flow has in essence its own queue in this service class, and we're using information from that to help schedule the packets in ways that minimize latency to the user. So in this case, a single algorithm is acting over a bunch of flows in a single class of service, and both scheduling packets among the flows, and signalling TCP flows appropriately when they should "slow down". So I think you and I are on close to the same page (but have been burned badly in the past by terminology issues getting in the way). On HTTP/1.1 we wasted probably > 2 years talking past each other because we didn't have clear and concise terminology that we all understood the same way. And I don't claim I have the right terminology for all this stuff, either (even in this mail). Which is why I was loathe to suggest exact text... - Jim > > At 19:50 05/12/2013, Jim Gettys wrote: > > > > 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 > _______________________________________________ > aqm mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/aqm > > > _______________________________________________ > aqm mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/aqm > > ________________________________________________________________ > Bob Briscoe, BT > > _______________________________________________ > aqm mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/aqm > >
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