[Ecn-sane] [tsvwg] Comments on L4S drafts

[Black, David]

Two comments as an individual, not as a WG chair:

> Mostly, they're things that an end-host algorithm needs
> to do in order to behave nicely, that might be good things anyways
> without regard to L4S in the network (coexist w/ Reno, avoid RTT bias,
> work well w/ small RTT, be robust to reordering).  I am curious which
> ones you think are too rigid ... maybe they can be loosened?

[1] I have profoundly objected to L4S's RACK-like requirement (use time to detect loss, and in particular do not use 3DupACK) in public on multiple occasions, because in reliable transport space, that forces use of TCP Prague, a protocol with which we have little to no deployment or operational experience.  Moreover, that requirement raises the bar for other protocols in a fashion that impacts endpoint firmware, and possibly hardware in some important (IMHO) environments where investing in those changes delivers little to no benefit.  The environments that I have in mind include a lot of data centers.  Process wise, I'm ok with addressing this objection via some sort of "controlled environment" escape clause text that makes this RACK-like requirement inapplicable in a "controlled environment" that does not need that behavior (e.g., where 3DupACK does not cause problems and is not expected to cause problems).  

For clarity, I understand the multi-lane link design rationale behind the RACK-like requirement and would agree with that requirement in a perfect world ... BUT ... this world is not perfect ... e.g., 3DupACK will not vanish from "running code" anytime soon.

> > So to me, it goes back to slamming the door shut, or not, on L4S's usage
> > of ect(1) as a too easily gamed e2e identifier. As I don't think it and
> > all the dependent code and algorithms can possibly scale past a single
> > physical layer tech, I'd like to see it move to a DSCP codepoint, worst
> > case... and certainly remain "experimental" in scope until anyone
> > independent can attempt to evaluate it.
> 
> That seems good to discuss in regard to the L4S ID draft.  There is a
> section (5.2) there already discussing DSCP, and why it alone isn't
> feasible.  There's also more detailed description of the relation and
> interworking in
> https://tools.ietf.org/html/draft-briscoe-tsvwg-l4s-diffserv-02

[2] We probably should pay more attention to that draft.  One of the things that I think is important in that draft is a requirement that operators can enable/disable L4S behavior of ECT(1) on a per-DSCP basis - the rationale for that functionality starts with incremental deployment.   This technique may also have the potential to provide a means for L4S and SCE to coexist via use of different DSCPs for L4S vs. SCE traffic (there are some subtleties here, e.g., interaction with operator bleaching of DSCPs to zero at network boundaries).

To be clear on what I have in mind:
	o Unacceptable: All traffic marked with ECT(1) goes into the L4S queue, independent of what DSCP it is marked with.
	o Acceptable:  There's an operator-configurable list of DSCPs that support an L4S service - traffic marked with ECT(1) goes into the L4S queue if and only if that traffic is also marked with a DSCP that is on the operator's DSCPs-for-L4S list.

Reminder: This entire message is posted as an individual, not as a WG chair.

Thanks, --David

> -----Original Message-----
> From: tsvwg <tsvwg-bounces at ietf.org> On Behalf Of Wesley Eddy
> Sent: Friday, July 19, 2019 2:34 PM
> To: Dave Taht; De Schepper, Koen (Nokia - BE/Antwerp)
> Cc: ecn-sane at lists.bufferbloat.net; tsvwg at ietf.org
> Subject: Re: [tsvwg] [Ecn-sane] Comments on L4S drafts
> 
> 
> [EXTERNAL EMAIL]
> 
> On 7/19/2019 11:37 AM, Dave Taht wrote:
> > It's the common-q with AQM **+ ECN** that's the sticking point. I'm
> > perfectly satisfied with the behavior of every ietf approved single
> > queued AQM without ecn enabled. Let's deploy more of those!
> 
> Hi Dave, I'm just trying to make sure I'm reading into your message
> correctly ... if I'm understanding it, then you're not in favor of
> either SCE or L4S at all?  With small queues and without ECN, loss
> becomes the only congestion signal, which is not desirable, IMHO, or am
> I totally misunderstanding something?
> 
> 
> > If we could somehow create a neutral poll in the general networking
> > community outside the ietf (nanog, bsd, linux, dcs, bigcos, routercos,
> > ISPs small and large) , and do it much like your classic "vote for a
> > political measure" thing, with a single point/counterpoint section,
> > maybe we'd get somewhere.
> 
> While I agree that would be really useful, it's kind of an "I want a
> pony" statement.  As a TSVWG chair where we're doing this work, we've
> been getting inputs from people that have a foot in many of the
> communities you mention, but always looking for more.
> 
> 
> > In particular conflating "low latency" really confounds the subject
> > matter, and has for years. FQ gives "low latency" for the vast
> > majority of flows running below their fair share. L4S promises "low
> > latency" for a rigidly defined set of congestion controls in a
> > specialized queue, and otherwise tosses all flows into a higher latency
> > queue when one flow is greedy.
> 
> I don't think this is a correct statement.  Packets have to be from a
> "scalable congestion control" to get access to the L4S queue.  There are
> some draft requirements for using the L4S ID, but they seem pretty
> flexible to me.  Mostly, they're things that an end-host algorithm needs
> to do in order to behave nicely, that might be good things anyways
> without regard to L4S in the network (coexist w/ Reno, avoid RTT bias,
> work well w/ small RTT, be robust to reordering).  I am curious which
> ones you think are too rigid ... maybe they can be loosened?
> 
> Also, I don't think the "tosses all flows into a higher latency queue
> when one flow is greedy" characterization is correct.  The other queue
> is for classic/non-scalable traffic, and not necessarily higher latency
> for a given flow, nor is winding up there related to whether another
> flow is greedy.
> 
> 
> > So to me, it goes back to slamming the door shut, or not, on L4S's usage
> > of ect(1) as a too easily gamed e2e identifier. As I don't think it and
> > all the dependent code and algorithms can possibly scale past a single
> > physical layer tech, I'd like to see it move to a DSCP codepoint, worst
> > case... and certainly remain "experimental" in scope until anyone
> > independent can attempt to evaluate it.
> 
> That seems good to discuss in regard to the L4S ID draft.  There is a
> section (5.2) there already discussing DSCP, and why it alone isn't
> feasible.  There's also more detailed description of the relation and
> interworking in
> https://tools.ietf.org/html/draft-briscoe-tsvwg-l4s-diffserv-02
> 
> 
> > I'd really all the tcp-go-fast-at-any-cost people to take a year off to
> > dogfood their designs, and go live somewhere with a congested network
> to
> > deal with daily, like a railway or airport, or on 3G network on a
> > sailboat or beach somewhere. It's not a bad life... REALLY.
> >
> Fortunately, at least in the IETF, I don't think there have been
> initiatives in the direction of going fast at any cost in recent
> history, and they would be unlikely to be well accepted if there were!
> That is at least one place that there seems to be strong consensus.
>