Re: [tcpm] [tsvwg] New Version Notification for draft-grimes-tcpm-tcpsce-00.txt

"Rodney W. Grimes" <4bone@gndrsh.dnsmgr.net> Fri, 26 July 2019 08:55 UTC

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From: "Rodney W. Grimes" <4bone@gndrsh.dnsmgr.net>
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To: Bob Briscoe <ietf@bobbriscoe.net>
Date: Fri, 26 Jul 2019 01:54:57 -0700 (PDT)
CC: Jonathan Morton <chromatix99@gmail.com>, "tcpm@ietf.org" <tcpm@ietf.org>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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Subject: Re: [tcpm] [tsvwg] New Version Notification for draft-grimes-tcpm-tcpsce-00.txt
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Bob,

Responding to specifics of draft-grimes-tcpm-tcpsce-00.txt
conversation in this thread as its author.

> Jonathan,
> 
> On 25/07/2019 14:53, Jonathan Morton wrote:
> >> On 25 Jul, 2019, at 2:20 pm, Bob Briscoe <in@bobbriscoe.net>; wrote:
> >>
> >> The idea was to have a generic wire protocol with a dumb receiver, so that the same feedback protocol could support multiple needs for feedback by different TCP congestion control algorithms.
> >>
> >> So a fairly inefficient re-use of the 'NS' TCP header flag for one particular experiment is very unlikely to fly, particularly when the experiment it supports doesn't satisfy all the requirements in 7560.
> > AccECN burns the same bit to provide higher fidelity feedback of CE, without addressing our need to feed back the distinction between ECT(1) and ECT(0) at all (unless the TCP Option is used).  Since higher fidelity feedback of CE is not useful for SCE, using NS in this way is actually more efficient for us.
> As you say, AccECN does support SCE - with the option. That was because 
> it was generally agreed that accuracy of the CE signal was paramount, 
> rather than trying to do two things with 3 bits and under-achieving for 
> both.

It supports TCPSCE only in the sense that once AccECN TWH fails
to find a L4S endpoint it does fall back to ECN as defined in
RFC3168 which TCPSCE builds upon by reuse of the NS bit which
was freed up by RFC8311 (to provide a feedback path for the
higher fadelity SCE forward mark.)

Further more by using this fall back position it means that
none of the remaining AccENC TWH code points that exist today
would need to be consumed in an attempt to negotiate this use.

Given that many of the 8 code points created by doing the L4S
TWH modifications are already consumed by L4S and that if you
do the modified L4S handshake you gain access to all of the
ECN TCP layer bits I feel it would be wasteful to consume that
also limited resource for a single bit access to NS vs a possible
future 3 bit access to (NS, ECE, CWR)

> 
> We did have another scheme with 5 & 3 codepoints in the 3 bits for two 
> counters - look back over the draft history. But the WG decided 
> simplicity was also important in a CC protocol, cos lack of bugs is also 
> important.

Intresting end phrase "lack of bugs is also important", I shall simply
state that SCE and SCETCP as running code is a significantly small amount
of code and the fact that a prototype working implementation was completed
in ~24 man hours speaks to its simplicty and elagance of design.

Though I would not qualify that as Internet deployable code, it was
adaquate such that we could conduct experiments and debug the IP
layer SCE marking code being implemented in a Linux versioned middle box.

> >   Happily, AccECN and SCE can coexist on different flows, thanks to the fact that AccECN does have a negotiation phase which SCE can naturally reject.
> You will find there is resistance to starting to use the NS flag after 
> having negotiated RFC3168 ECN feedback, but without any explicit 
> negotiation. You will probably be asked how a future experiment would be 
> able to use the NS flag if your experiment fails (assuming it is adopted 
> as IETF work in the first place). You are walking into a world of bit 
> scarcity.

See above, we are leaving the remaining AccECN TCP TWH code points
open for future override of NS/ACE, ECE, and CWR rather than consume
the code points left in that TWH to consome 1 bit in what is effectely
an unused code point in the TWH (fall back to RFC3168 ECN).


> 
> >
> >> For instance, I think the reason the tcpsce draft discusses multiple ways of doing the feedback is that, in the presence of pure ACK loss (which is often due to deliberate ACK thinning), none of the three solutions preserve reliable delivery of the ACK signal.
> > In SCE, *reliable* feedback of SCE signals is not actually required, both because the control loop is naturally stable, and because RFC-3168's CE feedback *is* reliable and thus offers a safe fallback.  Of course we understand that the design constraints for L4S' feedback mechanism were different.
> My point wasn't so much about safety, it was about about accuracy and 
> particularly biased error. The essence of low latency congestion control 
> is keeping the queue low without losing utilization, which requires 
> accuracy. If your feedback has a biased but unknown error, it's really 
> hard to accurately converging towards a target.

We have found no issues in our testing thus far that would indicated
that either of the (small) error amount or bias direction has a significant
impact on the convergence rate or target of the closed loop system.
Including doing a random drop of ack packet test.  I can agree that the
higher the accuracy of this feedback the better, but the solution is robust
as designed.  Much as you do not need super high accuracy input to
a steering wheel to keep a car centered in its curving lane.

I would not qualify this as "really hard to accurately converge",
if it was most closed loop control loop systems would fail.  If
on the other hand it was open loop then the accuracy would need
to be near perfect, is that the issue your trying to raise?

> 
> >
> > Ack thinning is also something we have explicitly considered, given that Cake includes an optional ack-filter which does exactly that.  (We have, for example, added consideration of the NS bit to Cake's ack-filter, which was a trivial patch.)  Mathematically, the most extreme errors possible in either direction, due to ack thinning, are easily corrected during subsequent RTTs.
> Exactly - if your feedback has a biased error, you will usually miss the 
> target and try again in the next round. No point continuing this 
> discussion - I'm just saying you'll need to see what happens in reality 
> when the feedback is thinned.

We are aware of the ack thinning concerns, and do plan to fully evaluate
that as a potential problem, though to date our experiments indicate it
is minimally impacting.  There is also all the discussion going on about
maybe ack thinning is not such a grand idea on its own merits.

> >   - Jonathan Morton
> Bob Briscoe                               http://bobbriscoe.net/
-- 
Rod Grimes                                                 rgrimes@freebsd.org