Re: [aqm] ECT(1)

[Bob Briscoe <[e-mail]>]

John,

There was much discussion about this identifier issue in Prague. Many 
people are well-aware that the sticking point is availability of an 
identifier, and its associated politics.


On 04/08/15 05:08, John Leslie wrote:
> Bob Briscoe <[e-mail]> wrote:
>> I do not believe an IP (v4) option or a v6 extension would be necessary.
>> If ECT(1) were used that would surely be sufficient alone.
>     Alas, we're facing a political question: not just a technical one.
>
>     I think folks are ready to deprecate ECN Nonce; but I'm not
> optimistic that folks are ready to embrace "Low-Latency, Low-Loss and
> Scalable" (L4S) service, as introduced in draft-briscoe-aqm-dualq-coupled
> (Has this draft been posted?).
>
>     (IMHO, this work promises to be very valuable for the _many_ uses
> that are latency-sensitive; but adoption is going to be a major
> challenge!)
>
>     We may indeed eventually get to where Bob is thinking today; but
> I don't see a clear path to IETF-wide consensus yet. Even getting an
> Experimental RFC approved which re-purposes ECT(1) strikes me as a
> very significant challenge. :^(
>
>> I believe the main criteria for an identifier for this new service are:
>> 1. preferably orthogonal to Diffserv classes.
>     IMHO, Diffserv classes are poison!
>
>     There are a number of good folks pursing a Less-than-Best-Effort
> diffserv class. I wish them luck! But I'd be amazed if they succeed.
>
>     Diffserv classes are the private preserve of a _large_ number of
> network-service-providers. Best-Effort is the only one with universal
> agreement what it means.
>
>     All the others are subject to non-documented shuffling at the
> boundaries between providers (and "bleaching" to Best-Effort at many
> points within providers' networks).
DSCPs are only poison in standards. As you say, ISPs use DSCPs for many 
internal services already.

(BTW, the request for a global scope DSCP for less-than-BE is very 
different to this case, because bleaching LBE (remarking to the 
all-zeros DSCP at borders) always /increases/ its priority.)

>> 2. preferably end-to-end in scope
>     I'm really not sure how we can make L4S useful if it lacks an
> End-to-End meaning. The signal must enter at the sender and mostly
> survive all the way to the receiver in order that the receiver
> (by whatever magic) can tell the sender about any congestion.

ISPs could go ahead with using a local DSCP now for L4S for their own 
premium services. A large proportion of traffic these days is served 
from within the same ISP as the user is connected to (esp using CDNs), 
so this would be very "useful". Using a DSCP for alternate ECN semantics 
is already recommended in RFC4774, so the IETF would not really need to 
do anything to get L4S started.

The IETF might want to head-off possible interop problems by assigning a 
global-scope DSCP for the alternate ECN semantics, which we all know are 
in short supply. If we did use a global-scope DSCP it would be solely 
for a migration period, which is a double-edged sword:
* if the migration period were truly short-lived, the global codepoint 
would become available soon.
* if the migration period took longer, fears that burning a global DSCP 
for a just a brief migration period would have proved unfounded.

If local-only L4S usage became widespread, it /could/ be used between 
domains by simply ignoring the DSCP and only using ECN as the 
classifier. But that all depends what the take-up of classic ECN is in 
the meantime, and whether all the major classic ECN host implementations 
migrate to L4S. (In the TCP Prague Bar BoF, Andrew McGregor made the 
point that all the major OS developers who control what nearly all the 
Internet's traffic looks like were in the room.) Then the IETF could 
come along afterwards and standardise the new ECN semantics.


>> 3. preferably classic (RFC168) ECN and 'L4S' ECN would not permanently
>>     burn two codepoints, since it seems that 'L4S' could eventually
>>     subsume classic ECN (a fork would not be needed, because classic
>>     ECN doesn't seem to do anything that L4S cannot do).
>>
>     This is "nice to have", I suppose; but it seems too optimistic
> to take seriously. Deployment of L4S will take at least five years;
> and nobody's crystal-ball is good enough to see beyond that.
Deployment of something that enables new valuable apps and products can 
take 2yrs.

Nonetheless, your general point is true.
>
>     Furthermore, I don't see how we can _ever_ entirely eradicate the
> RFC 3168 behavior of "same as drop".
<Flippant> According to measurement studies RFC3168 behaviour is 
currently entirely eradicated. At least at such a low level that the two 
CE packets that seemed to exhibit the behaviour are probably a symptom 
of bugs.
</Flippant>

<Seriously>I know what you mean: RFC3168 behaviour is latent in 
ECN-enabled servers waiting for a client request.

I have proposed that L4S behaviour is associated with e2e negotiation of 
new Accurate ECN semantics. Nonetheless, even if an L4S client attempts 
to negotiate AccECN with a server, if the server only supports classic 
ECN, the session should{Note 1} fall back to classic ECN. So we will 
need to distinguish classic ECN from L4S ECN... unless we all agree that 
AccECN must fall back to drop, even if the other end says it supports 
classic ECN.

{Note 1} AccECN is yet to be specified by the IETF, but this is the 
current thinking, which seems reasonable.
</Seriously>

> Furthermore, L4S _can't_
> eliminate packet drops; and IMHO a packet-drop in an L4S stream
> must be treated _differently_ than a L4S congestion mark.
No-one is questioning that behaviour on drop needs to stay as in Reno or 
Cubic. The question is only over whether behaviour in response to ECN-CE 
should be distinct from drop behaviour.


>> *ECT(1) **
>> *Seems a good identifier, but it has the following problems:
>>
>> a) L4S traffic would need to be distinguished from classic ECN both
>>     when unmarked (ECT0 vs ECT1) and when marked (CE vs CE???).
>>     Ie. congestion experienced (CE) would have to be shared between
>>     the classes.
>
>     Actually, there are _two_ ways ECT(1) could be used:
> - ECT(1) could be set to request L4S forwarding rules marking CE
>    to indicate L4S congestion; or
> - L4S forwarders could change ECT(1) to ECT(0) (or vice-versa?),
>    to mark L4S congestion.
The latter doesn't work, I'm afraid. Reason:
* If all buffers on a path (say X, Y, Z) classify L4S and Classic by 
ECT(1) and ECT(0) resp.,
* and if buffer X indicates L4S congestion by changing some L4S packets 
from ECT(1) to ECT(0)
* then at subsequent buffers on the path (Y or Z), the L4S packets that 
X remarked to ECT(0) will get classified into the Classic queue at Y and Z.

Result: a proportion L4S packets will get 'demoted' into low latency 
queues, introducing intermittent re-ordering delay, thus increasing the 
effective delay of the low latency L4S service to that of the classic 
queues.

>
>>     It would not be so problematic if all queues classified all CE
>>     packets as the lowest latency class (L4S); CE packets from classic
>>     flows would then be delivered early out of order, requiring some
>>     buffering, but probably no more buffering than is already needed
>>     for retransmissions, and at least they would never be late out of
>>     order. See also {Note 1}.
>     I'm trying to follow this...
>
>     What exactly does Bob mean by "all queues"? Mostly we think of
> queues as part of the forwarding action. But some forwarders choose
> their action upon packet entry to the queue; other at packet exit.
> And, AFAIR, no forwarder takes an action based upon the packet being
> CE-marked when it arrives.
I didn't intend to say anything about whether actions are on entry or on 
exit to the queue - I don't think that's relevant here.
Again, I was thinking about the problem of one queue remarking some 
packets in both L4S and Classic ECN to CE, then how those CE packets 
would get classified at subsequent queues on the path. I'll try again 
with a picture...


,------------. ,------------. ,---------------. ,------------.
L4S source --> ECT(1)\    |      ECT(1)|-->| L4S      |\CE & ECT(1) |    
CE & ECT(1)|==>|   L4S      |
                       \   |  L3        | `------------' \ | L3       
      |   `------------'
                        ==>| Classifier|     Bottleneck    
==========>|Classifier    |
                       /   |            | ,------------. / | 
               | ,------------.
Classic src--> ECT(0)/    |      ECT(0)|-->| Classic   |/CE & ECT(0) | 
ECT(0)|-->|  Classic   |
                           `------------' 
`------------'`---------------' `------------'


>
>> b) ECT(1) is the last available ECN codepoint (for both v4 & v6).
>>     Using ECT(1) for L4S and ECT(0) for Classic ECN would burn the last
>>     codepoint just for migration purposes (contravening my criterion
>>     #3). If we could predict that migration might one day finish, we
>>     could foresee a time when ECT(0) might become available again.
>>     But that's a long shot.
>     This is a political problem, more than a technical one.
>
>     We've painted ourselves into a corner, where there aren't spare
> bits -- and the "spare bits" in IPv6 turn out to be unusable. (We
> seem to have done this quite deliberatly -- I don't understand why!)
>
>     Nonetheless, we have a major need to mark incipient congestion, so
> that we can avoid over-filling buffers at forwarding nodes. The fact
> that we have only half-a-bit left to do this is the inevitable result
> of our refusal to allocate enough bits in the first place (or if you
> prefer, our insistence on using six bits for DSCP, defined in such a
> way as to prevent end-to-end meaning of them).
>
>     (Personally, I'd love to reclaim a few bits from DSCP; but to propose
> this would label me a clueless kook, so I won't.)
>
>     ECT(1) is there! It's allocated for ECN use. Refusing to define it
> with an ECN meaning is simply irrational.
>
>     Furthermore: there _is_ another bit! See RFC 3514. ;^)
>
>> c) For the record, the following uses of ECT(1) have been proposed by
>>     the IETF and by researchers:
>>   * receiver cheat detection (the ECN nonce [RFC3540] - experimental)
>>   * ECN path testing (ECN for RTP [RFC6679] - standards track)
>>   * various intermediate congestion level proposals (including PCN
>>     [RFC6660] - standards track)
>>   * various fast-start proposals (in research, e.g. VCP)
>     IMHO, only RFCs count as "proposals".
>
>     RFC 3540 is ripe for deprecation, IMHO.
>
>     RFC 6679 covers "ECN for RTP over UDP". Somehow I missed it coming
> out in 2012 (though I must have been listening to the IESG telechat
> where it was approved). Mea culpa!
>
>     It's not an easy read (58 pages, heavy with RTP details)! At first
> blush, I don't see what it's trying to do with ECT(1). It references
> RFC 3168 for the meaning of ECT(1); it keeps separate counters for
> ECT(0) and ECT(1); and it has a "random" mode (not RECOMMENDED) which
> is supposed to randomize whether ECT(0) or ECT(1) is sent.
>
>     The overall impression is that it tries to define feedback for all
> possible ECN cases: thus supporting ECN Nonce use as well as all other
> uses known at the time it was written.
>
>     To deprecate ECN Nonce, we'd need to UPDATE RFC 6679 as well as
> RFC 3168; but I don't see any new issues introduced by 6679 (and the
> features of it are already appropriate for L4S.
>
>> d) PCN is defined for a controlled environment, so that's not a problem.
>>     The wording or RTP-ECN does not mandate the use of ECT(1), but it is
>>     not always clear that it is optional either.
>     Clearly, keeping separate counters for ECT(1) and ECT(0) is required;
> but sending ECT(1) vs ECT(0) is not specified within RFC 6679.
>
>>     So I am trying to find out whether any implementations have used
>>     ECT(1).
>     At first blush, it would appear that the only _current_ use of ECT(1)
> would be for ECN Nonce. But of course, RFC 6679 says nothing to prevent
> its use for L4S.
>
>>     Even if none of the IETF uses of ECT(1) are problematic in practice,
>>     we should think very carefully before burning ECT(1) for L4S,
>>     because there do appear to be new uses being proposed for it that
>>     address a new potentially important class of problems: getting up to
>>     speed fast.
>     Some citations, please...
>
>     (BTW, I think L4S could be _very_ helpful for "speeding up" slow-start.)
>
>> *DSCP**
>> *It might be better to distinguish L4S ECN from Classic ECN by using
>> only ECT(0) and CE, but also using a distinctive DS codepoint for L4S.
>> L4S could start off local-network only (e.g. for a network operator's
>> premium services), or a global DSCP could be burned so that hosts could
>> set it without needing to be configured for the network they happen to
>> be connected to at any one time.
>     I don's see L4S as useful in "local-network-only" mode.
>
>     Granted, there _are_ many cases where the benefit of L4S would be
> greatest at the first hop (DOCSIS box). But the expected "bleaching"
> could be very confusing as to the meaning of CE marking that could be
> generated farther along the path. There is no such thing as a condition
> where _only_ the first hop can experience congestion.
>
>> Then, assuming all Classic ECN might eventually migrate to L4S ECN,
>> a DSCP would no longer be needed as well as ECT(0) to identify L4S.
>> Then the ECN field alone could represent L4S end-to-end.
>     This is overly optimistic.
>
>> We all know that DSCP has the following problems:
>> a) Diffserv is not orthogonal to Diffserv (obviously), so multiple DSCPs
>>     might be needed for L4S in each DS class
>     That seems fatal...
>
>> b) DS is not end-to-end
>> c) few global DSCPs left, altho certainly there are more DS codepoints
>>     than ECN codepoints left.
>     Network operators don't believe in "global DSCPs" They bleach anyway.
>
>     (I would tend to support carving out part of the "Experimental"
> subset of DHCPs as "must propagate if not understood" -- and possibly
> in ten years there might be enough equipment out there that respected
> that... but for now, it _all_ gets bleached.)
>
>> *Summary**
>> *Combining ECT(0) and CE with a globally assigned DSCP solely during
>> initial deployment of L4S seems the least worst choice.
>     We certainly could Experiment with that; but I'm very pessimistic.
>
>     OTOH, Experimenting with ECT(1) seems likely to work. IMHO...
>
> --
> John Leslie <[e-mail]>
>
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-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/