Re: [tsvwg] WGLC: draft-ietf-tsvwg-nqb-14 - coexistence with L4S

[[e-mail]]

Hi Bob,

the extensions proposed by you are ok, if all drafts are extended as suggested.

What you write below should be stated as mandatory behaviour in both drafts.

[BB] .….We should only say what we know for sure, which is that applying both codepoints shouldn't allow packets:
* to bypass ECN marking (the NQB draft already says this by circuitous reference to a para in ecn-l4s-id - see below)
* to be subject to less stringent policing than they would with either codepoint alone (we ought to add this to §3.3 of NQB).

NQB statement referring to L4S:
   Compliance with the DualQ
   Coupled AQM requirements (Section 2.5 of
   [I-D.ietf-tsvwg-aqm-dualq-coupled<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-14#ref-I-D.ietf-tsvwg-aqm-dualq-coupled>]) is considered sufficient to
   support the NQB PHB requirement of fair allocation of bandwidth
   between the QB and NQB queues (Section 5<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-14#section-5>).

That’s no obvious requirement to me. “MUST comply” would be.

Further, you seem to assume that an NQB policer is mandatory here, but text on that isn’t clear in the NQB draft.
The abstract says, no policing and the NQB text requires some not clearly defined kind of traffic protection.

   To prevent this situation from harming the performance of the real
   NQB flows, network elements that support differentiating NQB traffic
   SHOULD support a "traffic protection" function that can identify QB
   flows that are mismarked as NQB, and either reclassify those flows/
   packets to the QB queue or discard the offending traffic.

As I’ve been asking in another mail, is there any requirement to further specify co-operation of
NQB and L4S? NQB so far also allows to change the packets DSCP. I found the following
text in https://www.ietf.org/archive/id/draft-ietf-tsvwg-aqm-dualq-coupled-25.txt:

      -  if the packet is Not-ECT, the appropriate action depends on
         whether some other function is protecting the L queue from
         misbehaving flows (e.g. per-flow queue protection
         [I-D.briscoe-docsis-q-protection] or latency policing):

         o  If separate queue protection is provided, the L AQM SHOULD
            ignore the packet and forward it unchanged, meaning it
            should not calculate whether to apply congestion
            notification and it should neither drop nor CE-mark the
            packet (for instance, the operator might classify EF traffic
            that is unresponsive to drop into the L queue, alongside
            responsive L4S-ECN traffic)

         o  if separate queue protection is not provided, the L AQM
            SHOULD apply drop using a drop probability appropriate to
            Classic congestion control and appropriate to the target
            delay in the L queue

That’s a drop, if the NQB “traffic protection” results in an inappropriate NQB traffic rate as
compared to L4S configuration (there’s no guidance given). I wonder, what’s the
point in remarking the NQB marked traffic, then.


https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-14#section-4.2 goes on like

  Nodes that support the NQB PHB may choose to aggregate other service
   classes into the NQB queue.  This is particularly useful in cases
   where specialized PHBs for these other service classes are not
   provided.  Candidate service classes for this aggregation would
   include those that carry inelastic traffic that has low to very-low
   tolerance for loss, latency and/or jitter as discussed in [RFC4594<https://datatracker.ietf.org/doc/html/rfc4594>].
   These could include Telephony (EF/VA), Signaling (CS5), Real-Time
   Interactive (CS4) and Broadcast Video (CS3).  Or, in some networks,
   equipment limitations may necessitate aggregating all traffic marked
   with DSCPs 40-47 (i.e., whose three MSBs are 0b101).  As noted in
   Section 4.1<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb-14#section-4.1>, the choice of the value 45 is motivated in part by the
   desire to make this aggregation simpler in network equipment that can
   classify packets via comparing the DSCP value to a range of
   configured values.

That reads like suggesting a cocktail of traffic being policed and remarked or dropped, once forwarded
by NQB sharing the L4S queue. I’m not sure, whether NQB “traffic protection” applies
to traffic marked DSCP 101 101 only and the remaining traffic of the PHBs mentioned above
might enter the L4S queue without passing the traffic protection – the NQB text isn’t precise to that.

I do not think, that draft NQB should progress on standards track without providing clear guidance
on how to operate the PHB (also in co-existence with all the other traffic types and PHBs it
may carry and co-exist with).

Regards,

Ruediger

Von: Bob Briscoe <[e-mail]>
Gesendet: Samstag, 5. November 2022 22:16
An: Geib, Rüdiger <[e-mail]>; [e-mail]; [e-mail]
Cc: [e-mail]
Betreff: Re: [tsvwg] WGLC: draft-ietf-tsvwg-nqb-14 - coexistence with L4S

Greg, Sebastian, Ruediger,

I've given my comments about this draft before, and I'm happy with the outcomes now.
Other than the one (and possibly two) additional pieces of text I suggest below, I support this draft going forward from the WG.
See [BB]
On 03/11/2022 13:21, [e-mail]<mailto:[e-mail]> wrote:

Dear colleagues,



thanks for exchanging views. My take from what I read



- there is no clear separating of the PHBs NQB and L4S in current standards.

- there is no standards text expressing whether or not setting NQB DSCP combined with L4S ECT 1 is conforming to current specs or not.



Sebastian, I'm having the impression too that an application combining NQB DSCP with L4S ECT 1 may gain unfair advantage over QB flows if competing for WiFi spectrum by DSCP NQB after having passed a coupled Queue scheduler by L4S ECT 1 scheduling. I note that NQB is a standards track RFC and I'd expect to have clarity, whether a new standard may cause harm for consumers, especially, if correct implementation of NQB conforming sender behaviour is left to application programmers. I'm not an application programmer and I wonder, whether application programmers generally are concerned with sender behaviour at all. In a majority of cases, I'd assume a transport protocol between network and application layers.



Regards,



Ruediger





Sebastian, RFC791 states:



Type of Service

........

    Precedence.  An independent measure of the importance of this

    datagram.



If you read the NQB draft, you'll additionally note that "importance" is frequently used there.







-----Ursprüngliche Nachricht-----

Von: tsvwg <[e-mail]><mailto:[e-mail]> Im Auftrag von Sebastian Moeller

Gesendet: Donnerstag, 3. November 2022 10:45

An: Greg White <[e-mail]><mailto:[e-mail]>

Cc: Bless, Roland (TM) <[e-mail]><mailto:[e-mail]>; [e-mail]<mailto:[e-mail]>

Betreff: Re: [tsvwg] WGLC: draft-ietf-tsvwg-nqb-14 - coexistence with L4S







On Nov 3, 2022, at 00:34, Greg White <[e-mail]><mailto:[e-mail]> wrote:



Hi all,



See my responses to the points that relate to the NQB draft below [GW].



-Greg



On 11/2/22, 10:24 AM, "tsvwg on behalf of Sebastian Moeller" <[e-mail] on behalf of [e-mail]><mailto:[e-mail][e-mail]> wrote:



   Hi Roland,





On Nov 2, 2022, at 16:32, Bless, Roland (TM) <[e-mail]><mailto:[e-mail]> wrote:



Hi Rüdiger,



see below.



On 02.11.22 at 15:47 [e-mail]<mailto:[e-mail]> wrote:



current draft text informs about a coexistence of NQB and L4S in the same LowDelay queue. Does this include that

       • NQB traffic should or could be marked by a DSCP and have ECN 1 set?

       • L4S traffic should or could have ECN 1 set and be marked by NQB DSCP?



The current text is not clear on that issue. As all marking is to be done by application programmers and this is a standards track doc, I’m interested for the draft to be crisp and clear to that point. Include or exclude simultaneous NQB&L4S  DSCP&ECN setting?

As far as I understand the draft, it is just an implementation detail:

NQB packets do not need to have ECT(1) set to be put into the L4S low-latency queue.

It is a node local mapping from the NQB DSCP(s) to the L4Slow-latency

queue, i.e., they just use the already existing low-latency queue.

If I understand section 3.1 correctly, L4S flows should not be marked

as NQB traffic since they violate the following condition:



" In contrast, Queue-Building (QB) flows include those that use TCP

or QUIC, with Cubic, Reno or other TCP congestion control algorithms

that probe for the link capacity and induce latency and loss as a result."



and especially L4S flows may also violate the (not so strict) condition of a 1Mbit/s limit.



    [SM] That is IMHO incorrect at least generally. Just because TCP allows a flow to search for the capacity limit applications can (and do) decide to send less than possible.



[GW] Sebastian's clarification here is correct.  An application could mark its packets with both ECT(1) and NQB if it meets the requirements for both.  This might be a relatively unusual case, but it isn't precluded.  To Ruediger's original request that the draft be crisp and clear on this point, we could add a statement in Section 3.3 that explains this.

[BB] I agree with Greg and Sebastian's later points that it's better not to say too much when we don't really understand why anyone would apply both markings. We should only say what we know for sure, which is that applying both codepoints shouldn't allow packets:
* to bypass ECN marking (the NQB draft already says this by circuitous reference to a para in ecn-l4s-id - see below)
* to be subject to less stringent policing than they would with either codepoint alone (we ought to add this to §3.3 of NQB).








  [SM] Let's see how unusual that is going to be, after all NQB will result in a noticeable better low-latency performance over the ubiquitous WiFi networks (with the cost of that choice invisible to the application employing NQB marking unless the same application also establishes a CS0 parallel flow to asses whether the WiFi network is NQB-aware and whether NQB usage does not result in an overall throughput hit for the WiFi network (less aggregation will result in a total throughput loss)).









             [SM]But I guess the bigger issue is that the L4S drafts have some words to say when and when not to set ECT(1):



   https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-ecn-l4s-id-29#page-14:





   In order to coexist safely with other Internet traffic, a scalable

      congestion control is not allowed to tag its packets with the ECT(1)

      codepoint unless it complies with the following numbered requirements

      and recommendations: [list skipped]



   So NQB flows would need a scalable congestion control to be allowed

to set ECT(1)





   However usage of a "scalable congestion control" is not mandatory for setting ECT(1):



   As a condition for a host to send packets with the L4S identifier

      (ECT(1)), it SHOULD implement a congestion control behaviour that

      ensures that, in steady state, the average duration between induced

      ECN marks does not increase as flow rate scales up, all other factors

      being equal.  This is termed a scalable congestion control.





   I would respectfully argue that that SHOULD is mighty weak...





   BUT that L4S draft hasd the following ot say about NQB:



   To identify packets for just the scheduling treatment, it would be

      inappropriate to use the L4S ECT(1) identifier, because such traffic

      is unresponsive to ECN marking.  Examples of relevant non-ECN

      identifiers are:



      *  address ranges of specific applications or hosts configured to be,

         or known to be, safe, e.g. hard-coded IoT devices sending low

         intensity traffic;



      *  certain low data-volume applications or protocols (e.g. ARP,

DNS);



      *  specific Diffserv codepoints that indicate traffic with limited

         burstiness such as the EF (Expedited Forwarding [RFC3246]), Voice-

         Admit [RFC5865] or proposed NQB (Non-Queue-

         Building [I-D.ietf-tsvwg-nqb]) service classes or equivalent

         local-use DSCPs (see [I-D.briscoe-tsvwg-l4s-diffserv]).

[BB] 3 paras later (still in §5.4.1.1 of l4s-ecn-id), there's text that is more specifically relevant to this potential case of both ECT1 & NQB marking:

   A packet that carries one of these non-ECN identifiers to classify it

   into the L queue would not be subject to the L4S ECN marking

   treatment, unless it also carried an ECT(1) or CE codepoint.

We wrote this in ecn-l4s-id, not because we could envisage a case where both NQB and ECT1 markings would make sense, but because network equipment has to know what to do if it encounters a packet that has both markings.

Nonetheless, Sebastian suggested a believable scenario where senders might be incentivized to apply both when using L4S but also trying to get best performance from non-L4S WiFi.








   It makes a ton of sense to mirror this in the NQB draft explicitly to

   at least keep these two consistent.



[GW] I'd argue that mirroring this text in NQB isn't warranted, but I would agree that referencing it in NQB section 3.3 would be appropriate (and it's easier to ensure consistency that way).



  [SM2] I agree, mirroring was bad phrasing, what I meant is describe the same recommendation (only use ECT(1) for "scalable protocols", otherwise use the NQB DSCP for other flows fulfilling the L4S requirements) and add a reference. I see that "mirroring" sounds like I wanted to propose to copy the text verbatim, which I agree would be too much.

In the brief section about the relationship to L4S in §3.3 of the NQB draft, it says:

   Compliance with the DualQ

   Coupled AQM requirements (Section 2.5 of

   [I-D.ietf-tsvwg-aqm-dualq-coupled<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb#ref-I-D.ietf-tsvwg-aqm-dualq-coupled>]) is considered sufficient to

   support the NQB PHB requirement of fair allocation of bandwidth

   between the QB and NQB queues (Section 5<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-nqb#section-5>).

In that section (§2.5 of the DualQ draft), it invokes the whole of §5 of ecn-l4s-id as a mega-requirement.

   A Dual Queue Coupled AQM implementation MUST comply with the

   prerequisite L4S behaviours for any L4S network node (not just a

   DualQ) as specified in section 5 of [I-D.ietf-tsvwg-ecn-l4s-id<https://datatracker.ietf.org/doc/html/draft-ietf-tsvwg-aqm-dualq-coupled-25#ref-I-D.ietf-tsvwg-ecn-l4s-id>].

So, the NQB draft already references the text you (Sebastian) want it to reference, albeit in an indirect way. It looks like NQB references DualQ (rather than just §5 of ecn-l4s-id directly) in order to include the superset of other requirements in the DualQ draft as well.

It wouldn't harm to add an extra sentence to §3.3. of NQB saying sthg like
Note that these requirements in turn require compliance with all the
requirements in Section 5. of [I-D.ietf-tsvwg-ecn-l4s-id].



Bob














So my interpretation of the draft is that the answer to both

questions is no, i.e., either NQB DSCP is set or ECT(1) is set.

However, there should be a hint what to do in case both are set

(following the L4S argument of letting DiffServ being orthogonal, ECT(1) should take precedence and the DSCP should be kept nevertheless).



[GW] While both the NQB draft and the RFC-to-be on ECN-L4S-ID talk about NQB and L4S traffic sharing a low latency queue, it isn't precluded that they be queued separately (i.e. an NQB queue and an L4S queue).  If they were to be queued separately in a node that supports both, I think I agree with you that ECT(1) should take precedence.  The only situation where I can imagine a debate about that would be if a node implemented Traffic Protection for the NQB queue, but didn't implement such a feature for the L4S queue. That said, I'm not super comfortable with starting down the path of making recommendations for this separation of L4S and NQB, since I think it opens a can of worms. For example, is the NQB queue given equal forwarding preference compared to the "Classic" queue in the L4S dual-queue, or is it given equal forwarding preference compared to the L4S/Classic dual-queue combination?



  [SM2] I read "preference" as a synonym for the more usual "priority" here... why call it preference? (Genuine question: is this a usual nomenclature used in the field?)



And, would we then want to also describe the use of 4 queues (instead of 3) to handle all four combinations independently (in which case there is no precedence between the two markings)?



  [SM2] I would respectfully propose that until we implement and test such a beast our description will be hardly better than what an implementer will be able to figure out in actual testing/use, so why try to describe something where at best we can offer theoretical musings? Fine if the observations are either trivial or obvious (but in which case not making them should cause little harm), but what if we recommend something on purely theoretical grounds which then in practice fails to meet its goals? IMHO RFCs should try to not speculate too much and stick to what has been supported by data and facts.



Regards

  Sebastian







    [SM] The bigger issue is that L4S AQMs are not mandated to check at all whether any marked traffic actually fulfills the requirements. So honestly it matters little whether a flow uses NQB of L4S to gain access to L4S high-priority queue. Without actual policing/enforcement any requirements in RFC are reduced to "requirements".



   Kind Regards

    Sebastian



   P.S.: I am convinced that this draft is not (yet) ready and should not pass last call right now.







Regards,



Roland





Von: tsvwg <[e-mail]><mailto:[e-mail]> Im Auftrag von Black, David

Gesendet: Mittwoch, 2. November 2022 04:47

An: tsvwg IETF list <[e-mail]><mailto:[e-mail]>

Betreff: [tsvwg] WGLC: draft-ietf-tsvwg-nqb-14, closes 21 November

2022



This email announces a TSVWG Working Group Last Call (WGLC) on:



A Non-Queue-Building Per-Hop Behavior (NQB PHB) for Differentiated Services

                       draft-ietf-tsvwg-nqb-14

https://datatracker.ietf.org/doc/draft-ietf-tsvwg-nqb/



This draft is intended to become a Proposed Standard RFC.



This WGLC will run through the end of the day on Monday, November 21.

The WG will meet in London on Monday, November 7, while this WGLC is

in progress.



Comments should be sent to the [e-mail]<mailto:[e-mail]> list, although purely

editorial comments may be sent directly to the authors. Please cc:

the WG chairs at [e-mail]<mailto:[e-mail]>  if you would like the chairs

to track such editorial comments as part of the WGLC process.



No IPR disclosures have been submitted directly on this draft.



Thanks,

David, Gorry and Marten

(TSVWG Co-Chairs)





--

________________________________________________________________

Bob Briscoe                               http://bobbriscoe.net/