Re: [tsvwg] Some comments on NQB (part 2)

[Gorry Fairhurst <[e-mail]>]

On 29/04/2022 20:34, Greg White wrote:
> Thanks Ruediger.
>
> Glad to hear that we are converging, though it wasn't clear to me which version of the new text you preferred.  For now, I'll stick with the version that I'd sent on April 4, but let me know if I've misunderstood you.
> Hopefully others find this text change acceptable.
>
> N.B. I don't have any issue with your bigger picture idea, but it is beyond the scope of the NQB draft.   So, if you want to pursue documenting it in an RFC, it probably should be proposed separately.
>
> So, for the NQB draft, are folks ok with replacing:
>
> To support NQB, networks MUST preserve a DSCP marking distinction between NQB traffic and Default traffic when forwarding via an interconnect from or to another network.
> To facilitate the default treatment of NQB traffic in backbones and core networks discussed in the previous section (where IP Precedence may be deployed), networks that support NQB SHOULD NOT use the value 45 for NQB at network interconnects unless that usage is explicitly documented in the TCA (Traffic Conditioning Agreement, see [RFC2475]) for that interconnection.
> Rather, networks SHOULD remap NQB traffic to DSCP 5 prior to interconnection, unless agreed otherwise between the interconnecting partners.
> To be clear, interconnecting networks are not precluded from negotiating (via an SLA, TCA, or some other agreement) a different DSCP to use to signal NQB across an interconnect.
> Additionally, the fact that this PHB is intended for end-to-end usage does not preclude networks from mapping the NQB DSCP to a value other than 45 or 5 for internal usage, as long as the appropriate NQB DSCP is restored when forwarding to another network.
>   
>
> With [notes in square brackets added to help those trying to compare against the above]:
>
> To support NQB, networks MUST preserve a DSCP marking distinction between NQB traffic and Default traffic when forwarding via an interconnect from or to another network.    [no change]
> Networks that support NQB SHOULD support the ability to re-mark NQB traffic prior to such an interconnection.    [new recommendation]

GF: I think "networks that support" is slightly awkard for me. Does this actually mean "support the PHB"?
(if not, can we understand if we can differentiate "carry traffic marked with an NQB DSCP"... To me, ""support the PHB" is something I would configure with respect to an SLA, whereas "carry traffic marked with an NQB DSCP" is something which I could do without configuring, e.g. forward the DSCP using the "default PHB").

> It is RECOMMENDED that interconnecting networks negotiate the use of the DSCP value 45 to indicate NQB traffic across their interconnections (thus avoiding the need to re-mark traffic), however, local DSCP usage by either network could require the use of a different value.   [new recommendation]
> To be clear, interconnecting networks are not precluded from negotiating (via an SLA, TCA, or some other agreement) a different DSCP than 45 to use to mark NQB traffic across an interconnect.  [only editorial change]
> In situations where negotiation of a DSCP between interconnection partners is infeasible, networks that support NQB SHOULD NOT use the value 45 for NQB at network interconnects, but rather SHOULD re-mark NQB traffic to DSCP 5 prior to interconnection.  [limited the applicability of this recommendation]
GF: For me, I don't read the above sentence as being quite what I had 
hoped ... As above, I think there may be a distinction between an 
interconnection that *negotiates* a DSCP usage (and likely deploys a PHB 
mapping), and one that does not. I suspect many current networks/routers 
*do not* negotiate the usage of DSCPs for traffic. So I wonder whether 
we will need to split the advice two-ways:

- I see the case as mentioned that likely applies to ISPs who think 
decide to enforce a SLA:  "negotiation to use DSCP 45 between 
interconnection partners is infeasible" -> which I would see as a good 
to recommend  "SHOULD NOT use the value 45 for NQB at network 
interconnects, but rather SHOULD re-mark NQB traffic to DSCP 5 prior to 
interconnection"

- However, I also would like clarity for the case where there is no 
specific negoiation SLA, then I think the traffic SHOULD use DSCP 45, 
and ought to then pass this codepoint unmodified through this diffserv 
domain. My rationale is that we ought to avoid adding a remarking 
behaviour for routers that could transparently forward the traffic.
> This is intended to facilitate the default treatment of NQB traffic in backbones and core networks discussed in the previous section (where it is possible that IP Precedence may still be deployed).  [only editorial change]
GF: "may still be deployed" seems to miss an opportunity to also add "is 
deprecated" - which I think we should.
> Additionally, the fact that this PHB is intended for end-to-end usage does not preclude networks from mapping the NQB DSCP to a value other than 45 or 5 for internal usage, as long as the appropriate NQB DSCP is restored when forwarding to another network.  [no change]
>
> In addition to Ruediger, I'd like to specifically hear from David Black and Gorry, since two of the original sentences came from David, and Gorry was the OP raising a concern about those sentences.
>
>
> -Greg
>
The discussion looks really close on this point, I hope this helps,

Gorry

>
> On 4/29/22, 3:27 AM, "[e-mail]" <[e-mail]> wrote:
>
>      Hi Greg,
>
>      Thanks. My bigger picture: RFC 8100 is intended to support interconnection policies like:
>
>      If DSCP in range <0-7>      # with a negotiated SLA, different ranges may apply for different backbone PHBs and Codepoint rewrites...
>          then PHB=default
>
>      To me, no SLA negotiation is necessary if forwarding expected by a backbone is "default". In addition, I prefer the interconnection QoS policy to be as simple as possible, if no QoS SLA is negotiated:
>
>      (PHB=default)
>      If DSCP in range <8-63>
>          set DSCP=000 000
>
>      I appreciate your suggested text which allows that; no DSCP 45 traffic should be received at interconnections without negotiated QoS SLA, if the above is deployed. If DiffServ Standards were changed to support the above, 8 DSCP are available for PHBs whose differentiating support is most useful and can be decided upon at the access. I think that would be beneficial for
>      - Lower Effort PHB
>      - L4S / NQB (I think, any DSCP can be rewritten at access and may be at a home gateway, and if a standard proposes a value like 45, the better).
>
>      What I suggest to avoid at interconnection (and will not deploy, where I'm in charge) is (e.g.):
>
>      If <InterconnectionPartnerX> then
>          If DSCP <a>
>              then PHB=default
>          If DSCP <b>
>              then PHB=default AND set DSCP=000 001
>          If DSCP <c>
>              then PHB=EF
>          If DSCP <d>
>              then PHB=AF4 AND set DSCP=001 010
>          If DSCP <e>
>              then PHB=AF4 AND set DSCP=001 100
>          If DSCP <f>
>              then PHB=default AND set DSCP=000 101
>            ....
>          If <no match>
>              then PHB=default AND set DSCP=000 000
>
>      Rather than individual per interconnection partner combined with per DSCP policies at interconnection, I'm looking for simplistic, easily comprehensible and to the extent possible generic Behaviour Aggregate classification. That holds for (range based DSCP) remarking at interconnection too.
>
>      Regards,
>
>      Ruediger
>
>
>
>
>
>      -----Ursprüngliche Nachricht-----
>      Von: Greg White <[e-mail]>
>      Gesendet: Freitag, 29. April 2022 01:12
>      An: Geib, Rüdiger <[e-mail]>
>      Cc: [e-mail]
>      Betreff: Re: [tsvwg] Some comments on NQB (part 2)
>
>      Hi Ruediger,
>
>      Thanks for responding.   See my responses [GW] below.
>
>      -Greg
>
>
>      On 4/26/22, 8:14 AM, "[e-mail]" <[e-mail]> wrote:
>
>          Greg,
>
>          Sorry for the late response.
>
>          <snip>
>
>          You wrote:
>          What I offered for consideration below was that the [DSCP] value 45 be recommended across interconnections in cases where the two interconnecting partners are NQB-aware and they negotiate DSCP markings.
>
>          RG: To me, that's an additional concept. My take was, NQB doesn't require more than default transport in the backbone and at interconnection. If the later holds, negotiation of NQB is no issue to me, but an appropriately picked DSCP is important (it should unambiguously indicate "default forwarding" at interconnection).
>          If a QoS SLA is negotiated, in principle any negotiated DSCP does (it is well known that I prefer RFC 8100 at wholesale and interconnection interfaces, as this simplifies deployment and operation).
>
>      [GW] It seems to me that there isn't such a thing as a DSCP (other than possibly 0) that unambiguously indicates default forwarding at interconnection.  I quickly re-read RFC8100 and also don't see mention of it there (it refers to DSCP=0 as being default and seems to recommend that any traffic classified into the Default / Elastic Treatment Aggregate be re-marked to 0). As I understand it, the practice of aggregating traffic based on the IPP bits (top 3 bits) is not universal. If I'm right in that, then it seems that recommending NQB-aware networks re-mark NQB traffic to 5 and not use 45 at *all* interconnections might be unnecessary (and it was apparently concerning to some).
>      In my post on April 4: https://mailarchive.ietf.org/arch/msg/tsvwg/PKTrfNdTCEXmwoovSkqec6cOJZc/ in response to Gorry's concerns, I had suggested softening this to (paraphrasing here):
>      - If negotiating a DSCP to use at interconnection, recommend 45, but the parties can negotiate whichever value they want.
>      - If negotiation isn't possible, the sending network SHOULD NOT use 45, and instead SHOULD use 5.
>      What about this do you not like?  It seems to me that you're saying that you wouldn't negotiate a DSCP for NQB.  So, based on the proposed text, your interconnection partners SHOULD use 5.
>      Would it make you happier if the first statement were replaced with:
>      - If negotiating a DSCP to use at interconnection, recommend the use of either 5 or 45, but the parties can negotiate whichever value they want.
>
>
>
>          You wrote:
>          The data from Ana Custura and Gorry indicates that, unless something changes in regards to bleaching of the upper 3 bits by some networks, any future assignments of the values 13, 21, 29, 37, 53, 61 would do well to keep in mind that any traffic so marked could end up being aggregated with NQB traffic.  That said, this sort of bleaching is non-compliant with the definition of the DSCP field, and is already problematic for EF, VA, and all of the CS codepoints (which aggregate in incompatible ways), so (as was commented in the last meeting) we may want to consider identifying the routers that continue to do this, and try to work with the associated network operators to change the behavior.
>
>          RG: I'd appreciate a concise reference for your claim "this sort of bleaching is non-compliant with the definition of the DSCP field".
>
>      [GW] I probably didn't choose my words as carefully as I could have, and I made that statement (without doing the appropriate research) based on comments others had made.  RFC2474 Section 3 seems to imply to me that selectively bleaching certain bits of the field is not what was intended, but it does allow that "Nodes MAY rewrite the DS field as needed to provide a desired local or end-to-end service."  So, I don’t see any requirement statement that is violated.
>
>
>          RG: If you are interested, I can sketch examples where single sided changes were made to well  negotiated EF deployments and the interesting consequences caused by that. That's not what mean by "problematic for EF", it rather shows what happens if a QoS design isn't well agreed with all parties responsible for QoS aware network sections and policy points in an operational end-to-end production chain.
>
>          Regards,
>
>          Ruediger
>
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