Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext

Bob Briscoe <ietf@bobbriscoe.net> Mon, 08 March 2021 16:02 UTC

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To: Markku Kojo <kojo@cs.helsinki.fi>
Cc: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>, "tsvwg-chairs@ietf.org" <tsvwg-chairs@ietf.org>, Joe Touch <touch@strayalpha.com>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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From: Bob Briscoe <ietf@bobbriscoe.net>
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Date: Mon, 8 Mar 2021 16:01:33 +0000
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Subject: Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext
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Markku,

Thx. Unfortunately, this draft is coming up in the mtg this afternoon.
I take some of the blame -  only re-posting the draft a couple of hours 
ago, which presumably reminded you that you were going to think on this.

inline tagged [BB]

On 08/03/2021 13:48, Markku Kojo wrote:
> Hi Bob,
>
> this issue and text on it seems acceptable to me.
>
> However, the other issue with the two contradictory SHOULD's - that I 
> now notice I have never replied to - seems not ok, I think. 

[BB] The question is whether we have to solve this now.

I have a solution to resolve the contradiction. At the risk of 
prolonging the progress of this draft, I'll say it now. But if we can't 
resolve this in the next couple of days, I think we should go ahead with 
the two contradictory SHOULDs.

For the list, here's the two contradictory paras:

    Congestion indications SHOULD be propagated on the basis that an
    encapsulator or decapsulator SHOULD approximately preserve the
    proportion of PDUs with congestion indications arriving and leaving.

    The mechanism for propagating congestion indications SHOULD ensure
    that any incoming congestion indication is propagated immediately,
    not held awaiting the possibility of further congestion indications
    to be sufficient to indicate congestion on an outgoing PDU.


Possible resolution of the contradiction: the "SHOULD approximately 
preserve the proportion" is a rough long term average goal while "SHOULD 
ensure that incoming congestion indication is propagated immediately" is 
a requirement for after there has been some period (TBD) without any 
marking.

The big question is where would an implementer set that timescale? It 
needs to be a "typical RTT in the deployment environment" or some such 
get-out clause. I guess this is best left to the implementer.



Bob


> I'm now occupied for the next few hours, so I'll come back with more 
> detailed reasoning after the tsvwg meeting today.
>
> Cheers,
>
> /Markku
>
> On Mon, 8 Mar 2021, Bob Briscoe wrote:
>
>> Markku, chairs, all,
>>
>> Having reached agreement on the text last Dec, I then went and 
>> dropped the ball and
>> forgot all about this draft,... and ecn-encap-guidelines.
>>
>> == ECN-ENCAP-GUIDELINES ==
>>
>> I shall upload a new rev shortly with the following single diff that 
>> I noticed
>> during the meeting last Nov, and said I would do at the next rev:
>>
>>  4.6.  Reframing and Congestion Markings
>>
>>     The guidance in this section is worded in terms of framing
>>     boundaries, but it applies equally whether the protocol data units
>> -   are frames, cells, packets or fragments.
>> +   are frames, cells or packets.
>>
>> == RFC6040UPDATE-SHIM ==
>>
>> I shall upload a new rev shortly, with the following 3 paras at the 
>> end of S.5 on
>> ECN fragmentation/reassembly:
>>
>> Para 1 is just moved up from the end but otherwise unchanged.
>> Para 2 is unchanged.
>> Para 3 is the text agreed on this list last Dec subject to further 
>> checking, with
>> one exception: I removed the citation of RFC3168 after "equivalent".
>>     Reason: The citation of RFC6040 at the end is the relevant one, 
>> 'cos it
>> introduced the mechanism for ECT(0) and ECT(1) to be either 
>> equivalent or two
>> severity levels. In this respect it updated RFC3168. So it would not 
>> be appropriate
>> to cite RFC3168, which only said the two were equivalent. If we cited 
>> RFC3168 here,
>> it could be interpreted as if we're saying two RFC give conflicting 
>> definitions.
>>
>>     Section 5.3 of [RFC3168] defines the process that a tunnel egress
>>     follows to reassemble sets of outer fragments
>>     [I-D.ietf-intarea-tunnels] into packets.
>>
>>     During reassembly of outer fragments [I-D.ietf-intarea-tunnels], if
>>     the ECN fields of the outer headers being reassembled into a single
>>     packet consist of a mixture of Not-ECT and other ECN codepoints, the
>>     packet MUST be discarded.
>>
>> +   If there is mix of ECT(0) and ECT(1) fragments, then the reassembled
>> +   packet MUST be set to either ECT(0) or ECT(1).  In this case,
>> +   reassembly SHOULD take into account that the RFC series has so far
>> +   ensured that ECT(0) and ECT(1) can either be considered equivalent,
>> +   or they can provide 2 levels of congestion severity, where the
>> +   ranking of severity from highest to lowest is CE, ECT(1), ECT(0)
>> +   [RFC6040].
>>
>> If any of this isn't acceptable, I'll have to post another rev, but I 
>> think it's
>> what was agreed.
>>
>> Cheers
>>
>>
>>
>> Bob
>>
>>
>> On 14/12/2020 15:44, Markku Kojo wrote:
>>       Hi Bob, all,
>>
>>       apologies for the delay, now catching up again.
>>
>>       yes, handling mix of ECT(0)/ECT(1) like in the new proposed 
>> text below
>>       seems reasonable choice (for now).
>>
>>       I'll come back shortly with the issue in the other thread. It 
>> seems
>>       less clear, actually seems quite difficult to handle correctly 
>> for all
>>       foreseen cases.
>>
>>       /Markku
>>
>>       On Thu, 3 Dec 2020, Bob Briscoe wrote:
>>
>>             Markku, all,
>>
>>             I am also only now catching up with the list...
>>
>>             On 17/11/2019 08:46, Markku Kojo wrote:
>>                   Hi Dave, Joe, All,
>>
>>                   Catching up ...
>>
>>                   I agree with the modified new text as well as
>>             treatment of an ECT(0)/ECT(1)
>>                   mix as "any".
>>
>>
>>             [BB] Thanks. For the list, the current text that Markku is
>>             agreeing with is here:
>> https://tools.ietf.org/html/draft-ietf-tsvwg-rfc6040update-shim-11#section-5
>>
>>             Regarding reassembly of a mix of ECT(0)/ECT(1). I agree
>>             with David that the current text
>>             should handle this case that 3168 doesn't address.
>>             And I agree with Joe that an interim way of handling it is
>>             needed, not just punting until
>>             later.
>>
>>             I see that all of Jonathan, David and you Markku are happy
>>             with reassembling a mix of
>>             ECT(0) and ECT(1) to result in either ECT(0) or ECT(1).
>>             (for now). I think we can go one
>>             better than that, still without precluding a more specific
>>             RFC later. Here's proposed
>>             text:
>>
>>             After the following para:
>>
>>                During reassembly of outer fragments
>>             [I-D.ietf-intarea-tunnels], if
>>                the ECN fields of the outer headers being reassembled
>>             into a single
>>                packet consist of a mixture of Not-ECT and other ECN
>>             codepoints, the
>>                packet MUST be discarded.
>>
>>             Add:
>>
>>                   If there is mix of ECT(0) and ECT(1) fragments, then
>>             the reassembled packet
>>                   MUST be set to either ECT(0) or ECT(1). In this case,
>>             reassembly SHOULD take
>>                   into account that the RFC series has so far ensured
>>             that ECT(0) and ECT(1)
>>                   can either be considered equivalent [RFC3168], or
>>             they can provide 2 levels
>>                   of congestion severity, where the ranking of severity
>>             from highest to lowest
>>                   is CE, ECT(1), ECT(0) [RFC6040].
>>
>>
>>             Rationale: This avoids constraining future RFCs, but at
>>             least lays out all the
>>             interoperabilityrequirements we already have for handling
>>             this mixture. Then if an
>>             implementer wants to just default to choosing one, it hints
>>             that they should choose
>>             ECT(1).
>>
>>
>>
>>                   I also want to repeat my comment that
>>
>>                    draft-ietf-tsvwg-ecn-encap-guidelines-13
>>
>>                   added similar new text that alters RFC 3168, and it
>>             should be modified
>>                   accordingly.
>>
>>
>>             [BB] I'll start another thread for this, rather than make
>>             this thread too unweildy.
>>
>>
>>
>>             Bob
>>
>>
>>
>>                   Thanks,
>>
>>                   /Markku
>>
>>                   PS. I missed Bob's response to my comment at the
>>             time, but will reply it
>>                   separately at some point.
>>
>>
>>                   On Wed, 9 Oct 2019, David Black wrote:
>>
>>                         At this juncture, for an ECT(0)/ECT(1) mix
>>             across a set of
>>                         fragments being reassembled, I would suggest
>>             using "any" (i.e.,
>>                         either is ok) at this juncture to avoid
>>             constraining what we may
>>                         do in the future; in particular, this allows
>>             use of the value in
>>                         the first or last fragment, both of which are
>>             likely to be
>>                         convenient approaches for some implementations.
>>
>>                         Thanks, --David
>>
>>                               -----Original Message-----
>>                               From: Joe Touch <touch@strayalpha.com>
>>                               Sent: Wednesday, October 9, 2019 10:29 AM
>>                               To: Black, David
>>                               Cc: Jonathan Morton; tsvwg@ietf.org
>>                               Subject: Re: [tsvwg]
>> draft-ietf-tsvwg-rfc6040update-shim:
>>             Suggested
>>                               Fragmentation/Reassembly text
>>
>>
>>                               [EXTERNAL EMAIL]
>>
>>                               Hi, all,
>>
>>                               I disagree with the suggestion below.
>>
>>                               Pushing this “under the rug” for an
>>             indeterminate
>>                               later date only serves to
>>                               undermine the importance of this issue.
>>
>>                               At a MINIMUM, there needs to be direct
>>             guidance in
>>                               place until a “better”
>>                               solution can be developed. For now, that
>>             would mean
>>                               one of the following:
>>                               - use the max of the frag code point
>>             values
>>                               - use the min of the frag code point
>>             values
>>                               - use “any” of the frag code point values
>>                               - pick some other way (first, the one in
>>             the initial
>>                               fragment i.e., offset 0), etc.
>>
>>                               One of these needs to be *included at
>>             this time*.
>>
>>                               If a clean up doc needs to be issued, it
>>             can override
>>                               individual “scattered”
>>                               recommendations later.
>>
>>                               Joe
>>
>>                                     On Oct 9, 2019, at 6:33 AM, Black,
>>             David
>>                                     <David.Black@dell.com> wrote:
>>
>>                                           The one case this doesn't
>>                                           really cover is what happens
>>                                           when a fragment
>>
>>                               set
>>                                           has a mixture of ECT(0) and
>>                                           ECT(1) codepoints. This
>>                                           probably isn't very
>>                                           relevant to current ECN
>>                                           usage, but may become
>>                                           relevant with SCE, in
>>
>>                               which
>>                                           middleboxes on the tunnel
>>                                           path may introduce such a
>>                                           mixture to formerly
>>                                           "pure" packets.  From my
>>                                           perspective, a likely
>>                                           RFC-3168 compliant
>>                                           implementation of arbitrarily
>>                                           choosing one fragment's ECN
>>                                           codepoint as
>>                                           authoritative (where it
>>                                           doesn't conflict with other
>>                                           rules) is acceptable, but
>>                                           this doesn't currently seem
>>                                           to be mandatory.
>>
>>                                           With the above language, it
>>                                           should be sufficient to
>>                                           update RFC-3168 to
>>
>>                               cover
>>                                           this case at an appropriate
>>                                           time, rather than scattering
>>                                           further
>>
>>                               requirements
>>                                           in many documents.
>>
>>
>>                                     I would concur that using a
>>             separate
>>                                     draft to cover that case at the
>>
>>                               appropriate time would be the better
>>             course of
>>                               action.
>>
>>                                     Thanks, --David
>>
>>                                           -----Original Message-----
>>                                           From: Jonathan Morton
>> <chromatix99@gmail.com>
>>                                           Sent: Tuesday, October 8,
>>                                           2019 6:55 PM
>>                                           To: Black, David
>>                                           Cc: tsvwg@ietf.org
>>                                           Subject: Re: [tsvwg]
>>
>>             draft-ietf-tsvwg-rfc6040update-shim:
>>                                           Suggested
>> Fragmentation/Reassembly text
>>
>>
>>                                           [EXTERNAL EMAIL]
>>
>>                                                 On 8 Oct, 2019,
>>                                                 at 10:51 pm,
>>                                                 Black, David
>> <David.Black@dell.com>
>>                                                 wrote:
>>
>>                                                 **NEW**: Beyond
>>                                                 those first two
>>                                                 paragraphs, I
>>                                                 suggest deleting
>>                                                 the
>>
>>                               rest
>>                                           of Section 5 of the
>>                                           rfc6040update-shim draft and
>>                                           substituting the
>>
>>                               following
>>                                           paragraph:
>>
>>                                                   As a tunnel
>>                                                 egress
>>                                                 reassembles sets
>>                                                 of outer
>>                                                 fragments
>>
>>
>>             [I-D.ietf-intarea-tunnels]
>>                                                 into packets, it
>>                                                 MUST comply with
>>                                                   the reassembly
>>                                                 requirements in
>>                                                 Section 5.3 of
>>                                                 RFC 3168 in
>>                                                   order to ensure
>>                                                 that indications
>>                                                 of congestion are
>>                                                 not lost.
>>
>>                                                 It is certainly
>>                                                 possible to
>>                                                 continue from
>>                                                 that text to
>>                                                 paraphrase part
>>                                                 or all
>>
>>                               of
>>                                           Section 5.3 of RFC 3168, but
>>                                           I think the above text
>>                                           crisply addresses the
>>                                           problem, and avoids
>>                                           possibilities of subtle
>>                                           divergence.  I do like the
>>                                           “reassembles sets of outer
>>                                           fragments” lead-in text
>>                                           (which I copied from
>>
>>                               the
>>                                           current rfc6040shim-update
>>                                           draft) because that text
>>                                           makes it clear that
>>                                           reassembly logically precedes
>>                                           decapsulation at the tunnel
>>                                           egress.
>>
>>                                                 Comments?
>>
>>
>>                                           Looks good to me.
>>
>>                                           The one case this doesn't
>>                                           really cover is what happens
>>                                           when a fragment
>>
>>                               set
>>                                           has a mixture of ECT(0) and
>>                                           ECT(1) codepoints. This
>>                                           probably isn't very
>>                                           relevant to current ECN
>>                                           usage, but may become
>>                                           relevant with SCE, in
>>
>>                               which
>>                                           middleboxes on the tunnel
>>                                           path may introduce such a
>>                                           mixture to formerly
>>                                           "pure" packets.  From my
>>                                           perspective, a likely
>>                                           RFC-3168 compliant
>>                                           implementation of arbitrarily
>>                                           choosing one fragment's ECN
>>                                           codepoint as
>>                                           authoritative (where it
>>                                           doesn't conflict with other
>>                                           rules) is acceptable, but
>>                                           this doesn't currently seem
>>                                           to be mandatory.
>>
>>                                           With the above language, it
>>                                           should be sufficient to
>>                                           update RFC-3168 to
>>
>>                               cover
>>                                           this case at an appropriate
>>                                           time, rather than scattering
>>                                           further
>>
>>                               requirements
>>                                           in many documents.
>>
>>                                           - Jonathan Morton
>>
>>
>>
>>
>>
>>             --
>> ________________________________________________________________
>>             Bob Briscoe
>>             http://bobbriscoe.net/
>>                            PRIVILEGED AND CONFIDENTIAL
>>
>>
>>
>> -- 
>> ________________________________________________________________
>> Bob Briscoe                               http://bobbriscoe.net/
>>
>>

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/