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

Markku Kojo <kojo@cs.helsinki.fi> Mon, 14 December 2020 15:44 UTC

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Date: Mon, 14 Dec 2020 17:44:23 +0200 (EET)
From: Markku Kojo <kojo@cs.helsinki.fi>
To: Bob Briscoe <in@bobbriscoe.net>
cc: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>, David Black <David.Black@dell.com>, Joe Touch <touch@strayalpha.com>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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Subject: Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: SuggestedFragmentation/Reassemblytext
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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
> 
>