Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext
"Black, David" <David.Black@dell.com> Tue, 09 March 2021 17:25 UTC
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From: "Black, David" <David.Black@dell.com>
To: Bob Briscoe <ietf@bobbriscoe.net>, Markku Kojo <kojo@cs.helsinki.fi>
CC: Markku Kojo <kojo=40cs.helsinki.fi@dmarc.ietf.org>, Joe Touch <touch@strayalpha.com>, "tsvwg@ietf.org" <tsvwg@ietf.org>, "Black, David" <David.Black@dell.com>
Thread-Topic: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext
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Date: Tue, 09 Mar 2021 17:24:43 +0000
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsvwg/L2kHqqqlS7yPdkRVucdgufvdQlo>
Subject: Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext
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Writing as shepherd for both of the ECN encapsulation drafts (ecn-encap-guidelines, rfc6040update-shim), my intent is to look at both drafts shortly after this IETF meeting week (next week if possible, definitely by end of the month), so there's time to resolve the tension between the two "SHOULD" statements. After any text changes, I'll want to put my head together with Bob to come up with a summary of how we got here suitable for the shepherd write-up. Thanks, --David From: Bob Briscoe <ietf@bobbriscoe.net> Sent: Monday, March 8, 2021 11:02 AM To: Markku Kojo Cc: Markku Kojo; tsvwg-chairs@ietf.org; Joe Touch; tsvwg@ietf.org Subject: Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:SuggestedFragmentation/Reassemblytext [EXTERNAL EMAIL] 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><mailto:touch@strayalpha.com> Sent: Wednesday, October 9, 2019 10:29 AM To: Black, David Cc: Jonathan Morton; tsvwg@ietf.org<mailto: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><mailto: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><mailto:chromatix99@gmail.com> Sent: Tuesday, October 8, 2019 6:55 PM To: Black, David Cc: tsvwg@ietf.org<mailto: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><mailto: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/
- [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: Sugg… Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Jonathan Morton
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Joe Touch
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Jonathan Morton
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Markku Kojo
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Bob Briscoe
- [tsvwg] ecn-encap: (was: draft-ietf-tsvwg-rfc6040… Bob Briscoe
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Jonathan Morton
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Markku Kojo
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim: … Bob Briscoe
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Markku Kojo
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Bob Briscoe
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Markku Kojo
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Bob Briscoe
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Jonathan Morton
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Bob Briscoe
- [tsvwg] ecn-encap-guidelines reframing section Bob Briscoe
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Black, David
- Re: [tsvwg] ecn-encap-guidelines reframing section Black, David
- Re: [tsvwg] ecn-encap-guidelines reframing section Bob Briscoe
- Re: [tsvwg] ecn-encap-guidelines reframing section Jonathan Morton
- Re: [tsvwg] ecn-encap-guidelines reframing section Jonathan Morton
- Re: [tsvwg] ecn-encap-guidelines reframing section Black, David
- Re: [tsvwg] draft-ietf-tsvwg-rfc6040update-shim:S… Markku Kojo
- Re: [tsvwg] ecn-encap-guidelines reframing section Markku Kojo
- Re: [tsvwg] ecn-encap-guidelines reframing section Markku Kojo