Re: [trill] Adam Roach's Discuss on draft-ietf-trill-ecn-support-05: (with DISCUSS and COMMENT)

[Bob Briscoe <[e-mail]>]

@Adam, Thx for your patience. (It turned out I didn't see your DISCUSS 
cos I had created a bug in my own e-mail filters - sry for that).

@Donald (as editor), Adam's right that we risk implementers doing their 
own thing if they judge that Table 3 might be wrong, because its 
apparent illogicality is not explained without reading the references. 
So, I've suggested one further change to text, inline...


On 20/02/18 22:22, Adam Roach wrote:
> Bob --
>
> Thanks for taking the time to explain the history and rationale of 
> Table 3 to me. I'm going to clear my DISCUSS based on this 
> explanation, since it seems that the technical solution in the 
> document is correct, if a bit non-obvious. Some additional comments 
> inline.
>
> [For those of you who did not see Bob's response: it appears that my 
> original DISCUSS email did not reach him,  so he replied to a reduced 
> audience. I have restored the traditional IESG review distribution to 
> this email. For this reason, I am eliding less of Bob's response than 
> I usually would]
>
> On 2/20/18 12:51 PM, Bob Briscoe wrote:
>>
>>>>>
>>>>>         On Thu, Feb 8, 2018 at 2:15 PM, Adam Roach
>>>>>         <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>
>>>>>             On 2/8/18 11:53 AM, Donald Eastlake wrote:
>>>>>>             Hi Adam,
>>>>>>
>>>>>>             On Wed, Feb 7, 2018 at 8:12 PM, Adam Roach
>>>>>>             <[e-mail] <mailto:[e-mail]>> wrote:
>>>>>>             > Adam Roach has entered the following ballot
>>>>>>             position for
>>>>>>             > draft-ietf-trill-ecn-support-05: Discuss
>>>>>>             >
>>>>>>             > ...
>>>>>>             >
>>>>>>             >
>>>>>>             ----------------------------------------------------------------------
>>>>>>             > DISCUSS:
>>>>>>             >
>>>>>>             ----------------------------------------------------------------------
>>>>>>             >
>>>>>>             > Thanks to the authors, chairs, shepherd, and
>>>>>>             working group for the effort that
>>>>>>             > has been put into this document.
>>>>>>             >
>>>>>>             > I have concerns about some ambiguity and/or
>>>>>>             self-contradiction in this
>>>>>>             > specification, but I suspect these should be easy
>>>>>>             to fix. In particular, the
>>>>>>             > behavior defined in Table 3 doesn't seem to be
>>>>>>             consistent with the behavior
>>>>>>             > described in the prose.
>>>>>>             >
>>>>>>             > For easy reference, I've copied Table 3 here:
>>>>>>             >
>>>>>>             >>
>>>>>>             +---------+----------------------------------------------+
>>>>>>             >>       | Inner   |  Arriving TRILL 3-bit ECN
>>>>>>             Codepoint Name   |
>>>>>>             >>       | Native
>>>>>>              +---------+------------+------------+----------+
>>>>>>             >>       | Header  | Not-ECT | ECT(0)     | ECT(1) |
>>>>>>                 CE   |
>>>>>>             >>
>>>>>>             +---------+---------+------------+------------+----------+
>>>>>>             >>       | Not-ECT | Not-ECT | Not-ECT(*) |
>>>>>>             Not-ECT(*) |  <drop>  |
>>>>>>             >>       |  ECT(0) |  ECT(0) |  ECT(0)    |  ECT(1)  
>>>>>>              | CE   |
>>>>>>             >>       |  ECT(1) |  ECT(1) |  ECT(1)(*) |  ECT(1)  
>>>>>>              | CE   |
>>>>>>             >>       |    CE   |  CE |      CE  |      CE(*) |  
>>>>>>               CE   |
>>>>>>             >>
>>>>>>             +---------+---------+------------+------------+----------+
>>>>>>             >>
>>>>>>             >>  Table 3. Egress ECN Behavior
>>>>>>             >>
>>>>>>             >>  An asterisk in the above table indicates a
>>>>>>             currently unused
>>>>>>             >>  combination that SHOULD be logged. In contrast to
>>>>>>             [RFC6040], in TRILL
>>>>>>             >>  the drop condition is the result of a valid
>>>>>>             combination of events and
>>>>>>             >>  need not be logged.
>>>>>>             >
>>>>>>             > The prose in this document indicates:
>>>>>>             >
>>>>>>             >  1. Ingress gateway either copies the native header
>>>>>>             value to the TRILL ECN
>>>>>>             >     codepoint (resulting in any of the four values
>>>>>>             above) or doesn't insert
>>>>>>             >     any ECN information in the TRILL header.
>>>>>>             >
>>>>>>             >  2. Intermediate gateways can set the CCE flag,
>>>>>>             resulting in "CE" in the
>>>>>>             >     table above.
>>>>>>             >
>>>>>>             > Based on the above, a packet arriving at an egress
>>>>>>             gateway can only be in one of
>>>>>>             > the following states:
>>>>>>             >
>>>>>>             >  A. TRILL header is Not-ECT because no TRILL node
>>>>>>             inserted ECN information.
>>>>>>             >
>>>>>>             >  B. TRILL header value == Native header value
>>>>>>             because the ingress gateway
>>>>>>             >     copied it from native to TRILL.
>>>>>>             >
>>>>>>             >  C. TRILL header is "CE" because an intermediate
>>>>>>             node indicated congestion.
>>>>>>
>>>>>>             Sort of... But note that the TRILL header ECN bit s
>>>>>>             can indicate non-ECT while the CCE bit is set making
>>>>>>             the overall TRILL Header "CE".
>>>>>
>>>>>             Right. That's part of case C. My states above assume
>>>>>             application of Table 2 has already taken place.
>>>>>
>>>>>>
>>>>>>             > If that's correct, I would think that any state
>>>>>>             other than those three needs
>>>>>>             > to be marked with an (*). In particular, these two
>>>>>>             states fall into that
>>>>>>             > classification, and seem to require an asterisk:
>>>>>
>> [BB] These states are not tagged (*) cos they can arise with variants 
>> of ECN marking behaviour referred to in Section 4 (explained beneath 
>> each bullet below). By design (explained further below), a consistent 
>> encap and decap behaviour is intended to support all variants of ECN, 
>> because subnet ingress and egress nodes will very probably be 
>> deployed independently of each other and of intermediate nodes and/or 
>> L4 endpoints.
>>
>> Nonetheless, you're right that this is completely non-obvious to a 
>> reader coming to this new. So let's change the text:
>> CURRENT:
>>     An asterisk in the above table indicates a currently unused
>>     combination that SHOULD be logged. In contrast to [RFC6040 <https://tools.ietf.org/html/rfc6040>], in TRILL
>>     the drop condition is the result of a valid combination of events and
>>     need not be logged.
SUGGESTED (2nd attempt):

    An asterisk in the above table indicates a combination that is currently
    unused in all variants of ECN marking (see Section 4) and therefore
    SHOULD be logged.

    With one exception, the mappings in Table 3 are consistent with those
    for IP-in-IP tunnels [RFC6040], which ensures backward compatibility with
    all current and past variants of ECN marking (see Section 4). It also
    ensures forward compatibility with any future form of ECN marking that
    complies with the guidelines in [ECNencapGuide], including cases where
    ECT(1) represents a second level of marking severity below CE.

    The one exception is that the drop condition in Table 3 need not be
    logged because, with TRILL, it is the result of a valid combination of
    events.


>
> Thanks. That definitely helps; but see my comments below.
>
>>>>>>             >
>>>>>>             >   - Native==CE && TRILL==ECT(0)
>>>>>
>> [BB] You have indeed highlighted an awkward wrinkle here - we could 
>> have tagged this combination with '(*)', cos it is strictly currently 
>> unused by trill. However, I'm going to argue against a '(*)'...
>>
>> My explanation starts with some history: Back in 2001 when ECN in IP 
>> was first defined [RFC3168], an IP-in-IP tunnel ingress set the outer 
>> to ECT(0) if ECN in the native header was anything but Not-ECT 
>> (0b00). This was intended to close off a perceived covert channel in 
>> IPsec. However, since then the security area agreed that this added 
>> over-paranoid complexity. So, since [RFC6040] an IP-in-IP tunnel 
>> ingress now just copies the native ECN to the outer, and TRILL ECN 
>> follows this new simpler model.
>>
>> Altho this spec does not allow a TRILL ingress RBrIdge to exhibit the 
>> old IP-in-IP legacy behaviour, I think it best not to tag this 
>> combination as 'currently unused'. Because, wherever possible, we 
>> want trill's ECN encap and decap  to mimic IP-in-IP (a principle 
>> explained further down this email).
>>
>> A more specific reason: there is already a congestion monitoring 
>> technique being proposed that fakes the legacy IP-in-IP behaviour to 
>> measure congestion across a tunnel 
>> (draft-ietf-tsvwg-tunnel-congestion-feedback). Before it becomes an 
>> RFC, I wouldn't be surprised if that draft gets extended from 
>> IP-in-IP to IP-in-TRILL and other L2 protocols as well. If that 
>> likely event happened, we would have to update this trill-ecn spec to 
>> remove the '(*)' again. In the mean time, there's little harm in not 
>> logging a combination that is not harmful, even tho it is stricty 
>> 'currently unused'.
>>
>> (sry about the triple negative!)
>
> This is a great explanation. I understand the risks of putting 
> forward-looking speculation in RFCs, but could I convince you to at 
> least explain in the document that the reason for this state not being 
> considered out of the ordinary is to mimic legacy IP-in-IP ECN 
> behavior? Any acknowledgement of why this case is being treated the 
> way it is would be sufficient to prevent implementors from deciding 
> that the table is wrong.
>
>>
>>>>>>             >
>>>>>>             >   - Native==ECT(0) && TRILL==ECT(1)
>>>>>
>> [BB] The standards track PCN variant of ECN [RFC6660] uses ECT(1) as 
>> a lower severity of congestion notification than CE. Hence the 
>> asymmetry where a native ECT(0) header can have an ECT(1) outer, but 
>> not vice versa.
>
> As with the case above, it would be good (inasmuch as it would serve 
> to un-confuse implementors) to include an explanation that this case 
> makes sense due to RFC 6660's handling of ECT(0) versus ECT(1). Such a 
> comment would also address my concern quoted below.
>
>>
>>>>>>
>>>>>>             I would defer to my co-author Bob Briscoe but it
>>>>>>             looks to me like you have a good point.
>>>>>
>>>>>             Thanks; I'll wait to hear from Bob.
>>>>>>             > In addition, the behavior this table defines for
>>>>>>             Native==ECT(0) && TRILL==ECT(1)
>>>>>>             > is somewhat perplexing: for this case, the value in
>>>>>>             the TRILL header takes
>>>>>>             > precedence; however, when Native==ECT(1) &&
>>>>>>             TRILL==ECT(0) the Native header
>>>>>>             > takes precedence. Or, put another way, this table
>>>>>>             defines ECT(1) to always
>>>>>>             > override ECT(0). I don't find any prose in here to
>>>>>>             indicate why this needs to be
>>>>>>             > treated differentially, so I'm left to conclude
>>>>>>             that this is a typographical
>>>>>>             > error. If that's not the case, please add
>>>>>>             motivating text to Table 3 explaining
>>>>>>             > why ECT(1) is treated differently than ECT(0) for
>>>>>>             baseline ECN behavior.
>>>>>>
>>>>>>             As noted in Section 4.1, there is an ECN variation
>>>>>>             where ECT(0) just indicates ECT while ECT(1)
>>>>>>             indicates congestion of a lesser severity level has
>>>>>>             been encountered than that indicated by CE. I believe
>>>>>>             the dominance of ECT(1) over ECT(0) is designed to
>>>>>>             not interfere with this variation.
>>>>>
>>>>>             Yes, and section 4 opens with the explanation that
>>>>>             "Section 3 specifies interworking between TRILL and
>>>>>             the original standardized form of ECN in IP
>>>>>             [RFC3168]." Beyond that, I wouldn't expect any of the
>>>>>             text in non-normative section 4 or its subsections to
>>>>>             have bearing on the normative table in Section 3.
>>>>>
>> [BB]
>> * PCN is standards track, hence PCN-specific combinations are not 
>> tagged 'currently unused' in (normative) Section 3.
>> * L4S is experimental track, but it doesn't use any different 
>> combinations of ECN than standard ECN, so it doesn't alter the CU 
>> asterisks in section 3.
>>
>> Section 4 is informative, only because it doesn't define the protocol 
>> spec; it merely explains how the normative spec in Section 3 allows 
>> for ECN variants (whether stds track or experimental).
>
> Thanks. The answer to this that I find compelling is your explanation 
> below about RFC 6040's relative ordering of ECN markings, which I 
> think should be at least mentioned in passing here.
>
>>
>>
>>>>>
>>>>>             My reading of RFC 8311 is that it contemplates a
>>>>>             series of experiments beyond those currently under
>>>>>             development. It may well be that the current
>>>>>             experiments consider ECT(1) to have a higher severity
>>>>>             than ECT(0), but that this may not make sense for
>>>>>             future experiments. Even if it does, I don't see
>>>>>             guidance in RFC 8311 (or any other update to RFC 3168)
>>>>>             that suggests such a relationship between ECT(0) and
>>>>>             ECT(1) exists.
>>>>>
>> As above, the possibility that ECT(1) is a higher severity than 
>> ECT(0) is already on the standards track (PCN [RFC6660]).
>>
>> As far as possible, we want trill subnet endpoints to follow the same 
>> ECN behaviour as IP-in-IP tunnel endpoints [RFC6040]. The explicitly 
>> stated philosophy of RFC6040 and [ECNencapGuide] (which is referred 
>> to from the Intro to this trill spec), is for respectively all tunnel 
>> endpoints and subnet endpoints to have the same mechanical, dumb ECN 
>> behaviour, whatever the variant of ECN in use. Because in the 
>> incrementally deployed public Internet, one cannot know what tunnels 
>> and subnets might be encountered across different paths.
>>
>>
>>>>>
>>>>>             On top of this (as implied by the existence of section
>>>>>             4), the TRILL handling for ECN will need to vary from
>>>>>             experiment to experiment. It seems reasonable that
>>>>>             part of this variability would include different
>>>>>             mapping of ECN bits by the egress gateway.
>>>>>
>> Nope. By design, all variants of ECN use the same encap and decap 
>> behaviours, otherwise they would be undeployable to all intents and 
>> purposes. Perhaps we should make that clearer in this sentence 
>> introducing Section 4:
>>
>> CURRENT:
>>     The ECN wire protocol for TRILL (Section 2 
>> <https://tools.ietf.org/html/draft-ietf-trill-ecn-support-05#section-2>) has been designed to
>>     support the other known variants of ECN,
>> SUGGESTED:
>>     The ECN wire protocol for TRILL (Section 2 
>> <https://tools.ietf.org/html/draft-ietf-trill-ecn-support-05#section-2>) and the ingress (Section 3.1)
>>     and egress (Section 3.3) ECN behaviours have been designed to
>>     support the other known variants of ECN,
>
> This sounds good. It might be worth mentioning that this behavior is 
> intended to also be forward-compatible with future ECN variants, as 
> long as they conform to RFC 6040's notion of relative codepoint 
> priorities.
>
>>
>>>>>
>>>>>             Basically, I see two ways this can be resolved,
>>>>>             although I'm happy to hear alternatives so long as
>>>>>             they end up with the ECN and TRILL documents being in
>>>>>             a consistent and future-proof state:
>>>>>
>>>>>             Approach 1: Revise Table 3 so that ECT(0) and ECT(1)
>>>>>             are treated the same (i.e., pick one of "TRILL header
>>>>>             wins" or "Native header wins" -- I would suggest
>>>>>             "Native header wins," for maximal compatibility with
>>>>>             older ECN clients but I'm not dogmatic on this point),
>>>>>             and then include a normative statement that allows RFC
>>>>>             8311 experiments to override this mapping as makes
>>>>>             sense for their design.
>>>>>
>>>>>             Approach 2: Leave the table as is, but add an
>>>>>             explanation of why ECT(1) is given preferential
>>>>>             treatment over ECT(0).
>>>>>
>> [BB] I hope you're happy with the additional text I've suggested for 
>> why Table 3 was like it was. In summary, consistency with IP-in-IP is 
>> paramount wherever possible.
>
> That's exactly what the additional text should say. :)  I don't see 
> that in your current proposal.
>
>>
>>>>>             Add a normative dependency from this document to a new
>>>>>             document that updates RFC 8311 to add a requirement
>>>>>             that any experiments that treat ECT(0) differently
>>>>>             than ECT(1) MUST be designed such ECT(0) always
>>>>>             indicates a lower severity of congestion than ECT(1).
>>>>>             I presume that this new document would need to be done
>>>>>             in coordination with TSVWG.
>>>>>
>> [BB] That's already catered for in [RFC6040] (stds track) for 
>> IP-in-IP, quoting:
>>     o  In all other cases where the inner supports ECN, the decapsulator
>>        MUST set the outgoing ECN field to the more severe marking of the
>>        outer and inner ECN fields, where the ranking of severity from
>>        highest to lowest is CE, ECT(1), ECT(0), Not-ECT.  This in no way
>>        precludes cases where ECT(1) and ECT(0) have the same severity;
>>
>> and in [ECNencapGuide] (intended BCP) for IP-in-any-L2, quoting:
>>     4.  Congestion indications may be encoded by a severity level.  For
>>         instance increasing levels of congestion might be encoded by
>>         numerically increasing indications, e.g. pre-congestion
>>         notification (PCN) can be encoded in each PDU at three severity
>>         levels in IP or MPLS [RFC6660 <https://tools.ietf.org/html/rfc6660>].
>>
>>         If the arriving inner header is an ECN-PDU, where the inner and
>>         outer headers carry indications of congestion of different
>>         severity, the more severe indication SHOULD be forwarded in
>>         preference to the less severe.
>>
>>
>>>>>
>>>>>             I think Approach 1 is more straightforward, but if
>>>>>             there's a feeling in the working group that we need
>>>>>             default egress behavior that is forwards-compatible
>>>>>             with yet-undesigned experiments, then I think Approach
>>>>>             2 is what you need.
>>>>>
>>>>>             /a
>>>>>
>> The way ECN encap & decap has been designed already follows your 
>> 'approach 2'; where the same consistent behaviour at all encaps and 
>> decaps can be exploited in both backward and forward compatible ways.
>
> Given your explanation above, I agree that such is the case. I think 
> including enough of that explanation that implementors don't find 
> themselves double-guessing the specification in Table 3 would 
> ultimately aid in interoperability.
>
>>
>> Indeed, here's proof that this forward compatibility is not just 
>> theoretical...
>>
>> L4S came along after all this, and required drop probability to be 
>> the square of the marking probability, but only for one of the ECN 
>> codepoints. We managed to contrive the marking behaviour of an 
>> intermediate trill RBridge that supports L4S so that any trill egress 
>> will achieve this precise squaring, even an egress without any ECN or 
>> L4S logic, and no squaring logic either (see Appendix A).
>>
>> Cheers, and thx again for noticing all this.
>>
>>
>>
>> Bob
>
>
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-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/