Re: [mpls] [spring] RtgDir Early review: draft-ietf-spring-segment-routing-mpls-13

Rob Shakir <robjs@google.com> Tue, 20 November 2018 04:17 UTC

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From: Rob Shakir <robjs@google.com>
Date: Mon, 19 Nov 2018 20:17:09 -0800
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To: Alex Bogdanov <bogdanov@google.com>
Cc: Przemyslaw Krol <pkrol=40google.com@dmarc.ietf.org>, Shraddha Hegde <shraddha@juniper.net>, rtg-dir@ietf.org, spring@ietf.org, mpls@ietf.org, spring-chairs@ietf.org, jonathan.hardwick@metaswitch.com, draft-ietf-spring-segment-routing-mpls.authors@ietf.org
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Subject: Re: [mpls] [spring] RtgDir Early review: draft-ietf-spring-segment-routing-mpls-13
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Folks,

Thanks for the feedback here -- given that there were relatively late
changes to this draft in its lifecycle to absorb some of the material on
conflicts, if the consensus is that we need to make additions (it seems to
be pointing that way from this small sample), then I (personally, no chair
hat) think we should not worry too much about needing another (short) WGLC.

The alternatives of having undefined cases, or worse, needing an addendum
document very soon after this is published -- seem much worse than a short
delay.

Cheers,
r.

On Mon, Nov 19, 2018 at 10:47 AM Alex Bogdanov <bogdanov@google.com> wrote:

> Hello Shraddha, I think it's an important recommendation to include.
>
> In the absence of another obvious draft/RFC, I would lean towards my
> original proposal of including it as a section in
> draft-ietf-spring-segment-routing-mpls.
>
> Cheers,
>
> Alex
>
>
>
> On Mon, Nov 19, 2018 at 8:02 AM Przemyslaw Krol <pkrol=
> 40google.com@dmarc.ietf.org> wrote:
>
>> Hi Shraddha
>>
>> I think this would be very helpful.
>>
>> pk
>>
>
>> On Sun, Nov 18, 2018 at 8:39 PM Shraddha Hegde <shraddha@juniper.net>
>> wrote:
>>
>>> Hi all,
>>>
>>>
>>>
>>> I am preparing the shepherd write-up and noticed that the topic in below
>>> e-mail thread is an
>>>
>>> Open item. My personal opinion is to add a new section to this draft to
>>> address below cases
>>>
>>> > more than one node advertising the same IPv4/6 PREFIX and both have
>>> the same prefix-SID value with "N" flag
>>>
>>> > where an anycast prefix is advertised with a prefix-SID sub-TLV by
>>> some (but not all) of the nodes that advertise that prefix.
>>>
>>>
>>>
>>> This draft is addressing incoming label collision and resulting behavior
>>> and also describes other aspects like different SIDs for same prefix so it
>>> seems reasonable to add above two cases to this draft.
>>>
>>> WG members, if you have an opinion, pls respond on the list.
>>>
>>>
>>>
>>> Rgds
>>>
>>> Shraddha
>>>
>> *From:* Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
>>>
>> *Sent:* Sunday, November 4, 2018 9:37 PM
>>> *To:* Ahmed Bashandy <abashandy.ietf@gmail.com>
>>>
>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org>rg>; '
>>> adrian@olddog.co.uk' <adrian@olddog.co.uk>uk>; Jonathan Hardwick (
>>> Jonathan.Hardwick@metaswitch.com) <jonathan.hardwick@metaswitch.com>om>;
>>> spring@ietf.org; spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf..org; Shraddha
>>> Hegde <shraddha@juniper.net>
>>>
>>
>>> *Subject:* RE: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>
>>>
>>> Ahmed,
>>>
>>> Apologies for a delayed response.
>>>
>>> I fully agree that advertising the same prefix SID as the Node SID by
>>> two different nodes in the SR domain is “a clear violation of the SR
>>> architecture RFC (8402)”.
>>>
>>> But I do not think that the SR-MPLS draft can silently ignore this
>>> violation just because it “is not an incoming label collision”.
>>>
>>> The same applies to the controversy in advertising at the same prefix as
>>> Anycast by some nodes but not as Anycast (or even as a Node SID) by some
>>> other nodes.
>>>
>>> Your reference to these being just control plane issues and therefore
>>> not related to SR-MPLS is not valid - because the drafts dealing with the
>>> SR control plane to which you refer in this draft are strictly
>>> MPLS-oriented: they define how to advertise *SID labels* or *indices*
>>> that are translated into *SID labels*, and neither of these mechanisms
>>> is relevant fore SRV6 IMHO. (I do not have to remind you that a draft that
>>> defines SRV6 extensions for ISIS
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__datatracker.ietf.org_doc_draft-2Dbashandy-2Disis-2Dsrv6-2Dextensions_-3Finclude-5Ftext-3D1&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=ko-3eF8yySF1exH64SoeyEP0ett4gjsHmmOCvj9zCvQ&s=_AZSiqmTUTMKFS9DAqboueo_GnvvAcFxARWF820HnTA&e=>
>>> exists, and deals with other issues).
>>>
>>> My 2c,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302 <+972%203-926-6302>
>>>
>>> Cell:      +972-549266302 <+972%2054-926-6302>
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>> *From:* Ahmed Bashandy [mailto:abashandy.ietf@gmail.com
>>> <abashandy.ietf@gmail.com>]
>>> *Sent:* Sunday, October 28, 2018 1:01 AM
>>> *To:* Shraddha Hegde <shraddha@juniper.net>et>; Alexander Vainshtein <
>>> Alexander.Vainshtein@ecitele.com>
>>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org>rg>; '
>>> adrian@olddog.co.uk' <adrian@olddog.co.uk>uk>; Jonathan Hardwick (
>>> Jonathan.Hardwick@metaswitch.com) <jonathan.hardwick@metaswitch.com>om>;
>>> spring@ietf.org; spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>> *Subject:* Re: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>> Thanks for the comments
>>>
>>> While it is a clear violation of the SR architecture RFC (8402), more
>>> than one node advertising the same IPv4/6 PREFIX and both have the same
>>> prefix-SID value with "N" flag is not an incoming label collision because
>>> the label is associated with the same FEC, which is the prefix.
>>>
>>> Hence handling such violation is not an SR-MPLS problem because there is
>>> no incoming label collision and hence it it is outside the scope of this
>>> draft
>>>
>>>
>>>
>>> The second issue is which SID to choose for an SR-policy (be it a policy
>>> for TE, ti-lfa, uloop avoidance, security,..., etc). That is strictly a
>>> control layer functionality and is not specific to SR-MPLS. Hence it is
>>> outside the scope of this draft
>>>
>>>
>>>
>>> The third issue is the case where an anycast prefix is advertised with a
>>> prefix-SID sub-TLV by some (but not all) of the nodes that advertise that
>>> prefix. Again this is not an incoming label collision because the label is
>>> associated with a single FEC, which is the anycast prefix.
>>>
>>>
>>>
>>> On 7/19/18 8:30 PM, Shraddha Hegde wrote:
>>>
>> Hi Ahmed,
>>>
>>>
>>>
>>> The Node-SIDs are expected to be unique to a node.
>>>
>>> “
>>>
>>>    An IGP Node-SID MUST NOT be associated with a prefix that is owned by
>>>
>>>    more than one router within the same routing domain.”
>>>
>>>
>>>
>>> If two different nodes advertise same Node-SID,
>>>
>>>          For Example Node A and B both advertise prefix 1.1.1.1 and
>>> associate a  SID 1000 with N bit set.
>>>
>>> There is an anomaly here and IMO, this draft should address how to
>>> handle this anomaly and whether TI-LFA and other
>>>
>>> Applications can use this SID as a Node-SID.
>>>
>>> Another slight variation of this case is a scenario where A and B both
>>> advertise a prefix 1.1.1.1 and A assigns a Node-Sid
>>>
>>> Of 1000 and B does not assign any SID.
>>>
>>>
>>>
>>> Rgds
>>>
>>> Shraddha
>>>
>>>
>>>
>>> *From:* Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
>>> <Alexander.Vainshtein@ecitele.com>
>>> *Sent:* Thursday, July 19, 2018 10:05 PM
>>> *To:* Ahmed Bashandy <abashandy.ietf@gmail.com>
>>> <abashandy.ietf@gmail.com>
>>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org> <mpls@ietf.org>rg>;
>>> 'adrian@olddog.co.uk' <adrian@olddog.co.uk> <adrian@olddog.co.uk>uk>;
>>> Jonathan Hardwick (Jonathan.Hardwick@metaswitch.com)
>>> <jonathan.hardwick@metaswitch.com> <jonathan.hardwick@metaswitch.com>om>;
>>> Shraddha Hegde <shraddha@juniper.net> <shraddha@juniper.net>et>;
>>> spring@ietf.org; spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>> *Subject:* RE: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>> Ahmed hi!
>>>
>>> Lots of thanks for your response.
>>>
>>> Of course Node SIDs are not different from any other Prefix SIDs when it
>>> comes to the MPLS forwarding plane.
>>>
>>> But, IMHO, strictly speaking, this is correct for any other SID as well.
>>>
>>> You seem to ignore the difference between SR-MPLS and SRv6 with regard
>>> to the life span of prefix SIDs in general and Node SIDs in particular.
>>> From my POV this difference should be discussed in the draft.
>>>
>>> So it seems that we can only “agree to disagree” on the need to say
>>> something specific about Node SIDs in the draft, and let the WG to decide
>>> what to do about it.
>>>
>>> Regards,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302 <+972%203-926-6302>
>>>
>>> Cell:      +972-549266302 <+972%2054-926-6302>
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>> *From:* Ahmed Bashandy [mailto:abashandy.ietf@gmail.com
>>> <abashandy.ietf@gmail.com>]
>>> *Sent:* Thursday, July 19, 2018 7:13 PM
>>> *To:* Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>
>>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org>rg>; '
>>> adrian@olddog.co.uk' <adrian@olddog.co.uk>uk>; Jonathan Hardwick (
>>> Jonathan.Hardwick@metaswitch.com) <jonathan.hardwick@metaswitch.com>om>;
>>> shraddha@juniper.net; spring@ietf.org; spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>> *Subject:* Re: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>> Thanks for the reply
>>>
>>> See inline
>>>
>>> Ahmed
>>>
>>>
>>>
>>> On 7/12/18 12:22 AM, Alexander Vainshtein wrote:
>>>
>>> Ahmed and all,
>>>
>>> I would like to expand on my comments (and your responses) about the
>>> role of Node SIDs in SR-MPLS.
>>>
>>> I would like to bring your attention two points:
>>>
>>> 1.       Node SIDs (and, in general, Prefix SIDs) in MPLS-SR are
>>> different from the same in SRv6 because they require explicit configuration
>>> action by the operator of SR domain. I.e., it is not enough for a node to
>>> own some /32 or /128 prefix that is advertised by IGP. The operator must
>>> explicitly configure the node to use such a prefix as  Node SID and to
>>> assign to it a specific index that is unique in the SR domain. From my POV,
>>> this difference alone would qualify Node SIDs as a topic to be discussed in
>>> the MPLS-SR Architecture
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_draft-2Dietf-2Dspring-2Dsegment-2Drouting-2Dmpls-2D14&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=q6djpRXlamUzKZlGIuXTtBcsnwevHwddqvStZrSFMnE&e=>
>>> draft.
>>>
>>> #Ahmed: I disagree with your POV. From the forwarding plane perspective
>>> it does not make any difference whether a the label at the top of an MPLS
>>> packet (representing the prefix-SID) identifies a node or not. So from the
>>> SR-mpls forwarding point of view there is no difference between a
>>> prefix-SID and a node-SID. If there is any place in the SR-mpls draft where
>>> there is a need to handle a node-SID different from a prefix SID, it would
>>> be great to point it out
>>>
>>> 2.      In addition, quite a few constructs associated with SR-MPLS
>>> implicitly assume that each node in the SR-MPLS domain is assigned with at
>>> least one Node SID. One example can be found in the TI-LFA
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_draft-2Dbashandy-2Drtgwg-2Dsegment-2Drouting-2Dti-2Dlfa-2D04&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=jbH0DSYYo2UYymWZrlvAt7qUWVXsYKuCtMiEyoe-DWE&e=>
>>> draft. This draft says in Section 4.2:
>>>
>>>
>>>
>>> 4.2
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_draft-2Dbashandy-2Drtgwg-2Dsegment-2Drouting-2Dti-2Dlfa-2D04-23section-2D4.2&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=sAi3KCWUwGS3D93t8ic64W_46xm9y8Oacs7ozcAweS8&e=>..
>>> The repair node is a PQ node
>>>
>>>
>>>
>>>
>>>
>>>    When the repair node is in P(S,X), the repair list is made of a
>>>
>>>    single node segment to the repair node.
>>>
>>> In the scope of this section, the repair node is not adjacent to the
>>> PLR, and therefore, to the best of my understanding,  “a single node
>>> segment to the repair node” can be only the Node SID of the repair
>>> node. Since repair nodes are computed dynamically, this entire scheme
>>> depends on all nodes in the MPLS=SR domain  having at least one Node SID
>>> each
>>>
>>> #Ahmed: The choice of the SID to identify an intermediate or exit
>>> node(s) in an SR-policy is a control plane behavior, irrespective of reason
>>> such policy is created (be it ti-lfa explicit path, uloop avoidance
>>> explicit path, or some SR-TE explicit path). SR-Policy as well as Ti-LFA
>>> and uloop avoidance are handled in separate drafts. So just like the
>>> response to your previous comment, from forwarding plane perspective it
>>> does not make any difference whether the label at the top of an MPLS packet
>>> identifies a single or multiple nodes.
>>>
>>> .
>>>
>>>
>>>
>>> Hopefully these notes clarify my position on the subject.
>>>
>>>
>>>
>>> Regards,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302 <+972%203-926-6302>
>>>
>>> Cell:      +972-549266302 <+972%2054-926-6302>
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>> *From:* Alexander Vainshtein
>>> *Sent:* Wednesday, July 11, 2018 12:02 PM
>>> *To:* Ahmed Bashandy <abashandy.ietf@gmail.com>
>>> <abashandy.ietf@gmail.com>
>>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org> <mpls@ietf.org>rg>;
>>> 'adrian@olddog.co.uk' <adrian@olddog.co.uk> <adrian@olddog.co.uk>uk>;
>>> Jonathan Hardwick (Jonathan.Hardwick@metaswitch.com)
>>> <jonathan.hardwick@metaswitch.com> <jonathan.hardwick@metaswitch.com>om>;
>>> shraddha@juniper.net; spring@ietf.org; spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>> *Subject:* RE: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>> Ahmed, and all,
>>>
>>> Lots of thanks for a detailed response to my comments.
>>>
>>> Please see *inline below* my position on each of them.
>>>
>>>
>>>
>>> Regards,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302 <+972%203-926-6302>
>>>
>>> Cell:      +972-549266302 <+972%2054-926-6302>
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>> *From:* Ahmed Bashandy [mailto:abashandy.ietf@gmail.com
>>> <abashandy.ietf@gmail.com>]
>>> *Sent:* Wednesday, July 11, 2018 4:42 AM
>>> *To:* Alexander Vainshtein <Alexander.Vainshtein@ecitele.com>om>;
>>> spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>> *Cc:* rtg-dir@ietf.org; 'mpls@ietf.org' <mpls@ietf.org>rg>; '
>>> adrian@olddog.co.uk' <adrian@olddog.co.uk>uk>; Jonathan Hardwick (
>>> Jonathan.Hardwick@metaswitch.com) <jonathan.hardwick@metaswitch.com>om>;
>>> shraddha@juniper.net; spring@ietf.org
>>> *Subject:* Re: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>> Thanks for thorough (and VERY clear) the review
>>>
>>> See inline #Ahmed
>>>
>>>
>>>
>>> Ahmed
>>>
>>>
>>>
>>>
>>>
>>> On 6/15/18 11:08 PM, Alexander Vainshtein wrote:
>>>
>>> Re-sending to  correct SPRING WG list.
>>>
>>> Sincere apologies for the delay caused by a typo.
>>>
>>> Thumb typed by Sasha Vainshtein
>>>
>>>
>>> ------------------------------
>>>
>>> *From:* Alexander Vainshtein
>>> *Sent:* Sunday, June 10, 2018 10:43:52 AM
>>> *To:* spring-chairs@ietf.org;
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org
>>>
>>> *Cc:* spring@ietf..com <spring@ietf.com>om>; rtg-dir@ietf.org; '
>>> mpls@ietf.org'#39;; 'adrian@olddog.co.uk'.uk'; Jonathan Hardwick (
>>> Jonathan.Hardwick@metaswitch.com); shraddha@juniper.net
>>>
>>>
>>> *Subject:* RE: RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>> Explicitly adding Shraddha  who is the shepherd of this draft.
>>>
>>>
>>>
>>> Regards,
>>>
>>> Sasha
>>>
>>>
>>>
>>> Office: +972-39266302 <+972%203-926-6302>
>>>
>>> Cell:      +972-549266302 <+972%2054-926-6302>
>>>
>>> Email:   Alexander.Vainshtein@ecitele.com
>>>
>>>
>>>
>>> *From:* Alexander Vainshtein
>>> *Sent:* Friday, June 8, 2018 5:43 PM
>>> *To:* 'spring-chairs@ietf.org' <spring-chairs@ietf.org>
>>> <spring-chairs@ietf.org>rg>; '
>>> draft-ietf-spring-segment-routing-mpls.authors@ietf.org'
>>> <draft-ietf-spring-segment-routing-mpls.authors@ietf.org>
>>> <draft-ietf-spring-segment-routing-mpls.authors@ietf.org>
>>> *Cc:* 'spring@ietf.com' <spring@ietf.com> <spring@ietf.com>om>;
>>> rtg-dir@ietf.org; mpls@ietf.org; 'adrian@olddog.co.uk'
>>> <adrian@olddog.co.uk> <adrian@olddog.co.uk>
>>> *Subject:* RtgDir Early review:
>>> draft-ietf-spring-segment-routing-mpls-13
>>>
>>>
>>>
>>>
>>>
>>> Hello,
>>>
>>> I have been selected to do a routing directorate “early” review of this
>>> draft:
>>> https://datatracker.ietf.org/doc/draft-ietf-spring-segment-routing-mpls/
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__datatracker.ietf.org_doc_draft-2Dietf-2Dspring-2Dsegment-2Drouting-2Dmpls_&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=Cxbaaf9U0kj6_meVSobSkRLQW1SwI8MJvgHpuYp0QOM&e=>
>>>
>>>
>>>
>>> The routing directorate will, on request from the working group chair,
>>> perform an “early” review of a draft before it is submitted for publication
>>> to the IESG. The early review can be performed at any time during the
>>> draft’s lifetime as a working group document. The purpose of the early
>>> review depends on the stage that the document has reached. As this document
>>> is currently in the WG Last call, my focus for the review was to determine
>>> whether the document is ready to be published. Please consider my comments
>>> along with the other working group last call comments.
>>>
>>>
>>>
>>> For more information about the Routing Directorate, please see ​
>>> http://trac.tools.ietf.org/area/rtg/trac/wiki/RtgDir
>>> <https://urldefense.proofpoint.com/v2/url?u=http-3A__trac.tools.ietf.org_area_rtg_trac_wiki_RtgDir&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=6pnI7l82ewwzoxgOXqTKrbKuQidt6-KBsZdsXFnoQCg&e=>
>>>
>>>
>>>
>>> *Document*: draft-ietf-spring-segment-routing-mpls-13
>>>
>>> *Reviewer*: Alexander (“Sasha”) Vainshtein (
>>> alexander.vainshtein@ecitele.com)
>>>
>>> *Review Date*: 08-Jun-18
>>>
>>> *Intended Status*: Proposed Standard.
>>>
>>>
>>>
>>> *Summary*:
>>>
>>>
>>>
>>> I have some minor concerns about this document that I think should be
>>> resolved before it is submitted to the IESG.
>>>
>>>
>>>
>>> *Comments*:
>>>
>>>
>>>
>>> I consider this draft as an important  companion document to the Segment
>>> Routing Architecture
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_draft-2Dietf-2Dspring-2Dsegment-2Drouting-2D15&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=iJShh7e7yyVkt44v1O5pyCOMfHCpAvfBNGgFr5lk130&e=>
>>> draft that, ideally, should augment definitions of the Segment Routing (SR)
>>> notions and constructs given there with details specific for the SR
>>> instantiation that uses  the MPLS data plane (SR-MPLS).  Many issues raised
>>> in my review reflect either gaps that should be, but have not been, closed,
>>> or inconsistencies between the two drafts.
>>>
>>>
>>>
>>>
>>>
>>> Since RFC 8287
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_rfc8287&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=y7jp3UYNTtcmm9HOulzqPTrMURTrsMiO26rWlNZN5Ws&e=>
>>> is already published as a Standards Track RFC, I expect such augmentation
>>> to be backward compatible with this document (or to provide clear
>>> indications of required updates to this document). And I include the MPLS
>>> WG into distribution list.
>>>
>>>
>>>
>>> This draft was not easy reading for me. In particular, the style of
>>> Section 2.5 that discusses at length and in some detail multiple “corner
>>> cases” resulting, presumably, from misconfiguration, before it explains the
>>> basic (and relatively simple) “normal” behavior, looks problematic to me.
>>>
>>>
>>>
>>> The WG Last Call has been extended by one week. Nevertheless, I am
>>> sending out my comments
>>>
>>>
>>>
>>> *Major Issues*: None found
>>>
>>> #Ahmed: thanks a lot
>>>
>>>
>>>
>>> *Minor Issues*: Quite a few but, hopefully, easy to resolve.
>>>
>>>
>>>
>>> 1.    *Encapsulation of SR-MPLS packets*:
>>>
>>> a.    RFC 3032 (referenced by the draft) and RFC 5332 (*not mentioned
>>> in the draft*) depend two encapsulations of labeled packets - one for
>>> Downstream-allocated labels and another for Upstream-allocated ones.
>>>
>>> #Ahmed: RFC5332 is for multicast. As mentioned in Section 6 of
>>> draft-ietf-spring-segment-routing-15, multicast is outside the scope of SR.
>>> Hence the RFC was not referred to in the SR-MPLS draft
>>>
>>> *[[Sasha]] I would be satisfied with this response, would it not be for
>>> the following text I see in Section 2.2 of the* *SR Policy Architecture*
>>> <https://urldefense.proofpoint.com/v2/url?u=https-3A__tools.ietf.org_html_draft-2Dietf-2Dspring-2Dsegment-2Drouting-2Dpolicy-2D01&d=DwMGaQ&c=HAkYuh63rsuhr6Scbfh0UjBXeMK-ndb3voDTXcWzoCI&r=NyjLsr7JA7mvpCJa0YmPdVKcmMXJ31bpbBaNqzCNrng&m=CBn46-tTjZrFup0dR-EGAtt4QFq9Pi27RaO5rQCk1Qw&s=4f0H68LTvkp7N-bYTVLOhWqiEbHaCsOQR1z_Qzz3Wf4&e=>
>>> *draft:*
>>>
>>>    A variation of SR Policy can be used for packet replication.  A
>>>
>>>    candidate path could comprise multiple SID-Lists; one for each
>>>
>>>    replication path.  In such a scenario, packets are actually
>>>
>>>    replicated through each SID List of the SR Policy to realize a point-
>>>
>>>    to-multipoint service delivery.
>>>
>>>
>>>
>>> *This looks to me as being very much multicast in SR, and, unless you
>>> want to say that it is limited to SRv6, makes my question relevant IMHO.*
>>>
>>> b.    From my POV the ST-MPLS only uses Downstream-allocated labels –
>>> but I expect the draft to state that explicitly, one way or another. (If
>>> Upstream-allocated labels are relevant for SR-MPLS, I would see it as a
>>> major gap, so I hope that this is not the case).
>>>
>>> #Ahmed: I will add a statement in section 2.2 to mention that it is
>>> down-stream allocated as you mentioned
>>>
>>> *[[Sasha]] This is quite unambiguous and, once added, would resolve my
>>> comment in full – the previous comment notwithstanding. In particular, it
>>> would imply that even labels representing BSIDs of a SR Replication
>>> policies will be downstream-allocated. *
>>>
>>>
>>>
>>> 2.    *Label spaces in SR-MPLS*:
>>>
>>> a.    RFC 3031 (referenced by the draft) defines per-platform and
>>> per-interface label spaces, and RFC 5331 (*not mentioned in the draft*)
>>> adds context-specific label spaces and context labels.
>>>
>>> b.    The draft does not say which of these are or are not relevant for
>>> SR-MPLS
>>>
>>> c.    From my POV:
>>>
>>>                                          i.    Labels representing all
>>> kinds of SIDs mentioned in the draft MUST be allocated from the
>>> per-platform label space only
>>>
>>>                                         ii.    At the same time,
>>> instantiation of Mirror Segment IDs defined in Section 5.1 of the Segment
>>> Routing Architecture draft using MPLS data plane clearly calls for context
>>> labels and context-specific label spaces
>>>
>>> d.    I expect the draft to provide a clear-cut position on these
>>> aspects of SR-MPLS.
>>>
>>> #Ahmed: I will add a statement to section 2.2 to say that the it is
>>> per-platform. Regarding the function "mirroring", SR attaches a *function*
>>> to each SID. The "mirroring" function is already described in Section 5.1
>>> of draft-ietf-spring-segment-routing and is not specific to the MPLS
>>> forwarding plane. Hence there is no need to re-mention it here because this
>>> document is trying to be as specific as possible to the MPLS forwarding
>>> plane. General functions attached to SID are described in the segment
>>> routing architecture document or future documents. Furture documents
>>> proposing new SR function must be as specific and clear as possible
>>>
>>> *[[Sasha]] Looks OK to me.*
>>>
>>>
>>>
>>> 3.    *SR-MPLS and hierarchical LSPs*:
>>>
>>> a.    SR LSPs that include more than one segment are hierarchical LSPs
>>> from the POV of the MPLS data plane. Therefore some (possibly, all) of the
>>> models for handling TTL and TC bits that have been defined in RFC 3443 (*not
>>> mentioned in the draft*) should apply to SR-MPLS
>>>
>>> b.    RFC 8287 (*not referenced in the draft*) specifically discussed
>>> operation of the LSP Traceroute function for SR LSPs in the case when
>>> Pipe/Short Pipe model for TTL handling is used
>>>
>>> c.    I expect the draft to provide at least some guidelines regarding
>>> applicability of each specific model defined in RFC 3443 (separately for
>>> TTL and TC bits) to SR-MPLS.
>>>
>>> #Ahmed: BY design, the instantiation of SR over the MPLS forwarding
>>> plane (and hence this draft) does not modify the MPLS forwarding plan
>>> behavior as it is mentioned in the first sentence in Section 1. So the TTL
>>> behavior specified in rfc3443 is already implied and there is no need to
>>> re-mention it here just like all aspects of MPLS forwarding. RFC8287 is
>>> OAM-specific.  SR-OAM is handled in a separate document so is outside the
>>> scope of this draft
>>>
>>> *[[Sasha]] Unfortunately I do not think this is good enough. Let me ask
>>> a specific question reflecting my concerns:*
>>>
>>> *The head-end node sends SR-MPLS packets across a path defined by an
>>> ordered set of SIDs with more than one SID in the list. Each SID is
>>> represented by a label stack entry (LSE) in the MPLS label stack, and the
>>> label field in each LSE is the label that matches the corresponding SID.
>>> However, each LSE also includes the TTL and TC fields. How does the
>>> head-end node set these fields in each of the LSEs following the top one?
>>> This clearly depends on the model (Uniform vs. Pipe/Short Pipe) implemented
>>> in each node that that performs Next operation on the packet along the path
>>> – but the head-end node usually is not aware of that. *
>>>
>>> *RFC 8287 is relevant as an example here IMHO because it recommends the
>>> following setting of TTL in Traceroute packets:*
>>>
>>> -          *Set the TTL of all the labels above one that represents the
>>> segment you are currently tracing to maximum*
>>>
>>> -          *Set the TTL of the label one that represents the segment
>>> you are currently tracing to the desired value (to be incremented until end
>>> of segment is reached*
>>>
>>> -          *Set the TTL of all the labels below one that represents the
>>> segment you are currently tracing to 0.*
>>>
>>> *I expect the draft to provide some recommendations for traffic
>>> (non-OAM) packets as well.*
>>>
>>>
>>>
>>> 4.    *Inferring network layer protocol in SR-MPLS*:
>>>
>>> a.    I wonder if the draft could provide any details on the situation
>>> when a label that represents some kind of SID is the bottom-of-stack label
>>> to be popped by the egress LER
>>>
>>> #ahmed: This is part of the "Next" function. It is described in detail
>>> in this document.
>>>
>>> *[[Sasha]] NEXT function is mentioned in several places in the document.
>>> Can you please point to the specific text that is relevant for my question?*
>>>
>>>
>>>
>>> b.    For the reference, RFC 3032 says that “the identity of the
>>> network layer protocol  must be inferable from the value of the label which
>>> is popped from  the bottom of the stack, possibly along with the contents
>>> of the network layer header itself”
>>>
>>> c.    From my POV the following scenario indicates relevance of this
>>> expectation for SR-MPLS:
>>>
>>>                                          i.    IS-IS is used for
>>> distributing both IPv4 and IPv6 reachability in a given domain
>>>
>>>                                         ii.    An IS-IS adjacency over
>>> some dual-stack link is established, and a single Adj-SID for this
>>> adjacency is advertised
>>>
>>>                                        iii.    The node that has
>>> assigned and advertised this Adj-SID receives a labeled packet with the
>>> label representing this Adj-SID being both the top and bottom-of-stack label
>>>
>>>                                        iv.    The implementers must be
>>> given unambiguous instructions for forwarding the unlabeled packet via the
>>> dual-stack link as an Ipv4 or an IPv6 packet.
>>>
>>> #Ahmed: If you take a look at the SR-ISIS , SR-OSPFv2 and SR-OSFv3
>>> drafts, you will see all 3 protocol advertise different adj-SIDS for IPv4
>>> next-hop and IPv6 next-hop. For example, ISIS uses the "F-Flag" (section
>>> 2.2.1 in draft-ietf-isis-segment-routing-extensions-18) to specify whether
>>> the adj-SID is for IPv4 and IPv6. Similarly, the SR-ISIS draft attaches a
>>> prefix-SID to the prefix advertisement and hence implies the identity of
>>> the protocol underneath the bottom most label. For any other "function"
>>> attached to a SID, it is part of the specification of this function to
>>> describe what happens when the SID is represented by a label in the MPLS
>>> forwarding plane and this label is the bottom most label
>>>
>>> *[[Sasha]] OK, got it. This issue is resolved.*
>>>
>>>
>>>
>>> 5.    *Resolution* *of Conflicts*: Are the
>>>
>>> a.    Are the conflict resolution procedures listed in section 2.5
>>> mandatory to implement?
>>>
>>> b.    If they are mandatory to implement, are they also mandatory to
>>> deploy, or can the operators simply treat any detected conflict as
>>> requiring human intervention and preventing normal operation of SR-MPLS?
>>>
>>> #Ahmed: They are recommended. I will modify the paragraph after the
>>> first 3 bullets in Section 2.5 to say that it is recommeded.
>>>
>>> *[[Sasha]] OK. However, it would be nice if you could refer separately
>>> for “RECOMMENDED to implement” and “RECOMMENDED to deploy”.  The latter
>>> probably requires a configuration knob for enabling conflict resolution
>>> rules (if they are implemented). *
>>>
>>> c.    For the reference, the IETF capitalized MUST appears just in a
>>> few places in Section 2.5, and each appearance has very narrow context:
>>>
>>>                                          i.    For MCCs where the
>>> "Topology" and/or "Algorithm" fields are not defined, the numerical value
>>> of zero MUST be used for these two fields
>>>
>>>                                         ii.    If the same set of FECs
>>> are attached to the same label "L1", then the tie-breaking rules MUST
>>> always select the same FEC irrespective of the order in which the FECs and
>>> the label "L1" are received. In other words, the tie-breaking rule MUST be
>>> deterministic.
>>>
>>>                                        iii.    An implementation of
>>> explicit SID assignment MUST guarantee collision freeness on the same router
>>>
>>> From my POV, it is not possible to infer the answer to my question from
>>> these statements. Some explicit statement is required.
>>>
>>> #Ahmed: I agree with you POV and as mentioned in my reply to items (a)
>>> and (b), I will modify the paragraph to say that it is RECOMMENDED to
>>> answer you questions in items (a) and (b)
>>>
>>> d.    The tie-breaking rules in section 2.5.1 include some specific
>>> values for encoding FEC types and address families – but these values are
>>> not supposed to appear in any IANA registries (because the draft does not
>>> request any IANA actions). Can you please clarify what is so special about
>>> these values?
>>>
>>>