Re: [RTG-DIR] [spring] Alvaro Retana's Discuss on draft-ietf-spring-segment-routing-mpls-19: (with DISCUSS and COMMENT)

[Alexander Vainshtein <[e-mail]>]

Alvaro, Ahmed and all,
As the twice RTG-DIR reviewer of this draft I should probably have noticed this earlier, but...

I fully agree with Alvaro that the drafts that define SR extensions to IS-IS and OSPF do not say anything about these protocols installing SR-related forwarding entries in the MPLS data plane. What's more, I do not think that such functionality should be specified in these drafts. From my POV, whether SR-related forwarding entries are installed in the MPLS DP by SR extension to the IGP, or by a different entity is a local implementation issue.  I do not think there is any way for an external observer to decide whether these forwarding entries are installed by SR extension to IGP or by some other internal entity. 

I am aware of some SR-MPLS implementations where SR-related forwarding entries are installed in the MPLS DP by dedicated internal entities and not by SR extension to IGP. So far this fact did not affect in any way successful participation of these implementations in public interoperability tests. 

Regards, and apologies for missing this important point in my reviews,
Sasha

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-----Original Message-----
From: spring <[e-mail]> On Behalf Of Ahmed Bashandy
Sent: Tuesday, April 16, 2019 6:14 PM
To: Alvaro Retana <[e-mail]>; The IESG <[e-mail]>
Cc: [e-mail]; [e-mail]; [e-mail]; Shraddha Hegde <[e-mail]>
Subject: Re: [spring] Alvaro Retana's Discuss on draft-ietf-spring-segment-routing-mpls-19: (with DISCUSS and COMMENT)

thanks a lot for the comments (very clear and to the point)

I am taking a look right now and will start discussion with authors of the IGP drafts.

Ahmed



On 4/10/19 1:25 PM, Alvaro Retana via Datatracker wrote:
> Alvaro Retana has entered the following ballot position for
> draft-ietf-spring-segment-routing-mpls-19: Discuss
>
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> Please refer to 
> https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
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>
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-spring-segment-routing-mpl
> s/
>
>
>
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> (1) This first point is a cross-document DISCUSS.  In short, the 
> assumptions in this document about what an MCC is responsible for are 
> not in line with the corresponding IGP drafts for OSPF [1][2] and 
> IS-IS [3].  This misalignment must be resolved before any of these documents are published.
>
> [Note: I'll start a thread with the corresponding WGS, Authors, 
> Shepherds, Chairs and ADs.  Let's please discuss this point there.]
>
> This document uses the following definition in §2: "We call "MPLS 
> Control Plane Client (MCC)" any control plane entity installing 
> forwarding entries in the MPLS data plane.  IGPs with SR extensions...are examples of MCCs."
>
> The focus of the IGP drafts is on the transport of the SR information, 
> and not on other functions (see below).  Which component is 
> responsible for what is the point that needs clarification -- either 
> in this document, the IGP drafts, or both.
>
> These are some specific cases:
>
> (1.1) §2.4 (Mapping a SID Index to an MPLS label): "The following 
> rules MUST be applied by the MCC when calculating the MPLS label value 
> corresponding the SID index value "I"."  There's nothing in the IGP 
> extension documents that point at this set of rules, and only a 
> passing reference in the OSPF documents about outgoing labels.
>
> (1.2) §2.5 (Incoming Label Collision) also assumes more functions from 
> an MCC than what the IGP documents do.  For example: "Within an MCC, 
> apply tie-breaking rules to select one FEC only and assign the label to it."
>
> (1.3) §2.8 also expects work by the IGPs: "the MCC is responsible for 
> downloading the correct label value to FIB"...in this case not just 
> calculating the label, but installing it in the FIB.
>
> (1.4) §2.10.1: "The method by which the MCC on router "R0" determines 
> that PUSH or CONTINUE operation must be applied using the SID "Si" is 
> beyond the scope of this document. An example of a method to determine 
> the SID "Si" for PUSH operation is the case where IS-IS 
> [I-D.ietf-isis-segment-routing-extensions]..." Note that the IS-IS 
> draft (or the OSPF ones, for that matter) don't talk about how to 
> determine the operation
> -- if that is out of scope of this document, then where is it specified?
>
> (1.5) From §2:
>
>     An implementation SHOULD check that an IGP node-SID is not associated
>     with a prefix that is owned by more than one router within the same
>     routing domain. If so, it SHOULD NOT use this Node-SID, MAY use
>     another one if available, and SHOULD log an error.
>
> rfc8402 reads (§3.2): "An IGP Node-SID MUST NOT be associated with a 
> prefix that is owned by more than one router within the same routing 
> domain."  The text above is not in line with that (MUST NOT vs 
> SHOULD).  Also, how can "SHOULD check" be Normatively enforced?
>
> Both sentences above seem to be trying to specify a behavior for the IGPs.
>
> [1] 
> https://tools.ietf.org/html/draft-ietf-ospf-segment-routing-extensions
> [2]
> https://tools.ietf.org/html/draft-ietf-ospf-ospfv3-segment-routing-ext
> ensions [3] 
> https://tools.ietf.org/html/draft-ietf-isis-segment-routing-extensions
>
> (2) §2.5.1: According to §2.5, a "tie-breaking rule MUST be deterministic".
> However, the specification of the default rules are not: the first 
> step uses the administrative distance, but the specification says that 
> "the FEC types are ordered using the default administrative distance 
> ordering defined by the implementation"...and later that the "user 
> SHOULD ensure that the same administrative distance preference is used 
> on all routers".  The combination of different implementations and the 
> lack of an absolute requirement to ensure consistency can easily be non-deterministic.
>
> This point is related to the text in §2.6 which talks about how "the 
> ingress node MUST resolve" collisions the same way.  Because of the 
> lack of an absolute requirement for consistency, this "MUST" doesn't guarantee the same result.
>
> Also related is this text in §2.5.1: "All routers in a routing domain 
> SHOULD use the same tie-breaking rules to maximize forwarding 
> consistency."  When would all routers not use the same rules?  It 
> seems to me that forwarding consistency is very important and would want to be maximized all the time.
> IOW, why not use MUST?
>
> I'm making this point a DISCUSS item because it is directly related to 
> the ability of multiple implementations to interoperate.
>
>
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> (1) §2.2: "A global segment MUST be a label, or an index which may be 
> mapped to an MPLS label within the Segment Routing Global Block 
> (SRGB)..."  I don't think this sentence fragment is clear: the intent 
> is surely to say that the global segment MUST be mapped within the 
> SRGB (and not that it "MUST be a label"), right?  Suggestion: s/A 
> global segment MUST be a label, or an index which may be mapped/A 
> global segment is a label, or an index which MUST be mapped
>
> (2) §2.5: "Suppose an anycast prefix...the advertisement of the 
> prefix-SID by some, but not all, of advertising nodes SHOULD NOT be 
> treated as a label collision."  I'm not sure how the receiver knows if 
> the SID was advertised "by some, but not all"...or even if the prefix 
> is being used as anycast.  Given the Normative language, please 
> explain.  IOW, please clarify the difference between a duplicate 
> prefix-SID and an anycast prefix.  The use of "SHOULD NOT" above seems 
> to imply that there are cases when the situation should be treated as a label collision...what are those cases?
>
> (3) §2.5: "The remaining FECs with the default algorithm...are 
> installed in the FIB...without any incoming labels..."  What will these entries be used for?
> Given that we're talking about an MPLS network, there may be no 
> traffic that matches the FEC (the traffic should be labeled)...if that 
> is the case, then why install in the FIB at all?  OTOH, if there is a 
> possibility that unlabeled traffic is received, then this entry (meant 
> for a different purpose) could be used...also not an ideal situation.
>
> §2.6 makes the case that in order "to minimize the chance of 
> misforwarding, a FEC that loses its incoming label...MUST NOT be 
> installed in FIB".  This inconsistency adds strength to my questions above.
>
> (4) §2.5.1: "if more than one competing FEC remains after step 1, 
> select the smallest numerical FEC value"  What value?  Are you 
> referring to the FEC type (introduced later in this section)?  If so, please be explicit and consistent.
>
> (5) §2.5.2.1: The illustration seems incomplete as the rules in §2.5.2 
> say that "the receiving instance MUST compute its local label", but in 
> this case "B decides not to advertise any index".  The second part of 
> the example (in
> §2.5.2.2) seems to complete the scenario.  It seems confusing to me 
> that the first part shows an incomplete case...or am I misinterpreting the rules?
>
> (6) §2.7: "PUSH, NEXT, and CONTINUE...The specifications of these 
> operations can be found in [RFC8402]. This sub-section specifies how 
> to implement each of these operations in the MPLS forwarding plane."  
> It seems contradictory that the specification is in two places...  In 
> any case, I think that this section is unnecessary as it doesn't seem 
> to add anything from what rfc8402 already explains.
>
> (7) Nits...
>
> s/flooding mechanisms of link state IGPs fits/flooding mechanisms of 
> link state IGPs fit
>
> s/to have a node segment to reach the node/to have a node segment 
> reach the node
>
> s/per routing instance, topology, algorithm/per routing instance, 
> topology, or algorithm
>
> s/except rule/except the rule
>
> s/local label serves/a local label serves
>
> s/subTLVs/sub-TLVs
>
> s/Remaining FECs/The remaining FECs
>
> s/installed in FIB/installed in the FIB
>
> s/lowest value SHOULD be/lowest value SHOULD be:
>
> s/SR Algorithm,)/SR Algorithm)
>
>

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