[mpls] draft-ietf-mpls-spring-lsp-ping: resolution of issues raised in my RTG-DIR review

[Alexander Vainshtein <[e-mail]>]

Deborah and all,

I have read the latest (-11) version of the draft in order to check to which extent my RTG-DIR review comments (that referred to the -06 version of the draft) have been addressed.

Here are the results of this check:

Major Issue #1 in the -06 version of the draft:
Section 7.4 "Segment ID Check" of the draft claims to update "the procedure defined in Step 6 of Section 4.4  of [RFC8029]". However, I have failed to integrate the logic defined by the pseudocode in this section with the logic defined by the referenced pseudocode.

-          I have suspected that the newly introduced logic should be part of the Egress FEC Validation logic defined in Section 4.4.1 of RFC 8029.  This suspicion has been acknowledged by the authors, and they plan to fix the wrong reference in the next revision of the draft.

-          From my POV the best way to avoid possible misinterpretation by the implementers would be inclusion of the updated version of the pseudocode (with explicit marking of the new logic) directly in the document. I have suggested this to the authors, but I did not get their response so far.

Status in the -11 version of the draft:
The problematic text has been replaced with pseudocode that integrates the logic that has been defined in Section 4.4.1 of RFC 8029 with the new, SR-specific pseudocode. So, in a way, the original issue has been resolved. However, I have several issues with the current text:

-          From my POV, the pseudocode could be made much more readable if it were better structured. One possible way to do so could be defining how IGP extensions for SR can be treated as distributing if they distributed labels (including Implicit Null) for Node Segment IDs, and using the results in the original pseudocode of RFC 8029

-          The proposed pseudocode seems to lack the check of Label-L matching the Node Segment ID in the SRGB of the responder

-          It is not obvious how the proposed pseudocode treats previously defined FECs in the target FEC stack

-          Interface-I (the ingress interface of the received LSP Ping request) is only mentioned in the check of Adjacency SID. It is also not clear to me why the check expects its match with the Remote Interface ID in the Adjacency SID TLV.
I did not raise these issues in my original review exactly because it was impossible to integrate the new validation logic with the logic defined in RFC 8029. Please consider these issues as my IESG LC comments.

Major Issue #2 in the -06 version of the draft:
I agree with the previous reviewer regarding the mismatch between the (presumed) claim and the actual scope covered by the draft. To the best of my understanding, the authors have already agreed to fix this in the next revision of the draft.
Status in the -11 version of the draft:
The title of the draft has been changed to reflect its actual scope, and both the Abstract and Introduction clearly indicate that only IGP IP Prefix and Adjacency SIDs are covered by the draft. From my POV, this is sufficient, and this issue is resolved.

Minor Issue #1 in the -06 version of the draft:
I wonder why the authors place such an emphasis on Service labels (which, AFAIK, have not been defined in any level of detail anywhere so far) - these labels are mentioned in Sections 4.2, 5 (that mentions them as "Service segments")  and 8, while so many types of Segment IDs that are already well defined both in the SRPING WG documents and in the extensions of the relevant routing protocols are left out of scope of the draft.  The authors have acknowledged this and plan to fix this issue (probably by removing the Service labels completely).
Status in the -11 version of the draft:
Service labels and service segments do not appear in the new version of the draft.

Minor Issue #2 in the -06 version of the draft:
The description of possible data plane malfunctions in section 4.1 seems to assume that all the nodes in the referenced topology (shown in Figure 1) us the same SRGB. If this assumption (which is not explicitly mentioned) is not correct, the LSP Ping Echo request packets would not be "delivered to their expected destinations but not via the expected path" (as the text in this section claims). I suggest making any such assumptions explicit in the text.
Status in the -11 version of the draft:
The new text of section 4.1 explicitly mentions the assumption about same SRGB.

Minor Issue #3 in the -06 version of the draft:
I am not sure if "OSPF" and ISIS" (without any reference to Segment Routing) are the proper names for the label distribution protocols in the proposed new IANA registry in Section 10.2. From my POV "OSPF/ISIS SR Extensions" would be more appropriate. The authors disagree with this proposal claiming that OSPF and ISIS are not used for label distribution in any other way. (RFC 8029 that encodes the label distribution protocol in the Downstream  Detailed Mapping TLV did not request a dedicated registry for this purpose, and I think that the authors have been right to request such a registry).
Status in the -11 version of the draft:
The new version uses the same protocol names as the previous one. Meanwhile, I have found that OSPF and IS-IS can be used for distriburing label ranges for BIER (see OSPF Extensions for BIER<https://datatracker.ietf.org/doc/draft-ietf-bier-ospf-bier-extensions/?include_text=1> and a similar draft for ISIS). From my POV, this supports my recommendation to change the names of protocols to differentiate between multiple extensions to OSPF and IS-IS that distribute label ranges. Please consider this as my IESG LC WG comment.

Minor Issue #4 in the -06 version of the draft:
I doubt the draft needs address any FRR issues even in future (as mentioned in the last statement in Section 5), since, to the best of my understanding, any form of FRR:

-          Is operated locally by the node that is upstream to the failure without passing any information to the initiator of LSP-Ping Request packets

-          In any case is just a transient state in the (presumably short) interval between the failure being detected by the upstream node and re-convergence of IGP
I also think that references to "future versions" are not quite appropriate in the document that is going to the IETF LC. I recommend removing any such references from the document.
Status in the -11 version of the draft:
All mention of FRR has been removed from the new version, as well as any references to future versions.

Minor Issue #5 in the -06 version of the draft:
Section 9 "Backward Compatibility with non-Segment Routing devices" defines the behavior for the two slightly different use cases:
-              LSP Ping Echo Request packets are initiated by an SR-capable node (and therefore their target FEC stack contains SR-related FECs), while the responder is legacy device that is not SR-capable. In this case the proposed solution (respond with the FEC-return-code "Replying router has no mapping for the FEC at stack-depth") looks as the only possibility since the legacy device cannot parse SR FECs in any case
-              LSP Ping Echo Request packets are initiated by a legacy device while the responder is SR-capable. The draft defines the same behavior in this case - but, IMHO and FWIW, the responder could provide slightly better diagnostics since it can parse the "old" target FEC and recognize the equivalent Node ID. An additional return code would be required to implement this behavior.
This issue has not been discussed with the authors due to lack of time.
Status in the -11 version of the draft:
The authors have responded that a legacy (LDP-only) initiator of LSP Pings request messages in any case would not be able to parse a new return code. I agree with this comment, and withdraw the issue.

All the nits I've noticed also have been resolved. In particular, early IANA allocations that have expired on 15.09.17, have been extended to one more year.


Regards,
Sasha

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