Hi Peter, Thanks for the follow-up.
Please see inline. Cheers, Med De : Peter Psenak <[email protected]> Envoyé : lundi 22 septembre 2025 17:11 À : BOUCADAIR Mohamed INNOV/NET <[email protected]>; The IESG <[email protected]> Cc : [email protected]; [email protected]; [email protected]; [email protected] Objet : Re: Mohamed Boucadair's Discuss on draft-ietf-lsr-igp-ureach-prefix-announce-09: (with DISCUSS and COMMENT) Hi Med, thanks for the comments, please see inline (##PP): On 21/09/2025 12:21, Mohamed Boucadair via Datatracker wrote: Mohamed Boucadair has entered the following ballot position for draft-ietf-lsr-igp-ureach-prefix-announce-09: Discuss When responding, please keep the subject line intact and reply to all email addresses included in the To and CC lines. (Feel free to cut this introductory paragraph, however.) Please refer to https://www.ietf.org/about/groups/iesg/statements/handling-ballot-positions/ for more information about how to handle DISCUSS and COMMENT positions. The document, along with other ballot positions, can be found here: https://datatracker.ietf.org/doc/draft-ietf-lsr-igp-ureach-prefix-announce/ ---------------------------------------------------------------------- DISCUSS: ---------------------------------------------------------------------- Hi Peter, Clarence, Daniel, Shraddha, and Gyan, Thank you for the effort put into this specification. I like the approach adopted here as it leverages exiting features (which is good for backward compatibility) and thus eases incremental deployment. # Meta comment The flow of the document can be made better by introducing the explicit signaling part early in the document as that is what actually demux the use defined in the draft vs. any other future uses of the specific metric. The current flow reveals that the signaling part was added as an after though, not as part of the design. ##PP the document has been re-ordered several times based on the comments during various stages of the review process. I'm a bit reluctant to change it again, as different people have a different priorities, and it's not possible to make everyone happy. In the end, what we are specifying is rather simple, so the ordering does not play that significant role IMHO. [Med] I understand, but I still think this is important to fix. FWIW, as part of this review, I have checked implementations available out there and I was puzzled by one UPA implementation that says "UPA flags are not supported". The current structure reveals this issue: the support/advertisement/propagation are specified separately than signaling (which is the core of the extension and which justifies at the first place the PS status). The signaling part should be integrated in the "Supporting ..." part. 3. Supporting UPA in IS-IS . . . . . . . . . . . . . . . . . . . 5 3.1. Advertisement of UPA in IS-IS . . . . . . . . . . . . . . 5 3.2. Propagation of UPA in IS-IS . . . . . . . . . . . . . . . 6 4. Supporting UPA in OSPF . . . . . . . . . . . . . . . . . . . 6 4.1. Advertisement of UPA in OSPF . . . . . . . . . . . . . . 7 4.2. Propagation of UPA in OSPF . . . . . . . . . . . . . . . 8 5. Signaling UPA . . . . . . . . . . . . . . . . . . . . . . . . 8 5.1. Signaling UPA in IS-IS . . . . . . . . . . . . . . . . . 8 5.2. Signaling UPA in OSPF . . . . . . . . . . . . . . . . . . 9 5.2.1. Signaling UPA in OSPFv2 . . . . . . . . . . . . . . . 9 5.2.2. Signaling UPA in OSPFv3 . . . . . . . . . . . . . . . 9 Please find below some DICUSS points: # Planned Maintenance I expect that adequate reconfiguration will be put in place to isolate a node for PM purposes. The document does not explain how the signal defined here is needed or solves an issue. I appreciate that the introduction include a short mention of the use of overload bit, though. I'm also asking the question because there is no ** normative ** discussion (at least I missed it) about how that can be triggered at the originating ABR. Also, unlike a random failure, a planned failure is associated with an expected start and end times. Signaling the event without those characteristic questions the value of tagging a loss as an NP. ##PP I'm not sure what is being asked. This document does not specify the PM procedure of any kind. This document specifies the UPA signaling, including the bit that signals that the UPA was originated on the ABR/ASBR as a result of the PM in its source area/domain. UPA helps in propagating the information about the connectivity loss caused by the beginning of the PM outside of the area/domain. That's all. [Med] The main point is what issue are we solving by indicating a planned failure, compared to sole use of U-flag? What we lose if the UP-flag is removed? # Configuration Efficiency Section 2 Implementations MAY limit the UPA generation to specific prefixes, e.g. host prefixes, SRv6 locators, or similar. Such filtering is optional and MAY be controlled via configuration. I'm afraid that for the limit to be useful, it should be tweaked based on local operator policy and not on some magic that is internal to the implementation. I suggest the second MAY to be changed to SHOULD when such limit is supported. ##PP done [Med] Thanks. Idem for the following: Implementation MAY provide a configuration option to specify the UPA lifetime at the originating ABR or ASBR. I suggest we update it to: UPA implementations SHOULD provide a configuration option to specify the UPA ^^^^^^^^^^^^^^^^^^^ lifetime at the originating ABR or ASBR. ##PP done [Med] Thanks. The reasoning for this change is that a failure may last long and that an operator would like to maintain that loss state longer (than allowed by a default limit) to accommodate how the specific application consuming the loss signal. Please note that you say that the validity depends on the use case: CURRENT (S.2): The time the UPA is kept in the network SHOULD also reflect the intended use-case for which the UPA was advertised. # Withdrawal Section 2 says: UPA advertisements SHOULD therefore be withdrawn after some amount of time, that would provides sufficient time for UPA to be flooded network-wide and acted upon by receiving nodes, but limits the presence of UPA in the network. The time the UPA is kept in the network SHOULD also reflect the intended use-case for which the UPA was advertised. ... ABR or ASBR MUST withdraw the previously advertised UPA when the reason for which the UPA was generated ceases - e.g. prefix reachability was restored or its metric has changed such that it is below the configured threshold value. The second MUST is not useful if that reason disappears before the timeout that triggers the SHOULD. I think a simple text reorder would fix this. For example, NEW: ABR or ASBR MUST withdraw the previously advertised UPA when the reason for which the UPA was generated ceases - e.g. prefix reachability was restored or its metric has changed such that it is below the configured threshold value. Even if the reasons persist, UPA advertisements SHOULD be withdrawn after some amount of time, that would provides sufficient time for UPA to be flooded network-wide and acted upon by receiving nodes, but limits the presence of UPA in the network. The time the UPA is kept in the network SHOULD also reflect the intended use-case for which the UPA was advertised. ##PP done [Med] ACK # Scalability control Section 2 says: It is also RECOMMENDED that implementations limit the number of UPA advertisements which can be originated at a given time. The benefits gained by summarization may be nullified if a large number of UPAs are injected. This recommendation is really great, but there is a need to expose a configuration parameter here. Pease NEW: UPA implementations SHOULD provide a configuration option to limit the number of such UPAs. ##PP done. [Med] ACK # Backward compatibility Check CURRENT (3.2/4.2): Such requirement of reachability MUST NOT be applied for UPAs, as they are propagating unreachability. Does this mean that an ABR that don't support UPA might discard it? ##PP it will not discard it, bit it will not propagate it between areas. [Med] OK. This confirms what I had in mind: for the feature to be useful, all ABRs should upgraded. Can you please indicate this in the operational considerations section? Thanks. # Avoid redefining exiting behaviors CURRENT (Section 4): In addition, NU-bit is defined for OSPFv3 [RFC5340]. Prefixes having the NU-bit set in their PrefixOptions field SHOULD NOT be included in the routing calculation. The SHOULD NOT part is already defined in 5340. What is this redefined again? ##PP I changed to: "Prefixes having the NU-bit set in their PrefixOptions field are not included in the routing calculation." Is that ok? Without any text it's not clear what's the relevance of the NU bit here. [Med] It is better. You can add a pointer to Section 4.8.1 of [RFC5340]. # Check on "UP-Flag" is useless 5.1.1 The prefix that is advertised with U-Flag or UP-Flag MUST have the metric set to a value larger than 0xFE000000. If the prefix metric is less than or equal 0xFE000000, both of these flags MUST be ignored. 5.2.2 The prefix that is advertised with U-Flag or UP-Flag MUST have the NU-bit set in the PrefixOptions of the parent TLV. The "or UP-Flag" part of both MUSTs is useless as U-Flag will be set in such case as well. The case where only UP-Flag is set is invalid and will be ignored. Unless I missed a subtle thing here, please update these two. ##PP good catch, fixed it. We changed the UP flag from standalone one to be suplementary, but we forgot to update the text. [Med] Thanks. # Service stability The document declares the applications that consume the signal out of scope. Which is fine. However, there might be some negative implications due to how the loss signal (or it withdrawal/absence) is interpreted. For example, add a warning to insist that withdrawal does not mean revert to a nominal state. Having few sentences for these service to take appropriate measure before reverting. ##PP The usage of the UPA depends on the application. As we may not know what applications are going to use it, I would be reluctant to specify the application behavior. [Med] This is not about specifying the behavior but indicating precaution that these applications need to take care of. For some applications, the presence of the UPA may be required, before the application itself detects the loss later. Section 2 says: "ABR or ASBR MUST withdraw the previously advertised UPA when the reason for which the UPA was generated ceases" In such case the withdrawal could mean that the prefix for which the UPA was generated became reachable again. [Med] but we also withdraw UPA after a timer even if the reachability is not OK. Depending solely on the absence of UPA to infer that reachability is back, is not deterministic. UPA is a one time signal for applications that there might be some loss of connectivity. Application itself must use its own mechanism to verify/detect the loss eventually. In the meantime it may decide to do some proactive measures, but UPA does not dictates the application behavior. [Med] We just need to say so in the OPS consideration section. ---------------------------------------------------------------------- COMMENT: ---------------------------------------------------------------------- # Topology-dependent matters Abstract: This enables fast convergence by steering traffic away from the node which owns the prefix and is no longer reachable. "steering.." part of the text is only possible when there alternate routes for that prefix. Otherwise, the traffic will be dropped anyway. Pleas reword. For example, saying "when applicable" or similar would be just fine. ##PP done [Med] Thanks. # Why now? Summarization was there since decades. A mention about what exacerbates the need for this new feature now (and was not considered as a major issue in past) would be helpful. ##PP is it really required? I was not planning to mention it in the text . But if you insist... [Med] As a reader, it is helpful to understand this. It's the SRv6 that brings summarization back in life for underlay (IGPs). With MPLS summarization was not possible. [Med] I see that Robert/Joel have some comments on this. I trust that some motivation that captures the discussion will be recorded in the draft. Thank you. # "Egress" depends on the traffic directionality Section 1: Similarly, when an egress router needs to be taken out of service for maintenance, the traffic is drained from the node before taking it down. A router may behave as ingress/egress as a function of traffic direction. I would delete "egress" here. ##PP done [Med] ACK # (ni) There might be many ABRs per area, not single one Please make this change in Section 1 (and similar): OLD: When prefixes from such node are summarized by the Area Border Router (ABR) or Autonomous System Boundary Router (ASBR), NEW: When prefixes from such node are summarized by an Area Border Router (ABR) or Autonomous System Boundary Router (ASBR), ##PP done [Med] ACK # (Operational Considerations) Many signals, distinct expected uses Section 1 has the following: This document does not define how to advertise prefix that is not reachable for routing. That has been defined for IS-IS in [RFC5305] and [RFC5308], for OSPF in [RFC2328], and for OSPFv3 in [RFC5340]. I wonder whether we can list the signals available out there (explicit prefer advertisement, prefix with a specific metric, etc.) and remind the intend use scope for each of them. This can be record in the Operational Considerations. ##PP that is what the beginning of section 3 and 4 are doing for ISIS and OSPFv2/v3 respectively. [Med] Yeah, but that is lost in the mass. Moving/extending that part is as a helpful operational considerations to clarify the scope for each of these tools and this help those who will deploy. I'm raising this point, especially that the text right after says the following without appropriate scoping: CURRENT: This document defines two new flags in IS-IS, OSPF, and OSPFv3. These flags, together with the existing protocol mechanisms, provide the support for advertising prefix unreachability , together with the ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ reason for which the unreachability is advertised ##PP what do you mean by "without appropriate scoping"? [Med] Saying that « the support for advertising prefix unreachability" may be interpreted that we are advertising explicit prefix reachability, while we are not. # Unreachable metric?! Section 1 has the following: This document defines a method to signal a specific reason for which the prefix with unreachable metric was advertised. Unless, I'm mistaken there is no such "unreachable metric" as a thing in IS-IS. ##PP Please see the section 3 of this document that mentions [RFC5305<https://www.rfc-editor.org/info/rfc5305>]. That RFC defines a metric that makes the prefix "not reachable" for the purpose of the routing. [Med] I'm not sure to follow here. I interpret that metric as exclude it from SPF calculation, not more than that. We are reusing such metric value for UPA. # (Operational Considerations) Activation default Section 2 has the following: UPA MAY be generated by the ABR or ASBR for a prefix that is summarized by the summary address originated by the ABR or ASBR in the following cases: Can we say something about whether the use of UPA be enabled or disabled by default? ##PP I added the following text as a response to one of the Ketan's comments: "Generation of the UPA at the ABR or ASBR is optional and SHOULD be controlled by a configuration knob." I would leave the default behavior for the implementations to decide. I see no reason why an RFC should mandate any specific default behavior. [Med] For this specific case, UPA will be useful of enabled in all ABRs. I would this expect this to be disabled by default. # Threshold Section 2 says: - the metric to reach the prefix from the ABR or ASBR crosses the configured threshold. Can we explicit the threshold we are referring to here + add reference where to look at? ##PP there is no explicit threshold here, it's any value of the metric that the operator defines. For example, if for the planned maintenance the cost of router's links is set to some high value X, the operator may set the threshold on the ABR to X + something. [Med] Thanks. May be s/the configured threshold/a configured threshold # Explicit references in Section 3 OLD: [RFC5305] defines the encoding for advertising IPv4 prefixes using 4 octets of metric information. Section 4 specifies: .. Similarly, [RFC5308] defines the encoding for advertising IPv6 prefixes using 4 octets of metric information. Section 2 states: NEW: [RFC5305] defines the encoding for advertising IPv4 prefixes using 4 octets of metric information. Section 4 of [RFC5305] specifies: .. Similarly, [RFC5308] defines the encoding for advertising IPv6 prefixes using 4 octets of metric information. Section 2 of [RFC5308] states: ##PP Ketan made a similar comment and the text has been updated already. [Med] ACK. # "Advertisement of UPA in IS-IS" CURENT (S3.1) Recognition of the advertisement as UPA is only required on routers which have a valid use case for this information. I would delete this sentence as this section is about the originator. ##PP done [Med] Thanks. # Section 6 Consider moving that section right after current Section 7 as a subsection of an Operational Consideration section. ##PP done [Med] ACK ## Multiple ABRs That section may also discuss whether there are specific consideration to take into account, e.g., presence of multiple ABRs with announces UPAs for a set of prefixes in an area and measures to prevent routing stability. If you don't see any risk out there, saying that as well would be useful. ##PP there is no special risk with multiple ABRs - if the egress PE goes down they will all equally see it and generate the UPA. The problem is when one ABR sees the egress PE reachable and other as unreachable. This can only happen when the area partitions. That problem is described in section 6. [Med] I like explanation with PEs, better. The case I had in mind is, e.g., a multi-ABR case with anycast prefixes injected via distinct PEs but a failure only occurs at one PE leg. Probably there is no issue at all. ## Consider adding any implications (or absence of) explicit withdrawal of an UPA. ##PP I'm not sure what do you mean by explicit withdrawal. UPA must be withdraw by the originator in all cases. [Med] I understand that by withdrawal we mean "to not advertise". Either based on the timeout, or because the reason for which it was generated does not exist anymore, whatever comes first. thanks, Peter Cheers, Med ____________________________________________________________________________________________________________ Ce message et ses pieces jointes peuvent contenir des informations confidentielles ou privilegiees et ne doivent donc pas etre diffuses, exploites ou copies sans autorisation. Si vous avez recu ce message par erreur, veuillez le signaler a l'expediteur et le detruire ainsi que les pieces jointes. Les messages electroniques etant susceptibles d'alteration, Orange decline toute responsabilite si ce message a ete altere, deforme ou falsifie. Merci. This message and its attachments may contain confidential or privileged information that may be protected by law; they should not be distributed, used or copied without authorisation. 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