Hi Changwang, thank you for your prompt response to my comments at the IETF-117. I think that this draft is also relevant for the work of the BFD WG and I have invited its experts to the discussion. I agree with the base premise of the draft that it is essential to control the reverse path of an x-BFD session. But I also believe that such control is easier to realize for BFD sessions in Asynchronous mode, as defined in RFC 5880. Please find my notes below tagged GIM>>.
Regards, Greg On Mon, Jul 31, 2023 at 7:30 AM linchangwang <linchangwang.04...@h3c.com> wrote: > > > Hello Greg, > > > > From minutes-117-spring/: > > 10:15 S-BFD Path Consistency over SRv6 (10 mins) > > Presenter: Changwang Lin > draft-lin-sbfd-path-consistency-over-srv6 > <https://datatracker.ietf.org/doc/draft-lin-sbfd-path-consistency-over-srv6/> > > Greg Mirsky: Current version of the WG draft about the U-BFD, U-BFD is > only applicable to single hop. > > Not sure it is applicable to your scenario which has more than > single link. > > How this mapping between Segment List1 and Segment List2 occurs on a > system (reflector or echo-reflector) that receives a BFD packet? > > All the processing is in the forwarding plane, so in fact the BFD is not > involved. > > Joel: More details, complicated, so we need to take it to the mailing list. > > > > Thank you for your comments ，here is my response： > > 1. Regarding question 1: It is true that the current version of the > [ietf-bfd-unaffiliated-echo] draft only specifies the single hop scenario. > > However, it is worth noting that U-BFD can support multiple hops, for > example, by setting the TTL to 64. Therefore, U-BFD can also be used to > detect the seglist path in an SR policy. > GIM>> Let me quote from Section 2 of draft-ietf-bfd-unaffiliated-echo <https://datatracker.ietf.org/doc/draft-ietf-bfd-unaffiliated-echo/>: All Unaffiliated BFD Echo packets for the session MUST be sent with a Time to Live (TTL) or Hop Limit value of 255, and received with a TTL or Hop Limit value of 254, otherwise the received packets MUST be dropped [RFC5082]. I don't see how, as you suggest, a conformant U-BFD implementation can set TTL/Hop Limit to any value other than 255 or not drop the received packet if the value of its TTL/Hop Limit field is anything but 254. Or, perhaps you are planning to update the current U-BFD specification? > > > 2. Regarding question 2: > > [draft-ietf-idr-sr-policy-path-segment] extends BGP SR Policy to > distribute SR policies with carrying Path Segment and bidirectional path > > information. Through this extension, when distributing SR policy to the > headend, reverse path information and path segment of segment list could be > carried > > together. > > For example： > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > Using path segment and reverse path segment to establish a mapping table. > > Using the mapping table to get segment list by reverse Path segment. > > > > 1) Regarding SBFD, at the reflection end, the reverse seglist can be > obtained through the path-segment mechanism. > GIM>> As I understand RFC 7880, its goal is not to create any state related to SBFDReflector. On the other hand, mapping between a particular Path Segment SID and the Reverse Path Segment List does create such state and, as a result, defeats the idea of RFC 7880. At the same time, binding the reverse path of a BFD session in the Asynchronous mode in SR-MPLS can be easily achieved using, for example, MPLS LSP Ping extensions defined in draft-ietf-mpls-bfd-directed <https://datatracker.ietf.org/doc/draft-ietf-mpls-bfd-directed/> and draft-ietf-spring-bfd <https://datatracker.ietf.org/doc/draft-ietf-spring-bfd/>. > 2) For U-BFD, the complete reverse segment list can be distributed to the > head node along with the segment list. > > This reverse segment list can be used to specify the return path for > U-BFD. Consequently, the return path can be encapsulated at the head end. > GIM>> As I've noted earlier, I believe that U-BFD, as it is defined at this time, cannot be used in SRv6 as suggested in draft-lin-sbfd-path-consistency-over-srv6. > > > > > 3. Regarding question 3： > > By utilizing the extension for SR Policy defined in 【 > draft-ietf-idr-sr-policy-path-segment】: > > 1) By using Path Segment and Segment-Based Forwarding (SBFD) to > encapsulate and forward packets along a forward path, the path-segment is > included in the encapsulation.At the reflector node, this path-segment > information is used to lookup the reverse path-segment, which helps to > ensure the bidirectional consistency of the seglist . This provides a means > to implement SBFD detection with bidirectional path consistency > requirements. > > 2) As for U-BFD, since the complete return information has already > been encapsulated at the head end, there is no need for additional BFD > processing at the reflection system. The packet can simply be forwarded > back along the return path. > > In Summary, when encapsulating SBFD packets with the path-segment, BFD > processing is required at the reflection end. > > On the other hand, U-BFD packets are encapsulated with the complete > reverse seglist and do not require the path-segment. > > Therefore, U-BFD does not need any processing at the reflection end. > GIM>> I cannot agree with your suggestion that the remote end can be simply traversed by a U-BFD Control message. If that is the case, then I don't see how your implementation of U-BFD conforms to the specification (quoted above): All Unaffiliated BFD Echo packets for the session MUST be sent with a Time to Live (TTL) or Hop Limit value of 255, and received with a TTL or Hop Limit value of 254, otherwise the received packets MUST be dropped [RFC5082]. > > > For more detailed encapsulation formats, please refer to > https://datatracker.ietf.org/meeting/117/materials/slides-117-spring-bfd-path-consistency-over-sr-00.pdf > . > > Thanks again! > > > > > > Thanks, > > Changwang > > > > ------------------------------------------------------------------------------------------------------------------------------------- > 本邮件及其附件含有新华三集团的保密信息，仅限于发送给上面地址中列出 > 的个人或群组。禁止任何其他人以任何形式使用（包括但不限于全部或部分地泄露、复制、 > 或散发）本邮件中的信息。如果您错收了本邮件，请您立即电话或邮件通知发件人并删除本 > 邮件！ > This e-mail and its attachments contain confidential information from New > H3C, which is > intended only for the person or entity whose address is listed above. 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