[Lsvr] Fwd: Error Handling for BGP-LS with Segment Routing
Alvaro Retana <aretana.ietf@gmail.com> Wed, 19 December 2018 18:00 UTC
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From: Alvaro Retana <aretana.ietf@gmail.com>
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Subject: [Lsvr] Fwd: Error Handling for BGP-LS with Segment Routing
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Hi! I sent this message to the idr and spring WGs yesterday. It was suggested that this WG should be interested in the topic as well. I’m hoping that most of you are also subscribed to the idr list; if possible, please reply to the original thread: https://mailarchive.ietf.org/arch/msg/idr/0qsjuVaEhroKInHZMohP6HnsWd8 Thanks! Alvaro. On December 18, 2018 at 4:09:07 PM, Alvaro Retana (aretana.ietf@gmail.com) wrote: Dear idr and spring WGs: tl;dr I don't think that BGP-LS, with error handling as specified ("attribute discard"), can provide the robustness that an application (like SR), with direct impact on the forwarding in the network, needs. [Jump to the bottom for discussion.] The BGP-LS extensions for SR (e.g. draft-ietf-idr-bgp-ls-segment-routing-ext) are, as explained in that draft, used so that "an external component (e.g., a controller) then can collect SR information from across an SR domain and construct the end-to-end path (with its associated SIDs) that need to be applied to an incoming packet to achieve the desired end-to-end forwarding." To me, that obviously implies that use of BGP-LS for SR has a direct effect on how traffic is forwarded in the network. Does any one see it differently? The error handling mechanism specified in rfc7752 is "attribute discard" [rfc7606]. If an error is detected, then the information in the controller may be, at best, incomplete, but it could also be out of date...resulting in "segment routes" that don't follow the best available path or that may even end in a black hole. It seems clear to me that this is one of the cases that rfc7606 warned about: o Attribute discard: In this approach, the malformed attribute MUST be discarded and the UPDATE message continues to be processed. This approach MUST NOT be used except in the case of an attribute that has no effect on route selection or installation. ... For any malformed attribute that is handled by the "attribute discard" instead of the "treat-as-withdraw" approach, it is critical to consider the potential impact of doing so. In particular, if the attribute in question has or may have an effect on route selection or installation, the presumption is that discarding it is unsafe unless careful analysis proves otherwise. The analysis should take into account the tradeoff between preserving connectivity and potential side effects. There was a related discussion as a result of my AD review of draft-ietf-idr-ls-distribution (= rfc7606) [1][2]. At that time (2015), the consensus on the list was (paraphrasing): if there's a malformed attribute we won't be able to recover, but that's ok because BGP-LS is "purely application-level data that has no immediate corresponding forwarding state impact", and there won't be an impact on critical AFI/SAFI for network operations. No one else argued against that...so I ended up in the rough... I think the situation has now changed because BGP-LS is carrying SR information that is used to define paths in the network -- even if isolation exists, as described in rfc7752: ... Furthermore, it is anticipated that distribution of this NLRI will be handled by dedicated route reflectors providing a level of isolation and fault containment between different NLRI types. ...the BGP-LS information could still be incomplete, stale, etc.. After all that... I don't think that BGP-LS, with error handling as specified ("attribute discard"), can provide the robustness that an application (like SR), with direct impact on the forwarding in the network, needs. What now? I see several potential paths forward (there are probably more): (1) "fix" BGP-LS to mandate (MUST) isolation and change the error handling approach (2) change the error handling approach...maybe just when used with SR (3) the controller should only use the SR information received from routing protocols (IGP/BGP, e.g. draft-ietf-idr-bgp-prefix-sid) (4) ..?? I didn't find a specific discussion about this topic in the archive...but I may have missed it in between other related ones. If I did, please point me to it. Thoughts/ideas/comments? Thanks! Alvaro. [1] https://mailarchive.ietf.org/arch/msg/idr/FomvQV2DqjaaRiAcLYLn3LcIdYM [2] https://mailarchive.ietf.org/arch/msg/idr/wbPNQ-HM2NeR75gR2Or948J9o1I
- [Lsvr] Fwd: Error Handling for BGP-LS with Segmen… Alvaro Retana