Robert – The paragraph you quote below has to do with BGP behavior in the event “BFD session does not transition to the Up state”. There is no disagreement about what the protocol (BGP or OSPF) should do in this case. The point of strict-mode is to “wait-for-BFD”.
You, however, are trying to introduce some additional requirements. To this end you said: “What I find missing in the draft is a mutually (between OSPF peers) timer fired after BFD session is up which in OSPF could hold on allowing BFD to do some more testing before declaring adj to be established. I think just bringing OSPF adj immediately after the BFD session is up is not a good thing.” So apparently you want BFD to signal UP – but have the protocol do nothing until BFD completes some additional testing. What then was the point of BFD signaling UP to OSPF? And since you want the additional testing to be done by BFD, what new signal should BFD send to OSPF when this is done? The point of BFD sending UP to its clients is to indicate that BFD thinks the link has been verified from the BFD perspective. I do not see the point of sending two such signals. If you think current BFD testing is inadequate please ask for extensions to BFD (in the BFD WG). You also said: “BFD is a great tool to tell you if the end to end path is UP or DOWN. It was not designed to give you any characteristics or metrics for the path quality.” I agree. But if you are now proposing that protocol adjacencies should not come up until certain link quality metrics are met (e.g., link loss, delay) – you are moving into an area that is completely out of scope of this draft. I won’t dig deeper into what could be a very lengthy discussion. If you really want to pursue this idea, I suggest you write a new draft. Les From: Robert Raszuk <rob...@raszuk.net> Sent: Monday, January 31, 2022 6:59 AM To: Albert Fu <af...@bloomberg.net>; Les Ginsberg (ginsberg) <ginsb...@cisco.com>; Ketan Talaulikar <ketant.i...@gmail.com> Cc: Acee Lindem (acee) <a...@cisco.com>; draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org; lsr <lsr@ietf.org> Subject: Re: [Lsr] Working Group Last Call for "OSPF Strict-Mode for BFD" - draft-ietf-lsr-ospf-bfd-strict-mode-04 Les & Ketan Nowadays, it is also common to see the "break-in-middle" failures. we use BFD to detect this sort of failure within sub-second. And to dampen this sort of break-in-middle failures, we will need to use BFD holdtime/dampening. Another data point to the above and this discussion which Albert is co-author of. Ref: https://datatracker.ietf.org/doc/html/draft-ietf-idr-bgp-bfd-strict-mode Please see the below paragraph which clearly says BGP BFD Hold time: If the BFD session does not transition to the Up state, and the HoldTimer has been negotiated to a non-zero value, the BGP FSM will close the session appropriately. If the HoldTimer has been negotiated to a zero value, the session should be closed after a time of X. This time X is referred as "BGP BFD Hold time". The proposed default BGP BFD Hold time value is 30 seconds. The BGP BFD Hold time value is configurable. To me it is clear that BGP BFD Hold time is on the client side and here affects BGP FSM. Thx, Robert. From: ginsb...@cisco.com<mailto:ginsb...@cisco.com> At: 01/30/22 14:38:37 UTC-5:00 To: rob...@raszuk.net<mailto:rob...@raszuk.net>, ketant.i...@gmail.com<mailto:ketant.i...@gmail.com> Cc: Albert Fu (BLOOMBERG/ 120 PARK ) <mailto:af...@bloomberg.net> , a...@cisco.com<mailto:a...@cisco.com>, draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org<mailto:draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org>, lsr@ietf.org<mailto:lsr@ietf.org> Subject: RE: [Lsr] Working Group Last Call for "OSPF Strict-Mode for BFD" - draft-ietf-lsr-ospf-bfd-strict-mode-04 Robert – Here is what you said (emphasis added): <snip> But the timer I am suggesting is not related to BFD operation, but to OSPF (and/or ISIS). It is not about BFD sessions being UP or DOWN. It is about allowing BFD for more testing (with various parameters (for example increasing test packet size in some discrete steps) before OSPF is happy to bring the adj. up. <end snip> Point #1: If you want BFD to do more testing (such as MTU testing) then clearly you need extensions to BFD (such as https://datatracker.ietf.org/doc/draft-ietf-bfd-large-packets/ ) Point #2: The existing timers (as Ketan points out are mentioned in Section 5) are applied today at the OSPF level precisely because OSPF does not currently have strict-mode operation. So in a flapping scenario you could see the following behavior: a)BFD goes down b)OSPF goes down in response to BFD c)OSPF comes back up d)Link is still unstable – so traffic is being dropped some of the time – but perhaps OSPF adjacency stays up (i.e., OSPF hellos get through often enough to keep the OSPF adjacency up) So some implementations have chosen to insert a delay following “b”. This doesn’t guarantee stability, but hopefully makes it less likely. And because OSPF today does NOT wait for BFD to come up, the delay has to be implemented at the OSPF level. Once you have strict mode support, the sequence becomes: a)BFD goes down b)OSPF goes down in response to BFD c)BFD comes back up d)OSPF comes back up Now, if the concern is that BFD comes back up while the link is still unstable, the way to address that is to put a delay either before BFD attempts to bring up a new session or a delay after achieving UP state before it signals UP to its clients – such as OSPF. This is a better solution because all BFD clients benefit from this. Ad if the link is still unstable, it is more likely that the BFD session will go down during the delay period than it would be for OSPF because the BFD timers are significantly more aggressive. (BTW, this behavior can be done w/o a BFD protocol extension – it is purely an implementation choice.) From a design perspective, dampening is always best done at the lowest layer possible. In most cases, interface layer dampening is best. If that is not reliable for some reason, then move one layer up – not two layers up. Les From: Robert Raszuk <rob...@raszuk.net<mailto:rob...@raszuk.net>> Sent: Sunday, January 30, 2022 10:05 AM To: Ketan Talaulikar <ketant.i...@gmail.com<mailto:ketant.i...@gmail.com>> Cc: Les Ginsberg (ginsberg) <ginsb...@cisco.com<mailto:ginsb...@cisco.com>>; Acee Lindem (acee) <a...@cisco.com<mailto:a...@cisco.com>>; draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org<mailto:draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org>; Albert Fu <af...@bloomberg.net<mailto:af...@bloomberg.net>>; lsr <lsr@ietf.org<mailto:lsr@ietf.org>> Subject: Re: [Lsr] Working Group Last Call for "OSPF Strict-Mode for BFD" - draft-ietf-lsr-ospf-bfd-strict-mode-04 Hi Ketan, I would like to point out that the draft discusses the BFD "dampening" or "hold-down" mechanism in Sec 5. We are aware of BFD implementations that include such mechanisms in a protocol-agnostic manner. BFD dampening or hold-time are completely orthogonal to my point. Both have nothing to do with it. Those timers only fire when BFD goes down. In my example BFD does not go down. But we want to bring up the client adj. only after X ms/sec/min etc ...of normal BFD operation if no failure is detected during that timer. This draft indicates that OSPF adjacency will "advance" in the neighbor FSM only after BFD reports UP. And that is exactly too soon. In fact if you do that today without waiting some time (if you retire the current OSPF timer) you will not help at all in the case you are trying to address. Reason being that perhaps 200 ms after BFD UP it will go down, but OSPF adj. will get already established. It is really pretty simple. Thx, Robert. PS. And yes I think ISIS should also get fixed in that respect.
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