Albert – We are in full agreement.
Delays in bringing BFD backup after a previous failure may well be warranted in the break-in-middle scenarios. I am not convinced this needs to be standardized – seems quite appropriate as an implementation choice. But if any discussion were to occur in RFCs, I think it should be in some BFD document. As this draft is focused on OSPF protocol extensions, I don’t think BFD dampening needs to be discussed. In any case it should not alter the interaction between BFD and protocols. If it takes longer for BFD to come up that just means the OSPF adjacency will not come up either – which is exactly the behavior that is desired. Les From: Albert Fu (BLOOMBERG/ 120 PARK) <af...@bloomberg.net> Sent: Monday, January 31, 2022 6:50 AM To: Les Ginsberg (ginsberg) <ginsb...@cisco.com>; ketant.i...@gmail.com; rob...@raszuk.net Cc: Acee Lindem (acee) <a...@cisco.com>; draft-ietf-lsr-ospf-bfd-strict-m...@ietf.org; 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 Les, Your scenario below is indeed something we have encountered in our production network in the non-strict scenario, due to "flapping" links, where routing protocol could come up before BFD due to "break-in-middle" link issue (interface stayed up, so routing protocol remained active). Strict mode will address this issue. Another point to add is that we do have as a standard on our interfaces to safeguard against flapping link by configuring interface hold-time/carrier-delay. However, this is only useful in situations where the link physically goes down (and fast detection is automatic in most implementation). 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. Thanks Albert 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.
_______________________________________________ Lsr mailing list Lsr@ietf.org https://www.ietf.org/mailman/listinfo/lsr