Sorry, while I respect you both I don’t agree.
Base protocol specifications have controls on the rate of generation of LSAs –
those apply here as they do to all protocol advertisements.
A “BFD Reflector” is defined in S-BFD Base draft as:
“SBFDReflector - an S-BFD session on a network node that listens
for incoming S-BFD control packets to local entities and generates
response S-BFD control packets.”
As far as advertisements of S-BFD discriminators it would not matter whether
the Reflector flaps – it would require a change in the assignment of S-BFD
Discriminator on that node – which is as likely as reconfiguration of a node
address.
Please explain why this rare event represents something which is of concern to
the operation of an IGP.
I do not like cluttering normative specifications with discussions of points
that do not reflect real operational concerns – so the argument “what harm
could it do to discuss this” doesn’t carry much weight with me.
Les
From: rtg-dir [mailto:[email protected]] On Behalf Of Adrian Farrel
Sent: Thursday, April 28, 2016 6:00 AM
To: Acee Lindem (acee); 'Manav Bhatia'; 'Adrian Farrel'
Cc: [email protected]; [email protected];
[email protected]; 'OSPF WG List'
Subject: Re: [RTG-DIR] Rtg Dir review of
draft-ietf-ospf-sbfd-discriminator-04.txt
Acee has it right.
While (of course) stuff can be done in implementations to mitigate the effects,
the protocol extensions here increase the size of LSA and increase the amount
of flooding. Since the LSAs have to be stored (in some form), it is reasonable
to describe the amount of extra information that reflects across a network -
maybe express it as "LSA data" and leave it up to an implementation to choose
how to store it. Since the number of LSA updates impacts the routing plane
processing and bits on the wire, it is reasonable to ask what impact that might
have.
I am interested to hear whether turning Reflectors on and off could be a
feature that could cause LSAs to flap and so create flooding ripples in the
network.
Adrian
From: Acee Lindem (acee) [mailto:[email protected]]
Sent: 28 April 2016 10:21
To: Manav Bhatia; Adrian Farrel
Cc: <[email protected]<mailto:[email protected]>>;
[email protected]<mailto:[email protected]>;
[email protected]<mailto:[email protected]>;
OSPF WG List
Subject: Re: Rtg Dir review of draft-ietf-ospf-sbfd-discriminator-04.txt
Hi Manav,
From: Manav Bhatia <[email protected]<mailto:[email protected]>>
Date: Thursday, April 28, 2016 at 1:31 AM
To: Adrian Farrel <[email protected]<mailto:[email protected]>>
Cc: "<[email protected]<mailto:[email protected]>>"
<[email protected]<mailto:[email protected]>>, Routing Directorate
<[email protected]<mailto:[email protected]>>,
"[email protected]<mailto:[email protected]>"
<[email protected]<mailto:[email protected]>>,
OSPF WG List <[email protected]<mailto:[email protected]>>
Subject: Re: Rtg Dir review of draft-ietf-ospf-sbfd-discriminator-04.txt
Hi Adrian,
Thanks for the extensive review. I have a minor comment on a minor issue that
you raised.
Minor Issues:
I should like to see some small amount of text on the scaling impact on
OSPF. 1. How much additional information will implementations have to
store per node/link in the network? 2. What is the expected churn in
LSAs introduced by this mechanism (especially when the Reflector is
turned on and off)?
Isnt this implementation specific? This is what will differentiate one vendor
implementation from the other.
I am not sure how we can quantify this -- any ideas?
This is akin to saying that IS-IS, in contrast to OSPFv2, is more attuned for
partial SPF runs because the node information is cleanly separated from the
reachability information. However, this isnt entirely true. While i concede
that node information is mixed with prefix information in OSPFv2, there still
are ways in which clever implementations could separate the two and do exactly
what IS-IS does.
I took this rather circuitous approach to drive home the point that
scalability, churn, overheads on the system are in many cases dependent on the
protocol implementation and by that token outside the scope of the IETF drafts.
I believe what is being requested is a discussion of how often the S-BFD TLV is
likely to change, the effects on flooding, and, if required, recommendations
for any rate-limiting or other measures to prevent churn.
Thanks,
Acee
You *do* have...
A change in information in the S-BFD Discriminator TLV MUST NOT
trigger any SPF computation at a receiving router.
...which is a help.
I would be alarmed if an implementation in an absence of this pedantic note
triggered SPF runs each time an S-BFD disc changed ! I mean if you understand
the idea being discussed then you also understand that a change in this TLV has
no bearing on the reachability anywhere. And that knowledge should be enough to
prevent SPF runs in most cases !
I know that we have added this note but if we need to explicitly spell such
things out in all standards then we clearly have bigger problems ! :-)
Cheers, Manav
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