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Re: [Lsr] Dynamic flow control for flooding

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Wed, 24 July 2019 19:00 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: "henk.ietf@xs4all.nl" <henk.ietf@xs4all.nl>
CC: "stephane.litkowski@orange.com" <stephane.litkowski@orange.com>, Tony Li <tony.li@tony.li>, "lsr@ietf.org" <lsr@ietf.org>
Thread-Topic: [Lsr] Dynamic flow control for flooding
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Date: Wed, 24 Jul 2019 19:00:20 +0000
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Subject: Re: [Lsr] Dynamic flow control for flooding
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Henk -

Welcome to the discussion.

Inline.

> -----Original Message-----
> From: henk.ietf@xs4all.nl <henk.ietf@xs4all.nl>
> Sent: Wednesday, July 24, 2019 5:34 AM
> To: Les Ginsberg (ginsberg) <ginsberg@cisco.com>
> Cc: stephane.litkowski@orange.com; Tony Li <tony.li@tony.li>; lsr@ietf.org
> Subject: Re: [Lsr] Dynamic flow control for flooding
> 
> 
> Hello Les,
> 
> Les Ginsberg (ginsberg) schreef op 2019-07-24 07:17:
> >
> > There is something to be said for simply “flooding fast” and not
> > worrying about flow control at all (regardless of whether TX or RX
> > mechanisms would be used). Some packets would be dropped, but
> > retransmission timers will insure that the flooding eventually
> > succeeds and retransmit timers are long (5 seconds by default). (I am
> > not the only one mentioning this BTW…)
> 
> Why do we have initial waits and exponential backoffs for LSP-generation
> and SPF-computations ? Why not react immediately ? Why not react
> constantly ?
> 
[Les:] I think you know what I am about to say.. 😊

Initial wait exists in order to minimize premature SPF calculation. Topology changes always entail multiple LSP updates (often 2 for a single link failure - more for a node failure or line card failure).
We wait a short while to increase the likelihood that we have received all of the LSP updates associated w the topology event.

Backoff is used so that if the network has an extended period of instability we do not invest cycles w diminishing returns i.e., the updates we are making to forwarding are likely stale before we even complete the update.

> We have a lot of bandwidth and cpu-power now. Isn't simple always better
> than "overly complex stuff" like exponential backoffs ? If you have more
> cpu-power, more memory and more bandwidth, why invent new algorithms ?

[Les:] Base spec never defined an algorithm for dynamically changing flooding speed (unless you consider retransmit timers). So this is "inventing" - not "reinventing".

   Les
> 
> henk.
> 
> 
> I hope it is clear to everyone that these are not serious questions. I'm
> just
> saying: "sometimes fast is slow". I am sure that if we ask the "old
> guys", they
> can come up with many stories how these problems are sometimes
> counter-intuitive.
> And how networks have melted because "fast is slow". I could tell at
> least 2
> of those stories myself.

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