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CORRECTION - [RRG] Re: BGP path hunting, MRAI timer and Path Length Damping
Correction:
"The latest recommendation per RFC 4271 is not to apply
MRAI to withdrawals." was a typo on my part.
It should read:
"The latest recommendation per RFC 4271 is to apply
MRAI to withdrawals also (in addition to route advertisements)."
Sorry about this typo.
Sriram
Quoting ksriram@nist.gov:
> I have learned the following about MRAI since I sent my email yesterday.
> It appears that the recommendation about whether or not to
> apply MRAI to withdrawals went through a change from
> RFC 1771 to RFC 4271 (4271 obsoletes 1771).
> The reasons for the change are not clear to me.
> The latest recommendation per RFC 4271 is to apply
> MRAI to withdrawals also. RFC 4271 was issued relatively recently
> in January 2006. And as late as June 2005 another RFC 4098
> (on benchmarking BGP convergence) cited RFC 1771 and
> stated that MRAI did not apply to explicit withdrawals (in Section 3.12):
>
> 3.12. MinRouteAdvertisementInterval (MRAI)
>
> Definition:
> (Paraphrased from RFC 1771) The MRAI timer determines the minimum
> time between advertisements of routes to a particular destination
> (prefix) from a single BGP device. The timer is applied on a
> pre-prefix basis, although the timer is set on a per-BGP device
> basis.
>
> Discussion:
> Given that a BGP instance may manage in excess of 100,000 routes,
> RFC 1771 allows for a degree of optimization in order to limit the
> number of timers needed. The MRAI does not apply to routes
> received from BGP speakers in the same AS or to explicit
> withdrawals. RFC 1771 also recommends that random jitter is
> applied to MRAI in an attempt to avoid synchronization effects
> between the BGP instances in a network. In this document, we
> define routing plane convergence by measuring from the time an
> NLRI is advertised to the DUT to the time it is advertised from
> the DUT. Clearly any delay inserted by the MRAI will have a
> significant effect on this measurement.
>
> Based on this one might infer that BGP routers have implemented
> "not applying MRAI to withdrawals" for a fairly long time (prior to RFC
> 4271).
> To what extent have router vendors updated and deployed the MRAI
> per RFC 4271 (so that MRAI does indeed apply to withdrawals)?
> Tony: Can you please share with us your thoughts on this?
>
> I certainly like the PLD proposal and also Geoff's additional suggestions
> in his recent paper, and suggest that a careful further
> investigation be done of these methods to see if there are
> any inter-dependencies with the way MRAI is implemented.
> I like to see "MRAI not applied to withdrawals" for reasons I
> stated in my previous email (although it goes against RFC 4271).
> I also think that Robin's explanations about inherent MRAI-based damping
> of AA+ type of updates is further strengthened if
> MRAI did not apply to withdrawals.
>
> Sriram
> -------------------------------------------------------------
> Quoting Robin Whittle <rw@firstpr.com.au>:
>
> > I am finalising some technical writing explaining path hunting,
> > based on the understanding I initially gained from Geoff's
> > article - but looking more closely, my understanding of what
> > would happen doesn't match Geoff's diagram.
> >
> > I don't see how Geoff's example, with my understanding of BGP
> > and its MRAI timer, would lead to something resembling Tony's
> > explanation either.
> >
> > I am keen to establish whether Geoff's explanation is realistic
> > or not. My understanding of Sriram's message is that he thinks
> > it is not.
> >
> > Tony, can you advise on this? Anyone else?
> >
> > The second question is whether the MRAI timer applies to
> > withdrawals. I think it does (as I explain below), but Sriram
> > thinks not.
> >
> >
>
>
>
>
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