[RRG] Re: BGP path hunting, MRAI timer and Path Length Damping

[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 not to apply
MRAI to withdrawals. 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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