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Re: [RRG] Re: map change due to a path failure?
On Mon, Mar 24, 2008 at 10:14 PM, Stephen Sprunk <stephen@sprunk.org> wrote:
> My understanding is that a temporary path failure would _not_ require a
> mapping change in a protocol that provided the complete set of potential
> EID-to-RLOC mappings to the ITR and featured reachability tests. That is,
> after all, one of the prime reasons to go with multiple mappings.
Hi Stephen,
Depends. What reachability criteria does the ITR use to determine that
the ETR is still there and still has a valid path to the EID? Can't
depend on the ETR to tell you; it may have turned in to a black hole.
At the moment there are two mechanisms that have been validated which
guarantee that knowledge about reachability reaches the ITR. One is
monolithic push through the map or route system where every change in
ETR availability is distributed to every ITR. The other is pulled
cache with a TTL where the ITR expects to periodically request an
update from the origin map server. Either way it's going to take a
while for all interested parties to get the message.
If you'd like to propose another mechanism which guarantees the
propagation of reachability knowledge, browse on over to
http://bill.herrin.us/network/statechange.html and then send me some
text and analysis that I can add.
> > 2a. What's the maximum acceptable time in which one EID may be
> > inaccessible to another during change propagation due to a path
> > failure?
>
> A second or two at most, ideally, because folks are going to try to run VoIP
> over this system. The good news is that it's not "zero" or "50ms" like it
> would have been a decade ago, as cell phones have conditioned people to
> expect occasional audio loss they wouldn't have accepted when accustomed to
> the general high reliability of the wireline PSTN.
Two seconds is extremely ambitious for any architecture. BGP takes
anywhere from 30 to 120 seconds under normal conditions. I'm not sure
its possible to reliably detect that a path has failed in two seconds,
let alone propagate state change knowledge.
>
> S
>
> Stephen Sprunk "God does not play dice." --Albert Einstein
> CCIE #3723 "God is an inveterate gambler, and He throws the
> K5SSS dice at every possible opportunity." --Stephen Hawking
>
>
--
William D. Herrin ................ herrin@dirtside.com bill@herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004
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