On Tue, Mar 10, 2009 at 02:04:11PM +1100, Robin Whittle wrote: > Hi Dave, > > I have replied to part of your message in another thread on LISP-ALT > scaling. "Level 1" means an ALT router at the lowest level of the > highly aggregated inverted tree ALT hierarchy. > > You wrote, in part: > > >> How can an ALT router, such as this busy level 1 > >> aggregation router, detect the failure or > >> unreachability of a Map-Server? I guess it would > >> find out when it tries to send a Map-Request message > >> to the Map-Server via the GRE tunnel. But is there > >> a keepalive arrangement so the level 1 router could > >> detect the disappearance of this Map-Server before > >> it sends a Map-Request? > >> > >> More on potential problems with caching Map-Resolvers. > > > > > > Regarding "upness": You can make the same argument for > > BGP peers in the DFZ (or DNS if you like). In the case of > > BGP, we use BGP keepalives, BFD (if you like), and the > > existence of the route in the RIB/FIB as basically a > > bunch of heuristics, which work pretty well (when was the > > last time you couldn't get to Google because of a routing > > problem on the Internet?). Hence we don't need to > > calculate some exact value of "upness" (which is a good thing). > > > > BTW, the question "is entity X up" really reduces to > > Zeno's paradox (think: what is the granularity of > > "upness", and how often do I need to ask?). And we know > > what it took to solve Zeno's paradox. But as mentioned > > above, we can do a fine job (within our engineering > > constraints) with clever heuristics. The operational > > Internet is pretty much an existence proof of that. > > Your answer is pretty general and doesn't seem to apply to the > specific situation of the Map-Server and ETR.
You didn't ask a question. I was just noting that the
question as to whether or not something is "up" is not
well formed.
Dave
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