[e2e] fast restoration/protection

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Thu Jun 15 07:42:51 PDT 2006

so while this is correct, i think, initially, i conjecture that once one
had a bit of experience with this, one could start 
i) to look at
triggering the selection of alternate FIBs on local detection of
outage only
ii) examining the cascade of convergence as the knowledge of the
chance percolated
iii) trigger LSAs only (no periodic update) so that the selection is
made in < 1 rtt remotely too...

In missive <Pine.LNX.4.63.0606151604370.22620 at kuusi.ifi.uio.no>, Amund Kvalbein

 >>If I understand your idea correctly, what you would gain with respect to 
 >>restoration time is to eliminate the time it takes to compute a new FIB 
 >>after a link state change. That is fine, but running dijkstra constitutes 
 >>only a small part of the total convergence time (what dominates is the 
 >>time it takes to load the new FIB to the line cards, see paper by Francois 
 >>et al. in CCR July 2005).
 >>Also, you would still have the problem of transient loops if the routers 
 >>do not switch to the new FIB synchronously. Allowing the detecting router 
 >>to locally switch to an alternate path while avoiding forwarding loops is 
 >>an important goal for fast restoration schemes.
 >>On Thu, 15 Jun 2006, Jon Crowcroft wrote:
 >>> to clarify - what happens in a link state protocol is that you
 >>> distribute states each epoch to all routers who construct a RIB
 >>> and do a dijkstra to compute a local FIB - what i am saying is
 >>> save the output of the dijkstra for all cases you see, and then
 >>> when you get a link state delta that _matches_ the FIB for a previous
 >>> case, load the FIB immediately
 >>> for distance or path vector, this is harder as you need to add info
 >>> (BGP in practice already has loadsa add ons to achieve some of these
 >>> effects in an ad hoc way, but it might be nice to do it in some sort
 >>> of principled fashion:-)
 >>> In missive <E1FqqRl-0000uA-00 at mta1.cl.cam.ac.uk>, Jon Crowcroft typed:
 >>> >>there's loads and loads of fancy algorithms for computing alternate
 >>> >>routes and building k-resiliency etc etc, but does anyone know if
 >>> >>there's a paper on the obvious - if routers were to log routing
 >>> >>tables, and then one correlated this with faults, one could simply
 >>> >>_load_ the alternate forwarding table directly after a link or node
 >>> >>outage from the previous time there had been an outage at that point -
 >>> >>i.e. simply use the fact that the routing system is a distributed
 >>> >>algoriithm for path finding, but give it some memory:)
 >>> >>
 >>> >>j.
 >>> cheers
 >>>   jon



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