Hi all,
even though I put OSPF-FN draft in the subject it is the framework
approach FN-FRWK which draws more questions. At the very first line it
reads:
This document describes an architectural work that competes with the
IP Fast Re-Route (IPFRR) solution
Lets compare speed of traffic restoration between the two. So,
traditional OSPF convergence time consists of the sum of:
1. Failure detection
2. LSA origination
3. Per-hop flooding
4. SPF (delay and calculation itself)
5. RIB/FIB/hardware update
3, 4 and 5 all can be significant depending on network size, number of
routes etc.
FRR (both MPLS TE FRR and IPFRR) address 2-5. With good implementation
FRR should be by order of magnitude as fast as 1.
FN addresses only 3. It doesn't address 4 and 5. As I wrote above in
many networks they are at least as significant as 3.
So, by the speed of convergence FN doesn't look to come anywhere close
to FRR.
Now, lets look at FN from another perspective. Router announcing
failure doesn't benefit from FN. Its immediate neighbors do not benefit
from FN either - 1 hop traditional flooding should be as fast as 1 hop
FN flooding. It is only distant routers who benefit from the FN - and
the farther is router from the failure the bigger is gain.
On the other hand, if there exists path alternative to the failed
one then _typically_ it is not too far (in terms of hops) from the
failing one. I.e. from point of view of router which is 15 hops away
from the point of failure it is less likely that routes will change.
BTW, ordered FIB approach builds on concept that 'old' routes on remote
routers do not cause traffic blackholing or loops.
The big problem with FN approach is that routers remote from the
point of failure benefit most - but at the same time their convergence
is the least important for end-to-end traffic restoration.
The worst case network for FN is fully meshed area. Since each
router is 1 hop away from every other one FN will give no benefits.
The best case network for FN is an area consisting of one big ring.
In this case alternative path is on diametrically opposite end of the
network and convergence of remote routers is crucial.
So yeah, FN will help remote routers to converge faster. But how
much this will improve end-to-end traffic restoration in real networks?
I suspect not much. Some degree of meshiness in network topology is the
norm.
FN is an interesting proposal but it is very far from being
convincing. Pitching FN against FRR is a mistake.
--
Anton
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