I think that the new text is correct:
In cost terms a router (P) is in extended P-space if the shortest path
cost N->P is strictly less than the shortest path cost N->S->E->P. In
other words, once the packet it forced to N by S, it is lower cost for
it to continue on to P by any path except one that takes it back to S
and then across the S->E link.
As far as I can see this works for case 1 and case 2 below since the
only case where 1 applies is when N == Y, but that comes out in the wash
since D_opt(N,Y) == 0
I prefer not to include the D-opt notation at this stage of the text
unless we have to. There is a full set of cost based pseudocode in 4.2.3
which defines the D_opt term.
- Stewart
On 30/07/2013 13:08, Chris Bowers wrote:
It appears that the cost-based formulation for determining the
extended P-space in draft-ietf-rtgwg-remote-lfa-02 is incorrect. In
particular, section 4.2.2 says:
“In cost terms a router is in extended P-space if the shortest path
cost S-N->P is strictly less than the shortest path cost S-E->P.”
Instead, the requirement should be that the shortest path cost N->P is
strictly less than cost N-S-E->P.
I propose the following text for section 4.2.2.
“In terms of cost, a router Y is in the extended P-space of S with
respect to the failure of link S-E, if either of the two following
conditions is true:
(1) D_opt(S,Y) < D_opt(S,E) + D_opt(E,Y)
OR
(2) S has a neighbor N satisfying the inequality:
D_opt(N,Y) < D_opt(N,S) + D_opt(S,E) + D_opt(E,Y)”
This text corrects the error noted above, and it puts both of the
criteria for determining extended P-space in terms of cost in one
place in the text.
Chris
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