Subject: issue with cost-based formulation of extended P-space in draft-ietf-rtgwg-remote-lfa-02

From: Chris Bowers <[email-masked]>
Date: Tue, 30 Jul 2013 12:08:47 +0000

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