Subject: Re: draft-ietf-rtgwg-remote-lfa - ready for WGLC? - some comments and questions

From: Stewart Bryant <[email-masked]>
Date: Tue, 02 Jul 2013 18:07:28 +0100

On 26/06/2013 14:13, Levente Csikor wrote:

Levente

In each case below the conditions are surely fulfilled if n=e
thus I think that in each case the condition needs to be
changed to :

some node n!={s or e}
> In order to not to read our full papers and searching the answers, I 
> copied here the relevant parts:
> *Link-protecting rLFA condition:*
> For source /s/, destination /d/, and next-hop /e/, some node /n/ /!= 
> s,d/ is a link-protecting remote LFA for the /s-d/ pair  if and only if
> /dist(s, n) < dist(s, e) + dist(e, n)/  (1)
> /dist(n, d) < dist(n, s) + dist(s, d)/  (2)
> In these equations, one can easily see, that (1) defines the P-space, 
> while (2) is the condition of Q-space. Furthermore, with these 
> formalized conditions, one can easily observe, that (2) is actually 
> the basic loop-free criterion of pure LFA.
>
> *Link-protecting rLFA condition with extended P-space:*
> For source /s/, destination /d/, and next-hop /e/, some node /n != 
> s,d/ is an extended link-protecting remote LFA for the /s-d/ pair if 
> and only if
> /?v ? neigh(s) : dist(v, n) < dist(v, s) + dist(s, e) + dist(e, n)///
> /dist(n, d) < dist(n, s) + dist(s, d) . /
>
> *Node-protecting rLFA condition:*
> For source /s/, destination /d/, and next-hop /e/, some /n != s,d/ is 
> a node-protecting remote LFA for the /s-d/ pair if and only if
> /dist(s,n) < dist(s,e) + dist(e,n)///     (3)
> /dist(n,d) < dist(n,e) + dist(e,d) /    (4)
> As it was in the case of link protection, here, (3) defines the 
> P-space, while (4) characterize the Q-space.
> Here, two important observations can be made, which are the followings:
>  - P-space does not depend on the protection scheme (i.e., link or 
> node protection)
SB> That falls directly out of the definition of P-space
SB> since in link you cannot traverse the link to e and in
SB> node only get to e if you traverse the link to e
SB> thus the exclusion of the link to e applies in both cases
>  - (4) again is the basic node-protecting loop-free criterion of pure LFA.
>
> *Node-protecting rLFA condition with extended P-space:*
> For source /s/, destination /d/, and next-hop /e/, some node /n != 
> s,d/ is an extended node-protecting remote LFA for the /s-d/ pair if 
> and only if
> /?v ? neigh(s) : dist(v, n) < dist(v, e) + dist(e, n)/
> /dist(n, d) < dist(n, e) + dist(e, d) ./
>
> Despite the fact that we only considered unit cost networks, the 
> formal definitions above are *true for any arbitrary weighted network.*
SB> I do not see where the unit costs come into the text above. It looks
SB> like it is already expressed in terms of arbitrary cost.

SB> Additionally in order to limit the number of SPFs to a practical
SB> level, we normally suggest that the repair target in not d, but
SB> instead is e (in the link case) or next hop of e (node case).

SB> Anyway I will work on some text.

- Stewart

---

From: Levente Csikor <[email-masked]>
Date: Tue, 02 Jul 2013 22:03:24 +0200

Dear Stewart,
thanks for comments, see minecomments inline.
> thus I think that in each case the condition needs to be
LC> Yes, you have right that e should be excluded from the set of 
examined nodes, but in the first case below, it does not surely fulfill 
the condition,
LC> but it would conflict, since assume that n=e := X, then we would get 
that
LC> dist(s,X) < dist(s,X) + dist(X,X), where obviously the last 
component is zero!
LC> Moreover, if next-hop e (,which  is not the destination) became 
unreachable because of the failed link, then node s could not tunnel the 
traffic to e
LC> again, since it would use the failed link. If we could do that, we 
need one more label in the label stack, since we would need to reach 
some other
LC> node t, then we would need a tunnel from node t to node e, and then 
the remaining shortest path from node e to the destination.

LC> On the other hand, I think that node d has to remain in the set of 
"non-analyzed" nodes, i.e., some node n!={s,e,d}, since according to the 
draft, if
LC> we could tunnel the traffic directly to the destination (the case 
when n=d) "then conventional LFA would have been sufficient to effect 
the repair."

> SB> thus the exclusion of the link to e applies in both cases
LC> Yes it is kind of obvious observation, which falls out of the 
definition of P-space, but I think that this could be noted in the 
draft, since if someone tries
LC> to determine how much SPF,rSPF or even all-pair SP calculations are 
needed, then it is good to remember that for P-space, we could deal with 
the
LC> same amount of calculations in both protection cases.

> SB> like it is already expressed in terms of arbitrary cost.
LC> Since in our papers, we only dealt with unit cost networks, 
therefore I wanted to emphasize that these results are true for any 
arbitrary weighted network. (However, you have right about the fact that 
if the conditions are expressed by distance functions, they will 
definitely true with any link cost settings :) )

> SB> Anyway I will work on some text.
LC> Thanks.
> - Stewart

Regards,
Levente