Hi Mike,
From: Mike Shand
Date: Friday, October 2, 2015 at 2:30 PM
To: Pushpasis Sarkar
Cc: "[email protected]<mailto:[email protected]>", 'Jon Hudson'
Subject: Re: Routing directorate QA review for
draft-ietf-rtgwg-rlfa-node-protection
On 01/10/2015 18:49, Pushpasis Sarkar wrote:
Note that E itself is also in S's extended P space which makes this
>rather a strange example. It might be better to devise one where ONLY R3
>is in extended P space in order to avoid confusion.
[Pushpasis] But then E should not be in Q-Space(S-E) and hence it will not be a
PQ-node. The goal of this example was to show that while PQ node R2 provides
node protection for destination R3 and D2(in the text it says R1, I will
rectify it) the PQ-node R3 does not provide node-protection for the same
destinations R3 and D2. This example was specifically carved to show that not
only the path from PQ-node to destination must avoid primary next hop E, but
also all paths from S to the PQ-node as well.
Sorry, I don't understand. Surely E IS in Q-Space wrt S-E by definition.
[Pushpasis] Unless the text in RFC-7490 is not wrong, E is not included in
Q-Space(S-E). Here is the excerpt from section 5.2.1.3 that says E is not
included in Q-Space of (S-E) :-)
“
5.2.1.3. Q-space
The set of routers from which the node E can be reached, by normal
forwarding without traversing the link S-E, is termed the Q-space of
E with respect to the link S-E. The Q-space can be obtained by
computing a reverse Shortest Path Tree (rSPT) rooted at E, with the
subtree that might traverse the protected link S-E excised (i.e.,
those nodes that would send the packet via S-E plus those nodes that
have an ECMP set to E with one or more members of that ECMP set
traversing the protected link S-E). The rSPT uses the cost towards
the root rather than from it and yields the best paths towards the
root from other nodes in the network. In the case of Figure 1, the
Q-space of E with respect to S-E comprises nodes C and D only.
Expressed in cost terms, the set of routers {Q} are those for which
the shortest path cost Q<-E is strictly less than the shortest path
cost Q<-S<-E. In Figure 1, the intersection of the E's Q-space with
respect to S-E with S's P-space with respect to S-E defines the set
of viable repair tunnel endpoints, known as "PQ nodes". As can be
seen in the case of Figure 1, there is no common node and hence no
viable repair tunnel endpoint. However, when the extended P-space
(Section 5.2.1.2) at S with respect to S-E is considered, a suitable
intersection is found at C.
"
Thanks and Regards,
-Pushpasis
Yes, I understand the purpose of the example, and it serves that very well. I
just thought it may be confusing that the set of potential PQ nodes is greater
than you had described. That doesn't actually affect the point you are making
though.
Mike
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