Dear All,
we've been analyzing and dealing with Remote LFA (rLFA for short) more
than 1 year ago. As a first approach, we analyzed only unit costs
networks with single link failures, and our results are summarized in
[1]. Furthermore, we extended our work with node failures, which can be
found in [2]. Since, in the first version of the rLFA Draft, it was not
clearly explained that whether the usage of extended P-space involves
additional complexity or not, we divided our analyses into two cases. In
the first case, /only P-space/ is available for finding possible
PQ-nodes (link-protecting remote LFA), while in the second case,
/extended P-space/ was also considered (extended link-protecting remote
LFA). Since we found that /remote LFA with extended P-spaces provides
100% failure case coverage in all unit cost network against single link
failures/, our separated analyses were also helpful. (However, we proved
that if node protection is also considered, then unit cost networks are
not fully protected.)
As Alia commented recently on the algorithm description in Sec. 4.2.1
and Sec. 4.2.2. before, we also think that a more formal algorithm
description is necessary, especially, when P-spaces and Q-space are
defined with cost functions similarly as defined in LFA FRR. From a
routing point of view, I think, it would be easier to understand and
calculate these /spaces/ (P,ext.P,Q) if they were defined with distance
functions.
Fortunately, the authors of Remote LFA Draft do not have to resolve
these issues on their own, since we already defined them in our relevant
papers.
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)
- (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.*
I have to emphasize and thank the authors that in the second version of
the rLFA draft, it became more clear how MPLS LDP label stack can
provide the necessary "tunneling" for reaching remote LFA staging
points, which answered me a lot of questions.
However, there is something in which I still not sure. As far as I
know(, from the draft), reaching PQ-nodes in MPLS/LDP enabled network,
two labels are necessary for the source node to avoid the failed
component. An outer label, which is the label of S's neighbor for
sending traffic to the PQ-node, while the inner label is the PQ node's
label for reaching the destination. However, in this MPLS/LDP point of
view, is there any difference between the label stacks used to reach a
PQ-node in the case of simple P-space or extended P-space, or the are
the same?
On the other hand, according to the MPLS LDP specification (RFC 5036),
the targeted LDP capability may be disabled at the routers, which
indicates that the simple label stacks cannot be always used to destine
packets to a PQ-node, therefore IP tunneling (e.g., IP-in-IP, GRE)
should be used to reach the same end. Is it an issue nowadays, or these
are already addressed?
Additionally, for Sec. 9.3., we also have measured coverages of rLFA in
real-world topologies inferred from Rocketfuel dataset, SNDLib and
Topology-zoo, and we compared them to pure LFA coverages as well. These
results can be found in our papers, which may be used in this draft.
We think that our results could be a crystallization for some aspects of
remote LFA and may be used in the draft.
We also believe that the above mentioned more formal descriptions of P-
and Q-spaces and their connectivities to pure LFA can definitely ease
the understanding of how remote LFA works, especially for those who has
experience in simple LFA.
If you decide to use our results, please mention our works somehow in
the document (e.g., references, acknowledgements)
Best regards,
Levente Csikor
Ph.D. Student
MTA-BME Future Internet Research Group
High Speed Networks Laboratory
Dept. of Telecommunications and Media Informatics
Budapest University of Technology and Economics,
Hungary.
[1] L. Csikor, G. Retvari, "IP Fast Reroute with Remote Loop-Free
Alternates: the Unit Link Cost Case", In: Proceeding of RNDM 2012 4th
International Workshop on Reliable Networks Design and Modeling,
pp:16-22. 2012.
[2] L. Csikor, G. Retvari, "On Providing Fast Protection with Remote
Loop-Free Alternates: Analyzing and Optimizing Unit Cost Networks",
submitted to Telecommunications Systems Journal, 2012.
P.S.: I already tried to contact one of the authors directly (by email),
but no answers were received. That's why I send this message to the
list. I have never post any message to these mailing lists and I hope I
did not spamming it.
On 06/25/2013 06:53 PM, Acee Lindem wrote:
Hi Stewart,
See inline.
On 6/20/13 6:59 AM, "Stewart Bryant" <[email protected]> wrote:
I write this email as duty editor of draft-ietf-rtgwg-remote-lfa
I recently updated this draft and think that it is ready for WGLC.
When the WG adopted the draft Alia made a number of comments which
I address below:
AA> First, if the intent is to restrict this mechanism to ONLY link
AA> protection, that belongs at a minimum prominently in the abstract and
AA> introduction. It is currently first mentioned only in Section 3.
This has been addressed.
AA> Second, the algorithm description Sec 4.2.1 and Sec 4.2.2 needs
AA> significant expansion into a more formal algorithm description, such
AA> as is in the LFA spec, RFC 5286. A brief description of the
AA> computational complexity would be useful, but the critical part is
AA> having it specified clearly.
Mike and I have included some cost metric explanations which I think
is adequate for ensuring the expected behavior. I am not convinced
that the level of formalism in RFC5286 is required. The requirement
is that an implementation will function correctly, neither breaking
the network, nor surprising the operator.
I have to say that I agree with Alia here. I've read the document several
times and what you have in sections 4.2.1-4.2.3 is a heuristic for
determining a single remote LFA for a specific failure (S-E failure in
figure 1) rather than any kind of general case algorithm.
Thanks,
Acee
AA> Third, in Sec 4.2.3, there is no preferred or even specified method
AA> for selecting among the different PQ options that might be available.
AA> Such a method should be specified; as the draft says, there is an
AA> advantage from the network management perspective to consistency. It
AA> is also required to have agreement on the output of analysis to
AA> compare/contrast methods.
We have added some text
AA> Fourth, one issue described in RFC 5286 is what happens when a worse
AA> failure occurs than the LFA was computed to handle - i.e. if a node
AA> failure happens instead of a link failure. In that case, traffic
AA> looping can occur. With Remote LFA, I believe that the same issue can
AA> exist - but made even worse because there is no effort to look for
AA> node-protecting Remote LFAs. This concern needs some description in
AA> the draft. Additionally, the equivalent of the Downstream Paths
AA> condition should be specified, if possible, that allows such traffic
AA> looping to be avoided. Finally, since the argument for Remote LFA is
AA> the improved coverage over LFAs, I would like to see the coverage
AA> analysis based on simulation to show the coverage when the Downstream
AA> Paths equivalent requirement is met vs. when it is relaxed (as
AA> currently in the draft).
We added some test to section 6 to cover the worse than expected case.
AA> Fifth, for a better understanding of realistic behavior, I would like
AA> to see the analysis extended to show the min, average, and max
AA> coverage for the 11 specified topologies after each single failure has
AA> occurred in the topology. (Of course, the computation should
AA> recognize that protecting cut-links isn't feasible and not include
AA> those failures.)
I don't think that this draft is the place to provide a greater analysis
of coverage. However I do think such a draft needs to be written.
I understand that Mike has plans to publish a draft before Berlin
to address this.
AA> While I recognize that link failures are significantly more common
AA> than node failures, I believe that fast-reroute techniques should be
AA> able to cover node failures as well. Technically, I think that Remote
AA> LFA can, of course, be extended to provide node coverage - at the cost
AA> of computing a reverse-SPF from each next-next-hop of the computing
AA> router.
There are drafts covering the node case that are being proposed
by members of the WG.
AA> As I said early, at a minimum, the abstract and introduction
AA> need to clearly specify that Remote LFA only provides link protection
AA> and the traffic looping concerns need to be addressed.
Done
- Stewart
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