Agree
*From:*Alexander Vainshtein [mailto:[email protected]]
*Sent:* Monday, July 16, 2018 10:58
*To:* LITKOWSKI Stephane OBS/OINIS
*Cc:* Robert Raszuk; [email protected]; [email protected];
[email protected]; [email protected];
[email protected]; DECRAENE Bruno IMT/OLN
*Subject:* Re: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Stephane,
Lots of thanks for a promot response.
It wiuld be nice to see this in the draft.
Thumb typed by Sasha Vainshtein
------------------------------------------------------------------------
*From:*[email protected] <[email protected]>
*Sent:* Monday, July 16, 2018 10:46:56 AM
*To:* Alexander Vainshtein
*Cc:* Robert Raszuk; [email protected]; [email protected];
[email protected]; [email protected];
[email protected]; DECRAENE Bruno IMT/OLN
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Hi,
> would it be correct to assume that using the post-convergence path in
TI-LFA resolves some of operational issues with LFA selection that are
mentioned in Section 3 of RFC 7916 because these issues presumably
have been taken by the network operator as part of its original
network engineering? E.g., using PE routers to protect against core
failures, or selecting links with low BW while links with high BW are
available?
Exactly !
Brgds,
Stephane
*From:*Alexander Vainshtein [mailto:[email protected]]
*Sent:* Sunday, July 15, 2018 10:01
*To:* LITKOWSKI Stephane OBS/OINIS
*Cc:* Robert Raszuk; [email protected]; [email protected];
[email protected]; [email protected];
[email protected]; DECRAENE Bruno IMT/OLN
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Stephane,
Lots of thanks for a highly informative response.
Leaving aside the potential to use repair paths theoretically provided
by SR but not available for IP FRR, would it be correct to assume that
using the post-convergence path in TI-LFA resolves some of operational
issues with LFA selection that are mentioned in Section 3 of RFC 7916
because these issues presumably have been taken by the network
operator as part of its original network engineering? E.g., using PE
routers to protect against core failures, or selecting links with low
BW while links with high BW are available?
Such a claim looks reasonable to me – especially since, as you have
written, “TILFA perfectly fits the criteria: lowest IGP metric” that
is one of the mandatory criteria in 7916.
I admit that I did not understand that from just reading TI-LFA draft,
not in the least because RFC 7916 is neither mentioned nor referenced
there.
Regards,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: [email protected]
*From:*[email protected]
[mailto:[email protected]]
*Sent:* Friday, July 13, 2018 3:54 PM
*To:* Alexander Vainshtein <[email protected]>
*Cc:* Robert Raszuk <[email protected]>; [email protected];
[email protected];
[email protected]; [email protected];
[email protected]; DECRAENE Bruno IMT/OLN <[email protected]>
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
RFC7916 was written at a time when TILFA did not exist. LFA and RLFA
provide a set of candidate paths where we need to pick one or more to
be installed.
With segment routing, we bring the ability to use any alternate path
compared to LFA and RLFA which have a limited amount of candidate
paths. As a consequence, we are not exactly in the same backup path
selection logic as for LFA and RLFA. If we try to mimic RFC7916 logic
with SR, we need to consider the list of candidate paths to be all
alternate paths available through the network (so many many !) which
may provide some scaling concern especially if we try to involve path
attribute collection for the candidate paths. That’s why the logic of
TILFA is to focus on the postconvergence path.
To answer your question, IMO, TILFA perfectly fits the criteria:
lowest IGP metric.
We could think (as a theoretical exercice) of TILFA using other
criterias than the lowest IGP metric. Note that if we think of TILFA
in the context of flexalgo, the backup path may require to fit the
constraint defined by the flexalgo.
*From:*Alexander Vainshtein [mailto:[email protected]]
*Sent:* Thursday, July 12, 2018 15:16
*To:* LITKOWSKI Stephane OBS/OINIS
*Cc:* Robert Raszuk; [email protected]
<mailto:[email protected]>; [email protected]
<mailto:[email protected]>;
[email protected]
<mailto:[email protected]>;
[email protected] <mailto:[email protected]>; [email protected]
<mailto:[email protected]>; DECRAENE Bruno IMT/OLN
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Stephane,
Lots of thanks for a prompt response.
We seem to agree on at least one of my objections and the need to
remove the associated text from the draft.
Regarding the other one:
1.Lots of thanks for pointing to RFC 7916 that describes problems with
LFA/RLFA selection. BTW, this RFC is not referenced by the TI-LFA draft
2.One of the mandatory criteria in RFC 7916 is lowest IGP metric used
to reach the destination:
a.Is this equivalent to giving precedence to post-convergence paths?
b.This is just one of mandatory criteria in RFC 7916 with some
recommended criteria as well. These criteria should be evaluated based
on their preferences.
3.One modification of the draft that I can think about could be:
a.Provide a Normative reference to 7916. In particular, clarify that
all mandatory criteria listed in this RFC MUST be supported
b.RECOMMEND giving highest preference to the criteria of lowest IGP
metric to the destination.
Regards,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: [email protected]
<mailto:[email protected]>
*From:*[email protected]
<mailto:[email protected]>
[mailto:[email protected]]
*Sent:* Thursday, July 12, 2018 3:48 PM
*To:* Alexander Vainshtein <[email protected]
<mailto:[email protected]>>; DECRAENE Bruno IMT/OLN
<[email protected] <mailto:[email protected]>>
*Cc:* Robert Raszuk <[email protected] <mailto:[email protected]>>;
[email protected] <mailto:[email protected]>; [email protected]
<mailto:[email protected]>;
[email protected]
<mailto:[email protected]>;
[email protected] <mailto:[email protected]>; [email protected]
<mailto:[email protected]>
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Hi Sasha,
> This flow will experience two path changes (pre-convergence--> FRR
and FRR --> post-convergence
+1, I think that the current statement in the draft is more a
“marketing” one rather than a reality and IMO it may be worth removing it.
As Stewart and you pointed, from an end-to-end point of view the path
may change (so the statement is wrong), a node upstream from the
failure may reroute the traffic out of the FRR path. And in anyway,
the label stack used will change (except in one case) even if the hop
by hop logical path does not.
> Post-convergence path is taken into account in the operator’s panning
(e.g., by allocating sufficient resources for traffic flows on both
pre-convergence and post-convergence paths).
This argument is worth to mention. First of all, the draft does not
say that TILFA is magic and prevents the requirement of additional
tuning. It says : “there is much less need for the operator
to tune the decision among which protection path to choose.”.
This statement is perfectly true. With LFA and rLFA, you have high
chance to pick a P-PE to protect a core link and depending on your
topology, a lot of tuning and policies is required (see RFC7916) to
ensure you get a good backup (or sometimes we prefer not having a backup).
TILFA helps here as it can use a loopfree IGP metric optimized path
which requires less tuning. I do not say that there will never be a
requirement for tuning but it is unlikely.
Brgds,
Stephane
*From:*rtgwg [mailto:[email protected]] *On Behalf Of *Alexander
Vainshtein
*Sent:* Thursday, July 12, 2018 13:26
*To:* DECRAENE Bruno IMT/OLN
*Cc:* Robert Raszuk; [email protected]
<mailto:[email protected]>; [email protected]
<mailto:[email protected]>;
[email protected]
<mailto:[email protected]>;
[email protected] <mailto:[email protected]>; [email protected]
<mailto:[email protected]>
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Bruno,
It seems there is some misunderstanding, and I will try to clarify it.
To the best of my understanding, the following text in Section 1 of
the draft presents the benefits of using post-convergence path for FRR:
As the capacity of the post-convergence path is typically planned by
the operator to support the post-convergence routing of the traffic
for any expected failure, there is much less need for the operator
to tune the decision among which protection path to choose. The
protection path will automatically follow the natural backup path
that would be used after local convergence. This also helps to
reduce the amount of path changes and hence service transients: one
transition (pre-convergence to post-convergence) instead of two
(pre-convergence to FRR and then post-convergence).
I see two different claims of benefits from using post-convergence
path in this test fragment
1.One path change and therefore one service transient instead of two
2.Post-convergence path is taken into account in the operator’s
panning (e.g., by allocating sufficient resources for traffic flows on
both pre-convergence and post-convergence paths).
Speaking just for myself, I think that neither of these claims is
justified for traffic flows that do not originate at the PLR.
E.g., consider Stewart’s example and the traffic flow from A to E
1.This flow will experience two path changes (pre-convergence--> FRR
and FRR --> post-convergence
2.The network operator will not take links C-F, F-G and G-D for
consideration in its planning of pre-convergence and post-convergence
paths for this flow.
Did I miss something substantial?
Regards,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: [email protected]
<mailto:[email protected]>
*From:*[email protected] <mailto:[email protected]>
[mailto:[email protected]]
*Sent:* Thursday, July 12, 2018 12:49 PM
*To:* Stewart Bryant <[email protected]
<mailto:[email protected]>>
*Cc:* [email protected] <mailto:[email protected]>;
[email protected] <mailto:[email protected]>;
[email protected]
<mailto:[email protected]>;
[email protected] <mailto:[email protected]>; [email protected]
<mailto:[email protected]>; Ahmed Bashandy <[email protected]
<mailto:[email protected]>>; Alexander Vainshtein
<[email protected]
<mailto:[email protected]>>; Robert Raszuk
<[email protected] <mailto:[email protected]>>; Chris Bowers
<[email protected] <mailto:[email protected]>>
*Subject:* RE: Request for RTGWG Working Group adoption for
draft-bashandy-rtgwg-segment-routing-ti-lfa
Stewart,
Please see 1 comment inline [Bruno]
Trimming the text to ease the focus on this point
*From:*Stewart Bryant [mailto:[email protected]]
*Sent:* Tuesday, July 10, 2018 2:40 PM
On 09/07/2018 20:53, Ahmed Bashandy wrote:
[…]
b.*Selecting the post-convergence path *(inheritance from
draft-francois) does not provide for any benefits for traffic that
will not pass via the PLR */after convergence/*.
i.The authors claim to have addressed this issue by stating that
“Protection applies to traffic which traverses the Point of Local
Repair (PLR). Traffic which does NOT traverse the PLR remains
unaffected.”
SB> It is not as simple as that, and I think that the draft needs to
provide greater clarity.
I think there will be better examples, but consider
12
+--------------+
| |
A-----B-----C---//---D----E
10 | |
F--------G
Traffic injected at C will initially go C-D-E at cost 2, will be
repaired C-F-G-D-E at cost 4 and will remain on that path post
convergence. This congruence of path is what TI-LFA claims.
However, a long standing concern about traffic starting further back
in the network needs to be more clearly addressed in the draft to
clearly demonstrate the scope of applicability.
For traffic starting at A, before failure the path is A-B-C-D-E cost 13
TI-LFA will repair to make the path A-B-C-F-G-D-E cost 15 because
TI-LFA optimises based on local repairs computed at C.
After repair the path will be A-B-D-E cost 14.
[Bruno] The draft is about IP Fast ReRoute (FRR).
FRR is a local reaction to failure, so by hypothesis, all nodes but
the PLR are not aware about the failure. This includes all upstream
nodes which do keep forwarding traffic through the same path, i.e. via
the PLR.
The argument that the path would have been shorter if upstream node
were aware of the failure to reroute before (or that the PLR should
send the packet back in time) is not relevant.
The only question which matter is: from the PLR to the destination,
which is the best path to use?
I, and the draft, argue that the best path in IP routing, is the IGP
shortest path. Whichever type of metric you choose (e.g. bandwidth,
latency, cost…). Do you disagree on this?
Now, eventually we can narrow down the discussion to the choice of
terms. We can discuss about the term “post-convergence paths from the
point of local repair »,which you don’t think to like. Although, the
term seems technically true to me, I would also be fine with changing
from “post-convergence path” to “optimal IGP shortest path”
So the draft needs to make it clear to the reader that TI-LFA only
provides benefit to traffic which traverses the PLR before and after
failure.
[Bruno] No, that is not true. cf above.
--Bruno
Traffic which does not pass through the PLR after the failure will
need to be traffic engineered separately from traffic that passes
though the PLR in both cases.
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