Stephane,
Lots of thanks for a promot response.
It wiuld be nice to see this in the draft.
Thumb typed by Sasha Vainshtein
________________________________
From: stephane.litkow...@orange.com <stephane.litkow...@orange.com>
Sent: Monday, July 16, 2018 10:46:56 AM
To: Alexander Vainshtein
Cc: Robert Raszuk; rtgwg-cha...@ietf.org; pfr...@gmail.com;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org; rtgwg@ietf.org;
daniel.vo...@bell.ca; 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:alexander.vainsht...@ecitele.com]
Sent: Sunday, July 15, 2018 10:01
To: LITKOWSKI Stephane OBS/OINIS
Cc: Robert Raszuk; rtgwg-cha...@ietf.org; pfr...@gmail.com;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org; rtgwg@ietf.org;
daniel.vo...@bell.ca; 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: alexander.vainsht...@ecitele.com
From: stephane.litkow...@orange.com [mailto:stephane.litkow...@orange.com]
Sent: Friday, July 13, 2018 3:54 PM
To: Alexander Vainshtein <alexander.vainsht...@ecitele.com>
Cc: Robert Raszuk <rob...@raszuk.net>; rtgwg-cha...@ietf.org;
pfr...@gmail..com; draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org;
rtgwg@ietf.org; daniel.vo...@bell.ca; DECRAENE Bruno IMT/OLN
<bruno.decra...@orange.com>
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:alexander.vainsht...@ecitele.com]
Sent: Thursday, July 12, 2018 15:16
To: LITKOWSKI Stephane OBS/OINIS
Cc: Robert Raszuk; rtgwg-cha...@ietf.org<mailto:rtgwg-cha...@ietf.org>;
pfr...@gmail.com<mailto:pfr...@gmail.com>;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org<mailto:draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org>;
rtgwg@ietf.org<mailto:rtgwg@ietf.org>;
daniel.vo...@bell.ca<mailto:daniel.vo...@bell.ca>; 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:
alexander.vainsht...@ecitele.com<mailto:alexander.vainsht...@ecitele.com>
From: stephane.litkow...@orange.com<mailto:stephane.litkow...@orange.com>
[mailto:stephane.litkow...@orange.com]
Sent: Thursday, July 12, 2018 3:48 PM
To: Alexander Vainshtein
<alexander.vainsht...@ecitele.com<mailto:alexander.vainsht...@ecitele.com>>;
DECRAENE Bruno IMT/OLN
<bruno.decra...@orange.com<mailto:bruno.decra...@orange.com>>
Cc: Robert Raszuk <rob...@raszuk.net<mailto:rob...@raszuk.net>>;
rtgwg-cha...@ietf.org<mailto:rtgwg-cha...@ietf.org>;
pfr...@gmail.com<mailto:pfr...@gmail.com>;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org<mailto:draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org>;
rtgwg@ietf.org<mailto:rtgwg@ietf.org>;
daniel.vo...@bell.ca<mailto:daniel.vo...@bell.ca>
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:rtgwg-boun...@ietf.org] On Behalf Of Alexander Vainshtein
Sent: Thursday, July 12, 2018 13:26
To: DECRAENE Bruno IMT/OLN
Cc: Robert Raszuk; rtgwg-cha...@ietf.org<mailto:rtgwg-cha...@ietf.org>;
pfr...@gmail.com<mailto:pfr...@gmail.com>;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org<mailto:draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org>;
rtgwg@ietf.org<mailto:rtgwg@ietf.org>;
daniel.vo...@bell.ca<mailto:daniel.vo...@bell.ca>
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:
alexander.vainsht...@ecitele.com<mailto:alexander.vainsht...@ecitele.com>
From: bruno.decra...@orange.com<mailto:bruno.decra...@orange.com>
[mailto:bruno.decra...@orange.com]
Sent: Thursday, July 12, 2018 12:49 PM
To: Stewart Bryant <stewart.bry...@gmail.com<mailto:stewart.bry...@gmail.com>>
Cc: rtgwg-cha...@ietf.org<mailto:rtgwg-cha...@ietf.org>;
pfr...@gmail.com<mailto:pfr...@gmail.com>;
draft-bashandy-rtgwg-segment-routing-ti-...@ietf..org<mailto:draft-bashandy-rtgwg-segment-routing-ti-...@ietf.org>;
daniel.vo...@bell.ca<mailto:daniel.vo...@bell.ca>;
rtgwg@ietf.org<mailto:rtgwg@ietf.org>; Ahmed Bashandy
<abashandy.i...@gmail.com<mailto:abashandy.i...@gmail.com>>; Alexander
Vainshtein
<alexander.vainsht...@ecitele.com<mailto:alexander.vainsht...@ecitele.com>>;
Robert Raszuk <rob...@raszuk.net<mailto:rob...@raszuk.net>>; Chris Bowers
<cbow...@juniper.net<mailto:cbow...@juniper.net>>
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:stewart.bry...@gmail.com]
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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