Dear authors,
<cb> This seems to be a circular argument at this point. As I said in the
> preso and Tony has stated multiple times, if a network employs protection
> (FRR, LFA, TI-LFA, …) we assume sufficient capacity is available for the
> backups. If it is not, the operator should not enable TTE … and they
> probably shouldn’t use protection either.
>
Well please let's keep in mind that FRR protection mechanism are in many
cases recommended to be deployed with honoring QoS marking of flows under
protection.
So if my link/node fails and FRR gets employed it is ok to drop best effort
on the protected bypass path just to deliver high priority flows.
You seem to be going by assumption that FRR (PIC Core) is applicable only
to the networks where max 50% link utilization. I am not saying this is
impossible, but this assumption needs to be clearly spelled out in the
document (IMHO).
> <cb> We support it on any link but it will only work [be activated] if a
TTE tunnel exists. The current
> implementation leverages TI-LFA backup routes thus bounded by the IGP
domain.
PIC Edge is a flavour of FRR so clearly what you are supporting today is
not cases covered by PIC Edge or for that matter 50 ms protection on PHP
node to PE.
> <cb> Its worth noting also that some implementations can and do account
for BW along the
> protected path during bypass placement.
We are talking about protection established before _unexpected_ network
load. So I am not sure how can you predict the future.
To reiterate, let me restate my major concern with the proposal. Consider
well build ECMP core network:
___ ___ P1 ___ P3 __
/ \ / | \ _ / | \
CEs ------- PEs | / \ | PEe -- CEe
\ ___ / \ ___ P2 ___ P4___ /
Load from CEs to CEe is nicely ECMPs on PEs across north path (P1,P3) and
south path (P2, P4).
Imagine the load increases beyond planned and is not shaped/policed on
ingress by PEs.
Assume there is either lot's of elephant flows or lot's of small short
lived flows - law of large numbers applies.
ECMP is done well and all load is nicely spread across north and south
path.
So P1 to P3 links and P2 to P4 indicate congestion and nodes P1 and P2
apply TTE.
All happens symmetrical.
So we just gained nothing here but only increased RTT of the TTE selected
flows.
Kind regards,
R.
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