Thanks. I had not understood that the point was to specify the
"reverse" policy to the node that is being given the "forward" policy.
So that the forward node can specify the reverse in the message it
sends. That is understandable. And probably in the draft? (I was
asking based on the email.)
Again, thanks for explaining,
Joel
On 11/15/2021 11:26 AM, Mike Koldychev (mkoldych) wrote:
Hi Joel,
The context could be Circuit-Style SR Policy, or even just sending OAM packets
on a particular path and looping it back on the exact same path/paths in
reverse, by putting both forward and reverse label stacks (with or without any
SR Policy at the opposite end).
Note that I'm not talking about SR Policy POL1 being reverse of POL2. I'm
talking about individual Segment Lists (SLs) being reverses of other SLs.
The meaning of "reverse SL" is simple when both SLs are expressed as
adjacencies. But it's currently not well defined when Node SIDs are involved. For
example, the reverse of one Node-SID SL may require multiple Adjacency-SID SLs to cover
all ECMP paths.
So, I'm just bringing up this topic. I tend to agree with Cheng that we should
sync this with PCE/IDR/SPRING/etc.
Thanks,
Mike.
-----Original Message-----
From: Joel M. Halpern <j...@joelhalpern.com>
Sent: Friday, November 12, 2021 10:53 PM
To: Mike Koldychev (mkoldych) <mkold...@cisco.com>; SPRING WG <spring@ietf.org>
Cc: Rakesh Gandhi (rgandhi) <rgan...@cisco.com>; Chengli (Cheng Li)
<c...@huawei.com>
Subject: Re: [spring] SR Policy: per-SL reverse
I am missing something. In what context is it important to say that policy 2
is intended to represent the reverse of policy 1?
Yours,
Joel
On 11/12/2021 9:09 PM, Mike Koldychev (mkoldych) wrote:
Hi SPRING WG,
During the PCE session there was a presentation about signaling per-SL
(Segment List) reverse paths, see
https://datatracker.ietf.org/doc/html/draft-ietf-pce-multipath-03#sect
ion-4.5
<https://datatracker.ietf.org/doc/html/draft-ietf-pce-multipath-03#section-4.5>.
I received comments to bring this up in the SPRING WG.
In the simplest case, you have two SR Policies in opposite directions,
something like this:
SR policy POL1 <headend = PE1, endpoint = PE2>
Candidate-path CP1
SID-List = <ABC>
SR policy POL2 <headend = PE2, endpoint = PE1>
Candidate-path CP1
SID-List = <CBA>
Where <ABC> and <CBA> are two segment lists that can be considered
“opposites” of each other, maybe traversing the same links in reverse,
or maybe just the same nodes, etc.
However, if the SR Policies have multiple segment lists, it gets more
complicated:
SR policy POL1 <headend = PE1, endpoint = PE2>
Candidate-path CP1
SID-List = <ABC>
SID-List = <DEF>
SR policy POL2 <headend = PE2, endpoint = PE1>
Candidate-path CP1
SID-List = <CBA>
SID-List = <FED>
Where <ABC> and <CBA> are opposites, also <DEF> and <FED> are opposites.
REQ 1: It should be possible to express that multiple reverse SLs
correspond to the same forward SL. For example, if the forward SL is
using Node Segment(s) with ECMP and reverse SLs use Adjacency Segments
to cover multiple ECMP paths in reverse.
REQ 2: It should be possible to express that SL 1 is a reverse of SL
2, but SL 2 is **not** a reverse of SL 1. I.e., not mutually reverse.
REQ 3: Having a set of reverse SL(s) associated to every forward SL is
useful even if there is no actual SR Policy in the reverse direction.
I.e., if there’s just a unidirectional “forward” SR Policy that needs
to know the return paths for each of its SLs.
Currently SR Policy Architecture does not talk about reverse SLs. I’m
requesting the WG to review the proposal and decide if we should
standardize this.
Thanks,
Mike.
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