Hi Mike, I have a question about the possible use of the reverse SR path association and much appreciate your help understanding its use in OAM. In draft-ietf-spring-bfd <https://datatracker.ietf.org/doc/draft-ietf-spring-bfd/> we propose a new Non-FEC TLV to specify the SR policy for the reverse direction of a BFD session bootstrapped via LSP ping. Also, we propose that the same Non-FEC TLV can be used to control the path of the LSP Echo Reply sent in response to the LSP Echo Request that includes the Non-FEC TLV. What other use cases for the SR reverse association do you see?
Regards, Greg On Mon, Nov 15, 2021 at 8:26 AM Mike Koldychev (mkoldych) <mkoldych= 40cisco....@dmarc.ietf.org> 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. > > > > > > _______________________________________________ > > spring mailing list > > spring@ietf.org > > https://www.ietf.org/mailman/listinfo/spring > > > _______________________________________________ > spring mailing list > spring@ietf.org > https://www.ietf.org/mailman/listinfo/spring >
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