Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions

Rob Shakir <rjs@rob.sh> Mon, 29 December 2014 09:08 UTC

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To: Peter Psenak <ppsenak@cisco.com>, Shraddha Hegde <shraddha@juniper.net>
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Subject: Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions
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Peter, Shraddha,

Primarily —  I don’t think that use of the ‘B’ flag in the Adj-SID implies that there MUST be a backup route installed, it merely indicates that the Adj-SID MAY be subject to re-routing (and hence strict placement on an adjacency may not be honoured during link failures).

For me, I’m unclear on what the practical use of not requesting backup for a {Node,Prefix}-SID could be — its very nature (“the shortest path to X” where X is a node/prefix) means that it is not well defined in terms of a route through the network, and hence is not well defined in terms of performance. This (to me) says that we cannot really rely on such a SID for performance-sensitive traffic, and hence must always be able to tolerate events such as FRR paths during protection.

The fact that AdjSID maps deterministically to a particular link, about which the calculating entity (PCE/iLER) can know details of, means that performance can be inferred - and hence strict affinity to that path (and/or failure when it is not available) is of utility.

Kind regards,
r.


> On 29 Dec 2014, at 08:56, Peter Psenak <ppsenak@cisco.com> wrote:
> 
> Shraddha,
> 
> I do not see how an originator of the node-sid can mandate a protection for the prefix on other routers. What if there is no backup available on a certain node along the path?
> 
> The parallel with the B-flag in adj-sids is not right - in case of adj-sid the originator has the knowledge about the local adjacency protection and as such can signal it it it's LSA.
> 
> thanks,
> Peter
> 
> 
> On 12/29/14 09:47 , Shraddha Hegde wrote:
>> Peter,
>> 
>> 
>> Pls see inline.
>> 
>> Rgds
>> Shraddha
>> 
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Monday, December 29, 2014 2:02 PM
>> To: Shraddha Hegde; draft-ietf-ospf-segment-routing-extensions@tools.ietf.org; draft-ietf-isis-segment-routing-extensions@tools.ietf.org
>> Cc: ospf@ietf.org; isis-wg@ietf.org
>> Subject: Re: [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions
>> 
>> Shraddha,
>> 
>> I do not see how an originator can set any flag regarding the protection of the locally attached prefix.
>> <Shraddha> The originator advertises 2 node-sids. One with p flag set and the other without the p-flag set.
>> 
>>  It's all the routers on the path towards such prefix that need to deal with the protection.
>> <Shraddha> The receiving nodes will download protected path for the node-sid with p-flag set and download
>> Unprotected path for the node-sid with p-flag unset.
>> 
>> Signaling anything from the originator seems useless.
>> <Shraddha>  For node-sids it's the others who need to build the forwarding plane but it's only the originator who can signal which of
>>                         Sid need to be built with protection and which not. Other routers on the path cannot signal this information.
> 
> 
> 
>> 
>> With this you have two paths for the node. One is protected and the other is unprotected. This meets the requirement of having an un-protected path.
>> 
>> It's very much in parallel to B-flag in adj-sids. It is similar to advertising multiple adj-sids one with B-flag on and other with b-flag off , to get protected and unprotected
>> Adj-sids.
>> 
>> thanks,
>> Peter
>> 
>> On 12/29/14 09:26 , Shraddha Hegde wrote:
>>> Yes.You are right.
>>> 
>>> Lets say a prefix sid has a flag "p flag". If this is on it means build a path and provide protection.
>>> If this is off it means build a path with no protection.
>>> The receivers of the prefix-sid will build forwarding plane based on this flag.
>>> 
>>> The applications building the paths will either use prefix-sids with p flag on or off based on the need of the service.
>>> Rgds
>>> Shraddha
>>> 
>>> 
>>> -----Original Message-----
>>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>>> Sent: Monday, December 29, 2014 1:49 PM
>>> To: Shraddha Hegde;
>>> draft-ietf-ospf-segment-routing-extensions@tools.ietf.org;
>>> draft-ietf-isis-segment-routing-extensions@tools.ietf.org
>>> Cc: ospf@ietf.org; isis-wg@ietf.org
>>> Subject: Re: [Isis-wg] Mail regarding
>>> draft-ietf-ospf-segment-routing-extensions
>>> 
>>> Shraddha,
>>> 
>>> the problem is that the node that is advertising the node-sid can not advertise any data regarding the protection of such prefix, because the prefix is locally attached.
>>> 
>>> thanks,
>>> Peter
>>> 
>>> On 12/29/14 09:15 , Shraddha Hegde wrote:
>>>> Peter,
>>>> 
>>>> If there is a service which has to use un-protected path and while
>>>> building such a path if the node-sids Need to be used (one reason
>>>> could be label stack compression) , then there has to be unprotected node-sid that this service can make use of.
>>>> 
>>>> Prefix -sids could also be used to represent different service
>>>> endpoints which makes it even more relevant to have A means of representing  unprotected paths.
>>>> 
>>>> Would be good to hear from others on this, especially operators.
>>>> 
>>>> Rgds
>>>> Shraddha
>>>> 
>>>> 
>>>> -----Original Message-----
>>>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>>>> Sent: Monday, December 29, 2014 1:35 PM
>>>> To: Shraddha Hegde;
>>>> draft-ietf-ospf-segment-routing-extensions@tools.ietf.org;
>>>> draft-ietf-isis-segment-routing-extensions@tools.ietf.org
>>>> Cc: ospf@ietf.org; isis-wg@ietf.org
>>>> Subject: Re: [Isis-wg] Mail regarding
>>>> draft-ietf-ospf-segment-routing-extensions
>>>> 
>>>> Shraddha,
>>>> 
>>>> node-SID is advertised by the router for the prefix that is directly attached to it. Protection for such local prefix does not mean much.
>>>> 
>>>> thanks,
>>>> Peter
>>>> 
>>>> On 12/24/14 11:57 , Shraddha Hegde wrote:
>>>>> Authors,
>>>>> We have a "backup flag" in adjacency sid to indicate whether the
>>>>> label is protected or not.
>>>>> Similarly. I think we need a flag in prefix-sid as well to indicate
>>>>> whether the node-sid is to be protected or not.
>>>>> Any thoughts on this?
>>>>> Rgds
>>>>> Shraddha
>>>>> 
>>>>> 
>>>>> _______________________________________________
>>>>> Isis-wg mailing list
>>>>> Isis-wg@ietf.org
>>>>> https://www.ietf.org/mailman/listinfo/isis-wg
>>>>> 
>>>> 
>>>> .
>>>> 
>>> 
>>> .
>>> 
>> 
>> .
>> 
> 
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