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

Peter Psenak <ppsenak@cisco.com> Sat, 03 January 2015 10:20 UTC

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Date: Sat, 03 Jan 2015 11:20:52 +0100
From: Peter Psenak <ppsenak@cisco.com>
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To: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>, Shraddha Hegde <shraddha@juniper.net>, "draft-ietf-ospf-segment-routing-extensions@tools.ietf.org" <draft-ietf-ospf-segment-routing-extensions@tools.ietf.org>, "draft-ietf-isis-segment-routing-extensions@tools.ietf.org" <draft-ietf-isis-segment-routing-extensions@tools.ietf.org>
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Subject: Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions
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Hi Les,

On 1/2/15 17:35 , Les Ginsberg (ginsberg) wrote:
> Peter -
>
> The requirement Shraddha specified was to not allow a particular class of service ("heavy bandwidth services" was the example provided) to use certain links in the topology.

my understanding was different. I thought Shraddha simply wanted to 
avoid calculating a backup for the prefix.

thanks,
Peter

> My point is that advertising a flag for a given prefix which says "do not calculate a repair path for this prefix" does not help achieve this. Once the network reconverges following the failure of one of the links on which "heavy bandwidth services" is allowed/preferred it is quite likely that the new best path will be over a link on which "heavy bandwidth services" is NOT allowed/preferred. This will happen whether you have the new flag or not - so the flag will have no lasting effect. It would only affect traffic flow during the brief period during which the network is reconverging.
>
> I think you and I are actually in agreement - I am simply sending a stronger negative message - not only do I think the flag is not useful - I think it does not achieve the goal Shraddha has in mind.
>
>     Les
>
>
> -----Original Message-----
> From: Peter Psenak (ppsenak)
> Sent: Friday, January 02, 2015 12:18 AM
> To: Les Ginsberg (ginsberg); 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
>
> Hi Les,
>
> I believe the idea is not to exclude any particular link, it's actually much simpler - do not calculate backup for the prefix if the flag is set.
>
> I'm still not quite sure how useful above is, but technically it is possible.
>
> thanks,
> Peter
>
> On 12/30/14 17:22 , Les Ginsberg (ginsberg) wrote:
>> Shraddha -
>>
>> When performing a best path calculation whether a given link is in the set of best paths (to be protectedED) or not (could be used as a protectING path) is a function of the topology - not the link.  If there is a topology change it is quite likely that a given link will change from being a protectED link to being a protectING link (or vice versa). So what you propose regarding node-SIDs would not work.
>>
>> In the use case you mention below if you don't want a certain class of traffic to flow on a given link it requires a link attribute which is persistent across topology changes. There are ways to do that - using Adj-SIDs is one of them. But using node-SIDs in the way you propose is NOT.
>>
>>      Les
>>
>> -----Original Message-----
>> From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of Shraddha Hegde
>> Sent: Monday, December 29, 2014 10:12 PM
>> To: Peter Psenak (ppsenak);
>> 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: [OSPF] [Isis-wg] Mail regarding
>> draft-ietf-ospf-segment-routing-extensions
>>
>> Peter,
>>
>>> The requirement here is to get an un-protected path for services which do not want to divert the traffic on protected path in any case.
>>
>>> can you give an example of such a service and a reasoning why such service would want to avoid local protection along the path?
>>
>> Heavy bandwidth services are potential candidates.  The network is well planned and well provisioned for primary path but same is not true for backup paths.
>> Diverting heavy bandwidth services along protection path can disrupt the other services on that path, they are better-off un-protected so that an event in the network Would result in disconnection and a retry for such services.
>>
>> Rgds
>> Shraddha
>>
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Monday, December 29, 2014 4:35 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,
>>
>> On 12/29/14 10:06 , Shraddha Hegde wrote:
>>> Peter,
>>>
>>> The requirement here is to get an un-protected path for services which do not want to divert the traffic on protected path in any case.
>>
>> can you give an example of such a service and a reasoning why such service would want to avoid local protection along the path?
>>
>> thanks,
>> Peter
>>
>>> So when the originator of node-sid signals un-protected path requirement, there is always an unprotected path.
>>>
>>> Regarding the protected path, it is the default behavior as it exists today. You get protection if it's available otherwise you don't get protection.
>>>
>>> In fact, you can have the new flag to say "NP flag" meaning non-protected flag which can be set for the unprotected path.
>>> By default it remains off and gives the behavior as it exists today.
>>>
>>>
>>> Rgds
>>> Shraddha
>>>
>>> -----Original Message-----
>>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>>> Sent: Monday, December 29, 2014 2:26 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 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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