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

[Shraddha Hegde <[e-mail]>]

Les,


I agree with the point you made that the requirement of not protecting certain services can be met today using the
LFA-manageability draft. Deploying the solution using LFA manageability requires that the 

1. Services be represented by different prefixes
2. Come up with a policy that makes sure no backup path is downloaded for the prefix
3. Configure  the policy in each node in the network.
4. Repeat the process whenever a new service with similar characteristic comes up


The advantage of having an unprotected path to each node is the ease of deployment.
LFA-manageability has its own advantage of fine tuning the backup paths and I am not denying that.
I am trying to say that for certain use-cases it is easy to have unprotected paths in the network for each node
And use those path for services that need such paths.

If someone wants to simply have a unprotected path for certain node and use it for all the services
Which don't need protection, that flexibility should be available in the protocol. 
That is the reason I am saying that we should have "No protection" flag in the prefix-SID.

	
Rgds
Shraddha	

-----Original Message-----
From: Les Ginsberg (ginsberg) [mailto:[e-mail]] 
Sent: Monday, January 05, 2015 3:37 AM
To: Pushpasis Sarkar; Shraddha Hegde; Peter Psenak (ppsenak); [e-mail]; [e-mail]
Cc: [e-mail]; [e-mail]
Subject: RE: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions

Pushpasis -

I don't agree.

The use of one node-sid vs another has nothing whatever to do with the request Shraddha has made i.e. should we introduce a flag indicating whether a particular prefix should be protected or not. A node-sid only dictates what (intermediate) node traffic should be sent to - not what link(s) are used to reach that node.

Adjacency-sids have a different semantic - they identify the link over which traffic is to be forwarded. Identifying an adjacency-sid as unprotected means traffic will NEVER flow over a different link. There is no equivalent behavior w a node-sid - which is what this discussion has been about.

   Les


-----Original Message-----
From: Pushpasis Sarkar [mailto:[e-mail]]
Sent: Sunday, January 04, 2015 8:51 AM
To: Les Ginsberg (ginsberg); Shraddha Hegde; Peter Psenak (ppsenak); [e-mail]; [e-mail]
Cc: [e-mail]; [e-mail]
Subject: Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions

Hi Les,

I think the requirement Shraddha is referring is about the choice of exact node-sid to use while constructing the label-stack for a explicit-LSP on the ingress router, which will be typically done after running some CSPF on the SPRING topology. And not the IGP on ingress or transit routers.

Thanks
-Pushpasis

On 1/3/15, 3:10 AM, "Les Ginsberg (ginsberg)" <[e-mail]> wrote:

>Shraddha -
>
>IGPs today do NOT perform constraint based SPFs - so I don't know why 
>you believe that the primary SPF will meet a set of constraints that an 
>LFA calculation will not. In fact , it is the opposite which is true 
>because implementations today do support preferences in choosing LFAs 
>based on various configured policy - something which is NOT done for primary SPF.
>
>If you want a certain class of traffic to avoid a subset of the links 
>in the topology then you need to have a way of identifying the links 
>(NOT the node addresses) and a way of calculating a path which only 
>uses the links which meet the constraints of that class of service.
>Identifying a particular prefix as protected or unprotected won't achieve that.
>
>   Les
>
>-----Original Message-----
>From: Shraddha Hegde [mailto:[e-mail]]
>Sent: Friday, January 02, 2015 10:54 AM
>To: Les Ginsberg (ginsberg); Peter Psenak (ppsenak); 
>[e-mail];
>[e-mail]
>Cc: [e-mail]; [e-mail]
>Subject: RE: [Isis-wg] Mail regarding
>draft-ietf-ospf-segment-routing-extensions
>
>Hi Les/Peter,
>
>      When reconvergence happens, the primary path will be calculated 
>based on all constriants.
>This is not true with the protection path.Protection path is calculated 
>locally (LFA/RLFA)  and does not consider the characteristics of the 
>services running on that path.
>It's easier for some services to pick the unprotected path when the 
>nature of the service is that it can be restarted  when there is a 
>disconnection.
>
>Rgds
>Shraddha
>-----Original Message-----
>From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
>Sent: Friday, January 02, 2015 10:06 PM
>To: Peter Psenak (ppsenak); Shraddha Hegde; 
>[e-mail];
>[e-mail]
>Cc: [e-mail]; [e-mail]
>Subject: RE: [Isis-wg] Mail regarding
>draft-ietf-ospf-segment-routing-extensions
>
>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 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; 
>[e-mail];
>[e-mail]
>Cc: [e-mail]; [e-mail]
>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:[e-mail]] On Behalf Of Shraddha Hegde
>> Sent: Monday, December 29, 2014 10:12 PM
>> To: Peter Psenak (ppsenak);
>> [e-mail];
>> [e-mail]
>> Cc: [e-mail]; [e-mail]
>> 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:[e-mail]]
>> Sent: Monday, December 29, 2014 4:35 PM
>> To: Shraddha Hegde;
>> [e-mail];
>> [e-mail]
>> Cc: [e-mail]; [e-mail]
>> 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:[e-mail]]
>>> Sent: Monday, December 29, 2014 2:26 PM
>>> To: Shraddha Hegde;
>>> [e-mail];
>>> [e-mail]
>>> Cc: [e-mail]; [e-mail]
>>> 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:[e-mail]]
>>>> Sent: Monday, December 29, 2014 2:02 PM
>>>> To: Shraddha Hegde;
>>>> [e-mail];
>>>> [e-mail]
>>>> Cc: [e-mail]; [e-mail]
>>>> 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:[e-mail]]
>>>>> Sent: Monday, December 29, 2014 1:49 PM
>>>>> To: Shraddha Hegde;
>>>>> [e-mail];
>>>>> [e-mail]
>>>>> Cc: [e-mail]; [e-mail]
>>>>> 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:[e-mail]]
>>>>>> Sent: Monday, December 29, 2014 1:35 PM
>>>>>> To: Shraddha Hegde;
>>>>>> [e-mail];
>>>>>> [e-mail]
>>>>>> Cc: [e-mail]; [e-mail]
>>>>>> 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
>>>>>>> [e-mail]
>>>>>>> https://www.ietf.org/mailman/listinfo/isis-wg
>>>>>>>
>>>>>>
>>>>>> .
>>>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>>> .
>>>>
>>>
>>> .
>>>
>>
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