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

"Les Ginsberg (ginsberg)" <ginsberg@cisco.com> Mon, 05 January 2015 05:53 UTC

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From: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>
To: Pushpasis Sarkar <psarkar@juniper.net>, Shraddha Hegde <shraddha@juniper.net>, "Peter Psenak (ppsenak)" <ppsenak@cisco.com>, "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>, Hannes Gredler <hannes@juniper.net>
Thread-Topic: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions
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Date: Mon, 5 Jan 2015 05:53:18 +0000
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Subject: Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions
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Pushpasis -

The key point is that the proposal does not have any lasting impact on traffic flow. A simple topology should suffice to illustrate this.


A----B----C----D
          |       |
          E----F

(All links have the same cost)

Suppose we wish to have traffic entering at A flow along the path A-B-C-D - but if the link B---C fails we do NOT want traffic to take the path B--E--F--C.

You propose to have C advertise an address with two node-sids - one which allows protection - call it C(P) - and one which does NOT allow protection - call it C(NP).

If the label stack specifies C(NP) - then while the link B--C is UP everything works as desired (primary path to C(NP) on Node B is via link B-C).
However, when the link B--C goes down, the network will reconverge and in a modest amount of time the new primary path to C(NP) on node B will be via link B-E.

The existence of C(NP) therefore only affects traffic flow during the reconvergence period i.e. if we assume B did NOT install a repair path for C(NP) traffic will be dropped only until a new primary path is calculated. I don’t see the value in this.

As a (somewhat dangerous) aside, the functionality you are looking for is more akin to "not-via" as defined in RFC 6981 - though I am quick to add that I am NOT proposing to pursue that. :-)
But reading that RFC might give you more insight into why simply setting "don't protect" for a prefix isn't useful for the purpose you have in mind.

   Les



-----Original Message-----
From: Pushpasis Sarkar [mailto:psarkar@juniper.net] 
Sent: Sunday, January 04, 2015 8:34 PM
To: Les Ginsberg (ginsberg); Shraddha Hegde; Peter Psenak (ppsenak); draft-ietf-ospf-segment-routing-extensions@tools.ietf.org; draft-ietf-isis-segment-routing-extensions@tools.ietf.org; Hannes Gredler
Cc: ospf@ietf.org; isis-wg@ietf.org
Subject: Re: [OSPF] [Isis-wg] Mail regarding draft-ietf-ospf-segment-routing-extensions

Hi Les,

Please find comments inline..

Authors, 

Here is my proposal. Please let me know if this sounds reasonable or not.

- A new ŒNo-Potection-Required¹ or ŒNP¹ flag be added to the Prefix-SID Sub-TLV/TLV. Setting this flag means none of the transit routers should try to protect this node-segment.
- Let nodes advertise two node-sid-index each (per address-family), one without and one with ŒNP¹ flag set. For node-sid advertised with ŒNP¹ flag 0, routers same behave the same way as today. But when they receive a node-sid with ŒNP¹ flag set, they avoid/skip finding a backup for that segment.
- Finally ingress servers or TE-applications may use these 'node-sids with NP-flag set¹ for use cases where it is better to drop traffic on topology outages rather than diverting it to some other paths. For such cases ingress router or TE-applications should look for node-sids with ŒNP¹ flag set and not the regular node-sids. For all other normal use cases(including L3VPN/6VPE etc) traffic should be carried using node-sid without ŒNP‹flag set.

Thanks and Regards,
-Pushpasis

On 1/5/15, 3:37 AM, "Les Ginsberg (ginsberg)" <ginsberg@cisco.com> wrote:

>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.
[Pushpasis] This is not about which links to take. It is about wether transit routers should try to protect the node-segment to the this node-sid or not. I think this opens up a lot many number of possibilities on the ingress router and TE controller-based applications.

>
>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.
[Pushpasis] I am not trying to draw a parallel between this new flag and the ŒB¹ flag in Adj-Sid SubTlv. Like said before

>
>   Les
>
>
>-----Original Message-----
>From: Pushpasis Sarkar [mailto:psarkar@juniper.net]
>Sent: Sunday, January 04, 2015 8:51 AM
>To: Les Ginsberg (ginsberg); Shraddha Hegde; Peter Psenak (ppsenak); 
>draft-ietf-ospf-segment-routing-extensions@tools.ietf.orgorg;
>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
>
>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)" <ginsberg@cisco.com> 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:shraddha@juniper.net]
>>Sent: Friday, January 02, 2015 10:54 AM
>>To: Les Ginsberg (ginsberg); Peter Psenak (ppsenak); 
>>draft-ietf-ospf-segment-routing-extensions@tools.ietf.orgtf.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/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:ginsberg@cisco.com]
>>Sent: Friday, January 02, 2015 10:06 PM
>>To: Peter Psenak (ppsenak); Shraddha Hegde; 
>>draft-ietf-ospf-segment-routing-extensions@tools.ietf.orgtf.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
>>
>>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; 
>>draft-ietf-ospf-segment-routing-extensions@tools.ietf.orgtf.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
>>>>>>>>
>>>>>>>
>>>>>>> .
>>>>>>>
>>>>>>
>>>>>> .
>>>>>>
>>>>>
>>>>> .
>>>>>
>>>>
>>>> .
>>>>
>>>
>>> _______________________________________________
>>> OSPF mailing list
>>> OSPF@ietf.org
>>> https://www.ietf.org/mailman/listinfo/ospf
>>> .
>>>
>>
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