Re: [spring] meaning of "Strict Shortest Path" algorithm in draft-ietf-spring-segment-routing-09

2016-09-19 Thread Stefano Previdi (sprevidi) 
Chris, Jeff, Alex,

strict-SPF behavior has been intended as the forwarding of the packet according 
to spf, without any form of policy. 

It is true that ecmp is a matter of local implementation so we could extend the 
behavior description to:

 forwarding of the packet according to spf, 
 without any form of policy and according 
 to ecmp capability of the node.

Now, if you intentionally (through configuration) reduce the number of ecmp 
members, isn’t this fit the definition of a policy ?

The strict-spf behavior has been defined for exactly that purpose: allow an 
instruction to override any policy decision.

Note well, I’m not opposed to relax the constraint and allow ecmp differences 
in the “strict-spf” behavior. It’s just that at this stage I’m not (yet) 
convinced it’s a good thing.

s.


> On Sep 19, 2016, at 2:27 PM, Alexander Vainshtein 
>  wrote:
> 
> Jeff,
> I fully agree with what you say: from my POV restrictions on the number of 
> ECMP next hops do not make an SPF less strict.
>  
> Regards,
> Sasha
>  
> Office: +972-39266302
> Cell:  +972-549266302
> Email:   alexander.vainsht...@ecitele.com
>  
> From: Jeff Tantsura [mailto:jefftant.i...@gmail.com] 
> Sent: Monday, September 19, 2016 3:09 PM
> To: Stefano Previdi (sprevidi) 
> Cc: Alexander Vainshtein ; spring@ietf.org; 
> Chris Bowers 
> Subject: Re: [spring] meaning of "Strict Shortest Path" algorithm in 
> draft-ietf-spring-segment-routing-09
>  
> Number if ECMP paths is an implementation subject and would differ from 
> platform to platform. The way subset of ECMP paths is chosen is local to the 
> implementation.
> If you limit number of paths/size of ECMP bundle - it doesn't make it less 
> SPF-strict as long as SPF(Dijkstra) has been applied to compute.
> 
> Regards,
> Jeff
> 
> On Sep 19, 2016, at 12:21 PM, Stefano Previdi (sprevidi)  
> wrote:
> 
> sorry. What I meant is that if you restrict the number of ecmp path you have 
> computed, it is not what the definition of strict-spf is.
> 
> IOW, strict-spf means that you forward according to what SPF algorithm has 
> computed without applying any sort of constraint/policy/hack.
> 
> s.
> 
> 
> 
> On Sep 19, 2016, at 12:17 PM, Alexander Vainshtein 
>  wrote:
>  
> Stefano, Chris and all,
> I have to admit that I am completely confused:
>- to the best of my understanding, Chris has asked whether a policy that 
> puts a limit on max. number of ECMP next hops is not compatible with the 
> Strict SPF algorithm
>- Stefano says that "Yes, this policy is a good example when Strict SPF 
> algorithm can be advertised".
>  
>  
> What do I miss?
> Regards,
> Sasha
>  
> Office: +972-39266302
> Cell:  +972-549266302
> Email:   alexander.vainsht...@ecitele.com
>  
> -Original Message-
> From: spring [mailto:spring-boun...@ietf.org] On Behalf Of Stefano Previdi 
> (sprevidi)
> Sent: Monday, September 19, 2016 12:43 PM
> To: Chris Bowers 
> Cc: spring@ietf.org
> Subject: Re: [spring] meaning of "Strict Shortest Path" algorithm in 
> draft-ietf-spring-segment-routing-09
>  
>  
> On Sep 14, 2016, at 7:06 PM, Chris Bowers  wrote:
>  
> SPRING WG,
>  
> The current text in draft-ietf-spring-segment-routing-09 regarding the
> "Strict Shortest Path" algorithm reads as follows.
>  
>  o  "Strict Shortest Path": This algorithm mandates that the packet is
> forwarded according to ECMP-aware SPF algorithm and instruct any
> router in the path to ignore any possible local policy overriding
> SPF decision.  The SID advertised with "Strict Shortest Path"
> algorithm ensures that the path the packet is going to take is the
> expected, and not altered, SPF path.
>  
> One example of a local policy that overrides the ECMP-aware SPF
> algorithm decision is a limit on the number of ECMP next-hops.  The
> text above implies that if a router places any limit on the number of
> ECMP forwarding next-hops then it would be wrong for it to advertise the 
> “Strict Shortest Path” algorithm capability.
>  
> Is this the intended interpretation?
>  
>  
> well, yes. Your example is a good one for the “strict-SPF” behavior.
>  
> s.
>  
>  
>  
> If not, what is the intended interpretation?
>  
> Thanks,
> Chris
>  
> ___
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Re: [spring] meaning of "Strict Shortest Path" algorithm in draft-ietf-spring-segment-routing-09

2016-09-19 Thread Stefano Previdi (sprevidi) 

> On Sep 14, 2016, at 7:06 PM, Chris Bowers  wrote:
> 
> SPRING WG,
>  
> The current text in draft-ietf-spring-segment-routing-09 regarding the 
> "Strict Shortest Path" algorithm reads as follows.  
>  
>o  "Strict Shortest Path": This algorithm mandates that the packet is
>   forwarded according to ECMP-aware SPF algorithm and instruct any
>   router in the path to ignore any possible local policy overriding
>   SPF decision.  The SID advertised with "Strict Shortest Path"
>   algorithm ensures that the path the packet is going to take is the
>   expected, and not altered, SPF path.
>  
> One example of a local policy that overrides the ECMP-aware SPF algorithm 
> decision is a limit 
> on the number of ECMP next-hops.  The text above implies that if a router 
> places any 
> limit on the number of ECMP forwarding next-hops then it would be wrong for 
> it to advertise 
> the “Strict Shortest Path” algorithm capability.  
>  
> Is this the intended interpretation?


well, yes. Your example is a good one for the “strict-SPF” behavior.

s.


>  
> If not, what is the intended interpretation?
>  
> Thanks,
> Chris
>  
> ___
> spring mailing list
> spring@ietf.org
> https://www.ietf.org/mailman/listinfo/spring

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Re: [spring] meaning of "Strict Shortest Path" algorithm in draft-ietf-spring-segment-routing-09

2016-09-19 Thread Stefano Previdi (sprevidi) 
sorry. What I meant is that if you restrict the number of ecmp path you have 
computed, it is not what the definition of strict-spf is.

IOW, strict-spf means that you forward according to what SPF algorithm has 
computed without applying any sort of constraint/policy/hack.

s.


> On Sep 19, 2016, at 12:17 PM, Alexander Vainshtein 
>  wrote:
> 
> Stefano, Chris and all,
> I have to admit that I am completely confused:
>   - to the best of my understanding, Chris has asked whether a policy 
> that puts a limit on max. number of ECMP next hops is not compatible with the 
> Strict SPF algorithm
>   - Stefano says that "Yes, this policy is a good example when Strict SPF 
> algorithm can be advertised".
> 
> 
> What do I miss?
> Regards,
> Sasha
> 
> Office: +972-39266302
> Cell:  +972-549266302
> Email:   alexander.vainsht...@ecitele.com
> 
> -Original Message-
> From: spring [mailto:spring-boun...@ietf.org] On Behalf Of Stefano Previdi 
> (sprevidi)
> Sent: Monday, September 19, 2016 12:43 PM
> To: Chris Bowers 
> Cc: spring@ietf.org
> Subject: Re: [spring] meaning of "Strict Shortest Path" algorithm in 
> draft-ietf-spring-segment-routing-09
> 
> 
>> On Sep 14, 2016, at 7:06 PM, Chris Bowers  wrote:
>> 
>> SPRING WG,
>> 
>> The current text in draft-ietf-spring-segment-routing-09 regarding the 
>> "Strict Shortest Path" algorithm reads as follows.
>> 
>>   o  "Strict Shortest Path": This algorithm mandates that the packet is
>>  forwarded according to ECMP-aware SPF algorithm and instruct any
>>  router in the path to ignore any possible local policy overriding
>>  SPF decision.  The SID advertised with "Strict Shortest Path"
>>  algorithm ensures that the path the packet is going to take is the
>>  expected, and not altered, SPF path.
>> 
>> One example of a local policy that overrides the ECMP-aware SPF 
>> algorithm decision is a limit on the number of ECMP next-hops.  The 
>> text above implies that if a router places any limit on the number of 
>> ECMP forwarding next-hops then it would be wrong for it to advertise the 
>> “Strict Shortest Path” algorithm capability.
>> 
>> Is this the intended interpretation?
> 
> 
> well, yes. Your example is a good one for the “strict-SPF” behavior.
> 
> s.
> 
> 
>> 
>> If not, what is the intended interpretation?
>> 
>> Thanks,
>> Chris
>> 
>> ___
>> 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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