Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01

"Acee Lindem (acee)" <acee@cisco.com> Thu, 08 October 2015 21:07 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: Shraddha Hegde <shraddha@juniper.net>, Pushpasis Sarkar <psarkar@juniper.net>
Thread-Topic: OSPF Link Overload - draft-hegde-ospf-link-overload-01
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Date: Thu, 8 Oct 2015 21:07:09 +0000
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Cc: Hannes Gredler <hannes@gredler.at>, OSPF WG List <ospf@ietf.org>, Mohan Nanduri <mnanduri@microsoft.com>, "Jalil, Luay" <luay.jalil@verizon.com>
Subject: Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01
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Hi Shraddha, 
If this is truly TE, why would you use the OSPF prefix/link attribute
instead of the actual TE metric specified in RFC 3630?
Thanks,
Acee 


On 9/29/15, 1:05 PM, "Shraddha Hegde" <shraddha@juniper.net> wrote:

>Acee,
>
>I am not sure if I am able to convey what I mean by the "controller use
>case" in the previous mail thread. Here is another attempt to explain the
>use case.
>
>With metric change there is no guarantee that LSP will move to a
>different path. If the current path satisfies all constraints of the LSP
>and there is no better path
>Satisfying the constraints then the LSP would remain up and very much on
>the link that is going to be replaced. I mentioned in another mail
>thread, the high metric is
>Usable metric and does not mean "link down".
>
>Link maintenance is a special scenario. The LSP MUST move out of the
>link. Controller can take special actions if it knows the link is in
>overload state
>For Ex: Relax certain constraints of the LSP for the duration of
>maintenance and move the LSP on a different path.
>All these activities should happen in a non- disruptive fashion for the
>service and that’s the reason the link metric cannot be changed to
>max-metric (0xffffffff)
>
>If the "link overload" information remains at the link level, controller
>needs to take action based on metric alone.
>It might work for most cases assuming there are better alternate paths
>satisfying same constraints but we cannot guarantee
>LSPs will move from the link in all cases. If we consider a case when
>multiple links in the network go for maintenance/replacement
>simultaneously
>then there is higher probability that alternate paths satisfying the
>constraints can't be found and controller needs to perform special
>actions to
>move the LSPs around.
>
>IMHO, "link overload" is a characteristic of the link just like color,
>bandwidth etc and it makes sense to flood it area wide just like other
>attributes of the link.
>
>Rgds
>Shraddha
>
>-----Original Message-----
>From: Pushpasis Sarkar
>Sent: Tuesday, September 29, 2015 8:27 PM
>To: Acee Lindem (acee) <acee@cisco.com>
>Cc: Shraddha Hegde <shraddha@juniper.net>et>; OSPF WG List <ospf@ietf.org>rg>;
>Hannes Gredler <hannes@gredler.at>at>; Mohan Nanduri
><mnanduri@microsoft.com>t.com>; Jalil, Luay <luay.jalil@verizon.com>
>Subject: Re: OSPF Link Overload - draft-hegde-ospf-link-overload-01
>
>Hi Acee,
>
>
>
>
>On 9/29/15, 8:15 PM, "Acee Lindem (acee)" <acee@cisco.com> wrote:
>
>
>>I apologize if I offended you. I just wanted to avoid the circular
>>discussions and repetition of information having no bearing on the
>>issues raised.  
>[Pushpasis] No no. You have not offended me in any ways. So we are good
>then. I was worried that I might have offended you instead. :)
>>
>>
>>> [Pushpasis] Like mentioned already, and again in my opinion, this will
>>>help the controller deal with scenarios where it needs to distinguish
>>>between situations in which a link has been administratively put into
>>>‘out-of-order’ from situations where the link has degraded to a
>>>‘malfunctioning’ state and needs attention. Unfortunately I cannot come
>>>up with a use-cases how this distinction can be used (other than
>>>diverting service traffics away from the links). Perhaps some of the
>>>operators may throw more light.
>>
>>I’d like to hear from the operators (especially the authors Luay and
>>Mohan).
>[Pushpasis] Me too :)
>> 
>>
>>> 
>>> 
>>> Hoping I have not failed to communicate once more. If you still feel
>>>so, please let me know. And I will refrain myself from answering on
>>>this thread further.
>>
>>I think we are communicating now - the main question is what does this
>>link-maintenance condition needs to be flooded throughout the OSPF
>>routing domain when it seems that link-local signaling would offer a
>>much more straight-forward solution. The response so far has been, “For
>>the controller use-case” without any explanation of why increasing the
>>forward and reverse metrics isn’t enough (especially since you are doing
>>this anyway for backward compatibility). Les Ginsberg raised the same
>>point.
>[Pushpasis] I will not further exaggerate my already-expressed reasoning
>as I do not have a definite use case in hand. Hoping some operators in
>the working group may have more solid use-cases for this.
>
>Thanks and Regards,
>-Pushpasis
>
>>
>>Thanks,
>>Acee