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

Shraddha Hegde <shraddha@juniper.net> Mon, 28 September 2015 05:13 UTC

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From: Shraddha Hegde <shraddha@juniper.net>
To: "Acee Lindem (acee)" <acee@cisco.com>, OSPF WG List <ospf@ietf.org>
Thread-Topic: OSPF Link Overload - draft-hegde-ospf-link-overload-01
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Date: Mon, 28 Sep 2015 05:13:31 +0000
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Cc: Hannes Gredler <hannes@gredler.at>, "Jalil, Luay" <luay.jalil@verizon.com>, 'Mohan Nanduri' <mnanduri@microsoft.com>
Subject: Re: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01
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Acee,

Thanks for picking up the draft for adoption.

I believe this draft is very useful in automating the link upgrade process and software upgrade process in overlay deployments and
hence support WG adoption as co-author.

I would like to  take this opportunity to discuss  few of the points raised during Prague meeting.

1. Whether to keep the "Link overload" advertisement at area level or at link level.

In controller based deployments, it's useful to advertise the impending maintenance of the link to the controller so that controller can take
Special actions based on the information. The use case is described in sec 5.2 in  the draft. 
The draft advocates increasing the metric to usable high metric on both ends of the link. This is for backwards compatibility and to avoid need of flag
Day upgrade on all nodes.

 Controller cannot assign special meaning to the metric  for ex: Metric XXXX means the link going for maintenance and take different actions based on metric. 

For a completely automated upgrade process, controller would need a fine grained and specific information that the link is going for maintenance so that the services that use the particular link find a different path forcefully while keeping the entire process non-disruptive.


2. Use of high metric  on either side of the link  to divert the traffic.

As I already mentioned before, draft advocates raising the reverse metric to a high metric  but that is for backwards compatibility and to avoid 
Need for flag-day upgrade. There were suggestions at the Prague meeting to use lower bandwidth advertisements as well as removal of
Link characteristics to force the services on different path. These mechanisms would be disruptive and defeats the purpose of the draft.

3.  Backward compatibility

"Link-overload"  is a new information attached to a link and is very similar to a new constraint being added to the link.
This information is non-invasive in the sense that services that do not want to look at the new constraint (link overload)
May depend only on the metric to take specific actions.

Whereas services that have specialized requirement of providing non-disruptive upgrades can do so by processing the new constraint.

Section 4 in the draft talks about backwards compatibility.
I'll add more clarifications in the coming days.

Rgds
Shraddha


-----Original Message-----
From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of Acee Lindem (acee)
Sent: Saturday, September 26, 2015 6:05 AM
To: OSPF WG List <ospf@ietf.org>
Subject: [OSPF] OSPF Link Overload - draft-hegde-ospf-link-overload-01

In Prague, there was consensus in the room that this use case was not covered by existing mechanisms and that it was a problem the WG should solve. There were differing opinions as to the exact solution but that should not preclude OSPF WG adoption.

Please indicate your support (or concerns) for adopting this as a WG Document. 


Thanks,
Acee 


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