Re: [OSPF] Follow-up discussion on draft-zzhang-ospf-two-part-metric

"Acee Lindem (acee)" <acee@cisco.com> Fri, 03 October 2014 14:21 UTC

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From: "Acee Lindem (acee)" <acee@cisco.com>
To: "Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net>, "Peter Psenak (ppsenak)" <ppsenak@cisco.com>, "ospf@ietf.org" <ospf@ietf.org>
Thread-Topic: [OSPF] Follow-up discussion on draft-zzhang-ospf-two-part-metric
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Date: Fri, 3 Oct 2014 14:21:53 +0000
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Subject: Re: [OSPF] Follow-up discussion on draft-zzhang-ospf-two-part-metric
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Hi Guys, 
With respect to OSPFv3 Extended LSAs, the direction we have taken is to
keep the E-Network-LSA free of metrics and attributes. There are two
reasons for that: 
       1. Migration Simplicity - the E-Network-LSA remains as Network-LSA
is today advertising only the routers connecting to the pseudo-node.
       2. Size constraints - Given that the LSA-ID of the Network-LSA and
E-Network-LSA is DR¹s interface ID, we are limited to a single instance of
the Network-LSA and E-Network-LSA.

Of course, I¹d be willing to revisit the E-Network-LSA encoding, but my
opinion is that all requirements can be satisfied with sub-TLVs in the
E-Router-LSA. 

Thanks,
Acee 


     

On 10/3/14, 7:54 AM, "Jeffrey (Zhaohui) Zhang" <zzhang@juniper.net> wrote:

>Peter,
>
>Let's say we have router A on the network. The metric from the network to
>A is encoded in router A's own extended LSA, so no neighbor information
>is needed. You can imagine that it is AS IF the metric was encoded next
>to the normal to-network metric side by side.
>
>We did consider to optionally encode that in the extended network LSA for
>OSPFv3, so that if the underlying network has the ability for the DR to
>learn the from-network metric for each neighbor, then it'll be able
>encode those metrics in the network LSA, further reducing the churning
>when an affect-all event happens. That was in -02, but it was deemed too
>dependent on the underlying network (to let the DR know those metrics)
>and deviating from the normal OSPF, so it was take out.
>
>Thanks.
>Jeffrey
>
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Friday, October 03, 2014 7:37 AM
>> To: Jeffrey (Zhaohui) Zhang; ospf@ietf.org
>> Cc: vibhor.julka@l-3Com.com; Dave.Dubois@gdc4s.com; tom.mcmillan@l-
>> 3com.com; Lili Wang
>> Subject: Re: [OSPF] Follow-up discussion on draft-zzhang-ospf-two-part-
>> metric
>> 
>> Jeffrey,
>> 
>> the encoding you proposed would not work.
>> 
>> OSPFv2:
>> 
>> - you added Network-to-Router Metric Sub-TLV in Extended Link TLV, but
>> Extended Link TLV does not have any neighbor identification. Please look
>> at LAN Adj-SID Sub-TLV defined in
>> draft-ietf-ospf-segment-routing-extensions - you need something similar.
>> 
>> OSPFv3:
>> 
>> - you added Network-to-Router Metric Sub-TLV to Router-Link TLV of
>> E-Router-LSA. That is not the right place. You should put the metric
>> from DR to neighbors inside OSPFv3 E-Network-LSA and include neighbor
>> identifier in it.
>> 
>> thanks,
>> Peter
>> 
>> 
>> On 10/2/14 20:42 , Jeffrey (Zhaohui) Zhang wrote:
>> > Peter, and all,
>> >
>> > I have posted a new revision, which uses the following to encode the
>> from-network metric.
>> >
>> >> 1. OSPFv2: Extended Link LSA
>> >> 2. OSPFv3: E-Router LSAs
>> >
>> > http://www.ietf.org/id/draft-zzhang-ospf-two-part-metric-03.txt
>> >
>> > Please review and comment.
>> >
>> > Thanks!
>> > Jeffrey
>> >
>> >
>> >> -----Original Message-----
>> >> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> >> Sent: Monday, July 21, 2014 5:00 PM
>> >> To: Jeffrey (Zhaohui) Zhang; ospf@ietf.org
>> >> Cc: vibhor.julka@l-3Com.com; Dave.Dubois@gdc4s.com; tom.mcmillan@l-
>> >> 3com.com
>> >> Subject: Re: [OSPF] Follow-up discussion on draft-zzhang-ospf-two-
>> part-
>> >> metric
>> >>
>> >> Hi Jeffrey,
>> >>
>> >> please see inline:
>> >>
>> >>
>> >> On 7/21/14 15:24 , Jeffrey (Zhaohui) Zhang wrote:
>> >>> Hi,
>> >>>
>> >>> In today's OSPF session there were mainly two questions/comments
>> >> during my presentation:
>> >>>
>> >>> 1. Acee: more discussion on mailing list about whether this is a
>> >> general problem/solution that the WG should be taking on
>> >>> 2. Peter: should we use OSPFv3 Extended LSA for a cleaner encoding
>> >>
>> >> my comment was not specific to OSPFv3.
>> >> I propose to use following extensions to encode metric from DR to
>> >> attached router:
>> >>
>> >> 1. OSPFv2: Extended Link LSA
>> >> 2. OSPFv3: E-Router LSAs
>> >>
>> >>>
>> >>> I want to repeat and add some comments/answers here as a starting
>> >> point for more discussions on the mailing list.
>> >>>
>> >>> For #1:
>> >>>
>> >>> - The described problem is real for some large scale and mission
>> >> critical networks
>> >>> - The problem and solution are not only applicable to the above
>> >> mentioned example network, but also general to any broadcast network
>> >> that have different costs between different pair of routers. As long
>> as
>> >> the router-to-router costs can be presented as a to-network and a
>> from-
>> >> network part, then the simple solution applies
>> >>> - The two-part-metric concept is a natural extension of the SPF
>> graph
>> >> theory - we're just changing the previously zero from-network cost to
>> >> none-zero.
>> >>>
>> >>> For #2, there are pros and cons with each approach.
>> >>>
>> >>> - The stub-link based approach does not depend on the in-progress
>> LSA
>> >> Extensibility work. This has larger impact on implementation - the
>> >> feature can be supported w/o big changes to support extended LSA
>> format.
>> >>
>> >> though the stub-link approach is simpler to implement, it's a bit of
>> a
>> >> "hack", as you are using a standard encoding for a stub link to
>> >> advertise a metric for a neighbor on a broadcast link.
>> >>
>> >>> - The LSA Extensibility work is only applicable for OSPFv3. That
>> means
>> >> OSPFv2 would need a different approach for the problem. Acee also
>> >> mentioned that it would be good to have consistent approaches between
>> >> OSPFv2 and OSPFv3.
>> >>
>> >> what I proposed is consistent - uses new extended LSAs in both OSPFv2
>> >> and OSPFv3.
>> >>
>> >> thanks,
>> >> Peter
>> >>
>> >>> - As a result at this time we would prefer the stub-link approach.
>> >>>
>> >>> The authors would like to hear more of your opinions/suggestions.
>> >> Hopefully we can reach consensus on the applicability of the problem
>> &
>> >> solution so that it can become a WG item, and choose the best
>> encoding
>> >> approach.
>> >>>
>> >>> Thanks!
>> >>> Jeffrey
>> >>>
>> >>> _______________________________________________
>> >>> OSPF mailing list
>> >>> OSPF@ietf.org
>> >>> https://www.ietf.org/mailman/listinfo/ospf
>> >>>
>> >
>> > .
>> >
>
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