Re: [OSPF] I-D Action: draft-ietf-ospf-link-overload-05.txt

Peter Psenak <ppsenak@cisco.com> Fri, 21 April 2017 11:17 UTC

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Date: Fri, 21 Apr 2017 13:17:31 +0200
From: Peter Psenak <ppsenak@cisco.com>
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To: Shraddha Hegde <shraddha@juniper.net>, "Acee Lindem (acee)" <acee@cisco.com>
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Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-link-overload-05.txt
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Hi Shraddha,

please see inline:

On 21/04/17 12:53 , Shraddha Hegde wrote:
> Hi Peter,
>
> Thanks for the detailed review. Pls see inline..
>
> -----Original Message-----
> From: Peter Psenak [mailto:ppsenak@cisco.com]
> Sent: Friday, April 21, 2017 1:38 PM
> To: Shraddha Hegde <shraddha@juniper.net>; Acee Lindem (acee) <acee@cisco.com>
> Cc: ospf@ietf.org
> Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-link-overload-05.txt
>
> Hi Shraddha,
>
> please find my comments below:
>
> The draft defines two mechanisms:
>
> a) signaling the link overload to the neighbor. The purpose is to advertise the link with max-metric from both directions.
>
> b) flooding the Link-Overload sub-TLV inside the area. The purpose is to let "LSP ingress routers/controllers can learn of the impending maintenance activity"
>
> 1. Why do we need two mechanisms? Why is (b) needed, given that (a) results in link being advertised with max-metric in both directions?
>
> How is treatement of remote link having max-metric different to the treatment of a link that has the Link-Overload sub-TLV? I would understand the difference if you say that the link having the Link-Overload sub-TLV must not be used during SPF, but nothing like that is mentioned in the draft and I understand why.
>
> Is (b) needed to cover the case, where the signaling defined in (a) is not understood by the neighbor on the other side of the link? If yes, please state it in the draft.
> <Shraddha> Metric alone cannot be used as an indication for impending maintenance activity. When other nodes like ingress/controller need to understand the impending maintenance activity, area level advertisement would be needed. Application specific to this is described in sec 7.2

no argument about the need of area level flooding - Router LSA with the 
max metric will be flooded within the area.

I have read the section 7.2 several times, but I still do not understand 
what is the purpose of the Link-Overload sub-TLV there. What is the 
controller going to do when it receives the area scoped Link-Overload 
sub-TLV and how it is different to the case where the link is advertised 
in the Router LSA with max-metric in both directions?

>
> 2. For the signaling defined in (a)-  using the Router Information LSA for signaling something to the direct neighbor is a very dirty hack. As the name of the LSA says, it has been defined to signal capability of the node, which has nothing to do with what you are trying to use it for. We have to stop polluting the protocol with such hacks. RFC5613 defines a Link-Local Signaling mechanism for OSPF and that is the one we should use for siganling between neighbors.
> <Shraddha>  LLS is a good mechanism to use for signaling link level information that are useful before the adjacency is established. Section 2 RFC 5613  states that the LLS is not expected to be used for use-cases which cause routing changes. Link-overload does result into routing changes and is best handled using link local scope LSAs.

- LLS can be used to signal information prior to adjacency bringup as 
well as when the adjacency is FULL state. There are existing LLS TLVs 
that are send when adjacency is in FULL state.

- in your case the use of LLS would be to change the metric on the link 
in the reverse direction. That is not resulting in any routing changes 
directly (look at it as the remote configuration request). It's the 
max-metric in the Router LSA that is going to change the routing. So 
using LLS to signal what you need is perfectly valid.

I just can not see how we can standardize the use of RI LSA for what you 
are proposing to use it - it's completely wrong IMHO.

thanks,
Peter

>
> thanks,
> Peter
>
>
>
> On 19/04/17 15:08 , Shraddha Hegde wrote:
>> Hi Acee,
>>
>> New version draft-ietf-ospf-link-overload-06 is posted where the remote-ipv4 addr is moved to a new sub-TLV.
>> Pls review.
>>
>> The authors of the draft believe that draft has undergone multiple revisions/reviews and is ready for WG last call.
>>
>> Rgds
>> Shraddha
>>
>>
>> -----Original Message-----
>> From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of Acee Lindem
>> (acee)
>> Sent: Saturday, March 18, 2017 2:28 AM
>> Cc: ospf@ietf.org
>> Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-link-overload-05.txt
>>
>> Hi Shraddha, et al,
>>
>> With respect to section 4.1, I agree that matching link endpoints in
>> OSPFv2 requires more information. However, this is a general problem
>> and the remote address should be a separate OSPFv2 Link Attribute LSA
>> TLV rather than overloading the link overload TLV ;^)
>>
>> Thanks,
>> Acee
>>
>> On 2/23/17, 11:18 AM, "OSPF on behalf of internet-drafts@ietf.org"
>> <ospf-bounces@ietf.org on behalf of internet-drafts@ietf.org> wrote:
>>
>>>
>>> A New Internet-Draft is available from the on-line Internet-Drafts
>>> directories.
>>> This draft is a work item of the Open Shortest Path First IGP of the IETF.
>>>
>>>          Title           : OSPF Link Overload
>>>          Authors         : Shraddha Hegde
>>>                            Pushpasis Sarkar
>>>                            Hannes Gredler
>>>                            Mohan Nanduri
>>>                            Luay Jalil
>>> 	Filename        : draft-ietf-ospf-link-overload-05.txt
>>> 	Pages           : 13
>>> 	Date            : 2017-02-23
>>>
>>> Abstract:
>>>     When a link is being prepared to be taken out of service, the traffic
>>>     needs to be diverted from both ends of the link.  Increasing the
>>>     metric to the highest metric on one side of the link is not
>>>     sufficient to divert the traffic flowing in the other direction.
>>>
>>>     It is useful for routers in an OSPFv2 or OSPFv3 routing domain to be
>>>     able to advertise a link being in an overload state to indicate
>>>     impending maintenance activity on the link.  This information can be
>>>     used by the network devices to re-route the traffic effectively.
>>>
>>>     This document describes the protocol extensions to disseminate link-
>>>     overload information in OSPFv2 and OSPFv3.
>>>
>>>
>>>
>>> The IETF datatracker status page for this draft is:
>>> https://datatracker.ietf.org/doc/draft-ietf-ospf-link-overload/
>>>
>>> There's also a htmlized version available at:
>>> https://tools.ietf.org/html/draft-ietf-ospf-link-overload-05
>>>
>>> A diff from the previous version is available at:
>>> https://www.ietf.org/rfcdiff?url2=draft-ietf-ospf-link-overload-05
>>>
>>>
>>> Please note that it may take a couple of minutes from the time of
>>> submission until the htmlized version and diff are available at
>>> tools.ietf.org.
>>>
>>> Internet-Drafts are also available by anonymous FTP at:
>>> ftp://ftp.ietf.org/internet-drafts/
>>>
>>> _______________________________________________
>>> OSPF mailing list
>>> OSPF@ietf.org
>>> https://www.ietf.org/mailman/listinfo/ospf
>>
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>
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