Re: [OSPF] I-D Action: draft-ietf-ospf-segment-routing-extensions-13.txt

Peter Psenak <ppsenak@cisco.com> Thu, 11 May 2017 09:38 UTC

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Date: Thu, 11 May 2017 11:38:10 +0200
From: Peter Psenak <ppsenak@cisco.com>
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To: Shraddha Hegde <shraddha@juniper.net>, "internet-drafts@ietf.org" <internet-drafts@ietf.org>, "i-d-announce@ietf.org" <i-d-announce@ietf.org>
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Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-segment-routing-extensions-13.txt
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Hi Shraddha,

On 11/05/17 11:30 , Shraddha Hegde wrote:
> Peter,
>
> Inter-area/external prefixes with A-flag re-set is the only scenario
> I can think of where SRMS SIDs should not do PHP.
> Is there any other case?

- Intra-area route, where the downstream neighbor is not the originator 
or the prefix

- Inter-area prefixes, for which the downstream neighbor is not the 
originator or the prefix

- External prefix for which the downstream neighbor is not the 
originator or the prefix

- Inter-area prefixes, where originator is not advertising Extended 
Prefix TLV

- External prefix , where originator is not advertising Extended Prefix TLV

thanks,
Peter





>
>> "For all other cases, when SID is coming from SRMS, PHM MUST not be done"
> I suggest the text to be more specific to the cases since we do
> not have many scenarios to list.
>
> Rgds
> Shraddha
> -----Original Message-----
> From: Peter Psenak [mailto:ppsenak@cisco.com]
> Sent: Thursday, May 11, 2017 2:46 PM
> To: Shraddha Hegde <shraddha@juniper.net>et>; internet-drafts@ietf.org; i-d-announce@ietf.org
> Cc: ospf@ietf.org
> Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-segment-routing-extensions-13.txt
>
> Hi Shraddha
>
> please see inline:
>
> On 11/05/17 08:49 , Shraddha Hegde wrote:
>> Peter,
>>
>> It is clearly specified that ABR originating prefixes from other areas
>> should have NP Bit set.
>>
>> "The NP-Flag (No-PHP) MUST be set for Prefix-SIDs allocated to inter-
>>      area prefixes that are originated by the ABR based on intra-area or
>>      inter-area reachability between areas.  When the inter-area prefix is
>>      generated based on a prefix which is directly attached to the ABR,
>>      the NP-Flag SHOULD NOT be set."
>>
>>
>> The same behavior should apply to mapping server advertised advertisements as well.
>
> when SRMS advertises the SID, it can not set the NP-Flag, so we can not apply the exact same behavior there.
>
>>
>> " As the Mapping Server does not specify the originator of a prefix
>>>       advertisement, it is not possible to determine PHP behavior solely
>>>       based on the Mapping Server advertisement.  However, PHP behavior
>>>       SHOULD be done in following cases:
>>>
>>>          The Prefix is intra-area type and the downstream neighbor is the
>>>          originator of the prefix.
>>>
>>>          The Prefix is inter-area type and downstream neighbor is an ABR,
>>>          which is advertising prefix reachability and is also generating
>>>          the Extended Prefix TLV with the A-flag set for this prefix as
>>>          described in section 2.1 of [RFC7684]."
>>
>>
>> While the text above captures the case of directly attached prefixes
>> it does not cover the Case of re-distributed prefixes for mapping server advertisements.
>
> there is a text in the draft right after the above mention text that talks about the redistribution case:
>
>         "The Prefix is external type and downstream neighbor is an ASBR,
>         which is advertising prefix reachability and is also generating
>         the Extended Prefix TLV with the A-flag set for this prefix as
>         described in section 2.1 of [RFC7684]."
>
>
>>
>> Suggest to add below text.
>>
>>           "The Prefix is inter-area type and downstream neighbor is an ABR,
>>           which is advertising prefix reachability and is also generating
>>           the Extended Prefix TLV with the A-flag re-set for this prefix as
>>           described in section 2.1 of [RFC7684] then PHP MUST not be done"
>
>
> the draft says when PHP should be done when the SID is coming from the SRMS. It assumes that in all other cases PHP is not done.
>
> If we are going to say when the PHP must not be done for SID coming from SRMS, we need to list all cases, not only one of them.
>
> So I would say we either not say anything, or we say:
>
> "For all other cases, when SID is coming from SRMS, PHM MUST not be done"
>
> thanks,
> Peter
>
>
>> Rgds
>> Shraddha
>>
>> -----Original Message-----
>> From: Peter Psenak [mailto:ppsenak@cisco.com]
>> Sent: Wednesday, May 10, 2017 12:33 PM
>> To: Shraddha Hegde <shraddha@juniper.net>et>; internet-drafts@ietf.org;
>> i-d-announce@ietf.org
>> Cc: ospf@ietf.org
>> Subject: Re: [OSPF] I-D Action:
>> draft-ietf-ospf-segment-routing-extensions-13.txt
>>
>> Hi Shraddha,
>>
>> please see inline:
>>
>> On 10/05/17 07:34 , Shraddha Hegde wrote:
>>> Authors,
>>>
>>> Apologies for being late with this comment in the process of standardization.
>>>
>>> The below section 5 describes the PHP for mapping server
>>>
>>>
>>> " As the Mapping Server does not specify the originator of a prefix
>>>       advertisement, it is not possible to determine PHP behavior solely
>>>       based on the Mapping Server advertisement.  However, PHP behavior
>>>       SHOULD be done in following cases:
>>>
>>>          The Prefix is intra-area type and the downstream neighbor is the
>>>          originator of the prefix.
>>>
>>>          The Prefix is inter-area type and downstream neighbor is an ABR,
>>>          which is advertising prefix reachability and is also generating
>>>          the Extended Prefix TLV with the A-flag set for this prefix as
>>>          described in section 2.1 of [RFC7684]."
>>>
>>>
>>> The text says "PHP behavior" should be done in following cases.
>>> In the second case here it's an ABR re-advertising a prefix and SID
>>> being advertised for this Prefix from a mapping server. If we interpret "PHP behavior should be done"
>>> As the penultimate router removing the label and forwarding the
>>> packet to ABR, It does not work since the inner labels gets exposed at the ABR.
>>
>> above texts clearly specifies that PHP is done only for case where ABR is originating a prefix, not propagating it from other area. You can distinguish between the two based on the A-flag in the Extended Prefix TLV as specified in RFC7684, which the above text mentions.
>>
>> thanks,
>> Peter
>>>
>>> Request authors to add clarification text around "PHP behavior".
>>>
>>> Rgds
>>> Shraddha
>>>
>>> -----Original Message-----
>>> From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of
>>> internet-drafts@ietf.org
>>> Sent: Thursday, May 4, 2017 3:28 PM
>>> To: i-d-announce@ietf.org
>>> Cc: ospf@ietf.org
>>> Subject: [OSPF] I-D Action:
>>> draft-ietf-ospf-segment-routing-extensions-13.txt
>>>
>>>
>>> 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 Extensions for Segment Routing
>>>            Authors         : Peter Psenak
>>>                              Stefano Previdi
>>>                              Clarence Filsfils
>>>                              Hannes Gredler
>>>                              Rob Shakir
>>>                              Wim Henderickx
>>>                              Jeff Tantsura
>>> 	Filename        : draft-ietf-ospf-segment-routing-extensions-13.txt
>>> 	Pages           : 35
>>> 	Date            : 2017-05-04
>>>
>>> Abstract:
>>>       Segment Routing (SR) allows a flexible definition of end-to-end paths
>>>       within IGP topologies by encoding paths as sequences of topological
>>>       sub-paths, called "segments".  These segments are advertised by the
>>>       link-state routing protocols (IS-IS and OSPF).
>>>
>>>       This draft describes the OSPF extensions required for Segment
>>>       Routing.
>>>
>>>
>>>
>>> The IETF datatracker status page for this draft is:
>>> https://datatracker.ietf.org/doc/draft-ietf-ospf-segment-routing-exte
>>> n
>>> sions/
>>>
>>> There are also htmlized versions available at:
>>> https://tools.ietf.org/html/draft-ietf-ospf-segment-routing-extension
>>> s
>>> -13
>>> https://datatracker.ietf.org/doc/html/draft-ietf-ospf-segment-routing
>>> -
>>> extensions-13
>>>
>>> A diff from the previous version is available at:
>>> https://www.ietf.org/rfcdiff?url2=draft-ietf-ospf-segment-routing-ext
>>> e
>>> nsions-13
>>>
>>>
>>> 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
>>>
>>> _______________________________________________
>>> OSPF mailing list
>>> OSPF@ietf.org
>>> https://www.ietf.org/mailman/listinfo/ospf
>>> .
>>>
>>
>> .
>>
>
> .
>