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

Shraddha Hegde <shraddha@juniper.net> Thu, 11 May 2017 06:50 UTC

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From: Shraddha Hegde <shraddha@juniper.net>
To: Peter Psenak <ppsenak@cisco.com>, "internet-drafts@ietf.org" <internet-drafts@ietf.org>, "i-d-announce@ietf.org" <i-d-announce@ietf.org>
CC: "ospf@ietf.org" <ospf@ietf.org>
Thread-Topic: [OSPF] I-D Action: draft-ietf-ospf-segment-routing-extensions-13.txt
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Subject: Re: [OSPF] I-D Action: draft-ietf-ospf-segment-routing-extensions-13.txt
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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.

" 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.

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"
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-exten
> sions/
>
> There are also htmlized versions available at:
> https://tools.ietf.org/html/draft-ietf-ospf-segment-routing-extensions
> -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-exte
> 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/
>
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