Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00

Chris Bowers <cbowers@juniper.net> Wed, 21 October 2015 17:20 UTC

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From: Chris Bowers <cbowers@juniper.net>
To: "Acee Lindem (acee)" <acee@cisco.com>, Shraddha Hegde <shraddha@juniper.net>, OSPF WG List <ospf@ietf.org>
Thread-Topic: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00
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Date: Wed, 21 Oct 2015 17:20:23 +0000
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Subject: Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00
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In my opinion the backwards compatibility problems introduced by this proposal outweigh potential gains.

As a concrete example, there is at least one existing implementation of remote LFA where policy is used to select a backup tunnel that does not share an SRLG with the failed link.  This SRLG information is carried in the TE Opaque LSA.

As it currently reads, I think the proposal in  draft-ppsenak-ospf-te-link-attr-reuse has the potential to break existing standards-compliant implementations.

I might be OK with having the proposal only apply to sub-TLVs  that get defined in the future.  However, I think that taking TLVs that were  standardized over ten years ago, and selectively moving them or copying them to a different LSA based on a set of rules that is subject to interpretation is going to create confusion and interoperability headaches.

Chris

From: OSPF [mailto:ospf-bounces@ietf.org] On Behalf Of Acee Lindem (acee)
Sent: Wednesday, October 21, 2015 6:48 AM
To: Shraddha Hegde <shraddha@juniper.net>; OSPF WG List <ospf@ietf.org>
Subject: Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00

Hi Shraddha,

From: OSPF <ospf-bounces@ietf.org<mailto:ospf-bounces@ietf.org>> on behalf of Shraddha Hegde <shraddha@juniper.net<mailto:shraddha@juniper.net>>
Date: Wednesday, October 21, 2015 at 1:20 AM
To: OSPF WG List <ospf@ietf.org<mailto:ospf@ietf.org>>
Subject: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00

Hi All,


draft-ppsenak-ospf-te-link-attr-reuse-00 proposes moving and/or copying TLVs from the TE Opaque LSA to the Extended Link Opaque LSA. The draft lists the problems that the draft is trying to solve.  I have reproduced that list of problems below, with each problem followed by what I believe to be a better and simpler solution.

   1.  Whenever the link is advertised in a TE Opaque LSA, the link
       becomes a part of the TE topology, which may not match IP routed
       topology.  By making the link part of the TE topology, remote
       nodes may mistakenly believe that the link is available for MPLS
       TE or GMPLS, when, in fact, MPLS is not enabled on the link.

To address this issue, we simply need to define a new sub-TLV in the TE Link LSAto say whether MPLS/GMPLS/RSVP is enabled on the link instead of moving the TLVs around into different LSAs.

   2.  The TE Opaque LSA carries link attributes that are not used or
       required by MPLS TE or GMPLS.  There is no mechanism in TE Opaque
       LSA to indicate which of the link attributes should be passed to
       MPLS TE application and which should be used by OSPFv2 and other
       applications.

OSPF database is a container and OSPF can use any of the LSAS for its own use including the TE LSAs.  As far as the TE database goes, it contains data from TE LSAs as well as non-TE LSAs (Network LSA) today so the reasoning described here doesn’t make sense.

   3.  Link attributes used for non-TE purposes is partitioned across
       multiple LSAs - the TE Opaque LSA and the Extended Link Opaque
       LSA.  This partitioning will require implementations to lookup
       multiple LSAs to extract link attributes for a single link,
       bringing needless complexity to the OSPFv2 implementations.

There will be nodes in the network which will run older software which send these attributes via TE LSAs so the problem of looking into the TE LSAs for TE relatedinformation doesn’t get solved with this draft.  Rather it makes it more complicated. With this draft, the multiple LSA lookup will only increase.  An implementation will first have to find if Extended link LSA contains the required info, if not it will need to look up the info in TE.LSA.

The applications using the TE parameters for non-TE use-cases will use the OSPF Prefix/Link attributes for these use cases. Hence, there is no requirement to lookup the LSAs in multiple places. Backward compatibility will be covered in the specifications of these applications.

Thanks,
Acee






Looking up multiple LSAs for information is an implementation issue and I am sure there will be implementations that will handle this  gracefully so that it doesn’t cause
delays in critical paths. It doesn’t seem reasonable to come up with protocol extensions to solve implementation issues.


Rgds
Shraddha