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

Peter Psenak <> Wed, 21 October 2015 17:40 UTC

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Date: Wed, 21 Oct 2015 19:40:06 +0200
From: Peter Psenak <>
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To: Chris Bowers <>, "Acee Lindem (acee)" <>, Shraddha Hegde <>, OSPF WG List <>
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Subject: Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00
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Hi Chris,

On 10/21/15 19:20 , Chris Bowers wrote:
> In my opinion the backwards compatibility problems introduced by this
> proposal outweigh potential gains.

there is no backwards compatibility problem with the draft.

> 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.

that is fine, you are free to do that if the link is TE enabled, there 
is no problem. If the link is not TE enabled and you use TE Opaque LSA 
to flood the SRLG data for such link, you are going against the current 
specification. There is no way to do that today, because any router that 
would receive such TE Opaque LSA must assume such link is TE enabled.

> 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 don't believe so.

> 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.

What we propose is the way to advertise link attributes without making 
the link part of TE topology. We simply do not have a way to do that 
today. I do not see any problem in doing so, because we do not change 
anything on the TE Opaque LSA side, we are defining something new.


> Chris
> *From:*OSPF [] *On Behalf Of *Acee Lindem (acee)
> *Sent:* Wednesday, October 21, 2015 6:48 AM
> *To:* Shraddha Hegde <>; OSPF WG List <>
> *Subject:* Re: [OSPF] Regarding draft-ppsenak-ospf-te-link-attr-reuse-00
> Hi Shraddha,
> *From: *OSPF < <>> on
> behalf of Shraddha Hegde <
> <>>
> *Date: *Wednesday, October 21, 2015 at 1:20 AM
> *To: *OSPF WG List < <>>
> *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 thereasoning 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 lookup 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
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