Re: [OSPF] Comment on draft-ppsenak-ospf-te-link-attr-reuse-01

[Peter Psenak <[e-mail]>]

Hi Rob, Julien,

please see inline:

On 2/27/16 00:56 , Rob Shakir wrote:
> Hi Acee, Peter, Julien,
>
> On 23 February, 2016 at 12:06:10 PM, Acee Lindem (acee) ([e-mail]
> <mailto:[e-mail]>) wrote:
>
>> ##PP
>> I'm still not sure we want to enforce that even on the router that
>> generates these. Different apps may use different values of certain link
>> attribute. Take an SRLG as an example. You may have SRLG values on the
>> link used only by GMPLS/optical plane. These values have meaning only in
>> the context of the GMPLS/optical. You do not want LFA to use these
>> values, because their meaning is different and irrelevant for LFA, you
>> may define a different set of values used by LFA.
>
> I tend to agree with Julien here, this sounds problematic from a
> management perspective. If I something that is a physical property of
> the link (which a SRLG is); and I run “TE” and “non-TE” applications on
> my network - i.e., a LFA and RSVP-TE tunnels, then I now need to
> maintain both the lists being identical, such that the extended link
> attribute SRLG classification matches the TE attribute’s values. This
> sounds complex (what happens when they get out of sync); with no huge
> up-side to duplicating them. Even if we do want to have one application
> work differently to another w.r.t SRLGs, then why would we not do this
> with the policy that is used when making a path placement decision,
> rather than by splitting them into different attributes?
>
> IMHO, a nice solution here is that we have each set of information
> maintained in one place in the protocol; if this has historically been
> in the TE attribute, then I do not necessarily see a good reason to try
> and move it. Going forward, we should discuss for new extensions whether
> these attributes are solely useful for traffic engineering; or whether
> they are more general purpose, Metric gives us a good example here - the
> TE metric is *only* relevant to traffic engineering path placement,
> whereas the metric is relevant to LFA, standard IP applications, and can
> be relevant to TE.
>
> I would also rather see this than duplicating the same content of
> attributes across two different LSAs. This creates ambiguity as to which
> one was actually used by the consuming application on the network
> element for a particular protocol.

In general, enforcing the same values to be carried for the same link 
attribute in two different LSAs may become a limitation in the future. 
We already have a precedence with IGP/TE metric. At the end we are 
talking about two independent LSAs/TLVs, we are just sharing the same 
TLV format.

In terms of SRLG, there are real deployments, where optical plane uses 
SRLGs that are associated with a larger areas like "city" or "district" 
that are used for disaster recovery purposes and such values would have 
not meaning in the context of the LFA - one would still want to use the 
LFA that is in such SRLG.

There should be no ambiguity really - information carried in TE Opaque 
LSA is used by TE/GMPLS application. What is carried in Extended Link 
LSA is used by other applications.

thanks,
Peter






>
> Best,
>
> r.
>
>
>
> _______________________________________________
> OSPF mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/ospf
>