[OSPF] Comments on draft-ietf-ospf-te-link-attr-reuse-01.txt and draft-ietf-isis-te-app-01.txt

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Dear authors,

Please find below a comment on both draft-ietf-ospf-te-link-attr-reuse-01.txt and draft-ietf-isis-te-app-01.txt.

I consider the use case of bandwidth reservation. I know this is not the most common use case, but the one I known well. The context is that of an operator who would setup some RSVP-TE tunnels and simultaneously SR-TE paths with bandwidth reservation. In this particular case, it is not possible to manage both reservation with the drafts as they are.

Indeed, in OSPF draft, it is not proposed to advertised the usual bandwidth parameters as defined in RFC3630 and in ISIS, it is proposed to duplicate these parameters per application. The main problem arises from the fact that each application, in this case SR-TE and RSVP-TE, independently compute a path and therefore reserve bandwidth on their respective set of parameters. However, this will lead at a some point to bandwidth overbooking, which exactly what an operator wants to avoid by performing bandwidth reservation. Even if a PCE can be used to handle both the RSVP-TE tunnels and SR-TE paths, the same problem arises because each path computation is performed on a different set of bandwidth parameters i.e. one TED per application whereas these information relate to the same links. Of course a central entity like a PCE might try to reconcile the information into a single TED, but this will greatly increase the complexity of the PCE with a risk that the TE information will
never be up to date, so at the end unnecessary.

So, for me there are only 2 possibles solutions to avoid this overbooking problem:

1/ Split and partition network resources to avoid conflicts. But, this leads into a poor network usage. Indeed, if an application like RSVP-TE uses less bandwidth than its budget, why the SR-TE application could not reuse them if it has reached its threshold ? The under utilization of network resources will increase proportionally with the number of applications. Imagine if we want to use this principle for network Slicing. I understand the advantage for vendors, but I'm on the operator side ;-)

2/ Each time an application reserved some bandwidth, the routers concerned by this new path must update the bandwidth parameters of the concerned link not only to the given application, but also to all others. For example, when RSVP-TE setup a tunnel, Unreserved Bandwidth parameters must be updated in the standard RFC3630 set, but also in SR-TE parameters set. But, in this case, why duplicate TE parameters if at the end all set carry the same values, apart wasting CPU and bandwidth ?

In summary, duplicate TE information is only relevant for the added metrics i.e. delay, loss, jitter ... but unusable for concave metrics i.e. bandwidth.

Can you explain me how you intend to solve this issue as both possible solutions are not suitable for an operator.

Best Regards

Olivier

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