[Nsi-wg] UvA/TUD topology exchange proposal
Henrik Thostrup Jensen
htj at nordu.net
Thu Sep 18 09:48:37 EDT 2014
Hi again (sorry for the slow response time)
On Mon, 8 Sep 2014, Ralph Koning wrote:
> Hi Henrik,
>
> Thanks for your comments; it seems that some things are left unclear and
> let me elaborate on this. Naturally, we will improve the text in the
> next document version. As both Miroslav and me will be present at the
> Nordunet conference we can further elaborate and clarify the model.
>
> #0 The document is a proposal for a topology exchange
> - It is not a requirements document (upcoming/promised by Chin/John)
> - It is not a comparison of routing techniques
>
> #1 Topology exchange != routing
> We distinguish between 'topology exchange' and 'path calculation'.
You cannot separate these. The requirements inherently affect what
information is published/exhanges between networks/NSA. Similarly the
routing algorithm affects how the information gets passed around.
The basic shortcoming of the system is that it is based around a single
representation of each network (the NML way of thinking). However this is
practially never the case.
You can try and encode switching capabilities into each network
description and do pathfinding (the current approach for some in the NSI
group), but this falls to the ground when there are restrictive policies
about re-transit (i.e., I am allowed to transit a service into another
network, but the entity I am selling to is not allowed to re-transit).
I wish more people would look at BGP. There is a notion in the group, that
BGP is somehow bad or restrictive. However, the reason BGP is successfull
it because it allows representing the underlying policy of the network AND
being able to change up/downstream. This is completely impossible with
the single description-per-network idea of NML (which mainly seems to come
out of multi-layer pathfinding research, and if you are just interesting
in hardware capabilities that approach is fine). It is not BGP that
creates the policy, it just exposes it.
Best regards, Henrik
Henrik Thostrup Jensen <htj at nordu.net>
Software Developer, NORDUnet
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