[Nsi-wg] route selecting (path finding)

[Tomohiro Kudoh]

Hi Radek,

In the slide, the coordinator NSA finds a path, according to the
topology and availability information. It can find all the/multiple
paths at once, or it can use Dijkstra. The algorithm the coordinator NSA
uses is an implementation issue.

In my diagram, static information (topology and capability) are
described in the topology description, and may be obtained through
Information Service, and availability information is gathered by queries
to leaf NSAs. But I agree there can be other models.

One thing what I think we should consider is that if a domain is
operated by a commercial operator, the operator may not disclose all the
availability information. 

Kind regards,

Tomohiro



On Wed, 19 Aug 2009 14:03:25 +0200
"Radek Krzywania" <radek.krzywania at man.poznan.pl> wrote:

> Hi Tomohiro,
> According to your slides, it seems that pathfinder finds all/multiple
> inter-domain paths at once. If I am wrong, you may ignore the rest of this
> email ;) If I am right, and pathfinder is about to result in more than one
> path, then my experience is that this is quite difficult to implement. The
> issues are as follows:
> - you need to define how many paths will be returned (e.g. why 5, not 6 or
> 100), otherwise you will find all of them, which may be thousands.
> - Algorithms for graph searching allows you to find one path (e.g. Dijkstra)
> or all paths (some graphs searching algorithm). There are some k-Dijkstra
> algorithms but they relay on some constraints which we may not accept.
> - if you found first path, what are the constraints for the second one?
> Avoid all links from the first path, don't use one of the link on the path
> (then which one should be avoided), etc. Guessing which links should be
> avoided for n-th search is IMHO not the most optimal way of doing that. You
> can always remove the most crowded links, but still, you don't know why the
> previous (e.g. 1st) path will be refused, so you have no guarantee the next
> one (e.g. 2nd) will be optimal in this particular case.
> 
> Searching multiple path makes the pathfinding process more complex and
> includes some decision mechanisms. Thus I have a question if we are going to
> get into it. In AutoBAHN we did it simple - we use Dijkstra to find a sigle
> path. If it fails for some reason, we know where (e.g. insufficient
> bandwidth on link x or domain y refused the reservation). We temporary
> remove x or y from topology and run Dijkstra once again. So we have
> alternative path, which is expected to be still optimal (according to
> current network condition on x and y). AutoBAHN interface for pathfinder
> allows to get multiple path, just in case. But this feature is not used in
> practice due to implementation complexity.
> 
> Best regards
> Radek
> 
> -----Original Message-----
> From: nsi-wg-bounces at ogf.org [mailto:nsi-wg-bounces at ogf.org] On Behalf Of
> Tomohiro Kudoh
> Sent: Wednesday, August 19, 2009 1:44 PM
> To: NSI WG
> Subject: [Nsi-wg] route selecting (path finding)
> 
> Hi all,
> 
> Attached slide shows my view of route selectiong (path finding) wrt NSI.
> 
> I am sorry but I must leave today's call at around 10am EDT.
> 
> Tomohiro
>