[Nsi-wg] Example routing policies in NSI

[John MacAuley]

I wil add these to the slide pack.  Questions below…

On 2015-02-05, at 9:09 AM, Henrik Thostrup Jensen <htj at nordu.net> wrote:

> Hi
> 
> On Wed, 4 Feb 2015, John MacAuley wrote:
> 
>> Here is a slide package capturing my understanding of the routing policies described in Uppsala.  Not sure if I missed any.
> 
> I just had a quick look at it, but from the top of my head:
> 
> Link preferral. Many networks are connected through multiple links, but can also send traffic over backbonetransit networks. And there can be more of these. Being able to priotize between these is pretty important.

This sounds like a stander link "cost" type of issue where you want some links to have a lower cost, and hence, get preferred over higher cost links.

A -- B -- C
|    |    |
\--- D ---/

In this example lets say D is the transit network and the preferred path from A to C.  If we used OSPF then it is a 50-50 split on which path gets selected since both routes are of equal distance (link cost == 1).  If we increase the link cost of the non-transit links, say A-B to a cost of 2, then the A-D-C path will be selected until it is no longer a viable path.

Obviously, this type of cost based mechanism can be tricky in that now we have  path A-D-B the same cost as path A-B, but using some of our other policies can stop us from routing to B via D.

Have you seen this addressed a different way?

> 
> There are no cases with selective transit (well, only trivial ones). For instance with something likes this:
> 
> A - B - C - D
>      \ E
> 
> A can reach C & E, but not D.

"There are no cases" - do you mean this example does not need to be supported?

In this case is B a transit with no termination option for A?

Whom is enforcing this policy?  Is this a policy on source network A or is it policed on B/C?  The reason I ask is if 

> 
> Furthermore we are currently discussing this case with another network/community:
> 
> A - B - C - D
> 
> A should be able to reach B+D
> B should be able reach all.
> C should be able to reach B+C
> D should be able to reach A+B+C.
> (at least it is symmetric)
> 
> That is, A and C are not exchanging traffic, but C provides transit to D, and D has transit to A through B and C. Yay!

Are all networks aware of these policies, or is it selectively enforced by a subset?

> 
> I'd like to stress that it is not NORDUnet bringing the insanity here, but a case of conflicting link AUPs and this issue: https://honestnetworker.wordpress.com/2013/06/28/5/
> 
> I'll take a closer look at this and the other slides next week.
> 
> 
>    Best regards, Henrik
> 
> Henrik Thostrup Jensen <htj at nordu.net>
> Software Developer, NORDUnet
> 
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