Subject: Re: How to set the link costs ?

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Wed, 30 Oct 2002 15:12:05 -0500

NLSP (designed by Novell, basically support for IPX within IS-IS)
had a method of measuring round trip delay and bandwidth upon link
startup (pt-to-pt links) and throwing it into a formula to come up
with a number. For LANs, they just had a number for the type of link,
I believe.

At Digital, before I got there (so it was late 70's) there was some
tool where you input the traffic matrix and topology
it came up with link costs
to optimize the traffic flow. I didn't think this was a good idea...how
do you know the traffic matrix? What if the topology changes? Would
those artificial link costs optimized around a particular topology possibly
yield really really horrible routes if a link went down and so it was
operating with different topology?

Mostly I think link costs aren't a big deal..anything fairly sensible
will calculate fairly sensible paths.

Radia

        MIME-Version: 1.0
        From: mukul goyal <[email-masked]>
        Subject: Re: How to set the link costs ?
        To: [email-masked]

        Mikkel Thorup of AT&T Research has looked into the issue:
        www.research.att.com/~mthorup/PAPERS/papers.html

        Thanks,
        Mukul

        On Wed, 30 Oct 2002, Yasuhiro Ohara wrote:

        > Is there any way to calculate/produce the link cost, if topology is
        > given ? I know that depends on the topology and how the network
        > administrators want traffic to go through, but I think there should be
        > some generic guidance, to realize the policy if one is given. (I mean
        > "how traffic go through" by "policy".)
        >
        > Is there any related work, tools or papers ?
        > How do you parameterize the costs on your network ?
        >
        > I know the type of the link is a way.
        > I want another if there's.
        >
        > Waiting for your reply.
        >
        > regards,
        > yasu
        >

---

From: Andrew Lange <[email-masked]>
Date: Wed, 30 Oct 2002 12:23:42 -0800

Radia,

Good points.  In an attempt to address them at least partly:

How do you know the traffic matrix?

   You measure it.  Specifically you need to differentiate between the
   traffic matrix and the demand matrix.  The former including the
   next-hop AS, and the latter including the pop-to-pop flows.  Having
   both helps model under failure.  Netflow with correlation to BGP
   tables, both with NEXT_HOP, for the demand matrix, and with the
   peer as for the traffic matrix will help get this data.  Admittedly,
   in an ISP environment you'll have to sample, but the results should
   be valid.

What if the topology changes?  Would those artificial link costs
optimized around a particular topology possibly yield really really
horrible routes if a link went down and so it was operating with a
different topology?

   Well, the approach we have taken is to model for worst-case
   single failure.  As you have pointed out, modeling for byzantine
   failure quickly renders the problem unsolveable.  You make sure
   that you are not loading the network such that, even under worst-
   case single failure, you do not saturate any links.

   I think you're right in that in an arbitary topology there is a
   good chance that the failure paths will be awful.  But, from
   some of the work out there, it would appear that in real life
   ISP topologies at least, IGP Optimization yields good results.

Implied question:  Is it worth the effort?

  That would depend on your resources and the costs associated with
  the bandwidth.  In most corporate environments, I wouldn't think
  it would be worth the trouble.  In a big ISP environment, I think
  it is.

Andrew