Subject: Re: [rrg] Summary of architectural solution space

From: [email-masked]
Date: Tue, 23 Dec 2008 03:35:09 EST

In einer eMail vom 23.12.2008 00:16:53 Westeuropäische Normalzeit schreibt  
[email-masked]:

On Mon,  Dec 22, 2008 at 5:55 PM,  <[email-masked]> wrote:
> I  think there are enough folks around who are both experts in BGP and  
OSPF.
> (1):
> My question is, what sort of policy/QoS/etc.  routing can only be enabled by
> a distance vector protocol and can in  principle not be enabled by a
> link-state protocol ? (note, but I don't  need an instruction that 
link-state
> is intra-domain, DV is  inter-domain)

Address and direction sensitive link state. That is, for  some
combinations of source, destination and direction of transit the  link
is up, but for other combinations of combinations of  source,
destination and direction of transit the link is  down.

Distance-vector protocols can provide this for a limited but  very
useful subset of the possible  combinations.

And so can link.state protocols, right ? !
They just do it in a different way. Also, you may color the nodes and links  
such that you can even get Multiple Topologies (see OSPF-MT).
 
If you want real substantial progress in routing technology then you need  to 
improve Dijkstra (what I did) and not extend DV.

Heiner
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---

From: "William Herrin" <[email-masked]>
Date: Tue, 23 Dec 2008 09:25:31 -0500

On Tue, Dec 23, 2008 at 3:35 AM,  <[email-masked]> wrote:
>> My question is, what sort of policy/QoS/etc. routing can only be enabled
>> by a distance vector protocol

> such that you can even get Multiple Topologies (see OSPF-MT).

Go try it.

Build yourself a random network of 100 nodes with a total of 200 to
300 links between those nodes. For each direction on each link, assign
10 or so ranges of source+destination address for which the link is
considered "up." Let me give an example of what I mean by that:

Network:   A-B

Link direction AB is UP for source 10.0.0.0/8, destination any
Link direction BA is UP for source any, destination 10.0.0.0/8
Links AB and BA are DOWN for all other combinations of source and destination

Once you've built your 100-node network "on paper," do the following:

1. Calculate the size of your link-state RIB.

2. Formulate an algorithm, presumably using Dijkstra, that processes
that RIB into a next-hop FIB at a given node.

3. Calculate the size of the resulting next-hop FIB.

What you'll find when you do this is that the addition of direction
and source address to the RIB may square its size versus a comparable
distance-vector RIB. Worse, your Dijkstra calculation won't be able to
process out the impact of the source address on the FIB calculation so
you'll have to carry the source address into the FIB as well. This may
square the size of the FIB you need to carry versus a distance vector
protocol.

With DV, the source address factor is subtractive. It's handled by
reducing the database entries propagated over particular links,
removing the ones that aren't acceptable for that link. With
link-state it's multiplicative, vastly increasing the database size.

Regards,
Bill Herrin

-- 
William D. Herrin ................ [email-masked]  [email-masked]
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004
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