Subject: Implications of Route Optimization

From: "Kuntal Chowdhury" <[email-masked]>
Date: Tue, 23 Jul 2002 13:54:25 -0500

Note: I am posting this as a new thread since we are not
discussing about the issue of mandatory processing of HAO in the IPv6 CN
anymore.

Pekka Savola wrote:
> Assuming CN is in ISP 1's network, MN is vising ISP 2's 
> network and HA is 
> in ISP 3's network.
> 
> Your argument was, if I got it right, that route optimizations is bad 
> because traffic is between ISP 1 and ISP 2 so ISP 3 does not 
> get revenue.
> 
> This seems ridiculous.  
> 

Consider the scenario where ISP 3 (Home domain of the user) does volume 
based accounting i.e. byte counts. The visited domain (ISP 2) does
not support volume based accounting. Therefore the home domain assigns 
an AAA client in a router (which may well be the HA) which is guaranteed to 
be in the data path. Reverse tunneling is enforced by the home domain to 
ensure that the all the  data to/from the user pass through the home 
domain. Now the user performs RO with the CN. The CN starts sending data
directly to the MN bypassing the home domain. Therefore the byte counts
in the AAA client in the home domain will not be accurate.

> > If TE is used to route the traffic via such slow paths the difference is

> > noticeable (at least, without measurement tools), the network is
designed 
> > and operated badly, period.  Nobody would want to pay for that kind of 
> > service.
> > 
> 
> What makes you think that the TEed route between the CN and the MN 
> (i.e. Chicago -> Miami -> Dallas) will be a slow path? 

> The speed of light is a constant.

It seems the assumption is that the link between CN and the MN is an optical

link. Let me explain what determines the speed and quality of an optical
link. A few important factors for a light path are: 
1. Fiber type (DSF, USF, NZDF etc.)
2. EDFAs and Ramans amplifiers.
3. Number of 3Rs (Retiming, Reshaping, Reamplifying function).
4. Transmission systems (OC-48/192/768 etc.)
5. Network type i.e. Ring or Mesh.

Say CN is in site A, MN in site B and HA in site C. Say the link between 
site A and B is OC-48, older fiber type 'USF' and no EDFAs. However the
A -> C -> B is OC-192, newer fiber 'NZDF' with EDFAs. Also the number of 3Rs
between A -> B is more than the number of 3Rs for the link A -> C -> B. 
Therefore the link speed, quality and available bandwidth will be far better

for A -> C -> B than A -> B. 
Now if the MN performs RO, then it will end up shifting the traffic from
a better link (A -> C -> B) to a worse link (A -> B). This will be the case
when the link A -> B is not congested. 
When the link between A -> B is congested, and TE is implemented, the
traffic may be shifted back to the original link A- > C -> B. That will have
the opposite effect of RO.
I hope it is clear now that the speed of light is not the limiting factor
for a 
light path. Moreover performing RO w/o knowledge of the network topology
and constraints does not guarantee good results.

> > Do you think the shortest path is ALWAYS the best/fastest path?

> Almost always, yes.  When it's not, it's almost best, which is roughly the

> same thing.  All of this amounts to advantages by route optimization.

> (I'm disregarding stuff like extremely severe network congestion here, as 
> we're talking about stable scenarios here.)

The shortest path may not be the best/fastest path even in stable scenarios.
It depends on the factors listed above for the light paths.

---

From: Pekka Savola <[email-masked]>
Date: Wed, 24 Jul 2002 09:09:40 +0300 (EEST)

> in the AAA client in the home domain will not be accurate.

What is the purpose of byte counts?  Most often to see how many bytes have 
been transferred to the node in the specified network.  In Route 
Optimization case, such byte counts are close to zero.  I see no reason to 
artificially try to create these byte counts.  Only the node and the 
networks it visit know how much traffic is used.

> link. 

Similar restrictios apply to non-optical links too.  The main factor is 
the length of the link.

[...]
> light path. 

If the sites choose to prefer A -> B path instead of A -> C -> B (and they 
can easily do so, if they want), it's their choice.

> Moreover performing RO w/o knowledge of the network topology

I agree that RO is sometimes not worth the effort, but where optimized 
path would be worse seem to be just corner cases to me.

> It depends on the factors listed above for the light paths.

By 'shortest path' I mean the path that the network admins have chosen to 
be the best path to other site X.  It does not necessarily have to 
physically the shortest one, of course.  But it does not usually go 
through third parties either, though.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords

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