Subject: RE: RE: Policy based ISP selection - Redirects

From: "marcelo" <[email-masked]>
Date: Mon, 9 Jul 2001 20:13:44 +0200

I agree with you in both issues, we must keep in mind that routing policies
are routers concern and that routing headers spirit is to override existing
policies.

However, current ISP based address aggregation implies that hosts mechanisms
such as source address selection play a relevant role in route selection.
(Let´s consider solutions for this case, where ISP based address aggregation
is used)
In order to provide a coherent path, the outbound path must be selected
accordingly to the selected source address.
Available options are :
Routing based on source address, so that routers take routing decisions
based on destination address and source address. We really like this option
but it would be quite complex to adopt.
Routing based on traffic class field: For example, we assign a traffic class
code for each available ISP and then host can choose which one to use
marking the packet.
Routing based on flow label. I didn´t explore very much yet.
Routing based on ports: this implies that routers consider transport layer
information which I think should be avoided. Besides, this solution needs
changes in actual routing mechanisms.
Routing based in routing headers. I really don´t find this solution very
attractive but it is the only one which can be implemented easily, only some
modifications in the source address selection mechanism are needed.

Actually, the solution that I find more interesting is to use the traffic
class field. This allows that both the applications and the host routing
mechanism select the ISP. Besides routers can re-mark performing policing.
The main problem, as I have already mentioned is that more modifications are
needed. So, perhaps an initial solution could be to use routing headers and
then introduce the traffic class usage.

Best regards, marcelo

-----Mensaje original-----
De: [email-masked] [mailto:[email-masked]]En
nombre de Tony Hain
Enviado el: jueves, 05 de julio de 2001 20:30
Para: [email-masked]; [email-masked]; [email-masked];
[email-masked]
CC: [email-masked]; [email-masked]
Asunto: RE: RE: Policy based ISP selection - Redirects

Marcelo wrote:
> There are some issues when using redirect,i believe.
> The main problem that I found is that redirects must be with
> local scope. This implies that the first hop must redirect
> the packet to the correct ISP.
Yes a redirect must be to another router on the local link. In your drawing
this is the case, but it is clear from this note you are looking at a
different problem.

> Then , if there are several sub-networks in the site, all the
> routers must know when to send the redirects.
No, all the routers must know the routing policy, but that is their role in
life. Also as you note redirects are only applicable on link.

> This last statement means that:
> - All the routers must inspect the port information in all
> the packets which may arise some performance issues
> - All the routers and all the hosts must know the policies,
> which makes management more complex
Routers already need to know routing policy, so it is the desire for some
host to have a different policy that adds complexity.

> In the case that routing headers are used, only hosts must
> have policies knowledge
Overriding the routing policy of the underlying infrastructure is the reason
the routing header was created. In general the use of a routing header has
nothing to do with address selection rules. Even if you tried to tie them,
there is nothing preventing policy at host B to override that by using a
routing header to cause all of its packets to be sent through ISP-B.

Tony