Subject: routing PATH messages

From: alfred <[email-masked]>
Date: Thu, 10 Aug 2000 15:17:49 -0300

I have a question regarding the treatment of PATH messages in a router.

A PATH message arrives at a router over interface I1. The preferred
interface over which this packet will be forwarded is I2 (as determined
by the nextHop in the routing table). DId the inventors of RSVP envision

this as being the only way to determine the interface over which the
packet
will be forwarded?  Did they preclude the use of other mechanisms
(like policy based routing or access-lists) to override the preferred
nextHop
from the routing table and forward the packet over some other interface
I3.

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From: Lixia Zhang <[email-masked]>
Date: Thu, 10 Aug 2000 20:25:52 -0700

At 11:17 AM 8/10/00 , alfred wrote:
>packet will be forwarded? Did they preclude the use of other mechanisms
>nextHop from the routing table and forward the packet over some other
>interface I3.

The inventors of RSVP envisioned that routing would be a complex issue.
They decided that RSVP PATH messages should follow whatever paths, as
determined by the routing protocol (plus policy if you have), to the
destinations.
The next-hop in the forwarding table is the result of routing protocol
processing (possibly with policy).

Lixia

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From: ajd1 <[email-masked]>
Date: Mon, 14 Aug 2000 11:06:09 -0300

Thanks for your response to my earlier question. I have a followup
question:
We have 2 paths from a source router to a destination router -
ths ospf determined preferred path and an alternate path.
We have identical policies in place to force BOTH rsvp signalling
messages and associated udp data along the alternate path.
The routers obey the policy for udp data and send it along the
alternate path but ignore the policy for rsvp signalling  and
send it over the ospf determined path.
The routers accept the policy commands for protocol 46, I can see that
the policy gets installed in the router, but the
policy for protocol 46 gets ignored in the implementation.
The router vendor claims that rfc 2205 dictates that the RSVP signalling
messages MUST follow the ospf determined path (the nextHop with the
best metric) which I am trying to dispute. Am I correct in assuming that
your response to my earlier question supports my stance on this issue?

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From: Lixia Zhang <[email-masked]>
Date: Fri, 18 Aug 2000 17:06:57 -0700

>your response to my earlier question supports my stance on this issue?

Disclaimer: my opinions are my personal opinions.

Having said that: I am afraid that the vendor is right.  The PATH messages
should be forwarded according to whatever the routing protocol says.

What you have done is to tweak the forwarding so that UDP data packets go
somewhere else instead of following the forwarding table.  I am not sure
how good a design that is. If packets can go arbitrary places that are
inconsistent with what routing says, based on all kinds of different polies
at various places, how would one be able to control/manage such a network
(especially a large one)?

What I think you should have done, to support policies, is to integrate
your policy into routing, to make the forwarding table pointing to the
direction you wanted (for the data packets you wanted).

my 2 cents,
Lixia