Subject: RE: A question regarding RFC2205

From: "Feng, Mark" <[email-masked]>
Date: Mon, 23 Jul 2001 14:53:20 -0700

I have a similar question. 

RFC 2205 describes the usage of the SCOPE object. It seems that the object
is used for both multicast and unicast. In RFC 2209, it is only described
for multicast.

My question is, for unicast, can auto-refresh loop happen, i.e. is SCOPE
object needed for unicast?

Thanks in advance.

- Mark

---

From: David Charlap <[email-masked]>
Date: Mon, 23 Jul 2001 18:14:49 -0400

"Feng, Mark" wrote:
> RFC 2205 describes the usage of the SCOPE object. It seems that the
> object is used for both multicast and unicast. In RFC 2209, it is
> only described for multicast.
> 
> My question is, for unicast, can auto-refresh loop happen, i.e. is
> SCOPE object needed for unicast?

This is well beyond the scope of MPLS (no pun intended.)  But I'll try
to answer anyway.

SCOPE only makes sense with WF style reservations and multicast.

With unicast flows, there is a clear sense of direction from sender(s)
to receiver.  Which means that Path and Resv messages will only flow in
one direction along any particular link.  Resv messages won't loop
because they will only flow along the back-path from the received Path
messages.

Similarly with SE and FF styles there is no issue.  With these styles,
the Resv contains a list of the senders that the Resv message must
arrive at.  (Part of the flow descriptor list).  With respect to any
particular sender, it is always known where the corresponding Path
message came from so there is no issue.

When WF style is used, however, the Resv message has no list of senders
in its flow descriptor.  (That's the whole point of WF style - to
"wildcard" in everybody.)  Therefore, an incoming Resv must be sent out
all interfaces that have received Path messages for the session.  If
there are multiple receivers in the session (ie. multicast), then it is
very possible for a router to receive the same Resv from multiple
places.  The SCOPE object serves to keep this tangled mess of Resv
messages from exploding into a datastorm.

If you try to use SCOPE on FF or SE reservations, I suspect many
implementations will reject the Resv message, even though it is a
theoretically valid (although useless) object in that situation.

Using SCOPE on a unicast WF-style reservation should probably work on
any router that supports WF-style reservations.  But as far as I can
tell, it will not be terribly useful.

-- David