The MPLS WG Archive

[David Charlap <david.charlap@marconi.com>]

Raymond Law wrote:
> 
> If I understand correctly, the new LSP is reserved with the same
> ingress and egress LSR but with different amount of reserved
> resources such that both LSPs combine to match the new bw
> requirement of the traffic flows.

That's not the way SE-style reservations work.

With SE style reservation, all senders (ie all LSPs) in a single session
always have identical resource reservations.  When the egress sends out
a Resv for the second LSP with different resources, the routers will
change the reservation for the first LSP as well.  If the two LSPs have
divergent paths, Resv messages will be sent along both paths, in order
to keep the two LSPs in sync.

> I can see how the above works if the old LSP needs more bw, then
> it just sets up a new LSP with some more bw to accommodate the
> traffic (no double bw).  What if the old LSP is actually allocated
> more than enough bw and you want to take away some bw from it to
> give it to other LSPs?  How would you set up the new LSP with less
> bw than the old one?

Same as before.  Use make-before-break.  Create a new LSP in the same
session.  The egress sends back an SE-style Resv message for both LSPs
with the new amount of resources.  All the transit routers update their
reservations.  When the Resv hits the ingress, it can shunt traffic onto
the new LSP, and signal a teardown for the old one.

> Is the new LSP set up like a regular LSP or is it set up using the
> existing knowledge of the old LSP so as to lower the overhead
> associated with setting up a whole new LSP.

There is no real difference here.  If the new and old ones have features
in common, the ingress can choose to reuse what it knows.  Or it can
choose not to.  This is an implementation issue.  Either way, the Path
message leaving the router will be the same.

-- David