The MPLS WG Archive

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

John Sparr wrote:
> 
> Hello,
> 
> I have a question about reservation style in RSVP:
> 
> How does the receiver decide the reservation style?
> Since the sender has no effect for the style.

There is a flag in the SESSION_ATTRIBUTES object which an ingress node
can use to request SE style of an egress node.  But the choice of style
is advisory, not mandatory (note the use of SHOULD in the draft, not
MUST).

If an egress node only supports one style, then it just uses that style
- this is legal.  If both FF and SE are used, then the choice should be
based on the SESSION_ATTRIBUTES flag.

The reason for requesting SE style is for doing make-before-break
rerouting.  It is more efficient when done with SE style vs. FF style. 
While make-before-break rerouting with FF is possible, it requires that
extra resources (double the amount required by the LSP) be available on
nodes that are shared between the original and new LSPs.  When done with
SE style, the two LSPs share resources and the problem is avoided.

-- David