The MPLS WG Archive

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

Curtis Villamizar wrote:
> 
> First of all:  RSVP/TE != explicit-path
> 
> They are related but not equal.  RSVP/TE paths can be determined by
> the ingress based on the CSPF computation.  Explicit-path means the
> path is predetermined and fixed.  RSVP/TE supports explicit-path as an
> option.  When determined by the CSPF (explicit-path is not being
> used), the path is in a sense nailed down, at least until the
> adaptivity timer fires and a later CSPF decides there is a better path
> now.

RSVP-TE's definition of "explicit" can be rather fluid, depending on the 
nature of the ERO object and how it is generated.

- An LSP may be signaled without an ERO object.  In which case, it 
follows the IGP's route to the egress address.  The route table may be 
populated by the result of CSPF, ECMP, or other mechanisms.  When the 
route table changes, the LSP reroutes to follow it.  This is obviously 
not in any way explicit.

- The path described by an ERO may terminate before the egress node is 
reached, in which case the remaining hops are determined using the route 
table and the egress address.  This is only explicit up to the end of 
the ERO.

- An ERO object may contain loose hops.  The next-hop chosen in response 
to a loose hop is a function of the routing table.  This is explicit in 
the sense that the LSP will pass through certain nodes, but the route 
taken between those nodes is not explicit.

- Since RSVP ERO hop addresses identify nodes, not links (except when 
the GMPLS IF_ID/unnumbered HOP extensions are used), the local route 
table is used to pick a link when multiple links exist between a pair of 
nodes along the path.

- Finally, the ERO used by the ingress node may be manually generated by 
an operator, or it may be computed by software external to RSVP.  If it 
is computed, then the ERO-computation software may choose to generate a 
new ERO and reroute the LSP as the network topology changes.  This is 
explicit from RSVP's perspective, but not necessarily from the 
operator's perspective.

RSVP's path is only fixed in the intuitive sense if an ERO consisting 
entirely of strict subobjects is used, and these subobjects lead all the 
way to the egress node, and the entity that computes the ERO (human or 
software) chooses not to reroute the LSP as the network changes.

-- David