Generalized MPLS, Bundling, and bi-directional LSPs with uni-directional interfaces

["Atkinson, Stephen" <steve.atkinson@netplane.com>]

To quote from draft-ietf-mpls-generalized-rsvp-te-04 section  8.1,
"In all cases but bundling [MPLS-BUNDLE] the upstream interface is implied
by the downstream interface.  For bundling, the path sender explicitly
identifies the component interface used in each direction.".

I'm missing something here. This means, in the absence of Bundling, 
interfaces must always be bi-directional if used for setting up
bi-directional LSPs. 

Why is Bundling different in this regard? Is there something specific to
bundling requiring the ability to setup bi-directional LSPs on
uni-directional interfaces, which isn't required in the absence of bundling?

For example Node A and Node B are connected by 4 fibers. Two fibers
on Node A are outgoing and two are incoming. In both cases below we're
setting up a bi-directional LSP.

Case 1: The fibers are bundled as per MPLS-BUNDLE. In this case the 
             component to use in each direction is specified by
DOWNSTREAM_COMPONENT_INTERFACE_ID
             and UPSTREAM_COMPONENT_INTERFACE_ID as per MPLS-BUNDLE.

Case 2: The fibers are not bundled. In this case we can use the Interface_ID
TLV as per section 9.1
             of draft-ietf-mpls-generalized-signaling-05 (Type = 1, 2 or 3)
to specify the downstream fiber
             to use. There is no mechanism for specifying the upstream
fiber. 

Case 2 won't work unless there's an apriori coupling of an incoming and
outgoing fiber. If this is the
case, why is this coupling not used in the presence of bundling?

Thanks,
steve atkinson