Extended the paragraph comparing the use of C/D-mode as follows (extensions are
in the second part of the paragraph):

   It is possible to mix these two modes along a path.  This allows, for
   example, the use of D-mode at the edges of the network and C-mode in
   the core of the network.  Such combinations may make operation more
   efficient for mobile endpoints, while allowing multiplexing of
   signalling messages across shared security associations and transport
   connections between core routers.  The setup for these protocols
   imposes an initialisation cost for the use of C-mode, but in the long
   term this cost can be shared over all signalling sessions between
   peers; once the transport layer state exists, retransmission
   algorithms can operate much more aggressively than would be possible
   in a pure D-mode design.

The assertion that C-mode allows fast state setup in the face of packet loss
needs to be justified taking into account the cost of setting up the messaging
association protocols in the first place.