The Routing module implements the Network Routing layer described by the
   Digital Network Architecture.

   The entities that constitute the ROUTING  module are listed below. The
   indention indicates the hierarchical relationships between the entities.

                                Routing
                                   Destination area
                                   Destination node
                                   IP destination address
                                   Port
                                   Permitted neighbor
                                   EGP group
                                      EGP neighbor
                                   Circuit
                                      Adjacency
                                      IP address translation
                                      IP adjacency
                                      IP reachable address
                                      Reachable address

   The Routing module routes messages in the network and manages the message
   packet flow.  The Routing module components provide the following functions:

   o  Routing determines packet paths.  A path is the sequence of connected
      nodes and links between a source node and a destination node. The combined
      knowledge of all the network Routing layer modules of all the nodes in a
      network is used to determine the existence of a path, and route the packet
      to its destination.  The routing component at a routing node has the
      following specific functions:

      -  Extracts and interprets the route header in a packet.
      -  Performs packet forwarding based on the destination address.
      -  Performs packet fragmentation where necessary.
      -  Manages the characteristics of the path and if a node or link fails on
         a path, finds an alternate route.
      -  Interfaces with the Network Routing Subnetwork Dependent sublayer to
         receive reports concerning a circuit or node that has failed or the
         subsequent recovery of a circuit or node.
      -  Performs packet reassembly at the destination.
      -  Returns error reports to the source where necessary, for instance when
         the destination is unreachable or when the packet would have needed to
         be fragmented but segmentation permitted was not set in the packet.
         Segmentation permitted is always set in data packets generated by DNA
         nodes.  However, non-DNA nodes may do otherwise.

   o  Congestion control manages the resources used at each packet switching
      node each node that permits route-through.

   o  Packet lifetime control bounds the amount of time a packet can exist in
      the network.

   o  Initialization identifies the adjacent node and the adjacent node's
      network routing layer.  It also performs node verification, if required.

   o  Dynamic circuit management -- determines when to dial calls, when to hang
      up calls, and on dynamically assigned circuits which DTE address to dial.
      It exists only on dynamically established data links.

   Whether or not a particular attribute or event of a routing module entity is
   supported often depends on the type of node on which the routing module is
   operating. The description of an attribute or event indicates the type of
   node for which the attribute or event is supported, using the following key:

   L1    - The attribute is supported only for Level 1 routers.

   L2    - The attribute is supported only for Level 2 routers.

   L1,L2 - The attribute is supported only for Level 1 or Level 2 routers.

   End   - The attribute is supported only for end nodes.

   IP    - The attribute is supported only for systems that support dual Routing
           that is, both OSI and IP routing.

   All   - The attribute is supported by all implementations.

   The routing entity is the top-level entity in the Routing module hierarchy
   of entities.  The Routing module controls the operation of network routing
   within a node.