The Routing module implements the Network Routing layer described
by the Digital Network Architecture. It 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.
1 – Subentities
The entity hierarchy for the Routing module is:
_Destination_Area
| _Adjacency
| |_IP_Address_Translation
|_Circuit_______________|_IP_Adjacency
| |_IP_Reachable_Address
| |_Reachable_Address
Node___Routing___|
|_EGP_Group_______________EGP_Neighbor
|_Destination_Node
|_IP_Destination_Address
|_Port
|_Permitted_Neighbor
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.
A ROUTING DESTINATION AREA entity contains information about a
destination area or reachable address prefix. These entities are
created and deleted dynamically by the Routing module.
Destination areas exist only on nodes that are level 2 routers.
The address-prefix is the destination area managed by this
command.
A ROUTING CIRCUIT entity represents a data link to another node.
The circuit-name refers the circuit managed by this command.
A ROUTING CIRCUIT ADJACENCY entity describes an adjacency,
which is a neighboring node that is accessible via a particular
circuit. The circuit-name refers to the circuit associated
with the specified adjacency. The adjacency-name refers to the
adjacency managed by this command.
The create and delete commands are allowed only if circuit
is csma-cd and type is L1router or L2router. In addition, the
delete command is allowed on end systems only for x25 da circuit
adjacencies.
A ROUTING CIRCUIT IP ADDRESS TRANSLATION entity describes the
mapping between the IP address of an IP adjacency on a broadcast
circuit and its LAN address. This entity is supported only
on systems that support dual routing. ip address translation
entities are created automatically, but can be deleted manually.
A ROUTING CIRCUIT IP REACHABLE ADDRESS entity describes a
manually entered subnet address that is accessible by way of
a specified circuit. An IP reachable address allows a level 2
router at the boundary of a routing domain to include information
about the address and reachability of subnetworks outside the
domain. IP reachable addresses exist only on level 2 routers that
support dual routing.
A ROUTING CIRCUIT REACHABLE ADDRESS entity describes information
about a manually entered address prefix accessible over that circuit.
It exists only on L2 routers and end nodes. On L2 routers the
type may be outbound or inbound.
A reachable address of type outbound (default) describes address
prefixes in an external domain that are reachable by outbound
traffic over this circuit. For end systems, the circuit can be
either an X.25 DA circuit or a broadcast circuit on L2 routers.
The routing information contained in the reachable address is
entered directly into the L2 decision process. When
ManualL2Algorithm has the value routing vector, only reachable
addresses with address prefixes corresponding to Phase IV areas are
fed into the decision process.
On an L2 router, an inbound reachable address describes address
prefixes corresponding to Phase IV areas which are reachable
through the local node by inbound traffic over this circuit. The
routing information contained in the reachable address (area and cost)
is entered into a Phase IV routing vector message, which is
transmitted periodically over this circuit.
On an end system the type may be outbound or (for a broadcast
circuit only) filter. A reachable address of type outbound behaves
in a similar way to that on an L2 router except that the routing
information is used to control the operation of the ES cache. A
reachable address of type filter (for a broadcast circuit only)
specifies the permitted LAN addresses of routers on the LAN that
will be used by the cache algorithm.
For either outbound or filter type, the mapping attribute should be
set to manual because the default is X.121.
A ROUTING DESTINATION NODE entity contains information about a
particular destination node within the same area as this node.
These entities are created and deleted automatically by the
Routing module. Destination nodes exist only on nodes that are
level 1 or level 2 routers.
A ROUTING EGP GROUP entity defines a set of systems in the
same autonomous system with which this system may exchange
EGP messages. This entity is supported only on level 2 routers
that support dual routing (and, in particular, the EGP routing
protocol).
A ROUTING EGP GROUP EGP NEIGHBOR entity defines one of the
systems in the autonomous group defined by the owning egp
group entity. This entity is supported only on level 2 routers
that support dual routing (and, in particular, the EGP routing
protocol).
A ROUTING IP DESTINATION ADDRESS entity describes IP routing
information in the shortest paths database. This entity is
supported only on routers that support dual routing.
A ROUTING PERMITTED NEIGHBOR entity represents a neighboring node
on a nonbroadcast circuit that is permitted to connect to this
node. The neighbor-name is the name of the permitted neighbor
managed by this command.
A ROUTING PORT entity represents a client of the Routing module,
and presents information associated with that client. The port-
name refers to the port managed by this command.