RFC 1581 (rfc1581) - Page 2 of 4
Protocol Analysis for Extensions to RIP to Support Demand Circuits
Alternative Format: Original Text Document
RFC 1581 Demand RIP February 1994
A demand RIP implementation runs standard RIP on Local Area Networks
(LANs) allowing them to interoperate transparently with
implementations adhering to the original specifications.
3. Key Features
The proposal shares the same basic algorithms as RIP or RIP-2 when
running on LANs or fixed point-to-point links; Packet formats,
broadcast frequency, triggered update operation and database timeouts
are all unmodified.
The new features operate on WANs which use switched circuits on
demand to achieve intermittent connectivity. Instead of using
periodic 'broadcasts', information is only sent as triggered updates.
The proposal makes use of features of the underlying connection
oriented service to provide feedback on connectivity.
3.1 Triggered Updates
Updates are only sent on the WAN when an event changes the routing
database. Each update is retransmitted until acknowledged.
Information received in an update is not timed out.
The packet format of a RIP response is modified (with a different
unique command field) to include sequence and fragment number
information. An acknowledgement packet is also defined.
3.2 Circuit Manager
The circuit manager running below the IP network layer is responsible
for establishing a circuit to the next hop router whenever there is
data (or a routing update) to transfer. After a period of inactivity
the circuit will be closed by the circuit manager.
If the circuit manager fails to make a connection a circuit down
indication is sent to the routing application. The circuit manager
will then attempt at (increasing) intervals to establish a
connection. When successful a circuit up indication is sent to the
routing application.
3.3 Presumption of Reachability
In a stable network there is no requirement to propagate routing
information on a circuit, so if no routing information is (being)
received on a circuit it is assumed that:
Meyer