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Keywords: routing, Protocol
Network Working Group G. Meyer
Request for Comments: 1581 Spider Systems
Category: Informational February 1994
Protocol Analysis for Extensions to RIP to Support Demand Circuits
Status of this Memo
This document provides information for the Internet community. This
document does not specify an Internet standard of any kind.
Distribution of this document is unlimited.
Abstract
As required by Routing Protocol Criteria [1], this report documents
the key features of Routing over Demand Circuits on Wide Area
Networks - RIP [2] and the current implementation experience.
Acknowledgements
I would like to thank colleagues at Spider, in particular Richard
Edmonstone and Alan Turland who developed Spider's IP RIP and IPX RIP
and SAP implementations.
1. Protocol Documents
"Extensions to RIP to Support Demand Circuits" [2] suggests an
enhancement to the "Routing Internet Protocol" (RIP) [3] and "RIP-2"
[4] to allow them to run more cost-effectively on Wide Area Networks
(WANs). Network management extensions for Demand RIP are described
in RIP Version 2 MIB Extensions [5].
2. Applicability
Demand RIP requires that there is an underlying mechanism for
determining unreachability in a finite predictable period.
The demand extensions to RIP are particularly appropriate for WANs
where the cost - either financial or packet overhead - would make
periodic transmission of routing (or service advertising) updates
unacceptable:
o Connection oriented Public Data Networks - for example X.25 packet
switched networks or ISDN.
o Point-to-point links supporting PPP link quality monitoring or
echo request to determine link failure.
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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:
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RFC 1581 Demand RIP February 1994
o The most recently received information is accurate.
o The intervening path is operational (although there may be no
current connection).
If the circuit manager determines that the intervening path is NOT
operational routing information previously received on that circuit
is timed out. It is worth stressing that it can be ANY routed
datagram which triggers the event.
When the circuit manager re-establishes a connection, the application
exchanges full routing information with its peer.
3.4 Routing Information Flow Control
If the circuit manager reports a circuit as down, the routing
application is flow controlled from sending further information on
the circuit.
To prevent transmit queue overflow and also to avoid 'predictable'
circuit down messages, the routing application can also optionally
limit the rate of sending routing messages to an interface.
4. Implementations
At this stage there is only believed to be one completed
implementation.
The Spider Systems' implementation supports all the features outlined
for IP RIP-1, IPX RIP and IPX SAP. RIP-2 is not currently supported.
It has been tested against itself on X.25 and ISDN WANs. It has also
been tested in operation with various router and host RIP-1, IPX RIP
and IPX SAP implementations on Ethernet LANs.
Two other Novell-only implementations are known to be under
development.
5. Restrictions
Demand RIP relies on the ability to place a call in either direction.
Some dialup services - for example DTR dialing - allow calls to be
made in one direction only.
Demand RIP can not operate with third-party advertisement of routes
on the WAN. The next hop IP address in RIP-2 should always be
0.0.0.0 for any routes advertised on the WAN.
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RFC 1581 Demand RIP February 1994
6. Security Considerations
Security is provided through authentication of the logical and
physical address of the sender of the routing update. Incoming call
requests are matched by the circuit manager against a list of
physical addresses (used to make outgoing calls). The routing
application makes a further check against the logical address of an
incoming update.
Additional security can be provided by RIP-2 authentication [2] where
appropriate.
7. References
[1] Hinden, R., "Internet Engineering Task Force Internet Routing
Protocol Standardization Criteria", RFC 1264, Bolt Beranek and
Newman, Inc, October 1991.
[2] Meyer. G., "Extensions to RIP to Support Demand Circuits", RFC
1582, Spider Systems, February 1994.
[3] Hedrick. C., "Routing Information Protocol", STD 34, RFC 1058,
Rutgers University, June 1988.
[4] Malkin. G., "RIP Version 2 - Carrying Additional Information",
RFC 1388, Xylogics, January 1993.
[5] Malkin. G., and F. Baker, "RIP Version 2 MIB Extensions", RFC
1389, Xylogics, ACC, January 1993.
Author's Address
Gerry Meyer
Spider Systems
Stanwell Street
Edinburgh EH6 5NG
Scotland, UK
Phone: (UK) 31 554 9424
Fax: (UK) 31 554 0649
EMail: gerry@spider.co.uk
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