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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  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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  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: [email protected]










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