RFC 1582 (rfc1582) - Page 3 of 29

Extensions to RIP to Support Demand Circuits

RFC 1582                       Demand RIP                  February 1994


          7.1. Database Timer ................................... 24
          7.2. Retransmission Timer ............................. 25
          7.3. Reassembly Timer ................................. 26
      8. Implementation Considerations ...........................27
      9. Security Considerations ................................ 27
     10. References ............................................. 28
     11. Author's Address ....................................... 29

1. Introduction

   Routers are used on connection oriented networks, such as X.25 packet
   switched networks and ISDN networks, to allow potential connectivity
   to a large number of remote destinations.  Circuits on the Wide Area
   Network (WAN) are established on demand and are relinquished when the
   traffic subsides.  Depending on the application, the connection
   between any two sites for user data might actually be short and
   relatively infrequent.

   Practical experience of routing shows that periodic 'broadcasting' of
   routing updates on the WAN is unsatisfactory on several counts:

   o  Running a routing protocol like RIP is expensive if the standard
      form of transmitting routing information to every next hop router
      every 30 seconds is adhered to.  The more routers there are
      wishing to exchange information the worse the problem.  If there
      are N routers on the WAN, N * (N - 1) routing updates are sent over
      N * (N - 1)/2 connections every broadcast period.

      The expense arises because a circuit has to be kept open to each
      destination to which routing information is to be sent.  Routing
      updates are sufficiently frequent that little benefit is obtainable
      on most networks by attempting to set up a call purely for
      the duration of the routing update. (There are often minimum call
      charges, or there is a charge to set up a call irrespective of
      what data is sent.)

      The option of reducing the 'broadcast' frequency, while reducing
      the cost, would make the system less responsive.

   o  The number of networks to be connected (N) on the WAN can easily
      exceed the number of simultaneous calls (M) which the interface
      can support.  If this happens the routing 'broadcast' will only
      reach M next hop routers, and (N - M) next hop routers will not
      receive the routing update.

      A basic rate ISDN interface can support 2 simultaneous calls, and
      even the number of logical channels most users subscribe to on an
      X.25 network is not large.  There is no fundamental reason why



Meyer