Dial-on-demand routing

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In computing, Dial-on-demand routing (DDR) is a technique whereby a host or router will automatically initiate a dial-up connection over an ISDN or regular public switched telephone network line when network access is required, and close the connection when no more data needs to be transmitted or received. The line will only be used when needed, which reduces costs where circuit-switched (telephone) connections are billed by the minute.

DDR is commonly used by home PC users when the computer automatically dials out to an Internet service provider whenever a program requests a TCP/IP connection. More advanced setups may feature a router set up to provide a similar function for an entire computer network. In other situations, a router may be configured to use dial-out lines for backup connections if the primary routes of communication have somehow been severed.

Care must be taken to ensure only relevant packets cause the connection to be set up. To this end, DDR routers typically have special filter lists applied to the demand-dial function, e.g. one which prevents broadcast packets from triggering the connection process. Some networks, e.g. IPX, additionally require protocol spoofing techniques to be used to accommodate periodic but mandatory traffic.