Introduction to Demand-Dial Routing

Demand-dial routing is the forwarding of packets across a Point-to-Point Protocol (PPP) link. The PPP link is represented inside the Windows 2000 router as a demand-dial interface. Demand-dial interfaces can be used to create on-demand connections across dial-up, non-permanent or persistent media.

With local area network (LAN) and permanent wide area network (WAN) links, the interface that is being used to forward the packet is always in an active or connected state. The packet can be forwarded without having to create the physical or logical connection. However, the demand-dial interface can either be in a connected state or a disconnected state. If in a disconnected state when the packet is being forwarded, the demand-dial interface must be changed to a connected state before the packet can be forwarded.

The connection establishment process, consisting of creating a physical connection or a logical connection and a PPP connection, introduces a delay in the forwarding of the packet called the connection establishment delay. The length of the connection establishment delay varies for the type of physical or logical connection being established. For example, the connection establishment delay for analog phone lines or X.25 dialing in to a packet assembler-disassembler (PAD) can be 10 to 20 seconds or more. For Integrated Services Digital Network (ISDN) lines, the connection establishment delay can be as small as 3 to 5 seconds.

The connection establishment delay is an important consideration for applications being used across a demand-dial connection. There are two behaviors of applications to consider:

• How long it takes for the application to abandon the attempt to establish network communications, also known as application time-out. If the application time-out is longer than the connection establishment delay, then the application fails to establish communications and presents an error message to the user.
• How many times it attempts to establish network communications. On the first attempt, network traffic is forwarded to the demand-dial router which begins the connection establishment process. Due to the size of a finite buffer in the router, additional packets to be forwarded across the demand-dial connection that arrive during the connection establishment process might overwrite the initial application connection attempt packet. If the application tries to establish communications multiple times, then there is a better chance of forwarding an application connection attempt packet once the connection is established.

Applications that have long time-outs or multiple retries might not fail while waiting for the link to become available. Interactive applications such as Web browsers and Telnet might fail when first connecting. However, when the user retries the connection attempt, it succeeds because the first connection attempt created the demand-dial connection.

Once the connection is established, packets are forwarded across the demand-dial connection. Because the costs of demand-dial connections are typically time sensitive, after a configured amount of idle time the demand-dial link is terminated. Demand-dial connections have the benefit of allowing the user to use cheaper dial-up WAN links and only pay for the link when it is being used.

Demand-Dial Routing and Remote Access

Demand-dial routing is not the same as remote access; remote access connects a single user to a network, and demand-dial routing connects networks together. However, both remote access and demand-dial routing use PPP as the protocol mechanism to negotiate and authenticate the connection and encapsulate data sent on the connection. As implemented in the Windows 2000 Routing and Remote Access service, both remote access and demand-dial connections can be enabled separately but share the same:

• Behavior of the dial-in properties of user accounts.
• Security, including authentication protocols and encryption.
• Use of remote access policies.
• Use of Windows or Remote Authentication Dial-In User Service (RADIUS) as authentication providers.
• IP and Internetwork Packet Exchange (IPX) address allocation configuration.
• Use of PPP features such as Microsoft Point-to-Point Compression (MPPC), Multilink, and Bandwidth Allocation Protocol (BAP).
• Troubleshooting facilities including event logging, Windows or RADIUS authentication and accounting logging, and tracing.

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