4. Control functions

This section describes administrative controls. These are not part of the protocol per se. However, experience with existing networks suggests that they are important. Because they are not a necessary part of the protocol, they are considered optional. However, we strongly recommend that at least some of them be included in every implementation.

These controls are intended primarily to allow RIP to be connected to networks whose routing may be unstable or subject to errors. Here are some examples:

It is sometimes desirable to limit the hosts and gateways from which information will be accepted. On occasion, hosts have been misconfigured in such a way that they begin sending inappropriate information.

A number of sites limit the set of networks that they allow in update messages. Organization A may have a connection to organization B that they use for direct communication. For security or performance reasons A may not be willing to give other organizations access to that connection. In such cases, A should not include B's networks in updates that A sends to third parties.

Here are some typical controls. Note, however, that the RIP protocol does not require these or any other controls.

• a neighbor list - the network administrator should be able to define a list of neighbors for each host. A host would accept response messages only from hosts on its list of neighbors.
• allowing or disallowing specific destinations - the network administrator should be able to specify a list of destination addresses to allow or disallow. The list would be associated with a particular interface in the incoming or outgoing direction. Only allowed networks would be mentioned in response messages going out or processed in response messages coming in. If a list of allowed addresses is specified, all other addresses are disallowed. If a list of disallowed addresses is specified, all other addresses are allowed.