3.5. Output Processing

This section describes the processing used to create response messages that contain all or part of the routing table. This processing may be triggered in any of the following ways:

• by input processing when a request is seen. In this case, the resulting message is sent to only one destination. - by the regular routing update. Every 30 seconds, a response containing the whole routing table is sent to every neighboring gateway. (See section 3.3.)
• by triggered updates. Whenever the metric for a route is changed, an update is triggered. (The update may be delayed; see below.)

Before describing the way a message is generated for each directly- connected network, we will comment on how the destinations are chosen for the latter two cases. Normally, when a response is to be sent to all destinations (that is, either the regular update or a triggered update is being prepared), a response is sent to the host at the opposite end of each connected point-to-point link, and a response is broadcast on all connected networks that support broadcasting. Thus, one response is prepared for each directly-connected network and sent to the corresponding (destination or broadcast) address. In most cases, this reaches all neighboring gateways. However, there are some cases where this may not be good enough. This may involve a network that does not support broadcast (e.g., the ARPANET), or a situation involving dumb gateways. In such cases, it may be necessary to specify an actual list of neighboring hosts and gateways, and send a datagram to each one explicitly. It is left to the implementor to determine whether such a mechanism is needed, and to define how the list is specified.

Triggered updates require special handling for two reasons. First, experience shows that triggered updates can cause excessive loads on networks with limited capacity or with many gateways on them. Thus the protocol requires that implementors include provisions to limit the frequency of triggered updates. After a triggered update is sent, a timer should be set for a random time between 1 and 5 seconds. If other changes that would trigger updates occur before the timer expires, a single update is triggered when the timer expires, and the timer is then set to another random value between 1 and 5 seconds. Triggered updates may be suppressed if a regular update is due by the time the triggered update would be sent.

Second, triggered updates do not need to include the entire routing table. In principle, only those routes that have changed need to be included. Thus messages generated as part of a triggered update must include at least those routes that have their route change flag set. They may include additional routes, or all routes, at the discretion of the implementor; however, when full routing updates require multiple packets, sending all routes is strongly discouraged. When a triggered update is processed, messages should be generated for every directly-connected network. Split horizon processing is done when generating triggered updates as well as normal updates (see below). If, after split horizon processing, a changed route will appear identical on a network as it did previously, the route need not be sent; if, as a result, no routes need be sent, the update may be omitted on that network. (If a route had only a metric change, or uses a new gateway that is on the same network as the old gateway, the route will be sent to the network of the old gateway with a metric of infinity both before and after the change.) Once all of the triggered updates have been generated, the route change flags should be cleared.

If input processing is allowed while output is being generated, appropriate interlocking must be done. The route change flags should not be changed as a result of processing input while a triggered update message is being generated.

The only difference between a triggered update and other update messages is the possible omission of routes that have not changed. The rest of the mechanisms about to be described must all apply to triggered updates.

Here is how a response datagram is generated for a particular directly-connected network:

The IP source address must be the sending host's address on that network. This is important because the source address is put into routing tables in other hosts. If an incorrect source address is used, other hosts may be unable to route datagrams. Sometimes gateways are set up with multiple IP addresses on a single physical interface. Normally, this means that several logical IP networks are being carried over one physical medium. In such cases, a separate update message must be sent for each address, with that address as the IP source address.

Set the version number to the current version of RIP. (The version described in this document is 1.) Set the command to response. Set the bytes labeled "must be zero" to zero. Now start filling in entries.

To fill in the entries, go down all the routes in the internal routing table. Recall that the maximum datagram size is 512 bytes. When there is no more space in the datagram, send the current message and start a new one. If a triggered update is being generated, only entries whose route change flags are set need be included.

See the description in Section 3.2 for a discussion of problems raised by subnet and host routes. Routes to subnets will be meaningless outside the network, and must be omitted if the destination is not on the same subnetted network; they should be replaced with a single route to the network of which the subnets are a part. Similarly, routes to hosts must be eliminated if they are subsumed by a network route, as described in the discussion in Section 3.2.

If the route passes these tests, then the destination and metric are put into the entry in the output datagram. Routes must be included in the datagram even if their metrics are infinite. If the gateway for the route is on the network for which the datagram is being prepared, the metric in the entry is set to 16, or the entire entry is omitted. Omitting the entry is simple split horizon. Including an entry with metric 16 is split horizon with poisoned reverse. See Section 2.2 for a more complete discussion of these alternatives.