IPv6用RIPng（タイマ）

   This section describes all events that are triggered by timers.

   Every 30 seconds, the RIPng process is awakened to send an
   unsolicited Response message, containing the complete routing table
   (see section 2.6 on Split Horizon), to every neighboring router.
   When there are many routers on a single network, there is a tendency
   for them to synchronize with each other such that they all issue
   updates at the same time.  This can happen whenever the 30 second
   timer is affected by the processing load on the system.  It is
   undesirable for the update messages to become synchronized, since it
   can lead to unnecessary collisions on broadcast networks (see [13]
   for more details).  Therefore, implementations are required to take
   one of two precautions:

   - The 30-second updates are triggered by a clock whose rate is not
     affected by system load or the time required to service the
     previous update timer.

   - The 30-second timer is offset by a small random time (+/- 0 to 15
     seconds) each time it is set.  The offset is derived from: 0.5 *
     the update period (i.e. 30).

   There are two timers associated with each route, a "timeout" and a
   "garbage-collection time."  Upon expiration of the timeout, the route
   is no longer valid; however, it is retained in the routing table for
   a short time so that neighbors can be notified that the route has
   been dropped.  Upon expiration of the garbage-collection timer, the
   route is finally removed from the routing table.

   The timeout is initialized when a route is established, and any time
   an update message is received for the route.  If 180 seconds elapse
   from the last time the timeout was initialized, the route is
   considered to have expired, and the deletion process described below
   begins for that route.

   Deletions can occur for one of two reasons: the timeout expires, or
   the metric is set to 16 because of an update received from the
   current router (see section 2.4.2 for a discussion of processing
   updates from other routers).  In either case, the following events
   happen:

   - The garbage-collection timer is set for 120 seconds.

   - The metric for the route is set to 16 (infinity).  This causes the
     route to be removed from service.

   - The route change flag is to indicate that this entry has been
     changed.

   - The output process is signalled to trigger a response.

   Until the garbage-collection timer expires, the route is included in
   all updates sent by this router.  When the garbage-collection timer
   expires, the route is deleted from the routing table.

   Should a new route to this network be established while the garbage-
   collection timer is running, the new route will replace the one that
   is about to be deleted.  In this case the garbage-collection timer
   must be cleared.

   Triggered updates also use a small timer; however, this is best
   described in section 2.5.1.