ICMPv4 Router Advertisement and Router Solicitation Messages
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In the overview topic describing IP routing fundamentals, I discussed a critical aspect of IP internetwork design: the difference between the roles of a router and that of a host with regard to routing. Routers are charged with the job of routing datagrams, and therefore, of knowing routes and exchanging route information. Hosts generally do not know a great deal about routes; they rely on routers to convey datagrams intended for destinations outside the local network.
This dependence means that before a host can really participate on an internetwork, it needs to know the identity of at least one router on the local network. One way to ensure that this is the case is to just manually configure each host with the address of a local router as its default router. This method is simple, but has the typical drawbacks associated with manual processesit is time-consuming to set up, difficult to maintain, and inflexible.
It would be better if there were some method whereby a host could automatically discover the identity of local routers, and learn important information about them. In IP, this process is called router discovery, and was first defined in RFC 1256, ICMP Router Discovery Messages. The messages referenced in the RFC title are the ICMP Router Advertisement message and the Router Solicitation message, and were added to the ICMP message types defined in earlier standards such as RFC 792.
Routers are responsible for sending Router Advertisement messages. These messages tell listening devices that the router exists, and provide important information about the router such as its address (or addresses, if it has more than one) and how long the host should retain information about the router. Routine Router Advertisement messages are sent on a regular basis, with the time between messages administrator-configurable (usually between 7 and 10 minutes). Hosts listen for these messages; when an advertisement is received, the host processes it and adds the information about the router to its routing table.
A host that has no manually-configured routing information will have no knowledge of routers when it first powers on. Having it sit for many minutes looking for a routine Router Advertisement message is inefficient. Instead of waiting, the host may send a Router Solicitation message on its local network(s). This will prompt any router that hears it to immediately send out an extra Router Advertisement message directly to that host.
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