Error And Control Messages; Router Discovery Protocol - Novell NETWARE 6-DOCUMENTATION Manual

Error and Control Messages

Router Discovery Protocol

Another protocol in the TCP/IP suite is the Internet Control Message Protocol
(ICMP). ICMP packets contain information about errors and control on the
network: inoperative nodes and gateways, packet congestion at a gateway, and
so on. The IP software, rather than the application, interprets an ICMP
message. The IP software then takes the appropriate action with respect to the
ICMP message, independently of the application. Because an ICMP message
might need to travel across several networks to reach its destination, it is
encapsulated in the data portion of an IP datagram.
ICMP is also used to test connectivity between two nodes. The originating
node uses PING to send an ICMP echo request and waits for an ICMP echo
response from the destination.
The Router Discovery Protocol, an extension to Internet Control Message
Protocol (ICMP), allows hosts to discover routers on their networks and
determine which router to use as the default router. When a host needs to send
a packet to another network, it first sends the packet to a router that forwards
the packet toward the destination. To accomplish this, the host needs to know
where the routers are on its network and which one to send packets to.
When you configure the router discovery mechanism, the router advertises
itself with periodic ICMP router advertisement messages. Then the host
listens to this message and decides whether to use a router as the default router.
You can configure the host to solicit the router advertisement on attached
networks. All participating routers then reply to the inquiry. By collecting
those replies, the host discovers the routers on the network and determines
which router to use.
A host might not select the best router (the router with the optimal path) to
forward packets to a specific destination. When a router receives a packet
from a host that is better forwarded to another router on the network, the router
uses an ICMP Redirect message to notify the host of the optimal path.
NetWare routing software provides both host and router implementations of
the Router Discovery Protocol. The mode of operation of the Router
Discovery Protocol is determined by whether the IP Packet Forwarding
parameter is enabled. If IP Packet Forwarding is enabled, the Router
Discovery Protocol will send Router Advertisement messages. If IP Packet
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Understanding