Hello, everyone!
Today, I would like to share with you the ICMP Router Discovery Messages.
That one way to initialize a routing table is with static routes specified in configuration files. This is often used to set a default entry. A newer way is to use the ICMP router advertisement and solicitation messages.
The general concept is that after bootstrapping, a host broadcasts or multicasts a router solicitation message. One or more routers respond with a router advertisement message. Additionally, the routers periodically broadcast or multicast their router advertisements, allowing any hosts that are listening to update their routing table accordingly.
RFC 1256 [Deering 1991] specifies the format of these two ICMP messages. Figure 9.6 shows the format of the ICMP router solicitation message. Figure 9.7 shows the format of the ICMP router advertisement message sent by routers
Multiple addresses can be advertised by a router in a single message. A number of addresses is the number. Address entry size is the number of 32-bit words for each router address and is always 2. Lifetime is the number of seconds that the advertised addresses can be considered valid.
One or more pairs of an IP address and a preference then follow. The IP address must be one of the sending router's IP addresses. The preference level is a signed 32-bit integer indicating the preference of this address as a default router address, relative to other router addresses on the same subnet. Larger values imply more preferable addresses. The preference level 0x80000000 means the corresponding address, although advertised, is not to be used by the receiver as a default router address. The default value of the preference is normally 0.
Router Operation
When a router starts up it transmits periodic advertisements on all interfaces capable of broadcasting or multicasting. These advertisements are not exactly periodic, but are randomized, to reduce the probability of synchronization with other routers on the same subnet. The normal time interval between advertisements is between 450 and 600 seconds. The default lifetime for a given advertisement is 30 minutes.
Another use of the lifetime field occurs when an interface on a router is disabled. In that case, the router can transmit a final
advertisement on the interface with the lifetime set to 0.
In addition to the periodic, unsolicited advertisements, a router also listens for solicitations from hosts. It responds to these solicitations with a router advertisement.
If there are multiple routers on a given subnet, it is up to the system administrator to configure the preference level for each router as appropriate. For example, the primary default router would have a higher preference than a backup.
Host Operation
Upon bootstrap, a host normally transmits three router solicitations, 3 seconds apart. As soon as a valid advertisement is received, the solicitations stop.
A host also listens for advertisements from adjacent routers. These advertisements can cause the host's default router to change. Also, if an advertisement is not received for the current default, that default can time out.
As long as the normal default router stays up, that router will send advertisements every 10 minutes, with a lifetime of 30 minutes. This means the host's default entry won't time out, even if one or two advertisements are lost.
Implementation
The router discovery messages are normally generated by and processed by a user process (a daemon). This adds yet another program updating the routing table, although it would only add or delete a default entry. The daemon would have to be configured to act as a router or a host.
These two ICMP messages are new and not supported by all systems. Solaris 2.x is the only system in our network that supports these messages (the in.rdisc daemon). Although the RFC recommends using IP multicasting whenever possible, router discovery can work using broadcast messages also.
Welcome to leave your comments below the post,
I hope it will be helpful to you. Thank you!
