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Address Mask Request and Address Mask Reply query messages

RFC 792 defined a variety of host-configuration messages, allowing diskless systems to obtain IP addresses and other data during the boot process by using ICMP messages. However, the primary ICMP query messages used for this-the Information Request and Information Reply query messages-have since been deprecated and are now obsolete. The Address Mask Request and Address Mask Reply query messages are also somewhat obsolete, although their usage has not been deprecated as of yet.

Essentially, the Address Mask Request and Address Mask Reply query messages allow a host to determine the subnet mask in use on the local network. Once a host has obtained an IP address (which could be obtained via Reverse ARP, BOOTP, or DHCP), they could then send an Address Mask Request message to the local broadcast address of 255.255.255.255. An Address Mask server on the network would then respond with an Address Mask Reply query message that is unicast back to the requesting device directly, providing the host with a subnet mask for the IP address being used.

RFC 1122 states that the Address Mask Request and Address Mask Reply query messages are entirely optional. Indeed, only a handful of systems use these messages today.

For more information on the Address Request and Address Reply query messages, refer to "Address Mask Request and Address Mask Reply" later in this chapter.

Router Solicitation and Router Advertisement query messages

RFC 1256 introduced the concept of a Router Discovery protocol that allows the devices on a network to locate routers using ICMP query messages, rather than having to be configured to use static routing entries or run a full-fledged routing protocol like RIP or OSPF.

The Router Discovery protocol consists of a Router Solicitation query message, which is issued by hosts when they first become active on the network (sent to the all-routers multicast address of 224.0.0.2). Each router on the network should then respond to the Router Solicitation query messages with a unicast Router Advertisement query message, informing the querying device directly of the IP addresses that can be used for packet forwarding. In addition, routers will also issue unsolicited Router Advertisement messages on a periodic basis (sent to the all-hosts multicast address of 224.0.0.1), allowing hosts both to change their routing tables as higher-priority routers become available, and to expire old routers if they are no longer being advertised after a certain length of time.

According to RFC 1256, unsolicited Router Advertisement query messages should be sent to the all-hosts multicast address of 224.0.0.1 through every network interface in use by the router, although directed Router Advertisements that result from Router Solicitations are only unicast back to the requesting device, through the most appropriate interface. Each of these advertisement messages must contain the IP address in use on the local interface, as well as a Preference value for the IP address being published. Network devices then use the IP address with the highest Preference (the lowest number) as their default router.

This process is illustrated in Figure 5-8. In that example, Ferret issues an ICMP Router Solicitation query message to the all-routers multicast group address of 224.0.0.2, which both Sasquatch and Canary respond to (using the ICMP Router Advertisement query message, also sent to the all-routers multicast address of 224.0.0.2). Each response also contains a Preference value for that specific router, with Sasquatch having the highest preference (this router will become the default router for Ferret).

Figure 5-8: Router Discovery using Router Solicitation and Advertisement query messages

Note that routers do not advertise the networks or paths that they can route for. Devices must discover (and build) their own routing tables dynamically, by way of the ICMP Redirect error message discussed earlier in "Redirect error messages." In that model, the Router Discovery client uses the Router Solicitation query message to choose a default route, and then relies on the Redirect error messages whenever the host tries to send a datagram to the default router for delivery, but the router knows of a better path for the specified destination system.

This concept is illustrated in Figure 5-9. As we saw in Figure 5-8, Sasquatch had the strongest preference, and was chosen as the default router by Ferret. If the host wants to send a datagram to 192.168.30.10 (a non-local device), it would send the packets to Sasquatch for delivery. However, Sasquatch would know that Canary was the better router for that destination, so it would return an ICMP Redirect error message back to Ferret, informing it that all traffic for the 192.168.30.0 network should go to Canary. Sasquatch would then forward on the packets it had already received, while Ferret would make note of the new path, and (hopefully) start to use that router for any future datagrams for that destination network.

Figure 5-9: Router Discovery using Redirect error messages

On small or isolated networks that have only one or two routers on any given segment, the process of learning about the entire network will only take a few moments and is not nearly as complicated as it sounds here. Conversely, on large and complex networks with many different routers, this process can be significantly more complicated than it sounds here .

For more information on the Router Solicitation and Router Advertisement query messages, refer to "Router Solicitation" and "Router Advertisement," both later in this chapter.