Router Advertisements

Routers advertise their presence on a link and provide the information necessary for a node to configure itself. The RA is multicast to the link-scope all-nodes multicast group.

An RA is an ICMP packet of type 134. Its source IP address is the link-local address of the sending router, and the destination address is either the address of a node that sent a Router Solicitation or the link-scope all-nodes multicast address. The hop limit must be set to 255. The hop limit is not used, in this case, to stop routers from forwarding the packet. A value of 1 ensures that the packet does not get forwarded, because a router that receives the packet decrements the hop limit and drops the packet when the hop limit reaches 0. The value of 255 ensures that no off-link device sends RAs in an attempt to disrupt traffic flow. If an off-link device does send an RA, the RA traverses a router, which automatically decrements the hop-limit value, rendering the packet invalid. One of the ways that the receiving node validates the packet is by verifying that the hop limit is 255. IPv4 does not use this method of ensuring that the packet could not possibly have traversed a router.

An RA contains a Router Lifetime. The Router Lifetime informs nodes how long they should consider the router as a default. The time is in units of seconds, with a maximum value of 18.2 hours. A value of 0 means that the router is not a default candidate and should not appear on any host's default router list.

A host receiving RAs builds a default router list. All routers that advertised RAs with nonzero valued Router Lifetimes appear in the default router list. The entry for a router's Router Lifetime value in the default list is updated with each subsequent RA received. If an RA contains a zero-valued Router Lifetime for an already listed router, the host immediately removes the router from the default list (an improvement over IPv4). IPv4 hosts have to be manually configured with default router lists. Some IPv4 hosts run a routing protocol, such as RIP, to dynamically learn this information, and some run the ICMP Router Discovery Protocol (IRDP). Neither RIP nor IRDP are implemented on all IPv4 hosts, however.

An RA also contains a Reachable Time and a Retransmit Timer. The Reachable Time informs hosts how long to assume a neighbor is alive after receiving a reachability confirmation from that neighbor. This information is used in the Neighbor Unreachability detection process. The Retransmit Timer is the time, in milliseconds, between subsequent

Neighbor Solicitation messages. It is used in the address resolution and the Neighbor Unreachability detection processes.

Two bits found in the RA packet, the Managed Address (M) bit and the Other Stateful Configuration (O) bit, inform a host how it should configure itself. If the M bit is set, the host configures its address using the stateful autoconfiguration protocol, such as DHCP, in addition to any addresses configured with stateless autoconfiguration. If the O bit is set, hosts use the stateful autoconfiguration protocol to configure other information besides the address. IPv4 hosts are manually configured to indicate whether they should learn their IP configuration information via DHCP. Automatically providing this information to hosts on a link via router advertisements minimizes the amount of static configuration information contained in hosts, easing future reconfiguration efforts. The autoconfiguration methods are discussed in the section "Autoconfiguration."

The options that may be present in the RA are the source link layer address, the MTU, and prefix information. Including the source link layer address of the router in the RA eliminates the need for hosts to perform the address resolution protocol on default routers. A router may elect not to include the link layer address. The MTU option enables centralized control of the MTU that hosts on a link use. This option is used mainly for links with a variable MTU but may be used on other links. The value is set in the router, which then enables the configuration of all the hosts on the link. The prefix information is used to inform other nodes of on-link prefixes and for address autoconfiguration. A host that knows of all the prefixes that are configured on a link forwards traffic more knowledgeably. A mul-tihomed host can choose the closest interface to any known on-link destination prefix. A nonmultihomed host uses the prefix list to assist in next-hop detection.

The prefix information option contains data that is used for both on-link determination and stateless autoconfiguration. It contains the actual prefix and the length of the prefix, which is always from 1 to 128 bits. It also contains bits that indicate whether the prefix is to be used for on-link determination or for address configuration. When the L bit is set, you can use the prefix for on-link determination. When it is not set, you can determine no information about on-link or off-link. The A bit, when set, indicates that you can use the prefix for stateless address configuration.

The prefix option also contains a Valid Lifetime value and a Preferred Lifetime value. The Valid Lifetime indicates, in seconds, how long a prefix is valid for purposes of on-link determination. The lifetime is relative to the time the packet was sent. An advertised Valid Lifetime value of zero indicates that the prefix is no longer valid. The Preferred Lifetime is the number of seconds that the address automatically configured from the prefix can remain "preferred." A preferred address on an interface is one that any node can actively use for communication. A Preferred Lifetime of zero means the addresses configured with the prefix must be deprecated. A deprecated address is one that is used to maintain existing connections, but it should not be used to initiate new connections if a preferred address exists. A lifetime of all ones indicates infinity. You can use a prefix for both on-link determination and configuration. The two types of addresses are discussed further in the section "Autoconfiguration."

Continue reading here: Neighbor Unreachability Detection

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