HP-UX Reference (11i v2 07/12) - 7 Device (Special) Files, 9 General Information, Index (vol 10)

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ndp(7P) ndp(7P)
• Processing router advertisements.
This rule discusses what actions should be taken on receipt of router advertisements.
• Timing out prefixes and default routers.
Whenever routers send router advertisements, they include the lifetime of the router as well as the
prefixes that they advertise. NDP specifies what actions the host should take when these lifetimes
expire.
• Selecting a default router.
When there is more than one router in the link, the default router selection algorithm comes into
effect. This algorithm helps select the default router based on factors like reachability, etc.
• Sending a router solicitation.
When an interface is enabled, a host need not wait for the unsolicited router advertisement.
Instead, it can send a router solicitation and get a router advertisement as a response. This will
help in receiving the default router and prefix information as soon as the interface is enabled.
3. Algorithm for Sending a Packet
Any IPv6 host is required to maintain some data structures that will be used by the algorithm for
sending a packet. These data structures are:
Neighbor Cache
A set of entries that will maintain IPv6 Address to link-layer address mappings for neighbors to
which a packet has been sent recently. In addition to that it maintains information needed for
neighbor unreachability detection like the reachability state, etc.
Destination Cache
A set of entries for hosts to whom packets have been sent recently. This includes hosts which
are both on-link and off-link. It contains a level of indirection to the neighbor cache.
Prefix List
This is a list of prefixes which define the set of IPv6 address that are on-link. This information is
maintained on a per interface basis. Typically this list is built from Router Advertisements
received from the router.
Default Router List
A list of routers which will forward packets on behalf of this host. This list will again have a
pointer to a neighbor cache entry for the respective router.
A host will use the above data structures while sending a packet to a host. Following is the conceptual
algorithm for sending a packet to a unicast destination.
a. Before a packet is sent out, the next hop should be determined. Normally, next hop determina-
tion is not done on all packets. The results of a next hop determination are stored in the destina-
tion cache. The host should first check the destination cache for any entry that matches with the
current destination address. If it finds a match, then it proceeds to step c, below.
b. If there is no entry for the destination in the destination cache, a longest prefix match is made
with all prefixes in the prefix list. If there is a match, the destination is determined to be on-link
and the destination address will be considered as the next hop. Otherwise, the next hop is deter-
mined from the routing table.
c. Once the next hop is determined, the address resolution process and neighbor unreachability
detection are done for the next hop. This process is explained in the next section.
d. Once the neighbor is known to be reachable, the packet is sent to that destination.
4. Address Resolution and Neighbor Unreachability Detection
Address resolution is a process used to determine the link-layer address of a neighbor. The IPv6
Address to link-layer address mapping found through this process is cached in the Neighbor Cache.
Following are the steps involved in Address Resolution.
a. First, the neighbor cache is checked for an entry which matches the current destination address.
If the entry is not present, the host sends a Neighbor Solicitation Message to the solicited-node
multicast group. This multicast address is derived based on the destination IPv6 address and all
nodes with the particular IPv6 address are required to join that group.
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