HP-UX Reference (11i v1 05/09) - 7 Device (Special) Files, 9 General Information, Index (vol 10)
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NDP(7P) NDP(7P)
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.
b. If a host with the specified IPv6 address is present in the network, it will reply this solicitation
with a Neighbor Advertisement Message.
c. On receiving the Neighbor Advertisement the node will search for an entry in the neighbor cache
for the sender’s IPv6 address. A new entry is created in the neighbor cache and the reachability
flag is set to REACHABLE.
Once the Address resolution is completed, neighbor unreachability detection will be performed. This
process depends on the reachability field of the neighbor cache. An entry in the neighbor cache can
have any of the following states:
a. INCOMPLETE. It means that the address resolution is in progress and the link-layer address of
the destination is yet to be determined.
b. REACHABLE. It means that the destination is reachable until recently.
c. STALE. The destination is no longer known to be reachable, but reachability detection need not
be made until a packet has to be sent to that destination.
d. DELAY. This state is an optimization that gives additional time for the upper layer protocols to
provide the reachability confirmation.
e. PROBE. This state means that a reachability confirmation is actively requested by repeatedly
sending Neighbor Solicitations.
During neighbor unreachability detection, the node checks for the state in the neighbor cache. If the
state for the destination is REACHABLE, the packet is sent. Otherwise, the following are steps
involved:
a. When an address resolution is made on a destination, an entry is created in the neighbor cache
for that destination and the reachability state will be set to INCOMPLETE. If the address reso-
lution fails, the entry is deleted.
b. When the address resolution passes, the entry will be filled with the destination’s link-layer
address and the state will be set to REACHABLE.
c. There is a timer maintained called the Reachability timer which will expire the state of an entry
in the neighbor cache. Once this timer expires the reachability state changes from REACH-
ABLE to STALE.
d. When a packet is being sent to a destination whose state is STALE in the neighbor cache, the
node sets the state to DELAY and starts a timer associated with that state. By the time the
timer expires if the node received reachability confirmation, the state is set to REACHABLE.
Otherwise it is set to PROBE.
e. Once the entry’s state is in PROBE, the node sends unicast neighbor solicitations to the link-
layer address specified in the entry. If it receives a neighbor advertisement in response the state
is set to REACHABLE. This solicitation will be sent repeatedly and the maximum number of
times is configurable. If the reachability confirmation is not received after maximum solicita-
tions, the entry is deleted from the neighbor cache and the address resolution is done again.
Note: Entries in the neighbor cache can also be created as a result of node receiving unsolicited
Neighbor Advertisements, Router Advertisements and Router Solicitations etc. However, for the
entry created under these circumstances the reachability state will always be set to STALE.
5. Redirect function
A router will send a host a redirect message when it finds that there is a better next-hop router on the
same link. This is a requirement for a router.
Hosts on receiving a router redirect message, should update its destination cache with the new next
hop address.
AUTHOR
NDP was developed by the IPng Working Group of the Internet Engineering Task Force.
HP-UX 11i Version 1: September 2005 − 3 − Hewlett-Packard Company Section 7−−85