ndp.7p (2010 09)

ManualsBrandsHP ManualsSoftwareHP-UX Manpages

ndp(7P) ndp(7P)

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.

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:

INCOMPLETE The address resolution is in progress and the link-layer address of the destination is

yet to be determined.

REACHABLE The destination is reachable until recently.

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.

DELAY This state is an optimization that gives additional time for the upper layer protocols

to provide the reachability conﬁrmation.

PROBE 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 steps are

taken:

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

resolution 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

REACHABLE 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; 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.

On receiving a router redirect message, a host 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.