TCP/IP Tutorial and Technical Overview - IBM Redbooks

Option 2 (target link layer address)
Gives link address of router B so that node X can reach it
without a neighbor solicitation.
Option 4 (redirected header)
Includes the original packet sent by node X, full IP
header, and as much of the data that will fit so that the
total size of the redirect message does not exceed 576
bytes.
Neighbor unreachability detection
An additional responsibility of the neighbor discovery function of ICMPv6 is
neighbor unreachability detection (NUD).
A node actively tracks the reachability state of the neighbors to which it is
sending packets. It can do this in two ways: either by monitoring the upper layer
protocols to see if a connection is making forward progress (for example, TCP
acknowledgments are being received), or issuing specific neighbor solicitations
to check that the path to a target host is still available. When a path to a neighbor
appears to be failing, appropriate action is taken to try and recover the link. This
includes restarting the address resolution process or deleting a neighbor cache
entry so that a new router can be tried in order to find a working path to the
target.
NUD is used for all paths between nodes, including host-to-host, host-to-router,
and router-to-host. NUD can also be used for router-to-router communication if
the routing protocol being used does not already include a similar mechanism.
For further information about neighbor unreachability detection, refer to RFC
2461.
Stateless address autoconfiguration
Although the 128-bit address field of IPv6 solves a number of problems inherent
in IPv4, the size of the address itself represents a potential problem to the
TCP/IP administrator. Because of this, IPv6 has been designed with the
capability to automatically assign an address to an interface at initialization time,
with the intention that a network can become operational with minimal to no
action on the part of the TCP/IP administrator. IPv6 nodes generally use
autoconfiguration to obtain their IPv6 address. This can be achieved using
DHCP (see 9.5, “DHCP in IPv6” on page 371), which is known as stateful
autoconfiguration, or by stateless autoconfiguration, which is a new feature of
IPv6 and relies on ICMPv6.
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