TCP/IPv6 Configuration and Management Manual (G06.24+)

IPv6 Fundamentals
HP NonStop TCP/IPv6 Configuration and Management Manual—524523-008
A-16
Neighbor Discovery Protocol
Neighbor solicitation messages can also determine if more than one node has been
assigned the same unicast address.
Neighbor unreachability detection detects the failure of a neighbor or the failure of the
forward path to the neighbor. Doing so requires positive confirmation that packets sent
to a neighbor are actually reaching that neighbor and being processed properly by its
IP layer. Neighbor unreachability detection uses confirmation from two sources. When
possible, upper-layer protocols provide a positive confirmation that a connection is
making forward progress, that is, previously sent data is known to have been delivered
correctly (if, for example, new acknowledgments were received recently). When
positive confirmation is not forthcoming through such hints, a node sends unicast
neighbor solicitation messages that solicit neighbor advertisements as reachability
confirmation from the next hop. To reduce unnecessary network traffic, probe
messages are only sent to neighbors to which the node is actively sending packets.
In addition to addressing the above general problems, neighbor discovery also handles
these situations:
•
Link-layer address change
A node that knows its link-layer address has changed can multicast a few
(unsolicited) neighbor advertisement packets to all nodes to quickly update cached
link-layer addresses that have become invalid. Note that the sending of unsolicited
advertisements is a performance enhancement only (that is, doing so is
unreliable). The Neighbor unreachability detection algorithm ensures that all nodes
will reliably discover the new address, though the delay might be somewhat longer.
•
Inbound load balancing
Nodes with replicated interfaces might want to load balance the reception of
incoming packets across multiple network interfaces on the same link. Such nodes
have multiple link-layer addresses assigned to the same interface. For example, a
single network driver could represent multiple network interface cards as a single
logical interface having multiple link-layer addresses.
Load balancing is handled by allowing routers to omit the source link-layer address
from router advertisement packets, thereby forcing neighbors to use neighbor
solicitation messages to learn the link-layer addresses of routers. Returned
neighbor advertisement messages can then contain link-layer addresses that differ
depending on who issued the solicitation.
•
Proxy advertisements
A router willing to accept packets on behalf of a target address that cannot respond
to neighbor solicitations can issue non-override neighbor advertisements. There is
currently no specified use of proxy, but proxy advertising could potentially be used
to handle cases like mobile nodes that have moved off-link. However, proxy
advertising is not intended as a general mechanism to handle nodes that, for
example, do not implement this protocol.