Neighbor Solicitation and Unreachability

Neighbor Discovery uses neighbor solicitation messages to determine if more than one node is assigned the same unicast address. Neighbor unreachability detection detects the failure of a neighbor or the failure of the forward path to the neighbor. This detection requires positive confirmation that packets that are sent to a neighbor are actually reaching that neighbor. Neighbor unreachability detection also determines that packets are being processed properly by the node's IP layer.

Neighbor unreachability detection uses confirmation from two sources: upper-layer protocols and neighbor solicitation messages. When possible, upper-layer protocols provide a positive confirmation that a connection is making forward progress. For example, when new TCP acknowledgments are received, it is confirmed that previously sent data has been delivered correctly.

When a node does not get positive confirmation from upper-layer protocols, the node sends unicast neighbor solicitation messages. These messages solicit neighbor advertisements as reachability confirmation from the next hop. To reduce unnecessary network traffic, probe messages are sent only to neighbors to which the node is actively sending packets.

Duplicate Address Detection Algorithm

To ensure that all configured addresses are likely to be unique on a particular link, nodes run a duplicate address detection algorithm on addresses. The nodes must run the algorithm before assigning the addresses to an interface. The duplicate address detection algorithm is performed on all addresses.

The autoconfiguration process that is described in this section applies only to hosts, and not routers. Because host autoconfiguration uses information that is advertised by routers, routers need to be configured by some other means. However, routers generate link-local addresses by using the mechanism that is described in this chapter. In addition, routers are expected to successfully pass the duplicate address detection algorithm on all addresses prior to assigning the address to an interface.

Proxy Advertisements

A router that accepts packets on behalf of a target address can issue non-override neighbor advertisements. The router can accept packets for a target address that is unable to respond to neighbor solicitations. Currently, the use of proxy is not specified. However, proxy advertising can potentially be used to handle cases such as mobile nodes that have moved off-link. Note that the use of proxy is not intended as a general mechanism to handle nodes that do not implement this protocol.

Inbound Load Balancing

Nodes with replicated interfaces might need to load balance the reception of incoming packets across multiple network interfaces on the same link. Such nodes have multiple link-local addresses assigned to the same interface. For example, a single network driver can represent multiple network interface cards as a single logical interface that has multiple link-local addresses.

Load balancing is handled by allowing routers to omit the source link-local address from router advertisement packets. Consequently, neighbors must use neighbor solicitation messages to learn link-local addresses of routers. Returned neighbor advertisement messages can then contain link-local addresses that differ, depending on which issued the solicitation.

Link-Local Address Change

A node that knows its link-local address has been changed can send out multicast unsolicited, neighbor advertisement packets. The node can send multicast packets to all nodes to update cached link-local addresses that have become invalid. The sending of unsolicited advertisements is a performance enhancement only. The detection algorithm for neighbor unreachability ensures that all nodes reliably discover the new address, though the delay might be somewhat longer.

Comparison of Neighbor Discovery to ARP and Related IPv4 Protocols

The functionality of the IPv6 Neighbor Discovery protocol corresponds to a combination of the IPv4 protocols: Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP) Router Discovery, and ICMP Redirect. IPv4 does not have a generally agreed on protocol or mechanism for neighbor unreachability detection. However, host requirements do specify some possible algorithms for dead gateway detection. Dead gateway detection is a subset of the problems that neighbor unreachability detection solves.

The following list compares the Neighbor Discovery protocol to the related set of IPv4 protocols.

• Router discovery is part of the base IPv6 protocol set. IPv6 hosts do not need to snoop the routing protocols to find a router. IPv4 uses ARP, ICMP router discovery, and ICMP redirect for router discovery.

• IPv6 router advertisements carry link-local addresses. No additional packet exchange is needed to resolve the router's link-local address.

• Router advertisements carry site prefixes for a link. A separate mechanism is not needed to configure the netmask, as is the case with IPv4.

• Router advertisements enable address autoconfiguration. Autoconfiguration is not implemented in IPv4.

• Neighbor Discovery enables IPv6 routers to advertise an MTU for hosts to use on the link. Consequently, all nodes use the same MTU value on links that lack a well-defined MTU. IPv4 hosts on the same network might have different MTUs.

• Unlike IPv4 broadcast addresses, IPv6 address resolution multicasts are spread over 4 billion (2^32) multicast addresses, greatly reducing address resolution-related interrupts on nodes other than the target. Moreover, non-IPv6 machines should not be interrupted at all.

• IPv6 redirects contain the link-local address of the new first hop. Separate address resolution is not needed on receiving a redirect.

• Multiple site prefixes can be associated with the same IPv6 network. By default, hosts learn all local site prefixes from router advertisements. However, routers can be configured to omit some or all prefixes from router advertisements. In such instances, hosts assume that destinations are on remote networks. Consequently, hosts send the traffic to routers. A router can then issue redirects, as appropriate.

• Unlike IPv4, the recipient of an IPv6 redirect message assumes that the new next-hop is on the local network. In IPv4, a host ignores redirect messages that specify a next-hop that is not on the local network, according to the network mask. The IPv6 redirect mechanism is analogous to the XRedirect facility in IPv4. The redirect mechanism is useful on non-broadcast and shared media links. On these networks, nodes should not check for all prefixes for local link destinations.

• IPv6 neighbor unreachability detection improves packet delivery in the presence of failing routers. This capability improves packet delivery over partially failing or partitioned links. This capability also improves packet delivery over nodes that change their link-local addresses. For example, mobile nodes can move off the local network without losing any connectivity because of stale ARP caches. IPv4 has no corresponding method for neighbor unreachability detection.

• Unlike ARP, Neighbor Discovery detects half-link failures by using neighbor unreachability detection. Neighbor Discovery avoids sending traffic to neighbors when two-way connectivity is absent.

• By using link-local addresses to uniquely identify routers, IPv6 hosts can maintain the router associations. The ability to identify routers is required for router advertisements and for redirect messages. Hosts need to maintain router associations if the site uses new global prefixes. IPv4 does not have a comparable method for identifying routers.

• Because Neighbor Discovery messages have a hop limit of 255 upon receipt, the protocol is immune to spoofing attacks originating from off-link nodes. In contrast, IPv4 off-link nodes can send ICMP redirect messages. IPv4 off-link nodes can also send router advertisement messages.

• By placing address resolution at the ICMP layer, Neighbor Discovery becomes more media independent than ARP. Consequently, standard IP authentication and security mechanisms can be used.

IPv6 Routing

Routing in IPv6 is almost identical to IPv4 routing under Classless Inter-Domain Routing (CIDR). The only difference is that the addresses are 128-bit IPv6 addresses instead of 32-bit IPv4 addresses. With very straightforward extensions, all of IPv4's routing algorithms, such as OSPF, RIP, IDRP, and IS-IS, can be used to route IPv6.

IPv6 also includes simple routing extensions that support powerful new routing capabilities. The following list describes the new routing capabilities:

• Provider selection that is based on policy, performance, cost, and so on

• Host mobility, route to current location

• Auto-readdressing, route to new address

You obtain the new routing capabilities by creating sequences of IPv6 addresses that use the IPv6 routing option. An IPv6 source uses the routing option to list one or more intermediate nodes, or topological group, to be visited on the way to a packet's destination. This function is very similar in function to IPv4's loose source and record route option.

To make address sequences a general function, IPv6 hosts are required, in most instances, to reverse routes in a packet that a host receives. The packet must be successfully authenticated by using the IPv6 authentication header. The packet must contain address sequences in order to return the packet to its originator. This technique forces IPv6 host implementations to support the handling and reversal of source routes. The handling and reversal of source routes is the key that enables providers to work with hosts that implement the new IPv6 capabilities such as provider selection and extended addresses.

Router Advertisement

On multicast-capable links and point-to-point links, each router periodically sends to the multicast group a router advertisement packet that announces its availability. A host receives router advertisements from all routers, building a list of default routers. Routers generate router advertisements frequently enough so that hosts learn of their presence within a few minutes. However, routers do not advertise frequently enough to rely on an absence of advertisements to detect router failure. A separate detection algorithm that determines neighbor unreachability provides failure detection.

Router Advertisement Prefixes

Router advertisements contain a list of subnet prefixes that is used to determine if a host is on the same link (on-link) as the router. The list of prefixes is also used for autonomous address configuration. Flags that are associated with the prefixes specify the intended uses of a particular prefix. Hosts use the advertised on-link prefixes to build and maintain a list that is used to decide when a packet's destination is on-link or beyond a router. A destination can be on-link even though the destination is not covered by any advertised on-link prefix. In such instances, a router can send a redirect. The redirect informs the sender that the destination is a neighbor.

Router advertisements, and per-prefix flags, enable routers to inform hosts how to perform stateless address autoconfiguration.

Router Advertisement Messages

Router advertisement messages also contain Internet parameters, such as the hop limit, that hosts should use in outgoing packets. Optionally, router advertisement messages also contain link parameters, such as the link MTU. This feature enables the centralized administration of critical parameters. The parameters can be set on routers and automatically propagated to all hosts that are attached.

Nodes accomplish address resolution by sending to the multicast group a neighbor solicitation that asks the target node to return its link-layer address. Multicast neighbor solicitation messages are sent to the solicited-node multicast address of the target address. The target returns its link-layer address in a unicast neighbor advertisement message. A single request-response pair of packets is sufficient for both the initiator and the target to resolve each other's link-layer addresses. The initiator includes its link-layer address in the neighbor solicitation.