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| Italiano | ngnet.it home >> What is IPv6 >> The IPv6 Protocol >> The Neighbor Discovery protocol | ||||
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The Neighbor Discovery protocol The Neighbor Discovery protocol [RFC2461] is used to manage the interactions between different nodes by exchanging messages. These messages make it possible for hosts to communicate with each other and implement autoconfiguration. The procedures involved in autoconfiguration are as follows:
2. Address Configuration, used for automatic configuration of an interface's addresses. 3. Duplicate Address Detection: an algorithm used to check that an address to be assigned is not already in use.
2. Prefix Discovery: the process by which a host discovers the set of on-link prefixes, or in other words the prefixes that can be reached directly at the data link layer. 3. Address Resolution: the process by which an IP address is resolved to its data link-layer address. 4. Next-hop Determination: the algorithm used to determine the IP address of the neighbor which must forward packets to the final destination. The next-hop can be a router or the destination itself. 5. Neighbor Unreachability Detection: used to check whether a neighbor can be reached. 6. Redirect: : the process whereby a router informs a host of a better first-hop node to reach a particular destination.
An interface can be configured manually by the network administrator, or configuration can be performed automatically. This latter possibility has become extremely important in IPv6 because of address length and the need to renumber sites more frequently.
In IPv6, each address is associated with a period of validity so that network topology
can be changed automatically. To reduce the impact of renumbering on applications, two
timers are used for each address. Initially, an address is classified as preferred, meaning
that it can be used without restrictions as either a source or destination address. Once the
preferred lifetime expires, the address goes to the deprecated state, i.e., the address is
still valid but its use is discouraged for new communication. When the second timer, called
the valid lifetime, expires, the address becomes invalid and can no longer be used. In this
way, it is extremely likely that the applications which use an address that has gone to the
deprecated state will end before the address becomes invalid.
The first step is to create the link-local address and check that it is unique (using DAD).
The link-local address is generated by concatenating the prefix FE80 which identifies this
type of address with an identifier which is unique on the link (the MAC address, for example)
and is connected to the interface to be configured. If the result provided by this operation
is an address that has already been assigned to another interface on the same link, the
administrator must either perform manual configuration, or restart the automatic configuration
process. Once the link-local address has been generated, a verification procedure starts to
check that routers are present on the link. If routers are present, they respond with
Router Advertisement messages (see Figure 35), in which the flags M
(Managed Address Configuration) and O (Other Configuration)
specify the type of autoconfiguration to be used.
The stateless and stateful autoconfiguration mechanisms concern only the hosts, and can be used at the same time. Once a station has obtained an address in one of the ways described above, it must check that the address is unique using the DAD mechanism before assigning it to an interface. Stateless AutoconfigurationStateless autoconfiguration [RFC2462] takes place automatically as soon as the interface is enabled and follows link-local address generation as described in the preceding paragraph. The station starts the autoconfiguration process by registering in the multicast group FF02::1 which identifies all of the nodes in the same link so that it can receive messages originating from the routers using this destination address. The station then sends a Router Solicitation message (see Figure 34) whose destination address field is set to the all-routers multicast address FF02::2. In this message, the station can specify only the source link-layer address option shown in Figure 36, which contains the physical address of the sending host. The routers respond with a Router Advertisement in which the destination address is determined from the station's link-layer address and the source address is the associated link-local address. In addition, one or more prefix options can be specified. This option (see Figure 30) makes it possible to define a prefix (coded as a 128-bit address) whose length is written in prefix length and whose period of validity is, to use the terminology discussed earlier, is in Valid and preferred lifetime. If flag A (Autonomous configuration) is set to 1, the host can use the stateless configuration protocol to build the address by concatenating the address supplied to it by the prefix option with the interface ID. This only applies if the prefix is not too long, and otherwise will be ignored. Flag L indicates whether the prefix concerned is to be considered as directly reachable, i.e., on-link.
An example of what we have just described is shown in Figure 31.  After a station has obtained a unicast address in one of the ways described above, it must check that it is unique before assigning it to the interface. To do so, the station sends a Neighbor Solicitation message (Figure 32) in which the source address field is set to the unspecified address and the destination address field is set to the solicited-node multicast address. The latter is a special type of multicast address obtained by concatenating the 96-bit prefix FE02:0:0:0:0::1 and the last 32 bits of the interface's IPv6 address. If the same unicast address has already been assigned to another node, the latter will respond with a Neighbor Advertisement (Figure 33). When the node that initiated the DAD procedure receives this message, it disables the use of the address that has just been obtained. Address ResolutionThis mechanism consists of sending a Neighbor Solicitation message (Figure 32) whose destination is given by the solicited-node multicast address described above. The packet's target address field contains the addresses of the neighbors to be solicited. The host will respond with a Neighbor Advertisement, indicating its IPv6 address in the target address field and its physical address in the target link-layer address option (this option has the same format as the source link shown in Figure 36). When a Neighbor Advertisement (Figure 33) is received in response, the link-layer address is stored in a special table called the neighbor cache.
 Each node manages four tables which it consults when it has to send a packet. These tables are as follows:
When a node has to transmit a packet, the first thing it must do is find the next hop for the destination concerned. The next hop is a node directly connected to the link with which the source is associated. In many cases the source will have sent a packet to the destination in question on an earlier occasion, so that the next-hop address will already be stored in the destination cache; consequently, the source consults this cache first. If it does not contain the IPv6 address of the next hop, the next-hop determination procedure must be used. This procedure operates as follows. The node which must transmit the packet performs a longest prefix match 2 against the prefix list to determine whether the node is on-link. If the destination is on-link, it is also the next hop. Otherwise, the sender selects a router from the router list and uses it as the next hop for the destination concerned. It stores this router's IP address in the destination cache so that it can be used again for the subsequent packets. The station does not use a special criterion of any kind in selecting the next-hop router. Consequently, the selected router may not always represent the best route to the destination. In such cases, the router sends a special message called a redirect message to inform the source that there is a better next-hop to the destination. At this point, the neighbor's IPv6 address is known, but its link-layer address must still be determined in order to send the packet. The link-layer address is stored in the neighbor cache. If the address is not present in the cache, it can be found by means of the Address Resolution procedure. Once the next-hop link-layer address is known, the source can send the packet. Information about the routers present on the link or about the prefixes which a host can reach directly without passing through a router (i.e., on-link prefixes) is obtained by exchanging the Router Solicitation and Advertisement messages whose format is shown in Figures 34 and 35.  
Routers periodically sent multicast Router Advertisement messages to all nodes on the
network to announce their availability to forward packets and specify several options,
including:
  The Neighbor Unreachability Detection procedure is accomplished by analyzing the replies that a node can receive from higher level applications after sending a packet (e.g. TCP acknowledgement messages). If there are no replies (e.g., UDP), the entries in the neighbor cache are subject to timeout, and a Neighbor Solicitation message is sent when they expire. If the destination responds, the corresponding entry is retained; if not, it is deleted from the cache. In general, this procedure is invoked to verify the validity of information which has been labeled as stale (see Figure 38), meaning that it has been present in the table for a long time, though it has not yet expired. One of the important items of information obtained from this mechanism is the indication that a router has changed to a host. When this occurs, the R flag in the Neighbor Advertisement is set to zero (rather than 1 to indicate a router). 4 Comparison with IPv4From the foregoing discussion, it should be clear that the Neighbor Discovery protocol corresponds to a combination of the IPv4 protocols ARP, ICMPv4 Router Discovery and ICMP Redirect. Neighbor Discovery provides a multitude of improvements over the IPv4 set of protocols:
[*] Longest prefix match is the process of determining which prefix covers a given IPv6 address. When multiple prefixes cover an address, the longest prefix is the one that matches.
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