neighbor discovery in ipv6

Neighbor Discovery in IPv61.1 IntroductionThe lack of internet (IP) addresses in the Internet community today have forced a development of a new extended addressing protocol, namely IPv6. IPv6 has been developed with an address space of 128 bit as opposed to the older currently used version IPv4 with a 32 bit addressing space. Thereby solving the shortage of IP addresses problem on the Internet. When improving the Internet protocol IP the internet community also made a lot of other additions and improvements that is supported by IPv6. One of these improvements are Neighbor Discovery. In IP and mobile IP version 6 a very important component is Neighbor discovery, some of the features in Neighbor discovery are new others are renamed and altered compared to the earlier version of IP (IPv4).1.1.2 Neighbor discovery is used by nodes to:Determine the link-layer addresses for neighboring nodes (nodes residing on the same link) and quickly remove cached values we they become invalid.Find routers willing to forward their packets, that is acting as a default router.Keep track of which nodes are reachable and on which path they can be reached.1.1.3 Functions in Neighbor discoveryTo do this a number of functions are introduced:Router discovery: How to locate routers on the link.Prefix discovery: How to determine the network prefixes on the link. The prefixes are used to distinguish between nodes on the same link as node(neighbors) and nodes reached through a router.Parameter discovery: To find link parameters such as MTU, the value to put in the hop limit field.Address resolution: To determine neighbors link-layer address given its IP address.Next-hop determination: To decide next hop for packets going through a router.Neighbor unreachability detection: To detect that a neighbor no longer is reachable.Duplicate Address Detection: To determine that a nodes address is unique.Redirect: When a router informs the node of the better next hop.1.1.4 ICMP types providing the Neighbor discovery functionsThe neighbor functions are provided using five different ICMP (version 6) message types:Router solicitations: Requests from host to a router for router advertisements.Router advertisements: For routers to advertise their presence and parameters.Neighbor solicitations: To determine the link-layer address of a neighbor, to determine if the neighbor is still reachable and to detect duplicate addresses.Neighbor Advertisement: for nodes to announce their link-layer address.Redirect: For routers to inform hosts of better next hop.2.1 Requirements on hostsAll these features set requirements on the hosts. They are required to have a Neighbor cache, destination cache, a prefix list and a default router list.2.1.2 The Neighbor cacheThe Neighbor cache must have a set of entries about the neighbors such as their link-layer address, IP addresses, a flag indicating if the node is a router or a host, a pointer to queued packets waiting for the Address Resolution to complete. It also contains information regarding nodes reachability state, numbers of unanswered probes and next time a Neighbor Unreachability Detection is to take place.Each entry in this cache must have one of 5 different sates, stated below. These states will help the node to keep track of the status of each neighbor. Each entry also contains a timer so that old values change state when no update has occurred for a while.The different states in the neighbor cache are:Incomplete: The address resolution is in progress trying to obtain the link-layer Address Reachable: The entry is complete, the node is assumed reachable.Stale: Entry is known to be reachable but no verification attempt should be tried until traffic is sent to the neighbor.Delay: The reachability is uncertain but traffic is sent and solicitations are delayed. Probe: A probe message is sent to verify reachability2.1.3 The Destination cacheThe Destination cache contains destination information about recently sent traffic. IP addresses and a pointer to the next hop address stored in the neighbor cache. The cache is updated by Redirect messages from routers. This cache contains information about nodes residing both on the same link (neighbors) and on others as opposed to the destination cache that only contains information about nodes on the same link. 2.1.4 The prefix listThe prefix list contains entries for every prefix on the link. The hosts to determine if the destination is on the same link use this information. Thereby knowing if the packet should be sent directly or via the default router.2.1.5 Default router listA list containing the routers on the link that are willing to act like a default router for the node.3.1 Packet transmissionA node about to send a packet will use the three above described caches. When a packet is transmitted the node looks into the Destination cache, the prefix list and the default router to determine the IP address. This is done via a longest prefix match among the prefixes stored in the prefix list. This match will show if the node is on the same link or should be sent via the default router. Once the IP address is known the Node looks into the Neighbor cache to get the link-layer address for a suitable next hop. If the node doesn’t have an entry in the Neighbor cache, it creates an entry and starts the address resolution, while this process is running packets to that address are queued.Every time the Neighbor cache is used the reachability information is checked and if the status isn’t reachable suitable measures are being made, these according to what is stated in the Unreacability Detection function, described further down.4.1 Router and prefix discoveryRouter discovery is used to find neighboring routers, prefixes and configuration parameters such as the default value of the Hop limit or MTU on the present link. Routers periodically send a multicast router advertisement or as a response of a router solicitation. The nodes on the link receiving this advertisement use it to build up their list of default routers (the router can choose not to be listed to enable load balancing). The prefixes the nodes can learn from the advertisement are used to the get the IP-addresses of the nodes that are currently on the link. The prefix information is sent as an option. Moreover the Advertisement contains a set of flags that informs the routers about how to perform the Autoconfiguration (i.e. stateful or stateless)5.1 The Address resolutionAfter the prefix discovery the nodes has to determine or verify the link-layer addresses on the link. This is discovered by using Neighbor solicitations and Neighbor Advertisements. A node starts with sending a Neighbor Solicitation message to make the wanted node to return its link-layer address and at the same time providing the nodes own link-layer address. The solicitation is sent as a multicast message if an address needs to be resolved and unicast when an address needs to be verified.When positive acknowledgement does not verify that sent data is delivered neighbor solicitation can be sent to find out about the nodes reachability this will force the receiver to send an advertisement that confirms its reachability. The solicitation messages are also used to discover duplicate addresses, to see if more then one node is using the same address.6.1 Neighbor UnreachabilityThe frequency of router advertisements is not high enough to detect unreachability problems. Therefore the Neighbor Unreachability detection procedure is introduced. When the reachability isn’t confirmed by upper layer protocols (such as TCP) the node must probe to test the reachability. This is done via using Neighbor Solicitations.A node that has been regarded as reachable will after a certain time change state to the Stale state, when a packet is to be sent the state enters the delay state and a probe issent. Once a confirmation (neighbor solicitation) is received the state is changed to reachable.If a node changes its link layer address the neighbor unreachability detection will eventually deal with this problem. To improve the performance the neighbor discovery states that a node that does a link layer change shall send a few Neighbor advertisements.7.1 Redirect functionWhen a packet is transmitted to a node that appears not to be on the same like, it is sent via a router. If the chosen next hop is not an optimal one the router will transmit a redirect message to the source. This to present a better next hop alternative. The redirect message is trigged to be sent when the router forwards a packet on the same interface as it was received. When a node receives a redirection message it assumes that the node it is redirected to resides on the same link as it self. This will result in a neighbor or solicitation to get the link-layer address. This will give the nodes the ability to directly exchange messages to links on different subnets without passing through a router.8.1 Duplicate address detectionA node send a Neighbor Solicitation message to check if it’s own address is in use. If a response is received then we have a address duplication problem that the network administrator have to solve.9.1 Ipv4 comparison:Table 1.1 below states the Ipv6 Neighbor discovery functions and the corresponding IPv4 functionalities:IPv6 Neighbor Discovery Functions Similar IPv4 FunctionalityRouter Discovery ICMP Router DiscoveryPrefix Discovery DHCP or manual Configuration Parameter Discovery Manual configurationAddress resolution ARPNext hop determination Routing table searchesNeighbor Unreachability Detection No standard mechanismDuplicate Address detection No standard mechanismRedirect ICMP RedirectTable 1.1 Ipv6 Neighbor discovery functions and the corresponding IPv4 functionalities.10.1 Extensions made for mobile nodesA mobile node with its mobile behavior has different characteristics compared to regular fixed nodes. There are several additions and changes done to introduce mobility support in the neighbor discovery process. The Mobile IPv6 is still under development and just drafted yet, so what I’m describing may change in the near future.The Router Advertisment have an additional flag which makes it possible for the router to state whether it’s serving as a home agent or not. The prefix option information is extended so that the router is providing its global address and not just the link-local address. This extension is essential so that the mobile node can establish a forward service from it’s previous location. An other option has been added whichgives their router the ability to state the interval at which unsolicited Router Advertisements are sent. A second option extension is allows the home agent to advertise information such as Home agent life time and a preference value which indicates how preferable this Home agent is in case of several on the same link. To support mobile nodes movement detection (described below) the Advertisements are allowed to be sent more frequently.Router Solicitations are, when the node is not connected to its home link, for the same reason (make movement detection easier) allowed to be sent more frequently then in mobile IPv6.10.2 Movement detection using neighbor discoveryWhen a mobile node for example has moved out of range of wireless access point and is in range of another this must be detected. When this is detected the mobile node can configure a new care-of address from a new router that advertises.Some of the neighbor discovery functions are involved when a mobile node is to detect that it has moved. The entries from the nodes different caches contain information about routers and their prefixes. One of these entries defines the router that is the default router.To detect that the default router is unreachable the mobile node uses Neighbor Unreachability Detection described above. The router is also continually probed with Neighbor Solicitation messages and a failure in response can be seen, as the router is unreachable. Any received packet from the router will be taking as an acknowledgement that the router is still reachable. This can be exploit on some link when the node is set in a promiscuous mode so that all packets on the link will be received and not just those addressed to the node itself.However according to the Mobility support draft[1] any combination of movement detection that the mobile node can perform is allowed. This means that the movement detection with the neighbor discovery processes can be supplemented with for instance lower layer detection schemes. Lower layer schemes could be measuring the signal strength and when it’s to low from the current link change to another with higher, even if router advertisements still are getting thorough.11 Future – will Ipv6 be used?There are already several IPv6 networks in operation, 6bone run by NTT in Japan and MCI Worldcom in the USA. But the major driving force in introducing IPv6 is the Mobile industry. Companies as Ericsson and Nokia are running large research projects regarding Mobile IPv6, which probably will be introduced in the third generation of mobile networks, UMTS.The question of introducing IPv6 is also a regional issue. Asia has a lot higher demand for IPv4 addresses then America since they have more assigned to them. This means that USA is more reluctant to introducing IPv6 since all upgrades are costly. The transitation between IPv4 and IPv6 will not happened over a night. There are schemes like the NAT (network address translator) that will make a coexistence of IPv4 and IPv6 possible for a period. For a while we will have to cope with a dual stack system with both IPv4 and IPv6.However the lack of addresses and the mobile industries efforts will sooner or later force the Internet community to make the transitation to Mobile IPv6.12 References:[1] David B Johnson, Charles Perkins, Mobility Support in IPv6, 2 July 2001.[2]T. Narten, E. Nordmark, W. Simpson, Neighbor Discovery for IP Version 6(IPv6) RFC 2461, December 1998.[3]James D. Solomon, Mobile IP The internet unplugged, Prentice Hall PTR1998.[4][5]。