IPv6 ND Host-Host Communication Functions: Address Resolution, Next-Hop Determination, Neighbor Unreachability Detection and Duplicate Address Detection
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The delivery of datagrams in IP can be divided into two methods: direct and indirect. Indirect datagram delivery requires that routers provide help to hosts, which leads to the host-router discovery functions we examined in the previous topic. Direct delivery of datagrams is performed from one host to another on the same network. This doesn't require the use of routers, but necessitates other IPv6 ND protocol functions that involve Next-Hop Determination communication directly between local hosts.
The first task that any host must perform when it wants to send a datagram is Next-Hop Determination. This is the process by which a device looks at the destination address in a datagram and decides whether direct or indirect delivery are required. In early IPv4 this was done by looking at the class of the address, and later on, by using the subnet mask. In IPv6, the prefix information obtained from local routers is compared to the destination of the datagram to determine if the destination device is local or distant. If it is local, the next hop is the same as the destination address; if it is non-local, the next hop is chosen from the device's list of local routers (which are determined either by manual configuration or using the host-router discovery features of ND.)
For efficiency purposes, hosts do not perform this next-hop determination for each and every datagram. They maintain a destination cache that contains information about what the next hop should be for recent devices to which datagrams have been sent. Each time a next hop determination is performed for a particular destination, information from that determination is entered into the cache, so it can be used the next time datagrams are sent to that device.
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