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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP IPv6 Neighbor Discovery Protocol (ND)

TCP/IP IPv6 Neighbor Discovery Protocol (ND) 1 2 IPv6 ND General Operational Overview: ND Functions, Functional Groups and Message Types

The purpose of network layer protocols like the Internet Protocol is to provide a means of connecting together individual local networks to create a much larger internetwork. To higher protocol layers and to users, this internetwork behaves in most respects as if it were a single large network, because the lower layers hide the details that “glue” together the individual networks. Any device can send information to any other, regardless of where it is located, and “like magic”, it will work. At least, most of the time.

The existence of an internetwork means that devices can treat all other devices as peers, at least from the perspective of higher protocol layers and applications. From the standpoint of lower layers, however, there is a very important difference between devices that are on a host's local network and those that are elsewhere. In a general sense, most devices have a more important relationship with the devices that are on its local network than those that are far away. Some of the most obvious tasks that a device must perform specifically with other devices on its local network include:

• Direct Datagram Delivery: Devices deliver data directly to other devices on their local network, while data going to distant devices must be indirectly delivered (routed).
  
  
• Layer Two Addressing: To facilitate direct delivery, devices need to know the layer two addresses of the other devices on the local network; they don't need to know them for non-local devices.
  
  
• Router Identification: To deliver indirectly, a device needs to find a router on its local network that it can talk to.
  
  
• Router Communication: The local router must communicate information to each of the local hosts using it so they know how best to use it.
  
  
• Configuration: Hosts will usually look to information provided by local devices to let them perform configuration tasks such as determining their own IP address.

To support these and other requirements, several special protocols and functions were developed along with the original Internet Protocol (version 4). The IP addressing scheme lets devices differentiate local addresses from distant ones. The Address Resolution Protocol (ARP) lets devices determine layer two addresses from layer three addresses. The Internet Control Message Protocol (ICMP) provides a messaging system to support various communication requirements between local devices, including the ability of a host to find a local router and the router to provide information to local hosts.

These features all work properly in IPv4, but they were developed sort of in an ad hoc manner. They are defined not in a single place, but rather in a variety of different Internet standards. There were also some limitations with the way these “local device” functions were implemented.

TCP/IP IPv6 Neighbor Discovery Protocol (ND) 1 2 IPv6 ND General Operational Overview: ND Functions, Functional Groups and Message Types