IPv6 ND Overview, History, Motivation and Standards
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Formalizing Local Network Functions: The Concept of Neighbors
IP version 6 represents the biggest change in decades to not just the Internet Protocol itself, but the entire TCP/IP suite. It thus provided an ideal opportunity to formalize and integrate the many disparate functions and tasks related to communication between local devices. The result was the creation of a new protocol: Neighbor Discovery for IP Version 6, also commonly called the IPv6 Neighbor Discovery protocol. Since this protocol is new in version 6, there is no IPv4 version of it, so the name is usually just seen as the Neighbor Discovery (ND) protocol with no further qualifications; its use with IPv6 is implied.
The term neighbor is one that has been used for years in various networking standards and technologies to refer to devices that are local to each other. In the context of our current discussion, two devices are neighbors if they are on the same local network, meaning that they can send information to each other directly; the term can refer to either a regular host or a router. I think this is a good analogy to the way humans refer to those who live or work nearby. Just as most of us have a special relationship with people who are our neighbors and communicate more with them than with those who are far away, so do IP devices.
Since a neighbor is a local device, the name of the Neighbor Discovery protocol would seem to indicate that ND is all about how neighbors discover each other's existence. In the context of this protocol, however, the term discovery has a much more generic meaning: it refers to discovering not just who our neighbors are but also important information about them. In addition to letting devices identify their neighbors, ND facilitates all the tasks in the bullet list above, including such functions as address resolution, parameter communication, autoconfiguration and much more as we will see in the next few topics.
The Neighbor Discovery protocol was originally defined in RFC 1970, published in August 1996, and revised in the current defining standard, RFC 2461, published December 1998. Most of the functions of the ND protocol are implemented using a set of five special ICMPv6 control messages. Thus, to some extent, the operation of ND is partially described by the ICMPv6 standard, RFC 2463. Where ICMPv4 can be considered IPv4's administrative assistant, IPv6 really has two such assistants working closely together: ICMPv6 and the IPv6 ND protocol. I discuss more of the differences between the ways IPv4 and IPv6 implement ND's functions later in this section.
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