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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)

IPv6 ND General Operational Overview: ND Functions, Functional Groups and Message Types 1 IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration

The IPv6 Neighbor Discovery protocol has the distinction of being the only truly new protocol created as part of the core of Internet Protocol version 6; there is no “NDv4” at all. Of course, most of the services that ND provides to IPv6 were also required in version 4 of the Internet Protocol. They were just provided in a rather diverse set of protocols and standards that the ND protocol has formalized, integrated and improved.

What this means is that while ND is new, the jobs it does are equivalent to the tasks performed by several other protocols in IPv4. Specifically, the bulk of ND functions correspond to the following set of standards, features and message types in IPv4:

• ICMPv4 Router Discovery: Most of the functions associated with identifying and obtaining information from routers in ND are based on the use of ICMPv4 Router Advertisement and Router Solicitation messages as defined in RFC 1256.
  
  
• Address Resolution Protocol: ND provides enhanced address resolution capabilities that are similar to the functions provided in IPv4 by ARP.
  
  
• ICMPv4 Redirect: ND's redirect function and Redirect messages are based on similar functionality defined in IPv4 and ICMPv4.

Of course, there are other aspects of ND that only somewhat correlate to how things work in IPv4. There are also improvements or new functionality compared to how these IPv4 functions work. Some of these are in fact due to differences in how IPv6 itself operates compared to IPv4. For example, prefix discovery in ND is sort of related to the Address Mask Request and Address Mask Reply messaging in ICMPv4.

Overall, ND represents a substantial improvement compared to the way its job was done in IP version 4. Like IPv6 itself, ND is generally better suited to the needs of modern networks than the older protocols. Some of the more important specific improvements made in ND compared to how its job was done in IPv4 include the following:

• Formalizing Of Router Discovery: In IPv4 the process of router discovery and solicitation was arguably an “afterthought”; ND formalizes this process and makes it part of the core of the TCP/IP protocol suite.
  
  
• Formalizing Of Address Resolution: In a similar manner, address resolution is handled in a superior way in ND. ND functions at layer three and is tightly tied to IP just like ICMP is. There is no more need for an “ambiguously-layered” protocol like ARP, whose implementation is very dependent on the underlying physical and data link layers.
  
  
• Ability To Perform Functions Securely: ND operates at the network layer, so it can make use of the authentication and encryption capabilities of IPSec for tasks such as address resolution or router discovery.
  
  
• Autoconfiguration: In combination with features built into IPv6, ND allows many devices to automatically configure themselves even without the need for something like a DHCP server (though DHCPv6 does also exist.)
  
  
• Dynamic Router Selection: Devices use ND to detect if neighbors are reachable or not. If a device is using a router that stops being reachable it will detect this and automatically switch to another one.
  
  
• Multicast-Based Address Resolution: Address resolution is performed using special multicast addresses instead of broadcasts, reducing unnecessary disruption of “innocent bystanders” when resolution messages must be sent.
  
  
• Better Redirection: Improvements have been made to the method by which redirects are generated and used.

IPv6 ND General Operational Overview: ND Functions, Functional Groups and Message Types 1 IPv6 ND Host-Router Discovery Functions: Router Discovery, Prefix Discovery, Parameter Discovery and Address Autoconfiguration