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  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)                 9  ICMP Message Types and Formats                      9  ICMP Version 6 (ICMPv6) Informational Message Types and Formats

ICMPv6 Redirect Messages 1 2 ICMPv6 Informational Message Options

One of the more interesting decisions made in IPv6 was the selection of a very large 128-bit address size. This provides an address space far larger than what humans are ever likely to need—and probably larger than was needed for IPv6, strictly speaking. What this wealth of bits provides is the flexibility to assign meaning to different bits in the address structure. This in turn serves as the basis for important features such as the autoconfiguration and automated renumbering of IPv6 addresses.

The renumbering feature in IPv6 is of particular interest to network administrators, since it has the potential to make large network migrations and merges much simpler. In August 2000, the IETF published RFC 2894, Router Renumbering for IPv6, which describes a similar technique to allow routers in an autonomous system to be renumbered, by giving them new prefixes (network identifiers).

Router renumbering is actually a fairly simple process—especially if we avoid the gory details, which is exactly what I intend to do. A network administrator uses a device on the internetwork to generate one or more Router Renumbering Command messages. These messages provide a list of prefixes of routers that are to be renumbered. Each router processes these messages to see if the addresses on any of their interfaces match the specified prefixes. If so, they change the matched prefixes to the new ones specified in the message. Additional information is also included in the Router Renumbering Command as appropriate to control how and when the renumbering is done.

If the Command message requests it, each router processing the message will respond with a Router Renumbering Result message. This message serves as feedback to let the originator of the Command know whether the renumbering was successful, and what changes, if any, were made.

The router renumbering standard also defines a few important management features. Many of these reflect the great power of something that can mass-renumber routers—and hence, the potential for such power to be abused. It is possible to send commands in a “test mode”, where they are processed but the renumbering not actually executed. Messages include a sequence number to guard against replay attacks, and a special Sequence Number Reset message can be used to reset the sequence number information routers have previously stored. For added security, the standard specifies that messages be authenticated and identity-checked.

ICMPv6 Redirect Messages 1 2 ICMPv6 Informational Message Options