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IPv6 ND Functions Compared to Equivalent IPv4 Functions
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
- 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
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.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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