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DHCP For IP Version 6 (DHCPv6)
(Page 2 of 2)
DHCPv6 Operation Overview
The operation of DHCPv6 is similar
to that of DHCPv4, but the protocol itself has been completely rewritten.
It is not based on the older DHCP or on BOOTP, except in conceptual
terms. It still uses UDP but uses new port numbers, a new message format,
and restructured options. All of this means that the new protocol is
not strictly compatible with DHCPv4 or BOOTP, though I believe work
is underway on a method to allow DHCPv6 servers to work with IPv4 devices.
Key Concept: Since DHCP works with IP addresses and other configuration parameters, the change from IPv4 to IPv6 requires a new version of DHCP commonly called DHCPv6. This new DHCP represents a significant change from the original DHCP, and is still under development. DHCPv6 is used for IPv6 stateful autoconfiguration; the alternative is stateless autoconfiguration, a feature of IPv6 that allows a client to determine its IP address without need for a server.
DHCPv6 is also oriented around IPv6
methods of addressing, especially the use of link-local
scoped multicast addresses. This allows
efficient communication even before a client has been assigned an IP
address. Once a client has an address and knows the identity of a server
it may communicate with the server directly using unicast addressing.
DHCP Message Exchanges
There are two basic client/server
message exchanges that are used in DHCPv6: the four-message exchange
and the two-message exchange. The former is used when a client
needs to obtain an IPv6 address and other parameters. This process is
similar to the
regular DHCP address allocation process;
highly simplified, it involves these steps:
- The client sends a multicast Solicit
message to find a DHCPv6 server and ask for a lease.
- Any server that can fulfill the client's
request responds to it with an Advertise message.
- The client chooses one of the servers
and sends a Request message to it asking to confirm the offered
address and other parameters.
- The server responds with a Reply
message to finalize the process.
There is also a shorter variation
of the four-message process above, where a client sends a Solicit
message and indicates that a server should respond back immediately
with a Reply message.
If the client already has an IP address,
either assigned manually or obtained in some other way, a simpler process
can be undertaken, similar to how
in regular DHCP the DHCPINFORM
message is used:
- The client multicasts an Information-Request
- A server with configuration information
for the client sends back a Reply message.
As in regular DHCP, a DHCPv6 client
renews its lease after a period of time by sending a Renew message.
DHCPv6 also supports relay
agent functionality as in DHCPv4.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
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