Mobile IPv6 A.01.00 Administrator's Guide

Chapter 1

Introducing HP-UX Mobile IPv6

Dynamic Home Agent Address Discovery

When attached to a Foreign Network, a Mobile Node might not know the address of its Home Agent. For

example, a router operating as the Mobile Node's Home Agent might get reconﬁgured or replaced by a

different node serving as a Home Agent on the home network.

With Dynamic Home Agent Address Discovery (DHAAD), a Mobile Node only needs a home network preﬁx

conﬁgured and it can dynamically ﬁnd the address of a Home Agent on its home network when it needs to

The following is an overview of how DHAAD works:

• Each Home Agent listens to Router Advertisement messages sent by the other Home Agents, and records

the addresses of these Home Agents.

• The Home Agents listen for Dynamic Home Agent Address Discovery requests (or DHAAD requests) from

the Mobile Nodes on a special IPv6 subnet anycast address for the home network, the Mobile IPv6

Home-Agent’s anycast address (seven-bit anycast identiﬁer: 0x7E).

• The Mobile Node attaches to a foreign network and needs to send a Binding Update to a Home Agent on

its home network, but does not have an address for a Home Agent.

• The Mobile Node sends an ICMPv6 DHAAD request message to the Mobile IPv6 Home-Agents subnet

anycast address for its home network.

• The Home Agents reply to the Mobile Node with ICMPv6 DHAAD response messages that include a list of

global unicast addresses for the Home Agents on the home network. The response includes Home Agent

addresses based-on their Home Agent preference level. For each Home Agent, the

AdvHomeAgentPreference parameter in the Router Advertisement Daemon conﬁguration ﬁle

(/etc/rtradvd.conf by default) determines the preference level for each Home Agent. Refer to “The

Router Advertisement Daemon Conﬁguration File: /etc/rtradvd.conf” on page 39 for more information.

• The Mobile Node then sends a Binding Update with its Care-of Address to a Home Agent on the home

network. The Mobile Node determines which Home Agent to send its Binding Update to based on the

Home Agent preference value included in the Dynamic Home Agent Address Discovery response message.

If the Mobile Node does not receive a response, it might send a Binding Update to another Home Agent.