[support] Why does HA do ndisc_do_dad action?

[tony chen]

I find some desciption in the book "Mobile inter-networking with IPv6"

I don't know whether the information is meaningful or pointless.

7.3 Binding Update
7.3.1 Home Agent Actions for Receiving Binding Updates
7.3.1.3 Intercepting Packets Addressed to the Mobile Node
The result of arranging the proper care-of address in the Binding Cache
entry for the mobile node is to enable tunneling packets to the mobile node,
using IPv6-within-IPv6 encapsulation.In order to get those packets in the
first place, the home agent has to intercept them on the home network.
Typically, the home agent does that by causing its own MAC address to be
associated with the home address of the mobile node in the Neighbor Cache
entries of all nodes on the home network that maintain such entries for the
mobile node.

In IPv6, the process of resolving an IP address to a local MAC address is
done by the process of Neighbor Discovery [8], which we studied in Chapter
2. The two relevant messages here are Neighbor Solicitation and Neighbor
Advertisement.Typically, a node locally broadcasts a Neighbor Solicitation
in order to acquire the layer-2 address for some destination of interest on
the local ne1:work. The target node then responds with a Neighbor
Advertisement message.

However, the home agent needs to generate the correct association between
its own layer-2 address and the mobile node’s IPv6 home address, even though
it does not receive a Neighbor Solicitation message corresponding to the
home address of the mobile node. For this purpose, the home agent sends a
Neighbor Advertisement message to every node on the home network, by using
the link-local all-nodes multicast address which is FF02:: I (see Section
2.4).

The Neighbor Advertisement multicast is constructed as follows:
 The Source IP address of the IPv6 header is set to the IPv6 address of the
home agent.
 The Destination IP address of the IPv6 header is set the all-nodes
link-local address.
 The Target Address field of the Neighbor Advertisement is set to the IPv6
home address of the mobile node.
 The home agent’s layer-2 address is located in the Target Link-Layer
Address option of the Advertisement message so that it will be associated
with the mobile node’s IPv6 address
 The ’R’ bit of the Advertisement is set to zero, since the home agent is
not advertising that the mobile node is a router.
 The ’0’ bit of the Advertisement is set, because the home agent is
instructing every recipient of the multicast message to override ,any
existing Neighbor Cache entry that may already be present for the mobile
node.
 the ’S’ bit is set to zero, since the multicast message was not in response
to any Neighbor Solicitation message (in other words, the multicast message
is unsolicited).

If the home agent receives a Neighbor Solicitation message for the mobile
node, it responds with an appropriately constructed Neighbor Advertisement,
with the following differences from the multicast message just described:
The ’S’ bit is set.
The Destination IPv6 address in the IPv6 header is the a.ddress of the
soliciting node.

As just described, the home agent utilizes Neighbor Discovery protocol
messages to enable its interception of packets that are addressed to the
mobile node. These same messages are also used for the process of address
autoconfiguration in IPv6 (see RFC 2460 [3] for details). The basic idea for
stateless address autoconfiguration in
IPv6 is that an IPv6 node can construct a candidate address for its own use;
before committing that address for use as its endpoint identifier, the node
has to make sure that it is not already in use. The way to do this is to
issue a Neighbor Solicitation for that address. If any other node is already
using the candidate address, the node will then receive a Neighbor
Advertisement from the node already claiming the address; then another
candidate address formed and the process repeated.

We have seen earlier in Chapter 2 that this process of detecting whether a
candidate address is already in use is called Duplicate Address Detection
(DAD). It has received much attention in the context of use by mobile nodes,
as we will discuss in several other parts of this book.

2009/7/31 Arnaud Ebalard <arno at natisbad.org>

> Hi,
>
> tony chen <aoc90058 at gmail.com> writes:
>
> > Thank you ,
> > so the HA perform the DAD action is in order to check the HoA is not
> > already used on the link again.
>
> Yes. One would say that this is pointless in most cases and only creates
> a 1 sec latency. I haven't check the spec on that aspect to see if it is
> required.
>
> Just out of curiosity, does the behavior bothers in some way?
>
> Cheers,
>
> a+
>
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