What

Detect if a certain address is already in use
Only need to check within subnet, since it either concerns a link-local address, or a global scope address, but only your subnet uses the prefix provided to/by your router

Why

To prevent duplicate addresses caused by:

• Duplicate MAC
• Duplicate random output
• Manual config errors

Even with a DHCPv6 server, IPv6 still mandates the use of DAD as a backup

Concepts

• Tentative address
  • The address currently being checked for use
  • Cannot yet be used for normal traffic
• Neighbour Solicitation (NS): ICMPv6 message asking if the address is in use
• Neighbour Advertisement (NA): ICMPv6 message indicating that the address is in use
• Unspecified source: for the NS, the source :: is used, since the source does not yet have a usable address

How

1. Generate address, as specified in SLAAC
2. Based on the last 24 bits (last 1.5 segments), join the appropriate solicited-node multicast group
   1. E.g. 2001:db8:1::a4b3:8f21:c9d2:1e5f -> d2:1e5f, join multicast group ff02::1:ffd2:1e5f (all nodes group + last bits)
3. In said group send a Neighbour Solicitation with
   1. Source address: ::, since there is no usable address yet
   2. Destination: multicast group address, e.g. ff02::1:ffd2:1e5f
   3. Target address: the address being tested, e.g. 2001:db8:1::a4b3:8f21:c9d2:1e5f
4. Wait for response
   1. Duration set by RetransTimer
5. No response received: address is free and can be used
   1. Transition from tentative to preferred
6. Response received: address is in use
   1. If using SLAAC, the device can try again
   2. If manually configured, manual steps are needed