Information Technology Reference
In-Depth Information
Current DNS implementations need to be able to support A (for IPv4), and AAAA re-
source records, with type A having the highest priority and AAAA the lowest.
For hosts that suppor t dual-stack (IP v4 and IP v6) the applic ation dec ides w ith stack to use
and accordingly requests an AAAA or A record. As shown in Figure 9-7 below the client
device requests the AAAA record of the destination IPv6 server. The DNS server returns
the IPv6 address. Note that this is the same DNS server that supports IPv4 addresses; no
separate DNS servers are needed for IPv6 networks.
AAAA?
IPv6
Address
Local
DNS
IPv6
Network
Client
IPv6
Server
Figure 9-7
IPv6 DNS AAAA Request
Path MTU Discovery
IPv6 does not allow packet fragmentation throughout the internetwork. Only sending
hosts are allowed to fragment. Routers are not allowed to fragment packets. RFC 2460
specifies that the MTU of every link in an IPv6 must be 1280 bytes or greater. RFC 1981
recommends that nodes should implement IPv6 path MTU discovery to determine
whether any paths are greater than 1280 bytes. ICMPv6 packet-too-big error messages de-
termine the path MTU. Nodes along the path send the ICMPv6 packet-too-big message to
the sending host if the packet is larger than the outgoing interface MTU.
Figure 9-8 shows a host sending a 2000-byte packet. Because the outgoing interface MTU
is 1500 bytes, Router A sends an ICMPv6 packet-too-big error message back to Host A.
The sending host then sends a 1500-byte packet. The outgoing interface MTU at Router B
is 1300 bytes. Router B sends an ICMPv6 packet-too-big error message to Host A. Host A
then sends the packet with 1300 bytes.
IPv6 Address-Assignment Strategies
Assignment of IPv6 addresses to a host can be static or dynamically configured. Static
IPv6 address assignment just involves manual configuration on the host's configuration
files. Dynamic IPv6 address assignment can be in one of three ways.
Stateless autoconfiguration of link-local address
■
Key
To p i c
Stateless autoconfiguration of globally unique address
■
Stateful configuration with DHCPv6
■
Link-Local Address (Stateless Autoconfiguration)
method, without DHCP. Hosts obtain their link-local addresses automatically as an
