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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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9.8.2 Tunneling<br />

DNS plays a key role here. As described in Table 12-2 on page 438, a new<br />

resource record type, AAAA, is defined for mapping host names to <strong>IP</strong>v6<br />

addresses. The results of a name server lookup determine how a node will<br />

attempt to communicate with that system. The records found in the DNS for a<br />

node depend on which protocols it is running:<br />

► <strong>IP</strong>v4-only nodes only have A records containing <strong>IP</strong>v4 addresses in the DNS.<br />

► <strong>IP</strong>v6/<strong>IP</strong>v4 nodes that can interoperate with <strong>IP</strong>v4-only nodes have AAAA<br />

records containing <strong>IP</strong>v4-compatible <strong>IP</strong>v6 addresses <strong>and</strong> A records containing<br />

the equivalent <strong>IP</strong>v4 addresses.<br />

► <strong>IP</strong>v6-only nodes that cannot interoperate with <strong>IP</strong>v4-only nodes have only<br />

AAAA records containing <strong>IP</strong>v6 addresses.<br />

Because <strong>IP</strong>v6/<strong>IP</strong>v4 nodes make decisions about which protocols to use based<br />

on the information returned by the DNS, the incorporation of AAAA records in the<br />

DNS is a prerequisite to interoperability between <strong>IP</strong>v6 <strong>and</strong> <strong>IP</strong>v4 systems. Note<br />

that name servers do not necessarily need to use an <strong>IP</strong>v6-capable protocol<br />

stack, but they must support the additional record type.<br />

When <strong>IP</strong>v6 or <strong>IP</strong>v6/<strong>IP</strong>v4 systems are separated from other similar systems with<br />

which they want to communicate by older <strong>IP</strong>v4 networks, <strong>IP</strong>v6 packets must be<br />

tunneled through the <strong>IP</strong>v4 network.<br />

<strong>IP</strong>v6 packets are tunnelled over <strong>IP</strong>v4 very simply: The <strong>IP</strong>v6 packet is<br />

encapsulated in an <strong>IP</strong>v4 datagram, or in other words, a complete <strong>IP</strong>v4 header is<br />

added to the <strong>IP</strong>v6 packet. The presence of the <strong>IP</strong>v6 packet within the <strong>IP</strong>v4<br />

datagram is indicated by a protocol value of 41 in the <strong>IP</strong>v4 header.<br />

There are two kinds of tunneling of <strong>IP</strong>v6 packets over <strong>IP</strong>v4 networks: automatic<br />

<strong>and</strong> configured.<br />

Automatic tunneling<br />

Automatic tunneling relies on <strong>IP</strong>v4-compatible addresses. The decision of when<br />

to tunnel is made by an <strong>IP</strong>v6/<strong>IP</strong>v4 host that has a packet to send across an<br />

<strong>IP</strong>v4-routed network area, <strong>and</strong> it follows the following rules:<br />

► If the destination is an <strong>IP</strong>v4 or an <strong>IP</strong>v4-mapped address, send the packet<br />

using <strong>IP</strong>v4 because the recipient is not <strong>IP</strong>v6-capable. Otherwise, if the<br />

destination is on the same subnet, send it using <strong>IP</strong>v6, because the recipient is<br />

<strong>IP</strong>v6-capable.<br />

Chapter 9. <strong>IP</strong> version 6 381

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