DSTM (Dual Stack Transition Mechanism) [Bou05] is a tunnelling solution for IPv6-only networks, where IPv4 applications are still needed on dual-stack hosts within an IPv6-only infrastructure. IPv4 traffic is tunnelled over the IPv6-only domain until it reaches an IPv6/IPv4 gateway, which is in charge of packet encapsulation/decapsulation and forwarding between the IPv6-only and IPv4-only domains. The solution proposed by DSTM is transparent to any type of IPv4 application and allows the use of layer 3 security.

Usually with a tunnelling scheme, one IPv4 address is required for every host wishing to connect to the IPv4 Internet. DSTM reduces this constraint by dynamically allocating addresses only for the duration of the communication making it possible for several hosts to share the same address on a large time scale.

DSTM can be implemented if a network infrastructure only supports IPv6, but some of the nodes on the network have dual-stack capability (and make use of IPv4-only applications). DSTM consists of three components:

1. a DSTM server,
2. a DSTM gateway or TEP (Tunnel End Point) and
3. a dual-stack host (called a “DSTM client”) wishing to communicate using IPv4.

For the sake of simplicity, we have decided to present the server and the gateway as different equipment, but in actual deployments, these two functionalities are co-located on the same equipment.

As long as communications can take place in native IPv6, none of the capacities of DSTM are required. This applies to protocols like HTTP or SMTP, where the use of ALGs (Application Level Gateways) is to be preferred. When a DSTM node detects the need of an IPv4 address by a query to the DNS resulting in an IPv4 destination address or an application opening an IPv4 socket, the DSTM process is launched.

When the first IPv4 packet needs to be sent, the DSTM client asks the server for an address (step 1 in Figure 5-4). A number of protocols (DHCPv6 [RFC3315], TSP [BP02], RPC) have already been proposed to perform this task. Native IPv6 transport is the only restriction in this matter. DSTM developers are currently discussing whether a single method can be agreed for all DSTM implementations.