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https://www.7key.jp/rfc/2185/rfc2185_3.html#source
https://www.7key.jp/rfc/2185/rfc2185_3.html#translation
In the basic dual-IP-layer transition scheme, routers may independently support IPv4 and IPv6 routing. Other parts of the transition, such as DNS support, and selection by the source host of which packet format to transmit (IPv4 or IPv6) are discussed in [1]. Forwarding of IPv4 packets is based on routes learned through running IPv4-specific routing protocols. Similarly, forwarding of IPv6 packets (including IPv6-packets with IPv4-compatible addresses) is based on routes learned through running IPv6-specific routing protocols. This implies that separate instances of routing protocols are used for IPv4 and for IPv6 (although note that this could consist of two instances of OSPF and/or two instances of RIP, since both OSPF and RIP are capable of supporting both IPv4 and IPv6 routing).
A minor enhancement would be to use an single instance of an integrated routing protocol to support routing for both IPv4 and IPv6. At the time that this is written there is no protocol which has yet been enhanced to support this. This minor enhancement does not change the basic dual-IP-layer nature of the transition.
For initial testing of IPv6 with IPv4-compatible addresses, it may be useful to allow forwarding of IPv6 packets without running any IPv6- compatible routing protocol. In this case, a dual (IPv4 and IPv6) router could run routing protocols for IPv4 only. It then forwards IPv4 packets based on routes learned from IPv4 routing protocols. Also, it forwards IPv6 packets with an IPv4-compatible destination address based on the route for the associated IPv4 address. There are a couple of drawbacks with this approach: (i) It does not specifically allow for routing of IPv6 packets via IPv6-capable routers while avoiding and routing around IPv4-only routers; (ii) It does not produce routes for "non-compatible" IPv6 addresses. With this method the routing protocol does not tell the router whether neighboring routers are IPv6-compatible. However, neighbor discovery may be used to determine this. Then if an IPv6 packet needs to be forwarded to an IPv4-only router it can be encapsulated to the destination host.
Tunneling techniques are already widely deployed for bridging non-IP network layer protocols (e.g. AppleTalk, CLNP, IPX) over IPv4 routed infrastructure. IPv4 tunneling is an encapsulation of arbitrary packets inside IPv4 datagrams that are forwarded over IPv4 infrastructure between tunnel endpoints. For a tunneled protocol, a tunnel appears as a single-hop link (i.e. routers that establish a tunnel over a network layer infrastructure can inter-operate over the tunnel as if it were a one-hop, point-to-point link). Once a tunnel is established, routers at the tunnel endpoints can establish routing adjacencies and exchange routing information. Describing the protocols for performing encapsulation is outside the scope of this paper (see [1]). Static point-to-point tunnels may also be established between a host and a router, or between two hosts. Again, each manually configured point-to-point tunnel is treated as if it was a simple point-to-point link.
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