ルーティング面でのIPv6移行（配布と概略）

   This document gives an overview of the routing aspects of IPv4 to
   IPv6 transition. The approach outlined here is designed to be
   compatible with the existing mechanisms for IPv6 transition [1].

   During an extended IPv4-to-IPv6 transition period, IPv6-based systems
   must coexist with the installed base of IPv4 systems. In such a dual
   internetworking protocol environment, both IPv4 and IPv6 routing
   infrastructure will be present. Initially, deployed IPv6-capable
   domains might not be globally interconnected via IPv6-capable
   internet infrastructure and therefore may need to communicate across
   IPv4-only routing regions. In order to achieve dynamic routing in
   such a mixed environment, there need to be mechanisms to globally
   distribute IPv6 network layer reachability information between
   dispersed IPv6 routing regions. The same techniques can be used in
   later stages of IPv4-to-IPv6 transition to route IPv4 packets between
   isolated IPv4-only routing region over IPv6 infrastructure.

   The IPng transition provides a dual-IP-layer transition, augmented by
   use of encapsulation where necessary and appropriate. Routing issues
   related to this transition include:

   (1) Routing for IPv4 packets

   (2) Routing for IPv6 packets
           (2a) IPv6 packets with IPv6-native addresses
           (2b) IPv6 packets with IPv4-compatible addresses

   (3) Operation of manually configured static tunnels

   (4) Operation of automatic encapsulation
           (4a) Locating encapsulators
           (4b) Ensuring that routing is consist with
               encapsulation

   Basic mechanisms required to accomplish these goals include: (i)
   Dual-IP-layer Route Computation; (ii) Manual configuration of point-
   to-point tunnels; and (iii) Route leaking to support automatic
   encapsulation.

   The basic mechanism for routing of IPv4 and IPv6 involves dual-IP-
   layer routing. This implies that routes are separately calculated for
   IPv4 addresses and for IPv6 addressing. This is discussed in more
   detail in section 3.1.

   Tunnels (either IPv4 over IPv6, or IPv6 over IPv4) may be manually
   configured. For example, in the early stages of transition this may
   be used to allow two IPv6 domains to interact over an IPv4
   infrastructure. Manually configured static tunnels are treated as if
   they were a normal data link. This is discussed in more detail in
   section 3.2.

   Use of automatic encapsulation, where the IPv4 tunnel endpoint
   address is determined from the IPv4 address embedded in the IPv4-
   compatible destination address of IPv6 packet, requires consistency
   of routes between IPv4 and IPv6 routing domains for destinations
   using IPv4-compatible addresses. For example, consider a packet which
   starts off as an IPv6 packet, but then is encapsulated in an IPv4
   packet in the middle of its path from source to destination. This
   packet must locate an encapsulator at the correct part of its path.
   Also, this packet has to follow a consistent route for the entire
   path from source to destination. This is discussed in more detail in
   section 3.3.

   The mechanisms for tunneling IPv6 over IPv4 are defined in the
   transition mechanisms specification [1].