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+Network Working Group                                          R. Callon
+Request for Comments: 2185                    Cascade Communications Co.
+Category: Informational                                        D. Haskin
+                                                       Bay Networks Inc.
+                                                          September 1997
+
+
+                   Routing Aspects Of IPv6 Transition
+
+Status of this memo
+
+   This memo provides information for the Internet community.  This memo
+   does not specify an Internet standard of any kind.  Distribution of
+   this memo is unlimited.
+
+Abstract
+
+   This document gives an overview of the routing aspects of the IPv6
+   transition.  It is based on the protocols defined in the document
+   "Transition Mechanisms for IPv6 Hosts and Routers" [1].  Readers
+   should be familiar with the transition mechanisms before reading this
+   document.
+
+   The proposals contained in this document are based on the work of the
+   Ngtrans working group.
+
+1. TERMINOLOGY
+
+   This paper uses the following terminology:
+
+   node      - a protocol module that implements IPv4 or IPv6.
+
+   router    - a node that forwards packets not explicitly
+               addressed to itself.
+
+   host      - any node that is not a router.
+
+   border router - a router that forwards packets across
+               routing domain boundaries.
+
+   link      - a communication facility or medium over which
+               nodes can communicate at the link layer, i.e., the layer
+               immediately below internet layer.
+
+   interface - a node's attachment to a link.
+
+   address   - an network layer identifier for an interface or
+               a group of interfaces.
+
+
+
+Callon & Haskin              Informational                      [Page 1]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   neighbors - nodes attached to the same link.
+
+   routing domain - a collection of routers which coordinate
+               routing knowledge using a single routing protocol.
+
+   routing region (or just "region")  - a collection of routers
+               interconnected by a single internet protocol (e.g. IPv6)
+               and coordinating their routing knowledge using routing
+               protocols from a single internet protocol stack. A
+               routing region may be a superset of a routing domain.
+
+   tunneling  - encapsulation of protocol A within protocol B,
+               such that A treats B as though it were a datalink layer.
+
+   reachability information - information describing the set of
+               reachable destinations that can be used for packet
+               forwarding decisions.
+
+   routing information - same as reachability information.
+
+   address prefix - the high-order bits in an address.
+
+   routing prefix - address prefix that expresses destinations
+               which have addresses with the matching address prefixes.
+               It is used by routers to advertise what systems they are
+               capable of reaching.
+
+   route leaking - advertisement of network layer reachability
+               information across routing region boundaries.
+
+2. ISSUES AND OUTLINE
+
+   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.
+
+
+
+Callon & Haskin              Informational                      [Page 2]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   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].
+
+
+
+Callon & Haskin              Informational                      [Page 3]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+3. MORE DETAIL OF BASIC APPROACHES
+
+3.1 Basic Dual-IP-layer Operation
+
+   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.
+
+3.2 Manually Configured Static Tunnels
+
+   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
+
+
+
+Callon & Haskin              Informational                      [Page 4]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   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.
+
+3.3  Automatic Tunnels
+
+   Automatic tunneling may be used when both the sending and destination
+   nodes are connected by IPv4 routing.  In order for automatic
+   tunneling to work, both nodes must be assigned IPv4-compatible IPv6
+   addresses.  Automatic tunneling can be especially useful where either
+   source or destination hosts (or both) do not have any adjacent IPv6-
+   capable router.  Note that by "adjacent router", this includes
+   routers which are logically adjacent by virtue of a manually
+   configured point-to-point tunnel (which is treated as if it is a
+   simple point-to-point link).
+
+   With automatic tunneling, the resulting IPv4 packet is forwarded by
+   IPv4 routers as a normal IPv4 packet, using IPv4 routes learned from
+   routing protocols. There are therefore no special issues related to
+   IPv4 routing in this case. There are however routing issues relating
+   to how IPv6 routing works in a manner which is compatible with
+   automatic tunneling, and how tunnel endpoint addresses are selected
+   during the encapsulation process.  Automatic tunneling is useful from
+   a source host to the destination host, from a source host to a
+   router, and from a router to the destination host. Mechanisms for
+   automatic tunneling from a router to another router are not currently
+   defined.
+
+3.3.1 Host to Host Automatic Tunneling
+
+   If both source and destination hosts make use of IPv4-compatible IPv6
+   addresses, then it is possible for automatic tunneling to be used for
+   the entire path from the source host to the destination host. In this
+   case, the IPv6 packet is encapsulated in an IPv4 packet by the source
+   host, and is forwarded by routers as an IPv4 packet all the way to
+   the destination host. This allows initial deployment of IPv6-capable
+   hosts to be done prior to the update of any routers.
+
+
+
+
+
+
+
+
+Callon & Haskin              Informational                      [Page 5]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   A source host may make use of Host to Host automatic tunneling
+   provided that the following are both true:
+
+     - the source address is an IPv4-compatible IPv6 address.
+     - the destination address is an IPv4-compatible IPv6 address.
+     - the source host does know of one or more neighboring IPv4-
+       capable routers, or the source and destination are on the
+       same subnet.
+
+   If all of these requirements are true, then the source host may
+   encapsulate the IPv6 packet in an IPv4 packet, using a source IPv4
+   address which is extracted from the associated source IPv6 address,
+   and using a destination IPv4 address which is extracted from the
+   associated destination IPv6 address.
+
+   Where host to host automatic tunneling is used, the packet is
+   forwarded as a normal IPv4 packet for its entire path, and is
+   decapsulated (i.e., the IPv4 header is removed) only by the
+   destination host.
+
+3.3.2 Host to Router Configured Default Tunneling
+
+   In some cases "configured default" tunneling may be used to
+   encapsulate the IPv6 packet for transmission from the source host to
+   an IPv6-backbone. However, this requires that the source host be
+   configured with an IPv4 address to use for tunneling to the backbone.
+
+   Configured default tunneling is particularly useful if the source
+   host does not know of any local IPv6-capable router (implying that
+   the packet cannot be forwarded as a normal IPv6 packet directly over
+   the link layer), and when the destination host does not have an
+   IPv4-compatible IPv6 address (implying that host to host tunneling
+   cannot be used).
+
+   Host to router configured default tunneling may optionally also be
+   used even when the host does know of a local IPv6 router. In this
+   case it is a policy decision whether the host prefers to send a
+   native IPv6 packet to the IPv6-capable router or prefers to send an
+   encapsulated packet to the configured tunnel endpoint.
+
+   Similarly host to router default configured tunneling may be used
+   even when the destination address is an IPv4-compatible IPv6 address.
+   In this case for example a policy decision may be made to prefer
+   tunneling for part of the path and native IPv6 for part of the path,
+   or alternatively to use tunneling for the entire path from source
+   host to destination host.
+
+
+
+
+
+Callon & Haskin              Informational                      [Page 6]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   A source host may make use of host to router configured default
+   tunneling provided that ALL of the following are true:
+
+     - the source address is an IPv4-compatible IPv6 address.
+     - the source host does know of one or more neighboring IPv4-
+       capable routers
+     - the source host has been configured with an IPv4 address of
+       an dual router which can serve as the tunnel endpoint.
+
+   If all of these requirements are true, then the source host may
+   encapsulate the IPv6 packet in an IPv4 packet, using a source IPv4
+   address which is extracted from the associated source IPv6 address,
+   and using a destination IPv4 address which corresponds to the
+   configured address of the dual router which is serving as the tunnel
+   endpoint.
+
+   When host to router configured default tunneling is used, the packet
+   is forwarded as a normal IPv4 packet from the source host to the dual
+   router serving as tunnel endpoint, is decapsulated by the dual
+   router, and is then forwarded as a normal IPv6 packet by the tunnel
+   endpoint.
+
+3.3.2.1 Routing to the Endpoint for the Configured Default Tunnel
+
+   The dual router which is serving as the end point of the host to
+   router configured default tunnel must advertise reachability into
+   IPv4 routing sufficient to cause the encapsulated packet to be
+   forwarded to it.
+
+   The simplest approach is for a single IPv4 address to be assigned for
+   use as a tunnel endpoint.  One or more dual routers,  which have
+   connectivity to the IPv6 backbone and which are capable of serving as
+   tunnel endpoint,  advertise a host route to this address into IPv4
+   routing in the IPv4-only region.  Each dual host in the associated
+   IPv4-only region is configured with the address of this tunnel
+   endpoint and selects a route to this address for forwarding
+   encapsulated packet to a tunnel end point  (for example, the nearest
+   tunnel end point, based on whatever metric(s) the local routing
+   protocol is using).
+
+   Finally, in some cases there may be some reason for specific hosts to
+   prefer one of several tunnel endpoints, while allowing all potential
+   tunnel endpoints to serve as backups in case the preferred endpoint
+   is not reachable. In this case, each dual router with IPv6 backbone
+   connectivity which is serving as potential tunnel endpoint is given a
+   unique IPv4 address taken from a single IPv4 address block (where the
+   IPv4 address block is assigned either to the organization
+   administering the IPv4-only region, or to the organization
+
+
+
+Callon & Haskin              Informational                      [Page 7]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   administering the local part of the IPv6 backbone). In the likely
+   case that there are much less than 250 such dual routers serving as
+   tunnel endpoints, we suggest using multiple IPv4 addresses selected
+   from a single 24-bit IPv4 address prefix for this purpose. Each dual
+   router then advertises two routes into the IPv4 region: A host route
+   corresponding to the tunnel endpoint address specifically assigned to
+   it, and also a standard (prefix) route to the associated IPv4 address
+   block. Each dual host in the IPv4-only region is configured with a
+   tunnel endpoint address which corresponds to the preferred tunnel
+   endpoint for it to use. If the associated dual router is operating,
+   then the packet will be delivered to it based upon the host route
+   that it is advertising into the IPv4-only region. However, if the
+   associated dual router is down, but some other dual router serving as
+   a potential tunnel endpoint is operating, then the packet will be
+   delivered to the nearest operating tunnel endpoint.
+
+3.3.3 Router to Host Automatic Tunneling
+
+   In some cases the source host may have direct connectivity to one or
+   more IPv6-capable routers,  but the destination host might not have
+   direct connectivity to any IPv6-capable router. In this case,
+   provided that the destination host has an IPv4-compatible IPv6
+   address, normal IPv6 forwarding may be used for part of the packet's
+   path, and router to host tunneling may be used to get the packet from
+   an encapsulating dual router to the destination host.
+
+   In this case, the hard part is the IPv6 routing required to deliver
+   the IPv6 packet from the source host to the encapsulating router. For
+   this to happen, the encapsulating router has to advertise
+   reachability for the appropriate IPv4-compatible IPv6 addresses into
+   the IPv6 routing region.  With this approach, all IPv6 packets
+   (including those with IPv4-compatible addresses) are routed using
+   routes calculated  from native IPv6 routing. This implies that
+   encapsulating routers need to advertise into IPv6 routing specific
+   route entries corresponding to any IPv4-compatible IPv6 addresses
+   that belong to dual hosts which can be reached in an neighboring
+   IPv4-only region. This requires manual configuration of the
+   encapsulating routers to control which routes are to be injected into
+   IPv6 routing protocols.  Nodes in the IPv6 routing region would use
+   such a route to forward IPv6 packets along the routed path toward the
+   router that injected (leaked) the route, at which point packets are
+   encapsulated and forwarded to the destination host using normal IPv4
+   routing.
+
+   Depending upon the extent of the IPv4-only and dual routing regions,
+   the leaking of routes may be relatively simple or may be more
+   complex.  For example, consider a dual Internet backbone, connected
+   via one or two dual routers to an IPv4-only stub routing domain. In
+
+
+
+Callon & Haskin              Informational                      [Page 8]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   this case, it is likely that there is already one summary address
+   prefix which is being advertised into the Internet backbone in order
+   to summarize IPv4 reachability to the stub domain.  In such a case,
+   the border routers would be configured to announce the IPv4 address
+   prefix into the IPv4 routing within the backbone, and also announce
+   the corresponding IPv4-compatible IPv6 address prefix into IPv6
+   routing within the backbone.
+
+   A more difficult case involves the border between a major Internet
+   backbone which is IPv4-only, and a major Internet backbone which
+   supports both IPv4 and IPv6. In this case, it requires that either
+   (i) the entire IPv4 routing table be fed into IPv6 routing in the
+   dual routing domain (implying a doubling of the size of the routing
+   tables in the dual domain); or (ii) Manual configuration is required
+   to determine which of the addresses contained in the Internet routing
+   table include one or more IPv6-capable systems, and only these
+   addresses be advertised into IPv6 routing in the dual domain.
+
+3.3.4 Example of How Automatic Tunnels May be Combined
+
+   Clearly tunneling is useful only if communication can be achieved in
+   both directions. However, different forms of tunneling may be used in
+   each direction, depending upon the local environment, the form of
+   address of the two hosts which are exchanging IPv6 packets, and the
+   policies in use.
+
+   Table 1 summarizes the form of tunneling that will result given each
+   possible combination of host capabilities, and given one possible set
+   of policy decisions. This table is derived directly from the
+   requirements for automatic tunneling discussed above.
+
+   The example in table 1 uses a specific set of policy decisions: It is
+   assumed in table 1 that the source host will transmit a native IPv6
+   where possible in preference over encapsulation. It is also assumed
+   that where tunneling is needed, host to host tunneling will be
+   preferred over host to router tunneling. Other combinations are
+   therefore possible if other policies are used.
+
+   Due to a specific policy choice, the default sending rules in [1] may
+   not be followed.
+
+   Note that IPv6-capable hosts which do not have any local IPv6 router
+   must be given an IPv4-compatible v6 address in order to make use of
+   their IPv6 capabilities. Thus, there are no entries for IPv6-capable
+   hosts which have an incompatible IPv6 address and which also do not
+   have any connectivity to any local IPv6 router. In fact, such hosts
+   could communicate with other IPv6 hosts on the same local network
+   without the use of a router.  However, since this document focuses on
+
+
+
+Callon & Haskin              Informational                      [Page 9]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   routing and router implications of IPv6 transition, direct
+   communication between two hosts on the same local network without any
+   intervening router is outside the scope of this document.
+
+   Also, table 1 does not consider manually configured point-to-point
+   tunnels.  Such tunnels are treated as if they were normal point-to-
+   point links. Thus any two IPv6-capable devices which have a manually
+   configured tunnel between them may be considered to be directly
+   connected.
+
+  -----------------+------------------+--------------------------
+  Host A           | Host B           | Result
+  -----------------+------------------+--------------------------
+  v4-compat. addr. | v4-compat. addr. | host to host tunneling
+  no local v6 rtr. | no local v6 rtr. | in both directions
+  -----------------+------------------+--------------------------
+  v4-compat. addr. | v4-compat. addr. | A->B: host to host tunnel
+  no local v6 rtr. | local v6 rtr.    | B->A: v6 forwarding plus
+                   |                  |       rtr->host tunnel
+  -----------------+------------------+--------------------------
+  v4-compat. addr. | incompat. addr.  | A->B: host to rtr tunnel
+  no local v6 rtr. | local v6 rtr.    |       plus v6 forwarding
+                   |                  | B->A: v6 forwarding plus
+                   |                  |       rtr to host tunnel
+  -----------------+------------------+--------------------------
+  v4-compat. addr. | v4-compat. addr. | end to end native v6
+  local v6 rtr.    | local v6 rtr.    | in both directions
+  -----------------+------------------+--------------------------
+  v4-compat. addr. | incompat. addr.  | end to end native v6
+  local v6 rtr.    | local v6 rtr.    | in both directions
+  -----------------+------------------+--------------------------
+  incompat. addr.  | incompat. addr.  | end to end native v6
+  local v6 rtr.    | local v6 rtr.    | in both directions
+  -----------------+------------------+--------------------------
+
+          Table 1: Summary of Automatic Tunneling Combinations
+
+3.3.5 Example
+
+   Figure 2 illustrates an example network with two regions A and B.
+   Region A is dual, meaning that the routers within region A are
+   capable of forwarding both IPv4 and IPv6. Region B is IPv4-only,
+   implying that the routers within region B are capable of routing only
+   IPv4. The illustrated routers R1 through R4 are dual. The illustrated
+   routers r5 through r9 are IPv4-only. Also assume that hosts H3
+   through H8 are dual. Thus H7 and H8 have been upgraded to be IPv6-
+   capable, even though they exist in a region in which the routers are
+   not IPv6-capable. However, host h1 and h2 are IPv4-only.
+
+
+
+Callon & Haskin              Informational                     [Page 10]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+     .........................       .......................
+     .                       .       .                     .
+     .       h1              .       .              |-h2   .
+     .       |               .       .              |      .
+     .  H3---R1--------R2---------------r5----r9----+      .
+     .       |         |     .       .        |     |-H7   .
+     .       |         |     .       .        |            .
+     .       |         |     .       .        |            .
+     .  H4---R3--------R4---------------r6----r8-----H8    .
+     .                       .       .                     .
+     .........................       .......................
+      Region A (Dual Routers)        Region B (IPv4-only Rtrs)
+
+                Figure 2: Example of Automatic Tunneling
+
+   Consider a packet from h1 to H8. In this case, since h1 is IPv4-only,
+   it will send an IPv4 packet. This packet will traverse regions A and
+   B as a normal IPv4 packet for the entire path. Routing will take
+   place using normal IPv4 routing methods, with no change from the
+   operation of the current IPv4 Internet (modulo normal advances in the
+   operation of IPv4, of course). Similarly, consider a return packet
+   from H8 to h1. Here again H8 will transmit an IPv4 packet, which will
+   be forwarded as a normal IPv4 packet for the entire path.
+
+   Consider a packet from H3 to H8. In this case, since H8 is in an
+   IPv4-only routing domain, we can assume that H8 uses an IPv4-
+   compatible IPv6 address. Since both source and destination are IPv6-
+   capable, H3 may transmit an IPv6 packet destined to H8. The packet
+   will be forwarded as far as R2 (or R4) as an IPv6 packet.
+
+   Router R2 (or R4) will then encapsulate the full IPv6 packet in an
+   IPv4 header for delivery to H8. In this case it is necessary for
+   routing of IPv6 within region A to be capable of delivering this
+   packet correctly to R2 (or R4). As explained in section 3.3, routers
+   R2 and R4 may inject routes to IPv4-compatible IPv6 addresses into
+   the IPv6 routing used within region A corresponding to the routes
+   which are available via IPv4 routing within region B.
+
+   Consider a return packet from H8 to H3. Again, since both source and
+   destination are IPv6-capable, a IPv6 packet may be transmitted by H8.
+   However, since H8 does not have any direct connectivity to an IPv6-
+   capable router, H8 must make use of an automatic tunnel.  Which form
+   of automatic tunnel will be used depends upon the type of address
+   assigned to H3.
+
+
+
+
+
+
+
+Callon & Haskin              Informational                     [Page 11]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   If H3 is assigned an IPv4-compatible address, then the requirements
+   specified in section 3.3.1 will all be satisfied. In this case host
+   H8 may encapsulate the full IPv6 packet in an IPv4 header using a
+   source IPv4 address extracted from the IPv6 address of H8, and using
+   a destination IPv4 address extracted from the IPv6 address of H3.
+
+   If H3 has an IPv6-only address, then it is not possible for H8 to
+   extract an IPv4 address to use as the destination tunnel address from
+   the IPv6 address of H3.  In this case H8 must use host to router
+   tunneling, as specified in section 3.3.2. In this case one or both of
+   R2 and R4 must have been configured with a tunnel endpoint IPv4
+   address (R2 and R4 may use either the same address or different
+   addresses for this purpose).  R2 and/or R4 therefore advertise
+   reachability to the tunnel endpoint address to r5 and r6
+   (respectively), which advertise this reachability information into
+   region B. Also, H8 must have been configured to know which tunnel
+   endpoint address to use for host to router tunneling. This will
+   result in the IPv6 packet, encapsulated in an IPv4 header, to be
+   transmitted as far as the border router R2 or R4. The border router
+   will then strip off the IPv4 header, and forward the remaining IPv6
+   packet as a normal IPv6 packet using the normal IPv6 routing used in
+   region A.
+
+4. SECURITY CONSIDERATIONS
+
+   Use of tunneling may violate firewalls of underlying routing
+   infrastructure.
+
+   No other security issues are discussed in this paper.
+
+5. REFERENCES
+
+   [1] Gilligan, B. and E. Nordmark. Transition Mechanisms for IPv6
+       Hosts and Routers, Sun Microsystems, RFC 1933,  April 1996.
+
+
+6. AUTHORS' ADDRESSES
+
+   Ross Callon
+   Cascade Communications Co.
+   5 Carlisle Road
+   Westford, MA 01886
+   email: rcallon@casc.com
+
+
+
+
+
+
+
+
+Callon & Haskin              Informational                     [Page 12]
+
+RFC 2185           Routing Aspects Of IPv6 Transition     September 1997
+
+
+   Dimitry Haskin
+   Bay Networks, Inc.
+   2 Federal Street
+   Billerica, MA 01821
+   email: dhaskin@baynetworks.com
+
+
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+Callon & Haskin              Informational                     [Page 13]
+