





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.



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   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.



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   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].



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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



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   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.








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   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.





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   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



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   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



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   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



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   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.



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     .........................       .......................
     .                       .       .                     .
     .       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.







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   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








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   Dimitry Haskin
   Bay Networks, Inc.
   2 Federal Street
   Billerica, MA 01821
   email: dhaskin@baynetworks.com














































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