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Network Working Group                                          R. CallonRequest for Comments: 2185                    Cascade Communications Co.Category: Informational                                        D. Haskin                                                       Bay Networks Inc.                                                          September 1997Routing Aspects Of IPv6 TransitionStatus 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 insection 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 insection 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 insection 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 19973. MORE DETAIL OF BASIC APPROACHES3.1 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 aCallon & 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 organizationCallon & 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. InCallon & 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 onCallon & 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 Combinations3.3.5 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 insection 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 insection 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 insection 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.comCallon & 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.comCallon & Haskin              Informational                     [Page 13]