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rfc:rfc2185

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:
  1. the source address is an IPv4-compatible IPv6 address.
  2. the destination address is an IPv4-compatible IPv6 address.
  3. 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:
  1. the source address is an IPv4-compatible IPv6 address.
  2. the source host does know of one or more neighboring IPv4-

capable routers

  1. 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.
  1. —————-+——————+————————–

Host A | Host B | Result

  1. —————-+——————+————————–

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

Callon & Haskin Informational [Page 13]

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