INTERNET-DRAFT                                           Fred L. Templin
                                                       SRI International
                                                        12 March
                                                       17 May 2001

        Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
           (Formerly: Connecting IPv6 Nodes within IPv4 Sites)

                            Copyright Notice

                    Placeholder for ISOC copyright.




   This document specifies a method an intra-site automatic tunneling protocol
   (ISATAP) for connecting IPv6 hosts and routers (nodes) within
   predominantly IPv4-based sites. networks. This method is based on an IPv6-IPv4 compatibility IPv6
   aggregatable global unicast address format (described herein) that
   embeds the IPv4 address of a node within the EUI-64 format interface identifier of an IPv6
   identifier.  This document assumes that, during the IPv4 to IPv6 co-
   existence and transition phase, many sites will deploy IPv6
   incrementally within their IPv4 interior routing domains; especially
   those sites which have large and complex pre-existing IPv4
   infrastructures. Within such sites, the address format and methods
   described in this document will enable IPv6 deployment for nodes that
   do not share a common multiple access datalink data link with an IPv6 gateway
    within for their site.

   While other works in progress in the NGTRANS working group propose
   mechanisms for assigning globally-unique IPv6 address prefixes to
   sites and methods for inter-domain routing between such sites, the
   approach outlined in this memo enables large-scale incremental
   deployment of IPv6 for nodes within a site's pre-existing IPv4
   infrastructure without incurring aggregation scaling issues at the
   border gateways nor requiring site-wide deployment of special IPv4
   services such as multicast. The approach proposed by this document
   supports IPv6 routing within both the site-local and global IPv6
   routing domains as well as automatic IPv6 in IPv4 tunneling across
   portions of a site's IPv4 infrastructure which have no native IPv6
   support. Moreover, Additionally, this approach supports automatic tunneling
   within sites which use non globally-unique IPv4 address assignments,
   such as when Network Address Translation [NAT] is used.

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

1.  Introduction

   The IETF NGTRANS working group anticipates an heterogeneous IPv4/IPv6
   infrastructure in the near future and thus is chartered to develop
   mechanisms to support IPv4/IPv6 coexistence and transition toward
   global IPv6 deployment. For the most part, existing NGTRANS
   approaches focus on inter-domain routing between IPv6 "islands" islands using
   the existing global IPv4 backbone as transit. But, these islands may
   themselves consist of comprise complex heterogeneous IPv4/IPv6 networks (e.g.
   large academic or commercial campus "intranets") intranets) that require intra-
   domain IPv4 to IPv6 transition mechanisms and strategies as well. In
   order to address this requirement, this document presents a simple
   and scalable approach that enables incremental intra-site deployment of IPv6
   nodes within predominantly IPv4-based intranets. We refer to this
   approach as the Intra-Site Automatic Tunnel Addressing Protocol, or
   ISATAP (pronounced: "ice-a-tap").

   The ISATAP approach outlined in this document is based on a new an aggregatable global unicast
   address format that carries a standard 64-bit IPv6 address prefix
   [ADDR][AGGR] with a specially-constructed 64-bit EUI-
    64 EUI-64 Interface
   Identifier [EUI64]. The 64-bit  This address prefix used by
    this format is fully compatible with all existing and emerging prefix
   both native IPv6 and inter-domain NGTRANS routing practices (e.g. [6to4],[6BONE]).
   But, the interface identifier in an ISATAP address employs a special
   construction using (using the IEEE Organizationally Unique Identifier (OUI)
   reserved by the Internet Assigned Numbers Authority [IANA] along with a "type" field
    to indicate [IANA]) that the identifier
   encapsulates an IPv4 address suitable for automatic intra-domain IPv6-in-IPv4 tunneling.  As such, tun-
   neling. Since tunneling occurs only within the site-level prefix of
   the ISATAP address, the embedded IPv4 address NEED NOT be globally
   unique; rather, it need only be topologically correct for (and unique
   within) the context of
    that the site.

   This approach allows dual-stack nodes that do not share a common
    multiple access
   datalink with an IPv6 gateway to join the global IPv6 network by
   automatically tunneling IPv6 messages through the IPv4 routing
   infrastructure within the their site. Two methods for automatic discovery
   of an off-link IPv6 gateway within the site for ISATAP address autoconfiguration are
   provided. This approach allows large-scale intra-site deployment
   without incur-
    ring incurring aggregation scaling issues at the border gateways,
   since only a single IPv6 address prefix is used for the entire site.
   Finally, this approach supports intranets which use non-globally
   unique IPv4 addresses, such as when private address allocations
   [PRIVATE] and/or Network Address Translation [NAT] are used; even when multiple levels
    of NAT occur within used.

2.  Changes

   Major changes from version -00 to version -01:

     - Revised draft to require *different* /64 prefixs for ISATAP
       addresses and native IPv6 addresses. Thus, a given site.

    In node's ISATAP
       interface is assigned a /64 prefix that is distinct from the following sections, we present our proposed IPv6-IPv4 compati-
    bility address format in detail. We further discuss technical con-
    siderations for
       prefixes assigned to any other interfaces attached to the application
       node - be they physical or logical interfaces. This approach
       eliminates ISATAP-specific sending rules presented in earlier
       draft versions.

     - Changed sense of IPv6-IPv4 compatibility addresses 'u/l' bit in the ISATAP address interface
       identifier to facilitate incremental deployment of IPv6 indicate "local scope", since ISATAP interface
       identifiers are unique only within predominantly
    IPv4-based Intranets.

2.  Changes the scope of the ISATAP
       prefix. (See section 4.)

   Major changes from version 01 personal draft to NGTRANS WG version 02: -00:

     - Title change to provide higher-level description of field of
       use addressed by this draft. Removed other extraneous text.

     - Major new section on automatic discovery of off-link IPv6 routers
       when IPv6-IPv4 compatibility addresses are used.

3.  IPv6-IPv4 Compatibility Address Format

    In sections 3.1 and 3.2, we will motivate our proposed extensions  Terminology

   The terminology of [IPv6] applies to this document. Additionally, the existing IEEE OUI
   following terms are used extensively throughout this document:

   ISATAP prefix:
     Any globally aggregatable 64-bit IPv6 routing prefix (whether from a
     native IPv6 assigned numbers authority or from a special-purpose numbering
     scheme such as [6BONE][6TO4]) reserved by IANA a local network administrator
     specifically for ISATAP purposes. ISATAP prefixes are used to configure
     ISATAP addresses ONLY; native IPv6 addresses SHOULD NOT be configured
     using an ISATAP prefix.

   ISATAP address:
     An IPv6 address with an ISATAP prefix and having an IPv4 address
     embedded in the interface identifier in the manner described in
     section 4 below.

   ISATAP pseudo-interface:
     ISATAP encapsulation of IPv6 packets inside IPv4 packets occurs
     at a point that is logically equivalent to an IPv6 interface,
     with the link layer being the IPv4 unicast network.  This point
     is referred to as a pseudo-interface. An ISATAP pseudo-interface
     is assigned an ISATAP address through address autoconfiguration.

   ISATAP router:
     An IPv6 router supporting an ISATAP pseudo-interface. It is normally
     an interior router within an heterogeneous IPv6/IPv4 network.

   ISATAP host:
     An IPv6 host which has an ISATAP pseudo-interface.

4.  ISATAP Address Format

   In sections 4.1 and 4.2, we will motivate our proposed extensions of
   the existing IEEE OUI reserved by IANA to support IEEE EUI-64 format
   addresses.  While these proposed extensions are necessary to intended support
    our IPv6-IPv4 compatibility the
   ISATAP address format, they also provide a flex-
    ible flexible framework for
   future IANA use.  Therefore, we believe the exten-
    sions extensions proposed in sections 3.1 4.1
   and 3.2 4.2 may provide beneficial future use to the IANA beyond the scope of IPv6-IPv4 compatibility
   ISATAP addresses. We present our IPv6-IPv4 compatibility the ISATAP address format pro-
    posal itself in sections 3.3 sec-
   tions 4.3 and 3.4 and conclude this section with some
    notes on deployment considerations.

    3.1. 4.4.

   4.1.  IEEE EUI-64 Interface Identifiers in IPv6 Addresses

   IPv6 aggregatable global and local-use unicast addresses [ADDR]
   include a 64-bit interface identifier in IEEE EUI-64 format [EUI64],
   which is specified as the concatenation of a 24-bit company_id value
   (also known as the OUI) assigned by the IEEE Registration Authority
   (IEEE/RAC) and a 40-bit extension identifier assigned by the organi-
    zation owning address-
   ing authority for that OUI. (Normally, the addressing authority is
   the organization to which the IEEE EUI-64 has allocated the OUI). IEEE EUI-
   64 interface identifiers are for-
    matted formatted as follows:

    |0              1|1              3|3              4|4              6|
    |0              5|6              1|2              7|8              3|

   Where 'c' are the company-specific bits of the OUI, 'u' is the
   universal/local bit, 'g' is the individual/group bit and 'm' are the
   extension identifier bits. (NOTE: [ADDR] specifies that the 'u' bit
   is inverted from its normal sense in the IEEE context; therefore u=1
   indicates global scope and u=0 indicates local scope).

   In order to support encapsulation of legacy IEEE EUI-48 (24-bit)
   extension identifier values, [EUI64] specifies that the first two
   octets of the EUI-64 40-bit extension identifier (bits 24 through 39
   of the EUI-64 address itself) SHALL BE 0xFFFE if the extension iden-
   tifier encapsulates an EUI-48 value. [EUI64] further specifies that
   the first two octets of the extension identifier SHALL NOT be 0xFFFF,
   since this value is reserved by the IEEE/RAC. However, all other 40-bit 40-
   bit extension identifier values are available for assignment by the
   OUI addressing authority responsible for a given OUI.

    3.2. authority.

   4.2.  An EUI-64 Interface Identifier Format for IANA

   The IANA owns IEEE OUI: 0x00005E (also written as: 00-00-5E), 00-00-5E, and [IANA] specifies EUI-48 format
   (24-bit) interface identifier assign-
    ments assignments within that OUI. But,
   [IANA] does not specify how these legacy EUI-48 assignments will be
   written in EUI-64 format, nor does it specify a format for future
   40-bit extension identifier assignments. We propose the following
   format for EUI-64 addresses within IANA's OUI reservation:

    |0                      2|2      3|3      3|4                      6|
    |0                      3|4      1|2      9|0                      3|
    |  OUI ("00-00-5E"+u+g)  |  TYPE  |  TSE   |          TSD           |

   Where the fields are:

      OUI     IANA's OUI: 00-00-5E with 'u' and 'g' bits (3 octets)

      TYPE    Type field; indicates how (TSE, TSD) are interpreted (1 octet)

      TSE     Type-Specific Extension (1 octet)

      TSD     Type-Specific Data (3 octets)
   And the following interpretations are defined based on TYPE:

      TYPE         (TSE, TSD) Interpretation
      ----         -------------------------
      0x00-0xFD    RESERVED for future IANA use
      0xFE         (TSE, TSD) together contain an embedded IPv4 address
      0xFF         TSD is interpreted based on TSE as follows:

                   TSE          TSD Interpretation
                   ---          ------------------
                   0x00-0xFD    RESERVED for future IANA use
                   0xFE         TSD contains 24-bit EUI-48 intf identif-
    ier id
                   0xFF         RESERVED by IEEE/RAC

   Essentially, if TYPE=0xFE, TSE is treated as an extension of TSD. If
   TYPE=0xFF, TSE is treated as an extension of TYPE. Other values for
   TYPE (and hence, other interpretations of TSE, TSD) are reserved for
   future IANA use. This format conforms to all requirements specified
   in [EUI64] and supports encapsulation of EUI-48 interface identifiers
   in the manner described by that document. For example, an existing
   IANA EUI-48 format multicast address such as:


   would be written in the IANA EUI-64 format as:


   But, this proposed format also provides a special TYPE (0xFE) for
   embedding IPv4 addresses within the IANA 40-bit extension identifier.
   This special TYPE forms the basis for our IPv6-IPv4 compatibility
    aggregatable global unicast the ISATAP address format proposal as
   described in the following sections.

    3.3.  IPv6-IPv4 Compatibility

   4.3.  ISATAP Address Construction

   Using the proposed IANA-specific method for interface identifier con-
   struction discussed in sections 3.1 4.1 and 3.2 4.2 (with TYPE=0xFE), and
   with reference to [ADDR], we can construct IPv6-IPv4 compatibility
    aggregatable global unicast addresses. Using this methodology, we
    propose an IPv6 address format with embedded IPv4 ISATAP address in the
    EUI-64 interface identifier. The following diagram shows the con-
    struction: as fol-

    | 3|  13 | 8 |   24   |   16   | 8 | 8 | 8 | 8 |    32 bits     |
    |FP| TLA |RES|  NLA   |  SLA   | 0x| 0x| 0x| 0x|  IPv4 Address  |
    |  | ID  |   |  ID    |  ID    | 02| 00| 00| 5E| FE|   of Endpoint  |

   (NOTE: since ISATAP address interface identifiers are interpreted
   only within the local scope of the /64 ISATAP prefix, we set the u/l
   bit in the least significant octet of the OUI in the interface iden-
    tifier is 0x02 instead of 0x00 since u=1 for global to '0' to indicate
   local scope.)

   By way of example, an existing node with IPv4 address
   might be assigned an IPv6 64-bit prefix of 3FFE:1a05:510:200::/64. We
   can then construct an IPv6-IPv4 compatibility aggregatable global
    unicast ISATAP address for this node as:



   or (perhaps more appropriately) written as the alternative form for
   an IPv6 address with embedded IPv4 address found in [ADDR]:



   Similarly, we can construct the link-local and site-local variants
   (respectively) of the IPv6-IPv4 compatibility ISATAP address as:




   4.4.  Advantages

   By embedding an IPv4 address in the interface identifier portion of
   an IPv6 address as described in section 3.3, 4.3, we can construct aggre-
   gatable global unicast IPv6 addresses that can either be routed
    globally glo-
   bally via the IPv6 infrastructure or automatically tunneled locally
   across portions of a site's IPv4 infrastructure which have no native
   IPv6 routing support.  Thus the addressing scheme supports
    heterogeneous IPv6/IPv4 infrastructures in transition with incremen-
    tal deployment of IPv6 at the site level.  Additionally, a node with
    such an IPv6-IPv4 compatibility ISATAP address could act
   as a gateway for nodes with native IPv6 addresses connected to the same with which it
   shares a common physical link, since it the ISATAP node could automatically automati-
   cally tunnel messages across a site's IPv4 domain to
    reach a border IPv6 gateway for the site on behalf of such the
   native IPv6 nodes.  An example would be deployment of IPv6 on some
   subset of the hosts attached to a workgroup's Ethernet LAN. In this case, one
   host would receive could configure an IPv6-IPv4 compatibility ISATAP address and act as a gateway for the other
   hosts on the LAN which receive use native IPv6 addresses.

   An additional advantage for our proposed method of embedding an IPv4
   address in the interface identifier portion of an IPv6 address not
   found in other approaches such as [6TO4] is that large numbers of
    IPv6-IPv4 compatibility
   ISATAP addresses could be assigned within a common IPv6 routing prefix, pre-
   fix, thus providing maximal aggregation at the border gateways. For
   example, the single 64-bit IPv6 prefix:


   could include literally millions of nodes with IPv6-IPv4 compatibil-
    ity ISATAP addresses.

   This feature would allow a "sparse mode" IPv6 deploy-
    ment deployment such as the
   deployment of sparse populations of IPv6 hosts on large numbers of
   independent links throughout a large corporate Intranet.

   A final important advantage is that this method supports both sites
   that use globally unique IPv4 address assignments and those that use
   non-globally unique IPv4 addresses, such as when private address
   assignments and/or Network Address Translation are used. By way of
   analogy to the US Postal system, inter-domain transition approaches
   such as [6TO4] provide means for routing messages "cross-country" to
   the "street address" of a distant site while the approach outlined in
   this document provides localized routing information to reach a
   specific (mailstop, apartment number, post office box, etc) WITHIN
   that site.  Thus, the site-level routing information need not have
   relevance outside the scope of that site.


5.  ISATAP Deployment Considerations

    IPv6-IPv4 compatibility

   ISATAP addresses should only be used by nodes which do not share a
   common multiple access datalink with an a native IPv6 router. At least one ISATAP router
    for their site. But, there are numerous cases in which such "iso-
    lated" nodes may occur within an heterogeneous IPv6/IPv4 Intranet.

    Two such examples are:

      - A researcher wishes to run IPv6 on his existing IPv4-based works-
      tation. One or more IPv6 routers are
   must be configured within the
      researchers site, but the network administrators have not yet con-
      figured site which advertises an IPv6 router for the LAN that connects the researcher's

      - A network administrator within a large corporate network wishes
   administratively- assigned ISATAP prefix in response to configure IPv6 on the existing IPv4 subnets under their jurisd-
      iction, but these subnets are separated an Rtsol mes-
   sage from an off-link host. Such off-link hosts will configure an
   ISATAP pseudo-interface and assign it an address using the IPv6 border gate-
      way for the corporation by other IPv4 subnets which are not ready
      for IPv6 deployment.

    In both examples, intra-site IPv6-in-IPv4 tunneling can be used to
    span the "gaps" ISATAP
   prefix it receives in IPv6 coverage. The IPv6-IPv4 compatibility an Rtadv message solicited from an ISATAP

   Following ISATAP address
    format described in the previous subsections provides a means for
    isolated nodes to configuration, ISATAP hosts automatically
   and transparently communicate the IPv4 address of their *own* end of
   the ISATAP tunnel to an off-link IPv6
    gateway. any ISATAP host or router which uses the same
   ISATAP prefix.  While such nodes may optionally use stateful configuration
   to set an ISATAP prefix and a "default" route that points to the off-link gateway, an ISA-
   TAP router, a greatly preferred alternative is to provide for
   automatic intra-site IPv6 router discovery and stateless address
   autoconfiguration [DIS-
    CUSS]. [DISCUSS].  The following section presents a means
   for the automatic discovery of off-link IPv6 ISATAP routers.


5.1.  Automatic Discovery of Off-link IPv6 ISATAP Routers

   As described in [AUTO], a node that does not share a common multiple
   access datalink with an IPv6 router will NOT receive unsolicited
   Router Advertisements (Rtadv's), nor will Router Solicitations
   (Rtsol's) from that node reach an IPv6 router on the local link. But,
   the node may still be able to connect to the global IPv6 Internet if
   an ISATAP router for the site exists. Hence, a means for off-link IPv6 ISATAP
   router discovery is required.  We present the following procedure for
   a node to initiate off-link IPv6 ISATAP router discovery (and for an off-link IPv6 ISATAP router
   to respond) when
    IPv6-IPv4 compatibility addresses are used: an on-link IPv6 router is not available:

     - The node constructs an IPv6-IPv4 compatibility ISATAP link local address for itself
       (as described in section 3.) 4.) as:



     - The node discovers the IPv4 address for an off-link IPv6 ISATAP router
       as: V4ADDR_RTR (**)

     - The node sends an Rtsol to the IPv6 "all-routers-multicast" address
       tunneled through the IPv4 infrastructure to the off-link IPv6 ISATAP router's
       IPv4 address. The addresses used in the IPv6 and IPv4 headers are:

         ipv6_src:  FE80::0200:5EFE:V4ADDR_NODE  FE80::0:5EFE:V4ADDR_NODE
         ipv6_dst:  FF02::2
         ipv4_src:  V4ADDR_NODE
         ipv4_dst:  V4ADDR_RTR

     - Upon receiving the tunneled Rtsol, the off-link IPv6 ISATAP router sends
       a unicast Rtadv to the unicast address of the node which sent the
       Rtsol; again, by tunneling the Rtadv through IPv4. The addresses
       used in the IPv6 and IPv4 headers are:

         ipv6_src:  FE80::0200:5EFE:V4ADDR_RTR  FE80::0:5EFE:V4ADDR_RTR
         ipv6_dst:  FE80::0200:5EFE:V4ADDR_NODE  FE80::0:5EFE:V4ADDR_NODE
         ipv4_src:  V4ADDR_RTR
         ipv4_dst:  V4ADDR_NODE

     - Upon receiving the Rtsol, the originating node performs address
       autoconfiguration as described in [AUTO] and constructs:

       - a fully-qualified IPv6-IPv4 compatibility ISATAP address for use as the source address
         for IPv6 packets an ISATAP pseudo-interface

       - a default route that points to the off-link IPv6 router's
          IPv6-IPv4 compatibility link-local address ISATAP router

   Note (**) that the above procedure assumes a means for discovering
   V4ADDR_RTR. We present two alternative methods for the automatic
   discovery of V4ADDR_RTR:


5.2.  DNS Well-Known Service Name

   The first method for discovering V4ADDR_RTR employs a new DNS Well-
   Known Service (WKS) name [DNS1,DNS2]. With the establishment of a new
   well-known service name (e.g. "V6V4GW"), "ISATAPGW"), administrators could publish pub-
   lish the IPv4 address of a gateway which implementations could use to
   discover V4ADDR_RTR. This method has the advantage that it can be
   deployed immediately using existing mechanisms. However, it requires
   name service lookups and may not always provide the optimum
   V4ADDR_RTR resolution for isolated hosts which use IPv6-IPv4 compati-
    bility addresses.

4.2. if multiple ISATAP routers
   are available.

5.3.  IPv4 Anycast for Intra-domain IPv6 router ISATAP routers

   [6TO4ANY] proposes an IPv4 anycast prefix for 6to4 relay routers.
   The proposal suggests an IPv4 prefix assignment 'x.x.x.0/nn' ('nn' is
   currently proposed as 16) where the single address 'x.x.x.1' is
   assigned as the "6to4 IPv6 relay anycast address". We propose analo-
   gous assignments for the purpose of an "IPv6-IPv4 compatibility "ISATAP router anycast
   address". (Whether the reservation of a second /32 assignment from
   the 6to4 IPv4 anycast prefix proposed in [6TO4ANY] would be possible,
   or a separate prefix assignment would be required is a matter of
   debate and TBD.)

    Any IPv6 router capable of providing an IPv6-IPv4 compatibility
    address-based tunnel endpoint as described in the previous sections

   ISATAP routers would advertise the IPv6-IPv4 compatibility ISATAP router anycast prefix via
   the intra-domain IPv4 routing infrastructure. Isolated IPv6 nodes
   would then use the IPv6-IPv4 compatibility ISATAP router anycast address as the V4ADDR_RTR
   IPv4 destination for off-link Rtsol's. This approach has the significant signifi-
   cant advantages that:

     - implementations could hard-code the well-known V6V4Compat ISATAP
       anycast address, thus avoiding service discovery via DNS

     - an optimum path to an off-link IPv6 ISATAP router would be ensured
       by intra-domain IPv4 routing

   As described above, the IPv4 anycast method for locating intra-domain ISATAP
   routers that support IPv6-IPv4 compatibility address-based tunneling provides significant functional advantages over the DNS
   approach, while the DNS approach can be implemented immediately pending pend-
   ing the registration of a WKS name with IANA. While either method
   will work, the decision of which to push for standardization is TBD
   pending dis-
    cussion discussion at upcoming NGTRANS WG meetings.


6.  Sending Rules and Routing Considerations

    The sending rule for a host or router that sends an IPv6 packet to

   Since each node will be assigned an
    IPv6-IPv4 compatibility destination address is simple and direct:

      "If the 64-bit IPv6 prefix of the IPv6-IPv4 compatibility
      destination address matches the 64-bit IPv6 ISATAP prefix of one of my
      network interfaces, tunnel the packet through IPv4 - else, route
      the packet through IPv6."

    From the above rule, a sender that does NOT have an interface which
    shares a common 64-bit routing prefix with the packet's IPv6-IPv4
    compatibility destination address simply sends the packet to the
    next-hop gateway determined is adminis-
   tratively reserved for use ONLY by an ordinary IPv6 routing table lookup.
    In short, when a ISATAP nodes, no special sending node does not have an interface which shares
   rules are needed.  In particular, correspondent nodes that share a
   common 64-bit (site-level) routing ISATAP prefix with an IPv6-IPv4 compa-
    tibility destination address, the sending rule is identical to will always exchange messages using their ISATAP
   pseudo-interfaces, whereas nodes that
    for a native IPv6 destination address. This decision is independent
    of whether the sender has an IPv6-IPv4 compatibility address itself,
    or whether the sender even comprises a dual-stack configuration.
    Indeed, the sender can simply be do not share a native common ISATAP
   prefix will always exchange messages via standard IPv6 node with no legacy
    IPv4 support. routing. When
   sending a sender has message on an interface which shares a common 64-bit routing
    prefix with ISATAP pseudo-interface, an IPv6-IPv4 compatibility destination address, however,
    the sender must assume implementation
   SHOULD verify that the IPv6 destination is NOT directly reachable
    at the datalink level - even though address employs the shared site-level routing
    prefix implies otherwise. Instead, if the sender comprises a dual-
    stack configuration, it should automatically tunnel the IPv6 packet
    (via IPv6-in-IPv4 tunneling as described in [MECH]) to the IPv4
    address embedded within the IPv6-IPv4 compatibility destination
    address' interface identifier. If the sender is an IPv6-only node
    that DOES NOT comprise a dual-stack configuration, however, it has no
    means for automatically tunneling the packet via IPv4. In this case:

      - If the sender is the host that originates the packet, it should
        send the packet to a router that lists the 64-bit prefix in its
        router advertisements. If no such router exists, the sender should
        drop the packet and return a "No route to host" error indication
        to the originating application.

      - If the sender is a router that forwards the packet, it should drop
        the packet and send an ICMPv6 "Destination Unreachable" message to
        the source

    By implication, the scheme breaks down if a packet with an IPv6-IPv4
    compatibility destination ISATAP
   address reaches an IPv6-only router that
    has an interface which shares a common 64-bit routing prefix with the
    destination address. Additional mechanisms to address this issue
    might be possible, such as allowing dual-stack routers to advertise
    96-bit prefixes which incorporate the special 32-bit EUI-64 interface
    identifier prefix: 0200:5EFE. A sender could then interpret such an
    advertisement to mean that the advertising router comprises a dual
    stack and is capable of intra-site IPv6-in-IPv4 tunneling. But a
    reasonable argument could be made to the effect that:

      "By the time IPv6-only routers begin to proliferate throughout a
      site, nodes within the site should no longer be using IPv6-IPv4
      compatibility addresses."

    In fact, the advent of IPv6-only routers within a site would serve as
    a strong indication that the site is no longer a predominantly IPv4-
    based infrastructure in transition, but rather that the transition is
    either complete or nearly complete. Therefore, IPv6-IPv4 compatibil-
    ity addresses should no longer be used.

6.  Address Selection

    Other works in progress ([6TO4] and [SELECT]) have begun to explore
    the subject of address selection when multiple IPv6 destination
    address alternatives are available. These address selection policies
    deal with the 64-bit IPv6 routing prefix and thus can be applied
    independently of whether/not the destination address alternatives are
    constructed as construction rules described in this document. However, section 4 in order to
    ensure efficient routing within the destination's site, we propose
    the following simple "second-tier" address selection policy for deal-
    ing with IPv6-IPv4 compatibility addresses:

      "If multiple alternatives remain after detect
   mis-configured addresses. No other sending rules are necessary.

7.  Address Selection

   No special address selection has been
      applied on the 64-bit routing prefixes, and if at least one of the
      remaining alternatives is constructed with a native IPv6 interface
      identifier (one that does NOT contain an embedded IPv4 address as
      described in this document), select a native IPv6 address.  Other-
      wise, select an IPv6-IPv4 compatible address."

    This policy decision is in keeping with the concept that NGTRANS
    transition mechanisms should remain in place ONLY as long as needed
    and should be disabled as soon as native IPv6 mechanisms become

7. rules are necessary.

8.  Automatic Deprecation

    IPv6-IPv4 compatibility

   ISATAP addresses constructed in the manner described
    in this document are intended for use only by nodes which do not
   receive router advertisements native IPv6 Rtadv's due to not sharing a common multiple
    access datalink with
   an IPv6 router. When router advertisements native IPv6 Rtadv's become available (such as
   when an IPv6 router is deployed on a common
    multiple access datalink shared by the node), node's datalink), the node
   should discon-
    tinue use of its IPv6-IPv4 compatibility address and adopt construct a normal non-ISATAP aggregatable global IPv4 IPv6 unicast
   address using address auto-
    configuration auto-configuration [AUTO] for a non-ISATAP IPv6
   prefix discovered through normal means [DISC].  After the node's
   native IPv6 address is populated in the DNS, the node should eventu-
   ally cease sending Rtsol's to the ISATAP router
    discovery [DISC] means. and discontinue use
   of its ISATAP pseudo-interface.  In this way, IPv6-IPv4 compatibility ISATAP addresses will
   gradually (and automatically) disappear as IPv6 routers become are widely
   deployed within a site.

8. sites.

9.  Multicast Considerations

   Other works in progress [6TO4MULTI] are currently investigating IPv4-mapped mul-
   ticast addressing issues. issues for [6TO4]. The address format discussed in
   this docu-
    ment document is expected to be compatible with those emerging

9.  Relation to other works in progress

    The IPv6-IPv4 compatibility address format and routing policy deci-
    sions presented in this draft evolved from SRI contractual works out-
    side the scope of the NGTRANS working group. Additionally, the
    mechanisms presented in this draft were developed by the author with
    no prior knowledge of the activities in NGTRANS. The author recog-
    nizes that other works in progress seek to address very similar
    IPv4-IPv6 transition issues as those targeted by this draft. However,
    the approach described in this draft presents a number of unique
    advantages for NGTRANS that supplement the other works in progress.
    (Most specifically, advantages for incremental deployment of IPv6
    nodes at the intra-domain level.)

10.  IANA considerations

   In order to support the EUI-64 address form described in this docu-
   ment, we propose that IANA adopt the EUI-64 Interface Identifier for-
   mat specified in section 3.2 4.2 for the existing 00-00-5E OUI owned by
   IANA. No other actions are required by the IANA.

11.  Security considerations

   The IPv6-IPv4 compatibility ISATAP address format does not support privacy extensions for
   stateless address autoconfiguration [PRIVACY].  How-
    ever,  However, such privacy
   extensions are intended primarily to avoid reveal-
    ing revealing one's MAC
   address, and the IPv6-IPv4 compatibility ISATAP address format described in this document
   accomplishes this same goal.

   Additional security issues are called out in [6TO4] and probably
   apply here as well.

12.  Implementation status

   The author has implemented the mechanisms described in this draft
   through modifications to the FreeBSD 3.2-RELEASE [FBSD] operating
   system with the INRIA [INRIA] IPv6 distribution. These modifications
    implement the sending rules and routing considerations as described
    in section 5. The source code A Linux implementa-
   tion is not yet ready planned for public distribu-
    tion, but the author would be happy to discuss details with
    interested parties. June, 2001 timeframe.

   Additionally, Windows XP RC1 will implement elements of the mechanism
   proposed in this paper.


   The original ideas presented in this draft were derived from SRI contractual con-
   tractual work. The author recognizes that ideas similar to those in
   this document may have already been presented by others and wishes to ack-
   acknowledge any other such authors. The author also wishes to ack-
   nowledge the government contract administrators who sponsored the
   projects from which these works derived as well as his SRI colleagues
   with whom he has discussed and reviewed this work, including Monica
   Farah-Stapleton, Dr. Mike Frankel, J. Peter Marcotullio, Lou Rodriguez, Rodri-
   guez, and Dr. Ambatipudi Sas-
    try. Sastry.

   The author acknowledges discussions with Alain Durand and Keith Moore
    during valuable input from numerous members of the IETF 48 conference in Pittsburgh, PA.
   NGTRANS community which has helped
    motivate ideas on restructuring this document guide the direction of the draft.
   The list of contributors is too long to enumerate, but the input from
   the first version. community has been vital to the draft's evolution. Alain Durand
   deserves special mention for contributing the title of this draft and
   the ISATAP acronym.

   The author further finally wishes to provide special acknowledgement to Dave
   Thaler, Art Shelest, Richard Draves, and their colleagues others at Microsoft Research
   for their ideas on automatic discovery of off-link IPv6 routers. Much
   of the text in that section on deployment considerations derives directly
   from discussions with Dave,
    Art Art, Rich and others.


   [AGGR]     Hinden., R, O'Dell, M., and Deering, S., "An IPv6
              Aggregatable Global Unicast Address Format",
              RFC 2374, July 1998.

   [ADDR]     Hinden, R., and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

   [AUTO]     Thomson, S., and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.

   [DISC]     Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [DNS1]     Mockapetris, P. "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [DNS2]     Mockapetris, P. "Domain names - Implementation and Specif-
              STD 13, RFC 1035, November 1987.

   [DNSSRV]   Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [EUI64]    IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
              Registration Authority",
              March 1997

   [IANA]     Reynolds, J., and J. Postel, "Assigned Numbers", STD 2,
              USC/Information Sciences Institute, October 1994.

   [IPV4]     Postel, J., "Internet Protocol", RFC 791

   [IPV6]     Deering, S., and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460

   [6TO4]     Carpenter, B., and K. Moore, "Connection of IPv6 Domains
              via IPv4 Clouds", RFC 3056, February 2001.

   [6TO4ANY]  Huitema, C., "An anycast prefix for 6to4 relay routers",
              draft-ietf-ngtrans-6to4anycsat-02.txt (work in progress)

   [6TO4MULTI] Thaler, D., "Support for Multicsat over 6to4 Networks",
              draft-ietf-ngtrans-6to4-multicast-00.txt (work in pro-

   [MECH]     Gilligan, R., and E. Nordmark, "Transition Mechanisms for
              IPv6 Hosts and Routers", RFC 2893, August 2000.

   [SELECT]   Draves, R., Default Address Selection for IPv6, draft-
              ipngwg-default-addr-select-00.txt (work in progress)



   [6BONE]    Rockell, R., and R. Fink, RFC 2772, February 2000.

   [PRIVATE]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
              and E. Lear, "Address Allocation for Private Internets",
              RFC 1918, February 1996.

   [PRIVACY]  Narten, T., R. Draves, "Privacy Extensions for Stateless
              Autoconfiguration in IPv6", RFC 3041, January 2001.

   [NAT]      Egevang, K., and P. Francis, "The IP Network Address
              Translator (NAT)", RFC 1631, May 1994.

   [DISCUSS]  private discussions with Dave Thaler, Art Shelest, et al.

Authors Addresses

      Fred L. Templin
      SRI International
      333 Ravenswood Ave.
      Menlo Park, CA 94025, USA


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