    Network Working Group                                         F. Templin
    Request for Comments: 4214                                         Nokia
    Category: Experimental                                        T. Gleeson
                                                          Cisco Systems K.K.
                                                                   M. Talwar
                                                                   D. Thaler
                                                       Microsoft Corporation
                                                                October 2005

            Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)

    Status of This Memo

       This memo defines an Experimental Protocol for the Internet
       community.  It does not specify an Internet standard of any kind.
       Discussion and suggestions for improvement are requested.
       Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2005).

    IESG Note

       The content of this RFC was at one time considered by the IETF, and
       therefore it may resemble a current IETF work in progress or a
       published IETF work.  This RFC is not a candidate for any level of
       Internet Standard.  The IETF disclaims any knowledge of the fitness
       of this RFC for any purpose, and in particular notes that the
       decision to publish is not based on IETF review for such things as
       security, congestion control or inappropriate interaction with
       deployed protocols.  The RFC Editor has chosen to publish this
       document at its discretion.  Readers of this RFC should exercise
       caution in evaluating its value for implementation and deployment.
       See RFC 3932 for more information.

    Abstract

       The Intra-Site Automatic Tunnel Addressing Protocol (ISATAP) connects
       IPv6 hosts/routers over IPv4 networks.  ISATAP views the IPv4 network
       as a link layer for IPv6 and views other nodes on the network as
       potential IPv6 hosts/routers.  ISATAP supports an automatic tunneling
       abstraction similar to the Non-Broadcast Multiple Access (NBMA)
       model.

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    1.  Introduction

       This document specifies a simple mechanism called the Intra-Site
       Automatic Tunnel Addressing Protocol (ISATAP) that connects IPv6
       hosts/routers over IPv4 networks.  Dual-stack (IPv6/IPv4) nodes use
       ISATAP to automatically tunnel IPv6 packets in IPv4, i.e., ISATAP
       views the IPv4 network as a link layer for IPv6 and views other nodes
       on the network as potential IPv6 hosts/routers.

       ISATAP enables automatic tunneling whether global or private IPv4
       addresses are used, and presents a Non-Broadcast Multiple Access
       (NBMA) abstraction similar to [RFC2491][RFC2492][RFC2529][RFC3056].

       The main objectives of this document are to: 1) describe the domain
       of applicability, 2) specify addressing requirements, 3) specify
       automatic tunneling using ISATAP, 4) specify the operation of IPv6
       Neighbor Discovery over ISATAP interfaces, and 5) discuss Site
       Administration, Security, and IANA considerations.

    2.  Requirements

       The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
       SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
       document, are to be interpreted as described in [BCP14].

       This document also uses internal conceptual variables to describe
       protocol behavior and external variables that an implementation must
       allow system administrators to change.  The specific variable names,
       how their values change, and how their settings influence protocol
       behavior are provided in order to demonstrate protocol behavior.  An
       implementation is not required to have them in the exact form
       described here, as long as its external behavior is consistent with
       that described in this document.

    3.  Terminology

       The terminology of [RFC2460][RFC2461] applies to this document.  The
       following additional terms are defined:

       ISATAP node:
          A node that implements the specifications in this document.

       ISATAP interface:
          An ISATAP node's Non-Broadcast Multi-Access (NBMA) IPv6 interface,
          used for automatic tunneling of IPv6 packets in IPv4.

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       ISATAP interface identifier:
          An IPv6 interface identifier with an embedded IPv4 address
          constructed as specified in Section 6.1.

       ISATAP address:
          An IPv6 unicast address that matches an on-link prefix on an
          ISATAP interface of the node, and that includes an ISATAP
          interface identifier.

       locator:
          An IPv4 address-to-interface mapping; i.e., a node's IPv4 address
          and its associated interface.

       locator set:
          A set of locators associated with an ISATAP interface.  Each
          locator in the set belongs to the same site.

    4.  Domain of Applicability

       The domain of applicability for this technical specification is
       automatic tunneling of IPv6 packets in IPv4 for ISATAP nodes within
       sites that observe the security considerations found in this
       document, including host-to-router, router-to-host, and host-to-host
       automatic tunneling in certain enterprise networks and 3GPP/3GPP2
       wireless operator networks.  (Other scenarios with a sufficient trust
       basis ensured by the mechanisms specified in this document also fall
       within this domain of applicability.)

       Extensions to the above domain of applicability (e.g., by combining
       the mechanisms in this document with those in other technical
       specifications) are out of the scope of this document.

    5.  Node Requirements

       ISATAP nodes observe the common functionality requirements for IPv6
       nodes found in [NODEREQ] and the requirements for dual IP layer
       operation found in ([MECH], Section 2).  They also implement the
       additional features specified in this document.

    6.  Addressing Requirements

    6.1.  ISATAP Interface Identifiers

       ISATAP interface identifiers are constructed in Modified EUI-64
       format ([RFC3513], Section 2.5.1 and Appendix A) by concatenating the
       24-bit IANA OUI (00-00-5E), the 8-bit hexadecimal value 0xFE, and a
       32-bit IPv4 address in network byte order as follows:

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       |0              1|1              3|3                              6|
       |0              5|6              1|2                              3|
       +----------------+----------------+--------------------------------+
       |000000ug00000000|0101111011111110|mmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm|
       +----------------+----------------+--------------------------------+

       When the IPv4 address is known to be globally unique, the "u" bit
       (universal/local) is set to 1; otherwise, the "u" bit is set to 0.
       "g" is the individual/group bit, and "m" are the bits of the IPv4
       address.

    6.2.  ISATAP Interface Address Configuration

       Each ISATAP interface configures a set of locators consisting of IPv4
       address-to-interface mappings from a single site; i.e., an ISATAP
       interface's locator set MUST NOT span multiple sites.

       When an IPv4 address is removed from an interface, the corresponding
       locator SHOULD be removed from its associated locator set(s).  When a
       new IPv4 address is assigned to an interface, the corresponding
       locator MAY be added to the appropriate locator set(s).

       ISATAP interfaces form ISATAP interface identifiers from IPv4
       addresses in their locator set and use them to create link-local
       ISATAP addresses ([RFC2462], Section 5.3).

    6.3.  Multicast/Anycast

       It is not possible to assume the general availability of wide-area
       IPv4 multicast, so (unlike 6over4 [RFC2529]) ISATAP must assume that
       its underlying IPv4 carrier network only has unicast capability.
       Support for IPv6 multicast over ISATAP interfaces is not described in
       this document.

       Similarly, support for Reserved IPv6 Subnet Anycast Addresses is not
       described in this document.

    7.  Automatic Tunneling

       ISATAP interfaces use the basic tunneling mechanisms specified in
       ([MECH], Section 3).  The following sub-sections describe additional
       specifications.

    7.1.  Encapsulation

       ISATAP addresses are mapped to a link-layer address by a static
       computation; i.e., the last four octets are treated as an IPv4
       address.

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    7.2.  Handling ICMPv4 Errors

       ISATAP interfaces SHOULD process ARP failures and persistent ICMPv4
       errors as link-specific information indicating that a path to a
       neighbor may have failed ([RFC2461], Section 7.3.3).

    7.3.  Decapsulation

       The specification in ([MECH], Section 3.6) is used.  Additionally,
       when an ISATAP node receives an IPv4 protocol 41 datagram that does
       not belong to a configured tunnel interface, it determines whether
       the packet's IPv4 destination address and arrival interface match a
       locator configured in an ISATAP interface's locator set.

       If an ISATAP interface that configures a matching locator is found,
       the decapsulator MUST verify that the packet's IPv4 source address is
       correct for the encapsulated IPv6 source address.  The IPv4 source
       address is correct if:

          -  the IPv6 source address is an ISATAP address that embeds the
             IPv4 source address in its interface identifier, or

          -  the IPv4 source address is a member of the Potential Router
             List (see Section 8.1).

       Packets for which the IPv4 source address is incorrect for this
       ISATAP interface are checked to determine whether they belong to
       another tunnel interface.

    7.4.  Link-Local Addresses

       ISATAP interfaces use link-local addresses constructed as specified
       in Section 6 of this document.

    7.5.  Neighbor Discovery over Tunnels

       ISATAP interfaces use the specifications for neighbor discovery found
       in the following section of this document.

    8.  Neighbor Discovery for ISATAP Interfaces

       ISATAP interfaces use the neighbor discovery mechanisms specified in
       [RFC2461].  The following sub-sections describe specifications that
       are also implemented.

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    8.1.  Conceptual Model of a Host

       To the list of Conceptual Data Structures ([RFC2461], Section 5.1),
       ISATAP interfaces add the following:

          Potential Router List (PRL)
          A set of entries about potential routers; used to support router
          and prefix discovery.  Each entry ("PRL(i)") has an associated
          timer ("TIMER(i)"), and an IPv4 address ("V4ADDR(i)") that
          represents a router's advertising ISATAP interface.

    8.2.  Router and Prefix Discovery - Router Specification

       Advertising ISATAP interfaces send Solicited Router Advertisement
       messages as specified in ([RFC2461], Section 6.2.6) except that the
       messages are sent directly to the soliciting node; i.e., they might
       not be received by other nodes on the link.

    8.3.  Router and Prefix Discovery - Host Specification

       The Host Specification in ([RFC2461], Section 6.3) is used.  The
       following sub-sections describe specifications added by ISATAP
       interfaces.

    8.3.1.  Host Variables

       To the list of host variables ([RFC2461], Section 6.3.2), ISATAP
       interfaces add the following:

       PrlRefreshInterval
          Time in seconds between successive refreshments of the PRL after
          initialization.  The designated value of all ones (0xffffffff)
          represents infinity.
          Default: 3600 seconds

       MinRouterSolicitInterval
          Minimum time in seconds between successive solicitations of the
          same advertising ISATAP interface.  The designated value of all
          ones (0xffffffff) represents infinity.

    8.3.2.  Potential Router List Initialization

       ISATAP nodes initialize an ISATAP interface's PRL with IPv4 addresses
       discovered via manual configuration, a DNS Fully Qualified Domain
       Name (FQDN) [STD13], a DHCPv4 option, a DHCPv4 vendor-specific
       option, or an unspecified alternate method.  FQDNs are established
       via manual configuration or an unspecified alternate method.  FQDNs
       are resolved into IPv4 addresses through a static host file lookup,

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       querying the DNS service, querying a site-specific name service, or
       with an unspecified alternate method.

       After initializing an ISATAP interface's PRL, the node sets a timer
       for the interface to PrlRefreshInterval seconds and re-initializes
       the interface's PRL as specified above when the timer expires.  When
       an FQDN is used, and when it is resolved via a service that includes
       TTLs with the IPv4 addresses returned (e.g., DNS 'A' resource records
       [STD13]), the timer SHOULD be set to the minimum of
       PrlRefreshInterval and the minimum TTL returned.  (Zero-valued TTLs
       are interpreted to mean that the PRL is re-initialized before each
       Router Solicitation event; see Section 8.3.4.)

    8.3.3.  Processing Received Router Advertisements

       To the list of checks for validating Router Advertisement messages
       ([RFC2461], Section 6.1.1), ISATAP interfaces add the following:

          -  IP Source Address is a link-local ISATAP address that embeds
             V4ADDR(i) for some PRL(i).

       Valid Router Advertisements received on an ISATAP interface are
       processed as specified in ([RFC2461], Section 6.3.4).

    8.3.4.  Sending Router Solicitations

       To the list of events after which Router Solicitation messages may be
       sent ([RFC2461], Section 6.3.7), ISATAP interfaces add the following:

          -  TIMER(i) for some PRL(i) expires.

       Since unsolicited Router Advertisements may be incomplete and/or
       absent, ISATAP nodes MAY schedule periodic Router Solicitation events
       for certain PRL(i)s by setting the corresponding TIMER(i).

       When periodic Router Solicitation events are scheduled, the node
       SHOULD set TIMER(i) so that the next event will refresh remaining
       lifetimes stored for PRL(i) before they expire, including the Router
       Lifetime, Valid Lifetimes received in Prefix Information Options, and
       Route Lifetimes received in Route Information Options [DEFLT].
       TIMER(i) MUST be set to no less than MinRouterSolicitInterval seconds
       where MinRouterSolicitInterval is configurable for the node, or for a
       specific PRL(i), with a conservative default value (e.g., 2 minutes).

       When TIMER(i) expires, the node sends Router Solicitation messages as
       specified in ([RFC2461], Section 6.3.7) except that the messages are
       sent directly to PRL(i); i.e., they might not be received by other
       routers.  While the node continues to require periodic Router

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       Solicitation events for PRL(i), and while PRL(i) continues to act as
       a router, the node resets TIMER(i) after each expiration event as
       described above.

    8.4.  Neighbor Unreachability Detection

       Hosts SHOULD perform Neighbor Unreachability Detection ([RFC2461],
       Section 7.3).  Routers MAY perform neighbor unreachability detection,
       but this might not scale in all environments.

       After address resolution, hosts SHOULD perform an initial
       reachability confirmation by sending Neighbor Solicitation messages
       and receiving a Neighbor Advertisement message.  Routers MAY perform
       this initial reachability confirmation, but this might not scale in
       all environments.

    9.  Site Administration Considerations

       Site administrators maintain a Potential Router List (PRL) of IPv4
       addresses representing advertising ISATAP interfaces of routers.

       The PRL is commonly maintained as an FQDN for the ISATAP service in
       the site's name service (see Section 8.3.2).  There are no mandatory
       rules for the selection of the FQDN, but site administrators are
       encouraged to use the convention "isatap.domainname" (e.g.,
       isatap.example.com).

       When the site's name service includes TTLs with the IPv4 addresses
       returned, site administrators SHOULD configure the TTLs with
       conservative values to minimize control traffic.

    10.  Security Considerations

       Implementors should be aware that, in addition to possible attacks
       against IPv6, security attacks against IPv4 must also be considered.
       Use of IP security at both IPv4 and IPv6 levels should nevertheless
       be avoided, for efficiency reasons.  For example, if IPv6 is running
       encrypted, encryption of IPv4 would be redundant unless traffic
       analysis is felt to be a threat.  If IPv6 is running authenticated,
       then authentication of IPv4 will add little.  Conversely, IPv4
       security will not protect IPv6 traffic once it leaves the ISATAP
       domain.  Therefore, implementing IPv6 security is required even if
       IPv4 security is available.

       The threats associated with IPv6 Neighbor Discovery are described in
       [RFC3756].

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       There is a possible spoofing attack in which spurious ip-protocol-41
       packets are injected into an ISATAP link from outside.  Since an
       ISATAP link spans an entire IPv4 site, restricting access to the link
       can be achieved by restricting access to the site; i.e., by having
       site border routers implement IPv4 ingress filtering and ip-
       protocol-41 filtering.

       Another possible spoofing attack involves spurious ip-protocol-41
       packets injected from within an ISATAP link by a node pretending to
       be a router.  The Potential Router List (PRL) provides a list of IPv4
       addresses representing advertising ISATAP interfaces of routers that
       hosts use in filtering decisions.  Site administrators should ensure
       that the PRL is kept up to date, and that the resolution mechanism
       (see Section 9) cannot be subverted.

       The use of temporary addresses [RFC3041] and Cryptographically
       Generated Addresses [CGA] on ISATAP interfaces is outside the scope
       of this specification.

    11.  IANA Considerations

       The IANA has specified the format for Modified EUI-64 address
       construction ([RFC3513], Appendix A) in the IANA Ethernet Address
       Block.  The text in Appendix A of this document has been offered as
       an example specification.  The current version of the IANA registry
       for Ether Types can be accessed at:

          http://www.iana.org/assignments/ethernet-numbers

    12.  Acknowledgements

       The ideas in this document are not original, and the authors
       acknowledge the original architects.  Portions of this work were
       sponsored through SRI International internal projects and government
       contracts.  Government sponsors include Monica Farah-Stapleton and
       Russell Langan (U.S. Army CECOM ASEO), and Dr. Allen Moshfegh (U.S.
       Office of Naval Research).  SRI International sponsors include Dr.
       Mike Frankel, J. Peter Marcotullio, Lou Rodriguez, and Dr. Ambatipudi
       Sastry.

       The following are acknowledged for providing peer review input: Jim
       Bound, Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader,
       Ole Troan, and Vlad Yasevich.

       The following are acknowledged for their significant contributions:
       Alain Durand, Hannu Flinck, Jason Goldschmidt, Nathan Lutchansky,
       Karen Nielsen, Mohan Parthasarathy, Chirayu Patel, Art Shelest,
       Markku Savela, Pekka Savola, Margaret Wasserman, and Brian Zill.

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       The authors acknowledge the work of Quang Nguyen on "Virtual
       Ethernet", under the guidance of Dr. Lixia Zhang, that proposed very
       similar ideas to those that appear in this document.  This work was
       first brought to the authors' attention on September 20, 2002.

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    Appendix A.  Modified EUI-64 Addresses in the IANA Ethernet Address
                 Block

       Modified EUI-64 addresses ([RFC3513], Section 2.5.1 and Appendix A)
       in the IANA Ethernet Address Block are formed by concatenating the
       24-bit IANA OUI (00-00-5E) with a 40-bit extension identifier and
       inverting the "u" bit; i.e., the "u" bit is set to one (1) to
       indicate universal scope and set to zero (0) to indicate local scope.

       Modified EUI-64 addresses have the following appearance in memory
       (bits transmitted right-to-left within octets, octets transmitted
       left-to-right):

       0                       23                                        63
       |        OUI            |            extension identifier         |
       000000ug00000000 01011110xxxxxxxx xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx

       When the first two octets of the extension identifier encode the
       hexadecimal value 0xFFFE, the remainder of the extension identifier
       encodes a 24-bit vendor-supplied id as follows:

       0                       23               39                       63
       |        OUI            |     0xFFFE     |   vendor-supplied id   |
       000000ug00000000 0101111011111111 11111110xxxxxxxx xxxxxxxxxxxxxxxx

       When the first octet of the extension identifier encodes the
       hexadecimal value 0xFE, the remainder of the extension identifier
       encodes a 32-bit IPv4 address as follows:

       0                       23      31                                63
       |        OUI            |  0xFE |           IPv4 address          |
       000000ug00000000 0101111011111110 xxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxx

    Normative References

       [BCP14]    Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [STD13]    Mockapetris, P., "Domain names - implementation and
                  specification", STD 13, RFC 1035, November 1987.

       [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
                  (IPv6) Specification", RFC 2460, December 1998.

       [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
                  Discovery for IP Version 6 (IPv6)", RFC 2461, December
                  1998.

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    RFC 4214                         ISATAP                     October 2005

       [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
                  Autoconfiguration", RFC 2462, December 1998.

       [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
                  (IPv6) Addressing Architecture", RFC 3513, April 2003.

       [MECH]     Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
                  for IPv6 Hosts and Routers", RFC 4213, October 2005.

    Informative References

       [RFC2491]  Armitage, G., Schulter, P., Jork, M., and G. Harter, "IPv6
                  over Non-Broadcast Multiple Access (NBMA) networks", RFC
                  2491, January 1999.

       [RFC2492]  Armitage, G., Schulter, P., and M. Jork, "IPv6 over ATM
                  Networks", RFC 2492, January 1999.

       [RFC2529]  Carpenter, B. and C. Jung, "Transmission of IPv6 over IPv4
                  Domains without Explicit Tunnels", RFC 2529, March 1999.

       [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
                  Stateless Address Autoconfiguration in IPv6", RFC 3041,
                  January 2001.

       [RFC3056]  Carpenter, B. and K. Moore, "Connection of IPv6 Domains
                  via IPv4 Clouds", RFC 3056, February 2001.

       [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
                  Discovery (ND) Trust Models and Threats", RFC 3756, May
                  2004.

       [CGA]      Aura, T., "Cryptographically Generated Addresses (CGA)",
                  RFC 3972, March 2005.

       [DEFLT]    Draves, R. and D. Thaler, "Default Router Preferences and
                  More-Specific Routes", Work in Progress, December 2003.

       [NODEREQ]  Loughney, J., Ed., "IPv6 Node Requirements", Work in
                  Progress, May 2004.

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    RFC 4214                         ISATAP                     October 2005

    Authors' Addresses

       Fred L. Templin
       Nokia
       313 Fairchild Drive
       Mountain View, CA  94110
       US

       EMail: fltemplin@acm.org

       Tim Gleeson
       Cisco Systems K.K.
       Shinjuku Mitsui Building
       2-1-1 Nishishinjuku, Shinjuku-ku
       Tokyo  163-0409
       Japan

       EMail: tgleeson@cisco.com

       Mohit Talwar
       Microsoft Corporation
       One Microsoft Way
       Redmond, WA  98052-6399
       US

       Phone: +1 425 705 3131
       EMail: mohitt@microsoft.com

       Dave Thaler
       Microsoft Corporation
       One Microsoft Way
       Redmond, WA  98052-6399
       US

       Phone: +1 425 703 8835
       EMail: dthaler@microsoft.com

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    RFC 4214                         ISATAP                     October 2005

    Full Copyright Statement

       Copyright (C) The Internet Society (2005).

       This document is subject to the rights, licenses and restrictions
       contained in BCP 78 and at www.rfc-editor.org/copyright.html, and
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    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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