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RFC 4214 - Intra-Site Automatic Tunnel Addressing Protocol (ISAT

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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).


   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.


   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)

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

   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.

   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.

      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:

   |0              1|1              3|3                              6|
   |0              5|6              1|2                              3|

   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

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

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

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.

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

8.3.1.  Host Variables

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

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

      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,

   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

   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.,

   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

   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:


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

   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.

   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.

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

   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

   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

   [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

   [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.

Authors' Addresses

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

   EMail: fltemplin@acm.org

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

   EMail: tgleeson@cisco.com

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

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

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

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

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