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Network Working Group                                         F. Templin
Internet-Draft                                                     Nokia
Expires: July 25, 2003                                        T. Gleeson
                                                      Cisco Systems K.K.
                                                               M. Talwar
                                                               D. Thaler
                                                   Microsoft Corporation
                                                        January 24, 2003


        Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
                    draft-ietf-ngtrans-isatap-12.txt

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
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   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
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   The list of current Internet-Drafts can be accessed at http://
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   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 25, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

   This document specifies an Intra-Site Automatic Tunnel Addressing
   Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4
   sites.  ISATAP treats the site's IPv4 infrastructure as a link layer
   for IPv6 with no requirement for IPv4 multicast.  ISATAP enables
   intra-site automatic IPv6-in-IPv4 tunneling whether globally assigned
   or private IPv4 addresses are used.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  3
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . .  4
   6.  Automatic Tunneling  . . . . . . . . . . . . . . . . . . . . .  5
   7.  Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . .  7
   8.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Security considerations  . . . . . . . . . . . . . . . . . . . 11
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
       Normative References . . . . . . . . . . . . . . . . . . . . . 12
       Informative References . . . . . . . . . . . . . . . . . . . . 13
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
   A.  Major Changes  . . . . . . . . . . . . . . . . . . . . . . . . 16
   B.  Rationale for Interface Identifier Construction  . . . . . . . 17
   C.  ISATAP Interface MTU Considerations  . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 23































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1. Introduction

   This document presents a simple approach called the Intra-Site
   Automatic Tunnel Addressing Protocol (ISATAP) that enables
   incremental deployment of IPv6 [1] within IPv4 [2] sites.  ISATAP
   allows dual-stack nodes that do not share a physical link with an
   IPv6 router to automatically tunnel packets to the IPv6 next-hop
   address through IPv4, i.e., the site's IPv4 infrastructure is treated
   as a link layer for IPv6.

   Specific details for the operation of IPv6 and automatic tunneling
   over ISATAP links are given, including an interface identifier format
   that embeds an IPv4 address.  This format supports IPv6 address
   configuration and simple link-layer address mapping.  Also specified
   is the operation of IPv6 Neighbor Discovery and deployment/security
   considerations.

2. Applicability Statement

   ISATAP provides the following features:

   o  treats site's IPv4 infrastructure as a link layer for IPv6 using
      automatic IPv6-in-IPv4 tunneling

   o  enables incremental deployment of IPv6 hosts within IPv4 sites
      with no aggregation scaling issues at border gateways

   o  requires no special IPv4 services within the site (e.g.,
      multicast)

   o  supports both stateless address autoconfiguration and manual
      configuration

   o  supports networks that use non-globally unique IPv4 addresses
      (e.g., when private address allocations [10] are used)

   o  compatible with other NGTRANS mechanisms (e.g., 6to4 [11])


3. 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 [3].

   This document also makes use of internal conceptual variables to
   describe protocol behavior and external variables that an
   implementation must allow system administrators to change.  The



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   specific variable names, how their values change, and how their
   settings influence protocol behavior are provided to demonstrate
   protocol behavior.  An implementation is not required to have them in
   the exact form described here, so long as its external behavior is
   consistent with that described in this document.

4. Terminology

   The terminology of RFC 2460 [1] applies to this document.  The
   following additional terms are defined:

   link, on-link, off-link:
      same definitions as ([4], section 2.1).

   underlying link:
      a link layer that supports IPv4 (for ISATAP), and MAY also support
      IPv6 natively.

   ISATAP link:
      one or more underlying links used for tunneling.  The IPv4 network
      layer addresses of the underlying links are used as link-layer
      addresses on the ISATAP link.

   ISATAP interface:
      a node's attachment to an ISATAP link.

   advertising ISATAP interface:
      same meaning as "advertising interface" in ([4], section 6.2.2).

   ISATAP address:
      an on-link address on an ISATAP interface and with an interface
      identifier constructed as specified in Section 5.1


5. Basic IPv6 Operation

   ISATAP links transmit IPv6 packets via automatic tunnels using the
   site's IPv4 infrastructure as a link layer for IPv6, i.e., IPv6
   treats the site's IPv4 infrastructure as a Non-Broadcast, Multiple
   Access (NBMA) link layer.  The following considerations for IPv6 on
   ISATAP links are noted:

5.1 Interface Identifiers and Unicast Addresses

   ISATAP interface identifiers use "modified EUI-64" format ([5],
   section 2.5.1) and are formed by appending an IPv4 address on the
   ISATAP link to the 32-bit string '00-00-5E-FE'.  Appendix B includes
   non-normative rationale for this construction rule.



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   With reference to ([5], sections 2.5.4, 2.5.6), global and local-use
   ISATAP addresses are constructed as follows:

    |           64 bits            |     32 bits   |    32 bits     |
    +------------------------------+---------------+----------------+
    | global or local-use unicast  |   0000:5EFE   |  IPv4 Address  |
    |            prefix            |               | of ISATAP link |
    +------------------------------+---------------+----------------+


5.2 ISATAP Link/Interface Configuration

   ISATAP links consist of one or more underlying links that support
   IPv4 for tunneling within a site.

   ISATAP interfaces are configured over ISATAP links; each IPv4 address
   assigned to an underlying link is seen as a link-layer address for
   ISATAP.

   Neighbor discovery on ISATAP links (see: Section 7) provides the
   functional equivalent of unicast virtual circuits (VCs) required for
   other NBMA media types ([6], section 4.6).  Neighbor state
   information MAY be kept in the Conceptual Neighbor Cache ([4],
   section 5.1).

5.3 Link Layer Address Options

   With reference to ([6], section 5.2), when the [NTL] and [STL] fields
   in an ISATAP link layer address option encode 0, the [NBMA Number]
   field encodes a 4-octet IPv4 address.

5.4 Multicast and Anycast

   ISATAP interfaces recognize a node's required addresses as specified
   in ([5], section 2.8).

   Mechanisms for multicast/anycast emulation on ISATAP links (e.g.,
   adaptations of MLD [12], PIM-SM [13], MARS [14], etc.) are subject
   for future companion document(s).

6. Automatic Tunneling

   The common tunneling mechanisms specified in ([7], sections 2 and 3)
   are used, with the following noted considerations for ISATAP:

6.1 Dual IP Layer Operation

   ISATAP uses the same specification found in ([7], section 2).  That



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   is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
   are able to send and receive both IPv4 and IPv6 packets.

   Address configuration and DNS considerations are the same as ([7],
   sections 2.1 through 2.3).

6.2 Encapsulation/Decapsulation

   The specifications in ([7], sections 3.1 and 3.6) are used.
   Additionally, the IPv6 next-hop address for packets encapsulated on
   an ISATAP link MUST be an ISATAP address; other packets are discarded
   and an ICMPv6 destination unreachable indication with code 3 (Address
   Unreachable) ([8], section 3.1) is returned to the source.

6.3 Tunnel MTU and Fragmentation

   ISATAP automatic tunnel interfaces may be configured over multiple
   underlying links with diverse maximum transmission units (MTUs).  The
   minimum MTU for IPv6 interfaces is 1280 bytes ([1], Section 5), but
   the following considerations apply for ISATAP interfaces:

   o  Nearly all IPv4 nodes connect to physical links with MTUs of 1500
      bytes or larger (e.g., Ethernet)

   o  Sub-IPv4 layer encapsulations (e.g., VPN) may occur on some paths

   o  Commonly-deployed VPN interfaces use an MTU of 1400 bytes

   To maximize efficiency and minimize IPv4 fragmentation for the
   predominant deployment case, the ISATAP interface MTU, or "LinkMTU"
   (see: [4], Section 6.3.2 ), SHOULD be set to no more than 1380 bytes
   (1400 minus 20 bytes for IPv4 encapsulation).  LinkMTU MAY be set to
   larger values when a dynamic link layer MTU discovery mechanism is
   used or when a static MTU assignment is used and additional
   fragmentation in the site's IPv4 network is deemed acceptable.  See
   Appendix C for non-normative ISATAP interface MTU considerations.

   When a dynamic MTU discovery mechanism is not used, the ISATAP link
   layer encapsulates IPv6 packets with the Don't Fragment (DF) bit not
   set in the encapsualting IPv4 header.

6.4 Handling IPv4 ICMP Errors

   IPv4 ICMP errors and ARP failures are processed as link error
   notifications.

6.5 Local-Use IPv6 Unicast Addresses




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   The specification in ([7], section 3.7) is not used.  Instead, local
   use IPv6 unicast addresses are formed as specified in Section 5.1.

6.6 Ingress Filtering

   The specification in ([7], section 3.9) is used.  In particular,
   ISATAP nodes that forward decapsulated packets MUST verify the tunnel
   source address is acceptable.

7. Neighbor Discovery

   The specification in ([7], section 3.8) applies only to configured
   tunnels.  RFC 2461 [4] provides the following guidelines for
   non-broadcast multiple access (NBMA) link support:

      "Redirect, Neighbor Unreachability Detection and next-hop
      determination should be implemented as described in this document.
      Address resolution and the mechanism for delivering Router
      Solicitations and Advertisements on NBMA links is not specified in
      this document."

   ISATAP links SHOULD implement Redirect, Neighbor Unreachability
   Detection, and next-hop determination exactly as specified in [4].
   Address resolution and the mechanisms for delivering Router
   Solicitations and Advertisements for ISATAP links are not specified
   by [4]; instead, they are specified in the following sections of this
   document.

7.1 Address Resolution and Neighbor Unreachability Detection

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

   Following static address resolution, hosts SHOULD perform an initial
   reachability confirmation by sending Neighbor Solicitation (NS)
   message(s) and receiving a Neighbor Advertisement (NA) message using
   the mechanisms specified in ([4], section 7.2.).  When the ISATAP
   interface provides a multicast emulation mechanism (see: Section 5.4)
   solicitations are sent to the solicited-node multicast address
   corresponding to the target address.  Otherwise, the solicitation is
   sent to the target's unicast address.

   Hosts SHOULD additionally perform Neighbor Unreachability Detection
   (NUD) as specified in ([4], section 7.3).  Routers MAY perform the
   above-specified reachability detection and NUD procedures, but this
   might not scale in all environments.




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   All ISATAP nodes MUST send solicited neighbor advertisements ([4],
   section 7.2.4).

7.2 Duplicate Address Detection

   Duplicate Address Detection ([9], section 5.4) is not required for
   ISATAP addresses, since duplicate address detection is assumed
   already performed for the IPv4 addresses from which they derive.

7.3 Router and Prefix Discovery

   The following sections describe mechanisms to support the router and
   prefix discovery process ([4], section 6) on ISATAP links:

7.3.1 Conceptual Data Structures

   ISATAP nodes use the conceptual data structures Prefix List and
   Default Router List exactly as in ([4], section 5.1).  ISATAP links
   add a new conceptual data structure "Potential Router List" (PRL) and
   the following new configuration variable:

   PrlRefreshInterval
      Time in seconds between successive refreshments of the PRL after
      initialization.  SHOULD be no less than 3,600 seconds.

      Default: 3,600 seconds

   A PRL is associated with every ISATAP link.  Each entry in the PRL
   ("PRL(i)") has an IPv4 address ("V4ADDR(i)") that represents an
   advertising ISATAP interface and an associated timer ("TIMER(i)").
   The process for initializing and refreshing the PRL is described
   below:

   When a node enables an ISATAP link, it initializes the PRL with IPv4
   addresses.  The addresses MAY be discovered via a DHCPv4 [15] option
   for ISATAP (option code TBD), manual configuration, or an unspecified
   alternate method (e.g., DHCPv4 vendor-specific option).

   When no other mechanisms are available, a DNS fully-qualified domain
   name (FQDN) [16] established by an out-of-band method (e.g., DHCPv4,
   manual configuration, etc.) MAY be used.  The FQDN is resolved into
   IPv4 addresses for the PRL through a static host file, a
   site-specific name service, querying a DNS server within the site, or
   an unspecified alternate method.  There are no mandatory rules for
   the selection of a FQDN, but manual configuration MUST be supported.
   When DNS is used, client resolvers use the IPv4 transport.

   After initialization, nodes periodically refresh the PRL (i.e., using



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   one or more of the methods described above) after PrlRefreshInterval.

7.3.2 Validation of Router Advertisements Messages

   The specification in ([4], section 6.1.2) is used.

   Additionally, received RA messages that contain Prefix Information
   options and/or encode non-zero values in the Cur Hop Limit, Router
   Lifetime, Reachable Time, or Retrans Timer fields (see: [4], section
   4.2) MUST satisfy the following validity check for ISATAP:

   o  the network-layer (IPv6) source address is an ISATAP address and
      embeds V4ADDR(i) for some PRL(i)


7.3.3 Router Specification

   Routers with advertising ISATAP interfaces behave the same as
   described in ([4], section 6.2).  As permitted by ([4], section
   6.2.6), advertising ISATAP interfaces SHOULD send unicast RA messages
   to a soliciting host's address when the solicitation's source address
   is not the unspecified address.

7.3.4 Host Specification

   When no unsolicited RA messages containing prefix information options
   and/or non-zero router lifetime values are received, hosts MAY send
   Router Solicitation (RS) messages using the specification in Section
   7.3.4.1.  RA messages (whether solicited or unsolicited) are
   processed using the specification in Section 7.3.4.2.

7.3.4.1 Sending Router Solicitations

   All PRL(i)'s are assumed to represent active advertising ISATAP
   interfaces within the site, i.e., the PRL provides trust basis only;
   not reachability detection.  Hosts periodically solicit information
   from one or more PRL(i) by sending Router Solicitation (RS) messages.
   The manner of selecting a PRL(i) for solicitation and/or deprecating
   a previously-selected PRL(i) is outside the scope of this
   specification.  Hosts add the following variable to support the
   solicitation process:

   MinRouterSolicitInterval
      Minimum time in seconds between successive solicitations of the
      same advertising ISATAP interface.  SHOULD be no less than 900
      seconds.

      Default: 900 seconds



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   Solicitation consists of sending RS messages using the interface's
   link-local unicast addresses as the source address.  When the ISATAP
   interface provides a multicast emulation mechanism (see: Section
   5.4), RS messages are sent to the All-Routers multicast address.
   Otherwise, they are sent to the link-local ISATAP address constructed
   from V4ADDR(i) for some PRL(i) selected for solicitation.  The RS
   messages are otherwise sent exactly as in ([4], section 6.3.7).

7.3.4.2 Processing Router Advertisements

   Hosts process received RA messages exactly as in ([4], section 6.3.4)
   and ([9], section 5.5.3).  (But, see Appendix C for non-normative
   considerations for RA messages containing MTU options.)

   When the source address of the RA message is an ISATAP address that
   embeds V4ADDR(i) for some PRL(i) selected for solicitation, hosts
   additionally reset TIMER(i).  Let "MIN_LIFETIME" be the minimum value
   in the router lifetime or the lifetime(s) encoded in options included
   in the RA message.  Then, TIMER(i) is reset to:

      MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)


8. Deployment Considerations

8.1 Host And Router Deployment Considerations

   For hosts, if an underlying link supports both IPv4 (over which
   ISATAP is implemented) and also supports IPv6 natively, then ISATAP
   MAY be enabled if the native IPv6 layer does not receive Router
   Advertisements (i.e., does not have connection with an IPv6 router).
   After a non-link-local address has been configured and a default
   router acquired on the native link, the host SHOULD discontinue the
   router solicitation process described in the Host Specification
   (Section 7.3.4) and allow existing ISATAP address configurations to
   expire as specified in ([4], section 5.3) and ([9], section 5.5.4).
   Any ISATAP addresses added to the DNS for this host should also be
   removed.  In this way, ISATAP use will gradually diminish as IPv6
   routers are widely deployed throughout the site.

   Routers MAY configure both a native IPv6 and ISATAP interface over
   the same physical link.  Routing will operate as usual between these
   two domains.  Note that the prefixes used on the ISATAP and native
   IPv6 interfaces will be distinct.  The IPv4 address(es) configured on
   a router's advertising ISATAP interface(s) SHOULD be added (either
   automatically or manually) to the site's address records for
   advertising ISATAP interfaces.




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8.2 Site Administration Considerations

   The following considerations are noted for sites that deploy ISATAP:

   o  ISATAP links are administratively defined by a set of advertising
      ISATAP interfaces and set of nodes which discover those interface
      addresses.  Thus, ISATAP links are defined by administrative (not
      physical) boundaries.

   o  Hosts and routers that use ISATAP can be deployed in an ad-hoc
      fashion.  In particular, hosts can be deployed with little/no
      advanced knowledge of existing routers, and routers can be
      deployed with no reconfiguration requirements for hosts.

   o  Site administrators maintain a list of IPv4 addresses representing
      advertising ISATAP interfaces and make them available via one or
      more of the mechanisms described in Section 7.3.1.  ISATAP nodes
      use this list to initialize and periodically refresh the PRL.
      Responsible site administration can reduce the control traffic.
      At a minimum, administrators SHOULD ensure that dynamically
      advertised information for the site's PRL is well maintained.


9. IANA Considerations

   A DHCPv4 option code for ISATAP (TBD) [17] may be requested in the
   event that this document or a derivative thereof is moved to
   standards track.

   Modifications to the IANA "ethernet-numbers" registry (e.g., based on
   text in Appendix B) may be requested in the event that this document
   or a derivative thereof is moved to standards track.

10. Security considerations

   ISATAP site border routers and firewalls MUST implement IPv6 ingress
   filtering and MUST NOT forward packets with site-local source and/or
   destination addresses outside of the site [18].

   In addition to possible attacks against IPv6, security attacks
   against IPv4 must also be considered.  In particular, border routers
   and firewalls MUST implement IPv4 ingress filtering and
   ip-protocol-41 filtering.

   Even with IPv4 and IPv6 ingress filtering, reflection attacks can
   originate from compromised nodes within an ISATAP site that spoof
   IPv6 source addresses.  Security mechanisms for reflection attack
   mitigation (e.g., [19], [20], etc.) SHOULD be used in routers with



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   advertising ISATAP interfaces.  At a minimum, ISATAP site border
   gateways MUST log potential source address spoofing cases.

   IPv6 Neighbor Discovery trust models and threats [21] apply also to
   ISATAP.  However, ([21], section 4.4.) shows that most of these
   threats are mitigated in corporate networks that implement site
   security mechanisms, i.e., the applicability space for ISATAP.

   ISATAP addresses do not support privacy extensions for stateless
   address autoconfiguration [22].  However, since the ISATAP interface
   identifier is derived from the node's IPv4 address, ISATAP addresses
   do not have the same level of privacy concerns as IPv6 addresses that
   use an interface identifier derived from the MAC address.  (This is
   especially true when private address allocations [10] are used.)

11. Acknowledgements

   Some of the ideas presented in this draft were derived from work at
   SRI with internal funds and contractual support.  Government sponsors
   who supported the work include Monica Farah-Stapleton and Russell
   Langan from U.S.  Army CECOM ASEO, and Dr.  Allen Moshfegh from U.S.
   Office of Naval Research.  Within SRI, Dr.  Mike Frankel, J.  Peter
   Marcotullio, Lou Rodriguez, and Dr.  Ambatipudi Sastry supported the
   work and helped foster early interest.

   The following peer reviewers are acknowledged for taking the time to
   review a pre-release of this document and provide input: Jim Bound,
   Rich Draves, Cyndi Jung, Ambatipudi Sastry, Aaron Schrader, Ole
   Troan, Vlad Yasevich.

   The authors acknowledge members of the NGTRANS community who have
   made significant contributions to this effort, including Rich Draves,
   Alain Durand, Nathan Lutchansky, Karen Nielsen, Art Shelest, Margaret
   Wasserman, and Brian Zill.

   The authors also wish to acknowledge the work of Quang Nguyen [23]
   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.

Normative References

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

   [2]  Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.

   [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement



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        Levels", BCP 14, RFC 2119, March 1997.

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

   [5]  Hinden, R. and S. Deering, "IP Version 6 Addressing
        Architecture", draft-ietf-ipngwg-addr-arch-v3-11 (work in
        progress), October 2002.

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

   [7]  Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms for
        IPv6 Hosts and Routers", draft-ietf-ngtrans-mech-v2-01 (work in
        progress), November 2002.

   [8]  Conta, A. and S. Deering, "Internet Control Message Protocol
        (ICMPv6) for the Internet Protocol Version 6 (IPv6)
        Specification", RFC 2463, December 1998.

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

Informative References

   [10]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
         Lear, "Address Allocation for Private Internets", BCP 5, RFC
         1918, February 1996.

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

   [12]  Deering, S., Fenner, W. and B. Haberman, "Multicast Listener
         Discovery (MLD) for IPv6", RFC 2710, October 1999.

   [13]  Estrin, D., Farinacci, D., Helmy, A., Thaler, D., Deering, S.,
         Handley, M. and V. Jacobson, "Protocol Independent
         Multicast-Sparse Mode (PIM-SM): Protocol Specification", RFC
         2362, June 1998.

   [14]  Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
         Networks", RFC 2022, November 1996.

   [15]  Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
         March 1997.

   [16]  Mockapetris, P., "Domain names - implementation and



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         specification", STD 13, RFC 1035, November 1987.

   [17]  Droms, R., "Procedures and IANA Guidelines for Definition of
         New DHCP Options and Message Types", BCP 43, RFC 2939,
         September 2000.

   [18]  Hinden, R., "IPv6 Globally Unique Site-Local Addresses",
         draft-hinden-ipv6-global-site-local-00 (work in progress),
         December 2002.

   [19]  Savola, P., "Security Considerations for 6to4",
         draft-savola-ngtrans-6to4-security-01 (work in progress), March
         2002.

   [20]  Bellovin, S., Leech, M. and T. Taylor, "ICMP Traceback
         Messages", draft-ietf-itrace-03 (work in progress), January
         2003.

   [21]  Nikander, P., "IPv6 Neighbor Discovery trust models and
         threats", draft-ietf-send-psreq-01 (work in progress), January
         2003.

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

   [23]  Nguyen, Q., "http://irl.cs.ucla.edu/vet/report.ps", spring
         1998.

   [24]  Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

   [25]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
         November 1990.

   [26]  Postel, J., "Internet Control Message Protocol", STD 5, RFC
         792, September 1981.

   [27]  Baker, F., "Requirements for IP Version 4 Routers", RFC 1812,
         June 1995.

   [28]  Lahey, K., "TCP Problems with Path MTU Discovery", RFC 2923,
         September 2000.

   [29]  McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for
         IP version 6", RFC 1981, August 1996.

   [30]  Jacobson, V., Braden, B. and D. Borman, "TCP Extensions for
         High Performance", RFC 1323, May 1992.



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   [31]  Templin, F., "Neighbor Affiliation Protocol for
         IPv6-over-(foo)-over-IPv4", draft-templin-v6v4-ndisc-01 (work
         in progress), November 2002.


Authors' Addresses

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

   Phone: +1 650 625 2331
   EMail: ftemplin@iprg.nokia.com


   Tim Gleeson
   Cisco Systems K.K.
   Shinjuku Mitsu 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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Appendix A. Major Changes

   changes from version 11 to version 12:

   o  Added comments from co-authors

   o  Revised PRL initialization

   o  Updated MTU section

   changes from version 10 to version 11:

   o  Added multicast/anycast subsection

   o  Revised PRL initialization

   o  Updated neighbor discovery, security consideration sections

   o  Updated MTU section

   changes from version 09 to version 10:

   o  Rearranged/revised sections 5, 6, 7

   o  updated MTU section

   changes from version 08 to version 09:

   o  Added stateful autoconfiguration mechanism

   o  Normative references to RFC 2491, RFC 2462

   o  Moved non-normative MTU text to appendix C

   changes from version 07 to version 08:

   o  updated MTU section

   changes from version 06 to version 07:

   o  clarified address resolution, Neighbor Unreachability Detection

   o  specified MTU/MRU requirements

   changes from earlier versions to version 06:

   o  Addressed operational issues identified in 05 based on discussion
      between co-authors



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   o  Clarified ambiguous text per comments from Hannu Flinck; Jason
      Goldschmidt

   o  Moved historical text in section 4.1 to Appendix B in response to
      comments from Pekka Savola

   o  Identified operational issues for anticipated deployment scenarios

   o  Included reference to Quang Nguyen work


Appendix B. Rationale for Interface Identifier Construction

   ISATAP specifies an EUI64-format address construction for the
   Organizationally-Unique Identifier (OUI) owned by the Internet
   Assigned Numbers Authority (IANA).  This format (given below) is used
   to construct both native EUI64 addresses for general use and modified
   EUI-64 format interface identifiers for IPv6 unicast addresses:

   |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; specifies use of (TSE, TSD) (1 octet)

      TSE     Type-Specific Extension (1 octet)

      TSD     Type-Specific Data (3 octets)

   And the following interpretations are specified 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 id
                   0xFF         RESERVED by IEEE/RAC



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   Thus, if TYPE=0xFE, TSE is an extension of TSD.  If TYPE=0xFF, TSE is
   an extension of TYPE.  Other values for TYPE (thus, other
   interpretations of TSE, TSD) are reserved for future IANA use.

   The above specification is compatible with all aspects of EUI64,
   including support for encapsulating legacy EUI-48 interface
   identifiers (e.g., an IANA EUI-48 format multicast address such as:
   '01-00-5E-01-02-03' is encapsulated as: '01-00-5E-FF-FE-01-02-03').
   But, the specification also provides a special TYPE (0xFE) to
   indicate an IPv4 address is embedded.  Thus, when the first four
   octets of an IPv6 interface identifier are: '00-00-5E-FE' (note: the
   'u/l' bit MUST be 0) the interface identifier is said to be in
   "ISATAP format" and the next four octets embed an IPv4 address
   encoded in network byte order.

Appendix C. ISATAP Interface MTU Considerations

   ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
   separated by multiple link layer (IPv4) forwarding hops.  Thus, the
   path MTU of the underlying IPv4 network may determine the uni-
   directional IPv6 per-neighbor MTU from the encapsulator to the
   decapsulator.  (Note that this constitutes the MTU of only one hop in
   what may be a multiple-hop IPv6 path.) When the encapsulator's ISATAP
   interface configures a large LinkMTU value (see: Section 6.3),
   special considerations apply as described in the following
   non-normative sections:

C.1 Stateless (Static) MTU Assignment

   Nodes that connect to the Internet should be able to reassemble and/
   or discard IPv4 packets up to 64KB in length when the DF bit is not
   set in the encapsulating IPv4 header.  Nodes that cannot reassemble/
   discard maximum-length IPv4 packets are vulnerable to buffer overrun
   attacks.  This issue may be obviated for nodes that are accessed only
   within a site (i.e., do not connect directly to the Internet) since
   site border gateways, etc.  can filter and discard fragments of large
   packets before they reach constrained node(s).

   When the ISATAP encapsulator does not implement a dynamic link layer
   mechanism to determine per-neighbor MTUs, all IPv6 packets are
   encapsulated with the DF bit not set in the IPv4 header.
   Additionally, LinkMTU may be set to a value that is no more than the
   smallest Effective MTU to Receive (EMTU_R) (see: RFC 1122 [24],
   section 3.3.2) for all potential decapsulators in the site.  The
   value chosen for LinkMTU must be at least 1280 bytes (the minimum
   IPv6 MTU) and such that the potential worst-case level of
   fragmentation in the underlying IPv4 network is deemed "acceptable"
   by the site's standards.



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   For example, when all decapsulators in the site are known to have an
   EMTU_R of 10KB and the site's IPv4 routers are optimized for IPv4
   fragmentation, encapsulators may be able to use LinkMTU values as
   large as 10KB (minus 20 bytes for IPv4 encapsulation).  Conversely,
   when IPv4 fragmentation causes performance degradation along some
   paths, LinkMTU should be set to a smaller value.

   Nodes that use a static MTU assignment SHOULD copy the value in an
   MTU option received in any Router Advertisement message into LinkMTU
   for the ISATAP interface as specified in ([4], section 6.3.4).

C.2 Stateful (Dynamic) MTU Determination

   When the encapsulator implements a dynamic MTU determination
   mechanism it keeps a link layer cache of per-neighbor MTU values
   (e.g., as ancillary data in the IPv6 neighbor cache, in the IPv4 path
   MTU discovery cache, etc.).  IPv4 path MTU discovery [25] uses ICMPv4
   "fragmentation needed" messages, but these generally do not provide
   enough information for stateless translation to ICMPv6 "packet too
   big" messages (see: RFC 792 [26] and RFC 1812 [27], section 4.3.2.3).
   Additionally, ICMPv4 "fragmentation needed" messages can be spoofed,
   filtered, or not sent at all by some forwarding nodes.  Thus, IPv4
   Path MTU discovery used alone may be inadequate and can result in
   black holes that are difficult to diagnose [28].

   Alternate methods for determining per-neighbor MTUs should be used
   when RFC 1191 path MTU discovery is deemed inadequate.  In these
   methods, the encapsulator uses periodic and/or on-demand probing of
   the IPv4 path to the decapsulator to initialize and update cache
   entries.  The following three probing methods (among others) are
   possible:

   1.  Encapsulator-driven - the encapsulator periodically sends probe
       packets with the DF bit set in the IPv4 header and waits for a
       positive acknowledgement from the decapsulator that the probe was
       received

   2.  Decapsulator-driven - the encapsulator sends all packets with the
       DF bit NOT set in the IPv4 header unless and until the
       decapsulator sends a "Fragmentation Experienced" indication(s)

   3.  Hybrid - the encapsulator and decapsulator engage in a dialogue
       and use "intelligent" probing to monitor the path MTU

   These methods are discussed in detail in the following subsections:

C.2.1 Encapsulator-driven Method




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   In this method, the encapsulator sets the DF bit in the IPv4 header
   of probe packets.  Probe packets may be sent either when the
   encapsulator's link layer forwards a large data packet to the
   decapsulator (i.e., on-demand) or when the path MTU for the
   decapsulator has not been verified for some time (i.e., periodic).
   IPv6 Neighbor Solicitation (NS) or ICMPv6 ECHO_REQUEST packets with
   padding bytes added could be used for this purpose, since successful
   delivery results in a positive acknowledgement that the probe
   succeeded vis-a-vis a response from the decapsulator.

   While probing, the encapsulator maintains a queue of packets that
   have the decapsulator as the IPv6 next-hop address.  If the probe
   succeeds, packets in the queue that are no larger than the probe size
   are sent to the decapsulator.  If the probe fails, packets that are
   larger than the last known successful probe are dropped and an ICMPv6
   "packet too big" message returned to the sender [29].  The queue
   should be large enough to buffer the (delay*bandwidth) product for
   the round-trip time to the decapsulator.  When smaller queues are
   used, loss of packets that are too big for the yet-to-be-determined
   path MTU may occur with no ICMPv6 "packet too big" message returned.
   Such loss may occur only in rare instances, but may result in
   unpredictable behavior in senders that base their adaptation solely
   on ICMPv6 "packet too big" messages.

   This method has the advantage that the decapsulator need not
   implement any special mechanisms, since standard IPv6 request/
   response mechanisms are used.  Additionally, the encapsulator is
   assured that any packets that are too large for the decapsulator to
   receive will be dropped by the network.  Disadvantages for this
   method include the fact that probe packets do not carry data and thus
   consume network resources.  Additionally, queues may become large on
   Long, Fat Networks (LFNs) (see: RFC 1323 [30]).

C.2.2 Decapsulator-driven Method

   In this method, the encapsulator sends all packets with the DF bit
   NOT set in the IPv4 header with the expectation that the decapsulator
   will send a "Fragmentation Experienced" indication if the IPv4
   network fragments packets.  In other words, the decapsulator simply
   sends all packets that are no larger than LinkMTU unless and until it
   receives "Fragmentation Experienced" messages from the decapsulator.
   The decapsulator can use IPv6 Router Advertisement (RA) messages with
   an MTU option as the means for both reporting fragmentation and
   informing the encapsulator of a new MTU value to use.

   This method has the advantage that the data packets themselves are
   used as probes and no queuing on the encapsulator is necessary.
   (When large data packets for probing are not available, smaller data



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   packets can be null-padded to the desired probe size by artificially
   inflating the length field in the IPv4 header; leaving the IPv6
   length unchanged.) An additional advantage is that fewer packets will
   be lost since the decapsulator will quite often be able to reassemble
   packets fragmented by the network.  The primary disadvantage for this
   method is that, using the current specifications, the encapsulator
   has no way of knowing whether a particular decapsulator implements
   the "fragmentation experienced" signaling capability.  However, the
   "fragmentation experienced" indication can be trivially implemented
   in an application on the decapsulator that uses the Berkeley Packet
   Filter (aka, libpcap) to listen for fragmented packets from
   encapsulators.

   When fragmented packets arrive, the decapsulator sends IPv6 RA
   messages with an MTU option to inform the encapsulator that
   fragmentation has been experienced and a new value for the neighbor's
   MTU should be used.  The decapsulator additionally sends ICMPv6
   "packet too big" messages to the original source when a fragmented
   packet is not correctly reassembled.  This function need not be built
   into the decapsulator's operating system and can be added as an
   after-market feature.  Finally, simply adding an extra bit in a
   neighbor discovery message header would provide a means for the
   decapsulator to inform the encapsulator that dynamic MTU discovery is
   supported.

C.2.3 Hybrid Method

   In this method, the encapsulator and decapsulator engage in a
   "neighbor affiliation" protocol to negotiate link-layer parameters
   such as MTU.  (See: [31] for an example of such an approach.) This
   approach has the advantage that bi-directional links are used and
   both ends of the link have unambiguous knowledge that the other end
   implements the protocol.  However, the signaling protocol between the
   endpoints is complicated and additional state is required in both the
   encapsulator and decapsultor.  The hybrid method seems best suited to
   implementation in a reliable transport-layer protocol rather than at
   the network/link layer.

C.2.4 Additional Notes

   o  In all dynamic methods, some packet loss due to link/buffer
      restrictions may occur with no ICMPv6 "packet too big" message
      returned to the sender.  Unenlightened senders will interpret such
      loss as loss due to congestion, which may result in longer
      convergence to the actual path MTU.  Enlightened senders will
      interpret the loss as due to link/buffer restrictions and
      immediately reduce their MTU estimate.




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   o  In all dynamic methods, when a Router Advertisement (RA) message
      includes an MTU option hosts SHOULD NOT copy the option's value
      into LinkMTU for the ISATAP interface.  Instead, when the ISATAP
      interface uses a per-neighbor path MTU cache, hosts SHOULD copy
      the MTU option's value into the cache entry for the neighbor that
      sent the RA message.  This leaves an ambiguous interpretation for
      processing received RA messages which could be eliminated if [4]
      were modified to allow Neighbor Advertisement (NA) messages to
      carry MTU options.

   o  In all methods, a "minimum MTU" must be supported by all nodes for
      multicast (i.e., even when multicast is emulated on the NBMA IPv4
      network.) The mechanisms described above speak only to the unicast
      case for MTU determination.

   o  To avoid denial-of-service attacks that would cause superfluous
      probing based on counting down/up by small increments, plateau
      tables (e.g., [25], section 7) should be used when the actual MTU
      value is indeterminant.

   o  ICMPv4 "fragmentation needed" messages may result when a link
      restriction is encountered but may also come from denial of
      service attacks.  Implementations should treat ICMPv4
      "fragmentation needed" messages as "tentative" negative
      acknowledgments and apply heuristics to determine when to suspect
      an actual link restriction and when to ignore the messages.  IPv6
      packets lost due actual link restrictions are perceived as lost
      due to congestion by the original source, but robust
      implementations minimize instances of such packet loss without
      ICMPv6 "packet too big" messages returned to the sender.





















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