Network Working Group                                         F. Templin
Internet-Draft                                                     Nokia
Expires: March 10, 2004                                       T. Gleeson
                                                      Cisco Systems K.K.
                                                               M. Talwar
                                                               D. Thaler
                                                   Microsoft Corporation
                                                      September 10, 2003

        Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at http://

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

   This Internet-Draft will expire on March 10, 2004.

Copyright Notice

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


   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 unicast infrastructure as a link
   layer for IPv6. ISATAP enables intra-site automatic IPv6-in-IPv4
   tunneling whether globally assigned or private IPv4 addresses are

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Basic IPv6 Operation . . . . . . . . . . . . . . . . . . . . .  4
   5.  Automatic Tunneling  . . . . . . . . . . . . . . . . . . . . .  5
   6.  Neighbor Discovery . . . . . . . . . . . . . . . . . . . . . .  6
   7.  Address Autoconfiguration  . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   9.  Security considerations  . . . . . . . . . . . . . . . . . . .  9
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
       Normative References . . . . . . . . . . . . . . . . . . . . . 10
       Informative References . . . . . . . . . . . . . . . . . . . . 11
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
   A.  Major Changes  . . . . . . . . . . . . . . . . . . . . . . . . 12
   B.  Rationale for Interface Identifier Construction  . . . . . . . 14
   C.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 15
   D.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 15
       Intellectual Property and Copyright Statements . . . . . . . . 17

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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 [RFC2460] within IPv4 [RFC0791] sites.
   ISATAP allows dual-stack nodes that do not share a 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
   using ISATAP 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

2. Requirements

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119].

   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
   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.

3. Terminology

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

      same as defined in [RFC3582], which is intended to be equivalent
      to "enterprise" as defined in [RFC1918].

   link, on-link, off-link:
      same as defined in ([RFC2461], section 2.1).

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

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   ISATAP interface:
      an interface configured over one or more underlying links.

   advertising ISATAP interface:
      same meaning as "advertising interface" in ([RFC2461], section

   ISATAP address:
      an address with an on-link prefix assigned on an ISATAP interface
      and with an interface identifier constructed as specified in
      Section 4.1.

4. Basic IPv6 Operation

   ISATAP interfaces automatically tunnel IPv6 packets 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, with properties similar to [RFC2491]. The following
   ISATAP-specific considerations are noted for basic IPv6 operation:

4.1 Interface Identifiers and Unicast Addresses

   ISATAP interface identifiers use "modified EUI-64" format ([RFC3513],
   section 2.5.1) and are formed by appending an IPv4 address assigned
   to an underlying link to the 32-bit string '00-00-5E-FE'. Appendix B
   includes non-normative rationale for this construction rule.

   IPv6 global and local-use ([RFC3513], sections 2.5.4, 2.5.6) ISATAP
   addresses are constructed as follows:

    |           64 bits            |     32 bits   |    32 bits     |
    | global/local unicast prefix  |   0000:5EFE   |  IPv4 Address  |

4.2 ISATAP Interface Configuration

   ISATAP interfaces are configured over one or more underlying links
   that support IPv4 for tunneling within a site; each IPv4 address
   assigned to an underlying link is seen as a link-layer address for

4.3 Multicast and Anycast

   ISATAP interfaces recognize an IPv6 node's required addresses
   ([RFC3513], section 2.8), including certain multicast/anycast

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   Mechanisms for multicast/anycast emulation on ISATAP interfaces
   (e.g., MARS [RFC2022], etc.) are out of scope.

5. Automatic Tunneling

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

5.1 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 ([RFC2460], 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, LinkMTU ([RFC2461], Section 6.3.2) for
   the ISATAP interface 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 the anticipated/measured level of fragmentation in the site's
   IPv4 network is deemed acceptable.

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

5.2 Handling IPv4 ICMP Errors

   ARP failures and persistent ICMPv4 errors SHOULD be processed as
   link-specific information indicating that a path to a neighbor has
   failed ([RFC2461], section 7.3.3).

5.3 Local-Use IPv6 Unicast Addresses

   The specification in ([MECH], section 3.7) is not used; the
   specification in Section 4.1 is used instead.

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5.4 Ingress Filtering

   The specification in ([MECH], section 3.9) is used.

   Additionally, packets received on an ISATAP interface with an ISATAP
   network-layer (IPv6) source address that does not embed the
   link-layer (IPv4) source address in the interface identifier are
   silently discarded.

6. Neighbor Discovery

   The specification in ([MECH], section 3.8) applies only to configured
   tunnels. [RFC2461] 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 interfaces SHOULD implement Redirect, Neighbor Unreachability
   Detection, and next-hop determination exactly as specified in
   [RFC2461]. Address resolution and the mechanisms for delivering
   Router Solicitations and Advertisements are not specified by
   [RFC2461]; instead, they are specified in the following sections of
   this document.

6.1 Address Resolution and Neighbor Unreachability Detection

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

   Hosts SHOULD perform an initial reachability confirmation by sending
   Neighbor Solicitation (NS) message(s) and receiving a Neighbor
   Advertisement (NA) message as specified in ([RFC2461], section 7.2).
   Unless otherwise specified in a future document, solicitations are
   sent to the target's unicast address.

   Hosts SHOULD additionally perform Neighbor Unreachability Detection
   (NUD) as specified in ([RFC2461], section 7.3). Routers MAY perform
   these reachability confirmation and NUD procedures, but this might
   not scale in all environments.

   All ISATAP nodes MUST send solicited neighbor advertisements
   ([RFC2461], section 7.2.4).

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6.2 Duplicate Address Detection

   Duplicate Address Detection ([RFC2462], section 5.4) is not required
   for ISATAP addresses, since duplicate address detection is assumed to
   have been already performed for the IPv4 addresses from which they

6.3 Router and Prefix Discovery

   The following sections describe mechanisms to support the router and
   prefix discovery process ([RFC2461], section 6):

6.3.1 Conceptual Data Structures

   ISATAP nodes use the conceptual data structures Prefix List and
   Default Router List exactly as in ([RFC2461], section 5.1). ISATAP
   adds a new conceptual data structure "Potential Router List" (PRL)
   and the following new configuration variable:

      Time in seconds between successive refreshments of the PRL after
      initialization. It SHOULD be no less than 3,600 seconds. The
      designated value of all 1's (0xffffffff) represents infinity.

      Default: 3,600 seconds

   A PRL is associated with every ISATAP interface and supports the
   mechanisms specified in Section 6.3.4. Each entry in the PRL
   ("PRL(i)") has an associated timer ("TIMER(i)"), and an IPv4 address
   ("V4ADDR(i)") that represents a site border router's advertising
   ISATAP interface.

   When a node enables an ISATAP interface, it initializes the PRL with
   IPv4 addresses. The addresses MAY be discovered via a DHCPv4
   [RFC2131] option for ISATAP, manual configuration, or an unspecified
   alternate method (e.g., DHCPv4 vendor-specific option, etc.).

   When no other mechanisms are available, a DNS fully-qualified domain
   name (FQDN) [RFC1035] 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
   one or more of the methods described above) after PrlRefreshInterval.

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6.3.2 Validation of Router Advertisements Messages

   The specification in ([RFC2461], section 6.1.2) is used.

6.3.3 Router Specification

   Routers with advertising ISATAP interfaces behave the same as
   described in ([RFC2461], section 6.2). As permitted by ([RFC2461],
   section 6.2.6), advertising ISATAP interfaces SHOULD send unicast RA
   messages to a soliciting host's unicast address when the
   solicitation's source address is not the unspecified address.

6.3.4 Host Specification

   Hosts behave the same as described in ([RFC2461], section 6.3) with
   the following additional considerations for ISATAP: Soliciting Router Advertisements

   Hosts solicit Router Advertisements (RAs) by sending Router
   Solicitations (RSs) to advertising ISATAP interfaces in the PRL. The
   manner of selecting PRL(i)'s for solicitation is up to the
   implementation. Hosts add the following variable to support the
   solicitation process:

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

      Default: 900 seconds

   RS messages use a link-local unicast address from the ISATAP
   interface as the IPv6 source address. Router Advertisement Processing

   RAs received from a member of the PRL (i.e., RAs with an ISATAP IPv6
   source address that embeds V4ADDR(i) for some PRL(i)) are processed
   exactly as specified in ([RFC2461], section 6.3.4). Additionally,
   hosts reset TIMER(i) to schedule the next solicitation event (see:
   Section 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 as follows:

      TIMER(i) = MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)

   RAs received from a router other than a member of the PRL are

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   processed as specified in ([RFC2461], section 6.3.4) except that any
   RA contents ([RFC2461], section 6.2.3) that would alter ISATAP link
   parameters are silently ignored. In particular, non-zero values in
   the Router Lifetime, M and O flags, Cur Hop Limit, Reachable Time,
   and Retrans Timer as well as prefix options with the L and/or A bits
   set are ignored. If the MTU option is present, the option's value
   SHOULD be stored in a per-neighbor cache entry for the source of the
   RA; it MUST NOT be copied into LinkMTU for the ISATAP link.

7. Address Autoconfiguration

   Hosts invoke stateless address autoconfiguration under the conditions
   specified in ([RFC2462], sections 5.5).

   Hosts invoke stateful address autoconfiguration under the conditions
   specified in ([RFC2462], section 5.5). When DHCPv6 [RFC3315] is used,
   hosts send messages to the "All_DHCP_Relay_Agents_and_Servers"
   multicast address ([RFC3315], sections 1.2 and 1.3). Sending
   implementations map the "All_DHCP_Relay_Agents_and_Servers" multicast
   address to a link-layer (IPv4) address by selecting V4ADDR(i) for
   some PRL(i).

   When the site supports the DHCPv6 service, the server/relay function
   MUST be deployed equally on each router that is a member of the PRL.

8. IANA Considerations

   The IANA is advised to specify construction rules for IEEE EUI-64
   addresses formed from the Organizationally Unique Identifier (OUI)
   "00-00-5E" in the IANA "ethernet-numbers" registry. The non-normative
   text in Appendix B is offered as an example specification.

9. Security considerations

   ISATAP site border routers MUST implement IPv6 and IPv4 ingress
   filtering and in particular MUST discard any packets originating from
   outside of the site that use an IP address from the site as the
   source address. Additionally, site border routers MUST implement
   ip-protocol-41 filtering by not allowing packets for that protocol in
   and out of the site. Finally, site border routers MUST NOT forward
   any packets with local-use source or destination addresses outside of
   the site ([RFC3513], section 2.5.6).

   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 SHOULD be used in routers with advertising ISATAP
   interfaces. At a minimum, site border routers SHOULD log potential

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   source address spoofing cases.

   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 6.3.1. The list can include
   IPv4 anycast address(es) but administrators are advised to consider
   operational implications of anycast (e.g., see: [RFC1546]).

   ISATAP addresses do not support privacy extensions for stateless
   address autoconfiguration [RFC3041].

10. Acknowledgements

   Portions of this work were derived from SRI International internal
   funds 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, Vlad Yasevich.

   The following additional individuals are acknowledged for their
   contributions: Rich Draves, Alain Durand, Nathan Lutchansky, Karen
   Nielsen, Mohan Parthasarathy, Art Shelest, Pekka Savola, Margaret
   Wasserman, Brian Zill.

   The authors also acknowledge the work of Quang Nguyen [VET] 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

   [MECH]     Gilligan, R. and E. Nordmark, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", draft-ietf-v6ops-mech-v2-00
              (work in progress), February 2003.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

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

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   [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.

Informative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1546]  Partridge, C., Mendez, T. and W. Milliken, "Host
              Anycasting Service", RFC 1546, November 1993.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC2022]  Armitage, G., "Support for Multicast over UNI 3.0/3.1
              based ATM Networks", RFC 2022, November 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
              2131, March 1997.

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

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

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and
              M. Carney, "Dynamic Host Configuration Protocol for IPv6
              (DHCPv6)", RFC 3315, July 2003.

   [RFC3582]  Abley, J., Black, B. and V. Gill, "Goals for IPv6
              Site-Multihoming Architectures", RFC 3582, August 2003.

   [VET]      Nguyen, Q., "", spring

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Authors' Addresses

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

   Phone: +1 650 625 2331

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


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

   Phone: +1 425 705 3131

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

   Phone: +1 425 703 8835

Appendix A. Major Changes

   changes from version 14 to version 15:

   o  several editorial changes

   o  revised Security; IANA considerations

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   o  revised Section

   o  added new section on ingress filtering

   o  revised stateful autoconfiguration and moved to new section

   o  removed overly-restrictive text at end of Section

   changes from version 13 to version 14:

   o  removed applicability statement; applicability TBD by v6ops

   o  updated deployment/site admin sections; moved to appendices

   o  new text on "L" bit in prefix options in section

   o  removed extraneous text in Security Considerations

   o  fixed "layering bug" in section

   o  revised "ISATAP address" definition

   o  updated references for RFC 3315; 3513

   changes from earlier versions to version 13:

   o  Revised ISATAP interface/link terminology

   o  Returned to using symbolic reference names

   o  Revised MTU section; moved non-normative MTU text to separate

   o  Added multicast/anycast subsection

   o  Revised PRL initialization

   o  Updated neighbor discovery, security consideration sections

   o  Rearranged/revised sections 5, 6, 7

   o  Added stateful autoconfiguration mechanism

   o  Normative references to RFC 2491, RFC 2462

   o  Moved non-normative MTU text to appendix C

   o  clarified address resolution, Neighbor Unreachability Detection

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   o  specified MTU/MRU requirements

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

   o  Clarified ambiguous text per comments from Hannu Flinck; Jason

   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:

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

   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. Deployment Considerations

   Hosts can enable ISATAP, e.g., when native IPv6 service is
   unavailable. When native IPv6 service is acquired, hosts can
   discontinue the ISATAP router solicitation process (Section 6.3.4)
   and/or allow associated state to expire (see: [RFC2461], section 5.3
   and [RFC2462], section 5.5.4). In this case, any associated addresses
   added to the DNS should also be removed.

   Routers can configure both native IPv6 and ISATAP interfaces over the
   same physical link. The prefixes used on each interface will be
   distinct, and normal IPv6 routing between the interfaces can occur.

   Routers can obtain IPv6 prefix delegations from a server via an
   ISATAP interface and advertise the delegated prefix(es) on other IPv6

   Responsible administration can reduce control traffic overhead
   associated with router and prefix discovery.

Appendix D. Other Considerations

   The Potential Router List (PRL) contains the IPv4 addresses of
   advertising ISATAP interfaces on site border routers, and the
   specification mandates that nodes only accept Router Advertisement
   (RA) parameters that alter the ISATAP link (e.g., default router
   list, on-link prefix list, LinkMTU, etc.) if they are sent by a

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   member of the PRL. However, the specification allows any node on the
   ISATAP link to send "other" parameters in RAs and also allows any
   node on the ISATAP link to act as a (non-default) IPv6 router, e.g.,
   if the node is configured as a router for its other IPv6 links.

   These aspects of the specification allow useful functionality,
   including the ability for ISATAP nodes other than PRL members to
   serve as routers for "stub" IPv6 networks, the ability for ISATAP
   nodes to send IPv6 packets with non-ISATAP source addresses (e.g.,
   RFC 3401 privacy addresses), etc. But, allowing this functionality
   prevents ISATAP nodes from perform effective ingress filtering for
   IPv6 source addresses in packets they receive. Instead, the nodes
   must trust that: 1) site border routers are performing ingress
   filtering, and 2) malicious nodes are effectively denied access to
   the link.

   Additionally, the specification expects that that IPv4 addresses are
   uniquely assigned within the ISATAP site.

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