NGTRANS
Network Working Group                                         F. Templin
Internet-Draft                                                     Nokia
Expires: July 4, 2002 18, 2003                                        T. Gleeson
                                                      Cisco Systems K.K.
                                                               M. Talwar
                                                               D. Thaler
                                                   Microsoft Corporation
                                                        January 03, 2002 17, 2003

        Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
                    draft-ietf-ngtrans-isatap-10.txt
                    draft-ietf-ngtrans-isatap-11.txt

Status of this Memo

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

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   This Internet-Draft will expire on July 4, 2002. 18, 2003.

Copyright Notice

   Copyright (C) The Internet Society (2002). (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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  3
   3.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . .  3
   4.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.    Non-Broadcast, Multiple Access (NBMA)  Basic IPv6 Operation . . . . . .  4
   5.1   Multicast  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   5.2   Interface Identifiers and Address Construction . . . . . . .  5
   5.3   ISATAP Link/Interface Configuration  . . . . . . . . . . . .  5
   5.4   Link Layer Address Options . . . . . . . . . . . . . . . . .  6  4
   6.  Automatic Tunneling  . . . . . . . . . . . . . . . . . . . .  6
   6.1   Dual IP Layer Operation  . . . . . . . . . . . . . . . . . .  6
   6.2   Encapsulation  . . . . . . . . . . . . . . . . .  5
   7.  Neighbor Discovery . . . . . .  6
   6.3   Tunnel MTU and Fragmentation . . . . . . . . . . . . . . . .  7
   6.4   Handling IPv4 ICMP Errors  . . . . . . . . . . . . . . . . .  8
   6.5   Local-Use IPv6 Unicast Addresses .
   8.  Deployment Considerations  . . . . . . . . . . . . .  8
   6.6   Ingress Filtering . . . . . 10
   9.  IANA Considerations  . . . . . . . . . . . . . . . .  8
   7.    Neighbor Discovery for ISATAP Links . . . . . 11
   10. Security considerations  . . . . . . .  8
   7.1   Address Resolution . . . . . . . . . . . . 11
   11. Acknowledgements . . . . . . . . .  9
   7.2   Router and Prefix Discovery . . . . . . . . . . . . . . 12
       Normative References . .  9
   7.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . .  9
   7.2.2 Validity Checks for Router Advertisements . . 12
       Informative References . . . . . . . 10
   7.2.3 Router Specification . . . . . . . . . . . . . 13
       Authors' Addresses . . . . . . . 11
   7.2.4 Host Specification . . . . . . . . . . . . . . . 14
   A.  Major Changes  . . . . . . 11
   8.    ISATAP Deployment Considerations . . . . . . . . . . . . . . 12
   8.1   Host And Router Deployment Considerations . . . . 15
   B.  Rationale for Interface Identifier Construction  . . . . . 12
   8.2   Site Administration Considerations . . 17
   C.  Dynamic MTU Discovery  . . . . . . . . . . . 12
   9.    IANA Considerations . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . . . . 13
   10.   Security considerations  . . . . . . . . . . . . . . . . . . 13
   11.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
         Normative References . . . . . . . . . . . . . . . . . . . . 14
         Informative References . . . . . . . . . . . . . . . . . . . 15
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 16
   A.    Major Changes  . . . . . . . . . . . . . . . . . . . . . . . 17
   B.    Rationale for Interface Identifier Construction  . . . . . . 18
   C.    Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 19
         Intellectual Property and Copyright Statements . . . . . . . 21 22

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-based IPv4 [2] sites.  We refer to this
   approach as the Intra-Site Automatic Tunnel Addressing Protocol
   (ISATAP).  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.

   This document specifies

   Specific details for the operation of IPv6 and automatic tunneling
   over ISATAP links (i.e., automatic IPv6-in-IPv4 tunneling), are given, including an interface identifier format
   that embeds an IPv4 address.  This format supports IPv6 protocol mechanisms for address
   configuration as well
   as and simple link-layer address mapping.  Also specified in this
   document
   is the operation of IPv6 Neighbor Discovery for ISATAP.  The
   document finally presents deployment and security 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 (i.e., no configured tunnel
      state)

   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 [3] [18] are used), but does
      not allow the virtual ISATAP link to span a Network Address
      Translator [4] used)

   o  compatible with other NGTRANS mechanisms (e.g., 6to4 [19])

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 [5]. [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
   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 ([6], ([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 address:
      an on-link 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 5.2

   ISATAP router:
      an IPv6 node that has an ISATAP interface over which it forwards
      packets not explicitly addressed to itself.

   ISATAP host:
      any node that has an ISATAP interface and is not an ISATAP router.

5. Non-Broadcast, Multiple Access (NBMA) 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.  RFC 2491 [7] provides a general
   architecture for IPv6 over NBMA networks that forms the basis for
   companion documents such as the present.  The following subsections
   present NBMA considerations for IPv6 on
   ISATAP links:

5.1 Multicast

   ISATAP links most closely meet the description for connectionless
   service found in the last paragraph of ([7], section 1), i.e., ISATAP
   addresses provide the sender with an NBMA destination address to
   which it can transmit packets whenever it desires.  Thus, multicast
   emulation mechanisms are not required to support host-side operation
   of the IPv6 neighbor discovery protocol.

5.2 noted:

5.1 Interface Identifiers and Address Construction

   ([7], section 5.1) requires companion documents to specify the exact
   mechanism for generating Unicast Addresses

   ISATAP interface tokens (i.e., identifiers).
   Interface identifiers for ISATAP are compatible with the EUI-64
   identifier use "modified EUI-64" format ([8], ([5],
   section 2.5.1), 2.5.1) and are constructed formed by appending an IPv4 address on the
   ISATAP link to the 32-bit string '00-00-5E-FE'.  (Appendix  Appendix B includes
   non-normative text explaining
   the rationale for this construction rule.)

   Global rule.

   With reference to ([5], sections 2.5.4, 2.5.6), global and Local-use 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 |
    +------------------------------+---------------+----------------+

                                Figure 1

   For example, the global unicast address:

      3FFE:1A05:510:1111:0:5EFE:8CAD:8108

   has a prefix of '3FFE:1A05:510:1111::/64' and an ISATAP interface
   identifier with embedded IPv4 address: '140.173.129.8'.  The address
   is alternately written as:

      3FFE:1A05:510:1111:0:5EFE:140.173.129.8

   Examples for local-use addresses are obvious from the above and with
   reference to ([8], section 2.5.6).

5.3

5.2 ISATAP Link/Interface Configuration

   ISATAP Link/Interface configuration is consistent with ([7], sections
   5.1.1 and 5.1.2).

   An ISATAP link consists 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.

5.4

   At least one link-layer address per advertising ISATAP interface
   SHOULD be added to the Potential Routers List (see Section 7.3.1).

5.3 Link Layer Address Options

   ([7], section 5.2) requires companion documents

   With reference to specify the
   contents of ([6], section 5.2), when the [NTL], [STL], [NBMA Number] [NTL] and [NBMA Subaddress] [STL] fields for
   in an ISATAP link layer address options.  For ISATAP links:

   o  the [NTL] and [STL] fields MUST be zero

   o option encode 0, the [NBMA Number]
   field encodes a 4-octet IPv4 address

   o  the [NBMA Subaddress] field is omitted

   ([7], section 5.2) does NOT require companion documents to specify
   the value address.

5.4 Multicast and Anycast

   As for [Length], i.e., the total length of the option in 8
   octets.  Senders may therefore set [Length] to any value between 1
   and 255; when [Length] IPv6 interface, an ISATAP interface is greater than 1, receivers treat any bytes
   that follow the [NBMA Number] required to
   recognize certain IPv6 multicast and anycast addresses ([5], section
   2.8).  Mechanisms for sending multicast and anycast packets (e.g.,
   [20]) are left as null-padding. future work.

6. Automatic Tunneling

   The common tunneling mechanisms specified in ([9], ([7], sections 2 and 3)
   are used, with the following noted specific considerations for ISATAP:

6.1 Dual IP Layer Operation

   ISATAP uses the same specification found in ([9], ([7], section 2).  That
   is, ISATAP nodes provide complete IPv4 and IPv6 implementations and
   are able to send and receive both IPv4 and IPv6 packets.  ISATAP
   nodes operate with both their IPv4 and IPv6 stacks enabled.

   Address configuration considerations are the same as for ([9],
   section 2.1).  Additionally, ISATAP nodes require that IPv4 address
   configuration take place on at least one underlying link prior to
   IPv6 address configuration on an ISATAP link. and DNS considerations are the same as ([9], ([7],
   sections 2.2 and 2.1 through 2.3).

6.2 Encapsulation

   The specification in ([9], ([7], section 3.1) is used.  Additionally, the
   IPv6 next-hop address for packets sent on an ISATAP link MUST be an
   ISATAP address; other packets are discarded and an ICMPv6 destination
   unreachable indication with code 3 (Address Unreachable) [10] [8] is
   returned to the source.

6.3 Tunnel MTU and Fragmentation

   The specification in ([9], section 3.2) is NOT used; the
   specification in this section is used instead.

   ISATAP uses automatic tunnel interfaces that 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 the MTU of ISATAP
   interfaces apply: 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 VPNs VPN interfaces use an MTU of 1400 bytes

   Unless

   To maximize efficiency and minimize IPv4 fragmentation for the
   predominant deployment case, ISATAP interfaces that do not use a
   dynamic per-neighbor MTU discovery mechanism is implemented,
   ISATAP interfaces MUST use an MTU (ISATAP_MTU) of SHOULD set LinkMTU ([4], Section
   6.3.2 ) to no more than 1380 bytes (1400 minus 20 bytes for IPv4 encapsulation) to maximize
   efficiency and minimize IPv4 fragmentation for the predominant
   deployment case.  ISATAP_MTU
   encapsulation).  LinkMTU MAY be set to a larger value when the
   encapsulator implements values on ISATAP
   interfaces that use a dynamic per-neighbor MTU discovery
   mechanism, but this value SHOULD NOT exceed the largest MTU of all
   underlying links (minus 20 bytes for IPv4 encapsulation). mechanism.  Appendix C
   provides non-normative considerations for dynamic per-neighbor MTU discovery.

   The network layer (IPv6) forwards packets of size ISATAP_MTU or
   smaller to the ISATAP interface.  All other packets are dropped, and
   an ICMPv6 "packet too big" message with MTU = ISATAP_MTU is returned
   to the source [11].  The ISATAP link layer encapsulates packets of size 1380 bytes or smaller
   with the Don't Fragment (DF) bit NOT not set in the encapsualting IPv4
   header.

   Nodes that configure ISATAP interfaces MUST have IPv4 reassembly
   buffers large enough to receive packets with the DF bit not set in
   the encapsulating IPv4 header.  RFC 1122 [12], section 3.3.2
   specifies that the Effective MTU to Receive (EMTU_R) for IPv4 nodes:

      "...MUST be greater than or equal to 576, SHOULD be either
      configurable or indefinite, and SHOULD be greater than or equal to
      the MTU of the connected network(s)".

   With reference to this specification, the EMTU_R for nodes that
   configure ISATAP interfaces MUST be greater than or equal to 1500
   bytes (i.e., the predominant deployment case for connected IPv4
   networks) and SHOULD be either configurable or indefinite.

6.4 Handling

6.4 Handling IPv4 ICMP Errors

   The specification in ([9], section 3.4) MAY be used.

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

6.5 Local-Use IPv6 Unicast Addresses

   The specification in ([9], ([7], section 3.7) is NOT not used.  Instead, local
   use IPv6 unicast addresses are formed exactly as specified in ([8],
   section 2.5.6). Section 5.1.

6.6 Ingress Filtering

   The specification in ([9], ([7], section 3.9) is used on used.  In particular,
   ISATAP router
   interfaces.  (ISATAP host interfaces silently discard any nodes that forward decapsulated packets
   received MUST be configured
   with a foreign IPv6 destination address, i.e., an address
   not configured on the local IPv6 stack.)

   Additionally, packets received on ISATAP host and router interfaces
   MUST satisfy at least one (i.e., one or both) list of the following
   validity checks:

   o  the network-layer (IPv6) source address is an on-link ISATAP
      address with an interface identifier that embeds the link-layer
      (IPv4) source address

   o  the link-layer (IPv4) source IPv4 address is in the Potential Routers
      List (see Section 7.2.1)

   Packets prefixes that do not satisfy the above conditions are silently
   discarded. acceptable.

7. Neighbor Discovery for ISATAP Links

   RFC 2461 [6] [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 [6]. [4].
   Address resolution and the mechanisms for delivering Router
   Solicitations and Advertisements for ISATAP links are not specified
   by [6]; [4]; instead, they are specified in this document.  (Note that
   these mechanisms MAY potentially apply to other types of NBMA links
   in the future.)

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, ISATAP hosts SHOULD perform an initial
   reachability confirmation by sending unicast Neighbor Solicitations
   (NSs) and receiving a Neighbor Advertisement using the mechanisms
   specified in ([6], ([4], sections 7.2.2-7.2.8).  (Note that
   implementations MAY omit the source/target link layer options in NS/
   NA messages when unicast is used.)

   ISATAP hosts

   Hosts SHOULD additionally perform Neighbor Unreachability Detection
   (NUD) as specified in ([6], ([4], section 7.3).  ISATAP routers  Routers MAY perform the
   above-specified reachability detection and NUD procedures, but this
   might not scale in all environments.

   All ISATAP nodes MUST send solicited neighbor advertisements ([6], ([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

   Since NBMA multicast emulation mechanisms are not used, ISATAP nodes will typically not receive unsolicited multicast
   Router Advertisements.  Thus,
   alternate Advertisements, unicast mechanisms are required and as specified
   below:

7.2.1

7.3.1 Conceptual Data Structures

   ISATAP nodes use the conceptual data structures Prefix List and
   Default Router List exactly as in ([6], ([4], section 5.1).  ISATAP links
   add a new conceptual data structures structure "Potential Router List" and the
   following new configuration variable:

   ResolveInterval
      Time between name service resolutions.  Default and suggested
      minimum: 1hr

   A Potential Router List (PRL) is associated with every ISATAP link.
   The PRL provides a trust basis for router validation (see security
   considerations).
   Each entry in the PRL has an IPv4 address and an associated timer.
   The IPv4 address represents a router's ISATAP
   interface (likely to be an "advertising interface"), advertising ISATAP interface, and is
   used to construct the ISATAP link-local ISATAP address for that interface.
   The following sections specify the process for initializing the PRL:

   When a node enables an ISATAP link, it first discovers IPv4 addresses for
   the PRL.  The addresses SHOULD MAY be established by a DHCPv4 [13] [10] option
   for ISATAP (option code TBD), by manual configuration, or by an unspecified
   alternate method (e.g., DHCPv4 vendor-specific option).

   When no other mechanisms are available, a DNS fully-qualified domain
   name (FQDN) [20] [21] established by an out-of-band method (e.g., DHCPv4,
   manual configuration, etc.) MAY be used.  In this case, the  The FQDN is resolved into
   IPv4 addresses for the PRL through a static host file, a site-specific name
   service, or by querying an IPv4-based a DNS server.
   Unspecified server within the site, or an unspecified
   alternate methods for domain name resolution may also be
   used. method.  The following notes apply:

   1.  Site administrators maintain a list of IPv4 addresses
       representing advertising ISATAP router interfaces and make them
       available via one or more of the mechanisms described above.

   2.  There are no mandatory rules for the selection of a FQDN, but
       administrators are encouraged to use the convention
       "isatap.domainname" (e.g., isatap.example.com).
       manual configuration MUST be supported.

   3.  After initialization, nodes periodically re-initialize the PRL
       (after
       (e.g., after ResolveInterval).  When DNS is used, client DNS
       resolvers use the IPv4 transport to resolve the names and follow the cache
       invalidation procedures in [20] when the DNS time-to-live
       expires.

7.2.2 Validity Checks for transport.

7.3.2 Validation of Router Advertisements

   A node MUST silently discard any Router Advertisement Messages

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

   Additionally, received RA messages it
   receives that do not satisfy both the validity checks contain Prefix Information
   options and/or encode non-zero values in ([6], the Cur Hop Limit, Router
   Lifetime, Reachable Time, or Retrans Timer fields (see: [4], section 6.1.2) and
   4.2) MUST satisfy the following additional validity check for ISATAP:

   o  the network-layer (IPv6) source address is an ISATAP address and
      embeds an IPv4 address from the PRL

7.2.3

7.3.3 Router Specification

   Advertising

   Routers with advertising ISATAP interfaces of routers behave the same as
   advertising interfaces
   described in ([6], ([4], section 6.2).  However,
   periodic unsolicited multicast Router Advertisements are not used,
   thus the "interval timer" associated with advertising interfaces is
   not used for that purpose.

   When an ISATAP router receives a valid Router Solicitation on an
   advertising  Advertising ISATAP interface, it replies with a unicast Router
   Advertisement interfaces send
   RA messages to the address of the node which sent the Router
   Solicitation.  The source address of the Router Advertisement is a
   link-local node's unicast address associated with the interface.  This MAY
   be the same address, as the destination address of the Router Solicitation.
   ISATAP routers MAY engage in the solicitation process described under
   Host Specification below, e.g., if Router Advertisement consistency
   verification ([6], permitted by ([4],
   section 6.2.7) is desired.

7.2.4 6.2.6).

7.3.4 Host Specification

7.3.4.1 Sending Router Solicitations

   All entries in the PRL are assumed to represent active advertising
   ISATAP routers interfaces within the site, i.e., the PRL provides trust basis
   only; not reachability detection.  ISATAP nodes SHOULD use stateful
   configuration to assign IPv6 prefixes and default router information.
   When stateful configuration is not available, hosts MAY  Hosts periodically solicit
   information from one or more entries in the PRL ("PRL(i)") by sending
   unicast Router Solicitation (RS) messages using the PRL(i)'s IPv4 address
   ("V4ADDR_PRL(i)") and associated timer in ("TIMER(i)").  The manner of
   selecting a PRL(i) for solicitation and/or deprecating a
   previously-selected PRL(i) is outside the entry. scope of this
   specification.  Hosts add the following variable to support the
   solicitation process:

   MinRouterSolicitInterval
      Minimum time between sending Router Solicitations to any router. Solicitations.  Default and
      suggested minimum: 15min.

   When a PRL(i) is selected, the host sets its associated timer TIMER(i) to
   MinRouterSolicitInterval and initiates solicitation following a short
   delay as in ([6], section 6.3.7).  The manner of choosing particular
   routers in the PRL for solicitation is outside the scope of this
   specification.  The solicitation process repeats when the associated
   timer expires.
   delay.  Solicitation consists of sending Router Solicitations RS messages to the ISATAP
   link-local address constructed from the entry's IPv4 address, V4ADDR_PRL(i), i.e., they are
   sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers
   multicast'.
   'All-Routers-multicast'.  They are otherwise sent exactly as in ([6], ([4],
   section 6.3.7).

7.3.4.2 Processing Router Advertisements

   Hosts process received Router Advertisements RA messages exactly as in ([6], ([4], section 6.3.4).  Hosts additionally reset 6.3.4)
   and ([9], section 5.5.3) except that, when an RA message contains an
   MTU option, hosts SHOULD NOT copy the timer associated with option's value into the V4ADDR_PRL(i) embedded in ISATAP
   interface LinkMTU.  Instead, when the ISATAP link layer implements a
   per-neighbor path MTU cache, hosts SHOULD copy the MTU option's value
   into the cache entry for the router that sent the RA message (see:
   Appendix C).

   When the network-layer source address in
   each solicited Router Advertisement received.  The timer an RA message is an ISATAP
   address that embeds V4ADDR_PRL(i) for some PRL(i) selected for
   solicitation, hosts additionally reset to
   either 0.5 * (the TIMER(i).  Let "MIN_LIFETIME"
   be the minimum value in the router lifetime or valid lifetime of any on-link
   prefixes received advertised in the advertisement) or
   MinRouterSolicitInterval; whichever RA message.  Then, TIMER(i) is longer. reset to:

      MAX((0.5 * MIN_LIFETIME), MinRouterSolicitInterval)

8. ISATAP 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 and
   allow existing ISATAP address configurations to expire as specified
   in ([6], ([4], section 5.3) and ([14], ([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 an interface to simultaneously support both a native IPv6, IPv6 and also ISATAP (over IPv4). 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 router interfaces.

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

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

   o  When stateful autoconfiguration is not available, ISATAP nodes MAY
      periodically send unicast Router Solicitations to and receive
      unicast Router Advertisements from to one or more members of the
      potential router list.  A well-deployed stateful autoconfiguration
      service within the site can minimize and/or eliminate the need for
      periodic solicitation.

   o  ISATAP nodes periodically refresh the entries on 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) [21] is requested in [22] may be requested in the
   event that the IESG recommends this document for (or a derivative thereof) is moved to
   standards track.

10. Security considerations

   Site administrators are advised that, in

   ISATAP site border routers and firewalls MUST implement IPv6 ingress
   filtering and MUST NOT allow packets with site-local source and/or
   destination addresses (i.e., addresses with prefix FEC0::/10) to
   enter or leave the site.

   In addition to possible attacks against IPv6, security attacks
   against IPv4 MUST must also be considered.

   Responsible IPv4 site security management is strongly encouraged.  In particular, border gateways SHOULD routers
   and firewalls MUST implement filtering to detect
   spoofed IPv4 source addresses at a minimum; ip-protocol-41 ingress filtering
   SHOULD also be implemented.

   If and
   ip-protocol-41 filtering.

   Even with IPv4 source address filtering is not correctly implemented, the and IPv6 ingress filtering, reflection attacks can
   originate from nodes within an ISATAP validity checks will not be effective in preventing site that spoof IPv6 source address spoofing.

   If filtering
   addresses.  Security mechanisms for ip-protocol-41 is not correctly implemented, IPv6
   source address spoofing is clearly possible, but this can reflection attack mitigation
   (e.g., [11], [12], etc.) SHOULD be
   eliminated if both IPv4 used in routers with advertising
   ISATAP interfaces.  At a minimum, ISATAP site border gateways MUST
   log potential source address filtering, and the ISATAP
   validity checks are implemented. spoofing cases.

   (RFC 2461 [6]), [4], section 6.1.2 6.1.2) implies that nodes trust received
   Router
   Advertisements they receive Advertisement (RA) messages from on-link routers, as indicated
   by a value of 255 in the IPv6 'hop-limit' field.  Since this field is not
   decremented when ip-protocol-41 packets traverse multiple IPv4 hops
   ([9], section 3),  ISATAP links
   require a different trust model.  In
   particular, ONLY those Router Advertisements an additional validation check for received from a member
   of the Potential Routers List are trusted; all others are silently
   discarded.  This trust model is predicated on IPv4 source address
   filtering, as described above.

   The RA messages (see:
   Section 7.3.2).

   ISATAP address format does addresses do not support privacy extensions for stateless
   address autoconfiguration [22]. [23].  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 issue is the same for NAT'd addresses.)
   especially true when private address allocations [18] 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] [24]
   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]   Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G. and E.
         Lear, "Address Allocation for Private Internets", BCP 5, RFC
         1918, February 1996.

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

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

   [6]

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

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

   [8]   Hinden, R.

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

   [9]

   [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.

   [10]

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

   [11]  McCann, J., Deering,

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

   [12]  Braden, R., "Requirements for Internet Hosts - Communication
         Layers", STD 3, T. Narten, "IPv6 Stateless Address
         Autoconfiguration", RFC 1122, October 1989.

   [13] 2462, December 1998.

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

   [14]  Thomson, S.

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

   [12]  Bellovin, S., Leech, M. and T. Narten, "IPv6 Stateless Address
         Autoconfiguration", RFC 2462, December 1998.

   [15] Taylor, "ICMP Traceback
         Messages", draft-ietf-itrace-03 (work in progress), January
         2003.

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

   [16]

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

   [17]

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

   [18]  Droms,

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

   [17]  Braden, R., "Dynamic Host Configuration Protocol "Requirements for IPv6
         (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
         November 2002. Internet Hosts - Communication
         Layers", STD 3, RFC 1122, October 1989.

Informative References

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

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

   [20]  Thaler, D., "Support for Multicast over 6to4 Networks",
         draft-ietf-ngtrans-6to4-multicast-01 (work in progress), July
         2002.

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

   [21]

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

   [22]

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

   [23]

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

   [24]

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

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

   [27]  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

Appendix A. Major Changes

   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

   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

                                Figure 2

   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. Dynamic Per-neighbor MTU Discovery

   ISATAP encapsulators and decapsulators are IPv6 neighbors that may be
   separated by multiple link layer (IPv4) forwarding hops.  When
   ISATAP_MTU is an
   encapsulator's interface configures a LinkMTU ([4], Section 6.3.2)
   value larger than 1380 bytes, the encapsulator must implement a dynamic link layer (IPv4) mechanism
   is required to discover per-neighbor path MTUs.  The following text
   gives non-normative considerations for dynamic MTU discovery.

   IPv4 path MTU discovery [15] relies on [13] uses ICMPv4 "fragmentation needed"
   messages, but these generally do not provide enough information for
   stateless translation into to ICMPv6 "packet too big" messages (see: RFC
   792 [16] [14] and RFC 1812 [17], [15], 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 is may be inadequate and can result in black holes that are
   difficult to diagnose [24].

   The ISATAP encapsulator may implement an alternate [25].

   Alternate methods for determining per-neighbor MTUs should be used
   when RFC 1191 path MTU discovery mechanism, e.g., is deemed inadequate.  In any
   method, the encapsulator uses periodic and/or on-demand probing of
   the IPv4 path to the decapsulator.  Probing consists of sending packets
   larger than 1380 bytes to the neighbor and receiving positive
   confirmation of receipt.  Two  The following three methods are
   possible:

   In the first method,

   1.  Encapsulator-driven - the encapsulator does NOT set periodically sends probe
       packets with the DF bit set in the IPv4 header of probe packets.  In this case, the encapsulator must
   have a priori knowledge of the decapsulator's reassembly buffer size and should have waits for a priori knowledge of the decapsulator's link MTU.
   This method has
       positive acknowledgement from the advantage decapsulator that the probe packets will be delivered
   even if was
       received

   2.  Decapsulator-driven - the network performs fragmentation, thus ordinary data encapsulator sends all packets may be used for probing resulting with the
       DF bit NOT set in greater efficiency.
   Disadvantages for this method include:

   o  special mechanisms required on both encapsulator the IPv4 header unless and until the
       decapsulator

   o  extra state required on both sends a "Fragmentation Experienced" indication(s)

   3.  Hybrid - the encapsulator and decapsulator

   o  complex protocol signalling between encapsulator engage in a dialogue
       and decapsulator

   o  possible extended periods of network fragmentation
   In use "intelligent" probing to monitor the path MTU

   These methods are discussed in detail in the second (and preferred) following subsections:

C.1 Encapsulator-driven Method

   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 should could be used for this purpose, since successful
   delivery results in a positive acknowledgement that the probe succeeded, i.e., in the form of
   succeeded vis-a-vis a Neighbor Advertisement
   (NA) response from the decapsulator.  Setting

   While the DF bit prevents the network
   from fragmenting decapsulator is being probed, the packets and protects decapsulators from
   receiving encapsulator maintains a
   queue of packets that might overrun have the IPv4 reassembly buffer.
   Additionally, special mechanisms and state are needed only on decapsulator as the
   encapsulator, and IPv6 next-hop
   address.  The queue should be large enough to buffer the
   (delay*bandwidth) product for the round-trip time (RTT) to the
   decapsulator.  If the probe succeeds, packets in the queue that are
   no complex protocol signalling between larger than the
   encapsulator probe size are sent to the decapsulator.  If the
   probe fails, packets larger than the last known successful probe are
   dropped and an ICMPv6 "packet too big" message returned to the sender
   [16].

   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 required.
   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, packet queues may become
   large on Long, Fat Networks (LFNs) (see: RFC 1323 [26]).

C.2 Decapsulator-driven Method

   In either this method, implementations may choose to couple the probing
   process encapsulator sends all packets with neighbor cache management procedures ([6], section 7),
   e.g.  to maintain timers, state variables and/or the DF bit
   NOT set in the IPv4 header with the expectation that the decapsulator
   will send a queue of "Fragmentation Experienced" indication if the IPv4
   network fragments packets.  In other words, the decapsulator simply
   sends all packets
   waiting 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 probes both reporting fragmentation and
   informing the encapsulator of a new MTU value to complete.  Packets retained on use.

   This method has the queue distinct advantages that the data packets
   themselves are
   forwarded when used as probes succeed, and provide state no queueing on the encapsulator is
   necessary.  Additionally, fewer packets will be lost since the
   decapsulator will quite often be able to reassemble packets
   fragmented by the network.  The primary disadvantage is that, using
   the current specifications, the encapsulator has no way of knowing
   whether a particular decapsulator implements the "fragmentation
   experienced" signalling 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 sending fragmented packets from encapsulators.

   When fragmented packets arrive, the application 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 application additionally sends ICMPv6
   "packet too big" messages to the original source when probes fail.
   Implementations may choose 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 the RA
   message header ([4], section 4.2) would provide a means for the
   decapsulator to store per-neighbor inform the encapsulator that dynamic MTU information discovery is
   supported.

C.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: [27] for an example of such an approach.) This
   approach has the IPv4 path MTU discovery cache, in advantage that bi-directional links are used and
   both ends of the ISATAP link layer's private
   data structures, etc.

   Additional notes:

   1.  Per-neighbor MTUs may vary dynamically due to fluctuations in have unambiguous knowledge that the
       IPv4 forwarding path and/or multipath routing (e.g., when QoS
       routing other end
   implements the protocol.  However, the signalling protocol between
   the endpoints is used complicated and additional state is required in both
   the IPv4 network).  For such neighbors,
       encapsulators should detect a "losing battle" encapsulator and reduce decapsultor.

C.4 Summary

   In summary, the
       per-neighbor MTU size to no more than 1380 bytes.

   2.  When not probing, encapsulators may send packets to a neighbor
       with MTU greater than 1380 bytes either decapsulator-based approach in Appendix C.2 has
   distinct efficiency advantages over methods that engage the
   encapsulator.  Additionally, probing methods which use IPv4
   encapsulation with the DF bit NOT set or
       not set.  When the DF bit is set, undetected packet loss may
       occur in use LinkMTU values for the network if
   ISATAP link that exceed the neighbor's underlying link MTU decreases.  When the
       DF bit size.  Experimental
   verification is NOT set, undetected called for which may eventually result in a
   recommendation for proposed standard.

C.5 Additional Notes

   o  In all methods, some packet loss is less likely but due to link/buffer restrictions
      may occur either 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 network or at
      actual path MTU.  Enlightened senders will interpret the neighbor's reassembly
       buffer.

   3. loss as
      loss due to link/buffer restrictions and immediately reduce their
      MTU estimate.

   o  To avoid denial-of-service attacks that would cause superfluous
      probing based on counting down/up by small increments, plateau
      tables (e.g., [13], 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.

   o  Nodes that connect to the Internet are expected to be able to
      reassemble 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 or discard maximum-length IPv4 packets are
      vulnerable to attacks such as the "ping-of-death".

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