NGTRANS Working Group                                         F. Templin
Internet-Draft                                                     Nokia
Expires: June 19, July 1, 2003                                         T. Gleeson
                                                      Cisco Systems K.K.
                                                               M. Talwar
                                                               D. Thaler
                                                   Microsoft Corporation
                                                       December 19, 31, 2002

        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.

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Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.


   This document specifies an Intra-Site Automatic Tunnel Addressing
   Protocol (ISATAP) that connects IPv6 hosts and routers within IPv4
   sites.  ISATAP is a transition mechanism that treats the site's IPv4 infrastructure as a Non-Broadcast Multiple Access (NBMA) 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.    Terminology    Requirements . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.    Transmission of IPv6 Packets on ISATAP Links    Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.    Basic IPv6 Operation on ISATAP Interface Identifier Construction Links . . . . . . . . . .  4
   4.2   Stateless Autoconfiguration . .  5
   5.1   Interface Identifiers and Link-Local Addresses Address Construction . . . . . . .  5
   5.2   ISATAP Link/Interface Configuration  . . . . . . . . . . . .  5
   4.4   Sending Rules
   5.3   Dual Stack Operation and Address Mapping Configuration . . . . . . .  6
   5.4   Tunneling Mechanisms . . . . . . . . . . . . . . . . . . . .  6
   4.5   Validity Checks for Received Packets
   5.4.1 Encapsulation  . . . . . . . . . . . . . . . . . . . . . . .  6
   5.4.2 Tunnel MTU and Fragmentation . . . . . . . . . . . . . . . .  6
   5.    Neighbor Discovery for ISATAP Links
   5.4.3 Handling IPv4 ICMP Errors  . . . . . . . . . . . .  9
   5.1   Address Resolution . . . . .  7
   5.4.4 Decapsulation  . . . . . . . . . . . . . . . .  9
   5.2   Router and Prefix . . . . . . .  7
   5.4.5 Link-Local Addresses . . . . . . . . . . . . . . . . . . . .  7
   5.4.6 Ingress Filtering  . . . . . . . . . . . . . . . . . . . . .  7
   6.    Neighbor Discovery and Address Autoconfiguration . . . . . .  8
   6.1   Address Resolution . . . . . . . . . . 10
   5.2.1 . . . . . . . . . . .  8
   6.2   Address Autoconfiguration and Router Discovery . . . . . . .  9
   6.2.1 Conceptual Data Structures . . . . . . . . . . . . . . . . . 10
   5.2.2  9
   6.2.2 Validity Checks for Router Advertisements  . . . . . . . . . 11
   5.2.3 10
   6.2.3 Router Specification . . . . . . . . . . . . . . . . . . . . 12
   5.2.4 10
   6.2.4 Host Specification . . . . . . . . . . . . . . . . . . . . . 12
   6. 11
   7.    ISATAP Deployment Considerations . . . . . . . . . . . . . . 13
   6.1 12
   7.1   Host And Router Deployment Considerations  . . . . . . . . . 13
   6.2 12
   7.2   Site Administration Considerations . . . . . . . . . . . . . 13
   7. 12
   8.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 14
   8. 13
   9.    Security considerations  . . . . . . . . . . . . . . . . . . 14
   9. 13
   10.   Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 14
         Normative References . . . . . . . . . . . . . . . . . . . . 16 14
         Informative References . . . . . . . . . . . . . . . . . . . 17 15
         Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 17 16
   A.    Major Changes  . . . . . . . . . . . . . . . . . . . . . . . 18 17
   B.    Rationale for Interface Identifier Construction Rules  . . . 21
   C.    INTELLECTUAL PROPERTY  . . . . . . 18
   C.    Dynamic Per-neighbor MTU Discovery . . . . . . . . . . . . . 22 19
         Intellectual Property and Copyright Statements . . . . . . . 23 21

1. Introduction

   This document presents a simple approach that enables incremental
   deployment of IPv6 [1] within IPv4-based [2] sites in a manner that
   is compatible with inter-domain transition tunneling mechanisms, e.g., RFC 3056
   (6to4) [19]. [18].  We refer to this approach as the Intra-Site Automatic
   Tunnel Addressing Protocol, or ISATAP (pronounced: "ice-a-tap"). Protocol (ISATAP).  ISATAP allows dual-stack nodes
   that do not share a common 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 an NBMA link layer.
   layer for IPv6.

   This document specifies details for the transmission of IPv6 packets
   over ISATAP links (i.e., automatic IPv6-in-IPv4 tunneling), including
   a new EUI-64 based
   an interface identifier format [3][4][5] that embeds an IPv4 address.  This
   format supports IPv6 protocol mechanisms for address configuration of global,
   site-local and link-local addresses as specified in RFC 2462 [6] as
   well as simple link-layer address mapping.  Simple validity checks
   for received packets are given.  Also specified in this document is
   the operation of IPv6 Neighbor Discovery for ISATAP, as permitted for
   NBMA links by RFC 2461 [7]. ISATAP.  The document
   finally presents deployment and security considerations for ISATAP.

2. Applicability Statement

   ISATAP provides the following features:

   o  treats site's IPv4 infrastructure as an NBMA link layer for IPv6 using
      automatic IPv6-in-IPv4 tunneling (i.e., no configured tunnel

   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.,

   o  supports both stateless address autoconfiguration and manual

   o  supports networks that use non-globally unique IPv4 addresses
      (e.g., when private address allocations [8] [3] are used), but does
      not allow the virtual ISATAP link to span a Network Address
      Translator [9] [4]

   o  compatible with other NGTRANS mechanisms (e.g., 6to4 [19]) [18])

3. Requirements

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [5].

   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:
      same definition definitions as [6][7]. ([6], 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.

   ISATAP prefix:
      a prefix used to configure an address on the ISATAP interface.
      This prefix is administratively assigned to the ISATAP link and
      MUST NOT be duplicated on native IPv6 links.

   ISATAP address:
      an IPv6 on-link address with on an ISATAP prefix interface and with an ISATAP format interface
      identifier constructed as specified in section 4. Section 5.1

   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.

4. Transmission of

5. Basic IPv6 Packets Operation on ISATAP Links

   ISATAP links transmit IPv6 packets via automatic tunnels using the
   site's IPv4 infrastructure as an NBMA a link layer. layer for IPv6, i.e., the site's
   IPv4 ICMP errors
   and ARP failures may be processed infrastructure is treated as a Non-Broadcast, Multiple Access
   (NBMA) link error notifications, as
   allowed by RFC 2461 [7]. layer.  The common tunneling mechanisms specified
   in Section 3 of RFC 2893 [10] are used, with the following noted
   specific considerations subsections outline basic
   operational details for IPv6 on ISATAP links and automatic tunnels:

4.1 ISATAP links:

5.1 Interface Identifier Identifiers and Address Construction
   IPv6 unicast addresses [3][4] include a 64-bit

   (RFC2491 [7], section 5.1) requires companion documents to specify
   the exact mechanism for generating interface identifier
   field in "modified EUI-64 format", based on tokens (i.e.,
   identifiers).  Interface identifiers for ISATAP are compatible with
   the IEEE EUI-64 [5]
   specification.  (Modified EUI-64 identifier format inverts the sense of the 'u/
   l' bit from its specification in [5], i.e., 'u/l' = 0 indicates
   local-use.) ISATAP interface identifiers ([8], section 2.5.1), and are
   constructed by
   prepending the 32-bit string '00-00-5E-FE' with appending an IPv4 address (see on the following section for examples).  Appendix ISATAP link to the
   32-bit string '00-00-5E-FE'.  (Appendix B includes non-normative text
   explaining the rationale for this construction rule.

4.2 Stateless Autoconfiguration rule.)

   Global and Link-Local Addresses Local-use ISATAP addresses are unicast addresses that use ISATAP format
   interface identifiers constructed as follows:

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

                                Figure 1

   Link-local, site-local, and global ISATAP addresses can be created
   exactly as specified in [3], (e.g., by auto-configuration [6] or
   manual configuration).

   For example, the IPv6 global unicast address:


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


   The link-local

   (Similar examples for local-use addresses are made obvious by the
   above and site-local variants (respectively) are:



4.3 with reference to the IPv6 addressing architecture

5.2 ISATAP Link/Interface Configuration


   ISATAP Link/Interface configuration is consistent with (RFC2491 [7],
   sections 5.1.1 and 5.1.2).

   Using the terminology of Section 4, 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

   At least one link-layer address per each

5.3 Dual Stack Operation and Address Configuration

   ISATAP router interface
   SHOULD be added to uses the Potential Routers List (see Section 5.2.1).

4.4 Sending Rules same specification found in ([9], section 2).  That
   is, ISATAP nodes implement "IPv6/IPv4" or "dual-stack" configurations
   and operate with both stacks enabled.  Address Mapping configuration and DNS
   considerations are the same as for ([9], sections 2.1 and 2.2)

5.4 Tunneling Mechanisms

   The common tunneling mechanisms specified in ([9], sections 3.1
   through 3.7) are used, with the following noted specific

5.4.1 Encapsulation

   The specification in ([9], section 3.1) is used.  Additionally, the
   IPv6 next-hop address for packets sent on an ISATAP link MUST be an
   ISATAP address.  Packets that do not satisfy this constraint MUST
   be address; other packets are discarded (i.e., not encapsulated)
   and an ICMPv6 destination unreachable indication with code 3 (Address
   Unreachable) [11] MUST be returned.  No other sending
   rules are necessary.

   The procedure for mapping unicast addresses into link-layer addresses [10] is returned to simply treat the last four octets of source.

5.4.2 Tunnel MTU and Fragmentation

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

   ISATAP address as an
   IPv4 address (in network byte order).  No multicast address mappings
   are specified.

4.5 Validity Checks uses automatic tunnel interfaces that may be configured over
   multiple underlying links with diverse maximum transmission units
   (MTUs).  The minimum MTU for Received Packets

   Packets received on ISATAP IPv6 interfaces MUST satisfy at least one
   (i.e., one or both) of is 1280 bytes ([1],
   Section 5), but the following validity checks:

   o  the network-layer (IPv6) source address has considerations apply when IPv4 is used
   as a prefix configured link layer for IPv6:

   o  nearly all IPv4 nodes accept unfragmented packets up to 1500 bytes

   o  sub-IPv4 layer encapsulations (e.g., VPN) may occur on
      the ISATAP interface and an ISATAP-format interface identifier
      that embeds the link-layer (IPv4) source address, i.e., source is
      on-link some paths

   o  the link-layer (IPv4) source address is in the Potential Routers
      List (see Section 5.2.1), i.e., previous hop is  commonly-deployed VPNs use an on-link ISATAP

   Packets that do not satisfy at least one of the above checks are
   silently discarded.

4.6 Tunnel MTU and Fragmentation of 1400 bytes

   Thus, ISATAP interfaces implement automatic tunnels that may be configured
   over multiple underlying links with diverse MTUs.  The ISATAP
   interface SHOULD use an MTU (ISATAP_MTU) SHOULD of 1380 bytes
   (1400 minus 20 bytes for IPv4 encapsulation) to maximize efficiency
   and minimize IPv4 fragmentation.

   ISATAP_MTU MAY be set to larger values when the encapsulator uses
   dynamic per-neighbor MTU discovery.  When larger values are used,
   ISATAP_MTU SHOULD NOT exceed the largest maximum MTU of all underlying links (LINK_MTU)
   minus 120 20 bytes for possible link-layer
   encapsulations (see note 1).  The minimum value (ISATAP_MINMTU) MUST
   be at least 1280 bytes [1], but SHOULD be set to 1380 bytes (see note

   IPv6 path MTU discovery [14] is required link layer encapsulation.  (Appendix C provides
   non-normative considerations for dynamic per-neighbor MTU discovery.)

   As with ordinary IPv6 interfaces that
   send interfaces, the network layer (IPv6) forwards
   packets larger than 1280 bytes.  The following considerations
   for of size ISATAP_MTU or smaller to the ISATAP interfaces interface.  All
   other packets are noted:

   o  ISATAP encapsulators dropped, and decapsulators are IPv6 neighbors since
      they share a common link layer, i.e., the ISATAP link

   o  ISATAP neighbors may be separated by multiple IPv4 hops requiring
      IPv4 path an ICMPv6 "packet too big" message
   with MTU discovery [15] = ISATAP_MTU is returned to establish per-neighbor MTUs

   o  NBR_MTU information is stored as link-layer (IPv4) information
      (e.g., in the IPv4 path MTU discovery cache), thus it may not be
      visible to upper layers in all implementations

   o  NBR_MTU information may not always be available for each neighbor
      due to finite storage limitations

   o  IPv4 path MTU discovery delivers ICMPv4 "fragmentation needed"
      messages, but these do not provide enough state for translation
      into ICMPv6 "packet too big" messages (see: RFC 792 [12] and RFC
      1812 [13], section

   IPv6 sees the source [11].

   ISATAP interface as an ordinary IPv6 interface with a
   fixed MTU (ISATAP_MTU), i.e., only those IPv6 interfaces send all packets of size
   ISATAP_MTU 1380 bytes or smaller are accepted.  All other packets are dropped,
   and an IPv6 ICMP "packet too big" message with MTU = ISATAP_MTU is
   returned.  ISATAP interfaces SHOULD implement the following
   link-layer algorithm to determine when to perform IPv6-in-IPv4
   encapsulation and when to return an ICMPv6 "packet too big" message:

   Determine per-neighbor LINK_MTU; NBR_MTU, e.g., by consulting IPv4
   forwarding table and/or IPv4 path MTU discovery cache, then:

   if NBR_MTU information exists
   	if packet is larger than NBR_MTU - 120 and packet
   	  is larger than ISATAP_MINMTU
   		Send IPv6 ICMP "packet too big" with
   		Drop packet
   		if packet is larger than ISATAP_MINMTU
   			Encapsulate and set
   the Don't Fragment
                     	  flag (DF) bit NOT set in the encapsualting IPv4 header
   			Encapsulate but do not set the Don't
   			  Fragment flag header.

5.4.3 Handling IPv4 ICMP Errors

   The specification in the ([9], section 3.4) MAY be used.  IPv4 header

   	if packet is larger than LINK_MTU - 120 ICMP
   errors and packet ARP failures are otherwise processed as link error

5.4.4 Decapsulation

   The specification in ([9], section 3.6) is
             larger than ISATAP_MINMTU
   		Send IPv6 ICMP "packet too big" with
   		Drop packet
   		if IPv6 neighbor used.

5.4.5 Link-Local Addresses

   The specification in ([9], section 3.7) is also NOT used.  Instead,
   link-local addresses are formed by appending an IPv4 neighbor on interface identifier,
   as defined in Section 5.1, to the underlying link, or prefix FE80::/64.

5.4.6 Ingress Filtering

   The network layer (IPv6) destination address of a packet received on
   an ISATAP interface is less than
                     or == ISATAP_MINMTU
   			Encapsulate but do not set the Don't
   			  Fragment flag in either local (i.e., matches an address
   configured on the IPv4 header
   			send ICMPv6 "packet too big" local IPv6 stack) or foreign.  The decapsulator
   MUST be configured with
   			Drop packet

                                Figure 2

   Encapsulators MAY maintain per-neighbor MTU (NBR_MTU) values by
   periodically probing the a list of IPv4 path, e.g., by sending address prefixes that are
   acceptable, i.e., an ingress filter list (default deny all).  For
   packets larger
   than ISATAP_MINMTU with foreign network layer (IPv6) destination addresses, the DF bit set in the IPv4 header.  Large
   data packets and/or Neighbor Solicitation (NS) packets with padding
   bytes added (up to a total length of ISATAP_MTU) may
   link layer (IPv4) source address MUST be used for explicitly allowed by
   ingress filtering.  Packets that do not satisfy this
   purpose.  (NS packets condition are preferred, since successful delivery
   results in a positive acknowledgement from the decapsulator.)
   When probing, implementations SHOULD maintain state for translating
   ICMPv4 "fragmentation needed" messages into ICMPv6 "packet too big"
   messages for
   silently discarded.

   Additionally, all packets (whether foreign or local) MUST satisfy at
   least one (i.e., one or both) of the round-trip time (RTT) between following validity checks:

   o  the
   encapsulator and decapsulator 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 address is in the Potential Routers
      List (see note 3).  Implementations SHOULD
   repeat Section 6.2.1)

   Packets that do not satisfy the above conditions are silently

6. Neighbor Discovery and Address Autoconfiguration

   RFC 2491 [7] provides a general architecture for IPv6 over NBMA
   networks, including multicast mechanisms to support host-side
   operation of the IPv6 neighbor discovery protocol.  ISATAP links most
   closely meet the polling process within REACHABLE_TIME description for connectionless service found in the
   last paragraph of ([7], section 10) to
   detect link MTU restrictions.


   1. 1), i.e., ISATAP requires 20 bytes for link-layer (IPv4) encapsulation.
       However, sub-IPv4 layer encapsulation (e.g., for VPNs) may occur
       on some paths.  Commonly-deployed VPNs on Ethernet use addresses provide
   the sender with an MTU of
       1400 bytes, thus 100 bytes (1500 minums 1400) are reserverd for
       sub-IPv4 layer encapsulation.

   2.  Nearly all IPv4 routers NBMA destination address to which it can forward 1500 byte transmit
   packets without
       fragmentation. whenever it desires.  Thus, 1380 bytes (1500 minus 100 minus 20) is

   3.  ICMPv4 "fragmentation needed" messages can be injected by
       malicious nodes, but this same problem exists in IPv4.  Using
       Neighbor Solicitation messages the RFC 2491 multicast mechanisms
   are not required for probing address resolution and receiving a
       positive acknowledgement from a trusted decapsulator MAY help
       encapsulators recognize spoofed ICMPv4 "fragmentation needed"

5. Neighbor Discovery for not otherwise implemented
   on ISATAP Links

   Section 3.2 of links due to traffic scaling considerations (i.e., ISATAP
   links are unicast-only).

   RFC 2461 [7] [6] 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 [7]. [6].
   Address resolution and the mechanisms for delivering Router
   Solicitations and Advertisements for on ISATAP links are not specified
   by [7]; instead, they are specified use the
   specifications found in this document.  (Note that
   these mechanisms MAY potentially apply to other types of NBMA links
   in the future.)


6.1 Address Resolution
   Protocol addresses (IPv6) in

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

   ISATAP hosts SHOULD enhance  ([7], section 5.2) requires companion documents to specify
   the static format for link layer address options, however, link layer
   address options are not needed for address resolution computation
   with in ISATAP.
   Thus, no format is specified and the following specification from
   ([9], section 3.8) applies:

      "This means that a unicast Neighbor Solicitation(NS)/Neighbor Advertisement(NA)
   exchange to ensure IPv6 level reachability sender of Neighbor Discovery packets

      *  SHOULD NOT include Source Link Layer Address options or Target
         Link Layer Address options on the neighbor and also tunnel link.

      *  MUST silently ignore any received SLLA or TLLA options on the
         tunnel link."

   Following static address resolution, ISATAP hosts SHOULD implement
   the reachability confirmation specifications in [6], sections
   7.2.2-7.2.8 that apply when unicast Neighbor Solicitations (NS) are
   used.  ISATAP hosts SHOULD additionally perform Neighbor
   Unreachability Detection (NUD) as specified in (RFC 2461 [7], [6], section
   7.3).  ISATAP routers MAY implement perform the
   enhanced address resolution above-specified reachability
   detection and NUD, NUD procedures, but this might not scale in all

   All ISATAP nodes MUST send solicited neighbor advertisements ([7], ([6],
   section 7.2.4).

   Link-layer address options ([7], section 4.6.1) for this
   specification MUST have Length = 1 and a six-octet interface
   identifier consisting of two zero octets followed

   ISATAP links disable Duplicate Address Detection, as permitted by a four-octet
   IPv4 address.  Options of this form SHOULD NOT be sent in NS/NA
   ([12], section 4).

6.2 Address Autoconfiguration and MUST be silently ignored in received NS/NA messages.

5.2 Router and Prefix Discovery

   Since the site's IPv4 infrastructure is treated as an NBMA link
   layer, unsolicited Router Advertisements do multicast emulation mechanisms are not provide sufficient
   means for router discovery used on ISATAP links.  Thus,
   links, nodes will not receive unsolicited multicast Router
   Advertisements.  (RFC 2462 [12], section 5.5.2) requires that hosts
   use stateful autoconfiguration (i.e., DHCPv6 [13]) in the absence of
   Router Advertisements.  When statelful autoconfiguration is not
   available, nodes use alternate mechanisms are required (described below) for
   router and specified below:

5.2.1 prefix discovery.

6.2.1 Conceptual Data Structures

   ISATAP nodes use the conceptual data structures Prefix List and
   Default Router List exactly as in ([7], ([6], section 5.1).  ISATAP links
   add a two new conceptual data structure structures "Potential Router List" and the
   following new configuration variable:

      Time between name service resolutions.  Default and suggested
      minimum: 1hr
   "Stateful Autoconfiguration Server List".

   A Potential Router List (PRL) and Stateful Autoconfiguration Server
   List (SASL) 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"), and is used to construct the ISATAP
   link-local address for that interface.  Similarly, each entry in the
   SASL has an IPv4 address and associated timer.  The
   following sections specify IPv4 address
   represents a DHCPv6 server attached to the process for initializing ISATAP link, and is used
   to construct the PRL: ISATAP link-local address for that DHCPv6 server.

   When a node enables an ISATAP link, it first discovers a DNS (RFC
   1035 [22]) fully-qualified domain name IPv4 addresses
   for the site's ISATAP service. PRL and SASL.  The domain name addresses MAY be established by a DHCPv4 [17]
   [14] option for ISATAP (option code TBD, see IANA Considerations), TBD), by manual configuration, or
   by an unspecified alternative method.  The method (e.g., DHCPv4 option for
   ISATAP is implemented exactly as in RFC 3361 [18] with the following
   noted exceptions:

   o  the DHCP option code for ISATAP (TBD) is used

   o  the encoding byte MUST be 0, i.e.; only FQDNs are accepted

   o  if multiple vendor-specific
   option; DNS ([19]) fully-qualified domain names occur, only the first is used

   Next, the node initializes the link's PRL with IPv4 addresses
   associated with the names).

   When DNS fully-qualified domain name discovered above. names are used, IPv4 addresses for
   the PRL and SASL are discovered through manual config a static host file or by
   querying the name service an IPv4-based DNS server to
   resolving resolve the domain name names into
   address records (e.g., DNS 'A' resource records) containing IPv4
   addresses.  Unspecified alternative methods for domain name
   resolution may also be used.

   Notes:  The following notes apply when DNS
   fully-qualified domain names are used:

   1.  Site administrators maintain a domain name for the ISATAP service names and a list of IPv4 addresses representing for
       the PRL and SASL for the site's ISATAP router
       interfaces (normally service, e.g., as address
       records in the site's name
       service). service.  Administrators may also
       advertise the domain name names in a DHCPv4 option for ISATAP.

   2.  There are no mandatory rules for the selection of a domain name, names,
       but administrators are encouraged to use the convention
       "(list_name).isatap.domainname" (e.g.,

   3.  After initialization, nodes periodically re-initialize the PRL
       (after ResolveInterval).
       and SASL, e.g., once per hour.  When DNS is used, nodes MUST client DNS
       resolvers use the IPv4 transport to resolve the names and follow
       the cache invalidation procedures in [22] [19] when the DNS
       time-to-live expires.


6.2.2 Validity Checks for Router Advertisements

   A node MUST silently discard any Router Advertisement messages it
   receives that do not satisfy both the validity checks in ([7], ([6],
   section 6.1.2) and the following additional validity check for

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


6.2.3 Router Specification

   Advertising ISATAP interfaces of routers behave the same as
   advertising interfaces described in ([7], ([6], section 6.2).  However,
   periodic unsolicited multicast Router Advertisements are not
   required, 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 ISATAP interface, it replies with a unicast Router
   Advertisement to the address of the node which sent the Router
   Solicitation.  The source address of the Router Advertisement is a
   link-local unicast address associated with the interface.  This MAY
   be the same 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 ([7], ([6], section 6.2.7) is desired.


6.2.4 Host Specification

   All entries in the PRL are assumed to represent active ISATAP routers
   within the site, i.e., the PRL provides trust basis only; not
   reachability detection.  When stateful autoconfiguration is available
   (i.e., when the SASL is non-null and at least one DHCPv6 server is
   reachable), hosts may send unicast messages directly to the DHCPv6
   server as specified in ([13], section 1.1).  Hosts SHOULD attempt
   stateful autoconfiguration for each entry in the SASL (i.e., until an
   attempt succeeds) before concluding that stateful autoconfiguration
   is unavailable.

   When stateful autoconfiguration is unavailable, hosts MAY
   periodically solicit information from one or more entries in the PRL
   ("PRL(i)") by sending unicast Router Solicitation messages using the
   IPv4 address ("V4ADDR_PRL(i)") and associated timer in the entry.
   Hosts add the following variable to support the solicitation process:

      Minimum time between sending Router Solicitations to any router.
      Default and suggested minimum: 15min minimum 800,000 milliseconds (15min).

   When a PRL(i) is selected, the host sets its associated timer to
   MinRouterSolicitInterval and initiates solicitation following a short
   delay as in ([7], ([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.

   Solicitation consists of sending Router Solicitations to the ISATAP
   link-local address constructed from the entry's IPv4 address, i.e.,
   they are sent to 'FE80::0:5EFE:V4ADDR_PRL(i)' instead of 'All-Routers
   multicast'.  They are otherwise sent exactly as in ([7], ([6], section

   Hosts process received Router Advertisements exactly as in ([7], ([6],
   section 6.3.4).  Hosts additionally reset the timer associated with
   the V4ADDR_PRL(i) embedded in the network-layer source address in
   each received Router Advertisement.  The timer is reset to either 0.5
   * (the minimum value in the router lifetime or valid lifetime of any
   on-link prefixes advertised) or MinRouterSolicitInterval; whichever
   is longer.

   ([7], section 6.3.4) includes the following specification:

      "To limit the storage needed for the Default Router List, a host
      MAY choose not to store all of the router addresses discovered via
      advertisements.  However, a host MUST retain at least two
      addresses and SHOULD retain more."

   The router solicitation process for ISATAP nodes is analogous to
   choosing which router addresses to store as in the above text.
   ISATAP nodes may wish to consider the control traffic overhead of
   this process when choosing how many routers to solict.  The manner of
   choosing particular routers in the PRL for solicitation network-layer source address in
   each solicited Router Advertisement received.  The timer is outside reset to
   either 0.5 * (the minimum value in the scope router lifetime or valid
   lifetime of this specification.

6. any on-link prefixes received in the advertisement) or
   MinRouterSolicitInterval; whichever is longer.

7. ISATAP Deployment Considerations


7.1 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 ([7], ([6], section 5.3) and ([6], ([12], 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
   native IPv6, and also ISATAP (over IPv4).  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 ISATAP interface(s) SHOULD be
   added (either automatically or manually) to the site's address
   records for ISATAP router interfaces.


7.2 Site Administration Considerations

   The following considerations are noted for sites that deploy ISATAP:

   o  ISATAP links are administratively defined by a set of router
      interfaces, a set of stateful autoconfiguration servers, and set
      of nodes which have discover those interface and server addresses
      in their potential router lists. Thus,
      ISATAP links are defined by administrative (not physical)

   o  ISATAP hosts and routers 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 (RS) to and receive
      unicast Router Advertisements from to one or more members of the
      potential router list.  When Router
      Advertisements (RAs) are received,  A well-deployed stateful autoconfiguration
      service within the Router Lifetime value
      provides a timer for site can minimize and/or eliminate the next RS to be sent.  Worst-case is need for
      small values of Router Lifetime which is bounded by
      periodic solicitation.

   o  ISATAP nodes periodically refresh the entries on the PRL,
      typically by querying the DNS. PRL and SASL.
      Responsible site administration can reduce the control traffic.
      At a minimum, administrators SHOULD ensure that dynamically
      advertised information for the site's address records for ISATAP router
      interfaces PRL and SASL are well


8. IANA Considerations

   A DHCPv4 option assignment code for ISATAP (TBD) [20] is requested, as outlined requested in the procedures found in RFC 2939 [23], section 3.

   Appendix B proposes a specification for managing event
   that the IEEE OUI
   assigned to IANA for EUI-64 interface identifier construction.  This
   specification is made freely available to IANA IESG recommends this document for any purpose they
   may find useful.

8. standards track.

9. Security considerations

   Site administrators are advised that, in addition to possible attacks
   against IPv6, security attacks against IPv4 MUST also be considered.
   Many security considerations in RFC 2529 [20], section 9 apply also
   to ISATAP.

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

   If IPv4 source address filtering is not correctly implemented, the
   ISATAP validity checks will not be effective in preventing IPv6
   source address spoofing.

   If filtering for ip-protocol-41 is not correctly implemented, IPv6
   source address spoofing is clearly possible, but this can be
   eliminated if both IPv4 source address filtering, and the ISATAP
   validity checks are implemented.

   (RFC 2461 [7]), [6]), section 6.1.2 implies that nodes trust Router
   Advertisements they receive 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 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 ISATAP address format does not support privacy extensions for
   stateless address autoconfiguration [21].  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.)


10. 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 [24] [22]
   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

   [3]   Hinden, R. and S. Deering, "IP Version 6 Addressing
         Architecture", RFC 2373, July 1998.

   [4]   Hinden, R. and S. Deering, "An IPv6 Aggregatable Global Unicast
         Address Format", RFC 2374, July 1998.

   [5]   IEEE, "
         EUI64.html", March 1997.

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

   [7]   Narten, T., Nordmark, E.
   brought to the authors' attention on September 20, 2002.

Normative References

   [1]   Deering, S. and W. Simpson, "Neighbor Discovery
         for IP R. Hinden, "Internet Protocol, Version 6 (IPv6)", (IPv6)
         Specification", RFC 2461, 2460, December 1998.


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

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

   [9]   Gilligan, R. and E. Nordmark, "Transition "Basic Transition Mechanisms for
         IPv6 Hosts and Routers", RFC 2893, August 2000.

   [11] draft-ietf-ngtrans-mech-v2-01 (work in
         progress), November 2002.

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

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

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


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

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

   [13]  Droms, R., "Dynamic Host Configuration Protocol for IPv6
         (DHCPv6)", draft-ietf-dhc-dhcpv6-28 (work in progress),
         November 2002.

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

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

   [16]  Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer,
         H., Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
         "Stream  Postel, J., "Internet Control Transmission Message Protocol", STD 5, RFC 2960, October 2000.
         792, September 1981.

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

   [18]  Schulzrinne, H., "Dynamic Host Configuration Protocol
         (DHCP-for-IPv4) Option  Baker, F., "Requirements for Session Initiation Protocol (SIP)
         Servers", IP Version 4 Routers", RFC 3361, August 2002. 1812,
         June 1995.

Informative References


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

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

   [21]  Narten, T. and R. Draves, "Privacy Extensions for Stateless
         Address Autoconfiguration in IPv6", Clouds", RFC 3041, January 3056, February 2001.


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


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


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

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

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

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 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 version 05 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
   changes from version 04 to version 05:

   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 SRI IPR statement and contact information

   o  Included reference to Quang Nguyen work

   changes from version 03 to version 04:

   o  Re-wrote section on Potential Router List initialization to
      reference existing precedence in other documents

   o  several minor wording changes based on feedback from the community

   changes from version 02 to version 03:

   o  Added contributing co-authors

   o  RSs are now sent to unicast addresses rather than

   o  Brought draft into better alignment with other IPv6
      standards-track documents

   o  Added applicability statement

   changes from version 01 to version 02:

   o  Cleaned up text and tightened up terminology

   o  Changed "IPv6 destination address" to "IPv6 next-hop address"
      under "sending rules"

   o  Changed definition of ISATAP prefix to include link and site-local

   o  Changed language in sections 4 and 5

   changes from version 00 to version 01:

   o  Revised draft to require different /64 prefixes for ISATAP
      addresses and native IPv6 addresses.  Thus, a node's ISATAP
      interface is assigned a /64 prefix that is distinct from the
      prefixes assigned to any other interfaces attached to the node -
      be they physical or logical interfaces.  This approach eliminates
      ISATAP-specific sending rules presented in earlier draft versions.

   o  Changed sense of 'u/l' bit in the ISATAP address interface
      identifier to indicate "local scope", since ISATAP interface
      identifiers are unique only within the scope of the ISATAP prefix.
      (See section 4.)

   changes from personal draft 4.1 to version 00:

   o  Title change Appendix B in response to provide higher-level description of field of use
      addressed by this draft.  Removed other extraneous text.
      comments from Pekka Savola

   o  Major new section on automatic discovery of off-link IPv6 routers
      when IPv6-IPv4 compatibility addresses are used.  Identified operational issues for anticipated deployment scenarios

   o  Included reference to Quang Nguyen work

Appendix B. Rationale for Interface Identifier Construction Rules

   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 use in 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 interpretation 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 3 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 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 larger than 1380 bytes, the encapsulator must implement
   a dynamic link layer mechanism to discover per-neighbor MTUs.

   IPv4 path MTU discovery [15] relies on ICMPv4 "fragmentation needed"
   messages, but these do not provide enough information for stateless
   translation into ICMPv6 "packet too big" messages (see: RFC 792 [16]
   and RFC 1812 [17], section  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 inadequate and can result in black holes that are difficult
   to diagnose [23].

   The ISATAP encapsulator may implement an alternate per-neighbor MTU
   discovery mechanism, e.g., periodic and/or on-demand probing of the
   IPv4 path to the decapsulator.  Probing consists of sending packets
   larger than 1380 bytes with the DF bit set in the IPv4 header.
   Neighbor Solicitation (NS) packets with padding bytes added should be
   used for this purpose, since successful delivery results in a
   positive acknowledgement that the probe succeeded, i.e., in the form
   of a special TYPE (0xFE) to
   indicate Neighbor Advertisement (NA) from the decapsulator.  (NB: Setting
   the DF bit prevents decapsulators from receiving probe packets that
   would overrun the receive buffer on an IPv4 address underlying link, thus no
   maximum receive unit (MRU) is embedded.  Thus, when required.)

   Implementations may choose to couple the first four
   octets probing process with
   neighbor cache management procedures ([6], section 7), e.g.  to
   maintain timers, state variables and/or a queue of an IPv6 interface identifier are: '00-00-5E-FE' (note: packets waiting
   for probes to complete.  Packets retained on the
   'u/l' bit MUST be 0) queue are forwarded
   when probes succeed, and provide state for sending ICMPv6 "packet too
   big" messages to the interface identifier is said source when probes fail.  Implementations may
   choose to be store per-neighbor MTU information in
   "ISATAP format" and the next four octets embed an IPv4 address
   encoded path MTU
   discovery cache, in network byte order.


   SRI International has notified the IETF of IPR considerations for
   some aspects ISATAP link layer's private data structures,

   ICMPv4 "fragmentation needed" messages may result when a link
   restriction is encountered but may also come from denial of this specification.  For more information consult 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
   online list 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 claimed rights. such packet loss without
   ICMPv6 "packet too big" messages returned to the sender.

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