BEHAVE WG                                                   M. Wasserman
Internet-Draft                                     Sandstorm Enterprises
Intended status: Standards Track                                F. Baker
Expires: May 5, 2009                                       Cisco Systems
                                                           November 2008

            IPv6-to-IPv6 Network Address Translation (NAT66)

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   Copyright (c) 2008 IETF Trust and the persons identified as the
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   This document describes a stateless, transport-agnostic IPv6-to-IPv6
   Network Address Translation (NAT66) function that provides the

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   address independence benefit associated with IPv4-to-IPv4 NAT (NAT44)
   while minimizing, but not completely eliminating, the problems
   associated with NAT44.

Table of Contents

   1.  Requirements Terminology . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  What is Address Independence?  . . . . . . . . . . . . . . . .  3
   4.  NAT66 Applicability  . . . . . . . . . . . . . . . . . . . . .  4
   5.  NAT66 Overview . . . . . . . . . . . . . . . . . . . . . . . .  5
   6.  NAT66 Address Mapping  . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Checksum-Neutral Mapping . . . . . . . . . . . . . . . . .  8
     6.2.  Address Mapping Example  . . . . . . . . . . . . . . . . .  9
   7.  Prefixes for Internal Addressing . . . . . . . . . . . . . . . 11
   8.  NAT Behavioral Requirements  . . . . . . . . . . . . . . . . . 11
   9.  A Note on Port Mapping . . . . . . . . . . . . . . . . . . . . 11
   10. SAF Considerations . . . . . . . . . . . . . . . . . . . . . . 12
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   12. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   14. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     14.1. Changes Between -00 and -01  . . . . . . . . . . . . . . . 13
     14.2. Changes between -01 and -02  . . . . . . . . . . . . . . . 13
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 14
     15.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15

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1.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Introduction

   This document describes a stateless, transport-agnostic IPv6-to-IPv6
   Network Address Translation (NAT66) function that provides the
   address independence benefit associated with IPv4-to-IPv4 NAT (NAT44)
   while minimizing, but not completely eliminating, the problems
   associated with NAT44.

3.  What is Address Independence?

   As used in this document, IPv6 Address Independence consists of the
   following set of local network properties:

   o  The IPv6 addresses in use inside the local network (for nodes,
      ACLs, logs) do not need to be renumbered if the ISP changes a
      site's external address prefix.

   o  The IPv6 addresses in use inside the local network (for nodes,
      ACLs, logs) do not need to be renumbered when a site changes ISPs.

   o  It is not necessary for an administrator to convince an ISP to
      route his or her internal IPv6 addresses.

   This address independence requirement has been a primary driver for
   IPv4 NAT deployment in medium to large-sized enterprise networks,
   including NAT deployments in enterprises that have plenty of IPv4
   provider-independent address space (from IPv4 "swamp space").

   The Local Network Protection document [RFC4864] discusses a related
   concept called "Address Autonomy" as a benefit of NAT44.  RFC 4864
   indicates that address autonomy can be achieved by the simultaneous
   use of global addresses on all nodes within a site that need external
   connectivity, and Unique Local Addresses (ULAs) [RFC4193] for all
   internal communication.  However, this solution fails to meet the
   requirement for address independence, because if an ISP renumbering
   event occurs, all of the hosts, routers, DHCP servers, ACLs,
   firewalls and other internal systems that are configured with global
   addresses from the ISP will need to be renumbered before global
   connectivity is fully restored.

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   The use of IPv6 Provider Independent (PI) addresses has also been
   suggested as a means to fulfill the address independence requirement.
   However, this solution requires that an enterprise qualify to receive
   a PI assignment and persuade their ISP to install specific routes for
   the enterprise's PI addresses.  There are a number of practical
   issues with this approach, especially if there is a desire to route
   to a number of geographically and topologically diverse set of sites,
   which can sometimes involve coordinating with several ISPs to route
   portions of a single PI prefix.  These problems have caused numerous
   enterprises with plenty of IPv4 swamp space to choose to use IPv4 NAT
   for part, or substantially all, of their internal network instead of
   using their provider-independent address space.

4.  NAT66 Applicability

   NAT66 provides a simple and compelling solution to meet the Address
   Independence requirement in IPv6.  The address independence benefit
   stems directly from the translation function of the network address
   translator.  To avoid as many of the issues associated with NAT44 as
   possible, NAT66 is defined to include a two-way, checksum-neutral,
   algorithmic translation function, and nothing else.

   NAT66 does not include a port mapping function, and the defined
   address mapping mechanism is checksum-neutral.  This avoids the need
   for a NAT66 device to re-write transport layer headers, making it
   feasible to deploy new or improved transport layer protocols without
   upgrading NAT66 devices.  Because NAT66 does not involve re-writing
   transport-layer headers, NAT66 will not interfere with encrypting the
   full IP payload in many cases.

   The default NAT66 address mapping mechanism is purely algorithmic, so
   NAT66 devices do not need to maintain per-node or per-connection
   state, allowing deployment of more robust and adaptive networks than
   can be deployed using NAT44.  Since the default NAT66 mapping can be
   performed in either direction, it does not interfere with inbound
   connection establishment, thus allowing internal nodes to participate
   in direct peer-to-peer applications.

   Although NAT66 compares favorably to NAT44 in several ways, it does
   not eliminate all of the architectural problems associated with IPv4
   NAT, as described in [RFC2993].  NAT66 involves modifying IP headers
   in transit, so it is not compatible with security mechanisms that
   involve end-to-end encryption of the IP header, or mechanisms, such
   as AH, that provide integrity protection for the IP header.  NAT66
   may interfere with the use of application protocols that transmit IP
   addresses in the application-specific portion of the IP packet.
   These applications currently require application layer gateways

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   (ALGs) to work correctly through NAT44 devices, and similar ALGs may
   be required for these applications to work through NAT66 devices.
   The use of separate internal and external address prefixes creates
   complexity for DNS deployment, due the desire for internal nodes to
   communicate with other internal nodes using internal addresses, while
   external nodes need to obtain external addresses to communicate with
   the same nodes.  Typically, this results in the deployment of "split
   DNS", which mad add complexity to network configuration.

   There are significant technical impactss associated with the
   deployment of any address translation mechanism, including NAT66, and
   we strongly encourage anyone who is considering the implementation or
   deployment of NAT66 to read RFC 4864 [RFC4864], and to carefully
   consider the alternatives described in that document, many of which
   will cause fewer problems than NAT66.

5.  NAT66 Overview

   NAT66 may be implemented in an IPv6 router to map one IPv6 address
   prefix to another IPv6 address prefix as each IPv6 packet transits
   the router.  A router that implements a NAT66 function is referred to
   as a NAT66 device.

   In its simplest form, a NAT66 device will be attached to two network
   links, one of which is an "internal" network link attached to a leaf
   network within a single administrative domain, and the other of which
   is an "external" network with connectivity to the global Internet.
   All of the hosts on the internal network will use addresses from a
   single, locally-routed prefix, and those addresses will be translated
   to/from addresses in a globally-routable prefix as IP packets transit
   the NAT66 device.

   The following picture shows a NAT66 device attached to two networks.
   In this example, the internal network uses IPv6 Unique Local
   Addresses (ULAs) [RFC4193] to represent the internal IPv6 nodes, and
   the external network uses globally routable IPv6 addresses to
   represent the same nodes.

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        External Network:  Prefix = 2001:0DB8:0001:/48
                         |  NAT66  |
                         |  Device |
        Internal Netowrk:  Prefix = FD01:0203:0405:/48

   When a NAT66 device forwards packets in the "outbound" direction,
   from the internal network to the external network, NAT66 overwrites
   the IPv6 source address (in the IPv6 header) with a corresponding
   address from the external prefix.  When packets are forwarded in the
   "inbound" direction, from the external network to the internal
   network, the IPv6 destination address is overwritten with a
   corresponding address in the internal prefix.  Using the prefixes
   shown in the diagram above, as an IP packet passes through the NAT66
   device in the outbound direction, the source address prefix (FD01:
   0203:0405:/48) will be overwritten with the external address prefix
   (2001:0DB8:0001:/48).  In an inbound packet, the destination prefix
   (2001:0DB8:0001:/48) will be overwritten with the internal network
   prefix (FD01:0203:0405:/48).  In both cases, it is the local IPv6
   address that is overwritten; the remote IPv6 address remains
   unchanged.  Nodes on the the mapped network are said to be "behind"
   the NAT66 device.

   NAT66 can also be used between two private networks.  In these cases,
   both networks may use ULA prefixes, with each subnet in one network
   mapped into a corresponding subnet in the other network, and vice
   versa.  Or, each network may use ULA prefixes for internal addressing
   and global unicast addresses on the other network.

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            Internal Prefix = FD01:4444:5555:/48
                 V            |      External Prefix
                 V            |      2001:0DB8:6666:/48
                 V        +---------+      ^
                 V        |  NAT66  |      ^
                 V        |  Device |      ^
                 V        +---------+      ^
        External Prefix       |            ^
        2001:0DB8:0001:/48    |            ^
            Internal Prefix = FD01:0203:0405:/48

   In some cases, more than one NAT66 device may be attached to a
   network.  In this case, they two NAT66 devices may be configured with
   the same internal and external prefixes, or they may be configured
   with the same internal prefix and different external prefixes.

        External Network:  Prefix = 2001:0DB8:0001:/48
                 |                      |
                 |                      |
            +---------+            +---------+
            |  NAT66  |            |  NAT66  |
            |  Device |            |  Device |
            |   #1    |            |   #2    |
            +---------+            +---------+
                 |                      |
                 |                      |
        Internal Netowrk:  Prefix = FD01:0203:0405:/48

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      External Network #1:          External Network #2:
   Prefix = 2001:0DB8:0001:/48    Prefix = 2001:0DB8:5555:/48
   ---------------------------    --------------------------
                 |                          |
                 |                          |
            +---------+                +---------+
            |  NAT66  |                |  NAT66  |
            |  Device |                |  Device |
            |   #1    |                |   #2    |
            +---------+                +---------+
                 |                          |
                 |                          |
        Internal Netowrk:  Prefix = FD01:0203:0405:/48

6.  NAT66 Address Mapping

   When NAT66 is used as described in this document, no per-node or per-
   flow state is maintained in the NAT66 device.  Both inbound and
   outbound packets are translated algorithmically, using only
   information found in the IPv6 header.  Due to this property, NAT66's
   two-way, algorithmic address mapping can support both outbound and
   inbound connection establishment without the need for state-priming
   or rendevous mechanisms.  This is a significant improvement over
   NAT44 devices, but it also has significant security implications
   which are described in the Security Considerations section.

6.1.  Checksum-Neutral Mapping

   Note: There has been some discussion of whether it is necessary or
   even desireable to have NAT66 use a checksum-neutral mapping.

   When a change is made to one of the IP header fields in the IPv6
   pseudo-header checksum (such as the IP addresses), the checksum field
   in the transport layer header may become invalid.  Fortunately, an
   incremental change in the area covered by the Internet standard
   checksum [RFC1071] will result in a well-defined change to the
   checksum value [RFC1624].  So, a checksum change caused by modifying
   one part of the area covered by the checksum can be eliminated by
   making a complementary change to a different 16-bit field covered by
   the same checksum.

   The NAT66 address mapping mechanism described in this document is
   checksum-neutral, which means that it results in IP headers that will
   generate the same pseudo-header checksum when the checksum is

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   calculated using the standard Internet checksum [RFC1071].  Any
   changes that are made during translation of the IPv6 prefix are
   offset by changes to other parts of the IPv6 address.  This results
   in the transport layers that use the Internet checksum (such as TCP
   and UDP) calculating the same IPv6 pseudo header checksum for both
   the internal and external forms of the same packet, which avoids the
   need for the NAT66 device to modify those transport layer headers.

   To produce a checksum neutral transformation, the NAT66 device
   calculates the 16-bit one's complement sum of the internal and
   external IPv6 prefixes.  The difference between the original and
   mapped prefix checksums is calculated using 16-bit one's complement
   arithmetic, and the difference is added to the 16-bit value in bits
   49-64 of the mapped IPv6 address.  If the resulting value is 0xFFFF,
   it is changed to 0x0000.  Use of this algorithm results in an IPv6
   header that will have the same pseudo-header checksum as the original
   header.  Although the same mechanism is used to ensure that the NAT66
   mapping is checksum-neutral, the corresponding change may be made in
   different locations depending upon the prefix length.

6.2.  Address Mapping Example

   The NAT66 two-way algorithmic address mapping described in this
   document MUST be implemented on all NAT66 devices.  This mapping
   consists of mapping an internal IPv6 prefix, possibly a ULA, to/from
   an external prefix, typically a globally-routable unicast address,
   and making a complementary modification to the local IPv6 addresss.
   The same transformation is performed in both the inbound and outbound
   directions, so the only state that is needed on the NAT66 box to
   peform this transformation is knowledge of the internal and external
   address prefixes in use.

   For the network shown in the first example diagram in the NAT66
   Overview section above, we might have the following example:

   Internal Prefix: FD01:0203:0405:/48 External Prefix: 2001:0DB8:

   If a node with internal address FD01:0203:0405:0001::1234 sends an
   outbound packet through the NAT66 device, the resulting external
   address will be 2001:0DB8:0001:D550::1234.  The resulting address is
   obtained by calculating the checksum of both the internal and
   external 48-bit prefixes, sutracting the internal prefix from the
   external prefix using one's complement arithmetic and adding the
   result to the 16-bit subnet field (in this case 0x0001).

   To show the work:

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   The one's complement checksum of FD01:0203:0405 is 0xFCF5.  The one's
   complement checksum of 2001:0DB8:0001 is 0xD245.  Using one's
   complement math, 0xD245 - 0xFCF5 = 0xD54F. The subnet mask in the
   original packet is 0x0001.  Using one's complement math, 0x0001 +
   0xD54F = 0xD550.  Since 0xD550 != 0xFFFF, it is not changed to

   So, the value 0xD550 is written in the 16-bit subnet mask area,
   resulting in a mapped external address of 2001:0DB8:0001:D550::1234.

   When a response packet is received, it will contain the destination
   address 2001:0DB8:0001:D550::0001, which will be mapped using the
   same mapping algorithm, back to FD01:0203:0405:0001::1234.

   In this case, the difference between the two prefixes will be
   calculated as follows:

   Using one's complement math, 0xFCF5 - 0xD245 = 0x2AB0.  The subnet
   mask in the original packet = 0xD550.  Using one's complement math,
   0xD550 + 0x2AB0 = 0x0001.  Since 0x0001 != 0xFFFF, it is not changed
   to 0x0000.

   So the value 0x0001 is written into the subnet field, and the
   internal value of the subnet field is properly restored.

   This mapping results in no modification of the Interface Identifier
   (IID), which is held in the lower half of the IPv6 address, so it
   will not interfere with future protocols that may use unique IIDs for
   node identification.

   Use of this mapping is restricted to cases where both the internal
   and external prefixes are 48 bits long (a /48) or shorter, leaving at
   least 16 subnet bits that can be modified to ensure checksum
   neutrality.  This may not be a significant limitation in practice,
   because it is expected that most NAT66 devices will be used to map
   between a provider-allocated external prefix of /48 or shorter and a
   ULA that uses the same prefix length as the external prefix.  If
   necessary, however, there are several options for performing checksum
   correction in the IID portion of addresses that have a shorter

   Furthermore, to avoid issues with one's complement arithmetic, subnet
   masks with all ones (0xFFFF) in bits 49-64 cannot be used on networks
   behind a NAT66 device.

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7.  Prefixes for Internal Addressing

   NAT66 devices MUST support manual configuration of internal and
   external address prefixes, and MUST NOT place any restrictions on
   those prefixes except that they be valid IPv6 unicast address
   prefixes, as described in [RFC4291], and that they are /48 or

8.  NAT Behavioral Requirements

   NAT66 devices MUST support hairpinning behavior, as defined in the
   NAT Behavioral Requirements for UDP document [RFC4787].  This means
   that when a NAT66 device receives a packet on the internal interface
   that has a destination address that matches the site's external
   prefix, it will translate the packet and forward it internally.  This
   allows internal nodes to reach other internal nodes using their
   external, global addresses when necessary.

   Because NAT66 does not perform port mapping and uses a one-to-one,
   reversible mapping algorithm, it is not clear that any of the other
   NAT behavioral requirements apploy to NAT66.  However, this topic
   should be discussed in more detail.

   NAT66 devices that do not have a manually configured internal prefix
   SHOULD randomly generate a /48 ULA prefix for the internal network
   and advertise that prefix in router advertisements.  NAT66 boxes with
   more than one internal interface SHOULD assign a (non-0xFFFF) subnet
   number to each link, and include the subnet number in router
   advertisements on the corresponding link.  NAT66 devices that
   generate a ULA prefix MUST generate the prefix using a random number
   as described in RFC4291 [RFC4193], and SHOULD store the randomly
   generated prefix is non-volatile storage for continued use.

9.  A Note on Port Mapping

   In addition to overwriting IP addresses when packets are forwarded,
   NAT44 devices often overwrite the source port number in outbound
   traffic, and the destination port number in inbound traffic.  This
   mechanism is called "port mapping".

   The major benefit of port mapping is that it allows multiple
   computers to share a single IPv4 address.  A large number of internal
   IPv4 addresses (typically from the prefix) can be mapped
   into a single external, globally routable IPv4 address, with the
   local port number used to identify which internal node should receive
   each inbound packet.  This address amplification feature should not

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   be needed in IPv6, where every attached network should be assigned at
   least a /48 prefix, leaving room for 16 subnet bits and a 64 bit
   Interface Identifier [RFC3587].

   Since port mapping requires re-writing a portion of the transport
   layer header, it requires NAT66 devices to be aware of all of the
   transport protocols that they forward, thus stifling the development
   of new and improved transport protocols.  Modifying the transport
   layer header is incompatible with security mechanisms that encrypt
   the full IP payload, and restricts the NAT66 device to forwarding
   transport layers that use weak checksum algorithms that are easily
   recalculated in routers.  Since there is significant detriment caused
   by modifying transport layer headers and very little, if any, benefit
   to the use of port mapping in IPv6, NAT66 devices that comply with
   this specification MUST NOT perform port mapping.

10.  SAF Considerations

   There should be some discussion of how this document relates to the
   requirements listed in the IPv6 SAF Considerations draft
   [I-D.thaler-ipv6-saf].  Because NAT66 defines a fully reversible
   mapping, it would only be necessary for a host to get access to the
   external prefix that is being used for communication in order to
   perform a local mapping.  However, it is not clear how that could be
   accomplished while maintaining the requirement for Address

11.  Security Considerations

   When NAT66 is deployed using the two-way, algorithmic address mapping
   defined in the document, it allows direct inbound connections to
   internal nodes.  While this can be viewed as a benefit of NAT66 vs.
   NAT44, it does open internal nodes to attacks that would not be
   possible in a NAT44 network.  Although this situation is not
   substantially worse, from a security standpoint, than running IPv6
   with no NAT, some enterprises may assume that a NAT66 device will
   offer similar protection to a NAT44 device.  For this reason, it is
   RECOMMENDED that NAT66 devices include an IPv6 firewall function, and
   the firewall function SHOULD be configured by default to block all
   incoming connections.  Administrators could then enable inbound
   connectivity for specific ports by reconfiguring the firewall.

12.  IANA Considerations

   This document has no IANA considerations.

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13.  Acknowledgements

   The checksum-neutral algorithmic address mapping described in this
   document is based on e-mail written by Iljtsch Van Beijnum.  A
   similar mapping mechanism to the one described in this document was
   previously described in a document that can be found here: http://
   [TBD, move to an informative reference].

   The following people provided advice or review comments that
   substantially improved this document: Jari Arrko, Iljtsch Van
   Beijnum, Remi Depres, Tony Hain, Ed Jankiewicz, Dave Thaler, Mark

   This document was written using the xml2rfc tool described in RFC
   2629 [RFC2629].

14.  Change Log

14.1.  Changes Between -00 and -01

   There were several minor changes made between the -00 and -01
   versions of this draft:

   o  Added Fred Baker as a co-author.

   o  Minor mathematical corrections.

   o  Added AH to paragraph on NAT security issues.

   o  Added additional NAT topologies to overview (diagrams TBD).

14.2.  Changes between -01 and -02

   There were further changes made between -01 and -02:

   o  Removed topology hiding mechanism.

   o  Added diagrams.

   o  Made minor updates based on mailing list feedback.

   o  Added discussion of IPv6 SAF document.

   o  Added applicability section.

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   o  Added discussion of Address Independence requirement.

   o  Added hairpining requirement and discussion of applicability of
      other NAT behavioral requirements.

   Note: There were also some more major mailing list comments, but it
   hasn't been clear where consensus lies on a lot of the issues that
   were raised.  So, changes of that magnitude will probably have to
   wait until there is a WG to reach consensus and chairs to determine
   when it has been reached.

15.  References

15.1.  Normative References

   [RFC1071]  Braden, R., Borman, D., Partridge, C., and W. Plummer,
              "Computing the Internet checksum", RFC 1071,
              September 1988.

   [RFC1624]  Rijsinghani, A., "Computation of the Internet Checksum via
              Incremental Update", RFC 1624, May 1994.

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

   [RFC3587]  Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
              Unicast Address Format", RFC 3587, August 2003.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4787]  Audet, F. and C. Jennings, "Network Address Translation
              (NAT) Behavioral Requirements for Unicast UDP", BCP 127,
              RFC 4787, January 2007.

15.2.  Informative References

              Thaler, D., "Source Address Finding (SAF) for IPv6
              Translation Mechanisms", draft-thaler-ipv6-saf-01 (work in
              progress), February 2009.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              June 1999.

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   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              November 2000.

   [RFC4864]  Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
              E. Klein, "Local Network Protection for IPv6", RFC 4864,
              May 2007.

Authors' Addresses

   Margaret Wasserman
   Sandstorm Enterprises
   14 Summer Street
   Malden, MA  02148

   Phone: +1 781 333 3200

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117

   Phone: +1-408-526-4257

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