BEHAVE WG                                                   M. Wasserman
Internet-Draft                                     Sandstorm Enterprises
Expires: April 30, 2009                                 October 27, 2008

            IPv6-to-IPv6 Network Address Translation (NAT66)

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   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.

   This document also describes an address mapping option for NAT66 that
   offers the topology hiding benefit associated with NAT44 at the cost
   of additional state in the NAT66 device.

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Table of Contents

   1.  Requirements Terminology . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Motivations  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  NAT66 Overview . . . . . . . . . . . . . . . . . . . . . . . .  4
   5.  NAT66 Address Mapping Mechanisms . . . . . . . . . . . . . . .  5
     5.1.  Checksum-Neutral Mapping . . . . . . . . . . . . . . . . .  6
       5.1.1.  Two-Way Algorithmic Address Mapping  . . . . . . . . .  7
       5.1.2.  Topology Hiding Option . . . . . . . . . . . . . . . .  8
   6.  Prefixes for Internal Addressing . . . . . . . . . . . . . . . 10
   7.  A Note on Port Mapping . . . . . . . . . . . . . . . . . . . . 10
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
     11.2. Informative References . . . . . . . . . . . . . . . . . . 12
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13

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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.

   NAT66 does not include a port mapping function, and both of the
   defined address mapping mechanisms use checksum-neutral algorithms.
   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.

   This document also defines an address mapping option for NAT66 that
   offers the topology hiding benefit associated with NAT44.  This
   mechanism involves the configuration or dynamic maintenance of some
   per-node state in the NAT66 device.  So, when used with this optional
   address mapping mechanisms, NAT66 will have greater negative impact
   on direct peer-to-peer applications, and on the robustness and
   reliability of the network.  These trade-offs are discussed later in
   the document.

   Although NAT66 compares favorably to NAT44 in several ways, it does
   not eliminate all of the architectural problems associated with IPv4
   NAT.  [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.  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 (ALGs) to work correctly through

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   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 has
   it's own set of architectural implications [Ref Needed].

3.  Motivations

   In defining the NAT66 mechanism, it is not our goal to encourage the
   implementation or use of NAT66.  There are significant technical
   problems associated with the deployment of any type of NAT, and the
   IETF does not recommend the use of NAT.  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.

   We are documenting NAT66 because we believe that some people will
   choose to implement and deploy IPv6 NAT, in spite of our
   recommendation not to do so.  Some enterprises may choose to deploy
   IPv6 NAT to gain provider independent internal addressing or to
   simplify site multihoming.  Others may consider the trade-offs and
   choose IPv6 NAT as a topology hiding mechanism.  In other cases,
   administrators may choose to deploy IPv6 NAT to parallel their IPv4
   NAT-based network architecture.  Our goal is to define an IPv6-to-
   IPv6 NAT mechanism, NAT66, that will minimize the negative impacts of
   IPv6 NAT, in the event that some implementers do choose to implement
   an IPv6 NAT mechanism, and some network administrators do choose to
   deploy it.

4.  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

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   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.

        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 internal network are said to be "behind" the
   NAT66 device.

5.  NAT66 Address Mapping Mechanisms

   This document defines two address mapping functions that can be used
   in NAT66 devices.  To comply with this specification, NAT66 devices

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   MUST implement the Two-Way Algorithmic Address Mapping.  NAT66
   devices SHOULD implement the Topology Hiding Option.

   The Two-Way Algorithmic Address Mapping mechanism and the Topology
   Hiding Option both use 1:1 (one-to-one) mappings, meaning that a
   given internal address is always mapped to the same external address.

   When the Two-Way Algorithmic mapping is used, no per-node or per-flow
   state is maintained in the NAT66 box.  Both inbound and outbound
   packets are translated algorithmically, using only information found
   in the IPv6 header.  Due to this property, the 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.

   The Topology Hiding Option is intended to obscure the subnet
   information found in the internal IPv6 address prefix from external
   view, so that an external node cannot determine the structure of the
   internal network by looking at traffic outside of the NAT66 device.
   This features is called "topology hiding", and it is one of the
   benefits associated with NAT44.  Because it is not possible to derive
   the full internal address simply by looking at the external address,
   the NAT66 device needs to maintain state in order to copy the correct
   internal address into inbound packets.  This also means that inbound
   connection establishment will not work properly unless special
   provisions are made to enable inbound connectivity, such as
   configuring static state in the NAT66 device.

5.1.  Checksum-Neutral Mapping

   The NAT66 address mapping mechanisms described in this document are
   checksum-neutral, which means that they result in IP headers that
   will generate the same pseudo-header checksum when the checksum is
   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 (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 the transport layer headers.

   The NAT66 address mapping mechanisms both use the same technique to
   ensure that they produce checksum-neutral transformations.  When a
   changes is made to one of the fields in the IPv6 pseudo-header
   checksum, the checksum field in the transport layer header may become
   invalid.  Fortunately, an incremental change in the area covered by

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   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.

   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 a 16-bit value in another
   area of the local IPv6 address, thus resulting 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 both of the NAT66
   mappings are checksum-neutral, there are differences in which parts
   of the IPv6 header are mapped and where the complementary change is

5.1.1.  Two-Way Algorithmic Address Mapping

   The Two-Way Algorithmic Address Mapping MUST be implemented on all
   NAT66 devices.  This mapping consists of mapping an internal IPv6
   prefix, typically a ULA, to/from an external prefix, typically a
   globally-routable unicast address, and making a complementary
   modification to 16 subnet bits in bits 49 through 64 of 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 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:

   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

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   complement math, 0xD245 - 0xFCF5 = 0xD54F. The subnet mask in the
   original packet is 0x0001.  Using one's complement math, 0x0001 +
   0xD54F = 0xD550.

   So, the value D54F is written in the 16-bit subnet mask area,
   resulting in a mapped external address of 2001:0DB8:0001:D54F::0001.

   When a response packet is received, it will contain the destination
   address 2001:0DB8:0001:D54F::0001, which will be mapped using the
   same mapping algorithm, back to FD01:0203:0405:0000::0001.

   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 = 0x0000.

   So 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.  In
   cases where one or both prefixes are longer than a /48, the Topology
   Hiding Option can be used.

5.1.2.  Topology Hiding Option

   The topology hiding option SHOULD be implemented on all NAT66
   devices.  It is very similar to the Two-Way Algorithmic Address
   mapping, except that the subnet bits in the destination address are
   mapped to zero in the outbound direction and are restored to their
   original value in the inbound direction.  To remove the restriction
   on prefixes that have at least 16 bits of subnet space available, the
   checksum adjustment is made in the last 16 bits of the IP header,
   thus modifying the IPv6 Interface Identifier.  Because the Interface
   Identifier may no longer be unique, the "u" bit [RFC4291] is cleared
   in the IID.  This changes is also taken into account in the checksum

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   For the network shown in the 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 2001:0DB8:0001:0000:0002::E782.  The resulting
   address is obtained by calculating the checksum of both the internal
   and external 48-bit prefixes and subtracting the internal prefix
   checsum from the external prefix checksum.  If the "u" bit is cleared
   in the original address (the IID has universal scope), set the bit in
   the mapped address and add 0xFFFD to the checksum difference
   calculated above.  Then, add the checksum difference to the value o
   the last 16 bits of the IPv6 address.

   To show the work:

   The one's complement checksum of FD01:0203:0405:0001 is 0xFCF4.  The
   one's complement checksum of 2001:0DB8:0001:0000 is 0xD245.  Using
   one's complement math, 0xD245 - 0xFCF4 = 0xD550.  The original
   address has the "u" bit clear, so 0xD550 + 0xFFFD = 0xD54E. The last
   16 bits of the original address are 0x1234.  Using one's complement
   math, 0x1234 + 0xD54E = 0xE782.

   So, when the prefix is mapped, the "u" bit is set in the IID, and the
   value 0xE782 is written into the last 16 bits of the address, this
   results in a mapped external address of 2001:0DB8:0001:0000:0002::

   When a response packet is received, it will contain the destination
   address 2001:0DB8:0001:0000:0002::E782.  Unfortunately that address
   does not contain enough information to do an algorithmic reverse
   transformation, as the subnet bits were zeroed out when the external
   address was selected.  Therefore, the NAT66 will need to consult its
   internal state to perform the reverse address mapping.

   The internal state used for this mapping could consist of dynamic
   per-node mapping state, as is maintained in most NAT44 devices today,
   or it could consist of a static mapping of external addresses to
   internal addresses.  If dynamic state is used, inbound connections to
   nodes that have not yet communicated externally will fail, because
   there will be no state to perform the inbound mapping.  NAT66
   implementations SHOULD provide a means for network administrators to
   configure static mapping state to allow inbound mapping when the
   Topology Hiding Option is in use.

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   Note: We could place the checksum adjustment in the 16-bit subnet
   field, if the prefixes are /48 or less, thus avoiding the need to
   modify the IID in those cases.  Is that worth doing?  We can't
   blindly overwrite the 16-bit following prefix no matter where they
   are, because of the need to maintain the "u" and "g" bits in the 7th
   and 8th bits of the IID.

6.  Prefixes for Internal Addressing

   IPv6 includes a form of local addressing called Unique Local
   Addresses (ULAs) [RFC4193], and it is RECOMMENDED that ULAs be used
   to address network nodes that are located on an internal network
   serviced by a NAT66 device.

   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 meet the
   requirements outlined in this document.

   NAT66 devices that do not have a manually configured internal prefix
   SHOULD randomly generate a ULA prefix for the internal network and
   advertise that prefix in router advertisements.  NAT66 boxes with
   more than one internal interface SHOULD assign a 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.

7.  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, and perhaps its only significant
   benefit, 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 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].

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   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
   trasnport 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.

8.  Security Considerations

   When NAT66 is deployed using the two-way, algorithmic address
   mapping, 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.  For this reason, it is important for IPv6 networks that use
   NAT66 with the two-way, algorithmic address mapping to deploy a
   firewall to block undesired inbound traffic.  To avoid situations
   where network administrators are surprised by IPv6 inbound
   connections, NAT66 devices SHOULD include an IPv6 firewall function,
   and the firewall function SHOULD be configured by default to block
   all incoming connections.

9.  IANA Considerations

   This document has no IANA considerations.

10.  Acknowledgements

   The checksum-neutral algorithmic address mapping described in this
   document is based on e-mail written by Iljtsch Van Beijnum.

   The following people provided advice or review comments that
   substantially improved this document: TBD.

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

11.  References

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11.1.  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.

11.2.  Informative References

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

   [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.

Author's Address

   Margaret Wasserman
   Sandstorm Enterprises
   14 Summer Street
   Malden, MA  02148

   Phone: +1 781 333 3200

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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
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   retain all their rights.

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