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RFC 6296 - IPv6-to-IPv6 Network Prefix Translation


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Internet Engineering Task Force (IETF)                      M. Wasserman
Request for Comments: 6296                             Painless Security
Category: Experimental                                          F. Baker
ISSN: 2070-1721                                            Cisco Systems
                                                               June 2011

                IPv6-to-IPv6 Network Prefix Translation

Abstract

   This document describes a stateless, transport-agnostic IPv6-to-IPv6
   Network Prefix Translation (NPTv6) function that provides the
   address-independence benefit associated with IPv4-to-IPv4 NAT
   (NAPT44) and provides a 1:1 relationship between addresses in the
   "inside" and "outside" prefixes, preserving end-to-end reachability
   at the network layer.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6296.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  What is Address Independence?  . . . . . . . . . . . . . .  4
     1.2.  NPTv6 Applicability  . . . . . . . . . . . . . . . . . . .  5
     1.3.  Requirements Terminology . . . . . . . . . . . . . . . . .  7
   2.  NPTv6 Overview . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  NPTv6: The Simplest Case . . . . . . . . . . . . . . . . .  7
     2.2.  NPTv6 between Peer Networks  . . . . . . . . . . . . . . .  8
     2.3.  NPTv6 Redundancy and Load Sharing  . . . . . . . . . . . .  9
     2.4.  NPTv6 Multihoming  . . . . . . . . . . . . . . . . . . . .  9
     2.5.  Mapping with No Per-Flow State . . . . . . . . . . . . . . 10
     2.6.  Checksum-Neutral Mapping . . . . . . . . . . . . . . . . . 10
   3.  NPTv6 Algorithmic Specification  . . . . . . . . . . . . . . . 11
     3.1.  NPTv6 Configuration Calculations . . . . . . . . . . . . . 11
     3.2.  NPTv6 Translation, Internal Network to External Network  . 12
     3.3.  NPTv6 Translation, External Network to Internal Network  . 12
     3.4.  NPTv6 with a /48 or Shorter Prefix . . . . . . . . . . . . 12
     3.5.  NPTv6 with a /49 or Longer Prefix  . . . . . . . . . . . . 13
     3.6.  /48 Prefix Mapping Example . . . . . . . . . . . . . . . . 13
     3.7.  Address Mapping for Longer Prefixes  . . . . . . . . . . . 14
   4.  Implications of Network Address Translator Behavioral
       Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Prefix Configuration and Generation  . . . . . . . . . . . 15
     4.2.  Subnet Numbering . . . . . . . . . . . . . . . . . . . . . 15
     4.3.  NAT Behavioral Requirements  . . . . . . . . . . . . . . . 15
   5.  Implications for Applications  . . . . . . . . . . . . . . . . 16
     5.1.  Recommendation for Network Planners Considering Use of
           NPTv6 Translation  . . . . . . . . . . . . . . . . . . . . 17
     5.2.  Recommendations for Application Writers  . . . . . . . . . 18
     5.3.  Recommendation for Future Work . . . . . . . . . . . . . . 18
   6.  A Note on Port Mapping . . . . . . . . . . . . . . . . . . . . 18
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 20
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Why GSE?  . . . . . . . . . . . . . . . . . . . . . . 23
   Appendix B.  Verification Code . . . . . . . . . . . . . . . . . . 25

1.  Introduction

   This document describes a stateless IPv6-to-IPv6 Network Prefix
   Translation (NPTv6) function, designed to provide address
   independence to the edge network.  It is transport-agnostic with
   respect to transports that do not checksum the IP header, such as
   SCTP, and to transports that use the TCP/UDP/DCCP (Datagram
   Congestion Control Protocol) pseudo-header and checksum [RFC1071].

   For reasons discussed in [RFC2993] and Section 5, the IETF does not
   recommend the use of Network Address Translation technology for IPv6.
   Where translation is implemented, however, this specification
   provides a mechanism that has fewer architectural problems than
   merely implementing a traditional stateful Network Address Translator
   in an IPv6 environment.  It also provides a useful alternative to the
   complexities and costs imposed by multihoming using provider-
   independent addressing and the routing and network management issues
   of overlaid ISP address space.  Some problems remain, however.  The
   reader should consider the alternatives suggested in [RFC4864] and
   the considerations of [RFC5902] for improved approaches.

   The stateless approach described in this document has several
   ramifications:

   o  Any security benefit that NAPT44 might offer is not present in
      NPTv6, necessitating the use of a firewall to obtain those
      benefits if desired.  An example of such a firewall is described
      in [RFC6092].

   o  End-to-end reachability is preserved, although the address used
      "inside" the edge network differs from the address used "outside"
      the edge network.  This has implications for application referrals
      and other uses of Internet layer addresses.

   o  If there are multiple identically configured prefix translators
      between two networks, there is no need for them to exchange
      dynamic state, as there is no dynamic state -- the algorithmic
      translation will be identical across each of them.  The network
      can therefore asymmetrically route, load share, and fail-over
      among them without issue.

   o  Since translation is 1:1 at the network layer, there is no need to
      modify port numbers or other transport parameters.

   o  TCP sessions that authenticate peers using the TCP Authentication
      Option [RFC5925] cannot have their addresses translated, as the
      addresses are used in the calculation of the Message
      Authentication Code.  This consideration applies in general to any

      UNilateral Self-Address Fixing (UNSAF) [RFC3424] Protocol, which
      the IAB recommends against the deployment of in an environment
      that changes Internet addresses.

   o  Applications using the Internet Key Exchange Protocol Version 2
      (IKEv2) [RFC5996] should, at least in theory, detect the presence
      of the translator; while no NAT traversal solution is required,
      [RFC5996] would require such sessions to use UDP.

1.1.  What is Address Independence?

   For the purposes of this document, IPv6 address independence consists
   of the following set of properties:

   From the perspective of the edge network:

      *  The IPv6 addresses used inside the local network (for
         interfaces, access lists, and logs) do not need to be
         renumbered if the global prefix(es) assigned for use by the
         edge network are changed.

      *  The IPv6 addresses used inside the edge network (for
         interfaces, access lists, and logs) or within other upstream
         networks (such as when multihoming) do not need to be
         renumbered when a site adds, drops, or changes upstream
         networks.

      *  It is not necessary for an administration to convince an
         upstream network to route its internal IPv6 prefixes or for it
         to advertise prefixes derived from other upstream networks into
         it.

      *  Unless it wants to optimize routing between multiple upstream
         networks in the process of multihoming, there is no need for a
         BGP exchange with the upstream network.

   From the perspective of the upstream network:

      *  IPv6 addresses used by the edge network are guaranteed to have
         a provider-allocated prefix, eliminating the need and concern
         for BCP 38 [RFC2827] ingress filtering and the advertisement of
         customer-specific prefixes.

   Thus, address independence has ramifications for the edge network,
   networks it directly connects with (especially its upstream
   networks), and the Internet as a whole.  The desire for address
   independence 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").  It has also been a driver for edge
   networks to become members of Regional Internet Registry (RIR)
   communities, seeking to obtain BGP Autonomous System Numbers and
   provider-independent prefixes, and as a result has been one of the
   drivers of the explosion of the IPv4 route table.  Service providers
   have stated that the lack of address independence from their
   customers has been a negative incentive to deployment, due to the
   impact of customer routing expected in their networks.

   The Local Network Protection [RFC4864] document discusses a related
   concept called "Address Autonomy" as a benefit of NAPT44.  [RFC4864]
   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, Access Control
   Lists (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.

   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 its 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 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.

1.2.  NPTv6 Applicability

   NPTv6 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 prefix
   translator.  To avoid as many of the issues associated with NAPT44 as
   possible, NPTv6 is defined to include a two-way, checksum-neutral,
   algorithmic translation function, and nothing else.

   The fact that NPTv6 does not map ports and is checksum-neutral avoids
   the need for an NPTv6 Translator to rewrite transport layer headers.
   This makes it feasible to deploy new or improved transport layer

   protocols without upgrading NPTv6 Translators.  Similarly, since
   NPTv6 does not rewrite transport layer headers, NPTv6 will not
   interfere with encryption of the full IP payload in many cases.

   The default NPTv6 address-mapping mechanism is purely algorithmic, so
   NPTv6 translators do not need to maintain per-node or per-connection
   state, allowing deployment of more robust and adaptive networks than
   can be deployed using NAPT44.  Since the default NPTv6 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 without the application layer
   overhead one finds in many IPv4 Peer-to-Peer applications.

   Although NPTv6 compares favorably to NAPT44 in several ways, it does
   not eliminate all of the architectural problems associated with IPv4
   NAT, as described in [RFC2993].  NPTv6 involves modifying IP headers
   in transit, so it is not compatible with security mechanisms, such as
   the IPsec Authentication Header, that provide integrity protection
   for the IP header.  NPTv6 may interfere with the use of application
   protocols that transmit IP addresses in the application-specific
   portion of the IP datagram.  These applications currently require
   Application Layer Gateways (ALGs) to work correctly through NAPT44
   devices, and similar ALGs may be required for these applications to
   work through NPTv6 Translators.  The use of separate internal and
   external prefixes creates complexity for DNS deployment, due to 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.  This
   frequently results in the deployment of "split DNS", which may add
   complexity to network configuration.

   The choice of address within the edge network bears consideration.
   One could use a ULA, which maximizes address independence.  That
   could also be considered a misuse of the ULA; if the expectation is
   that a ULA prevents access to a system from outside the range of the
   ULA, NPTv6 overrides that.  On the other hand, the administration is
   aware that it has made that choice and could deploy a second ULA for
   the purpose of privacy if it desired; the only prefix that will be
   translated is one that has an NPTv6 Translator configured to
   translate to or from it.  Also, using any other global-scope address
   format makes one either obtain a PI prefix or be at the mercy of the
   agency from which it was allocated.

   There are significant technical impacts associated with the
   deployment of any prefix translation mechanism, including NPTv6, and
   we strongly encourage anyone who is considering the implementation or

   deployment of NPTv6 to read [RFC4864] and [RFC5902], and to carefully
   consider the alternatives described in that document, some of which
   may cause fewer problems than NPTv6.

1.3.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  NPTv6 Overview

   NPTv6 may be implemented in an IPv6 router to map one IPv6 address
   prefix to another IPv6 prefix as each IPv6 datagram transits the
   router.  A router that implements an NPTv6 prefix translation
   function is referred to as an NPTv6 Translator.

2.1.  NPTv6: The Simplest Case

   In its simplest form, an NPTv6 Translator interconnects 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 datagrams
   transit the NPTv6 Translator.  The lengths of these two prefixes will
   be functionally the same; if they differ, the longer of the two will
   limit the ability to use subnets in the shorter.

               External Network:  Prefix = 2001:0DB8:0001:/48
                   --------------------------------------
                                     |
                                     |
                              +-------------+
                              |     NPTv6   |
                              |  Translator |
                              +-------------+
                                     |
                                     |
                   --------------------------------------
               Internal Network:  Prefix = FD01:0203:0405:/48

                       Figure 1: A Simple Translator

   Figure 1 shows an NPTv6 Translator 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.

   When an NPTv6 Translator forwards datagrams in the "outbound"
   direction, from the internal network to the external network, NPTv6
   overwrites the IPv6 source prefix (in the IPv6 header) with a
   corresponding external prefix.  When datagrams are forwarded in the
   "inbound" direction, from the external network to the internal
   network, the IPv6 destination prefix is overwritten with a
   corresponding internal prefix.  Using the prefixes shown in the
   diagram above, as an IP datagram passes through the NPTv6 Translator
   in the outbound direction, the source prefix (FD01:0203:0405:/48)
   will be overwritten with the external prefix (2001:0DB8:0001:/48).
   In an inbound datagram, the destination prefix (2001:0DB8:0001:/48)
   will be overwritten with the internal prefix (FD01:0203:0405:/48).
   In both cases, it is the local IPv6 prefix that is overwritten; the
   remote IPv6 prefix remains unchanged.  Nodes on the internal network
   are said to be "behind" the NPTv6 Translator.

2.2.  NPTv6 between Peer Networks

   NPTv6 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.

                  Internal Prefix = FD01:4444:5555:/48
                  --------------------------------------
                       V            |      External Prefix
                       V            |      2001:0DB8:6666:/48
                       V        +---------+      ^
                       V        |  NPTv6  |      ^
                       V        |  Device |      ^
                       V        +---------+      ^
              External Prefix       |            ^
              2001:0DB8:0001:/48    |            ^
                  --------------------------------------
                  Internal Prefix = FD01:0203:0405:/48

               Figure 2: Flow of Information in Translation

2.3.  NPTv6 Redundancy and Load Sharing

   In some cases, more than one NPTv6 Translator may be attached to a
   network, as shown in Figure 3.  In such cases, NPTv6 Translators are
   configured with the same internal and external prefixes.  Since there
   is only one translation, even though there are multiple translators,
   they map only one external address (prefix and Interface Identifier
   (IID)) to the internal address.

               External Network:  Prefix = 2001:0DB8:0001:/48
                   --------------------------------------
                          |                      |
                          |                      |
                   +-------------+        +-------------+
                   |  NPTv6      |        |  NPTv6      |
                   |  Translator |        |  Translator |
                   |   #1        |        |   #2        |
                   +-------------+        +-------------+
                          |                      |
                          |                      |
                   --------------------------------------
               Internal Network:  Prefix = FD01:0203:0405:/48

                      Figure 3: Parallel Translators

2.4.  NPTv6 Multihoming

            External Network #1:          External Network #2:
         Prefix = 2001:0DB8:0001:/48    Prefix = 2001:0DB8:5555:/48
         ---------------------------    --------------------------
                         |                      |
                         |                      |
                  +-------------+        +-------------+
                  |  NPTv6      |        |  NPTv6      |
                  |  Translator |        |  Translator |
                  |   #1        |        |   #2        |
                  +-------------+        +-------------+
                         |                      |
                         |                      |
                  --------------------------------------
              Internal Network:  Prefix = FD01:0203:0405:/48

      Figure 4: Parallel Translators with Different Upstream Networks

   When multihoming, NPTv6 Translators are attached to an internal
   network, as shown in Figure 4, but are connected to different
   external networks.  In such cases, NPTv6 Translators are configured
   with the same internal prefix but different external prefixes.  Since

   there are multiple translations, they map multiple external addresses
   (prefix and IID) to the common internal address.  A system within the
   edge network is unable to determine which external address it is
   using apart from services such as Session Traversal Utilities for NAT
   (STUN) [RFC5389].

   Multihoming in this sense has one negative feature as compared with
   multihoming with a provider-independent address: when routes change
   between NPTv6 Translators, the translated prefix can change since the
   upstream network changes.  This causes sessions and referrals
   dependent on it to fail as well.  This is not expected to be a major
   issue, however, in networks where routing is generally stable.

2.5.  Mapping with No Per-Flow State

   When NPTv6 is used as described in this document, no per-node or per-
   flow state is maintained in the NPTv6 Translator.  Both inbound and
   outbound datagrams are translated algorithmically, using only
   information found in the IPv6 header.  Due to this property, NPTv6's
   two-way, algorithmic address mapping can support both outbound and
   inbound connection establishment without the need for maintenance of
   mapping state or for state-priming or rendezvous mechanisms.  This is
   a significant improvement over NAPT44 devices, but it also has
   significant security implications, which are described in Section 7.

2.6.  Checksum-Neutral Mapping

   When a change is made to one of the IP header fields in the IPv6
   pseudo-header checksum (such as one of 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 part of the area covered by the checksum can be
   corrected by making a complementary change to a different 16-bit
   field covered by the same checksum.

   The NPTv6 mapping mechanisms described in this document are checksum-
   neutral, which means that they result in IP headers that will
   generate the same IPv6 pseudo-header checksum when the checksum is
   calculated using the standard Internet checksum algorithm [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 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 datagram, which avoids the
   need for the NPTv6 Translator to modify those transport layer headers
   to correct the checksum value.

   The outgoing checksum correction is achieved by making a change to a
   16-bit section of the source address that is not used for routing in
   the external network.  Due to the nature of checksum arithmetic, when
   the corresponding correction is applied to the same bits of
   destination address of the inbound packet, the Destination Address
   (DA) is returned to the correct internal value.

   As noted in Section 4.2, this mapping results in an edge network
   using a /48 external prefix to be unable to use subnet 0xFFFF.

3.  NPTv6 Algorithmic Specification

   The [RFC4291] IPv6 Address is reproduced for clarity in Figure 5.

      0    15 16   31 32   47 48   63 64   79 80   95 96  111 112  127
     +-------+-------+-------+-------+-------+-------+-------+-------+
     |     Routing Prefix    | Subnet|   Interface Identifier (IID)  |
     +-------+-------+-------+-------+-------+-------+-------+-------+

            Figure 5: Enumeration of the IPv6 Address [RFC4291]

3.1.  NPTv6 Configuration Calculations

   When an NPTv6 Translation function is configured, it is configured
   with

   o  one or more "internal" interfaces with their "internal" routing
      domain prefixes, and

   o  one or more "external" interfaces with their "external" routing
      domain prefixes.

   In the simple case, there is one of each.  If a single router
   provides NPTv6 translation services between a multiplicity of domains
   (as might be true when multihoming), each internal/external pair must
   be thought of as a separate NPTv6 Translator from the perspective of
   this specification.

   When an NPTv6 Translator is configured, the translation function
   first ensures that the internal and external prefixes are the same
   length, extending the shorter of the two with zeroes if necessary.
   These two prefixes will be used in the prefix translation function
   described in Sections 3.2 and 3.3.

   They are then zero-extended to /64 for the purposes of a calculation.
   The translation function calculates the one's complement sum of the
   16-bit words of the /64 external prefix and the /64 internal prefix.
   It then calculates the difference between these values: internal

   minus external.  This value, called the "adjustment", is effectively
   constant for the lifetime of the NPTv6 Translator configuration and
   is used in per-datagram processing.

3.2.  NPTv6 Translation, Internal Network to External Network

   When a datagram passes through the NPTv6 Translator from an internal
   to an external network, its IPv6 Source Address is either changed in
   two ways or results in the datagram being discarded:

   o  If the internal subnet number has no mapping, such as being 0xFFFF
      or simply not mapped, discard the datagram.  This SHOULD result in
      an ICMP Destination Unreachable.

   o  The internal prefix is overwritten with the external prefix, in
      effect subtracting the difference between the two checksums (the
      adjustment) from the pseudo-header's checksum, and

   o  A 16-bit word of the address has the adjustment added to it using
      one's complement arithmetic.  If the result is 0xFFFF, it is
      overwritten as zero.  The choice of word is as specified in
      Sections 3.4 or 3.5 as appropriate.

3.3.  NPTv6 Translation, External Network to Internal Network

   When a datagram passes through the NPTv6 Translator from an external
   to an internal network, its IPv6 Destination Address is changed in
   two ways:

   o  The external prefix is overwritten with the internal prefix, in
      effect adding the difference between the two checksums (the
      adjustment) to the pseudo-header's checksum, and

   o  A 16-bit word of the address has the adjustment subtracted from it
      (bitwise inverted and added to it) using one's complement
      arithmetic.  If the result is 0xFFFF, it is overwritten as zero.
      The choice of word is as specified in Section 3.4 or Section 3.5
      as appropriate.

3.4.  NPTv6 with a /48 or Shorter Prefix

   When an NPTv6 Translator is configured with internal and external
   prefixes that are 48 bits in length (a /48) or shorter, the
   adjustment MUST be added to or subtracted from bits 48..63 of the
   address.

   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.

   NPTv6 Translator implementations MUST implement the /48 mapping.

3.5.  NPTv6 with a /49 or Longer Prefix

   When an NPTv6 Translator is configured with internal and external
   prefixes that are longer than 48 bits in length (such as a /52, /56,
   or /60), the adjustment must be added to or subtracted from one of
   the words in bits 64..79, 80..95, 96..111, or 112..127 of the
   address.  While the choice of word is immaterial as long as it is
   consistent, these words MUST be inspected in that sequence and the
   first that is not initially 0xFFFF chosen, for consistency's sake.

   NPTv6 Translator implementations SHOULD implement the mapping for
   longer prefixes.

3.6.  /48 Prefix Mapping Example

   For the network shown in Figure 1, the Internal Prefix is FD01:0203:
   0405:/48, and the External Prefix is 2001:0DB8:0001:/48.

   If a node with internal address FD01:0203:0405:0001::1234 sends an
   outbound datagram through the NPTv6 Translator, 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, subtracting the internal prefix from
   the external prefix using one's complement arithmetic to calculate
   the "adjustment", and adding the adjustment 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
   complement arithmetic, 0xD245 - 0xFCF5 = 0xD54F.  The subnet in the
   original datagram is 0x0001.  Using one's complement arithmetic,
   0x0001 + 0xD54F = 0xD550.  Since 0xD550 != 0xFFFF, it is not changed
   to 0x0000.

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

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

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

   Using one's complement arithmetic, 0xFCF5 - 0xD245 = 0x2AB0.  The
   subnet in the original datagram = 0xD550.  Using one's complement
   arithmetic, 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.

3.7.  Address Mapping for Longer Prefixes

   If the prefix being mapped is longer than 48 bits, the algorithm is
   slightly more complex.  A common case will be that the internal and
   external prefixes are of different lengths.  In such a case, the
   shorter prefix is zero-extended to the length of the longer as
   described in Section 3.1 for the purposes of overwriting the prefix.
   Then, they are both zero-extended to 64 bits to facilitate one's
   complement arithmetic.  The "adjustment" is calculated using those
   64-bit prefixes.

   For example, if the internal prefix is a /48 ULA and the external
   prefix is a /56 provider-allocated prefix, the ULA becomes a /56 with
   zeros in bits 48..55.  For purposes of one's complement arithmetic,
   they are then both zero-extended to 64 bits.  A side effect of this
   is that a subset of the subnets possible in the shorter prefix is
   untranslatable.  While the security value of this is debatable, the
   administration may choose to use them for subnets that it knows need
   no external accessibility.

   We then find the first word in the IID that does not have the value
   0xFFFF, trying bits 64..79, and then 80..95, 96..111, and finally
   112..127.  We perform the same calculation (with the same proof of
   correctness) as in Section 3.6 but apply it to that word.

   Although any 16-bit portion of an IPv6 IID could contain 0xFFFF, an
   IID of all-ones is a reserved anycast identifier that should not be
   used on the network [RFC2526].  If an NPTv6 Translator discovers a
   datagram with an IID of all-zeros while performing address mapping,
   that datagram MUST be dropped, and an ICMPv6 Parameter Problem error
   SHOULD be generated [RFC4443].

   Note: This mechanism does involve modification of the IID; it may not
   be compatible with future mechanisms that use unique IIDs for node
   identification.

4.  Implications of Network Address Translator Behavioral Requirements

4.1.  Prefix Configuration and Generation

   NPTv6 Translators MUST support manual configuration of internal and
   external prefixes and MUST NOT place any restrictions on those
   prefixes except that they be valid IPv6 unicast prefixes as described
   in [RFC4291].  They MAY also support random generation of ULA
   addresses on command.  Since the most common place anticipated for
   the implementation of an NPTv6 Translator is a Customer Premises
   Equipment (CPE) router, the reader is urged to consider the
   requirements of [RFC6204].

4.2.  Subnet Numbering

   For reasons detailed in Appendix B, a network using NPTv6 Translation
   and a /48 external prefix MUST NOT use the value 0xFFFF to designate
   a subnet that it expects to be translated.

4.3.  NAT Behavioral Requirements

   NPTv6 Translators MUST support hairpinning behavior, as defined in
   the NAT Behavioral Requirements for UDP document [RFC4787].  This
   means that when an NPTv6 Translator receives a datagram on the
   internal interface that has a destination address that matches the
   site's external prefix, it will translate the datagram and forward it
   internally.  This allows internal nodes to reach other internal nodes
   using their external, global addresses when necessary.

   Conceptually, the datagram leaves the domain (is translated as
   described in Section 3.2) and returns (is again translated as
   described in Section 3.3).  As a result, the datagram exchange will
   be through the NPTv6 Translator in both directions for the lifetime
   of the session.  The alternative would be to require the NPTv6
   Translator to drop the datagram, forcing the sender to use the
   correct internal prefix for its peer.  Performing only the external-
   to-internal translation results in the datagram being sent from the
   untranslated internal address of the source to the translated and
   therefore internal address of its peer, which would enable the
   session to bypass the NPTv6 Translator for future datagrams.  It
   would also mean that the original sender would be unlikely to
   recognize the response when it arrived.

   Because NPTv6 does not perform port mapping and uses a one-to-one,
   reversible-mapping algorithm, none of the other NAT behavioral
   requirements apply to NPTv6.

5.  Implications for Applications

   NPTv6 Translation does not create several of the problems known to
   exist with other kinds of NATs as discussed in [RFC2993].  In
   particular, NPTv6 Translation is stateless, so a "reset" or brief
   outage of an NPTv6 Translator does not break connections that
   traverse the translation function, and if multiple NPTv6 Translators
   exist between the same two networks, the load can shift or be
   dynamically load shared among them.  Also, an NPTv6 Translator does
   not aggregate traffic for several hosts/interfaces behind a fewer
   number of external addresses, so there is no inherent expectation for
   an NPTv6 Translator to block new inbound flows from external hosts
   and no issue with a filter or blacklist associated with one prefix
   within the domain affecting another.  A firewall can, of course, be
   used in conjunction with an NPTv6 Translator; this would allow the
   network administrator more flexibility to specify security policy
   than would be possible with a traditional NAT.

   However, NPTv6 Translation does create difficulties for some kinds of
   applications.  Some examples include:

   o  An application instance "behind" an NPTv6 Translator will see a
      different address for its connections than its peers "outside" the
      NPTv6 Translator.

   o  An application instance "outside" an NPTv6 Translator will see a
      different address for its connections than any peer "inside" an
      NPTv6 Translator.

   o  An application instance wishing to establish communication with a
      peer "behind" an NPTv6 Translator may need to use a different
      address to reach that peer depending on whether the instance is
      behind the same NPTv6 Translator or external to it.  Since an
      NPTv6 Translator implements hairpinning (Section 4.3), it suffices
      for applications to always use their external addresses.  However,
      this creates inefficiencies in the local network and may also
      complicate implementation of the NPTv6 Translator.  [RFC3484] also
      would prefer the private address in such a case in order to reduce
      those inefficiencies.

   o  An application instance that moves from a realm "behind" an NPTv6
      Translator to a realm that is "outside" the network, or vice
      versa, may find that it is no longer able to reach its peers at
      the same addresses it was previously able to use.

   o  An application instance that is intermittently communicating with
      a peer that moves from behind an NPTv6 Translator to "outside" of
      it, or vice versa, may find that it is no longer able to reach
      that peer at the same address that it had previously used.

   Many, but not all, of the applications that are adversely affected by
   NPTv6 Translation are those that do "referrals" -- where an
   application instance passes its own addresses, and/or addresses of
   its peers, to other peers.  (Some believe referrals are inherently
   undesirable; others believe that they are necessary in some
   circumstances.  A discussion of the merits of referrals, or lack
   thereof, is beyond the scope of this document.)

   To some extent, the incidence of these difficulties can be reduced by
   DNS hacks that attempt to expose addresses "behind" an NPTv6
   Translator only to hosts that are also behind the same NPTv6
   Translator and perhaps to also expose only the "internal" addresses
   of hosts behind the NPTv6 Translator to other hosts behind the same
   NPTv6 Translator.  However, this cannot be a complete solution.  A
   full discussion of these issues is out of scope for this document,
   but briefly: (a) reliance on DNS to solve this problem depends on
   hosts always making queries from DNS servers in the same realm as
   they are (or on DNS interception proxies, which create their own
   problems) and on mobile hosts/applications not caching those results;
   (b) reliance on DNS to solve this problem depends on network
   administrators on all networks using such applications to reliably
   and accurately maintain current DNS entries for every host using
   those applications; and (c) reliance on DNS to solve this problem
   depends on applications always using DNS names, even though they
   often must run in environments where DNS names are not reliably
   maintained for every host.  Other issues are that there is often no
   single distinguished name for a host and no reliable way for a host
   to determine what DNS names are associated with it and which names
   are appropriate to use in which contexts.

5.1.  Recommendation for Network Planners Considering Use of NPTv6
      Translation

   In light of the above, network planners considering the use of NPTv6
   translation should carefully consider the kinds of applications that
   they will need to run in the future and determine whether the
   address-stability and provider-independence benefits are consistent
   with their application requirements.

5.2.  Recommendations for Application Writers

   Several mechanisms (e.g., STUN [RFC5389], Traversal Using Relays
   around NAT (TURN) [RFC5766], and Interactive Connectivity
   Establishment (ICE) [RFC5245]) have been used with traditional IPv4
   NAT to circumvent some of the limitations of such devices.  Similar
   mechanisms could also be applied to circumvent some of the issues
   with an NPTv6 Translator.  However, all of these require the
   assistance of an external server or a function co-located with the
   translator that can tell an "internal" host what its "external"
   addresses are.

5.3.  Recommendation for Future Work

   It might be desirable to define a general mechanism that would allow
   hosts within a translation domain to determine their external
   addresses and/or request that inbound traffic be permitted.  If such
   a mechanism were to be defined, it would ideally be general enough to
   also accommodate other types of NAT likely to be encountered by IPV6
   applications, in particular IPv4/IPv6 Translation [RFC6144] [RFC6147]
   [RFC6145] [RFC6146] [RFC6052].  For this and other reasons, such a
   mechanism is beyond the scope of this document.

6.  A Note on Port Mapping

   In addition to overwriting IP addresses when datagrams are forwarded,
   NAPT44 devices 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 one of the [RFC1918] private address
   spaces) 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 datagram.  This address-
   amplification feature is not generally foreseen as a necessity at
   this time.

   Since port mapping requires rewriting a portion of the transport
   layer header, it requires NAPT44 devices to be aware of all of the
   transport protocols that they forward, thus stifling the development
   of new and improved transport protocols and preventing the use of
   IPsec encryption.  Modifying the transport layer header is
   incompatible with security mechanisms that encrypt the full IP
   payload and restricts the NAPT44 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, NPTv6 Translators that comply with this
   specification MUST NOT perform port mapping.

7.  Security Considerations

   When NPTv6 is deployed using either of the two-way, algorithmic
   mappings defined in this document, it allows direct inbound
   connections to internal nodes.  While this can be viewed as a benefit
   of NPTv6 versus NAPT44, it does open internal nodes to attacks that
   would be more difficult in a NAPT44 network.  From a security
   standpoint, although this situation is not substantially worse than
   running IPv6 with no NAT, some enterprises may assume that an NPTv6
   Translator will offer similar protection to a NAPT44 device.

   The port mapping mechanism in NAPT44 implementations requires that
   state be created in both directions.  This has lead to an industry-
   wide perception that NAT functionality is the same as a stateful
   firewall.  It is not.  The translation function of the NAT only
   creates dynamic state in one direction and has no policy.  For this
   reason, it is RECOMMENDED that NPTv6 Translators also implement
   firewall functionality such as described in [RFC6092], with
   appropriate configuration options including turning it on or off.

   When [RFC4864] talks about randomizing the subnet identifier, the
   idea is to make it harder for worms to guess a valid subnet
   identifier at an advertised network prefix.  This should not be
   interpreted as endorsing concealment of the subnet identifier behind
   the obfuscating function of a translator such as NPTv6.  [RFC4864]
   specifically talks about how to obtain the desired properties of
   concealment without using a translator.  Topology hiding when using
   NAT is often ineffective in environments where the topology is
   visible in application layer messaging protocols such as DNS, SIP,
   SMTP, etc.  If the information were not available through the
   application layer, [RFC2993] would not be valid.

   Due to the potential interactions with IKEv2/IPsec NAT traversal, it
   would be valuable to test interactions of NPTv6 with various aspects
   of current-day IKEv2/IPsec NAT traversal.

8.  Acknowledgements

   The checksum-neutral algorithmic address mapping described in this
   document is based on email written by Iljtsch van Beijnum.

   The following people provided advice or review comments that
   substantially improved this document: Allison Mankin, Christian
   Huitema, Dave Thaler, Ed Jankiewicz, Eric Kline, Iljtsch van Beijnum,
   Jari Arkko, Keith Moore, Mark Townsley, Merike Kaeo, Ralph Droms,
   Remi Despres, Steve Blake, and Tony Hain.

9.  References

9.1.  Normative References

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

   [RFC2526]  Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
              Addresses", RFC 2526, March 1999.

   [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.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

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

9.2.  Informative References

   [GSE]      O'Dell, M., "GSE - An Alternate Addressing Architecture
              for IPv6", Work in Progress, February 1997.

   [NIST]     NIST, "Draft NIST Framework and Roadmap for Smart Grid
              Interoperability Standards, Release 1.0", September 2009.

   [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.

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

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, May 2000.

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

   [RFC3424]  Daigle, L. and IAB, "IAB Considerations for UNilateral
              Self-Address Fixing (UNSAF) Across Network Address
              Translation", RFC 3424, November 2002.

   [RFC3484]  Draves, R., "Default Address Selection for Internet
              Protocol version 6 (IPv6)", RFC 3484, February 2003.

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

   [RFC5245]  Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address Translator (NAT)
              Traversal for Offer/Answer Protocols", RFC 5245,
              April 2010.

   [RFC5389]  Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
              "Session Traversal Utilities for NAT (STUN)", RFC 5389,
              October 2008.

   [RFC5766]  Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
              Relays around NAT (TURN): Relay Extensions to Session
              Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.

   [RFC5902]  Thaler, D., Zhang, L., and G. Lebovitz, "IAB Thoughts on
              IPv6 Network Address Translation", RFC 5902, July 2010.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, June 2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              October 2010.

   [RFC6092]  Woodyatt, J., "Recommended Simple Security Capabilities in
              Customer Premises Equipment (CPE) for Providing
              Residential IPv6 Internet Service", RFC 6092,
              January 2011.

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, April 2011.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, April 2011.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, April 2011.

   [RFC6147]  Bagnulo, M., Sullivan, A., Matthews, P., and I. van
              Beijnum, "DNS64: DNS Extensions for Network Address
              Translation from IPv6 Clients to IPv4 Servers", RFC 6147,
              April 2011.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011.

Appendix A.  Why GSE?

   For the purpose of this discussion, let us oversimplify the
   Internet's structure by distinguishing between two broad classes of
   networks: transit and edge.  A "transit network", in this context, is
   a network that provides connectivity services to other networks.  Its
   Autonomous System (AS) number may show up in a non-final position in
   BGP AS paths, or in the case of mobile and residential broadband
   networks, it may offer network services to smaller networks that
   cannot justify RIR membership.  An "edge network", in contrast, is
   any network that is not a transit network; it is the ultimate
   customer, and while it provides internal connectivity for its own
   use, it is a consumer of transit services in other respects.  In
   terms of routing, a network in the transit domain generally needs
   some way to make choices about how it routes to other networks; an
   edge network is generally quite satisfied with a simple default
   route.

   The [GSE] proposal, and as a result this proposal (which is similar
   to GSE in most respects and inspired by it), responds directly to
   current concerns in the RIR communities.  Edge networks are used to
   an environment in IPv4 in which their addressing is disjoint from
   that of their upstream transit networks; it is either provider
   independent, or a network prefix translator makes their external
   address distinct from their internal address, and they like the
   distinction.  In IPv6, there is a mantra that edge network addresses
   should be derived from their upstream, and if they have multiple
   upstreams, edge networks are expected to design their networks to use
   all of those prefixes equivalently.  They see this as unnecessary and
   unwanted operational complexity and, as a result, are pushing very
   hard in the RIR communities for provider-independent addressing.

   Widespread use of provider-independent addressing has a natural and
   perhaps unavoidable side effect that is likely to be very expensive
   in the long term.  With widespread PI addressing, the routing table
   will enumerate the networks at the edge of the transit domain, the
   edge networks, rather than enumerate the transit domain.  Per the BGP
   Update Report of 17 December 2010, there are currently over 36,000
   Autonomous Systems being advertised in BGP, of which over 15,000
   advertise only one prefix.  There are in the neighborhood of 5000 ASs
   that show up in a non-final position in AS paths, and perhaps another
   5000 networks whose AS numbers are terminal in more than one AS path.
   In other words, we have prefixes for some 36,000 transit and edge
   networks in the route table now, many of which arguably need an
   Autonomous System number only for multihoming.  The vast majority of
   networks (2/3) having the tools necessary to multihome are not

   visibly doing so and would be well served by any solution that gives
   them address independence without the overhead of RIR membership and
   BGP routing.

   Current growth estimates suggest that we could easily see that be on
   the order of 10,000,000 within fifteen years.  Tens of thousands of
   entries in the route table are very survivable; while our protocols
   and computers will likely do quite well with tens of millions of
   routes, the heat produced and power consumed by those routers, and
   the inevitable impact on the cost of those routers, is not a good
   outcome.  To avoid having a massive and unscalable route table, we
   need to find a way that is politically acceptable and returns us to
   enumerating the transit domain, not the edge.

   There have been a number of proposals.  As described, Shim6 moves the
   complexity to the edge, and the edge is rebelling.  Geographic
   addressing in essence forces ISPs to "own" geographic territory from
   a routing perspective, as otherwise there is no clue in the address
   as to what network a datagram should be delivered to in order to
   reach it.  Metropolitan Addressing can imply regulatory authority
   and, even if it is implemented using internet exchange consortia,
   visits a great deal of complexity on the transit networks that
   directly serve the edge.  The one that is likely to be most
   acceptable is any proposal that enables an edge network to be
   operationally independent of its upstreams, with no obligation to
   renumber when it adds, drops, or changes ISPs, and with no additional
   burden placed either on the ISP or the edge network as a result.
   From an application perspective, an additional operational
   requirement in the words of the Roadmap for the Smart Grid [NIST] is
   that

      "...the network should provide the capability to enable an
      application in a particular domain to communicate with an
      application in any other domain over the information network, with
      proper management control as to who and where applications can be
      inter-connected."

   In other words, the structure of the network should allow for and
   enable appropriate access control, but the structure of the network
   should not inherently limit access.

   The GSE model, by statelessly translating the prefix between an edge
   network and its upstream transit network, accomplishes that with a
   minimum of fuss and bother.  Stated in the simplest terms, it enables
   the edge network to behave as if it has a provider-independent prefix
   from a multihoming and renumbering perspective without the overhead
   of RIR membership or maintenance of BGP connectivity, and it enables

   the transit networks to aggressively aggregate what are from their
   perspective provider-allocated customer prefixes, to maintain a
   rational-sized routing table.

Appendix B.  Verification Code

   This non-normative appendix is presented as a proof of concept; it is
   in no sense optimized.  For example, one's complement arithmetic is
   implemented in portable subroutines, where operational
   implementations might use one's complement arithmetic instructions
   through a pragma; such implementations probably need to explicitly
   force 0xFFFF to 0x0000, as the instruction will not.  The original
   purpose of the code was to verify whether or not it was necessary to
   suppress 0xFFFF by overwriting with zero and whether predicted issues
   with subnet numbering were real.

   The point is to

   o  demonstrate that if one or the other representation of zero is not
      used in the word in which the checksum is updated, the program
      maps inner and outer addresses in a manner that is,
      mathematically, 1:1 and onto (each inner address maps to a unique
      outer address, and that outer address maps back to exactly the
      same inner address), and

   o  give guidance on the suppression of 0xFFFF checksums.

   In short, in one's complement arithmetic, x-x=0 but will take the
   negative representation of zero.  If 0xFFFF results are forced to the
   value 0x0000, as is recommended in [RFC1071], the word the checksum
   is adjusted in cannot be initially 0xFFFF, as on the return it will
   be forced to 0.  If 0xFFFF results are not forced to the value 0x0000
   as is recommended in [RFC1071], the word the checksum is adjusted in
   cannot be initially 0, as on the return it will be calculated as
   0+(~0) = 0xFFFF.  We chose to follow [RFC1071]'s recommendations,
   which implies a requirement to not use 0xFFFF as a subnet number in
   networks with a /48 external prefix.

  /*
   * Copyright (c) 2011 IETF Trust and the persons identified as
   * authors of the code.  All rights reserved.
   *
   * Redistribution and use in source and binary forms, with or without
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  #include "stdio.h"
  #include "assert.h"
  /*
   * program to verify the NPTv6 algorithm
   *
   * argument:
   *    Perform negative zero suppression: boolean
   *
   * method:
   *    We specify an internal and an external prefix.  The prefix
   *    length is presumed to be the common length of both and, for
   *    this, is a /48.  We perform the three algorithms specified.
   *    The "datagram" address is in effect the source address
   *    internal->external and the destination address
   *    external->internal.
   */
  unsigned short  inner_init[] = {
      0xFD01, 0x0203, 0x0405, 1, 2, 3, 4, 5};
  unsigned short  outer_init[] = {
      0x2001, 0x0db8, 0x0001, 1, 2, 3, 4, 5};
  unsigned short  inner[8];
  unsigned short  datagram[8];
  unsigned char   checksum[65536] = {0};

  unsigned short  outer[8];
  unsigned short  adjustment;
  unsigned short  suppress;
  /*
   * One's complement sum.
   * return number1 + number2
   */
  unsigned short
  add1(number1, number2)
      unsigned short  number1;
      unsigned short  number2;
  {
      unsigned int    result;

      result = number1;
      result += number2;
      if (suppress) {
          while (0xFFFF <= result) {
              result = result + 1 - 0x10000;
          }
      } else {
          while (0xFFFF < result) {
              result = result + 1 - 0x10000;
          }
      }
      return result;
  }

  /*
   * One's complement difference
   * return number1 - number2
   */
  unsigned short
  sub1(number1, number2)
      unsigned short  number1;
      unsigned short  number2;
  {
      return add1(number1, ~number2);
  }

  /*
   * return one's complement sum of an array of numbers
   */
  unsigned short
  sum1(numbers, count)
      unsigned short *numbers;
      int             count;
  {

      unsigned int    result;

      result = *numbers++;
      while (--count > 0) {
          result += *numbers++;
      }

      if (suppress) {
          while (0xFFFF <= result) {
              result = result + 1 - 0x10000;
          }
      } else {
          while (0xFFFF < result) {
              result = result + 1 - 0x10000;
          }
      }
      return result;
  }

  /*
   * NPTv6 initialization: Section 3.1 assuming Section 3.4
   *
   * Create the /48, a source address in internal format, and a
   * source address in external format.  Calculate the adjustment
   * if one /48 is overwritten with the other.
   */
  void
  nptv6_initialization(subnet)
      unsigned short  subnet;
  {
      int             i;
      unsigned short  inner48;
      unsigned short  outer48;

      /* Initialize the internal and external prefixes. */
      for (i = 0; i < 8; i++) {
          inner[i] = inner_init[i];
          outer[i] = outer_init[i];
      }
      inner[3] = subnet;
      outer[3] = subnet;
      /* Calculate the checksum adjustment. */
      inner48 = sum1(inner, 3);
      outer48 = sum1(outer, 3);
      adjustment = sub1(inner48, outer48);
  }

  /*

   * NPTv6 datagram from edge to transit: Section 3.2 assuming
   * Section 3.4
   *
   * Overwrite the prefix in the source address with the outer
   * prefix and adjust the checksum.
   */
  void
  nptv6_inner_to_outer()
  {
      int             i;

      /* Let's get the source address into the datagram. */
      for (i = 0; i < 8; i++) {
          datagram[i] = inner[i];
      }

      /* Overwrite the prefix with the outer prefix. */
      for (i = 0; i < 3; i++) {
          datagram[i] = outer[i];
      }

      /* Adjust the checksum. */
      datagram[3] = add1(datagram[3], adjustment);
  }

  /*
   * NPTv6 datagram from transit to edge: Section 3.3 assuming
   * Section 3.4
   *
   * Overwrite the prefix in the destination address with the
   * inner prefix and adjust the checksum.
   */
  void
  nptv6_outer_to_inner()
  {
      int             i;

      /* Overwrite the prefix with the outer prefix. */
      for (i = 0; i < 3; i++) {
          datagram[i] = inner[i];
      }

      /* Adjust the checksum. */
      datagram[3] = sub1(datagram[3], adjustment);
  }

  /*
   * Main program

   */
  main(argc, argv)
      int             argc;
      char          **argv;
  {
      unsigned        subnet;
      int             i;

      if (argc < 2) {
             fprintf(stderr, "usage: nptv6 supression\n");
             assert(0);
         }
         suppress = atoi(argv[1]);
         assert(suppress <= 1);

         for (subnet = 0; subnet < 0x10000; subnet++) {
             /* Section 3.1: initialize the system */
             nptv6_initialization(subnet);

             /* Section 3.2: take a datagram from inside to outside */
             nptv6_inner_to_outer();

             /* The resulting checksum value should be unique. */
             if (checksum[subnet]) {
                  printf("inner->outer duplicated checksum: "
                         "inner: %x:%x:%x:%x:%x:%x:%x:%x(%x) "
                         "calculated: %x:%x:%x:%x:%x:%x:%x:%x(%x)\n",
                         inner[0], inner[1], inner[2], inner[3],
                         inner[4], inner[5], inner[6], inner[7],
                         sum1(inner, 8), datagram[0], datagram[1],
                         datagram[2], datagram[3], datagram[4],
                         datagram[5], datagram[6], datagram[7],
                         sum1(datagram, 8));
          }

          checksum[subnet] = 1;

          /*
           * The resulting checksum should be the same as the inner
           * address's checksum.
           */
          if (sum1(datagram, 8) != sum1(inner, 8)) {
              printf("inner->outer incorrect: "
                     "inner: %x:%x:%x:%x:%x:%x:%x:%x(%x) "
                     "calculated: %x:%x:%x:%x:%x:%x:%x:%x(%x)\n",
                     inner[0], inner[1], inner[2], inner[3],
                     inner[4], inner[5], inner[6], inner[7],
                     sum1(inner, 8),

                     datagram[0], datagram[1], datagram[2], datagram[3],
                     datagram[4], datagram[5], datagram[6], datagram[7],
                     sum1(datagram, 8));
          }

          /* Section 3.3: take a datagram from outside to inside */
          nptv6_outer_to_inner();

          /*
           * The returning datagram should have the same checksum it
           * left with.
           */
          if (sum1(datagram, 8) != sum1(inner, 8)) {
              printf("outer->inner checksum incorrect: "
                     "calculated: %x:%x:%x:%x:%x:%x:%x:%x(%x) "
                     "inner: %x:%x:%x:%x:%x:%x:%x:%x(%x)\n",
                     datagram[0], datagram[1], datagram[2], datagram[3],
                     datagram[4], datagram[5], datagram[6], datagram[7],
                     sum1(datagram, 8), inner[0], inner[1], inner[2],
                     inner[3], inner[4], inner[5], inner[6], inner[7],
                     sum1(inner, 8));
          }

          /*
           * And every octet should calculate back to the same inner
           * value.
           */
          for (i = 0; i < 8; i++) {
              if (inner[i] != datagram[i]) {
                  printf("outer->inner different: "
                         "calculated: %x:%x:%x:%x:%x:%x:%x:%x "
                         "inner: %x:%x:%x:%x:%x:%x:%x:%x\n",
                         datagram[0], datagram[1], datagram[2],
                         datagram[3], datagram[4], datagram[5],
                         datagram[6], datagram[7], inner[0], inner[1],
                         inner[2], inner[3], inner[4], inner[5],
                         inner[6], inner[7]);
                  break;
              }
          }
      }
  }

Authors' Addresses

   Margaret Wasserman
   Painless Security
   North Andover, MA  01845
   USA

   Phone: +1 781 405 7464
   EMail: mrw@painless-security.com
   URI:   http://www.painless-security.com

   Fred Baker
   Cisco Systems
   Santa Barbara, California  93117
   USA

   Phone: +1-408-526-4257
   EMail: fred@cisco.com

 

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