Explicit Address Mappings for Stateless IP/ICMP Translation
RFC 7757

Internet Engineering Task Force (IETF)                       T. Anderson
Request for Comments: 7757                                Redpill Linpro
Updates: 6145                                            A. Leiva Popper
Category: Standards Track                                     NIC Mexico
ISSN: 2070-1721                                            February 2016

      Explicit Address Mappings for Stateless IP/ICMP Translation

Abstract

   This document extends the Stateless IP/ICMP Translation Algorithm
   (SIIT) with an Explicit Address Mapping (EAM) algorithm and formally
   updates RFC 6145.  The EAM algorithm facilitates stateless IP/ICMP
   translation between arbitrary (non-IPv4-translatable) IPv6 endpoints
   and IPv4.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc7757.

Copyright Notice

   Copyright (c) 2016 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Explicit Address Mapping Algorithm  . . . . . . . . . . . . .   5
     3.1.  Explicit Address Mapping Table  . . . . . . . . . . . . .   5
     3.2.  Explicit Address Mapping Specification  . . . . . . . . .   6
     3.3.  IP Address Translation Procedure  . . . . . . . . . . . .   6
       3.3.1.  Address Translation Steps: IPv4 to IPv6 . . . . . . .   7
       3.3.2.  Address Translation Steps: IPv6 to IPv4 . . . . . . .   7
   4.  Hairpinning of IPv6 Traffic . . . . . . . . . . . . . . . . .   8
     4.1.  Problem Statement . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Recommendation  . . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Simple Hairpinning Support  . . . . . . . . . . . . .   9
       4.2.2.  Intrinsic Hairpinning Support . . . . . . . . . . . .   9
   5.  Overlapping Explicit Address Mappings . . . . . . . . . . . .  10
   6.  Lack of Checksum Neutrality . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Use Cases  . . . . . . . . . . . . . . . . . . . . .  14
     A.1.  464XLAT . . . . . . . . . . . . . . . . . . . . . . . . .  14
     A.2.  IVI . . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     A.3.  SIIT-DC . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Appendix B.  Example IP Address Translations  . . . . . . . . . .  15
     B.1.  Hairpinning Examples  . . . . . . . . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   The Stateless IP/ICMP Translation Algorithm (SIIT) [RFC6145]
   specifies that when translating IPv4 addresses to IPv6 and vice
   versa, all addresses must be translated using the algorithm specified
   in [RFC6052].  This document specifies an alternative to the
   algorithm specified in [RFC6052], where IP addresses are translated
   according to a table of Explicit Address Mappings configured on the
   stateless translator.  This removes the previous constraint that IPv6
   nodes that communicate with IPv4 nodes through SIIT must be
   configured with IPv4-translatable IPv6 addresses.

   Translation using the Explicit Address Mapping Table does not replace
   [RFC6052].  For most use cases, it is expected that both algorithms
   are used in concert.  The Explicit Address Mapping algorithm is used
   only when a mapping matching the address to be translated exists.  If
   no matching mapping exists, the algorithm specified in [RFC6052] will

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   be used instead.  Thus, when translating an individual IP packet, an
   SIIT implementation might translate one of the two IP address fields
   according to an EAM, while the other IP address field is translated
   according to [RFC6052].

1.1.  Terminology

   This document makes use of the following terms:

   EAM:
      An Explicit Address Mapping, as specified in Section 3.2.

   EAMT:
      The Explicit Address Mapping Table, as specified in Section 3.1.

   Inner (header or address):
      Refers to an IP header located inside the payload of an ICMP error
      packet or to an IP address within that header.  Compare with
      "Outer".

   Outer (header or address):
      Refers to the first IP header in a packet or to an IP address
      within that header.  In other words, an IP header or address that
      is NOT "Inner".  If a reference is made to an IP header or address
      without the "Inner" or "Outer" qualifier, it should be considered
      as "Outer".

   SIIT:
      The Stateless IP/ICMP Translation Algorithm, as specified in
      [RFC6145].

   XLAT:
      Short for "translation".

   IPv4-Converted IPv6 Addresses:
      As defined in Section 1.3 of [RFC6052].

   IPv4-Translatable IPv6 Addresses:
      As defined in Section 1.3 of [RFC6052].

   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 [RFC2119].

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2.  Problem Statement

   Section 3.2.1 of [RFC6144] notes that "stateless translation
   mechanisms typically put constraints on what IPv6 addresses can be
   assigned to IPv6 nodes that want to communicate with IPv4
   destinations using an algorithmic mapping."  In practice, this means
   that the IPv6 nodes must be configured with IPv4-translatable IPv6
   addresses.  For the reasons discussed below, some environments may
   find that the use of IPv4-translatable IPv6 addresses is not desired
   or even possible.

   Limited availability:
      The number of IPv4-translatable IPv6 addresses available to an
      operator is equal to the number of IPv4 addresses that is assigned
      to the SIIT function.  IPv4 addresses are scarce, and as a result,
      an operator might not have enough IPv4-translatable IPv6 addresses
      to number the entire IPv6 infrastructure.

   Restricted format:
      IPv4-translatable IPv6 addresses must conform to the format
      specified in Section 2.2 of [RFC6052].  This format is not
      compatible with other common IPv6 address formats, such as the
      IPv6 address format based on the 64-bit Extended Unique Identifier
      (EUI-64) and used by IPv6 Stateless Address Autoconfiguration
      [RFC4862].

   An operator could overcome the above two problems by building an IPv6
   network using regular (non-IPv4-translatable) IPv6 addresses and
   assigning IPv4-translatable IPv6 addresses as secondary addresses on
   the nodes that want to communicate with IPv4 nodes through SIIT only.
   However, doing so may result in a new set of undesired consequences:

   Routing complexity:
      The IPv4-translatable IPv6 addresses must be routed throughout the
      IPv6 network separately from the primary (non-IPv4-translatable)
      IPv6 addresses used by the nodes.  It might be impossible to
      aggregate these routes, as two adjacent IPv4-translatable IPv6
      addresses might not be assigned to two adjacent IPv6 nodes.  As a
      result, in order to support SIIT, the IPv6 network might need to
      carry a large number of extraneous routes.  These routes must be
      separately injected into the IPv6 routing topology somehow.  Any
      intermediate devices in the IPv6 network such as a firewall might
      require special configuration in order to treat the
      IPv4-translatable IPv6 address the same as the primary IPv6
      address, for example, by requiring that any Access Control List
      (ACL) entries involving the primary IPv6 address of a node must be
      duplicated.

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   Operational complexity:
      The IPv4-translatable IPv6 addresses not only have to be assigned
      to the IPv6 nodes participating in SIIT, but also all applications
      and services on those nodes must be configured to use them.  For
      example, if the IPv6 node is a load balancer, it might require a
      separate virtual server definition using the IPv4-translatable
      IPv6 address in addition to one using the service's primary IPv6
      address.  A web server might require specific configuration to
      listen for connections on both the IPv4-translatable and the
      primary IPv6 address.  A high-availability cluster service must be
      set up to fail over both addresses between cluster nodes, and
      depending on how the IPv6 network learns the location of the
      IPv4-translatable IPv6 address, the fail-over mechanism used for
      the two addresses might be completely different.  Service
      monitoring must be done for both the IPv4-translatable and the
      primary IPv6 address, and any troubleshooting procedures must be
      extended to involve both addresses.  Finally, the Default Address
      Selection Policy Table [RFC6724] on the IPv6 nodes might need to
      be altered in order to ensure that outbound sessions towards the
      IPv4 Internet are sourced from an IPv4-translatable IPv6 address.

   In short, the use of IPv4-translatable IPv6 addresses in parallel
   with regular IPv6 addresses is in many ways analogous to the use of
   dual stack [RFC4213].  While no actual IPv4 packets are used, the
   IPv4-translatable IPv6 addresses create a secondary "stack" in the
   infrastructure that must be treated and operated separately from the
   primary one.  This increases the complexity of the overall
   infrastructure, in turn increasing operational overhead and reducing
   reliability.  An operator who for such reasons finds the use of dual
   stack unappealing might feel the same way about using SIIT with
   IPv4-translatable IPv6 addresses.

3.  Explicit Address Mapping Algorithm

   This normative section defines the EAM algorithm and formally updates
   Sections 4.1 and 5.1 of [RFC6145].  Specifically, when the EAM
   algorithm is applied, it supplants the requirement in [RFC6145] that
   states that a translator operating in the stateless mode must
   translate the Source Address and Destination Address IP header fields
   according to Section 2.3 of [RFC6052].

3.1.  Explicit Address Mapping Table

   An SIIT implementation includes an EAMT, a conceptual table in which
   each row represents an EAM.  Each EAM describes a mapping between
   IPv4 and IPv6 prefixes/addresses.  An operator populates the EAMT to
   provide the mappings between the two address families.

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   The EAMT consists of the following columns:

   o  IPv4 Prefix

   o  IPv6 Prefix

   SIIT implementations MAY include other columns in order to support
   proprietary extensions to the EAM algorithm.

   Throughout this document, figures representing the EAMT contain an
   Index column using the pound sign as the header.  This column is not
   a required part of this specification; it is included only as a
   convenience to the reader.

3.2.  Explicit Address Mapping Specification

   An EAM consists of an IPv4 prefix and an IPv6 prefix.  The prefix
   length MAY be omitted, in which case the implementation MUST assume
   it to be 32 for IPv4 and 128 for IPv6.  Figure 1 illustrates an EAMT
   containing examples of valid EAMs.

               +---+----------------+----------------------+
               | # |  IPv4 Prefix   |     IPv6 Prefix      |
               +---+----------------+----------------------+
               | 1 | 192.0.2.1      | 2001:db8:aaaa::      |
               | 2 | 192.0.2.2/32   | 2001:db8:bbbb::b/128 |
               | 3 | 192.0.2.16/28  | 2001:db8:cccc::/124  |
               | 4 | 192.0.2.128/26 | 2001:db8:dddd::/64   |
               | 5 | 192.0.2.192/29 | 2001:db8:eeee:8::/62 |
               | 6 | 192.0.2.224/31 | 64:ff9b::/127        |
               +---+----------------+----------------------+

                          Figure 1: Example EAMT

   An EAM's IPv4 prefix value MUST have an identical or smaller number
   of suffix bits than its corresponding IPv6 prefix value.

   Unless otherwise specified in Section 4, an SIIT implementation MUST
   individually translate each IP address it encounters in the packet's
   IP headers (including any IP headers contained within ICMP errors)
   according to Section 3.3.

3.3.  IP Address Translation Procedure

   This section describes step by step how an SIIT implementation
   translates addresses between IPv4 and IPv6.  Only the outcome of the
   algorithm described should be considered normative, that is, an SIIT
   implementation may implement the exact procedure differently than

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   what is described here, but the outcome of the algorithm MUST be the
   same.

   For concrete examples of IP address translations, refer to
   Appendix B.

3.3.1.  Address Translation Steps: IPv4 to IPv6

   1.  The IPv4 prefix column of the EAMT is searched for the EAM entry
       that shares the longest common prefix with the IPv4 address being
       translated.  The IPv4 prefix and IPv6 prefix values of the EAM
       entry found is from now on referred to as EAM4 and EAM6,
       respectively.

   2.  If no matching EAM entry is found, the EAM algorithm is aborted.
       The SIIT implementation MUST proceed to translate the address in
       accordance with [RFC6145] (and its updates).

   3.  The prefix bits of EAM4 are removed from the IPv4 address being
       translated.  The remaining suffix bits from the IPv4 address
       being translated are stored in a temporary buffer.

   4.  The prefix bits of EAM6 are prepended to the temporary buffer.

   5.  If the temporary buffer at this point does not contain a 128-bit
       value, it is padded with trailing zeros so that it reaches a
       length of 128 bits.

   6.  The contents of the temporary buffer is the translated IPv6
       address.

3.3.2.  Address Translation Steps: IPv6 to IPv4

   1.  The IPv6 prefix column of the EAMT is searched for the EAM entry
       that shares the longest common prefix with the IPv6 address being
       translated.  The IPv4 prefix and IPv6 prefix values of the EAM
       entry found is from now on referred to as EAM4 and EAM6,
       respectively.

   2.  If no matching EAM entry is found, the EAM algorithm is aborted.
       The SIIT implementation MUST proceed to translate the address in
       accordance with [RFC6145] (and its updates).

   3.  The prefix bits of EAM6 are removed from the IPv6 address being
       translated.  The remaining suffix bits from the IPv6 address
       being translated are stored in a temporary buffer.

   4.  The prefix bits of EAM4 are prepended to the temporary buffer.

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   5.  If the temporary buffer at this point does not contain a 32-bit
       value, any trailing bits are discarded so that the buffer is
       reduced to a length of 32 bits.

   6.  The contents of the temporary buffer is the translated IPv4
       address.

4.  Hairpinning of IPv6 Traffic

4.1.  Problem Statement

   Two IPv6 nodes that are both covered by EAMs might in certain
   circumstances attempt to communicate through a stateless translator
   rather than using native IPv6 directly.  This happens if one of the
   nodes initiates traffic towards the IPv4-converted IPv6 address whose
   embedded IPv4 address matches an EAM that covers the other node.
   Special consideration is required in order to make this communication
   pattern work in a bidirectional fashion.  This is illustrated by the
   example below.

   Assume that a stateless translator is configured with a translation
   prefix of 64:ff9b::/96 (per [RFC6052]) and the EAMT shown in
   Figure 1.  The IPv6 node 2001:db8:aaaa:: transmits an IPv6 packet
   towards 64:ff9b::192.0.2.2, which reaches the translator and is
   translated into an IPv4 packet with source address 192.0.2.1 and
   destination address 192.0.2.2.  This destination address is found in
   the EAMT, so the packet loops back into the translation function and
   is translated back to an IPv6 packet with source address
   2001:db8:aaaa:: and destination address 2001:db8:bbbb::b.

   While this packet will reach its destination just fine, a problem
   will occur when 2001:db8:bbbb::b responds to it.  The response packet
   will have a source address of 2001:db8:bbbb::b and a destination
   address of 2001:db8:aaaa:: and will be routed directly to its
   destination without being subjected to any form of translation.
   Because the source address of this response packet (2001:db8:bbbb::b)
   is not equal to the destination address of the initial outgoing
   packet (64:ff9b::192.0.2.2), the packet will most likely be discarded
   by 2001:db8:aaaa::, and bidirectional communication will most likely
   fail.

   The above scenario could be made to work by ensuring that the
   stateless translator is hairpinning the traffic in both directions.
   Section 4.2 describes how this is accomplished.  The resulting
   address translations are demonstrated step by step in Appendix B.1.

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4.2.  Recommendation

   An SIIT implementation SHOULD include a feature that ensures that
   hairpinned IPv6 traffic is supported.  The feature SHOULD be enabled
   by default.  The following two subsections describe two alternate
   ways to implement this feature.  An implementation MAY support both
   approaches.

4.2.1.  Simple Hairpinning Support

   When the simple hairpinning feature is enabled, the translator
   employs the following rules when translating from IPv4 to IPv6:

   1.  If the packet is not an ICMPv4 error: The EAM algorithm MUST NOT
       be used in order to translate the source address in the IPv4
       header.

   2.  If the packet is an ICMPv4 error: The EAM algorithm MUST NOT be
       used when translating the destination address in the inner IPv4
       header.

   3.  If the packet is an ICMPv4 error whose outer IPv4 source address
       is equal to its inner IPv4 destination address: The EAM algorithm
       MUST NOT be used in order to translate the source address in the
       outer IPv4 header.

   Rules #2 and #3 are cumulative.

   The addresses in question MUST instead be translated according to
   [RFC6145], as if they did not match any EAM.

4.2.2.  Intrinsic Hairpinning Support

   When the intrinsic hairpinning feature is enabled, the translator
   employs the following rules after having translated an IPv6 packet to
   IPv4:

   If all the conditions in either of the two sets below are true, the
   packet is to be hairpinned.  The implementation MUST immediately
   (i.e., prior to forwarding it to the IPv4 network) translate the
   packet back to IPv6.  During the second translation pass, the
   behavior specified in Section 4.2.1 MUST be applied, and the Hop
   Limit field SHOULD NOT be decremented.

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   Condition set A:

      A1.  The packet is not an ICMPv4 error.

      A2.  The destination address was translated using the algorithm in
           [RFC6052].

      A3.  The destination address is found in the EAMT.

   Condition set B:

      B1.  The packet is an ICMPv4 error.

      B2.  The inner source address was translated using the algorithm
           in [RFC6052].

      B3.  The inner source address is found in the EAMT.

5.  Overlapping Explicit Address Mappings

   The algorithm specified in Section 3 relies on making a lookup in the
   EAMT in order to find the EAM entry that shares the longest common
   prefix with the address being translated.  Operators should note that
   configuring EAMs with overlapping or identical IPv4 or IPv6 prefixes
   in the EAMT may create configurations where the IPv4-to-IPv6 and
   IPv6-to-IPv4 address translations will not be symmetric.  This may in
   some cases make bidirectional communication impossible.

   EAM #1 in the example EAMT (Figure 2) could be thought of as
   implementing IVI (Appendix A.2), while EAM #2 introduces a single
   exception in the style of SIIT-DC (Appendix A.3).  The IPv4 prefixes
   of the two EAMs overlap, while the IPv6 prefixes do not.  This
   results in a situation where the IPv6 address
   2001:db8:ffc6:3364:4000:: will be translated (according to EAM #1) to
   the IPv4 address 198.51.100.64.  However, when this IPv4 address is
   translated back to IPv6, it will be translated (according to EAM #2)
   to the IPv6 address 2001:db8::abcd.  Because the IPv4-to-IPv6
   translation in this example does not mirror the corresponding IPv6-
   to-IPv4 translation, bidirectional communication involving the IPv6
   address 2001:db8:ffc6:3364:4000:: might fail.  In order to help avoid
   such situations, implementations MAY warn the operator when a new EAM
   that overlaps with a previously existing one is inserted into the
   EAMT.

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               +---+------------------+--------------------+
               | # |   IPv4 Prefix    |    IPv6 Prefix     |
               +---+------------------+--------------------+
               | 1 | 0.0.0.0/0        | 2001:db8:ff00::/40 |
               | 2 | 198.51.100.64/32 | 2001:db8::abcd/128 |
               +---+------------------+--------------------+

            Figure 2: EAMT Containing Overlapping IPv4 Prefixes

   In Figure 3, the IPv6 prefixes of the two EAMs are identical.  The
   behavior of the stateless translator when translating an IPv6 packet
   that contains the address 2001:db8::1 to IPv4 is in this case
   unspecified.  In order to prevent this situation from occurring,
   implementations MAY refuse to insert a new EAM, whose IPv4 or IPv6
   prefix value is identical to that of an already existing EAM, into
   the EAMT.

                 +---+-----------------+-----------------+
                 | # |   IPv4 Prefix   |   IPv6 Prefix   |
                 +---+-----------------+-----------------+
                 | 1 | 198.51.100.8/32 | 2001:db8::1/128 |
                 | 2 | 198.51.100.9/32 | 2001:db8::1/128 |
                 +---+-----------------+-----------------+

             Figure 3: EAMT Containing Identical IPv6 Prefixes

6.  Lack of Checksum Neutrality

   When one or both of the address fields in an IP/ICMP packet are
   translated according to the EAM algorithm, the translation cannot be
   relied upon to be checksum neutral, even if the well-known prefix
   64:ff9b::/96 is used.  This consideration is discussed in more detail
   in Section 4.1 of [RFC6052].

7.  Security Considerations

   The EAM algorithm does not introduce any new security issues beyond
   those that are already discussed in Section 7 of [RFC6145].

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8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
              Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
              DOI 10.17487/RFC6052, October 2010,
              <http://www.rfc-editor.org/info/rfc6052>.

   [RFC6145]  Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
              Algorithm", RFC 6145, DOI 10.17487/RFC6145, April 2011,
              <http://www.rfc-editor.org/info/rfc6145>.

8.2.  Informative References

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <http://www.rfc-editor.org/info/rfc4213>.

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,
              <http://www.rfc-editor.org/info/rfc4862>.

   [RFC6144]  Baker, F., Li, X., Bao, C., and K. Yin, "Framework for
              IPv4/IPv6 Translation", RFC 6144, DOI 10.17487/RFC6144,
              April 2011, <http://www.rfc-editor.org/info/rfc6144>.

   [RFC6219]  Li, X., Bao, C., Chen, M., Zhang, H., and J. Wu, "The
              China Education and Research Network (CERNET) IVI
              Translation Design and Deployment for the IPv4/IPv6
              Coexistence and Transition", RFC 6219,
              DOI 10.17487/RFC6219, May 2011,
              <http://www.rfc-editor.org/info/rfc6219>.

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
              <http://www.rfc-editor.org/info/rfc6724>.

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   [RFC6791]  Li, X., Bao, C., Wing, D., Vaithianathan, R., and G.
              Huston, "Stateless Source Address Mapping for ICMPv6
              Packets", RFC 6791, DOI 10.17487/RFC6791, November 2012,
              <http://www.rfc-editor.org/info/rfc6791>.

   [RFC6877]  Mawatari, M., Kawashima, M., and C. Byrne, "464XLAT:
              Combination of Stateful and Stateless Translation",
              RFC 6877, DOI 10.17487/RFC6877, April 2013,
              <http://www.rfc-editor.org/info/rfc6877>.

   [RFC7335]  Byrne, C., "IPv4 Service Continuity Prefix", RFC 7335,
              DOI 10.17487/RFC7335, August 2014,
              <http://www.rfc-editor.org/info/rfc7335>.

   [RFC7755]  Anderson, T., "SIIT-DC: Stateless IP/ICMP Translation for
              IPv6 Data Center Environments", RFC 7755,
              DOI 10.17487/RFC7755, February 2016,
              <http://www.rfc-editor.org/info/rfc7755>.

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Appendix A.  Use Cases

   The following subsections describe some use cases that at the time of
   writing leverage SIIT with the EAM algorithm.

A.1.  464XLAT

   When the customer-side translator (CLAT) component in the 464XLAT
   [RFC6877] architecture does not have a dedicated IPv6 prefix
   assigned, it may instead use "one interface IPv6 address that is
   claimed by the CLAT."  This IPv6 address might not be
   IPv4-translatable.  If this is the case, the CLAT essentially
   implements the EAM algorithm using an EAMT as follows (assuming the
   CLAT's IPv4 address is picked from the IPv4 Service Continuity Prefix
   [RFC7335]):

           +---+--------------+-------------------------------+
           | # | IPv4 Prefix  |          IPv6 Prefix          |
           +---+--------------+-------------------------------+
           | 1 | 192.0.0.1/32 | CLAT_claimed_IPv6_address/128 |
           +---+--------------+-------------------------------+

                 Figure 4: Example EAMT for a 464XLAT CLAT

   In this particular use case, the EAM algorithm is used to translate
   IPv6 destination addresses to IPv4, and conversely, IPv4 source
   addresses to IPv6.  Other addresses are translated using [RFC6052].

A.2.  IVI

   IVI [RFC6219] describes a stateless translation model that embeds
   IPv4 addresses in a 40-bit translation prefix where bits 33-40 are
   required to be 1.  The embedded IPv4 address is located in bits 41-72
   of the IPv6 address.  Bits 73-128 are required to be 0.

   The location of the eight least significant IPv4 address bits makes
   the IVI address mapping differ from [RFC6052].

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                 +---+-------------+--------------------+
                 | # | IPv4 Prefix |    IPv6 Prefix     |
                 +---+-------------+--------------------+
                 | 1 | 0.0.0.0/0   | 2001:db8:ff00::/40 |
                 +---+-------------+--------------------+

                      Figure 5: Example EAMT for IVI

   In this particular use case, all addresses are translated according
   to the EAM algorithm.  In other words, [RFC6052] mapping is not used
   at all.

A.3.  SIIT-DC

   SIIT-DC [RFC7755] describes the use of SIIT to facilitate
   connectivity from the IPv4 Internet to services hosted in an
   IPv6-only data center.  In order to avoid the constraints relating to
   the use of IPv4-translatable IPv6 addresses discussed in Section 2,
   the stateless IPv4/IPv6 translators are provisioned with an EAMT
   containing one entry per IPv6-only service that are to be made
   available from the IPv4 Internet, for example (assuming
   2001:db8:aaaa::1 and 2001:db8:bbbb::1 are assigned to load balancers
   or servers that provide the IPv6-only services in question):

               +---+----------------+----------------------+
               | # |  IPv4 Prefix   |     IPv6 Prefix      |
               +---+----------------+----------------------+
               | 1 | 203.0.113.1/32 | 2001:db8:aaaa::1/128 |
               | 2 | 203.0.113.2/32 | 2001:db8:bbbb::1/128 |
               +---+----------------+----------------------+

                    Figure 6: Example EAMT for SIIT-DC

   In this particular use case, the EAM algorithm is used to translate
   IPv4 destination addresses to IPv6, and conversely, IPv6 source
   addresses to IPv4.  Other addresses are translated using [RFC6052].

Appendix B.  Example IP Address Translations

   Figure 7 demonstrates how a set of example IP addresses are
   translated given the example EAMT in Figure 1.  Implementors may use
   the examples given to develop test cases to validate correct
   operation.  Note that the address translations are bidirectional, so
   a single row in the table describes two address translations: IPv4 to
   IPv6 and IPv6 to IPv4.

   It is also assumed that the translation prefix is configured to be
   64:ff9b::/96 (per [RFC6052]).

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     +--------------+------------------------+-----------------------+
     | IPv4 Address |      IPv6 Address      |        Comment        |
     +--------------+------------------------+-----------------------+
     | 192.0.2.1    | 2001:db8:aaaa::        | According to EAM #1   |
     | 192.0.2.2    | 2001:db8:bbbb::b       | According to EAM #2   |
     | 192.0.2.16   | 2001:db8:cccc::        | According to EAM #3   |
     | 192.0.2.24   | 2001:db8:cccc::8       | According to EAM #3   |
     | 192.0.2.31   | 2001:db8:cccc::f       | According to EAM #3   |
     | 192.0.2.128  | 2001:db8:dddd::        | According to EAM #4   |
     | 192.0.2.152  | 2001:db8:dddd:0:6000:: | According to EAM #4   |
     | 192.0.2.183  | 2001:db8:dddd:0:dc00:: | According to EAM #4   |
     | 192.0.2.191  | 2001:db8:dddd:0:fc00:: | According to EAM #4   |
     | 192.0.2.195  | 2001:db8:eeee:9:8000:: | According to EAM #5   |
     | 192.0.2.225  | 64:ff9b::1             | According to EAM #6   |
     | 192.0.2.248  | 64:ff9b::c000:2f8      | According to RFC 6052 |
     +--------------+------------------------+-----------------------+

                 Figure 7: Example IP Address Translations

B.1.  Hairpinning Examples

   The following examples show how hairpinned IPv6 packets between the
   IPv6 nodes 2001:db8:aaaa:: and 2001:db8:bbbb::b are translated
   according to Section 4.  As in Appendix B, the EAMT in Figure 1 is
   used, and the translation prefix is 64:ff9b::/96 (per [RFC6052]).  In
   addition, the [RFC6791] pool is assumed to contain only the single
   address 198.51.100.1.

        +--------------+--------------------+---------------------+
        |  XLAT Stage  |   Source Address   | Destination Address |
        +--------------+--------------------+---------------------+
        | Initial      | 2001:db8:aaaa::    | 64:ff9b::192.0.2.2  |
        +--------------+--------------------+---------------------+
        | Intermediate | 192.0.2.1          | 192.0.2.2           |
        +--------------+--------------------+---------------------+
        | Final        | 64:ff9b::192.0.2.1 | 2001:db8:bbbb::b    |
        +--------------+--------------------+---------------------+

               Figure 8: Hairpinning of a Normal IPv6 Packet

   Figure 8 illustrates how a normal (i.e., not an ICMP error) IPv6
   packet sent from 2001:db8:aaaa:: towards 64:ff9b::192.0.2.2 is
   hairpinned.  In this example, rule #1 in Section 4.2.1 was applied in
   order to disable the EAM algorithm when translating the intermediate
   IPv4 source address to IPv6.

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   +--------------+-------+-----------------------+--------------------+
   |  XLAT Stage  | Loc.  |    Source Address     | Destination Addr.  |
   +--------------+-------+-----------------------+--------------------+
   | Initial      | Outer | 2001:db8::1234        | 64:ff9b::192.0.2.1 |
   |              | Inner | 64:ff9b::192.0.2.1    | 2001:db8:bbbb::b   |
   +--------------+-------+-----------------------+--------------------+
   | Intermediate | Outer | 198.51.100.1          | 192.0.2.1          |
   |              | Inner | 192.0.2.1             | 192.0.2.2          |
   +--------------+-------+-----------------------+--------------------+
   | Final        | Outer | 64:ff9b::198.51.100.1 | 2001:db8:aaaa::    |
   |              | Inner | 2001:db8:aaaa::       | 64:ff9b::192.0.2.2 |
   +--------------+-------+-----------------------+--------------------+

         Figure 9: Hairpinning of a Router-Originated ICMPv6 Error

   Figure 9 illustrates the hairpinning of an ICMPv6 error sent by an
   arbitrary IPv6 router (2001:db8::1234) in response to the packet in
   Figure 8.  In this example, rule #2 in Section 4.2.1 was applied in
   order to disable the EAM algorithm when translating the intermediate
   inner IPv4 destination address to IPv6.

    +--------------+-------+--------------------+--------------------+
    |  XLAT Stage  | Loc.  |   Source Address   | Destination Addr.  |
    +--------------+-------+--------------------+--------------------+
    | Initial      | Outer | 2001:db8:bbbb::b   | 64:ff9b::192.0.2.1 |
    |              | Inner | 64:ff9b::192.0.2.1 | 2001:db8:bbbb::b   |
    +--------------+-------+--------------------+--------------------+
    | Intermediate | Outer | 192.0.2.2          | 192.0.2.1          |
    |              | Inner | 192.0.2.1          | 192.0.2.2          |
    +--------------+-------+--------------------+--------------------+
    | Final        | Outer | 64:ff9b::192.0.2.2 | 2001:db8:aaaa::    |
    |              | Inner | 2001:db8:aaaa::    | 64:ff9b::192.0.2.2 |
    +--------------+-------+--------------------+--------------------+

         Figure 10: Hairpinning of a Host-Originated ICMPv6 Error

   Figure 10 illustrates the hairpinning of an ICMPv6 error sent by the
   original destination host itself in response to the packet in
   Figure 8.  In this example, rules #2 and #3 in Section 4.2.1 were
   both applied in order to disable the EAM algorithm when translating
   the intermediate inner IPv4 destination address and the intermediate
   outer IPv4 source address to IPv6.

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        +--------------+--------------------+---------------------+
        |  XLAT Stage  |   Source Address   | Destination Address |
        +--------------+--------------------+---------------------+
        | Initial      | 2001:db8:bbbb::b   | 64:ff9b::192.0.2.1  |
        +--------------+--------------------+---------------------+
        | Intermediate | 192.0.2.2          | 192.0.2.1           |
        +--------------+--------------------+---------------------+
        | Final        | 64:ff9b::192.0.2.2 | 2001:db8:aaaa::     |
        +--------------+--------------------+---------------------+

             Figure 11: Hairpinning of Normal Response Packet

   Figure 11 illustrates how the response from 2001:db8:bbbb::b to the
   packet in Figure 8 is hairpinned in the exact same fashion as the
   initial packet.  Again, rule #1 in Section 4.2.1 was applied in order
   to disable the EAM algorithm when translating the intermediate IPv4
   source address to IPv6.  The example is included in order to
   illustrate how the addresses in the packet initially sent by
   2001:db8:aaaa:: match those in the translated response packet sent by
   2001:db8:bbbb::b, thus facilitating bidirectional communication.

Acknowledgements

   This document was conceived due to comments made by Dave Thaler in
   the V6OPS session at IETF 91 as well as email discussions between
   Fred Baker and the authors.

   Valuable reviews, suggestions, and other feedback was given by Fred
   Baker, Mohamed Boucadair, Cameron Byrne, Brian E.  Carpenter, Brian
   Haberman, Ray Hunter, Alvaro Retana, Michael Richardson, Dan
   Romascanu, Hemant Singh, and Andrew Yourtchenko.

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Authors' Addresses

   Tore Anderson
   Redpill Linpro
   Vitaminveien 1A
   0485 Oslo
   Norway

   Phone: +47 959 31 212
   Email: tore@redpill-linpro.com
   URI:   http://www.redpill-linpro.com

   Alberto Leiva Popper
   NIC Mexico
   Av. Eugenio Garza Sada 427 L4-6
   Monterrey, Nuevo Leon  64840
   Mexico

   Email: ydahhrk@gmail.com
   URI:   http://www.nicmexico.mx/

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