Stateless IP/ICMP Translation for IPv6 Internet Data Center Environments (SIIT-DC): Dual Translation Mode

Internet Engineering Task Force (IETF)                       T. Anderson
Request for Comments: 7756                                Redpill Linpro
Category: Informational                                      S. Steffann
ISSN: 2070-1721                              S.J.M. Steffann Consultancy
                                                           February 2016


      Stateless IP/ICMP Translation for IPv6 Internet Data Center
             Environments (SIIT-DC): Dual Translation Mode

Abstract

   This document describes an extension of the Stateless IP/ICMP
   Translation for IPv6 Internet Data Center Environments (SIIT-DC)
   architecture, which allows applications, protocols, or nodes that are
   incompatible with IPv6 and/or Network Address Translation to operate
   correctly with SIIT-DC.  This is accomplished by introducing a new
   component called an SIIT-DC Edge Relay, which reverses the
   translations made by an SIIT-DC Border Relay.  The application and/or
   node is thus provided with seemingly native IPv4 connectivity that
   provides end-to-end address transparency.

   The reader is expected to be familiar with the SIIT-DC architecture
   described in RFC 7755.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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/rfc7756.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Edge Relay Description  . . . . . . . . . . . . . . . . . . .   5
     3.1.  Node-Based Edge Relay . . . . . . . . . . . . . . . . . .   6
     3.2.  Network-Based Edge Relay  . . . . . . . . . . . . . . . .   7
       3.2.1.  Edge Relay "on a Stick" . . . . . . . . . . . . . . .   8
       3.2.2.  Edge Relay That Bridges IPv6 Packets  . . . . . . . .   9
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   9
     4.1.  IPv6 Path MTU . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  IPv4 MTU  . . . . . . . . . . . . . . . . . . . . . . . .  10
     4.3.  IPv4 Identification Header  . . . . . . . . . . . . . . .  10
   5.  Intra-IDC IPv4 Communication  . . . . . . . . . . . . . . . .  10
     5.1.  Hairpinning by the SIIT-DC Border Relay . . . . . . . . .  11
     5.2.  Additional EAMs Configured in Edge Relay  . . . . . . . .  12
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Examples: Network-Based IPv4 Connectivity  . . . . .  16
     A.1.  Subnet with IPv4 Service Addresses  . . . . . . . . . . .  16
     A.2.  Subnet with Unrouted IPv4 Addresses . . . . . . . . . . .  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   SIIT-DC [RFC7755] describes an architecture where IPv4-only users can
   access IPv6-only services through a stateless translator called an
   SIIT-DC Border Relay (BR).  This approach has certain limitations,
   however.  In particular, the following cases will work poorly or not
   at all:

   o  Application protocols that do not support NAT (i.e., the lack of
      end-to-end transparency of IP addresses).

   o  Nodes that cannot connect to IPv6 networks at all or that can only
      connect such networks if they also provide IPv4 connectivity
      (i.e., dual-stacked networks).

   o  Application software that makes use of legacy IPv4-only APIs or
      otherwise makes assumptions that IPv4 connectivity is available.

   By extending the SIIT-DC architecture with a new component called an
   Edge Relay (ER), all of the above can be made to work correctly in an
   otherwise IPv6-only network environment using SIIT-DC.

   The purpose of the ER is to reverse the IPv4-to-IPv6 packet
   translations previously done by the BR for traffic arriving from IPv4
   clients and forward this as "native" IPv4 to the node or application.
   In the reverse direction, IPv4 packets transmitted by the node or
   application are intercepted by the ER, which translates them to IPv6
   before they are forwarded to the BR, which in turn will reverse the
   translations and forward them to the IPv4 client.  The node or
   application is thus provided with "virtual" IPv4 Internet
   connectivity that retains end-to-end transparency for the IPv4
   addresses.

2.  Terminology

   This document makes use of the following terms:

   SIIT-DC Border Relay (BR):
      A device or a logical function that performs stateless protocol
      translation between IPv4 and IPv6.  It MUST do so in accordance
      with [RFC6145] and [RFC7757].

   SIIT-DC Edge Relay (ER):
      A device or logical function that provides "native" IPv4
      connectivity to IPv4-only devices or application software.  It is
      very similar in function to a BR but is typically located close to
      the IPv4-only component(s) it is supporting rather than on the
      outer network border of the Internet Data Center (IDC).  An ER may
      be either node based (Section 3.1) or network based (Section 3.2).

   IPv4 Service Address:
      An IPv4 address representing a node or service located in an IPv6
      network.  It is coupled with an IPv6 Service Address using an
      Explicit Address Mapping (EAM).  Packets sent to this address are
      translated to IPv6 by the BR, and possibly back to IPv4 by an ER,
      before reaching the node or service.

   IPv6 Service Address:
      An IPv6 address assigned to an application, node, or service
      either directly or indirectly (through an ER).  It is coupled with
      an IPv4 Service Address using an EAM.  IPv4-only clients
      communicate with the IPv6 Service Address through SIIT-DC.

   Explicit Address Mapping (EAM):
      A bidirectional coupling between an IPv4 Service Address and an
      IPv6 Service Address configured in a BR or ER.  When translating
      between IPv4 and IPv6, the BR/ER changes the address fields in the
      translated packet's IP header according to any matching EAM.  The
      EAM algorithm is specified in [RFC7757].

   Translation Prefix:
      An IPv6 prefix into which the entire IPv4 address space is mapped,
      according to the algorithm in [RFC6052].  The translation prefix
      is routed to the BR's IPv6 interface.  When translating between
      IPv4 and IPv6, a BR/ER will insert/remove the translation prefix
      into/from the address fields in the translated packet's IP header,
      unless an EAM exists for the IP address that is being translated.

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

   IDC:
      Short for "Internet Data Center"; a data center whose main purpose
      is to deliver services to the public Internet.  SIIT-DC is
      primarily targeted at being deployed in an IDC.  An IDC is
      typically operated by an Internet Content Provider or a Managed
      Services Provider.

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

   XLAT:
      Short for "Translation".  Used in figures to indicate where a BR/
      ER uses SIIT [RFC6145] to translate IPv4 packets to IPv6 and vice
      versa.

   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].

3.  Edge Relay Description

   An ER is at its core an implementation of the Stateless IP/ICMP
   Translation Algorithm [RFC6145] that supports Explicit Address
   Mappings [RFC7757].  It provides virtual IPv4 connectivity for nodes
   or applications that require this to operate correctly with SIIT-DC.

   Packets from the IPv4 Internet destined for an IPv4 Service Address
   are first translated to IPv6 by a BR.  The resulting IPv6 packets are
   subsequently forwarded to the ER that owns the IPv6 Service Address
   the translated packets are addressed to.  The ER then translates them
   back to IPv4 before forwarding them to the IPv4 application or node.
   In the other direction, the exact same translations happen, only in
   reverse.  This process provides end-to-end transparency of IPv4
   addresses.

   An ER may handle an arbitrary number of IPv4/IPv6 Service Addresses.
   All the EAMs configured in the BR that involve the IPv4/IPv6 Service
   Addresses handled by an ER MUST also be present in the ER's
   configuration.

   An ER may be implemented in two distinct ways: as a software-based
   service residing inside an otherwise IPv6-only node or as a network-
   based service that provides an isolated IPv4 network segment to which
   nodes that require IPv4 can connect.  In both cases, native IPv6
   connectivity may be provided simultaneously with the virtual IPv4
   connectivity.  Thus, dual-stack connectivity is facilitated in case
   the node or application supports it.

   The choice between a node- or network-based ER is made on a per-
   service or per-node basis.  An arbitrary number of each type of ER
   may co-exist in an SIIT-DC architecture.

   This section describes the different approaches and discusses which
   approach fits best for the various use cases.

3.1.  Node-Based Edge Relay

    [IPv4 Internet]  [IPv6 Internet]
          |            |
    +-----|-----+      |
    | (BR/XLAT) |      |
    +-----|-----+      |
          |            |      +-----<IPv6-only node/server>----------+
    [IPv6-only IDC network]   |                    +----------------+|
       |                      |  /--(ER/XLAT)--AF_INET  Dual-stack  ||
       \-------------------------+                 |    application ||
                              |  \------------AF_INET6  software    ||
                              |                    +----------------+|
                              +--------------------------------------+

                     Figure 1: A Node-Based Edge Relay

   A node-based ER is typically implemented as a logical software
   function that runs inside the operating system of an IPv6 node.  It
   provides applications running on the same node with IPv4
   connectivity.  Its IPv4 Service Address SHOULD be considered a
   regular local address that allows applications running on the same
   node to use it with IPv4-only API calls, e.g., to create AF_INET
   sockets that listen for and accept incoming connections to its IPv4
   Service Address.  An ER may accomplish this by creating a virtual
   network adapter to which it assigns the IPv4 Service Address and
   points a default IPv4 route.  This approach is similar to the
   "Bump-in-the-Stack" approach discussed in [RFC6535]; however, it does
   not include an Extension Name Resolver.

   As shown in Figure 1, if the application supports dual-stack
   operation, IPv6 clients will be able to communicate with it directly
   using native IPv6.  Neither the BR nor the ER will intercept this
   communication.  Support for IPv6 in the application is, however, not
   a requirement; the application may opt not to establish any IPv6
   sockets.  Foregoing IPv6 in this manner will simply preclude
   connectivity to the service from IPv6-only clients; connectivity to
   the service from IPv4 clients (through the BR) will continue work in
   the same way.

   The ER requires a dedicated IPv6 Service Address for each IPv4
   Service Address it has configured.  The IPv6 network MUST forward
   traffic to these IPv6 Service Addresses to the node, whose operating
   system MUST in turn forward them to the ER.  This document does not
   attempt to fully explore the multitude of ways this could be
   accomplished; however, considering that the IPv6 protocol is designed
   for having multiple addresses assigned to a single node, one
   particularly straight-forward way would be to assign the ER's IPv6
   Service Addresses as secondary IPv6 addresses on the node itself so
   that the upstream router learns of their location using the IPv6
   Neighbor Discovery Protocol [RFC4861].

3.2.  Network-Based Edge Relay

         [IPv4 Internet]  [IPv6 Internet]
               |             |
         +-----|-----+       |
         | (BR/XLAT) |       |
         +-----|-----+       |
               |             |
          [IPv6-only IDC network]   +--<IPv4-only node/server>--+
               |                    |         +----------------+|
         +-----|-----+   [v4-only]  |         |    IPv4-only   ||
         | (ER/XLAT)-----[network]--------AF_INET  application ||
         +-----------+   [segment]  |         |    software    ||
                                    |         +----------------+|
                                    +---------------------------+

                Figure 2: A Basic Network-Based Edge Relay

   A network-based ER functions the exact same way as a node-based ER
   does, only that instead of assigning the IPv4 Service Addresses to an
   internal-only virtual network adapter, traffic destined for them are
   forwarded onto a network segment to which nodes that require IPv4
   connectivity connect to.  The ER also functions as the default IPv4
   router for the nodes on this network segment.

   Each node on the IPv4 network segment MUST acquire and assign an IPv4
   Service Address to a local network interface.  While this document
   does not attempt to explore all the various methods by which this
   could be accomplished, some examples are provided in Appendix A.

   The basic ER illustrated in Figure 2 establishes an IPv4-only network
   segment between itself and the IPv4-only nodes it serves.  This is
   fine if the nodes it provides IPv4 access to have no support for IPv6
   whatsoever; however, if they are dual-stack capable, it would not be
   ideal to take away their IPv6 connectivity in this manner.  While it
   is RECOMMENDED to use a node-based ER in this case, appropriate
   implementations of a node-based ER might not be available for every
   node.  If the application protocol in question does not work
   correctly in a NAT environment, standard SIIT-DC cannot be used
   either, which leaves a network-based ER as the only remaining
   solution.  The following subsections contain examples on how the ER
   could be implemented in a way that provides IPv6 connectivity for
   dual-stack capable nodes.

3.2.1.  Edge Relay "on a Stick"

       [IPv4 Internet]  [IPv6 Internet]
             |             |
       +-----|-----+       |
       | (BR/XLAT) |       |
       +-----|-----+       |
             |             |
        [IPv6-only IDC network]
          |
          |  +-------------+
          |  |  _IPv6_     |
          |  | /      \    |
          +====  (ER/XLAT) |
          |  | \_    _/    |
          |  |   IPv4      |          +--<Dual-stack node/server>--+
          |  +-------------+          |          +----------------+|
          |                           |  /---AF_INET  Dual-stack  ||
        [Dual-stack network segment]----<        |    application ||
                                      |  \--AF_INET6  software    ||
                                      |          +----------------+|
                                      +----------------------------+

             Figure 3: A Network-Based Edge Relay "on a Stick"

   The ER "on a stick" approach illustrated in Figure 3 ensures that the
   dual-stack capable node retains native IPv6 connectivity by
   connecting the ER's IPv4 and IPv6 interfaces to the same network
   segment, alternatively by using a single dual-stacked interface.
   Native IPv6 traffic between the IDC network and the node bypasses the
   ER entirely, while IPv4 traffic from the node will be routed directly
   to the ER (because it acts as its default IPv4 router), where it is
   translated to IPv6 before being transmitted to the upstream default
   IPv6 router.  The ER could attract inbound traffic to the IPv6
   Service Addresses by responding to the upstream router's IPv6
   Neighbor Discovery [RFC4861] messages for them.

3.2.2.  Edge Relay That Bridges IPv6 Packets

       [IPv4 Internet]  [IPv6 Internet]
             |             |
       +-----|-----+       |
       | (BR/XLAT) |       |
       +-----|-----+       |
             |             |
        [IPv6-only IDC network]
                   |
       +-----------|--------------+
       |      ____/ \_IPv6_       |
       |     /             \      |
       | (IPv6 Bridge)  (ER/XLAT) |
       |     \____   _    _/      |
       |          \ / IPv4        |   +--<Dual-stack node/server>--+
       +-----------|--------------+   |          +----------------+|
                   |                  |  /---AF_INET  Dual-stack  ||
        [Dual-stack network segment]----<        |    application ||
                                      |  \--AF_INET6  software    ||
                                      |          +----------------+|
                                      +----------------------------+

      Figure 4: A Network-Based Edge Relay Containing an IPv6 Bridge

   The ER illustrated in Figure 4 will transparently bridge IPv6 frames
   between its upstream and downstream interfaces.  IPv6 packets sent
   from the upstream IDC network to an IPv6 Service Address are
   intercepted by the ER (e.g., by responding to IPv6 Neighbor Discovery
   [RFC4861] messages for them) and routed through the translation
   function before being forwarded out the ER's downstream interface as
   IPv4 packets.  The downstream network segment thus becomes dual
   stacked.

4.  Deployment Considerations

4.1.  IPv6 Path MTU

   The IPv6 Path MTU between the ER and the BR will typically be larger
   than the default value defined in Section 4 of [RFC6145] (1280
   bytes), as it will typically be contained within a single
   administrative domain.  Therefore, it is RECOMMENDED that the IPv6
   Path MTU configured in the ER be raised accordingly.  It is
   RECOMMENDED that the ER and the BR use identical configured IPv6 Path
   MTU values.

4.2.  IPv4 MTU

   In order to avoid IPv6 fragmentation, an ER SHOULD ensure that the
   IPv4 MTU used by applications or nodes is equal to the configured
   IPv6 Path MTU - 20 so that a maximum-sized IPv4 packet can fit in an
   unfragmented IPv6 packet.  This ensures that the application may do
   its part in avoiding IP-level fragmentation from occurring, e.g., by
   segmenting/fragmenting outbound packets at the application layer, and
   advertising the maximum size its peer may use for inbound packets
   (e.g., through the use of the TCP Maximum Segment Size (MSS) option).

   A node-based ER could accomplish this by configuring this MTU value
   on the virtual network adapter, while a network-based ER could do so
   by advertising the MTU to its downstream nodes using the DHCPv4
   Interface MTU option [RFC2132].

4.3.  IPv4 Identification Header

   If the generation of IPv6 Atomic Fragments is disabled, the value of
   the IPv4 Identification header will be lost during the translation.
   Conversely, enabling the generation of IPv6 Atomic Fragments will
   ensure that the IPv4 Identification header will be carried end to
   end.  Note that for this to work bidirectionally, IPv6 Atomic
   Fragment generation MUST be enabled on both the BR and the ER.

   Apart from certain diagnostic tools, there are few (if any)
   application protocols that make use of the IPv4 Identification
   header.  Therefore, the loss of the IPv4 Identification value will
   generally not cause any problems.

   IPv6 Atomic Fragments and their impact on the IPv4 Identification
   header is further discussed in Section 4.9.2 of [RFC7755].

5.  Intra-IDC IPv4 Communication

   Although SIIT-DC is primarily intended to facilitate communication
   between IPv4-only nodes on the Internet and services located in an
   IPv6-only IDC network, an IPv4-only node or application located
   behind an ER might need to communicate with other nodes or services
   in the IDC.  The IPv4-only node or application will need to go
   through the ER, as it will typically be incapable of contacting IPv6
   destinations directly.  The following subsections discuss various
   methods on how to facilitate such communication.

5.1.  Hairpinning by the SIIT-DC Border Relay

   If the BR supports hairpinning as described in Section 4.2 of
   [RFC7757], the easiest solution is to make the target service
   available through SIIT-DC in the normal way; that is, by provisioning
   an EAM to the BR that assigns an IPv4 Service Address with the target
   service's IPv6 Service Address.

   This allows the IPv4-only node or application to transmit packets
   destined for the target service's IPv4 Service Address, which the ER
   will then translate to a corresponding IPv4-converted IPv6 address by
   inserting the translation prefix [RFC6052].  When this IPv6 packet
   reaches the BR, it will be hairpinned and transmitted back to the
   target service's IPv6 Service Address (where it could possibly pass
   through another ER before reaching the target service).  Return
   traffic from the target service will be hairpinned in the same
   fashion.

   +-[Pkt#1: IPv4]-+             +--[Pkt#2: IPv6]-------------+
   | SRC 192.0.2.1 |  (XLAT#1)   | SRC 2001:db8:a::           |
   | DST 192.0.2.2 |--(@ ER A)-->| DST 2001:db8:46::192.0.2.2 |---\
   +---------------+             +----------------------------+   |
                                                                (XLAT#2)
   +-[Pkt#4: IPv4]-+             +--[Pkt#3: IPv6]-------------+ ( @ BR )
   | SRC 192.0.2.1 |   (XLAT#3)  | SRC 2001:db8:46::192.0.2.1 |   |
   | DST 192.0.2.2 |<--(@ ER B)--| DST 2001:db8:b::           |<--/
   +---------------+             +----------------------------+

                Figure 5: Hairpinned IPv4-IPv4 Packet Flow

   Figure 5 illustrates the flow of a hairpinned packet sent from the
   IPv4-only node/app behind ER A towards an IPv6-only node/app behind
   ER B.  ER A is configured with the EAM {192.0.2.1,2001:db8:a::} and
   ER B with {192.0.2.2,2001:db8:b::}.  The BR is configured with both
   EAMs and supports hairpinning.  Note that if the target service had
   not been located behind an ER, the third and final translation
   (XLAT#3) would not have happened, i.e., the target service/node would
   have received and responded to packet #3 directly.

   If the IPv4-only nodes/services do not need connectivity with the
   public IPv4 Internet, private IPv4 addresses [RFC1918] could be used
   as their IPv4 Service Addresses in order to conserve the IDC
   operator's pool of public IPv4 addresses.

5.2.  Additional EAMs Configured in Edge Relay

   If the BR does not support hairpinning, or if the hairpinning
   solution is not desired for some other reason, intra-IDC IPv4 traffic
   may be facilitated by configuring additional EAMs on the ER for each
   service the IPv4-only node or application needs to communicate with.
   This makes the IPv6 traffic between the ER and the target service's
   IPv6 Service Address follow the direct path through the IPv6 network.
   The traffic does not pass the BR, which means that this solution
   might yield better latency than the hairpinning approach.

   The additional EAM configured in the ER consists of the target's IPv6
   Service Address and an IPv4 Service Address.  The IPv4-only node or
   application will contact the target's assigned IPv4 Service Address
   using its own IPv4 Service Address as the source.  The ER will then
   proceed to translate the original IPv4 packet to an IPv6 packet.  The
   source address of the resulting IPv6 packet will be the IPv6 Service
   Address of the local node or application, while the destination
   address will be the IPv6 Service Address of the target.  Any replies
   from the target will undergo identical translation, only in reverse.

   If the target service is located behind another ER, that other ER
   MUST also be provisioned with an additional EAM that contains the
   IPv4 and IPv6 Service Addresses of the origin IPv4-only node or
   application.  Otherwise, the target service's ER will be unable to
   translate the source address of the incoming packets.

            +-[Pkt#1: IPv4]-+             +--[Pkt#2: IPv6]---+
            | SRC 192.0.2.1 |  (XLAT#1)   | SRC 2001:db8:a:: |
            | DST 192.0.2.2 |--(@ ER A)-->| DST 2001:db8:b:: |
            +---------------+             +------------------+
                                                   |
            +-[Pkt#3: IPv4]-+                      |
            | SRC 192.0.2.1 |        (XLAT#2)      |
            | DST 192.0.2.2 |<-------(@ ER B)------/
            +---------------+

              Figure 6: Non-hairpinned IPv4-IPv4 Packet Flow

   Figure 6 illustrates the flow of a packet carrying intra-IDC IPv4
   traffic between two IPv4-only nodes/applications that are both
   located behind ERs.  Both ER A and ER B are configured with two EAMs:
   {192.0.2.1,2001:db8:a::} and {192.0.2.2,2001:db8:b::}.  The packet
   will follow the regular routing path through the IPv6 IDC network;
   the BR is not involved, and the packet will not be hairpinned.

   The above approach is not mutually exclusive with the hairpinning
   approach described in Section 5.1: If both EAMs above are also
   configured on the BR, both 192.0.2.1 and 192.0.2.2 would be reachable
   from other IPv4-only services/nodes using the hairpinning approach.
   They would also be reachable from the IPv4 Internet.

   Note that if the target service in this example was not located
   behind an ER, but instead was a native IPv6 service listening on
   2001:db8:b::, the second translation step in Figure 6 would not
   occur; the target service would receive and respond to packet #2
   directly.

   As with the hairpinning approach, if the IPv4-only nodes/services do
   not need connectivity to/from the public IPv4 Internet, private IPv4
   addresses [RFC1918] could be used as their IPv4 Service Addresses.
   Alternatively, in the case where the target service is on native
   IPv6, the target's assigned IPv4 Service Address has only local
   significance behind the ER.  It could therefore be assigned from the
   IPv4 Service Continuity Prefix [RFC7335].

6.  Security Considerations

   This section discusses security considerations specific to the use of
   an ER.  See the Security Considerations section in [RFC7755] for
   security considerations applicable to the SIIT-DC architecture in
   general.

   If the ER receives an IPv4 packet from the application/node from a
   source address it does not have an EAM for, both the source and
   destination addresses will be rewritten according to [RFC6052].
   After undergoing the reverse translation in the BR, the resulting
   IPv4 packet routed to the IPv4 network will have a spoofed IPv4
   source address.  The ER SHOULD therefore ensure that ingress
   filtering [RFC2827] is used on the ER's IPv4 interface so that such
   packets are immediately discarded.

   If the ER receives an IPv6 packet with both the source and
   destination address equal to one of its local IPv6 Service Addresses,
   the resulting packet would appear to the IPv4-only application/node
   as locally generated, as both the source address and the destination
   address will be the same address.  This could trick the application
   into believing the packet came from a trusted source (itself).  To
   prevent this, the ER SHOULD discard any received IPv6 packets that
   have a source address that is either 1) equal to any of its local
   IPv6 Service Addresses or 2) after translation from IPv6 to IPv4,
   equal to any of its local IPv4 Service Addresses.

7.  References

7.1.  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>.

   [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>.

   [RFC7757]  Anderson, T. and A. Leiva, "Explicit Address Mappings for
              Stateless IP/ICMP Translation", RFC 7757,
              DOI 10.17487/RFC7757, February 2016,
              <http://www.rfc-editor.org/info/rfc7757>.

7.2.  Informative References

   [RFC826]   Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <http://www.rfc-editor.org/info/rfc826>.

   [RFC1918]  Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
              and E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996,
              <http://www.rfc-editor.org/info/rfc1918>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <http://www.rfc-editor.org/info/rfc2132>.

   [RFC2827]  Ferguson, P. and D. Senie, "Network Ingress Filtering:
              Defeating Denial of Service Attacks which employ IP Source
              Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827,
              May 2000, <http://www.rfc-editor.org/info/rfc2827>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <http://www.rfc-editor.org/info/rfc4861>.

   [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>.

   [RFC6535]  Huang, B., Deng, H., and T. Savolainen, "Dual-Stack Hosts
              Using "Bump-in-the-Host" (BIH)", RFC 6535,
              DOI 10.17487/RFC6535, February 2012,
              <http://www.rfc-editor.org/info/rfc6535>.

   [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>.

   [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>.

Appendix A.  Examples: Network-Based IPv4 Connectivity

A.1.  Subnet with IPv4 Service Addresses

   One relatively straight-forward way to provide IPv4 connectivity
   between a network-based ER and the IPv4 node(s) it serves is to
   ensure the IPv4 Service Address(es) can be enclosed within a larger
   IPv4 prefix.  The ER may then claim one address in this prefix for
   itself and use it to provide an IPv4 default router address and
   assign the IPv4 Service Address(es) to its downstream node(s) using
   DHCPv4 [RFC2131].  For example, if the IPv4 Service Addresses are
   192.0.2.26 and 192.0.2.27, the ER would configure the address
   192.0.2.25/29 on its IPv4-facing interface and would add the two IPv4
   Service Addresses to its DHCPv4 pool.

   One disadvantage of this method is that IPv4 communication between
   the IPv4 node(s) behind the ER and other services made available
   through SIIT-DC becomes impossible, if those other services are
   assigned IPv4 Service Addresses that also are covered by the same
   IPv4 prefix (e.g., 192.0.2.28).  This happens because the IPv4 nodes
   will mistakenly believe they have an on-link route to the entire
   prefix and attempt to resolve the addresses using ARP [RFC826],
   instead of sending them to the ER for translation to IPv6.  This
   problem could, however, be overcome by avoiding assigning IPv4
   Service Addresses that overlap with an IPv4 prefix handled by an ER
   (at the expense of wasting some potential IPv4 Service Addresses) or
   by ensuring that the overlapping IPv4 Service Addresses are only
   assigned to services that do not need to communicate with the IPv4
   node(s) behind the ER.  A third way to avoid this problem is
   discussed in Appendix A.2.

A.2.  Subnet with Unrouted IPv4 Addresses

   In order to avoid the problem discussed in Appendix A.1, a private
   unrouted IPv4 network that does not encompass the IPv4 Service
   Address(es) could be used to provide connectivity between the ER and
   the IPv4-only node(s) it serves.  An IPv4-only node must then assign
   its IPv4 Service Address as a secondary local address, while the ER
   routes each of the IPv4 Service Addresses to its assigned node using
   that node's private on-link IPv4 address as the next hop.  This
   approach would ensure there are no overlaps with IPv4 Service
   Addresses elsewhere in the infrastructure, but on the other hand, it
   would preclude the use of DHCPv4 [RFC2131] for assigning the IPv4
   Service Addresses.

   This approach creates a need to ensure that the IPv4 application is
   selecting the IPv4 Service Address (as opposed to its private on-link
   IPv4 address) as its source address when initiating outbound
   connections.  This could be accomplished by altering the Default
   Address Selection Policy Table [RFC6724] on the IPv4 node.

Acknowledgements

   The authors would like to especially thank the authors of 464XLAT
   [RFC6877]: Masataka Mawatari, Masanobu Kawashima, and Cameron Byrne.
   The architecture described by this document is merely an adaptation
   of their work to a data center environment and could not have
   happened without them.

   The authors would like also to thank the following individuals for
   their contributions, suggestions, corrections, and criticisms: Fred
   Baker, Tobias Brox, Olafur Gudmundsson, Christer Holmberg, Ray
   Hunter, Shucheng LIU (Will), and Andrew Yourtchenko.

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


   Sander Steffann
   S.J.M. Steffann Consultancy
   Tienwoningenweg 46
   Apeldoorn, Gelderland  7312 DN
   The Netherlands

   Email: sander@steffann.nl