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Versions: (draft-anderson-v6ops-siit-dc) 00 01 02 03 RFC 7755

IPv6 Operations                                              T. Anderson
Internet-Draft                                            Redpill Linpro
Intended status: Standards Track                       December 18, 2014
Expires: June 21, 2015


SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Centre Environments
                      draft-ietf-v6ops-siit-dc-00

Abstract

   This document describes SIIT-DC, an extension to the Stateless IP/
   ICMP Translation (SIIT) algorithm, that makes it ideally suited for
   use in IPv6 data centre environments.  SIIT-DC simultaneously
   facilitates IPv6 deployment and IPv4 address conservation.  The
   overall SIIT-DC architecture is described, as well as guidelines for
   operators.  Finally, the normative implementation requirements are
   described, as a list of additions and changes to SIIT.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
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   This Internet-Draft will expire on June 21, 2015.

Copyright Notice

   Copyright (c) 2014 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
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   publication of this document.  Please review these documents
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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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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
     1.1.  Motivation and Goals  . . . . . . . . . . . . . . . . . .   3
       1.1.1.  Single Stack IPv6 Operation . . . . . . . . . . . . .   4
       1.1.2.  Stateless Operation . . . . . . . . . . . . . . . . .   4
       1.1.3.  IPv4 Address Conservation . . . . . . . . . . . . . .   5
       1.1.4.  No Loss of End User's IPv4 Source Address . . . . . .   5
       1.1.5.  Compatible with Standard IPv6 Implementations . . . .   5
       1.1.6.  No Architectural Dependency on IPv4 . . . . . . . . .   6
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   7
     3.1.  DNS Configuration . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Packet Flow . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Deployment Guidelines . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Application Support for NAT . . . . . . . . . . . . . . .  12
     4.2.  Application Support for IPv6  . . . . . . . . . . . . . .  12
     4.3.  Application Communication Pattern . . . . . . . . . . . .  12
     4.4.  Choice of Translation Prefix  . . . . . . . . . . . . . .  13
     4.5.  Routing Considerations  . . . . . . . . . . . . . . . . .  13
     4.6.  Location of the SIIT-DC Gateways  . . . . . . . . . . . .  14
     4.7.  Migration from Dual Stack . . . . . . . . . . . . . . . .  15
     4.8.  Packet Size and Fragmentation Considerations  . . . . . .  15
       4.8.1.  IPv4/IPv6 Header Size Difference  . . . . . . . . . .  16
       4.8.2.  IPv6 Atomic Fragments . . . . . . . . . . . . . . . .  16
       4.8.3.  Minimum Path MTU Difference Between IPv4 and IPv6 . .  17
   5.  Implementation Requirements . . . . . . . . . . . . . . . . .  18
     5.1.  Compliance with RFC6145 and RFC6052 . . . . . . . . . . .  18
     5.2.  Static Address Mapping Function . . . . . . . . . . . . .  19
     5.3.  Support for Increasing the IPv6 Path MTU  . . . . . . . .  19
     5.4.  Loop Prevention Mechanism . . . . . . . . . . . . . . . .  20
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Requirements Language . . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  20
     9.1.  Mistaking the Translation Prefix for a Trusted Network  .  21
     9.2.  Packets Looping Through the SIIT-DC Function  . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Complete SIIT-DC topology example  . . . . . . . . .  23
   Appendix B.  Comparison to Other Deployment Approaches  . . . . .  26
     B.1.  IPv4-only . . . . . . . . . . . . . . . . . . . . . . . .  26
     B.2.  IPv4-only + NAPT44  . . . . . . . . . . . . . . . . . . .  26
     B.3.  IPv4-only + NAT64 . . . . . . . . . . . . . . . . . . . .  28



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     B.4.  Dual Stack  . . . . . . . . . . . . . . . . . . . . . . .  29
     B.5.  Partial Dual Stack (IPv6-only back-end) . . . . . . . . .  30
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  31

1.  Introduction

   SIIT-DC is an extension of SIIT [RFC6145] that provides a network-
   centric stateless translation service that allows a data centre
   operator or Internet Content Provider (ICP) to run a data centre
   network, servers, and applications using exclusively IPv6, while at
   the same time ensuring that end users that have only IPv4
   connectivity will be able to continue to access the services and
   applications.

1.1.  Motivation and Goals

   Historically, dual stack [RFC4213] [RFC6883] has been the recommended
   way to transition from an IPv4-only environment to one capable of
   serving IPv6 users.  However, for data centre operators and Internet
   content providers, dual stack operation has a number of disadvantages
   compared to single stack operation.  In particular, running two
   protocols rather than one results in increased complexity and
   operational overhead, with a very low expected return on investment
   in the short to medium term while few end-users only have
   connectivity to the IPv6 Internet.  Furthermore, the dual stack
   approach does not in any way help with the depletion of the IPv4
   address space.

   Therefore, some operators may prefer an approach in which they only
   need to operate one protocol in the data centre as they prepare for
   the future.  The design goals are:

   o  Promote the deployment of native IPv6 services (cf. [RFC6540]).

   o  Provide IPv4 service availability for legacy users with no loss of
      performance or functionality.

   o  To ensure that that the legacy users' IPv4 addresses remain
      available to the servers and applications.

   o  To conserve and maximise the utilisation of IPv4 addresses.

   o  To avoid introducing more complexity than absolutely necessary,
      especially on the servers and applications.

   o  To be easy to scale and deploy in a fault-tolerant manner.





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   The following subsections elaborates on how SIIT-DC meets these
   goals.

1.1.1.  Single Stack IPv6 Operation

   SIIT-DC allows an operator to build their applications on an
   IPv6-only foundation.  IPv4 end-user connectivity becomes a service
   provided by the network, which systems administration and application
   development staff do not need to concern themselves with.

   Obviously, this will promote universal IPv6 deployment for all of the
   provider's services and applications.

   It is worth noting that SIIT-DC requires no special support or change
   from the underlying IPv6 infrastructure, it will work with any kind
   of IPv6 network.  Traffic between IPv6-enabled end users and
   IPv6-enabled services will always be native, and SIIT-DC will not be
   involved in it at all.

1.1.2.  Stateless Operation

   Unlike other solutions that provide either dual stack availability to
   single-stack services (e.g., Stateful NAT64 [RFC6146] and Layer-4/7
   proxies), or that provide conservation of IPv4 addresses (e.g.,
   NAPT44 [RFC3022]), a SIIT-DC Gateway does not keep any state between
   each packet in a single connection or flow.  In this sense it
   operates exactly like a normal IP router, and has similar scaling
   properties - the limiting factors are packets per second and
   bandwidth.  The number of concurrent flows and flow initiation rates
   are irrelevant for performance.

   This not only allows individual SIIT-DC Gateways to easily attain
   "line rate" performance, it also allows for per-packet load balancing
   between multiple SIIT-DC Gateways using Equal-Cost Multipath Routing
   [RFC2991].  Asymmetric routing is also acceptable, which makes it
   easy to avoid sub-optimal traffic patterns; the prefixes involved may
   be anycasted from all the SIIT-DC Gateways in the provider's network,
   thus ensuring that the most optimal path through the network is used,
   even where the optimal path in one direction differs from the optimal
   path in the opposite direction.

   Finally, stateless operation means that high availability is easily
   achieved.  If an SIIT-DC Gateway should fail, its traffic can be re-
   routed onto another SIIT-DC Gateway using a standard IP routing
   protocol.  This does not impact existing flows any more than what any
   other IP re-routing event would.





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1.1.3.  IPv4 Address Conservation

   In most parts of the world, it is difficult or even impossible to
   obtain generously sized IPv4 allocations from the Regional Internet
   Registries.  The resulting scarcity in turn impacts individual end
   users and operators, which might be forced to purchase IPv4 addresses
   from other operators in order to cover their needs.  This process can
   be risky to business continuity, in the case no suitable block for
   sale can be located, and/or turn out to be prohibitively expensive.
   Even so, a data centre operator will find that providing IPv4 service
   is essential, as a large share of the Internet users still does not
   have IPv6 connectivity.

   A key goal of SIIT-DC is to help reduce a data centre operator's IPv4
   address requirement to the absolute minimum, by allowing the operator
   to remove them entirely from components that do not need to
   communicate with endpoints in the IPv4 Internet.  One example would
   be servers that are operating in a supporting/back-end role and only
   communicates with to other servers (database servers, file servers,
   and so on).  Another example would be the network infrastructure
   itself (router-to-router links, loopback addresses, and so on).
   Furthermore, as LAN prefix sizes must always be rounded up to the
   nearest power of two (or larger, if one reserves space for future
   growth), even more IPv4 addresses will often end up being wasted
   without even being used.

   With SIIT-DC, the operator can remove these valuable IPv4 addresses
   from his back-end servers and network infrastructure, and reassign
   them to the SIIT-DC service as IPv4 Service Addresses.  There is no
   requirement that IPv4 Service Addresses are assigned in an aggregated
   manner, so there is nothing lost due to infrastructure overhead;
   every single IPv4 address assigned to SIIT-DC can be used an IPv4
   Service Address.

1.1.4.  No Loss of End User's IPv4 Source Address

   SIIT-DC will map the entire end-user's IPv4 source address into an
   predefined IPv6 translation prefix.  This ensures that there is no
   loss of information; the end-user's IPv4 source address remains
   available to the server/application, allowing it to perform tasks
   like Geo-Location, logging, abuse handling, and so forth.

1.1.5.  Compatible with Standard IPv6 Implementations

   Except for the introduction of the SIIT-DC Gateways themselves, no
   change to the network, servers, applications, or anything else is
   required in order to support SIIT-DC.  SIIT-DC is practically
   invisible from the point of view of the the IPv4 clients, the IPv6



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   servers, the IPv6 data centre network, and the IPv4 Internet.  SIIT-
   DC interoperates with all standards-compliant IPv4 or IPv6 stacks.

1.1.6.  No Architectural Dependency on IPv4

   SIIT-DC will allow an ICP or data centre operator to build
   infrastructure and applications entirely on IPv6.  This means that
   when the day comes to discontinue support for IPv4, no change needs
   to be made to the overall architecture - it's only a matter of
   shutting off the SIIT-DC Gateways.  Therefore, by deploying native
   IPv6 along with SIIT-DC, operators will avoid future migration or
   deployment projects relating to IPv6 roll-out and/or IPv4 sun-
   setting.

2.  Terminology

   This document makes use of the following terms:

   IPv4 Service Address  A public IPv4 address with which IPv4-only
      clients will communicate.  This communication will be translated
      to IPv6 by the SIIT-DC Gateway.

   IPv4 Service Address Pool  One or more IPv4 prefixes routed to the
      SIIT-DC Gateway's IPv4 interface.  IPv4 Service Addresses are
      allocated from this pool.  Note that this does not necessarily
      have to be a "pool" per se, as it could also be one or more host
      routes (whose prefix length is equal to /32).  The primary purpose
      of using a pool rather than host routes is to facilitate IPv4
      route aggregation and ease provisioning of new IPv4 Service
      Addresses.

   IPv6 Service Address  A public IPv6 address assigned to a server or
      application in the IPv6 network.  IPv6-only and dual stacked
      clients communicates with this address directly without invoking
      SIIT-DC.  IPv4-only clients also communicates with this address
      through the SIIT-DC Gateway and via an IPv4 Service Address.

   SIIT-DC Host Agent  A logical function very similar to an SIIT-DC
      Gateway that resides on a server and provides virtual IPv4
      connectivity to applications, by reversing the translations done
      by the SIIT-DC Gateway.  It is an optional component of the SIIT-
      DC architecture, that may be used to increase application support.
      See [I-D.anderson-v6ops-siit-dc-2xlat].

   SIIT-DC Gateway  A device or a logical function that translates
      between IPv4 and IPv6 in accordance with Section 5.





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   Static Address Mapping  A bi-directional mapping between an IPv4
      Service Address and an IPv6 Service Address configured in the
      SIIT-DC Gateway.  When translating between IPv4 and IPv6, the
      SIIT-DC Gateway changes the address fields in the translated
      packet's IP header according to any matching Static Address
      Mapping.

   Translation Prefix  An IPv6 prefix into which the entire IPv4 address
      space is mapped.  This prefix is routed to the SIIT-DC Gateway's
      IPv6 interface.  It is either an Network-Specific Prefix or a
      Well-Known Prefix as specified in [RFC6052].  When translating
      between IPv4 and IPv6, the SIIT-DC Gateway prepends or strips the
      Translation Prefix from the address fields in the translated
      packet's IP header, unless a Static Address Mapping exists for the
      IP address in question.

3.  Architectural Overview

   This section describes the basic SIIT-DC architecture.

                           SIIT-DC Architecture






























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             +-------------------+         +----------------+
             | IPv6-capable user |         | IPv4-only user |
             | ================= |         | ============== |
             |                   |         |                |
             +-<2001:db8::ab:cd>-+         +-<203.0.113.50>-+
                 |                                  |
              (the IPv6 internet)         (the IPv4 Internet)
                 |                                  |
                 |        +------------------<192.0.2.0/24>-+
                 |        |                                 |
                 |        |         SIIT-DC Gateway         |
                 |        |         ===============         |
                 |        |                                 |
                 |        |       Translation Prefix:       |
                 |        |         2001:db8:46::/96        |
                 |        |                                 |
                 |        |     Static Address Mapping:     |
                 |        | 192.0.2.1 <=> 2001:db8:12:34::1 |
                 |        |                                 |
                 |        +--------------<2001:db8:46::/96>-+
                 |                               |
                (the IPv6-only data centre network)
                 |                               |
                 | ------------------------------/
                 |/
                 |
           +--<2001:db8:12:34::1>------------------------------+
           |     |                                             |
           |     |          IPv6-only server                   |
           |     |          ================                   |
           |     |                                             |
           | +-[2001:db8:12:34::1]---------------------------+ |
           | |      AF_INET6                                 | |
           | |                                               | |
           | |             IPv6-only application             | |
           | |                                               | |
           | +-----------------------------------------------+ |
           +---------------------------------------------------+

                                 Figure 1

   In this example, 192.0.2.0/24 is allocated as an IPv4 Service Address
   Pool.  Individual IPv4 Service Addresses are assigned from this pool.
   The provider must route this prefix to the SIIT-DC Gateway's IPv4
   interface.  Note that there are no restrictions on how many IPv4
   Service Address Pools are used or their prefix length, as long as
   they are all routed to the SIIT-DC Gateway's IPv4 interface.




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   The Static Address Mapping list is used when translating an IPv4
   Service Address (here 192.0.2.1) to its corresponding IPv6 Service
   Address (here 2001:db8:12:34::1) and vice versa.  When the SIIT-DC
   Gateway translates an IPv4 packet to IPv6, any IPv4 Service Address
   found in the original IPv4 header will be replaced with the
   corresponding IPv6 Service Address in the resulting IPv6 header, and
   vice versa when translating an IPv6 packet to IPv4.

   2001:db8:46::/96 is the Translation Prefix into which the entire IPv4
   address space is mapped.  It is used for translation of the end
   user's IPv4 address to IPv6 and vice versa according to the algorithm
   defined in Section 2.2 of RFC6052 [RFC6052].  This algorithmic
   mapping has a lower precedence than the configured Static Address
   Mappings.

   The SIIT-DC Gateway itself can be either a separate device or a
   logical function in another multi-purpose device, for example an IP
   router.  Any number of SIIT-DC Gateways may exist simultaneously in
   an operators infrastructure, as long as they all have the same
   translation prefix and list of Static Mappings configured.

3.1.  DNS Configuration

   The IPv6 Service Address of should be registered in DNS using an "IN
   AAAA" record, while its corresponding IPv4 Service Address should be
   registered using an "IN A" record.  This results in the following DNS
   records:

              DNS Configuration for a SIIT-DC enabled service

               www.example.com.  IN AAAA  2001:db8:12:34::1
               www.example.com.  IN A     192.0.2.1

                                 Figure 2

3.2.  Packet Flow

   In this example, "IPv4-only user" initiates a request to the
   application running on the IPv6-only server.  He starts by looking up
   the "IN A" record of "www.example.org" in DNS, and attempts to
   connect to this address on the service by transmitting the following
   IPv4 packet destined for the IPv4 Service Address:

                      Stage 1: Client -> Server, IPv4







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            +------------------------------------------------+
            | IP Version:           4                        |
            | Source Address:       203.0.113.50             |
            | Destination Address:  192.0.2.1                |
            | Protocol:             TCP                      |
            |------------------------------------------------|
            | TCP SYN [...]                                  |
            +------------------------------------------------+

                                 Figure 3

   This packet is then routed over the Internet to the (nearest) SIIT-DC
   Gateway, which translates it into the following IPv6 packet and
   forward it into the IPv6 network:

                  Stage 2: Client -> Server request, IPv4

            +-------------------------------------------------+
            | IP Version:           6                         |
            | Source Address:       2001:db8:46::203.0.113.50 |
            | Destination Address:  2001:db8:12:34::1         |
            | Next Header:          TCP                       |
            |-------------------------------------------------|
            | TCP SYN [...]                                   |
            +-------------------------------------------------+

                                 Figure 4

   The destination address field was translated to the IPv6 Service
   Address according to the configured Static Address Mapping, while the
   source address was field translated according to the [RFC6052]
   mapping using the Translation Prefix (because it did not match any
   Static Address Mapping).  The rest of the IP header was translated
   according to [RFC6145].  The Layer 4 payload is copied verbatim, with
   the exception of the TCP checksum being recalculated.

   Note that the IPv6 address 2001:db8:46::203.0.113.50 may also be
   expressed as 2001:db8:46::cb00:7132, cf. Section 2.2 of RFC4291
   [RFC4291].

   Next, the application receives receives this IPv6 packet and responds
   to it like it would with any other IPv6 packet:

                 Stage 3: Server -> Client response, IPv6







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            +-------------------------------------------------+
            | IP Version:           6                         |
            | Source Address:       2001:db8:12:34::1         |
            | Destination Address:  2001:db8:46::203.0.113.50 |
            | Next Header:          TCP                       |
            |-------------------------------------------------|
            | TCP SYN+ACK [...]                               |
            +-------------------------------------------------+

                                 Figure 5

   The response packet is routed to the (nearest) SIIT-DC Gateway's IPv6
   interface, which will translate it back to IPv4 as follows:

                 Stage 4: Server -> Client response, IPv4

            +------------------------------------------------+
            | IP Version:           4                        |
            | Source Address:       192.0.2.2                |
            | Destination Address:  203.0.113.50             |
            | Protocol:             TCP                      |
            |------------------------------------------------|
            | TCP SYN+ACK [...]                              |
            +------------------------------------------------+

                                 Figure 6

   This time, the source address matched the Static Address Mapping and
   was translated accordingly, while the destination address did not,
   and was therefore translated according to [RFC6052] by having the
   Translation Prefix stripped.  The rest of the packet was translated
   according to [RFC6145].

   The resulting IPv4 packet is transmitted back to the end user over
   the IPv4 Internet.  Subsequent packets in the flow will follow the
   exact same translation pattern.  They may or may not cross the same
   translators as earlier packets in the same flow.

   The end user's IPv4 stack has no idea that it is communicating with
   an IPv6 server, nor does the server's IPv6 stack have any idea that
   is is communicating with an IPv4 client.  To them, it's just plain
   IPv4 or IPv6, respectively.  However, the applications running on the
   server may optionally be updated to recognise and strip the
   Translation Prefix, so that the end user's IPv4 address may be used
   for logging, Geo-Location, abuse handling, and so forth.

4.  Deployment Guidelines




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   In this section, we list recommendations and guidelines for operators
   who would like to deploy a SIIT-DC service in their data centre
   network.

4.1.  Application Support for NAT

   Not all application protocols are able to operate in a network
   environment where rewriting of IP addresses occur.  An operator
   should therefore carefully evaluate the applications he would like to
   make available for IPv4 users through SIIT-DC, to ensure they do not
   fall in this category.  In general, if an application layer protocol
   works correctly through standard NAT44 (see [RFC3235]), it will most
   likely work correctly through SIIT-DC as well.

   Higher-level protocols that embed IP addresses as part of their
   payload are especially problematic, as noted in [RFC2663], [RFC2993],
   and [RFC3022].  Such protocols will most likely not work through any
   form of address translation, including SIIT-DC.  One well-known
   example of such a protocol is FTP [RFC0959].

   The SIIT-DC architecture may be extended with a Host Agent that
   reverses the translation performed by the SIIT-DC Gateway before
   passing the packets to the application software.  This allows the
   problematic application protocols described above to work correctly
   in an SIIT-DC environment as well.  See
   [I-D.anderson-v6ops-siit-dc-2xlat] for a description of this
   extension.

4.2.  Application Support for IPv6

   SIIT-DC requires that the application software supports IPv6
   networking, and that it has no dependency on IPv4 networking.  If
   this is not the case, the approach described in
   [I-D.anderson-v6ops-siit-dc-2xlat] may be used, as it provides the
   application with seemingly native IPv4 connectivity.  This allows
   IPv4-only applications to work correctly in an otherwise IPv6-only
   environment.

4.3.  Application Communication Pattern

   SIIT-DC is ideally suited for applications where IPv4-only nodes on
   the Internet initiate traffic towards the IPv6-only services, which
   in turn are only passively listening for inbound traffic and
   responding as necessary.  One well-known example of such a protocol
   is HTTP [RFC7230].  This is due to the fact that in this case, an
   IPv4 user looks exactly like an ordinary IPv6 user from the host and
   application's point of view, and requires no special treatment.




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   It is possible to combine SIIT-DC with DNS64 [RFC6147] in order to
   allow an IPv6-only application to initiate communication with
   IPv4-only nodes through an SIIT-DC Gateway.  However, in this case,
   care must be taken so that all outgoing communication is sourced from
   the IPv6 Service Address that has a Static Mapping configured on the
   SIIT-DC Gateway.  If another unmapped address is used, the SIIT-DC
   Gateway will discard the packet.

   An alternative approach to the above would be to make use of an SIIT-
   DC Host Agent as described in [I-D.anderson-v6ops-siit-dc-2xlat].
   This provides the application with seemingly native IPv4
   connectivity, which it may use for both inbound and outbound
   communication without requiring the application to select a specific
   source address for its outbound communications.

4.4.  Choice of Translation Prefix

   Either a Network-Specific Prefix (NSP) from the provider's own IPv6
   address space or the IANA-allocated Well-Known Prefix 64:ff9b::/96
   (WKP) may be used.  From a technical point of view, both should work
   equally well, however as only a single WKP exists, if a provider
   would like to deploy more than one instance of SIIT-DC in his
   network, or Stateful NAT64 [RFC6146], an NSP must be used anyway for
   all but one of those deployments.

   Furthermore, the WKP cannot be used in inter-domain routing.  By
   using an NSP, a provider will have the possibility to provide SIIT-DC
   service to other operators across Autonomous System borders.

   For these reasons, this document recommends that an NSP is used.
   Section 3.3 of [RFC6052] discusses the choice of translation prefix
   in more detail.

   The Translation Prefix may use any of the lengths described in
   Section 2.2 of RFC6052 [RFC6052], but /96 has two distinct advantages
   over the others.  First, converting it to IPv4 can be done in a
   single operation by simply stripping off the first 96 bits; second,
   it allows for IPv4 addresses to be embedded directly into the text
   representation of an IPv6 address using the familiar dotted quad
   notation, e.g., "2001:db8::198.51.100.10" (cf. Section 2.4 of RFC6052
   [RFC6052])), instead of being converted to hexadecimal notation.
   This makes it easier to write IPv6 ACLs and similar that match
   translated endpoints in the IPv4 Internet.  Use of a /96 prefix
   length is therefore recommended.

4.5.  Routing Considerations





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   The prefixes that constitute the IPv4 Service Address Pool and the
   IPv6 Translation Prefix may be routed to the SIIT-DC Gateway(s) as
   any other IPv4 or IPv6 route in the provider's network.

   If more than one SIIT-DC Gateway is being deployed, it is recommended
   that a dynamic routing protocol (such as BGP, IS-IS, or OSPF) is
   being used to advertise the routes within the provider's network.
   This will ensure that the traffic that is to be translated will reach
   the closest SIIT-DC Gateway, reducing or eliminating sub-optimal
   traffic patterns, as well as provide high availability - if one SIIT-
   DC Gateway fails, the dynamic routing protocol will automatically
   redirect the traffic to the next-best translator.

4.6.  Location of the SIIT-DC Gateways

   The goal of SIIT-DC is to facilitate a true IPv6-only application and
   network architecture, with the sole exception being the IPv4
   interfaces of the SIIT-DC Gateways and the network infrastructure
   required to connect them to the IPv4 Internet.  Therefore, the SIIT-
   DC Gateways should be located somewhere between the IPv4 Internet and
   the application delivery stack.  This should be understood to include
   all servers, load balancers, firewalls, intrusion detection systems,
   and similar devices that are processing traffic to a greater extent
   than merely forwarding it.

   It is optimal to place the SIIT-DC Gateways as close as possible to
   the direct path between the servers and the end users.  If the
   closest translator is located a long way from the optimal path, all
   packets in both directions must make a detour.  This would increase
   the RTT between the server and the end user by by two times the extra
   latency incurred by the detour, as well as cause unnecessary load on
   the network links on the detour path.

   Where possible, it is beneficial to implement the SIIT-DC Gateways as
   a logical function within the routers would have handled the traffic
   anyway, had the topology been dual stacked.  This way, the
   translation service would not need to be assigned separate networks
   ports (which might become saturated and impact the service quality),
   nor would it require extra rack space and energy.  Some particularly
   good choices of the location could be within a data centre's access
   routers, or within the provider's border routers.  When every single
   application in the data centre or the provider's network eventually
   runs on single-stack IPv6, there would no need to run IPv4 on the
   inside of the SIIT-DC Gateway.  This reduces complexity, and allows
   the operator to reclaim IPv4 addresses from the network
   infrastructure that may instead be used as IPv4 Service Address
   Pools.




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   Finally, another possibility is that the data centre operator
   outsources the SIIT-DC service to another entity, for example his
   upstream ISP.  Doing so allows the data centre operator to build a
   true IPv6-only infrastructure.  However, in this case, care must be
   taken to ensure that the path between the data centre and the SIIT-DC
   operator has a stable and known MTU, and that the SIIT-DC Gateways
   are not too far away from the data centre (otherwise, translated
   traffic could incur a latency penalty).

4.7.  Migration from Dual Stack

   While this document discusses the use of IPv6-only servers and
   applications, there is no technical requirement that the servers are
   IPv4 free.  SIIT-DC works equally well for dual stacked servers,
   which makes migration easy - after setting up the translation
   function, the DNS "IN A" record for the service is updated to point
   to the IPv4 address that will be translated to IPv6, the previously
   used IPv4 service address may continue to be assigned to the server.
   This makes roll-back to dual stack easy, as it is only a matter of
   changing the DNS record back to what it was before.

   It is also possible to use DNS Round Robin to gradually migrate a
   dual-stacked service's IPv4 traffic from native to SIIT-DC.  This is
   done by configuring multiple DNS "IN A" records for the service's
   hostname, and pointing one portion of them to the service's native
   IPv4 addresses and another portion to IPv4 Service Addresses handled
   by SIIT-DC.  The distribution of "IN A" records determines how much
   of the client traffic will pass through the SIIT-DC Gateway and how
   much will remain native.  This operator may then gradually increase
   the share of SIIT-DC "IN A" DNS records until no native addresses
   remain.

   When all client traffic is handled by SIIT-DC, the operator may
   proceed to remove the (now unused) IPv4 addresses assigned to the
   servers in question.  They could then potentially be recycled as
   another IPv4 Service Address Pool assigned to SIIT-DC.

4.8.  Packet Size and Fragmentation Considerations

   There are some key differences between IPv4 and IPv6 relating to
   packet sizes and fragmentation that one should consider when
   deploying SIIT-DC.  They result in a few problematic corner cases,
   which can be dealt with in a few different ways.  The following
   subsections will discuss these in detail, and provide operational
   guidance.

   In particular, the operator may find that relying on fragmentation in
   the IPv6 domain is undesired or even operationally impossible



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   [I-D.taylor-v6ops-fragdrop].  For this reason, the recommendations in
   this section seeks to minimise the use of IPv6 fragmentation.

   Unless otherwise stated, the following subsections assume that the
   MTU in both the IPv4 and IPv6 domains is 1500 bytes.

4.8.1.  IPv4/IPv6 Header Size Difference

   The IPv6 header is up to 20 bytes larger than the IPv4 header.  This
   means that a full-size 1500 bytes large IPv4 packet cannot be
   translated to IPv6 without being fragmented, otherwise it would
   likely have resulted in a 1520 bytes large IPv6 packet.

   If the transport protocol used is TCP, this is generally not a
   problem, as the IPv6 server will advertise a TCP MSS of 1440 bytes.
   This causes the client to never send larger packets than what can be
   translated to a single full-size IPv6 packet, eliminating any need
   for fragmentation.

   For other transport protocols, full-size IPv4 packets with the DF
   flag cleared will need to be fragmented by the SIIT-DC Gateway.  The
   only way to avoid this is to increase the Path MTU between the SIIT-
   DC Gateway and the servers to 1520 bytes.  Note that the servers' MTU
   SHOULD NOT be increased accordingly, as that would cause them to
   undergo Path MTU Discovery for most native IPv6 destinations.
   However, the servers would need to be able to accept and process
   incoming packets larger than their own MTU.  If the server's IPv6
   implementation allows the MTU to be set differently for specific
   destinations, it could be increased to 1520 for destinations within
   the Translation Prefix specifically.

4.8.2.  IPv6 Atomic Fragments

   In keeping with the fifth paragraph of Section 4 of RFC6145
   [RFC6145], an SIIT-DC Gateway will by default add an IPv6
   Fragmentation header to the resulting IPv6 packet when translating an
   IPv4 packet with the Don't Fragment flag set to 0.

   This happens even though the resulting IPv6 packet isn't actually
   fragmented into several pieces, resulting in an IPv6 Atomic Fragment
   [RFC6946].  These Atomic Fragments are generally not useful in a data
   centre environment, and it is therefore recommended that this
   behaviour is disabled in the SIIT-DC Gateways.  To this end,
   Section 4 of RFC6145 [RFC6145] notes that the "translator MAY provide
   a configuration function that allows the translator not to include
   the Fragment Header for the non-fragmented IPv6 packets".





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   Note that [I-D.ietf-6man-deprecate-atomfrag-generation] seeks to
   update [RFC6145], making the functionality described above as the
   standard and only mode of operation.

   In IPv6, the Identification value is located inside the Fragmentation
   header.  That means that if the generation of IPv6 Atomic Fragments
   is disabled, the IPv4 Identification value will be lost during
   translation to IPv6.  This could potentially confuse some diagnostic
   tools.

4.8.3.  Minimum Path MTU Difference Between IPv4 and IPv6

   Section 5 of RFC2460 [RFC2460] specifies that the minimum IPv6 link
   MTU is 1280 bytes.  Therefore, an IPv6 node can reasonably assume
   that if it transmits an IPv6 packet that is 1280 bytes or smaller, it
   is guaranteed to reach its destination without requiring
   fragmentation or invoking the Path MTU Discovery algorithm [RFC1981].
   However, this assumption fails if the destination is an IPv4 node
   reached through a protocol translator such as an SIIT-DC Gateway, as
   the minimum IPv4 link MTU is 68 bytes.  See Section 3.2 of RFC791
   [RFC0791].

   Section 5.1 of RFC6145 [RFC6145] specifies that an SIIT-DC Gateway
   should set the IPv4 Don't Fragment flag to 1 when it translates a
   non-fragmented IPv6 packet to IPv4.  This means that when the path to
   the destination IPv4 node contains an IPv4 link with an MTU smaller
   than 1260 bytes (which corresponds to an IPv6 MTU smaller than 1280
   bytes, cf. Section 4.8.1), the Path MTU Discovery algorithm will be
   invoked, even if the original IPv6 packet was only 1280 bytes large.
   This happens as a result of the IPv4 router connecting to the IPv4
   link with the small MTU returning an ICMPv4 Need To Fragment error
   with an MTU value smaller than 1260, which in turns is translated by
   the SIIT-DC Gateway to an ICMPv6 Packet Too Big error with an MTU
   value smaller than 1280 which is then transmitted to the origin IPv6
   node.

   When an IPv6 node receives an ICMPv6 Packet Too Big error indicating
   an MTU value smaller than 1280, the last paragraph of Section 5 of
   RFC2460 [RFC2460] gives it two choices on how to proceed:

   o  It may reduce its Path MTU value to the value indicated in the
      Packet Too Big, i.e., limit the size of subsequent packets
      transmitted to that destination to the indicated value.  This
      approach causes no problems for the SIIT-DC function, as it simply
      allows Path MTU Discovery to work transparently across the SIIT-DC
      Gateway.





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   o  It may reduce its Path MTU value to exactly 1280, and in addition
      include a Fragmentation header in subsequent packets sent to that
      destination.  In other words, the IPv6 node will start emitting
      Atomic Fragments.  The Fragmentation header signals to the the
      SIIT-DC Gateway that the Don't Fragment flag should be set to 0 in
      the resulting IPv4 packet, and it also provides the Identification
      value.

   If the use of the IPv6 Fragmentation header is problematic, and the
   operator has IPv6 nodes that implement the second option above, the
   operator should consider enabling the functionality described as the
   "second approach" in Section 6 of RFC6145 [RFC6145].  This
   functionality changes the SIIT-DC Gateway's behaviour as follows:

   o  When translating ICMPv4 Need To Fragment to ICMPv6 Packet Too Big,
      the resulting packet will never contain an MTU value lower than
      1280.  This prevents the IPv6 nodes from generating Atomic
      Fragments.

   o  When translating IPv6 packets smaller than or equal to 1280 bytes,
      the Don't Fragment flag in the resulting IPv4 packet will be set
      to 0.  This ensures that in the eventuality that the path contains
      an IPv4 link with an MTU smaller than 1260, the IPv4 router
      connected to that link will have the responsibility to fragment
      the packet before forwarding it towards its destination.

   In summary, this approach could be seen as prompting the IPv4
   protocol itself to provide the "link-specific fragmentation and
   reassembly at a layer below IPv6" required for links that "cannot
   convey a 1280-octet packet in one piece", to paraphrase Section 5 of
   RFC2460 [RFC2460].  Note that
   [I-D.ietf-6man-deprecate-atomfrag-generation] seeks to update
   [RFC6145], making the approach described above as the standard and
   only mode of operation.

5.  Implementation Requirements

   This normative section specifies the SIIT-DC protocol that is
   implemented by an SIIT-DC Gateway.  Because SIIT-DC builds on and
   closely resembles SIIT [RFC6145], this section should be read as a
   set of additions and changes that are applied to an implementation
   already compliant to SIIT [RFC6145].  Each of the following
   subsections discuss how the requirement relates to with any
   corresponding requirements in SIIT [RFC6145].

5.1.  Compliance with RFC6145 and RFC6052





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   Unless otherwise stated in the following sections, an SIIT-DC
   implementation MUST comply fully with [RFC6145].  It must also
   implement the algorithmic address mapping defined in [RFC6052].

5.2.  Static Address Mapping Function

   The implementation MUST allow the operator to configure an arbitrary
   number of Static Address Mappings which override the default
   [RFC6052] algorithm.  It SHOULD be possible to specify a single bi-
   directional mapping that will be used in both the IPv4=>IPv6 and
   IPv6=>IPv4 directions, but it MAY additionally (or alternatively)
   support unidirectional mappings.

   An example of such a bidirectional Static Address Mapping would be:

   o  192.0.2.1 <=> 2001:db8:12:34::1

   To accomplish the same using unidirectional mappings, the following
   two mappings must instead be configured:

   o  192.0.2.1 => 2001:db8:12:34::1

   o  2001:db8:12:34::1 => 192.0.2.1

   In both cases, if the SIIT-DC Gateway receives an IPv6 packet that
   has the value 2001:db8:12:34::1 in either the source or destination
   field of the IPv6 header, it MUST rewrite this field to 192 0.2.1
   when translating to IPv4.  Similarly, if the SIIT-DC Gateway receives
   an IPv4 packet that has the value 192.0.2.1 as the either the source
   or destination field of the IPv4 header, it MUST rewrite this field
   to 2001:db8:12:34::1 when translating to IPv6.  For all IPv4 or IPv6
   source or destination field values for which there are no matching
   Static Address Mapping, [RFC6052] compliant mapping MUST be used
   instead.

   Relation to [RFC6145]: The Static Address Mapping is a novel feature
   feature that is not discussed in [RFC6145].  It conflicts with the
   [RFC6145] requirement that all addresses must be translated according
   to the [RFC6052] algorithm.

5.3.  Support for Increasing the IPv6 Path MTU

   The SIIT-DC Gateway MUST provide a configuration function for the
   network administrator to adjust the threshold of the minimum IPv6 MTU
   to a value that reflects the real value of the minimum IPv6 MTU in
   the network (greater than 1280 bytes).  This will help reduce the
   chance of including the Fragment Header in the resulting IPv6
   packets.



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   Relation to [RFC6145]: This strengthens the corresponding "MAY"
   requirement located in Section 4 of RFC6145 [RFC6145] to a "MUST".

5.4.  Loop Prevention Mechanism

   As noted in Section 9.2, there is a potential for packets looping
   through the SIIT-DC function if it receives an IPv4 packet for which
   there is no Static Address Mapping.  It is therefore RECOMMENDED that
   the implementation has a mechanism that automatically prevents this
   behaviour.  One way this could be accomplished would be to discard
   any IPv4 packets that would be translated into an IPv6 packet that
   would be routed straight back into the SIIT-DC function.

   If such a mechanism isn't provided, the implementation MUST provide a
   way to manually filter or null-route the destination addresses that
   would otherwise cause loops.

   Relation to [RFC6145]: This security consideration applies only when
   an SIIT-DC Gateway translates a packet in "pure" SIIT [RFC6145] mode
   (i.e., when both address fields are translated according to
   [RFC6052]).  This consideration is in other words not specific to
   SIIT-DC, it is inherited from [RFC6145].  In spite of this, [RFC6145]
   does not describe this consideration or any methods of prevention.
   The requirements in this section is therefore novel to SIIT-DC, even
   though they apply equally to [RFC6145].

6.  Acknowledgements

   The author would like to thank the following individuals for their
   contributions, suggestions, corrections, and criticisms: Fred Baker,
   Cameron Byrne, Brian E Carpenter, Ross Chandler, Dagfinn Ilmari
   Mannsaaker, Lars Olafsen, Stig Sandbeck Mathisen, Knut A. Syed,
   Andrew Yourtchenko.

7.  Requirements Language

   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].

8.  IANA Considerations

   This draft makes no request of the IANA.  The RFC Editor may remove
   this section prior to publication.

9.  Security Considerations





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9.1.  Mistaking the Translation Prefix for a Trusted Network

   If a Network-Specific Prefix from the provider's own address space is
   chosen for the translation prefix, as is recommended, care must be
   taken if the translation service is used in front of services that
   have application-level ACLs that distinguish between the operator's
   own networks and the Internet at large, as the translated IPv4 end
   users on the Internet will appear to be located within the provider's
   own IPv6 address space.  It is therefore important that the
   translation prefix is treated the same as the Internet at large,
   rather than as a trusted network.

9.2.  Packets Looping Through the SIIT-DC Function

   If the SIIT-DC Gateway receives an IPv4 packet destined to an address
   for which there is no Static Address Mapping, its destination address
   will be rewritten according to [RFC6052], making the resulting IPv6
   packet have a destination address within the translation prefix,
   which is likely routed to back to the SIIT-DC function.  This will
   cause the packet to loop until its Time To Live / Hop Limit reaches
   zero, potentially creating a Denial Of Service vulnerability.

   To avoid this, it should be ensured that packets sent to IPv4
   destinations addresses for which there are no Static Address
   Mappings, or whose resulting IPv6 address does not have a more-
   specific route to the IPv6 network, are immediately discarded.

10.  References

10.1.  Normative References

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

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

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

10.2.  Informative References

   [I-D.anderson-v6ops-siit-dc-2xlat]
              tore, t., "SIIT-DC: Dual Translation Mode", draft-
              anderson-v6ops-siit-dc-2xlat-00 (work in progress),
              September 2014.




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   [I-D.ietf-6man-deprecate-atomfrag-generation]
              Gont, F., Will, W., and t. tore, "Deprecating the
              Generation of IPv6 Atomic Fragments", draft-ietf-6man-
              deprecate-atomfrag-generation-00 (work in progress),
              November 2014.

   [I-D.taylor-v6ops-fragdrop]
              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", draft-taylor-v6ops-fragdrop-02 (work in
              progress), December 2013.

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September
              1981.

   [RFC0959]  Postel, J. and J. Reynolds, "File Transfer Protocol", STD
              9, RFC 959, October 1985.

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations", RFC
              2663, August 1999.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991, November 2000.

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

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022, January
              2001.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235, January 2002.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.




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   [RFC4217]  Ford-Hutchinson, P., "Securing FTP with TLS", RFC 4217,
              October 2005.

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

   [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.

   [RFC6540]  George, W., Donley, C., Liljenstolpe, C., and L. Howard,
              "IPv6 Support Required for All IP-Capable Nodes", BCP 177,
              RFC 6540, April 2012.

   [RFC6883]  Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
              Content Providers and Application Service Providers", RFC
              6883, March 2013.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments", RFC
              6946, May 2013.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7269]  Chen, G., Cao, Z., Xie, C., and D. Binet, "NAT64
              Deployment Options and Experience", RFC 7269, June 2014.

Appendix A.  Complete SIIT-DC topology example

   This figure shows a more complete SIIT-DC topology, in order to
   better demonstrate the beneficial properties it has.  In particular,
   it tries to highlight the following:

   o  Stateless operation: Any number of SIIT-DC Gateways may be
      deployed side-by side, or indeed anywhere in the IPv6 network, as
      any standard routing mechanism may be used to direct traffic to
      them (shown here with BGP on the IPv4 side and ECMP on the IPv6
      side).  This in turn leads to high availability, should one of the
      SIIT-DC Gateways fail or become unavailable, those standard
      routing mechanisms will ensure that traffic is automatically
      redirect one of the remaining SIIT-DC Gateways.




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   o  IPv4 address conservation: Even though the to customers in the
      example have several hundred servers, most of them are not used
      for externally available services, and thus do not require an IPv4
      address.  The network between the servers and the SIIT-DC Gateways
      require no IPv4 addresses, either.  Furthermore, the IPv4
      addresses that are used do not have to be assigned to customers in
      the form of aggregated blocks or prefixes; which makes it easy to
      achieve 100% effective utilisation of the IPv4 service address
      pools.

   o  Application support: The translation-friendly applications HTTP
      and SMTP will work through SIIT-DC without requiring any special
      customisation.  Furthermore, translation-unfriendly applications
      such as FTP will also work if an host agent in present, cf.
      [I-D.anderson-v6ops-siit-dc-2xlat].

   o  Native IPv6 as the foundation: Every server, application, and
      network component has access to native and untranslated IPv6
      connectivity to each other and to the Internet.  Traffic through
      the SIIT-DC Gateways will diminish over time as IPv6 is deployed
      throughout the Internet.  Eventually they may be shut down
      entirely, which causes no disruption to the application stacks'
      ability to deliver their services over native IPv6.

                Example data centre topology using SIIT-DC


























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                   /--------------------------------\ /---------------\
                   |          IPv4 Internet         | | IPv6 Internet |
                   \-+----------------------------+-/ \--------+------/
                     |                            |            |
                     | <----------[BGP]---------> |            |
                     |                            |            |
   +---------<192.0.2.0/24>----------+ +---<192.0.2.0/24>---+  |
   |                                 | |                    |  |
   |        SIIT-DC Gateway 1        | | SIIT-DC Gateway 2  |  |
   |        =================        | | =================  |  |
   |                                 | |                    |  |
   |       Translation Prefix:       | |                    |  |
   |         2001:db8:46::/96        | |                    |  |
   |                                 | |                    |  |
   |     Static Address Mappings:    | | Exactly the same   |  |
   | 192.0.2.1 <=> 2001:db8:12:34::1 | | configuration as   |  |
   | 192.0.2.2 <=> 2001:db8:12:34::2 | | SIIT-DC Gateway 1  |  |
   | 192.0.2.3 <=> 2001:db8:fe:dc::1 | |                    |  |
   | 192.0.2.4 <=> 2001:db8:12:34::4 | |                    |  |
   | [...]                           | |                    |  |
   |                                 | |                    |  |
   +--------<2001:db8:46::/96>-------+ +-<2001:db8:46::/96>-+  |
                     |                            |            |
                     | <---------[ECMP]---------> |            |
                     |                            |            |
   /-----------------+----------------------------+-\          |
   |            IPv6 data centre network            +----------+
   \-+-----------------------------------+----------/
     |                                   |
     | Customer A's server LAN           | Customer B's server LAN
     | 2001:db8:12:34::/64               | 2001:db8:fe:dc::/64
     |                                   |
     |                                   |
     +-- www      ::1 (IPv6+SIIT-DC)     +-- www ::1 (IPv6+SIIT-DC)
     |                                   |
     |                                   +-- file01 ::f:01 (IPv6)
     +-- mta      ::2 (IPv6+SIIT-DC)     |   [...]
     |                                   +-- file99 ::f:99 (IPv6)
     +-- ftp      ::3 (IPv6)
     |            ::4 (SIIT-DC/Host Agent)
     |
     +-- app01 ::a:01 (IPv6)
     |   [...]
     +-  app99 ::a:99 (IPv6)
     |
     +-- db01  ::d:01 (IPv6)
     |   [..]
     +-- db99  ::d:99 (IPv6)



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                                 Figure 7

Appendix B.  Comparison to Other Deployment Approaches

   There are a number of alternative deployment strategies a data centre
   operator may follow.  They each have different properties and helps
   solve a different set of challenges.  This section aims to compare
   the SIIT-DC approach with each of the most common ones, by
   highlighting the benefits and disadvantages of each.

B.1.  IPv4-only

   At the time of writing, IPv4-only operation remains the status quo
   for most operators.  As such, it is well understood and supported.
   An operator can reasonably expect everything to work correctly in an
   IPv4-only environment.

   Benefits of IPv4-only operation compared to SIIT-DC include:

   o  No translation occurs, the end-to-end principle is intact.

   o  Compatible with all common application protocols.

   o  Compatible with IPv4-only devices.

   o  Compatible with IPv4-only application software, without requiring
      a host agent.

   Disadvantages of IPv4-only operation compared to SIIT-DC include:

   o  Does not provide any form of IPv6 connectivity.

   o  Does not alleviate IPv4 address scarcity.

B.2.  IPv4-only + NAPT44

   An operator who would otherwise chose a traditional IPv4-only
   approach, but cannot due to having insufficient public IPv4 addresses
   available, could chose to deploy using a combination of private IPv4
   addresses [RFC1918] and NAPT44 [RFC3022] devices which will translate
   between a smaller number of public IPv4 addresses and the private
   addresses assigned to the servers that provide public services to the
   Internet.

   Benefits of IPv4-only + NAPT44 operation compared to SIIT-DC include:

   o  Compatible with IPv4-only devices.




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   o  Compatible with IPv4-only application software, without requiring
      a host agent.

   Disadvantages of IPv4-only + NAPT44 operation compared to SIIT-DC
   include:

   o  Does not provide any form of IPv6 availability.

   o  Requires network devices that track all flow state, which may
      create a performance bottleneck and be an easy target for Denial
      of Service attacks.

   o  Limits routing flexibility (prevents closest exit routing), as
      outbound traffic must pass across the same NAPT44 device that
      handled the inbound traffic.

   o  Limited potential for horizontal scaling, as packets cannot be
      load-balanced across multiple NAT devices.

   o  Depending on whether or not the NAPT44 device rewrites source
      addresses in order to attract the return traffic to itself:

   o

      *  Obscures the true source address of the user from the server/
         application, preventing it from e.g. performing geo-location
         lookups, or:

      *  Requires an IPv4 default route to be pointed to the NAPT44
         device, also attracting native traffic that does not need to
         undergo translation.

   In addition, application compatibility is a consideration with both
   NAPT44 and SIIT-DC, but the exact nature depends from application to
   application, so it is hard to objectively quantify if there is a
   clear advantage to either approach here.  Some translation-unfriendly
   application protocols may work without host modifications through the
   use of Application Layer Gateway support in the NAPT44 device (e.g.,
   FTP [RFC0959]), or in the SIIT-DC architecture when a host agent is
   being used [I-D.anderson-v6ops-siit-dc-2xlat].  Other application
   protocols might not work with NAPT44 at all, but will work in the
   SIIT-DC if a host agent is being used (e.g., FTP/TLS [RFC4217]).

   In summary, the most accurate statement would be to say that an
   NAPT44 architecture is more compatible with translation-unfriendly
   protocols than plain SIIT-DC, while SIIT-DC is more compatible than
   NAPT44 if a host agent is used.




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   For a more complete discussion of potential issues with running
   NAPT44, see Architectural Implications of NAT [RFC2993].

B.3.  IPv4-only + NAT64

   An operator who would otherwise chose a traditional IPv4-only
   approach, but would in addition like to provide service availability
   for IPv6 end users, could use Stateful NAT64 [RFC6146] to accomplish
   this.  In a sense, this would be the mirror image of an SIIT-DC
   architecture: The infrastructure and servers remains single-stacked,
   while connectivity to the other IP stack is provided through a
   translation system.  Further information about operating Stateful
   NAT64 is found in [RFC7269].

   Note that Stateful NAT64 can be deployed with or without NAPT44.
   With the exception that IPv6 service availability is being provided,
   the discussion in the previous two sections fully applies to an
   IPv4-only environment that includes NAT64.

   Benefits of IPv4-only + NAT64 operation compared to SIIT-DC include:

   o  Compatible with IPv4-only devices.

   o  Compatible with IPv4-only application software, without requiring
      a host agent.

   Disadvantages of IPv4-only + NAT64 operation compared to SIIT-DC
   include:

   o  Does not alleviate IPv4 address scarcity (assuming NAPT44 isn't
      used).

   o  Requires network devices that track all flow state, which may
      create a performance bottleneck and be an easy target for Denial
      of Service attacks.

   o  Limits routing flexibility (prevents closest exit routing), as
      outbound traffic must pass across the same NAT64 device that
      handled the inbound traffic.

   o  Limited potential for horizontal scaling, as packets cannot be
      load-balanced across multiple NAT devices.

   o  Obscures the true source address of the user from the server/
      application, preventing it from e.g. performing geo-location
      lookups.





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   o  The traffic levels on the Stateful NAT64 routers will increase
      over time, in lockstep with the increased deployment of IPv6 in
      the Internet.  For this reason, Section 3.2 of RFC7269 [RFC7269]
      notes that the use of Stateful NAT64 in a data centre environment
      "is only reasonable at an early stage".  With SIIT-DC, the inverse
      is true; the traffic levels on the SIIT-DC Gateways will decrease
      over time, as end users will prefer to use native IPv6 once it is
      available to them.

B.4.  Dual Stack

   Dual Stack [RFC4213] [RFC6883] could be used both with or without
   NAPT44 to handle IPv4.  In general, the benefits and disadvantages
   are equal to the corresponding IPv4-only option, except for the fact
   that Dual Stack does provides IPv6 connectivity.  Therefore, his
   section only lists the benefits and disadvantages which are unique to
   a Dual Stack environment.

   Benefits of Dual Stack operation compared to SIIT-DC include:

   o  No translation occurring, the end-to-end principle is intact
      (assuming NAPT44 isn't used).

   o  Compatible with all common application protocols (assuming NAPT44
      isn't used).

   o  Compatible with IPv4-only devices.

   o  Compatible with IPv4-only application software, without requiring
      a host agent.

   Disadvantages of Dual Stack operation compared to SIIT-DC include:

   o  Does not alleviate IPv4 address scarcity (assuming NAPT44 isn't
      used).

   o  Increases the complexity of the infrastructure, as many things
      must done twice (once for IPv4 and once for IPv6).  Examples of
      things that must be duplicated in this manner under Dual Stack
      include: Firewall rules/ACLs, IGP topology, monitoring,
      troubleshooting.

   o  Encourages software developers, systems administrators, etc. to
      build architectures that cannot operate correctly without IPv4.
      This in turn makes it difficult to make use of Dual Stack as a
      short term transitional stage, rather than a near-permanent end
      state.




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   o  Increases the amount of things that can encounter failures, and
      increases the time required to locate and fix such failures.  This
      reduces reliability.

B.5.  Partial Dual Stack (IPv6-only back-end)

   It is possible to use the Dual Stack deployment strategy for front-
   end services only.  That is, the front-end servers (or load
   balancers) that serves public Internet-available services are
   provisioned with both native IPv4 and native IPv6 connectivity on
   their Internet-facing interfaces, while the interfaces facing the
   back-end infrastructure are IPv6 only.  All back-end servers that do
   not communicate directly with Internet clients are IPv6-only.  All
   communication between back-end servers as well as all traffic between
   the back-end servers and the front-end servers will therefore use
   only IPv6.

   One variation of this approach is to have a two separate sets of
   front-end servers, where one set has IPv4-only Internet-facing
   interfaces, while the other set has IPv6-only Internet-facing
   interfaces.  However, both sets must have IPv6-only interfaces facing
   the back-end infrastructure.

   Benefits of Partial Dual Stack operation compared to SIIT-DC include:

   o  No translation occurring, the end-to-end principle is intact.

   o  Compatible with all common application protocols.

   o  Compatible with IPv4-only devices (front-end only).

   o  Compatible with IPv4-only application software (front-end only).

   Disadvantages of Partial Dual Stack operation compared to SIIT-DC
   include:

   o  Increases the complexity of the front-end infrastructure, as many
      things must done twice (once for IPv4 and once for IPv6).
      Examples of things that must be duplicated in this manner under
      Partial Dual Stack include: Firewall rules/ACLs, IGP topology,
      monitoring, troubleshooting.

   o  Can not support any IPv4-only devices or application software in
      the back-end infrastructure.

   In addition, Partial Dual Stack will alleviate IPv4 address scarcity
   compared to the normal Dual Stack approach, but not quite to the same
   extent as SIIT-DC.  This is primarily due to the data centre network



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   infrastructure having to be dual-stacked in order to provide native
   IPv4 addressing to the front-end servers, and because the front-end
   server LANs must be rounded up in size to the nearest CIDR boundary
   which may result in IPv4 addresses being unused.  However, depending
   on the exact circumstances, this difference in IPv4 address
   consumption between SIIT-DC and Partial Dual Stack may be negligible.

Author's Address

   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

































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