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RFC 7755

Informational
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SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Center Environments

 


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Internet Engineering Task Force (IETF)                       T. Anderson
Request for Comments: 7755                                Redpill Linpro
Category: Informational                                    February 2016
ISSN: 2070-1721


SIIT-DC: Stateless IP/ICMP Translation for IPv6 Data Center Environments

Abstract

   This document describes the use of the Stateless IP/ICMP Translation
   Algorithm (SIIT) in an IPv6 Internet Data Center (IDC).  In this
   deployment model, traffic from legacy IPv4-only clients on the
   Internet is translated to IPv6 upon reaching the IDC operator's
   network infrastructure.  From that point on, it may be treated the
   same as traffic from native IPv6 end users.  The IPv6 endpoints may
   be numbered using arbitrary (non-IPv4-translatable) IPv6 addresses.
   This facilitates a single-stack IPv6-only network infrastructure, as
   well as efficient utilization of public IPv4 addresses.

   The primary audience is IDC operators who are deploying IPv6, running
   out of available IPv4 addresses, and/or feeling that dual stack
   causes undesirable operational complexity.

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/rfc7755.

Page 2 
Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Single-Stack IPv6 Operation . . . . . . . . . . . . . . .   3
     1.2.  Stateless Operation . . . . . . . . . . . . . . . . . . .   4
     1.3.  IPv4 Address Conservation . . . . . . . . . . . . . . . .   4
     1.4.  Clients' IPv4 Source Addresses Visible to Applications  .   5
     1.5.  Compatible with Standard IPv4 and IPv6 Stacks . . . . . .   5
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Architectural Overview  . . . . . . . . . . . . . . . . . . .   8
     3.1.  Packet Flow . . . . . . . . . . . . . . . . . . . . . . .   9
   4.  Deployment Considerations and Guidelines  . . . . . . . . . .  10
     4.1.  Application/Device Support for IPv6 . . . . . . . . . . .  10
     4.2.  Application Support for NAT . . . . . . . . . . . . . . .  10
     4.3.  Application Communication Pattern . . . . . . . . . . . .  10
     4.4.  Choice of Translation Prefix  . . . . . . . . . . . . . .  11
     4.5.  Routing Considerations  . . . . . . . . . . . . . . . . .  12
     4.6.  Location of the SIIT-DC Border Relays . . . . . . . . . .  12
     4.7.  Migration from Dual Stack . . . . . . . . . . . . . . . .  13
     4.8.  Translation of ICMPv6 Errors to IPv4  . . . . . . . . . .  13
     4.9.  MTU and Fragmentation . . . . . . . . . . . . . . . . . .  13
       4.9.1.  IPv4/IPv6 Header Size Difference  . . . . . . . . . .  14
       4.9.2.  IPv6 Atomic Fragments . . . . . . . . . . . . . . . .  14
       4.9.3.  Minimum Path MTU Difference between IPv4 and IPv6 . .  15
     4.10. IPv4-Translatable IPv6 Service Addresses  . . . . . . . .  16
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     5.1.  Mistaking the Translation Prefix for a Trusted Network  .  17
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Complete SIIT-DC IDC Topology Example  . . . . . . .  21
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  24
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

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1.  Introduction

   Historically, dual stack [RFC4213] [RFC6883] has been the recommended
   way to transition from a legacy IPv4-only environment to one capable
   of serving IPv6 users.  However, for IDC operators, 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 little
   return on investment for as long as large parts of the public
   Internet remains predominantly IPv4 only.  Furthermore, the dual-
   stack approach does not in any way help with the depletion of the
   IPv4 address space, which at the time of writing is a pressing
   concern in most parts of the world.

   Therefore, some IDC operators may instead prefer an approach in which
   they only need to operate one protocol in the data center as they
   prepare for the future.  Stateless IP/ICMP Translation for IPv6 Data
   Center Environments (SIIT-DC) is one such approach.  Its design goals
   include:

   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  Ensure that the legacy users' IPv4 addresses remain visible to the
      nodes and applications located in the IPv6 network.

   o  Conserve and maximize the utilization of the operator's public
      IPv4 addresses.

   o  Avoid introducing more complexity than absolutely necessary,
      especially on the nodes and applications.

   o  Easy to scale and deploy in a fault-tolerant manner.

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

1.1.  Single-Stack IPv6 Operation

   SIIT-DC allows IDC operators to build their infrastructure and
   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.  This promotes universal IPv6 deployment for
   the IDC operator's services and applications.

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   SIIT-DC requires no special support or change from the underlying
   IPv6 infrastructure; it is compatible with all standard IPv6
   networks.  Traffic between IPv6-enabled end users and IPv6-enabled
   services will always be transported native end to end; SIIT-DC does
   not intercept or handle native IPv6 traffic at all.

   When the day comes to discontinue all support for IPv4, no change
   needs to be made to the overall architecture -- it's only a matter of
   shutting off the SIIT-DC Border Relays (BRs).  Operators who deploy
   native IPv6 along with SIIT-DC will thus avoid requiring any future
   migration or deployment projects relating to IPv6 deployment and/or
   IPv4 sunsetting.

1.2.  Stateless Operation

   Unlike other solutions that provide either dual-stack availability to
   single-stack services (e.g., Stateful Network Address and Protocol
   Translation from IPv6 Clients to IPv4 Servers (NAT64) [RFC6146] and
   Layer 4/7 proxies) or conservation of IPv4 addresses (e.g., IPv4
   address translation (NAPT44) [RFC3022]), SIIT-DC does not maintain
   any state associated with individual connections or flows.  In this
   sense, it operates exactly like a regular 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 BRs to easily attain "line-rate"
   performance, but it also allows for per-packet load balancing between
   multiple BRs using Equal-Cost Multipath Routing [RFC2991].
   Asymmetric routing is also acceptable, which makes it easy to avoid
   suboptimal traffic patterns; the prefixes involved may be anycasted
   from all the BRs 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 a BR should fail, its traffic can be rerouted onto
   another BR using a standard IP routing protocol.  This does not
   impact existing flows any more than what any other IP rerouting event
   would.

1.3.  IPv4 Address Conservation

   In most parts of the world, it is difficult or even impossible to
   obtain generously sized IPv4 delegations from the Internet Numbers
   Registry System [RFC7020].  The resulting scarcity in turn impacts
   individual end users and operators, whom might be forced to purchase

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   IPv4 addresses from other operators in order to cover their needs.
   This process can be risky to business continuity, in the case where
   no suitable block for sale can be located, and/or turn out to be
   prohibitively expensive.  In spite of this, an IDC operator will find
   that providing IPv4 service remains essential, as a large share of
   the Internet end users still do not have IPv6 connectivity.

   A key goal of SIIT-DC is to help reduce a data center operator's IPv4
   address requirement to the absolute minimum by allowing the operator
   to remove them entirely from nodes and applications that do not need
   to communicate with endpoints in the IPv4 Internet.  One example
   would be servers that are operating in a supporting/backend role and
   only communicating with 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 backend servers and network infrastructure and reassign them
   to the SIIT-DC service as IPv4 Service Addresses.  There exists no
   requirement that IPv4 Service Addresses are to be 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
   as an IPv4 Service Address.

1.4.  Clients' IPv4 Source Addresses Visible to Applications

   SIIT-DC uses the [RFC6052] algorithm to map the entire end-user's
   IPv4 source address into a predefined IPv6 translation prefix.  This
   ensures that there is no loss of information; the end-user's IPv4
   source address remains available to the application located in the
   IPv6 network, allowing it to perform tasks like geolocation, logging,
   abuse handling, and so forth.

1.5.  Compatible with Standard IPv4 and IPv6 Stacks

   Except for the introduction of the BRs themselves, no change to the
   network, nodes, applications, or anything else is required in order
   to support SIIT-DC.  SIIT-DC is practically invisible from the point
   of view of the IPv4 clients, the IPv6 nodes, the IPv6 data center
   network, and the IPv4 Internet.  SIIT-DC interoperates with all
   standards-compliant IPv4 or IPv6 stacks.

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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
      IDC's outer network border.  The ER is an optional component of
      SIIT-DC.  It is discussed in more detail in [RFC7756].

   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.

   IPv4 Service Address Pool:
      One or more IPv4 prefixes routed to the BR's IPv4 interface.  IPv4
      Service Addresses are allocated from this pool.  This does not
      necessarily have to be a "pool" per se, as it could also be one or
      more host routes (whose prefix lengths are equal to /32).  The
      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:
      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].

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   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-Translatable 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].

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3.  Architectural Overview

   This section describes the basic SIIT-DC architecture.

              IPv6-capable user      IPv4-only user
              <2001:db8::ab:cd>      <203.0.113.50>
                |                          |
             (the IPv6 Internet)   (the IPv4 Internet)
                |                          |
                |  +-[BR]---------<192.0.2.0/24>--------------+
                |  |                                          |
                |  | EAM #1:      192.0.2.1,2001:db8:12:34::1 |
                |  | EAM #2..#n:  [...]                       |
                |  | XLAT Prefix: 2001:db8:46::/96            |
                |  |                                          |
                |  +------------<2001:db8:46::/96>------------+
                |                        |
               (the IPv6-only data center network)
                           |
            +--<2001:db8:12:34::1>--[v6-only server]-+
            |     |                                  |
            | +-[2001:db8:12:34::1]--[v6-only app]-+ |
            | |   AF_INET6 socket                  | |
            | +------------------------------------+ |
            +----------------------------------------+

                      Figure 1: SIIT-DC Architecture

   In Figure 1, 192.0.2.0/24 is the IPv4 Service Address Pool.
   Individual IPv4 Service Addresses are assigned from this prefix, and
   traffic destined for it is routed to the BR's IPv4-facing network
   interface.  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 BR's IPv4-facing network interface.

   When translating packets between IPv4 and IPv6, the BR uses EAM #1 to
   replace any occurrence of the IPv4 Service Address (192.0.2.1) with
   its corresponding IPv6 Service Address (2001:db8:12:34::1).
   Addresses that do not match any EAM configured in the BR are
   translated by inserting or removing the translation prefix
   (2001:db8:46::/96); cf. Section 2.2 of [RFC6052].

   The BR can be deployed as a separate device or as a logical function
   in another multipurpose device, such as an IP router.  Any number of
   BRs may exist simultaneously in the IDC's network infrastructure, as
   long as they are all configured with the same translation prefix and
   an identical EAM Table.

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   The IPv6 Service Address 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 ensures that IPv6-capable
   clients access the application/service directly using native IPv6 end
   to end, while IP4-only clients will access it through SIIT-DC.

3.1.  Packet Flow

   In this example, the "IPv4-only user" from Figure 1 initiates a
   connection to the application running on the IPv6-only server.  After
   first having looked up the "IN A" record in DNS, the user starts by
   transmitting a TCP SYN packet to the IPv4 Service Address.  This IPv4
   packet is routed to the BR and is there translated to IPv6 as
   follows:

        +--[IPv4]----------+     +--[IPv6]-----------------------+
        | SRC 203.0.113.50 |     | SRC 2001:db8:46::203.0.113.50 |
        | DST 192.0.2.1    | --> | DST 2001:db8:12:34::1         |
        | TCP SYN [..]     |     | TCP SYN [..]                  |
        +------------------+     +-------------------------------+

                    Figure 2: IPv4-to-IPv6 Translation

   The resulting IPv6 packet is routed to the IPv6-only server, which
   processes and responds to it as if it had been a native IPv6 packet
   all along.  The server's IPv6 response packet is then routed back to
   the BR, where it is translated back to IPv4 as follows:

        +--[IPv6]-----------------------+      +--[IPv4]----------+
        | SRC 2001:db8:12:34::1         |      | SRC 192.0.2.1    |
        | DST 2001:db8:46::203.0.113.50 | -->  | DST 203.0.113.50 |
        | TCP SYN/ACK [..]              |      | TCP SYN/ACK [..] |
        +-------------------------------+      +------------------+

                    Figure 3: IPv6-to-IPv4 Translation

   It is important to note that neither the IPv4 client nor the IPv6
   server/application need any special support to participate in
   SIIT-DC.  However, the application may optionally be taught to
   extract the embedded IPv4 source address from incoming IPv6 packets
   with source addresses within the translation prefix.  This will allow
   it to perform IPv4-specific tasks such as geolocation, logging, abuse
   handling, and so on.

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4.  Deployment Considerations and Guidelines

4.1.  Application/Device Support for IPv6

   SIIT-DC as described in this document requires that the application
   (and/or the node the application is located on) supports IPv6
   networking and that it has no dependency on local IPv4 network
   connectivity.

   SIIT-DC can, however, support legacy IPv4-dependent applications and
   nodes through the introduction of an ER.  The ER provides the legacy
   application or node with seemingly native IPv4 Internet connectivity,
   so that it may operate correctly in an otherwise IPv6-only network
   environment.  This approach is described in more detail in [RFC7756].

4.2.  Application Support for NAT

   The operator should carefully examine whether or not the application
   protocols he would like to use SIIT-DC with are able to operate in a
   network environment where rewriting of IP addresses occurs.  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 particularly problematic [RFC2663] [RFC2993] [RFC3022].
   One well-known example of such a protocol is FTP [RFC959].  Such
   protocols can be made to work with SIIT-DC through the introduction
   of an ER, which provides end-to-end IPv4 address transparency by
   reversing the translations performed by the BR before passing the
   packets to the NAT-incompatible application.  This approach is
   described in more detail in [RFC7756].

4.3.  Application Communication Pattern

   SIIT-DC is best suited for traditional client/server applications
   where IPv4-only clients on the Internet initiate traffic towards an
   IPv6-only service, which in turn is passively listening for inbound
   traffic and responding as necessary.  In this case, an IPv4 client
   looks exactly like a native IPv6 client from the IPv6 service's point
   of view and thus does not require any special treatment.  One
   particularly common application protocol that follows this client/
   server communication pattern, and thus is ideally suited for use with
   SIIT-DC, is HTTP [RFC7230].

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   It is also possible to combine SIIT-DC with DNS64 [RFC6147] in order
   to allow an IPv6-only application to initiate communication with
   IPv4-only nodes through SIIT-DC.  However, in this case, care must be
   taken so that all outgoing communication is sourced from an IPv6
   Service Address that is found in an EAM configured in the BR.  If
   another address is used, the BR will most likely be unable to
   translate it to IPv4, causing the packet to be discarded.  This could
   be prevented by altering the Default Address Selection Policy
   Table [RFC6724] on the IPv6 node.

   An alternative approach to the above would be to place an ER in front
   of the application in question, as described in [RFC7756].  This
   provides the application with seemingly native IPv4 connectivity,
   which it may use freely for bidirectional communication with the IPv4
   Internet.  An application or node located behind an ER does not need
   to worry about selecting a specific source address, as it will only
   have valid options available.

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 (WKP)
   64:ff9b::/96 may be used.  From a technical point of view, both work
   equally well.  However, only a single WKP exists, so if a provider
   would like to deploy more than one instance of SIIT-DC in his
   network, or another translation technology such as Stateful NAT64
   [RFC6146], the operator will be forced to use an NSP for all but one
   of those deployments.

   Another consideration is that the WKP cannot be used in inter-domain
   routing.  By using an NSP instead, SIIT-DC will support a deployment
   where the BR and the IPv6 Service Address are located in different
   Autonomous Systems.

   The translation prefix may use any of the lengths described in
   Section 2.2 of [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]), instead of being converted to hexadecimal notation.  This
   makes it easier to write literal IPv6 addresses (e.g., in ACLs) that
   correspond to translated endpoints in the IPv4 Internet.

   For the reasons discussed above, this document recommends that an NSP
   with a prefix length of /96 be used.  Section 3.3 of [RFC6052]
   discusses the choice of the translation prefix in more detail.

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4.5.  Routing Considerations

   The prefixes that constitute the IPv4 Service Address Pool and the
   IPv6 translation prefix may be routed to the BRs like any other IPv4
   or IPv6 route in the provider's network.  If more than one BR is
   being deployed, it is recommended that a routing protocol (IGP) be
   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 BR, reducing or eliminating suboptimal traffic patterns as
   well as providing high availability: should one BR fail, the IGP will
   automatically redirect the traffic to the closest alternate BR.

4.6.  Location of the SIIT-DC Border Relays

   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 BRs and the network infrastructure required to
   connect the BRs to the IPv4 Internet.  Therefore, the BRs must be
   located somewhere between the IPv4 Internet and the application
   delivery stack, which includes 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 BRs as close as possible to the direct
   path between the location of the IPv6 Service Address and the end
   users.  If the closest BR was located a long way from the direct
   path, all packets in both directions must make a detour in order to
   traverse the BR.  This would increase the RTT between the service and
   the end user 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 BRs as a logical
   function within the routers that also handle the native IPv6 traffic
   between the IPv6 Service Address and the IPv6 Internet.  This way, an
   SIIT-DC deployment does not require separate networks ports (which
   might become saturated and impact the service quality) nor will it
   require extra rack space and energy.  Some particularly good choices
   for the location could be within the IDC's access routers or within
   the Autonomous System's border routers.

   Finally, another possibility is that the IDC operator outsources the
   SIIT-DC service to another entity, for example, his upstream ISP.
   Doing so allows the IDC operator to build a true IPv6-only
   infrastructure.

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4.7.  Migration from Dual Stack

   While this document mainly discusses the use of IPv6-only nodes and
   applications, it is important to note that SIIT-DC is fully
   compatible with dual-stack infrastructures, including dual-stack
   nodes and applications.

   Thus, migrating a dual-stacked service to an IPv6-only one where
   SIIT-DC provides the IPv4 Internet connectivity is easy.  The
   operator would start out by designating the service's current native
   IPv6 address as the IPv6 Service Address and assigning it a
   corresponding IPv4 Service Address.  At this point, the service will
   respond on both its old (native) IPv4 address and the SIIT-DC IPv4
   Service Address.  The operator may now move traffic from the former
   to the latter by changing the service's "IN A" DNS record.  Once all
   IPv4 traffic has been successfully moved to SIIT-DC, the old IPv4
   address may be reclaimed.

4.8.  Translation of ICMPv6 Errors to IPv4

   In response to an IPv4 packet subsequently translated to IPv6 by the
   BR, an IPv6 router in the IDC network may need to transmit an ICMPv6
   error back to the origin IPv4 node.  By default, such an ICMPv6 error
   will most likely be discarded by the BR, unless the source address of
   the ICMPv6 error happens to be an IPv4-translatable IPv6 address or
   covered by an EAM.

   To facilitate reliable delivery of such ICMPv6 errors, an SIIT-DC
   operator SHOULD implement the recommendations in [RFC6791] in the
   BRs.

4.9.  MTU and Fragmentation

   There are some key differences between IPv4 and IPv6 relating to
   packet sizes and fragmentation that one MUST 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
   [FRAGMENTS].  For this reason, the recommendations in this section
   seek to minimize the use of IPv6 fragmentation.

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

Top      ToC       Page 14 
4.9.1.  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; the IPv6 node will advertise a TCP Maximum Segment Size
   (MSS) of 1440 bytes during the initial TCP handshake.  This causes
   the IPv4 clients 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 Don't
   Fragment (DF) flag cleared will need to be fragmented by the BR.
   This may be avoided by increasing the Path MTU between the BR and the
   IPv6 nodes to 1520 bytes or greater.  If this is done, the MTU on the
   IPv6 nodes themselves SHOULD NOT be increased accordingly, as doing
   so would cause them to undergo Path MTU Discovery for all
   destinations on the IPv6 Internet.  The nodes MUST, however, be able
   to accept and process incoming packets larger than their own MTU.  If
   the nodes' IPv6 implementation allows the initial Path MTU to be set
   differently for specific destinations, it MAY be increased to 1520
   for destinations within the translation prefix specifically.

4.9.2.  IPv6 Atomic Fragments

   In keeping with the fifth paragraph of Section 4 of [RFC6145], a
   stateless translator like a BR will by default add an IPv6
   Fragmentation header to the resulting IPv6 packet when translating an
   IPv4 packet with the DF 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 an IDC environment, and it is
   therefore recommended that this behavior be disabled in the BRs.  To
   this end, Section 4 of [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."

   Note that work is currently in progress (in [RFC6145bis]) to
   deprecate IPv6 Atomic Fragments.  As a result, a BR that conforms to
   that document is required to behave as recommended above.

   In IPv6, the Identification value is located inside the Fragmentation
   header.  That means that if the generation of IPv6 Atomic Fragments

Top      ToC       Page 15 
   is disabled, the IPv4 Identification value will be lost during
   translation to IPv6.  This could potentially confuse some diagnostic
   tools.

4.9.3.  Minimum Path MTU Difference between IPv4 and IPv6

   Section 5 of [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 might prove false if the destination is an IPv4 node
   reached through a protocol translator such as a BR, as the minimum
   IPv4 link MTU is 68 bytes.  See Section 3.2 of [RFC791].

   Section 5.1 of [RFC6145] specifies that a stateless translator 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 4.9.1), 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 turn is translated by
   the BR 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, it is not allowed to reduce its Path
   MTU estimation to the indicated value.  It must instead include a
   Fragmentation header in subsequent packets sent on that path
   [RFC1981].  In other words, the IPv6 node will start emitting Atomic
   Fragments.  The Fragmentation header signals to the BR 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, the
   operator should consider enabling the functionality described as the
   "second approach" in Section 6 of [RFC6145].  This functionality
   changes the BR's behavior 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.

Top      ToC       Page 16 
   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].

   Note that work is currently in progress (in [RFC6145bis]) to
   deprecate IPv6 Atomic Fragments.  As a result, a BR that conforms to
   that document is required to behave as suggested above.

4.10.  IPv4-Translatable IPv6 Service Addresses

   SIIT-DC is designed so that the IPv6 Service Addresses are not
   required to be IPv4-translatable IPv6 addresses.  Section 2 of
   [RFC7757] discusses why it is desirable to avoid requiring the use of
   IPv4-translatable IPv6 addresses.

   It is, however, quite possible to deploy SIIT-DC in combination with
   IPv4-translatable IPv6 Service Addresses.  The primary benefits in
   doing so are:

   o  The operator is not required to provision EAMs for
      IPv4-translatable IPv6 Service Addresses onto the BR/ERs.

   o  [RFC6145] translation can be performed in a checksum-neutral
      manner; cf. Section 4.1 of [RFC6052].

   The trade-off is that the IPv4-translatable IPv6 Service Addresses
   must be configured on the IPv6 nodes, and the applications must be
   set up to use them -- likely in addition to their primary
   (non-IPv4-translatable) IPv6 addresses.  The IPv4-translatable IPv6
   Service Addresses must also be routed from the BR through the IDC's
   IPv6 network infrastructure to the nodes on which they are assigned.
   This essentially requires the entire IPv6 infrastructure to be made
   aware of and handle translated IPv4 traffic as a special case, which
   significantly increases complexity.  As previously described in
   Section 1.1, avoiding such drawbacks is a design goal of SIIT-DC.
   The use of IPv4-translatable IPv6 Service Addresses is therefore
   discouraged.

Top      ToC       Page 17 
5.  Security Considerations

5.1.  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 recommended in Section 4.4,
   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 traffic
   from translated IPv4 end users on the Internet might appear to be
   originating from the provider's own network.  It is therefore
   important that the translation prefix be treated the same as the
   Internet at large rather than as a trusted network.

   In order to alleviate this problem, the operator may opt to use a
   translation prefix that is distinct from and not a subset of the IPv6
   prefixes used elsewhere in the network infrastructure.

6.  References

6.1.  Normative References

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

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

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

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

   [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>.

Top      ToC       Page 18 
6.2.  Informative References

   [FRAGMENTS]
              Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
              M., and T. Taylor, "Why Operators Filter Fragments and
              What It Implies", Work in Progress, draft-taylor-v6ops-
              fragdrop-02, December 2013.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

   [RFC959]   Postel, J. and J. Reynolds, "File Transfer Protocol",
              STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985,
              <http://www.rfc-editor.org/info/rfc959>.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
              December 1998, <http://www.rfc-editor.org/info/rfc2460>.

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, DOI 10.17487/RFC2663, August 1999,
              <http://www.rfc-editor.org/info/rfc2663>.

   [RFC2991]  Thaler, D. and C. Hopps, "Multipath Issues in Unicast and
              Multicast Next-Hop Selection", RFC 2991,
              DOI 10.17487/RFC2991, November 2000,
              <http://www.rfc-editor.org/info/rfc2991>.

   [RFC2993]  Hain, T., "Architectural Implications of NAT", RFC 2993,
              DOI 10.17487/RFC2993, November 2000,
              <http://www.rfc-editor.org/info/rfc2993>.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              DOI 10.17487/RFC3022, January 2001,
              <http://www.rfc-editor.org/info/rfc3022>.

   [RFC3235]  Senie, D., "Network Address Translator (NAT)-Friendly
              Application Design Guidelines", RFC 3235,
              DOI 10.17487/RFC3235, January 2002,
              <http://www.rfc-editor.org/info/rfc3235>.

Top      ToC       Page 19 
   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213,
              DOI 10.17487/RFC4213, October 2005,
              <http://www.rfc-editor.org/info/rfc4213>.

   [RFC6145bis]
              Bao, C., Li, X., Baker, F., Anderson, T., and F. Gont,
              "IP/ICMP Translation Algorithm (rfc6145bis)", Work in
              Progress, draft-bao-v6ops-rfc6145bis-05, January 2016.

   [RFC6146]  Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
              NAT64: Network Address and Protocol Translation from IPv6
              Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146,
              April 2011, <http://www.rfc-editor.org/info/rfc6146>.

   [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,
              DOI 10.17487/RFC6147, April 2011,
              <http://www.rfc-editor.org/info/rfc6147>.

   [RFC6540]  George, W., Donley, C., Liljenstolpe, C., and L. Howard,
              "IPv6 Support Required for All IP-Capable Nodes", BCP 177,
              RFC 6540, DOI 10.17487/RFC6540, April 2012,
              <http://www.rfc-editor.org/info/rfc6540>.

   [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>.

   [RFC6883]  Carpenter, B. and S. Jiang, "IPv6 Guidance for Internet
              Content Providers and Application Service Providers",
              RFC 6883, DOI 10.17487/RFC6883, March 2013,
              <http://www.rfc-editor.org/info/rfc6883>.

   [RFC6946]  Gont, F., "Processing of IPv6 "Atomic" Fragments",
              RFC 6946, DOI 10.17487/RFC6946, May 2013,
              <http://www.rfc-editor.org/info/rfc6946>.

   [RFC7020]  Housley, R., Curran, J., Huston, G., and D. Conrad, "The
              Internet Numbers Registry System", RFC 7020,
              DOI 10.17487/RFC7020, August 2013,
              <http://www.rfc-editor.org/info/rfc7020>.

Top      ToC       Page 20 
   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7756]  Anderson, T. and S. Steffann, "Stateless IP/ICMP
              Translation for IPv6 Internet Data Center Environments
              (SIIT-DC): Dual Translation Mode", RFC 7756,
              DOI 10.17487/RFC7756, February 2016,
              <http://www.rfc-editor.org/info/rfc7756>.

Top      ToC       Page 21 
Appendix A.  Complete SIIT-DC IDC Topology Example

   Figure 4 attempts to "tie it all together" and show a more complete
   SIIT-DC topology, in order to better demonstrate its advantageous
   properties discussed in Section 1.  These are discussed in more
   detail below.

Top      ToC       Page 22 
                  /--------------------------------\ /---------------\
                  |          IPv4 Internet         | | IPv6 Internet |
                  \-+----------------------------+-/ \--------+------/
                    |                            |            |
                    | <----------[BGP]---------> |          [BGP]
                    |                            |            |
     +-------<192.0.2.0/24>---------+ +---<192.0.2.0/24>---+  |
     |                        BR #1 | | BR #2              |  |
     | EAM Table:                   | |                    |  |
     | ==========                   | |                    |  |
     | 192.0.2.1,2001:db8:12:34::1  | |                    |  |
     | 192.0.2.2,2001:db8:12:34::2  | |  Exactly the same  |  |
     | 192.0.2.3,2001:db8:fe:dc::1  | |  configuration as  |  |
     | 192.0.2.4,2001:db8:12:34::4  | |  BR #1             |  |
     | 192.0.2.5,2001:db8:fe:dc::e  | |                    |  |
     |                              | |                    |  |
     | XLAT Prefix 2001:db8:46::/96 | |                    |  |
     |                              | |                    |  |
     +--------<2001:db8:46::/96>----+ +-<2001:db8:46::/96>-+  |
                       |                      |               |
                       | <------[ECMP]------> |               |
                       |                      |               |
     /-----------------+----------------------+--\            |
     |         IPv6 IDC network w/ OSPFv3        +------------/
     \-+--------------------------------+--------/
       |                                |
       | Tenant A's server LAN          | Tenant B's server LAN
       | 2001:db8:12:34::/64            | 2001:db8:fe:dc::/64
       |                                |
       +-- www      ::1 (IPv6+SIIT-DC)  +-- www-lb ::1 (IPv6+SIIT-DC)
       |                                |
       +-- mta      ::2 (IPv6+SIIT-DC)  +-- web ::80:01 (IPv6 only)
       |                                |   [...]
       +-- ftp      ::3 (IPv6)          +-- web ::80:99 (IPv6 only)
       |            ::4 (IPv4, via ER)  |
       |                                |         +----+
       +-- app01 ::a:01 (IPv6 only)     \---- ::e | ER | --\
       |   [...]                                  +----+   |
       +-  app99 ::a:99 (IPv6 only)                        |
       |                                  ftp 192.0.2.5 ---/
       +-- db01  ::d:01 (IPv6 only)
       |   [..]
       \-- db99  ::d:99 (IPv6 only)

                  Figure 4: Example SIIT-DC IDC Topology

Top      ToC       Page 23 
   Single-Stack IPv6 Operation:
      As discussed in Section 1.1, SIIT-DC facilitates an IPv6-only IDC
      network infrastructure.  The only places where IPv4 is absolutely
      required are between the BRs and the IPv4 Internet and between any
      ERs and the IPv4-only applications or devices they are serving
      (illustrated here as the two tenants' FTP servers).  The figure
      also illustrates how SIIT-DC does not interfere with native IPv6;
      when there is no longer a need to support IPv4 clients, the BRs
      may be decommissioned without causing any impact to native IPv6
      traffic.

   Stateless Operation:
      As discussed in Section 1.2, SIIT-DC operates in a stateless
      fashion.  In the illustration, both BRs are simultaneously
      advertising (i.e., anycasting) the IPv4 Service Address Pool and
      the IPv6 translation prefix, so incoming traffic from the IPv4
      Internet may arrive at either of the BRs, while outgoing IPv6
      traffic destined for IPv4 endpoints are load balanced between them
      using Equal-Cost Multipath Routing.  No continuous state
      synchronization between the two BRs occurs.  Should one of the BRs
      fail, the BGP and OSPF protocols will ensure that traffic
      converges on the remaining BR.  Existing sessions will not be
      disrupted beyond any disruption caused by the BGP/OSPF convergence
      process itself.

   IPv4 Address Conservation:
      As discussed in Section 1.3, SIIT-DC conserves the IDC operator's
      IPv4 address space.  Even though the two customers in the example
      above have several hundred servers, the majority of the servers
      are not used for running services made available directly from the
      Internet and therefore do not need to consume IPv4 addresses.  The
      IDC network infrastructure consumes no IPv4 addresses, either.
      Finally, the IPv4 addresses that are assigned to the SIIT-DC
      function as IPv4 Service Address Pools may be assigned with 100%
      efficiency, one address at a time; there is no requirement to
      assign multiple addresses to a single customer in a contiguous
      block.

   Application Support:
      As discussed in Section 1.5, as long as the application protocol
      is translation friendly (illustrated here with HTTP and SMTP), it
      will work with SIIT-DC without requiring any special adaptation.
      Furthermore, translation-unfriendly applications (illustrated here
      with FTP) will also work when located behind an ER [RFC7756].
      Tenant A's FTP server illustrates how an ER may be located in the
      networking stack of a node, while Tenant B's FTP server

Top      ToC       Page 24 
      illustrates how the ER may be deployed as a network service.  The
      latter approach enables SIIT-DC to support IPv4-only
      nodes/devices.

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, Tobias Gondrom,
   Christer Holmberg, Dagfinn Ilmari Mannsaaker, Lars Olafsen, Stig
   Sandbeck Mathisen, Knut A. Syed, Qin Wu, and Andrew Yourtchenko.

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