Softwire WG                                                 M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                               I. Farrer
Expires: April 3, 2017                                  Deutsche Telekom
                                                      September 30, 2016

    Unified IPv4-in-IPv6 Softwire CPE: A DHCPv6-based Prioritization


   In IPv6-only provider networks, transporting IPv4 packets
   encapsulated in IPv6 is a common solution to the problem of IPv4
   service continuity.  A number of differing functional approaches have
   been developed for this, each having their own specific
   characteristics.  As these approaches share a similar functional
   architecture and use the same data plane mechanisms, this memo
   specifies a DHCPv6 option whereby a single CPE can interwork with all
   of the standardized and proposed approaches to providing encapsulated
   IPv4 in IPv6 services by providing a prioritization mechanism.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 3, 2017.

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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
   ( 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Rationale . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  DHCPv6 S46 Priority Option  . . . . . . . . . . . . . . .   4
     1.4.  DHCPv6 Client Behavior  . . . . . . . . . . . . . . . . .   5
     1.5.  DHCPv6 Server Behavior  . . . . . . . . . . . . . . . . .   6
   2.  Operator Deployment Considerations for Deploying     Multiple
       Sotfwire Mechanisms . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Client Address Planning . . . . . . . . . . . . . . . . .   6
     2.2.  Backwards Compatability with Existing Softwire Clients  .   7
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  S46 Mechanisms and their Identifying Option Codes . . . .   8
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   IPv4 service continuity is one of the major technical challenges
   which must be considered during IPv6 migration.  Over the past few
   years, a number of different approaches have been developed to assist
   with this problem (e.g., [RFC6333], [RFC7596], or [RFC7597]).  These
   approaches, referred to as 'S46 mechanisms' in this document, exist
   in order to meet the particular deployment, scaling, addressing and
   other requirements of different service provider's networks.

   A common feature shared between all of the differing modes is the
   integration of softwire tunnel end-point functionality into the

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   Customer Premise Equipment (CPE) router.  Due to this inherent data
   plane similarity, a single CPE may be capable of supporting several
   different approaches.  Users may also wish to configure a specific
   mode of operation.

   A service provider's network may also have more than one S46
   mechanism enabled in order to support a diverse CPE population with
   differing client functionality, such as during a migration between
   mechanisms, or where services require specific supporting softwire

   For softwire based services to be successfully established, it is
   essential that the customer end-node, the service provider end-node
   and provisioning systems are able to indicate their capabilities and
   preferred mode of operation.

   A number of DHCPv6 options for the provisioning of softwires have
   been standardized:

   RFC6334 Defines DHCPv6 option 64 for configuring Basic Bridging
           BroadBand (B4, [RFC6333]) elements with the IPv6 address of
           the Address Family Transition Router (AFTR, [RFC6333]).
   RFC7341 Defines DHCPv6 option 88 for configuring the address of a
           DHCPv4 over DHCPv6 server, which can then be used by a
           softwire client for obtaining further configuration.
   RFC7598 Defines DHCPv6 options 94, 95 and 96 for provisioning Mapping
           of Address and Port with Encapsulation (MAP-E, [RFC7597]),
           Mapping of Address and Port using Translation (MAP-T,
           [RFC7599]), and Lightweight 4over6 [RFC7596] respectively.

   This document describes a DHCPv6 based prioritization method whereby
   a CPE which supports several S46 mechanisms and receives
   configuration for more than one can prioritise which mechanism to
   use.  The method requires no server side logic to be implemented and
   only uses a simple S46 mechanism prioritization to be implemented in
   the CPE.

   The prioritization method as described here does not provide
   redundancy between S46 mechanisms for the client.  I.e.  If the
   highest priority S46 mechanism which has been provisioned to the
   client is not available for any reason, the means for identifying
   this and falling back to the S46 mechanism with the next highest
   priority is not in the scope of this document.

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1.1.  Terminology

   This document makes use of the following terms:

   o  Address Family Transition Router (AFTR): is the IPv4-in-IPv6
      tunnel termination point and the NAT44 function deployed in the
      operator's network [RFC6333].
   o  Border Relay (BR): a MAP-enabled router managed by the service
      provider at the edge of a MAP domain.  A BR has at least an
      IPv6-enabled interface and an IPv4 interface connected to the
      native IPv4 network [RFC7597].
   o  Customer Premise Equipment (CPE): denotes the equipment at the
      customer edge that terminates the customer end of an IPv6
      transitional tunnel.  In some documents (e.g., [RFC7597]), this
      functional entity is called CE (Customer Edge).

1.2.  Rationale

   The following rationale has been adopted for this document:

   (1)  Simplify solution migration paths: Define unified CPE behavior,
        allowing for smooth migration between the different s46
   (2)  Deterministic CPE co-existence behavior: Specify the behavior
        when several S46 mechanisms co-exist in the CPE.
   (3)  Deterministic service provider co-existence behavior: Specify
        the behavior when several modes co-exist in the service
        providers network.
   (4)  Re-usability: Maximize the re-use of existing functional blocks
        including tunnel end-points, port restricted NAPT44, forwarding
        behavior, etc.
   (5)  Solution agnostic: Adopt neutral terminology and avoid (as far
        as possible) overloading the document with solution-specific
   (6)  Flexibility: Allow operators to compile CPE software only for
        the mode(s) necessary for their chosen deployment context(s).
   (7)  Simplicity: Provide a model that allows operators to only
        implement the specific mode(s) that they require without the
        additional complexity of unneeded modes.

1.3.  DHCPv6 S46 Priority Option

   The S46 Priority Option is used to convey a priority order of IPv4
   service continuity mechanisms.  Figure 1 shows the format of the S46
   Priority Option.

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      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     |      OPTION_S46_PRIORITY      |         option-length         |
     |        s46-option-code        |        s46-option-code        |
     |              ...              |        s46-option-code        |

                       Figure 1: S46 Priority Option

   o  option-code: OPTION_S46_PRIORITY (TBD)
   o  option-length: >=2 and a multiple of 2, in octets.
   o  s46-option-code: 16-bits long IANA registered option code of the
      DHCPv6 option which is used to identify the softwire mechanism.
      S46 mechanism are prioritized in the appearance order in the S46
      Priority Option.

   Codes in OPTION_S46_PRIORITY are processed in order; in the event
   that a client receives more than one s46-option-code with a
   particular value, this should be considered as invalid.  DHCP servers
   MAY validate the list of s46-option-code values to detect invalid
   values and duplicates.  The option MUST contain at least one s46-

1.4.  DHCPv6 Client Behavior

   Clients MAY request option OPTION_S46_PRIORITY, as defined in
   [RFC3315], Sections 17.1.1, 18.1.1, 18.1.3, 18.1.4, 18.1.5, and 22.7.
   As a convenience to the reader, we mention here that the client
   includes requested option codes in the Option Request Option.

   Upon receipt of a DHCPv6 Advertise message from the server containing
   OPTION_S46_PRIORITY the client performs the following steps:

   1.  Check the contents of the DHCPv6 message for options containing
       valid S46 mechanism configuration.  A candidate list of possible
       S46 mechanisms is created from these option codes.
   2.  Check the contents of OPTION_S46_PRIORITY for the DHCPv6 option
       codes contained in the included s46-option-code fields.  From
       this, an S46 mechanism priority list is created, ordered from
       highest to lowest following the appearance order.
   3.  Sequentially check the priority list against the candidate list
       until a match is found.
   4.  When a match is found, the client MUST configure the resulting
       S46 mechanism.

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   In the event that no match is found between the priority list and the
   candidate list, the client MAY proceed with configuring one or more
   of the provisioned S46 softwire mechanism(s).  In this case, which
   mechanism(s) are chosen by the client is implementation-specific and
   not defined here.

   If an invalid OPTION_S46_PRIORITY option is received, the client MAY
   proceed with configuring the provisioned S46 mechanisms as if
   OPTION_S46_PRIORITY had not been received.

   If an unknown option code is received in OPTION_S46_PRIORITY option,
   the client MUST skip it and continue processing other listed option
   codes if they exist.  The initial option codes that are allowed to be
   included in a OPTION_S46_PRIORITY option are listed in Section 4.1.

1.5.  DHCPv6 Server Behavior

   Sections 17.2.2 and 18.2 of [RFC3315] govern server operation in
   regards to option assignment.  As a convenience to the reader, we
   mention here that the server will send a particular option code only
   if configured with specific values for that option code and if the
   client requested it.

   Option OPTION_S46_PRIORITY is a singleton.  Servers MUST NOT send
   more than one instance of the OPTION_S46_PRIORITY option.

2.  Operator Deployment Considerations for Deploying Multiple Sotfwire

   The following sub-sections describe some considerations for operators
   who are planning on implementing multiple softwire mechanisms in
   their network (e.g., during a migration between mechanisms).

2.1.  Client Address Planning

   As an operator's available IPv4 resources are likely to be limited,
   it may be desirable to use a common range of IPv4 addresses across
   all of the active Softwire mechanisms.  However, this is likely to
   result in difficulties in routing ingress IPv4 traffic to the correct
   Border Relay (BR)/AFTR instance which is actively serving a given CE.
   For example, a client which is configured to use MAP-E may send its
   traffic to the MAP-E BR, but on the return path, the ingress IP
   traffic gets routed to a MAP-T BR.  The resulting translated packet
   that gets forwarded to the MAP-E client will be dropped.

   Therefore, operators are advised to use separate IPv4 pools for each
   of the different mechanisms to simplify planning and IPv4 routing.

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   For IPv6 planning there is less of a constraint as the BR/AFTR
   elements for the different mechanisms can contain configuration for
   overlapping client's IPv6 addresses, providing only one mechanism is
   actively serving a given client at a time.  However, the IPv6 address
   that is used as the tunnel concentrator's endpoint (BR/AFTR address)
   needs to be different for each mechanisms to ensure correct

2.2.  Backwards Compatability with Existing Softwire Clients

   Deployed clients which can support multiple softwire mechanisms, but
   do not implement the prioritization mechanism described here may
   require additional planning.  In this scenario, the CPE would request
   configuration for all of the supported softwire mechanisms in its
   DHCPv6 Option Request Option (ORO), but would not request
   OPTION_S46_PRIORITY.  By default, the DHCPv6 server will respond with
   configuration for all of the requested mechanisms which could result
   in unpredictable and unwanted client configuration.

   In this scenario, it may be necessary for the operator to implement
   logic within the DHCPv6 server to identify such clients and only
   provision them with configuration for a single softwire mechanism.
   It should be noted that this can lead to complexity and reduced
   scalability in the DHCPv6 server implementation due to the addition
   DHCPv6 message processing overhead.

3.  Security Considerations

   Security considerations discussed in [RFC6334] and [RFC7598] apply
   for this document.

   Misbehaving intermediate nodes may alter the content of the S46
   Priority Option.  This may lead to setting a different IPv4 service
   continuity mechanism than the one initially preferred by the network
   side.  Also, a misbehaving node may alter the content of the S46
   Priority Option and other DHCPv6 options (e.g., DHCPv6 Option #64 or
   #90) so that the traffic is intercepted by an illegitimate node.
   Those attacks are not unique to the S46 Priority Option but are
   applicable to any DHCPv6 option that can be altered by a misbehaving
   intermediate node.

4.  IANA Considerations

   IANA is kindly requested to allocate the following DHCPv6 option


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   All values should be added to the DHCPv6 option code space defined in
   Section 24.3 of [RFC3315].

4.1.  S46 Mechanisms and their Identifying Option Codes

   This document requests that IANA create a new registry entitled
   "Option Codes permitted in the S46 Priority Option".  This registry
   will enumerate the set of DHCPv6 Option Codes that can be included in
   OPTION_S46_PRIORITY option.  Options may be added to this list using
   the IETF Review process described in Section 4.1 of [RFC5226].

   The following table shows the option codes which are currently
   defined and the S46 mechanisms which they represent.  The contents of
   this table shows the format and the initial values for the new
   registry.  Option codes that have not been requested to be added
   according to the stated procedure should not be mentioned at all in
   the table, and should not be listed as "reserved" or "unassigned".
   The valid range of values for the registry is the range of DHCPv6
   Option Codes (1-65535).

             | Option Code |   S46 Mechanism    | Reference |
             |      64     |      DS-Lite       | [RFC6334] |
             |      88     | DHCPv4 over DHCPv6 | [RFC7341] |
             |      94     |       MAP-E        | [RFC7598] |
             |      95     |       MAP-T        | [RFC7598] |
             |      96     | Lightweight 4over6 | [RFC7598] |

             Table 1: DHCPv6 Option to S46 Mechanism Mappings

5.  Acknowledgements

   Many thanks to O.  Troan, S.  Barth.  A.  Yourtchenko, B.  Volz, T.
   Mrugalski, J.  Scudder, P.  Kyzivat, F.  Baker, and B.  Campbell for
   their input and suggestions.

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,

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   [RFC3315]  Droms, R., Ed., Bound, J., Volz, B., Lemon, T., Perkins,
              C., and M. Carney, "Dynamic Host Configuration Protocol
              for IPv6 (DHCPv6)", RFC 3315, DOI 10.17487/RFC3315, July
              2003, <>.

   [RFC6334]  Hankins, D. and T. Mrugalski, "Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite",
              RFC 6334, DOI 10.17487/RFC6334, August 2011,

   [RFC7341]  Sun, Q., Cui, Y., Siodelski, M., Krishnan, S., and I.
              Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport",
              RFC 7341, DOI 10.17487/RFC7341, August 2014,

   [RFC7598]  Mrugalski, T., Troan, O., Farrer, I., Perreault, S., Dec,
              W., Bao, C., Yeh, L., and X. Deng, "DHCPv6 Options for
              Configuration of Softwire Address and Port-Mapped
              Clients", RFC 7598, DOI 10.17487/RFC7598, July 2015,

6.2.  Informative References

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, DOI 10.17487/RFC6333, August 2011,

   [RFC7596]  Cui, Y., Sun, Q., Boucadair, M., Tsou, T., Lee, Y., and I.
              Farrer, "Lightweight 4over6: An Extension to the Dual-
              Stack Lite Architecture", RFC 7596, DOI 10.17487/RFC7596,
              July 2015, <>.

   [RFC7597]  Troan, O., Ed., Dec, W., Li, X., Bao, C., Matsushima, S.,
              Murakami, T., and T. Taylor, Ed., "Mapping of Address and
              Port with Encapsulation (MAP-E)", RFC 7597,
              DOI 10.17487/RFC7597, July 2015,

   [RFC7599]  Li, X., Bao, C., Dec, W., Ed., Troan, O., Matsushima, S.,
              and T. Murakami, "Mapping of Address and Port using
              Translation (MAP-T)", RFC 7599, DOI 10.17487/RFC7599, July
              2015, <>.

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Authors' Addresses

   Mohamed Boucadair


   Ian Farrer
   Deutsche Telekom


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