Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 8026 Orange
Category: Standards Track I. Farrer
ISSN: 2070-1721 Deutsche Telekom AG
 November 2016
 Unified IPv4-in-IPv6 Softwire Customer Premises Equipment (CPE):
 A DHCPv6-Based Prioritization Mechanism
Abstract
 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 instance of Customer
 Premises Equipment (CPE) can interwork with all of the standardized
 and proposed approaches to providing encapsulated IPv4-in-IPv6
 services by providing a prioritization mechanism.
Status of This Memo
 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 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/rfc8026.
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
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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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
 1.1.1. Requirements Language . . . . . . . . . . . . . . . . 4
 1.2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . 4
 1.3. DHCPv6 S46 Priority Option . . . . . . . . . . . . . . . 5
 1.4. DHCPv6 Client Behavior . . . . . . . . . . . . . . . . . 6
 1.5. DHCPv6 Server Behavior . . . . . . . . . . . . . . . . . 7
 2. Operator Deployment Considerations for Deploying Multiple
 Softwire Mechanisms . . . . . . . . . . . . . . . . . . . . . 7
 2.1. Client Address Planning . . . . . . . . . . . . . . . . . 7
 2.2. Backwards Compatability with Existing Softwire Clients . 7
 3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
 4.1. S46 Mechanisms and Their Identifying Option Codes . . . . 8
 5. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
 5.1. Normative References . . . . . . . . . . . . . . . . . . 9
 5.2. Informative References . . . . . . . . . . . . . . . . . 10
 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
 IPv4 service continuity is one of the major technical challenges that
 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., as described in [RFC6333], [RFC7596], and
 [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
 providers' networks.
 A common feature shared among all of the differing modes is the
 integration of softwire tunnel endpoint functionality into the
 Customer Premises 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
 architectures.
 For softwire-based services to be successfully established, it is
 essential that the customer's end node and the service provider's 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:
 RFC 6334 Defines DHCPv6 option 64 for configuring Basic Bridging
 BroadBand (B4) [RFC6333] elements with the IPv6 address of
 the Address Family Transition Router (AFTR) [RFC6333].
 RFC 7341 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.
 RFC 7598 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 that supports several S46 mechanisms and receives configuration
 for more than one can prioritize 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. That is, if the
 highest priority S46 mechanism that 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.
1.1. Terminology
 This document makes use of the following terms:
 o Address Family Transition Router (AFTR): The IPv4-in-IPv6 tunnel
 termination point and the Network Address Translator IPv4/IPv4
 (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 Premises 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 the Customer Edge (CE).
1.1.1. 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].
1.2. Rationale
 The following rationale has been adopted for this document:
 (1) Simplified solution migration paths: Define unified CPE
 behavior, allowing for smooth migration between the different
 S46 mechanisms.
 (2) Deterministic CPE coexistence behavior: Specify the behavior
 when several S46 mechanisms coexist in the CPE.
 (3) Deterministic service provider coexistence behavior: Specify the
 behavior when several modes coexist in the service providers
 network.
 (4) Reusability: Maximize the reuse of existing functional blocks
 including tunnel endpoints, the port-restricted Network Address
 Port Translator IPv4/IPv4 (NAPT44), forwarding behavior, etc.
 (5) Solution agnostic: Adopt neutral terminology and avoid (as far
 as possible) overloading the document with solution-specific
 terms.
 (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.
 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 (111)
 o option-length: >=2 and a multiple of 2, in octets.
 o s46-option-code: 16-bit IANA-registered option code of the DHCPv6
 option that is used to identify the softwire mechanism. S46
 mechanisms are prioritized in the appearance order in the S46
 Priority Option.
 Codes in OPTION_S46_PRIORITY are processed in order; if a client
 receives more than one s46-option-code with a particular value, it
 should consider this case to be 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-option-code.
1.4. DHCPv6 Client Behavior
 Clients MAY request the OPTION_S46_PRIORITY option, 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.
 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 the 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 an 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
 regard 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 Softwire
 Mechanisms
 The following subsections 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 that is configured to use MAP-E may send
 its traffic to the MAP-E BR; however, 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.
 For IPv6 planning, there is less of a constraint as the BR/AFTR
 elements for the different mechanisms can contain configuration for
 overlapping the client's IPv6 addresses, provided that 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 mechanism to ensure correct
 operation.
2.2. Backwards Compatability with Existing Softwire Clients
 Deployed clients that 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 additional
 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 has allocated the following DHCPv6 option code:
 111 OPTION_S46_PRIORITY
 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
 IANA has created a new registry titled "Option Codes permitted in the
 S46 Priority Option". This registry enumerates the set of DHCPv6
 option codes that can be included in the 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 that are currently defined
 and the S46 mechanisms that 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
 they 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. References
5.1. 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>.
 [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, <http://www.rfc-editor.org/info/rfc3315>.
 [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,
 <http://www.rfc-editor.org/info/rfc6334>.
 [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,
 <http://www.rfc-editor.org/info/rfc7341>.
 [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,
 <http://www.rfc-editor.org/info/rfc7598>.
5.2. 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,
 <http://www.rfc-editor.org/info/rfc5226>.
 [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,
 <http://www.rfc-editor.org/info/rfc6333>.
 [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, <http://www.rfc-editor.org/info/rfc7596>.
 [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,
 <http://www.rfc-editor.org/info/rfc7597>.
 [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, <http://www.rfc-editor.org/info/rfc7599>.
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.
Authors' Addresses
 Mohamed Boucadair
 Orange
 Rennes
 France
 Email: mohamed.boucadair@orange.com
 Ian Farrer
 Deutsche Telekom AG
 CTO-ATI, Landgrabenweg 151
 Bonn, NRW 53227
 Germany
 Email: ian.farrer@telekom.de