Softwire WG M. Boucadair
Internet-Draft France Telecom
Intended status: Standards Track I. Farrer
Expires: April 21, 2016 Deutsche Telekom
October 19, 2015

Unified IPv4-in-IPv6 Softwire CPE


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 describes a specification whereby a single CPE can interwork with all of the standardized and proposed approaches to providing encapsulated IPv4 in IPv6 services.

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 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 working documents as Internet-Drafts. The list of current Internet-Drafts is at

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This Internet-Draft will expire on April 21, 2016.

Copyright Notice

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Table of Contents

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. 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 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 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:

Defines DHCPv6 option 64 for configuring B4 elements with the IPv6 address of the AFTR.
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.
Defines DHCPv6 options 94, 95 and 96 for provisioning MAP-E, MAP-T and lw4o6 respectively.

This document describes a DHCPv6 based prioritisation 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 prioritisation 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.

1.1. Rationale

The following rationale has been adopted for this document:

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

1.2. 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_V6_S46_PRIORITY      |         option-length         |
|        s46-option-code        |  s46-priority   |      option-|
|code |        s46-option-code        |  s46-priority   |       .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+       .
.                                                               .

Figure 1: S46 Priority Option

The fields s46_option_code and s46_priority are a key-value pair. Both fields MUST appear together sequentially.

Each defined s46_option_code, and its associated s46_priority field MAY appear exactly once within the list of S46 option codes. The option MUST contain at least two s46_option_code/s46_priority pairs, and MAY contain more as necessary.

DISCUSSION: The prioritisation field is included to offer additional flexibility, making it possible to give the same weight to two or more mechanisms to instruct the client to configure them simultaneously. If this is not seen to be a requirement (i.e. only one active mechanism is needed), then the prioritisation field could be removed. The prioritization would then be done as an ordered list of option codes as described in section 17 of RFC7227.

The s46_priority field associated with each s46_option_code is an 8-bit integer, which tells the recipient the priority of the associated S46 mechanism. A higher s46_priority is taken by the client to be a higher priority. If two or more s46_option_codes with the same s46_priority value are received, then the client should attempt to configure these mechanisms simultaneously. As this document is solely concerned with provisioning, issues related to the client's usage of multiple active softwires are out of scope.

1.3. Client Behavior

Clients MAY request option OPTION_V6_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 Offer message from the server containing OPTION_V6_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_V6_S46_PRIORITY for the DHCPv6 option codes contained in the included s46_option_code fields and their related s46_priorities. From this, an S46 mechanism priority list is created, ordered from highest to lowest s46_priority value.
  3. Sequentially check the priority list against the candidate list until a match is found.
  4. When a match is found, the client SHOULD configure the resulting S46 mechanism. Configuration for other S46 mechanisms MUST be discarded.

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.

In the event that the client receives OPTION_V6_S46_PRIORITY with the following errors, it MUST be discarded:

If an invalid OPTION_V6_S46_PRIORITY option is received, the client MAY proceed with configuring the provisioned S46 mechanisms as if OPTION_V6_S46_PRIORITY had not been received.

In the event that a client receives an OPTION_V6_S46_PRIORITY option containing a value in s46-option-code representing an S46 mechanism which the client has not implemented, this is not considered an error.

1.4. 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 option foo only if configured with specific values for foo and if the client requested it.

Option OPTION_V6_S46_PRIORITY is a singleton. Servers MUST NOT send more than one instance of the OPTION_V6_S46_PRIORITY option.

1.5. S46 Mechanisms and their Identifying Option Codes

The following table shows the currently defined option codes and the S46 mechanisms which they represent. This list is complete at the time of writing, but should not be considered definitive as new S46 mechanisms may be defined in the future.

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

2. Security Considerations

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

Misbehaving intermediate nodes may alter the content of the S46 option. This may lead to setting a different IPv4 service continuity mechanism than the one initially preferred by the network side.

3. IANA Considerations

IANA is kindly requested to allocate the following DHCPv6 option code:[RFC3315].


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

4. Acknowledgements

Many thanks to O. Troan, S. Barth. A. Yourtchenko, B. Volz T. Mrugalski for their input and suggestions.

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.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998.
[RFC3315] Droms, R., 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.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006.
[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.
[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.
[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.
[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., Dec, W., Li, X., Bao, C., Matsushima, S., Murakami, T. and T. Taylor, "Mapping of Address and Port with Encapsulation (MAP-E)", RFC 7597, DOI 10.17487/RFC7597, July 2015.
[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.

5.2. Informative References

[RFC7040] Cui, Y., Wu, J., Wu, P., Vautrin, O. and Y. Lee, "Public IPv4-over-IPv6 Access Network", RFC 7040, DOI 10.17487/RFC7040, November 2013.
[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.

Authors' Addresses

Mohamed Boucadair France Telecom Rennes, France EMail:
Ian Farrer Deutsche Telekom Germany EMail: