Dynamic Host Configuration Protocol Option for Softwires
draft-dhankins-softwire-tunnel-option-01

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Abstract

This document describes how Softwires configuration can be obtained via DHCPv6.

Table of Contents

1.  Requirements Language
2.  Introduction
3.  Softwires DHCPv6 Option
4.  Security Considerations
5.  IANA Considerations
6.  Normative References
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Requirements Language

In this document, the key words "MAY", "MUST, "MUST NOT", "OPTIONAL", "RECOMMENDED", "SHOULD", and "SHOULD NOT", are to be interpreted as described in BCP 14, RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

2.  Introduction

Softwires (Droms, R. and B. Haberman, “Softwires Network Address Translation,” July 2008.) [draft‑droms‑softwires‑snat‑01] is a method to extend IPv4 access to an IPv6-only addressed host. One of its key components is an IPv4-over-IPv6 tunnel, but a host will not know if the network it is attached to offers Softwires support, and if it did would not know the remote end of the tunnel to establish a connection.

These are two separate pieces of information; 1) Should I shut down my dual-stack IPv4 side, and use the Softwires tunnel exclusively? 2) At what IPv6 address will I find the softwires service?

These two questions can be answered with one DHCPv6 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] option.

DISCUSSION: It can be argued that if you inform a client it should perform Softwires, but fail to deliver an IPv6 tunnel endpoint, then its IPv4 access is certainly broken. If you give the client an IPv6 tunnel endpoint but fail to inform it that it must use Softwires support, then again its access is likely broken, or at least it isn't using Softwires as intended by the operator. So the presence of a tunnel address also indicates the intent to use it.

3.  Softwires DHCPv6 Option

The Softwires DHCPv6 Option is simply an IPv6 address.

The Softwires Option Format follows:

     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_SOFTWIRES (TBD)    |           length (16)        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                           IPv6 Address                        |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The code for this option is TBD. The length is precisely 16. The IPv6 Address field is an IPv6 address.

The Softwires option MAY appear in the root scope of a DHCPv6 packet. It MUST NOT appear inside any IA_NA, IA_TA, IA_PD, IAADDR, or similar.

If configured with a value, DHCPv6 servers MUST include the Softwires option if it appears on the client's OPTION_ORO. A server SHOULD NOT include the option otherwise.

A client that supports Softwires MUST include OPTION_SOFTWIRES on its OPTION_ORO. There is no reasonable expectation that a server will reply with the Softwires option if it has not been requested.

If the client receives a Softwires Option, it MUST verify the option length is precisely 16 octets, and ignore the option otherwise. Provided it is of valid length, the client SHOULD terminate or withdraw any DHCPv4 (Droms, R., “Dynamic Host Configuration Protocol,” March 1997.) [RFC2131] configuration on the same interface. If DHCPv4 configuration has concluded, the client SHOULD perform a DHCPRELEASE as it tears down its IPv4 configuration. The client SHOULD establish a Softwires tunnel to the included address.

DISCUSSION: The author's best understanding of the current epistemology on IPv6 multihoming is that the client will have IPv6 addresses on multiple different IPv6 prefixes. If a host is multihomed, then, it is strange enough to wonder how DHCPv6 configuration will work as most DHCPv6 clients will attach to only one DHCPv6 server. It is even stranger to wonder how the client would react if all of its multiple homes wished to provide IPv4 access via Softwires. Would a client establish more than one tunnel? Perhaps this option should permit multiple IPv6 addresses?

4.  Security Considerations

This document does not present any new security issues, but as with all DHCPv6-derived configuration state, it is completely possible that the configuration is being delivered by a third party (Man In The Middle). As such, there is no basis to trust that the access the Softwires connection represents can be trusted, and it should not therefore bypass any security mechanisms such as IP firewalls.

5.  IANA Considerations

IANA is requested to allocate one DHCPv6 Option code, referencing this document.

6. Normative References

Author's Address

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