Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Options for Dual-Stack Lite
draft-ietf-softwire-ds-lite-tunnel-option-04

Abstract

This document specifies two DHCPv6 options which are meant to be used by a Dual-Stack Lite client (Basic Bridging BroadBand element, B4) to discover its Address Family Transition Router (AFTR) address.

Status of this Memo

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Table of Contents

1.  Requirements Language
2.  Introduction
3.  The Dual-Stack Lite Address DHCPv6 Option
4.  The Dual-Stack Lite Name DHCPv6 Option
5.  DHCPv6 Server Behavior
6.  DHCPv6 Client Behavior
7.  Security Considerations
8.  IANA Considerations
9.  Acknowledgements
10.  Normative References
§  Authors' Addresses

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 RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

2.  Introduction

Dual-Stack Lite (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .) [I‑D.softwire‑ds‑lite] is a solution to offer both IPv4 and IPv6 connectivity to customers which are addressed only with an IPv6 prefix (no IPv4 address is assigned to the attachment device). One of its key components is an IPv4-over-IPv6 tunnel, commonly referred to as a Softwire, but a DS-Lite Basic Bridging BroadBand (B4) will not know if the network it is attached to offers Dual-Stack Lite support, and if it did would not know the remote end of the tunnel to establish a connection.

To inform the B4 of the Address Family Transition Router's (AFTR) location, either an IPv6 address, Fully Qualified Domain Name (FQDN) or both may be used. Once this information is conveyed, the presence of the configuration indicating the AFTR's location also informs a host to initiate Dual-Stack Lite (DS-Lite) service and become a Softwire Initiator.

To provide the conveyance of the configuration information, two DHCPv6 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] options are used; one in the case where the host receives an IPv6 address, and one in the case where the host receives an FQDN in order to derive an IPv6 address.

The details of how the B4 establishes an IPv4-in-IPv6 tunnel to the AFTR are out of scope for this document.

3.  The Dual-Stack Lite Address DHCPv6 Option

The Dual-Stack Lite Address option consists of option-code and option-len fields (common for all DHCPv6 options), and a 128 bit tunnel-endpoint-addr field, containing one IPv6 address. The tunnel-endpoint-addr specifies the location of the remote tunnel endpoint, expected to be located at an AFTR.

The DS-Lite Address 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. Any DS-Lite Address option received inside any other option MUST be ignored.

The DS-Lite Address option MUST NOT appear more than once in a message. Client that receives more than one DS-Lite Address option MUST discard all instances of that option.

The format of the Dual-Stack Lite Address option is shown in the following figure:

     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_DS_LITE_ADDR (TBD)   |         option-len: 16        |
    +-------------------------------+-------------------------------+
    |                                                               |
    |               tunnel-endpoint-addr (IPv6 Address)             |
    |                                                               |
    |                                                               |
    +---------------------------------------------------------------+

             option-code: OPTION_DS_LITE_ADDR (TBD)

              option-len: Length of the tunnel-endpoint-addr field,
                          which is precisely 16 octets.

    tunnel-endpoint-addr: A single IPv6 address in binary
                          representation of the remote tunnel
                          endpoint, located at the DS-Lite AFTR.

The client validates the DS-Lite Address option by confirming the option length is exaclty 16 octets. The client MUST ignore any DS-Lite Address option that has length other than 16 octets.

Because this option conveys the tunnel-endpoint-addr value, no further processing is required of the client.

This option conveys a single IPv6 address, as the Dual-Stack Lite specification (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .) [I‑D.softwire‑ds‑lite] defines only one Softwire connection between a B4 and any AFTR. Multiple connections or endpoints are undefined. For more information, see Section 7.2 "High Availability" of [I‑D.softwire‑ds‑lite] (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .).

4.  The Dual-Stack Lite Name DHCPv6 Option

The Dual-Stack Lite Name option consists of option-code and option-len fields (common for all DHCPv6 options), and a variable length tunnel-endpoint-name field, containing a Fully Qualified Domain Name that refers to the AFTR the client is requested to establish a connection with.

The DS-Lite Name 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. Any DS-Lite Name option received inside any other option MUST be ignored.

The DS-Lite Name option MUST NOT appear more than once in a message. Client that receives more than one DS-Lite Name option MUST discard all instances of that option.

The format of the Dual-Stack Lite Name option is shown in the following figure:

     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_DS_LITE_NAME (TBD)   |           option-len          |
    +-------------------------------+-------------------------------+
    |                   tunnel-endpoint-name (FQDN)                 |
    +---------------------------------------------------------------+

             option-code: OPTION_DS_LITE_NAME (TBD)

              option-len: Length in octets of the tunnel-endpoint-
                          name field.

    tunnel-endpoint-name: A single Fully Qualified Domain Name of the
                          remote tunnel endpoint, located at the
                          DS-Lite AFTR.

The tunnel-endpoint-name field is formatted as required in DHCPv6 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] Section 8 ("Representation and Use of Domain Names"). Briefly, the format described is using a single octet noting the length of one DNS label (limited to at most 64 octets), followed by the label. This repeats until all labels in the FQDN are exhausted. The root label (or the end of the FQDN) is denoted as a zero length label. Any possible updates to Section 8 of DHCPv6 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] also apply to encoding of this FQDN field. An example FQDN format for this option is shown in Figure 3 (Example tunnel-endpoint-name.).

    +------+------+------+------+------+------+------+------+------+
    | 0x07 |   e  |   x  |   a  |   m  |   p  |   l  |   e  | 0x03 |
    +------+------+------+------+------+------+------+------+------+
    |   i  |   s  |   c  | 0x03 |   o  |   r  |   g  | 0x00 |
    +------+------+------+------+------+------+------+------+

Note that in the specific case of the example tunnel-endpoint-name, (Example tunnel-endpoint-name.) the length of the option is 17 octets, and so an option-len field value of 17 would be used.

The client (B4) validates the option in this format by first confirming that the option length is greater than 3, that the option data can be contained by the option length (that the option length does not run off the end of the packet), and that the tunnel-endpoint-name is of valid format as described in DHCPv6 Section 8 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315]; there are no compression tags, there is at least one label of nonzero length.

The client (B4) determines a value for the tunnel-endpoint-addr from the tunnel-endpoint-name using standard DNS resolution, as defined in [RFC3596] (Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, “DNS Extensions to Support IP Version 6,” October 2003.). If the DNS response contains more than one IPv6 address, the client picks only one IPv6 address and uses it as a remote tunnel endpoint. The client MUST NOT establish more than one DS-Lite tunnel at the same time. For a redundancy and high availability discussion, see Section 7.2 "High availability" of [I‑D.softwire‑ds‑lite] (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .).

5.  DHCPv6 Server Behavior

DHCP servers must translate user input from their own specific and unique operator's interfaces into configuration state for the client. The server MUST provide a way to configure the OPTION_DS_LITE_ADDR. As a convenience to the operator, the server MAY allow the operator to enter a Fully Qualified Domain Name, upon which the server performs DNS Resolution to assemble its OPTION_DS_LITE_ADDR contents. The server MAY either provide the same Fully Qualified Domain Name as OPTION_DS_LITE_NAME contents, or simply provide a distinct method of configuring it.

If configured with values, DHCPv6 servers will include the DS-Lite Address and/or Name options if either or both appear on the client's Option Request Option (OPTION_ORO). RFC 3315 Section 17.2.2 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] describes how a DHCPv6 client and server negotiate configuration values using the ORO.

A DHCPv6 server MUST NOT send either option if it has not been explicitly requested by the client.

A DHCPv6 server MUST NOT send more than one DS-Lite Address option. A DHCPv6 server MUST NOT send more than one DS-Lite Name option.

A DHCPv6 server MUST NOT send DS-Lite Address or DS-Lite Name options as suboptions in other options.

If the server is configured with an FQDN as the tunnel endpoint locator, the configured FQDN value MUST contain a resolvable Fully Qualified Domain Name, having appropriate delegations from the root, and having a AAAA record locating the Softwire Concentrator.

If OPTION_DS_LITE_NAME is being configured, the server MUST be configured to provide OPTION_DNS_SERVERS defined in [RFC3646] (Droms, R., “DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” December 2003.) together with the DS-Lite Name option, so that clients will be able to ask for DNS servers locations to resolve the domain name provided in the DS-Lite Name option.

6.  DHCPv6 Client Behavior

A client that supports B4 functionality of DS-Lite (defined in [I‑D.softwire‑ds‑lite] (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .)) and conforms to this specification MUST include OPTION_DS_LITE_ADDR on its OPTION_ORO, and MAY include OPTION_DS_LITE_NAME at its option and ability.

If requesting the OPTION_DS_LITE_NAME option, the client also SHOULD request OPTION_DNS_SERVERS defined in [RFC3646] (Droms, R., “DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” December 2003.) to be able to resolve any received domain name.

If the client receives either DS-Lite option, it MUST verify the option contents as described in Section 3 (The Dual-Stack Lite Address DHCPv6 Option) and Section 4 (The Dual-Stack Lite Name DHCPv6 Option). The client (B4) is expected to establish a softwire tunnel to the tunnel-endpoint-addr IPv6 address it determines from either of these options.

If the client requests and receives both the OPTION_DS_LITE_ADDR and the OPTION_DS_LITE_NAME options, it MUST proceed with resolving the OPTION_DS_LITE_NAME.

7.  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 DS-Lite Softwire connection represents can be trusted, and it should not therefore bypass any security mechanisms such as IP firewalls.

RFC 3315 (Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M. Carney, “Dynamic Host Configuration Protocol for IPv6 (DHCPv6),” July 2003.) [RFC3315] discusses DHCPv6-related security issues.

[I‑D.softwire‑ds‑lite] (Durand, A., Ed., “Dual-stack lite broadband deployments post IPv4 exhaustion,” .) discusses DS-Lite related security issues.

8.  IANA Considerations

IANA is requested to allocate two DHCPv6 option codes referencing this document. One delineating OPTION_DS_LITE_ADDR, and one delineating OPTION_DS_LITE_NAME.

9.  Acknowledgements

Authors would like to thank Alain Durand, Rob Austein, Dave Thaler and Paul Selkirk for their valuable feedback and suggestions.

10. Normative References

Authors' Addresses