Softwire WG I. Farrer
Internet-Draft Deutsche Telekom AG
Intended status: Standards Track Q. Sun
Expires: January 5, 2016 Y. Cui
Tsinghua University
July 4, 2015

DHCPv4 over DHCPv6 Source Address Option


DHCPv4 over DHCPv6 [RFC7341] describes a mechanism for dynamically configuring IPv4 over an IPv6-only network. For DHCPv4 over DHCPv6 to function with some IPv4-over-IPv6 softwire mechanisms and deployment scenarios, the operator must learn the /128 IPv6 address that the client will use as the source of IPv4-in-IPv6 tunnel. This address, in conjunction with the IPv4 address and the Port Set ID allocated to the DHCP 4o6 client are used to create a binding table entry in the softwire tunnel concentrator. This memo defines two DHCPv6 options used to communicate the source tunnel IPv6 address between the DHCP 4o6 client and server. It is designed to work in conjunction with the IPv4 address allocation process.

Status of This Memo

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Table of Contents

1. Introduction

Deterministic IPv4-over-IPv6 transition technologies require that elements are pre-configured with binding rules for routing traffic to clients. This places a constraint on the location of the client's tunnel endpoint: The tunnel endpoint has to be a pre-determined prefix which is usually be configured on the home gateway device. [I-D.ietf-softwire-map-dhcp] describes a DHCPv6 based mechanism for provisioning such deterministic softwires.

If a dynamic provisioning model is used, such as using DHCPv4 over DHCPv6, then pre-configuation of the softwire elements is not possible and client rules must be created/deleted in line with the allocation of IPv4 addresses to clients. This has the benefit of removing the fixed address constraint for the client's tunnel endpoint, as the address that the client will use can be learnt when the tunnel is provisioned. The operator's tunnel concentrator(s) can then be configured with the binding rule.

This document describes a mechanism for informing the service provider of the binding between the dynamically allocated IPv4 address and Port Set ID (learnt through DHCPv4 over DHCPv6) and the IPv6 address that the softwire Initiator will use for accessing IPv4-over-IPv6 services. It is used with DHCPv4 over DHCPv6 [RFC7341] message flows to communicate the binding over the IPv6-only network. The service provider can then use this binding information to provision other functional elements in their network accordingly, e.g. using the client's binding information to synchronise the binding table in the border router.

The mechanism allows much more flexibility in the location of the IPv4-over-IPv6 tunnel endpoint, as the IPv6 address is dynamically signalled back to the service provider. The DHCP 4o6 client and tunnel client could be run on end devices attached to any routable IPv6 prefix allocated to an end-user, located anywhere within an arbitrary home network topology.

2. Applicability

The mechanism described in this document is only suitable for use for provisioning softwire clients via DHCP 4o6. The options described here are only applicable within the DHCP 4o6 message exchange process. Current mechanisms suitable for extending to incorporate DHCPv4 over DHCPv6 with dynamic IPv4 address leasing include [I-D.ietf-softwire-map] and [I-D.ietf-softwire-lw4over6].

3. 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].

4. Solution Overview

The solution in this document is intended for the dynamic establishment of IPv4-over-IPv6 softwires. DHCP 4o6 [RFC7341] supports dynamically allocating (shared) IPv4 address. For a softwire to be successfully created, the IPv4 address has to be linked to the client's IPv6 tunnel source address. Within this process, the DHCP 4o6 client uses a DHCPv6 option to signal its tunnel source IPv6 address to the DHCP 4o6 server so that the operator's provisioning system can create the binding and configure the tunnel concentrator accordingly.

Two new DHCPv6 options are defined in this memo: OPTION_DHCP4O6_SADDR_HINT and OPTION_DHCP4O6_SADDR. They are intended to be used alongside the normal DHCPv4 IPv4 address allocation message flow in the context of DHCPv4 over DHCPv6 [RFC7341]. If a DHCP 4o6 client supports this mechanism, it MUST include the code of OPTION_DHCP4O6_SADDR_HINT in the Option Request Option (ORO) [RFC3315] when requesting IPv4 configuration through DHCP 4o6.

The communication of parameters between the client and server is a two-way process: OPTION_DHCP4O6_SADDR_HINT is optionally used by the DHCP 4o6 server to indicate to the client a preferred IPv6 prefix for binding the received IPv4 configuration and sourcing tunnel traffic. This may be necessary if there are multiple IPv6 prefixes in use in the customer network (e.g. ULAs), or if the specific IPv4-over-IPv6 transition mechanism requires the use of a particular prefix for any reason. When the client has selected an IPv6 address to bind the IPv4 configuration to, it passes the address back to the DHCP 4o6 server through OPTION_DHCP4O6_SADDR.

A softwire initiator also requires the IPv6 address of the border router (i.e. softwire tunnel concentrator). In the dynamic mode, it SHOULD acquire an IPv6 prefix of the BR through OPTION_BR_PREFIX. Then the /128 border router address is constructed in the same manner as described in [I-D.ietf-softwire-map], by concatenating the OPTION_BR_PREFIX with IPv4 address and PSID.

To configure a softwire with DHCP 4o6, the DHCP client requests the 4o6 Server Address option using DHCPv6. If the DHCPv6 server includes the DHCP 4o6 Server Address option in its response, then DHCPv4 over DHCPv6 can be enabled, as in [RFC7341]. If the IPv6 source address of the client changes (such as IPv6 lease expiration, etc.), the client follows the Section 9 of [RFC7341] to re-enable the DHCPv4-over-DHCPv6 function.

5. IPv6/IPv4 Binding Message Flow

The following diagram shows the client/server message flow and how the options defined in this document are used. In each step, the relevant DHCPv4 message is given above the arrow and the relevant options below the arrow. The DHCPv4 messages are encapsulated in DHCPv4-query or DHCPv4-response messages, and those options are included in the 'options' field of the DHCPv4-query or DHCPv4-response message.

        DHCP 4o6                                              DHCP 4o6
         Client                                                Server
           |                DHCPDISCOVER (DHCPv4)                 |
    Step 1 |----------------------------------------------------->|
           |             ORO with OPTION_BR_PREFIX,               |
           |          OPTION_DHCP4O6_SADDR_HINT(DHCPv6)           |
           |                                                      |
           |                 DHCPOFFER (DHCPv4)                   |
    Step 2 |<-----------------------------------------------------|
           |    (cipv6-prefix-hint with service provider's        |
           |           preferred prefix) (DHCPv6)                 |
           |                                                      |
           |                 DHCPREQUEST (DHCPv4)                 |
    Step 3 |----------------------------------------------------->|
           |               OPTION_BR_PREFIX,                      |
           |    OPTION_DHCP4O6_SADDR (cipv6-bound-prefix with     |
           |     client's bound /128 IPv6 address) (DHCPv6)       |
           |                                                      |
           |                   DHCPACK (DHCPv4)                   |
    Step 4 |<-----------------------------------------------------|
           |               OPTION_BR_PREFIX,                      |
           |    OPTION_DHCP4O6_SADDR (cipv6-bound-prefix with     |
           |      client's bound /128 IPv6 prefix) (DHCPv6)       |
           |                                                      |


IPv6/IPv4 Binding Message Flow

A client attempting dynamic softwire configuration includes the option code for OPTION_BR_PREFIX, OPTION_DHCP4O6_SADDR_HINT in the DHCPv6 ORO in all DHCPv4-query messages it sends.

When a DHCP 4o6 Server replies with a DHCPOFFER message, it SHOULD include an OPTION_BR_PREFIX. It MAY also include OPTION_DHCP4O6_SADDR_HINT, which is used to indicate a preferred prefix that the client should use to bind IPv4 configuration to. If this option is received, the client MUST perform a longest prefix match between cipv6-prefix-hint and all prefixes/addresses in use on the device. If a match is found, the selected prefix MUST then be used to bind the received IPv4 configuration to. If the client doesn't receive OPTION_DHCP4O6_SADDR_HINT the client can select any valid /128 IPv6 prefix to use.

OPTION_DHCP4O6_SADDR is used by the client to inform the DHCP 4o6 Server which IPv6 address the IPv4 configuration has been bound to. The client MUST put the selected IPv6 address into this option and include it in the DHCPv4-response message when it sends the DHCPREQUEST message.

6. DHCPv6 Options

6.1. DHCPv4 over DHCPv6 Source Address Hint 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_DHCP4O6_SADDR_HINT   |         option-length         |
        |cipv6-hintlen  |                                               |
        +-+-+-+-+-+-+-+-+          cipv6-prefix-hint                    .
        .                          (variable length)                    .

6.2. DHCPv4 over DHCPv6 Source Address Option

The format of DHCPv4 over DHCPv6 Source address option is defined as 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_DHCP4O6_SADDR      |         option-length         |
        |                                                               |
        +                        cipv6-src-address                      +
        .                           (128 bits)                          .

6.3. Border Router Prefix Option

The format of Border Router Prefix option is defined as 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_BR_PREFIX        |         option-length         |
        |br-prefix6-len |                                               |
        +-+-+-+-+-+-+-+-+        br-ipv6-prefix                         +
        .                      (variable length)                        .

This option provisions the softwire initiator with an IPv6 prefix of BR. If the prefix length is /128, the softwire initiator takes this /128 IPv6 address as the BR's tunnel endpoint address. If the prefix length is /64, the softwire initiator MUST create the BR's /128 tunnel endpoint address by concatenating that prefix, its IPv4 address and PSID. This is similar to the initiator creating its own IPv6 tunnel endpoint address [I-D.ietf-softwire-map].

    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
   |            Prefix from the OPTION_BR_PREFIX                   |
   |                        (64-bits)                              |
   |         Zero Padding          |         IPv4 Address          |
   |       IPv4 Addr cont.         |             PSID              |

7. Security Considerations


8. IANA Considerations

IANA is requested to allocate the DHCPv6 option codes: OPTION_DHCP4O6_SADDR_HINT, OPTION_DHCP4O6_SADDR and OPTION_BR_PREFIX.

9. Acknowledgements

The authors would like to thank Ted Lemon and Lishan Li for their contributions.

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC7341] Sun, Q., Cui, Y., Siodelski, M., Krishnan, S. and I. Farrer, "DHCPv4-over-DHCPv6 (DHCP 4o6) Transport", RFC 7341, August 2014.

10.2. Informative References

[I-D.farrer-softwire-br-multiendpoints] Farrer, I. and Q. Sun, "Multiple Tunnel Endpoints on Border Router", Internet-Draft draft-farrer-softwire-br-multiendpoints-00, March 2015.
[I-D.ietf-softwire-lw4over6] Cui, Y., Qiong, Q., Boucadair, M., Tsou, T., Lee, Y. and I. Farrer, "Lightweight 4over6: An Extension to the DS-Lite Architecture", Internet-Draft draft-ietf-softwire-lw4over6-13, November 2014.
[I-D.ietf-softwire-map] Troan, O., Dec, W., Li, X., Bao, C., Matsushima, S., Murakami, T. and T. Taylor, "Mapping of Address and Port with Encapsulation (MAP)", Internet-Draft draft-ietf-softwire-map-13, March 2015.
[I-D.ietf-softwire-map-dhcp] 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", Internet-Draft draft-ietf-softwire-map-dhcp-12, March 2015.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC3315] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C. and M. Carney, "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

Authors' Addresses

Ian Farrer Deutsche Telekom AG CTO-ATI, Landgrabenweg 151 Bonn, NRW 53227 Germany EMail:
Qi Sun Tsinghua University Beijing, 100084 P.R. China Phone: +86-10-6278-5822 EMail:
Yong Cui Tsinghua University Beijing, 100084 P.R. China Phone: +86-10-6260-3059 EMail: