________     __________
             |        |   |          |
             | DHCPv6 |   | DHCP 4o6 |
             | Server |   |  Server  |
             |________|   |__________|
                 ^       /            \
               1 |    2 /              \ 3a/b
              ___v____ /                \ ________
             |        |                  |        |
             |  lwB4  |<---------------->| lwAFTR |
             |________|    Data Plane    |________|
           

The numbers corresponding to each of the provisioning flows are described in more detail below.

Figure 1: Dynamic lw4o6 Provisioning Model

3.1. Flow 1: lwB4's IPv6 Addressing and DHCPv6 Configuration

Before attempting the DHCP 4o6 configuration process to obtain IPv4 configuration, the lwB4 requires an IPv6 address of a suitable scope to allow communication with the lwAFTR (e.g. a link-local address cannot be used). This IPv6 address can be configured using any applicable method (e.g. SLAAC, DHCPv6, etc.).

To enable DHCPv4 over DHCPv6 transport, the lwB4 needs to perform DHCPv6 to retrieve the DHCP 4o6 server's IPv6 address. The option code OPTION_DHCP4_O_DHCP6_SERVER (88) is included in the client's ORO. The DHCPv6 server responds the DHCP 4o6 server's IPv6 address by carrying the addresses in DHCP 4o6 Server Address option as defined in [RFC7341].

3.2. Flow 2: DHCP 4o6 Function

Once the lwB4 has acquired the IPv6 address of the DHCP 4o6 server, stateful configuration using DHCP 4o6 is performed to obtain an IPv4 address and (optionally) a port set. The lwB4 sends a DHCPv4 DISCOVER message in a DHCPv4-query Message to the DHCP 4o6 server(s) to activate the DHCP 4o6 transport. To obtain a shared IPv4 address, the lwB4 also has to include Parameter Request List option with the option code OPTION_V4_PORTPARAMS (159) as described in [RFC7618].

To obtain the IPv6 address of lwAFTR and inform the DHCP4o6 server of the lwB4's IPv6 tunnle source address, the message flow described in section 5 of [I-D.fsc-softwire-dhcp4o6-saddr-opt] is followed by the lwB4.

Once successfully completed, the client has been provisioned with the IPv6 address of the lwAFTR, an IPv4 address and (optionally) a range of source ports. The server has the /128 IPv6 address that the client will use its tunnel source associated with the IPv4 lease.

3.3. Flow 3: lwAFTR Binding Table Maintainence

As stated in [RFC7596], the lwAFTR MUST synchronize the binding information with the port-restricted address provisioning process. In the dynamic provisioning model described in this document, once the lwB4's provisioning process is completed and the DHCP 4o6 server holds the client's /128 IPv6 tunnel endpoint address, this binding information can be syncronized with the lwAFTR. The method for this synchronization is dependent on whether the DHCP 4o6 and lwAFTR functions are co-located on the same physical host.

3.3.1. Flow 3a: Binding Table Maintenance for Co-located lwAFTR/DHCP 4o6 Functions

Here, the lwAFTR maintains its binding table as per section 6.1 of [RFC7596] and is synchronized with DHCP 4o6 process. The following DHCP 4o6 messages trigger binding table modification:

DHCPACK:

Generated by the DHCP 4o6 server, triggers lwAFTR to add a new entry or modify an existing entry.

DHCPRELEASE:

Generated by lwB4, triggers lwAFTR to delete an existing entry.

When the DHCP 4o6 server generates a DHCPACK message, information about the new lease including the client's IPv6 tunnel endpoint address and IPv4 address/PSID tuple is sent to the lwAFTR process. The lwAFTR performs a check that this new binding does not match an existing binding (both the v6 and v4 information must be checked independently to ensure no conflicts). If no conflicts are found the lwAFTR creates a new binding table entry for the client.

If there an existing entry is found, the lwAFTR updates the IPv6 address and lifetime fields of the entry.

When the DHCP 4o6 server receives a DHCPRELEASE message, the lwAFTR looks up the binding table using the lwB4's IPv6 address, IPv4 address and PSID as index. The lwAFTR deletes the entry either by removing it from the binding table or by marking the lifetime field with an invalid value (e.g. 0).

3.3.2. Flow 3b: Binding Table Maintenance for Distributed lwAFTR/DHCP 4o6 Functions

With this architecture, NETCONF [RFC6241] is used for syncronising client DHCP 4o6 provisioning and the lwAFTR binding table. A YANG model for lw4o6 is defined in [I-D.sun-softwire-yang]. In this deployment model, the DHCP 4o6 server and lwAFTR also implements a NETCONF server. When an IPv4 leasing event occurs (DHCPACK/DHCPRELEASE messages, or an active lease expiring), the DHCP 4o6 server informs the operator's centralised configuration database of the change.

The operator's configuration database will then use NETCONF to update the lwAFTR of the relevant change by adding or removing the binding table entry which matches he DHCP 4o6 server's IPv4 address lease.

4. Security Considerations

Security considerations in [RFC7596] and [RFC7341] are also relevant here.

The DHCP message triggered binding table maintenance may be used by an attacker to send fake DHCP messages to lwAFTR. The operator network should deploy [RFC2827] to prevent this kind of attack.

4.1. Data Retention Requirements

In some countries, regulations require a service providers to retain the necessary information to link IP allocatoin information to a specific customer at a specific point in time.

With a deterministic provisioning model, any individual client will always receive a pre-determined set of IPv4 provisioning requirements. In this scenario, the logging requirement may be met by retaining information on how the DHCPv6 server has been pre-provisioned, with timestamp information on when changes to the pre-provisioning have come into effect.

The dynamic provisioning model that is described in this document brings an additional logging requirement to the service provider: The retention logs holding allocated IPv4 address and ports, the associated IPv6 tunnel endpoint and timestamps marking the start and end of the lease. This is a higher logging overheard than deterministic provisioning, but is in line with the amount of logging that service providers currently have.

5. IANA Considerations

This document does not include an IANA request.

6. References

6.1. Normative References

[I-D.fsc-softwire-dhcp4o6-saddr-opt]	Farrer, I., Sun, Q. and Y. Cui, "DHCPv4 over DHCPv6 Source Address Option", Internet-Draft draft-fsc-softwire-dhcp4o6-saddr-opt-05, June 2016.
[RFC2827]	Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, DOI 10.17487/RFC2827, May 2000.
[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.
[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.
[RFC7618]	Cui, Y., Sun, Q., Farrer, I., Lee, Y., Sun, Q. and M. Boucadair, "Dynamic Allocation of Shared IPv4 Addresses", RFC 7618, DOI 10.17487/RFC7618, August 2015.

6.2. Informative References

[I-D.sun-softwire-yang]	Sun, Q., Wang, H., Cui, Y., Farrer, I., Boucadair, M. and R. Asati, "YANG Data Model for IPv4-in-IPv6 Softwire", Internet-Draft draft-sun-softwire-yang-04, October 2015.
[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.
[RFC6241]	Enns, R., Bjorklund, M., Schoenwaelder, J. and A. Bierman, "Network Configuration Protocol (NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011.
[RFC6887]	Wing, D., Cheshire, S., Boucadair, M., Penno, R. and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, DOI 10.17487/RFC6887, April 2013.
[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.

Authors' Addresses