0     8     16    24    32    40    48
   +-----+-----+-----+-----+-----+-----+--
   |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 | ...
   +-----+-----+-----+-----+-----+-----+--
        IPv4 Address 1          IPv4 Address 2 ...

The description of the fields is as follows:

• Code: OPTION_V4_PCP_SERVER (TBA, see Section 9.2);
• Length: Length of all included data in octets. The minimum length is 5.
• List-Length: Length of the "List of PCP server IPv4 Addresses" field in octets; MUST be a multiple of 4.
• List of PCP server IPv4 Addresses: Contains one or more IPv4 addresses of the PCP server to be used by the PCP client. The format of this field is shown in Figure 3.
• OPTION_V4_PCP_SERVER can include multiple lists of PCP server IPv4 addresses; each list is treated as a separate PCP server. When several lists of PCP server IPv4 addresses are to be included, "List-Length" and "PCP server IPv4 Addresses" fields are repeated.

This format assumes that an IPv4 address is encoded as a1.a2.a3.a4.

Figure 3: Format of the List of PCP server IPv4 Addresses

[RFC3396] MUST be used if OPTION_V4_PCP_SERVER exceeds the maximum DHCPv4 option size of 255 octets.

4.2. DHCPv4 Client Behavior

To discover one or more PCP servers, the DHCPv4 client requests PCP server IP addresses by including OPTION_V4_PCP_SERVER in a Parameter Request List Option [RFC2132].

The DHCPv4 client MUST be prepared to receive multiple lists of PCP server IPv4 addresses in the same DHCPv4 PCP server option; each list is to be treated as a separate PCP server.

The DHCPv4 client MUST silently discard multicast and host loopback addresses [RFC6890] conveyed in OPTION_V4_PCP_SERVER.

5. DHCP Server Configuration Guidelines

DHCP servers supporting the DHCP PCP server option can be configured with a list of IP addresses of the PCP server(s). If multiple IP addresses are configured, the DHCP server MUST be explicitly configured whether all or some of these addresses refer to:

1. the same PCP server: the DHCP server returns multiple addresses in the same instance of the DHCP PCP server option.
2. distinct PCP servers: the DHCP server returns multiple lists of PCP server IP addresses to the requesting DHCP client (encoded as multiple OPTION_V6_PCP_SERVER or in the same OPTION_V4_PCP_SERVER); each list is referring to a distinct PCP server. For example, multiple PCP servers may be configured to a PCP client in some deployment contexts such as multi-homing. It is out of scope of this document to enumerate all deployment scenarios that require multiple PCP servers to be returned.

Precisely how DHCP servers are configured to separate lists of IP addresses according to which PCP server they address is out of scope for this document. However, DHCP servers MUST NOT treat IP addresses returned from a single Fully Qualified Domain Name (FQDN) lookup as belonging to more than one PCP server.

DHCPv6 servers that implement this option and that can populate the option by resolving FQDNs will need a mechanism for indicating whether to query for A records or only AAAA records. When a query returns A records, the IP addresses in those records are returned in the DHCPv6 response as IPv4-mapped IPv6 addresses.

• Discussion: The motivation for this design is to accommodate deployment cases where an IPv4 connectivity service is provided while only DHCPv6 is in use (e.g., an IPv4-only PCP server in a DS-Lite context [RFC6333]).

Since this option requires support for IPv4-mapped IPv6 addresses, a DHCPv6 server implementation will not be complete if it does not query for A records and represent any that are returned as IPv4-mapped IPv6 addresses in DHCPv6 responses. This behavior is neither required nor suggested for DHCPv6 options in general: it is specific to OPTION_V6_PCP_SERVER. The mechanism whereby DHCPv6 implementations provide this functionality is beyond the scope of this document.

For guidelines on providing context-specific configuration information (e.g., returning a regional-based configuration), and information on how a DHCP server might be configured with FQDNs that get resolved on demand, see [I-D.ietf-dhc-topo-conf].

6. Dual-Stack Hosts

A Dual-Stack host might receive PCP server option via both DHCPv4 and DHCPv6. For guidance on how a DHCP client can handle PCP server IP lists for the same network but obtained via different mechanisms, see [I-D.ietf-pcp-server-selection].

7. Hosts with Multiple Interfaces

A host may have multiple network interfaces (e.g, 3G, IEEE 802.11, etc.); each configured differently. Each PCP server learned MUST be associated with the interface via which it was learned.

Refer to [I-D.ietf-pcp-server-selection] and Section 8.4 of [RFC6887] for more discussion on multi-interface considerations.

8. Security Considerations

The security considerations in [RFC2131] and [RFC3315] are to be considered. PCP-related security considerations are discussed in [RFC6887].

The PCP Server option targets mainly the simple threat model (Section 18.1 of [RFC6887]). It is out of scope of this document to discuss potential implications of the use of this option in the advanced threat model (Section 18.2 of [RFC6887]).

9. IANA Considerations

9.1. DHCPv6 Option

IANA is requested to assign the following new DHCPv6 Option Code in the registry maintained in http://www.iana.org/assignments/dhcpv6-parameters:

Option Name	Value
OPTION_V6_PCP_SERVER	TBA

9.2. DHCPv4 Option

IANA is requested to assign the following new DHCPv4 Option Code in the registry maintained in http://www.iana.org/assignments/bootp-dhcp-parameters/:

Option Name	Value	Data length	Meaning
OPTION_V4_PCP_SERVER	TBA	Variable; the minimum length is 5.	Includes one or multiple lists of PCP server IP addresses; each list is treated as a separate PCP server.

10. Acknowledgements

Many thanks to C. Jacquenet, R. Maglione, D. Thaler, T. Mrugalski, T. Reddy, S. Cheshire, M. Wasserman, C. Holmberg, A. Farrel, S. Farrel, B. Haberman, and P. Resnick for their review and comments.

Special thanks to T. Lemon and B. Volz for the review and their effort to enhance this specification.

11. References

11.1. Normative References

[RFC2119]	Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, 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.
[RFC2131]	Droms, R., "Dynamic Host Configuration Protocol", RFC 2131, March 1997.
[RFC4291]	Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC2132]	Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor Extensions", RFC 2132, March 1997.
[RFC6887]	Wing, D., Cheshire, S., Boucadair, M., Penno, R. and P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, April 2013.
[RFC6890]	Cotton, M., Vegoda, L., Bonica, R. and B. Haberman, "Special-Purpose IP Address Registries", BCP 153, RFC 6890, April 2013.
[RFC3396]	Lemon, T. and S. Cheshire, "Encoding Long Options in the Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396, November 2002.

11.2. Informative References

[I-D.ietf-dhc-topo-conf]	Lemon, T. and T. Mrugalski, "Customizing DHCP Configuration on the Basis of Network Topology", Internet-Draft draft-ietf-dhc-topo-conf-01, February 2014.
[RFC6052]	Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010.
[I-D.ietf-pcp-server-selection]	Boucadair, M., Penno, R., Wing, D., Patil, P. and T. Reddy, "PCP Server Selection", Internet-Draft draft-ietf-pcp-server-selection-02, January 2014.
[RFC6333]	Durand, A., Droms, R., Woodyatt, J. and Y. Lee, "Dual-Stack Lite Broadband Deployments Following IPv4 Exhaustion", RFC 6333, August 2011.

Authors' Addresses