RFC 7291

DHCP Options for the Port Control Protocol (PCP)

📅 published July 2014, from ietf

DHCP Options for the Port Control Protocol (PCP) 🎀

okay bestie, the basics first 💅 RFC 7291 is a Standards Track document (ISSN 2070-1721) from July 2014. The authors are M. Boucadair (France Telecom), R. Penno (Cisco), and D. Wing (Cisco) 🫶. It doesn't obsolete or update anything, so it's a fresh drop, no cap ✨

Abstract ✨

so the big idea is that this doc defines DHCP options for both IPv4 and IPv6 🦋 so hosts can get the IP addresses of their Port Control Protocol (PCP) servers. Whether you use DHCPv4 or DHCPv6 depends on your PCP deployment scenario, and which scenarios work with which version is out of scope here 💀

Status of This Memo 📜

This is an Internet Standards Track document 🔥

It's a product of the Internet Engineering Task Force (IETF) and represents the consensus of the IETF community. It got public review and was approved for publication by the Internet Engineering Steering Group (IESG). More on Internet Standards is in Section 2 of RFC 5741.

The current status, any errata, and how to send feedback all live at http://www.rfc-editor.org/info/rfc7291 💖

Copyright Notice ©️

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) as in effect on the date of publication. Please read those carefully, because they spell out your rights and restrictions for this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions, and they're provided without warranty as described in the Simplified BSD License 📚

Table of Contents 🗂️

1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
  1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
3.  DHCPv6 PCP Server Option  . . . . . . . . . . . . . . . . . .   3
  3.1.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   3
  3.2.  DHCPv6 Client Behavior  . . . . . . . . . . . . . . . . .   4
4.  DHCPv4 PCP Option . . . . . . . . . . . . . . . . . . . . . .   5
  4.1.  Format  . . . . . . . . . . . . . . . . . . . . . . . . .   5
  4.2.  DHCPv4 Client Behavior  . . . . . . . . . . . . . . . . .   6
5.  DHCP Server Configuration Guidelines  . . . . . . . . . . . .   6
6.  Dual-Stack Hosts  . . . . . . . . . . . . . . . . . . . . . .   8
7.  Hosts with Multiple Interfaces  . . . . . . . . . . . . . . .   8
8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
  9.1.  DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . .   8
  9.2.  DHCPv4 Option . . . . . . . . . . . . . . . . . . . . . .   9
10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
  11.1.  Normative References . . . . . . . . . . . . . . . . . .   9
  11.2.  Informative References . . . . . . . . . . . . . . . . .  10

1. Introduction ✨

this doc defines DHCPv4 [RFC2131] and DHCPv6 [RFC3315] options that hand hosts their PCP server [RFC6887] IP addresses 🧚

it assumes a PCP server can be reached with one or multiple IP addresses, so a list of IP addresses can come back in the DHCP PCP server option 🍓

it also lets you return one or several lists of PCP server IP addresses. That's a hint for the PCP client so it knows whether to send PCP requests to every server (multi-homing, multiple PCP-controlled devices providing distinct services, etc.) or to use just one address from the list (redundancy group scenario, proxy-based model, etc.). For a deep dive on PCP deployment scenarios, see [PCP-DEPLOYMENT] 💜

and for how a PCP client can juggle multiple IP addresses and multiple PCP servers, see [PCP-SERVER] 🦋

1.1. 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 [RFC2119] 📖

2. Terminology 📖

here's the vocab you need:

"PCP server" is the functional element that receives and processes PCP requests from a PCP client. A PCP server can be co-located with the function it controls (e.g., NAT, Firewall) or separate from it. Refer to [RFC6887].
"PCP client" is the PCP software instance that issues PCP requests to a PCP server. Refer to [RFC6887].
"DHCP" refers to both DHCPv4 [RFC2131] and DHCPv6 [RFC3315].
"DHCP client" is a node that starts requests to get configuration parameters from one or more DHCP servers.
"DHCP server" is a node that responds to requests from DHCP clients.

3. DHCPv6 PCP Server Option 🦋

3.1. Format

the DHCPv6 PCP server option is how you configure a list of IPv6 addresses for a PCP server 💖

the layout is shown in Figure 1:

 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_V6_PCP_SERVER      |         Option-length         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                                                               |
|                     PCP Server IPv6 Address                   |
|                                                               |
|                                                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              ...                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: DHCPv6 PCP Server Option

here's what the fields mean:

Option-code: OPTION_V6_PCP_SERVER (86; see Section 9.1)
Option-length: the length of the "PCP Server IPv6 Address(es)" field in octets. It MUST be a multiple of 16.
PCP Server IPv6 Addresses: one or more IPv6 addresses [RFC4291] of the PCP server for the PCP client to use. Heads up, IPv4-mapped IPv6 addresses (Section 2.5.5.2 of [RFC4291]) are allowed in this option too 🙈

to return more than one PCP server to the DHCPv6 client (as opposed to more than one address for a single PCP server), the DHCPv6 server returns multiple instances of OPTION_V6_PCP_SERVER 🌸

3.2. DHCPv6 Client Behavior

to find one or more PCP servers, the DHCPv6 client asks for PCP server IP addresses by including OPTION_V6_PCP_SERVER in an Option Request Option (ORO), as described in Section 22.7 of [RFC3315] 🔍

The DHCPv6 client MUST be prepared to receive multiple instances of OPTION_V6_PCP_SERVER. Each instance is treated as a separate PCP server 🙌

if an IPv4-mapped IPv6 address shows up in OPTION_V6_PCP_SERVER, it means the PCP server has the matching IPv4 address 🧠

Note: When presented with the IPv4-mapped prefix, current versions of Windows and Mac OS generate IPv4 packets but won't send IPv6 packets [RFC6052]. Representing IPv4 addresses as IPv4-mapped IPv6 addresses follows the same logic as in Section 5 of [RFC6887]. 🙃

The DHCPv6 client MUST silently discard multicast and host loopback addresses [RFC6890] conveyed in OPTION_V6_PCP_SERVER 🚫

4. DHCPv4 PCP Option 🌈

4.1. Format

the DHCPv4 PCP server option is how you configure a list of IPv4 addresses for a PCP server. Figure 2 shows how it's built 🎀

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Code         |     Length    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | List-Length   |   List of     |
     +-+-+-+-+-+-+-+-+  PCP Server   |
     /              IPv4 Addresses   /
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ---
     | List-Length   |   List of     |    |
     +-+-+-+-+-+-+-+-+  PCP Server   |    |
     /              IPv4 Addresses   /    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
     .             ...               . optional
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+    |
     | List-Length   |   List of     |    |
     +-+-+-+-+-+-+-+-+  PCP Server   |    |
     /              IPv4 Addresses   /    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   ---

Figure 2: DHCPv4 PCP Server Option

here are the field descriptions:

Code: OPTION_V4_PCP_SERVER (158; see Section 9.2);
Length: the length of all included data in octets. The minimum length is 5.
List-Length: the length of the "List of PCP Server IPv4 Addresses" field in octets. It MUST be a multiple of 4.
List of PCP Server IPv4 Addresses: one or more IPv4 addresses of the PCP server for the PCP client to use. Figure 3 shows the format of this field 🍑

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

this format assumes an IPv4 address is encoded as a1.a2.a3.a4 🧮

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

OPTION_V4_PCP_SERVER can include multiple lists of PCP server IPv4 addresses, and each list is treated as a separate PCP server. When you include several lists, the "List-Length" and "List of PCP Server IPv4 Addresses" fields get repeated 🔁

OPTION_V4_PCP_SERVER is a concatenation-requiring option. As such, the mechanism specified in [RFC3396] MUST be used if OPTION_V4_PCP_SERVER exceeds the maximum DHCPv4 option size of 255 octets 📏

4.2. DHCPv4 Client Behavior

to find one or more PCP servers, the DHCPv4 client asks for 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 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 ✨

okay so DHCP servers that support the DHCP PCP server option can be set up with a list of IP addresses for the PCP server(s) 🎀. If you configure more than one IP address, the DHCP server MUST be explicitly configured to say whether all or some of those addresses are:

the same PCP server: the DHCP server returns multiple addresses in the same instance of the DHCP PCP server option.
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_SERVERs or in the same OPTION_V4_PCP_SERVER). Each list refers to a distinct PCP server. For example, multiple PCP servers may be configured for a PCP client in some deployment contexts, like multi-homing 🌈. Listing every deployment scenario that needs multiple PCP servers returned is out of the scope of this document, so we're skipping that 🫶.

How DHCP servers get configured to separate lists of IP addresses by the PCP server they point to is out of the scope of this document too 💅. Still, DHCP servers MUST NOT combine the IP addresses of multiple PCP servers and return them to the DHCP client as if they belong to a single PCP server. DHCP servers also MUST NOT separate the addresses of a single PCP server and return them as if they belong to distinct PCP servers 😭. For example, say an administrator configures the DHCP server with a Fully Qualified Domain Name (FQDN) for a PCP server. Even if that FQDN resolves to multiple addresses, the DHCP server MUST deliver them all within a single server address block 🧚.

DHCPv6 servers that implement this option and can fill it in by resolving FQDNs will need a way to indicate 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 support deployments where an IPv4 connectivity service is offered while only DHCPv6 is in use. An example is an IPv4-only PCP server in a Dual-Stack Lite (DS-Lite) context [RFC6333] 🌸.

Since this option requires support for IPv4-mapped IPv6 addresses, a DHCPv6 server implementation is not complete if it doesn't query for A records and represent any that come back 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 ✨. How DHCPv6 implementations provide this functionality is beyond the scope of this document, so that's a no from us 🙈.

For guidelines on providing context-specific configuration information (for example, returning a regional-based configuration) and on how a DHCP server might be configured with FQDNs that get resolved on demand, check out [DHC-CONFIG] 👑.

6. Dual-Stack Hosts

okay, a dual-stack host might get a PCP server option through both DHCPv4 and DHCPv6 🔥. For guidance on how a DHCP client can handle PCP server IP lists that are for the same network but come from different mechanisms, see [PCP-SERVER] 💜.

7. Hosts with Multiple Interfaces

a host may have multiple network interfaces (like 3G, IEEE 802.11, etc.), and each one can be configured differently 📱. Each PCP server learned MUST be associated with the interface it was learned through 🎀.

For more on multi-interface considerations, see [PCP-SERVER] and Section 8.4 of [RFC6887] 🦋.

8. Security Considerations

the security considerations in [RFC2131] and [RFC3315] still apply, so don't skip them 🚨. PCP-related security considerations are covered in [RFC6887] 🔐.

The PCP server option defined here applies when you're operating under the simple threat model (Section 18.1 of [RFC6887]) 🛡️. Operating under the advanced threat model (Section 18.2 of [RFC6887]) may or may not be appropriate, and analysing that question is out of the scope of this document 💀. Be careful out there, bestie 😭💖.

9. IANA Considerations

9.1. DHCPv6 Option

IANA has assigned the following new DHCPv6 Option Code in the registry maintained at http://www.iana.org/assignments/dhcpv6-parameters 🌈:

                    Option Name          Value
                    -------------------- -----
                    OPTION_V6_PCP_SERVER 86

9.2. DHCPv4 Option

IANA has assigned the following new DHCPv4 Option Code in the registry maintained at http://www.iana.org/assignments/bootp-dhcp-parameters 🍓:

Option Name          Tag Data Length Meaning
-------------------- --- ----------- --------------------------------
OPTION_V4_PCP_SERVER 158 Variable;   Includes one or multiple lists
                         the minimum of PCP server IP addresses; each
                         length is   list is treated as a separate
                         5.          PCP server.

10. Acknowledgements

big 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 reviewing and commenting 🫶✨.

Special thanks to T. Lemon and B. Volz for their reviews and their work to make this spec even better 👑💕.

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.

[RFC2131]  Droms, R., "Dynamic Host Configuration Protocol", RFC
           2131, March 1997.

[RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
           Extensions", RFC 2132, 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.

[RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
           Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
           November 2002.

[RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
           Architecture", RFC 4291, February 2006.

[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.

11.2. Informative References

[DHC-CONFIG]
           Lemon, T. and T. Mrugalski, "Customizing DHCP
           Configuration on the Basis of Network Topology", Work in
           Progress, February 2014.

[PCP-DEPLOYMENT]
           Boucadair, M., "Port Control Protocol (PCP) Deployment
           Models", Work in Progress, April 2014.

[PCP-SERVER]
           Boucadair, M., Penno, R., Wing, D., Patil, P., and T.
           Reddy, "PCP Server Selection", Work in Progress, April
           2014.

[RFC6052]  Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
           Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
           October 2010.

[RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
           Stack Lite Broadband Deployments Following IPv4
           Exhaustion", RFC 6333, August 2011.

Authors' Addresses

Mohamed Boucadair
France Telecom
Rennes  35000
France

EMail: mohamed.boucadair@orange.com


Reinaldo Penno
Cisco
USA

EMail: repenno@cisco.com


Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California  95134
USA

EMail: dwing@cisco.com

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