PCP DHCP Options: DHCP Options for the Port Control Protocol (PCP)
How a DHCP server can tell a host where its Port Control Protocol server lives, over both IPv4 and IPv6.
Boucadair, M., Penno, R., and D. Wing, "DHCP Options for the Port Control Protocol (PCP)", RFC 7291, DOI 10.17487/RFC7291, July 2014, <https://www.rfc-editor.org/info/rfc7291>.
@misc{rfc7291,
series = {Request for Comments},
number = 7291,
howpublished = {RFC 7291},
publisher = {RFC Editor},
doi = {10.17487/RFC7291},
url = {https://www.rfc-editor.org/info/rfc7291},
author = {M. Boucadair and R. Penno and D. Wing},
title = {{DHCP Options for the Port Control Protocol (PCP)}},
pagetotal = 11,
year = 2014,
month = jul,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7291 France Telecom
Category: Standards Track R. Penno
ISSN: 2070-1721 D. Wing
Cisco
July 2014
DHCP Options for the Port Control Protocol (PCP)
Abstract
This document specifies DHCP (IPv4 and IPv6) options to configure
hosts with Port Control Protocol (PCP) server IP addresses. The use
of DHCPv4 or DHCPv6 depends on the PCP deployment scenarios. The set
of deployment scenarios to which DHCPv4 or DHCPv6 can be applied is
outside the scope of this document.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7291.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to 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 are 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 document defines DHCPv4 [RFC2131] and DHCPv6 [RFC3315] options
that can be used to configure hosts with PCP server [RFC6887] IP
addresses.
This specification assumes a PCP server is reachable with one or
multiple IP addresses. As such, a list of IP addresses can be
returned in the DHCP PCP server option.
This specification allows returning one or multiple lists of PCP
server IP addresses. This is used as a hint to guide the PCP client
when determining whether to send PCP requests to one or multiple PCP
servers. Concretely, the PCP client needs an indication to decide
whether entries need to be instantiated in all PCP servers (e.g.,
multi-homing, multiple PCP-controlled devices providing distinct
services, etc.) or use one IP address from the list (e.g., redundancy
group scenario, proxy-based model, etc.). Refer to [PCP-DEPLOYMENT]
for a discussion on PCP deployment scenarios.
For guidelines on how a PCP client can use 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
This document makes use of the following terms:
o "PCP server" denotes a functional element that receives and
processes PCP requests from a PCP client. A PCP server can be
co-located with or be separated from the function (e.g., NAT,
Firewall) it controls. Refer to [RFC6887].
o "PCP client" denotes a PCP software instance responsible for
issuing PCP requests to a PCP server. Refer to [RFC6887].
o "DHCP" refers to both DHCPv4 [RFC2131] and DHCPv6 [RFC3315].
o "DHCP client" denotes a node that initiates requests to obtain
configuration parameters from one or more DHCP servers.
o "DHCP server" refers to a node that responds to requests from DHCP
clients.
3. DHCPv6 PCP Server Option
3.1. Format
The DHCPv6 PCP server option can be used to configure a list of IPv6
addresses of a PCP server.
The format of this option 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
The fields of the option shown in Figure 1 are as follows:
o Option-code: OPTION_V6_PCP_SERVER (86; see Section 9.1)
o Option-length: Length of the "PCP Server IPv6 Address(es)" field
in octets. MUST be a multiple of 16.
o PCP Server IPv6 Addresses: Includes one or more IPv6 addresses
[RFC4291] of the PCP server to be used by the PCP client. Note,
IPv4-mapped IPv6 addresses (Section 2.5.5.2 of [RFC4291]) are
allowed to be included in this option.
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 discover one or more PCP servers, the DHCPv6 client requests 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 to be treated as a separate
PCP server.
If an IPv4-mapped IPv6 address is received in OPTION_V6_PCP_SERVER,
it indicates that the PCP server has the corresponding IPv4 address.
Note: When presented with the IPv4-mapped prefix, current versions
of Windows and Mac OS generate IPv4 packets but will not 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 can be used to configure a list of IPv4
addresses of a PCP server. The format of this option is illustrated
in Figure 2.
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
The descriptions of the fields are as follows:
o Code: OPTION_V4_PCP_SERVER (158; see Section 9.2);
o Length: Length of all included data in octets. The minimum length
is 5.
o List-Length: Length of the "List of PCP Server IPv4 Addresses"
field in octets. MUST be a multiple of 4.
o 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.
0 8 16 24 32 40 48
+-----+-----+-----+-----+-----+-----+--
| a1 | a2 | a3 | a4 | a1 | a2 | ...
+-----+-----+-----+-----+-----+-----+--
IPv4 Address 1 IPv4 Address 2 ...
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
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, the
"List-Length" and "List of PCP Server IPv4 Addresses" fields are
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 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_SERVERs 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 the 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 the
scope of this document. However, 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, and DHCP servers
MUST NOT separate the addresses of a single PCP server and return
them as if they belong to distinct PCP servers. For example, if an
administrator configures the DHCP server by providing a Fully
Qualified Domain Name (FQDN) for a PCP server, even if that FQDN
resolves to multiple addresses, the DHCP server MUST deliver them
within a single server address block.
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
Dual-Stack Lite (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 [DHC-CONFIG].
6. Dual-Stack Hosts
A dual-stack host might receive a 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 [PCP-SERVER].
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 [PCP-SERVER] 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 defined here is applicable when operating under
the simple threat model (Section 18.1 of [RFC6887]). Operation under
the advanced threat model (Section 18.2 of [RFC6887]) may or may not
be appropriate; analysis of this question is out of the scope of this
document.
9. IANA Considerations
9.1. DHCPv6 Option
IANA has assigned 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 86
9.2. DHCPv4 Option
IANA has assigned the following new DHCPv4 Option Code in the
registry maintained in
<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
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 their reviews and their
efforts 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.
[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
AI ELI5
On this page
§1What the Options Are For
This RFC defines options for DHCPv4 (RFC 2131) and DHCPv6 (RFC 3315) that configure a host with the IP addresses of a PCP server. PCP itself is defined in RFC 6887. A PCP server is the element that receives and processes PCP requests, and it may be separate from the NAT or firewall it controls.
A PCP server may be reachable at several IP addresses, so the option carries a list of addresses. The option can also carry several lists. That grouping is a hint to the PCP client: it needs to know whether to create entries on all the PCP servers (for example with multi-homing, or several PCP-controlled devices that provide different services) or to use just one address from the list (for example a redundancy group or a proxy-based model).
A network can hand your computer a short list of helpers to ask. Sometimes you must ask every helper on the list, and sometimes any one of them will do.
§3The DHCPv6 PCP Server Option
The DHCPv6 option has an Option-code of OPTION_V6_PCP_SERVER (86), an Option-length and then one or more IPv6 addresses. The length counts the address field in octets and MUST be a multiple of 16. IPv4-mapped IPv6 addresses are allowed, and a client that receives one reads it as meaning the PCP server has that IPv4 address.
To return more than one PCP server, as opposed to more than one address for a single server, the DHCPv6 server sends several instances of the option. The client MUST be ready to receive several instances and treats each as a separate PCP server.
To ask for the option, the client lists OPTION_V6_PCP_SERVER in an Option Request Option. The client MUST silently discard multicast and host loopback addresses in the option.
On IPv6 the network sends one note per helper. Each note holds one or more addresses for that helper, and a computer ignores addresses that cannot possibly be a real helper.
§4The DHCPv4 PCP Option
The DHCPv4 option has a Code of OPTION_V4_PCP_SERVER (158) and a Length with a minimum of 5. After that come one or more lists, each made of a List-Length byte (which MUST be a multiple of 4) followed by that many bytes of IPv4 addresses.
Here the several servers are packed inside a single option, so each list is treated as a separate PCP server. Because a DHCPv4 option is limited to 255 octets, an option that grows beyond that MUST use the long-option mechanism of RFC 3396.
The client asks for the option in a Parameter Request List option (RFC 2132), MUST be ready to receive several lists, and MUST silently discard multicast and host loopback addresses.
On IPv4 all the helpers go on one big note. Each helper gets its own short list, and each list starts with a number telling how long it is.
| Detail | DHCPv6 | DHCPv4 |
|---|---|---|
| Option name | OPTION_V6_PCP_SERVER | OPTION_V4_PCP_SERVER |
| Code | 86 | 158 |
| Address size | 16 octets (multiple of 16) | 4 octets (List-Length multiple of 4) |
| Several servers | Several instances of the option | Several lists in one option |
| Client asks via | Option Request Option | Parameter Request List |
§5Rules for DHCP Servers
If an administrator configures several addresses, the DHCP server MUST be told explicitly whether they belong to the same PCP server or to distinct ones. Same server means they go in one instance (or one list) of the option. Distinct servers means one list per server.
Two things are forbidden. A DHCP server MUST NOT merge the addresses of several PCP servers into one list, and MUST NOT split the addresses of one server into several lists. The RFC gives an example: when a name resolves to several addresses, they all belong together and go into a single block.
DHCPv6 servers that fill the option by looking up names need a way to say whether to query for A records or only AAAA records. Addresses from A records are returned as IPv4-mapped IPv6 addresses. The stated reason is deployments where an IPv4-only PCP server is used while only DHCPv6 is available, such as Dual-Stack Lite (RFC 6333).
The person running the network must say which addresses are the same helper and which are different helpers. The server must never mix them up.
§6Dual-Stack Hosts and Several Interfaces
A dual-stack host may get a PCP server option over both DHCPv4 and DHCPv6. This RFC does not say how to combine them and points to the work in progress on PCP server selection.
A host may also have several network interfaces, such as 3G and IEEE 802.11, each configured differently. Each PCP server a host learns MUST be associated with the interface it was learned on.
If your device is on two networks at once, it must remember which network told it about which helper.
§8Security Considerations
The section is short. It says the security considerations of RFC 2131 and RFC 3315 apply, and that PCP security is discussed in RFC 6887.
The option is stated to be applicable under the simple threat model of RFC 6887. Whether it is appropriate under the advanced threat model is left open, and the analysis is out of scope.
The note from the network is only as trustworthy as the network itself. This RFC says it is meant for simple situations and leaves the harder cases for someone else to study.