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IETF RFC 7488
Last modified on Wednesday, March 11th, 2015
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Internet Engineering Task Force (IETF) M. Boucadair
Request for Comments: 7488 France Telecom
Updates: 6887 R. Penno
Category: Standards Track D. Wing
ISSN: 2070-1721 P. Patil
T. Reddy
Cisco
March 2015
Port Control Protocol (PCP) Server Selection
 Abstract
This document specifies the behavior to be followed by a Port Control
Protocol (PCP) client to contact its PCP server(s) when one or
several PCP server IP addresses are configured.
This document updates RFC 6887.
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/RFC 7488.
Copyright Notice
Copyright (c) 2015 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.
Boucadair, et al. Standards Track PAGE 1 
RFC 7488 PCP Server Selection March 2015
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology and Conventions . . . . . . . . . . . . . . . . . 3
2.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
3. IP Address Selection: PCP Server with Multiple IP Addresses . 3
4. IP Address Selection: Multiple PCP Servers . . . . . . . . . 4
5. Example: Multiple PCP Servers on a Single Interface . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Multihoming . . . . . . . . . . . . . . . . . . . . 9
A.1. IPv6 Multihoming . . . . . . . . . . . . . . . . . . . . 9
A.2. IPv4 Multihoming . . . . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
A host may have multiple network interfaces (e.g., 3G, IEEE 802.11,
etc.), each configured with different PCP servers. Each PCP server
learned must be associated with the interface on which it was
learned. Generic multi-interface considerations are documented in
Section 8.4 of [RFC 6887]. Multiple PCP server IP addresses may be
configured on a PCP client in some deployment contexts such as
multihoming (see Appendix A). A PCP server may also have multiple IP
addresses associated with it. It is out of the scope of this
document to enumerate all deployment scenarios that require multiple
PCP server IP addresses to be configured.
If a PCP client discovers multiple PCP server IP addresses, it needs
to determine which actions it needs to undertake (e.g., whether PCP
entries are to be installed in all or a subset of discovered IP
addresses, whether some PCP entries are to be removed, etc.). This
document makes the following assumptions:
o There is no requirement that multiple PCP servers configured on
the same interface have the same capabilities.
o PCP requests to different PCP servers are independent, the result
of a PCP request to one PCP server does not influence another.
o The configuration mechanism must distinguish IP addresses that
belong to the same PCP server.
Boucadair, et al. Standards Track PAGE 2
RFC 7488 PCP Server Selection March 2015
This document specifies the behavior to be followed by a PCP client
[RFC 6887] to contact its PCP server(s) [RFC 6887] when it is
configured with one or several PCP server IP addresses (e.g., using
DHCP [RFC 7291]). This document does not make any assumption on the
type of these IP addresses (i.e., unicast/anycast).
2. Terminology and Conventions
2.1. 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 [RFC 2119].
2.2. Terminology
o PCP client: denotes a PCP software instance responsible for
issuing PCP requests to a PCP server. Refer to [RFC 6887].
o PCP server: denotes a software instance that receives and
processes PCP requests from a PCP client. A PCP server can be co-
located with or be separated from the function it controls (e.g.,
Network Address Translation (NAT) or firewall). Refer to
[RFC 6887].
3. IP Address Selection: PCP Server with Multiple IP Addresses
This section describes the behavior a PCP client follows to contact
its PCP server when the PCP client has multiple IP addresses for a
single PCP server.
1. A PCP client should construct a set of candidate source addresses
(see Section 4 of [RFC 6724]) based on application input and PCP
[RFC 6887] constraints. For example, when sending a PEER or a MAP
with a FILTER request for an existing TCP connection, the only
candidate source address is the source address used for the
existing TCP connection. But when sending a MAP request for a
service that will accept incoming connections, the candidate
source addresses may be all of the node's IP addresses or some
subset of IP addresses on which the service is configured to
listen.
2. The PCP client then sorts the PCP server IP addresses as per
Section 6 of [RFC 6724] using the candidate source addresses
selected in the previous step as input to the destination address
selection algorithm.
Boucadair, et al. Standards Track PAGE 3
RFC 7488 PCP Server Selection March 2015
3. The PCP client initializes its Maximum Retransmission Count (MRC)
to 4.
4. The PCP client sends its PCP messages following the
retransmission procedure specified in Section 8.1.1 of [RFC 6887].
If no response is received after MRC attempts, the PCP client
retries the procedure with the next IP address in the sorted
list.
The PCP client may receive a response from an IP address after
exhausting MRC attempts for that particular IP address. The PCP
client SHOULD ignore such a response because receiving a delayed
response after exhausting four retransmissions sent with
exponentially increasing intervals is an indication that problems
are to be encountered in the corresponding forwarding path and/or
when processing subsequent requests by that PCP server instance.
If, when sending PCP requests, the PCP client receives a hard
ICMP error [RFC 1122], it MUST immediately try the next IP address
from the list of PCP server IP addresses.
5. If the PCP client has exhausted all IP addresses configured for a
given PCP server, the procedure SHOULD be repeated every 15
minutes until the PCP request is successfully answered.
6. Once the PCP client has successfully received a response from a
PCP server's IP address, all subsequent PCP requests to that PCP
server are sent on the same IP address until that IP address
becomes unresponsive. In case the IP address becomes
unresponsive, the PCP client clears the cache of sorted
destination addresses and follows the steps described above to
contact the PCP server again.
For efficiency, the PCP client SHOULD use the same Mapping Nonce for
requests sent to all IP addresses belonging to the same PCP server.
As a reminder, nonce validation checks are performed when operating
in the Simple Threat Model (see Section 18.1 of [RFC 6887]) to defend
against some off-path attacks.
4. IP Address Selection: Multiple PCP Servers
This section describes the behavior a PCP client follows to contact
multiple PCP servers, with each PCP server reachable on a list of IP
addresses. There is no requirement that these multiple PCP servers
have the same capabilities.
Boucadair, et al. Standards Track PAGE 4
RFC 7488 PCP Server Selection March 2015
Note that how PCP clients are configured to separate lists of IP
addresses of each PCP server is implementation specific and
deployment specific. For example, a PCP client can be configured
using DHCP with multiple lists of PCP server IP addresses; each
list is referring to a distinct PCP server [RFC 7291].
If several PCP servers are configured, each with multiple IP
addresses, the PCP client contacts all PCP servers using the
procedure described in Section 3.
As specified in Sections 11.2 and 12.2 of [RFC 6887], the PCP client
must use a different Mapping Nonce for each PCP server with which it
communicates.
If the PCP client is configured, using some means, with the
capabilities of each PCP server, a PCP client may choose to contact
all PCP servers simultaneously or iterate through them with a delay.
This procedure may result in a PCP client instantiating multiple
mappings maintained by distinct PCP servers. The decision to use all
these mappings or delete some of them depends on the purpose of the
PCP request. For example, if the PCP servers are configuring
firewall (not NAT) functionality, then the client would, by default
(i.e., unless it knows that they all replicate state among them),
need to use all the PCP servers.
5. Example: Multiple PCP Servers on a Single Interface
Figure 1 depicts an example that is used to illustrate the server
selection procedure specified in Sections 3 and 4. In this example,
PCP servers (A and B) are co-located with edge routers (rtr1 and
rtr2) with each PCP server controlling its own device.
Boucadair, et al. Standards Track PAGE 5
RFC 7488 PCP Server Selection March 2015
ISP Network
| |
.........................................................
| | Subscriber Network
+----------+-----+ +-----+----------+
| PCP-Server-A | | PCP-Server-B |
| (rtr1) | | (rtr2) |
+-------+--------+ +--+-------------+
192.0.2.1 | | 198.51.100.1
2001:db8:1111::1 | | 2001:db8:2222::1
| |
| |
-------+-------+------+-----------
|
| 203.0.113.0
| 2001:db8:3333::1
+---+---+
| Host |
+-------+
Edge Routers (rtr1, rtr2)
Figure 1: Single Uplink, Multiple PCP Servers
The example describes behavior when a single IP address for one PCP
server is not responsive. The PCP client is configured with two PCP
servers for the same interface, PCP-Server-A and PCP-Server-B, each
of which have two IP addresses: an IPv4 address and an IPv6 address.
The PCP client wants an IPv4 mapping, so it orders the addresses as
follows:
o PCP-Server-A:
* 192.0.2.1
* 2001:db8:1111::1
o PCP-Server-B:
* 198.51.100.1
* 2001:db8:2222::1
Boucadair, et al. Standards Track PAGE 6
RFC 7488 PCP Server Selection March 2015
Suppose that:
o The path to reach 192.0.2.1 is broken
o The path to reach 2001:db8:1111::1 is working
o The path to reach 198.51.100.1 is working
o The path to reach 2001:db8:2222::1 is working
It sends two PCP requests at the same time, the first to 192.0.2.1
(corresponding to PCP-Server-A) and the second to 198.51.100.1
(corresponding to PCP-Server-B). The path to 198.51.100.1 is
working, so a PCP response is received. Because the path to
192.0.2.1 is broken, no PCP response is received. The PCP client
retries four times to elicit a response from 192.0.2.1 and finally
gives up on that address and sends a PCP message to 2001::db8:1111:1.
That path is working, and a response is received. Thereafter, the
PCP client should continue using that responsive IP address for PCP-
Server-A (2001:db8:1111::1). In this particular case, it will have
to use the THIRD_PARTY option for IPv4 mappings.
6. Security Considerations
PCP-related security considerations are discussed in [RFC 6887].
This document does not specify how PCP server addresses are
provisioned on the PCP client. It is the responsibility of PCP
server provisioning document(s) to elaborate on security
considerations to discover legitimate PCP servers.
7. References
7.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/RFC 2119>.
[RFC 6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, September 2012,
<http://www.rfc-editor.org/info/RFC 6724>.
[RFC 6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, April
2013, <http://www.rfc-editor.org/info/RFC 6887>.
Boucadair, et al. Standards Track PAGE 7
RFC 7488 PCP Server Selection March 2015
7.2. Informative References
[RFC 1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989,
<http://www.rfc-editor.org/info/RFC 1122>.
[RFC 4116] Abley, J., Lindqvist, K., Davies, E., Black, B., and V.
Gill, "IPv4 Multihoming Practices and Limitations", RFC
4116, July 2005, <http://www.rfc-editor.org/info/RFC 4116>.
[RFC 7291] Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
the Port Control Protocol (PCP)", RFC 7291, July 2014,
<http://www.rfc-editor.org/info/RFC 7291>.
Boucadair, et al. Standards Track PAGE 8
RFC 7488 PCP Server Selection March 2015
Appendix A. Multihoming
The main problem of a PCP multihoming situation can be succinctly
described as "one PCP client, multiple PCP servers." As described in
Section 3, if a PCP client discovers multiple PCP servers, it should
send requests to all of them with assumptions described in Section 1.
The following sub-sections describe multihoming examples to
illustrate the PCP client behavior.
A.1. IPv6 Multihoming
In this example of an IPv6 multihomed network, two or more routers
co-located with firewalls are present on a single link shared with
the host(s). Each router is, in turn, connected to a different
service provider network, and the host in this environment would be
offered multiple prefixes and advertised multiple DNS servers.
Consider a scenario in which firewalls within an IPv6 multihoming
environment also implement a PCP server. The PCP client learns the
available PCP servers using DHCP [RFC 7291] or any other provisioning
mechanism. In reference to Figure 2, a typical model is to embed
DHCP servers in rtr1 and rtr2. A host located behind rtr1 and rtr2
can contact these two DHCP servers and retrieve from each server the
IP address(es) of the corresponding PCP server.
The PCP client will send PCP requests in parallel to each of the PCP
servers.
Boucadair, et al. Standards Track PAGE 9
RFC 7488 PCP Server Selection March 2015
==================
| Internet |
==================
| |
| |
+----+-+ +-+----+
| ISP1 | | ISP2 |
+----+-+ +-+----+ ISP Network
| |
.........................................................
| |
| | Subscriber Network
+-------+---+ +----+------+
| rtr1 with | | rtr2 with |
| FW1 | | FW2 |
+-------+---+ +----+------+
| |
| |
-------+----------+------
|
+---+---+
| Host |
+-------+
Figure 2: IPv6 Multihoming
A.2. IPv4 Multihoming
In this example of an IPv4 multihomed network described in "NAT- or
RFC 2260-based Multihoming" (Section 3.3 of [RFC 4116]), the gateway
router is connected to different service provider networks. This
method uses Provider-Aggregatable (PA) addresses assigned by each
transit provider to which the site is connected. The site uses NAT
to translate the various provider addresses into a single set of
private-use addresses within the site. In such a case, two PCP
servers might have to be present to configure NAT to each of the
transit providers. The PCP client learns the available PCP servers
using DHCP [RFC 7291] or any other provisioning mechanism. In
reference to Figure 3, a typical model is to embed the DHCP server
and the PCP servers in rtr1. A host located behind rtr1 can contact
the DHCP server to obtain IP addresses of the PCP servers. The PCP
client will send PCP requests in parallel to each of the PCP servers.
Boucadair, et al. Standards Track PAGE 10
RFC 7488 PCP Server Selection March 2015
=====================
| Internet |
=====================
| |
| |
+----+--------+ +-+------------+
| ISP1 | | ISP2 |
| | | |
+----+--------+ +-+------------+ ISP Network
| |
| |
..............................................................
| |
| Port1 | Port2 Subscriber Network
| |
+----+--------------+----+
|rtr1: NAT & PCP servers |
| GW Router |
+----+-------------------+
|
|
|
-----+--------------
|
+-+-----+
| Host | (private address space)
+-------+
Figure 3: IPv4 Multihoming
Acknowledgements
Many thanks to Dave Thaler, Simon Perreault, Hassnaa Moustafa, Ted
Lemon, Chris Inacio, and Brian Haberman for their reviews and
comments.
Boucadair, et al. Standards Track PAGE 11
RFC 7488 PCP Server Selection March 2015
Authors' Addresses
Mohamed Boucadair
France Telecom
Rennes 35000
France
EMail: mohamed.boucadair@orange.com
Reinaldo Penno
Cisco Systems, Inc.
United States
EMail: repenno@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
United States
EMail: dwing@cisco.com
Prashanth Patil
Cisco Systems, Inc.
Bangalore
India
EMail: praspati@cisco.com
Tirumaleswar Reddy
Cisco Systems, Inc.
Cessna Business Park, Varthur Hobli
Sarjapur Marathalli Outer Ring Road
Bangalore, Karnataka 560103
India
EMail: tireddy@cisco.com
Boucadair, et al. Standards Track PAGE 12
RFC TOTAL SIZE: 22577 bytes
PUBLICATION DATE: Wednesday, March 11th, 2015
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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