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RFC 7488 - Port Control Protocol (PCP) Server Selection

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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
                                                              March 2015

              Port Control Protocol (PCP) Server Selection


   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

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.

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 [RFC6887].  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.

   This document specifies the behavior to be followed by a PCP client
   [RFC6887] to contact its PCP server(s) [RFC6887] when it is
   configured with one or several PCP server IP addresses (e.g., using
   DHCP [RFC7291]).  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",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.2.  Terminology

   o  PCP client: denotes a PCP software instance responsible for
      issuing PCP requests to a PCP server.  Refer to [RFC6887].

   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

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 [RFC6724]) based on application input and PCP
       [RFC6887] 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

   2.  The PCP client then sorts the PCP server IP addresses as per
       Section 6 of [RFC6724] using the candidate source addresses
       selected in the previous step as input to the destination address
       selection algorithm.

   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 [RFC6887].
       If no response is received after MRC attempts, the PCP client
       retries the procedure with the next IP address in the sorted

       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 [RFC1122], 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 [RFC6887]) 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.

      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 [RFC7291].

   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 [RFC6887], the PCP client
   must use a different Mapping Nonce for each PCP server with which it

   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.

                                ISP Network
                              |              |
                              |              |        Subscriber Network
                   +----------+-----+  +-----+----------+
                   | PCP-Server-A   |  | PCP-Server-B   |
                   |    (rtr1)      |  |   (rtr2)       |
                   +-------+--------+  +--+-------------+
        |              |
          2001:db8:1111::1 |              |     2001:db8:2222::1
                           |              |
                           |              |
                                   |    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

   o  PCP-Server-A:


      *  2001:db8:1111::1

   o  PCP-Server-B:


      *  2001:db8:2222::1

   Suppose that:

   o  The path to reach is broken

   o  The path to reach 2001:db8:1111::1 is working

   o  The path to reach 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
   (corresponding to PCP-Server-A) and the second to
   (corresponding to PCP-Server-B).  The path to is
   working, so a PCP response is received.  Because the path to is broken, no PCP response is received.  The PCP client
   retries four times to elicit a response from 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 [RFC6887].

   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

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997,

   [RFC6724]  Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
              "Default Address Selection for Internet Protocol Version 6
              (IPv6)", RFC 6724, September 2012,

   [RFC6887]  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/rfc6887>.

7.2.  Informative References

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122, October 1989,

   [RFC4116]  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/rfc4116>.

   [RFC7291]  Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
              the Port Control Protocol (PCP)", RFC 7291, July 2014,

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 [RFC7291] 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

                          |    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
   RFC2260-based Multihoming" (Section 3.3 of [RFC4116]), 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 [RFC7291] 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.

                        |    Internet       |
                           |              |
                           |              |
                      +----+--------+   +-+------------+
                      | ISP1        |   | ISP2         |
                      |             |   |              |
                      +----+--------+   +-+------------+ ISP Network
                           |              |
                           |              |
                           |              |
                           | Port1        | Port2    Subscriber Network
                           |              |
                      |rtr1: NAT & PCP servers |
                      |       GW Router        |
                         | Host  |  (private address space)

                        Figure 3: IPv4 Multihoming


   Many thanks to Dave Thaler, Simon Perreault, Hassnaa Moustafa, Ted
   Lemon, Chris Inacio, and Brian Haberman for their reviews and

Authors' Addresses

   Mohamed Boucadair
   France Telecom
   Rennes  35000

   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.

   EMail: praspati@cisco.com

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   EMail: tireddy@cisco.com


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