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Internet Engineering Task Force (IETF)                         S. Kiesel
Request for Comments: 7723                       University of Stuttgart
Category: Standards Track                                       R. Penno
ISSN: 2070-1721                                      Cisco Systems, Inc.
                                                            January 2016

             Port Control Protocol (PCP) Anycast Addresses


   The Port Control Protocol (PCP) anycast addresses enable PCP clients
   to transmit signaling messages to their closest PCP-aware on-path
   NAT, firewall, or other middlebox without having to learn the IP
   address of that middlebox via some external channel.  This document
   establishes one well-known IPv4 address and one well-known IPv6
   address to be used as PCP anycast addresses.

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) 2016 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.

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RFC 7723                  PCP Anycast Addresses             January 2016

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  PCP Server Discovery Based on Well-Known IP Address . . . . .   3
     2.1.  PCP Discovery Client Behavior . . . . . . . . . . . . . .   3
     2.2.  PCP Discovery Server Behavior . . . . . . . . . . . . . .   3
   3.  Deployment Considerations . . . . . . . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Registration of an IPv4 Special-Purpose Address . . . . .   5
     4.2.  Registration of an IPv6 Special-Purpose Address . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
     5.1.  Information Leakage through Anycast . . . . . . . . . . .   6
     5.2.  Hijacking of PCP Messages Sent to Anycast Addresses . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The Port Control Protocol (PCP) [RFC6887] provides a mechanism to
   control how incoming packets are forwarded by upstream devices such
   as Network Address and Protocol Translation from IPv6 Clients to IPv4
   Servers (NAT64), Network Address Translation from IPv4 to IPv4
   (NAT44), and IPv6 and IPv4 firewall devices.  Furthermore, it
   provides a mechanism to reduce application keepalive traffic
   [PCP-OPTIMIZE].  The PCP base protocol document [RFC6887] specifies
   the message formats used, but the address to which a client sends its
   request is either assumed to be the default router (which is
   appropriate in a typical single-link residential network) or has to
   be configured otherwise via some external mechanism, such as a
   configuration file or a DHCP option [RFC7291].

   This document follows a different approach: it establishes two well-
   known anycast addresses for the PCP server, one IPv4 address and one
   IPv6 address.  PCP clients usually send PCP requests to these well-
   known addresses if no other PCP server addresses are known or after
   communication attempts to such other addresses have failed.  The
   anycast addresses are allocated from pools of special-purpose IP
   addresses (see Section 4), in accordance with Section 3.4 of
   [RFC4085].  Yet, a means to disable or override these well-known
   addresses (e.g., a configuration file option) should be available in

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   Using an anycast address is particularly useful in larger network
   topologies.  For example, if the PCP-enabled NAT/firewall function is
   not located on the client's default gateway but further upstream in a
   Carrier-Grade NAT (CGN), sending PCP requests to the default
   gateway's IP address will not have the desired effect.  When using a
   configuration file or the DHCP option to learn the PCP server's IP
   address, this file or the DHCP server configuration must reflect the
   network topology and the router and CGN configuration.  This may be
   cumbersome to achieve and maintain.  If there is more than one
   upstream CGN and traffic is routed using a dynamic routing protocol
   such as OSPF, this approach may not be feasible at all, as it cannot
   provide timely information regarding which CGN to interact with.  In
   contrast, when using the PCP anycast address, the PCP request will
   travel through the network like any other packet (i.e., without any
   special support from DNS, DHCP, other routers, or anything else)
   until it reaches the PCP-capable device that receives it, handles it,
   and sends back a reply.  A further advantage of using an anycast
   address instead of a DHCP option is that the anycast address can be
   hard-coded into the application.  There is no need for an application
   programming interface that passes the PCP server's address from the
   operating system's DHCP client to the application.  For further
   discussion of deployment considerations, see Section 3.

2.  PCP Server Discovery Based on Well-Known IP Address

2.1.  PCP Discovery Client Behavior

   PCP clients can add the PCP anycast addresses, which are defined in
   Sections 4.1 and 4.2, after the default router list (for IPv4 and
   IPv6) to the list of PCP server(s) (see step 2 in Section 8.1 of
   [RFC6887]).  This list is processed as specified in [RFC7488].

   Note: If, in some specific scenario, it was desirable to use only the
   anycast address (and not the default router), this could be achieved
   by putting the anycast address into the configuration file or DHCP

2.2.  PCP Discovery Server Behavior

   PCP servers can be configured to listen on the anycast addresses for
   incoming PCP requests.  When a PCP server receives a PCP request
   destined for an anycast address it supports, it sends the
   corresponding PCP replies using that same anycast address as the
   source address (see the "How UDP and TCP Use Anycasting" section of
   [RFC1546] for further discussion).

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3.  Deployment Considerations

   For general recommendations regarding operation of anycast services,
   see [RFC4786].  Architectural considerations of IP anycast are
   discussed in [RFC7094].

   In some deployment scenarios, using PCP anycasting may have certain
   limitations that can be overcome by using additional mechanisms or by
   using other PCP server discovery methods instead, such as DHCP
   [RFC7291] or a configuration file.

   One important example is a network topology in which a network is
   connected to one or more upstream network(s) via several parallel
   firewalls, each individually controlled by its own PCP server.  Even
   if all of these PCP servers are configured for anycasting, only one
   will receive the messages sent by a given client, depending on the
   state of the routing tables.

   As long as routing is always symmetric, i.e., all upstream and
   downstream packets from/to that client are routed through this very
   same firewall, communication will be possible as expected.  If there
   is a routing change, a PCP client using PCP anycasting might start
   interacting with a different PCP server.  From the PCP client's point
   of view, this would be the same as a PCP server reboot and the client
   could detect it by examining the Epoch field during the next PCP
   response or ANNOUNCE message.  The client would re-establish the
   firewall rules and packet flows could resume.

   If, however, routing is asymmetric, upstream packets from a client
   traverse a different firewall than the downstream packets to that
   client.  Establishing policy rules in only one of these two firewalls
   by means of PCP anycasting will not have the desired result of
   allowing bidirectional connectivity.  One solution approach to
   overcome this problem is an implementation-specific mechanism to
   synchronize state between all firewalls at the border of a network,
   i.e., a PEER message sent to any of these PCP servers would establish
   rules in all firewalls.  Another approach would be to use a different
   discovery mechanism (e.g., DHCP or a configuration file) that allows
   a PCP client to acquire a list of all PCP servers controlling the
   parallel firewalls and configure each of them individually.

   PCP anycast as such allows a PCP client to communicate only with its
   closest upstream PCP server.  However, it may be used in conjunction
   with the PCP proxy function [RFC7648], in order to support scenarios
   with cascaded PCP-enabled NATs or firewalls.

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4.  IANA Considerations

4.1.  Registration of an IPv4 Special-Purpose Address

   IANA has assigned a single IPv4 address from the prefix
   and registered it in the "IANA IPv4 Special-Purpose Address Registry"

   | Attribute            | Value                                     |
   | Address Block        |                              |
   | Name                 | Port Control Protocol Anycast             |
   | RFC                  | RFC 7723 (this document)                  |
   | Allocation Date      | October 2015                              |
   | Termination Date     | N/A                                       |
   | Source               | True                                      |
   | Destination          | True                                      |
   | Forwardable          | True                                      |
   | Global               | True                                      |
   | Reserved-by-Protocol | False                                     |

4.2.  Registration of an IPv6 Special-Purpose Address

   IANA has assigned a single IPv6 address from the 2001:0000::/23
   prefix and registered it in the "IANA IPv6 Special-Purpose Address
   Registry" [RFC6890].

   | Attribute            | Value                                     |
   | Address Block        | 2001:1::1/128                             |
   | Name                 | Port Control Protocol Anycast             |
   | RFC                  | RFC 7723 (this document)                  |
   | Allocation Date      | October 2015                              |
   | Termination Date     | N/A                                       |
   | Source               | True                                      |
   | Destination          | True                                      |
   | Forwardable          | True                                      |
   | Global               | True                                      |
   | Reserved-by-Protocol | False                                     |

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5.  Security Considerations

   In addition to the security considerations in [RFC6887], [RFC4786],
   and [RFC7094], two further security issues are considered here.

5.1.  Information Leakage through Anycast

   In a network without any border gateway, NAT, or firewall that is
   aware of the PCP anycast address, outgoing PCP requests could leak
   out onto the external Internet, possibly revealing information about
   internal devices.

   Using an IANA-assigned, well-known PCP anycast address enables border
   gateways to block such outgoing packets.  In the default-free zone,
   routers should be configured to drop such packets.  Such
   configuration can occur naturally via BGP messages advertising that
   no route exists to said address.

   Sensitive clients that do not wish to leak information about their
   presence can set an IP TTL on their PCP requests that limits how far
   they can travel towards the public Internet.  However, methods for
   choosing an appropriate TTL value, e.g., based on the assumed radius
   of the trusted network domain, is beyond the scope of this document.

   Before sending PCP requests with possibly privacy-sensitive
   parameters (e.g., IP addresses and port numbers) to the PCP anycast
   addresses, PCP clients can send an ANNOUNCE request (without
   parameters; see Section 14.1 of [RFC6887]) in order to probe whether
   a PCP server consumes and processes PCP requests sent to that anycast

5.2.  Hijacking of PCP Messages Sent to Anycast Addresses

   The anycast addresses are treated by normal host operating systems as
   normal unicast addresses, i.e., packets destined for an anycast
   address are sent to the default router for processing and forwarding.
   Hijacking such packets in the first network segment would effectively
   require the attacker to impersonate the default router, e.g., by
   means of ARP spoofing in an Ethernet network.  Once an anycast
   message is forwarded closer to the core network, routing will likely
   become subject to dynamic routing protocols such as OSPF or BGP.
   Anycast messages could be hijacked by announcing counterfeited
   messages in these routing protocols.  When analyzing the risk and
   possible consequences of such attacks in a given network scenario,
   the probable impacts on PCP signaling need to be put into proportion
   with probable impacts on other protocols such as the actual
   application protocols.

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RFC 7723                  PCP Anycast Addresses             January 2016

   In addition to following best current practices in first-hop security
   and routing-protocol security, PCP authentication [RFC7652] may be
   useful in some scenarios.  However, the effort needed for a proper
   setup of this authentication mechanism (e.g., installing the right
   shared secrets or cryptographic keys on all involved systems) may
   thwart the goal of fully automatic configuration by using PCP
   anycast.  Therefore, this approach may be less suitable for scenarios
   with high trust between the operator of the PCP-controlled middlebox
   and all users (e.g., a residential gateway used only by family
   members) or in scenarios in which there is rather limited trust that
   the middlebox will behave correctly (e.g., the Wi-Fi in an airport
   lounge).  In contrast, this scheme may be highly useful in scenarios
   with many users and a trusted network operator, such as a large
   corporate network or a university campus network, which uses several
   parallel NATs or firewalls to connect to the Internet.  Therefore, a
   thorough analysis of the benefits and costs of using PCP
   authentication in a given network scenario is recommended.

6.  References

6.1.  Normative References

   [RFC6887]  Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
              P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
              DOI 10.17487/RFC6887, April 2013,

   [RFC6890]  Cotton, M., Vegoda, L., Bonica, R., Ed., and B. Haberman,
              "Special-Purpose IP Address Registries", BCP 153,
              RFC 6890, DOI 10.17487/RFC6890, April 2013,

   [RFC7488]  Boucadair, M., Penno, R., Wing, D., Patil, P., and T.
              Reddy, "Port Control Protocol (PCP) Server Selection",
              RFC 7488, DOI 10.17487/RFC7488, March 2015,

6.2.  Informative References

              Reddy, T., Patil, P., Isomaki, M., and D. Wing,
              "Optimizing NAT and Firewall Keepalives Using Port Control
              Protocol (PCP)", Work in Progress,
              draft-ietf-pcp-optimize-keepalives-06, May 2015.

   [RFC1546]  Partridge, C., Mendez, T., and W. Milliken, "Host
              Anycasting Service", RFC 1546, DOI 10.17487/RFC1546,
              November 1993, <http://www.rfc-editor.org/info/rfc1546>.

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RFC 7723                  PCP Anycast Addresses             January 2016

   [RFC4085]  Plonka, D., "Embedding Globally-Routable Internet
              Addresses Considered Harmful", BCP 105, RFC 4085,
              DOI 10.17487/RFC4085, June 2005,

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <http://www.rfc-editor.org/info/rfc4786>.

   [RFC7094]  McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
              "Architectural Considerations of IP Anycast", RFC 7094,
              DOI 10.17487/RFC7094, January 2014,

   [RFC7291]  Boucadair, M., Penno, R., and D. Wing, "DHCP Options for
              the Port Control Protocol (PCP)", RFC 7291,
              DOI 10.17487/RFC7291, July 2014,

   [RFC7648]  Perreault, S., Boucadair, M., Penno, R., Wing, D., and S.
              Cheshire, "Port Control Protocol (PCP) Proxy Function",
              RFC 7648, DOI 10.17487/RFC7648, September 2015,

   [RFC7652]  Cullen, M., Hartman, S., Zhang, D., and T. Reddy, "Port
              Control Protocol (PCP) Authentication Mechanism",
              RFC 7652, DOI 10.17487/RFC7652, September 2015,


   The authors would like to thank the members of the PCP working group
   for contributions and feedback, in particular, Mohamed Boucadair,
   Charles Eckel, Simon Perreault, Tirumaleswar Reddy, Markus Stenberg,
   Dave Thaler, and Dan Wing.

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RFC 7723                  PCP Anycast Addresses             January 2016

Authors' Addresses

   Sebastian Kiesel
   University of Stuttgart Information Center
   Networks and Communication Systems Department
   Allmandring 30
   Stuttgart  70550

   Email: ietf-pcp@skiesel.de

   Reinaldo Penno
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   United States

   Email: repenno@cisco.com

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