path: root/doc/rfc/rfc6887.txt

Internet Engineering Task Force (IETF)                      D. Wing, Ed.
Request for Comments: 6887                                         Cisco
Category: Standards Track                                    S. Cheshire
ISSN: 2070-1721                                                    Apple
                                                            M. Boucadair
                                                          France Telecom
                                                                R. Penno
                                                                   Cisco
                                                              P. Selkirk
                                                                     ISC
                                                              April 2013


                      Port Control Protocol (PCP)

Abstract

   The Port Control Protocol allows an IPv6 or IPv4 host to control how
   incoming IPv6 or IPv4 packets are translated and forwarded by a
   Network Address Translator (NAT) or simple firewall, and also allows
   a host to optimize its outgoing NAT keepalive messages.

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/rfc6887.

Copyright Notice

   Copyright (c) 2013 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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Deployment Scenarios . . . . . . . . . . . . . . . . . . .  5
     2.2.  Supported Protocols  . . . . . . . . . . . . . . . . . . .  5
     2.3.  Single-Homed Customer Premises Network . . . . . . . . . .  5
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Relationship between PCP Server and Its PCP-Controlled
       Device . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Note on Fixed-Size Addresses . . . . . . . . . . . . . . . . . 10
   6.  Protocol Design Note . . . . . . . . . . . . . . . . . . . . . 11
   7.  Common Request and Response Header Format  . . . . . . . . . . 13
     7.1.  Request Header . . . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Response Header  . . . . . . . . . . . . . . . . . . . . . 15
     7.3.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     7.4.  Result Codes . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  General PCP Operation  . . . . . . . . . . . . . . . . . . . . 20
     8.1.  General PCP Client: Generating a Request . . . . . . . . . 21
       8.1.1.  PCP Client Retransmission  . . . . . . . . . . . . . . 22
     8.2.  General PCP Server: Processing a Request . . . . . . . . . 24
     8.3.  General PCP Client: Processing a Response  . . . . . . . . 25
     8.4.  Multi-Interface Issues . . . . . . . . . . . . . . . . . . 27
     8.5.  Epoch  . . . . . . . . . . . . . . . . . . . . . . . . . . 27
   9.  Version Negotiation  . . . . . . . . . . . . . . . . . . . . . 29
   10. Introduction to MAP and PEER Opcodes . . . . . . . . . . . . . 30
     10.1. For Operating a Server . . . . . . . . . . . . . . . . . . 33
     10.2. For Operating a Symmetric Client/Server  . . . . . . . . . 35
     10.3. For Reducing NAT or Firewall Keepalive Messages  . . . . . 37
     10.4. For Restoring Lost Implicit TCP Dynamic Mapping State  . . 38
   11. MAP Opcode . . . . . . . . . . . . . . . . . . . . . . . . . . 39
     11.1. MAP Operation Packet Formats . . . . . . . . . . . . . . . 40
     11.2. Generating a MAP Request . . . . . . . . . . . . . . . . . 43
       11.2.1. Renewing a Mapping . . . . . . . . . . . . . . . . . . 44
     11.3. Processing a MAP Request . . . . . . . . . . . . . . . . . 44
     11.4. Processing a MAP Response  . . . . . . . . . . . . . . . . 48
     11.5. Address Change Events  . . . . . . . . . . . . . . . . . . 49
     11.6. Learning the External IP Address Alone . . . . . . . . . . 50
   12. PEER Opcode  . . . . . . . . . . . . . . . . . . . . . . . . . 50
     12.1. PEER Operation Packet Formats  . . . . . . . . . . . . . . 51
     12.2. Generating a PEER Request  . . . . . . . . . . . . . . . . 54
     12.3. Processing a PEER Request  . . . . . . . . . . . . . . . . 55
     12.4. Processing a PEER Response . . . . . . . . . . . . . . . . 56





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   13. Options for MAP and PEER Opcodes . . . . . . . . . . . . . . . 57
     13.1. THIRD_PARTY Option for MAP and PEER Opcodes  . . . . . . . 57
     13.2. PREFER_FAILURE Option for MAP Opcode . . . . . . . . . . . 59
     13.3. FILTER Option for MAP Opcode . . . . . . . . . . . . . . . 61
   14. Rapid Recovery . . . . . . . . . . . . . . . . . . . . . . . . 63
     14.1. ANNOUNCE Opcode  . . . . . . . . . . . . . . . . . . . . . 64
       14.1.1. ANNOUNCE Operation . . . . . . . . . . . . . . . . . . 65
       14.1.2. Generating and Processing a Solicited ANNOUNCE
               Message  . . . . . . . . . . . . . . . . . . . . . . . 65
       14.1.3. Generating and Processing an Unsolicited ANNOUNCE
               Message  . . . . . . . . . . . . . . . . . . . . . . . 66
     14.2. PCP Mapping Update . . . . . . . . . . . . . . . . . . . . 67
   15. Mapping Lifetime and Deletion  . . . . . . . . . . . . . . . . 69
     15.1. Lifetime Processing for the MAP Opcode . . . . . . . . . . 71
   16. Implementation Considerations  . . . . . . . . . . . . . . . . 72
     16.1. Implementing MAP with EDM Port-Mapping NAT . . . . . . . . 72
     16.2. Lifetime of Explicit and Implicit Dynamic Mappings . . . . 72
     16.3. PCP Failure Recovery . . . . . . . . . . . . . . . . . . . 72
       16.3.1. Recreating Mappings  . . . . . . . . . . . . . . . . . 73
       16.3.2. Maintaining Mappings . . . . . . . . . . . . . . . . . 73
       16.3.3. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . 74
     16.4. Source Address Replicated in PCP Header  . . . . . . . . . 75
     16.5. State Diagram  . . . . . . . . . . . . . . . . . . . . . . 76
   17. Deployment Considerations  . . . . . . . . . . . . . . . . . . 77
     17.1. Ingress Filtering  . . . . . . . . . . . . . . . . . . . . 77
     17.2. Mapping Quota  . . . . . . . . . . . . . . . . . . . . . . 77
   18. Security Considerations  . . . . . . . . . . . . . . . . . . . 78
     18.1. Simple Threat Model  . . . . . . . . . . . . . . . . . . . 78
       18.1.1. Attacks Considered . . . . . . . . . . . . . . . . . . 79
       18.1.2. Deployment Examples Supporting the Simple Threat
               Model  . . . . . . . . . . . . . . . . . . . . . . . . 79
     18.2. Advanced Threat Model  . . . . . . . . . . . . . . . . . . 80
     18.3. Residual Threats . . . . . . . . . . . . . . . . . . . . . 80
       18.3.1. Denial of Service  . . . . . . . . . . . . . . . . . . 80
       18.3.2. Ingress Filtering  . . . . . . . . . . . . . . . . . . 81
       18.3.3. Mapping Theft  . . . . . . . . . . . . . . . . . . . . 81
       18.3.4. Attacks against Server Discovery . . . . . . . . . . . 81
   19. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 82
     19.1. Port Number  . . . . . . . . . . . . . . . . . . . . . . . 82
     19.2. Opcodes  . . . . . . . . . . . . . . . . . . . . . . . . . 82
     19.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 82
     19.4. Options  . . . . . . . . . . . . . . . . . . . . . . . . . 82
   20. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 83
   21. References . . . . . . . . . . . . . . . . . . . . . . . . . . 84
     21.1. Normative References . . . . . . . . . . . . . . . . . . . 84
     21.2. Informative References . . . . . . . . . . . . . . . . . . 84
   Appendix A. NAT-PMP Transition . . . . . . . . . . . . . . . . . . 87




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1.  Introduction

   The Port Control Protocol (PCP) provides a mechanism to control how
   incoming packets are forwarded by upstream devices such as Network
   Address Translator IPv6/IPv4 (NAT64), Network Address Translator
   IPv4/IPv4 (NAT44), and IPv6 and IPv4 firewall devices, and a
   mechanism to reduce application keepalive traffic.  PCP is designed
   to be implemented in the context of Carrier-Grade NATs (CGNs) and
   small NATs (e.g., residential NATs), as well as with dual-stack and
   IPv6-only Customer Premises Equipment (CPE) routers, and all of the
   currently known transition scenarios towards IPv6-only CPE routers.
   PCP allows hosts to operate servers for a long time (e.g., a network-
   attached home security camera) or a short time (e.g., while playing a
   game or on a phone call) when behind a NAT device, including when
   behind a CGN operated by their Internet service provider or an IPv6
   firewall integrated in their CPE router.

   PCP allows applications to create mappings from an external IP
   address, protocol, and port to an internal IP address, protocol, and
   port.  These mappings are required for successful inbound
   communications destined to machines located behind a NAT or a
   firewall.

   After creating a mapping for incoming connections, it is necessary to
   inform remote computers about the IP address, protocol, and port for
   the incoming connection.  This is usually done in an application-
   specific manner.  For example, a computer game might use a rendezvous
   server specific to that game (or specific to that game developer), a
   SIP phone would use a SIP proxy, and a client using DNS-Based Service
   Discovery [RFC6763] would use DNS Update [RFC2136] [RFC3007].  PCP
   does not provide this rendezvous function.  The rendezvous function
   may support IPv4, IPv6, or both.  Depending on that support and the
   application's support of IPv4 or IPv6, the PCP client may need an
   IPv4 mapping, an IPv6 mapping, or both.

   Many NAT-friendly applications send frequent application-level
   messages to ensure that their session will not be timed out by a NAT.
   These are commonly called "NAT keepalive" messages, even though they
   are not sent to the NAT itself (rather, they are sent 'through' the
   NAT).  These applications can reduce the frequency of such NAT
   keepalive messages by using PCP to learn (and influence) the NAT
   mapping lifetime.  This helps reduce bandwidth on the subscriber's
   access network, traffic to the server, and battery consumption on
   mobile devices.

   Many NATs and firewalls include Application Layer Gateways (ALGs) to
   create mappings for applications that establish additional streams or
   accept incoming connections.  ALGs incorporated into NATs may also



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   modify the application payload.  Industry experience has shown that
   these ALGs are detrimental to protocol evolution.  PCP allows an
   application to create its own mappings in NATs and firewalls,
   reducing the incentive to deploy ALGs in NATs and firewalls.

2.  Scope

2.1.  Deployment Scenarios

   PCP can be used in various deployment scenarios, including:

   o  Basic NAT [RFC3022]

   o  Network Address and Port Translation [RFC3022], such as commonly
      deployed in residential NAT devices

   o  Carrier-Grade NAT [RFC6888]

   o  Dual-Stack Lite (DS-Lite) [RFC6333]

   o  NAT that is Layer-2 Aware [L2NAT]

   o  Dual-Stack Extra Lite [RFC6619]

   o  NAT64, both Stateless [RFC6145] and Stateful [RFC6146]

   o  IPv4 and IPv6 simple firewall control [RFC6092]

   o  IPv6-to-IPv6 Network Prefix Translation (NPTv6) [RFC6296]

2.2.  Supported Protocols

   The PCP Opcodes defined in this document are designed to support
   transport-layer protocols that use a 16-bit port number (e.g., TCP,
   UDP, Stream Control Transmission Protocol (SCTP) [RFC4960], and
   Datagram Congestion Control Protocol (DCCP) [RFC4340]).  Protocols
   that do not use a port number (e.g., Resource Reservation Protocol
   (RSVP), IP Encapsulating Security Payload (ESP) [RFC4303], ICMP, and
   ICMPv6) are supported for IPv4 firewall, IPv6 firewall, and NPTv6
   functions, but are out of scope for any NAT functions.

2.3.  Single-Homed Customer Premises Network

   PCP assumes a single-homed IP address model.  That is, for a given IP
   address of a host, only one default route exists to reach other hosts
   on the Internet from that source IP address.  This is important
   because after a PCP mapping is created and an inbound packet (e.g.,
   TCP SYN) is rewritten and delivered to a host, the outbound response



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   (e.g., TCP SYNACK) has to go through the same (reverse) path so it
   passes through the same NAT to have the necessary inverse rewrite
   performed.  This restriction exists because otherwise there would
   need to be a PCP-enabled NAT for every egress (because the host could
   not reliably determine which egress path packets would take), and the
   client would need to be able to reliably make the same internal/
   external mapping in every NAT gateway, which in general is not
   possible (because the other NATs might already have the necessary
   external port mapped to another host).

3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

   Internal Host:
      A host served by a NAT gateway, or protected by a firewall.  This
      is the host that will receive incoming traffic resulting from a
      PCP mapping request, or the host that initiated an implicit
      dynamic outbound mapping (e.g., by sending a TCP SYN) across a
      firewall or a NAT.

   Remote Peer Host:
      A host with which an internal host is communicating.  This can
      include another internal host (or even the same internal host); if
      a NAT is involved, the NAT would need to hairpin the traffic
      [RFC4787].

   Internal Address:
      The address of an internal host served by a NAT gateway or
      protected by a firewall.

   External Address:
      The address of an internal host as seen by other remote peers on
      the Internet with which the internal host is communicating, after
      translation by any NAT gateways on the path.  An external address
      is generally a public routable (i.e., non-private) address.  In
      the case of an internal host protected by a pure firewall, with no
      address translation on the path, its external address is the same
      as its internal address.

   Endpoint-Dependent Mapping (EDM):  A term applied to NAT operation
      where an implicit mapping created by outgoing traffic (e.g., TCP
      SYN) from a single internal address, protocol, and port to
      different remote peers and ports may be assigned different
      external ports, and a subsequent PCP mapping request for that



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      internal address, protocol, and port may be assigned yet another
      different external port.  This term encompasses both Address-
      Dependent Mapping and Address and Port-Dependent Mapping
      [RFC4787].

   Endpoint-Independent Mapping (EIM):  A term applied to NAT operation
      where all mappings from a single internal address, protocol, and
      port to different remote peers and ports are all assigned the same
      external address and port.

   Remote Peer Address:
      The address of a remote peer, as seen by the internal host.  A
      remote address is generally a publicly routable address.  In the
      case of a remote peer that is itself served by a NAT gateway, the
      remote address may in fact be the remote peer's external address,
      but since this remote translation is generally invisible to
      software running on the internal host, the distinction can safely
      be ignored for the purposes of this document.

   Third Party:
      In the common case, an internal host manages its own mappings
      using PCP requests, and the internal address of those mappings is
      the same as the source IP address of the PCP request packet.

      In the case where one device is managing mappings on behalf of
      some other device that does not implement PCP, the presence of the
      THIRD_PARTY option in the MAP request signifies that the specified
      address, rather than the source IP address of the PCP request
      packet, should be used as the internal address for the mapping.

   Mapping, Port Mapping, Port Forwarding:
      A NAT mapping creates a relationship between an internal IP
      address, protocol, and port, and an external IP address, protocol,
      and port.  More specifically, it creates a translation rule where
      packets destined *to* the external IP address, protocol, and port
      have their destination address and port translated to the internal
      address and port, and conversely, packets *from* the internal IP
      address, protocol, and port have their source address and port
      translated to the external address and port.  In the case of a
      pure firewall, the "mapping" is the identity function, translating
      an internal IP address, protocol, and port number to the same
      external IP address, protocol, and port number.  Firewall
      filtering, applied in addition to that identity mapping function,
      is separate from the mapping itself.







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   Mapping Types:
      There are three dimensions to classifying mapping types: how they
      are created (implicitly/explicitly), their primary purpose
      (outbound/inbound), and how they are deleted (dynamic/static).
      Implicit mappings are created as a side effect of some other
      operation; explicit mappings are created by a mechanism explicitly
      dealing with mappings.  Outbound mappings exist primarily to
      facilitate outbound communication; inbound mappings exist
      primarily to facilitate inbound communication.  Dynamic mappings
      are deleted when their lifetime expires, or through other protocol
      action; static mappings are permanent until the user chooses to
      delete them.

      *  Implicit dynamic mappings are created implicitly as a side
         effect of traffic such as an outgoing TCP SYN or outgoing UDP
         packet.  Such packets were not originally designed explicitly
         for creating NAT (or firewall) state, but they can have that
         effect when they pass through a NAT (or firewall) device.
         Implicit dynamic mappings usually have a finite lifetime,
         though this lifetime is generally not known to the client using
         them.

      *  Explicit dynamic mappings are created as a result of explicit
         PCP MAP and PEER requests.  Like a DHCP address lease, explicit
         dynamic mappings have a finite lifetime, and this lifetime is
         communicated to the client.  As with a DHCP address lease, if
         the client wants a mapping to persist the client must prove
         that it is still present by periodically renewing the mapping
         to prevent it from expiring.  If a PCP client goes away, then
         any mappings it created will be automatically cleaned up when
         they expire.

      *  Explicit static mappings are created by manual configuration
         (e.g., via command-line interface or other user interface) and
         persist until the user changes that manual configuration.

      Both implicit and explicit dynamic mappings are dynamic in the
      sense that they are created on demand, as requested (implicitly or
      explicitly) by the internal host, and have a lifetime.  After the
      lifetime, the mapping is deleted unless the lifetime is extended
      by action by the internal host (e.g., sending more traffic or
      sending another PCP request).

      Static mappings are, by their nature, always explicit.  Static
      mappings differ from explicit dynamic mappings in that their
      lifetime is effectively infinite (they exist until manually
      removed), but otherwise they behave exactly the same as explicit
      MAP mappings.



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      While all mappings are, by necessity, bidirectional (most Internet
      communication requires information to flow in both directions for
      successful operation), when talking about mappings, it can be
      helpful to identify them loosely according to their 'primary'
      purpose.

      *  Outbound mappings exist primarily to enable outbound
         communication.  For example, when a host calls connect() to
         make an outbound connection, a NAT gateway will create an
         implicit dynamic outbound mapping to facilitate that outbound
         communication.

      *  Inbound mappings exist primarily to enable listening servers to
         receive inbound connections.  Generally, when a client calls
         listen() to listen for inbound connections, a NAT gateway will
         not implicitly create any mapping to facilitate that inbound
         communication.  A PCP MAP request can be used explicitly to
         create a dynamic inbound mapping to enable the desired inbound
         communication.

      Explicit static (manual) mappings and explicit dynamic (MAP)
      mappings both allow internal hosts to receive inbound traffic that
      is not in direct response to any immediately preceding outbound
      communication (i.e., to allow internal hosts to operate a "server"
      that is accessible to other hosts on the Internet).

   PCP Client:
      A PCP software instance responsible for issuing PCP requests to a
      PCP server.  Several independent PCP clients can exist on the same
      host.  Several PCP clients can be located in the same local
      network.  A PCP client can issue PCP requests on behalf of a
      third-party device for which it is authorized to do so.  An
      interworking function from Universal Plug and Play Internet
      Gateway Device (UPnP IGDv1 [IGDv1]) to PCP is another example of a
      PCP client.  A PCP server in a NAT gateway that is itself a client
      of another NAT gateway (nested NAT) may itself act as a PCP client
      to the upstream NAT.

   PCP-Controlled Device:
      A NAT or firewall that controls or rewrites packet flows between
      internal hosts and remote peer hosts.  PCP manages the mappings on
      this device.

   PCP Server:
      A PCP software instance that resides on the PCP-Controlled Device
      that receives PCP requests from the PCP client and creates
      appropriate state in response to that request.




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   Subscriber:
      The unit of billing for a commercial ISP.  A subscriber may have a
      single IP address from the commercial ISP (which can be shared
      among multiple hosts using a NAT gateway, thereby making them
      appear to be a single host to the ISP) or may have multiple IP
      addresses provided by the commercial ISP.  In either case, the IP
      address or addresses provided by the ISP may themselves be further
      translated by a Carrier-Grade NAT (CGN) operated by the ISP.

4.  Relationship between PCP Server and Its PCP-Controlled Device

   The PCP server receives and responds to PCP requests.  The PCP server
   functionality is typically a capability of a NAT or firewall device,
   as shown in Figure 1.  It is also possible for the PCP functionality
   to be provided by some other device, which communicates with the
   actual NAT(s) or firewall(s) via some other proprietary mechanism, as
   long as from the PCP client's perspective such split operation is
   indistinguishable from the integrated case.

                                  +-----------------+
         +------------+           | NAT or firewall |
         | PCP client |-<network>-+      with       +---<Internet>
         +------------+           |    PCP server   |
                                  +-----------------+

                   Figure 1: PCP-Enabled NAT or Firewall

   A NAT or firewall device, between the PCP client and the Internet,
   might implement simple or advanced firewall functionality.  This may
   be a side effect of the technology implemented by the device (e.g., a
   network address and port translator, by virtue of its port rewriting,
   normally requires connections to be initiated from an inside host
   towards the Internet), or this might be an explicit firewall policy
   to deny unsolicited traffic from the Internet.  Some firewall devices
   deny certain unsolicited traffic from the Internet (e.g., TCP, UDP to
   most ports) but allow certain other unsolicited traffic from the
   Internet (e.g., UDP port 500 and IP ESP) [RFC6092].  Such default
   filtering (or lack thereof) is out of scope of PCP itself.  If a
   client device wants to receive traffic and supports PCP, and does not
   possess prior knowledge of such default filtering policy, it SHOULD
   use PCP to request the necessary mappings to receive the desired
   traffic.

5.  Note on Fixed-Size Addresses

   For simplicity in building and parsing request and response packets,
   PCP always uses fixed-size 128-bit IP address fields for both IPv6
   addresses and IPv4 addresses.



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   When the address field holds an IPv6 address, the fixed-size 128-bit
   IP address field holds the IPv6 address stored as is.

   When the address field holds an IPv4 address, an IPv4-mapped IPv6
   address [RFC4291] is used (::ffff:0:0/96).  This has the first 80
   bits set to zero and the next 16 set to one, while its last 32 bits
   are filled with the IPv4 address.  This is unambiguously
   distinguishable from a native IPv6 address, because an IPv4-mapped
   IPv6 address [RFC4291] would not be valid for a mapping.

   When checking for an IPv4-mapped IPv6 address, all of the first 96
   bits MUST be checked for the pattern -- it is not sufficient to check
   for ones in bits 81-96.

   The all-zeros IPv6 address MUST be expressed by filling the
   fixed-size 128-bit IP address field with all zeros (::).

   The all-zeros IPv4 address MUST be expressed by 80 bits of zeros,
   16 bits of ones, and 32 bits of zeros (::ffff:0:0).

6.  Protocol Design Note

   PCP can be viewed as a request/response protocol, much like many
   other UDP-based request/response protocols, and can be implemented
   perfectly well as such.  It can also be viewed as what might be
   called a hint/notification protocol, and this observation can help
   simplify implementations.

   Rather than viewing the message streams between PCP client and PCP
   server as following a strict request/response pattern, where every
   response is associated with exactly one request, the message flows
   can be viewed as two somewhat independent streams carrying
   information in opposite directions:

   o  A stream of hints flowing from PCP client to PCP server, where the
      client indicates to the server what it would like the state of its
      mappings to be, and

   o  A stream of notifications flowing from PCP server to PCP client,
      where the server informs the clients what the state of its
      mappings actually is.

   To an extent, some of this approach is required anyway in a UDP-based
   request/response protocol, since UDP packets can be lost, duplicated,
   or reordered.






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   In this view of the protocol, the client transmits hints to the
   server at various intervals signaling its desires, and the server
   transmits notifications to the client signaling the actual state of
   its mappings.  These two message flows are loosely correlated in that
   a client request (hint) usually elicits a server response
   (notification), but only loosely, in that a client request may result
   in no server response (in the case of packet loss), and a server
   response may be generated gratuitously without an immediately
   preceding client request (in the case where server configuration
   change, e.g., change of external IP address on a NAT gateway, results
   in a change of mapping state).

   The exact times that client requests are sent are influenced by a
   client timing state machine taking into account whether (i) the
   client has not yet received a response from the server for a prior
   request (retransmission), or (ii) the client has previously received
   a response from the server saying how long the indicated mapping
   would remain active (renewal).  This design philosophy is the reason
   why PCP's retransmissions and renewals are exactly the same packet on
   the wire.  Typically, retransmissions are sent with exponentially
   increasing intervals as the client waits for the server to respond,
   whereas renewals are sent with exponentially decreasing intervals as
   the expiry time approaches, but, from the server's point of view,
   both packets are identical, and both signal the client's desire that
   the stated mapping exist or continue to exist.

   A PCP server usually sends responses as a direct result of client
   requests, but not always.  For example, if a server is too overloaded
   to respond, it is allowed to silently ignore a request message and
   let the client retransmit.  Also, if external factors cause a NAT
   gateway or firewall's configuration to change, then the PCP server
   can send unsolicited responses to clients informing them of the new
   state of their mappings.  Such reconfigurations are expected to be
   rare, because of the disruption they can cause to clients, but should
   they happen, PCP provides a way for servers to communicate the new
   state to clients promptly, without having to wait for the next
   periodic renewal request.

   This design goal helps explain why PCP request and response messages
   have no transaction ID, because such a transaction ID is unnecessary,
   and would unnecessarily limit the protocol and unnecessarily
   complicate implementations.  A PCP server response (i.e.,
   notification) is self-describing and complete.  It communicates the
   internal and external addresses, protocol, and ports for a mapping,
   and its remaining lifetime.  If the client does in fact currently
   want such a mapping to exist, then it can identify the mapping in
   question from the internal address, protocol, and port, and update
   its state to reflect the current external address and port, and



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   remaining lifetime.  If a client does not currently want such a
   mapping to exist, then it can safely ignore the message.  No client
   action is required for unexpected mapping notifications.  In today's
   world, a NAT gateway can have a static mapping, and the client device
   has no explicit knowledge of this, and no way to change the fact.
   Also, in today's world, a client device can be connected directly to
   the public Internet, with a globally routable IP address, and, in
   this case, it effectively has "mappings" for all of its listening
   ports.  Such a device has to be responsible for its own security and
   cannot rely on assuming that some other network device will be
   blocking all incoming packets.

7.  Common Request and Response Header Format

   All PCP messages are sent over UDP, with a maximum UDP payload length
   of 1100 octets.  The PCP messages contain a request or response
   header containing an Opcode, any relevant Opcode-specific
   information, and zero or more options.  All numeric quantities larger
   than a single octet (e.g., result codes, lifetimes, Epoch times,
   etc.) are represented in conventional IETF network order, i.e., most
   significant octet first.  Non-numeric quantities are represented as
   is on all platforms, with no byte swapping (e.g., IP addresses and
   ports are placed in PCP messages using the same representation as
   when placed in IP or TCP headers).

   The packet layout for the common header, and operation of the PCP
   client and PCP server, are described in the following sections.  The
   information in this section applies to all Opcodes.  Behavior of the
   Opcodes defined in this document is described in Sections 10, 11, and
   12.





















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7.1.  Request Header

   All requests have the following format:

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Version = 2  |R|   Opcode    |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                 Requested Lifetime (32 bits)                  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |            PCP Client's IP Address (128 bits)                 |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     :             (optional) Opcode-specific information            :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     :             (optional) PCP Options                            :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: Common Request Packet Format

   These fields are described below:

   Version:  This document specifies protocol version 2.  PCP clients
      and servers compliant with this document use the value 2.  This
      field is used for version negotiation as described in Section 9.

   R: Indicates Request (0) or Response (1).

   Opcode:  A 7-bit value specifying the operation to be performed.  MAP
      and PEER Opcodes are defined in Sections 11 and 12.

   Reserved:  16 reserved bits.  MUST be zero on transmission and MUST
      be ignored on reception.

   Requested Lifetime:  An unsigned 32-bit integer, in seconds, ranging
      from 0 to 2^32-1 seconds.  This is used by the MAP and PEER
      Opcodes defined in this document for their requested lifetime.







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   PCP Client's IP Address:  The source IPv4 or IPv6 address in the IP
      header used by the PCP client when sending this PCP request.  An
      IPv4 address is represented using an IPv4-mapped IPv6 address.
      The PCP Client IP Address in the PCP message header is used to
      detect an unexpected NAT on the path between the PCP client and
      the PCP-controlled NAT or firewall device.  See Section 8.1.

   Opcode-specific information:  Payload data for this Opcode.  The
      length of this data is determined by the Opcode definition.

   PCP Options:  Zero, one, or more options that are legal for both a
      PCP request and for this Opcode.  See Section 7.3.

7.2.  Response Header

   All responses have the following format:

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Version = 2  |R|   Opcode    |   Reserved    |  Result Code  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Lifetime (32 bits)                       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Epoch Time (32 bits)                      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                      Reserved (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     :             (optional) Opcode-specific response data          :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :             (optional) Options                                :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: Common Response Packet Format

   These fields are described below:

   Version:  Responses from servers compliant with this specification
      MUST use version 2.  This is set by the server.

   R: Indicates Request (0) or Response (1).  All Responses MUST use 1.
      This is set by the server.





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   Opcode:  The 7-bit Opcode value.  The server copies this value from
      the request.

   Reserved:  8 reserved bits, MUST be sent as 0, MUST be ignored when
      received.  This is set by the server.

   Result Code:  The result code for this response.  See Section 7.4 for
      values.  This is set by the server.

   Lifetime:  An unsigned 32-bit integer, in seconds, ranging from 0 to
      2^32-1 seconds.  On an error response, this indicates how long
      clients should assume they'll get the same error response from
      that PCP server if they repeat the same request.  On a success
      response for the PCP Opcodes that create a mapping (MAP and PEER),
      the Lifetime field indicates the lifetime for this mapping.  This
      is set by the server.

   Epoch Time:  The server's Epoch Time value.  See Section 8.5 for
      discussion.  This value is set by the server, in both success and
      error responses.

   Reserved:  96 reserved bits.  For requests that were successfully
      parsed, this MUST be sent as 0, MUST be ignored when received.
      This is set by the server.  For requests that were not
      successfully parsed, the server copies the last 96 bits of the PCP
      Client's IP Address field from the request message into this
      corresponding 96-bit field of the response.

   Opcode-specific information:  Payload data for this Opcode.  The
      length of this data is determined by the Opcode definition.

   PCP Options:  Zero, one, or more options that are legal for both a
      PCP response and for this Opcode.  See Section 7.3.

7.3.  Options

   A PCP Opcode can be extended with one or more options.  Options can
   be used in requests and responses.  The design decisions in this
   specification about whether to include a given piece of information
   in the base Opcode format or in an option were an engineering trade-
   off between packet size and code complexity.  For information that is
   usually (or always) required, placing it in the fixed Opcode data
   results in simpler code to generate and parse the packet, because the
   information is a fixed location in the Opcode data, but wastes space
   in the packet in the event that field is all zeros because the
   information is not needed or not relevant.  For information that is
   required less often, placing it in an option results in slightly more
   complicated code to generate and parse packets containing that



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   option, but saves space in the packet when that information is not
   needed.  Placing information in an option also means that an
   implementation that never uses that information doesn't even need to
   implement code to generate and parse it.  For example, a client that
   never requests mappings on behalf of some other device doesn't need
   to implement code to generate the THIRD_PARTY option, and a PCP
   server that doesn't implement the necessary security measures to
   create third-party mappings safely doesn't need to implement code to
   parse the THIRD_PARTY option.

   Options use the following Type-Length-Value format:

      0                   1                   2                   3
      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 Code  |  Reserved     |       Option Length           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                       (optional) Data                         :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: Options Header

   The description of the fields is as follows:

   Option Code:  8 bits.  Its most significant bit indicates if this
      option is mandatory (0) or optional (1) to process.

   Reserved:  8 bits.  MUST be set to 0 on transmission and MUST be
      ignored on reception.

   Option Length:  16 bits.  Indicates the length of the enclosed data,
      in octets.  Options with length of 0 are allowed.  Options that
      are not a multiple of 4 octets long are followed by one, two, or
      three 0 octets to pad their effective length in the packet to be a
      multiple of 4 octets.  The Option Length reflects the semantic
      length of the option, not including any padding octets.

   Data:  Option data.

   If several options are included in a PCP request, they MAY be encoded
   in any order by the PCP client, but MUST be processed by the PCP
   server in the order in which they appear.  It is the responsibility
   of the PCP client to ensure that the server has sufficient room to
   reply without exceeding the 1100-octet size limit; if its reply would
   exceed that size, the server generates an error.






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   If, while processing a PCP request, including its options, an error
   is encountered that causes a PCP error response to be generated, the
   PCP request MUST cause no state change in the PCP server or the
   PCP-controlled device (i.e., it rolls back any tentative changes it
   might have made while processing the request).  Such an error
   response MUST consist of a complete copy of the request packet with
   the error code and other appropriate fields set in the header.

   An option MAY appear more than once in a request or in a response, if
   permitted by the definition of the option.  If the option's
   definition allows the option to appear only once but it appears more
   than once in a request, and the option is understood by the PCP
   server, the PCP server MUST respond with the MALFORMED_OPTION result
   code.  If the PCP server encounters an invalid option (e.g., PCP
   option length is longer than the UDP packet length), the error
   MALFORMED_OPTION SHOULD be returned (rather than MALFORMED_REQUEST),
   as that helps the client better understand how the packet was
   malformed.  If a PCP response would have exceeded the maximum PCP
   message size, the PCP server SHOULD respond with MALFORMED_REQUEST.

   If the overall option structure of a request cannot successfully be
   parsed (e.g., a nonsensical option length), the PCP server MUST
   generate an error response with code MALFORMED_OPTION.

   If the overall option structure of a request is valid, then how each
   individual option is handled is determined by the most significant
   bit in the option code.  If the most significant bit is set, handling
   this option is optional, and a PCP server MAY process or ignore this
   option, entirely at its discretion.  If the most significant bit is
   clear, handling this option is mandatory, and a PCP server MUST
   return the error MALFORMED_OPTION if the option contents are
   malformed, or UNSUPP_OPTION if the option is unrecognized,
   unimplemented, or disabled, or if the client is not authorized to use
   the option.  In error responses, all options are returned.  In
   success responses, all processed options are included and unprocessed
   options are not included.

   Because the PCP client cannot reject a response containing an Option,
   PCP clients MUST ignore Options that they do not understand that
   appear in responses, including Options in the mandatory-to-process
   range.  Naturally, if a client explicitly requests an Option where
   correct execution of that Option requires processing the Option data
   in the response, that client SHOULD implement code to do that.








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   Different options are valid for different Opcodes.  For example:

   o  The THIRD_PARTY option is valid for both MAP and PEER Opcodes.

   o  The FILTER option is valid only for the MAP Opcode (for the PEER
      Opcode it would have no meaning).

   o  The PREFER_FAILURE option is valid only for the MAP Opcode (for
      the PEER Opcode, similar semantics are automatically implied).

7.4.  Result Codes

   The following result codes may be returned as a result of any Opcode
   received by the PCP server.  The only success result code is 0; other
   values indicate an error.  If a PCP server encounters multiple errors
   during processing of a request, it SHOULD use the most specific error
   message.  Each error code below is classified as either a 'long
   lifetime' error or a 'short lifetime' error, which provides guidance
   to PCP server developers for the value of the Lifetime field for
   these errors.  It is RECOMMENDED that short lifetime errors use a
   30-second lifetime and long lifetime errors use a 30-minute lifetime.

   0  SUCCESS: Success.

   1  UNSUPP_VERSION: The version number at the start of the PCP Request
      header is not recognized by this PCP server.  This is a long
      lifetime error.  This document describes PCP version 2.

   2  NOT_AUTHORIZED: The requested operation is disabled for this PCP
      client, or the PCP client requested an operation that cannot be
      fulfilled by the PCP server's security policy.  This is a long
      lifetime error.

   3  MALFORMED_REQUEST: The request could not be successfully parsed.
      This is a long lifetime error.

   4  UNSUPP_OPCODE: Unsupported Opcode.  This is a long lifetime error.

   5  UNSUPP_OPTION: Unsupported option.  This error only occurs if the
      option is in the mandatory-to-process range.  This is a long
      lifetime error.

   6  MALFORMED_OPTION: Malformed option (e.g., appears too many times,
      invalid length).  This is a long lifetime error.

   7  NETWORK_FAILURE: The PCP server or the device it controls is
      experiencing a network failure of some sort (e.g., has not yet
      obtained an external IP address).  This is a short lifetime error.



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   8  NO_RESOURCES: Request is well-formed and valid, but the server has
      insufficient resources to complete the requested operation at this
      time.  For example, the NAT device cannot create more mappings at
      this time, is short of CPU cycles or memory, or is unable to
      handle the request due to some other temporary condition.  The
      same request may succeed in the future.  This is a system-wide
      error, different from USER_EX_QUOTA.  This can be used as a catch-
      all error, should no other error message be suitable.  This is a
      short lifetime error.

   9  UNSUPP_PROTOCOL: Unsupported transport protocol, e.g., SCTP in a
      NAT that handles only UDP and TCP.  This is a long lifetime error.

   10 USER_EX_QUOTA: This attempt to create a new mapping would exceed
      this subscriber's port quota.  This is a short lifetime error.

   11 CANNOT_PROVIDE_EXTERNAL: The suggested external port and/or
      external address cannot be provided.  This error MUST only be
      returned for:
      *  MAP requests that included the PREFER_FAILURE option
         (normal MAP requests will return an available external port)
      *  MAP requests for the SCTP protocol (PREFER_FAILURE is implied)
      *  PEER requests

      See Section 13.2 for details of the PREFER_FAILURE Option.  The
      error lifetime depends on the reason for the failure.

   12 ADDRESS_MISMATCH: The source IP address of the request packet does
      not match the contents of the PCP Client's IP Address field, due
      to an unexpected NAT on the path between the PCP client and the
      PCP-controlled NAT or firewall.  This is a long lifetime error.

   13 EXCESSIVE_REMOTE_PEERS: The PCP server was not able to create the
      filters in this request.  This result code MUST only be returned
      if the MAP request contained the FILTER option.  See Section 13.3
      for details of the FILTER Option.  This is a long lifetime error.

8.  General PCP Operation

   PCP messages MUST be sent over UDP [RFC0768].  Every PCP request
   generates at least one response, so PCP does not need to run over a
   reliable transport protocol.

   When receiving multiple identical requests, the PCP server will
   generally generate identical responses -- barring cases where the PCP
   server's state changes between those requests due to other activity.
   As an example of how such a state change could happen, a request
   could be received while the PCP-controlled device has no mappings



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   available, and the PCP server will generate an error response.  If
   mappings become available and then another copy of that same request
   arrives (perhaps duplicated in transit in the network), the PCP
   server will allocate a mapping and generate a non-error response.  A
   PCP client MUST handle such updated responses for any request it
   sends, most notably to support rapid recovery (Section 14).  Also see
   the Protocol Design Note (Section 6).

8.1.  General PCP Client: Generating a Request

   This section details operation specific to a PCP client, for any
   Opcode.  Procedures specific to the MAP Opcode are described in
   Section 11, and procedures specific to the PEER Opcode are described
   in Section 12.

   Prior to sending its first PCP message, the PCP client determines
   which server to use.  The PCP client performs the following steps to
   determine its PCP server:

   1.  if a PCP server is configured (e.g., in a configuration file or
       via DHCP), that single configuration source is used as the list
       of PCP server(s), else

   2.  the default router list (for IPv4 and IPv6) is used as the list
       of PCP server(s).  Thus, if a PCP client has both an IPv4 and
       IPv6 address, it will have an IPv4 PCP server (its IPv4 default
       router) for its IPv4 mappings, and an IPv6 PCP server (its IPv6
       default router) for its IPv6 mappings.

   For the purposes of this document, only a single PCP server address
   is supported.  Should future specifications define configuration
   methods that provide a longer list of PCP server addresses, those
   specifications will define how clients select one or more addresses
   from that list.

   With that PCP server address, the PCP client formulates its PCP
   request.  The PCP request contains a PCP common header, PCP Opcode
   and payload, and (possibly) options.  As with all UDP client software
   on any operating system, when several independent PCP clients exist
   on the same host, each uses a distinct source port number to
   disambiguate their requests and replies.  The PCP client's source
   port SHOULD be randomly generated [RFC6056].

   The PCP client MUST include the source IP address of the PCP message
   in the PCP request.  This is typically its own IP address; see
   Section 16.4 for how this can be coded.  This is used to detect an
   unexpected NAT on the path between the PCP client and the
   PCP-controlled NAT or firewall device, to avoid wasting resources on



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   the PCP-controlled NAT creating pointless non-functional mappings.
   When such an intervening non-PCP-aware inner NAT is detected,
   mappings must first be created by some other means in the inner NAT,
   before mappings can be usefully created in the outer PCP-controlled
   NAT.  Having created mappings in the inner NAT by some other means,
   the PCP client should then use the inner NAT's external address as
   the client IP address, to signal to the outer PCP-controlled NAT that
   the client is aware of the inner NAT, and has taken steps to create
   mappings in it by some other means, so that mappings created in the
   outer NAT will not be a pointless waste of resources.

8.1.1.  PCP Client Retransmission

   PCP clients are responsible for reliable delivery of PCP request
   messages.  If a PCP client fails to receive an expected response from
   a server, the client must retransmit its message.  The
   retransmissions MUST use the same Mapping Nonce value (see Sections
   11.1 and 12.1).  The client begins the message exchange by
   transmitting a message to the server.  The message exchange continues
   for as long as the client wishes to maintain the mapping, and
   terminates when the PCP client is no longer interested in the PCP
   transaction (e.g., the application that requested the mapping is no
   longer interested in the mapping) or (optionally) when the message
   exchange is considered to have failed according to the retransmission
   mechanism described below.

   The client retransmission behavior is controlled and described by the
   following variables:

     RT:   Retransmission timeout, calculated as described below

    IRT:   Initial retransmission time, SHOULD be 3 seconds

    MRC:   Maximum retransmission count, SHOULD be 0 (0 indicates no
           maximum)

    MRT:   Maximum retransmission time, SHOULD be 1024 seconds

    MRD:   Maximum retransmission duration, SHOULD be 0 (0 indicates no
           maximum)

   RAND:   Randomization factor, calculated as described below

   With each message transmission or retransmission, the client sets RT
   according to the rules given below.  If RT expires before a response
   is received, the client retransmits the request and computes a new
   RT.




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   Each of the computations of a new RT include a new randomization
   factor (RAND), which is a random number chosen with a uniform
   distribution between -0.1 and +0.1.  The randomization factor is
   included to minimize synchronization of messages transmitted by PCP
   clients.  The algorithm for choosing a random number does not need to
   be cryptographically sound.  The algorithm SHOULD produce a different
   sequence of random numbers from each invocation of the PCP client.

   The RT value is initialized based on IRT:

      RT = (1 + RAND) * IRT

   RT for each subsequent message transmission is based on the previous
   value of RT, subject to the upper bound on the value of RT specified
   by MRT.  If MRT has a value of 0, there is no upper limit on the
   value of RT, and MRT is treated as "infinity".  The new value of RT
   is calculated as shown below, where RTprev is the current value of
   RT:

      RT = (1 + RAND) * MIN (2 * RTprev, MRT)

   MRC specifies an upper bound on the number of times a client may
   retransmit a message.  Unless MRC is zero, the message exchange fails
   once the client has transmitted the message MRC times.

   MRD specifies an upper bound on the length of time a client may
   retransmit a message.  Unless MRD is zero, the message exchange fails
   once MRD seconds have elapsed since the client first transmitted the
   message.

   If both MRC and MRD are non-zero, the message exchange fails whenever
   either of the conditions specified in the previous two paragraphs are
   met.  If both MRC and MRD are zero, the client continues to transmit
   the message until it receives a response or the client no longer
   wants a mapping.

   Once a PCP client has successfully received a response from a PCP
   server on that interface, it resets RT to a value randomly selected
   in the range 1/2 to 5/8 of the mapping lifetime, as described in
   Section 11.2.1, "Renewing a Mapping", and sends subsequent PCP
   requests for that mapping to that same server.

      Note: If the server's state changes between retransmissions and
      the server's response is delayed or lost, the state in the PCP
      client and server may not be synchronized.  This is not unique to
      PCP, but also occurs with other network protocols (e.g., TCP).  In
      the unlikely event that such de-synchronization occurs, PCP heals
      itself after lifetime seconds.



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8.2.  General PCP Server: Processing a Request

   This section details operation specific to a PCP server.  Processing
   SHOULD be performed in the order of the following paragraphs.

   A PCP server MUST only accept normal (non-THIRD_PARTY) PCP requests
   from a client on the same interface from which it would normally
   receive packets from that client, and it MUST silently ignore PCP
   requests arriving on any other interface.  For example, a residential
   NAT gateway accepts PCP requests only when they arrive on its (LAN)
   interface connecting to the internal network, and silently ignores
   any PCP requests arriving on its external (WAN) interface.  A PCP
   server that supports THIRD_PARTY requests MAY be configured to accept
   THIRD_PARTY requests on other configured interfaces (see Section 13.1
   for details on the THIRD_PARTY Option).

   Upon receiving a request, the PCP server parses and validates it.  A
   valid request contains a valid PCP common header, one valid PCP
   Opcode, and zero or more options (which the server might or might not
   comprehend).  If an error is encountered during processing, the
   server generates an error response that is sent back to the PCP
   client.  Processing of an Opcode and its options is specific to each
   Opcode.

   Error responses have the same packet layout as success responses,
   with certain fields from the request copied into the response, and
   other fields assigned by the PCP server set as indicated in Figure 3.

   Copying request fields into the response is important because this is
   what enables a client to identify to which request a given response
   pertains.  For Opcodes that are understood by the PCP server, it
   follows the requirements of that Opcode to copy the appropriate
   fields.  For Opcodes that are not understood by the PCP server, it
   simply generates the UNSUPP_OPCODE response and copies fields from
   the PCP header and copies the rest of the PCP payload as is (without
   attempting to interpret it).

   All responses (both error and success) contain the same Opcode as the
   request, but with the "R" bit set.

   Any error response has a non-zero result code, and is created by:

   o  Copying the entire UDP payload, or 1100 octets, whichever is less,
      and zero-padding the response to a multiple of 4 octets if
      necessary
   o  Setting the R bit
   o  Setting the result code
   o  Setting the Lifetime, Epoch Time, and Reserved fields



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   o  Updating other fields in the response, as indicated by 'set by the
      server' in the PCP response field description

   A success response has a zero result code, and is created by:

   o  Copying the first 4 octets of request packet header
   o  Setting the R bit
   o  Setting the result code to zero
   o  Setting the Lifetime, Epoch Time, and Reserved fields
   o  Possibly setting Opcode-specific response data if appropriate
   o  Adding any processed options to the response message

   If the received PCP request message is less than 2 octets long, it is
   silently dropped.

   If the R bit is set, the message is silently dropped.

   If the first octet (version) is a version that is not supported, a
   response is generated with the UNSUPP_VERSION result code, and the
   Version Negotiation steps detailed in Section 9 are followed.

   Otherwise, if the version is supported but the received message is
   shorter than 24 octets, the message is silently dropped.

   If the server is overloaded by requests (from a particular client or
   from all clients), it MAY simply silently discard requests, as the
   requests will be retried by PCP clients, or it MAY generate the
   NO_RESOURCES error response.

   If the length of the message exceeds 1100 octets, is not a multiple
   of 4 octets, or is too short for the Opcode in question, it is
   invalid and a MALFORMED_REQUEST response is generated, and the
   response message is truncated to 1100 octets.

   The PCP server compares the source IP address (from the received IP
   header) with the field PCP Client IP Address.  If they do not match,
   the error ADDRESS_MISMATCH MUST be returned.  This is done to detect
   and prevent accidental use of PCP where a non-PCP-aware NAT exists
   between the PCP client and PCP server.  If the PCP client wants such
   a mapping, it needs to ensure that the PCP field matches its apparent
   IP address from the perspective of the PCP server.

8.3.  General PCP Client: Processing a Response

   The PCP client receives the response and verifies that the source IP
   address and port belong to the PCP server of a previously sent PCP
   request.  If not, the response is silently dropped.




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   If the received PCP response message is less than 4 octets long, it
   is silently dropped.

   If the R bit is clear, the message is silently dropped.

   If the error code is UNSUPP_VERSION, Version Negotiation processing
   continues as described in Section 9.

   Responses shorter than 24 octets, longer than 1100 octets, or not a
   multiple of 4 octets are invalid and ignored.

   The PCP client then validates that the Opcode matches a previous PCP
   request.  If the response does not match a previous PCP request, the
   response is ignored.  The response is further matched by comparing
   fields in the response Opcode-specific data to fields in the request
   Opcode-specific data, as described by the processing for that Opcode.
   If that fails, the response is ignored.

   After these matches are successful, the PCP client checks the Epoch
   Time field (see Section 8.5) to determine if it needs to restore its
   state to the PCP server.  A PCP client SHOULD be prepared to receive
   multiple responses from the PCP server at any time after a single
   request is sent.  This allows the PCP server to inform the client of
   mapping changes such as an update or deletion.  For example, a PCP
   server might send a SUCCESS response and, after a configuration
   change on the PCP server, later send a NOT_AUTHORIZED response.  A
   PCP client MUST be prepared to receive responses for requests it
   never sent (which could have been sent by a previous PCP instance on
   this same host, or by a previous host that used the same client IP
   address, or by a malicious attacker) by simply ignoring those
   unexpected messages.

   If the error ADDRESS_MISMATCH is received, it indicates the presence
   of a NAT between the PCP client and PCP server.  Procedures to
   resolve this problem are beyond the scope of this document.

   For both success and error responses, a Lifetime value is returned.
   The lifetime indicates how long this response should be considered
   valid by the client (i.e for success results, how long the mapping
   will last, and for failure results how long the same failure
   condition should be expected to persist).  The PCP client SHOULD
   impose an upper limit on this returned value (to protect against
   absurdly large values, e.g., 5 years), detailed in Section 15,
   "Mapping Lifetime and Deletion".

   If the result code is 0 (SUCCESS), the request succeeded.





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   If the result code is not 0, the request failed, and the PCP client
   SHOULD NOT resend the same request for the indicated lifetime of the
   error (as limited by the sanity checking detailed in Section 15).

   If the PCP client has discovered a new PCP server (e.g., connected to
   a new network), the PCP client MAY immediately begin communicating
   with this PCP server, without regard to hold times from communicating
   with a previous PCP server.

8.4.  Multi-Interface Issues

   Hosts that desire a PCP mapping might be multi-interfaced (i.e., own
   several logical/physical interfaces).  Indeed, a host can be
   configured with several IPv4 addresses (e.g., WiFi and Ethernet) or
   dual-stacked.  These IP addresses may have distinct reachability
   scopes (e.g., if IPv6, they might have global reachability scope as
   is the case for a Global Unicast Address (GUA) [RFC3587] or limited
   scope as is the case for a Unique Local Address (ULA) [RFC4193]).

   IPv6 addresses with global reachability (e.g., GUAs) SHOULD be used
   as the source address when generating a PCP request.  IPv6 addresses
   without global reachability (e.g., ULAs) SHOULD NOT be used as the
   source interface when generating a PCP request.  If IPv6 privacy
   addresses [RFC4941] are used for PCP mappings, a new PCP request will
   need to be issued whenever the IPv6 privacy address is changed.  This
   PCP request SHOULD be sent from the IPv6 privacy address itself.  It
   is RECOMMENDED that the client delete its mappings to the previous
   privacy address after it no longer needs those old mappings.

   Due to the ubiquity of IPv4 NAT, IPv4 addresses with limited scope
   (e.g., private addresses [RFC1918]) MAY be used as the source
   interface when generating a PCP request.

8.5.  Epoch

   Every PCP response sent by the PCP server includes an Epoch Time
   field.  This time field increments by one every second.  Anomalies in
   the received Epoch Time value provide a hint to PCP clients that a
   PCP server state loss may have occurred.  Clients respond to such
   state loss hints by promptly renewing their mappings, so as to
   quickly restore any lost state at the PCP server.

   If the PCP server resets or loses the state of its explicit dynamic
   mappings (that is, those mappings created by PCP requests), due to
   reboot, power failure, or any other reason, it MUST reset its Epoch
   time to its initial starting value (usually zero) to provide this
   hint to PCP clients.  After resetting its Epoch time, the PCP server
   resumes incrementing the Epoch Time value by one every second.



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   Similarly, if the external IP address(es) of the NAT (controlled by
   the PCP server) changes, the Epoch time MUST be reset.  A PCP server
   MAY maintain one Epoch Time value for all PCP clients or MAY maintain
   distinct Epoch Time values (per PCP client, per interface, or based
   on other criteria); this choice is implementation-dependent.

   Whenever a client receives a PCP response, the client validates the
   received Epoch Time value according to the procedure below, using
   integer arithmetic:

   o  If this is the first PCP response the client has received from
      this PCP server, the Epoch Time value is treated as necessarily
      valid, otherwise

      *  If the current PCP server Epoch time (curr_server_time) is less
         than the previously received PCP server Epoch time
         (prev_server_time) by more than one second, then the client
         treats the Epoch time as obviously invalid (time should not go
         backwards).  The server Epoch time apparently going backwards
         by *up to* one second is not deemed invalid, so that minor
         packet reordering on the path from PCP server to PCP client
         does not trigger a cascade of unnecessary mapping renewals.  If
         the server Epoch time passes this check, then further
         validation checks are performed:

         +  The client computes the difference between its
            current local time (curr_client_time) and the
            time the previous PCP response was received from this PCP
            server (prev_client_time):
            client_delta = curr_client_time - prev_client_time;

         +  The client computes the difference between the
            current PCP server Epoch time (curr_server_time) and the
            previously received Epoch time (prev_server_time):
            server_delta = curr_server_time - prev_server_time;

         +  If client_delta+2 < server_delta - server_delta/16
            or server_delta+2 < client_delta - client_delta/16,
            then the client treats the Epoch Time value as invalid,
            else the client treats the Epoch Time value as valid.

   o  The client records the current time values for use in its next
      comparison:
      prev_client_time = curr_client_time
      prev_server_time = curr_server_time






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   If the PCP client determined that the Epoch Time value it received
   was invalid, then it concludes that the PCP server may have lost
   state, and promptly renews all its active port mapping leases
   following the mapping recreation procedure described in
   Section 16.3.1.

   Notes:

   o  The client clock MUST never go backwards.  If curr_client_time is
      found to be less than prev_client_time, then this is a client bug,
      and how the client deals with this client bug is implementation
      specific.

   o  The calculations above are constructed to allow client_delta and
      server_delta to be computed as unsigned integer values.

   o  The "+2" in the calculations above is to accommodate quantization
      errors in client and server clocks (up to one-second quantization
      error each in server and client time intervals).

   o  The "/16" in the calculations above is to accommodate inaccurate
      clocks in low-cost devices.  This allows for a total discrepancy
      of up to 1/16 (6.25%) to be considered benign; e.g., if one clock
      were to run too fast by 3% while the other clock ran too slow by
      3%, then the client would not consider this difference to be
      anomalous or indicative of a restart having occurred.  This
      tolerance is strict enough to be effective at detecting reboots,
      while not being so strict as to generate false alarms.

9.  Version Negotiation

   A PCP client sends its requests using PCP version number 2.  Should
   later updates to this document specify different message formats with
   a version number greater than 2, it is expected that PCP servers will
   still support version 2 in addition to the newer version(s).
   However, in the event that a server returns a response with result
   code UNSUPP_VERSION, the client MAY log an error message to inform
   the user that it is too old to work with this server.

   Should later updates to this document specify different message
   formats with a version number greater than 2, and backwards
   compatibility be desired, this first octet can be used for forward
   and backward compatibility.








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   If future PCP versions greater than 2 are specified, version
   negotiation proceeds as follows:

   1.  The client sends its first request using the highest
       (i.e., presumably 'best') version number it supports.

   2.  If the server supports that version, it responds normally.

   3.  If the server does not support that version, it replies giving a
       result containing the result code UNSUPP_VERSION, and the closest
       version number it does support (if the server supports a range of
       versions higher than the client's requested version, the server
       returns the lowest of that supported range; if the server
       supports a range of versions lower than the client's requested
       version, the server returns the highest of that supported range).

   4.  If the client receives an UNSUPP_VERSION result containing a
       version it does support, it records this fact and proceeds to use
       this message version for subsequent communication with this PCP
       server (until a possible future UNSUPP_VERSION response if the
       server is later updated, at which point the version negotiation
       process repeats).  If the version number in the UNSUPP_VERSION
       response is zero then that means this is a NAT-PMP server
       [RFC6886], and a client MAY choose to communicate with it using
       the older NAT-PMP protocol, as described in Appendix A.

   5.  If the client receives an UNSUPP_VERSION result containing a
       version it does not support, then the client SHOULD try the next-
       lower version supported by the client.  The attempt to use the
       next-lower version repeats until the client has tried version 2.
       If using version 2 fails, the client MAY log an error message to
       inform the user that it is too old to work with this server, and
       the client SHOULD set a timer to retry its request in 30 minutes
       or the returned Lifetime value, whichever is smaller.  By
       automatically retrying in 30 minutes, the protocol accommodates
       an upgrade of the PCP server.

10.  Introduction to MAP and PEER Opcodes

   There are four uses for the MAP and PEER Opcodes defined in this
   document:

   o  a host operating a server and wanting an incoming connection
      (Section 10.1);

   o  a host operating a client and server on the same port
      (Section 10.2);




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   o  a host operating a client and wanting to optimize the application
      keepalive traffic (Section 10.3); and

   o  a host operating a client and wanting to restore lost state in its
      NAT (Section 10.4).

   These are discussed in the following sections, and a (non-normative)
   state diagram is provided in Section 16.5.

   When operating a server (see Sections 10.1 and 10.2), the PCP client
   knows if it wants an IPv4 listener, IPv6 listener, or both on the
   Internet.  The PCP client also knows if it has an IPv4 address or
   IPv6 address configured on one of its interfaces.  It takes the union
   of this knowledge to decide to which of its PCP servers to send the
   request (e.g., an IPv4 address or an IPv6 address), and whether to
   send one or two MAP requests for each of its interfaces (e.g., if the
   PCP client has only an IPv4 address but wants both IPv6 and IPv4
   listeners, it sends a MAP request containing the all-zeros IPv6
   address in the Suggested External Address field, and sends a second
   MAP request containing the all-zeros IPv4 address in the Suggested
   External Address field).  If the PCP client has both an IPv4 and IPv6
   address, and only wants an IPv4 listener, it sends one MAP request
   from its IPv4 address (if the PCP server supports NAT44 or IPv4
   firewall) or one MAP request from its IPv6 address (if the PCP server
   supports NAT64).  The PCP client can simply request the desired
   mapping to determine if the PCP server supports the desired mapping.
   Applications that embed IP addresses in payloads (e.g., FTP, SIP)
   will find it beneficial to avoid address family translation, if
   possible.

   The MAP and PEER requests include a Suggested External IP Address
   field.  Some PCP-controlled devices, especially CGN but also multi-
   homed NPTv6 networks, have a pool of public-facing IP addresses.  PCP
   allows the client to indicate if it wants a mapping assigned on a
   specific address of that pool or any address of that pool.  Some
   applications will break if mappings are created on different IP
   addresses (e.g., active mode FTP), so applications should carefully
   consider the implications of using this capability.  Static mappings
   for that internal address (e.g., those created by a command-line
   interface on the PCP server or PCP-controlled device) may exist to a
   certain external address, and if the suggested external IP address is
   the IPv4 or IPv6 all-zeros address, PCP SHOULD assign its mappings to
   the same external address, as this can also help applications using a
   mix of both static mappings and PCP-created mappings.  If, on the
   other hand, the suggested external IP address contains a non-zero IP
   address the PCP server SHOULD create a mapping to that external
   address, even if there are other mappings from that same internal
   address to a different external address.  Once an internal address



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   has no implicit dynamic mappings and no explicit dynamic mappings in
   the PCP-controlled device, a subsequent implicit or explicit mapping
   for that internal address MAY be assigned to a different External
   address.  Generally, this reassignment would occur when a CGN device
   is load balancing newly seen internal addresses to its public pool of
   external addresses.

   The following table summarizes how various common PCP deployments use
   IPv6 and IPv4 addresses.

   The 'internal' address is implicitly the same as the source IP
   address of the PCP request, except when the THIRD_PARTY option is
   used.

   The 'external' address is the Suggested External Address field of the
   MAP or PEER request, and its address family is usually the same as
   the 'internal' address family, except when technologies like NAT64
   are used.

   The 'remote peer' address is the remote peer IP address of the PEER
   request or the FILTER option of the MAP request, and is always the
   same address family as the 'internal' address, even when NAT64 is
   used.  In NAT64, the IPv6 PCP client is not necessarily aware of the
   NAT64 or aware of the actual IPv4 address of the remote peer, so it
   expresses the IPv6 address from its perspective, as shown in Figure
   5.

                 internal  external  PCP remote peer  actual remote peer
                 --------  -------   ---------------  ------------------
   IPv4 firewall   IPv4      IPv4         IPv4              IPv4
   IPv6 firewall   IPv6      IPv6         IPv6              IPv6
           NAT44   IPv4      IPv4         IPv4              IPv4
           NAT46   IPv4      IPv6         IPv4              IPv6
           NAT64   IPv6      IPv4         IPv6              IPv4
           NPTv6   IPv6      IPv6         IPv6              IPv6

               Figure 5: Address Families with MAP and PEER

   Note that the internal address and the remote peer address are always
   the same address family, and the external address and the actual
   remote peer address are always the same address family.










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10.1.  For Operating a Server

   A host operating a server (e.g., a web server) listens for traffic on
   a port, but the server never initiates traffic from that port.  For
   this to work across a NAT or a firewall, the host needs to (a) create
   a mapping from a public IP address, protocol, and port to itself
   using the MAP Opcode, as described in Section 11; (b) publish that
   public IP address, protocol, and port via some sort of rendezvous
   server (e.g., DNS, a SIP message, or a proprietary protocol); and
   (c) ensure that any other non-PCP-speaking packet filtering
   middleboxes on the path (e.g., host-based firewall, network-based
   firewall, or other NATs) will also allow the incoming traffic.
   Publishing the public IP address and port is out of scope of this
   specification.  To accomplish (a), the host follows the procedures
   described in this section.

   As normal, the application needs to begin listening on a port.  Then,
   the application constructs a PCP message with the MAP Opcode, with
   the external address set to the appropriate all-zeros address,
   depending on whether it wants a public IPv4 or IPv6 address.































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   The following pseudocode shows how PCP can be reliably used to
   operate a server:

    /* start listening on the local server port */
    int s = socket(...);
    bind(s, ...);
    listen(s, ...);

    getsockname(s, &internal_sockaddr, ...);
    bzero(&external_sockaddr, sizeof(external_sockaddr));

    while (1)
        {
        /* Note: The "time_to_send_pcp_request()" check below includes:
         * 1. Sending the first request
         * 2. Retransmitting requests due to packet loss
         * 3. Resending a request due to impending lease expiration
         * 4. Resending a request due to server state loss
         * The PCP packet sent is identical in all four cases; from
         * the PCP server's point of view they are the same operation.
         * The suggested external address and port may be updated
         * repeatedly during the lifetime of the mapping.
         * Other fields in the packet generally remain unchanged.
         */
        if (time_to_send_pcp_request())
            pcp_send_map_request(internal_sockaddr.sin_port,
                internal_sockaddr.sin_addr,
                &external_sockaddr, /* will be zero the first time */
                requested_lifetime, &assigned_lifetime);

        if (pcp_response_received())
            update_rendezvous_server("Client Ident", external_sockaddr);

        if (received_incoming_connection_or_packet())
            process_it(s);

        if (other_work_to_do())
            do_it();

        /* ... */

        block_until_we_need_to_do_something_else();
        }

          Figure 6: Pseudocode for Using PCP to Operate a Server






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10.2.  For Operating a Symmetric Client/Server

   A host operating a client and server on the same port (e.g.,
   Symmetric RTP [RFC4961] or SIP Symmetric Response Routing (rport)
   [RFC3581]) first establishes a local listener, (usually) sends the
   local and public IP addresses, protocol, and ports to a rendezvous
   service (which is out of scope of this document), and initiates an
   outbound connection from that same source address and same port.  To
   accomplish this, the application uses the procedure described in this
   section.

   An application that is using the same port for outgoing connections
   as well as incoming connections MUST first signal its operation of a
   server using the PCP MAP Opcode, as described in Section 11, and
   receive a positive PCP response before it sends any packets from that
   port.

      Discussion: In general, a PCP client doesn't know in advance if it
      is behind a NAT or firewall.  On detecting that the host has
      connected to a new network, the PCP client can attempt to request
      a mapping using PCP; if that succeeds, then the client knows it
      has successfully created a mapping.  If, after multiple retries,
      it has received no PCP response, then either the client is *not*
      behind a NAT or firewall and has unfettered connectivity or the
      client *is* behind a NAT or firewall that doesn't support PCP (and
      the client may still have working connectivity by virtue of static
      mappings previously created manually by the user).  Retransmitting
      PCP requests multiple times before giving up and assuming
      unfettered connectivity adds delay in that case.  Initiating
      outbound TCP connections immediately without waiting for PCP
      avoids this delay, and will work if the NAT has endpoint-
      independent mapping (EIM) behavior, but may fail if the NAT has
      endpoint-dependent mapping (EDM) behavior.  Waiting enough time to
      allow an explicit PCP MAP mapping to be created (if possible)
      first ensures that the same external port will then be used for
      all subsequent implicit dynamic mappings (e.g., TCP SYNs) sent
      from the specified internal address, protocol, and port.  PCP
      supports both EIM and EDM NATs, so clients need to assume they may
      be dealing with an EDM NAT.  In this case, the client will
      experience more reliable connectivity if it attempts explicit PCP
      MAP requests first, before initiating any outbound TCP connections
      from that internal address and port.  For further information on
      using PCP with EDM NATs, see Section 16.1.








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   The following pseudocode shows how PCP can be used to operate a
   symmetric client and server:

    /* start listening on the local server port */
    int s = socket(...);
    bind(s, ...);
    listen(s, ...);

    getsockname(s, &internal_sockaddr, ...);
    bzero(&external_sockaddr, sizeof(external_sockaddr));

    while (1)
        {
        /* Note: The "time_to_send_pcp_request()" check below includes:
         * 1. Sending the first request
         * 2. Retransmitting requests due to packet loss
         * 3. Resending a request due to impending lease expiration
         * 4. Resending a request due to server state loss
         */
        if (time_to_send_pcp_request())
            pcp_send_map_request(internal_sockaddr.sin_port,
                internal_sockaddr.sin_addr,
                &external_sockaddr, /* will be zero the first time */
                requested_lifetime, &assigned_lifetime);

        if (pcp_response_received())
            update_rendezvous_server("Client Ident", external_sockaddr);

        if (received_incoming_connection_or_packet())
            process_it(s);

        if (need_to_make_outgoing_connection())
            make_outgoing_connection(s, ...);

        if (data_to_send())
            send_it(s);

        if (other_work_to_do())
            do_it();

        /* ... */

        block_until_we_need_to_do_something_else();
        }

              Figure 7: Pseudocode for Using PCP to Operate a
                          Symmetric Client/Server




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10.3.  For Reducing NAT or Firewall Keepalive Messages

   A host operating a client (e.g., XMPP client, SIP client) sends from
   a port, and may receive responses, but never accepts incoming
   connections from other remote peers on this port.  It wants to ensure
   that the flow to its remote peer is not terminated (due to
   inactivity) by an on-path NAT or firewall.  To accomplish this, the
   application uses the procedure described in this section.

   Middleboxes, such as NATs or firewalls, generally need to see
   occasional traffic or they will terminate their session state,
   causing application failures.  To avoid this, many applications
   routinely generate keepalive traffic for the primary (or sole)
   purpose of maintaining state with such middleboxes.  Applications can
   reduce such application keepalive traffic by using PCP.

      Note: For reasons beyond NAT, an application may find it useful to
      perform application-level keepalives, such as to detect a broken
      path between the client and server, keep state alive on the remote
      peer, or detect a powered-down client.  These keepalives are not
      related to maintaining middlebox state, and PCP cannot do anything
      useful to reduce those keepalives.

   To use PCP for this function, the application first connects to its
   server, as normal.  Afterwards, it issues a PCP request with the PEER
   Opcode as described in Section 12 to learn and/or extend the lifetime
   of its mapping.
























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   The following pseudocode shows how PCP can be reliably used with a
   dynamic socket, for the purposes of reducing application keepalive
   messages:

    /* make outgoing connection to server */
    int s = socket(...);
    connect(s, &remote_peer, ...);

    getsockname(s, &internal_sockaddr, ...);
    bzero(&external_sockaddr, sizeof(external_sockaddr));

    while (1)
        {
        /* Note: The "time_to_send_pcp_request()" check below includes:
         * 1. Sending the first request
         * 2. Retransmitting requests due to packet loss
         * 3. Resending a request due to impending lease expiration
         * 4. Resending a request due to server state loss
         */
        if (time_to_send_pcp_request())
            pcp_send_peer_request(internal_sockaddr.sin_port,
                internal_sockaddr.sin_addr,
                &external_sockaddr, /* will be zero the first time */
                remote_peer, requested_lifetime, &assigned_lifetime);

        if (data_to_send())
            send_it(s);

        if (received_incoming_data())
            process_it(s);

        if (other_work_to_do())
            do_it();

        /* ... */

        block_until_we_need_to_do_something_else();
        }

           Figure 8: Pseudocode Using PCP with a Dynamic Socket

10.4.  For Restoring Lost Implicit TCP Dynamic Mapping State

   After a NAT loses state (e.g., because of a crash or power failure),
   it is useful for clients to re-establish TCP mappings on the NAT.
   This allows servers on the Internet to see traffic from the same IP
   address and port, so that sessions can be resumed exactly where they
   were left off.  This can be useful for long-lived connections



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   (e.g., instant messaging) or for connections transferring a lot of
   data (e.g., FTP).  This can be accomplished by first establishing a
   TCP connection normally and then sending a PEER request/response and
   remembering the external address and external port.  Later, when the
   NAT has lost state, the client can send a PEER request with the
   suggested external port and suggested external address remembered
   from the previous session, which will create a mapping in the NAT
   that functions exactly as an implicit dynamic mapping.  The client
   then resumes sending TCP data to the server.

      Note: This procedure works well for TCP, provided:

         (i) the NAT creates a new implicit dynamic outbound mapping
         only for outbound TCP segments with the SYN bit set (i.e., the
         newly booted NAT silently drops outbound data segments from the
         client when the NAT does not have an active mapping for those
         segments), and

         (ii) the newly booted NAT does not send a TCP RST in response
         to receiving unexpected inbound TCP segments.

      This procedure works less well for UDP, because as soon as
      outbound UDP traffic is seen by the NAT, a new UDP implicit
      dynamic outbound mapping will be created (probably on a different
      port).

11.  MAP Opcode

   This section defines an Opcode that controls inbound forwarding from
   a NAT (or firewall) to an internal host.

   MAP:  Create an explicit dynamic mapping between an Internal Address
           + Port and an External Address + Port.

   PCP servers SHOULD provide a configuration option to allow
   administrators to disable MAP support if they wish.

   Mappings created by PCP MAP requests are, by definition, endpoint-
   independent mappings (EIMs) with endpoint-independent filtering (EIF)
   (unless the FILTER option is used), even on a NAT that usually
   creates endpoint-dependent mapping (EDM) or endpoint-dependent
   filtering (EDF) for outgoing connections, since the purpose of an
   (unfiltered) MAP mapping is to receive inbound traffic from any
   remote endpoint, not from only one specific remote endpoint.

   Note also that all NAT mappings (created by PCP or otherwise) are by
   necessity bidirectional and symmetric.  For any packet going in one
   direction (in or out) that is translated by the NAT, a reply going in



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   the opposite direction needs to have the corresponding opposite
   translation done so that the reply arrives at the right endpoint.
   This means that if a client creates a MAP mapping, and then later
   sends an outgoing packet using the mapping's internal address,
   protocol, and port, the NAT should translate that packet's internal
   address and port to the mapping's external address and port, so that
   replies addressed to the external address and port are correctly
   translated back to the mapping's internal address and port.

   On operating systems that allow multiple listening servers to bind to
   the same internal address, protocol, and port, servers MUST ensure
   that they have exclusive use of that internal address, protocol, and
   port (e.g., by binding the port using INADDR_ANY, or using
   SO_EXCLUSIVEADDRUSE or similar) before sending their PCP MAP request,
   to ensure that no other PCP clients on the same machine are also
   listening on the same internal protocol and internal port.

   As a side effect of creating a mapping, ICMP messages associated with
   the mapping MUST be forwarded (and also translated, if appropriate)
   for the duration of the mapping's lifetime.  This is done to ensure
   that ICMP messages can still be used by hosts, without application
   programmers or PCP client implementations needing to use PCP
   separately to create ICMP mappings for those flows.

   The operation of the MAP Opcode is described in this section.

11.1.  MAP Operation Packet Formats

   The MAP Opcode has a similar packet layout for both requests and
   responses.  If the assigned external IP address and port in the PCP
   response always match the internal IP address and port from the PCP
   request, then the functionality is purely a firewall; otherwise, the
   functionality is a Network Address Translator that might also perform
   firewall-like functions.

















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   The following diagram shows the format of the Opcode-specific
   information in a request for the MAP Opcode.

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Mapping Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Suggested External Port    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |           Suggested External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 9: MAP Opcode Request

   These fields are described below:

   Requested lifetime (in common header):  Requested lifetime of this
      mapping, in seconds.  The value 0 indicates "delete".

   Mapping Nonce:  Random value chosen by the PCP client.  See
      Section 11.2, "Generating a MAP Request".  Zero is a legal value
      (but unlikely, occurring in roughly one in 2^96 requests).

   Protocol:  Upper-layer protocol associated with this Opcode.  Values
      are taken from the IANA protocol registry [proto_numbers].  For
      example, this field contains 6 (TCP) if the Opcode is intended to
      create a TCP mapping.  This field contains 17 (UDP) if the Opcode
      is intended to create a UDP mapping.  The value 0 has a special
      meaning for 'all protocols'.

   Reserved:  24 reserved bits, MUST be sent as 0 and MUST be ignored
      when received.

   Internal Port:  Internal port for the mapping.  The value 0 indicates
      'all ports', and is legal when the lifetime is zero (a delete
      request), if the protocol does not use 16-bit port numbers, or the
      client is requesting 'all ports'.  If the protocol is zero
      (meaning 'all protocols'), then internal port MUST be zero on
      transmission and MUST be ignored on reception.




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   Suggested External Port:  Suggested external port for the mapping.
      This is useful for refreshing a mapping, especially after the PCP
      server loses state.  If the PCP client does not know the external
      port, or does not have a preference, it MUST use 0.

   Suggested External IP Address:  Suggested external IPv4 or IPv6
      address.  This is useful for refreshing a mapping, especially
      after the PCP server loses state.  If the PCP client does not know
      the external address, or does not have a preference, it MUST use
      the address-family-specific all-zeros address (see Section 5).

   The internal address for the request is the source IP address of the
   PCP request message itself, unless the THIRD_PARTY option is used.

   The following diagram shows the format of Opcode-specific information
   in a response packet for the MAP Opcode:

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Mapping Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Assigned External Port     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |            Assigned External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 10: MAP Opcode Response

   These fields are described below:

   Lifetime (in common header):  On an error response, this indicates
      how long clients should assume they'll get the same error response
      from the PCP server if they repeat the same request.  On a success
      response, this indicates the lifetime for this mapping, in
      seconds.

   Mapping Nonce:  Copied from the request.

   Protocol:  Copied from the request.




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   Reserved:  24 reserved bits, MUST be sent as 0 and MUST be ignored
      when received.

   Internal Port:  Copied from the request.

   Assigned External Port:  On a success response, this is the assigned
      external port for the mapping.  On an error response, the
      suggested external port is copied from the request.

   Assigned External IP Address:  On a success response, this is the
      assigned external IPv4 or IPv6 address for the mapping.  An IPv4
      address is encoded using IPv4-mapped IPv6 address.  On an error
      response, the suggested external IP address is copied from the
      request.

11.2.  Generating a MAP Request

   This section describes the operation of a PCP client when sending
   requests with the MAP Opcode.

   The request MAY contain values in the Suggested External Port and
   Suggested External IP Address fields.  This allows the PCP client to
   attempt to rebuild lost state on the PCP server, which improves the
   chances of existing connections surviving, and helps the PCP client
   avoid having to change information maintained at its rendezvous
   server.  Of course, due to other activity on the network (e.g., by
   other users or network renumbering), the PCP server may not be able
   to grant the suggested external IP address, protocol, and port, and
   in that case it will assign a different external IP address and port.

   A PCP client MUST be written assuming that it may *never* be assigned
   the external port it suggests.  In the case of recreating state after
   a NAT gateway crash, the suggested external port, being one that was
   previously allocated to this client, is likely to be available for
   this client to continue using.  In all other cases, the client MUST
   assume that it is unlikely that its suggested external port will be
   granted.  For example, when many subscribers are sharing a Carrier-
   Grade NAT, popular ports such as 80, 443, and 8080 are likely to be
   in high demand.  At most one client can have each of those popular
   ports for each external IP address, and all the other clients will be
   assigned other, dynamically allocated, external ports.  Indeed, some
   ISPs may, by policy, choose not to grant those external ports to
   *anyone*, so that none of their clients are *ever* assigned external
   ports 80, 443, or 8080.

   If the protocol does not use 16-bit port numbers (e.g., RSVP, IP
   protocol number 46), the port number MUST be zero.  This will cause
   all traffic matching that protocol to be mapped.



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   If the client wants all protocols mapped, it uses protocol 0 (zero)
   and internal port 0 (zero).

   The Mapping Nonce value is randomly chosen by the PCP client,
   following accepted practices for generating unguessable random
   numbers [RFC4086], and is used as part of the validation of PCP
   responses (see below) by the PCP client, and validation for mapping
   refreshes by the PCP server.  The client MUST use a different mapping
   nonce for each PCP server it communicates with, and it is RECOMMENDED
   to choose a new random mapping nonce whenever the PCP client is
   initialized.  The client MAY use a different mapping nonce for every
   mapping.

11.2.1.  Renewing a Mapping

   An existing mapping SHOULD have its lifetime extended by the PCP
   client for as long as the client wishes to have that mapping continue
   to exist.  To do this, the PCP client sends a new MAP request
   indicating the internal port.  The PCP MAP request SHOULD also
   include the currently assigned external IP address and port in the
   Suggested External IP Address and Suggested External Port fields, so
   if the PCP server has lost state it can recreate the lost mapping
   with the same parameters.

   The PCP client SHOULD renew the mapping before its expiry time;
   otherwise, it will be removed by the PCP server (see Section 15,
   "Mapping Lifetime and Deletion").  To reduce the risk of inadvertent
   synchronization of renewal requests, a random jitter component should
   be included.  It is RECOMMENDED that PCP clients send a single
   renewal request packet at a time chosen with uniform random
   distribution in the range 1/2 to 5/8 of expiration time.  If no
   SUCCESS response is received, then the next renewal request should be
   sent 3/4 to 3/4 + 1/16 to expiration, and then another 7/8 to 7/8 +
   1/32 to expiration, and so on, subject to the constraint that renewal
   requests MUST NOT be sent less than four seconds apart (a PCP client
   MUST NOT send a flood of ever-closer-together requests in the last
   few seconds before a mapping expires).

11.3.  Processing a MAP Request

   This section describes the operation of a PCP server when processing
   a request with the MAP Opcode.  Processing SHOULD be performed in the
   order of the following paragraphs.

   The Protocol, Internal Port, and Mapping Nonce fields from the MAP
   request are copied into the MAP response.  The THIRD_PARTY option, if
   present, and processed by the PCP server, is also copied into the MAP
   response.



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   If the requested lifetime is non-zero, then:

   o  If both the protocol and internal port are non-zero, it indicates
      a request to create a mapping or extend the lifetime of an
      existing mapping.  If the PCP server or PCP-controlled device does
      not support the protocol, the UNSUPP_PROTOCOL error MUST be
      returned.

   o  If the protocol is non-zero and the internal port is zero, it
      indicates a request to create or extend a mapping for all incoming
      traffic for that entire protocol -- a 'wildcard' (all-ports)
      mapping for that protocol.  If this request cannot be fulfilled in
      its entirety, the UNSUPP_PROTOCOL error MUST be returned.

   o  If both the protocol and internal port are zero, it indicates a
      request to create or extend a mapping for all incoming traffic for
      all protocols (commonly called a 'DMZ host').  If this request
      cannot be fulfilled in its entirety, the UNSUPP_PROTOCOL error
      MUST be returned.

   o  If the protocol is zero and the internal port is non-zero, then
      the request is invalid and the PCP server MUST return a
      MALFORMED_REQUEST error to the client.

   If the requested lifetime is zero, it indicates a request to delete
   an existing mapping.

   Further processing of the lifetime is described in Section 15,
   "Mapping Lifetime and Deletion".

   If operating in the Simple Threat Model (Section 18.1), and the
   internal port, protocol, and internal address match an existing
   explicit dynamic mapping, but the mapping nonce does not match, the
   request MUST be rejected with a NOT_AUTHORIZED error with the
   lifetime of the error indicating duration of that existing mapping.
   The PCP server only needs to remember one Mapping Nonce value for
   each explicit dynamic mapping.  This specification makes no statement
   about mapping nonce with the Advanced Threat Model.

   If the internal port, protocol, and internal address match an
   existing static mapping (which will have no nonce), then a PCP reply
   is sent giving the external address and port of that static mapping,
   using the nonce from the PCP request.  The server does not record the
   nonce.







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   If an option with value less than 128 exists (i.e., mandatory to
   process) but that option does not make sense (e.g., the
   PREFER_FAILURE option is included in a request with lifetime=0), the
   request is invalid and generates a MALFORMED_OPTION error.

   If the PCP-controlled device is stateless (that is, it does not
   establish any per-flow state, and simply rewrites the address and/or
   port in a purely algorithmic fashion, including no rewriting), the
   PCP server simply returns an answer indicating the external IP
   address and port yielded by this stateless algorithmic translation.
   This allows the PCP client to learn its external IP address and port
   as seen by remote peers.  Examples of stateless translators include
   stateless NAT64, 1:1 NAT44, and NPTv6 [RFC6296], all of which modify
   addresses but not port numbers, and pure firewalls, which modify
   neither the address nor the port.

   It is possible that a mapping might already exist for a requested
   internal address, protocol, and port.  If so, the PCP server takes
   the following actions:

   1.  If the MAP request contains the PREFER_FAILURE option, but the
       suggested external address and port do not match the external
       address and port of the existing mapping, the PCP server MUST
       return CANNOT_PROVIDE_EXTERNAL.

   2.  If the existing mapping is static (created outside of PCP), the
       PCP server MUST return the external address and port of the
       existing mapping in its response and SHOULD indicate a lifetime
       of 2^32-1 seconds, regardless of the suggested external address
       and port in the request.

   3.  If the existing mapping is explicit dynamic inbound (created by a
       previous MAP request), the PCP server MUST return the existing
       external address and port in its response, regardless of the
       suggested external address and port in the request.
       Additionally, the PCP server MUST update the lifetime of the
       existing mapping, in accordance with Section 15, "Mapping
       Lifetime and Deletion".

   4.  If the existing mapping is dynamic outbound (created by outgoing
       traffic or a previous PEER request), the PCP server SHOULD create
       a new explicit inbound mapping, replicating the ports and
       addresses from the outbound mapping (but the outbound mapping
       continues to exist, and remains in effect if the explicit inbound
       mapping is later deleted).

   If no mapping exists for the internal address, protocol, and port,
   and the PCP server is able to create a mapping using the suggested



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   external address and port, it SHOULD do so.  This is beneficial for
   re-establishing state lost in the PCP server (e.g., due to a reboot).
   There are, however, cases where the PCP server is not able to create
   a new mapping using the suggested external address and port:

   o  The suggested external address, protocol, and port is already
      assigned to another existing explicit or implicit mapping
      (i.e., is already forwarding traffic to some other internal
      address and port).

   o  The suggested external address, protocol, and port is already used
      by the NAT gateway for one of its own services, for example, TCP
      port 80 for the NAT gateway's own configuration web pages, or UDP
      ports 5350 and 5351, used by PCP itself.  A PCP server MUST NOT
      create client mappings for external UDP ports 5350 or 5351.

   o  The suggested external address, protocol, and port is otherwise
      prohibited by the PCP server's policy.

   o  The suggested external IP address, protocol, or suggested port are
      invalid or invalid combinations (e.g., external address 127.0.0.1,
      ::1, a multicast address, or the suggested port is not valid for
      the protocol).

   o  The suggested external address does not belong to the NAT gateway.

   o  The suggested external address is not configured to be used as an
      external address of the firewall or NAT gateway.

   If the PCP server cannot assign the suggested external address,
   protocol, and port, then:

   o  If the request contained the PREFER_FAILURE option, then the PCP
      server MUST return CANNOT_PROVIDE_EXTERNAL.

   o  If the request did not contain the PREFER_FAILURE option, and the
      PCP server can assign some other external address and port for
      that protocol, then the PCP server MUST do so and return the newly
      assigned external address and port in the response.  In no case is
      the client penalized for a 'poor' choice of suggested external
      address and port.  The suggested external address and port may be
      used by the server to guide its choice of what external address
      and port to assign, but in no case do they cause the server to
      fail to allocate an external address and port where otherwise it
      would have succeeded.  The presence of a non-zero suggested
      external address or port is merely a hint; it never does any harm.





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   A PCP-controlled device MUST NOT create mappings for a protocol not
   indicated in the request.  For example, if the request was for a TCP
   mapping, an additional corresponding UDP mapping MUST NOT be
   automatically created.

   Mappings typically consume state on the PCP-controlled device, and it
   is RECOMMENDED that a per-host and/or per-subscriber limit be
   enforced by the PCP server to prevent exhausting the mapping state.
   If this limit is exceeded, the result code USER_EX_QUOTA is returned.

   If all of the preceding operations were successful (did not generate
   an error response), then the requested mapping is created or
   refreshed as described in the request and a SUCCESS response is
   built.

11.4.  Processing a MAP Response

   This section describes the operation of the PCP client when it
   receives a PCP response for the MAP Opcode.

   After performing common PCP response processing, the response is
   further matched with a previously sent MAP request by comparing the
   internal IP address (the destination IP address of the PCP response,
   or other IP address specified via the THIRD_PARTY option), the
   protocol, the internal port, and the mapping nonce.  Other fields are
   not compared, because the PCP server sets those fields.  The PCP
   server will send a Mapping Update (Section 14.2) if the mapping
   changes (e.g., due to IP renumbering).

   If the result code is NO_RESOURCES and the request was for the
   creation or renewal of a mapping, then the PCP client SHOULD NOT send
   further requests for any new mappings to that PCP server for the
   (limited) value of the lifetime.  If the result code is NO_RESOURCES
   and the request was for the deletion of a mapping, then the PCP
   client SHOULD NOT send further requests of *any kind* to that PCP
   server for the (limited) value of the lifetime.

   On a success response, the PCP client can use the external IP address
   and port as needed.  Typically, the PCP client will communicate the
   external IP address and port to another host on the Internet using an
   application-specific rendezvous mechanism such as DNS SRV records.

   After a success response, for as long as renewal is desired, the PCP
   client MUST set a timer or otherwise schedule an event to renew the
   mapping before its lifetime expires.  Renewing a mapping is performed
   by sending another MAP request, exactly as described in Section 11.2,
   except that the suggested external address and port SHOULD be set to
   the values received in the response.  From the PCP server's point of



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   view a MAP request to renew a mapping is identical to a MAP request
   to create a new mapping, and is handled identically.  Indeed, in the
   event of PCP server state loss, a renewal request from a PCP client
   will appear to the server to be a request to create a new mapping,
   with a particular suggested external address and port, which happen
   to be what the PCP server previously assigned.  See also
   Section 16.3.1, "Recreating Mappings".

   On an error response, the client SHOULD NOT repeat the same request
   to the same PCP server within the lifetime returned in the response.

11.5.  Address Change Events

   A customer premises router might obtain a new external IP address,
   for a variety of reasons including a reboot, power outage, DHCP lease
   expiry, or other action by the ISP.  If this occurs, traffic
   forwarded to a host's previous address might be delivered to another
   host that now has that address.  This affects all mapping types,
   whether implicit or explicit.  This same problem already occurs today
   when a host's IP address is reassigned, without PCP and without an
   ISP-operated CGN.  The solution is the same as today: the problems
   associated with host renumbering are caused by host renumbering, and
   are eliminated if host renumbering is avoided.  PCP defined in this
   document does not provide machinery to reduce the host renumbering
   problem.

   When an internal host changes its internal IP address (e.g., by
   having a different address assigned by the DHCP server), the NAT (or
   firewall) will continue to send traffic to the old IP address.
   Typically, the internal host will no longer receive traffic sent to
   that old IP address.  Assuming the internal host wants to continue
   receiving traffic, it needs to install new mappings for its new IP
   address.  The Suggested External Port field will not be fulfilled by
   the PCP server, in all likelihood, because it is still being
   forwarded to the old IP address.  Thus, a mapping is likely to be
   assigned a new external port number and/or external IP address.  Note
   that such host renumbering is not expected to happen routinely on a
   regular basis for most hosts, since most hosts renew their DHCP
   leases before they expire (or re-request the same address after
   reboot) and most DHCP servers honor such requests and grant the host
   the same address it was previously using before the reboot.

   A host might gain or lose interfaces while existing mappings are
   active (e.g., Ethernet cable plugged in or removed, joining/leaving a
   WiFi network).  Because of this, if the PCP client is sending a PCP
   request to maintain state in the PCP server, it SHOULD ensure that
   those PCP requests continue to use the same interface (e.g., when
   refreshing mappings).  If the PCP client is sending a PCP request to



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   create new state in the PCP server, it MAY use a different source
   interface or different source address.

11.6.  Learning the External IP Address Alone

   NAT-PMP [RFC6886] includes a mechanism to allow clients to learn the
   external IP address alone, without also requesting a port mapping.
   NAT-PMP was designed for residential NAT gateways, where such an
   operation makes sense because a typical residential NAT gateway has
   only one external IP address.  PCP has broader scope, and also
   supports Carrier-Grade NATs (CGNs) that may have a pool of external
   IP addresses, not just one.  A client may not be assigned any
   particular external IP address from that pool until it has at least
   one implicit, explicit, or static port mapping, and even then only
   for as long as that mapping remains valid.  Client software that just
   wishes to display the user's external IP address for cosmetic
   purposes can achieve that by requesting a short-lived mapping (e.g.,
   to the Discard service (TCP/9 or UDP/9) or some other port) and then
   displaying the resulting external IP address.  However, once that
   mapping expires a subsequent implicit or explicit dynamic mapping
   might be mapped to a different external IP address.

12.  PEER Opcode

   This section defines an Opcode for controlling dynamic outbound
   mappings.

   PEER: Create a new dynamic outbound mapping to a remote peer's IP
         address and port, or extend the lifetime of an existing
         outbound mapping.

   The use of this Opcodes is described in this section.

   PCP servers SHOULD provide a configuration option to allow
   administrators to disable PEER support if they wish.

   Because a mapping created or managed by PEER behaves almost exactly
   like an implicit dynamic outbound mapping created as a side effect of
   a packet (e.g., TCP SYN) sent by the host, mappings created or
   managed using PCP PEER requests may be endpoint-independent mapping
   (EIM) or endpoint-dependent mapping (EDM), with endpoint-independent
   filtering (EIF) or endpoint-dependent filtering (EDF), consistent
   with the existing behavior of the NAT gateway or firewall in question
   for implicit outbound mappings it creates automatically as a result
   of observing outgoing traffic from internal hosts.






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12.1.  PEER Operation Packet Formats

   The PEER Opcode allows a PCP client to create a new explicit dynamic
   outbound mapping (which functions similarly to an outbound mapping
   created implicitly when a host sends an outbound TCP SYN) or to
   extend the lifetime of an existing outbound mapping.

   The following diagram shows the Opcode layout for the PEER Opcode.
   The formats for the PEER request and response packets are aligned so
   that related fields fall at the same offsets in the packet.

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Mapping Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Suggested External Port    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |           Suggested External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Remote Peer Port        |     Reserved (16 bits)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |               Remote Peer IP Address (128 bits)               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 11: PEER Opcode Request

   These fields are described below:

   Requested Lifetime (in common header):  Requested lifetime of this
      mapping, in seconds.  Note that it is not possible to reduce the
      lifetime of a mapping (or delete it, with requested lifetime=0)
      using PEER.

   Mapping Nonce:  Random value chosen by the PCP client.  See
      Section 12.2, "Generating a PEER Request".  Zero is a legal value
      (but unlikely, occurring in roughly one in 2^96 requests).




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   Protocol:  Upper-layer protocol associated with this Opcode.  Values
      are taken from the IANA protocol registry [proto_numbers].  For
      example, this field contains 6 (TCP) if the Opcode is describing a
      TCP mapping.  This field contains 17 (UDP) if the Opcode is
      describing a UDP mapping.  Protocol MUST NOT be zero.

   Reserved:  24 reserved bits, MUST be set to 0 on transmission and
      MUST be ignored on reception.

   Internal Port:  Internal port for the mapping.  Internal port MUST
      NOT be zero.

   Suggested External Port:  Suggested external port for the mapping.
      If the PCP client does not know the external port, or does not
      have a preference, it MUST use 0.

   Suggested External IP Address:  Suggested external IP address for the
      mapping.  If the PCP client does not know the external address, or
      does not have a preference, it MUST use the address-family-
      specific all-zeros address (see Section 5).

   Remote Peer Port:  Remote peer's port for the mapping.  Remote peer
      port MUST NOT be zero.

   Reserved:  16 reserved bits, MUST be set to 0 on transmission and
      MUST be ignored on reception.

   Remote Peer IP Address:  Remote peer's IP address.  This is from the
      perspective of the PCP client, so that the PCP client does not
      need to concern itself with NAT64 or NAT46 (which both cause the
      client's idea of the remote peer's IP address to differ from the
      remote peer's actual IP address).  This field allows the PCP
      client and PCP server to disambiguate multiple connections from
      the same port on the internal host to different servers.  An IPv6
      address is represented directly, and an IPv4 address is
      represented using the IPv4-mapped address syntax (Section 5).

   When attempting to re-create a lost mapping, the suggested external
   IP address and port are set to the External IP Address and Port
   fields received in a previous PEER response from the PCP server.  On
   an initial PEER request, the external IP address and port are set to
   zero.

   Note that semantics similar to the PREFER_FAILURE option are
   automatically implied by PEER requests.  If the Suggested External IP
   Address or Suggested External Port fields are non-zero, and the PCP
   server is unable to honor the suggested external IP address,
   protocol, or port, then the PCP server MUST return a



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   CANNOT_PROVIDE_EXTERNAL error response.  The PREFER_FAILURE option is
   neither required nor allowed in PEER requests, and if a PCP server
   receives a PEER request containing the PREFER_FAILURE option it MUST
   return a MALFORMED_REQUEST error response.

   The following diagram shows the Opcode response for the PEER Opcode:

      0                   1                   2                   3
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                 Mapping Nonce (96 bits)                       |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Assigned External Port     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |            Assigned External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |       Remote Peer Port        |     Reserved (16 bits)        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |               Remote Peer IP Address (128 bits)               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 12: PEER Opcode Response

   Lifetime (in common header):  On a success response, this indicates
      the lifetime for this mapping, in seconds.  On an error response,
      this indicates how long clients should assume they'll get the same
      error response from the PCP server if they repeat the same
      request.

   Mapping Nonce:  Copied from the request.

   Protocol:  Copied from the request.

   Reserved:  24 reserved bits, MUST be set to 0 on transmission, MUST
      be ignored on reception.






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   Internal Port:  Copied from request.

   Assigned External Port:  On a success response, this is the assigned
      external port for the mapping.  On an error response, the
      suggested external port is copied from the request.

   Assigned External IP Address:  On a success response, this is the
      assigned external IPv4 or IPv6 address for the mapping.  On an
      error response, the suggested external IP address is copied from
      the request.

   Remote Peer Port:  Copied from request.

   Reserved:  16 reserved bits, MUST be set to 0 on transmission, MUST
      be ignored on reception.

   Remote Peer IP Address:  Copied from the request.

12.2.  Generating a PEER Request

   This section describes the operation of a client when generating a
   message with the PEER Opcode.

   The PEER Opcode MAY be sent before or after establishing
   bidirectional communication with the remote peer.

   If sent before, this is considered a PEER-created mapping that
   creates a new dynamic outbound mapping in the PCP-controlled device.

   If sent after, this allows the PCP client to learn the IP address,
   port, and lifetime of the assigned external address and port for the
   existing implicit dynamic outbound mapping, and potentially to extend
   this lifetime (for reducing NAT or firewall keepalive messages, as
   described in Section 10.3).

   PEER requests are also useful for restoring mappings after a NAT has
   lost its mapping state (e.g., due to a crash).

   The Mapping Nonce value is randomly chosen by the PCP client,
   following accepted practices for generating unguessable random
   numbers [RFC4086], and is used as part of the validation of PCP
   responses (see below) by the PCP client, and validation for mapping
   refreshes by the PCP server.  The client MUST use a different mapping
   nonce for each PCP server it communicates with, and it is RECOMMENDED
   to choose a new random mapping nonce whenever the PCP client is
   initialized.  The client MAY use a different mapping nonce for every
   mapping.




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   The PEER Opcode contains a Remote Peer Address field, which is always
   from the perspective of the PCP client.  Note that when the
   PCP-controlled device is performing address family translation (NAT46
   or NAT64), the remote peer address from the perspective of the PCP
   client is different from the remote peer address on the other side of
   the address family translation device.

12.3.  Processing a PEER Request

   This section describes the operation of a server when receiving a
   request with the PEER Opcode.  Processing SHOULD be performed in the
   order of the following paragraphs.

   The following fields from a PEER request are copied into the
   response: Protocol, Internal Port, Remote Peer IP Address, Remote
   Peer Port, and Mapping Nonce.

   When an implicit dynamic mapping is created, some NATs and firewalls
   validate destination addresses and will not create an implicit
   dynamic mapping if the destination address is invalid (e.g.,
   127.0.0.1).  If a PCP-controlled device does such validation for
   implicit dynamic mappings, it SHOULD also do a similar validation of
   the remote peer IP address, protocol, and port for PEER-created
   explicit dynamic mappings.  If the validation determines the remote
   peer IP address of a PEER request is invalid, then no mapping is
   created, and a MALFORMED_REQUEST error result is returned.

   On receiving the PEER Opcode, the PCP server examines the mapping
   table for a matching five-tuple { Protocol, Internal Address,
   Internal Port, Remote Peer Address, Remote Peer Port }.

   If no matching mapping is found, and the suggested external address
   and port are either zero or can be honored for the specified
   Protocol, a new mapping is created.  By having the PEER create such a
   mapping, we avoid a race condition between the PEER request and the
   initial outgoing packet arriving at the NAT or firewall device first,
   and allow PEER to be used to recreate a lost outbound dynamic mapping
   (see Section 16.3.1, "Recreating Mappings").  Thereafter, this PEER-
   created mapping is treated as if it was an implicit dynamic outbound
   mapping (e.g., as if the PCP client sent a TCP SYN) and a lifetime
   appropriate to such a mapping is returned (note: on many NATs and
   firewalls, such mapping lifetimes are very short until bidirectional
   traffic is seen by the NAT or firewall).

   If no matching mapping is found, and the suggested external address
   and port cannot be honored, then no new state is created, and the
   error CANNOT_PROVIDE_EXTERNAL is returned.




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   If a matching mapping is found, and no previous PEER Opcode was
   successfully processed for this mapping, then the Suggested External
   Address and Port values in the request are ignored, the lifetime of
   that mapping is adjusted as described below, and information about
   the existing mapping is returned.  This allows a client to explicitly
   extend the lifetime of an existing mapping and/or to learn an
   existing mapping's external address, port, and lifetime.  The mapping
   nonce is remembered for this mapping.

   If operating in the Simple Threat Model (Section 18.1), and the
   internal port, protocol, and internal address match a mapping that
   already exists, but the mapping nonce does not match (that is, a
   previous PEER request was processed), the request MUST be rejected
   with a NOT_AUTHORIZED error with the lifetime of the error indicating
   duration of that existing mapping.  The PCP server only needs to
   remember one Mapping Nonce value for each mapping.  This
   specification makes no statement about mapping nonce with the
   Advanced Threat Model.

   Processing the Lifetime value of the PEER Opcode is described in
   Section 15, "Mapping Lifetime and Deletion".  Sending a PEER request
   with a very short requested lifetime can be used to query the
   lifetime of an existing mapping.  So that PCP clients can reduce the
   frequency of their NAT and firewall keepalive messages, it is
   RECOMMENDED that lifetimes of mappings created or lengthened with
   PEER be longer than the lifetimes of implicitly created mappings.

   If all of the preceding operations were successful (did not generate
   an error response), then a SUCCESS response is generated, with the
   Lifetime field containing the lifetime of the mapping.

   If a PEER-created or PEER-managed mapping is not renewed using PEER,
   then it reverts to the NAT's usual behavior for implicit mappings.
   For example, continued outbound traffic keeps the mapping alive, as
   per the NAT or firewall device's existing policy.  A PEER-created or
   PEER-managed mapping may be terminated at any time by action of the
   TCP client or server (e.g., due to TCP FIN or TCP RST), as per the
   NAT or firewall device's existing policy.

12.4.  Processing a PEER Response

   This section describes the operation of a client when processing a
   response with the PEER Opcode.

   After performing common PCP response processing, the response is
   further matched with an outstanding PEER request by comparing the
   internal IP address (the destination IP address of the PCP response,
   or other IP address specified via the THIRD_PARTY option), the



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   protocol, the internal port, the remote peer address, the remote peer
   port, and the mapping nonce.  Other fields are not compared, because
   the PCP server sets those fields to provide information about the
   mapping created by the Opcode.  The PCP server will send a Mapping
   Update (Section 14.2) if the mapping changes (e.g., due to IP
   renumbering).

   If the result code is NO_RESOURCES and the request was for the
   creation or renewal of a mapping, then the PCP client SHOULD NOT send
   further requests for any new mappings to that PCP server for the
   (limited) value of the lifetime.

   On a successful response, the application can use the assigned
   Lifetime value to reduce its frequency of application keepalives for
   that particular NAT mapping.  Of course, there may be other reasons,
   specific to the application, to use more frequent application
   keepalives.  For example, the PCP assigned lifetime could be one hour
   but the application may want to maintain state on its server (e.g.,
   "busy" / "away") more frequently than once an hour.  If the response
   indicates an unexpected IP address or port (e.g., due to IP
   renumbering), the PCP client will want to re-establish its connection
   to its remote server.

   If the PCP client wishes to keep this mapping alive beyond the
   indicated lifetime, it MAY rely on continued inside-to-outside
   traffic to ensure that the mapping will continue to exist, or it MAY
   issue a new PCP request prior to the expiration.  The recommended
   timings for renewing PEER mappings are the same as for MAP mappings,
   as described in Section 11.2.1.

      Note: Implementations need to expect the PEER response may contain
      an external IP address with a different family than the remote
      peer IP address, e.g., when NAT64 or NAT46 are being used.

13.  Options for MAP and PEER Opcodes

   This section describes options for the MAP and PEER Opcodes.  These
   options MUST NOT appear with other Opcodes, unless permitted by those
   other Opcodes.

13.1.  THIRD_PARTY Option for MAP and PEER Opcodes

   This option is used when a PCP client wants to control a mapping to
   an internal host other than itself.  This is used with both MAP and
   PEER Opcodes.

   Due to security concerns with the THIRD_PARTY option, this option
   MUST NOT be implemented or used unless the network on which the PCP



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   messages are to be sent is fully trusted.  For example, if access
   control lists (ACLs) are installed on the PCP client, PCP server, and
   the network between them, so those ACLs allow only communications
   from a trusted PCP client to the PCP server.

   A management device would use this option to control a PCP server on
   behalf of users.  For example, a management device located in a
   network operations center, which presents a user interface to end
   users or to network operations staff, and issues PCP requests with
   the THIRD_PARTY option to the appropriate PCP server.

   The THIRD_PARTY option is formatted as follows:

    0                   1                   2                   3
    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 Code=1 |  Reserved     |   Option Length=16            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                Internal IP Address (128 bits)                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 13: THIRD_PARTY Option

   The fields are described below:

   Internal IP Address:  Internal IP address for this mapping.

      Option Name: THIRD_PARTY
      Number: 1
      Purpose: Indicates the MAP or PEER request is for a host other
      than the host sending the PCP option.
      Valid for Opcodes: MAP, PEER
      Length: 16 octets
      May appear in: request.  May appear in response only if it
      appeared in the associated request.
      Maximum occurrences: 1

   A THIRD_PARTY option MUST NOT contain the same address as the source
   address of the packet.  This is because many PCP servers may not
   implement the THIRD_PARTY option at all, and with those servers a
   client redundantly using the THIRD_PARTY option to specify its own IP
   address would cause such mapping requests to fail where they would
   otherwise have succeeded.  A PCP server receiving a THIRD_PARTY
   option specifying the same address as the source address of the
   packet MUST return a MALFORMED_REQUEST result code.



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   A PCP server MAY be configured to permit or to prohibit the use of
   the THIRD_PARTY option.  If this option is permitted, properly
   authorized clients may perform these operations on behalf of other
   hosts.  If this option is prohibited, and a PCP server receives a PCP
   MAP request with a THIRD_PARTY option, it MUST generate a
   UNSUPP_OPTION response.

   It is RECOMMENDED that customer premises equipment implementing a PCP
   server be configured to prohibit third-party mappings by default.
   With this default, if a user wants to create a third-party mapping,
   the user needs to interact out-of-band with their customer premises
   router (e.g., using its HTTP administrative interface).

   It is RECOMMENDED that service provider NAT and firewall devices
   implementing a PCP server be configured to permit the THIRD_PARTY
   option, when sent by a properly authorized host.  If the packet
   arrives from an unauthorized host, the PCP server MUST generate an
   UNSUPP_OPTION error.

   Note that the THIRD_PARTY option is not needed for today's common
   scenario of an ISP offering a single IP address to a customer who is
   using NAT to share that address locally, since in this scenario all
   the customer's hosts appear, from the point of view of the ISP, to be
   a single host.

   When a PCP client is using the THIRD_PARTY option to make and
   maintain mappings on behalf of some other device, it may be
   beneficial if, where possible, the PCP client verifies that the other
   device is actually present and active on the network.  Otherwise, the
   PCP client risks maintaining those mappings forever, long after the
   device that required them has gone.  This would defeat the purpose of
   PCP mappings having a finite lifetime so that they can be
   automatically deleted after they are no longer needed.

13.2.  PREFER_FAILURE Option for MAP Opcode

   This option is only used with the MAP Opcode.

   This option indicates that if the PCP server is unable to map both
   the suggested external port and suggested external address, the PCP
   server should not create a mapping.  This differs from the behavior
   without this option, which is to create a mapping.

   PREFER_FAILURE is never necessary for a PCP client to manage mappings
   for itself, and its use causes additional work in the PCP client and
   in the PCP server.  This option exists for interworking with non-PCP
   mapping protocols that have different semantics than PCP (e.g., UPnP
   IGDv1 interworking [PNP-IGD-PCP], where the semantics of UPnP IGDv1



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   only allow the UPnP IGDv1 client to dictate mapping a specific port),
   or separate port allocation systems that allocate ports to a
   subscriber (e.g., a subscriber-accessed web portal operated by the
   same ISP that operates the PCP server).  A PCP server MAY support
   this option, if its designers wish to support such downstream devices
   or separate port allocation systems.  PCP servers that are not
   intended to interface with such systems are not required to support
   this option.  PCP clients other than UPnP IGDv1 interworking clients
   or other than a separate port allocation system SHOULD NOT use this
   option because it results in inefficient operation, and they cannot
   safely assume that all PCP servers will implement it.  It is
   anticipated that this option will be deprecated in the future as more
   clients adopt PCP natively and the need for this option declines.

   The PREFER_FAILURE option is formatted as follows:

    0                   1                   2                   3
    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 Code=2 |  Reserved     |   Option Length=0             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 14: PREFER_FAILURE Option

      Option Name: PREFER_FAILURE
      Number: 2
      Purpose: indicates that the PCP server should not create an
      alternative mapping if the suggested external port and address
      cannot be mapped.
      Valid for Opcodes: MAP
      Length: 0
      May appear in: request.  May appear in response only if it
      appeared in the associated request.
      Maximum occurrences: 1

   The result code CANNOT_PROVIDE_EXTERNAL is returned if the suggested
   external address, protocol, and port cannot be mapped.  This can
   occur because the external port is already mapped to another host's
   outbound dynamic mapping, an inbound dynamic mapping, a static
   mapping, or the same internal address, protocol, and port already
   have an outbound dynamic mapping that is mapped to a different
   external port than suggested.  This can also occur because the
   external address is no longer available (e.g., due to renumbering).
   The server MAY set the lifetime in the response to the remaining
   lifetime of the conflicting mapping + TIME_WAIT [RFC0793], rounded up
   to the next larger integer number of seconds.





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   If a PCP request contains the PREFER_FAILURE option and has zero in
   the Suggested External Port field, then it is invalid.  The PCP
   server MUST reject such a message with the MALFORMED_OPTION error
   code.

   PCP servers MAY choose to rate-limit their handling of PREFER_FAILURE
   requests, to protect themselves from a rapid flurry of 65535
   consecutive PREFER_FAILURE requests from clients probing to discover
   which external ports are available.

   There can exist a race condition between the MAP Opcode using the
   PREFER_FAILURE option and Mapping Update (Section 14.2).  For
   example, a previous host on the local network could have previously
   had the same internal address, with a mapping for the same internal
   port.  At about the same moment that the current host sends a MAP
   Request using the PREFER_FAILURE option, the PCP server could send a
   spontaneous Mapping Update for the old mapping due to an external
   configuration change, which could appear to be a reply to the new
   mapping request.  Because of this, the PCP client MUST validate that
   the external IP address, protocol, port, and nonce in a success
   response match the associated suggested values from the request.  If
   they do not match, it is because the Mapping Update was sent before
   the MAP request was processed.

13.3.  FILTER Option for MAP Opcode

   This option is only used with the MAP Opcode.

   This option indicates that filtering incoming packets is desired.
   The protocol being filtered is indicated by the Protocol field in the
   MAP Request, and the remote peer IP address and remote peer port of
   the FILTER option indicate the permitted remote peer's source IP
   address and source port for packets from the Internet; other traffic
   from other addresses is blocked.  The remote peer prefix length
   indicates the length of the remote peer's IP address that is
   significant; this allows a single option to permit an entire subnet.
   After processing this MAP request containing the FILTER option and
   generating a successful response, the PCP-controlled device will drop
   packets received on its public-facing interface that don't match the
   filter fields.  After dropping the packet, if its security policy
   allows, the PCP-controlled device MAY also generate an ICMP error in
   response to the dropped packet.

   The use of the FILTER option can be seen as a performance
   optimization.  Since all software using PCP to receive incoming
   connections also has to deal with the case where it may be directly
   connected to the Internet and receive unrestricted incoming TCP
   connections and UDP packets, if it wishes to restrict incoming



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   traffic to a specific source address or group of source addresses,
   such software already needs to check the source address of incoming
   traffic and reject unwanted traffic.  However, the FILTER option is a
   particularly useful performance optimization for battery powered
   wireless devices, because it can enable them to conserve battery
   power by not having to wake up just to reject unwanted traffic.

   The FILTER option is formatted as follows:

      0                   1                   2                   3
      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 Code=3 |  Reserved     |   Option Length=20            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Reserved   | Prefix Length |      Remote Peer Port         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |               Remote Peer IP address (128 bits)               |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 15: FILTER Option Layout

   These fields are described below:

   Reserved:  8 reserved bits, MUST be sent as 0 and MUST be ignored
      when received.

   Prefix Length:  indicates how many bits of the IPv4 or IPv6 address
      are relevant for this filter.  The value 0 indicates "no filter",
      and will remove all previous filters.  See below for detail.

   Remote Peer Port:  the port number of the remote peer.  The value 0
      indicates "all ports".

   Remote Peer IP address:  The IP address of the remote peer.

      Option Name: FILTER
      Number: 3
      Purpose: specifies a filter for incoming packets
      Valid for Opcodes: MAP
      Length: 20 octets
      May appear in: request.  May appear in response only if it
      appeared in the associated request.
      Maximum occurrences: as many as fit within maximum PCP message
      size




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   The Prefix Length indicates how many bits of the address are used for
   the filter.  For IPv4 addresses (which are encoded using the
   IPv4-mapped address format (::FFFF:0:0/96)), this means valid prefix
   lengths are between 96 and 128 bits, inclusive.  That is, add 96 to
   the IPv4 prefix length.  For IPv6 addresses, valid prefix lengths are
   between 0 and 128 bits, inclusive.  Values outside those ranges cause
   the PCP server to return the MALFORMED_OPTION result code.

   If multiple occurrences of the FILTER option exist in the same MAP
   request, they are processed in the order received (as per normal PCP
   option processing), and they MAY overlap the filtering requested.  If
   there is an existing mapping (with or without a filter) and the
   server receives a MAP request with FILTER, the filters indicated in
   the new request are added to any existing filters.  If a MAP request
   has a lifetime of 0 and contains the FILTER option, the error
   MALFORMED_OPTION is returned.

   If any occurrences of the FILTER option in a request packet are not
   successfully processed then an error is returned (e.g.,
   MALFORMED_OPTION if one of the options was malformed) and as with
   other PCP errors, returning an error causes no state to be changed in
   the PCP server or in the PCP-controlled device.

   To remove all existing filters, the Prefix Length 0 is used.  There
   is no mechanism to remove a specific filter.

   To change an existing filter, the PCP client sends a MAP request
   containing two FILTER options, the first option containing a prefix
   length of 0 (to delete all existing filters) and the second
   containing the new remote peer's IP address, protocol, and port.
   Other FILTER options in that PCP request, if any, add more allowed
   remote peers.

   The PCP server or the PCP-controlled device is expected to have a
   limit on the number of remote peers it can support.  This limit might
   be as small as one.  If a MAP request would exceed this limit, the
   entire MAP request is rejected with the result code
   EXCESSIVE_REMOTE_PEERS, and the state on the PCP server is unchanged.

   All PCP servers MUST support at least one filter per MAP mapping.

14.  Rapid Recovery

   PCP includes a rapid recovery feature, which allows PCP clients to
   repair failed mappings within seconds, rather than the minutes or
   hours it might take if they relied solely on waiting for the next
   routine renewal of the mapping.  Mapping failures may occur when a
   NAT gateway is rebooted and loses its mapping state, or when a NAT



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   gateway has its external IP address changed so that its current
   mapping state becomes invalid.

   The PCP rapid recovery feature enables users to, for example, connect
   to remote machines using ssh, and then reboot their NAT or firewall
   device (or even replace it with completely new hardware) without
   losing their established ssh connections.

   Use of PCP rapid recovery is a performance optimization to PCP's
   routine self-healing.  Without rapid recovery, PCP clients will still
   recreate their correct state when they next renew their mappings, but
   this routine self-healing process may take hours rather than seconds,
   and will probably not happen fast enough to prevent active TCP
   connections from timing out.

   There are two mechanisms to perform rapid recovery, described below.
   Failing to implement and deploy a rapid recovery mechanism will
   encourage application developers to feel the need to refresh their
   PCP state more frequently than necessary, causing more network
   traffic.  Therefore, a PCP server that can lose state (e.g., due to
   reboot) or might have a mapping change (e.g., due to IP renumbering)
   MUST implement either the Announce Opcode or the Mapping Update
   mechanism and SHOULD implement both mechanisms.

14.1.  ANNOUNCE Opcode

   This rapid recovery mechanism uses the ANNOUNCE Opcode.  When the PCP
   server loses its state (e.g., it lost its state when rebooted), it
   resets its Epoch time to its initial starting value (usually zero)
   and sends the ANNOUNCE response to the link-scoped multicast address
   (specific address explained below) if a multicast network exists on
   its local interface, or, if configured with the IP address(es) and
   port(s) of PCP client(s), it sends unicast ANNOUNCE responses to
   those address(es) and port(s).  This means ANNOUNCE may not be
   available on all networks (such as networks without a multicast link
   between the PCP server and its PCP clients).  Additionally, an
   ANNOUNCE request can be sent (unicast) by a PCP client that elicits a
   unicast ANNOUNCE response like any other Opcode.

   Upon receiving PCP response packets with an anomalous Epoch time,
   clients deduce that the PCP server lost state and recreate their lost
   mappings.









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14.1.1.  ANNOUNCE Operation

   The PCP ANNOUNCE Opcode requests and responses have no
   Opcode-specific payload (that is, the length of the Opcode-specific
   data is zero).  The Requested Lifetime field of requests and Lifetime
   field of responses are both set to 0 on transmission and ignored on
   reception.

   If a PCP server receives an ANNOUNCE request, it first parses it and
   generates a SUCCESS if parsing and processing of ANNOUNCE is
   successful.  An error is generated if the client's IP Address field
   does not match the packet source address, or the request packet is
   otherwise malformed, such as packet length less than 24 octets.  Note
   that, in the future, options MAY be sent with the PCP ANNOUNCE
   Opcode; PCP clients and servers need to be prepared to receive
   options with the ANNOUNCE Opcode.

      Discussion: Client-to-server request messages are sent, from any
      client source port, to listening UDP port 5351 on the server;
      server-to-client multicast notifications are sent from the
      server's UDP port (5351) to listening UDP port 5350 on the client.
      The reason the same listening UDP port is not used for both
      purposes is that a single device may have multiple roles.  For
      example, a multi-function home gateway that provides NAT service
      (PCP server) may also provide printer sharing (which wants a PCP
      client), or a home computer (PCP client) may also provide
      "Internet Sharing" (NAT) functionality (which needs to offer PCP
      service).  Such devices need to act as both a PCP server and a PCP
      client at the same time, and the software that implements the PCP
      server on the device may not be the same software component that
      implements the PCP client.  The software that implements the PCP
      server needs to listen for unicast client requests, whereas the
      software that implements the PCP client needs to listen for
      multicast restart announcements.  In many networking APIs it is
      difficult or impossible to have two independent clients listening
      for both unicasts and multicasts on the same port at the same
      time.  For this reason, two ports are used.

14.1.2.  Generating and Processing a Solicited ANNOUNCE Message

   The PCP ANNOUNCE Opcode MAY be sent (unicast) by a PCP client.  The
   Requested Lifetime value MUST be set to zero.

   When the PCP server receives the ANNOUNCE Opcode and successfully
   parses and processes it, it generates SUCCESS response with an
   assigned lifetime of zero.





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   This functionality allows a PCP client to determine a server's Epoch,
   or to determine if a PCP server is running, without changing the
   server's state.

14.1.3.  Generating and Processing an Unsolicited ANNOUNCE Message

   When sending unsolicited responses, the ANNOUNCE Opcode MUST have
   result code equal to zero (SUCCESS), and the packet MUST be sent from
   the unicast IP address and UDP port number on which PCP requests are
   received (so that the PCP response processing described in
   Section 8.3 will accept the message).  This message is most typically
   multicast, but can also be unicast.  Multicast PCP restart
   announcements are sent to 224.0.0.1:5350 and/or [ff02::1]:5350, as
   described below.  Sending PCP restart announcements via unicast
   requires that the PCP server know the IP address(es) and port(s) of
   its listening clients, which means that sending PCP restart
   announcements via unicast is only applicable to PCP servers that
   retain knowledge of the IP address(es) and port(s) of their clients
   even after they otherwise lose the rest of their state.

   When a PCP server device that implements this functionality reboots,
   restarts its NAT engine, or otherwise enters a state where it may
   have lost some or all of its previous mapping state (or enters a
   state where it doesn't even know whether it may have had prior state
   that it lost), it MUST inform PCP clients of this fact by unicasting
   or multicasting a gratuitous PCP ANNOUNCE Opcode response packet, as
   shown below, via paths over which it accepts PCP requests.  If
   sending a multicast ANNOUNCE message, a PCP server device that
   accepts PCP requests over IPv4 sends the Restart Announcement to the
   IPv4 multicast address 224.0.0.1:5350 (224.0.0.1 is the All Hosts
   multicast group address), and a PCP server device that accepts PCP
   requests over IPv6 sends the Restart Announcement to the IPv6
   multicast address [ff02::1]:5350 (ff02::1 is for all nodes on the
   local segment).  A PCP server device that accepts PCP requests over
   both IPv4 and IPv6 sends a pair of Restart Announcements, one to each
   multicast address.  If sending a unicast ANNOUNCE messages, it sends
   ANNOUNCE response message to the IP address(es) and port(s) of its
   PCP clients.  To accommodate packet loss, the PCP server device MAY
   transmit such packets (or packet pairs) up to ten times (with an
   appropriate Epoch Time value in each to reflect the passage of time
   between transmissions) provided that the interval between the first
   two notifications is at least 250 ms, and the interval between
   subsequent notification at least doubles.

   A PCP client that sends PCP requests to a PCP server via a multicast-
   capable path, and implements the Restart Announcement feature, and
   wishes to receive these announcements, MUST listen to receive these
   PCP Restart Announcements (gratuitous PCP ANNOUNCE Opcode response



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   packets) on the appropriate multicast-capable interfaces on which it
   sends PCP requests, and MAY also listen for unicast announcements
   from the server too, (using the UDP port it already uses to issue
   unicast PCP requests to, and receive unicast PCP responses from, that
   server).  A PCP client device that sends PCP requests using IPv4
   listens for packets sent to the IPv4 multicast address
   224.0.0.1:5350.  A PCP client device that sends PCP requests using
   IPv6 listens for packets sent to the IPv6 multicast address
   [ff02::1]:5350.  A PCP client device that sends PCP requests using
   both IPv4 and IPv6 listens for both types of Restart Announcement.
   The SO_REUSEPORT socket option or equivalent should be used for the
   multicast UDP port, if required by the host OS to permit multiple
   independent listeners on the same multicast UDP port.

   Upon receiving a unicasted or multicasted PCP ANNOUNCE Opcode
   response packet, a PCP client MUST (as it does with all received PCP
   response packets) inspect the announcement's source IP address, and
   if the Epoch Time value is outside the expected range for that
   server, it MUST wait a random amount of time between 0 and 5 seconds
   (to prevent synchronization of all PCP clients), then for all PCP
   mappings it made at that server address the client issues new PCP
   requests to recreate any lost mapping state.  The use of the
   Suggested External IP Address and Suggested External Port fields in
   the client's renewal requests allows the client to remind the
   restarted PCP server device of what mappings the client had
   previously been given, so that in many cases the prior state can be
   recreated.  For PCP server devices that reboot relatively quickly it
   is usually possible to reconstruct lost mapping state fast enough
   that existing TCP connections and UDP communications do not time out,
   and continue without failure.  As for all PCP response messages, if
   the Epoch Time value is within the expected range for that server,
   the PCP client does not recreate its mappings.  As for all PCP
   response messages, after receiving and validating the ANNOUNCE
   message, the client updates its own Epoch time for that server, as
   described in Section 8.5.

14.2.  PCP Mapping Update

   This rapid recovery mechanism is used when the PCP server remembers
   its state and determines its existing mappings are invalid (e.g., IP
   renumbering changes the external IP address of a PCP-controlled NAT).

   It is anticipated that servers that are routinely reconfigured by an
   administrator or have their WAN address changed frequently will
   implement this feature (e.g., residential CPE routers).  It is
   anticipated that servers that are not routinely reconfigured will not
   implement this feature (e.g., service provider-operated CGN).




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   If a PCP server device has not forgotten its mapping state, but for
   some other reason has determined that some or all of its mappings
   have become unusable (e.g., when a home gateway is assigned a
   different external IPv4 address by the upstream DHCP server), then
   the PCP server device automatically repairs its mappings and notifies
   its clients by following the procedure described below.

   For PCP-managed mappings, for each one the PCP server device should
   update the external IP address and external port to appropriate
   available values, and then send unicast PCP MAP or PEER responses (as
   appropriate for the mapping) to inform the PCP client of the new
   external IP address and external port.  Such unsolicited responses
   are identical to the MAP or PEER responses normally returned in
   response to client MAP or PEER requests, containing newly updated
   External IP Address and External Port values, and are sent to the
   same client IP address and port that the PCP server used to send the
   prior response for that mapping.  If the earlier associated request
   contained the THIRD_PARTY option, the THIRD_PARTY option MUST also
   appear in the Mapping Update as it is necessary for the PCP client to
   disambiguate the response.  If the earlier associated request
   contained the PREFER_FAILURE option, and the same external IP
   address, protocol, and port cannot be provided, the error
   CANNOT_PROVIDE_EXTERNAL SHOULD be sent.  If the earlier associated
   request contained the FILTER option, the filters are moved to the new
   mapping and the FILTER option is sent in the Mapping Update response.
   Non-mandatory options SHOULD NOT be sent in the Mapping Update
   response.

      Discussion: It could have been possible to design this so that the
      PCP server (1) sent an ANNOUNCE Opcode to the PCP client, the PCP
      client reacted by (2) sending a new MAP request and (3) receiving
      a MAP response.  Instead, the server can create a shortcut for
      that design by simply sending the message it would have sent in
      (3).

   To accommodate packet loss, the PCP server device SHOULD transmit
   such packets three times, with an appropriate Epoch Time value in
   each to reflect the passage of time between transmissions.  The
   interval between the first two notifications MUST be at least 250 ms,
   and the third packet after a 500-ms interval.  Once the PCP server
   has received a refreshed state for that mapping, the PCP server
   SHOULD cease those retransmissions for that mapping, as it serves no
   further purpose to continue sending messages regarding that mapping.

   Upon receipt of such an updated MAP or PEER response, a PCP client
   uses the information in the response to adjust rendezvous servers or
   reconnect to servers, respectively.  For MAP, this would mean
   updating the DNS entries or other address and port information



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   recorded with some kind of application-specific rendezvous server.
   For PEER responses giving a CANNOT_PROVIDE_EXTERNAL error, this would
   typically mean establishing new connections to servers.  Anytime the
   external address or port changes, existing TCP and UDP connections
   will be lost; PCP can't avoid that, but does provide immediate
   notification of the event to lessen the impact.

15.  Mapping Lifetime and Deletion

   The PCP client requests a certain lifetime, and the PCP server
   responds with the assigned lifetime.  The PCP server MAY grant a
   lifetime smaller or larger than the requested lifetime.  The PCP
   server SHOULD be configurable for permitted minimum and maximum
   lifetime, and the minimum value SHOULD be 120 seconds.  The maximum
   value SHOULD be the remaining lifetime of the IP address assigned to
   the PCP client if that information is available (e.g., from the DHCP
   server), or half the lifetime of IP address assignments on that
   network if the remaining lifetime is not available, or 24 hours.
   Excessively long lifetimes can cause consumption of ports even if the
   internal host is no longer interested in receiving the traffic or is
   no longer connected to the network.  These recommendations are not
   strict, and deployments should evaluate the trade-offs to determine
   their own minimum and maximum Lifetime values.

   Once a PCP server has responded positively to a MAP request for a
   certain lifetime, the port mapping is active for the duration of the
   lifetime unless the lifetime is reduced by the PCP client (to a
   shorter lifetime or to zero) or until the PCP server loses its state
   (e.g., crashes).  Mappings created by PCP MAP requests are not
   special or different from mappings created in other ways.  In
   particular, it is implementation-dependent if outgoing traffic
   extends the lifetime of such mappings beyond the PCP-assigned
   lifetime.  PCP clients MUST NOT depend on this behavior to keep
   mappings active, and MUST explicitly renew their mappings as required
   by the Lifetime field in PCP response messages.

   Upon receipt of a PCP response with an absurdly long assigned
   lifetime, the PCP client SHOULD behave as if it received a more sane
   value (e.g., 24 hours), and renew the mapping accordingly, to ensure
   that if the static mapping is removed, the client will continue to
   maintain the mapping it desires.

   An application that forgets its PCP-assigned mappings (e.g., the
   application or OS crashes) will request new PCP mappings.  This may
   consume port mappings, if the application binds to a different
   internal port every time it runs.  The application will also likely
   initiate new outbound TCP connections, which create implicit dynamic
   outbound mappings without using PCP, which will also consume port



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   mappings.  If there is a port mapping quota for the internal host,
   frequent restarts such as this may exhaust the quota.

   To help clean PCP state, when the PCP-controlled device is collocated
   with the address assignment (DHCP) server, such as in a typical
   residential CPE, it is RECOMMENDED that when an IP address becomes
   invalid (e.g., the DHCP lease expires, or the DHCP client sends an
   explicit DHCP RELEASE) the PCP-controlled device SHOULD also discard
   any dynamic mapping state relating to that expired IP address.

   When using NAT, the same external port may be assigned for use by
   different internal hosts at different times.  For example, if an
   internal host using an external port ceases sending traffic using
   that port, then its mapping may expire, and then later the same
   external port may be assigned to a new internal host.  The new
   internal host could then receive incoming traffic that was intended
   for the previous internal host.  This generally happens
   inadvertently, and this reassignment of the external port only
   happens after the current holder of the external port has ceased
   using it for some period of time.  It would be unacceptable if an
   attacker could use PCP to intentionally speed up this reassignment of
   the external port in order to deliberately steal traffic intended for
   the current holder, by (i) spoofing PCP requests using the current
   holder's source IP address and mapping nonce to fraudulently delete
   the mapping or shorten its lifetime, and then (ii) subsequently
   claiming the external port for itself.

   Therefore, in the simple security model, to protect against this
   attack, PCP MUST NOT allow a PCP request (even a PCP request that
   appears to come from the current holder of the mapping) to cause a
   mapping to expire sooner than it would naturally have expired
   otherwise by virtue of outbound traffic keeping the mapping active.
   A PCP server MUST set the lifetime of a mapping to no less than the
   remaining time before the mapping would expire if no further outbound
   traffic is seen for that mapping.  This means a MAP or PEER request
   with lifetime of 0 will only set the assigned lifetime to 0 (i.e.,
   delete the mapping) if the internal host had not sent a packet using
   that mapping for the idle-timeout time, otherwise the assigned
   lifetime will be the remaining idle-timeout time.

   Finally, to reduce unwanted traffic and data corruption for both TCP
   and UDP, the assigned external port created by the MAP Opcode or PEER
   Opcode SHOULD NOT be reused for an interval equal to the reuse time
   limit enforced by the NAT for its implicit dynamic mappings
   (typically, the maximum TCP segment lifetime of 2 minutes [RFC0793]).
   Furthermore, to reduce port stealing attacks, the assigned external
   port also SHOULD NOT be reused for an interval equal to the time the
   PCP- controlled device would normally maintain an idle (no traffic)



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   implicit dynamic mapping (e.g., 2 minutes for UDP [RFC4787] and 124
   minutes for TCP [RFC5382]).  However, within these time windows, the
   PCP server SHOULD allow an external port to be reclaimed by the same
   client, where "same client" means "same internal IP address, internal
   port, and mapping nonce".

15.1.  Lifetime Processing for the MAP Opcode

   If the requested lifetime is zero then:

   o  If both the protocol and internal port are non-zero, it indicates
      a request to delete the indicated mapping immediately.

   o  If the protocol is non-zero and the internal port is zero, it
      indicates a request to delete a previous 'wildcard' (all-ports)
      mapping for that protocol.  The nonce MUST match the nonce used to
      create the 'wildcard' mapping.

   o  If both the protocol and internal port are zero, it indicates a
      request to delete a previous 'DMZ host' (all incoming traffic for
      all protocols) mapping.  The nonce MUST match the nonce used to
      create the 'DMZ host' mapping.

   o  If the protocol is zero and the internal port is non-zero, then
      the request is invalid and the PCP server MUST return a
      MALFORMED_REQUEST error to the client.

   In requests where the requested Lifetime is 0, the Suggested External
   Address and Suggested External Port fields MUST be set to zero on
   transmission and MUST be ignored on reception, and these fields MUST
   be copied into the assigned external IP address and assigned external
   port of the response.

   PCP MAP requests can only delete or shorten lifetimes of MAP-created
   mappings.  If the PCP client attempts to delete a static mapping
   (i.e., a mapping created outside of PCP itself), or an outbound
   (implicit or PEER-created) mapping, the PCP server MUST return
   NOT_AUTHORIZED.  If the PCP client attempts to delete a mapping that
   does not exist, the SUCCESS result code is returned (this is
   necessary for PCP to return the same response for retransmissions or
   duplications of the same request).  If the deletion request was
   properly formatted and successfully processed, a SUCCESS response is
   generated with the protocol and internal port number copied from the
   request, and the response lifetime set to zero.  An inbound mapping
   (i.e., static mapping or MAP-created dynamic mapping) MUST NOT have
   its lifetime reduced by transport protocol messages (e.g., TCP RST,
   TCP FIN).  Note the THIRD_PARTY option (Section 13.1), if authorized,
   can also delete PCP-created MAP mappings.



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16.  Implementation Considerations

   Section 16 provides non-normative guidance that may be useful to
   implementers.

16.1.  Implementing MAP with EDM Port-Mapping NAT

   For implicit dynamic outbound mappings, some existing NAT devices
   have endpoint-independent mapping (EIM) behavior while other NAT
   devices have endpoint-dependent mapping (EDM) behavior.  NATs that
   have EIM behavior do not suffer from the problem described in this
   section.  The IETF strongly encourages EIM behavior
   [RFC4787][RFC5382].

   In EDM NAT devices, the same external port may be used by an outbound
   dynamic mapping and an inbound dynamic mapping (from the same
   internal host or from a different internal host).  This complicates
   the interaction with the MAP Opcode.  With such NAT devices, there
   are two ways envisioned to implement the MAP Opcode:

   1.  Have outbound mappings use a different set of external ports than
       inbound mappings (e.g., those created with MAP), thus reducing
       the interaction problem between them; or

   2.  On arrival of a packet (inbound from the Internet or outbound
       from an internal host), first attempt to use a dynamic outbound
       mapping to process that packet.  If none match, attempt to use an
       inbound mapping to process that packet.  This effectively
       'prioritizes' outbound mappings above inbound mappings.

16.2.  Lifetime of Explicit and Implicit Dynamic Mappings

   No matter if a NAT is EIM or EDM, it is possible that one (or more)
   outbound mappings, using the same internal port on the internal host,
   might be created before or after a MAP request.  When this occurs, it
   is important that the NAT honor the lifetime returned in the MAP
   response.  Specifically, if an inbound mapping was created with the
   MAP Opcode, the implementation needs to ensure that termination of an
   outbound mapping (e.g., via a TCP FIN handshake) does not prematurely
   destroy the MAP-created inbound mapping.

16.3.  PCP Failure Recovery

   If an event occurs that causes the PCP server to lose dynamic mapping
   state (such as a crash or power outage), the mappings created by PCP
   are lost.  Occasional loss of state may be unavoidable in a
   residential NAT device that does not write transient information to
   non-volatile memory.  Loss of state is expected to be rare in a



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   service provider environment (due to redundant power, disk drives for
   storage, etc.).  Of course, due to outright failure of service
   provider equipment (e.g., software malfunction), state may still be
   lost.

   The Epoch time allows a client to deduce when a PCP server may have
   lost its state.  When the Epoch Time value is observed to be outside
   the expected range, the PCP client can attempt to recreate the
   mappings following the procedures described in this section.

   Further analysis of PCP failure scenarios is planned for a future
   document [PCP-FAIL].

16.3.1.  Recreating Mappings

   A mapping renewal packet is formatted identically to an original
   mapping request; from the point of view of the client, it is a
   renewal of an existing mapping; however, from the point of view of a
   newly rebooted PCP server, it appears as a new mapping request.  In
   the normal process of routinely renewing its mappings before they
   expire, a PCP client will automatically recreate all its lost
   mappings.

   When the PCP server loses state and begins processing new PCP
   messages, its Epoch time is reset and begins counting again.  As the
   result of receiving a packet where the Epoch Time field is outside
   the expected range (Section 8.5), indicating that a reboot or similar
   loss of state has occurred, the client can renew its port mappings
   sooner, without waiting for the normal routine renewal time.

16.3.2.  Maintaining Mappings

   A PCP client refreshes a mapping by sending a new PCP request
   containing information learned from the earlier PCP response.  The
   PCP server will respond indicating the new lifetime.  It is possible,
   due to reconfiguration or failure of the PCP server, that the
   external IP address and/or external port, or the PCP server itself,
   has changed (due to a new route to a different PCP server).  Such
   events are rare, but not an error.  The PCP server will simply return
   a new external address and/or external port to the client, and the
   client should record this new external address and port with its
   rendezvous service.  To detect such events more quickly, a server
   that requires extremely high availability may find it beneficial to
   use shorter lifetimes in its PCP mappings requests, so that it
   communicates with the PCP server more often.  This is an engineering
   trade-off based on (i) the acceptable downtime for the service in
   question, (ii) the expected likelihood of NAT or firewall state loss,
   and (iii) the amount of PCP maintenance traffic that is acceptable.



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   If the PCP client has several mappings, the Epoch Time value only
   needs to be retrieved for one of them to determine whether or not it
   appears the PCP server may have suffered a catastrophic loss of
   state.  If the client wishes to check the PCP server's Epoch time, it
   sends a PCP request for any one of the client's mappings.  This will
   return the current Epoch Time value.  In that request, the PCP client
   could extend the mapping lifetime (by asking for more time) or
   maintain the current lifetime (by asking for the same number of
   seconds that it knows are remaining of the lifetime).

   If a PCP client changes its internal IP address (e.g., because the
   internal host has moved to a new network), and the PCP client wishes
   to still receive incoming traffic, it needs create new mappings on
   that new network.  New mappings will typically also require an update
   to the application-specific rendezvous server if the external address
   or port is different from the previous values (see Sections 10.1 and
   11.5).

16.3.3.  SCTP

   Although SCTP has port numbers like TCP and UDP, SCTP works
   differently when behind an address-sharing NAT, in that SCTP port
   numbers are not changed [SCTPNAT].  Outbound dynamic SCTP mappings
   use the verification tag of the association instead of the local and
   remote peer port numbers.  As with TCP, explicit outbound mappings
   can be made to reduce keepalive intervals, and explicit inbound
   mappings can be made by passive listeners expecting to receive new
   associations at the external port.

   Because an SCTP-aware NAT does not (currently) rewrite SCTP port
   numbers, it will not be able to assign an external port that is
   different from the client's internal port.  A PCP client making a MAP
   request for SCTP should be aware of this restriction.  The PCP client
   SHOULD make its SCTP MAP request just as it would for a TCP MAP
   request: in its initial PCP MAP request it SHOULD specify zero for
   the external address and port, and then in subsequent renewals it
   SHOULD echo the assigned external address and port.  However, since a
   current SCTP-aware NAT can only assign an external port that is the
   same as the internal port, it may not be able to do that if the
   external port is already assigned to a different PCP client.  This is
   likely if there is more than one instance of a given SCTP service on
   the local network, since both instances are likely to listen on the
   same well-known SCTP port for that service on their respective hosts,
   but they can't both have the same external port on the NAT gateway's
   external address.  A particular external port may not be assignable
   for other reasons, such as when it is already in use by the NAT
   device itself, or otherwise prohibited by policy, as described in
   Section 11.3, "Processing a MAP Request".  In the event that the



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   external port matching the internal port cannot be assigned (and the
   SCTP-aware NAT does not perform SCTP port rewriting), the SCTP-aware
   NAT MUST return a CANNOT_PROVIDE_EXTERNAL error to the requesting PCP
   client.  Note that this restriction places an extra burden on the
   SCTP server whose MAP request failed, because it then has to tear
   down its exiting listening socket and try again with a different
   internal port, repeatedly until it is successful in finding an
   external port it can use.

   The SCTP complications described above occur because of address
   sharing.  The SCTP complications are avoided when address sharing is
   avoided (e.g., 1:1 NAT, firewall).

16.4.  Source Address Replicated in PCP Header

   All PCP requests include the PCP client's IP address replicated in
   the PCP header.  This is used to detect unexpected address rewriting
   (NAT) on the path between the PCP client and its PCP server.  On
   operating systems that support the sockets API, the following steps
   are RECOMMENDED for a PCP client to insert the correct source address
   in the PCP header:

   1.  Create a UDP socket.
   2.  Call "connect" on this UDP socket using the address and port of
       the desired PCP server.
   3.  Call the getsockname() function to retrieve a sockaddr containing
       the source address the kernel will use for UDP packets sent
       through this socket.
   4.  If the IP address is an IPv4 address, encode the address into an
       IPv4-mapped IPv6 address.  Place the IPv4-mapped IPv6 address or
       the native IPv6 address into the PCP Client's IP Address field in
       the PCP header.
   5.  Send PCP requests using this connected UDP socket.


















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16.5.  State Diagram

   Each mapping entry of the PCP-controlled device would go through the
   state machine shown below.  This state diagram is non-normative.

       CLOSE_MSG or
      (NO_TRAFFIC and EXPIRY)   +---------+  NO_TRAFFIC and EXPIRY
                +-------------->|         |<------------+
                |               |NO_ENTRY |             |
                |   +-----------|         |---------+   |
                |   |           +---------+         |   |
                |   |              ^  |             |   |
                |   |   NO_TRAFFIC |  |             |   |
                |   |           or |  |             |   |
                |   |   CLOSE_MSGS |  |             |   |
                |   |              |  |             |   |
                |   |PEER request  |  |  MAP request|   |
                |   V              |  |             V   |
             +---------+           |  |         +---------+
         +-->|  "P",   |           |  |    M-R  |  "M",   |<--+
     P-R |   | PEER    |-----------|--|-------->| MAP     |   | M-R or
         +---|  mapping|           |  |         |  mapping|---+ P-R or
             +---------+           |  |         +---------+  CLOSE_MSGS
                |   ^              |  |             ^   |
                |   |PEER request  |  |  MAP request|   |
                |   |              |  |             |   |
                |   |              |  |             |   |
                |   |              |  |             |   |
                |   |              |  | outbound    |   |
                |   |              |  | TRAFFIC     |   |
                |   |              |  V             |   |
                |   |           +---------+         |   |
                |   +-----------| "I",    |---------+   |
                |               | implicit|             |
                +-------------->| mapping |<------------+
            TRAFFIC and EXPIRY  +---------+  TRAFFIC and EXPIRY

                       Figure 16: PCP State Diagram

   The meanings of the states and events are:

      NO_ENTRY:  Invalid state represents Entry does not exist.  This is
            the only possible start state.

      M-R:  MAP request

      P-R:  PEER request




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      M:    Mapping entry when created by MAP request

      P:    Mapping entry when created/managed by PEER request

      I:    Implicit mapping created by an outgoing packet from the
            client (e.g., TCP SYN), and also the state when a
            PCP-created mapping's lifetime expires while there is still
            active traffic.

      EXPIRY: PEER or MAP lifetime expired

      TRAFFIC:  Traffic seen by PCP-controlled device using this entry
            within the expiry time for that entry.  This traffic may be
            inbound or outbound.

      NO_TRAFFIC:  Indicates that there is no TRAFFIC.

      CLOSE_MSG:  Protocol messages from the client or server to close
            the session (e.g., TCP FIN or TCP RST), as per the NAT or
            firewall device's handling of such protocol messages.

   Notes on the diagram:

   1.  The 'and' clause indicates the events on either side of 'and' are
       required for the state-transition.  The 'or' clause indicates
       either one of the events are enough for the state-transition.

   2.  Transition from state M to state I is implementation dependent.

17.  Deployment Considerations

17.1.  Ingress Filtering

   As with implicit dynamic mappings created by outgoing TCP SYN
   packets, explicit dynamic mappings created via PCP use the source IP
   address of the packet as the internal address for the mappings.
   Therefore, ingress filtering [RFC2827] SHOULD be used on the path
   between the internal host and the PCP server to prevent the injection
   of spoofed packets onto that path.

17.2.  Mapping Quota

   On PCP-controlled devices that create state when a mapping is created
   (e.g., NAT), the PCP server SHOULD maintain per-host and/or per-
   subscriber quotas for mappings.  It is implementation specific
   whether the PCP server uses a separate quotas for implicit, explicit,
   and static mappings, a combined quota for all of them, or some other
   policy.



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18.  Security Considerations

   The goal of the PCP protocol is to improve the ability of end nodes
   to control their associated NAT state, and to improve the efficiency
   and error handling of NAT mappings when compared to existing implicit
   mapping mechanisms in NAT boxes and stateful firewalls.  It is the
   security goal of the PCP protocol to limit any new denial-of-service
   opportunities, and to avoid introducing new attacks that can result
   in unauthorized changes to mapping state.  One of the most serious
   consequences of unauthorized changes in mapping state is traffic
   theft.  All mappings that could be created by a specific host using
   implicit mapping mechanisms are inherently considered to be
   authorized.  Confidentiality of mappings is not a requirement, even
   in cases where the PCP messages may transit paths that would not be
   traveled by the mapped traffic.

18.1.  Simple Threat Model

   PCP servers are secure against off-path attackers who cannot spoof a
   packet that the PCP server will view as a packet received from the
   internal network.  PCP clients are secure against off-path attackers
   who can spoof the PCP server's IP address.

   Defending against attackers who can modify or drop packets between
   the internal network and the PCP server, or who can inject spoofed
   packets that appear to come from the internal network is out of
   scope.  Such an attacker can redirect traffic to a host of their
   choosing.

   A PCP server is secure under this threat model if the PCP server is
   constrained so that it does not configure any explicit mapping that
   it would not configure implicitly.  In most cases, this means that
   PCP servers running on NAT boxes or stateful firewalls that support
   the PEER and MAP Opcodes can be secure under this threat model if
   (1) all of their hosts are within a single administrative domain (or
   if the internal hosts can be securely partitioned into separate
   administrative domains, as in the DS-Lite B4 case), (2) explicit
   mappings are created with the same lifetime as implicit mappings, and
   (3) the THIRD_PARTY option is not supported.  PCP servers can also
   securely support the MAP Opcode under this threat model if the
   security policy on the device running the PCP server would permit
   endpoint-independent filtering of implicit mappings.

   PCP servers that comply with the Simple Threat Model and do not
   implement a PCP security mechanism described in Section 18.2 MUST
   enforce the constraints described in the paragraph above.





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18.1.1.  Attacks Considered

   o  If you allow multiple administrative domains to send PCP requests
      to a single PCP server that does not enforce a boundary between
      the domains, it is possible for a node in one domain to perform a
      denial-of-service attack on other domains or to capture traffic
      that is intended for a node in another domain.

   o  If explicit mappings have longer lifetimes than implicit mappings,
      it makes it easier to perpetrate a denial-of-service attack than
      it would be if the PCP server was not present.

   o  If the PCP server supports deleting or reducing the lifetime of
      existing mappings, this allows an attacking node to steal an
      existing mapping and receive traffic that was intended for another
      node.

   o  If the THIRD_PARTY option is supported, this also allows an
      attacker to open a window for an external node to attack an
      internal node, allows an attacker to steal traffic that was
      intended for another node, or may facilitate a denial-of-service
      attack.  One example of how the THIRD_PARTY option could grant an
      attacker more capability than a spoofed implicit mapping is that
      the PCP server (especially if it is running in a service
      provider's network) may not be aware of internal filtering that
      would prevent spoofing an equivalent implicit mapping, such as
      filtering between a guest and corporate network.

   o  If the MAP Opcode is supported by the PCP server in cases where
      the security policy would not support endpoint-independent
      filtering of implicit mappings, then the MAP Opcode changes the
      security properties of the device running the PCP server by
      allowing explicit mappings that violate the security policy.

18.1.2.  Deployment Examples Supporting the Simple Threat Model

   This section offers two examples of how the Simple Threat Model can
   be supported in real-world deployment scenarios.

18.1.2.1.  Residential Gateway Deployment

   Parity with many currently deployed residential gateways can be
   achieved using a PCP server that is constrained as described in
   Section 18.1 above.







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18.2.  Advanced Threat Model

   In the Advanced Threat Model, the PCP protocol ensures that attackers
   (on- or off-path) cannot create unauthorized mappings or make
   unauthorized changes to existing mappings.  The protocol must also
   limit the opportunity for on- or off-path attackers to perpetrate
   denial-of-service attacks.

   The Advanced Threat Model security model will be needed in the
   following cases:

   o  Security infrastructure equipment, such as corporate firewalls,
      that does not create implicit mappings.

   o  Equipment (such as CGNs or service provider firewalls) that serves
      multiple administrative domains and does not have a mechanism to
      securely partition traffic from those domains.

   o  Any implementation that wants to be more permissive in authorizing
      explicit mappings than it is in authorizing implicit mappings.

   o  Implementations that wish to support any deployment scenario that
      does not meet the constraints described in Section 18.1.

   To protect against attacks under this threat model, a PCP security
   mechanism that provides an authenticated, integrity-protected
   signaling channel would need to be specified.

   PCP servers that implement a PCP security mechanism MAY accept
   unauthenticated requests.  In their default configuration, PCP
   servers implementing the PCP security mechanism MUST still enforce
   the constraints described in Section 18.1 when processing
   unauthenticated requests.

18.3.  Residual Threats

   This section describes some threats that are not addressed in either
   of the above threat models and recommends appropriate mitigation
   strategies.

18.3.1.  Denial of Service

   Because of the state created in a NAT or firewall, a per-host and/or
   per-subscriber quota will likely exist for both implicit dynamic
   mappings and explicit dynamic mappings.  A host might make an
   excessive number of implicit or explicit dynamic mappings, consuming





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   an inordinate number of ports, causing a denial of service to other
   hosts.  Thus, Section 17.2 recommends that hosts be limited to a
   reasonable number of explicit dynamic mappings.

   An attacker, on the path between the PCP client and PCP server, can
   drop PCP requests, drop PCP responses, or spoof a PCP error, all of
   which will effectively deny service.  Through such actions, the PCP
   client might not be aware the PCP server might have actually
   processed the PCP request.  An attacker sending a NO_RESOURCES error
   can cause the PCP client to not send messages to that server for a
   while.  There is no mitigation to this on-path attacker.

18.3.2.  Ingress Filtering

   It is important to prevent a host from fraudulently creating,
   deleting, or refreshing a mapping (or filtering) for another host,
   because this can expose the other host to unwanted traffic, prevent
   it from receiving wanted traffic, or consume the other host's mapping
   quota.  Both implicit and explicit dynamic mappings are created based
   on the source IP address in the packet, and hence depend on ingress
   filtering to guard against spoof source IP addresses.

18.3.3.  Mapping Theft

   In the time between when a PCP server loses state and the PCP client
   notices the lower-than-expected Epoch Time value, it is possible that
   the PCP client's mapping will be acquired by another host (via an
   explicit dynamic mapping or implicit dynamic mapping).  This means
   incoming traffic will be sent to a different host ("theft").  Rapid
   recovery reduces this interval, but does not completely eliminate
   this threat.  The PCP client can reduce this interval by using a
   relatively short lifetime; however, this increases the amount of PCP
   chatter.  This threat is reduced by using persistent storage of
   explicit dynamic mappings in the PCP server (so it does not lose
   explicit dynamic mapping state), or by ensuring that the previous
   external IP address, protocol, and port cannot be used by another
   host (e.g., by using a different IP address pool).

18.3.4.  Attacks against Server Discovery

   This document does not specify server discovery, beyond contacting
   the default gateway.









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19.  IANA Considerations

   IANA has performed the following actions.

19.1.  Port Number

   PCP uses ports 5350 and 5351, previously assigned by IANA to NAT-PMP
   [RFC6886].  IANA has reassigned those ports to PCP.

19.2.  Opcodes

   IANA has created a new protocol registry for PCP Opcodes, numbered
   0-127, initially populated with the values:

           Value            Opcode
           -----            -------------------------
           0                ANNOUNCE
           1                MAP
           2                PEER
           3-31             Standards Action [RFC5226]
           32-63            Specification Required [RFC5226]
           96-126           Reserved for Private Use [RFC5226]
           127              Reserved, Standards Action [RFC5226]

   The value 127 is Reserved and may be assigned via Standards Action
   [RFC5226].  The values in the range 3-31 can be assigned via
   Standards Action [RFC5226], 32-63 via Specification Required
   [RFC5226], and the range 96-126 is for Private Use [RFC5226].

19.3.  Result Codes

   IANA has created a new registry for PCP result codes, numbered 0-255,
   initially populated with the result codes from Section 7.4.  The
   value 255 is Reserved and may be assigned via Standards Action
   [RFC5226].

   The values in the range 14-127 can be assigned via Standards Action
   [RFC5226], 128-191 via Specification Required [RFC5226], and the
   range 191-254 is for Private Use [RFC5226].

19.4.  Options

   IANA has created a new registry for PCP options, numbered 0-255, each
   with an associated mnemonic.  The values 0-127 are mandatory to
   process, and 128-255 are optional to process.  The initial registry
   contains the options described in Section 13.  The option values 0,
   127, and 255 are Reserved and may be assigned via Standards Action
   [RFC5226].



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   Additional PCP option codes in the ranges 4-63 and 128-191 can be
   created via Standards Action [RFC5226], the ranges 64-95 and 192-223
   are for Specification Required [RFC5226], and the ranges 96-126 and
   224-254 are for Private Use [RFC5226].

   Documents describing an option should describe the processing for
   both the PCP client and server, and the information below:

      Option Name: <mnemonic>
      Number: <value>
      Purpose: <textual description>
      Valid for Opcodes: <list of Opcodes>
      Length: <rules for length>
      May appear in: <requests/responses/both>
      Maximum occurrences: <count>

20.  Acknowledgments

   Thanks to Xiaohong Deng, Alain Durand, Christian Jacquenet, Jacni
   Qin, Simon Perreault, James Yu, Tina TSOU (Ting ZOU), Felipe Miranda
   Costa, James Woodyatt, Dave Thaler, Masataka Ohta, Vijay K. Gurbani,
   Loa Andersson, Richard Barnes, Russ Housley, Adrian Farrel, Pete
   Resnick, Pasi Sarolahti, Robert Sparks, Wesley Eddy, Dan Harkins,
   Peter Saint-Andre, Stephen Farrell, Ralph Droms, Felipe Miranda
   Costa, Amit Jain, and Wim Henderickx for their comments and review.

   Thanks to Simon Perreault for highlighting the interaction of dynamic
   connections with PCP-created mappings and for many other review
   comments.

   Thanks to Francis Dupont for his several thorough reviews of the
   specification, which improved the protocol significantly.

   Thanks to T. S. Ranganathan for the state diagram.

   Thanks to Peter Lothberg for clock skew information, which guided the
   choice of tolerance levels for deciding when an Epoch time should be
   considered to be anomalous.

   Thanks to Margaret Wasserman and Sam Hartman for writing the Security
   Considerations section.

   Thanks to authors of DHCPv6 for retransmission text.








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21.  References

21.1.  Normative References

   [RFC0768]        Postel, J., "User Datagram Protocol", STD 6,
                    RFC 768, August 1980.

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

   [RFC2827]        Ferguson, P. and D. Senie, "Network Ingress
                    Filtering: Defeating Denial of Service Attacks which
                    employ IP Source Address Spoofing", BCP 38,
                    RFC 2827, May 2000.

   [RFC4086]        Eastlake, D., Schiller, J., and S. Crocker,
                    "Randomness Requirements for Security", BCP 106,
                    RFC 4086, June 2005.

   [RFC4193]        Hinden, R. and B. Haberman, "Unique Local IPv6
                    Unicast Addresses", RFC 4193, October 2005.

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

   [RFC5226]        Narten, T. and H. Alvestrand, "Guidelines for
                    Writing an IANA Considerations Section in RFCs",
                    BCP 26, RFC 5226, May 2008.

   [RFC6056]        Larsen, M. and F. Gont, "Recommendations for
                    Transport-Protocol Port Randomization", BCP 156,
                    RFC 6056, January 2011.

   [proto_numbers]  IANA, "Protocol Numbers", 2011,
                    <http://www.iana.org/assignments/protocol-numbers>.

21.2.  Informative References

   [IGDv1]          UPnP Gateway Committee, "WANIPConnection:1",
                    November 2001, <http://upnp.org/specs/gw/
                    UPnP-gw-WANIPConnection-v1-Service.pdf>.

   [L2NAT]          Miles, D. and M. Townsley, "Layer2-Aware NAT", Work
                    in Progress, March 2009.

   [PCP-FAIL]       Boucadair, M., Dupont, F., and R. Penno, "Port
                    Control Protocol (PCP) Failure Scenarios", Work
                    in Progress, August 2012.



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   [PNP-IGD-PCP]    Boucadair, M., Penno, R., and D. Wing, "Universal
                    Plug and Play (UPnP) Internet Gateway Device (IGD)-
                    Port Control Protocol (PCP) Interworking Function",
                    Work in Progress, December 2012.

   [RFC0793]        Postel, J., "Transmission Control Protocol", STD 7,
                    RFC 793, September 1981.

   [RFC1918]        Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot,
                    G., and E. Lear, "Address Allocation for Private
                    Internets", BCP 5, RFC 1918, February 1996.

   [RFC2136]        Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
                    "Dynamic Updates in the Domain Name System (DNS
                    UPDATE)", RFC 2136, April 1997.

   [RFC3007]        Wellington, B., "Secure Domain Name System (DNS)
                    Dynamic Update", RFC 3007, November 2000.

   [RFC3022]        Srisuresh, P. and K. Egevang, "Traditional IP
                    Network Address Translator (Traditional NAT)",
                    RFC 3022, January 2001.

   [RFC3581]        Rosenberg, J. and H. Schulzrinne, "An Extension to
                    the Session Initiation Protocol (SIP) for Symmetric
                    Response Routing", RFC 3581, August 2003.

   [RFC3587]        Hinden, R., Deering, S., and E. Nordmark, "IPv6
                    Global Unicast Address Format", RFC 3587,
                    August 2003.

   [RFC4303]        Kent, S., "IP Encapsulating Security Payload (ESP)",
                    RFC 4303, December 2005.

   [RFC4340]        Kohler, E., Handley, M., and S. Floyd, "Datagram
                    Congestion Control Protocol (DCCP)", RFC 4340,
                    March 2006.

   [RFC4787]        Audet, F. and C. Jennings, "Network Address
                    Translation (NAT) Behavioral Requirements for
                    Unicast UDP", BCP 127, RFC 4787, January 2007.

   [RFC4941]        Narten, T., Draves, R., and S. Krishnan, "Privacy
                    Extensions for Stateless Address Autoconfiguration
                    in IPv6", RFC 4941, September 2007.

   [RFC4960]        Stewart, R., "Stream Control Transmission Protocol",
                    RFC 4960, September 2007.



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   [RFC4961]        Wing, D., "Symmetric RTP / RTP Control Protocol
                    (RTCP)", BCP 131, RFC 4961, July 2007.

   [RFC5382]        Guha, S., Biswas, K., Ford, B., Sivakumar, S., and
                    P. Srisuresh, "NAT Behavioral Requirements for TCP",
                    BCP 142, RFC 5382, October 2008.

   [RFC6092]        Woodyatt, J., "Recommended Simple Security
                    Capabilities in Customer Premises Equipment (CPE)
                    for Providing Residential IPv6 Internet Service",
                    RFC 6092, January 2011.

   [RFC6145]        Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
                    Algorithm", RFC 6145, April 2011.

   [RFC6146]        Bagnulo, M., Matthews, P., and I. van Beijnum,
                    "Stateful NAT64: Network Address and Protocol
                    Translation from IPv6 Clients to IPv4 Servers",
                    RFC 6146, April 2011.

   [RFC6296]        Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network
                    Prefix Translation", RFC 6296, June 2011.

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

   [RFC6619]        Arkko, J., Eggert, L., and M. Townsley, "Scalable
                    Operation of Address Translators with Per-Interface
                    Bindings", RFC 6619, June 2012.

   [RFC6763]        Cheshire, S. and M. Krochmal, "DNS-Based Service
                    Discovery", RFC 6763, February 2013.

   [RFC6886]        Cheshire, S. and M. Krochmal, "NAT Port Mapping
                    Protocol (NAT-PMP)", RFC 6886, April 2013.

   [RFC6888]        Perreault, S., Ed., Yamagata, I., Miyakawa, S.,
                    Nakagawa, A., and H. Ashida, "Common Requirements
                    for Carrier-Grade NATs (CGNs)", BCP 127, RFC 6888,
                    April 2013.

   [SCTPNAT]        Stewart, R., Tuexen, M., and I. Ruengeler, "Stream
                    Control Transmission Protocol (SCTP) Network Address
                    Translation", Work in Progress, February 2013.






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RFC 6887               Port Control Protocol (PCP)            April 2013


Appendix A.  NAT-PMP Transition

   The Port Control Protocol (PCP) is a successor to the NAT Port
   Mapping Protocol, NAT-PMP [RFC6886], and shares similar semantics,
   concepts, and packet formats.  Because of this, NAT-PMP and PCP both
   use the same port and use NAT-PMP and PCP's version negotiation
   capabilities to determine which version to use.  This section
   describes how an orderly transition from NAT-PMP to PCP may be
   achieved.

   A client supporting both NAT-PMP and PCP SHOULD send its request
   using the PCP packet format.  This will be received by a NAT-PMP
   server or a PCP server.  If received by a NAT-PMP server, the
   response will be UNSUPP_VERSION, as indicated by the NAT-PMP
   specification [RFC6886], which will cause the client to downgrade to
   NAT-PMP and resend its request in NAT-PMP format.  If received by a
   PCP server, the response will be as described by this document and
   processing continues as expected.

   A PCP server supporting both NAT-PMP and PCP can handle requests in
   either format.  The first octet of the packet indicates if it is
   NAT-PMP (first octet zero) or PCP (first octet non-zero).

   A PCP-only gateway receiving a NAT-PMP request (identified by the
   first octet being zero) will interpret the request as a version
   mismatch.  Normal PCP processing will emit a PCP response that is
   compatible with NAT-PMP, without any special handling by the PCP
   server.























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RFC 6887               Port Control Protocol (PCP)            April 2013


Authors' Addresses

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

   EMail: dwing@cisco.com


   Stuart Cheshire
   Apple Inc.
   1 Infinite Loop
   Cupertino, California  95014
   USA

   Phone: +1 408 974 3207
   EMail: cheshire@apple.com


   Mohamed Boucadair
   France Telecom
   Rennes  35000
   France

   EMail: mohamed.boucadair@orange.com


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

   EMail: repenno@cisco.com


   Paul Selkirk
   Internet Systems Consortium
   950 Charter Street
   Redwood City, California  94063
   USA

   EMail: pselkirk@isc.org






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