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Versions: (draft-wing-pcp-base) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 RFC 6887

PCP working group                                           D. Wing, Ed.
Internet-Draft                                                     Cisco
Intended status: Standards Track                             S. Cheshire
Expires: June 21, 2012                                             Apple
                                                            M. Boucadair
                                                          France Telecom
                                                                R. Penno
                                                        Juniper Networks
                                                              P. Selkirk
                                                                     ISC
                                                       December 19, 2011


                      Port Control Protocol (PCP)
                         draft-ietf-pcp-base-19

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 21, 2012.

Copyright Notice

   Copyright (c) 2011 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



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Deployment Scenarios . . . . . . . . . . . . . . . . . . .  6
     2.2.  Supported Protocols  . . . . . . . . . . . . . . . . . . .  6
     2.3.  Single-homed Customer Premises Network . . . . . . . . . .  6
   3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   4.  Relationship between PCP Server and its NAT/firewall . . . . . 11
   5.  Note on Fixed-Size Addresses . . . . . . . . . . . . . . . . . 11
   6.  Common Request and Response Header Format  . . . . . . . . . . 12
     6.1.  Request Header . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  Response Header  . . . . . . . . . . . . . . . . . . . . . 14
     6.3.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . 15
     6.4.  Result Codes . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  General PCP Operation  . . . . . . . . . . . . . . . . . . . . 19
     7.1.  General PCP Client: Generating a Request . . . . . . . . . 20
     7.2.  General PCP Server: Processing a Request . . . . . . . . . 21
     7.3.  General PCP Client: Processing a Response  . . . . . . . . 23
     7.4.  Multi-Interface Issues . . . . . . . . . . . . . . . . . . 24
     7.5.  Epoch  . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   8.  Version Negotiation  . . . . . . . . . . . . . . . . . . . . . 26
   9.  Introduction to MAP and PEER Opcodes . . . . . . . . . . . . . 27
     9.1.  For Operating a Server . . . . . . . . . . . . . . . . . . 29
     9.2.  For Operating a Symmetric Client/Server  . . . . . . . . . 31
     9.3.  For Reducing NAT Keepalive Messages  . . . . . . . . . . . 33
     9.4.  For Restoring Lost Implicit TCP Dynamic Mapping State  . . 34
   10. MAP Opcode . . . . . . . . . . . . . . . . . . . . . . . . . . 35
     10.1. MAP Operation Packet Formats . . . . . . . . . . . . . . . 36
     10.2. Generating a MAP Request . . . . . . . . . . . . . . . . . 38
       10.2.1.  Renewing a Mapping  . . . . . . . . . . . . . . . . . 39
     10.3. Processing a MAP Request . . . . . . . . . . . . . . . . . 39
     10.4. Processing a MAP Response  . . . . . . . . . . . . . . . . 41
     10.5. Mapping Lifetime and Deletion  . . . . . . . . . . . . . . 42
     10.6. Address Change Events  . . . . . . . . . . . . . . . . . . 44
     10.7. Learning the External IP Address Alone . . . . . . . . . . 45
   11. PEER Opcode  . . . . . . . . . . . . . . . . . . . . . . . . . 46
     11.1. PEER Operation Packet Formats  . . . . . . . . . . . . . . 47
     11.2. Generating a PEER Request  . . . . . . . . . . . . . . . . 50
     11.3. Processing a PEER Request  . . . . . . . . . . . . . . . . 50



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     11.4. Processing a PEER Response . . . . . . . . . . . . . . . . 51
   12. Options for MAP and PEER Opcodes . . . . . . . . . . . . . . . 52
     12.1. THIRD_PARTY Option for MAP and PEER Opcodes  . . . . . . . 52
     12.2. PREFER_FAILURE Option for MAP Opcode . . . . . . . . . . . 54
     12.3. FILTER Option for MAP Opcode . . . . . . . . . . . . . . . 56
   13. Rapid Recovery . . . . . . . . . . . . . . . . . . . . . . . . 58
     13.1. ANNOUNCE Opcode  . . . . . . . . . . . . . . . . . . . . . 59
       13.1.1.  ANNOUNCE operation packet format  . . . . . . . . . . 59
       13.1.2.  Generating and Processing an ANNOUNCE request . . . . 59
       13.1.3.  Generating an ANNOUNCE response . . . . . . . . . . . 60
       13.1.4.  Processing a PEER Response  . . . . . . . . . . . . . 61
     13.2. PCP Mapping Update . . . . . . . . . . . . . . . . . . . . 61
   14. Implementation Considerations  . . . . . . . . . . . . . . . . 62
     14.1. Implementing MAP with EDM port-mapping NAT . . . . . . . . 62
     14.2. Lifetime of Explicit and Implicit Dynamic Mappings . . . . 63
     14.3. PCP Failure Scenarios  . . . . . . . . . . . . . . . . . . 63
       14.3.1.  Recreating Mappings . . . . . . . . . . . . . . . . . 64
       14.3.2.  Maintaining Mappings  . . . . . . . . . . . . . . . . 64
       14.3.3.  SCTP  . . . . . . . . . . . . . . . . . . . . . . . . 65
     14.4. Source Address Replicated in PCP Header  . . . . . . . . . 65
     14.5. State Diagram  . . . . . . . . . . . . . . . . . . . . . . 66
   15. Deployment Considerations  . . . . . . . . . . . . . . . . . . 67
     15.1. Ingress Filtering  . . . . . . . . . . . . . . . . . . . . 67
     15.2. Mapping Quota  . . . . . . . . . . . . . . . . . . . . . . 67
   16. Security Considerations  . . . . . . . . . . . . . . . . . . . 68
     16.1. Simple Threat Model  . . . . . . . . . . . . . . . . . . . 68
       16.1.1.  Attacks Considered  . . . . . . . . . . . . . . . . . 69
       16.1.2.  Deployment Examples Supporting the Simple Threat
                Model . . . . . . . . . . . . . . . . . . . . . . . . 69
     16.2. Advanced Threat Model  . . . . . . . . . . . . . . . . . . 70
     16.3. Residual Threats . . . . . . . . . . . . . . . . . . . . . 71
       16.3.1.  Denial of Service . . . . . . . . . . . . . . . . . . 71
       16.3.2.  Ingress Filtering . . . . . . . . . . . . . . . . . . 71
       16.3.3.  Mapping Theft . . . . . . . . . . . . . . . . . . . . 71
       16.3.4.  Attacks Against Server Discovery  . . . . . . . . . . 72
   17. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 72
     17.1. Port Number  . . . . . . . . . . . . . . . . . . . . . . . 72
     17.2. Opcodes  . . . . . . . . . . . . . . . . . . . . . . . . . 72
     17.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 73
     17.4. Options  . . . . . . . . . . . . . . . . . . . . . . . . . 73
   18. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 73
   19. References . . . . . . . . . . . . . . . . . . . . . . . . . . 73
     19.1. Normative References . . . . . . . . . . . . . . . . . . . 73
     19.2. Informative References . . . . . . . . . . . . . . . . . . 74
   Appendix A.  NAT-PMP Transition  . . . . . . . . . . . . . . . . . 76
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 77
     B.1.  Changes from draft-ietf-pcp-base-18 to -19 . . . . . . . . 77
     B.2.  Changes from draft-ietf-pcp-base-17 to -18 . . . . . . . . 78



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     B.3.  Changes from draft-ietf-pcp-base-16 to -17 . . . . . . . . 78
     B.4.  Changes from draft-ietf-pcp-base-15 to -16 . . . . . . . . 78
     B.5.  Changes from draft-ietf-pcp-base-14 to -15 . . . . . . . . 79
     B.6.  Changes from draft-ietf-pcp-base-13 to -14 . . . . . . . . 79
     B.7.  Changes from draft-ietf-pcp-base-12 to -13 . . . . . . . . 80
     B.8.  Changes from draft-ietf-pcp-base-11 to -12 . . . . . . . . 80
     B.9.  Changes from draft-ietf-pcp-base-10 to -11 . . . . . . . . 81
     B.10. Changes from draft-ietf-pcp-base-09 to -10 . . . . . . . . 81
     B.11. Changes from draft-ietf-pcp-base-08 to -09 . . . . . . . . 81
     B.12. Changes from draft-ietf-pcp-base-07 to -08 . . . . . . . . 82
     B.13. Changes from draft-ietf-pcp-base-06 to -07 . . . . . . . . 83
     B.14. Changes from draft-ietf-pcp-base-05 to -06 . . . . . . . . 84
     B.15. Changes from draft-ietf-pcp-base-04 to -05 . . . . . . . . 86
     B.16. Changes from draft-ietf-pcp-base-03 to -04 . . . . . . . . 86
     B.17. Changes from draft-ietf-pcp-base-02 to -03 . . . . . . . . 87
     B.18. Changes from draft-ietf-pcp-base-01 to -02 . . . . . . . . 87
     B.19. Changes from draft-ietf-pcp-base-00 to -01 . . . . . . . . 88
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 88

































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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 NAT64,
   NAT44, 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), small NATs (e.g.,
   residential NATs), as well as with dual-stack and IPv6-only 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 webcam) 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 and port to an internal IP address 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 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 [I-D.cheshire-dnsext-dns-sd] 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 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
   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,



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   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 [I-D.ietf-behave-lsn-requirements]

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

   o  Layer-2 Aware NAT [I-D.miles-behave-l2nat]

   o  Dual-Stack Extra Lite [I-D.arkko-dual-stack-extra-lite]

   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, SCTP, DCCP).  Protocols that do not use a port number (e.g.,
   RSVP, IPsec ESP, ICMP, 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 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)
   arrives at the host, the outbound response (e.g., TCP SYNACK) has to
   go through the same path so it is seen by the firewall or rewritten
   by the NAT.  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



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   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 have the necessary port mapped
   to another host).


3.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in "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 receives the incoming traffic resulting from a
      PCP MAP 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.

   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 and Port to different Remote
      Peers and Ports may be assigned different External Ports, and a
      subsequent PCP MAP request for that Internal Address and Port may
      be assigned yet another different External Port.  This term
      encompasses both Address-Dependent Mapping and Address and Port-
      Dependent Mapping [RFC4787].





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   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 and port are translated to the
      internal IP and port, and vice versa.  In the case of a pure
      firewall, the "Mapping" is the identity function, translating an
      internal IP address and port number to the same external IP
      address and port number.  Firewall filtering, applied in addition
      to that identity mapping function, is separate from the mapping
      itself.

   Mapping Types:
      There are two dimensions to classifying mapping types: how they
      are created (implicitly/explicitly) and their primary purpose
      (outbound/inbound).

      *  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.

      *  Explicit dynamic mappings are created as a result of explicit
         PCP MAP and PEER requests.  Like a DHCP address lease, explicit
         dynamic mappings have finite lifetime, and, as with a DHCP
         address lease, if a client wants a mapping to persist the
         client must prove that it is still present by periodically



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         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 web-based 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 a new PCP request).

      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.

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

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

      *  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
         communcation.  A PCP MAP request can be used explicitly to
         create a dynamic inbound mapping to enable the desired inbound
         communcation.

      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).






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   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 (just as several independent web browsers 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 implements the server side of the PCP
      protocol, via which PCP clients request and manage explicit
      mappings.  See also Section 4.

   Interworking Function:
      A functional element responsible for translating or proxying
      another protocol to PCP.  For example interworking UPnP IGDv1
      [IGDv1] with PCP.

   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 large-scale NAT operated by the ISP.















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4.  Relationship between PCP Server and its NAT/firewall

   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 or firewall 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 IPsec ESP) [RFC6092].  Such default
   filtering (or lack thereof) is out of scope of PCP itself.  If a
   device supports PCP and wants to receive traffic, and does not
   possess knowledge of such filtering, it SHOULD use PCP to create 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.

   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, IPv4-mapped IPv6
   addresses [RFC4291] are 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 IPv4-mapped IPv6



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   address [RFC4291] are not used as either the source or destination
   address of actual IPv6 packets.

   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-zeroes IPv6 address is expressed by filling the fixed-size
   128-bit IP address field with all zeroes (::).

   The all-zeroes IPv4 address is expressed as: 80 bits of zeros, 16
   bits of ones, and 32 bits of zeros (::ffff:0:0).


6.  Common Request and Response Header Format

   All PCP messages contain a request (or response) header containing an
   Opcode, any relevant Opcode-specific information, and zero or more
   Options.  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
   Section 10 and Section 11.




























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6.1.  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 = 1  |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 1.  This value
      MUST be 1 when sending, and MUST be 1 when receiving.  This field
      is used for version negotiation as described in Section 8.

   R: Indicates Request (0) or Response (1).  All Requests MUST use 0.

   Opcode:  A seven-bit value specifying the operation to be performed.
      Opcodes are defined in Section 10 and Section 11.

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

   Requested Lifetime:  An unsigned 32-bit integer, in seconds, ranging
      from 0 to 4,294,967,295 seconds.  This is used by the MAP and PEER
      Opcodes defined in this document for their requested lifetime.
      Future Opcodes which don't need this field MUST set the field to
      zero on transmission and ignore it on reception.





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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.  IPv4
      is represented using an IPv4-mapped IPv6 address.

6.2.  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 = 1  |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 MUST use version 1.  This is set by the server.

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

   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 6.4 for
      values.  This is set by the server.






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   Lifetime:  An unsigned 32-bit integer, in seconds, ranging from 0 to
      4,294,967,295 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 currently-defined PCP Opcodes -- MAP and
      PEER -- this indicates the lifetime for this mapping.  If future
      Opcodes are defined that do not have a lifetime associated with
      them, then in success responses for those Opcodes the Lifetime
      MUST be set to zero on transmission and MUST be ignored on
      reception.  This is set by the server.

   Epoch Time:  The server's Epoch time value.  See Section 7.5 for
      discussion.  This value is set by the server, in both success and
      error responses.

   Reserved:  96 reserved bits, MUST be sent as 0, MUST be ignored when
      received.  This is set by the server.  This padding exists so that
      for unrecognized requests, the server can blindly copy an entire
      request message into a response message and have that response
      make some kind of reasonable sense to the recipient.

6.3.  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-zeroes 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
   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.







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   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 four octets long are followed by one, two,
      or three zero octets to pad their effective length in the packet
      to be a multiple of four octets.  The Option Length reflects the
      semantic length of the option, not including any padding octets.

   data:  Option data.

   The handling of an Option by the PCP client and PCP server MUST be
   specified in an appropriate document, which MUST state whether the
   PCP Option can appear in a request and/or response, whether it can
   appear more than once, and indicate what sort of Option data it
   conveys.  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 the server has sufficient
   room to reply without exceeding the 1024-byte size limit.

   If, while processing an Option, 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 changes it might have made while processing the
   request).  The 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



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   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 this occurs in a response, the PCP client processes
   the first occurrence and MAY log an error.  If the PCP server
   encounters an invalid option (e.g., option length extends beyond the
   length of the PCP Opcode itself), 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.

   The most significant bit in the Option Code indicates if its
   processing is optional or mandatory.  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 process this Option or return an error code if it
   cannot.  If the PCP server does not implement this Option, or cannot
   perform the function indicated by this Option (e.g., due to a parsing
   error with the Option), it MUST generate an error response with code
   UNSUPP_OPTION or MALFORMED_OPTION (as appropriate).

   PCP clients are free to ignore any or all Options included in
   responses, although naturally if a client explicitly requests an
   Option where correct execution of that Option requires processing the
   Option data in the response, that client is expected to implement
   code to do that.

   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).

   Option definitions MUST include 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>





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      Maximum occurrences: <count>

6.4.  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: Unsupported protocol version.

   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.

   4  UNSUPP_OPCODE: Unsupported Opcode.

   5  UNSUPP_OPTION: Unsupported Option.  This error only occurs if the
      Option is in the mandatory-to-process range.

   6  MALFORMED_OPTION: Malformed Option (e.g., appears too many times,
      invalid length).

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

   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 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
      is a short lifetime error.  This can be used as a catch-all error,
      should no other error message be suitable.






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   9  UNSUPP_PROTOCOL: Unsupported Protocol.  This is a long lifetime
      error.

   10 USER_EX_QUOTA: Mapping would exceed user'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 PEER requests, for MAP requests that included the
      PREFER_FAILURE Option (because otherwise a new external port could
      have been assigned), or MAP requests for the SCTP protocol.  See
      Section 12.2 for processing details.  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.

   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 12.3
      for processing information.  This is a long lifetime error.


7.  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.

   PCP is idempotent, meaning that if the PCP client sends the same
   request multiple times (or the PCP client sends the request once and
   it is duplicated by the network), and the PCP server processes those
   requests multiple times, the result is the same as if the PCP server
   had processed only one of those duplicate requests.  Note this
   doesn't mean that all responses are the same if there was a state
   change in the PCP-controlled device.  For example, if a request is
   received while the PCP-controlled device has no mappings available,
   it will generate an error response.  If mappings become available and
   then a (duplicated or re-transmitted) request is seen by the server,
   it will generate a non-error response.  Of course, this sort of
   behavior is common to all UDP-based protocols and is not unique to
   PCP.  A PCP client will need to properly handle such updated
   responses for any request it sends, most notably to support Mapping
   Update (Section 13.2).







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7.1.  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 10, and procedures specific to the PEER Opcode are described
   in Section 11.

   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).

   For the purposes of this document, only a single PCP server address
   is supported.  Should future specifications define configuration
   methods that provide a 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 or TCP 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].

   To assist with detecting an on-path NAT, 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 14.4 for how this can be
   coded.

   When attempting to contact a PCP server, the PCP client sends a PCP
   message to the first server in its list of PCP servers, and
   initializes a timer to 2 seconds.

   For as long as the client still desires the indicated mapping, if no
   response is received before the timer expires then the request is re-
   transmitted and the timer is doubled (to 4 seconds).  This process
   repeats (until either a successful response is received or the client
   no longer desires the indicated mapping) with the timer being doubled
   each time, up to a maximum retransmission interval of fifteen
   minutes, at which point the retransmission continue with a fifteen



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   minute interval (until a successful response is received or the
   client no longer desires the indicated mapping).

   Once a PCP client has successfully received a response from a PCP
   server on that interface, it sends subsequent PCP requests to that
   same server, with a retransmission timer of 2 seconds.  If, after 2
   seconds, a response is not received from that PCP server, the same
   back-off algorithm described above is performed.

7.2.  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 it would normally receive packets
   from that client, and 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 which
   supports THIRD_PARTY requests MAY be configured to accept THIRD_PARTY
   requests on other interfaces from properly authorized clients.

   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 which is sent back to the PCP
   client.  Processing an Opcode and the Options are 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
   (including the ANNOUNCE Opcode), 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.



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   Any error response has a nonzero Result Code, and is created by:
   o  Copying the entire request packet, or 1024 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
   o  Possibly updating other fields in the response if appropriate

   A success response has a zero Result Code, and is created by:
   o  Copying the first four 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 two 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
   other steps detailed in Section 8 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 1024 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.

   The PCP server compares the source IP address (from the received IP
   header) with the field PCP Client IP Adddress.  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 the PCP field matches its apparent IP
   address from the perspective of the PCP server.






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7.3.  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.

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

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

   If the error code is UNSUPP_VERSION processing continues as described
   in Section 8.

   The PCP client then validates that the Opcode matches a previous PCP
   request.  Responses shorter than 24 octets, longer than 1024 octets,
   or not a multiple of 4 octets are invalid and ignored, likely causing
   the request to be re-transmitted.  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.  After these matches are successful, the PCP client
   checks the Epoch Time field to determine if it needs to restore its
   state to the PCP server (see Section 7.5).  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.

   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.

   If the result code is 0 (SUCCESS), the PCP client knows the request
   was successful.

   If the result code is not 0, the request failed.  If the result code
   is NO_RESOURCES, PCP client SHOULD NOT send *any* further requests of
   any kind to that PCP server for the indicated error lifetime.  For
   other error result codes, the PCP client SHOULD NOT resend the same
   request for the indicated error lifetime.  If the PCP server
   indicates an error lifetime in excess of 30 minutes, the PCP client
   MAY choose to set its retry timer to 30 minutes.

   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



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   with a previous PCP server.

7.4.  Multi-Interface Issues

   Hosts which 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
   for Global Unicast Address (GUA, [RFC3587]) or limited scope as for
   Unique Local Address (ULA) [RFC4193]).

   IPv6 addresses with global reachability (e.g., GUA) SHOULD be used as
   the source address when generating a PCP request.  IPv6 addresses
   without global reachability (e.g., ULA [RFC4193]), 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 mappings to the previous
   privacy address be deleted.

   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.

   As mentioned in Section 2.3, only single-homed CP routers are in
   scope.  Therefore, there is no viable scenario where a host located
   behind a CP router is assigned two Global Unicast Addresses belonging
   to different global IPv6 prefixes.

7.5.  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.
   Similarly, if the public IP address(es) of the NAT (controlled by the
   PCP server) changes, the Epoch time MUST be reset.  A PCP server MAY



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   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 value
         (current_server_time) is less than the previously received PCP
         server Epoch time value (previous_server_time) then the client
         treats the Epoch time value as obviously invalid (time should
         not go backwards), else

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

         +  The client computes the difference between the
            current PCP server Epoch time value (current_server_time)
            and the
            previously received Epoch time value (previous_server_time):
            server_delta = current_server_time - previous_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:
      previous_client_time = current_client_time
      previous_server_time = current_server_time

   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 as
   described in Section 14.3.1.







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   Notes:

   o  The client clock MUST never go backwards.  If current_client_time
      is found to be less than previous_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 accomodate 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 accomodate 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.


8.  Version Negotiation

   A PCP client sends its requests using PCP version number 1.  Should
   later updates to this document specify different message formats with
   a version number greater than 1 it is expected that PCP servers will
   still support version 1 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 1, and backwards
   compatibility is desired, this first octet can be used for forward
   and backward compatibility.

   If future PCP versions greater than 1 are specified, version
   negotiation proceeds as follows:

   1.  If a client or server supports more than one version it SHOULD
       support a contiguous range of versions -- i.e., a lowest version
       and a highest version and all versions in between.

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



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   3.  If the server supports that version it responds normally.

   4.  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).

   5.  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).

   6.  If the client receives an UNSUPP_VERSION result containing a
       version it does not support then 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.


9.  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 9.1);

   o  a host operating a client and server on the same port
      (Section 9.2);

   o  a host operating a client and wanting to optimize the application
      keepalive traffic (Section 9.3);

   o  and a host operating a client and wanting to restore lost state in
      its NAT (Section 9.4).

   These are discussed in the following sections.

   When operating a server (Section 9.1 and Section 9.2) 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



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   request (e.g., a PCP server on its IPv4 interface or its IPv6
   interface), and if 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 interface (if the PCP server supports
   NAT44 or IPv4 firewall) or one MAP request from its IPv6 interface
   (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.

   It is REQUIRED that the PCP-controlled device assign the same
   external IP address to PCP-created explicit dynamic mappings and to
   implicit dynamic mappings for a given Internal Address.  In the
   absence of a PCP option indicating otherwise, it is REQUIRED that all
   PCP-created explicit dynamic mappings be assigned the same external
   IP address.  It is RECOMMENDED that static mappings for that Internal
   Address (e.g., those created by a command-line interface on the PCP
   server or PCP-controlled device) also be assigned to the same IP
   address.  Once an Internal Address has no implicit dynamic mappings
   and no explicit dynamic mappings in the PCP-controlled device, a
   subsequent PCP request for that Internal Address MAY be assigned to a
   different External Address.  Generally, this re-assignment would
   occur when a CGN device is load balancing newly-seen hosts to its
   public IPv4 address pool.





















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   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 is 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 the table.

                                  internal  external  remote peer
                                  --------  -------   -----------
                    IPv4 firewall   IPv4      IPv4      IPv4
                    IPv6 firewall   IPv6      IPv6      IPv6
                            NAT44   IPv4      IPv4      IPv4
                            NAT64   IPv6      IPv4      IPv6
                            NPTv6   IPv6      IPv6      IPv6

               Figure 5: Address Families with MAP and PEER

   PCP MAP and PEER requests cannot delete or shorten the lifetime of an
   existing implicit outbound mapping for the indicated internal address
   and port.  PCP MAP requests can delete or shorten the lifetime of an
   existing explicit dynamic inbound mapping, but cannot affect the
   lifetime of an existing explicit static inbound mapping.

9.1.  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 and port to itself as described in
   Section 10 and (b) publish that public IP address and port via some
   sort of rendezvous server (e.g., DNS, a SIP message, a proprietary
   protocol).  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-zeroes address,
   depending on whether it wants a public IPv4 or IPv6 address.





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   The following pseudo-code 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 cases, apart from the
         * Suggested External Address and Port which may differ between
         * (1), (2), and (3).
         */
        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: Pseudo-code for using PCP to operate a server








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9.2.  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 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 10, 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 the host has connected
      to a new network, the PCP client can attempt to request a mapping
      using PCP, and 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 which 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 TCP SYNs sent from the specified Internal Address
      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.  See also
      Section 14.1.









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   The following pseudo-code 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
         * The PCP packet sent is identical in all cases, apart from the
         * Suggested External Address and Port which may differ between
         * (1), (2), and (3).
         */
        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();
        }




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    Figure 7: Pseudo-code for using PCP to operate a symmetric client/
                                  server

9.3.  For Reducing NAT 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
   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 need to see occasional traffic
   or 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 11.






















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

    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
         * The PCP packet sent is identical in all cases, apart from the
         * Suggested External Address and Port which may differ between
         * (1), (2), and (3).
         */
        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 (other_work_to_do())
            do_it();

        /* ... */

        block_until_we_need_to_do_something_else();
        }

           Figure 8: Pseudo-code using PCP with a dynamic socket

9.4.  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 (e.g.,
   instant messaging) or for connections transferring a lot of data



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   (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 the NAT creates
      a new implicit dynamic outbound mapping only for TCP segments with
      the SYN bit set (i.e., the newly-booted NAT drops the re-
      transmitted data segments from the client because the NAT does not
      have an active mapping for those segments), and if the server is
      not sending data that elicits a RST from the NAT.  This is not the
      case for UDP, because a new UDP mapping will be created (probably
      on a different port) as soon as UDP traffic is seen by the NAT.


10.  MAP Opcode

   This section defines an Opcode which controls 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 (EIM) with Endpoint Independent Filtering (EIF)
   (unless the FILTER Option is used), even on a NAT that usually
   creates Endpoint Dependent Mappings (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
   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 source 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 to the



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   mapping's Internal Address and Port.

   On Operating Systems that allow multiple listening clients to bind to
   the same Internal Port, clients MUST ensure that they have exclusive
   use of that Internal Port (e.g., by binding the port using
   INADDR_ANY, or using SO_EXCLUSIVEADDRUSE or similar) before sending
   their MAP request, to ensure that no other clients on the same
   machine are also listening on the same 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.

10.1.  MAP Operation Packet Formats

   The MAP Opcode has a similar packet layout for both requests and
   responses.  If the Assigned External IP address and Assigned External
   Port in the PCP response always match the Internal IP Address and
   Port in the PCP request, then the functionality is purely a firewall;
   otherwise it pertains to a network address translator which might
   also perform firewall-like functions.

   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Suggested External Port    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |           Suggested External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 9: MAP Opcode Request Packet Format

   These fields are described below:





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

   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.  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
      Protocol is 0 (meaning 'all protocols')

   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-zeroes 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Protocol    |          Reserved (24 bits)                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |        Internal Port          |    Assigned External Port     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |            Assigned External IP Address (128 bits)            |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: MAP Opcode Response Packet Format



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   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.  The PCP client SHOULD impose an upper limit on this
      returned Assigned Lifetime value, and 24 hours is RECOMMENDED.
      This means if the PCP server returns an absurdly long Assigned
      Lifetime (e.g., 5 years), the PCP client will behave as if it
      received a more sane value (e.g., 24 hours).

   Protocol:  Copied from the request.

   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.

10.2.  Generating a MAP Request

   This section and Section 10.5 describe 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
   grant the suggested External IP Address and Port, and in that case it
   will assign a different External IP Address and Port.

   If the Protocol does not use 16-bit port numbers (e.g., RSVP), the
   port number MUST be 0.  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).

10.2.1.  Renewing a Mapping

   An existing mapping can have its lifetime extended by the PCP client.
   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 as the suggested external IP
   address and port, so that if the NAT gateway 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 10.5).
   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).

10.3.  Processing a MAP Request

   This section and Section 10.5 describe 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 and Internal Port fields from the MAP request are copied
   into the MAP response.  If present and processed by the PCP server
   the THIRD_PARTY Option is also copied into the MAP response.

   If the Requested Lifetime is 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, it MUST generate an UNSUPP_PROTOCOL error.  If the
   requested Lifetime is non-zero, the Internal Port is zero, and the
   Protocol is non-zero, it indicates a request to map all incoming
   traffic for that entire Protocol.  If this request cannot be
   fulfilled in its entirety, the error NO_RESOURCES MUST be returned.
   If the requested Lifetime is non-zero, the Internal Port is zero, and
   the Protocol is zero, it indicates a request to map all incoming
   traffic for all protocols.  If this request cannot be fulfilled in
   its entirety, the error NO_RESOURCES MUST be returned.  If the



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   Protocol is 0 but the Internal Port is non-zero, the error
   MALFORMED_REQUEST MUST be returned.

   If the requested lifetime is zero, it indicates a request to delete
   an existing mapping or set of mappings.  Processing of the lifetime
   is described in Section 10.5.

   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), 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.

   It is possible that a mapping might already exist for a requested
   Internal Address and Port, for example because of a previous MAP
   request or an implicit dynamic mapping or other reasons.  If a
   mapping already exists for the requested Internal Address and Port
   and the PREFER_FAILURE Option is not present, the PCP server MUST
   refresh the lifetime of that already-existing mapping, and return the
   already-existing External Address and Port in its response,
   regardless of the Suggested External Address and Port in the request.
   If a mapping already exists for the requested Internal Address and
   Port and the request contains the PREFER_FAILURE Option, but the
   Suggested External Address and Port do not match the actual External
   Address and Port of the already existing mapping, the error
   CANNOT_PROVIDE_EXTERNAL is returned.  If an implicit outbound mapping
   already exists for the requested Internal Address and Port, a new
   explicit mapping should be made replicating the ports and addresses
   from the implicit outbound mapping (but the implicit outbound mapping
   continues to exist, and remains in effect if the explicit mapping is
   later deleted).

   If no mapping exists for the Internal Address and Port, and the PCP
   server is able to create a mapping using the Suggested 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).
   If the PCP server cannot assign the Suggested External Address and
   Port but can assign some other External Address and Port (and the
   request did not contain the PREFER_FAILURE Option) the PCP server



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   MUST do so and return the newly assigned External Address and Port in
   the response.  Cases where a PCP server or PCP-controlled device
   cannot assign the Suggested External Address and Port include:

   o  The Suggested External Address and Port is already assigned to
      another existing explicit, implicit, or static mapping (i.e., is
      already forwarding traffic to some other internal address and
      port).

   o  The Suggested External Address and Port is already used by the NAT
      gateway for one of its own services (e.g., port 80 for the NAT
      gateway's own configuration pages).

   o  The Suggested External Address and Port is otherwise prohibited by
      the PCP server's policy.

   o  The Suggested External Address or port is invalid (e.g.,
      127.0.0.1, ::1, multicast address, or the port is not valid for
      the indicated 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.

   o  The PREFER_FAILURE option is included in the request and the
      Suggested External Address and Port are not assignable to the PCP
      client, which returns the CANNOT_PROVIDE_EXTERNAL error.

   By default, 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, a UDP mapping MUST NOT be 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.

10.4.  Processing a MAP Response

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




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   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 and the Internal Port.  Other fields are not compared,
   because the PCP server sets those fields.  Note that if the PCP
   server supports Mapping Update (Section 13.2) the PCP server will
   send additional MAP responses if the mapping changes (e.g., due to IP
   renumbering).

   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.

   As long as renewal is desired, the PCP client MUST also 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 10.2, 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 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 happens to be what the PCP
   server previously assigned.  See also Section 14.3.1.

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

10.5.  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 RECOMMENDED values are 120 seconds for the minimum
   value and 24 hours for the maximum.  It is RECOMMENDED that the
   server be configurable to restrict lifetimes to less than 24 hours,
   because mappings will consume 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



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   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.

   If a PCP client sends a PCP MAP request to create a mapping that
   already exists as a static mapping, the PCP server will return a
   successful result, confirming that the requested mapping exists.  The
   lifetime the PCP server returns for such a static mapping SHOULD be
   4294967295 (0xFFFFFFFF).  Upon receipt of such a MAP 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.

   If the requested lifetime is zero then:

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

   o  If both the internal port and protocol are zero, it indicates a
      request to delete all mappings for this Internal Address for all
      transport protocols.  This is useful when a host reboots or joins
      a new network, to clear out prior stale state from the NAT gateway
      before beginning to install new mappings.

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

   o  If the internal port is non-zero and the protocol is 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.

   If the PCP client attempts to delete a single static mapping (i.e., a
   mapping created outside of PCP itself), the error NOT_AUTHORIZED is
   returned.  If the PCP client attempts to delete a mapping that does



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   not exist, the SUCCESS result code is returned (this is necessary for
   PCP to be idempotent).  If the PCP MAP request was for port=0
   (indicating 'all ports'), the PCP server deletes all of the explicit
   dynamic mappings it can (but not any implicit or static mappings),
   and returns a SUCCESS response.  If the deletion request was properly
   formatted and successfully processed, a SUCCESS response is generated
   with lifetime of 0 and the server copies the protocol and internal
   port number from the request into the response.  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).

   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 implicit dynamic outbound mappings without using PCP,
   which will also consume port mappings.  If there is a port mapping
   quota for the Internal Host, frequent restarts such as this may
   exhaust the quota.  PCP provides some protections against such port
   consumption: When a PCP client first acquires a new IP address (e.g.,
   reboots or joins a new network), it SHOULD remove mappings that may
   already be instantiated for that new Internal Address.  To do this,
   the PCP client sends a MAP request with protocol, internal port, and
   lifetime set to 0.  Some port mapping APIs (e.g., the
   "DNSServiceNATPortMappingCreate" API provided by Apple's Bonjour on
   Mac OS X, iOS, Windows, Linux [Bonjour]) automatically monitor for
   process exit (including application crashes) and automatically send
   port mapping deletion requests if the process that requested them
   goes away without explicitly relinquishing them.

   To reduce unwanted traffic and data corruption, External UDP and TCP
   ports SHOULD NOT be re-used for an interval (TIME_WAIT interval
   [RFC0793]).  However, the PCP server SHOULD allow the previous user
   of an External Port to re-acquire the same port during that interval.

10.6.  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 the host's previous address might be delivered to
   another host which now has that address.  This affects both implicit
   dynamic mappings and explicit dynamic mappings.  However, this same
   problem already occurs today when a host's IP address is re-assigned,
   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



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   avoided.  PCP defined in this document does not provide machinery to
   reduce the host renumbering problem.

   When an Internal Host changes its 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 public 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 those
   PCP requests continue to use the same interface (e.g., when
   refreshing mappings).  If the PCP client is sending a PCP request to
   create new state in the PCP server, it MAY use a different source
   interface or different source address.

10.7.  Learning the External IP Address Alone

   NAT-PMP [I-D.cheshire-nat-pmp] includes a mechanism to allow clients
   to learn the External IP Address alone, without also requesting a
   port mapping.  In the case of PCP, this operation no longer makes
   sense.  PCP supports Large Scale NATs (CGN) which 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.







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11.  PEER Opcode

   This section defines an Opcode for controlling dynamic 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 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 Mappings (EIM) or
   Endpoint Dependent Mappings (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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11.1.  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 request packet format for the PEER
   Opcode.  This packet format is aligned with the response packet
   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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   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 Packet Format

   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.

   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.







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

   Internal Port:  Internal port for the mapping.

   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-zeroes address (see Section 5).

   Remote Peer Port:  Remote peer's port for the mapping.

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







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   The following diagram shows the response packet format 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   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 Packet Format

   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.

   Protocol:  Copied from the request.

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

   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; IPv4 or
      IPv6 address is indicated by the Opcode.  On an error response,
      the Suggested External IP Address is copied from the request.





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   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.

11.2.  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 bi-
   directional communication with the remote peer.

   If sent before, this is considered a PEER-created mapping which
   creates a new dynamic outbound mapping in the PCP-controlled device,
   which will be used for translating traffic to and from the remote
   peer; this mapping functions the same as an implicit dynamic outbound
   mapping (e.g., from an outbound TCP SYN from the client).  This is
   useful for restoring a mapping after a NAT has lost its mapping state
   (e.g., due to a crash).

   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 the purpose described in Section 9.3).

   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.

11.3.  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, and Remote
   Peer Port.

   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.,



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   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 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.  If the requested mapping does not yet exist, and the
   Suggested External Address and Port can be honored, the mapping is
   created.  By having PEER create such a mapping, we avoid a race
   condition between the PEER request or the initial outgoing packet
   arriving at the NAT or firewall device first, and allow PEER to be
   used to recreate an outbound dynamic mapping (see last paragraph of
   Section 14.3.1).  If the requested mapping does not yet exist, and
   Suggested External Address and Port cannot be honored, the error
   CANNOT_PROVIDE_EXTERNAL is returned.  If the requested mapping
   already exists, it is a request to modify the lifetime of that
   existing mapping.

   The PEER Opcode can extend the lifetime of an existing dynamic
   outbound mapping.  The PCP server may grant the client's requested
   lifetime, or may grant a value higher or lower, depending on local
   policy.  The PEER Opcode MUST NOT ever reduce the lifetime of an
   existing outbound mapping.  If the Requested Lifetime is less than
   the lifetime of the existing mapping, it is treated as a request for
   the lifetime of the mapping (this can be used to query the lifetime
   of an existing mapping).

   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,
   e.g., 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.

11.4.  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



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   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, the Remote Peer Address and the Remote
   Peer Port.  Other fields are not compared, because the PCP server
   sets those fields to provide information about the mapping created by
   the Opcode.  Note that if the PCP server supports Mapping Update
   (Section 13.2) the PCP server will send additional PEER responses if
   the mapping changes (e.g., due to IP renumbering).

   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 issue a new PCP request prior to the
   expiration, or it MAY rely on continued inside-to-outside traffic to
   ensure the mapping will continue to exist.  See Section 10.2.1 for
   recommended renewal timing.

      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.


12.  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.

12.1.  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.









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   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 or 0       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   |                Internal IP Address (128 bits)                 |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 13: THIRD_PARTY Option packet format

   The fields are described below:

   Option Length:  The only valid option lengths are 0 and 16.  The
      length 0 has special meaning (see 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: 0 or 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.  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.  This is because many PCP
   servers may not implement the THIRD_PARTY Option at all, and a client
   using the THIRD_PARTY Option to specify the same address as the
   source address of the packet will cause mapping requests to fail
   where they would otherwise have succeeded.

   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.




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   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.

   Determining which PCP clients are authorized to use the THIRD_PARTY
   Option for which other hosts is deployment-dependent.  For example,
   an ISP using Dual-Stack Lite could choose to allow a client
   connecting over a given IPv6 tunnel to manage mappings for any other
   host connecting over the same IPv6 tunnel, or the ISP could choose to
   allow only the DS-Lite B4 element to manage mappings for other hosts
   connecting over the same IPv6 tunnel.  A cryptographic authentication
   and authorization model is outside the scope of this specification.
   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 to be a single host from the point of
   view of the ISP.

   Where possible, it may beneficial if a client using the THIRD_PARTY
   Option to create and maintain mappings on behalf of some other device
   can take steps to verify that the other device is still present and
   active on the network.  Otherwise the client using the THIRD_PARTY
   Option to maintain mappings on behalf of some other device 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.

   A PCP client can delete all PCP-created explicit dynamic inbound
   mappings (i.e., those created by PCP MAP requests) that it is
   authorized to delete by sending a PCP MAP request including a zero-
   length THIRD_PARTY Option, zero in the Internal Port field, and zero
   in the Protocol field.

12.2.  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



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   server should not create a mapping.  This differs from the behavior
   without this Option, which is to create a mapping.

   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 packet format

      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 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 and Port already has an outbound dynamic
   mapping which 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, rounded up to the next larger integer number of seconds.

   This Option exists solely for use by UPnP IGDv1 interworking
   [I-D.bpw-pcp-upnp-igd-interworking], where the semantics of UPnP
   IGDv1 only allow the UPnP IGDv1 client to dictate mapping a specific
   port.  A PCP server MAY support this Option, if its designers wish to
   support downstream devices that perform UPnP IGDv1 interworking.  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.  PCP servers that are not
   intended to support downstream devices that perform UPnP IGDv1
   interworking are not required to support this Option.  PCP clients
   other than UPnP IGDv1 interworking clients SHOULD NOT use this Option
   because it results in inefficient operation, and they cannot safely



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   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 UPnP IGDv1 interworking declines.

   There can exist a race condition between the MAP Opcode using the
   PREFER_FAILURE option and Mapping Update (Section 13.2).  Because of
   this, the PCP client MUST validate that the External IP Address and
   Port in a success response matches the associated sugggested values
   from the request.  If they don't match, it is because the Mapping
   Update was sent before the MAP request was processed.  If the PCP
   server has no use for this new mapping, it SHOULD delete the mapping.

12.3.  FILTER Option for MAP Opcode

   This Option is only used with the MAP Opcode.

   This Option indicates that filtering incoming packets is desired.
   The Remote Peer Port and Remote Peer IP Address indicate the
   permitted remote peer's source IP address and port for packets from
   the Internet.  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 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:




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   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

   The Prefix Length indicates how many bits of the IPv6 address or IPv4
   address are used for the filter.  For IPv4 addresses, which are
   represented using the IPv4-mapped address format (::FFFF:0:0/96), the
   value of the Prefix Length pertains only to to the IPv4 portion of
   the address.  Thus, a Prefix Length of 32 with an IPv4-mapped address
   indicates "only this address".  With IPv4-mapped addresses, the
   minimum Prefix length value is 0 and the maximum is 32; for IPv6
   addresses the minimum value is 0 and the maximum is 128.  Values
   outside those range 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
   an existing mapping exists (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.




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   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 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.

   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
   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 a unwanted traffic.


13.  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
   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



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   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.

13.1.  ANNOUNCE Opcode

   This rapid recovery mechanism is used when the PCP server loses its
   state (e.g., it lost its state when rebooted), and there is a
   multicast network or multicast link between the PCP server and its
   PCP clients.  Additionally, the Opcode defined in this section can be
   sent in a request by a PCP client.

13.1.1.  ANNOUNCE operation packet format

   The PCP ANNOUNCE Opcode has no Opcode-specific payload (that is, the
   length of the Opcode-specific response data is zero), so a packet
   layout is not shown.

   On responses, the ANNOUNCE Opcode MUST have Result Code equal to zero
   (SUCCESS).  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: A separate port is used for server-to-client Restart
      Announcements (5350) than client-to-server requests (5351) because
      a single device may have multiple roles.  For example, multi-
      function home gateway may also provide printer sharing, or a home
      computer may also provide "Internet Sharing" (NAT) functionality.
      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 is 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.

13.1.2.  Generating and Processing an ANNOUNCE request

   The PCP ANNOUNCE Opcode MAY be sent (unicast) by a PCP client.  When
   received by the PCP server, it generates a SUCCESS response.

   This functionality can be useful for a PCP client wishing to
   determine a server's Epoch, or to determine if a PCP server is



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   running without changing the server's state.

13.1.3.  Generating an ANNOUNCE response

   Generating a PCP restart announcement is an optional feature of PCP.

   PCP Servers that accept PCP requests from multicast-reachable clients
   (e.g. from clients directly attached via Ethernet or Wi-Fi) SHOULD
   send the PCP ANNOUNCE Opcode response packet described below to
   signal restarts.  PCP Clients that send PCP requests to a PCP Server
   via a multicast-capable path (e.g. to a PCP server attached to the
   same Ethernet or Wi-Fi link) SHOULD listen for and process these
   Restart Announcements sent by PCP Servers.

   When a PCP server device that implements Restart Announcement
   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
   multicasting a gratuitous PCP ANNOUNCE Opcode response packet, as
   shown below, via all multicast-capable paths via which it accepts PCP
   requests.  A PCP server device which 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).
   A PCP server device which 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
   which accepts PCP requests over both IPv4 and IPv6 sends a pair of
   Restart Announcements, one to each multicast address.  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 250ms, 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
   packets) on the appropriate multicast-capable interfaces on which it
   sends PCP requests.  A PCP client device which sends PCP requests
   using IPv4 listens for packets sent to the IPv4 multicast address
   224.0.0.1:5350.  A PCP client device which sends PCP requests using
   IPv6 listens for packets sent to the IPv6 multicast address [FF02::
   1]:5350.  A PCP client device which 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



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   multicast UDP port, if required by the host OS to permit multiple
   independent listeners on the same multicast UDP port.)

13.1.4.  Processing a PEER Response

   Upon receiving a 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, then for all PCP mappings
   it made at that server address the client should issue new PCP
   requests to to recreate any lost mapping state.  The use of the
   Suggested External IP Address and Suggested External Port fields in
   the client's subsequent 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.

13.2.  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 public IP address of a PCP-controlled NAT).

   Implementation of the feature described in this section is optional
   for PCP servers.  It is anticipated that servers which 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 which are not routinely
   reconfigured will not implement this feature (e.g., service provider-
   operated CGN).

   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 MAY chose to remedy this situation by automatically
   repairing its mappings and notifying its clients by following the
   procedure described below.

   For MAP-created and PEER-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 MAP
   responses are identical to the MAP or PEER responses normally



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   returned in response to client MAP or PEER requests containing newly
   updated External IP Address and External Port values.

      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, that design is short-cutted by the
      server simply sending the message it would have sent in (3).

   To accommodate packet loss, the PCP server device MAY transmit such
   packets 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 250ms,
   and the interval between subsequent notification at least doubles.

   Upon receipt of such MAP or PEER response, a PCP client uses the
   information in the response to adjust rendezvous servers or re-
   connect to servers, respectively.  For MAP, this would means updating
   the DNS entries or other address and port information recorded with
   some kind of application-specific rendezvous server.  For PEER
   responses indicating the external port or address changed, this would
   typically mean re-establishing connections to servers.  Any time 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 lesson the impact.


14.  Implementation Considerations

14.1.  Implementing MAP with EDM port-mapping NAT

   This section provides non-normative guidance that may be useful to
   implementers.

   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 which
   have EIM behavior do not suffer from the problem described in this
   section.  The IETF strongly encourages EIM behavior
   [RFC4787][RFC5382].

   In such 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:





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   1.  Have outbound mappings use a different set of public 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.

14.2.  Lifetime of Explicit and Implicit Dynamic Mappings

   This section provides non-normative guidance that may be useful to
   implementers.

   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 a 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.

14.3.  PCP Failure Scenarios

   This section provides non-normative guidance that may be useful to
   implementers.

   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 which does not write transient information to
   non-volatile memory.  Loss of state is expected to be rare in a
   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 in
   [I-D.boucadair-pcp-failure].





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14.3.1.  Recreating Mappings

   This section provides non-normative guidance that may be useful to
   implementers.

   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, but 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 7.5), 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.

14.3.2.  Maintaining Mappings

   This section provides non-normative guidance that may be useful to
   implementers.

   A PCP client refreshes a mapping by sending a new PCP request
   containing information 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 public IP
   address and/or public port, or the PCP server itself, has changed
   (due to a new route to a different PCP server).  Such events are 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, the PCP client may find it beneficial to
   use shorter lifetimes (so that it communicates with the PCP server
   more often).

   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



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   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 are different to the previous values (see Section 9.1 and
   Section 10.6).

14.3.3.  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 [I-D.ietf-behave-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 rewrite SCTP port numbers, a PCP
   MAP or PEER request for an SCTP mapping SHOULD provide the same
   Internal Port and Suggested External Port.  If the PCP server
   supports SCTP, and the suggested external port cannot be provided in
   an explicit dynamic SCTP mapping, then the error
   CANNOT_PROVIDE_EXTERNAL is returned.  This places an extra burden on
   the SCTP client because it then has to tear down its listening socket
   and try again with a different Internal Port, repeatedly until it is
   successful in finding a 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).

14.4.  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 address rewriting (NAT)
   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 and port to include
   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.




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   4.  If the IP address is an IPv4 address, encode the address into an
       IPv4-mapped IPv6 address.  Place the native IPv6 address or IPv4-
       mapped IPv6 address into the PCP Client's IP Address field in the
       PCP header.
   5.  Send PCP requests using this connected UDP socket.

14.5.  State Diagram

   Each mapping entry of the PCP-controlled device would go through the
   state machine shown below.

       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:





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

   M-R:  MAP request

   P-R:  PEER request

   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.

   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.


15.  Deployment Considerations

15.1.  Ingress Filtering

   As with implicit dynamic mappings created by outgoing TCP 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.

15.2.  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



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   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.


16.  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
   travelled by the mapped traffic.

16.1.  Simple Threat Model

   PCP is secure against off-path attackers who cannot spoof a packet
   that the PCP Server will view as a packet received from the internal
   network.

   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.

   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 Opcode can be secure under this threat model if 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), explicit mappings are created
   with the same lifetime as implicit mappings, the PCP server does not
   support deleting or reducing the lifetime of existing mappings, and
   the PCP server does not support the third party option.  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



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   implement a PCP security mechanism described in Section 16.2 MUST
   enforce the constraints described in the paragraph above.

16.1.1.  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.

16.1.2.  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.

16.1.2.1.  Residential Gateway Deployment

   Parity with many currently-deployed residential gateways can be
   achieved using a PCP Server that is constrained as described in
   Section 16.1.1 above.




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16.1.2.2.  DS-Lite Deployment

   A DS-Lite deployment could be secure under the Simple Threat Model,
   even if the B4 device makes PCP mapping requests on behalf of
   internal clients using the THIRD_PARTY option.  In this case the DS-
   Lite PCP server MUST be configured to only allow the B4 device to
   make THIRD_PARTY requests, and only on behalf of other Internal Hosts
   sharing the same DS-Lite IPv6 tunnel.  The B4 device MUST guard
   against spoofed packets being injected into the IPv6 tunnel using the
   B4 device's IPv4 source address, so the DS-Lite PCP Server can trust
   that packets received over the DS-Lite IPv6 tunnel with the B4
   device's source IPv4 address do in fact originate from the B4 device.
   The B4 device is in a position to enforce this requirement, because
   it is the DS-Lite IPv6 tunnel endpoint.

   Allowing the B4 device to use the THIRD_PARTY Option to create
   mappings for hosts reached via the IPv6 tunnel terminated by the B4
   device is acceptable, because the B4 device is capable of creating
   these mappings implicitly and can prevent others from spoofing these
   mappings.

16.2.  Advanced Threat Model

   In the Advanced Threat Model the PCP protocol must be ensure 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 serve
      multiple administrative domains and do 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 support the THIRD_PARTY Option (unless they
      can meet the constraints outlined in Section 16.1.2.2).

   o  Implementations that wish to support any deployment scenario that
      does not meet the constraints described in Section 16.1.




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

   PCP Servers that implement a PCP security mechanism MAY accept
   unauthenticated requests.  PCP Servers implementing the PCP security
   mechanism MUST enforce the constraints described in Section 16.1
   above, in their default configuration, when processing
   unauthenticated requests.

16.3.  Residual Threats

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

16.3.1.  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
   an inordinate number of ports, causing a denial of service to other
   hosts.  Thus, Section 15.2 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 effective deny service.  Through such actions, the PCP
   client might not be aware the PCP server might have actually
   processed the PCP request.

16.3.2.  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.

16.3.3.  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



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   incoming traffic will be sent to a different host ("theft").  Rapid
   Recovery reduces this interval, but would 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 the previous external
   IP address and port cannot be used by another host (e.g., by using a
   different IP address pool).

16.3.4.  Attacks Against Server Discovery

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


17.  IANA Considerations

   IANA is requested to perform the following actions:

17.1.  Port Number

   PCP will use ports 5350 and 5351 (currently assigned by IANA to NAT-
   PMP [I-D.cheshire-nat-pmp]).  We request that IANA re-assign those
   ports to PCP, and relinquish UDP port 44323.

   [Note to RFC Editor: Please remove the text about relinquishing port
   44323 prior to publication.]

17.2.  Opcodes

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

                 value            Opcode
                 -----            -------------------------
                 0                ANNOUNCE
                 1                MAP
                 2                PEER
                 3-95             (specification required)
                 96-126           (private use)
                 127              Reserved

   The value 127 is Reserved and may be assigned via Standards Action
   [RFC5226].  The values in the range 3-95 can be assigned via
   Specification Required [RFC5226], and the range 96-126 is for Private
   Use [RFC5226].




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17.3.  Result Codes

   IANA shall create a new registry for PCP result codes, numbered
   0-255, initially populated with the result codes from Section 6.4.
   The value 255 is Reserved and may be assigned via Standards Action
   [RFC5226].

   Result Codes in the range 13-191 can be assigned via Specification
   Required [RFC5226], and the range 192-254 is for Private Use
   [RFC5226].

17.4.  Options

   IANA shall create a new registry for PCP Options, numbered 0-255 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 12.  The Option values 0, 127 and
   255 are Reserved and may be assigned via Standards Action [RFC5226].

   Additional PCP Option codes in the ranges 4-63 and 128-191 can be
   created via Specification Required [RFC5226], and the ranges 64-126
   and 192-254 are for Private Use [RFC5226].


18.  Acknowledgments

   Thanks to Xiaohong Deng, Alain Durand, Christian Jacquenet, Jacni
   Qin, Simon Perreault, James Yu, Tina TSOU (Ting ZOU), Felipe Miranda
   Costa, and James Woodyatt for their comments and review.  Thanks to
   Simon Perreault for highlighting the interaction of dynamic
   connections with PCP-created mappings.

   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 Margaret Wasserman for writing the Security Considerations
   section.


19.  References

19.1.  Normative References

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




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

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

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

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

   [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/protocol-numbers.xml>.

19.2.  Informative References

   [Bonjour]  "Bonjour",
              <http://en.wikipedia.org/wiki/Bonjour_(software)>.

   [I-D.arkko-dual-stack-extra-lite]
              Arkko, J., Eggert, L., and M. Townsley, "Scalable
              Operation of Address Translators with Per-Interface
              Bindings", draft-arkko-dual-stack-extra-lite-05 (work in
              progress), February 2011.

   [I-D.boucadair-pcp-failure]
              Boucadair, M., Dupont, F., and R. Penno, "Port Control
              Protocol (PCP) Failure Scenarios",
              draft-boucadair-pcp-failure-02 (work in progress),
              September 2011.

   [I-D.bpw-pcp-upnp-igd-interworking]
              Boucadair, M., Penno, R., Wing, D., and F. Dupont,



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              "Universal Plug and Play (UPnP) Internet Gateway Device
              (IGD)-Port Control Protocol (PCP) Interworking Function",
              draft-bpw-pcp-upnp-igd-interworking-02 (work in progress),
              February 2011.

   [I-D.cheshire-dnsext-dns-sd]
              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
              progress), December 2011.

   [I-D.cheshire-nat-pmp]
              Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
              draft-cheshire-nat-pmp-03 (work in progress), April 2008.

   [I-D.ietf-behave-lsn-requirements]
              Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common requirements for Carrier Grade NATs
              (CGNs)", draft-ietf-behave-lsn-requirements-05 (work in
              progress), November 2011.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",
              draft-ietf-behave-sctpnat-05 (work in progress),
              June 2011.

   [I-D.miles-behave-l2nat]
              Miles, D. and M. Townsley, "Layer2-Aware NAT",
              draft-miles-behave-l2nat-00 (work in progress),
              March 2009.

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

   [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.

   [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



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              Routing", RFC 3581, August 2003.

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

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

   [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.


Appendix A.  NAT-PMP Transition

   The Port Control Protocol (PCP) is a successor to the NAT Port
   Mapping Protocol, NAT-PMP [I-D.cheshire-nat-pmp], and shares similar
   semantics, concepts, and packet formats.  Because of this NAT-PMP and



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   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 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 as indicated by the NAT-PMP specification
   [I-D.cheshire-nat-pmp], which will cause the client to downgrade to
   NAT-PMP and re-send 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.


Appendix B.  Change History

   [Note to RFC Editor: Please remove this section prior to
   publication.]

B.1.  Changes from draft-ietf-pcp-base-18 to -19

   o  Described race condition with MAP containing PREFER_FAILURE and
      Mapping Update.

   o  Added state machine (Section 14.5).

   o  Fully integrated Rapid Recovery, with a separate Opcode having its
      own processing description.  Moved detail about why separate ports
      for multicast/unicast to its own section.

   o  Clarified that due to Mapping Update, a single MAP or PEER request
      can receive multiple responses, each updating the previous
      request, and that the PCP client needs to handle MAP updates or
      PEER updates accordingly.







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B.2.  Changes from draft-ietf-pcp-base-17 to -18

   o  Removed UNPROCESSED option.  Instead, unprocessed options are
      simply not included in responses.

   o  Updated terminology section for Implicit/Explicit & Outbound/
      Inbound.

   o  PEER requests cannot delete or shorten the lifetime of a mapping.

   o  Clarified that PCP clients only retransmit mapping requests for as
      long as they actually want the mapping.

   o  Revised Epoch time calculations and explanation.

   o  Renamed the announcement opcode from No-Op to ANNOUNCE.

B.3.  Changes from draft-ietf-pcp-base-16 to -17

   o  suggest acquiring a mapping to the Discard port if there is a
      desire to show the user their external address (Section 10.7).

   o  Added Restart Announcement.

   o  Tweaked terminology.

   o  Detailed how error responses are generated.

B.4.  Changes from draft-ietf-pcp-base-15 to -16

   o  fixed mistake in PCP request format (had 32 bits of extraneous
      fields)

   o  Allow MAP to request all ports (port=0) for a specific protocol
      (protocol!=0), for the same reason we added support for all ports
      (port=0) and all protocols (protocol=0) in -15

   o  corrected text on Client Processing a Response related to
      receiving ADDRESS_MISMATCH error.

   o  updated Epoch text.

   o  Added text that MALFORMED_REQUEST is generated for MAP if Protocol
      is zero but Internal Port is non-zero.







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B.5.  Changes from draft-ietf-pcp-base-14 to -15

   o  Softened and removed text that was normatively explaining how PEER
      is implemented within a NAT.

   o  Allow a MAP request for protocol=0, which means "all protocols".
      This can work for an IPv6 or IPv4 firewall.  Its use with a NAPT
      is undefined.

   o  combined SERVER_OVERLOADED and NO_RESOURCES into one error code,
      NO_RESOURCES.

   o  SCTP mappings have to use same internal and suggested external
      ports, and have implied PREFER_FAILURE semantics.

   o  Re-instated ADDRESS_MISMATCH error, which only checks the client
      address (not its port).

B.6.  Changes from draft-ietf-pcp-base-13 to -14

   o  Moved discussion of socket operations for PCP source address into
      Implementation Considerations section.

   o  Integrated numerous WGLC comments.

   o  NPTv6 in scope.

   o  Re-written security considerations section.  Thanks, Margaret!

   o  Reduced PEER4 and PEER6 Opcodes to just a single Opcode, PEER.

   o  Reduced MAP4 and MAP6 Opcodes to just a single Opcode, MAP.

   o  Rearranged the PEER packet formats to align with MAP.

   o  Removed discussion of the "O" bit for Options, which was
      confusing.  Now the text just discusses the most significant bit
      of the Option code which indicates mandatory/optional, so it is
      clearer the field is 8 bits.

   o  The THIRD_PARTY Option from an unauthorized host generates
      UNSUPP_OPTION, so the PCP server doesn't disclose it knows how to
      process THIRD_PARTY Option.

   o  Added table to show which fields of MAP or PEER need IPv6/IPv4
      addresses for IPv4 firewall, DS-Lite, NAT64, NAT44, etc.





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   o  Accommodate the server's Epoch going up or down, to better detect
      switching to a different PCP server.

   o  Removed ADDRESS_MISMATCH; the server always includes its idea of
      the Client's IP Address and Port, and it's up to the client to
      detect a mismatch (and rectify it).

B.7.  Changes from draft-ietf-pcp-base-12 to -13

   o  All addresses are 128 bits.  IPv4 addresses are represented by
      IPv4-mapped IPv6 addresses (::FFFF/96)

   o  PCP request header now includes PCP client's port (in addition to
      the client's IP address, which was in -12).

   o  new ADDRESS_MISMATCH error.

   o  removed PROCESSING_ERROR error, which was too similar to
      MALFORMED_REQUEST.

   o  Tweaked text describing how PCP client deals with multiple PCP
      server addresses (Section 7.1)

   o  clarified that when overloaded, the server can send
      SERVER_OVERLOADED (and drop requests) or simply drop requests.

   o  Clarified how PCP client chooses MAP4 or MAP6, depending on the
      presence of its own IPv6 or IPv4 interfaces (Section 9).

   o  compliant PCP server MUST support MAPx and PEERx, SHOULD support
      ability to disable support.

   o  clarified that MAP-created mappings have no filtering, and PEER-
      created mappings have whatever filtering and mapping behavior is
      normal for that particular NAT / firewall.

   o  Integrated WGLC feedback (small changes to abstract, definitions,
      and small edits throughout the document)

   o  allow new Options to be defined with a specification (rather than
      standards action)

B.8.  Changes from draft-ietf-pcp-base-11 to -12

   o  added implementation note that MAP and implicit dynamic mappings
      have independent mapping lifetimes.





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B.9.  Changes from draft-ietf-pcp-base-10 to -11

   o  clarified what can cause CANNOT_PROVIDE_EXTERNAL error to be
      generated.

B.10.  Changes from draft-ietf-pcp-base-09 to -10

   o  Added External_AF field to PEER requests.  Made PEER's Suggested
      External IP Address and Assigned External IP Address always be 128
      bits long.

B.11.  Changes from draft-ietf-pcp-base-08 to -09

   o  Clarified in PEER Opcode introduction (Section 11) that they can
      also create mappings.

   o  More clearly explained how PEER can re-create an implicit dynamic
      mapping, for purposes of rebuilding state to maintain an existing
      session (e.g., long-lived TCP connection to a server).

   o  Added Suggested External IP Address to the PEER Opcodes, to allow
      more robust rebuilding of connections.  Added related text to the
      PEER server processing section.

   o  Removed text encouraging PCP server to statefully remember its
      mappings from Section 14.3.1, as it didn't belong there.  Text in
      Security Considerations already encourages persistent storage.

   o  More clearly discussed how PEER is used to re-establish TCP
      mapping state.  Moved it to a new section, as well (it is now
      Section 9.4).

   o  MAP errors now copy the Suggested Address (and port) fields to
      Assigned IP Address (and port), to allow PCP client to distinguish
      among many outstanding requests when using PREFER_FAILURE.

   o  Mapping theft can also be mitigated by ensuring hosts can't re-use
      same IP address or port after state loss.

   o  the UNPROCESSED option is renumbered to 0 (zero), which ensures no
      other option will be given 0 and be unable to be expressed by the
      UNPROCESSED option (due to its 0 padding).

   o  created new Implementation Considerations section (Section 14)
      which discusses non-normative things that might be useful to
      implementers.  Some new text is in here, and the Failure Scenarios
      text (Section 14.3) has been moved to here.




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   o  Tweaked wording of EDM NATs in Section 14.1 to clarify the problem
      occurs both inside->outside and outside->inside.

   o  removed "Interference by Other Applications on Same Host" section
      from security considerations.

   o  fixed zero/non-zero text in Section 10.5.

   o  removed duplicate text saying MAP is allowed to delete an implicit
      dynamic mapping.  It is still allowed to do that, but it didn't
      need to be said twice in the same paragraph.

   o  Renamed error from UNAUTH_TARGET_ADDRESS to
      UNAUTH_THIRD_PARTY_INTERNAL_ADDRESS.

   o  for FILTER option, removed unnecessary detail on how FILTER would
      be bad for PEER, as it is only allowed for MAP anyway.

   o  In Security Considerations, explain that PEER can create a mapping
      which makes its security considerations the same as MAP.

B.12.  Changes from draft-ietf-pcp-base-07 to -08

   o  moved all MAP4-, MAP6-, and PEER-specific options into a single
      section.

   o  discussed NAT port-overloading and its impact on MAP (new section
      Section 14.1), which allowed removing the IMPLICIT_MAPPING_EXISTS
      error.

   o  eliminated NONEXIST_PEER error (which was returned if a PEER
      request was received without an implicit dynamic mapping already
      being created), and adjusted PEER so that it creates an implicit
      dynamic mapping.

   o  Removed Deployment Scenarios section (which detailed NAT64, NAT44,
      Dual-Stack Lite, etc.).

   o  Added Client's IP Address to PCP common header.  This allows
      server to refuse a PCP request if there is a mismatch with the
      source IP address, such as when a non-PCP-aware NAT was on the
      path.  This should reduce failure situations where PCP is deployed
      in conjunction with a non-PCP-aware NAT.  This addition was
      consensus at IETF80.

   o  Changed UNSPECIFIED_ERROR to PROCESSING_ERROR.  Clarified that
      MALFORMED_REQUEST is for malformed requests (and not related to
      failed attempts to process the request).



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   o  Removed MISORDERED_OPTIONS.  Consensus of IETF80.

   o  SERVER_OVERLOADED is now a common PCP error (instead of specific
      to MAP).

   o  Tweaked PCP retransmit/retry algorithm again, to allow more
      aggressive PCP discovery if an implementation wants to do that.

   o  Version negotiation text tweaked to soften NAT-PMP reference, and
      more clearly explain exactly what UNSUPP_VERSION should return.

   o  PCP now uses NAT-PMP's UDP port, 5351.  There are no normative
      changes to NAT-PMP or PCP to allow them both to use the same port
      number.

   o  New Appendix A to discuss NAT-PMP / PCP interworking.

   o  improved pseudocode to be non-blocking.

   o  clarified that PCP cannot delete a static mapping (i.e., a mapping
      created by CLI or other non-PCP means).

   o  moved theft of mapping discussion from Epoch section to Security
      Considerations.

B.13.  Changes from draft-ietf-pcp-base-06 to -07

   o  tightened up THIRD_PARTY security discussion.  Removed "highest
      numbered address", and left it as simply "the CPE's IP address".

   o  removed UNABLE_TO_DELETE_ALL error.

   o  renumbered Opcodes

   o  renumbered some error codes

   o  assigned value to IMPLICIT_MAPPING_EXISTS.

   o  UNPROCESSED can include arbitrary number of option codes.

   o  Moved lifetime fields into common request/response headers

   o  We've noticed we're having to repeatedly explain to people that
      the "requested port" is merely a hint, and the NAT gateway is free
      to ignore it.  Changed name to "suggested port" to better convey
      this intention.





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   o  Added NAT-PMP transition section

   o  Separated Internal Address, External Address, Remote Peer Address
      definition

   o  Unified Mapping, Port Mapping, Port Forwarding definition

   o  adjusted so DHCP configuration is non-normative.

   o  mentioned PCP refreshes need to be sent over the same interface.

   o  renamed the REMOTE_PEER_FILTER option to FILTER.

   o  Clarified FILTER option to allow sending an ICMP error if policy
      allows.

   o  for MAP, clarified that if the PCP client changed its IP address
      and still wants to receive traffic, it needs to send a new MAP
      request.

   o  clarified that PEER requests have to be sent from same interface
      as the connection itself.

   o  for MAP opcode, text now requires mapping be deleted when lifetime
      expires (per consensus on 8-Mar interim meeting)

   o  PEER Opcode: better description of remote peer's IP address,
      specifically that it does not control or establish any filtering,
      and explaining why it is 'from the PCP client's perspective'.

   o  Removed latent text allowing DMZ for 'all protocols' (protocol=0).
      Which wouldn't have been legal, anyway, as protocol 0 is assigned
      by IANA to HOPOPT (thanks to James Yu for catching that one).

   o  clarified that PCP server only listens on its internal interface.

   o  abandoned 'target' term and reverted to simplier 'internal' term.

B.14.  Changes from draft-ietf-pcp-base-05 to -06

   o  Dual-Stack Lite: consensus was encapsulation mode.  Included a
      suggestion that the B4 will need to proxy PCP-to-PCP and UPnP-to-
      PCP.

   o  defined THIRD_PARTY Option to work with the PEER Opcode, too.
      This meant moving it to its own section, and having both MAP and
      PEER Opcodes reference that common section.




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   o  used "target" instead of "internal", in the hopes that clarifies
      internal address used by PCP itself (for sending its packets)
      versus the address for MAPpings.

   o  Options are now required to be ordered in requests, and ordering
      has to be validated by the server.  Intent is to ease server
      processing of mandatory-to-implement options.

   o  Swapped Option values for the mandatory- and optional-to-process
      Options, so we can have a simple lowest..highest ordering.

   o  added MISORDERED_OPTIONS error.

   o  re-ordered some error messages to cause MALFORMED_REQUEST (which
      is PCP's most general error response) to be error 1, instead of
      buried in the middle of the error numbers.

   o  clarified that, after successfully using a PCP server, that PCP
      server is declared to be non-responsive after 5 failed
      retransmissions.

   o  tightened up text (which was inaccurate) about how long general
      PCP processing is to delay when receiving an error and if it
      should honor Opcode-specific error lifetime.  Useful for MAP
      errors which have an error lifetime.  (This all feels awkward to
      have only some errors with a lifetime.)

   o  Added better discussion of multiple interfaces, including
      highlighting WiFi+Ethernet.  Added discussion of using IPv6
      Privacy Addresses and RFC1918 as source addresses for PCP
      requests.  This should finish the section on multi-interface
      issues.

   o  added some text about why server might send SERVER_OVERLOADED, or
      might simply discard packets.

   o  Dis-allow internal-port=0, which means we dis-allow using PCP as a
      DMZ-like function.  Instead, ports have to be mapped individually.

   o  Text describing server's processing of PEER is tightened up.

   o  Server's processing of PEER now says it is implementation-specific
      if a PCP server continues to allow the mapping to exist after a
      PEER message.  Client's processing of PEER says that if client
      wants mapping to continue to exist, client has to continue to send
      recurring PEER messages.





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B.15.  Changes from draft-ietf-pcp-base-04 to -05

   o  tweaked PCP common header packet layout.

   o  Re-added port=0 (all ports).

   o  minimum size is 12 octets (missed that change in -04).

   o  removed Lifetime from PCP common header.

   o  for MAP error responses, the lifetime indicates how long the
      server wants the client to avoid retrying the request.

   o  More clearly indicated which fields are filled by the server on
      success responses and error responses.

   o  Removed UPnP interworking section from this document.  It will
      appear in [I-D.bpw-pcp-upnp-igd-interworking].

B.16.  Changes from draft-ietf-pcp-base-03 to -04

   o  "Pinhole" and "PIN" changed to "mapping" and "MAP".

   o  Reduced from four MAP Opcodes to two.  This was done by implicitly
      using the address family of the PCP message itself.

   o  New option THIRD_PARTY, to more carefully split out the case where
      a mapping is created to a different host within the home.

   o  Integrated a lot of editorial changes from Stuart and Francis.

   o  Removed nested NAT text into another document, including the IANA-
      registered IP addresses for the PCP server.

   o  Removed suggestion (MAY) that PCP server reserve UDP when it maps
      TCP.  Nobody seems to need that.

   o  Clearly added NAT and NAPT, such as in residential NATs, as within
      scope for PCP.

   o  HONOR_EXTERNAL_PORT renamed to PREFER_FAILURE

   o  Added 'Lifetime' field to the common PCP header, which replaces
      the functions of the 'temporary' and 'permanent' error types of
      the previous version.

   o  Allow arbitrary Options to be included in PCP response, so that
      PCP server can indicate un-supported PCP Options.  Satisfies PCP



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      Issue #19

   o  Reduced scope to only deal with mapping protocols that have port
      numbers.

   o  Reduced scope to not support DMZ-style forwarding.

   o  Clarified version negotiation.

B.17.  Changes from draft-ietf-pcp-base-02 to -03

   o  Adjusted abstract and introduction to make it clear PCP is
      intended to forward ports and intended to reduce application
      keepalives.

   o  First bit in PCP common header is set.  This allows DTLS and non-
      DTLS to be multiplexed on same port, should a future update to
      this specification add DTLS support.

   o  Moved subscriber identity from common PCP section to MAP* section.

   o  made clearer that PCP client can reduce mapping lifetime if it
      wishes.

   o  Added discussion of host running a server, client, or symmetric
      client+server.

   o  Introduced PEER4 and PEER6 Opcodes.

   o  Removed REMOTE_PEER Option, as its function has been replaced by
      the new PEER Opcodes.

   o  IANA assigned port 44323 to PCP.

   o  Removed AMBIGUOUS error code, which is no longer needed.

B.18.  Changes from draft-ietf-pcp-base-01 to -02

   o  more error codes

   o  PCP client source port number should be random

   o  PCP message minimum 8 octets, maximum 1024 octets.

   o  tweaked a lot of text in section 7.4, "Opcode-Specific Server
      Operation".





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   o  opening a mapping also allows ICMP messages associated with that
      mapping.

   o  PREFER_FAILURE value changed to the mandatory-to-process range.

   o  added text recommending applications that are crashing obtain
      short lifetimes, to avoid consuming subscriber's port quota.

B.19.  Changes from draft-ietf-pcp-base-00 to -01

   o  Significant document reorganization, primarily to split base PCP
      operation from Opcode operation.

   o  packet format changed to move 'protocol' outside of PCP common
      header and into the MAP* opcodes

   o  Renamed Informational Elements (IE) to Options.

   o  Added REMOTE_PEER (for disambiguation with dynamic ports),
      REMOTE_PEER_FILTER (for simple packet filtering), and
      PREFER_FAILURE (to optimize UPnP IGDv1 interworking) options.

   o  Is NAT or router behind B4 in scope?

   o  PCP option MAY be included in a request, in which case it MUST
      appear in a response.  It MUST NOT appear in a response if it was
      not in the request.

   o  Result code most significant bit now indicates permanent/temporary
      error

   o  PCP Options are split into mandatory-to-process ("P" bit), and
      into Specification Required and Private Use.

   o  Epoch discussion simplified.


Authors' Addresses

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

   Email: dwing@cisco.com





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   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-ftgroup.com


   Reinaldo Penno
   Juniper Networks
   1194 N Mathilda Avenue
   Sunnyvale, California  94089
   USA

   Email: rpenno@juniper.net


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

   Email: pselkirk@isc.org

















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