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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: April 21, 2012                                            Apple
                                                            M. Boucadair
                                                          France Telecom
                                                                R. Penno
                                                        Juniper Networks
                                                              P. Selkirk
                                                                     ISC
                                                        October 19, 2011


                      Port Control Protocol (PCP)
                         draft-ietf-pcp-base-15

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 April 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 . . . . . 10
   5.  Note on Fixed-Size Addresses . . . . . . . . . . . . . . . . . 10
   6.  Common Request and Response Header Format  . . . . . . . . . . 11
     6.1.  Request Header . . . . . . . . . . . . . . . . . . . . . . 12
     6.2.  Response Header  . . . . . . . . . . . . . . . . . . . . . 13
     6.3.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . 14
     6.4.  Result Codes . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  General PCP Operation  . . . . . . . . . . . . . . . . . . . . 18
     7.1.  General PCP Client: Generating a Request . . . . . . . . . 18
     7.2.  General PCP Server: Processing a Request . . . . . . . . . 19
     7.3.  General PCP Client: Processing a Response  . . . . . . . . 21
     7.4.  Multi-Interface Issues . . . . . . . . . . . . . . . . . . 22
     7.5.  Epoch  . . . . . . . . . . . . . . . . . . . . . . . . . . 22
     7.6.  Version Negotiation  . . . . . . . . . . . . . . . . . . . 23
     7.7.  General PCP Option . . . . . . . . . . . . . . . . . . . . 24
       7.7.1.  UNPROCESSED Option . . . . . . . . . . . . . . . . . . 24
   8.  Introduction to MAP and PEER Opcodes . . . . . . . . . . . . . 25
     8.1.  For Operating a Server . . . . . . . . . . . . . . . . . . 27
     8.2.  For Operating a Symmetric Client/Server  . . . . . . . . . 28
     8.3.  For Reducing NAT Keepalive Messages  . . . . . . . . . . . 31
     8.4.  For Restoring Lost Implicit TCP Dynamic Mapping State  . . 32
   9.  MAP Opcode . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     9.1.  MAP Operation Packet Formats . . . . . . . . . . . . . . . 34
     9.2.  Generating a MAP Request . . . . . . . . . . . . . . . . . 36
       9.2.1.  Renewing a Mapping . . . . . . . . . . . . . . . . . . 36
     9.3.  Processing a MAP Request . . . . . . . . . . . . . . . . . 37
     9.4.  Processing a MAP Response  . . . . . . . . . . . . . . . . 39
     9.5.  Mapping Lifetime and Deletion  . . . . . . . . . . . . . . 40
     9.6.  Address Change Events  . . . . . . . . . . . . . . . . . . 42
     9.7.  Learning the External IP Address Alone . . . . . . . . . . 43
   10. PEER Opcode  . . . . . . . . . . . . . . . . . . . . . . . . . 43
     10.1. PEER Operation Packet Formats  . . . . . . . . . . . . . . 44



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     10.2. Generating a PEER Request  . . . . . . . . . . . . . . . . 47
     10.3. Processing a PEER Request  . . . . . . . . . . . . . . . . 47
     10.4. Processing a PEER Response . . . . . . . . . . . . . . . . 49
   11. Options for MAP and PEER Opcodes . . . . . . . . . . . . . . . 49
     11.1. THIRD_PARTY Option for MAP and PEER Opcodes  . . . . . . . 50
     11.2. PREFER_FAILURE Option for MAP Opcode . . . . . . . . . . . 52
     11.3. FILTER Option for MAP Opcode . . . . . . . . . . . . . . . 53
   12. Implementation Considerations  . . . . . . . . . . . . . . . . 55
     12.1. Implementing MAP with EDM port-mapping NAT . . . . . . . . 55
     12.2. Lifetime of Explicit and Implicit Dynamic Mappings . . . . 56
     12.3. PCP Failure Scenarios  . . . . . . . . . . . . . . . . . . 56
       12.3.1. Recreating Mappings  . . . . . . . . . . . . . . . . . 57
       12.3.2. Maintaining Mappings . . . . . . . . . . . . . . . . . 57
       12.3.3. SCTP . . . . . . . . . . . . . . . . . . . . . . . . . 58
     12.4. Source Address and Port in PCP Header  . . . . . . . . . . 58
   13. Deployment Considerations  . . . . . . . . . . . . . . . . . . 58
     13.1. Ingress Filtering  . . . . . . . . . . . . . . . . . . . . 58
     13.2. Mapping Quota  . . . . . . . . . . . . . . . . . . . . . . 59
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 59
     14.1. Simple Threat Model  . . . . . . . . . . . . . . . . . . . 59
       14.1.1. Attacks Considered . . . . . . . . . . . . . . . . . . 60
       14.1.2. Deployment Examples Supporting the Simple Threat
               Model  . . . . . . . . . . . . . . . . . . . . . . . . 61
     14.2. Advanced Threat Model  . . . . . . . . . . . . . . . . . . 61
     14.3. Residual Threats . . . . . . . . . . . . . . . . . . . . . 62
       14.3.1. Denial of Service  . . . . . . . . . . . . . . . . . . 62
       14.3.2. Ingress Filtering  . . . . . . . . . . . . . . . . . . 63
       14.3.3. Mapping Theft  . . . . . . . . . . . . . . . . . . . . 63
       14.3.4. Attacks Against Server Discovery . . . . . . . . . . . 63
   15. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 63
     15.1. Port Number  . . . . . . . . . . . . . . . . . . . . . . . 63
     15.2. Opcodes  . . . . . . . . . . . . . . . . . . . . . . . . . 64
     15.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 64
     15.4. Options  . . . . . . . . . . . . . . . . . . . . . . . . . 64
   16. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 64
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 65
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 65
     17.2. Informative References . . . . . . . . . . . . . . . . . . 66
   Appendix A.  NAT-PMP Transition  . . . . . . . . . . . . . . . . . 68
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 69
     B.1.  Changes from draft-ietf-pcp-base-14 to -15 . . . . . . . . 69
     B.2.  Changes from draft-ietf-pcp-base-13 to -14 . . . . . . . . 69
     B.3.  Changes from draft-ietf-pcp-base-12 to -13 . . . . . . . . 70
     B.4.  Changes from draft-ietf-pcp-base-11 to -12 . . . . . . . . 71
     B.5.  Changes from draft-ietf-pcp-base-10 to -11 . . . . . . . . 71
     B.6.  Changes from draft-ietf-pcp-base-09 to -10 . . . . . . . . 71
     B.7.  Changes from draft-ietf-pcp-base-08 to -09 . . . . . . . . 71
     B.8.  Changes from draft-ietf-pcp-base-07 to -08 . . . . . . . . 72



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     B.9.  Changes from draft-ietf-pcp-base-06 to -07 . . . . . . . . 73
     B.10. Changes from draft-ietf-pcp-base-05 to -06 . . . . . . . . 74
     B.11. Changes from draft-ietf-pcp-base-04 to -05 . . . . . . . . 76
     B.12. Changes from draft-ietf-pcp-base-03 to -04 . . . . . . . . 76
     B.13. Changes from draft-ietf-pcp-base-02 to -03 . . . . . . . . 77
     B.14. Changes from draft-ietf-pcp-base-01 to -02 . . . . . . . . 77
     B.15. Changes from draft-ietf-pcp-base-00 to -01 . . . . . . . . 78
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 78











































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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  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.  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 client would



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   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
      mapping (e.g., by sending a TCP SYN) across a firewall or a NAT.

   Remote 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 from [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 to that
      identity function, is separate from the mapping itself.

   Mapping Types:
      There are three different ways to create mappings: implicit
      dynamic mappings, explicit dynamic mappings, and static mappings.
      Implicit dynamic mappings are created as a result of a TCP SYN or
      outgoing UDP packet or a PCP PEER request, and allow Internal
      Hosts to receive replies to their outbound packets.  Explicit
      dynamic mappings are created as a result of a PCP MAP request.
      Static mappings are created by manual configuration (e.g., via
      command-line interface or web page).  Explicit and static mappings
      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).  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



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      the Internal Host (e.g., sending more traffic or sending a new PCP
      MAP request).  Static mappings differ from dynamic mappings in
      that their lifetime is effectively infinite (they exist until
      manually removed) but otherwise they behave exactly the same as an
      explicit dynamic mapping.

   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 IGD, [IGD]) 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 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.  This is conceptually separate from the NAT or firewall
      itself, but is typically implemented as a capability of the PCP-
      controlled device.  See also Section 4.

   Interworking Function:
      A functional element responsible for translating or proxying
      another protocol to PCP.  For example interworking between UPnP
      IGD [IGD] 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 as described in
   [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



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   distinguishable from a legal IPv6 address, because IPv4-mapped IPv6
   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 0xFF 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 9 and Section 10.



























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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)                 |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    PCP Client's Port          |    Reserved (16 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 7.6.

   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 9 and Section 10.

   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.

   Reserved:  16 reserved bits, MUST be sent as 0 and MUST be ignored
      when received.

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 (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.

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

   Opcode:  The 7-bit Opcode value, copied 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.

   Epoch:  The server's Epoch 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.

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 and 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 octets of zeros 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 the
      padding octets.

   data:  Option data.  The Option data MUST end on a 32-bit boundary,
      padded with 0's when necessary.

   The handling of an Option by the PCP client and PCP server MUST be
   specified in an appropriate document, which MUST include 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 with an error including UNPROCESSED Options; this can
   be achieved by sending messages that don't exceed 1024-
   4*number_of_options octets.

   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 encode the Options in the same order as



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   received in the request.  Additional Options included in the response
   (if any) MUST be included at the end.  An Option MAY appear more than
   once in a request or in a response, if permitted by the definition of
   the Option.  If the Option's definition allows the Option to appear
   only once but it appears more than once in a request, and the Option
   is understood by the PCP server, the PCP server MUST respond with the
   MALFORMED_OPTION result code; if this occurs in a response, the PCP
   client processes the first occurrence and MAY log an error.  If an
   invalid option length is encountered (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.  The UNPROCESSED option MUST NOT appear in a request; if
   it does, it causes a MALFORMED_REQUEST error.  If a PCP response
   would have exceeded the maximum PCP message size, the PCP server MAY
   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) and MUST include
   the UNPROCESSED Option in the response (see Section 7.7.1).

   PCP clients are free to ignore any or all Options included in
   responses, although naturally if a client explicitly requests an
   Option where correct handling 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, the
   UNPROCESSED option is valid for all Opcodes, but only in response
   messages.  The THIRD_PARTY Option is valid for both MAP and PEER
   Opcodes.  The PREFER_FAILURE option is valid only for the MAP Opcode
   (for the PEER Opcode, its semantics are implied).  The FILTER option
   is valid only for the MAP Opcode (for the PEER Opcode it would have
   no meaning).

   Option definitions MUST include the information below:







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

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.



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      This is a system-wide error, and 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.

   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_PORT: the requested external port 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 PEER
      or MAP requests for the SCTP protcool.  See Section 11.2 for
      processing details.  The error lifetime depends on the reason for
      the failure.

   12 ADDRESS_MISMATCH: the source IP address or port of the request
      packet does not match the contents of the PCP Client's IP Address
      or UDP port.

   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 11.3
      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 a 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.

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

   Prior to sending its first PCP message, the PCP client determines



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   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 12.4 for how this can be
   coded.

   When attempting to contact a PCP server, the PCP client initializes a
   timer to 2 seconds.  The PCP client sends a PCP message to the first
   server in its list of PCP servers.  If no response is received before
   the timer expires, the timer is doubled (to 4 seconds) and the
   request is re-transmitted.  If no response is received before the
   timer expires, the timer is doubled again (to 8 seconds) and the
   request is re-transmitted.

   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.




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   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.

   If the received message is at least two octets long but 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 7.6 are followed.

   Otherwise, if the version is supported but the received message is
   shorter than 4 octets or has the R bit set, the message is silently
   dropped.

   If the server is overloaded by requests (from a particular client or
   from all clients), it MAY simply 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 or is not a multiple
   of 4 octets, it is invalid.  Invalid requests are handled by copying
   up to 1024 octets of the request into the response, setting the
   result code to MALFORMED_REQUEST, and zero-padding the response to a
   multiple of 4 octets if necessary.

   The PCP server compares the IP address (from the 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 the IP address
   from the perspective of the PCP server.

   Error responses have the same packet layout as success responses,
   with fields from the request copied into the response, and fields



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   assigned by the PCP server set as indicated in Figure 3.

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

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 an outstanding PCP
   request.  It validates that the Opcode matches an outstanding 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 field to determine if it needs to restore its state
   to the PCP server (see Section 7.5).

   If the PCP Client's IP Address and PCP Client's Port fields of the
   PCP response header do not match the source address and port of the
   request, it indicates the presence of a NAT between the PCP client
   and PCP server.  If they don't match, then the PCP client (or the
   user on the client host) MUST ensure that an appropriate NAT mapping
   is created on the intervening NAT(s) (e.g., using UPnP IGD, NAT-PMP,
   or manual configuration), otherwise, the PCP-installed mapping will
   be ineffective.

   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 UNSUPP_VERSION, processing continues as described in Section 7.6.
   If the result code is NO_RESOURCES, PCP client SHOULD NOT send *any*
   further requests 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



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   with this PCP server, without regard to hold times from communicating
   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 field.
   This field increments by 1 every second, and is used by the PCP
   client to determine if PCP state needs to be restored.  If the PCP
   server resets or loses the state of its explicit dynamic Mappings
   (that is, those mappings created by PCP MAP requests), due to reboot,
   power failure, or any other reason, it MUST reset its Epoch time to
   zero or adjust its Epoch time (up or down) by at least 87840 seconds
   (24 hours).  After resetting its Epoch time, it resumes incrementing
   the Epoch value by one every second.  Similarly, if the public IP
   address(es) of the NAT (controlled by the PCP server) changes, the
   Epoch MUST be reset.  A PCP server MAY maintain one Epoch value for
   all PCP clients, or MAY maintain distinct Epoch values (per PCP
   client, per interface, or based on other criteria); this choice is
   implementation-dependent.



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   Whenever a client receives a PCP response, the client computes an
   expected Epoch value based on the Epoch value in the last packet it
   received from that PCP server plus the time elapsed since that packet
   was received.  If the received Epoch value is less then 7/8 (0.875%)
   seconds below the expected Epoch value, or is more than 3660 seconds
   (one hour) above or below the expected Epoch value, the PCP client
   concludes that the PCP server lost state.  After concluding the PCP
   server lost state, the client immediately renews all its active port
   mapping leases as described in Section 12.3.1.

7.6.  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, these first two octets 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 first request using highest (i.e., presumably
       'best') version number it supports.

   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



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       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.

7.7.  General PCP Option

   The following Option can appear in certain PCP responses, without
   regard to the Opcode.

7.7.1.  UNPROCESSED Option

   If the PCP server cannot process a mandatory-to-process Option, for
   whatever reason, it includes the UNPROCESSED Option in the response,
   shown in Figure 5.  This helps with debugging interactions between
   the PCP client and PCP server.  This Option MUST NOT appear more than
   once in a PCP response.  The unprocessed Options are listed once, and
   the Option data is zero-filled to the necessary 32 bit boundary.  If
   a certain Option appeared more than once in the PCP request, that
   Option value MAY appear once or as many times as it occurred in the
   request.  The order of the Options in the PCP request has no
   relationship with the order of the Option values in this UNPROCESSED
   Option.  This Option MUST NOT appear in a response unless the
   associated request contained at least one mandatory-to-process
   Option.

   The UNPROCESSED 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=0 |  Reserved     |   Option Length=variable      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Option-code-1 | ... additional option-codes as necessary      |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 5: UNPROCESSED option









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      Option Name: UNPROCESSED
      Number: 0
      Purpose: indicates which PCP Options in the request were not
      processed by the PCP server
      Valid for Opcodes: all
      Length: 1 octet or more
      May appear in: responses, and only if the result code is non-zero.
      Maximum occurrences: 1


8.  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 8.1);

   o  a host operating a client and server on the same port
      (Section 8.2);

   o  a host operating a client and wanting to optimize the application
      keepalive traffic (Section 8.3);

   o  and a host operating a client and wanting to restore lost state in
      its NAT (Section 8.4).

   These are discussed in the following sections.

   When operating a server (Section 8.1 and Section 8.2) the PCP client
   knows if it wants an IPv4 listener, IPv6 listener, or both on the
   Internet.  The PCP client also knows if it has an IPv4 address or
   IPv6 address configured on one of its interfaces.  It takes the union
   of this knowledge to decide to which of its PCP servers to send the
   request (e.g., 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 Requested External Address field,
   and sends a second MAP request containing the all-zeros IPv4 address
   in the Requested 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



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   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 Host.  In the absence
   of a PCP option indicating otherwise, it is REQUIRED that PCP-created
   explicit dynamic mappings be assigned the same external IP address.
   It is RECOMMENDED that static mappings for that Internal Host (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 all
   internal addresses assigned to a given Internal Host have no implicit
   dynamic mappings and have 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.

   The following table summarizes how various common PCP deployments use
   IPv6 and IPv4 addresses.  The 'source' is the source address of the
   PCP packet itself, 'internal' is the Internal IP Address field of the
   THIRD_PARTY Option (if present) or the same as the source address of
   the PCP packet iself (if the THIRD_PARTY Option is not present),
   'external' is the Requested External Address field of the MAP or PEER
   request, the 'remote peer' is the Remote Peer IP Address of the PEER
   request or the FILTER option of the MAP request.

                              source   internal  external  remote peer
          IPv4 firewall        IPv4     IPv4      IPv4       IPv4
          IPv6 firewall        IPv6     IPv6      IPv6       IPv6
          NAT44                IPv4     IPv4      IPv4       IPv4
          Dual-Stack Lite (1)  IPv6     IPv4      IPv4       IPv4
          Dual-Stack Lite (2)  IPv4     IPv4      IPv4       IPv4
          NAT64 (3)            IPv6     IPv6      IPv4       IPv6
          NPTv6                IPv6     IPv6      IPv6       IPv6

               Figure 6: Address Families with MAP and PEER

   In (1) and (2), 'source' refers to the PCP messaging between the
   Dual-Stack Lite B4 element and the AFTR element, with (1) showing
   Dual-Stack Lite plain mode and (2) showing Dual-Stack Lite
   Encapsulation mode [I-D.dupont-pcp-dslite].  In a Dual-Stack Lite
   environment within the subscriber's network from a host to the B4
   element, the PCP messaging is IPv4 firewall, IPv6 firewall, or NAT44.
   In (3), 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.

   Note that PCP requests containing the MAP or PEER Opcodes cannot



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   delete or shorten the lifetime of an existing implicit mapping for
   the indicated internal address and port.  Conceptually implicit and
   explicit mappings are different "layers" in the NAT forwarding state
   database.

8.1.  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 9 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
         * 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 7: Pseudo-code for using PCP to operate a server

8.2.  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



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   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 9, 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 12.1.














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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
         * 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();
        }

    Figure 8: Pseudo-code for using PCP to operate a symmetric client/



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                                  server

8.3.  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 10.























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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
         * 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 9: Pseudo-code using PCP with a dynamic socket

8.4.  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
   (e.g., FTP).  This can be accomplished by first establishing a TCP



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   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 only
      creates a new implicit dynamic mapping 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.


9.  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 and an External IP address.

   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
   mapping's Internal Address and Port.



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   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.

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

9.1.  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 10: MAP Opcode Request Packet Format

   These fields are described below:

   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'.





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   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 11: MAP Opcode Response Packet Format

   These fields are described below:

   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.




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   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.

9.2.  Generating a MAP Request

   This section and Section 9.5 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.

   If the client wants all protocols mapped it uses Protocol 0 (zero)
   and Internal Port 0 (zero).

9.2.1.  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.




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   The PCP client SHOULD renew the mapping before its expiry time,
   otherwise it will be removed by the PCP server (see Section 9.5).  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).

   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).

9.3.  Processing a MAP Request

   This section and Section 9.5 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 following fields from the MAP request are copied into the MAP
   response: Protocol, Internal Port, Requested External Address, and
   (if present and processed by the PCP server) the THIRD_PARTY Option.

   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
   or cannot create a mapping for the Protocol (e.g., because the
   request is for a NAT mapping instead of a firewall mapping and the
   PCP-controlled device is not a NAT or does not support NATting that
   specific Protocol), it MUST generate an UNSUPP_PROTOCOL error.  If
   the Protocol is supported and the Internal Port is zero but the
   protocol uses a 16 bit port number (e.g., TCP, SCTP), it generates a
   MALFORMED_REQUEST error.  If the Protocol is supported and does not
   use 16 bit port number (e.g., RSVP), the Internal Port and the
   Requested External Port are ignored, all packets with that Protocol
   number are mapped to the internal host, and these ignored field
   values are copied into the response.  If the Protocol and Internal
   Port are both 0 (zero), all protocols and all ports, without any
   exception, are mapped; if this request cannot be fulfilled, the error
   UNSUPP_PROTOCOL MUST be returned.

   If the requested lifetime is zero, it indicates a request to delete



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   an existing mapping or set of mappings.  Processing of the lifetime
   is described in Section 9.5.

   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.

   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 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 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_PORT is returned.  If an implicit mapping
   already exists for the requested Internal Address and Port, the
   mapping SHOULD be upgraded to an explicit mapping.

   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
   MUST do so and return the newly assigned External Address and Port in
   the response.  Cases where a NAT gateway 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).





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   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 0 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_PORT 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.  This SUCCESS response contains the same Opcode as the
   request, but with the "R" bit set.

9.4.  Processing a MAP Response

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

   After performing common PCP response processing, the response is
   further matched with an outstanding request by comparing the
   protocol, internal IP address, and internal port.  Other fields are
   not compared, because the PCP server sets those fields.

   On a success response, the PCP client can use the External IP Address
   and Port as desired.  Typically the PCP client will communicate the
   External IP Address and Port to another host on the Internet using an



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   application-specific rendezvous mechanism such as DNS SRV records.

   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 9.2, 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 request 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 for a new mapping, with a particular Suggested External
   Address and Port, which happens to be what the PCP server previously
   assigned.  See also Section 12.3.2.

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

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

   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



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   lifetime the PCP server returns for such a static mapping SHOULD be
   4294967295 (0xFFFFFFFF).  There is no need for a PCP client to renew
   a static mapping.

   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, or 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
   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 explicit 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 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



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   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.

   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 signal PCP
   separately to create ICMP mappings for those flows.

9.6.  Address Change Events

   A customer premises router might obtain a new 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
   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



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   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.

9.7.  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 made
   at least one implicit or explicit port mapping, and even then only
   for as long as that implicit or explicit port 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 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.


10.  PEER Opcode

   This section defines an Opcode for controlling dynamic mappings.

     PEER: Create an implicit dynamic mapping, or set or query an
           existing implicit dynamic mapping to a remote peer's IPv4
           address and port.

   The use of these 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
   as if an implicit dynamic mapping were created by a packet sent by
   the host (e.g., TCP SYN sent by the host), mappings created or



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   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 mappings it creates automatically as a result of
   observing outgoing traffic from Internal Hosts.

10.1.  PEER Operation Packet Formats

   The PEER Opcode allows the PCP client to create an implicit dynamic
   mapping (which functions similar to the host sending a TCP SYN), and
   allows the PCP client to manage an implicit dynamic mapping by
   extending its lifetime.

   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 12: PEER Opcode Request Packet Format

   These fields are described below:

   Requested Lifetime (in common header):  Requested lifetime of this
      mapping, in seconds.  Note that, depending on the implementation
      of the PCP-controlled device, it may not be possible to reduce the
      lifetime of a mapping (or delete it, with requested lifetime=0)
      using PEER.




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   Protocol:  Upper-layer protocol associated with this Opcode.  Values
      are taken from the IANA protocol registry [proto_numbers].  For
      example, this field contains 6 (TCP) if the Opcode is describing a
      TCP mapping.

   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, and does
      not create or adjust the filtering associated with the mapping
      (for that, the FILTER option is used, Section 11.3).  An IPv6
      address is represented directly, and an IPv4 address is
      represented using the IPv4-mapped address syntax (80 bits of
      zeros, 16 bits of ones, and 32 bits of the IPv4 address).

   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 the PREFER_FAILURE semantics 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_PORT error response.  The



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   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.

   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 13: 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.





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   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.

   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.

10.2.  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 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 if an implicit dynamic mapping
      were created (e.g., because of a TCP SYN from the client).  This
      is useful for restoring a mapping after a NAT has lost its
      implicit mapping state (e.g., due to a crash).  Note that some PCP
      servers and some PCP-controlled devices are expected to not
      support this functionality and will respond with a PCP error.

      If sent after, this is considered an "implicit dynamic mapping".
      This allows the client to learn the IP address, port, and lifetime
      of the assigned External Address and Port for the implicit
      mapping, and to extend this lifetime (for the purpose described in
      Section 8.3).

   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.

10.3.  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.



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   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.,
   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 implicit 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 gateway first, and allow PEER to be used to
   recreate an implicit dynamic mapping (see last paragraph of
   Section 12.3.1).  If the requested mapping does not yet exist, but
   Suggested External Address and Port cannot be honored, the error
   CANNOT_PROVIDE_EXTERNAL_PORT is returned.  If the requested mapping
   already exists, it is a request to modify that existing mapping.

   The PEER Opcode MAY reduce the lifetime of an existing implicit
   dynamic mapping created by PEER; this is implementation-dependent.

   If the PCP-controlled device can extend the lifetime of a mapping,
   the PCP server uses the smaller of its configured maximum lifetime
   value and the requested lifetime from the PEER request, and sets the
   lifetime to that value.

   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.

   After a successful PEER response is sent, it is implementation-
   specific if the PCP-controlled device destroys the mapping when the
   lifetime expires, or if the PCP-controlled device's implementation
   allows traffic to keep the mapping alive.  Thus, if the PCP client
   wants the mapping to persist beyond the lifetime reported in the
   response, it MUST refresh the mapping (by sending another PEER
   message) prior to the expiration of the lifetime.  If the mapping is
   terminated by the TCP client or server (e.g., TCP FIN or TCP RST),
   the mapping will be destroyed normally; the mapping will not persist
   for the time indicated by Lifetime.  This means the Lifetime in a



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   PEER response indicates how long the mapping will persist in the
   absence of a transport termination message (e.g., TCP RST).

   Some transport protocols signal the end of a connection (e.g., TCP
   FIN, TCP RST, SCTP SHUTDOWN).  After a successful PEER response is
   sent, the receipt of such a transport-specific message MUST NOT cause
   the mapping to be destroyed.  Rather, the mapping is maintained until
   the PEER-signaled lifetime expires.  If the PCP client wishes to
   terminate the mapping prior to this, it will send a PEER request with
   Lifetime set to 0, which MAY be honored by the PCP server; as stated
   earlier, that is implementation-dependent.

10.4.  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 a request by comparing the protocol, internal IP
   address, internal port, remote peer address and remote peer port.
   Other fields are not compared, because the PCP server changes those
   fields to provide information about the mapping created by the
   Opcode.

   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 PCP client wishes to keep this mapping alive beyond the
   indicated lifetime, it SHOULD issue a new PCP request prior to the
   expiration.  That is, inside->outside traffic is not sufficient to
   ensure the mapping will continue to exist.  See Section 9.2.1 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.


11.  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.



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11.1.  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.

   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 14: THIRD_PARTY Option packet format

   The fields are described below:

   Internal IP Address:  Internal IP address for this mapping.  If the
      Option Length is zero, there is no Internal IP address for this
      mapping and this indicates "all Internal IPv4 and IPv6 Addresses
      for which this client is authorized" which is used to delete all
      pre-existing mappings with the MAP Opcode.

      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.




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

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

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

   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 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.




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11.2.  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 the
   Suggested External Port, the PCP server should not map an external
   port.  This differs from the behavior without this Option, which is
   to map a different external port.

   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 15: 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 are
      not available.
      Valid for Opcodes: MAP
      Length: 0
      May appear in: requests
      Maximum occurrences: 1

   The result code CANNOT_PROVIDE_EXTERNAL_PORT is returned if the
   Suggested External Port cannot be mapped.  This can occur because the
   External Port is already mapped to another host's implicit dynamic
   mapping, an explicit dynamic mapping, a static mapping, or the same
   Internal Address and Port has an implicit dynamic mapping which is
   mapped to a different External Port than requested.  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 IGD interworking
   [I-D.bpw-pcp-upnp-igd-interworking], where the semantics of UPnP IGD
   version 1 only allow the UPnP IGD 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 IGD
   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



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   not intended to support downstream devices that perform UPnP IGD
   interworking are not required to support this Option.  PCP clients
   other than UPnP IGD interworking clients SHOULD NOT use this Option
   because it results in inefficient operation, and they cannot safely
   assume that all PCP servers will implement it.  It is anticipated
   that this Option will be deprecated in the future as more clients
   adopt PCP natively and the need for UPnP IGD interworking declines.

11.3.  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 16: FILTER Option layout

   These fields are described below:

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





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   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: requests, and MUST appear in successfully-processed
      responses
      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 same 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 of occurrences of the FILTER Option in a request packet are
   not successfully processed then an error is returned (e.g.,
   MALFORMED_OPTION if one of the Options was malformed) and as with
   other PCP errors, returning an error causes no state to be changed in
   the PCP server or in the PCP-controlled device.

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




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   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 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.


12.  Implementation Considerations

12.1.  Implementing MAP with EDM port-mapping NAT

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

   For implicit dynamic 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
   implicit dynamic mapping (from the same Internal Host or from a
   different Internal Host) and an explicit dynamic mapping.  This
   complicates the interaction with the MAP Opcode.  With such NAT
   devices, there are two ways envisioned to implement the MAP Opcode:

   1.  Have implicit dynamic mappings use a different set of public
       ports than explicit dynamic mappings (e.g., those created with



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       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 an implicit dynamic
       mapping to process that packet.  If none match, then the incoming
       packet should use the explicit dynamic mapping to process that
       packet.  This effectively 'prioritizes' implicit dynamic mappings
       above explicit dynamic mappings.

12.2.  Lifetime of Explicit and Implicit Dynamic Mappings

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

   No matter if a NAT is EIM or EDM, it is possible that one (or more)
   implicit dynamic 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
   implicit dynamic mapping (e.g., via a TCP FIN handshake) does not
   prematurely destroy the MAP-created mapping.  On a NAT that
   implements endpoint-independent mapping with endpoint-independent
   filtering, this could be implemented by extending the lifetime of the
   implicit dynamic mapping to the lifetime of the explicit dynamic
   mapping.

12.3.  PCP Failure Scenarios

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

   If an event occurs that causes the PCP server to lose explicit
   dynamic mapping state (such as a crash or power outage), the mappings
   created by PCP are lost.  Such loss of state is expected to be rare
   in a service provider environment (due to redundant power, disk
   drives for storage, etc.), but more common in a residential NAT
   device which does not write this information to non-volatile memory.
   Of course, due to outright failure of service provider equipment
   (e.g., software malfunction), state may still be lost.

   The Epoch allows a client to deduce when a PCP server may have lost
   its state.  When the Epoch value is observed to be smaller than
   expected, 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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12.3.1.  Recreating Mappings

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

   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 is reset and begins counting again from zero (per
   the procedure of Section 7.5).  As the result of receiving a packet
   where the Epoch field indicates 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.

12.3.2.  Maintaining Mappings

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

   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).  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 value only
   needs to be retrieved for one of them to verify the PCP server has
   not lost explicit dynamic mapping state.

   If the client wishes to check the PCP server's Epoch, it sends a PCP
   request for any one of the client's mappings.  This will return the
   current Epoch value.  In that request the PCP client could extend the
   mapping lifetime (by asking for more time) or maintain the current
   lifetime (by asking for the same number of seconds that it knows are
   remaining of the lifetime).

   If a PCP client changes its Internal IP Address (e.g., because the
   Internal Host has moved to a new network), and the PCP client wishes
   to still receive incoming traffic, it needs create new mappings on
   that new network.  New mappings will typically also require an update
   to the application-specific rendezvous server if the External Address



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   or Port are different to the previous values (see Section 8.1 and
   Section 9.6).

12.3.3.  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].  Because implicit
   dynamic SCTP mappings use the verification tag of the association
   instead of the local and remote peer port numbers, explicit dynamic
   SCTP mappings need only be established by passive listeners expecting
   to receive new associations at the external port.

   Because an SCTP-aware NAT does not rewrite SCTP port numbers (and
   firewalls never do), a PCP MAP or PEER request for an SCTP mapping
   SHOULD provide the same Internal Port and Requested External Port.
   If the PCP server supports SCTP, and the requested external port
   cannot be provided in an explicit dynamic SCTP mapping, then the
   error CANNOT_PROVIDE_EXTERNAL_PORT is returned.

12.4.  Source Address and Port in PCP Header

   All PCP requests include the PCP client's IP address 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.  Bind the UDP socket.
   3.  Call the getsockname() function to retrieve a sockaddr containing
       the source address and port the kernel will use for UDP packets
       sent through this socket.
   4.  If the IP address is an IPv4 address, encode the address into an
       IPv4-mapped IPv6 address.  Place the 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 bound UDP socket.


13.  Deployment Considerations

13.1.  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



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   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.

13.2.  Mapping Quota

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


14.  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.

14.1.  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



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   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
   implement a PCP security mechanism described in Section 14.2 MUST
   enforce the constraints described in the paragraph above.

14.1.1.  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.






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14.1.2.  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.

14.1.2.1.  Residential Gateway Deployment

   Parity with many currently-deployed residential gateways can be
   achieved using a PCP Server that is constrained as described in
   Section 14.1.1 above.

14.1.2.2.  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.

   DS-Lite's security policies may also permit use of the MAP Opcode.

14.2.  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.




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   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 14.1.2.2).

   o  Implementations that wish to support any deployment scenario that
      does not meet the constraints described in Section 14.1.

   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 14.1
   above, in their default configuration, when processing
   unauthenticated requests.

14.3.  Residual Threats

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

14.3.1.  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 13.2 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 would not be aware the PCP server might have actually
   processed the PCP request.







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14.3.2.  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.

14.3.3.  Mapping Theft

   In the time between when a PCP server loses state and the PCP client
   notices the lower than expected Epoch value, it is possible that the
   PCP client's mapping will be acquired by another host (via an
   explicit dynamic mapping or implicit dynamic mapping).  This means
   incoming traffic will be sent to a different host ("theft").  A
   mechanism to immediately inform the PCP client of state loss would
   reduce this interval, but would not 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).

14.3.4.  Attacks Against Server Discovery

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


15.  IANA Considerations

   IANA is requested to perform the following actions:

15.1.  Port Number

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

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







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15.2.  Opcodes

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

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

   The values 0 and 127 are 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].

15.3.  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].

15.4.  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 7.7.1 and Section 11.  The Option
   values 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].


16.  Acknowledgments

   Thanks to Xiaohong Deng, Alain Durand, Christian Jacquenet, Jacni
   Qin, Simon Perreault, James Yu, Tina TSOU (Ting ZOU), Felipe Miranda



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   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 Margaret Wasserman for writing the Security Considerations
   section.


17.  References

17.1.  Normative References

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

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

   [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>.





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17.2.  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,
              "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-10 (work in
              progress), February 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.dupont-pcp-dslite]
              Dupont, F., Tsou, T., and J. Qin, "The Port Control
              Protocol in Dual-Stack Lite environments",
              draft-dupont-pcp-dslite-00 (work in progress),
              August 2011.

   [I-D.ietf-behave-lsn-requirements]
              Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common requirements for Carrier Grade NAT
              (CGN)", draft-ietf-behave-lsn-requirements-03 (work in
              progress), August 2011.

   [I-D.ietf-behave-sctpnat]
              Stewart, R., Tuexen, M., and I. Ruengeler, "Stream Control
              Transmission Protocol (SCTP) Network Address Translation",



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              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.

   [IGD]      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
              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.




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   [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
   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.




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Appendix B.  Change History

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

B.1.  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 requested external
      ports, and have implied PREFER_FAILURE semantics.

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

B.2.  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.



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

   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.3.  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 8).

   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)







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B.4.  Changes from draft-ietf-pcp-base-11 to -12

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

B.5.  Changes from draft-ietf-pcp-base-10 to -11

   o  clarified what can cause CANNOT_PROVIDE_EXTERNAL_PORT error to be
      generated.

B.6.  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.7.  Changes from draft-ietf-pcp-base-08 to -09

   o  Clarified in PEER Opcode introduction (Section 10) 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 12.3.1, 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 8.4).

   o  MAP errors now copy the Requested IP 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).




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   o  created new Implementation Considerations section (Section 12)
      which discusses non-normative things that might be useful to
      implementors.  Some new text is in here, and the Failure Scenarios
      text (Section 12.3) has been moved to here.

   o  Tweaked wording of EDM NATs in Section 12.1 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 9.5.

   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.8.  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 12.1), 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



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      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).

   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.9.  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




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   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.

   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.10.  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.



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   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.

   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



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      wants mapping to continue to exist, client has to continue to send
      recurring PEER messages.

B.11.  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.12.  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.



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   o  Allow arbitrary Options to be included in PCP response, so that
      PCP server can indicate un-supported PCP Options.  Satisfies PCP
      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.13.  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.14.  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.15.  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 IGD 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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