One document matched: draft-ietf-pcp-base-24.xml
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<rfc category="std" docName="draft-ietf-pcp-base-24" ipr="trust200902">
<front>
<title abbrev="Port Control Protocol (PCP)">Port Control Protocol
(PCP)</title>
<author fullname="Dan Wing" initials="D." role="editor" surname="Wing">
<organization abbrev="Cisco">Cisco Systems, Inc.</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<city>San Jose</city>
<region>California</region>
<code>95134</code>
<country>USA</country>
</postal>
<email>dwing@cisco.com</email>
</address>
</author>
<author fullname="Stuart Cheshire" initials="S." surname="Cheshire">
<organization abbrev="Apple">Apple Inc.</organization>
<address>
<postal>
<street>1 Infinite Loop</street>
<city>Cupertino</city>
<region>California</region>
<code>95014</code>
<country>USA</country>
</postal>
<phone>+1 408 974 3207</phone>
<email>cheshire@apple.com</email>
</address>
</author>
<author fullname="Mohamed Boucadair" initials="M." surname="Boucadair">
<organization>France Telecom</organization>
<address>
<postal>
<street></street>
<city>Rennes</city>
<region></region>
<code>35000</code>
<country>France</country>
</postal>
<email>mohamed.boucadair@orange-ftgroup.com</email>
</address>
</author>
<author fullname="Reinaldo Penno" initials="R." surname="Penno">
<organization>Juniper Networks</organization>
<address>
<postal>
<street>1194 N Mathilda Avenue</street>
<city>Sunnyvale</city>
<region>California</region>
<code>94089</code>
<country>USA</country>
</postal>
<email>rpenno@juniper.net</email>
</address>
</author>
<author fullname="Paul Selkirk" initials="P." surname="Selkirk">
<organization abbrev="ISC">Internet Systems Consortium</organization>
<address>
<postal>
<street>950 Charter Street</street>
<city>Redwood City</city>
<region>California</region>
<code>94063</code>
<country>USA</country>
</postal>
<email>pselkirk@isc.org</email>
</address>
</author>
<date />
<workgroup>PCP working group</workgroup>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="introduction" title="Introduction">
<t>The Port Control Protocol (PCP) provides a mechanism to
control how incoming packets are forwarded by upstream devices
such as Network Address Translator IPv6/IPv4 (NAT64), Network
Address Translator IPv4/IPv4 (NAT44), 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 Customer Premise
Equipment (CPE) routers, and all of the currently-known
transition scenarios towards IPv6-only CPE routers. PCP allows
hosts to operate servers for a long time (e.g., a 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.</t>
<t>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.</t>
<t>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
<xref target="I-D.cheshire-dnsext-dns-sd"></xref> would use DNS Update
<xref target="RFC2136"></xref> <xref target="RFC3007"></xref>. 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.</t>
<t>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.</t>
<t>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, reducing
the incentive to deploy ALGs in NATs and firewalls.</t>
</section>
<section title="Scope">
<section title="Deployment Scenarios">
<t>PCP can be used in various deployment scenarios, including:<list
style="symbols">
<t><xref target="RFC3022">Basic NAT</xref></t>
<t><xref target="RFC3022">Network Address and Port
Translation</xref>, such as commonly deployed in residential NAT
devices</t>
<t><xref target="I-D.ietf-behave-lsn-requirements">Carrier-Grade
NAT</xref></t>
<t><xref target="RFC6333">Dual-Stack Lite (DS-Lite)</xref></t>
<t><xref target="I-D.miles-behave-l2nat">Layer-2 Aware
NAT</xref></t>
<t><xref target="I-D.arkko-dual-stack-extra-lite">Dual-Stack Extra
Lite</xref></t>
<t>NAT64, both <xref target="RFC6145">Stateless</xref> and
<xref target="RFC6146">Stateful</xref></t>
<t>IPv4 and <xref target="RFC6092">IPv6 simple firewall
control</xref></t>
<t><xref target="RFC6296">IPv6-to-IPv6 Network Prefix Translation (NPTv6)</xref></t>
</list></t>
</section>
<section title="Supported Protocols">
<t>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, <xref target="RFC4960">SCTP</xref>,
<xref target="RFC4340">DCCP</xref>). Protocols that do not use
a port number (e.g., RSVP, <xref target="RFC4303">IPsec ESP</xref>, ICMP, ICMPv6) are
supported for IPv4 firewall, IPv6 firewall, and NPTv6
functions, but are out of scope for any NAT functions.</t>
</section>
<section anchor="single_homed" title="Single-homed Customer Premises Network">
<t>PCP assumes a single-homed IP address model. That is, for a given
IP address of a host, only one default route exists to reach the
Internet from that source IP address.
This is important because after a PCP mapping is created and
an inbound packet (e.g., TCP SYN) arrives at the host, the outbound
response (e.g., TCP SYNACK) has to go through the same path so it is
seen by the firewall or rewritten by the NAT. This restriction exists
because otherwise there would need to be a PCP-enabled NAT for every
egress (because the host could not reliably determine which egress
path packets would take) and the client would need to be able to
reliably make the same internal/external mapping in every NAT gateway,
which in general is not possible (because the other NATs might have
the necessary port mapped to another host).</t>
</section>
</section>
<section anchor="terminology" title="Terminology">
<t>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
<xref target="RFC2119">"Key words for use in RFCs to Indicate Requirement
Levels"</xref>.</t>
<t><list style="hanging">
<t hangText="Internal Host:"><vspace blankLines="0" />A host served
by a NAT gateway, or protected by a firewall. This is the host
that will receive incoming traffic resulting from a PCP mapping request, or
the host that initiated an implicit dynamic outbound mapping (e.g., by
sending a TCP SYN) across a firewall or a NAT.</t>
<t hangText="Remote Peer Host:"><vspace blankLines="0" />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 (see Section 6 of <xref target="RFC4787"></xref>
for definition of hairpin).</t>
<t hangText="Internal Address:"><vspace blankLines="0" />The address
of an Internal Host served by a NAT gateway or protected by a
firewall.</t>
<t hangText="External Address:"><vspace blankLines="0" />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.</t>
<t hangText="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 mapping
request for that Internal Address, Protocol, and Port may be assigned
yet another different External Port. This term encompasses
both Address-Dependent Mapping and Address and
Port-Dependent Mapping <xref target="RFC4787"></xref>.</t>
<t hangText="Remote Peer Address:"><vspace blankLines="0" />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.</t>
<t hangText="Third Party:"><vspace blankLines="0" />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. <vspace blankLines="1" /> 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.</t>
<t hangText="Mapping, Port Mapping, Port Forwarding:"><vspace
blankLines="0" />A NAT mapping creates a relationship between an
internal IP address, protocol, and port, and an external IP address,
protocol, and port. More specifically, it creates a translation rule
where packets destined to the external IP and port are translated to
the internal IP and port, and vice versa. In the case of a pure
firewall, the "Mapping" is the identity function, translating an
internal IP address and port number to the same external IP address
and port number. Firewall filtering, applied in addition to that identity
mapping function, is separate from the mapping itself.</t>
<t hangText="Mapping Types:"><vspace blankLines="0" />
There are two dimensions to classifying mapping types: how they are
created (implicitly/explicitly) and their primary purpose
(outbound/inbound).<list style="symbols">
<t>Implicit dynamic mappings are created implicitly as a
side-effect of traffic such as an outgoing TCP SYN or
outgoing UDP packet. Such packets were not originally
designed explicitly for creating NAT (or firewall) state, but they can
have that effect when they pass through a NAT (or firewall) device.</t>
<t>Explicit dynamic mappings are created as a result of
explicit PCP MAP and PEER requests.
Like a DHCP address lease, explicit dynamic mappings have
finite lifetime, and, as with a DHCP address lease, if a
client wants a mapping to persist the client must prove that
it is still present by periodically renewing the mapping to
prevent it from expiring. If a PCP client goes away, then
any mappings it created will be automatically cleaned up
when they expire.</t>
<t>Explicit static mappings are created by manual configuration
(e.g., via command-line interface or other user
interface) and persist until the user changes that manual
configuration.</t>
</list>
Both implicit and explicit dynamic mappings are dynamic in the
sense that they are created on demand, as requested
(implicitly or explicitly) by the Internal Host, and have a
lifetime. After the lifetime, the mapping is deleted unless
the lifetime is extended by action by the Internal Host (e.g.,
sending more traffic or sending a new PCP request).
<vspace blankLines="1" />
Explicit static mappings differ from explicit dynamic mappings in that
their lifetime is effectively infinite (they exist until manually removed)
but otherwise they behave exactly the same as explicit MAP mappings.
<vspace blankLines="1" />
While all mappings are by necessity bidirectional (most Internet communication
requires information to flow in both directions for successful operation) it
can be helpful when talking about mappings to identify them loosely according
to their 'primary' purpose.
<list style="symbols">
<t>Outbound mappings exist primarily to enable outbound communication. For example, when a host calls connect() to make an outbound connection, a
NAT gateway will create an implicit dynamic outbound mapping to facilitate that
outbound communication.</t>
<t>Inbound mappings exist primarily to enable listening servers to receive inbound
connections. Generally, when a client calls listen() to listen for inbound
connections, a NAT gateway will not implicitly create any mapping to facilitate
that inbound communication. A PCP MAP request can be used explicitly to create
a dynamic inbound mapping to enable the desired inbound communication.</t>
</list>
Explicit static (manual) mappings and explicit dynamic (MAP) mappings
both allow Internal Hosts to receive inbound traffic that is not in direct
response to any immediately preceding outbound communication (i.e., to
allow Internal Hosts to operate a "server" that is accessible to other
hosts on the Internet).</t>
<t hangText="PCP Client:"><vspace blankLines="0" />A PCP software
instance responsible for issuing PCP requests to a PCP server.
Unlike some other NAT and firewall control applications which
have to be embedded in the underlying operating system or framework,
several independent PCP Clients can exist on the same host.
Several PCP Clients can be located in the same local network. A PCP
Client can issue PCP requests on behalf of a third party device for
which it is authorized to do so. An interworking function from
Universal Plug and Play Internet Gateway Device (UPnP IGDv1
<xref target="IGDv1"></xref>) 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.</t>
<t hangText="PCP-Controlled Device:"><vspace blankLines="0" />A
NAT or firewall that controls or rewrites packet flows between
internal hosts and remote peer hosts. PCP manages the Mappings
on this device.</t>
<t hangText="PCP Server:"><vspace blankLines="0" />A PCP software
instance that resides on the NAT or firewall that receives PCP
requests from the PCP client and creates appropriate state in
response to that request.</t>
<!--
<t hangText="Interworking Function:"><vspace blankLines="0" />A
functional element responsible for translating or proxying another
protocol to PCP. For example interworking
<xref target="IGDv1">UPnP IGDv1</xref> with PCP.</t>
-->
<t hangText="Subscriber:"><vspace blankLines="0" /> 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.</t>
<!--
<t hangText="5-tuple"><vspace blankLines="0" />The 5 pieces
of information that uniquely identify a flow at a given
place and time: transport protocol, source IP address and
port, destination IP address and port. When NAT is being
used, addresses and ports for a given flow are rewritten as
packets pass through the NAT, so for the same flow Internal
Hosts see Internal Addresses, and Remote Peers see External
Addresses. Most NATs configure their Internal Hosts via DHCP
to have different Internal Addresses. Some deployment
scenarios (e.g., Dual-Stack Lite, Layer 2-Aware NAT) allow multiple hosts to
use the same Internal Address at the same time and use
additional information (such as IPv6 tunnel identifier or
VLAN identifier) to distinguish 5-tuples.</t>
-->
</list></t>
</section>
<?rfc needLines="15" ?>
<section anchor="relationship" title="Relationship between PCP Server and its NAT/firewall">
<t>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 <xref target="diagram_pcp_server"></xref>. It is also possible
for the PCP functionality to be provided by some other device, which
communicates with the actual NAT(s) or firewall(s) via some other proprietary
mechanism, as long as from the PCP client's perspective such split
operation is indistinguishable from the integrated case.</t>
<figure anchor="diagram_pcp_server" title="PCP-Enabled NAT or Firewall">
<artwork align="center"><![CDATA[
+-----------------+
+------------+ | NAT or firewall |
| PCP client |-<network>-+ with +---<Internet>
+------------+ | PCP server |
+-----------------+]]></artwork>
</figure>
<t>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)
<xref target="RFC6092"></xref>. 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.</t>
</section>
<section anchor="fixed-size-addr" title="Note on Fixed-Size Addresses">
<t>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.</t>
<t>When the address field holds an IPv6 address, the fixed-size
128-bit IP address field holds the IPv6 address stored as-is.</t>
<t>When the address field holds an IPv4
address, <xref target="RFC4291">IPv4-mapped IPv6
addresses</xref> are used (::ffff:0:0/96). This has the first
80 bits set to zero and the next 16 set to one, while its
last 32 bits are filled with the IPv4 address. This is
unambiguously distinguishable from a native IPv6 address,
because <xref target="RFC4291">IPv4-mapped IPv6
address</xref> would not be used for mappings.</t>
<t>When checking for an IPv4-mapped IPv6 address, all of the
first 96 bits MUST be checked for the pattern -- it is not
sufficient to check for ones in bits 81-96.</t>
<t>The all-zeroes IPv6 address MUST be expressed by filling the
fixed-size 128-bit IP address field with all zeroes (::).</t>
<t>The all-zeroes IPv4 address MUST be expressed by 80 bits of
zeros, 16 bits of ones, and 32 bits of zeros (::ffff:0:0).</t>
</section>
<section title="Protocol Design Note" anchor="protocol_design_note">
<t>PCP can be viewed as a request/response protocol, much like many other
UDP-based request/response protocols, and can be implemented perfectly well as such.
It can also be viewed as what might be called a hint/notification protocol,
and this observation can help simplify implementations.</t>
<t>Rather than viewing the message streams between PCP client and PCP server
as following a strict request/response pattern, where every response is associated
with exactly one request, the message flows can be viewed as two somewhat independent
streams carrying information in opposite directions:
<list style="symbols">
<t>A stream of hints flowing from PCP client to PCP server, where the client
indicates to the server what it would like the state of its mappings to be, and</t>
<t>A stream of notifications flowing from PCP server to PCP client, where the server
informs the clients what the state of its mappings actually is.</t>
</list>
To an extent, some of this approach is required anyway in a UDP-based request/response
protocol, since UDP packets can be lost, duplicated, or reordered.</t>
<t>In this view of the protocol, the client transmits hints to the
server at various intervals signaling its desires, and the server
transmits notifications to the client signaling the actual state of its
mappings. These two message flows are loosely correlated in that
a client request (hint) usually elicits a server response (notification),
but only loosely, in that a client request may result in no server response
(in the case of packet loss) and a server response may be generated gratuitously
without an immediately preceding client request (in the case where server
configuration change, e.g. change of external IP address on a NAT gateway,
results in a change of mapping state).</t>
<t>The exact times that client requests are sent are influenced by a
client timing state machine taking into account
(i) if the client has not yet received a response from the server for
a prior request (retransmission), and
(ii) if the client has previously received a response from the server
saying how long the indicated mapping would remain active (renewal).
This design philosophy is the reason why PCP's retransmissions and renewals are
exactly the same packet on the wire. Typically, retransmissions are sent with
exponentially increasing intervals as the client waits for the server to respond,
whereas renewals are sent with exponentially decreasing intervals as the
expiry time approaches, but from the server's point of view both packets
are identical, and both signal the client's desire that the stated
mapping exist or continue to exist.</t>
<t>A PCP server usually sends responses as a direct result of client requests,
but not always. For example, if a server is too overloaded to respond, it is allowed
to silently ignore a request message and let the client retransmit. Also, if
external factors cause a NAT gateway or firewall's configuration to change,
then the PCP server can send unsolicited responses to clients informing them
of the new state of their mappings. Such reconfigurations are expected to be rare,
because of the disruption they can cause to clients, but should they happen,
PCP provides a way for servers, at their discretion, to communicate the new state
to clients promptly, without having to wait for the next periodic renewal request.
</t>
<t>This design goal helps explain why PCP request and response messages
have no transaction ID, because such a transaction ID is unnecessary,
and would unnecessarily limit the protocol and unnecessarily complicate
implementations. A PCP server response (i.e. notification) is self-describing
and complete. It communicates the internal and external addresses and ports
for a mapping, and its remaining lifetime. If the client does in fact
currently want such a mapping to exist then it can identify the mapping in question
from the internal address, protocol, and port, and update its state to reflect the
current external address and port, and remaining lifetime. If a client does not
currently want such a mapping to exist then it can safely ignore the message.
No client action is required for unexpected mapping notifications. In today's
world a NAT gateway can have a static mapping, and the client device has
no explicit knowledge of this, and no way to change the fact. Also, in today's
world a client device can be connected directly to the public Internet, with
a globally-routable IP address, and in this case it effectively has "mappings"
for all of its listening ports. Such a device has to be responsible for its own
security, and cannot have its security rely on assuming that some other network
device will be blocking all incoming packets.</t>
</section>
<section title="Common Request and Response Header Format">
<t>All PCP messages are sent over UDP, with a maximum length of
1024 bytes. The 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
<xref target="map_opcodes"></xref> and
<xref target="peer_opcodes"></xref>.</t>
<section title="Request Header">
<figure align="left" anchor="common_request" title="Common Request Packet Format">
<preamble>All requests have the following format:</preamble>
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| Opcode | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Lifetime (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Client's IP Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) Opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Version:">This document specifies protocol
version 1. PCP clients and servers compliant with this
document use the value 1. This field is used for version
negotiation as described in
<xref target="version"></xref>.</t>
<t hangText="R:">Indicates Request (0) or Response (1).</t>
<t hangText="Opcode:">A seven-bit value specifying the operation
to be performed. Opcodes are defined in
<xref target="map_opcodes"></xref> and
<xref target="peer_opcodes"></xref>.</t>
<!--
<t hangText="PCP Client's Port:">The source port in the UDP header
used by the PCP client when sending this PCP request.</t>
-->
<t hangText="Reserved:">16 reserved bits. MUST be 0 on transmission and
MUST be ignored on reception.</t>
<t hangText="Requested Lifetime:">An unsigned 32-bit
integer, in seconds, ranging from 0 to 2^32-1
seconds. This is used by the MAP and PEER Opcodes defined
in this document for their requested lifetime.</t>
<t hangText="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. This is used to detect an on-path NAT, see
<xref target="pcp_processing_response"></xref>.</t>
<t hangText="Opcode-specific information:">Payload data for
this Opcode. The length of this data is determined by the Opcode
definition.</t>
<t hangText="PCP Options:">Zero, one, or more Options that
are legal for both a PCP request and for this Opcode. See
<xref target="options"></xref>.</t>
</list></t>
</section>
<section title="Response Header">
<figure align="left" anchor="common_response" title="Common Response Packet Format">
<preamble>All responses have the following format:</preamble>
<artwork align="center"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| Opcode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch Time (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Reserved (96 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) Opcode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Options :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Version:">Responses MUST use version 1. This is set by the server.</t>
<t hangText="R:">Indicates Request (0) or Response (1). All
Responses MUST use 1. This is set by the server.</t>
<t hangText="Opcode:">The 7-bit Opcode value. The server copies this value from the
request.</t>
<t hangText="Reserved:">8 reserved bits, MUST be sent as 0, MUST
be ignored when received. This is set by the server.</t>
<t hangText="Result Code:">The result code for this response. See
<xref target="result_codes"></xref> for values. This is set by the
server.</t>
<t hangText="Lifetime:">An unsigned 32-bit integer, in
seconds, ranging from 0 to 2^32-1 seconds. On an
error response, this indicates how long clients should
assume they'll get the same error response from that PCP
server if they repeat the same request. On a success
response for the PCP Opcodes that create a mapping (MAP
and PEER), the Lifetime field indicates the lifetime for
this mapping. This is set by the server.</t>
<t hangText="Epoch Time:">The server's Epoch time value. See
<xref target="epoch"></xref> for discussion. This value is set by the server, in both success and error responses.</t>
<t hangText="Reserved:">96 reserved bits. For requests
that were successfully parsed, this MUST be sent as 0,
MUST be ignored when received. This is set by the server.
For requests that were not successfully parsed, the server
copies the last 96 bits of the PCP Client's IP Address
field from the request message into this corresponding 96
bit field of the response.</t>
<t hangText="Opcode-specific information:">Payload data for
this Opcode. The length of this data is determined by the Opcode
definition.</t>
<t hangText="PCP Options:">Zero, one, or more Options that
are legal for both a PCP response and for this Opcode. See
<xref target="options"></xref>.</t>
</list></t>
</section>
<section anchor="options" title="Options">
<t>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.</t>
<!--
(However, note that in the case of THIRD_PARTY, for requests
containing the THIRD_PARTY Option a PCP server that doesn't
implement the THIRD_PARTY Option still needs to return a
NOT_AUTHORIZED error rather than UNSUPP_OPCODE. This is to avoid
leaking the capabilities of the PCP server to unauthorized clients.)</t>
-->
<figure align="left" anchor="options-layout" title="Options Header">
<preamble>Options use the following Type-Length-Value
format:</preamble>
<artwork align="center"><![CDATA[
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 :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The description of the fields is as follows:<list style="hanging">
<t hangText="Option Code:">8 bits. Its most significant bit
indicates if this Option is mandatory (0) or optional (1) to
process.</t>
<t hangText="Reserved:">8 bits. MUST be set to 0 on transmission
and MUST be ignored on reception.</t>
<t hangText="Option Length:">16 bits. Indicates the length of the
enclosed data, in octets. Options with length of 0 are allowed.
Options that are not a multiple of four octets long are followed
by one, two, or three zero octets to pad their effective length
in the packet to be a multiple of four octets. The Option Length
reflects the semantic length of the option, not including any
padding octets.</t>
<t hangText="data:">Option data.</t>
</list></t>
<t>If several Options are
included in a PCP request, they MAY be encoded in any order by
the PCP client, but MUST be processed by the PCP server in the
order in which they appear. It is the responsibility of the
PCP client to ensure the server has sufficient room to reply
without exceeding the 1024-byte size limit; if its reply would
exceed that size, the server generates an error.</t>
<t>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). Such an error response MUST consist of a complete copy of the request
packet with the error code and other appropriate fields set in the header.
An Option MAY appear more than once in a request or in a
response, if permitted by the definition of the Option. If the
Option's definition allows the Option to appear only once but it
appears more than once in a request, and the Option is understood
by the PCP server, the PCP server MUST respond with
the MALFORMED_OPTION result code.
If the PCP server encounters an invalid option
(e.g., option length is longer than the UDP packet length)
the error MALFORMED_OPTION SHOULD
be returned (rather than MALFORMED_REQUEST), as that helps the
client better understand how the packet was malformed.
If a PCP response would have
exceeded the maximum PCP message size, the PCP server SHOULD
respond with MALFORMED_REQUEST.</t>
<t>If an Option cannot successfully be parsed, the PCP server
MUST generate an error response with code MALFORMED_OPTION.
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 return the error
UNSUPP_OPTION if the Option is unrecognized, unimplemented, or
disabled, or if the client is not authorized to use the
Option. All processed options are included in successful
responses, but unprocessed options are not included in
successful responses. All options are returned in error
responses.</t>
<t>PCP clients are free to ignore any or all Options included in
responses, although naturally if a client explicitly requests an
Option where correct execution of that Option requires processing the
Option data in the response, that client is expected to implement code
to do that.</t>
<t>Different options are valid for different Opcodes. For example:
<list style="symbols">
<t>The THIRD_PARTY Option is valid for both MAP and PEER Opcodes.</t>
<t>The FILTER Option is valid only for the MAP Opcode
(for the PEER Opcode it would have no meaning).</t>
<t>The PREFER_FAILURE Option is valid
only for the MAP Opcode (for the PEER Opcode, similar semantics
are automatically implied).</t>
</list>
</t>
</section>
<?rfc needLines="15" ?>
<section anchor="result_codes" title="Result Codes">
<t>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.
<list style="hanging">
<t hangText="0">SUCCESS: Success.</t>
<t hangText="1">UNSUPP_VERSION: Unsupported protocol version. This is a long lifetime error.</t>
<t hangText="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.</t>
<!--
NOT_AUTHORIZED MUST be
returned if the client uses the THIRD_PARTY Option and the PCP
Server (a) does not support THIRD_PARTY requests, or (b) supports
THIRD_PARTY requests but has be configured to disallow them, or
(c) supports THIRD_PARTY requests and has be configured to allow
them for some clients but this client did not pass the PCP
Server's authorization check.</t>
-->
<t hangText="3">MALFORMED_REQUEST: The request could
not be successfully parsed. This is a long lifetime error.</t>
<t hangText="4">UNSUPP_OPCODE: Unsupported Opcode. This is a long lifetime error.</t>
<t hangText="5">UNSUPP_OPTION: Unsupported Option. This error only
occurs if the Option is in the mandatory-to-process range. This is a long lifetime error.</t>
<t hangText="6">MALFORMED_OPTION: Malformed Option (e.g., appears
too many times, invalid length). This is a long lifetime error.</t>
<t hangText="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.</t>
<t hangText="8">NO_RESOURCES: Request is well-formed and
valid, but the server has insufficient resources to
complete the requested operation at this time. For
example, the NAT device cannot create more mappings at
this time, is short of CPU cycles or memory, or due to
some other temporary condition. The same request may
succeed in the future. This is a system-wide error,
different from USER_EX_QUOTA.
This can be used as a catch-all error, should no
other error message be suitable. This is a short lifetime error.</t>
<t hangText="9">UNSUPP_PROTOCOL: Unsupported Protocol. This is a
long lifetime error.</t>
<t hangText="10">USER_EX_QUOTA: This attempt to create a
new mapping would exceed this subscriber's port quota. This
is a short lifetime error.</t>
<t hangText="11">CANNOT_PROVIDE_EXTERNAL: the suggested
external port and/or external address cannot be provided.
This error MUST only be returned for PEER requests, for
MAP requests that included the PREFER_FAILURE Option
(because otherwise a new external port could have been
assigned), or MAP requests for the SCTP protocol.
See <xref target="prefer_failure"></xref> for processing
details. The error lifetime depends on the reason for the
failure.</t>
<t hangText="12">ADDRESS_MISMATCH: the source IP address of
the request packet does not match the contents of the PCP
Client's IP Address field. This is a long lifetime error.</t>
<t hangText="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 <xref target="filter"></xref> for processing
information. This is a long lifetime error.</t>
</list></t>
</section>
</section>
<section anchor="general" title="General PCP Operation">
<t>PCP messages MUST be sent
over <xref target="RFC0768">UDP</xref>.
Every PCP request generates at least one response, so PCP does
not need to run over a reliable transport protocol.</t>
<t>When receiving multiple identical requests, the PCP server will
generate identical responses, provided the PCP server's state
did not change between those requests due to other activity.
For example, if a request is received while the PCP-controlled
device has no mappings available, it will generate an error
response. If mappings become available and then a (duplicated
or re-transmitted) request is seen by the server, it will
generate a non-error response. A PCP client will need to
properly handle such updated responses for any request it sends,
most notably to support Mapping Update
(<xref target="update"></xref>). Also
see <xref target="protocol_design_note"></xref>.</t>
<section title="General PCP Client: Generating a Request" anchor="general_generate_request">
<t>This section details operation specific to a PCP client, for any
Opcode. Procedures specific to the MAP Opcode are described in
<xref target="map_opcodes"></xref>, and procedures specific to the PEER
Opcode are described in <xref target="peer_opcodes"></xref>.</t>
<t>Prior to sending its first PCP message, the PCP client determines
which server to use. The PCP client performs the following steps to
determine its PCP server: <list style="numbers">
<t>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;</t>
<t>the default router list (for IPv4 and IPv6) is used as
the list of PCP Server(s). Thus, if a PCP client has both
an IPv4 and IPv6 address, it will have an IPv4 PCP server
(its IPv4 default router) for its IPv4 mappings, and an
IPv6 PCP server (its IPv6 default router) for its IPv6
mappings.</t>
</list></t>
<t>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.</t>
<t>With that PCP server address, the PCP client formulates its PCP
request. The PCP request contains a PCP common header, PCP Opcode and
payload, and (possibly) Options. As with all UDP client
software on any operating system, when several independent PCP clients
exist on the same host, each uses a distinct source port number to
disambiguate their requests and replies. The PCP client's source port
SHOULD be randomly generated <xref target="RFC6056"></xref>.</t>
<t>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 <xref target="implement_source_address"></xref> for how
this can be coded.</t>
<!--
<t>When attempting to contact a PCP server, the PCP client
sends a PCP message to the first server in its list of PCP
servers, and SHOULD initialize a timer to a random value
between 2 and 3 seconds.</t>
-->
<section title="PCP Client Retransmission">
<t>PCP clients are responsible for reliable delivery of PCP request
messages. If a PCP client fails to receive an expected response
from a server, the client must retransmit its message. The client
begins the message exchange by transmitting a message to the
server. The message exchange terminates when either the client
successfully receives the appropriate response or responses from the
server, or when the message exchange is considered to
have failed according to the retransmission mechanism described
below.</t>
<t>The client retransmission behavior is controlled and described by the
following variables:
<list style="hanging" hangIndent="8">
<t hangText=" RT">Retransmission timeout, calculated as described below</t>
<t hangText=" IRT">Initial retransmission time, SHOULD be 3 seconds</t>
<t hangText=" MRC">Maximum retransmission count, SHOULD be 0 (0 indicates no maximum)</t>
<t hangText=" MRT">Maximum retransmission time, SHOULD be 1024 seconds</t>
<t hangText=" MRD">Maximum retransmission duration, SHOULD be 0 (0 indicates no maximum)</t>
<t hangText=" RAND">Randomization factor, calculated as described below</t>
</list></t>
<t>With each message transmission or retransmission, the client sets RT
according to the rules given below. If RT expires before a response
is received, the client recomputes RT and retransmits the
request.</t>
<t>Each of the computations of a new RT include a new randomization factor
(RAND), which is a random number chosen with a uniform distribution
between -0.1 and +0.1. The randomization factor is included to
minimize synchronization of messages transmitted by PCP clients.
The algorithm for choosing a random number does not need to be
cryptographically sound. The algorithm SHOULD produce a different
sequence of random numbers from each invocation of the PCP
client.</t>
<t>The RT value is initialized based on IRT:
<list style="empty"><t>RT = IRT + RAND*IRT</t></list></t>
<t>RT for each subsequent message transmission is based on the previous
value of RT:
<list style="empty"><t>RT = 2*RTprev + RAND*RTprev</t></list></t>
<t>MRT specifies an upper bound on the value of RT (disregarding the
randomization added by the use of RAND). If MRT has a value of 0,
there is no upper limit on the value of RT. Otherwise:</t>
<figure>
<artwork>
if (RT > MRT)
RT = MRT + RAND*MRT
</artwork>
</figure>
<t>MRC specifies an upper bound on the number of times a client may
retransmit a message. Unless MRC is zero, the message exchange fails
once the client has transmitted the message MRC times.</t>
<t>MRD specifies an upper bound on the length of time a client may
retransmit a message. Unless MRD is zero, the message exchange fails
once MRD seconds have elapsed since the client first transmitted the
message.</t>
<t>If both MRC and MRD are non-zero, the message exchange fails
whenever either of the conditions specified in the previous two
paragraphs are met. If both MRC and MRD are zero, the client
continues to transmit the message until it receives a response.</t>
<!--
<t>For as long as the client still desires the indicated
mapping, if no response is received before the timer expires
then the request is re-transmitted. These re-transmissions
are sent at a random value between 1.5 and 2.5 times the previous
transmission interval. The randomization helps avoid client
synchronization. This repeats until a maximum retransmission
interval between 512 and 1024 seconds, at which point the
retransmission continue at that interval, until a successful
response is received or the client no longer desires the
indicated mapping.</t>
-->
<t>Once a PCP client has successfully received a response from a PCP
server on that interface, it resets RT to its initial value and
sends subsequent PCP requests to that same server.</t>
<!--
<t>If, during its transmissions to a PCP server, the PCP client
receives a hard or soft ICMP error (<xref target="RFC5461"></xref>),
the ICMP error SHOULD be ignored.</t>
<t>Upon receiving a response (success or error), the PCP client does
not change to a different PCP server. That is, it does not "shop
around" trying to find a PCP server to service its (same) request.</t>
-->
</section>
</section>
<section title="General PCP Server: Processing a Request">
<t>This section details operation specific to a PCP server. Processing
SHOULD be performed in the order of the following paragraphs.</t>
<t>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.</t>
<t>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.</t>
<t>Error responses have the same packet layout as success
responses, with certain fields from the request copied into the
response, and other fields assigned by the PCP server set as
indicated in <xref target="common_response"></xref>.</t>
<t>Copying request fields into the response is important because this is what
enables a client to identify to which request a given
response pertains. For Opcodes that are understood by the PCP
server, it follows the requirements of that Opcode to copy the
appropriate fields. For Opcodes that are not understood by the
PCP server (including the ANNOUNCE Opcode), it simply generates the UNSUPP_OPCODE response and
copies fields from the PCP header and copies the rest of the
PCP payload as-is (without attempting to interpret it).</t>
<t>All responses (both error and success) contain the same Opcode
as the request, but with the "R" bit set.</t>
<?rfc needLines="11" ?>
<t>Any error response has a nonzero Result Code, and is created by:
<?rfc subcompact="yes"?>
<list style="symbols">
<t>Copying the entire request packet, or 1024 octets,
whichever is less, and zero-padding the response to a multiple
of 4 octets if necessary</t>
<t>Setting the R bit</t>
<t>Setting the Result Code</t>
<t>Setting the Lifetime, Epoch Time and Reserved fields</t>
<t>Updating other fields in the response, as indicated by 'set
by the server' in the PCP response field description.</t>
<?rfc subcompact="no"?>
</list>
</t>
<?rfc needLines="11" ?>
<t>A success response has a zero Result Code, and is created by:
<?rfc subcompact="yes"?>
<list style="symbols">
<t>Copying the first four octets of request packet header</t>
<t>Setting the R bit</t>
<t>Setting the Result Code to zero</t>
<t>Setting the Lifetime, Epoch Time and Reserved fields</t>
<t>Possibly setting opcode-specific response data if appropriate</t>
<t>Adding any processed options to the response message</t>
<?rfc subcompact="no"?>
</list>
</t>
<t>If the received PCP request message is less than two octets long it is silently dropped.</t>
<t>If the R bit is set the message is silently dropped.</t>
<t>If the first octet (version) is a version that is not supported,
a response is generated with the UNSUPP_VERSION result code, and the
other steps detailed in <xref target="version"/> are followed.</t>
<t>Otherwise, if the version is supported but the received
message is shorter than 24 octets, the message is silently dropped.</t>
<t>If the server is overloaded by requests (from a particular client
or from all clients), it MAY simply silently discard requests, as the requests
will be retried by PCP clients, or it MAY generate the
NO_RESOURCES error response.</t>
<t>If the length of the message exceeds 1024 octets, is not a
multiple of 4 octets, or is too short for the opcode in
question, it is invalid and a MALFORMED_REQUEST response is
generated, and the response message is truncated to 1024 octets.</t>
<t>The PCP server compares the source IP address (from the
received IP header) with the field PCP Client IP Address. If
they do not match, the error ADDRESS_MISMATCH MUST be returned.
This is done to detect and prevent accidental use of PCP where a
non-PCP-aware NAT exists between the PCP client and PCP server.
If the PCP client wants such a mapping it needs to ensure the
PCP field matches its apparent IP address from the perspective
of the PCP server.</t>
</section>
<section title="General PCP Client: Processing a Response" anchor="pcp_processing_response">
<t>The PCP client receives the response and verifies that the source
IP address and port belong to the PCP server of a previously-sent PCP
request. If not, the response is silently dropped.</t>
<t>If the received PCP response message is less than four octets
long it is silently dropped.</t>
<t>If the R bit is clear the message is silently dropped.</t>
<t>If the error code is UNSUPP_VERSION processing continues as
described in <xref target="version"></xref>.</t>
<t>The PCP client then validates that the Opcode matches a
previous PCP request. Responses shorter than 24 octets,
longer than 1024 octets, or not a multiple of 4 octets are
invalid and ignored, likely causing the request to be
re-transmitted. The response is further matched by comparing
fields in the response Opcode-specific data to fields in the
request Opcode-specific data, as described by the processing
for that Opcode. After these matches are successful, the PCP
client checks the Epoch Time field to determine if it needs to
restore its state to the PCP server
(see <xref target="epoch"></xref>). A PCP client SHOULD be
prepared to receive multiple responses from the PCP Server at
any time after a single request is sent.
This allows the PCP
server to inform the client of mapping changes such as an
update or deletion. For example, a PCP Server might send a
SUCCESS response and, after a configuration change on the PCP
Server, later send a NOT_AUTHORIZED response. A PCP client MUST
be prepared to receive responses for requests it never sent
(which could have been sent by a previous PCP instance on this
same host, or by a previous host that used the same client IP
address) by simply ignoring those unexpected messages.</t>
<t>If the error ADDRESS_MISMATCH is received, it indicates the
presence of a NAT between the PCP client and PCP server.
Procedures to resolve this problem are beyond the scope of
this document.</t>
<t>For both success and error responses a Lifetime value is
returned. The Lifetime indicates how long this request is
considered valid by the server. The PCP client SHOULD impose
an upper limit on this returned value (to protect against
absurdly large values, e.g., 5 years), detailed
in <xref target="lifetime"></xref>.</t>
<t>If the result code is 0 (SUCCESS), the request succeeded.</t>
<t>If the result code is not 0, the request failed. If the
result code is NO_RESOURCES, the PCP client SHOULD NOT send
further requests for new mappings to that PCP server for the
value of the Lifetime (limited by the sanity checking detailed
in <xref target="lifetime"></xref>). If a request for renewal or
deletion of an existing mapping results in NO_RESOURCES, the
PCP client SHOULD NOT send further requests of any kind to
that PCP server for the (limited) value of the Lifetime. For
other error result codes, the PCP client SHOULD NOT resend the
same request for the (limited) value of the Lifetime.</t>
<t>If the PCP client has discovered a new PCP server (e.g., connected
to a new network), the PCP client MAY immediately begin communicating
with this PCP server, without regard to hold times from communicating
with a previous PCP server.</t>
</section>
<section anchor="mif" title="Multi-Interface Issues">
<t>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, <xref target="RFC3587"></xref>) or limited scope
as for <xref target="RFC4193">Unique Local Address (ULA)</xref>).</t>
<t>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., <xref target="RFC4193">ULA</xref>), SHOULD NOT be used
as the source interface when generating a PCP
request. If <xref target="RFC4941">IPv6 privacy
addresses</xref> are used for PCP mappings, a new PCP request
will need to be issued whenever the IPv6 privacy address is
changed. This PCP request SHOULD be sent from the IPv6 privacy
address itself. It is RECOMMENDED that the client delete its
mappings to the previous privacy address after it no longer needs
those old mappings.</t>
<t>Due to the ubiquity of IPv4 NAT, IPv4 addresses with limited scope
(e.g., <xref target="RFC1918">private addresses</xref>) MAY be used as
the source interface when generating a PCP request.</t>
<!--
<t>As mentioned in <xref target="single_homed"></xref>, 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.</t>
-->
<!--
Section 2.4, "Change of the Internal IP Address"
When a new IP address is assigned to a host implementing a PCP Client,
the PCP Client MUST install on the PCP server all the mappings it
manages, using the new assigned IP address as the internal IP
address.
This needs some additional discussion. Let's take an example where
I have a computer with both WiFi and wired Ethernet connections
and I plug and unplug the Ethernet connection, or go in and out
of range of WiFi. When do we want PCP to do something about
that change of connectivity? Does the answer depend on IPv4,
IPv6, or dual stack running on that interface if we're doing PIN44,
PIN64, PIN46, PIN66?
Med: Good point. IMHO, the address selection should be done according to the-be-completed
section: http://tools.ietf.org/html/draft-ietf-pcp-base-02#section-6.1.3. Based on the
output of that process, the PCP Client can be triggered accordingly.
Or ... just recommend the internal IP address not be changed!
-->
</section>
<section anchor="epoch" title="Epoch">
<t>Every PCP response sent by the PCP server includes an Epoch
time field. This time field increments by one every second.
Anomalies in the received Epoch time value provide a hint to PCP
clients that a PCP server state loss may have occurred. Clients
respond to such state loss hints by promptly renewing their mappings,
so as to quickly restore any lost state at the PCP server.</t>
<t>If the PCP server resets or loses the state of its
explicit dynamic Mappings (that is, those mappings created by
PCP requests), due to reboot, power failure, or any other
reason, it MUST reset its Epoch time to its initial starting
value (usually zero) to provide this hint to PCP clients.
After resetting its Epoch time, the PCP server resumes incrementing the
Epoch time value by one every second. Similarly, if the public IP
address(es) of the NAT (controlled by the PCP server) changes,
the Epoch time MUST be reset. A PCP server MAY maintain one Epoch time
value for all PCP clients, or MAY maintain distinct Epoch time
values (per PCP client, per interface, or based on other
criteria); this choice is implementation-dependent.</t>
<t>Whenever a client receives a PCP response, the client
validates the received Epoch time value according to the
procedure below, using integer arithmetic:
<list style="symbols">
<t>If this is the first PCP response the client has received from
this PCP server, the Epoch time value is treated as necessarily valid, otherwise
<list style="symbols">
<t>If the current PCP server Epoch time value (current_server_time)
is less than the previously received PCP server Epoch time value
(previous_server_time) then the client treats the Epoch time
value as obviously invalid (time should not go backwards), else
<list style="symbols">
<t>The client computes the difference between its
<vspace />
current local time value (current_client_time) and the
<vspace />
time the previous PCP response was received from this PCP
server (previous_client_time):
<vspace />
client_delta = current_client_time - previous_client_time;</t>
<t>The client computes the difference between the
<vspace />
current PCP server Epoch time value (current_server_time) and the
<vspace />
previously received Epoch time value (previous_server_time):
<vspace />
server_delta = current_server_time - previous_server_time;</t>
<t>If client_delta+2 < server_delta - server_delta/16
<vspace />
or server_delta+2 < client_delta - client_delta/16
<vspace />
then the client treats the Epoch time value as invalid,
<vspace />
else the client treats the Epoch time value as valid</t>
</list></t>
</list></t>
<t>The client records the current time values for use in its next comparison:
<vspace />
previous_client_time = current_client_time
<vspace />
previous_server_time = current_server_time</t>
</list></t>
<t>If the PCP client determined that the Epoch time value it
received was invalid then it concludes that the PCP server may
have lost state, and promptly renews all its active port mapping
leases as described in <xref target="reboot"></xref>.</t>
<?rfc needLines="8" ?>
<t>Notes:<list style="symbols">
<t>The client clock MUST never go backwards. If current_client_time is found
to be less than previous_client_time then this is a client bug, and how
the client deals with this client bug is implementation specific.</t>
<t>The calculations above are constructed to allow
client_delta and server_delta to be computed as unsigned
integer values.</t>
<t>The "+2" in the calculations above is to accommodate
quantization errors in client and server clocks (up to one
second quantization error each in server and client time intervals).</t>
<t>The "/16" in the calculations above is to accommodate inaccurate clocks in
low-cost devices. This allows for a total discrepancy of up to 1/16 (6.25%)
to be considered benign, e.g., if one clock were to run too fast by 3% while the
other clock ran too slow by 3% then the client would not consider this
difference to be anomalous or indicative of a restart having occurred.
This tolerance is strict enough to be effective at detecting reboots,
while not being so strict as to generate false alarms.</t>
</list></t>
</section>
</section>
<?rfc needLines="7" ?>
<section anchor="version" title="Version Negotiation">
<t>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.</t>
<t>Should later updates to this document specify different
message formats with a version number greater than 1, and
backwards compatibility is desired, this first octet
can be used for forward and backward compatibility.</t>
<t>If future PCP versions greater than 1 are specified, version
negotiation proceeds as follows: <list style="numbers">
<!--
<t>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.</t>
-->
<t>The client sends its first request using the highest (i.e., presumably
'best') version number it supports.</t>
<t>If the server supports that version it responds normally.</t>
<t>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).</t>
<t>If the client receives an UNSUPP_VERSION result containing a
version it does support, it records this fact and proceeds to use
this message version for subsequent communication with this PCP
server (until a possible future UNSUPP_VERSION response if the
server is later updated, at which point the version negotiation
process repeats).</t>
<!--
<t>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. By automatically retrying in 30 minutes, the protocol
accomodates an upgrade of the PCP server.</t>
-->
<t>If the client receives an UNSUPP_VERSION result
containing a version it does not support then the client
SHOULD try the next-lower version supported by the client.
The attempt to use the next-lower version repeats until
the client has tried version 1. If using version 1 fails,
the client MAY log an error message to inform the user
that it is too old to work with this server, and the
client SHOULD set a timer to retry its request in 30
minutes or the returned Lifetime value, whichever is
smaller. By automatically retrying in 30 minutes, the
protocol accomodates an upgrade of the PCP server.</t>
</list></t>
</section>
<?rfc needLines="8" ?>
<section anchor="opcode_introduction" title="Introduction to MAP and PEER Opcodes">
<t>There are four uses for the MAP and PEER Opcodes defined in this
document:<list style="symbols">
<t>a host operating a server and wanting an incoming connection
(<xref target="operating_a_server"></xref>);</t>
<t>a host operating a client and server on the same port
(<xref target="pcp_symmetric"></xref>);</t>
<t>a host operating a client and wanting to optimize the application
keepalive traffic (<xref target="keepalives"></xref>);</t>
<t>and a host operating a client and wanting to restore lost state
in its NAT (<xref target="restoring"></xref>).</t>
</list></t>
<t>These are discussed in the following sections, and a
(non-normative) state diagram is provided
in <xref target="state_section"></xref>.</t>
<t>When operating a server
(<xref target="operating_a_server"></xref>
and <xref target="pcp_symmetric"></xref>) the PCP client knows
if it wants an IPv4 listener, IPv6 listener, or both on the
Internet. The PCP client also knows if it has an IPv4 address or
IPv6 address configured on one of its interfaces. It takes the
union of this knowledge to decide to which of its PCP servers to
send the request (e.g., an IPv4 address or an IPv6 address), and
if to send one or two MAP requests for
each of its interfaces (e.g., if the PCP client has only an IPv4
address but wants both IPv6 and IPv4 listeners, it sends a MAP
request containing the all-zeros IPv6 address in the Suggested
External Address field, and sends a second MAP request containing
the all-zeros IPv4 address in the Suggested External Address field.
If the PCP
client has both an IPv4 and IPv6 address, and only wants an IPv4
listener, it sends one MAP request from its IPv4 address (if
the PCP server supports NAT44 or IPv4 firewall) or one MAP
request from its IPv6 address (if the PCP server supports
NAT64)). The PCP client can simply request the desired mapping
to determine if the PCP server supports the desired
mapping. Applications that embed IP addresses in payloads (e.g.,
FTP, SIP) will find it beneficial to avoid address family
translation, if possible.</t>
<t>The MAP and PEER requests include a Suggested External IP
Address field. Some PCP-controlled devices, especially CGN but
also multi-homed NPTv6 networks, have a pool of public-facing IP
addresses. PCP allows the client to indicate if it wants a
mapping assigned on a specific address of that pool or any
address of that pool. Some applications will break if mappings
are created on different IP addresses (e.g., active mode FTP),
so applications should carefully consider the implications of
using this capability. Static mappings for that Internal
Address (e.g., those created by a command-line interface on the
PCP server or PCP-controlled device) may exist to a certain
External Address, and if the Suggested External IP Address is
the all-zeros address, PCP SHOULD assign its mappings to the same
External Address, as this can also help applications using a mix
of both static mappings and PCP-created mappings. If, on the other
hand, the Suggested External IP Address contains an IP address
the PCP Server SHOULD create a mapping to that external address, even if
there are other mappings from that same Internal Address to a
different External Address.
Once an
Internal Address has no implicit dynamic mappings and no
explicit dynamic mappings in the PCP-controlled device, a
subsequent implicit or explicit mapping for that Internal
Address MAY be assigned to a different External
Address. Generally, this re-assignment would occur when a CGN
device is load balancing newly-seen Internal Addresses to its
public pool of External Addresses.</t>
<!--
<t>On many common platforms (i.e., those making use of the BSD sockets
API), using PCP for purposes of application keepalives by issuing a
PCP request followed by a dynamic connection to the server is
difficult to implement properly. This is therefore NOT RECOMMENDED.
Instead, client applications SHOULD first establish a dynamic
connection to a server, and then issue a PCP request related to that
connection, including the REMOTE_PEER Option.</t>
-->
<?rfc needLines="30" ?>
<figure anchor="AF_diagram" title="Address Families with MAP and PEER">
<preamble>The following table summarizes how various common PCP deployments use
IPv6 and IPv4 addresses. The 'internal' address is implicitly the
same as the source IP address of the PCP request, except when the
THIRD_PARTY option is used. The 'external' address is the Suggested
External Address field of the MAP or PEER request, and is address
family is usually the same as the 'internal' address family, except
when technologies like NAT64 are used. The 'remote peer' address is
the Remote Peer IP Address of the PEER request or the FILTER option
of the MAP request, and is always the same address family as the
'internal' address, even when NAT64 is used.
In NAT64, the IPv6 PCP client is not necessarily aware of the NAT64
or aware of the actual IPv4 address of the remote peer, so it
expresses the IPv6 address from its perspective as shown in the
table.</preamble>
<artwork align="center">
internal external remote peer
-------- ------- -----------
IPv4 firewall IPv4 IPv4 IPv4
IPv6 firewall IPv6 IPv6 IPv6
NAT44 IPv4 IPv4 IPv4
NAT64 IPv6 IPv4 IPv6
NPTv6 IPv6 IPv6 IPv6
</artwork>
</figure>
<!-- moved
<t>PCP MAP and PEER requests cannot delete or shorten the lifetime of an
existing implicit outbound mapping for the indicated internal address and port.
PCP MAP requests can delete or shorten
the lifetime of an existing explicit dynamic inbound mapping, but cannot affect
the lifetime of an existing explicit static inbound mapping.</t>
-->
<section anchor="operating_a_server" title="For Operating a Server">
<t>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
<xref target="map_opcodes"></xref> 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.</t>
<t>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.</t>
<figure anchor="fig_operate_server" title="Pseudo-code for using PCP to operate a server">
<preamble>The following pseudo-code shows how PCP can be reliably
used to operate a server:</preamble>
<artwork align="center"><![CDATA[
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* 4. Resending a request due to server state loss
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_map_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received())
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet())
process_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}]]></artwork>
</figure>
</section>
<section anchor="pcp_symmetric" title="For Operating a Symmetric Client/Server">
<t>A host operating a client and server on the same port (e.g.,
<xref target="RFC4961">Symmetric RTP</xref> or <xref target="RFC3581">SIP
Symmetric Response Routing (rport)</xref>) first establishes a local
listener, (usually) sends the local and public IP addresses and ports
to a rendezvous service (which is out of scope of this document), and
initiates an outbound connection from that same source address and
same port. To accomplish this, the application uses the procedure
described in this section.</t>
<t>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
<xref target="map_opcodes"></xref>, and receive a positive PCP response
before it sends any packets from that port. <list style="empty">
<t>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
implicit dynamic mappings (e.g., TCP SYNs) sent from the specified Internal Address, Protocol, and
Port. PCP supports both EIM and EDM NATs, so clients need
to assume they may be dealing with an EDM NAT. In this
case, the client will experience more reliable
connectivity if it attempts explicit PCP MAP requests
first, before initiating any outbound TCP connections from
that Internal Address and Port. See
also <xref target="EDM"></xref>.</t>
</list></t>
<?rfc needLines="47" ?>
<figure anchor="fig_pseudocode_symmetric" title="Pseudo-code for using PCP to operate a symmetric client/server">
<preamble>The following pseudo-code shows how PCP can be used to
operate a symmetric client and server:</preamble>
<artwork align="center"><![CDATA[
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* 4. Resending a request due to server state loss
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_map_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received())
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet())
process_it(s);
if (need_to_make_outgoing_connection())
make_outgoing_connection(s, ...);
if (data_to_send())
send_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}]]></artwork>
<postamble></postamble>
</figure>
</section>
<section anchor="keepalives" title="For Reducing NAT or Firewall Keepalive Messages">
<t>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.</t>
<t>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. <list style="empty">
<t>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.</t>
</list></t>
<t>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 <xref target="peer_opcodes"></xref>.</t>
<figure anchor="fig_keepalive_pseudocode" title="Pseudo-code using PCP with a dynamic socket">
<preamble>The following pseudo-code shows how PCP can be reliably
used with a dynamic socket, for the purposes of reducing application
keepalive messages:</preamble>
<artwork align="center"><![CDATA[
int s = socket(...);
connect(s, &remote_peer, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* 4. Resending a request due to server state loss
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_peer_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
remote_peer, requested_lifetime, &assigned_lifetime);
if (data_to_send())
send_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}]]></artwork>
</figure>
</section>
<section anchor="restoring" title="For Restoring Lost Implicit TCP Dynamic Mapping State">
<t>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
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. <list style="empty">
<t>Note: This procedure works well for TCP, provided the NAT
creates a new implicit dynamic outbound mapping only for TCP segments with the
SYN bit set (i.e., the newly-booted NAT drops the re-transmitted
data segments from the client because the NAT does not have an
active mapping for those segments), and if the server is not
sending data that elicits a RST from the NAT. This is not the case
for UDP, because a new UDP mapping will be created (probably on a
different port) as soon as UDP traffic is seen by the NAT.</t>
</list></t>
</section>
</section>
<section anchor="map_opcodes" title="MAP Opcode">
<t>This section defines an Opcode which controls forwarding from a NAT
(or firewall) to an Internal Host.<list hangIndent="8" style="hanging">
<t hangText=" MAP:">Create an explicit dynamic mapping between an
Internal Address + Port and an External Address + Port.</t>
</list></t>
<t>PCP Servers SHOULD
provide a configuration option to allow administrators to disable MAP
support if they wish.</t>
<t>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.</t>
<t>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, Protocol, 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.</t>
<t>On Operating Systems that allow multiple listening servers to
bind to the same Internal Port, servers 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 PCP MAP request, to ensure that no other
PCP clients on the same machine are also listening on the same
Internal Port.</t>
<t>As a side-effect of creating a mapping, ICMP messages associated
with the mapping MUST be forwarded (and also translated, if
appropriate) for the duration of the mapping's lifetime. This is done
to ensure that ICMP messages can still be used by hosts, without
application programmers or PCP client implementations needing to
use PCP separately to create ICMP mappings for those flows.</t>
<t>The operation of the MAP Opcode is described in this section.</t>
<section title="MAP Operation Packet Formats">
<t>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.</t>
<figure anchor="map_request" title="MAP Opcode Request">
<preamble>The following diagram shows the format of the
Opcode-specific information in a request for the MAP Opcode.
</preamble>
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Requested lifetime (in common header):">Requested
lifetime of this mapping, in seconds. The value 0 indicates
"delete".</t>
<t hangText="Protocol:">Upper-layer protocol associated with this
Opcode. Values are taken from the
<xref target="proto_numbers">IANA protocol registry</xref>. 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'.</t>
<t hangText="Reserved:">24 reserved bits, MUST be sent as 0 and
MUST be ignored when received.</t>
<t hangText="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')</t>
<t hangText="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.</t>
<t hangText="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
<xref target="fixed-size-addr"/>).</t>
</list></t>
<t>The internal address for the request is the source IP address of
the PCP request message itself, unless the THIRD_PARTY Option is
used.</t>
<figure anchor="map_response" title="MAP Opcode Response">
<preamble>The following diagram shows the format of Opcode-specific
information in a response packet for the MAP Opcode:</preamble>
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Lifetime (in common header):">On an error
response, this indicates how long clients should assume
they'll get the same error response from the PCP server if
they repeat the same request. On a success response, this
indicates the lifetime for this mapping, in seconds.</t>
<t hangText="Protocol:">Copied from the request.</t>
<t hangText="Reserved:">24 reserved bits, MUST be sent as 0 and
MUST be ignored when received.</t>
<t hangText="Internal Port:">Copied from the request.</t>
<t hangText="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.</t>
<t hangText="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.</t>
</list></t>
</section>
<section anchor="map-opcode_client_operation" title="Generating a MAP Request">
<t>This section and <xref target="lifetime"></xref> describe the
operation of a PCP client when sending requests with the MAP Opcode.</t>
<t>The request MAY contain values in the Suggested External Port and
Suggested External IP Address fields. This allows the PCP client to
attempt to rebuild lost state on the PCP server, which improves the
chances of existing connections surviving, and helps the PCP client
avoid having to change information maintained at its rendezvous
server. Of course, due to other activity on the network (e.g., by
other users or network renumbering), the PCP server may not be able to
grant the suggested External IP Address and Port, and in that case it
will assign a different External IP Address and Port.</t>
<t>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.</t>
<t>If the client wants all protocols mapped it uses Protocol 0 (zero)
and Internal Port 0 (zero).</t>
<section anchor="renewal" title="Renewing a Mapping">
<t>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 in the Suggested
External IP address and Suggested External Port fields, so if the PCP server has lost
state it can recreate the lost mapping with the same parameters.</t>
<t>The PCP client SHOULD renew the mapping before its expiry
time, otherwise it will be removed by the PCP server
(see <xref target="lifetime"></xref>). 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).</t>
</section>
</section>
<section anchor="map-opcode_server_operation" title="Processing a MAP Request">
<t>This section and <xref target="lifetime"></xref> 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.</t>
<t>The Protocol and Internal Port fields from the MAP request
are copied into the MAP response. If present and processed by the PCP server
the THIRD_PARTY Option is also copied into the MAP response.</t>
<t>If the Requested Lifetime is non-zero, it indicates a
request to create a mapping or extend the lifetime of an
existing mapping. If the PCP server or PCP-controlled device
does not support the Protocol,
it MUST generate an UNSUPP_PROTOCOL error. If the requested
Lifetime is non-zero, the Internal Port is zero, and the
Protocol is non-zero, it indicates a request to map all
incoming traffic for that entire Protocol. If this request
cannot be fulfilled in its entirety, the error UNSUPP_PROTOCOL
MUST be returned. If the requested Lifetime is non-zero, the
Internal Port is zero, and the Protocol is zero, it indicates
a request to map all incoming traffic for all protocols. If
this request cannot be fulfilled in its entirety, the error
UNSUPP_PROTOCOL MUST be returned. If the Protocol is zero but
the Internal Port is non-zero, the error MALFORMED_REQUEST
MUST be returned.</t>
<t>If the requested lifetime is zero, it indicates a request to
delete an existing mapping or set of mappings. Processing of the
lifetime is described in <xref target="lifetime"></xref>.</t>
<t>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.</t>
<t>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
<xref target="RFC6296">NPTv6</xref>, all of which modify addresses but
not port numbers.</t>
<!-- text from -22 is here, commented out.
<t>It is possible that a mapping might already exist for a
requested Internal Address and Port, for example because of a
previous MAP request, or an implicit dynamic mapping (e.g.,
created by TCP SYN), or other
reasons. If an explicit inbound mapping (created by MAP) already exists for the requested
Internal Address and Port and the PREFER_FAILURE Option is not
present, the PCP server makes this a MAP-managed mapping and
MUST refresh (i.e., extend) the lifetime of that
already-existing mapping, and return the already-existing
External Address and Port in its response, regardless of the
Suggested External Address and Port in the request. If a
mapping already exists for the requested Internal Address and
Port and the request contains the PREFER_FAILURE Option, but
the Suggested External Address and Port do not match the
actual External Address and Port of the already existing
mapping, the error CANNOT_PROVIDE_EXTERNAL is returned. If an
implicit outbound mapping (e.g., created by PEER or created by
a TCP SYN) already exists for the requested
Internal Address and Port, a new explicit mapping should be
made replicating the ports and addresses from the implicit
outbound mapping (but the implicit outbound mapping continues
to exist, and remains in effect if the explicit mapping is
later deleted).</t>
-->
<t>
It is possible that a mapping might already exist for a requested
Internal Address, Protocol, and Port. If so, the PCP server takes the
following actions:
<list style="numbers">
<t>If the MAP request contains the PREFER_FAILURE Option, but the
Suggested External Address and Port do not match the External
Address and Port of the existing mapping, the PCP server MUST
return CANNOT_PROVIDE_EXTERNAL.</t>
<t>If the existing mapping is static (created outside of PCP), the PCP
server MUST return the External Address and Port of the existing
mapping in its response and SHOULD indicate a Lifetime of 2^32-1
seconds, regardless of the Suggested External Address and
Port in the request.</t>
<t>If the existing mapping is explicit dynamic inbound (created by a
previous MAP request), the PCP server MUST return the existing
External Address and Port in its response, regardless of the
Suggested External Address and Port in the request. Additionally,
the PCP server MUST update the lifetime of the existing mapping, in
accordance with section 10.5.</t>
<t>If the existing mapping is dynamic outbound (created by outgoing
traffic or a previous PEER request), the PCP server SHOULD create a
new explicit inbound mapping, replicating the ports and addresses
from the outbound mapping (but the outbound mapping continues to
exist, and remains in effect if the explicit inbound mapping is
later deleted).</t>
</list>
</t>
<t>The PCP server MUST NOT create mappings for the PCP ports
themselves (5350 and 5351), and SHOULD have a policy control to
deny mappings for other ports. In these cases, the error
NOT_AUTHORIZED SHOULD be returned.</t>
<t>If no mapping exists for the Internal Address, Protocol, 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 PCP server or PCP-controlled device
cannot assign the Suggested External Address and Port
include: <list style="symbols">
<t>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, protocol, and port).</t>
<t>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).</t>
<t>The Suggested External Address and Port is otherwise
prohibited by the PCP server's policy.</t>
<t>The Suggested External Address or port is invalid (e.g.,
127.0.0.1, ::1, multicast address, or the port is not
valid for the indicated protocol).</t>
<t>The Suggested External Address does not belong to the
NAT gateway.</t>
<t>The Suggested External Address is not configured to be
used as an external address of the firewall or NAT gateway.</t>
<t>The PREFER_FAILURE option is included in the request
and the Suggested External Address and Port are not
assignable to the PCP client, which returns the
CANNOT_PROVIDE_EXTERNAL error.</t>
</list></t>
<t>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.</t>
<!--
All of this is discussed in the section for THIRD_PARTY option;
shouldn't be here.
<t>If the THIRD_PARTY Option is not present in the request, the source
IP address of the PCP packet is used as the Internal Address for the
mapping. If the THIRD_PARTY Option is present, the PCP server
validates that the client is authorized to make mappings on behalf of
the indicated Internal IP Address. This validation depends on the PCP
deployment scenario; see <xref target="DSLiteDeployment"></xref> for
an example validation procedure. If the source IP address of the PCP
request is not authorized to make mappings on behalf of the indicated
internal IP address, an error response MUST be generated with result
code NOT_AUTHORIZED.</t>
-->
<t>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.</t>
<t>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.</t>
</section>
<section title="Processing a MAP Response">
<t>This section describes the operation of the PCP client when it
receives a PCP response for the MAP Opcode.</t>
<t>After performing common PCP response processing, the
response is further matched with a previously-sent MAP request
by comparing the Internal IP Address (the destination IP
address of the PCP response, or other IP address specified via
the THIRD_PARTY option), the Protocol and the Internal Port.
Other fields are not compared, because the PCP server sets
those fields. Note that if the PCP server
supports <xref target="update">Mapping Update
</xref> the PCP server will send additional MAP responses if
the mapping changes (e.g., due to IP renumbering).</t>
<t>On a success response, the PCP client can use the External
IP Address and Port as needed. Typically the PCP client will
communicate the External IP Address and Port to another host
on the Internet using an application-specific rendezvous
mechanism such as DNS SRV records.</t>
<t>As long as renewal is desired, the PCP client MUST also set a timer or otherwise schedule an event
to renew the mapping before its lifetime expires. Renewing a mapping
is performed by sending another MAP request, exactly as described
in <xref target="map-opcode_client_operation"></xref>, except that the
Suggested External Address and Port SHOULD be set to the values
received in the response. From the PCP server's point of view a MAP
request to renew a mapping is identical to a MAP request to create a
new mapping, and is handled identically. Indeed, in the event of PCP
server state loss, a renewal request from a PCP client will appear to
the server to be a request to create a new mapping, with a particular
Suggested External Address and Port, which happens to be what the PCP
server previously assigned. See also <xref target="reboot"></xref>.</t>
<t>On an error response, the client SHOULD NOT repeat the same request
to the same PCP server within the lifetime returned in the
response.</t>
</section>
<section anchor="lifetime" title="Mapping Lifetime and Deletion">
<t>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.</t>
<t>Once a PCP server has responded positively to a MAP request
for a certain lifetime, the port mapping is active for the
duration of the lifetime unless the lifetime is reduced
by the PCP client (to a
shorter lifetime or to zero) or until the PCP server loses its
state (e.g., crashes). Mappings created by PCP MAP requests
are not special or different from mappings created in other
ways. In particular, it is implementation-dependent if
outgoing traffic extends the lifetime of such mappings beyond
the PCP-assigned lifetime. PCP clients MUST NOT depend on this
behavior to keep mappings active, and MUST explicitly renew
their mappings as required by the Lifetime field in PCP
response messages.</t>
<!-- removed, becuase it is duplicate
<t>If a PCP client sends a PCP MAP request to create a mapping
that already exists as a static mapping, the PCP server will
return a successful result, confirming that the requested
mapping exists. The lifetime the PCP server returns for such a
static mapping SHOULD be 2^32-1.
-->
<t>Upon receipt of a MAP or PEER response with an absurdly long Assigned Lifetime the
PCP client SHOULD behave as if it received a more sane value
(e.g., 24 hours), and renew the mapping accordingly, to ensure
that if the static mapping is removed the client will continue
to maintain the mapping it desires.</t>
<t>If the requested lifetime is zero then: <list style="symbols">
<t>If both the internal port and protocol are non-zero, it
indicates a request to delete the indicated mapping immediately.</t>
<t>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.</t>
<t>If the internal port is zero and the protocol is non-zero, it
indicates a request to delete a previous 'wildcard' (all-ports)
mapping for that protocol.</t>
<t>If the internal port is non-zero and the protocol is zero,
then the request is invalid and the PCP Server MUST return
a MALFORMED_REQUEST error to the client.</t>
</list></t>
<t>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.</t>
<t>PCP PEER requests cannot delete or shorten lifetimes of any
mappings. If a PEER request matches an existing mapping, and the
requested lifetime is less than that of the existing mapping, the
PCP server returns SUCCESS with the lifetime of the existing
mapping.</t>
<t>PCP MAP requests can only delete or shorten lifetimes of MAP-created
mappings. If the PCP client attempts to delete a static mapping
(i.e., a mapping created outside of PCP itself), or an outbound
(implicit or PEER-created) mapping, the PCP server MUST return
NOT_AUTHORIZED. If the PCP client attempts to delete a mapping that
does not exist, the SUCCESS result code is returned (this is
necessary for PCP to return the same response for the same request). If the PCP MAP request was for
port=0 (indicating 'all ports'), the PCP server deletes all of the
explicit dynamic mappings it can (but not any implicit or static
mappings), and returns a SUCCESS response. If the deletion request
was properly formatted and successfully processed, a SUCCESS response
is generated with lifetime of 0 and the server copies the protocol
and internal port number from the request into the response. An
inbound mapping (i.e., static mapping or MAP- created dynamic
mapping) MUST NOT have its lifetime reduced by transport protocol
messages (e.g., TCP RST, TCP FIN). Note the THIRD_PARTY Option, if
authorized, can also delete PCP-created mappings
(see <xref target="third_party"></xref>).</t>
<t>An application that forgets its PCP-assigned mappings (e.g., the
application or OS crashes) will request new PCP mappings. This may
consume port mappings, if the application binds to a different
Internal Port every time it runs. The application will also likely
initiate new implicit dynamic outbound mappings without using PCP, which will
also consume port mappings. If there is a port mapping quota for the
Internal Host, frequent restarts such as this may exhaust the quota.
PCP provides some protections against such port consumption: When a
PCP client first acquires a new IP address (e.g., reboots or joins a
new network), it SHOULD remove mappings that may already be
instantiated for that new Internal Address. To do this, the PCP client
sends a MAP request with protocol, internal port, and lifetime set to
0. Some port mapping APIs (e.g., the "DNSServiceNATPortMappingCreate"
API provided by Apple's Bonjour on Mac OS X, iOS, Windows, Linux
<xref target="Bonjour"></xref>) 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.</t>
<t>To reduce unwanted traffic and data corruption, External UDP and
TCP ports SHOULD NOT be re-used for an interval (TIME_WAIT interval
<xref target="RFC0793"></xref>). However, the PCP server SHOULD allow
the previous user of an External Port to re-acquire the same port
during that interval.</t>
</section>
<section anchor="renumbering" title="Address Change Events">
<t>A customer premises router might obtain a new External IP address, for a
variety of reasons including a reboot, power outage, DHCP lease
expiry, or other action by the ISP. If this occurs, traffic forwarded
to the host's previous address might be delivered to another host
which now has that address. This affects both implicit dynamic
mappings and explicit dynamic mappings. However, this same problem
already occurs today when a host's IP address is re-assigned, without
PCP and without an ISP-operated CGN. The solution is the same as
today: the problems associated with host renumbering are caused by
host renumbering and are eliminated if host renumbering is avoided.
PCP defined in this document does not provide machinery to reduce the
host renumbering problem.</t>
<t>When an Internal Host changes its IP address (e.g., by having a
different address assigned by the DHCP server) the NAT (or firewall)
will continue to send traffic to the old IP address. Typically, the
Internal Host will no longer receive traffic sent to that old IP
address. Assuming the Internal Host wants to continue receiving
traffic, it needs to install new mappings for its new IP address. The
suggested external port field will not be fulfilled by the PCP server,
in all likelihood, because it is still being forwarded to the old IP
address. Thus, a mapping is likely to be assigned a new external port
number and/or public IP address. Note that such host renumbering is
not expected to happen routinely on a regular basis for most hosts,
since most hosts renew their DHCP leases before they expire (or
re-request the same address after reboot) and most DHCP servers honor
such requests and grant the host the same address it was previously
using before the reboot.</t>
<t>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.</t>
</section>
<section title="Learning the External IP Address Alone" anchor="alone">
<t><xref target="I-D.cheshire-nat-pmp">NAT-PMP</xref> includes
a mechanism to allow clients to learn the External IP Address
alone, without also requesting a port mapping. In the case of
PCP, this operation no longer makes sense. PCP supports Large
Scale NATs (CGN) which may have a pool of External IP Addresses,
not just one. A client may not be assigned any particular
External IP Address from that pool until it has at least
one implicit, explicit or static port mapping, and even
then only for as long as that mapping remains valid.
Client software that just wishes to display the user's External
IP Address for cosmetic purposes can achieve that by requesting
a short-lived mapping
(e.g., to the Discard service (TCP/9 or UDP/9) or some other port)
and then displaying the resulting External
IP Address. However, once that mapping expires a subsequent
implicit or explicit dynamic mapping might be mapped to a
different external IP address.</t>
</section>
</section>
<section anchor="peer_opcodes" title="PEER Opcode">
<t>This section defines an Opcode for controlling dynamic mappings.
<list hangIndent="8" style="hanging">
<t hangText=" PEER:">Create a new dynamic
outbound mapping to a remote peer's IP address and port,
or extend the lifetime of an existing outbound mapping.</t>
</list>The use of this Opcodes is described in this section.</t>
<t>PCP Servers SHOULD
provide a configuration option to allow administrators to disable PEER
support if they wish.</t>
<t>Because a mapping created or managed by PEER behaves
almost exactly like an implicit dynamic mapping created as a side-effect of
a packet (e.g., TCP SYN) sent by the host,
mappings created or managed using PCP PEER requests may be
Endpoint Independent Mappings (EIM) or Endpoint Dependent Mappings
(EDM), with Endpoint Independent Filtering (EIF) or Endpoint Dependent
Filtering (EDF), consistent with the existing behavior of the NAT
gateway or firewall in question for implicit outbound mappings it creates
automatically as a result of observing outgoing traffic from Internal
Hosts.</t>
<?rfc needLines="30" ?>
<section title="PEER Operation">
<t>The PEER Opcode allows a PCP client to create a new explicit
dynamic outbound mapping (which functions similarly to an outbound mapping
created implicitly when a host sends an outbound TCP SYN) or to
extend the lifetime of an existing outbound mapping.</t>
<figure anchor="peer_request" title="PEER Opcode Request">
<preamble>The following diagram shows the Opcode layout for
the PEER Opcode. This packet format is aligned with the response
packet format:</preamble>
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Requested Lifetime (in common header):">Requested
lifetime of this mapping, in seconds. Note that it is not
possible to reduce the lifetime of a mapping (or delete it, with
requested lifetime=0) using PEER.</t>
<t hangText="Protocol:">Upper-layer protocol associated with this
Opcode. Values are taken from the
<xref target="proto_numbers">IANA protocol registry</xref>. For example,
this field contains 6 (TCP) if the Opcode is describing a TCP
mapping.</t>
<t hangText="Reserved:">24 reserved bits, MUST be set to 0 on
transmission and MUST be ignored on reception.</t>
<t hangText="Internal Port:">Internal port for the mapping.</t>
<t hangText="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.</t>
<t hangText="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
<xref target="fixed-size-addr"/>).</t>
<t hangText="Remote Peer Port:">Remote peer's port for the
mapping.</t>
<t hangText="Reserved:">16 reserved bits, MUST be set to 0 on
transmission and MUST be ignored on reception.</t>
<t hangText="Remote Peer IP Address:">Remote peer's IP
address from the perspective of the PCP client, so that the PCP
client does not need to concern itself with NAT64 or NAT46 (which
both cause the client's idea of the remote peer's IP address to
differ from the remote peer's actual IP address). This field
allows the PCP client and PCP server to disambiguate multiple
connections from the same port on the Internal Host to different
servers. An IPv6 address is
represented directly, and an IPv4 address is represented
using the IPv4-mapped address syntax
(<xref target="fixed-size-addr"></xref>).</t>
</list></t>
<t>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.</t>
<t>Note that semantics similar to the PREFER_FAILURE option are automatically
implied by PEER requests. If the Suggested External IP
Address or Suggested External Port fields are non-zero, and
the PCP server is unable to honor the Suggested External IP
Address or Port, then the PCP server MUST return a
CANNOT_PROVIDE_EXTERNAL error response. The
PREFER_FAILURE Option is neither required nor allowed in PEER
requests, and if PCP server receives a PEER request containing
the PREFER_FAILURE Option it MUST return a MALFORMED_REQUEST
error response.</t>
<figure anchor="peer_response" title="PEER Opcode Response">
<preamble>The following diagram shows the Opcode response for
the PEER Opcode:</preamble>
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t><list style="hanging">
<t hangText="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.</t>
<t hangText="Protocol:">Copied from the request.</t>
<t hangText="Reserved:">24 reserved bits, MUST be set to 0 on
transmission, MUST be ignored on reception.</t>
<t hangText="Internal Port:">Copied from request.</t>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="Remote Peer port:">Copied from request.</t>
<t hangText="Reserved:">16 reserved bits, MUST be set to 0 on
transmission, MUST be ignored on reception.</t>
<t hangText="Remote Peer IP Address:">Copied from the request.</t>
</list></t>
</section>
<section anchor="peer_opcode_client_operation" title="Generating a PEER Request">
<t>This section describes the operation of a client when generating
a message with the PEER Opcode.</t>
<t>The PEER Opcode MAY be sent before or after
establishing bi-directional communication with the remote peer.</t>
<t>If sent before, this is considered a PEER-created mapping
which creates a new dynamic outbound mapping in the
PCP-controlled device. This is useful for restoring a
mapping after a NAT has lost its mapping state (e.g., due to a
crash).</t>
<t>If sent after, this allows the PCP client to learn the IP address,
port, and lifetime of the assigned External Address and Port
for the existing implicit dynamic outbound mapping,
and potentially to extend this lifetime (for the
purpose described in <xref target="keepalives"></xref>).</t>
<t>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.</t>
</section>
<section anchor="peer-opcode_server_operation" title="Processing a PEER Request">
<t>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.</t>
<t>The following fields from a PEER request are copied into
the response: Protocol, Internal Port, Remote Peer IP Address,
and Remote Peer Port.</t>
<t>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 explicit dynamic mappings. If the validation determines
the Remote Peer IP Address of a PEER request is
invalid, then no mapping is created, and a MALFORMED_REQUEST
error result is returned.</t>
<t>On receiving the PEER Opcode, the PCP server examines the
mapping table. If the requested mapping does not yet exist,
and the Suggested External Address and Port can be honored,
the mapping is created. By having PEER create such a mapping,
we avoid a race condition between the PEER request or the
initial outgoing packet arriving at the NAT or firewall device
first, and allow PEER to be used to recreate an outbound
dynamic mapping (see last paragraph of
<xref target="reboot"></xref>). Thereafter, this PEER-created
mapping is treated as if it was an implicit dynamic outbound
mapping (e.g., as if the PCP client sent a TCP SYN) and a
Lifetime appropriate to such a mapping is returned (note: on
many NATs and firewalls, such mapping lifetimes are very short
until the bi-directional traffic is seen by the NAT or
firewall). If the requested mapping does not yet exist, and
Suggested External Address and Port cannot be honored, the
error CANNOT_PROVIDE_EXTERNAL is returned. If the requested
mapping already exists, it is a request to modify the lifetime
of that existing mapping.</t>
<t>The PEER Opcode can extend the lifetime of an existing
dynamic outbound mapping. The PCP server may grant the client's
requested lifetime, or may grant a value higher or lower,
depending on local policy. The PEER Opcode MUST NOT ever reduce the
lifetime of an existing outbound mapping. If the Requested
Lifetime is less than the lifetime of the existing mapping, it
is treated as a request for the lifetime of the mapping (this can
be used to query the lifetime of an existing mapping).</t>
<t>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.</t>
<t>If a PEER-created or PEER-managed mapping is not renewed
using PEER, then it reverts to the NAT's usual behavior for
implicit mappings, e.g., continued outbound traffic keeps the
mapping alive, as per the NAT or firewall device's existing policy.
A PEER-created or PEER-managed mapping may be
terminated at any time by action of the TCP client or server
(e.g., due to TCP FIN or TCP RST), as per the NAT or firewall
device's existing policy.</t>
</section>
<section title="Processing a PEER Response">
<t>This section describes the operation of a client when processing a
response with the PEER Opcode.</t>
<t>After performing common PCP response processing, the response
is further matched with an outstanding PEER request by comparing the
Internal IP Address (the destination IP address of the PCP
response, or other IP address specified via the THIRD_PARTY
option), the Protocol, the Internal Port, the Remote Peer
Address and the Remote Peer Port. Other fields are not compared,
because the PCP server sets those fields to provide
information about the mapping created by the Opcode.
Note that if the PCP server
supports <xref target="update">Mapping Update
</xref> the PCP server will send additional PEER responses if
the mapping changes (e.g., due to IP renumbering).</t>
<t>On a successful response, the application can use the assigned
lifetime value to reduce its frequency of application keepalives for
that particular NAT mapping. Of course, there may be other reasons,
specific to the application, to use more frequent application
keepalives. For example, the PCP assigned lifetime could be one hour
but the application may want to maintain state on its server (e.g.,
"busy" / "away") more frequently than once an hour. If the response
indicates an unexpected IP address or port (e.g., due to IP
renumbering), the PCP client will want to re-establish its
connection to its remote server.</t>
<t>If the PCP client wishes to keep this mapping alive beyond the
indicated lifetime, it MAY issue a new PCP request prior to the
expiration, or it MAY rely on continued inside-to-outside traffic
to ensure the mapping will continue to exist. See
<xref target="renewal"/> for recommended renewal timing.</t>
<t><list style="empty">
<t>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.</t>
</list></t>
</section>
</section>
<section anchor="map_peer_options" title="Options for MAP and PEER Opcodes">
<t>This section describes Options for the MAP and PEER
Opcodes. These Options MUST NOT appear with other Opcodes, unless
permitted by those other Opcodes.</t>
<section anchor="third_party" title="THIRD_PARTY Option for MAP and PEER Opcodes">
<t>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.</t>
<t>A management device would use this Option to control a PCP
server on behalf of users. For example, a management device
located in a network operations center, which presents a user
interface to end users or to network operations staff, and
issues PCP requests with the THIRD_PARTY option to the appropriate
PCP server.</t>
<figure anchor="fig_third_party" title="THIRD_PARTY Option">
<preamble>The THIRD_PARTY Option is formatted as follows:</preamble>
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code=1 | Reserved | Option Length=16 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| Internal IP Address (128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The fields are described below:<list style="hanging">
<t hangText="Internal IP Address:">Internal IP address for this
mapping. </t>
</list></t>
<?rfc needLines="7" ?>
<t><list style="empty">
<?rfc subcompact="yes"?>
<t>Option Name: THIRD_PARTY</t>
<t>Number: 1</t>
<t>Purpose: Indicates the MAP or PEER request is for a host other
than the host sending the PCP Option.</t>
<t>Valid for Opcodes: MAP, PEER</t>
<t>Length: 16 octets</t>
<t>May appear in: request. May appear in response only if it
appeared in the associated request.</t>
<t>Maximum occurrences: 1</t>
<?rfc subcompact="no"?>
</list></t>
<t>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.</t>
<t>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.</t>
<t>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).</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>A PCP client can delete all PCP-created explicit dynamic
inbound mappings (i.e., those created by PCP MAP requests)
that it is authorized to delete by sending a PCP MAP request
with zero in the Internal Port field, zero in the Protocol
field, and a THIRD_PARTY Option with the all-zeros address in
the Internal IP Address field. Upon receipt of such a request
from an authorized PCP client, the PCP server MUST delete all
described mappings the PCP client is authorized to delete, and
return SUCCESS. SUCCESS is returned if zero
or more mappings were deleted.</t>
</section>
<section anchor="prefer_failure" title="PREFER_FAILURE Option for MAP Opcode">
<t>This Option is only used with the MAP Opcode.</t>
<t>This Option indicates that if the PCP server is unable to
map both the Suggested External Port and Suggested External
Address, the PCP server should not create a mapping. This
differs from the behavior without this Option, which is to
create a mapping.</t>
<figure anchor="fig_prefer_failure" title="PREFER_FAILURE Option">
<preamble>The PREFER_FAILURE Option is formatted as follows:</preamble>
<artwork><![CDATA[
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<?rfc needLines="7" ?>
<t><list style="empty">
<?rfc subcompact="yes"?>
<t>Option Name: PREFER_FAILURE</t>
<t>Number: 2</t>
<t>Purpose: indicates that the PCP server should not create an
alternative mapping if the suggested external port and address
cannot be mapped.</t>
<t>Valid for Opcodes: MAP</t>
<t>Length: 0</t>
<t>May appear in: request. May appear in response only if it
appeared in the associated request.</t>
<t>Maximum occurrences: 1</t>
<?rfc subcompact="no"?>
</list></t>
<t>The result code CANNOT_PROVIDE_EXTERNAL is returned if
the Suggested External Address and Port
cannot be mapped. This can occur because the External Port is
already mapped to another host's outbound dynamic mapping, an
inbound dynamic mapping, a static mapping, or the same
Internal Address, Protocol, and Port already has an outbound dynamic mapping
which is mapped to a different External Port than
suggested. This can also occur because the External Address is
no longer available (e.g., due to renumbering). The server
MAY set the Lifetime in the response to the remaining lifetime
of the conflicting mapping, rounded up to the next larger
integer number of seconds.</t>
<t>This Option exists for interworking with non-PCP mapping
protocols that have different semantics than PCP (e.g.,
<xref target="I-D.bpw-pcp-upnp-igd-interworking">UPnP IGDv1
interworking</xref>, where the semantics of UPnP IGDv1 only
allow the UPnP IGDv1 client to dictate mapping a specific
port), or separate port allocation systems which allocate
ports to a subscriber (e.g., a subscriber-accessed web portal
operated by the same ISP that operates the PCP server). A PCP
server MAY support this Option, if its designers wish to
support such downstream devices or separate port allocation
systems. PCP servers that are not intended to interface with
such systems are not required to support this Option. PCP
clients other than UPnP IGDv1 interworking clients or other
than a separate port allocation system SHOULD NOT use this
Option because it results in inefficient operation, and they
cannot safely assume that all PCP servers will implement
it. It is anticipated that this Option will be deprecated in
the future as more clients adopt PCP natively and the need for
this Option declines.</t>
<t>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.</t>
<t>There can exist a race condition between the MAP
Opcode using the PREFER_FAILURE option and
<xref target="update">Mapping Update</xref>. Because of this,
the PCP client MUST validate that the External IP Address and Port
in a success response matches the associated suggested values
from the request. If they don't match, it is because the
Mapping Update was sent before the MAP request was processed.
If the PCP server has no use for this new mapping, it SHOULD
delete the mapping.</t>
</section>
<section anchor="filter" title="FILTER Option for MAP Opcode">
<t>This Option is only used with the MAP Opcode.</t>
<t>This Option indicates that filtering incoming packets is desired.
The protocol being filtered is indicated by the MAP Opcode, and
the Remote Peer Port and Remote Peer IP Address of the FILTER Option indicate the permitted
remote peer's source IP address and port for packets from the
Internet; other traffic from other addresses is blocked. The remote peer prefix length indicates the length of the
remote peer's IP address that is significant; this allows a single
Option to permit an entire subnet. After processing this MAP request
containing the FILTER Option and generating a successful response, the
PCP-controlled device will drop packets received on its public-facing
interface that don't match the filter fields. After dropping the
packet, if its security policy allows, the PCP-controlled device MAY
also generate an ICMP error in response to the dropped packet.</t>
<figure anchor="fig_filter" title="FILTER Option layout">
<preamble>The FILTER Option is formatted as follows:</preamble>
<artwork align="center"><![CDATA[
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) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>These fields are described below:<list style="hanging">
<t hangText="Reserved:">8 reserved bits, MUST be sent as 0 and
MUST be ignored when received.</t>
<t hangText="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.</t>
<t hangText="Remote Peer Port:">the port number of the remote
peer. The value 0 indicates "all ports".</t>
<t hangText="Remote Peer IP address:">The IP address of the remote peer.</t>
</list></t>
<?rfc needLines="7" ?>
<t><list style="empty">
<?rfc subcompact="yes"?>
<t>Option Name: FILTER</t>
<t>Number: 3</t>
<t>Purpose: specifies a filter for incoming packets</t>
<t>Valid for Opcodes: MAP</t>
<t>Length: 20 octets</t>
<t>May appear in: request. May appear in response only if it
appeared in the associated request.</t>
<t>Maximum occurrences: as many as fit within maximum PCP message
size</t>
<?rfc subcompact="no"?>
</list></t>
<!--
This paragraph is detail justifying why FILTER is complicated with
PEER. FILTER is only defined for MAP, so this detail is unnecessary.
<t>On networks with a NAT, there can be an interactions with
EDM NATs and this Opcode
(see <xref target="EDM"></xref>). Because of this, the
FILTER Option MUST only be used by a client that is operating
a server (that is, using the MAP Opcode), as this ensures that
no other application will be assigned the same port for its
outgoing connection. It is RECOMMENDED that the PCP client
avoid other use, because it will cause some UNSAF NAT
traversal mechanisms <xref target="RFC3424"></xref> to fail
where they would have otherwise succeeded, breaking other
applications running on this same host. Networks without a
NAT do not have these restrictions.</t>
-->
<t>The Prefix Length indicates how many bits of the address
are used for the filter. For IPv4 addresses (which are
encoded using the IPv4-mapped address format (::FFFF:0:0/96)),
this means valid prefix lengths are between 96 and 128 bits,
inclusive. That is, add 96 to the IPv4 prefix length. For
IPv6 addresses, valid prefix lengths are between 0 and 128
bits, inclusive. Values outside those ranges cause the PCP
server to return the MALFORMED_OPTION result code.</t>
<t>If multiple occurrences of the FILTER Option exist in the
same MAP request, they are processed in the order
received (as per normal PCP Option processing) and they MAY
overlap the filtering requested. If an existing mapping exists
(with or without a filter) and the server receives a MAP
request with FILTER, the filters indicated in the new request
are added to any existing filters. If a MAP request has a
lifetime of 0 and contains the FILTER Option, the error
MALFORMED_OPTION is returned.</t>
<t>If any occurrences of the FILTER Option in a request packet are
not successfully processed then an error is returned (e.g.,
MALFORMED_OPTION if one of the Options was malformed) and as with
other PCP errors, returning an error causes no state to be changed in
the PCP server or in the PCP-controlled device.</t>
<t>To remove all existing filters, the Prefix Length 0 is used. There
is no mechanism to remove a specific filter.</t>
<t>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.</t>
<t>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.</t>
<t>All PCP servers MUST support at least one filter per MAP
mapping.</t>
<t>The use of the FILTER Option can be seen as a performance
optimization. Since all software using PCP to receive incoming
connections also has to deal with the case where it may be directly
connected to the Internet and receive unrestricted incoming TCP
connections and UDP packets, if it wishes to restrict incoming traffic
to a specific source address or group of source addresses such
software already needs to check the source address of incoming traffic
and reject unwanted traffic. However, the FILTER Option is a
particularly useful performance optimization for battery powered
wireless devices, because it can enable them to conserve battery power
by not having to wake up just to reject a unwanted traffic.</t>
</section>
<!--
<section title="EVEN_PLUS_ONE">
<t><xref target="RFC3550">RTP</xref> has historically used an
even-numbered port, and RTCP the next-higher port (often abbreviated
as "port + 1"). Some equipment still expects RTP/RTCP on adjacent
ports. To accommodate that, the PCP Option EVEN_PLUS_ONE can be used
by the PCP client and server, with the following procedure.</t>
<t><list style="empty">
<t>[Ed. Note: Are there any other protocols that need adjacent
ports?]</t>
</list></t>
<t>The procedure is for the PCP client to bind to two ports on its
local interface. It then sends a PCP request for external port 0
(indicating it will accept any port from the server) for one of those
internal ports and includes the PCP Option EVEN_PLUS_ONE with this
request. If the server understands this Option, it will choose an
even-numbered external port and will reserve the next-higher port with
a short timeout (suggested duration is 10 seconds) for this same PCP
client to allocate in a subsequent PCP request. The PCP server sends a
response and indicates it performed that operation by including the
PCP Option EVEN_PLUS_ONE in the response. The PCP client then sends a
second PCP request for the next-higher external port. Refreshing and
deleting the ports works as normal.</t>
<figure anchor="even_plus_one_message_flow" title="Message Flow, EVEN_PLUS_ONE">
<preamble>A diagram of the message flow:</preamble>
<artwork align="center"><![CDATA[
PCP Client PCP server
| |
|....request ext-port=0, EVEN_PLUS_ONE....>|
|<.response ext-port=23456, EVEN_PLUS_ONE..|
|....request ext-port=23457...............>|
|<-response ext-port=23457.................|
| |
]]></artwork>
</figure>
<t>The PCP Option EVEN_PLUS_ONE always has an Option Length of 0. When
the PCP client wants an even-numbered port and the next-higher
adjacent port, it includes the PCP Option EVEN_PLUS_ONE in its PCP
request. If the server understands the Option, has successfully mapped
an even-numbered external-port, and temporarily reserved the
next-higher port for this PCP client, the server MUST include the
EVEN_PLUS_ONE Option in its response. If the server understands the
Option, but was unsuccessful at mapping an even-numbered port or
unsuccessful at temporarily reserving the next-higher port for this
PCP client, the server MUST NOT include the EVEN_PLUS_ONE Option in
its response. If the server receives a PCP request with the
EVEN_PLUS_ONE Option for an odd-numbered suggested-external-port, it
MUST return an error.</t>
<t>This Option is permitted with the following Opcodes: MAP44, MAP64,
MAP46, MAP66.</t>
</section>
-->
<!--
<section title="REMOTE_PEER">
<t>Due to a pre-existing dynamic mapping, a PCP server may not be
able to instantiate a filter on a specific port. It is thus
recommended that applications bind to the source port (to prevent
other applications from using that same source port) prior to using
this PCP Option.</t>
<t>In those situations, the Option REMOTE_PEER is necessary to
disambiguate the mappings. This Option does not change filtering
behavior of the NAT or firewall.</t>
<figure>
<artwork align="center"><![CDATA[ 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Remote Peer Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if MAP44 or MAP64, :
: 1 28 bits if MAP46 or MAP66) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+]]></artwork>
</figure>
<t><list style="empty">
<t>This Option:<list style="empty">
<t>value: 129</t>
<t>is valid for Opcodes: MAP44, MAP64, MAP46, or MAP66</t>
<t>is included in responses: MUST</t>
<t>length: 8 or 20</t>
<t>maximum occurrences: 1</t>
</list></t>
</list></t>
</section>
-->
</section>
<!--
<section title="NAT-PMP Transition">
<t>Port Control Protocol (PCP) is a successor to NAT Port
Mapping Protocol (NAT-PMP), and shares similar semantics,
concepts, and packet formats. Because of this NAT-PMP and PCP
can both use the same port, and use the NAT-PMP and PCP's
version negotiation capabilities to determine which version to
use. This section describes how an orderly transition may be
achieved.</t>
<section title="NAT-PMP Clients Updated to Add PCP Support">
<t>A client supporting both NAT-PMP and PCP SHOULD optimistically
assume that the gateway supports PCP, since we expect that this will
rapidly become the case, and we want to optimize for better
performance in this case. A dual-mode client SHOULD send all its
requests first using PCP packet format. If the gateway responds with a
packet four or more octets long, containing the following (NAT-PMP
format) data in the first four octets, then the dual-mode client SHOULD
conclude that this NAT gateway supports only NAT-PMP, and SHOULD retry
its request in the older NAT-PMP format.</t>
<figure align="left" anchor="PMPGate" title="NAT-PMP Gateway Response to PCP Request">
<preamble>NAT-PMP gateways respond to PCP requests with the
following packet. The first octet (supported version) is zero. The
second octet (Opcode) echoes back the request Opcode, with the top
bit set. The third octet (high octet of the NAT-PMP result code) is
zero. The fourth octet is 1 (NAT-PMP and PCP result code "Unsupported
Version").</preamble>
<artwork align="center"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Version = 0 |R| OP = any | Zero | Result = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
<section title="NAT-PMP Gateways Updated to Add PCP Support">
<t>A gateway supporting both NAT-PMP and PCP is able to handle and
respond to requests using both packet formats. If the first octet of
the packet is zero, a dual-mode gateway SHOULD parse the request as a
NAT-PMP-format message and reply using a NAT-PMP-format response.
Otherwise it should parse the request as a PCP-format message and
respond accordingly.</t>
<t>A PCP-only gateway receiving a NAT-PMP request (identified by the
first octet being zero) MUST reply with the packet shown below, so that
the NAT-PMP may log an error message informing the user that they need
to update to a PCP-capable client.</t>
<figure align="left" anchor="PCPGate" title="PCP Gateway Response to NAT-PMP Request">
<preamble>PCP gateways respond to NAT-PMP requests (identified by
the first octet being zero) with the following packet. The first octet
(supported version) is 1. The second octet (Opcode) echoes back the
request Opcode, with the top bit set. The third octet (high octet of
the NAT-PMP result code) is zero. The fourth octet is 1 (NAT-PMP and
PCP result code "Unsupported Version").</preamble>
<artwork align="center"><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Version = 1 |R| OP = any | Zero | Result = 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</section>
</section>
-->
<section anchor="Rapid Recovery" title="Rapid Recovery">
<t>PCP includes a rapid recovery feature, which allows PCP clients
to repair failed mappings within seconds, rather than the minutes
or hours it might take if they relied solely on waiting for the
next routine renewal of the mapping. Mapping failures may occur
when a NAT gateway is rebooted and loses its mapping state, or
when a NAT gateway has its external IP address changed so that its
current mapping state becomes invalid.</t>
<t>The PCP rapid recovery feature enables users to, for
example, connect to remote machines using ssh, and then reboot
their NAT or firewall device (or even replace it with completely
new hardware) without losing their established ssh
connections.</t>
<t>Use of PCP rapid recovery is a performance optimization to
PCP's routine self-healing. Without rapid recovery, PCP clients
will still recreate their correct state when they next renew
their mappings, but this routine self-healing process may take
hours rather than seconds, and will probably not happen fast
enough to prevent active TCP connections from timing out.</t>
<t>There are two mechanisms to perform rapid recovery, described
below.</t>
<section anchor="announcement" title="ANNOUNCE Opcode">
<t>This rapid recovery mechanism uses the ANNOUNCE Opcode. When the
PCP server loses its state (e.g., it lost its state when rebooted), it
sends the ANNOUNCE response to the link-scoped multicast address
(specific address explained below) if a multicast network exists on
its local interface or, if configured with the IP address of PCP
client(s), sends unicast ANNOUNCE responses to those address(es). This
means ANNOUNCE may not be available on all networks (such as networks
without a multicast link between the PCP server and its PCP clients).
Additionally, an ANNOUNCE request can be sent (unicast) by a PCP
client which elicits a unicast ANNOUNCE response like any other
Opcode.</t>
<section title="ANNOUNCE Operation">
<t>The PCP ANNOUNCE Opcode requests and respones have no
Opcode-specific payload (that is, the length of the Opcode-specific
data is zero), so a packet layout is not shown. The Requested
Lifetime field of requests and Lifetime field of responses are both
set to 0 on transmission and ignored on reception.</t>
<t>If a PCP server receives an
ANNOUNCE request, it first parses it and generates a SUCCESS if
parsing and processing of ANNOUNCE is successful (i.e., an error
is generated if the request packet is mal-formed, such as invalid
length). Note that, in the future, Options MAY be sent with the PCP
ANNOUNCE Opcode; PCP clients and servers need to be prepared to
receive Options with the ANNOUNCE Opcode.</t>
<t><list style="empty"><t>Discussion: A separate port is used
when the server sends an unsolicited ANNOUNCE OpCode (5350) than
normal client-initiated requests (5351) because a single device
may have multiple roles. For example, a multi-function home
gateway may also provide printer sharing, or a home computer may
also provide "Internet Sharing" (NAT) functionality. Such
devices need to act as both a PCP Server and a PCP Client at the
same time, and the software that implements the PCP Server on
the device may not be the same software component that
implements the PCP Client. The software that implements the PCP
Server needs to listen for unicast client requests, whereas the
software that implements the PCP Client needs to listen for
multicast restart announcements. In many networking APIs it is
difficult or impossible to have two independent clients
listening for both unicasts and multicasts on the same port at
the same time. For this reason, two ports are used.</t>
</list></t>
</section>
<section title="Generating and Processing a Solicited ANNOUNCE Message">
<t>The PCP ANNOUNCE Opcode MAY be sent (unicast) by a PCP client. The
Requested Lifetime value MUST be set to zero.</t>
<t>When the PCP server receives the ANNOUNCE Opcode and successfully
parses and processes it, it generates SUCCESS response with an Assigned
Lifetime of zero.</t>
<t>This functionality allows a PCP client to determine a server's
Epoch, or to determine if a PCP server is running, without changing the
server's state.</t>
</section>
<section title="Generating and Processing an Unsolicited ANNOUNCE Message">
<t>Generating a PCP restart announcement, described in this
section, is an optional feature of PCP. This message is most
typically multicast, but can also be unicast. Unicast requires
the PCP server send the message to the PCP client's IP
address(es), which it may not know after it loses state. When
sending unsolicited unicast ANNOUNCE responses, they are sent to
port 5351. When sending unsolicited multicast ANNOUNCE responses,
they are sent to port 5350. When sending unsolicited
responses, the ANNOUNCE Opcode MUST have Result Code equal to
zero (SUCCESS).</t>
<!--
<t>PCP Servers that accept PCP requests from multicast-reachable
clients (e.g., from clients directly attached via Ethernet or
Wi-Fi) SHOULD send the PCP ANNOUNCE Opcode response packet
described below to signal restarts. PCP Clients that send PCP
requests to a PCP Server via a multicast-capable path (e.g., to a
PCP server attached to the same Ethernet or Wi-Fi link) SHOULD
listen for and process these Restart Announcements sent by PCP
Servers. If a multicast-capable path is not available, PCP
Clients MAY listen on port 5350 for these Restart Announcements.</t>
-->
<t>When a PCP server device that implements this functionality
reboots, restarts its NAT engine, or otherwise enters a state
where it may have lost some or all of its previous mapping state
(or enters a state where it doesn't even know whether it may
have had prior state that it lost) it MUST inform PCP clients of
this fact by unicasting or multicasting a gratuitous PCP
ANNOUNCE Opcode response packet, as shown below, via paths over
which it accepts PCP requests. If sending a multicast ANNOUNCE
message, a PCP server device which accepts PCP requests over
IPv4 sends the Restart Announcement to the IPv4 multicast
address 224.0.0.1:5350 (224.0.0.1 is the All Hosts multicast
group address). If sending a multicast ANNOUNCE message, a PCP
server device which accepts PCP requests over IPv6 sends the
Restart Announcement to the IPv6 multicast address
[ff02::1]:5350 (ff02::1 is for all nodes on the local segment).
A PCP server device which accepts PCP requests over both IPv4
and IPv6 sends a pair of Restart Announcements, one to each
multicast address. If sending unicast ANNOUNCE messages, it
sends ANNOUNCE response message to the IP address(es) of the PCP
clients with destination port 5350. To accommodate packet loss,
the PCP server device MAY transmit such packets (or packet pairs)
up to ten times (with an appropriate Epoch time value in each to
reflect the passage of time between transmissions) provided that
the interval between the first two notifications is at least
250ms, and the interval between subsequent notification at least
doubles.</t>
<t>A PCP client that sends PCP requests to a PCP Server via a
multicast-capable path, and implements the Restart Announcement
feature, and wishes to receive these announcements, MUST listen
to receive these PCP Restart Announcements (gratuitous PCP
ANNOUNCE Opcode response packets) on the appropriate
multicast-capable interfaces on which it sends PCP requests,
and MAY listen to receive those on their unicast address.
A PCP client device which sends PCP requests using IPv4 listens
for packets sent to the IPv4 multicast address 224.0.0.1:5350.
A PCP client device which sends PCP requests using IPv6 listens
for packets sent to the IPv6 multicast address [ff02::1]:5350.
A PCP client device which sends PCP requests using both IPv4 and
IPv6 listens for both types of Restart Announcement. (The
SO_REUSEPORT socket option or equivalent should be used for the
multicast UDP port, if required by the host OS to permit
multiple independent listeners on the same multicast UDP
port.)</t>
<t>Upon receiving a unicasted or multicasted PCP ANNOUNCE Opcode response packet, a PCP
client MUST (as it does with all received PCP response packets)
inspect the Announcement's source IP address, and if the Epoch
time value is outside the expected range for that server, it MUST
wait a random amount of time between 0 and 5 seconds (to prevent
synchronization of all PCP clients), then
for all PCP mappings it made at that server address the client
issues new PCP requests to recreate any lost mapping
state. The use of the Suggested External IP Address and
Suggested External Port fields in the client's
renewal requests allows the client to remind the restarted PCP
server device of what mappings the client had previously been
given, so that in many cases the prior state can be
recreated. For PCP server devices that reboot relatively quickly
it is usually possible to reconstruct lost mapping state fast
enough that existing TCP connections and UDP communications do
not time out and continue without failure.</t>
</section>
</section>
<section anchor="update" title="PCP Mapping Update">
<t>This rapid recovery mechanism is used when the PCP server
remembers its state and determines its existing mappings are
invalid (e.g., IP renumbering changes the public IP address of
a PCP-controlled NAT).</t>
<t>Implementation of the feature described in this section is
optional for PCP servers. It is anticipated that servers which
are routinely reconfigured by an administrator or have their WAN
address changed frequently will implement this feature (e.g.,
residential CPE routers). It is anticipated that servers which
are not routinely reconfigured will not implement this
feature (e.g., service provider-operated CGN).</t>
<t>If a PCP server device has not forgotten its mapping state,
but for some other reason has determined that some or all of its
mappings have become unusable (e.g., when a home gateway is
assigned a different external IPv4 address by the upstream DHCP
server) then the PCP server device MAY chose to remedy this
situation by automatically repairing its mappings and notifying
its clients by following the procedure described below.</t>
<t>For MAP-created and PEER-managed mappings, for each one the
PCP server device should update the External IP Address and
External Port to appropriate available values, and then send
unicast PCP MAP or PEER responses (as appropriate for the
mapping) to inform the PCP client of the new External IP Address
and External Port. Such unsolicited responses are identical to
the MAP or PEER responses normally returned in response to
client MAP or PEER requests containing newly updated External IP
Address and External Port values. If the earlier associated
request contained the THIRD_PARTY Option, the THIRD_PARTY Option
MUST also appear in the Mapping Update as it is necessary for
the PCP client to disambiguate the response. If the earlier
associated request contained the PREFER_FAILURE option, and the
same external IP address and port cannot be provided, the error
CANNOT_PROVIDE_EXTERNAL SHOULD be sent. If the earlier associated
request contained the FILTER option, the filters are moved to
the new mapping and the FILTER Option is sent in the Mapping
Update response. Non-mandatory Options SHOULD NOT be sent in
the Mapping Update response.</t>
<t><list style="empty">
<t>Discussion: It could have been possible to design this
so that the PCP server (1) sent an ANNOUNCE Opcode to the
PCP client, the PCP client reacted by (2) sending a new
MAP request and (3) receiving a MAP response. Instead,
that design is short-cutted by the server simply sending
the message it would have sent in (3).</t></list></t>
<t>To accommodate packet loss, the PCP server device MAY
transmit such packets up to ten times (with an appropriate Epoch
time value in each to reflect the passage of time between
transmissions) provided that the interval between the first two
notifications is at least 250ms, and the interval between
subsequent notification at least doubles.</t>
<t>Upon receipt of such MAP or PEER response, a PCP client uses
the information in the response to adjust rendezvous servers or
re-connect to servers, respectively. For MAP, this would
means updating the DNS entries or other address and
port information recorded with some kind of application-specific
rendezvous server. For PEER responses indicating the external
port or address changed, this would typically mean
re-establishing connections to servers. Any time the external
address or port changes, existing TCP and UDP connections will
be lost; PCP can't avoid that, but does provide immediate
notification of the event to lessen the impact.</t>
</section>
</section>
<section anchor="implementation_considerations" title="Implementation Considerations">
<section anchor="EDM" title="Implementing MAP with EDM port-mapping NAT">
<t>This section provides non-normative guidance that may be useful to
implementers.</t>
<t>For implicit dynamic outbound mappings, some existing NAT devices have
endpoint-independent mapping (EIM) behavior while other NAT devices
have endpoint-dependent mapping (EDM) behavior. NATs which have EIM
behavior do not suffer from the problem described in this section. The
IETF strongly encourages EIM behavior
<xref target="RFC4787"></xref><xref target="RFC5382"></xref>.</t>
<t>In such EDM NAT devices, the same external port may be used
by an outbound dynamic mapping and an inbound dynamic mapping
(from the same Internal Host or from a different Internal Host).
This complicates the interaction with the MAP Opcode. With such
NAT devices, there are two ways envisioned to implement the MAP
Opcode: <list style="numbers">
<t>Have outbound mappings use a different set of public
ports than inbound mappings (e.g., those created with MAP),
thus reducing the interaction problem between them; or</t>
<t>On arrival of a packet (inbound from the Internet or outbound from an
Internal Host), first attempt to use a dynamic outbound mapping to
process that packet. If none match, attempt to use an inbound mapping to
process that packet. This effectively 'prioritizes' outbound mappings
above inbound mappings.</t>
</list></t>
</section>
<section title="Lifetime of Explicit and Implicit Dynamic Mappings">
<t>This section provides non-normative guidance that may be useful to
implementers.</t>
<t>No matter if a NAT is EIM or EDM, it is possible that one (or more)
outbound mappings, using the same internal port on the
Internal Host, might be created before or after a MAP request. When
this occurs, it is important that the NAT honor the Lifetime returned
in the MAP response. Specifically, if a mapping was created with the
MAP Opcode, the implementation needs to ensure that termination of an
outbound mapping (e.g., via a TCP FIN handshake) does not
prematurely destroy the MAP-created inbound mapping.</t>
</section>
<section anchor="failure" title="PCP Failure Scenarios">
<t>This section provides non-normative guidance that may be useful to
implementers.</t>
<t>If an event occurs that causes the PCP server to lose
dynamic mapping state (such as a crash or power outage), the mappings
created by PCP are lost. Occasional loss of state may be unavoidable
in a residential NAT device which does not write transient
information to non-volatile memory. Loss of state is expected
to be rare in a service provider environment (due to redundant
power, disk drives for storage, etc.). Of course, due to
outright failure of service provider equipment (e.g., software
malfunction), state may still be lost.</t>
<t>The Epoch Time allows a client to deduce when a PCP server may have
lost its state. When the Epoch Time value is observed to be outside the
expected range, the PCP client can attempt to recreate the mappings
following the procedures described in this section.</t>
<t>Further analysis of PCP failure scenarios is in
<xref target="I-D.boucadair-pcp-failure"></xref>.</t>
<section title="Recreating Mappings" anchor="reboot">
<!--
<t>The PCP server SHOULD store mappings in persistent storage so
when it is powered off or rebooted, it remembers the port mapping
state of the network. Due to the physical architecture of some PCP
servers, this is not always achievable (e.g., some non-volatile
memory can withstand only a certain number of writes, so writing PCP
mappings to such memory is generally avoided).</t>
<t>However, maintaining this state is not essential for correct
operation.
-->
<t>This section provides non-normative guidance that may be useful to
implementers.</t>
<t>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.</t>
<t>When the PCP server loses state and begins processing new PCP
messages, its Epoch time is reset and begins counting again. As the result
of receiving a packet where the Epoch time field is outside the
expected range (<xref target="epoch"></xref>), indicating that a reboot or similar loss of
state has occurred, the client can renew its port mappings
sooner, without waiting for the normal routine renewal time.</t>
</section>
<section title="Maintaining Mappings" anchor="maintaining_mappings">
<t>This section provides non-normative guidance that may be useful to
implementers.</t>
<t>A PCP client refreshes a mapping by sending a new PCP request
containing information from the earlier PCP response. The PCP server
will respond indicating the new lifetime. It is possible, due to
reconfiguration or failure of the PCP server, that the public IP
address and/or public port, or the PCP server itself, has changed
(due to a new route to a different PCP server). Such events are
not an error. The PCP server will simply return a new External
Address and/or External Port to the client, and the client should
record this new External Address and Port with its rendezvous service.
To detect such events more quickly, the PCP client may find it
beneficial to use shorter lifetimes (so that it communicates
with the PCP server more often).</t>
<t>If the PCP client has several mappings, the Epoch Time
value only needs to be retrieved for one of them to determine
whether or not it appears the PCP server may have suffered a
catastrophic loss of state.
If the client wishes to check the PCP server's Epoch Time, it sends a
PCP request for any one of the client's mappings. This will return
the current Epoch Time value. In that request the PCP client could extend
the mapping lifetime (by asking for more time) or maintain the
current lifetime (by asking for the same number of seconds that it
knows are remaining of the lifetime).</t>
<t>If a PCP client changes its Internal IP Address (e.g., because
the Internal Host has moved to a new network), and the PCP client
wishes to still receive incoming traffic, it needs create new
mappings on that new network. New mappings will typically also
require an update to the application-specific rendezvous server if
the External Address or Port are different to the previous values
(see <xref target="operating_a_server"></xref> and
<xref target="renumbering"></xref>).</t>
</section>
<section title="SCTP">
<t>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 <xref target="I-D.ietf-behave-sctpnat"></xref>.
Outbound dynamic SCTP mappings use the verification tag of the
association instead of the local and remote peer port numbers. As
with TCP, explicit outbound mappings can be made to reduce keepalive
intervals, and explicit inbound mappings can be made by passive
listeners expecting to receive new associations at the external
port.</t>
<t>Because an SCTP-aware NAT does not rewrite SCTP port numbers, a
PCP MAP or PEER request for an SCTP mapping SHOULD provide the same
Internal Port and Suggested External Port. If the PCP server
supports SCTP, and the suggested external port cannot be provided in
an explicit dynamic SCTP mapping, then the error
CANNOT_PROVIDE_EXTERNAL is returned. This places an extra burden on
the SCTP client because it then has to tear down its listening socket
and try again with a different Internal Port, repeatedly until it is
successful in finding a port it can use.</t>
<t>The SCTP complications described above occur because of
address sharing. The SCTP complications are avoided when
address sharing is avoided (e.g., 1:1 NAT, firewall).</t>
</section>
</section>
<section title="Source Address Replicated in PCP Header" anchor="implement_source_address">
<t>All PCP requests include the PCP client's IP address replicated in
the PCP header. This is used to detect address rewriting (NAT)
between the PCP client and its PCP server. On operating systems that
support the sockets API, the following steps are RECOMMENDED for a PCP
client to insert the correct source address and port to include in the
PCP header:</t>
<t><list style="numbers">
<?rfc subcompact="yes"?>
<t>Create a UDP socket.</t>
<t>Call "connect" on this UDP socket using the
address and port of the desired PCP server.</t>
<t>Call the getsockname() function to retrieve a sockaddr
containing the source address the kernel will use
for UDP packets sent through this socket.</t>
<t>If the IP address is an IPv4 address, encode the
address into an IPv4-mapped IPv6 address. Place the
native IPv6 address or IPv4-mapped IPv6 address into
the PCP Client's IP Address field in the PCP header.</t>
<t>Send PCP requests using this connected UDP socket.</t>
<?rfc subcompact="no"?>
</list></t>
</section>
<section anchor="state_section" title="State Diagram">
<t>Each mapping entry of the PCP-controlled device would go through the
state machine shown below. This state diagram is non-normative.</t>
<figure anchor="state_diagram" title="PCP State Diagram">
<artwork align="center"><![CDATA[
CLOSE_MSG or
(NO_TRAFFIC and EXPIRY) +---------+ NO_TRAFFIC and EXPIRY
+-------------->| |<------------+
| |NO_ENTRY | |
| +-----------| |---------+ |
| | +---------+ | |
| | ^ | | |
| | NO_TRAFFIC | | | |
| | or | | | |
| | CLOSE_MSGS | | | |
| | | | | |
| |PEER request | | MAP request| |
| V | | V |
+---------+ | | +---------+
+-->| "P", | | | M-R | "M", |<--+
P-R | | PEER |-----------|--|-------->| MAP | | M-R or
+---| mapping| | | | mapping|---+ P-R or
+---------+ | | +---------+ CLOSE_MSGS
| ^ | | ^ |
| |PEER request | | MAP request| |
| | | | | |
| | | | | |
| | | | | |
| | | | outbound | |
| | | | TRAFFIC | |
| | | V | |
| | +---------+ | |
| +-----------| "I", |---------+ |
| | implicit| |
+-------------->| mapping |<------------+
TRAFFIC and EXPIRY +---------+ TRAFFIC and EXPIRY
]]></artwork>
</figure>
<?rfc needLines="15" ?>
<t>The meanings of the states and events are:
<list style="hanging" hangIndent="8">
<t hangText=" NO_ENTRY:">Invalid state represents Entry does not
exist. This is the only possible start state.</t>
<t hangText=" M-R:">MAP request</t>
<t hangText=" P-R:">PEER request</t>
<t hangText=" M:">Mapping entry when created by MAP request</t>
<t hangText=" P:">Mapping entry when created/managed by PEER request</t>
<t hangText=" I:">Implicit mapping created by an outgoing packet from
the client (e.g., TCP SYN), and also the state when a PCP-created
mapping's lifetime expires while there is still active traffic.</t>
<t hangText=" EXPIRY:">PEER or MAP lifetime expired</t>
<t hangText=" TRAFFIC:">Traffic seen by PCP-controlled device using
this entry within the expiry time for that entry. This
traffic may be inbound or outbound.</t>
<t hangText=" NO_TRAFFIC:">Indicates that there is no TRAFFIC.</t>
<t hangText=" CLOSE_MSG:">Protocol messages from the client or server
to close the session (e.g., TCP FIN or TCP RST), as per the NAT
or firewall device's handling of such protocol messages.</t>
</list></t>
<t>Notes on the diagram:<list style="numbers">
<t>The 'and' clause indicates the events on either side of 'and' are
required for the state-transition. The 'or' clause indicates
either one of the events are enough for the state-transition.</t>
<t>Transition from state M to state I is implementation dependent.</t>
</list></t>
</section>
</section>
<!--
<section anchor="scenarios" title="Deployment Scenarios">
<section title="Dual-Stack Lite">
<t>The interesting components in a Dual-Stack Lite deployment are the
B4 element (which is the customer premises router) and the
Address Family Transition Router (AFTR) element. The AFTR element
terminates the IPv6-over-IPv4 tunnel and also implements a
Carrier-Grade NAT44 function. The B4 element does not need to perform
a NAT function (and usually does not perform a NAT function), but it
does operate its own DHCP server and is the local network's default
router.</t>
<t>Various PCP deployment scenarios can be considered to control the
PCP server in the AFTR element:<list style="numbers">
<t>If UPnP IGDv1 and NAT-PMP
<xref target="I-D.cheshire-nat-pmp"></xref> are used in the LAN: an
interworking function is required to be implemented by the B4
element to ensure interworking between the protocol used in the
LAN and PCP. Details of this interworking function are left for future work.</t>
<t>Hosts behind the B4 element will either include a PCP client, UPnP IGDv1 client, NAT-PMP client, or combination of these three. <list style="letters">
<t>if a UPnP IGDv1 client, the B4 element will need to include
an interworking function from UPnP IGDv1 to PCP.</t>
<t>if a PCP client, the PCP client will communicate
directly with its nearest PCP server (e.g., built
into its B4 element).</t>
<t>if a NAT-PMP client, the NAT-PMP client will communicate with its default gateway (its B4 element), which will proxy NAT-PMP to the Dual-Stack Lite AFTR element. Details of this proxy function are left for future work.</t>
</list></t>
<t>The B4 element includes a PCP client which is invoked by an
HTTP-based configuration (as is common today). The internal IP
address field in the PCP payload would be the Internal Host used
in the port forwarding configuration.</t>
</list></t>
<t>In Dual-Stack Lite, the B4 element encapsulates its PCP messages
into the IPv6 tunnel towards the AFTR element. When proxying
for other hosts, the B4 element will use the THIRD_PARTY
Option with the MAP and PEER Opcodes.</t>
</section>
<section title="NAT64">
<t>Hosts behind a NAT64 device can make use of PCP in order to
perform port reservation (to get a publicly routable IPv4
port). For example, in such situations, the IPv6 host can use
the MAP4 Opcode to operate a server that listens on the IPv4
Internet.</t>
<t>If the IANA-assigned IP address is used for the discovery of the
PCP server, that IPv4 address can be placed into the IPv6 destination
address following that particular network's well-known prefix or
network-specific prefix, per <xref target="RFC6052"></xref>.</t>
</section>
<section title="NAT44 and NAT444">
<t>Residential subscribers in NAT44 (and NAT444) deployments are
usually given one IPv4 address, but may also be given several IPv4
addresses. These addresses are not routable on the IPv4 Internet, but
are routable between the subscriber's home and the ISP's CGN. To
accommodate multiple hosts within a home, especially when provided
insufficient IPv4 addresses for the number of devices in the home,
subscribers operate a NAT device. When this occurs in conjunction
with an upstream NAT44, this is nicknamed "NAT444".</t>
<list style="empty">
<t>[Ed. Note: Does PCP need a mechanism to detect a
non-PCP-supporting NAT between a PCP client and a PCP server? Or
can that problem be detected by relying on the failure of PCP
server Discovery? This is tracked as PCP Issue #25
<xref target="PCP-Issues"></xref>.]</t>
</list></t>
</section>
<section title="IPv6 Simple Firewall">
<t>Many IPv6 deployments will include a <xref target="RFC6092">simple
firewall</xref>, which permits outgoing packets to initiate
bi-directional communication but blocks unsolicited incoming packets,
which is similar to PCP's security model that allows a host to create
a mapping to itself. In many situations, especially residential
networks that lack an IT staff, the security provided by an IPv6
simple firewall and the security provided by PCP are compatible. In
such situations, the IPv6 simple firewall and the IPv6 host can use
the MAP6 Opcode to allow unsolicited incoming packets, so the host can
operate a server.</t>
</section>
</section>
-->
<!--
<section anchor="upnp-interworking" title="Interworking with UPnP IGD 1.0 and 2.0">
<t><list style="empty">
<t>[Ed. Note: This UPnP IGD Interworking section will likely be
moved to a separate document which will fully describe how a proxy
needs to translate UPnP IGD messages into PCP messages. This is
tracked as PCP Issue #28 <xref target="PCP-Issues"></xref>.]</t>
</list></t>
<t>The following diagram shows how UPnP IGD can be interworked with PCP,
using an interworking function (IWF).</t>
<figure align="center" anchor="igd-interworking-function" title="Network Diagram, Interworking UPnP IGD and PCP">
<artwork><![CDATA[
xxx
]]></artwork>
</figure>
<section title="UPnP IGD 1.0 with AddPortMapping Action">
<t>In <xref target="IGDv1">UPnP IGD 1.0</xref> it is only possible to
request a specific port using the AddPortMapping action. Requiring a
specific port is incompatible with both (1) a Carrier-Grade NAT and
with (2) widely-deployed applications. Regarding (1), another
subscriber is likely to already be using the same port, so it will be
unavailable to this application to operate a server. Regarding (2), if
the same popular application exists on two devices behind the same
NAT, they cannot both get the same port. PCP cannot correct this
behavior of UPnP IGD:1, but PCP does work with this behavior.</t>
<t>Due to this incompatibility with address sharing and popular
applications, future hosts and applications will either support UPnP
IGD 2.0's AddAnyPortMapping method (see
<xref target="upnp-2-interworking"></xref>) or, more likely, will support
PCP natively.</t>
<t>When a requested port assignment fails, most UPnP IGD control
points will retry the port assignment requesting the next higher port
or requesting a random port. These UPnP IGD requests are translated to
PCP requests and sent to the PCP server. The requests include the
PREFER_FAILURE Option, which causes the PCP server to return an error
if it cannot allocate the suggested port. The interworking function
translates the PCP error response to a UPnP IGD error response. This
repeats until the UPnP IGD client gives up or until the PCP server is
able to return the suggested port.</t>
<figure align="left" anchor="message-flow-upnp-1"
title="Message Flow: Interworking from UPnP IGD 1.0 AddPortMapping action to PCP">
<preamble>Message flow would be similar to this:</preamble>
<artwork align="center"><![CDATA[
UPnP Control Point in-home CPE PCP server
xxx
]]></artwork>
</figure>
</section>
<section anchor="upnp-2-interworking" title="UPnP IGD 2.0 with AddAnyPortMapping Action">
<t>If the UPnP IGD control point and the UPnP IGD interworking
function both implement <xref target="IGD-2">UPnP IGD 2.0</xref> and
the UPnP IGD control point uses IGD 2's new AddAnyPortMapping action,
only one round-trip is necessary. This is because AddAnyPortMapping
has semantics similar to PCP's semantics, allowing the PCP server to
assign any port.</t>
<figure align="left" anchor="message-flow-upnp-2"
title="Message Flow: Interworking from UPnP IGD 2.0 AddAnyPortMapping action to PCP">
<preamble>Message flow would be similar to this:</preamble>
<artwork align="center"><![CDATA[
UPnP Control Point in-home CPE PCP server
xxx
]]></artwork>
</figure>
</section>
<section title="Lifetime Maintenance">
<t>UPnP IGD 1.0 and 2.0 provide a lifetime (PortMappingLeaseDuration),
but it is seldom used by UPnP IGD control points, and does not allow
the UPnP IGD to override the requested duration. Thus, the UPnP
IGD/PCP interworking function is responsible for extending the
lifetime of mappings that are still interesting to the UPnP IGD
control point.</t>
<t><list style="empty">
<t>Note: It can be an implementation advantage, where possible,
for the UPnP IGD/PCP interworking function to request a port
mapping lifetime only while that client is active and connected.
For example, creating a PCP mapping that is equal to the client's
remaining DHCP lifetime is one useful approach.</t>
</list></t>
</section>
</section>
-->
<?rfc needLines="6" ?>
<section title="Deployment Considerations">
<section title="Ingress Filtering" anchor="ingress_filtering">
<t>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
<xref target="RFC2827">ingress filtering</xref> SHOULD be used on the path
between the Internal Host and the PCP Server to prevent the injection
of spoofed packets onto that path.</t>
</section>
<section anchor="quota" title="Mapping Quota">
<t>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.</t>
</section>
</section>
<?rfc needLines="6" ?>
<section anchor="security" title="Security Considerations">
<t>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.</t>
<section anchor="SimpleThreat" title="Simple Threat Model">
<t>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.</t>
<t>Defending against attackers who can modify or drop packets
between the internal network and the PCP server, or who can
inject spoofed packets that appear to come from the internal
network is out of scope. Such an attacker can re-direct traffic
to a host of their choosing.</t>
<t>A PCP Server is secure under this threat model if the PCP
Server is constrained so that it does not configure any explicit
mapping that it would not configure implicitly. In most cases,
this means that PCP Servers running on NAT boxes or stateful
firewalls that support the PEER Opcode can be secure
under this threat model if all of their hosts are within a
single administrative domain (or if the internal hosts can be
securely partitioned into separate administrative domains, as in
the DS-Lite B4 case), explicit mappings are created with the
same lifetime as implicit mappings, the PCP server does not
support deleting or reducing the lifetime of existing mappings,
and the PCP server does not support the third party option. PCP
Servers can also securely support the MAP Opcode
under this threat model if the security policy on the device
running the PCP Server would permit endpoint independent
filtering of implicit mappings.</t>
<t>PCP Servers that comply with the Simple Threat Model and do
not implement a PCP security mechanism described in
<xref target="AdvancedThreat"/> MUST enforce the constraints
described in the paragraph above.</t>
<?rfc needLines="6" ?>
<section title="Attacks Considered" anchor="AttacksConsidered">
<t><list style='symbols'>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
</section>
<section title="Deployment Examples Supporting the Simple Threat Model">
<t>This section offers two examples of how the Simple Threat
Model can be supported in real-world deployment scenarios.</t>
<section title="Residential Gateway Deployment">
<t>Parity with many currently-deployed residential gateways
can be achieved using a PCP Server that is constrained as
described in <xref target="AttacksConsidered"/> above.</t>
</section>
<section anchor="DSLiteDeployment" title="DS-Lite Deployment">
<t>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.</t>
<t>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.</t>
</section>
</section>
</section>
<section anchor="AdvancedThreat" title="Advanced Threat Model">
<t>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.</t>
<t>The Advanced Threat Model security model will be needed in
the following cases:</t>
<t><list style='symbols'>
<t>Security infrastructure equipment, such as corporate
firewalls, that does not create implicit mappings.</t>
<t>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.</t>
<t>Any implementation that wants to be more permissive in
authorizing explicit mappings than it is in authorizing
implicit mappings.</t>
<t>Implementations that support the THIRD_PARTY Option (unless
they can meet the constraints outlined in
<xref target="DSLiteDeployment"/>).</t>
<t>Implementations that wish to support any deployment
scenario that does not meet the constraints described in
<xref target="SimpleThreat"/>.</t>
</list></t>
<t>To protect against attacks under this threat model, a
PCP security mechanism that
provides an authenticated, integrity protected signaling
channel would need to be specified.</t>
<t>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
<xref target="SimpleThreat"/> above, in their default
configuration, when processing unauthenticated requests.</t>
</section>
<section title="Residual Threats">
<t>This section describes some threats that are not addressed in
either of the above threat models, and recommends appropriate
mitigation strategies.</t>
<section title="Denial of Service">
<t>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, <xref target="quota"/> recommends that hosts be limited
to a reasonable number of explicit dynamic mappings.</t>
<t>An attacker, on the path between the PCP client and PCP
server, can drop PCP requests, drop PCP responses, or spoof
a PCP error, all of which will effectively deny service.
Through such actions, the PCP client might not be aware the
PCP server might have actually processed the PCP request.
An attacker sending a NO_RESOURCES error can cause the PCP
client to not send messages to that server for a while. There
is no mitigation to this on-path attacker.</t>
</section>
<section title="Ingress Filtering">
<t>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.</t>
</section>
<section title="Mapping Theft">
<t>In the time between when a PCP server loses state and the
PCP client notices the lower than expected Epoch Time value, it is
possible that the PCP client's mapping will be acquired by
another host (via an explicit dynamic mapping or implicit
dynamic mapping). This means incoming traffic will be sent to
a different host ("theft"). Rapid Recovery reduces
this interval, but would not completely eliminate this threat.
The PCP client can reduce this interval by using a relatively
short lifetime; however, this increases the amount of PCP
chatter. This threat is reduced by using persistent storage
of explicit dynamic mappings in the PCP server (so it does not
lose explicit dynamic mapping state), or by ensuring the
previous external IP address and port cannot be used by
another host (e.g., by using a different IP address pool).</t>
</section>
<section title="Attacks Against Server Discovery">
<t>This document does not specify server discovery, beyond contacting
the default gateway.</t>
</section>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<t>IANA is requested to perform the following actions:</t>
<section anchor="iana_port" title="Port Number">
<t>PCP will use ports 5350 and 5351 (currently assigned by IANA to
<xref target="I-D.cheshire-nat-pmp">NAT-PMP</xref>). We request that IANA
re-assign those ports to PCP, and relinquish UDP port 44323.</t>
<t>[Note to RFC Editor: Please remove the text about relinquishing
port 44323 prior to publication.]</t>
</section>
<section anchor="iana_opcodes" title="Opcodes">
<t>IANA shall create a new protocol registry for PCP Opcodes,
numbered 0-127, initially populated with the values:</t>
<figure>
<artwork align="center"><![CDATA[
value Opcode
----- -------------------------
0 ANNOUNCE
1 MAP
2 PEER
3-31 Standards Action [RFC5226]
32-63 Specification Required [RFC5226]
96-126 Private Use [RFC5226]
127 Reserved, Standards Action [RFC5226]]]></artwork>
</figure>
<t>The value 127 is Reserved and may be assigned via
<xref target="RFC5226">Standards Action</xref>.
The values in the
range 3-31 can be assigned via <xref target="RFC5226">Standards
Action</xref>, 32-63 via <xref target="RFC5226">Specification
Required</xref>, and 96-126 is for <xref target="RFC5226">Private
Use</xref>.</t>
</section>
<section anchor="iana_result_codes" title="Result Codes">
<t>IANA shall create a new registry for PCP result codes, numbered
0-255, initially populated with the result codes from
<xref target="result_codes"></xref>. The value 255 is Reserved
and may be assigned via <xref target="RFC5226">Standards
Action</xref>.</t>
<t>The values in the
range 14-127 can be assigned via <xref target="RFC5226">Standards
Action</xref>, 128-191 via <xref target="RFC5226">Specification
Required</xref>, and 191-254 is for <xref target="RFC5226">Private
Use</xref>.</t>
</section>
<section anchor="iana_ie" title="Options">
<t>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
<xref target="map_peer_options"/>. The Option values 0, 127 and
255 are Reserved and may be assigned
via <xref target="RFC5226">Standards Action</xref>.</t>
<t>Additional PCP Option codes in the ranges 4-63 and 128-191
can be created via <xref target="RFC5226">Standards Action
</xref>, the ranges 64-95 and 192-223 are for
<xref target="RFC5226">Specification Required</xref> and the ranges
96-126 and 224-254 are for <xref target="RFC5226">Private
Use</xref>.</t>
<t>Documents describing an Option should describe if the
processing for both the PCP client and server and the
information below: <list style="empty">
<?rfc subcompact="yes"?>
<t>Option Name: <mnemonic></t>
<t>Number: <value></t>
<t>Purpose: <textual description></t>
<t>Valid for Opcodes: <list of Opcodes></t>
<t>Length: <rules for length></t>
<t>May appear in: <requests/responses/both></t>
<t>Maximum occurrences: <count></t>
<?rfc subcompact="no"?>
</list></t>
</section>
</section>
<section title="Acknowledgments">
<t>Thanks to Xiaohong Deng, Alain Durand, Christian Jacquenet,
Jacni Qin, Simon Perreault, James Yu, Tina TSOU (Ting ZOU),
Felipe Miranda Costa, James Woodyatt, Dave Thaler, Masataka
Ohta, Vijay K. Gurbani, Loa Andersson, Richard Barnes, Russ
Housley, Adrian Farrel, Pete Resnick, Pasi Sarolahti, Robert
Sparks, Wesley Eddy, Dan Harkins, Peter Saint-Andre, Stephen
Farrel, Ralph Droms, for their comments and review. Thanks to
Simon Perreault for highlighting the interaction of dynamic
connections with PCP-created mappings.</t>
<t>Thanks to Francis Dupont for his several thorough reviews of the
specification, which improved the protocol significantly.</t>
<t>Thanks to T. S. Ranganathan for the state diagram.</t>
<t>Thanks to Margaret Wasserman for writing the Security
Considerations section.</t>
<t>Thanks to authors of DHCPv6 for retransmission text.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.0768"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2827"?>
<?rfc include='reference.RFC.4193'?>
<?rfc include="reference.RFC.5226"?>
<?rfc include='reference.RFC.6056'?>
<reference anchor="proto_numbers"
target="http://www.iana.org/assignments/protocol-numbers/protocol-numbers.xml">
<front>
<title>Protocol Numbers</title>
<author fullname="IANA" surname="IANA">
<organization></organization>
</author>
<date year="2011" />
</front>
</reference>
</references>
<references title="Informative References">
<?rfc include='reference.RFC.3007'?>
<?rfc include='reference.RFC.2136'?>
<?rfc include='reference.RFC.0793'?>
<?rfc include='reference.RFC.4303'?>
<?rfc include='reference.RFC.1918'?>
<?rfc include='reference.RFC.3022'?>
<?rfc include='reference.RFC.3581'?>
<?rfc include='reference.RFC.3587'?>
<?rfc include='reference.RFC.4291'?>
<?rfc include='reference.RFC.4787'?>
<?rfc include='reference.RFC.4941'?>
<?rfc include='reference.RFC.4961'?>
<?rfc include='reference.RFC.5382'?>
<?rfc include='reference.RFC.6092'?>
<?rfc include='reference.RFC.6296'?>
<?rfc include='reference.RFC.6145'?>
<?rfc include='reference.RFC.6146'?>
<?rfc include='reference.RFC.6333'?>
<?rfc include='reference.RFC.4960'?>
<?rfc include='reference.RFC.4340'?>
<?rfc include='reference.I-D.miles-behave-l2nat'?>
<!-- <?rfc include='reference.I-D.penno-pcp-zones'?> -->
<?rfc include='reference.I-D.ietf-behave-sctpnat'?>
<?rfc include='reference.I-D.bpw-pcp-upnp-igd-interworking'?>
<?rfc include='reference.I-D.ietf-behave-lsn-requirements'?>
<?rfc include='reference.I-D.cheshire-nat-pmp'?>
<!--<?rfc include='reference.I-D.cheshire-pcp-recovery'?>-->
<?rfc include='reference.I-D.arkko-dual-stack-extra-lite'?>
<?rfc include='reference.I-D.boucadair-pcp-failure'?>
<!-- <?rfc include='reference.I-D.dupont-pcp-dslite'?> -->
<!--
<?rfc include='reference.RFC.5461'?>
<?rfc include='reference.RFC.3424'?>
-->
<?rfc include='reference.I-D.cheshire-dnsext-dns-sd'?>
<reference anchor="Bonjour"
target="http://en.wikipedia.org/wiki/Bonjour_(software)">
<front>
<title>Bonjour</title>
<author></author>
<date />
</front>
</reference>
<reference anchor="IGDv1"
target="http://upnp.org/specs/gw/UPnP-gw-WANIPConnection-v1-Service.pdf">
<front>
<title>WANIPConnection:1</title>
<author fullname="UPnP Gateway Committee"
surname="UPnP Gateway Committee">
<organization>UPnP Forum</organization>
</author>
<date month="November" year="2001" />
</front>
</reference>
<!--
<reference anchor="IGD-2"
target="http://upnp.org/specs/gw/UPnP-gw-WANIPConnection-v2-Service.pdf">
<front>
<title>Internet Gateway Device (IGD) V 2.0</title>
<author fullname="UPnP Gateway Committee"
surname="UPnP Gateway Committee">
<organization>UPnP Forum</organization>
</author>
<date month="September" year="2010" />
</front>
</reference>
-->
<!--
<reference anchor="PCP-Issues"
target="http://trac.tools.ietf.org/wg/pcp/trac/report/1">
<front>
<title>PCP Active Tickets</title>
<author fullname="PCP Working Group" surname="PCP Working Group">
<organization>IETF</organization>
</author>
<date month="January" year="2011" />
</front>
</reference>
-->
<!--
<reference anchor="I-D.bpw-pcp-proxy">
<front>
<title>Port Control Protocol (PCP) Proxy Function</title>
<author fullname="Mohammed Boucadair" initials="M"
surname="Boucadair">
<organization></organization>
</author>
<author fullname="Reinaldo Penno" initials="R" surname="Penno">
<organization></organization>
</author>
<author fullname="Dan Wing" initials="D" surname="Wing">
<organization></organization>
</author>
<author fullname="Francis Dupont" initials="F" surname="Dupont">
<organization></organization>
</author>
<date day="27" month="February" year="2011" />
<abstract>
<t>This document specifies the behavior of a PCP Proxy element,
for instance implemented in Customer Premises routers.</t>
</abstract>
</front>
<seriesInfo name="Internet-Draft" value="draft-bpw-pcp-proxy-00" />
<format target="http://www.ietf.org/internet-drafts/draft-bpw-pcp-proxy-00.txt"
type="TXT" />
</reference>
-->
<!--
<reference anchor="I-D.wing-pcp-nested-nat">
<front>
<title>Using PCP With One Nested NAT</title>
<author fullname="Dan Wing" initials="D" surname="Wing">
<organization></organization>
</author>
<date month="January" year="2011" />
</front>
<seriesInfo name="Internet-Draft" value="draft-wing-pcp-nested-nat-00" />
<format target="http://www.ietf.org/internet-drafts/draft-wing-pcp-nested-nat-00.txt"
type="TXT" />
</reference>
-->
</references>
<section title="NAT-PMP Transition">
<t>The Port Control Protocol (PCP) is a successor to the NAT Port
Mapping Protocol, <xref target="I-D.cheshire-nat-pmp">NAT-PMP</xref>,
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.</t>
<t>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 <xref target="I-D.cheshire-nat-pmp">the NAT-PMP
specification</xref>, 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.</t>
<t>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).</t>
<t>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.</t>
</section>
<section title="Change History">
<t>[Note to RFC Editor: Please remove this section prior to
publication.]</t>
<section title="Changes from draft-ietf-pcp-base-23 to -24">
<t><list style="symbols">
<t>Explained common questions regarding PCP's design, such as lack of
transction identifiers and its request/response semantics and operation
(<xref target="protocol_design_note">Protocol Design Note</xref>).</t>
<t>added MUST for all-zeros IPv6 and IPv4 address formats.</t>
<t>included field definitions for Opcode-specific information
and PCP options under both <xref target="common_request"></xref>
and <xref target="common_response"></xref>.</t>
<t>adopted retransmission mechanism from DHCPv6.</t>
<t>1024 message size limit described in PCP message restriction.</t>
<t>Explained PCP server list, with example of host with IPv4 and IPv6
addresses having two PCP servers (one IPv4 PCP server for IPv4
mappings and one IPv6 PCP server for IPv6 mappings).</t>
<t>mention PCP client needs to expect unsolicited PCP responses from
previous incarnations of itself (on the same host) or of this host (using
same IP address as another PCP client).</t>
<t>eliminated overuse of 'packet format' when it was 'opcode format'.</t>
<t>for IANA registries, added code points assignable via Standards Action
(previously was just Specification Required).</t>
<t>Version negotiation, added explanation that retrying after 30 minutes
makes the protocol self-healing if the PCP server is upgraded.</t>
<t>Version negotiation now accomodates non-contiguous version numbers.</t>
<t>Tweaked definition of VERSION field (that "1" is for this version,
but other values could of course appear in the future).</t>
<t>when receiving unsolicited ANNOUNCE, PCP client now waits random
0-5 seconds.</t>
<t>Removed 'interworking function' from list of terminology because
we no longer use the term in this document.</t>
<t>tightened definitions of 'PCP client' and 'PCP server'.</t>
<t>For 'Requested Lifetime' definitions, removed text requiring its
value be 0 for not-yet-defined opcodes.</t>
<t>Removed some unnecessary text suggesting logging (is an implementation
detail).</t>
<t>Added active-mode FTP as example protocol that can break with mappings
to different IP addresses.</t>
<t>Clarified that if PCP request contains a Suggested External Address,
the PCP server should try to create a mapping to that address even if
other mappings already exist to a different external address.</t>
<t>Changed "internal address and port" to "internal address, protocol,
and port" in several places.</t>
<t>Clarified which 96 bits are copied into error response. Clarified that
only error responses are copied verbatim from request.</t>
<t>a single PCP server can control multiple NATs or multiple firewalls (<xref
target="relationship"></xref>).</t>
<t>Clarified that sending unsolicited multicast ANNOUNCE is not always available on all networks.</t>
<t>Clarified option length error example is when option length exceeds UDP length</t>
<t>Explained that an on-path attacker that can spoof packets can re-direct
traffic to a host of their choosing.</t>
<t>Instead of saying IPv4-mapped addresses won't appear on the wire,
say they aren't used for mappings.</t>
<t>THIRD_PARTY is useful for management device (e.g., in a network operations
center).</t>
<t>Clarified PCP responses have fields updated as indicated with 'set by
the server' from field definitions.</t>
<t>Disallow using MAP to the PCP ports themselves and encourage implementations
have policy control for other ports.</t>
<t>Instead of 'idempotent', now says 'identical requests generate
identical response'.</t>
<t>Described which Options are included when sending Mapping Update (unsolicited responses), <xref target="update"></xref>.</t>
<t>Dropped <xref target="RFC2136"></xref> and <xref target="RFC3007"></xref> to informative references.</t>
<t>Updated from 'should' to 'SHOULD' in <xref target="ingress_filtering"></xref>.</t>
<t>Described 'hairpin' in terminology section.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-22 to -23">
<t><list style="symbols">
<t>Instead of returning error NO_RESOURCES when requesting a MAP
for all protocols or for all ports, return UNSUPP_PROTOCOL.</t>
<t>Clarify that PEER-created mappings are treated as if it
was implicit dynamic outbound mapping
(<xref target="peer-opcode_server_operation"></xref>).</t>
<t>Point out that PEER-created mappings may be very short until
bi-directional traffic is seen by the PCP-managed device.</t>
<t>Clairification that an existing implicit mapping (created e.g., by TCP SYN)
can become managed by a MAP request (<xref target="map-opcode_server_operation"></xref>.</t>
<t>Clarified the ANNOUNCE Opcode is being defined in <xref target="announcement"></xref>, and that the length of requests (as well as responses) is zero.</t>
<t>Clarify that ANNOUNCE has Lifetime=0 for requests and responses.</t>
<t>Clarify ANNOUNCE can be sent unicast by the client (to solicit a
response), or can be multicasted (unsolicited) by the server.</t>
<t>Allow ANNOUNCE to be sent unicast by the server, to accomodate case
where PCP server fails but knows the IP address of a PCP client (e.g.,
web portal).</t>
<t>Clarified ports used for unicast and multicast unsolicited ANNOUNCE.</t>
<t>Tweaked NO_RESOURCES handling, to just disallow *new* mappings.</t>
<t>State diagram is now non-normative, because it overly simplifies
that implicit mappings become MAP (when they actually still retain
their previous behavior when the MAP expires).</t>
<t>In section <xref target="lifetime"></xref>, clarified that PEER
cannot delete or shorten any lifetime, and that MAP can only shorten
or delete lifetimes of MAP-created mappings.</t>
<t>Clarified handling of MAP when mapping already exists (4 steps).</t>
<t>2^32-1</t>
<t>Randomize retry interval (1.5-2.5), and maximum retry interval is
now 1024 seconds (was 15 minutes).</t>
<t>Remove MUST be 0 for Reserved field when sending error responses for
un-parseable message.</t>
<t>Whenever PCP client includes Suggested IP Address (in MAP or PEER),
the PCP server should try to fulfill that request, even if creating a
mapping on that IP address means the internal host will have mappings
on different IP addresses and ports.</t>
<t>For NO_RESOURCES error, the PCP client can attempt to renew and
attempt to delete mappings (as they can help shed load) -- it just
can't try to create new ones.</t>
<t>Removed the overly simplistic normative text regarding honoring
Suggested External Address
from <xref target="opcode_introduction"></xref> in favor of
the text in <xref target="map-opcode_server_operation"></xref> which has
significantly more detail.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-21 to -22">
<t><list style="symbols">
<t>Removed paragraph discussing multiple addresses on the same
(physical) interface; those will work with PCP.</t>
<t>The FILTER Option's Prefix Length field redefined to simply be
a count of the relevant bits (rather than 0-32 for IPv4-mapped
addresses).</t>
<t>Point out NO_RESOURCES attack vector in security considerations.</t>
<t>Tighten up recommendation for client handling long Lifetimes, and
moved from the MAP-specific section to the General PCP Processing
section. Client should normalize to 24 hours maximum for success and
30 minute maximum for errors.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-20 to -21">
<t><list style="symbols">
<t>To delete all mappings using THIRD_PARTY, use the
all-zeros IP address (rather than previous text which used length=0).</t>
<t>added normative text for what PCP server does when it receives
all-zeros IP address in THIRD_PARTY option.</t>
<t>PREFER_FAILURE allowed for use by web portal.</t>
<t>clarifications to mandatory option processing.</t>
<t>cleanup and wordsmithing of the THIRD_PARTY text.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-19 to -20">
<t><list style="symbols">
<t>clarify if Options are included in responses.</t>
<t>clarify when External Address can be ignored by the
PCP server / PCP-controlled device</t>
<t>added 'Transition from state M to state I is
implementation dependent' to state diagram</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-18 to -19">
<t><list style="symbols">
<t>Described race condition with MAP containing PREFER_FAILURE
and Mapping Update.</t>
<t>Added state machine (<xref target="state_section"></xref>).</t>
<t>Fully integrated Rapid Recovery, with a separate Opcode
having its own processing description.</t>
<t>Clarified that due to Mapping Update, a single MAP or PEER request
can receive multiple responses, each updating the previous request,
and that the PCP client needs to handle MAP updates or PEER updates
accordingly.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-17 to -18">
<t><list style="symbols">
<t>Removed UNPROCESSED option. Instead, unprocessed options
are simply not included in responses.</t>
<t>Updated terminology section for Implicit/Explicit and Outbound/Inbound.</t>
<t>PEER requests cannot delete or shorten the lifetime of a mapping.</t>
<t>Clarified that PCP clients only retransmit mapping requests for
as long as they actually want the mapping.</t>
<t>Revised Epoch time calculations and explanation.</t>
<t>Renamed the announcement opcode from No-Op to ANNOUNCE.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-16 to -17">
<t><list style="symbols">
<t>suggest acquiring a mapping to the Discard port if
there is a desire to show the user their external
address (<xref target="alone"></xref>).</t>
<t>Added Restart Announcement.</t>
<t>Tweaked terminology.</t>
<t>Detailed how error responses are generated.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-15 to -16">
<t><list style="symbols">
<t>fixed mistake in PCP request format (had 32 bits of extraneous fields)</t>
<t>Allow MAP to request all ports (port=0) for a specific protocol
(protocol!=0), for the same reason we added support for all ports
(port=0) and all protocols (protocol=0) in -15</t>
<t>corrected text on Client Processing a Response related to receiving
ADDRESS_MISMATCH error.</t>
<t>updated Epoch text.</t>
<t>Added text that MALFORMED_REQUEST is generated for MAP if Protocol is
zero but Internal Port is non-zero.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-14 to -15">
<t><list style="symbols">
<t>Softened and removed text that was normatively explaining
how PEER is implemented within a NAT.</t>
<t>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.</t>
<t>combined SERVER_OVERLOADED and NO_RESOURCES into one error code,
NO_RESOURCES.</t>
<t>SCTP mappings have to use same internal and suggested
external ports, and have implied PREFER_FAILURE semantics.</t>
<t>Re-instated ADDRESS_MISMATCH error, which only checks the
client address (not its port).</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-13 to -14">
<t><list style="symbols">
<t>Moved discussion of socket operations for PCP source
address into Implementation Considerations section.</t>
<t>Integrated numerous WGLC comments.</t>
<t>NPTv6 in scope.</t>
<t>Re-written security considerations section. Thanks, Margaret!</t>
<t>Reduced PEER4 and PEER6 Opcodes to just a single Opcode, PEER.</t>
<t>Reduced MAP4 and MAP6 Opcodes to just a single Opcode, MAP.</t>
<t>Rearranged the PEER packet formats to align with MAP.</t>
<t>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.</t>
<t>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.</t>
<t>Added table to show which fields of MAP or PEER need
IPv6/IPv4 addresses for IPv4 firewall, DS-Lite, NAT64,
NAT44, etc.</t>
<t>Accommodate the server's Epoch going up or down, to better
detect switching to a different PCP server.</t>
<t>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).</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-12 to -13">
<t><list style="symbols">
<t>All addresses are 128 bits. IPv4 addresses are
represented by IPv4-mapped IPv6 addresses (::FFFF/96)</t>
<t>PCP request header now includes PCP client's port (in addition
to the client's IP address, which was in -12).</t>
<t>new ADDRESS_MISMATCH error.</t>
<t>removed PROCESSING_ERROR error, which was too similar to
MALFORMED_REQUEST.</t>
<t>Tweaked text describing how PCP client deals with multiple PCP
server addresses (<xref target="general_generate_request"></xref>)</t>
<t>clarified that when overloaded, the server can send
SERVER_OVERLOADED (and drop requests) or simply drop requests.</t>
<t>Clarified how PCP client chooses MAP4 or MAP6, depending on the
presence of its own IPv6 or IPv4 interfaces
(<xref target="opcode_introduction"></xref>).</t>
<t>compliant PCP server MUST support MAPx and PEERx, SHOULD support
ability to disable support.</t>
<t>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.</t>
<t>Integrated WGLC feedback (small changes to abstract,
definitions, and small edits throughout the document)</t>
<t>allow new Options to be defined with a specification
(rather than standards action)</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-11 to -12">
<t><list style="symbols">
<t>added implementation note that MAP and implicit dynamic
mappings have independent mapping lifetimes.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-10 to -11">
<t><list style="symbols">
<t>clarified what can cause CANNOT_PROVIDE_EXTERNAL error to
be generated.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-09 to -10">
<t><list style="symbols">
<t>Added External_AF field to PEER requests. Made PEER's Suggested
External IP Address and Assigned External IP Address always be 128
bits long.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-08 to -09">
<t><list style="symbols">
<t>Clarified in PEER Opcode introduction (<xref
target="peer_opcodes"></xref>) that they can also create mappings.</t>
<t>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).</t>
<t>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.</t>
<t>Removed text encouraging PCP server to statefully remember its
mappings from <xref target="reboot"></xref>, as it didn't belong
there. Text in Security Considerations already encourages
persistent storage.</t>
<t>More clearly discussed how PEER is used to re-establish TCP
mapping state. Moved it to a new section, as well (it is now
<xref target="restoring"></xref>).</t>
<t>MAP errors now copy the Suggested Address (and port) fields
to Assigned IP Address (and port), to allow PCP client to
distinguish among many outstanding requests when using
PREFER_FAILURE.</t>
<t>Mapping theft can also be mitigated by ensuring hosts can't
re-use same IP address or port after state loss.</t>
<t>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).</t>
<t>created new Implementation Considerations section (<xref
target="implementation_considerations"></xref>) which discusses
non-normative things that might be useful to implementers. Some
new text is in here, and the Failure Scenarios text (<xref
target="failure"></xref>) has been moved to here.</t>
<t>Tweaked wording of EDM NATs in <xref target="EDM"></xref> to
clarify the problem occurs both inside->outside and
outside->inside.</t>
<t>removed "Interference by Other Applications on Same Host"
section from security considerations.</t>
<t>fixed zero/non-zero text in
<xref target="lifetime"></xref>.</t>
<t>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.</t>
<t>Renamed error from UNAUTH_TARGET_ADDRESS to
UNAUTH_THIRD_PARTY_INTERNAL_ADDRESS.</t>
<t>for FILTER option, removed unnecessary detail on how FILTER
would be bad for PEER, as it is only allowed for MAP anyway.</t>
<t>In Security Considerations, explain that PEER can create a
mapping which makes its security considerations the same as
MAP.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-07 to -08">
<t><list style="symbols">
<t>moved all MAP4-, MAP6-, and PEER-specific options into a single
section.</t>
<t>discussed NAT port-overloading and its impact on MAP (new
section <xref target="EDM"></xref>), which allowed removing the
IMPLICIT_MAPPING_EXISTS error.</t>
<t>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.</t>
<t>Removed Deployment Scenarios section (which detailed NAT64,
NAT44, Dual-Stack Lite, etc.).</t>
<t>Added Client's IP Address to PCP common header. This allows
server to refuse a PCP request if there is a mismatch with the
source IP address, such as when a non-PCP-aware NAT was on the
path. This should reduce failure situations where PCP is deployed
in conjunction with a non-PCP-aware NAT. This addition was
consensus at IETF80.</t>
<t>Changed UNSPECIFIED_ERROR to PROCESSING_ERROR. Clarified that
MALFORMED_REQUEST is for malformed requests (and not related to
failed attempts to process the request).</t>
<t>Removed MISORDERED_OPTIONS. Consensus of IETF80.</t>
<t>SERVER_OVERLOADED is now a common PCP error (instead of
specific to MAP).</t>
<t>Tweaked PCP retransmit/retry algorithm again, to allow more
aggressive PCP discovery if an implementation wants to do
that.</t>
<t>Version negotiation text tweaked to soften NAT-PMP reference,
and more clearly explain exactly what UNSUPP_VERSION should
return.</t>
<t>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.</t>
<t>New Appendix A to discuss NAT-PMP / PCP interworking.</t>
<t>improved pseudocode to be non-blocking.</t>
<t>clarified that PCP cannot delete a static mapping (i.e., a
mapping created by CLI or other non-PCP means).</t>
<t>moved theft of mapping discussion from Epoch section to
Security Considerations.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-06 to -07">
<t><list style="symbols">
<t>tightened up THIRD_PARTY security discussion. Removed "highest
numbered address", and left it as simply "the CPE's IP
address".</t>
<t>removed UNABLE_TO_DELETE_ALL error.</t>
<t>renumbered Opcodes</t>
<t>renumbered some error codes</t>
<t>assigned value to IMPLICIT_MAPPING_EXISTS.</t>
<t>UNPROCESSED can include arbitrary number of option codes.</t>
<t>Moved lifetime fields into common request/response headers</t>
<t>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.</t>
<t>Added NAT-PMP transition section</t>
<t>Separated Internal Address, External Address, Remote Peer
Address definition</t>
<t>Unified Mapping, Port Mapping, Port Forwarding definition</t>
<t>adjusted so DHCP configuration is non-normative.</t>
<t>mentioned PCP refreshes need to be sent over the same
interface.</t>
<t>renamed the REMOTE_PEER_FILTER option to FILTER.</t>
<t>Clarified FILTER option to allow sending an ICMP error if
policy allows.</t>
<t>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.</t>
<t>clarified that PEER requests have to be sent from same
interface as the connection itself.</t>
<t>for MAP opcode, text now requires mapping be deleted when
lifetime expires (per consensus on 8-Mar interim meeting)</t>
<t>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'.</t>
<t>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).</t>
<t>clarified that PCP server only listens on its internal
interface.</t>
<t>abandoned 'target' term and reverted to simplier 'internal'
term.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-05 to -06">
<t><list style="symbols">
<t>Dual-Stack Lite: consensus was encapsulation mode. Included a
suggestion that the B4 will need to proxy PCP-to-PCP and
UPnP-to-PCP.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>Swapped Option values for the mandatory- and
optional-to-process Options, so we can have a simple
lowest..highest ordering.</t>
<t>added MISORDERED_OPTIONS error.</t>
<t>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.</t>
<t>clarified that, after successfully using a PCP server, that PCP
server is declared to be non-responsive after 5 failed
retransmissions.</t>
<t>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.)</t>
<t>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.</t>
<t>added some text about why server might send SERVER_OVERLOADED,
or might simply discard packets.</t>
<t>Dis-allow internal-port=0, which means we dis-allow using PCP
as a DMZ-like function. Instead, ports have to be mapped
individually.</t>
<t>Text describing server's processing of PEER is tightened
up.</t>
<t>Server's processing of PEER now says it is
implementation-specific if a PCP server continues to allow the
mapping to exist after a PEER message. Client's processing of PEER
says that if client wants mapping to continue to exist, client has
to continue to send recurring PEER messages.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-04 to -05">
<t><list style="symbols">
<t>tweaked PCP common header packet layout.</t>
<t>Re-added port=0 (all ports).</t>
<t>minimum size is 12 octets (missed that change in -04).</t>
<t>removed Lifetime from PCP common header.</t>
<t>for MAP error responses, the lifetime indicates how long the
server wants the client to avoid retrying the request.</t>
<t>More clearly indicated which fields are filled by the server on
success responses and error responses.</t>
<t>Removed UPnP interworking section from this document. It will
appear in
<xref target="I-D.bpw-pcp-upnp-igd-interworking"></xref>.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-03 to -04">
<t><list style="symbols">
<t>"Pinhole" and "PIN" changed to "mapping" and "MAP".</t>
<t>Reduced from four MAP Opcodes to two. This was done by
implicitly using the address family of the PCP message itself.</t>
<t>New option THIRD_PARTY, to more carefully split out the case
where a mapping is created to a different host within the
home.</t>
<t>Integrated a lot of editorial changes from Stuart and
Francis.</t>
<t>Removed nested NAT text into another document, including the
IANA-registered IP addresses for the PCP server.</t>
<t>Removed suggestion (MAY) that PCP server reserve UDP when it
maps TCP. Nobody seems to need that.</t>
<t>Clearly added NAT and NAPT, such as in residential NATs, as
within scope for PCP.</t>
<t>HONOR_EXTERNAL_PORT renamed to PREFER_FAILURE</t>
<t>Added 'Lifetime' field to the common PCP header, which replaces
the functions of the 'temporary' and 'permanent' error types of
the previous version.</t>
<t>Allow arbitrary Options to be included in PCP response, so that
PCP server can indicate un-supported PCP Options. Satisfies PCP
Issue #19</t>
<t>Reduced scope to only deal with mapping protocols that have
port numbers.</t>
<t>Reduced scope to not support DMZ-style forwarding.</t>
<t>Clarified version negotiation.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-02 to -03">
<t><list style="symbols">
<t>Adjusted abstract and introduction to make it clear PCP is
intended to forward ports and intended to reduce application
keepalives.</t>
<t>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.</t>
<t>Moved subscriber identity from common PCP section to MAP*
section.</t>
<t>made clearer that PCP client can reduce mapping lifetime if it
wishes.</t>
<t>Added discussion of host running a server, client, or symmetric
client+server.</t>
<t>Introduced PEER4 and PEER6 Opcodes.</t>
<t>Removed REMOTE_PEER Option, as its function has been replaced
by the new PEER Opcodes.</t>
<t>IANA assigned port 44323 to PCP.</t>
<t>Removed AMBIGUOUS error code, which is no longer needed.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-01 to -02">
<t><list style="symbols">
<t>more error codes</t>
<t>PCP client source port number should be random</t>
<t>PCP message minimum 8 octets, maximum 1024 octets.</t>
<t>tweaked a lot of text in section 7.4, "Opcode-Specific Server
Operation".</t>
<t>opening a mapping also allows ICMP messages associated with
that mapping.</t>
<t>PREFER_FAILURE value changed to the mandatory-to-process
range.</t>
<t>added text recommending applications that are crashing obtain
short lifetimes, to avoid consuming subscriber's port quota.</t>
</list></t>
</section>
<section title="Changes from draft-ietf-pcp-base-00 to -01">
<t><list style="symbols">
<t>Significant document reorganization, primarily to split base
PCP operation from Opcode operation.</t>
<t>packet format changed to move 'protocol' outside of PCP common
header and into the MAP* opcodes</t>
<t>Renamed Informational Elements (IE) to Options.</t>
<t>Added REMOTE_PEER (for disambiguation with dynamic ports),
REMOTE_PEER_FILTER (for simple packet filtering), and
PREFER_FAILURE (to optimize UPnP IGDv1 interworking) options.</t>
<t>Is NAT or router behind B4 in scope?</t>
<t>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.</t>
<t>Result code most significant bit now indicates
permanent/temporary error</t>
<t>PCP Options are split into mandatory-to-process ("P" bit), and
into Specification Required and Private Use.</t>
<t>Epoch discussion simplified.</t>
</list></t>
</section>
</section>
<!--
<section anchor="discovery_analysis" title="Analysis of Techniques to Discover PCP Server">
<t><list style="empty">
<t>[Ed. Note: This Appendix will be removed in a later version of
this document. It is included here for reference and discussion
purposes. This is tracked as PCP Issue #8
<xref target="PCP-Issues"></xref>.]</t>
</list></t>
<t><list style="empty">
<t>Discussion Note: A deployment model is a non-PCP aware NAT in a
home, and a PCP-aware CGN operated by the ISP. For example, the home
users purchased a NAT last year at a computer shop (to extend their
home's WiFi network). This could work by having the host use UPnP
IGDv1 with the in-home NAT, and the host use PCP with the LSN. But
this deployment model is impossible with several of the mechanisms
below. Is this deployment model important, or can we wait for PCP to
be enabled on CPE?</t>
</list></t>
<t>Several mechanisms for discovering the PCP server can be envisaged as
listed below:</t>
<t><list style="numbers">
<t>A special-purpose IPv4 or IPv6 address, assigned by IANA, which
is routed normally until it hits a PCP server, which responds. <list
style="empty">
<t>Analysis: This solution can be deployed in the context of
DS-Lite architecture. Concretely, a well-known IPv4 address can
be used to reach a PCP server in the device that provides
the AFTR capabilities. Since all IPv4 messages issued by a
DS-Lite CP router will be encapsulated in IPv6, no state
synchronisation issues will be experienced because PCP messages
will be handled by the appropriate PCP server.</t>
<t>In some deployment scenarios (e.g., deployment of several
stateful NAT64/NAT46 in the same domain), the use of this
address is not recommended since PCP messages, issued by a given
host, may be handled by a PCP server in a NAT node
which is not involved to handle IP packets issued from that
host. The use of this special-purpose IP address may induce
session failures and therefore the customer may experience
troubles when accessing its services.</t>
<t>Consequently, the use of a special-purpose IPv4 address is
suitable for DS-Lite NAT44. As for NAT46/NAT64, this is left to
the Service Providers according to their deployment
configuration.</t>
<t>The special-use address MUST NOT be advertised in the global
routing table. Packets with that destination address SHOULD be
filtered so they are not transmitted on the Internet.</t>
</list></t>
<t>Assume the default router is a PCP server, and send PCP packets
to the IP address of the default router. <list style="empty">
<t>Analysis: This solution is not suitable for DS-Lite NAT44 nor
for all variants of NAT64/NAT46. <list style="empty">
<t>In the context of DS-Lite: There is no default IPv4
router configured in the CP router. All outgoing IPv4
traffic is encapsulated in IPv6 and then forwarded to a
pre-configured DS-Lite AFTR device. Furthermore, if IPv6 is
used to reach the PCP server, the first router may not be
the one which implements the AFTR.</t>
<t>For NAT64/NAT46 scenarios: The NAT function is not
implemented by the first router, therefore this solution
candidate does not allow to discover a valid PCP server.</t>
</list></t>
<t>Therefore, this alternative is not recommended.</t>
</list></t>
<t>Service Location Protocol (<xref
target="RFC2608">SLP</xref>).<list style="empty">
<t>Analysis: This solution is not suitable in scenarios where
multicast is not enabled. SLP is a chatty protocol. Also, as
with UPnP IGDv1's use of SSDP, SLP will discover NAT gateways
which exist on the local network, but are not actually on the
path that packets will take from the Internal Host to the
Internet, leading to the situation where Internal Hosts create
apparently-successful mappings, which are in fact completely
worthless for the purpose of establishing useful communication
with Remote Hosts on the Internet. This alternative is not
recommended.</t>
</list></t>
<t>NAPTR. The host would issue a DNS query for a NAPTR record,
formed from some bits of the host's IPv4 or IPv6 address. For
example, a host with the IPv6 address 2001:db8:1:2:3:4:567:89ab
would first send an NAPTR query for
3.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA (20 elements,
representing a /64 network prefix), which returns the PCP server's
IPv6 address. A similar scheme can be used with IPv4 using, for
example, the first 24 bits of the IPv4 address.<list style="empty">
<t>Analysis: This solution candidate requires more configuration
effort by the Service Provider so as to redirect a given client
to the appropriate PCP server. Any change of the engineering
policies (e.g., introduce new LSN device, load-based
dimensioning, load-balancing, etc.) would require to update the
zone configuration. This would be a hurdle for the flexibility
of the operational networks. Adherence to DNS is not encouraged
and means which allows for more flexibility are to be
promoted.</t>
<t>Therefore, this mechanism is not recommended.</t>
</list></t>
<t>New DHCPv6/DHCP option and/or a RA option to convey an FQDN of a
PCP server.<list style="empty">
<t>Analysis: Since DS-Lite and NAT64/NAT46 are likely to be
deployed in provider-provisioned environments, DHCP (both DHCPv6
and IPv4 DHCP) is convenient to provision the address/FQDN of
the PCP server.</t>
</list></t>
</list></t>
</section>
-->
<!--
<section title="Contributors' Addresses">
<t>The following individuals contributed substantial text to this
document and are listed in alphabetical order:</t>
<figure>
<artwork align="left"><![CDATA[
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
Francis Dupont
Internet Systems Consortium
Email: fdupont@isc.org
]]></artwork>
</figure>
</section>
-->
</back>
</rfc>
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