PCP working group D. Wing, Ed.
Internet-Draft Cisco
Intended status: Standards Track S. Cheshire
Expires: October 22, 2011 Apple
M. Boucadair
France Telecom
R. Penno
Juniper Networks
April 20, 2011
Port Control Protocol (PCP)
draft-ietf-pcp-base-08
Abstract
Port Control Protocol allows a host to control how incoming IPv6 or
IPv4 packets are translated and forwarded by a network address
translator (NAT) or simple firewall to an IPv6 or IPv4 host, and also
allows a host to optimize its NAT keepalive messages.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 22, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 5
2.2. Supported Protocols . . . . . . . . . . . . . . . . . . . 5
2.3. Single-homed Customer Premises Network . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Relationship between PCP Server and its NAT/firewall . . . . . 8
5. Common Request and Response Header Format . . . . . . . . . . 9
5.1. Request Header . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Response Header . . . . . . . . . . . . . . . . . . . . . 11
5.3. Options . . . . . . . . . . . . . . . . . . . . . . . . . 12
5.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 14
6. General PCP Operation . . . . . . . . . . . . . . . . . . . . 15
6.1. General PCP Client: Generating a Request . . . . . . . . . 15
6.2. General PCP Server: Processing a Request . . . . . . . . . 16
6.3. General PCP Client: Processing a Response . . . . . . . . 17
6.4. Multi-Interface Issues . . . . . . . . . . . . . . . . . . 18
6.5. Epoch . . . . . . . . . . . . . . . . . . . . . . . . . . 19
6.6. Version Negotiation . . . . . . . . . . . . . . . . . . . 19
6.7. General PCP Option . . . . . . . . . . . . . . . . . . . . 20
6.7.1. UNPROCESSED Option . . . . . . . . . . . . . . . . . . 20
7. Introduction to MAP and PEER OpCodes . . . . . . . . . . . . . 21
7.1. For Operating a Server . . . . . . . . . . . . . . . . . . 22
7.2. For Reducing NAT Keepalive Messages . . . . . . . . . . . 23
7.3. For Operating a Symmetric Client/Server . . . . . . . . . 25
8. MAP OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 28
8.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 28
8.2. OpCode-Specific Result Codes . . . . . . . . . . . . . . . 30
8.3. OpCode-Specific Client: Generating a Request . . . . . . . 31
8.4. OpCode-Specific Server: Processing a Request . . . . . . . 32
8.5. OpCode-Specific Client: Processing a Response . . . . . . 34
8.6. Mapping Lifetime and Deletion . . . . . . . . . . . . . . 34
8.7. Subscriber Renumbering and Address Change Events . . . . . 36
8.8. PCP Failure Scenarios . . . . . . . . . . . . . . . . . . 37
8.8.1. Recreating Mappings . . . . . . . . . . . . . . . . . 37
8.8.2. Maintaining Mappings . . . . . . . . . . . . . . . . . 38
8.9. Implementing MAP with non-EIM NATs . . . . . . . . . . . . 39
9. PEER OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 39
9.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 40
9.2. OpCode-Specific Client: Generating a Request . . . . . . . 43
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9.3. OpCode-Specific Server: Processing a Request . . . . . . . 43
9.4. OpCode-Specific Client: Processing a Response . . . . . . 44
10. Options for MAP and PEER OpCodes . . . . . . . . . . . . . . . 45
10.1. THIRD_PARTY Option for MAP and PEER OpCodes . . . . . . . 45
10.2. PREFER_FAILURE Option for MAP OpCodes . . . . . . . . . . 48
10.3. FILTER Option for MAP OpCodes . . . . . . . . . . . . . . 49
11. Deployment Considerations . . . . . . . . . . . . . . . . . . 51
11.1. Ingress Filtering . . . . . . . . . . . . . . . . . . . . 51
11.2. Per-Subscriber Explicit Dynamic Mapping Quota . . . . . . 51
12. Security Considerations . . . . . . . . . . . . . . . . . . . 51
12.1. Denial of Service . . . . . . . . . . . . . . . . . . . . 52
12.2. Ingress Filtering . . . . . . . . . . . . . . . . . . . . 52
12.3. Validating THIRD_PARTY Internal Address . . . . . . . . . 52
12.4. Interference by Other Applications on Same Host . . . . . 53
12.5. Theft of mapping . . . . . . . . . . . . . . . . . . . . . 53
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 53
13.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 53
13.2. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 53
13.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 54
13.4. Options . . . . . . . . . . . . . . . . . . . . . . . . . 54
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 54
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54
15.1. Normative References . . . . . . . . . . . . . . . . . . . 54
15.2. Informative References . . . . . . . . . . . . . . . . . . 55
Appendix A. NAT-PMP Transition . . . . . . . . . . . . . . . . . 57
Appendix B. Change History . . . . . . . . . . . . . . . . . . . 57
B.1. Changes from draft-ietf-pcp-base-07 to -08 . . . . . . . . 58
B.2. Changes from draft-ietf-pcp-base-06 to -07 . . . . . . . . 59
B.3. Changes from draft-ietf-pcp-base-05 to -06 . . . . . . . . 60
B.4. Changes from draft-ietf-pcp-base-04 to -05 . . . . . . . . 61
B.5. Changes from draft-ietf-pcp-base-03 to -04 . . . . . . . . 62
B.6. Changes from draft-ietf-pcp-base-02 to -03 . . . . . . . . 62
B.7. Changes from draft-ietf-pcp-base-01 to -02 . . . . . . . . 63
B.8. Changes from draft-ietf-pcp-base-00 to -01 . . . . . . . . 63
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 64
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1. Introduction
The Port Control Protocol (PCP) provides a mechanism to control how
incoming packets are forwarded by upstream devices such as NAT64,
NAT44, and firewall devices, and a mechanism to reduce application
keepalive traffic. PCP is primarily designed to be implemented in
the context of both Carrier-Grade NATs (CGN) and small NATs (e.g.,
residential NATs). PCP allows hosts to operate server 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.
PCP allows applications to create mappings from an external IP
address and port to an internal IP address and port. These mappings
are required for successful inbound communications destined to
machines located behind a NAT or a firewall.
After creating a mapping for incoming connections, it is necessary to
inform remote computers about the IP address and port for the
incoming connection. This is usually done in an application-specific
manner. For example, a computer game would use a rendezvous server
specific to that game (or specific to that game developer), and a SIP
phone would use a SIP proxy. PCP does not provide this rendezvous
function. The rendezvous function will support IPv4, IPv6, or both.
Depending on that support and the application's support of IPv4 or
IPv6, the PCP client will need an IPv4 mapping, an IPv6 mapping, or
both.
Many NAT-friendly applications send frequent application-level
messages to ensure their session will not be timed out by a NAT.
These are commonly called "NAT keepalive" messages, even though they
are not sent to the NAT itself (rather, they are sent 'through' the
NAT). These applications can reduce the frequency of those NAT
keepalive messages by using PCP to learn (and influence) the NAT
mapping lifetime. This helps reduce bandwidth on the subscriber's
access network, traffic to the server, and battery consumption on
mobile devices.
Many NATs and firewalls have included application layer gateways
(ALGs) to create mappings for applications that establish additional
streams or accept incoming connections. ALGs incorporated into NATs
additionally 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.
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2. Scope
2.1. Deployment Scenarios
PCP can be used in various deployment scenarios, including:
o Dual-Stack Lite (DS-Lite) [I-D.ietf-softwire-dual-stack-lite],
and;
o NAT64, both Stateful [I-D.ietf-behave-v6v4-xlate-stateful] and
Stateless [I-D.ietf-behave-v6v4-xlate], and;
o Carrier-Grade NAT [I-D.ietf-behave-lsn-requirements], and;
o Basic NAT [RFC3022], and;
o Network Address and Port Translation (NAPT) [RFC3022], such as
commonly deployed in residential NAT devices, and;
o Layer-2 aware NAT [I-D.miles-behave-l2nat] and Dual-Stack Extra
Lite [I-D.arkko-dual-stack-extra-lite], and;
o IPv4 and IPv6 simple firewall control [RFC6092].
2.2. Supported Protocols
The PCP OpCodes defined in this document are designed to support
transport-layer protocols that use a 16-bit port number (e.g., TCP,
UDP, SCTP, DCCP). Protocols that do not use a port number (e.g.,
IPsec ESP), and the ability to use PCP to forward all traffic to a
single default host (often nicknamed a "DMZ"), are beyond the scope
of this document.
2.3. Single-homed Customer Premises Network
The PCP machinery assumes a single-homed host model. That is, for a
given IP version, only one default route exists to reach the
Internet. This is important because after a PCP mapping is created
and an inbound packet (e.g., TCP SYN) arrives at the host, the
outbound response (e.g., TCP SYNACK) has to go through the same path
so it is seen by the firewall or rewritten by the NAT. This
restriction exists because otherwise there would need to be one PCP
server for each egress, because the host could not reliably determine
which egress path packets would take, so 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
would likely have the necessary port mapped to another host).
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3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in "Key words for use in
RFCs to Indicate Requirement Levels" [RFC2119].
Internal Host:
A host served by a NAT gateway, or protected by a firewall. This
is the host that receives the incoming traffic created by a PCP
MAP request, or the host that initiated an implicit dynamic
mapping (e.g., by sending a TCP SYN) across a firewall or a NAT.
Remote Host:
A host with which an Internal Host is communicating.
Internal Address:
The address of an Internal Host served by a NAT gateway (typically
a private address [RFC1918]) or protected by a firewall.
External Address:
The address of an Internal Host as seen by other Remote Hosts 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.
Remote Peer Address:
The address of a Remote Host, as seen by the Internal Host. A
Remote Address is generally a public routable address. In the
case of a Remote Host that is itself served by a NAT gateway, the
Remote Address may in fact be the Remote Host's External Address,
but since this remote translation is generally invisible to
software running on the Internal Host, the distinction can safely
be ignored for the purposes of this document.
Third Party:
In the common case, an Internal Host manages its own Mappings
using PCP requests, and the Internal Address of those Mappings is
the same as the source IP address of the PCP request packet.
In the case where one device is managing Mappings on behalf of
some other device, the presence of the THIRD_PARTY option in the
MAP request signifies that the specified address, not the source
IP address of the PCP request packet, should be used as the
Internal Address for the Mapping. For example, this can occur if
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the internal host does not implement PCP.
Mapping, Port Mapping, Port Forwarding:
A NAT mapping creates a relationship between an internal IP
address, protocol, and port and an external IP address, protocol,
and port. More specifically, it creates a translation rule where
packets destined to the external IP and port are translated to the
internal IP and port, and vice versa. In the case of a pure
firewall, the "Mapping" is the identity function, translating an
internal port number to the same external port number, and this
"Mapping" indicates to the firewall that traffic to and from this
internal port number is permitted to pass.
Mapping Types:
There are three different ways to create mappings: implicit
dynamic mappings, explicit dynamic mappings, and static mappings.
Implicit dynamic mappings are created as a result of a TCP SYN or
outgoing UDP packet, and allow Internal Hosts to receive replies
to their outbound packets. Explicit dynamic mappings are created
as a result of PCP MAP requests. Static mappings are created by
manual configuration (e.g., command-line interface or web page).
Explicit and static mappings allow Internal Hosts to receive
inbound traffic that is not in direct response to any immediately
preceding outbound communication (i.e., allow Internal Hosts to
operate a "server" that is accessible to other hosts on the
Internet). Both implicit and explicit dynamic mappings are
dynamic in the sense that they are created on demand, as requested
(implicitly or explicitly) by the Internal Host, and have a
lifetime. After the lifetime, the mapping is deleted unless the
lifetime is extended by action by the Internal Host (e.g., sending
more traffic or sending a new PCP MAP request). Static mappings
differ from dynamic mappings in that their lifetime is typically
infinite (they exist until manually removed) but otherwise they
behave exactly the same as an explicit dynamic mapping with an
infinite lifetime. For example, a PCP MAP request to create a
mapping that already exists as a static mapping will return a
successful result, confirming that the requested mapping exists.
PCP Client:
A PCP software instance responsible for issuing PCP requests to a
PCP server. One or several PCP Clients can be embedded in the
same host. Several PCP Clients can be located in the same local
network. A PCP Client can issue PCP request on behalf of a third
party device for which it is authorized to do so. An interworking
function from Universal Plug and Play Internet Gateway Device
(UPnP IGD, [IGD]) to PCP is another example of a PCP Client. A
PCP server in a NAT gateway that is itself a client of another NAT
gateway (nested NAT) may itself act as a PCP client to the
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upstream NAT.
PCP Server:
A network element which receives and processes PCP requests from a
PCP client. Generally this is a PCP-capable NAT gateway or
firewall. A NAT gateway creates mappings determining how it
translates packets it forwards, and PCP enables clients to
communicate with the NAT gateway about those mappings. In
principle it is also possible for the PCP server to be some other
device, which in turn communicates with the NAT gateway using some
other network protocol, but this introduces additional complexity
and fragility into the system, and is a deployment detail which
should be implemented in a way that is invisible to the PCP
client. See also Section 4.
Interworking Function:
a functional element responsible for interworking another protocol
with PCP. For example interworking between UPnP IGD [IGD] with
PCP.
subscriber:
an entity provided access to the network. In the case of a
commercial ISP, this is typically a single home.
5-tuple The 5 pieces of information that fully identify a flow, from
the perspective of a subscriber: source IP address, destination IP
address, protocol, source port number, destination port number.
From the perspective of a NAPT device, in certain deployments an
additional piece of information is necessary to distinguish
subscribers with overlapping IP addresses. This additional
information depends on the deployment scenario, but examples of
the information include the subscriber's IPv6 address (for the
subscriber's Dual-Stack Lite tunnel) or the subscriber's Virtual
LAN number ([I-D.miles-behave-l2nat]), or other similar
identifier.
4. Relationship between PCP Server and its NAT/firewall
The PCP server receives PCP requests. The PCP server might be
integrated within the NAT or firewall device (as shown in Figure 1)
which is expected to be a common deployment.
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+-----------------+
+------------+ | NAT or firewall |
| PCP client |-<network>-+ with +---<Internet>
+------------+ | PCP server |
+-----------------+
Figure 1: NAT or Firewall with Embedded PCP Server
It is also possible to operate the PCP server in a separate device
from the NAT, so long as such operation is indistinguishable from the
PCP client's perspective.
5. Common Request and Response Header Format
All PCP messages contain a request (or response) header containing an
opcode, any relevant opcode-specific information, and zero or more
options. The packet layout for the common header, and operation of
the PCP client and PCP server are described in the following
sections. The information in this section applies to all OpCodes.
Behavior of the OpCodes defined in this document is described in
Section 8 and Section 9.
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5.1. Request Header
All requests have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| OpCode | Reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Client's IP address (always 128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Common Request Packet Format
These fields are described below:
Version: This document specifies protocol version 1. NAT-PMP, a
precursor to PCP, specified protocol version 0. Should later
updates to this document specify different message formats with a
version number greater than 1, the first two bytes of those new
message formats will continue to contain the version number and
opcode as shown here, so that a PCP server receiving a message
format newer or older than the version(s) it understands can still
parse enough of the message to correctly identify the version
number, and determine whether the problem is that this server is
too old and needs to be updated to work with the PCP client, or
whether the PCP client is too old and needs to be updated to work
with this server.
R: Indicates Request (0) or Response (1). All Requests MUST use 0.
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OpCode: Opcodes are defined in Section 8 and Section 9.
Reserved: 16 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
Requested Lifetime: The Requested Lifetime field is an unsigned 32-
bit integer, in seconds, ranging from 0 to 4,294,967,295 seconds.
This is used by the MAP and PEER OpCodes defined in this document
for their requested lifetime. Future OpCodes which don't need
this field MUST set the field to zero on transmission and ignore
on reception.
Reserved: 32 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
PCP Client's IP Address: The IP address of the PCP client, from the
PCP client's perspective. If IPv4, only the first 32 bits are
used, the other bits MUST be set to 0.
5.2. Response Header
All responses have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version = 1 |R| OpCode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Client's IP address (always 128 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) OpCode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Options :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Common Response Packet Format
These fields are described below:
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Version: Responses MUST use version 1.
R: Indicates Request (0) or Response (1). All Responses MUST use 1.
OpCode: The OpCode value, copied from the request.
Reserved: 8 reserved bits, MUST be sent as 0, MUST be ignored when
received. This is set by the server.
Result Code: The result code for this response. See Section 5.4 for
values. This is set by the server.
Lifetime: The Lifetime field is an unsigned 32-bit integer, in
seconds, ranging from 0 to 4,294,967,295 seconds. On an error
response, this indicates how long clients should assume they'll
get the same error response from that PCP server if they repeat
the same request. On a success response for the currently-defined
PCP OpCodes -- MAP and PEER -- this indicates the lifetime for
this mapping. If future OpCodes are defined that do not have a
lifetime associated with them, then in success responses for those
OpCodes the Lifetime MUST be set to zero on transmission and MUST
be ignored on reception.
Epoch: The server's Epoch value. See Section 6.5 for discussion.
This value is set in both success and error responses.
PCP Client's IP Address: The IP address of the PCP client, from the
PCP server's perspective. If IPv4, only the first 32 bits are
used, the other bits MUST be set to 0.
5.3. Options
A PCP OpCode can be extended with an Option. Options can be used in
requests and responses. The decision about whether to include a
given piece of information in the base opcode format or in an option
is 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 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
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other device doesn't need to implement code to generate the
THIRD_PARTY option, and a PCP server that doesn't implement the
necessary security measures to create third-party mappings safely
doesn't need to implement code to parse the THIRD_PARTY option.
Options use the following Type-Length-Value format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Options Header
The description of the fields is as follows:
Option Code: 8 bits. Its highest bit is the "O" bit and indicates
if this Option is mandatory (0) or optional (1) to process.
Reserved: 8 bits. MUST be set to 0 on transmission and MUST be
ignored on reception.
Option-Length: 16 bits. Indicates the length of the enclosed data
in octets. Options with length of 0 are allowed.
data: Option data. The option data MUST end on a 32-bit boundary,
padded with 0's when necessary.
A given Option MAY be included in a request containing a specific
OpCode. The handling of an Option by the PCP client and PCP server
MUST be specified in an appropriate document and MUST include whether
the PCP Option can appear (one or more times) in a request and/or
response, and indicate the contents of the Option in the request and
in the response. If several Options are included in a PCP request or
response, they MAY be encoded in any order by the PCP client and are
processed in the order received.
If, while processing an option, an error is encountered that causes a
PCP error response to be generated, the PCP request MUST cause no
state change in the PCP server or the PCP-controlled device (i.e., it
rolls back any changes it might have made while processing the
request). The response MUST encode the Options in the same order,
but may omit some PCP Options in the response, as is necessary to
indicate the PCP server does not understand that Option or that
Option is not permitted to be included in responses by the definition
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of the Option itself. Additional Options included in the response
(if any) MUST be included at the end. A certain Option MAY appear
more than once in a request or in a response, if permitted by the
definition of the Option itself. If the Option's definition allows
the Option to appear only once but it appears more than once in a
request, the PCP server MUST respond with the MALFORMED_OPTION result
code; if this occurs in a response, the PCP client processes the
first occurrence and ignores the other occurrences as if they were
not present.
If the "O" bit (high bit) in the OpCode is clear,
o the PCP server MUST only generate a positive PCP response if it
can successfully process the PCP request and this Option.
o if the PCP server does not implement this Option, or cannot
perform the function indicated by this Option (e.g., due to a
parsing error with the option), it MUST generate a failure
response with code UNSUPP_OPTION or MALFORMED_OPTION (as
appropriate) and include the UNPROCESSED option in the response
(Section 6.7.1).
If the "O" bit is set, the PCP server MAY process or ignore this
Option, entirely at its discretion.
Option definitions MUST include the information below:
This Option:
name: <mnemonic>
number: <value>
purpose: <textual description>
is valid for OpCodes: <list of OpCodes>
length: <rules for length>
may appear in: <requests/responses/both>
maximum occurrences: <count>
5.4. Result Codes
The following result codes may be returned as a result of any OpCode
received by the PCP server. The only success result code is 0, other
values indicate an error. If a PCP server has encountered multiple
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errors during processing of a request, it SHOULD use the most
specific error message.
0 SUCCESS, success
1 UNSUPP_VERSION, unsupported version.
2 MALFORMED_REQUEST, indicating the request could not be
successfully parsed.
3 UNSUPP_OPCODE, unsupported OpCode.
4 UNSUPP_OPTION, unsupported Option. This error only occurs if the
Option is in the mandatory-to-process range.
5 MALFORMED_OPTION, malformed Option (e.g., exists too many times,
invalid length).
6 PROCESSING_ERROR, server encountered an error after parsing while
attempting to process a request.
7 SERVER_OVERLOADED, server is processing too many PCP requests from
this client or from other clients, and requests this client delay
sending any other requests for the time indicated in Lifetime.
Additional result codes, specific to the OpCodes and Options defined
in this document, are listed in Section 8.2 and Section 10.1.
6. General PCP Operation
PCP messages MUST be sent over UDP [RFC0768]. Every PCP request
generates a response, so PCP does not need to run over a reliable
transport protocol.
PCP is idempotent, so if the PCP client sends the same request
multiple times and the PCP server processes those requests, the same
result occurs. The order of operation is that a PCP client generates
and sends a request to the PCP server, which processes the request
and generates a response back to the PCP client.
6.1. General PCP Client: Generating a Request
This section details operation specific to a PCP client, for any
OpCode. Procedures specific to the MAP OpCodes are described in
Section 8, and procedures specific to the PEER OpCodes are described
in Section 9.
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Prior to sending its first PCP message, the PCP client determines
which servers to use. The PCP client performs the following steps to
determine its PCP server(s):
1. if a PCP server is configured (e.g., in a configuration file or
DHCP), that single configuration source is used as the list of
PCP server(s), else;
2. the address of the default router is used as the PCP server.
With that list of PCP servers, the PCP client formulates its PCP
request. The PCP request contains a PCP common header, PCP OpCode
and payload, and (possibly) Options. As with all UDP or TCP clients
on any operating system, when several PCP clients are embedded in the
same host, each uses a distinct source port number to disambiguate
their requests and replies. The PCP client's source port SHOULD be
randomly generated [RFC6056].
When attempting to contact a PCP server, the PCP client initializes a
timer to 2 seconds. The PCP client sends a PCP message the first
server in its list of PCP servers. If no response is received before
the timer expires, the timer is doubled (to 4 seconds) and the
request is re-transmitted. If no response is received before the
timer expires, the timer is doubled again (to 8 seconds) and the
request is re-transmitted. This procedure is repeated in parallel or
in series to each PCP server in the list, on each interface, until a
response is received from a PCP server. If the requests are sent in
parallel and responses from multiple PCP servers are received, only
the PCP server closest to the top of the list, on that interface, is
used for subsequent requests; PCP requests which received a positive
response and create state (e.g., MAP) SHOULD have their state cleared
(e.g., lifetime set to 0).
Once a PCP client has successfully received a response from a PCP
server on that interface, it sends subsequent PCP requests to that
same server, with a retransmission timer of 2 seconds. If, after 2
seconds, a response is not received from that PCP server, the same
back-off algorithm described above is performed.
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.
6.2. General PCP Server: Processing a Request
This section details operation specific to a PCP server. Processing
SHOULD be performed in the order of the following paragraphs.
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A PCP server processes incoming requests on the PCP port from clients
or an operator-configured interface (e.g., from the ISP's network
operations center). The PCP server MUST drop (ignore) requests that
arrive from elsewhere (e.g., the Internet).
Upon receiving a message, the PCP server parses and validates it. A
valid request contains a valid PCP common header, one valid PCP
Opcode, and zero or more Options (which the server might or might not
comprehend). If an error is encountered during processing, the
server generates an error response which is sent back to the PCP
client. Processing an OpCode and the Options are specific to each
OpCode.
If the received message is shorter than 4 octets or has the R bit set
the message is simply dropped. If the length of the request exceeds
1024 octets or is not a multiple of 4 octets, it is invalid. Invalid
requests are handled by copying up to 1024 octets of the request into
the response, setting the result code to MALFORMED_REQUEST, and zero-
padding the response to a multiple of 4 octets if necessary. If the
version number is not supported, a response is generated with the
UNSUPP_VERSION result code and the other steps detailed in
Section 6.6. If the OpCode is not supported, a response is generated
with the UNSUPP_OPCODE result code.
If the source IP address of the received packet does not match the
contents of the PCP Client IP Address field, a response is generated
with the ADDRESS_MISMATCH result code. This is done to detect and
prevent accidental use of PCP where a non-PCP-aware NAT or NAPT
exists between the PCP client and PCP server.
Error responses have the same packet layout as success responses,
with fields from the request copied into the response, and fields
assigned by the PCP server are set as indicated in Figure 3
6.3. General PCP Client: Processing a Response
The PCP client receives the response and verifies the source IP
address and port belong to the PCP server of an outstanding PCP
request. It validates the OpCode matches an outstanding PCP request.
Responses shorter than 12 octets, longer than 1024 octets, or not a
multiple of 4 octets are invalid and ignored, likely causing the
request to be re-transmitted. The response is further matched by
comparing fields in the response OpCode-specific data to fields in
the request OpCode-specific data, as described by the processing for
that OpCode. After these matches are successful, the PCP client
checks the Epoch field to determine if it needs to restore its state
to the PCP server (see Section 6.5).
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If the result code is 0, the PCP client knows the request was
successful.
If the result code is not 0, the request failed. If the result code
is UNSUPP_VERSION, processing continues as described in Section 6.6.
If the result code is SERVER_OVERLOADED, clients SHOULD NOT send
*any* further requests to that PCP server for the indicated error
lifetime. For other error result codes, The PCP client SHOULD NOT
resend the same request for the indicated error lifetime. If a PCP
server indicates an error lifetime in excess of 30 minutes, A PCP
client MAY choose to set its retry timer to 30 minutes.
If the PCP client has discovered a new PCP server (e.g., connected to
a new network), the PCP client MAY immediately begin communicating
with this PCP server, without regard to hold times from communicating
with a previous PCP server.
6.4. Multi-Interface Issues
Hosts which desire a PCP mapping might be multi-interfaced (i.e., own
several logical/physical interfaces). Indeed, a host can be
configured with several IPv4 addresses (e.g., WiFi and Ethernet) or
dual-stacked. These IP addresses may have distinct reachability
scopes (e.g., if IPv6 they might have global reachability scope as
for Global Unicast Address (GUA, [RFC3587]) or limited scope as for
Unique Local Address (ULA) [RFC4193]).
IPv6 addresses with global reachability (e.g., GUA) SHOULD be used as
the source address when generating a PCP request. IPv6 addresses
without global reachability (e.g., ULA [RFC4193]), SHOULD NOT be used
as the source interface when generating a PCP request. If IPv6
privacy addresses [RFC4941] are used for PCP mappings, a new PCP
request will need to be issued whenever the IPv6 privacy address is
changed. This PCP request SHOULD be sent from the IPv6 privacy
address itself. It is RECOMMENDED that mappings to the previous
privacy address be deleted.
Due to the ubiquity of IPv4 NAT, IPv4 addresses with limited scope
(e.g., private addresses [RFC1918]) MAY be used as the source
interface when generating a PCP request.
As mentioned in Section 2.3, only single-homed CP routers are in
scope. Therefore, there is no viable scenario where a host located
behind a CP router is assigned with two Global Unicast Addresses
belonging to different global IPv6 prefixes.
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6.5. Epoch
Every PCP response sent by the PCP server includes an Epoch field.
This field increments by 1 every second, and is used by the PCP
client to determine if PCP state needs to be restored. If the PCP
server resets or loses the state of its explicit dynamic Mappings
(that is, those mappings created by PCP MAP requests), due to reboot,
power failure, or any other reason, it MUST reset its Epoch time to
0. Similarly, if the public IP address(es) of the NAT (controlled by
the PCP server) changes, the Epoch MUST be reset to 0. A PCP server
MAY maintain one Epoch value for all PCP clients, or MAY maintain
distinct Epoch values for each PCP client; this choice is
implementation-dependent.
Whenever a client receives a PCP response, the client computes its
own conservative estimate of the expected Epoch value by taking the
Epoch value in the last packet it received from the gateway and
adding 7/8 (87.5%) of the time elapsed since that packet was
received. If the Epoch value in the newly received packet is less
than the client's conservative estimate by more than one second, then
the client concludes that the PCP server lost state, and the client
MUST immediately renew all its active port mapping leases as
described in Section 8.8.1.
When a client notices that the PCP server reduced its Epoch value,
the PCP clients will send PCP requests to refresh their mappings.
The PCP server needs to be scaled appropriately to accommodate this
traffic. Because PCP lacks a mechanism to simultaneously inform all
PCP clients of the Epoch value, the PCP clients will only flood the
PCP server simultaneously when a power outage and restoration event
causes state loss in both the PCP clients and PCP server.
6.6. Version Negotiation
A PCP client sends its requests using PCP version number 1. Should
later updates to this document specify different message formats with
a version number greater than 1 it is expected that PCP servers will
still support version 1 in addition to the newer version(s).
However, in the event that a server returns a response with error
code UNSUPP_VERSION, the client MAY log an error message to inform
the user that it is too old to work with this server.
When sending a response containing the UNSUPP_VERSION result code,
the PCP message MUST be 12 octets long.
If future PCP versions greater than 1 are specified, version
negotiation is expected to proceed as follows:
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1. If a client or server supports more than one version it SHOULD
support a contiguous range of versions -- i.e., a lowest version
and a highest version and all versions in between.
2. Client sends first request using highest (i.e., presumably
'best') version number it supports.
3. If server supports that version it responds normally.
4. If server does not support that version it replies giving a
result containing the error code UNSUPP_VERSION, and the closest
version number it does support (if the server supports a range of
versions higher than the client's requested version, the server
returns the lowest of that supported range; if the server
supports a range of versions lower than the client's requested
version, the server returns the highest of that supported range).
5. If the client receives an UNSUPP_VERSION result containing a
version it does support, it records this fact and proceeds to use
this message version for subsequent communication with this PCP
server (until a possible future UNSUPP_VERSION response if the
server is later updated, at which point the version negotiation
process repeats).
6. If the client receives an UNSUPP_VERSION result containing a
version it does not support then the client MAY log an error
message to inform the user that it is too old to work with this
server, and the client SHOULD set a timer to retry its request in
30 minutes or the returned Lifetime value, whichever is smaller.
6.7. General PCP Option
The following option can appear in certain PCP responses, without
regard to the OpCode.
6.7.1. UNPROCESSED Option
If the PCP server cannot process a mandatory-to-process option, for
whatever reason, it includes the UNPROCESSED Option in the response,
shown in Figure 5. This helps with debugging interactions between
the PCP client and PCP server. This option MUST NOT appear more than
once in a PCP response. The unprocessed options are listed once, and
the option data is zero-filled to the necessary 32 bit boundary. If
a certain Option appeared more than once in the PCP request, that
Option value can appear once or as many times as it occurred in the
request. The order of the Options in the PCP request has no
relationship with the order of the Option values in this UNPROCESSED
Option. This Option MUST NOT appear in a response unless the
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associated request contained at least one mandatory-to-process
Option.
The UNPROCESSED option is formatted as follows, showing an example of
two option codes that were unprocessed:
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 | option-code-2 | padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: UNPROCESSED option
Padding: 0, 1, 2, or 3 octets. If the number of option-codes is not
a multiple of 4, padding is used to make it 32-bit aligned. The
padding MUST be on on sending, and MUST be ignored by the receiver.
This Option:
name: UNPROCESSED
number: 1
purpose: indicates which PCP options in the request are not
supported by the PCP server
is valid for OpCodes: all
length: 1 or more
may appear in: responses, and only if the result code is non-
zero.
maximum occurrences: 1
7. Introduction to MAP and PEER OpCodes
There are three uses for the MAP and PEER OpCodes defined in this
document: a host operating a server (and wanting an incoming
connection), a host operating a client (and wanting to optimize the
application keepalive traffic), and a host operating a client and
server on the same port. These are discussed in the following
sections.
When operating a server (Section 7.1 and Section 7.3) the PCP client
knows if it wants an IPv4 listener, IPv6 listener, or both on the
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Internet. The PCP client also knows if it has an IPv4 address on
itself or an IPv6 interface on itself. It takes the union of this
knowledge to decide to send a one or two MAP requests for each of its
interfaces. Applications that embed IP addresses in payloads (e.g.,
FTP, SIP) will find it beneficial to avoid address family
translation, if possible.
It is REQUIRED that the PCP-controlled device assign the same
external IP address to PCP-created explicit dynamic mappings and to
implicit dynamic mappings. It is RECOMMENDED that static mappings
(e.g., those created by a command-line interface on the PCP server or
PCP-controlled device) also be assigned to the same IP address. Once
all internal addresses belonging to a given subscriber have no
implicit dynamic mappings and have no explicit dynamic mappings in
the PCP-controlled device, a subsequent PCP request for that internal
address MAY be assigned to a different external IP address.
Generally, this re-assignment would occur when a CGN device is load
balancing newly-seen hosts to its public IPv4 address pool.
7.1. For Operating a Server
A host operating a server (e.g., a web server) listens for traffic on
a port, but the server never initiates traffic from that port. For
this to work across a NAT or a firewall, the host needs to (a) create
a mapping from a public IP address and port to itself as described in
Section 8 and (b) publish that public IP address and port via some
sort of rendezvous server (e.g., DNS, a SIP message, a proprietary
protocol). Publishing the public IP address and port is out of scope
of this specification. To accomplish (a), the host follows the
procedures described in this section.
As normal, the application needs to begin listening on a port. Then,
the application constructs a PCP message with the appropriate MAP
OpCode depending on if it is listening on an IPv4 or IPv6 address and
if it wants a public IPv4 or IPv6 address.
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The following pseudo-code shows how PCP can be reliably used to
operate a server:
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
external_sockaddr = 0;
while (1)
{
/* Note: the "time_to_send_pcp_request" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may change over time
*/
if (time_to_send_pcp_request)
pcp_send_map_request(internal_sockaddr.sin_port,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received)
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet)
process_it(s);
if (other_work_to_do)
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
Figure 6: Pseudo-code for using PCP to operate a server
7.2. For Reducing NAT Keepalive Messages
A host operating a client (e.g., XMPP client, SIP client) sends from
a port but never accepts incoming connections on this port. It wants
to ensure the flow to its server 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.
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Middleboxes such as NATs or firewalls need to see occasional traffic
or will terminate their session state, causing application failures.
To avoid this, many applications routinely generate keepalive traffic
for the primary (or sole) purpose of maintaining state with such
middleboxes. Applications can reduce such application keepalive
traffic by using PCP.
Note: For reasons beyond NAT, an application may find it useful to
perform application-level keepalives, such as to detect a broken
path between the client and server, detect a crashed server, or
detect a powered-down client. These keepalives are not related to
maintaining middlebox state, and PCP cannot do anything useful to
reduce those keepalives.
To use PCP for this function, the application first connects to its
server, as normal. Afterwards, it issues a PCP request with the
PEER4 or PEER6 OpCode as described in Section 9. The PEER4 OpCode is
used if the host is using IPv4 for its communication to its peer;
PEER6 if using IPv6. The same 5-tuple as used for the connection to
the server is placed into the PEER4 or PEER6 payload.
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The following pseudo-code shows how PCP can be reliably used with a
dynamic socket, for the purposes of reducing application keepalive
messages:
int s = socket(...);
connect(s, &remote_peer, ...);
getsockname(s, &internal_sockaddr, ...);
external_sockaddr = 0;
while (1)
{
/* Note: the "time_to_send_pcp_request" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may change over time
*/
if (time_to_send_pcp_request)
pcp_send_peer_request(internal_sockaddr.sin_port,
&external_sockaddr, /* will be zero the first time */
remote_peer, requested_lifetime, &assigned_lifetime);
if (data_to_send)
send_it(s);
if (other_work_to_do)
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
Figure 7: Pseudo-code using PCP with a dynamic socket
7.3. For Operating a Symmetric Client/Server
A host operating a client and server on the same port (e.g.,
Symmetric RTP [RFC4961] or SIP Symmetric Response Routing (rport)
[RFC3581]) first establishes a local listener, (usually) sends the
local and public IP addresses and ports to a rendezvous service
(which is out of scope of this document), and initiates an outbound
connection from that same source address and same port. To
accomplish this, the application uses the procedure described in this
section.
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An application that is using the same port for outgoing connections
as well as incoming connections MUST first signal its operation of a
server using the PCP MAP OpCode, as described in Section 8, and
receive a positive PCP response before it sends any packets from that
port.
Discussion: Although reversing those steps is tempting (to
eliminate the PCP round trip before a packet can be sent from that
port) and will work if the NAT has endpoint-independent mapping
(EIM) behavior, reversing the steps will fail if the NAT does not
have EIM behavior. With a non-EIM NAT, the implicit mapping
created by an outgoing TCP SYN and the explicit mapping created
using the MAP OpCode will cause different ports to be assigned
(which is not desirable; after all, the application is using the
same port for outgoing and incoming traffic on purpose) and they
will generally also have different lifetimes. PCP does not
attempt to change or dictate how a NAT creates its mappings
(endpoint independent mapping, or otherwise) so there is no
assurance that an implicit mapping will be EIM or non-EIM. Thus,
it is necessary for an application to first signal its operation
of a server using the PCP MAP OpCode. See also Section 8.9.
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The following pseudo-code shows how PCP can be used to operate a
symmetric client and server:
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
external_sockaddr = 0;
while (1)
{
/* Note: the "time_to_send_pcp_request" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may change over time
*/
if (time_to_send_pcp_request)
pcp_send_map_request(internal_sockaddr.sin_port,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received)
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet)
process_it(s);
if (need_to_make_outgoing_connection)
make_outgoing_connection(s, ...);
if (data_to_send)
send_it(s);
if (other_work_to_do)
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
Figure 8: Pseudo-code for using PCP to operate a symmetric client/
server
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8. MAP OpCodes
This section defines two OpCodes which control forwarding from a NAT
(or firewall) to an internal host. They are:
MAP4=1: create a mapping between an internal address and external
IPv4 address (e.g., NAT44, NAT64, or firewall)
MAP6=2: create a mapping between an internal target address and
external IPv6 address (e.g., NAT46, or firewall)
The internal address is the source IP address of the PCP request
message itself, unless the THIRD_PARTY option is used.
Note that all mappings created by PCP MAP requests are, by
definition, Endpoint Independent Mappings (even on a NAT that usually
creates Endpoint Dependent Mappings for outgoing connections) since
the purpose of a MAP mapping is to receive inbound traffic from any
remote endpoint, not from only one specific remote endpoint.
Note also that all NAT mappings (created by PCP or otherwise) are by
necessity bidirectional and symmetrical. For any packet going in one
direction (in or out) that is translated by the NAT, a reply going in
the opposite direction needs to have the corresponding opposite
translation done so that the reply arrives at the right endpoint.
This means that if a client creates a MAP mapping, and then later
sends an outgoing packet using the mapping's internal source port,
the NAT should translate that packet's Internal Address and Port to
the mapping's External Address and Port, so that replies addressed to
the External Address and Port are correctly translated to the
mapping's Internal Address and Port.
The operation of the MAP OpCodes is described in this section.
8.1. OpCode Packet Formats
The two MAP OpCodes (MAP4, MAP6) share a similar packet layout for
both requests and responses. Because of this similarity, they are
shown together. For both of the MAP OpCodes, if the assigned
external IP address and assigned external port match the request's
Internal IP address and port, the functionality is purely a firewall;
otherwise it pertains to a network address translator which might
also perform firewall-like functions.
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The following diagram shows the OpCode-specific information format in
a request for the MAP4 and MAP6 OpCodes.
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 (32 or 128, depending on OpCode):
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: MAP OpCode Request Packet Format
These fields are described below:
Requested lifetime (in common header): Requested lifetime of this
mapping, in seconds. The value 0 indicates "delete".
Protocol: indicates upper-layer protocol associated with this
OpCode. Values are taken from the IANA protocol registry
[proto_numbers]. For example, this field contains 6 (TCP) if the
opcode is intended to create a TCP mapping. The value 0 has a
special meaning for 'all protocols', and is used only for delete
requests. This means that HOPOPT (which is assigned by IANA as
protocol 0) cannot have a mapping deleted by PCP.
Reserved: 24 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
Internal Port: Internal port for the mapping. The value 0 indicates
"all ports", and is only legal in a request if lifetime=0.
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 uses 0.
Suggested External IP Address: Suggested external IP address. This
is useful for refreshing a mapping, especially after the PCP
server loses state. If the PCP server can fulfill the request, it
will do so. If the PCP client does not know the external address,
or does not have a preference, it MUST use 0.
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The following diagram shows the OpCode-specific information format in
a response packet for the MAP4 and MAP6 OpCodes:
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 (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: MAP OpCode Response Packet Format
These fields are described below:
Lifetime (in common header): On a success response, this indicates
the lifetime for this mapping, in seconds. On an error response,
this indicates how long clients should assume they'll get the same
error response from the that PCP server if they repeat the same
request.
Protocol: Copied from the request
Reserved: 24 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
Assigned External IP Address: On success responses, this is the
assigned external IPv4 or IPv6 address for the mapping; IPv4 or
IPv6 address is indicated by the OpCode. On error responses, this
MUST be 0.
Internal Port: Internal port for the mapping, copied from request.
Assigned External Port: On success responses, this is the assigned
external port for the mapping. On error responses, the Assigned
External Port MUST be 0.
8.2. OpCode-Specific Result Codes
In addition to the general PCP result codes (Section 5.4), the
following additional result codes may be returned as a result of the
four MAP OpCodes received by the PCP server. These errors are
considered 'long lifetime' or 'short lifetime', which provides
guidance to PCP server developers for the value of the Lifetime field
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for these errors. It is RECOMMENDED that short lifetime errors use
30 second lifetime and long lifetime errors use 30 minute lifetime.
20 NETWORK_FAILURE, PCP server or the device it controls are
experiencing a network failure of some sort (e.g., has not
obtained an IP address). This is a short lifetime error.
21 NO_RESOURCES, e.g., NAT device cannot create more mappings at this
time. This is a system-wide error, and different from
USER_EX_QUOTA. This is a short lifetime error.
22 UNSUPP_PROTOCOL, unsupported Protocol. This is a long lifetime
error.
23 NOT_AUTHORIZED, e.g., PCP server supports mapping, but the feature
is disabled for this PCP client, or the PCP client requested a
mapping that cannot be fulfilled by the PCP server's security
policy. This is a long lifetime error.
24 USER_EX_QUOTA, mapping would exceed user's port quota. This is a
short lifetime error.
25 CANNOT_PROVIDE_EXTERNAL_PORT, indicates the port is already in use
(e.g. already allocated to a previous PCP client) or otherwise
temporarily unavailable. This error is only returned if the
request included the Option PREFER_FAILURE. This is a short
lifetime error.
26 EXCESSIVE_REMOTE_PEERS, indicates 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 REMOTE_FILTER Option.
This is a long lifetime error.
Additional result codes may be returned if the THIRD_PARTY option is
used, see Section 10.1.
8.3. OpCode-Specific Client: Generating a Request
This section describes the operation of a PCP client when sending
requests with OpCodes MAP4 and MAP6.
The request MAY contain values in the suggested-external-ip-address
and suggested-external-port fields. This allows the PCP client to
attempt to rebuild the PCP server's state, so that the PCP client
could avoid having to change information maintained at the 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 fulfill the request.
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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
allocated external IP address and port as the suggested external IP
address and port, so that if the NAT gateway has lost state it can
recreate the lost mapping with the same parameters.
The PCP client SHOULD renew the mapping before its expiry time,
otherwise it will be removed by the PCP server (see Section 8.6). In
order to prevent excessive PCP chatter, it is RECOMMENDED to send a
single renewal request packet when a mapping is halfway to expiration
time, then, if no SUCCESS result is received, another single renewal
request 3/4 of the way to expiration time, and then another at 7/8 of
the way to expiration time, 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 ever-closer-together requests in the last few
seconds before a mapping expires).
8.4. OpCode-Specific Server: Processing a Request
This section describes the operation of a PCP server when processing
a request with the OpCodes MAP4 or MAP6. Processing SHOULD be
performed in the order of the following paragraphs.
If the server is overloaded by requests (from a particular client or
from all clients), it MAY simply discard requests, as the requests
will be retried by PCP clients, or MAY generate the SERVER_OVERLOADED
error response, or both.
If the request contains internal-port=0 and the lifetime is non-zero,
the server MUST generate a MALFORMED_REQUEST error.
If the requested lifetime is not zero, it indicates a request to
create a mapping or extend the lifetime of an existing mapping.
Processing of the lifetime is described in Section 8.6.
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 and 1:1
NAT44, both of which modify addresses but not port numbers.
If an Option with value less than 128 exists (i.e. mandatory to
process) but that option does not make sense (e.g., the
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PREFER_FAILURE option is included in a request with lifetime=0), the
request is invalid and generates a MALFORMED_OPTION error.
If the PCP server can allocate the suggested external port, and the
request did not contain the PREFER_FAILURE Option, it SHOULD do so.
This is beneficial for re-establishing state lost when the PCP server
loses its state (e.g., due to a reboot). If the PCP server cannot
allocate the suggested external port but can allocate some other port
and the request did not contain the PREFER_FAILURE Option, the PCP
server MUST do so and return the allocated port in the response.
Cases where a NAT gateway cannot allocate the suggested external port
include:
o Where the suggested external port is already allocated to another
existing explicit, implicit, or static mapping, or already
forwarding traffic to some other internal address:port, or;
o Where the suggested external 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), or;
o When the suggested external port is otherwise prohibited by the
PCP server's policy.
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.
If the THIRD_PARTY option is not present in the request, the source
IP address of the PCP packet is used when creating 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 Section 12.3 for the validation procedure. If the
internal IP address in 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.
Mappings typically consume state on the PCP-controlled device, and it
is RECOMMENDED that a per-subscriber or per-host limit be enforced by
the PCP server to prevent exhausting the mapping state. If this
limit is exceeded, the result code USER_EX_QUOTA is returned.
If all of the proceeding operations were successful (did not generate
an error response), then the requested mappings are created or
refreshed as described in the request and a SUCCESS response is
built. This SUCCESS response contains the same OpCode as the
request, but with the "R" bit set.
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8.5. OpCode-Specific Client: Processing a Response
This section describes the operation of the PCP client when it
receives a PCP response for the OpCodes MAP4 or MAP6.
After performing common PCP response processing, the response is
further matched with an outstanding request by comparing the
protocol, internal IP address, and internal port. Other fields are
not compared, because the PCP server sets those fields.
If a successful response, the PCP client can use the external IP
address and port(s) as desired. Typically the PCP client will
communicate the external IP address and port(s) to another host on
the Internet using an application-specific rendezvous mechanism such
as DNS SRV records.
On an error response, clients SHOULD NOT repeat the same request to
the same PCP server within the lifetime returned in the response.
8.6. Mapping Lifetime and Deletion
The PCP client requests a certain lifetime, and the PCP server
responds with the assigned lifetime. The PCP server MAY grant a
lifetime smaller or larger than the requested lifetime. The PCP
server SHOULD be configurable for permitted minimum and maximum
lifetime, and the RECOMMENDED values are 120 seconds for the minimum
value and 24 hours for the maximum. It is RECOMMENDED that the
server restrict lifetimes to less than 24 hours, because they will
consume ports even if the internal host is no longer interested in
receiving the traffic or no longer connected to the network.
Once a PCP server has responded positively to a mapping request for a
certain lifetime, the port forwarding 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 PEER and MAP requests are
not special or different to mappings created other ways. In
particular, it is implementation-dependent if outgoing traffic
extends the lifetime of such mappings. PCP clients MUST NOT depend
on this behavior to keep mappings active, and MUST explicitly renew
their mappings as required by the Lifetime field in PCP response
messages.
If the requested lifetime is 0 then:
o If the internal port and protocol both are non-zero, it indicates
a request to delete the indicated mapping immediately.
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o If the internal port is non-zero and the protocol is zero, it
indicates a request to delete all mappings for this Internal
Address for the given internal port for all transport protocols.
o If the internal port and protocol both 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.
The suggested external address and port fields are ignored in
requests where the requested lifetime is 0.
A PCP MAP request can delete an implicit dynamic mapping (e.g., a
mapping created by a TCP SYN) or an explicit dynamic mapping (e.g., a
mapping created by PCP MAP). If the PCP client attempts to delete a
single static mapping (i.e., a mapping created outside of PCP
itself), the error NOT_AUTHORIZED is returned. If the PCP client
attempts to delete an implicit dynamic mapping (e.g., created by a
TCP SYN), the PCP server deletes the mapping and responds with the
SUCCESS result code. If the PCP client attempts to delete a mapping
that does not exist, the SUCCESS result code is returned (this is
necessary for PCP to be idempotent). If the PCP MAP request was for
port=0 (indicating 'all ports'), the PCP server deletes all of the
explicit dynamic mappings it can (but not any implicit mappings), and
returns a SUCCESS response. If the deletion request was properly
formatted and successfully processed, a SUCCESS response is generated
with lifetime of 0 and the server copies the protocol and internal
port number from the request into the response. An explicit dynamic
mapping MUST NOT have its lifetime reduced by transport protocol
messages (e.g., TCP RST, TCP FIN).
An application that forgets its PCP-assigned mappings (e.g., the
application or OS crashes) will request new PCP mappings. This may
consume port mappings, if the application binds to a different
Internal Port every time it runs. The application will also likely
initiate new implicit dynamic mappings (e.g., TCP connections)
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) automatically
monitor for process exit (including application crashes) and
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automatically send port mapping deletion requests if the process that
requested them goes away without explicitly relinquishing them.
To reduce unwanted traffic and data corruption, UDP and TCP ports
should not be immediately re-used for an interval (TIME_WAIT interval
as discussed in [RFC0793]). However, the PCP server MUST allow the
same subscriber and same internal address to re-acquire the same port
during that interval.
As a side-effect of creating a mapping, ICMP messages associated with
the mapping MUST be forwarded (and also translated, if appropriate)
for the duration of the mapping's lifetime. This is done to ensure
that ICMP messages can still be used by hosts, without application
programmers or PCP client implementations needing to signal PCP
separately to create ICMP mappings for those flows.
The following list summarizes the sentinel values when deleting a
mapping using lifetime=0:
* all ports, all protocols, all Internal Addresses for which the
client is authorized: internal address=0, via the THIRD_PARTY option
* all ports, all protocols: internal port=0, protocol=0
* all ports, specific protocol: internal port=0, protocol={protocol
value} (e.g., protocol=6 for TCP)
* one port, specific protocol: internal port={port number},
protocol={protocol value} (e.g., port=12345, protocol=6 for TCP)
8.7. Subscriber Renumbering and Address Change Events
The customer premises router might obtain a new IP address. This can
occur because of 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 subscriber might be delivered to
another customer who now has that address. This affects both
implicit dynamic mappings and explicit dynamic mappings. However,
this same problem occurs today when a subscriber'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 subscriber
renumbering are caused by subscriber renumbering and are eliminated
if subscriber renumbering is avoided. PCP defined in this document
does not provide machinery to reduce the subscriber renumbering
problem.
When a new Internal Address is assigned to a host embedding a PCP
client, the NAT (or firewall) controlled by the PCP server will
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continue to send traffic to the old IP address. Typically, the PCP
client will no longer receive traffic sent to that old IP address.
Assuming the PCP client 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 this scenario is not
expected to happen routinely on a regular basis for most hosts, since
most hosts renew their DHCP leases before they expire (or re-request
the same address after reboot) and most DHCP servers honor such
requests and grant the host the same address it was previously using
before the reboot.
A host might gain or lose interfaces while existing mappings are
active (e.g., Ethernet cable plugged in or removed, joining/leaving a
WiFi network). Because of this, if the PCP client is sending a PCP
request to maintain state in the PCP server, it SHOULD ensure those
PCP requests continue to use the same interface (e.g., when
refreshing mappings). If the PCP client is sending a PCP request to
create new state in the PCP server, it MAY use a different source
interface or different source address.
8.8. PCP Failure Scenarios
If an event occurs that causes the PCP server to lose explicit
dynamic mapping state (such as a crash or power outage), the mappings
created by PCP are lost. Such loss of state is rare in a service
provider environment (due to redundant power, disk drives for
storage, etc.), but more common in a residential NAT device which
does not write information to its non-volatile memory. Of course,
due to outright failure of service provider equipment (e.g., software
malfunction), state may still be lost.
The Epoch allows a client to deduce when a PCP server may have lost
its state. When the Epoch value is smaller than expected, the PCP
client can attempt to recreate the mappings following the procedures
described in this section.
8.8.1. Recreating Mappings
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).
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However, maintaining this state is not essential for correct
operation. When the PCP server loses state and begins processing new
PCP messages, its Epoch is reset to zero (per the procedure of
Section 6.5).
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 the 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.
In addition, as the result of receiving a packet where the Epoch
field indicates that a reboot or similar loss of state has occurred,
the client can renew its port mappings sooner, without waiting for
the normal routine renewal time.
The discussion in this section focuses on recreating inbound port
mappings after loss of PCP server state, because that is the more
serious problem. Losing port mappings for outgoing connections
destroys those currently active connections, but does not prevent
clients from establishing new outgoing connections. In contrast,
losing inbound port mappings not only destroys all existing inbound
connections, but also prevents the reception of any new inbound
connections until the port mapping is recreated. Accordingly, we
consider recovery of inbound port mappings the more important
priority. However, clients that want outgoing connections to survive
a NAT gateway reboot can also achieve that using PCP. After
initiating an outbound TCP connection (which will cause the NAT
gateway to establish an implicit port mapping) the client should send
the NAT gateway a PEER request for the source port of its TCP
connection, which will cause the NAT gateway to send a response
giving the external port it allocated for that mapping. The client
can then store this information, and use it later to recreate the
mapping if it determines that the NAT gateway has lost its mapping
state.
8.8.2. Maintaining Mappings
A PCP client can refresh 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
failure of the PCP server, that the public IP address and/or public
port, or the PCP server itself, has changed (due to a new route to a
different PCP server). To detect such events more quickly, the PCP
client may find it beneficial to use shorter lifetimes (so that it
communicates with the PCP server more often). If the PCP client has
several mappings, the Epoch value only needs to be retrieved for one
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of them to verify the PCP server has not lost port forwarding state.
If the client wishes to check the PCP server's Epoch, it sends a PCP
request for any one of the client's mappings. This will return the
current Epoch value. In that request the PCP client could extend the
mapping lifetime (by asking for more time) or maintain the current
lifetime (by asking for the same number of seconds that it knows are
remaining of the lifetime).
If an internal IP address is no longer valid (e.g., because the
internal host has moved to a new network), and the PCP client wishes
to still receive incoming traffic, it MUST create a new mapping on
that new network. A new mapping will also require an update to the
application-specific rendezvous server (see Section 7.1 and
Section 8.7).
8.9. Implementing MAP with non-EIM NATs
For implicit dynamic mappings, some existing NAT devices have
endpoint-independent mapping (EIM) behavior while other NAT devices
have non-endpoint-independent mapping (non-EIM) behavior. NATs which
have EIM behavior do not suffer from the problem described in this
section. EIM behavior is strongly encouraged by both [RFC4787] and
[RFC5382].
In such non-EIM NAT devices, the same external port may be used by
connections from different internal hosts. This complicates the
interaction with the MAP4 and MAP6 OpCodes. With such NAT devices,
there are two ways envisioned to implement the MAP4 and MAP6 OpCodes:
1. have implicit dynamic mappings (e.g., TCP SYN) use a different
set of public ports than explicit dynamic mappings (e.g., those
created with MAP4 or MAP6), thus avoiding the interaction problem
between them.
2. on arrival of an incoming packet from the Internet, first attempt
to process implicit dynamic mappings (as done today by existing
port overload code), and if none of those can be processed then
the incoming packet is for the explicit dynamic mapping. This
effectively 'prioritizes' implicit dynamic mappings above
explicit dynamic mappings.
9. PEER OpCodes
This section defines two OpCodes for controlling dynamic connections.
They are:
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PEER4=3: Set (or query) the lifetime for flow to a remote peer's
IPv4 address.
PEER6=4: Set (or query) the lifetime for flow to a remote peer's
IPv6 address.
The operation of these OpCodes is described in this section.
9.1. OpCode Packet Formats
The PEER OpCodes provide a single function: the ability for the PCP
client to query and (possibly) extend the lifetime of an existing
mapping.
The two PEER OpCodes (PEER4 and PEER6) share a similar packet layout
for both requests and responses. Because of this similarity, they
are shown together.
The following diagram shows the request packet format for PEER4 and
PEER6. This packet format is aligned with the response packet
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | Reserved (24 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Port | Suggested External Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Peer Port | Reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP Address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Reserved (128 bits) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: PEER OpCode Request Packet Format
These fields are described below:
Requested Lifetime (in common header): Requested lifetime of this
mapping, in seconds. Note that, depending on the implementation
of the PCP-controlled device, it may not be possible to reduce the
lifetime of a mapping (or delete it, with requested lifetime=0)
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using PEER.
Protocol: indicates upper-level protocol associated with this
OpCode. Values are taken from the IANA protocol registry
[proto_numbers]. For example, this field contains 6 (TCP) if the
OpCode is describing a TCP peer.
Reserved: 24 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
Internal Port: Internal port of the 5-tuple.
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 server can fulfill the request, it
will do so. If the PCP client does not know the external port, or
does not have a preference, it uses 0.
Remote Peer Port: Remote peer's port of the 5-tuple.
Reserved: 16 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
Remote Peer IP Address: This is the 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. Note
this field has no bearing whatsoever on any filtering associated
with the mapping.
Reserved: 128 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
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The following diagram shows the response packet format for PEER4 and
PEER6:
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 | External_AF | Reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Internal Port | External Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Remote Peer Port | Reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP Address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: External IP Address (32 bits if External_AF indicates IPv4 :
: 128 bits if External_AF indicates IPv6) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: PEER OpCode Response Packet Format
Lifetime (in common header): On a success response, this indicates
the lifetime for this mapping, in seconds. On an error response,
this indicates how long clients should assume they'll get the same
error response from the PCP server if they repeat the same
request.
Protocol: Copied from the request.
External_AF: For success responses, this contains the address family
of the external IP address associated with this peer connection,
to properly decode the External IP Address. This field is
necessary because the Remote Peer's IP Address is from the PCP
client's perspective, whereas the External_AF and External IP
Address are from the PCP-controlled device's perspective. As an
example, if the PCP-controlled device is a NAT64, the PCP client
only knows the remote peer's IPv6 address, whereas the NAT64 knows
the remote peer's IPv4 address. Values are from IANA's address
family numbers (IPv4 is 1, IPv6 is 2). For error responses, the
value MUST be 1.
Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be
ignored on reception.
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Internal Port: copied from request.
External Port: For success responses, this is the external port
number, assigned by the NAT (or firewall) to this mapping. If
firewall or 1:1 NAT, this will match the internal port. For error
responses, this MUST be 0.
Remote Peer port: Copied from request.
Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be
ignored on reception.
Remote Peer IP Address: Copied from the request.
External IP Address: For success responses, this contains the
external IP address, assigned by the NAT (or firewall) to this
mapping. This field allows the PCP client and its remote peer to
determine if there is another NAT between the PCP-controlled NAT
and remote peer. If the PCP-controlled device is a firewall, this
will match the internal IP address. This field is 128 bits long
if External_AF indicates IPv6, or 32 bits long if External_AF
indicates IPv4.
9.2. OpCode-Specific Client: Generating a Request
This section describes the operation of a client when generating the
OpCodes PEER4 or PEER6.
The PEER4 or PEER6 OpCodes MAY be sent before or after establishing
bi-directional communication with the remote peer. If sent before,
PEER4 or PEER6 OpCodes will create a mapping in the PCP-controlled
device that functions exactly as if an implicit dynamic connection
were made (e.g., TCP SYN). If sent after, the PEER4 or PEER6 OpCodes
query (and control) the implicit dynamic mapping.
The PEER4 and PEER6 OpCodes contain a description of the remote peer
address, from the perspective of the PCP client. This is important
when the PCP-controlled device is performing address family
translation (NAT46 or NAT64), because the destination address from
the perspective of the PCP client is different from the destination
address on the other side of the address family translation device.
For this reason, the PEER4 and PEER6 responses contain an External_AF
field.
9.3. OpCode-Specific Server: Processing a Request
This section describes the operation of a server when receiving a
request with the OpCode PEER4 or PEER6. Processing SHOULD be
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performed in the order of the following paragraphs.
On receiving the PEER4 or PEER6 OpCode, the PCP server examines the
mapping table. If the described mapping does not exist yet, it is
created, just as it would be for an outgoing UDP packet or TCP SYN.
This avoids a race condition between the PEER request or the initial
outgoing packet arriving at the NAT gateway first.
The PEER4 or PEER6 OpCode MAY reduce the lifetime of an existing
mapping; this is implementation-dependent.
If the PCP-controlled device can extend the lifetime of a mapping,
the PCP server uses the smaller of its configured maximum lifetime
value and the requested lifetime from the PEER request, and sets the
lifetime to that value.
If all of the proceeding operations were successful (did not generate
an error response), then a SUCCESS response is generated, with the
Lifetime field containing the lifetime of the mapping.
After a successful PEER response is sent, it is implementation-
specific if the PCP-controlled device destroys the mapping when the
lifetime expires, or if the PCP-controlled device's implementation
allows traffic to keep the mapping alive. Thus, if the PCP client
wants the mapping to persist beyond the lifetime, it MUST refresh the
mapping (by sending another PEER message) prior to the expiration of
the lifetime. If the mapping is terminated by the TCP client or
server (e.g., TCP FIN or TCP RST), the mapping will be destroyed
normally; the mapping will not persist for the time indicated by
Lifetime. This means the Lifetime in a PEER response indicates how
long the mapping will persist in the absence of a transport
termination message (e.g., TCP RST).
9.4. OpCode-Specific Client: Processing a Response
This section describes the operation of a client when processing a
response with the OpCode PEER4 or PEER6.
After performing common PCP response processing, the response is
further matched with a request by comparing the protocol, external
AF, internal IP address, internal port, remote peer address and
remote peer port. Other fields are not compared, because the PCP
server changes those fields to provide information about the mapping
created by the OpCode.
If 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,
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specific to the application, to use more frequent application
keepalives. For example, the PCP assigned lifetime could be one hour
but the application may want to maintain state on its server (e.g.,
"busy" / "away") more frequently than once an hour.
If the PCP client wishes to keep this mapping alive beyond the
indicated lifetime, it SHOULD issue a new PCP request prior to the
expiration. That is, inside->outside traffic is not sufficient to
ensure the mapping will continue to exist. It is RECOMMENDED to send
a single renewal request packet when a mapping is halfway to
expiration time, then, if no SUCCESS response is received, another
single renewal request 3/4 of the way to expiration time, and then
another at 7/8 of the way to expiration time, and so on, subject to
the constraint that renewal requests MUST NOT be sent less than four
seconds apart (a PCP client MUST NOT ever-closer-together requests in
the last few seconds before a mapping expires).
10. Options for MAP and PEER OpCodes
This section describes Options for the MAP4, MAP6, PEER OpCodes.
These Options MUST NOT appear with other OpCodes, unless permitted by
those OpCodes.
10.1. THIRD_PARTY Option for MAP and PEER OpCodes
This Option is used when a PCP client wants to control a mapping to
an internal host other than itself. This is used with both MAP and
PEER OpCodes.
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.
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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Internal IP Address (32 bits of 128 bits, depending :
: on Option length) :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
THIRD_PARTY option packet format
The fields are described below:
Internal IP Address: IP address of this mapping. If the length of
this Option is 4, this is a 32-bit IPv4 address. If the length of
this Option is 16, this is a 128-bit IPv6 address. This can
contain the special value "0" (all zeros), which indicates "all
Internal Addresses for which this client is authorized" which is
used to delete all pre-existing mappings with the MAP Opcode.
This Option:
name: THIRD_PARTY
number: 4
purpose: Indicates the MAP or PEER request is for a host other
than the host sending the PCP option.
is valid for OpCodes: MAP4, MAP6, PEER4, PEER6
length: 4 if Internal IP Address is IPv4, 16 if Internal IP
Address is IPv6.
may appear in: request. May appear in response only if it
appeared in the associated request.
maximum occurrences: 1
The following additional result codes may be returned as a result of
using this Option.
51 UNAUTH_TARGET_ADDRESS, indicating the internal IP address
specified is not permitted (e.g., client is not authorized to make
mappings for this Internal Address, or is otherwise prohibited.).
This error can be returned for both MAP and PEER requests. If
this is a MAP request, this is a long-term error.
A PCP server MAY be configured to permit or to restrict the use of
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the THIRD_PARTY option. If this option is permitted, any host can
create, modify, or destroy mappings for another host on the
subscriber's network. If third party mappings are restricted, only
authorized clients can perform these operations. If a PCP server is
configured to restrict third party mappings, and receives a PCP MAP
request with a THIRD_PARTY option, it MUST generate a
UNAUTH_TARGET_ADDRESS response. Determining which PCP clients are
authorized to use the THIRD_PARTY option depends on the deployment
scenario. For Dual-Stack Lite deployments, the PCP server only
supports this option if the source IPv6 address is the B4's source IP
address. For home deployments (where the PCP server is embedded in
the NAT device), this option MUST NOT be processed. For scenarios
where the subscriber has only one IP address (e.g., typical
residential ISP service) this Option serves no purpose (and will only
generate error messages from the server). If a subscriber has more
than one IP address the ISP MUST determine its own policy for how to
identify the trusted device within the subscriber's home. This might
be, for example, the lowest- or highest-numbered host address for
that user's IPv4 prefix. A cryptographic authentication and
authorization model is outside the scope of this specification.
It is RECOMMENDED that PCP servers embedded into customer premise
equipment be configured to refuse 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 premise
router (e.g., using its HTTP administrative interface).
It is RECOMMENDED that PCP servers embedded into service provider NAT
and firewall devices be configured to permit the THIRD_PARTY option,
when sent by the customer premise router. With this configuration,
if a user wants to create an explicit dynamic mapping or query an
implicit dynamic mapping for another host within their network, the
user needs to interact out-of-band with their customer premise router
(e.g., using its HTTP administrative interface). To accomplish this,
the PCP server in the ISP's network processes requests with the
THIRD_PARTY option if they arrived from the IP address of the
customer premise router. In deployments with only one IP address
(e.g., which is common in residential networks), the PCP messages
will -- by necessity -- arrive from the IP address of the customer
premise router router. In networks where users have multiple IPv4 or
multiple IPv6 addresses, the PCP server MUST only allow the
THIRD_PARTY option if the PCP message was sent by the IP address of
the subscriber's customer premise router. In Dual-Stack Lite, this
would be the B4 element's IPv6 address. If the packet arrived from a
different address, the PCP server MUST generate an
UNAUTH_TARGET_ADDRESS error.
If authorized to do so, a PCP client can delete all the PCP-created
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explicit dynamic mappings (i.e., those created by PCP MAP requests)
for all hosts belonging to the same subscriber. This is done by
sending a PCP MAP request including the THIRD_PARTY option with its
Internal Address field set to 0.
10.2. PREFER_FAILURE Option for MAP OpCodes
This option is only used with the MAP4 and MAP6 OpCodes.
This option indicates that if the PCP server is unable to MAP the
suggested port, then instead of returning an available port that it
*can* allocate, the PCP server should instead allocate no port and
return result code CANNOT_PROVIDE_EXTERNAL_PORT.
This option is intended solely for use by UPnP IGD interworking
[I-D.bpw-pcp-upnp-igd-interworking], where the semantics of UPnP IGD
version 1 do not provide any way to indicate to an UPnP IGD client
that any port is available other than the one it wanted. A PCP
server MAY support this option, if its designers wish to support
downstream devices that perform UPnP IGD interworking. PCP servers
MAY choose to rate-limit their handling of PREFER_FAILURE requests,
to protect themselves from a rapid flurry of 65535 consecutive
PREFER_FAILURE requests from clients probing to discover which
external ports are available. PCP servers that are not intended to
support downstream devices that perform UPnP IGD interworking are not
required to support this option. PCP clients other than UPnP IGD
interworking clients SHOULD NOT use this option because it results in
inefficient operation, and they cannot safely assume that all PCP
servers will implement it. It is anticipated that this option will
be deprecated in the future as more clients adopt PCP natively and
the need for UPnP IGD interworking declines.
This Option:
name: PREFER_FAILURE
number: 3
is valid for OpCodes: MAP4, MAP6
is included in responses: MUST
length: 0
may appear in: requests
maximum occurrences: no
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10.3. FILTER Option for MAP OpCodes
This Option indicates filtering incoming packets is desired. The
remote peer port and remote peer IP Address indicate the permitted
remote peer's source IP address and port for packets from the
Internet. The remote peer prefix length indicates the length of the
remote peer's IP address that is significant; this allows a single
Option to permit an entire subnet. After processing this MAP request
containing the FILTER option and generating a successful response,
the PCP-controlled device will drop packets received on its public-
facing interface that don't match the filter fields. After dropping
the packet, if its security policy allows, the PCP-controlled device
MAY also generate an ICMP error in response to the dropped packet.
The FILTER packet layout is described below:
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 | prefix-length | Remote Peer Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if MAP4, :
: 128 bits if MAP6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: FILTER option layout
These fields are described below:
Reserved: 8 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
prefix-length: indicates how many bits of the IPv4 or IPv6 address
are relevant for this filter. The value 0 indicates "no filter",
and will remove all previous filters. See below for detail.
Remote Peer Port: the port number of the remote peer. The value 0
indicates "all ports"
Remote Peer IP address: The IP address of the remote peer.
This Option:
name: FILTER
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number: 2
is valid for OpCodes: MAP4, MAP6
is included in responses: MUST, if it appeared in the request
length: 2 if used with MAP4, 5 if used with MAP6
may appear in: requests, and MUST appear in successfully-processed
responses
maximum occurrences: as many as fit within maximum PCP message
size
Because of interactions with dynamic ports this 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 ephemeral port for its outgoing connection. It is
RECOMMENDED that the PCP client avoid other use, because it will
cause some UNSAF NAT traversal mechanisms [RFC3424] to fail where
they would have otherwise succeeded, breaking other applications
running on this same host.
The prefix-length indicates how many bits of the IPv6 address or IPv4
address are used for the filter. For MAP4, a prefix-length of 32
indicates the entire IPv4 address is used. For MAP6, a prefix-length
of 128 indicates the entire IPv6 address is used. For MAP4 the
minimum prefix-length value is 0 and the maximum value is 32. For
MAP6 the minimum prefix-length value is 0 and the maximum value is
128. Values outside those range cause an MALFORMED_OPTION result
code.
If multiple occurrences of the FILTER option exist in the same MAP
request, they are processed in the same order received, and they MUST
all be successfully processed or return an error (e.g.,
MALFORMED_OPTION if one of the options was malformed), and they MAY
overlap the filtering requested. As with other PCP errors, returning
an error causes no state to be changed in the PCP server or in the
PCP-controlled device. 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.
To remove all existing filters, the prefix-length 0 is used. There
is no mechanism to remove a specific filter.
To change an existing filter, the PCP client sends a MAP request
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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 hosts.
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.
11. Deployment Considerations
11.1. Ingress Filtering
To prevent spoofing of PCP requests, ingress filtering [RFC2827] MUST
be performed by devices between the PCP clients and PCP server. For
example, with a PCP server integrated into a customer premise router,
the Ethernet switch needs to perform ingress filtering. As another
example, with a PCP server deployed by a service provider, the
service provider's aggregation router (the first device connecting to
subscribers) needs to do ingress filtering.
11.2. Per-Subscriber Explicit Dynamic Mapping Quota
On PCP-controlled devices that create state when a mapping is created
(e.g., NAPT), the PCP server SHOULD maintain a per-subscriber quota
for explicit dynamic mappings. It is implementation-specific if the
PCP server has a separate or combined quota for both implicit dynamic
mappings (e.g., created by TCP SYNs) and explicit dynamic mappings
(created using PCP).
12. Security Considerations
This document defines Port Control Protocol and two types of OpCodes,
PEER and MAP. The PEER OpCode allows querying and extending (if
permitted) the lifetime of an existing implicit dynamic mapping, so a
host can reduce its keepalive messages. The MAP OpCode allows
creating a mapping so a host can receive incoming unsolicited
connections from the Internet in order to run a server.
The PEER OpCode does not introduce any new security considerations,
unless the THIRD_PARTY Option is included. Discussion of the
THIRD_PARTY Option is below.
With the exception of wireless providers (who are interested in
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protecting their radio access network), Internet service providers do
not typically filter traffic from the Internet towards their
subscribers. However, when an ISP introduces stateful address
sharing with a NAPT device, such filtering will occur as a side
effect of the NAPT device. Filtering will also occur with an IPv6
CPE [RFC6092]. The MAP OpCode allows a PCP client to create a
mapping so that a host can receive inbound traffic and operate a
server. Security considerations for the MAP OpCode are described in
the following sections.
12.1. Denial of Service
Because of the state created in a NAPT or firewall, a per-subscriber
quota will likely exist for both implicit dynamic mappings (e.g.,
outgoing TCP connections) and explicit dynamic mappings (PCP). A
subscriber might make an excessive number of implicit or explicit
dynamic mappings, consuming an inordinate number of ports, causing a
denial of service to other subscribers. Thus, Section 11.2
recommends that subscribers be limited to a reasonable number of
explicit dynamic mappings.
12.2. Ingress Filtering
It is important to prevent a subscriber from creating a mapping for
another subscriber (or for another host), because this allows
incoming packets from the Internet and consumes the other user's
mapping quota. Both implicit dynamic mappings (e.g., outgoing TCP
connections) and explicit dynamic mappings (PCP) need ingress
filtering. Thus, PCP relies on the same ingress filtering as
implicit dynamic mappings and does not create a new requirement for
ingress filtering.
12.3. Validating THIRD_PARTY Internal Address
The THIRD_PARTY Option contains a Internal Address field, which
allows a PCP client to create, extend, or delete an implicit or
explicit dynamic mapping for another host.
In scenarios where the subscriber has one IP address (e.g., as
commonly occurs with IPv4 residential deployments) or the subscriber
has multiple IP addresses and a CP router enforces a PCP policy (by
operating its own PCP server or performing filtering [RFC6092]), the
PCP server in both the CP router and the ISP's equipment will both
reject any message containing THIRD_PARTY. Thus, PCP cannot be used
by a host to create, modify, or delete mappings of other hosts,
except by using the administrative interface of the customer premise
router (e.g., HTTP interface), as described in Section 10.1.
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In other scenarios, where the subscriber has multiple IP addresses
and the subscriber CP router is not filtering, but the ISP is
providing filtering, the ISP should only accept PCP messages
containing the THIRD_PARTY Option from the IP address of the
customer's router, as described in Section 10.1.
12.4. Interference by Other Applications on Same Host
An application running on a host can send PCP messages which create
or delete mappings for ports related to other applications on that
same host. This is by design. To reduce interference with other
applications, it is strongly RECOMMENDED that applications
implementing PCP themselves refrain from performing the delete-all
MAP function (lifetime=0, port=0). Instead, it is RECOMMENDED that
the MAP delete-all function be performed by the underlying operating
system.
12.5. Theft of mapping
In the time between when a PCP server loses state and the PCP client
notices the lower-than-expected Epoch value, it is possible that the
PCP client's mapping will be acquired by another host (via an
explicit dynamic mapping or implicit dynamic mapping). This means
incoming traffic will be sent to a different host ("theft"). A
mechanism to immediately inform the PCP client of state loss would
reduce this interval, but would not eliminate this threat. The PCP
client can reduce this interval by using a relatively short lifetime;
however, this increases the amount of PCP chatter. This threat is
eliminated by using persistent storage of explicit dynamic mappings
in the PCP server (so it does not lose explicit dynamic mapping
state). This threat can be mitigated by authenticating the data
connection between the hosts (e.g., using TLS).
13. IANA Considerations
IANA is requested to perform the following actions:
13.1. Port Number
PCP will use port 5351 (currently assigned by IANA to NAT-PMP). We
request that IANA re-assign that same port number to PCP, and
relinquish UDP port 44323.
13.2. OpCodes
IANA shall create a new protocol registry for PCP OpCodes, initially
populated with the values in Section 8 and Section 9. The values 0
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and 127 are reserved.
Additional OpCodes in the range 4-95 can be created via Standards
Action [RFC5226], and the range 96-126 is for Private Use [RFC5226].
13.3. Result Codes
IANA shall create a new registry for PCP result codes, numbered
0-255, initially populated with the result codes from Section 5.4,
Section 8.2, Section 10.3, and Section 10.1. The values 0 and 255
are reserved.
Additional Result Codes can be defined via Specification Required
[RFC5226].
13.4. Options
IANA shall create a new registry for PCP Options, numbered 0-255 with
an associated mnemonic. The values 0-127 are mandatory-to-process,
and 128-255 are optional-to-process. The initial registry contains
the options described in Section 10 and Section 10.1. The option
values 0, 127, and 255 are reserved.
Additional PCP option codes in the ranges 5-63 and 128-191 can be
created via Standards Action [RFC5226], and the ranges 64-126 and
192-254 are for Private Use [RFC5226].
14. Acknowledgments
Thanks to Alain Durand, Christian Jacquenet, Jacni Qin, Simon
Perreault, Paul Selkirk, and James Yu for their comments and review.
Thanks to Simon Perreault for highlighting the interaction of dynamic
connections with PCP-created mappings.
15. References
15.1. Normative References
[I-D.ietf-behave-v6v4-xlate]
Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
Algorithm", draft-ietf-behave-v6v4-xlate-23 (work in
progress), September 2010.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport-
Protocol Port Randomization", BCP 156, RFC 6056,
January 2011.
[proto_numbers]
IANA, "Protocol Numbers", 2010, <http://www.iana.org/
assignments/protocol-numbers/protocol-numbers.xml>.
15.2. Informative References
[]
Arkko, J., Eggert, L., and M. Townsley, "Scalable
Operation of Address Translators with Per-Interface
Bindings", draft-arkko-dual-stack-extra-lite-05 (work in
progress), February 2011.
[I-D.bpw-pcp-upnp-igd-interworking]
Boucadair, M., Penno, R., Wing, D., and F. Dupont,
"Universal Plug and Play (UPnP) Internet Gateway Device
(IGD)-Port Control Protocol (PCP) Interworking Function",
draft-bpw-pcp-upnp-igd-interworking-02 (work in progress),
February 2011.
[I-D.cheshire-nat-pmp]
Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)",
draft-cheshire-nat-pmp-03 (work in progress), April 2008.
[I-D.ietf-behave-lsn-requirements]
Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
and H. Ashida, "Common requirements for IP address sharing
schemes", draft-ietf-behave-lsn-requirements-01 (work in
progress), March 2011.
[I-D.ietf-behave-v6v4-xlate-stateful]
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Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers",
draft-ietf-behave-v6v4-xlate-stateful-12 (work in
progress), July 2010.
[I-D.ietf-softwire-dual-stack-lite]
Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
Stack Lite Broadband Deployments Following IPv4
Exhaustion", draft-ietf-softwire-dual-stack-lite-07 (work
in progress), March 2011.
[I-D.miles-behave-l2nat]
Miles, D. and M. Townsley, "Layer2-Aware NAT",
draft-miles-behave-l2nat-00 (work in progress),
March 2009.
[IGD] UPnP Gateway Committee, "WANIPConnection:1",
November 2001, <http://upnp.org/specs/gw/
UPnP-gw-WANIPConnection-v1-Service.pdf>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network
Address Translator (Traditional NAT)", RFC 3022,
January 2001.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC3581] Rosenberg, J. and H. Schulzrinne, "An Extension to the
Session Initiation Protocol (SIP) for Symmetric Response
Routing", RFC 3581, August 2003.
[RFC3587] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
Unicast Address Format", RFC 3587, August 2003.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy
Wing, et al. Expires October 22, 2011 [Page 56]
Internet-Draft Port Control Protocol (PCP) April 2011
Extensions for Stateless Address Autoconfiguration in
IPv6", RFC 4941, September 2007.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, July 2007.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment (CPE) for Providing
Residential IPv6 Internet Service", RFC 6092,
January 2011.
Appendix A. NAT-PMP Transition
The Port Control Protocol (PCP) is a successor to the NAT Port
Mapping Protocol (NAT-PMP), and shares similar semantics, concepts,
and packet formats. Because of this NAT-PMP and PCP 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.
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 [I-D.cheshire-nat-pmp], which will
cause the client to downgrade to NAT-PMP and re-send its request in
NAT-PMP format. If received by a PCP server, the response will be as
described by this document and processing continues as expected.
A PCP server supporting both NAT-PMP and PCP can respond to requests
in either format. The first byte of the packet indicates if it is
NAT-PMP (first byte zero) or PCP (first byte non-zero).
A PCP-only gateway receiving a NAT-PMP request (identified by the
first byte being zero) will interpret the request as a version
mismatch. Normal PCP processing will emit a PCP response that is
compatible with NAT-PMP, without any special handling by the PCP
server.
Appendix B. Change History
[Note to RFC Editor: Please remove this section prior to
publication.]
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B.1. Changes from draft-ietf-pcp-base-07 to -08
o moved all MAP4-, MAP6-, and PEER-specific options into a single
section.
o discussed NAPT port-overloading and its impact on MAP (new section
Section 8.9), which allowed removing the IMPLICIT_MAPPING_EXISTS
error.
o eliminated NONEXIST_PEER error (which was returned if a PEER
request was received without an implicit dynamic mapping already
being created), and adjusted PEER so that it creates an implicit
dynamic mapping.
o Removed Deployment Scenarios section (which detailed NAT64, NAT44,
Dual-Stack Lite, etc.).
o Added Client's IP Address to PCP common header. This allows
server to refuse a PCP request if there is a mismatch with the
source IP address, such as when a non-PCP-aware NAT was on the
path. This should reduce failure situations where PCP is deployed
in conjunction with a non-PCP-aware NAT. This addition was
consensus at IETF80.
o Changed UNSPECIFIED_ERROR to PROCESSING_ERROR. Clarified that
MALFORMED_REQUEST is for malformed requests (and not related to
failed attempts to process the request).
o Removed MISORDERED_OPTIONS. Consensus of IETF80.
o SERVER_OVERLOADED is now a common PCP error (instead of specific
to MAP).
o Tweaked PCP retransmit/retry algorithm again, to allow more
aggressive PCP discovery if an implementation wants to do that.
o Version negotation text tweaked to soften NAT-PMP reference, and
more clearly explain exactly what UNSUPP_VERSION should return.
o PCP now uses NAT-PMP's UDP port, 5351. There are no normative
changes to NAT-PMP or PCP to allow them both to use the same port
number.
o New Appendix A to discuss NAT-PMP / PCP interworking.
o improved pseudocode to be non-blocking.
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o clarified that PCP cannot delete a static mapping (i.e., a mapping
created by CLI or other non-PCP means).
o moved theft of mapping discussion from Epoch section to Security
Considerations, (Section 12.5).
B.2. Changes from draft-ietf-pcp-base-06 to -07
o tightened up THIRD_PARTY security discussion. Removed "highest
numbered address", and left it as simply "the CPE's IP address".
o removed UNABLE_TO_DELETE_ALL error.
o renumbered Opcodes
o renumbered some error codes
o assigned value to IMPLICIT_MAPPING_EXISTS.
o UNPROCESSED can include arbitrary number of option codes.
o Moved lifetime fields into common request/response headers
o We've noticed we're having to repeatedly explain to people that
the "requested port" is merely a hint, and the NAT gateway is free
to ignore it. Changed name to "suggested port" to better convey
this intention.
o Added NAT-PMP transition section
o Separated Internal Address, External Address, Remote Peer Address
definition
o Unified Mapping, Port Mapping, Port Forwarding definition
o adjusted so DHCP configuration is non-normative.
o mentioned PCP refreshes need to be sent over the same interface.
o renamed the REMOTE_PEER_FILTER option to FILTER.
o Clarified FILTER option to allow sending an ICMP error if policy
allows.
o for MAP, clarified that if the PCP client changed its IP address
and still wants to receive traffic, it needs to send a new MAP
request.
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o clarified that PEER requests have to be sent from same interface
as the connection itself.
o for MAP opcode, text now requires mapping be deleted when lifetime
expires (per consensus on 8-Mar interim meeting)
o PEER OpCode: better description of remote peer's IP address,
specifically that it does not control or establish any filtering,
and explaining why it is 'from the PCP client's perspective'.
o Removed latent text allowing DMZ for 'all protocols' (protocol=0).
Which wouldn't have been legal, anyway, as protocol 0 is assigned
by IANA to HOPOPT (thanks to James Yu for catching that one).
o clarified that PCP server only listens on its internal interface.
o abandoned 'target' term and reverted to simplier 'internal' term.
B.3. Changes from draft-ietf-pcp-base-05 to -06
o Dual-Stack Lite: consensus was encapsulation mode. Included a
suggestion that the B4 will need to proxy PCP-to-PCP and UPnP-to-
PCP.
o defined THIRD_PARTY option to work with the PEER OpCode, too.
This meant moving it to its own section, and having both MAP and
PEER OpCodes reference that common section.
o used "target" instead of "internal", in the hopes that clarifies
internal address used by PCP itself (for sending its packets)
versus the address for MAPpings.
o Options are now required to be ordered in requests, and ordering
has to be validated by the server. Intent is to ease server
processing of mandatory-to-implement options.
o Swapped Option values for the mandatory- and optional-to-process
Options, so we can have a simple lowest..highest ordering.
o added MISORDERED_OPTIONS error.
o re-ordered some error messages to cause MALFORMED_REQUEST (which
is PCP's most general error response) to be error 1, instead of
buried in the middle of the error numbers.
o clarified that, after successfully using a PCP server, that PCP
server is declared to be non-responsive after 5 failed
retransmissions.
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o tightened up text (which was inaccurate) about how long general
PCP processing is to delay when receiving an error and if it
should honor OpCode-specific error lifetime. Useful for MAP
errors which have an error lifetime. (This all feels awkward to
have only some errors with a lifetime.)
o Added better discussion of multiple interfaces, including
highlighting WiFi+Ethernet. Added discussion of using IPv6
Privacy Addresses and RFC1918 as source addresses for PCP
requests. This should finish the section on multi-interface
issues.
o added some text about why server might send SERVER_OVERLOADED, or
might simply discard packets.
o Dis-allow internal-port=0, which means we dis-allow using PCP as a
DMZ-like function. Instead, ports have to be mapped individually.
o Text describing server's processing of PEER is tightened up.
o Server's processing of PEER now says it is implementation-specific
if a PCP server continues to allow the mapping to exist after a
PEER message. Client's processing of PEER says that if client
wants mapping to continue to exist, client has to continue to send
recurring PEER messages.
B.4. Changes from draft-ietf-pcp-base-04 to -05
o tweaked PCP common header packet layout.
o Re-added port=0 (all ports).
o minimum size is 12 octets (missed that change in -04).
o removed Lifetime from PCP common header.
o for MAP error responses, the lifetime indicates how long the
server wants the client to avoid retrying the request.
o More clearly indicated which fields are filled by the server on
success responses and error responses.
o Removed UPnP interworking section from this document. It will
appear in [I-D.bpw-pcp-upnp-igd-interworking].
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B.5. Changes from draft-ietf-pcp-base-03 to -04
o "Pinhole" and "PIN" changed to "mapping" and "MAP".
o Reduced from four MAP OpCodes to two. This was done by implicitly
using the address family of the PCP message itself.
o New option THIRD_PARTY, to more carefully split out the case where
a mapping is created to a different host within the home.
o Integrated a lot of editorial changes from Stuart and Francis.
o Removed nested NAT text into another document, including the IANA-
registered IP addresses for the PCP server.
o Removed suggestion (MAY) that PCP server reserve UDP when it maps
TCP. Nobody seems to need that.
o Clearly added NAT and NAPT, such as in residential NATs, as within
scope for PCP.
o HONOR_EXTERNAL_PORT renamed to PREFER_FAILURE
o Added 'Lifetime' field to the common PCP header, which replaces
the functions of the 'temporary' and 'permanent' error types of
the previous version.
o Allow arbitrary Options to be included in PCP response, so that
PCP server can indicate un-supported PCP Options. Satisfies PCP
Issue #19
o Reduced scope to only deal with mapping protocols that have port
numbers.
o Reduced scope to not support DMZ-style forwarding.
o Clarified version negotiation.
B.6. Changes from draft-ietf-pcp-base-02 to -03
o Adjusted abstract and introduction to make it clear PCP is
intended to forward ports and intended to reduce application
keepalives.
o First bit in PCP common header is set. This allows DTLS and non-
DTLS to be multiplexed on same port, should a future update to
this specification add DTLS support.
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o Moved subscriber identity from common PCP section to MAP* section.
o made clearer that PCP client can reduce mapping lifetime if it
wishes.
o Added discussion of host running a server, client, or symmetric
client+server.
o Introduced PEER4 and PEER6 OpCodes.
o Removed REMOTE_PEER Option, as its function has been replaced by
the new PEER OpCodes.
o IANA assigned port 44323 to PCP.
o Removed AMBIGUOUS error code, which is no longer needed.
B.7. Changes from draft-ietf-pcp-base-01 to -02
o more error codes
o PCP client source port number should be random
o PCP message minimum 8 octets, maximum 1024 octets.
o tweaked a lot of text in section 7.4, "Opcode-Specific Server
Operation".
o opening a mapping also allows ICMP messages associated with that
mapping.
o PREFER_FAILURE value changed to the mandatory-to-process range.
o added text recommending applications that are crashing obtain
short lifetimes, to avoid consuming subscriber's port quota.
B.8. Changes from draft-ietf-pcp-base-00 to -01
o Significant document reorganization, primarily to split base PCP
operation from OpCode operation.
o packet format changed to move 'protocol' outside of PCP common
header and into the MAP* opcodes
o Renamed Informational Elements (IE) to Options.
o Added REMOTE_PEER (for disambiguation with dynamic ports),
REMOTE_PEER_FILTER (for simple packet filtering), and
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PREFER_FAILURE (to optimize UPnP IGD interworking) options.
o Is NAT or router behind B4 in scope?
o PCP option MAY be included in a request, in which case it MUST
appear in a response. It MUST NOT appear in a response if it was
not in the request.
o Result code most significant bit now indicates permanent/temporary
error
o PCP Options are split into mandatory-to-process ("P" bit), and
into Specification Required and Private Use.
o Epoch discussion simplified.
Authors' Addresses
Dan Wing (editor)
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
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
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Reinaldo Penno
Juniper Networks
1194 N Mathilda Avenue
Sunnyvale, California 94089
USA
Email: rpenno@juniper.net
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