PCP working group D. Wing, Ed.
Internet-Draft Cisco
Intended status: Standards Track February 7, 2011
Expires: August 11, 2011
Port Control Protocol (PCP)
draft-ietf-pcp-base-04
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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Copyright Notice
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Copyright (c) 2011 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 4
2.2. Supported Transport Protocols . . . . . . . . . . . . . . 5
2.3. Single-homed Customer Premises Network . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Relationship of PCP Server and its NAT . . . . . . . . . . . . 8
5. Common Request and Response Header Format . . . . . . . . . . 8
5.1. Request Header . . . . . . . . . . . . . . . . . . . . . . 9
5.2. Response Header . . . . . . . . . . . . . . . . . . . . . 10
5.3. Options . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 13
6. General PCP Operation . . . . . . . . . . . . . . . . . . . . 14
6.1. General PCP Client Operation . . . . . . . . . . . . . . . 14
6.1.1. Generating and Sending a Request . . . . . . . . . . . 14
6.1.2. Processing a Response . . . . . . . . . . . . . . . . 15
6.1.3. Multi-interface Issues . . . . . . . . . . . . . . . . 16
6.1.4. Epoch . . . . . . . . . . . . . . . . . . . . . . . . 17
6.2. General PCP Server Operation . . . . . . . . . . . . . . . 17
6.3. General PCP Options . . . . . . . . . . . . . . . . . . . 18
6.3.1. UNPROCESSED . . . . . . . . . . . . . . . . . . . . . 18
7. Introduction to MAP and PEER OpCodes . . . . . . . . . . . . . 19
7.1. For Operating a Server . . . . . . . . . . . . . . . . . . 20
7.2. For Reducing NAT Keepalive Messages . . . . . . . . . . . 20
7.3. For Operating a Symmetric Client/Server . . . . . . . . . 21
8. MAP OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 23
8.2. OpCode-Specific Result Codes . . . . . . . . . . . . . . . 25
8.3. OpCode-Specific Client Operation, Processing a Response . 26
8.4. OpCode-Specific Server Operation . . . . . . . . . . . . . 27
8.4.1. Maintaining Same External IP Address . . . . . . . . . 29
8.4.2. Mapping Lifetime . . . . . . . . . . . . . . . . . . . 29
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8.4.3. Mapping Deletion . . . . . . . . . . . . . . . . . . . 30
8.4.4. Subscriber Renumbering . . . . . . . . . . . . . . . . 30
8.5. PCP Options for MAP OpCodes . . . . . . . . . . . . . . . 31
8.5.1. THIRD_PARTY . . . . . . . . . . . . . . . . . . . . . 31
8.5.2. REMOTE_PEER_FILTER . . . . . . . . . . . . . . . . . . 32
8.5.3. PREFER_FAILURE . . . . . . . . . . . . . . . . . . . . 33
8.6. PCP Mapping State Maintenance . . . . . . . . . . . . . . 34
8.6.1. Recreating Mappings . . . . . . . . . . . . . . . . . 34
8.6.2. Maintaining Mappings . . . . . . . . . . . . . . . . . 35
9. PEER OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 36
9.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 36
9.2. OpCode-Specific Result Codes . . . . . . . . . . . . . . . 39
9.3. OpCode-Specific Client Operation, Processing a Response . 39
9.4. OpCode-Specific Server Operation . . . . . . . . . . . . . 40
10. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 40
10.1. Dual Stack-Lite . . . . . . . . . . . . . . . . . . . . . 40
10.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 41
10.1.2. Encapsulation Mode . . . . . . . . . . . . . . . . . . 41
10.1.3. Plain IPv6 Mode . . . . . . . . . . . . . . . . . . . 41
10.2. NAT64 . . . . . . . . . . . . . . . . . . . . . . . . . . 42
10.3. NAT44 and NAT444 . . . . . . . . . . . . . . . . . . . . . 42
10.4. IPv6 Firewall . . . . . . . . . . . . . . . . . . . . . . 42
10.5. Subscriber Identification . . . . . . . . . . . . . . . . 42
11. Interworking with UPnP IGD 1.0 and 2.0 . . . . . . . . . . . . 44
11.1. UPnP IGD 1.0 with AddPortMapping Action . . . . . . . . . 44
11.2. UPnP IGD 2.0 with AddAnyPortMapping Action . . . . . . . . 47
11.3. Lifetime Maintenance . . . . . . . . . . . . . . . . . . . 47
12. Security Considerations . . . . . . . . . . . . . . . . . . . 47
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48
13.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 48
13.2. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 48
13.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 48
13.4. Options . . . . . . . . . . . . . . . . . . . . . . . . . 48
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49
15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49
15.1. Normative References . . . . . . . . . . . . . . . . . . . 49
15.2. Informative References . . . . . . . . . . . . . . . . . . 50
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 51
A.1. Changes from draft-ietf-pcp-base-03 to -04 . . . . . . . . 51
A.2. Changes from draft-ietf-pcp-base-02 to -03 . . . . . . . . 52
A.3. Changes from draft-ietf-pcp-base-01 to -02 . . . . . . . . 53
A.4. Changes from draft-ietf-pcp-base-00 to -01 . . . . . . . . 53
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 54
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1. Introduction
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. If the PCP-
controlled device is a NAT, a mapping is created; if the PCP-
controlled device is a firewall, a mapping is created in the
firewall. These mappings are required for successful inbound
communications destined to machines located behind a NAT.
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.
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 (or control) the NAT mapping
lifetime. This helps reduce bandwidth on the subscriber's access
network, traffic to the server, and battery consumption on mobile
devices.
2. Scope
2.1. Deployment Scenarios
PCP can be used in various deployment scenarios, including:
o Dual Stack-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;
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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 IPv6 firewall control [RFC6092].
2.2. Supported Transport Protocols
The PCP OpCodes defined in this document are designed to support
transport protocols that use a 16-bit port number (e.g., TCP, UDP,
SCTP, DCCP). Transport 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 "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 the proper address rewriting takes place on that outbound response
packet. 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.
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 RFC 2119 [RFC2119].
Internal Host:
A host served by a NAT gateway, or protected by a firewall.
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Remote Host:
A host with which an Internal Host is communicating.
Internal Address, External Address, Remote Address:
An Internal Address is the address of an Internal Host served by a
NAT gateway (typically a private address [RFC1918]) or an Internal
Host protected by a firewall.
An External Address is 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.
A Remote Address is 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 my in fact be the Remote Host's
External Address, but since this is remote translation generally
invisible to software running on the Internal Host, the
distinction can safely be ignored for the purposes of this
document.
Target Address:
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 which does not implement PCP, the presence of a
Target Address option in the PCP request signifies that the
specified Target Address, not the source IP address of the PCP
request packet, should be used as the Internal Address for the
Mapping. When PCP requests are used with a Target Address option,
appropriate security measures MUST be used to ensure that the
device creating, renewing, or deleting mappings is authorized to
do so on behalf of the given Target Address.
Mapping:
A NAT mapping creates a relationship between an internal IP
transport address and an external IP transport address. More
specifically, it creates a translation rule where packets destined
to the external IP and port are translated to the internal IP and
port. 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 vice versa, and this "Mapping" indicates
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to the firewall that traffic to and from this internal port number
is permitted to pass. See also Port Forwarding.
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. Explicit dynamic mappings are created as a
result of PCP requests. 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 which they are automatically deleted unless
the lifetime is extended by action by the Internal Host. Static
mappings are created by manual configuration (e.g., command
language interface or web page) and differ from dynamic mappings
in that their lifetime is typically infinite (they exist until
manually removed) but otherwise they behave exactly the same as
dynamic mappings. E.g. a PCP request to create a mapping that
already exists as a static mapping will return a successful
result, confirming that the requested mapping exists.
Port Forwarding, Port Mapping:
Port forwarding (or port mapping) allows a host to receive traffic
sent to a specific IP address and port.
In the context of a NAT or NAPT, the Internal Address and External
Address are different. In the context of a pure firewall, the
Internal Address and External Address are the same. In both
contexts, if an internal host is listening to connections on a
specific port (that is, operating as a server), the external IP
address and port number need to be port forwarded to the internal
IP address and port number, which may be the same, in the case of
a pure firewall. In the context of a NAPT, it is possible that
both the IP address and port are modified. For example with a
NAPT, a webcam might be listening on port 80 on its internal
address 192.168.1.1, while its publicly-accessible external
address is 192.0.2.1 and port is 12345. The NAT does 'port
forwarding' of one to the other.
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 of a given subscriber. A PCP Client can issue PCP request
on behalf of a third party device of the same subscriber. An
interworking function, from UPnP IGD to PCP, or from NAT-PMP
[I-D.cheshire-nat-pmp] is another example of a PCP Client. A PCP
server in a NAT gateway that is itself a client of another NAT
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gateway (nested NAT) may itself act as a PCP client to the
upstream NAT.
PCP Server:
A network element which receives and processes PCP requests from a
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 or NAT-PMP [I-D.cheshire-nat-pmp] and PCP.
subscriber:
an entity provided access to the network. In the case of a
commercial ISP, this is typically a single home.
host:
a device which can have packets sent to it, as a result of PCP
operations. A host is not necessarily a PCP client.
5-tuple The 5 pieces of information that fully identify a flow:
source IP address, destination IP address, protocol, source port
number, destination port number.
4. Relationship of PCP Server and its NAT
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.
+-----------------+
+------------+ | 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
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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.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|Ver=0|R| OpCode | |
+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Reserved (84 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Common Request Packet Format
These fields are described below:
1 A single bit set to 1. This allows DTLS and non-DTLS to be
multiplexed on same port, should a future update to this
specification add DTLS support.
Ver: This document specifies protocol version 0. Should later
updates to this document specify different message formats with a
version number greater than zero, the first two bytes of those new
message formats will still 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.
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R: Indicates Request (0) or Response (1). All Requests MUST use 0.
OpCode: Opcodes are defined in Section 8 and Section 9. New OpCodes
can be defined per rules described in Section 13.
Reserved: 84 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|Ver=0|R| Opcode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) OpCode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Options :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Common Response Packet Format
These fields are described below:
Ver: This document specifies protocol version 0. Should later
updates to this document specify different message formats with a
version number greater than zero, the first four bytes of those
new message formats will still contain the version number, opcode,
and result code, as shown here, so that a PCP client 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
client is too old and needs to be updated to work with the PCP
server, or whether the PCP server is too old and needs to be
updated to work with this client.
R: Indicates Request (0) or Response (1). All Responses MUST use 1.
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OpCode: The OpCode value from the request.
Reserved: 12 reserved bits, MUST be sent as 0, MUST be ignored when
received.
Result Code: The result code for this response. See Section 5.4 for
values.
Lifetime: The value is in seconds. On a success response this
indicates the lifetime of the successful mapping. If a client
wishes to maintain its mapping beyond this lifetime it MUST renew
the mapping *before* it expires (typically halfway to expiry,
analogous to how clients renew a DHCP lease). 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. Clients SHOULD NOT repeat the same request to the
same PCP server within the lifetime given in the error response.
In the case of the SERVER_OVERLOADED error response, clients
SHOULD NOT send *any* further requests to the that PCP server
within the lifetime given in the SERVER_OVERLOADED error response.
Epoch: The server's Epoch value. See Section 6.1.4 for discussion.
5.3. Options
A PCP OpCode can be extended with an Option. Options can be used in
requests and responses. It is anticipated that Options will include
information which are associated with the normal function of an
OpCode. For example, an Option could indicate DSCP [RFC2474]
markings to apply to incoming or outgoing traffic associated with a
PCP mapping, or an Open could include descriptive text (e.g., "for my
webcam").
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:
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Option Code: Option code, 8 bits. The first bit of the option code
is the "P" bit, described below. Option codes MUST be registered
with IANA following the procedure described in Section 13.
Reserved: MUST be set to 0 on transmission and MUST be ignored on
reception.
Option-Length: Indicates in units of 4 octets the length of the
enclosed data. 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 or a response, as
permitted by that Option. If a given Option was included in a
request, and understood and processed by the PCP server, it MUST be
included in the response. 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 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 can be encoded in any order by the PCP
client. The response MUST encode them in the same order, but may
omit some PCP Options in the response (e.g., omitting them is
necessary to indicate the PCP server does not understand that Option,
or simply because that Option is not included in responses), and
additional options (if any) are included at the end. A single Option
MAY appear more than once, if permitted by the definition of the
Option itself. If the Option's definition allows the Option to
appear once but it appears more than once, the PCP server MUST
respond with the MALFORMED_OPTION response code.
If the "P" bit in the OpCode is set,
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 and include the UNPROCESSED option in the response
(Section 6.3.1).
If the "P" bit is clear, the PCP server MAY process or ignore this
Option.
To enhance interoperability, newly defined Options should avoid
interdependencies with each other.
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New Options MUST include the information below:
This Option:
name: <mnemonic>
number: <value>
purpose:
is valid for OpCodes: <list of OpCodes>
has length: <rules for length>
may appear in requests or responses: <requests/responses/both>
may appear more than once: <yes/no>
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.
0 SUCCESS, success
1 UNSUPP_VERSION, unsupported version
2 UNSUPP_OPCODE, unsupported OpCode
3 UNSUPP_OPTION, unsupported Option
4 MALFORMED_OPTION, malformed Option (e.g., exists too many times,
invalid length)
5 UNSPECIFIED_ERROR, server encountered unspecified error
6 MALFORMED_REQUEST
7 SERVER_OVERLOADED. Client should wait for at least the time
specified in the "Lifetime" field before sending any further PCP
requests to this server.
Additional result codes, specific to the OpCodes defined in this
document, are listed in Section 8.2 and Section 9.2 .
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6. General PCP Operation
PCP messages MUST be sent over UDP, and the PCP server MUST listen
for PCP requests on the PCP port number (Section 13.1). 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 Operation
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.
6.1.1. Generating and Sending a Request
Prior to sending its first PCP message, the PCP client determines
which servers to use. The PCP client tries the following to get a
list of PCP servers:
1. if a PCP server is configured (e.g., in a configuration file),
the address(es) of the PCP server(s) are used as the list of PCP
server(s), else;
2. if DHCP indicates the PCP server(s), the address(es) of the
indicated PCP server(s) are used as the list of PCP server(s),
else;
3. 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. It initializes a retransmission
timer to 4 seconds. (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 sends a PCP message to each server in
sequence, waiting for a response until its timer expires. Once a PCP
client has successfully communicated with a PCP server, it continues
communicating with that PCP server until that PCP server becomes non-
responsive, which causes the PCP client to attempt to re-iterate the
procedure starting with the first PCP server on its list. If a hard
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ICMP error is received the PCP client SHOULD immediately abort trying
to contact that PCP server (see Section 2 of [RFC5461] for discussion
of ICMP and ICMPv6 hard errors). If no response is received from any
of those servers, it doubles its retransmission timer and tries each
server again. This is repeated 4 times (for a total of 5
transmissions to each server). If, after these transmissions, the
PCP client has still not received a response, the PCP client SHOULD
abort the procedure.
[Ed. Note: We need to define precisely what we mean by "non-
responsive". No response after some number of retransmissions?
How many? No response within what time period? -- SC]
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.
6.1.2. 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 request.
It validates the version number and OpCode matches an outstanding
request. Responses shorter than 8 octets, longer than 1024 octets,
or not a multiple of 4 octets are invalid and ignored. The response
is further matched by comparing fields in the response OpCode-
specific data to fields in the request OpCode-specific data. After a
successful match with an outstanding request, the PCP client checks
the Epoch field to determine if it needs to restore its state to the
PCP server (see Section 6.1.4).
If the response code is 0, the PCP client knows the request was
successful.
If the response code is not 0, the request failed. The PCP client
MAY resend the same message but MUST first wait until the smaller of
30 minutes or the value of the Lifetime field. If the PCP client has
re-discovered a new PCP server (e.g., connected to a new network),
the PCP client is not restricted from communicating immediately with
its new PCP server.
A PCP client sends its requests using PCP version number 0. Should
later updates to this document specify different message formats with
a version number greater than zero it is expected that PCP servers
will still support version 0 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, and the client
SHOULD set a timer to retry its request in 30 minutes (in case this
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was a temorary condition and the server configuration is changed to
rectify the situation).
If future PCP versions greater than zero are specified, version
negotiation is expected to proceed as follows:
1. If a client or server supports more than one version it SHOULD
support a contiguous range of versions -- i.e. a lowest version
and a highest version and all versions in between.
2. 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
response 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 response 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 response 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.
6.1.3. 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 dual-
stack or be configured with several IP addresses. These IP addresses
may have distinct reachability scopes (e.g., if IPv6 they might have
global reachability scope as for GUA (Global Unicast Address) or
limited scope such as ULA (Unique Local Address, [RFC4193])).
IPv6 addresses with global reachability scope SHOULD be used as
internal IP address when requesting a PCP mapping in a PCP-controlled
device. IPv6 addresses with limited scope (e.g., ULA), SHOULD NOT be
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indicated as internal IP address in a PCP message.
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 GUA addresses belonging to
the same global IPv6 prefix.
[Ed. Note: text regarding multi-homing needs work.]
6.1.4. Epoch
Every PCP response sent by the PCP server includes an Epoch field.
This field increments by 1 every second, and indicates to the PCP
client if PCP state needs to be restored. If the PCP server resets
or loses the state of its Mappings, 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 8.6.1.
[Ed. Note: comment from Dave Thaler: "So spoofed packets with
Epoch=0 that look like they come from the server could result in a
big amplification attack on the PCP server. How is this
mitigated?". This is tracked as PCP Issue #21, [PCP-Issues].]
6.2. General PCP Server Operation
This section details operation specific to a PCP server.
Upon receiving a PCP request 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.
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If the received message is shorter than 4 octets, has the R bit set,
or the first bit is clear, the request is simply dropped. If the
version number is not supported, a response is generated containing
the UNSUPP_VERSION response code and the protocol version which the
server does understand (if the server understands a range of protocol
versions then it returns the supported version closest to the version
in the request).
If the OpCode is not supported, a response is generated with the
UNSUPP_OPCODE response code. 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 response code to MALFORMED_REQUEST, and
zero-padding the response to a multiple of 4 octets if necessary.
Error responses have the same packet layout as success responses,
with fields copied from the request copied into the response, and
other fields assigned by the PCP server set to 0.
[Ed. Note: Need more detail around how an error response is
formed, and what it contains.]
6.3. General PCP Options
The following options can appear in certain PCP responses.
6.3.1. UNPROCESSED
If the PCP server cannot process a mandatory-to-process option, for
whatever reason, it includes this Option in the response. This helps
with debugging interactions between the PCP client and PCP server.
For simplicity, no more than 4 options can be encoded. This option
MUST NOT appear more than once in a PCP response, no matter how many
PCP options appeared in the request and were unprocessed by the PCP
server. If only one Option code was unprocessed, that option code it
is placed in option-code-1 (and the other three fields are set to
zero), if two Option codes were unprocessed, their option codes are
placed in option-code-1 and option-code-2, and so on. If a certain
Option appeared more than once in the PCP request, that Option value
only appears once in the option-code fields. The order of the
Options in the PCP request has no relationship with the order of the
Option values in this UNPROCESSED Option. This Option MUST NOT
appear in a response unless the associated request contained at least
one mandatory-to-process Option.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code-1 | option-code-2 | option-code-3 | option-code-4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: UNPROCESSED option
This Option:
name: UNPROCESSED
number: TBD
purpose: indicates which PCP options in the request are not
supported by the PCP server
is valid for OpCodes: all
has length: 1
may appear in requests or responses: responses, and only if the
response code is non-zero.
may appear more than once: no
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
Internet. The PCP client also knows if it has an IPv4 interface 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.
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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 application 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
application follows the procedures described in this section.
As normal, the application needs to begin listening to a port, and to
ensure that it can get exclusive use of that port it needs to choose
a port that is not in the operating system's ephemeral port range.
Then, the application constructs a PCP message with the appropriate
MAP OpCode depending on if it is listening on an IPv4 or IPv6
interface and if it wants a public IPv4 or IPv6 address.
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(...);
internal_sockaddr = ...;
bind(s, &internal_sockaddr, ...);
listen(s, ...);
requested_external_sockaddr = 0;
pcp_send_map_request(internal_sockaddr,
requested_external_sockaddr, &assigned_external_sockaddr,
requested_lifetime, &assigned_lifetime);
update_rendezvous_server("Client 12345", assigned_external_sockaddr);
while (1) {
int c = accept(s, ...);
/* ... */
}
Figure 6: Pseudo-code for using PCP to operate a server
7.2. For Reducing NAT Keepalive Messages
[Ed. Note: This section creates a difference between an
implicitly-created mapping, which PCP then tries to query/control
using the PEER OpCode, and a explicitly-created mapping which was
created with a MAP OpCode. This section attempts to address PCP
Issue #9 and PCP Issue #35.]
A host operating a client (e.g., XMPP client, SIP client) sends from
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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
applications uses the procedure described in this section.
Middleboxes such as NATs or firewalls need to see occasional traffic
or will terminate their session state, causing application failures.
To avoid this, many applications routinely generate keepalive traffic
for the primary (or sole) purpose of maintaining state with such
middleboxes. Applications can reduce such application keepalive
traffic by using PCP.
Note: For reasons beyond NAT, an application may find it useful to
perform application-level keepalives, such as to detect a broken
path between the client and server, 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 applications 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.
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_address, ...);
external_address = NUL;
pcp_send_peer_request(internal_address,
requested_external_address, &assigned_external_address,
remote_peer, requested_lifetime, &assigned_lifetime);
Figure 7: Pseudo-code using PCP with a dynamic socket
7.3. For Operating a Symmetric Client/Server
[Ed. Note: The PEER4 and PEER6 OpCodes, discussed here, are
intended to resolve PCP Issue #35.]
A host operating a client and server on the same port (e.g.,
Symmetric RTP [RFC4961] or SIP Symmetric Response Routing (rport)
[RFC3581]) first establishes a local listener, (usually) sends the
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local and public IP addresses and ports to a rendezvous service
(which is out of scope of this document), and (usually) initiates
outbound connections from that same source address. To accomplish
this, the application uses the procedure described in this section.
An application that is using the same port for outgoing connections
as well as incoming connections MUST first signal its operation of a
server using the PCP MAP OpCode, as described in Section 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 mappings
(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 applications to first signal its operation of
a server using hte PCP MAP OpCode.
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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(...);
internal_sockaddr = ...;
bind(s, &internal_sockaddr, ...);
listen(s, ...);
requested_external_sockaddr = 0;
pcp_send_map_request(internal_sockaddr,
requested_external_sockaddr, &assigned_external_sockaddr,
requested_lifetime, &assigned_lifetime);
update_rendezvous_server("Client 12345", assigned_external_sockaddr);
send_packet(s, "Hello World");
while (1) {
int c = accept(s, ...);
/* ... */
}
Figure 8: Pseudo-code for using PCP to operate a symmetric client/
server
8. MAP OpCodes
This section defines four OpCodes which control forwarding from a NAT
(or firewall) to an internal host. They are:
MAP4=0: create a mapping between an internal address and public
IPv4 address (e.g., NAT44, NAT64, or firewall)
MAP6=1: create a mapping between an internal address and public
IPv6 address (e.g., NAT46, NAT66, or firewall)
The operation of these 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 both match the
request's source IP address and MAP OpCode's internal IP address, the
functionality is purely a firewall; otherwise it pertains to a
network address translator which might also perform firewall
functions.
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The following diagram shows the request packet format for MAP4 and
MAP6. 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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Requested external IP address (32 or 128, depending on OpCode):
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | requested external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: MAP OpCode Request Packet Format
These fields are described below:
Requested External IP Address: Requested 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 doesn't know the external address,
or doesn't have a preference, it MUST use 0.
Requested lifetime: Requested lifetime of this mapping, in seconds.
Internal port: Internal port for the mapping.
Requested external port: requested 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 doesn't know the external port, or
doesn't have a preference, it uses 0.
The following diagram shows the response packet format for MAP4 and
MAP6 OpCodes:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Assigned external IP address (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | assigned external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: MAP OpCode Response Packet Format
These fields are described below:
Assigned external IP address: Assigned external IPv4 or IPv6 address
for the mapping. IPv4 or IPv6 address is indicated by the OpCode
Internal port: Internal port for the mapping, copied from request.
Assigned external port: assigned external port for the mapping.
IPv4 or IPv6 address is indicated by the OpCode. If the NAT
gateway can allocate the requested external port it SHOULD do so.
This is beneficial for re-establishing state lost when a NAT
gateway fails or loses its state due to reboot. If the NAT
gateway cannot allocate the requested external port but can
allocate some other port, it MUST do so and return the allocated
port in the response. Cases where a NAT gateway cannot allocate
the requested external port include when the requested external
port is prohibited by policy, already used by the NAT gateway for
one of its own services (e.g. port 80 for the NAT gateway's own
configuration pages) already allocated to another explicit mapping
(by static manual allocation or by a prior PCP request by a
different Internal Host) or the rare case where the requested
external port was already allocated to an implicit mapping which
cannot be 'promoted' to an explicit mapping for this Internal Host
(a different Internal Host already made a prior outbound
connection for which the NAT gateway happened to assign the
external port requested in this explicit PCP request).
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.
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20 NETWORK_FAILURE, e.g., NAT device has not obtained a DHCP lease
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.
22 UNSUPP_PROTOCOL, unsupported Protocol
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.
24 USER_EX_QUOTA, mapping would exceed user's port quota
25 CANNOT_PROVIDE_EXTERNAL_PORT, indicates the port is already in use
or otherwise unavailable (e.g., special port that cannot be
allocated by the server's policy). This error is only returned if
the request included the Option PREFER_FAILURE.
26 UNABLE_TO_DELETE_ALL, indicates the PCP server was not able to
delete all mappings.
Other result codes are defined following the procedure in
Section 13.3.
8.3. OpCode-Specific Client Operation, Processing a Response
This section describes the operation of a PCP client when sending
requests with OpCodes MAP4 and MAP6, and processing those responses.
A PCP client can delete a mapping immediately by sending the
appropriate MAP OpCode with a lifetime of 0. The PCP server responds
by returning a MAP Response with a lifetime of 0.
To delete all mappings for all hosts associated with this subscriber,
an all-zero internal IP address is used.
The request MAY contain values in the requested-external-ip-address
and requested-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.
An existing mapping can have its lifetime extended by the PCP client.
To do this, the PCP client sends a new PCP map request to the server
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indicating the internal IP address and port(s).
The PCP client SHOULD renew the mapping before its expiry time,
otherwise it will be removed by the PCP server (see Section 8.4.3).
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 positive 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 send an infinite number of ever-
closer-together requests in the last few seconds before a mapping
expires).
To delete a mapping, lifetime=0 is used. To delete a mapping for a
specific protocol and port, the MAP request contains that specific
internal port and protocol. To delete all mappings for a particular
protocol, port 0 is used to indicate a wildcard.
A response is matched with a request by comparing the protocol,
internal IP address, internal port, external address and external
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 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.
When a PCP client first acquires a new IP address, it may want to
remove mappings that may have been instantiated for a previous host.
To do this, the PCP client sends a MAP request with external port,
internal port, and lifetime set to 0.
8.4. OpCode-Specific Server Operation
This section describes the operation of a PCP server when processing
the OpCodes MAP4 or MAP6.
If the requested lifetime is 0, it indicates a request to delete the
mapping immediately. The contents of the protocol field or the
internal-port field can be zero, indicating a wildcard. If the
protocol field is 0, it indicates all protocols (and the internal-
port field is ignored). If the internal-port is 0, it means all
ports for the particular protocol. On a deletion request, the
requested external port field is ignored by the server. If the
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deletion request was properly formatted, and the associated mapping
(if present) is deleted, a positive response is generated with
lifetime of 0 and copies the protocol and internal port number from
the request into the response. If the PCP client attempts to delete
a port mapping which was manually assigned by some kind of
configuration tool, the PCP server MUST respond with
UNABLE_TO_DELETE_ALL result code, but the other fields are encoded as
described above. If the PCP client attempts to delete a mapping that
does not exist, the success response code is returned. If the PCP
client is not authorized to delete this mapping, NOT_AUTHORIZED is
returned.
If the requested lifetime is not zero, it indicates a request to
create a mapping or extend the lifetime of an existing mapping.
If the PCP request contains protocol=255, it indicates the PCP client
wants to map all protocols. If this cannot be fulfilled by the PCP-
controlled device, UNSUPP_PROTOCOL is returned.
See Section 8.4.2 for processing the lifetime.
If the requested external port is 0, and the PCP-controlled device
does not change port numbers (that is, it does not do port
translation) the PCP server MUST return a response indicating that
the assigned external port is the same as the internal port.
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 greater than 128 exists but that option does
not make sense (e.g., the PREFER_FAILURE option is included in a
request with lifetime=0 (indicating a delete request)), the request
is invalid and generates a MALFORMED_OPTION error.
By default, a PCP-controlled device MUST NOT create mappings for a
protocol not indicated in the request. For example, if the request
was for a TCP mapping, a UDP mapping MUST NOT be created.
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 then validates that
internal IP address indicated in that option belongs to the same
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subscriber. This validation depends on the PCP deployment scenario;
see Section 10.5 for the validation procedure. If the internal IP
address in the PCP request does not belong to the subscriber, an
error response MUST be generated with result code NOT_AUTHORIZED.
If all of the proceeding operations were successful (did not generate
an error response), then the requested mappings are created as
described in the request and a positive response is built. This
positive result contains the same OpCode as the request, but with the
"R" bit set.
As a side-effect of creating a mapping, ICMP messages associated with
the mapping are also translated (if appropriate) and forwarded 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.
8.4.1. Maintaining Same External IP Address
If there are active mappings associated with a given subscriber (see
Section 10.5) -- created via dynamic assignment, by PCP or any other
means -- subsequent PCP mapping requests belonging to the same
subscriber MUST use the same external IP address. This follows the
intent of REQ-1 of [I-D.ietf-behave-lsn-requirements].
Once an internal host has no active mapping in the PCP-controlled
device, a subsequent PCP request for that host MAY be assigned to a
different external IP address.
8.4.2. Mapping Lifetime
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 NOT RECOMMENDED that the
server allow lifetimes exceeding 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).
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An application that forgets its PCP-assigned mappings (e.g., the
application or OS crashes) will request new PCP mappings (consuming
the user's port quota (if there is a quota) and the resource limit
for number of mappings), and the application will also probably
initiate dynamic connections to servers without using PCP (also
consuming the user's port quota). PCP provides no explicit
protection against such port consumption. In such environments, it
is RECOMMENDED that applications use shorter PCP lifetimes to reduce
the impact of consuming the user's port quota.
An OS that issues a "delete all" request on reboot protects itself
against this resource exhaustion by voluntarily relinquishing all of
its old mappings before beginning to request new ones. The PCP
server MAY chose to allocate the same (recently relinquished)
mappings when mappings are re-requested by the booting OS.
Some port mapping APIs (such as the "DNSServiceNATPortMappingCreate"
API provided by Apple's Bonjour on Mac OS X, iOS, Windows, Linux,
etc.) automatically monitor for process exit (including application
crashes) and automatically send port mapping deletion requests if the
process that requested them goes away without explicitly
relinquishing them.
8.4.3. Mapping Deletion
A mapping MUST be deleted by the PCP server upon the expiry of its
lifetime, or upon request from the PCP client.
In order to prevent another subscriber from receiving unwanted
traffic, the PCP server SHOULD NOT assign that same external port to
another host for 120 seconds (MSL, [RFC0793]). The PCP server MUST
allow the same host to re-acquire the same port during that same
interval.
8.4.4. Subscriber Renumbering
The customer premises router might obtain a new IPv4 address or new
IPv6 prefix. 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 --
both traffic mapped with MAP requests and dynamic traffic. This same
problem can occur if an IP address is re-assigned today, without PCP
and without an ISP-operated CGN. The solution is the same as today:
the problems associated with subscriber renumbering are eliminated if
the ISP avoids re-assigning IP addresses to different subscribers.
When a new Internal Address is assigned to a host embedding a PCP
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client, the NAT (or firewall) controlled by the PCP server will
continue to send traffic to the old IP address. Assuming the PCP
client wants to continue receiving traffic, it needs to install new
mappings for its new IP address. The requested 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.
8.5. PCP Options for MAP OpCodes
8.5.1. THIRD_PARTY
This Option is used when a PCP client wants to control a mapping to
another host. A PCP server will only support this option if sent by
an authorized PCP client, which depends on the deployment scenario.
For Dual-Stack Lite deployments, the PCP server only supports this
option if the source IPv4 address is the B4's source IP address. For
other scenarios, the subscriber has only one IPv4 address and this
Option serves no purpose. If a subscriber has more than one IPv4
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.
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) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Mapping Internal IP address (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Protocol: indicates 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.
Reserved: 24 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
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Mapping Internal IP Address: Internal IP address of the mapping.
This can be IPv4 or IPv6, depending on the OpCode.
8.5.2. REMOTE_PEER_FILTER
This Option indicates packet filtering 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. That is,
packets with a source IP address, transport, or port that do not
match those fields of the PCP request are dropped by the PCP server-
controlled NAT/firewall device.
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, :
: 1 28 bits if MAP6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This Option:
name: REMOTE_PEER_FILTER
number: 128
is valid for OpCodes: MAP44, MAP64, MAP46, or MAP66
is included in responses: MUST
has length: 2 or 5
may appear in requests or responses: requests
may appear more than once: no
Because of interactions with dynamic ports this Option MUST only be
used by a client that is operating a server, as this ensures that no
other application will be assigned the same ephemeral port for its
outgoing connection. Other use by a PCP client is NOT RECOMMENDED
and 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
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indicates the entire IPv4 address is used; a prefix-length of 0
indicates none of the IPv4 address is used (which is effectively the
same as not adding a filter at all). For MAP4 the minimum value is 0
and the maximum value is 32; for MAP6 the minimum value is 0 and the
maximum value is 128. Values outside that range cause an
MALFORMED_OPTION response code.
[Ed. Note: How do we want to remove a filter? Do we want to allow
removing a filter at all -- is there a use-case for that or can
the application just create a new mapping? If we have a use-case,
perhaps use 0.0.0.0 as the remote IP address to remove all
filters? This is tracked as PCP Issue #10 [PCP-Issues].]
8.5.3. PREFER_FAILURE
This option indicates that if the PCP server is unable to allocate
the requested 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_HONOR_EXTERNAL_PORT.
This option is intended solely for use by UPnP IGD interworking
(Section 11), where the semantics of IGD version 1 do not provide any
way to indicate to an IGD client that any port is available other
than the one it requested. A PCP server MAY support this option, if
its designers wish to support downstream devices that perform 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 IGD
interworking are not required to support this option. PCP clients
other than IGD interworking clients MUST NOT use this option because
it results in inefficient operation, and they cannot safely assume
that all PCP servers will implement it. The option is provided only
because the semantics of IGD version 1 offer no viable alternative
way to implement an IGD interworking function. It is anticipated
that this option will be deprecated in the future as more clients
adopt PCP natively and the need for IGD interworking declines.
This Option:
name: PREFER_FAILURE
number: 130
is valid for OpCodes: MAP4, MAP6
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is included in responses: MUST
has length: 0
may appear in requests or responses: requests
may appear more than once: no
8.6. PCP Mapping State Maintenance
If an event occurs that causes the PCP server to lose 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 such loss of
state is more common in a residential NAT device which does not write
information to its non-volatile memory.
The Epoch allows a client to deduce when a PCP server may have lost
its state. If this occurs, the PCP client can attempt to recreate
the mappings following the procedures described in this section.
8.6.1. 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).
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 6.1.4).
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.
This self-healing property of the protocol is very important.
When a client renews its port mappings as the result of receiving a
packet where the Epoch field indicates that a reboot or similar loss
of state has occurred, the client MUST first delay by a random amount
of time selected with uniform random distribution in the range 0 to 5
seconds, and then send its first PCP request. After that request is
acknowledged by the PCP server, the client may then send its second
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request, and so on, as rapidly as the gateway allows. The requests
SHOULD be issued serially, one at a time; the client SHOULD NOT issue
multiple requests simultaneously in parallel.
[Ed. Note: the paragraph above is copied from NAT-PMP, and seems
to be advice specific to receiving asynchronous notification that
the Epoch was reset. Asynchronous notification needs the delay
described (in fact, it probably needs greater delay than 0-5
seconds if on a larger network) and also needs to discourage
sending multiple PCP requests serially. However, PCP does not
have asynchronous notification (yet), and has different needs/
requirements for pacing. In short: the above paragraph needs some
discussion. This is tracked as PCP Issue #11 [PCP-Issues].]
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 port mapping 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.
8.6.2. 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
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
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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).
9. PEER OpCodes
This section defines two OpCodes for controlling dynamic connections.
They are:
PEER4=2: Set or query lifetime for flow from IPv4 address to a
remote peer's IPv4 address.
PEER6=3: Set or query lifetime for flow from IPv6 address to a
remote peer's IPv6 address.
The operation of these OpCodes is described in this section.
9.1. OpCode Packet Formats
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. For both of the PEER OpCodes, if the internal IP
address and internal port fields of the request both match the
external IP address and external port fields of the response, the IP
addresses and ports are not changed and thus the functionality is
purely a firewall; otherwise it pertains to a network address
translator which might also perform firewall functions.
The following diagram shows the request packet format for PEER4 and
PEER6. This packet format is aligned with the response packet
format:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol | Reserved (24 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Internal IP address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Reserved (128 bits) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| remote peer port | reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: PEER OpCode Request Packet Format
These fields are described below:
Protocol: indicates 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.
Mapping Internal IP Address: Internal IP address of the 5-tuple.
This can be 32 bits long (if OpCode is PEER4) or 128 bits long (if
OpCode is PEER6).
Remote Peer IP Address: Remote peer's IP address, from the
perspective of the PCP client.
Reserved: 128 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
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Requested lifetime: Requested lifetime of this mapping, in seconds.
Unlike the MAP OpCode, there is no special meaning of 0.
internal port: Internal port for the of the 5-tuple.
Reserved: 16 reserved bits, MUST be 0 on transmission and MUST be
ignored on reception.
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.
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 IP address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if PEER4, 128 bits if PEER6) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: External IP address (always 128 bits) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| remote peer port | reserved (16 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: PEER OpCode Request Packet Format
Protocol: Copied from the request.
External_AF The address family of the external IP address associated
with this peer connection. Values are from IANA's address family
numbers (IPv4 is 1, IPv6 is 2).
Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be
ignored on reception.
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Internal IP address Copied from the request.
remote Peer IP address Copied from the request.
External IP Address External IP address, assigned by the NAT (or
firewall) to this mapping. If firewall, this will match the
internal IP address. This field is always 128 bits long. If
External_AF indicates IPv4, the IPv4 address is encoded in the
first 32 bits of the External IP Address field and the remaining
96 bits are zero.
internal port: copied from request.
external port: External port number, assigned by the NAT (or
firewall) to this mapping. If firewall or 1:1 NAT, this will
match the internal port.
remote peer port: Copied from request.
Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be
ignored on reception.
9.2. 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
two PEER OpCodes received by the PCP server.
50 NONEXIST_PEER, the connection to that peer does not exist in the
mapping table
Other result codes are defined following the procedure in
Section 13.3.
9.3. OpCode-Specific Client Operation, Processing a Response
This section describes the operation of a client when sending the
OpCodes PEER4 or PEER6.
After connecting to a server using UDP or TCP, the PCP client sends a
PCP request containing the PEER4 or PEER6 OpCode.
A response is 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.
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If a successful response, the PCP client uses the assigned lifetime
value to reduce its frequency of application keepalives for the NAT.
Of course, there may be other reasons, specific to the application,
to use more frequent application keepalives. For example, the PCP
assigned-lifetime could be one hour but the application may want to
ensure the server is still accessible (e.g., has not crashed) more
frequently than once an hour.
9.4. OpCode-Specific Server Operation
This section describes the operation of a client when processing the
OpCodes PEER4 or PEER6.
The PEER OpCodes provide a single function: the ability for the PCP
client to query and (possibly) extend the lifetime of an existing
mapping.
On receiving the PEER4 or PEER6 OpCode, the PCP server examines the
mapping table. If a mapping does not exist, the NONEXIST_PEER error
is returned. Otherwise, the PCP server chooses the smaller of the
requested lifetime and its configured maximum lifetime value, and
sets the lifetime of the existing mapping. This means that a PEER4
or PEER6 request does not reduce the lifetime of an existing mapping,
nor can the PEER OpCodes delete a mapping. If the mapping is
terminated by the TCP client or server (e.g., TCP FIN or TCP RST),
the mapping will eventually be destroyed normally; the earlier use of
PEER does not extend the lifetime in that case.
If all of the proceeding operations were successful (did not generate
an error response), then a SUCCESS response is generated, with the
assigned-lifetime containing the lifetime of the mapping.
10. Deployment Scenarios
10.1. Dual Stack-Lite
The interesting components in a Dual-Stack Lite deployment are the B4
element (which is the customer premises router) and the AFTR element
(which is the device that both terminates the IPv6-over-IPv4 tunnel
and also implements the Carrier-Grade NAT44 function). The B4
element does not need to perform a NAT function (and usually does not
perform a NAT function), but it does operate its own DHCP server and
is the local network's default router.
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10.1.1. Overview
Various PCP deployment scenarios can be considered to control the PCP
server embedded in the AFTR element:
1. UPnP IGD and NAT-PMP [I-D.cheshire-nat-pmp] are used in the LAN:
an interworking function is required to be embedded in the B4
element to ensure interworking between the protocol used in the
LAN and PCP. UPnP IGD-PCP Interworking Function is described in
Section 11.
2. Hosts behind the B4 element will either include a PCP client or
UPnP IGD client, or both.
A. if a UPnP IGD client, the B4 element will need to include an
interworking function from UPnP IGD to PCP.
B. if a PCP client, the PCP client will communicate directly
with the PCP server.
3. The B4 element includes a PCP client which is invoked by an HTTP-
based configuration (as is common today). The internal IP
address field in the PCP payload would be the internal host used
in the port forwarding configuration.
Two modes are identified to forward PCP packets to a PCP server
controlling the provisioned AFTR as described in the following sub-
sections.
[Ed. Note: We need to decide on Encapsulation Mode or Plain IPv6
Mode. This is tracked as PCP Issue #13 [PCP-Issues].]
10.1.2. Encapsulation Mode
In this mode, B4 element does no processing at all of the PCP
messages, and forwards them as any other UDP traffic. With DS-Lite,
this means that IPv4 PCP messages issued by internal PCP clients are
encapsulated into the IPv6 tunnel sent to the AFTR as for any other
IPv4 packets. The IPv6 address used as source address MUST be the
same as the one used by the B4 element. The AFTR decapsulates the
IPv4 packets and processes the PCP requests (because the destination
IPv4 address points to the PCP server embedded in the AFTR).
10.1.3. Plain IPv6 Mode
Another alternative for deployment of PCP in a DS-Lite context is to
rely on a PCP Proxy in the B4 element. Protocol exchanges between
the PCP Proxy and the PCP server are conveyed using plain IPv6 (no
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tunnelling is used). Nevertheless, the IPv6 address used as source
address by the PCP Proxy MUST be the same as the one used by the B4
element.
10.2. NAT64
Hosts behind a NAT64 device can make use of PCP in order to perform
port reservation (to get a publicly routable IPv4 port).
If the IANA-assigned IP address is used for the discovery of the PCP
server, that IPv4 address can be placed into the IPv6 destination
address following that particular network's well-known prefix or
network-specific prefix, per [RFC6052].
10.3. NAT44 and NAT444
Residential subscribers in NAT44 (and NAT444) deployments are usually
given one IPv4 address, but may also be given several IPv4 addresses.
These addresses are not routable on the IPv4 Internet, but are
routable between the subscriber's home and the ISP's CGN. To
accommodate multiple hosts within a home, especially when provided
insufficient IPv4 addresses for the number of devices in the home,
subscribers operate a NAPT device. When this occurs in conjunction
with an upstream NAT44, this is nicknamed "NAT444".
[Ed. Note: Does PCP need a mechanism to detect a non-PCP-
supporting NAT between a PCP client and a PCP server? Or can that
problem be detected by relying on the failure of PCP server
Discovery? This is tracked as PCP Issue #25 [PCP-Issues].]
10.4. IPv6 Firewall
See Section 8.5.2.
[Ed. Note: this IPv6 firewall section needs more text. This is
tracked as PCP Issue #10 [PCP-Issues].]
10.5. Subscriber Identification
The MAP OpCodes require subscriber identification because they
allocate resources or adjust resources allocated to a subscriber.
For the MAP OpCode, it is permitted for a PCP client to create a
mapping on behalf of a third party device (e.g., a computer can
create PCP mappings on behalf of a webcam). However, a PCP client
cannot open mappings for a different subscriber. The mechanism to
identify "same subscriber" depends on the sort of NAT on this
network:
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o If the PCP-controlled device is a NAT64: the internal IP address
indicated in the PCP message and the source IPv6 address of
received PCP request MUST belong to the same IPv6 prefix. The
length of the IPv6 prefix is the same as the length assigned to
each subscriber on that particular network (e.g., /64), and that
length must be configurable by the network operator.
o If the PCP-controlled device is a DS-Lite AFTR: DS-Lite (Section
11 of [I-D.ietf-softwire-dual-stack-lite]) already requires the
tunnel transport source address be validated, and that same
address is used by PCP to assign the tunnel-ID to the requested
mapping (see Section 10.1.2 and Section 10.1.3). Thus, PCP
acquires the same security properties as DS-Lite. If address
validation is implemented correctly, the PCP client can not
instruct mappings on behalf of devices of another subscriber.
o If LSN with a routed network (NAT44), each subscriber has a known
set of IPv4 address (usually one IPv4 address) and all PCP
requests MUST be sent from only one of the subscriber's IP
addresses and MUST only open mappings towards the subscriber's own
IP address.
o If IPv6 firewall: the internal IP address indicated in the PCP
message and the source IPv6 address of received PCP request MUST
belong to the same IPv6 prefix. The length of the IPv6 prefix is
the same as the length assigned to each subscriber on that
particular network (e.g., /64), and that length must be
configurable by the network operator.
PCP-controlled devices can be a DS-Lite AFTR or an IPv4-IPv6
interconnection node such as NAT46 or NAT64. These nodes are
deployed by Service Providers to deliver global connectivity service
to their customers. Appropriate functions to restrict the use of
these resources (e.g., LSN facility) to only subscribed users should
be supported by these devices. Access control can be implicit or
explicit:
o It is said to be explicit if an authorisation procedure is
required for a user to be granted access to such resources. For
such variant of PCP-controlled device, a subscriber can be
identified by an IPv6 address, an IPv4 address, a MAC address, or
any other information.
o For other scenarios, such as plain IPv4-in-IPv6 encapsulation for
a DS-Lite architecture, the access to the service is based on the
source IPv6 prefix. No per-user polices is pre-configured in the
PCP-controlled device.
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11. Interworking with UPnP IGD 1.0 and 2.0
[Ed. Note: This UPnP IGD Interworking section will likely be moved
to a separate document which will fully describe how a proxy needs
to translate UPnP IGD messages into PCP messages. This is tracked
as PCP Issue #28 [PCP-Issues].]
The following diagram shows how UPnP IGD can be interworked with PCP,
using an interworking function (IWF).
+-------------+
| IGD Control |
| Point |-----+
+-------------+ | +---------+ +--------+
+---| IGD-PCP | | PCP |
| IWF +-------+ Server |--<Internet>
+---| | | |
+-------------+ | +---------+ +--------+
| Local Host |-----+
+-------------+ | |
| |
LAN Side | WAN side |
<======UPnP IGD=============>|<========PCP=====>|
Figure 13: Network Diagram, Interworking UPnP IGD and PCP
11.1. UPnP IGD 1.0 with AddPortMapping Action
In UPnP IGD 1.0 [IGD] it is only possible to request a specific port
using the AddPortMapping action. Requiring a specific port is
incompatible with both (1) a Carrier-Grade NAT and with (2) widely-
deployed applications. Regarding (1), another subscriber is likely
to already be using the same port, so it will be unavailable to this
application to operate a server. Regarding (2), if the same popular
application exists on two devices behind the same NAPT, they cannot
both get the same port. PCP cannot correct this behavior of UPnP
IGD:1, but PCP does work with this behavior.
Due to this incompatibility with address sharing and popular
applications, future hosts and applications will either support UPnP
IGD 2.0's AddAnyPortMapping method (see Section 11.2) or, more
likely, will support PCP natively.
When a requested port assignment fails, most UPnP IGD control points
will retry the port assignment requesting the next higher port or
requesting a random port. These UPnP IGD requests are translated to
PCP requests and sent to the PCP server. The requests include the
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PREFER_FAILURE option, which causes the PCP server to return an error
if it cannot allocate the requested port. The interworking function
translates the PCP error response to a UPnP IGD error response. This
repeats until the UPnP IGD client gives up or until the PCP server is
able to return the requested port.
Message flow would be similar to this:
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UPnP Control Point in-home CPE PCP server
| | |
|-UPnP:AddPortMapping(80)--->| |
| |-PCP:request port 80------>|
| | PREFER_FAILURE |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
|-UPnP:AddPortMapping(81)--->| |
| |-PCP:request port 81------>|
| | PREFER_FAILURE |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
| | |
|-UPnP:AddPortMapping(82)--->| |
| |-PCP:request port 82------>|
| | PREFER_FAILURE |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
| | |
|-UPnP:AddPortMapping(83)--->| |
| |-PCP:request port 83------>|
| | PREFER_FAILURE |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
... ... ... 84 ...
... ... ... 85 ...
... ... ... .. ...
... ... ... 96 ...
... ... ... 97 ...
| | |
|-UPnP:AddPortMapping(98)--->| |
| |-PCP:request port 98------>|
| | PREFER_FAILURE |
| | |
| |<-PCP:ok, port 98----------|
|<-UPnP: ok, port 98---------| |
| | |
Figure 14: Message Flow: Interworking from UPnP IGD 1.0
AddPortMapping action to PCP
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11.2. UPnP IGD 2.0 with AddAnyPortMapping Action
If the UPnP IGD control point and the UPnP IGD interworking function
both implement UPnP IGD 2.0 [IGD-2] and the UPnP IGD control point
uses IGD 2's new AddAnyPortMapping action, only one round-trip is
necessary. This is because AddAnyPortMapping has semantics similar
to PCP's semantics, allowing the PCP server to assign any port.
Message flow would be similar to this:
UPnP Control Point in-home CPE PCP server
| | |
|-UPnP:AddAnyPortMapping()->| |
| |-PCP:external port 0----->|
| |<-PCP:external port=12345-|
|<-UPnP:port=12345----------| |
| | |
Figure 15: Message Flow: Interworking from UPnP IGD 2.0
AddAnyPortMapping action to PCP
11.3. Lifetime Maintenance
UPnP IGD 1.0 and 2.0 provide a lifetime (PortMappingLeaseDuration),
but it is seldom used by UPnP IGD control points, and does not allow
the UPnP IGD to override the requested duration. Thus, the UPnP IGD/
PCP interworking function is responsible for extending the lifetime
of mappings that are still interesting to the UPnP IGD control point.
Note: It can be an implementation advantage, where possible, for
the UPnP IGD/PCP interworking function to request a port mapping
lifetime only while that client is active and connected. For
example, creating a PCP mapping that is equal to the client's
remaining DHCP lifetime is one useful approach.
12. Security Considerations
The PCP client's source port SHOULD be randomly generated as per
[I-D.ietf-tsvwg-port-randomization].
On today's Internet, ISPs do not typically filter incoming traffic
for their subscribers. However, when ISP introduce stateful address
sharing with NAPT devices, such filtering will occur as a side
effect. PCP allows controlling that filtering, and PCP allows
indicating the 'inside' IP address that should have the filtering
removed. It is important that PCP allows removing the filtering for
hosts belonging to one subscriber, but not hosts belonging to another
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subscriber. This is done in different ways depending on the
architecture of the address sharing device and how subscribers are
identified behind that device, and described in detail in
Section 10.5.
Because of the state created in a NAPT or firewall, it is anticipated
that port forwarding (MAP OpCodes) will have a quota applied to each
subscriber. If the quota is small and the maximum lifetime is large,
a faulty or disconnected PCP client could cause a denial of service
for other PCP clients belonging to that same subscriber. To prevent
this problem, if a PCP server is configured for a small per-
subscriber quota (e.g., less than 15 ports) then it is RECOMMENDED it
also be be configured for a short maximum lifetime (e.g., 5 minutes).
13. IANA Considerations
IANA is requested to perform the following actions:
13.1. Port Number
IANA has assigned UDP port 44323 for PCP.
13.2. OpCodes
IANA shall create a new protocol registry for PCP OpCodes, initially
populated with the values in Section 8 and Section 9.
New OpCodes in the range 1-95 can be created via Standards Action
[RFC5226], and the range 96-128 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, and Section 9.2.
New Result Codes can be created 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 optional-to-process,
and 128-255 are mandatory-to-process. The initial registry contains
the options described in Section 8.5, and the option values 0 and 255
are reserved.
New PCP option codes in the range 0-63 and 128-192 can be created via
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Standards Action [RFC5226], and the range 64-127 and 192-255 is for
Private Use [RFC5226].
14. Acknowledgments
Thanks to Alain Durand, Christian Jacquenet, and Simon Perreault 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.
[I-D.ietf-behave-v6v4-xlate-stateful]
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-06 (work
in progress), August 2010.
[I-D.ietf-tsvwg-port-randomization]
Larsen, M. and F. Gont, "Transport Protocol Port
Randomization Recommendations",
draft-ietf-tsvwg-port-randomization-09 (work in progress),
August 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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,
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May 2008.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052,
October 2010.
[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.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]
Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida,
"Common requirements for IP address sharing schemes",
draft-ietf-behave-lsn-requirements-00 (work in progress),
October 2010.
[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>.
[IGD-2] UPnP Gateway Committee, "Internet Gateway Device (IGD) V
2.0", September 2010, <http://upnp.org/specs/gw/
UPnP-gw-WANIPConnection-v2-Service.pdf>.
[PCP-Issues]
PCP Working Group, "PCP Active Tickets", January 2011,
<http://trac.tools.ietf.org/wg/pcp/trac/report/1>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
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[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[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.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, July 2007.
[RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
February 2009.
[RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment (CPE) for Providing
Residential IPv6 Internet Service", RFC 6092,
January 2011.
Appendix A. Changes
A.1. 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.
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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.
A.2. Changes from draft-ietf-pcp-base-02 to -03
o Adjusted abstract and introduction to make it clear PCP is
intended to forward ports and intended to reduce application
keepalives.
o First bit in PCP common header is set. This allows DTLS and non-
DTLS to be multiplexed on same port, should a future update to
this specification add DTLS support.
o Moved subscriber identity from common PCP section to MAP* section.
o made clearer that PCP client can reduce mapping lifetime if it
wishes.
o Added discussion of host running a server, client, or symmetric
client+server.
o Introduced PEER4 and PEER6 OpCodes.
o Removed REMOTE_PEER Option, as its function has been replaced by
the new PEER OpCodes.
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o IANA assigned port 44323 to PCP.
o Removed AMBIGUOUS error code, which is no longer needed.
A.3. 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.
A.4. Changes from draft-ietf-pcp-base-00 to -01
o Significant document reorganization, primarily to split base PCP
operation from OpCode operation.
o packet format changed to move 'protocol' outside of PCP common
header and into the MAP* opcodes
o Renamed Informational Elements (IE) to Options.
o Added REMOTE_PEER (for disambiguation with dynamic ports),
REMOTE_PEER_FILTER (for simple packet filtering), and
PREFER_FAILURE (to optimize UPnP IGD interworking) options.
o Is NAT or router behind B4 in scope?
o PCP option MAY be included in a request, in which case it MUST
appear in a response. It MUST NOT appear in a response if it
wasn't in the request.
o Response 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.
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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
Reinaldo Penno
Juniper Networks
1194 N Mathilda Avenue
Sunnyvale, California 94089
USA
Email: rpenno@juniper.net
Francis Dupont
Internet Systems Consortium
Email: fdupont@isc.org
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