PCP working group D. Wing
Internet-Draft Cisco
Intended status: Standards Track December 15, 2010
Expires: June 18, 2011
Port Control Protocol (PCP)
draft-ietf-pcp-base-01
Abstract
Port Control Protocol is an address-family independent mechanism to
control how incoming packets are forwarded by upstream devices such
as network address translators (NATs) and simple IPv6 firewalls.
Status of this Memo
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Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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to this document. Code Components extracted from this document must
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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 Premise Network . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Relationship of PCP Server and its NAT . . . . . . . . . . . . 6
5. Common Request and Response Header Format . . . . . . . . . . 7
5.1. Request Header . . . . . . . . . . . . . . . . . . . . . . 8
5.2. Response Header . . . . . . . . . . . . . . . . . . . . . 9
5.3. Options . . . . . . . . . . . . . . . . . . . . . . . . . 9
5.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 11
6. General Operation . . . . . . . . . . . . . . . . . . . . . . 12
6.1. General PCP Client Operation . . . . . . . . . . . . . . . 12
6.1.1. Generating and Sending a Request . . . . . . . . . . . 12
6.1.2. Processing a Response . . . . . . . . . . . . . . . . 13
6.1.3. Multi-interface Issues . . . . . . . . . . . . . . . . 13
6.2. General PCP Server Operation . . . . . . . . . . . . . . . 14
7. Pinhole Operation . . . . . . . . . . . . . . . . . . . . . . 14
7.1. OpCode Packet Format . . . . . . . . . . . . . . . . . . . 14
7.2. OpCode-Specific Error Codes . . . . . . . . . . . . . . . 17
7.3. OpCode-Specific Client Operation, Processing a Response . 17
7.4. OpCode-Specific Server Operation . . . . . . . . . . . . . 18
7.4.1. Maintaining Same External IP Address . . . . . . . . . 19
7.4.2. Pinhole Lifetime . . . . . . . . . . . . . . . . . . . 19
7.4.3. Pinhole deletion . . . . . . . . . . . . . . . . . . . 20
7.4.4. Subscriber Identification . . . . . . . . . . . . . . 20
7.4.5. Renumbering . . . . . . . . . . . . . . . . . . . . . 21
7.4.6. Disambiguating Remote Peer . . . . . . . . . . . . . . 21
7.5. PCP Options for PIN OpCodes . . . . . . . . . . . . . . . 22
7.5.1. REMOTE_PEER_FILTER . . . . . . . . . . . . . . . . . . 22
7.5.2. REMOTE_PEER . . . . . . . . . . . . . . . . . . . . . 23
7.5.3. HONOR_EXTERNAL_PORT . . . . . . . . . . . . . . . . . 24
7.6. Interaction with Dynamic Connections . . . . . . . . . . . 24
7.7. Using PCP to Eliminate Application Keepalive Messages . . 25
7.8. PCP Mapping State Maintenance . . . . . . . . . . . . . . 25
7.8.1. Epoch . . . . . . . . . . . . . . . . . . . . . . . . 26
7.8.2. Recreating Pinholes . . . . . . . . . . . . . . . . . 26
7.8.3. Maintaining Pinholes . . . . . . . . . . . . . . . . . 27
7.9. Nested NATs . . . . . . . . . . . . . . . . . . . . . . . 28
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7.10. Pinhole Deletion . . . . . . . . . . . . . . . . . . . . . 28
7.11. Pinhole Lifetime Extension . . . . . . . . . . . . . . . . 28
8. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 29
8.1. Dual Stack-Lite . . . . . . . . . . . . . . . . . . . . . 29
8.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 29
8.1.2. Encapsulation Mode . . . . . . . . . . . . . . . . . . 29
8.1.3. Plain IPv6 Mode . . . . . . . . . . . . . . . . . . . 30
8.2. NAT64 . . . . . . . . . . . . . . . . . . . . . . . . . . 30
8.3. NAT44 and NAT444 . . . . . . . . . . . . . . . . . . . . . 30
8.4. IPv6 Firewall . . . . . . . . . . . . . . . . . . . . . . 30
9. Interworking with UPnP IGD 1.0 and 2.0 . . . . . . . . . . . . 31
9.1. UPnP IGD 1.0 with AddPortMapping Action . . . . . . . . . 31
9.2. UPnP IGD 2.0 with AddAnyPortMapping Action . . . . . . . . 32
9.3. Lifetime Maintenance . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . 33
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
11.1. PCP Server IP address . . . . . . . . . . . . . . . . . . 33
11.2. Port Number . . . . . . . . . . . . . . . . . . . . . . . 34
11.3. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 34
11.5. Options . . . . . . . . . . . . . . . . . . . . . . . . . 34
12. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 35
14.1. Normative References . . . . . . . . . . . . . . . . . . . 35
14.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 37
A.1. Changes from -00 to -01 . . . . . . . . . . . . . . . . . 37
Appendix B. Analysis of Techniques to Discover PCP Server . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
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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 NATs and
firewalls. PCP is primarily designed to be implemented in the
context of both large scale NAT and small-scale NAT (e.g.,
residential NATs). PCP allows hosts to operate servers permanently
(e.g., a webcam) or temporarily (e.g., while playing a game) when
behind one or more NAT devices, including when behind a large-scale
NAT operated by their Internet service provider.
PCP allows applications to create pinholes from an external IP
address 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 pinhole is created in the firewall. These pinholes
are required for successful inbound communications destined to
machines located behind a NAT.
After creating a pinhole 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.
2. Scope
2.1. Deployment Scenarios
PCP can be used in various deployment scenarios, including:
o DS-Lite [I-D.ietf-softwire-dual-stack-lite], including a NAT and
including a router behind the B4 element, and;
[Ed. Note: Do we want 'NAT behind the B4 element' in our
scope? This affects PCP server discovery, among other things.
Howeve, users deploy these NATs. Do we want PCP to work with
this, detect this, or quietly fail??]
o NAT64, both Stateful [I-D.ietf-behave-v6v4-xlate-stateful] and
Stateless [I-D.ietf-behave-v6v4-xlate], and;
o Large-Scale NAT44 [I-D.ietf-behave-lsn-requirements], including
nested NATs ("NAT444"), and;
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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 [I-D.ietf-v6ops-cpe-simple-security].
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) can be wildcarded (allowing any traffic with that
protocol to pass), provided of course the upstream device being
controlled by PCP supports that functionality, or new PCP OpCodes can
be defined to support those protocols.
2.3. Single-homed Customer Premise Network
The PCP machinery assumes a single-homed subscriber model. That is,
for a given IP version, only one default route exists to reach the
Internet. This is important because after a PCP pinhole 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.
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].
Port Forwarding:
Port forwarding 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 firewall,
the internal address and external address are different. 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. In the context of
a NAPT, it is possible that both the IP address and port are
modfiied. For example with a NAPT, a webcam might be listening
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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.
In the context of a firewall, the internal address, external IP
address, and ports are not changed.
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 PCP to PCP (for nested NATs) is another example of a PCP
client.
PCP Server:
A network element which receives and processes PCP requests
from a PCP Client. See also Section 4.
Mapping:
See also Port Forwarding. A mapping exists only in the context
of a Network Address Translator. 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.
Mapping Types:
There are three different ways to create mappings: dynamic
(e.g., outgoing TCP SYN), PCP, and static configured (e.g., CLI
or web page). These mappings are one and the same but their
attributes such as lifetime or filtering might be different.
Interworking Function: a functional element responsible for
interworking another protocol with PCP. For example interworking
between UPnP IGD [IGD] with PCP.
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)
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which is expected to be a common deployment.
+-----------------+
+------------+ | NAT or firewall |
| PCP Client |-<network>-+ +---<Internet>
+------------+ | with embedded |
| PCP server |
+-----------------+
Figure 1: device with Embedded PCP Server
However, it is useful to also model a separation of the PCP server
from the NAT, as shown below (Figure 2). The PCP server would
communicate with the NAT using a protocol beyond the scope of this
document, such as a proprietary protocol or any suitable standard
protocol for NAT control.
+-----------------+
+--+ NAT or firewall +---<Internet>
/ +-----------------+
+------------+ / ^
| PCP Client +-<network> | any suitable control protocol
+------------+ \ v
\ +------------+
+--+ PCP Server |
+------------+
Figure 2: NAT with Separate PCP Server
5. Common Request and Response Header Format
All PCP messages contain a request (or response) header, opcode-
specific information, and (optional) informational elements. 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
specific OpCodes defined in this document is described in Section 7,
and behavior of new OpCodes are described in their own documents
following a similar format as in Section 7.
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5.1. Request Header
All requests have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver=0 |reserve|R| OpCode | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) PCP Options :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Common Request Packet Format
These fields are described below:
Ver: Version is 0
reserve: 4 reserved bits, MUST be sent as 0 and MUST be ignored when
received.
R: Indicates Request (0) or Response (1). All Requests MUST use 0.
OpCode: Opcodes are defined in Figure 7. More can be defined per
rules described in Section 11.
Reserved: 16 reserved bits, MUST be sent as 0 and MUST be ignored
when received.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver=1 |reserve|R| Opcode | Reserved | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) OpCode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Informational Elements :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: Common Response Packet Format
These fields are described below:
Ver: Version is 1
reserve: 4 reserved bits, MUST be sent as 0, MUST be ignored when
received.
R: Indicates Request (0) or Response (1). All Responses MUST use 1.
OpCode+128: The OpCode value from the request.
Protocol: Protocol field, echoed exactly from the request
Result Code: The result code for this response. See Section 5.4 for
values.
Epoch: The server's Epoch value. The same value is used for all PCP
clients. See Section 7.8.1 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 markings to apply
to incoming or outgoing traffic associated with a PCP pinhole, or an
Open could include descriptive text (e.g., "for my webcam").
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Options use the following Type-Length-Value format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Code | Reserved | Option-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Options Header
The description of the fields is as follows:
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 11.
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. An Option MUST NOT
appear in a PCP response unless solicited in the associated request.
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 Server MUST be
specified in a 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 response.
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, it MUST generate
a failure response without including this Option in the response.
o If the PCP server does implement this Option, but cannot perform
the function indicated by this Option, it MUST generate a failure
response and include this Option (and its arguments) in the
response.
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If the "P" bit is clear, the PCP server MAY process or ignore this
Option.
If several Options are to be included in a PCP request or response,
the Options MAY 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., to indicate the PCP server does not
understand that Option, or that Option is not included in responses).
A single Option MAY appear more than once, if permitted by the
defintion of the Option itself. If the Option's definition allows
the Option to appear only once, the PCP server MUST respond to such a
PCP request with an error message [[which error?]]
To enhance interoperability, newly defined Options should avoid
interdependencies with each other.
New Options MUST include the information below:
This Option:
number:
is valid for OpCodes: <list of OpCodes>
is included in responses: [MUST | MUST NOT]
has length: <rules for length>
appear more than once: ["yes" | "no"]
5.4. Result Codes
The following response codes may be returned as a result of any
OpCode received by the PCP server. Result codes are 8 bits long and
the most significant bit indicates if the error is transient or
permanent. A transient error is one that might resolve itself, such
that an identical PCP request submitted later would be successful
(e.g., a resource is consumed and might become available later). A
permanent error is one that cannot reasonably be successful if an
identical PCP request is submitted (e.g., unsupported opcode).
Future result codes should follow this same format.
0 - Success
128 - Unsupported version
129 - Unsupported OpCode
Figure 6: PCP Result Codes
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Other result codes, specific to the OpCodes defined in this document,
are listed in Section 7.2.
6. General Operation
PCP messages MUST be sent over UDP, and the PCP Server MUST listen
for PCP requests on the PCP port number (Section 11.2). Every PCP
request generates a response, so PCP does not need to run over a
reliable transport protocol.
6.1. General PCP Client Operation
This section details operation specific to a PCP client, for any
OpCode. Procedures specific to the PIN OpCodes are described in
Section 7.
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 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. if the PCP working group decides an IANA-allocated address for
the PCP server is suitable, it is added to the list of PCP
servers, else;
4. the address of the default router, it is added to the list of PCP
servers.
[Ed. Note: more discussion around these methods is necessary to
reach consensus on the best approach(es) for PCP. Also see
Appendix B for further analysis.]
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 optional Information Elements. It initializes a
retransmission timer to 4 seconds. 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
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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 ICMP error is received (e.g., port
unreachable, network unreachable) the PCP client SHOULD immediately
abort trying to contact that PCP server. 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. If, after
repeating the procedure 4 times, no PCP server has responded the PCP
client SHOULD abort the procedure.
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. The response is further matched by comparing fields in the
response OpCode-specific data to fields in the request OpCode-
specific data. This matching is described by the OpCode itself, see
Section 7.3. 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 7.8.1).
If it is an error response (>0), the request failed. If the error
response is less than 128, retrying the same request sometime in the
future might be successful. If the error response is greater or
equal to 128, retrying the same request is unlikely to be successful.
If it is the success response (0), the PCP client knows the request
was successful.
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 instructing a PCP mapping in a PCP-
controlled device. IPv6 addresses with limited scope (e.g., ULA),
SHOULD NOT be indicated as internal IP address in a PCP message.
As mentioned in Section 2.3, only mono-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
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global IPv6 prefix.
[Ed. Note: text regarding multi-homing needs work.]
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 optional 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.
After successful parsing of the message, the PCP server validates
that the internal IP address in the PCP request belongs to that
subscriber. This validation depends on the PCP deployment scenario;
see Section 7.4.4. 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.
7. Pinhole Operation
This document defines four OpCodes for forwarding ports and
controlling dynamic connections. These four OpCodes share a similar
packet layout for requests and responses. They are:
PIN44 = 0 = IPv4 address to IPv4 address (e.g., NAT44 or firewall)
PIN46 = 1 = IPv4 address to IPv6 address (e.g., NAT46 or firewall)
PIN64 = 2 = IPv6 address to IPv4 address (e.g., NAT64 or firewall)
PIN66 = 3 = IPv6 address to IPv6 address (e.g., NAT66 or firewall)
Figure 7: OpCodes
These operation of these OpCodes is described in this section.
7.1. OpCode Packet Format
The four PIN OpCodes (PIN44, PIN46, PIN64, PIN66) share a similar
packet layout for both requests and responses. Because of this
similarity, they are shown together. For all of the PIN OpCodes, if
the request's internal-ip-address and internal-port matches the
response's external-ip-address and external-port, 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
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might also perform firewall functions.
The following diagram shows the request packet format for PIN44,
PIN46, PIN64, and PIN66. 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Pinhole Internal IP address (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Requested external IP address (32 or 128, depending on OpCode):
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Requested lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | requested external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: PIN 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 intended to
create a TCP mapping. If a particular OpCode does not need the
field, it MUST sent as 0 and MUST be ignored when received.
Reserved: MUST be 0 on transmission and MUST be ignored on
reception.
Pinhole Internal IP Address: Internal IP address of the pinhole.
This can be IPv4 or IPv6, depending on the OpCode.
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.
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Requested lifetime: Requested lifetime of this pinhole, in seconds.
internal port: Internal port for the pinhole.
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 MUST use 0.
The following diagram shows the response packet format for PIN44,
PIN46, PIN64, and PIN66:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Pinhole Internal IP address (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Assigned external IP address (32 or 128, depending on OpCode) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Assigned lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| internal port | assigned external port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: PIN OpCode Response Packet Format
These fields are described below:
Protocol: Copied from the request.
Reserved: MUST be 0 on transmission, MUST be ignored on reception.
Pinhole Internal IP address: Copied from the request. IPv4 or IPv6
address is indicated by the OpCode.
Assigned external IP address: Assigned external IPv4 or IPv6 address
for the pinhole. IPv4 or IPv6 address is indicated by the OpCode
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Assigned Lifetime: Lifetime for this mapping, in seconds
Internal port: Internal port for the pinhole, copied from request.
Assigned external port: requested external port for the mapping.
IPv4 or IPv6 address is indicated by the OpCode
7.2. OpCode-Specific Error Codes
In addition to the response codes described in Figure 6, the
following additional response codes may be returned as a result of
the four PIN OpCodes received by the PCP server.
130 - NOT_AUTHORIZED
(e.g., PCP server supports mapping, but feature is disabled)
3 - Network Failure
(e.g., NAT device has not obtained a DHCP lease)
4 - Out of resources
(e.g., NAT device cannot create more mappings at this time)
[Ed. Note: probably need an OpCode indicating that the
REMOTE_PEER option is necessary to disambiguate a
connection. Do we require the PCP server to figure
out there is an ambiguity (multiple connections using
the same port)? Probably.]
Figure 10: Result Codes specific to PIN OpCodes
Other OpCodes are in result codes are defined following the procedure
in Section 11.4.
7.3. OpCode-Specific Client Operation, Processing a Response
[Ed. Note: describe how PCP client sends a PIN* request, and might
get an Invalid Opcode response, and what it does.]
This section describes the operation of a client when sending the
OpCodes PIN44, PIN64, PIN46, or PIN66.
A response is matched with a request by comparing the protocol,
internal IP address, internal port, remote peer address and remote
peer port. Other fields are not compared, because the PCP server
changes those fields to provide information about the pinhole 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
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the Internet using an application-specific rendezvous mechanism.
If an unsucessful response code greater than or equal to 128, the PCP
client SHOULD NOT re-issue the same request. If a response code less
than or equal to 127, the PCP client MAY, if it wishes, resend the
same message after a delay of at least 5 seconds.
7.4. OpCode-Specific Server Operation
This section describes the operation of a client when processing the
OpCodes PIN44, PIN64, PIN46, or PIN66.
If the requested lifetime is 0, it indicates a Delete request. This
means the pinhole described by the internal IP address should be
deleted, and requested external port field is ignored by the server
(that is, it isn't validated). If the deletion request was
successful, a positive response generated containing external port of
0 and lifetime of 0. If the deletion request was unsusccessful a
non-zero result code is returned and the lifetime is undefined. If
the client attempts to delete a port mapping which was manually
assigned by some kind of configuration tool, the PCP server MUST
respond with a 'Not Authorized' error (result code 2).
[Ed. Note: Should we similarly return an error if attempting to
delete mappings that were created dynamically (e.g., TCP SYN)?]
[Ed. Note: Issue #15: Should ICMP messages, associated with this
pinhole, be allowed by default. Consensus appears to be 'yes',
because that is what dynamic mappings on NATs do. And if we don't
do this, we will have PCP clients forget (or avoid) requesting an
ICMP pinhole, further degrading ICMP operation on the Internet.]
[Ed. Note: The following text needs discussion. Keep it or
discard it?] When a mapping is destroyed as a result of its
lifetime expiring or for any other reason, if the NAT gateway's
internal state indicates that there are still active TCP
connections traversing that now- defunct mapping, then the NAT
gateway SHOULD send appropriately- constructed TCP RST (reset)
packets both to the local client and to the remote peer on the
Internet to terminate that TCP connection.
A client can request the explicit deletion of all its UDP or TCP
mappings by sending the same deletion request to the NAT gateway with
external port, internal port, and lifetime set to 0. A client MAY
choose to do this when it first acquires a new IP address in order to
protect itself from port mappings that were performed by a previous
owner of the IP address. After receiving such a deletion request,
the PCP server and NAT MUST delete all the port mappings. The PCP
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server responds to such a deletion request with a response as
described above, with the internal port set to zero. If the PCP
server is unable to delete a port mapping, for example, because the
mapping was manually configured by a configuration tooll, the gateway
MUST still delete as many port mappings as possible, but respond with
a non-zero result code. The exact result code to return depends on
the cause of the failure. If the gateway is able to successfully
delete all port mappings as requested, it MUST respond with a result
code of 0.
The PCP-controlled device MAY reduce the lifetime that was requested
by the PCP Client. The PCP-controlled device SHOULD NOT offer a
lease lifetime greater than that requested by the PCP Client. The
RECOMMENDED lifetime assigned by the server is 7200 seconds (i.e.,
two hours).
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.
Nevertheless, a configurable feature MAY be supported by the PCP-
controlled device, which MAY reserve (but not forward) the companion
port so the same PCP Client can request it in the future.
If all of the proceeding operations were successful (did not generate
an error response), then the requested pinholes are created as
described in the request and a positive response is built. This
positive response contains the same OpCode as the request plus 128.
7.4.1. Maintaining Same External IP Address
If there are active mappings for a particular PCP Client -- created
via dynamic assignment or created by PCP -- subsequent mapping
requests from that same PCP Client MUST use the same external IP
address. This is necessary because some protocols require using the
same IP address for several ports, and follows REQ-1 of
[I-D.ietf-behave-lsn-requirements]. Once a client has no active
dynamic mappings and no PCP pinholes, a subsequent dynamic mapping or
PCP request MAY be assigned to a different external IP address.
7.4.2. Pinhole Lifetime
Once a PCP server has responded positively to a pinhole request for a
certain lifetime, the port forwarding is active for the duration of
the lifetime unless deleted by the PCP client (or until the PCP
server crashes). This has interactions with dynamically-created
mappings, see Sections Section 7.6.
It is NOT RECOMMENDED that the server allow long lifetimes (exceeding
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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.
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.
7.4.3. Pinhole deletion
A pinhole 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]).
[Ed. Note: it should (MUST?) allow the same host to re-acquire
the same port, though.]
7.4.4. Subscriber Identification
The PIN OpCodes require subscriber identification because they
allocate resources to a subscriber. A PCP Client can create mappings
in a PCP-controlled device 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 pinholes for a different subscriber. The
mechanism to identify "same subscriber" depends on the sort of NAT on
this network:
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 8.1.2 and Section 8.1.3). 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
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requests MUST be sent from only one of the subscriber's IP
addresses and MUST only open pinholes towards the subscriber's own
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.
7.4.5. Renumbering
The customer premise router might obtain a new IPv6 prefix, either
due to a reboot, power outage, DHCPv6 lease expiry, or other action.
If this occurs, the ports reserved using PCP might be delivered to
another customer who now has that (old) address. This same problem
can occur if an IP address is re-assigned today, without PCP. The
solution is the same as today: ISPs should not re-assign IP
addresses.
7.4.6. Disambiguating Remote Peer
When operating a UDP or TCP server, one application is listening on a
port and no other application will be assigned that port by the
operating system. However, ephemeral connections (that is, outgoing
connections) to different servers can share the same ephemeral port,
even between applications. If a PCP request with a PIN OpCode is
sent after such a dynamic connection is established (potentially by
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another application), the PCP server cannot reliable distinguish if
the PIN request is referring to the current connection or to a new
connection. To eliminate this ambiguity, the remote peer's IP
address and port can be specified in the REMOTE_PEER Option.
This Option does not create or modify filtering on the NAT (for that,
use REMOTE_PEER_FILTER) and have no effect on the NAT or firewall's
normal filtering which will be any combination of Endpoint-
Independent Filtering, Address-Dependent Filtering, or Address and
Port-Dependent Filtering (these terms are originally defined and
explained in [RFC4787]).
7.5. PCP Options for PIN OpCodes
7.5.1. 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. As a consequence of dropping the
packet, an ICMP port unreachable error SHOULD be generated, as long
as it does not violate the security policy of the NAT/firewall.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Remote Peer Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if PIN44 or PIN64, :
: 1 28 bits if PIN46 or PIN66) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This Option:
value: 128
is valid for OpCodes: PIN44, PIN64, PIN46, or PIN66
is included in responses: MUST
has length: 64 or 160
may appear more than once: no
Because of interactions with dynamic ports (Section 7.6), this Option
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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.
[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?
7.5.2. REMOTE_PEER
Due to a pre-existing dynamic mapping, a PCP server may not be able
to instantiate a filter on a specific port. It is thus recommended
that applications bind to the source port (to prevent other
applications from using that same source port) prior to using this
PCP Option.
In those situations, the Option REMOTE_PEER is necessary to
disambiguate the mappings. This option does not change filtering
behavior of the NAT or firewall.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Remote Peer Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: Remote Peer IP address (32 bits if PIN44 or PIN64, :
: 1 28 bits if PIN46 or PIN66) :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This Option:
value: 129
is valid for OpCodes: PIN44, PIN64, PIN46, or PIN66
is included in responses: MUST
has length: 64 or 160
may appear more than once: no
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7.5.3. HONOR_EXTERNAL_PORT
This option indicates the PCP client needs to have the indicated
requested-eternal-port allocated. If the PCP server cannot provide
that port for the exclusive use of the PCP client, the PCP server
MUST return an error [which code should we use?].
This option is intended for use by UPnP IGD interworking (Section 9).
This Option:
value: 6
is valid for OpCodes: PIN44, PIN64, PIN46, or PIN66
is included in responses: MUST
has length: 0
may appear more than once: no
7.6. Interaction with Dynamic Connections
Dynamic connections are created by hosts sending a TCP or UDP packet
across their NAT. PCP can be used with such dynamic connections.
There are two interesting cases: PCP before a dynamic connection, and
PCP after a dynamic connection.
If two applications on the same host communicate with different
destinations, it is possible for them to be allocated the same
ephemeral source port -- without either of the applications using the
REUSEADDR socket option. Thus, one application's use of PCP (to
adjust the NAT or firewalls behavior) can affect an unrelated
application on the same host. This is why the four PIN OpCodes
include the remote peer fields. The remote peer fields allow the PCP
server to to disambiguate which mapping applies to the PIN OpCode
message.
Because PCP is not intended to adjust NAT behavior for dynamically-
created connections, it is possible (and reasonable) for that second
dynamic connection to be mapped to a different public port. Even if
that occurs, the other application can issue its own PCP request (to
determine the mapping lifetime) following the same procedure
described in this section.
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7.7. Using PCP to Eliminate Application Keepalive Messages
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 avoid such application keepalive
traffic by using PCP.
To use PCP for this function, the applications first connects to its
server, as normal. Afterwards, it issues a PCP PIN request (PIN44,
PIN64, PIN46, or PIN66, as appropriate) specifying the same 5-tuple
as that connection in the PCP PIN request (internal IP address and
port, transport protocol, and remote peer IP address and port), with
requested-external-address and requested-external-port both set to 0,
and includes the REMOTE_PEER Option indicating the remote peer's IP
address is from the perspective of the PCP client. The PCP response
will indicate the lifetime for that session.
On many common platforms (i.e. those making use of the BSD sockets
API), using PCP for purposes of application keepalives by issuing a
PCP request followed by a dynamic connection to the server is
difficult to implement properly. This is therefore NOT RECOMMENDED.
Instead, client applications SHOULD first establish a dynamic
connection to a server, and then issue a PCP request related to that
connection, including the REMOTE_PEER option.
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, ...);
pcp_send_request(internal_address, 0, remote_peer, lifetime);
/* external address is set to zero */
Figure 11: Pseudo-code for using PCP to with a dynamic socket
7.8. PCP Mapping State Maintenance
If an event occurs that causes the PCP server to lose state (such as
a crash or power outage), the pinholes 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.
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The Epoch indicates if the PCP server has lost its state. If this
occurs, the PCP client can attempt to recreate the pinholes following
the procedures described in this section.
7.8.1. 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 port mapping table, due to reboot, power
failure, or any other reason, it MUST reset its Epoch time to 0.
Similarly, if the public IP address of the NAT (controlled by the PCP
server) changes, the Epoch MUST be reset to 0.
Whenever a client receives a PCP response, the client computes its
own conservative estimate of the expected Epoch value by taking the
Epoc 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 7.8.2.
[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?"]
7.8.2. Recreating Pinholes
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 7.8.1).
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.
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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
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.]
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.
7.8.3. Maintaining Pinholes
A PCP client can refresh a pinhole 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
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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 pinholes, 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
request for any one of the client's pinholes. 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).
7.9. Nested NATs
[Ed. Note: Nested NATs will be discussed in a later version of
this document or, more likely, in a separate document. The
mechanism to support nested NATs is to have each NAT implement a
PCP server (to service devices and NATs behind it) and a PCP
client (to communicate with NATs above it). This allows a PCP
client to be unaware of the PCP communications going on between
upstream NATs.]
7.10. Pinhole Deletion
A PCP Client can delete a pinhole immediately by sending the
appropriate PIN OpCode with a lifetime of 0. The PCP server responds
by returning a PIN Response with a lifetime of 0.
To delete all pinholes for all hosts associated with this subscriber,
an all-zero internal IP address is used.
7.11. Pinhole Lifetime Extension
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
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 7.4.3).
In order to prevent excessive PCP chatter, it is RECOMMENDED to renew
only 60 seconds before expiration time (to account for
retransmissions that might be necessary due to packet loss, clock
synchronization between PCP client and PCP server, and so on).
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8. Deployment Scenarios
8.1. Dual Stack-Lite
The interesting components in a Dual-Stack Lite deployment are the B4
element (which is the customer premise router) and the AFTR element
(which is the device that both terminates the IPv6-over-IPv4 tunnel
and also implements the large-scale 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.
8.1.1. Overview
Various PCP deployment scenarios can be considered to control the PCP
server embedded in the AFTR element:
1. UPnP IGD and NAT-PMP 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 9.
2. Hosts behind the B4 element with either include a PCP client or
UPnP IGD client.
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 include a PCP Client which is invoked by an HTTP-
based configuration (as is common today). The internal-IP-
address 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.]
8.1.2. 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
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encapsulated into the IPv6 tunnel sent to the AFTR as for any other
IPv4 packets. 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).
8.1.3. 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
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.
8.2. 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].
8.3. 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 large scale NAT
(LSN). To accomodate 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?]
8.4. IPv6 Firewall
See Section 7.5.1.
[Ed. Note: this IPv6 firewall section needs more text.]
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9. 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.]
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 12: Network Diagram, Interworking UPnP IGD and PCP
9.1. 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 large-scale 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 large-scale address sharing and
popular applications, future hosts and applications will either
support UPnP IGD 2.0's AddAnyPort Mapping (see Section 9.2) or will
support PCP.
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
HONOR_EXTERNAL_PORT option, which causes the PCP server to return an
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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:
UPnP Control Point in-home CPE PCP server
| | |
|-UPnP:AddPortMapping(80)--->| |
| |-PCP:request port 80------>|
| | HONOR_EXTERNAL_PORT |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
|-UPnP:AddPortMapping(3213)->| |
| |-PCP:request port 3213---->|
| | HONOR_EXTERNAL_PORT |
| | |
| |<-PCP:error----------------|
|<-UPnP: unavailable---------| |
| | |
... ... ... ...
| | |
|-UPnP:AddPortMapping(8921)->| |
| |-PCP:request port 8921---->|
| | HONOR_EXTERNAL_PORT |
| | |
| |<-PCP:ok, port 8921--------|
|<-UPnP: ok, port 8921-------| |
| | |
Figure 13: Message Flow: Interworking from UPnP IGD 1.0
AddPortMapping action to PCP
9.2. 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.
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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 14: Message Flow: Interworking from UPnP IGD 2.0
AddAnyPortMapping action to PCP
9.3. Lifetime Maintenance
Neither UPnP IGD 1.0 nor 2.0 provide a lifetime. 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. The UPnP IGD/PCP
interworking function is responsible for renewing the PCP lifetime
as necessary. For clients not using DHCP, other mechanisms to
check on the client host's liveliness can also be useful (e.g.,
ping, ARP, or WiFi association) can be used to discern liveliness
of the UPnP IGD control point. However, it is NOT RECOMMENDED to
attempt to connect to the TCP or UDP port opened on the control
point to determine if the host still wants to receive packets, as
the server could be temporarily down when tested, causing a false
negative.
10. Security Considerations
11. IANA Considerations
IANA is requested to perform the following actions:
11.1. PCP Server IP address
IANA shall assign an IPv4 and an IPv6 address for PCP discovery.
[Ed. Note: perhaps we can use the AFTR element's IPv4 address? But
still need an IPv6 address assigned for PIN64 and PIN66.]
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11.2. Port Number
Need a UDP port allocated.
11.3. OpCodes
IANA shall create a new protocol registry for PCP OpCodes, initially
populared with the values in Figure 7.
New OpCodes can be created via Standards Action [RFC5226].
11.4. Result Codes
IANA shall create a new registry for PCP result codes, numbered
0-255, initially populated with the error codes from both Figure 6
and Figure 10.
New Result Codes can be created via Specification Required [RFC5226].
11.5. 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 7.5.
New information elements in the range 0-64 and 128-192 can be created
via Standards Action [RFC5226], and the range 64-127 and 192-255 is
for Private Use [RFC5226].
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12. Authors
The following individuals contributed substantial text to this
document and are listed in alphabetical order:
Mohamed Boucadair
France Telecom
Email: mohamed.boucadair@orange-ftgroup.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, California 95014 USA
Phone: +1 408 974 3207
EMail: cheshire@apple.com
Francis Dupont
ISC
Email: Francis.Dupont@fdupont.fr
Reinaldo Penno
Juniper Networks
Email: rpenno@juniper.net
13. 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
pinholes.
14. References
14.1. 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.
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[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.
[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,
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>.
14.2. Informative References
[]
Arkko, J. and L. Eggert, "Scalable Operation of Address
Translators with Per-Interface Bindings",
draft-arkko-dual-stack-extra-lite-03 (work in progress),
October 2010.
[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.ietf-v6ops-cpe-simple-security]
Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment for Providing Residential IPv6
Internet Service", draft-ietf-v6ops-cpe-simple-security-16
(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.
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[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/igd2>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2608] Guttman, E., Perkins, C., Veizades, J., and M. Day,
"Service Location Protocol, Version 2", RFC 2608,
June 1999.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
Appendix A. Changes
A.1. Changes from -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 PIN* opcodes
o Renamed Informational Elements (IE) to Options.
o Added REMOTE_PEER, REMOTE_PEER_FILTER, and HONOR_EXTERNAL_PORT
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
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o PCP Options are split into mandatory-to-process ("P" bit), and
into Specification Required and Private Use.
o Epoch discussion simplified.
Appendix B. Analysis of Techniques to Discover PCP Server
[Ed. Note: This Appendix will be removed in a later version of
this document. It is included here for reference and discussion
purposes.]
Discussion Note: A deployment model is a non-PCP aware NAT in a
home, and a PCP-aware large scale NAT (LSN) operated by the ISP.
This could work by having the host use UPnP IGD with the in-home
NAT, and the host use PCP with the LSN. But this deployment model
is impossible with several of the mechanisms below. Is this
deployment model important, or can we wait for PCP to be enabled
on CPE?
Several mechanisms for discovering the PCP Server can be envisaged as
listed below:
1. A special-purpose IPv4 or IPv6 address, assigned by IANA, which
is routed normally until it hits a PCP Server, which responds.
Analysis: This solution can be deployed in the context of DS-
Lite architecture. Concretely, a well-known IPv4 address can
be used to reach a PCP Server embedded in the device that
embeds the AFTR capabilities. Since all IPv4 messages issued
by a DS-Lite CP router will be encapsulated in IPv6, no state
synchronisation issues will be experienced because PCP
messages will be handled by the appropriate PCP Server.
In some deployment scenarios (e.g., deployment of several
stateful NAT64/NAT46 in the same domain), the use of this
address is not recommended since PCP messages, issued by a
given host, may be handled by a PCP Server embedded in a NAT
node which is not involved to handle IP packets issued from
that host. The use of this special-purpose IP address may
induce session failures and therefore the customer may
experience troubles when accessing its services.
Consequently, the use of a special-purpose IPv4 address is
suitable for DS-Lite NAT44. As for NAT46/NAT64, this is left
to the Service Providers according to their deployment
configuration.
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The special-use address MUST NOT be advertised in the global
routing table. Packets with that destination address SHOULD
be filtered so they are not transmitted on the Internet.
2. Assume the default router is a PCP Server, and send PCP packets
to the IP address of the default router.
Analysis: This solution is not suitable for DS-Lite NAT44 nor
for all variants of NAT64/NAT46.
In the context of DS-Lite: There is no default IPv4 router
configured in the CP router. All outgoing IPv4 traffic is
encapsulated in IPv6 and then forwarded to a pre-configured
DS-Lite AFTR device. Furthermore, if IPv6 is used to reach
the PCP Server, the first router may not be the one which
embeds the AFTR.
For NAT64/NAT46 scenarios: The NAT function is not embedded
in the first router, therefore this solution candidate does
not allow to discover a valid PCP Server.
Therefore, this alternative is not recommended.
3. Service Location Protocol (SLP [RFC2608]).
Analysis: This solution is not suitable in scenarios where
multicast is not enabled. SLP is a chatty protocol. This
alternative is not recommended.
4. NAPTR. The host would issue a DNS query for a NAPTR record,
formed from some bits of the host's IPv4 or IPv6 address. For
example, a host with the IPv6 address 2001:db8:1:2:3:4:567:89ab
would first send an NAPTR query for
3.0.0.0.2.0.0.0.1.0.0.0.8.b.d.0.1.0.0.2.IP6.ARPA (20 elements,
representing a /64 network prefix), which returns the PCP
Server's IPv6 address. A similar scheme can be used with IPv4
using, for example, the first 24 bits of the IPv4 address.
Analysis: This solution candidate requires more configuration
effort by the Service Provider so as to redirect a given
client to the appropriate PCP Server. Any change of the
engineering policies (e.g., introduce new LSN device, load-
based dimensioning, load-balancing, etc.) would require to
update the zone configuration. This would be a hurdle for the
flexibility of the operational networks. Adherence to DNS is
not encouraged and means which allows for more flexibility are
to be promoted.
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Therefore, this mechanism is not recommended.
5. New DHCPv6/DHCP option and/or a RA option to convey an FQDN of a
PCP Server.
Analysis: Since DS-Lite and NAT64/NAT46 are likely to be
deployed in provider-provisioned environments, DHCP (both
DHCPv6 and IPv4 DHCP) is convenient to provision the address/
FQDN of the PCP Server.
Author's Address
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, California 95134
USA
Email: dwing@cisco.com
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