PCP working group D. Wing
Internet-Draft Cisco
Intended status: Standards Track October 25, 2010
Expires: April 28, 2011
Pinhole Control Protocol (PCP)
draft-wing-pcp-base-01
Abstract
Pinhole Control Protocol is an address-family independent mechanism
to control how incoming packets are forwarded by upstream devices
such as IPv4 NAT devices, NAT64 devices, and 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
Provisions Relating to IETF Documents
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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. PCP Server Discovery . . . . . . . . . . . . . . . . . . . . . 6
5. Common Request and Response Header Format . . . . . . . . . . 7
5.1. Request Header . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Response Header . . . . . . . . . . . . . . . . . . . . . 8
5.3. Information Elements . . . . . . . . . . . . . . . . . . . 9
5.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 10
6. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. PIN OpCodes . . . . . . . . . . . . . . . . . . . . . . . 11
7. PCP Mapping State Maintenance . . . . . . . . . . . . . . . . 14
7.1. Epoch . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Recreating Pinholes . . . . . . . . . . . . . . . . . . . 15
7.3. Maintaining Pinholes . . . . . . . . . . . . . . . . . . . 17
7.4. Nested NATs . . . . . . . . . . . . . . . . . . . . . . . 17
8. Processing Pinhole Requests and Responses . . . . . . . . . . 18
8.1. Generating and Sending a Request . . . . . . . . . . . . . 18
8.2. Processing a Request and Generating the Response . . . . . 18
8.3. Processing a Response . . . . . . . . . . . . . . . . . . 20
9. PCP Client Operation . . . . . . . . . . . . . . . . . . . . . 20
9.1. Pinhole Lifetime Extension . . . . . . . . . . . . . . . . 20
9.2. Pinhole Deletion . . . . . . . . . . . . . . . . . . . . . 20
9.3. Multi-interface Issues . . . . . . . . . . . . . . . . . . 21
9.4. Renumbering . . . . . . . . . . . . . . . . . . . . . . . 21
10. PCP Server Operation . . . . . . . . . . . . . . . . . . . . . 21
10.1. Relationship of PCP Server and its NAT . . . . . . . . . . 21
10.2. Pinhole Lifetime . . . . . . . . . . . . . . . . . . . . . 22
10.3. Pinhole deletion . . . . . . . . . . . . . . . . . . . . . 22
10.4. Subscriber Identification . . . . . . . . . . . . . . . . 23
10.5. External IP Address . . . . . . . . . . . . . . . . . . . 24
11. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 24
11.1. Dual Stack-Lite . . . . . . . . . . . . . . . . . . . . . 24
11.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . 24
11.1.2. Encapsulation Mode . . . . . . . . . . . . . . . . . 25
11.1.3. Plain IPv6 Mode . . . . . . . . . . . . . . . . . . . 25
11.2. NAT64 . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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11.3. NAT44 and NAT444 . . . . . . . . . . . . . . . . . . . . . 26
11.4. IPv6 Firewall . . . . . . . . . . . . . . . . . . . . . . 26
12. Adjacent Port Procedure . . . . . . . . . . . . . . . . . . . 26
13. Interworking with UPnP IGD . . . . . . . . . . . . . . . . . . 27
13.1. UPnP IGD 1.0 with AddPortMapping Action . . . . . . . . . 27
13.2. UPnP IGD 2.0 with AddAnyPortMapping Action . . . . . . . . 29
13.3. Lifetime Maintenance . . . . . . . . . . . . . . . . . . . 30
14. NAT-PMP Backwards Compatibility . . . . . . . . . . . . . . . 30
15. Security Considerations . . . . . . . . . . . . . . . . . . . 31
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
16.1. PCP Server IP address . . . . . . . . . . . . . . . . . . 31
16.2. Port Number . . . . . . . . . . . . . . . . . . . . . . . 31
16.3. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 31
16.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 31
16.5. Information Elements . . . . . . . . . . . . . . . . . . . 31
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
18.1. Normative References . . . . . . . . . . . . . . . . . . . 32
18.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Analysis of Techniques to Discover PCP Server . . . . 34
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
Pinhole 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 low-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 rendzevous 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], and;
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;
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].
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2.2. Supported Transport Protocols
The PCP OpCodes defined in this document are designed to support
transport protocols that uses a 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.
In this document, only TCP and UDP are defined.
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, much as there is only one default route for a dynamic
connection (e.g., TCP SYN) towards the Internet. 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 with internal and external IP
addresses, 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 (also called
"mapped") to the internal IP address and port number. The
internal and external IP addresses are different, and a key
point is that the internal and external transport destination
port numbers could be different. For example, 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.
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In the context of a firewall, the internal and external IP
addresses (and ports) are not changed.
PCP Client:
The network element that sends PCP requests to the PCP Server.
This network element could be an application running on a host,
embedded in the host's OS or libraries, or running on a network
device (such as a customer premise router).
PCP Server:
A network element which receives and processes PCP requests
from a PCP Client. See also Section 10.1.
Mapping:
In the context of Network Address Translation a 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 PCP Interworking Function denotes a
functional element which is responsible for another protocol with
PCP, for example interworking with UPnP IGD [IGD] described in
Section 13.
4. PCP Server Discovery
There are several possible methods to discover a PCP Service:
o sending the PCP message to the default router. This requires the
default router to support PCP.
o a fixed IPv4 and a fixed IPv6 address, to be assigned by IANA.
This follows the same routing path as other Internet-bound
traffic.
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[Ed. Note: For an IPv4 address, would the AFTR element's IPv4
address, 192.0.0.1, be suitable as this address for DS-Lite
deployments? Would that same address be suitable for all PCP
deployment scenarios?]
o New DHCP option. This requires the local network's DHCP server
support the new option.
[Ed. Note: more discussion around these methods is necessary to
reach consensus on the best approach(es)s for PCP.]
5. Common Request and Response Header Format
PCP is intended to be backwards compatible with NAT-PMP so that a
NAT-PMP client (or server) will receive an error message when sending
a request to a PCP server (or client).
All PCP messages contain a request (or response) header, opcode-
specific information, and (optional) informational elements. These
are described in the following sections.
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=1 |reserve| OpCode | Protocol | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved (32 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) opcode-specific information :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Informational Elements :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Common Request Packet Format
These fields are described below:
Ver: Version is 1
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reserve: 4 reserved bits, MUST be sent as 0, MUST be ignored when
received.
OpCode: defined in Figure 5.
Protocol: indicates protocol associated with this opcode. For
example, this field contains 6 (TCP) if the opcode is intended to
create a TCP mapping. Values are taken from the IANA protocol
registry [proto_numbers]. If a particular OpCode does not need
the field, it MUST sent as 0 and MUST be ignored when received.
Reserved: The reserved fields 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver=1 |reserve| Opcode+128 | Protocol | Result Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Epoch |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: (optional) OpCode-specific response data :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: (optional) Informational Elements :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: 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.
OpCode+128: The OpCode value from the request plus 128.
Protocol: Protocol field, echoed exactly from the request
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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 7.1 for discussion.
5.3. Information Elements
The Informational Elements (IE) allow extending PCP, without defining
a new PCP version and without consuming additional opcodes. IEs can
be used in requests and responses. It is anticipated that IEs will
include information which are associated with the normal function of
an OpCode, such as one of the PIN OpCodes defined in this document.
For example, an IE could indicate DSCP markings to apply to incoming
or outgoing traffic associated with a PCP pinhole, or an IE could
include descriptive text (e.g., "for my webcam").
IEs 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| IE Code | Reserved | IE-Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Informational Element header
The description of the fields is as follows:
IE Code: IE codes MUST be registered with IANA following the
procedure described in Section 16.5.
Reserved: MUST be set to 0 on transmission and MUST be ignored on
reception.
IE-Length: Indicates in units of 4 octets the length of the enclosed
data. IEs MUST be padded when necessary to 32 bits boundaries.
IEs with length of 0 are allowed.
A given IE MAY be included in the request and/or the response. The
handling of an IE at the PCP Client and the PCP Server sides MUST be
specified in dedicated document(s).
[Ed. Note: Do we want to allow an unsolicited IE to appear in a
response?]
If several IEs are to be included in a PCP request or response, IEs
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MAY be encoded in any order by the PCP Client or the PCP Server.
[Ed. Note: There are two proposals to handle unsupported IEs on
the server: (1) return a notification in the response with the
Code(s) of unsupported IEs, (2) every IE that appears in a request
will cause an IE to appear in the response if the server
understood the request' IE(s). Consensus is needed.]
[Ed. Note: There is a proposal to define a Mandatory bit, so that
the PCP server will not process a PCP request at all if it
encounters an unsupported IE with the Mandatory bit set. This
diverges from IE being "informational", but may have some value.
Consensus is needed.]
New IEs are defined in companion documents and MUST follow the format
shown in Figure 1. To avoid errors and increased complexity, it is
RECOMMENDED to define one single IE which carry all the required
information for a given usage instead of defining several IEs to be
included simultaneously in the same PCP message. Interdependence
between IEs SHOULD be avoided at maximum.
5.4. Result Codes
The following response codes are defined:
0 - Success
1 - Unsupported Version
2 - Not Authorized/Refused
(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)
5 - Unsupported opcode
Figure 4: PCP Result Codes
Additional result codes are defined following the procedure in
Section 16.4.
6. OpCodes
This document defines four OpCodes which share a similar packet
layout for requests and responses. For these OpCodes, the request
and response packet formats take the same space and layout. New
OpCodes MAY choose to follow the same format. The OpCodes defined in
this document are:
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PIN44 = 0 = IPv4 address to IPv4 address (NAT44 or IPv4 firewall)
PIN46 = 1 = IPv4 address to IPv6 address (NAT46)
PIN64 = 2 = IPv6 address to IPv4 address (NAT64)
PIN66 = 3 = IPv6 address to IPv6 address (NAT66 or IPv6 firewall)
Figure 5: OpCodes
6.1. PIN OpCodes
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 internal-ip-address and internal-port matches (requested)
external-ip-address and (requested) external-port, the (request or)
response pertains to a firewall; otherwise it pertains to a NAT.
The following diagram shows the request 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| |
| Reserved (always 160 bits) |
| |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: 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 6: PIN OpCode Request Packet Format
These fields are described below:
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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 MAY place any value into this
field including 0. If the Pinhole Internal IP Address equals the
Requested External IP Address, it indicates the PCP client wants
firewall (versus NAT) behavior.
Requested lifetime: Requested lifetime of this pinhole, in seconds.
internal port: Internal port for the pinhole.
requested external port: requested external port.
internal port: Internal port for the pinhole, copied from request.
Assigned 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 SHOULD use 0.
[Ed. Note: for firewall, we need to add fields indicating the
remote peer address (address family will match the address family
of the requsted IP address). Addition permission for multiple
remote peers is possible (by sending multiple PCP requests, one
for each remote peer's IP address). Deleting a single permission
would require a new OpCode. Should perhaps firewall use different
OpCodes than NAT??]
The following diagram shows the response packet format for PIN44,
PIN46, PIN64, and PIN66:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PCP Request Address Family | PCP Request Port |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| PCP Request IP Address (always 128 bits) |
| (first 32 bits are XOR'd) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: 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 7: PIN OpCode Response Packet Format
These fields are described below:
PCP Request Address Family: The IP address family of the PCP
request, as received in the IP header by the PCP server. Will
usually be 1 (IPv4) or 2 (IPv6). This is used by the PCP client
to determine if there is a NAT between the PCP client and PCP
server (see Section 7.4).
PCP Request Port: The port of the PCP request, as received in the
UDP header by the PCP server. This is used by the PCP client to
determine if there is a NAT between the PCP client and PCP server
(see Section 7.4).
PCP Request IP Address: The IPv4 or IPv6 address of the PCP request,
as received in the IP header by the PCP server. This is used by
the PCP client to determine if there is a NAT between the PCP
client and PCP server (see Section 7.4). As some NATs rewrite
IPv4 packets containing the NAT's public IPv4 address in the UDP
payload, the first 32 bits of the address are XOR'd with
0xFFFFFFFF if it contains an IPv4 address; IPv6 addresses are not
XOR'd.
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Pinhole Internal IP address Copied from 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
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. PCP Mapping State Maintenance
If an event occurs that causes the PCP server and NAT 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.
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.
7.1. Epoch
Every packet sent by the PCP Server includes a "Seconds since start
of epoch" field (SSSOE). The PCP Server MUST set its Epoch time to
zero when it is ready to accept new connections. 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 and
begin counting SSSOE from 0 again. Whenever a client receives any
packet from the PCP Server, either gratuitously or in response to a
client request, the client computes its own conservative estimate of
the expected SSSOE value by taking the SSSOE 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 SSSOE in the newly
received packet is less than the client's conservative estimate by
more than one second, then the client concludes that the NAT gateway
has undergone a reboot or other loss of port mapping state, and the
client MUST immediately renew all its active port mapping leases as
described in Section 7.2.
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7.2. Recreating Pinholes
The NAT gateway MAY store mappings in persistent storage so when it
is powered off or rebooted, it remembers the port mapping state of
the network.
However, maintaining this state is not essential for correct
operation. When the NAT gateway powers on or clears its port mapping
state as the result of a configuration change, it MUST reset the
epoch time.
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 freshly-
rebooted NAT gateway it appears as a new mapping request.
This self-healing property of the protocol is very important.
The remarkable reliability of the Internet as a whole derives in
large part from the fact that important state is held in the
endpoints, not in the network itself [Saltzer84]]. Power-cycling an
Ethernet switch results only in a brief interruption in the flow of
packets; established TCP connections through that switch are not
broken, merely delayed for a few seconds. Indeed, an old Ethernet
switch can even be replaced with a new one, and as long as the cables
are transferred over reasonably quickly, after the upgrade all the
TCP connections that were previously going though the old switch will
be unbroken and now going through the new one. The same is true of
IP routers, wireless base stations, etc. The one exception is NAT
gateways. Because the port mapping state is required for the NAT
gateway to know where to forward inbound packets, loss of that state
breaks connectivity through the NAT gateway. By allowing clients to
detect when this loss of NAT gateway state has occurred, and recreate
it on demand, we turn hard state in the network into soft state, and
allow it to be recovered automatically when needed.
Without this automatic recreation of soft state in the NAT gateway,
reliable long-term networking would not be achieved. As mentioned
above, the reliability of the Internet does not come from trying to
build a perfect network in which errors never happen, but from
accepting that in any sufficiently large system there will always be
some component somewhere that's failing, and designing mechanisms
that can handle those failures and recover. To illustrate this point
with an example, consider the following scenario: Imagine a network
security camera that has a web interface and accepts incoming
connections from web browser clients. Imagine this network security
camera uses NAT-PMP or a similar protocol to set up an inbound port
mapping in the NAT gateway so that it can receive incoming
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connections from clients the other side of the NAT gateway. Now,
this camera may well operate for weeks, months, or even years.
During that time it's possible that the NAT gateway could experience
a power failure or be rebooted. The user could upgrade the NAT
gateway's firmware, or even replace the entire NAT gateway device
with a newer model. The general point is that if the camera operates
for a long enough period of time, some kind of disruption to the NAT
gateway becomes inevitable. The question is not whether the NAT
gateway will lose its port mappings, but when, and how often. If the
network camera and devices like it on the network can detect when the
NAT gateway has lost its port mappings, and recreate them
automatically, then these disruptions are self-correcting and largely
invisible to the end user. If, on the other hand, the disruptions
are not self-correcting, and after a NAT gateway reboot the user has
to manually reset or reboot all the other devices on the network too,
then these disruptions are *very* visible to the end user. This
aspect of the design is what makes the difference between a protocol
that keeps on working indefinitely over a time scale of months or
years, and a protocol that works in brief testing, but in the real
world is continually failing and requiring manual intervention to get
it going again.
When a client renews its port mappings as the result of receiving a
packet where the "Seconds since start of epoch" field (SSSoE)
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 port mapping request. After that request is
acknowledged by the gateway, 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 focusses on recreating inbound port
mappings after loss of NAT gateway 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,
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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 NAT-PMP. 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 later to recreate the
mapping if it determines that the NAT gateway has lost its mapping
state.
7.3. 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
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.
7.4. Nested NATs
A PCP Client can detect the presence of a NAT on the path between the
PCP client and PCP server by sending a PCP request to the PCP server
and comparing fields in the PCP response. If the request's IP
address family, IP address, and source port match the information in
the PCP response's payload (PCP Request Address Family, PCP Request
Port, and PCP Request XOR'd IP Address), there is no NAT on the path.
If they differ, there is a NAT on the path.
Note: this provides a function similar to STUN [RFC5389]. Being
integrated within PCP itself provides the advantage of checking
the path to the PCP server, which may be a different path than to
the STUN server.
After determining a NAT is on the path, the PCP application can take
appropriate action based on this information. This action would
require using another mechanism to open pinholes in the intervening
NATs (e.g., UPnP IGD, NAT-PMP) or returning an error to the user.
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8. Processing Pinhole Requests and Responses
PCP messages MUST be sent over UDP, and the PCP Server MUST listen
for PCP requests on the PCP port number (Section 16.2). Every PCP
request generates a response, so PCP does not need to run over a
reliable transport protocol.
8.1. Generating and Sending a Request
To create a pinhole, the PCP client generates a PCP request for the
appropriate address family of the internal host and the desired
public mapping. The PCP request contains a PCP common header, PCP
OpCode and payload, and optional Information Elements.
The PCP client determines its PCP server using the procedure
described in Section 4. It initializes its retransmission timer,
RETRY_TIMER, to the round trip time between the PCP client and PCP
server. If this value is unknown, 250ms is RECOMMENDED. The PCP
Client sends its PCP message to the PCP server and waits RETRY_TIMER
for a response. If no response is received, it doubles the value of
RETRY_TIMER, sends another (identical) PCP message and waits
RETRY_TIMER*2. This procedure is repeated three times, doubling the
value of RETRY_TIMER each time. If no response is received after 4
attempts, the PCP client tries with the next IP address in its list
of PCP servers. If it has exhausted its list, it SHOULD abort the
procedure. If, when sending PCP requests the PCP Client receives an
ICMP error (e.g., port unreachable, network unreachable) it SHOULD
immediately abort the procedure. 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.
8.2. Processing a Request and Generating the Response
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 Informational Elements (which the
server might or might not comprehend). If an error is encountered
during processing, an error response is generated and 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 deployment scenario; see
Section 10.4. If the internal IP address in the PCP request does not
belong to the subscriber, an error response MUST be generated with
error-code=2.
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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, apositive 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)?]
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
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-
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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.
8.3. Processing a Response
The PCP client receives the response and checks that the response
matches one of its outstanding requests. If it is an error response,
the PCP client knows that none of the requested pinholes were
created, and can attempt to resolve the problem based on the error
code and error subcode.
If it is an positive response, the PCP client knows the request was
entirely successful and 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 mechanism.
9. PCP Client Operation
This section details operation specific to a PCP client.
9.1. 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 10.3).
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).
9.2. Pinhole Deletion
A PCP Client MAY delete a pinhole prior to its natural expiration by
sending a PCP Map Request with a lifetime of 0. The PCP server
responds by returning a PCP Map Response with a lifetime of 0.
To delete all pinholes for all ports, the "W" (wildcard) bit is set,
and no internal port/external port is included in the PCP request.
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To delete all pinholes for all hosts associated with this subscriber,
an all-zero internal IP address is used.
9.3. 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
global IPv6 prefix.
9.4. 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.
10. PCP Server Operation
This section details operation specific to a PCP server.
10.1. Relationship of PCP Server and its NAT
The PCP server receives PCP requests. The PCP server might be
integrated within the NAT device (as shown in Figure 8) which is
expected to be a common deployment.
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+-----------------+
+------------+ | NAT or firewall |
| PCP Client |-<network>-+ +---<Internet>
+------------+ | with embedded |
| PCP server |
+-----------------+
Figure 8: 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 9). 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> | suitable NAT control protocol
+------------+ \ v
\ +------------+
+--+ PCP Server |
+------------+
Figure 9: NAT with Separate PCP Server
10.2. 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. Also see XXX.
It is NOT RECOMMENDED that the server allow long 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.
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.
10.3. 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
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another host for 120 seconds (MSL, [RFC0793]).
[Ed. Note: it should (MUST?) allow the same host to re-acquire
the same port, though.]
10.4. Subscriber Identification
Subscribers identification is required for several reasons such as
the following:
o Allow access to the network resources;
o Configure service profiles such as a bandwidth and/or port usage
quotas for fairness service usage among all subscribers;
o Blacklist a subscriber because of abuse or non-payment of service
fee, etc.
o Legal requirements such as legal intercept or legal storage.
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, it is not desirable for a PCP client
to 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.
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 11.1.2 and Section 11.1.3). 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 CGN with a routed network, each subscriber has one IPv4 address
and all PCP requests MUST be sent from only that IP address and
MUST only open pinholes towards its own IP address.
PCP-controlled devices can be a DS-Lite AFTR or an IPv4-IPv6
interconnection node such as NAT46 or NAT64. These nodes are
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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.
10.5. 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]. Additionally, all PCP-mapped
requests MUST also use the same external IP address. 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.
11. Deployment Scenarios
11.1. 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.
11.1.1. Overview
Various PCP deployment scenarios can be considered to control the PCP
server embedded in the AFTR element:
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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 13.
2. Hosts behind the B4 element include a PCP Client, and communicate
directly with the PCP server. No interworking function is
required to be embedded in the B4 element.
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.]
11.1.2. 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 AFTR decapsulates the IPv4 packets and processes
the PCP requests (because the destination IPv4 address points to the
PCP Server embedded in the AFTR).
11.1.3. 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.
11.2. 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
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network-specific prefix, per [I-D.ietf-behave-address-format].
11.3. NAT44 and NAT444
Subscribers are given only one IPv4 address. To accomodate multiple
hosts within the home, subscribers operate a NAPT device. When this
occurs in conjunction with an upstream NAT44, this is nicknamed
"NAT444".
In either environment (with or without a NAPT in the home), the
service provider and PCP server see only one IPv4 address from each
subscriber.
PCP includes a function to detect a NAT between the PCP client and
PCP server, described in Section 7.4.
11.4. IPv6 Firewall
[Ed. Note: PCP packet format needs changes to support IPv6 firewall,
or we need additional OpCodes for IPv6 firewall.]
12. Adjacent Port Procedure
RTP and RTCP have historically run on adjacent ports, and some
existing equipment still expects them to be on adjacent ports. To
accomodate that, a procedure can be used rather than adding
complexity to the protocol or to the server implementation.
[Ed. Note: Are there any other referencable protocols that need
adjacent ports?]
The procedure is for the PCP client to bind to two ports on its local
interface. It then sends a PCP request for external port 0
(indicating it will accept any port from the server) for one of those
internal ports. This request can have a short lifetime (e.g., 5
seconds) to avoid the need to delete the pinhole. It receives the
PCP response indicating it now has external port N. The PCP client
then attempts to obtain a port on either side of this external port.
It sends two PCP requests, using the same internal port number in
both requests, for external port N-1 and for external port N+1. The
adjacent external ports N-1 and N+1 are either (a) not available, (b)
only one is available, or (c) both are available. If (a), an
unrelated port will be assigned and the procedure can be repeated.
If (b) the procedure was successful. Case (c) is also successful,
because the PCP client cannot distinguish it from case (b), because
the PCP server maps an specific internal IP address and internal port
to a single external IP address and port.
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[Ed. Note: Add message flow diagram showing adjacent port
procedure]
13. Interworking with UPnP IGD
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 10: Network Diagram, Interworking UPnP IGD and PCP
13.1. 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. Requesting a specific port is
incompatible with both (1) a large-scale NAT and with (2) successful
applications. Regarding (1), other subscribers are likely to also be
running the same application, all demanding (or desiring) the same
port number. Regarding (2), a popular application will exist on
multiple devices within the home. Thus, PCP is not designed to
optimize for this behavior of requesting a particular port as it
cannot work well in address sharing environments; but PCP will work
with this behavior using the suggested procedure below.
Due to this incompatibility with large-scale address sharing and
popular applications, future hosts and applications will either
support UPnP IGD 2.0 (which has improved behavior, see Section 13.2)
or will support PCP.
To interwork from UPnP IGD to PCP, our recommendation is that every
UPnP request be forwarded to the PCP server -- this works no matter
if the UPnP IGD control point is randomizing or incrementing each
port number when its requests fail. When a requested port assignment
fails, most UPnP IGD control points will retry the port assignment
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requesting the next higher port or requesting a random port. In
either case, the described procedure will work. The UPnP IGD/PCP
interworking function would request very short leases (e.g., 5
seconds) in order to avoid the chatter of a Delete message
(lifetime=0). Once a port can be allocated, its lifetime is
extended. When interworking with UPnP IGD, the in-home CPE limits
itself to sending one PCP message a second, which ensures there are
only 5 outstanding PCP reservations at a time; this avoids consuming
all of that subscriber's NAT mappings while trying to find an
available port via the UPnP IGD->PCP interworking function).
Note: for this to work successfully, the PCP server (large NAT)
needs to honor the requested-external-port field in the PCP
request. Which is the purpose of that field, of course.
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Message flow would be similar to this:
UPnP CP in-home CPE PCP server
| | |
|-UPnP:give me port 80--->| |
| |-PCP:request port 80------>|
| | with lifetime=5 seconds |
| |<-PCP:here is port 51389---|
|<-UPnP: unavailable------| |
| | |
| (allow port 51389 to naturally expire, |
| or actively Delete it) |
| | |
|-UPnP:give me port 3213->| |
| |-PCP:request port 3213---->|
| | with lifetime=5 seconds |
| |<-PCP:here is port 23831---|
|<-UPnP: unavailable------| |
| | |
| (allow port 23831 to naturally expire, |
| or actively Delete it) |
| | |
... ... ... ...
| | |
|-UPnP:give me port 8921->| |
| |-PCP:request port 8921---->|
| | with lifetime=5 seconds |
| |<-PCP:here is port 8921----|
| | |
| |-PCP:life=1 hour,port=8921>|
| |<-PCP:ok-------------------|
| | |
|<-UPnP: ok, port 8921----| |
| | |
Figure 11: Message Flow: Interworking from UPnP IGD 1.0
AddPortMapping action to PCP
13.2. 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 the IGD 2's new AddAnyPortMapping message, 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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13.3. Lifetime Maintenance
UPnP IGD does not provide a lifetime, so the UPnP IGD/PCP
interworking function is responsible for extending the lifetime of
mappings that are still interesting to the UPnP IGD device.
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. As long as client renews its DHCP lease, the PCP
lifetime should also be extended. 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; the server could be temporarily down when
tested, causing a false negative.
14. NAT-PMP Backwards Compatibility
Because NAT-PMP and PCP share the same port, it is important that a
NAT-PMP client receive a NAT-PMP error message. This is done by
examining the version number of the incoming PCP message; if it is
zero, the message is from a NAT-PMP client. A valid NAT-PMP response
(rather than a PCP response) is necessary, shown below.
A server which supports both NAT-PMP and PCP would be able to process
both NAT-PMP and PCP requests normally, and (if necessary) proxy
between the protocols.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 | OP = 128 + x | Response Code=1 (unsupp. ver.)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
| 0 (96 bits) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: NAT-PMP response
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[Ed. Note: More analysis is necessary on NAT-PMP backward
compatibility, including checking if NAT-PMP clients are compliant
with [I-D.cheshire-nat-pmp]] regarding error handling.
15. Security Considerations
[Ed. Note: to be completed.]
16. IANA Considerations
IANA is requested to perform the following actions:
16.1. 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.]
16.2. Port Number
Re-use NAT-PMP port number, UDP/5351.
16.3. OpCodes
IANA shall create a new protocol registry for PCP OpCodes, initially
populared with the values in Figure 5.
New OpCodes can be created via Standards Action [RFC2434].
16.4. Result Codes
IANA shall create a new registry for PCP result codes, numbered
0-255, initially populated with the error codes from Figure 4.
New Result Codes can be created via Specification Required [RFC2434].
16.5. Information Elements
IANA shall create a new registry for PCP Information Elements,
numbered 0-255 with associated mnemonic.
New information elements in the range 0-127 can be created via
Standards Action [RFC2434], new information elements in the range
128-192 can be created with Expert Review [RFC2434], and the range
193-255 is for Private Use [RFC2434].
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17. Acknowledgments
Thanks to Alain Durand and Christian Jacquenet for their comments and
review.
Thanks to Mohamed Boucadair, Francis Dupont, and Reinaldo Penno for
significant contributions. Thanks to Stuart Cheshire for writing
NAT-PMP [I-D.cheshire-nat-pmp] and for his contributions to this
document.
18. References
18.1. 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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[proto_numbers]
IANA, "Protocol Numbers", 2010, <http://www.iana.org/
assignments/protocol-numbers/protocol-numbers.xml>.
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18.2. 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.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-address-format]
Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X.
Li, "IPv6 Addressing of IPv4/IPv6 Translators",
draft-ietf-behave-address-format-10 (work in progress),
August 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.
[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.
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[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[Saltzer84]
Saltzer, J., Reed, D., and D. Clark, "End-to-end arguments
in system design", 1984, <http://web.mit.edu/Saltzer/www/
publications/endtoend/endtoend.pdf>.
Appendix A. 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.]]
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.
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.
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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.
Therefore, this mechanism is not recommended.
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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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