| PCP working group | D. Wing, Ed. |
| Internet-Draft | Cisco |
| Intended status: Standards Track | S. Cheshire |
| Expires: April 22, 2012 | Apple |
| M. Boucadair | |
| France Telecom | |
| R. Penno | |
| Juniper Networks | |
| P. Selkirk | |
| ISC | |
| October 20, 2011 |
Port Control Protocol (PCP)
draft-ietf-pcp-base-15
The Port Control Protocol allows an IPv6 or IPv4 host to control how incoming IPv6 or IPv4 packets are translated and forwarded by a network address translator (NAT) or simple firewall, and also allows a host to optimize its outgoing NAT keepalive messages.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 22, 2012.
Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
The Port Control Protocol (PCP) provides a mechanism to control how incoming packets are forwarded by upstream devices such as NAT64, NAT44, IPv6 and IPv4 firewall devices, and a mechanism to reduce application keepalive traffic. PCP is designed to be implemented in the context of Carrier-Grade NATs (CGNs), small NATs (e.g., residential NATs), as well as with dual-stack and IPv6-only CPE routers, and all of the currently-known transition scenarios towards IPv6-only CPE routers. PCP allows hosts to operate servers for a long time (e.g., a webcam) or a short time (e.g., while playing a game or on a phone call) when behind a NAT device, including when behind a CGN operated by their Internet service provider or an IPv6 firewall integrated in their CPE router.
PCP allows applications to create mappings from an external IP address and port to an internal IP address and port. These mappings are required for successful inbound communications destined to machines located behind a NAT or a firewall.
After creating a mapping for incoming connections, it is necessary to inform remote computers about the IP address and port for the incoming connection. This is usually done in an application-specific manner. For example, a computer game might use a rendezvous server specific to that game (or specific to that game developer), a SIP phone would use a SIP proxy, and a client using DNS-Based Service Discovery [I-D.cheshire-dnsext-dns-sd] would use DNS Update [RFC2136] [RFC3007]. PCP does not provide this rendezvous function. The rendezvous function may support IPv4, IPv6, or both. Depending on that support and the application's support of IPv4 or IPv6, the PCP client may need an IPv4 mapping, an IPv6 mapping, or both.
Many NAT-friendly applications send frequent application-level messages to ensure their session will not be timed out by a NAT. These are commonly called "NAT keepalive" messages, even though they are not sent to the NAT itself (rather, they are sent 'through' the NAT). These applications can reduce the frequency of such NAT keepalive messages by using PCP to learn (and influence) the NAT mapping lifetime. This helps reduce bandwidth on the subscriber's access network, traffic to the server, and battery consumption on mobile devices.
Many NATs and firewalls include application layer gateways (ALGs) to create mappings for applications that establish additional streams or accept incoming connections. ALGs incorporated into NATs may also modify the application payload. Industry experience has shown that these ALGs are detrimental to protocol evolution. PCP allows an application to create its own mappings in NATs and firewalls, reducing the incentive to deploy ALGs in NATs and firewalls.
PCP can be used in various deployment scenarios, including:
The PCP Opcodes defined in this document are designed to support transport-layer protocols that use a 16-bit port number (e.g., TCP, UDP, SCTP, DCCP). Protocols that do not use a port number (e.g., RSVP, IPsec ESP, ICMP, ICMPv6) are supported for IPv4 firewall, IPv6 firewall, and NPTv6 functions, but are out of scope for any NAT functions.
PCP assumes a single-homed IP address model. That is, for a given IP address of a host, only one default route exists to reach the Internet. This is important because after a PCP mapping is created and an inbound packet (e.g., TCP SYN) arrives at the host, the outbound response (e.g., TCP SYNACK) has to go through the same path so it is seen by the firewall or rewritten by the NAT. This restriction exists because otherwise there would need to be a PCP-enabled NAT for every egress (because the host could not reliably determine which egress path packets would take) and the client would need to be able to reliably make the same internal/external mapping in every NAT gateway, which in general is not possible (because the other NATs might have the necessary port mapped to another host).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
The PCP server receives and responds to PCP requests. The PCP server functionality is typically a capability of a NAT or firewall device, as shown in Figure 1. It is also possible for the PCP functionality to be provided by some other device, which communicates with the actual NAT or firewall via some other proprietary mechanism, as long as from the PCP client's perspective such split operation is indistinguishable from the integrated case.
+-----------------+
+------------+ | NAT or firewall |
| PCP client |-<network>-+ with +---<Internet>
+------------+ | PCP server |
+-----------------+
A NAT or firewall device, between the PCP client and the Internet, might implement simple or advanced firewall functionality. This may be a side-effect of the technology implemented by the device (e.g., a network address and port translator, by virtue of its port rewriting, normally requires connections to be initiated from an inside host towards the Internet), or this might be an explicit firewall policy to deny unsolicited traffic from the Internet. Some firewall devices deny certain unsolicited traffic from the Internet (e.g., TCP, UDP to most ports) but allow certain other unsolicited traffic from the Internet (e.g., UDP port 500 and IPsec ESP as described in [RFC6092]). Such default filtering (or lack thereof) is out of scope of PCP itself. If a device supports PCP and wants to receive traffic, and does not possess knowledge of such filtering, it SHOULD use PCP to create the necessary mappings to receive the desired traffic.
For simplicity in building and parsing request and response packets, PCP always uses fixed-size 128-bit IP address fields for both IPv6 addresses and IPv4 addresses.
When the address field holds an IPv6 address, the fixed-size 128-bit IP address field holds the IPv6 address stored as-is.
When the address field holds an IPv4 address, IPv4-mapped IPv6 addresses [RFC4291] are used (::ffff:0:0/96). This has the first 80 bits set to zero and the next 16 set to one, while its last 32 bits are filled with the IPv4 address. This is unambiguously distinguishable from a legal IPv6 address, because IPv4-mapped IPv6 address [RFC4291] are not used as either the source or destination address of actual IPv6 packets.
When checking for an IPv4-mapped IPv6 address, all of the first 96 bits MUST be checked for the pattern -- it is not sufficient to check for 0xFF in bits 81-96.
The all-zeroes IPv6 address is expressed by filling the fixed-size 128-bit IP address field with all zeroes (::).
The all-zeroes IPv4 address is expressed as: 80 bits of zeros, 16 bits of ones, and 32 bits of zeros (::ffff:0:0).
All PCP messages contain a request (or response) header containing an Opcode, any relevant Opcode-specific information, and zero or more Options. The packet layout for the common header, and operation of the PCP client and PCP server, are described in the following sections. The information in this section applies to all Opcodes. Behavior of the Opcodes defined in this document is described in Section 9 and Section 10.
All requests have the following format:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version = 1 |R| Opcode | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Requested Lifetime (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | PCP Client's IP address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PCP Client's Port | Reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) Opcode-specific information : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) PCP Options : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
All responses have the following format:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version = 1 |R| Opcode | Reserved | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Epoch (32 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Reserved (96 bits) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) Opcode-specific response data : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : (optional) Options : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
A PCP Opcode can be extended with one or more Options. Options can be used in requests and responses. The design decisions in this specification about whether to include a given piece of information in the base Opcode format or in an Option were an engineering trade-off between packet size and code complexity. For information that is usually (or always) required, placing it in the fixed Opcode data results in simpler code to generate and parse the packet, because the information is a fixed location in the Opcode data, but wastes space in the packet in the event that field is all-zeroes because the information is not needed or not relevant. For information that is required less often, placing it in an Option results in slightly more complicated code to generate and parse packets containing that Option, but saves space in the packet when that information is not needed. Placing information in an Option also means that an implementation that never uses that information doesn't even need to implement code to generate and parse it. For example, a client that never requests mappings on behalf of some other device doesn't need to implement code to generate the THIRD_PARTY Option, and a PCP server that doesn't implement the necessary security measures to create third-party mappings safely doesn't need to implement code to parse the THIRD_PARTY Option.
Options use the following Type-Length-Value format:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code | Reserved | Option Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : (optional) data : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The description of the fields is as follows:
The handling of an Option by the PCP client and PCP server MUST be specified in an appropriate document, which MUST include whether the PCP Option can appear in a request and/or response, whether it can appear more than once, and indicate what sort of Option data it conveys. If several Options are included in a PCP request, they MAY be encoded in any order by the PCP client, but MUST be processed by the PCP server in the order in which they appear. It is the responsibility of the PCP client to ensure the server has sufficient room to reply with an error including UNPROCESSED Options; this can be achieved by sending messages that don't exceed 1024-4*number_of_options octets.
If, while processing an Option, an error is encountered that causes a PCP error response to be generated, the PCP request MUST cause no state change in the PCP server or the PCP-controlled device (i.e., it rolls back any changes it might have made while processing the request). The response MUST encode the Options in the same order as received in the request. Additional Options included in the response (if any) MUST be included at the end. An Option MAY appear more than once in a request or in a response, if permitted by the definition of the Option. If the Option's definition allows the Option to appear only once but it appears more than once in a request, and the Option is understood by the PCP server, the PCP server MUST respond with the MALFORMED_OPTION result code; if this occurs in a response, the PCP client processes the first occurrence and MAY log an error. If an invalid option length is encountered (e.g., option length extends beyond the length of the PCP Opcode itself), the error MALFORMED_OPTION SHOULD be returned (rather than MALFORMED_REQUEST), as that helps the client better understand how the packet was malformed. The UNPROCESSED option MUST NOT appear in a request; if it does, it causes a MALFORMED_REQUEST error. If a PCP response would have exceeded the maximum PCP message size, the PCP server MAY respond with MALFORMED_REQUEST.
The most significant bit in the Option Code indicates if its processing is optional or mandatory. If the most significant bit is set, handling this Option is optional, and a PCP server MAY process or ignore this Option, entirely at its discretion. If the most significant bit is clear, handling this Option is mandatory, and a PCP server MUST process this Option or return an error code if it cannot. If the PCP server does not implement this Option, or cannot perform the function indicated by this Option (e.g., due to a parsing error with the Option), it MUST generate an error response with code UNSUPP_OPTION or MALFORMED_OPTION (as appropriate) and MUST include the UNPROCESSED Option in the response (see Section 7.7.1).
PCP clients are free to ignore any or all Options included in responses, although naturally if a client explicitly requests an Option where correct handling of that Option requires processing the Option data in the response, that client is expected to implement code to do that.
Different options are valid for different Opcodes. For example, the UNPROCESSED option is valid for all Opcodes, but only in response messages. The THIRD_PARTY Option is valid for both MAP and PEER Opcodes. The PREFER_FAILURE option is valid only for the MAP Opcode (for the PEER Opcode, its semantics are implied). The FILTER option is valid only for the MAP Opcode (for the PEER Opcode it would have no meaning).
Option definitions MUST include the information below:
The following result codes may be returned as a result of any Opcode received by the PCP server. The only success result code is 0; other values indicate an error. If a PCP server encounters multiple errors during processing of a request, it SHOULD use the most specific error message. Each error code below is classified as either a 'long lifetime' error or a 'short lifetime' error, which provides guidance to PCP server developers for the value of the Lifetime field for these errors. It is RECOMMENDED that short lifetime errors use a 30 second lifetime and long lifetime errors use a 30 minute lifetime.
PCP messages MUST be sent over UDP [RFC0768]. Every PCP request generates a response, so PCP does not need to run over a reliable transport protocol.
PCP is idempotent, meaning that if the PCP client sends the same request multiple times (or the PCP client sends the request once and it is duplicated by the network), and the PCP server processes those requests multiple times, the result is the same as if the PCP server had processed only one of those duplicate requests.
This section details operation specific to a PCP client, for any Opcode. Procedures specific to the MAP Opcode are described in Section 9, and procedures specific to the PEER Opcode are described in Section 10.
Prior to sending its first PCP message, the PCP client determines which server to use. The PCP client performs the following steps to determine its PCP server:
For the purposes of this document, only a single PCP server address is supported. Should future specifications define configuration methods that provide a list of PCP server addresses, those specifications will define how clients select one or more addresses from that list.
With that PCP server address, the PCP client formulates its PCP request. The PCP request contains a PCP common header, PCP Opcode and payload, and (possibly) Options. As with all UDP or TCP client software on any operating system, when several independent PCP clients exist on the same host, each uses a distinct source port number to disambiguate their requests and replies. The PCP client's source port SHOULD be randomly generated [RFC6056].
To assist with detecting an on-path NAT, the PCP client MUST include the source IP address of the PCP message in the PCP request. This is typically its own IP address; see Section 12.4 for how this can be coded.
When attempting to contact a PCP server, the PCP client initializes a timer to 2 seconds. The PCP client sends a PCP message to the first server in its list of PCP servers. If no response is received before the timer expires, the timer is doubled (to 4 seconds) and the request is re-transmitted. If no response is received before the timer expires, the timer is doubled again (to 8 seconds) and the request is re-transmitted.
Once a PCP client has successfully received a response from a PCP server on that interface, it sends subsequent PCP requests to that same server, with a retransmission timer of 2 seconds. If, after 2 seconds, a response is not received from that PCP server, the same back-off algorithm described above is performed.
This section details operation specific to a PCP server. Processing SHOULD be performed in the order of the following paragraphs.
A PCP server MUST only accept normal (non-THIRD_PARTY) PCP requests from a client on the same interface it would normally receive packets from that client, and MUST silently ignore PCP requests arriving on any other interface. For example, a residential NAT gateway accepts PCP requests only when they arrive on its (LAN) interface connecting to the internal network, and silently ignores any PCP requests arriving on its external (WAN) interface. A PCP server which supports THIRD_PARTY requests MAY be configured to accept THIRD_PARTY requests on other interfaces from properly authorized clients.
Upon receiving a request, the PCP server parses and validates it. A valid request contains a valid PCP common header, one valid PCP Opcode, and zero or more Options (which the server might or might not comprehend). If an error is encountered during processing, the server generates an error response which is sent back to the PCP client. Processing an Opcode and the Options are specific to each Opcode.
If the received message is at least two octets long but the first octet (version) is a version that is not supported, a response is generated with the UNSUPP_VERSION result code, and the other steps detailed in Section 7.6 are followed.
Otherwise, if the version is supported but the received message is shorter than 4 octets or has the R bit set, the message is silently dropped.
If the server is overloaded by requests (from a particular client or from all clients), it MAY simply discard requests, as the requests will be retried by PCP clients, or it MAY generate the NO_RESOURCES error response.
If the length of the message exceeds 1024 octets or is not a multiple of 4 octets, it is invalid. Invalid requests are handled by copying up to 1024 octets of the request into the response, setting the result code to MALFORMED_REQUEST, and zero-padding the response to a multiple of 4 octets if necessary.
The PCP server compares the IP address (from the IP header) with the field PCP Client IP Adddress. If they do not match, the error ADDRESS_MISMATCH MUST be returned. This is done to detect and prevent accidental use of PCP where a non-PCP-aware NAT exists between the PCP client and PCP server. If the PCP client wants such a mapping it needs to ensure the PCP field matches the IP address from the perspective of the PCP server.
Error responses have the same packet layout as success responses, with fields from the request copied into the response, and fields assigned by the PCP server set as indicated in Figure 3.
Copying request fields are important because this is what enables a client to identify to which request a given error response pertains. For OpCodes that are understood by the PCP server, it follows the requirements of that OpCode to copy the appropriate fields. For OpCodes that are not understood by the PCP server, it simply generates the UNSUPP_OPCODE response and copies fields from the PCP header and copies the rest of the PCP payload as-is (without attempting to interpret it).
The PCP client receives the response and verifies that the source IP address and port belong to the PCP server of an outstanding PCP request. It validates that the Opcode matches an outstanding PCP request. Responses shorter than 24 octets, longer than 1024 octets, or not a multiple of 4 octets are invalid and ignored, likely causing the request to be re-transmitted. The response is further matched by comparing fields in the response Opcode-specific data to fields in the request Opcode-specific data, as described by the processing for that Opcode. After these matches are successful, the PCP client checks the Epoch field to determine if it needs to restore its state to the PCP server (see Section 7.5).
If the PCP Client's IP Address and PCP Client's Port fields of the PCP response header do not match the source address and port of the request, it indicates the presence of a NAT between the PCP client and PCP server. If they don't match, then the PCP client (or the user on the client host) MUST ensure that an appropriate NAT mapping is created on the intervening NAT(s) (e.g., using UPnP IGD, NAT-PMP, or manual configuration), otherwise, the PCP-installed mapping will be ineffective.
If the result code is 0 (SUCCESS), the PCP client knows the request was successful.
If the result code is not 0, the request failed. If the result code is UNSUPP_VERSION, processing continues as described in Section 7.6. If the result code is NO_RESOURCES, PCP client SHOULD NOT send *any* further requests to that PCP server for the indicated error lifetime. For other error result codes, the PCP client SHOULD NOT resend the same request for the indicated error lifetime. If the PCP server indicates an error lifetime in excess of 30 minutes, the PCP client MAY choose to set its retry timer to 30 minutes.
If the PCP client has discovered a new PCP server (e.g., connected to a new network), the PCP client MAY immediately begin communicating with this PCP server, without regard to hold times from communicating with a previous PCP server.
Hosts which desire a PCP mapping might be multi-interfaced (i.e., own several logical/physical interfaces). Indeed, a host can be configured with several IPv4 addresses (e.g., WiFi and Ethernet) or dual-stacked. These IP addresses may have distinct reachability scopes (e.g., if IPv6 they might have global reachability scope as for Global Unicast Address (GUA, [RFC3587]) or limited scope as for Unique Local Address (ULA) [RFC4193]).
IPv6 addresses with global reachability (e.g., GUA) SHOULD be used as the source address when generating a PCP request. IPv6 addresses without global reachability (e.g., ULA [RFC4193]), SHOULD NOT be used as the source interface when generating a PCP request. If IPv6 privacy addresses [RFC4941] are used for PCP mappings, a new PCP request will need to be issued whenever the IPv6 privacy address is changed. This PCP request SHOULD be sent from the IPv6 privacy address itself. It is RECOMMENDED that mappings to the previous privacy address be deleted.
Due to the ubiquity of IPv4 NAT, IPv4 addresses with limited scope (e.g., private addresses [RFC1918]) MAY be used as the source interface when generating a PCP request.
As mentioned in Section 2.3, only single-homed CP routers are in scope. Therefore, there is no viable scenario where a host located behind a CP router is assigned two Global Unicast Addresses belonging to different global IPv6 prefixes.
Every PCP response sent by the PCP server includes an Epoch field. This field increments by 1 every second, and is used by the PCP client to determine if PCP state needs to be restored. If the PCP server resets or loses the state of its explicit dynamic Mappings (that is, those mappings created by PCP MAP requests), due to reboot, power failure, or any other reason, it MUST reset its Epoch time to zero or adjust its Epoch time (up or down) by at least 87840 seconds (24 hours). After resetting its Epoch time, it resumes incrementing the Epoch value by one every second. Similarly, if the public IP address(es) of the NAT (controlled by the PCP server) changes, the Epoch MUST be reset. A PCP server MAY maintain one Epoch value for all PCP clients, or MAY maintain distinct Epoch values (per PCP client, per interface, or based on other criteria); this choice is implementation-dependent.
Whenever a client receives a PCP response, the client computes an expected Epoch value based on the Epoch value in the last packet it received from that PCP server plus the time elapsed since that packet was received. If the received Epoch value is less then 7/8 (0.875%) seconds below the expected Epoch value, or is more than 3660 seconds (one hour) above or below the expected Epoch value, the PCP client concludes that the PCP server lost state. After concluding the PCP server lost state, the client immediately renews all its active port mapping leases as described in Section 12.3.1.
A PCP client sends its requests using PCP version number 1. Should later updates to this document specify different message formats with a version number greater than 1 it is expected that PCP servers will still support version 1 in addition to the newer version(s). However, in the event that a server returns a response with result code UNSUPP_VERSION, the client MAY log an error message to inform the user that it is too old to work with this server.
Should later updates to this document specify different message formats with a version number greater than 1, and backwards compatibility is desired, these first two octets can be used for forward and backward compatibility.
If future PCP versions greater than 1 are specified, version negotiation proceeds as follows:
The following Option can appear in certain PCP responses, without regard to the Opcode.
If the PCP server cannot process a mandatory-to-process Option, for whatever reason, it includes the UNPROCESSED Option in the response, shown in Figure 5. This helps with debugging interactions between the PCP client and PCP server. This Option MUST NOT appear more than once in a PCP response. The unprocessed Options are listed once, and the Option data is zero-filled to the necessary 32 bit boundary. If a certain Option appeared more than once in the PCP request, that Option value MAY appear once or as many times as it occurred in the request. The order of the Options in the PCP request has no relationship with the order of the Option values in this UNPROCESSED Option. This Option MUST NOT appear in a response unless the associated request contained at least one mandatory-to-process Option.
The UNPROCESSED Option is formatted as follows:
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=0 | Reserved | Option Length=variable | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option-code-1 | ... additional option-codes as necessary | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are four uses for the MAP and PEER Opcodes defined in this document:
These are discussed in the following sections.
When operating a server (Section 8.1 and Section 8.2) the PCP client knows if it wants an IPv4 listener, IPv6 listener, or both on the Internet. The PCP client also knows if it has an IPv4 address or IPv6 address configured on one of its interfaces. It takes the union of this knowledge to decide to which of its PCP servers to send the request (e.g., a PCP server on its IPv4 interface or its IPv6 interface), and if to send one or two MAP requests for each of its interfaces (e.g., if the PCP client has only an IPv4 address but wants both IPv6 and IPv4 listeners, it sends a MAP request containing the all-zeros IPv6 address in the Requested External Address field, and sends a second MAP request containing the all-zeros IPv4 address in the Requested External Address field. If the PCP client has both an IPv4 and IPv6 address, and only wants an IPv4 listener, it sends one MAP request from its IPv4 interface (if the PCP server supports NAT44 or IPv4 firewall) or one MAP request from its IPv6 interface (if the PCP server supports NAT64)). The PCP client can simply request the desired mapping to determine if the PCP server supports the desired mapping. Applications that embed IP addresses in payloads (e.g., FTP, SIP) will find it beneficial to avoid address family translation, if possible.
It is REQUIRED that the PCP-controlled device assign the same external IP address to PCP-created explicit dynamic mappings and to implicit dynamic mappings for a given Internal Host. In the absence of a PCP option indicating otherwise, it is REQUIRED that PCP-created explicit dynamic mappings be assigned the same external IP address. It is RECOMMENDED that static mappings for that Internal Host (e.g., those created by a command-line interface on the PCP server or PCP-controlled device) also be assigned to the same IP address. Once all internal addresses assigned to a given Internal Host have no implicit dynamic mappings and have no explicit dynamic mappings in the PCP-controlled device, a subsequent PCP request for that Internal Address MAY be assigned to a different External Address. Generally, this re-assignment would occur when a CGN device is load balancing newly-seen hosts to its public IPv4 address pool.
The following table summarizes how various common PCP deployments use IPv6 and IPv4 addresses. The 'source' is the source address of the PCP packet itself, 'internal' is the Internal IP Address field of the THIRD_PARTY Option (if present) or the same as the source address of the PCP packet iself (if the THIRD_PARTY Option is not present), 'external' is the Requested External Address field of the MAP or PEER request, the 'remote peer' is the Remote Peer IP Address of the PEER request or the FILTER option of the MAP request.
source internal external remote peer
IPv4 firewall IPv4 IPv4 IPv4 IPv4
IPv6 firewall IPv6 IPv6 IPv6 IPv6
NAT44 IPv4 IPv4 IPv4 IPv4
Dual-Stack Lite (1) IPv6 IPv4 IPv4 IPv4
Dual-Stack Lite (2) IPv4 IPv4 IPv4 IPv4
NAT64 (3) IPv6 IPv6 IPv4 IPv6
NPTv6 IPv6 IPv6 IPv6 IPv6
In (1) and (2), 'source' refers to the PCP messaging between the Dual-Stack Lite B4 element and the AFTR element, with (1) showing Dual-Stack Lite plain mode and (2) showing Dual-Stack Lite Encapsulation mode [I-D.dupont-pcp-dslite]. In a Dual-Stack Lite environment within the subscriber's network from a host to the B4 element, the PCP messaging is IPv4 firewall, IPv6 firewall, or NAT44. In (3), the IPv6 PCP client is not necessarily aware of the NAT64 or aware of the actual IPv4 address of the remote peer, so it expresses the IPv6 address from its perspective as shown in the table.
Note that PCP requests containing the MAP or PEER Opcodes cannot delete or shorten the lifetime of an existing implicit mapping for the indicated internal address and port. Conceptually implicit and explicit mappings are different "layers" in the NAT forwarding state database.
A host operating a server (e.g., a web server) listens for traffic on a port, but the server never initiates traffic from that port. For this to work across a NAT or a firewall, the host needs to (a) create a mapping from a public IP address and port to itself as described in Section 9 and (b) publish that public IP address and port via some sort of rendezvous server (e.g., DNS, a SIP message, a proprietary protocol). Publishing the public IP address and port is out of scope of this specification. To accomplish (a), the host follows the procedures described in this section.
As normal, the application needs to begin listening on a port. Then, the application constructs a PCP message with the MAP Opcode, with the external address set to the appropriate all-zeroes address, depending on whether it wants a public IPv4 or IPv6 address.
The following pseudo-code shows how PCP can be reliably used to operate a server:
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_map_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received())
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet())
process_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
A host operating a client and server on the same port (e.g., Symmetric RTP [RFC4961] or SIP Symmetric Response Routing (rport) [RFC3581]) first establishes a local listener, (usually) sends the local and public IP addresses and ports to a rendezvous service (which is out of scope of this document), and initiates an outbound connection from that same source address and same port. To accomplish this, the application uses the procedure described in this section.
An application that is using the same port for outgoing connections as well as incoming connections MUST first signal its operation of a server using the PCP MAP Opcode, as described in Section 9, and receive a positive PCP response before it sends any packets from that port.
The following pseudo-code shows how PCP can be used to operate a symmetric client and server:
/* start listening on the local server port */
int s = socket(...);
bind(s, ...);
listen(s, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_map_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
requested_lifetime, &assigned_lifetime);
if (pcp_response_received())
update_rendezvous_server("Client Ident", external_sockaddr);
if (received_incoming_connection_or_packet())
process_it(s);
if (need_to_make_outgoing_connection())
make_outgoing_connection(s, ...);
if (data_to_send())
send_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
A host operating a client (e.g., XMPP client, SIP client) sends from a port, and may receive responses, but never accepts incoming connections from other Remote Peers on this port. It wants to ensure the flow to its Remote Peer is not terminated (due to inactivity) by an on-path NAT or firewall. To accomplish this, the application uses the procedure described in this section.
Middleboxes such as NATs or firewalls need to see occasional traffic or will terminate their session state, causing application failures. To avoid this, many applications routinely generate keepalive traffic for the primary (or sole) purpose of maintaining state with such middleboxes. Applications can reduce such application keepalive traffic by using PCP.
To use PCP for this function, the application first connects to its server, as normal. Afterwards, it issues a PCP request with the PEER Opcode as described in Section 10.
The following pseudo-code shows how PCP can be reliably used with a dynamic socket, for the purposes of reducing application keepalive messages:
int s = socket(...);
connect(s, &remote_peer, ...);
getsockname(s, &internal_sockaddr, ...);
bzero(&external_sockaddr, sizeof(external_sockaddr));
while (1)
{
/* Note: the "time_to_send_pcp_request()" check below includes:
* 1. Sending the first request
* 2. Retransmitting requests due to packet loss
* 3. Resending a request due to impending lease expiration
* The PCP packet sent is identical in all cases, apart from the
* Suggested External Address and Port which may differ between
* (1), (2), and (3).
*/
if (time_to_send_pcp_request())
pcp_send_peer_request(internal_sockaddr.sin_port,
internal_sockaddr.sin_addr,
&external_sockaddr, /* will be zero the first time */
remote_peer, requested_lifetime, &assigned_lifetime);
if (data_to_send())
send_it(s);
if (other_work_to_do())
do_it();
/* ... */
block_until_we_need_to_do_something_else();
}
After a NAT loses state (e.g., because of a crash or power failure), it is useful for clients to re-establish TCP mappings on the NAT. This allows servers on the Internet to see traffic from the same IP address and port, so that sessions can be resumed exactly where they were left off. This can be useful for long-lived connections (e.g., instant messaging) or for connections transferring a lot of data (e.g., FTP). This can be accomplished by first establishing a TCP connection normally and then sending a PEER request/response and remembering the External Address and External Port. Later, when the NAT has lost state, the client can send a PEER request with the Suggested External Port and Suggested External Address remembered from the previous session, which will create a mapping in the NAT that functions exactly as an implicit dynamic mapping. The client then resumes sending TCP data to the server.
This section defines an Opcode which controls forwarding from a NAT (or firewall) to an Internal Host.
PCP Servers SHOULD provide a configuration option to allow administrators to disable MAP support if they wish.
Mappings created by PCP MAP requests are, by definition, Endpoint Independent Mappings (EIM) with Endpoint Independent Filtering (EIF) (unless the FILTER Option is used), even on a NAT that usually creates Endpoint Dependent Mappings (EDM) or Endpoint Dependent Filtering (EDF) for outgoing connections, since the purpose of an (unfiltered) MAP mapping is to receive inbound traffic from any remote endpoint, not from only one specific remote endpoint.
Note also that all NAT mappings (created by PCP or otherwise) are by necessity bidirectional and symmetric. For any packet going in one direction (in or out) that is translated by the NAT, a reply going in the opposite direction needs to have the corresponding opposite translation done so that the reply arrives at the right endpoint. This means that if a client creates a MAP mapping, and then later sends an outgoing packet using the mapping's internal source port, the NAT should translate that packet's Internal Address and Port to the mapping's External Address and Port, so that replies addressed to the External Address and Port are correctly translated to the mapping's Internal Address and Port.
On Operating Systems that allow multiple listening clients to bind to the same Internal Port, clients MUST ensure that they have exclusive use of that Internal Port (e.g., by binding the port using INADDR_ANY, or using SO_EXCLUSIVEADDRUSE or similar) before sending their MAP request, to ensure that no other clients on the same machine are also listening on the same Internal Port.
The operation of the MAP Opcode is described in this section.
The MAP Opcode has a similar packet layout for both requests and responses. If the Assigned External IP address and Assigned External Port in the PCP response always match the Internal IP Address and Port in the PCP request, then the functionality is purely a firewall; otherwise it pertains to a network address translator which might also perform firewall-like functions.
The following diagram shows the format of the Opcode-specific information in a request for the MAP Opcode.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Port | Suggested External Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Suggested External IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
The internal address for the request is the source IP address of the PCP request message itself, unless the THIRD_PARTY Option is used.
The following diagram shows the format of Opcode-specific information in a response packet for the MAP Opcode:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Port | Assigned External Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Assigned External IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
This section and Section 9.5 describe the operation of a PCP client when sending requests with the MAP Opcode.
The request MAY contain values in the Suggested External Port and Suggested External IP Address fields. This allows the PCP client to attempt to rebuild lost state on the PCP server, which improves the chances of existing connections surviving, and helps the PCP client avoid having to change information maintained at its rendezvous server. Of course, due to other activity on the network (e.g., by other users or network renumbering), the PCP server may not be able grant the suggested External IP Address and Port, and in that case it will assign a different External IP Address and Port.
If the Protocol does not use 16-bit port numbers (e.g., RSVP), the port number MUST be 0. This will cause all traffic matching that protocol to be mapped.
If the client wants all protocols mapped it uses Protocol 0 (zero) and Internal Port 0 (zero).
An existing mapping can have its lifetime extended by the PCP client. To do this, the PCP client sends a new MAP request indicating the internal port. The PCP MAP request SHOULD also include the currently assigned external IP address and port as the suggested external IP address and port, so that if the NAT gateway has lost state it can recreate the lost mapping with the same parameters.
The PCP client SHOULD renew the mapping before its expiry time, otherwise it will be removed by the PCP server (see Section 9.5). To reduce the risk of inadvertent synchronization of renewal requests, a random jitter component should be included. It is RECOMMENDED that PCP clients send a single renewal request packet at a time chosen with uniform random distribution in the range 1/2 to 5/8 of expiration time. If no SUCCESS response is received, then the next renewal request should be sent 3/4 to 3/4 + 1/16 to expiration, and then another 7/8 to 7/8 + 1/32 to expiration, and so on, subject to the constraint that renewal requests MUST NOT be sent less than four seconds apart (a PCP client MUST NOT send a flood of ever-closer-together requests in the last few seconds before a mapping expires).
The PCP client SHOULD impose an upper limit on this returned Assigned Lifetime value, and 24 hours is RECOMMENDED. This means if the PCP server returns an absurdly long Assigned Lifetime (e.g., 5 years), the PCP client will behave as if it received a more sane value (e.g., 24 hours).
This section and Section 9.5 describe the operation of a PCP server when processing a request with the MAP Opcode. Processing SHOULD be performed in the order of the following paragraphs.
The following fields from the MAP request are copied into the MAP response: Protocol, Internal Port, Requested External Address, and (if present and processed by the PCP server) the THIRD_PARTY Option.
If the requested lifetime is non-zero, it indicates a request to create a mapping or extend the lifetime of an existing mapping. If the PCP server or PCP-controlled device does not support the Protocol or cannot create a mapping for the Protocol (e.g., because the request is for a NAT mapping instead of a firewall mapping and the PCP-controlled device is not a NAT or does not support NATting that specific Protocol), it MUST generate an UNSUPP_PROTOCOL error. If the Protocol is supported and the Internal Port is zero but the protocol uses a 16 bit port number (e.g., TCP, SCTP), it generates a MALFORMED_REQUEST error. If the Protocol is supported and does not use 16 bit port number (e.g., RSVP), the Internal Port and the Requested External Port are ignored, all packets with that Protocol number are mapped to the internal host, and these ignored field values are copied into the response. If the Protocol and Internal Port are both 0 (zero), all protocols and all ports, without any exception, are mapped; if this request cannot be fulfilled, the error UNSUPP_PROTOCOL MUST be returned.
If the requested lifetime is zero, it indicates a request to delete an existing mapping or set of mappings. Processing of the lifetime is described in Section 9.5.
If the PCP-controlled device is stateless (that is, it does not establish any per-flow state, and simply rewrites the address and/or port in a purely algorithmic fashion), the PCP server simply returns an answer indicating the external IP address and port yielded by this stateless algorithmic translation. This allows the PCP client to learn its external IP address and port as seen by remote peers. Examples of stateless translators include stateless NAT64, 1:1 NAT44, and NPTv6 [RFC6296], all of which modify addresses but not port numbers.
If an Option with value less than 128 exists (i.e., mandatory to process) but that Option does not make sense (e.g., the PREFER_FAILURE Option is included in a request with lifetime=0), the request is invalid and generates a MALFORMED_OPTION error.
If a mapping already exists for the requested Internal Address and Port and the PREFER_FAILURE Option is not present, the PCP server MUST refresh the lifetime of that already-existing mapping, and return the already-existing External Address and Port in its response, regardless of the Suggested External Address and Port in the request. If a mapping already exists for the requested Internal Address and Port the request contains the PREFER_FAILURE Option, but the Suggested External Address and Port do not match the actual External Address and Port of the already existing mapping, the error CANNOT_PROVIDE_EXTERNAL_PORT is returned. If an implicit mapping already exists for the requested Internal Address and Port, the mapping SHOULD be upgraded to an explicit mapping.
If no mapping exists for the Internal Address and Port, and the PCP server is able to create a mapping using the Suggested External Address and Port, it SHOULD do so. This is beneficial for re-establishing state lost in the PCP server (e.g., due to a reboot). If the PCP server cannot assign the Suggested External Address and Port but can assign some other External Address and Port (and the request did not contain the PREFER_FAILURE Option) the PCP server MUST do so and return the newly assigned External Address and Port in the response. Cases where a NAT gateway cannot assign the Suggested External Address and Port include:
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.
Mappings typically consume state on the PCP-controlled device, and it is RECOMMENDED that a per-host and/or per-subscriber limit be enforced by the PCP server to prevent exhausting the mapping state. If this limit is exceeded, the result code USER_EX_QUOTA is returned.
If all of the preceding operations were successful (did not generate an error response), then the requested mapping is created or refreshed as described in the request and a SUCCESS response is built. This SUCCESS response contains the same Opcode as the request, but with the "R" bit set.
This section describes the operation of the PCP client when it receives a PCP response for the MAP Opcode.
After performing common PCP response processing, the response is further matched with an outstanding request by comparing the protocol, internal IP address, and internal port. Other fields are not compared, because the PCP server sets those fields.
On a success response, the PCP client can use the External IP Address and Port as desired. Typically the PCP client will communicate the External IP Address and Port to another host on the Internet using an application-specific rendezvous mechanism such as DNS SRV records.
The PCP client MUST also set a timer or otherwise schedule an event to renew the mapping before its lifetime expires. Renewing a mapping is performed by sending another MAP request, exactly as described in Section 9.2, except that the Suggested External Address and Port SHOULD be set to the values received in the response. From the PCP server's point of view a MAP request to renew a mapping is identical to a MAP request to request a new mapping, and is handled identically. Indeed, in the event of PCP server state loss, a renewal request from a PCP client will appear to the server to be a request for a new mapping, with a particular Suggested External Address and Port, which happens to be what the PCP server previously assigned. See also Section 12.3.2.
On an error response, the client SHOULD NOT repeat the same request to the same PCP server within the lifetime returned in the response.
The PCP client requests a certain lifetime, and the PCP server responds with the assigned lifetime. The PCP server MAY grant a lifetime smaller or larger than the requested lifetime. The PCP server SHOULD be configurable for permitted minimum and maximum lifetime, and the RECOMMENDED values are 120 seconds for the minimum value and 24 hours for the maximum. It is RECOMMENDED that the server be configurable to restrict lifetimes to less than 24 hours, because mappings will consume ports even if the Internal Host is no longer interested in receiving the traffic or is no longer connected to the network. These recommendations are not strict, and deployments should evaluate the trade offs to determine their own minimum and maximum lifetime values.
Once a PCP server has responded positively to a mapping request for a certain lifetime, the port mapping is active for the duration of the lifetime unless the lifetime is reduced by the PCP client (to a shorter lifetime or to zero) or until the PCP server loses its state (e.g., crashes). Mappings created by PCP MAP requests are not special or different from mappings created in other ways. In particular, it is implementation-dependent if outgoing traffic extends the lifetime of such mappings beyond the PCP-assigned lifetime. PCP clients MUST NOT depend on this behavior to keep mappings active, and MUST explicitly renew their mappings as required by the Lifetime field in PCP response messages.
If a PCP client sends a PCP MAP request to create a mapping that already exists as a static mapping, the PCP server will return a successful result, confirming that the requested mapping exists. The lifetime the PCP server returns for such a static mapping SHOULD be 4294967295 (0xFFFFFFFF). There is no need for a PCP client to renew a static mapping.
If the requested lifetime is zero then:
In requests where the requested Lifetime is 0, the Suggested External Address and Suggested External Port fields MUST be set to zero on transmission and MUST be ignored on reception, and these fields MUST be copied into the Assigned External IP Address and Assigned External Port of the response.
If the PCP client attempts to delete a single static mapping (i.e., a mapping created outside of PCP itself), the error NOT_AUTHORIZED is returned. If the PCP client attempts to delete a mapping that does not exist, the SUCCESS result code is returned (this is necessary for PCP to be idempotent). If the PCP MAP request was for port=0 (indicating 'all ports'), the PCP server deletes all of the explicit dynamic mappings it can (but not any implicit or static mappings), and returns a SUCCESS response. If the deletion request was properly formatted and successfully processed, a SUCCESS response is generated with lifetime of 0 and the server copies the protocol and internal port number from the request into the response. An explicit dynamic mapping MUST NOT have its lifetime reduced by transport protocol messages (e.g., TCP RST, TCP FIN).
An application that forgets its PCP-assigned mappings (e.g., the application or OS crashes) will request new PCP mappings. This may consume port mappings, if the application binds to a different Internal Port every time it runs. The application will also likely initiate new implicit dynamic mappings without using PCP, which will also consume port mappings. If there is a port mapping quota for the Internal Host, frequent restarts such as this may exhaust the quota. PCP provides some protections against such port consumption: When a PCP client first acquires a new IP address (e.g., reboots or joins a new network), it SHOULD remove mappings that may already be instantiated for that new Internal Address. To do this, the PCP client sends a MAP request with protocol, internal port, and lifetime set to 0. Some port mapping APIs (e.g., the "DNSServiceNATPortMappingCreate" API provided by Apple's Bonjour on Mac OS X, iOS, Windows, Linux [Bonjour]) automatically monitor for process exit (including application crashes) and automatically send port mapping deletion requests if the process that requested them goes away without explicitly relinquishing them.
To reduce unwanted traffic and data corruption, External UDP and TCP ports SHOULD NOT be re-used for an interval (TIME_WAIT interval [RFC0793]). However, the PCP server SHOULD allow the previous user of an External Port to re-acquire the same port during that interval.
As a side-effect of creating a mapping, ICMP messages associated with the mapping MUST be forwarded (and also translated, if appropriate) for the duration of the mapping's lifetime. This is done to ensure that ICMP messages can still be used by hosts, without application programmers or PCP client implementations needing to signal PCP separately to create ICMP mappings for those flows.
A customer premises router might obtain a new IP address, for a variety of reasons including a reboot, power outage, DHCP lease expiry, or other action by the ISP. If this occurs, traffic forwarded to the host's previous address might be delivered to another host which now has that address. This affects both implicit dynamic mappings and explicit dynamic mappings. However, this same problem already occurs today when a host's IP address is re-assigned, without PCP and without an ISP-operated CGN. The solution is the same as today: the problems associated with host renumbering are caused by host renumbering and are eliminated if host renumbering is avoided. PCP defined in this document does not provide machinery to reduce the host renumbering problem.
When an Internal Host changes its IP address (e.g., by having a different address assigned by the DHCP server) the NAT (or firewall) will continue to send traffic to the old IP address. Typically, the Internal Host will no longer receive traffic sent to that old IP address. Assuming the Internal Host wants to continue receiving traffic, it needs to install new mappings for its new IP address. The suggested external port field will not be fulfilled by the PCP server, in all likelihood, because it is still being forwarded to the old IP address. Thus, a mapping is likely to be assigned a new external port number and/or public IP address. Note that such host renumbering is not expected to happen routinely on a regular basis for most hosts, since most hosts renew their DHCP leases before they expire (or re-request the same address after reboot) and most DHCP servers honor such requests and grant the host the same address it was previously using before the reboot.
A host might gain or lose interfaces while existing mappings are active (e.g., Ethernet cable plugged in or removed, joining/leaving a WiFi network). Because of this, if the PCP client is sending a PCP request to maintain state in the PCP server, it SHOULD ensure those PCP requests continue to use the same interface (e.g., when refreshing mappings). If the PCP client is sending a PCP request to create new state in the PCP server, it MAY use a different source interface or different source address.
NAT-PMP [I-D.cheshire-nat-pmp] includes a mechanism to allow clients to learn the External IP Address alone, without also requesting a port mapping. In the case of PCP, this operation no longer makes sense. PCP supports Large Scale NATs (CGN) which may have a pool of External IP Addresses, not just one. A client may not be assigned any particular External IP Address from that pool until it has made at least one implicit or explicit port mapping, and even then only for as long as that implicit or explicit port mapping remains valid. Client software that just wishes to display the user's External IP Address for cosmetic purposes can achieve that by requesting a short-lived mapping and then displaying the resulting External IP Address. However, once that mapping expires a subsequent implicit or explicit dynamic mapping might be mapped to a different external IP address.
This section defines an Opcode for controlling dynamic mappings.
The use of these Opcodes is described in this section.
PCP Servers SHOULD provide a configuration option to allow administrators to disable PEER support if they wish.
Because a mapping created or managed by PEER behaves almost exactly as if an implicit dynamic mapping were created by a packet sent by the host (e.g., TCP SYN sent by the host), mappings created or managed using PCP PEER requests may be Endpoint Independent Mappings (EIM) or Endpoint Dependent Mappings (EDM), with Endpoint Independent Filtering (EIF) or Endpoint Dependent Filtering (EDF), consistent with the existing behavior of the NAT gateway or firewall in question for implicit mappings it creates automatically as a result of observing outgoing traffic from Internal Hosts.
The PEER Opcode allows the PCP client to create an implicit dynamic mapping (which functions similar to the host sending a TCP SYN), and allows the PCP client to manage an implicit dynamic mapping by extending its lifetime.
The following diagram shows the request packet format for the PEER Opcode. This packet format is aligned with the response packet format:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Port | Suggested External Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Suggested External IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Peer Port | Reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Remote Peer IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
When attempting to re-create a lost mapping, the Suggested External IP Address and Port are set to the External IP Address and Port fields received in a previous PEER response from the PCP server. On an initial PEER request, the External IP Address and Port are set to zero.
Note that the PREFER_FAILURE semantics are automatically implied by PEER requests. If the Suggested External IP Address or Suggested External Port fields are non-zero, and the PCP server is unable to honor the Suggested External IP Address or Port, then the PCP server MUST return a CANNOT_PROVIDE_EXTERNAL_PORT error response. The PREFER_FAILURE Option is neither required nor allowed in PEER requests, and if PCP server receives a PEER request containing the PREFER_FAILURE Option it MUST return a MALFORMED_REQUEST error response.
The following diagram shows the response packet format for the PEER Opcode:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Internal Port | Assigned External Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Assigned External IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Remote Peer Port | Reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Remote Peer IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This section describes the operation of a client when generating a message with the PEER Opcode.
The PEER Opcode MAY be sent before or after establishing bi-directional communication with the remote peer.
The PEER Opcode contains a Remote Peer Address field, which is always from the perspective of the PCP client. Note that when the PCP-controlled device is performing address family translation (NAT46 or NAT64), the remote peer address from the perspective of the PCP client is different from the remote peer address on the other side of the address family translation device.
This section describes the operation of a server when receiving a request with the PEER Opcode. Processing SHOULD be performed in the order of the following paragraphs.
The following fields from a PEER request are copied into the response: Protocol, Internal Port, Remote Peer IP Address, and Remote Peer Port.
When an implicit dynamic mapping is created, some NATs and firewalls validate destination addresses and will not create an implicit dynamic mapping if the destination address is invalid (e.g., 127.0.0.1). If a PCP-controlled device does such validation for implicit dynamic mappings, it SHOULD also do a similar validation of the Remote Peer IP Address and Port for PEER-created implicit dynamic mappings. If the validation determines the Remote Peer IP Address of a PEER request is invalid, then no mapping is created, and a MALFORMED_REQUEST error result is returned.
On receiving the PEER Opcode, the PCP server examines the mapping table. If the requested mapping does not yet exist, and the Suggested External Address and Port can be honored, the mapping is created. By having PEER create such a mapping, we avoid a race condition between the PEER request or the initial outgoing packet arriving at the NAT gateway first, and allow PEER to be used to recreate an implicit dynamic mapping (see last paragraph of Section 12.3.1). If the requested mapping does not yet exist, but Suggested External Address and Port cannot be honored, the error CANNOT_PROVIDE_EXTERNAL_PORT is returned. If the requested mapping already exists, it is a request to modify that existing mapping.
The PEER Opcode MAY reduce the lifetime of an existing implicit dynamic mapping created by PEER; this is implementation-dependent.
If the PCP-controlled device can extend the lifetime of a mapping, the PCP server uses the smaller of its configured maximum lifetime value and the requested lifetime from the PEER request, and sets the lifetime to that value.
If all of the preceding operations were successful (did not generate an error response), then a SUCCESS response is generated, with the Lifetime field containing the lifetime of the mapping.
After a successful PEER response is sent, it is implementation-specific if the PCP-controlled device destroys the mapping when the lifetime expires, or if the PCP-controlled device's implementation allows traffic to keep the mapping alive. Thus, if the PCP client wants the mapping to persist beyond the lifetime reported in the response, it MUST refresh the mapping (by sending another PEER message) prior to the expiration of the lifetime. If the mapping is terminated by the TCP client or server (e.g., TCP FIN or TCP RST), the mapping will be destroyed normally; the mapping will not persist for the time indicated by Lifetime. This means the Lifetime in a PEER response indicates how long the mapping will persist in the absence of a transport termination message (e.g., TCP RST).
Some transport protocols signal the end of a connection (e.g., TCP FIN, TCP RST, SCTP SHUTDOWN). After a successful PEER response is sent, the receipt of such a transport-specific message MUST NOT cause the mapping to be destroyed. Rather, the mapping is maintained until the PEER-signaled lifetime expires. If the PCP client wishes to terminate the mapping prior to this, it will send a PEER request with Lifetime set to 0, which MAY be honored by the PCP server; as stated earlier, that is implementation-dependent.
This section describes the operation of a client when processing a response with the PEER Opcode.
After performing common PCP response processing, the response is further 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 mapping created by the Opcode.
On a successful response, the application can use the assigned lifetime value to reduce its frequency of application keepalives for that particular NAT mapping. Of course, there may be other reasons, specific to the application, to use more frequent application keepalives. For example, the PCP assigned lifetime could be one hour but the application may want to maintain state on its server (e.g., "busy" / "away") more frequently than once an hour.
If the PCP client wishes to keep this mapping alive beyond the indicated lifetime, it SHOULD issue a new PCP request prior to the expiration. That is, inside->outside traffic is not sufficient to ensure the mapping will continue to exist. See Section 9.2.1 for recommended renewal timing.
This section describes Options for the MAP and PEER Opcodes. These Options MUST NOT appear with other Opcodes, unless permitted by those other Opcodes.
This Option is used when a PCP client wants to control a mapping to an Internal Host other than itself. This is used with both MAP and PEER Opcodes.
The THIRD_PARTY Option is formatted as follows:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code=1 | Reserved | Option Length=16 or 0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Internal IP Address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The fields are described below:
A THIRD_PARTY Option MUST NOT contain the same address as the source address of the packet. A PCP server receiving a THIRD_PARTY Option specifying the same address as the source address of the packet MUST return a MALFORMED_REQUEST result code. This is because many PCP servers may not implement the THIRD_PARTY Option at all, and a client using the THIRD_PARTY Option to specify the same address as the source address of the packet will cause mapping requests to fail where they would otherwise have succeeded.
A PCP server MAY be configured to permit or to prohibit the use of the THIRD_PARTY Option. If this Option is permitted, properly authorized clients may perform these operations on behalf of other hosts. If this Option is prohibited, and a PCP server receives a PCP MAP request with a THIRD_PARTY Option, it MUST generate a UNSUPP_OPTION response.
It is RECOMMENDED that customer premises equipment implementing a PCP Server be configured to prohibit third party mappings by default. With this default, if a user wants to create a third party mapping, the user needs to interact out-of-band with their customer premises router (e.g., using its HTTP administrative interface).
It is RECOMMENDED that service provider NAT and firewall devices implementing a PCP Server be configured to permit the THIRD_PARTY Option, when sent by a properly authorized host. If the packet arrives from an unauthorized host, the PCP server MUST generate an UNSUPP_OPTION error.
Determining which PCP clients are authorized to use the THIRD_PARTY Option for which other hosts is deployment-dependent. For example, an ISP using Dual-Stack Lite could choose to allow a client connecting over a given IPv6 tunnel to manage mappings for any other host connecting over the same IPv6 tunnel, or the ISP could choose to allow only the DS-Lite B4 element to manage mappings for other hosts connecting over the same IPv6 tunnel. A cryptographic authentication and authorization model is outside the scope of this specification. Note that the THIRD_PARTY Option is not needed for today's common scenario of an ISP offering a single IP address to a customer who is using NAT to share that address locally, since in this scenario all the customer's hosts appear to be a single host from the point of view of the ISP.
Where possible, it may beneficial if a client using the THIRD_PARTY Option to create and maintain mappings on behalf of some other device can take steps to verify that the other device is still present and active on the network. Otherwise the client using the THIRD_PARTY Option to maintain mappings on behalf of some other device risks maintaining those mappings forever, long after the device that required them has gone. This would defeat the purpose of PCP mappings having a finite lifetime so that they can be automatically deleted after they are no longer needed.
A PCP client can delete all PCP-created explicit dynamic mappings (i.e., those created by PCP MAP requests) that it is authorized to delete by sending a PCP MAP request including a zero-length THIRD_PARTY Option.
This Option is only used with the MAP Opcode.
This Option indicates that if the PCP server is unable to map the Suggested External Port, the PCP server should not map an external port. This differs from the behavior without this Option, which is to map a different external port.
The PREFER_FAILURE Option is formatted as follows:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code=2 | Reserved | Option Length=0 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The result code CANNOT_PROVIDE_EXTERNAL_PORT is returned if the Suggested External Port cannot be mapped. This can occur because the External Port is already mapped to another host's implicit dynamic mapping, an explicit dynamic mapping, a static mapping, or the same Internal Address and Port has an implicit dynamic mapping which is mapped to a different External Port than requested. The server MAY set the Lifetime in the response to the remaining lifetime of the conflicting mapping, rounded up to the next larger integer number of seconds.
This Option exists solely for use by UPnP IGD interworking [I-D.bpw-pcp-upnp-igd-interworking], where the semantics of UPnP IGD version 1 only allow the UPnP IGD client to dictate mapping a specific port. A PCP server MAY support this Option, if its designers wish to support downstream devices that perform UPnP IGD interworking. PCP servers MAY choose to rate-limit their handling of PREFER_FAILURE requests, to protect themselves from a rapid flurry of 65535 consecutive PREFER_FAILURE requests from clients probing to discover which external ports are available. PCP servers that are not intended to support downstream devices that perform UPnP IGD interworking are not required to support this Option. PCP clients other than UPnP IGD interworking clients SHOULD NOT use this Option because it results in inefficient operation, and they cannot safely assume that all PCP servers will implement it. It is anticipated that this Option will be deprecated in the future as more clients adopt PCP natively and the need for UPnP IGD interworking declines.
This Option is only used with the MAP Opcode.
This Option indicates that filtering incoming packets is desired. The Remote Peer Port and Remote Peer IP Address indicate the permitted remote peer's source IP address and port for packets from the Internet. The remote peer prefix length indicates the length of the remote peer's IP address that is significant; this allows a single Option to permit an entire subnet. After processing this MAP request containing the FILTER Option and generating a successful response, the PCP-controlled device will drop packets received on its public-facing interface that don't match the filter fields. After dropping the packet, if its security policy allows, the PCP-controlled device MAY also generate an ICMP error in response to the dropped packet.
The FILTER Option is formatted as follows:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code=3 | Reserved | Option Length=20 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Prefix Length | Remote Peer Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | Remote Peer IP address (128 bits) | | | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
These fields are described below:
The Prefix Length indicates how many bits of the IPv6 address or IPv4 address are used for the filter. For IPv4 addresses, which are represented using the IPv4-mapped address format (::FFFF:0:0/96), the value of the Prefix Length pertains only to to the IPv4 portion of the address. Thus, a Prefix Length of 32 with an IPv4-mapped address indicates "only this address". With IPv4-mapped addresses, the minimum Prefix length value is 0 and the maximum is 32; for IPv6 addresses the minimum value is 0 and the maximum is 128. Values outside those range cause the PCP server to return the MALFORMED_OPTION result code.
If multiple occurrences of the FILTER Option exist in the same MAP request, they are processed in the same order received (as per normal PCP Option processing) and they MAY overlap the filtering requested. If an existing mapping exists (with or without a filter) and the server receives a MAP request with FILTER, the filters indicated in the new request are added to any existing filters. If a MAP request has a lifetime of 0 and contains the FILTER Option, the error MALFORMED_OPTION is returned.
If any of occurrences of the FILTER Option in a request packet are not successfully processed then an error is returned (e.g., MALFORMED_OPTION if one of the Options was malformed) and as with other PCP errors, returning an error causes no state to be changed in the PCP server or in the PCP-controlled device.
To remove all existing filters, the Prefix Length 0 is used. There is no mechanism to remove a specific filter.
To change an existing filter, the PCP client sends a MAP request containing two FILTER Options, the first Option containing a Prefix Length of 0 (to delete all existing filters) and the second containing the new remote peer's IP address and port. Other FILTER Options in that PCP request, if any, add more allowed Remote Peers.
The PCP server or the PCP-controlled device is expected to have a limit on the number of remote peers it can support. This limit might be as small as one. If a MAP request would exceed this limit, the entire MAP request is rejected with the result code EXCESSIVE_REMOTE_PEERS, and the state on the PCP server is unchanged.
All PCP servers MUST support at least one filter per MAP mapping.
The use of the FILTER Option can be seen as a performance optimization. Since all software using PCP to receive incoming connections also has to deal with the case where may be directly connected to the Internet and receive unrestricted incoming TCP connections and UDP packets, if it wishes to restrict incoming traffic to a specific source address or group of source addresses such software already needs to check the source address of incoming traffic and reject unwanted traffic. However, the FILTER Option is a particularly useful performance optimization for battery powered wireless devices, because it can enable them to conserve battery power by not having to wake up just to reject a unwanted traffic.
This section provides non-normative guidance that may be useful to implementors.
For implicit dynamic mappings, some existing NAT devices have endpoint-independent mapping (EIM) behavior while other NAT devices have endpoint-dependent mapping (EDM) behavior. NATs which have EIM behavior do not suffer from the problem described in this section. The IETF strongly encourages EIM behavior [RFC4787][RFC5382].
In such EDM NAT devices, the same external port may be used by an implicit dynamic mapping (from the same Internal Host or from a different Internal Host) and an explicit dynamic mapping. This complicates the interaction with the MAP Opcode. With such NAT devices, there are two ways envisioned to implement the MAP Opcode:
This section provides non-normative guidance that may be useful to implementors.
No matter if a NAT is EIM or EDM, it is possible that one (or more) implicit dynamic mappings, using the same internal port on the Internal Host, might be created before or after a MAP request. When this occurs, it is important that the NAT honor the Lifetime returned in the MAP response. Specifically, if a mapping was created with the MAP Opcode, the implementation needs to ensure that termination of an implicit dynamic mapping (e.g., via a TCP FIN handshake) does not prematurely destroy the MAP-created mapping. On a NAT that implements endpoint-independent mapping with endpoint-independent filtering, this could be implemented by extending the lifetime of the implicit dynamic mapping to the lifetime of the explicit dynamic mapping.
This section provides non-normative guidance that may be useful to implementors.
If an event occurs that causes the PCP server to lose explicit dynamic mapping state (such as a crash or power outage), the mappings created by PCP are lost. Such loss of state is expected to be rare in a service provider environment (due to redundant power, disk drives for storage, etc.), but more common in a residential NAT device which does not write this information to non-volatile memory. Of course, due to outright failure of service provider equipment (e.g., software malfunction), state may still be lost.
The Epoch allows a client to deduce when a PCP server may have lost its state. When the Epoch value is observed to be smaller than expected, the PCP client can attempt to recreate the mappings following the procedures described in this section.
Further analysis of PCP failure scenarios is in [I-D.boucadair-pcp-failure].
This section provides non-normative guidance that may be useful to implementors.
A mapping renewal packet is formatted identically to an original mapping request; from the point of view of the client it is a renewal of an existing mapping, but from the point of view of a newly rebooted PCP server it appears as a new mapping request. In the normal process of routinely renewing its mappings before they expire, a PCP client will automatically recreate all its lost mappings.
When the PCP server loses state and begins processing new PCP messages, its Epoch is reset and begins counting again from zero (per the procedure of Section 7.5). As the result of receiving a packet where the Epoch field indicates that a reboot or similar loss of state has occurred, the client can renew its port mappings sooner, without waiting for the normal routine renewal time.
This section provides non-normative guidance that may be useful to implementors.
A PCP client refreshes a mapping by sending a new PCP request containing information from the earlier PCP response. The PCP server will respond indicating the new lifetime. It is possible, due to reconfiguration or failure of the PCP server, that the public IP address and/or public port, or the PCP server itself, has changed (due to a new route to a different PCP server). To detect such events more quickly, the PCP client may find it beneficial to use shorter lifetimes (so that it communicates with the PCP server more often). If the PCP client has several mappings, the Epoch value only needs to be retrieved for one of them to verify the PCP server has not lost explicit dynamic mapping state.
If the client wishes to check the PCP server's Epoch, it sends a PCP request for any one of the client's mappings. This will return the current Epoch value. In that request the PCP client could extend the mapping lifetime (by asking for more time) or maintain the current lifetime (by asking for the same number of seconds that it knows are remaining of the lifetime).
If a PCP client changes its Internal IP Address (e.g., because the Internal Host has moved to a new network), and the PCP client wishes to still receive incoming traffic, it needs create new mappings on that new network. New mappings will typically also require an update to the application-specific rendezvous server if the External Address or Port are different to the previous values (see Section 8.1 and Section 9.6).
Although SCTP has port numbers like TCP and UDP, SCTP works differently when behind an address-sharing NAT, in that SCTP port numbers are not changed [I-D.ietf-behave-sctpnat]. Because implicit dynamic SCTP mappings use the verification tag of the association instead of the local and remote peer port numbers, explicit dynamic SCTP mappings need only be established by passive listeners expecting to receive new associations at the external port.
Because an SCTP-aware NAT does not rewrite SCTP port numbers (and firewalls never do), a PCP MAP or PEER request for an SCTP mapping SHOULD provide the same Internal Port and Requested External Port. If the PCP server supports SCTP, and the requested external port cannot be provided in an explicit dynamic SCTP mapping, then the error CANNOT_PROVIDE_EXTERNAL_PORT is returned.
All PCP requests include the PCP client's IP address in the PCP header. This is used to detect address rewriting (NAT) between the PCP client and its PCP server. On operating systems that support the sockets API, the following steps are RECOMMENDED for a PCP client to insert the correct source address and port to include in the PCP header:
As with implicit dynamic mappings created by outgoing TCP packets, explicit dynamic mappings created via PCP use the source IP address of the packet as the Internal Address for the mappings. Therefore ingress filtering [RFC2827] should be used on the path between the Internal Host and the PCP Server to prevent the injection of spoofed packets onto that path.
On PCP-controlled devices that create state when a mapping is created (e.g., NAT), the PCP server SHOULD maintain per-host and/or per-subscriber quotas for mappings. It is implementation-specific whether the PCP server uses a separate quotas for implicit, explicit, and static mappings, a combined quota for all of them, or some other policy.
The goal of the PCP protocol is to improve the ability of end nodes to control their associated NAT state, and to improve the efficiency and error handling of NAT mappings when compared to existing implicit mapping mechanisms in NAT boxes and stateful firewalls. It is the security goal of the PCP protocol to limit any new denial of service opportunities, and to avoid introducing new attacks that can result in unauthorized changes to mapping state. One of the most serious consequences of unauthorized changes in mapping state is traffic theft. All mappings that could be created by a specific host using implicit mapping mechanisms are inherently considered to be authorized. Confidentiality of mappings is not a requirement, even in cases where the PCP messages may transit paths that would not be travelled by the mapped traffic.
PCP is secure against off-path attackers who cannot spoof a packet that the PCP Server will view as a packet received from the internal network.
Defending against attackers who can modify or drop packets between the internal network and the PCP server, or who can inject spoofed packets that appear to come from the internal network is out-of-scope.
A PCP Server is secure under this threat model if the PCP Server is constrained so that it does not configure any explicit mapping that it would not configure implicitly. In most cases, this means that PCP Servers running on NAT boxes or stateful firewalls that support the PEER Opcode can be secure under this threat model if all of their hosts are within a single administrative domain (or if the internal hosts can be securely partitioned into separate administrative domains, as in the DS-Lite B4 case), explicit mappings are created with the same lifetime as implicit mappings, the PCP server does not support deleting or reducing the lifetime of existing mappings, and the PCP server does not support the third party option. PCP Servers can also securely support the MAP Opcode under this threat model if the security policy on the device running the PCP Server would permit endpoint independent filtering of implicit mappings.
PCP Servers that comply with the Simple Threat Model and do not implement a PCP security mechanism described in Section 14.2 MUST enforce the constraints described in the paragraph above.
This section offers two examples of how the Simple Threat Model can be supported in real-world deployment scenarios.
Parity with many currently-deployed residential gateways can be achieved using a PCP Server that is constrained as described in Section 14.1.1 above.
A DS-Lite deployment could be secure under the Simple Threat Model, even if the B4 device makes PCP mapping requests on behalf of internal clients using the THIRD_PARTY option. In this case the DS-Lite PCP server MUST be configured to only allow the B4 device to make THIRD_PARTY requests, and only on behalf of other Internal Hosts sharing the same DS-Lite IPv6 tunnel. The B4 device MUST guard against spoofed packets being injected into the IPv6 tunnel using the B4 device's IPv4 source address, so the DS-Lite PCP Server can trust that packets received over the DS-Lite IPv6 tunnel with the B4 device's source IPv4 address do in fact originate from the B4 device. The B4 device is in a position to enforce this requirement, because it is the DS-Lite IPv6 tunnel endpoint.
Allowing the B4 device to use the THIRD_PARTY Option to create mappings for hosts reached via the IPv6 tunnel terminated by the B4 device is acceptable, because the B4 device is capable of creating these mappings implicitly and can prevent others from spoofing these mappings.
DS-Lite's security policies may also permit use of the MAP Opcode.
In the Advanced Threat Model the PCP protocol must be ensure that attackers (on- or off-path) cannot create unauthorized mappings or make unauthorized changes to existing mappings. The protocol must also limit the opportunity for on- or off-path attackers to perpetrate denial of service attacks.
The Advanced Threat Model security model will be needed in the following cases:
To protect against attacks under this threat model, a PCP security mechanism which provides an authenticated, integrity protected signaling channel would need to be specified.
PCP Servers that implement a PCP security mechanism MAY accept unauthenticated requests. PCP Servers implementing the PCP security mechanism MUST enforce the constraints described in Section 14.1 above, in their default configuration, when processing unauthenticated requests.
This section describes some threats that are not addressed in either of the above threat models, and recommends appropriate mitigation strategies.
Because of the state created in a NAT or firewall, a per-host and/or per-subscriber quota will likely exist for both implicit dynamic mappings and explicit dynamic mappings. A host might make an excessive number of implicit or explicit dynamic mappings, consuming an inordinate number of ports, causing a denial of service to other hosts. Thus, Section 13.2 recommends that hosts be limited to a reasonable number of explicit dynamic mappings.
An attacker, on the path between the PCP client and PCP server, can drop PCP requests, drop PCP responses, or spoof a PCP error, all of which will effective deny service. Through such actions, the PCP client would not be aware the PCP server might have actually processed the PCP request.
It is important to prevent a host from fraudulently creating, deleting, or refreshing a mapping (or filtering) for another host, because this can expose the other host to unwanted traffic, prevent it from receiving wanted traffic, or consume the other host's mapping quota. Both implicit and explicit dynamic mappings are created based on the source IP address in the packet, and hence depend on ingress filtering to guard against spoof source IP addresses.
In the time between when a PCP server loses state and the PCP client notices the lower than expected Epoch value, it is possible that the PCP client's mapping will be acquired by another host (via an explicit dynamic mapping or implicit dynamic mapping). This means incoming traffic will be sent to a different host ("theft"). A mechanism to immediately inform the PCP client of state loss would reduce this interval, but would not eliminate this threat. The PCP client can reduce this interval by using a relatively short lifetime; however, this increases the amount of PCP chatter. This threat is reduced by using persistent storage of explicit dynamic mappings in the PCP server (so it does not lose explicit dynamic mapping state), or by ensuring the previous external IP address and port cannot be used by another host (e.g., by using a different IP address pool).
This document does not specify server discovery, beyond contacting the default gateway.
IANA is requested to perform the following actions:
PCP will use port 5351 (currently assigned by IANA to NAT-PMP [I-D.cheshire-nat-pmp]). We request that IANA re-assign that same port number to PCP, and relinquish UDP port 44323.
[Note to RFC Editor: Please remove the text about relinquishing port 44323 prior to publication.]
IANA shall create a new protocol registry for PCP Opcodes, numbered 0-127, initially populated with the values:
value Opcode ----- ------------------------- 0 Reserved 1 MAP 2 PEER 3-95 (specification required) 96-126 (private use) 127 Reserved
The values 0 and 127 are Reserved and may be assigned via Standards Action [RFC5226]. The values in the range 3-95 can be assigned via Specification Required [RFC5226], and the range 96-126 is for Private Use [RFC5226].
IANA shall create a new registry for PCP result codes, numbered 0-255, initially populated with the result codes from Section 6.4. The value 255 is Reserved and may be assigned via Standards Action [RFC5226].
Result Codes in the range 13-191 can be assigned via Specification Required [RFC5226], and the range 192-254 is for Private Use [RFC5226].
IANA shall create a new registry for PCP Options, numbered 0-255 with an associated mnemonic. The values 0-127 are mandatory-to-process, and 128-255 are optional to process. The initial registry contains the Options described in Section 7.7.1 and Section 11. The Option values 127 and 255 are Reserved and may be assigned via Standards Action [RFC5226].
Additional PCP Option codes in the ranges 4-63 and 128-191 can be created via Specification Required [RFC5226], and the ranges 64-126 and 192-254 are for Private Use [RFC5226].
Thanks to Xiaohong Deng, Alain Durand, Christian Jacquenet, Jacni Qin, Simon Perreault, James Yu, Tina TSOU (Ting ZOU), Felipe Miranda Costa, and James Woodyatt for their comments and review. Thanks to Simon Perreault for highlighting the interaction of dynamic connections with PCP-created mappings.
Thanks to Francis Dupont for his several thorough reviews of the specification, which improved the protocol significantly.
Thanks to Margaret Wasserman for writing the Security Considerations section.
The Port Control Protocol (PCP) is a successor to the NAT Port Mapping Protocol, NAT-PMP [I-D.cheshire-nat-pmp], and shares similar semantics, concepts, and packet formats. Because of this NAT-PMP and PCP both use the same port, and use NAT-PMP and PCP's version negotiation capabilities to determine which version to use. This section describes how an orderly transition may be achieved.
A client supporting both NAT-PMP and PCP SHOULD send its request using the PCP packet format. This will be received by a NAT-PMP server or a PCP server. If received by a NAT-PMP server, the response will be as indicated by the NAT-PMP specification [I-D.cheshire-nat-pmp], which will cause the client to downgrade to NAT-PMP and re-send its request in NAT-PMP format. If received by a PCP server, the response will be as described by this document and processing continues as expected.
A PCP server supporting both NAT-PMP and PCP can handle requests in either format. The first octet of the packet indicates if it is NAT-PMP (first octet zero) or PCP (first octet non-zero).
A PCP-only gateway receiving a NAT-PMP request (identified by the first octet being zero) will interpret the request as a version mismatch. Normal PCP processing will emit a PCP response that is compatible with NAT-PMP, without any special handling by the PCP server.
[Note to RFC Editor: Please remove this section prior to publication.]