PCP working group D. Wing, Ed. Internet-Draft Cisco Intended status: Standards Track February 7, 2011 Expires: August 11, 2011 Port Control Protocol (PCP) draft-ietf-pcp-base-04 Abstract Port Control Protocol allows a host to control how incoming IPv6 or IPv4 packets are translated and forwarded by a network address translator (NAT) or simple firewall to an IPv6 or IPv4 host, and also allows a host to optimize its NAT keepalive messages. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on August 11, 2011. Copyright Notice Wing, et al. Expires August 11, 2011 [Page 1] Internet-Draft Port Control Protocol (PCP) February 2011 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Deployment Scenarios . . . . . . . . . . . . . . . . . . . 4 2.2. Supported Transport Protocols . . . . . . . . . . . . . . 5 2.3. Single-homed Customer Premises Network . . . . . . . . . . 5 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Relationship of PCP Server and its NAT . . . . . . . . . . . . 8 5. Common Request and Response Header Format . . . . . . . . . . 8 5.1. Request Header . . . . . . . . . . . . . . . . . . . . . . 9 5.2. Response Header . . . . . . . . . . . . . . . . . . . . . 10 5.3. Options . . . . . . . . . . . . . . . . . . . . . . . . . 11 5.4. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 13 6. General PCP Operation . . . . . . . . . . . . . . . . . . . . 14 6.1. General PCP Client Operation . . . . . . . . . . . . . . . 14 6.1.1. Generating and Sending a Request . . . . . . . . . . . 14 6.1.2. Processing a Response . . . . . . . . . . . . . . . . 15 6.1.3. Multi-interface Issues . . . . . . . . . . . . . . . . 16 6.1.4. Epoch . . . . . . . . . . . . . . . . . . . . . . . . 17 6.2. General PCP Server Operation . . . . . . . . . . . . . . . 17 6.3. General PCP Options . . . . . . . . . . . . . . . . . . . 18 6.3.1. UNPROCESSED . . . . . . . . . . . . . . . . . . . . . 18 7. Introduction to MAP and PEER OpCodes . . . . . . . . . . . . . 19 7.1. For Operating a Server . . . . . . . . . . . . . . . . . . 20 7.2. For Reducing NAT Keepalive Messages . . . . . . . . . . . 20 7.3. For Operating a Symmetric Client/Server . . . . . . . . . 21 8. MAP OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 23 8.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 23 8.2. OpCode-Specific Result Codes . . . . . . . . . . . . . . . 25 8.3. OpCode-Specific Client Operation, Processing a Response . 26 8.4. OpCode-Specific Server Operation . . . . . . . . . . . . . 27 8.4.1. Maintaining Same External IP Address . . . . . . . . . 29 8.4.2. Mapping Lifetime . . . . . . . . . . . . . . . . . . . 29 Wing, et al. Expires August 11, 2011 [Page 2] Internet-Draft Port Control Protocol (PCP) February 2011 8.4.3. Mapping Deletion . . . . . . . . . . . . . . . . . . . 30 8.4.4. Subscriber Renumbering . . . . . . . . . . . . . . . . 30 8.5. PCP Options for MAP OpCodes . . . . . . . . . . . . . . . 31 8.5.1. THIRD_PARTY . . . . . . . . . . . . . . . . . . . . . 31 8.5.2. REMOTE_PEER_FILTER . . . . . . . . . . . . . . . . . . 32 8.5.3. PREFER_FAILURE . . . . . . . . . . . . . . . . . . . . 33 8.6. PCP Mapping State Maintenance . . . . . . . . . . . . . . 34 8.6.1. Recreating Mappings . . . . . . . . . . . . . . . . . 34 8.6.2. Maintaining Mappings . . . . . . . . . . . . . . . . . 35 9. PEER OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 36 9.1. OpCode Packet Formats . . . . . . . . . . . . . . . . . . 36 9.2. OpCode-Specific Result Codes . . . . . . . . . . . . . . . 39 9.3. OpCode-Specific Client Operation, Processing a Response . 39 9.4. OpCode-Specific Server Operation . . . . . . . . . . . . . 40 10. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 40 10.1. Dual Stack-Lite . . . . . . . . . . . . . . . . . . . . . 40 10.1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 41 10.1.2. Encapsulation Mode . . . . . . . . . . . . . . . . . . 41 10.1.3. Plain IPv6 Mode . . . . . . . . . . . . . . . . . . . 41 10.2. NAT64 . . . . . . . . . . . . . . . . . . . . . . . . . . 42 10.3. NAT44 and NAT444 . . . . . . . . . . . . . . . . . . . . . 42 10.4. IPv6 Firewall . . . . . . . . . . . . . . . . . . . . . . 42 10.5. Subscriber Identification . . . . . . . . . . . . . . . . 42 11. Interworking with UPnP IGD 1.0 and 2.0 . . . . . . . . . . . . 44 11.1. UPnP IGD 1.0 with AddPortMapping Action . . . . . . . . . 44 11.2. UPnP IGD 2.0 with AddAnyPortMapping Action . . . . . . . . 47 11.3. Lifetime Maintenance . . . . . . . . . . . . . . . . . . . 47 12. Security Considerations . . . . . . . . . . . . . . . . . . . 47 13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 48 13.1. Port Number . . . . . . . . . . . . . . . . . . . . . . . 48 13.2. OpCodes . . . . . . . . . . . . . . . . . . . . . . . . . 48 13.3. Result Codes . . . . . . . . . . . . . . . . . . . . . . . 48 13.4. Options . . . . . . . . . . . . . . . . . . . . . . . . . 48 14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 49 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 49 15.1. Normative References . . . . . . . . . . . . . . . . . . . 49 15.2. Informative References . . . . . . . . . . . . . . . . . . 50 Appendix A. Changes . . . . . . . . . . . . . . . . . . . . . . . 51 A.1. Changes from draft-ietf-pcp-base-03 to -04 . . . . . . . . 51 A.2. Changes from draft-ietf-pcp-base-02 to -03 . . . . . . . . 52 A.3. Changes from draft-ietf-pcp-base-01 to -02 . . . . . . . . 53 A.4. Changes from draft-ietf-pcp-base-00 to -01 . . . . . . . . 53 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 54 Wing, et al. Expires August 11, 2011 [Page 3] Internet-Draft Port Control Protocol (PCP) February 2011 1. Introduction Port Control Protocol (PCP) provides a mechanism to control how incoming packets are forwarded by upstream devices such as NAT64, NAT44, and firewall devices, and a mechanism to reduce application keepalive traffic. PCP is primarily designed to be implemented in the context of both Carrier-Grade NATs (CGN) and small NATs (e.g., residential NATs). PCP allows hosts to operate server for a long time (e.g., a webcam) or a short time (e.g., while playing a game or on a phone call) when behind a NAT device, including when behind a CGN operated by their Internet service provider. PCP allows applications to create mappings from an external IP address and port to an internal IP address and port. If the PCP- controlled device is a NAT, a mapping is created; if the PCP- controlled device is a firewall, a mapping is created in the firewall. These mappings are required for successful inbound communications destined to machines located behind a NAT. After creating a mapping for incoming connections, it is necessary to inform remote computers about the IP address and port for the incoming connection. This is usually done in an application-specific manner. For example, a computer game would use a rendezvous server specific to that game (or specific to that game developer), and a SIP phone would use a SIP proxy. PCP does not provide this rendezvous function. Many NAT-friendly applications send frequent application-level messages to ensure their session will not be timed out by a NAT. These are commonly called "NAT keepalive" messages, even though they are not sent to the NAT itself (rather, they are sent 'through' the NAT). These applications can reduce the frequency of those NAT keepalive messages by using PCP to learn (or control) the NAT mapping lifetime. This helps reduce bandwidth on the subscriber's access network, traffic to the server, and battery consumption on mobile devices. 2. Scope 2.1. Deployment Scenarios PCP can be used in various deployment scenarios, including: o Dual Stack-Lite [I-D.ietf-softwire-dual-stack-lite], and; o NAT64, both Stateful [I-D.ietf-behave-v6v4-xlate-stateful] and Stateless [I-D.ietf-behave-v6v4-xlate], and; Wing, et al. Expires August 11, 2011 [Page 4] Internet-Draft Port Control Protocol (PCP) February 2011 o Carrier-Grade NAT [I-D.ietf-behave-lsn-requirements], and; o Basic NAT [RFC3022], and; o Network Address and Port Translation (NAPT) [RFC3022], such as commonly deployed in residential NAT devices, and; o Layer-2 aware NAT [I-D.miles-behave-l2nat] and Dual-Stack Extra Lite [I-D.arkko-dual-stack-extra-lite], and; o IPv6 firewall control [RFC6092]. 2.2. Supported Transport Protocols The PCP OpCodes defined in this document are designed to support transport protocols that use a 16-bit port number (e.g., TCP, UDP, SCTP, DCCP). Transport protocols that do not use a port number (e.g., IPsec ESP), and the ability to use PCP to forward all traffic to a single default host (often nicknamed "DMZ"), are beyond the scope of this document. 2.3. Single-homed Customer Premises Network The PCP machinery assumes a single-homed host model. That is, for a given IP version, only one default route exists to reach the Internet. This is important because after a PCP mapping is created and an inbound packet (e.g., TCP SYN) arrives at the host the outbound response (e.g., TCP SYNACK) has to go through the same path so the proper address rewriting takes place on that outbound response packet. This restriction exists because otherwise there would need to be one PCP server for each egress, because the host could not reliably determine which egress path packets would take, so the client would need to be able to reliably make the same internal/ external mapping in every NAT gateway, which in general is not possible. 3. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. Internal Host: A host served by a NAT gateway, or protected by a firewall. Wing, et al. Expires August 11, 2011 [Page 5] Internet-Draft Port Control Protocol (PCP) February 2011 Remote Host: A host with which an Internal Host is communicating. Internal Address, External Address, Remote Address: An Internal Address is the address of an Internal Host served by a NAT gateway (typically a private address [RFC1918]) or an Internal Host protected by a firewall. An External Address is the address of an Internal Host as seen by other Remote Hosts on the Internet with which the Internal Host is communicating, after translation by any NAT gateways on the path. An External Address is generally a public routable (i.e. non- private) address. In the case of an Internal Host protected by a pure firewall, with no address translation on the path, its External Address is the same as its Internal Address. A Remote Address is the address of a Remote Host, as seen by the Internal Host. A Remote Address is generally a public routable address. In the case of a Remote Host that is itself served by a NAT gateway, the Remote Address my in fact be the Remote Host's External Address, but since this is remote translation generally invisible to software running on the Internal Host, the distinction can safely be ignored for the purposes of this document. Target Address: In the common case, an Internal Host manages its own Mappings using PCP requests, and the Internal Address of those Mappings is the same as the source IP address of the PCP request packet. In the case where one device is managing Mappings on behalf of some other device which does not implement PCP, the presence of a Target Address option in the PCP request signifies that the specified Target Address, not the source IP address of the PCP request packet, should be used as the Internal Address for the Mapping. When PCP requests are used with a Target Address option, appropriate security measures MUST be used to ensure that the device creating, renewing, or deleting mappings is authorized to do so on behalf of the given Target Address. Mapping: A NAT mapping creates a relationship between an internal IP transport address and an external IP transport address. More specifically, it creates a translation rule where packets destined to the external IP and port are translated to the internal IP and port. In the case of a pure firewall, the "Mapping" is the identity function, translating an internal port number to the same external port number and vice versa, and this "Mapping" indicates Wing, et al. Expires August 11, 2011 [Page 6] Internet-Draft Port Control Protocol (PCP) February 2011 to the firewall that traffic to and from this internal port number is permitted to pass. See also Port Forwarding. Mapping Types: There are three different ways to create mappings: implicit dynamic mappings, explicit dynamic mappings, and static mappings. Implicit dynamic mappings are created as a result of a TCP SYN or outgoing UDP packet. Explicit dynamic mappings are created as a result of PCP requests. Both implicit and explicit dynamic mappings are dynamic in the sense that they are created on demand, as requested (implicitly or explicitly) by the Internal Host, and have a lifetime, after which they are automatically deleted unless the lifetime is extended by action by the Internal Host. Static mappings are created by manual configuration (e.g., command language interface or web page) and differ from dynamic mappings in that their lifetime is typically infinite (they exist until manually removed) but otherwise they behave exactly the same as dynamic mappings. E.g. a PCP request to create a mapping that already exists as a static mapping will return a successful result, confirming that the requested mapping exists. Port Forwarding, Port Mapping: Port forwarding (or port mapping) allows a host to receive traffic sent to a specific IP address and port. In the context of a NAT or NAPT, the Internal Address and External Address are different. In the context of a pure firewall, the Internal Address and External Address are the same. In both contexts, if an internal host is listening to connections on a specific port (that is, operating as a server), the external IP address and port number need to be port forwarded to the internal IP address and port number, which may be the same, in the case of a pure firewall. In the context of a NAPT, it is possible that both the IP address and port are modified. For example with a NAPT, a webcam might be listening on port 80 on its internal address 192.168.1.1, while its publicly-accessible external address is 192.0.2.1 and port is 12345. The NAT does 'port forwarding' of one to the other. PCP Client: A PCP software instance responsible for issuing PCP requests to a PCP server. One or several PCP Clients can be embedded in the same host. Several PCP Clients can be located in the same local network of a given subscriber. A PCP Client can issue PCP request on behalf of a third party device of the same subscriber. An interworking function, from UPnP IGD to PCP, or from NAT-PMP [I-D.cheshire-nat-pmp] is another example of a PCP Client. A PCP server in a NAT gateway that is itself a client of another NAT Wing, et al. Expires August 11, 2011 [Page 7] Internet-Draft Port Control Protocol (PCP) February 2011 gateway (nested NAT) may itself act as a PCP client to the upstream NAT. PCP Server: A network element which receives and processes PCP requests from a PCP client. See also Section 4. Interworking Function: a functional element responsible for interworking another protocol with PCP. For example interworking between UPnP IGD [IGD] with PCP or NAT-PMP [I-D.cheshire-nat-pmp] and PCP. subscriber: an entity provided access to the network. In the case of a commercial ISP, this is typically a single home. host: a device which can have packets sent to it, as a result of PCP operations. A host is not necessarily a PCP client. 5-tuple The 5 pieces of information that fully identify a flow: source IP address, destination IP address, protocol, source port number, destination port number. 4. Relationship of PCP Server and its NAT The PCP server receives PCP requests. The PCP server might be integrated within the NAT or firewall device (as shown in Figure 1) which is expected to be a common deployment. +-----------------+ +------------+ | NAT or firewall | | PCP client |--+ with +--- +------------+ | PCP server | +-----------------+ Figure 1: NAT or Firewall with Embedded PCP Server It is also possible to operate the PCP server in a separate device from the NAT, so long as such operation is indistinguishable from the PCP client's perspective. 5. Common Request and Response Header Format All PCP messages contain a request (or response) header containing an opcode, any relevant opcode-specific information, and zero or more Wing, et al. Expires August 11, 2011 [Page 8] Internet-Draft Port Control Protocol (PCP) February 2011 options. The packet layout for the common header, and operation of the PCP client and PCP server are described in the following sections. The information in this section applies to all OpCodes. Behavior of the OpCodes defined in this document is described in Section 8 and Section 9. 5.1. Request Header All requests have the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|Ver=0|R| OpCode | | +-+-+-+-+-+-+-+-+-+-+-+-+ | | Reserved (84 bits) | | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) opcode-specific information : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) PCP Options : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Common Request Packet Format These fields are described below: 1 A single bit set to 1. This allows DTLS and non-DTLS to be multiplexed on same port, should a future update to this specification add DTLS support. Ver: This document specifies protocol version 0. Should later updates to this document specify different message formats with a version number greater than zero, the first two bytes of those new message formats will still contain the version number and opcode as shown here, so that a PCP server receiving a message format newer or older than the version(s) it understands can still parse enough of the message to correctly identify the version number, and determine whether the problem is that this server is too old and needs to be updated to work with the PCP client, or whether the PCP client is too old and needs to be updated to work with this server. Wing, et al. Expires August 11, 2011 [Page 9] Internet-Draft Port Control Protocol (PCP) February 2011 R: Indicates Request (0) or Response (1). All Requests MUST use 0. OpCode: Opcodes are defined in Section 8 and Section 9. New OpCodes can be defined per rules described in Section 13. Reserved: 84 reserved bits, MUST be sent as 0 and MUST be ignored when received. 5.2. Response Header All responses have the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1|Ver=0|R| Opcode | Reserved | Result Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Epoch | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : (optional) OpCode-specific response data : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : (optional) Options : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Common Response Packet Format These fields are described below: Ver: This document specifies protocol version 0. Should later updates to this document specify different message formats with a version number greater than zero, the first four bytes of those new message formats will still contain the version number, opcode, and result code, as shown here, so that a PCP client receiving a message format newer or older than the version(s) it understands can still parse enough of the message to correctly identify the version number, and determine whether the problem is that this client is too old and needs to be updated to work with the PCP server, or whether the PCP server is too old and needs to be updated to work with this client. R: Indicates Request (0) or Response (1). All Responses MUST use 1. Wing, et al. Expires August 11, 2011 [Page 10] Internet-Draft Port Control Protocol (PCP) February 2011 OpCode: The OpCode value from the request. Reserved: 12 reserved bits, MUST be sent as 0, MUST be ignored when received. Result Code: The result code for this response. See Section 5.4 for values. Lifetime: The value is in seconds. On a success response this indicates the lifetime of the successful mapping. If a client wishes to maintain its mapping beyond this lifetime it MUST renew the mapping *before* it expires (typically halfway to expiry, analogous to how clients renew a DHCP lease). On an error response, this indicates how long clients should assume they'll get the same error response from the PCP server if they repeat the same request. Clients SHOULD NOT repeat the same request to the same PCP server within the lifetime given in the error response. In the case of the SERVER_OVERLOADED error response, clients SHOULD NOT send *any* further requests to the that PCP server within the lifetime given in the SERVER_OVERLOADED error response. Epoch: The server's Epoch value. See Section 6.1.4 for discussion. 5.3. Options A PCP OpCode can be extended with an Option. Options can be used in requests and responses. It is anticipated that Options will include information which are associated with the normal function of an OpCode. For example, an Option could indicate DSCP [RFC2474] markings to apply to incoming or outgoing traffic associated with a PCP mapping, or an Open could include descriptive text (e.g., "for my webcam"). Options use the following Type-Length-Value format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Code | Reserved | Option-Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : (optional) data : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Options Header The description of the fields is as follows: Wing, et al. Expires August 11, 2011 [Page 11] Internet-Draft Port Control Protocol (PCP) February 2011 Option Code: Option code, 8 bits. The first bit of the option code is the "P" bit, described below. Option codes MUST be registered with IANA following the procedure described in Section 13. Reserved: MUST be set to 0 on transmission and MUST be ignored on reception. Option-Length: Indicates in units of 4 octets the length of the enclosed data. Options with length of 0 are allowed. data: Option data. The option data MUST end on a 32 bit boundary, padded with 0's when necessary. A given Option MAY be included in a request or a response, as permitted by that Option. If a given Option was included in a request, and understood and processed by the PCP server, it MUST be included in the response. The handling of an Option by the PCP client and PCP server MUST be specified in an appropriate document and must include whether the PCP Option can appear (one or more times) in a request, and indicate the contents of the Option in the request and in the response. If several Options are included in a PCP request or response, they can be encoded in any order by the PCP client. The response MUST encode them in the same order, but may omit some PCP Options in the response (e.g., omitting them is necessary to indicate the PCP server does not understand that Option, or simply because that Option is not included in responses), and additional options (if any) are included at the end. A single Option MAY appear more than once, if permitted by the definition of the Option itself. If the Option's definition allows the Option to appear once but it appears more than once, the PCP server MUST respond with the MALFORMED_OPTION response code. If the "P" bit in the OpCode is set, o the PCP server MUST only generate a positive PCP response if it can successfully process the PCP request and this Option. o if the PCP server does not implement this Option, or cannot perform the function indicated by this Option (e.g., due to a parsing error with the option), it MUST generate a failure response and include the UNPROCESSED option in the response (Section 6.3.1). If the "P" bit is clear, the PCP server MAY process or ignore this Option. To enhance interoperability, newly defined Options should avoid interdependencies with each other. Wing, et al. Expires August 11, 2011 [Page 12] Internet-Draft Port Control Protocol (PCP) February 2011 New Options MUST include the information below: This Option: name: number: purpose: is valid for OpCodes: has length: may appear in requests or responses: may appear more than once: 5.4. Result Codes The following result codes may be returned as a result of any OpCode received by the PCP server. The only success result code is 0, other values indicate an error. 0 SUCCESS, success 1 UNSUPP_VERSION, unsupported version 2 UNSUPP_OPCODE, unsupported OpCode 3 UNSUPP_OPTION, unsupported Option 4 MALFORMED_OPTION, malformed Option (e.g., exists too many times, invalid length) 5 UNSPECIFIED_ERROR, server encountered unspecified error 6 MALFORMED_REQUEST 7 SERVER_OVERLOADED. Client should wait for at least the time specified in the "Lifetime" field before sending any further PCP requests to this server. Additional result codes, specific to the OpCodes defined in this document, are listed in Section 8.2 and Section 9.2 . Wing, et al. Expires August 11, 2011 [Page 13] Internet-Draft Port Control Protocol (PCP) February 2011 6. General PCP Operation PCP messages MUST be sent over UDP, and the PCP server MUST listen for PCP requests on the PCP port number (Section 13.1). Every PCP request generates a response, so PCP does not need to run over a reliable transport protocol. PCP is idempotent, so if the PCP client sends the same request multiple times and the PCP server processes those requests, the same result occurs. The order of operation is that a PCP client generates and sends a request to the PCP server which processes the request and generates a response back to the PCP client. 6.1. General PCP Client Operation This section details operation specific to a PCP client, for any OpCode. Procedures specific to the MAP OpCodes are described in Section 8, and procedures specific to the PEER OpCodes are described in Section 9. 6.1.1. Generating and Sending a Request Prior to sending its first PCP message, the PCP client determines which servers to use. The PCP client tries the following to get a list of PCP servers: 1. if a PCP server is configured (e.g., in a configuration file), the address(es) of the PCP server(s) are used as the list of PCP server(s), else; 2. if DHCP indicates the PCP server(s), the address(es) of the indicated PCP server(s) are used as the list of PCP server(s), else; 3. the address of the default router is used as the PCP server. With that list of PCP servers, the PCP client formulates its PCP request. The PCP request contains a PCP common header, PCP OpCode and payload, and (possibly) Options. It initializes a retransmission timer to 4 seconds. (As with all UDP or TCP clients on any operating system, when several PCP clients are embedded in the same host, each uses a distinct source port number to disambiguate their requests and replies.) The PCP client sends a PCP message to each server in sequence, waiting for a response until its timer expires. Once a PCP client has successfully communicated with a PCP server, it continues communicating with that PCP server until that PCP server becomes non- responsive, which causes the PCP client to attempt to re-iterate the procedure starting with the first PCP server on its list. If a hard Wing, et al. Expires August 11, 2011 [Page 14] Internet-Draft Port Control Protocol (PCP) February 2011 ICMP error is received the PCP client SHOULD immediately abort trying to contact that PCP server (see Section 2 of [RFC5461] for discussion of ICMP and ICMPv6 hard errors). If no response is received from any of those servers, it doubles its retransmission timer and tries each server again. This is repeated 4 times (for a total of 5 transmissions to each server). If, after these transmissions, the PCP client has still not received a response, the PCP client SHOULD abort the procedure. [Ed. Note: We need to define precisely what we mean by "non- responsive". No response after some number of retransmissions? How many? No response within what time period? -- SC] Upon receiving a response (success or error), the PCP client does not change to a different PCP server. That is, it does not "shop around" trying to find a PCP server to service its (same) request. 6.1.2. Processing a Response The PCP client receives the response and verifies the source IP address and port belong to the PCP server of an outstanding request. It validates the version number and OpCode matches an outstanding request. Responses shorter than 8 octets, longer than 1024 octets, or not a multiple of 4 octets are invalid and ignored. The response is further matched by comparing fields in the response OpCode- specific data to fields in the request OpCode-specific data. After a successful match with an outstanding request, the PCP client checks the Epoch field to determine if it needs to restore its state to the PCP server (see Section 6.1.4). If the response code is 0, the PCP client knows the request was successful. If the response code is not 0, the request failed. The PCP client MAY resend the same message but MUST first wait until the smaller of 30 minutes or the value of the Lifetime field. If the PCP client has re-discovered a new PCP server (e.g., connected to a new network), the PCP client is not restricted from communicating immediately with its new PCP server. A PCP client sends its requests using PCP version number 0. Should later updates to this document specify different message formats with a version number greater than zero it is expected that PCP servers will still support version 0 in addition to the newer version(s). However, in the event that a server returns a response with error code UNSUPP_VERSION, the client MAY log an error message to inform the user that it is too old to work with this server, and the client SHOULD set a timer to retry its request in 30 minutes (in case this Wing, et al. Expires August 11, 2011 [Page 15] Internet-Draft Port Control Protocol (PCP) February 2011 was a temorary condition and the server configuration is changed to rectify the situation). If future PCP versions greater than zero are specified, version negotiation is expected to proceed as follows: 1. If a client or server supports more than one version it SHOULD support a contiguous range of versions -- i.e. a lowest version and a highest version and all versions in between. 2. Client sends first request using highest (i.e. presumably 'best') version number it supports. 3. If server supports that version it responds normally. 4. If server does not support that version it replies giving a response containing the error code UNSUPP_VERSION, and the closest version number it does support (if the server supports a range of versions higher than the client's requested version, the server returns the lowest of that supported range; if the server supports a range of versions lower than the client's requested version, the server returns the highest of that supported range). 5. If the client receives an UNSUPP_VERSION response containing a version it does support, it records this fact and proceeds to use this message version for subsequent communication with this PCP server (until a possible future UNSUPP_VERSION response if the server is later updated, at which point the version negotiation process repeats). 6. If the client receives an UNSUPP_VERSION response containing a version it does not support then the client MAY log an error message to inform the user that it is too old to work with this server, and the client SHOULD set a timer to retry its request in 30 minutes. 6.1.3. Multi-interface Issues Hosts which desire a PCP mapping might be multi-interfaced (i.e., own several logical/physical interfaces). Indeed, a host can be dual- stack or be configured with several IP addresses. These IP addresses may have distinct reachability scopes (e.g., if IPv6 they might have global reachability scope as for GUA (Global Unicast Address) or limited scope such as ULA (Unique Local Address, [RFC4193])). IPv6 addresses with global reachability scope SHOULD be used as internal IP address when requesting a PCP mapping in a PCP-controlled device. IPv6 addresses with limited scope (e.g., ULA), SHOULD NOT be Wing, et al. Expires August 11, 2011 [Page 16] Internet-Draft Port Control Protocol (PCP) February 2011 indicated as internal IP address in a PCP message. As mentioned in Section 2.3, only single-homed CP routers are in scope. Therefore, there is no viable scenario where a host located behind a CP router is assigned with two GUA addresses belonging to the same global IPv6 prefix. [Ed. Note: text regarding multi-homing needs work.] 6.1.4. Epoch Every PCP response sent by the PCP server includes an Epoch field. This field increments by 1 every second, and indicates to the PCP client if PCP state needs to be restored. If the PCP server resets or loses the state of its Mappings, due to reboot, power failure, or any other reason, it MUST reset its Epoch time to 0. Similarly, if the public IP address(es) of the NAT (controlled by the PCP server) changes, the Epoch MUST be reset to 0. A PCP server MAY maintain one Epoch value for all PCP clients, or MAY maintain distinct Epoch values for each PCP client; this choice is implementation-dependent. Whenever a client receives a PCP response, the client computes its own conservative estimate of the expected Epoch value by taking the Epoch value in the last packet it received from the gateway and adding 7/8 (87.5%) of the time elapsed since that packet was received. If the Epoch value in the newly received packet is less than the client's conservative estimate by more than one second, then the client concludes that the PCP server lost state, and the client MUST immediately renew all its active port mapping leases as described in Section 8.6.1. [Ed. Note: comment from Dave Thaler: "So spoofed packets with Epoch=0 that look like they come from the server could result in a big amplification attack on the PCP server. How is this mitigated?". This is tracked as PCP Issue #21, [PCP-Issues].] 6.2. General PCP Server Operation This section details operation specific to a PCP server. Upon receiving a PCP request message, the PCP server parses and validates it. A valid request contains a valid PCP common header, one valid PCP Opcode, and zero or more Options (which the server might or might not comprehend). If an error is encountered during processing, the server generates an error response which is sent back to the PCP client. Processing an OpCode and the Options are specific to each OpCode. Wing, et al. Expires August 11, 2011 [Page 17] Internet-Draft Port Control Protocol (PCP) February 2011 If the received message is shorter than 4 octets, has the R bit set, or the first bit is clear, the request is simply dropped. If the version number is not supported, a response is generated containing the UNSUPP_VERSION response code and the protocol version which the server does understand (if the server understands a range of protocol versions then it returns the supported version closest to the version in the request). If the OpCode is not supported, a response is generated with the UNSUPP_OPCODE response code. If the length of the request exceeds 1024 octets or is not a multiple of 4 octets, it is invalid. Invalid requests are handled by copying up to 1024 octets of the request into the response, setting the response code to MALFORMED_REQUEST, and zero-padding the response to a multiple of 4 octets if necessary. Error responses have the same packet layout as success responses, with fields copied from the request copied into the response, and other fields assigned by the PCP server set to 0. [Ed. Note: Need more detail around how an error response is formed, and what it contains.] 6.3. General PCP Options The following options can appear in certain PCP responses. 6.3.1. UNPROCESSED If the PCP server cannot process a mandatory-to-process option, for whatever reason, it includes this Option in the response. This helps with debugging interactions between the PCP client and PCP server. For simplicity, no more than 4 options can be encoded. This option MUST NOT appear more than once in a PCP response, no matter how many PCP options appeared in the request and were unprocessed by the PCP server. If only one Option code was unprocessed, that option code it is placed in option-code-1 (and the other three fields are set to zero), if two Option codes were unprocessed, their option codes are placed in option-code-1 and option-code-2, and so on. If a certain Option appeared more than once in the PCP request, that Option value only appears once in the option-code fields. The order of the Options in the PCP request has no relationship with the order of the Option values in this UNPROCESSED Option. This Option MUST NOT appear in a response unless the associated request contained at least one mandatory-to-process Option. Wing, et al. Expires August 11, 2011 [Page 18] Internet-Draft Port Control Protocol (PCP) February 2011 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | option-code-1 | option-code-2 | option-code-3 | option-code-4 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: UNPROCESSED option This Option: name: UNPROCESSED number: TBD purpose: indicates which PCP options in the request are not supported by the PCP server is valid for OpCodes: all has length: 1 may appear in requests or responses: responses, and only if the response code is non-zero. may appear more than once: no 7. Introduction to MAP and PEER OpCodes There are three uses for the MAP and PEER OpCodes defined in this document: a host operating a server (and wanting an incoming connection), a host operating a client (and wanting to optimize the application keepalive traffic), and a host operating a client and server on the same port. These are discussed in the following sections. When operating a server (Section 7.1 and Section 7.3) the PCP client knows if it wants an IPv4 listener, IPv6 listener, or both on the Internet. The PCP client also knows if it has an IPv4 interface on itself or an IPv6 interface on itself. It takes the union of this knowledge to decide to send a one or two MAP requests for each of its interfaces. Applications that embed IP addresses in payloads (e.g., FTP, SIP) will find it beneficial to avoid address family translation, if possible. Wing, et al. Expires August 11, 2011 [Page 19] Internet-Draft Port Control Protocol (PCP) February 2011 7.1. For Operating a Server A host operating a server (e.g., a web server) listens for traffic on a port, but the server never initiates traffic from that port. For this to work across a NAT or a firewall, the application needs to (a) create a mapping from a public IP address and port to itself as described in Section 8 and (b) publish that public IP address and port via some sort of rendezvous server (e.g., DNS, a SIP message, a proprietary protocol). Publishing the public IP address and port is out of scope of this specification. To accomplish (a), the application follows the procedures described in this section. As normal, the application needs to begin listening to a port, and to ensure that it can get exclusive use of that port it needs to choose a port that is not in the operating system's ephemeral port range. Then, the application constructs a PCP message with the appropriate MAP OpCode depending on if it is listening on an IPv4 or IPv6 interface and if it wants a public IPv4 or IPv6 address. The following pseudo-code shows how PCP can be reliably used to operate a server: /* start listening on the local server port */ int s = socket(...); internal_sockaddr = ...; bind(s, &internal_sockaddr, ...); listen(s, ...); requested_external_sockaddr = 0; pcp_send_map_request(internal_sockaddr, requested_external_sockaddr, &assigned_external_sockaddr, requested_lifetime, &assigned_lifetime); update_rendezvous_server("Client 12345", assigned_external_sockaddr); while (1) { int c = accept(s, ...); /* ... */ } Figure 6: Pseudo-code for using PCP to operate a server 7.2. For Reducing NAT Keepalive Messages [Ed. Note: This section creates a difference between an implicitly-created mapping, which PCP then tries to query/control using the PEER OpCode, and a explicitly-created mapping which was created with a MAP OpCode. This section attempts to address PCP Issue #9 and PCP Issue #35.] A host operating a client (e.g., XMPP client, SIP client) sends from Wing, et al. Expires August 11, 2011 [Page 20] Internet-Draft Port Control Protocol (PCP) February 2011 a port but never accepts incoming connections on this port. It wants to ensure the flow to its server is not terminated (due to inactivity) by an on-path NAT or firewall. To accomplish this, the applications uses the procedure described in this section. Middleboxes such as NATs or firewalls need to see occasional traffic or will terminate their session state, causing application failures. To avoid this, many applications routinely generate keepalive traffic for the primary (or sole) purpose of maintaining state with such middleboxes. Applications can reduce such application keepalive traffic by using PCP. Note: For reasons beyond NAT, an application may find it useful to perform application-level keepalives, such as to detect a broken path between the client and server, detect a crashed server, or detect a powered-down client. These keepalives are not related to maintaining middlebox state, and PCP cannot do anything useful to reduce those keepalives. To use PCP for this function, the applications first connects to its server, as normal. Afterwards, it issues a PCP request with the PEER4 or PEER6 OpCode as described in Section 9. The PEER4 OpCode is used if the host is using IPv4 for its communication to its peer; PEER6 if using IPv6. The same 5-tuple as used for the connection to the server is placed into the PEER4 or PEER6 payload. The following pseudo-code shows how PCP can be reliably used with a dynamic socket, for the purposes of reducing application keepalive messages: int s = socket(...); connect(s, &remote_peer, ...); getsockname(s, &internal_address, ...); external_address = NUL; pcp_send_peer_request(internal_address, requested_external_address, &assigned_external_address, remote_peer, requested_lifetime, &assigned_lifetime); Figure 7: Pseudo-code using PCP with a dynamic socket 7.3. For Operating a Symmetric Client/Server [Ed. Note: The PEER4 and PEER6 OpCodes, discussed here, are intended to resolve PCP Issue #35.] A host operating a client and server on the same port (e.g., Symmetric RTP [RFC4961] or SIP Symmetric Response Routing (rport) [RFC3581]) first establishes a local listener, (usually) sends the Wing, et al. Expires August 11, 2011 [Page 21] Internet-Draft Port Control Protocol (PCP) February 2011 local and public IP addresses and ports to a rendezvous service (which is out of scope of this document), and (usually) initiates outbound connections from that same source address. To accomplish this, the application uses the procedure described in this section. An application that is using the same port for outgoing connections as well as incoming connections MUST first signal its operation of a server using the PCP MAP OpCode, as described in Section 8, and receive a positive PCP response before it sends any packets from that port. Discussion: Although reversing those steps is tempting (to eliminate the PCP round trip before a packet can be sent from that port) and will work if the NAT has endpoint-independent mappings (EIM) behavior, reversing the steps will fail if the NAT does not have EIM behavior. With a non-EIM NAT, the implicit mapping created by an outgoing TCP SYN and the explicit mapping created using the MAP OpCode will cause different ports to be assigned (which is not desirable; after all, the application is using the same port for outgoing and incoming traffic on purpose) and they will generally also have different lifetimes. PCP does not attempt to change or dictate how a NAT creates its mappings (endpoint independent mapping, or otherwise) so there is no assurance that an implicit mapping will be EIM or non-EIM. Thus, it is necessary for applications to first signal its operation of a server using hte PCP MAP OpCode. Wing, et al. Expires August 11, 2011 [Page 22] Internet-Draft Port Control Protocol (PCP) February 2011 The following pseudo-code shows how PCP can be used to operate a symmetric client and server: /* start listening on the local server port */ int s = socket(...); internal_sockaddr = ...; bind(s, &internal_sockaddr, ...); listen(s, ...); requested_external_sockaddr = 0; pcp_send_map_request(internal_sockaddr, requested_external_sockaddr, &assigned_external_sockaddr, requested_lifetime, &assigned_lifetime); update_rendezvous_server("Client 12345", assigned_external_sockaddr); send_packet(s, "Hello World"); while (1) { int c = accept(s, ...); /* ... */ } Figure 8: Pseudo-code for using PCP to operate a symmetric client/ server 8. MAP OpCodes This section defines four OpCodes which control forwarding from a NAT (or firewall) to an internal host. They are: MAP4=0: create a mapping between an internal address and public IPv4 address (e.g., NAT44, NAT64, or firewall) MAP6=1: create a mapping between an internal address and public IPv6 address (e.g., NAT46, NAT66, or firewall) The operation of these OpCodes is described in this section. 8.1. OpCode Packet Formats The two MAP OpCodes (MAP4, MAP6) share a similar packet layout for both requests and responses. Because of this similarity, they are shown together. For both of the MAP OpCodes, if the assigned external IP address and assigned external port both match the request's source IP address and MAP OpCode's internal IP address, the functionality is purely a firewall; otherwise it pertains to a network address translator which might also perform firewall functions. Wing, et al. Expires August 11, 2011 [Page 23] Internet-Draft Port Control Protocol (PCP) February 2011 The following diagram shows the request packet format for MAP4 and MAP6. This packet format is aligned with the response packet format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Requested external IP address (32 or 128, depending on OpCode): : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Requested lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | internal port | requested external port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: MAP OpCode Request Packet Format These fields are described below: Requested External IP Address: Requested external IP address. This is useful for refreshing a mapping, especially after the PCP server loses state. If the PCP server can fulfill the request, it will do so. If the PCP client doesn't know the external address, or doesn't have a preference, it MUST use 0. Requested lifetime: Requested lifetime of this mapping, in seconds. Internal port: Internal port for the mapping. Requested external port: requested external port for the mapping. This is useful for refreshing a mapping, especially after the PCP server loses state. If the PCP server can fulfill the request, it will do so. If the PCP client doesn't know the external port, or doesn't have a preference, it uses 0. The following diagram shows the response packet format for MAP4 and MAP6 OpCodes: Wing, et al. Expires August 11, 2011 [Page 24] Internet-Draft Port Control Protocol (PCP) February 2011 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Assigned external IP address (32 or 128, depending on OpCode) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | internal port | assigned external port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: MAP OpCode Response Packet Format These fields are described below: Assigned external IP address: Assigned external IPv4 or IPv6 address for the mapping. IPv4 or IPv6 address is indicated by the OpCode Internal port: Internal port for the mapping, copied from request. Assigned external port: assigned external port for the mapping. IPv4 or IPv6 address is indicated by the OpCode. If the NAT gateway can allocate the requested external port it SHOULD do so. This is beneficial for re-establishing state lost when a NAT gateway fails or loses its state due to reboot. If the NAT gateway cannot allocate the requested external port but can allocate some other port, it MUST do so and return the allocated port in the response. Cases where a NAT gateway cannot allocate the requested external port include when the requested external port is prohibited by policy, already used by the NAT gateway for one of its own services (e.g. port 80 for the NAT gateway's own configuration pages) already allocated to another explicit mapping (by static manual allocation or by a prior PCP request by a different Internal Host) or the rare case where the requested external port was already allocated to an implicit mapping which cannot be 'promoted' to an explicit mapping for this Internal Host (a different Internal Host already made a prior outbound connection for which the NAT gateway happened to assign the external port requested in this explicit PCP request). 8.2. OpCode-Specific Result Codes In addition to the general PCP result codes (Section 5.4), the following additional result codes may be returned as a result of the four MAP OpCodes received by the PCP server. Wing, et al. Expires August 11, 2011 [Page 25] Internet-Draft Port Control Protocol (PCP) February 2011 20 NETWORK_FAILURE, e.g., NAT device has not obtained a DHCP lease 21 NO_RESOURCES, e.g., NAT device cannot create more mappings at this time. This is a system-wide error, and different from USER_EX_QUOTA. 22 UNSUPP_PROTOCOL, unsupported Protocol 23 NOT_AUTHORIZED, e.g., PCP server supports mapping, but the feature is disabled for this PCP client, or the PCP client requested a mapping that cannot be fulfilled by the PCP server's security policy. 24 USER_EX_QUOTA, mapping would exceed user's port quota 25 CANNOT_PROVIDE_EXTERNAL_PORT, indicates the port is already in use or otherwise unavailable (e.g., special port that cannot be allocated by the server's policy). This error is only returned if the request included the Option PREFER_FAILURE. 26 UNABLE_TO_DELETE_ALL, indicates the PCP server was not able to delete all mappings. Other result codes are defined following the procedure in Section 13.3. 8.3. OpCode-Specific Client Operation, Processing a Response This section describes the operation of a PCP client when sending requests with OpCodes MAP4 and MAP6, and processing those responses. A PCP client can delete a mapping immediately by sending the appropriate MAP OpCode with a lifetime of 0. The PCP server responds by returning a MAP Response with a lifetime of 0. To delete all mappings for all hosts associated with this subscriber, an all-zero internal IP address is used. The request MAY contain values in the requested-external-ip-address and requested-external-port fields. This allows the PCP client to attempt to rebuild the PCP server's state, so that the PCP client could avoid having to change information maintained at the rendezvous server. Of course, due to other activity on the network (e.g., by other users or network renumbering), the PCP server may not be able to fulfill the request. An existing mapping can have its lifetime extended by the PCP client. To do this, the PCP client sends a new PCP map request to the server Wing, et al. Expires August 11, 2011 [Page 26] Internet-Draft Port Control Protocol (PCP) February 2011 indicating the internal IP address and port(s). The PCP client SHOULD renew the mapping before its expiry time, otherwise it will be removed by the PCP server (see Section 8.4.3). In order to prevent excessive PCP chatter, it is RECOMMENDED to send a single renewal request packet when a mapping is halfway to expiration time, then, if no positive response is received, another single renewal request 3/4 of the way to expiration time, and then another at 7/8 of the way to expiration time, and so on, subject to the constraint that renewal requests MUST NOT be sent less than four seconds apart (a PCP client MUST NOT send an infinite number of ever- closer-together requests in the last few seconds before a mapping expires). To delete a mapping, lifetime=0 is used. To delete a mapping for a specific protocol and port, the MAP request contains that specific internal port and protocol. To delete all mappings for a particular protocol, port 0 is used to indicate a wildcard. A response is matched with a request by comparing the protocol, internal IP address, internal port, external address and external port. Other fields are not compared, because the PCP server changes those fields to provide information about the mapping created by the OpCode. If a successful response, the PCP client can use the external IP address and port(s) as desired. Typically the PCP client will communicate the external IP address and port(s) to another host on the Internet using an application-specific rendezvous mechanism such as DNS SRV records. When a PCP client first acquires a new IP address, it may want to remove mappings that may have been instantiated for a previous host. To do this, the PCP client sends a MAP request with external port, internal port, and lifetime set to 0. 8.4. OpCode-Specific Server Operation This section describes the operation of a PCP server when processing the OpCodes MAP4 or MAP6. If the requested lifetime is 0, it indicates a request to delete the mapping immediately. The contents of the protocol field or the internal-port field can be zero, indicating a wildcard. If the protocol field is 0, it indicates all protocols (and the internal- port field is ignored). If the internal-port is 0, it means all ports for the particular protocol. On a deletion request, the requested external port field is ignored by the server. If the Wing, et al. Expires August 11, 2011 [Page 27] Internet-Draft Port Control Protocol (PCP) February 2011 deletion request was properly formatted, and the associated mapping (if present) is deleted, a positive response is generated with lifetime of 0 and copies the protocol and internal port number from the request into the response. If the PCP client attempts to delete a port mapping which was manually assigned by some kind of configuration tool, the PCP server MUST respond with UNABLE_TO_DELETE_ALL result code, but the other fields are encoded as described above. If the PCP client attempts to delete a mapping that does not exist, the success response code is returned. If the PCP client is not authorized to delete this mapping, NOT_AUTHORIZED is returned. If the requested lifetime is not zero, it indicates a request to create a mapping or extend the lifetime of an existing mapping. If the PCP request contains protocol=255, it indicates the PCP client wants to map all protocols. If this cannot be fulfilled by the PCP- controlled device, UNSUPP_PROTOCOL is returned. See Section 8.4.2 for processing the lifetime. If the requested external port is 0, and the PCP-controlled device does not change port numbers (that is, it does not do port translation) the PCP server MUST return a response indicating that the assigned external port is the same as the internal port. If the PCP-controlled device is stateless (that is, it does not establish any per-flow state, and simply rewrites the address and/or port in a purely algorithmic fashion), the PCP server simply returns an answer indicating the external IP address and port yielded by this stateless algorithmic translation. This allows the PCP client to learn its external IP address and port as seen by remote peers. Examples of stateless translators include stateless NAT64 and 1:1 NAT44 (both of which modify addresses but not port numbers). If an option with value greater than 128 exists but that option does not make sense (e.g., the PREFER_FAILURE option is included in a request with lifetime=0 (indicating a delete request)), the request is invalid and generates a MALFORMED_OPTION error. By default, a PCP-controlled device MUST NOT create mappings for a protocol not indicated in the request. For example, if the request was for a TCP mapping, a UDP mapping MUST NOT be created. If the THIRD_PARTY option is not present in the request, the source IP address of the PCP packet is used when creating the mapping. If the THIRD_PARTY option is present, the PCP server then validates that internal IP address indicated in that option belongs to the same Wing, et al. Expires August 11, 2011 [Page 28] Internet-Draft Port Control Protocol (PCP) February 2011 subscriber. This validation depends on the PCP deployment scenario; see Section 10.5 for the validation procedure. If the internal IP address in the PCP request does not belong to the subscriber, an error response MUST be generated with result code NOT_AUTHORIZED. If all of the proceeding operations were successful (did not generate an error response), then the requested mappings are created as described in the request and a positive response is built. This positive result contains the same OpCode as the request, but with the "R" bit set. As a side-effect of creating a mapping, ICMP messages associated with the mapping are also translated (if appropriate) and forwarded for the duration of the mapping's lifetime. This is done to ensure that ICMP messages can still be used by hosts, without application programmers or PCP client implementations needing to signal PCP separately to create ICMP mappings for those flows. 8.4.1. Maintaining Same External IP Address If there are active mappings associated with a given subscriber (see Section 10.5) -- created via dynamic assignment, by PCP or any other means -- subsequent PCP mapping requests belonging to the same subscriber MUST use the same external IP address. This follows the intent of REQ-1 of [I-D.ietf-behave-lsn-requirements]. Once an internal host has no active mapping in the PCP-controlled device, a subsequent PCP request for that host MAY be assigned to a different external IP address. 8.4.2. Mapping Lifetime The PCP client requests a certain lifetime, and the PCP server responds with the assigned lifetime. The PCP server MAY grant a lifetime smaller or larger than the requested lifetime. The PCP server SHOULD be configurable for permitted minimum and maximum lifetime, and the RECOMMENDED values are 120 seconds for the minimum value and 24 hours for the maximum. It is NOT RECOMMENDED that the server allow lifetimes exceeding 24 hours, because they will consume ports even if the internal host is no longer interested in receiving the traffic or no longer connected to the network. Once a PCP server has responded positively to a mapping request for a certain lifetime, the port forwarding is active for the duration of the lifetime unless the lifetime is reduced by the PCP client (to a shorter lifetime or to zero) or until the PCP server loses its state (e.g., crashes). Wing, et al. Expires August 11, 2011 [Page 29] Internet-Draft Port Control Protocol (PCP) February 2011 An application that forgets its PCP-assigned mappings (e.g., the application or OS crashes) will request new PCP mappings (consuming the user's port quota (if there is a quota) and the resource limit for number of mappings), and the application will also probably initiate dynamic connections to servers without using PCP (also consuming the user's port quota). PCP provides no explicit protection against such port consumption. In such environments, it is RECOMMENDED that applications use shorter PCP lifetimes to reduce the impact of consuming the user's port quota. An OS that issues a "delete all" request on reboot protects itself against this resource exhaustion by voluntarily relinquishing all of its old mappings before beginning to request new ones. The PCP server MAY chose to allocate the same (recently relinquished) mappings when mappings are re-requested by the booting OS. Some port mapping APIs (such as the "DNSServiceNATPortMappingCreate" API provided by Apple's Bonjour on Mac OS X, iOS, Windows, Linux, etc.) automatically monitor for process exit (including application crashes) and automatically send port mapping deletion requests if the process that requested them goes away without explicitly relinquishing them. 8.4.3. Mapping Deletion A mapping MUST be deleted by the PCP server upon the expiry of its lifetime, or upon request from the PCP client. In order to prevent another subscriber from receiving unwanted traffic, the PCP server SHOULD NOT assign that same external port to another host for 120 seconds (MSL, [RFC0793]). The PCP server MUST allow the same host to re-acquire the same port during that same interval. 8.4.4. Subscriber Renumbering The customer premises router might obtain a new IPv4 address or new IPv6 prefix. This can occur because of a variety of reasons including a reboot, power outage, DHCP lease expiry, or other action by the ISP. If this occurs, traffic forwarded to the subscriber might be delivered to another customer who now has that address -- both traffic mapped with MAP requests and dynamic traffic. This same problem can occur if an IP address is re-assigned today, without PCP and without an ISP-operated CGN. The solution is the same as today: the problems associated with subscriber renumbering are eliminated if the ISP avoids re-assigning IP addresses to different subscribers. When a new Internal Address is assigned to a host embedding a PCP Wing, et al. Expires August 11, 2011 [Page 30] Internet-Draft Port Control Protocol (PCP) February 2011 client, the NAT (or firewall) controlled by the PCP server will continue to send traffic to the old IP address. Assuming the PCP client wants to continue receiving traffic, it needs to install new mappings for its new IP address. The requested external port field will not be fulfilled by the PCP server, in all likelihood, because it is still being forwarded to the old IP address. Thus, a mapping is likely to be assigned a new external port number and/or public IP address. Note that this scenario is not expected to happen routinely on a regular basis for most hosts, since most hosts renew their DHCP leases before they expire (or re-request the same address after reboot) and most DHCP servers honor such requests and grant the host the same address it was previously using before the reboot. 8.5. PCP Options for MAP OpCodes 8.5.1. THIRD_PARTY This Option is used when a PCP client wants to control a mapping to another host. A PCP server will only support this option if sent by an authorized PCP client, which depends on the deployment scenario. For Dual-Stack Lite deployments, the PCP server only supports this option if the source IPv4 address is the B4's source IP address. For other scenarios, the subscriber has only one IPv4 address and this Option serves no purpose. If a subscriber has more than one IPv4 address, the ISP MUST determine its own policy for how to identify the trusted device within the subscriber's home. This might be, for example, the lowest- or highest-numbered host address for that user's IPv4 prefix. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Mapping Internal IP address (32 or 128, depending on OpCode) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Protocol: indicates protocol associated with this OpCode. Values are taken from the IANA protocol registry [proto_numbers]. For example, this field contains 6 (TCP) if the opcode is intended to create a TCP mapping. Reserved: 24 reserved bits, MUST be 0 on transmission and MUST be ignored on reception. Wing, et al. Expires August 11, 2011 [Page 31] Internet-Draft Port Control Protocol (PCP) February 2011 Mapping Internal IP Address: Internal IP address of the mapping. This can be IPv4 or IPv6, depending on the OpCode. 8.5.2. REMOTE_PEER_FILTER This Option indicates packet filtering is desired. The remote peer port and remote peer IP Address indicate the permitted remote peer's source IP address and port for packets from the Internet. That is, packets with a source IP address, transport, or port that do not match those fields of the PCP request are dropped by the PCP server- controlled NAT/firewall device. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | prefix-length | Remote Peer Port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Remote Peer IP address (32 bits if MAP4, : : 1 28 bits if MAP6) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This Option: name: REMOTE_PEER_FILTER number: 128 is valid for OpCodes: MAP44, MAP64, MAP46, or MAP66 is included in responses: MUST has length: 2 or 5 may appear in requests or responses: requests may appear more than once: no Because of interactions with dynamic ports this Option MUST only be used by a client that is operating a server, as this ensures that no other application will be assigned the same ephemeral port for its outgoing connection. Other use by a PCP client is NOT RECOMMENDED and will cause some UNSAF NAT traversal mechanisms [RFC3424] to fail where they would have otherwise succeeded, breaking other applications running on this same host. The prefix-length indicates how many bits of the IPv6 address or IPv4 address are used for the filter. For MAP4, a prefix-length of 32 Wing, et al. Expires August 11, 2011 [Page 32] Internet-Draft Port Control Protocol (PCP) February 2011 indicates the entire IPv4 address is used; a prefix-length of 0 indicates none of the IPv4 address is used (which is effectively the same as not adding a filter at all). For MAP4 the minimum value is 0 and the maximum value is 32; for MAP6 the minimum value is 0 and the maximum value is 128. Values outside that range cause an MALFORMED_OPTION response code. [Ed. Note: How do we want to remove a filter? Do we want to allow removing a filter at all -- is there a use-case for that or can the application just create a new mapping? If we have a use-case, perhaps use 0.0.0.0 as the remote IP address to remove all filters? This is tracked as PCP Issue #10 [PCP-Issues].] 8.5.3. PREFER_FAILURE This option indicates that if the PCP server is unable to allocate the requested port, then instead of returning an available port that it *can* allocate, the PCP server should instead allocate no port and return result code CANNOT_HONOR_EXTERNAL_PORT. This option is intended solely for use by UPnP IGD interworking (Section 11), where the semantics of IGD version 1 do not provide any way to indicate to an IGD client that any port is available other than the one it requested. A PCP server MAY support this option, if its designers wish to support downstream devices that perform IGD interworking. PCP servers MAY choose to rate-limit their handling of PREFER_FAILURE requests, to protect themselves from a rapid flurry of 65535 consecutive PREFER_FAILURE requests from clients probing to discover which external ports are available. PCP servers that are not intended to support downstream devices that perform IGD interworking are not required to support this option. PCP clients other than IGD interworking clients MUST NOT use this option because it results in inefficient operation, and they cannot safely assume that all PCP servers will implement it. The option is provided only because the semantics of IGD version 1 offer no viable alternative way to implement an IGD interworking function. It is anticipated that this option will be deprecated in the future as more clients adopt PCP natively and the need for IGD interworking declines. This Option: name: PREFER_FAILURE number: 130 is valid for OpCodes: MAP4, MAP6 Wing, et al. Expires August 11, 2011 [Page 33] Internet-Draft Port Control Protocol (PCP) February 2011 is included in responses: MUST has length: 0 may appear in requests or responses: requests may appear more than once: no 8.6. PCP Mapping State Maintenance If an event occurs that causes the PCP server to lose state (such as a crash or power outage), the mappings created by PCP are lost. Such loss of state is rare in a service provider environment (due to redundant power, disk drives for storage, etc.). But such loss of state is more common in a residential NAT device which does not write information to its non-volatile memory. The Epoch allows a client to deduce when a PCP server may have lost its state. If this occurs, the PCP client can attempt to recreate the mappings following the procedures described in this section. 8.6.1. Recreating Mappings The PCP server SHOULD store mappings in persistent storage so when it is powered off or rebooted, it remembers the port mapping state of the network. Due to the physical architecture of some PCP servers, this is not always achievable (e.g., some non-volatile memory can withstand only a certain number of writes, so writing PCP mappings to such memory is generally avoided). However, maintaining this state is not essential for correct operation. When the PCP server loses state and begins processing new PCP messages, its Epoch is reset to zero (per the procedure of Section 6.1.4). A mapping renewal packet is formatted identically to an original mapping request; from the point of view of the client it is a renewal of an existing mapping, but from the point of view of the PCP server it appears as a new mapping request. This self-healing property of the protocol is very important. When a client renews its port mappings as the result of receiving a packet where the Epoch field indicates that a reboot or similar loss of state has occurred, the client MUST first delay by a random amount of time selected with uniform random distribution in the range 0 to 5 seconds, and then send its first PCP request. After that request is acknowledged by the PCP server, the client may then send its second Wing, et al. Expires August 11, 2011 [Page 34] Internet-Draft Port Control Protocol (PCP) February 2011 request, and so on, as rapidly as the gateway allows. The requests SHOULD be issued serially, one at a time; the client SHOULD NOT issue multiple requests simultaneously in parallel. [Ed. Note: the paragraph above is copied from NAT-PMP, and seems to be advice specific to receiving asynchronous notification that the Epoch was reset. Asynchronous notification needs the delay described (in fact, it probably needs greater delay than 0-5 seconds if on a larger network) and also needs to discourage sending multiple PCP requests serially. However, PCP does not have asynchronous notification (yet), and has different needs/ requirements for pacing. In short: the above paragraph needs some discussion. This is tracked as PCP Issue #11 [PCP-Issues].] The discussion in this section focuses on recreating inbound port mappings after loss of PCP server state, because that is the more serious problem. Losing port mappings for outgoing connections destroys those currently active connections, but does not prevent clients from establishing new outgoing connections. In contrast, losing inbound port mappings not only destroys all existing inbound connections, but also prevents the reception of any new inbound connections until the port mapping is recreated. Accordingly, we consider recovery of inbound port mappings the more important priority. However, clients that want outgoing connections to survive a NAT gateway reboot can also achieve that using PCP. After initiating an outbound TCP connection (which will cause the NAT gateway to establish an implicit port mapping) the client should send the NAT gateway a port mapping request for the source port of its TCP connection, which will cause the NAT gateway to send a response giving the external port it allocated for that mapping. The client can then store this information, and use it later to recreate the mapping if it determines that the NAT gateway has lost its mapping state. 8.6.2. Maintaining Mappings A PCP client can refresh a mapping by sending a new PCP request containing information from the earlier PCP response. The PCP server will respond indicating the new lifetime. It is possible, due to failure of the PCP server, that the public IP address and/or public port, or the PCP server itself, has changed (due to a new route to a different PCP server). To detect such events more quickly, the PCP client may find it beneficial to use shorter lifetimes (so that it communicates with the PCP server more often). If the PCP client has several mappings, the Epoch value only needs to be retrieved for one of them to verify the PCP server has not lost port forwarding state. If the client wishes to check the PCP server's Epoch, it sends a PCP Wing, et al. Expires August 11, 2011 [Page 35] Internet-Draft Port Control Protocol (PCP) February 2011 request for any one of the client's mappings. This will return the current Epoch value. In that request the PCP client could extend the mapping lifetime (by asking for more time) or maintain the current lifetime (by asking for the same number of seconds that it knows are remaining of the lifetime). 9. PEER OpCodes This section defines two OpCodes for controlling dynamic connections. They are: PEER4=2: Set or query lifetime for flow from IPv4 address to a remote peer's IPv4 address. PEER6=3: Set or query lifetime for flow from IPv6 address to a remote peer's IPv6 address. The operation of these OpCodes is described in this section. 9.1. OpCode Packet Formats The two PEER OpCodes (PEER4 and PEER6) share a similar packet layout for both requests and responses. Because of this similarity, they are shown together. For both of the PEER OpCodes, if the internal IP address and internal port fields of the request both match the external IP address and external port fields of the response, the IP addresses and ports are not changed and thus the functionality is purely a firewall; otherwise it pertains to a network address translator which might also perform firewall functions. The following diagram shows the request packet format for PEER4 and PEER6. This packet format is aligned with the response packet format: Wing, et al. Expires August 11, 2011 [Page 36] Internet-Draft Port Control Protocol (PCP) February 2011 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | Reserved (24 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Internal IP address (32 bits if PEER4, 128 bits if PEER6) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Remote Peer IP address (32 bits if PEER4, 128 bits if PEER6) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Reserved (128 bits) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Requested lifetime | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | internal port | reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | remote peer port | reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: PEER OpCode Request Packet Format These fields are described below: Protocol: indicates protocol associated with this OpCode. Values are taken from the IANA protocol registry [proto_numbers]. For example, this field contains 6 (TCP) if the OpCode is describing a TCP peer. Reserved: 24 reserved bits, MUST be 0 on transmission and MUST be ignored on reception. Mapping Internal IP Address: Internal IP address of the 5-tuple. This can be 32 bits long (if OpCode is PEER4) or 128 bits long (if OpCode is PEER6). Remote Peer IP Address: Remote peer's IP address, from the perspective of the PCP client. Reserved: 128 reserved bits, MUST be 0 on transmission and MUST be ignored on reception. Wing, et al. Expires August 11, 2011 [Page 37] Internet-Draft Port Control Protocol (PCP) February 2011 Requested lifetime: Requested lifetime of this mapping, in seconds. Unlike the MAP OpCode, there is no special meaning of 0. internal port: Internal port for the of the 5-tuple. Reserved: 16 reserved bits, MUST be 0 on transmission and MUST be ignored on reception. Remote Peer Port: Remote peer's port of the 5-tuple. Reserved: 16 reserved bits, MUST be 0 on transmission and MUST be ignored on reception. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Protocol | External_AF | Reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Internal IP address (32 bits if PEER4, 128 bits if PEER6) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : Remote Peer IP address (32 bits if PEER4, 128 bits if PEER6) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : : External IP address (always 128 bits) : : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | internal port | external port | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | remote peer port | reserved (16 bits) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: PEER OpCode Request Packet Format Protocol: Copied from the request. External_AF The address family of the external IP address associated with this peer connection. Values are from IANA's address family numbers (IPv4 is 1, IPv6 is 2). Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be ignored on reception. Wing, et al. Expires August 11, 2011 [Page 38] Internet-Draft Port Control Protocol (PCP) February 2011 Internal IP address Copied from the request. remote Peer IP address Copied from the request. External IP Address External IP address, assigned by the NAT (or firewall) to this mapping. If firewall, this will match the internal IP address. This field is always 128 bits long. If External_AF indicates IPv4, the IPv4 address is encoded in the first 32 bits of the External IP Address field and the remaining 96 bits are zero. internal port: copied from request. external port: External port number, assigned by the NAT (or firewall) to this mapping. If firewall or 1:1 NAT, this will match the internal port. remote peer port: Copied from request. Reserved: 16 reserved bits, MUST be 0 on transmission, MUST be ignored on reception. 9.2. OpCode-Specific Result Codes In addition to the general PCP result codes (Section 5.4) the following additional result codes may be returned as a result of the two PEER OpCodes received by the PCP server. 50 NONEXIST_PEER, the connection to that peer does not exist in the mapping table Other result codes are defined following the procedure in Section 13.3. 9.3. OpCode-Specific Client Operation, Processing a Response This section describes the operation of a client when sending the OpCodes PEER4 or PEER6. After connecting to a server using UDP or TCP, the PCP client sends a PCP request containing the PEER4 or PEER6 OpCode. A response is matched with a request by comparing the protocol, external AF, internal IP address, internal port, remote peer address and remote peer port. Other fields are not compared, because the PCP server changes those fields to provide information about the mapping created by the OpCode. Wing, et al. Expires August 11, 2011 [Page 39] Internet-Draft Port Control Protocol (PCP) February 2011 If a successful response, the PCP client uses the assigned lifetime value to reduce its frequency of application keepalives for the NAT. Of course, there may be other reasons, specific to the application, to use more frequent application keepalives. For example, the PCP assigned-lifetime could be one hour but the application may want to ensure the server is still accessible (e.g., has not crashed) more frequently than once an hour. 9.4. OpCode-Specific Server Operation This section describes the operation of a client when processing the OpCodes PEER4 or PEER6. The PEER OpCodes provide a single function: the ability for the PCP client to query and (possibly) extend the lifetime of an existing mapping. On receiving the PEER4 or PEER6 OpCode, the PCP server examines the mapping table. If a mapping does not exist, the NONEXIST_PEER error is returned. Otherwise, the PCP server chooses the smaller of the requested lifetime and its configured maximum lifetime value, and sets the lifetime of the existing mapping. This means that a PEER4 or PEER6 request does not reduce the lifetime of an existing mapping, nor can the PEER OpCodes delete a mapping. If the mapping is terminated by the TCP client or server (e.g., TCP FIN or TCP RST), the mapping will eventually be destroyed normally; the earlier use of PEER does not extend the lifetime in that case. If all of the proceeding operations were successful (did not generate an error response), then a SUCCESS response is generated, with the assigned-lifetime containing the lifetime of the mapping. 10. Deployment Scenarios 10.1. Dual Stack-Lite The interesting components in a Dual-Stack Lite deployment are the B4 element (which is the customer premises router) and the AFTR element (which is the device that both terminates the IPv6-over-IPv4 tunnel and also implements the Carrier-Grade NAT44 function). The B4 element does not need to perform a NAT function (and usually does not perform a NAT function), but it does operate its own DHCP server and is the local network's default router. Wing, et al. Expires August 11, 2011 [Page 40] Internet-Draft Port Control Protocol (PCP) February 2011 10.1.1. Overview Various PCP deployment scenarios can be considered to control the PCP server embedded in the AFTR element: 1. UPnP IGD and NAT-PMP [I-D.cheshire-nat-pmp] are used in the LAN: an interworking function is required to be embedded in the B4 element to ensure interworking between the protocol used in the LAN and PCP. UPnP IGD-PCP Interworking Function is described in Section 11. 2. Hosts behind the B4 element will either include a PCP client or UPnP IGD client, or both. A. if a UPnP IGD client, the B4 element will need to include an interworking function from UPnP IGD to PCP. B. if a PCP client, the PCP client will communicate directly with the PCP server. 3. The B4 element includes a PCP client which is invoked by an HTTP- based configuration (as is common today). The internal IP address field in the PCP payload would be the internal host used in the port forwarding configuration. Two modes are identified to forward PCP packets to a PCP server controlling the provisioned AFTR as described in the following sub- sections. [Ed. Note: We need to decide on Encapsulation Mode or Plain IPv6 Mode. This is tracked as PCP Issue #13 [PCP-Issues].] 10.1.2. Encapsulation Mode In this mode, B4 element does no processing at all of the PCP messages, and forwards them as any other UDP traffic. With DS-Lite, this means that IPv4 PCP messages issued by internal PCP clients are encapsulated into the IPv6 tunnel sent to the AFTR as for any other IPv4 packets. The IPv6 address used as source address MUST be the same as the one used by the B4 element. The AFTR decapsulates the IPv4 packets and processes the PCP requests (because the destination IPv4 address points to the PCP server embedded in the AFTR). 10.1.3. Plain IPv6 Mode Another alternative for deployment of PCP in a DS-Lite context is to rely on a PCP Proxy in the B4 element. Protocol exchanges between the PCP Proxy and the PCP server are conveyed using plain IPv6 (no Wing, et al. Expires August 11, 2011 [Page 41] Internet-Draft Port Control Protocol (PCP) February 2011 tunnelling is used). Nevertheless, the IPv6 address used as source address by the PCP Proxy MUST be the same as the one used by the B4 element. 10.2. NAT64 Hosts behind a NAT64 device can make use of PCP in order to perform port reservation (to get a publicly routable IPv4 port). If the IANA-assigned IP address is used for the discovery of the PCP server, that IPv4 address can be placed into the IPv6 destination address following that particular network's well-known prefix or network-specific prefix, per [RFC6052]. 10.3. NAT44 and NAT444 Residential subscribers in NAT44 (and NAT444) deployments are usually given one IPv4 address, but may also be given several IPv4 addresses. These addresses are not routable on the IPv4 Internet, but are routable between the subscriber's home and the ISP's CGN. To accommodate multiple hosts within a home, especially when provided insufficient IPv4 addresses for the number of devices in the home, subscribers operate a NAPT device. When this occurs in conjunction with an upstream NAT44, this is nicknamed "NAT444". [Ed. Note: Does PCP need a mechanism to detect a non-PCP- supporting NAT between a PCP client and a PCP server? Or can that problem be detected by relying on the failure of PCP server Discovery? This is tracked as PCP Issue #25 [PCP-Issues].] 10.4. IPv6 Firewall See Section 8.5.2. [Ed. Note: this IPv6 firewall section needs more text. This is tracked as PCP Issue #10 [PCP-Issues].] 10.5. Subscriber Identification The MAP OpCodes require subscriber identification because they allocate resources or adjust resources allocated to a subscriber. For the MAP OpCode, it is permitted for a PCP client to create a mapping on behalf of a third party device (e.g., a computer can create PCP mappings on behalf of a webcam). However, a PCP client cannot open mappings for a different subscriber. The mechanism to identify "same subscriber" depends on the sort of NAT on this network: Wing, et al. Expires August 11, 2011 [Page 42] Internet-Draft Port Control Protocol (PCP) February 2011 o If the PCP-controlled device is a NAT64: the internal IP address indicated in the PCP message and the source IPv6 address of received PCP request MUST belong to the same IPv6 prefix. The length of the IPv6 prefix is the same as the length assigned to each subscriber on that particular network (e.g., /64), and that length must be configurable by the network operator. o If the PCP-controlled device is a DS-Lite AFTR: DS-Lite (Section 11 of [I-D.ietf-softwire-dual-stack-lite]) already requires the tunnel transport source address be validated, and that same address is used by PCP to assign the tunnel-ID to the requested mapping (see Section 10.1.2 and Section 10.1.3). Thus, PCP acquires the same security properties as DS-Lite. If address validation is implemented correctly, the PCP client can not instruct mappings on behalf of devices of another subscriber. o If LSN with a routed network (NAT44), each subscriber has a known set of IPv4 address (usually one IPv4 address) and all PCP requests MUST be sent from only one of the subscriber's IP addresses and MUST only open mappings towards the subscriber's own IP address. o If IPv6 firewall: the internal IP address indicated in the PCP message and the source IPv6 address of received PCP request MUST belong to the same IPv6 prefix. The length of the IPv6 prefix is the same as the length assigned to each subscriber on that particular network (e.g., /64), and that length must be configurable by the network operator. PCP-controlled devices can be a DS-Lite AFTR or an IPv4-IPv6 interconnection node such as NAT46 or NAT64. These nodes are deployed by Service Providers to deliver global connectivity service to their customers. Appropriate functions to restrict the use of these resources (e.g., LSN facility) to only subscribed users should be supported by these devices. Access control can be implicit or explicit: o It is said to be explicit if an authorisation procedure is required for a user to be granted access to such resources. For such variant of PCP-controlled device, a subscriber can be identified by an IPv6 address, an IPv4 address, a MAC address, or any other information. o For other scenarios, such as plain IPv4-in-IPv6 encapsulation for a DS-Lite architecture, the access to the service is based on the source IPv6 prefix. No per-user polices is pre-configured in the PCP-controlled device. Wing, et al. Expires August 11, 2011 [Page 43] Internet-Draft Port Control Protocol (PCP) February 2011 11. Interworking with UPnP IGD 1.0 and 2.0 [Ed. Note: This UPnP IGD Interworking section will likely be moved to a separate document which will fully describe how a proxy needs to translate UPnP IGD messages into PCP messages. This is tracked as PCP Issue #28 [PCP-Issues].] The following diagram shows how UPnP IGD can be interworked with PCP, using an interworking function (IWF). +-------------+ | IGD Control | | Point |-----+ +-------------+ | +---------+ +--------+ +---| IGD-PCP | | PCP | | IWF +-------+ Server |-- +---| | | | +-------------+ | +---------+ +--------+ | Local Host |-----+ +-------------+ | | | | LAN Side | WAN side | <======UPnP IGD=============>|<========PCP=====>| Figure 13: Network Diagram, Interworking UPnP IGD and PCP 11.1. UPnP IGD 1.0 with AddPortMapping Action In UPnP IGD 1.0 [IGD] it is only possible to request a specific port using the AddPortMapping action. Requiring a specific port is incompatible with both (1) a Carrier-Grade NAT and with (2) widely- deployed applications. Regarding (1), another subscriber is likely to already be using the same port, so it will be unavailable to this application to operate a server. Regarding (2), if the same popular application exists on two devices behind the same NAPT, they cannot both get the same port. PCP cannot correct this behavior of UPnP IGD:1, but PCP does work with this behavior. Due to this incompatibility with address sharing and popular applications, future hosts and applications will either support UPnP IGD 2.0's AddAnyPortMapping method (see Section 11.2) or, more likely, will support PCP natively. When a requested port assignment fails, most UPnP IGD control points will retry the port assignment requesting the next higher port or requesting a random port. These UPnP IGD requests are translated to PCP requests and sent to the PCP server. The requests include the Wing, et al. Expires August 11, 2011 [Page 44] Internet-Draft Port Control Protocol (PCP) February 2011 PREFER_FAILURE option, which causes the PCP server to return an error if it cannot allocate the requested port. The interworking function translates the PCP error response to a UPnP IGD error response. This repeats until the UPnP IGD client gives up or until the PCP server is able to return the requested port. Message flow would be similar to this: Wing, et al. Expires August 11, 2011 [Page 45] Internet-Draft Port Control Protocol (PCP) February 2011 UPnP Control Point in-home CPE PCP server | | | |-UPnP:AddPortMapping(80)--->| | | |-PCP:request port 80------>| | | PREFER_FAILURE | | | | | |<-PCP:error----------------| |<-UPnP: unavailable---------| | | | | |-UPnP:AddPortMapping(81)--->| | | |-PCP:request port 81------>| | | PREFER_FAILURE | | | | | |<-PCP:error----------------| |<-UPnP: unavailable---------| | | | | | | | |-UPnP:AddPortMapping(82)--->| | | |-PCP:request port 82------>| | | PREFER_FAILURE | | | | | |<-PCP:error----------------| |<-UPnP: unavailable---------| | | | | | | | |-UPnP:AddPortMapping(83)--->| | | |-PCP:request port 83------>| | | PREFER_FAILURE | | | | | |<-PCP:error----------------| |<-UPnP: unavailable---------| | | | | ... ... ... 84 ... ... ... ... 85 ... ... ... ... .. ... ... ... ... 96 ... ... ... ... 97 ... | | | |-UPnP:AddPortMapping(98)--->| | | |-PCP:request port 98------>| | | PREFER_FAILURE | | | | | |<-PCP:ok, port 98----------| |<-UPnP: ok, port 98---------| | | | | Figure 14: Message Flow: Interworking from UPnP IGD 1.0 AddPortMapping action to PCP Wing, et al. Expires August 11, 2011 [Page 46] Internet-Draft Port Control Protocol (PCP) February 2011 11.2. UPnP IGD 2.0 with AddAnyPortMapping Action If the UPnP IGD control point and the UPnP IGD interworking function both implement UPnP IGD 2.0 [IGD-2] and the UPnP IGD control point uses IGD 2's new AddAnyPortMapping action, only one round-trip is necessary. This is because AddAnyPortMapping has semantics similar to PCP's semantics, allowing the PCP server to assign any port. Message flow would be similar to this: UPnP Control Point in-home CPE PCP server | | | |-UPnP:AddAnyPortMapping()->| | | |-PCP:external port 0----->| | |<-PCP:external port=12345-| |<-UPnP:port=12345----------| | | | | Figure 15: Message Flow: Interworking from UPnP IGD 2.0 AddAnyPortMapping action to PCP 11.3. Lifetime Maintenance UPnP IGD 1.0 and 2.0 provide a lifetime (PortMappingLeaseDuration), but it is seldom used by UPnP IGD control points, and does not allow the UPnP IGD to override the requested duration. Thus, the UPnP IGD/ PCP interworking function is responsible for extending the lifetime of mappings that are still interesting to the UPnP IGD control point. Note: It can be an implementation advantage, where possible, for the UPnP IGD/PCP interworking function to request a port mapping lifetime only while that client is active and connected. For example, creating a PCP mapping that is equal to the client's remaining DHCP lifetime is one useful approach. 12. Security Considerations The PCP client's source port SHOULD be randomly generated as per [I-D.ietf-tsvwg-port-randomization]. On today's Internet, ISPs do not typically filter incoming traffic for their subscribers. However, when ISP introduce stateful address sharing with NAPT devices, such filtering will occur as a side effect. PCP allows controlling that filtering, and PCP allows indicating the 'inside' IP address that should have the filtering removed. It is important that PCP allows removing the filtering for hosts belonging to one subscriber, but not hosts belonging to another Wing, et al. Expires August 11, 2011 [Page 47] Internet-Draft Port Control Protocol (PCP) February 2011 subscriber. This is done in different ways depending on the architecture of the address sharing device and how subscribers are identified behind that device, and described in detail in Section 10.5. Because of the state created in a NAPT or firewall, it is anticipated that port forwarding (MAP OpCodes) will have a quota applied to each subscriber. If the quota is small and the maximum lifetime is large, a faulty or disconnected PCP client could cause a denial of service for other PCP clients belonging to that same subscriber. To prevent this problem, if a PCP server is configured for a small per- subscriber quota (e.g., less than 15 ports) then it is RECOMMENDED it also be be configured for a short maximum lifetime (e.g., 5 minutes). 13. IANA Considerations IANA is requested to perform the following actions: 13.1. Port Number IANA has assigned UDP port 44323 for PCP. 13.2. OpCodes IANA shall create a new protocol registry for PCP OpCodes, initially populated with the values in Section 8 and Section 9. New OpCodes in the range 1-95 can be created via Standards Action [RFC5226], and the range 96-128 is for Private Use [RFC5226]. 13.3. Result Codes IANA shall create a new registry for PCP result codes, numbered 0-255, initially populated with the result codes from Section 5.4, Section 8.2, and Section 9.2. New Result Codes can be created via Specification Required [RFC5226]. 13.4. Options IANA shall create a new registry for PCP Options, numbered 0-255 with an associated mnemonic. The values 0-127 are optional-to-process, and 128-255 are mandatory-to-process. The initial registry contains the options described in Section 8.5, and the option values 0 and 255 are reserved. New PCP option codes in the range 0-63 and 128-192 can be created via Wing, et al. Expires August 11, 2011 [Page 48] Internet-Draft Port Control Protocol (PCP) February 2011 Standards Action [RFC5226], and the range 64-127 and 192-255 is for Private Use [RFC5226]. 14. Acknowledgments Thanks to Alain Durand, Christian Jacquenet, and Simon Perreault for their comments and review. Thanks to Simon Perreault for highlighting the interaction of dynamic connections with PCP-created mappings. 15. References 15.1. Normative References [I-D.ietf-behave-v6v4-xlate] Li, X., Bao, C., and F. Baker, "IP/ICMP Translation Algorithm", draft-ietf-behave-v6v4-xlate-23 (work in progress), September 2010. [I-D.ietf-behave-v6v4-xlate-stateful] Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", draft-ietf-behave-v6v4-xlate-stateful-12 (work in progress), July 2010. [I-D.ietf-softwire-dual-stack-lite] Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual- Stack Lite Broadband Deployments Following IPv4 Exhaustion", draft-ietf-softwire-dual-stack-lite-06 (work in progress), August 2010. [I-D.ietf-tsvwg-port-randomization] Larsen, M. and F. Gont, "Transport Protocol Port Randomization Recommendations", draft-ietf-tsvwg-port-randomization-09 (work in progress), August 2010. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, October 2005. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, Wing, et al. Expires August 11, 2011 [Page 49] Internet-Draft Port Control Protocol (PCP) February 2011 May 2008. [RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M., and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, October 2010. [proto_numbers] IANA, "Protocol Numbers", 2010, . 15.2. Informative References [I-D.arkko-dual-stack-extra-lite] Arkko, J., Eggert, L., and M. Townsley, "Scalable Operation of Address Translators with Per-Interface Bindings", draft-arkko-dual-stack-extra-lite-05 (work in progress), February 2011. [I-D.cheshire-nat-pmp] Cheshire, S., "NAT Port Mapping Protocol (NAT-PMP)", draft-cheshire-nat-pmp-03 (work in progress), April 2008. [I-D.ietf-behave-lsn-requirements] Yamagata, I., Miyakawa, S., Nakagawa, A., and H. Ashida, "Common requirements for IP address sharing schemes", draft-ietf-behave-lsn-requirements-00 (work in progress), October 2010. [I-D.miles-behave-l2nat] Miles, D. and M. Townsley, "Layer2-Aware NAT", draft-miles-behave-l2nat-00 (work in progress), March 2009. [IGD] UPnP Gateway Committee, "WANIPConnection:1", November 2001, . [IGD-2] UPnP Gateway Committee, "Internet Gateway Device (IGD) V 2.0", September 2010, . [PCP-Issues] PCP Working Group, "PCP Active Tickets", January 2011, . [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. Wing, et al. Expires August 11, 2011 [Page 50] Internet-Draft Port Control Protocol (PCP) February 2011 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral Self-Address Fixing (UNSAF) Across Network Address Translation", RFC 3424, November 2002. [RFC3581] Rosenberg, J. and H. Schulzrinne, "An Extension to the Session Initiation Protocol (SIP) for Symmetric Response Routing", RFC 3581, August 2003. [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", BCP 131, RFC 4961, July 2007. [RFC5461] Gont, F., "TCP's Reaction to Soft Errors", RFC 5461, February 2009. [RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in Customer Premises Equipment (CPE) for Providing Residential IPv6 Internet Service", RFC 6092, January 2011. Appendix A. Changes A.1. Changes from draft-ietf-pcp-base-03 to -04 o "Pinhole" and "PIN" changed to "mapping" and "MAP". o Reduced from four MAP OpCodes to two. This was done by implicitly using the address family of the PCP message itself. o New option THIRD_PARTY, to more carefully split out the case where a mapping is created to a different host within the home. o Integrated a lot of editorial changes from Stuart and Francis. Wing, et al. Expires August 11, 2011 [Page 51] Internet-Draft Port Control Protocol (PCP) February 2011 o Removed nested NAT text into another document, including the IANA- registered IP addresses for the PCP server. o Removed suggestion (MAY) that PCP server reserve UDP when it maps TCP. Nobody seems to need that. o Clearly added NAT and NAPT, such as in residential NATs, as within scope for PCP. o HONOR_EXTERNAL_PORT renamed to PREFER_FAILURE o Added 'Lifetime' field to the common PCP header, which replaces the functions of the 'temporary' and 'permanent' error types of the previous version. o Allow arbitrary Options to be included in PCP response, so that PCP server can indicate un-supported PCP Options. Satisfies PCP Issue #19 o Reduced scope to only deal with mapping protocols that have port numbers. o Reduced scope to not support DMZ-style forwarding. o Clarified version negotiation. A.2. Changes from draft-ietf-pcp-base-02 to -03 o Adjusted abstract and introduction to make it clear PCP is intended to forward ports and intended to reduce application keepalives. o First bit in PCP common header is set. This allows DTLS and non- DTLS to be multiplexed on same port, should a future update to this specification add DTLS support. o Moved subscriber identity from common PCP section to MAP* section. o made clearer that PCP client can reduce mapping lifetime if it wishes. o Added discussion of host running a server, client, or symmetric client+server. o Introduced PEER4 and PEER6 OpCodes. o Removed REMOTE_PEER Option, as its function has been replaced by the new PEER OpCodes. Wing, et al. Expires August 11, 2011 [Page 52] Internet-Draft Port Control Protocol (PCP) February 2011 o IANA assigned port 44323 to PCP. o Removed AMBIGUOUS error code, which is no longer needed. A.3. Changes from draft-ietf-pcp-base-01 to -02 o more error codes o PCP client source port number should be random o PCP message minimum 8 octets, maximum 1024 octets. o tweaked a lot of text in section 7.4, "Opcode-Specific Server Operation". o opening a mapping also allows ICMP messages associated with that mapping. o PREFER_FAILURE value changed to the mandatory-to-process range. o added text recommending applications that are crashing obtain short lifetimes, to avoid consuming subscriber's port quota. A.4. Changes from draft-ietf-pcp-base-00 to -01 o Significant document reorganization, primarily to split base PCP operation from OpCode operation. o packet format changed to move 'protocol' outside of PCP common header and into the MAP* opcodes o Renamed Informational Elements (IE) to Options. o Added REMOTE_PEER (for disambiguation with dynamic ports), REMOTE_PEER_FILTER (for simple packet filtering), and PREFER_FAILURE (to optimize UPnP IGD interworking) options. o Is NAT or router behind B4 in scope? o PCP option MAY be included in a request, in which case it MUST appear in a response. It MUST NOT appear in a response if it wasn't in the request. o Response code most significant bit now indicates permanent/ temporary error o PCP Options are split into mandatory-to-process ("P" bit), and into Specification Required and Private Use. Wing, et al. Expires August 11, 2011 [Page 53] Internet-Draft Port Control Protocol (PCP) February 2011 o Epoch discussion simplified. Authors' Addresses Dan Wing (editor) Cisco Systems, Inc. 170 West Tasman Drive San Jose, California 95134 USA Email: dwing@cisco.com Stuart Cheshire Apple, Inc. 1 Infinite Loop Cupertino, California 95014 USA Phone: +1 408 974 3207 Email: cheshire@apple.com Mohamed Boucadair France Telecom Rennes, 35000 France Email: mohamed.boucadair@orange-ftgroup.com Reinaldo Penno Juniper Networks 1194 N Mathilda Avenue Sunnyvale, California 94089 USA Email: rpenno@juniper.net Francis Dupont Internet Systems Consortium Email: fdupont@isc.org Wing, et al. Expires August 11, 2011 [Page 54]