    Internet Engineering Task Force (IETF)                      F. Brockners
    Request for Comments: 6736                                   S. Bhandari
    Category: Standards Track                                          Cisco
    ISSN: 2070-1721                                                 V. Singh

                                                                  V. Fajardo
                                                      Telcordia Technologies
                                                                October 2012

       Diameter Network Address and Port Translation Control Application

    Abstract

       This document describes the framework, messages, and procedures for
       the Diameter Network address and port translation Control
       Application.  This Diameter application allows per-endpoint control
       of Network Address Translators and Network Address and Port
       Translators, which are added to networks to cope with IPv4 address
       space depletion.  This Diameter application allows external devices
       to configure and manage a Network Address Translator device --
       expanding the existing Diameter-based Authentication, Authorization,
       and Accounting (AAA) and policy control capabilities with a Network
       Address Translator and Network Address and Port Translator control
       component.  These external devices can be network elements in the
       data plane such as a Network Access Server, or can be more
       centralized control plane devices such as AAA-servers.  This Diameter
       application establishes a context to commonly identify and manage
       endpoints on a gateway or server and a Network Address Translator and
       Network Address and Port Translator device.  This includes, for
       example, the control of the total number of Network Address
       Translator bindings allowed or the allocation of a specific Network
       Address Translator binding for a particular endpoint.  In addition,
       it allows Network Address Translator devices to provide information
       relevant to accounting purposes.

    Status of This Memo

       This is an Internet Standards Track document.

       This document is a product of the Internet Engineering Task Force
       (IETF).  It represents the consensus of the IETF community.  It has
       received public review and has been approved for publication by the
       Internet Engineering Steering Group (IESG).  Further information on
       Internet Standards is available in Section 2 of RFC 5741.

       Information about the current status of this document, any errata,
       and how to provide feedback on it may be obtained at
       http://www.rfc-editor.org/info/rfc6736.

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    Copyright Notice

       Copyright (c) 2012 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. Conventions .....................................................6
       3. Deployment Framework ............................................7
          3.1. Deployment Scenario ........................................7
          3.2. Diameter NAPT Control Application Overview .................9
          3.3. Deployment Scenarios for DNCA .............................10
       4. DNCA Session Establishment and Management ......................12
          4.1. Session Establishment .....................................13
          4.2. Session Update ............................................16
          4.3. Session and Binding Query .................................18
          4.4. Session Termination .......................................20
          4.5. Session Abort .............................................21
          4.6. Failure Cases of the DNCA Diameter Peers ..................22
       5. Use of the Diameter Base Protocol ..............................23
          5.1. Securing Diameter Messages ................................23
          5.2. Accounting Functionality ..................................24
          5.3. Use of Sessions ...........................................24
          5.4. Routing Considerations ....................................24
          5.5. Advertising Application Support ...........................24
       6. DNCA Commands ..................................................25
          6.1. NAT-Control-Request (NCR) Command .........................25
          6.2. NAT-Control-Answer (NCA) Command ..........................26
       7. NAT Control Application Session State Machine ..................26
       8. DNCA AVPs ......................................................29
          8.1. Reused Base Protocol AVPs .................................29
          8.2. Additional Result-Code AVP Values .........................30
               8.2.1. Success ............................................30
               8.2.2. Transient Failures .................................30
               8.2.3. Permanent Failures .................................31

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          8.3. Reused NASREQ Diameter Application AVPs ...................32
          8.4. Reused AVPs from RFC 4675 .................................33
          8.5. Reused AVPs from Diameter QoS Application .................33
          8.6. Reused AVPs from ETSI ES 283 034, e4 Diameter
               Application ...............................................34
          8.7. DNCA-Defined AVPs .........................................35
               8.7.1. NC-Request-Type AVP ................................36
               8.7.2. NAT-Control-Install AVP ............................36
               8.7.3. NAT-Control-Remove AVP .............................37
               8.7.4. NAT-Control-Definition AVP .........................37
               8.7.5. NAT-Internal-Address AVP ...........................38
               8.7.6. NAT-External-Address AVP ...........................38
               8.7.7. Max-NAT-Bindings ...................................39
               8.7.8. NAT-Control-Binding-Template AVP ...................39
               8.7.9. Duplicate-Session-Id AVP ...........................39
               8.7.10. NAT-External-Port-Style AVP .......................39
       9. Accounting Commands ............................................40
          9.1. NAT Control Accounting Messages ...........................40
          9.2. NAT Control Accounting AVPs ...............................40
               9.2.1. NAT-Control-Record .................................41
               9.2.2. NAT-Control-Binding-Status .........................41
               9.2.3. Current-NAT-Bindings ...............................41
       10. AVP Occurrence Tables .........................................41
          10.1. DNCA AVP Table for NAT Control Initial and Update
                Requests .................................................42
          10.2. DNCA AVP Table for Session Query Requests ................43
          10.3. DNCA AVP Table for Accounting Messages ...................43
       11. IANA Considerations ...........................................44
          11.1. Application Identifier ...................................44
          11.2. Command Codes ............................................44
          11.3. AVP Codes ................................................44
          11.4. Result-Code AVP Values ...................................44
          11.5. NC-Request-Type AVP ......................................44
          11.6. NAT-External-Port-Style AVP ..............................45
          11.7. NAT-Control-Binding-Status AVP ...........................45
       12. Security Considerations .......................................45
       13. Examples ......................................................47
          13.1. DNCA Session Establishment Example .......................47
          13.2. DNCA Session Update with Port Style Example ..............50
          13.3. DNCA Session Query Example ...............................51
          13.4. DNCA Session Termination Example .........................53
       14. Acknowledgements ..............................................55
       15. References ....................................................55
          15.1. Normative References .....................................55
          15.2. Informative References ...................................56

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    1.  Introduction

       Internet service providers deploy Network Address Translators (NATs)
       and Network Address and Port Translators (NAPTs) [RFC3022] in their
       networks.  A key motivation for doing so is the depletion of
       available public IPv4 addresses.  This document defines a Diameter
       application allowing providers to control the behavior of NAT and
       NAPT devices that implement IPv4-to-IPv4 network address and port
       translation [RFC2663] as well as stateful IPv6-to-IPv4 address family
       translation as defined in [RFC2663], [RFC6145], and [RFC6146].  The
       use of a Diameter application allows for simple integration into the
       existing Authentication, Authorization, and Accounting (AAA)
       environment of a provider.

       The Diameter Network address and port translation Control Application
       (DNCA) offers the following capabilities:

       1.  Limits or defines the number of NAPT/NAT-bindings made available
           to an individual endpoint.  The main motivation for restricting
           the number of bindings on a per-endpoint basis is to protect the
           service of the service provider against denial-of-service (DoS)
           attacks.  If multiple endpoints share a single public IP address,
           these endpoints can share fate.  If one endpoint would (either
           intentionally, or due to misbehavior, misconfiguration, malware,
           etc.) be able to consume all available bindings for a given
           single public IP address, service would be hampered (or might
           even become unavailable) for those other endpoints sharing the
           same public IP address.  The efficiency of a NAPT deployment
           depends on the maximum number of bindings an endpoint could use.
           Given that the typical number of bindings an endpoint uses
           depends on the type of endpoint (e.g., a personal computer of a
           broadband user is expected to use a higher number of bindings
           than a simple mobile phone) and a NAPT device is often shared by
           different types of endpoints, it is desirable to actively manage
           the maximum number of bindings.  This requirement is specified in
           REQ-3 of [CGN-REQS].

       2.  Supports the allocation of specific NAPT/NAT-bindings.  Two types
           of specific bindings can be distinguished:

           *  Allocation of a predefined NAT-binding: The internal and
              external IP addresses as well as the port pair are specified
              within the request.  Some deployment cases, such as access to
              a web-server within a user's home network with IP address and
              port, benefit from statically configured bindings.

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           *  Allocation of an external IP address for a given internal IP
              address: The allocated external IP address is reported back to
              the requester.  In some deployment scenarios, the application
              requires immediate knowledge of the allocated binding for a
              given internal IP address but does not control the allocation
              of the external IP address; for example, SIP-proxy server
              deployments.

       3.  Defines the external address pool(s) to be used for allocating an
           external IP address: External address pools can be either pre-
           assigned at the NAPT/NAT device or specified within a request.
           If pre-assigned address pools are used, a request needs to
           include a reference to identify the pool.  Otherwise, the request
           contains a description of the IP address pool(s) to be used, for
           example, a list of IP-subnets.  Such external address pools can
           be used to select the external IP address in NAPT/NAT-bindings
           for multiple subscribers.

       4.  Generates reports and accounting records: Reports established
           bindings for a particular endpoint.  The collected information is
           used by accounting systems for statistical purposes.

       5.  Queries and retrieves details about bindings on demand: This
           feature complements the previously mentioned accounting
           functionality (see item 4).  This feature can be used by an
           entity to find NAT-bindings belonging to one or multiple
           endpoints on the NAT device.  The entity is not required to
           create a DNCA control session to perform the query but would,
           obviously, still need to create a Diameter session complying to
           the security requirements.

       6.  Identifies a subscriber or endpoint on multiple network devices
           (NAT/NAPT device, the AAA-server, or the Network Access Server
           (NAS)): Endpoint identification is facilitated through a Global
           Endpoint ID.  Endpoints are identified through a single
           classifier or a set of classifiers, such as IP address, Virtual
           Local Area Network (VLAN) identifier, or interface identifier
           that uniquely identify the traffic associated with a particular
           global endpoint.

       With the above capabilities, DNCA qualifies as a Middlebox
       Communications (MIDCOM) protocol [RFC3303], [RFC3304], [RFC5189] for
       middleboxes that perform NAT.  The MIDCOM protocol evaluation
       [RFC4097] evaluated Diameter as a candidate protocol for MIDCOM.
       DNCA provides the extensions to the Diameter base protocol [RFC6733]
       following the MIDCOM protocol requirements, such as the support of
       NAT-specific rule transport, support for oddity of mapped ports, as
       well as support for consecutive range port numbers.  DNCA adds to the

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       MIDCOM protocol capabilities in that it allows the maintenance of the
       reference to an endpoint representing a user or subscriber in the
       control operation, enabling the control of the behavior of a NAT
       device on a per-endpoint basis.  Following the requirements of
       different operators and deployments, different management protocols
       are employed.  Examples include, for example, Simple Network
       Management Protocol (SNMP) [RFC3411] and Network Configuration
       (NETCONF) [RFC6241], which can both be used for device configuration.
       Similarly, DNCA complements existing MIDCOM implementations, offering
       a MIDCOM protocol option for operators with an operational
       environment that is Diameter focused that desire the use of Diameter
       to perform per-endpoint NAT control.  Note that in case an operator
       uses multiple methods and protocols to configure a NAT device, such
       as, for example, command line interface (CLI), SNMP, NETCONF, or Port
       Control Protocol (PCP), along with DNCA specified in this document,
       the operator MUST ensure that the configurations performed using the
       different methods and protocols do not conflict in order to ensure a
       proper operation of the NAT service.

       This document is structured as follows: Section 2 lists terminology,
       while Section 3 provides an introduction to DNCA and its overall
       deployment framework.  Sections 3.2 to 8 cover DNCA specifics, with
       Section 3.2 describing session management, Section 5 the use of the
       Diameter base protocol, Section 6 new commands, Section 8 Attribute
       Value Pairs (AVPs) used, and Section 9 accounting aspects.
       Section 10 presents AVP occurrence tables.  IANA and security
       considerations are addressed in Sections 11 and 12, respectively.

    2.  Conventions

       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 [RFC2119].

       Abbreviations and terminology used in this document:

          AAA: Authentication, Authorization, Accounting

          DNCA: Diameter Network address and port translation Control
          Application

          Endpoint: Managed entity of the DNCA.  An endpoint represents a
          network element or device, associated with a subscriber, a user,
          or a group of users.  An endpoint is represented by a single
          access-session on a NAS.  DNCA assumes a 1:1 relationship between
          an endpoint, the access-session it represents, and the associated
          DNCA session.

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          NAPT: Network Address and Port Translation, see also [RFC3022].

          NAT: Network Address Translation (NAT and NAPT are used in this
          document interchangeably)

          NAT-binding or binding: Association of two IP address/port pairs
          (with one IP address typically being private and the other one
          public) to facilitate NAT

          NAT-binding predefined template: A policy template or
          configuration that is predefined at the NAT device.  It may
          contain NAT-bindings, IP address pools for allocating the external
          IP address of a NAT-binding, the maximum number of allowed NAT-
          bindings for endpoints, etc.

          NAT device: Network Address Translator or Network Address and Port
          Translator: An entity performing NAT or NAPT.

          NAT controller: Entity controlling the behavior of a NAT device.

          NAS: Network Access Server

          NCR: NAT-Control-Request

          NCA: NAT-Control-Answer

          NAT44: IPv4-to-IPv4 NAPT, see [RFC2663]

          NAT64: IPv6-to-IPv4 address family translation, see [RFC6145] and
          [RFC6146]

          PPP: Point-to-Point Protocol [RFC1661]

    3.  Deployment Framework

    3.1.  Deployment Scenario

       Figure 1 shows a typical network deployment for IPv4 Internet access.
       A user's IPv4 host (i.e., endpoint) gains access to the Internet
       though a NAS, which facilitates the authentication of the endpoint
       and configures the endpoint's connection according to the
       authorization and configuration data received from the AAA-server
       upon successful authentication.  Public IPv4 addresses are used
       throughout the network.  DNCA manages an endpoint that represents a
       network element or device or an IPv4 host, associated with a
       subscriber, a user or a group of users.  An endpoint is represented

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       by a single access-session on a NAS.  DNCA assumes a 1:1:1
       relationship between an endpoint, the access-session it represents,
       and the associated DNCA session.

                             +---------+
                             |         |
                             |   AAA   |
                             |         |
                             +---------+
                                  |
                                  |
                                  |
                                  |
        +---------+          +---------+             +----------+
        |  IPv4   |          |         |             |  IPv4    |
        |  Host   |----------|   NAS   |-------------| Internet |
        |         |          |         |             |          |
        +---------+          +---------+             +----------+

        <-------------------- Public IPv4 ---------------------->

             Figure 1: Typical Network Deployment for Internet Access

       Figure 2 depicts the deployment scenario where a service provider
       places a NAT between the host and the public Internet.  The objective
       is to provide the customer with connectivity to the public IPv4
       Internet.  The NAT device performs network address and port (and
       optionally address family) translation, depending on whether the
       access network uses private IPv4 addresses or public IPv6 addresses
       to public IPv4 addresses.  Note that there may be more than one NAS,
       NAT device, or AAA-entity in a deployment, although the figures only
       depict one entity each for clarity.

       If the NAT device would be put in place without any endpoint
       awareness, the service offerings of the service provider could be
       impacted as detailed in [CGN-REQS].  This includes cases like the
       following:

       o  Provisioning static NAT-bindings for particular endpoints

       o  Using different public IP address pools for a different set of
          endpoints (for example, residential or business customers)

       o  Reporting allocated bindings on a per-endpoint basis

       o  Integrate control of the NAT device into the already existing per-
          endpoint management infrastructure of the service provider

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                       +---------+
                       |         |
                       |   AAA   |
                       |         |
                       +---------+
                            |
                            |
                            |
                            |
         +--------+    +---------+    +--------+    +----------+
         |  IPv4  |----|         |----|  NAT-  |----| IPv4     |
         |  Host  |    |   NAS   |    | device |    | Internet |
         |        |    |         |    |        |    |          |
         +--------+    +---------+    +--------+    +----------+

       For NAT44 deployments (IPv4 host):
            <----- Private IPv4 ----------><--- Public IPv4 --->

       For NAT64 deployments (IPv6 host):
            <----- Public  IPv6 ----------><--- Public IPv4 --->

                   Figure 2: Access Network Deployment with NAT

       Figure 2 shows a typical deployment for IPv4 Internet access
       involving a NAT device within the service provider network.  The
       figure describes two scenarios: one where an IPv4 host (with a
       private IPv4 address) accesses the IPv4 Internet, as well as one
       where an IPv6-host accesses the IPv4 Internet.

    3.2.  Diameter NAPT Control Application Overview

       DNCA runs between two DNCA Diameter peers.  One DNCA Diameter peer
       resides within the NAT device, the other DNCA Diameter peer resides
       within a NAT controller (discussed in Section 3.3).  DNCA allows per-
       endpoint control and management of NAT within the NAT device.  Based
       on Diameter, DNCA integrates well with the suite of Diameter
       applications deployed for per-endpoint authentication, authorization,
       accounting, and policy control in service provider networks.

       DNCA offers:

       o  Request and answer commands to control the allowed number of NAT-
          bindings per endpoint, to request the allocation of specific
          bindings for an endpoint, to define the address pool to be used
          for an endpoint.

       o  Per-endpoint reporting of the allocated NAT-bindings.

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       o  Unique identification of an endpoint on a NAT device, AAA-server,
          and NAS to simplify correlation of accounting data streams.

       DNCA allows controlling the behavior of a NAT device on a per-
       endpoint basis during initial session establishment and at later
       stages by providing an update procedure for already established
       sessions.  Using DNCA, per-endpoint NAT-binding information can be
       retrieved using either accounting mechanisms or an explicit session
       query to the NAT.

    3.3.  Deployment Scenarios for DNCA

       DNCA can be deployed in different ways.  DNCA supports deployments
       with "n" NAT controllers and "m" NAT devices, with n and m equal to
       or greater than 1.  From a DNCA perspective, an operator should
       ensure that the session representing a particular endpoint is atomic.
       Any deployment MUST ensure that, for any given endpoint, only a
       single DNCA NAT controller and is active at any point in time.  This
       is to ensure that NAT devices controlled by multiple NAT controllers
       do not receive conflicting control requests for a particular endpoint
       or that they would not be unclear about to which NAT controller to
       send accounting information.  Operational considerations MAY require
       an operator to use alternate control mechanisms or protocols such as
       SNMP or manual configuration via a CLI to apply per-endpoint NAT-
       specific configuration, for example, static NAT-bindings.  For these
       cases, the NAT device MUST allow the operator to configure a policy
       on how configuration conflicts are resolved.  Such a policy could
       specify, for example, that manually configured NAT-bindings using the
       CLI always take precedence over those configured using DNCA.

       Two common deployment scenarios are outlined in Figure 3 ("Integrated
       Deployment") and Figure 4 ("Autonomous Deployment").  Per the note
       above, multiple instances of NAT controllers and NAT devices could be
       deployed.  The figures only show single instances for reasons of
       clarity.  The two shown scenarios differ in which entity fulfills the
       role of the NAT controller.  Within the figures, (C) denotes the
       network element performing the role of the NAT controller.

       The integrated deployment approach hides the existence of the NAT
       device from external servers, such as the AAA-server.  It is suited
       for environments where minimal changes to the existing AAA deployment
       are desired.  The NAS and the NAT device are Diameter peers
       supporting the DNCA.  The Diameter peer within the NAS, performing
       the role of the NAT controller, initiates and manages sessions with
       the NAT device, exchanges NAT-specific configuration information, and
       handles reporting and accounting information.  The NAS receives
       reporting and accounting information from the NAT device.  With this

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       information, the NAS can provide a single accounting record for the
       endpoint.  A system correlating the accounting information received
       from the NAS and NAT device would not be needed.

       An example network attachment for an integrated NAT deployment can be
       described as follows: an endpoint connects to the network, with the
       NAS being the point of attachment.  After successful authentication,
       the NAS receives endpoint-related authorization data from the AAA-
       server.  A portion of the authorization data applies to per-endpoint
       configuration on the NAS itself, another portion describes
       authorization and configuration information for NAT control aimed at
       the NAT device.  The NAS initiates a DNCA session to the NAT device
       and sends relevant authorization and configuration information for
       the particular endpoint to the NAT device.  This can comprise NAT-
       bindings, which have to be pre-established for the endpoint, or
       management-related configuration, such as the maximum number of NAT-
       bindings allowed for the endpoint.  The NAT device sends its per-
       endpoint accounting information to the NAS, which aggregates the
       accounting information received from the NAT device with its local
       accounting information for the endpoint into a single accounting
       stream towards the AAA-server.

                       +---------+
                       |         |
                       |   AAA   |
                       |         |
                       +---------+
                            |
                            |
                            |
         +--------+    +---------+    +--------+    +----------+
         |        |    |   (C)   |    |        |    |          |
         |  Host  |----|   NAS   |----|  NAT-  |----| IPv4     |
         |        |    |         |    | device |    | Internet |
         +--------+    +---------+    +--------+    +----------+

       For NAT44 deployments (IPv4 host):
            <----- Private IPv4 ----------><--- Public IPv4 --->

       For NAT64 deployments (IPv6 host):
            <----- Public  IPv6 ----------><--- Public IPv4 --->

              Figure 3: NAT Control Deployment: Integrated Deployment

       Figure 3 shows examples of integrated deployments.  It illustrates
       two scenarios: one where an IPv4 host (with a private IPv4 address)
       accesses the IPv4 Internet and another where an IPv6 host accesses
       the IPv4 Internet.

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       The autonomous deployment approach decouples endpoint management on
       the NAS and NAT device.  In the autonomous deployment approach, the
       AAA-system and the NAT device are the Diameter peers running the
       DNCA.  The AAA-system also serves as NAT controller.  It manages the
       connection to the NAT device, controls the per-endpoint
       configuration, and receives accounting and reporting information from
       the NAT device.  Different from the integrated deployment scenario,
       the autonomous deployment scenario does not "hide" the existence of
       the NAT device from the AAA infrastructure.  Here, two accounting
       streams are received by the AAA-server for one particular endpoint:
       one from the NAS and one from the NAT device.

                       +---------+
                       |   (C)   |
                       |   AAA   |---------
                       |         |         |
                       +---------+         |
                            |              |
                            |              |
                            |              |
         +--------+    +---------+    +---------+    +----------+
         |  IPv4/ |    |         |    |         |    |  IPv4    |
         |  IPv6  |----|   NAS   |----|  NAT-   |----| Internet |
         |  Host  |    |         |    | device  |    |          |
         +--------+    +---------+    +---------+    +----------+

       For NAT44 deployments (IPv4 host):
            <----- Private IPv4 ----------><--- Public IPv4 --->

       For NAT64 deployments (IPv6 host):
            <----- Public  IPv6 ----------><--- Public IPv4 --->

              Figure 4: NAT Control Deployment: Autonomous Deployment

       Figure 4 shows examples of autonomous deployments.  It illustrates
       two scenarios: one where an IPv4 host (with a private IPv4 address)
       accesses the IPv4 Internet and another where an IPv6 host accesses
       the IPv4 Internet.

    4.  DNCA Session Establishment and Management

       Note that from this section on, there are references to some of the
       commands and AVPs defined for DNCA.  Please refer to Sections 6 and 8
       for details.  DNCA runs between a Diameter peer residing in a NAT
       controller and a Diameter peer residing in a NAT device.  Note that,
       per what was already mentioned above, each DNCA session between
       Diameter peers in a NAT controller and a NAT device represents a
       single endpoint, with an endpoint being either a network element, a

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       device, or an IPv4 host associated with a subscriber, a user, or a
       group of users.  The Diameter peer within the NAT controller is
       always the control-requesting entity: it initiates, updates, or
       terminates the sessions.  Sessions are initiated when the NAT
       controller learns about a new endpoint (i.e., host) that requires a
       NAT service.  This could be due to, for example, the entity hosting
       the NAT controller receiving authentication, authorization, or
       accounting requests for or from the endpoint.  Alternate methods that
       could trigger session setup include local configuration, receipt of a
       packet from a formerly unknown IP address, etc.

    4.1.  Session Establishment

       The DNCA Diameter peer within the NAT controller establishes a
       session with the DNCA Diameter peer within the NAT device to control
       the behavior of the NAT function within the NAT device.  During
       session establishment, the DNCA Diameter peer within the NAT
       controller passes along configuration information to the DNCA
       Diameter peer within the NAT device.  The session configuration
       information comprises the maximum number of bindings allowed for the
       endpoint associated with this session, a set of predefined NAT-
       bindings to be established for this endpoint, or a description of the
       address pool, from which external addresses are to be allocated.

       The DNCA Diameter peer within the NAT controller generates a NAT-
       Control-Request (NCR) message to the DNCA Diameter peer within the
       NAT device with the NC-Request-Type AVP set to INITIAL_REQUEST to
       initiate a Diameter NAT control session.  On receipt of an NCR, the
       DNCA Diameter peer within the NAT device sets up a new session for
       the endpoint associated with the endpoint classifier(s) contained in
       the NCR.  The DNCA Diameter peer within the NAT device notifies its
       DNCA Diameter peer within the NAT controller about successful session
       setup using a NAT-Control-Answer (NCA) message with the Result-Code
       set to DIAMETER_SUCCESS.  Figure 5 shows the initial protocol
       interaction between the two DNCA Diameter peers.

       The initial NAT-Control-Request MAY contain configuration information
       for the session, which specifies the behavior of the NAT device for
       the session.  The configuration information that MAY be included,
       comprises:

       o  A list of NAT-bindings, which should be pre-allocated for the
          session; for example, in case an endpoint requires a fixed
          external IP address/port pair for an application.

       o  The maximum number of NAT-bindings allowed for an endpoint.

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    RFC 6736            Diameter NAT Control Application        October 2012

       o  A description of the external IP address pool(s) to be used for
          the session.

       o  A reference to a NAT-binding Predefined template on the NAT
          device, which is applied to the session.  Such a NAT-binding
          Predefined template on the NAT device may contain, for example,
          the name of the IP address pool from which external IP addresses
          should be allocated, the maximum number of bindings permitted for
          the endpoint, etc.

       In certain cases, the NAT device may not be able to perform the tasks
       requested within the NCR.  These include the following:

       o  If a DNCA Diameter peer within the NAT device receives an NCR from
          a DNCA Diameter peer within a NAT controller with the NC-Request-
          Type AVP set to INITIAL_REQUEST that identifies an already
          existing session, that is, the endpoint identifier matches an
          already existing session, the DNCA Diameter peer within the NAT
          device MUST return an NCA with the Result-Code set to
          SESSION_EXISTS and provide the Session-Id of the existing session
          in the Duplicate-Session-Id AVP.

       o  If a DNCA Diameter peer within the NAT device receives an NCR from
          a DNCA Diameter peer within a NAT controller with the NC-Request-
          Type AVP set to INITIAL_REQUEST that matches more than one of the
          already existing sessions, that is, the DNCA Diameter peer and
          endpoint identifier match already existing sessions, the DNCA
          Diameter peer within the NAT device MUST return an NCA with the
          Result-Code set to INSUFFICIENT-CLASSIFIERS.  In case a DNCA
          Diameter peer receives an NCA that reports Insufficient-
          Classifiers, it MAY choose to retry establishing a new session
          using additional or more specific classifiers.

       o  If the NCR contains a NAT-binding Predefined template not defined
          on the NAT device, the DNCA Diameter peer within the NAT device
          MUST return an NCA with the Result-Code AVP set to
          UNKNOWN_BINDING_TEMPLATE_NAME.

       o  In case the NAT device is unable to establish all of the bindings
          requested in the NCR, the DNCA Diameter peer MUST return an NCA
          with the Result-Code set to BINDING_FAILURE.  A DNCA Diameter peer
          within a NAT device MUST treat an NCR as an atomic operation;
          hence, none of the requested bindings will be established by the
          NAT device.  Either all requested actions within an NCR MUST be
          completed successfully or the entire request fails.

    Brockners, et al.            Standards Track                   [Page 14]

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       o  If a NAT device cannot conform to a request to set the maximum
          number of NAT-bindings allowed for a session, the DNCA Diameter
          peer in the NAT device MUST return an NCA with the Result-Code AVP
          set to MAX_BINDINGS_SET_FAILURE.  Such a condition can, for
          example, occur if the operator specified the maximum number of
          NAT-bindings through another mechanism, which, per the operator's
          policy, takes precedence over DNCA.

       o  If a NAT device does not have sufficient resources to process a
          request, the DNCA Diameter peer MUST return an NCA with the
          Result-Code set to RESOURCE_FAILURE.

       o  In the case where Max-NAT-Bindings, NAT-Control-Definition, and
          NAT-Control-Binding-Template are included in the NCR, and the
          values in Max-NAT-Bindings and NAT-Control-Definition contradict
          those specified in the pre-provisioned template on the NAT device
          that NAT-Control-Binding-Template references, Max-NAT-Bindings and
          NAT-Control-Definition MUST override the values specified in the
          template to which NAT-Control-Binding-Template refers.

       NAT controller (DNCA Diameter peer)   NAT device (DNCA Diameter peer)
                   |                                           |
                   |                                           |
                   |                                           |
                Trigger                                        |
                   |                                           |
                   |                   NCR                     |
                   |------------------------------------------>|
                   |                                           |
                   |                                           |
                   |                                           |
                   |                                           |
                   |                                 If able to comply
                   |                                 with request, then
                   |                                 create session state
                   |                                           |
                   |                                           |
                   |                     NCA                   |
                   |<------------------------------------------|
                   |                                           |
                   |                                           |

          Figure 5: Initial NAT-Control-Request and Session Establishment

       Note: The DNCA Diameter peer within the NAT device creates session
       state only if it is able to comply with the NCR.  On success, it will
       reply with an NCA with the Result-Code set to DIAMETER_SUCCESS.

    Brockners, et al.            Standards Track                   [Page 15]

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    4.2.  Session Update

       A session update is performed if the NAT controller desires to change
       the behavior of the NAT device for an existing session.  A session
       update could be used, for example, to change the number of allowed
       bindings for a particular session or establish or remove a predefined
       binding.

       The DNCA Diameter peer within the NAT controller generates an NCR
       message to the DNCA Diameter peer within the NAT device with the NC-
       Request-Type AVP set to UPDATE_REQUEST upon receiving a trigger
       signal.  If the session is updated successfully, the DNCA Diameter
       peer within the NAT device notifies the DNCA Diameter peer within the
       NAT controller about the successful session update using a NAT-
       Control-Answer (NCA) message with the Result-Code set to
       DIAMETER_SUCCESS.  Figure 6 shows the protocol interaction between
       the two DNCA Diameter peers.

       In certain cases, the NAT device may not be able to perform the tasks
       requested within the NCR.  These include the following:

       o  If a DNCA Diameter peer within a NAT device receives an NCR update
          or query request for a non-existent session, it MUST set the
          Result-Code in the answer to DIAMETER_UNKNOWN_SESSION_ID.

       o  If the NCR contains a NAT-binding Predefined template not defined
          on the NAT device, an NCA with the Result-Code AVP set to
          UNKNOWN_BINDING_TEMPLATE_NAME MUST be returned.

       o  If the NAT device cannot establish the requested binding because
          the maximum number of allowed bindings has been reached for the
          endpoint classifier, an NCA with the Result-Code AVP set to
          MAXIMUM_BINDINGS_REACHED_FOR_ENDPOINT MUST be returned to the DNCA
          Diameter peer.

       o  If the NAT device cannot establish some or all of the bindings
          requested in an NCR, but has not yet reached the maximum number of
          allowed bindings for the endpoint, an NCA with the Result-Code set
          to BINDING_FAILURE MUST be returned.  As already noted, the DNCA
          Diameter peer in a NAT device MUST treat an NCR as an atomic
          operation.  Hence, none of the requested bindings will be
          established by the NAT device in case of failure.  Actions
          requested within an NCR are either all successful or all fail.

       o  If the NAT device cannot conform to a request to set the maximum
          number of bindings allowed for a session as specified by the Max-
          NAT-Bindings, the DNCA Diameter peer in the NAT device MUST return
          an NCA with the Result-Code AVP set to MAX_BINDINGS_SET_FAILURE.

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       o  If the NAT device does not have sufficient resources to process a
          request, an NCA with the Result-Code set to RESOURCE_FAILURE MUST
          be returned.

       o  If an NCR changes the maximum number of NAT-bindings allowed for
          the endpoint defined through an earlier NCR, the new value MUST
          override any previously defined limit on the maximum number of
          NAT-bindings set through the DNCA.  Note that, prior to
          overwriting an existing value, the NAT device MUST check whether
          the overwrite action conforms to the locally configured policy.
          Deployment dependent, an existing value could have been set by a
          protocol or mechanism different from DNCA and with higher
          priority.  In which case, the NAT device will refuse the change
          and the DNCA Diameter peer in the NAT device MUST return an NCA
          with the Result-Code AVP set to MAX_BINDINGS_SET_FAILURE.  It
          depends on the implementation of the NAT device on how the NAT
          device copes with a case where the new value is lower than the
          actual number of allocated bindings.  The NAT device SHOULD
          refrain from enforcing the new limit immediately (that is,
          actively remove bindings), but rather disallows the establishment
          of new bindings until the current number of bindings is lower than
          the newly established maximum number of allowed bindings.

       o  If an NCR specifies a new NAT-binding Predefined template on the
          NAT device, the NAT-binding Predefined template overrides any
          previously defined rule for the session.  Existing NAT-bindings
          SHOULD NOT be impacted by the change of templates.

       o  In case Max-NAT-Bindings, NAT-Control-Definition, and NAT-Control-
          Binding-Template are included in the NCR, and the values in Max-
          NAT-Bindings and NAT-Control-Definition contradict those specified
          in the pre-provisioned template on the NAT device that NAT-
          Control-Binding-Template references, Max-NAT-Bindings and NAT-
          Control-Definition MUST override the values specified in the
          template to which the NAT-Control-Binding-Template refers.

       Note: Already established bindings for the session SHOULD NOT be
       affected in case the tasks requested within the NCR cannot be
       completed.

    Brockners, et al.            Standards Track                   [Page 17]

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       NAT controller (DNCA Diameter peer)   NAT device (DNCA Diameter peer)
                   |                                           |
                   |                                           |
                   |                                           |
            Change of session                                  |
               attributes                                      |
                   |                                           |
                   |                   NCR                     |
                   |------------------------------------------>|
                   |                                           |
                   |                                           |
                   |                                   If able to comply
                   |                                   with the request:
                   |                                  update session state
                   |                                           |
                   |                                           |
                   |                     NCA                   |
                   |<------------------------------------------|
                   |                                           |

                 Figure 6: NAT-Control-Request for Session Update

    4.3.  Session and Binding Query

       A session and NAT-binding query MAY be used by the DNCA Diameter peer
       within the NAT controller either to retrieve information on the
       current bindings for a particular session at the NAT device or to
       discover the session identifier for a particular external IP address/
       port pair.

       A DNCA Diameter peer within the NAT controller starts a session query
       by sending an NCR message with NC-Request-Type AVP set to
       QUERY_REQUEST.  Figure 7 shows the protocol interaction between the
       DNCA Diameter peers.

       Two types of query requests exist.  The first type of query request
       uses the Session-Id as input parameter to the query.  It is to allow
       the DNCA Diameter peer within the NAT controller to retrieve the
       current set of bindings for a specific session.  The second type of
       query request is used to retrieve the session identifiers, along with
       the associated bindings, matching a criteria.  This enables the DNCA
       Diameter peer within the NAT controller to find those sessions, which
       utilize a specific external or internal IP address.

       1.  Request a list of currently allocated NAT-bindings for a
           particular session: On receiving an NCR, the NAT device SHOULD
           look up the session information for the Session-Id contained in
           the NCR and report all currently active NAT-bindings for the

    Brockners, et al.            Standards Track                   [Page 18]

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           session using an NCA message with the Result-Code set to
           DIAMETER_SUCCESS.  In this case, the NCR MUST NOT contain a NAT-
           Control-Definition AVP.  Each NAT-binding is reported in a NAT-
           Control-Definition AVP.  In case the Session-Id is unknown, the
           DNCA Diameter peer within the NAT device MUST return an NCA
           message with the Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.

       2.  Retrieve Session-Ids and bindings for internal IP address or one
           or multiple external IP address/port pairs: If the DNCA Diameter
           peer within the NAT controller wishes to retrieve the Session-
           Id(s) for an internal IP address or one or multiple external IP
           address/port pairs, it MUST include the internal IP address as
           part of the Framed-IP-Address AVP or external IP address/port
           pair(s) as part of the NAT-External-Address AVP of the NCR.  The
           external IP address/port pair(s) are known in advance by the
           controller via configuration, AAA interactions, or other means.
           The Session-Id is not included in the NCR or the NCA for this
           type of a query.  The DNCA Diameter peer within the NAT device
           SHOULD report the NAT-bindings and associated Session-Ids
           corresponding to the internal IP address or external IP address/
           port pairs in an NCA message using one or multiple instances of
           the NAT-Control-Definition AVP.  The Result-Code is set to
           DIAMETER_SUCCESS.  In case an external IP address/port pair has
           no associated existing NAT-binding, the NAT-Control-Definition
           AVP contained in the reply just contains the NAT-External-Address
           AVP.

    Brockners, et al.            Standards Track                   [Page 19]

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       NAT controller (DNCA Diameter peer)   NAT device (DNCA Diameter peer)
                   |                                           |
                   |                                           |
                   |                                           |
         DNCA Session Established                              |
                   |                                           |
                   |                   NCR                     |
                   |------------------------------------------>|
                   |                                           |
                   |                                           |
                   |                                           |
                   |                                           |
                   |                          Look up corresponding session
                   |                            and associated NAT-bindings
                   |                                           |
                   |                   NCA                     |
                   |<------------------------------------------|
                   |                                           |
                   |                                           |
                   |                                           |

                              Figure 7: Session Query

    4.4.  Session Termination

       Similar to session initiation, session tear down MUST be initiated by
       the DNCA Diameter peer within the NAT controller.  The DNCA Diameter
       peer sends a Session-Termination-Request (STR) message to its peer
       within the NAT device upon receiving a trigger signal.  The source of
       the trigger signal is outside the scope of this document.  As part of
       STR-message processing, the DNCA Diameter peer within the NAT device
       MAY send an accounting stop record reporting all bindings.  All the
       NAT-bindings belonging to the session MUST be removed, and the
       session state MUST be cleaned up.  The DNCA Diameter peer within the
       NAT device MUST notify its DNCA Diameter peer in the NAT controller
       about successful session termination using a Session-Termination-
       Answer (STA) message with Result-Code set to DIAMETER_SUCCESS.
       Figure 8 shows the protocol interaction between the two DNCA Diameter
       peers.

       If a DNCA Diameter peer within a NAT device receives an STR and fails
       to find a matching session, the DNCA Diameter peer MUST return an STA
       with the Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.

    Brockners, et al.            Standards Track                   [Page 20]

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       NAT controller (DNCA Diameter peer)   NAT device (DNCA Diameter peer)
                   |                                            |
                   |                                            |
                Trigger                                         |
                   |                                            |
                   |                   STR                      |
                   |------------------------------------------->|
                   |                                            |
                   |                                            |
                   |                                            |
                   |                                            |
                   |                                            |
                   |           Send accounting stop             |
                   |<-------------------------------------------|
                   |       reporting all session bindings       |
                   |                                            |
                   |                                            |
                   |                                  Remove NAT-bindings
                   |                                       of session
                   |                                            |
                   |                                  Terminate session /
                   |                                 Remove session state
                   |                                            |
                   |                                            |
                   |                                            |
                   |                  STA                       |
                   |<-------------------------------------------|
                   |                                            |
                   |                                            |

                      Figure 8: Terminate NAT Control Session

    4.5.  Session Abort

       An Abort-Session-Request (ASR) message is sent from the DNCA Diameter
       peer within the NAT device to the DNCA Diameter peer within the NAT
       controller when it is unable to maintain a session due to resource
       limitations.  The DNCA Diameter peer within the NAT controller MUST
       acknowledge a successful session abort using an Abort-Session-Answer
       (ASA) message with the Result-Code set to DIAMETER_SUCCESS.  Figure 9
       shows the protocol interaction between the DNCA Diameter peers.  The
       DNCA Diameter peers will start a session termination procedure as
       described in Section 4.4 following an ASA with the Result-Code set to
       DIAMETER_SUCCESS.

       If the DNCA Diameter peer within a NAT controller receives an ASR but
       fails to find a matching session, it MUST return an ASA with the
       Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.  If the DNCA Diameter

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       peer within the NAT controller is unable to comply with the ASR for
       any other reason, an ASA with the Result-Code set to
       DIAMETER_UNABLE_TO_COMPLY MUST be returned.

       NAT controller (DNCA Diameter peer)   NAT device (DNCA Diameter peer)
                   |                                            |
                   |                                            |
                   |                                         Trigger
                   |                                            |
                   |                   ASR                      |
                   |<-------------------------------------------|
                   |                                            |
                   |                                            |
                   |                                            |
                   |                  ASA                       |
                   |------------------------------------------->|
                   |                                            |
                   |                                            |
                   |                                            |
                   |           On successful ASA                |
                   |<------Session Termination Procedure------->|

                        Figure 9: Abort NAT Control Session

    4.6.  Failure Cases of the DNCA Diameter Peers

       This document does not specify the behavior in case the NAT device
       and NAT controller, or their respective DNCA Diameter peers, are out
       of sync or lose state.  This could happen, for example, if one of the
       entities restarts, in case of a (temporary) loss of network
       connectivity, etc.  Example failure cases include the following:

       o  NAT controller and the DNCA Diameter peer within the NAT
          controller lose state (e.g., due to a restart).  In this case:

          *  the DNCA Diameter peer within the NAT device MAY receive an NCR
             with the NC-Request-Type AVP set to INITIAL_REQUEST that
             matches an existing session of the DNCA Diameter peer within
             the NAT device.  The DNCA Diameter peer within the NAT device
             MUST return a Result-Code that contains a Duplicate-Session-Id
             AVP to report the Session-Id of the existing session.  The DNCA
             Diameter peer within the NAT controller MAY send an explicit
             Session-Termination-Request (STR) for the older session, which
             was lost.

          *  a DNCA Diameter peer MAY receive accounting records for a
             session that does not exist.  The DNCA Diameter peer sends an
             accounting answer with the Result-Code set to

    Brockners, et al.            Standards Track                   [Page 22]

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             DIAMETER_UNKNOWN_SESSION_ID in response.  On receiving the
             response, the DNCA Diameter peer SHOULD clear the session and
             remove associated session state.

       o  The NAT device and the DNCA Diameter peer within NAT device lose
          state.  In such a case, the DNCA Diameter peer MAY receive an NCR
          with the NC-Request-Type AVP set to UPDATE_REQUEST for a non-
          existent session.  The DNCA Diameter peer MUST return an NCA with
          the Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.  When a DNCA
          application within a NAT controller receives this NCA with the
          Result-Code set to DIAMETER_UNKNOWN_SESSION_ID, it MAY try to re-
          establish DNCA session or disconnect corresponding access session.

       o  The DNCA Diameter peer within the NAT controller is unreachable,
          for example, it is detected by Diameter device watchdog messages
          (as defined in Section 5.5 of [RFC6733]) or accounting requests
          from the DNCA Diameter peer fail to get a response, NAT-bindings
          and NAT device state pertaining to that session MUST be cleaned up
          after a grace period that is configurable on the NAT device.  The
          grace period can be configured as zero or higher, depending on
          operator preference.

       o  The DNCA Diameter peer within the NAT device is unreachable or
          down and the NCR fails to get a response.  Handling of this case
          depends on the actual service offering of the service provider.
          The service provider could, for example, choose to stop offering
          connectivity service.

       o  A discussion of the mechanisms used for a NAT device to clean up
          state in case the DNCA Diameter peer within the NAT device crashes
          is outside the scope of this document.  Implementers of NAT
          devices could choose from a variety of options such as coupling
          the state (e.g., NAT-bindings) to timers that require periodic
          refresh, or time out otherwise, operating system watchdogs for
          applications, etc.

    5.  Use of the Diameter Base Protocol

       The Diameter base protocol [RFC6733] applies with the clarifications
       listed in the present specification.

    5.1.  Securing Diameter Messages

       For secure transport of Diameter messages, the recommendations in
       [RFC6733] apply.

       DNCA Diameter peers SHOULD verify their identity during the
       Capabilities Exchange Request procedure.

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       A DNCA Diameter peer within the NAT device SHOULD verify that a DNCA
       Diameter peer that issues an NCR command is allowed to do so based
       on:

       o  The identity of the DNCA Diameter peer

       o  The type of NCR Command

       o  The content of the NCR Command

       o  Any combination of the above

    5.2.  Accounting Functionality

       Accounting functionality (the accounting session state machine,
       related Command Codes and AVPs) is defined in Section 9.

    5.3.  Use of Sessions

       Each DNCA session MUST have a globally unique Session-Id, as defined
       in [RFC6733], which MUST NOT be changed during the lifetime of the
       DNCA session.  The Diameter Session-Id serves as the global endpoint
       identifier.  The DNCA Diameter peers maintain state associated with
       the Session-Id.  This globally unique Session-Id is used for
       updating, accounting, and terminating the session.  A DNCA session
       MUST NOT have more than one outstanding request at any given time.  A
       DNCA Diameter peer sends an Abort-Session-Request as defined in
       [RFC6733] if it is unable to maintain sessions due to resource
       limitation.

    5.4.  Routing Considerations

       It is assumed that the DNCA Diameter peer within a NAT controller
       knows the DiameterIdentity of the Diameter peer within a NAT device
       for a given endpoint.  Both the Destination-Realm and Destination-
       Host AVPs are present in the request from a DNCA Diameter peer within
       a NAT controller to a DNCA Diameter peer within a NAT device.

    5.5.  Advertising Application Support

       Diameter nodes conforming to this specification MUST advertise
       support for DNCA by including the value of 12 in the Auth-
       Application-Id of the Capabilities-Exchange-Request and Capabilities-
       Exchange-Answer commands [RFC6733].

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    RFC 6736            Diameter NAT Control Application        October 2012

    6.  DNCA Commands

       The following commands are used to establish, maintain, and query
       NAT-bindings.

    6.1.  NAT-Control-Request (NCR) Command

       The NAT-Control-Request (NCR) command, indicated by the command field
       set to 330 and the 'R' bit set in the Command Flags field, is sent
       from the DNCA Diameter peer within the NAT controller to the DNCA
       Diameter peer within the NAT device in order to install NAT-bindings.

       User-Name, Logical-Access-Id, Physical-Access-ID, Framed-IP-Address,
       Framed-IPv6-Prefix, Framed-Interface-Id, EGRESS-VLANID, NAS-Port-ID,
       Address-Realm, and Calling-Station-ID AVPs serve as identifiers for
       the endpoint.

       Message format:
          < NC-Request > ::= < Diameter Header: 330, REQ, PXY>
                           { Auth-Application-Id }
                           { Origin-Host }
                           { Origin-Realm }
                           { Destination-Realm }
                           { Destination-Host }
                           { NC-Request-Type }
                           [ Session-Id ]
                           [ Origin-State-Id ]
                        *1 [ NAT-Control-Remove ]
                        *1 [ NAT-Control-Install ]
                           [ NAT-External-Address ]
                           [ User-Name ]
                           [ Logical-Access-Id ]
                           [ Physical-Access-ID ]
                           [ Framed-IP-Address ]
                           [ Framed-IPv6-Prefix ]
                           [ Framed-Interface-Id ]
                           [ EGRESS-VLANID]
                           [ NAS-Port-ID]
                           [ Address-Realm ]
                           [ Calling-Station-ID ]
                         * [ Proxy-Info ]
                         * [ Route-Record ]
                         * [ AVP ]

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    RFC 6736            Diameter NAT Control Application        October 2012

    6.2.  NAT-Control-Answer (NCA) Command

       The NAT-Control-Answer (NCA) command, indicated by the Command Code
       field set to 330 and the 'R' bit cleared in the Command Flags field,
       is sent by the DNCA Diameter peer within the NAT device in response
       to the NAT-Control-Request command.

       Message format:
          <NC-Answer> ::= < Diameter Header: 330, PXY >
                          { Origin-Host }
                          { Origin-Realm }
                          { Result-Code }
                          [ Session-Id ]
                          [ NC-Request-Type ]
                        * [ NAT-Control-Definition ]
                          [ Current-NAT-Bindings ]
                          [ Origin-State-Id ]
                          [ Error-Message ]
                          [ Error-Reporting-Host ]
                        * [ Failed-AVP ]
                        * [ Proxy-Info ]
                          [ Duplicate-Session-Id ]
                        * [ Redirect-Host]
                          [ Redirect-Host-Usage ]
                          [ Redirect-Max-Cache-Time ]
                        * [ Proxy-Info ]
                        * [ Route-Record ]
                        * [ Failed-AVP ]
                        * [ AVP ]

    7.  NAT Control Application Session State Machine

       This section contains a set of finite state machines, representing
       the life cycle of a DNCA session, which MUST be observed by all
       implementations of the DNCA Diameter application.  The DNCA Diameter
       peers are stateful and the state machine maintained is similar to the
       stateful client and server authorization state machine described in
       [RFC6733].  When a session is moved to the Idle state, any resources
       that were allocated for the particular session must be released.  Any
       event not listed in the state machines MUST be considered an error
       condition, and an answer, if applicable, MUST be returned to the
       originator of the message.

       In the state table, the event "Failure to send NCR" means that the
       DNCA Diameter peer within the NAT controller is unable to send the
       NCR command to the desired destination.  This could be due to the

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    RFC 6736            Diameter NAT Control Application        October 2012

       peer being down or due to the peer sending back the transient failure
       or temporary protocol error notification DIAMETER_TOO_BUSY or
       DIAMETER_LOOP_DETECTED in the Result-Code AVP of an NCA.

       In the state table, "FAILED NCA" means that the DNCA Diameter peer
       within the NAT device was not able to honor the corresponding NCR.
       This can happen due to any transient or permanent error at the NAT
       device or its associated DNCA Diameter peer within indicated by the
       following error Result-Code values: RESOURCE_FAILURE,
       UNKNOWN_BINDING_TEMPLATE_NAME, MAX_BINDINGS_SET_FAILURE,
       BINDING_FAILURE, MAXIMUM_BINDINGS_REACHED_FOR_ENDPOINT,
       SESSION_EXISTS, INSUFFICIENT_CLASSIFIERS.

       The following state machine is observed by a DNCA Diameter peer
       within a NAT controller.  The state machine description uses the term
       "access session" to describe the connectivity service offered to the
       endpoint or host.  "Access session" should not be confused with the
       Diameter session.

                 DNCA Diameter peer within a NAT controller
          State     Event                          Action     New State
          -------------------------------------------------------------
          Idle      New endpoint detected that     Send        Pending
                    requires NAT control           NCR
                                                   Initial
                                                   Request

          Idle      ASR received                   Send ASA    Idle
                    for unknown session            with
                                                   Result-Code
                                                   = UNKNOWN_
                                                   SESSION_ID

          Pending   Successful NCA                 Setup       Open
                    received                       complete

          Pending   Successful NCA                 Send STR    Discon
                    received,
                    but peer unable to provide
                    service

          Pending   Error processing successful    Send STR    Discon
                    NCA

          Pending   Failed                         Clean up    Idle
                    NCA received

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          Open      NAT control                    Send        Open
                    update required                NCR update
                                                   request
          Open      Successful                                 Open
                    NCA received

          Open      Failed                         Clean up    Idle
                    NCA received

          Open      Access session end detected    Send STR    Discon

          Open      ASR received,                  Send ASA    Discon
                    access session will be         with
                    terminated                     Result-Code
                                                   = SUCCESS,
                                                   Send STR

          Open      ASR received,                  Send ASA    Open
                    access session will not        with
                    be terminated                  Result-Code
                                                   != SUCCESS

          Discon    ASR Received                   Send ASA    Idle

          Discon    STA Received                   Discon.     Idle
                                                   endpoint

       The following state machine is observed by a DNCA Diameter peer
       within a NAT device.

                 DNCA Diameter peer within a NAT device
          State     Event                          Action     New State
          -------------------------------------------------------------
          Idle      NCR query request              Send       Idle
                    received, and                  successful
                    able to provide requested      NCA
                    NAT-binding report

          Idle      NCR received                   Send       Open
                    and able to                    successful
                    provide requested              NCA
                    NAT control service

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          Idle      NCR request                    Send       Idle
                    received, and                  failed
                    unable to provide requested    NCA
                    NAT control service

          Open      NCR request                    Send       Open
                    received, and                  successful
                    able to provide requested      NCA
                    NAT control service

          Open      NCR request                    Send       Idle
                    received, and                  failed
                    unable to provide requested    NCA,
                    NAT control service            Clean up

          Open      Unable to continue             Send ASR   Discon
                    providing requested
                    NAT control service

          Open      Unplanned loss of session/     Clean up   Idle
                    connection to DNCA Diameter
                    peer in NAT controller
                    detected (e.g., due to Diameter
                    watchdog notification)

          Discon    Failure to send ASR            Wait,      Discon
                                                   resend ASR

          Discon    ASR successfully sent and      Clean up   Idle
                    ASA received with Result-Code

          Not       ASA received                   None       No change
          Discon

          Any       STR received                   Send STA,  Idle
                                                   Clean up

    8.  DNCA AVPs

    8.1.  Reused Base Protocol AVPs

       The following table describes the AVPs reused from the Diameter base
       protocol [RFC6733]; their AVP Code values, types, and possible flag
       values and whether the AVP MAY be encrypted.  [RFC6733] specifies the
       AVP Flag rules for AVPs in Section 4.5.  The Diameter AVP rules are
       defined in [RFC6733], Section 4.

    Brockners, et al.            Standards Track                   [Page 29]

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                                                       +---------+
                                                       |  AVP    |
                                                       |  Flag   |
                                                       |  rules  |
       +-----------------------------------------------|-----+---+---------+
       |                           AVP                 |     |   |         |
       | Attribute Name            Code     Data Type  |MUST |MAY|   Encr  |
       +-----------------------------------------------+-----+---+---------+
       |Acct-Interim-Interval      85       Unsigned32 | M   | P |    Y    |
       |Auth-Application-Id        258      Unsigned32 | M   | P |    N    |
       |Destination-Host           293      DiamIdent  | M   | P |    N    |
       |Destination-Realm          283      DiamIdent  | M   | P |    N    |
       |Error-Message              281      UTF8String | M   | P |    N    |
       |Error-Reporting-Host       294      DiamIdent  | M   | P |    N    |
       |Failed-AVP                 279      Grouped    | M   | P |    N    |
       |Origin-Host                264      DiamIdent  | M   | P |    N    |
       |Origin-Realm               296      DiamIdent  | M   | P |    N    |
       |Origin-State-Id            278      Unsigned32 | M   | P |    N    |
       |Proxy-Info                 284      Grouped    | M   | P |    N    |
       |Result-Code                268      Unsigned32 | M   | P |    N    |
       |Route-Record               282      DiamIdent  | M   |   |    N    |
       |Session-Id                 263      UTF8String | M   | P |    Y    |
       |User-Name                  1        UTF8String | M   | P |    Y    |
       +-----------------------------------------------+-----+---+---------+
       Table 1: DIAMETER AVPs from the Diameter Base Protocol

       The Auth-Application-Id AVP (AVP Code 258) is assigned by IANA to
       Diameter applications.  The value of the Auth-Application-Id for the
       Diameter NAT Control Application is 12.  Please refer to [RFC6733]
       for the definition of the Diameter AVP flag rules and the associated
       abbreviations used in the table.

    8.2.  Additional Result-Code AVP Values

       This section defines new values for the Result-Code AVP that SHALL be
       supported by all Diameter implementations that conform to the present
       document.

    8.2.1.  Success

       No new Result-Code AVP value is defined within this category.

    8.2.2.  Transient Failures

       Result-Code AVP values that fall within the transient failures
       category are those used to inform a peer that the request could not
       be satisfied at the time that it was received.  The request may be
       able to be satisfied in the future.

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       The following new values of the Result-Code AVP are defined:

          RESOURCE_FAILURE (4014)

             The DNCA Diameter peer within the NAT device indicates that the
             binding could not be installed or a new session could not be
             created due to resource shortage.

    8.2.3.  Permanent Failures

       The Result-Code AVP values, which fall within the permanent failures
       category are used to inform the peer that the request failed and
       should not be attempted again.  The request may be able to be
       satisfied in the future.

       The following new values of the Result-Code AVP are defined:

          UNKNOWN_BINDING_TEMPLATE_NAME (5042)

             The DNCA Diameter peer within the NAT device indicates that the
             binding could not be installed or a new session could not be
             created because the specified NAT-Control-Binding-Template AVP,
             which refers to a predefined policy template in the NAT device,
             is unknown.

          BINDING_FAILURE (5043)

             The DNCA Diameter peer within the NAT device indicates that the
             requested binding(s) could not be installed.  For example,
             Requested ports are already in use.

          MAX_BINDINGS_SET_FAILURE (5044)

             The DNCA Diameter peer within the NAT device indicates that it
             failed to conform to a request to configure the maximum number
             of bindings for a session.  For example, an operator defined
             the maximum number of bindings on the NAT device using a method
             or protocol that takes precedence over DNCA.

          MAXIMUM_BINDINGS_REACHED_FOR_ENDPOINT (5045)

             The DNCA Diameter peer within the NAT device denies the request
             because the maximum number of allowed bindings has been reached
             for the specified endpoint classifier.

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          SESSION_EXISTS (5046)

             The DNCA Diameter peer within the NAT device denies a request
             to initialize a new session, if it already has a DNCA session
             that uses the same set of classifiers as indicated by the DNCA
             Diameter peer within the NAT controller in the new session
             initialization request.

          INSUFFICIENT_CLASSIFIERS (5047)

             The DNCA Diameter peer within the NAT device requests to
             initialize a new session, if the classifiers in the request
             match more than one of the existing sessions on the DNCA
             Diameter peer within the NAT device.

    8.3.  Reused NASREQ Diameter Application AVPs

       The following table describes the AVPs reused from the Diameter
       Network Access Server Application [RFC4005]; their AVP Code values,
       types, and possible flag values; and whether the AVP MAY be
       encrypted.  The [RFC6733] specifies the AVP Flag rules for AVPs in
       Section 4.5.  The Diameter AVP rules are defined in the [RFC6733],
       Section 4.
                                              +---------------------+
                                              |    AVP Flag Rules   |
       +------------------+------+------------|----+-----+----+-----|----+
       |                  | AVP  |            |    |     |SHLD| MUST|    |
       | Attribute Name   | Code |  Value Type|MUST| MAY | NOT|  NOT|Encr|
       |------------------|------|------------|----+-----+----+-----|----|
       | NAS-Port         |   5  | Unsigned32 | M  |  P  |    |  V  | Y  |
       | NAS-Port-Id      |  87  | UTF8String | M  |  P  |    |  V  | Y  |
       | Calling-Station- |  31  | UTF8String | M  |  P  |    |  V  | Y  |
       |   Id             |      |            |    |     |    |     |    |
       | Framed-IP-Address|   8  | OctetString| M  |  P  |    |  V  | Y  |
       | Framed-Interface-|  96  | Unsigned64 | M  |  P  |    |  V  | Y  |
       |   Id             |      |            |    |     |    |     |    |
       | Framed-IPv6-     |  97  | OctetString| M  |  P  |    |  V  | Y  |
       |  Prefix          |      |            |    |     |    |     |    |
       +------------------+------+------------|----+-----+----+-----|----+
       Table 2: Reused NASREQ Diameter application AVPs.  Please refer to
       [RFC6733] for the definition of the Diameter AVP Flag rules and the
       associated abbreviations used in the table.

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    8.4.  Reused AVPs from RFC 4675

       The following table describes the AVPs reused from "RADIUS Attributes
       for Virtual LAN and Priority Support" [RFC4675]; their AVP Code
       values, types, and possible flag values; and whether the AVP MAY be
       encrypted.  [RFC6733] specifies the AVP Flag rules for AVPs in
       Section 4.5.  The Diameter AVP rules are defined in [RFC6733],
       Section 4.
                                              +---------------------+
                                              |    AVP Flag Rules   |
       +------------------+------+------------|----+-----+----+-----|----+
       |                  | AVP  |            |    |     |SHLD| MUST|    |
       | Attribute Name   | Code |  Value Type|MUST| MAY | NOT|  NOT|Encr|
       |------------------|------|------------|----+-----+----+-----|----|
       | Egress-VLANID    |  56  | OctetString| M  |  P  |    |  V  | Y  |
       +------------------+------+------------|----+-----+----+-----|----+
       Table 3: Reused attributes from [RFC4675].  Please refer to [RFC6733]
       for the definition of the Diameter AVP Flag rules and the associated
       abbreviations used in the table.

    8.5.  Reused AVPs from Diameter QoS Application

       The following table describes the AVPs reused from the "Traffic
       Classification and Quality of Service (QoS) Attributes for Diameter"
       [RFC5777]; their AVP Code values, types, and possible flag values;
       and whether the AVP MAY be encrypted.  [RFC6733] specifies the AVP
       Flag rules for AVPs in Section 4.5.  The Diameter AVP rules are
       defined in [RFC6733], Section 4.
                                                       +---------+
                                                       |  AVP    |
                                                       |  Flag   |
                                                       |  Rules  |
       +-----------------------------------------------|-----+---+---------+
       |                           AVP                 |     |   |         |
       | Attribute Name            Code     Data Type  |MUST |MAY|   Encr  |
       +-----------------------------------------------+-----+---+---------+
       |Port                       530     Integer32   |  M  | P |    Y    |
       |Protocol                   513     Enumerated  |  M  | P |    Y    |
       |Direction                  514     Enumerated  |  M  | P |    Y    |
       +-----------------------------------------------+-----+---+---------+

       Table 4: Reused QoS-attributes.  Please refer to [RFC6733] for the
       definition of the Diameter AVP Flag rules and the associated
       abbreviations used in the table.

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    8.6.  Reused AVPs from ETSI ES 283 034, e4 Diameter Application

       The following table describes the AVPs reused from the Diameter e4
       Application [ETSIES283034]; their AVP Code values, types, and
       possible flag values; and whether the AVP MAY be encrypted.
       [RFC6733] specifies the AVP Flag rules for AVPs in Section 4.5.  The
       Diameter AVP rules are defined in [RFC6733], Section 4.  The
       Vendor-ID field in these AVP header will be set to ETSI (13019).

                                                       +---------+
                                                       |  AVP    |
                                                       |  Flag   |
                                                       |  Rules  |
       +-----------------------------------------------|-----+---+---------+
       |                           AVP                 |     |   |         |
       | Attribute Name            Code     Data Type  |MUST |MAY|   Encr  |
       +-----------------------------------------------+-----+---+---------+
       |Address-Realm              301     OctetString | M,V |   |    Y    |
       |Logical-Access-Id          302     OctetString |   V | M |    Y    |
       |Physical-Access-ID         313     UTF8String  |   V | M |    Y    |
       +-----------------------------------------------+-----+---+---------+

       Table 5: Reused AVPs from the Diameter e4 application.  Please refer
       to [RFC6733] for the definition of the Diameter AVP Flag rules and
       the associated abbreviations used in the table.

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    8.7.  DNCA-Defined AVPs

       The following table describes the new Diameter AVPs defined in this
       document; their AVP Code values, types, and possible flag values; and
       whether the AVP MAY be encrypted.  [RFC6733] specifies the AVP Flag
       rules for AVPs in Section 4.5.  The Diameter AVP rules are defined in
       [RFC6733], Section 4.  The AVPs defined here MUST NOT have the 'V'
       bit in the AVP Flags field set.

                                                          +---------+
                                                          |  AVP    |
                                                          |  Flag   |
                                                          |  Rules  |
       +--------------------------------------------------|-----+---+------+
       |                       AVP                        |     |   |      |
       | Attribute Name        Code    Sect.   Data Type  |MUST |MAY| Encr |
       +--------------------------------------------------+-----+---+------+
       |NC-Request-Type        595     8.7.1   Enumerated | M   | P |  Y   |
       |NAT-Control-Install    596     8.7.2   Grouped    | M   | P |  Y   |
       |NAT-Control-Remove     597     8.7.3   Grouped    | M   | P |  Y   |
       |NAT-Control-Definition 598     8.7.4   Grouped    | M   | P |  Y   |
       |NAT-Internal-Address   599     8.7.5   Grouped    | M   | P |  Y   |
       |NAT-External-Address   600     8.7.6   Grouped    | M   | P |  Y   |
       |Max-NAT-Bindings       601     8.7.7   Unsigned32 | M   | P |  Y   |
       |NAT-Control-           602     8.7.8   OctetString| M   | P |  Y   |
       | Binding-Template                                 |     |   |      |
       |Duplicate-             603     8.7.9   UTF8String | M   | P |  Y   |
       | Session-Id                                       |     |   |      |
       |NAT-External-Port-     604     8.7.10  Enumerated | M   | P |  Y   |
       | Style                                            |     |   |      |
       |NAT-Control-Record     605     9.2.1   Grouped    | M   | P |  Y   |
       |NAT-Control-           606     9.2.2   Enumerated | M   | P |  Y   |
       | Binding-Status                                   |     |   |      |
       |Current-NAT-Bindings   607     9.2.3   Unsigned32 | M   | P |  Y   |
       +--------------------------------------------------+-----+---+------+

       Table 6: New Diameter AVPs.  Please refer to [RFC6733] for the
       definition of the Diameter AVP Flag rules and the associated
       abbreviations used in the table.

    Brockners, et al.            Standards Track                   [Page 35]

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    8.7.1.  NC-Request-Type AVP

       The NC-Request-Type AVP (AVP Code 595) is of type Enumerated and
       contains the reason for sending the NAT-Control-Request command.  It
       shall be present in all NAT-Control-Request messages.

       The following values are defined:

          INITIAL_REQUEST (1)

             An Initial Request is to initiate a Diameter NAT control
             session between the DNCA Diameter peers.

          UPDATE_REQUEST (2)

             An Update Request is used to update bindings previously
             installed on a given access session, to add new binding on a
             given access session, or to remove one or several binding(s)
             activated on a given access session.

          QUERY_REQUEST (3)

             Query Request is used to query a NAT device about the currently
             installed bindings for an endpoint classifier.

    8.7.2.  NAT-Control-Install AVP

       The NAT-Control-Install AVP (AVP code 596) is of type Grouped, and it
       is used to activate or install NAT-bindings.  It also contains Max-
       NAT-Bindings that defines the maximum number of NAT-bindings allowed
       for an endpoint and the NAT-Control-Binding-Template that references
       a predefined template on the NAT device that may contain static
       binding, a maximum number of bindings allowed, an IP address pool
       from which external binding addresses should be allocated, etc.  If
       the NAT-External-Port-Style AVP is present, then the NAT device MUST
       select the external ports for the NAT-bindings, per the style
       specified.  The NAT-External-Port-Style is applicable for NAT-
       bindings defined by the NAT-Control-Definition AVPs whose NAT-
       External-Address or Port AVPs within the NAT-External-Address are
       unspecified.

       AVP format:
         NAT-Control-Install ::= < AVP Header: 596 >
                                  * [ NAT-Control-Definition ]
                                    [ NAT-Control-Binding-Template ]
                                    [ Max-NAT-Bindings ]
                                    [ NAT-External-Port-Style ]
                                  * [ AVP ]

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    8.7.3.  NAT-Control-Remove AVP

       The NAT-Control-Remove AVP (AVP code 597) is of type Grouped, and it
       is used to deactivate or remove NAT-bindings.  At least one of the
       two AVPs (NAT-Control-Definition AVP or NAT-Control-Binding-Template
       AVP) SHOULD be present in the NAT-Control-Remove AVP.

       AVP format:
         NAT-Control-Remove ::= < AVP Header: 597 >
                                 * [ NAT-Control-Definition ]
                                   [ NAT-Control-Binding-Template ]
                                 * [ AVP ]

    8.7.4.  NAT-Control-Definition AVP

       The NAT-Control-Definition AVP (AVP code 598) is of type Grouped, and
       it describes a binding.

       The NAT-Control-Definition AVP uniquely identifies the binding
       between the DNCA Diameter peers.

       If both the NAT-Internal-Address and NAT-External-Address AVP(s) are
       supplied, it is a predefined binding.

       If the NAT-External-Address AVP is not specified, then the NAT device
       MUST select the external port as per the NAT-External-Port-Style AVP,
       if present in the NAT-Control-Definition AVP.

       The Protocol AVP describes the transport protocol for the binding.
       The NAT-Control-Definition AVP can contain either zero or one
       Protocol AVP.  If the Protocol AVP is omitted and if both internal
       and external IP addresses are specified, then the binding reserves
       the IP addresses for all transport protocols.

       The Direction AVP is of type Enumerated.  It specifies the direction
       for the binding.  The values of the enumeration applicable in this
       context are: "IN","OUT".  If Direction AVP is OUT or absent, the NAT-
       Internal-Address refers to the IP address of the endpoint that needs
       to be translated.  If Direction AVP is "IN", NAT-Internal-Address is
       the destination IP address that has to be translated.

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       AVP format:
         NAT-Control-Definition ::= < AVP Header: 598 >
                                     { NAT-Internal-Address }
                                     [ Protocol ]
                                     [ Direction ]
                                     [ NAT-External-Address ]
                                     [ Session-Id ]
                                   * [ AVP ]

    8.7.5.  NAT-Internal-Address AVP

       The NAT-Internal-Address AVP (AVP code 599) is of type Grouped.  It
       describes the internal IP address and port for a binding.  Framed-
       IPV6-Prefix and Framed-IP-Address AVPs are mutually exclusive.  The
       endpoint identifier Framed-IP-Address, Framed-IPv6-Prefix, and the
       internal address in this NAT-Internal-Address AVP to install NAT-
       bindings for the session MUST match.

       AVP format:
         NAT-Internal-Address ::= < AVP Header: 599 >
                                   [ Framed-IP-Address ]
                                   [ Framed-IPv6-Prefix ]
                                   [ Port]
                                 * [ AVP ]

    8.7.6.  NAT-External-Address AVP

       The NAT-External-Address AVP (AVP code 600) is of type Grouped, and
       it describes the external IP address and port for a binding.  The
       external IP address specified in this attribute can be reused for
       multiple endpoints by specifying the same address in the respective
       NAT-External-Address AVPs.  If the external IP address is not
       specified and the NAT-External-Port-Style AVP is specified in the
       NAT-Control-Definition AVP, then the NAT device MUST select an
       external port as per the NAT-External-Port-Style AVP.

       AVP format:
         NAT-External-Address ::= < AVP Header: 600 >
                                   [ Framed-IP-Address ]
                                   [ Port ]
                                 * [ AVP ]

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    8.7.7.  Max-NAT-Bindings

       The Max-NAT-Bindings AVP (AVP code 601) is of type Unsigned32.  It
       indicates the maximum number of NAT-bindings allowed for a particular
       endpoint.

    8.7.8.  NAT-Control-Binding-Template AVP

       The NAT-Control-Binding-Template AVP (AVP code 602) is of type
       OctetString.  It defines a name for a policy template that is
       predefined at the NAT device.  Details on the contents and structure
       of the template and configuration are outside the scope of this
       document.  The policy to which this AVP refers may contain NAT-
       bindings, an IP address pool for allocating the external IP address
       of a NAT-binding, and a maximum number of allowed NAT-bindings.  Such
       a policy template can be reused by specifying the same NAT-Control-
       Binding-Template AVP in the corresponding NAT-Control-Install AVPs of
       multiple endpoints.

    8.7.9.  Duplicate-Session-Id AVP

       The Duplicate-Session-Id AVP (AVP Code 603) is of type UTF8String.
       It is used to report errors and contains the Session-Id of an
       existing session.

    8.7.10.  NAT-External-Port-Style AVP

       The NAT-External-Port-Style AVP (AVP Code 604) is of type Enumerated
       and contains the style to be followed while selecting the external
       port for a NAT-binding relative to the internal port.

       The following values are defined:

          FOLLOW_INTERNAL_PORT_STYLE (1)

             External port numbers selected MUST follow the same sequence
             and oddity as the internal ports of the NAT-bindings.  The port
             oddity is required to support protocols like RTP and RTCP as
             defined in [RFC3550].  If for example the internal port in a
             requested NAT-binding is odd numbered, then the external port
             allocated MUST also be odd numbered, and vice versa for an even
             numbered port.  In addition, the sequence of port numbering is
             maintained: if internal ports are consecutive, then the NAT
             device MUST choose consecutive external ports for the NAT-
             bindings.

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    9.  Accounting Commands

       The DNCA reuses session-based accounting as defined in the Diameter
       base protocol [RFC6733] to report the bindings per endpoint.  This
       reporting is achieved by sending Diameter Accounting-Request (ACR)
       commands [Start, Interim, and Stop] from the DNCA Diameter peer
       within the NAT device to its associated DNCA Diameter peer within the
       NAT controller.

       The DNCA Diameter peer within the NAT device sends an ACR Start on
       receiving an NCR with NC-Request-Type AVP set to INITIAL_REQUEST for
       a session or on creation of the first binding for a session requested
       in an earlier NCR.  DNCA may send ACR Interim updates, if required,
       either due to a change in bindings resulting from an NCR with NC-
       Request-Type AVP set to UPDATE_REQUEST, periodically as specified in
       Acct-Interim-Interval by the DNCA Diameter peer within the NAT
       controller, or when it creates or tears down bindings.  An ACR Stop
       is sent by the DNCA Diameter peer within the NAT device on receiving
       an STR message.

       The function of correlating the multiple bindings used by an endpoint
       at any given time is relegated to the post processor.

       The DNCA Diameter peer within the NAT device may trigger an Interim
       accounting record when the maximum number of bindings, if received in
       an NCR, is reached.

    9.1.  NAT Control Accounting Messages

       The ACR and ACA messages are reused as defined in the Diameter base
       protocol [RFC6733] for exchanging endpoint NAT-binding details
       between the DNCA Diameter peers.  The DNCA Application ID is used in
       the accounting commands.  The ACR contains one or more optional NAT-
       Control-Record AVPs to report the bindings.  The NAT device indicates
       the number of allocated NAT-bindings to the NAT controller using the
       Current-NAT-Bindings AVP.  This number needs to match the number of
       bindings identified as active within the NAT-Control-Record AVP.

    9.2.  NAT Control Accounting AVPs

       In addition to AVPs for ACR specified in [RFC6733], the DNCA Diameter
       peer within the NAT device must add the NAT-Control-Record AVP.

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    9.2.1.  NAT-Control-Record

       The NAT-Control-Record AVP (AVP code 605) is of type Grouped.  It
       describes a binding and its status.  If NAT-Control-Binding-Status is
       set to Created, Event-Timestamp indicates the binding creation time.
       If NAT-Control-Binding-Status is set to Removed, Event-Timestamp
       indicates the binding removal time.  If NAT-Control-Binding-Status is
       active, Event-Timestamp need not be present; if a value is present,
       it indicates that binding is active at the given time.
         NAT-Control-Record ::= < AVP Header: 605 >
                                { NAT-Control-Definition }
                                { NAT-Control-Binding-Status }
                                [ Event-Timestamp ]

    9.2.2.  NAT-Control-Binding-Status

       The NAT-Control-Binding-Status AVP (AVP code 606) is of type
       enumerated.  It indicates the status of the binding: created,
       removed, or active.

       The following values are defined:

          Created (1)

             NAT-binding is created.

          Active (2)

             NAT-binding is active.

          Removed (3)

             NAT-binding was removed.

    9.2.3.  Current-NAT-Bindings

       The Current-NAT-Bindings AVP (AVP code 607) is of type Unsigned32.
       It indicates the number of NAT-bindings active on the NAT device.

    10.  AVP Occurrence Tables

       The following sections present the AVPs defined in this document and
       specify the Diameter messages in which they can be present.  Note:
       AVPs that can only be present within a Grouped AVP are not
       represented in this table.

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       The table uses the following symbols:

          0         The AVP MUST NOT be present in the message.

          0+        Zero or more instances of the AVP can be present in the
                    message.

          0-1       Zero or one instance of the AVP can be present in the
                    message.  It is considered an error if there is more
                    than one instance of the AVP.

          1         One instance of the AVP MUST be present in the message.

          1+        At least one instance of the AVP MUST be present in the
                    message.

    10.1.  DNCA AVP Table for NAT Control Initial and Update Requests

       The following table lists DNCA-specific AVPs that have to be present
       in NCRs and NCAs with the NC-Request-Type set to INITIAL_REQUEST or
       UPDATE_REQUEST.

                                           +-------------------+
                                           |  Command Code     |
       +-----------------------------------+-------------------+
       | Attribute Name                        NCR    NCA      |
       +-------------------------------------------------------+
       |NC-Request-Type                         1      1       |
       |NAT-Control-Install                    0-1     0       |
       |NAT-Control-Remove                     0-1     0       |
       |NAT-Control-Definition                  0      0       |
       |Current-NAT-Bindings                    0      0       |
       |Duplicate-Session-Id                    0     0-1      |
       +-------------------------------------------------------+

       Note that any combination of NAT-Control-Install and NAT-Control-
       Remove AVPs could be present in an update or initial requests.
       Consider the following examples:

          Neither the NAT-Control-Install AVP nor the NAT-Control-Remove AVP
          is present: This could, for example, be the case if the NAT
          controller would only want to receive accounting information but
          not control NAT-bindings.

          Only NAT-Control-Install AVP is present: This could, for example,
          be the case if a new NAT-binding is installed for an existing
          session.

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          Only NAT-Control-Remove AVP is present: This could, for example,
          be the case if a new NAT-binding is removed from an existing
          session.

          Both, NAT-Control-Install AVP and NAT-Control-Remove AVP are
          present: This could, for example. be the case if a formerly
          created NAT-binding is removed and a new NAT-binding is
          established within the same request.

    10.2.  DNCA AVP Table for Session Query Requests

       The following table lists DNCA-specific AVPs that have to be present
       in NCRs and NCAs with the NC-Request-Type set to QUERY_REQUEST.

                                           +-------------------+
                                           |  Command Code     |
       +-----------------------------------+-------------------+
       | Attribute Name                        NCR    NCA      |
       +-------------------------------------------------------+
       |NC-Request-Type                         1      1       |
       |NAT-Control-Install                     0      0       |
       |NAT-Control-Remove                      0      0       |
       |NAT-Control-Definition                  0      0+      |
       |NAT-External-Address                    0+     0       |
       |Current-NAT-Bindings                    0      1       |
       |Duplicate-Session-Id                    0      0       |
       +-------------------------------------------------------+

    10.3.  DNCA AVP Table for Accounting Messages

       The following table lists DNCA-specific AVPs, which may or may not be
       present in ACR and ACA messages.
                                           +-------------------+
                                           |  Command Code     |
       +-----------------------------------+-------------------+
       | Attribute Name                        ACR    ACA      |
       +-------------------------------------------------------+
       |NAT-Control-Record                      0+     0       |
       |Current-NAT-Bindings                    1      0       |
       +-------------------------------------------------------+

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    11.  IANA Considerations

       This section contains either the namespaces that have been created in
       this specification or the values assigned to existing namespaces
       managed by IANA.

       In the subsections below, when we speak about review by a Designated
       Expert [RFC5226], please note that the Designated Expert will be
       assigned by the IESG.  Initially, such Expert discussions take place
       on the AAA WG mailing list.

    11.1.  Application Identifier

       This specification assigns the value 12, 'Diameter NAT Control
       Application', to the Application Identifier namespace defined in
       [RFC6733].  See Section 4 for more information.

    11.2.  Command Codes

       This specification uses the value 330 from the Command code namespace
       defined in [RFC6733] for the NAT-Control-Request (NCR) and NAT-
       Control-Answer (NCA) commands.  See Section 6.1 and Section 6.2 for
       more information on these commands.

    11.3.  AVP Codes

       This specification assigns the values 595-607 from the AVP Code
       namespace defined in [RFC6733].  See Section 8.7 for the assignment
       of the namespace in this specification.

    11.4.  Result-Code AVP Values

       This specification assigns the values 4014 and 5042-5047 from the
       Result-Code AVP value namespace defined in [RFC6733].  See
       Section 8.2 for the assignment of the namespace in this
       specification.

    11.5.  NC-Request-Type AVP

       As defined in Section 8.7.1, the NC-Request-Type AVP includes
       Enumerated type values 1-3.  IANA has created and is maintaining a
       namespace for this AVP.  All remaining values are available for
       assignment by a Designated Expert [RFC5226].

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    11.6.  NAT-External-Port-Style AVP

       As defined in Section 8.7.10, the NAT-External-Port-Style AVP
       includes Enumerated type value 1.  IANA has created and is
       maintaining a namespace for this AVP.  All remaining values are
       available for assignment by a Designated Expert [RFC5226].

    11.7.  NAT-Control-Binding-Status AVP

       As defined in Section 8.7.1, the NAT-Control-Binding-Status AVP
       includes Enumerated type values 1-3.  IANA has created and is
       maintaining a namespace for this AVP.  All remaining values are
       available for assignment by a Designated Expert [RFC5226].

    12.  Security Considerations

       This document describes procedures for controlling NAT-related
       attributes and parameters by an entity, which is non-local to the
       device performing NAT.  This section discusses security
       considerations for DNCA.  This includes the interactions between the
       Diameter peers within a NAT controller and a NAT device as well as
       general considerations for a NAT-control in a service provider
       network.

       Security between a NAT controller and a NAT device has a number of
       components: authentication, authorization, integrity, and
       confidentiality.

       "Authentication" refers to confirming the identity of an originator
       for all datagrams received from the originator.  Lack of
       authentication of Diameter messages between the Diameter peers can
       jeopardize the fundamental service of the peering network elements.
       A consequence of not authenticating the message sender by the
       recipient would be that an attacker could spoof the identity of a
       "legitimate" authorizing entity in order to change the behavior of
       the receiver.  An attacker could, for example, launch a DoS attack by
       setting the maximum number of bindings for a session on the NAT
       device to zero; provisioning bindings on a NAT device that includes
       IP addresses already in use in other parts of the network; or
       requesting session termination of the Diameter session and hampering
       an endpoint's (i.e., a user's) connectivity.  Lack of authentication
       of a NAT device to a NAT controller could lead to situations where
       the NAT device could provide a wrong view of the resources (i.e.,
       NAT-bindings).  In addition, a NAT-binding Predefined template on the
       NAT device could be configured differently than expected by the NAT
       controller.  If either of the two DNCA Diameter peers fail to provide
       the required credentials, the failure should be subject to logging.
       The corresponding logging infrastructure of the operator SHOULD be

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       built in a way that it can mitigate potential DoS attacks resulting
       from large amounts of logging events.  This could include proper
       dimensioning of the logging infrastructure combined with policing the
       maximum amount of logging events accepted by the logging system to a
       threshold which the system is known to be able to handle.

       "Authorization" refers to whether a particular authorizing entity is
       authorized to signal a network element request for one or more
       applications, adhering to a certain policy profile.  Failing the
       authorization process might indicate a resource theft attempt or
       failure due to administrative and/or credential deficiencies.  In
       either case, the network element should take the proper measures to
       log such attempts.

       Integrity is required to ensure that a Diameter message exchanged
       between the Diameter peers has not been maliciously altered by
       intermediate devices.  The result of a lack of data integrity
       enforcement in an untrusted environment could be that an impostor
       will alter the messages exchanged between the peers.  This could
       cause a change of behavior of the peers, including the potential of a
       DoS.

       Confidentiality protection of Diameter messages ensures that the
       signaling data is accessible only to the authorized entities.  When
       signaling messages between the DNCA Diameter peers traverse untrusted
       networks, lack of confidentiality will allow eavesdropping and
       traffic analysis.

       Diameter offers security mechanisms to deal with the functionality
       demanded above.  DNCA makes use of the capabilities offered by
       Diameter and the underlying transport protocols to deliver these
       requirements (see Section 5.1).  If the DNCA communication traverses
       untrusted networks, messages between DNCA Diameter peers SHOULD be
       secured using either IPsec or TLS.  Please refer to [RFC6733],
       Section 13 for details.  DNCA Diameter peers SHOULD perform bilateral
       authentication, authorization, as well as procedures to ensure
       integrity and confidentiality of the information exchange.  In
       addition, the Session-Id chosen for a particular Diameter session
       SHOULD be chosen in a way that it is hard to guess in order to
       mitigate issues through potential message replay.

       DNCA Diameter peers SHOULD have a mutual trust setup.  This document
       does not specify a mechanism for authorization between the DNCA
       Diameter peers.  The DNCA Diameter peers SHOULD be provided with
       sufficient information to make an authorization decision.  The
       information can come from various sources, for example, the peering
       devices could store local authentication policy, listing the
       identities of authorized peers.

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       Any mechanism or protocol providing control of a NAT device, and DNCA
       is an example of such a control mechanism, could allow for misuse of
       the NAT device given that it enables the definition of per-
       destination or per-source rules.  Misuse could include anti-
       competitive practices among providers, censorship, crime, etc.  NAT-
       control could be used as a tool for preventing or redirecting access
       to particular sites.  For instance, by controlling the NAT-bindings,
       one could ensure that endpoints aren't able to receive particular
       flows, or that those flows are redirected to a relay that snoops or
       tampers with traffic instead of directly forwarding the traffic to
       the intended endpoint.  In addition, one could set up a binding in a
       way that the source IP address used is one of a relay so that traffic
       coming back can be snooped on or interfered with.  The operator also
       needs to consider security threats resulting from unplanned
       termination of the DNCA session.  Unplanned session termination,
       which could happen due to, e.g., an attacker taking down the NAT
       controller, leads to the NAT device cleaning up the state associated
       with this session after a grace period.  If the grace period is set
       to zero, the endpoint will experience an immediate loss of
       connectivity to services reachable through the NAT device following
       the termination of the DNCA session.The protections on DNCA and its
       Diameter protocol exchanges don't prevent such abuses of NAT-control.
       Prevention of misuse or misconfiguration of a NAT device by an
       authorized NAT controller is beyond the scope of this protocol
       specification.  A service provider deploying DNCA needs to make sure
       that higher-layer processes and procedures are put in place that
       allow them to detect and mitigate misuses.

    13.  Examples

       This section shows example DNCA message content and exchange.

    13.1.  DNCA Session Establishment Example

       Figure 15 depicts a typical call flow for DNCA session establishment.

       In this example, the NAT controller does the following:

       a.  requests a maximum of 100 NAT-bindings for the endpoint.

       b.  defines a static binding for a TCP connection that associates the
           internal IP Address:Port 192.0.2.1:80 with the external IP
           Address:Port 198.51.100.1:80 for the endpoint.

       c.  requests the use of a preconfigured template called "local-
           policy" while creating NAT-bindings for the endpoint.

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       endpoint             NAT controller (within NAS)           NAT device
          |                            |                               |
          |                            |                               |
          |      1. Trigger            |                               |
          |--------------------------->|                               |
          |       +-------------------------------------+              |
          |       |  2. Determine that NAT control      |              |
          |       |     is required for the endpoint    |              |
          |       +-------------------------------------+              |
          |                            |                               |
          |                            |                               |
          |                           ...................................
          |                           .|   3. Diameter Base CER/CEA    |.
          |                           .|<----------------------------->|.
          |                           ...................................
          |                            |                               |
          |                            |                               |
          |                            |         4.  NCR               |
          |                            |------------------------------>|
          |                            |                               |
          |                            |                     5. DNCA session
          |                            |                        established
          |                            |                               |
          |                            |         6.  NCA               |
          |                            |<------------------------------|
          |                            |                               |
          |                            |                               |
          |                  7. Data traffic                           |
          |----------------------------------------------------------->|
          |                            |                               |
          |                            |                               |
          |                            |                    8. NAT-bindings
          |                            |                     created as per
          |                            |                   directives in the
          |                            |                       DNCA session
          |                            |                               |

                    Figure 15: Initial NAT-Control-Request and
                           Session Establishment Example

       Detailed description of the steps shown in Figure 15:

       1.  The NAT controller (co-located with the NAS here) creates state
           for an endpoint based on a trigger.  This could, for example, be
           the successful establishment of a Point-to-Point Protocol (PPP)
           [RFC1661] access session.

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       2.  Based on the configuration of the DNCA Diameter peer within the
           NAT controller, the NAT controller determines that NAT-control is
           required and is to be enforced at a NAT device.

       3.  If there is no Diameter session already established with the DNCA
           Diameter peer within NAT device, a Diameter connection is
           established and Diameter Base CER/CEA are exchanged.

       4.  The NAT-Controller creates an NCR message (see below) and sends
           it to the NAT device.  This example shows IPv4 to IPv4 address
           and port translation.  For IPv6 to IPv4 translation, the Framed-
           IP-Address AVP would be replaced by the Framed-IPv6-Address AVP
           with the value set to the IPv6 address of the endpoint.

         < NC-Request > ::= < Diameter Header: 330, REQ, PXY>
                          Session-Id =  "natC.example.com:33041;23432;"
                          Auth-Application-Id = <DNCA Application ID>
                          Origin-Host = "natC.example.com"
                          Origin-Realm = "example.com"
                          Destination-Realm = "example.com"
                          Destination-Host = "nat-device.example.com"
                          NC-Request-Type = INITIAL_REQUEST
                          User-Name = "subscriber_example1"
                          Framed-IP-Address = "192.0.2.1"
                          NAT-Control-Install = {
                               NAT-Control-Definition = {
                                  Protocol = TCP
                                  Direction = OUT
                                  NAT-Internal-Address = {
                                       Framed-IP-Address = "192.0.2.1"
                                       Port = 80
                                  }
                                  NAT-External-Address = {
                                       Framed-IP-Address = "198.51.100.1"
                                       Port = 80
                                  }
                               }
                               Max-NAT-Bindings = 100
                               NAT-Control-Binding-Template = "local-policy"
                          }

       5.  The NAT device establishes a DNCA session as it is able to comply
           with the request.

       6.  The NAT device sends an NCA to indicate the successful completion
           of the request.

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          <NC-Answer> ::= < Diameter Header: 330, PXY >
                           Session-Id =  "natC.example.com:33041;23432;"
                           Origin-Host = "nat-device.example.com"
                           Origin-Realm = "example.com"
                           NC-Request-Type = INITIAL_REQUEST
                           Result-Code = DIAMETER_SUCCESS

       7.  The endpoint sends packets that reach the NAT device.

       8.  The NAT device performs NAT for traffic received from the
           endpoint with source address 192.0.2.1.  Traffic with source IP
           address 192.0.2.1 and port 80 are translated to the external IP
           address 198.51.100.1 and port 80.  Traffic with source IP address
           192.0.2.1 and a source port different from 80 will be translated
           to IP address 198.51.100.1 and a port chosen by the NAT device.
           Note that this example assumes that the NAT device follows
           typical binding allocation rules for endpoints, in that only a
           single external IP address is used for all traffic received from
           a single IP address of an endpoint.  The NAT device will allow a
           maximum of 100 NAT-bindings be created for the endpoint.

    13.2.  DNCA Session Update with Port Style Example

       This section gives an example for a DNCA session update: A new set of
       NAT-bindings is requested for an existing session.  The request
       contains a directive ( the "NAT-External-Port-Style" AVP set to
       FOLLOW_INTERNAL_PORT_STYLE) that directs the NAT device to maintain
       port-sequence and port-oddity for the newly created NAT-bindings.  In
       the example shown, the internal ports are UDP port 1036 and 1037.
       The NAT device follows the directive selects the external ports
       accordingly.  The NAT device would, for example, create a mapping of
       192.0.2.1:1036 to 198.51.100.1:5056 and 192.0.2.1:1037 to
       198.51.100.1:5057, thereby maintaining port oddity (1036->5056,
       1037->5057) and sequence ( the consecutive internal ports 1036 and
       1037 map to the consecutive external ports 5056 and 5057).

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          < NC-Request > ::= < Diameter Header: 330, REQ, PXY>
                           Session-Id =  "natC.example.com:33041;23432;"
                           Auth-Application-Id = <DNCA Application ID>
                           Origin-Host = "natC.example.com"
                           Origin-Realm = "example.com"
                           Destination-Realm = "example.com"
                           Destination-Host = "nat-device.example.com"
                           NC-Request-Type = UPDATE_REQUEST
                           NAT-Control-Install = {
                               NAT-Control-Definition = {
                                   Protocol = UDP
                                   Direction = OUT
                                   NAT-Internal-Address = {
                                        Framed-IP-Address = "192.0.2.1"
                                        Port = 1035
                                   }
                               }
                               NAT-Control-Definition = {
                                   Protocol = UDP
                                   Direction = OUT
                                   NAT-Internal-Address = {
                                        Framed-IP-Address = "192.0.2.1"
                                        Port = 1036
                                   }
                               }
                               NAT-External-Port-
                                      Style = FOLLOW_INTERNAL_PORT_STYLE
                           }

    13.3.  DNCA Session Query Example

       This section shows an example for DNCA session query for a subscriber
       whose internal IP Address is 192.0.2.1.
          < NC-Request > ::= < Diameter Header: 330, REQ, PXY>
                           Auth-Application-Id = <DNCA Application ID>
                           Origin-Host = "natC.example.com"
                           Origin-Realm = "example.com"
                           Destination-Realm = "example.com"
                           Destination-Host = "nat-device.example.com"
                           NC-Request-Type = QUERY_REQUEST
                           Framed-IP-Address = "192.0.2.1"

       The NAT device constructs an NCA to report all currently active NAT-
       bindings whose internal address is 192.0.2.1.

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          <NC-Answer> ::= < Diameter Header: 330, PXY >
                        Origin-Host = "nat-device.example.com"
                        Origin-Realm = "example.com"
                        NC-Request-Type = QUERY_REQUEST
                        NAT-Control-Definition = {
                                Protocol = TCP
                                Direction = OUT
                                NAT-Internal-Address = {
                                    Framed-IP-Address = "192.0.2.1"
                                    Port = 80
                                   }
                                NAT-External-Address = {
                                     Framed-IP-Address = "198.51.100.1"
                                     Port = 80
                                   }
                                Session-Id = "natC.example.com:33041;23432;"
                        }
                        NAT-Control-Definition = {
                                Protocol = TCP
                                Direction = OUT
                                NAT-Internal-Address = {
                                    Framed-IP-Address = "192.0.2.1"
                                    Port = 1036
                                   }
                                NAT-External-Address = {
                                     Framed-IP-Address = "198.51.100.1"
                                     Port = 5056
                                   }
                                Session-Id = "natC.example.com:33041;23432;"
                        }
                        NAT-Control-Definition = {
                                Protocol = TCP
                                Direction = OUT
                                NAT-Internal-Address = {
                                    Framed-IP-Address = "192.0.2.1"
                                    Port = 1037
                                   }
                                NAT-External-Address = {
                                     Framed-IP-Address = "198.51.100.1"
                                     Port = 5057
                                   }
                                Session-Id = "natC.example.com:33041;23432;"
                           }

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    13.4.  DNCA Session Termination Example

       In this example the NAT controller decides to terminate the
       previously established DNCA session.  This could, for example, be the
       case as a result of an access session (e.g., a PPP session)
       associated with an endpoint having been torn down.

           NAT controller                            NAT device
                 |                                       |
                 |                                       |
        +--------------+                                 |
        |  1. Trigger  |                                 |
        +--------------+                                 |
                 |                                       |
                 |                                       |
                 |             2.  STR                   |
                 |-------------------------------------->|
                 |                                       |
                 |                             3. DNCA session
                 |                                   lookup
                 |             4.  ACR                   |
                 |<--------------------------------------|
                 |                                       |
                 |             5.  ACA                   |
                 |-------------------------------------->|
                 |                                       |
                 |                                       |
                 |                             6. DNCA bindings
                 |                            and session cleanup
                 |                                       |
                 |             7.  STA                   |
                 |<--------------------------------------|
                 |                                       |

                Figure 20:  NAT Control Session Termination Example

       The following steps describe the sequence of events for tearing down
       the DNCA session in the example above:

       1.  The NAT controller receives a trigger that a DNCA session
           associated with a specific endpoint should be terminated.  An
           example event could be the termination of the PPP [RFC1661]
           access session to an endpoint in a NAS.  The NAS correspondingly
           triggers the NAT controller request to tear down the associated
           DNCA session.

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       2.  The NAT controller creates the required NCR message and sends it
           to the NAT device:

          < STR >     ::= < Diameter Header: 275, REQ, PXY>
                           Session-Id =  "natC.example.com:33041;23432;"
                           Auth-Application-Id = <DNCA Application ID>
                           Origin-Host = "natC.example.com"
                           Origin-Realm = "example.com"
                           Destination-Realm = "example.com"
                           Destination-Host = "nat-device.example.com"
                           Termination-Cause = DIAMETER_LOGOUT

       3.  The NAT device looks up the DNCA session based on the Session-Id
           AVP and finds a previously established active session.

       4.  The NAT device reports all NAT-bindings established for that
           subscriber using an ACR:
          < ACR >     ::= < Diameter Header: 271, REQ, PXY>
                           Session-Id =  "natC.example.com:33041;23432;"
                           Auth-Application-Id = <DNCA Application ID>
                           Origin-Host = "nat-device.example.com"
                           Origin-Realm = "example.com"
                           Destination-Realm = "example.com"
                           Destination-Host = "natC.example.com"
                           Accounting-Record-Type = STOP_RECORD
                           Accounting-Record-Number = 1
                           NAT-Control-Record = {
                               NAT-Control-Definition = {
                                   Protocol = TCP
                                   Direction = OUT
                                   NAT-Internal-Address = {
                                       Framed-IP-Address = "192.0.2.1"
                                       Port = 5001
                                      }
                                   NAT-External-Address = {
                                        Framed-IP-Address = "198.51.100.1"
                                        Port = 7777
                                      }
                                  }
                                 NAT-Control-Binding-Status = Removed
                              }

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       5.  The NAT controller receives and processes the ACR as per its
           configuration.  It responds with an ACA to the NAT device.

          <ACA>      ::= < Diameter Header: 271, PXY >
                           Session-Id =  "natC.example.com:33041;23432;"
                           Origin-Host = "natC.example.com"
                           Origin-Realm = "example.com"
                           Result-Code = DIAMETER_SUCCESS
                           Accounting-Record-Type = STOP_RECORD
                           Accounting-Record-Number = 1

       6.  On receipt of the ACA the NAT device cleans up all NAT-bindings
           and associated session state for the endpoint.

       7.  NAT device sends an STA.  On receipt of the STA the NAT
           controller will clean up the corresponding session state.
          <STA>      ::= < Diameter Header: 275, PXY >
                           Session-Id =  "natC.example.com:33041;23432;"
                           Origin-Host = "nat-device.example.com"
                           Origin-Realm = "example.com"
                           Result-Code = DIAMETER_SUCCESS

    14.  Acknowledgements

       The authors would like to thank Jari Arkko, Wesley Eddy, Stephen
       Farrell, Miguel A. Garcia, David Harrington, Jouni Korhonen, Matt
       Lepinski, Avi Lior, Chris Metz, Pallavi Mishra, Lionel Morand, Robert
       Sparks, Martin Stiemerling, Dave Thaler, Hannes Tschofenig, Sean
       Turner, Shashank Vikram, Greg Weber, and Glen Zorn for their input on
       this document.

    15.  References

    15.1.  Normative References

       [ETSIES283034]  ETSI, "Telecommunications and Internet Converged
                       Services and Protocols for Advanced Networks
                       (TISPAN), Network Attachment Sub-System (NASS), e4
                       interface based on the Diameter protocol.",
                       September 2008.

       [RFC2119]       Bradner, S., "Key words for use in RFCs to Indicate
                       Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC4005]       Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
                       "Diameter Network Access Server Application",
                       RFC 4005, August 2005.

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    RFC 6736            Diameter NAT Control Application        October 2012

       [RFC4675]       Congdon, P., Sanchez, M., and B. Aboba, "RADIUS
                       Attributes for Virtual LAN and Priority Support",
                       RFC 4675, September 2006.

       [RFC5226]       Narten, T. and H. Alvestrand, "Guidelines for Writing
                       an IANA Considerations Section in RFCs", BCP 26,
                       RFC 5226, May 2008.

       [RFC5777]       Korhonen, J., Tschofenig, H., Arumaithurai, M.,
                       Jones, M., and A. Lior, "Traffic Classification and
                       Quality of Service (QoS) Attributes for Diameter",
                       RFC 5777, February 2010.

       [RFC6733]       Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
                       "Diameter Base Protocol", RFC 6733, October 2012.

    15.2.  Informative References

       [CGN-REQS]      Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa,
                       A., and H. Ashida, "Common requirements for Carrier
                       Grade NATs (CGNs)", Work in Progress, September 2012.

       [RFC1661]       Simpson, W., "The Point-to-Point Protocol (PPP)",
                       STD 51, RFC 1661, July 1994.

       [RFC2663]       Srisuresh, P. and M. Holdrege, "IP Network Address
                       Translator (NAT) Terminology and Considerations",
                       RFC 2663, August 1999.

       [RFC3022]       Srisuresh, P. and K. Egevang, "Traditional IP Network
                       Address Translator (Traditional NAT)", RFC 3022,
                       January 2001.

       [RFC3303]       Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor,
                       A., and A. Rayhan, "Middlebox communication
                       architecture and framework", RFC 3303, August 2002.

       [RFC3304]       Swale, R., Mart, P., Sijben, P., Brim, S., and M.
                       Shore, "Middlebox Communications (midcom) Protocol
                       Requirements", RFC 3304, August 2002.

       [RFC3411]       Harrington, D., Presuhn, R., and B. Wijnen, "An
                       Architecture for Describing Simple Network Management
                       Protocol (SNMP) Management Frameworks", STD 62,
                       RFC 3411, December 2002.

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    RFC 6736            Diameter NAT Control Application        October 2012

       [RFC3550]       Schulzrinne, H., Casner, S., Frederick, R., and V.
                       Jacobson, "RTP: A Transport Protocol for Real-Time
                       Applications", STD 64, RFC 3550, July 2003.

       [RFC4097]       Barnes, M., "Middlebox Communications (MIDCOM)
                       Protocol Evaluation", RFC 4097, June 2005.

       [RFC5189]       Stiemerling, M., Quittek, J., and T. Taylor,
                       "Middlebox Communication (MIDCOM) Protocol
                       Semantics", RFC 5189, March 2008.

       [RFC6145]       Li, X., Bao, C., and F. Baker, "IP/ICMP Translation
                       Algorithm", RFC 6145, April 2011.

       [RFC6146]       Bagnulo, M., Matthews, P., and I. van Beijnum,
                       "Stateful NAT64: Network Address and Protocol
                       Translation from IPv6 Clients to IPv4 Servers",
                       RFC 6146, April 2011.

       [RFC6241]       Enns, R., Bjorklund, M., Schoenwaelder, J., and A.
                       Bierman, "Network Configuration Protocol (NETCONF)",
                       RFC 6241, June 2011.

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    RFC 6736            Diameter NAT Control Application        October 2012

    Authors' Addresses

       Frank Brockners
       Cisco
       Hansaallee 249, 3rd Floor
       Duesseldorf, Nordrhein-Westfalen  40549
       Germany

       EMail: fbrockne@cisco.com

       Shwetha Bhandari
       Cisco
       Cessna Business Park, Sarjapura Marathalli Outer Ring Road
       Bangalore, Karnataka 560 087
       India

       EMail: shwethab@cisco.com

       Vaneeta Singh
       18, Cambridge Road
       Bangalore 560008
       India

       EMail: vaneeta.singh@gmail.com

       Victor Fajardo
       Telcordia Technologies
       1 Telcordia Drive #1S-222
       Piscataway, NJ 08854
       USA

       EMail: vf0213@gmail.com

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