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RFC 6736

 
 
 

Diameter Network Address and Port Translation Control Application

Part 2 of 3, p. 12 to 39
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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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   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.

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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.

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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.

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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

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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.

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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.

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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

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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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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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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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   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.

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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.

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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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