Internet Engineering Task Force F. Brockners
Internet-Draft S. Bhandari
Intended status: Standards Track Cisco
Expires: January 11, 2012 V. Singh
V. Fajardo
Telcordia Technologies
July 10, 2011

Diameter Network Address and Port Translation Control Application
draft-ietf-dime-nat-control-09

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 AAA and policy control capabilities with a Network Address Translators and Network Address and Port Translators 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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 11, 2012.

Copyright Notice

Copyright (c) 2011 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.


Table of Contents

1. Introduction

Internet service providers have started to deploy Network Address Translators (NATs) and Network Address and Port Translators (NAPTs) in their networks to deal with the depletion of available public IPv4 addresses. This document defines a Diameter application allowing providers to control the behavior of these NAT and NAPT devices. 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 end point or user. The main motivation for restricting the number of bindings on a per end point basis is to protect the service of the service provider against denial of service attacks. If multiple end points share a single public IP address, these end points can share fate. If one end point would (either intentionally, or due to mis-behavior, mis-configuration, mal-ware, 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 end points sharing the same public IP address. The efficiency of a NAPT deployment depends on the maximum number of bindings an end point could use. Given that the typical number of bindings an end point uses depends on the type of end point (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 end points, it is desirable to actively manage the maximum number of bindings.
  2. Supports the allocation of specific NAPT/NAT bindings. Two types of specific bindings can be distinguished:
  3. Defines the external address pool(s) to be used for allocating an external IP address: External address pools can either be 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 user. 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).
  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 or a set of classifiers, such as IP address, Virtual Local Area Network (VLAN) identifier, or interface identifier which uniquely identify the traffic associated with a particular global endpoint.

This document is structured as follows: Section 2 lists terminology, while Section 3 provides an introduction to DNCA and its overall deployment framework. Sections 4 to 8 cover DNCA specifics, with Section 4 describing session management, Section 5 the use of the Diameter base protocol, Section 6 new commands, Section 7 AVPs used, and Section 8 accounting aspects. Section 9 presents an AVP occurence table. IANA and security considerations are addressed in Sections 10 and 11.

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 used in this document:

3. Deployment Framework

3.1. Deployment Scenario

Figure 1 shows a typical network deployment for Internet access. A user’s IPv4 host gains access to the Internet though a NAS, which facilitates the authentication of the endpoint and configures the user’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.

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

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

Figure 2 depicts the deployment scenario when a service provider introduces a NAT-device to increase the efficiency of the global IPv4 address pool utilization. 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. If the NAT-device would be put in place without any endpoint awareness, the service offerings of the service provider could be impacted. This includes cases like:

                +---------+
                |         |
                |   AAA   |
                |         |
                +---------+
                     |
                     |
                     |
                     |
  +--------+    +---------+    +--------+    +----------+
  |  IPv4/ |    |         |    |        |    |  IPv4    |
  |  IPv6  |----|   NAS   |----|  NAT-  |----| Internet |
  |  Host  |    |         |    | device |    |          |
  +--------+    +---------+    +--------+    +----------+
  <-------- Private IPv4 ----------><--- Public IPv4 --->
  <-------- Public  IPv6 ----------><--- Public IPv4 --->

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 the NAT-Controller. 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:

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 either using accounting mechanisms or through an explicit session query to the NAT.

3.3. Deployment Scenarios For DNCA

DNCA can be deployed in different ways. Two common deployment scenarios are outlined in Figure 3 (“integrated deployment”) and Figure 4 (“autonomous deployment”). The two 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 NAT-device. With this information, the NAS can provide a single accounting record for the endpoint. A system correlating the accounting information received from 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   |
                |         |
                +---------+
                     |
                     |
                     |
  +--------+    +---------+    +--------+    +----------+
  |  IPv4/ |    |   (C)   |    |        |    |  IPv4    |
  |  IPv6  |----|   NAS   |----|  NAT-  |----| Internet |
  |  Host  |    |         |    | device |    |          |
  +--------+    +---------+    +--------+    +----------+
  <-------- Public  IPv6 ----------><--- Public IPv4 --->
  <-------- Private IPv4 ----------><--- Public IPv4 --->

The autonomous deployment approach decouples user management on 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 also 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  |    |          |
    +--------+    +---------+    +---------+    +----------+
    <-------- Public  IPv6 ----------><---- Public IPv4 --->
    <-------- Private IPv4 ----------><---- Public IPv4 --->

4. DNCA Session Establishment and Management

Note that this section forward references some of the commands and AVPs defined for DNCA. Please refer to Section 6 and Section 8 for details. DNCA runs between a Diameter peer residing in a NAT-controller and a Diameter peer residing in a NAT-device. 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 for example be due to the entity hosting the NAT-controller receiving authentication, authorization, or accounting requests for or from the endpoint. Alternate methods that could trigger session set up 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 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 pre-defined NAT bindings to be established for this endpoint, or a description of the address pool, that external addresses are to be allocated from.

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 NC-Request-Type AVP set to INITIAL_REQUEST to initiate a Diameter NAT control session. On receipt of a 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 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 which may be included, comprises:

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

NAT-controller (DNCA Diameter peer)   NAT-device (DNCA Diameter peer)
            |                                           |
            |                                           |
            |                                           |
         Trigger                                        |
            |                                           |
            |                   NCR                     |
            |------------------------------------------>|
            | (INITIAL_REQUEST, endpoint classifier,    |
            |     session id, NAT control config data)  |
            |                                           |
            |                                           |
            |                                           |
            |                                           |
            |                                 If Able to comply
            |                                 with Request then
            |                                 Create session state
            |                                           |
            |                                           |
            |                     NCA                   |
            |<------------------------------------------|
            |                 (result code)             |
            |                                           |
            |                                           |

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 a NCA with Result-Code set to DIAMETER_SUCCESS.

4.2. Session Re-Authorization

Session re-authorization is performed if the NAT-controller desires to change the behavior of the NAT-device for an existing session. Session re-authorization could be used, for example, to change the number of allowed bindings for a particular session, or establish or remove a pre-defined binding.

The DNCA Diameter peer within the NAT-controller generates a NCR message to DNCA Diameter peer within NAT-device with 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 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:

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

NAT-controller (DNCA Diameter peer)   NAT-device (DNCA Diameter peer)
            |                                           |
            |                                           |
            |                                           |
     Change of session                                  |
        attributes                                      |
            |                                           |
            |                   NCR                     |
            |------------------------------------------>|
            |       (UPDATE_REQUEST session id,         |
            |         NAT control config data)          |
            |                                           |
            |                                           |
            |                                   If able to comply 
            |                                   with the request:
            |                                  Update session state
            |                                           |
            |                                           |
            |                     NCA                   |
            |<------------------------------------------|
            |                 (result code)             |
            |                                           |
            |                                           |

4.3. Session and Binding Query

Session query can be used by the DNCA Diameter peer within the NAT-controller to either retrieve information on the current bindings for a particular session at the NAT-device or 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 a 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 IP-address.

  1. Request a list of currently allocated NAT bindings for a particular session: On receiving a NCR, the NAT-device looks up the session information for the session ID contained in the NCR, and reports all currently active NAT-bindings for the session using a NCA message with 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 returns NCA with Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.
  2. Retrieve session IDs and internal IP address/port pairs for 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 one or multiple external IP-address/port pairs, it MUST include the external IP-address/port pair(s) as part of the NAT-Control-Definition AVP of the NCR. 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 reports the NAT-bindings and associated session IDs corresponding to the external IP-address/port pairs in a NCA message with Result-Code set to DIAMETER_SUCCESS with the same session ID, which was used in NCR. 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.
NAT-controller (DNCA Diameter peer)   NAT-device (DNCA Diameter peer)
            |                                           |
            |                                           |
            |                                           |
  DNCA Session Established                              |
            |                                           |
            |                   NCR                     |
            |------------------------------------------>|
            |              (QUERY_REQUEST)              |
            |                                           |
            |                                           |
            |                                           |
            |                          Look up corresponding session
            |                            and associated NAT-bindings
            |                                           |
            |                   NCA                     |
            |<------------------------------------------|
            |              (Result-Code)                |
            |                                           |
            |                                           |

4.4. Session Termination

Similar to session initiation, session tear down is always initiated by the DNCA Diameter peer within the NAT-controller. The DNCA Diameter peer sends a Session Terminate 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. In response, the DNCA Diameter peer within the NAT-device sends an accounting stop record reporting all bindings and notifies its DNCA Diameter peer about successful session termination using a Session Terminate 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 a STR and fails to find a matching session, the DNCA Diameter peer returns a STA with Result-Code set to DIAMETER_UNKNOWN_SESSION_ID.

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

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 acknowledges successful session abort using a Abort Session Answer (ASA) message with 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 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 returns an ASA with Result-Code set to DIAMETER_UNKNOWN_SESSION_ID. If the DNCA Diameter peer within the NAT-controller is unable to comply with the ASR for any other reason, an ASA with Result-Code set to DIAMETER_UNABLE_TO_COMPLY is returned.

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

4.6. Failure cases of the DNCA Diameter peers

This document does not specify the behavior in case NAT-device and NAT-controller, or their respective DNCA Diameter peers are out of sync. This could happen for example if one of the entities restarts, in case of a (temporary) loss of network connectivity etc. The peering entities MUST have built-in redundancy support to recover state in case of failure.

Example failure cases include the following:

5. Use Of The Diameter Base Protocol

The Diameter Base Protocol defined by [RFC3588] applies with the clarifications listed in the present specification.

5.1. Securing Diameter Messages

For secure transport of Diameter messages recommendations in [RFC3588] apply.

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

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

5.2. Accounting Functionality

Accounting functionality (accounting session state machine, related command codes and AVPs) is defined in Section 9 below.

5.3. Use Of Sessions

Each DNCA session MUST have a globally unique Session-ID as defined in [RFC3588], which MUST NOT be changed during the lifetime of a 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. DNCA session MUST NOT have more than one outstanding request at any given instant. A DNCA Diameter peer sends an Abort-Session-Request as defined in [RFC3588] 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 TBD in the Auth-Application-Id of the Capabilities-Exchange-Request and Capabilities-Exchange-Answer command[RFC3588].

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 TBD 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, Calling-Station-ID AVPs serve as identifiers for the endpoint.

Message format:

   < NC-Request > ::= < Diameter Header: TBD, REQ, PXY>
                    [ Session-Id ]
                    { Auth-Application-Id }
                    { Origin-Host }
                    { Origin-Realm }
                    { Destination-Realm }
                    { Destination-Host }
                    { NC-Request-Type }
                    [ Origin-State-Id ]
                 *1 [ NAT-Control-Remove ]
                 *1 [ NAT-Control-Install ]
                    [ 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 ]

6.2. NAT-Control Answer (NCA) Command

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

Message format:

   <NC-Answer> ::= < Diameter Header: TBD, PXY >
                   [ Session-Id ]
                   { Origin-Host }
                   { Origin-Realm }
                   { Result-Code }
                   [ 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 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 [RFC3588]. 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 as 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 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 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 and 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_RULE_NAME, 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 ID.

          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                 Sent STR   Discon
             received
             but peer unable to provide
             service

   Pending   Error processing successful    Sent STR   Discon
             NCA

   Pending   Failed                         Cleanup    Idle
             NCA received

   Open      NAT control                    Send       Open
             update required                NCR Update
                                            Request

   Open      Successful                                Open
             NCA received  

   Open      Failed                         Cleanup    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
                                            user/device

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

   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            Cleanup

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

   Discon    Failure to send ASR            Wait,      Discon
                                            resend ASR

   Discon    ASR successfully sent and      Cleanup    Idle
             ASA Received with Result-Code

   Not       ASA Received                   None       No change
   Discon

   Any       STR Received                   Send STA,  Idle
                                            Cleanup.

8. DNCA AVPs

8.1. Reused Base Protocol AVPs

AVPs reused from Diameter Base Protocol [RFC3588] are listed below.

                                                +-------------------+
                                                |  AVP Flag rules   |
+-----------------------------------------------|-----+---+---------+
|                           AVP                 |     |   |   May   |
| Attribute Name            Code     Data Type  |MUST |MAY| encrypt |
+-----------------------------------------------+-----+---+---------+
|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    |
+-----------------------------------------------+-----+---+---------+
|M - Mandatory bit. An AVP with "M" bit set and its value MUST be   |
|    supported and recognized by a Diameter entity in order the     |
|    message, which carries this AVP, to be accepted.               |
|P - Indicates the need for encryption for end-to-end security.     |
+-------------------------------------------------------------------+

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 TBD.

8.2. Additional Result-Code AVP Values

This section defines new values for the Result-Code AVP which 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.

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

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:

8.3. Reused NASREQ Diameter Application AVPs

The following AVPs are reused from Diameter Network Access Server Application [RFC4005].

                                       +---------------------+
                                       |    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          |      |            |    |     |    |     |    |
+------------------+------+------------|----+-----+----+-----|----+

8.4. Reused AVPs from RFC 4675

The following AVPs are reused from "RADIUS Attributes for Virtual LAN and Priority Support" specification [RFC4675].

                                       +---------------------+
                                       |    AVP Flag rules   |
+------------------+------+------------|----+-----+----+-----|----+
|                  | AVP  |            |    |     |SHLD| MUST|    |
| Attribute Name   | Code |  Value Type|MUST| MAY | NOT|  NOT|Encr|
|------------------|------|------------|----+-----+----+-----|----|
| Egress-VLANID    |  56  | OctetString| M  |  P  |    |  V  | Y  |
+------------------+------+------------|----+-----+----+-----|----+

8.5. Reused AVPs from Diameter QoS Application

The following AVPs are reused from the Traffic Classification and Quality of Service (QoS) Attributes for Diameter [RFC5777].

                                                +-------------------+
                                                |  AVP Flag rules   |
+-----------------------------------------------|-----+---+---------+
|                           AVP                 |     |   |   May   |
| Attribute Name            Code     Data Type  |MUST |MAY| encrypt |
+-----------------------------------------------+-----+---+---------+
|Port                       TBD     Integer32   |  M  | P |    Y    |
|IP-Address-Mask            TBD     Grouped     |  M  | P |    Y    |
|Protocol                   TBD     Enumerated  |  M  | P |    Y    |
|Direction                  TBD     Enumerated  |  M  | P |    Y    |
+-----------------------------------------------+-----+---+---------+
|M - Mandatory bit. An AVP with "M" bit set and its value MUST be   |
|    supported and recognized by a Diameter entity in order the     |
|    message, which carries this AVP, to be accepted.               |
|P - Indicates the need for encryption for end-to-end security.     |
+-------------------------------------------------------------------+

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

The following AVPs are reused from the Diameter e4 Application [ETSIES283034].

                                                +-------------------+
                                                |  AVP Flag rules   |
+-----------------------------------------------|-----+---+---------+
|                           AVP                 |     |   |   May   |
| Attribute Name            Code     Data Type  |MUST |MAY| encrypt |
+-----------------------------------------------+-----+---+---------+
|Address-Realm              301     OctetString | M,V |   |    Y    |
|Logical-Access-Id          302     OctetString |   V | M |    Y    |
|Physical-Access-ID         313     UTF8String  |   V | M |    Y    |
+-----------------------------------------------+-----+---+---------+
|M - Mandatory bit. An AVP with "M" bit set and its value MUST be   |
|    supported and recognized by a Diameter entity in order the     |
|    message, which carries this AVP, to be accepted.               |
|P - Indicates the need for encryption for end-to-end security.     |
|V - Indicates whether the optional Vendor-ID field is present      |
|    in the AVP header. Vendor-Id header of all AVPs in             |
|    this table will be set to ETSI (13019).                        |
+-------------------------------------------------------------------+

8.7. DNCA Defined AVPs

The following table describes the new Diameter AVPs used in this document.

                                                +-------------------+
                                                |  AVP Flag rules   |
+-----------------------------------------------|-----+---+---------+
|                       AVP  Section            |     |   |   May   |
| Attribute Name        Code Defined Data Type  |MUST |MAY| encrypt |
+-----------------------------------------------+-----+---+---------+
|NC-Request-Type        TBD  8.7.1   Enumerated | M   | P |    Y    |
|NAT-Control-Install    TBD  8.7.2   Grouped    | M   | P |    Y    |
|NAT-Control-Remove     TBD  8.7.3   Grouped    | M   | P |    Y    |
|NAT-Control-Definition TBD  8.7.4   Grouped    | M   | P |    Y    |
|NAT-Internal-Address   TBD  8.7.5   Grouped    | M   | P |    Y    |
|NAT-External-Address   TBD  8.7.6   Grouped    | M   | P |    Y    |
|Max-NAT-Bindings       TBD  8.7.7   Unsigned32 | M   | P |    Y    |
|NAT-Control-           TBD  8.7.8   OctetString| M   | P |    Y    |
| Binding-Rule                                  |     |   |         |
|Duplicate-             TBD  8.7.9   UTF8String | M   | P |    Y    |
| Session-ID                                    |     |   |         |
|NAT-Control-Record     TBD  9.2.1   Grouped    | M   | P |    Y    |
|NAT-Control-           TBD  9.2.2   Enumerated | M   | P |    Y    |
| Binding-Status                                |     |   |         |
|Current-NAT-Bindings   TBD  9.2.3   Unsigned32 | M   | P |    Y    |
+-----------------------------------------------+-----+---+---------+
|M - Mandatory bit. An AVP with "M" bit set and its value MUST be   |
|    supported and recognized by a Diameter entity in order the     |
|    message, which carries this AVP, to be accepted.               |
|P - Indicates the need for encryption for end-to-end security.     |
+-------------------------------------------------------------------+

8.7.1. NC-Request-Type AVP

The NC-Request-Type AVP (AVP Code TBD) 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:

8.7.2. NAT-Control-Install AVP

The NAT-Control AVP (AVP code TBD) 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 to be allowed for a subscriber and the NAT-Control-Binding-Rule that references a predefined policy 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.

AVP format:

  NAT-Control-Install ::= < AVP Header: TBD >
                           * [ NAT-Control-Definition ]
                             [ NAT-Control-Binding-Rule ]
                             [ Max-NAT-Bindings]
                           * [ AVP ]

8.7.3. NAT-Control-Remove AVP

The NAT-Control-Remove AVP (AVP code TBD) is of type Grouped, and it is used to deactivate or remove NAT-bindings.

AVP format:

  NAT-Control-Remove ::= < AVP Header: TBD >
                          * [ NAT-Control-Definition ]
                            [ NAT-Control-Binding-Rule ]
                          * [ AVP ]

8.7.4. NAT-Control-Definition AVP

The NAT-Control-Definition AVP (AVP code TBD) 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 pre-defined binding.

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-address 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.

AVP format:

  NAT-Control-Definition ::= < AVP Header: TBD >
                              { 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 TBD) 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.

AVP format:

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

8.7.6. NAT-External-Address AVP

The NAT-External-Address AVP (AVP code TBD) is of type Grouped, and it describes the external IP-address and port for a binding. IP-Address-Mask AVP can only be specified when the Framed-IP-Address AVP is present. 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.

AVP format:

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

8.7.7. Max-NAT-Bindings

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

8.7.8. NAT-Control-Binding-Rule AVP

The NAT-Control-Binding-Rule AVP (AVP code TBD) 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 to may contain NAT-bindings, IP-address pool for allocating the external IP-address of a NAT-binding, and maximum number of allowed NAT-bindings. Such policy template can be reused by specifying the same NAT-Control-Binding-Rule 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 TBD) is of type UTF8String. It is used to report errors and contains the Session-Id of an existing session.

9. Accounting Commands

The DNCA reuses session based accounting as defined in the Diameter Base Protocol[RFC3588] to report the bindings per endpoint. This reporting is achieved by sending Diameter Accounting Requests (ACR) [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 a 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 a NCR with NC-Request-Type AVP set to UPDATE_REQUEST, or 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 STR.

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 maximum number of bindings, if received in NCR, is reached.

9.1. NAT Control Accounting Messages

The ACR and ACA messages are reused as defined in Diameter Base Protocol [RFC3588] for exchanging endpoint NAT binding details between the DNCA Diameter peers. DNCA Application ID is used in the accounting commands. ACR contains one or more optional NAT-Control-Record AVP to report the bindings. The NAT-device indicates the number of allocated NAT bindings to 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 [RFC3588], the DNCA Diameter peer within the NAT-device must add the NAT-Control-Record AVP.

9.2.1. NAT-Control-Record

The NAT-Control-Record AVP (AVP code TBD) 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: TBD >
                         { NAT-Control-Definition }
                         { NAT-Control-Binding-Status } 
                         [ Event-Timestamp ]

9.2.2. NAT-Control-Binding-Status

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

The following values are defined:

9.2.3. Current-NAT-Bindings

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

10. AVP Occurrence Table

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

The table uses the following symbols:

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 NCR and NCA with 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     |
+-------------------------------------------------------+

10.2. DNCA AVP Table for Session Query request

The following table lists DNCA specific AVPs that have to be present in NCR and NCA with 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+      |
|Current-NAT-Bindings                    0      1       |
|Duplicate-Session-Id                    0      0       |
+-------------------------------------------------------+

10.3. DNCA AVP Table for Accounting Message

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       |
+-------------------------------------------------------+

11. IANA Considerations

This section contains the namespaces that have either 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, 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 <TBD>, 'Diameter NAT Control Application', to the Application Identifier namespace defined in [RFC3588]. See Section 4 for more information.

11.2. Command Codes

This specification uses the value <TBD> from the Command code namespace defined in [RFC3588] for the NAT-Control-Request (NCR), 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 <TBD> from the AVP code namespace defined in [RFC3588]. See Figure 19for the assignment of the namespace in this specification.

11.4. Result-Code AVP Values

This specification assigns the values <TBD> (4xxx, 5xxx, 5xxx, 5xxx, 5xxx,5xxx) from the Result-Code AVP value namespace defined in [RFC3588]. 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].

11.6. 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 NAT-control in a service provider network.

Security between NAT-controller and 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 denial of service attack by setting the maximum number of bindings for a session on the NAT-device to zero; provision bindings on a NAT-device which include IP-addresses already in use in other parts of the network; or request session termination of the Diameter session and hamper 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, templates on the NAT-device specifying pre-defined binding rules could be configured differently than expected by the NAT-controller. Failing of any of the two DNCA Diameter peers to provide the required credentials should be subject to logging.

Authorization refers to whether a particular authorizing entity is authorized to signal a network element requests 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 denial of service.

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, it is assumed that messages between DNCA Diameter peers are secured using either IPsec or TLS. Please refer to [RFC3588], section 13 for details. DNCA Diameter peers MAY perform bilateral authentication, authorization as well as procedures to ensure integrity and confidentiality of the information exchange.

It is assumed that the DNCA Diameter peers are typically in the same domain and have a mutual trust set up. This document does not specify a mechanisms for authorization between the DNCA Diameter peers. It is assumed that the DNCA Diameter peers are 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.

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 end points 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 end point. In addition one could setup 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 protections on DNCA and its Diameter protocol exchanges don't prevent such abuses of NAT-control. A service provider deploying DNCA needs to make sure that higher layer processes and procedures are put in place which allow them to detect and mitigate misuses.

13. Acknowledgements

The authors would like to thank Wesley Eddy, Miguel A. Garcia, Jouni Korhonen, Matt Lepinski, Avi Lior, Chris Metz, Pallavi Mishra, Lionel Morand, Hannes Tschofenig, Shashank Vikram, Greg Weber, and Glen Zorn for their input on this document.

14. Change History (to be removed prior to publication as an RFC)

Changes from -00 to -01

  1. new values for Result-Code AVP used - instead of Experimental-Result AVP
  2. added support for transport specific binding (UDP/TCP)
  3. added support for twice-NAT
  4. clarified the use of the two different types of query-requests

Changes from -01 to -02

  1. Reference to pull mode removed, session initiation event clarified in section 4.1
  2. added Redirect-* AVPs in NCA command
  3. Removed reference to Called-Station-Id AVP in NCR command
  4. Editorial changes
  5. added support for bindings providing AFT (NAT64)

Changes from -02 to -03

  1. Editorial changes

Changes from -03 to -04

  1. Editorial changes suggested in WG last call review
  2. Removed NCR Request type terminate and replaced with STR
  3. All references to Auth-Session-State are removed and a new section to describe FSM for Manager and Agent has been added
  4. Clarified reuse of External address and address pools among multiple subscribers

Changes from -04 to -05

  1. Removed references to Large Scale NAT as per review comments

Changes from -05 to -06

  1. Editorial changes

Changes from -06 to -07

  1. Added a note in section 4.3 stating the state of pre-existing bindings on update failure
  2. Security considerations are made consistent between sections 5.1 and 12
  3. Editorial changes

Changes from -07 to -08

  1. Added section 4.6 to describe session abort
  2. Editorial changes
  3. Nomenclature change: From DNCA Agent/Manager to DNCA Diameter peers identifying the location where they reside (NAT-controller or NAT-device)
  4. IANA consideration Section format changes
  5. Updated security section (included considerations directly, rather than referring to Diameter QoS similarities).

Changes from -08 to -09

  1. expanded on the need for an SP controlling the maximum number of bindings of an end point (see introduction section)
  2. added a paragraph in the security section outlining general mis-uses of NAT-control (non specific to DNCA), with DNCA being an example of such a NAT-control protocol
  3. editorial changes

15. References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G. and J. Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[RFC4675] Congdon, P., Sanchez, M. and B. Aboba, "RADIUS Attributes for Virtual LAN and Priority Support", RFC 4675, September 2006.
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, May 2008.
[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.
[RFC4005] Calhoun, P., Zorn, G., Spence, D. and D. Mitton, "Diameter Network Access Server Application", RFC 4005, August 2005.

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