Internet Engineering Task Force (IETF) F. Brockners

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

V. Fajardo
Telcordia Technologies
October 2012

Diameter Network Address and Port Translation Control Application

Abstract

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

Status of This Memo

This is an Internet Standards Track document.

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

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

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RFC 6736 Diameter NAT Control Application October 2012

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.

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

Table of Contents

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

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

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RFC 6736 Diameter NAT Control Application October 2012

1. Introduction

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

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

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

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

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

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

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

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

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

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

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

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

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

2. Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].

Abbreviations and terminology used in this document:

AAA: Authentication, Authorization, Accounting

DNCA: Diameter Network address and port translation Control
Application

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

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

NAS: Network Access Server

NCR: NAT-Control-Request

NCA: NAT-Control-Answer

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

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

PPP: Point-to-Point Protocol [RFC1661]

3. Deployment Framework

3.1. Deployment Scenario

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

Figure 1: Typical Network Deployment for Internet Access

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

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

o Provisioning static NAT-bindings for particular endpoints

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

o Reporting allocated bindings on a per-endpoint basis

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

Figure 2: Access Network Deployment with NAT

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

3.2. Diameter NAPT Control Application Overview

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

DNCA offers:

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

3.3. Deployment Scenarios for DNCA

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

Figure 3: NAT Control Deployment: Integrated Deployment

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

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

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

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

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

Figure 4: NAT Control Deployment: Autonomous Deployment

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

4. DNCA Session Establishment and Management

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

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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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

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

Figure 9: Abort NAT Control Session

4.6. Failure Cases of the DNCA Diameter Peers

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

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

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

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

Brockners, et al. Standards Track [Page 22]

RFC 6736 Diameter NAT Control Application October 2012

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.

Brockners, et al. Standards Track [Page 23]

RFC 6736 Diameter NAT Control Application October 2012

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].

Brockners, et al. Standards Track [Page 24]

RFC 6736 Diameter NAT Control Application October 2012

6. DNCA Commands

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

6.1. NAT-Control-Request (NCR) Command

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

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

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

Brockners, et al. Standards Track [Page 25]

RFC 6736 Diameter NAT Control Application October 2012

6.2. NAT-Control-Answer (NCA) Command

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

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

7. NAT Control Application Session State Machine

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

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

Brockners, et al. Standards Track [Page 26]

RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

Pending Successful NCA Setup Open
received complete

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

Pending Error processing successful Send STR Discon
NCA

Pending Failed Clean up Idle
NCA received

Brockners, et al. Standards Track [Page 27]

RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

Discon Failure to send ASR Wait, Discon
resend ASR

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

Not ASA received None No change
Discon

Any STR received Send STA, Idle
Clean up

8. DNCA AVPs

8.1. Reused Base Protocol AVPs

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

Brockners, et al. Standards Track [Page 29]

RFC 6736 Diameter NAT Control Application October 2012

+---------+
| 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.

Brockners, et al. Standards Track [Page 30]

RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

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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RFC 6736 Diameter NAT Control Application October 2012

9. Accounting Commands

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

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

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

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

9.1. NAT Control Accounting Messages

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

9.2. NAT Control Accounting AVPs

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

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RFC 6736 Diameter NAT Control Application October 2012

9.2.1. NAT-Control-Record

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

9.2.2. NAT-Control-Binding-Status

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

The following values are defined:

Created (1)

NAT-binding is created.

Active (2)

NAT-binding is active.

Removed (3)

NAT-binding was removed.

9.2.3. Current-NAT-Bindings

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

10. AVP Occurrence Tables

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

Brockners, et al. Standards Track [Page 41]

RFC 6736 Diameter NAT Control Application October 2012

The table uses the following symbols:

0 The AVP MUST NOT be present in the message.

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

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

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

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

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

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

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

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

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

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

Brockners, et al. Standards Track [Page 42]

RFC 6736 Diameter NAT Control Application October 2012

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

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

10.2. DNCA AVP Table for Session Query Requests

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

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

10.3. DNCA AVP Table for Accounting Messages

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

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11. IANA Considerations

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

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

11.1. Application Identifier

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

11.2. Command Codes

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

11.3. AVP Codes

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

11.4. Result-Code AVP Values

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

11.5. NC-Request-Type AVP

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

Brockners, et al. Standards Track [Page 44]

RFC 6736 Diameter NAT Control Application October 2012

11.6. NAT-External-Port-Style AVP

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

11.7. NAT-Control-Binding-Status AVP

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

12. Security Considerations

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

13. Examples

This section shows example DNCA message content and exchange.

13.1. DNCA Session Establishment Example

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

In this example, the NAT controller does the following:

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

Detailed description of the steps shown in Figure 15:

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

13.2. DNCA Session Update with Port Style Example

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

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RFC 6736 Diameter NAT Control Application October 2012

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

13.3. DNCA Session Query Example

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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RFC 6736 Diameter NAT Control Application October 2012

13.4. DNCA Session Termination Example

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

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

Figure 20: NAT Control Session Termination Example

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

14. Acknowledgements

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

15. References

15.1. Normative References

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

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

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

Brockners, et al. Standards Track [Page 55]

RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

15.2. Informative References

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

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

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

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

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

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

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

Brockners, et al. Standards Track [Page 56]

RFC 6736 Diameter NAT Control Application October 2012

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

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

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

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

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

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

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RFC 6736 Diameter NAT Control Application October 2012

Authors' Addresses

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

EMail: [email protected]

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

EMail: [email protected]

Vaneeta Singh
18, Cambridge Road
Bangalore 560008
India

EMail: [email protected]

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

EMail: [email protected]

Brockners, et al. Standards Track [Page 58]