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RFC 7289

Informational
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Carrier-Grade NAT (CGN) Deployment with BGP/MPLS IP VPNs

 


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Internet Engineering Task Force (IETF)                 V. Kuarsingh, Ed.
Request for Comments: 7289                                 J. Cianfarani
Category: Informational                            Rogers Communications
ISSN: 2070-1721                                                June 2014


        Carrier-Grade NAT (CGN) Deployment with BGP/MPLS IP VPNs

Abstract

   This document specifies a framework to integrate a Network Address
   Translation (NAT) layer into an operator's network to function as a
   Carrier-Grade NAT (also known as CGN or Large-Scale NAT).  The CGN
   infrastructure will often form a NAT444 environment as the subscriber
   home network will likely also maintain a subscriber-side NAT
   function.  Exhaustion of the IPv4 address pool is a major driver
   compelling some operators to implement CGN.  Although operators may
   wish to deploy IPv6 to strategically overcome IPv4 exhaustion, near-
   term needs may not be satisfied with an IPv6 deployment alone.  This
   document provides a practical integration model that allows the CGN
   platform to be integrated into the network, meeting the connectivity
   needs of the subscriber while being mindful of not disrupting
   existing services and meeting the technical challenges that CGN
   brings.  The model included in this document utilizes BGP/MPLS IP
   VPNs, which allow for virtual routing separation, helping ease the
   CGN's impact on the network.  This document does not intend to defend
   the merits of CGN.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see 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/rfc7289.

Page 2 
Copyright Notice

   Copyright (c) 2014 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.

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Table of Contents

   1. Introduction ....................................................4
      1.1. Acronyms and Terms .........................................4
   2. Existing Network Considerations .................................5
   3. CGN Network Deployment Requirements .............................5
      3.1. Centralized versus Distributed Deployment ..................6
      3.2. CGN and Traditional IPv4 Service Coexistence ...............7
      3.3. CGN Bypass .................................................7
      3.4. Routing Plane Separation ...................................8
      3.5. Flexible Deployment Options ................................8
      3.6. IPv4 Overlap Space .........................................9
      3.7. Transactional Logging for CGN Systems ......................9
      3.8. Base CGN Requirements ......................................9
   4. BGP/MPLS IP VPN-Based CGN Framework .............................9
      4.1. Service Separation ........................................11
      4.2. Internal Service Delivery .................................12
           4.2.1. Dual-Stack Operation ...............................14
      4.3. Deployment Flexibility ....................................16
      4.4. Comparison of BGP/MPLS IP VPN Option versus Other
           CGN Attachment Options ....................................16
           4.4.1. Policy-Based Routing ...............................16
           4.4.2. Traffic Engineering ................................17
           4.4.3. Multiple Routing Topologies ........................17
      4.5. Multicast Considerations ..................................17
   5. Experiences ....................................................17
      5.1. Basic Integration and Requirements Support ................17
      5.2. Performance ...............................................18
   6. Security Considerations ........................................18
   7. BGP/MPLS IP VPN CGN Framework Discussion .......................18
   8. Acknowledgements ...............................................19
   9. References .....................................................19
      9.1. Normative Reference .......................................19
      9.2. Informative References ....................................19

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1.  Introduction

   Operators are faced with near-term IPv4 address-exhaustion
   challenges.  Many operators may not have a sufficient amount of IPv4
   addresses in the future to satisfy the needs of their growing
   subscriber base.  This challenge may also be present before or during
   an active transition to IPv6, somewhat complicating the overall
   problem space.

   To face this challenge, operators may need to deploy CGN (Carrier-
   Grade NAT) as described in [RFC6888] to help extend the connectivity
   matrix once IPv4 address caches run out on the local operator.  CGN
   deployments will most often be added into operator networks that
   already have active IPv4 and/or IPv6 services.

   The addition of the CGN introduces a translation layer that is
   controlled and administered by an operator and that should be added
   in a manner that minimizes disruption to existing services.  The CGN
   system addition may also include interworking in a dual-stack
   environment where the IPv4 path requires translation.

   This document shows how BGP/MPLS IP VPNs as described in [RFC4364]
   can be used to integrate the CGN infrastructure solving key
   integration challenges faced by the operator.  This model has also
   been tested and validated in real production-network models and
   allows fluid operation with existing IPv4 and IPv6 services.

1.1.  Acronyms and Terms

   Acronyms and terms used in this document are defined in the list
   below.

      CGN - Carrier-Grade NAT

      DOCSIS - Data Over Cable Service Interface Specification

      CMTS - Cable Modem Termination System

      DSL - Digital Subscriber Line

      BRAS - Broadband Remote Access Server

      GGSN - Gateway GPRS Support Node

      GPRS - General Packet Radio Service

      ASN-GW - Access Service Network Gateway

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      GRT - Global Routing Table

      Internal Realm - Addressing and/or network zone between the
      Customer Premises Equipment (CPE) and CGN as specified in
      [RFC6888]

      External Realm - Public-side network zone and addressing on the
      Internet-facing side of the CGN as specified in [RFC6888]

2.  Existing Network Considerations

   The selection of CGN may be made by an operator based on a number of
   factors.  The overall driver to use CGN may be the depletion of IPv4
   address pools, which leaves little to no addresses for a growing IPv4
   service or connection demand growth.  IPv6 is considered the
   strategic answer for IPv4 address depletion; however, the operator
   may independently decide that CGN is needed to supplement IPv6 and
   address their particular IPv4 service deployment needs.

   If the operator has chosen to deploy CGN, they should do this in a
   manner as not to negatively impact the existing IPv4 or IPv6
   subscriber base.  This will include solving a number of challenges
   since subscribers whose connections require translation will have
   network routing and flow needs that are different from legacy IPv4
   connections.

3.  CGN Network Deployment Requirements

   If a service provider is considering a CGN deployment with a provider
   NAT44 function, there are a number of basic architectural
   requirements that are of importance.  Preliminary architectural
   requirements may require all or some of those captured in the list
   below.  Each of the architectural requirement items listed is
   expanded upon in the following subsections.  It should be noted that
   architectural CGN requirements are additive to base CGN functional
   requirements contained in [RFC6888].  The assessed architectural
   requirements for deployment are:

   -  Support distributed (sparse) and centralized (dense) deployment
      models.  See Section 3.1

   -  Allow coexistence with traditional IPv4-based deployments, which
      provide global-scoped IPv4 addresses to CPEs.  See Section 3.2.

   -  Provide a framework for CGN bypass supporting non-translated flows
      between endpoints within a provider's network.  See Section 3.3.

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   -  Provide a routing framework that allows the segmentation of
      routing control and forwarding paths between CGN-mediated and non-
      CGN-mediated flows.  See Section 3.4.

   -  Provide flexibility for operators to modify their deployments over
      time as translation demands change (connections, bandwidth,
      translation realms/zones, and other vectors).  See Section 3.5.

   -  Flexibility should include integration options for common access
      technologies such as DSL (BRAS), DOCSIS (CMTS), Mobile (GGSN/PGW/
      ASN-GW), and direct Ethernet.  See Section 3.5.

   -  Support deployment modes that allow for IPv4 address overlap
      within the operator's network (between various translation realms
      or zones).  See Section 3.6.

   -  Allow for evolution to future dual-stack and IPv4/IPv6 transition
      deployment modes.  See Section 3.5.

   -  Transactional logging and export capabilities to support auxiliary
      functions, including abuse mitigation.  See Section 3.7.

   -  Support for stateful connection synchronization between
      translation instances/elements (redundancy).  See Section 3.8.

   -  Support for CGN Shared Address Space [RFC6598] deployment modes if
      applicable.  See Section 3.6.

   -  Allow for the enablement of CGN functionality (if required) while
      still minimizing costs and subscriber impact to the best extend
      possible.  See Section 3.8.

   Other requirements may be assessed on an operator-by-operator basis,
   but those listed above may be considered for any given deployment
   architecture.

3.1.  Centralized versus Distributed Deployment

   Centralized deployments of CGN (longer proximity to end user and/or
   higher densities of subscribers/connections to CGN instances) differ
   from distributed deployments of CGN (closer proximity to end user
   and/or lower densities of subscribers/connections to CGN instances).
   Service providers may likely deploy CGN translation points more
   centrally during initial phases if the early system demand is low.
   Early deployments may see light loading on these new systems since
   legacy IPv4 services will continue to operate with most endpoints
   using globally unique IPv4 addresses.  Exceptional cases that may
   drive heavy usage in initial stages may include operators that

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   already translate a significant portion of their IPv4 traffic,
   operators that may transition to a CGN implementation from legacy
   translation mechanisms (i.e., traditional firewalls), or operators
   that build a greenfield deployment that may see quick growth in the
   number of new IPv4 endpoints that require Internet connectivity.

   Over time, some providers may need to expand and possibly distribute
   the translation points if demand for the CGN system increases.  The
   extent of the expansion of the CGN infrastructure will depend on
   factors such as growth in the number of IPv4 endpoints, status of
   IPv6 content on the Internet, and the overall progress globally to an
   IPv6-dominate Internet (reducing the demand for IPv4 connectivity).
   The overall demand for CGN resources will probably follow a bell-like
   curve with a growth, peak, and decline period.

3.2.  CGN and Traditional IPv4 Service Coexistence

   Newer CGN-serviced endpoints will exist alongside endpoints served by
   traditional IPv4 globally routed addresses.  Operators will need to
   rationalize these environments since both have distinct forwarding
   needs.  Traditional IPv4 services will likely require (or be best
   served by) direct forwarding toward Internet peering points while
   CGN-mediated flows require access to a translator.  CGN-mediated and
   non-CGN-mediated flows pose two fundamentally different forwarding
   needs.

   The new CGN environments should not negatively impact the existing
   IPv4 service base by forcing all traffic to translation-enabled
   network points since many flows do not require translation and this
   would reduce performance of the existing flows.  This would also
   require massive scaling of the CGN, which is a cost and efficiency
   concern as well.

   Efficiency of traffic flow and forwarding is considered important
   since networks are under considerable demand to deliver more and more
   bandwidth without the luxury of needless inefficiencies that can be
   introduced with CGN.

3.3.  CGN Bypass

   The CGN environment is only needed for flows with translation
   requirements.  Many flows that remain within the operator's network
   do not require translation.  Such services include operator-offered
   DNS services, DHCP services, NTP services, web caching, email, news,
   and other services that are local to the operator's network.

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   The operator may want to leverage opportunities to offer third
   parties a platform to also provide services without translation.  CGN
   bypass can be accomplished in many ways, but a simplistic,
   deterministic, and scalable model is preferred.

3.4.  Routing Plane Separation

   Many operators will want to engineer traffic separately for CGN flows
   versus flows that are part of the more traditional IPv4 environment.
   Many times, the routing of these two major flow types differ;
   therefore, route separation may be required.

   Routing-plane separation also allows the operator to utilize other
   addressing techniques, which may not be feasible on a single routing
   plane.  Such examples include the use of overlapping private address
   space [RFC1918], Shared Address Space [RFC6598], or other IPv4 space
   that may overlap globally within the operator's network.

3.5.  Flexible Deployment Options

   Service providers operate complex routing environments and offer a
   variety of IPv4-based services.  Many operator environments utilize
   distributed external routing infrastructures for transit and peering,
   and these may span large geographical areas.  A CGN solution should
   offer operators the ability to place CGN translation points at
   various points within their network.

   The CGN deployment should also be flexible enough to change over time
   as demand for translation services increase or change as noted in
   [RFC6264].  In turn, the deployment will need to then adapt as
   translation demand decreases due to the transition of flows to IPv6.
   Translation points should be able to be placed and moved with as
   little re-engineering effort as possible, minimizing the risks to the
   subscriber base.

   Depending on hardware capabilities, security practices, and IPv4
   address availability, the translation environments may need to be
   segmented and/or scaled over time to meet organic IPv4 demand growth.
   Operators may also want to choose models that support transition to
   other translation environments such as Dual-Stack Lite (DS-Lite)
   [RFC6333] and/or Network Address and Protocol Translation from IPv6
   Clients to IPv4 Servers (NAT64) [RFC6146].  Operators will want to
   seek deployment models that are conducive to meeting these goals as
   well.

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3.6.  IPv4 Overlap Space

   IPv4 address overlap for CGN translation realms may be required if
   insufficient IPv4 addresses are available within the operator
   environment to assign internally unique IPv4 addresses to the CGN
   subscriber base.  The CGN deployment should provide mechanisms to
   manage IPv4 overlap if required.

3.7.  Transactional Logging for CGN Systems

   CGNs may require transactional logging since the source IP and
   related transport-protocol information are not easily visible to
   external hosts and system.

   If needed, CGN systems should be able to generate logs that identify
   internal-realm host parameters (i.e., IP/Port) and associate them to
   external-realm parameters imposed by the translator.  The logged
   information should be stored on the CGN hardware and/or exported to
   another system for processing.  The operator may choose to also
   enable mechanisms to help reduce logging, such as block allocation of
   UDP and TCP ports or deterministic translation options, e.g.,
   [CGN-DEPLOYMENTS].

   Operators may be legally obligated to keep track of translation
   information.  The operator may need to utilize their standard
   practices in handling sensitive customer data when storing and/or
   transporting such data.  Further information regarding CGN logging
   requirements can be found in Section 4 of [RFC6888].

3.8.  Base CGN Requirements

   Whereas the requirements above represent assessed architectural
   requirements, the CGN platform will also need to meet the base CGN
   requirements of a CGN function.  Base requirements include functions
   such as Bulk Port Allocation and other CGN device-specific functions.
   These base CGN platform requirements are captured in [RFC6888].

4.  BGP/MPLS IP VPN-Based CGN Framework

   The BGP/MPLS IP VPN [RFC4364] framework for CGN segregates the
   internal realms within the service-provider space into Layer 3 MPLS-
   based VPNs.  The operator can deploy a single realm for all CGN-based
   flows or can deploy multiple realms based on translation demand and
   other factors such as geographical proximity.  A realm in this model
   refers to a "VPN", which shares a unique Route Distinguisher / Route
   Target (RD/RT) combination, routing plane, and forwarding behaviors.

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   The BGP/MPLS IP VPN infrastructure provides control-plane and
   forwarding separation for the traditional IPv4 service environment
   and CGN environment(s).  The separation allows for routing
   information (such as default routes) to be propagated separately for
   CGN-based and non-CGN-based subscriber flows.  Traffic can be
   efficiently routed to the Internet for normal flows and routed
   directly to translators for CGN-mediated flows.  Although many
   operators may run a "default-route-free" core, IPv4 flows that
   require translation must obviously be routed first to a translator,
   so a default route is acceptable for the internal realms.

   The physical location of the Virtual Routing and Forwarding (VRF)
   termination point for a BGP/MPLS IP VPN-enabled CGN can vary and be
   located anywhere within the operator's network.  This model fully
   virtualizes the translation service from the base IPv4 forwarding
   environment that will likely be carrying Internet-bound traffic.  The
   base IPv4 environment can continue to service traditional IPv4
   subscriber flows plus post translated CGN flows.

   Figure 1 provides a view of the basic model.  The access node
   provides CPE access to either the CGN VRF or the Global Routing
   Table (GRT), depending on whether the subscriber receives a private
   or public IP.  Translator-mediated traffic follows an MPLS Label
   Switched Path (LSP) that can be set up dynamically and can span one
   hop or many hops (with no need for complex routing policies).
   Traffic is then forwarded to the translator, which can be an external
   appliance or integrated into the VRF Termination (Provider Edge)
   router.  Once traffic is translated, it is forwarded to the GRT for
   general Internet forwarding.  The GRT can also be a separate VRF
   (Internet access VPN/VRF) should the provider choose to implement
   their Internet-based services in that fashion.  The translation
   services are effectively overlaid onto the network but are maintained
   within a separate forwarding and control plane.

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                   Access Node     VRF Termination        CGN
                  +-----------+     +-----------+    +-----------+
                  |           |     |           |    |           |
          CPE     | +-------+ |     | +-------+ |    | +-------+ |
         +----+   | |       | | LSP | |       | | IP | |       | |
         |  --+---+-+->VRF--+-+-----+-+->VRF--+-+----+-+->     | |
         +----+   | |       | |     | |       | |    | |       | |
                  | +-------+ |     | +-------+ |    | |       | |
                  |           |     |           |    | | XLATE | |
                  |           |     |           |    | |       | |
         CPE-CGN  | +-------+ |     | +-------+ |    | |       | |
         +----+   | |       | |     | |       | | IP | |       | |
         |  --+---+-+->GRT  | |     | |  GRT<-+-+----+-+--     | |
         +----+   | |   |   | |     | |   |   | |    | |       | |
                  | +---+---+ |     | +---+---+ |    | +-------+ |
                  +-----+-----+     +-----+-----+    +-----------+
                        |                 |
                        |                 |                IPv4
                        |                 |   IP       +---------+
                        |                 +------------+->       |
                        |                     IP       |    GRT  |
                        +------------------------------+->       |
                                                       +---------+

                 Figure 1: Basic BGP/MPLS IP VPN CGN Model

   If more then one VRF (translation realm) is used within the
   operator's network, each VPN instance can manage CGN flows
   independently for the respective realm.  The described architecture
   does not prescribe a single redundancy model that ensures network
   availability as a result of CGN failure.  Deployments are able to
   select a redundancy model that fits best with their network design.
   If state information needs to be passed or maintained between
   hardware instances, the vendor would need to enable this feature in a
   suitable manner.

4.1.  Service Separation

   The MPLS/VPN CGN framework supports route separation.  The
   traditional IPv4 flows can be separated at the access node (initial
   Layer 3 service point) from those that require translation.  This
   type of service separation is possible on common technologies used
   for Internet access within many operator networks.  Service
   separation can be accomplished on common access technology, including
   those used for DOCSIS (CMTS), Ethernet access, DSL (BRAS), and mobile
   access (GGSN/ASN-GW) architectures.

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4.2.  Internal Service Delivery

   Internal services can be delivered directly to the privately
   addressed endpoint within the CGN domain without translation.  This
   can be accomplished in one of two methods.  The first method is the
   use of "route leaking", a method of exchanging routes between the CGN
   VRF and GRT; this method may also include reducing the overall number
   of VRFs in the system and exposing services in the GRT.  The second
   method, which is described in detail within this section, is the use
   of a Services VRF.  The second model is a more traditional extranet
   services model but requires more system resources to implement.

   Using direct route exchange (import/export) between the CGN VRFs and
   the Services VRFs creates reachability using the aforementioned
   extranet model available in the BGP/MPLS IP VPN structure.  This
   model allows the provider to maintain separate forwarding rules for
   translated flows, which require a pass through the translator to
   reach external network entities, versus those flows that need to
   access internal services.  This operational detail can be
   advantageous for a number of reasons, such as service-access policies
   and endpoint identification.  First, the provider can reduce the load
   on the translator since internal services do not need to be factored
   into the scaling of the CGN hardware (which may be quite large).
   Second, more direct forwarding paths can be maintained to provide
   better network efficiency.  Third, geographic locations of the
   translators and the services infrastructure can be deployed in
   locations in an independent manner.  Additionally, the operator can
   allow CGN subject endpoints to be accessible via an untranslated
   path, reducing the complexities of provider-initiated management
   flows.  This last point is of key interest since NAT removes
   transparency to the end device in normal cases.

   Figure 2 below shows how internal services are provided untranslated
   since flows are sent directly from the access node to the services
   node/VRF via an MPLS LSP.  This traffic is not forwarded to the CGN
   translator and therefore is not subject to problematic behaviors
   related to NAT.  The Services VRF contains routing information that
   can be "imported" into the access node VRF, and the CGN VRF routing
   information can be "imported" into the Services VRF.

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              Access Node     VRF Termination     CGN
            +-------------+    +-----------+  +----------+
            |             |    |           |  |          |
     CPE    | +---------+ |    | +-------+ |  | +------+ |
   +-----+  | |         | |    | |       | |  | |      | |
   |   --+--+-+-> VRF --+-+--+ | |  VRF  | |  | |      | |
   +-----+  | |         | |  | | |       | |  | |      | |
            | +---------+ |  | | +-------+ |  | |      | |
            |             |  | |           |  | |XLATE | |
            |             |  | |           |  | |      | |
   CPE-CGN  | +---------+ |  | | +-------+ |  | |      | |
   +-----+  | |         | |  | | |       | |  | |      | |
   |   --+--+-+-> GRT   | |  | | |  GRT  | |  | |      | |
   +-----+  | |    |    | |  | | |       | |  | |      | |
            | +----+----+ |  | | +-------+ |  | +------+ |
            +------+------+  | +-----------+  +----------+
                   |         |
                   |         |                    IPv4
                   |         |               +-----------+
                   |         +---------------+->Services |
                   |                         |    VRF    |
                   .-------------------------+->   |     |
                                             +-----+-----+
                                                   |
                                             +-----V-----+
                                             |           |
                                             |   Local   |
                                             |  Content  |
                                             +-----------+

                Figure 2: Internal Services and CGN Bypass

   An extension to the services delivery LSP is the ability to also
   provide direct subscriber-to-subscriber traffic flows between CGN
   zones.  Each zone or realm may be fitted with separate CGN resources,
   but the subtending subscribers don't necessarily need to be mediated
   (translated) by the CGN translators.  This option, as shown in
   Figure 3, is easy to implement and can only be enabled if no IPv4
   address overlap is used between communicating CGN zones.

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             Access Node-1     VRF Termination   CGN-1
            +-------------+    +-----------+  +----------+
            |             |    |           |  |          |
     CPE-1  | +---------+ |    | +-------+ |  | +------+ |
   +-----+  | |         | |    | |       | |  | |      | |
   |   --+--+-+-  VRF --+-+-+  | |  VRF  | |  | |      | |
   +-----+  | |         | | |  | |       | |  | |      | |
            | +---------+ | |  | +-------+ |  | |      | |
            |             | |  |           |  | |XLATE | |
            |             | |  |           |  | |      | |
   CPE-2-CGN| +---------+ | |  | +-------+ |  | |      | |
   +-----+  | |         | | |  | |       | |  | |      | |
   |   --+--+-+-  GRT   | | |  | |  GRT  | |  | |      | |
   +-----+  | |         | | |  | |       | |  | |      | |
            | +---------+ | |  | +-------+ |  | +------+ |
            +-------------+ |  +-----------+  +----------+
                            |
                        LSP |
                            |
             Access Node-2  |  VRF Termination   CGN-2
            +-------------+ |  +-----------+  +----------+
            |             | |  |           |  |          |
   CPE-3-CGN| +---------+ | |  | +-------+ |  | +------+ |
   +-----+  | |         | | |  | |       | |  | |      | |
   |   --+--+-+-- VRF --+-+-+  | |  VRF  | |  | |      | |
   +-----+  | |         | |    | |       | |  | |      | |
            | +---------+ |    | +-------+ |  | |      | |
            |             |    |           |  | |XLATE | |
            |             |    |           |  | |      | |
     CPE-4  | +---------+ |    | +-------+ |  | |      | |
   +-----+  | |         | |    | |       | |  | |      | |
   |   --+--+-+-  GRT   | |    | |  GRT  | |  | |      | |
   +-----+  | |         | |    | |       | |  | |      | |
            | +---------+ |    | +-------+ |  | +------+ |
            +-------------+    +-----------+  +----------+

               Figure 3: Subscriber-to-Subscriber CGN Bypass

   The inherent capabilities of the BGP/MPLS IP VPN model demonstrates
   the ability to offer CGN bypass in a standard and deterministic
   manner without the need of policy-based routing or traffic
   engineering.

4.2.1.  Dual-Stack Operation

   The BGP/MPLS IP VPN CGN model can also be used in conjunction with
   IPv4/IPv6 dual-stack service modes.  Since many providers will use
   CGNs on an interim basis while IPv6 matures within the global

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   Internet or due to technical constraints, a dual-stack option is of
   strategic importance.  Operators can offer this dual-stack service
   for both traditional IPv4 (global IP) endpoints and CGN-mediated
   endpoints.

   Operators can separate the IP flows for IPv4 and IPv6 traffic, or
   they can use other routing techniques to move IPv6-based flows toward
   the GRT (Global Routing Table) while allowing IPv4 flows to remain
   within the IPv4 CGN VRF for translator services.

   Figure 4 shows how IPv4 translation services can be provided
   alongside IPv6-based services.  This model allows the provider to
   enable CGN to manage IPv4 flows (translated), and IPv6 flows are
   routed without translation efficiently toward the Internet.  Once
   again, forwarding of flows to the translator does not impact IPv6
   flows, which do not require this service.

             Access Node   VRF Termination        CGN
            +-----------+   +-----------+    +-----------+
            |           |   |           |    |           |
    CPE-CG  | +-------+ |   | +-------+ |    | +-------+ |
   +-----+  | |       | |LSP| |       | | IP | |       | |
   |   --+--+-+->VRF--+-+---+-+->VRF--+-+----+-+>      | |
   |IPv4 |  | |       | |   | |       | |    | |       | |
   |     |  | +-------+ |   | +-------+ |    | |       | |
   +-----|  |           |   |           |    | | XLATE | |
   |IPv6 |  |           |   |           |    | |       | |
   |     |  | +-------+ |   | +-------+ |    | |       | |
   |     |  | |  IPv6 | |   | |  IPv4 | | IP | |       | |
   |   --+--+-+->GRT  | |   | |  GRT<-+-+----+-+--     | |
   +-----+  | |   |   | |   | |   |   | |    | |       | |
            | +---+---+ |   | +---+---+ |    | +-------+ |
            +-----+-----+   +-----+-----+    +-----------+
                  |               |
                  |               |          +-----------+
                  |               |    IP    |    IPv4   |
                  |               +----------+->  GRT    |
                  |                          +-----------+
                  |
                  |
                  |
                  |               IP         +-----------+
                  +--------------------------+->  IPv6   |
                                             |    GRT    |
                                             +-----------+

               Figure 4: CGN with IPv6 Dual-Stack Operation

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4.3.  Deployment Flexibility

   The CGN translator services can be moved, separated or segmented (new
   translation realms) without the need to change the overall
   translation design.  Since dynamic LSPs are used to forward traffic
   from the access nodes to the translation points, the physical
   location of the VRF termination points can vary and be changed
   easily.

   This type of flexibility allows the service provider to initially
   deploy more centralized translation services based on relatively low
   loading factors and distribute the translation points over time to
   improve network-traffic efficiencies and support higher translation
   load.

   Although traffic-engineered paths are not required within the MPLS/
   VPN deployment model, nothing precludes an operator from using
   technologies like MPLS with Traffic Engineering [RFC3031].
   Additional routing mechanisms can be used as desired by the provider
   and can be seen as independent.  There is no specific need to
   diversify the existing infrastructure in most cases.

4.4.  Comparison of BGP/MPLS IP VPN Option versus Other CGN Attachment
      Options

   Other integration architecture options exist that can attach CGN
   based service flows to a translator instance.  Alternate options that
   can be used to attach such services include:

   -  policy-based routing (static) to direct translation-bound traffic
      to a network-based translator;

   -  traffic engineering; and

   -  multiple routing topologies.

4.4.1.  Policy-Based Routing

   Policy-based routing (PBR) provides another option to direct CGN-
   mediated flows to a translator.  PBR options, although possible, are
   difficult to maintain (due to static policy) and must be configured
   throughout the network with considerable maintenance overhead.

   More centralized deployments may be difficult or too onerous to
   deploy using policy-based routing methods.  Policy-based routing
   would not achieve route separation (unless used with others options)
   and may add complexities to the providers' routing environment.

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4.4.2.  Traffic Engineering

   Traffic engineering can also be used to direct traffic from an access
   node toward a translator.  Traffic engineering, like MPLS-TE, may be
   difficult to set up and maintain.  Traffic engineering provides
   additional benefits if used with MPLS by adding potential for faster
   path re-convergence.  Traffic engineering paths would need to be
   updated and redefined over time as CGN translation points are
   augmented or moved.

4.4.3.  Multiple Routing Topologies

   Multiple routing topologies can be used to direct CGN-based flows to
   translators.  This option would achieve the same basic goal as the
   MPLS/VPN option but with additional implementation overhead and
   platform configuration complexity.  Since operator based translation
   is expected to have an unknown lifecycle, and may see various degrees
   of demand (dependent on operator IPv4 Global space availability and
   shift of traffic to IPv6), it may be too large of an undertaking for
   the provider to enabled this as their primary option for CGN.

4.5.  Multicast Considerations

   When deploying BGP/MPLS IP VPNs as a service method for user-plane
   traffic to access CGN, one needs to be cognizant of current or future
   IP multicast requirements.  User-plane IP Multicast that may
   originate outside of the VRF requires specific consideration.  Adding
   the requirement for user plane IP multicast can potentially cause
   additional complexity related to importing and exporting the IP
   multicast routes in addition to suboptimal scaling and bandwidth
   utilization.

   It is recommended to reference best practice and designs from
   [RFC6037], [RFC6513], and [RFC5332].

5.  Experiences

5.1.  Basic Integration and Requirements Support

   The MPLS/VPN CGN environment has been successfully integrated into
   real network environments utilizing existing network service delivery
   mechanisms.  It solves many issues related to provider-based
   translation environments while still subject to problematic behaviors
   inherent within NAT.

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   The key issues that are solved or managed with the MPLS/VPN option
   include:

   -  Support for the centralized and distributed deployment model

   -  Routing plane separation for CGN flows versus traditional IPv4
      flows

   -  Flexible translation point design (can relocate translators and
      split translation zones easily)

   -  Low maintenance overhead (dynamic routing environment with little
      maintenance of separate routing infrastructure other than
      management of MPLS/VPNs)

   -  CGN bypass options (for internal and third-party services that
      exist within the provider domain)

   -  IPv4 translation realm overlap support (can reuse IP addresses
      between zones with some impact to extranet service model)

   -  Simple failover techniques can be implemented with redundant
      translators, such as using a second default route

5.2.  Performance

   The MPLS/VPN CGN model was observed to support basic functions that
   are typically used by subscribers within an operator environment.  A
   full review of the observed impacts related to CGN (NAT444) are
   covered in [RFC7021].

6.  Security Considerations

   An operator implementing CGN using BGP/MPLS IP VPNs should refer to
   Section 7 of [RFC6888] for security considerations related to CGN
   deployments.  The operator should continue to employ the standard
   security methods in place for their standard MPLS deployment and can
   also refer to the Security Considerations section in [RFC4364], which
   discusses both control-plane and data-plane security.

7.  BGP/MPLS IP VPN CGN Framework Discussion

   The MPLS/VPN delivery method for a CGN deployment is an effective and
   scalable way to deliver mass translation services.  The architecture
   avoids the complex requirements of traffic engineering and policy-
   based routing when combining these new service flows to existing IPv4

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   operation.  This is advantageous since the NAT444/CGN environments
   should be introduced with as little impact as possible, and these
   environments are expected to change over time.

   The MPLS/VPN-based CGN architecture solves many of the issues related
   to deploying this technology in existing operator networks.

8.  Acknowledgements

   Thanks to the following people for their comments and feedback: Dan
   Wing, Chris Metz, Chris Donley, Tina TSOU, Christopher Liljenstolpe,
   and Tom Taylor.

   Thanks to the following people for their participation in integrating
   and testing the CGN environment and for their guidance in regard to
   IPv6 transition: Syd Alam, Richard Lawson, John E. Spence, John Jason
   Brzozowski, Chris Donley, Jason Weil, Lee Howard, and Jean-Francois
   Tremblay.

9.  References

9.1.  Normative Reference

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.

9.2.  Informative References

   [CGN-DEPLOYMENTS]
              Donley, C., Grundemann, C., Sarawat, V., Sundaresan, K.,
              and O. Vautrin, "Deterministic Address Mapping to Reduce
              Logging in Carrier Grade NAT Deployments", Work in
              Progress, January 2014.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets", BCP
              5, RFC 1918, February 1996.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

   [RFC5332]  Eckert, T., Rosen, E., Aggarwal, R., and Y. Rekhter, "MPLS
              Multicast Encapsulations", RFC 5332, August 2008.

   [RFC6037]  Rosen, E., Cai, Y., and IJ. Wijnands, "Cisco Systems'
              Solution for Multicast in BGP/MPLS IP VPNs", RFC 6037,
              October 2010.

Top      ToC       Page 20 
   [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.

   [RFC6264]  Jiang, S., Guo, D., and B. Carpenter, "An Incremental
              Carrier-Grade NAT (CGN) for IPv6 Transition", RFC 6264,
              June 2011.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

   [RFC6513]  Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
              VPNs", RFC 6513, February 2012.

   [RFC6598]  Weil, J., Kuarsingh, V., Donley, C., Liljenstolpe, C., and
              M. Azinger, "IANA-Reserved IPv4 Prefix for Shared Address
              Space", BCP 153, RFC 6598, April 2012.

   [RFC6888]  Perreault, S., Yamagata, I., Miyakawa, S., Nakagawa, A.,
              and H. Ashida, "Common Requirements for Carrier-Grade NATs
              (CGNs)", BCP 127, RFC 6888, April 2013.

   [RFC7021]  Donley, C., Howard, L., Kuarsingh, V., Berg, J., and J.
              Doshi, "Assessing the Impact of Carrier-Grade NAT on
              Network Applications", RFC 7021, September 2013.

Authors' Addresses

   Victor Kuarsingh (editor)
   Rogers Communications
   8200 Dixie Road
   Brampton, Ontario  L6T 0C1
   Canada

   EMail: victor@jvknet.com
   URI:   http://www.rogers.com


   John Cianfarani
   Rogers Communications
   8200 Dixie Road
   Brampton, Ontario  L6T 0C1
   Canada

   EMail: john.cianfarani@rci.rogers.com
   URI:   http://www.rogers.com