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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


 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

Kuarsingh & Cianfarani Informational [Page 1] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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
 ( 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.

Kuarsingh & Cianfarani Informational [Page 2] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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

Kuarsingh & Cianfarani Informational [Page 3] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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
    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

Kuarsingh & Cianfarani Informational [Page 4] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

    GRT - Global Routing Table
    Internal Realm - Addressing and/or network zone between the
    Customer Premises Equipment (CPE) and CGN as specified in
    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

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:
  1. Support distributed (sparse) and centralized (dense) deployment

models. See Section 3.1

  1. Allow coexistence with traditional IPv4-based deployments, which

provide global-scoped IPv4 addresses to CPEs. See Section 3.2.

  1. Provide a framework for CGN bypass supporting non-translated flows

between endpoints within a provider's network. See Section 3.3.

Kuarsingh & Cianfarani Informational [Page 5] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

  1. 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.
  1. 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.
  1. 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.
  1. Support deployment modes that allow for IPv4 address overlap

within the operator's network (between various translation realms

    or zones).  See Section 3.6.
  1. Allow for evolution to future dual-stack and IPv4/IPv6 transition

deployment modes. See Section 3.5.

  1. Transactional logging and export capabilities to support auxiliary

functions, including abuse mitigation. See Section 3.7.

  1. Support for stateful connection synchronization between

translation instances/elements (redundancy). See Section 3.8.

  1. Support for CGN Shared Address Space [RFC6598] deployment modes if

applicable. See Section 3.6.

  1. 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

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

Kuarsingh & Cianfarani Informational [Page 6] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 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
 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.

Kuarsingh & Cianfarani Informational [Page 7] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 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

Kuarsingh & Cianfarani Informational [Page 8] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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.,
 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.

Kuarsingh & Cianfarani Informational [Page 9] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 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.

Kuarsingh & Cianfarani Informational [Page 10] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

                 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.

Kuarsingh & Cianfarani Informational [Page 11] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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.

Kuarsingh & Cianfarani Informational [Page 12] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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

Kuarsingh & Cianfarani Informational [Page 13] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

           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

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

Kuarsingh & Cianfarani Informational [Page 14] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 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
 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

Kuarsingh & Cianfarani Informational [Page 15] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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
 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
 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

 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:
  1. policy-based routing (static) to direct translation-bound traffic

to a network-based translator;

  1. traffic engineering; and
  1. 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.

Kuarsingh & Cianfarani Informational [Page 16] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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
 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.

Kuarsingh & Cianfarani Informational [Page 17] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 The key issues that are solved or managed with the MPLS/VPN option
  1. Support for the centralized and distributed deployment model
  1. Routing plane separation for CGN flows versus traditional IPv4


  1. Flexible translation point design (can relocate translators and

split translation zones easily)

  1. Low maintenance overhead (dynamic routing environment with little

maintenance of separate routing infrastructure other than

    management of MPLS/VPNs)
  1. CGN bypass options (for internal and third-party services that

exist within the provider domain)

  1. IPv4 translation realm overlap support (can reuse IP addresses

between zones with some impact to extranet service model)

  1. 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

Kuarsingh & Cianfarani Informational [Page 18] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

 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

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

            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.

Kuarsingh & Cianfarani Informational [Page 19] RFC 7289 CGN Deployment with BGP/MPLS IP VPNs June 2014

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

Kuarsingh & Cianfarani Informational [Page 20]

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