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Versions: (draft-kuarsingh-lsn-deployment) 00 01 02 03 04 05 06

OPSAWG                                                 V. Kuarsingh, Ed.
Internet-Draft                                             J. Cianfarani
Intended status: Informational                     Rogers Communications
Expires: October 15, 2014                                 April 13, 2014


                  CGN Deployment with BGP/MPLS IP VPNs
                  draft-ietf-opsawg-lsn-deployment-06

Abstract

   This document specifies a framework to integrate a Network Address
   Translation 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 which 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 CGNs
   impact on the network.  This document does not intend to defend the
   merits of CGN.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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

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

   This Internet-Draft will expire on October 15, 2014.






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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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terms . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Existing Network Considerations . . . . . . . . . . . . . . .   4
   3.  CGN Network Deployment Requirements . . . . . . . . . . . . .   4
     3.1.  Centralized versus Distributed Deployment . . . . . . . .   5
     3.2.  CGN and Traditional IPv4 Service Co-existence . . . . . .   6
     3.3.  CGN By-Pass . . . . . . . . . . . . . . . . . . . . . . .   6
     3.4.  Routing Plane Separation  . . . . . . . . . . . . . . . .   7
     3.5.  Flexible Deployment Options . . . . . . . . . . . . . . .   7
     3.6.  IPv4 Overlap Space  . . . . . . . . . . . . . . . . . . .   7
     3.7.  Transactional Logging for CGN Systems . . . . . . . . . .   8
     3.8.  Base CGN Requirements . . . . . . . . . . . . . . . . . .   8
   4.  BGP/MPLS IP VPN based CGN Framework . . . . . . . . . . . . .   8
     4.1.  Service Separation  . . . . . . . . . . . . . . . . . . .  10
     4.2.  Internal Service Delivery . . . . . . . . . . . . . . . .  11
       4.2.1.  Dual Stack Operation  . . . . . . . . . . . . . . . .  13
     4.3.  Deployment Flexibility  . . . . . . . . . . . . . . . . .  15
     4.4.  Comparison of BGP/MPLS IP VPN Option versus other CGN
           Attachment Options  . . . . . . . . . . . . . . . . . . .  15
       4.4.1.  Policy Based Routing  . . . . . . . . . . . . . . . .  15
       4.4.2.  Traffic Engineering . . . . . . . . . . . . . . . . .  16
       4.4.3.  Multiple Routing Topologies . . . . . . . . . . . . .  16
     4.5.  Multicast Considerations  . . . . . . . . . . . . . . . .  16
   5.  Experiences . . . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Basic Integration and Requirements Support  . . . . . . .  16
     5.2.  Performance . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   8.  BGP/MPLS IP VPN CGN Framework Discussion  . . . . . . . . . .  17
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18



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     10.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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 local operator.
   CGN deployments will most often be added into operator networks which
   already have active IPv4 and/or IPv6 services.

   The addition of the CGN introduces an operator controlled and
   administered translation layer which should be added in a manner
   which 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.  Terms

   A list of acronyms used throughout this document are defined in 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




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      GPRS - General Packet Radio Service

      ASN-GW - Access Service Network Gateway

      GRT - Global Routing Table

      Internal Realm - Addressing and/or network zone been the 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 which 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 which 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 are
   expanded upon in the following subsections.  It should be noted that
   architectural CGN requirements add additive to base CGN functional
   requirements in [RFC6888].  The assessed architectural requirements
   for deployment are:

      - Support distributed (sparse) and centralized (dense) deployment
      models;

      - Allow co-existence with traditional IPv4 based deployments,
      which provide global scoped IPv4 addresses to CPEs;





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      - Provide a framework for CGN by-pass supporting non-translated
      flows between endpoints within a provider's network;

      - Provide a routing framework which allows the segmentation of
      routing control and forwarding paths between CGN and non-CGN
      mediated flows;

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

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

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

      - Allow for evolution to future dual-stack and IPv4/IPv6
      transition deployment modes;

      - Transactional logging and export capabilities to support
      auxiliary functions including abuse mitigation;

      - Support for stateful connection synchronization between
      translation instances/elements (redundancy);

      - Support for CGN Shared Space [RFC6598] deployment modes if
      applicable;

      - Allows for the enablement of CGN functionality (if required)
      while still minimizing costs and subscriber impact to the best
      extend possible;

   Other requirements may be assessed on a 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



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   legacy IPv4 services will continue to operate with most endpoints
   using globally unique IPv4 addresses.  Exceptional cases which may
   drive heavy usage in initial stages may include operators who already
   translate a significant portion of their IPv4 traffic; may transition
   to a CGN implementation from legacy translation mechanisms (i.e.
   traditional firewalls); or build a green field deployment which may
   see quick growth in the number of new IPv4 endpoints which 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 Co-existence

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

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

3.3.  CGN By-Pass

   The CGN environment is only needed for flows with translation
   requirements.  Many flows which remain within the operator's network,
   do not require translation.  Such services include operator offered
   DNS Services, DHCP Services, NTP Services, Web Caching, E-Mail, News
   and other services which 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
   by-pass 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 which 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 use of other IPv4
   space which 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 peering infrastructures for transit and peering and these
   may span large geographical areas and regions.  A CGN solution should
   offer the operator an 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 caused by 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 DS-Lite [RFC6333] and/or NAT64
   [RFC6146].  Operators will want to seek deployment models which are
   conducive to meeting these goals as well.

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



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   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 is not easily visible to
   external hosts and system.

   If needed, the CGN systems should be able to generate logs which
   identify internal realm host parameters (i.e. IP/Port) and associated
   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
   such as [I-D.donley-behave-deterministic-cgn].

   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 can be found in
   [RFC6888] with respect to CGN logging requirements (Logging section).

3.8.  Base CGN Requirements

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

   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 and non-CGN based subscriber flows.  Traffic can be efficiently



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   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 which 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 which 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, depending on whether the subscriber receives a private or
   public IP.  Translator mediated traffic follows an MPLS Label-
   switched Path (LSP) which can be setup dynamically and can span one
   hop, or many hops (with no need for complex routing policies).
   Traffic is then forwarded to the translator (shown below) which can
   be an external appliance or integrated into the VRF Termination
   (Provider Edge) router.  Once traffic is translated, it is forwarded
   to the global routing table for general Internet forwarding.  The
   Global Routing table 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 which 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 may
   include reducing the overall number of VRFs in the system and
   exposing services in the GRT along with a method of exchanging routes
   between the CGN VRF and GRT called route leaking.  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 reachablity 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 which 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).  Secondly, more direct
   forwarding paths can be maintained providing better network
   efficiency.  Thirdly, 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 behaviours
   related to NAT.  The services VRF contains routing information which
   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 By-Pass

   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 below, 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  | |  | |      | |
          +-----+  | |         | |    | |       | |  | |      | |
                   | +---------+ |    | +-------+ |  | +------+ |
                   +-------------+    +-----------+  +----------+

   The inherent capabilities of the BGP/MPLS IP VPN model demonstrates
   the ability to offer CGN By-Pass 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
   Internet or due to technical constraints, a dual stack option is of
   strategic importance.  Operators can offer this dual stack service



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   for both traditional IPv4 (global IP) endpoints and CGN mediated
   endpoints.

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

   The Figure 4 below shows how IPv4 translation services can be
   provided alongside IPv6 based services.  The model shown allows the
   provider to enable CGN to manage IPv4 flows (translated) and IPv6
   flows are routed without translation efficiently towards 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 which can attach CGN
   based service flows to a translator instance.  Alternate options
   which can be used to attach such services include:

      - Policy Based Routing (Static) to direct translation bound
      traffic to a network based translator;

      - Traffic Engineering or;

      - 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 (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 towards a translator.  Traffic Engineering, like MPLS-TE, may be
   difficult to setup and maintain.  Traffic Engineering provides
   additional benefits if used with MPLS by adding potentials for faster
   path re-convergence.  Traffic Engineering paths would need to be
   updated and redefined overtime 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 (dependant 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 VPN's as an 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 which may
   originate outside of the VRF requires more consideration specific
   consideration.  Adding the requirement for user plane IP multicast
   can potentially cause additional complexity related to import and
   exporting the IP multicast routes in addition to sub optimal 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
   behaviours inherent within NAT.

   Key issues which are solved or managed with the MPLS/VPN option
   include:




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      - Centralized and Distributed Deployment model support

      - 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 then
      management of MPLS/VPNs)

      - CGN By-pass options (for internal and third party services which
      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 which
   are typically used by subscribes within an operator environment.  A
   full review of the observed impacts related to CGN (NAT444) are
   covered in [RFC7021].

6.  IANA Considerations

   This document has no IANA actions.

7.  Security Considerations

   An operator implementing CGN using BGP/MPLS IP VPNs should refer to
   [RFC6888] section 7 for security considerations related to CGN
   deployments.  The operator should continue to employ 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.

8.  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
   operation.  This is advantageous since the NAT44/CGN environments



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   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 this issues
   related to deploying this technology in existing operator networks.

9.  Acknowledgements

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

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

10.  References

10.1.  Normative References

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

10.2.  Informative References

   [I-D.donley-behave-deterministic-cgn]
              Donley, C., Grundemann, C., Sarawat, V., Sundaresan, K.,
              and O. Vautrin, "Deterministic Address Mapping to Reduce
              Logging in Carrier Grade NAT Deployments", draft-donley-
              behave-deterministic-cgn-07 (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.

   [RFC5969]  Townsley, W. and O. Troan, "IPv6 Rapid Deployment on IPv4
              Infrastructures (6rd) -- Protocol Specification", RFC
              5969, August 2010.




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

   [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










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

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











































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