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.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Acronyms and 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 Coexistence . . . . . . 6
3.3. CGN Bypass . . . . . . . . . . . . . . . . . . . . . . . 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. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. BGP/MPLS IP VPN CGN Framework Discussion . . . . . . . . . . 17
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . 18
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
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.
- 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 3.8. 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
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.
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 peering external routing infrastructures for transit and peering,
and these may span large geographical areas and regions. 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.
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.
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.
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.
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.
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.
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
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
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.
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.
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
operation. This is advantageous since the NAT44/CGN 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 References
[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.
[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