RFC 8049
YANG Data Model for L3VPN Service Delivery
Internet Engineering Task Force (IETF) S. Litkowski Request for Comments: 8049 Orange Business Services Category: Standards Track L. Tomotaki ISSN: 2070-1721 Verizon K. Ogaki KDDI Corporation February 2017 YANG Data Model for L3VPN Service DeliveryAbstract
This document defines a YANG data model that can be used for communication between customers and network operators and to deliver a Layer 3 provider-provisioned VPN service. This document is limited to BGP PE-based VPNs as described in RFCs 4026, 4110, and 4364. This model is intended to be instantiated at the management system to deliver the overall service. It is not a configuration model to be used directly on network elements. This model provides an abstracted view of the Layer 3 IP VPN service configuration components. It will be up to the management system to take this model as input and use specific configuration models to configure the different network elements to deliver the service. How the configuration of network elements is done is out of scope for this document. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. 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/rfc8049.
Copyright Notice Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................4 1.1. Terminology ................................................4 1.2. Requirements Language ......................................5 1.3. Tree Diagrams ..............................................5 2. Acronyms ........................................................5 3. Definitions .....................................................7 4. Layer 3 IP VPN Service Model ....................................8 5. Service Data Model Usage ........................................9 6. Design of the Data Model .......................................10 6.1. Features and Augmentation .................................18 6.2. VPN Service Overview ......................................18 6.2.1. VPN Service Topology ...............................18 6.2.1.1. Route Target Allocation ...................19 6.2.1.2. Any-to-Any ................................20 6.2.1.3. Hub and Spoke .............................20 6.2.1.4. Hub and Spoke Disjoint ....................21 6.2.2. Cloud Access .......................................22 6.2.3. Multicast Service ..................................24 6.2.4. Extranet VPNs ......................................26 6.3. Site Overview .............................................27 6.3.1. Devices and Locations ..............................29 6.3.2. Site Network Accesses ..............................30 6.3.2.1. Bearer ....................................30 6.3.2.2. Connection ................................31 6.3.2.3. Inheritance of Parameters Defined at Site Level and Site Network Access Level ..32 6.4. Site Role .................................................32
6.5. Site Belonging to Multiple VPNs ...........................33
6.5.1. Site VPN Flavor ....................................33
6.5.1.1. Single VPN Attachment:
site-vpn-flavor-single ....................33
6.5.1.2. MultiVPN Attachment:
site-vpn-flavor-multi .....................33
6.5.1.3. SubVPN Attachment: site-vpn-flavor-sub ....34
6.5.1.4. NNI: site-vpn-flavor-nni ..................36
6.5.2. Attaching a Site to a VPN ..........................37
6.5.2.1. Referencing a VPN .........................37
6.5.2.2. VPN Policy ................................38
6.6. Deciding Where to Connect the Site ........................40
6.6.1. Constraint: Device .................................41
6.6.2. Constraint/Parameter: Site Location ................41
6.6.3. Constraint/Parameter: Access Type ..................42
6.6.4. Constraint: Access Diversity .......................43
6.6.5. Infeasible Access Placement ........................49
6.6.6. Examples of Access Placement .......................50
6.6.6.1. Multihoming ...............................50
6.6.6.2. Site Offload ..............................53
6.6.6.3. Parallel Links ............................59
6.6.6.4. SubVPN with Multihoming ...................60
6.6.7. Route Distinguisher and VRF Allocation .............64
6.7. Site Network Access Availability ..........................64
6.8. Traffic Protection ........................................66
6.9. Security ..................................................66
6.9.1. Authentication .....................................67
6.9.2. Encryption .........................................67
6.10. Management ...............................................68
6.11. Routing Protocols ........................................68
6.11.1. Handling of Dual Stack ............................69
6.11.2. LAN Directly Connected to SP Network ..............70
6.11.3. LAN Directly Connected to SP Network with
Redundancy ........................................70
6.11.4. Static Routing ....................................70
6.11.5. RIP Routing .......................................71
6.11.6. OSPF Routing ......................................71
6.11.7. BGP Routing .......................................73
6.12. Service ..................................................75
6.12.1. Bandwidth .........................................75
6.12.2. QoS ...............................................75
6.12.2.1. QoS Classification .......................75
6.12.2.2. QoS Profile ..............................78
6.12.3. Multicast .........................................81
6.13. Enhanced VPN Features ....................................82
6.13.1. Carriers' Carriers ................................82
6.14. External ID References ...................................83
6.15. Defining NNIs ............................................83
6.15.1. Defining an NNI with the Option A Flavor ..........85
6.15.2. Defining an NNI with the Option B Flavor ..........88
6.15.3. Defining an NNI with the Option C Flavor ..........91
7. Service Model Usage Example ....................................92
8. Interaction with Other YANG Modules ............................98
9. YANG Module ...................................................102
10. Security Considerations ......................................154
11. IANA Considerations ..........................................155
12. References ...................................................155
12.1. Normative References ....................................155
12.2. Informative References ..................................157
Acknowledgements .................................................157
Contributors .....................................................157
Authors' Addresses ...............................................157
1. Introduction
This document defines a Layer 3 VPN service data model written in
YANG. The model defines service configuration elements that can be
used in communication protocols between customers and network
operators. Those elements can also be used as input to automated
control and configuration applications.
1.1. Terminology
The following terms are defined in [RFC6241] and are not redefined
here:
o client
o configuration data
o server
o state data
The following terms are defined in [RFC7950] and are not redefined
here:
o augment
o data model
o data node
The terminology for describing YANG data models is found in [RFC7950]. This document presents some configuration examples using XML representation.1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].1.3. Tree Diagrams
A simplified graphical representation of the data model is presented in Section 6. The meanings of the symbols in these diagrams are as follows: o Brackets "[" and "]" enclose list keys. o Curly braces "{" and "}" contain names of optional features that make the corresponding node conditional. o Abbreviations before data node names: "rw" means configuration data (read-write), and "ro" means state data (read-only). o Symbols after data node names: "?" means an optional node, and "*" denotes a "list" or "leaf-list". o Parentheses enclose choice and case nodes, and case nodes are also marked with a colon (":"). o Ellipsis ("...") stands for contents of subtrees that are not shown.2. Acronyms
AAA: Authentication, Authorization, and Accounting. ACL: Access Control List. ADSL: Asymmetric DSL. AH: Authentication Header. AS: Autonomous System.
ASBR: Autonomous System Border Router. ASM: Any-Source Multicast. BAS: Broadband Access Switch. BFD: Bidirectional Forwarding Detection. BGP: Border Gateway Protocol. BSR: Bootstrap Router. CE: Customer Edge. CLI: Command Line Interface. CsC: Carriers' Carriers. CSP: Cloud Service Provider. DHCP: Dynamic Host Configuration Protocol. DSLAM: Digital Subscriber Line Access Multiplexer. ESP: Encapsulating Security Payload. GRE: Generic Routing Encapsulation. IGMP: Internet Group Management Protocol. LAN: Local Area Network. MLD: Multicast Listener Discovery. MTU: Maximum Transmission Unit. NAT: Network Address Translation. NETCONF: Network Configuration Protocol. NNI: Network-to-Network Interface. OAM: Operations, Administration, and Maintenance. OSPF: Open Shortest Path First. OSS: Operations Support System.
PE: Provider Edge. PIM: Protocol Independent Multicast. POP: Point of Presence. QoS: Quality of Service. RD: Route Distinguisher. RIP: Routing Information Protocol. RP: Rendezvous Point. RT: Route Target. SFTP: Secure FTP. SLA: Service Level Agreement. SLAAC: Stateless Address Autoconfiguration. SP: Service Provider. SPT: Shortest Path Tree. SSM: Source-Specific Multicast. VM: Virtual Machine. VPN: Virtual Private Network. VRF: VPN Routing and Forwarding. VRRP: Virtual Router Redundancy Protocol.3. Definitions
Customer Edge (CE) Device: A CE is equipment dedicated to a particular customer; it is directly connected (at Layer 3) to one or more PE devices via attachment circuits. A CE is usually located at the customer premises and is usually dedicated to a single VPN, although it may support multiple VPNs if each one has separate attachment circuits.
Provider Edge (PE) Device: A PE is equipment managed by the SP; it can support multiple VPNs for different customers and is directly connected (at Layer 3) to one or more CE devices via attachment circuits. A PE is usually located at an SP point of presence (POP) and is managed by the SP. PE-Based VPNs: The PE devices know that certain traffic is VPN traffic. They forward the traffic (through tunnels) based on the destination IP address of the packet and, optionally, based on other information in the IP header of the packet. The PE devices are themselves the tunnel endpoints. The tunnels may make use of various encapsulations to send traffic over the SP network (such as, but not restricted to, GRE, IP-in-IP, IPsec, or MPLS tunnels).4. Layer 3 IP VPN Service Model
A Layer 3 IP VPN service is a collection of sites that are authorized to exchange traffic between each other over a shared IP infrastructure. This Layer 3 VPN service model aims at providing a common understanding of how the corresponding IP VPN service is to be deployed over the shared infrastructure. This service model is limited to BGP PE-based VPNs as described in [RFC4026], [RFC4110], and [RFC4364].
5. Service Data Model Usage
l3vpn-svc | Model | | +------------------+ +-----+ | Orchestration | < --- > | OSS | +------------------+ +-----+ | | +----------------+ | | Config manager | | +----------------+ | | | | NETCONF/CLI ... | | +------------------------------------------------+ Network +++++++ + AAA + +++++++ ++++++++ Bearer ++++++++ ++++++++ ++++++++ + CE A + ----------- + PE A + + PE B + ---- + CE B + ++++++++ Connection ++++++++ ++++++++ ++++++++ Site A Site B The idea of the L3 IP VPN service model is to propose an abstracted interface between customers and network operators to manage configuration of components of an L3VPN service. A typical scenario would be to use this model as an input for an orchestration layer that will be responsible for translating it to an orchestrated configuration of network elements that will be part of the service. The network elements can be routers but can also be servers (like AAA); the network's configuration is not limited to these examples. The configuration of network elements can be done via the CLI, NETCONF/RESTCONF [RFC6241] [RFC8040] coupled with YANG data models of a specific configuration (BGP, VRF, BFD, etc.), or some other technique, as preferred by the operator. The usage of this service model is not limited to this example; it can be used by any component of the management system but not directly by network elements.
6. Design of the Data Model
The YANG module is divided into two main containers: "vpn-services" and "sites". The "vpn-service" list under the vpn-services container defines global parameters for the VPN service for a specific customer. A "site" is composed of at least one "site-network-access" and, in the case of multihoming, may have multiple site-network-access points. The site-network-access attachment is done through a "bearer" with an "ip-connection" on top. The bearer refers to properties of the attachment that are below Layer 3, while the connection refers to properties oriented to the Layer 3 protocol. The bearer may be allocated dynamically by the SP, and the customer may provide some constraints or parameters to drive the placement of the access. Authorization of traffic exchange is done through what we call a VPN policy or VPN service topology defining routing exchange rules between sites. The figure below describes the overall structure of the YANG module: module: ietf-l3vpn-svc +--rw l3vpn-svc +--rw vpn-services | +--rw vpn-service* [vpn-id] | +--rw vpn-id svc-id | +--rw customer-name? string | +--rw vpn-service-topology? identityref | +--rw cloud-accesses {cloud-access}? | | +--rw cloud-access* [cloud-identifier] | | +--rw cloud-identifier string | | +--rw (list-flavor)? | | | +--:(permit-any) | | | | +--rw permit-any? empty | | | +--:(deny-any-except) | | | | +--rw permit-site* leafref | | | +--:(permit-any-except) | | | +--rw deny-site* leafref | | +--rw authorized-sites | | | +--rw authorized-site* [site-id] | | | +--rw site-id leafref | | +--rw denied-sites | | | +--rw denied-site* [site-id] | | | +--rw site-id leafref | | +--rw address-translation
| | +--rw nat44
| | +--rw enabled? boolean
| | +--rw nat44-customer-address? inet:ipv4-address
| +--rw multicast {multicast}?
| | +--rw enabled? boolean
| | +--rw customer-tree-flavors
| | | +--rw tree-flavor* identityref
| | +--rw rp
| | +--rw rp-group-mappings
| | | +--rw rp-group-mapping* [id]
| | | +--rw id uint16
| | | +--rw provider-managed
| | | | +--rw enabled? boolean
| | | | +--rw rp-redundancy? boolean
| | | | +--rw optimal-traffic-delivery? boolean
| | | +--rw rp-address? inet:ip-address
| | | +--rw groups
| | | +--rw group* [id]
| | | +--rw id uint16
| | | +--rw (group-format)?
| | | +--:(startend)
| | | | +--rw group-start? inet:ip-address
| | | | +--rw group-end? inet:ip-address
| | | +--:(singleaddress)
| | | +--rw group-address? inet:ip-address
| | +--rw rp-discovery
| | +--rw rp-discovery-type? identityref
| | +--rw bsr-candidates
| | +--rw bsr-candidate-address* inet:ip-address
| +--rw carrierscarrier? boolean {carrierscarrier}?
| +--rw extranet-vpns {extranet-vpn}?
| +--rw extranet-vpn* [vpn-id]
| +--rw vpn-id svc-id
| +--rw local-sites-role? identityref
+--rw sites
+--rw site* [site-id]
+--rw site-id svc-id
+--rw requested-site-start? yang:date-and-time
+--rw requested-site-stop? yang:date-and-time
+--rw locations
| +--rw location* [location-id]
| +--rw location-id svc-id
| +--rw address? string
| +--rw postal-code? string
| +--rw state? string
| +--rw city? string
| +--rw country-code? string
+--rw devices
| +--rw device* [device-id]
| +--rw device-id svc-id
| +--rw location? leafref
| +--rw management
| +--rw address-family? address-family
| +--rw address? inet:ip-address
+--rw site-diversity {site-diversity}?
| +--rw groups
| +--rw group* [group-id]
| +--rw group-id string
+--rw management
| +--rw type? identityref
+--rw vpn-policies
| +--rw vpn-policy* [vpn-policy-id]
| +--rw vpn-policy-id svc-id
| +--rw entries* [id]
| +--rw id svc-id
| +--rw filter
| | +--rw (lan)?
| | +--:(prefixes)
| | | +--rw ipv4-lan-prefix* inet:ipv4-prefix {ipv4}?
| | | +--rw ipv6-lan-prefix* inet:ipv6-prefix {ipv6}?
| | +--:(lan-tag)
| | +--rw lan-tag* string
| +--rw vpn
| +--rw vpn-id leafref
| +--rw site-role? identityref
+--rw site-vpn-flavor? identityref
+--rw maximum-routes
| +--rw address-family* [af]
| +--rw af address-family
| +--rw maximum-routes? uint32
+--rw security
| +--rw authentication
| +--rw encryption {encryption}?
| +--rw enabled? boolean
| +--rw layer enumeration
| +--rw encryption-profile
| +--rw (profile)?
| +--:(provider-profile)
| | +--rw profile-name? string
| +--:(customer-profile)
| +--rw algorithm? string
| +--rw (key-type)?
| +--:(psk)
| | +--rw preshared-key? string
| +--:(pki)
+--rw service
| +--rw qos {qos}?
| | +--rw qos-classification-policy
| | | +--rw rule* [id]
| | | +--rw id uint16
| | | +--rw (match-type)?
| | | | +--:(match-flow)
| | | | | +--rw match-flow
| | | | | +--rw dscp? inet:dscp
| | | | | +--rw dot1p? uint8
| | | | | +--rw ipv4-src-prefix? inet:ipv4-prefix
| | | | | +--rw ipv6-src-prefix? inet:ipv6-prefix
| | | | | +--rw ipv4-dst-prefix? inet:ipv4-prefix
| | | | | +--rw ipv6-dst-prefix? inet:ipv6-prefix
| | | | | +--rw l4-src-port? inet:port-number
| | | | | +--rw target-sites* svc-id
| | | | | +--rw l4-src-port-range
| | | | | | +--rw lower-port? inet:port-number
| | | | | | +--rw upper-port? inet:port-number
| | | | | +--rw l4-dst-port? inet:port-number
| | | | | +--rw l4-dst-port-range
| | | | | | +--rw lower-port? inet:port-number
| | | | | | +--rw upper-port? inet:port-number
| | | | | +--rw protocol-field? union
| | | | +--:(match-application)
| | | | +--rw match-application? identityref
| | | +--rw target-class-id? string
| | +--rw qos-profile
| | +--rw (qos-profile)?
| | +--:(standard)
| | | +--rw profile? string
| | +--:(custom)
| | +--rw classes {qos-custom}?
| | +--rw class* [class-id]
| | +--rw class-id string
| | +--rw rate-limit? uint8
| | +--rw latency
| | | +--rw (flavor)?
| | | ...
| | +--rw jitter
| | | +--rw (flavor)?
| | | ...
| | +--rw bandwidth
| | +--rw guaranteed-bw-percent? uint8
| | +--rw end-to-end? empty
| +--rw carrierscarrier {carrierscarrier}?
| | +--rw signalling-type? enumeration
| +--rw multicast {multicast}?
| +--rw multicast-site-type? enumeration
| +--rw multicast-address-family
| | +--rw ipv4? boolean {ipv4}?
| | +--rw ipv6? boolean {ipv6}?
| +--rw protocol-type? enumeration
+--rw traffic-protection {fast-reroute}?
| +--rw enabled? boolean
+--rw routing-protocols
| +--rw routing-protocol* [type]
| +--rw type identityref
| +--rw ospf {rtg-ospf}?
| | +--rw address-family* address-family
| | +--rw area-address? yang:dotted-quad
| | +--rw metric? uint16
| | +--rw sham-links {rtg-ospf-sham-link}?
| | +--rw sham-link* [target-site]
| | +--rw target-site svc-id
| | +--rw metric? uint16
| +--rw bgp {rtg-bgp}?
| | +--rw autonomous-system? uint32
| | +--rw address-family* address-family
| +--rw static
| | +--rw cascaded-lan-prefixes
| | +--rw ipv4-lan-prefixes* [lan next-hop] {ipv4}?
| | | +--rw lan inet:ipv4-prefix
| | | +--rw lan-tag? string
| | | +--rw next-hop inet:ipv4-address
| | +--rw ipv6-lan-prefixes* [lan next-hop] {ipv6}?
| | +--rw lan inet:ipv6-prefix
| | +--rw lan-tag? string
| | +--rw next-hop inet:ipv6-address
| +--rw rip {rtg-rip}?
| | +--rw address-family* address-family
| +--rw vrrp {rtg-vrrp}?
| +--rw address-family* address-family
+--ro actual-site-start? yang:date-and-time
+--ro actual-site-stop? yang:date-and-time
+--rw site-network-accesses
+--rw site-network-access* [site-network-access-id]
+--rw site-network-access-id svc-id
+--rw site-network-access-type? identityref
+--rw (location-flavor)
| +--:(location)
| | +--rw location-reference? leafref
| +--:(device)
| +--rw device-reference? leafref
+--rw access-diversity {site-diversity}?
| +--rw groups
| | +--rw group* [group-id]
| | +--rw group-id string
| +--rw constraints
| +--rw constraint* [constraint-type]
| +--rw constraint-type identityref
| +--rw target
| +--rw (target-flavor)?
| +--:(id)
| | +--rw group* [group-id]
| | ...
| +--:(all-accesses)
| | +--rw all-other-accesses? empty
| +--:(all-groups)
| +--rw all-other-groups? empty
+--rw bearer
| +--rw requested-type {requested-type}?
| | +--rw requested-type? string
| | +--rw strict? boolean
| +--rw always-on? boolean {always-on}?
| +--rw bearer-reference? string {bearer-reference}?
+--rw ip-connection
| +--rw ipv4 {ipv4}?
| | +--rw address-allocation-type? identityref
| | +--rw number-of-dynamic-address? uint8
| | +--rw dhcp-relay
| | | +--rw customer-dhcp-servers
| | | +--rw server-ip-address* inet:ipv4-address
| | +--rw addresses
| | +--rw provider-address? inet:ipv4-address
| | +--rw customer-address? inet:ipv4-address
| | +--rw mask? uint8
| +--rw ipv6 {ipv6}?
| | +--rw address-allocation-type? identityref
| | +--rw number-of-dynamic-address? uint8
| | +--rw dhcp-relay
| | | +--rw customer-dhcp-servers
| | | +--rw server-ip-address* inet:ipv6-address
| | +--rw addresses
| | +--rw provider-address? inet:ipv6-address
| | +--rw customer-address? inet:ipv6-address
| | +--rw mask? uint8
| +--rw oam
| +--rw bfd {bfd}?
| +--rw enabled? boolean
| +--rw (holdtime)?
| +--:(profile)
| | +--rw profile-name? string
| +--:(fixed)
| +--rw fixed-value? uint32
+--rw security
| +--rw authentication
| +--rw encryption {encryption}?
| +--rw enabled? boolean
| +--rw layer enumeration
| +--rw encryption-profile
| +--rw (profile)?
| +--:(provider-profile)
| | +--rw profile-name? string
| +--:(customer-profile)
| +--rw algorithm? string
| +--rw (key-type)?
| +--:(psk)
| | ...
| +--:(pki)
+--rw service
| +--rw svc-input-bandwidth? uint32
| +--rw svc-output-bandwidth? uint32
| +--rw svc-mtu? uint16
| +--rw qos {qos}?
| | +--rw qos-classification-policy
| | | +--rw rule* [id]
| | | +--rw id uint16
| | | +--rw (match-type)?
| | | | +--:(match-flow)
| | | | | +--rw match-flow
| | | | | ...
| | | | +--:(match-application)
| | | | +--rw match-application? identityref
| | | +--rw target-class-id? string
| | +--rw qos-profile
| | +--rw (qos-profile)?
| | +--:(standard)
| | | +--rw profile? string
| | +--:(custom)
| | +--rw classes {qos-custom}?
| | +--rw class* [class-id]
| | ...
| +--rw carrierscarrier {carrierscarrier}?
| | +--rw signalling-type? enumeration
| +--rw multicast {multicast}?
| +--rw multicast-site-type? enumeration
| +--rw multicast-address-family
| | +--rw ipv4? boolean {ipv4}?
| | +--rw ipv6? boolean {ipv6}?
| +--rw protocol-type? enumeration
+--rw routing-protocols
| +--rw routing-protocol* [type]
| +--rw type identityref
| +--rw ospf {rtg-ospf}?
| | +--rw address-family* address-family
| | +--rw area-address? yang:dotted-quad
| | +--rw metric? uint16
| | +--rw sham-links {rtg-ospf-sham-link}?
| | +--rw sham-link* [target-site]
| | +--rw target-site svc-id
| | +--rw metric? uint16
| +--rw bgp {rtg-bgp}?
| | +--rw autonomous-system? uint32
| | +--rw address-family* address-family
| +--rw static
| | +--rw cascaded-lan-prefixes
| | +--rw ipv4-lan-prefixes* [lan next-hop] {ipv4}?
| | | +--rw lan inet:ipv4-prefix
| | | +--rw lan-tag? string
| | | +--rw next-hop inet:ipv4-address
| | +--rw ipv6-lan-prefixes* [lan next-hop] {ipv6}?
| | +--rw lan inet:ipv6-prefix
| | +--rw lan-tag? string
| | +--rw next-hop inet:ipv6-address
| +--rw rip {rtg-rip}?
| | +--rw address-family* address-family
| +--rw vrrp {rtg-vrrp}?
| +--rw address-family* address-family
+--rw availability
| +--rw access-priority? uint32
+--rw vpn-attachment
+--rw (attachment-flavor)
+--:(vpn-policy-id)
| +--rw vpn-policy-id? leafref
+--:(vpn-id)
+--rw vpn-id? leafref
+--rw site-role? identityref
6.1. Features and Augmentation
The model defined in this document implements many features that allow implementations to be modular. As an example, an implementation may support only IPv4 VPNs (IPv4 feature), IPv6 VPNs (IPv6 feature), or both (by advertising both features). The routing protocols proposed to the customer may also be enabled through features. This model also proposes some features for options that are more advanced, such as support for extranet VPNs (Section 6.2.4), site diversity (Section 6.6), and QoS (Section 6.12.2). In addition, as for any YANG model, this service model can be augmented to implement new behaviors or specific features. For example, this model proposes different options for IP address assignments; if those options do not fulfill all requirements, new options can be added through augmentation.
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