Network Function Virtualization - Learning Module

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Virtual Network Functions: Design and Implementation

Virtual Network Functions: The Software Building Blocks of Modern Networks

Having established the foundational concept of NFV, we now turn our attention to the entities at its heart: Virtual Network Functions (VNFs). If NFV is the architectural paradigm, VNFs are its building blocks—the software implementations of network capabilities that replace traditional hardware appliances.

A VNF is not simply "network software running in a VM." It is a carefully engineered software entity designed to deliver network functionality with carrier-grade performance, reliability, and manageability. The difference between a prototype network application and a production-ready VNF is as significant as the difference between a student project and enterprise software.

This page explores VNFs in depth: their architecture, design principles, lifecycle management, and the characteristics that separate mature VNF implementations from problematic ones.

What You Will Learn

By the end of this page, you will understand the anatomy of Virtual Network Functions, VNF component architecture and design patterns, the distinction between VNFC and VNF, VNF descriptors and packaging standards, lifecycle management from instantiation to termination, and the key characteristics of well-designed VNFs.

Anatomy of a Virtual Network Function

A Virtual Network Function is more than just software—it's a complete package that includes the application logic, configuration, dependencies, and metadata needed for automated deployment and management.

The Formal Definition:

According to ETSI GS NFV 003, a Virtual Network Function (VNF) is defined as:

"A virtualization of a network function in a legacy non-virtualized network. For practical purposes, a VNF is a software implementation of a network function that is capable of running over the NFV Infrastructure (NFVI)."

Key Characteristics of VNFs:

Defining Characteristics of VNFs

•Software-Implemented: The network function is delivered entirely as software, with no dependency on specialized hardware (though it may leverage hardware acceleration when available).
•Infrastructure-Agnostic: VNFs are designed to run on standard NFV Infrastructure without requiring specific hardware configurations (within defined constraints).
•Self-Contained: A VNF package includes all necessary components—application binaries, libraries, configuration templates, and operational scripts.
•Manageable via Standard APIs: VNFs expose well-defined interfaces for lifecycle management, configuration, and monitoring.
•Composable: VNFs can be chained together to create complex network services, each providing a specific function.
•Scalable: VNFs can be instantiated multiple times and scaled horizontally or vertically to meet demand.

VNF vs. VNFC:

A critical distinction in the ETSI framework is between a VNF and its constituent VNF Components (VNFCs):

VNF Component (VNFC):

A single VM (or container) implementing part of a VNF
The smallest unit of a VNF that can be individually deployed
Runs on a single hypervisor or container host
May be replicated for redundancy or scaling

VNF (Virtual Network Function):

May consist of one or more VNFCs
Represents the complete functional unit
VNFCs within a VNF communicate via internal links
The VNF is the unit of lifecycle management

Example: Multi-VNFC Architecture

Consider a virtualized Evolved Packet Core (vEPC) VNF:

vEPC VNF
├── VNFC-1: MME (Mobility Management Entity)
├── VNFC-2: SGW-C (Serving Gateway Control Plane)
├── VNFC-3: SGW-U (Serving Gateway User Plane)
├── VNFC-4: PGW-C (PDN Gateway Control Plane)
├── VNFC-5: PGW-U (PDN Gateway User Plane)
└── VNFC-6: HSS (Home Subscriber Server)

Each VNFC is a separate VM with specific resource requirements and scaling characteristics, but together they form a single VNF representing the complete EPC functionality.

Practical VNFC Deployment

While the multi-VNFC model matches traditional telecom architecture, many modern VNFs consolidate functionality into fewer components to reduce complexity. The optimal VNFC decomposition balances independent scaling needs, fault isolation requirements, and operational simplicity.

VNF Internal Architecture

Within a VNF (or VNFC), the internal architecture determines performance, reliability, and manageability. Well-designed VNF architectures share common patterns that address the unique challenges of network function software.

The Data Plane / Control Plane Split:

Network functions typically separate into two fundamental processing paths:

Data Plane (Fast Path):

Handles actual network traffic (packets, flows)
Performance-critical: must achieve line-rate processing
Stateless or minimal state per-packet
Often bypasses kernel using DPDK, SR-IOV
Optimized for throughput and latency

Control Plane (Slow Path):

Handles signaling, configuration, management
Less performance-sensitive but more complex
Maintains substantial state (session tables, routing tables)
May use standard kernel networking
Prioritizes functionality and reliability

Converting Mermaid diagram...

Common VNF Architectural Patterns:

Pattern 1: Monolithic VNF

Single process/binary implementing all functionality
Simpler deployment and management
Limited scalability (vertical only)
Common for simple VNFs (DNS, DHCP)

Pattern 2: Multi-Process VNF

Separate processes for data plane and control plane
Can scale data plane independently
Inter-process communication overhead
Common for complex VNFs (firewalls, routers)

Pattern 3: Microservices VNF

Control plane decomposed into microservices
Data plane as high-performance service
Maximum flexibility and scalability
Highest operational complexity
Common in cloud-native VNFs (CNFs)

VNF Architectural Pattern Comparison
Aspect	Monolithic	Multi-Process	Microservices
Complexity	Low	Medium	High
Scalability	Vertical only	Component-level	Fine-grained
Fault Isolation	Limited	Good	Excellent
Resource Efficiency	Good	Moderate	Variable
Deployment Complexity	Simple	Moderate	Complex
Update Granularity	Full VNF	Per-component	Per-service
State Management	Simple	Moderate	Complex (distributed)
Typical Use Case	Simple functions	Traditional telecom	Cloud-native

VNF Networking Considerations:

VNFs require carefully designed networking that addresses:

Internal Connectivity:

Inter-VNFC communication within a VNF
Often uses internal virtual networks
Low-latency, high-bandwidth requirements

External Connectivity:

Data plane ports for traffic processing
Management interfaces for lifecycle control
May require multiple network segments (public, private, management)

Virtual Interface Types:

VirtIO: Standard virtualized NIC, software-based, moderate performance
SR-IOV VF: Hardware-accelerated, near-native performance, limited features
OVS-DPDK Port: Accelerated virtual switching, good balance
Paravirtualized: Enhanced VirtIO with backend optimization

Network Interface Selection

For data plane VNFs requiring high throughput, SR-IOV provides the best raw performance but limits live migration and some virtualization features. OVS-DPDK offers a balance—better performance than standard VirtIO while preserving management flexibility. Choose based on your specific performance requirements and operational constraints.

VNF Descriptors and Packaging

For VNFs to be deployable in an automated, standardized manner, they must be packaged with comprehensive metadata describing their requirements, dependencies, and operational parameters. ETSI NFV defines standard formats for VNF packaging.

The VNF Package (VNFD):

A VNF Descriptor (VNFD) is a deployment template that describes a VNF in terms of its:

Deployment and operational behavior
Resource requirements
Software images
Virtual links and connection points
Lifecycle management scripts
Configuration parameters

vnf-descriptor-example.yaml
YAML
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# Example VNFD following ETSI SOL001 TOSCA-based specification
 
tosca_definitions_version: tosca_simple_yaml_1_3
 
description: Virtual Firewall VNF Descriptor
 
metadata:
  template_name: vFirewall
  template_version: "2.0.0"
  template_author: NetworkVendor
 
topology_template:
  
  # Input parameters for customization
  inputs:
    vnf_name:
      type: string
      description: Name of the VNF instance
      default: vFirewall-prod
    
    admin_password:
      type: string
      description: Admin password for management access
      sensitive: true
  
  # Node definitions (VNFCs)
  node_templates:
    
    # Primary firewall VNFC
    vFirewall_VNFC:
      type: tosca.nodes.nfv.Vdu.Compute
      properties:
        name: firewall-primary
        description: Primary firewall processing node
        sw_image_data:
          name: vFirewall-image-v2.0
          version: "2.0.0"
          checksum:
            algorithm: sha-256
            hash: abc123...
          size: 4 GB
          operating_system: Ubuntu 22.04 LTS
        vdu_profile:
          min_number_of_instances: 1
          max_number_of_instances: 10
      capabilities:
        virtual_compute:
          properties:
            virtual_cpu:
              num_virtual_cpu: 8
              cpu_architecture: x86_64
            virtual_memory:
              virtual_mem_size: 16 GB
              numa_enabled: true
            virtual_local_storage:
              - size_of_storage: 100 GB
      requirements:
        - virtual_storage: vFirewall_storage
    
    # Storage for logs and state
    vFirewall_storage:
      type: tosca.nodes.nfv.Vdu.VirtualBlockStorage
      properties:
        size_of_storage: 500 GB
        type_of_storage: ssd
    
    # External connection points
    ext_cpd_mgmt:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: management
        description: Management interface for VNF configuration
      requirements:
        - virtual_binding: vFirewall_VNFC
    
    ext_cpd_data_ingress:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: data_ingress
        description: Data plane ingress interface
        bitrate_requirement: 40 Gbps
      requirements:
        - virtual_binding: vFirewall_VNFC
    
    ext_cpd_data_egress:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: data_egress
        description: Data plane egress interface
        bitrate_requirement: 40 Gbps
      requirements:
        - virtual_binding: vFirewall_VNFC
  
  # Policies for VNF behavior
  policies:
    - anti_affinity_policy:
        type: tosca.policies.nfv.AntiAffinityRule
        targets: [vFirewall_VNFC]
        properties:
          scope: NFVI_node
    
    - scaling_policy:
        type: tosca.policies.nfv.ScalingAspects
        targets: [vFirewall_VNFC]
        properties:
          aspects:
            firewall_scale:
              name: Firewall scaling aspect
              description: Scale firewall instances based on throughput
              max_scale_level: 9
              step_deltas:
                - delta_1
 

VNF Package Structure:

ETSI SOL004 defines the structure of a VNF Package (CSAR format):

vFirewall-pkg.csar
├── TOSCA-Metadata/
│   └── TOSCA.meta                 # Package metadata
├── Definitions/
│   ├── vnfd.yaml                  # Main VNF descriptor
│   └── types.yaml                 # Custom type definitions
├── Files/
│   └── scripts/
│       ├── install.sh             # Installation script
│       ├── configure.sh           # Configuration script
│       ├── start.sh               # Start script
│       ├── stop.sh                # Stop script
│       ├── scale_out.sh           # Scale-out script
│       └── health_check.sh        # Health check script
├── Images/
│   └── vFirewall-2.0.qcow2        # VNF software image  
├── Artifacts/
│   ├── ansible/                   # Ansible playbooks
│   └── helm/                      # Helm charts (for CNFs)
└── manifest.mf                    # Package manifest with checksums

Key VNFD Elements:

VNFD Core Components

•VDU (Virtualized Deployment Unit): Defines each VNFC—compute resources, software images, and storage requirements.
•Virtual Links (VL): Internal networks connecting VDUs within the VNF.
•Connection Points (CP): Interfaces where the VNF connects to external networks.
•Deployment Flavours: Predefined configurations for different deployment scenarios (small, medium, large).
•Scaling Aspects: Rules for horizontal and vertical scaling.
•Lifecycle Management Scripts: Hooks for automated operations at each lifecycle stage.
•Monitoring Parameters: Metrics the VNF exposes for health monitoring.

ETSI SOL001/004 Compliance

ETSI SOL001 defines the TOSCA-based VNFD specification, while SOL004 defines packaging. Compliance with these standards ensures VNF portability across different MANO implementations. However, many vendor VNFs use proprietary extensions that limit portability—always verify interoperability when selecting VNFs.

VNF Lifecycle Management

Unlike static hardware appliances, VNFs are dynamic entities with defined lifecycles. The VNF Manager (VNFM) orchestrates these lifecycle transitions, ensuring VNFs are properly instantiated, configured, scaled, healed, and terminated.

The VNF Lifecycle States:

A VNF instance progresses through several well-defined states:

Not Instantiated: VNF package is onboarded but no instance exists
Instantiated - Not Configured: VNF resources are allocated but function isn't operational
Instantiated - Configured: VNF is fully configured and ready for traffic
Operating: VNF is actively processing traffic
Terminated: VNF instance has been destroyed

Converting Mermaid diagram...

Lifecycle Management Operations:

Instantiate:

VNFM creates all VNF resources (VMs, networks, storage)
Based on VNFD deployment flavour and instantiation parameters
Executes instantiation scripts
Allocates virtual resources from VIM
Creates internal networks and attaches connection points

Configure:

Applies initial configuration to VNF
May use cloud-init, Ansible, or vendor-specific mechanisms
Configures network parameters (IP addresses, routes)
Loads security policies or processing rules
Integrates with external systems (AAA, NMS)

Scale:

Scale Out: Add VNFC instances to increase capacity
Scale In: Remove VNFC instances to reduce resources
Scale Up: Increase resources of existing VNFCs
Scale Down: Decrease resources of existing VNFCs

Heal:

Detect VNFC failures through health monitoring
Restart failed VNFCs
Migrate VNFCs from failing hosts
Automatic recovery without operator intervention

Update/Upgrade:

Apply software updates to running VNFs
May be in-service or require brief outage
Rolling updates across VNFC instances
Rollback capability on failure

Terminate:

Graceful shutdown of VNF operation
Release all allocated resources
Clean up associated state and logs
Remove from service chains

lifecycle-scripts-example.sh
Bash
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#!/bin/bash
# Example VNF Lifecycle Scripts
 
# ===========================================
# INSTANTIATE SCRIPT
# Called when VNF resources are allocated
# ===========================================
instantiate() {
    echo "[$(date)] Starting VNF instantiation..."
    
    # Wait for cloud-init to complete
    cloud-init status --wait
    
    # Configure network interfaces
    configure_network_interfaces
    
    # Initialize data plane (DPDK)
    initialize_dpdk
    
    # Start core services
    systemctl start vnf-control-plane
    systemctl start vnf-data-plane
    
    # Register with management system
    register_with_ems
    
    echo "[$(date)] VNF instantiation complete"
}
 
# ===========================================
# CONFIGURE SCRIPT  
# Called to apply operational configuration
# ===========================================
configure() {
    local config_file=$1
    echo "[$(date)] Applying VNF configuration from $config_file..."
    
    # Validate configuration syntax
    if ! vnf-config-validator "$config_file"; then
        echo "ERROR: Invalid configuration"
        exit 1
    fi
    
    # Apply configuration atomically
    vnf-config-apply --atomic "$config_file"
    
    # Verify configuration was applied
    vnf-config-verify
    
    echo "[$(date)] Configuration applied successfully"
}
 
# ===========================================
# SCALE OUT SCRIPT
# Called when new instances are added
# ===========================================
scale_out() {
    local new_instance_id=$1
    echo "[$(date)] Scaling out: adding instance $new_instance_id..."
    
    # Wait for new instance to be ready
    wait_for_instance_ready "$new_instance_id"
    
    # Add to load balancer pool
    add_to_lb_pool "$new_instance_id"
    
    # Synchronize state to new instance  
    sync_state_to_instance "$new_instance_id"
    
    # Enable traffic to new instance
    enable_traffic "$new_instance_id"
    
    echo "[$(date)] Scale out complete: instance $new_instance_id active"
}
 
# ===========================================
# HEAL SCRIPT
# Called when instance failure is detected
# ===========================================
heal() {
    local failed_instance_id=$1
    echo "[$(date)] Healing failed instance $failed_instance_id..."
    
    # Remove from traffic distribution
    disable_traffic "$failed_instance_id"
    
    # Attempt recovery
    if attempt_in_place_recovery "$failed_instance_id"; then
        enable_traffic "$failed_instance_id"
        echo "[$(date)] In-place recovery successful"
    else
        # Request VNFM to replace instance
        request_instance_replacement "$failed_instance_id"
        echo "[$(date)] Requested instance replacement"
    fi
}
 
# ===========================================
# HEALTH CHECK SCRIPT
# Called periodically by VNFM
# ===========================================
health_check() {
    echo "[$(date)] Executing health check..."
    
    # Check control plane
    if ! systemctl is-active vnf-control-plane; then
        echo "UNHEALTHY: Control plane not running"
        exit 1
    fi
    
    # Check data plane
    if ! check_dpdk_ports; then
        echo "UNHEALTHY: Data plane ports down"
        exit 1
    fi
    
    # Check processing capacity
    if ! check_processing_capacity; then
        echo "DEGRADED: High resource utilization"
        exit 2  # Degraded but not failed
    fi
    
    echo "HEALTHY"
    exit 0
}

Lifecycle Script Quality

The quality of lifecycle scripts directly impacts operational reliability. Poorly written scripts that don't handle edge cases, lack proper error handling, or assume specific infrastructure behaviors are a common source of VNF deployment failures. Always test lifecycle scripts thoroughly across different failure scenarios.

VNF State Management

One of the most challenging aspects of VNF design is managing state in a dynamic, virtualized environment. Unlike hardware appliances that maintain state in local memory for the device's lifetime, VNFs must handle state in ways that support scaling, healing, and migration.

Categories of VNF State:

Configuration State:

Static operational parameters
Typically loaded at instantiation
Changes infrequently
Examples: firewall rules, routing policies, security certificates

Session State:

Per-connection or per-flow information
Changes frequently during operation
Examples: TCP connection tracking, NAT mappings, user sessions

Processing State:

Transient state during packet processing
Extremely short-lived
Examples: packet buffers, in-flight transactions

VNF State Categories and Handling Strategies
State Category	Lifetime	Size	Sharing Approach	Persistence
Configuration	Instance lifetime	KB to MB	Distributed at config time	Persisted in config store
Session/Flow	Seconds to hours	MB to GB	Shared state store or sync	Optional (checkpoint)
Processing	Microseconds	Bytes to KB	Not shared	Never persisted
Metrics/Logs	Hours to days	MB to GB	Push to external systems	Time-series DB / log store

State Management Patterns:

Pattern 1: Stateless VNF

The simplest approach: design VNFs with no persistent state.

All state stored externally (databases, caches)
VNF instances are interchangeable
Easy scaling: add/remove instances freely
Easy healing: restart without state recovery
Example: Load balancers using external session stores

Pattern 2: Sticky Sessions (Affinity)

Route related traffic to the same VNF instance.

State kept locally within each instance
Traffic affinity ensures consistency
Scaling limited by affinity constraints
Instance failure loses affected sessions
Example: SIP session border controllers

Pattern 3: State Replication

Synchronize state across multiple VNF instances.

Active-standby or active-active replication
Enables fast failover
Replication overhead impacts performance
Consistency challenges with async replication
Example: Stateful firewalls with HA pairs

Pattern 4: External State Store

Store state in a dedicated, highly available data store.

VNF instances query state store as needed
State survives instance failures
Store becomes performance bottleneck
Network latency for state access
Example: Cloud-native VNFs using Redis/Memcached

Converting Mermaid diagram...

Design for Statelessness When Possible

Whenever feasible, design VNFs to be stateless or minimize local state. Stateless VNFs are inherently more cloud-native—they scale effortlessly, heal instantly, and avoid complex state synchronization challenges. If state is required, externalize it to purpose-built data stores designed for high availability.

VNF Types and Examples

The diverse landscape of network functions translates into a diverse VNF ecosystem. Different VNF categories have different performance profiles, scaling characteristics, and architectural requirements.

Security VNFs:

Security VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Firewall	Packet filtering, inspection	High-throughput deep inspection	Palo Alto VM-Series, Fortinet FortiGate-VM
vIDS/vIPS	Intrusion detection/prevention	Real-time signature matching	Snort, Suricata, Cisco Firepower
vWAF	Web application firewall	HTTP/HTTPS termination	F5 BIG-IP VE, Imperva WAF
vVPN Gateway	IPsec/SSL VPN termination	Crypto acceleration	Cisco CSR 1000v, strongSwan

Traffic Management VNFs:

Traffic Management VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Load Balancer	Traffic distribution	Session persistence at scale	HAProxy, NGINX Plus, F5 BIG-IP VE
vWAN Optimizer	Compression, deduplication	Stateful optimization	Riverbed SteelHead, Silver Peak
vDPI	Deep packet inspection	Line-rate analysis	Sandvine Policy TS, Procera
vCGNAT	Carrier-grade NAT	Millions of translations	A10 vThunder, Cisco CGv6

Routing and Switching VNFs:

Routing and Switching VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Router	IP routing	Line-rate forwarding	Cisco CSR 1000v, Juniper vMX, VPP
Virtual Switch	L2 switching	Millions of MAC entries	Open vSwitch, Contrail vRouter
vBNG	Broadband network gateway	Subscriber management	Cisco Staros, Casa CBRS
vPE	Provider edge router	MPLS/VRF support	Nokia VSR, IOS-XRv

Telecom Core VNFs:

Telecom Core VNF Examples
VNF Type	Function	Key Challenge	Example Products
vEPC	4G core network	Ultra-high availability	Cisco Ultra Services, Nokia CloudBand
vIMS	VoLTE/VoIP core	SIP session handling	Metaswitch, Oracle OCSBC
vSBC	Session border control	Media transcoding	Ribbon SBC, Oracle Enterprise SBC
vPCRF	Policy and charging	Real-time decisions	Huawei PCRF, Oracle Policy

Cloud-Native Network Functions (CNFs)

The latest evolution is Cloud-Native Network Functions (CNFs)—VNFs designed from the ground up for container platforms like Kubernetes. CNFs use microservices architecture, are packaged as Helm charts, and leverage Kubernetes primitives for scaling and healing. Major telecom vendors now offer CNF versions of their VNF products for 5G deployments.

VNF Design Best Practices

The difference between a successful VNF deployment and a problematic one often comes down to VNF design quality. Well-designed VNFs are easier to deploy, more reliable in operation, and less costly to maintain.

Design Principles:

Essential VNF Design Principles

•Design for Failure: Assume any component can fail at any time. VNFs should detect and recover from failures without external intervention.
•Embrace Statelessness: Minimize local state. When state is required, externalize it to resilient stores or implement robust replication.
•Support Horizontal Scaling: Design VNFs to scale out (more instances) rather than up (bigger instances). Horizontal scaling provides better fault isolation and elasticity.
•Implement Graceful Degradation: When overloaded, VNFs should shed load gracefully rather than failing completely. Implement backpressure and admission control.
•Expose Health Metrics: VNFs must expose comprehensive health and performance metrics. Silent failures are the enemy of reliable operations.
•Automate Everything: Every lifecycle operation should be automatable. Manual steps introduce errors and slow deployment.
•Be Infrastructure-Agnostic: Minimize dependencies on specific VIM versions, hypervisors, or hardware. Portability enables operational flexibility.
•Secure by Default: Ship with secure defaults. Require explicit action to weaken security posture, not to strengthen it.

Anti-Patterns to Avoid:

VNF Anti-Patterns

•Hardware Assumptions: Assuming specific CPU features, memory configurations, or NIC types without checking
•Hardcoded IPs: Embedding IP addresses instead of using config or discovery
•Single Points of Failure: Critical functions without redundancy
•Unbounded Resources: No limits on memory growth, connection counts, or queue depths
•Poor Logging: Missing logs for important events, or logging so much it impacts performance

Better Approaches

•Feature Detection: Check for required features at startup, fail fast with clear messages
•Configuration-Driven: All parameters from config files, environment variables, or metadata service
•Active-Active Design: Distribute function across multiple instances with no single point of failure
•Resource Governance: Explicit limits, admission control, and graceful degradation under pressure
•Structured Logging: Consistent log format, appropriate levels, sampled under load

Performance Engineering:

VNF performance requires deliberate engineering:

Data Plane Optimization:

User-space networking (DPDK or similar)
Zero-copy packet processing
Batch processing to amortize overhead
Lock-free data structures
Memory pool pre-allocation

Resource Efficiency:

CPU pinning for critical threads
NUMA-aware memory allocation
Huge page usage for large memory
Efficient inter-core communication
Minimized context switching

Observability:

Per-component metrics (CPU, memory, queue depths)
Latency histograms, not just averages
Distributed tracing support
Alert thresholds and anomaly detection
Capacity planning data

The 'Lifted and Shifted' VNF Problem

Many early VNFs were simply existing appliance software running in a VM—'lifted and shifted' without redesign. These VNFs often exhibit poor performance, limited scalability, and brittle failure handling. True VNFs are designed from the ground up for virtualized operation, not ported as an afterthought.

Summary: Virtual Network Functions

We've comprehensively explored Virtual Network Functions—the software building blocks of NFV. Let's consolidate the key concepts:

Key Takeaways

•VNFs are more than software in VMs — They are complete packages including application code, configuration, metadata, and lifecycle scripts designed for automated deployment and management.
•VNFs consist of VNF Components (VNFCs) — Each VNFC is a deployable unit (VM or container), while the VNF is the complete functional entity potentially comprising multiple VNFCs.
•Architecture matters enormously — The choice between monolithic, multi-process, and microservices architectures impacts scalability, reliability, and operational complexity.
•VNF Descriptors enable automation — TOSCA-based VNFDs in CSAR packages describe everything needed for automated deployment, standardizing VNF portability.
•Lifecycle management is continuous — VNFs progress through instantiate, configure, scale, heal, update, and terminate operations managed by the VNFM.
•State management is a core challenge — Design decisions around state (stateless, sticky, replicated, externalized) fundamentally impact VNF scalability and resilience.
•Different VNF types have different needs — Security VNFs, traffic management VNFs, routing VNFs, and telecom core VNFs each have unique performance and architectural requirements.
•Design quality determines success — Following best practices and avoiding anti-patterns separates VNFs that work in production from those that fail.

What's Next:

With VNFs understood, we'll next examine the NFV Architecture in detail—the complete framework that enables VNF deployment and operation. We'll explore NFVI, MANO (Management and Orchestration), and the reference points that connect NFV components.

Page Complete

You now understand Virtual Network Functions comprehensively—their architecture, packaging, lifecycle, state management, and design principles. This knowledge prepares you to evaluate VNF products, design VNF-friendly infrastructure, and understand the NFV architecture that supports them.

Virtual Network Functions: Design and Implementation

Virtual Network Functions: The Software Building Blocks of Modern Networks

This page explores VNFs in depth: their architecture, design principles, lifecycle management, and the characteristics that separate mature VNF implementations from problematic ones.

What You Will Learn

Anatomy of a Virtual Network Function

The Formal Definition:

According to ETSI GS NFV 003, a Virtual Network Function (VNF) is defined as:

Key Characteristics of VNFs:

Defining Characteristics of VNFs

•Software-Implemented: The network function is delivered entirely as software, with no dependency on specialized hardware (though it may leverage hardware acceleration when available).
•Infrastructure-Agnostic: VNFs are designed to run on standard NFV Infrastructure without requiring specific hardware configurations (within defined constraints).
•Self-Contained: A VNF package includes all necessary components—application binaries, libraries, configuration templates, and operational scripts.
•Manageable via Standard APIs: VNFs expose well-defined interfaces for lifecycle management, configuration, and monitoring.
•Composable: VNFs can be chained together to create complex network services, each providing a specific function.
•Scalable: VNFs can be instantiated multiple times and scaled horizontally or vertically to meet demand.

VNF vs. VNFC:

A critical distinction in the ETSI framework is between a VNF and its constituent VNF Components (VNFCs):

VNF Component (VNFC):

A single VM (or container) implementing part of a VNF
The smallest unit of a VNF that can be individually deployed
Runs on a single hypervisor or container host
May be replicated for redundancy or scaling

VNF (Virtual Network Function):

May consist of one or more VNFCs
Represents the complete functional unit
VNFCs within a VNF communicate via internal links
The VNF is the unit of lifecycle management

Example: Multi-VNFC Architecture

Consider a virtualized Evolved Packet Core (vEPC) VNF:

vEPC VNF
├── VNFC-1: MME (Mobility Management Entity)
├── VNFC-2: SGW-C (Serving Gateway Control Plane)
├── VNFC-3: SGW-U (Serving Gateway User Plane)
├── VNFC-4: PGW-C (PDN Gateway Control Plane)
├── VNFC-5: PGW-U (PDN Gateway User Plane)
└── VNFC-6: HSS (Home Subscriber Server)

Each VNFC is a separate VM with specific resource requirements and scaling characteristics, but together they form a single VNF representing the complete EPC functionality.

Practical VNFC Deployment

VNF Internal Architecture

The Data Plane / Control Plane Split:

Network functions typically separate into two fundamental processing paths:

Data Plane (Fast Path):

Handles actual network traffic (packets, flows)
Performance-critical: must achieve line-rate processing
Stateless or minimal state per-packet
Often bypasses kernel using DPDK, SR-IOV
Optimized for throughput and latency

Control Plane (Slow Path):

Handles signaling, configuration, management
Less performance-sensitive but more complex
Maintains substantial state (session tables, routing tables)
May use standard kernel networking
Prioritizes functionality and reliability

Converting Mermaid diagram...

Common VNF Architectural Patterns:

Pattern 1: Monolithic VNF

Single process/binary implementing all functionality
Simpler deployment and management
Limited scalability (vertical only)
Common for simple VNFs (DNS, DHCP)

Pattern 2: Multi-Process VNF

Separate processes for data plane and control plane
Can scale data plane independently
Inter-process communication overhead
Common for complex VNFs (firewalls, routers)

Pattern 3: Microservices VNF

Control plane decomposed into microservices
Data plane as high-performance service
Maximum flexibility and scalability
Highest operational complexity
Common in cloud-native VNFs (CNFs)

VNF Architectural Pattern Comparison
Aspect	Monolithic	Multi-Process	Microservices
Complexity	Low	Medium	High
Scalability	Vertical only	Component-level	Fine-grained
Fault Isolation	Limited	Good	Excellent
Resource Efficiency	Good	Moderate	Variable
Deployment Complexity	Simple	Moderate	Complex
Update Granularity	Full VNF	Per-component	Per-service
State Management	Simple	Moderate	Complex (distributed)
Typical Use Case	Simple functions	Traditional telecom	Cloud-native

VNF Networking Considerations:

VNFs require carefully designed networking that addresses:

Internal Connectivity:

Inter-VNFC communication within a VNF
Often uses internal virtual networks
Low-latency, high-bandwidth requirements

External Connectivity:

Data plane ports for traffic processing
Management interfaces for lifecycle control
May require multiple network segments (public, private, management)

Virtual Interface Types:

VirtIO: Standard virtualized NIC, software-based, moderate performance
SR-IOV VF: Hardware-accelerated, near-native performance, limited features
OVS-DPDK Port: Accelerated virtual switching, good balance
Paravirtualized: Enhanced VirtIO with backend optimization

Network Interface Selection

VNF Descriptors and Packaging

The VNF Package (VNFD):

A VNF Descriptor (VNFD) is a deployment template that describes a VNF in terms of its:

Deployment and operational behavior
Resource requirements
Software images
Virtual links and connection points
Lifecycle management scripts
Configuration parameters

vnf-descriptor-example.yaml
YAML
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# Example VNFD following ETSI SOL001 TOSCA-based specification
 
tosca_definitions_version: tosca_simple_yaml_1_3
 
description: Virtual Firewall VNF Descriptor
 
metadata:
  template_name: vFirewall
  template_version: "2.0.0"
  template_author: NetworkVendor
 
topology_template:
  
  # Input parameters for customization
  inputs:
    vnf_name:
      type: string
      description: Name of the VNF instance
      default: vFirewall-prod
    
    admin_password:
      type: string
      description: Admin password for management access
      sensitive: true
  
  # Node definitions (VNFCs)
  node_templates:
    
    # Primary firewall VNFC
    vFirewall_VNFC:
      type: tosca.nodes.nfv.Vdu.Compute
      properties:
        name: firewall-primary
        description: Primary firewall processing node
        sw_image_data:
          name: vFirewall-image-v2.0
          version: "2.0.0"
          checksum:
            algorithm: sha-256
            hash: abc123...
          size: 4 GB
          operating_system: Ubuntu 22.04 LTS
        vdu_profile:
          min_number_of_instances: 1
          max_number_of_instances: 10
      capabilities:
        virtual_compute:
          properties:
            virtual_cpu:
              num_virtual_cpu: 8
              cpu_architecture: x86_64
            virtual_memory:
              virtual_mem_size: 16 GB
              numa_enabled: true
            virtual_local_storage:
              - size_of_storage: 100 GB
      requirements:
        - virtual_storage: vFirewall_storage
    
    # Storage for logs and state
    vFirewall_storage:
      type: tosca.nodes.nfv.Vdu.VirtualBlockStorage
      properties:
        size_of_storage: 500 GB
        type_of_storage: ssd
    
    # External connection points
    ext_cpd_mgmt:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: management
        description: Management interface for VNF configuration
      requirements:
        - virtual_binding: vFirewall_VNFC
    
    ext_cpd_data_ingress:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: data_ingress
        description: Data plane ingress interface
        bitrate_requirement: 40 Gbps
      requirements:
        - virtual_binding: vFirewall_VNFC
    
    ext_cpd_data_egress:
      type: tosca.nodes.nfv.VduCpd
      properties:
        layer_protocols: [ipv4]
        role: data_egress
        description: Data plane egress interface
        bitrate_requirement: 40 Gbps
      requirements:
        - virtual_binding: vFirewall_VNFC
  
  # Policies for VNF behavior
  policies:
    - anti_affinity_policy:
        type: tosca.policies.nfv.AntiAffinityRule
        targets: [vFirewall_VNFC]
        properties:
          scope: NFVI_node
    
    - scaling_policy:
        type: tosca.policies.nfv.ScalingAspects
        targets: [vFirewall_VNFC]
        properties:
          aspects:
            firewall_scale:
              name: Firewall scaling aspect
              description: Scale firewall instances based on throughput
              max_scale_level: 9
              step_deltas:
                - delta_1
 

VNF Package Structure:

ETSI SOL004 defines the structure of a VNF Package (CSAR format):

vFirewall-pkg.csar
├── TOSCA-Metadata/
│   └── TOSCA.meta                 # Package metadata
├── Definitions/
│   ├── vnfd.yaml                  # Main VNF descriptor
│   └── types.yaml                 # Custom type definitions
├── Files/
│   └── scripts/
│       ├── install.sh             # Installation script
│       ├── configure.sh           # Configuration script
│       ├── start.sh               # Start script
│       ├── stop.sh                # Stop script
│       ├── scale_out.sh           # Scale-out script
│       └── health_check.sh        # Health check script
├── Images/
│   └── vFirewall-2.0.qcow2        # VNF software image  
├── Artifacts/
│   ├── ansible/                   # Ansible playbooks
│   └── helm/                      # Helm charts (for CNFs)
└── manifest.mf                    # Package manifest with checksums

Key VNFD Elements:

VNFD Core Components

•VDU (Virtualized Deployment Unit): Defines each VNFC—compute resources, software images, and storage requirements.
•Virtual Links (VL): Internal networks connecting VDUs within the VNF.
•Connection Points (CP): Interfaces where the VNF connects to external networks.
•Deployment Flavours: Predefined configurations for different deployment scenarios (small, medium, large).
•Scaling Aspects: Rules for horizontal and vertical scaling.
•Lifecycle Management Scripts: Hooks for automated operations at each lifecycle stage.
•Monitoring Parameters: Metrics the VNF exposes for health monitoring.

ETSI SOL001/004 Compliance

VNF Lifecycle Management

The VNF Lifecycle States:

A VNF instance progresses through several well-defined states:

Not Instantiated: VNF package is onboarded but no instance exists
Instantiated - Not Configured: VNF resources are allocated but function isn't operational
Instantiated - Configured: VNF is fully configured and ready for traffic
Operating: VNF is actively processing traffic
Terminated: VNF instance has been destroyed

Converting Mermaid diagram...

Lifecycle Management Operations:

Instantiate:

VNFM creates all VNF resources (VMs, networks, storage)
Based on VNFD deployment flavour and instantiation parameters
Executes instantiation scripts
Allocates virtual resources from VIM
Creates internal networks and attaches connection points

Configure:

Applies initial configuration to VNF
May use cloud-init, Ansible, or vendor-specific mechanisms
Configures network parameters (IP addresses, routes)
Loads security policies or processing rules
Integrates with external systems (AAA, NMS)

Scale:

Scale Out: Add VNFC instances to increase capacity
Scale In: Remove VNFC instances to reduce resources
Scale Up: Increase resources of existing VNFCs
Scale Down: Decrease resources of existing VNFCs

Heal:

Detect VNFC failures through health monitoring
Restart failed VNFCs
Migrate VNFCs from failing hosts
Automatic recovery without operator intervention

Update/Upgrade:

Apply software updates to running VNFs
May be in-service or require brief outage
Rolling updates across VNFC instances
Rollback capability on failure

Terminate:

Graceful shutdown of VNF operation
Release all allocated resources
Clean up associated state and logs
Remove from service chains

lifecycle-scripts-example.sh
Bash
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#!/bin/bash
# Example VNF Lifecycle Scripts
 
# ===========================================
# INSTANTIATE SCRIPT
# Called when VNF resources are allocated
# ===========================================
instantiate() {
    echo "[$(date)] Starting VNF instantiation..."
    
    # Wait for cloud-init to complete
    cloud-init status --wait
    
    # Configure network interfaces
    configure_network_interfaces
    
    # Initialize data plane (DPDK)
    initialize_dpdk
    
    # Start core services
    systemctl start vnf-control-plane
    systemctl start vnf-data-plane
    
    # Register with management system
    register_with_ems
    
    echo "[$(date)] VNF instantiation complete"
}
 
# ===========================================
# CONFIGURE SCRIPT  
# Called to apply operational configuration
# ===========================================
configure() {
    local config_file=$1
    echo "[$(date)] Applying VNF configuration from $config_file..."
    
    # Validate configuration syntax
    if ! vnf-config-validator "$config_file"; then
        echo "ERROR: Invalid configuration"
        exit 1
    fi
    
    # Apply configuration atomically
    vnf-config-apply --atomic "$config_file"
    
    # Verify configuration was applied
    vnf-config-verify
    
    echo "[$(date)] Configuration applied successfully"
}
 
# ===========================================
# SCALE OUT SCRIPT
# Called when new instances are added
# ===========================================
scale_out() {
    local new_instance_id=$1
    echo "[$(date)] Scaling out: adding instance $new_instance_id..."
    
    # Wait for new instance to be ready
    wait_for_instance_ready "$new_instance_id"
    
    # Add to load balancer pool
    add_to_lb_pool "$new_instance_id"
    
    # Synchronize state to new instance  
    sync_state_to_instance "$new_instance_id"
    
    # Enable traffic to new instance
    enable_traffic "$new_instance_id"
    
    echo "[$(date)] Scale out complete: instance $new_instance_id active"
}
 
# ===========================================
# HEAL SCRIPT
# Called when instance failure is detected
# ===========================================
heal() {
    local failed_instance_id=$1
    echo "[$(date)] Healing failed instance $failed_instance_id..."
    
    # Remove from traffic distribution
    disable_traffic "$failed_instance_id"
    
    # Attempt recovery
    if attempt_in_place_recovery "$failed_instance_id"; then
        enable_traffic "$failed_instance_id"
        echo "[$(date)] In-place recovery successful"
    else
        # Request VNFM to replace instance
        request_instance_replacement "$failed_instance_id"
        echo "[$(date)] Requested instance replacement"
    fi
}
 
# ===========================================
# HEALTH CHECK SCRIPT
# Called periodically by VNFM
# ===========================================
health_check() {
    echo "[$(date)] Executing health check..."
    
    # Check control plane
    if ! systemctl is-active vnf-control-plane; then
        echo "UNHEALTHY: Control plane not running"
        exit 1
    fi
    
    # Check data plane
    if ! check_dpdk_ports; then
        echo "UNHEALTHY: Data plane ports down"
        exit 1
    fi
    
    # Check processing capacity
    if ! check_processing_capacity; then
        echo "DEGRADED: High resource utilization"
        exit 2  # Degraded but not failed
    fi
    
    echo "HEALTHY"
    exit 0
}

Lifecycle Script Quality

VNF State Management

Categories of VNF State:

Configuration State:

Static operational parameters
Typically loaded at instantiation
Changes infrequently
Examples: firewall rules, routing policies, security certificates

Session State:

Per-connection or per-flow information
Changes frequently during operation
Examples: TCP connection tracking, NAT mappings, user sessions

Processing State:

Transient state during packet processing
Extremely short-lived
Examples: packet buffers, in-flight transactions

VNF State Categories and Handling Strategies
State Category	Lifetime	Size	Sharing Approach	Persistence
Configuration	Instance lifetime	KB to MB	Distributed at config time	Persisted in config store
Session/Flow	Seconds to hours	MB to GB	Shared state store or sync	Optional (checkpoint)
Processing	Microseconds	Bytes to KB	Not shared	Never persisted
Metrics/Logs	Hours to days	MB to GB	Push to external systems	Time-series DB / log store

State Management Patterns:

Pattern 1: Stateless VNF

The simplest approach: design VNFs with no persistent state.

All state stored externally (databases, caches)
VNF instances are interchangeable
Easy scaling: add/remove instances freely
Easy healing: restart without state recovery
Example: Load balancers using external session stores

Pattern 2: Sticky Sessions (Affinity)

Route related traffic to the same VNF instance.

State kept locally within each instance
Traffic affinity ensures consistency
Scaling limited by affinity constraints
Instance failure loses affected sessions
Example: SIP session border controllers

Pattern 3: State Replication

Synchronize state across multiple VNF instances.

Active-standby or active-active replication
Enables fast failover
Replication overhead impacts performance
Consistency challenges with async replication
Example: Stateful firewalls with HA pairs

Pattern 4: External State Store

Store state in a dedicated, highly available data store.

VNF instances query state store as needed
State survives instance failures
Store becomes performance bottleneck
Network latency for state access
Example: Cloud-native VNFs using Redis/Memcached

Converting Mermaid diagram...

Design for Statelessness When Possible

VNF Types and Examples

Security VNFs:

Security VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Firewall	Packet filtering, inspection	High-throughput deep inspection	Palo Alto VM-Series, Fortinet FortiGate-VM
vIDS/vIPS	Intrusion detection/prevention	Real-time signature matching	Snort, Suricata, Cisco Firepower
vWAF	Web application firewall	HTTP/HTTPS termination	F5 BIG-IP VE, Imperva WAF
vVPN Gateway	IPsec/SSL VPN termination	Crypto acceleration	Cisco CSR 1000v, strongSwan

Traffic Management VNFs:

Traffic Management VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Load Balancer	Traffic distribution	Session persistence at scale	HAProxy, NGINX Plus, F5 BIG-IP VE
vWAN Optimizer	Compression, deduplication	Stateful optimization	Riverbed SteelHead, Silver Peak
vDPI	Deep packet inspection	Line-rate analysis	Sandvine Policy TS, Procera
vCGNAT	Carrier-grade NAT	Millions of translations	A10 vThunder, Cisco CGv6

Routing and Switching VNFs:

Routing and Switching VNF Examples
VNF Type	Function	Key Challenge	Example Products
Virtual Router	IP routing	Line-rate forwarding	Cisco CSR 1000v, Juniper vMX, VPP
Virtual Switch	L2 switching	Millions of MAC entries	Open vSwitch, Contrail vRouter
vBNG	Broadband network gateway	Subscriber management	Cisco Staros, Casa CBRS
vPE	Provider edge router	MPLS/VRF support	Nokia VSR, IOS-XRv

Telecom Core VNFs:

Telecom Core VNF Examples
VNF Type	Function	Key Challenge	Example Products
vEPC	4G core network	Ultra-high availability	Cisco Ultra Services, Nokia CloudBand
vIMS	VoLTE/VoIP core	SIP session handling	Metaswitch, Oracle OCSBC
vSBC	Session border control	Media transcoding	Ribbon SBC, Oracle Enterprise SBC
vPCRF	Policy and charging	Real-time decisions	Huawei PCRF, Oracle Policy

Cloud-Native Network Functions (CNFs)

VNF Design Best Practices

Design Principles:

Essential VNF Design Principles

•Design for Failure: Assume any component can fail at any time. VNFs should detect and recover from failures without external intervention.
•Embrace Statelessness: Minimize local state. When state is required, externalize it to resilient stores or implement robust replication.
•Support Horizontal Scaling: Design VNFs to scale out (more instances) rather than up (bigger instances). Horizontal scaling provides better fault isolation and elasticity.
•Implement Graceful Degradation: When overloaded, VNFs should shed load gracefully rather than failing completely. Implement backpressure and admission control.
•Expose Health Metrics: VNFs must expose comprehensive health and performance metrics. Silent failures are the enemy of reliable operations.
•Automate Everything: Every lifecycle operation should be automatable. Manual steps introduce errors and slow deployment.
•Be Infrastructure-Agnostic: Minimize dependencies on specific VIM versions, hypervisors, or hardware. Portability enables operational flexibility.
•Secure by Default: Ship with secure defaults. Require explicit action to weaken security posture, not to strengthen it.

Anti-Patterns to Avoid:

VNF Anti-Patterns

•Hardware Assumptions: Assuming specific CPU features, memory configurations, or NIC types without checking
•Hardcoded IPs: Embedding IP addresses instead of using config or discovery
•Single Points of Failure: Critical functions without redundancy
•Unbounded Resources: No limits on memory growth, connection counts, or queue depths
•Poor Logging: Missing logs for important events, or logging so much it impacts performance

Better Approaches

•Feature Detection: Check for required features at startup, fail fast with clear messages
•Configuration-Driven: All parameters from config files, environment variables, or metadata service
•Active-Active Design: Distribute function across multiple instances with no single point of failure
•Resource Governance: Explicit limits, admission control, and graceful degradation under pressure
•Structured Logging: Consistent log format, appropriate levels, sampled under load

Performance Engineering:

VNF performance requires deliberate engineering:

Data Plane Optimization:

User-space networking (DPDK or similar)
Zero-copy packet processing
Batch processing to amortize overhead
Lock-free data structures
Memory pool pre-allocation

Resource Efficiency:

CPU pinning for critical threads
NUMA-aware memory allocation
Huge page usage for large memory
Efficient inter-core communication
Minimized context switching

Observability:

Per-component metrics (CPU, memory, queue depths)
Latency histograms, not just averages
Distributed tracing support
Alert thresholds and anomaly detection
Capacity planning data

The 'Lifted and Shifted' VNF Problem

Summary: Virtual Network Functions

We've comprehensively explored Virtual Network Functions—the software building blocks of NFV. Let's consolidate the key concepts:

Key Takeaways

•VNFs are more than software in VMs — They are complete packages including application code, configuration, metadata, and lifecycle scripts designed for automated deployment and management.
•VNFs consist of VNF Components (VNFCs) — Each VNFC is a deployable unit (VM or container), while the VNF is the complete functional entity potentially comprising multiple VNFCs.
•Architecture matters enormously — The choice between monolithic, multi-process, and microservices architectures impacts scalability, reliability, and operational complexity.
•VNF Descriptors enable automation — TOSCA-based VNFDs in CSAR packages describe everything needed for automated deployment, standardizing VNF portability.
•Lifecycle management is continuous — VNFs progress through instantiate, configure, scale, heal, update, and terminate operations managed by the VNFM.
•State management is a core challenge — Design decisions around state (stateless, sticky, replicated, externalized) fundamentally impact VNF scalability and resilience.
•Different VNF types have different needs — Security VNFs, traffic management VNFs, routing VNFs, and telecom core VNFs each have unique performance and architectural requirements.
•Design quality determines success — Following best practices and avoiding anti-patterns separates VNFs that work in production from those that fail.

What's Next:

Page Complete