SDN — Software-Defined Networking

Overview

SDN is a network architecture that separates the control plane (routing decisions) from the data plane (packet forwarding). A centralized controller programs network devices, enabling dynamic, manageable, and cost-effective networks.

Traditional vs SDN Architecture

graph TD
    subgraph "Traditional Networking"
        T1[Switch 1<br>Control + Data] --- T2[Switch 2<br>Control + Data]
        T2 --- T3[Switch 3<br>Control + Data]
        T3 --- T1
    end
    subgraph "SDN Architecture"
        C[SDN Controller<br>Control Plane]
        S1[Switch 1<br>Data Plane Only]
        S2[Switch 2<br>Data Plane Only]
        S3[Switch 3<br>Data Plane Only]
        C -->|OpenFlow| S1
        C -->|OpenFlow| S2
        C -->|OpenFlow| S3
    end

SDN Layers

graph TD
    subgraph "Application Plane"
        A1[Network Monitoring]
        A2[Load Balancing]
        A3[Firewall Rules]
        A4[Traffic Engineering]
    end
    subgraph "Control Plane"
        C[SDN Controller<br>OpenDaylight, ONOS, Floodlight]
    end
    subgraph "Data Plane"
        D1[Switch 1]
        D2[Switch 2]
        D3[Switch 3]
    end
    A1 --> C
    A2 --> C
    A3 --> C
    A4 --> C
    C --> D1
    C --> D2
    C --> D3

OpenFlow Protocol

OpenFlow is the standard SDN protocol between controller and switches:

Flow Table Entry

OpenFlow Messages

sequenceDiagram
    participant C as Controller
    participant S as Switch
    C->>S: FlowMod (add/modify/delete flow)
    S->>C: PacketIn (no matching flow)
    C->>S: PacketOut (send packet)
    S->>C: FlowRemoved (flow expired)
    S->>C: PortStatus (link up/down)
    C->>S: MultipartRequest (statistics)
    S->>C: MultipartReply

Example Flow Rules

# Forward HTTP traffic to web server
Match: IP dst=10.0.0.1, TCP dst_port=80
Action: output:port3

# Drop SSH from external
Match: IP src=203.0.113.0/24, TCP dst_port=22
Action: drop

# Default rule (send to controller)
Match: *
Action: send_to_controller

SDN Controllers

SDN Benefits

SDN Use Cases

graph TD
    A[SDN Use Cases] --> B[Data Center Networking]
    A --> C[Wide Area Networks]
    A --> D[Network Virtualization]
    A --> E[Security]
    A --> F[traffic Engineering]
    B --> G[Google B4 WAN]
    B --> H[VMware NSX]
    C --> I[SD-WAN]
    D --> J[Overlay networks]
    E --> K[Dynamic firewalling]
    F --> L[Load balancing]

SD-WAN

SD-WAN applies SDN principles to WAN connections:

graph TD
    subgraph "Branch Office"
        SDWAN[SD-WAN Appliance]
    end
    subgraph "Transport Options"
        MPLS[MPLS Link]
        INET[Internet Link]
        LTE[LTE/5G Link]
    end
    subgraph "Cloud / HQ"
        GW[SD-WAN Gateway]
    end
    SDWAN --> MPLS --> GW
    SDWAN --> INET --> GW
    SDWAN --> LTE --> GW

Benefits: Application-aware routing, cost savings (use internet instead of MPLS), centralized management, automatic failover.

Interview Questions

1. Q: What is SDN and why is it important? A: SDN separates the control plane (routing decisions) from the data plane (packet forwarding). A centralized controller programs network devices. Benefits: centralized management, programmability, vendor independence, and rapid innovation.

2. Q: What is OpenFlow? A: The standard SDN protocol between controller and switches. It defines how the controller installs flow rules (match + action) in switches. When a packet doesn’t match any rule, it’s sent to the controller (PacketIn).

3. Q: What’s the difference between traditional and SDN networking? A: Traditional: each device has its own control plane (distributed). SDN: centralized control plane manages all devices. Traditional requires CLI/SNMP per device. SDN uses APIs for programmatic control.

4. Q: What is SD-WAN? A: SD-WAN applies SDN to WAN connections. It aggregates multiple transport links (MPLS, internet, LTE) and routes traffic based on application requirements. Benefits: cost savings, application-aware routing, centralized management.

5. Q: What are the challenges of SDN? A: 1) Single point of failure (controller), 2) Scalability (controller must handle all decisions), 3) Security (controller is a high-value target), 4) Latency (controller-switch communication), 5) Vendor lock-in (despite open standards).

Common Mistakes

• Confusing SDN (architecture) with OpenFlow (protocol)
• Assuming SDN eliminates all hardware (data plane still needs switches)
• Not understanding that SDN controller is a single point of failure
• Forgetting that SDN requires well-defined APIs between layers
• Confusing SDN with NFV (complementary but different)

Summary

SDN separates control and data planes, enabling centralized, programmable network management. OpenFlow is the standard protocol. SD-WAN applies SDN to WAN. Benefits include programmability, vendor independence, and dynamic configuration.

Cross-References

• Wireless Overview
• NFV — Complementary technology
• 5G — 5G uses SDN principles
• Load Balancing — SDN use case

Cross References

• NFV
• Load Balancing
• Cloud Virtualization