Software defined WAN (SD-WAN) is foreseen to bring a landscape change in the enterprise businesses. As per the latest predictions from Gartner, by the end of 2019, 30% of the enterprises will already have SD-WAN deployed in their branch locations. Market forecasts for SD-WAN are also promising enough to create a buzz in the networking world. Latest survey from IDC forecasts SD-WAN infrastructures to show 40.4% compound annual growth rate and is estimated to reach $4.5 billion by 2022. These trends have been triggering a hot debate on how SD-WAN compares to the already proven MPLS technology and how it promises to solve the inherent challenges faced by the enterprises. This article intends to shed some light on the set of challenges faced by MPLS consumers and how SD-WAN promises to rescue them out of these.

Looking back at MPLS
 

Running applications with high reliability and assured SLA has always been the prime need for enterprises. The traditional networking model of enterprises has all users on the same LAN while they are using applications.  But with enterprises adding distributed branches and applications moving to the cloud, the user of an application and the application itself might be located anywhere on the globe. In order to provide the same LAN-like performance to its users, enterprises have started relying on private access lines using MPLS. With private VPN connectivity, MPLS service providers have been offering 99.999% uptime and SLAs guaranteeing latency, packet delivery and availability numbers. Thus, each branch office would have an MPLS link to the main office and use that to access all enterprise data and applications.

Key challenges of MPLS 
 

Enterprises achieve the desired performance and reliability using these private WAN links. But this is achieved by paying high access costs. Enterprises end up paying as high as $100-$300 per Mbps, per month. Due to lack of enough competition in the MPLS market and the amount of manual work required in setting up MPLS circuits, the cost of MPLS service ceases to decline. Cost factor is becoming more alarming with easy availability of cheap broadband Internet connections. The cost per bit has been declining steadily for broadband.

This model is changing now. The enterprises have started moving their applications to the cloud. Collaboration with branch offices happens through data warehouses and applications hosted in the cloud. This has triggered a tremendous shift in the enterprises data traffic patterns. Due to fewer applications residing in the data center and majority of them running in the cloud (SaaS, PaaS, IaaS), static MPLS architecture is posing challenges. Traffic is typically backhauled from branch offices to the main headquarters and is then sent to the Internet using a secure gateway. Enterprises end up requiring more bandwidth of an already expensive pipe. The growth of IoT devices in recent years have added more to the ever growing bandwidth requirements for the enterprises. 

Setting up MPLS for distributed sites is time consuming and involves considerable manual configurations. A new site could have routers, WAN appliances, firewall. Skilled staff is required for configuring the networking fabrics at sites and the underlying network. Any minor change in service delivery requires a re-configuration at each step. Monitoring and analysing of sites and the connecting links are done individually to discover the stress points in the network. 
All the above factors are triggering the Enterprises to look for alternatives to MPLS. 

Software Defined WAN (SD-WAN)
 

Due to cost factors, there is now a strong urge to use cheaper broadband links and still achieve MPLS-like performance. One such option is to use overlay tunnels over heterogenous WAN links and add the ability to control traffic steering on these links. Traffic steering rules could be used to control the behaviour of different classes of traffic on each site. This way the enterprises could use any of the available WAN links based on business needs.

SD-WAN is part of the WAN-edge suite of technologies, where edge devices can process packets at line speed based on template rules provided by a central SD-WAN controller. SD-WAN thus has a common heritage with SDN concepts. As in SDN, the control plane (defining the traffic steering rules) is moved to a centralized controller and the data plane (executing the traffic steering rules) stays at the customer edge devices. Applying SDN concepts to WAN connections led to the term “Software defined WAN (SD-WAN)”. Thus, a centralized SD-WAN controller is used to control the SD-WAN devices installed in the customer premises or sites. Because SD-WAN does not require configuring of routers within the WAN (which is required in MPLS, in order to do resource reservation on a per-label basis) and the development of fast packet processing engines based on content-addressable memory, SD-WAN and other WAN edge technologies are an order of magnitude cheaper than MPLS, both in terms of device cost and operationally.

How SD-WAN helps
 

Enterprises are increasingly deploying SD-WAN solutions to address the challenges faced with MPLS. SD-WAN is transport agnostic. SD-WAN overlay tunnels can be created over any WAN link (MPLS, Internet, LTE, 4G) of the enterprise site.

To save on the bandwidth consumption of the expensive MPLS links, SD-WAN moves the less critical applications on SD-WAN tunnels of the alternate WAN link (Internet). Enterprises can define the traffic steering rules for each application. Using the centralized SD-WAN controller, these traffic steering rules are circulated to the SD-WAN devices. Data traffic of all traffic classes pass through this SD-WAN device and is handled as per the configured rules. For example, SD-WAN controller might configure a site to use MPLS for VoIP traffic and Internet connection for O365 applications.

To support faster access to applications running in the cloud, SD-WAN can configure a traffic steering rule to use the Internet link for these applications. Thus, the cloud traffic goes directly from the branch office to Internet instead of backhauling to the headquarters. Some SD-WAN operators offer direct access to cloud data-centers (for example AWS or Microsoft Azure) from their gateways, improving performance and reliability of applications hosted on these clouds.

Further, SD-WAN offers WAN optimizations like data duplication, data compression and forward error correction. These optimizations reduce the WAN bandwidth requirements and provide better QoS. These WAN optimizations come as value-added services in most of the SD-WAN solutions.

Ease of Management with SD-WAN
 

SD-WAN solutions provide centralized monitoring and analytics support. Application performances in terms of desired metrics is measured on each SD-WAN tunnel and site. These metrics are reported back to the SD-WAN controller. Using the configured traffic steering rules and the reported metrics, the controller can identify anomalies in the network. Accordingly, the traffic steering rules can be modified to iteratively improve the application performances. Analytics support can be used to forecast the potential policy changes or recommend them based on real-time data.

SD-WAN solutions generally have single pane of glass management. Apart from the monitoring feature, this pane is also used for automatic configuration of the SD-WAN devices and for orchestration of the SD-WAN tunnels. Adding SD-WAN capabilities to a site simply implies - power up a SD-WAN device on the site and connect it to the centralized controller using Internet. The device automatically retrieves the images, configurations and traffic steering rules from the controller. 
 

From the single pane of glass management, multiple devices can be upgraded, or policies can be changed on them. SD-WAN no more requires any trained IT guy for configuration and monitoring of each site. IT team can create the profile templates defining the policies and can use the same template to configure multiple devices or sites. This Zero touch provisioning in SD-WAN fastens and simplifies the site deployment and management.
 

Cloud based SD-WAN gateways are flexible and scalable. There is no need for all services to be running. On-demand network services like security or firewall can be instantiated from the SD-WAN controller. No data center redesign is required. Enterprises can use any COTS server and create the SD-WAN virtual edge devices. These devices can then be configured for services and policies using the centralized SD-WAN controller.

SD-WAN standardization
 

Rising interest in SD-WAN triggered standardization needs across industry. Metro Ethernet Forum (MEF) has published its first paper in July 2017 introducing the SD-WAN components of an end to end SD-WAN solution. MEF is also coming up with service specifications defining frameworks and APIs for all interfaces of the SD-WAN components.

Conclusion
 

The transport agnostic SD-WAN tunnels and traffic steering capabilities that SD-WAN supports, helps enterprises in saving the expensive MPLS bandwidth. Automation capabilities and centralized monitoring of SD-WAN reduces the deployment and management complexities of enterprise sites. But, SD-WAN might not truly replace MPLS. Enterprises might still prefer MPLS for critical traffic. Hybrid WAN architecture (SD-WAN and MPLS working together) is the most sought after solution today. Standardization efforts in SD-WAN will further enhance the confidence of enterprises to incorporate SD-WAN in their businesses