SD-WAN is an acronym for software-defined networking in a wide area network (WAN). An SD-WAN simplifies the management and operation of a WAN by decoupling (separating) the networking hardware from its control mechanism. This concept is similar to how software-defined networking implements virtualization technology to improve data center management and operation.
A key application of an SD-WAN is to allow companies to build higher-performance WANs using lower-cost and commercially available Internet access, enabling businesses to partially or wholly replace more expensive private WAN connection technologies such as MPLS.
- 1 Overview
- 2 History
- 3 Required characteristics
- 4 Form factors
- 5 Features
- 6 Complementary technology
- 7 Testing & validation
- 8 Marketplace
- 9 References
WANs allow companies to extend their computer networks over large distances, to connect remote branch offices to data centers and each other, and deliver the applications and services required to perform business functions. When companies extend networks over greater distances and sometimes across multiple carriers' networks, they face operational challenges including network congestion, jitter, packet loss, and even service outages. Modern applications such as VoIP calling, videoconferencing, streaming media, and virtualized applications and desktops require low latency. Bandwidth requirements are also increasing, especially for applications featuring high-definition video. It can be expensive and difficult to expand WAN capability, with corresponding difficulties related to network management and troubleshooting.
SD-WAN products are designed to address these network problems. By enhancing or even replacing traditional branch routers with virtualization appliances that can control application-level policies and offer a network overlay, less expensive consumer-grade Internet links can act more like a dedicated circuit. This simplifies the setup process for branch personnel. SD-WAN products can be physical appliances or virtual appliances, and are placed in small remote and branch offices, larger offices, corporate data centers, and increasingly on cloud platforms.
A centralized controller is used to set policies and prioritize traffic. The SD-WAN takes into account these policies and the availability of network bandwidth to route traffic. This helps ensure that application performance meets service level agreements (SLAs).
SD-WAN consists of several technologies combined with newer enhancements. Redundant telecommunication links connecting remote sites date back to the 1970s with X.25 links used for remote mainframe terminal access. Central management of those links with a greater focus on application delivery across the WAN started to become popular in the mid-2000s. SD-WAN combines the two, and adds the ability to dynamically share network bandwidth across the connection points. Additional enhancements include central controllers, integrated analytics and on-demand circuit provisioning, with some network intelligence based in the cloud, allowing centralized policy management and security.
Networking publications started using the term SD-WAN to describe this new networking trend as early as 2014.
Research firm Gartner has defined an SD-WAN as having four required characteristics:
- The ability to support multiple connection types, such as MPLS, frame relay and higher speed LTE wireless communications
- The ability to do dynamic path selection, for load sharing and resiliency purposes
- A simple interface that is easy to configure and manage
- The ability to support VPNs, and third party services such as WAN optimization controllers, firewalls and web gateways
SD-WAN products can be physical appliances or software based.
Features of SD-WANs include resilience, security and quality of service (QoS), with flexible deployment options and simplified administration and troubleshooting.
A resilient SD-WAN reduces network downtime. The technology must feature real time detection of outages and automatic switch over to working links.
Quality of service
SD-WAN technology supports quality of service by having application level awareness, giving bandwidth priority to the most critical applications. This may include dynamic path selection, sending an application on a faster link, or even splitting an application between two paths to improve performance by delivering it faster.
Most SD-WAN products are available as pre-configured appliances, placed at the network edge in data centers, branch offices and other remote locations. There are also virtual appliances that can work on existing network hardware, or the appliance can be deployed as a virtual appliance on the cloud in environments such as Amazon Web Services (AWS). This allows enterprises to benefit from SD-WAN services as they migrate application delivery from corporate servers to cloud based services such as Salesforce.com and Google apps.
Administration and troubleshooting
Management simplicity is a key requirement for SD-WANs, per Gartner. As with network equipment in general, GUIs may be preferred to command line interface (CLI) methods of configuration and control. Other beneficial administrative features include automatic path selection, the ability to centrally configure each end appliance by pushing configuration changes out, and even a true software defined networking approach that lets all appliances and virtual appliances be configured centrally based on application needs rather than underlying hardware.
Online traffic engineering
With a global view of network status, a controller that manages SD-WAN can perform careful and adaptive traffic engineering by assigning new transfer requests according to current usage of resources (links). For example, this can be achieved by performing central calculation of transmission rates at the controller and rate-limiting at the senders (end-points) according to such rates.
SD-WAN versus WAN Optimization
There are some similarities between SD-WAN and WAN optimization, the name given to the collection of techniques used to increase data-transfer efficiencies across WANs. The goal of each is to accelerate application delivery between branch offices and data centers, but SD-WAN technology focuses additionally on cost savings and efficiency, specifically by allowing lower cost network links to perform the work of more expensive leased lines, whereas WAN Optimization focuses squarely on improving packet delivery. An SD-WAN utilizing virtualization techniques assisted with WAN Optimization traffic control allows network bandwidth to dynamically grow or shrink as needed. SD-WAN technology and WAN optimization can be used separately or together, and some SD-WAN vendors are adding WAN optimization features to their products.
WAN edge routers
A WAN edge router is a device that routes data packets between different WAN locations, giving an enterprise access to a carrier network. Also called a boundary router, it is unlike a core router, which only sends packets within a single network. SD-WANs can work as an overlay to simplify the management of existing WAN edge routers, by lowering dependence on routing protocols. SD-WAN can also potentially be an alternative to WAN Edge routers.
SD-WAN versus hybrid WAN
SD-WANs are similar to hybrid WANs, and sometimes the terms are used interchangeably, but they are not identical. A hybrid WAN consists of different types of connection types, and may have a software defined network (SDN) component, but doesn't have to.
Testing & validation
As there is no standard algorithm for SD-WAN controllers, device manufacturers each use their own proprietary algorithm in the transmission of data. These algorithms determine which traffic to direct over which link and when to switch traffic from one link to another. Given the breadth of options available in relation to both software and hardware SD-WAN control solutions, it's imperative they be tested and validated under real-world conditions within a lab setting prior to deployment.
There are multiple solutions available for testing purposes, ranging from purpose-built network emulation appliances which can apply specified network impairments to the network being tested in order to reliably validate performance, to software-based solutions.
IT website Network World divides the SD-WAN vendor market into three groups: established networking vendors who are adding SD-WAN products to their offerings, WAN specialists who are starting to integrate SD-WAN functionality into their products, and startups focused specifically on the SD-WAN market.
Alternatively, a market overview by Nemertes Research groups SD-WAN vendors into categories based on their original technology space, and which are "Pure-play SD-WAN providers," "WAN optimization vendors," "Link-aggregation vendors," and "General network vendors." While Network World's second category (startups focused specifically on the SD-WAN market), is generally equivalent to Nemertes' "Pure-play SD-WAN providers" category, Nemertes offers a more detailed view of the preexisting WAN and overall networking providers.
Additionally, Nemertes Research also describes the in-net side of the SD-WAN market, describing the go-to-market strategy of connectivity providers entering the SD-WAN market. These providers include "Network-as-a-service vendors", "Carriers or telcos", "Content delivery networks" and "Secure WAN providers".
Several online resources, including the networking technology podcast "Packet Pushers", keep an updated list of existing SD-WAN vendors. Network World also recently named 10 hot SD-WAN startups. 
- "SD-WAN: What it is and why you'll use it one day". networkworld.com. 2016-02-10. Retrieved 2016-06-27.
- "Predicting SD-WAN Adoption". gartner.com. 2015-12-15. Retrieved 2016-06-27.
- "How to address WAN jitter issues for real-time applications". networkworld.com. 2012-10-22. Retrieved 2016-06-27.
- "What's slowing down your network and how to fix it". computerweekly.com. 2015-04-01. Retrieved 2016-06-27.
- "Low-latency networks aren't just for Wall Street anymore". gigaom.com. 2012-04-06. Retrieved 2016-06-27.
- "How fast should my Internet connection be to watch streaming HD movies?". HowStuffWorks.com. 2011-10-31. Retrieved 2016-06-27.
- "Software-Defined WAN: A Primer". networkcomputing.com. 2014-09-09. Retrieved 2016-06-27.
- "SD-What? Understanding SD-WAN". techtarget.com. Retrieved 2016-06-28.
- "SD-WAN Vendors Making A Splash". networkcomputing.com. 2015-08-27. Retrieved 2016-06-28.
- "A Brief History of the Enterprise WAN". networkworld.com. 2012-04-06. Retrieved 2016-06-28.
- "Managing the WAN". networkworld.com. 2006-03-13. Retrieved 2016-06-28.
- "SD-WAN: The Killer App For Enterprise SDN?". networkcomputing.com. 2015-07-22. Retrieved 2016-06-28.
- "SD-WAN: Bringing WAN Sexy Back". nojitter.com. 2015-11-02. Retrieved 2016-06-29.
- "Do wide area networks need to get software-defined?". techtarget.com. Retrieved 2015-05-13.
- "How IPsec provides secure communications". techtarget.com. Retrieved 2016-06-29.
- "List of SD-WAN Vendors". packetpushers.net. Retrieved 2016-06-29.
- "Difference Between GUI and Command Line". differencebtw.com. 2015-05-29. Retrieved 2016-06-29.
- C. Hong; et al. (2013). "Achieving High Utilization with Software-Driven WAN" (PDF). ACM SIGCOMM.
- S. Kandula; et al. (2014). "Calendaring for Wide Area Networks" (PDF). ACM SIGCOMM.
- H. Zhang; et al. (2015). "Guaranteeing Deadlines for Inter-Datacenter Transfers" (PDF). ACM EUROSYS.
- M. Noormohammadpour; et al. (2017). "DCCast: Efficient Point to Multipoint Transfers Across Datacenters". USENIX HOTCLOUD.
- M. Noormohammadpour; et al. (2018). "QuickCast: Fast and Efficient Inter-Datacenter Transfers using Forwarding Tree Cohorts". IEEE INFOCOM. Retrieved January 23, 2018.
- "Why SD-WAN is the next breed of WAN optimization". techtarget.com. Retrieved 2016-06-29.
- "Citrix Combines SD-WAN, WAN Optimization In Single Appliance". packetpushers.com. 2016-03-16. Retrieved 2016-06-29.
- "Definition: edge router". techtarget.com. Retrieved 2016-06-29.
- "Definition hybrid WAN". techtarget.com. Retrieved 2016-08-22.
- "SD-WAN providers: Consider the small with the big". SearchSDN. Retrieved 2017-11-01.
10 hot SD-WAN startups to watch