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Industry Insights

Sophisticated Path Selection Capabilities Underpin SD-WAN Performance

By Versa Staff
Versa Networks
August 13, 2019

SD-WAN has many capabilities that address the challenges associated with complex network edge infrastructure. Specifically, central configuration and management, zero-touch branch deployment, and automatic monitoring and path selection.

SD-WAN’s ability to optimize pathways for different traffic types is a key advantage, which benefits IT with greater efficiencies, and unprecedented WAN flexibility, availability, reliability and performance. Today’s advanced cloud-managed SD-WAN solutions give enterprises the ability to dynamically connect branch offices, on-premises data centers, public/private cloud and SaaS, on a global scale. Removed are the carrier dependencies and resource restrictions that used to limit the way enterprises consume applications.

Leveraging multiple links of any type, any location and any service provider, offers multiple benefits, including lower cost, time savings, and the agility required to accelerate business, by rapidly taking advantage of dynamic opportunities. SD-WAN automatically determines congestion issues based on policies, and proactively allocates diverse traffic types to the most appropriate paths.

Basic SD-WAN path selection provides a foundation for link redundancy and failover. When the WAN becomes virtualized, multiple links of any type become a single network, with the entire aggregated bandwidth managed and segmented based on business policies.

But the market has moved way beyond table stakes for MPLS cost arbitrage. A new generation of SD-WAN solutions is more intelligent, alloy more metrics and criteria and optimize traffic delivery with greater levels of path selection and control.

How sophisticated path selection capabilities work

Next generation SD-WAN’s offer more sophisticated path selection capabilities on the edge devices that continuously monitor links, transport paths and application performance on a per traffic-class basis, using real-time data traffic to calculate performance. The common metrics and criteria these SD-WANs use for path detection and switching are packet loss, latency, jitter, MOS, PESQ and hard-down.

Each SD-WAN edge device monitors round-trip delay for a service frame, which includes delay variation, loss ratio (the percentage of service frames that are not delivered), and the availability, as measured by the percentage of time the path was in a connected state. Active monitoring provides sub-second path failover and recovery.

These SD-WAN solutions can also conduct path selection monitoring for SaaS applications, using both active and passive probes. Dynamic traffic engineering and application-specific link selection can be based upon:

• Local SD-WAN traffic steering policy configuration
• Local application QoS configuration
• Access circuit state and status
• Information about latency, jitter and packet loss

By utilizing information about latency, jitter and packet loss for non-VPN sites, like SaaS, and other sites, such as YouTube and Netflix, over various access circuits, each branch device builds a database with key traffic engineering information. Paths through which SLA responses are not received are

considered to be path-down and are made non-available for SD-WAN forwarding. The edge network reacts in real-time, based on the defined SLA requirements of the applications.

An advanced SD-WAN will also include voice and video Codecs to analyze the real user experience of each voice and video session and supports RTP and SRTP-based voice and video applications; that class of information provides ongoing database updates of application identification signatures and Codecs.

A composite path selection score is used, that takes into consideration TCP parameters, MOS-like scoring, round-trip-time, round-trip-delay, jitter, delay, loss and application performance metrics. Machine learning-based scoring for application policies can also be applied. In doing so, the SD-WAN will learn the network characterizes and anomalies, and continuously optimize the path selection capabilities.

Progressive SD-WAN solutions have self-healing features, and architectures that virtualize edge networking and security functions within the enterprise WAN and multi-cloud networks; they are application-aware and user-experience driven, monitoring applications and the network to optimize traffic delivery, and provide a robust security posture. Automation is the key to responding to policy-based changes, and dynamically adapting WAN infrastructure, to deliver consistent uptime, application reliability and an optimal user experience.

Key capabilities of advanced SD-WAN solutions with sophisticated path selection:
SLA Monitoring

SD-WAN branch devices continuously monitor the performance of all paths. A branch-to-branch path is any valid transport tunnel between the two branches. For example, if two branches have two broadband links each, and the branches are in a single transport domain, there are four paths between those branches.

Adaptive Monitoring

When SLA monitoring is configured on a WAN interface, the monitoring of paths to every neighbor link learned through Multiprotocol BGP starts automatically, which in the case of a full-mesh topology with numerous branches, such monitoring can result in a large amount of SLA traffic. To reduce the amount of SLA traffic monitoring on the network, adaptive monitoring will perform SLA monitoring only to neighbors that are actively passing traffic

Data-Driven SLA Monitoring

Data-driven SLA monitoring is an extension of adaptive SLA monitoring that regulates the amount of traffic monitoring between branches. It accomplishes this by creating and deleting SLA-monitoring contexts, based on whether traffic is flowing toward a remote site.

Alternate Path

When data-driven SLA monitoring is enabled, an alternate path to a destination branch is specified, accomplished by assigning one branch device as a hub that forwards traffic between branches. While a new SLA monitoring context is being created on the direct path between two branches, the alternate path is used to send the initial packets of a flow towards the destination branch.


Packet replication improves the quality of voice traffic, and other mission-critical application traffic. The SD-WAN nodes mirror packets among two or more paths. If a packet is lost on one link, the mirrored packet is delivered on secondary links. If the remote device receives more than one copy of the packet, it sends the first received packet toward the LAN, and drops subsequent packets.

Forward Error Correction

Forward error correction (FEC) controls errors in data transmission that occur over unreliable or noisy communication channels. The sender encodes the message using an error-correcting code (ECC) and does so in a redundant manner. The redundancy allows the receiver to correct errors without having to request, over a reverse channel, that the sender retransmit the lost data. For FEC to work, the sender generates an FEC parity packet for every N packet it sends. On the sender, administrators configure the frequency at which FEC parity packets are generated. The receiver uses this parity packet to recover any lost packets. In this way, FEC minimizes packet loss at the receiving end, improving the end user’s quality of experience.

A modernized WAN makes business more effective

SD-WAN gives administrators the ability to centrally distribute rules, policies and configurations across a network of distributed branches of any size, within seconds. Path selection provides the assurance that the network will automatically take action to avoid congestion and failure issues.

When we modernize WANs with sophisticated path selection capabilities, enterprises empower IT to operate edge network connectivity to meet business requirements, rather than managing network equipment to meet functional requirements.

Leveraging SD-WAN solutions that use dynamic path selection policies to optimize how traffic moves between branches, data centers and clouds is critical to accelerating digital transformation strategies and migration to multi-cloud and public Internet transport mechanisms.. Rather than reacting to network problems, SD-WAN proactively monitors and automatically selects the best path based on business policies to maximize availability, reliability and performance.