Considerations for deploying a multi-CDN strategy on your video platform

As online video consumption continues its ascent, new challenges for content publishers are always around the corner. In a highly competitive market where streaming failures make headlines, redundancy and quality of experience have never been more important.

To ensure consistent high-quality video, an increasing number of broadcasters are turning to multi-CDN strategies, deploying several CDNs in their video delivery workflow.

Why opt for a multi-CDN strategy?

While multi-CDN has become the norm across large streaming services, there are several different reasons content owners choose to implement such a solution.

Improve performance: Using multiple CDNs can help ensure the best coverage by opting for the CDN that performs best in each country, region or ISP. In addition to using a CDN in the region it is strongest, a multi-CDN strategy can also help alleviate local network congestion and help ensure viewers receive their video from the best source available. Performance-based strategies vary in sophistication from static geography-based switchers that choose CDNs based on viewer location, to tools that make data-driven choices based on real-time quality statistics or even local device feedback. By routing traffic to the most efficient content source, these systems can improve availability, latency, start-up times, and throughput on a large scale.

Redundancy: When the press is more than happy to publicize service outages, diversity has never been so important, especially for high-profile events that can attract hundreds of thousands or millions of viewers. Using several CDNs strengthens reliability and can offer an efficient failover mechanism: if one CDN fails, another one can take over to provide continuity.

Cost management: Multi-CDN can help broadcasters optimize commit usage with each of their CDN vendors by being able to switch from one provider to another when thresholds are met. Moreover, the option of sending traffic to different CDNs can help encourage content delivery networks to compete on a performance basis for publishers’ business.

How to implement a multi-CDN strategy

Whatever your reasons are for adopting a multi-CDN strategy, following the steps below can help you avoid potential pitfalls and get the most out of your video delivery workflow.

First, remove any dependencies that tie you to a specific CDN. Make sure to have a unified origin that can be used by different CDNs, which are configured as edges only. Then, whether it’s DRMs, special video players or transcoders, your entire workflow needs to be compatible with any CDN. At this stage, you will also want to make sure your multi-CDN switching tool can work with different tokens.

Next, plan your switching strategy.

The policies that govern CDN selection and switching is the most important part of a successful multi-CDN strategy. Establishing the right rules for your use case involves looking holistically at your goals: business criteria, cost considerations and quality expectations. Your multi-CDN strategy can be based on different types of decision criteria:

Static business rules statically determine what CDN is used for specific stream types, geographies, ISPs or devices. These rules may be applied at all times or can vary depending on the time of day, season, or other criteria.

Commit-based rules switch CDNs according to the bandwidth consumed during the reference period, taking into consideration overage rates and volumes remaining to reach the commit. Applying these criteria requires either having access into APIs into CDN traffic data or managing switches manually. When an API is not available, broadcasters may opt to set up approximate usage thresholds for each CDN in advance.

Global QoS-based rules choose the best performing CDN by querying a database of QoS statistics. Depending on the source, these can come from real-time CDN metrics such as latency, RTT, and errors, or player metrics such as buffering, bitrate and startup time. The dataset can come from either the global data of an analytics provider, or from a custom set of QOS data from your own audience.

Global information can be a low-cost option for those with tight budgets. However, it can include aggregation biases that are not relevant to your specific traffic and can therefore prove highly inaccurate. Broadcasters with significant traffic and the right budget should opt for statistics from on their own audience.

Per-device QoS-based rules take switching to the next level. As its name suggests, global QoS-based rules make switching decisions according to an average of how a CDN handles millions of different videos under different (and changing) conditions, geographies, ISPs, infrastructure, and times. This average doesn’t necessarily provide a meaningful insight when it comes to an individual viewer’s experience. By contrast, multi-CDN switchers that reside on the client-side – within the player – are able to make independent switching decisions according to actual QoS feedback from the device, allowing you to optimize delivery for each user.

Once you identify your decision criteria, you’ll need to prioritize them. Should switches be first based on price? Quality of service? Geography? Should different regions apply different policies? Should different devices or stream types use different CDNs? A decision tree as in the following example can be a good way to articulate your strategy:

Types of multi-CDN switching tools

Although there are several solutions on the market, some broadcasters choose to create their own tool. This in-house approach allows for manual, or in some cases, automated switching, and can do the trick when applying a simple multi-CDN policy. However, many content publishers do not have the resources to create and maintain a sophisticated solution that can handle complex rules balancing multiple dynamic criteria. Switchers built in-house are most often used to apply a cost-based policy, to switch between CDN vendors according to commit, or to implement basic geographical rules.

Whether you opt for an in-house or a third-party solution, it’s important to understand how various types of multi-CDN switchers work and how they differ from one another.

Multi-CDN aggregators centralize multiple CDNs and manage all aspects of the decision-making through a dashboard. This method can work for publishers who don’t have the time or resources to dedicate to administering a dynamic CDN policy, who don’t require a lot of customization and who are willing to give up control on how switching decisions are made.

DNS solutions switch CDNs by changing the DNS entry for the stream URL. These solutions are simple to implement, as the video source URL remains constant. In this case, the switch delay depends on the DNS entry TTL (time-to-live), meaning that this is not necessarily the best choice for QoE. It is also not ideal from a redundancy perspective: DNS-based switching simply replaces one single-point-of-failure – the CDN – with another – the DNS resolver. If this service fails, everything else can go down with it.

The on-the-fly manifest rewrite with a proxy method switches CDNs by rewriting the manifest in real-time. The manifest is edited on-the-go and sent to the viewer’s player whenever there is a switch, changing the video segment URL within the manifest according to the current best CDN. This solution can enable midstream switching in certain use cases. It also eliminates the risk of a cascade effect over your video workflow that can be caused by thousands of simultaneous session resets. However, similarly to DNS-based solutions, here too we replace one single point of failure with another. In addition, rewriting the manifest can be risky as any error can be detrimental to your stream. And although the switch is done instream, it is not completely seamless; it can take more than a few seconds for the server to learn (from the client) about the unavailability of a CDN.

Server-side API switchers reside server-side in your back office. Each time your webpage loads, your backend sends the most appropriate CDN according to static internal geography, business or cost rules, or dynamic third-party QoS data. These solutions are easier to control since any changes can be done directly on your server; however, they can add some delay in page load time. They also make less accurate QoS-based switching decisions, as they act based on aggregated data and not individual QoS feedback from the device.

Client-side CDN switchers sit within the video player. They make QoS-based decisions based on real-time conditions perceived by the individual user device. Unlike the other solutions mentioned, they also enable midstream switching on a segment-by-segment basis, which can offer a seamless viewing experience for the viewer and no need for a hard refresh in the event of a CDN failure. This method offers the most personalization when it comes to QoS; however, it requires an integration into the video player and is not something that is easily built in-house, as the decision-making algorithm requires detailed planning, experimenting, and tweaking. Among other things, you’ll need to decide, for example, what bitrate is tolerable for the specific device and player size, how many CDN errors should trigger a reroute, etc.

Choosing the right multi-CDN switching tool is the final and most important step, as it relies greatly on your business needs and priorities. However, even when these are well defined, the challenge remains; as you can see, it is not always simple to find a solution that ticks all the boxes.

For those looking for a simple yet effective tool that optimizes quality of service even midstream, Lumen has built CDN Load Balancer, a client-side midstream CDN switcher. This unique tool applies broadcasters’ business policies in tandem with individual device-based QoS load-balancing.

Building on Lumen’s expertise in CDN and media delivery solutions. CDN Load Balancer can dynamically select the best video source according to specific user device metrics, personalizing delivery to the individual viewer throughout the video session.

For more information on CDN Load Balancer, check out our product page or peruse our technical data sheet. For a live demo or to try it free of charge* on your video platform, drop us an email at application.delivery@lumen.com.

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