Search
Close this search box.

Blog

SD-WAN and Forward Error Correction: Mitigating Packet Loss

Rich Vidil looks at how Forward Error Correction works and describes how leading SD-WAN platforms utilize it to mitigate packet loss.
October 29, 2020

SD-WAN and Forward Error Correction: Mitigating Packet Loss

SD-WAN improves the performance and reliability of IP networks, and one of the techniques it employs is Forward Error Correction (FEC). This blog post explains how FEC works and describes how leading SD-WAN platforms utilize it to mitigate packet loss.

We begin with a reminder: packet delivery on IP networks is best effort, not guaranteed. This was a deliberate design choice by the inventors of TCP/IP to minimize router cost and maximize efficiency; if an IP network experiences congestion and router buffers fill up, packets may be dropped, and impacted applications must respond appropriately.

Dealing with Packet Loss

One way that applications deal with packet loss is by implementing a bidirectional protocol between senders and receivers wherein packets are marked with sequence numbers by the sender (i.e., “packet 1,” “packet 2,” “packet 3,” …). This makes it easy for a receiver to identify which packets failed to arrive, and it can send messages back to the sender requesting those packets be resent. The Transport Control Protocol (TCP) portion of TCP/IP is a general-purpose implementation of this scheme, and it works well for many applications, including mainstays like World Wide Web (HTTP) and Email (SMTP).

Unfortunately, there are some applications for which this approach doesn’t work, notably real-time voice and video. For these applications, delivery is time-sensitive; there is no opportunity to resend. When voice or video packets are lost, the results are garbled speech and pixelated video. Can anything be done to mitigate this? Is there some scheme by which a receiver could recover a lost packet in a timely fashion, without waiting for the sender to resend it? The answer is yes, and that scheme is FEC.

How FEC Works

FEC is not a new concept. In fact, its origins go back more than seventy years to the early days of digital communications, where it was developed to detect, and correct bit errors caused by noisy communications channels such as analog telephone lines. It accomplished this by inserting additional error-correcting bits into the data stream, and provided certain conditions were met, receivers could use these additional bits to detect and correct bit errors in the data.

FEC applied to IP networking uses a similar approach: senders generate and send special error-correcting packets along with the regular data packets, and provided certain conditions are met, these additional packets enable receivers to regenerate lost data packets without retransmission. This sounds complicated, but it’s surprisingly easy to implement.

FEC in SD-WAN

One leading SD-WAN platform generates one error-correcting packet for every four consecutive data packets by default. The contents of this error-correcting packet are the bitwise exclusive OR (XOR) across the associated data packets. This implements an even parity scheme, and the error-correcting packet is sometimes referred to as a parity packet. Depending upon network availability and conditions, the parity packet might be sent using the same underlay network as its associated data packets, or it might be sent using a different underlay network. As with TCP, the data packets are marked with sequence numbers to make it easy to detect missing packets.

On the receiving end, the SD-WAN platform examines the sequence numbers of the arriving data packets to determine if any are missing. If all of them were delivered, the corresponding parity packet is silently discarded, and the data packets are delivered to the LAN. But if one of the data packets is missing, it is quickly regenerated by performing the same bitwise XOR operation used by the sender, this time across the remaining three data packets plus the parity packet.

Adaptive FEC

This parity scheme can recover one lost packet in an associated group of data packets; if two or more packets in the group are missing, they cannot be recovered this way. For this reason, the ratio of data packets to parity packets must be small enough so that multiple missing packets in a group is unlikely. At the same time, it should be large enough to minimize the bandwidth overhead of the parity packets. Leading SD-WAN platforms adjust this ratio dynamically based on measured packet loss of their underlay networks. This adaptive approach allows SD-WAN to strike an optimal balance between effective error correction and bandwidth overhead.

Conclusion

FEC is a great example of SD-WAN improving the performance of IP networks by applying application-layer intelligence to IP routing. During packet loss conditions in its underlay network(s), FEC is automatically invoked, and the effective rate of packet loss experienced by applications is significantly reduced. This translates into better audio and video quality and improved application throughput.

About GTT

GTT connects people across organizations, around the world, and to every application in the cloud. Our clients benefit from an outstanding service experience built on our core values of simplicity, speed, and agility. GTT owns and operates a global Tier 1 internet network and provides a comprehensive suite of cloud networking services. We also offer a complementary portfolio of managed services, including managed SD-WAN from leading technology vendors.

Related RESOURCES

Blog

5 Common Ways Cybercriminals Spread Malware (and How To Outsmart Them)

There are currently around 1.2 billion malicious programs and potentially unwanted applications in existence, with threat actors deploying an average of 200,454 unique malware scripts ...
How is ESG part of working at GTT?
Blog

How is ESG part of working at GTT?

At GTT, we are passionate about supporting people – and it is that passion that drives our commitment to enhancing and growing our environmental, social ...
The Power of Training, Mentoring and Learning at GTT
Blog

Unlocking My Full Potential: The Power of Training, Mentoring and Learning at GTT

In a world where the pursuit of personal and professional excellence is a constant endeavor, I’m proud to work at GTT. Its mentoring and learning ...
DDoS attacks
Blog

DDoS Attacks in the First Half of 2024

Recent DDoS attacks including the Port of Seattle and Microsoft are showing that Cybercriminals are becoming more bold and attacking organizations regardless of size or ...
Young female engineer concept. GUI (Graphical User Interface).
Blog

How to Choose The Right SD-WAN Managed Service Provider for Your Business

Many factors can influence an enterprise's decision regarding which MSP to work with for SD-WAN, SSE and a SASE framework. But most agree on which ...
Blog

Heightened DDoS Activity: A Growing Concern in 2024

In January and February of 2024, GTT’s Security Operations Center along with Corero’s Advanced Research Team have jointly recorded a significant surge in distributed denial-of-service ...
Blog

What’s Next For Networking?

The combo of faster & more efficient connectivity with new networking technologies & insights, productivity, network performance, & efficiency will all improve, providing additional cost ...
Blog

Global Tier 1 IP Networks: Everything You Need To Know

Tier 1 IP networks are the first choice for global enterprises for dedicated, reliable & secure internet access. Learn why GTT is the ideal Tier ...
Blog

The Future of IT in Manufacturing: Managed SD-WAN Solutions

GTT offers cutting-edge Managed SD-WAN for manufacturing companies. Discover how you can take a step toward the future of Manufacturing IT and networking.

OUR GARTNER RATING

Gartner Peer Insights logo
4.2


62 Reviews

88%
as of the last 12 months

Talk to an Expert

Interested in learning more about GTT products & services? Please complete this short form to schedule a call with one of our sales consultants.

Thank you for your information. One of our sales consultants will be in touch with you.

Scroll to Top