In the early days of IP WAN’s, bandwidth was limited and expensive, and a vibrant WAN optimization industry arose to improve WAN efficiency and throughput. In today’s era of Internet-based SD-WAN, bandwidth has become plentiful and economical, lessening the need for optimization techniques such as data deduplication (caching) and compression. And many other WAN optimization features have been incorporated into SD-WAN platforms, including traffic shaping, rate limiting, and forward error correction. This begs the question of whether WAN optimization platforms are still relevant today. The answer depends upon how the WAN is utilized and the characteristics of the data transferred over it.
Distance Introduces Latency
Although the modern WAN landscape is quite different than it was in the 1990’s, one thing hasn’t changed: WAN’s can span large distances, and distance introduces latency. Consider a hypothetical company with offices in Boston and Los Angeles, about 4200 km apart. The theoretical minimum round trip delay over that distance is about 42 milliseconds (assuming a perfectly straight path and limited only by the speed of light in an optical fiber). Suppose I want to transfer a 25 GB video file between these locations. If I have a 1 Gbps WAN link, assuming I can use 100% of that bandwidth, I could transfer the file in about 200 seconds (less than 4 minutes). If I have a 10 Gbps link, I could do it in 20 seconds. If I had a 100 Gbps link, I’d only need 2 seconds. Sure, there would be a 21 millisecond delay before the first packet arrived regardless of the available bandwidth, but who cares about that? Throughput depends only on bandwidth, doesn’t it? Depending upon how one transfers the file, there can be a surprising linkage between throughput and latency.
Latency Can Impact Throughput
Perhaps the most common example of this linkage is the TCP protocol, which will only transfer a certain amount of data before receiving an acknowledgement from the receiver. This amount is referred to as the TCP “window,” and it can have a surprising throttling effect on WAN throughput due to latency. If I use Microsoft Windows based FTP to transfer the file, the default TCP window size is 65,536 bytes = 524,288 bits. Dividing this by the round trip delay gives us the maximum throughput I can obtain: 524,288 bits / 0.042 seconds = 12,483,047 bits/second, i.e., just under 12.5 Mbps. At that speed it will take more than 4 hours to transfer the file, even if I had a 100 Gb link!
Dealing With Latency Through Protocol Optimization
WAN optimization platforms recognize protocols like TCP and can intelligently optimize their performance in a manner that’s totally transparent to senders and receivers. Rather than waiting for the far end receiver to acknowledge receipt of a window’s worth of data, the local WAN optimizer on the sender’s side acknowledges it immediately, and this frees the sender to begin transmitting the next window of data. The WAN optimizer then bundles up multiple smaller windows into larger blocks of data and transmits those blocks making full use of the available bandwidth. On the receiver’s side the local WAN optimizer breaks up the larger blocks back into smaller windows for delivery to the receiver. The improvement can be dramatic. Suppose a local acknowledgement can be given in 0.1 milliseconds. This increases the maximum throughput to 524,288 bits / 0.0001 seconds = 5.24 Gbps. Assuming I have a 10 Gb or 100 Gb link I can transfer the file in about 38 seconds.
Today’s enterprises are increasingly global, and locations can be scattered across multiple continents. Left unmitigated, latency due to large inter-site distances can substantially degrade the performance of certain applications and protocols. WAN optimization technology can complement SD-WAN to mitigate these issues, taking full advantage of all available WAN bandwidth to maximize throughput and performance.
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