Last week, I explained the growing importance of understanding TCP basics -- specifically, TCP windowing. As companies deal with ever larger amounts of data that need to move across the WAN, whether between premises or between a premise and the cloud, it becomes increasingly important to optimize the basics to achieve the best performance. To ease this process, a variety of networking vendors sell so-called WAN accelerators. However, they're no panacea, suitable to only certain environments.
The truth is that these vendors and their customers are often plagued by a general lack of understanding about what the devices can and can't do. You might even assume WAN accelerators to be the snake oil of the Internet age -- surely you can't get more than 10Mbps of throughput on a 10Mbps circuit, right? On the other hand, you often hear the notion that WAN accelerators are techno-magic devices that can make anything faster. As is usually the case, neither extreme turns out to be true.
WAN accelerators are great in some situations and at best unhelpful in others. Furthermore, not all WAN acceleration devices and software stacks do the same things. Read on to understand when WAN accelerators really deliver faster WAN networking.
WAN acceleration basics
The first thing to understand is that most WAN acceleration devices are deployed in pairs, one at each of the sites between which the bandwidth is to be optimized. You need a device at each site because WAN acceleration devices optimize traffic in a way that renders such traffic unintelligible to the original destination. It's like encryption, where you need a companion decryptor at the other end to make the encrypted data intelligible. Thus, a WAN accelerator must be present at the receiving end to return the network traffic to its original form.
When you're trying to accelerate network traffic between two sites, you'll have these WAN accelerators at each gateway. But you can also implement WAN acceleration on users' PCs, to accelerate the traffic from the office to wherever they are. Likewise, for acceleration between your corporate network and the cloud, you'd have a WAN accelerator deployed in the cloud service as a virtual appliance or use the native WAN acceleration support offered by some cloud service providers.
In these scenarios, you need a WAN accelerator on each end of the connection. That means WAN acceleration will do little to nothing to speed up general Internet access. It's meant for speeding up specific network connections.
Protocol optimization
One of the most basic features a WAN accelerator can implement is TCP optimization. WAN accelerators can overcome inappropriate endpoint TCP configurations by terminating the TCP connections crossing them locally at each end and using separate, optimally configured connections in between instead.
Optimizations typically include using very large TCP windows through receive-side scaling, appropriately implemented selective acknowledgements, and optimized congestion-avoidance response mechanisms.
Selective acknowledgements let the destination station acknowledge portions of a TCP window without having received all of it. When you're using very large TCP windows, selective acknowledgement prevents the entire window from having to be resent if there is packet loss. It can also prevent the entire window from being "filled," so the sending station can transmit data constantly -- diminishing the impact of link latency.