Since reporting on Netflix data that shows months-long declines in streaming performance on Verizon and Comcast, we've heard from several readers who say they've eliminated video problems by using VPN services.

"Netflix on Comcast tanked for us in the past month or two," one commenter wrote. "We would spend more time buffering than actually watching video. Once we set up a VPN with UnblockUs it magically went away."

Another reader sent us bandwidth statistics from his home that compared traffic on Comcast to traffic over a VPN, saying the poor Comcast performance proves that "they throttle Netflix heavily." Similar accusations have been leveled against Verizon.

No one has actually shown beyond a shadow of a doubt that ISPs directly throttle Netflix traffic. Throttling may be legal in the US now that Verizon has convinced a court to strike down net neutrality rules, and Netflix has criticized ISPs, but it hasn't gone so far as to allege throttling. Netflix performance does vary by ISP, but we can't rule out the possibility that Netflix's own practices are causing bad outcomes for consumers. It's also possible that no one is doing anything particularly nefarious.

What's clear is that somewhere in the path from Netflix servers to consumers' homes, there is congestion. It's also clear that some people have improved their own streaming video by using VPNs (virtual private networks) or third-party DNS (Domain Name System) services. (UnblockUs, mentioned above, is not technically a VPN service but it achieves a similar effect by changing your DNS settings.)

There's no guarantee that using a VPN will improve things—it could even make them worse. But if you're desperate, it may be worth a try.

Routing around congestion

Sandvine cofounder and CTO Don Bowman talked to Ars this week about why VPN and DNS services can help relieve congestion for certain users. He also talked about the types of shady tactics both video streaming providers and ISPs could employ in their ongoing battles and how that could harm home Internet users.

Sandvine is a maker of equipment that helps consumer broadband providers manage network congestion, a controversial practice several years back when it came to Comcast's use of Sandvine products to limit peer-to-peer file sharing traffic, which led to a 2008 agreement settling the matter between Comcast and BitTorrent. Sandvine also publishes research on which applications drive growth in Internet usage, showing that Netflix and YouTube together account for more than half of peak downstream traffic on North American fixed networks.

VPN services route an Internet user's traffic through data centers in different locations and are usually sold to people who seek anonymity or to evade country-specific blocks (such as video content blocked in certain countries).

As a side effect, a VPN may route your traffic away from congested servers and links that would normally serve up video to your home. Netflix and YouTube store video caches in many locations, and data can take multiple paths to its final destination.

"Imagine you're in the US and that you are on a carrier that existed in multiple states and time zones, you can VPN to the West Coast from the East Coast and end up getting the idle servers that are there, just sitting and waiting for people to get out of school and off work and so on," Bowman said.

BTGuard and Private Tunnel are examples of VPN services. If you consistently get better performance with a VPN than without, it could be a sign of a "systematic dispute between vendors upstream." In the past, Verizon and Comcast have each had disputes with bandwidth providers over whether the ISPs should be paid to carry Netflix traffic.

These disputes can lead to the links between networks not being upgraded quickly enough. This affects all traffic, but is more noticeable for bandwidth-hungry streaming video.

"If a VPN consistently behaves better, it's proof that there is something along the path that doesn't have enough capacity," Bowman said. "It might be a link [between providers] or it might be a server. There may not be enough servers in the server farm, there may not be enough capacity between the two carriers."

This would not be proof that Verizon or Comcast is throttling Netflix or any specific application.

"The only way you could draw that conclusion is if you guaranteed [traffic] came from the same path and server but that somehow the VPN made it look different," Bowman said. "If you could VPN to the same server you were getting the streaming from, using exactly the same path and then it behaves better, that would be pretty conclusive proof."

It's important to know that Netflix and your ISP aren't the only two companies responsible for carrying traffic. Bits have to travel across numerous networks, and problems could crop up in any of them.

"Streaming content exists on a set of CDN (content delivery network) providers from more than one company, and they in turn have many servers in each location, and they have multiple locations, and each of those locations has a set of intermediate ISPs which connect to a set of intermediate ISPs which connect to the eyeball ISP you're on," Bowman said. "It's very difficult for even networking professionals who are in those locations to know how a packet will flow."

CDNs can take multiple paths to a consumer ISP and will hit transit networks on the way there. The receiver of content is responsible for showing how it can be reached, while the sender is responsible for how it reaches the endpoint when there are multiple choices, Bowman explained. Using a VPN can force the sender to make different choices than it otherwise would. Sandvine research suggests that there are generally no systemic problems within consumer ISP networks—it's the interconnects between networks where trouble arises.

Using a VPN takes your traffic away from the shortest path by distance, but may be faster in cases when the path would otherwise be congested. Networks generally aren't intelligent enough to automatically route around congestion.

"A network is based around packet switching, and every packet is treated independently," Bowman said. "At each location it's got a set of ways to get to its next location. It doesn't know that two hops down it gets busy."

In the future, it may be easier to pinpoint the source of problems. Speed test tools from Netflix and Google today show averages across wide swaths of users. Bowman said he'd like to see more advanced tools that benchmark the interconnections between specific network providers.

You're rolling the dice by using a VPN, betting that you might get a faster path. That bet may backfire. "In general it makes things worse for the user," he said. "There's a reason they're trying to do this optimization, to give you the best experience, and you're breaking it."