We’ve written a lot about how Netflix takes up a gigantic share of Internet traffic. During peak viewing hours, Netflix accounts for about a third of all bits sent to Internet users in North America on “fixed” connections—that is, cable, DSL, fiber, or satellite, but not cellular.

But Netflix users also send a ton of data upstream, so much so that Sandvine’s latest Internet Phenomena Report puts Netflix at 9.48 percent of all peak upstream traffic on North American fixed Internet services, second only to BitTorrent's 25.49 percent. Sandvine, a maker of equipment that helps consumer broadband providers manage network congestion, defines “peak” hours as those when network usage is within 95 percent of its daily maximum, typically from 7 to 11pm.

It isn’t new that Netflix is both an upload and download monster. But for some reason, its share of uploads went up substantially in the latest measurement while downloads remained level. The twice-annual report had Netflix accounting for 6.44 percent of peak upstream traffic and 34.21 percent of downstream traffic in the first half of this year, while the newest report has Netflix at 9.48 percent of upstream and 34.89 percent of downstream:

The change may not be a permanent thing, as it has fluctuated before. For example, Netflix’s upstream rate was as high as 7.7 percent back in fall 2011, while the service’s downstream rate was 32.69 percent.

Sandvine spokesperson Dan Deeth said he’d prefer to see a major change happen in consecutive reports “before blowing the whistle on anything too crazy.”

Download traffic is much heavier than upload traffic on the whole, but they both increased at about the same rate this year. The average Internet subscribing household in North America uploaded 7.6GB of data per month in Sandvine's report in the first half of this year, increasing to 8.5GB in the latest report, a boost of 11.8 percent. The average household's monthly downloads increased from 43.8GB to 48.9GB, a boost of 11.6 percent.

Peer-to-peer Netflix? Not quite

Netflix has researched peer-to-peer technology that could dramatically boost the amount of data its customers send, but hasn’t rolled anything out. Netflix has also facetiously suggested that if its users uploaded as much data as they downloaded, Internet service providers would be forced to offer the company free network interconnection.

So how do Netflix customers send so much data today? The answer is mostly in “ACK packets,” Deeth said. Signifying “acknowledgement” that data has been received, ACK packets are part of the TCP’s (Transmission Control Protocol’s) three-way handshake that connects client devices to servers over the Internet. The third step of the process has the client (i.e. a Netflix subscriber) sending acknowledgement back to Netflix’s servers.

These ACK packets are so numerous that they can sometimes interfere with downloads. Internet service plans with upload speeds that are much smaller than download speeds exacerbate this problem, Sandvine wrote in its fall 2011 report.

“If upstream is so heavily utilized that ACKs fail to reach the sender, then TCP responds by backing off its sending rate, which to the subscriber means reduced downstream speeds and will ultimately manifest as a downgrade in video quality,” Sandvine wrote.

As mentioned earlier, Netflix was responsible for about 7.7 percent of peak upstream traffic in North America at the time of the fall 2011 report. “Netflix transmits little (or no) data on the upstream, so this 7.7 percent is purely in the form of TCP ACKs,” Sandvine wrote. “If these ACKs are unable to quickly return to the originating server, then the TCP streams carrying the subscriber’s video will slow down. In turn, this will manifest to the subscriber as a downgrade in their quality of experience.”

We asked Netflix if there’s any apparent reason for its increased upload rates, but the answer was no.

“After a full day of going back and forth with the engineers, the best we can say is, we don't know why the upstream number is higher,” Netflix spokesperson Anne Marie Squeo told Ars. “We haven't meaningfully changed how we deliver content.”

Each Sandvine report is based on a month's worth of anonymized data collected from a "representative cross-section of the world’s leading fixed and mobile communications service providers."

The numbers are quite a bit different during peak hours on North American cellular networks. There, Netflix takes up 4.51 percent of download traffic and 0.92 percent of upstream.

Netflix streamed 6.5 billion hours of video worldwide in the first three months of 2014, the company reportedly said earlier this year. Netflix download speeds range from 0.5Mbps to 25Mbps, with the average stream around 3Mbps on top-performing ISPs. Overall, Netflix distributes multiple terabits per second.

Netflix doesn't operate in all countries. In Europe, it accounted for just 3.44 percent of peak downstream traffic on fixed networks. Some users watch Netflix by using software that masks their location, however. Sandvine found that "on [one] fixed network in Australasia, where Netflix isn’t even available yet, approximately 2.5 percent of subscribers are accessing the service and it comprises as much as 4 percent of peak downstream traffic."

BitTorrent declines, Amazon Prime rises

Streaming video is continuing to shine compared to peer-to-peer file sharing in North America. While BitTorrent accounted for 25.49 percent of uploads on North American fixed networks, its 2.8 percent share of the much bigger download category resulted in an aggregate traffic share of 5.03 percent.

"As observed in previous reports, BitTorrent continues to lose share and now accounts for just 5 percent of traffic during peak periods," Sandvine wrote. "In our last report, we revealed that file sharing as a whole accounted for less than 9 percent of total daily traffic, and that trend continues with file sharing now responsible for just 7 percent of daily network traffic. This demonstrates a sharp decline in share from the 31 percent of total traffic we had revealed in our 2008 report."

UPDATE: BitTorrent, Inc. spokesperson Christian Averill contacted Ars and pointed out that BitTorrent's share of Internet traffic has declined at least partly because of the Micro Transport Protocol (µTP) deployed in 2008. "µTP is a congestion aware protocol that senses when the network is overloaded and yields traffic to other applications," Averill said. "It’s not a good thing to be #1 on this report, it means you're hogging bandwidth and probably doing so inefficiently. But it is a good thing that BitTorrent traffic went from 30 percent of traffic during peak hours in 2008 down to 5 percent during peak hours in 2014. It means µTP is working as intended."

Meanwhile, there's good news for Amazon. "Amazon Instant Video has established itself as the second largest paid streaming video service in North America," despite not officially being available in Canada, Sandvine said. "While still only accounting for 2.6 percent of downstream traffic, its share has more than doubled in the past 18 months."