(Credit: PJ McDonnell via Shutterstock)
Starlink is well-known for avoiding hard data caps. But SpaceX is disclosing a technical caveat that can disrupt a user’s online activities during periods of abnormally high usage.
A new support page warns that Residential and Roam plan subscribers could experience degraded performance if they open too many active sessions on their internet connection.
“Starlink Residential and Roam plans use Carrier Grade Network Address Translation (CGNAT). These plans have a limit of 1,200 concurrent sessions. A session is a connection using either UDP or TCP,” the page says. “When the 1,200-session limit is reached, new sessions automatically cause the oldest sessions to be dropped.”
The page goes on to warn that interruptions can occur “especially with applications that use many simultaneous connections,” such as VoIP calls, video conferencing, online gaming, and VPNs. “Many modern applications, especially real-time communication tools, require a high number of active sessions. When the limit is exceeded, important connections can be terminated without warning,” the support document adds.
(Credit: Starlink.com)Starlink’s use of CGNAT isn’t new. In the past, a few users have posted about Starlink customer support telling them about the 1,200-connection limit. But now the company is openly acknowledging the issue, even though it’s rare for ISPs to mention the constraint.
“What appears to be new is that Starlink is documenting it explicitly on its support page, not necessarily the limit itself, as I believe that Starlink likely already had such a limit in place before,” says Jinwei Zhao, a PhD student at the University of Victoria in British Columbia, Canada, who studies Starlink and satellite internet systems.
CGNAT was created as a workaround to address the IPv4 address shortage. ISPs need to assign IP addresses to user devices to route internet traffic, and CGNAT creates a way for all kinds of internet service providers to share a single IPv4 address across multiple subscribers. Starlink also supports newer IPv6 addresses, but IPv6 isn't backward-compatible with IPv4.
"Many websites still lack IPv6 support. A notable example is GitHub, the largest developer platform," along with older legacy sites, Zhao says. (A Starlink support page also notes that subscribers on the Starlink Residential and Roam plans will use the 100.64.0.0/10 prefix as their default IPv4 configuration.)
(Credit: putilich via Getty Images)The problem is that CGNAT comes with some limitations. The standard’s documentation encourages ISPs to limit active sessions to prevent users from hogging network resources. Hence, Starlink’s 1,200-active session limit.
Daryll Swer, a networking expert and consultant at Swer Networks, added that CGNAT is “required to scale limited v4 address space resource to millions of customers on large ISP networks.” For example, a small ISP may have only a few hundred IPv4 addresses to allocate among 50,000 subscribers, he said.
Still, Swer noted that it would be rare for a Starlink subscriber to hit 1,200 active sessions. He estimates that about 80% of the internet traffic for an average Starlink customer could route through the newer IPv6 protocol. That’s because most of the world’s internet traffic is flowing through content delivery networks such as Cloudflare, Amazon Web Services, and Google Cloud, which are "natively IPv6-enabled." (Netflix has built its own content delivery network for video streaming while HBO Max uses a multi-CDN approach.)
“This means that ~20% of traffic is IPv4, and it's extremely rare for an average household to have 1,200 TCP/UDP sessions active every minute, 24/7, even with 40+ devices on the LAN,” he told PCMag in an email.
That said, reaching 1,200 active sessions is still “very much doable,” given that a single application can have multiple internet connections opened, Swer said. “If some application or all of them in totality had 1,200 sockets opened, then that's 1,200 sockets used."
A year ago, one Starlink user mentioned hitting the 1,200 limit due to having “~35 devices running in my house, from cameras to smart devices, desktops, phones, my NAS running docker and a Fedora box running Nagios. I started killing things off, and eventually all of my apps started working correctly.”
So, if you're a power user with an unconventional setup, you might encounter the session limit. A page from Cloudflare also indicates that about 48% of Starlink user traffic in North America occurring over internet browsers has been heading to IPv4 sites, the remainder going to IPv6. In Asia, the Starlink IPv4 traffic is even higher at 80%, suggesting far more websites in the region lack IPv6 support.
On why, Swer suspects some Starlink customers could be using their own, older Wi-Fi routers that lack the IPv6 support. "I've deployed Starlinks worldwide for 'residential-like' use cases, and we often saw that ~80% (IPv6) marker as the norm," he added.
(Cloudflare Radar)The other issue with Starlink’s use of CGNAT is that it doesn’t support the port forwarding required for a public-facing server, since customers aren’t given a public IP address.
A Starlink subscriber named Robert Hawkins told PCMag that it’s been a problem since he’d like to stream movies from his home network. “It is difficult to host due to the use of CGNAT,” he said, later adding, “Port forwarding won't work with my server because Starlink doesn't give a publicly routable IP address.”
The CGNAT use at ISPs is often why it's flagged as a complaint when it comes to hosting. However, Starlink’s support page notes it can assign a customer a public IP address, but only if they subscribe to the more expensive, business-focused Priority plans, which feature a cap on high-speed data. “Priority plans include a public IP address and do not have the session limitations associated with CGNAT,” the page adds.
SpaceX also has a "fair use policy" for Starlink that allows the company to take other measures to curb excessive usage.
Editor's note: This story has been corrected to note Netflix uses its own CDN for video streaming and HBO Max uses a multi-CDN approach.