VPN Overhead Calculator for MTU and MSS

There’s a chance that the tunnel you’ve built looks great on paper, but now your video conference has stalled out into pixelated garbage and your speed test tells you that you’re still pulling full gigabit speeds. It turns out that the problem isn’t realy with the encryption at all: it’s the invisible tax of each additional header that gets tacked onto your data packets. Each additional layer of transport add bytes, and so does encryption, until your nice clean one thousand five hundred byte payload just won’t go through standard network door.

Plug in the constraints on your particular path and protocol into calculator above and it’ll do the math for you. No more guesswork about how much headroom you might need.

How to Fix VPN Speed Problems

The problem is that all VPN protocols puts their own headers atop whatever IP traffic they’re carrying. For example, with moddern AES-GCM encryption and no NAT traversal, an IPsec tunnel will tack on about seventy-four bytes. Turning on UDP encapsulation for NAT traversal support make that number go up significantly, especially if you also use old-school CBC mode different than GCM. Because OpenVPN travels atop TLS records, it’s typically bulkier: It adds almost a hundred bytes in most cases. Surprisingly, WireGuard is quite slim due to its stripped-down design and frequently come in below eighty bytes, but even then, when you’re trying to push thousands of packet per second over a busy link, every byte counts. Know how big those headers are, and set your expectations accordingly.

By default most networks has a maximum transmission unit set to 1,500 bytes, sufficient until you throw in some encapsulation. Say your provider is good with it but the VPN adds an extra 80. Now your packet’s too big for the wire to carry. No problem, right? But that is not the case. Unless there is path MTU discovery or fragmentation, those oversized packets will be silent dropped. This lead to intermittent connectivity problems that are famously hard to troubleshoot.

The page has a nice table breaking this down by scenario (and then IPsec, WireGuard, and OpenVPN each cut into your available bandwidth in their own way). Treat the physical path limit as a hard ceiling; figure out what all needs removing from it, and don’t send anything larger than that through tunnel.

Most admins fail here though because they also don’t consider the inner packet headers in their TCP maximum segment size. That’s the MSS value, which describe the amount of actual application data that will fit within a TCP segment (minus the TCP header and the IP header). To calculate your recommended MSS value, start with your tunnel MTU. Subtract 20 for the IPv4 header and another 20 for the TCP header to get a solid baseline. Make it larger, and you risk increasing the likelihood of fragmentation. Go smaller, and you’re forcing more round trips than are needed. Which means higher latency. It’s a fine line between reliability and efficiency, and it comes down completeley to your own stack.

In the real world, you’ll seldom see clean networks with uniform MTUs. Carrier networks often adds their own encapsulation and reduce the connection to less than a thousand four hundred bytes on mobile devices. PPPoE cuts eight bytes directly from DSL lines, and who knows what’s left after you layer a VPN over an already-reduced space? Unless you design for constraints of your access link, your tunnel packets may get dropped or fragmented based off your firewall rules’ strictness.

Adding some safety margin. Say, sixteen or thirty-two bytes. Accounts for any surprises along the way and is considered conservative.

As the number of packets per second increases, overhead as a fraction of every packet have a noticeable impact on efficiency. The more packets, the smaller they tend to be, which makes the header tax a meaningful part of the overall traffic. Instead of using bandwidth to move data, it is being used to move metadata. Tuning your MSS clamp or choosing a less bloated protocol (such as WireGuard) become worthwhile for certain use cases if you understand this tradeoff.

You are doing all this to make sure data arrives in one piece, but you are also tuning it so it travels efficienty down a limited pipe. Get your numbers wrong, and you might of ended up constantly poking holes in your tunnel to fix problems downstream.