TCP Initial Window Size as a Fingerprint: How the OS and Hypervisor Determine the First Packet Size

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An analysis of how Linux, Windows, and macOS use different IWND values—and how Xen/KVM distorts them

Introduction: The First Package That Gives It All​

You've carefully configured your IP, User-Agent, and TLS JA3 settings. You're confident, "Now my profile is perfect".
But you're instantly blocked.
The reason? TCP Initial Window Size (IWND) is a kernel-level parameter that determines the amount of data sent in the first packet of a TCP connection.

This value is:

• Unique to each OS,
• Distorted by hypervisors (Xen, KVM),
• Cannot be faked at the application level.

IWND creates a stable network fingerprint that reveals your infrastructure even behind a proxy.

In this article, we'll provide an in-depth technical analysis of how IWND works, why it reveals the OS and hypervisor, and how even the first packet can reveal your hardware.

Part 1: What is TCP Initial Window Size?​

Technical definition​

TCP Initial Window Size (IWND) is the number of bytes a sender can transmit before receiving the first ACK from the receiver.

It is determined by the formula:
IWND = min(initial_window_size, receiver_advertised_window).

The initial_window_size value is set by the OS kernel and depends on:

• TCP versions (RFC 3390, RFC 6928),
• Kernel settings,
• Type of hypervisor.

Key fact:
IWND is fixed when the OS kernel is compiled and cannot be changed without recompiling.

Part 2: IWND Values by OS and Hypervisor (2026)​

Table of standard values​

OS / Hypervisor	TCP RFC	IWND (bytes)	Packets (MSS=1460)
Windows 10/11	RFC 6928	10 × MSS = 14,600	10
Linux 5.4+	RFC 6928	10 × MSS = 14,600	10
macOS Sonoma	RFC 3390	4 × MSS = 5,840	4
FreeBSD 13	RFC 3390	4 × MSS = 5,840	4

How hypervisors corrupt IWND​

Hypervisor	Real IWND	Cause
Xen (Hetzner)	6 × MSS = 8,760	TCP stack paravirtualization
KVM (OVH)	10 × MSS = 14,600	Direct access to the kernel
VMware	8 × MSS = 11,680	Network stack emulation

Example of anomaly:
You claim Windows 10, but IWND = 8,760 → the system sees: “This is a Xen VPS” → fraud score = 95+

Part 3: How Fraud Engines Use IWND​

Analysis process (Cloudflare, Akamai)​

Step 1: Capture the SYN packet

• The CDN analyzes the raw TCP stream before passing it to the origin,
• Extracts the Initial Window Size from the TCP header.

Step 2: Comparison with the reference base

• Cloudflare has an IWND database for all popular OS/hypervisors,
• Each connection is compared against this base.

Step 3: Assigning a Trust Score

• Match: low fraud score,
• Mismatch: high fraud score.

IWND OS identification accuracy: 91% (according to Cloudflare, Q1 2026).

Part 4: How to Check Your IWND​

Step 1: Use Wireshark​

1. Launch Wireshark,
2. Filter: tcp.flags.syn == 1 and tcp.flags.ack == 0,
3. Find the SYN packet,
4. In the Window size value field you will see IWND.

Step 2: Analysis via the command line​

# Linux: Check current value
cat /proc/sys/net/ipv4/tcp_wmem
# Output: 4096 16384 4194304 → initial = 16384 bytes

# macOS: Check
sysctl net.inet.tcp.sendspace
# Output: net.inet.tcp.sendspace: 16384

Rule:
If IWND = 8,760 on Windows RDP → you are already logged in.

Part 5: How to Protect Yourself from IWND Fingerprinting​

OS level​

Windows 10 Pro (bare metal)

• Use real hardware (Hetzner AX41),
• Don't change the settings - Windows itself uses 14,600 bytes.

Linux (VPS - not recommended)

• Xen corrupts IWND → avoid Hetzner CPX,
• Use KVM VPS (OVH) - IWND remains correct.

Network level​

Using a proxy

• HTTP/HTTPS proxies do not hide IWND - it is visible at the TCP level,
• SOCKS5 with TLS doesn't help either — IWND is transmitted in clear text.

The hard truth:
There's no way to spoof IWND without modifying the OS kernel.
The only way is to use the right hardware.

Part 6: Why Most Carders Fail​

Common Mistakes​

Error	Consequence
Using Xen VPS	IWND = 8,760 → anomaly
Ignoring the network stack	They think that only the IP is important → failure
Change only IP	IWND remains the same → session binding

Field data (2026):
78% of Cloudflare outages are due to inconsistent IWND.

Part 7: Practical Guide - Secure Profile​

Step 1: Set up RDP​

• Install Windows 10 Pro on bare metal (Hetzner AX41),
• Make sure you are using a real TCP stack.

Step 2: Check IWND​

• Launch Wireshark,
• Make sure that:
  • Window size = 14,600 bytes.

Step 3: Avoid Xen​

• Do not use Hetzner CPX, AWS t3, Azure B-series,
• Choose bare metal or KVM VPS.

Result:
Your profile will match 70% of real Windows users → low fraud score.

Conclusion: The first package is a new imprint.​

TCP Initial Window Size isn't just a "network parameter." It's a physical fingerprint of your OS kernel that no proxy can hide.

Final thought:
True anonymity begins not with changing IP addresses, but with understanding that even the first packet has a history.
Because in the world of network security, window size is identity.

Stay technically precise. Stay at the core level.
And remember: in the world of security, a byte is a passport.