Commands.page Logo

Ubuntu Command Download File Set TCP Window Size

This article explains how to download files on Ubuntu while managing TCP window sizes to optimize network performance. While there is no single command flag that directly sets the TCP window size during a download, you can use specific options with curl or adjust system parameters to control socket buffers that influence the window size.

Direct Command Options

Standard download utilities like wget do not offer a flag to set the TCP window size. However, curl provides options to adjust the socket buffer size, which directly influences the maximum TCP receive window. To download a file and set the receive buffer size, use the following command:

curl --recv-buffer 65536 -O https://example.com/file.zip

In this example, --recv-buffer 65536 sets the socket receive buffer to 64KB. This limits the TCP window size advertised to the server, which can help stabilize connections on high-latency networks.

System-Wide TCP Configuration

For more permanent control over TCP window sizes, you should configure the Linux kernel parameters using sysctl. This affects all network connections rather than just a single download command. To view current TCP memory settings, run:

sysctl net.ipv4.tcp_rmem

To temporarily increase the maximum TCP window size, use:

sudo sysctl -w net.ipv4.tcp_rmem="4096 87380 67108864"

This setting adjusts the minimum, default, and maximum receive buffer sizes used by the TCP stack. For permanent changes, add the configuration to /etc/sysctl.conf and run sudo sysctl -p.

Summary of Best Practices

  1. Use Curl for Per-Download Tuning: Utilize --recv-buffer or --send-buffer flags when specific buffer sizes are needed for a single task.
  2. Use Sysctl for Global Tuning: Modify kernel parameters if you consistently require larger TCP windows for all connections.
  3. Verify Network Conditions: Increasing window sizes is most beneficial on high-bandwidth, high-latency networks (Long Fat Networks).

By combining curl options for specific tasks and sysctl for system-wide optimization, you can effectively manage TCP window behavior on Ubuntu.