"In a world of big files (video clips, entire DVDs, etc.), fast network connections (ADSL, VDSL, Cable, fiber to the home), and global distribution, the traditional TCP default configuration was hitting the socket buffer limit. Because TCP offers reliable data transport it has to keep a buffer of data sent until the remote end has acknowledged reception of it. It takes the ACK a full round trip (RTT, as seen with ping) to make it back. Thus for fast connections over large distances, like from U.S.A. to Europe or Asia, we need large socket buffer to keep all the unacknowledged data around. FreeBSD had a default 32 K send socket buffer. This supports a maximal transfer rate of only slightly more than 2 Mbit/s on a 100 ms RTT trans-continental link. Or at 200 ms just above 1 Mbit/s. With TCP send buffer auto scaling in its default settings it supports 20 Mbit/s at 100 ms and 10 Mbit/s at 200 ms (socket buffer at 256 KB) per TCP connection. That's an improvement of factor 10, or 1000%. If you have very fast Internet connections very far apart you may want to further adjust the defaults upwards. The nice thing about socket buffer auto-tuning is the conservation of kernel memory which is in somewhat limited supply. The tuning happens based on actual measured connection parameters and are adjusted dynamically. For example a SSH session on a 20 Mbit/s 100 ms link will not adjust upwards because the initial default parameters are completely sufficient and do not slow down the session. On the other hand a 1GB file transfer on the same connection will cause the tuning to kick in and to quickly increase the socket buffers to the max."