Photo by Jani Brumat on Unsplash

Photo by Jani Brumat on Unsplash

Enlarging the TCP initial window

Open the Floodgates

Article from ADMIN 53/2019
Linux on-board resources let admins open the floodgates for high-volume TCP packages.

Most Internet services rely on the Transmission Control Protocol (TCP), an interprocess communication protocol that dates back to the 1980s, an era when data streams were more like trickles by today's standards. TCP includes the SMTP protocol for email and HTTP/1.1 and /2.0; therefore, all TCP stack optimizations have a positive effect on performance. In this article, I take a look at the TCP initial window, which defines the size of the first packet, and thus of all other packets, sent over TCP.

The first Request for Comments on TCP (RFC 793) dates back to 1981 [1]. One of the important aspects of TCP has always been that it maximizes the available network bandwidth. However, it avoids overloading the individual components in the connection or their buffers by splitting the payload to be transferred (e.g., an HTML file) into small packets and transferring them one by one. The receiver gives the sender regular feedback on how many packets it has received. If one is missing, the transmitter resends it. TCP uses sequence numbers to ensure the correct sequence of packets.

In the early days of the Internet, modems (acoustic couplers) commonly had a transmission rate of 300bps. Today, many private homes in large cities use fiber optics with transmission rates of 1GBps. However, the basic concept of TCP has not changed in all these years. TCP does not know the maximum transfer rate of a network connection, so it does not send the complete file to be transferred, just a small piece of it.

The TCP initial window (IW), on the other hand, has changed over the years and now has 10 segments, one of which can contain 1500 bytes, with 40 bytes lost as overhead. The size of the first data packet from the server to the receiver reflects this,

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