Editing Transmission Control Protocol (section)

===Forcing data delivery===
Normally, TCP waits for 200&nbsp;ms for a full packet of data to send ([[Nagle's Algorithm]] tries to group small messages into a single packet). This wait creates small, but potentially serious delays if repeated constantly during a file transfer. For example, a typical send block would be 4&nbsp;KB, a typical MSS is 1460, so 2 packets go out on a 10&nbsp;Mbit/s Ethernet taking ~1.2&nbsp;ms each followed by a third carrying the remaining 1176 after a 197&nbsp;ms pause because TCP is waiting for a full buffer. In the case of telnet, each user keystroke is echoed back by the server before the user can see it on the screen. This delay would become very annoying.

Setting the [[network socket|socket]] option <code>TCP_NODELAY</code> overrides the default 200&nbsp;ms send delay. Application programs use this socket option to force output to be sent after writing a character or line of characters.

The {{harvtxt|RFC 793}} defines the <code>PSH</code> push bit as "a message to the receiving TCP stack to send this data immediately up to the receiving application".<ref name=comer/> There is no way to indicate or control it in [[user space]] using [[Berkeley sockets]]; it is controlled by the [[protocol stack]] only.<ref name="Stevens2006">{{cite book|author=Richard W. Stevens|title=TCP/IP Illustrated. Vol. 1, The protocols|url=https://archive.org/details/tcpipillustrated00stev|url-access=registration|date=November 2011|publisher=Addison-Wesley|isbn=978-0-201-63346-7|pages=Chapter 20}}</ref>