Opening a circuit

Applications communicate with each other using the virtual circuits provided by TCP. These circuits are established on an as-needed basis, getting created and destroyed as requested by the applications in use. Whenever an application needs to communicate with another application somewhere on the network, it will ask the local TCP provider to establish a virtual circuit on its behalf.

There are two methods for requesting that a virtual circuit be opened: either a client will request an open so that data can be sent immediately, or a server will open a port in “listen” mode, waiting for a connection request to arrive from a client.

The simplest of the two methods is the "passive open,” which is the form used by servers that want to listen for incoming connections. A passive open indicates that the server is willing to accept incoming connection requests from other systems, and that it does not want to initiate an outbound connection. Typically, a passive open is "unqualified,” meaning the server can accept an incoming connection from anybody. However, some security-sensitive applications will accept connections only from predefined entities, a condition known as a “qualified passive open.” This type is most often seen with corporate web servers, ISP news servers, and other restricted-access systems.

When a publicly accessible server first gets started, it will request that TCP open a well-known port in passive mode, offering connectivity to any node that sends in a ...

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