This illustration shows three desktop computers and three protocol stacks side-by-side. Each computer is connected to a cloud representing the Internet. Each protocol stack appears as:

In each stack, the "Transport" layer is highlighted in blue to indicate it as a socket. Four separate processes are depicted.

In the center protocol stack, the "Application" layer includes two processes, labeled as P1 and P2. P1, on the left side of the stack, has a blue arrow extending from P1 in the application layer, traveling down through the all the other layers, through the cloud representing the Internet, and up through all the layers in the protocol stack of the computer on its left, ending in that computer's "Application" layer and ending at the label P3. This arrow points to and from P1 to P3.

The center protocol stack also has an arrow extending from P2, on the right side of the stack, down through the layers, through the Internet and up the protocol stack associated with the computer to the right. This arrow also points in both directions, and ends at P4.