In this illustration, we see four desktop computers arranged as four points of a square; the computer at top-left is labeled "Host A," the computer at top-right is labeled "Host B," the computer at bottom-right is labeled "Host C," and the computer at bottom-left is labeled "Host D." Each host appears next to its protocol stack. Each host is also connected to a link-layer switch, and between every host is a router-the router between Host A and Host B is labeled "R1;" the router between Host B and Host C is labeled "R2;" the router between Host C and Host D is labeled "R3;" and the router between Host D and Host A is labeled "R4."

Each host is sending data to one host and receiving data from another. Host A is sending data to Host C. This data originates in the top level of Host A's protocol stack, which is labeled as "(lambda)in: original data." A second data point originates in the next layer down, and this one is labeled "(lambda)'in: original data, plus retransmitted data." A blue arrow indicating the data's path travels down through the protocol stack, through R1, through Host B's link-layer switch, through R2, through Host C's link-layer switch, and up through Host C's protocol stack. Host B is similarly sending data to Host D. This data also has two origin points in Host B's protocol stack, and travels a similar path as Host A's data, passing through R2, through Host C's link-layer switch, through R3, through Host D's link-layer switch, and up through Host D's protocol stack. Host C is sending data to Host A. Host D is sending data to Host B. This data travels a similar path, ending at the top layer of Host B's protocol stack, at a point labeled "(lambda)out."