This illustration is similar to figure 4.43. In it, we see four desktop computers positioned side-by-side. From left to right, they are labeled Host A, Host B, Host C, and Host D. Each computer appears next to its protocol stack. Hosts A and B and connected to a link-layer switch, which is connected to a central router. Hosts C and D are also connected to a link-layer switch, which is connected to the same central router. This router is labeled "Finite shared output link buffers."

Host A is sending data to Host D. This data originates in the top level of Host A's protocol stack, which is labeled as "(lambda)in: original data." A second data origin point appears in the next layer down, labeled "(lambda)'in: original data, plus retransmitted data." A blue arrow indicating the data's path travels vertically down through Host A's protocol stack, passes through the central router, and up through Host D's protocol stack.

Host B is sending data to Host C. This data also starts in the top level of Host B's protocol stack, travels vertically down through it, passes through the central router, and goes up through Host C's protocol stack to the top level, which is labeled as "(lambda)out."