This illustration is similar to figure 3.49, in which two computers-Host A and Host B-appear side-by-side, with a network in between, represented by a cloud containing a series of routers. Each Host appears next to its protocol stack. Host A is sending data to Host B, which originates in the top layer of Host A's protocol stack, travels down through it, passes through one router and then a second. At the second router, the data signal picks up a blue rectangle labeled "ECN=11." A note further indicates: "ECN bits set in IP datagram header at congested router." From here, the data continues through a third router and travels upwards through Host B's protocol stack. In the second-to-top layer of the protocol stack, a message is sent to the same point in Host A's protocol stack, and includes a blue rectangle, which now is labeled "ECN Echo = 1." A note further explains: "ECN Echo bit in set in receiver-to-sender TCP ACK segment."