Up to this point, we have focused primarily on the functional aspects of network. Like any computer system, however, computer networks are also expected to perform well. This is because the effectiveness of computations distributed over the network often depends directly on the efficiency with which the network delivers the computation's data. While the old programming adage “first get it right and then make it fast” is valid in many settings, in networking it is usually necessary to “design for performance.” It is therefore important to understand the various factors that impact network performance.