The fundamental aspect of the Internet architecture that distinguishes it from other network technologies (such as X.25 and ATM) is that it is connectionless (vs. connection-oriented) and stateless (vs. stateful). The heated debate of whether connection-oriented or connectionless architecture is better has lasted for several decades. Proponents of the connectionless architecture point out the great robustness and scalability properties of the architecture, as demonstrated by the Internet. One well-known articulation of this philosophy is the "End-to-End Arguments". Opponents argue, rightfully, that there is no known solution that can provide quantitative performance assurances or guaranteed QoS in a connectionless network. It has been widely recognized that QoS is a must-have feature as the Internet technology evolves to the next stage. However, all existing solutions that provide guaranteed QoS require routers to maintain per flow (another name for connection used by the Internet community) state, which is the fundamental element of a connection-oriented architecture. The apparent conflicting goals of having a stateless network and supporting QoS have presented a great dilemma for Internet architects. As an example, Dave Clark, one of the most respected Internet architects and the author of the famous "End-to-End Arguments" paper, was also a key designer of the Internet Integrated Services Architecture that requires routers to maintain per flow state.