CHAPTER Application layer systems 9 APPLICATION LAYER The application layer is responsible for implementing distributed applications and their protocols. This layer implements functionality accessed by end users. When considering distributed applications that use the network for communication, numer- ous examples come to mind: electronic mail, access to Web documents, interactive audio, streaming video, real-time gaming, etc. Permitting such an incredible diversity of applications at this layer is one of the main successes of layered Internet architec- ture. With only a small number of protocols at the transport layer and only a single protocol at the network layer, all these applications can be supported. As discussed in the Appendix, the hourglass architecture of Internet protocol stack is at the core of this design.