Ericsson Cisco Webtide As technologies that enable bidirectional communication over HTTP have become more pervasive, interest has grown in (1) defining improved mechanisms for support of existing technologies (short-term solutions such as new HTTP headers) and (2) laying the foundation for development of new bidirectional protocols (long-term solutions such as WebSocket, Bidirectional Web Transfer Protocol, and Reverse HTTP). In order to provide context for such work, this document provides a first tentative map of the design space for bidirectional HTTP, with special attention to deployed infrastructure (e.g. web clients, intermediaries, firewalls, NATs, web servers) and programming environments.