Networking applications today already have access to a great amount of Inter-Provider network topology information. For example, views of the Internet routing table are easily available at looking glass servers and entirely practical to be downloaded by clients. What is missing is knowledge of the underlying network topology from the ISP or Content Provider (henceforth referred as Provider) point of view. In other words, what a Provider prefers in terms of traffic optimization -- and a way to distribute it. The ALTO Service provides network information (e.g., basic network location structure, preferences of network paths) with the goal of modifying network resource consumption patterns while maintaining or improving application performance. The basic information of ALTO is based on abstract maps of a network. These maps provide simplified, yet enough information of a network for applications to effectively utilize. Additional services are built on top the maps. This document describes a protocol implementing the ALTO Service. Although the ALTO service would primarily be provided by the network (i.e., the ISP), content providers and third parties could also operate this service. Applications that could use this service are those that have a choice in connection endpoints. Examples of such applications are peer-to-peer (P2P) and content delivery networks.