Peer w. ALTO cli. Tracker ALTO Server --------+-------- --------+-------- --------+-------- | F1 Tracker query | | |======================>| | | F2 Tracker reply | | |<======================| | | F3 ALTO query | | |---------------------------------------------->| | F4 ALTO reply | | |<----------------------------------------------| | | | ==== Application protocol (i.e., tracker-based P2P app protocol) ---- ALTO protocol