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