Peer             Tracker w. ALTO cli.       ALTO Server
--------+--------       --------+--------       --------+--------
        | F1 Tracker query      |                       |
        |======================>|                       |
        |                       | F2 ALTO query         |
        |                       |---------------------->|
        |                       | F3 ALTO reply         |
        |                       |<----------------------|
        | F4 Tracker reply      |                       |
        |<======================|                       |
        |                       |                       |

====  Application protocol (i.e., tracker-based P2P app protocol)
----  ALTO protocol