|  |  |  |  |  |  |  |  | 
   
    | • | A
    sender and a receiver communicate 
 |  | 
   
    |  | 
   
    | • | Buffer
    size is 1: each message must be 
 |  | 
   
    |  | acknowledged
    before next is sent 
 |  | 
   
    |  | 
   
    | • | If
    acknowledgement does not arrive in a 
 |  | 
   
    |  | reasonable
    time, message is resent 
 |  | 
   
    |  | 
   
    | • | The
    communication network may lose but not 
 | 
   
    |  | corrupt
    messages 
 |  |