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IDLE— Connection not established yet.
WAITING— CONNECT has been executed and CALL REQUEST sent. QUEUED— A CALL REQUEST has arrived; no LISTEN yet. ESTABLISHED— The connection has been established.
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SENDING— The user is waiting for permission to send a packet. RECEIVING— A RECEIVE has been done.
DISCONNECTING— A DISCONNECT has been done locally.
答：该传输实体有可能死锁。当双方同时执行RECEIVE 时就会进入死锁状态。
6-10 Out of curiosity, the implementer of the transport entity of Fig. 6-20 has decided to put counters inside the sleep procedure to collect statistics about the conn array. Among these are the number of connections in each of the seven possible states, ni (i = 1, ... ,7). After writing a massive FORTRAN program to analyze the data, our implementer discovers that the relation Sni = MAX_CONN appears to always be true. Are there any other invariants involving only these seven variables?
答：有， n2?+n3+n6+?n7=1 ????
因为状态listening（n2）、waiting（n 3）、sending（n6）和receiving（n7）都意味着用户被封锁，因此当处在其中的一个状态时，就不可能是在另一个状态。
6-11 What happens when the user of the transport entity given in Fig. 6-20 sends a zero-length message? Discuss the significance of your answer.
答：长度为零的报文被另一边接收。这种报文的发送可以被用来表示文件结束的信号。 6-12 For each event that can potentially occur in the transport entity of Fig. 6-20, tell whether it is legal when the user is sleeping in sending state.
答：因为文件处于封锁状态，所有的传输层原语都不可能执行。因此，仅分组到达事件是可能的，而且还不是所有的到达事件。事实上，仅仅跟呼叫请求、清除请求、数据分组和信用量分组这几个分组到达有关的事件是合法的。
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