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Review Questions: 3. Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number x and destination port number y. What are the source and destination port numbers for the segments traveling from Host B to Host A? The source port number is y, the destination port number is x. 5. Why is it that voice and video traffic is often sent over TCP rather than UDP in today’s Internet. (Hint: The answer we are looking for has nothing to do with TCP’s congestion-control mechanism.) 因为UDP虽然传输速度快，不需要建立对等连接，但是容易丢包，而且数据包顺序容易混乱；TCP传输更稳定，文件没有丢失或者损失。 6. Is it possible for an application to enjoy reliable data transfer even when the application runs over UDP? If so, how? 是的，只需要在UDP的协议上添加验证的数据，比如给每个包传输时加个头，并且设置返回值，还有很多其他方法在此不一一列举了。 7. Suppose a process in Host C has a UDP socket with port number 6789. Suppose both Host A and Host B each sends a UDP segment to Host C with destination port number 6789. Will both of these segments be directed to the same socket at Host C? If so, how will the process at Host C know that these two segments originated from two different hosts? 正解，两个部分将针对同一接口。在套接字接口，对于每个收到的片段，操作系统将提供过程与IP地址确定各段的起源。 8. Suppose that a Web server runs in Host C on port 80. Suppose this Web server uses persistent connections, and is currently receiving requests from two different Hosts, A and B. Are all of the requests being sent through the same socket at Host C? If they are being passed through different sockets, do both of the sockets have port 80? Discuss and explain. 对于每个坚持的连接，web服务器会创建一个连接套接字，每个连接套接字由4个数组定义。当主机C收到一个请求的时候，它会检查数据报和段中的4个区域，以此决定用哪个区域来接收TCP段。因此，A和B传输的套接字是不一样的。每一个套接字的标识符都使用80端口作为目的的，但是不同的资源有不同的IP地址。和UDP不同，当传输层传输一个TCP段至应用层的时候，它并不需要明确指定IP地址，因为套接字标识符会帮它指定。 14. True or false? (F) a. Host A is sending Host B a large file over a TCP connection. Assume Host B has no data to send Host A. Host B will not send acknowledgments to Host A because Host B cannot piggyback the acknowledgments