一、TCP连接的几种状态：
下图为状态表，
TCP protocol operations may be divided into three phases. Connections must be properly established in a multi-step handshake process (connection establishment) before entering the data transfer phase. After data transmission is completed, the connection termination closes established virtual circuits and releases all allocated resources.
A TCP connection is managed by an operating system through a programming interface that represents the local end-point for communications, the Internet socket. During the lifetime of a TCP connection the local end-point undergoes a series of state changes.
操作系统通过代表通信本地端的一个叫网络socket的一个可编程接口来管理TCP连接。在ＴＣＰ连接周期中，本地端会经历一系列状态变化。
LISTEN
(server) represents waiting for a connection request from any remote TCP and port.
SYN-SENT
(client) represents waiting for a matching connection request after having sent a connection request.
SYN-RECEIVED
(server) represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.
ESTABLISHED
(both server and client) represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
FIN-WAIT-1
(both server and client) represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.
FIN-WAIT-2
(both server and client) represents waiting for a connection termination request from the remote TCP.
CLOSE-WAIT
(both server and client) represents waiting for a connection termination request from the local user.
CLOSING
(both server and client) represents waiting for a connection termination request acknowledgment from the remote TCP.
LAST-ACK
(both server and client) represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request).
TIME-WAIT
(either server or client) represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request. a connection can stay in TIME-WAIT for a maximum of four minutes known as two MSL (maximum segment lifetime).]
CLOSED
(both server and client) represents no connection state at all.
二、怎么样建立连接（３次握手）：
SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment's sequence number to a random value A.
SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.
ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A+1, and the acknowledgement number is set to one more than the received sequence number i.e. B+1.
At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.
三、连接终止(4次挥手)：
The connection termination phase uses a four-way handshake（４次挥手）, with each side of the connection terminating independently. When an endpoint wishes to stop its half of the connection, it transmits a FIN packet, which the other end acknowledges（告知已收到） with an ACK. Therefore, a typical tear-down requires a pair of FIN and ACK segments from each TCP endpoint. After the side that sent the first FIN has responded with the final ACK, it waits for a timeout before finally closing the connection, during which time the local port is unavailable for new connections; this prevents confusion due to delayed packets being delivered during subsequent connections.
A connection can be "half-open", in which case one side has terminated its end, but the other has not. The side that has terminated can no longer send any data into the connection, but the other side can. The terminating side should continue reading the data until the other side terminates as well.
It is also possible to terminate the connection by a 3-way handshake, when host A sends a FIN and host B replies with a FIN & ACK (merely combines 2 steps into one) and host A replies with an ACK.
Some host TCP stacks may implement a half-duplex close sequence, as Linux or HP-UX do. If such a host actively closes a connection but still has not read all the incoming data the stack already received from the link, this host sends a RST instead of a FIN . This allows a TCP application to be sure the remote application has read all the data the former sent—waiting the FIN from the remote side, when it actively closes the connection. But the remote TCP stack cannot distinguish between a Connection Aborting RST and Data Loss RST. Both cause the remote stack to lose all the data received.
Some application protocols may violate the OSI model layers, using the TCP open/close handshaking for the application protocol open/close handshaking — these may find the RST problem on active close. As an example:
s = connect(remote); send(s, data); close(s);
For a usual program flow like above, a TCP/IP stack like that described above does not guarantee that all the data arrives to the other application.
四、中文解释３次握手，４次挥手
http://www.52im.net/thread-258-1-1.html（网上找的一篇文章）
五、为什么TIME_WAIT状态还需要等待2MSL才能回到CLOSED状态？或者为什么TCP要引入TIME_WAIT状态？
当TCP执行一个主动关闭，并发回最后一个ACK后，该连接必须在TIME_WAIT状态停留的时间为2倍的MSL，这样可以让TCP再次发送最后的ACK以防止这个ACK丢失（另一端超时重发最后的FIN）。MSL（Maximum Segment Lifetime）即最大生存时间，常用值为30秒、1分钟或者2分钟。
