Linux服务器上11种网络连接状态
通常情况下:一个正常的TCP连接，都会有三个阶段:1、TCP三次握手;2、数据传送;3、TCP四次挥手
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注:以下说明最好能结合”图:TCP的状态机”来理解。
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SYN:(同步序列编号,Synchronize Sequence Numbers)该标志仅在三次握手建立TCP连接时有效。表示一个新的TCP连接请求。
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ACK:(确认编号,Acknowledgement Number)是对TCP请求的确认标志,同时提示对端系统已经成功接收所有数据。
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FIN:(结束标志,FINish)用来结束一个TCP回话.但对应端口仍处于开放状态,准备接收后续数据。
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1)、LISTEN:首先服务端需要打开一个socket进行监听，状态为LISTEN. /* The socket is listening for incoming connections. 侦听来自远方TCP端口的连接请求 */
2)、SYN_SENT:客户端通过应用程序调用connect进行active open.于是客户端tcp发送一个SYN以请求建立一个连接.之后状态置为SYN_SENT. /*The socket is actively attempting to establish a connection. 在发送连接请求后等待匹配的连接请求 */
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3)、SYN_RECV:服务端应发出ACK确认客户端的SYN,同时自己向客户端发送一个SYN. 之后状态置为SYN_RECV? /* A connection request has been received from the network. 在收到和发送一个连接请求后等待对连接请求的确认 */
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4)、ESTABLISHED: 代表一个打开的连接，双方可以进行或已经在数据交互了。/* The socket has an established connection. 代表一个打开的连接，数据可以传送给用户 */
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5)、FIN_WAIT1:主动关闭(active close)端应用程序调用close，于是其TCP发出FIN请求主动关闭连接，之后进入FIN_WAIT1状态./* The socket is closed, and the connection is shutting down. 等待远程TCP的连接中断请求，或先前的连接中断请求的确认 */
6)、CLOSE_WAIT:被动关闭(passive close)端TCP接到FIN后，就发出ACK以回应FIN请求(它的接收也作为文件结束符传递给上层应用程序),并进入CLOSE_WAIT. /* The remote end has shut down, waiting for the socket to close. 等待从本地用户发来的连接中断请求 */
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7)、FIN_WAIT2:主动关闭端接到ACK后，就进入了FIN-WAIT-2 ./* Connection is closed, and the socket is waiting for a shutdown from the remote end. 从远程TCP等待连接中断请求 */
8)、LAST_ACK:被动关闭端一段时间后，接收到文件结束符的应用程序将调用CLOSE关闭连接。这导致它的TCP也发送一个 FIN,等待对方的ACK.就进入了LAST-ACK . /* The remote end has shut down, and the socket is closed. Waiting for acknowledgement. 等待原来发向远程TCP的连接中断请求的确认 */
9)、TIME_WAIT:在主动关闭端接收到FIN后，TCP就发送ACK包，并进入TIME-WAIT状态。/* The socket is waiting after close to handle packets still in the network.等待足够的时间以确保远程TCP接收到连接中断请求的确认 */
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10)、CLOSING: 比较少见./* Both sockets are shut down but we still don't have all our data sent. 等待远程TCP对连接中断的确认 */
11)、CLOSED: 被动关闭端在接受到ACK包后，就进入了closed的状态。连接结束./* The socket is not being used. 没有任何连接状态 */
TIME_WAIT状态的形成只发生在主动关闭连接的一方。
??? 主动关闭方在接收到被动关闭方的FIN请求后，发送成功给对方一个ACK后,将自己的状态由FIN_WAIT2修改为TIME_WAIT，而必须再等2倍 的MSL(Maximum Segment Lifetime,MSL是一个数据报在internetwork中能存在的时间)时间之后双方才能把状态 都改为CLOSED以关闭连接。目前RHEL里保持TIME_WAIT状态的时间为60秒。
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当然上述很多TCP状态在系统里都有对应的解释或设置,可见man tcp
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二、关于长连接和短连接:
?????? 通俗点讲:短连接就是一次TCP请求得到结果后,连接马上结束.而长连接并不马上断开,而一直保持着,直到长连接TIMEOUT(具体程序都有相关参数说明).长连接可以避免不断的进行TCP三次握手和四次挥手.
?????? 长连接(keepalive)是需要靠双方不断的发送探测包来维持的,keepalive期间服务端和客户端的TCP连接状态是ESTABLISHED.目前http 1.1版本里默认都是keepalive(1.0版本默认是不keepalive的)，ie6/7/8和firefox都默认用的是http 1.1版本了(如何查看当前浏览器用的是哪个版本，这里不再赘述)。Apache,java
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一个应用至于到底是该使用短连接还是长连接，应该视具体情况而定。一般的应用应该使用长连接。
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??? 1、Linux的相关keepalive参数
a、 tcp_keepalive_time - INTEGER
??? How often TCP sends out keepalive messages when keepalive is enabled.
??? Default: 2hours.
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b、 tcp_keepalive_probes - INTEGER
??? How many keepalive probes TCP sends out, until it decides that the
??? connection is broken. Default value: 9.
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c、 tcp_keepalive_intvl - INTEGER
??? How frequently the probes are send out. Multiplied by
??? tcp_keepalive_probes it is time to kill not responding connection,
??? after probes started. Default value: 75sec i.e. connection
??? will be aborted after ~11 minutes of retries.
??? 2、F5负载均衡上的相关参数说明
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a、Keep Alive Interval
Specifies, when enabled, how frequently the system sends data over an idle TCP connection, to determine whether the connection is still valid.
???? Specify: Specifies the interval at which the system sends data over an idle connection, to determine whether the connection is still valid. The default is 1800 milliseconds.
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b、Time Wait
Specifies the length of time that a TCP connection remains in the TIME-WAIT state before entering the CLOSED state.
?????Specify: Specifies the number of milliseconds that a TCP connection can remain in the TIME-WAIT state. The default is 2000.
c、Idle Timeout
Specifies the length of time that a connection is idle (has no traffic) before the connection is eligible for deletion.
????? Specify: Specifies a number of seconds that the TCP connection can remain idle before the system deletes it. The default is 300 seconds.
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?? 3、Apache的相关参数说明
??? 以下是Apache/2.0.61版本的默认参数和说明
a、KeepAlive:
? default On.Whether or not to allow persistent connections (more than
one request per connection). Set to "Off" to deactivate.
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b、MaxKeepAliveRequests:
? default 100.The maximum number of requests to allow
?during a persistent connection. Set to 0 to allow an unlimited amount.
?We recommend you leave this number high, for maximum performance.
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c、KeepAliveTimeout:
? default 15. Number of seconds to wait for the next request from the
same client on the same connection.
4、JAVA1.6的相关参数说明:
a、http.keepAlive=<boolean>default: true
Indicates if keep alive (persistent) connections should be supported.
b、http.maxConnections=<int>default: 5
Indicates the maximum number of connections per destination to be kept alive at any given time
