The TCP ( Transfer Control Protocol ) transport control protocol
One, two transmission modes:
| TCP for connection |
reliable |
Commonly used in Point to Point |
Micro letter |
| UDP a connectionless |
high speed |
Commonly used in the opposite point |
Live |
Second, the data direction: in the communication of the internet, the client is always actively connected to the server
Third, the port role : a virtual computer interface for data communication
Fourth, the port type:
| Physical port |
For connecting a physical device, such as cable, the USB , etc. |
| Virtual port |
For data transmission, for example: 80/21/22 etc. |
Added: Virtual Port range: 1-65535
Five, three-way handshake:
SEQ ID SEQ : accounting . 4 bytes for the data segment order mark, the TCP all data bytes transmitted are connected to a programmed number, the first byte of the random number generated by the local; ed to bytes after the serial number, give each segment is assigned a number; the sequence number seq is the first byte of the data segment number.
No acknowledgment ACK : accounting . 4 bytes, the next expects to receive the other of the first data byte of a segment number; first byte of the segment sequence number indicates the number of data carrying; numbers refer confirmed It is expected to be received next byte number; therefore the current number of the last byte of the segment +1 is the acknowledgment number.
Confirmed the ACK : accounting . 1 bit only when ACK = 1 , the acknowledgment number field is valid. ACK = 0 , the acknowledgment number is invalid.
Synchronizing the SYN : for synchronizing serial connection establishment. When the SYN =. 1 , the ACK = 0 indicates when: It is a connection request packet segments. If the agreed connection, in the response message segments such that the SYN =. 1 , the ACK =. 1 . Accordingly, the SYN =. 1 indicates that this is a connection request, or the connection acceptance message. SYN This flag only TCP when connections will build production is set to 1 , after the completion of the handshake SYN flag is set to 0 .
Terminating the FIN : for releasing a connection. FIN = 1 represents: a data sender of this segment has been transmitted, and for the release of the transport connection.
Note: the ACK , the SYN and FIN these capitalized word representation flag, whose value is either 1 or a 0 ; ACK , SEQ lowercase number word representation.
1 , the handshake process:
| 00 |
SYN = 0, ACK = O |
Logogram |
No operation |
| 10 |
SYN = 1, ACK = 0 |
SYN=1 |
Connection request |
| 11 |
SYN = 1, ACK = 1 |
SYN = 1, ACK = 1 |
Request confirmation |
| 01 |
SYN = 0, ACK = 1 |
ACK=1 |
Connection confirmation |
The first handshake: connection is established, the client sends syn packets ( syn = J ) to the server, and enters the SYN_SENT state, waiting for the server to confirm; the SYN : synchronization sequence number ( the Synchronize Sequence Numbers ).
Second handshake: server receives syn packets, must confirm the customer's SYN ( ACK = + J. 1 ), while themselves transmit a SYN packet ( syn = K ), i.e. SYN + ACK packet, then the server enters SYN_RECV state.
Third handshake: a client server receives the SYN + ACK packet, the server sends an acknowledgment packet to the ACK (ACK = K +. 1 ), this packet is sent, the client and the server into the ESTABLISHED ( the TCP connection succeeds) state, complete the three handshake.
2 , field meanings:
| Field |
meaning |
| URG |
The urgent pointer is valid. Is 1 , it represents a bit needs to be a priority processing |
| ACK |
Represents the response acknowledgment number is valid, is generally set to 1 . |
| PA |
Expressed DATA data transmission, the receiver application immediately prompts from TCP to read data buffer away. |
| RST |
Other requirements to re-establish a connection reset. |
| SYN |
Request to establish a connection and an initial value setting its sequence number in a sequence number field. Establish a connection, is set to 1 . |
| END |
Want to disconnect disables the connection |
3 Why do you need three-way handshake:
| A handshake |
Break the TCP reliability |
| The second handshake |
Server wait |
4 , English terminology:
| LISTEN |
monitor |
| SYN-SENT |
SYN Sent |
| SYN-RCVD |
SYN Received |
| ESTABLISHED |
Already connected |
Six, four off
Disconnection process:
| 00 |
FINE = 0, ACK = 0 |
Logogram |
No operation |
| 10 |
FINE = 1, ACK = 0 |
END = 1 |
Disconnect request |
| 11 |
FINE = 1, ACK = 1 |
FINE = 1, ACK = 1 |
Request confirmation |
| 01 |
FINE = 0, ACK = 1 |
ACK=1 |
断开确认 |
1）客户端进程发出连接释放报文，并且停止发送数据。释放数据报文首部，FIN=1，其序列号为seq=u（等于前面已经传送过来的数据的最后一个字节的序号加1），此时，客户端进入FIN-WAIT-1（终止等待1）状态。 TCP规定，FIN报文段即使不携带数据，也要消耗一个序号。
2）服务器收到连接释放报文，发出确认报文，ACK=1，ack=u+1，并且带上自己的序列号seq=v，此时，服务端就进入了CLOSE-WAIT（关闭等待）状态。TCP服务器通知高层的应用进程，客户端向服务器的方向就释放了，这时候处于半关闭状态，即客户端已经没有数据要发送了，但是服务器若发送数据，客户端依然要接受。这个状态还要持续一段时间，也就是整个CLOSE-WAIT状态持续的时间。
3）客户端收到服务器的确认请求后，此时，客户端就进入FIN-WAIT-2（终止等待2）状态，等待服务器发送连接释放报文（在这之前还需要接受服务器发送的最后的数据）。
4）服务器将最后的数据发送完毕后，就向客户端发送连接释放报文，FIN=1，ack=u+1，由于在半关闭状态，服务器很可能又发送了一些数据，假定此时的序列号为seq=w，此时，服务器就进入了LAST-ACK（最后确认）状态，等待客户端的确认。
5）客户端收到服务器的连接释放报文后，必须发出确认，ACK=1，ack=w+1，而自己的序列号是seq=u+1，此时，客户端就进入了TIME-WAIT（时间等待）状态。注意此时TCP连接还没有释放，必须经过2∗∗MSL（最长报文段寿命）的时间后，当客户端撤销相应的TCB后，才进入CLOSED状态。
6）服务器只要收到了客户端发出的确认，立即进入CLOSED状态。同样，撤销TCB后，就结束了这次的TCP连接。可以看到，服务器结束TCP连接的时间要比客户端早一些。
常见面试题
【问题1】为什么连接的时候是三次握手，关闭的时候却是四次握手？
答：因为当Server端收到Client端的SYN连接请求报文后，可以直接发送SYN+ACK报文。其中ACK报文是用来应答的，SYN报文是用来同步的。但是关闭连接时，当Server端收到FIN报文时，很可能并不会立即关闭SOCKET，所以只能先回复一个ACK报文，告诉Client端，"你发的FIN报文我收到了"。只有等到我Server端所有的报文都发送完了，我才能发送FIN报文，因此不能一起发送。故需要四步握手。
【问题2】为什么TIME_WAIT状态需要经过2MSL(最大报文段生存时间)才能返回到CLOSE状态？
答：虽然按道理，四个报文都发送完毕，我们可以直接进入CLOSE状态了，但是我们必须假象网络是不可靠的，有可以最后一个ACK丢失。所以TIME_WAIT状态就是用来重发可能丢失的ACK报文。在Client发送出最后的ACK回复，但该ACK可能丢失。Server如果没有收到ACK，将不断重复发送FIN片段。所以Client不能立即关闭，它必须确认Server接收到了该ACK。Client会在发送出ACK之后进入到TIME_WAIT状态。Client会设置一个计时器，等待2MSL的时间。如果在该时间内再次收到FIN，那么Client会重发ACK并再次等待2MSL。所谓的2MSL是两倍的MSL(Maximum Segment Lifetime)。MSL指一个片段在网络中最大的存活时间，2MSL就是一个发送和一个回复所需的最大时间。如果直到2MSL，Client都没有再次收到FIN，那么Client推断ACK已经被成功接收，则结束TCP连接。
【问题3】为什么不能用两次握手进行连接？
答：3次握手完成两个重要的功能，既要双方做好发送数据的准备工作(双方都知道彼此已准备好)，也要允许双方就初始序列号进行协商，这个序列号在握手过程中被发送和确认。
现在把三次握手改成仅需要两次握手，死锁是可能发生的。作为例子，考虑计算机S和C之间的通信，假定C给S发送一个连接请求分组，S收到了这个分组，并发 送了确认应答分组。按照两次握手的协定，S认为连接已经成功地建立了，可以开始发送数据分组。可是，C在S的应答分组在传输中被丢失的情况下，将不知道S 是否已准备好，不知道S建立什么样的序列号，C甚至怀疑S是否收到自己的连接请求分组。在这种情况下，C认为连接还未建立成功，将忽略S发来的任何数据分 组，只等待连接确认应答分组。而S在发出的分组超时后，重复发送同样的分组。这样就形成了死锁。
【问题4】如果已经建立了连接，但是客户端突然出现故障了怎么办？
TCP还设有一个保活计时器，显然，客户端如果出现故障，服务器不能一直等下去，白白浪费资源。服务器每收到一次客户端的请求后都会重新复位这个计时器，时间通常是设置为2小时，若两小时还没有收到客户端的任何数据，服务器就会发送一个探测报文段，以后每隔75秒钟发送一次。若一连发送10个探测报文仍然没反应，服务器就认为客户端出了故障，接着就关闭连接。