Transport layer protocol introduced three important TCP / four-way handshake (the theoretical part, knock on the blackboard!)

The I and small partners to share the transport layer of the network in seven, I will be divided into the following steps for everyone decomposition Description:
1.TCP protocol describes
2.TCP packet format
3.TCP three-way handshake
4.TCP four handshake
5.UDP protocol describes
6. common protocol and port

And a UDP protocol .TCP

1.TCP is connection-oriented, reliable protocol process to process communication
2.TCP provide full duplex service, i.e., at the same time data can be bidirectional transmission
3.TCP segment (up to 1500 characters Festival, 1.5kb)

• The TCP packet a plurality of bytes is called segment (Segment)
• TCP packet is encapsulated in an IP datagram
  TCP segment:
  
  1. Source port number (16)
  (16) 2. The destination port number
  3. The number (32): the transmitting side for each byte are numbered for the convenience of correct recombinant
  4. acknowledgment number (32): a confirmation information transmitting terminal
  5. the header length (4)
  6 reserved (. 6)
  7.URG (bit urgent pointer): 1 represents is turned on, 0 for off is not necessary after the buffer layer is directly submitted to the application, for example as follows:
  after all of the data sent from the b end then the two machines a and b respectively have a cache, this time a to b will first transmit data originally acetate buffer, transport to the application layer buffer
  8.ACK (acknowledgment): acknowledge bit
  (quickly receive transmitted to the application layer) 9.PSH: 1 followed by the representative open, is 0 for off, to go through the cache, and a number of segments directly attendance transmitting the application layer, no queuing
  10.RST (resend connection): reset bit
  11.SYN (request to establish a connection flag): 1 for establishing a connection,
  12.FIN (disconnection request flag):
  13. window size ( 16): the number of data segments for explaining the local may receive , The window size is variable, the larger the faster the transmission window
  14. Checksum (16): error checking
  15. Urgent Pointer (16)
  16. options: Option
  two .TCP establishment of a connection establishment process is three handshake: (important)
  example: At this point we have a client PC1 / PC2 server
  
  process is as follows:
  1. PC1 transmits a connection request SYN
  2. After receiving ACK PC2 will send confirmation, and sends a connection request SYN (bidirectional exchange)
  3. PC1 transmits acknowledgment ACK

三.TCP断开连接的四次握手：（重要）
举例：此时我们有客户端PC1/服务端PC2

具体过程如下：

1. PC1发送FIN断开连接请求，并发送ACK确认位
2. PC2加以确认ACK
3. PC2发送FIN断开连接请求，和ACK确认位请求
4. PC1发送ACK确认位
   但是这里需要注意的是在TCP断开连接过程中，有一个半关闭的概念。TCP一方可以终止发送数据，但仍然可以接收数据，称为半关闭。
   此处我们举例说明例如：迅雷下载东西，在页面点击下载之后使用迅雷下载，此时退出页面之后迅雷仍会继续下载
   
   具体过程如下：
5. PC1客户端发送FIN报文段，半关闭此连接
6. PC2服务器发送ACK报文段接受半关闭，然后继续发送数据，而客户端只发送ACK确认，不再发送任何数据
7. PC2服务器把数据都发送完成时，就发送FIN报文段，
8. PC1发送ACK报文段，这样就关闭了连接

常用的TCP端口号及其功能：

我们来分别说一下各个端口表示的含义：

• 21端口：FTP（20端口，数据连接/21端口，控制连接）
• 23端口：TELNET：明文访问
• 22端口：SSH：密文访问
• 25端口SMTP：服务器开放的端口，用于发送邮件
• 80端口：HTTP：超文本传输协议
• 443端口：HTTPS：安全传输协议
• 110端口：POP3，用于邮件的接收
• 143端口：IAMP4：用于邮件的接收
• 53端口：DNS

四.UDP协议：
1.无连接、不可靠的传输协议
2.花费的开销小

UDP报文的首部格式：

UDP长度：用来指出UDP的总长度，为首部加上数据
校验和：用来完成对UDP数据的差错校验，它是UDP协议提供的唯一可靠机制
常用的UDP端口号及其功能：

These are the specific transport layer protocol process related presentations and important TCP three-way and four-way handshake, there are some common ports and corresponding to the representative of the meaning, thank you!