TCP/IP

三次握手

第一次A->B: SYN=1; seq = x
(表示A到B的连接请求) (表示A发送从序号x开始的报文)
第二次B<-A: SYN=1; seq = y; ACK=1; ack = x+1
(表示B到A的连接请求)(表示B发送从序号y开始的报文)(ack有效)
(ack：已经接受到A序号为x的请求，期待从x+1开始的)
第三次A->B: ACK=1; ack = y+1; seq = x+1

4-time close

client -> server
1. active closer: FIN(seq = x)
2. passive closer: ACK(seq = v; ack = x+1) the sequence number for ACK is v
3. passive closer: FIN(seq = w; ack = x+1) the sequence number for FIN is w
4. active closer: ACK(seq = x+1; ack = w+1)

 

difference between 0.0.0.0 and 127.0.0.1 

• 0.0.0.0 is used for binding to all available network interfaces.
• 127.0.0.1 is used for communication with the local machine, and it always refers to the loopback interface.

When configuring a service, the choice between 0.0.0.0 and 127.0.0.1 depends on whether you want the service to be accessible only locally (using 127.0.0.1) or from any network interface on the machine (using 0.0.0.0).