Transport layer protocol tcp/udp

One 、UDP agreement

UDP Segment format ：

The range of port numbers is 0-2^16-1, among 0-1023 It is occupied by many software .

1.ephemeral Port number

Many services have well-known Port number , Then the port number of the client program does not have to be well-known Of , Usually, the system automatically assigns an idle port number each time the client program , Use it and release it .

2.UDP The protocol is not connection oriented , Nor does it guarantee transmission reliability

① Sending end UDP The protocol only encapsulates the data transmitted from the application layer into segments and gives them to IP The protocol layer completes the task , If the segment cannot be sent to the other party due to network failure ,UDP The protocol layer does not return any error messages to the application layer .

② On the receiving end UDP The protocol layer just hands over the received data to the responding application according to the port number, which is regarded as the completion of the task , If the sender sends multiple packets and passes through different routes on the network , When arriving at the receiving end, the order is out of order ,UDP The protocol does not guarantee that it will be delivered to the application layer in the order in which it is sent .

③ Usually at the receiving end UDP The protocol layer puts the received data into a fixed size buffer and waits for the application to extract and process , If the application fetches and exits slowly , And the sending end is very fast , You'll lose your bag ,UDP The protocol layer does not report such an error as high .

So consider ： Waiting for an answer , Timeout retransmission , Number the packet , Flow control, etc .

characteristic ： Simple communication , Trust them , I'm so stupid , Do things differently .

Adaptive scene ：

① Less resources are needed , In the intranet with good network conditions or applications that are not sensitive to packet loss .

② Don't communicate one-on-one , Establishing a connection , It's an application that can be broadcast .

③ It needs to be fast , Low latency , It can tolerate a few packet losses , But even if the network is congested , And never flinch , When you are indomitable .

Business case ：

① Streaming protocol ： Live delivery

② Real time games ： Instant seckill

③loT The Internet of things ： Embedded relatively dull

④ Mobile communication field ： Mobile traffic Internet data face protocol GTP-U Is based on UDP Of

Two 、TCP agreement

TCP Segment format ：

Source port number and target port number are indispensable , and UDP equally .

1. Why is the package numbered ？

In order to solve the disorder problem .

It should also determine the serial number . The bag sent out should have confirmation , Otherwise, it is impossible to know whether the other party has received . If you don't receive it, you should resend it , Know delivery . Solve the problem of no packet loss .

2. Status bit

Such as SYN Initiate a connection ,ACK Yes reply ,RST It's about reconnecting ,FIN Is to end the connection etc .

TCP It's connection-oriented , So both parties need to maintain the state of the connection , Sending of these packets with status bits , Will cause a change in the status of both parties .

3. Window size

TCP Do flow control , Each side of the communication declares a window , Indicate your current processing capability , Don't send too fast , Don't be too slow .

Through to TCP The analysis of the head , Focus on the following issues ：

Sequence problem , Steady and steady

Packet loss problem , Promise is reliable

Connection maintenance , Have a beginning and an end

flow control , Grasp the measure

Congestion control , Know how to advance, know how to retreat

4. Three handshakes

TCP Connection and connection , It's called three handshakes .