OSI 7 Layers, TCP, UDP

• OSI 7 Layers

Closer to the computer lower layer, closer to the human higher layer.

 

• TCP

- Origin

To prevent data loss, split the data into small pieces and send the data through multiple lines instead of communicate with only one physical line.

reference : https://evan-moon.github.io/2019/11/10/header-of-tcp/

 

- Two way, exchanging response signals between sender and receiver. 

Sender : Sends the data after split into small pieces

Reciever : Receives the data in sequence

 

- 3 Way Handshake

It is the process to check whether the packets are properly received between client and server.

 

S	SYN	연결을 생성할 때 클라이언트가 서버에 시퀀스 번호를 보내는 패킷
S.	SYN-ACK	시퀀스 번호를 받은 서버가 ACK 값을 생성하여 클라이언트에게 응답하는 패킷
.	ACK	ACK 값을 사용하여 응답하는 패킷

Line 1: The client sent the random sequence number to the server in the seq field.
Line 2: The server increases the sequence number sent by the client by 1 and sends it to the ack(acknowledge) field.
Line 3: The client increases the sequence number received from the server by 1 and sends it to its ack(acknowledge) field.

 

- HOLB

Head of Line Blocking, a bottleneck in the communication between sender and receiver by packet lost in the middle or slow parsing speed from the receiver.

 

• UDP

- One way, it does not check whether the packets are properly received between client and server, rapider than TCP.

- UDP Header

 

Client→input www.google.com→1. UDP connects to dns server

                                                        2. Send the UDP packet to dns server

                                                        3. Request IP address to dns server

                                                        4. dns server sends IP address to the client by UDP packet                    

                                                        ↓ Client : get ip address and connectable to www.google.com

 

reference : https://evan-moon.github.io/2019/10/08/what-is-http3/