[풀스택] Transport Layer 4 / Process-to-Process Delivery

 

 

 

Process-to-Process Delivery

• The data link layer is responsible for delivery of frames between two neighboring nodes over a link.
• This is called node-to-node delivery. -> 서로 다른 계층, ex) 1 - 2 계층 간 주고 받을 때 
• The network layer is responsible for delivery of datagram between two hosts.
• This is called host-to-host delivery. -> 동일한 계층끼리 주고받을 때 (host) 
  앞서 살펴본 1,2 그리고 3 계층 이야기

• The transport layer is responsible for process-to-process delivery, which is the delivery of a packet,
  part of a message, from one process to another.
  
• 프로세스 간 주고 받음 -> IPC에 해당한다고 한다.
  TCP Application Protocol is an IPC (Inter-process Communication) created to provide common remote interface for between applications.
  
• Two processes communicate in a client/server relationship.

 

• 4계층 간 통신 - 프로그램(프로세스) 간 통신을 의미
• 4계층의 가장 중요한 역할은 뭐냐? 
  * 에러 검출보다 중요한 것 
  도대체 어느 프로그램이 보낸거야? 누가 받아야 하는거야?
  즉, 송신/수신 프로그램의 식별 

 

 

 

Client/Server Paradigm

• The most common process-to-process communication is though the Client/Server Paradigm.
• Client : A process on the local host
• Server : A process on the remote host to provide services.
• Both processes (Client and Server) have the same name.
• Ex) day time Client process and day time Server process. For communication, we must define the following;
  • Local host
  • Local process
  • Remote host
  • Remote process 
    • 어느 컴퓨터의 / 무슨 프로그램 
      ip address / application address
      주요한 정보 4가지

 

 

 

Addressing

•  A frame in the data link layer needs a destination MAC address.
• At the network layer, we need an IP address.
• At the transport layer, we need a transport layer address, called a Port number to choose among multiple processes running on the host.
• Port number range : 0~65,535 (16-bit integer) Well-known port number
  : Universal port No. for server (fixed value, ranging : 0 ~1,023)
  • 고정되어 있으며, 사용해서는 안되는 well - known 포트 번호
  • 대부분 운영체제 서비스 
  • 이것 이상의 통신 프로그램을 띄울 수 없음
• Ephemeral port number : A port No.
  chosen randomly by the transport layer SW running on the client host.

 

• 결국 프로그램은 주소를 가져야 한다. 
  port number : 구멍의 번호
  프로그램이 운영체제 위에 구멍을 뚫고, 이를 통해 정보를 내리고 받음
  * cpp io stream -> file은 물리적인 위치에 바이트를 강물처럼 쏟아붇는 것
  port 구멍을 통해, 정보를 stream으로 내리는 것

 

• 가장 중요한 것은, 프로그램(프로세스)를 식별하는 것 
• IP Address vs Port number 
  • 아이피 주소(3)를 통해 컴퓨터 기기를 식별하고, 내부의 프로그램(프로세스)를 포트 넘버(4)를 통해 찾는다.

 

 

 

Socket Addresses

• 5계층 이상의 어플리케이션
  굳이 ip 주소와 포트 번호를 따로 줘야 할까?
  둘을 합쳐서 하나의 숫자로 표현하는 경우가 있다.
• socket address 라고 부르기도 하고, 둘을 합친 헥사 번호를 사용하기도 한다. 

 

• 소켓 - 콘센트 꽂는 것
  어플리케이션이 외부 통신을 위해
  운영체제에 통신을 위한 소켓을 꽂은 것이다.
  tcp udp 기반의 프로그래밍 == 소켓 프로그래밍
• IP 위에서 주소를 주고 받고, tcp 위에서 정보를 주고 받음

 

 

 

Connectionless vs Connection-Oriented Service

• Connectionless Service
  • The packets are not numbered; they may be delayed or connection release.
  • There is no acknowledgment either.
  • UDP protocol is connectionless in the Internet model.
  • 에러 검출 및 복구가 필요 없다면 UDP 써도 되는 것

 

• Connection-Oriented Service
  • A connection is first established between the sender and the receiver.
  • Data are transferred.
  • At the end, the connection is released.
  • TCP and SCTP are connection-Oriented protocols.
    

 

• osi 7 계층이 해야할 일
  • layer 4 - 서로 다른 컴퓨터 간 에러 검출, 복구, 흐름 제어
    이를 충실히 하는게 tcp
    충실히 안하는게 udp 
• 안해도 되냐? 통신은 안 할 수도 있음
• 에러 검출 및 복구 하려면 주고 받는 정보 매우 많다
  서로 준비 태세가 필요하다
  tcp - 연결 설정 과정을 한다 - 미리 정보를 주고 받아 보자
  준비 되었다면? 데이터를 쏘자! -> 커넥션이 먼저 준비되어야 한다. 
• 프로그래밍 코드 안에 이 과정을 확인할 수 있다. 

 

 

 

Reliable vs Unreliable

• Reliable Service (신뢰)
  • If the application layer program needs reliability,
    we use a reliable transport protocol such as TCP and SCTP.
  • This means a slower and more complex service.
• Unreliable Service
  • If the application layer program does not need reliability
    because it uses its own flow and error control mechanism 
    or it needs fast service
    or the nature of the service does not demand flow and error control (real-time application),
    then unreliable protocol such as UDP can be used.

• TCP 최대한 시도하다가 잘 모르겠다? -> 포기 보다는 reset
  연결 설정을 초기화한다. 왜? 
  프로토콜이 너무 복잡하기 때문에, 우리가 할 수 있는 최선은 Reset
• TCP 에러 검출 및 복구를 하지만,
  2계층과 마찬가지로
  4계층의 tcp도 최선을 다하는 것 -> 안되면 ? 포기

 

 

 

https://www.javatpoint.com/computer-network-transport-layer

 

 

 

• Do we need reliability control at the transport layer,
  even the data link layer(2계층) is reliable and has flow and error control?
• The answer is yes.
• The network layer in the Internet is unreliable (best- effort delivery),
  we need to implement reliability at the transport layer.
• 왜 2계층 -> 4계층도 또 에러검출 복구를 해야 해요? 
  1) 1-2계층에서 포기 가능 
  2) 3계층에서도 오류 날 수 있고, (라우터, 가능성은 거의 없지만)
  3) 정상적 오류 상황 (에러라고 보기는 어렵고)
   - 특정 포트로 트래픽이 몰리면 -> 버퍼 오버플로우 -> 패킷이 버려질 수 밖에 없다.