技術類文章

BDC CH2: Network Model

CH2: Network Model

  • Protocol Layering
    • 定義通訊上的規則
      • A protocol defines the rules that both the sender and receiver and all intermediate devices need to follow to be able to comunicate effectively
    • Same layers do the same thing (同一層做相同/相似的事)

Difference between TCP/IP Model and OSI Model

  • TCP/IP model has four different layers:
    • (對照到OSI Model)
    • Application(Application+Presentation+Session)
      • HTTP、FTP、TELNET、NTP、DHCP、PING
    • Transport(Transport)
      • TCP、UDP
    • Network(Network)
      • IR、ARP、ICMP
    • Network Interface(Data Link + Physical)
      • Ethernet、Fiber、Routers、Switches

How to make communication approach?

  • Cross layer en-capsulation and de-capsulation
    • 把上層送來的內容當成自己這層的payload再加上自己這層的header後包在一起往下層送
    • 收到下層送來的整包內容後先拆解掉屬於自己這層的header再把payload往上層送
    • 因此header會在過程中不斷被修改

TCP/IP Protocol Suite(僅列重點，詳細看講義)

Physical Layer

  • Data format: bit
  • Major duties:
    • topology
    • simplex、 half-duplex、full-duplex
    • direction of data flow…etc
  • Data format: frame
  • Major duties:
    • Framing and tagging
      • 可以僅讓擁有特定tag的通過
      • 增加tag到frame header中，可將第二層各內容進行分割
    • MAC Address(header)
      • 決定送給哪一個實體層裝置
      • 理論上唯一
    • Switching
      • 根據MAC位置決定轉發給誰
  • Address Resolution Protocol(ARP)
    • Find the link layer (physical) address of the device(找MAC位置)
      • 一開始以廣播方式尋找目的地之MAC位置
      • switch有紀錄功能，所以可依特定路徑傳送frame

Network Layer

  • Data format: datagram (packet，封包)
  • Major duties:
    • IP address
    • Routing (連接不同的子網路/網段)
    • Find the next hop
  • Internet Protocol(IP位置)
    • 路由器可透過路由協定去學習封包如何前往其他網域的路徑，並在自身維護一個路由表紀錄Next-hop資訊供IP routing所使用
    • 封包IP位置不在路由表內則被丟棄
    • 和自己不同子網域的IP電腦溝通需要依靠Layer IP routing

Transport Layer

  • Data format: TCP segment / UDP datagram
  • Major duties:
    • Service port mapping
    • Connection (TCP) or connectionless (UDP)
      • TCP會檢查是否有漏payload
    • Flow control
    • Error control
    • Retransmission
  • Some Important Number
    • FTP: 20、21
    • SMTP: 25
    • DNS: 53
    • HTTP: 80
    • SSH: 22
    • TELNET: 23
    • DHCP: 67
  • Details of each transmission protocol
    • Transmission Control Protocol (TCP)
      • Reliable-connection
      • Btye-oriented
      • Retransmission
    • User Datagram Protocol (UDP)
      • Unreliable-transmission
      • Message-oriented
    • Stream Control Transmission Protocol (SCTP)
      • Reliable-connection
      • Message-oriented
      • Combine the best features of TCP and UDP

Application Layer

  • Data format: Message
  • Common protocols: HTTP、FTP、SMTP…etc

Multiplexing and De-multiplexing

  • Message to Packet -> Multiplexing
  • Packet to Message -> De-multiplexing (還原訊息)
  • 依照目的地傳送封包
  • 大訊息可能被拆成數個封包進行傳送

OSI Reference Model

  • PDU (Protocol Data Unit)
    • ex. Data、Segement/Datagram、Packet、Frame、Bit/Symbol
  • SDU (Service Data Unit)
    • payload
  • PDU = SDU (payload) + Protocol Header
  • At the layer N, the SDU will be added “layer N header” to produce a layer N PDU. After that, it will be passed to layer N-1.
  • This process continues util reaching the layer 1.
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