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Notes on data Structures and Algorithms

Notes for geek Time iOS Developer Class

IOS large factory interview high frequency algorithm summary

Summary of iOS interview materials

Network interconnection model

  • In order to better promote the research and development of the Internet, the international organization for Standardization ISO developed the network interconnection model in 1985
    • OSI (Open System Interconnect Reference Model), with 7 layers structure

Request process

The network layer

The physical layer

  • The physical layer defines interface standards, cable standards, transmission rates, and transmission modes

Digital signal, analog signal

  • Analog Signal
    • Continuous signal suitable for long distance transmission
    • Poor anti-interference ability, waveform distortion is difficult to correct when disturbed

  • Digital Signal
    • Discrete signals, not suitable for long distance transmission
    • Strong anti-interference ability, waveform distortion can be repaired when disturbed

Data communication model

Channel (Channel)

  • Channel: The channel through which information is transmitted. There can be more than one channel on a transmission medium (such as a network cable)
  • Simplex communication:
    • A signal can only be transmitted in one direction, and the direction of transmission cannot be changed at any time
    • Such asRadio broadcast.Cable television broadcasting
  • Half duplex communication:
    • Signals can be transmitted in both directions, but they must be transmitted alternately, only in one direction at a time
    • Such asintercom
  • Full duplex communication:
    • Signals can be transmitted both ways simultaneously
    • Such asMobile phone

Data link layer

  • Link: A physical line (wired or wireless) from one node to an adjacent node, with no other switching nodes in between

  • Data link: When data is transmitted over a link, a corresponding communication protocol is required to control data transmission
    • Different types of data links may use different communication protocols
      • Broadcast channel: CSMA/CD protocol (e.g. network of coaxial cables, hubs, etc.)
      • Point-to-point channel: PPP (e.g. channel between 2 routers)
  • There are three basic problems with the data link layer:
    • Encapsulated into a frame
    • Transparent transmission
    • Error inspection

Encapsulated into a frame

  • The data part of a Frame
    • Is the Packet (IP Packet, Packet) transmitted from the network layer.
  • Maximum Transfer Unit (MTU)
    • Each data link layer protocol sets an upper limit on the data length of frames that can be transmitted
    • The MTU of the Ethernet is 1500 bytes

Transparent transmission

Error inspection

CSMA/CD protocol

  • CSMA/CD (Carrier Sense Multiple Access with Collision Detection)
    • Carrier sense multiple access/collision detection
  • Using theCSMA/CDThe network of is can be calledEthernet, which transmits Ethernet frames
    • Ethernet frames are in the following formats: Ethernet V2 standard, IEEE 802.3 standard
    • The most used is: Ethernet V2 standard
  • Ethernet frames are at least 64 bytes long in order to be able to detect whether a conflict has occurred with the frame being sent
  • The network consisting of switches already supports full-duplex communication and does not need to be usedCSMA/CDProtocol, but the frames it transmits are still Ethernet frames
    • So networks that use switch components are still called Ethernet

Ethernet V2 frame format

  • First:Source MAC address + Destination MAC Address + Network type (IPv4 or IPv6)
  • Ethernet frames:Head + data + FCS
  • The length of the data is at least:64 - 6 - 6 - 2 - 4 = 46 bytes

Standard Ethernet V2

  • When the length of the data part is less than46Byte time
    • The data link layer adds some byte padding after the data
    • The receiver will strip out the added bytes

  • Length summary:
    • Ethernet frame data length:46~1500bytes
    • Ethernet frame length:64~1518bytes(Destination MAC address + Source MAC address + Network type + Data + FCS)

The network card

  • When a nic receives a frame, it checks the frame for errors. If the frame passes the check, it receives the frame. Otherwise, it discards the frame
  • WiresharkNo frames caughtFCS, because it catches the frame that passed the error checkFCSWill be removed by hardware)
  • WiresharkFailed frame for error check could not be caught

The PPP protocol

  • AddressFields: The values in the figure are0xFFPoint-to-point channels do not require source MAC addresses or destination MAC addresses
  • ControlFields: The values in the figure are0x03What is the current effect
  • ProtocolField: Protocol type used internally
  • Frame start, frame end:0x7E

Byte filling

  • will0x7Ereplace0x7D5E
  • will0x7Dreplace0x7D5D