CS144 Lec2

  • gitk: UI for git

Reliability

Reliable retrieval a piece of data (get data)

  • Datagram is an unreliable abstraction (TCP/UDP Layer!)
  • How to build reliable abstraction on the top of datagram(unreliable): Protocol
    • client -> request -> server
    • client wait for xx time to get reply
      • if get reply -> done!
      • it time expires, no answer -> repeat send request
    • After yy times failed timeout, return an error to the user
  • Abstraction: Client, Server,Request, Reply
  • How to deal with " delivered but byte changed"?
    • Add some checkSum/ Redundant information: "The Sum of all byteds in my datagram == 57"
  • Idemopotent/幂等的: It doesn't matter how many times the operation happens

Reliable action (Change data)

  • Abstraction: State
  • "Fire the first available torpedo" - is none-idemopotent
    • "Fire torpedo #3" - is idemopotent: has same result
  • General Solution:
    • "My instruction #3 is xxxxxxx" - idemopotent
    • The server should remeber the number of instruction

TCP

  • How to send reliable byte stream

    • eg: ByteStream "ABCDEF"
    • index from 0, 1, 2, 3, 4, 5
    • The instructions will be: [lab 1,2,3,4]
      • "Byte indexs 0-2 is 'ABC' "
      • "Byte indexs 3-5 is 'DEF' "
      • "Byte stream finished at index #6"

  • RFC: request for comments

  • Internet Datagram: [IP layer ? ]

  • IP Datagram header

    • IHL: Header Length
    • Internet Datagram: from program to program ?

  • How to distinguish with datagram belongs to which application

    • protocol (1 byte)

    • Multiplexing. How to multiplexing? Using port number.

    • UDP: (Put in the "message" part of Internet Datagram) [TCP layer]

    • UDP: from program to program / Port number distinguishes diferent programs

    • IP Datagram header

    •   // send message to port 12345
  UDPSocket sock;
  Address destination{"1.2.3.4", "12345"};
  sock.sendto(destination, "Hello");
    •   // listen on port 12345
  UDPSocket sick;
  // 0.0.0.0 listen all the request from port 12345
  Address myaddress("0.0.0.0", "12345");  //port number 12345
  sock.bind(my_address);
  while(true){
      auto income_datagram = sock.recv();
      cout << income_datagram.source_address.to_string() << endl;
    //reply
    sock.sendto(income_datagram.source_address, "Thanks for your info!");
  }

  • package and datagram When we're talking about a datagram, we're specifically talking about connectionless communication and an unreliable service over a packet-switched network. So, we refer to "IP datagrams" and UDP datagrams" -- neither IP nor UDP set up connections or guarantee reliability.