[UCSD CSE120]网络-networks

本文是我在上UCSD的 CSE 120: Principles of Operating Systems (Winter 2020) 整理的笔记,这一课主要介绍了网络的概念,包括网络的类型和协议的概念。并且简单介绍了一下OSI7层模型,包括addressing和routing的过程。


  1. What is a Network?

    • Network

      • Set of computing nodes
      • Connected by communication links
      • Allow data transfer by a sender to a receiver
    • Internetwork: a network of networks

      • The “Internet” is a global internetwork
      • Nodes communicate using IP (Internet Protocol)
  2. Types of Networks

    • By topology: ring, star, bus, graph

    • By geographic coverage

      • LAN: local area network (spanning floor, building)
      • WAN: wide area network (spanning state, country)
  3. Circuit-switch vs. Packet switching

    • Circuit switching: establish path, send data

      • Reserve resources provide performance control
      • Example: telephone system

    • Packet switching: forward packets hop to hop

      • Fair sharing despite burst, statistical multiplexing
      • Example: postal system


  1. What is protocol?

    • Goal: get message from sender to receiver
    • Protocol

      • agreed message format and transfer procedure
    • Multiparty, so no central thread of control

      • Sender and reciver are separate processes
    • Expectations of operation

      • first you do x, then I do y, then you do z,…
      • If you do q, I’ll do p
  2. Message

    • Message: contains header and data

      • Similar terms: packet, datagram, frame
    • Data: what sender wants to receiver to know

    • Header: information to support protocol

      • Source and destination addresses
      • State of protocol operation
      • Error control (to check intergity of received data)
    • Example:

      • Assume: Ann sends message to Bob locally (same city)

        • Message format: (from, to), message contents
        • Transfer procedure: post on refrigerator

      • What if Ann sends message to Bob in different cities

        • Message format: address(es) on envelope, letter
        • Transfer procedure: postal system

    • Summary (Layering: Separation of Functions)

      • Ann and Bob

        • Don’t have to know about delivery
        • However, aid postal system by providing addresses
      • Postal System

        • Only has to know addresses and how to deliver
        • Doesn’t care about “data”: Ann, Bob, letter

7 Layers of OSI (Open Systems Interconnection) Refernce Model

layer name function
7 Application application protocol, e.g., HTTP
6 Presentation syntax, network format
5 Session start/stop/manage connections
4 Transport segment, reliability, flow control
3 Network logical addressing, routing
2 Link physical addressing, framing
1 Physical 0’s and 1’s over a wire
  1. Internet Protocol Stack

    • “Hourglass” design
    • Application: Email, Web
    • Session: sockets
    • Transport: TCP, UDP, …
    • Network: IP (Only one kind of protocol in this layer, which makes Internet Internet!)
    • Link: Ethernet, ATM, …
    • Physical
  2. Encapsulation

    • Higher level n with lower level n - 1
    • Can also have level within a level: tunneling
    • Multiplexing and de-multiplexing
  3. Addresses

    • Generally, three levels of addresses

      • Domain names: cs.ucsd.edu
      • Logical addresses (IP):
      • Physical addresses (Ethernet): 0x27A5BB17019D
    • Address resolution

      • Mapping higher level name to lower level name
      • Techniques: table lookup, formula, protocol
    • Sizes of address spaces

      • IPv4 (version 4, current/past)

        • 32 bit addresses
        • $2^{32}$ = 4 billion addresses
      • IPv6 (version 6, future/current)

        • 128 bit addresses
        • $2^{128} = 2^8\times (2^{10})^{12} = 256 \times (10^3)^{12} = 2.56 \times 10^{38}$
  4. Routing

    • Routing: how to get packet from A to B

      • A forwards to X; X to Y; Y to Z; Z to B

    • Each intermediate node can be a decision point

      • Static: always make the same decision
      • Dynamic: decision can change (e.g., based on state)
  5. Scalability

    • How well does system grow

      • in terms of performance, reliability, etc
    • Ramifications of adding node or link

      • Local effects vs. global effects
    • Information growth: import to reduce

      • Amount stored at nodes
      • Amount exchanged between nodes
  6. Error Control

    • Parity: even, odd, two-dimesional
    • CRC (cyclic redundance code)
    • Checksum
    • Automatic repeat request (ARQ)
  7. The two general’s problem