[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。

Basics

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

       

Protocol

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): 128.53.27.92
     • 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