chapter one-Computer networks an

总览.png

1、What is the Internet？

Two angles

• basic hardware and software components that make up the Internet(physics)
  hosts(end systems),communication links, packet switches(router:used in the network core, link-layer switch:used in access networks),ISPs(Internet Service Providers)

• networking infrastructure that provides services to distributed applications(function)
  End systems attached to the Internet provide an Application Programming Interface (API) that specifies how a program running on one end system asks the Internet infrastructure to deliver data to a specific destination program running on another end system

What is a protocol?

A protocol defines the format and the order of messages exchanged between two or more communicating entities, as well as the actions taken on the transmission and/or receipt of a message or other event.

2、The Network Edge

• end system(sit at the edge of the Internet)/host(clients and servers)
• access networks
  network that physically connects an end system to the first router(edge router )
  将端系统连接到其边缘路由器的物理链路

3、The Network Core

packet switching

message: sources that end systems exchange with each other
packet: breaks long messages into smaller chunks of data known as packets
Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link(如果链路传输速率为R bit/秒，传输L bits的packets，传输时间为L/R秒)

• Store-and-Forward Transmission
  packet switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link.(receive, store before forward)
• Queuing Delays and Packet Loss
  If an arriving packet needs to be transmitted onto a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer.----suffer queuing delays
  If the amount of buffer space is finite, an arriving packet may find that the buffer is completely full----packet loss will occur
• Forwarding Tables and Routing Protocols
  a router uses a packet’s destination address to index a forwarding table and determine the appropriate outbound link
  routing protocols that are used to automatically set the forwarding tables
• Circuit Switching(another approach to moving data through a network of links and switches(besides packet switching))
  resources needed along a path (buffers, link transmission rate) to provide for communication between the end systems are reserved for the duration of the communication session between the end systems.
  frequency-division multiplexing(FDM)
  time-division multiplexing(TDM)
  With FDM, each circuit continuously gets a fraction of the bandwidth. With TDM, each circuit gets all of the bandwidth periodically during brief intervals of time (that is, during slots)
• Packet Switching Versus Circuit Switching
  better sharing of transmission capacity
  simpler, more efficient, and less costly to implement

A Network of Networks

A Network of Networks

ISPs themselves must be interconnected to achieve the goal that every end system can be connected with each other

4、Delay,Loss and Throughput in Packet-Switched Networks

Delay:

• nodal processing delay:examine the packet’s header and determine where to direct the packet(mainly, other factors such as checking the errors in the packet also belong to processing delay)
• queuing delay: wait to be transmitted onto the link
• transmission delay:because of Store-and-Forward Transmission ,the whole packet needed to be received before transmitted
• propagation delay :the time required to propagate from the beginning of the link to router B(depends on physical medium of the link)
• transmission delay VS propagation delay
  • transmission delay is the amount of time required for the router to push out the packet, it is a function of the packet's length and the transmission rate of the link, nothing to do with the distance between two routers;

  • propagation delay is the time it takes a bit to propagate from one router to the next; it is a function of the distance between the two routers;
    白话理解：(本书采用了一个汽车组经过收费站的例子)
    传输时延：集齐一个分组内所有比特再进行转发所等待的时间，如路由器A等待一个分组，从分组的第一个比特开始传输进A到该分组最后一个比特存储完毕，准备转发至路由器B
    传播时延：分组在链路的物理介质中传播的时间

    
    The nodal delay at router A.png

Queuing Delay and Packet Loss

• queuing delay is pretty complicated ( if 10 packets arrive at an empty queue at the same time, the first packet transmitted will suffer no queuing delay, while the last packet transmitted will suffer a relatively large queuing delay (while it waits for the other nine packets to be transmitted))

• estimating the extent of the queuing delay: traffic intensity
  Let a denote the average rate at which packets arrive at the queue (a is in units of packets/sec);R is the transmission rate(bits/sec), all packets consist of L bits
  La/R called traffic intensity
  白话理解：a为一秒钟内到达队列的分组个数，R为传输速率(每秒钟路由器可以推出去bit数)，每个分组内含有Lbit，那么La/R就是一个进入量/推出量

  
  Dependence of average queuing delay on traffic intensity.png
  • Packet Loss
    With no place to store such a packet, a router will drop that packet

  End-to-End Delay

Throughput in Computer Networks

• If the file consists of F bits and the transfer takes T seconds for Host B to receive all F bits, then the average throughput of the file transfer is F/T bits/sec(接受文件的速率)
• a network with N links between the server and the client, the throughput for a file transfer from server to client is min{R1, R2,..., RN}

5、Protocol Layers and Their Service Models

Layered Architecture

each layer provides its service by (1) performing certain actions within that layer and by(2) using the services of the layer directly below it.

The Internet protocol stack (a) and OSI reference model (b).png

• Application Layer
• Transport Layer
• Network Layer
• Link Layer
• Physical Layer

Encapsulation

6、Networks Under Attack

7、History of Computer Networking and the Internet

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掌握计算机网路领域知识的过程就是理解网络协议的构成、原理和工作方式的过程。
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