跳转至

1: Introduction

约 2535 个字 预计阅读时间 13 分钟

警告

前半部分仅删改小角龙笔记；因为很容易出概念题，后半部分摘录书上原文，作为补充。

小角龙

1.1 网络模型

  • OSI 七层模型：三个重要概念：服务，接口和协议
    • 物理层：在一条通信信道上传输比特
      • 要确定用什么电子信号表示0和1，一个比特持续多久，传输是否可以双向，连接如何建立等等
    • 数据链路层：发送方将输入数据拆分成若干数据帧(frame)，然后按照一定的顺序来发送这些数据帧
      • 接收方必须确认收到的每一帧，保证数据的顺序和完整性，然后发回确认帧
      • 为了防止发送方淹没慢速的接收方，需要对流量进行调节
      • 介质访问控制子层则是控制对共享信道的访问，没有拥塞控制的功能
    • 网络层：控制子网的运行，将数据包从源端发送到接收方，需要处理拥塞
      • 允许异构的网络相互连接成互联网络
    • 传输层：接受会话层的数据，将数据分割成小单元，然后传到网络层
      • 传输层是真正的端到端的层，始终将源数据从源端带到接收方
      • OSI模型中1-3层是链式连接的，而4-7层是端到端的
    • 会话层：负责管理主机间的会话进程，利用传输层提供的端到端的服务，向表示层提供增值服务，主要表现为表示层实体或者用户进程建立连接并在连接上传输数据
      • 这一过程也称为建立同步
    • 表示层：关注传递信息的语法和语义，主要是实现数据的格式转换和压缩
    • 应用层：包含了各种各样的协议，提供了用户和网络的接口
      • 软件开发基本都是应用层的工作 image-20200922231442750
  • TCP/IP
    • 原本是Linux/Unix下的一个网络通信栈，比较OSI七层模型简单
    • 没有表示层和会话层，物理层和数据链路层Host-to-network
    • 因为开放，所以用的人多，所以成为了互联网的标准
  • 混合模型 hybrid model：本课程讨论的模型
    • 物理层
    • 数据链接层
    • 网络层：可能分为内层和外层
    • 传输层
    • 应用层

1.2 协议架构

  • 数据传输的过程：
    • 在source machine中，数据自顶向下传输，每一层会给上一层传输的数据包添加一个header，如果传输的数据包过大则会把数据包进行拆分，但是header不会进行拆分
    • 在destination machine中，则一步步去掉底层的header，并将小的数据包合并

1.3 服务 Service

  • 服务类型

    • 面向连接的服务 connection-oriented
      • Reliable message stream 可信信息流，比如page的序列
      • Reliable byte stream 可信字节流，比如远程登录，TCP/IP协议向应用层发送的是字节流
      • Unreliable connection 不可信连接，比如数字化语音
    • 不需要连接的服务 connection-less
      • Unreliable datagram 不可信数据报，比如垃圾邮件
      • Acknowledged datagram 共识数据包，比如注册邮件
      • Request-reply 请求回复，比如数据库查询
    • QoS 服务的质量

  • 服务原语 Service Primitives

    • 面向连接服务的5个服务原语
    Primitive Meaning
    LISTEN 等待一个还没来的连接
    CONNECT 和一个等待的对等层进行连接
    RECEIVE 等待一个还没来的message
    SEND 向对等层发送一个message
    DISCONNECT 结束一个连接
    • Request--Indication--Response--Confirm

1.4 C/S模式

  • Client-Server Model 客户端主动连接，服务端被动连接
    • 客户端之间不能直接通信，客户端的通信也需要经过服务器
    • 新技术：服务器分配端口，让两个客户端直接通信，不需要所有数据经过服务器
    • 当客户端升级的时候需要更新整个客户端，手机APP就是C/S模式
  • Browser-Server Model 从C/S体系发展而来
    • 通过浏览器来实现用户和服务端的交互，只需要升级浏览器

  • Peer-to-peer Model(P2P模式)——对等网络

    • 每个客户端同时又是服务端，没有固定的客户端和服务端

  • wireless network 无线网络

    • 注意和移动计算mobile computing的区别，移动计算可以是有线的，可以是无线的
    • wifi就是一种无线网络

1.5 网络传输

  • 三种传输方式：
    • Broadcast links 广播式传输
      • 传输模型类似于总线，总分式结构
      • 以太网的原理都是基于广播的
    • Point-to-point links 点对点传输
      • 传输模型是一张复杂的网——网络拓扑结构
      • 一次只发给一台计算机
    • multicasting 多点广播
      • 传输模型是一个环，一次发给一组计算机
      • 不是天然存在的，需要通过软件系统来实现多点广播机制
  • 网络的类型：按照规模来划分
    • 个人区域网 Personal area network(PAN)
    • 局域网 Local area network(LAN)
    • 层域网Metropolitan area network(MAN)
      • 技术的可替代性比较强，局域网大一点就可以作为层域网，广域网小一点就可以作为层域网
    • 广域网Wide area network(WAN)
      • 特点是会有很多router和subnet
    • 最大的网络就是因特网 The Internet

原文摘录

1.1 Uses of Computer Networks

Computer network means a collection of interconnected, autonomous computing devices.

Internet is the most well-known example of a network of networks.

The client-server model is widely used and forms the basis of much network usage. The most popular realization is that of a Web application, where a server generates Web pages based on its database in response to client requests that may update the database.

Another popular model for accessing information is peer-to-peer communication. Every person can, in principle, communicate with one or more other people; there is no fixed division into clients and servers.

Ubiquitous computing entails computing that is embedded in everyday life.

Devices such as televisions can use power-line networks to send information throughout the house over the wires that carry electricity.

1.2 Types of Computer Networks

计算机网络的主要种类包含 Broadband Access Networks 和 Mobile and Wireless Access Networks。

Wireless network

  • Cellular networks operated by telephone companies
  • Wireless hotspots based on the 802.11 standard

Although wireless networking and mobile computing are not identical: fixed wireless and mobile wireless networks.

image-20250110182546733

Smartphones combine aspects of mobile phones and mobile computers, connect to wireless hotspots and automatically switch between networks to choose the best option for the user.

  • Text messaging or texting (SMS Short Message Service) over the cellular network was popular.
  • Mobile phones are being increasingly used in m-commerce (mobile-commerce).
    • Equipped with NFC (Near Field Communication), the mobile can act as an RFID smartcard and interact with a nearby reader for payment.
  • GPS (Global Positioning System) can directly locate a device, and mobile phones often also triangulate between Wi-Fi hotspots with known locations to determine their location. geo-tagging

Sensor networks have nodes that gather and relay information they sense about the state of the physical world.

Modern data center networks have hundreds of thousands or millions of servers in a single location, serving cloud computing and are designed to move large amounts of data between servers in the data center / between the data center and the rest of the Internet.

CDN (Content Delivery Network) is a large collection of servers that are geographically distributed in such a way that content is placed as close as possible to the users that are requesting it.

When the content provider and your ISP (Internet Service Provider) are not directly connected, they often rely on a transit network to carry the traffic between them. Transit networks are traditionally called backbone networks because they have had the role of carrying traffic between two endpoints.

Networks called VPNs (Virtual Private Networks) connect the individual networks at different sites into one logical network.

A computer network can provide a powerful communication medium among employees.

Telephone calls between employees may be carried by the computer network instead of by the phone company, this is called IP telephony or VoIP (Voice over IP).

Desktop sharing lets remote workers see and interact with a graphical computer screen.

1.3 Network Technologies

PANs (Personal Area Networks) let devices communicate over the range of a person. A common example is a wireless network that connects a computer with its peripherals.

A LAN (Local Area Network) is a private network that operates within and nearby a single building.

In wireless LANs (IEEE 802.11), each computer talks to a device called an AP (Access Point), wireless router, or base station.

Another common scenario entails nearby devices relaying packets for one another in a so-called mesh network configuration. In some cases, the relays are the same nodes as the endpoints, more commonly, a mesh network will include a separate collection of nodes whose sole responsibility is relaying traffic.

Many wired LANs comprise point-to-point wired links. IEEE 802.3, popularly called Ethernet, is by far the most common type of wired LAN.

  • A switch has multiple ports, each of which can connect to one other device, such as a computer or even another switch.

A MAN (Metropolitan Area Network) covers a city. The best-known examples of MANs are the cable television networks.

  • Both television signals and Internet are fed into the centralized cable head-end.

A collection of interconnected networks is called an internetwork or internet. We will use these terms in a generic sense, in contrast to the global Internet (which is one specific internet).

The device that connects two or more networks and provides the necessary translation, both in terms of hardware and software, is a gateway.

1.5 Network Protocols

1.5.1 Design Goals

Reliability

  • error detection & correction
  • routing: finding a working path through a network

Resource Allocation

  • Designs that continue to work well when the network gets large are said to be scalable.
  • statistical multiplexing: sharing based on the statistics of demand.
  • flow control: Feedback from the receiver to the sender.
  • congestion: the network is oversubscribed because too many computers want to send too much traffic.
  • real-time delivery
  • Quality of service (QoS) is the name given to mechanisms that reconcile these competing demands.

Evolvability

  • protocol layering: the key structuring mechanism used to support change by dividing the overall problem and hiding implementation details.
  • addressing (in low layers) or naming (in high layers): identifying the senders and receivers that are involved in a particular message.
  • internetworking: number messages & disassembling, transmitting, and then reassembling messages.

Security

  • confidentiality: defend eavesdropping on communications
  • authentication: prevent someone from impersonating someone else
  • integrity: prevent surreptitious changes to messages

1.5.2 Protocol Layering

To reduce their design complexity, most networks are organized as a stack of layers or levels.

A protocol is an agreement between the communicating parties on how communication is to proceed.

The entities comprising the corresponding layers on different machines are called peers, may be software processes, hardware devices, or even human beings.

  • peers communicate by using the protocol.

Between each pair of adjacent layers is an interface, it defines which primitive operations and services the lower layer makes available to the upper one.

A set of layers and protocols is called a network architecture.

A list of the protocols used by a certain system, one protocol per layer, is called a protocol stack.

1.5.3 Connections and Reliability

Connection-oriented service: when a connection is established, the sender, receiver, and subnet conduct a negotiation about the parameters to be used.

  • A circuit is another name for a connection with associated resources, such as a fixed bandwidth.

Connectionless service: Each message carries the full destination address, and each one is routed through the intermediate nodes inside the system independent of all the subsequent messages.

  • store-and-forward switching: the intermediate nodes receive a message in full before sending it on to the next node.
  • cut-through switching: the onward transmission of a message at a node starts before it is completely received by the node.

acknowledged datagram service: Used when not having to establish a connection to send one message is desired, but reliability is essential. It is like sending a registered letter and requesting a return receipt.

request-reply service: the sender transmits a single datagram containing a request; the reply contains the answer. client-server model.

image-20250110192906359

1.6 Reference Models

OSI失败的原因：Bad Timing、Bad Design、Bad Implementations、Bad Politics

1.9 Metric Units

image-20250110191834565