计网第一章笔记----绪论

文章整理自西安交通大学软件学院朱利老师的课堂讲解，仅供复习参考使用，请勿转载

这一章笔记当时上的是网课，根本不知道老师在讲啥，然后记的就很迷，感觉没有层次结构，而且还全是英文名，后面几章的会好一点，都是上课跟着老师的叙述记的中文了

电信网络：PSTN，打电话网络（public switch telephone network）

NAP：网络接入点，network access port

ISP：网络服务商，internet service provider

网络设计之初是军用发邮件的

什么是Internet

• 是一个世界范围内的计算机网络
• 是一个公用网络

架构

edges（边缘），core（内核），links（链路）

硬件

• Ends Systems
• Core: routers
• links

软件

• Protocols & App
• Networking OS

What’s the Internet: “nuts and bolts” view 基本

• millions of connected computing devices: hosts = end systems
  • running network apps
• communication links
  • fiber, copper, radio, satellite, LAN
  • transmission rate = bandwidth
• routers:（路由） forward packets (chunks of data)
• protocols（协议）control sending, forwarding,receiving of messages(报文)
  • ve.g., TCP, IP, HTTP, Skype, Ethernet
• Internet: “network of networks”
  • loosely hierarchical
  • public Internet versus private intranet（内部网，元区网）
• Internet standards
  • RFC: Request for comments（协议）
  • IETF: Internet Engineering Task Force（维护RFC的工程任务组）

What’s the Internet: a service view

• communication infrastructure enables distributed applications:
  • Web, VoIP（IP网的声音服务）, email, games, e-commerce, file sharing
• communication services provided to apps:
  • reliable data delivery from source to destination
    • 数据不丢
    • 无差错
    • 传输顺序不乱
  • “best effort” (unreliable) data delivery

What’s a protocol?

protocols define format, order of msgs and actions taken on msg transmission, receipt
$$\begin{array}{rl}Protocol& =\{format,order,actions\}\\ & =\{syntax,semantic,rules\}\\ & =\{语法，语义，规则\}\end{array}$$
network protocols:

• machines rather than humans
• all communication activity in Internet governed by protocols
• Protocols are running everywhere in the Internet, different protocols are used to accomplish different communication tasks

The network edge:

• end systems (hosts):
  • run application programs
  • e.g. Web, email
  • at “edge of network”
• client/server model(C/S)
  • client host requests, receives service from always-on server
  • e.g. Web browser/server; email client/server
• peer-peer model(p2p)
  • minimal (or no) use of dedicated servers
  • e.g. Skype, BitTorrent

Access networks and physical media

How to connect end systems to edge router?

• residential access nets(驻家接入方式)
• institutional access networks (school, company)（单位接入网络）
• mobile access networks（移动接入网络）

Keep in mind（关键点）:

• bandwidth (bits per second) of access network?
• shared or dedicated?
• fee

Dial-up Modem

拨号方式

Internet的通信方式为串行通信，使用的方式就是Modem（不知道啥意识）

• Uses existing telephony infrastructure
  • Home is connected to central office（电话网络的服务商）
• up to 56Kbps direct access to router (often less)
• Can’t surf and phone at same time: not “always on”（不能同时上网和打电话）

Asymmetric Digital Subscriber Line (ADSL)

不对称的数字用户线

DSLAM：数字用户线接入复用器

下行链路的带宽宽一点，下行链路的带宽窄一点

下行是从服务商到自己这里

上行是从自己这里到服务商

• Also uses existing telephone infrastructure
• up to 1 Mbps upstream (today typically < 256 kbps)
• up to 8 Mbps downstream (today typically < 1 Mbps)
• dedicated physical line to telephone central office

Residential access: HFC

混合光纤同轴电缆

CMTS：电缆modem终端系统

驻家使用的是同轴电缆，到服务商使用的是电缆

• HFC: hybrid fiber coax
  • asymmetric: up to 30Mbps downstream, 2 Mbps upstream（不是专用的，好多家共享的）
• network of cable and fiber attaches homes to ISP router
  • homes share access to router
  • unlike DSL, which has dedicated access

FTTH:Fiber to the Home

optical line termial：光学线终端

• Optical links from central office to the home
• Two competing optical technologies:
  • Passive Optical network:PON, no power supply（被动光网，无需供电）
  • Active Optical Network:AON（主动光网，需要供电）
• Much higher Internet rates; fiber also carries television and phone services

Ethernet Internet access

以太网

局域网里面的交换设备

Wireless access networks

无线局域网接入

• shared wireless access network connects end system to router
  • via base station aka “access point”

wifi：是access point

基站是base station

需要区分清楚

Physical Media

• Bit: propagates between
  transmitter/rcvr pairs
• physical link: what lies between transmitter & receiver
• guided media:
  • signals propagate in solid media: copper, fiber, coax
• unguided media:
  • signals propagate freely, e.g., radio

Twisted Pair (TP)

双绞线

• two insulated copper wires
  • Category 3: traditional phone wires, 10 Mbps Ethernet
  • C5, C5+, C6

Coaxial cable

同轴电缆

• two concentric copper conductors
• bidirectional
• baseband:
  • single channel on cable
  • legacy Ethernet
• broadband:
  • multiple channels on cable
  • HFC

Fiber optic cable

光缆

• glass fiber carrying light pulses, each pulse a bit
• high-speed operation:
  • high-speed point-to-point transmission (e.g., 10’s-100’s Gps)
• low error rate: repeaters spaced far apart ; immune to electromagnetic noise

需要使用光纤焊接机将两根光缆焊接起来

需要使用光缆接续盒将焊点保护起来

radio

• signal carried in electromagnetic spectrum
• no physical “wire”
• bidirectional
• propagation environment effects:
  • reflection
  • obstruction by objects
  • interference

The Network Core

• mesh of interconnected routers
• the fundamental question: how is data transferred through net?
• circuit switching(电路交换):
  • dedicated circuit per call: telephone net
• packet-switching(包交换):
  • data sent thru net in discrete “chunks”
  • 存储+转发

Circuit Switching

End-end resources reserved for “call”

拨号后相应的资源（带宽）就留好了，需要有一个呼叫建立（拨号）的过程

• link bandwidth, switch capacity
• dedicated resources: no sharing
• circuit-like (guaranteed) performance
• call setup required

network resources (e.g., bandwidth) divided into “pieces”

• pieces allocated to calls
• resource piece idle if not used by owning call (no sharing)

通信方式(电话网络使用的方式，不是internet使用的)

• FDM:frequency division multiplex（频率复用）
  • HDTV,ADSL,WLAN
• TDM:time division multiplex（时分复用）
  • telephone
• CDMA:code division multiplex access
• WDM（光缆通信）

FDM和TMD对比

例子

Packet Switching

each end-end data stream divided into packets

• user A, B packets share network resources
• each packet uses full link bandwidth
• resources used as needed

resource contention

• aggregate resource demand can exceed amount available
• congestion: packets queue, wait for link use
• store and forward: packets move one hop at a time
  • Node receives complete packet before forwarding

Statistical Multiplexing（统计复用）

A->D的最大带宽为1.6Mb/s

store-and-forward

• takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
• store and forward:
  • entire packet must arrive at router before it can be transmitted on next link
• delay = 3L/R (assuming zero propagation delay)

Example:

• L = 7.5 Mbits
• R = 1.5 Mbps
• transmission delay = 15 sec

3*7.5/1.5=15s

Packet switching versus circuit switching

ATM:异步传输模式(async transmit mode)

• 1 Mb/s link
• each user:
  • 100 kb/s when “active”
  • active 10% of time
• circuit-switching:
  • 10 users
• packet switching:
  • with 35 users, probability for 10 active at same time is less than 0.0004

分组交换网络的好处

• great for bursty data（适合突发数据）
• resource sharing/utilization better（更好地资源共享）
• simpler, no call setup（比较简单，没有拨号的过程）
• More users carried（能承载的用户量更大）

分组交换网络的缺点

• 不适合实时性较高的东西
• excessive congestion: packet delay and loss
  • protocols needed for reliable data transfer, congestion control

network of networks

网络是大致分层的

• at center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage
  • treat each other as equals
• Tier-2 ISPs: smaller (often regional) ISPs
  • Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
• Tier-3 ISPs and local ISPs
  • last hop (“access”) network (closest to end systems)

How do loss and delay occur?

packets queue in router buffers

• 链路的数据包到达率超过了输出链路容量
• packets queue, wait for turn

Delay in packet-switched networks

1. nodal processing: （节点处理）
   • check bit errors
   • determine output link，确定输出口
2. queueing（排队）
   • time waiting at output link for transmission （等待传输）
   • depends on congestion level of router 取决于路由器的拥塞级别
   • 是一个非线性关系
3. Transmission delay（传送延迟）
   • R=link bandwidth (bps)
   • L=packet length (bits)
   • time to send bits into link = L/R
4. Propagation delay（传播延迟）
   • d = length of physical link
   • s = propagation speed in medium (3x108 m/s)
   • propagation delay = d/s

Nodal delay

• $d_{proc}$ = processing delay
  • typically a few microsecs or less
• $d_{queue}$= queuing delay（最不能够忽略不计）
  • depends on congestion
• $d_{trans}$= transmission delay
  • = L/R, significant for low-speed links
• $d_{prop}$= propagation delay
  • a few microsecs to hundreds of msecs

Queueing delay (revisited)

队列延迟

• R=link bandwidth (bps)
• L=packet length (bits)
• a=average packet arrival rate

traffic intensity = La/R

• La/R ~ 0: average queueing delay small
• La/R -> 1: delays become large
• La/R > 1: more “work” arriving than can be serviced, average delay infinite!（可能丢包）

查看网络链接的命令

• ping hostname or IP address
• Tracert 202.117.1.13

For all i:

• sends three packets that will reach router i on path towards destination
• router i will return packets to sender
• sender records time interval between transmission and reply.

Throughput

互联网上丢包是一种常态

• 路由器发送端队列满了
• TTL寿命到0会被主动丢掉（垃圾包）
• 接收端的缓存满了

throughput: rate (bits/time unit) at which bits transferred between sender/receiver

instantaneous 瞬时速率: rate at given point in time

average 平均速率: rate over longer period of time

单位：bps

Rs(rate server)：服务器发送速率

Rc(rate client)：接收端接收速率

当服务器的带宽可以看为无穷时，传输速率是Rs和Rc的最小值

如果不能看为无穷时，实际传输速率是三者中的最小值，称为瓶颈

瓶颈链路(bottleneck link)：link on end-end path that constrains end-end throughput

• 有十组会话（一个Rs和Rc链路）
• 每个会话端对端吞吐率是min(Rc,Rs,R/10)
• in practice: R or Rs is often bottleneck

Protocal Layers 网络协议

Hierarchical Organizing 分层架构

好处

• 对某一个模块更新的时候不用考虑其他层
• 系统结构清晰

缺点

• 分层越多，通信效率越低

Internet protocol stack-TCP/IP

TCP/IP五层模型、Internet的五层模型

• 应用层：支持网络应用
  • FTP,SMTP,HTTP,apps
• 传输层：给应用层提供通信支撑
  • TCP（可靠的通信，transport control protocol）：不出差错，顺序不会乱
  • UDP（不可靠的通信，user datagram protocol）：有没有差错，顺序乱不乱不知道
• 网络层：数据报从源到目的地的路由
  • IP,routing protocol（路由协议）…
• 链路层：相邻网元之间的数据传输
  • PPP,（point to point protocol）
  • Ethernet ,ADSL,FTTH,WI-FI…
• 物理层：bits “on the wire”

物理层是第一层，是基础

沙漏模型（sandglass Model）：细腰，中间最细（传输层）

每向下走一层后，都会把上一层的整个放到下面一层的数据域中，然后后添加一个头，形成新的封装格式

报文，段，数据报，帧统称为PDU（protocol data unit协议数据单元）

每一层的PDU叫什么一定要记住

例如第四层的PDU叫做段

• 发送端
  • 将协议进行封装，封装完了称为报文（message）
  • 到了传输层后，会再次封装，形成 头 + 数据域的格式 ，称为段（segment）
  • 到了网络层之后，会再次风祖航，形成 头 + 数据域的格式，称为数据报（datagram），也叫做包（package）
  • 到了链路层，再加一个头，称为帧（frame）
  • 物理层的PDU是位（bit）
• 交换机
  • 只有两层，链路层和物理层
  • 到link层进行转发，称为层2转发
• 路由器
  • 有三层，物理层，链路层，网络层
  • 入口是物理层，进入后爬三层
  • 路由器是层3设备，必须在第三层才能进行转发
  • 每经过一个路由器，都必须爬三楼，下三楼
• 目的地
  • 每一层都会检查是否有差错
  • 如果没有问题就去掉头向上传，最后恢复为message

Internet不适合实时通信，对比电话线

ISO/OSI reference model

OSI七层模型

能说出来七层模型都是什么就行了，自顶向下顺序不能说错

ISO：internet standard organization国际标准化组织

OSI：open system interconnection 开放系统互联

• 应用层 application
  • 与五层模型的应用层不一样
• 表示层 presentation
  • 解释数据的含义
  • 加密，压缩（在五层模型中一般是在应用层用户自己进行的）
• 会话层 session
  • 发送端和接收端进行数据同步的（设置检查点，出现问题后从检查点重新传输）

分层越多，效率越低所以现在很少有采用这种结构的

lnternet history

1967：起源于美国的一个项目，ARPAnet ，主要用来传电子邮件

1983：development of TCP/IP

TCP/IP 中间的 / 不能省略，省略后仅代表两个协议，写在一起代表所有网络协议

练习

e，ae，d

a：同轴电缆

b：带滤网的双绞线

c：带屏蔽的双绞线

d：无屏蔽的双绞线

e：光缆

FTP和TFTP都在应用层

FTP：可靠的传输协议，传输层使用的是TCP

TFTP：不可靠的传输协议，速度比较快，传输层使用的是UDP

d，c，ace

• 为了克服局域网的扩展问题
• 为了解决局域网的距离限制问题
• 为了把网络连接起来