计算机网络-自顶向下方法【一】

Ch1-Introduction

2017-02-04 15:23:25 星期六

What is the Internet

hosts

hosts = end-systems,上面会running network apps
hosts大到服务器，小到PDA

communication links

fiber, copper, radio, satellite
transmission rate = bandwidth

routers

forward packets(数据块)

protocols

control sending, receiving of msgs
eg. TCP, IP,HTTP(浏览器),FTP(point-to-point),PPP

Intenet

network of networks
loosely hierarchical
public Internet versus
private intranet(使得建在不同地区的网络，看起来像建在同一个地区。)

Internet standards

RFC: Request for comments
IETF: Internet Engineering Task Force
IEEE

communication infrastructure

enable distributed applications: eg. web, email, sharing……

communication services provided to applications

connectionless（通信前不用建连线） eg. UDP,适合query, domain name server
connection-oriented（通信前要先建连线，就像打电话一样） eg. TCP

What is a protocol

all communication activity in Internet governed by protocols
protocols define

• format
• order
  order of messages sent and received among network entities
• action
  action taken on the ttransmisssion/receipt of a message

  Network edge

  applications and hosts 比如PC，PDA, 手机

  end systems(hosts)

• run application programs
• eg. Web, email
• at “edge of network”

  client/server model

  client提出需求，server回应
• client host requests, receives service from alsways-on server
• eg. Web browser/server; email client/server

  peer-peer model

• mminimal (or no) use of dedicated servers eg.Gnutella KaZaA
  没有server了，避免

1. server 负担过重
2. 被track，不容易被跟踪，抓到

   connection-oriented service

   data transfer between end systems

   handshaking

   数据传输前，先建立连线
   setup(prepare for)data transfer ahead of time

• Hello, Hello back human protocol
• set up “state” in two communicating hosts

  TCP

  Transmission Control Protocol
• Internet’s connection-oriented service

  reliable, in-order byte-stream data transfer

  Internet是一个不可靠的环境，可能发生丢包
  file transfer要求可靠，使命必达
  TCP内容要对，顺序也要对
• loss:acknowledgements and retransmissions
  接收方要确认，否则要重发

  flow control

• sender won’t overwhelm recevier
  recevier 要告诉sender可以送多快

  congestion control

  拥挤，堵塞
  如果网络拥挤，就放慢传送速度
  如何判断网络有多快？每次double越来越快地送packet，直到packet掉了。然后减半，再冲
• senders “slow down sending rate” when network congested
  eg. 每一个link都有一个频宽，当要嵩的信息量大于频宽的时候会queue在router里面，慢慢送

  connectionless service

  data transfer between end systems
• same as connection-oriented service

  UDP

  User Datagram Protocol
• unreliable data transfer 不可靠，不管对方有没有收到
• no flow control
• no congestion control

  App’s using TCP

• HTTP (Web), FTP (file transfer), Telnet (remote login)BBS, SMTP (email)

  App’s using UDP

  允许掉一点packet 影音啥的
• streaming media网络音乐, teleconferencing, DNS, Internet telephony

  Network core

  mesh(网状) of interconnected routers
• router
• network of networks

  circuit switching（电路交换）预分配 no sharing

  在每一段link上都预留一段频宽
  End-end resources reserved for “call”
• link bandwidth, switch capacity
• dedicated resources(专属的资源)：no sharing
• circuit-like(guaranteed) performance
• call setup required

  pieces

  network resources(e.g. bandwidth) divided into “pieces”
• pieces allocated to calls
• resource piece idle if not used by owning call(no sharing)

  dividing link bandwidth into “pieces”time division

• frequency division
• time division

  FDMA

  

  TDMA

  

  packet switching(分组交换)按需 sharing

  packets

  each end-end data stream divided into packets
• user A,Bpackets share network resources
• each packet uses full link bandwidth
• resources uses as needed (not reservation)

  contention

  resource contention:
• aggregate resource demand can exceed amount available
• congestion（拥挤）: packets queue, wait for link use
• store and forward（先存起来，再转送）: packets move one hop at a time

  transmit over link
  wait turn at next link

比如，如果A不用，B就全部把它用完

Packet Switching vs Circuit Switching

Packet switching

• allows more users ti use network

• Great for bursty(短暂而大量的) data

  resource sharing
  simpler, no call setup

• Excessive congestion: packet delay and loss

• protocols needed for reliable data transfer, congestion control

How to provide circuit-like behavior
(Internet 不保证bandwidth)

• bandwidth guarantees needed for audio/video apps

Packet Switching:Message Segmenting

• L = 7.5 Mbits
• R = 1.5 Mbps
• dealy = 15 sec

Now,Break the msgs into 5000 packets

• Each packet 1500 bits
• 1 msec to transmit packet on one link
• pipelinling each kink works in parallel (比如第二个packet开始送第一个L的时候，第一个packet可以开始送第二个L)
• Delay reduced from 15 sec to 5.002 sec

Packet-switched networks:forwarding

Goal

move packets through routers from source to destination

• we’ll study several path selection (i.e. routing) algorithms (chapter 4)
• router之间平时会交换资料，了解周边网络长什么样，决定packet走哪一条路

  datagram network（类比于信）

• destination address in packet determines next hop
• routes may change during session（中间交流的过程）
• analogy: driving, asking directions

  virtual circuit network(人工的方式走固定的路线)

• each packet carries tag (virtual circuit ID)（贴标签）, tag determines next hop
• fixed path determined at call setup time, remains fixed thru call
• routers maintain per-call state

  怎样通过分组交换网形成通路：
  因特网中，每个通过该网络传输的packet在它的首部包含了其目的地址。就像邮政地址一样，该地址是一种层次结构。当packet到达网络中的一台路由器时，该路由器检查packet的目的地址的一部分，并向相邻路由器转发该packet。
  更特别的是，每台路由器具有一个转发表（forwarding table)用于将目的地址（或目的地址的一部分）映射到输出链路。当packet到达一台路由器时，该路由器检查目的地址，并用这个目的地址搜索转发表，以找到合适的输出链路。然后路由器将该packet导向输出链路。

Network Taxonomy

• Datagram network is not either connection-oriented or connectionless.
• Internet provides both connection-oriented (TCP) and
  connectionless services (UDP) to apps.

  Network access and physical media

  communication links

  Access networks and physical media

  keep in mind

• bandwidth (bits per second) of access network?
• shared or dedicated? (共享的还是专属的)

Residential access: point to point access

Dialup via modem

• up to 54Kbps(often less)
• Can’t surf and phone at same time: can’t be “always on”

  ADSL(asymmetric digital subscriber line) 对Frequency做分割

• up to 1 Mbps upstream (today typically < 256 kbps)

• up to 8 Mbps downstream (today typically < 1 Mbps)

• FDM: 50 kHz - 1 MHz for downstream

4 kHz - 50 kHz for upstream
  0 kHz - 4 kHz for ordinary telephone

Residential access: cable modems

HFC: hybrid fiber coaxial cable（混合式）

• asymmetric: up to 30Mbps downstream, 2 Mbps upstream
• network of cable and fiber attaches homes to ISP router
• shared access to router among home
• issues: congestion（拥挤）, dimensioning（规模）
• deployment（部件）: available via cable companies, e.g., MediaOne

  Company access: local area networks （区域网络，快很多）

  company/university local area network (LAN) connects end system to edge router
  Ethernet(有线网络里面普及率最高的)

• mshared or dedicated link connects end system and router
• 10 Mbs, 100Mbps, Gigabit Ethernet
  deployment: institutions, home LANs happening now

Wireless access networks

shared wireless access network connects end system to router
via base station also known as “access point”

• wireless LANs:
• • 802.11b (WiFi): 11 Mbps
• • 802.11g: 54 Mbps
• • 802.11n: 108 Mbps
• wider-area wireless access（远距离无线通信）
• • provided by telcom operator
• • 3G ~ 384 kbps
• • 4G ~ 2 Mbps以上
• • WAP/GPRS in Europe
• • 802.16a/e (WiMax):

Home networks

Typical home network components:

• ADSL or cable modem
• router/firewall/NAT
• Ethernet
• wireless access

  Physical Media

  -Bit: propagates between
  transmitter/receiver pairs
• physical link: what lies between transmitter & receiver
• • guided media:有实体的材料，送电流
    signals propagate in solid media: copper, fiber, coax
• • unguided media:无线通信，送电磁波
    signals propagate freely, e.g., radio

    Twisted Pair (TP) 双绞线

    Coaxial cable:同轴电缆

• two concentric copper conductors
• bidirectional双向传输，送进去，电流往两边跑
• baseband:一个channel,大家抢，办公室里面用（送data)
• • single channel on cable
• • legacy Ethernet
• broadband:有线电视
• • multiple channels on cable
• • HFC

    Fiber optic cable:光纤（高速，不受电磁波干扰）

• glass fiber carrying light pulses, each pulse a bit
• high-speed operation:
• • high-speed point-to-point transmission (e.g., 5 Gps)
• low error rate: repeaters（用来放大信号） spaced far apart ; immune to electromagnetic noise

  Physical media: radio

• signal carried in electromagnetic spectrum
• no physical “wire”
• bidirectional
• propagation environment effects:
• • reflection
• • obstruction by objects
• • interference
    Radio link types:
• terrestrial microwave
• LAN (e.g., Wifi)
• wide-area (e.g., cellular) 3G,4G
• satellite
• • delay 不方便互动（实时视频互动啥的）

Internet structure and ISPs

Internet structure: network of networks
两台电脑相连，经过很多ISP转接

Dealy&loss in packet-switched networks

How do loss and delay occur

packets queue in router buffers

• packet arrival rate to link exceeds output link capacity
• packets queue, wait for turn

4 sources of packet delay

1. nodal processing（收到packet先检查是否正确）

• check bit errors
• determine output link
  2.queueing
• time waiting at output link for transmission
• depends on congestion level of router

1. Transmission delay:

• R = link bandwidth(bps)（把资料送上link的速度）
• L = packet length(bits)
• time to sen bits into link = L/R

1. Propagation delay

• d = length of physical link
• s = propagation speed in medium(~2x10的8次方 m/sec) （传递在link上的速度）
• propagation delay = d/s

1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!

dproc = processing delay

• typically a few microsecs or less
  dqueue = queuing delay
• depends on congestion
  dtrans = transmission delay
• m= L/R, significant for low-speed links
  dprop = propagation delay
• a few microsecs to hundreds of msecs

Queueing delay(revisited)

• (L · a)/R ~ 0: average queueing delay small
• (L · a)/R -> 1: delays become large
• (L · a)/R > 1: more “work” arriving than can be serviced, average delay infinite! (infinite queue length) –- or packet loss! (finite queue length)

“Real”Internet delays and routes

• Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination.

Packet loss

• queue (also known as buffer) preceding link in buffer has finite capacity
• when packet arrives to full queue, packet is dropped (also known as lost)
• lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all

Protocol layers, service models

layers:each layer implements a service

每一个layer都是利用下层的service来实现某些功能，提供给上一层

• via its own internal-layer actions
• relying on services provided by layer below

  Internet protocol stack

  

  • application:supporting network applications

• • FTP, SMTP(mail), HTTP
• transport:host-host data transfer
• • TCP（虽然网络不可靠，但要求提供给上层的服务是可靠的）, UDP
• network:routing of datagrams from source to destination（路径可能会经过很多点）（路径选择，将datagram送到destination, 可能会loss）
• • IP, routing protocols
• link:data transfer between neighboring network elements（直接相连的两点，送两个连接的node，一边送到一边）
• • PPP,Ethernet
• physical:bits “on the wire”

router：3层（physical, link, network) 看IP
switch:2层(physical link) 看MAC

each layer:

• distributed “entities” implement layer functions at each node
• entities perform actions, exchange messages with peers

  layering:physical communication

  

  Encapsulation

  上一层交给你的东西，对下一层来说，都是data,下一层加上自己的header或（头和尾）

  Hl:加MAC address（从哪来，到哪去）每经过一个link,更新一次
  Hn:加IP address（从哪来，到哪去）source IP 和 destination IP都不能换