计算机网络自顶向下Character1学习笔记

计算机网络自顶向下方法Character1

本文概述：

本文主要记录计算机网络自顶向下方法第一章节的知识点。
学习书籍为：计算机网络自顶向下方法
学习视频为：国立清华大学黄能富教授讲解的计算机网络自顶向下方法，需要的可以点击 这里
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What’s the Internet: nuts and blots view

• 终端(host) = end-systems: running network apps

• 通信线路(communication links): transmission rate = bandwidth 带宽

• 路由器(routers): forward packets

• Protocols(协议): control sending, receiving of messages: TCP/IP, HTTP, Skype, 802.11

• Internet: network of networks: Interconnected ISPs

• Internet standards: RFC: Request for comments/ IETF: Internet Engineering Task Force.

What’s the Internet: a service view

• communication infrastructure that provides services to application: Web, VoIP, email, games, e-commerce, social nets… 通信基础建设，分散式应用(enbales distributed)

• provides programming interface to apps:

  allow sending and receiving app programs to connect to Internet.

  provides service to options, analogous to postal service.(提供选项服务，类似于邮政服务。)

• communication services provided to application:
  connectionless / connection-oriented

What’s a protocol:

• Define: format, order of messages sent and received among network entities
• actions taken on message transmission, receipt of a message.
• machines rather than humans;
• all communication activity in Internet governed by protocols

A closer look at network structure:

network edge: applications and hosts e.g. phone
networl core: routers/ network of networks
access networks, physical media: communication links

The network edge:

• end systems(hosts)
  • run application programs: Web/ email
  • client/server model 主从结构
  • client host requests, receives. service from always-on server like web browser/server or email client/server
  • peer-peer(P2P) model(UDP): minimal(or no) use of dedicated servers eg: KaZaA 不使用server

connection-oriented service:

• Goal: data transfer between end systrems
  • handshaking(握手): set up(prepare for) data transfer ahead of time; set up “state” in two communicating hosts.
  • TCP- Transmission Control Protocol (TCP service): Internet’s connection-oriented service
    • reliable(内容正确，数量正确，顺序正确);
    • in-order byte-stream data transfer: problem: loss: acknowledgements(一定要回答收到) and retransmissions（如果没有回答，重新发送）.
    • flow control（流量控制） : sender won’t overwhelm receiver. 发送者不知道接收者的承载能力，一直发送，导致接收者崩溃，因此需要流量控制。receiver控制（告诉）sender能发多少。
    • congestion control（拥塞控制）: sender “slow down sending rate” when network congested. 发生拥挤，就放慢传送速度

conectionless service

• Goal: data transfer between end systems
  • UDP: User Datagram Protocol. Internet’s connctionless service: unreliable data transfer; no flow control; no congestion control

TCP/UDP

• App‘s using TCP: HTTP(web), FTP(file transfer), Telnet(remote login), SMTP(email)

  App’s using UDP: streaming media, teleconferencing, DNS (域名系统), Internet telephony.

The Network Core:

• mesh of inetrconnected routers (网状路由器)

• Q: The fundamental question: how is data transfered throught net?

  A: circuit switching： dedicated circuit per call: telephone net
  packet-switching: data sent thru (直通) net in discrete “chunks”（大块）

Circuit switching

• End-end resources reserved for “call”:

  • link bandwidth, switch capacity

  • dedicated resources: no sharing 每一个call都是专属的，专用的

  • circuit-like (guaranteed) performance

  • call setup required.

  • network resourcs divided into “pieces” (pieces allocated to calls; resource piece idle if not used by owning call (no sharing) 资源块空闲如果不被使用的话)

  • dividing link bandwidth into “pieces”. (frequency division/ time division) 时间和频率两种分割方式

Packet switching （不要固定的线路不要固定的时间）

• each end-end data stream divided into packets

  • user A, B packets share network resources

  • each packet used 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 (transmit over link; wait turn at next link)

  • Sequence of A & B packets does not have fixed pattern —> Statistical multiplexing

    In TDM each host gets some slot in revolving TDM frame.
    

  • Packet switching VS Circuit switching

  Packet switching allows more userd to use network!
  

  • Is packet switching a “slam dunk winner?” 不管什么时候，packet switching都是最好的吗？

    • Great for bursty data (爆发性短暂的data): resource shraing; simpler, no call setup
    • Excessive congestion: packet delay and loss: protocols needed for reliable data transfer, congestion control

  • How to provide circuit-like behavior?

    • bandwidth guarantees needed for audio/video apps
    • still an unsolved problem.

  • store-and-forward 存储转发
    
    Takes L/R seconds to transmit (push out) packet of L bits onto a link of R bps.

    Entire packet must arrive at router before it can be transmitted on next link: store and forward

  • Message Segmenting

    pipelining: each link works in parallel
    

  • Forwarding

    • Goal: moveing packets through routerd from source to destination
    • datagram network:
      • destination address(IP) in packet determines next hop
      • routes may change during session
      • analogy: driving, asking directions
    • virtual circuit network: 不是专用的线路，是固定的
      • Each packet carries tag (virtual circuit ID), tag determines next hop
      • Fixed path determined at call setup time (一定有建立联系的时间), remains fixed thru call(通过通话保持固定）
      • routers maintain per-call state

Network Taxonomy

固定路线：

FDM: 按频率分，不分享，专属resource

TDM: 按时间分，不分享，专属resource

不固定路线：

Networks with VCs: 不固定的情况下走固定的路线使用Virtual Circuits. 人工的方式走固定路线，tag

Datagram network: 每一个都带着destination address, 具体路线看router. router有一个很大的动态表。
Datagram network is not either connection-oriented or connectionless.

Internet provides both connection-oriented (TCP) and connectionless services (UDP) to apps.

Access networks and physical media:

• Q: How to connect end systems to edge router? 如何将终端系统连接到边缘路由器？
  • residential access nets
  • insititutional access networks
  • mobile access networks

Residential access: point to point access

• Dialup via modern: 56kbps; Can’t surf and phone at the same time: can’t be always on
• ADSL: asymmetric digital subscriber line非对称数字用户线: up to 1 Mbps upstream (today typically < 256 bps); up to 8 Mbps downstream(today typically < 1 Mps); FDM: 50 kHz ~ 1 Mhz for downstream/ 4 kHz ~ 50 kHz for upstream/ 0 kHz ~ 4 kHz for ordinary telephone 非对称性，上传下载。 线路上可以有很多的频道。

Residential access: cable modems同轴电缆

• HFC: hybrid fiber coaxial cable 光纤和同轴电缆混合在一起的结构
  • 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, dimensioning 规模大
• deployment部署: available via cable companies, e.g., MediaOne

Institutional access networks(School, company):

• company/university local area network (LAN) connects end system to edge router.
  

Wireless access networks: shared wireless access network connects end system to router

Home networks

• Typical home network components:

  • ADSL or cable modem

  • router/ firewall/ NAT

  • Ethernet

  • wireless access point

Physical Media

• Bit: propagates between transmitter/receiver pairs 在发射器/接收器对之间传播

• 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

    Category 5 TP: 100Mbps Ethernet

• Coaxial cable:双向传输

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

• Fiber optic cable:

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

• Radio:

  • signal carried in electromagnetic spectrum
  • no physical “wire”
  • bidirectional
  • propagation environment effects: reflection; obstruction by objects; interference
  • Link Types:
    • terrestrial microwave
    • LAN(e.g. WIFI)
    • wide-area(e.g. cellular)
    • satellite

Internet structure: network of networks

ISP: Internet Service Provider

• At center “Tier 1 ISPs”(e.g. UUNet,BBN/Genuity, Sprint, AT&t), national/international coverage
  

• “Tier-2 ISPS”: smaller (oftrn 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))
  

Delay & Loss

• How do loss and delay occur?

  packets queue in router buffera

  • Packet arrival rate to link exceeds output link capacity
  • Packets queue, wait for turn

Four source of packet delay

• nodal processing:

  • check bit errors
  • determine output link (查表检查）

• queueing

  • time waiting at output link for transmission
  • depends on congestion level of router

• Transmission delay:

  • R = link bandwidth(bps)
  • L = packet length(bits)
  • time to send bits into link = L/R

• Propagation delay(传播延时)

  • d = length of physical link
  • s = propagation speed in medium
  • propagation delay = d/s
  • **Note: **s and R are very different quantities
    

Nodal Delay:

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:

• R = link bandwidth (bps)

• L = packet length (bits)

• a = average packet arrival rate

• L * a = average bit arrival rate

• traffic intensity = (L * a)/R

  • traffic intensity ~ 0: average queueing delay small （包来的数量很少）
  • traffic intensity -> 1: delays become large (queue已经快满了）
  • traffic intensity > 1 : more “work” arriving than can be serviced, average delay infinite! (infinite queue length) – or packet loss! (finite queue length) (这意味着queue已经满了，出的比入的要少)

“Real” Internet delays and routes:

• Traceroute program (路径追踪)：provides delay measurement from source to router along end-end Internet path towards destination.

Protocol layers, servie models

Why layering?

Dealing with complex systems:

• explicit structure allows identification, relationship of complex system’s pieces; layered reference model for discussion
• modularization eases maintenance, updating of system
• change of implementation of layer’s service transparent to rest of system

Internet protocol stack

• application: supporting network applications (FTP, SMYP, HTTP)

• transport: host-host data transfer (application to application)

• TCP(用不可靠的环境提供可靠的传输），UDP

• network: routing of datagrams from source to destination 一个source送到一个目的地，可能会经过几十个router，路径选择，好坏，随时变换

  • IP， routing protocols

• link: data transfer between neighboring network elements 两点之间的传送

  • PPP, Ethernet

• physical: bit “on the wire”

Layering: logical communication (e.g. transport)

• take data from app
• add addressing, reliability check info to form “datagram”
• send datagram to peer
• wait for peer to ack receipt
• e.g. post office
• 资料要对，先后顺序要对，不能多也不能少，量也要对。

Layering: physical communication

这里就直接摆过程了

此过程中，较为重要的是当data到达路由器时进行的包装和拆装过程，即Encapsulation

如上图所示，当Hn放入source address 和 destination address, router会解析Hn；上一层交付的东西，整体为传输的data；HI中存储着网卡的位置和发送包的位置。