---
# System prepended metadata

title: Chapter 1 Introduction
tags: [network]

---

# Chapter 1 Introduction
### 1.1 what isthe Internet?
+ What’s the Internet: “**nuts and bolts**”view
    + billions of connected computing devices:
        + hosts =end systems
        + running network apps
    + communication links
        + 材質 fiber, copper, radio, satellite
        + **transmission rate: bandwidth**頻寬
    + packet switches:
        + forward packets (chunks of data)
        + routersand switches
    + Internet:
        + “network of networks”
        + Interconnected ISPs(Internet Service Provider) : 主要的功能是要將你家裡的電腦與網際網路之間搭起連線的橋樑
    + protocol: 路上一種可以彼此溝通的模式
        + scontrol sending, receiving of messages
        + e.g., TCP, IP, HTTP, Skype, 802.11
    + Internet standards(STD): 關於網際網路相關的技術與方法論的技術規範
        + RFC: Request for comments
        + IETF: Internet Engineering Task Force
+ What’s the Internet: a **service** view
    + infrastructure that provides services to applications:
    + provides programming interface to apps
+ What’s a protocol?
    + all communication activity in Internet governed by protocols
    + protocolsdefine format, order of messages sent and received among network entities, and actions take non message transmission, receipt.
---
### 1.2 network edge
+ network structure
    + network edge:
        + hosts: clients and servers
        + servers often in data centers
    + network core:
        + interconnected routers
    + access networks, physical media:
        + wired, wireless communication links
        :::info
        + Q: How to connect end systems to edge router?
            + residential access nets
            + institutional access networks (school, company)
            + mobile access networks
        :::
+ Access network: digital subscriber line (DSL)
    + use existing telephone line to central office DSLAM(Digital Subscriber Line Access Multiplexer )
        + data over DSL phone line goes to Internet
        + voice over DSL phone line goes to telephone net
    + < 2.5 Mbps upstream transmission rate (typically < 1 Mbps)
    + 24 Mbps downstream transmission rate (typically < 10 Mbps)
    + ![](https://i.imgur.com/Ugy3hGO.png)

+ Access network: cable network
    + HFC: hybrid fiber coax
    + network of cable, fiber attaches homes to ISP router
        + homes **share** access networkto cable headend
        + **DSL, which has dedicated access to central office**
    + ![](https://i.imgur.com/2FnF8uj.png)
+ Access network: home network
    + ![](https://i.imgur.com/I8zaReD.png)
+ Enterprise access networks (Ethernet)
    + today, end systems typically connect into Ethernet switch
    + ![](https://i.imgur.com/LqoqqNR.png)
+ Wireless access networks
    + shared wirelessaccess network connects end system to router(via  “access point”)
    + ![](https://i.imgur.com/1FVEnc6.png)
+ Host: sends packets of data
    + take app message
    + break in to small chunk known as packet of L bits
    + trans packet into access network at transmission rate R
        + link transmission rate, aka link capacity, aka link bandwidth
    + ![](https://i.imgur.com/peKCG8M.png)


---
### 1.3 network core
+ The network core
    + mesh of interconnected routers
    + packet-switching: hosts break application-layer messages into packets
        + each packet transmitted at full link capacity
        + forward packetsfrom one router to the next, across links on path from source to destination
+ Packet-switching: store-and-forward
    + ==store and forward==: entire packet must arrive at router before it can be transmittedon next link
    + end-end delay = 2L/R (assuming zero propagation delay)
        + one-hop numerical example
            + L = 7.5 Mbits R = 1.5 Mbps
            + one-hop transmission delay = 5 sec
+ Packet Switching: queueing delay, loss
    + if arrival rate (in bits) to link exceeds transmission rate of link for a period of time:
        + packets will queue, wait to be transmitted on link
        + packets can be dropped (lost) if memory (buffer) fills up
    + ![](https://i.imgur.com/2av55rn.png)
+ Two key network-core functions
    + routing: 
        + determines source-destination route taken by packets
        + by routing algorithms
    + forwarding:
        + move packets from router’s input to appropriate router output
        + for destination address in arriving packet’s header
+ Alternative core: circuit switching 電路交換
    + end-end resources allocated to, reserved for “call”between source & dest: 在兩通信端之間建立一條專用的 (dedicated) 實際路徑。
        + dedicated resources: no sharing
        + circuit segment idle 閒置 if not used by call (no sharing)
        + 直到通信結束之後，這條專用路徑才停止使用，並讓出供他人繼續使用。
        + used in traditional telephone networks
        + in diagram, each link has four circuits
        + 实时性强，时延小，交换设备成本低，但同时也会带来线路利用率低，电路接续时间长，通信效率低，不同类型终端用户不能进行通信等缺点。
+ Circuit switching: FDM versus TDM
    + 当发生拥塞等网络问题时， TDM 中的数据丢失可能只会是一部分，而FDM 中就可能是大部分或全部
    + FDM（频分复用Frequency Division Multiplexing)：将整个传输频带划分为若干个频率通道，每个用 户占用一个通道。频率通道之间留有防护频带
    + TDM（时分复用Time Division Multiplexing）：时间分割成小的时间片，每个时间片又分为若干个通道(时隙），每个用户占用一个通道传输数据。
    + ![](https://i.imgur.com/2JC6MQT.png)
+ Packet switching versus circuit switching
    + packet switching allows more users to use network!
    + excessive congestion possible: packet delay and loss
        + protocols needed for reliable data transfer, congestion control
    + packet switch is great for bursty 集送式數據 data
        + resource sharing.
        + simpler, no call setup
    :::info
    + Q:How to provide circuit-like behavior?
    + bandwidth guarantees needed for audio/video apps
    :::
+ Internet structure: network of networks
    + End systems connect to Internet via access ISPs (Internet Service Providers)
        + residential
        + company
        + university etc
    + Access ISPs(網際網路服務提供者) in turn must be interconnected互聯
    + ![](https://i.imgur.com/9BOjHHK.png)
    + ![](https://i.imgur.com/iBngz7k.png)

        + at center: small # of well-connected large networks
            + tier-1”commercial ISPs(e.g., Level 3, Sprint, AT&T, NTT), national & international coverage

---
### 1.4 delay, loss, throughput in networks
+ How do loss and delay occur?
    + 封包queue in buffer
        + 因為packet arrival rate to link > output link capacity
    + ![](https://i.imgur.com/3gHTnWV.png)
+ Four sources of packet delay
    + ![](https://i.imgur.com/J3mpltM.png)
    + dproc: nodal processing 節點處理延遲
        + 路由器**處理封包表頭（packet header）、檢查位元資料錯誤與尋找配送路徑等所花費的時間**。
        + check bit errors
        + determine output link
    + dqueue: queueing delay
        + 路由器因為某些因素無法立刻將封包傳送到網路上，造成**封包暫存在佇列（queue）中等待的時間**。 
        + time waiting at output link for transmission
        + depends on congestion level of router
    + dtrans: transmission delay:
        + **網路卡將資料傳送到網路線上（或從網路線上接收）所花的時間**，與**網路設備的傳送速度**有關（如高速乙太網路傳送速度為 100Mbps）
        + L: packet length (bits)
        + R: link bandwidth (bps)
        + dtrans= L/R
    + dprop: propagation delay:
        + 封包在**網路線上傳輸**所花費的時間，與網路線上電子訊號跑的速度有關
        + d: length of physical link
        + s: propagation speed (~2x108m/sec)
        + dprop= d/s
:::info 
#### hw1
+ 頻寬時延乘積（bandwidth-delay product) 稱頻寬延時乘積、頻寬延時積等
    + 指的是一個資料鏈路的能力（每秒位元）與來回通訊延遲（單位秒）的乘積。其結果是以位元（或位元組）為單位的一個資料總量，等同在任何特定時間該網**路線路上的最巨量資料量——已傳送但尚未確認的資料**。
    + **Propagation delay 乘上傳輸速率 = 該 link 會出現的最高 bits 數** **負載量**
    + bandwidth-delay product 即為在 link 中因為傳輸延遲可容納的 bits 數
    + ![](https://i.imgur.com/XP4af9z.png)
    + ![](https://i.imgur.com/K7f8NVv.png)
+ width of a bit
    + ![](https://i.imgur.com/9c7SkiS.png)
    + ![](https://i.imgur.com/yrl92xz.png)
    + 線的長度/線裡面的bit最高附載量(公式解)
    + a general expression
    + ![](https://i.imgur.com/tHYUXWk.png)
+ 假設傳輸速率可以改變 當傳輸速率是多少是 一個bit的寬度等同於線的長度
    + ![](https://i.imgur.com/uQc2Fna.png)
    + ![](https://i.imgur.com/903pTHR.png)
    + ![](https://i.imgur.com/7KWly2w.png)
    + bit per second
+ 計算傳輸檔案要花多少時間
    + 一個大檔案
        + ![](https://i.imgur.com/IhUR4QC.png)
        + ![](https://i.imgur.com/ABoTDBK.png)
    + 分割小檔案
        + ![](https://i.imgur.com/kpir3Sm.png)
        + 一個送完才能送下一個 所以要回傳回來才知道可不可以傳下一個 因此dprop*2
        + ![](https://i.imgur.com/jz7J6KD.png)
    + 比較兩者差別
        + ![](https://i.imgur.com/uu6Kyop.png)
+ 傳輸檔案問題
    + ![](https://i.imgur.com/XnXcZOo.png)
    + ![](https://i.imgur.com/dYigF9g.png)

    + without segementation
        + ![](https://i.imgur.com/lA6aL1k.png)
        + 到第一個router的時間
            + ![](https://i.imgur.com/BzNDyNx.png)
        + total time
            + ![](https://i.imgur.com/i9DkhqQ.png)
    + with segement
        + ![](https://i.imgur.com/o2TY0c9.png)
        + 第一個packet到router的時間
            + ![](https://i.imgur.com/69zrHiD.png)
        + 第一個到router後第二個才開始傳 第二個什麼時候才會傳完
            + ![](https://i.imgur.com/mxRWh6W.png)
        + total time
            + ![](https://i.imgur.com/7rLYJlb.png)
            + ![](https://i.imgur.com/bXBKsHI.png)
            + 傳完第一個是0.015 在這之後每0.05秒 一個packet傳到799個 
    + 除了可以減少傳輸時間為什麼要用segementaion
        + ![](https://i.imgur.com/9TadeVb.png)
        + 沒有分割要是有error整個大檔案都要重傳
    + segementation的缺點
        + ![](https://i.imgur.com/pPkAhIr.png)
        + 簡單來說浪費很多header
:::

+ traceroute 
    + ![](https://i.imgur.com/MkknOg9.png)
+ Packet loss
    + queue (aka buffer) preceding link in buffer has finite capacity
    + packet arriving to full queue dropped (aka lost)
    + ![](https://i.imgur.com/rG6KAh7.png)
+ Throughput
    + rate (bits/time unit) at which bits transferred between sender/receiver
        + instantaneous:rate at given point in time
        + average:rate over longer period of time
        + ![](https://i.imgur.com/ExFyMV9.png)
        + ![](https://i.imgur.com/cdHOPBZ.png)



---
### 1.5 protocol layers, service models
+ Why layering?
    + modularization eases maintenance, updating of system
        + e.g., change in gate procedure doesn’t affect rest of system
+ Internet protocol stack
    + application:supporting network applications
        + FTP, SMTP, HTTP
    + transport:process-process data transfer
        + TCP, UDP
    + network:routing of datagrams from source to destination
        + IP, routing protocols
    + link:data transfer between neighboring network elements
        + Ethernet, 802.111 (WiFi), PPP
    + physical:bits “on the wire”
    + ![](https://i.imgur.com/XaIKrXw.png)
+ ISO/OSI reference model
    + presentation:
        + allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific conventions
    + session:
        + synchronization, checkpointing, recovery of data exchange
    + ![](https://i.imgur.com/5fR7rNO.png)
+ Encapsulation
    + ![](https://i.imgur.com/tM1pppo.png)

---
### 1.6 networks under attack: security
+ Bad guys: put malware 惡意軟件 into hosts via Internet
    + virus: self-replicating infection by receiving/executing object (e.g., e-mail attachment
    + worm: self-replicating infection by passively receiving object that gets itself executed
    + spyware malwarecan record keystrokes, web sites visited, upload info to collection site
    + infected host can be enrolled in botnet殭屍網路,used for spam. DDoS attacks
+ Denial of Service (DoS):
    + attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic
        + select target
        + break into hosts around the network (see botnet)
        + send packets to target from compromised hosts
+ packet “sniffing ”:
    + promiscuous network interface reads/records all packets(e.g., including passwords!) passing by    
    + ![](https://i.imgur.com/Dghpo6P.png) 
+ IP spoofing:
    + send packet with false source address(fake addresses)
    + ![](https://i.imgur.com/mW4Zgx3.png)


---
### 1.7 history
[reference link]https://webcache.googleusercontent.com/search?q=cache:NozW-PkuFCkJ:https://zhidao.baidu.com/question/743958922749448252.html+&cd=4&hl=zh-TW&ct=clnk&gl=tw
[reference link]ISP https://arcobaleno18.pixnet.net/blog/post/203574046
[reference link]認識惡意威脅:https://blog.trendmicro.com.tw/?p=5795
###### tags: `network`
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