COMP211

2021-10-05

Lec 0 - Preliminaries

Computer Network

Course Aims:

To introduce networked computer systems in general and Internet in particular:
- The basic operating principles
- The design and organization priciples of successful computer networks
- The key protocols and technologies used in the Internet

Learning Outcomes:

be able to describe and justify the OSI Reference Model and the key protocols that govern the Internet
be able to program application and protocols for computer networks
be able to illustrate and debate the use and need of cryptographic techniques in network security

Assessment

Coursework: 30%
- 2 JAVA programming assignments, 10% each
  - A1 Deadline: 21 OCtober 2021
  - A2 Deadline: 4 Novermber 2021
- 1 class test, 10%
  - 22 Novermber 2021
Exam: 70%

Lec 1

Chapter goal:

Get “feel”, “big picture,” Intruduction to terminology
- more depth, detail later in course
Approach:
- use Internet as example
Overview/roadmap:
- What is the Internet?
- What is a protocol?
- Network edge: hosts, access network, physical media
- Network core: packet/circuit switching, internet structure
- Performace: loss, delay, throughput
- Security
- Protocol layers, service models
- History

The Internet: a “nuts and bolts” view

connected computing devices:
Packet switches
Communication links
Networks
Internet: “network of networks”
protocols are everywhere
Internet standards
- RFC: Request for Comments
- IETF: Internet Engineering Task Force

The Internet: a “service” view

为应用程序提供服务的基础设施
Provides programming interface to distributed applications:
- “hooks” allowing sending/receiving apps to “connect”to, use Internet transport service
- provides service options, analogous to postal service

PROTOCOL协议

Human protocols
- “what’s the time?”
- “I have a questin”
- introductions
Network protocols:
- computers(devices) rather than humans
- all communication activity in Internet governed by protocols
  Protocols define the format, orderof messages sent and received among network entities, and actions taken
  on msg transmission, receipt

A closer look at Internet structure

Network edge:
- hosts: clients and servers
- servers often in data centers
*Access networks, physical media:
- wired, wireless communication links
Network core:
- interconnected routers
- network of networks

Access networks and physical media

connect end systems to edge router
- residential access nets
- institutional access networks(school, company)
- mobile sccess networks(WiFi, 4G/5G)
寻找需求：
- transmission rate(bits per second) of acces network?
- shared or dedicated access among users?

cable-based access:
通过线缆连接从供应商到各户

frequency division multiplexing(FDM): different channels transmitted in different frequency bands

HFC: hybrid fiber coax 混合光纤同轴

asymmetric: up to 40 Mbps- 1.2Gbs Gbsdownstream transmission rate, 30-100 Mbps upstream transmission rate

network of cable, fiber attaches homes to ISP router

homes share access network to cable headend

digital subscriber line(DSL)：
电话上网

use existing telephone line to central office DSLAM

data over DSL phone line goes to Internet

voice over DSL phone line goes to telephone net

24-52 Mbps dedicated downstream transmission rate

3.5-16 Mbps dedicated upstream transmission rate

home networks

Wireless access networks Shared wirelessaccess network connects end system to router

via base station aka”access point”

Wireless local area networks
- typically within or around building(~30 m)
- 802.11b/g/n(WiFi): 11, 54, 450 Mbps transmission rate
Wide-area cellular access networks
- provided by mobile, cellular network operator(10’s)km
- 10’s Mbps
- 4G cellular networks(5G)

enterprise networks

companies, universities, etc.

mix of wired, wireless link technologies, connecting a mix of switches and routers (we’ll cover differences shortly)

Ethernet” wired access at 100Mbps, 1Gbps, 10Gbps

WiFi: wireless access points at 11, 54, 450 Mbps

Host: sends packets of data
host sending function:

takes application message
breaks into smaller chunks, known as packets, of length L bits
transmits packet into access network at transmission rate R
- link transmission rate, aka link capacity, aka link bandwidth

delay = L/R

Links: physical media

bit: propagates between transmitter/receiver pairs
physical link:
guided media:
unguided media:
Twisted pair(TP)网线接口
- physical media
  - Coaxial cable:
    - two concentric copper conductors
    - bidirectional
    - broadband:
      - multiple frequency channels on cable
      - 100’s Mbps per channel
  - Fiber optic cable:
    - glass fiber carrying light pulses, each pulse a bit
    - high-speed operation:
      - high-speed point-to-point transmission(10’s-100’s Gbps)
    - low error rate:
      - repeaters spaced far apart
      - immune to electromagnetic noise
  - Wireless radio:
    - signal carried in electromagnetic spectrum
    - no physical “wire”
    - propagation environment effects:
      - reflection
      - obstruction by objects
      - interference
  - Radio link types:
    - terrstrial microwave
    - Wireless LAN
    - wide-area
      - 4G cellular: ~ 10’s Mbps
    - satellite
      - up to 45 Mbps per channel
      - 270 msec end-end delay
      - geosynchronous versus low-earth-orbit

The network core

mesh of interconnected routers
packet-switching: hosts break application-layer messages into packets
- forward packetsfrom one router to the next, across links on path from source to destination
- each packet transmitted at full link capacityIntroduction: 1-25mobile networkhome network

Packet-switching:

store-and-forward

Transmission delay:takes L/Rseconds to transmit (push out) L-bit packet into link at Rbps 传输延迟等于 L/R
store and forward: entirepacket must arrive at router before it can be transmitted on next link 一存一进
End-end delay: 2L/R (above), assuming zero propagation delay (more on delay shortly) 端端延迟

queueing delay, loss

*packet queuing and loss: if arrival rate (in bps) to link exceeds transmission rate (bps) of link for a period of time:

packets will queue, waiting to be transmitted on output link 排队
packets can be dropped (lost) if memory (buffer) in router fills up 丢包

Two key network-core functions

Forwarding:
- local: move arriving packets from router’s input link to appropriate router output link
Routing:
- global action: determine source-destination paths taken by packets
- routing algorithms

Alternative to packet switching: circuit switching
end-end resources allocated to, reserved for “call” between source and destination

in diagram, each link has four circuits.
- call gets 2nd circuit in top link and 1stcircuit in right link.
dedicated resources: no sharing
- circuit-like(guaranteed)
  performance
circuit segment idle if not used by call
commonly used in trafitional telephone networks

Packet switching versus circuit switching

packet switching allows more users to use network!
circuit switching good at:
- resource sharing
- simpler, no call setup
excessive congestion possible: packet delay and loss due to buffer overflow
- protocols needed for reliable data transfer, congestion control
- How to provide circuit-like behavior?
  - bandwidth guarantess traditionally needed for audio/video applications

Internet strucutre: a “network of networks” 由小到大逐渐组成互联网

Hosts connect to Internet via access Internet Service Providers (ISPs)
- residential, enterprise (company, university, commercial) ISPs
Access ISPs in turn must be interconnected•so that any two hosts can send packets to each other
Resulting network of networks is very complex•evolution was driven by economicsand national policies
Let’s take a stepwise approach to describe current Internet structure

组成形式
global ISP
company ISP
IXP ( Internet exchange point): to connet each ISP
peering link
regional ISP
access ISP

Packet loss and delay
packets queue in router buffers

packets queue, wait for turn
arrival rate to link exceeds output link capacity: packet loss
Packet delay: four sources
- “transmission”, “propagation”, “nodal processing”, “queueing”
- d(nodal) = d(proc) + d(queue) + d(trans) + d(prop)

d(proc): nodal processing

check bit errors
determine output link
typically < msec

d(queue): queueing delay

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

d(tran): transmission delay:

L: packet length(bits)
R: link transmission rate(bps)
d(trans) = L/R

d(prop): propagatin delay:

d: length of physical link
s: propagation speed
d(prop) = d/s

packet loss 缓冲区无法容纳的就会丢失

queue(aka buffer) preceding link in buffer has finite capacity
packet arriving to full queue dropped(aka lost)
lost packet may be retransmitted by previous node, by source end system, or not at all

Throughput 吞吐量

throughput: rate(bits/time unit) at which bits are being sent from sender to receiver
- instantaneous: rate at given point in time
- average: rate over longer period of time
- Rs < Rc & Rs > Rc
  bottleneck link: link on end-end path that constrains end-end throughput
network scenario
- per-connection end-end throughput: min(Rc,Rs,R/10)
- in practice: Rc or Rs is often bottleneck

Network security

field of network security:
- under attacks
- defend attacks
- immune to attacks
Internet not originally designed with(much) security in mind
- 起初的互联网比较封闭安全
- protocol designers playing “cathc-up”
- security considerations in all layers
  Bad guys

malware

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 malware can record keystrokes, web sites visited, upload info to collection site
infected host can be enrolled in botnet,used for spam or distributed denial of service (DDoS) attacks

denial of service
Denial of Service(Dos): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic

select target
break into hosts
send packets to target from compromised hosts

packet interception
packet “sniffing”

broadcast media(shared Ethernet, wireless)
promiscuous network interface reads/records all packets(e.g.,including passwords!) passing by

fake identity
IP spoofing: send packet with false source address

Protocol “layers” and reference models

Networks are complex, with many “pieces”:
- hosts
- routers
- links of various media
- applications
- protocols
- hardware, software

Internet protocol stack

application
treansport
network
link
physical

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
Internet statck “missing” these layers!

Internet history