1: Introduction 1

Computer Network

张 辉

hzhang@buaa.edu.cn82338088

Textbook 计算机网络（第四版） Adrew S. Tanenbaum 清华大学出版社 Computer Networking: A Top-Down Approach Featuring the Internet by Kurose and Ross （高等教育出版社）

1: Introduction 2

教学内容

计算机网络是计算机技术与通信技术的交叉学科。本课程以计算机网络体系结构为总纲，系统、分层次地讲述计算机网络的基本概念和工作原理，包括物理层、数据链路层、网络层、传输层和应用层的功能、接口和主要协议，重点介绍 Internet 核心协议族 TCP/IP 。

1: Introduction 3

教学目标了解有关计算机网络的基本概念，掌

握计算机网络的基本工作原理和主要技术，学会计算机网络应用原理及其方法。

以计算机网络体系结构的分层模型为基 础 ， 重 点 掌 握 Internet 。 采 用 的TCP/IP 协议族的工作原理，为今后各种计算机网络及其相关应用、学习和研究打下基础。

1: Introduction 4

What Will We Cover? 网络简介 网络体系结构 网络物理层（传输媒介、接口、信号） 数据链路层（网络检错、同步、 HDLC 、 PPP ） 局域网技术（ Ethernet 、 Token Ring 、 Token

bus 、 ATM) 网络层（ IP 编址、 subnetting 、 VLSM 、 CIDR 、

IPv6 ） 路由原理（ RIP 、 OSPF 、 BGP ）、广域网 传输层（ TCP 、 UDP ） 流量控制、拥塞控制及网络性能 应用层（ SMTP 、 ftp 、 Web 、 DNS 等） 网络安全及网络管理 网络新技术 (MPLS 、 Multicasting 、 Grid 、 NGI 等 )

1: Introduction 5

Part I: Introductiongoal: overview, “feel” of

networking more depth, detail

later in course approach:

descriptive use Internet as

example

Overview: what’s the Internet what’s a protocol? network edge network core access net, physical media performance: loss, delay protocol layers, service

models backbones, NAPs, ISPs Network history

1: Introduction 6

计算机网络与其它网络的关系

1: Introduction 7

计算机网络的构成

计算机网络运行在通信传输子网之上，由用户资源子网和通信传输子网组成业务网。

1: Introduction 8

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

millions of connected computing devices: hosts, end-systems pc’s workstations, servers PDA’s phones

running network apps communication links

fiber, copper, radio, satellite

routers: forward packets of data thru network

local ISP

companynetwork

regional ISP

router workstation

servermobile

1: Introduction 9

What’s the Internet: “nuts and bolts” view protocols: control sending,

receiving of msgs e.g., TCP, IP, HTTP, FTP, PPP

Internet: “network of networks” loosely hierarchical public Internet versus private

intranet

Internet standards RFC: Request for comments IETF: Internet Engineering

Task Force

local ISP

companynetwork

regional ISP

router workstation

servermobile

1: Introduction 10

What’s the Internet: a service view

communication infrastructure enables distributed applications: WWW, email, games, e-

commerce, database., voting,

more?

communication services provided: connectionless connection-oriented

cyberspace [Gibson]:“a consensual hallucination

experienced daily by billions of operators, in every nation, ...."

1: Introduction 11

What’s a protocol?human protocols: “what’s the time?” “I have a question” introductions

… specific msgs sent… specific actions

taken when msgs received, or other events

network protocols: machines rather than

humans all communication

activity in Internet governed by protocols

protocols define format, order of msgs sent and

received among network entities, and actions taken on msg transmission, receipt

1: Introduction 12

What’s a protocol?a human protocol and a computer network protocol:

Q: Other human protocol?

Hi

Hi

Got thetime?

2:00

TCP connection req.

TCP connectionreply.Get http://www.buaa..edu.cn/index.htm

<file>time

1: Introduction 13

协议 protocol 协议：计算机网络同等层次中，通信双方进

行信息交换时必须遵守的规则。

协议的组成：1. 语法（ syntax) ：以二进制形式表示的命

令和相应的结构2. 语义（ semantics) ：由发出的命令请求，

完成的动作和回送的响应组成的集合3. 定时关系（ timing) ：有关事件顺序的说

明

1: Introduction 14

Who is Who on the Internet ? Internet Architecture Board (IAB): The IAB is responsible

for defining the overall architecture of the Internet, providing guidance and broad direction to the IETF.

Internet Engineering Task Force (IETF): The IETF is the protocol engineering and development arm of the Internet. Subdivided into many working groups, which specify Request For Comments or RFCs.

IRTF (Internet Research Task Force): The Internet Research Task Force is a composed of a number of focused, long-term and small Research Groups.

The Internet Engineering Steering Group (IESG): The IESG is responsible for technical management of IETF activities and the Internet standards process. Standards. Composed of the Area Directors of the IETF working groups.

1: Introduction 15

Internet Standardization Process

All standards of the Internet are published as RFC (Request for Comments). But not all RFCs are Internet Standards !

available: http://www.ietf.org A typical (but not only) way of standardization

is: Internet Drafts Proposed Standard Draft Standard (requires 2 working

implementation) Internet Standard (declared by IAB)

David Clark, MIT, 1992: "We reject: kings, presidents, and voting. We believe in: rough consensus and running code.”

1: Introduction 16

A closer look at network structure: network edge:

applications and hosts network core:

routers network of networks

access networks, physical media: communication links

1: Introduction 17

The network edge: end systems (hosts):

run application programs e.g., WWW, email at “edge of network”

client/server model client host requests,

receives service from server e.g., WWW client (browser)/

server; email client/server

peer-peer model: host interaction symmetric e.g.: teleconferencing

1: Introduction 18

Network edge: connection-oriented service

Goal: data transfer between end sys.

handshaking: 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

TCP service [RFC 793] reliable, in-order byte-

stream data transfer loss: acknowledgements

and retransmissions

flow control: sender won’t overwhelm

receiver

congestion control: senders “slow down

sending rate” when network congested

1: Introduction 19

Network edge: connectionless service

Goal: data transfer between end systems same as before!

UDP - User Datagram Protocol [RFC 768]: Internet’s connectionless service unreliable data

transfer no flow control no congestion

control

App’s using TCP: HTTP (WWW), FTP

(file transfer), Telnet (remote login), SMTP (email)

App’s using UDP: streaming media,

teleconferencing, Internet telephony

1: Introduction 20

The Network Core

mesh of interconnected routers

the fundamental question: how is data transferred through net? circuit switching:

dedicated circuit per call: telephone net

packet-switching: data sent thru net in discrete “chunks”

1: Introduction 21

network core

Communication networks

Broadcast networksEnd nodes share a common channel

(TV, radio…)

Switched networks end nodes send to one (or more) end nodes

Packet switchingData sent in discrete portions

(the Internet)

Circuit switchingDedicated circuit per call

(telephone, ISDN)

(physical)

Datagram networksEach packet switched

independently

Virtual circuit networksPre-established path

(logical)

1: Introduction 22

Network Core: Circuit SwitchingEnd-end resources

reserved for “call” link bandwidth, switch

capacity dedicated resources:

no sharing circuit-like

(guaranteed) performance

call setup required pieces allocated to calls resource piece idle if

not used by owning call (no sharing)

1: Introduction 23

Circuit Switching: FDM and TDM

FDM

frequency

time

TDM

frequency

time

4 users

Example:

1: Introduction 24

Circuit Switching

Three phases1. circuit establishment2. data transfer3. circuit termination

If circuit not available: “Busy signal”

Examples Telephone networks ISDN (Integrated Services Digital

Networks)

1: Introduction 25

Circuit Switching Telephone Network

Source“Caller”

Central Office“C.O.”

Destination“Callee”

Central Office“C.O.”

TrunkExchange

Each phone call is allocated 64kb/s. So, a 2.5Gb/s trunk line can carry about 39,000

calls.

1: Introduction 26

Circuit Switching

A node (switch) in a circuit switching network

incoming links outgoing linksNode

1: Introduction 27

Numerical example How long does it take to send a file of

80k bytes from host A to host B over a circuit-switched network? All links are 1.536 Mbps Each link uses TDM with 24 slots 500 msec to establish end-to-end circuit

Work it out!

Rate 1.536 Mbps /24 = 64,000 bps

Total time 500ms+ 80*8kb/64kbps=10500 ms

1: Introduction 28

Symbols

Mbpsmegabits per second 106 bits per second

MBpsmegabytes per second

Gbps, Kbps,bandwidth

the total information flow over a given time

1: Introduction 29

分组 packet

邮政系统 信件－收信人地址－发信人地址

数据报 (DataGram)

自带寻址信息 能独立地从数据源“行走”到目的终点的数据包 (packet)

1: Introduction 30

Network Core: 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:

资源可能供不应求 拥塞 : 分组排队 , 等

待链路资源 在路由器上存储转

发 : 分组一次移动一跳通过链路传输等待下一条链路

Bandwidth division into “pieces”Dedicated allocationResource reservation

1: Introduction 31

Packet Switching

A

R1

R2

R4

R3

B

Source Destination

It’s the method used by the Internet. Each packet is individually routed packet-by-packet,

using the router’s local routing table. The routers maintain no per-flow state. Different packets may take different paths. Several packets may arrive for the same output link at

the same time, therefore a packet switch has buffers.

1: Introduction 32

Network Core: Packet Switching

Packet-switching versus circuit switching: human restaurant analogy

other human analogies?

A

B

C10 MbsEthernet

1.5 Mbs

45 Mbs

D E

statistical multiplexing

queue of packetswaiting for output

link

1: Introduction 33

Data are sent as formatted bit-sequences, so-called packets.

Packets have the following structure:

• Header and Trailer carry control information (e.g., destination address, check sum)

Each packet is passed through the network from node to node along some path (Routing)

At each node the entire packet is received, stored briefly, and then forwarded to the next node (Store-and-Forward Networks)

Header Data Trailer

Packet Structure and switching

1: Introduction 34

Packet Switching

A node in a packet switching network

incoming links outgoing linksNode

Memory

1: Introduction 35

分组交换 vs. 电路交换

1 Mb/s 链路 每个用户 :

100Kb/s 当“激活” 激活时间为 10%

电路交换 : 10 用户

分组交换 : 对 35 个用户来说 , 概

率 : > 10 个用户同时激活小于 .0017

分组交换使得更多用户可“同时”使用网络 !

N users

1 Mbps link

n

ip p

i

n i i

0 1 0

1( )

1: Introduction 36

分组交换 vs. 电路交换

在突发性数据传输过程中表现优异资源共享无须事先建立连接

过度拥塞 : 导致分组延迟和丢失需要协议来保障可靠的数据传输 , 拥塞控制

Q: 如何在分组交换网中提供电路交换的性能 ?为音频 /视频（ audio/video ）应用提供带宽

保障仍然是一个需要解决的问题

分组交换是不是 “ winner”?

1: Introduction 37

Packet-switched networks: routing Goal: move packets among routers from source

to destination we’ll study several path selection algorithms

datagram network: destination address determines next hop routes may change during session analogy: driving, asking directions

virtual circuit network: each packet carries tag (virtual circuit ID), tag

determines next hop fixed path determined at call setup time, remains fixed

thru call routers maintain per-call state

1: Introduction 39

Virtual circuit network

A

BC

D

E

1

2

3

3

2

1

13

2

12

3

11

22

33

2

3

1

SVC 1

SVC 2

Note: Packet headers don’t need to contain the full destination

address of the packet - only contain virtual circuit identifier (VCI)

1: Introduction 40

Why does the Internet use packet switching?

1. Efficient use of expensive links: The links are assumed to be expensive and scarce. Packet switching allows many, bursty flows to share

the same link efficiently. “Circuit switching is rarely used for data

networks, ... because of very inefficient use of the links” - Gallager

2. Resilience to failure of links & routers: ”For high reliability, ... [the Internet] was to be a

datagram subnet, so if some lines and [routers] were destroyed, messages could be ... rerouted” - Tanenbaum

Source: Networking 101

1: Introduction 41

Access networks and physical media

Q: How to connection end systems to edge router?

residential access nets institutional access

networks (school, company)

mobile access networks

Keep in mind: bandwidth (bits per

second) of access network?

shared or dedicated?

1: Introduction 42

Residential access: point to point access

Dialup via modem up to 56Kbps direct access

to router (conceptually) ISDN: intergrated services

digital network: 128Kbps all-digital connect to router

ADSL: asymmetric digital subscriber line up to 1 Mbps home-to-

router up to 8 Mbps router-to-

home

1: Introduction 43

Residential access: cable modems

Diagram: http://www.cabledatacomnews.com/cmic/diagram.html

1: Introduction 44

Cable Network Architecture

home

cable headend

cable distributionnetwork (simplified)

Typically 500 to 5,000 homes

HFC: hybrid fiber coaxasymmetric: up to 10Mbps upstream, 1 Mbps downstream

1: Introduction 45

Institutional access: local area networks

company/univ local area network (LAN) connects end system to edge router

Ethernet: shared or dedicated

cable connects end system and router

10 Mbs, 100Mbps, Gigabit Ethernet

deployment: institutions, home LANs soon

1: Introduction 46

Wireless access networks

shared wireless access network connects end system to router

wireless LANs: radio spectrum replaces

wire e.g., Lucent Wavelan 10

Mbps

wider-area wireless access CDPD: wireless access

to ISP router via cellular network

basestation

mobilehosts

router

1: Introduction 47

Home network components ADSL or cable modem router/firewall/NAT (Network Address Translation)

NAT enables a LAN to use one set of IP addresses for internal traffic and a second set of addresses for external traffic. It makes all necessary IP address translations.

Ethernet wireless access point

wirelessaccess point

wirelesslaptops

router/firewall

cablemodem

to/fromcable

headend

Ethernet

1: Introduction 48

Physical Media

physical link: transmitted data bit propagates across link

guided media: signals propagate in

solid media: copper, fiber

unguided media: signals propagate

freely e.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps ethernet

Category 5 TP: 100Mbps ethernet

1: Introduction 49

Physical Media: coax, fiber

Coaxial cable: wire (signal carrier)

within a wire (shield) baseband: single

channel on cable broadband: multiple

channel on cable

bidirectional common use in

10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses high-speed operation:

100Mbps Ethernet high-speed point-to-

point transmission (e.g., 5 Gps)

low error rate

1: Introduction 50

Physical media: radio

signal carried in electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., waveLAN) 2Mbps, 11Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps

satellite up to 50Mbps channel (or multiple

smaller channels) 270 Msec end-end delay geosynchronous versus LEOS

1: Introduction 51

Delay in packet-switched networkspackets experience delay

on end-to-end path four sources of delay at

each hop

nodal processing: check bit errors determine output link

queueing time waiting at output

link for transmission depends on congestion

level of router

A

B

propagation

transmission

nodalprocessing queueing

1: Introduction 52

Delay in packet-switched networksTransmission delay: R=link bandwidth

(bps) L=packet length (bits) time to send bits into

link = L/R

Propagation delay: d = length of physical

link s = propagation speed in

medium (~2x108 m/sec) propagation delay = d/s

A

B

propagation

transmission

nodalprocessing queueing

Note: s and R are very different quantitites!

1: Introduction 53

Packet Switching

Host A

Host B

R1

R2

R3

A

R1

R2

R4

R3

B

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

Source Destination

“ Store-and-Forward” at each Router

( )i ii

TRANSP PROP Minimum end to end latency

1: Introduction 54

Packet SwitchingWhy not send the entire message in one packet?

Breaking message into packets allows parallel transmission across all links, reducing end to end latency. It also prevents a link from being “hogged” for a long time by one message.

Host A

Host B

R1

R2

R3

M/R

min/ ii

M R PROP Latency

Host A

Host B

R1

R2

R3

( / )i ii

PROP M R Latency

M/R

+n*m/R

N 为经过 R 的数量

1: Introduction 55

传播延迟忽略不计的情况

1: Introduction 56

Queueing delay

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!

1: Introduction 57

Little’s Theorem

: average customer arrival rate N: average number of customers in system T: average delay per customer in system

Little’s Theorem: System in steady-state

N T

N

T

1: Introduction 58

Queueing Theory

假设 :顾客总数无限，以保证到达速率不受影响；队列大小无限，保证无分组被丢弃；服务规则假设为 FIFO；分组以泊松到达；

u

N

u

T1

1: Introduction 59

习题一

顾客以 5 人 /分钟的速度到达一个 snack

bar ，他们平均需等待 5 分钟才可以拿到食物。顾客在店里用餐的概率为 0.5 ，而顾客带走食物不在店里用餐的概率也为0.5 。顾客平均用餐时间为 20 分钟，问该snack bar 中的顾客平均数目是多少？

1: Introduction 60

习题二已知： LAN 输出给路由器平均速度为 5 分组 /秒；平均分组长度为 144字节；从路由器到广域网的线路带宽为 9600bps, 求：1 、每个分组在路由器重的平均滞留时间？2 、路由器中平均有多少个分组？

u

N

LAN Router

WAN线路

= 1.5 分组

Ts=(144*8)/9600=0.12秒 =5 分组 /秒；

1: Introduction 61

Network Architecture

What is layering?

Why layering?

ISO/OSI layering and Internet layering

How to determine the layers: the end-

to-end arguments

Summary

1: Introduction 62

Protocol “Layers”Networks are

complex! many “pieces”:

hosts routers links of various

media applications protocols hardware,

software

Question: Is there any hope of

organizing structure of network?

Or at least our discussion of networks?

1: Introduction 63

Organization of air travel

a series of steps

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing

1: Introduction 64

Organization of air travel: a different view

Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below

ticket (purchase)

baggage (check)

gates (load)

runway takeoff

airplane routing

ticket (complain)

baggage (claim)

gates (unload)

runway landing

airplane routing

airplane routing