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2020COMPUTER NETWORKING
A TOP- DOWN APPROACH
SEVENTH EDITION
,Inhoudsopgave
CHAPTER 1 COMPUTER NETWORKS AND THE INTERNET 2
1 .1 WAT IS INTERNET? 3
1.1.1 A Nuts and Bolts 3
1.1.2. Services Description 3
1.1.3 What Is a Protocol? 4
1.2 THE NETWORK EDGE 5
Access Networks: 5
Home access 5
Access in the Enterprise (and the Home): Ethernet and WiFi 6
1.2.2 Physical Media 6
1.3 THE NETWORK CORE 8
1.3.1 Packet Switching 8
1.3.2 Circuit Switching 9
1.3.3 A Network of Networks 9
1.4 DELAY, LOSS AND THROUGHPUT IN PACKET SWITCHED NETWORKS 10
1.4.1 Overview of Delay 10
1.4.2 Queuing Delay and Packet loss 10
1.4.3 End to End Delay 11
1.4.4 Throughput in Computer Networks 11
1.5 PROTOCOL LAYER AND THEIR SERVICE MODELS 11
1.5.2 Encapsulation 14
1.6 NETWORKS UNDER ATTACK 15
CHAPTER 2 APPLICATION LAYER 16
2.1 PRINCIPLES OF NETWORK APPLICATIONS 17
2.1.1 Network Application Architectures 17
2.1.2 Processes Communicating 17
2.1.3 Transport Services Available to Application 18
2.1.4 Transport Services Provides by the Internet 19
2.1.5 Application Layer Protocol 20
2.2 THE WEB AND HTTP 20
2.2.1 Overview of HTTP 20
2.2.2 Non-Persistent and Persistent Connection 20
2.2.3 HTTP message Format 21
2.2.4 User server Interaction: Cookies 23
2.2.5 Web caching 23
2.3 ELECTRONIC MAIL IN THE INTERNET 25
2.3.1 SMTP 25
2.3.2 Comparisons with HTTP 25
2.3.3 Mail message Formats 26
2.3.4 Mail Access Protocols 26
2.4 DNS- THE INTERNET’S DIRECTORY SERVICE 27
2.4.2 Overview of How DNS works 27
2.4.3 DNS Records and Messages 29
2.5 PEER-TO-PEER APPLICATIONS 29
1
,Chapter 1 Computer Networks and the Internet
2
,1 .1 Wat is Internet?
We can describe the Internet in terms of Nuts and bolts (basic hardware and software
components) and networking infrastructure that provides services to distributed applications.
1.1.1 A Nuts and Bolts
The Internet is a computer network that Interconnects billions of computing devices
throughout the world. All of these devices are called hosts or end systems.
• Traditional: Desktop PC, Linux workstations and so-called servers.
• Nontraditional: Internet of things such as laptops, smartphones, tablets…
End systems are connected together by a network of communication links and packet
switches. Communication links are made up of different types of physical media with different
transmission rate (bits/ second):
• Coaxial cable
• Copper wire
• Optical fiber
• Radio Spectrum
The two most prominent types of packet switches are:
• Routers, used in network core
• Link-layer switches, used in access networks
End systems, packet switches and other pieces of the Internet runs protocols that controls the
sending and receiving of information within the Internet such as TCP/IP.
Internet standards are developed by the Internet Engineering Task Force (IETF), documents
are called RFCs. They define protocols such as TCP, IP, HTTP and SMTP.
Other bodies define standards for network components, most notably for network links. The
IEEE 802 LAN/MAN standards Committee specifies e.g. Ethernet and Wi-Fi.
1.1.2. Services Description
We can also describe the Internet as an infrastructure that provides services to applications.
In addition to traditional application such as e-mail and Web surfing. Internet application
include mobile and smartphone and tablet application.
The application are said to be distributed applications, since they involve multiple end system
that exchange data with each other, and they run on end systems.
How does one program running on one end system instruct the internet to deliver data to
another program running on another end system? End systems attached to the Internet
provide a socket Interface. Internet socket is a set of rules that the sending program must
follow so that the Internet can deliver the data to the destination program.
3
,1.1.3 What Is a Protocol?
Consider what you do when you want to ask someone for the time. First greeting than ask the
question. A network protocol is similar, except that we are talking about hardware or software
components of some devices.
Between two network interface cards; congestion control protocols in end systems control the
rate at which packets are transmitted between sender and receiver, protocols in routers
determine a packets path from source to destination.
What happens when you make a request to a webserver for example?
1) Your computer sends a connection request message and wait for replay.
2) The webserver sends a reply message after receiving your connection request.
3) Your computer sends the name of the webpage it wants to fetch in a Get message.
4) Finally, the Webserver return the Webpage(file) to your computer.
A protocol defines the format and the order of message exchanged between two or more
communication entitles, as well as the action take on the transmission and/or receipt of a
message or other event.
4
,1.2 The Network Edge
The computers and other devices are often referred to as a end system, because they sit at the edge
of the Internet. End systems are also referred to as hosts, because they host (that is, run) application
programs such as a web browser program. Host= end systems. Hosts are sometimes divided into two
categories; clients and servers.
Access Networks:
Home access
Digital subscriber line (DSL):
§ Internet access through telephone line (telephone company, telco).
§ Digital subscriber line access multiplexer (DSLAM) to exchange data
§ DSL modem translates digital data to high frequency tones
§ Physical media: Twisted pair copper wire
§ Asymmetric, the downstream and upstream rates are different.
§ Maximum rate is limited by distance between home and CO (5-10 miles), the gauge
of the twisted pair line and the degree of electrical interference.
Cable Internet access:
§ Internet access through existing cable television infrastructure
§ Fiber and coaxial cable, Hybrid fiber coax (HFC)
§ Required special modems, cable modems
§ As with a DSL modem connects though Ethernet port
§ Cable modem termination system (CMTS) turning analog signal sent from the cable
modem back to digital.
§ Cable modems divide the HFC into two channels a up and downstream.
§ Access is asymmetric
§ Shared broadcast medium, every packet travels every home. Simultaneously
downloading a video file influence the actual rate.
Fiber to the home (FTTH)
§ Provides higher speed, in the gigabits per second range
§ Provide an optical fiber path from the CO directly to the home
§ Several competing technologies from the CO to home, simplest is called direct fiber
§ Commonly each fiber leaving the Co is actually shared by many homes
§ competing optical distribution that perform the splitting Active optical networks
(AON)s and Passive optical networks (PONS)
§ AONS is essentially switched Ethernet
§ PONS: Splitter combines home onto a single, shared optical fiber, which connects to
optical line terminator (OLT) in the telco’s CO.
Satellite Link
§ A satellite can be used where DSL, Cable, and FTTH are not available
§ A satellite link connects a residence at speeds of more than1 Mbps
§ Star Band and HughesNet are expels of satellite link
Dial -up
§ Access over traditional phone line
5
, § Connects over a phone line to a modem in the ISP.
§ Slow 56kbps
Access in the Enterprise (and the Home): Ethernet and WiFi
A local area network is used to connect an end system to the edge router. Ethernet is the most
prevalent types of LAN technology used in corporate, university and home networks.
Increasingly people are accessing the internet wirelessly from different devices.
§ A wireless LAN user must be within few tens of meters of the access point
§ Wireless Lan access based on IEEE 802.11 technology, known as Wi-Fi.
§ 802.11 provides a shared transmission rate of up to more than 100 Mbps
§ Ethernet uses Copper wire
A Wide- Area Wireless Access are being used to exchange data while on the run. Devices
employ the same wireless infrastructure used for cellular telephony to send/receive packets
though a base station that is operated by the cellular network provider.
§ Un like WiFi a user need only be with a few tens of kilometers of the base station
§ Example are 3G, 4G and fifth generation 5G of wide area wireless networks
§ LTE (long Term Evolution has its roots in 3G technology, can achieve rates in excess of
10 Mbps.
1.2.2 Physical Media
From source to a destination a bit travels through a series of transmitters-receiver pairs. For
each transmitter-receiver pain, the bit is sent by propagating electromagnetic waves or optical
pulses across a physical medium (copper wire, coax, fiber, radio spectrum and satellite radio.
Physical medium falls in two categories:
§ Guided media, waves are guided along a solid medium
§ Unguided media, the waves propagate in the atmosphere and in outer space such as
wireless LAN or a digital satellite channel.
Twisted Pair Copper Wire
§ Least expensive
§ Guided transmission
§ Used by telephone networks e.g. at home
§ Shielded twisted pair (STP) twisted together in protective shield with foil or
mesh to reduce electrical interference
§ Unshielded twisted pair(UTP) are twisted together to reduce noise and
crosstalk. Commonly used for computer networks within a building, that is for
LAN’s
§ Modern twisted pair such as category 6a can achieve rates of 10 Gbps
Coaxial Cable
§ Like twisted pair, coaxial consists of two copper conductors
§ Conductors are concentric than parallel (unlike twisted pair)
§ Common in cable television
6
, § Access at rates of tens of Mbps
§ Can be used as a guided shared medium
Fiber Optics
§ Thin, flexible medium that conducts pulses of light
§ Each pulse representing a bit
§ A single optical fiber can support up to tens or even hundreds of Gbps
§ Immune to electromagnetic interferences
§ Very low signal attenuation up to 100 kilometers
§ Are very hard to tap
§ Fiber optics is prevalent in the backbone of the Internet
§ High cost
Terrestrial Radio Channels
§ Carry signal in the electromagnetic spectrum
§ No physical wire to be installed
§ Can penetrate walls
§ Long distance
§ Can be classified into three groups:
• Very short distance, with one or two meters (e.g. wireless headsets,
keyboards and medical devices)
• Local areas, from ten to a few hundred meters (Local area network,
LAN)
• Wide area, spanning tens of kilometer (the cellular access
technologies)
Satellite Radio Channels
A communication satellite links two or more Earth based microwave
transmitter/receivers, known as ground stations. Two types of satellites are
used in communications are:
§ Geostationary satellites
• Remains above the same spot on Earth
• In orbit at 36000 kilometers above the Earth’s surface
• Huge distance, delay of 280 milliseconds
• Used in areas without access to DSL or cable
§ Low earth orbiting (LEO) satellites
• Closer to Earth
• Not remain above on sport
• Rotate around Earth, just as the Moon does
7
,1.3 The Network Core
There are two fundamental approaches to moving data through a network of links and
switches. Packet switching and circuit switching.
1.3.1 Packet Switching
To send message the source breaks long messages into smaller chunks of data, known as
packets. Packets are transmitted over each communication link at rate equal to the full
transmission rate of the link. It is connection less, does not need to reserve a path.
Store and Forward Transmission
It means that the packet switch must receive the entire packet before it can begin to
transmit the first bit of the packet onto the outbound link. For example, the source
has three packets to send to the destination.
1. The source begins to transmit at time 0.
2. At time L/R seconds, the source has transmitted the entire packet and the
entire packet has been received and stored at the router (no propagation
delay).
3. At time 2L/R the router has transmitted the entire packet and its has been
received by the destination. Total delay for the first packet is 2L/R
4. At time L/R the source begins to send the second packet, so at the time 3L/R
the destination receives the second packet
5. Final at time 4L/R the destination has received all three packets.
Queuing Delays and Packet Loss
For each attacked link the packet switch has an output buffer (also call output queue).
Which stores the packet that the router is about to send into that link. Packet loss will
occur if an arriving packet find that the buffer is completely full of other packets
waiting for transmission.
Suppose A and B send their packet along 100 Mbps Ethernet link to the first router.
The router then directs these packets to the 15 Mbps link. If during a short interval of
time the arrival rate exceeds 15 Mbps, congestion will occur at the router.
Forwarding Tables and Routing Protocols
When a source sends a packet, the source includes the destination’s Ip address in the
packet’s header. When a packet arrives at a router, the router examines a portion of
the packet’s destination address and forwards to an adjacent router. Each router has
a forwarding table that maps destination address9 or a portion to that outbound links.
The Internet has a special routing protocols that are used to automatically set the
forwarding tables. A routing protocol may, for example determine the shortest path
from each router to each destination and us the shorted path result to configure the
forwarding tables in the routers.
8
, 1.3.2 Circuit Switching
In a circuit switched networks, the resources needed a long. A path (buffers, link transmission
rate) to provide for communication between the end systems are reserved for duration of the
communication. Traditional telephone networks are examples of circuit switched networks.
§ Establish Network (bona fide connection, switches maintain connection state,
called circuit). Dedicated end to end connection
§ The sender can transfer at the guaranteed constant rate because if reservation
of transmission link rate.
§ Connection oriented
§ Entire massage must follow the same route
§ Waste of bandwidth
§ Designed for voice
Multiplexing in Circuit Switched Networks
A circuit in a link is implemented with either FDM or TDM. With FDM, each circuit
continuously gets a fraction of the bandwidth. With TDM, each circuit get all of the
bandwidth periodically during brief intervals of time (that is, during slots).
§ Frequency division multiplexing (FDM): Frequency divided among the
connections (e.g. radio)
§ Time division multiplexing (TDM): Time is divided into frames of fixed duration;
each frame is divided into a fixed number of time slots.
Time to send a file with 640000 bits over circuit switching (TDM have a bit rate of 1536
Mbps with 24 slots and 0,5 seconds for establishing connection.
1536/24= 64 kbps
640000/64= 10 second +0,5 second
Total time= 10,5 seconds
1.3.3 A Network of Networks
The access ISP can provide either wired or wireless connectivity, using an array of
access technologies including DSL cable, FTTH, Wi-Fi and cellular. The access ISPs
themselves must be interconnected. This is done by creating a network of network.
Our first network structure, Network structure 1 interconnects all of the access ISPs
with a single global transit ISP.
Network structure 2 is building global transit ISPs. It’s a two-tier hierarchy with global
transit providers residing at the top tier and access ISPs at the bottom tier.
9
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