chap 1 (adeel nayyer) - copy

8/3/2019 Chap 1 (Adeel Nayyer) - Copy

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Internet is a network of networks. Internet is very complex and ever changing,

both in terms of its hardware and software components, as well as in the services it

provides.

1.1.1 Nuts-and-Bolts Description (Basic hardware and software

components)Hosts and End systems

The public Internet is a worldwide computer network, that is, the network that

connects millions of computing devices throughout the world.

In Internet jargon, all of these devices are called hosts or end systems.

Communication Links

End systems are connected together by communication links. There are many types

of communication links; coaxial cable, copper wire, fiber optics, and radio spectrum.Different links can transmit data at different rates, with the transmission rate of

a link measured in bits/second.

Packet Switches

End systems are indirectly connected to each other through intermediate switching

devices known as packet switches. A packet switch takes a chunk of information

arriving at one of its incoming communication links and forwards that chunk of

information on one of its outgoing communication links. Two most prominent types

of packet switches are routers and link-layer switches.

In the jargon of Internet, the chunk of information is called a packet.

Path

From the sending end system to the receiving end system, the sequence of

communication links and the packet switches traversed by a packet is known as a

route or a path through the network.

Packet Switching

The Internet uses a technique known as packet switching that allows multiple

communicating end systems to share a path, or parts of a path, at the same time.ISPs

End systems access the Internet through Internet Service Providers (ISPs).

Protocols



End systems, packet switches, and other pieces of Internet, run protocols that

control the sending and receiving of information within the Internet.

The Transmission Control Protocol (TCP) and the Internet Protocol (IP) are the

most important protocols in the Internet.

The IP protocol specifies the format of the packets that are sent and received

among the routers and end systems.The Internet’s principal protocols are collectively known as TCP/IP.

Internet Standards

Internet Standards are developed by the Internet Engineering Task Force (IETF).

The IETF documents are called requests for comments (RFCs).

Intranet

There are many private networks whose hosts cannot exchange messages with

hosts outside of the private network. These private networks are often referred toas intranets.

1.1.2 A Service Description (Networking Infrastructure)

The Internet allows distributed programs running on its end systems to exchange

data with each other. These applications include Web surfing, instant messaging,audio and video streaming, Internet telephony, distributed games, peer-to-peer

(P2P) file sharing, remote login, electronic mail etc.

Services Internet Provides

The Internet provides two services to its distributed applications: a connection-

oriented service and connectionless service. The connection-oriented reliable

service guarantees that data transmitted from a sender to a receiver will

eventually be delivered to the receiver in order and its entirety. The connectionless

service does not make any guarantees about eventual delivery.

1.1.3 What is a Protocol?

All activity in the Internet that involves two or more communicating remote

entities is governed by a protocol. Protocols are running everywhere in the

Internet.

A protocol defines the format and the order of messages exchanged between twoor more communicating entities, as well as the actions taken on the transmission

and/or receipt of a message or other event.

1.2.1 End systems, Clients, and Servers



End systems (Hosts)

In computer networking jargon, the computers connected to the Internet are often

referred to as end systems, as they sit at the edge of the Internet.

End systems are also referred to as hosts because they host application programs

such as a Web browser program, a Web server program, an e-mail reader program,or an e-mail server program.

Hosts are sometimes further subdivided into two categories: clients and servers.

Client-Server Model

A client program is a program running on one end system that requests and receives

a service from a server program running on another end system. The Web, e-mail,

file transfer, remote login, and many other popular applications adopt client/server

model.

Since a program typically runs on one computer and the server program runs on

another computer, client/server Internet applications are distributed applications.

Peer-Peer Model

The P2P file-sharing applications in the user’s end system acts as both a client

program and a server program. The program running in a peer acts as a client whenit requests a file from another peer; and the program acts as a server when it

sends a file to another peer. Internet telephony is one of examples of this model.

1.2.2 Connectionless and Connection-Oriented Service

Connection-Oriented Service

Control packets are sent before the actual data for the establishment of

connection. This process is called Handshaking process.

Reliable data transfer; transmission of information without errors and in proper

order. Acknowledgements and retransmissions.

Flow control; makes sure that neither side of a connection overwhelms the other

side by sending too much packets too fast.

Congestion control; service helps preventing the Internet from entering the state

of gridlock by forcing the end systems to decrease their rate of data transmission.

Transmission Control Protocol (TCP)Applications using TCP include remote login, e-mail, file-transfer, and Web etc.

Connectionless Service

No handshaking like in connection-oriented service by sending control packets

before actual data transferring. Since there is no handshaking process, data can bedelivered soon.



Ideal for simple transaction-oriented applications.

No reliability that the data transferred will be reach entirely or not.

No provisions for flow control and congestion control.

User Datagram Protocol (UDP)

Applications using this protocol are Internet phone, video conferencing, and

multimedia applications etc.

1.3.1 Circuit Switching and Packet Switching

Two fundamental approaches to build a network: circuit switching and packet

switching.

In circuit-switched networks, the resources needed along a path between the end

systems are reserved for the duration of the communication.

In packet-switched networks, these resources are not reserved. A session’s

messages use the resources on demand and as consequence may have to wait for

access to a communication link.

Multiplexing in Circuit-Switched Networks

A circuit in a link is implemented with either frequency-division multiplexing (FDM)

or time-division multiplexing (TDM).

With FDM, the frequency spectrum of a link is shared among the connections

established across the link. This link dedicates a frequency band, known asbandwidth, to each connection for the duration of the connection.

For a TDM link, time is divided into frames of fixed duration, and each frame is

divided into a number of time slots. When the network establishes a connection

across a link, the network dedicates one time slot in every frame to the connection.For TDM, the transmission rate of a circuit is equal to the frames rate multiplied

the number of bits in a slot.

Packet Switching

Applications exchange messages in accomplishing their task. Messages can contain

anything the protocol designer wants. Messages may perform a control function or

can contain data such as an e-mail message, a JPEG image, or an MP3 audio file.In modern computer networks, the source breaks large messages into smaller

chunks of data known aspackets

. Between source and destination, each of thesepackets travels through communication links and packet switches. Packets are

transmitted over each communication link at a rate equal to the full transmission

rate of the link.

Store-and-Forward Transmission



Most packet switches use store-and-forward transmission at the inputs of the

link. It means that the switch must receive the entire packet before it can begin

transmit the first bit of the packet onto the outbound link. So this technique

introduces store-and-forward delay at each switch on the path of packet. This

delay is proportional to the length of the packet in bits. Store-and-forward delay =

L (no. of bits) / R (bps) seconds.

Output Buffer

Each packet switch has an output buffer (output queue), which stores the packets

to be sent. If an arriving packet needs to be transmitted across a link but finds it

busy, then it must have to wait in the output buffer. In this way, packets suffer

queuing delay.

Packet Loss

In the case, that a packet has to wait in the output buffer and the buffer is filled

completely with other packets to be transmitted later, then this causes packet

loss. Either the arriving packet or one of the already-queued packets will be

dropped.

Packet Switching Versus Circuit Switching; Statistical Multiplexing

Critics of packet switching: packet switching is not suitable for real-time servicesbecause of its variable and unpredictable end-to-end delays.

Packet Switching proponents: packet switching offers better sharing of bandwidththan circuit switching. It is simpler, more efficient and less costly to implement

than circuit switching.The data transferred on a link-layer switch is not transmitted in any fixed pattern;

rather the bandwidth is shared on demand. It is called statistical multiplexing.

1.3.2 Packet Switched Networks: Datagram Networks and Virtual-

Circuit Networks

Two broad classes of packet-switched networks: datagram networks and virtual-

circuit networks. They differ in whether their switches use destination addressesor so-called virtual-circuit numbers to forward packets toward their destination.

Datagram Network

Any network that forwards packets according to the host destination addresses is

called datagram network. The routers in the Internet forward packets according

to the host destination addresses; hence Internet is a datagram network.



Analogous to postal service.

Each packet traversing the network contains the address of the packet’s

destination in its header.

Analogous to a car driver who does not use maps but instead prefers to ask for

directions.

In contrast with VC networks, datagram networks do not maintain connection-stateinformation in their switches.

Packet switches make forwarding decisions based upon the packet’s destination

address.

Virtual-Circuit Network

Any network that forwards packets according to virtual-circuit numbers is called a

virtual-circuit network. Examples: X.25, frame relay, ATM (asynchronous transfer

mode).

A virtual connection between a source and destination host.Setting up and maintaining this VC will involve not only the two end systems but

each and every switch along the VC’s source-to-destination path.

A virtual circuit identifier (VC ID) will be assigned to a VC when a VC is firstestablished between source and destination.

The source and destination of a VC are only indirectly identified through the VC ID;

the actual addresses of the source and destination are not needed to perform

switching.Packet switching can be performed quickly.

A switch in a VC network maintains state information for its ongoing connections.

Each time a new connection is established across a switch, a new connection entrymust be added to the switch’s translation table; and each time a connection is

released, an entry must be removed from the table.

1.4.1 Access Networks

Access networks are the physical link(s) that connect an end system to its edge

router, which is the first router on a path from the end system to any other

distant end system.

Three categories of access networks: residential access, company access, and

wireless access.

Residential Access

Residential access – connecting home end systems into the network.

One form: dial-up modem over an ordinary analog telephone (twisted-pair copper

wire) line into a residential ISP. 56 kbps.



Another form is the broadband residential access. There are two types of

broadband residential access: digital subscriber line (DSL) and hybrid fiber-

coaxial cable (HFC).

DSL access is typically provided by a telephone company. Much higher rates as

compared to the dial-up modems. DSL and HFC require special modems, called cable

modems. HFC is a shared broadcast medium. Every packet sent by the head end travels

downstream on every link to every home; and every packet sent by a home travels

on the upstream channel to the head end. As the downstream and upstream

channels are shared, a distributed multiple access protocol is needed to coordinatetransmissions and avoid collisions.

DSL transmission rate is dedicated rather than shared.

Always on service provided to subscribers. Telephone calls and Internet facility at

the same time.

Company Access

Local Area Network (LAN) is typically used to connect and end system to the edge

router.Ethernet technology is the most prevalent access technology. 10 Mbps to 10 Gbps.

It uses either twisted-pair copper wire or coaxial cable to connect a number of end

systems with each other and with an edge router.

Ethernet uses a shared medium, so that the end users share the transmission rateof the LAN.

Switched Ethernet uses multiple twisted-pair Ethernet segments connected at a

switch to allow the full transmission rate of an Ethernet to be delivered todifferent users on the same LAN simultaneously.

Wireless Access

In a wireless LAN, wireless users transmit/receive packets to/from a base station(wireless access point, WAP) within a radius of few tens of meters. The base station is

typically connected to a wired Internet and thus serves to connect wireless users to the

wired network.

In wide-area wireless access networks, the base station is managed by atelecommunications provider and serves users within a radius of tens of kilometers.

Wi-Fi provides a shared transmission rate of 11 Mbps.

1.4.2 Physical Media

A bit, when travelling from source to destination, passes through a series of

transmitter/receiver pairs. For each transmitter-receiver pair, the bit is sent by



propagating electromagnetic waves or optical pulses across a physical medium.

Examples: twisted-pair copper wire, coaxial cable, multimode fiber-optical cable,

terrestrial radio spectrum, and satellite radio spectrum.

Physical media fall into two categories: guided media and unguided media.

With guided media, the waves are guided along a solid medium, such as fiber-optical

cable, a twisted-pair copper wire, or a coaxial cable.With unguided media, the waves propagate in the atmosphere and in outer space,

such as in a wireless LAN or a digital satellite channel.

Twisted-Pair Copper WireLeast expensive and most commonly used.

Consists of two insulated copper wires, each about 1 mm thick, arranged in a regularspiral pattern.

Unshielded twisted pair (UTP) is commonly used for computer networks within a

building, that is, for LANs. Data rates for LANs using twisted pair today range

from 10 Mbps to 1 Gbps.

Coaxial Cable

Consists of two concentric copper conductors rather than parallel.

High bit rates.

Can be used as a guided shared medium.

Fiber Optics

An optical fiber is a thin, flexible medium that conducts pulses of light, with each

pulse representing a bit.

Can support ten or even hundreds of Gbps.

Have very low signal attenuation up to 100 kilometers.

Terrestrial Radio Channels

Carry signals in electromagnetic spectrum.

No need to install physical wire.

Can penetrate walls, provide connectivity to a mobile user, and can potentially carry

a signal for long distances.

Classified into two groups: those that operate in local areas, ten to a few hundred

meters coverage; and those that operate in wide area, spanning tens of kilometers.

Satellite Radio Channels

Links two or more Earth-based microwave transmitters/receivers, known as groundstations.

The satellite receives transmissions on one frequency band, regenerates the signal

using a repeater, and transmits the signal on another frequency.

Can provide transmission rates in the gigabits per second range.

Two kinds of satellites used: geostationary satellites and low-altitude satellites.



Geostationary satellites permanently remain above the same spot on Earth. These

are used at the backbone of the Internet.Low-altitudes satellites are placed much closer to the Earth and do not remain

permanently above one spot on Earth.

1.5 ISPs and Internet Backbones

In the public Internet, access networks situated at the edge of the Internet are

connected to the rest of the Internet through a tiered hierarchy of ISPs.

Tier-1 ISPs

At the very top of the hierarchy is a relatively small number of so-called tier-1

ISPs.

Their link speeds are often 622 Mbps or higher, with the large tier-1 ISPs having links in 2.5 to 10 Gbps range.

Directly connected to each of the other tier-1 ISPsConnected to a large number of tier-2 ISPs and other customer networks

International in coverageKnown as Internet backbone networks

No group officially sanctions tier-1 status

Tier-2 ISPs

Typically has regional or national coverage and is connected to only a few of the

tier-1 ISPs.

A tier-2 ISP is said to be a customer of the tier-1 ISP to which it is connected, andthe tier-1 ISP is said to be a provider to its customer.

A provider ISP charges its customers ISP a fee depending on the transmission rate

of the link connecting the two.

At the bottom of the hierarchy are the access ISPs.

Connection between ISPs

When two ISPs are directly connected to each other, they are said to peer with

each other.

Within the ISP’s network, the points at which the ISP connects to other ISPs are

known as Points of Presence (POPs). A POP is simply the group of one or morerouters in the ISP’s network at which routers in other ISPs or in the networks

belonging to the ISP’s customers can connect.

ISPs are often interconnect at Network Access Points (NAPs), each of which can

be owned and operated by either some third-party telecommunications company or

by an Internet backbone provider.



The trend is for tier-1 ISPs to interconnect with each other at private peering

points, and for tier-2 ISPs to interconnect with other tier-2 ISPs and with tier-1

ISPs at NAPs.

1.6 Delay and Loss in Packet-Switched Networks

The most important delays: nodal processing delay, queuing delay, transmission

delay, and propagation delay.

Processing Delay

The time required to examine the packet’s header and determine where to direct

the packet is part of the processing delay. It also includes the time needed to

check for bit-level errors in the packet.

Queuing Delay

At the queue, the packet experiences queuing delay as it waits to be transmitted

onto the link.Depends on the number of already queuing packets waiting for their transmission.

If there is no packet in the queue, the queuing delay is zero.

Transmission Delay

For a packet of size L bits and let the link rate be R bits/second.

It is also known as store-and-forward delay is denoted by L/R.

It is the amount of time to transmit the entire packet into the link.

Propagation Delay

The time required to propagate a bit from the beginning to the next switch is the

propagation delay. The bit propagates at the propagation speed of the link. Ranges

from 2 x 10^8 to 3 x 10^8 meters/sec.

It is the distance between the two routers divided by the propagation speed.

Nodal Delay

d(proc) = processing delay, d(queue) = queuing delay, d(trans) = transmitting delay,

d(prop) = propagation delay, then d(nodal) = node-to-node delay is:

d(nodal) = d(proc) + d(queue) + d(trans) + d(prop)

Comparing Transmission and Propagation Delay

The transmission delay is the amount of time required for the router to push out

the packet; it is a function of the packet’s length and the transmission rate of the

link, but has nothing to do with the distance between the two routers.



The propagation delay is the time it takes a bit to propagate from one router to the

next; it is a function of the distance between the two routers, but has nothing to

do with the packet’s length and the transmission rate of the link.

1.6.2 Queuing Delay and Packet Loss

If ‘a’ is the average rate at which packets arrive at the queue (in packets/sec), R isthe transmission rate (in bits/sec), and L is the size of all the packets (in bits).

Then, the ratio La/R is called the traffic intensity.

If traffic intensity > 1, then the queue size will increase continuously and the delay

will approach infinity. So always design your system so that traffic intensity < 1.

Packet Loss

If a packet arrives to find a full queue in a switch, with no place to store such a

packet, a router will drop that packet, that is, the packet will be lost.

The fraction of lost packets increases as the traffic intensity increases.

End-to-End Delay

Assuming there are (N-1) routers between the source and destination hosts, no

queuing delay, processing delay to be d(proc), transmitting delay to be L/R bits/sec,

and the propagating delay to be d(prop), then, end-to-end delay is accumulated to

d(end-end) = N ( d(proc) + d(trans) + d(prop) )

1.7 Protocol Layers and Their Service Models

The protocols of various layers taken together are called theprotocol stack

. TheInternet protocol stack consists of five layers: the physical layer, the data link

layer, the network layer, the transport layer, and the application layer.

Application Layer

Residence of network applications and their application-layer protocols.

HTTP, SMTP, FTP etc.

Transport Layer

Transports the application-layer messages between the client and server sides of

an application.TCP and UDP.

Transport layer message is referred to as a segment.

Network Layer

Responsible for moving network-layer packets known as datagrams from one host toanother.



It has a protocol (Internet Protocol, IP) that defines the fields in the datagram as

well as how the end systems and routers act on these fields.

It also has routing protocols that determine the routes that datagrams take

between sources and destinations.

Link Layer

To move a packet from one node to the next node in the route, the network layer

relies on the services of the link layer.

Examples: Ethernet and Point-to-point Protocol (PPP).

Link layer packets are referred to as frames.

Physical Layer

The job of the physical layer is to move the individual bits within a frame from one

node to the next.

Examples: twisted-pair copper wire, single-mode fiber optics etc.

chap 1 (adeel nayyer) - copy

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