MC Unit I and II Notes

8/20/2019 MC Unit I and II Notes

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Prepared by: Dr.A.Neela Madheswari, Asso Prof/CSE, MEC Page 1

Mobile Computing

Unit – I Introduction

Mobile Computing – Mobile Computing Vs wireless Networking – Mobile ComputingApplications – Characteristics of Mobile computing – Structure of Mobile ComputingApplication. MAC Protocols – Wireless MAC Issues – Fixed Assignment Schemes – RandomAssignment Schemes – Reservation Based Schemes.

Unit – II Mobile Internet Protocol and Transport Layer

Overview of Mobile IP – Features of Mobile IP – Key Mechanism in Mobile IP – routeOptimization. Overview of TCP/IP – Architecture of TCP/IP- Adaptation of TCP Window – Improvement in TCP Performance.

Unit – III Mobile Telecommunication System

Global System for Mobile Communication (GSM) – General Packet Radio Service(GPRS) – Universal Mobile Telecommunication System (UMTS)

Unit – IV Mobile Ad hoc Networks

Ad-Hoc Basic Concepts – Characteristics – Applications – Design Issues – Routing – Essential of Traditional Routing Protocols – Popular Routing Protocols – Vehicular Ad Hocnetworks ( VANET) – MANET Vs VANET – Security.

Unit – V Mobile Platforms and Applications

Mobile Device Operating Systems – Special Constrains & Requirements – CommercialMobileOperating Systems – Software Development Kit: iOS, Android, BlackBerry, WindowsPhone-M-commerece- – Structure – Pros & Cons – Mobile Payment System – Security Issues.

References:

Prasant Kumar Pattnaik, Rajib Mall, ―Fundamentals of Mobile Computing‖, PHI Learning Pvt.Ltd, New Delhi – 2012.

Jochen H. Schller, ―Mobile Communications‖, Second Edition, Pearson Education, New Delhi,2007.

Dharma Prakash Agarval, Qing and An Zeng, "Introduction to Wireless and Mobile systems",

Thomson Asia Pvt Ltd, 2005.


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Uwe Hansmann, Lothar Merk, Martin S. Nicklons and Thomas Stober, ―Principles of MobileComputing‖, Springer, 2003.

William.C.Y.Lee,―Mobile Cellular Telecommunications-Analog and Digital Systems‖, SecondEdition,Tata Mc Graw Hill Edition ,2006.

C.K.Toh, ―AdHoc Mobile Wireless Networks‖, First Edition, Pearson Education, 2002.

Android Developers : http://developer.android.com/index.html

Apple Developer : https://developer.apple.com/

Windows Phone Dev Center : http://developer.windowsphone.com

BlackBerry Developer : http://developer.blackberry.com/

http://developer.android.com/index.htmlhttp://developer.android.com/index.htmlhttp://developer.windowsphone.com/http://developer.windowsphone.com/http://developer.blackberry.com/http://developer.blackberry.com/http://developer.blackberry.com/http://developer.blackberry.com/http://developer.windowsphone.com/http://developer.android.com/index.html


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Unit – I Introduction

1.1 Mobile Computing

Mobile Computing is a technology that allows transmission of data, voice and video via a

computer or any other wireless enabled device without having to be connected to a fixed physical link.

The main concept involves − Mobile communication, Mobile hardware, Mobile software.

Mobile communication

The mobile communication refers to the infrastructure put in place to ensure that

seamless and reliable communication goes on.

It include devices such as protocols, services, bandwidth, and portals necessary to

facilitate and support the stated services.

The data format is also defined at this stage. This ensures that there is no collision with

other existing systems which offer the same service.

Since the media is unguided/unbounded, the overlaying infrastructure is basically radio

wave-oriented. That is, the signals are carried over the air to intended devices that are

capable of receiving and sending similar kinds of signals.

Mobile Hardware

Mobile hardware includes mobile devices or device components that receive or access the

service of mobility.

It would range from portable laptops, smartphones, tablet PCs, Personal Digital

Assistants.


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Mobile software

Mobile software is the actual program that runs on the mobile hardware.

It deals with the characteristics and requirements of mobile applications.

This is the engine of the mobile device.

In other terms, it is the operating system of the appliance.

It is the essential component that operates the mobile device.

Since portability is the main factor, this type of computing ensures that users are not tied

or pinned to a single physical location, but are able to operate from anywhere.

It incorporates all aspects of wireless communications.

Evolution of communication networks

i. Circuit switching network:

A dedicated channel has to be established before call is made. Channel is reserved

between two users till the connection is alive.

ii. Packet switching network:

No need to establish the connection initially. Channel is available to use by many

users.

iii. Mobile cellular network:

It is categorized into various generations such as 1g, 2g, 3g, etc.


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1g

It is the first generation of wireless telephone technology

Came during 1980 and 1990

Speed was up to 2.4kbps

Allows voice calls in one country

Use analog signal

Advanced mobile phone system (AMPS) was first launched in USA in 1g mobile systems

Drawbacks:

Poor voice quality

Poor battery life

No security

Limited capacity

2g

It is based on GSM (Global System for Mobile communications)

Launched in Finland during 1991

Use digital signals

Data speed up to 64 kbps

Features: Enables text messages, picture messages and MMS, better quality and capacity

Drawbacks:

Requires strong digital signals

No network coverage, digital signals would weak

Unable to handle complex data such as video

2.5g

It is the technology between 2g and 3g

It is called 2g cellular technology combined with GPRS (General packet radio service)

Features:

Send/receive email messages

Web browsing

Speed is 64 to 144 kbps

Camera phones

3g


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During 2000

Data transmission speed is 144 Kbps to 2 Mbps

Features:

Faster communication Send/receive large email messages

High speed web

TV streaming/mobile TV

4g

Started from late 2000s

Basic term to describe it is MAGIC. M stands for mobile multimedia, A stands for

Anytime Anywhere, G stands for Global mobility support, I stands for Integrated

Wireless solution and C stands for Customized Personal Services. High QoS and high security

Provide any kind of service at any time as per user requirements anywhere

Drawbacks:

Battery usage is more

Hard to implement

Need complicated hardware

Expensive equipment required to implement next generation network

5g

Started from late 2010s

Complete wireless communication with no limitations

Supportable to WWWW (wireless WWW)

Data rate is 1Gbps

Example mobile devices

1. Pager:

Receiver only Tiny displays

Used for simple text messages

2. Mobile phones:

Can transmit voice, data

Used for simple text displays

3. PDAs (Personal Digital Assistant):


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Has simple graphical displays

Character recognition

Simplified www

4. Palmtop:

Tiny keyboard

Simple versions of standard applications

5. Laptop:

Fully functional

Standard applications

1.2 Mobile Computing Vs wireless Networking

Why we need mobile computing?

Enable anywhere/anytime connectivity

Bring computer communications to areas without pre-existing infrastructure

Enable mobility

Enable new applications

An exciting new research area

Mobile computing is defined as the technology that allows transmission of data, voice and video

via computer or wireless devices. The main advantage of mobile computing is users no need to

sit in a particular place or location. Mobile computing denotes accessing information and

services while we are on move. Wireless networking provides basic communication

infrastructure to make mobile computing possible. Mobile computing is based on wireless

networking. Wireless networking forms one of the necessary components of mobile computing.

Wireless networks appear in various forms such as: i) WLANs, ii) mobile cellular networks, iii)

Personal Area Networks, iv) Adhoc networks.

Wireless networks appear in two forms such as: i) extension of wired networks with the existing

wired infrastructure, ii) ad hoc networks also known as MANET (Mobile Ad hoc Networks).

WSNS (Wireless Sensor Networks) are special type of wireless ad hoc networks.

Wireless networking Mobile computing Example

X X Stationary computersX √ Notebook in a hotel√ X Wireless LAN building√ √ PDAs


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1.3 Mobile Computing Applications

Applications of wireless and mobile devices

1. Transmission of news, road condition and weather

2.

Personal communication3. Position via GPS

4. Local adhoc network with vehicles to prevent accidents and act as a guidance system

5. Vehicle data (train) can be transmitted in advance for maintenance and controls

6. Early transmission of patient data to hospital, current status, first diagnosis

7. Replacement of fixed infrastructure in case of earthquakes, fire, etc

8. Mobile office

9. Direct access to customer files stored in a central location

10. Wireless LANs in buildings

11. Outdoor internet access

12.

Ad hoc networks for multi user games13. Automatic call forwarding, transmission of actual workspace to current location

14. Mobile information services such as current special offers, etc.

Limitations of mobile devices

1. Battery backup is low, limited computing power and low quality

2. Not efficient keyboard

3. Can prove to theft due to its size

4. Higher loss rates due to interference

5.

Limited user interfaces and limited memory6. Higher delays (low downloads for Internet)

7. Lower security and simpler active attacking since i) medium is accessible for everyone,

ii) base station can be simulated and attract calls from mobile phones

1.4 Characteristics of Mobile computing

The important characteristics of mobile computing are:

1. Ubiquity:

The dictionary meaning of ubiquity is present everywhere.

In the context of mobile computing, ubiquity means the ability of a user to

perform computations from anywhere and at anytime.

2. Local awareness:


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A hand-held device equipped with global positioning system can transparently

provide information about the current location of a user to a tracking station.

3. Adaptation:

It implies the ability of a system to adjust to bandwidth fluctuation withoutinconveniencing the user.

In mobile computing environment, adaptation is crucial because of intermittent

disconnections and bandwidth fluctuations that can arise due to a number of

factors such as handoff, obstacles, environment noise, etc.

4. Broadcast:

Due to this characteristic, mobile computing environment can efficiently deliver

data simultaneously to hundreds of mobile users.

5. Personalization:

Services in a mobile environment can be easily personalized according to a user‘s profile.

This is required to let the users easily avail information with their hand held

devices.

1.5 Structure of Mobile Computing Application

Three layered architecture diagram:


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1. Presentation tier:

The topmost level of a mobile computing application concerns the user interface.

A good user interface facilitates the users to issue requests and to present the

results to them meaningfully.

The programs at this layer run on the client machine.

This layer usually includes web browsers and customized client programs for

dissemination of information and for collection of data from the user.

2. Application tier:

This layer has the vital responsibility of making logical decisions and performing

calculations.

It also moves and processes data between the presentation and data layers.

It performs the processing of user input, obtaining information and then making

decisions.

This layer is implemented using technology like Java, .NET services, cold fusion,

etc.

The implementation of this layer and the functionality provided by this layer

should be database independent.

This layer of functionalities is usually implemented on a fixed server.

3. Data tier:

It is responsible for providing the basic facilities of data storage, access, and

manipulation.

Often this layer contains a database.

The information is stored and retrieved from this database.

This layer is also implemented on a fixed server.

1.6 MAC Protocols

The MAC layer is the "low" part of the second OSI layer, the layer of the "data link". In fact, the

IEEE divided this layer into two layers "above" is the control layer the logical connection

(Logical Link Control, LLC) and "down" the control layer, the medium access (MAC).


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The MAC layer is the "low" part of the second OSI layer, the layer of the "data link". In fact, the

IEEE divided this layer into two layers "above" is the control layer the logical connection

(Logical Link Control, LLC) and "down" the control layer, the medium access (MAC).

IEEE 802 refers to a family of IEEE standards dealing with LAN and MAN. It includes various

groups and some of them are active and some of them are inactive. It provides standard for physical and data link layer in OSI model.

Some of the IEEE standards used for MAC are:

1. IEEE 802.3 – Ethernet2. IEEE 802.4 – Token Bus3. IEEE 802.5 – Token Ring4. IEEE 802.11 – Wireless LAN and Wi-Fi5. IEEE 802.15 – Wireless PAN, Bluetooth and Zigbee Technologies

6.

IEEE 802.15.1 – Wireless PAN or Bluetooth operates under 3 classes: 100 mts, 10 mtsand 1 mt7. IEEE 802.15.4 – Zigbee8. IEEE 802.15.6 – Body Area networks9. IEEE 802.15.7 – Visible light communications10. IEEE 802.16 – WiMAX (World wide Interoperability for Microwave access)

For WAN, we have the standard named X.25 by ITU (International Telecommunication Union).

It relates to the lower three layers. X.25 at the physical layer denotes the specification for

electrical, mechanical, etc. At data link layer, it covers link access protocol i.e. LAPB (Link

Access Protocol Balanced) that defines how two devices establish error-free connection. Atnetwork layer, it specifies packet format, routing and multiplexing of transmission between

communicating devices.

We are having two categories of network according to the medium it is used namely: i) networks

that use dedicated link between one node to another, ii) networks that use shared communication

medium between them.

When we are using dedicated link between source and destination system, then there is no

problem. But when we use shared communication medium among multiple systems, the there

arise lot of issues: i) which device will have control over the shared medium, ii) how much time

the device will take control over the shared medium, iii) when two or more devices trying to use

the shared medium at the same time, what is the output?

MAC protocol will take care of these issues. The major classification of MAC protocols are:

i. Random assignment scheme

ii. Reservation based scheme


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iii. Fixed assignment scheme

1.7 Wireless MAC Issues

MAC Issues

MAC protocol in wireless channel is much more complex than wired channel.

A collision detection scheme is much more difficult to implement in a wireless

environment since collisions are hard to detect by the transmitting nodes.

In infrastructure-less networks the issue of hidden and exposed terminals make MAC

protocol inefficient unless special care is taken.

Hidden terminal problem

The hidden terminal problem arises when at least three nodes communicate among each other.

Consider nodes A, B and C. A and B are in same range of medium to communicate. B and C are

in same range of medium to communicate. Here A can communicate with B, C can communicate

with B, B can communicate with A and B. Also C cannot be reach from A and A cannot be

reached from C.

Consider A and C wants to communicate with B at the same time. Then there is collision occurs.

Here C does not know A‘s transmission since A is hidden for C and also A does not know C‘stransmission since C is hidden for A. This problem is referred as ‗hidden terminal problem‘.

Exposed terminal problem

The hidden terminal problem arises when at least three nodes communicate among each other.

Consider nodes A, B and C. A and B are in same range of medium to communicate. B and C are

in same range of medium to communicate. Here A can communicate with B, C can communicate

with B, B can communicate with A and B. Also C cannot be reach from A and A cannot be

reached from C.


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MAC protocols usually inhibit transmission when transmission from another terminal is

detected. Consider another node D comes in the range of C. i.e. D and C are in same range of

medium to communicate. Now C can able to reach B and D.

Consider B transmit data to A. During this time, C cannot be transmit to any other node since C

and B shares the same communication medium or in other words, B and C using same range of

medium to communicate. Even if node D wants to transmit to node C and it can be received

properly without any collision, the transmission cannot be happened. This is due to the

knowledge of C that B is communicating to A. The data transmission from B and A is exposed to

C and thus the transmission from C and D cannot be occur. This problem is referred as ‗exposedterminal problem‘.

1.8 Fixed Assignment Schemes

The fixed assignment schemes are usually called circuit-switched scheme. In this scheme, the

resources required for a call are assigned for the entire duration of the call. There are three

categories for fixed assignment scheme. They are:

a. FDMA (Frequency Division Multiple Access)

b. TDMA (Time Division Multiple Access)

c. CDMA (Code Division Multiple Access)

a. FDMA (Frequency Division Multiple Access)

Frequency is defined as the number of cycles per unit time. Usually the time unit referred is in

terms of seconds. The unit of frequency is Hertz. If we have 2 cycles per second, the frequency is

2 Hz. The range of frequencies human can hear is 12 to 14 Kilo Hertz.

Example: FM radios are come under this category. If we want to divide the frequency range of

20Hz to 120Hz into five divisions, then the frequency division is calculated and divide it by the

number of channels required. i.e. here frequency division is (120-20 = 100 Hz) / 5 = 20 Hz.

Hence the partition of the channel according to frequency is 20-40 Hz, 40-60 Hz, 60-80 Hz, 80-

100 Hz, 100-120 Hz.


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The graph is plotted with frequency in y axis and time in x axis for FDMA. The available

bandwidth of the channel is divided into many narrower frequency bands called channels. Each

user is allotted for a particular range of frequency. if we want to use full duplex communication,

then each user is allotted with two different frequencies, one for sending and the other for

receiving. When a call is undertaken by the user, the frequency used by that user will not be

allotted for other user. Hence it does not achieve high channel utilization. In the wired telephone

transmission system, this scheme is used.

Advantages of FDMA

• Channel bandwidth is relatively narrow• Simple algorithmically, and from a hardware standpoint • Fairly efficient when the number of stations is small and the traffic is uniformly constant • Capacity increase can be obtained by reducing the information bit rate and using efficientdigital code

• No need for network timing • No restriction regarding the type of baseband or type of modulation

Disadvantages of FDMA

• If channel is not in use, it sits idle• The presence of guard bands• Maximum bit rate per channel is fixed • Does not differ significantly from analog system

b. TDMA (Time Division Multiple Access)

The graph is plotted with frequency in y axis and time in x axis for TDMA. In TDMA, the same

physical channel is allotted for different users in different time slot. Each user is allocate to use

the channel in a round robin fashion. It is easily adapt to transmission of data and voice

communication. Example: GSM (Global System for Mobile Communications) used this

technique.


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Advantages of TDMA

• Flexible bit rate • No frequency guard band required •Easy for mobile or base stations to initiate and execute hands off

• Extended battery life •TDMA installations offer savings in base station equipment, space and maintenance •The most cost-effective technology for upgrading a current analog system to digital

Disadvantages of TDMA

• Requires network -wide timing synchronization• Requires signal processing for matched filtering and correlation detection • Demands high peak power on uplink in transient mode

• Multipath distortion

c. Code Division Multiple Access

In this technique, the graph is plotted with frequency in y axis and time in x axis and code in z

axis. Hence all the users can use the same physical medium at the same time. To differentiate

among them, a special code of signals is used for every user who are using the channel.

CDMA uses different codes for different users and all are multiplexed over the same physical

channel. Example: All mobile applications where we use encryption in the sending end and

decryption in the receiving end come under this type.

Advantages of CDMA

•Many users of CDMA use the same frequency, TDD or FDD may be used• No absolute limit on the number of users • Easy addition of more users • Impossible for hackers to decipher the code sent • Better signal quality

Disadvantages of CDMA

• As the number of users increases, the overall quality of service decreases• Self -jamming


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1.9 Random Assignment Schemes

Random assignment schemes are called packet-switched schemes. Here no resource reservation

is made. The nodes simply start to transmission as soon as they have packet to send.

The various random access MAC schemes are:

i. ALOHA

ii. Slotted ALOHA

iii. CSMA

iv. CSMA/CD

v. CSMA/CA

i. ALOHA: It is the simplest scheme. It is free for all. When a node needs to send, it does so. If a

frame successfully reaches the destination, then it sends the next frame. It listens for an amount

of time equal to the maximum round trip delay plus a fixed increment. If it hears an

acknowledgement, then there is no problem i.e. the packets are reached without any problem.

Otherwise, it resends after waiting a random amount of time. This type is suitable when small

number of users send data infrequently.

ii. Slotted ALOHA: It is similar to ALOHA, the exception is that the time is divided into equal-

sized slots in which a packet can be sent. Thus, the size of the packet is restricted. The slotted

ALOHA system employs beacon signals that are sent at precise intervals that mark the beginning

of a slot, at which point the nodes having data to send can start to transmit. This scheme also

does not work well when the number of stations used to send data is high.

iii. CSMA: CSMA stands for Carrier Sense Multiple Access. Every node attached to the systemthat implements CSMA is capable of sensing whether the communication medium is free to

transmit or not. Hence this scheme is called LBT (Listen Before Talking) i.e. the node senses the

medium before starting to transmit. If the node senses that some transmission is going on, then it

will not start its transmission.

iv. CSMA/CD: CSMA/CD stands for Carrier Sense Multiple Access with Collision Detection. In

this scheme, the sender starts to transmit if it senses the channel to be free. Even if it senses the

channel to be free, there can be a collision during transmission. This can be happens when two or

more nodes start sending the data at the same time. After sending data, node will sense whether

there is any collision. If there is any collision, then it will stop its transmission. Hence it is alsocalled LAT (Listen After Talking).

v. CSMA/CA: CSMA/CA stands for Carrier Sense Multiple Access with Collision Avoidance.

There are a multiple number of nodes want to transmit data at the same time whenever the

channel is free to transmit. But more than two systems want to use the shared communication

channel, then there will be a collision and hence the receiver cannot able to receive proper data.


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To avoid this occurrence of collision, in this scheme, every node will follow a random amount of

time delay to sense the channel is free or not after a particular wait time while checking for the

channel to be free. Hence this scheme is called as collision avoidance.

1.10 Reservation Based Schemes

Reservation based schemes are also known as packet-switched schemes. In this scheme, a node

makes explicit reservation of the channel for an entire call before transmitting. Hence it is

suitable for handling calls with widely varying traffic characteristics.

The basic form of the reservation scheme is the RTS/CTS scheme. In RTS/CTS scheme, a sender

transmits an RTS (Request To Send) packet to the receiver before the actual data transmission.

On receiving, the receiver sends a CTS (Clear To Send) packet and the actual data transfer will

commence only after that process.

A node wants to send a message first reserves the medium by using appropriate control message

(RTS) and the corresponding destination node accepting this request answers with a control

message to the sender (CTS). Every other nodes which senses these control messages in the

network will not perform data transmission. One of the examples of RTS-CTS based MAC

protocols is MACA.

MACA

MACA stands for Multiple Access Collision Avoidance. It solves the hidden and exposed

terminal problems. A node running MACA requests to use the medium by sending RTS to

receiver.

Consider three systems A, B and C such that A and B are in same communication range, B and C

are in same communication range. Similarly A can reach only to B, C can reach only to B, B can

reach to A and B.

Solution to Hidden terminal problem

The steps to be followed under MACA to avoid hidden terminal problem are:

i. A sends RTS (with some information) to B

ii. B receives RTS that contains sender‘s name, receiver‘s name (here it is A and B) and

the length of future transmission.iii. CTS is sent to A and C which contains sender‘s name, receiver‘s name (here it is A

and B) and the length of transmission.

iv. C understood now that B is sending CTS to A. Hence C will not start its transmission

for the time given in CTS.

v. Collision cannot occur for the time mentioned in CTS.


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Thus here hidden terminal problem is solved. But collision can occur during the sending of RTS.

i.e. A and C send RTS to B at the same time. In this case, B can resolve this problem by

acknowledging only for one station with CTS. No transmission will occur without an appropriate

CTS.

Solution to Exposed terminal problem

The steps to be followed under MACA to avoid exposed terminal problem are:

i. B needs to transmit to A and thus transmit RTS to A and C.

ii. RTS contains name of the sender and receiver, length of data transmission in time.

iii. C does not response to this RTS since it represents A only in the RTS message. But A

will respond with CTS to B.

iv. C does not receive this CTS, and conclude that A is outside the detection range.

v. C can start its transmission assuming that no collision would occur at A.

Here exposed terminal problem is solved.


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Unit – II Mobile Internet Protocol and Transport Layer

2.1 Overview of Mobile IP

Mobile IP (Internet Protocol) enables the transfer of information to and from mobile computers,

such as laptops and wireless communications. The mobile computer can change its location to a

foreign network and still access and communicate with and through the mobile computer's home

network.

When a mobile node moves from one network to another, it has to reconfigure its IP according to

the new network. It is a complicated task. i.e. whenever the mobile node moves, it has to

reconfigure its IP address according to the newer network it is added now. If it is done, then it

may lose its connectivity to the already existing network connection. To solve this issue, we have

to use mobile IP.

Mobile IP uses two IP addresses: i) fixed home address, ii) care-of address. Mobile IP enables

computer to roam freely on the Internet or organization‘s network while maintaining the samehome address.

Figure: Mobile IP Topology

Using Mobile IP topology, the following scenario shows how a datagram moves from

one point to another within the Mobile IP framework.

1. The Internet host sends a datagram to the mobile node using the mobile node's home

address (normal IP routing process).


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2. If the mobile node is on its home network, the datagram is delivered through the normal

IP process to the mobile node. Otherwise, the home agent picks up the datagram.

3. If the mobile node is on a foreign network, the home agent forwards the datagram to the

foreign agent.

4. The foreign agent delivers the datagram to the mobile node.

5. Datagrams from the mobile node to the Internet host are sent using normal IP routing

procedures.

6. If the mobile node is on a foreign network, the packets are delivered to the foreign agent.

The foreign agent forwards the datagram to the Internet host.

Mobile IP Functional Entities

Mobile IP introduces the following new functional entities:

1. Mobile Node (MN) – Host or router that changes its point of attachment from one

network to another.

2. Home Agent (HA) – Router on a mobile node's home network that intercepts datagrams

destined for the mobile node, and delivers them through the care-of address. The home

agent also maintains current location information for the mobile node.

3. Foreign Agent (FA) – Router on a mobile node's visited network that provides routingservices to the mobile node while the mobile node is registered.

How Mobile IP Works

Mobile IP enables routing of IP datagrams to mobile nodes.

The mobile node's home address always identifies the mobile node, regardless of its

current point of attachment to the Internet or an organization's network.

When away from home, a care-of address associates the mobile node with its home

address by providing information about the mobile node's current point of attachment to

the Internet or an organization's network.

Mobile IP uses a registration mechanism to register the care-of address with a home

agent.


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The home agent redirects datagrams from the home network to the care-of address by

constructing a new IP header that contains the mobile node's care-of address as the

destination IP address.

This new header then encapsulates the original IP datagram, causing the mobile node's

home address to have no effect on the encapsulated datagram's routing until it arrives atthe care-of address.

This type of encapsulation is also called tunneling. After arriving at the care-of address,

each datagram is de-encapsulated and then delivered to the mobile node.

Consider a mobile node is currently in network A, it has its associated network IP address. The

network is called home network. If the mobile node changes its position to any other network,

this new network is called foreign network and to get the connectivity to the new network, the

mobile node should be associated with new network‘s associated IP address. If we change the

mobile node‘s IP address according to the new network position, then it will lose its previousconnectivity. To solve these problems, mobile IP is used for mobile nodes. According to themobile IP, each mobile node is associated with a pair of IP address namely: i) home address and

ii) care-of-address.

Mobile IP agents

Mobile IP agents are routers that have been given additional programming to make them

―Mobile IP aware‖. The communication between a mobile node and the agent on its localnetwork is basically the same as the normal communication required between a device on an IP

network and its local router, except more information needs to be sent when the router is an

agent. The router in the home network called home agent and the router in the foreign network

called foreign agent are commonly called mobile IP agents. These home and foreign agents are

used to route the packets for mobile node.

Care-of address

There are two types of care-of addresses. They are:

i. Foreign agent care-of address: When the mobile node is in foreign network

and if the mobile node is not having its own IP address, then it should use IP

address of the foreign agent itself. Thus foreign agent‘s IP address is used as acare-of address for mobile node. This type of care-of address is called foreignagent care-of address. In this case, the mobile node is connected to the foreign

agent using the layer 2 protocol only. Any packets routing from home agent

will reach the foreign agent, then from foreign agent, it uses layer 2 protocol

to reach the mobile node. Consider the figure below for example.


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ii. Co-located care-of address: When the mobile node has its own IP address in

its foreign network, this address is referred as co-located care-of address. Any

packet to be routed to the mobile node in the foreign network, then it will be

routed to the mobile node‘s co-located care-of address. The mobile node canobtain this address by two methods: i) manually by enquiring the admin to

provide IP, ii) using DHCP protocol. Consider the figure below for example.


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2.2 Features of Mobile IP

1. Transparency:

During its movement mobile end system‘s communication should not be disrupted.

The IP address is to be managed transparently and there should not be effect of mobility

on any ongoing communication.

2. Compatibility:

Mobile IP should be compatible with the existing Internet protocols.

3. Security:

Mobile IP should as far as possible, provide users with secure communications overInternet.

4. Efficiency and Scalability:

There can be a large number of mobile systems in the whole Internet.

This should be results in large number of messages or it should not incur too much

computational overhead.

It should also be scalable to support billions of moving hosts worldwide.

2.3 Key Mechanism in Mobile IP

Agent discovery

When a mobile node is first turned on, it cannot assume that it is still ―at home‖ the way normalIP devices do. It must first determine where it is, and if it is not at home, begin the process of

setting up datagram forwarding from its home network. This process is accomplished by

communicating with a local router serving as an agent, through the process called agent

discovery.

The main goals of agent discovery include the following:

o Agent/Node Communication: Agent discovery is the method by which a mobile node

first establishes contact with an agent on the local network to which it is attached.


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i. Messages are sent from the agent to the node containing important

information about the agent. These messages are called agent advertisement

messages.

ii. A message can also be sent from the node to the agent asking for this

information to be sent. This message is called agent solicitation message (i.e.request for agent advertisement).

o Orientation: The node uses the agent discovery process to determine where it is.

Specifically, it learns whether it is on its home network or a foreign network by

identifying the agent that sends it messages.

o Care-Of Address Assignment: The agent discovery process is the method used to tell a

mobile node the care-of address it should use, when foreign agent care-of addressing is

used.

The messages used in the agent discovery process are:

o Agent Advertisement: This is a message transmitted regularly by a router acting as a

Mobile IP agent. It consists of a regular Router Advertisement message that has one or

more extensions added that contain Mobile-IP-specific information for mobile nodes.

o Agent Soli citation: This message can be sent by a mobile IP device to nudge a local

agent to send an Agent Advertisement .

Agent Registration

Once a mobile node has completed agent discovery, it knows whether it is on its home networkor a foreign network. If on its home network it communicates as a regular IP device, but if on a

foreign network it must activate Mobile IP. This requires that it communicate with its home

agent so information and instructions can be exchanged between the two. This process is called

home agent registration, or more simply, just registration.

The main purpose of registration is to actually start Mobile IP working. The mobile node must

contact the home agent and tell it that it is on a foreign network and request that datagram

forwarding be turned on. It also must let the home agent know its care-of address so the home

agent knows where to send the forwarded datagrams. The home agent in turn needs to

communicate various types of information back to the mobile node when registration is performed. Note that the foreign agent is not really involved in registration, except perhaps to

relay messages, as we will see.

Mobile Node Registration Events

Successful registration establishes what is called in the standard a mobility binding between a

home agent and a mobile node. For the duration of the registration, the mobile node's regular


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home address is tied to its current care-of address and the home agent will encapsulate and

forward datagrams addressed to the home address over to the care-of address. The mobile node is

supposed to manage its registration and handle various events using several actions:

o Registration: The mobile node initiates a registration when it first detects it has moved

from its home network to a foreign network.

o Deregistration: When the mobile node returns home, it should tell the home agent to

cancel forwarding, a process called deregistration.

o Reregistration: If the mobile node moves from one foreign network to another, or if its

care-of address changes, it must update its registration with the home agent. It also must

do so if its current registration is about to expire, even if it remains stationary on one

foreign network.

Each registration is established only for a specific length of time, which is why regular

reregistration is required whether the device moves or not. Registrations are time-limited to

ensure that they do not become stale. If, for example, a node forgets to de-register when it returns

home, the datagram forwarding will eventually stop when the registration expires.

Mobile IP Data encapsulation and Tunneling

Once a mobile node on a foreign network has completed a successful registration with its home

agent, the Mobile IP datagram forwarding process will be fully ―activated‖. The home agent willintercept datagrams intended for the mobile node as they are routed to its home network, and

forward them to the mobile node. This is done by encapsulating the datagrams and then sending

them to the node's care-of address.

Mobile IP Data Encapsulation Techniques

Encapsulation is required because each datagram we intercept and forward needs to be resent

over the network to the device's care-of address. The default encapsulation process used in

Mobile IP is called IP Encapsulation Within IP , defined in RFC 2003 and commonly abbreviated

IP-in-IP . It is a relatively simple method that describes how to take an IP datagram and make it

the payload of another IP datagram. In Mobile IP, the new headers specify how to send the

encapsulated datagram to the mobile node's care-of address.

In addition to IP-in-IP, two other encapsulation methods may be optionally used: Minimal Encapsulation Within IP , defined in RFC 2004, and Generic Routing Encapsulation (GRE),

defined in RFC 1701.

The Mobile IP Data Delivery Tunnel

The encapsulation process creates a logical construct called a tunnel between the device that

encapsulates and the one that decapsulates. This is the same idea of a tunnel used in discussions


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of virtual private networks (VPNs), IPSec tunnel mode, or the various other tunneling protocols

used for security.

The tunnel represents a conduit over which datagrams are forwarded across an arbitrary

internetwork, with the details of the encapsulated datagram (meaning the original IP headers)

temporarily hidden.

In Mobile IP, the start of the tunnel is the home agent, which does the encapsulation. The end of

the tunnel depends on what sort of care-of address is being used:

o Foreign Agent Care-Of Address: The foreign agent is the end of the tunnel. It receives

encapsulated messages from the home agent, strips off the outer IP header and then

delivers the datagram to the mobile node. This is generally done using layer two, because

the mobile node and foreign agent are on the same local network, and of course, the

mobile node does not have its own IP address on that network (it is using that of the

foreign agent.)

o Co-Located Care-Of Address: The mobile node itself is the end of the tunnel and strips

off the outer header.

Key mechanisms in mobile IP

Mobile IP is associated with the following three basic mechanisms:

i. Discovering the care-of address

ii. Registering the care-of address

iii. Tunneling to the care-of address

i. Discovering the care-of address

Each mobile node uses a discovery protocol to identify the respective home and foreign agents.

The discovery of the care-of address consists of the following steps:


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a. Mobile agents advertise their presence by periodically broadcasting the

agent advertisement messages.

b. The mobile node receiving the agent advertisement message observes

whether the message is from its own home agent and determines whether

it is on the home network or on a foreign network.

c.

If a mobile node does not wish to wait for the periodic advertisement, it

can send out agent solicitation messages that will be responded to by a

mobility agent.

Agent advertisement involves the following activities:

a. Foreign agents send messages to advertise the available care-of addresses.

b. Home agents send advertisements to make themselves known.

c. Mobile hosts can issue solicitations to actively seek information.

d. If a mobile host has not heard from the foreign agent to which its current care-of address

belongs, it takes up another care-of address.

ii. Registering the care-of address

If the mobile node discovers that it is on the home network, it operates in a normal IP routing

process. If the mobile node is in foreign network, then it had to register the care-of address of the

mobile node to the home agent and then only the home agent can able to redirect the packets

which are coming for mobile node to the home agent.

The registration process consists of the following steps:

a.

If the mobile node is on the foreign network, it registers with the foreign agent bysending a registration request message which includes the permanent IP address of the

mobile host and the IP address of its home agent.

b. The FA (foreign agent) send the registration request containing the permanent IP address

of the mobile node and the IP address of the foreign agent to the home agent.

c. When the HA (home agent) receives the registration request, it updates the mobility

binding by associating the care-of address of the mobile node with its home address. The

HA then sends an acknowledgement to the FA.

d. The FA in turn updates its visitors list by inserting the entry for the mobile node and

relays the reply to the mobile node.


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iii. Tunneling to the care-of address

In computer networks, a tunneling protocol allows a network user to access or provide a

network service that the underlying network does not support or provide directly. One important

use of a tunneling protocol is to allow a foreign protocol to run over a network that does notsupport that particular protocol; for example, running IPv6 over IPv4. Simply, tunneling is a

technology that enables one network to send its data via another network‘s connections.

Tunneling takes place to forward the datagram from the HA to the mobile node. The packets are

coming from the remote host to the mobile node. if the mobile node is in home network, then it

will follow the normal IP routing process. If the mobile node is in foreign network, then the

packets or datagrams from the remote host will reach to the HA, where it will tunnel the packets

to the foreign network. The tunneling is of two ways:

1. If the mobile node follows co-located care-of address, then the tunneling path is from

HA to the mobile node. Thus the packets are routed through the tunnel from HA to

the mobile node directly.

2. If the mobile node follows FA care-of address, then the tunneling path is from HA to

the FA. Thus the packets from HA will tunnel from HA to FA and from the FA, it


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will follows the Layer2 protocol to reach the mobile agent. Here tunneling can be

done from HA to FA only.

The steps involved in tunneling process are:

a.

When a HA receives a packet addressed to a mobile host, it forwards the packet to thecare-of address using IP-within-IP.

b. Using IP-within-IP, the HA inserts new IP header in front of the IP header of any

datagram.

c. Destination address is set to the care-of-address.

d. Source address is set to the HA‘s address. e. After stripping out the first header, IP processes the packet again.

Overview and Architecture of TCP/IP

OSI Reference Model

Most network protocol suites are viewed as structured in layers.

This is a result of the Open Systems Interconnect (OSI) Reference Model designed by the

International Standards Organization (ISO).

The OSI model describes network activities as having a structure of seven layers, each of

which has one or more protocols associated with it.

The layers represent data transfer operations common to all types of data transfers amongcooperating networks.

The protocol layers of the OSI Reference Model are traditionally listed from the top

(layer 7) to the bottom (layer 1) up, as shown in the following table.

The operations defined by the OSI model are conceptual and not unique to any particular

network protocol suite.

For example, the OSI network protocol suite implements all seven layers of the OSI

Reference Model.

TCP/IP uses some of OSI model layers and combines others. Other network protocols,

such as SNA, add an eighth layer.

Table: The Open Systems Interconnect Reference Model


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Layer

No. Layer Name Description

7 Application Consists of standard communication services and applications

that everyone can use

6 Presentation Ensures that information is delivered to the receiving machine

in a form that it can understand

5 Session Manages the connections and terminations between

cooperating computers

4 Transport Manages the transfer of data and assures that received and

transmitted data are identical

3 Network Manages data addressing and delivery between networks

2 Data Link Handles the transfer of data across the network media

1 Physical Defines the characteristics of the network hardware

TCP/IP Protocol Architecture Model

The OSI model describes an idealized network communications protocol family.

TCP/IP does not correspond to this model directly, as it either combines several OSI

layers into a single layer, or does not use certain layers at all.

The following table shows the layers of TCP/IP, listed from topmost layer (application)

to lowest (physical network).

The table shows the TCP/IP protocol layers, their OSI Model equivalents, and examples

of the protocols available at each level of the TCP/IP protocol stack.


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Each host involved in a communication transaction runs its own implementation of the

protocol stack.

Table: TCP/IP Protocol Stack

OSI Ref.

Layer No.

OSI Layer

Equivalent

TCP/IP

Layer TCP/IP Protocol Examples

5,6,7 Application, Session,

Presentation

Application NFS, NIS+, DNS, telnet, ftp, rlogin,

rsh, rcp, RIP, RDISC, SNMP, and

others

4 Transport Transport TCP, UDP

3 Network Internet IP, ARP, ICMP

2 Data Link Data Link PPP, IEEE 802.2

1 Physical Physical

Network

Ethernet (IEEE 802.3) Token Ring,

RS-232, others

Physical Network Layer

The physical network layer specifies the characteristics of the hardware to be used for the

network.

For example, it specifies the physical characteristics of the communications media.

The physical layer of TCP/IP describes hardware standards such as IEEE 802.3, the

specification for Ethernet network media, and RS-232, the specification for standard pin

connectors.


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Data-Link Layer

The data-link layer identifies the network protocol type of the packet, in this case TCP/IP.

It also provides error control and "framing."

Examples of data-link layer protocols are Ethernet IEEE 802.2 framing and Point-to-

Point Protocol (PPP) framing.

Internet Layer

This layer, also known as the network layer, accepts and delivers packets for the network.

It includes the powerful Internet protocol (IP), the Address Resolution Protocol (ARP)

protocol, Reverse Address Resolution Protocol (RARP), Internet Group Message

Protocol (IGMP), and the Internet Control Message Protocol (ICMP) protocol.

IP Protocol

The IP protocol and its associated routing protocols are possibly the most significant of the entire

TCP/IP suite. IP is responsible for:

IP addressing - The IP addressing conventions are part of the IP protocol.

Host-to-host communications - IP determines the path a packet must take, based on the

receiving host's IP address.

Packet formatting - IP assembles packets into units known as IP datagrams.

Fragmentation - If a packet is too large for transmission over the network media, IP on

the sending host breaks the packet into smaller fragments. IP on the receiving host then

reconstructs the fragments into the original packet.

ARP Protocol

The Address Resolution Protocol (ARP) conceptually exists between the data link and

Internet layers.

ARP assists IP in directing datagrams to the appropriate receiving host by mapping

Ethernet addresses (48 bits long) to known IP addresses (32 bits long).

RARP protocol

It is used by IP to find the IP address based on the physical address of a computer. The operation

is reverse process of the ARP protocol.


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ICMP Protocol

Internet Control Message Protocol (ICMP) is the protocol responsible for detecting network error

conditions and reporting on them. ICMP reports on:

Dropped packets (when packets are arriving too fast to be processed)

Connectivity failure (when a destination host can't be reached)

Redirection (which tells a sending host to use another router)

Transport Layer

The TCP/IP transport layer protocols ensure that packets arrive in sequence and without

error, by swapping acknowledgments of data reception, and retransmitting lost packets.

This type of communication is known as "end-to-end." Transport layer protocols at this

level are Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).

TCP Protocol

TCP enables applications to communicate with each other as though connected by a

physical circuit.

TCP sends data in a form that appears to be transmitted in a character-by-character

fashion, rather than as discreet packets.

This transmission consists of a starting point, which opens the connection, the entire

transmission in byte order, and an ending point, which closes the connection.

TCP attaches a header onto the transmitted data. This header contains a large number of

parameters that help processes on the sending machine connect to peer processes on the

receiving machine.

TCP confirms that a packet has reached its destination by establishing an end-to-end

connection between sending and receiving hosts.

TCP is therefore considered a "reliable, connection-oriented" protocol.

UDP Protocol

UDP, the other transport layer protocol, provides datagram delivery service.

It does not provide any means of verifying that connection was ever achieved between

receiving and sending hosts. Because UDP eliminates the processes of establishing and


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verifying connections, applications that send small amounts of data use it rather than

TCP.

Application Layer

The application layer defines standard Internet services and network applications thatanyone can use. These services work with the transport layer to send and receive data.

There are many applications layer protocols, some of which you probably already use.

Some of the protocols include:

1.

Standard TCP/IP services such as the ftp, tftp, and telnet commands

2. UNIX "r" commands, such as rlogin and rsh

3. Name services, such as NIS+ and Domain Name System (DNS)

4. File services, such as the NFS service

5. Simple Network Management Protocol (SNMP), which enables network management

6. RIP and other routing protocols

Adaptation of TCP Window

TCP window

TCP is a connection-oriented protocol; both ends of a connection keep strict track of all

data transmitted, so that any lost or jumbled segments can be retransmitted or reordered

as necessary to maintain reliable transport.

To compensate for limited buffer space (where received data is temporarily stored until

the appropriate application can process it), TCP hosts agree to limit the amount of

unacknowledged data that can be in transit at any given time.

This is referred to as the window size, and is communicated via a 16-bit field in the TCP

header.

Suppose we have two hosts, A and B, that form a TCP connection.

At the start of the connection, both hosts allocate 32 KB of buffer space for incoming

data, so the initial window size for each is 32,768.


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Host A needs to send data to host B. It can tell from host B's advertised window size that

it can transmit up to 32,768 bytes of data (in intervals of the maximum segment size, or

MSS) before it must pause and wait for an acknowledgment.

Assuming an MSS of 1460 bytes, host A can transmit 22 segments before exhausting

host B's receive window.

When acknowledging receipt of the data sent by host A, host B can adjust its window

size.

For example, if the upper-layer application has only processed half of the buffer, host B

would lower its window size to 16 KB.


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If the buffer was still entirely full, host B would set its window size to zero, indicating

that it cannot yet accept more data.

On a LAN with high bandwidth and extremely low delay, windows are rarely stressed as

there are typically very few segments in transit between two endpoints at any given time.

On a high-bandwidth, high-delay network, however, an interesting phenomenon occurs:

it is possible to max out the receive window of the destination host before receiving an

acknowledgment.

As an example, let's assume a TCP connection is established between two hosts

connected by a dedicated 10 Mbps path with a one-way delay of 80ms.

Both hosts advertise the maximum window size of 65,535 bytes (the maximum value of a

16-bit unsigned integer).

We can calculate the potential amount of data in transit in one direction at one point intime as bandwidth * delay: 10,000,000 bps divided by 8 bits per byte, multiplied by 0.08

seconds equals 100,000 bytes.

In other words, if host A begins transmitting to host B continuously, it will have sent

100,000 bytes before host B receives the first byte transmitted.


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However, because our maximum receive window is only 65,535 bytes, host A must stop

transmitting once this number has been reached and wait for an acknowledgment from

host B.

2.8 Improvement in TCP Performance

Congestion in a network may occur if the load on the network i.e. the number of packets sent to

the network is greater than the capacity of the network i.e. the number of packets a network can

handle.

Congestion control refers to the mechanisms and techniques to control the congestion and keeps

the load below the capacity.

Congestion

Congestion in a network occurs because routers and switches have queues-buffers that

hold the packets before and after processing.

A router for example, has an input queue and an output queue for each interface. When a

packet arrives at the incoming interface, it undergoes three steps before departing as:

i. The packet is put at the end of the input queue while waiting to be checked.

ii. The processing module of the router removes the packet from the input queue once it

reaches the front of the queue and uses its routing table and the destination address to

find the route.

iii.

The packet is put in the appropriate output queue and waits its turn to be send.

There are 2 issues: i) if the rate of packet arrival is higher than the packet processing rate,

the input queues become longer and longer, ii) if the packet departure rate is less than the

packet processing rate, the output queues become longer and longer.

TCP uses a sliding window to handle flow control.


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The sliding window of TCP is byte-oriented and is of variable size.

The window spans a portion of the buffer containing bytes received from the process.

The bytes inside the window are the bytes that can be in transit i.e. they can be sent

without worrying about acknowledgement.

The imaginary window has two walls named right and left.

The window is opened, closed or shrunk.

Opening a window means moving the right wall to the right. This allows more new bytes

in the buffer that are eligible for sending.

Closing the window means moving the left wall to right. This means that some bytes

have been acknowledged and the sender need not worry about them.

Shrinking the window means moving right wall to left. This should not be allowed since

it revoke the eligibility of some bytes for sending. The left wall is also cannot be moved

to left since that number of acknowledgement received already.

The size of the window at one end is determined by two values: receiver window (rwnd)

and the congestion window (cwnd).

The receiver window is the value advertised by the other end in a segment containing

acknowledgement. i.e. TCP segment header contains a field as window size.

In that field the receiver can set the number of bytes it can accept before its bufferoverflows. Congestion window value is determined by the network to avoid congestion.

Improvement in TCP performance for Traditional Networks


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TCP‘s policy for handling congestion is based on three phases:

i. Slow start: Sender starts with very slow rate of transmission, increases the rate rapidly

to reach a threshold.

ii.

Congestion avoidance: When the threshold is reached, the data rate is reduced toavoid congestion.

iii. Congestion detection: If congestion is detected, sender goes back to the slow-start or

congestion avoidance phase. Thus the congestion is detected.

1. Slow start – Exponential increase:

Slow start is one of the algorithms used in TCP congestion control.

It is based on the idea that the size of the cwnd starts with one maximum segment size

(MSS). This means that the sender can send only one segment.

After receipt of the acknowledgement for segment 1, the size of the congestion window is

increased by 1, i.e. cwnd is now 2.

Now two segments can be sent. When each acknowledgement is received, the size of the

window is increased by 1 MSS.

When all seven segments are acknowledged, cwnd = 8.


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If we look at the size of cwnd in terms of rounds, we find that the rate is exponential as shown

below:

Initially, cwnd = 1

After round1, cwnd = 21 = 2



Slow start cannot continue indefinitely.

There must be a threshold to stop this phase.

The sender keeps track of a variable named ssthresh (slow-start threshold).

When the size of window in bytes reaches this threshold, slow start stops and the next

phase starts.

In most implementations the value of ssthresh is 65,535 bytes.


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2. Congestion avoidance – Additive increase:

If we start with the slow-start algorithm, the size of the congestion window increases

exponentially.

To avoid congestion before it happens, we have to slow down this exponential growth.

TCP defines another algorithm called congestion avoidance, which undergoes an additive

increase instead of an exponential one.

When the size of the congestion window reaches the slow-start threshold, the slow-start

phase stops and the additive phase begins.

In this algorithm, each time the whole window of segments is acknowledged, the size of

the congestion window is increased by 1. It is shown in figure below.

In this case, after the sender has received acknowledgements for a complete window size

of segments, the size of the window is increased by one segment.


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If we look at the size of cwnd in terms of rounds, we find that the rate is additive as

shown below:

Initially, cwnd = 1

After round1, cwnd = 1+1 = 2



3. Congestion detection – Multiplicative decrease:

If congestion occurs, the congestion window size must be decreased.

The only way the sender can guess that congestion has occurred is by the need toretransmit a segment. However, retransmission can occur in one of the two cases: i) when

a timer times out, ii) when three ACKs are received.

In both the cases, the size of the threshold is dropped to one-half, a multiplicative

decrease.

If a time out occurs, there is a strong possibility of congestion, a segment has probably

been dropped in the network, and there is no news about the sent segments.

The steps taken are:

a. The value of threshold is set to one-half of the current window size.

b. cwnd is set to the size of one segment.

c. Starts the slow-start phase again.

If three ACKs are received, there is a weaker possibility of congestion, a segment may

have been dropped, but some segments after that may have arrived safely since three

ACKs are received.

This is called fast retransmission and fast recovery. The steps taken are:

a. The value of threshold is set to one-half of the current window size.

b. cwnd is set to the size of one segment.

c. Starts the congestion avoidance phase.


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When we receive three ACKs in TCP?

Every time data packet arrives at the receiving side, the receiver responds with an

acknowledgment, even if this sequence number has already been acknowledged.

Thus, when a packet arrives out of order — that is, TCP cannot yet acknowledge the datathe packet contains because earlier data has not yet arrived — TCP resends the sameacknowledgment it sent the last time. This second transmission of the same

acknowledgment is called a duplicate ACK .

When the sending side sees a duplicate ACK, it knows that the other side must have

received a packet out of order, which suggests that an earlier packet might have been lost.

Since it is also possible that the earlier packet has only been delayed rather than lost, the

sender waits until it sees some number of duplicate ACKs and then retransmits the

missing packet.

In practice, TCP waits until it has seen three duplicate ACKs before retransmitting the

packet. It is shown in figure below.


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In this example, the destination receives packets 1 and 2, but packet 3 is lost in the

network.

Thus, the destination will send a duplicate ACK for packet 2 when packet 4 arrives, again

when packet 5 arrives, and so on. (To simplify this example, we think in terms of packets

1, 2, 3, and so on, rather than worrying about the sequence numbers for each byte.)

When the sender sees the third duplicate ACK for packet 2 — the one sent because thereceiver had gotten packet 6 — it retransmits packet 3. Note that when the retransmittedcopy of packet 3 arrives at the destination, the receiver then sends a cumulative ACK for

everything up to and including packet 6 back to the source.

MC Unit I and II Notes

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