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https://www.tcetmumbai.in/EXTC/Syllabus/Autonomy/TE%20Sem%20VI_Syllabus.pdf

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��Computer Communication Networks � �

Module 1: Introduction

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Contents

Network Applications
Network Hardware
Network Software
Reference Models
Overview of TCP/IP
Layer Functions
Services
Sockets and ports
Encapsulation

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Basic introduction to a network

Network is simply defined as something that connects things together for a specific purpose
The term network is used in a variety of contexts, including telephone, television, computer, or even people networks

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Applications of Networks

Business applications

Resource sharing

Information sharing

VPNs

Email communication

IP telephony

Home applications

Internet access

Peer-to-peer communication
Social media
Instant messaging
E-commerce

Business-to-consumer
Business-to-business
Government-to-consumer
Consumer-to-consumer

Ubiquitous computing

Mobile users

Cellular networks

Communication through wireless hotspots

m-Commerce

Wearable devices

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Network Hardware �

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Network hardware (Contd..)

Hubs	Gateways	Router	Repeater	Bridge	Switch
They are generally used to connect computers in a LAN	A gateway is a network node that forms a passage between two networks operating with different transmission protocols	Routers are networking devices operating at layer 3 or a network layer of the OSI model	Its job is to regenerate the signal over the same network before the signal becomes too weak or corrupted so as to extend the length to which the signal can be transmitted over the same network	A bridge is a network device that connects multiple LANs (local area networks), that has a similar protocol, together to form a larger LAN	It is an intelligent network device that uses MAC addresses (addresses of medium access control sublayer) to send data packets to selected destination ports
A hub has many ports in it. A computer which intends to be connected to the network is plugged in to one of these ports	Gateway is located at the boundary of a network and manages all data that inflows or outflows from that network	They are responsible for receiving, analyzing, and forwarding data packets among the connected computer networks	Repeaters do not amplify the signal. When the signal becomes weak, they copy the signal bit by bit and regenerate it at the original strength	By joining multiple LANs, bridges help in multiplying the network capacity of a single LAN	It is supports unicast (one-to-one), multicast (one-to-many) and broadcast (one-to-all) communications
A hub cannot filter data. It is a non-intelligent network device that sends message to all ports	The feature that differentiates a gateway from other network devices is that it can operate at any layer of the OSI model	When a data packet arrives, the router inspects the destination address, consults its routing tables to decide the optimal route and then transfers the packet along this route
It primarily broadcasts messages
They are passive devices and don’t have any software associated with it

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Network hardware

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Network software

Most networks are organized as a stack of layers or levels, each one built upon the one below it
The number of layers, the name of each layer, the contents of each layer, and the function of each layer differ from network to network
The purpose of each layer is to offer certain services to the higher layers while shielding those layers from the details of how the offered services are implemented
In a sense, each layer is a kind of virtual machine, offering certain services to the layer above it
When layer n on one machine carries on a conversation with layer n on another machine, the rules and conventions used in this conversation are collectively known as the layer n protocol

Basically, a protocol is an agreement between the communicating parties on how communication is to proceed
Violating the protocol will make communication more difficult, if not completely impossible

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Network software (Contd..)

The entities comprising the corresponding layers on different machines are called peers
The peers may be software processes, hardware devices, or even human beings
In other words, it is the peers that communicate by using the protocol to talk to each other
No data are directly transferred from layer n on one machine to layer n on another machine
Instead, each layer passes data and control information to the layer immediately below it, until the lowest layer is reached
Below layer 1 is the physical medium through which actual communication occurs
Virtual communication is shown by dotted lines and physical communication by solid lines
Between each pair of adjacent layers is an interface.
The interface defines which primitive operations and services the lower layer makes available to the upper one
Interfaces minimize the amount of information that must be passed between layers
A set of layers and protocols is called a network architecture
The specification of an architecture must contain enough information to allow an implementer to write the program or build the hardware for each layer so that it will correctly obey the appropriate protocol
A list of the protocols used by a certain system, one protocol per layer, is called a protocol stack

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Reference models

There are two important network architectures

OSI (Open Systems Interconnections) reference model
TCP/IP reference model

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OSI model

An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture
The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software
The OSI model is not a protocol; it is a model for understanding and designing a network architecture that is flexible, robust, and interoperable
The OSI model is a layered framework for the design of network systems that allows communication between all types of computer systems
It consists of seven separate but related layers, each of which defines a part of the process of moving information across a network

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OSI model: Organization of the 7 layers

Physical, data link, network layers together acting as the ‘NETWORK SUPPORT LAYERS’

Session, Presentation, application layers together acting as the ‘USER SUPPORT LAYERS’

Transport

layer

Links the two subgroups and ensures that what the lower layers have transmitted is in a form that the upper layers can use

Deals with the physical aspects of moving data from one device to another

Allows interoperability between unrelated software systems

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OSI model: Details of each layer https://www.youtube.com/watch?v=vv4y_uOneC0�

Physical	Data link	Network	Transport	Session	Presentation	Application
Coordinates the movement of a bit stream	Makes the physical layer appear error-free to the upper layer	Responsible for source-to-destination delivery of packets	Responsible for process-to-process delivery of the message	Establishes, maintains and synchronizes the interaction between communicating systems	Concerned with the syntax and semantics of the information	Enables the user, whether human or software, to access the network
Defines the characteristics of the interface	Divides the stream of bits into frames	Performs logical addressing	Divides a given message into transmittable segments	Allows two systems to enter into a dialog	Responsible for interoperability between devices using different encoding systems	Supports services such as electronic mail, remote file access and transfer, shared database management etc.
Defines how 0s and 1s can be encoded	Imposes a flow control mechanism	Provides the mechanism of routing	Performs either connectionless or connection-oriented delivery	Allows a process to add checkpoints (synchronization points) into a stream of data	Assures privacy
Concerned with the connection of the devices to the media	Helps detect and retransmit damaged or lost frames as also recognize duplicate frames		Performs flow control but end to end rather than across a single link		Allows data compression while transmitting text, audio and video
Defines how the devices are connected to make a network	Determines which device has control over the link		Responsible for error control
Defines the direction of transmission

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Different network topologies

Bus topology

Star topology

Ring topology

Mesh topology

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TCP/IP reference model�

It’s a five-layer model with the layer names like the ones in the OSI model
Observations:

Two layers, session and presentation are missing
Application layer considered as combination of 3 layers in the OSI model

Some of the functionalities of the session layer are available in some of the transport layer protocols

Application layer is just not one piece of software but a medium where many applications can be developed;

Therefore, if some of the functionalities mentioned in the session and presentation are needed for a particular application, it can be included in the development of the application layer itself

Modules of TCP/IP are not interdependent

OSI model specifies which protocol belong to each of its layers; however the layers of TCP/IP contain relatively independent protocols that can be mixed and matched

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TCP/IP model: Details of each layer

Physical	Data link	Network	Transport	Application
TCP/IP does not define any specific protocol for the physical layer i.e., it supports all the standard and proprietary protocols	TCP/IP does not define any specific protocol for the data link layer	Also called as the internetwork layer	All nodes in a network need to have the network layer, only the two end computers need to have the transport layer	This layer is equivalent to the combined session, presentation and application layers in the OSI model
Unit of communication is a single bit	The unit of communication is a frame	TCP supports Internet Protocol (IP)	The unit of communication is a segment	The unit of communication at the application layer is a message
The responsibility of the physical layer is delivery of bits	At the data link layer, two nodes communicate logically but not physically	IP transports data in the form of packets called datagrams	Here again the communication is logical not physical	Communication at the application layer is end to end
The other functionalities matches with what is mentioned for the physical layer of the OSI model		Communication at network layer differs from the communication at the data link or physical layer wherein the communication at the network layer is end to end while the communication at the remaining two are node to node	The transport layer is represented by two protocols: UDP and TCP
		Here again the communication is logical not physical

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Communication at data link layer

Communication at network layer

Communication at transport layer

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Addressing in TCP/IP architecture

Physical address	Logical address	Port address	Application specific address
Also called as MAC address, it is a unique address provided by the device's vendor at the time of manufacturing and embedded in its NIC (Network Interface Card)	Also called as IP address, is the identifier that enables your device to send or receive data packets across the internet	Port number identifies a specific process to which an Internet or other network message is to be forwarded when it arrives at a server	User-friendly addresses that are designed for that specific application
It is 12 digit and 48 bits long	It is 32 bits long and is expressed in the form of four pairs	It is 16 bits long
E.g.: 00:0a:95:9d:67:16	E.g. 192.16.100.1	E.g. 80 for HTTP, 123 for NTP, 67 and 68 for DHCP traffic, 22 for SSH etc	E.g. e-mail address, Universal Resource Locator

Application-Specific addresses

Port addresses

Logical addresses

Physical addresses

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Services

A service is specified by a set of operations available to user processes to access the service
These operations tell the service to perform some action or report on an action taken by a peer entity
The set of operations available depends on the nature of the service being provided.
The operations for connection-oriented service are different from those of connectionless service
These operations might be used for a request-reply interaction in a client-server environment

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Services (Contd..)

The service defines what operations the layer is prepared to perform on behalf of its users, but it says nothing at all about how these operations are implemented while a protocol, in contrast, is a set of rules governing the format and meaning of the packets, or messages that are exchanged by the peer entities within a layer

Services relate to the interfaces between layers while protocols relate to the packets sent between peer entities on different machines

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Sockets and ports: Ports

The end objective of internet communication is a process communicating with another process
For more than one process to be received simultaneously a method of labeling different processes is required
In TCP/IP architecture, the labels assigned are called as port address (or port number)
Some important points to be remembered:

Transport layer is the layer providing process-to-process communication
In any client/server application, a server process waits for incoming requests by listening to a port

A port is an address that identifies which process is to receive a message

Client host and server host are defined using IP addresses, however processes running between these hosts is defined by another set of identifiers called as port numbers
In TCP/IP protocol suite, port numbers are integers between 0 and 65,535

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Sockets and ports: Ports

ICANN i.e. Internet Corporation for Assigned Names and Numbers has divided port numbers into three ranges:

Well-known ports: The ports ranging from 0 to 1023 are assigned and controlled by ICANN

TCP/IP has decided to use universal port numbers for servers

Registered ports: The ports ranging from 1024 to 49151 are not assigned or controlled by ICANN

These are only registered with ICANN to prevent duplication
Client end defines itself by the ephemeral port number

Dynamic ports: The ports ranging from 49152 to 65535 are neither controlled nor registered

They can be used as temporary or private port numbers
Client end defines itself by the ephemeral port number

https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt
IP address defines the host among the different hosts in the world and after the host has been selected, the port number defines one of the processes on this particular host

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Sockets and ports: Sockets

For a client process to communicate with a server process following needs to be done

A client process is nothing but a set of predefined instructions telling the client host what to do
This set of instructions is needed:

to tell the transport layer to open the connection
send data to other end
receive data from the other end
close the connection

This set of instructions designed for interaction between two entities is called an interface
Network communication at the application layer is dealt with by the Socket interface
The socket interface, as a set of instructions, is located between the operating system and the application program.
To access the services provided by the TCP/IP protocol suite, an application needs to use the instructions defined in the socket interface

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Sockets and ports: Sockets (Contd..)

The interaction between a process and the OS is done through a list of predefined functions:

The socket function

int socket(int family, int type, int protocol)

The bind function

int bind(int sockfd, const struct sockaddress* localAddress, socklen_t addreLen)

The connect function

int connect(int sockfd, const struct sockaddress* remoteAddress, socklen_t addreLen)

The listen function

int listen(int sockfd, int backlog)

The accept function

int accept(int sockfd, const struct sockaddress* clientAddr, socklen_t addreLen)

The fork function
The close function
The send and recv functions
The sendto and recvfrom functions
.
.

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Encapsulation

Involves appending additional information to the message at a given layer (say x) which is moving down from an immediate upper layer (x+1)
The appended information allows the receiver side layer x to deliver the message up to the appropriate application

The appended information might also include error detection bits that allows the receiver to determine whether bits in the message have been changed in the route

Encapsulation reduces the dependencies between adjacent layers to service definition only

I.e. layer x+1 as a user of the service provided by layer x is only interested in the correct execution of the service required to transfer its message

An exchange using the OSI model

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Encapsulation (Contd..)

Significance of encapsulation