Network Communicating Protocols, IP Address & OSI reference model

 

3.1.  Network Protocols

Protocols are needed for computer networks to communicate efficiently.

Network protocols are set of rules that enable data to flow from one NIC to another.

Protocols control the messages origination, the messages end, and the messages quantity in the network. Major Networking Protocols.

 

3.1.1.NetBEUI

Network Basic Input / Output System. A common network protocol that allows applications on different computers to communicate within a local area network (LAN). It was created by IBM for its early PC Network, and was adopted by Microsoft.  It does not support a routing mechanism. NetBEUI was developed by IBM for its LAN Manager product and has been adopted by Microsoft for its Windows NT, LAN Manager, and Windows for Workgroups products.

Advantages High speed on small networks, Ease of implementation, Small memory overhead, Self tuned (does not need configuration)

Disadvantages It cannot be routed between networks.

 

3.1.2.IPX/SPX

Internet  work Packet Exchange. It’s the Novell NetWare designed Protocol, which is the default for  all NetWare networks. Offers speed , works well with routers, and take up very little RAM. IPX/SPX packets can be routed from one network to another.

Advantages Ease of setup. Support for routing between networks.

Speeds greater than TCP/IP for NT.

Disadvantages Slower than NetBEUI. IPX/SPX is not a vendor neutral

 

3.1.3.TCP/IP

IP -  Internet protocol . default protocol for the Windows NT and UNIX networks. Lacks speed and takes up a large amount  of memory . however, it is universally supported, and is the protocol upon which the Internet is based. IP function at the Network layer of the OSI  model.

TCP – Transmission Control Protocol, the most common transport layer protocol used on Ethernet and the Internet. I t was developed by DARPA. TCP is built on top the Internet Protocol {IP} and is nearly always seen in the combination TCP/IP (TCP over IP). It adds reliable communication , flow , multiplexing and connection-oriented communication. It provides full-duplex, process-to-process connections.

TCP/IP Advantages and Disadvantages

Advantages Broad connectivity among all types of computers and servers

Direct access to the Internet

Disadvantages Difficulty of setup Slower than IPX & NetBEUI

 

 

3.1.4.UDP-  User Data gram Protocol, Internet standard network layer, transport layer and session layer protocols, which provide simple but unreliable data gram services. It adds a checksum and additional process-to process addressing information. UDP is a connectionless protocol , which , like TCP, is layered in top of IP. UDP neither guarantees delivery nor does it require a connection . As a result it is lightweight and efficient, but all error processing and retransmission must be take care of by the application program.

 

3.1.5.POP3 – Post Office Protocol version 3. POP3 allows  a client computer to retrieve electronic mail from a POP3 server via a TCP/IP or other connection. It does not provide for sending mail. POP is useful for computers without  a permanent network connection and which require a “post office “ (the POP server)  to hold their mail until they can retrieve it..

 

3.1.6.SMTP- Simple Mail Transfer Protocol , a protocol used to transfer electronic mail between computers, usually over Ethernet. It is a server-to-server protocol, so other protocols  are used to access the messages. The SMTP dialog usually happens in the background under the control of the messages transport system, but it is possible to interact with an SMTP server using telnet to connect to the normal SMTP port 25.

 

3.1.7.      SNMP- Simple Network Management  Protocol, the Internet standard protocol developed to manage nodes on an IP network. SNMP is not limited to TCP/IP  and can be used to manage and monitor all sorts of equipment including computers, routers , wiring hub, etc.

 

3.1.8.      FTP- File Transfer Protocol, a client-server protocol that allows a user on one computer  to transfer files to and from another computer over a TCP/IP network. Also the client program the user executes to transfer files.

 

3.1.9. HTTP- Hypertext Transfer Protocol, the client-server TCP/IP protocol used on the World Wide

           Web for the exchange of HTML documents. It conventionally uses port 80.

3.1.10. PPP- Point to Point Protocol, the Internet standard for transmitting  network  layer data  grams(e.g. IP Packets) over serial point-to-point links. PPP has a number of 9 advantages over SLIP; it is designed to operate both over asynchronous connection and bit-oriented synchronous systems and it can configure connections to a remote dynamically, and test that the link is usable. PPP can be configured to encapsulate different network layer protocol by using the appropriate

 Network Control Protocol ( NCP)

 

3.1.11.  SLIP- Serial Line Internet Protocol, software that allows the Internet Protocol(IP),normally used on Ethernet, to be used over a serial line, e.g. an RS-232 serial port connected to a modem. SLIP modifies a standard Internet data gram by appending a special SLIP END character to it, which allows data gram to distinguish separately. SLIP does not provide error detection.

 

3.1.12.  PPTP- Point to Point Tunneling Protocol, a protocol for connecting Windows NT clients and server over Remote Access Services (RAS). PPTP can be used to create a Virtual Private Network between computers running NT. It is an extension of PPP sponsored by Microsoft. Microsoft Point to Point Encryption may be used with PPTP to provide an encrypted connection, but PPTP   itself does not use encryption.

 

3.1.13. Telnet-  The Internet standard protocol for remote login. Runs on top of TCP/IP. Unix BSD networking software includes a program, telnet, which uses the protocol and acts as a terminal emulator for the remote login session.

 

3.2. IP Addressing Fundamentals

A host is a computer or device on a TCP/IP network. Every TCP/IP host is uniquely identified by its IP address. An IP address consists of network ID and a host ID .If two different hosts belong to the same network, they have the same network ID. The two hosts will have different host ID's and can communicate with each other locally without going through a router. if two hosts have different network ID's, they belong to different segments on the network. they must communicate with each other remotely through a router or default gateway.

An IP address consists of 32 binary bits, where each bit is either a 0 or 1.

it has 4 octets. Each octet  has 8 bits.

we first write the 32 bits into four 8-bit numbers (octets) separated by a periods(.). for Example: 11000001. 00001010. 00011110. 00000010.(IP address in binary form.

 

3.2.1. Address Classes

There are 5 different address classes. classes can be distinguished by decimal notation of the very first octet. The following Address Class table illustrates the class and address to which it  belongs.

A      1     -126     subnet mask   255.0.0.0       Available     N.H.H.H

B      128  - 191   subnet mask   255.255.0.0   Available      N.N.H.H

C       192 - 223   subnet mask  255.255.255.0 Available     N.N.N.H

D       224 - 239      N/A                          reserved for multicasting

E       240  - 255     N/A                         reserved

 

Note: 127 is reserved for loop back(127.0.0.1) and is used for internal testing on local machine.

Class A- No. of networks : 2^7 -2 = 128-2=126,

No. of hosts       : 2^24-2= 1677716 -2=1677714

Class B -No. of networks : 2^14-2=16384-2=16382

No. of hosts       : 2^16-2=65536-2=65534

Class C - No. of networks : 2^21-2=2097152-2=2097150

No. of hosts       : 2^8-2=256-2=254

3.2.2. Subnet mask

In the IP(internet protocol) addressing scheme, a group of selected bits identify a sub network. All the members of the sub network share the mask value. once the values are identified using the mask, members of this subnet can be referenced more easily. This is also known as an address mask.

Sub netting -  used in IP network to breakup larger network into smaller sub networks. reduce hosts and network traffics. it is easier to troubleshooting.

128 > 1 0 0 0 0 0 0 0

192 > 1 1 0 0 0 0 0 0

224 > 1 1 1 0 0 0 0 0

240 > 1 1 1 1 0 0 0 0

248 > 1 1 1 1 1 0 0 0

252 > 1 1 1 1 1 1 0 0

254 > 1 1 1 1 1 1 1 0

255>  1 1 1 1 1 1 1 1

No. of subnets : no. of (1's) power 2 minus 2

No. of hosts    : no. of (0's) power 2 minus 2

Examples :

Class C -                     

192.168.0.0

255.255.255.192 ,no. of subnet = 2^2-2=4-2=2,no. of host = 2^6-2=64-2=62

 2 subnets

62 hosts per subnet

 

3.3. The OSI Model

The open System Interconnection (OSI) Model is a seven layer model that helps designers of network operating systems and software to create relatively standardized software. This is useful when trying to get different operating systems to talk to each other. Although the model is only a blueprint, it is almost universally followed.

 

Fig 3.3   OSI Layers          

 

The model was developed by the International Organization for Standardization (ISO) in 1984. It   is now considered the primary architectural model for inter-computer communications.

• The Open Systems Interconnection (OSI) reference model is a descriptive network scheme. It ensures greater compatibility and interoperability between various types of network technologies.

 

• The OSI model describes how information or data makes its way from application programes (such as spreadsheets) through a network medium (such as wire) to another application program  located on another network.

        

• The OSI reference model divides the problem of moving information between computers over a network medium into SEVEN smaller and more manageable problems.

 

This separation into smaller more manageable functions is known as layering.

 

Use of OSI Reference model / Scope of OSI Reference Model

 

 

The OSI  Reference Model is composed of seven layers, each specifying particular network functions. The process of breaking up the functions or tasks of networking into layers reduces complexity and makes learning easier to understand. It breaks the network communication into smaller, simpler parts that are easier to develop.

It allows different types of hardware and software to communicate with each other. It prevents changes in one layer from affecting the other layers.

A layer should be created where a different level of  abstraction is needed.

• Each layer should perform a well defined function.

• The function of each layer should be chosen with an eye towards defining internationally standardized protocols.

• The layer boundaries should be chosen to minimize the  information flow across the interfaces.

• The number of layers should be large enough that distinct functions need not be thrown together in the same layer out of necessity, and small enough that the architecture does not become unwieldy.

 

Encapsulation.

           As the data flows down through the layers in the hierarchy, each layer adds some extra information to the data in the form of headers or tailors. This process of wrapping data with headers and tailors is called encapsulation.

 

These extra information are added:

• To enable the opposite corresponding layer to take the right operation on the data (to facilitate his work).
• To enable the network to transfer the data accurately from the source to the destination.
• Through these information each layer actually communicates with the opposite corresponding layer and this is called peer-to-peer communication.

Note:- At the receiver side De-Encapsulation take place.

 

Advantages of Reference Models

It divides the network communication process into smaller and simpler components, thus adding component development, design, and troubleshooting.

It allows multiple-vendor development through standardization of network components.

It encourages industry standardization by defining what functions occur at each layer of the model.

It allows various types of network hardware and software to communicate.

It prevents changes in one layer from affecting other layers, so it does not hamper development.

                           Fig 3.2  Different layers of OSI model

 

Now we will explain each layer in detail.

 

3.3.1.   Physical Layer.

The Physical layer defines all the electrical and physical specifications for devices.

The physical layer is the most basic network layer, providing only the means of transmitting raw bits.

Physical layer specifications define characteristics such as:

• Voltage levels
• Timing of voltage changes.
• Physical data rates.
• Maximum transmission distances
• Physical connectors.

Physical layer implementations can be categorized as either LAN or WAN specifications

The physical layer performs the functions required to transmit a bit stream over a physical medium. The major duties performed by physical layer are:

 

Physical characteristics of interface and media.

Defines the characteristics of the interface between the devices and the transmission media.

It also defines the type of transmission medium.

 

How Physical Layer works using Bits?

• Physical layer receives a steam of bits (sequence of 0s and 1s) without any interruption.
• To be transmitted, bits must be encoded into a signals – electrical or optical.
• The physical layer defined the type of representation ( how 0s and 1s are changed into signals).

 

Data rate:

The transmission rate – the number of bit per second- is also defined by the physical layer.

Physical layer protocols are encoding techniques (RZ, NRZ, Manchester etc.).

.                                   Fig 3.3.1 Physical Layer Transmission          

 

3.3.2.  Data Link Layer.

 Definition:-The data link layer is responsible for moving frames from one hop (node) to the next.

This layer organizes the data into frames, to be put on the physical medium.

The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers:

• Logical Link Control (LLC) sub layer.
• Media Access Control (MAC) sub layer.

The data link layer is responsible for moving frames from one hop (node) to the next.

The major duties of the data link layer are:

• Framing:

The data link layer divides the stream of bits steam from the network layer into manageable data units called frames.

• Physical addressing:

If frame is to be distributed to different systems on the network, the data link layer adds a header to the frame to define the sender and receiver of the frame.

Physical address is the MAC address, which is hard coded into NIC and is of 48-bit represented by Hexadecimal format.

• Flow control:

If the rate at which the data are absorbed by the receiver is less than the rate produced in the sender, the data link layer imposes a flow control mechanism to prevent overwhelming the receiver.

• Error control:

The data link layer adds reliability to the physical layer by adding mechanism to detect and retransmit damaged or lost frames.

It also uses a mechanism to prevent duplication of frames.

Error control is normally achieved through a trailer added to the end of the frame.

• Access control:

The data link layer protocol has to determine that how to get access to the link in case when two or more devices are connected to the same link.

 

Data Link layer protocols are CSMA/CD, CSMA/CA, Token Passing etc.

 

Fig 3.3.2   Data Link Layer Transmission          

 

 

3.3.3.  Network Layer.

The network layer is responsible for the source -to-destination delivery of a packet possibly across multiple networks. It two systems are connected to the same link, there is usually no need for a network layer. However, if the two systems are attached to different networks with connecting devices between the networks, there is need for the network layer to accomplish the delivery.

 

The major duties performed by the network layer are:

• Logical addressing:

The physical addressing implemented by the data link layer handles the addressing problem locally.

If a packet passes the network boundary, we need another addressing system to perform the source and destination delivery.

The network layer adds a header to the segment received from the session containing the logical addresses of the sender and receiver.

Logical address is also called IP address which is of 32-bits and represented in decimal format.

• Routing:

To route the packets from the source to destination in an internet work, the router uses network layer information.

 

Network layer protocols are IP, IPX, AppleTalk.

                                  Fig 3.3.3 Network  Layer Transmission          

 

3.3.4.  Transport Layer.

The transport layer is responsible for process-to-process delivery of the entire message.

The major duties performed by the transport layer are:

• Port address:

Each running process open a logical port on the computer.

The transport layer header must therefore include a type of address called port address.

The network layer gets each packet to the correct computer, the transport layer get the entire message to the correct process on that computer.

• Segmentation and reassembly:

A message received form the upper layers is divided into transmittable segments, each segment contains a sequence number.

These number enables the transport layer to reassemble the message correctly upon arrival at the destination and to identify and replace packets that were lost in the transmission.

• Connection Control:

The transport layer can be either connectionless or connection oriented.

A connection oriented transport layer makes a logical connection with the transport layer at the destination machine first before delivering the packets.

After all the data are transferred, the connection is terminated.

 

• Flow control:

Like data link layer, the transport layer is responsible for flow control.

However, flow control at this layer is performed end to end rather than across a single link.

• Error control:

Like data link layer, the transport layer is responsible for error control.

However, error control at this layer is performed end to end rather than across a single link.

Transport layer 4 protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol

                              Fig 3.3.4  Transport  Layer Transmission          

 

3.3.5.  Session Layer.

The Session layer controls the sessions between computers.

It establishes, manages and terminates the connections between the local and remote application. The session layer defines how to establish, maintaining and terminates session between two communication hosts.

The major duties of the session layer are:

Synchronization:

For lengthy transaction (file transfer), the user may choose to establish synchronization points associated with the transfer. If a fault develops during a transaction, the dialog may be restarted at an agreed synchronization point.

Dialog control:

Session layer determines that which role is to be played at any given time by a host.

Duplex: Two-way simultaneous.

Half-Duplex: Two-way alternate.

Simplex: One-way.

 

Session layer protocols are SQL, ASP(AppleTalk Session Protocol), Remote Procedure Call (RPC), X Window System.

                               Fig 3.3.5  Session Layer Transmission          

 

3.3.6.  Presentation Layer.

It provides coding and conversion functions to ensure that information sent from the application layer of one system would be readable by the application layer of another system.

Examples of Coding and Conversion functions:

 

Common data representation formats:

The use of standard image, sound and video formats.

Conversion Schemes:

To exchange information with systems by using different text and data representation., such as EBCDIC and ASCII.

The presentation layer is responsible for the delivery and formatting of information to the application layer for further processing or display.

 

 It is sometimes called the syntax layer

The major duties of the presentation layer are:

Format conversion:

Convert message from one format into another format .i.e. for ASCII to EBCEDIC or vice versa.

Compression.

Compress the message to take less bandwidth on the transmission media and less time for transmission.

Encryption:

Convert the message into a form that will not be readable by others.

Provides security to the message.

 

Protocols of the presentation layer are JPEG, MPEG, ASCII, EBCDIC etc.

                                  Fig 3.3.6  Presentation  Layer Transmission          

 3.3.7.  Application Layer.

The application layer is the OSI layer that is closest to the user. It provides network services to the user’s applications (.i.e. spreadsheet etc.). It provides a means for the user to access information on the network through an application. This layer interacts with software applications that implement a communicating component.

         Fig 3.3.7 Application Layer Transmission          

Application layers functions are:

Mail service:

It provides network services for the email application.

 

File transfer and Access:

It provides network services for a user to access files on a remote computer, to retrieve files from a remote computer for use in the local computer and to manage or control files in a remote computer locally.

 

World Wide Web:

It provides network services to access the World Wide Web.

 

Identifying communication partner.

Determines the identity and availability of communication partners for an application with data to transmit.

 

Determining resource availability:

Decide weather sufficient network resources for the requested communication exist.

 

Synchronize communication

All communication between applications requires cooperation that is managed by the application layer.

 

Some examples of application layer protocols :-

      Telnet

Typically used to provide user oriented command line login sessions between hosts on the Internet.

 

File Transfer Protocol (FTP)

It is a commonly used protocol for exchanging files over the network.

 

Simple Mail Transfer Protocol (SMTP)

It is the standard protocol for e-mail transmissions across the Internet.

 

Hypertext Transfer Protocol (HTTP)

It is a method used to transfer information on the World Wide Web. Its original purpose was to provide a way to publish and retrieve HTML pages.

 

3.4. Summary

There was no standard for networks in the early days and as a result it was difficult for networks to communicate with each other.

The International Organization for Standardization (ISO) recognized this. and researched various network schemes, and in 1984 introduced the Open Systems Interconnection (OSI) reference model.      

The OSI reference model has standards which ensure vendors greater compatibility and interoperability between various types of network technologies.

The OSI reference model organizes network functions into seven numbered layers.

Each layer provides a service to the layer above it in the protocol specification and communicates with the same layer’s software or hardware on other computers

 Summary of the Layers.