Basics of OSI Model and TCP/IP Model

OSI MODEL

OSI Layer

To run a network OSI (Open systems interconnection) model is just act as A guideline prepared by ISO (International organization for Standardization).

It provides a framework for building and implementing networking standards, devices and other interconnections. 

When a communication system is designed in this manner it is called Layered architecture. 

Now a days many devices work at the all seven layers of OSI model.

The OSI have seven layers. This is further divided in to two groups. The top three layers (Session, Presentation and Application) define how the application communicates with each other and with users. The down four layers (Physical, Datalink, Network and Transport) shows how data is communicated end to end. 

The Seven (7) Layers are as below;

7. Application Layer

6. Presentation Layer

5. Session Layer

4. Transport Layer

3. Network Layer

2. Data link Layer

1. Physical Layer

Now a days new model is working i.e TCP/IP model.

• Layers 7, 6, and 5 correspond to Application layer in TCP/IP model
• Layer 4 correspond to Transport layer in TCP/IP model
• Layer 3 corresponds to Internet layer in TCP/IP model
• Layer 2 and 1 correspond to Network Interface layer in TCP/IP model
• Protocols that were not originally part of the TCP/IP specifications are referred not by position in TCP/IP model but by OSI model.    

Protocols, their port numbers and layers in TCP/IP model :-

• Protocol number - is used to define a stream of data associated with a specific service.
• The transport is provided by TCP and UDP protocols.
• Internet layer protocols are ARP, IP and ICMP.
• HTTP - hypertext transfer protocol TCP port 80 (application layer).
• SSL - Secure socket layers TCP port 443.
• SMTP - TCP port 25. Files stored in LocalDrive:\Inetpub\Mailroot.
• SNMP - simple network management protocol used to provide information about TCP/IP hosts, UDP port 161.
• FTP - only basic authentication allowed, TCP port 20 (data) TCP port 21 (control). Files stored in LocalDrive:\Inetpub\Ftproot (application layer)
• POP - TCP port 110
• DNS - UDP port 53 (query) TCP port 53 (zone transfer)
• NNTP - TCP port 119. Files stored in LocalDrive:\Inetpub\Nntpfile\Root
• PPTP - Point to point tunneling protocol TCP port 1723; protocol number 47
• L2TP/IPSec - UDP ports 500, 1701 and 4500; protocol number 50
• ARP, ICMP and IP (internet layer).  

 TCP/IP:

• The TCP/IP model is the newer networking model, OSI Open System Interconnection model is an older model
• Network interface - is the layer in the communications process that describes standards for physical media, for example ethernet. In OSI model it is both Physical layer and Data link layer.
• Internet - is the layer in the communications process during which information is packaged, addressed and routed to other network destinations. ARP is used for address resolution, IP for addressing and routing data and ICMP for reporting errors and exchanging limited control/status information. In OSI model this layer is called the Network layer.
• Transport - is the layer in the communications process during which the standards of data transport are determined. TCP protocol with its guarantees of delivery and connectionless unguaranteed but fast UDP protocol. This layer has the same name in the OSI model.
• Application - is the layer in the communications process during which end user data is changed, packaged and sent to and from transport layer, for example telenet. In OSI we have three layers, Session, Presentation and Application.

.Problems with TCP/IP connections:

• Network diagnostics - is a graphical tool that administrator can access from help and support tools menu. Users can save output to a file for examination by network administrator.
• Netdiag -  is a command line tool that is used to run different network tests. Administrator needs to install the tool first from the Windows CD, the support tools file is called suptools.msi.
• Tracert - shows the path a packet takes to reach given destination, this is done by setting different TTL values in the IP header of ICMP echo requests. Up to 30 hops, tells administrator when connectivity stops.
• Pathping - as tracert but shows the path that a packet takes to reach a given destination, however it also shows detailed analysis of traffic. Used to troubleshoot erratic network behaviour such as packets being delayed, where tracert is used for network connectivity.
• Arp - used to show the Arp cache on the PC. Sometimes local network computers can have wrong MAC addresses of each other cached and thus cannot communicate, use arp to check whatever addresses are correct. To cleat arp cache use arp -d command. Arp -a is used to check hardware address mappings, if it checks out look for hardware problem
• If the administrator is able to ping loopback address, PC own address and the local gateway but no other PCs the problem is most likely with arp cache being corrupted.
• Troubleshooting steps: loopback, local PC, default gateway, remote host by IP, remote host by name.

 -DR