Protocols Introduction

Protocols Run on OSI, TCP/IP, & DOD Models

As I discussed in the previous post that OSI, TCP/IP & DoD Models are same according to functionality. Only the difference is there at the design of Layers: as TCP/IP converge 7 Layers of OSI into 5 Layers and DoD converges it into 4 Layers. Since all protocols run on one Model would run also the remaining two Models also. Now I discussed about some of the protocols run the OSI Model:

The Process Application Layer Protocols

The process Application Layer is same as the Application, Presentation, & Session Layers of OSI model, since all protocols used in OSI Model also used in TCP/IP. The Process Application Layer is Responsible for node-to-node application communication and control user interface. Different protocols and application used in the IP Network which discussed below.

• Telnet: Telnet is a terminal emulation and called chameleon of protocol. It allows a Telnet Client to access the resources of Telnet Server. This is achieved through polling on the Telnet Server, which making the Client Machine appears as it the part of the terminal directly attached to the local network. Users/ Clients start Telnet session through Telnet Client Software and login through this to Telnet Server.
•  FTP: File Transfer Protocol is a protocol actually done the job of files transfers between two machines. Beauty of FTP is that it not only a protocol but also a Program. As a protocol it is by applications, as a program it is employed by users to perform file tasks by hands. It allows for access to both directories and files, it can also done certain types of directory operation like reallocating to different directories. Through Telnet we can access to servers and then through FTP we done the jobs of file transfer or directories operations etc.
• TFTP: Trivial File Transfer Protocol is a stripped version of FTP. If one can know about directories that where to go, & what to do, & it is an easy job to done. Then TFTP will be best protocol. Since its also called protocol of choice. TFTP is just only for file transfers not for directories operations. It is not secure as it not uses the authentication as FTP uses.
• NFS: Network File System is a jewel of protocol especially for file sharing. The difference between FTP and NFS is that NFS is used for file shairing between two different operating systems. Lets a NFS server uses UNIX OS and NFS Client uses windows 98. The both system are unlike as different case sensitivity, filename length, security etc.  Since NFS allows NFS Server to allows some portion of the RAM which Will be used by windows 98.  Since normal communication starts between both OS.
• SMTP: Simple Mail Transfer Protocol defines a method for Email delivery. It uses a spooled or Queue for this purposes. When a message is sent to a destination, it is queued on some common queue between sender and reliever. Now te server software at destination regularly check this queue, when it detect this Email it further proceed this Email to its destination. 
• LPD: Line Printer Daemon is a protocol used for printer sharing in a local network. The LPD with the help of LPR(Line Printer) Program, allows print jobs to be queued and sent to the local network printer using TCP/IP protocol of Transport Layer.
• X Window: X Window defines a protocol for client/ server operation, it’s basically write applications based on GUI (Graphical User Interface). It allows a program called a client, to run on one computer window client and have it display things on other computer called window server.
• SNMP: SNMP is a network management protocol which collects valuable information through SNMP agents through continuous or fixed interval polling. The polling is done from management station. If all is well, SNMP receive something called “baseline” a healthy network report. This network protocol are also called watchdog over the network, quickly notify managers of any sudden turn of events. These watchdogs are called agents. Agents send an alert called trap to management station in case of any poll recieves.
• DNS: Domain Name Service is a protocol used to resolves hostnames and easy our lives over internet. This is used for assigning hostnames to IP addresses. An IP address identifies hosts on the network and the internet as well. For example www.yahoo.com is a hostname, but there is an IP address identifies it. Now let’s a yahoo wants to change its IP address. What will be happened if there were no DNS? We should remember the new IP address. But with DNS every change in the IP address will no effect on the hostname.
• DHCP: Dynamic Host Configuration Protocol assign IP addresses to hosts. It allows easer administration and work well in small to large network environment. Another beauty DHCP is that is supports all types of hardware to DHCP server. Through DHCP the hosts wastage is controlled and it support subneting. A DHCP assign IP address, Subnet Mask, Domain Name, Default Gateway, & DNS to a host. A client/host send a DHCP discover message in order to receive IP address and more

The Host-to-Host Layer Protocols

 Host-to-Host Layer is same as Transport Layer in OSI Reference Model. Since both protocols are same. The main purpose of Host-to Host layer is to distinguish between the upper layer applications from the complexities of lower network layers. TCP & UDP protocols are used in the Host-to-Host Layer.

• TCP: Transmission Control Protocol takes a large amount of data from upper layers and breaks them into segments. It numbers and sequences the segments so that at the receiving station it will reassemble and identify lost segments. For every segment TCP wait for acknowledge, those segments are lost are not acknowledge. Since TCP retransmit it. TCP creates a virtual circuit between sender and receiver & the communication called connection-oriented. It is also reliable. Since all these phenomena’s makes overload on a TCP header.
• UDP: User datagram Protocol is totally opposite of TCP. It is basically design for applications where reliability not consider. A common comparison between TCP and UDP are below:

TCP	UDP
Sequence	Unsequence
Reliable	Unreliable
Connection Oriented	Connection Less
Acknowledgments	No Acknowledgment
Windowing/ Flow Control	Not Windowing/ Flow Control
Virtual Circuit/ Overhead	Low Overhead

The Internet Layer Protocol

The Internet Layer is equalent to Network Layer of OSI. Since both models used the same protocols. It provides Routing and single network interface to upper layers. Protocols used are IP, ICMP, ARP, RARP, Proxy ARP.

• IP: Internet Protocol is essential for Network Layer. The other protocols are used to support IP. IP holds the big picture of Network and interconnected networks to it. Each device has a Logical address called IP address. IP address is exist in the header of each packet to determine the host and destination of the packet. Then routing table of each router decides with the help of IP address the best path to a destination. 
• ICMP: Internet Control Management Protocol is a management and control protocol for IP. It is often used by hosts and routers in Layer 3 to exchange Network/Internet Layer information and problem notification messages. These messages are sending within IP packets header only. ping, traceroute are examples of ICMP messages. 
• ARP: Address Resolution Protocol is used to find the Mac address of a host when IP address is already known. This is achieved through the dynamic discovery of MAC address. A broadcast address is send to all hosts in the network. Only the host which has the IP address will reply its MAC address through unicast ARP Reply to the host who request for ARP Request. Now we have both IP address and MAC address. 
• RARP: Reverse Address Resolution Protocol is the reverse operation of ARP. When one know about the MAC address and need to find IP address, RARP protocol should be used.
• Proxy ARP: Proxy ARP is used to find MAC address in different network with same subnetwork address. Since the host consider it as same network as her and sent ARP. In this scenario the router replies to ARP as behalf of destination host.