Comparison between TCP/IP and OSI Reference Model

Aim: Comparison Between TCP/IP and OSI Reference Model
First we discuss, what is TCP\IP and OSI model, why they are introduced and where they are used. In the following Section OSI Reference models explained with all layers as well as their responsibilities.

Open Systems Interconnection model (OSI model):
OSI reference model is now considered as a primary standard for internetworking and inter computing. Today many network communication protocols are based on the standards of OSI model. In the OSI model the network/data communication is defined into seven layers. These 7 layers further divide the tasks of moving the data across the network into subtask and hence complete one communication cycle between two computers or two network devices. Each layer is assigned a task and the task is completed independently. The OSI layers have the clear and independent characteristics and tasks. The OSI model is made up of the following layers: the physical, data link, network, transport, session, presentation and application. Together, these seven layers are collectively referred to as a stack. As a node receives data, each layer starting with the physical layer extracts the various portions of the packet and this process works its way up to the application layer. When data is sent, it begins at the application layer and travels down to the physical layer. The information is pushed to the next layer of the stack by means of commands called primitives. Each layer uses a peer protocol to encode the information, which ensures that the same layer on the receiving node will be able to understand the information.

Physical Layer
Beginning at the bottom, the first layer is the physical layer. It governs the actual voltages, type of electrical signals, mechanical connections and other items relating to the actual data transmission medium. This includes cabling types, distances and connectors, as well as protocols like CSMA/CD.

Data Link Layer
The next layer is the data link layer. This is the layer that actually constructs the frames, and it also performs error checking using CRC. It ensures that the frames are sent up to the next layer in the same order that they were received, providing an error free virtual path to the network layer. The data link layer consists of two sub layers; the logical link control (LLC) and the media access control (MAC), which provide reliable communications by ensuring the data link is not broken and also by examining packet address information. A bridge is an example of a device that works at this layer. A bridge learns, forwards and filters traffic by examining the layer 2 MAC address. This helps segment network traffic. More recently, bridges have been replaced by switches,

which perform the same functions as a bridge, but can do so on each port. To find out more about switches, visit the Products link on the left.

Network Layer
Moving up to the next layer in the stack we come to the network layer. This layer actually routes packets of data, finding a path (both physical and logical) to the receiving or destination computer. It provides a unique address for each node through address resolution. One of the most common protocols for routing information at this layer is the Internet Protocol (IP). An example of hardware that can operate at this layer is a router. Although routers are often used to allow a LAN to access a WAN, layer 3 switches can also provide routing capabilities, but often at full wire-speed.

Transport Layer
The transport layer makes sure that the data arrives without errors, in the proper sequence and in a reliable condition. It uses flow control to make sure that information is sent at the proper speed for the receiving device to be able to handle it, and it repackages large data into smaller messages and then back again at the receiving node. An example protocol at this layer is the Transmission Control Protocol (TCP). Layer 4 switches can use the port information found in the TCP header to provide QoS (Quality of Service) and load balancing. To learn more about multi-layer switches, visit the Products link.

Session Layer
The session layer establishes the link between two nodes and ensures that the link is maintained and then disconnected. This is referred to as the session. It also makes sure the session is orderly, establishing which node transmits first, how long it can transmit, and what to do in case of an error. It also handles the security of the session.

Presentation Layer
The presentation layer deals with the actual formatting of the data. It handles compression, encryption, as well as translation to make sure differences in formatting can be read by the receiving node. For example, data might be converted from EBCDIC to ASCII formatting so that the receiving node can understand it.

Application Layer
This brings us to the seventh and final layer, the application layer. It allows applications access to network services, such as file and printer sharing, as well as file transfer and management services. This would be the layer that a programmer uses to allow his application to access a network service, such as linking into a database. Although this explains the flow of data and what processes are performed by each layer starting with the physical layer and working to the top, or application, layer, the process would be the same, only reversed, for data flowing from the application layer and down to the bottom, or the physical layer.

Transmission Control Protocol/Internet Protocol (TCP/IP)
TCP and IP were developed by a Department of Defense (DOD) research project to connect a number different networks designed by different vendors into a network of networks (the "Internet"). It was initially successful because it delivered a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems. Several computers in a small department can use TCP/IP (along with other protocols) on a single LAN. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. On the battlefield a communications network will sustain damage, so the DOD designed TCP/IP to be robust and automatically recover from any node or phone line failure. This design allows the construction of very large networks with less central management. However, because of the automatic recovery, network problems can go undiagnosed and uncorrected for long periods of time.

Today, TCP/IP is often associated with the Internet. Its architecture and design are closely bound with Internet advances and growth. Since, however, there is no organization that owns the Internet, you might ask how this whole system is controlled. There are organizations that are responsible for setting up standards and controlling the advance of the TCP/IP technologies.

The physical layer
The Physical layer describes the physical characteristics of the communication, such as conventions about the nature of the medium used for communication (such as wires, fiber optic links or radio links), and all related details such as connectors, channel codes and modulation, signal strengths, wavelength, low-level synchronization and timing and maximum distances. The Internet protocol suite does not cover the physical layer of any network; see the articles on specific network technologies for detail on the physical layer of each particular technology.

The data-link layer
The data link layer specifies how packets are transported over the physical layer, including the framing (i.e. the special bit patterns which mark the start and end of packets). Ethernet, for example, includes fields in the packet header which specify which machine or machines on the network a packet is destined for. Examples of Datalink layer protocols are Ethernet, Wireless Ethernet, SLIP, Token Ring and ATM. PPP is a little more complex, as it was originally specified as a separate protocol which ran on top of another data link layer, HDLC/SDLC.

This layer is sometimes further subdivided into Logical Link Control and Media Access Control.

Transport Layer
In TCP/IP architecture, there are two Transport Layer protocols. The Transmission Control Protocol (TCP) guarantees information transmission. The User Datagram Protocol (UDP) transports datagram without end-to-end reliability checking. Both protocols are useful for different applications.

Network Layer
The Internet Protocol (IP) is the primary protocol in the TCP/IP Network Layer. All upper and lower layer communications must travel through IP as they are passed through the TCP/IP protocol stack. In addition, there are many supporting protocols in the Network Layer, such as ICMP, to facilitate and manage the routing process.

Application Layer
The Application layer provides applications the ability to access the services of the other layers and defines the protocols that applications use to exchange data. There are many Application layer protocols and new protocols are always being developed. The most widely-known Application layer protocols are those used for the exchange of user information:
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The Hypertext Transfer Protocol (HTTP) is used to transfer files that make up the Web pages of the World Wide Web. The File Transfer Protocol (FTP) is used for interactive file transfer. The Simple Mail Transfer Protocol (SMTP) is used for the transfer of mail messages and attachments. Telnet, a terminal emulation protocol, is used for logging on remotely to network hosts.

Comparison between TCP/IP and OSI
On OSI:
OSI's major contribution to networking theory is in its distinct separation between three fundamental concepts:
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1. Services: A service defines what a layer does, but abstracts the details of implementation from higher levels in the protocol stack. 2. Interfaces: The interface makes the layer available to higher layers. It defines the conventions of communication - what to send and what to expect, but also does not deal with implementation details. 3. Protocols: These are private methods of implementation which the higher layers have no access to or knowledge of. Thus, they can be changed (i.e. to allow adding support for a new hardware technology) without altering the basic functioning of higher layers.

It should be rather obvious to someone with a computer science background that these concepts are extremely similar to the design philosophy of object-oriented programming. The layer possesses a set of service methods which can be invoked through the layer interface to initiate internal protocols. This similarity also means that OSI would gain some of the main advantages of objectbased design: data encapsulation, modularity and reusability, and access protection. Because the protocols themselves are hidden, they can be changed as new and improved technologies become available, without compromising the integrity of the system, in a way transparent to higher layers (to say nothing of the end-users!). We might in hindsight consider that this was a good idea. Unfortunately, the designers of the OSI model built the reference model before the protocols existed and did not understand from an engineering perspective where various pieces would optimally fit. This is because the majority of the protocols they were designing this for hadn't even been invented yet!

On TCP/IP:
TCP/IP's differences from the OSI model stem from its design requirements:
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A common application set Implicit support for dynamic routing Connectionless networking level Implicit support for packet-switching Universal connectivity

It should be clearly noted that TCP/IP has no provisions for the object-like separation between service, interface, and protocol of the OSI model. Whereas OSI was an abstraction created before technologies existed, TCP/IP simply described the existing hardware from an engineer's perspective and gave little thought to ensuring the model made sense at a higher level. The layers fit the technology perfectly, of course, but dialogue was impossible with other non-TCP/IP-based networks! The main differences between OSI and TCP/IP exist in the higher layers (5-7) and in the Network layer (3). OSI supports connectionless as well as connection-oriented protocols at the network level (again, due to its inherently abstract design), but only connectionoriented communication at the transport layer. TCP/IP is connectionless at the network level and supports both modes at the transport level, where the user, who often has little knowledge of low-level networking hardware and protocols, has to select how his connection functions.

Problems with OSI
OSI was a poor performer in implementation, and there are definite flaws in the protocols. Flow control is a problem at every layer and error control must be implemented all layers as well. Network management is problematic and was actually omitted from the original OSI model. Semantic confusion about the Presentation and Application layers created so many major headaches that data security and encryption were eventually taken out altogether!

OSI was killed off because:
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Early slow and bug-filled, unusable implementations ruined its public image. OSI was thought to originate with the European Community and the U.S. federal government. Its probable market for use was proprietary. TCP/IP was bundled as part of Berkeley UNIX and was free. OSI is labyrinthine and full of almost bureaucratic levels of unnecessary complexity. The seven-layer model was somewhat arbitrary, and was basically done in an attempt to wrest control away from IBM's 7-layer SNATM protocol to a world standard controlled by a neutral organization (the ISO) rather than by a single corporation -- not to simplify actually using the model!

Problems with TCP/IP:
Far from blameless, TCP/IP has some problems as well, the primary one being that it speaks only its own language:
   

It can't be used to intelligently describe another type of protocol stack (like SNA). Its network layer is more of an interface than a true layer of its own. There is no distinction between the Physical and Data Link layers. This is a poor choice from an engineering standpoint. Many of the original protocol implementations were hacks (in the "oldskool" sense, of course) with very limited usefulness and arbitrary constraints based on hardware limitations or on simplifying the coding task.

Difference between OSI model and TCP/IP model
The Internet Protocol Suite also known as TCP/IP is the set of communications protocols used for the Internet and other similar networks. It is named from two of the most important protocols in it: the Transmission Control Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in this standard. IP networking represents a synthesis of several developments that began to evolve in the 1960s and 1970s, namely the Internet and LANs (Local Area Networks), which emerged in the mid- to late-1980s, together with the advent of the World Wide Web in early 1990s. The Internet Protocol Suite, like many protocol suites, may be viewed as a set of layers. Each layer solves a set of problems involving the transmission of data, and provides a well-defined service to the upper layer protocols based on using services from some lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lower layer protocols to translate data into forms that can eventually be physically transmitted.

The main differences between the two models are as follows:
1. OSI is a reference model and TCP/IP is an implementation of OSI model. 2.TCP/IP Protocols are considered to be standards around which the internet has developed. The OSI model however is a "generic, protocolindependent standard." 3. TCP/IP combines the presentation and session layer issues into its application layer. 4. TCP/IP combines the OSI data link and physical layers into the network access layer. 5. TCP/IP appears to be a simpler model and this is mainly due to the fact that it has fewer layers. 6. TCP/IP is considered to be a more credible model- This is mainly due to the fact because TCP/IP protocols are the standards around which the internet was developed therefore it mainly gains creditability due to this reason. Where as in contrast networks are not usually built around the OSI model as it is merely used as a guidance tool.

7. The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers 8. In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[6] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network. 9.The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right. 10.The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.