The Internet Protocol Suite Also Known as TCP

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, a nd 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 do es 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.

OSI Model Reference

TCP/IP Model Reference

Service, interface and protocol are not clearly defined. For example, the Service, interface only real services offered and protocol by the Internet layer are - Send IP Packet - Receive IP Packet Because models were invented before protocols, functionalities put in each layer are not very optimized. Seven layers, Network (Internet), Transport and Application layers being similar to TCP/IP Both connectionless and connection-oriented communication are supported in the network layer, but only connection-oriented communication in the transport layer.

Protocols in the OSI model are better hidden and can be replaced relatively easily as the technology changes, which is one of the main objective of layered protocols. In this case, the protocols have been invented before models, so the functionalities are perfectly described.

Functionalities

Numbers of layers

Only four layers.

Connectionless/ Connectionoriented communication

Only one mode in the network layer (connectionless) but both modes in the transport layer are supported, giving the users a choice.

Internet Protocol Suite:
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 th at can eventually be physically transmitted. The TCP/IP model consists of four layers (RFC 1122). From lowest to highest, these are the Link Layer, the Internet Layer, the Transport Layer, and the Application Layer.

OSI model:
The Open Systems Interconnection Reference Model also known to be OSI Reference Model or OSI Model is an abstract description for layered communications and computer network protocol design. It was developed as part of the Open Systems Interconnection (OSI) initiative. In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the OSI Seven Layer Model. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of the path. All aspects of OSI design evolved from experiences with the CYCLADES network, which also influenced Internet design. The new design was do cumented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for use by the layer above it. Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly describe the functionality provided to an ( N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host.
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Layers in the TCP/IP Suite: The TCP/IP suite uses encapsulation to provide abstraction of protocols and services. Such encapsulation usually is aligned with the division of the protocol suite into layers of general functionality. In general, an application (the highest level of the model ) uses a set of protocols to send its data down the layers, being further encapsulated at each level. This may be illustrated by an example network scenario, in which two Internet host computers communicate across local network boundaries constituted by their internetworking gateways (routers).

The functional groups of protocols and methods are the Application Layer, the Transport Layer, the Internet Layer, and the Link Layer (RFC 1122). It should be noted that this model was not intended to be a rigid reference model into which new protocols have to fit in order to be accepted as a standard. y Application Layer: DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP, POP3, SIP, SMTP,SMPP, SNMP, SSH, Telnet, Echo, RTP, PNRP, rlogin, ENRP Routing protocols like BGP and RIP which run over TCP/UDP, may also be considered part of the Internet Layer. y Transport Layer: TCP, UDP, DCCP, SCTP, IL, RUDP, RSVP y Internet Layer: IP (IPv4, IPv6) ICMP, IGMP, and ICMPv6 OSPF for IPv4 was initially considered IP layer protocol since it runs per IPsubnet, but has been placed on the Link since RFC 2740. y Link Layer: ARP, RARP, OSPF (IPv4/IPv6), IS -IS, NDP

Different authors have interpreted the RFCs differently regarding whether the Link Layer and the four-layer TCP/IP model covers physical layer issues or a hardware layer is assumed below the link layer. Some authors have tried to use other names for the link layer, such as Network interface layer, in effort to avoid confusion with the Data link layer of the seven-layer OSI model. Others have attempted to map the Internet Protocol model onto the seven-layer OSI Model. The mapping often results in a five-layer TCP/IP model, wherein the Link Layer is split into a Data Link Layer on top of a Physical Layer. Especially in literature with a bottom-up approach to computer networking, where physical layer issues are emphasized, an evolution towards a five-layer Internet model can be observed out of pedagogical reasons. The Internet Layer is usually directly mapped to the OSI's Network Layer. At the top of the hierarchy, the Transport Layer is always mapped directly into OSI Layer 4 of the same name. OSIs Application Layer, Presentation Layer, and Session Layer are collapsed into TCP/IP's Application Layer. As a result, these efforts r esult in either a four- or fivelayer scheme with a variety of layer names. This has caused considerable confusion in the application of these models. Other authors dispense with rigid pedagogy[17] focusing instead on functionality and behavior. The Internet protocol stack has never been altered by the Internet Engineering Task Force (IETF) from the four layers defined in RFC 1122. The IETF makes no effort to follow the seven-layer OSI model and does not refer to it in standards-track protocol specifications and other architectural documents. The IETF has repeatedly stated that Internet protocol and architecture development is not intended to be OSI -compliant.

Description of OSI layers:
Layer 1: Physical Layer: The Physical Layer defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of pins, voltages, cable specifications, Hubs, repeaters, network

adapters, Host Bus Adapters (HBAs used in Storage Area Networks) and more. To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Standards such as RS-232 do use physical wires to control access to the medium. Layer 2: Data Link Layer: The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-tomultipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work, in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages. WAN Protocol architecture: Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They also are capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. IEEE 802 LAN architecture: Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multi access media. Layer 3: Network Layer: The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks, while maintaining the quality of service requested by the Transport Layer. The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer²sending data throughout the extended network and making the Internet possible. This is a logical

addressing scheme ± values are chosen by the network engineer. The addressing scheme is hierarchical. The best-known example of a Layer 3 protocol is the Internet Protocol (IP). It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for fragmenting the packet into sufficiently small packets that the medium can accept. Layer 4: Transport Layer: The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, the best known examples of a Layer 4 protocol are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).[citation needed] Layer 5: Session Layer: The Session Layer controls the dialogues/connections (sessions) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full -duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedu res. The OSI model made this layer responsible for "graceful close" of sessions, which is a property of TCP, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls (RPCs). Layer 6: Presentation Layer: The Presentation Layer establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the Presentation Service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol Data Units, and moved down the stack. This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer. Layer 7: Application Layer:

The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP) , and Simple Mail Transfer Protocol (SMTP).

General Comparison with TCP/IP:
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. Even though the concept is different than in OSI, these layers are nevertheless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer, while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven -layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document. 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 rou ting 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. The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.

TCP/IP Model Vs OSI Model
This article is on TCP/IP model vs OSI model. It is meant to highlight the differences between the two set standards of the industry. TCP/IP model and the OSI model have been the two protocol suites on which communication industry heavily relies on.

Both, TCP/IP model and OSI model, work in very similar fashions. But they do have very subtle differences. Knowing these differences is crucial to learning computer networking. This article will try to show the comparison between TCP/IP model vs OSI model. A Background OSI reference model came into existence way before TCP/IP model was created. Advance research project agency (ARPA) created OSI reference model so that they can logically group the similarly working components of the network into various layers of the protocol. But after the advent of the Internet, there arose the need for a streamlined protocol suite, which would address the need of the ever growing Internet. So the Defense Advanced Research Project Agency (DARPA), decided to create TCP/IP protocol suite. This was going to address many, if not all the issues that had arisen with OSI reference model. TCP/IP Model Layers Explained So, what does TCP/IP stand for ? It is a suite of protocol which is named after its most significant pair of protocols. That is Transmission Control Protocol and Internet Protocol. TCP/IP are are made up of layers. Each layer is responsible for a set of computer network related tasks. Every layer provides service to the the layer above it. There are in all four layers in the TCP/IP reference model.

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Application Layer: This is the topmost layer of the TCP/IP suite. This is responsible for coding of the packet data. Transport layer: This layer monitors end to end path selections of the packets. It also provides service to the application layer. Internet Layer: This layer is responsible for sending packets through different networks. Link Layer: It is the closest layer to the network hardware. It provides service to Internet layer.

OSI Model Layers Explained In OSI reference model there seven layers of protocols. Again, in OSI reference model, each layer provides services to the layer above it. There are in all seven layers of in OSI. They are

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Physical Layer: It is the lower most layer of the OSI reference model. It is layer which is responsible for direct interaction of OSI model with hardware. The hardware provides service to the physical layer and it provides service to the datalink layer. Datalink Layer: There may be certain errors, which may occur at physical layer. If possible, these errors are corrected by datalink layer. Datalink layer provides the way by which various entities can transfer the data to the network. Network Layer: It does not allow the quality of the service to be degraded that was requested by transport layer. It is also responsible for data transfer sequence from source to destination. Transport Layer: The reliability of the data is ensured by the transport layer. It also retransmits those data that fail to reach the destination. Session Layer: The sessions layer is responsible for creating and terminating the connection. Management of such connection is taken care of by sessions layer.

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MODELS 2.The OSI model is a reference model. 2.The TCP/IP model is an implementation of the OSI model.
3.In OSI model,the protocols came after the model was described. 3.In TCP/TP model,the protocols came first,and the model was reall just a description of the existing protocols. 4.In OSI model,the protocols are better hidden. 4.In TCP/IP model ,the protocols are not hidden. 5.The OSI model has 7 layers. The TCP/IP model has only 4 layers. 6.The OSI model supports both connectionless and connection -oriented communication in the network layer,but only connection -oriented communication in transport layer. 6.The TCP/IP model supports both connectionless and connection -oriented communication in the transport layer.,giving users the choice.

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As it c be seen from the previous pages there are a number of comparisons which can be drawn between the two models This section will therefore be focusing on highlighting the similarities and differences between the OSI a TC /IP models nd

SIMILARITIES
The main similarities between the two models include the following: They share similar architecture. - Both of the models share a similar architecture. This can be illustrated by the fact that both of them are construct d with layers. e They share a common application layer. Both of the models share a common "application layer". However in practice this layer includes different services depending upon each model. Both models have comparable transport and networklayers.- This can be illustrated by the fact that whatever functions are performed between the presentation and network layer of the OSI model similar functions are performed at the Transport layer of the TCP/IP model. Knowledge of both models is required by networking professionals. According to article obtained from the internet networking professionals "need to know both models". (Source: Both models assume that packets are switched. Basically this means that individual packets may take differing paths in order to reach the same destination.

DIFFERENCES

The main differences between the two models are as follows: TCP/IP Protocols are considered to be standards around which the internet has developed. The OSI model however is a "generic protocol- independent standard." (www.n f .com/crs) TCP/IP combines the presentation and session layer issues into itsapplication layer. TCP/IP combines the OSI data link and physical layers into the network access layer. TCP/IP appears to be a more simpler model and this is mainly due to the fact that it has fewer layers. 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 OS model as it is merely used as a I guidance tool. The OSI model consists of 7 architectural layers whereas the TCP/IP only has 4 layers.
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OSI HISTORY

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The OSI (open Systems interconnection) model was developed by the International Standards Organization (ISO) in 1984 in an attempt to provide some standard to the way networking should work. It is a theoretical layered model in which the notion of networking is divided into several layers, each of which define specific functions and features. However it must be noted that this model only represents a general guideline for developing usable network interfaces and protocols. Sometimes it may become very difficult to distinguish between each layer as some systems do not rigorously adhere to the model. Despite all this, however the OSI model has earned the honour of being "the model" upon which all good network protocols are based. The OSI Model is based upon 7 layers (Application layer, Presentation Layer, Session Layer , Transport Layer, Network Layer, Data Link Layer and the Physical layer). Each one of these are used to perform particular tasks, which include "supporting end user processes, encrypting the data as-well as providing transparent data transfer over a network" (Croucher P, "Communications and Networks", 1999 British Library publication). The architectural framework of each layer is implemented throughout with software and hardware to establish how data can be transmitted transparently. Figure 1 below highlights the responsibility of each layer according to the OSI model.

Layer Application (Layer 7)

Responsibility

The application layer consists of application programs and user interfaces. It is at this layer that user's interact with al systems that the model defines. It supports many features that allow exchange of information across users. For exam provides application services for transfer of e-mails, and other network software services. The applications that use these s include FTP and Telnet which exist entirely on this layer. In Addition to this the layer focuses on aspects s "communication partners, quality of service, constraints on data syntax ,authentication and privacy". (www.in guide.co.uk/tcp-ip)

Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3)

The purpose of the presentation layer is to represent data in such a form that it can be exchanged through a network. A t example of this would be encrypting the data. This layer is also referred to as the syntax layer.

The Session layer is used for establishing, managing and terminating connections on a network. Additionally this layer concerned with setting up, coordinating, terminating and managing conversations between the user and network. It is level that the user and machine names are interpreted.

The Transport layer controls the quality and reliability of the data transmission. At this layer each data packet is sequenc acknowledged . In addition to this layer also provides transparent transfer of data between end users and the host. It responsible for end to end error recovery and flow control. The network layer is used for routing data across configured network nodes. This layer uses many technologies to enable the data to be transmitted which are known as switching and routing. These technologies create logical (known as virtual circuits) paths for transmitting data across a network from a node to node transmission. Additionally other functions of this layer include addressing, error handling, congestion control and finally packet sequencing.

Physical (Layer 1)

The Physical layer establishes the physical connection between a computer and the network. This layer also controls the transmission of information as it specifies the mechanical and electrical characteristics of the protocol in terms of conne pin assignments and voltage levels.
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TCP/IP HISTORY

The Transmission Control Protocol/Internet Protocol (TCP/IP) originated in the 1970 s.It was initially developed by the Department of Defence (DOD) in an attempt to connect a number of different networks Today however it is more commonly used as a standard communication protocol used in a number of networks the most common of which is the Internet.

FIGURE 2

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The Data link laye is primarily c ncerne with packaging and un-packaging data packets s that they can be enc dec d ed into bits which will enable the data packets to be transmitted through a network. The Data Link layer wo transmission protocol by supporting error handling in the physical layer and corruption (through check summing). Add is also known that the Data Link Layer is divided in to two sub-layers; these are known as The Media Access Contro Logical Link Control Layer . The MAC sub-layer controls the flow of how a computer sends data across a network an network will receive it. Apart from this it also gives permission to transmit data. Whilst the Logical Link Control Laye frame synchronization and flow control.

As it is depicted in Figure 2 above, the TCP/IP model itself consists of four layers and each layer is responsible for performing prescribed functions which will be discussed in detail below: Application Layer - This layer is basically concerned with defining the protocols. It consists of application programs and user interfaces. In practice it works by sending an unbroken "data stream" to the Transport layer . At this stage the stream is broken into packets , each of which is "framed" with a TCP header (which contains the sender's and recipient's addresses and error checking information). According to writers such as Buchanan it supports a number of protocols such as" SMTP, FTP and Telnet". (Buchanan. W, "Distributed Systems and Networks", McGraw- Hill, 2000) Transport Layer - This layer is responsible for providing communication session management between host computers. Generally speaking it defines the level of service and status of the connection used when transporting the data. The main protocols that are used in this layer include TCP, UDP and RTP. Internet Layer - This layer is basically concerned with the Packaging of data. Here the data is packaged into IP datagram's. which contain the source and destination address information. This is what is used to forward the datagram's between hosts and across networks and the, main protocols used in this layer are IP, ICMP, ARP as well as RARP. Network Interface Layer - This layer is responsible for specifying how the data will be sent through the network physically. This includes assessing "how bits are electrically signalled by the hardware devices that interface directly with a network medium" (Croucher P, "Communications and Networks", 1999 British Library publication). These devices generally include the types of caballing used such as the coaxial cable or the twisted pair copper wire. Typical examples of protocols used throughout this layer are the "Token Ring", "Ethernet" and FDDI.

TCP/IP and the OSI model
There is brief discussion on mapping the TCP/IP model onto the OSI model. Since the TCP/IP and OSI protocol suites do not match precisely, there is no one correct answer. The following diagram attempts to show where various TCP/IP and other protocols would reside in the original OSI model: The OSI Model: Application Presentation Session Transport Network Data Link Physical Commonly, the top three layers of the OSI model (Application, Presentation and Session) are considered as a single Application Layer in the TCP/IP suite. Because the TCP/IP suite has no unified session layer on which higher layers are built, these functions are

typically carried out (or ignored) by individual applications. The most notable difference between TCP/IP and OSI models is the Application layer, as TCP/IP integrates a few steps of the OSI model into its Application layer. A simplified TCP/IP interpretation of the stack is shown below Application "layer 7" e.g. HTTP, FTP, DNS 4 Transport e.g. TCP, UDP, RTP, SCTP 3 Network For TCP/IP this is the Internet Protocol (IP) 2 Data Link e.g. Ethernet, Token ring, etc. 1 Physical e.g. physical media, and encoding techniques. Professor Kevin Roark, Computer Science Department (2)

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 ect.), and all related details such as connectors, channel codes and modulation, and maximum distances. The Internet protocol suite does not cover the physical layer of any network.

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.

The network layer
As originally defined, the Network layer solved the problem of getting packets across a single network. With the advent of the concept of internetworking, additional functionality was added to this layer, such as getting data from the source network to the destination network. In the internet protocol suite, IP performs the basic task of getting packets of data from source to destination.

The transport layer
The protocols at the Transport layer can solve problems like reliability ("did the data reach the destination?") and ensure that data arrives in the correct order. In the TCP/IP protocol suite, transport protocols also determine which application any given data is intended for. The dynamic routing protocols which technically fit at this layer in the TCP/IP Protocol Suite (since they run over IP) are generally considered to be part of the Network layer TCP is a "reliable", connection-oriented, transport mechanism providing a reliable byte stream, which makes sure data arrives complete, undamaged, and in order. TCP tries to continuously measure how loaded the network is and throttles its sending rate in order to avoid overloading the network. Furthermore, TCP will attempt to deliver all data correctly in the specified sequence. UDP is a connectionless datagram protocol. It is a "best effort" or "unreliable" protocol not because it is particularly unreliable, but because it does not verify that packets have reached their destination, and gives no guarantee that they will arrive in order. If an Application requires these characteristics, it must provide them itself, or use TCP. Professor Kevin Roark, Computer Science Department (3) UDP is typically used for applications such as streaming media (audio and video, etc) where on-time arrival is more important than reliability, or for simple query/response applications like DNS lookups, where the overhead of setting up a reliable connection is

disproportionately large.

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The Application layer is the layer that most common network-aware programs interface use in order to communicate across a network with other programs. Processes that occur in this layer are application specific; data is passed from the network -aware program, in the format used internally by this application, and is encoded into a standard protocol. Some specific programs are considered to run in this layer. They provide services that directly support user applications. These programs and their corresponding protocols include: HTTP (The World Wide Web), FTP (File transport), SMTP (Email), SSH (Secure remote login), DNS (Name <-> IP Address lookups) and many others. Once the data from an application has been encoded into a standard application layer protocol it will be passed down to the next layer of the IP stack. At the Transport Layer, applications will most commonly make use of TCP or UDP, and are often associated with a well-known port number. The most common ports are listed below: ‡ File Transfer Protocol (FTP) on port 21 ‡ Secure Shell (SSH) on port 22 ‡ Telnet on port 23 ‡ Simple Mail Transport Protocol (SMTP) for outgoing e-mail on port 25 ‡ Domain Name System (DNS) lookups on UDP (or sometimes TCP) port 53 ‡ Dynamic Host Configuration Protocol (DHCP) on ports 67 and 68 ‡ Gopher on port 70 ‡ Finger on port 79 ‡ HTTP on TCP port 80 ‡ POP3 read e-mail on port 110 ‡ Network News Transfer Protocol (NNTP) on port 119 ‡ Network Time Protocol (NTP) on port 123 ‡ NetBIOS on port 139 ‡ IMAP read e-mail on port 143 ‡ Simple network management protocol (SNMP) on port 161 ‡ HTTPS secure HTTP on port 443 ‡ IMAPS on port 993 ‡ Universal Plug and Play (UPnP) on port 5000 ‡ IRC on port 6667

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OSI model

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7. Application Layer NNTP · SIP · SSI · DNS · FTP · Gopher · HTTP · NFS · NTP · SMPP · SMTP · SNMP · Telnet · DHCP · Netconf · RTP · ( ore) 6. Presentation Layer MIME · XDR · TLS · SSL 5. Session Layer Na ed Pipes · NetBIOS · SAP · L2TP · PPTP ·
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SPDY 4. Transport Layer TCP · UDP · SCTP · DCCP · SPX 3. Network Layer IP (IP 4, IP 6) · ICMP · IPsec · IGMP · IPX ·
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AppleTalk

2. Data Link Layer ATM · SDLC · HDLC · ARP · CSLIP · SLIP · GFP · PLIP · IEEE 802.3 · Fra e Relay · ITUT G.hn DLL · PPP · X.25 · Network Switch · 1. Physical Layer EIA/TIA-232 · EIA/TIA-449 · ITU-T V-Series · I.430 · I.431 · POTS · PDH · SONET/SDH · PON · OTN · DSL · IEEE 802.3 · IEEE 802.11 · IEEE 802.15 · IEEE 802.16 · IEEE 1394 · ITU—

T G.hn PHY · USB · Bluetooth · Hubs This box: view · tal · edit
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The Op i l (OSI model) was a product of the Open Systems Interconnection effort at the International Organization for Standardization It . is a way of sub-dividing a communications system into smaller parts called layers. Similar communication functions are grouped into logical layers. A layer provides services to its upper layer while receiving services from the layer below. On each layer, aninstance provides service to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to sendand receive packets that make up the contents of that path. Two instances at one layer are connected by a hori ontal connection on that layer.

Communication in the OSI-Model (Example with layers 3 to 5)

Contents
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1 History 2 Description of OSI layers o 2.1 Layer 1: Physical Layer o 2.2 Layer 2: Data Link Layer  2.2.1 WAN Protocol architecture  2.2.2 IEEE 802 LAN architecture o 2.3 Layer 3: Network Layer o 2.4 Layer 4: Transport Layer o 2.5 Layer 5: Session Layer o 2.6 Layer 6: Presentation Layer o 2.7 Layer 7: Application Layer 3 Cross-layer functions 4 Interfaces 5 Examples 6 Comparison with TCP/IP 7 See also 8 References 9 External links

[edit] History
Work on a layered model of network architecture was started and the International Organi ation for Standardi ation (ISO) began to develop its OSI framework architecture. OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. Note: The standard documents that described the OSI model could be freely downloaded from the ITU-T as the X.200-series of recommendations.[1] A number of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO, but only some of them at no charge.[2] The concept of a 7 layer model was provided by the work of Charles Bachman, Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN, CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it, and provided facilities for use by the layer above it. Protocols enabled an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host.

[edit] Description of OSI layers
Depending on to recommendation X.200, there are seven layers, each generically known as an N layer. An N+1 entity requests services from the N entity. At each level, two entities (N-entity peers) interact by means of the N protocol by transmitting protocol data units (PDU). A Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer to a lower layer, and which the lower layer has not yet encapsulated into a protocol data unit (PDU). An SDU is a set of data that is sent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user. The PDU at any given layer, layer N, is the SDU of the layer below, layer N-1. In effect the SDU is the 'payload' of a given PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by the lower layer. All the data contained in the SDU becomes encapsulated within the PDU. The layer N-1 adds headers or footers, or both, to the SDU, transforming it into a PDU of layer N-1. The added headers or footers are part of the process used to make it possible to get data from a source to a destination.
OSI Model Data unit Layer Function

7. Network process to application Application Data 6. Data representation, encryption and decryption, convert Presentation machine dependent data to machine independent data 5. Session Segments
Interhost communication

Host layers

4. Transport End-to-end connections and reliability, flow control

Packet/Datagram 3. Network Path determination and logical addressing Media Frame layers Bit 2. Data Link Physical addressing 1. Physical
Media, signal and binary transmission

Some orthogonal aspects, such as management and security, involve every layer. Security services are not related to a specific layer: they can be related by a number of layers, as defined by ITU-T X.800 Recommendation.[3]

These services are aimed to improve the CIA triad (i.e.confidentiality, integrity, availability) of transmitted data. Actually the availability of communication service is determined by network design and/or network management protocols. Appropriate choices for these are needed to protect against denial of service.

[edit] Layer 1: Physical Layer
The Physical Layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more. To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer. Think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, whereas the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. Standards such as RS-232 do use physical wires to control access to the medium. The major functions and services performed by the Physical Layer are:
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Establishment and termination of a connection to a co unications ediu . Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control. Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as token ring, FDDI, ITUT G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE 802.15.4.

[edit] Layer 2: Data Link Layer
The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and

acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages. The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete Data Link Layer which provides both error correction and flow control by means of a selective repeat Sliding Window Protocol. Both WAN and LAN service arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the FDDI bit stream is not used by the Layer. [edit] WAN Protocol architecture Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are also capable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC and HDLC, and derivatives of HDLC such as LAPB and LAPD. [edit] IEEE 802 LAN architecture Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3. This layer manages the interaction of devices with a shared medium, which is the function of a Media Access Control (MAC) sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media. While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescent MAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.

[edit] Layer 3: Network Layer
The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the Transport Layer (in contrast to the data link layer which connects hosts within the same network). The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer²sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme ± values are chosen by the network engineer. The addressing scheme is not hierarchical. Careful analysis of the Network Layer indicated that the Network Layer could have at least three sublayers:
1. Subnetwork Access that considers protocols that deal with the interface to networks, such as X.25
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2. Subnetwork Dependent Convergence when it is necessary to bring the level of a transit network up to the level of networks on either side 3. Subnetwork Independent Convergence which handles transfer across multiple networks.

The best example of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of erroneous packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses. A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.

[edit] Layer 4: Transport Layer
The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes are shown in the following table:[4]
Feature Na e Connection oriented network Connectionless network Concatenation and separation Segmentation and reassembly
g

f

TP0 TP1 TP2 TP3 TP4 Yes Yes Yes Yes Yes No No No No Yes No Yes Yes Yes Yes Yes Yes Yes Yes Yes

Error Recovery

No Yes Yes Yes Yes

Reinitiate connection (if an excessive number of PDUs are unacknowledged) No Yes No Yes No Multiplexing and demultiplexing over a single irtual circuit Explicit flow control Retransmission on timeout Reliable Transport Service
h

No No Yes Yes Yes No No Yes Yes Yes No No No No Yes No Yes No Yes Yes

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with the dispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the Transport Layer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-end encryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames inside transport packet.

[edit] Layer 5: Session Layer
The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.

[edit] Layer 6: Presentation Layer
The Presentation Layer establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack. This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.[5]

The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or seriali ation of objects and other data structures from and to XML.

[edit] Layer 7: Application Layer
The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations also include:
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On OSI stack: o FTAM File Transfer and Access Management Protocol o X.400 Mail o Co on anage ent infor ation protocol (CMIP) On TCP/IP stack: o Hypertext Transfer Protocol (HTTP), o File Transfer Protocol (FTP), o Si ple Mail Transfer Protocol (SMTP) o Si ple Network Manage ent Protocol (SNMP).
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[edit] Cross-layer functions
There are some functions or services that are not tied to a given layer, but they can affect more than one layer. Examples are
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security ser ice (teleco unication)[3] as defined by ITU-T X.800 Recommendation. management functions, i.e. functions that permit to configure, instantiate, monitor, terminate the communications of two or more entities: there is a specific application layer protocol Co on anage ent infor ation protocol (CMIP) and its corresponding service co on anage ent infor ation ser ice (CMIS), they need to interact with every layer in order to deal with their instances. MPLS operates at an OSI Model layer that is generally considered to lie between traditional definitions of Layer 2 (Data Link Layer) and Layer 3 (Network Layer), and thus is often referred to as a "Layer 2.5" protocol. It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a datagram service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. ARP is used to translate IPv4 addresses (OSI Layer 3) into Ethernet MAC addresses (OSI Layer 2)
o n n n n n nn n nn

[edit] Interfaces
Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific. For example Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface, are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfaces between the media (Layer 2) and the network protocol (Layer 3). Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications.

[edit] Examples
Layer # Na e
p

FTAM, X.400, X.500, Applicatio DAP, 7 n ROSE, RTSE, ACSE[7] CMIP[8]

NNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, DHCP, SMPP, SMTP, SNMP, Telnet, RIP, BGP

INAP, MAP, AFP, ZIP, RIP, TCAP, RTMP, SAP ISUP, NBP TUP

APPC

HL7, Modbus

ISO/IEC 88 Presentati 23, X.226, MIME, SSL, 6 ISO/IEC 95 TLS, XDR on 76-1, X.236

AFP

TDI, ASCII, EBCDIC, MIDI, MPEG Na ed pipes, NetBIOS, SAP, half duplex, full duplex, si plex,
p p

5 Session

Sockets. ISO/IEC 83 Session 27, X.225, establishm ISO/IEC 95 ent in TCP, 48-1, X.235 RTP

ASP, ADSP, PAP

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p

OSI protocols

Signali TCP/IP ng AppleTal protocols System k 7[6]

IPX

SNA

UMTS

Misc. exa ples

RPC ISO/IEC 80 73, TP0, TP1, TP2, TCP, UDP, 4 Transport TP3, TP4 SCTP, DCCP (X.224), ISO/IEC 86 02, X.234 ISO/IEC 82 08, X.25 (PLP), ISO/IEC 88 3 Network 78, X.223, ISO/IEC 84 73-1, CLNP X.233.

DDP, SPX

NBF

r

r

r

ISO/IEC 76 66, X.25 (LAPB), Token Bus, 2 Data Link X.222, ISO/IEC 88 02-2 LLC Type 1 and 2[9]

802.3 (Ethernet), 802.11a/b/ g/n MAC/LLC, 802.1Q (VLAN), ATM, HDP, FDDI, Fibre IEEE LocalTal Channel, 802.3 k, framin LLC (Logical Link Fra e AppleTal Relay, g, Control), MAC PPP, SLIP, MTP, k SDLC HDLC, ISL, PPTP, L2TP Q.710 Ethern (Media Access Re ote PPP, Q.921, et II Control) Access, Token Ring, fra in PPP CDP, NDP g ARP (maps layer 3 to layer 2 address), ITU-T G.hn DLL CRC, Bit stuffing, ARQ, Data

q

IP, IPsec, ICMP, IGMP, OSPF

ATP (TokenT SCCP, alk or IPX MTP EtherTal k)

RRC (Radio Resource Control) Packet Data Con ergence Protocol (PDCP) and BMC (Broadcast/Multi cast Control)

NBF, Q.931, IS-IS

Leaky bucket, token bucket

O er Cable Ser ice Interface Specificatio n (DOCSIS) RS-232, Full duplex, RJ45, V.35, V.34, I.430, I.431, T1, E1, 10BASET, 100BASETX, POTS, SONET, SDH, DSL, 802.11a/b/ g/n PHY, ITU-T G.hn PHY, Controller Area Network, Data O er Cable Ser ice Interface Specificatio n (DOCSIS)
s s s s

1 Physical

X.25 (X.21bis, EIA/TIA232, EIA/TIA449, EIA530, G.703)[9]

RS-232, RS-422, MTP, STP, Q.710 PhoneN et

Twin UMTS Physical ax Layer or L1

[edit] Comparison with TCP/IP
In the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.[10] 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. Even though the concept is different from the OSI model, these layers are nevert heless often compared with the OSI layering scheme in the following way: The Internet Application Layer includes the OSI Application Layer, Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful close function of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) is a subset of the OSI Network Layer (see above), while the Link Layer includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These

comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document. The presumably strict peer layering of the OSI model 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 Open Systems Interconnection ( OSI ) is a standard reference model for communication between two end users in a network. The model is used in developing products and understanding networks. Also see the notes below the figure.

Illustration republished with permission from The manual Page .

OSI divides telecommunication into seven layers. The layers are in two groups. The upper four layers are used whenever a message passes from or to a user. The lower three layers are used when any message passes through the host computer. Messages intended for this computer pass to the upper layers. Messages destined for some other host are not passed up to the upper layers but are forwarded to another host. The seven layers are: Layer 7: The application layer ...This is the layer at which communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. (This layer is not the application itself, although some applications may perform application layer functions.) Layer 6: The presentation layer ...This is a layer, usually part of an operating system, that converts incoming and outgoing data from one presentation format to another (for example, from a text stream into a popup window with the newly arrived text). Sometimes called the syntax layer. Layer 5: The session layer ...This layer sets up, coordinates, and terminates conversations, exchanges, and dialogs between the applications at each end. It deals with session and connection coordination. Layer 4: The transport layer ...This layer manages the end-to-end control (for example, determining whether all packets have arrived) and error-checking. It ensures complete data transfer. Layer 3: The network layer ...This layer handles the routing of the data (sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level). The network layer does routing and forwarding. Layer 2: The data-link layer ...This layer provides synchronization for the physical level and does bit-stuffing for strings of 1's in excess of 5. It furnishes transmission protocol knowledge and management. Layer 1: The physical layer ...This layer conveys the bit stream through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier.