Osi Model Layers

OSI Model Layers

Layer 7 6 5 4 3 2 1

Layer name

Main function

Used for application specifically Written to run over the network Translate from application to Presentation network format, and vice versa Establishes, maintains, and ends Session session across the network Additional connection below the Transport session layer Translate logical network address Network and names to their physical address Turns packets into raw bits and at Data link the receiving end turns bits into packets Transmits raw bit stream over Physical physical cable Application

Example Protocol FTP,DNS NFS, http

Network Component Gateway , Smart switch Gateway

Named pipes RPC

Gateway

TCP

Gateway

IP

router, ATM switch

Ethernet

Bridge, network interface card

IEEE 802

Hub

TCP/IP is an industry standard set of protocols developed by the U.S. Department of Defense Advanced Research Projects Agency (DARPA) in 1969. It maps TCP/IP protocols to a four-layer conceptual model known as the DARPA model. It is often compared to the still born OSI Protocol Layers. The four layers of the DARPA model are: 1. 2. 3. 4.

Application, Transport, Internet, Network Interface

Protocols define the procedures to use for the systems involved in the communication process. A data communication protocol is a set of rules that must be followed for the two electronic devices to communicate. These rules cover: Syntax - Data format and coding • Semantics - Control information and error handling Timing - Speed matching and sequencing •

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A data communication protocol is a set of rules that must be followed for the two electronic devices to communicate These rules cover: • •

Syntax - Data format and coding Semantics - Control information and error handling

•

Timing - Speed matching and sequencing

Protocols: • • •

Define the procedures to be used by systems involved in the communication process In data communications, are a set of rules that must be followed for devices to communicate Are implemented in software/firmware

Many protocols were developed to provide and support data communications. They form a communication architecture, sometimes referred as "protocol stack". For example TCP/IP family of protocols are often referred to as "TCP protocol stack". Each protocol provides for a function that is needed to make the data communication possible. Many protocols are used so that the problem can be broken into manageable pieces. Each software module that implements a protocol can be developed and updated independently of other modules, as long as the interface between modules remains constant. TCP/IP protocol stack consist of layers. You can think of layers in terms of object oriented programming (classes and subclasses) or in term of libraries. All layers of TCP-IP for so called protocol stack. Various organization influence the development of TCP-IP protocol stack. Among them: • • •

Internet Architecture Board (IAB), www.iab.org/ Internet Engineering Task Force (IETF), www.ietf.org/ Internet Society, www.isoc.org/

• • •

The Internet Corporation of Assigned Names and Numbers (ICANN), www.icann.org/ Internet Research Task Force (IRTF), www.irtf.org/ World Wide Web Consortium (W3C), www.w3.org/

The following mapping helps you better understand the terms used: • • •

Model = structure Layer = functions Protocol = rules

Some of the advantages of a layered model are similar to advantages of modular programming: • • • •

Diminish the complexity of networking by separating it into many functions or layers Introduction of changes or new features in one layer can be accomplished without changes in the other layers Provides a standard to follow, allowing inter-operability between software and hardware vendors Simplifies troubleshooting

TCP/IP model is a four-layered structure resting on a common hardware platform. It was initially developed for DARPA and sometimes called DARPA model. TCP/IP model has standards that are defined and described in Request for Comment (RFC) documents. The layers are as following: Application layer • •

Refers to standard network services like http, ftp, telnet as well as communication methods used by various application programs Also defines compatible representation of all data

Transport layer • •

Manages the transfer of data by using connection oriented (TCP) and connectionless (UDP) transport protocols Manages the connections between networked applications

Internet layer • •

Manages addressing of packets and delivery of packets between networks Fragments packets so that they can be dealt with by lower level layer (Network Interface layer Network)

Network Interface layer • •

Delivers data via physical link (Ethernet is the most common link level protocol ) Provides error detection and packet framing

RFCs are documents that describe various layers protocol architecture and functions. They are sequentially numbered and are freely accessible from http://www.ietf.org and several other sites (for example www.faqs.com ) Each layer in the DARPA model corresponds to one or more layers of the seven-layer Open Systems Interconnection (OSI) model. The latter is a purely bureaucratic invention by one of semi-useless international organizations and has neither theoretical value (being rather naive view on layers) or practical significance (TCP-IP is the king of the hill) but still each year students are tortured with this crap and need to memorize the correspondence between TCP-IP layers and OSI layers.

The main ideas about formal correspondence between TCP-IP layers and OSI layers can be easily deducted from the following diagram:

TCP/IP Protocol Layers (from Microsoft/TCP-IP Protocol Architecture) 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:

• • • •

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.

Additionally, the following Application layer protocols help facilitate the use and management of TCP/IP networks: • • •

The Domain Name System (DNS) is used to resolve a host name to an IP address. The Routing Information Protocol (RIP) is a routing protocol that routers use to exchange routing information. The Simple Network Management Protocol (SNMP) is used between a network management console and network devices (routers, bridges, intelligent hubs) to collect and exchange network management information.

Examples of Application layer interfaces for TCP/IP applications are Sockets and NetBIOS. Sockets provides a standard application programming interface (API) for interprocess communication via TCP/IP. NetBIOS is an industry standard interface for accessing protocol services such as sessions, datagrams, and name resolution. Transport Layer The Transport layer (also known as the Host-to-Host Transport layer) is responsible for providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). Either of these two protocols are used by the application layer process, the choice depends on the application's transmission reliability requirements.

The mechanisms used by the Transport layer to determine whether data has been correctly delivered are: • • •

Acknowledgement responses Sequencing Flow control

The Transport layer facilitates end-to-end data transfer. It supports multiple operations simultaneously. The layer is implemented by two protocols: the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP). TCP uses packets called segments, while UDP uses packets called datagrams. Both TCP and UDP are encapsulated inside Internet layer datagrams for transmission to the next node. The Transport layer facilitates two types of communication: • •

Connection-oriented (TCP) – A connection must be established at the Transport layer of both systems before the application can transmit any data. Connectionless (UDP) – All systems do not need to establish a connection with the recipient prior to data exchange. TCP is a more reliable form of data exchange than UDP.

TCP and UDP: •

•

TCP is a reliable, connection-oriented protocol that provides error checking and flow control through a virtual link that it establishes and finally terminates. TCP is responsible for the establishment of a TCP connection (TCP handshake), the sequencing and acknowledgment of packets sent, and the recovery of packets lost during transmission. UDP is an unreliable, connectionless protocol that provides data transport with lower network traffic overheads than TCP. UDP is used when the amount of data to be transferred is small (such as the data

that would fit into a single packet), or when the overhead of establishing a TCP connection is not desired or when the applications or upper layer protocols provide reliable delivery. UDP does not error check or offer any flow control, this is left to the application process. Still it can be used by protocols that provide reliable packet transmission like NFS. The Transport layer encompasses the responsibilities of the OSI Transport layer and some of the responsibilities of the OSI Session layer. Internet Layer This layer is responsible for addressing, packaging, and routing functions. It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes. The core protocols of the Internet layer are IP, ARP, ICMP, and IGMP. • • • •

The Internet Protocol (IP) is a routable protocol responsible for IP addressing, routing, and the fragmentation and reassembly of packets. The Address Resolution Protocol (ARP) is responsible for the resolution of the Internet layer address to the Network Interface layer address such as a hardware address. The Internet Control Message Protocol (ICMP) is responsible for providing diagnostic functions and reporting errors due to the unsuccessful delivery of IP packets. The Internet Group Management Protocol (IGMP) is responsible for the management of IP multicast groups.

The Internet layer is analogous to the Network layer of the OSI model. Network Interface Layer

The Network Interface layer (also called the Network Access layer) is responsible for placing TCP/IP packets on the network medium and receiving TCP/IP packets off the network medium. TCP/IP was designed to be independent of the network access method, frame format, and medium. In this way, TCP/IP can be used to connect differing network types. This layer include LAN technologies such as Ethernet and Token Ring and WAN technologies such as X.25 and Frame Relay. Independence from any specific network technology gives TCP/IP the ability to be adapted to new technologies such as Asynchronous Transfer Mode (ATM). It consists of combination of datalink and physical layers deals with pure hardware (wires, satellite links, network interface cards, etc.) and access methods such as CSMA/CD (carrier sensed multiple access with collision detection). Ethernet is the most popular network access layer protocol. Its hardware operates at the physical layer and its medium access control method (CSMA/CD) operates at the datalink layer. . The Network Interface layer encompasses the Data Link and Physical layers of the OSI model. Note that the Internet layer does not take advantage of sequencing and acknowledgment services that might be present in the Data-Link layer. An unreliable Network Interface layer is assumed, and reliable communications through session establishment and the sequencing and acknowledgment of packets is the responsibility of the Transport layer.