Network architecture
The network architecture of the Internet is predominantly expressed by its use of the Internet Protocol Suite, rather than a specific model for interconnecting networks or nodes in the network, or the usage of specific types of hardware links.
OSI network model
Physical layer
The physical layer defines the 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. Its main task is the transmission of a stream of bits over a communication channel.
Data-linking 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-multi point media, characteristic of wide area media in the telephone system. 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
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 layer—sending 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. It controls the operation of the sub-net and determine the routing strategies between IMP and ensures that all the packs are correctly received at the destination in the proper order.
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 re-transmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. Some transport layer protocols (such as TCP, but not UDP) support virtual circuits that provide connection-oriented communication over an underlying packet-oriented data-gram network, where it assures the delivery of packets in the order in which they were sent and that they are free of errors. The data-gram transportation deliver the packets randomly and broadcast it to multiple nodes.
The transport layer multiplexes several streams on to one physical channel. The transport headers indicate which message belongs to which connection.
Session layer
This layer provides a user interface to the network where the user negotiates to establish a connection. The user must provide the remote address to be contacted. The operation of setting up a session between two processes is known as “binding”. In some protocols, it is merged with the transport layer. Its main work is to transfer data from the other application to this application so this application is mainly used for transferred layer.
Presentation layer
The presentation layer establishes context between entities on the application layer, 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. The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASSN), with capabilities such as converting an BODICE-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.
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.
The TCP/IP Protocol Suite
The TCP/IP protocol suite maps to a four-layer conceptual model known as the DARPA model, which was named after the U.S. government agency that initially developed TCP/IP. The four layers of the DARPA model are: Application, Transport, Internet, and Network Interface. Each layer in the DARPA model corresponds to one or more layers of the seven-layer OSI model.
The TCP/IP protocol suite has two sets of protocols at the Internet layer:
- IPv4, also known as IP, is the Internet layer in common use today on private intranets and the Internet.
- IPv6 is the new Internet layer that will eventually replace the existing IPv4 Internet layer.
Network Interface Layer
The Network Interface layer (also called the Network Access layer) sends TCP/IP packets on the network medium and receives TCP/IP packets off the network medium. TCP/IP was designed to be independent of the network access method, frame format, and medium. Therefore, you can use TCP/IP to communicate across differing network types that use LAN technologies—such as Ethernet and 802.11 wireless LAN—and WAN technologies—such as Frame Relay and Asynchronous Transfer Mode (ATM). By being independent of any specific network technology, TCP/IP can be adapted to new technologies.
Transport Layer
The Transport layer (also known as the Host-to-Host Transport layer) provides the Application layer with session and datagram communication services. The Transport layer encompasses the responsibilities of the OSI Transport layer. The core protocols of the Transport layer are TCP and UDP.
TCP provides a one-to-one, connection-oriented, reliable communications service. TCP establishes connections, sequences and acknowledges packets sent, and recovers packets lost during transmission.
Application Layer
The Application layer allows applications to access the services of the other layers, and it defines the protocols that applications use to exchange data. The Application layer contains many protocols, and more are always being developed.
The most widely known Application layer protocols help users exchange information:
- The Hypertext Transfer Protocol (HTTP) transfers files that make up pages on the World Wide Web.
- The File Transfer Protocol (FTP) transfers individual files, typically for an interactive user session.
- The Simple Mail Transfer Protocol (SMTP) transfers mail messages and attachments.
Additionally, the following Application layer protocols help you use and manage TCP/IP networks:
- The Domain Name System (DNS) protocol resolves a host name, such as http://www.microsoft.com, to an IP address and copies name information between DNS servers.
- The Routing Information Protocol (RIP) is a protocol that routers use to exchange routing information on an IP network.
- The Simple Network Management Protocol (SNMP) collects and exchanges network management information between a network management console and network devices such as routers, bridges, and servers.
IPUNOTES 