Peerto Peer Communication

For data to travel from the source to the destination, each layer of the OSI reference model at the source must communicate with its peer layer at the destination. This form of communication is referred to as peer-to-peer communication. During this process, the protocols at each layer exchange information, called protocol data units (PDU), between peer layers, as shown in Figure 1-23.

Data packets on a network originate at a source and then travel to a destination. Each layer depends on the service function of the OSI layer below it. To provide this service, the lower layer uses encapsulation to put the PDU from the upper layer into its data field. It then adds whatever headers the layer needs to perform its function. As the data moves down through Layers 7 through 5 of the OSI reference model, additional headers are added. The grouping of data at the Layer 4 PDU is called a segment.

The network layer provides a service to the transport layer, and the transport layer presents data to the internetwork subsystem. The network layer moves the data through the internetwork by encapsulating the data and attaching a header to create a datagram (the Layer 3 PDU). The header contains information required to complete the transfer, such as source and destination logical addresses.

Figure 1-23 Peer-to-Peer Communication

Sender Receiver

Figure 1-23 Peer-to-Peer Communication

Sender Receiver

The data link layer provides a service to the network layer by encapsulating the network layer datagram in a frame (the Layer 2 PDU). The frame header contains the physical addresses required to complete the data link functions, and the frame trailer contains the FCS.

The physical layer provides a service to the data link layer, encoding the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually a wire) at Layer 1.

Network devices such as hubs, switches, and routers work at the lower three layers. Hubs are at Layer 1, switches are at Layer 2, and routers are at Layer 3.

The TCP/IP Protocol Stack

The TCP/IP suite is a layered model similar to the OSI reference model. Its name is actually a combination of two individual protocols, Transmission Control Protocol (TCP) and Internet Protocol (IP). It is divided into layers, each of which performs specific functions in the data communication process.

Both the OSI model and the TCP/IP stack were developed by different organizations at approximately the same time as a means to organize and communicate the components that guide the transmission of data.

Although the OSI reference model is universally recognized, the historical and technical open standard of the Internet is the TCP/IP protocol stack. The TCP/IP protocol stack, shown in Figure 1-24, varies slightly from the OSI reference model.

Figure 1-24 TCP/IP Protocol Stack

AppIication Transport Internet Network Access

The TCP/IP protocol stack has four layers. Note that although some of the layers in the TCP/IP protocol stack have the same names as layers in the OSI reference model, the layers have different functions in each model, as is described in the following list:

■ Application layer: The application layer handles high-level protocols, including issues of representation, encoding, and dialog control. The TCP/IP model combines all application-related issues into one layer and ensures that this data is properly packaged for the next layer.

■ Transport layer: The transport layer deals with QoS issues of reliability, flow control, and error correction. One of its protocols, TCP, provides for reliable network communications.

■ Internet layer: The purpose of the Internet layer is to send source datagrams from any network on the internetwork and have them arrive at the destination, regardless of the path they took to get there.

■ Network access layer: The name of this layer is broad and somewhat confusing. It is also called the host-to-network layer. It includes the LAN and WAN protocols and all the details in the OSI physical and data link layers.

OSI Model Versus TCP/IP Stack

Both similarities and differences exist between the TCP/IP protocol stack and the OSI reference model. Figure 1-25 offers a side-by-side comparison of the two models.

Similarities between the TCP/IP protocol stack and the OSI reference model include the following:

■ Both have application layers, though they include different services.

■ Both have comparable transport and network layers.

■ Both assume packet-switched technology, not circuit-switched. (Analog telephone calls are an example of circuit-switched technology.)

Figure 1-25 OSIModel Versus TCP/IP

TCP/IP Protocol Stack OSI Model

Figure 1-25 OSIModel Versus TCP/IP

TCP/IP Protocol Stack OSI Model

The differences that exist between the TCP/IP protocol stack and the OSI reference model include the following:

■ TCP/IP combines the presentation and session layers into its application layer.

■ TCP/IP combines the OSI data link and physical layers into the network access layer.

TCP/IP protocols are the standards around which the Internet developed, so the TCP/IP protocol stack gains credibility just because of its protocols. In contrast, networks are not typically built on the OSI reference model, even though the OSI reference model is used as a guide.

Summary of Understanding the Host-to-Host Communications Model

This following summarizes the host-to-host communications model key points:

■ The OSI reference model defines the network functions that occur at each layer.

■ The physical layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

■ The data link layer defines how data is formatted for transmission.

■ The network layer provides connectivity and path selection between two host systems that might be located on geographically separated networks.

■ The transport layer segments data from the system of the sending host and reassembles the data into a data stream on the system of the receiving host.

■ The session layer establishes, manages, and terminates sessions between two communicating hosts.

■ The presentation layer ensures that the information sent at the application layer of one system is readable by the application layer of another system.

■ The application layer provides network services, such as e-mail, file transfer, and web services, to applications of the users.

■ The information sent on a network is referred to as data or data packets. If one computer wants to send data to another computer, the data must first be packaged by a process called encapsulation.

■ When the remote device receives a sequence of bits, the physical layer at the remote devices passes the bits of data up the protocol stack for manipulation. This process is referred to as de-encapsulation.

■ TCP/IP is now the most widely used protocol for a number of reasons, including its flexible addressing scheme, usability by most operating systems and platforms, its many tools and utilities, and the need to be connected to the Internet.

■ The components of the TCP/IP stack are the network access, Internet, transport, and application layers.

■ The OSI reference model and the TCP/IP stack are similar in structure and function, with correlation at the physical, data link, network, and transport layers. The OSI reference model divides the application layer of the TCP/IP stack into three separate layers.

Continue reading here: Understanding TCPIPs Transport and Application Layers

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