We have so far discussed several types of networks and the topologies such networks can use. Whether the network is a LAN, WAN, or MAN, the mechanism for the basic communication of data is the transfer of a single bit from one point to another. However, as you might have guessed already, this process would be very tedious, so to move data in computer networks more quickly, there are programs that these networks use to provide a high-level communication interface between nodes and handle all low-level communica- tion details. These are the network protocols. A protocol is set of guidelines followed by two communicating entities.
For data to move between two network nodes, there are many details, func- tions, and interfaces that must be executed and exchanged. Because of these, the protocols that we introduced above are not one giant program, but a collection of them working in unison to accomplish one task, make one or more bits of data move from one network node to another. So these proto- cols are put in a bundle. These bundles are called protocol.suites. Suites are
designed to make the interaction between several protocols as efficient as possible. To work most efficiently, the protocols in these suites are layered
in what is referred to as a protocol stack. The protocols making these stacks are discussed next.
Server Laptop Laptop Laptop Server Server Figure.9..Tree.topology
Network.Communication.Stack.Layers
Application.Layer
This layer specifies how an application program is going to use the network.
It contains most of the top-layer use programs. The details of this layer are
how each application on a node makes a request to the server. It also specifies
how the requesting application will respond to the server.
Transport.Layer
This layer is actually a combination of two OSI layers, the presentation and session layers. However, under transport control protocol (TCP)/IP, it is referred to as the transport layer. In the TCP/IP suite, this layer has two standard protocols:..TCP. and user.datagram.protocol (UDP). TCP provides a connection-oriented service and it guarantees delivery of all application layer packets to their destinations. This guarantee is based on two mecha- nisms: congestion control, which throttles the transmission rate of the source
element when there is traffic congestion in the network, and the flow control
mechanism that tries to match sender and receiver speeds to synchronize the
flow rate and reduce the packet drop rate. While TCP offers guarantees of
delivery of the application layer packets, UDP offers no such guarantees. It provides a no-frills connectionless service with just delivery and no ac-
knowledgments. But it is much more efficient and a protocol of choice for
real-time data, such as streaming video and music. Transport layer delivers transport-layer packets and protocols to the network layer. Figure 13 shows the TCP data structure and Figure 14 shows an UDP data structure.
Presentation.Layer
This specifies how data is going to be presented. This is essential because
computers either present data as little.endian or big.endian. The presentation protocol, therefore, presents an interface to do the translation of the data.
Session.layer
This is responsible for establishing a communication session between two network nodes (Kizza, 2005).
Layer Delivery unit Protocols
Application Message • Handles all higher level protocols including: file transfer protocol (FTP), name server protocol (NSP), simple mail transfer protocol (SMTP), simple network management protocol (SNMP),
HTTP, remote file access (telnet), Remote file
server (NFS), name resolution (DNS), TFTP, SNMP, DHCP, and BOOTP
• Combines application, session, and presentation layers of the OSI model.
• Handles all high-level protocols
Transport Segment • Handles transport protocols including: TCP and UDP.
Network Datagram • Contains the following protocols: IP, Internet control message protocol (ICMP), and Internet group management protocol (IGMP). • Supports transmitting source packets from any
network on the internetwork and makes sure they arrive at the destination independent of the path and networks they took to get there.
• Best path determination and packet switching occur at this layer.
Data link Frame Contains protocols that require IP packet to cross a physical link from one device to another directly con- nected device. It included the following networks: • WAN
• LAN
Physical Bit stream All network card drivers
Table.3..TCP/IP.protocol.layers
Figure.12..Application.layer.data.frame
Network.Layer
This layer is responsible for moving packets, now called datagrams, from router to router along the path from a source host to the destination host. It supports a number of protocols including the IP, Internet.control.message.protocol (ICMP) and Internet.group.management.protocol (IGMP). IP is the most widely used network layer protocol. IP uses header information from the transport layer protocols that include datagram source and destination port numbers from IP addresses and other TCP header and IP information, in order to move datagrams from router to router through the network. Best routes are found in the network by using routing algorithms. Figure 15 shows an IP datagram structure. The standard IP address has been the so-called IPv4, a 32-bit addressing scheme. But with the rapid growth of the Internet, there was fear of running out of addresses, so a new IPv6, a 64-bit addressing scheme, was created. The network layer conveys the network layer protocols to the data link layer.
Data.Link.Layer
This layer provides the network with services that move packets from one packet switch like a router to the next over connecting links. This layer also offers reliable delivery of network layer packets over links. It is at the low-
32 bits
Source address Destination address Sequence number Acknowledgement number
Other control information Data
32 bits
Source address Destination address Other header control information UDP checksum Data
Figure.13..A.TCP.structure
est level of communication and it includes the network.NIC and operating. system (OS) protocols. The protocols in this layer include: Ethernet, ATM, and others such as frame relay. The data link layer protocol unit, the frame, may be moved over links from source to destination by different link layer protocols at different links along the way.
Physical.Layer
This layer is responsible for literally moving data link datagrams bit by bit over the links and between network elements. The protocols here depend on and use the characteristics of the link medium and the signals on the medium.
Network.Communication.Protocols.
Although there are many different protocol models in use in various networks, the most popular and widely used suites are: OSI by the International Stan- dards Organization (ISO) and TTCP together with IP, forming the TCP/IP suite. Each one of these is made up of several layers stacks.
Both OSI and TCP/IP models use two protocol stacks, one at the source ele- ment and the other at the destination element Let us now take a look at both starting with the ISO’s OSI model.
Figure.15..An.IP.datagram.structure
Other header con- trol information
Source port number Destination port number
Data
Layer number Protocol
7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data link 1 Physical Table.1..ISO.protocol.layers.and.corresponding.services
Open.System.Interconnection.(OSI).Protocol.Suite
The development of the OSI model was based on the secure premise that a communication task over a network can be broken into seven layers, where each layer represents a different portion of the task. Different layers of the protocol provide different services and ensure that each layer can commu- nicate only with its own neighboring layers. That is, the protocols in each layer are based on the protocols of the previous layers.
Starting from the top of the protocol stack, tasks and information move down from the top layers until they reach the bottom layer, where they are sent out over the network media from the source system to the destination. At the destination, the task or information rises back up through the layers until it reaches the top. Each layer is designed to accept work from the layer above it and to pass work down to the layer below it, and vice versa. To ease interlayer communication, the interfaces between layers are standardized. However, each layer remains independent and can be designed independently and each layer’s functionality should not affect the functionalities of other layers above and below it.
Table 1 shows an OSI model consisting of seven layers and the descriptions of the services provided in each layer.
In a peer-to-peer communication, the two communicating computers can initiate and receive tasks and data. The task and data initiated from each computer starts from the top in the application layer of the protocol stack on each computer. The tasks and data then move down from the top layers until they reach the bottom layer, where they are sent out over the network media from the source system to the destination. At the destination, the task and
Table.2..OSI.Datagrams.seen.in.each.layer.with.header.added
No header Data Application H1 Data Presentation H2 Data Session H3 Data Transport
H4 Data Network
H5 Data Data link No header Data Physical
data rise back up through the layers until the top. Each layer is designed to accept work from the layer above it and pass work down to the layer below it. As data passes from layer to layer of the sender machine, layer headers are appended to the data, causing the datagram to grow larger. Each layer header contains information for that layer’s peer on the remote system. That information may indicate how to route the packet through the network, or what should be done to the packet as it is handed back up the layers on the recipient computer.
Figure 11 shows a logical communication model between two peer comput- ers using the ISO model. Table 2 shows the datagram with added header information as it moves through the layers. Although the development of the OSI model was intended to offer a standard for all other proprietary models, and it was as encompassing of all existing models as possible, it never really replaced many of those rival models it was intended to replace. In fact, it is this “all in one” concept that led to market failure because it became too complex. Its late arrival on the market also prevented its much anticipated interoperability across networks.
Transport.Control.Protocol/Internet.Protocol.(TCP/IP).Model
Among OSI’s rivals was TCP/IP, which was far less complex and more his- torically established by the time OSI came on the market. The TCP/IP model does not exactly match the OSI model. For example, it has two to three fewer levels, than the seven layers of the OSI model. It was developed for the U.S. Department of Defense Advanced Research Project Agency (DARPA), but over the years had seen a phenomenal growth in popularity and is now the de facto standard for the Internet and many intranets. It consists of two major
Applcaton Presentaton Sesson Transport Network Datalnk Physcal Physcal Datalnk Network Transport Sesson Presentaton Applcaton Channel Machne A Machne B Figure.11..ISO.logical.peer.communication.model
protocols: the TCP and IP, hence the TCP/IP designation. Table 6.3 shows the layers and protocols in each layer.