Flow Models

A computer network is comprised of nodes corresponding to network elements such as workstations,routers and switches and links that connect those elements. A network now contains all the traffic originating at a node and destined for some other node in the network. Each how can in principle traverse a set of paths connecting its origin and destination, which is determined by the routing policy.

In computer networks, the low traffic is carried on packets, whose payload is expressed in bytes, while on road networks, the traffic is carried on vehicles. The volume of traffic measured on a link may refer to either the number of packets and/or the number of bytes in computer networks, and such data for a particular time interval -typically of the order of a couple of minutes- are available through queries using the Simple Network Management Protocol (SNMP) protocol.

 The volume of traffic on a 1link is the sum of volumes of all rows traversing that link. This produces highly aggregate data and the question of interest is to estimate various statistics of the underlying network.

Application Layer - OSI Model

At the very top of the OSI Reference Model stack of layers, the application layer. The application layer is the one that is used by network applications. These programs are what actually implement the functions performed by users to accomplish various tasks over the network.

It's important to understand that what the OSI model calls an “application” is not exactly the same as what we normally think of as an “application”. In the OSI model, the application layer provides services for user applications to employ. For example, when you use your Web browser, that actual software is an application running on your PC. It doesn't really “reside” at the application layer. Rather, it makes use of the services offered by a protocol that operates at the application layer, which is called the Hypertext Transfer Protocol (HTTP). The distinction between the browser and HTTP is subtle, but important.

The reason for pointing this out is because not all user applications use the application layer of the network in the same way. Sure, your Web browser does, and so does your e-mail client and your Usenet news reader. But if you use a text editor to open a file on another machine on your network, that editor is not using the application layer. In fact, it has no clue that the file you are using is on the network: it just sees a file addressed with a name that has been mapped to a network somewhere else. The operating system takes care of redirecting what the editor does, over the network.

Similarly, not all uses of the application layer are by applications. The operating system itself can (and does) use services directly at the application layer.

That caveat aside, under normal circumstances, whenever you interact with a program on your computer that is designed specifically for use on a network, you are dealing directly with the application layer. For example, sending an e-mail, firing up a Web browser, or using an IRC chat program—all of these involve protocols that reside at the application layer.

There are dozens of different application layer protocols that enable various functions at this layer. Some of the most popular ones include HTTP, FTP, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP and IRC. Lots of alphabet soup, sorry. J I describe all of these and more in the chapter on higher-layer protocols and applications.

As the “top of the stack” layer, the application layer is the only one that does not provide any services to the layer above it in the stack—there isn't one! Instead, it provides services to programs that want to use the network, and to you, the user. So the responsibilities at this layer are simply to implement the functions that are needed by users of the network. And, of course, to issue the appropriate commands to make use of the services provided by the lower layers.

DOD model

This model is sometimes called the DOD model since it was designed for the department of defense It is also called the TCP/IP four layer protocol, or the internet protocol. It has the following layers:

• Link - Device driver and interface card which maps to the data link and physical layer of the OSI model.
• Network - Corresponds to the network layer of the OSI model and includes the IP, ICMP, and IGMP protocols.
• Transport - Corresponds to the transport layer and includes the TCP and UDP protocols.
• Application - Corresponds to the OSI Session, Presentation and Application layers and includes FTP, Telnet, ping, Rlogin, rsh, TFTP, SMTP, SNMP, DNS, your program, etc.
• The four layer TCP/IP protocol. Each layer has a set of data that it generates.
• The Link layer corresponds to the hardware, including the device driver and interface card. The link layer has data packets associated with it depending on the type of network being used such as ARCnet, Token ring or ethernet. In our case, we will be talking about ethernet.
• The network layer manages the movement of packets around the network and includes IP, ICMP, and IGMP. It is responsible for making sure that packages reach their destinations, and if they don't, reporting errors.

The transport layer is the mechanism used for two computers to exchange data with regards to software. The two types of protocols that are the transport mechanisms are TCP and UDP.

The application layer refers to networking protocols that are used to support various services such as FTP, Telnet, BOOTP, etc. Note here to avoid confusion, that the application layer is generally referring to protocols such as FTP, telnet, ping, and other programs designed for specific purposes which are governed by a specific set of protocols defined with RFC's (request for comments). However a program that you may write can define its own data structure to send between your client and server program so long as the program you run on both the client and server machine understand your protocol. For example when your program opens a socket to another machine, it is using TCP protocol, but the data you send depends on how you structure it.

Simple Network Management Protocol - SNMP

Simple Network Management Protocol (SNMP) is a widely used protocol designed to facilitate the management of networked devices from a central location.  Designed originally for the management of devices such as routers and switches, its usage has grown rapidly to encompass the monitoring of nearly any electronic device one can think of.  SNMP is now  used to monitor and manage television broadcast studios, automated fare collection systems, airborne military platforms, energy distribution systems, emergency radio networks, and much more.

The SNMP architecture is composed of three major elements:

• Managers (software) are responsible for communicating with (and managing) network devices that implement SNMP Agents (also software).
• Agents reside in devices such as workstations, switches, routers, microwave radios, printers, and provide information to Managers.
• MIBs (Management Information Base) describe data objects to be managed by an Agent within a device. MIBs are actually just text files, and values of MIB data objects are the topic of conversation between Managers and Agents.

SNMP Standards and Versions:
SNMP Standards are described in Request for Comments (RFC) documents published by the Internet Engineering Task Force (IETF).  Standards Topics can generally be categorized into:

• Messaging protocols between Managers and Agents (which encompasses security issues)
• MIB syntax standards
• “Standard MIB” definitions