What data unit is associated with the open systems interconnection layer two?

7.3.2 The open systems interconnection model

The OSI model is important in IoT model operation. There are seven layers to the OSI model and these are discussed briefly here [18]. It consists of seven layers, namely:

Physical layer

Data link layer

Network layer

Transport layer

Session layer

Presentation layer

Application layer

Each stratum or layer is accountable for providing some kind of distinct and structured services to the associated end-to-end layer either upwards or downwards in the stack. While the division differentiates each layer with its separate function, they create the flow of data without disconnecting from each other by working independently and simultaneously (Fig. 7.5).

We now discuss each of the layers and their individual capacities:

OSI layer 7: This layer is known as the “application layer,” and starts at the top. It provides abstraction to the user level of the intrinsic details of all the layers below it. It also specifies protocols for sharing and the interface methods in use by the hosts within a network. This is a place where a user interacts with the network utilizing advanced level protocols [19]. The DNS, HTTP, Telnet, SSH, FTP, SMTP, RDP (remote desktop protocol), etc. are used at this layer.

OSI layer 6: This layer is known as the presentation layer, and it lies beneath the application layer. In this layer are OS services like Linux, Unix, Mac, Windows, etc. The presentation layer is committed to delivering and formatting of information in two steps toward the application layer, which may or may not perform additional processing. It takes care of any issues that may arise as a consequence of sending data from one node to another. The presentation layer removes the concern of the application layer in dealing with syntactical differences to represent data residing in end-user systems [20]. Examples include the translation of an EBCDIC (extended binary coded decimal interchange code)-coded file in the form of text to an ASCII (American Standard Code for Information Interchange)-coded file, while also adding some encryption for security, such as secure sockets layer (SSL) protocol.

OSI layer 5: Session layer is beneath the presentation layer in the OSI model. It has the task of dealing with the communications in order to establish a session among the two network nodes [21]. Examples include the establishment of a session between a computer and the server.

OSI layer 4: This layer is the transport layer. It helps to establish an endways communication through the endpoints. This utilizes the notion of windowing, in which one is responsible for the decision of the amount of information flow or the data to be sent at one point in time connecting the two nodes in a network [22].

OSI layer 3: The layer three is the network layer. The routers operate at this level. This layer helps to put the data into packets which we may call IP datagrams. They have IP address information of the source and destination address which is transmitted to the hosts and over the network. They also help in routing of IP datagrams that are marked with IP addresses. There is a specific routing protocol which enables routers to communicate with each other. The transfer of information among two nodes is enabled by this, and the routing algorithms help to specify the routes. Every router contains the prior information of the networks to which they are attached. Then there is a routing protocol, which helps to share the coming information to the immediate neighbors and then to the other ends within the network. The routers learn this way around the existing topology of the network they are working in. Though layers 3 and 4, that is, the network and transport layers, are different, they are closely knit together in practice. The name of the Internet protocol TCP/IP comes from the transport layer protocol (TCP) and network layer protocol (IP) [2–4]. Here the host enables the message transfer in the network by placing over it the data to be sent, marked with a destination address in the hope that it will be sent to the specified location. The packets containing messages could arrive in an altered order, and not as specified during the transfer. The next task of the higher layers that exist at the destination point is to rearrange the order of the packets and then supply them to the applications stratum that features applications functioning.

OSI layer 2: This stratum is known as the data link layer. It helps switches to work on the data link layer. This layer decides when to deal with the delivery of frames amid devices that are placed on the same LAN and uses media access control addresses. The frames are not allowed or they do not have the function to cross the borders of a local network. The internetwork routing is controlled by layer 3. This helps to permit the data link protocols to emphasize their attention on local carriage, giving the address scheme and media intercession. Therefore the data link layer is comparable to a local neighborhood traffic cop, which does the arbitration amid parties struggling for entry through the medium, without the input of the final destination. For instance, the data link protocols include the Ethernet used in multinode form in LAN and the point-to-point protocol [23].

OSI layer 1: This is the basic stratum of the OSI model and is called the physical layer. This physical layer describes the physical world interface with the virtual world using an electrical or mechanical interface for establishing the physical medium. This contains the idea of basic networking hardware transmission technologies. The wiring and cabling are part of this layer. This layer is committed to outlining the patterns of diffusing raw bits over a physical link that binds together network nodes, copper wires, fiberoptic cables, radio links, etc. The physical layer governs stream of bits and their placement from the data link layer on to the pins for a USB printer interface, an optical fiber transmitter, or a radio carrier, etc. [24].

2.1 Introduction

A number of interfacing standards are in daily use. Standards such as I2C or USB have been built into many components and systems. Typically, using them requires no hardware design and limited software design. Understanding how these interfaces work is very useful. Their principles also help us understand the role of interfaces in embedded systems.

The Open Systems Interconnection (OSI) model, shown in Fig. 2.1 is widely used to describe the design of computer networks. The model includes several layers that start at the most basic physical characteristics at layer 1 up to the application in layer 7. This chapter concentrates on the bottom three layers, known as the media layers:

Layer 1, the physical layer (PHY), includes the physical and electrical characteristics of the connection. Physical characteristics include, for example, the type of connector used. Electrical characteristics describe the signals used for data.

Layer 2, the data link layer, describes the basic transfer of data from one network node to another. The unit of transfer at this level is the frame. This layer is divided into two sublayers. This layer is divided into two sublayers: the media access control (MAC) layer controls how devices can gain access to the communications medium; the logical link control (LLC) layer identifies and encapsulate protocols at the network layer as well as managing error correction and frame synchronization.

Layer 3, the network layer, moves data sequences from one node to another, potentially across several different types of networks.

In this chapter, we will look at six different standard interfaces:

The RS-232 serial interface commonly used on personal computers.

The I2C interface which is used to communicate with devices along a relatively simple bus; we also discuss the I2S bus used for digital audio and the CAN bus used in automotive systems.

The Universal Serial Bus (USB), which has gone through several revisions as a common standard for PC interfacing.

WiFi, a wireless network widely used for PCs and also for IoT systems.

Zigbee, a wireless network designed for embedded systems.

Two wireless interfaces, Bluetooth and Bluetooth Low Energy (BLE). Despite sharing a common root name, these two interfaces vary in some interesting and important ways.

LoRaWAN, a low power wide area network.

In Section 2.9, we will consider Internet connections that rely on communications interfaces.

II.C TCP/IP Reference Model

While the OSI model was carefully designed, standardized, and then implemented, the Transmission Control Protocol (TCP)/Internet Protocol (IP) architecture was implemented first in the early 1980's and modeled afterwards. Well accepted in the university circles first, and later in the industry, TCP/IP, also called the Internet architecture, is nowadays one of the major architectures. However, the TCP/IP reference model is not general and consistent, but rather implementation bound, and it does not have a clear concept of services and protocols. The IP protocol serves as a joining point for many different networks, providing them with a method for communication.

TCP/IP consists of four layers, as shown in Fig. 2, which can be matched to some of the OSI model layers. The host-to-network layer is similar to the physical and data link layers. The internet layer with the IP protocol corresponds to the network layer. The main concept for TCP/IP, implemented on this layer, is a connectionless packet-switched network.

Above the internet layer is the transport layer with two main protocols, TCP and User Datagram Protocol (UDP). TCP is a reliable and connection-oriented, end-to-end protocol. TCP has error detection/correction, data retransmission, and flow control mechanisms and takes care of out-of-order messages. UDP in contrast is an unreliable, connectionless protocol providing support for audio and video applications. The last layer, the application layer, hosts a variety of high-level protocols such as File Transfer Protocol (FTP) or Simple Mail Transfer Protocol (SMTP).

1.2.4 Radio and networking

Combined wireless/network communications

Modern communications systems combine wireless and networking. As illustrated in Figure 1.9, radios carry digital information and are used to connect to networks. Those networks may be specialized, as in traditional cell phones, but increasingly radios are used as the physical layer in Internet protocol systems.

Networking

The Open Systems Interconnection (OSI) model [Sta97A] of the International Standards Organization (ISO) defines a seven-layer model for network services:

Physical: The electrical and physical connection

Data link: Access and error control across a single link

Network: Basic end-to-end service

Transport: Connection-oriented services

Session: Activity control, such as checkpointing

Presentation: Data exchange formats

Application: The interface between the application and the network

Although it may seem that embedded systems would be too simple to require use of the OSI model, the model is in fact quite useful. Even relatively simple embedded networks provide physical, data link, and network services. An increasing number of embedded systems provide Internet service that requires implementing the full range of functions in the OSI model.

Internetworking standard

The Internet is one example of a network that follows the OSI model. The Internet Protocol (IP) [Los97, Sta97A] is the fundamental protocol on the Internet. IP is used to internetwork between different types of networks. IP sits at the network layer in the OSI model. It does not provide guaranteed end-to-end service. It instead provides best-effort routing of packets. Higher-level protocols must be used to manage the stream of packets between source and destination.

Wireless

Wireless data communication is widely used. A data receiver, for example, must perform several tasks:

They must demodulate the signal down to the baseband.

They must detect the baseband signal to identify bits.

They must correct errors in the raw bit stream.

Software radio

Wireless data radios may be built from combinations of analog, hardwired digital, configurable, and programmable components. A software radio is, broadly speaking, a radio that can be programmed; the term software-defined radio (SDR) is often used to mean either a purely or partly programmable radio. Given the clock rates at which today’s digital processors operate, CPUs are used primarily for baseband operations. Some processors can run fast enough to be used for some of the radio-frequency processing.

Software radio tiers

The SDR Forum, a technical group for software radio, defines five tiers of software-defined radio [SDR05]:

Tier 0: A hardware radio cannot be programmed.

Tier 1: A software-controlled radio has some functions implemented in software but operations like modulation and filtering cannot be altered without changing hardware.

Tier 2: A software-defined radio may use multiple antennas for different bands but the radio can cover a wide range of frequencies and use multiple modulation techniques.

Tier 3: An ideal software-defined radio does not use analog amplification or heterodyne mixing before A/D conversion.

Tier 4: An ultimate software radio is lightweight, consumes very little power, and requires no external antenna.

Digital demodulation

Demodulation requires multiplying the received signal by a signal from an oscillator and filtering the result to select the lower-frequency version of the signal. The bit detection process depends somewhat on the modulation scheme, but digital communication mechanisms often rely on phase. High-data-rate systems often use multiple frequencies arranged in a constellation. The phases of the component frequencies of the signal can be modulated to create different symbols.

Error correction

Traditional error correction codes can be checked using combinational logic. For example, a convolutional coder can be used as an error correction coder. The convolutional coder convolves the input with itself according to a chosen polynomial. Figure 1.10 shows a fragment of a trellis that represents possible states of a decoder; the label on an edge shows the input bit and the produced output bits. Any bits in the transmission may have been corrupted; the decoder must determine the most likely sequence of data bits that could have produced the received sequence.

Several more powerful codes that require iterative decoding have recently become popular. Turbo codes use multiple encoders. The input data is encoded by two convolutional encoders, each of which uses a different but generally simple code. One of the coders is fed the input data directly. The other coder is fed a permuted version of the input stream. Both coded versions of the data are sent across the channel. The decoder uses two decoding units, one for each code. The two decoders are operated iteratively. At each iteration, the two decoders swap likelihood estimates of the decoded bits; each decoder uses the other’s likelihoods as a priori estimates for its own next iteration.

Low-density parity check (LDPC) codes also require multiple iterations to determine errors and corrections. An LDPC code can be defined using a bipartite graph like that shown in Figure 1.11; the codes are called low-density because their graphs are sparse. The left-hand nodes are called message nodes and the right-hand nodes are check nodes. Each check node defines a sum of message node values. The message nodes define the coordinates for codewords; a legal codeword is a set of message node values that sets all the check nodes to 1. During decoding, an LDPC decoding algorithm passes messages between the message nodes and check nodes. One approach is to pass probabilities for the data bit values as messages. Multiple iterations should cause the algorithm to settle onto a good estimate of the data bit values.

Radios and protocols

A radio may simply act as the physical layer of a standard network stack, but modern networks and radios are designed to take advantage of the inherent characteristics of wireless networks. For example, traditional wired networks have only a limited number of nodes connected to a link but radios inherently broadcast; broadcast can be used to improve network control, error correction, and security. Wireless networks are generally ad hoc in that the members of the network are not predetermined and nodes may enter or leave during network operation. Ad hoc networks require somewhat different network control than is used in fixed, wired networks.

The next example looks at a cell phone communication standard.

Example 1.1

CDMA2000

CDMA2000 [Van04] is a widely used standard for spread-spectrum-based cellular telephony. It uses direct sequence spread-spectrum transmission, which multiplies the data to be transmitted with a high-frequency, pseudorandom bit sequence. The pseudorandom sequence spreads the data over a broad range of frequencies. The data appears as noise unless the receiver knows the pseudorandom sequence. Several radios can use the same frequency band without interfering because the pseudorandom codes allow their signals to be separated.

A simplified diagram of the system looks like this:

The spreader modulates the data with the pseudorandom code. The interleaver transposes coded data blocks to make the code more resistant to burst errors. The transmitter’s power is controlled so that all signals have the same strength at the receiver.

The physical layer protocol defines a set of channels that can carry data or control. A forward channel goes from a base station to a mobile station, while a reverse channel goes from a mobile station to a base station. Pilot channels are used to acquire the CDMA signal and provide phase information; they also allow the mobile station to estimate the channel’s characteristics. A number of different types of channels are defined for data, control, power control, etc.

The link layer defines medium access control (MAC) and signaling link access control (LAC). The MAC layer multiplexes logical channels onto the physical medium, provides reliable transport of user traffic, and manages quality of service. The LAC layer provides a variety of services: authentication, integrity, segmentation and reassembly, etc.

The OSI Model

The first two layers of the OSI model involve both hardware and software. In the five upper layers (Layers 3 to 7), the OSI model typically is implemented via software only.

Crunch Time

The OSI model is represented as a stack because data that is sent across the network has to move through each layer at both the sending and receiving ends. The sending computer generally initiates the process at the application layer and the data is sent down the stack to the physical layer and across the network to the receiving computer. On the receiving end, the data is received at the physical layer and the data packet is sent up the stack to the application layer.

The application layer starts the process. Small pieces of information relative to the transmission of information are added to the data at each layer; this is called encapsulation. The process is then reversed on the receiving side to get back to just the data.

The Microsoft Model

With the release of  Windows NT 3.1, TCP/IP was built into the operating system, providing a seamless integration of networking functionality in the OS. Since that time, it has become standard to provide TCP/IP with the operating system since so many computers today connect to a network in one form or another.

DID YOU KNOW?

The Microsoft model provides a standard platform for application developers. This modular design enables the developer to rely upon the underlying services of the OS through the use of standard interfaces. These interfaces provide specific functionality developers that can be used as building blocks to develop an application. This makes development time shorter and provides common interfaces for users, making learning and using new applications easier.

Understanding the Function of Boundary Layers

The Microsoft model describes software and hardware components and the connections between them that facilitate computer networking. This modular approach both allows and encourages hardware and software vendors to develop products that work together through the Microsoft operating system. Boundary layers are interfaces that reside at the boundaries of functionality. They interact with the layer below and the layer above, providing an interface from one layer to the next.

The interfaces defined by Microsoft are as follows:

■

Network Device Interface Specification (NDIS) This layer maps to the data link layer of the OSI model and the network interface layer of the Defense Advanced Research Projects Agency (DARPA) model, which we will cover next. The NDIS layer is the boundary between the physical network and the higher level transport protocols. This layer provides the standardized functions that allow various transport protocols to use any network device driver that is compatible with the specifications of this layer, providing both flexibility and reliability to developers.

■

Transport Driver Interface (TDI) This layer provides a portal into the transport protocols for kernel mode components such as servers and redirectors. It acts as the gateway between the transport layer and the session layer in the OSI model, providing a common interface, which developers can use to access both transport and session layer functionalities.

■

Application Program Interface (API) This layer is the interface through which developers can access network infrastructure services such as various application layer protocols.

The Windows OS is divided into three primary areas: the User, the Executive, and the Kernel. The Kernel is the core of the Microsoft operating system architecture, and it manages the most basic operations, including interacting with the hardware abstraction layer that interacts with the hardware (CPU, memory, and so forth). The Kernel also synchronizes activities with the Executive level, which includes the Input/Output (I/O) Manager and the Process Manager. The User level interacts with the Executive level; this is the level at which most applications and user interfaces reside.

Figure 5.2 shows the relationship of these boundary layers to both the OSI model and to the Microsoft architecture.

FIGURE 5.2. The Microsoft model

1.7 The OSI Model

The open systems interconnection (OSI) model is used to provide a methodical way to approach how data traverses networks, systems, and operates with application used on those computers and networks. It is a helpful tool that seems to be timeless as it is continuously referenced and used today since its inception many years ago. Founded from the Department of Defense (DoD) four-layer model back when the Internet (ARPAnet) was first conceived, it serves as a way to help not only describe how data traverses systems and networks but also an outstanding tool that can be used to help troubleshoot problems.

When the data arrives at its destination, the receiving station’s physical layer picks it up and performs the reverse process (also known as decapsulation). The physical layer converts the bits back into frames to pass on to the data link layer. The data link layer removes its header and trailer and passes the data on to the network layer. Once again, this process repeats itself until the data reaches all the way to the application layer. In Figure 1.7, we see the layers of the OSI model.

The layers of the OSI model are described as follows:

Application layer: This topmost layer of the OSI model is responsible for managing communications between network applications. This layer is not the application itself, although some applications may perform application layer functions. Examples of application layer protocols include file transfer protocol (FTP), hypertext transfer protocol (HTTP), simple mail transfer protocol (SMTP), and Telnet.

Presentation layer: This layer is responsible for data presentation, encryption, and compression.

Session layer: The session layer is responsible for creating and managing sessions between end systems. The session layer protocol is often unused in many protocols. Examples of protocols at the session layer include NetBIOS and remote procedure call (RPC).

Transport layer: This layer is responsible for communication between programs or processes. Port or socket numbers are used to identify these unique processes. Examples of transport layer protocols include TCP, user datagram protocol (UDP), and SPX.

Network layer: This layer is responsible for addressing and delivering packets from the source node to the destination node. The network layer takes data from the transport layer and wraps it inside a packet or datagram. Logical network addresses are generally assigned to nodes at this layer. Examples of network layer protocols include IP and IPX.

Data link layer: This layer is responsible for delivering frames between NICs on the same physical segment. It is subdivided into the media access control (MAC) layer and the logical link control (LLC) layer. Communication at the data link layer is generally based on hardware addresses. The data link layer wraps data from the network layer inside a frame. Examples of data link layer protocols include Ethernet, the now almost defunct token ring, and point-to-point protocol (PPP). Devices that operate at this layer include bridges and switches.

Physical layer: This layer defines connectors, wiring, and the specifications on how voltage and bits pass over the wired (or wireless) media. Devices at this layer include repeaters, concentrators, and hubs. Devices that operate at the physical layer do not have an understanding of paths.

When using Wireshark, you must consider the methodologies used to troubleshoot with as well as how the data works on networks and systems. Knowing how to launch and run the tool is not enough! You need to specifically know where to place it, when to run it, and what it is you will capture. You will then need to analyze which tests your knowledge of networks, computers, applications, and systems.

2.6.2 Layered Structure

To facilitate the design of communication networks, the functionalities of networks can be organized into a stack of layers. Each layer takes charge of different tasks and communicates with adjacent layers. Certain protocols are also designed for the interfaces between two adjacent layers. The most popular definition of network layers is the Open Systems Interconnection (OSI) reference model developed by the International Standards Organization (ISO). Another typical model is the TCP/IP reference model. Both are illustrated in Fig. 2.8. The details of the models are explained as follows.

We first introduce the OSI model, in which the network is divided into seven layers. Since the layers for session and presentation are not used in most network designs, we focus on the remaining five layers, which have been widely used in the design and analysis of communication networks.

Physical layer: The physical layer is concerned with how to transmit information bits from a transmitter to a receiver. It mainly involves the modulation/demodulation, coding/decoding,1 and signal processing for transmission and reception. Most of our previous discussions relate to the physical layer.

Data link layer: This layer takes charge of tasks such as frame acknowledgment (ACK), flow regulation, and channel sharing. The latter, called medium access control (MAC), is the most important for wireless networks due to the broadcast nature of wireless transmissions, and is therefore usually considered as an independent layer. Essentially, the MAC layer addresses resource allocation, e.g., how to allocate different communication channels to different users, and scheduling, e.g., when there is competition among users, which user should obtain the priority to transmit.

Network layer: This layer determines how to find a route in the network from the source to the destination. For example, we need to design an addressing mechanism for routing. Moreover, when the addresses of the source and destination are known, we need to design algorithms for the network to find a path with the minimum cost (e.g., the number of hops) to the destination. When a path is broken by an emergency, the network layer needs to find a new path for the data flow.

Transport layer: This layer receives data from the application layer, splits it into smaller units if needed, and then passes it to the network layer. The main task of the transport layer is congestion control, i.e., how to control the source rate according to the congestion situation in the network.

Application layer: This provides various protocols for different applications. For example, the HyperText Transfer Protocol (HTTP) is used for websites.

Note that the physical, data link, and network layers are concerned with the intermediate nodes in the network, while the transport and application layers involve only the two ends of data flow, namely the source and destination.

In the TCP/IP reference model, the physical and data link layers are not well specified. They are considered as the host-to-network layer. The Internet and TCP layers in the TCP/IP reference model roughly correspond to the network and transport layers in the OSI model. More details can be found in Ref. [36].

Understanding the OSI model

The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model in the early 1980's to describe how network protocols and components work together. It divides network functions into seven layers, and each layer represents a group of related specifications, functions, and activities.

The layers of the OSI model are:

Application layer This topmost layer of the OSI model is responsible for managing communications between network applications. This layer is not the application program itself, although some applications may have the ability and the underlying protocols to perform application layer functions. For example, a Web browser is an application, but it is the underlying Hypertext Transfer Protocol (HTTP) protocol that provides the application layer functionality. Examples of application layer protocols include File Transfer Protocol (FTP), Simple Network Management Protocol (SNMP), Simple Mail Transfer Protocol (SMTP), and Telnet.

Presentation layer This layer is responsible for data presentation, encryption, and compression.

Session layer The session layer is responsible for creating and managing sessions between end systems. The session layer protocol is often unused in many protocols. Examples of protocols at the session layer include NetBIOS and Remote Procedure Call (RPC).

Transport layer This layer is responsible for communication between programs or processes. Port or socket numbers are used to identify these unique processes. Examples of transport layer protocols include: TCP, UDP, and Sequenced Packet Exchange (SPX).

Network layer This layer is responsible for addressing and delivering packets from the source computer to the destination computer. The network layer takes data from the transport layer and wraps it inside a packet or datagram. Logical network addresses are generally assigned to computers at this layer. Examples of network layer protocols include IP and Internetwork Packet Exchange (IPX). Devices that work at this layer are routers and Layer 3 switches.

Data link layer This layer is responsible for delivering frames between NICs on the same physical segment. Communication at the data link layer is generally based on MAC addresses. The data link layer wraps data from the network layer inside a frame. Examples of data link layer protocols include Ethernet, Token Ring, and Point-to-Point Protocol (PPP). Devices that operate at this layer include bridges and switches.

Physical layer This layer defines connectors, wiring, and the specifications on how voltage and bits pass over the cabled or wireless media. Devices at this layer include repeaters, concentrators, hubs, and cable taps. Devices that operate at the physical layer do not have an understanding of network paths.

The terms frame and packet tend to be used interchangeably when talking about network traffic. However, the difference lies in the various layers of the OSI model. A frame is a unit of transmission at the data link layer. A packet is a unit of transmission at the network layer, however many people use the term packet to refer to data at any layer.

The OSI model is very generic and can be used to explain virtually any network protocol. Various protocol suites are often mapped against the OSI model for this purpose. A solid understanding of the OSI model aids tremendously in network analysis, comparison, and troubleshooting. However, it is also important to remember that not all protocols map nicely to the OSI model. For example, TCP/IP was designed to map to the U.S. Department of Defense (DoD) model. In the 1970s, the DoD developed its four-layer model. The core Internet protocols adhere to this model.

The DoD model is merely a condensed version of the OSI model. Its four layers are:

Process layer This layer defines protocols that implement user-level applications such as mail delivery, remote login, and file transfer.

Host-to-host layer This layer handles the connection, data flow management, and retransmission of lost data.

Internet layer This layer is responsible for delivering data from source host to destination host across a set of different physical networks that connect the two machines.

Network access layer This layer handles the delivery of data over a particular hardware media.

Writing Your Own Sniffer

There is an excellent paper titled “Basic Packet-Sniffer Construction from the Ground Up” by Chad Renfro located at www.unixgeeks.org/security/newbie/security/sniffer/sniffer_construction.txt. In this paper he presented a very basic 28-line packet sniffer written in C, called sniff.c. Even if you aren't a programmer, Chad explains the program line by line in an easy to understand manner. The program demonstrates the use of the RAW_SOCKET device to read TCP packets from the network and print basic header information to std_out. For simplicity, the program operates in non-promiscuous mode, so you would first need to put your interface in promiscuous mode by using the ifconfig eth0 promisc command.

There is also a header file that has to be copied into the same directory as sniff.c. It provides standard structures to access the IP and TCP fields. The structures identify each field in the IP and TCP header. It contains more information than what the sniff.c actually uses, but it least it is there to build upon.

To run the program, copy the sniff.c and headers.h into the same directory, and enter the command gcc -o sniff sniff.c. This will compile the program and create and executable file called sniff, which can be run by typing ./sniff. The following text shows the output of the sniff program when I attempted a TELNET and FTP connection:

Once you are done capturing data, you can end the program by typing CTRL-C. You may also want to remove your interface from promiscuous mode by typing the command ifconfig eth0 -promisc.

What data unit is associated with open systems interconnection layer four?

Techopedia Explains Protocol Data Unit (PDU) PDU is a significant term related to the initial four layers of the OSI model. In Layer 1, PDU is a bit, in Layer 2 it is a frame, in Layer 3 it is a packet and in Layer 4 it is a segment. In Layer 5 and above, PDU is referred to as data.

Which of the following network connectivity devices operates at Layer 2 of the OSI model?

Switch: A network switch is a multiport network bridge that uses MAC addresses to forward data at the data link layer (layer 2) of the OSI model.

What is another name for the OSI data link layer?

The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer.

What is the purpose of the layer 2 LLC sublayer?

What is the purpose of the Layer 2 LLC sublayer? It is used to handle multiplexing, flow and error control, and reliability.