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CISSP Cert Guide: Telecommunications and Network Security

Table of contents for Data communications and networking / Behrouz A. Forouzan.

❶Provides an interface with the physical network. Concerned with circuit, message or packet switching.

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Seven Layers of OSI Model and functions of seven layers of OSI model
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Difference between OSI and TCP/IP Reference Model

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.

When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. The data and control information that is transmitted through internetworks takes a variety of forms. The terms used to refer to these information formats are not used consistently in the internetworking industry but sometimes are used interchangeably.

Common information formats include frames, packets, datagrams, segments, messages, cells, and data units. A frame is an information unit whose source and destination are data link layer entities. A frame is composed of the data link layer header and possibly a trailer and upper-layer data.

The header and trailer contain control information intended for the data link layer entity in the destination system. Data from upper-layer entities is encapsulated in the data link layer header and trailer. A packet is an information unit whose source and destination are network layer entities.

A packet is composed of the network layer header and possibly a trailer and upper-layer data. The header and trailer contain control information intended for the network layer entity in the destination system. Data from upper-layer entities is encapsulated in the network layer header and trailer.

The term datagram usually refers to an information unit whose source and destination are network layer entities that use connectionless network service. The term segment usually refers to an information unit whose source and destination are transport layer entities. A message is an information unit whose source and destination entities exist above the network layer often at the application layer. A cell is an information unit of a fixed size whose source and destination are data link layer entities.

A cell is composed of the header and payload. The header contains control information intended for the destination data link layer entity and is typically 5 bytes long. The payload contains upper-layer data that is encapsulated in the cell header and is typically 48 bytes long. Data unit is a generic term that refers to a variety of information units.

SDUs are information units from upper-layer protocols that define a service request to a lower-layer protocol. BPDUs are used by the spanning-tree algorithm as hello messages. Large networks typically are organized as hierarchies. A hierarchical organization provides such advantages as ease of management, flexibility, and a reduction in unnecessary traffic. Thus, the International Organization for Standardization ISO has adopted a number of terminology conventions for addressing network entities.

An ES is a network device that does not perform routing or other traffic forwarding functions. Typical ESs include such devices as terminals, personal computers, and printers. An IS is a network device that performs routing or other traffic-forwarding functions.

Typical ISs include such devices as routers, switches, and bridges. Two types of IS networks exist: An intradomain IS communicates within a single autonomous system, while an interdomain IS communicates within and between autonomous systems. An area is a logical group of network segments and their attached devices. Areas are subdivisions of autonomous systems AS's. An AS is a collection of networks under a common administration that share a common routing strategy.

Autonomous systems are subdivided into areas, and an AS is sometimes called a domain. A Hierarchical Network Contains Numerous Components illustrates a hierarchical network and its components.

In general, transport protocols can be characterized as being either connection-oriented or connectionless. Connection-oriented services must first establish a connection with the desired service before passing any data. A connectionless service can send the data without any need to establish a connection first. In general, connection-oriented services provide some level of delivery guarantee, whereas connectionless services do not. Connection-oriented service involves three phases: During connection establishment, the end nodes may reserve resources for the connection.

The end nodes also may negotiate and establish certain criteria for the transfer, such as a window size used in TCP connections. This resource reservation is one of the things exploited in some denial of service DOS attacks.

An attacking system will send many requests for establishing a connection but then will never complete the connection. The attacked computer is then left with resources allocated for many never-completed connections.

Then, when an end node tries to complete an actual connection, there are not enough resources for the valid connection. The data transfer phase occurs when the actual data is transmitted over the connection.

During data transfer, most connection-oriented services will monitor for lost packets and handle resending them. The protocol is generally also responsible for putting the packets in the right sequence before passing the data up the protocol stack. When the transfer of data is complete, the end nodes terminate the connection and release resources reserved for the connection. Connection-oriented network services have more overhead than connectionless ones.

Connection-oriented services must negotiate a connection, transfer data, and tear down the connection, whereas a connectionless transfer can simply send the data without the added overhead of creating and tearing down a connection. Each has its place in internetworks. Internetwork addresses identify devices separately or as members of a group.

Addressing schemes vary depending on the protocol family and the OSI layer. Three types of internetwork addresses are commonly used: A data link layer address uniquely identifies each physical network connection of a network device.

Data-link addresses sometimes are referred to as physical or hardware addresses. Data-link addresses usually exist within a flat address space and have a pre-established and typically fixed relationship to a specific device.

End systems generally have only one physical network connection and thus have only one data-link address. Routers and other internetworking devices typically have multiple physical network connections and therefore have multiple data-link addresses. Each Interface on a Device Is Uniquely Identified by a Data-Link Address illustrates how each interface on a device is uniquely identified by a data-link address.

MAC addresses are 48 bits in length and are expressed as 12 hexadecimal digits. The last 6 hexadecimal digits comprise the interface serial number, or another value administered by the specific vendor. Because internetworks generally use network addresses to route traffic around the network, there is a need to map network addresses to MAC addresses.

When the network layer has determined the destination station's network address, it must forward the information over a physical network using a MAC address. Different protocol suites use different methods to perform this mapping, but the most popular is Address Resolution Protocol ARP. Different protocol suites use different methods for determining the MAC address of a device. The following three methods are used most often.

The Hello protocol enables network devices to learn the MAC addresses of other network devices. MAC addresses either are embedded in the network layer address or are generated by an algorithm.

When a network device needs to send data to another device on the same network, it knows the source and destination network addresses for the data transfer. It must somehow map the destination address to a MAC address before forwarding the data. First, the sending station will check its ARP table to see if it has already discovered this destination station's MAC address.

If it has not, it will send a broadcast on the network with the destination station's IP address contained in the broadcast.

Every station on the network receives the broadcast and compares the embedded IP address to its own. Only the station with the matching IP address replies to the sending station with a packet containing the MAC address for the station. The first station then adds this information to its ARP table for future reference and proceeds to transfer the data.

When the destination device lies on a remote network, one beyond a router, the process is the same except that the sending station sends the ARP request for the MAC address of its default gateway. It then forwards the information to that device. The default gateway will then forward the information over whatever networks necessary to deliver the packet to the network on which the destination device resides.

The router on the destination device's network then uses ARP to obtain the MAC of the actual destination device and delivers the packet. The Hello protocol is a network layer protocol that enables network devices to identify one another and indicate that they are still functional.

When a new end system powers up, for example, it broadcasts hello messages onto the network. Devices on the network then return hello replies, and hello messages are also sent at specific intervals to indicate that they are still functional.

Network devices can learn the MAC addresses of other devices by examining Hello protocol packets. Three protocols use predictable MAC addresses. In these protocol suites, MAC addresses are predictable because the network layer either embeds the MAC address in the network layer address or uses an algorithm to determine the MAC address.

A network layer address identifies an entity at the network layer of the OSI layers. Network addresses usually exist within a hierarchical address space and sometimes are called virtual or logical addresses. The relationship between a network address and a device is logical and unfixed; it typically is based either on physical network characteristics the device is on a particular network segment or on groupings that have no physical basis the device is part of an AppleTalk zone.

End systems require one network layer address for each network layer protocol that they support. This assumes that the device has only one physical network connection. Routers and other internetworking devices require one network layer address per physical network connection for each network layer protocol supported.

The router therefore has nine network layer addresses. Each Network Interface Must Be Assigned a Network Address for Each Protocol Supported illustrates how each network interface must be assigned a network address for each protocol supported.

Internetwork address space typically takes one of two forms: A hierarchical address space is organized into numerous subgroups, each successively narrowing an address until it points to a single device in a manner similar to street addresses. A flat address space is organized into a single group in a manner similar to U.

Hierarchical addressing offers certain advantages over flat-addressing schemes. Address sorting and recall is simplified using comparison operations. For example, "Ireland" in a street address eliminates any other country as a possible location. Hierarchical and Flat Address Spaces Differ in Comparison Operations illustrates the difference between hierarchical and flat address spaces.

Addresses are assigned to devices as one of two types: Static addresses are assigned by a network administrator according to a preconceived internetwork addressing plan. A static address does not change until the network administrator manually changes it.

Dynamic addresses are obtained by devices when they attach to a network, by means of some protocol-specific process. A device using a dynamic address often has a different address each time that it connects to the network.

Some networks use a server to assign addresses. Server-assigned addresses are recycled for reuse as devices disconnect. A device is therefore likely to have a different address each time that it connects to the network. Internetwork devices usually have both a name and an address associated with them.

Internetwork names typically are location-independent and remain associated with a device wherever that device moves for example, from one building to another. Internetwork addresses usually are location-dependent and change when a device is moved although MAC addresses are an exception to this rule. As with network addresses being mapped to MAC addresses, names are usually mapped to network addresses through some protocol.

For example, it's easier for you to remember www. Therefore, you type www. Your computer performs a DNS lookup of the IP address for Cisco's web server and then communicates with it using the network address.

Flow control is a function that prevents network congestion by ensuring that transmitting devices do not overwhelm receiving devices with data.

A high-speed computer, for example, may generate traffic faster than the network can transfer it, or faster than the destination device can receive and process it. The three commonly used methods for handling network congestion are buffering, transmitting source-quench messages, and windowing. Buffering is used by network devices to temporarily store bursts of excess data in memory until they can be processed. Occasional data bursts are easily handled by buffering.

Excess data bursts can exhaust memory, however, forcing the device to discard any additional datagrams that arrive. Source-quench messages are used by receiving devices to help prevent their buffers from overflowing.

The receiving device sends source-quench messages to request that the source reduce its current rate of data transmission. First, the receiving device begins discarding received data due to overflowing buffers.

Second, the receiving device begins sending source-quench messages to the transmitting device at the rate of one message for each packet dropped. The source device receives the source-quench messages and lowers the data rate until it stops receiving the messages.

Finally, the source device then gradually increases the data rate as long as no further source-quench requests are received. Windowing is a flow-control scheme in which the source device requires an acknowledgement from the destination after a certain number of bytes have been transmitted. With a window size of , the source requires an acknowledgement after sending bytes, as follows. First, the source device sends bytes to the destination device. Then, after receiving the bytes, the destination device sends an acknowledgement to the source.

The source receives the acknowledgement and sends more bytes. If the destination does not receive one or more of the packets for some reason, such as overflowing buffers, it does not receive enough packets to send an acknowledgement. The source then retransmits the packets at a reduced transmission rate. Error-checking schemes determine whether transmitted data has become corrupt or otherwise damaged while traveling from the source to the destination.

Error checking is implemented at several of the OSI layers. One common error-checking scheme is the cyclic redundancy check CRC , which detects and discards corrupted data.

Error-correction functions such as data retransmission are left to higher-layer protocols. A CRC value is generated by a calculation that is performed at the source device. The destination device compares this value to its own calculation to determine whether errors occurred during transmission. First, the source device performs a predetermined set of calculations over the contents of the packet to be sent. Then, the source places the calculated value in the packet and sends the packet to the destination.

The destination performs the same predetermined set of calculations over the contents of the packet and then compares its computed value with that contained in the packet. If the values are equal, the packet is considered valid. If the values are unequal, the packet contains errors and is discarded. Multiplexing is a process in which multiple data channels are combined into a single data or physical channel at the source. Multiplexing can be implemented at any of the OSI layers.

Conversely, demultiplexing is the process of separating multiplexed data channels at the destination. The data link thus provides data transfer across the physical link. That transfer can be reliable or unreliable; many data-link protocols do not have acknowledgments of successful frame reception and acceptance, and some data-link protocols might not even have any form of checksum to check for transmission errors.

In those cases, higher-level protocols must provide flow control , error checking, and acknowledgments and retransmission. Within the semantics of the OSI network architecture, the data-link-layer protocols respond to service requests from the network layer and they perform their function by issuing service requests to the physical layer. The data link layer has two sublayers: The uppermost sublayer, LLC, multiplexes protocols running at the top of data link layer, and optionally provides flow control, acknowledgment, and error notification.

The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines. MAC may refer to the sublayer that determines who is allowed to access the media at any one time e.

Other times it refers to a frame structure delivered based on MAC addresses inside. There are generally two forms of media access control: In a network made up of people speaking, i.

The Media Access Control sublayer also determines where one frame of data ends and the next one starts — frame synchronization.

There are four means of frame synchronization: Beside framing, data link layers also include mechanisms to detect and even recover from transmission errors. For a receiver to detect transmission error, the sender must add redundant information in the form of bits as an error detection code to the frame sent. When the receiver obtains a frame with an error detection code it recomputes it and verifies whether the received error detection code matches the computed error detection code.

If they match the frame is considered to be valid. An error detection code can be defined as a function that computes the r amount of redundant bits corresponding to each string of N total number of bits. If there are two or more bits in error, the receiver may not be able to detect the transmission error. A simple example of how this works using metadata is transmitting the word "HELLO", by encoding each letter as its position in the alphabet.

Thus, the letter A is coded as 1, B as 2, and so on as shown in the table on the right. Finally, the "8 5 12 12 15 7" numbers sequence is transmitted, which the receiver will see on its end if there are no transmission errors. The receiver knows that the last number received is the error-detecting metadata and that all data before is the message, so the receiver can recalculate the above math and if the metadata matches it can be concluded that the data has been received error-free.

The link layer functionality was described in RFC and is defined differently than the Data Link Layer of OSI, and encompasses all methods that affect the local link. From Wikipedia, the free encyclopedia.

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View OSI Model Key Terms Table from IT IT at University of Phoenix. Sorniel M Davila IT/ Version 4 OSI Model Key Terms Table Term Physical layer Data link layer Network layer Transport.

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The OSI Network Model or Open System Interconnection Model is a model that has these set rules so computers can considerableaps.tk does each layer of the OSI Model. key to any successful considerableaps.tk the following table to define the key terms related to the OSI model. OSI Reference Model Layer Summary. To assist you in quickly comparing the layers of the OSI Reference Model, and understanding where they are different and how they relate to each other, I have created the summary chart shown in Table It shows each layer's name and number, describes its key responsibilities, talks about what type of .

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OSI Model Key Terms Table Use the table to define the key terms related to the OSI Model. Describe the functions of any hardware connectivity devices and tools listed.. Term Definition Function (if applicable) Physical layer This is the first and lowest layer in the seven-layer OSI model. Axia College MaterialAppendix C OSI Model Key Terms Table Use the table to define the key terms related to the OSI Model. Describe t.