Network+ 1.1 Compare the layers of the OSI and TCP/IP models

Layers

Logical groupings used in a reference model.

Network (Layer 3)

Manages device addressing, tracks the location of devices on the network, and determines the best way to move data.

Datagram

A basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order of arrival are not guaranteed by the network.

State Transition

A change in voltage from high to low and low to high.

Reference Model

A conceptual blueprint of how communciations should take place. It address all processes required for effective communication and divides these processes into logical groupings.

TCP/IP (DoD) Model

A condensed version of the OSI model that's composed of four layers: 1.) Process/Application Layer 2.) Host-to-Host Layer 3.) Internet Layer 4.) Network Access Layer

Data Link Header

A customized header used by the Data Link layer that contains the destination and source hardware address.

Frame

A data unit transferred across a network. These consist of several parts, such as the sending and receiving MAC addresses, the data being sent, and the frame check sequence.

Header

A part of the data packet and contains transparent information about the file or the transmission.

Buffer

A region of a physical memory storage used to temporarily store data while it is being moved from one place to another. Typically, the data is stored here as it is retrieved from a retriever or just before it is sent from a sender. May also be used when moving data between processes within a computer.

Positive Acknowledgement with Rentransmission

A technique that requires a receiving machine to communicate with the transmitting source by sending an acknowledgement message back to the sender when it receives data.

Protocol Data Unit (PDU)

A unit of data which is specified in a protocol of a given layer and which consists of protocol-control information and possibly user data of that layer. Allows communication and the exchange of information between layers. Usually attached to the header in front of the data field but can also be in the trailer, or end, of it.

Encoding

Allows the perceived item of use or interest to be converted into a construct that can be stored within the brain and recalled later from short term or long term memory.

Routing Table

Also known as a routing information base (RIB), is a data table stored in a router or a networked computer that lists the routes to particular network destinations, and in some cases, metrics (distances) associated with those routes. This data table contains information about the topology of the network immediately around it. The construction of routing tables is the primary goal of routing protocols.

Handshake

An automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer.

Cyclic Redundancy Check (CRC)

An error-detecting code commonly used in digital networks and storage devices to detect accidental changes to raw data. Blocks of data entering these systems get a short check value attached, based on the remainder of a polynomial division of their contents; on retrieval the calculation is repeated, and corrective action can be taken against presumed data corruption if the check values do not match.

Network Address

An identifier for a node or network interface of a telecommunications network. A router must maintain a routing table with these identifiers due to needing individual routing protocols because each routing protocol keeps track of a network with a different addressing scheme (IP and IPv6, for example).

Overhead

Any combination of excess or indirect computation time, memory, bandwidth, or other resources that are required to attain a particular goal.

OSI Model Upper Layers

Application (Layer 7), Presentation (Layer 6) and Session (Layer 5) Defines how the applications within the end stations will communicate with each other and with users.

Packet

Basic component of communication over a network. Group of bits of fixed maximum size and well-defined format that is switched and transmitted as a single entity through a network. Contains source and destination address, data, and control information.

Router Characteristics

By default, won't forward any broadcast or multicast packets Use the logical address in a Network layer header to determine the next-hop router to forward the packet to. Can use access lists, created by an administrator, to control security on the types of packets that are allowed to enter or exit an interface. Can provide Layer 2 bridging functions if needed and can simultaneously route through the same interface. Layer 3 devices provide connections between virtual LANs (VLANs) Can provide quality of service (QoS) for specific types of network traffic. Can be referred to as a Layer 3 Switch. Does not care about the location of a particular host. They're only concerned about where networks are located and the best way to reach them - including remote networks.

Three-Way Handshake

Connection-oriented session's virtual circuit (SYN - SYN/ACK - ACK) 1.) The first "connection agreement" segment is a request for synchronization - SYN. 2.) The next segments acknowledge the request and establish connection parameters - the rules - between hosts. These segments request that the receiver's sequencing is synchronized here as well so that a bidirectional connection is formed - SYN/ACK. 3.) The final segment is also an acknowledgement. It notifies the destination host that the connection agreement has been accepted and that the connection has been established. Data transfer can now begin - ACK.

TCP Segment

Consists of a segment header and a data section.

Internet Layer

Corresponds to the OSI's Network layer, designating the protocols relating to the logical transmission of packets over the entire network. It takes care of logical addressing of hosts by giving them an IP address, and it handles the routing of packets among multiple networks.

Presentation Layer Basic Functions

Data encryption, compression, and translation services.

Media Access Control (MAC)

Defines how packets are placed on the media. One of two sublayers of the IEEE Ethernet Data Link layer.

Physical Topology

Describes how devices are connected to the network through the actual cables that transmit data, or the physical structure of the network.

Logical Topology

Describes the arrangement of devices on a computer network and how they communicate with one another.

Session Layer Basic Functions

Dialog control

Transport Layer Basic Functions

End-to-end connection

Application Layer Basic Functions

File, print, message, database and application services.

Data Link Layer Basic Functions

Framing

Process/Application Layer

Integrates the various activities of the OSI's top three layers (Application, Presentation and Session). This layer defines protocols for node-to-node application communication and also controls user-interface specifications.

Application (Layer 7)

Marks the spot where users actually communicate with the computer. This layer chooses and determines the availability of communicating partners along with the resources necessary to make the connection, coordinates partnering applications, and forms a consensus on procedures for controlling data integrity and error recovery. Also responsible for identifying and establishing the availability of the intended communication partner and determining whether sufficient resources for the intended communication exist.

Network Access Layer

Monitors the data exchange between the host and the network. The equivalent of the Data Link and Physical layers of the OSI Model, this layer oversees hardware addressing and defines protocols for the physical transmission of data.

Data Packets

Packets used to transport user data through the internetwork.

Route-update Packets

Packets used to update neighboring routers about the networks connected to all routers within the internetwork. Used to help build and maintain routing tables on each router.

Host-to-Host Layer

Parallels the functions of the OSI's Transport Layer, defining protocols for setting up the level of transmission service for applications. It tackles issues such a creating reliable end-to-end communication and ensuring the error-free delivery of data. It handles packet sequencing and maintains data integrity.

Physical Layer Basic Functions

Physical topology

OSI Model Mnemonic

Please Do Not Throw Sausage Pizza Away

Presentation (Layer 6)

Presents data to the Application layer and is responsible for data translation and code formatting. This layer is essentially a translator and provides coding and conversion functions. Tasks like data compression, decompression, encryption, and decryption are associated with this layer.

Open Systems Interconnection (OSI) Model

Primary architectural model for networks. Comprises seven hierarchical layers that were developed to enable different networks to communicate reliably between disparate systems. This model describes how data and network information are communicated from an application on one computer through the network media to an application on another computer. A set of guidelines that application developers can use to create and implement applications that run on a network. It also provides a framework for creating and implementing networking standards, devices, and internetworking schemes. Comprised of seven layers: Application (Layer 7) Presentation (Layer 6) Session (Layer 5) Transport (Layer 4) Network (Layer 3) Data Link (Layer 2) Physical (Layer 1)

Routing Protocols

Protocols used to send route-update packets. Routing Information Protocol (RIP), RIPv2, Enchanced Interior Gateway Routing Protocol (EIGRP), and Open Shortest Path First (OSPF) are examples.

Routed Protocols

Protocols used to support data traffic. Internet Protocol (IP) and Internet Protocol version 6 (IPv6) are two examples.

Data Link (Layer 2)

Provides the physical transmission of the data and handles error notification, network topology and flow control. This means that this layer ensures that messages are delivered to the proper device on a LAN using hardware addresses and translates messages from the Network layer into bits for the Physical layer to transmit. Responsible for the unique identification of each device that resides on a local network.

Frame Check Sequence (FCS)

Refers to the extra error-detecting code added to a frame in a communications protocol. Frames are used to send upper-layer data and ultimately the application data from a source to a destination.

Logical Link Control (LLC)

Responsible for identifying Network layer protocols and then encapsulating them. This type of header tells the Data Link layer what to do with a packet once a frame is received.

Session (Layer 5)

Responsible for setting up, managing, and then tearing down sessions between Presentation layer entities. This layer also provides dialog control between devices, or nodes. It coordinates communication between systems and serves to organize their communication by offering three different modes: simplex, half duplex and full duplex. This layer keeps applications' data separate from other applications' data.

Network Layer Basic Functions

Routing

Transport (Layer 4)

Services located in this layer segment and reassemble data from upper-layer applications and unite it onto the same data stream. They provide end-to-end data transport services and can establish a logical connection between the sending host and destination host on an internetwork. Responsible for providing mechanisms for multiplexing upper-layer applications, establishing virtual connections, and then tearing down virtual circuits. This layer also hides details of any network-dependent information from the higher layers by providing transparent data transfer.

Flow Control

Takes place at the Transport layer. Provides a means for the receiver to govern the amount of data sent by the sender. It prevents a sending host on one side of the connection from overflowing the buffers in the receiving host - an event that can result in lost data.

Reliable Networking

Term used at the Transport layer that means that acknowledgements, sequencing, and flow control will be used. These are the connection-oriented protocols of the Transport layer.

Logical Address

The address at which an item (memory cell, storage element, network host) appears to reside from the perspective of an executing application program.

Virtual Circuit

The communication path created during connection-oriented communication between the sender's TCP process and the destination's TCP process.

Binding

The communication processes that are related to each other are bound, or grouped together, at a particular layer.

Interface

The exit of a routing table taken when destined for a specific network.

Hop Count

The number of routers a packet passes through en route to a remote network.

Encapsulation

The process of encoding data as it goes down the OSI stack. Order is data, segment, packet, frame, bits.

De-encapsulation

The process of pulling a packet from the frame and discarding what is left of the frame. The packet is then handed to the Network layer, where the address is checked. If the address matches, the segment is pulled from the packet and what's left of the packet is discarded. The segment is left to be processed at the Transport layer.

Window

The quantity of data segments (measured in bytes) that the transmitting machine is allowed to send without receiving an acknowledgement. These are used to control the amount of outstanding, unacknowledged data segments.

Physical (Layer 1)

This layer sends bits and receives bits. It communicates directly with the various types of actual communication media. Specifies the electrical, mechanical, procedural, and functional requirements for activating, maintaining, and deactivating a physical link between end systems. Converts data impulses that are designed for the wired or wireless medium being used on the attached segment. This is the layer where you identify the interface between the data terminal equipment (DTE) and the data communication equipment (DCE). Specifies the layout of the transmission media (its topology, in other words).

Metric

This value equals the distance to the remote network.

OSI Model Lower Layers

Transport (Layer 4), Network (Layer 3), Data Link (Layer 2) and Physical (Layer 1) Defines how data is transmitted through physical media, switches and routers. Also determines how to rebuild a data stream from a transmitting host to a desintation host's application.