Chapter 2, The OSI Model

OUI (Organizationally Unique Identifier)

A 24-bit character sequence assigned by IEEE NIC's manufacturer.

IP (Internet Protocol)

A core protocol in the TCP/IP suite that operates in the Network layer of the OSI model and provides information about how and where data should be delivered. IP is the subprotocol that enables TCP/IP to internetwork.

checksum

A method of error checking that determines if the contents of an arriving data unit match the contents of the data unit sent by the source.

token ring

A networking technology developed by IBM in the 1980s. It relies upon direct links between nodes and a ring topology, using tokens to allow nodes to transmit data.

Ethernet

A networking technology originally developed at Xerox in the 1970s and improved by Digital Equipment Corporation, Intel, and Xerox. Ethernet, which is the most common form of network transmission technology, follows the IEEE 802.3 standard.

IANA Internet Assigned Numbers Authority

A nonprofit, United States government- funded group that was established and charged with managing IP address allocation and the Domain Name System. IANA continues to perform Internet addressing and Domain Name System administration.

frame

A package for data that includes not only the raw data, or "payload," but also the sender's and recipient's addressing and control information. These are generated at the Data Link layer of the OSI model and are issued to the network at the Physical layer.

API (application programming interface)

A set of routines that make up part of a software application.

token

A special control frame that indicates to the rest of the network that a particular node has the right to transmit data.

TIA (Telecommunications Industry Association)

A subgroup of the EIA that focuses on standards for information technology, wireless, satellite, fiber optics, and telephone equipment.

three-way handshake

A three-step process in which Transport layer protocols establish a connection between nodes. The three steps are: Node A issues a SYN packet to node B, node B responds with SYN-ACK, and node A responds with ACK.

network address

A unique identifying number for a network node that follows a hierarchical addressing scheme and can be assigned through operating system software. Network addresses are added to data packets and interpreted by protocols at the Network layer of the OSI model.

extension identifier

A unique set of characters assigned to each NIC by its manufacturer. In the traditional, 48-bit physical addressing scheme, this identifier is24 bits long. In EUI-64, the extension identifier is 40 bits long.

PDU (protocol data unit)

A unit of data at any layer of the OSI model.

HTTP (Hypertext Transfer Protocol)

An Application layer protocol that formulates and interprets requests between Web clients and servers.

CRC (cyclic redundancy check)

An algorithm (or mathematical routine) used to verify the accuracy of data contained in a data frame.

IEEE (Institute of Electrical and Electronics Engineers)

An international society composed of engineering professionals.

ANSI American National Standards Institute

An organization composed of more than 1000 representatives from industry and government who together determine standards for the electronics industry in addition to other fields

IETF (Internet Engineering Task Force)

An organization that sets standards for how systems communicate over the Internet

EIA

Electronic Industries Alliance

ITU

International Telecommunication Union

ISOC

Internet Society

ISP

Internet service provider

EUI-64 (Extended Unique Identifier-64)

The IEEE standard defining 64-bit physical addresses. In the EUI-64 scheme, the OUI portion of an address is 24 bits in length. A 40-bit extension identifier makes up the rest of the physical address to total 64 bits.

IP address

The Network layer address assigned to nodes to uniquely identify them on a TCP/IP network.

FCS (frame check sequence)

The field in a frame responsible for ensuring that data carried by the frame arrives intact. It uses an algorithm, such as CRC, to accomplish this verification.

Session layer

The fifth layer in the OSI model. This layer establishes and maintains communication between two nodes on the network. It can be considered the "traffic cop" for communications, such as videoconferencing, that require precisely coordinated data exchange.

Transport layer

The fourth layer of the OSI model. In this layer, protocols ensure that data are transferred from point A to point B reliably and without errors. Layer services include flow control, acknowledgment, error correction, segmentation, reassembly, and sequencing.

MTU (maximum transmission unit)

The largest data unit a network (for example, Ethernet or token ring) will accept for transmission.

MAC (Media Access Control) sublayer

The lower sublayer of the Data Link layer. The MAC appends the physical address of the destination computer onto the frame.

ICANN Internet Corporation for Assigned Names and Numbers

The nonprofit corporation currently designated by the United States government to maintain and assign IP addresses.

sequencing

The process of assigning a placeholder to each piece of a data block to allow the receiving node's Transport layer to reassemble the data in the correct order.

encapsulate

The process of wrapping one layer's PDU with protocol information so that it can be interpreted by a lower layer.

Application layer

The seventh layer of the OSI model. This layer's protocols enable software programs to negotiate formatting, procedural, security, synchronization, and other requirements with the network.

Presentation layer

The sixth layer of the OSI model. Protocols in this layer translate between the application and the network. Here, data are formatted in a schema that the network can understand, with the format varying according to the type of network used. This layer also manages data encryption and decryption, such as the scrambling of system passwords.

LLC (Logical Link Control) sublayer

The upper sublayer in the Data Link layer. The LLC provides a common interface and supplies reliability and flow control services.

SYN (synchronization)

The packet one node sends to request a connection with another node on the network. The SYN packet is the first of three in the three-step process of establishing a connection.

fragmentation

A Network layer service that subdivides segments it receives from the Transport layer into smaller packets.

ISO (International Organization for Standardization)

A collection of standards organizations representing 162 countries with headquarters located in Geneva, Switzerland. Its goal is to establish international technological standards to facilitate the global exchange of information and barrier-free trade.

RIR (Regional Internet Registry)

A not-for-profit agency that manages the distribution of IP addresses to private and public entities.

ACK

A response generated at the Transport layer of the OSI model that confirms to a sender that its frame was received. This packet is the third of three in the three-step process of establishing a connection.

segment

A unit of data that results from subdividing a larger protocol data unit.

Physical layer

The lowest, or first, layer of the OSI model. Protocols in this layer generate and detect signals so as to transmit and receive data over a network medium. These protocols also set the data transmission rate and monitor data error rates, but do not provide error correction.

SYN-ACK (synchronization-acknowledgment)

The packet a node sends to acknowledge to another node that it has received a SYN request for connection.

Data Link layer

The second layer in the OSI model. This layer bridges the networking media with the Network layer. Its primary function is to divide the data it receives from the Network layer into frames that can then be transmitted by the Physical layer.

Network layer

The third layer in the OSI model. Protocols in this layer translate network addresses into their physical counterparts and decide how to route data from the sender to the receiver.