Open Systems Interconnection (OSI) Layers

The Open Systems Interconnection (OSI) model has seven layers that define network communication. For the CISSP exam, you need to understand the order of the layers, what layer number is assigned to each layer name, and the function of each layer. Each layer performs a defined function for the layer above it and communicates with its peer layer over an interface.

The OSI model is an ISO standard. Contrary to the acronym, ISO stands for International Organization of Standardization, not International Standards Organizations.

From bottom the layers and their Protocol Data Unit (PDU) are:

1. Physical – Bits
2. Data Link – Frame
3. Network – Packet
4. Transport – Segment
5. Session – Data
6. Presentation – Data
7. Application – Data

A handy way to remember the order is to remember the phrase “Please Do Not Throw Sausage Pizza Away”. When you come across the first OSI question on the exam, use this phrase to write down the order and layers on your scrap paper, then refer to that for any future questions.

Encapsulation

Encapsulation is an important concept with the OSI model. It is the process by which data moves between different PDU types. Each layer (or protocol) accepts a message from a layer (or protocol) above it and places its own header. Encapsulation occurs as data moves from higher to lower layers.

Data > Segments > Packets > Frames > Bits

Decapsulation is the reverse of encapsulation and occurs as data moves from the lower to higher layers.

Bits > Frames > Packets > Segments > Data

Layer 1 – Physical

This layer deals with the hardware and raw bit stream. It puts data onto the network media and takes it back off.  The Protocol Data Unit (PDU) of this layer is bits.

The physical layer moves the final completed frame from the computer’s memory location to the network transmission medium. It is not involved with any further packaging operations on the packet such as headers and control fields. Physical later protocols only deal with the mechanical, electrical, functional, and procedural aspects of this process.

If the network goes down because rats chewed through a cable, then this would be a “layer 1″ problem. Ethernet, ISDN, connectors, voltage, multiplexers, repeaters, and cable are terms associated with the physical layer.

Layer 2 – Data Link

The Data Link Layer is responsible for physically passing data from one node to another. The Protocol Data Unit (PDU) of this layer is the frame. The Data Link layer performs flow control, error detection, and control. This layer has two sub-layers, Media Access Control (MAC) and Logical Link Control (LLC).

The Data Link layer is the last layer in the protocol suite that treats the data as a logical data string held in the computer’s main memory and processed by the communications software. Final sequence numbering, addressing data, and the primary error control data must be provided before data can be passed to the Physical layer for actual transmission.

MAC address, ARP, RARP, PPP, and SLIP are terms associated with the Data Link layer.

Layer 3 – Network

The network layer performs addressing and routing. It defines the functions necessary to support communication between indirectly connected entities. It has the ability of forwarding messages from one Layer 3 entity to another, hop-by-hop, until the final destination is reached. It does this by routing data from one node to another. The Protocol Data Unit (PDU) of this layer is packet.

IP, OSPF, RIP, IGMP are terms associated with the Network Layer.

Layer 4 – Transport

The Transport Layer is concerned with getting layer 4 messages from source to destination in a reliable manner. This is an end-to-end communication, unlike the hop-by-hop communicaiton at layer 3. The Transport Layer is responsible for end-to-end integrity of data transmission. The Protocol Data Unit (PDU) of this layer is segment.

The Transport Layer provides the interface between the lower level physical networking controls and the higher levels that are concerned with logical application data handling. If the message is long, it may be partitioned into a series of smaller message usits.

TCP, UDP, and SPX are terms associated with the Transport Layer.

Layer 5 – Session

The Session Layer controls communication between applications on hosts. Synchronization of communicating applications comes into play when coordinating timing of corresponding events at the end points is important, such as in financial transactions. This layer is responsible for establishing and maintaining communications channels. The Protocol Data Unit (PDU) of this layer is data.

The Session Layer provides control over the orderly exchange of data during the period when the sender and receiver are communicating. Login passwords and the exchange of user IDs may be handled at the Session layer. Accounting operations, as well as aspects of flow control, may also be determined at this layer.

NFS, SQL, RPC, and NetBIOS are terms associated with the Session Layer.

Layer 6 – Presentation

The presentation layer performs any necessary data transformations or formatting required by the end applications, it provides control over the way the data will be encoded to allow proper handling when it is presented to the receiver’s application. Encryption, Data compression, and file formatting are performed at this layer. This layer is responsible for establishing and maintaining communications channels. The Protocol Data Unit (PDU) of this layer is data. The Presentation Layer

MPEG, ASCII, GIF, TIFF, and JPEG are terms associated with the Presentation Layer.

Layer 7 – Application

The Application Layer provides network services that directly support an application running on a host. This layer is closest to the end user and is responsible for program to program communication. The Protocol Data Unit (PDU) of this layer is data. The Application Layer is the layer that the end user sees and is familiar with.

FTP, TFTP, Telnet, SNMP, BOOTP, SMTP, and MIME are terms associated with the Application Layer.

Image courtesy Andrea Beggi

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