1.3.5 OSI Layer Facts

Application

The Application layer corresponds to the Session, Presentation, and Application layers of the OSI model. Protocols associated with the Application layer include FTP, HTTP, Telnet, SMTP, DNS, and SNMP.

Data Link (Layer 2)

The Data Link layer defines the rules and procedures for hosts as they access the Physical layer. These rules and procedures define: > How physical network devices are identified on the network by defining a unique hardware address (physical or MAC address). > How and when devices have access to the LAN and can transmit on the network medium (media access control and logical topology). > How to verify that the data received from the Physical layer is error free (parity and CRC). > How devices control the rate of data transmission between hosts (flow control). At the Data Link layer, segments are called frames. Switches, bridges and NICs, and WAPs function in layer 2. > Media Access Control (MAC) > Logical Link Control (LLC)

Host-to-Host

The Host-To-Host layer is comparable to the Transport layer of the OSI model. It is responsible for error checking and reliable packet delivery. Here, the data stream is broken into segments that must be assigned sequence numbers so they can be reassembled correctly on the remote side after they are transported. Protocols associated with the Host-To-Host layer include Transport Control Protocol (TCP) and User Datagram Protocol (UDP).

Internet

The Internet Layer is comparable to the Network layer of the OSI model. It is responsible for moving packets through a network. This involves addressing hosts and making routing decisions to identify how the packet traverses the network. Protocols associated with the Internet layer include Address Resolution Protocol (ARP), Internet Control Message Protocol (ICMP), and the Internet Group Management Protocol (IGMP).

Network Access

The Network Access layer corresponds to the Physical and Data Link Layers of the OSI model. It is responsible for describing the physical layout of the network and formatting messages on the transmission medium. *The TCP/IP model focuses specifically on the functions in the Internet layer and Host-to-Host layers. All other functions of the traditional OSI model are encompassed in the first and fourth layers.

Network (Layer 3)

The Network layer describes how data is routed across networks and on to the destination. Network layer functions include: > Identifying hosts and networks by using logical addresses. > Maintaining a list of known networks and neighboring routers. > Determining the next network point where data should be sent. Routers use a routing protocol that takes various factors into account, such as the number of hops in the path, link speed, and link reliability, to select the optimal path for data. At the Network layer, data segments are called packets.

Physical (Layer 1)

The Physical layer of the OSI model sets standards for sending and receiving electrical signals between devices. Protocols at the Physical layer identify: > How digital data (bits) are converted to electrical pulses, radio waves, or pulses of light and moved across network cables. > Specifications for cables and connectors. > The physical topology. At the Physical layer, data segments are called bits. NICs, repeaters, hubs, WAPs, and modems function in layer 1.

TCP/IP Model Layers

The TCP/IP model incorporates the general concepts and structure of the OSI model. The layers of the TCP/IP model are as follows:

Transport (Layer 4)

The Transport layer provides a transition between the upper and lower layers of the OSI model, making the upper and lower layers transparent from each other. Transport layer functions include: > End-to-end flow control. > Port and socket numbers. > Segmentation, sequencing, and combination. > Connection services, either reliable (connection-oriented) or unreliable (connectionless) delivery of data. At the Transport layer, data segments are called segments.

Application Layer (Layer 7)

The application layer integrates network functionality into the host operating system and enables communication between network clients and services. The Application Layer does not include specific applications that provide services, but rather provides the capability for services to operate on the network. Most application layer protocols operate at multiple layers down to the session and even transport layers. However, these protocols are classified as Application layer protocols because they start at the Application layer (the Application Layer is the highest layer where they operate). Services typically associated with the Application layer include: > HTTP > Telnet > FTP > TFTP > SNMP

Presentation Layer (Layer 6)

The presentation layer formats, or presents, data in a compatible form for receipt by the Application layer or the destination system. Specifically, the presentation layer ensures: > Formatting and translation of data between systems. > Negotiation of data transfer syntax between systems by converting character sets to the correct format. > Encapsulation of data into message envelopes by encryption and compression. > Restoration of data by decryption and decompression.

Session (Layer 5)

The session layer manages the sessions in which data are transferred. Session layer functions include: > Management of multiple sessions (each client connection is called a session). A server can concurrently maintain thousands of sessions. > Assignment of a session ID number to each session to keep data streams separate. > The setup, maintenance, and teardown of communication sessions.