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OSI Stack (Open Systems Interconnection)
 

OSI Stack (Open Systems Interconnection)

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Overview of OSI stack and protocol layering. ...

Overview of OSI stack and protocol layering.
OSI (Open System Interconnection) was an industry wide attempt to standardize the mushrooming number of incompatible networking protocol suites in the early days of the Internet. Despite backing from industry heavy-weights, the OSI protocol suite was not accepted by the market and was eventually supplanted by the Internet protocol suite around TCP/IP.
These days OSI is usually referred to as the reference protocol layer model with seven layers (physical, data link, network, transport, session, presentation and application). Typically the upper three layers are coalesced into one application layer.
In modern operating systems, layer 1 through 4 run in the kernel while the upper layers run in user space.

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    OSI Stack (Open Systems Interconnection) OSI Stack (Open Systems Interconnection) Presentation Transcript

    • OSI - Open System Interconnect indigoo.com • Contents 1. Layering model 2. The 7 layers of OSI 3. Where the OSI stack resides in a real system Application Layer Presentation Layer Presentation Layer Session Layer Session Layer Transport Layer Transport Layer Network Layer Network Layer Data Link Layer Data Link Layer Physical Layer © Peter R. Egli 2014 Application Layer Physical Layer 1/7 Rev. 3.60
    • OSI - Open System Interconnect indigoo.com 1. Layering model (1/2) Protocol layers logically communicate with their peers (horizontal communication). Physically data units are passed between protocol layers (vertical communication). Each protocol layer encapsulates an outbound data unit into a protocol specific packet, i.e. adds a protocol specific header. Some protocols also add a trailer that usually contains some kind of checksum. Application Layer APDU = Transport SDU APDU Application Layer TPDU = Network SDU (=segment) Transport Layer Network Layer Data Link Layer APDU Transport Layer Network PDU = Data Link SDU (=packet) NH TH APDU Network Layer TH Data Link PDU (=frame) DH NH TH APDU DT Data Link Layer Bit stream Physical Layer SDU PDU APDU TPDU Physical Layer Service Data Unit (data unit that a specific protocol layer provides as transport service) Protocol Data Unit (protocol header + payload = upper layer’s PDU) Application Protocol Data Unit Transport Protocol Data Unit © Peter R. Egli 2014 2/7 Rev. 3.60
    • indigoo.com OSI - Open System Interconnect 1. Layering model (2/2) Depending on the type of a host either the full stack or only the lower layers are used: Application Layer Application protocol Application Layer Presentation Layer Presentation protocol Presentation Layer Session Layer Session protocol Session Layer Transport Layer Transport protocol Transport Layer Network Layer Network Layer Data Link Layer Data Link Layer Physical Layer Host, gateway © Peter R. Egli 2014 Network Layer Network Layer Data Link Layer Data Link Layer Data Link Layer Physical Layer Physical Layer Physical Layer Physical Layer Router Switch, hub, bridge Router Host, gateway 3/7 Rev. 3.60
    • indigoo.com OSI - Open System Interconnect 2. The 7 layers of OSI (1/2)  OSI (ISO/IEC 7498) was an attempt to combat the diversity of concepts and protocols. The choice for 7 layers has many reasons, among others the fact that IBM‘s SNA also had 7 layers and that there were already 7 working groups at OSI so each group was assigned a layer for definition. OSI did not really catch on, but its basic concept of layering is useful for the taxonomy of protocols (at which layer is a protocol and thus what is its function). The session and presentation layers are almost always tightly coupled with the application protocol and thus may simply be omitted (collapsed with the application layer). OSI stack: Application Layer Presentation Layer Simplified OSI stack: Application Layer Session Layer Transport Layer Transport Layer Network Layer Network Layer Data Link Layer Data Link Layer Physical Layer Physical Layer © Peter R. Egli 2014 4/7 Rev. 3.60
    • OSI - Open System Interconnect indigoo.com 2. The 7 layers of OSI (2/2) Each of the OSI layers performs a specific set of functions. Application Layer Performs application-specific functions like the exchange and execution of application-commands. Examples: HTTP, FTP, SMTP. Presentation Layer Performs conversion from application-specific data formats into a format that can be understood by the remote application. Examples: ASN.1, XML, EBCDIC, ASCII. Session Layer Transport Layer Controls (establish, manage, terminate) the dialogues (connections) between the peers. Examples: RPC. Provides transparent transfer of data units, possibly with some kind of quality of service. Examples: TCP, UDP, SCTP. Network Layer Forwards a packet towards its destination based on a network address. Examples: IP, IPX, AppleTalk DDP, X.25 packet layer. Data Link Layer Defines procedures for media access and framing of bits (start and end marking of bit stream). Examples: Ethernet, WLAN, ATM, Frame Relay. Physical Layer Defines the characteristics of the bit stream on the physical medium (voltages, frequencies, connector pinouts etc.). Examples: RS-232, Ethernet, Bluetooth, SCSI. © Peter R. Egli 2014 5/7 Rev. 3.60
    • OSI - Open System Interconnect indigoo.com 3. Where the OSI stack resides in a real system (1/2) No system really implements protocols according to OSI. In reality the protocols (software modules processing a protocol) are integrated into the operating system (e.g. Windows). ~Layer 5 - 7 ~Layer 3 & 4 ~Layer 1 & 2 Source: www.microsoft.com © Peter R. Egli 2014 6/7 Rev. 3.60
    • indigoo.com OSI - Open System Interconnect 3. Where the OSI stack resides in a real system (2/2) Linux Network Stack Layer 5 - 7 Layer 4 Layer 3 Source: Understanding Linux Network Internals Christian Benvenuti, 2006, O’Reilly Layer 1 & 2 © Peter R. Egli 2014 7/7 Rev. 3.60