OSI Stack (Open Systems Interconnection)

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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)

  1. 1. © Peter R. Egli 2015 1/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com Peter R. Egli INDIGOO.COM OVERVIEW OF PROTOCOL LAYERING AND OSI MODEL OF NETWORK STACKS OSI OPEN SYSTEMS INTERCONNECTION
  2. 2. © Peter R. Egli 2015 2/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com Contents 1. Layering model 2. The 7 layers of OSI 3. Where the OSI stack resides in a real system
  3. 3. © Peter R. Egli 2015 3/8 Rev. 3.70 OSI - Open Systems Interconnection 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. Network Layer Data Link Layer Physical Layer Transport Layer Application Layer Network Layer Data Link Layer Physical Layer Transport Layer Application Layer Network PDU = Data Link SDU (=packet) TPDU = Network SDU (=segment) Data Link PDU (=frame) APDU = Transport SDU SDU Service Data Unit (data unit that a specific protocol layer provides as transport service) PDU Protocol Data Unit (protocol header + payload = upper layer’s PDU) APDU Application Protocol Data Unit TPDU Transport Protocol Data Unit Bit stream APDU APDUTH APDUTHNH DTDH APDUTHNH
  4. 4. © Peter R. Egli 2015 4/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com 1. Layering model (2/2) Depending on the type of a host either the full stack or only the lower layers are used: Network Layer Data Link Layer Physical Layer Transport Layer Session Layer Application Layer Presentation Layer Network Layer Data Link Layer Physical Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Network Layer Data Link Layer Physical Layer Transport Layer Session Layer Application Layer Presentation Layer Transport protocol Session protocol Presentation protocol Application protocol Host, gateway Router Switch, hub, bridge Router Host, gateway
  5. 5. © Peter R. Egli 2015 5/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com Network Layer Data Link Layer Physical Layer Transport Layer Session Layer Application Layer Presentation Layer Network Layer Transport Layer Application Layer 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: Simplified OSI stack: Data Link Layer Physical Layer
  6. 6. © Peter R. Egli 2015 6/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com 2. The 7 layers of OSI (2/2) Each of the OSI layers performs a specific set of functions. Network Layer Data Link Layer Physical Layer Transport Layer Session Layer Application Layer Presentation Layer Performs application-specific functions like the exchange and execution of application-commands. Examples: HTTP, FTP, SMTP. Performs conversion from application-specific data formats into a format that can be understood by the remote application. Examples: ASN.1, XML, EBCDIC, ASCII. 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. Forwards a packet towards its destination based on a network address. Examples: IP, IPX, AppleTalk DDP, X.25 packet layer. Defines procedures for media access and framing of bits (start and end marking of bit stream). Examples: Ethernet, WLAN, ATM, Frame Relay. Defines the characteristics of the bit stream on the physical medium (voltages, frequencies, connector pinouts etc.). Examples: RS-232, Ethernet, Bluetooth, SCSI.
  7. 7. © Peter R. Egli 2015 7/8 Rev. 3.70 OSI - Open Systems Interconnection 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 3 & 4 ~Layer 5 - 7 ~Layer 1 & 2 Source: www.microsoft.com
  8. 8. © Peter R. Egli 2015 8/8 Rev. 3.70 OSI - Open Systems Interconnection indigoo.com 3. Where the OSI stack resides in a real system (2/2) Linux Network Stack Layer 3 Layer 5 - 7 Layer 1 & 2 Layer 4 Source: Understanding Linux Network Internals, Christian Benvenuti, 2006, O’Reilly

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