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OSI model

ContentsArticles

OSI model 1Application Layer 9Presentation Layer 11Session Layer 13Transport Layer 14Network Layer 17Data Link Layer 19Physical Layer 23Internet Protocol Suite 25

ReferencesArticle Sources and Contributors 31Image Sources, Licenses and Contributors 33

Article LicensesLicense 34

OSI model 1

OSI modelThe Open Systems Interconnection model (OSI model) is a product of the Open Systems Interconnection effort atthe International Organization for Standardization. It is a way of sub-dividing a communications system into smallerparts called layers. A layer is a collection of conceptually similar functions that provide services to the layer above itand receives services from the layer below it. On each layer an instance provides services to the instances at the layerabove and requests service from the layer below.For example, a layer that provides error-free communications across a network provides the path needed byapplications above it, while it calls the next lower layer to send and receive packets that make up the contents of thepath. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.

Communication in the OSI-Model (Example with layers 3 to 5)

History

In 1978, work on a layered model ofnetwork architecture was started andthe International Organization forStandardization (ISO) began todevelop its OSI frameworkarchitecture. OSI has two majorcomponents: an abstract model ofnetworking, called the Basic ReferenceModel or seven-layer model, and a setof specific protocols.

Note: The standard documents thatdescribe the OSI model can be freelydownloaded from the ITU-T as theX.200-series of recommendations.[1] A number of the protocol specifications are also available as part of the ITU-TX series. The equivalent ISO and ISO/IEC standards for the OSI model are available from ISO, but only some ofthem at no charge.[2]

The concept of a 7 layer model was provided by the work of Charles Bachman, then of Honeywell. Various aspectsof OSI design evolved from experiences with the ARPANET, the fledgling Internet, NPLNET, EIN, CYCLADESnetwork and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various addenda. In thismodel, a networking system is divided into layers. Within each layer, one or more entities implement itsfunctionality. Each entity interacts directly only with the layer immediately beneath it, and provides facilities for useby the layer above it.Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host.Service definitions abstractly describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one ofthe seven layers of protocols operating in the local host.

OSI model 2

Description of OSI layersAccording to X.200 Recommendation, there are 7 layers, each one is generically called N layer. The N+1 entity askfor transmission services to the N entity.At each level two entities (N-entity) interacts by means of the (N) protocol by transmitting Protocol Data Units(PDU). Service Data Unit (SDU) is a specific unit of data that has been passed down from an OSI layer, to a lowerlayer, and has not yet been encapsulated into a Protocol data Unit (PDU), by the lower layer. It is a set of data that issent by a user of the services of a given layer, and is transmitted semantically unchanged to a peer service user . ThePDU at any given layer, layer 'n', is the SDU of the layer below, layer 'n-1'. In effect the SDU is the 'payload' of agiven PDU. That is, the process of changing a SDU to a PDU, consists of an encapsulation process, performed by thelower layer. All the data contained in the SDU becomes enapsulated within the PDU. The layer n-1 adds headers orfooters, or both, to the SDU, transforming it into a PDU of layer n-1. The added headers or footers are part of theprocess used to make it possible to get data from a source to a destination.

OSI Model

Data unit Layer Function

Hostlayers

Data 7. Application Network process to application

6. Presentation Data representation,encryption and decryption,convert machine dependent data to machine independent data

5. Session Interhost communication

Segments 4. Transport End-to-end connections and reliability,Flow control

Medialayers

Packet 3. Network Path determination and logical addressing

Frame 2. Data Link Physical addressing

Bit 1. Physical Media, signal and binary transmission

Some orthogonal aspects, such as management and security, involve every layer.Security services are not related to a specific layer: they can be related by a number of layers, as defined by ITU-TX.800 Recommendation.[3]

This services are aimed to improve the Cia triad (i.e.Confidentiality, Integrity, Availability) of transmitted data.Actually the availability of communication service is determined by network design and/or network managementprotocols. Appropriate choices for these are needed to protect against denial of service.

Layer 1: Physical LayerThe Physical Layer defines the electrical and physical specifications for devices. In particular, it defines therelationship between a device and a transmission medium, such as a copper or optical cable. This includes the layoutof pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storagearea networks) and more.To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer. Think of thePhysical Layer as concerned primarily with the interaction of a single device with a medium, whereas the Data LinkLayer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium.Standards such as RS-232 do use physical wires to control access to the medium.The major functions and services performed by the Physical Layer are:• Establishment and termination of a connection to a communications medium.• Participation in the process whereby the communication resources are effectively shared among multiple users.

For example, contention resolution and flow control.• Modulation, or conversion between the representation of digital data in user equipment and the corresponding

signals transmitted over a communications channel. These are signals operating over the physical cabling (such as

OSI model 3

copper and optical fiber) or over a radio link.Parallel SCSI buses operate in this layer, although it must be remembered that the logical SCSI protocol is aTransport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer;Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such astoken ring, FDDI, ITU-T G.hn and IEEE 802.11, as well as personal area networks such as Bluetooth and IEEE802.15.4.

Layer 2: Data Link LayerThe Data Link Layer provides the functional and procedural means to transfer data between network entities and todetect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended forpoint-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local areanetwork architecture, which included broadcast-capable multiaccess media, was developed independently of the ISOwork in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use.In modern practice, only error detection, not flow control using sliding window, is present in data link protocols suchas Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for mostprotocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms arerarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such asTCP, but is still used in niches where X.25 offers performance advantages.The ITU-T G.hn standard, which provides high-speed local area networking over existing wires (power lines, phonelines and coaxial cables), includes a complete Data Link Layer which provides both error correction and flow controlby means of a selective repeat Sliding Window Protocol.Both WAN and LAN service arrange bits, from the Physical Layer, into logical sequences called frames. Not allPhysical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layerfunctions. For example, every fifth bit of the FDDI bit stream is not used by the Layer.

WAN Protocol architecture

Connection-oriented WAN data link protocols, in addition to framing, detect and may correct errors. They are alsocapable of controlling the rate of transmission. A WAN Data Link Layer might implement a sliding window flowcontrol and acknowledgment mechanism to provide reliable delivery of frames; that is the case for SDLC andHDLC, and derivatives of HDLC such as LAPB and LAPD.

IEEE 802 LAN architecture

Practical, connectionless LANs began with the pre-IEEE Ethernet specification, which is the ancestor of IEEE 802.3.This layer manages the interaction of devices with a shared medium, which is the function of a Media AccessControl sublayer. Above this MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC)sublayer, which deals with addressing and multiplexing on multiaccess media.While IEEE 802.3 is the dominant wired LAN protocol and IEEE 802.11 the wireless LAN protocol, obsolescentMAC layers include Token Ring and FDDI. The MAC sublayer detects but does not correct errors.

Layer 3: Network LayerThe Network Layer provides the functional and procedural means of transferring variable length data sequencesfrom a source to a destination via one or more networks, while maintaining the quality of service requested by theTransport Layer. The Network Layer performs network routing functions, and might also perform fragmentation andreassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended networkand making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer.The addressing scheme is not hierarchical.

OSI model 4

Careful analysis of the Network Layer indicated that the Network Layer could have at least 3 sublayers:1.Subnetwork Access - that considers protocols that deal with the interface to networks, such as X.25; 2.SubnetworkDependent Convergence - when it is necessary to bring the level of a transit network up to the level of networks oneither side; 3.Subnetwork Independent Convergence - which handles transfer across multiple networks. The bestexample of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at atime, from end system to ingress router, router to router, and from egress router to destination end system. It is notresponsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded.In this scheme, IPv4 and IPv6 would have to be classed with X.25 as Subnet Access protocols because they carryinterface addresses rather than node addresses.A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to theNetwork Layer. These include routing protocols, multicast group management, Network Layer information anderror, and Network Layer address assignment. It is the function of the payload that makes these belong to theNetwork Layer, not the protocol that carries them.

Layer 4: Transport LayerThe Transport Layer provides transparent transfer of data between end users, providing reliable data transfer servicesto the upper layers. The Transport Layer controls the reliability of a given link through flow control,segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means thatthe Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also providesthe acknowledgement of the successful data transmission and if no error free data was transferred then sends the nextdata.Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of theTransport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User DatagramProtocol (UDP).Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0(which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks,similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provideerror-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, whichOSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data andpreservation of record boundaries, both of which TCP is incapable. Detailed characteristics of TP0-4 classes areshown in the following table:[4]

Feature Name TP0 TP1 TP2 TP3 TP4

Connection oriented network Yes Yes Yes Yes Yes

Connectionless network No No No No Yes

Concatenation and separation No Yes Yes Yes Yes

Segmentation and reassembly Yes Yes Yes Yes Yes

Error Recovery No Yes No Yes Yes

Reinitiate connection (if an excessive number of PDUs are unacknowledged) No Yes No Yes No

Multiplexing and demultiplexing over a single virtual circuit No No Yes Yes Yes

Explicit flow control No No Yes Yes Yes

Retransmission on timeout No No No No Yes

Reliable Transport Service No Yes No Yes Yes

OSI model 5

Perhaps an easy way to visualize the Transport Layer is to compare it with a Post Office, which deals with thedispatch and classification of mail and parcels sent. Do remember, however, that a post office manages the outerenvelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentationservices that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the TransportLayer, such as carrying non-IP protocols such as IBM's SNA or Novell's IPX over an IP network, or end-to-endencryption with IPsec. While Generic Routing Encapsulation (GRE) might seem to be a Network Layer protocol, ifthe encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that usesIP headers but contains complete frames or packets to deliver to an endpoint. L2TP carries PPP frames insidetransport packet.

Layer 5: Session LayerThe Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminatesthe connections between the local and remote application. It provides for full-duplex, half-duplex, or simplexoperation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model madethis layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, andalso for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The SessionLayer is commonly implemented explicitly in application environments that use remote procedure calls.

Layer 6: Presentation LayerThe Presentation Layer establishes context between Application Layer entities, in which the higher-layer entitiesmay use different syntax and semantics if the presentation service provides a mapping between them. If a mapping isavailable, presentation service data units are encapsulated into session protocol data units, and passed down thestack.This layer provides independence from data representation (e.g., encryption) by translating between application andnetwork formats. The presentation layer transforms data into the form that the application accepts. This layer formatsand encrypts data to be sent across a network. It is sometimes called the syntax layer.The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), withcapabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects andother data structures from and to XML.

Layer 7: Application LayerThe Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer andthe user interact directly with the software application. This layer interacts with software applications that implementa communicating component. Such application programs fall outside the scope of the OSI model. Application layerfunctions typically include identifying communication partners, determining resource availability, and synchronizingcommunication. When identifying communication partners, the application layer determines the identity andavailability of communication partners for an application with data to transmit. When determining resourceavailability, the application layer must decide whether sufficient network or the requested communication exist. Insynchronizing communication, all communication between applications requires cooperation that is managed by theapplication layer. Some examples of application layer implementations include Hypertext Transfer Protocol (HTTP),File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and X.400 Mail.

OSI model 6

InterfacesNeither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberatelyabstract service specifications. Protocol specifications precisely define the interfaces between different computers,but the software interfaces inside computers are implementation-specific.For example Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Interface,are interfaces between applications (Layer 5 and above) and the transport (Layer 4). NDIS and ODI are interfacesbetween the media (Layer 2) and the network protocol (Layer 3).Interface standards, except for the Physical Layer to media, are approximate implementations of OSI ServiceSpecifications.

Examples

Layer OSI protocols TCP/IPprotocols

SignalingSystem

7[5]

AppleTalk IPX SNA UMTS Misc. examples

# Name

7 Application FTAM, X.400,X.500, DAP,ROSE, RTSE,ACSE

NNTP, SIP,SSI, DNS,FTP, Gopher,HTTP, NFS,NTP, DHCP,SMPP, SMTP,SNMP, Telnet,RIP, BGP

INAP,MAP,TCAP,ISUP,TUP

AFP, ZIP,RTMP,NBP

RIP,SAP

APPC HL7, Modbus

6 Presentation ISO/IEC 8823,X.226,ISO/IEC 9576-1,X.236

MIME, SSL,TLS, XDR

AFP TDI, ASCII,EBCDIC, MIDI,MPEG

5 Session ISO/IEC 8327,X.225,ISO/IEC 9548-1,X.235

Sockets.Sessionestablishmentin TCP, SIP,RTP

ASP,ADSP, PAP

NWLink DLC? Named pipes,NetBIOS, SAP,half duplex, fullduplex, simplex,SDP, RPC

4 Transport ISO/IEC 8073,TP0, TP1, TP2,TP3, TP4 (X.224),ISO/IEC 8602,X.234

TCP, UDP,SCTP, DCCP

DDP,SPX

NBF

3 Network ISO/IEC 8208,X.25 (PLP),ISO/IEC 8878,X.223,ISO/IEC 8473-1,CLNP X.233.

IP, IPsec,ICMP, IGMP,OSPF

SCCP,MTP

ATP(TokenTalkorEtherTalk)

IPX RRC (Radio ResourceControl) Packet DataConvergence Protocol(PDCP) and BMC(Broadcast/MulticastControl)

NBF, Q.931, IS-ISLeaky bucket,token bucket

OSI model 7

2 Data Link ISO/IEC 7666,X.25 (LAPB),Token Bus,X.222,ISO/IEC 8802-2LLC Type 1 and2[6]

PPP, SLIP,PPTP, L2TP

MTP,Q.710

LocalTalk,AppleTalkRemoteAccess, PPP

IEEE802.3framing,EthernetIIframing

SDLC LLC (Logical LinkControl), MAC(Media AccessControl)

802.3 (Ethernet),802.11a/b/g/nMAC/LLC, 802.1Q(VLAN), ATM,HDP, FDDI, FibreChannel, FrameRelay, HDLC, ISL,PPP, Q.921, TokenRing, CDP, ARP(maps layer 3 tolayer 2 address),ITU-T G.hn DLLCRC, Bit stuffing,ARQ, Data OverCable ServiceInterfaceSpecification(DOCSIS)

1 Physical X.25 (X.21bis,EIA/TIA-232,EIA/TIA-449,EIA-530, G.703)[6]

MTP,Q.710

RS-232,RS-422,STP,PhoneNet

Twinax UMTS Physical Layeror L1

RS-232, Fullduplex, RJ45, V.35,V.34, I.430, I.431,T1, E1, 10BASE-T,100BASE-TX,POTS, SONET,SDH, DSL,802.11a/b/g/n PHY,ITU-T G.hn PHY,Controller AreaNetwork, DataOver Cable ServiceInterfaceSpecification(DOCSIS)

Comparison with TCP/IPIn the TCP/IP model of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSImodel.[7] RFC 3439 contains a section entitled "Layering considered harmful." However, TCP/IP does recognizefour broad layers of functionality which are derived from the operating scope of their contained protocols, namelythe scope of the software application, the end-to-end transport connection, the internetworking range, and lastly thescope of the direct links to other nodes on the local network.Even though the concept is different from the OSI model, these layers are nevertheless often compared with the OSIlayering scheme in the following way: The Internet Application Layer includes the OSI Application Layer,Presentation Layer, and most of the Session Layer. Its end-to-end Transport Layer includes the graceful closefunction of the OSI Session Layer as well as the OSI Transport Layer. The internetworking layer (Internet Layer) isa subset of the OSI Network Layer (see above), while the Link Layer includes the OSI Data Link and PhysicalLayers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocolmodel as defined in ISO 7498, rather than refinements in such things as the internal organization of the NetworkLayer document.The presumably strict peer layering of the OSI model as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application

OSI model 8

Layer protocol in its own right.

See also• Cognitive networks• Hierarchical internetworking model• Internet protocol suite• Layer 8• OSI protocol suite• Protocol stack• Service layer• TCP/IP model• X.25 protocol suite• WAP protocol suite

References[1] ITU-T X-Series Recommendations (http:/ / www. itu. int/ rec/ T-REC-X/ en).[2] "Publicly Available Standards" (http:/ / standards. iso. org/ ittf/ PubliclyAvailableStandards/ index. html). Standards.iso.org. 2010-07-30. .

Retrieved 2010-09-11.[3] X.800 : Security architecture for Open Systems Interconnection for CCITT applications (http:/ / www. itu. int/ rec/ T-REC-X. 800-199103-I/

e)[4] "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073" (http:/ / www. itu. int/ rec/ T-REC-X. 224-199511-I/ en/ ). .[5] ITU-T Recommendation Q.1400 (03/1993) (http:/ / www. itu. int/ rec/ T-REC-Q. 1400/ en/ ), Architecture framework for the development of

signaling and OA&M protocols using OSI concepts, pp 4, 7.[6] CISCO Cisco Systems, Inc. Internetworking Technology Handbook OSI Model Physical Layer (http:/ / www. cisco. com/ en/ US/ docs/

internetworking/ technology/ handbook/ Intro-to-Internet. html#wp1020669)[7] RFC 3439

External links• ISO/IEC standard 7498-1:1994 (http:/ / standards. iso. org/ ittf/ PubliclyAvailableStandards/

s020269_ISO_IEC_7498-1_1994(E). zip) (PDF document inside ZIP archive) (requires HTTP cookies in order toaccept licence agreement)

• ITU-T X.200 (the same contents as from ISO) (http:/ / www. itu. int/ rec/ dologin_pub. asp?lang=e&id=T-REC-X. 200-199407-I!!PDF-E& type=items)

• The ISO OSI Reference Model , Beluga graph of data units and groups of layers (http:/ / infchg. appspot. com/usr?at=1263939371)

• OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection (http:/ / www.comsoc. org/ livepubs/ 50_journals/ pdf/ RightsManagement_eid=136833. pdf)PDF (776 KB), HubertZimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425 – 432.

• Internetworking Basics (http:/ / www. cisco. com/ en/ US/ docs/ internetworking/ technology/ handbook/Intro-to-Internet. html)

Application Layer 9

Application LayerApplication Layer is a term used in categorizing protocols and methods in architectural models of computernetworking. Both the OSI model and the Internet Protocol Suite (TCP/IP) define application layers.In TCP/IP, the Application Layer contains all protocols and methods that fall into the realm of process-to-processcommunications via an Internet Protocol (IP) network using the Transport Layer protocols to establish underlyinghost-to-host connections.In the OSI model, the definition of its Application Layer is narrower in scope, explicitly distinguishing additionalfunctionality above the Transport Layer at two additional levels: Session Layer and Presentation Layer. OSIspecifies strict modular separation of functionality at these layers and provides protocol implementations for eachlayer.The common application layer services provide semantic conversion between associated application processes. Note:Examples of common application services of general interest include the virtual file, virtual terminal, and jobtransfer and manipulation protocols.

Protocol examples• 9P, Plan 9 from Bell Labs distributed file system protocol• AFP,• APPC, Advanced Program-to-Program Communication• AMQP, Advanced Message Queuing Protocol• BitTorrent• Atom Publishing Protocol• BOOTP, Bootstrap Protocol• CFDP, Coherent File Distribution Protocol• DDS, Data Distribution Service• DHCP, Dynamic Host Configuration Protocol• DeviceNet• DNS, Domain Name System (Service) Protocol• eDonkey• ENRP, Endpoint Handlespace Redundancy Protocol• FastTrack (KaZaa, Grokster, iMesh)• Finger, User Information Protocol• Freenet• FTAM, File Transfer Access and Management• FTP, File Transfer Protocol• Gopher, Gopher protocol• HL7, Health Level Seven• HTTP, HyperText Transfer Protocol• H.323, Packet-Based Multimedia Communications System• IMAP, IMAP4, Internet Message Access Protocol (version 4)• IRCP, Internet Relay Chat Protocol• Kademlia• LDAP, Lightweight Directory Access Protocol• LPD, Line Printer Daemon Protocol• MIME (S-MIME), Multipurpose Internet Mail Extensions and Secure MIME

Application Layer 10

• Modbus• Netconf• NFS, Network File System• NIS, Network Information Service• NNTP, Network News Transfer Protocol• NTCIP, National Transportation Communications for Intelligent Transportation System Protocol• NTP, Network Time Protocol• OSCAR, AOL Instant Messenger Protocol• PNRP, Peer Name Resolution Protocol• POP, POP3, Post Office Protocol (version 3)• RDP, Remote Desktop Protocol• Rlogin, Remote Login in UNIX Systems• RPC, Remote Procedure Call• RTMP Real Time Messaging Protocol• RTP, Real-time Transport Protocol• RTPS, Real Time Publish Subscribe• RTSP, Real Time Streaming Protocol• SAP, Session Announcement Protocol• SDP, Session Description Protocol• SIP, Session Initiation Protocol• SLP, Service Location Protocol• SMB, Server Message Block• SMTP, Simple Mail Transfer Protocol• SNMP, Simple Network Management Protocol• SNTP, Simple Network Time Protocol• SPTP, Secure Parallel Transfer Protocol• SSH, Secure Shell• SSMS, Secure SMS Messaging Protocol• TCAP, Transaction Capabilities Application Part• TDS, Tabular Data Stream• TELNET, Terminal Emulation Protocol of TCP/IP• TFTP, Trivial File Transfer Protocol• TSP, Time Stamp Protocol• VTP, Virtual Terminal Protocol• Waka, an HTTP replacement protocol• Whois (and RWhois), Remote Directory Access Protocol• WebDAV• X.400, Message Handling Service Protocol• X.500, Directory Access Protocol (DAP)• XMPP, Extensible Messaging and Presence Protocol

Application Layer 11

References

External links• How The Application Layer Works (http:/ / learn-networking. com/ tcp-ip/ how-the-application-layer-works)

(refers to the Internet Protocol Suite (aka "TCP/IP"))

Presentation LayerThe Presentation Layer is Layer 6 of the seven-layer OSI model of computer networking.The Presentation Layer is responsible for the delivery and formatting of information to the application layer forfurther processing or display. It relieves the application layer of concern regarding syntactical differences in datarepresentation within the end-user systems. Note: An example of a presentation service would be the conversion ofan EBCDIC-coded text file to an ASCII-coded file.The Presentation Layer is the lowest layer at which application programmers consider data structure andpresentation, instead of simply sending data in form of datagrams or packets between hosts. This layer deals withissues of string representation - whether they use the Pascal method (an integer length field followed by the specifiedamount of bytes) or the C/C++ method (null-terminated strings, i.e. "thisisastring\0"). The idea is that the applicationlayer should be able to point at the data to be moved, and the Presentation Layer will deal with the rest.Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can bethought of as the key functionality of the Presentation Layer.Encryption is typically done at this level too, although it can be done on the Application, Session, Transport, orNetwork Layers; each having its own advantages and disadvantages. Another example is representing structure,which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, morecomplicated things are standardized in this layer. Two common examples are 'objects' in object-orientedprogramming, and the exact way that streaming video is transmitted.In many widely used applications and protocols, no distinction is made between the presentation and applicationlayers. For example, HTTP, generally regarded as an application layer protocol, has Presentation Layer aspects suchas the ability to identify character encoding for proper conversion, which is then done in the Application Layer.Within the service layering semantics of the OSI network architecture, the Presentation Layer responds to servicerequests from the Application Layer and issues service requests to the Session Layer.

Services• Encryption• Compression

SublayersThe Presentation Layer is composed of two sublayers:• CASE (Common Application Service Element)• SASE (Specific Application Service Element)

Presentation Layer 12

CASEThe CASE sublayer provides services for the Application Layer and request services from the Session Layer. Itprovides support for common application services, such as:• ACSE (Association Control Service Element)• ROSE (Remote Operation Service Element)• CCR (Commitment Concurrency and Recovery)• RTSE (Reliable Transfer Service Element)

SASEThe SASE sublayer provides application specific services (protocols), such as• FTAM (File Transfer, Access and Manager)• VT (Virtual Terminal)• MOTIS (Message Oriented Text Interchange Standard)• CMIP (Common Management Information Protocol)• JTM (Job Transfer and Manipulation) a former OSI standard [1]

• MMS (Manufacturing Messaging Service)• RDA (Remote Database Access)• DTP (Distributed Transaction Processing)• Tel Net(a remote terminal access protocol)

Protocols• AFP, Apple Filing Protocol• ASCII, American Standard Code for Information Interchange• EBCDIC, Extended Binary Coded Decimal Interchange Code• ICA, Independent Computing Architecture, the Citrix system core protocol• LPP, Lightweight Presentation Protocol• NCP, NetWare Core Protocol• NDR, Network Data Representation• XDR, eXternal Data Representation• X.25 PAD, Packet Assembler/Disassembler Protocol

References[1] http:/ / www. furniss. co. uk/ jtm/ index. html

Session Layer 13

Session LayerThe Session Layer is Layer 5 of the seven-layer OSI model of computer networking.The Session Layer provides the mechanism for opening, closing and managing a session between end-userapplication processes, i.e. a semi-permanent dialogue. Communication sessions consist of requests and responsesthat occur between applications. Session Layer services are commonly used in application environments that makeuse of remote procedure calls (RPCs).An example of a Session Layer protocol is the OSI protocol suite Session Layer Protocol, also known as X.225 orISO 8327. In case of a connection loss this protocol may try to recover the connection. If a connection is not used fora long period, the Session Layer Protocol may close it and re-open it. It provides for either full duplex or half-duplexoperation and provides synchronization points in the stream of exchanged messages.[1]

Other examples of Session Layer implementations include Zone Information Protocol (ZIP) – the AppleTalkprotocol that coordinates the name binding process, and Session Control Protocol (SCP) – the DECnet Phase IVSession Layer protocol.Within the service layering semantics of the OSI network architecture, the Session Layer responds to servicerequests from the Presentation Layer and issues service requests to the Transport Layer.

Services• Authentication• Permissions• Session restoration (checkpointing and recovery)The Session Layer of the OSI model is responsible for session checkpointing and recovery. It allows information ofdifferent streams, perhaps originating from different sources, to be properly combined or synchronized.An example application is web conferencing, in which the streams of audio and video must be synchronous to avoidso-called lip synch problems. Floor control ensures that the person displayed on screen is the current speaker.Another application is in live TV programs, where streams of audio and video need to be seamlessly merged andtransitioned from one to the other to avoid silent airtime or excessive overlap.

Protocols• ADSP, AppleTalk Data Stream Protocol• ASP, AppleTalk Session Protocol• H.245, Call Control Protocol for Multimedia Communication• ISO-SP, OSI Session Layer Protocol (X.225, ISO 8327)• iSNS, Internet Storage Name Service• L2F, Layer 2 Forwarding Protocol• L2TP, Layer 2 Tunneling Protocol• NetBIOS, Network Basic Input Output System• PAP, Password Authentication Protocol• PPTP, Point-to-Point Tunneling Protocol• RPC, Remote Procedure Call Protocol• RTCP, Real-time Transport Control Protocol• SMPP, Short Message Peer-to-Peer• SCP, Secure Copy Protocol• SSH, Secure Shell

Session Layer 14

• ZIP, Zone Information Protocol• SDP, Sockets Direct Protocol

Comparison with TCP/IP modelThe TCP/IP reference model does not concern itself with the OSI model's details of application or transport protocolsemantics and therefore does not consider a Session Layer. OSI's session management in connection with the typicaltransport protocols (TCP, SCTP), is contained in the Transport Layer protocols, or otherwise considered the realm ofthe Application Layer protocols. TCP/IP's layers are descriptions of operating scopes (application, host-to-host,network, link) and not detailed prescriptions of operating procedures or data semantics.

See also• Session (computer science)

References[1] ITU-T Recommendation X.225 (http:/ / www. itu. int/ rec/ T-REC-X. 225/ en/ )

Transport LayerIn computer networking, the Transport Layer provides end-to-end communication services for applications[1]

within a layered architecture of network components and protocols. The transport layer provides convenient servicessuch as connection-oriented data stream support, reliability, flow control, and multiplexing.Transport layers are contained in both the TCP/IP model (RFC 1122),[2] which is the foundation of the Internet, andthe Open Systems Interconnection (OSI) model of general networking. The definitions of the Transport Layer areslightly different in these two models. This article primarily refers to the TCP/IP model, in which TCP is largely fora convenient application programming interface to internet hosts, as opposed to the OSI model definition of theTransport Layer.The most well-known transport protocol is the Transmission Control Protocol (TCP). It lent its name to the title ofthe entire Internet Protocol Suite, TCP/IP. It is used for connection-oriented transmissions, whereas theconnectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the morecomplex protocol, due to its stateful design incorporating reliable transmission and data stream services. Otherprominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream ControlTransmission Protocol (SCTP).

ServicesThere are many services that can be optionally provided by a Transport Layer protocol, and different protocols mayor may not implement them.• Connection-oriented communication: Interpreting the connection as a data stream can provide many benefits to

applications. It is normally easier to deal with than the underlying connection-less models, such as theTransmission Control Protocol's underlying Internet Protocol model of datagrams.

• Byte orientation: Rather than processing the messages in the underlying communication system format, it is ofteneasier for an application to process the data stream as a sequence of bytes. This simplification helps applicationswork with various underlying message formats.

• Same order delivery: The Network layer doesn't generally guarantee that packets of data will arrive in the same order that they were sent, but often this is a desirable feature. This is usually done through the use of segment

Transport Layer 15

numbering, with the receiver passing them to the application in order. This can cause head-of-line blocking.• Reliability: Packets may be lost during transport due to network congestion and errors. By means of an error

detection code, such as a checksum, the transport protocol may check that the data is not corrupted, and verifycorrect receipt by sending an ACK or NACK message to the sender. Automatic repeat request schemes may beused to retransmit lost or corrupted data.

• Flow control: The rate of data transmission between two nodes must sometimes be managed to prevent a fastsender from transmitting more data than can be supported by the receiving data buffer, causing a buffer overrun.This can also be used to improve efficiency by reducing buffer underrun.

• Congestion avoidance: Congestion control can control traffic entry into a telecommunications network, so as toavoid congestive collapse by attempting to avoid oversubscription of any of the processing or link capabilities ofthe intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sendingpackets. For example, automatic repeat requests may keep the network in a congested state; this situation can beavoided by adding congestion avoidance to the flow control, including slow-start. This keeps the bandwidthconsumption at a low level in the beginning of the transmission, or after packet retransmission.

• Multiplexing: Ports can provide multiple endpoints on a single node. For example, the name on a postal address isa kind of multiplexing, and distinguishes between different recipients of the same location. Computer applicationswill each listen for information on their own ports, which enables the use of more than one network service at thesame time. It is part of the Transport Layer in the TCP/IP model, but of the Session Layer in the OSI model.

AnalysisThe Transport Layer is responsible for delivering data to the appropriate application process on the host computers.This involves statistical multiplexing of data from different application processes, i.e. forming data packets, andadding source and destination port numbers in the header of each Transport Layer data packet. Together with thesource and destination IP address, the port numbers constitutes a network socket, i.e. an identification address of theprocess-to-process communication. In the OSI model, this function is supported by the Session Layer.Some Transport Layer protocols, for example TCP, but not UDP, support virtual circuits, i.e. provide connectionoriented communication over an underlying packet oriented datagram network. A byte-stream is delivered whilehiding the packet mode communication for the application processes. This involves connection establishment,dividing of the data stream into packets called segments, segment numbering and reordering of out-of order data.Finally, some Transport Layer protocols, for example TCP, but not UDP, provide end-to-end reliablecommunication, i.e. error recovery by means of error detecting code and automatic repeat request (ARQ) protocol.The ARQ protocol also provides flow control, which may be combined with congestion avoidance.UDP is a very simple protocol, and does not provide virtual circuits, nor reliable communication, delegating thesefunctions to the application program. UDP packets are called datagrams, rather than segments.TCP is used for many protocols, including HTTP web browsing and email transfer. UDP may be used formulticasting and broadcasting, since retransmissions are not possible to a large amount of hosts. UDP typically giveshigher throughput and shorter latency, and is therefore often used for real-time multimedia communication wherepacket loss occasionally can be accepted, for example IP-TV and IP-telephony, and for online computer games.In many non-IP-based networks, for example X.25, Frame Relay and ATM, the connection oriented communicationis implemented at network layer or data link layer rather than the Transport Layer. In X.25, in telephone networkmodems and in wireless communication systems, reliable node-to-node communication is implemented at lowerprotocol layers.The OSI model defines five classes of transport protocols: TP0, providing the least error recovery, to TP4, which isdesigned for less reliable networks.

Transport Layer 16

ProtocolsThe exact definition of what qualifies as a transport layer protocol is not firm. The following is a short list:• ATP, AppleTalk Transaction Protocol• CUDP, Cyclic UDP• DCCP, Datagram Congestion Control Protocol• FCP, Fiber Channel Protocol• IL, IL Protocol• NBF, NetBIOS Frames protocol• SCTP, Stream Control Transmission Protocol• SPX, Sequenced Packet Exchange• SST, Structured Stream Transport• TCP, Transmission Control Protocol• UDP, User Datagram Protocol• UDP Lite• µTP, Micro Transport Protocol

Comparison of TCP/IP transport protocols

Service / Attribute UDP TCP DCCP SCTP

Packet header size 8 Bytes 20-60Bytes

12 or 16bytes

12 Bytes + Variable ChunkHeader

Transport Layer packet entity Datagram Segment Datagram Datagram

Port numbering Yes Yes Yes Yes

Error detection Optional Yes Yes Yes

Reliability: Error recovery by automatic repeat request (ARQ) No Yes No Yes

Virtual circuits: Sequence numbering and reordering No Yes Yes Optional

Flow control No Yes No Yes

Congestion avoidance: Variable congestion window, slow start, timeouts

No Yes Yes Yes

Multiple streams No No No Yes

ECN support No Yes Yes Yes

NAT friendly Yes Yes Yes No

Comparison of OSI transport protocolsThe OSI model defines five classes of connection-mode transport protocols designated class 0 (TP0) to class 4(TP4). Class 0 contains no error recovery, and was designed for use on network layers that provide error-freeconnections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSIassigns to the Session Layer. All OSI connection-mode protocol classes provide expedited data and preservation ofrecord boundaries. Detailed characteristics of the classes are shown in the following table:[3]

Transport Layer 17

Service TP0 TP1 TP2 TP3 TP4

Connection oriented network Yes Yes Yes Yes Yes

Connectionless network No No No No Yes

Concatenation and separation No Yes Yes Yes Yes

Segmentation and reassembly Yes Yes Yes Yes Yes

Error Recovery No Yes No Yes Yes

Reinitiate connection (if an excessive number of PDUs are unacknowledged) No Yes No Yes No

multiplexing and demultiplexing over a single virtual circuit No No Yes Yes Yes

Explicit flow control No No Yes Yes Yes

Retransmission on timeout No No No No Yes

Reliable Transport Service No Yes No Yes Yes

References[1] RFC 1122, §1.1.3. "The transport layer provides end-to-end communication services for applications."[2] RFC 1122, Requirements for Internet Hosts -- Communication Layers, IETF, R. Braden (Editor), October 1989[3] "ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073" (http:/ / www. itu. int/ rec/ T-REC-X. 224-199511-I/ en/ ). .

Network LayerThe Network Layer is Layer 3 of the seven-layer OSI model of computer networking.The Network Layer is responsible for routing packets delivery including routing through intermediate routers,whereas the Data Link Layer is responsible for Media Access Control, Flow Control and Error Checking.The Network Layer provides the functional and procedural means of transferring variable length data sequencesfrom a source to a destination host via one or more networks while maintaining the quality of service functions.Functions of the Network Layer include:• Connection model: connectionless communication

For example, IP is connectionless, in that a frame can travel from a sender to a recipient without the recipienthaving to send an acknowledgement. Connection-oriented protocols exist higher at other layers of that model.

• Host addressingEvery host in the network needs to have a unique address which determines where it is. This address willnormally be assigned from a hierarchical system, so you can be "Fred Murphy" to people in your house, "FredMurphy, Main Street 1" to Dubliners, or "Fred Murphy, Main Street 1, Dublin" to people in Ireland, or "FredMurphy, Main Street 1, Dublin, Ireland" to people anywhere in the world. On the Internet, addresses areknown as Internet Protocol (IP) addresses.

• Message forwardingSince many networks are partitioned into subnetworks and connect to other networks for wide-areacommunications, networks use specialized hosts, called gateways or routers to forward packets betweennetworks. This is also of interest to mobile applications, where a user may move from one location to another,and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) was notdesigned with this feature in mind, although mobility extensions exist. IPv6 has a better designed solution.

Network Layer 18

Within the service layering semantics of the OSI network architecture the Network Layer responds to servicerequests from the Transport Layer and issues service requests to the Data Link Layer.

Protocols• IPv4/IPv6, Internet Protocol• DVMRP, Distance Vector Multicast Routing Protocol• ICMP, Internet Control Message Protocol• IGMP, Internet Group Multicast Protocol• PIM-SM, Protocol Independent Multicast Sparse Mode• PIM-DM, Protocol Independent Multicast Dense Mode• IPsec, Internet Protocol Security• IPX, Internetwork Packet Exchange• RIP, Routing Information Protocol• DDP, Datagram Delivery Protocol

Relation to TCP/IP modelThe TCP/IP model describes the protocol suite of the Internet (RFC 1122). This model has a layer called the InternetLayer, located above the Link Layer. In many text books and other secondary references the Internet Layer is oftenequated with OSI's Network Layer. However, this is misleading as the allowed characteristics of protocols (e.g.,whether they are connection-oriented or connection-less) placed into these layer are different in the two models. TheInternet Layer of TCP/IP is in fact only a subset of functionality of the Network Layer. It only describes one type ofnetwork architecture, the Internet.In general, direct or strict comparisons between these models should be avoided, since the layering in TCP/IP is not aprincipal design criterion and the Internet Engineering Task Force (IETF) considers it to be "harmful" (RFC 3439).

See also• Datakit• Router• DECnet• AppleTalk

References• RFC 1122• RFC 3439• Tanenbaum, Andrew S. (2003). Computer networks. Upper Saddle River, New Jersey: Prentice Hall.

ISBN 0-13-066102-3.

Network Layer 19

External links• OSI Reference Model—The ISO Model of Architecture for Open Systems Interconnection [1]PDF (776 KB),

Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp. 425-432.

References[1] http:/ / www. comsoc. org/ livepubs/ 50_journals/ pdf/ RightsManagement_eid=136833. pdf

Data Link LayerThe Data Link Layer is Layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is partof the link layer of the TCP/IP reference model.The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide areanetwork or between nodes on the same local area network segment[1] . The Data Link Layer provides the functionaland procedural means to transfer data between network entities and might provide the means to detect and possiblycorrect errors that may occur in the Physical Layer. Examples of data link protocols are Ethernet for local areanetworks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node)connections.The Data Link Layer is concerned with local delivery of frames between devices on the same LAN. Data Linkframes, as these protocol data units are called, do not cross the boundaries of a local network. Inter-network routingand global addressing are higher layer functions, allowing Data Link protocols to focus on local delivery, addressing,and media arbitration. In this way, the Data Link layer is analogous to a neighborhood traffic cop; it endeavors toarbitrate between parties contending for access to a medium.When devices attempt to use a medium simultaneously, frame collisions occur. Data Link protocols specify howdevices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them.Delivery of frames by layer 2 devices is affected through the use of unambiguous hardware addresses. A frame'sheader contains source and destination addresses that indicate which device originated the frame and which device isexpected to receive and process it. In contrast to the hierarchical and routable addresses of the network layer, layer 2addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to whichthe address belongs.The data link thus provides data transfer across the physical link. That transfer can be reliable or unreliable; manydata link protocols do not have acknowledgments of successful frame reception and acceptance, and some data linkprotocols might not even have any form of checksum to check for transmission errors. In those cases, higher-levelprotocols must provide flow control, error checking, and acknowledgments and retransmission.In some networks, such as IEEE 802 local area networks, the Data Link Layer is described in more detail with MediaAccess Control (MAC) and Logical Link Control (LLC) sublayers; this means that the IEEE 802.2 LLC protocol canbe used with all of the IEEE 802 MAC layers, such as Ethernet, token ring, IEEE 802.11, etc., as well as with somenon-802 MAC layers such as FDDI. Other Data Link Layer protocols, such as HDLC, are specified to include bothsublayers, although some other protocols, such as Cisco HDLC, use HDLC's low-level framing as a MAC layer incombination with a different LLC layer. In the ITU-T G.hn standard, which provides a way to create a high-speed(up to 1 Gigabit/s) Local area network using existing home wiring (power lines, phone lines and coaxial cables), theData Link Layer is divided into three sub-layers (Application Protocol Convergence, Logical Link Control andMedium Access Control).Within the semantics of the OSI network architecture, the Data Link Layer protocols respond to service requestsfrom the Network Layer and they perform their function by issuing service requests to the Physical Layer.

Data Link Layer 20

Models of communication

Sublayers of the Data Link Layer

Logical Link Control sublayerThe uppermost sublayer is Logical Link Control (LLC). This sublayer multiplexes protocols running atop the DataLink Layer, and optionally provides flow control, acknowledgment, and error notification. The LLC providesaddressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over thetransmission medium and for controlling the data exchanged between the originator and recipient machines.

Media Access Control sublayerThe sublayer below it is Media Access Control (MAC). Sometimes this refers to the sublayer that determines who isallowed to access the media at any one time (usually CSMA/CD). Other times it refers to a frame structure withMAC addresses inside.There are generally two forms of media access control: distributed and centralized. Both of these may be comparedto communication between people. In a network made up of people speaking, i.e. a conversation, we look for cluesfrom our fellow talkers to see if any of them appear to be about to speak. If two people speak at the same time, theywill back off and begin a long and elaborate game of saying "no, you first".The Media Access Control sublayer also determines where one frame of data ends and the next one starts -- framesynchronization. There are four means of frame synchronization: time based, character counting, byte stuffing andbit stuffing.• The time based approach simply puts a specified amount of time between frames. The major drawback of this is

that new gaps can be introduced or old gaps can be lost due to external influences.• Character counting simply notes the count of remaining characters in the frame's header. This method, however,

is easily disturbed if this field gets faulty in some way, thus making it hard to keep up synchronization.• Byte stuffing precedes the frame with a special byte sequence such as DLE STX and succeeds it with DLE ETX.

Appearances of DLE (byte value 0x10) has to be escaped with another DLE. The start and stop marks aredetected at the receiver and removed as well as the inserted DLE characters.

• Similarly, bit stuffing replaces these start and end marks with flag consisting of a special bit pattern (e.g. a 0, six 1bits and a 0). Occurrences of this bit pattern in the data to be transmitted is avoided by inserting a bit. To use theexample where the flag is 01111110, a 0 is inserted after 5 consecutive 1's in the data stream. The flags and theinserted 0's are removed at the receiving end. This makes for arbitrary long frames and easy synchronization forthe recipient. Note that this stuffed bit is added even if the following data bit is 0, which could not be mistaken fora sync sequence, so that the receiver can unambiguously distinguish stuffed bits from normal bits.

List of Data Link Layer services• Encapsulation of network layer data packets into frames• Frame synchronization• Logical link control (LLC) sublayer:

• Error control (automatic repeat request,ARQ), in addition to ARQ provided by some Transport layer protocols,to forward error correction (FEC) techniques provided on the Physical Layer, and to error-detection and packetcanceling provided at all layers, including the network layer. Data link layer error control (i.e. retransmissionof erroneous packets) is provided in wireless networks and V.42 telephone network modems, but not in LANprotocols such as Ethernet, since bit errors are so uncommon in short wires. In that case, only error detectionand canceling of erroneous packets are provided.

Data Link Layer 21

• Flow control, in addition to the one provided on the Transport layer. Data link layer error control is not used inLAN protocols such as Ethernet, but in modems and wireless networks.

• Media access control (MAC) sublayer:• Multiple access protocols for channel-access control, for example CSMA/CD protocols for collision detection

and retransmission in Ethernet bus networks and hub networks, or the CSMA/CA protocol for collisionavoidance in wireless networks.

• Physical addressing (MAC addressing)• LAN switching (packet switching) including MAC filtering and spanning tree protocol• Data packet queueing or scheduling• Store-and-forward switching or cut-through switching• Quality of Service (QoS) control• Virtual LANs (VLAN)

Protocol examples• ARCnet• ATM• Cisco Discovery Protocol (CDP)• Controller Area Network (CAN)• Econet• Ethernet• Ethernet Automatic Protection Switching (EAPS)• Fiber Distributed Data Interface (FDDI)• Frame Relay• High-Level Data Link Control (HDLC)• IEEE 802.2 (provides LLC functions to IEEE 802 MAC layers)• IEEE 802.11 wireless LAN• Link Access Procedures, D channel (LAPD)• LocalTalk• Multiprotocol Label Switching (MPLS)• Point-to-Point Protocol (PPP)• Serial Line Internet Protocol (SLIP) (obsolete)• Spanning tree protocol• StarLan• Token ring• Unidirectional Link Detection (UDLD)• and most forms of serial communication.

Data Link Layer 22

InterfacesThe Data Link Layer is often implemented in software as a "network card driver". The operating system will have adefined software interface between the data link and the network transport stack above. This interface is not a layeritself, but rather a definition for interfacing between layers.

Relation to TCP/IP modelIn the frame work of the TCP/IP (Internet Protocol Suite) model, OSI's Data Link Layer, in addition to othercomponents, is contained in TCP/IP's lowest layer, the Link Layer. The Internet Protocol's Link Layer only concernsitself with hardware issues to the point of obtaining hardware addresses for locating hosts on a physical network linkand transmitting data frames onto the link. Thus, the Link Layer is broader in scope and encompasses all methodsthat affect the local link, which is the group of connections that are limited in scope to other nodes on the localaccess network.The TCP/IP model is not a top/down comprehensive design reference for networks. It was formulated for thepurpose of illustrating the logical groups and scopes of functions needed in the design of the suite of internetworkingprotocols of TCP/IP, as needed for the operation of the Internet. In general, direct or strict comparisons of the OSIand TCP/IP models should be avoided, because the layering in TCP/IP is not a principal design criterion and ingeneral considered to be "harmful" (RFC 3439). In particular, TCP/IP does not dictate a strict hierarchical sequenceof encapsulation requirements, as is attributed to OSI protocols.

See also• ODI• NDIS• SANA-II - Standard Amiga Networking Architecture, version 2• S. Tanenbaum, Andrew (2005). Computer Networks (4th Edition ed.). 482,F.I.E., Patparganj, Delhi 110 092:

Dorling Kindersley(India)Pvt. Ltd.,licenses of Pearson Education in South Asia. ISBN 81-7758-165-1.

References[1] "What is Layer 2, and Why Should You Care?" (http:/ / www. accel-networks. com/ blog/ 2009/ 09/

what-is-layer-2-and-why-should-you-care. html). accel-networks.com. . Retrieved 2009-09-29.

Physical Layer 23

Physical LayerThe Physical Layer is the first and lowest layer in the seven-layer OSI model of computer networking. Theimplementation of this layer is often termed PHY.The Physical Layer consists of the basic hardware transmission technologies of a network. It is a fundamental layerunderlying the logical data structures of the higher level functions in a network. Due to the plethora of availablehardware technologies with widely varying characteristics, this is perhaps the most complex layer in the OSIarchitecture.The Physical Layer defines the means of transmitting raw bits rather than logical data packets over a physical linkconnecting network nodes. The bit stream may be grouped into code words or symbols and converted to a physicalsignal that is transmitted over a hardware transmission medium. The Physical Layer provides an electrical,mechanical, and procedural interface to the transmission medium. The shapes and properties of the electricalconnectors, the frequencies to broadcast on, the modulation scheme to use and similar low-level parameters, arespecified here.Within the semantics of the OSI network architecture, the Physical Layer translates logical communications requestsfrom the Data Link Layer into hardware-specific operations to affect transmission or reception of electronic signals.

Physical signaling sublayerIn a local area network (LAN) or a metropolitan area network (MAN) using open systems interconnection (OSI)architecture, the physical signaling sublayer is the portion of the Physical Layer that:• interfaces with the medium access control sublayer (MAC) which is a part of the Data Link Layer• performs character encoding, transmission, reception and decoding.• performs mandatory isolation functions.[1]

List of servicesThe major functions and services performed by the Physical Layer are:• Bit-by-bit or symbol-by-symbol delivery• Providing a standardized interface to physical transmission media, including

• Mechanical specification of electrical connectors and cables, for example maximum cable length• Electrical specification of transmission line signal level and impedance• Radio interface, including electromagnetic spectrum frequency allocation and specification of signal strength,

analog bandwidth, etc.• Specifications for IR over optical fiber or a wireless IR communication link

• Modulation• Line coding• Bit synchronization in synchronous serial communication• Start-stop signalling and flow control in asynchronous serial communication• Circuit switching• Multiplexing

• Establishment and termination of circuit switched connections• Carrier sense and collision detection utilized by some level 2 multiple access protocols• Equalization filtering, training sequences, pulse shaping and other signal processing of physical signals• Forward error correction[2] for example bitwise convolutional coding• Bit-interleaving and other channel coding

Physical Layer 24

The Physical Layer is also concerned with• Bit rate• Point-to-point, multipoint or point-to-multipoint line configuration• Physical network topology, for example bus, ring, mesh or star network• Serial or parallel communication• Simplex, half duplex or full duplex transmission mode• Autonegotiation

List of protocols• Telephone network modems- V.92• IRDA Physical Layer• USB Physical Layer• EIA RS-232, EIA-422, EIA-423, RS-449, RS-485• Ethernet physical layer Including 10BASE-T, 10BASE2, 10BASE5, 100BASE-TX, 100BASE-FX, 100BASE-T,

1000BASE-T, 1000BASE-SX and other varieties• Varieties of 802.11Wi-Fi Physical Layers• DSL• ISDN• T1 and other T-carrier links, and E1 and other E-carrier links• SONET/SDH• Optical Transport Network (OTN)• GSM Um radio interface physical layer• Bluetooth Physical Layer• ITU Recommendations: see ITU-T• Firewire• TransferJet Physical Layer• Etherloop• ARINC 818 Avionics Digital Video Bus• G.hn/G.9960 Physical Layer

Hardware equipment (network node) examples• Network adapter• Repeater• Network hub• Modem• Fiber Media Converter

Relation to TCP/IP modelThe TCP/IP model, defined in RFC 1122 and RFC 1123, is a high-level networking description used for the Internetand similar networks. It does not define an equivalent layer that deals exclusively with hardware-level specificationsand interfaces, as this model does not concern itself directly with physical interfaces. Several RFCs mention aphysical layer and data link layer, but that is in context of IEEE protocols. RFC 1122 and 1123 do not mention anyphysical layer functionality or physical layer standards.

Physical Layer 25

See also• Clock recovery• Ethernet physical layer• Data transmission• Digital communication• Digital modulation• Line code• Pulse shaping• Bit synchronization• Channel model

References[1] This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (http:/ /

www. its. bldrdoc. gov/ fs-1037/ fs-1037c. htm).[2] Bersekas, Dimitri; Gallager, Robert (1992). Data Networks. Prentice Hall. p. 61. ISBN 0-13-200916-1.

External links• http:/ / www. erg. abdn. ac. uk/ users/ gorry/ course/ phy-pages/ phy. html• http:/ / www. tcpipguide. com/ free/ t_PhysicalLayerLayer1. htm

Internet Protocol SuiteThe Internet Protocol Suite is the set of communications protocols used for the Internet and other similar networks.It is commonly also known as TCP/IP, named from two of the most important protocols in it: the TransmissionControl Protocol (TCP) and the Internet Protocol (IP), which were the first two networking protocols defined in thisstandard. Modern IP networking represents a synthesis of several developments that began to evolve in the 1960sand 1970s, namely the Internet and local area networks, which emerged during the 1980s, together with the advent ofthe World Wide Web in the early 1990s.The Internet Protocol Suite, like many protocol suites, is constructed as a set of layers. Each layer solves a set ofproblems involving the transmission of data. In particular, the layers define the operational scope of the protocolswithin.Often a component of a layer provides a well-defined service to the upper layer protocols and may be using servicesfrom the lower layers. Upper layers are logically closer to the user and deal with more abstract data, relying on lowerlayer protocols to translate data into forms that can eventually be physically transmitted.The TCP/IP model consists of four layers (RFC 1122).[1] [2] From lowest to highest, these are the Link Layer, theInternet Layer, the Transport Layer, and the Application Layer.

HistoryThe Internet Protocol Suite resulted from research and development conducted by the Defense Advanced Research Projects Agency (DARPA) in the early 1970s. After initiating the pioneering ARPANET in 1969, DARPA started work on a number of other data transmission technologies. In 1972, Robert E. Kahn joined the DARPA Information Processing Technology Office, where he worked on both satellite packet networks and ground-based radio packet networks, and recognized the value of being able to communicate across both. In the spring of 1973, Vinton Cerf, the developer of the existing ARPANET Network Control Program (NCP) protocol, joined Kahn to work on

Internet Protocol Suite 26

open-architecture interconnection models with the goal of designing the next protocol generation for the ARPANET.By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, where the differences betweennetwork protocols were hidden by using a common internetwork protocol, and, instead of the network beingresponsible for reliability, as in the ARPANET, the hosts became responsible. Cerf credits Hubert Zimmerman andLouis Pouzin, designer of the CYCLADES network, with important influences on this design.The design of the network included the recognition that it should provide only the functions of efficientlytransmitting and routing traffic between end nodes and that all other intelligence should be located at the edge of thenetwork, in the end nodes. Using a simple design, it became possible to connect almost any network to theARPANET, irrespective of their local characteristics, thereby solving Kahn's initial problem. One popular expressionis that TCP/IP, the eventual product of Cerf and Kahn's work, will run over "two tin cans and a string."A computer called a router (a name changed from gateway to avoid confusion with other types of gateways) isprovided with an interface to each network, and forwards packets back and forth between them. Requirements forrouters are defined in (Request for Comments 1812).[3]

The idea was worked out in more detailed form by Cerf's networking research group at Stanford in the 1973–74period, resulting in the first TCP specification.(Request for Comments 675) [4] (The early networking work at XeroxPARC, which produced the PARC Universal Packet protocol suite, much of which existed around the same period oftime, was also a significant technical influence; people moved between the two.)DARPA then contracted with BBN Technologies, Stanford University, and the University College London todevelop operational versions of the protocol on different hardware platforms. Four versions were developed: TCPv1, TCP v2, a split into TCP v3 and IP v3 in the spring of 1978, and then stability with TCP/IP v4 — the standardprotocol still in use on the Internet today.In 1975, a two-network TCP/IP communications test was performed between Stanford and University CollegeLondon (UCL). In November, 1977, a three-network TCP/IP test was conducted between sites in the US, UK, andNorway. Several other TCP/IP prototypes were developed at multiple research centres between 1978 and 1983. Themigration of the ARPANET to TCP/IP was officially completed on January 1, 1983, when the new protocols werepermanently activated.[5]

In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computernetworking.[6] In 1985, the Internet Architecture Board held a three day workshop on TCP/IP for the computerindustry, attended by 250 vendor representatives, promoting the protocol and leading to its increasing commercialuse.

Layers in the Internet Protocol Suite

The concept of layersThe TCP/IP suite uses encapsulation to provide abstraction of protocols and services. Such encapsulation usually isaligned with the division of the protocol suite into layers of general functionality. In general, an application (thehighest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at eachlevel.This may be illustrated by an example network scenario, in which two Internet host computers communicate acrosslocal network boundaries constituted by their internetworking gateways (routers).

Internet Protocol Suite 27

The functional groups of protocols and methods are the Application Layer, the Transport Layer, the Internet Layer,and the Link Layer (RFC 1122). This model was not intended to be a rigid reference model into which new protocolshave to fit in order to be accepted as a standard.The following table provides some examples of the protocols grouped in their respective layers.

Application DNS, TFTP, TLS/SSL, FTP, Gopher, HTTP, IMAP, IRC, NNTP, POP3, SIP, SMTP, SMPP, SNMP, SSH, Telnet, Echo, RTP,PNRP, rlogin, ENRP

Routing protocols like BGP and RIP which run over TCP/UDP, may also be considered part of the Internet Layer.

Transport TCP, UDP, DCCP, SCTP, IL, RUDP, RSVP

Internet IP (IPv4, IPv6), ICMP, IGMP, and ICMPv6

OSPF for IPv4 was initially considered IP layer protocol since it runs per IP-subnet, but has been placed on the Link since RFC 2740.

Link ARP, RARP, OSPF (IPv4/IPv6), IS-IS, NDP

Layer names and number of layers in the literatureThe following table shows the layer names and the number of layers of networking models presented in RFCs andtextbooks in widespread use in today's university computer networking courses.

RFC 1122 [7] Tanenbaum Cisco

Academy[8]

Kurose[9]

Forouzan[10]

Comer[11]

Kozierok[12]

Stallings[13] Arpanet Reference

Model 1982 (RFC 871)

Four layers[14]

Four layers [15] Four layers Five layers Four+one layers Five layers Three layers

"Internetmodel"

"TCP/IP referencemodel"[16]

"Internetmodel"

"Five-layer Internetmodel" or "TCP/IPprotocol suite"

"TCP/IP 5-layerreference model"

"TCP/IP model" "Arpanet reference model"

Application[14] [17]

Application Application Application Application Application Application/Process

Transport [14] Transport Transport Transport Transport Host-to-host ortransport

Host-to-host

Internet [14] Internet Internetwork Network Internet Internet

Link [14] Host-to-network Networkinterface

Data link Data link (Networkinterface)

Network access Network interface

Physical (Hardware) Physical

Internet Protocol Suite 28

These textbooks are secondary sources that may contravene the intent of RFC 1122 and other IETF primarysources.[18]

Different authors have interpreted the RFCs differently regarding the question whether the Link Layer (and theTCP/IP model) covers Physical Layer issues, or if a hardware layer is assumed below the Link Layer. Some authorshave tried to use other names for the Link Layer, such as network interface layer, in view to avoid confusion with theData Link Layer of the seven layer OSI model. Others have attempted to map the Internet Protocol model onto theOSI Model. The mapping often results in a model with five layers where the Link Layer is split into a Data LinkLayer on top of a Physical Layer. In literature with a bottom-up approach to Internet communication,[10] [11] [13] inwhich hardware issues are emphasized, those are often discussed in terms of Physical Layer and Data Link Layer.The Internet Layer is usually directly mapped into the OSI Model's Network Layer, a more general concept ofnetwork functionality. The Transport Layer of the TCP/IP model, sometimes also described as the host-to-host layer,is mapped to OSI Layer 4 (Transport Layer), sometimes also including aspects of OSI Layer 5 (Session Layer)functionality. OSI's Application Layer, Presentation Layer, and the remaining functionality of the Session Layer arecollapsed into TCP/IP's Application Layer. The argument is that these OSI layers do usually not exist as separateprocesses and protocols in Internet applications.However, the Internet protocol stack has never been altered by the Internet Engineering Task Force from the fourlayers defined in RFC 1122. The IETF makes no effort to follow the OSI model although RFCs sometimes refer toit. The IETF has repeatedly stated that Internet protocol and architecture development is not intended to beOSI-compliant.RFC 3439, addressing Internet architecture, contains a section entitled: "Layering Considered Harmful".[18]

ImplementationsMost computer operating systems in use today, including all consumer-targeted systems, include a TCP/IPimplementation.Minimally acceptable implementation includes implementation for (from most essential to the less essential) IP,ARP, ICMP, UDP, TCP and sometime IGMP. It is in principle possible to support only one of transport protocols(i.e. simple UDP), but it is rarely done, as it limits usage of the whole implementation. IPv6, beyond own version ofARP (NBP), and ICMP (ICMPv6), and IGMP (IGMPv6) have some additional required functionalities, and often isaccompanied with integrated IPSec security layer. Other protocols could be easily added later (often they can beimplemented entirely in the userspace), for example DNS for resolving domain names to IP addresses or DHCPclient for automatic configuration of network interfaces.Most of the IP implementations are accessible to the programmers using socket abstraction (usable also with otherprotocols) and proper API for most of the operations. This interface is known as BSD sockets and was used initiallyin C.Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems andKA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connectedvia serial lines.

Internet Protocol Suite 29

See also• List of network protocols• List of automation protocols• List of TCP and UDP port numbers• FLIP (Fast-Local-Internet-Protocol) another stack

References[1] RFC 1122, Requirements for Internet Hosts -- Communication Layers, R. Braden (ed.), October 1989[2] RFC 1123, Requirements for Internet Hosts -- Application and Support, R. Braden (ed.), October 1989[3] Baker, Fred, ed (June 1995). Requirements for IP Version 4 Routers (http:/ / tools. ietf. org/ html/ rfc1812). Internet Engineering Task Force,

Network Working Group. Request for Comments 1812. . Retrieved 2009-09-22[4] V.Cerf et al. (December 1974). "Specification of Internet Transmission Control Protocol" (http:/ / www. ietf. org/ rfc/ rfc0675. txt). .[5] Internet History (http:/ / www. livinginternet. com/ i/ ii. htm)[6] Ronda Hauben. "From the ARPANET to the Internet" (http:/ / www. columbia. edu/ ~rh120/ other/ tcpdigest_paper. txt). TCP Digest

(UUCP). . Retrieved 2007-07-05.[7] IETF, RFC 1122, p.7, "To communicate using the Internet system, a host must implement the layered set of protocols comprising the Internet

protocol suite. A host typically must implement at least one protocol from each layer."[8] Mark Dye, Mark A. Dye, Wendell, Network Fundamentals: CCNA Exploration Companion Guide, 2007, ISBN 1-58713-208-7[9] James F. Kurose, Keith W. Ross, Computer Networking: A Top-Down Approach, 2008, ISBN 0-321-49770-8 (http:/ / www.

pearsonhighered. com/ educator/ academic/ product/ 0,,0321497708,00+ en-USS_01DBC. html)[10] Behrouz A. Forouzan, Data Communications and Networking (http:/ / books. google. com/ books?id=U3Gcf65Pu9IC&

printsec=frontcover& dq=forouzan+ "computer+ networks"& ei=RPZ9SOCvMofctAO02di0AQ& hl=en&sig=ACfU3U2Hh_n83pPtf5uCreCih0HnWvNcxg#PPA29,M1)

[11] Douglas E. Comer, Internetworking with TCP/IP: Principles, Protocols and Architecture, Pearson Prentice Hall 2005, ISBN 0-13-187671-6(http:/ / books. google. com/ books?id=jonyuTASbWAC& pg=PA155& hl=sv& source=gbs_toc_r& cad=0_0&sig=ACfU3U18gHAia1pU_Pxn-rhkCnH1v70M6Q#PPA161,M1)

[12] Charles M. Kozierok, "The TCP/IP Guide", No Starch Press 2005 (http:/ / books. google. com/ books?id=Pm4RgYV2w4YC& pg=PA131&dq="TCP/ IP+ model+ layers"& lr=& hl=sv& sig=ACfU3U3ofMwYAbZfGz1BmAXc2oNNFC2b8A#PPA129,M1)

[13] William Stallings, Data and Computer Communications, Prentice Hall 2006, ISBN 0-13-243310-9 (http:/ / books. google. com/books?id=c_AWmhkovR0C& pg=PA35& dq="internet+ layer"+ "network+ access+ layer"& ei=-O99SI3EJo32sgOQpPThDw& hl=en&sig=ACfU3U38aXznzeAnQdbLcPFXfCgxAd4lFg)

[14] IETF, RFC 1122, p.7-8, "The protocol layers [...] are as follows [...]:[...] Application Layer [...] Transport Layer [...] Internet Layer [...] LinkLayer"

[15] Andrew Tanenbaum, Computer Networks, section 1.4.3, "[...] the OSI model has seven layers and the TCP/IP has four layers."[16] Tanenbaum, Andrew S. (2002). Computer Networks. Prentice Hall. p. 41. ISBN 0130661023. "1.4.2 The TCP/IP Reference Model" Excerpt

at Google Books (http:/ / books. google. com/ books?id=Pd-z64SJRBAC& pg=PA42& vq=internet+ layer& dq=networks& hl=sv&source=gbs_search_s& sig=ACfU3U3DHANeIz0sOsd5NK4VXSrgNFYVAw#PPA42,M1)

[17] IETF, RFC 1122, p.8, "The application layer is the top layer of the Internet protocol suite."[18] R. Bush; D. Meyer (December 2002). Some Internet Architectural Guidelines and Philosophy (http:/ / www. isi. edu/ in-notes/ rfc3439. txt).

Internet Engineering Task Force. . Retrieved 2007-11-20

Further reading• Douglas E. Comer. Internetworking with TCP/IP - Principles, Protocols and Architecture. ISBN 86-7991-142-9• Joseph G. Davies and Thomas F. Lee. Microsoft Windows Server 2003 TCP/IP Protocols and Services. ISBN

0-7356-1291-9• Forouzan, Behrouz A. (2003). TCP/IP Protocol Suite (2nd ed.). McGraw-Hill. ISBN 0-07-246060-1.• Craig Hunt TCP/IP Network Administration. O'Reilly (1998) ISBN 1-56592-322-7• Maufer, Thomas A. (1999). IP Fundamentals. Prentice Hall. ISBN 0-13-975483-0.• Ian McLean. Windows(R) 2000 TCP/IP Black Book. ISBN 1-57610-687-X• Ajit Mungale Pro .NET 1.1 Network Programming. ISBN 1-59059-345-6• W. Richard Stevens. TCP/IP Illustrated, Volume 1: The Protocols. ISBN 0-201-63346-9

Internet Protocol Suite 30

• W. Richard Stevens and Gary R. Wright. TCP/IP Illustrated, Volume 2: The Implementation. ISBN0-201-63354-X

• W. Richard Stevens. TCP/IP Illustrated, Volume 3: TCP for Transactions, HTTP, NNTP, and the UNIX DomainProtocols. ISBN 0-201-63495-3

• Andrew S. Tanenbaum. Computer Networks. ISBN 0-13-066102-3• David D. Clark, "The Design Philosophy of the DARPA Internet Protocols" (http:/ / groups. csail. mit. edu/ ana/

Publications/ PubPDFs/ The design philosophy of the DARPA internet protocols. pdf), ComputerCommunications Review 18:4, August 1988, pp. 106–114

External links• Internet History (http:/ / www. livinginternet. com/ i/ ii. htm) -- Pages on Robert Kahn, Vinton Cerf, and TCP/IP

(reviewed by Cerf and Kahn).• RFC 675 (http:/ / www. ietf. org/ rfc/ rfc0675. txt) - Specification of Internet Transmission Control Program,

December 1974 Version• TCP/IP State Transition Diagram (http:/ / www. night-ray. com/ TCPIP_State_Transition_Diagram. pdf) (PDF)• RFC 1180 A TCP/IP Tutorial - from the Internet Engineering Task Force (January 1991)• TCP/IP FAQ (http:/ / www. itprc. com/ tcpipfaq/ )• TCP/IP Resources List (http:/ / www. private. org. il/ tcpip_rl. html)• The TCP/IP Guide (http:/ / www. tcpipguide. com/ free/ ) - A comprehensive look at the protocols and the

procedures/processes involved• A Study of the ARPANET TCP/IP Digest (http:/ / www. columbia. edu/ ~rh120/ other/ tcpdigest_paper. txt)• TCP/IP Sequence Diagrams (http:/ / www. eventhelix. com/ RealtimeMantra/ Networking/ )• The Internet in Practice (http:/ / www. searchandgo. com/ articles/ internet/ internet-practice-4. php)• TCP/IP - Directory & Informational Resource (http:/ / softtechinfo. com/ network/ tcpip. html)• Daryl's TCP/IP Primer (http:/ / www. ipprimer. com) - Intro to TCP/IP LAN administration, conversational style• Introduction to TCP/IP (http:/ / www. linux-tutorial. info/ MContent-142)• TCP/IP commands from command prompt (http:/ / blog. webgk. com/ 2007/ 10/

dns-tcpip-commands-from-command-prompt. html)• cIPS (http:/ / sourceforge. net/ projects/ cipsuite/ ) — Robust TCP/IP stack for embedded devices without an

Operating System• TCP Server Client Program using Python (http:/ / pal-subbiah. spaces. live. com/ blog/ )

Article Sources and Contributors 31

Article Sources and ContributorsOSI model  Source: http://en.wikipedia.org/w/index.php?oldid=389772875  Contributors: 0612, 0x6D667061, 1337 JN, 24.12.199.xxx, 63.227.96.xxx, 7, 75th Trombone, 802geek, @modi,ABF, AK Auto, Abarry, Abune, Adamantios, Addshore, Adibob, Adityagaur 7, Adoniscik, Adrianwn, Advancedtelcotv, Ageekgal, Ahoerstemeier, Aitias, Ajo Mama, Ajw901, Alansohn,Albanaco, Aldie, Ale jrb, AlistairMcMillan, Alphachimp, Alucard 16, Alvestrand, Amillar, Amitbhatia76, Amtanoli, Andre Engels, Andybryant, Animum, Anjola, Anna Lincoln, Anonymousanonymous, Another-anomaly, Apocryphite, Apparition11, Arroww, Artur Perwenis, Arunachalammanohar, Ashutosh.mcse, Aslambasha09, Asn1tlv, AtomicDragon, Atreyu42, Audunv,AxelBoldt, Ayengar, B4hand, BACbKA, BDerrly, Bakilas, Balajia82, Bariswheel, Bdamokos, Beelaj, BenLiyanage, Beno1000, Biblbroks, Bjelleklang, Bletch, Blueskies238, Bmylez, Bobo192,Bogdangiusca, Boikej, Bojer, BommelDing, Bonobosarenicer, Booyabazooka, Borgx, Brambleclawx, Brandon, Brick Thrower, Brougham96, Bryan Derksen, BuickCenturyDriver, Bzimage.it,Bücherwürmlein, CDima, CIreland, CMBJ, Caerwine, Caesura, Caltas, CambridgeBayWeather, Camw, Can You Prove That You're Human, Can't sleep, clown will eat me, CanadianLinuxUser,Candc4, Caper13, Carre, Casey Abell, Causa sui, Cburnett, Cbustapeck, Cflm001, Charles Edward, Charm, Che090572, Chfalcao, Chimpex, ChiragPatnaik, Chrislk02, Chupon, Citicat,Closedmouth, Cokoli, Cometstyles, Conquest ace, Conversion script, Coriron, Courcelles, Cputrdoc, CraSH, CraigBox, Crasheral, Cs mat3, Ctbolt, Cxxl, CyborgTosser, Cyktsui, CynicalMe,DARTH SIDIOUS 2, DJPohly, DSParillo, Damian Yerrick, Daniel, Danlev, Dave2, David Edgar, David Gerard, David0811, DavidBarak, DavidLevinson, Davidjk, Dcooper, Dcovell, DeagleAP, Delfeye, Delldot, Demitsu, DennyColt, Dgtsyb, Dili, Dino.korah, Discospinster, Dispenser, Djchainz, Djib, Djmoa, DmitryKo, Doniago, Dpark, DrSpice, Drat, Dreish, Drwarpmind,Duey111, Dumbledad, Dzubint, EJDyksen, EJSawyer, ENeville, EagleOne, Eazy007, Ecw.technoid.dweeb, Ed g2s, EdH, Edivorce, Edward, ElKevbo, Eldiablo, Eleassar, Elfosardo, Eliezerb,Elipongo, Emperorbma, Enjoi4586, Enochlau, Epbr123, Eric Soyke, Everyking, Evillawngnome, Ewlyahoocom, Excirial, FF2010, Fang Aili, Feezo, Fiable.biz, Filemon, Finlay McWalter,Fjpanna, Fleg31789, Flewis, Flowanda, FrankTobia, Fred Bradstadt, Fredrik, Free Bear, FreshPrinz, Fresheneesz, Friday, Friedo, Friginator, Fullstop, Fumitol, Fuzheado, Fvw, Fæ, GDonato,Gadfium, Gafex, GarethGilson, Gary King, Gasp01, Gazpacho, Ghostalker, Giftlite, Gilliam, GlassCobra, Glenn, Goodnightmush, Graeme Bartlett, Graham.rellinger, Grendelkhan, Grubber, Gsl,Gurchzilla, Guy Harris, Gwernol, Gökhan, H2g2bob, H34d, Haakon, Hadal, HamatoKameko, HarisM, Hatch68, Hcberkowitz, Hdante, Helix84, Hellomarius, Henrikholm, Herbee, Heron, HesNikke, Hetar, HexaChord, Hgerstung, Hiddekel, Highpriority, Honeyman, IMSoP, IReceivedDeathThreats, Iambk, Ideoplex, Ifroggie, Ilario, Immunize, Inkhorn, Inkling, Insineratehymn, Intgr,Inversetime, InvisibleK, Iridescent, IronGargoyle, Ishikawa Minoru, Isofox, Isthisthingworking, Itpastorn, Itusg15q4user, Iviney, J.delanoy, JMatthews, JV Smithy, Jake Wartenberg, Jannetta,Jauerback, Jchristn, Jcw69, Jdrrmk, JeTataMe, Jeff G., Jeffrey Mall, Jetekus, Jhilving, JidGom, Jmorgan, Jnc, JoanneB, JodyB, Joebeone, John Hopley, John Vandenberg, John254, Johnblade,Johnleemk, Johnuniq, JonHarder, Jonathanwagner, Jonwatson, Joodas, Josef Sábl cz, Josh Parris, Jovianeye, Joy, Jpta, Jrodor, Jschoon4, Jusdafax, Kaaveh Ahangar, Kallaspriit, Karelklic,Kaszeta, Katalaveno, Kaz219, Kazrak, Kbrose, Kcordina, KerryVeenstra, Kesla, Kevin Rector, Kgrr, Khat17, Killiondude, Kingpin13, Kirill Lokshin, KnowledgeOfSelf, Kraftlos, Kramerino,Krampo, Krellis, Kuru, Kvng, Kyllys, LOL, LOTRrules, Lachlancooper, Lankiveil, Lawrence Cohen, Lazarus666, Leafyplant, Lear's Fool, Lectonar, Lee Carre, Lights, LittleOldMe, LizardJr8,Lockcole, Logictheo, Logthis, Lomn, Looxix, Lordeaswar, Lotje, Lulu of the Lotus-Eaters, Luna Santin, Lupin, Lynnallendaly, M, MBisanz, MER-C, MIT Trekkie, Maguscrowley, Mahanga,Majorly, Mange01, Manishar us, MarkSutton, MarkWahl, Markb, Markhurd, Markolinsky, MartinHarper, Marvin01, Mattalyst, Matthew Yeager, Mattjgalloway, Mattmill30, Mbc362,Mboverload, McGinnis, Mcnuttj, Mdd, Meepster, Mendel, Mephistophelian, Merlion444, Metaclassing, Micahcowan, Michael Hardy, Mike Rosoft, Mikel Ward, Mikeo, Mikeyh56, Milindm2255, Minimac, Mkweise, Mlewis000, Mmeerman, Mmernex, Mmmeg, Mobius R, Mohitjoshi999, Mohitsport, Mojalefa247, Morten, Moxfyre, Mr Elmo, Mr Stephen, Mr.ghlban, MrOllie,Mrankur, Mtd2006, MuZemike, Mulad, Mwtoews, Myanw, Myheadspinsincircles, N-Man, N5iln, Naishadh, Nanshu, Naohiro19, Naresh jangra, Nasa-verve, Nate Silva, NawlinWiki, Nbarth,Nbhatla, Nejko, Nemesis of Reason, Nethgirb, Netsnipe, Niaz, Nick, Nickshanks, Nicolas1981, Nisavid, Nitecruzr, Nivix, Nk, Noahspurrier, Nolyann, Nsaa, Nubiatech, NuclearWarfare, Nux,OSUKid7, Odie5533, Ogress, Oita2001, OlEnglish, Omicronpersei8, Originalharry, Ott, Ottosmo, Ouishoebean, Oxymoron83, PGWG, Pamri, Panser Born, Paparodo, Parakalo, Pastore Italy,Patch1103, Patrikor, Patstuart, Paul August, PaulWIKIJeffery, Pb30, Penno, Pethr, Phatom87, Phil Boswell, Philip Trueman, PhilipMW, Pluyo8989, Pmorkert, Postdlf, Postmortemjapan, Praggu,ProPuke, Pseudomonas, Psiphiorg, Public Menace, Puchiko, Puckly, PyreneesJIM, Pytom, RainbowOfLight, Ravikiran r, RazorICE, Rcannon100, Rebroad, Recognizance, RedWolf, Reedy,Rejax, Rettetast, Rfc1394, Rgilchrist, Rhobite, Rich Farmbrough, RichardVeryard, Rick Sidwell, Rjgodoy, Rjstinyc, Rlaager, Rnbc, RobEby, Robert K S, RobertL30, RockMFR, Rohwigan03,Ronz, RoscoMck, RossPatterson, Roux, Roux-HG, RoyBoy, Rsiddharth, Runis57, Runtux, Ryan au, Ryt, Ryulong, S, S3000, SMC, Saad ziyad, Saddy Dumpington, Safety Cap, Saintfiends,Sakurambo, SaxicolousOne, Scarian, Schumi555, Scientus, Scohoust, Seaphoto, Sesu Prime, Shadow1, Shadowjams, SharePointStacy, Shell Kinney, Shirik, Shoeofdeath, ShornAssociates,Shrofami, Sietse Snel, Simonfl, Simple Bob, SineChristoNon, Sir Nicholas de Mimsy-Porpington, Sir Stupidity, Skier Dude, Sliceofmiami, Slrobertson, Smalljim, Smokizzy, SnowFire, Snowolf,Soosed, SpaceFlight89, Speaker to Lampposts, SpeedyGonsales, Spitfire8520, SpuriousQ, Sridev, StaticGull, Stemonitis, Stephan Leeds, Stephen Gilbert, StephenFalken, Stevage, Steven Zhang,StuartBrady, Subfrowns, Sunilmalik1107, Suruena, Swapcouch, Syntaxsystem, TAS, THEN WHO WAS PHONE?, Tagishsimon, Tangotango, Tarekradi, Tbsdy lives, Tcncv, Techtoucian,Tedickey, Tellyaddict, Tempodivalse, The Anome, The Athlon Duster, The Haunted Angel, The Thing That Should Not Be, Therumakna, Thief12, Thingg, Think4amit, ThreeDee912,ThunderBird, Tide rolls, Tim Q. Wells, Tom harrison, TomPhil, Tommy2010, Tompsci, Tooki, Tpbradbury, Tpvibes, Tranzz, Travelbird, Tree Biting Conspiracy, Trevor MacInnis, Triona,TripleF, Triwbe, Troy 07, Turb0chrg, Tyler.szabo, UU, Umair ahmed123, Unkownkid123, Venu62, Versus22, VidGa, Vishnava, Visor, Vk anantha, Vmguruprasath, Voidxor, Waggers,Warrierrakesh, Wayfarer, Weregerbil, Whitejay251, WikiDan61, Wikipelli, William Avery, Willking1979, Wilson.canadian, Wily duck, Wire323, Wireless friend, Wishingtown, Wknight94,WoiKiCK, Woohookitty, Wrlee, Wrs1864, Wtmitchell, Yamamoto Ichiro, YamiKaitou, Yamike, Yms, YolanCh, Yuokool12, ZX81, ZachPruckowski, Zachary, Zoobee79, Žmogus, 2844anonymous edits

Application Layer  Source: http://en.wikipedia.org/w/index.php?oldid=389551543  Contributors: AS, Ahoerstemeier, AlistairMcMillan, Amillar, AndyHedges, Arunachalammanohar,Ashdurbat, B4hand, Brest, Butko, ChazBeckett, Cradel, DDR2Nite, DIonized, DeweyQ, Dogcow, Dominio, Eimsand, Ejabberd, ElKevbo, Enjoi4586, Fctoma, Frap, Fredrik, Geozapf, GermanX,Gilliam, Graham87, GrapeSteinbeck, Grapht, Gruzd, Harryboyles, Hawaiiboy99, Hede2000, Honcw, Hrvoje Simic, Hu12, Ipahophead, IvanLanin, James smith2, Jamie, Jauerback, Jaybeeunix,Jaymcjay, Jerome Charles Potts, Jnc, Johnuniq, Jorunn, Kbrose, Kbthompson, Kesac, Lababidi, LiDaobing, Looxix, Lost.goblin, Lulu of the Lotus-Eaters, Lysdexia, Mange01, MartinHarper,MattieTK, Mfloryan, Mhby87, Minesweeper, Morte, Mwtoews, Nhorton, Night Gyr, Nixdorf, Orphan Wiki, Oxymoron83, Panarchy, Pgallert, Reconsider the static, RedWolf, Rich Farmbrough,Rserpool, SatyrTN, Schlesselman, ScottDavis, Squideshi, Stephan Leeds, Suruena, Template namespace initialisation script, Tmopkisn, Useight, West.andrew.g, Wisamsafi, Wknight94,Wmasterj, Yacht, Yerpo, Zac439, Zfr, 144 ,ديسلا ىفطصم دمحأ anonymous edits

Presentation Layer  Source: http://en.wikipedia.org/w/index.php?oldid=387634468  Contributors: ACSE, Alan Pascoe, AlistairMcMillan, Anon lynx, Arastcp, B4hand, Bkell, Borgx, Butko,CyborgTosser, Danielcohn, Dgtsyb, Digisus, EagleOne, Enjoi4586, Facuq, FelipeVargasRigo, Francs2000, Gmlk, Guy Harris, Hede2000, Hetar, Itai, James smith2, Jelsova, Jengelh, Jnc, Jotel,Kbrose, Keegscee, Khendon, Kremso, LiDaobing, Looxix, MartinHarper, Michael Hardy, Modster, Nisiguti, Orderud, Ravensun, RedWolf, Rjgodoy, SatyrTN, ScottDavis, Sdfisher, Shanes,Stdazi, Template namespace initialisation script, Timsk, Ugur Basak, Underpants, Widefox, Yacht, 60 anonymous edits

Session Layer  Source: http://en.wikipedia.org/w/index.php?oldid=389152912  Contributors: Alan.tate, Aldie, AlistairMcMillan, Arastcp, Bettia, Borgx, Btornado, Butko, Conti, Danielcohn,Dgtsyb, Digisus, Dontopenyoureyes, Eleassar, Enjoi4586, FelipeVargasRigo, Fred Bradstadt, GLaDOS, Gbeeker, Giftlite, Guy Harris, Hede2000, J.Tame, JHolman, James smith2, Jamesooders,Jll, Jonathan Drain, Kbrose, Kris Schnee, LFaraone, LOLEDITING, LiDaobing, Looxix, Mange01, MartinHarper, Mav, MementoVivere, Nisiguti, Oli Filth, Pilif12p, RedWolf, Rick Sidwell,SatyrTN, ScottDavis, Shiro jdn, Stdazi, Template namespace initialisation script, Tinnitus97, Tobias Conradi, Underpants, Widefox, Yacht, 58 anonymous edits

Transport Layer  Source: http://en.wikipedia.org/w/index.php?oldid=389950337  Contributors: 1ForTheMoney, Adoniscik, Alansohn, Aldie, AlistairMcMillan, Altenmann, Alvestrand, AndrewHampe, Andreyf, BenBreen2003, Bigmantonyd, Borgx, Brettmeyers, Butko, Butlerm, Choudesh, Conan, Costello, Dgreen34, Dgtsyb, Eggnock, Enjoi4586, FatalError, Fmunshi, Fred Bradstadt,Gavinatkinson, GermanX, Giftlite, Guy Harris, Hadal, Hede2000, Hullbr3ach, Imcdnzl, Int21h, Ipatrol, J.delanoy, JHolman, Jec, Jimfbleak, Jnc, Johnuniq, Kadin2048, Kbrose, Kingpin13,Kungfuadam, Kwi, LiDaobing, Limbo socrates, Looxix, Lost.goblin, MTC, Mange01, Marcan, MartinHarper, Mirv, Mmmready, Nealmcb, Nixdorf, Ovy, Phantomsteve, Phatom87, PrologFan,Razorflame, Retroguy90, Rmhermen, Robth, Samjoopin, SatyrTN, ScottDavis, Suffusion of Yellow, Suruena, Tcmcfaul, Template namespace initialisation script, Tpvibes, Vipinhari, Wmasterj,Yacht, Yyy, Zac439, ZeroOne, Zoicon5, 120 anonymous edits

Network Layer  Source: http://en.wikipedia.org/w/index.php?oldid=384407933  Contributors: Abhi 4442003, Adrian M. H., Alfio, Arastcp, Arunachalammanohar, Bendykst, Borgx, Butko,Capricorn42, Colin Marquardt, Crawdaddio, Dgtsyb, Digisus, Dominio, Dribbleboy, Dthomsen8, Egret, ElKevbo, Enchanter, Enjoi4586, Enochlau, Epbr123, Feezo, Fredrik, Giftlite, Goatasaur,Gouldja, GringoCroco, Haakon, Hede2000, Immibis, Infrogmation, JCWilson, James smith2, Jgiam, Jnc, Jorge Stolfi, Josh Parris, Karih, Kbrose, Lankiveil, Looxix, Mange01, Mark7-2,MartinHarper, Mhby87, Muhandes, Nathan Hamblen, Niteowlneils, Nixdorf, Oscarthecat, Pepsi Lite, QuadrivialMind, RazorICE, Rick Sidwell, RobertMfromLI, SatyrTN, ScottDavis, Strake,Template namespace initialisation script, Tompagenet, Widefox, Yacht, Zac439, 113 anonymous edits

Data Link Layer  Source: http://en.wikipedia.org/w/index.php?oldid=384539507  Contributors: 1000Faces, 3DS Mike, AAA!, AGruntsJaggon, Aldie, AlexandriNo, Alfio, Amillar, Angela,Arastcp, B4hand, Barri, BigDunc, Blehfu, Butko, Cahoover, Canterbury Tail, Casey Abell, DanMS, DavidABraun, Dddddd2w32, Dgtsyb, Diogenes00, Docu, Dominio, Eekoo, Egil, Ehn, Emberof Light, Enjoi4586, Excirial, Falkonry, Gasheadsteve, Giftlite, Guy Harris, Hadal, Hede2000, Ibarrere, Intgr, Invictus42, Itusg15q4user, J.delanoy, James smith2, Jmkim dot com, Kannadigahavi,Kbrose, Ken l lee, Konstable, Kubanczyk, Lawrence Cohen, Looxix, Luís Felipe Braga, Mange01, Martijn Hoekstra, MartinHarper, Mathieumcguire, Michael Hardy, Mlpearc, Nixdorf,Nolispanmo, Numa, Oli Filth, Philip Trueman, Plasticup, Playstationman, Rabarberski, Ravensun, RedWolf, Remember the dot, Rick Sidwell, SatyrTN, ScottDavis, Shanel, Shiro jdn, Sin-man,SirPavlova ♥, SpaceFlight89, Stdazi, Suruena, Talinus, Template namespace initialisation script, TexasAndroid, TheFeds, Thierryc, Tide rolls, Timo Honkasalo, Tomchiukc, TripleF, Turian,TutterMouse, Udunuwara, Webgeek, Weregerbil, Why Not A Duck, Widefox, Willy on Wheels over Ethernet, Wplacek, Yacht, Yyy, ZeWrestler, Zfr, 155 anonymous edits

Physical Layer  Source: http://en.wikipedia.org/w/index.php?oldid=390284084  Contributors: 1exec1, AGruntsJaggon, Acdx, Alai, Alfio, Amaurea, Amillar, Amire80, Arastcp, Arnero, BertK, BjKa, Borgx, Bouquet, Brest, Butko, CONFIQ, CanadianLinuxUser, Carl.bunderson, Chriscandy, Clovis Sangrail, CosineKitty, DerHexer, Deville, Dgtsyb, Dicklyon, Digisus, Dkovacs, Dominio, Drieken, Dtgm, Eastlaw, EdH, Emperorbma, Enjoi4586, Epbr123, Europrobe, Extraordinary, Fahidka, Gary63, Gerixau, Giftlite, Giraffedata, Grafen, Grassynoel, Grendelkhan, Guy Harris, Harobikes34, Harp, Hede2000, Hetar, Hosterweis, IMC Networks, Iain99, Ibarrere, ImpossibleEcho, IntrigueBlue, Itai, Itpastorn, Itusg15q4user, James smith2, John Silvestri, Johnuniq,

Article Sources and Contributors 32

Jredmond, Kaaveh Ahangar, Kbrose, KelleyCook, Kevin Rector, Kremso, Kubanczyk, Lawrence Cohen, Linkminer, Looxix, Luckyherb, Mange01, Marianocecowski, Marketsnipers,MartinHarper, Mhby87, Mlewis000, Mosca, Nbarth, Nubiatech, NyAp, Pepsi Lite, Phantasee, Phoenix-forgotten, RexNL, Rick Sidwell, RockMFR, Rwwww, SatyrTN, Savannah Kaylee,Scootey, ScottDavis, Shashiranjan18, Sietse Snel, Slamminheads, Stdazi, Ta bu shi da yu, Tagishsimon, Template namespace initialisation script, TimothyJKeller, Tomchiukc, Tsuite, UU,Underpants, Violask81976, Welsley, Widefox, Yacht, Yyy, 142 anonymous edits

Internet Protocol Suite  Source: http://en.wikipedia.org/w/index.php?oldid=390277898  Contributors: 130.243.79.xxx, 203.109.250.xxx, 213.253.39.xxx, 66.169.238.xxx, A8UDI, AapoLaitinen, Abdull, Abdullais4u, Acceptus, Acroterion, Ahoerstemeier, Albanaco, Aldie, AliMaghrebi, Aliasptr, Alireza.usa, AlistairMcMillan, Amungale, Ana Couto, Anna Lincoln, Anororn,Arcenciel, ArchonMagnus, Arteitle, ArticCynda, Avant Guard, Axcess, AxelBoldt, B4hand, Barberio, Barnacle157, Bender235, Bernard François, Betterworld, Bezenek, Bhavin, Biot,Bloodshedder, Bmicomp, Branko, Brian.fsm, Brion VIBBER, Camw, Canthusus, CaptainVindaloo, Carnildo, Casey Abell, Cate, Cburnett, Chadernook, Cheesycow5, Ckatz, Coasting,Conversion script, Coolcaesar, Ctm314, Cynthia Rhoads, DARTH SIDIOUS 2, Damian Yerrick, Daniel Staal, DanielCD, Darkhalfactf, DavidDW, DavidDouthitt, DerekLaw, Dgtsyb, Dmeranda,Dnas, Dogcow, Doradus, Dorgan65, Doug Bell, Drphilharmonic, Duffman, EagleOne, Ed g2s, Edmilne, Edward, Edwardando, Eeekster, Ekashp, Ellywa, Elwood j blues, EncMstr, Enjoi4586,Epbr123, Eptin, Equendil, Ericl234, Erik Sandberg, Ethanthej, Etu, Evil Monkey, Evil saltine, Expensivehat, Falcon9x5, Ferkelparade, Fixman88, Fr34k, Freyr, GaelicWizard, Geneb1955,Gilliam, Glane23, Glenn, Globemasterthree, Golbez, GordonMcKinney, Graham87, Gringo.ch, Gsl, Guy Harris, Haakon, Hadal, Hairy Dude, HarisM, Hcberkowitz, Headbomb, Helix84, Here,Hoary, Holylampposts, Hpnguyen83, Hyad, IMSoP, Ilario, Imcdnzl, Imran, Indeterminate, Inhumandecency, Inomyabcs, Intgr, Itai, J.delanoy, JTN, Jackqu7, James Mohr, JamesBWatson,Jantangring, Jatkins, JesterXXV, Jimp, Jmdavid1789, Jnc, Joanjoc, John Vandenberg, Johnblade, JonHarder, Jorunn, Jrogern, Jsoon eu, Jusdafax, JustAGal, KYPark, Kaare, Kasperd, Kbrose,Kim Bruning, KnowledgeOfSelf, Konman72, Koyaanis Qatsi, Krauss, Krellis, Kungming2, Kusma, Kvng, Kyng, Labongo, Larree, Law, Layer, Leapfrog314, Lee Carre, Logictheo, Lova Falk,Luna Santin, Magister Mathematicae, Maltest, Mandarax, Mange01, Manop, Marcika, Martyman, Martyvis, Master Conjurer, Matt Dunn, Mattbrundage, Matthew Woodcraft, Matusz, Mav,Mckoss, Mendel, Merlissimo, Metaclassing, Michael Hardy, Miles, MilesMi, Mintguy, Mrzaius, Mukkakukaku, Mwarren us, Mzje, NMChico24, Nasz, Navedahmed123, NawlinWiki,Nealcardwell, Nealmcb, NewEnglandYankee, Ngriffeth, Nhorton, Nick C, Niteowlneils, Nivix, Nixdorf, Nknight, Nmacu, Nubiatech, Nv8200p, Obradovic Goran, Oheckmann, Olathe, Otets,OttoTheFish, Oxwil, Oxymoron83, Palfrey, Papadopa, Patilravi1985, Paul, Paul Koning, Paulkramer, Peripitus, Pfalstad, Pharaoh of the Wizards, Phgao, Piano non troppo, PioM, Plugwash,Pokeywiz, Poslfit, Pps, Public Menace, Punjabi101, Quinxorin, R'n'B, RaNo, Radagast83, Ramnath R Iyer, Rayward, RedWolf, Reliablesources, RevRagnarok, Rich Farmbrough, Rich257,Richardwhiuk, Rick Block, Rick Sidwell, Rjd0060, Rjwilmsi, RobEby, RobertG, RobertL30, Roberta F., Robost, Rodeosmurf, Ross Fraser, Rrelf, Rserpool, Runis57, Ruthherrin, SJP, STHayden,SWAdair, Samjoopin, SasiSasi, Sheldrake, Shii, Shiraun, Shiro jdn, Shizhao, Sietse Snel, Sjakkalle, Skullketon, Smartse, Snaxe920, Spmion, Stefan Milosevski, Stephan Leeds, Stephenb, Sully,SuperWiki, Suruena, Svick, Swhitehead, Swpb, Ta bu shi da yu, Tagishsimon, Tarquin, Techpro30, Template namespace initialisation script, Tfl, That Guy, From That Show!, Thatguyflint, TheAnome, The Nut, TheOtherJesse, Theresa knott, Thingg, Thumperward, Thunderboltz, Thw1309, Tide rolls, Tim Watson, Timwi, TinaSDCE, Tmaufer, Tmchk, Tobias Hoevekamp, TomPhil,Tr606, Tyler, Typhoon, Ukexpat, Unyoyega, Vanis314, Victor Liu, Violetriga, Wadamja, Waggers, Weregeek, Weylinp, Whereizben, Wiki104, Wikid77, Wikiklrsc, William Avery, Wimt,Wolfkeeper, Wonderstruck, Wrs1864, XJamRastafire, Xeesh, Xosé, Yakudza, Yas, Ydalal, Yudiweb, ZNott, Zac439, Zeerak88, Zfr, Zigger, Zoicon5, Zondor, Zundark, ^demon, 719 ,يتوص تاشanonymous edits

Image Sources, Licenses and Contributors 33

Image Sources, Licenses and ContributorsFile:OSI-model-Communication.svg  Source: http://en.wikipedia.org/w/index.php?title=File:OSI-model-Communication.svg  License: Public Domain  Contributors: User:RuntuxImage:IP stack connections.svg  Source: http://en.wikipedia.org/w/index.php?title=File:IP_stack_connections.svg  License: GNU Free Documentation License  Contributors: Cburnett, Kbrose,Mdd, WikipediaMaster, 2 anonymous editsImage:UDP encapsulation.svg  Source: http://en.wikipedia.org/w/index.php?title=File:UDP_encapsulation.svg  License: GNU Free Documentation License  Contributors: original work,colorization by

License 34

LicenseCreative Commons Attribution-Share Alike 3.0 Unportedhttp:/ / creativecommons. org/ licenses/ by-sa/ 3. 0/