This is a Quick Guide to the OSI Model and the TCP/IP model.
Last modified: 2011/Oct/09.
The Open Systems Interconnection model (OSI model) prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers in which an instance of a layer provides services to its upper layer instances while receiving services from the layer below.
|N°||Layer||PDU *||Functions||Protocols / Devices|
|7||Application||Data||This layer supports application and end-user processes. This layer provides application services for file transfers, e-mail, and other network software services. Everything at this layer is application-specific.||NNTP, SIP, SSI, DNS, FTP, Gopher, HTTP, NFS, NTP, SMPP, SMTP, SMB, SNMP, Telnet, DHCP, Netconf, RTP, SPDY, SSH and SCP, TFTP, more...|
|6||Presentation||Data||Data representation, encryption and decryption, convert machine dependent data to machine independent data. This layer transform data into the form that the application layer can accept.||MIME, XDR, TLS, SSL, XDR, ASN.1, SMB, AFP, NCP, MIDI, HTML, GIF, TIFF, JPEG, ASCII, EBCDIC|
|5||Session||Data||Interhost communication. This layer establishes, manages and terminates connections between applications.||Named Pipes, NetBIOS, SAP, L2TP, PPTP, SOCKS, TLS, SSH, X.225, RPC.|
|4||Transport||Segment||End-to-end connections, reliability and flow control. This layer provides transparent transfer of data between hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer when it must.||
Protocols: TCP, UDP, SCTP, DCCP, SPX TCP, UDP, RTP, DCCP, SCTP, SPX, ATP. Devices: Gateway, Advanced Cable Tester, Brouter
|3||Network||Packet||Path determination and logical addressing for network routing and forwarding functions. Also packet fragmentation and reassembly occurs at this layer, as well as internetworking, error handling, congestion control and packet sequencing.||
Protocols: IP (IPv4 - IPv6), ICMP, IPsec, IGMP, IPX, Netbeui, AppleTalk, HSRP, VRRPd, Routing Protocols: RIP, IGRP, EIGRP, OSPF, IS-IS, EGP, BGP, etc). Devices: Brouter, Router, Frame Relay Device, IPSwitch, ATM Switch, Advanced Cable Tester, DDP.
|2||Data Link||Frame||Physical addressing of the Network. Provides functional and procedural means to transfer data between network entities. There are two sublayers: the upper Logical Link Control (LLC) and the lower Media Access Control (MAC). LLC controls frame synchronization, flow control and error checking. MAC controls how a network host gains access to the data and permission to transmit it.||Protocols: ATM, SDLC, HDLC, ARP, CSLIP, SLIP, GFP, GLBP, PLIP, IEEE 802.3 (Ethernet), Frame Relay, ITU-T G.hn DLL, PPP, X.25, Devices: Bridges, Network Switch (and Virtual LANs), NIC, Routers.|
|1||Physical||Bit||Defines electrical and physical specifications for devices. Media, signal and binary transmission. This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.||Standards: EIA/TIA-232, EIA/TIA-449, ITU-T V-Series, I.430, I.431, POTS, PDH, SONET/SDH, PON, OTN, DSL, IEEE 802.3 (Ethernet), IEEE 802.11 (Wi-Fi), IEEE 802.15 (WPAN), IEEE 802.16 (WiMAX), IEEE 1394 (Firewire), ITU-T G.hn PHY, USB, Bluetooth, Devices: NIC, Twisted Pair, Coax, Fiber Optic, Wireless Media, Repeater, Multiplexer, Hubs, (Passive/Active), Router, TDR, Transciever, Oscilloscope, Amplifier, Carrier pigeon|
* PDU = Protocol Data Unit. In OSI layer 3, the PDU may be also called "Datagram" when the packet belongs to an upper layer "unreliable" service like UDP.
** Routing protocols, according to the OSI Routing framework, are layer management protocols for the network layer, regardless of their transport mechanism.
The TCP/IP model (also called The Internet Model) is a description framework for computer network protocols created in the 1970s by DARPA, an agency of the United States Department of Defense, to describe a set guidelines and implementations of specific networking protocols to enable computers to communicate over a network. TCP/IP provides end-to-end connectivity specifying how data should be formatted, addressed, transmitted, routed and received at the destination.
|OSI N°||OSI Model Equivalent||TCP/IP N°||TCP/IP Model||PDU||TCP/IP Protocols|
|7||Application||4||Application||Data||FTP, HTTP, POP3, IMAP, telnet, SMTP, DNS, TFTP|
|2||Data Link||1||Network Access||Frames||Down to DL/MAC Sublayer, lower layers are not explicitly defined, but implicitly assumed.|
The TCP/IP model is a high-level networking description that does not concern itself directly with physical interfaces, so it doesn't have actually a physical layer. The TCP/IP model includes specifications of translating the network addressing methods used in the Internet Protocol to data link addressing, such as Media Access Control (MAC), however all other aspects below that level (like a Physical layer) are implicitly assumed to exist in the Link Layer, but are not explicitly defined.