2 Layers in the Internet Protocol Suite
2.1 The concept of layers
2.2 Layer names and number of layers in the literature
4 See also
6 Further reading
7 External links
The 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 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 between network protocols were hidden by using a common internetwork protocol, and, instead of the network being responsible for reliability, as in the ARPANET, the hosts became responsible. Cerf credits Hubert Zimmerman and Louis Pouzin, designer of the CYCLADES network, with important influences on this design. The network's design included the recognition it should provide only the functions of efficiently transmitting and routing traffic between end nodes and that all other intelligence should be located at the edge of the network, in the end nodes. Using a simple design, it became possible to connect almost any network to the ARPANET, irrespective of their local characteristics, thereby solving Kahn's initial problem. One popular expression is 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, is provided with an interface to each network. It forwards packets back and forth between them. Originally a router was called gateway, but the term was changed to avoid confusion with other types of gateways. The idea was worked out in more detailed form by Cerf's networking research group at Stanford in the 1973–74 period, resulting in the first TCP specification. The early networking work at Xerox PARC, which produced the PARC Universal Packet protocol suite, much of which existed around the same period of time, was also a significant technical influence. DARPA then contracted with BBN Technologies, Stanford University, and the University College London to develop operational versions of the protocol on different hardware platforms. Four versions were developed: TCP v1, TCP v2, a split into TCP v3 and IP v3 in the spring of 1978, and then stability with TCP/IP v4 — the standard protocol still in use on the Internet today. In 1975, a two-network TCP/IP communications test was performed between Stanford and University College London (UCL). In November, 1977, a three-network TCP/IP test was conducted between sites in the US, UK, and Norway. Several other TCP/IP prototypes were developed at multiple research centres between 1978 and 1983. The migration of the ARPANET to TCP/IP was officially completed on flag day January 1, 1983, when the new protocols were permanently activated. In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking. In 1985, the Internet Architecture Board held a three day workshop on TCP/IP for the computer industry, attended by 250 vendor representatives, promoting the protocol and leading to its increasing commercial use. Layers in the Internet Protocol Suite
The concept of layers
Instantiations of the TCP/IP stack operating on two hosts each connected to its router on the Internet. Shown is the flow of user data through the layers used at each hop. The Internet Protocol Suite uses encapsulation to provide abstraction of protocols and services. Encapsulation is usually aligned with the division of the protocol suite into layers of general functionality. In general, an application (the highest level of the model) uses a set of protocols to send its data down the layers, being further encapsulated at each level. According to RFC 1122, the Internet Protocol Suite organizes the functional groups of protocols and methods into four layers, the Application Layer, the Transport Layer, the Internet Layer, and the Link Layer. This model was not intended to be a rigid reference model into which new protocols have to fit in order to be accepted as a standard. The role of layering in TCP/IP may be illustrated by an example network scenario (right-hand diagram), in which two Internet host computers communicate across local network boundaries constituted by their internetworking routers. The application on each host executes read and write operations as if the processes were directly connected to each other by some kind of data pipe, every other detail of the communication is hidden from each process. The underlying mechanisms that transmit data between the host computers are located in the lower protocol layers. The Transport Layer establishes host-to-host connectivity, meaning it handles the details of data transmission that are independent of the structure of user data and the logistics of exchanging information for any particular specific purpose. The layer simply establishes a basic data channel that an application uses in its task-specific data exchange. For this purpose the layer establishes the concept of the port, a numbered logical construct allocated specifically for each of the communication channels an application needs. For many types of services, these port numbers have been standardized so that client computers may address specific services of a server computer without the involvement of service announcements or directory services. The Transport Layer operates on top of the Internet Layer. The Internet Layer is not only agnostic of application data structures as the Transport Layer, but it also does not distinguish between operation of the various Transport Layer protocols. It only provides an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding the Transport Layer datagrams to an appropriate next-hop router for further relaying to its destination. With this functionality, the Internet Layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet. The Internet Protocol is the principal component of the Internet Layer, and it defines two addressing systems to identify network hosts computers, and to locate them on the network. The original address system of the ARPANET and its successor, the Internet, is Internet Protocol Version 4 (IPv4). It uses a 32-bit IP address and is therefore capable of identifying approximately four billion hosts. This limitation was eliminated by the standardization of Internet Protocol Version 6 (IPv6) in 1998, and beginning production implementations in approximately 2006. The lowest layer in the Internet Protocol Suite is the Link Layer. It comprises the tasks of specific networking requirements on the local link, the network segment that a hosts network interface is connected to. This involves interacting with the hardware-specific functions of network interfaces and specific transmission technologies.
Successive encapsulation of application data descending through the protocol stack before transmission on the local network link. As the user data, first manipulated and structured in the Application Layer, is passed through the descending layers of the protocol stack each layer adds encapsulation information as illustrated in the diagram (right). A receiving host reverses the encapsulation at each layer by extracting the higher level data and passing it up the stack to the receiving process. Layer names and number of layers in the literature
The following table shows the layer names and the number of layers of networking models presented in RFCs and textbooks in widespread use in today's university computer networking courses. RFC 1122 TanenbaumCisco AcademyKurose Forouzan Comer KozierokStallingsArpanet Reference Model 1982 (RFC 871) Four layers Four layers Four layersFive layersFour+one layersFive layersThree layers "Internet model""TCP/IP reference model""Internet model""Five-layer Internet model" or "TCP/IP protocol suite""TCP/IP 5-layer reference model""TCP/IP model""Arpanet reference model" Application ApplicationApplicationApplicationApplicationApplicationApplication/Process Transport TransportTransportTransportTransportHost-to-host or transportHost-to-host Internet InternetInternetworkNetworkInternetInternet Link Host-to-networkNetwork interfaceData linkData link (Network interface)Network accessNetwork interface Physical(Hardware)Physical
These textbooks are secondary sources that may contravene the intent of RFC 1122 and other IETF primary sources. Different authors have interpreted the RFCs differently regarding the question whether the Link Layer (and the TCP/IP model) covers Physical Layer issues, or if a hardware layer is assumed below the Link Layer. Some authors have tried to use other names for the Link Layer, such as network interface layer, in view to avoid confusion with the Data Link Layer of the seven layer OSI model. Others have attempted to map the Internet Protocol model onto the OSI Model. The mapping often results in a model with five layers where the Link Layer is split into a Data Link Layer on top of a Physical Layer. In literature with a bottom-up approach to Internet communication, in which 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 of network 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 are collapsed into TCP/IP's Application Layer. The argument is that these OSI layers do usually not exist as separate processes and protocols in Internet applications. However, the Internet protocol stack has never been altered by the Internet Engineering Task Force from the four layers defined in RFC 1122. The IETF makes no effort to follow the OSI model although RFCs sometimes refer to it. The IETF has repeatedly stated that Internet protocol and architecture development is not intended to be OSI-compliant. RFC 3439, addressing Internet architecture, contains a section entitled: "Layering Considered Harmful". Implementations
Most computer operating systems in use today, including all consumer-targeted systems, include a TCP/IP implementation. Minimally acceptable implementation includes implementation for (from most essential to the less essential) IP, ARP, ICMP, UDP, TCP and sometimes 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 of ARP (NBP), and ICMP (ICMPv6), and IGMP (IGMPv6) have some additional required functionalities, and often is accompanied with integrated IPSec security layer. Other protocols could be easily added later (often they can be implemented entirely in the userspace), for example DNS for resolving domain names to IP addresses or DHCP client for automatic configuration of network interfaces. Most of the IP implementations are accessible to the programmers using socket abstraction (usable also with other protocols) and proper API for most of the operations. This interface is known as BSD sockets and was used initially in C. Unique implementations include Lightweight TCP/IP, an open source stack designed for embedded systems and KA9Q NOS, a stack and associated protocols for amateur packet radio systems and personal computers connected via serial lines.