Internet Protocol Suite


The Internet protocol suite, commonly known as TCP/IP, is the set of s used in the and similar s. The current foundational protocols in the suite are the (TCP) and the (IP). During its development, versions of it were known as the Department of Defense (DoD) model because the development of the networking method was funded by the through . Its implementation is a . The Internet protocol suite provides specifying how data should be packetized, addressed, transmitted, , and received. This functionality is organized into four s, which classify all related protocols according to each protocol's scope of networking.RFC 1122, ''Requirements for Internet Hosts – Communication Layers'', R. Braden (ed.), October 1989.RFC 1123, ''Requirements for Internet Hosts – Application and Support'', R. Braden (ed.), October 1989 From lowest to highest, the layers are the , containing communication methods for data that remains within a single network segment (link); the , providing between independent networks; the , handling host-to-host communication; and the , providing process-to-process data exchange for applications. The s underlying the Internet protocol suite and its constituent protocols are maintained by the (IETF). The Internet protocol suite predates the , a more comprehensive reference framework for general networking systems.


Early research

The Internet protocol suite resulted from research and development conducted by the Defense Advanced Research Projects Agency () in the late 1960s. After initiating the pioneering in 1969, DARPA started work on a number of other data transmission technologies. In 1972, joined the DARPA , 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, , who helped develop the existing ARPANET (NCP) protocol, joined Kahn to work on interconnection models with the goal of designing the next protocol generation for the ARPANET. They drew on the experience from the ARPANET research community and the , which Cerf chaired. By the summer of 1973, Kahn and Cerf had worked out a fundamental reformulation, in which the differences between local network protocols were hidden by using a common , and, instead of the network being responsible for reliability, as in the existing ARPANET protocols, this function was delegated to the hosts. Cerf credits and , designer of the network, with important influences on this design. The new protocol was implemented as the in 1974., , Carl Sunshine (December 1974), , ''Specification of Internet Transmission Control Protocol'' (December 1974) Initially, the Transmission Control Program managed both transmissions and routing, but as experience with the protocol grew, collaborators recommended division of functionality into layers of distinct protocols. Advocates included of the University of Southern California's , who edited the (RFCs), the technical and strategic document series that has both documented and catalyzed Internet development,Internet Hall of Fame and the research group of at . Postel stated, "We are screwing up in our design of Internet protocols by violating the principle of layering." Encapsulation of different mechanisms was intended to create an environment where the upper layers could access only what was needed from the lower layers. A monolithic design would be inflexible and lead to scalability issues. In version 3 of TCP, written in 1978, the Transmission Control Program was split into two distinct protocols, the as connectionless layer and the as a reliable . The design of the network included the recognition that 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. This design is known as the . Using this design, it became possible to connect other networks to the ARPANET that used the same principle, irrespective of other local characteristics, thereby solving Kahn's initial internetworking problem. A popular expression is that TCP/IP, the eventual product of Cerf and Kahn's work, can run over "two tin cans and a string." Years later, as a joke, the formal protocol specification was created and successfully tested. DARPA contracted with , , and the to develop operational versions of the protocol on several hardware platforms. During development of the protocol the version number of the packet routing layer progressed from version 1 to version 4, the latter of which was installed in the ARPANET in 1983. It became known as ' (IPv4) as the protocol that is still in use in the Internet, alongside its current successor, (IPv6).

Early implementation

In 1975, a two-network IP communications test was performed between Stanford and University College London. In November 1977, a three-network IP test was conducted between sites in the US, the UK, and Norway. Several other IP prototypes were developed at multiple research centers between 1978 and 1983. Before the January 1, 1983 "Flag Day", the Internet used NCP instead of TCP as the transport layer protocol. A computer called a is provided with an interface to each network. It forwards s back and forth between them.RFC 1812, ''Requirements for IP Version 4 Routers'', F. Baker (June 1995) Originally a router was called ''gateway'', but the term was changed to avoid confusion with other types of .


In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking. In the same year, and 's research group at University College London adopted the protocol. The migration of the ARPANET to TCP/IP was officially completed on January 1, 1983, when the new protocols were permanently activated. In 1985, the Internet Advisory Board (later ) held a three-day TCP/IP workshop for the computer industry, attended by 250 vendor representatives, promoting the protocol and leading to its increasing commercial use. In 1985, the first conference focused on network interoperability by broader adoption of TCP/IP. The conference was founded by Dan Lynch, an early Internet activist. From the beginning, large corporations, such as IBM and DEC, attended the meeting. IBM, AT&T and DEC were the first major corporations to adopt TCP/IP, this despite having competing s. In IBM, from 1984, 's group did TCP/IP development. They navigated the corporate politics to get a stream of TCP/IP products for various IBM systems, including , , and . At the same time, several smaller companies, such as and the , began offering TCP/IP stacks for and .Wollongong
/ref> The first TCP/IP stack came from the University of Wisconsin. Some of the early TCP/IP stacks were written single-handedly by a few programmers. Jay Elinsky and of IBM Research wrote TCP/IP stacks for VM/CMS and OS/2, respectively. In 1984 Donald Gillies at MIT wrote a ''ntcp'' multi-connection TCP which ran atop the IP/PacketDriver layer maintained by John Romkey at MIT in 1983–4. Romkey leveraged this TCP in 1986 when FTP Software was founded. Starting in 1985, Phil Karn created a multi-connection TCP application for ham radio systems (KA9Q TCP).Phil Karn, ''KA9Q TCP Download Website'' The spread of TCP/IP was fueled further in June 1989, when the agreed to place the TCP/IP code developed for into the public domain. Various corporate vendors, including IBM, included this code in commercial TCP/IP software releases. Microsoft released a native TCP/IP stack in Windows 95. This event helped cement TCP/IP's dominance over other protocols on Microsoft-based networks, which included IBM's (SNA), and on other platforms such as 's , (OSI), and (XNS). Nonetheless, for a period in the late 1980s and early 1990s, engineers, organizations and nations were , the OSI model or the Internet protocol suite would result in the best and most robust computer networks.

Formal specification and standards

The s underlying the Internet protocol suite and its constituent protocols have been delegated to the (IETF). The characteristic architecture of the Internet Protocol Suite is its broad division into operating scopes for the protocols that constitute its core functionality. The defining specification of the suite is RFC 1122, which broadly outlines four s. These have stood the test of time, as the IETF has never modified this structure. As such a model of networking, the Internet Protocol Suite predates the OSI model, a more comprehensive reference framework for general networking systems.

Key architectural principles

The has evolved over time. Its original expression put the maintenance of state and overall intelligence at the edges, and assumed the Internet that connected the edges retained no state and concentrated on speed and simplicity. Real-world needs for firewalls, network address translators, web content caches and the like have forced changes in this principle.Rethinking the design of the Internet: The end-to-end arguments vs. the brave new world
Marjory S. Blumenthal, David D. Clark, August 2001
The states: "In general, an implementation must be conservative in its sending behavior, and liberal in its receiving behavior. That is, it must be careful to send well-formed datagrams, but must accept any datagram that it can interpret (e.g., not object to technical errors where the meaning is still clear)." "The second part of the principle is almost as important: software on other hosts may contain deficiencies that make it unwise to exploit legal but obscure protocol features." is used 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. The data is further encapsulated at each level. An early architectural document, , emphasizes architectural principles over layering. RFC 1122, titled ''Host Requirements'', is structured in paragraphs referring to layers, but the document refers to many other architectural principles and does not emphasize layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows: * The is the scope within which applications, or , create user data and communicate this data to other applications on another or the same host. The applications make use of the services provided by the underlying lower layers, especially the transport layer which provides ''pipes'' to other processes. The communications partners are characterized by the application architecture, such as the and networking. This is the layer in which all application protocols, such as SMTP, FTP, SSH, HTTP, operate. Processes are addressed via ports which essentially represent . * The performs host-to-host communications on either the local network or remote networks separated by routers. It provides a channel for the communication needs of applications. UDP is the basic transport layer protocol, providing an unreliable datagram service. The Transmission Control Protocol provides flow-control, connection establishment, and reliable transmission of data. * The exchanges datagrams across network boundaries. It provides a uniform networking interface that hides the actual topology (layout) of the underlying network connections. It is therefore also the layer that establishes internetworking. Indeed, it defines and establishes the Internet. This layer defines the addressing and routing structures used for the TCP/IP protocol suite. The primary protocol in this scope is the Internet Protocol, which defines es. Its function in routing is to transport datagrams to the next host, functioning as an IP router, that has the connectivity to a network closer to the final data destination. * The defines the networking methods within the scope of the local network link on which hosts communicate without intervening routers. This layer includes the protocols used to describe the local network topology and the interfaces needed to affect the transmission of Internet layer datagrams to next-neighbor hosts.

Link layer

The protocols of the operate within the scope of the local network connection to which a host is attached. This regime is called the ''link'' in TCP/IP parlance and is the lowest component layer of the suite. The link includes all hosts accessible without traversing a router. The size of the link is therefore determined by the networking hardware design. In principle, TCP/IP is designed to be hardware independent and may be implemented on top of virtually any link-layer technology. This includes not only hardware implementations, but also virtual link layers such as s and . The link layer is used to move packets between the Internet layer interfaces of two different hosts on the same link. The processes of transmitting and receiving packets on the link can be controlled in the for the , as well as in or by specialized . These perform functions, such as framing, to prepare the Internet layer packets for transmission, and finally transmit the frames to the and over a . The TCP/IP model includes specifications for translating the network addressing methods used in the Internet Protocol to link-layer addresses, such as (MAC) addresses. All other aspects below that level, however, are implicitly assumed to exist, and are not explicitly defined in the TCP/IP model. The link layer in the TCP/IP model has corresponding functions in Layer 2 of the OSI model.

Internet layer

requires sending data from the source network to the destination network. This process is called and is supported by host addressing and identification using the hierarchical ing system. The provides an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding datagrams to an appropriate next-hop router for further relaying to its destination. The internet layer has the responsibility of sending packets across potentially multiple networks. With this functionality, the internet layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet. The internet layer does not distinguish between the various transport layer protocols. IP carries data for a variety of different s. These protocols are each identified by a unique : for example, (ICMP) and (IGMP) are protocols 1 and 2, respectively. The Internet Protocol is the principal component of the internet layer, and it defines two addressing systems to identify network hosts and to locate them on the network. The original address system of the and its successor, the Internet, is (IPv4). It uses a 32-bit and is therefore capable of identifying approximately four billion hosts. This limitation was eliminated in 1998 by the standardization of (IPv6) which uses 128-bit addresses. IPv6 production implementations emerged in approximately 2006.

Transport layer

The transport layer establishes basic data channels that applications use for task-specific data exchange. The layer establishes host-to-host connectivity in the form of end-to-end message transfer services that are independent of the underlying network and independent of the structure of user data and the logistics of exchanging information. Connectivity at the transport layer can be categorized as either , implemented in TCP, or , implemented in UDP. The protocols in this layer may provide , , , , and application addressing (). For the purpose of providing process-specific transmission channels for applications, the layer establishes the concept of the . This is 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 or s. Because IP provides only a , some transport-layer protocols offer reliability. TCP is a connection-oriented protocol that addresses numerous reliability issues in providing a : * data arrives in-order * data has minimal error (i.e., correctness) * duplicate data is discarded * lost or discarded packets are resent * includes traffic congestion control The newer (SCTP) is also a reliable, connection-oriented transport mechanism. It is message-stream-oriented, not byte-stream-oriented like TCP, and provides multiple streams multiplexed over a single connection. It also provides support, in which a connection end can be represented by multiple IP addresses (representing multiple physical interfaces), such that if one fails, the connection is not interrupted. It was developed initially for telephony applications (to transport over IP). Reliability can also be achieved by running IP over a reliable data-link protocol such as the (HDLC). The (UDP) is a connectionless protocol. Like IP, it is a best-effort, unreliable protocol. Reliability is addressed through using a checksum algorithm. UDP is typically used for applications such as streaming media (audio, video, etc.) where on-time arrival is more important than reliability, or for simple query/response applications like lookups, where the overhead of setting up a reliable connection is disproportionately large. (RTP) is a datagram protocol that is used over UDP and is designed for real-time data such as . The applications at any given network address are distinguished by their TCP or UDP port. By convention, certain are associated with specific applications. The TCP/IP model's transport or host-to-host layer corresponds roughly to the fourth layer in the OSI model, also called the transport layer. is rapidly emerging as an alternative transport protocol. Whilst it is technically carried via UDP packets it seeks to offer enhanced transport connectivity relative to TCP. works exclusively via QUIC.

Application layer

The includes the protocols used by most applications for providing user services or exchanging application data over the network connections established by the lower level protocols. This may include some basic network support services such as s and host configuration. Examples of application layer protocols include the (HTTP), the (FTP), the (SMTP), and the (DHCP).
, W. Richard Stevens, February 1994
Data coded according to application layer protocols are into transport layer protocol units (such as TCP streams or UDP datagrams), which in turn use s to effect actual data transfer. The TCP/IP model does not consider the specifics of formatting and presenting data and does not define additional layers between the application and transport layers as in the OSI model (presentation and session layers). According to the TCP/IP model, such functions are the realm of and s. The application layer in the TCP/IP model is often compared to a combination of the fifth (session), sixth (presentation), and seventh (application) layers of the OSI model. Application layer protocols are often associated with particular applications, and common services have ''well-known'' port numbers reserved by the (IANA). For example, the uses server port 80 and uses server port 23. connecting to a service usually use s, i.e., port numbers assigned only for the duration of the transaction at random or from a specific range configured in the application. At the application layer, the TCP/IP model distinguishes between ''user protocols'' and ''support protocols''.RFC 1122
''Requirements for Internet Hosts – Communication Layers'', 1.1.3 ''Internet Protocol Suite'', 1989
Support protocols provide services to a system of network infrastructure. User protocols are used for actual user applications. For example, FTP is a user protocol and DNS is a support protocol. Although the applications are usually aware of key qualities of the transport layer connection such as the endpoint IP addresses and port numbers, application layer protocols generally treat the transport layer (and lower) protocols as es which provide a stable network connection across which to communicate. The transport layer and lower-level layers are unconcerned with the specifics of application layer protocols. Routers and do not typically examine the encapsulated traffic, rather they just provide a conduit for it. However, some and applications use to interpret application data. An example is the (RSVP). It is also sometimes necessary for to consider the application payload.

Layer names and number of layers in the literature

The following table shows various networking models. The number of layers varies between three and seven. Some of the networking models are from textbooks, which are secondary sources that may conflict with the intent of RFC 1122 and other primary sources.RFC 3439, ''Some Internet Architectural Guidelines and Philosophy'', R. Bush, D. Meyer (eds.), December 2002.

Comparison of TCP/IP and OSI layering

The three top layers in the OSI model, i.e. the application layer, the presentation layer and the session layer, are not distinguished separately in the TCP/IP model which only has an application layer above the transport layer. While some pure OSI protocol applications, such as , also combined them, there is no requirement that a TCP/IP protocol stack must impose monolithic architecture above the transport layer. For example, the NFS application protocol runs over the (XDR) presentation protocol, which, in turn, runs over a protocol called (RPC). RPC provides reliable record transmission, so it can safely use the best-effort UDP transport. Different authors have interpreted the TCP/IP model differently, and disagree whether the link layer, or any aspect of the TCP/IP model, covers OSI layer 1 () issues, or whether TCP/IP assumes a hardware layer exists below the link layer. Several authors have attempted to incorporate the OSI model's layers 1 and 2 into the TCP/IP model since these are commonly referred to in modern standards (for example, by and ). This often results in a model with five layers, where the link layer or network access layer is split into the OSI model's layers 1 and 2. The IETF protocol development effort is not concerned with strict layering. Some of its protocols may not fit cleanly into the OSI model, although RFCs sometimes refer to it and often use the old OSI layer numbers. The IETF has repeatedly stated that Internet Protocol and architecture development is not intended to be OSI-compliant. RFC 3439, referring to the internet architecture, contains a section entitled: "Layering Considered Harmful". For example, the session and presentation layers of the OSI suite are considered to be included in the application layer of the TCP/IP suite. The functionality of the session layer can be found in protocols like and and is more evident in protocols like and the (SIP). Session-layer functionality is also realized with the port numbering of the TCP and UDP protocols, which are included in the transport layer of the TCP/IP suite. Functions of the presentation layer are realized in the TCP/IP applications with the standard in data exchange. Another difference is in the treatment of . The OSI routing protocol belongs to the network layer, and does not depend on for delivering packets from one router to another, but defines its own layer-3 encapsulation. In contrast, , , and other routing protocols defined by the IETF are transported over IP, and, for the purpose of sending and receiving routing protocol packets, routers act as hosts. As a consequence, include routing protocols in the application layer. Some authors, such as Tanenbaum in ''Computer Networks'', describe routing protocols in the same layer as IP, reasoning that routing protocols inform decisions made by the forwarding process of routers. IETF protocols can be encapsulated recursively, as demonstrated by tunnelling protocols such as (GRE). GRE uses the same mechanism that OSI uses for tunnelling at the network layer.


The Internet protocol suite does not presume any specific hardware or software environment. It only requires that hardware and a software layer exists that is capable of sending and receiving packets on a computer network. As a result, the suite has been implemented on essentially every computing platform. A minimal implementation of TCP/IP includes the following: (IP), (ARP), (ICMP), (TCP), (UDP), and (IGMP). In addition to IP, ICMP, TCP, UDP, Internet Protocol version 6 requires (NDP), , and (MLD) and is often accompanied by an integrated security layer.

See also

* , an early layered network model * (fast local Internet protocol stack) * * * * *



* * * * * * * * * * * *
A Protocol for Packet Network Intercommunication, Cerf & Kahn, IEEE Trans on Comms, Vol Com-22, No 5 May 1974

External links

– Pages on Robert Kahn, Vinton Cerf, and TCP/IP (reviewed by Cerf and Kahn). * A TCP/IP Tutorial – from the Internet Engineering Task Force (January 1991)
The Ultimate Guide to TCP/IP

The TCP/IP Guide
– A comprehensive look at the protocols and the procedure and processes involved * {{citation , url= , archive-url= , archive-date=2021-12-04 , title=A Study of the ARPANET TCP/IP Digest
TCP/IP Sequence Diagrams

Daryl's TCP/IP Primer
– Intro to TCP/IP LAN administration, conversational style Reference models