TCP/IP provides end-to-end connectivity specifying how data should be packetized, addressed, transmitted,
routed and received at the destination. Indeed, this functionality is organized into four abstraction layers which are used to sort all related protocols according to the scope of networking involved.
[1][2] From lowest to highest, the layers are the
link layer, containing communication technologies for a single network segment (link); the
internet layer, connecting hosts across independent networks, thus establishing
internetworking; the
transport layer handling host-to-host communication; and the
application layer, which provides process-to-process application data exchange.
History[edit]
Early research[edit]
Diagram of the first internetworked connection
The Internet protocol suite resulted from research and development conducted by the Defense Advanced Research Projects Agency (
DARPA) in the late 1960s.
[3] 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, in which 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 Zimmermann and
Louis 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 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 the 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." (Years later, as a joke, the
IP over Avian Carriers formal protocol specification was created and successfully tested.)
A computer called a
router is provided with an interface to each network. It forwards
packets back and forth between them.
[4] Originally a router was called
gateway, but the term was changed to avoid confusion with other types of
gateways.
Specification[edit]
From 1973 to 1974, Cerf's networking research group at Stanford worked out details of the idea, resulting in the first TCP specification.
[5] A significant technical influence was the early networking work at
Xerox PARC, which produced the
PARC Universal Packet protocol suite, much of which existed around that time.
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, TCP v3 and IP v3, and TCP/IP v4. The last protocol is still in use 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, the UK, and Norway. Several other TCP/IP prototypes were developed at multiple research centers 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.
[6]
Adoption[edit]
In March 1982, the US Department of Defense declared TCP/IP as the standard for all military computer networking.
[7] In 1985, the Internet Advisory Board (later renamed 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.
In 1985, the first
Interop 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. Interoperability conferences have been held every year since then. Every year from 1985 through 1993, the number of attendees tripled.
[citation needed]
IBM, AT&T and DEC were the first major corporations to adopt TCP/IP, despite having competing internal protocols (
SNA,
XNS, etc.). In IBM, from 1984,
Barry Appelman's group did TCP/IP development. (Appelman later moved to
AOL to be the head of all its development efforts.) They navigated the corporate politics to get a stream of TCP/IP products for various IBM systems, including
MVS,
VM, and
OS/2. At the same time, several smaller companies began offering TCP/IP stacks for
DOS and
MS Windows, such as the company
FTP Software, and the
Wollongong Group.
[8] The first VM/CMS TCP/IP stack came from the University of Wisconsin.
[9]
Back then, most of these TCP/IP stacks were written single-handedly by a few talented programmers. For example,
John Romkey of
FTP Software was the author of the MIT PC/IP package.
[10] John Romkey's PC/IP implementation was the first IBM PC TCP/IP stack. Jay Elinsky and
Oleg Vishnepolsky of IBM Research wrote TCP/IP stacks for VM/CMS and OS/2, respectively.
[11]
The spread of TCP/IP was fueled further in June 1989, when AT&T agreed to place the TCP/IP code developed for
UNIX into the public domain. Various vendors, including IBM, included this code in their own TCP/IP stacks. Many companies sold TCP/IP stacks for Windows until Microsoft released a native TCP/IP stack in Windows 95. This event was a little late in the evolution of the Internet, but it cemented TCP/IP's dominance over other protocols, which eventually disappeared. These protocols included
IBM Systems Network Architecture (SNA),
Open Systems Interconnection (OSI), Microsoft's native
NetBIOS, and
Xerox Network Systems (XNS).
[citation needed]
Key architectural principles[edit]
An early architectural document,
RFC 1122, emphasizes architectural principles over layering.
[12]
- End-to-end principle: This principle 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.[13]
- Robustness Principle: "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)."[14] "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."[15]
Abstraction layers[edit]
Two Internet hosts connected via two routers and the corresponding layers used at each hop. 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.
|
Encapsulation of application data descending through the layers described in RFC 1122
|
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.
The layers of the protocol suite near the top are logically closer to the user application, while those near the bottom are logically closer to the physical transmission of the data. Viewing layers as providing or consuming a service is a method of
abstraction to isolate upper layer protocols from the details of transmitting bits over, for example,
Ethernet and
collision detection, while the lower layers avoid having to know the details of each and every application and its protocol.
Even when the layers are examined, the assorted architectural documents—there is no single architectural model such as ISO 7498, the
Open Systems Interconnection (OSI) model—have fewer and less rigidly defined layers than the OSI model, and thus provide an easier fit for real-world protocols. One frequently referenced document,
RFC 1958, does not contain a stack of layers. The lack of emphasis on layering is a major difference between the IETF and OSI approaches. It only refers to the existence of the internetworking layer and generally to
upper layers; this document was intended as a 1996 snapshot of the architecture: "The Internet and its architecture have grown in evolutionary fashion from modest beginnings, rather than from a Grand Plan. While this process of evolution is one of the main reasons for the technology's success, it nevertheless seems useful to record a snapshot of the current principles of the Internet architecture."
RFC 1122, entitled
Host Requirements, is structured in paragraphs referring to layers, but the document refers to many other architectural principles not emphasizing layering. It loosely defines a four-layer model, with the layers having names, not numbers, as follows:
- The Application layer is the scope within which applications create user data and communicate this data to other applications on another or the same host. The applications, or processes, make use of the services provided by the underlying, lower layers, especially the Transport Layer which provides reliable or unreliable pipes to other processes. The communications partners are characterized by the application architecture, such as the client-server model and peer-to-peer networking. This is the layer in which all higher level protocols, such as SMTP, FTP, SSH, HTTP, operate. Processes are addressed via ports which essentially represent services.
- The Transport Layer performs host-to-host communications on either the same or different hosts and 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 Internet layer has the task of exchanging 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 referred to as 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 IP addresses. Its function in routing is to transport datagrams to the next IP router that has the connectivity to a network closer to the final data destination.
- The Link layer 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 effect transmission of Internet layer datagrams to next-neighbor hosts.
The Internet protocol suite and the layered
protocol stack design were in use before the OSI model was established. Since then, the TCP/IP model has been compared with the OSI model in books and classrooms, which often results in confusion because the two models use different assumptions and goals, including the relative importance of strict layering.
This abstraction also allows upper layers to provide services that the lower layers do not provide. While the original OSI model was extended to include connectionless services (OSIRM CL),
[16] IP is not designed to be reliable and is a
best effort delivery protocol. This means that all transport layer implementations must choose whether or how to provide reliability. UDP provides data integrity via a
checksum but does not guarantee delivery; TCP provides both data integrity and delivery guarantee by retransmitting until the receiver acknowledges the reception of the packet.
This model lacks the formalism of the OSI model and associated documents, but the IETF does not use a formal model and does not consider this a limitation, as illustrated in the comment by
David D. Clark, "We reject: kings, presidents and voting. We believe in: rough consensus and running code." Criticisms of this model, which have been made with respect to the OSI model, often do not consider ISO's later extensions to that model.
For multiaccess links with their own addressing systems (e.g. Ethernet) an address mapping protocol is needed. Such protocols can be considered to be below IP but above the existing link system. While the IETF does not use the terminology, this is a subnetwork dependent convergence facility according to an extension to the OSI model, the internal organization of the network layer (IONL).
[17]
ICMP & IGMP operate on top of IP but do not transport data like UDP or TCP. Again, this functionality exists as layer management extensions to the OSI model, in its
Management Framework (OSIRM MF)
[18]
The SSL/TLS library operates above the transport layer (uses TCP) but below application protocols. Again, there was no intention, on the part of the designers of these protocols, to comply with OSI architecture.
The link is treated like a black box. The IETF explicitly does not intend to discuss transmission systems, which is a less academic
[citation needed] but practical alternative to the OSI model.
The following is a description of each layer in the TCP/IP networking model starting from the lowest level.
Link layer[edit]
The
link layer has the networking scope of the local network connection to which a host is attached. This regime is called the
link in TCP/IP literature. It is the lowest component layer of the Internet protocols, as TCP/IP is designed to be hardware independent. As a result TCP/IP may be implemented on top of virtually any hardware networking technology.
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 a given link can be controlled both in the
software device driver for the
network card, as well as on
firmware or specialized
chipsets. These perform
data link functions such as adding a
packet header to prepare it for transmission, then actually transmit the frame over a
physical medium. 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). All other aspects below that level, however, are implicitly assumed to exist in the link layer, but are not explicitly defined.
This is also the layer where packets may be selected to be sent over a
virtual private network or other
networking tunnel. In this scenario, the link layer data may be considered application data which traverses another instantiation of the IP stack for transmission or reception over another IP connection. Such a connection, or virtual link, may be established with a transport protocol or even an application scope protocol that serves as a
tunnel in the link layer of the protocol stack. Thus, the TCP/IP model does not dictate a strict hierarchical encapsulation sequence.
The TCP/IP model's link layer corresponds to the Open Systems Interconnection (OSI) model physical and data link layers, layers one and two of the OSI model.
Internet layer[edit]
The
internet layer has the responsibility of sending packets across potentially multiple networks.
Internetworking requires sending data from the source network to the destination network. This process is called
routing.
[19]
The Internet Protocol performs two basic functions:
- Host addressing and identification: This is accomplished with a hierarchical IP addressing system.
- Packet routing: This is the basic task of sending packets of data (datagrams) from source to destination by forwarding them to the next network router closer to the final destination.
Some of the protocols carried by IP, such as ICMP which is used to transmit diagnostic information, and IGMP which is used to manage
IP Multicast data, are layered on top of IP but perform internetworking functions. This illustrates the differences in the architecture of the TCP/IP stack of the Internet and the OSI model. The TCP/IP model's internet layer corresponds to layer three of the Open Systems Interconnection (OSI) model, where it is referred to as the network layer.
The internet layer provides only 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.
Transport layer[edit]
The transport layer establishes a basic data channel that an application uses in its task-specific data exchange. The layer establishes process-to-process connectivity, meaning it provides end-to-end services that are independent of the structure of user data and the logistics of exchanging information for any particular specific purpose. Its responsibility includes end-to-end message transfer independent of the underlying network, along with error control, segmentation, flow control, congestion control, and application addressing (port numbers). End-to-end message transmission or connecting applications at the transport layer can be categorized as either
connection-oriented, implemented in TCP, or
connectionless, implemented in UDP.
For the purpose of providing process-specific transmission channels for applications, the layer establishes the concept of the
port. 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 service announcements or directory services.
For example, the TCP is a connection-oriented protocol that addresses numerous reliability issues in providing a
reliable byte stream:
- 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
Stream Control Transmission Protocol (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
multi-homing 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
SS7 over IP), but can also be used for other applications.
The User Datagram Protocol is a connectionless
datagram protocol. Like IP, it is a best effort, "unreliable" protocol. Reliability is addressed through
error detection using a weak checksum algorithm. UDP is typically used for applications such as streaming media (audio, video,
Voice over IP etc.) where on-time arrival is more important than reliability, or for simple query/response applications like
DNS lookups, where the overhead of setting up a reliable connection is disproportionately large.
Real-time Transport Protocol (RTP) is a datagram protocol that is designed for real-time data such as
streaming audio and video.
The applications at any given network address are distinguished by their TCP or UDP port. By convention certain well known ports are associated with specific applications.
The TCP/IP model's transport or host-to-host layer corresponds to the fourth layer in the Open Systems Interconnection (OSI) model, also called the transport layer.
Application layer[edit]
The 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). Such functions are the realm of
libraries and
application programming interfaces.
Application layer protocols generally treat the transport layer (and lower) protocols as
black boxes which provide a stable network connection across which to communicate, although the applications are usually aware of key qualities of the transport layer connection such as the end point IP addresses and port numbers. Application layer protocols are often associated with particular
client–server applications, and common services have
well-known port numbers reserved by the
Internet Assigned Numbers Authority (IANA). For example, the
HyperText Transfer Protocol uses server port 80 and
Telnet uses server port 23.
Clients connecting to a service usually use
ephemeral ports, i.e., port numbers assigned only for the duration of the transaction at random or from a specific range configured in the application.
The transport layer and lower-level layers are unconcerned with the specifics of application layer protocols. Routers and
switches do not typically examine the encapsulated traffic, rather they just provide a conduit for it. However, some
firewall and
bandwidth throttling applications must interpret application data. An example is the
Resource Reservation Protocol (RSVP). It is also sometimes necessary for
network address translator (NAT) traversal to consider the application payload.
The application layer in the TCP/IP model is often compared as equivalent to a combination of the fifth (Session), sixth (Presentation), and the seventh (Application) layers of the Open Systems Interconnection (OSI) model.
Furthermore, the TCP/IP reference model distinguishes between
user protocols and
support protocols.
[21] Support protocols provide services to a system. User protocols are used for actual user applications. For example, FTP is a user protocol and DNS is a system protocol.
Layer names and number of layers in the literature[edit]
The following table shows various networking models. The number of layers varies between three and seven.
| RFC 1122, Internet STD 3 (1989) | Cisco Academy[22] | Kurose,[23] Forouzan[24] | Comer,[25]Kozierok[26] | Stallings[27] | Tanenbaum[28] | Mike Padlipsky's 1982 "Arpanet Reference Model" (RFC 871) | OSI model |
|---|
| Four layers | Four layers | Five layers | Four+one layers | Five layers | Five layers | Three layers | Seven layers |
| "Internet model" | "Internet model" | "Five-layer Internet model" or "TCP/IP protocol suite" | "TCP/IP 5-layer reference model" | "TCP/IP model" | "TCP/IP 5-layer reference model" | "Arpanet reference model" | OSI model |
| Application | Application | Application | Application | Application | Application | Application/Process | Application |
| Presentation |
| Session |
| Transport | Transport | Transport | Transport | Host-to-host or transport | Transport | Host-to-host | Transport |
| Internet | Internetwork | Network | Internet | Internet | Internet | Network |
| Link | Network interface | Data link | Data link (Network interface) | Network access | Data link | Network interface | Data link |
| (n/a) | | Physical | (Hardware) | Physical | Physical | | Physical |
Some of the networking models are from textbooks, which are secondary sources that may conflict with the intent of
RFC 1122 and other
IETF primary sources.
[29]
