HOME

TheInfoList




A communication protocol is a system of rules that allows two or more entities of a
communications system 400px, Communication system A communications system or communication system is a collection of individual telecommunications networks, transmission systems, relay stations, tributary stations, and terminal equipment usually capable of inte ...
to transmit
information Information is processed, organised and structured data Data (; ) are individual facts A fact is something that is truth, true. The usual test for a statement of fact is verifiability—that is whether it can be demonstrated to c ...

information
via any kind of variation of a
physical quantity A physical quantity is a physical property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''numerical value'' ...
. The protocol defines the rules,
syntax In linguistics Linguistics is the scientific study of language, meaning that it is a comprehensive, systematic, objective, and precise study of language. Linguistics encompasses the analysis of every aspect of language, as well as the ...

syntax
, semantics and
synchronization Synchronization is the coordination of events to operate a system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by ...

synchronization
of
communication Communication (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to b ...

communication
and possible error recovery methods. Protocols may be implemented by
hardware Hardware may refer to: Technology Computing and electronics * Computer hardware, physical parts of a computer * Digital electronics, electronics that operate on digital signals * Electronic component, device in an electronic system used to affect e ...

hardware
,
software Software is a collection of Instruction (computer science), instructions that tell a computer how to work. This is in contrast to Computer hardware, hardware, from which the system is built and actually performs the work. At the low level lang ...

software
, or a combination of both. Communicating systems use well-defined formats for exchanging various messages. Each message has an exact meaning intended to elicit a response from a range of possible responses pre-determined for that particular situation. The specified behavior is typically independent of how it is to be
implemented
implemented
. Communication protocols have to be agreed upon by the parties involved. To reach an agreement, a protocol may be developed into a
technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task which is applied to a common and repeated use of rules, conditions, guidelines or characteristics for products or related processes and produc ...
. A
programming language A programming language is a formal language In logic, mathematics, computer science, and linguistics, a formal language consists of string (computer science), words whose symbol (formal), letters are taken from an alphabet (computer science) ...

programming language
describes the same for computations, so there is a close analogy between protocols and programming languages: ''protocols are to communication what programming languages are to computations''.Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, "They (protocols) are to communication what programming languages are to computation" An alternate formulation states that ''protocols are to communication what
algorithm In and , an algorithm () is a finite sequence of , computer-implementable instructions, typically to solve a class of problems or to perform a computation. Algorithms are always and are used as specifications for performing s, , , and other ...

algorithm
s are to computation''.Comer 2000, Sect. 1.3 - Internet Services, p. 3, "Protocols are to communication what algorithms are to computation" Multiple protocols often describe different aspects of a single communication. A group of protocols designed to work together is known as a protocol suite; when implemented in software they are a
protocol stack The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protocols ...
. Internet communication protocols are published by the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is an open standards organization, which develops and promotes voluntary Internet standards, in particular the technical standards that comprise the Internet protocol suite (TCP/IP). It has no formal ...
(IETF). The
IEEE The Institute of Electrical and Electronics Engineers (IEEE) is a professional association for electronic engineering and electrical engineering (and associated disciplines) with its corporate office in New York City and its operations center i ...
(Institute of Electrical and Electronics Engineers) handles wired and wireless networking and the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization develops and publish ...
(ISO) handles other types. The
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPEG-4 AVC for video ...
handles telecommunication protocols and formats for the
public switched telephone network The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by nati ...
(PSTN). As the PSTN and Internet
converge Converge may refer to: * Converge (band), American hardcore punk band * Converge (Baptist denomination), American national evangelical Baptist body * Limit (mathematics) * Converge ICT, internet service provider in the Philippines See also

...
, the standards are also being driven towards convergence.


Communicating systems


History

One of the first uses of the term ''protocol'' in a data-commutation context occurs in a memorandum entitled ''A Protocol for Use in the
NPL Data Communications Network The NPL network or NPL Data Communications Network was a local area computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of ...
'' written by
Roger Scantlebury Roger Anthony Scantlebury (born August 1936) is a British computer scientist who worked at the National Physical Laboratory (NPL) and later at Logica. Scantlebury participated in pioneering work to develop packet switching In telecommunications ...
and Keith Bartlett in April 1967. On the
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switching In telecommunications, packet switching is a method of grouping data that is transmitted over a digital network into '' packets''. Packets are ...
, the starting point for host-to-host communication in 1969 was the 1822 protocol, which defined the transmission of messages to an IMP. The
Network Control Program The Network Control Program (NCP) provided the middle layers of the protocol stack running on host computers of the ARPANET, the predecessor to the modern Internet The Internet (Capitalization of Internet, or internet) is the global ...
for the ARPANET was first implemented in 1970. The NCP interface allowed
application software Application software (app for short) is computing software designed to carry out a specific task other than one relating to the operation of the computer itself, typically to be used by end-users. Examples of an application Application may refer ...
to connect across the ARPANET by implementing higher-level communication protocols, an early example of the ''protocol layering'' concept."NCP – Network Control Program"
Living Internet
Networking research in the early 1970s by
Robert E. Kahn
Robert E. Kahn
and
Vint Cerf Vinton Gray Cerf (; born June 23, 1943) is an American Internet pioneer and is recognized as one of "List of Internet pioneers, the fathers of the Internet", sharing this title with TCP/IP co-developer Bob Kahn. He has received honorary degrees ...

Vint Cerf
led to the formulation of the
Transmission Control Program The Internet Protocol (IP) is the network layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model A conceptual model is a representation of a system, made of the composition of concept Co ...
(TCP). Its specification was written by Cerf with
Yogen Dalal Instead of a single "inventor", the Internet The Internet (or internet) is the global system of interconnected s that uses the (TCP/IP) to communicate between networks and devices. It is a ' that consists of private, public, academic ...
and Carl Sunshine in December 1974, still a monolithic design at this time. The
International Networking Working GroupThe International Networking Working Group (INWG) was a group of prominent computer science researchers in the 1970s who studied and developed Standardization, standards and communication protocol, protocols for Computer network, computer networking. ...
agreed a connectionless
datagram A datagram is a basic transfer unit associated with a packet-switched network In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the quest ...
standard which was presented to the CCIT in 1975 but was not adopted by the ITU or by the ARPANET. International research, particularly the work of Rémi Després, contributed to the development of the
X.25 X.25 is an ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPE ...
standard, based on
virtual circuit A virtual circuit (VC) is a means of transporting data over a packet-switched network In telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers t ...
s by the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPEG-4 AVC for video ...
in 1976. Computer manufacturers developed
proprietary protocol In telecommunications, a proprietary protocol is a communications protocol owned by a single organization or individual. Intellectual property rights and enforcement Ownership by a single organization gives the owner the ability to place restrictio ...
s such as IBM's
Systems Network Architecture Systems Network Architecture (SNA) is IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 191 ...
(SNA), Digital Equipment Corporation's
DECnet DECnet is a suite of network protocols created by Digital Equipment Corporation. Originally released in 1975 in order to connect two PDP-11 minicomputers, it evolved into one of the first peer-to-peer network architectures, thus transforming DEC ...
and
Xerox Network Systems Xerox Network Systems (XNS) is a computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of techno ...
. TCP software was redesigned as a modular protocol stack. Originally referred to as ''IP/TCP'', it was installed on
SATNET SATNET, also known as the Atlantic Packet Satellite Network, was an early satellite network that formed an initial segment of the Internet The Internet (Capitalization of Internet, or internet) is the global system of interconnected ...
in 1982 and on the ARPANET in January 1983. The development of a complete protocol suite by 1989, as outlined in and , laid the foundation for the growth of
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A commu ...
as a comprehensive protocol suite as the core component of the emerging
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
."TCP/IP Internet Protocol"
Living Internet
International work on a reference model for communication standards led to the
OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without regard to its underlying internal structure and technology. Its goal is the interoperabil ...

OSI model
, published in 1984. For a period in the late 1980s and early 1990s, engineers, organizations and nations became polarized over the issue of which standard, the OSI model or the Internet protocol suite, would result in the best and most robust computer networks.


Concept

The information exchanged between devices through a network or other media is governed by rules and conventions that can be set out in communication protocol specifications. The nature of communication, the actual data exchanged and any
state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, Un ...
-dependent behaviors, is defined by these specifications. In digital computing systems, the rules can be expressed by
algorithm In and , an algorithm () is a finite sequence of , computer-implementable instructions, typically to solve a class of problems or to perform a computation. Algorithms are always and are used as specifications for performing s, , , and other ...

algorithm
s and
data structure In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of ...

data structure
s. Protocols are to communication what algorithms or programming languages are to computations. Operating systems usually contain a set of cooperating processes that manipulate shared data to communicate with each other. This communication is governed by well-understood protocols, which can be embedded in the process code itself.Ben-Ari 1982, chapter 2 - The concurrent programming abstraction, p. 18-19, states the same.Ben-Ari 1982, Section 2.7 - Summary, p. 27, summarizes the concurrent programming abstraction. In contrast, because there is no
shared memory In computer science, shared memory is random-access memory, memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Shared memory is an efficient means of pass ...

shared memory
, communicating systems have to communicate with each other using a shared
transmission medium A transmission medium is a system or substance that can mediate the propagation of signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such ...
. Transmission is not necessarily reliable, and individual systems may use different hardware or operating systems. To implement a networking protocol, the protocol software modules are interfaced with a framework implemented on the machine's operating system. This framework implements the networking functionality of the operating system. When protocol algorithms are expressed in a portable programming language the protocol software may be made
operating system An operating system (OS) is system software System software is software designed to provide a platform for other software. Examples of system software include operating systems (OS) like macOS, Linux, Android (operating system), Android and Mi ...

operating system
independent. The best-known frameworks are the
TCP/IP model The Internet protocol suite is the conceptual model A conceptual model is a representation of a system, made of the composition of concept Concepts are defined as abstract ideas or general notions that occur in the mind, in speech, or in tho ...
and the
OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without regard to its underlying internal structure and technology. Its goal is the interoperabil ...

OSI model
. At the time the Internet was developed,
abstraction layerIn computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes and development of both computer hardware , hardware and software. ...
ing had proven to be a successful design approach for both compiler and operating system design and, given the similarities between programming languages and communication protocols, the originally monolithic networking programs were decomposed into cooperating protocols. This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design.Sect. 11.10 - The Disadvantage Of Layering, p. 192, states: layering forms the basis for protocol design. Systems typically do not use a single protocol to handle a transmission. Instead they use a set of cooperating protocols, sometimes called a
protocol suite The protocol stack or network stack is an implementation Implementation is the realization of an application, or execution of a plan, idea, scientific modelling, model, design, specification, Standardization, standard, algorithm, or policy. Indu ...
.Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, states the same. Some of the best known protocol suites are
TCP/IP The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A commu ...
,
IPX/SPX IPX/SPX stands for Internetwork Packet Exchange/Sequenced Packet Exchange. IPX and SPX are networking protocol, networking protocols used initially on networks using the (since discontinued) Novell NetWare operating systems, and also became widel ...
,
X.25 X.25 is an ITU-T The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications and Information Communication Technology such as X.509 for cybersecurity, Y.3172 and Y.3173 for machine learning, and H.264/MPE ...
,
AX.25AX.25 (Amateur X.25) is a data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model A conceptual model is a representation of a ...
and
AppleTalk AppleTalk is a discontinued proprietary suite of networking protocols developed by Apple Inc. for their Macintosh computers. AppleTalk includes a number of features that allow local area network A local area network (LAN) is a computer ...
. The protocols can be arranged based on functionality in groups, for instance, there is a group of
transport protocol Transport (commonly used in the U.K.), or transportation (used in the U.S.), is the movement of humans, animals and goods from one location to another. In other words, the action of transport is defined as a particular movement of an organism ...
s. The functionalities are mapped onto the layers, each layer solving a distinct class of problems relating to, for instance: application-, transport-, internet- and network interface-functions.Comer 2000, Sect. 11.3 - The Conceptual Layers Of Protocol Software, p. 178, "Each layer takes responsibility for handling one part of the problem." To transmit a message, a protocol has to be selected from each layer. The selection of the next protocol is accomplished by extending the message with a protocol selector for each layer.


Types

There are two types of communication protocols, based on their representation of the content being carried: text-based and binary.


Text-based

A text-based protocol or plain text protocol represents its content in human-readable format, often in plain text. The immediate human readability stands in contrast to binary protocols which have inherent benefits for use in a computer environment (such as ease of mechanical
parsing Parsing, syntax analysis, or syntactic analysis is the process of analyzing a string String or strings may refer to: *String (structure), a long flexible structure made from threads twisted together, which is used to tie, bind, or hang other obje ...

parsing
and improved bandwidth utilization). Different network applications have different methods of encapsulating data. One method very common with Internet protocols is a text oriented representation that transmits requests and responses as lines of
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the stu ...
text, terminated by a newline character (and usually a carriage return character). Examples of protocols that use plain, human-readable text for its commands are FTP (
File Transfer Protocol The File Transfer Protocol (FTP) is a standard communication protocol A communication protocol is a system of rules that allows two or more entities of a communications system 400px, Communication system A communications system or com ...
), SMTP (
Simple Mail Transfer Protocol The Simple Mail Transfer Protocol (SMTP) is an Internet Standard, internet standard communication protocol for email, electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-le ...
), and the
finger protocol In computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by the Node (networking ...
. Text-based protocols are typically optimized for human parsing and interpretation, and are therefore suitable whenever human inspection of protocol contents is required, such as during
debugging In computer programming Computer programming is the process of designing and building an executable In computing, executable code, an executable file, or an executable program, sometimes simply referred to as an executable or binary, caus ...

debugging
and during early protocol development design phases.


Binary

A binary protocol utilizes all values of a
byte The byte is a unit of digital information that most commonly consists of eight bit The bit is a basic unit of information in computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It ...
, as opposed to a text-based protocol which only uses values corresponding to human-readable characters in
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding In computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the stu ...
encoding. Binary protocols are intended to be read by a machine rather than a human being. Binary protocols have the advantage of terseness, which translates into speed of transmission and interpretation. Binary have been used in the normative documents describing modern standards like
EbXML Electronic Business using eXtensible Markup Language, commonly known as e-business XML, or ebXML (pronounced ee-bee-ex-em-el, 'bi,eks,em'el as it is typically referred to, is a family of XML based standards sponsored by OASIS (organization), OAS ...
,
HTTP/2 HTTP/2 (originally named HTTP/2.0) is a major revision of the HTTP network protocol used by the World Wide Web. It was derived from the earlier experimental SPDY protocol, originally developed by Google. HTTP/2 was developed by the HTTP Working ...
,
HTTP/3 HTTP/3 is the third and upcoming major version of the Hypertext Transfer Protocol used to exchange information on the World Wide Web, alongside HTTP/1.1 and HTTP/2. HTTP/3 runs over QUIC, which is published as . HTTP semantics are consistent ac ...
and EDOC. An interface in UML may also be considered a binary protocol.


Basic requirements

Getting the data across a network is only part of the problem for a protocol. The data received has to be evaluated in the context of the progress of the conversation, so a protocol must include rules describing the context. These kind of rules are said to express the ''syntax'' of the communication. Other rules determine whether the data is meaningful for the context in which the exchange takes place. These kind of rules are said to express the ''semantics'' of the communication. Messages are sent and received on communicating systems to establish communication. Protocols should therefore specify rules governing the transmission. In general, much of the following should be addressed: ;Data formats for data exchange :Digital message bitstrings are exchanged. The bitstrings are divided in fields and each field carries information relevant to the protocol. Conceptually the bitstring is divided into two parts called the ''header'' and the ''payload''. The actual message is carried in the payload. The header area contains the fields with relevance to the operation of the protocol. Bitstrings longer than the
maximum transmission unit In computer networking A computer network is a set of computers sharing resources located on or provided by Node (networking), network nodes. The computers use common communication protocols over digital signal, digital Interconnection, inter ...
(MTU) are divided in pieces of appropriate size. ;Address formats for data exchange :Addresses are used to identify both the sender and the intended receiver(s). The addresses are carried in the header area of the bitstrings, allowing the receivers to determine whether the bitstrings are of interest and should be processed or should be ignored. A connection between a sender and a receiver can be identified using an address pair ''(sender address, receiver address)''. Usually, some address values have special meanings. An all-''1''s address could be taken to mean an addressing of all stations on the network, so sending to this address would result in a broadcast on the local network. The rules describing the meanings of the address value are collectively called an ''addressing scheme''. ;Address mapping :Sometimes protocols need to map addresses of one scheme on addresses of another scheme. For instance to translate a logical IP address specified by the application to an Ethernet MAC address. This is referred to as ''address mapping''. ;Routing :When systems are not directly connected, intermediary systems along the ''route'' to the intended receiver(s) need to forward messages on behalf of the sender. On the Internet, the networks are connected using routers. The interconnection of networks through routers is called ''
internetworking Internetworking is the practice of interconnecting multiple computer networks, such that any pair of Host (network), hosts in the connected networks can exchange messages irrespective of their hardware-level networking technology. The resulting s ...
''. ;Detection of transmission errors :Error detection is necessary on networks where data corruption is possible. In a common approach, a CRC of the data area is added to the end of packets, making it possible for the receiver to detect differences caused by corruption. The receiver rejects the packets on CRC differences and arranges somehow for retransmission. ;Acknowledgements :Acknowledgement of correct reception of packets is required for
connection-oriented communication Connection-oriented communication is a network communication mode in telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical A cylinder ...
. Acknowledgments are sent from receivers back to their respective senders. ;Loss of information - timeouts and retries :Packets may be lost on the network or be delayed in transit. To cope with this, under some protocols, a sender may expect an acknowledgment of correct reception from the receiver within a certain amount of time. Thus, on timeouts, the sender may need to retransmit the information. In case of a permanently broken link, the retransmission has no effect so the number of retransmissions is limited. Exceeding the retry limit is considered an error. ;Direction of information flow :Direction needs to be addressed if transmissions can only occur in one direction at a time as on
half-duplex A duplex communication system Image:Communication-System.png, 400px, Communication system A communications system or communication system is a collection of individual telecommunications networks, transmission (telecommunications), transmis ...
links or from one sender at a time as on a
shared medium In telecommunication Telecommunication is the transmission of information by various types of technologies over wire A wire is a single usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads or el ...
. This is known as
media access control In IEEE 802, IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MA ...
. Arrangements have to be made to accommodate the case of
collision In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. " ...
or contention where two parties respectively simultaneously transmit or wish to transmit. ;Sequence control :If long bitstrings are divided into pieces and then sent on the network individually, the pieces may get lost or delayed or, on some types of networks, take different routes to their destination. As a result, pieces may arrive out of sequence. Retransmissions can result in duplicate pieces. By marking the pieces with sequence information at the sender, the receiver can determine what was lost or duplicated, ask for necessary retransmissions and reassemble the original message. ;Flow control :Flow control is needed when the sender transmits faster than the receiver or intermediate network equipment can process the transmissions. Flow control can be implemented by messaging from receiver to sender. ;Queueing :Communicating processes or state machines employ queues (or "buffers"), usually FIFO queues, to deal with the messages in the order sent, and may sometimes have multiple queues with different prioritization


Protocol design

Systems engineering Systems engineering is an field of and that focuses on how to design, integrate, and manage s over their s. At its core, systems engineering utilizes principles to organize this body of knowledge. The individual outcome of such efforts, an e ...
principles have been applied to create a set of common network protocol design principles. The design of complex protocols often involves decomposition into simpler, cooperating protocols. Such a set of cooperating protocols is sometimes called a protocol family or a protocol suite, within a conceptual framework. Communicating systems operate concurrently. An important aspect of
concurrent programming Concurrent computing is a form of computing in which several computations are executed ''Concurrency (computer science), concurrently''—during overlapping time periods—instead of ''sequentially—''with one completing before the next starts. ...
is the synchronization of software for receiving and transmitting messages of communication in proper sequencing. Concurrent programming has traditionally been a topic in operating systems theory texts. Formal verification seems indispensable because concurrent programs are notorious for the hidden and sophisticated bugs they contain. A mathematical approach to the study of concurrency and communication is referred to as
communicating sequential processes In computer science Computer science deals with the theoretical foundations of information, algorithms and the architectures of its computation as well as practical techniques for their application. Computer science is the study of Algorit ...
(CSP). Concurrency can also be modeled using
finite state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation A model is an informative representation of an object, person or system. ...
s, such as
MealyIn the theory of computation, a Mealy machine is a finite-state machine whose output values are determined both by its current state (computer science), state and the current inputs. This is in contrast to a Moore machine, whose (Moore) output values ...
and
Moore machine In the theory of computation, a Moore machine is a finite-state machine whose output values are determined only by its current state (computer science), state. This is in contrast to a Mealy machine, whose output values are determined both by its cu ...

Moore machine
s. Mealy and Moore machines are in use as design tools in digital electronics systems encountered in the form of hardware used in telecommunication or electronic devices in general. The literature presents numerous analogies between computer communication and programming. In analogy, a transfer mechanism of a protocol is comparable to a central processing unit (CPU). The framework introduces rules that allow the programmer to design cooperating protocols independently of one another.


Layering

In modern protocol design, protocols are layered to form a protocol stack. Layering is a design principle that divides the protocol design task into smaller steps, each of which accomplishes a specific part, interacting with the other parts of the protocol only in a small number of well-defined ways. Layering allows the parts of a protocol to be designed and tested without a
combinatorial explosion In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
of cases, keeping each design relatively simple. The communication protocols in use on the
Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consist ...

Internet
are designed to function in diverse and complex settings. Internet protocols are designed for simplicity and modularity and fit into a coarse hierarchy of functional layers defined in the
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 Departm ...
. The first two cooperating protocols, the
Transmission Control Protocol The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite The Internet protocol suite, commonly known as TCP/IP, is the set of communications protocol A communication protocol is a system of rules ...
(TCP) and the
Internet Protocol The Internet Protocol (IP) is the network layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model A conceptual model is a representation of a system, made of the composition of concept C ...
(IP) resulted from the decomposition of the original Transmission Control Program, a monolithic communication protocol, into this layered communication suite. The
OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without regard to its underlying internal structure and technology. Its goal is the interoperabil ...

OSI model
was developed internationally based on experience with networks that predated the internet as a reference model for general communication with much stricter rules of protocol interaction and rigorous layering. Typically, application software is built upon a robust data transport layer. Underlying this transport layer is a datagram delivery and routing mechanism that is typically
connectionless Connectionless communication, often referred to as CL-mode communication,Information Processing Systems - Open Systems Interconnection, "Transport Service Definition - Addendum 1: Connectionless-mode Transmission", International Organization for St ...
in the Internet. Packet relaying across networks happens over another layer that involves only network link technologies, which are often specific to certain physical layer technologies, such as
Ethernet Ethernet () is a family of wired computer network A computer network is a set of s sharing resources located on or provided by . The computers use common s over to communicate with each other. These interconnections are made up of te ...

Ethernet
. Layering provides opportunities to exchange technologies when needed, for example, protocols are often stacked in a tunneling arrangement to accommodate the connection of dissimilar networks. For example, IP may be tunneled across an
Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM) is a telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It ...
(ATM) network.


Protocol layering

Protocol layering forms the basis of protocol design. It allows the decomposition of single, complex protocols into simpler, cooperating protocols.Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, introduces the decomposition in layers. The protocol layers each solve a distinct class of communication problems. Together, the layers make up a layering scheme or model. Computations deal with algorithms and data; Communication involves protocols and messages; So the analog of a
data flow diagram A data-flow diagram is a way of representing a flow of data through a process or a system (usually an information system An information system (IS) is a formal, sociotechnical, organizational system designed to collect, process, store, and distribu ...
is some kind of message flow diagram. To visualize protocol layering and protocol suites, a diagram of the message flows in and between two systems, A and B, is shown in figure 3. The systems, A and B, both make use of the same protocol suite. The vertical flows (and protocols) are in-system and the horizontal message flows (and protocols) are between systems. The message flows are governed by rules, and data formats specified by protocols. The blue lines mark the boundaries of the (horizontal) protocol layers.


Software layering

The software supporting protocols has a layered organization and its relationship with protocol layering is shown in figure 5. To send a message on system A, the top-layer software module interacts with the module directly below it and hands over the message to be encapsulated. The lower module fills in the header data in accordance with the protocol it implements and interacts with the bottom module which sends the message over the communications channel to the bottom module of system B. On the receiving system B the reverse happens, so ultimately the message gets delivered in its original form to the top module of system B.
Program translation A translator or programming language processor is a generic term that can refer to anything that converts code from one computer language into another. A program written in high-level language is called source program. These include translations ...
is divided into subproblems. As a result, the translation software is layered as well, allowing the software layers to be designed independently. The same approach can be seen in the TCP/IP layering.Comer 2000, Sect. 11.2 - The need for multiple protocols, p. 178, explains similarities protocol software and compiler, assembler, linker, loader. The modules below the application layer are generally considered part of the operating system. Passing data between these modules is much less expensive than passing data between an application program and the transport layer. The boundary between the application layer and the transport layer is called the operating system boundary.


Strict layering

Strictly adhering to a layered model, a practice known as strict layering, is not always the best approach to networking. Strict layering can have a negative impact on the performance of an implementation. While the use of protocol layering is today ubiquitous across the field of computer networking, it has been historically criticized by many researchers as abstracting the protocol stack in this way may cause a higher layer to duplicate the functionality of a lower layer, a prime example being error recovery on both a per-link basis and an end-to-end basis.


Design patterns

Commonly recurring problems in the design and implementation of communication protocols can be addressed by
software design pattern In software engineering, a software design pattern is a general, reusability, reusable solution to a commonly occurring problem within a given context in software design. It is not a finished design that can be transformed directly into Source code, ...
s.


Formal specification

Popular formal methods of describing communication syntax are
Abstract Syntax Notation One Abstract Syntax Notation One (ASN.1) is a standard interface description language An interface description language or interface definition language (IDL), is a specification language used to describe a Component-based software engineering, softwa ...
(an
ISO The International Organization for Standardization (ISO ) is an international standard An international standard is a technical standard A technical standard is an established norm (social), norm or requirement for a repeatable technical task w ...
standard) and
augmented Backus–Naur formIn computer science, augmented Backus–Naur form (ABNF) is a metalanguage based on Backus–Naur form (BNF), but consisting of its own syntax and derivation rules. The motive principle for ABNF is to describe a formal system of a language to be used ...
(an
IETF The Internet Engineering Task Force (IETF) is an open standards organization, which develops and promotes voluntary Internet standards, in particular the technical standards that comprise the Internet protocol suite (TCP/IP). It has no formal ...
standard).
Finite-state machine A finite-state machine (FSM) or finite-state automaton (FSA, plural: ''automata''), finite automaton, or simply a state machine, is a mathematical model of computation In computer science Computer science deals with the theoretical fou ...
models are used to formally describe the possible interactions of the protocol. and communicating finite-state machines


Protocol development

For communication to occur, protocols have to be selected. The rules can be expressed by algorithms and data structures. Hardware and operating system independence is enhanced by expressing the algorithms in a portable programming language. Source independence of the specification provides wider interoperability. Protocol standards are commonly created by obtaining the approval or support of a
standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...
, which initiates the standardization process. The members of the standards organization agree to adhere to the work result on a voluntary basis. Often the members are in control of large market-shares relevant to the protocol and in many cases, standards are enforced by law or the government because they are thought to serve an important public interest, so getting approval can be very important for the protocol.


The need for protocol standards

The need for protocol standards can be shown by looking at what happened to the bi-sync protocol (BSC) invented by
IBM International Business Machines Corporation (IBM) is an American multinational technology company headquartered in Armonk, New York, with operations in over 170 countries. The company began in 1911, founded in Endicott, New York, as the C ...

IBM
. BSC is an early link-level protocol used to connect two separate nodes. It was originally not intended to be used in a multinode network, but doing so revealed several deficiencies of the protocol. In the absence of standardization, manufacturers and organizations felt free to enhance the protocol, creating incompatible versions on their networks. In some cases, this was deliberately done to discourage users from using equipment from other manufacturers. There are more than 50 variants of the original bi-sync protocol. One can assume, that a standard would have prevented at least some of this from happening.Marsden 1986, Section 6.1 - Why are standards necessary?, p. 64-65, uses BSC as an example to show the need for both standard protocols and a standard framework. In some cases, protocols gain market dominance without going through a standardization process. Such protocols are referred to as ''
de facto standard A de facto standard is a custom or convention Convention may refer to: * Convention (norm), a custom or tradition, a standard of presentation or conduct ** Treaty, an agreement in international law * Convention (meeting), meeting of a (usually la ...
s''. De facto standards are common in emerging markets, niche markets, or markets that are (or oligopolized). They can hold a market in a very negative grip, especially when used to scare away competition. From a historical perspective, standardization should be seen as a measure to counteract the ill-effects of de facto standards. Positive exceptions exist; a de facto standard operating system like Linux does not have this negative grip on its market, because the sources are published and maintained in an open way, thus inviting competition.


Standards organizations

Some of the
standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary function is developing, coordinating, promulgating, revising, amending, reissuing, interpr ...
s of relevance for communication protocols are the
International Organization for Standardization The International Organization for Standardization (ISO ) is an international standard-setting body composed of representatives from various national standards organizations. Founded on 23 February 1947, the organization develops and publish ...
(ISO), the
International Telecommunication Union The International Telecommunication Union is a list of specialized agencies of the United Nations, specialized agency of the United Nations responsible for all matters related to information and communications technology, information and co ...

International Telecommunication Union
(ITU), the
Institute of Electrical and Electronics Engineers The Institute of Electrical and Electronics Engineers (IEEE) is a professional association A professional association (also called a professional body, professional organization, or professional society) usually seeks to further Further or F ...
(IEEE), and the
Internet Engineering Task Force The Internet Engineering Task Force (IETF) is an open standards organization, which develops and promotes voluntary Internet standards, in particular the technical standards that comprise the Internet protocol suite (TCP/IP). It has no formal ...
(IETF). The IETF maintains the protocols in use on the Internet. The IEEE controls many software and hardware protocols in the electronics industry for commercial and consumer devices. The ITU is an umbrella organization of telecommunication engineers designing the
public switched telephone network The public switched telephone network (PSTN) provides Communications infrastructure, infrastructure and services for public telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that are operated by nati ...
(PSTN), as well as many
radio Radio is the technology of signaling and communicating Communication (from Latin ''communicare'', meaning "to share") is the act of developing Semantics, meaning among Subject (philosophy), entities or Organization, groups through the use ...

radio
communication systems. For
marine electronicsMarine electronics refers to electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow ...
the NMEA standards are used. The
World Wide Web Consortium The World Wide Web Consortium (W3C) is the main international standards organization A standards organization, standards body, standards developing organization (SDO), or standards setting organization (SSO) is an organization whose primary ...
(W3C) produces protocols and standards for Web technologies. International standards organizations are supposed to be more impartial than local organizations with a national or commercial self-interest to consider. Standards organizations also do research and development for standards of the future. In practice, the standards organizations mentioned, cooperate closely with each other.


The standardization process

In the ISO, the standardization process starts off with the commissioning of a sub-committee workgroup. The workgroup issues working drafts and discussion documents to interested parties (including other standards bodies) in order to provoke discussion and comments. This will generate a lot of questions, much discussion and usually some disagreement. These comments are taken into account and a ''draft proposal'' is produced by the working group. After feedback, modification, and compromise the proposal reaches the status of a ''draft international standard'', and ultimately an ''international standard''. International standards are reissued periodically to handle the deficiencies and reflect changing views on the subject.


OSI standardization

A lesson learned from
ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switching In telecommunications, packet switching is a method of grouping data that is transmitted over a digital network into '' packets''. Packets are ...
, the predecessor of the Internet, was that protocols need a framework to operate. It is therefore important to develop a general-purpose, future-proof framework suitable for ''structured protocols'' (such as layered protocols) and their standardization. This would prevent protocol standards with overlapping functionality and would allow clear definition of the responsibilities of a protocol at the different levels (layers). This gave rise to the
Open Systems Interconnection model The Open Systems Interconnection model (OSI model) is a conceptual model that characterises and standardises the communication functions of a telecommunication or computing system without regard to its underlying internal structure and technolog ...
(OSI model), which is used as a framework for the design of standard protocols and services conforming to the various layer specifications. In the OSI model, communicating systems are assumed to be connected by an underlying physical medium providing a basic transmission mechanism. The layers above it are numbered. Each layer provides service to the layer above it using the services of the layer immediately below it. The top layer provides services to the application process. The layers communicate with each other by means of an interface, called a ''service access point''. Corresponding layers at each system are called ''peer entities''. To communicate, two peer entities at a given layer use a protocol specific to that layer which is implemented by using services of the layer below. For each layer, there are two types of standards: protocol standards defining how peer entities at a given layer communicate, and service standards defining how a given layer communicates with the layer above it. In the original version of the OSI model, the layers and their functionality are (from highest to lowest layer): * The ''
Application layer An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communications network. The application layer abstraction is used in b ...
'' may provide the following services to the application processes: identification of the intended communication partners, establishment of the necessary authority to communicate, determination of availability and authentication of the partners, agreement on privacy mechanisms for the communication, agreement on responsibility for error recovery and procedures for ensuring data integrity, synchronization between cooperating application processes, identification of any constraints on syntax (e.g. character sets and data structures), determination of cost and acceptable quality of service, selection of the dialogue discipline, including required logon and logoff procedures. * The ''
presentation layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model A conceptual model is a representation of a system, made of the composition of concept Concepts are defined as abstract ideas or general ...
'' may provide the following services to the application layer: a request for the establishment of a session, data transfer, negotiation of the syntax to be used between the application layers, any necessary syntax transformations, formatting and special purpose transformations (e.g. data compression and data encryption). * The ''
session layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that characterises and standardises the communication functions of a telecommunication or computing system without regard to its underlyi ...
'' may provide the following services to the presentation layer: establishment and release of session connections, normal and expedited data exchange, a quarantine service which allows the sending presentation entity to instruct the receiving session entity not to release data to its presentation entity without permission, interaction management so presentation entities can control whose turn it is to perform certain control functions, resynchronization of a session connection, reporting of unrecoverable exceptions to the presentation entity. * The ''
transport layer In computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by the Node (netw ...
'' provides reliable and transparent data transfer in a cost-effective way as required by the selected quality of service. It may support the multiplexing of several transport connections on to one network connection or split one transport connection into several network connections. * The ''
network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate Router (computing), routers. Functions The network layer provides t ...
'' does the setup, maintenance and release of network paths between transport peer entities. When relays are needed, routing and relay functions are provided by this layer. The quality of service is negotiated between network and transport entities at the time the connection is set up. This layer is also responsible for
network congestion Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking probab ...
control. * The ''
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without r ...
'' does the setup, maintenance and release of data link connections. Errors occurring in the physical layer are detected and may be corrected. Errors are reported to the network layer. The exchange of data link units (including flow control) is defined by this layer. * The ''
physical layer In the seven-layer OSI model The Open Systems Interconnection model (OSI model) is a that characterises and standardises the communication functions of a or computing system without regard to its underlying internal structure and technolo ...
'' describes details like the electrical characteristics of the physical connection, the transmission techniques used, and the setup, maintenance and clearing of physical connections. In contrast to the TCP/IP layering scheme, which assumes a connectionless network, RM/OSI assumed a connection-oriented network. Connection-oriented networks are more suitable for wide area networks and connectionless networks are more suitable for local area networks. Using connections to communicate implies some form of session and (virtual) circuits, hence the (in the TCP/IP model lacking) session layer. The constituent members of ISO were mostly concerned with wide area networks, so development of RM/OSI concentrated on connection-oriented networks and connectionless networks were only mentioned in an addendum to RM/OSI. At the time, the IETF had to cope with this and the fact that the Internet needed protocols that simply were not there. As a result, the IETF developed its own standardization process based on "rough consensus and running code". The standardization process is described b
RFC2026
Nowadays, the IETF has become a standards organization for the protocols in use on the Internet. RM/OSI has extended its model to include connectionless services and because of this, both TCP and IP could be developed into international standards.


Taxonomies

Classification schemes for protocols usually focus on the domain of use and function. As an example of domain of use,
connection-oriented protocol Connection-oriented communication is a network communication mode in telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What ...
s and
connectionless protocol Connectionless communication, often referred to as CL-mode communication,Information Processing Systems - Open Systems Interconnection, "Transport Service Definition - Addendum 1: Connectionless-mode Transmission", International Organization for St ...
s are used on connection-oriented networks and connectionless networks respectively. An example of function is a
tunneling protocol In computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by the Node (networking ...
, which is used to encapsulate packets in a high-level protocol so that the packets can be passed across a transport system using the high-level protocol. A ''layering scheme'' combines both function and domain of use. The dominant layering schemes are the ones proposed by the IETF and by ISO. Despite the fact that the underlying assumptions of the layering schemes are different enough to warrant distinguishing the two, it is a common practice to compare the two by relating common protocols to the layers of the two schemes.Comer 2000, Sect. 11.5.1 - The TCP/IP 5-Layer Reference Model, p. 183, states the same. The layering scheme from the IETF is called ''Internet layering'' or ''TCP/IP layering''. The layering scheme from ISO is called ''the OSI model'' or ''ISO layering''. In networking equipment configuration, a term-of-art distinction is often drawn: The term "protocol" strictly refers to the transport layer, and the term "service" refers to protocols utilizing a "protocol" for transport. In the common case of TCP and UDP, services are distinguished by port numbers. Conformance to these port numbers is voluntary, so in content inspection systems the term "service" strictly refers to port numbers, and the term "application" is often used to refer to protocols identified through inspection signatures.


See also

* Lists of network protocols


Notes


References


Bibliography

*
Radia Perlman Radia Joy Perlman (born December 18, 1951) is an American computer programmer and network engineer. She is most famous for her invention of the spanning-tree protocol (STP), which is fundamental to the operation of network bridge A network ...

Radia Perlman
: ''Interconnections: Bridges, Routers, Switches, and Internetworking Protocols.'' 2nd Edition. Addison-Wesley 1999, . In particular Ch. 18 on "network design folklore", which is also available online at http://www.informit.com/articles/article.aspx?p=20482 * Gerard J. Holzmann: ''Design and Validation of Computer Protocols.'' Prentice Hall, 1991, . Also available online at http://spinroot.com/spin/Doc/Book91.html * In particular Ch.11 Protocol layering. Also has a RFC guide and a Glossary of Internetworking Terms and Abbreviations. * Internet Engineering Task Force abbr. IETF (1989): ''RFC1122, Requirements for Internet Hosts -- Communication Layers, R. Braden (ed.)'', Available online at http://tools.ietf.org/html/rfc1122. Describes TCP/IP to the implementors of protocolsoftware. In particular the introduction gives an overview of the design goals of the suite. * M. Ben-Ari (1982): ''Principles of concurrent programming'' 10th Print. Prentice Hall International, . * C.A.R. Hoare (1985): ''Communicating sequential processes'' 10th Print. Prentice Hall International, . Available online via http://www.usingcsp.com * R.D. Tennent (1981): ''Principles of programming languages'' 10th Print. Prentice Hall International, . * Brian W Marsden (1986): ''Communication network protocols'' 2nd Edition. Chartwell Bratt, . *
Andrew S. Tanenbaum Andrew Stuart Tanenbaum (born March 16, 1944), sometimes referred to by the handle ast, is an American-Dutch computer scientist and professor emeritus of computer science Computer science deals with the theoretical foundations of information ...

Andrew S. Tanenbaum
(1984): ''Structured computer organization'' 10th Print. Prentice Hall International, .


Further reading

*
Radia Perlman Radia Joy Perlman (born December 18, 1951) is an American computer programmer and network engineer. She is most famous for her invention of the spanning-tree protocol (STP), which is fundamental to the operation of network bridge A network ...

Radia Perlman
, ''Interconnections: Bridges, Routers, Switches, and Internetworking Protocols (2nd Edition)''. Addison-Wesley 1999. . In particular Ch. 18 on "network design folklore". * Gerard J. Holzmann, ''Design and Validation of Computer Protocols''. Prentice Hall, 1991. . Also available online at http://spinroot.com/spin/Doc/Book91.html


External links


Javvin's Protocol Dictionary


{{Authority control Data transmission *