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{{short description|System for exchanging messages between computing systems}}
{{Use American English|date=March 2020}}
{{Use dmy dates|date=December 2022}}

A '''communication protocol''' is a system of rules that allows two or more entities of a [[communications system]] to transmit [[information]] via any variation of a [[physical quantity]]. The protocol defines the rules, [[syntax]], [[semantics (computer science)|semantics]], and [[synchronization]] of [[communication]] and possible [[Error detection and correction|error recovery methods]]. Protocols may be implemented by [[Computer hardware|hardware]], [[software]], or a combination of both.<ref>{{Cite patent|title=Wireless communication protocol|country=US|number=7529565|pubdate=2009-05-05|assign1=[[Starkey Hearing Technologies|Starkey Laboratories Inc.]]|assign2=[[Oticon|Oticon AS]]|inventor1-last=Hilpisch|inventor1-first=Robert E.|inventor2-last=Duchscher|inventor2-first=Rob|inventor3-last=Seel|inventor3-first=Mark|inventor4-last=Hansen|inventor4-first=Peter Soren Kirk}}</ref>

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 predetermined for that particular situation. The specified behavior is typically independent of how it is to be [[Implementation|implemented]]. Communication protocols have to be agreed upon by the parties involved.<ref>{{citation |url=https://www.britannica.com/EBchecked/topic/410357/protocol |title=Protocol |publisher=[[Encyclopædia Britannica]] |access-date=24 September 2012 |archive-date=12 September 2012 |archive-url=https://web.archive.org/web/20120912020227/http://www.britannica.com/EBchecked/topic/410357/protocol |url-status=live }}</ref> To reach an agreement, a protocol may be developed into a [[technical standard]]. A [[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''.<ref name="AnalogyII">Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, "They (protocols) are to communication what programming languages are to computation"</ref> An alternate formulation states that ''protocols are to communication what [[algorithm]]s are to computation''.<ref name="AnalogyI">Comer 2000, Sect. 1.3 - Internet Services, p. 3, "Protocols are to communication what algorithms are to computation"</ref>

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]].

Internet communication protocols are published by the [[Internet Engineering Task Force]] (IETF). The [[IEEE]] (Institute of Electrical and Electronics Engineers) handles wired and wireless networking and the [[International Organization for Standardization]] (ISO) handles other types. The [[ITU-T]] handles [[telecommunications]] protocols and formats for the [[public switched telephone network]] (PSTN). As the PSTN and Internet [[Technological convergence|converge]], the standards are also being driven towards convergence.

==Communicating systems==
=== History ===
{{Further|Protocol Wars}}
The first use of the term ''protocol'' in a modern data-commutation context occurs in April 1967 in a memorandum entitled ''A Protocol for Use in the NPL Data Communications Network.'' Under the direction of [[Donald Davies]], who pioneered [[packet switching]] at the [[National Physical Laboratory (United Kingdom)|National Physical Laboratory]] in the United Kingdom, it was written by [[Roger Scantlebury]] and Keith Bartlett for the [[NPL network]].<ref>{{Cite book|last=Naughton|first=John|url=https://books.google.com/books?id=bbonCgAAQBAJ&pg=PT290|title=A Brief History of the Future|date=24 September 2015|publisher=Orion|isbn=978-1-4746-0277-8|language=en}}</ref><ref>{{cite journal |last1=Campbell-Kelly |first1=Martin |title=Data Communications at the National Physical Laboratory (1965-1975) |journal=IEEE Annals of the History of Computing |date=July 1987 |volume=9 |issue=3 |pages=221–247 |doi=10.1109/MAHC.1987.10023 }}</ref><ref name="Pelkey6.1a">{{Cite book |last=Pelkey |first=James L. |title=Entrepreneurial Capitalism and Innovation: A History of Computer Communications 1968–1988 |chapter=6.1 The Communications Subnet: BBN 1969 |quote=As Kahn recalls: ... Paul Baran’s contributions ... I also think Paul was motivated almost entirely by voice considerations. If you look at what he wrote, he was talking about switches that were low-cost electronics. The idea of putting powerful computers in these locations hadn’t quite occurred to him as being cost effective. So the idea of computer switches was missing. The whole notion of protocols didn’t exist at that time. And the idea of computer-to-computer communications was really a secondary concern. |chapter-url=https://historyofcomputercommunications.info/section/6.1/the-communications-subnet-bbn-1969/}}</ref><ref name=":5a">{{Cite book |last=Waldrop |first=M. Mitchell |url=https://books.google.com/books?id=eRnBEAAAQBAJ&pg=PT285 |title=The Dream Machine |date=2018 |publisher=Stripe Press |isbn=978-1-953953-36-0 |pages=286 |language=en |quote=Baran had put more emphasis on digital voice communications than on computer communications.}}</ref><ref>{{Cite journal |last=Kleinrock |first=L. |date=1978 |title=Principles and lessons in packet communications |journal=Proceedings of the IEEE |volume=66 |issue=11 |pages=1320–1329 |doi=10.1109/PROC.1978.11143 |quote=Paul Baran ... focused on the routing procedures and on the survivability of distributed communication systems in a hostile  environment, but did not concentrate on the need for resource sharing in its form as we now understand it; indeed, the concept of a software switch was not present in his work. }}</ref>

On the [[ARPANET]], the starting point for host-to-host communication in 1969 was the [[1822 protocol]], written by [[Bob Kahn]], which defined the transmission of messages to an IMP.<ref>{{cite report|url=http://www.bitsavers.org/pdf/bbn/imp/BBN1822_Jan1976.pdf|title=Interface Message Processor: Specifications for the Interconnection of a Host and an IMP|id=Report No. 1822|publisher=[[Bolt Beranek and Newman]] (BBN)}}</ref> The [[Network Control Program (ARPANET)|Network Control Program]] (NCP) for the ARPANET, developed by [[Steve Crocker]] and other graduate students including [[Jon Postel]] and [[Vint Cerf]], was first implemented in 1970.<ref>{{Cite book|url=https://books.google.com/books?id=dRRDDwAAQBAJ&pg=PA319|title=UGC -NET/JRF/SET PTP & Guide Teaching and Research Aptitude: UGC -NET By HD|last=BOOKS|first=HIGH DEFINITION|publisher=High Definition Books|language=en}}</ref> The NCP interface allowed [[application software]] to connect across the ARPANET by implementing higher-level communication protocols, an early example of the ''protocol layering'' concept.<ref name="LINCP">{{cite web|url=https://livinginternet.com/i/ii_ncp.htm|title=NCP – Network Control Program|website=Living Internet|access-date=8 October 2022|archive-date=7 August 2022|archive-url=https://web.archive.org/web/20220807153805/https://livinginternet.com/i/ii_ncp.htm|url-status=live}}</ref>

The [[CYCLADES]] network, designed by [[Louis Pouzin]] in the early 1970s was the first to implement the [[end-to-end principle]], and make the [[Host (network)|hosts]] responsible for the reliable delivery of data on a packet-switched network, rather than this being a service of the network itself.<ref name="Bennett2009">{{cite web |last1=Bennett |first1=Richard |date=September 2009 |title=Designed for Change: End-to-End Arguments, Internet Innovation, and the Net Neutrality Debate |url=https://www.itif.org/files/2009-designed-for-change.pdf |access-date=11 September 2017 |publisher=Information Technology and Innovation Foundation |pages=7, 11}}</ref> His team was the first to tackle the highly complex problem of providing user applications with a reliable [[virtual circuit]] service while using a [[best-effort service]], an early contribution to what will be the [[Transmission Control Protocol]] (TCP).<ref>{{Cite book |last=Abbate |first=Janet |url=https://books.google.com/books?id=E2BdY6WQo4AC&pg=PA125 |title=Inventing the Internet |date=2000 |publisher=MIT Press |isbn=978-0-262-51115-5 |pages=124–127 |language=en |quote=In fact, CYCLADES, unlike ARPANET, had been explicitly designed to facilitate internetworking; it could, for instance, handle varying formats and varying levels of service |author-link=Janet Abbate}}</ref><ref name="Kim2005">{{cite book |last1=Kim |first1=Byung-Keun |url=https://books.google.com/books?id=lESrw3neDokC&pg=PA54 |title=Internationalising the Internet the Co-evolution of Influence and Technology |date=2005 |publisher=Edward Elgar |isbn=1845426754 |pages=51–55 |quote=In addition to the NPL Network and the ARPANET, CYCLADES, an academic and research experimental network, also played an important role in the development of computer networking technologies}}</ref><ref name=":7">{{Cite news |date=2013-11-30 |title=The internet's fifth man |url=https://www.economist.com/news/technology-quarterly/21590765-louis-pouzin-helped-create-internet-now-he-campaigning-ensure-its |access-date=2020-04-22 |newspaper=The Economist |quote=In the early 1970s Mr Pouzin created an innovative data network that linked locations in France, Italy and Britain. Its simplicity and efficiency pointed the way to a network that could connect not just dozens of machines, but millions of them. It captured the imagination of Dr Cerf and Dr Kahn, who included aspects of its design in the protocols that now power the internet.}}</ref>

[[Bob Metcalfe]] and others at [[Xerox PARC]] outlined the idea of [[Ethernet]] and the [[PARC Universal Packet]] (PUP) for internetworking.<ref name="Moschovitisp78-9">{{harvnb|Moschovitis|1999|p=[https://archive.org/details/historyofinterne0000unse/page/78/mode/2up 78-9]}}</ref>

Research in the early 1970s by Bob Kahn and Vint Cerf led to the formulation of the [[Transmission Control Program]] (TCP).<ref>{{cite journal |last1=Cerf |first1=V. |last2=Kahn |first2=R. |title=A Protocol for Packet Network Intercommunication |journal=IEEE Transactions on Communications |date=May 1974 |volume=22 |issue=5 |pages=637–648 |doi=10.1109/TCOM.1974.1092259 |quote=The authors wish to thank a number of colleagues for helpful comments during early discussions of international network protocols, especially R. Metcalfe, R. Scantlebury, D. Walden, and H. Zimmerman; D. Davies and L. Pouzin who constructively commented on the fragmentation and accounting issues; and S. Crocker who commented on the creation and destruction of associations. }}</ref> Its {{IETF RFC|675}} specification was written by Cerf with [[Yogen Dalal]] and Carl Sunshine in December 1974, still a monolithic design at this time.

The [[International Network Working Group]] agreed on a connectionless [[datagram]] standard which was presented to the [[CCITT]] in 1975 but was not adopted by the CCITT nor by the ARPANET.<ref name=":0">{{cite journal |last1=McKenzie |first1=Alexander |title=INWG and the Conception of the Internet: An Eyewitness Account |journal=IEEE Annals of the History of Computing |date=January 2011 |volume=33 |issue=1 |pages=66–71 |doi=10.1109/MAHC.2011.9 |bibcode=2011IAHC...33a..66M }}</ref> Separate international research, particularly the work of [[Rémi Després]], contributed to the development of the [[X.25]] standard, based on [[virtual circuit]]s, which was adopted by the CCITT in 1976.<ref>{{cite journal |last1=Schwartz |first1=Mischa |title=X.25 Virtual Circuits - TRANSPAC IN France - Pre-Internet Data Networking [History of communications] |journal=IEEE Communications Magazine |date=November 2010 |volume=48 |issue=11 |pages=40–46 |doi=10.1109/MCOM.2010.5621965 }}</ref><ref>{{cite journal |last1=Rybczynski |first1=Tony |title=Commercialization of packet switching (1975-1985): A Canadian perspective [History of Communications] |journal=IEEE Communications Magazine |date=December 2009 |volume=47 |issue=12 |pages=26–31 |doi=10.1109/MCOM.2009.5350364 }}</ref> Computer manufacturers developed [[proprietary protocol]]s such as IBM's [[Systems Network Architecture]] (SNA), Digital Equipment Corporation's [[DECnet]] and [[Xerox Network Systems]].<ref>{{Cite book|url=https://books.google.com/books?id=y-tFrTTweRIC&pg=PA337|title=The "Hidden" Prehistory of European Research Networking|publisher=Trafford Publishing|isbn=978-1-4669-3935-6|pages=354|language=en}}</ref>

TCP software was redesigned as a modular protocol stack, referred to as ''TCP/IP.'' This was installed on [[SATNET]] in 1982 and on the ARPANET in January 1983. The development of a complete [[Internet protocol suite]] by 1989, as outlined in {{IETF RFC|1122}} and {{IETF RFC|1123}}, laid the foundation for the growth of TCP/IP as a comprehensive protocol suite as the core component of the emerging [[Internet]].<ref name="LITCPIP">{{cite web|url=https://livinginternet.com/i/ii_tcpip.htm|title=TCP/IP Internet Protocol|website=Living Internet|access-date=8 October 2022|archive-date=1 September 2022|archive-url=https://web.archive.org/web/20220901071957/https://livinginternet.com/i/ii_tcpip.htm|url-status=live}}</ref>

International work on a reference model for communication standards led to the [[OSI model]], published in 1984. For a period in the late 1980s and early 1990s, engineers, organizations and nations became [[Protocol Wars|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.<ref name="ieee2017032">{{cite magazine|author=Andrew L. Russell|date=30 July 2013|title=OSI: The Internet That Wasn't|url=https://spectrum.ieee.org/osi-the-internet-that-wasnt|magazine=[[IEEE Spectrum]]|volume=50|issue=8}}</ref><ref>{{Cite web|url=https://www2.cs.duke.edu/courses/common/compsci092/papers/govern/consensus.pdf|title=Rough Consensus and Running Code' and the Internet-OSI Standards War|last=Russell|first=Andrew L.|publisher=IEEE Annals of the History of Computing|access-date=23 February 2020|archive-date=17 November 2019|archive-url=https://web.archive.org/web/20191117080112/https://www2.cs.duke.edu/courses/common/compsci092/papers/govern/consensus.pdf|url-status=live}}</ref><ref>{{Cite web|date=2006|title=Standards Wars|url=https://courses.cs.washington.edu/courses/csep590a/06au/projects/standards-wars.pdf|access-date=23 February 2020|archive-date=24 February 2021|archive-url=https://web.archive.org/web/20210224162416/https://courses.cs.washington.edu/courses/csep590a/06au/projects/standards-wars.pdf|url-status=live}}</ref>

=== 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 (computer science)|state]]-dependent behaviors, is defined by these specifications. In digital computing systems, the rules can be expressed by [[algorithm]]s and [[data structure]]s. Protocols are to communication what algorithms or programming languages are to computations.<ref name="AnalogyII"/><ref name="AnalogyI"/>

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.<ref name="Ben-Ari p.18">Ben-Ari 1982, chapter 2 - The concurrent programming abstraction, p. 18-19, states the same.</ref><ref name="Ben-Ari p.27">Ben-Ari 1982, Section 2.7 - Summary, p. 27, summarizes the concurrent programming abstraction.</ref> In contrast, because there is no [[shared memory]], communicating systems have to communicate with each other using a shared [[transmission medium]]. 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.<ref name="Marsden p.64"/> When protocol algorithms are expressed in a portable programming language the protocol software may be made [[operating system]] independent. The best-known frameworks are the [[TCP/IP model]] and the [[OSI model]].

At the time the Internet was developed, [[abstraction layer]]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.<ref>Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, explains this by drawing analogies between computer communication and programming languages.</ref> This gave rise to the concept of layered protocols which nowadays forms the basis of protocol design.<ref name="Sect p. 192">Sect. 11.10 - The Disadvantage Of Layering, p. 192, states: layering forms the basis for protocol design.</ref>

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]].<ref name="Sect p. 177">Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, states the same.</ref> Some of the best-known protocol suites are [[TCP/IP]], [[IPX/SPX]], [[X.25]], [[AX.25]] and [[AppleTalk]].

The protocols can be arranged based on functionality in groups, for instance, there is a group of [[transport protocol]]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.<ref name="Sect p. 178">Comer 2000, Sect. 11.3 - The Conceptual Layers Of Protocol Software, p. 178, "Each layer takes responsibility for handling one part of the problem."</ref> 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.<ref>Comer 2000, Sect. 11.11 - The Basic Idea Behind Multiplexing And Demultiplexing, p. 192, states the same.</ref>

==Types==
There are two types of communication protocols, based on their representation of the content being carried: text-based and binary.{{fact|date=July 2025}}

=== Text-based ===
A '''text-based protocol''' or '''plain text protocol''' represents its content in [[human-readable format]], often in [[plain text]] encoded in a machine-readable encoding such as [[ASCII]] or [[UTF-8]], or in structured text-based formats such as [[Intel hex format]], [[XML]] or [[JSON]].

The immediate human readability stands in contrast to native binary protocols which have inherent benefits for use in a computer environment (such as ease of mechanical [[parsing]] and [[Lossless compression|improved bandwidth utilization]]).

Network applications have various 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]] 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]]), SMTP ([[Simple Mail Transfer Protocol]]), early versions of HTTP ([[Hypertext Transfer Protocol]]), and the [[finger protocol]].<ref>{{cite web|last=Kirch|first=Olaf|date=16 January 2002|title=Text Based Protocols|url=http://www.lst.de/~okir/blackhats/node76.html|url-status=dead|archive-url=https://web.archive.org/web/20100530140215/http://www.lst.de/~okir/blackhats/node76.html|archive-date=30 May 2010|access-date=21 October 2014}}</ref>

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]] and during early protocol development design phases.

=== Binary ===
A '''binary protocol''' utilizes all values of a [[byte]], as opposed to a text-based protocol which only uses values corresponding to human-readable characters in [[ASCII]] 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.<ref>{{cite web|last=Kirch|first=Olaf|date=16 January 2002|title=Binary Representation Protocols|url=http://www.lst.de/~okir/blackhats/node77.html|url-status=dead|archive-url=https://web.archive.org/web/20100530225453/http://www.lst.de/~okir/blackhats/node77.html|archive-date=30 May 2010|access-date=4 May 2006}}</ref>

Binary have been used in the normative documents describing modern standards like [[EbXML]], [[HTTP/2]], [[HTTP/3]] and [[Enterprise Distributed Object Computing|EDOC]].<ref>{{cite web|last=Kirch|first=Olaf|date=16 January 2002|title=Binary Representation Protocols|url=http://www.omg.org/technology/documents/formal/edoc.htm|url-status=dead|archive-url=https://web.archive.org/web/20060305095832/http://www.omg.org/technology/documents/formal/edoc.htm|archive-date=5 March 2006|access-date=4 May 2006}}</ref> An interface in UML<ref>{{cite web|author=|date=|title=Welcome To UML Web Site!|url=https://www.uml.org/|website=Uml.org|accessdate=15 January 2017|archive-date=30 September 2019|archive-url=https://web.archive.org/web/20190930184152/https://www.uml.org/|url-status=live}}</ref> 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 kinds 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 kinds 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:<ref>Marsden 1986, Chapter 3 - Fundamental protocol concepts and problem areas, p. 26-42, explains much of the following.</ref>
;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]] (MTU) are divided in pieces of appropriate size.<ref>Comer 2000, Sect. 7.7.4 - Datagram Size, Network MTU, and Fragmentation, p. 104, Explains fragmentation and the effect on the header of the fragments.</ref>
;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''.<ref>Comer 2000, Chapter 4 - Classful Internet Addresses, p. 64-67;71.</ref>
;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''.<ref>Marsden 1986, Section 14.3 - Layering concepts and general definitions, p. 187, explains address mapping.</ref>
;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]]''.
;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.<ref>Marsden 1986, Section 3.2 - Detection and transmission errors, p. 27, explains the advantages of backward error correction.</ref>
;Acknowledgements
:Acknowledgement of correct reception of packets is required for [[connection-oriented communication]]. Acknowledgments are sent from receivers back to their respective senders.<ref>Marsden 1986, Section 3.3 - Acknowledgement, p. 28-33, explains the advantages of positive only acknowledgment and mentions datagram protocols as exceptions.</ref>
;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 [[Timeout (computing)|timeouts]], the sender may need to retransmit the information.{{efn|Failure to receive an acknowledgment indicates that either the original transmission or the acknowledgment was lost. The sender has no means to distinguish these cases and therefore, to ensure all data is received, must make the conservative assumption that the original transmission was lost.}} 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.<ref>Marsden 1986, Section 3.4 - Loss of information - timeouts and retries, p. 33-34.</ref>
;Direction of information flow
:Direction needs to be addressed if transmissions can only occur in one direction at a time as on [[half-duplex]] links or from one sender at a time as on a [[shared medium]]. This is known as [[media access control]]. Arrangements have to be made to accommodate the case of [[Collision (telecommunications)|collision]] or [[Contention (telecommunications)|contention]] where two parties respectively simultaneously transmit or wish to transmit.<ref>Marsden 1986, Section 3.5 - Direction of information flow, p. 34-35, explains master/slave and the negotiations to gain control.</ref>
;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.<ref>Marsden 1986, Section 3.6 - Sequence control, p. 35–36, explains how packets get lost and how sequencing solves this.</ref>
;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.<ref>Marsden 1986, Section 3.7 - Flow control, p. 36–38.</ref>
;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]] 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,<ref name="Sect p. 177"/> within a conceptual framework.

Communicating systems operate concurrently. An important aspect of [[concurrent programming]] 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.<ref>Ben-Ari 1982, in his preface, p. xiii.</ref> Formal verification seems indispensable because concurrent programs are notorious for the hidden and sophisticated bugs they contain.<ref>Ben-Ari 1982, in his preface, p. xiv.</ref> A mathematical approach to the study of concurrency and communication is referred to as [[communicating sequential processes]] (CSP).<ref>Hoare 1985, Chapter 4 - Communication, p. 133, deals with communication.</ref> Concurrency can also be modeled using [[finite-state machine]]s, such as [[Mealy machine|Mealy]] and [[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.<ref>{{citation |author=S. Srinivasan |publisher=NPTEL courses |title=Digital Circuits and Systems |url=http://nptel.iitm.ac.in/video.php?courseId=1005&p=3 |url-status=dead |archive-url=https://web.archive.org/web/20091227210642/http://nptel.iitm.ac.in/video.php?courseId=1005&p=3 |archive-date=27 December 2009 }}</ref>{{better source needed|date=September 2018}}

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===
[[File:Internet layering.svg|thumb|alt=Figure 2. Protocols in relation to the Internet layering scheme.|The TCP/IP model or Internet layering scheme and its relation to some common protocols.]]

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]] of cases, keeping each design relatively simple.

The communication protocols in use on the [[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]].<ref name="Comer decomposition"/> The first two cooperating protocols, the [[Transmission Control Protocol]] (TCP) and the [[Internet Protocol]] (IP) resulted from the decomposition of the original Transmission Control Program, a monolithic communication protocol, into this layered communication suite.

The [[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]] 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]]. Layering provides opportunities to exchange technologies when needed, for example, protocols are often stacked in a [[Tunneling protocol|tunneling]] arrangement to accommodate the connection of dissimilar networks. For example, IP may be tunneled across an [[Asynchronous Transfer Mode]] (ATM) network.

====Protocol layering====
[[File:Message flows.svg|thumb|alt=Figure 3. Message flows using a protocol suite.|Figure 3. Message flows using a protocol suite. Black loops show the actual messaging loops, red loops are the effective communication between layers enabled by the lower layers.]]

Protocol layering forms the basis of protocol design.<ref name="Sect p. 192"/> It allows the decomposition of single, complex protocols into simpler, cooperating protocols.<ref name="Comer decomposition">Comer 2000, Sect. 11.2 - The Need For Multiple Protocols, p. 177, introduces the decomposition in layers.</ref> 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]] is some kind of message flow diagram.<ref name="AnalogyI"/> 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====
[[File:Protocol and Software layering.svg|thumb |Figure 5: Protocol and software layering. The software modules implementing the protocols are represented by cubes. The information flow between the modules is represented by arrows. The (top two horizontal) red arrows are virtual. The blue lines mark the layer boundaries.]]

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.<ref>Comer 2000, Sect. 11.3 - The Conceptual Layers Of Protocol Software, p. 179, the first two paragraphs describe the sending of a message through successive layers.</ref>

[[Program translation]] 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.<ref name="Comer p. 178">Comer 2000, Sect. 11.2 - The need for multiple protocols, p. 178, explains similarities protocol software and compiler, assembler, linker, loader.</ref>

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.<ref>Comer 2000, Sect. 11.9.1 - Operating System Boundary, p. 192, describes the operating system boundary.</ref>

====Strict layering====
Strictly adhering to a layered model, a practice known as strict layering, is not always the best approach to networking.<ref>IETF 1989, Sect 1.3.1 - Organization, p. 15, 2nd paragraph: many design choices involve creative "breaking" of strict layering.</ref> Strict layering can have a negative impact on the performance of an implementation.<ref>Comer 2000, Sect. 11.10 - The Disadvantage Of Layering, p. 192, explains why "strict layering can be extremely inefficient" giving examples of optimizations.</ref>

Although the use of protocol layering is today ubiquitous across the field of computer networking, it has been historically criticized by many researchers<ref>{{cite journal| last = Wakeman| first = I| date = Jan 1992| title = Layering considered harmful| journal = IEEE Network| pages =20–24}}</ref> 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.<ref>{{cite book |last1=Kurose |first1=James | last2=Ross | first2=Keith |year= 2005 | title=Computer Networking: A Top-Down Approach |publisher=Pearson}}</ref>

===Design patterns===
Commonly recurring problems in the design and implementation of communication protocols can be addressed by [[software design pattern]]s.<ref>{{cite web|first1=Jorge Edison|last1=Lascano|first2=Stephen|last2=Clyde|first3=Ali|last3=Raza|title=Communication-protocol Design Patterns (CommDP) - COMMDP|url=http://commdp.serv.usu.edu/wiki/index.php/Communication-protocol_Design_Patterns_(CommDP)|archive-url=https://web.archive.org/web/20170318090654/http://commdp.serv.usu.edu/wiki/index.php/Communication-protocol_Design_Patterns_(CommDP)|archive-date=18 March 2017|access-date=17 March 2017|url-status=dead}}</ref><ref>{{cite conference|first1=J. E.|last1=Lascano|first2=S.|last2=Clyde|title=A Pattern Language for Application-level Communication Protocols|conference=ICSEA 2016, The Eleventh International Conference on Software Engineering Advances|date=2016|pages=22–30}}</ref><ref>{{cite book|first=R.|last=Daigneau|title=Service Design Patterns: Fundamental Design Solutions for SOAP/WSDL and RESTful Web Services|edition=1|location=Upper Saddle River, NJ|publisher=Addison-Wesley Professional|date=2011}}</ref><ref>{{cite book|first=M.|last=Fowler|author-link=Martin Fowler (software engineer)|title=Patterns of Enterprise Application Architecture|edition=1|location=Boston|publisher=Addison-Wesley Professional|date=2002|isbn=0-321-12742-0}}</ref><ref>[1]F. Buschmann, K. Henney, and D. C. Schmidt, Pattern-Oriented Software Architecture Volume 4: A Pattern Language for Distributed Computing, Volume 4 edition. Chichester England; New York: Wiley, 2007.</ref>

===Formal specification===
Popular formal methods of describing communication syntax are [[Abstract Syntax Notation One]] (an [[ISO]] standard) and [[augmented Backus–Naur form]] (an [[IETF]] standard).

[[Finite-state machine]] models are used to formally describe the possible interactions of the protocol.<ref>{{Cite journal | last1 = Bochmann | first1 = G. | title = Finite state description of communication protocols | doi = 10.1016/0376-5075(78)90015-6 | journal = Computer Networks | volume = 2 | issue = 4–5 | pages = 361–372| year = 1978 }}</ref><ref>Comer 2000, Glossary of Internetworking Terms and Abbreviations, p. 704, term protocol.</ref> and communicating finite-state machines<ref>{{cite journal |last1=Brand |first1=Daniel |last2=Zafiropulo |first2=Pitro |title=On Communicating Finite-State Machines |journal=Journal of the ACM |date=April 1983 |volume=30 |issue=2 |pages=323–342 |doi=10.1145/322374.322380 |doi-access=free }}</ref>

==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]], 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 [[Binary Synchronous Communications]] (BSC) protocol invented by [[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.<ref name="Marsden p.64">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.</ref>

In some cases, protocols gain market dominance without going through a standardization process. Such protocols are referred to as ''[[de facto standard]]s''. De facto standards are common in emerging markets, niche markets, or markets that are [[monopolized]] (or [[oligopoly|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]]s of relevance for communication protocols are the [[International Organization for Standardization]] (ISO), the [[International Telecommunication Union]] (ITU), the [[Institute of Electrical and Electronics Engineers]] (IEEE), and the [[Internet Engineering Task Force]] (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]] (PSTN), as well as many [[radio]] communication systems. For [[marine electronics]] the [[NMEA]] standards are used. The [[World Wide Web Consortium]] (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.<ref>Marsden 1986, Section 6.3 - Advantages of standardization, p. 66-67, states the same.</ref>

Multiple standards bodies may be involved in the development of a protocol. If they are uncoordinated, then the result may be multiple, incompatible definitions of a protocol, or multiple, incompatible interpretations of messages; important invariants in one definition (e.g., that [[time-to-live]] values are [[monotone decreasing]] to prevent stable [[routing loops]]) may not be respected in another.{{sfn|Bryant|Morrow|2009|p=4}}

===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.<ref>Marsden 1986, Section 6.4 - Some problems with standardisation, p. 67, follows HDLC to illustrate the process.</ref>

===OSI standardization===
{{OSI model|expanded=all}}

A lesson learned from [[ARPANET]], 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).<ref>Marsden 1986, Section 6.1 - Why are standards necessary?, p. 65, explains lessons learned from ARPANET.</ref> This gave rise to the [[Open Systems Interconnection model]] (OSI model), which is used as a framework for the design of standard protocols and services conforming to the various layer specifications.<ref>Marsden 1986, Section 14.1 - Introduction, p. 181, introduces OSI.</ref>

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.<ref>Marsden 1986, Section 14.3 - Layering concepts and general definitions, p. 183-185, explains terminology.</ref> 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 OSI model, the layers and their functionality are (from highest to lowest layer):
* The ''[[Application layer]]'' 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.<ref>Marsden 1986, Section 14.4 - The application layer, p. 188, explains this.</ref>
* The ''[[presentation layer]]'' 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).<ref>Marsden 1986, Section 14.5 - The presentation layer, p. 189, explains this.</ref>
* The ''[[session layer]]'' 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.<ref>Marsden 1986, Section 14.6 - The session layer, p. 190, explains this.</ref>
* The ''[[transport layer]]'' 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.<ref>Marsden 1986, Section 14.7 - The transport layer, p. 191, explains this.</ref>
* The ''[[network layer]]'' 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]] control.<ref>Marsden 1986, Section 14.8 - The network layer, p. 192, explains this.</ref>
* The ''[[data link layer]]'' 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.<ref>Marsden 1986, Section 14.9 - The data link layer, p. 194, explains this.</ref>
* The ''[[physical layer]]'' describes details like the electrical characteristics of the physical connection, the transmission techniques used, and the setup, maintenance and clearing of physical connections.<ref>Marsden 1986, Section 14.10 - The physical layer, p. 195, explains this.</ref>

In contrast to the [[#Software layering|TCP/IP layering scheme]], which assumes a connectionless network, RM/OSI assumed a connection-oriented network.<ref>{{cite ISO standard|csnumber=14252|title=ISO 7498:1984 – Information processing systems - Open Systems Interconnection - Basic Reference Model|page=5|quote=This Basic Reference Model of Open Systems Interconnection is based on the assumption that a connection is required for the transfer of data.}}</ref> Connection-oriented networks are more suitable for wide area networks and connectionless networks are more suitable for local area networks. Connection-oriented communication requires 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 the development of RM/OSI concentrated on connection-oriented networks and connectionless networks were first mentioned in an addendum to RM/OSI<ref>{{cite ISO standard|csnumber=14253|title=ISO 7498:1984/ADD 1:1987 – Information processing systems — Open Systems Interconnection — Basic Reference Model — Addendum 1}}</ref><ref>Marsden 1986, Section 14.11 - Connectionless mode and RM/OSI, p. 195, mentions this.</ref> and later incorporated into an update to RM/OSI.<ref>{{cite ISO standard|csnumber=20269|title=ISO 7498:1994 – Information processing systems - Open Systems Interconnection - Basic Reference Model}}</ref>

At the time,{{when|reason=RFC 791 for IP came out in 1981, the RM/OSI was updated to handle connectionless in 1987, and ISO 8473, CLNP, dates back to 1988.|date=March 2022}} the IETF had to cope with this and the fact that the Internet needed protocols that simply were not there.{{cn|date=March 2022}} As a result, the IETF developed its own standardization process based on "rough consensus and running code".<ref>Comer 2000, Section 1.9 - Internet Protocols And Standardization, p. 12, explains why the IETF did not use existing protocols.</ref> The standardization process is described by {{IETF RFC|2026}}.

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.{{cn|date=March 2022}}

== Wire image ==
{{Main|Wire data}}
The ''wire image'' of a protocol is the information that a non-participant observer is able to glean from observing the protocol messages, including both information explicitly given meaning by the protocol, but also inferences made by the observer.{{sfn|Trammell|Kuehlewind|2019|p=2}} Unencrypted protocol metadata is one source making up the wire image, and [[side-channel attack|side-channels]] including packet timing also contribute.{{sfn|Trammell|Kuehlewind|2019|p=3}} Different observers with different vantages may see different wire images.{{sfn|Trammell|Kuehlewind|2019|p=4}} 
The wire image is relevant to end-user [[Internet privacy|privacy]] and the [[extensibility]] of the protocol.{{sfn|Fairhurst|Perkins|2021|loc=7. Conclusions}}

If some portion of the wire image is not cryptographically [[authenticated]], it is subject to modification by intermediate parties (i.e., [[middleboxes]]), which can influence protocol operation.{{sfn|Trammell|Kuehlewind|2019|p=3}} Even if authenticated, if a portion is not encrypted, it will form part of the wire image, and intermediate parties may intervene depending on its content (e.g., dropping packets with particular flags). Signals deliberately intended for intermediary consumption may be left authenticated but unencrypted.{{sfn|Trammell|Kuehlewind|2019|p=5}}

The wire image can be deliberately engineered, encrypting parts that intermediaries should not be able to observe and providing signals for what they should be able to.{{sfn|Trammell|Kuehlewind|2019|p=6}} If provided signals are decoupled from the protocol's operation, they may become untrustworthy.{{sfn|Trammell|Kuehlewind|2019|p=7-8}} Benign network management and research are affected by metadata encryption; protocol designers must balance observability for operability and research against ossification resistance and end-user privacy.{{sfn|Fairhurst|Perkins|2021|loc=7. Conclusions}} The IETF announced in 2014 that it had determined that large-scale surveillance of protocol operations is an attack due to the ability to infer information from the wire image about users and their behaviour,{{sfn|Farrell|Tschofenig|2014|p=2}} and that the IETF would "work to mitigate pervasive monitoring" in its protocol designs;{{sfn|Farrell|Tschofenig|2014|p=3}} this had not been done systematically previously.{{sfn|Farrell|Tschofenig|2014|p=3}} The [[Internet Architecture Board]] recommended in 2023 that disclosure of information by a protocol to the network should be intentional,{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.1. Intentional Distribution}} performed with the agreement of both recipient and sender,{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.2. Control of the Distribution of Information}} authenticated to the degree possible and necessary,{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.3. Protecting Information and Authentication}} only acted upon to the degree of its trustworthiness,{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.5. Limiting Impact of Information}} and minimised and provided to a minimum number of entities.{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.4. Minimize Information}}{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=2.6. Minimum Set of Entities}} Engineering the wire image and controlling what signals are provided to network elements was a "developing field" in 2023, according to the IAB.{{sfn|Arkko|Hardie|Pauly|Kühlewind|2023|loc=3. Further Work}}

== Ossification ==
{{main|protocol ossification}}

[[Protocol ossification]] is the loss of flexibility, [[extensibility]] and evolvability of [[network protocols]]. This is largely due to [[middlebox]]es that are sensitive to the wire image of the protocol, and which can interrupt or interfere with messages that are valid but which the middlebox does not correctly recognize.{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=619}} This is a violation of the [[end-to-end principle]].{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=620}} Secondary causes include inflexibility in endpoint implementations of protocols.{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=620-621}}

Ossification is a major issue in [[Internet]] protocol design and deployment, as it can prevent new protocols or extensions from being deployed on the Internet, or place strictures on the design of new protocols; new protocols may have to be [[encapsulation (networking)|encapsulated]] in an already-deployed protocol or mimic the wire image of another protocol.{{sfn|Papastergiou|Fairhurst|Ros|Brunstrom|2017|p=623-4}} Because of ossification, the [[Transmission Control Protocol]] (TCP) and [[User Datagram Protocol]] (UDP) are the only practical choices for [[transport protocol]]s on the Internet,{{sfn|McQuistin|Perkins|Fayed|2016|p=1}} and TCP itself has significantly ossified, making extension or modification of the protocol difficult.{{sfn|Thomson|Pauly|2021|loc=A.5. TCP}}

Recommended methods of preventing ossification include [[encrypting]] protocol metadata,{{sfn|Hardie|2019|p=7-8}} and ensuring that extension points are exercised and wire image variability is exhibited as fully as possible;{{sfn|Thomson|Pauly|2021|loc=3. Active Use}} remedying existing ossification requires coordination across protocol participants.{{sfn|Thomson|Pauly|2021|loc=3.5. Restoring Active Use}} [[QUIC]] is the first [[IETF]] transport protocol to have been designed with deliberate anti-ossification properties.{{sfn|Trammell|Kuehlewind|2019|p=2}}

==Taxonomies==
Classification schemes for protocols usually focus on the domain of use and function. As an example of domain of use, [[connection-oriented protocol]]s and [[connectionless protocol]]s are used on connection-oriented networks and connectionless networks respectively. An example of function is a [[tunneling protocol]], 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 [[#Protocol layering|''layering scheme'']] combines both function and domain of use. The dominant layering schemes are the ones developed 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.<ref>Comer 2000, Sect. 11.5.1 - The TCP/IP 5-Layer Reference Model, p. 183, states the same.</ref> 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==
*{{annotated link|Cryptographic protocol}}
*{{annotated link|Lists of network protocols}}
*{{annotated link|Protocol Builder}}

==Notes==
{{Notelist}}

==References==
{{Reflist}}

===Bibliography===
* {{cite book|author=Radia Perlman|author-link=Radia Perlman|title=Interconnections: Bridges, Routers, Switches, and Internetworking Protocols|edition=2nd|publisher=Addison-Wesley|date=1999|isbn=0-201-63448-1}}. In particular Ch. 18 on "network design folklore", which is also [https://www.informit.com/articles/article.aspx?p=20482 available online]
* {{cite book|author=Gerard J. Holzmann|author-link=Gerard J. Holzmann|title=Design and Validation of Computer Protocols|publisher=Prentice Hall|date=1991|isbn=0-13-539925-4|url=http://spinroot.com/spin/Doc/Book91.html}}
* {{cite book |author=Douglas E. Comer |year=2000 |title=Internetworking with TCP/IP - Principles, Protocols and Architecture |edition=4th |publisher=Prentice Hall |isbn=0-13-018380-6|author-link=Douglas E. Comer }} In particular Ch.11 Protocol layering. Also has a RFC guide and a Glossary of Internetworking Terms and Abbreviations.
* {{cite IETF|publisher=Internet Engineering Task Force abbr. IETF|date=1989|rfc=1122|title=Requirements for Internet Hosts -- Communication Layers|editor=R. Braden|editor-link=Bob Braden}} Describes TCP/IP to the implementors of protocolsoftware. In particular the introduction gives an overview of the design goals of the suite.
* {{cite book|author=M. Ben-ari|author-link=M. Ben-Ari|date=1982|title=Principles of concurrent programming|edition=10th Print|publisher=Prentice Hall International|isbn=0-13-701078-8}}
* {{cite book|author=C.A.R. Hoare|author-link=Tony Hoare|date=1985|title=Communicating sequential processes|edition=10th Print|publisher=Prentice Hall International|isbn=0-13-153271-5|url=http://www.usingcsp.com}}
* {{cite book|author=R.D. Tennent|date=1981|title=Principles of programming languages|edition=10th Print|publisher=Prentice Hall International|isbn=0-13-709873-1}}
* {{cite book|author=Brian W Marsden|date=1986|title=Communication network protocols|edition=2nd|publisher=Chartwell Bratt|isbn=0-86238-106-1}}
* {{cite book|author=Andrew S. Tanenbaum|author-link=Andrew S. Tanenbaum|date=1984|title=Structured computer organization|edition=10th Print|publisher=Prentice Hall International|isbn=0-13-854605-3}}
* {{ cite ietf | rfc = 5704 | title = Uncoordinated Protocol Development Considered Harmful | date = November 2009 | editor1-last = Bryant | editor1-first = Stewart | editor2-last = Morrow | editor2-first = Monique }}
* {{ cite ietf | rfc = 7258 | title = Pervasive Monitoring Is an Attack | date = May 2014 | last1 = Farrell | first1 = Stephen | last2 = Tschofenig | first2 = Hannes }}
* {{ cite ietf | rfc = 8546 | title = The Wire Image of a Network Protocol | date = April 2019 | last1 = Trammell | first1 = Brian | last2 = Kuehlewind | first2 = Mirja }}
* {{ cite ietf | rfc = 8558 | title = Transport Protocol Path Signals | date = April 2019 | editor-last = Hardie | editor-first = Ted }}
* {{ cite ietf | rfc = 9065 | title = Considerations around Transport Header Confidentiality, Network Operations, and the Evolution of Internet Transport Protocols | date = July 2021 | last1 = Fairhurst | first1 = Gorry | last2 = Perkins | first2 = Colin }}
* {{ cite ietf | rfc = 9170 | title = Long-Term Viability of Protocol Extension Mechanisms | date = December 2021 | last1 = Thomson | first1 = Martin | last2 = Pauly | first2 = Tommy }}
* {{ cite ietf | rfc = 9419 | title = Considerations on Application - Network Collaboration Using Path Signals | date = July 2023 | last1 = Arkko | first1 = Jari | last2 = Hardie | first2 = Ted | last3 = Pauly | first3 = Tommy | last4 = Kühlewind | first4 = Mirja }}
* {{ cite conference | doi = 10.1145/2959424.2959443 | title = Implementing Real-Time Transport Services over an Ossified Network | date = July 2016 | conference = 2016 Applied Networking Research Workshop | last1 = McQuistin | first1 = Stephen | last2 = Perkins | first2 = Colin | last3 = Fayed | first3 = Marwan | hdl = 1893/26111 | hdl-access = free }}
* {{ cite journal | doi = 10.1109/COMST.2016.2626780 | title = De-Ossifying the Internet Transport Layer: A Survey and Future Perspectives | date = 2017 | journal = [[IEEE Communications Surveys & Tutorials]] | last1 = Papastergiou | first1 = Giorgos | last2 = Fairhurst | first2 = Gorry | last3 = Ros | first3 = David | last4 = Brunstrom | first4 = Anna | last5 = Grinnemo | first5 = Karl-Johan | last6 = Hurtig | first6 = Per | last7 = Khademi | first7 = Naeem | last8 = Tüxen | first8 = Michael | last9 = Welzl | first9 = Michael | last10 = Damjanovic | first10 = Dragana | last11 = Mangiante | first11 = Simone | volume = 19 | pages = 619–639 | hdl = 2164/8317 | hdl-access = free }}
* {{cite book |last1=Moschovitis |first1=Christos J. P. |title=History of the Internet: A Chronology, 1843 to the Present |date=1999 |publisher=ABC-CLIO |isbn=978-1-57607-118-2 |url=https://archive.org/details/historyofinterne0000unse }}

==External links==
* {{web archive |url=https://web.archive.org/web/20040610001039/http://javvin.com/protocolsuite.html |title=Javvin's Protocol Dictionary}}
* [https://www.ipcomm.de/protocols_en.html Overview of protocols in telecontrol field with OSI Reference Model]

{{Telecommunications}}
{{Computer science}}
{{Authority control}}

[[Category:Communications protocols| ]]
[[Category:Data transmission]]
[[Category:Network protocols|*]]