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Text of the page (random words):
ementing higher level communication protocols an early example of the protocol layering concept 11 the cyclades network designed by louis pouzin in the early 1970s was the first to implement the end to end principle and make the hosts responsible for the reliable delivery of data on a packet switched network rather than this being a service of the network itself 12 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 13 14 15 bob metcalfe and others at xerox parc outlined the idea of ethernet and the parc universal packet pup for internetworking 16 research in the early 1970s by bob kahn and vint cerf led to the formulation of the transmission control program tcp 17 its 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 18 separate international research particularly the work of rémi després contributed to the development of the x 25 standard based on virtual circuits which was adopted by the ccitt in 1976 19 20 computer manufacturers developed proprietary protocols such as ibm s systems network architecture sna digital equipment corporation s decnet and xerox network systems 21 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 rfc 1122 and rfc 1123 laid the foundation for the growth of tcp ip as a comprehensive protocol suite as the core component of the emerging internet 22 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 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 23 24 25 concept edit 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 dependent behaviors are defined by these specifications in digital computing systems the rules can be expressed by algorithms and data structures protocols are to communication what algorithms or programming languages are to computations 3 4 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 26 27 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 28 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 layering 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 29 this gave rise to the concept of layered protocols which nowadays forms the basis of protocol design 30 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 31 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 protocols 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 32 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 33 message encoding edit communication protocols define the representation of messages exchanged between communicating systems common approaches to message encoding use text or binary representations citation needed text based edit a text based protocol or plain text protocol represents its messages 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 with binary message representations which have inherent benefits for use in a computer environment such as ease of mechanical parsing and improved bandwidth utilization network applications have various methods of encapsulating data one method very common with internet protocols is a text oriented message 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 their commands are ftp file transfer protocol smtp simple mail transfer protocol early versions of http hypertext transfer protocol and the finger protocol 34 text based message representations are typically easier for humans to inspect and interpret and are therefore suitable whenever human inspection of protocol contents is required such as during debugging and during early protocol development design phases binary edit a binary protocol uses a message representation that may utilize all values of a byte as opposed to a text based representation which is limited to values corresponding to characters in a character encoding such as ascii or utf 8 binary message representations are intended to be processed by machines rather than read directly by humans binary protocols have the advantage of terseness which translates into speed of transmission and interpretation 35 binary message representations have been used in modern standards such as ebxml http 2 http 3 and edoc 36 an interface in uml 37 may also be considered a binary protocol basic requirements edit 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 38 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 relevant to the operation of the protocol bitstrings longer than the maximum transmission unit mtu are divided in pieces of appropriate size 39 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 40 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 41 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 42 acknowledgements acknowledgement of correct reception of packets is required for connection oriented communication acknowledgments are sent from receivers back to their respective senders 43 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 a 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 44 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 or contention where two parties simultaneously transmit or wish to transmit 45 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 46 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 47 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 edit 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 31 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 48 formal verification seems indispensable because concurrent programs are notorious for the hidden and sophisticated bugs they contain 49 a mathematical approach to the study of concurrency and communication is referred to as communicating sequential processes csp 50 concurrency can also be modeled using finite state machines such as mealy and moore machines 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 51 better source needed 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 edit 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 52 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 proto...
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