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Text of the page (random words):
d 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 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 atm network protocol layering edit 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 30 it allows the decomposition of single complex protocols into simpler cooperating protocols 52 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 4 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 edit 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 53 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 54 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 55 strict layering edit strictly adhering to a layered model a practice known as strict layering is not always the best approach to networking 56 strict layering can have a negative impact on the performance of an implementation 57 although the use of protocol layering is today ubiquitous across the field of computer networking it has been historically criticized by many researchers 58 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 59 design patterns edit commonly recurring problems in the design and implementation of communication protocols can be addressed by software design patterns 60 61 62 63 64 formal specification edit 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 65 66 and communicating finite state machines 67 protocol development edit 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 edit 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 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