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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 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 28 in some cases protocols gain market dominance without going through a standardization process such protocols are referred to as de facto standards de facto standards are common in emerging markets niche markets or markets that are monopolized or oligopolized they can hold a market in a very negative grip especially when used to scare away competition from a historical perspective standardization should be seen as a measure to counteract the ill effects of de facto standards positive exceptions exist a de facto standard operating system like linux does not have this negative grip on its market because the sources are published and maintained in an open way thus inviting competition standards organizations edit some of the standards organizations 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 68 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 69 the standardization process edit 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 address deficiencies and reflect changing views on the subject 70 osi standardization edit osi model by layer 7 application layer nntp sip ssi dns ftp gopher http http 3 nfs ntp smpp ssh smtp snmp telnet dhcp netconf more 6 presentation layer mime xdr asn 1 ascii tls pgp more 5 session layer named pipe netbios sap pptp rtp socks x 225 71 more 4 transport layer tcp udp sctp dccp quic spx more 3 network layer ip ipv4 ipv6 icmp icmpv6 ipsec igmp ipx is is appletalk x 25 plp more 2 data link layer atm arp sdlc hdlc cslip slip gfp plip ieee 802 llc mac l2tp frame relay itu t g hn dll ppp x 25 lapb q 922 lapf more 1 physical layer rs 232 rs 449 itu t v series i 430 i 431 pdh sonet sdh pon otn dsl ieee 802 ieee 1394 itu t g hn phy usb bluetooth x 21 more v t e 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 a clear def...
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