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{{Short description|Protocol acknowledgement capability}}
{{Use American English|date=January 2020}}
In [[computer networking]], a '''reliable''' protocol is a [[communication protocol]] that notifies the sender whether or not the delivery of data to intended recipients was successful. [[Reliability engineering|Reliability]] is a synonym for '''assurance''', which is the term used by the [[ITU]] and [[ATM Forum]], and leads to '''[[fault-tolerant]] messaging'''. ▼
▲In [[computer networking]], a '''reliable''' protocol is a [[communication protocol]] that notifies the sender whether or not the delivery of data to intended recipients was successful. Reliability is a synonym for '''assurance''', which is the term used by the [[ITU]] and [[ATM Forum]].
Reliable protocols typically incur more overhead than unreliable protocols, and as a result, function more slowly and with less scalability. This often is not an issue for [[unicast]] protocols, but it may become a problem for [[reliable multicast]] protocols.
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==Real-time systems==
There is, however, a problem with the definition of reliability as "delivery or notification of failure" in [[real-time computing]]. In such systems, failure to deliver the real-time data will adversely affect the performance of the systems, and some systems, e.g. [[safety-critical]], [[safety-involved]], and some secure [[mission-critical]] systems, must be [[formal methods|proved]] to perform at some specified minimum level. This, in turn, requires that a specified minimum reliability for the delivery of the critical data be met. Therefore, in these cases, it is only the delivery that matters; notification of the failure to deliver does ameliorate the failure. In [[hard real-time system]]s, all data must be delivered by the deadline or it is considered a system failure. In [[firm real-time system]]s, late data is still valueless but the system can tolerate some amount of late or missing data.<ref name = "Schneider et al 2001">S., Schneider, G., Pardo-Castellote, M., Hamilton.
There are a number of protocols that are capable of addressing real-time requirements for reliable delivery and timeliness:
[[MIL-STD-1553B]] and [[STANAG 3910]] are well-known examples of such timely and reliable protocols for [[avionics#Aircraft networks|avionic data buses]]. MIL-1553 uses a 1 Mbit/s shared media for the transmission of data and the control of these transmissions, and is widely used in federated military [[avionics]] systems.<ref name="Ekman_SAAB">{{citation |author=Mats Ekman |title=Avionic Architectures Trends and challenges |url=https://www.kth.se/polopoly_fs/1.146328!/Menu/general/column-content/attachment/3_Ekman_Saab.pdf |publisher=KTH |url-status=dead |archive-url=https://web.archive.org/web/20150203164824/https://www.kth.se/polopoly_fs/1.146328!/Menu/general/column-content/attachment/3_Ekman_Saab.pdf |archive-date=2015-02-03 |quote=Each system has its own computers performing its own functions}}</ref> It uses a bus controller (BC) to command the connected remote terminals (RTs) to receive or transmit this data. The BC can, therefore, ensure that there will be no [[network congestion|congestion]], and transfers are always timely. The MIL-1553 protocol also allows for automatic retries that can still ensure timely delivery and increase the reliability above that of the physical layer. STANAG 3910, also known as EFABus in its use on the [[Eurofighter Typhoon]], is, in effect, a version of MIL-1553 augmented with a 20 Mbit/s shared media bus for data transfers, retaining the 1 Mbit/s shared media bus for control purposes.
The [[Asynchronous Transfer Mode]] (ATM), the [[Avionics Full-Duplex Switched Ethernet]] (AFDX), and [[Time Triggered Ethernet]] (TTEthernet) are examples of packet-switched networks protocols where the timeliness and reliability of data transfers can be assured by the network. AFDX and TTEthernet are also based on IEEE 802.3 Ethernet, though not entirely compatible with it.
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ATM uses connection-oriented [[virtual channel]]s (VCs) which have fully deterministic paths through the network, and [[UPC and NPC|usage and network parameter control]] (UPC/NPC), which are implemented within the network, to limit the traffic on each VC separately. This allows the usage of the shared resources (switch buffers) in the network to be calculated from the parameters of the traffic to be carried in advance, i.e. at system design time. That they are implemented by the network means that these calculations remain valid even when other users of the network behave in unexpected ways, i.e. transmit more data than they are expected to. The calculated usages can then be compared with the capacities of these resources to show that, given the constraints on the routes and the bandwidths of these connections, the resource used for these transfers will never be over-subscribed. These transfers will therefore never be affected by congestion and there will be no losses due to this effect. Then, from the predicted maximum usages of the switch buffers, the maximum delay through the network can also be predicted. However, for the reliability and timeliness to be proved, and for the proofs to be tolerant of faults in and malicious actions by the equipment connected to the network, the calculations of these resource usages cannot be based on any parameters that are not actively enforced by the network, i.e. they cannot be based on what the sources of the traffic are expected to do or on statistical analyses of the traffic characteristics (see [[network calculus]]).<ref>{{cite journal| first1=Y. J. | last1=Kim | first2=S. C. | last2=Chang | first3=C. K. | last3=Un | first4=B. C. | last4=Shin | title=UPC/NPC algorithm for guaranteed QoS in ATM networks | journal=Computer Communications | volume=19 | number=3 | date=March 1996 | pages=216–225 | publisher=[[Elsevier Science Publishers]] | ___location=Amsterdam, the Netherlands | doi=10.1016/0140-3664(96)01063-8 }}</ref>
AFDX uses frequency ___domain bandwidth allocation and [[Traffic policing (communications)|traffic policing]], that allows the traffic on each virtual link to be limited so that the requirements for shared resources can be predicted and [[congestion avoidance|congestion prevented]] so it can be proved not to affect the critical data.<ref>
TTEthernet provides the lowest possible latency in transferring data across the network by using time-___domain control methods – each time triggered transfer is scheduled at a specific time so that contention for shared resources is controlled and thus the possibility of congestion is eliminated. The switches in the network enforce this timing to provide tolerance of faults in, and malicious actions on the part of, the other connected equipment. However, "synchronized local clocks are the fundamental prerequisite for time-triggered communication".<ref>Wilfried Steiner and Bruno Dutertre, "[https://web.archive.org/web/20230125090223/http://www.csl.sri.com/users/bruno/publis/fmics2010.pdf
However, low latency in transferring data over the bus or network does not necessarily translate into low transport delays between the application processes that source and sink this data. This is especially true where the transfers over the bus or network are cyclically scheduled (as is commonly the case with MIL-STD-1553B and STANAG 3910, and necessarily so with AFDX and TTEthernet) but the application processes are not synchronized with this schedule.
With both AFDX and TTEthernet, there are additional functions required of the interfaces, e.g. AFDX's Bandwidth Allocation Gap control, and TTEthernet's requirement for very close synchronization of the sources of time-triggered data, that make it difficult to use standard Ethernet interfaces. Other methods for control of the traffic in the network that would allow the use of such standard IEEE 802.3 network interfaces is a subject of current research.<ref name="Charlton et al 2013">{{citation |author=D. W. Charlton |display-authors=etal |title=An Avionic Gigabit Ethernet Network |work=Avionics, Fiber-Optics and Photonics Conference (AVFOP) |year=2013 |publisher=IEEE |pages=17–18 |doi=10.1109/AVFOP.2013.6661601|isbn=978-1-4244-7348-9 |s2cid=3162009 }}</ref>
==See also==
*{{anl|Robustness of complex networks}}
*{{anl|Efficiency (network science)}}
*{{anl|Cascading failure}}
==References==
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