Reliability (computer networking): Difference between revisions

Revision as of 19:16, 27 December 2020

Revision as of 09:36, 26 January 2021

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==History==

Building on the [[packet switching]] concepts proposed by [[Donald Davies]], the first [[communication protocol]] on the [[ARPANET]] was a reliable packet delivery procedure to connect its hosts via the [[BBN Report 1822|1822 interface]].<ref name="J. Gillies, R. Cailliau">{{cite book|last1=Gillies|first1=J.|url=https://books.google.com/books?id=pIH-JijUNS0C&pg=PA25|title=How the Web was Born: The Story of the World Wide Web|last2=Cailliau|first2=R.|date=2000|publisher=[[Oxford University Press]]|isbn=0192862073|pages=23–25}}</ref><ref name=":2">{{cite journal|last1=Roberts|first1=Dr. Lawrence G.|date=November 1978|title=The Evolution of Packet Switching|url=http://www.ismlab.usf.edu/dcom/Ch10_Roberts_EvolutionPacketSwitching_IEEE_1978.pdf|journal=IEEE Invited Paper|accessdateaccess-date=September 10, 2017|quote=In nearly all respects, Davies’ original proposal, developed in late 1965, was similar to the actual networks being built today.}}</ref> A host computer simply arranged the data in the correct packet format, inserted the address of the destination host computer, and sent the message across the interface to its connected [[Interface Message Processor]] (IMP). Once the message was delivered to the destination host, an acknowledgment was delivered to the sending host. If the network could not deliver the message, the IMP would send an error message back to the sending host.

Meanwhile, the developers of [[CYCLADES]] and of [[ALOHAnet]] demonstrated that it was possible to build an effective computer network without providing reliable packet transmission. This lesson was later embraced by the designers of [[Ethernet]].

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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 (VL) to be limited so that the requirements for shared resources can be predicted and [[congestion prevention|congestion prevented]] so it can be proved not to affect the critical data.<ref>AFDX Tutorial, {{cite web |url=http://www.techsat.com/fileadmin/media/pdf/infokiosk/TechSAT_TUT-AFDX-EN.pdf |title=Archived copy |accessdateaccess-date=2015-02-03 |url-status=dead |archive-url=https://web.archive.org/web/20150618140031/http://www.techsat.com/fileadmin/media/pdf/infokiosk/TechSAT_TUT-AFDX-EN.pdf |archive-date=2015-06-18 }}</ref> However, the techniques for predicting the resource requirements and proving that congestion is prevented are not part of the AFDX standard.

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, [http://www.csl.sri.com/users/bruno/publis/fmics2010.pdf ''SMT-Based Formal Verification of a ''TTEthernet'' Synchronization Function''], S. Kowalewski and M. Roveri (Eds.), FMICS 2010, LNCS 6371, pp. 148–163, 2010.</ref> This is because the sources of critical data will have to have the same view of time as the switch, in order that they can transmit at the correct time and the switch will see this as correct. This also requires that the sequence with which a critical transfer is scheduled has to be predictable to both source and switch. This, in turn, will limit the transmission schedule to a highly deterministic one, e.g. the [[cyclic executive]].