Reliability (computer networking): Difference between revisions

Revision as of 15:34, 9 September 2020

Revision as of 03:31, 26 September 2020

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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.co.ukcom/books?id=pIH-JijUNS0C&lpg=PA25&ots=MKZj0F7pJN&pg=PA25#v=onepage&q&f=false|title=How the Web was Born: The Story of the World Wide Web|last2=Cailliau|first2=R.|date=2000|publisher=[[Oxford University Press]]|ISBNisbn=0192862073|pages=23-2523–25}}</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|volume=|pages=|accessdate=September 10, 2017|quote=In nearly all respects, Davies’ original proposal, developed in late 1965, was similar to the actual networks being built today.|via=}}</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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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.

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 articlejournal| 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 [[Traffic policing (communications)|traffic policing]] or bandwidth allocation, 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 |accessdate=2015-02-03 |url-status=dead |archiveurl=https://web.archive.org/web/20150618140031/http://www.techsat.com/fileadmin/media/pdf/infokiosk/TechSAT_TUT-AFDX-EN.pdf |archivedate=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.