Table Of ContentDrago Hercog
Communication
Protocols
Principles, Methods and Specifications
Communication Protocols
Drago Hercog
Communication Protocols
fi
Principles, Methods and Speci cations
DragoHercog
UniversityofLjubljana
Ljubljana,Slovenia
ISBN978-3-030-50404-5 ISBN978-3-030-50405-2 (eBook)
https://doi.org/10.1007/978-3-030-50405-2
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Preface
The author of this book has been teaching the closely interrelated subjects of
communication protocols and communication networks for many years both atthe
UniversityofLjubljana,Slovenia,andatseveralhighengineeringschoolsinFrance.
Communication protocols have also been the main topic of the author's research
activities for many years. This book arises from these teaching and research expe-
riences. Although the areas of communication networks and communication pro-
tocolsstronglydependoneachother,Itriedtoavoidthetreatmentofcommunication
networksasmuchaspossibleinthisbook.Areadershallthereforehavetorelyon
theextensiveliteratureonthetopicsofcommunicationnetworkswhichisavailable
elsewhere.
Ittookalong,atoolongtimetowritethisbook,especiallybecauseIretiredwhile
working on it. However, due to the fact that the emphasis of this work is on the
principlesonwhichcommunicationprotocolsarebased,ratherthanonthedescrip-
tionofspecificprotocols,theauthorhopesthatthebookwillbeusefulforboththe
studentsofcommunicationsandtheengineerswhoarealreadyworkinginindustry,
butstillwanttodeepenandbroadentheirknowledgeofthisinterestingarea.Itmight
even be read by some individuals who are interested in communication protocols,
althoughtheirbasicareaofexpertiseisnotcommunicationtechnologies.
Nowadays, the great majority of communication devices are computers of dif-
ferent shapes, sizes and complexities; they communicate by exchanging messages.
Theexchangeofmessagesmustofcourseconformtocertainrules,whichmustbe
agreed upon in advance; otherwise communicating devices could not understand
each other. The rules for the exchange of communication messages are called
communication protocols. Hence, communication protocols are a kind of artificial
languages which allow different communication devices to communicate and to
understand each other. The field of communication protocols can therefore be
viewed as a topic that is related to communications, computer science, logic, even
linguistics,and,ofcourse,electricalengineering,asallcommunicatingdevicesare
electrical devices; one must also be aware that many problems which are being
solvedbycommunicationprotocolsareinfactelectrotechnicalproblems.
v
vi Preface
Allthetime,evernewcommunicationprotocolsarebeingdevelopedandputinto
use.Abookdescribingonlyspecificprotocolswouldthereforeveryquicklybecome
obsolete.Specificprotocols,however,arebasedoncommunicationmethodsthatare
muchmoregeneralandcanbeusedbydifferentprotocolsinslightlydifferentways
which are adapted to the conditions of use and the environment in which specific
protocolsaretooperate;inthissense,communicationprotocolscanbeviewedasthe
implementations of communication methods. Of course, existing communication
methods are also being modified and new communication methods are being
developed;thisprocessis,however,muchslowerthanthedevelopmentofspecific
protocols. The majority of this book is therefore devoted to the treatment of
communication methods on which specific communication protocols are based.
(We shouldremarkherethat manycommunication methods areoftenalsoreferred
toascommunicationprotocols.)Readerswhowillknowandunderstandcommuni-
cation methods will certainly have no difficulties to understand specific protocols
based on them, both those existing and used already and those that are yet to be
developed and put into use; it will also be much easier for them to study and
understandnewcommunicationmethodsthatareyettobedeveloped.Towardsthe
end of this book, however, some specific protocols are briefly discussed, which
should give readers some idea about how communication methods are used in
practice. Nowadays, the communication networks whose operationis based on the
protocolIParebecomingmoreandmorecommonandareexpectedtoprevailalsoin
thenearfuture;inthelastpartofthisbook,wethereforetreatmainlyprotocolsthat
areusedintheInternetandinIPnetworks ingeneral.Notmanyspecificprotocols
aredescribedandtheirdescriptionsarequiteshort;thesedescriptionsmustprimarily
be viewed as examples intended to additionally illustrate the main topics of this
book,hencethecommunicationmethods.
It was already told that communication protocols are artificial languages which
allow communication devices to communicate, hence to exchange information
between them.Aprotocol isimplementedintheformofahardware orasoftware,
which inside a communication device provides for the communication with other
communication devices. Such devices may be designed and manufactured by dif-
ferent producersandcancommunicate onlyifcommunication protocolsareunam-
biguously specified so that these specifications cannot be understood in different
ways, and the devices are designed and produced strictly in accordance with these
specifications. Specifications of protocols are therefore often defined in formal
specification languages which allow specifications to be unambiguous (often
standardisationorganisationsusesuchlanguages,too);besidesthis,formalspecifi-
cationscanserveasthestartingpointforthecorrectnessverificationorevenforthe
implementation of protocols. Nowadays, it is therefore advisable that a communi-
cations engineer knows and can understand at least one formal language. There
existsoneadditionalreasonwhytheknowledgeofformalismsforthespecification
ofprotocolsisimportant;astudentoranengineerwhoiscapabletoformallyspecify
aprotocolinoneoftheformal languageswill certainlyhavea much better idea of
how a protocol really functions and operates. Up to now, many specification
formalisms have already been defined and used; the choice of the formal language
Preface vii
tobepresentedandusedinthisbookwasthereforenotquiteeasy.Theformalisms
whicharebasedonthefinitestatemachinemodelareconsiderednowadaysbymany
authorstohaveatoohighimplementationbias(itwouldbedifficulttoopposethis
opinion) and also obsolete (which is not necessarily true), while those formalisms
thatarebasedonaprocessalgebraareoftenviewedasmodern.However,theformer
formalisms are more simple and easier to be understood and used by a practicing
engineer, which is the reason that they found their use in the everyday industrial
practice, while the latter formalisms are a favourite topic of research activities at
universitiesandinstitutes.Itistruethatitisnowadayspossibletoformallyprovethe
correctness of protocol specifications which are based on a process algebra; unfor-
tunately, this is in practice only feasible for the cases which are simple enough;
complexspecificationsarethereforestillcomposedwithformalismswhicharebased
onthefinitestatemachinemodel.Thesewerethereasonswhytheformallanguage
SDL,whichisbasedontheextendedfinitestatemachinemodel,waschosentobe
presentedandusedinthisbook;furthermore,manycomputer-aideddesigntoolsfor
protocol specification, verification (with simulation) and even implementation,
which are based on SDL, have been developed and are used in the industrial
environment. In this work, the language SDL is not only presented in a special
chapter but also used to formally specify numerous communication methods and
protocolswhicharediscussedhere.Inspiteofthis,someotherspecificationmodels
andformalismsarealsobrieflyoverviewedinthefirstpartofthebook.
Thecontentsofthebookarepartitionedintofourparts,andeachofthesepartsis
further divided into several chapters. Part I contains three chapters discussing the
specificationofsystemsandprotocols,includingthedescriptionofthespecification
language SDL; SDL is presented with the sufficient detail which should allow a
readertounderstandthespecificationsthatarepresentedinthecontinuationofthis
book. Part II consists of four chapters, which discuss a general definition and
descriptionofprotocols,theirroleinacommunicationsystem,andtheirproperties,
aswellastheconceptofprotocolstacks.AveryimportanttaskofPartsIandIIisto
acquaintareaderwithmanybasicconceptsandthetechnicalterminology,whichare
thenextensivelyusedinthecontinuationofthebook.ThesevenchaptersofPartIII
formthecore,aswellasthemostimportantandthelongestpartofthebook,asthey
describe and specify many communication methods, on which numerous specific
protocols are based. At last, some specific communication protocols are briefly
presented in twelve chapters of Part IV. A list of references is added which were
consulted by the author and which could also be used by readers, and also a short
summaryofSDLlanguageconstructs.Attheendofthebook,thereistheindexof
technicaltermswithpagenumbersforreference.
Anyworkunavoidablyreflectsitsauthor;thiscanbeseeninthestyleofwriting,
inthewaythecontentsarepresented,and,mostimportantly,inthechoiceoftopics
and the emphases which the topics and subtopics are given in the book. As was
already pointed out, the author of this book spent quite a lot of time researching
communicationprotocols;animportantemphasisoftheseresearchactivitieswasput
onthefieldofslidingwindowprotocols;theseresearchactivitiesalsoincludedthe
simulations of protocol performance. Prior to this, he was interested in the
viii Preface
methodologies for the design of complex systems. One must therefore not be
surprised to find out that the chapter devoted to sliding window protocols is quite
long andincludes some simulationresultsfor these protocols. All theperformance
simulators which were used to generate these results were designed and
implemented by the author. Also, the author could not resist the temptation to
include a discussion on system and protocol design methodologies in the first part
of the book. Some of the most important research achievements of the author are
includedbothintothecontentsofthebookandintothelistofreferences;however,
onlythosereferencesarelistedwhicharestronglyrelatedtothecontentsofthisbook
andtreatthetopicsthatcannotbefoundelsewhere.
As was already mentioned, many communication methods and communication
protocolsinthisbookareformallyspecifiedinthespecificationlanguageSDL.For
thispurpose,thesoftwareCinderellaSDL[41]wasemployedwhichincludesboth
the editor of SDL specifications and the logical simulator of SDL specifications.
While the editor verifies the syntactic correctness of a specification on the fly, the
simulatorsimulatesthebehaviourofthespecifiedsystemanddrawsthesimulation
results in the form of MSC diagrams, which allows a user to test the functional
correctness of a specification. Of course, a simulation is possible only if the
specification is completely formal. Furthermore, the whole system, including both
protocolentitiesandusers,mustbespecified;often,themodelofacommunication
channelalsohadtobespecifiedtomodelsomepeculiaritiesofthechannel,suchas
delays and losses. In the description of a system to be simulated, there may be no
informal specifications, no declarations may be missing. The syntactic and the
functional correctness of all specifications that are presented in the book were
verified with the editor and the simulator, respectively; for the presentation in the
book, however, some specifications were somehow simplified. In most cases, only
thespecificationsofprotocolentitiesareshown,asonlytheyarenecessarytospecify
the protocol, while the specifications of users and channels are needed only to be
able to simulate the system. Many specifications shown in the book are also
generalised in the sense that the definitions of some data types and constants are
eitherhiddeninthespecificationofthesystem(whichisnotshown)orreplacedwith
commentaries for thepurposeofthepresentationinthebook.Where theresults of
logic simulations are presented in the form of MSC diagrams, the original MSC
diagrams, as output by the SDL simulator, were also simplified by removing from
them those processes that are not mandatory to understand the communication
processandpreservingonlytheexchangeofmessagesbetweenprotocolentities.
Theprotocolswhichareusedinpracticearestandardisedinmostcases.Thereare
many organisations which develop and publish communication standards; some of
themostimportantamongthemareISO,ITU,IEEE,ETSIandIETF.Inmanycases,
the identifiers of standards are not listed, unless they also serve as the names of
protocols themselves. Of course, a reader can easily find the respective standards,
especially because the web addresses of the organisations for standardisation are
giveninthelistofreferences.
Towards the end of the book there is a list of important references which were
consulted when writing this book; the readers of this book are also invited to see
Preface ix
someofthesereferencestofindsomeadditionalinformationwhichcannotbefound
here, or to discover the topics of communication protocols from some other per-
spectiveasisofferedbytheauthorofthiswork.Thelistofreferencesthatisgiven
hereisofcoursenotexhaustive,asthereisahugeamountofliteratureoncommu-
nications and communication protocols available. Technical books are the most
numerous in the list, there are less technical papers and even less standards. This
should,however, posenoproblemforreaders,asmany search enginesontheweb
allownowadaysanybodytoeasilyfindthetechnicalliteratureofinterest.Although
theFrenchlanguageisbyfarnotaswidespreadastheEnglishlanguagenowadays,
onebookinFrench[5]isalsolisted(probablybecausetheauthorlikesthisbook,and
helikestheFrenchlanguageevenmore).
Forty-one references can be found in the list. They are arranged into several
groups.References[1]–[5]listsomeofthenumerousbookswhichtreat communi-
cationsystemsandwithinthisframealsocommunicationprotocols.Someofthese
booksareverypopular,e.g.[1]and[2],andalso[5];thelatteronecanunfortunately
not be read by everybody because of the language barrier. The above books have
alreadybeen published inmany editions;areader will ofcourse look for thelatest
one. References [6]–[10] are devoted to the special treatment of the theory of
communication protocols. In references [11]–[18], the models and languages for
the specification of protocols are discussed, while the language SDL is specially
treated in references [19]–[23]. The performance properties of protocols are
discussed in references [24]–[26], while reference [27] explains how the author
developedtheperformancesimulatorstogeneratetheresultswhichareincludedin
this book. The books [28] and [29] treat only the protocols which are used in IP
networks. The book [28] in particular is considered by many communications
engineers a kind of a »bible« for theworld of Internet protocols. The papers [30]–
[32] contain the results of the author’s research on the theory of sliding window
protocolswhicharealsoincludedinthisbook.References[33]–[37]refertotheweb
pages ofsome of themostimportant organisations for standardisation; these pages
are excellent starting points to search for various standards. In reference [38], the
specification language SDL is standardised, and in [39], the protocol PPP is
standardised.Theencyclopaedia»Wikipedia«[40]canalsobeusedwhenworking
in the field of communications, but with great care to distinguish between good
articlesandthosethatarelessgood(whichisofcoursetruewithallwebresources).
Finally,[41]referstothewebpageofthedeveloperofthesoftwareCinderellaSDL
which was used by the author for editing and simulating the SDL specifications
whichareincludedinthisbook.
This work is not a scientific paper. References to already published work are
thereforenotincludedinthetext.Themajorityofthematerialwhichispresentinthis
book can also be found elsewhere, and in particular in various books that treat
communication protocols. The research results of the author, which have not yet
beenmuchpublishedbutareyetincludedinthisbook,includethedefinitionofthe
relative efficiency of protocols [26], some topics in the theory of sliding window
protocolsthatarepublishedin[30],[31]and[32],andalsothemethodologyforthe
developmentoftheperformancesimulatorsforprotocolsspecifiedinSDL[27](this
x Preface
methodology isnot atopicof this book, butwas used todevelop thesimulatorsto
generatetheperformancesimulationresultswhichareinsertedinthebook).Allthe
SDLmodelswhicharepublishedinthisbookwerealsodevelopedbytheauthor.
The Index of acronyms appears before the main text of the book and lists the
acronymswhicharementionedinthistext.Althoughtoomanyacronymsareoften
employedintechnicaltexts,whichcansubstantiallydeterioratetheirreadability,the
useofacronymswasavoidedasmuchaspossibleinthisbook.Ofcourse,thereare
namesoftechnologiesandprotocolswhichwereoriginallycoinedasacronymsand
canthereforenotbeavoided.
The short summary of SDL language constructs (titled SDL Keywords and
Symbols) can serve as a quick reference to a reader when studying the SDL
specificationswhichareincludedintothisbook.
The Index of technical terms contains the alphabetical list of technical terms
whichareusedormentionedinthetext.Witheachterm,oneormorenumbersare
listedindicatingthepageswherethistermisdefinedorimportantlymentioned.With
the terms which are specially related to SDL constructs, the word SDL is added
withinparentheses.
The author enjoyed the time spent when writing this book. He hopes that the
readers who willread it will also enjoy it and profitfrom it, and thatthe bookwill
help them to more deeply understand the very interesting field of communication
protocols.
Thecontentsofthisbookaretheresultofmypedagogicandresearchworkinthe
areaofcommunicationprotocolsthroughmanyyears.Thefinalresultis,however,
also due to the endeavours of the publisher, the editors, the reviewers and the
graphical designers. Thanks to all of you! The errors (I am sure there are still too
many, in spite of my numerous and careful proofreadings) stay only my fault and
responsibility.
Iowethanksalso,andespecially,tomyfamilyfortheirpatienceandunderstand-
ing.Theworkisdedicatedtomythreesweetgrandchildren—Svit,MaticandTiara.
Ljubljana,Slovenia DragoHercog
