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YMODEM

-1-

XMODEM/YMODEMPROTOCOLREFERENCE

Acompendiumofdocumentsdescribingthe

XMODEMandYMODEM

FileTransferProtocols

Thisdocumentwasformatted8-4-87.

EditedbyChuckForsberg

Pleasedistributeaswidelyaspossible.

QuestionstoChuckForsberg

OmenTechnologyInc

TheHighReliabilitySoftware

17505-VSauvieIslandRoad

PortlandOregon97231

VOICE:

503-621-3406:

VOICE

Modem（TeleGodzilla）:

503-621-3746Speed19200,2400,1200,300

CompuServe:

70007,2304

GEnie:

CAF

UUCP:

...!

tektronix!

reed!

omen!

caf

-2-

1.TOWEROFBABEL

A"YMODEMTowerofBabel"hasdescendedonthemicrocomputingcommunity

bringingwithitconfusion,frustration,bloatedphonebills,andwasted

manhours.Sadly,I（ChuckForsberg）ampartlytoblameforthismess.

Asauthoroftheearly1980sbatchand1kXMODEMextensions,Iassumed

readersofearlierversionsofthisdocumentwouldimplementasmuchof

theYMODEMprotocolastheirprogrammingskillsandcomputingenvironments

wouldpermit.Thisprovedarathernaiveassumptionasprogrammers

motivatedbycompetitivepressureimplementedaslittleofYMODEMas

possible.SomehavetakenwhateverpartsofYMODEMthatappealedtothem,

appliedthemtoMODEM7Batch,Telink,XMODEMorwhatever,andcalledthe

resultYMODEM.

JeffGarbers（Crosstalkpackagedevelopmentdirector）saiditall:

"With

protocolsinthepublicdomain,anyonewhowantstodinkaroundwiththem

cangoahead."[1]

DocumentscontainingalteredexamplesderivedfromYMODEM.DOChaveadded

totheconfusion.Inoneinstance,theheadinginYMODEM.DOC'sFigure1

hasmutatedfrom"1024bytePackets"to"YMODEM/CRCFileTransfer

Protocol".NoneoftheXMODEMandYMODEMexamplesshowninonedocument

werecorrect.

Toputanendtothisconfusion,wemustmake"perfectlyclear"what

YMODEMstandsfor,asWardChristensendefineditinhis1985coiningof

theterm.

Tothemajorityofyouwhoread,understood,andrespectedWard's

definitionofYMODEM,Iapologizefortheinconvenience.

1.1Definitions

ARCARCisaprogramthatcompressesoneormorefilesintoanarchive

andextractsfilesfromsucharchives.

XMODEMreferstothefiletransferetiquetteintroducedbyWard

Christensen's1977MODEM.ASMprogram.ThenameXMODEMcomesfrom

KeithPetersen'sXMODEM.ASMprogram,anadaptationofMODEM.ASM

forRemoteCP/M（RCPM）systems.It'salsocalledtheMODEMor

MODEM2protocol.SomewhoareunawareofMODEM7'sunusualbatch

filemodecallitMODEM7.Otheraliasesinclude"CP/MUsers'

Group"and"TERMIIFTP3".ThenameXMODEMcaughtonpartly

becauseitisdistinctiveandpartlybecauseofmediainterestin

__________

1.PageC/12,PC-WEEKJuly12,1987

Chapter1

X/YMODEMProtocolReference08-03-873

bulletinboardandRCPMsystemswhereitwasaccessedwithan

"XMODEM"command.Thisprotocolissupportedbyeveryserious

communicationsprogrambecauseofitsuniversality,simplicity,

andreasonableperformance.

XMODEM/CRCreplacesXMODEM's1bytechecksumwithatwobyteCyclical

RedundancyCheck（CRC-16）,givingmodernerrordetection

protection.

XMODEM-1kReferstotheXMODEM/CRCprotocolwith1024bytedatablocks.

YMODEMReferstotheXMODEM/CRC（optional1kblocks）protocolwithbatch

transmissionasdescribedbelow.

TrueYMODEM（TM）InanattempttosortouttheYMODEMTowerofBabel,Omen

TechnologyhastrademarkedthetermTrueYMODEM（TM）torepresent

thecompleteYMODEMprotocoldescribedinthisdocument,including

pathname,length,andmodificationdatetransmittedinblock0.

PleasecontactOmenTechnologyaboutcertifyingprogramsforTrue

YMODEM（TM）compliance.

ZMODEMusesfamiliarXMODEM/CRCandYMODEMtechnologyinanewprotocol

thatprovidesreliability,throughput,filemanagement,anduser

amenitiesappropriatetocontemporarydatacommunications.

ZOOLikeARC,ZOOisaprogramthatcompressesoneormorefilesinto

a"zooarchive".ZOOsupportsmanydifferentoperatingsystems

includingUnixandVMS.

Chapter1

X/YMODEMProtocolReference08-03-874

2.YMODEMMINIMUMREQUIREMENTS

AllprogramsclaimingtosupportYMODEMmustmeetthefollowingminimum

requirements:

+Thesendingprogramshallsendthepathname（filename）inblock0.

+ThepathnameshallbeanullterminatedASCIIstringasdescribed

below.

+Thereceivingprogramshallusethispathnameforthereceivedfile

name,unlessexplicitlyoverridden.

+ThesendingprogramshalluseCRC-16inresponsetoa"C"pathname

nak,otherwiseuse8bitchecksum.

+Thereceivingprogrammustacceptanymixtureof128and1024byte

blocksitreceives.

+Thesendingprogrammustnotchangethelengthofanunacknowledged

block.

+Attheendofeachfile,thesendingprogramshallsendEOTuptoten

timesuntilitreceivesanACKcharacter.（Thisispartofthe

XMODEMspec.）

+Theendofatransfersessionshallbesignifiedbyanull（empty）

pathname.

ProgramsnotmeetingalloftheserequirementsarenotYMODEMcompatible.

Meetingtheminimumrequirementsdoesnotguaranteereliablefile

transfersunderstress.

Chapter2

X/YMODEMProtocolReference08-03-875

3.WHYYMODEM?

Sinceitsdevelopmenthalfadecadeago,theWardChristensenmodem

protocolhasenabledawidevarietyofcomputersystemstointerchange

data.Thereishardlyacommunicationsprogramthatdoesn'tatleast

claimtosupportthisprotocol.

Recentadvancesincomputing,modemsandnetworkinghaverevealedanumber

ofweaknessesintheoriginalprotocol:

+Theshortblocklengthcausedthroughputtosufferwhenusedwith

timesharingsystems,packetswitchednetworks,satellitecircuits,

andbuffered（errorcorrecting）modems.

+The8bitarithmeticchecksumandotheraspectsallowedline

impairmentstointerferewithdependable,accuratetransfers.

+Onlyonefilecouldbesentpercommand.Thefilenamehadtobe

giventwice,firsttothesendingprogramandthenagaintothe

receivingprogram.

+Thetransmittedfilecouldaccumulateasmanyas127extraneous

bytes.

+Themodificationdateofthefilewaslost.

Anumberofotherprotocolshavebeendevelopedovertheyears,butnone

havedisplacedXMODEMtodate:

+Lackofpublicdomaindocumentationandexampleprogramshavekept

proprietaryprotocolssuchasBlast,Relay,andotherstightlybound

tothefortunesoftheirsuppliers.

+ComplexitydiscouragesthewidespreadapplicationofBISYNC,SDLC,

HDLC,X.25,andX.PCprotocols.

+Performancecompromisesandcomplexityhavelimitedthepopularityof

theKermitprotocol,whichwasdevelopedtoallowfiletransfersin

environmentshostiletoXMODEM.

TheXMODEMprotocolextensionsandYMODEMBatchaddresssomeofthese

weaknesseswhilemaintainingmostofXMODEM'ssimplicity.

YMODEMissupportedbythepublicdomainprogramsYAM（CP/M）,

YAM（CP/M-86）,YAM（CCPM-86）,IMP（CP/M）,KMD（CP/M）,rz/sz（Unix,Xenix,

VMS,BerkeleyUnix,Venix,Xenix,Coherent,IDRIS,Regulus）.Commercial

implementationsincludeMIRROR,andProfessional-YAM.[1]Communications

Chapter3

X/YMODEMProtocolReference08-03-876

programssupportingtheseextensionshavebeeninusesince1981.

The1kblocklength（XMODEM-1k）describedbelowmaybeusedinconjunction

withYMODEMBatchProtocol,orwithsinglefiletransfersidenticaltothe

XMODEM/CRCprotocolexceptforminimalchangestosupport1kblocks.

AnotherextensionistheYMODEM-gprotocol.YMODEM-gprovidesmaximum

throughputwhenusedwithendtoenderrorcorrectingmedia,suchasX.PC

anderrorcorrectingmodems,including9600bpsunitsbyTeleBit,

U.S.Robotics,Hayes,ElectronicVaults,DataRace,andothers.

Tocompletethistome,editedversionsofWardChristensen'soriginal

protocoldocumentandJohnByrns'sCRC-16documentareincludedfor

reference.

ReferencestotheMODEMorMODEM7protocolhavebeenchangedtoXMODEMto

accommodatethevernacular.InAustralia,itisproperlycalledthe

ChristensenProtocol.

3.1SomeMessagesfromthePioneer

#:

130940S0/Communications25-Apr-8518:

38:

47

Sb:

myprotocol

Fm:

WardChristensen76703,302[2]

To:

all

Beawarethearticle[3]DIDquotemecorrectlyintermsofthephrases

like"notrobust",etc.

ItwasaquickhackIthrewtogether,veryunplanned（likeeverythingI

do）,tosatisfyapersonalneedtocommunicatewith"someother"people.

ONLYthefactthatitwasdonein8/77,andthatIputitinthepublic

domainimmediately,madeitbecomethestandardthatitis.

Ithinkitstimeformeto

（1）documentit;（peoplecallmeandsay"myproductisgoingtoinclude

it-whatcanI'reference'",or"I'mwritingapaperonit,whatdoIput

inthebibliography"）and

__________________________________________________________________________

1.AvailableforIBMPC,XT,AT,UnixandXenix

2.Editedfortypesettingappearance

3.InfoworldApril29p.16

Chapter3

X/YMODEMProtocolReference08-03-877

（2）proposean"incrementalextension"toit,whichmighttake"exactly"

theformofChuckForsberg'sYAMprotocol.HewroteYAMinCforCP/Mand

putitinthepublicdomain,andwroteabatchprotocolforUnix[4]called

rbandsb（receivebatch,sendbatch）,whichwasbasicallyXMODEMwith

（a）arecord0containingfilenamedatetimeandsize

（b）a1Kblocksizeoption

（c）CRC-16.

HedidsomecleverprogrammingtodetectfalseACKorEOT,butbasically

leftthemthesame.

PeoplewhosuggestImakeSIGNIFICANTchangestotheprotocol,suchas

"fullduplex","multipleoutstandingblocks","multipledestinations",etc

etcdon'tunderstandthattheincred