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RFC1483ATM上的多协议封装
来源：ciscobbs
2002-7-2610:25:00
rfc1483
Pressheretogoto
thetopoftherfc'tree'.
NetworkWorkingGroupJuhaHeinanen
ReguestforComments
:1483TelecomFinland
July1993
MultiprotocolEncaps
ulationoverATMAdaptation
Layer5
StatusofthisMemo
ThisRFCspecifiesanIABstanda
community,andrequestsdiscussiona
rdstrackprotocolfortheInternet
ndsuggestionsforimprovements.
Pleaserefertothecurrentedit
forthestandardizationstateandst
ionoftheIABOfficialProtocolStandards
atusofthisprotocol.
Distributionofthismemoisunlimited.
Abstract
Thismemodescribes
interconnecttrafficove
twoencapsulationsmethodsf
rATMAAL5.Thefirstmethod
orcarryingnetwork
allows
multiplexingofmultipleprotoco
thesecondmethodassumesthateach
lsoverasingleATMvirtualcircuitwhereas
protocoliscarriedover
aseparateATMvirtualcircuit.
1.Introduction
AsynchronousTransfe
forbothlocalandwide
rMode(ATM)basednetworks
areaapplications.Thismemo
areofincreasinginterest
describestwodiffer
entmethodsforcarryingcon
nectionlessnetwork
interconnecttraffic
ATMnetwork.Thefirstme
,routedandbridgedProtoco
thodallowsmultiplexingof
lDataUnits(PDUs),overan
multipleprotocolsoverasingle
ATMvirtualcircuit.Theprotocol
ofacarriedPDUisidentifiedb
yprefixingthePDUbyanIEEE802.2
LogicalLinkControl
Encapsulationandasubs
(LLC)header.Thismethodis
etofithasbeenearlier
inthefollowingcalledLLC
definedforSMDS[1]
multiplexingimplicitly
.Thesecondmethoddoeshigh
byATMVirtualCircuits(VCs
er-layerprotocol
).Itisinthe
followingcalledVCBasedMultiplexing.
ATMisacellbasedtransfermod
informationtobesegmentedandreas
ethatrequiresvariablelengthuser
sembledto/fromshort,fixed
lengthcells.Thisme
(SAR)methodforbridged
modoesn'tspecifyanewSeg
androutedPDUs.Instead,th
mentationAndReassembly
e
PDUsarecarriedin
(CPCS)PDUofATMAdapta
describeshowroutedand
thePayloadfieldofCommon
tionLayertype5(AAL5)[2]
bridgedPDUsare
PartConvergenceSublayer
.Notethatthismemoonly
carrieddirectlyovertheCPCSo
ConvergenceSublayer(SSCS)ofAAL5
AAL5July1993RelayServiceSpecific
inI.36x.1[3],isusedovertheCPC
carriedusingtheNLPIDmultiplexing
fAAL5,i.e.,whentheServiceSpecific
isempty.IfFrameRFC1483Multiprotocolover
ConvergenceSublayer(FR-SSCS),asdefined
SofAAL5,thenroutedandbridgedPDUsare
methoddescribedinRFC
1294[4].AppendixA
FR-SSCS-PDUaswellash
accordingtoRFC1294.
(whichisforinformationon
owIPandCLNPPDUsareenca
ly)showstheformatofthe
psulatedoverFR-SSCS
2.SelectionoftheMultiplexing
Multiplexingwillbedominantinenv
numbersofATMVCsisfastandecono
prevailin
MethodItisenvisionedthatVCBased
ironmentswheredynamiccreationoflarge
mical.Theseconditionsarelikelytofirst
privateATMnetworks
whenitisnotpractical
.LLCEncapsulation,ontheo
foronereasonoranothert
therhand,maybedesirable
ohave
aseparateVCforea
ATMnetworkonlysupport
dependsheavilyonthen
chcarriedprotocol.Thisis
s(semi)PermanentVirtualC
umberof
thecase,forexample,ifthe
ircuits(PVCs)orifcharging
simultaneousVCs.WhentwoATMst
ationswishtoexchangeconnectionlessnetwork
interconnecttraffic
manualconfiguration(in
,selectionofthemultiplex
caseofPVCs)orbyB-ISDN
ingmethodisdoneeitherby
signallingprocedures(incaseo
signallingarestillunderstudyin
fSwitchedVCs).ThedetailsofB-ISDN
CCITT[5].Itcan,however,
beassumedthatB-ISDNsignallin
informationelement,whichwillallo
gmessagesincludeaLowlayercompatibility
wnegotiationof
AAL5andthecarried
(encapsulation)protocol.
3.AAL5FrameFormat
Nomatterwhichmult
shallbeencapsulatedwi
AAL5CPCS-PDUisgivenb
iplexingmethodisselected,
thinthePayloadfieldofAA
elow:
routedandbridgedPDUs
L5CPCS-PDU.Theformatofthe
Heinanen[Page2]
RFC1483MultiprotocoloverAAL5J
uly1993
AAL5CPCS-PDUFormat
ThePayloadfieldcontainsuser
fieldpadstheCPCS-PDUtofitexact
octetcellpayloadcreatedbytheSA
rightjustifiedinthecell.TheCPCS
totransparentlytransferCPCSuser
underthemultiprotocolATMencapsul
toanyvalue.TheCPI(CommonPartIn
informationupto2^16-1octets.ThePAD
lyintotheATMcellssuchthatthelast48
RsublayerwillhavetheCPCS-PDUTrailer
-UU(User-to-Userindication)fieldisused
touserinformation.Thefieldhasnofunction
ationdescribedinthismemoandcanbeset
dicator)fieldalingstheCPCS-PDUtrailerto
ThePayloadfieldco
fieldpadstheCPCS-PDU
octetcellpayloadcreat
rightjustifiedinthec
totransparentlytransfe
underthemultiprotocol
toanyvalue.
ntainsuserinformationupt
tofitexactlyintotheATM
edbytheSARsublayerwill
ell.TheCPCS-UU(User-to-Us
rCPCSusertouserinformat
ATMencapsulationdescribed
o2^16-1octets.ThePAD
cellssuchthatthelast48
havetheCPCS-PDUTrailer
erindication)fieldisused
ion.Thefieldhasnofunction
inthismemoandcanbeset
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