Access aggregation is a viable option for service providers to boost bandwidth across the last mile in areas where it is too costly to increase the capacity of legacy access. Multipath TCP is ideal for access aggregation in the last mile as it is able to boost bandwidth significantly while simultaneously increasing reliability and ensuring seamless connectivity. Multipath TCP proxies are carrier-grade and optimized for high traffic throughput. They allow service providers to use Multipath TCP for access aggregation without the need for end devices and internet servers to be aware of it, and make it possible to implement traffic-steering policies for the use of available access networks in the most cost-effective and efficient way.

1. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 1
ERICSSON
TECHNOLOGY
... DSL ... LTE
nection speed over ...
... DSL/Wi-Fi + LTE
with Multipath TCP
C H A R T I N G T H E F U T U R E O F I N N O V A T I O N | # 0 8 ∙ 2 0 1 6
MULTIPATHTCP
&THELASTMILE

2. ✱ MULTIPATH TCP & THE LAST MILE
2 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
ROBERT SKOG,
DINAND ROELAND,
JAUME RIUS I RIU,
UWE HORN,
MICHAEL ERIKSSON
The rapid uptake of bandwidth-consuming services such as video on
demand and linear TV has many service providers struggling to keep pace
with ever increasing bandwidth demands. The problem is particularly acute on
the last mile: the segment of the network that delivers broadband services to
users’ homes and workplaces.
a s a n a lt e r n at i v e to building out
the physical communications infrastructure
– which in some geographical areas may
be too costly or time consuming – Ericsson
proposes an access aggregation solution
based on Multipath TCP. Our solution consists
of a carrier-grade Multipath TCP proxy that
allows the use of Multipath TCP across access
networks without the need to introduce it in
end devices or internet servers.
■Thelastmileisthepartofthetelecommunications
networkthatphysicallyreachesuserpremises,either
bywirelesstechnology(cellularnetworks)orwireline
technologysuchascable,fiberordigitalsubscriberline
(dsl).Theachievabledataratesforeachoftheseaccess
technologiesvary,butinmanycasesthebandwidth
dependsonthedistancebetweentheaccess
terminationpointintheserviceprovidernetwork
andthedeviceintheuserpremises.Thismeansthat
nomatterhowfasttheserviceisuptotheaccess
terminationpoint,theuserswhoarefarthestaway
fromitwillexperiencesignificantlyslowerservice
thantheoneswhoarecloser.
Forexample,althoughthemostrecently
standardizeddsltechnologiesallowbitratesofup
to1Gbps,mostsubscriberstodayarestillgettingless
than20Mbps.Thereasonforthisisthedependency
betweentheachievablebitrateandthelengthof
thecopperlineconnectingahouseholdtothedsl
accessmultiplexer(dslam).AsFigure1shows,ifthe
distancebetweentheuserpremisesandthedslam
The
lastmileWITH MULTIPATH TCP
BOLSTERING

3. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 3
exceeds2km,dslspeedfallsquicklybelow20Mbps.
Theobvioussolutionistoreducethelengthofthe
lastmile.Ifthecopperlinedistancecanbereduced
tolessthan250m,newtechnologiesandstandards
suchasvectoringandG.fastwillallowbitratesof
about1Gbps.However,reducingthecopperline
distanceiscostlybecauseitrequiresthedeployment
ofmorestreetcabinetsconnectedbyfiberlinestothe
backbonenetwork.Togetaroundthis,somefixed
broadbandserviceprovidershavestartedtolaunch
offeringsthatcombinedslwithlteasacheaperway
toboostthebitratefordslcustomersthandeploying
morefiber-connecteddslamstreetcabinets.
Similarly,lte/wi-fiaggregationisusefulasa
boosterformobilephones.Someoperatorshave
starteddeployingsolutionsthatcombineWi-Fiand
lteaccessesinareassuchasshoppingmallsandbig
eventvenuesasameanstoincreaseusercapacitywhile
atthesametimeoffloadingtheircellularnetworktraffic
tothefixednetworkswhenpossible.
Technologiesforaccessaggregation
Manystandardizedaggregationtechnologiesonly
supportusecasesinwhichlinksusingthesameaccess
typeareaggregated.Thisisknownasbonding,and
examplesincludethebondingofseveralEthernetlinks,
oroftwodslaccesslinks.Notableexceptionsareip
FlowMobilityandmultiple-accesspdnconnectivity
–bothdefinedby3gpp–whichareabletosupport
aggregationofmultipleaccesstypes[1].However,these
twotechnologieshavegainedlittletractionbecause
theirintroductiononmobiledeviceswouldrequirea
significantimplementationeffort,andeventheapps
runningonthemwouldrequiremodifications.
Multipathtcpasspecifiedbytheietf[2]canbe
deployedinexistingnetworksmoreeasilythanother
alternativesbecauseitisanevolutionoftcp[3]–the
mostwidelyusedprotocolintheinternettoday.This
guaranteesinteroperabilitybetweenequipmentfrom
differentvendors.Liketcp,Multipathtcpworks
ontopofip.Sinceipisthefoundationofallinternet
protocols,Multipathtcpcanbeusedacrossallkindsof
accessnetworks,providingarichtoolkitthatsupports
accessaggregationforusecasessuchasbandwidth
aggregation,reliabilityandseamlessconnectivity.
Inaddition,thereisanopensourcereference
implementationforMultipathtcpthatiscontinuously
developedandimprovedbyalargecommunityof
developers[4].
Figure2showstwoaccessaggregationscenarios
enabledbyMultipathtcp.Thefirstscenarioshows
dsl/lteaggregation,whereanexistingdslconnection
iscombinedwithlte.Ifthedsllinkprovides12Mbps
andtheltelinkprovides8Mbps,theaggregated
MULTIPATH TCP CAN BE
USED ACROSS ALL KINDS OF
ACCESS NETWORKS,
PROVIDING A RICH TOOLKIT
THAT SUPPORTS ACCESS
AGGREGATION FOR USE
CASES SUCH AS BANDWIDTH
AGGREGATION, RELIABILITY
AND SEAMLESS
CONNECTIVITY.
Terms and abbreviations
ACK — ACKnowledgment | CCA — Congestion Control Algorithm | CPE — customer premises equipment | CPU
— central processing unit | DPDK — Data Plane Development Kit | DSL — digital subscriber line | DSLAM — DSL
access multiplexer | IETF — Internet Engineering Task Force | MFDN — Media First Delivery Node | RNA — Radio
Network Aware | RTT — round-trip time | TCP RNA — TCP Radio Network Aware | VDSL – Very high-speed DSL

4. ✱ MULTIPATH TCP & THE LAST MILE
4 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
100
90
80
70
60
50
40
30
20
10
0
0 300 600
Source: Ofcom
900 1200 1500 1800 2100 2500 3000 3500 4000 4500 5200 5800 6200
Speed (Mbps)
VDSL2
VDSL
ADSL2+
ADSL2
ADSL
Distance (meters)
Figure 1 Speed versus
copper line length between
user premises and the
DSLAM for the most widely
deployed DSL technologies
Figure 2
Examples of access
aggregation enabled
by Multipath TCP
DSL + LTE
LTE + Wi-Fi
CPE
DSL
YouTube
Facebook
Twitter
GoogleLTE
LAN
Wi-Fi

5. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 5
bandwidththatcanbeobtainedviaMultipathtcpis
roughly20Mbps.
Thesecondscenarioshowslte/Wi-Fiaggregation,
whichfunctionsaccordingtothesameprinciple.
Togetherwithamobiledevicemanufacturer,Ericsson
hasperformedsuccessfulfieldtrialsinpubliclteand
Wi-Finetworksusingcommerciallyavailablemobile
devices.Onlythefirmwarewasmodifiedtosupport
Multipathtcp.
AlthoughthebenefitsofMultipathtcpareoften
presentedinthecontextoftwodifferentaccess
networks,thereisnolimitinMultipathtcpthat
wouldpreventtheuseofthree,fourormoreaccess
networks.Theaccessnetworkscouldevenbeoperated
bydifferentserviceproviders,whichisanadditional
benefitforusecasesaimingforimprovedresiliency.
Aggregatingbandwidth
Bandwidthaggregationreferstotheabilityof
Multipathtcptocombinethebandwidthofseveral
linksintoonelogicalconnection.Figure3shows
anexampleofhowMultipathtcpaddstogetherthe
bandwidthofdslandlte.Thisisequallyvalidforthe
lte+Wi-Fiscenariodepictedinthebottompartof
Figure2.
ThebandwidthaggregationfeaturesofMultipath
tcpapplytobothdownlinkanduplinkdirections.As
aresult,Multipathtcpalsohelpstoimproveuplink
speeds,whichareonlyafractionofthedownlinkspeed
inexisting(asymmetric)dslconsumerservices.For
instance,theuplinkspeedovera6Mbpsasymmetric
dslconnectionisusuallybelow1Mbps.Aggregating
dslandltemakesitpossibletoboosttheuplinkspeed
to10Mbpsandmore.
Examplesofservicesthatwouldbenefitfromthe
Multipathtcpbandwidthaggregationare:
〉〉 AuserwatchingHDTV(highdefinitionTV)overaDSL
accessconnectionthatisnotcapableofproviding
enoughbandwidth–MultipathTCPcanbeusedto
schedulesurplustrafficoverLTE(particularlyusefulfor
thedownlink).
〉〉 Auseruploadingdocumentsorphotostoaserver–when
theDSLuplinkcapacityisexceeded,MultipathTCPcan
addLTEcapacityforquickerupload.
Improvingreliability
Inthecontextofaccessaggregation,reliabilityrefers
totheabilitytomaintaindataexchangewithina
session,evenifoneorseveralaccesslinksbecome
unavailable.Figure4comparesthebehaviorofa
BANDWIDTH
AGGREGATION REFERS TO
THE ABILITY OF MULTIPATH
TCP TO COMBINE THE
BANDWIDTH OF SEVERAL
LINKS INTO ONE LOGICAL
CONNECTION.
Figure 3
DSL and LTE
bandwidth
aggregation with
Multipath TCP
... DSL ... LTE
Connection speed over ...
... DSL/Wi-Fi + LTE
with Multipath TCP

6. ✱ MULTIPATH TCP & THE LAST MILE
6 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
traditionalwanbackupsolutionwiththatofasolution
basedonMultipathtcp.Traditionalsolutionscannot
reactquicklytothedisappearanceandreappearance
ofaccesslinks.Wheneveralinkdisappears,sessions
breakandneedtobereestablished,whichcanleadto
datalossandtheneedforhumanintervention.
Multipathtcpisabletoreactmorequicklytoaccess
linksdisappearingandreappearing.Andaslongas
atleastoneaccesslinkisupandrunning,aMultipath
tcpenabledsessionwillcontinuewithoutinterruption
–albeitatalowerbitrate.Likewise,ifanaccesslink
reappears,thebitrategoesup.Theconnectionalways
runsatanoptimalspeedinrelationtotheavailabilityof
thelinksinvolved.
Achievingseamlessconnectivity
Theconceptofseamlessconnectivityisrelatedto
reliability,referringmorespecificallytotheabilityof
Multipathtcptoswitchfromoneaccesstoanother
withouthavinganyimpactontheapplication.A
typicalusecasewouldbeasessionstartedover
Wi-Fi.IfthemobiledeviceleavesWi-Ficoverageand
entersmobilebroadbandcoverage,thesessionwill
breakandneedtobereestablished.Thiscanbequite
annoyingandtimeconsumingfortheuser,especially
iftwo-factorauthenticationisinvolved.With
Multipathtcp,thesessiondoesnotgetinterrupted
duetothechangeofaccess.
Changingfromoneaccesstoanothercanalsobe
triggeredbyserviceproviderpolicies.Forexample,
aserviceprovidercouldhaveapolicytouselteby
default,butmovesometraffictoWi-Fiwhenthereis
goodcoverageandavailablecapacity.Or,alternatively,
theserviceprovidercouldsetapolicywhereWi-Fiis
usedbydefaultandlteisusedtoprovidewide-area
coverage.Inallcases,theuseofMultipathtcpprevents
sessionsfrombeinginterruptedifandwhenaccess
systemschange.
HowMultipathTCPworks
tcp[3]isoneofthemainprotocolsintheipsuite,
providingareliablemeansofcommunicationbetween
twoendpoints.Onceatcpconnectionhasbeensetup,
bothendpointscansendadatastreamtoeachother.
tcpisdesignedtocopewithdatathatisdamaged,lost,
duplicatedordeliveredoutoforder.Furthermore,
itprovidesameanstoperformflowcontrol.Upon
receivingdata,thereceiversendsanacknowledgment
Figure 4 Improved
connection resiliency
with Multipath TCP
Several
seconds
Ongoing sessions break down,
service interrupted for several seconds Session continuation at optimal speed
Several
seconds
Bandwidth
Bandwidth
Time
DSL
DSL failure DSL back
DSL
LTE
Traditional WAN backup solution WAN backup solution with Multipath TCP
Time
DSL
DSL failure DSL back
DSL + LTE DSL + LTE
LTE

7. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 7
(ack)backtothesender.Suchanackcontainsa
“window,”whichindicatesthemaximumnumberof
bytesthesenderisallowedtotransmitbeforereceiving
furtherpermission.Thisway,thereceivercontrols
theamountofdatatransferredbythesender.Finally,
thereceiptornon-receiptofacksguidesthetcp
CongestionControlAlgorithm(cca)todeterminethe
paceatwhichdatamaybesent.
Today,manyendpointshavemultipledata
communicationinterfacesandthereforemultipleip
addresses.Forexample,alaptopisoftenequipped
withbothawiredandawirelessinterface,anda
smartphoneoftenhasthecapabilitytousemultiple
wirelesscommunicationtechnologies.Usingregular
tcp,thesedevicesarecapableofestablishingmultiple
simultaneoustcpconnections,witheachconnection
tiedtoonespecificipinterface.Inotherwords,each
tcpconnectionisboundtoasinglepathdefined
bytheipaddressesoftheconnection’sendpoints.
Note,however,thatapathisdefinedhereintermsof
endpointidentifiers;itisnotthesameastheroutethat
individualpacketstakeontheirwayfromoneendpoint
totheother.
Multipathtcp[2]isasetofextensionstostandard
tcpthatallowsconnectionstousemultiplepaths
simultaneously.Multipleregulartcpconnections,
alsoknownassubflows,areaggregatedintoasingle
Multipathtcpconnection.Figure5comparesthe
protocolsstackofregulartcpwiththatofMultipathtcp.
Inregulartcp,anapplicationinitiates
communicationbyopeningaconnectionviaan
applicationprogramminginterface(api)providedby
theoperatingsystem.Thetcplayercommunicatesin
itsturnwiththeiplayer.InMultipathtcp,thetcplayer
hasbeenextended.Upwards,theMultipathtcplayer
exposesaninterfacethatisperceivedasregulartcpby
theapplication.Downwards,theMultipathtcplayer
maysetupmultipleregulartcpconnections.These
maybeboundtodifferentiplayers.InFigure5,the
hostisequippedwithmultipledatacommunication
interfaces.Eachoneisassociatedwithitsownip
address.TheMultipathtcplayeraggregatesthe
multipletcpconnectionsintoasingleMultipathtcp
connection.Theapplicationdoesnotneedtobeaware
ofwhichprotocolstackisused.
Figure6showsanexampleofhowaMultipath
tcpconnectioncanbeestablished.Itstartswiththe
setupofafirstsubflow(steps2-4).Thesestepsconsist
Figure 5 Protocol
stack for TCP and
Multipath TCP
Application
TCP
IP
L2/L1
Application
Multipath TCP
TCP
subflow #1
IP #1
L2/L1 #1
TCP
subflow #n
IP #n
L2/L1 #n
TCP
subflow #2
IP #2
L2/L1 #2
TCP networking API TCP networking API

8. ✱ MULTIPATH TCP & THE LAST MILE
8 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
ofathree-wayhandshake,similartotheprocessin
regulartcp.TheonlydifferenceforMultipathtcpis
thatanmp_capableoptionisusedinthetcpheader.
Withthisoption,thedeviceindicatestoitspeerthat
itisMultipathtcpcapableandwantstouseit(step2).
IfthepeerisalsoabletouseMultipathtcp,itreplies
withasimilarcapabilityindication(step3).Aspartof
thethree-wayhandshake,theendpointsalsoexchange
securitykeys.Aftersettingupthefirstsubflow,both
endpointscanexchangedataovertheconnection
(steps6–7).
OnceaMultipathtcpconnectionhasbeen
established,eachendpointmayinitiatethesetupofan
additionalsubflow.IntheexampleshowninFigure6,
thedevicehastwonetworkinterfaces.Eachinterface
isassociatedwithitsownipaddress.Here,thedevice
takestheinitiativetoestablishasecondsubflowvia
itssecondinterface.Again,athree-wayhandshakeis
usedtoachievethis.Butthistimetheoptionmp_join
isusedtoindicatethatthisisanewsubflowthatistobe
joinedtoanexistingMultipathtcpconnection.Atoken
(step9),derivedfromtheearlierreceivedkey(step3),
isusedtocorrectlybindthetwosubflows.Additional
authenticationinformationisalsoexchangedtoensure
theauthenticityofbothendpoints.
Oncethenewsubflowhasbeenestablished,both
endpointscanuseittosendandreceivedata.Inour
example,thedevicesendsdatatoitspeer(step14).
Notethatthedeviceneedstotakeanactivedecision
regardingwhichsubflowtouse(step13).Howthis
decisionismadeisnotdefinedinthestandard,which
givesthedesignerthefreedomtoimplementthe
schedulingpolicythatismostappropriateforeachcase.
Subflowsmaycomeandgoforvariousreasons,such
asconnectivityproblems.Toensurereliable,in-order
deliverytotheapplication,Multipathtcpusesadata
sequencenumberthatiscarriedinaDataSequence
Signaloption(steps6-7and14-15).Asidefrom
ensuringin-orderdelivery,thisnumbercanbeused
incombinationwiththesequencenumbersusedby
regulartcpatsubflowleveltoexecuteretransmissions
ondifferentsubflows,ifneeded.Multipathtcpcanalso
synchronizecongestioncontroloversubflowsinorder
toavoidunfairnesstosingle-pathusers[5].
AnadditionalbenefitofMultipathtcpisthatit
canbeintroducedincrementally.Inparticular,ifthe
receiverofthefirstsubflow’stcpsyndoesnotsupport
Multipathtcp,itwillsimplydiscardthecapability
option.Itwillreplywithatcpsynack,butwithout
addingthemp_capableoption,andtheconnection
willbemadewithstandardtcp.
Theproxy-basedapproachtoMultipathTCP
accessaggregation
Proxiesmakeitpossibletoachievethebenefitsof
Multipathtcpforaccessaggregationwithoutrequiring
Multipathtcpsupportinallenddevicesandinternet
servers.Anadditionalbenefitofproxiesisthattheygive
theserviceprovidercontrolovertheschedulingofthe
traffic.Inthisway,serviceproviderscanensurethat
theavailableaccessalternativesareusedinthemost
efficientandcost-effectiveway.Theuseofproxieshas
alreadybeenrecognizedbytheindustry,andworkhas
beendoneandpublishedbytheBroadbandForum
definingthearchitecture[6].Ericssoniscontributing
activelytothiswork.
Figure7providesahigh-leveloverviewoftheproxy-
basedapproachtoMultipathtcpaccessaggregation.
Therearetwoproxiesinvolved:anetworkproxyand
acustomerpremisesequipment(cpe)proxy.The
User space
Incomputerdesign,adistinctionismadebetweenkernelspaceanduserspace.Kernelspaceiswheretheoperating
systemcoderuns–hardwaredevicedrivers,memorymanagementandprotocolstacks,forexample.Userspaceis
whereordinaryprogramsrun.IndesigningourMultipathTCPsolution,wechosetoplaceaprotocolstack(MPTCP)in
userspaceratherthaninkernelspace.Thisresultsinfasterpacketprocessing,becausepacketsdon’tneedtotravel
fromkernelspacetouserspace.Instead,theygodirectlyfromthehardwareinterfacetouserspace.

9. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 9
1: Establishment of a subflow via interface A
5: Example of a data exchange
8: Adding a subflow via interface B
12: Example of a data exchange
13: Make scheduling decision
2: TCP/IP SYN
Network
interface A
Device
Network
interface B Peer
header option MP_CAPABLE: device’s key
3: TCP/IP SYN ACK
header option MP_CAPABLE: peer’s key
4: TCP/IP ACK
header option MP_CAPABLE: peer’s key, device’s key
6: TCP/IP data
header option DSS
7: TCP/IP ACK
header option DSS
14: TCP/IP data
header option DSS
(via subflow A or subflow B)
15: TCP/IP ACK
header option DSS
9: TCP/IP SYN
header option MP_JOIN: peer’s token, authentication information
10: TCP/IP SYN ACK
header option MP_JOIN: authentication information
11a: TCP/IP ACK
header option MP_JOIN: authentication information
11b: TCP/IP ACK
ACK
Figure 6 Establishment of a Multipath TCP connection

10. ✱ MULTIPATH TCP & THE LAST MILE
10 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
networkproxyislocatedintheserviceprovider’s
networkandconvertstcpsessionsfrominternet
serversintoMultipathtcpsessionsthatoperateacross
multipleaccessnetworks.Similarly,thecpeproxy
convertsaMultipathtcpsessionwiththenetwork
proxybackintoatcpsession.
Enddeviceswithbuilt-inMultipathtcpsupport
couldalsoconnectdirectlytothenetworkproxy.There
arealreadysomesmartphonesonthemarketwithbuilt-
inMultipathtcpsupportthatcanbeusedtoaggregate
lteandWi-Fi.Ericssonhasrunteststhatprove
thefeasibilityofthissetupinpubliclteandWi-Fi
networks.
Theproxiescanbeusedtoenhancestandard
Multipathtcpviaadditionaltraffic-steering
capabilitiesthatareoptimizedforthespecific
applicationscenario.Forinstance,aserviceprovider
mightwanttoensurethatthedslpipeisfilled
firstbeforeusingthescarcerltebandwidth.This
traffic-steeringapproachisoftenreferredtoasa
cheapest-link-firstpolicy.Serviceprovidersmightalso
wanttodefinepoliciestopreventorallowtheuseof
heterogeneousaccessforspecificservices,ortoforce
selectedservicestouseonlyoneoftheavailableaccess
links.AllofthisispossiblewithMultipathtcp,asthe
ietfstandarddoesnotprescribeaspecifictraffic-
steeringmethod.
Inanimplementation,theoptionalcpeproxywillbe
integratedinacpesuchasahomeorofficerouter.This
setupcanbeusedinaresidentialorenterprisesetting,
andwhenitisinplace,alldevicesconnectingtothe
routerwillreceiveafasterandmorereliableinternet
connection.Trafficsteeringcanalsobeappliedat
thecpeproxyleveltocontrolthetrafficintheuplink
direction.
Ericssonispartneringwithcpevendorsand
chipsetmanufacturerssuchasInteltoensureefficient
implementationoftheMultipathtcpcpeproxy.We
alsoofferareferencedesignandatestlabenvironment
forcpevendors.
Carrier-gradeMultipathTCPproxy
implementation
OneimportantrequirementforaMultipathtcpproxy
intheserviceprovidernetworkistheabilitytosupport
ahigh-performance,carrier-gradeipsolutionfor
trafficaggregation.Figure8illustrateshowEricsson’s
solutioncanbeusedasaMultipathtcpnetworkproxy,
whichcanbedeployedineitheravirtualizedor
non-virtualizedenvironment.
Allcomponents–includingMultipathtcp
functionality–areimplementedinuserspace[7]to
meetthecapacityrequirements.Thetcptrafficcanbe
accesseddirectlyfromhardwareusingaDataPlane
DevelopmentKit(dpdk)[8].Thepacketdistribution
functionisresponsibleforsendingtraffictothe
Multipathtcpprotocolstack,locatedintheuserspace
ononeorseveralcentralprocessingunit(cpu)cores.
TheEricssonsolutionimplementsMultipathtcp
functionalityasspecifiedbytheietf[2],combined
withaspecificallydesignedtcpccacalledtcprna
(RadioNetworkAware).tcprnaisdesignedto
utilizethemobileraninanoptimalway,andsolvesthe
equationsforthecorrectcongestionwindowbyusing
measurementsofthespeedofthearrivingtcpacksin
conjunctionwithreactionsoflosttcpsegments.The
benefitsoftcprnaare:
〉〉 maximumutilizationofavailablebandwidthforbothuplink
anddownlink
〉〉 reducedretransmissionsusingtrafficshaping
〉〉 controllablelatency
〉〉 avoidingbufferbloat.
Thissolutionishighlyconfigurableandcanbetailored
tosupportmultipleMultipathtcpusecasesperaccess
network.Thetraffic-steeringsettingsarepolicydriven.
ERICSSON IS
PARTNERING WITH CPE
VENDORS AND CHIPSET
MANUFACTURERS SUCH AS
INTEL TO ENSURE EFFICIENT
IMPLEMENTATION OF THE
MULTIPATH TCP CPE PROXY.

11. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 11
OneconfigurationexampleistosendMultipathtcp
trafficononepreferredsubflow,suchasthedsllink.
Whenthedsllinkhasreacheditslimit,anysurplus
Multipathtcptrafficwillbesentonanothersubflow–
mostcommonlytheltelink.
Anotherconfigurationexampleaimstooptimize
radiousageonasystem-widelevel.IfMultipathtcp
trafficissharingradiospectrumwithothernon-
Multipathtcptraffic–fromlte-onlymobilephones,
forexample–itmightbepreferabletoavoidexcessive
useoftheltelinkfromMultipathtcptraffic.Thiscan
beachievedbyconfiguringthetcprnaforthelte
linktobehavelikebackgrounddelivery.Theresult
isthatMultipathtcptrafficwillbackoffwhen
tcprnadetectsthatthecelliscongested,infavorof
lte-onlytraffic.
Attimes,itmightbedesirabletoconfigure
Multipathtcpformaximumthroughput–when
combiningltewithWi-Fiaccessforfastfile
download,forexample.Insuchascenario,thesolution
canbeconfiguredtouseround-trip-time-based(rtt-
based)trafficsteering.Suchtrafficsteeringisachieved
bysendingdataoverthesubflowwiththelowestrtt.
Ifthatlinkreachesitscapacitylimitandthereismore
datatosend,therestofthedataissentovertheother
subflow.Ifonesubflowcanhandleallthedata,onlythe
linkwiththelowestrttwillbeused.
Conclusions
Accessaggregationisaviableoptionforservice
providerstoboostbandwidthacrossthelastmilein
areaswhereitistoocostlytoincreasethecapacityof
legacyaccess.Typicalaccessaggregationscenariosare
thecombinationofdslwithlteorthecombination
Figure 7 Proxy-based approach for Multipath TCP access aggregation
Any fixed or Wi-Fi
connected device
Servers
Device with
Multipath TCP support
(a smartphone, for example)
DSL
LTE
LTE
Wi-Fi
Wi-Fi
Ethernet
Internet
CPE
proxy
Network
proxy
TCPMultipath TCPTCP

12. ✱ MULTIPATH TCP & THE LAST MILE
12 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
Figure 8 Multipath TCP network proxy
CPU Core 1
CPU Core 2
CPU Core N
Multipath TCP
Multipath TCP TCP
TCP RNA
IP stack
Packet
distributor
Load balancer
DPDK

13. MULTIPATH TCP & THE LAST MILE ✱
OCTOBER 24, 2016 ✱ ERICSSON TECHNOLOGY REVIEW 13
ofltewithWi-Fi.Multipathtcp,asspecifiedbythe
ietf,isidealforaccessaggregationinthelastmile,
asitisabletoboostbandwidthsignificantly,while
simultaneouslyincreasingreliabilityandensuring
seamlessconnectivity.
Multipathtcpcomesasasetofextensionsto
standardtcp.Itleveragesallofthebenefitsoftcp
suchasfairness,flowcontrolandreliability,aswellas
allowingtheuseofmultiplepathsthroughanetwork
simultaneously.Multipathtcpproxiesallowservice
providerstouseMultipathtcpforaccessaggregation
withouttheneedforenddevicesandinternetservers
tobeawareofit.
EricssonhascreatedaMultipathtcpproxythatis
tailoredtothespecificneedsofserviceproviders.Itis
carrier-grade,optimizedforhightrafficthroughput
andallowsserviceproviderstoimplementtraffic-
steeringpoliciesfortheuseofavailableaccess
networksinthemostcost-effectiveandefficientway.
References:
1. 3GPP TS 23.402, Architecture enhancements for non-3GPP accesses, available at:
www.3gpp.org/DynaReport/23402.htm
2. IETF RFC 6824, TCP Extensions for Multipath Operation with Multiple Addresses, available at:
https://tools.ietf.org/html/rfc6824
3. IETF RFC 793, Transmission Control Protocol, available at:
https://tools.ietf.org/html/rfc793
4. Linux Kernel Multipath TCP Project, available at:
http://www.multipath-tcp.org/
5. IETF RFC 6356, Coupled Congestion Control for Multipath Transport Protocols, available at:
https://tools.ietf.org/html/rfc6356
6. Broadband Forum, Hybrid Access Broadband Network Architecture (TR-348), available at:
https://www.broadband-forum.org/technical/download/TR-348.pdf
7. Jonathan Corbet, Alessandro Rubini, Greg Kroah-Hartman, Linux Device Drivers, 3rd Edition.
Nutshell Handbooks, 2005.
8. DPDK – Data Plane Development Kit, available at:
https://en.wikipedia.org/wiki/Data_Plane_Development_Kit

14. ✱ MULTIPATH TCP & THE LAST MILE
14 ERICSSON TECHNOLOGY REVIEW ✱ OCTOBER 24, 2016
Robert Skog
◆ is a senior expert in the
field of media delivery.
After earning an M.Sc.
in electrical engineering
from KTH Royal Institute
of Technology in
Stockholm in 1989, he
joined Ericsson’s two-
year trainee program for
system engineers. Since
then, he has mainly worked
in the service layer and
media delivery areas, with
everything from the first
WAP solutions to today’s
advanced media delivery
solutions. In 2005, Skog
won Ericsson’s prestigious
Inventor of the Year Award.
Dinand Roeland
◆ joined Ericsson in 2000
as a systems manager for
core network products. At
Ericsson Research since
2007, he is currently a
senior specialist in core
network architectures
and features. He has been
a key contributor to the
standardization of multi-
access support in the
3GPP EPC architecture,
especially in Wi-Fi.
Roeland holds an M.Sc.
cum laude in computer
architecture from the
University of Groningen,
the Netherlands.
Jaume Rius i Riu
◆ joined Ericsson in 2004
and has been principal
researcher in connectivity
architectures at Ericsson
Research since 2014. His
work focuses mainly on
the standardization of
fixed-mobile convergence
networking technologies
(hybrid access, mobile
transport, converged
policy control, IPv6,
network controlled Wi-
Fi and so on) and proof
of concept development.
Rius i Riu holds an M.Sc.
in physics from UAB
Autonomic University of
Barcelona and a Ph.D. in
experimental physics from
KTH Royal Institute of
Technology in Stockholm.
Uwe Horn
◆ is a solutions director
within Ericsson’s Global
Customer Unit Vodafone.
He has worked in the
telecommunications
industry for more
than 15 years and held
various positions in R&D,
consulting, marketing
and sales. For the past
10 years, he has worked
closely with Tier-1 service
providers to develop
new solutions based on
the latest technologies.
Horn holds a Ph.D. in
telecommunication
engineering from Friedrich-
Alexander-Universität
Erlangen-Nürnberg in
Germany and a diploma
in computer science from
the University of Bonn,
Germany.
Michael Eriksson
◆ is a senior researcher at
Ericsson Research. During
most of his more than 20
years with Ericsson, his
research has focused on
the areas of computer
science and networking.
His current focus is on the
design and implementation
of advanced networking
prototypes. Eriksson
holds an M.Sc. in electrical
engineering from
KTH Royal Institute of
Technology in Stockholm.
theauthors

15. ISSN 0014-0171
284 23-3292 | Uen
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