TCP Performance over Current Cellular Access: A Comprehensive Analysis

Transcription

1 TCP Performane over Current Cellular Aess: A Comprehensve Analyss Eneko Atxuteg 1(B), Åke Arvdsson 2, Fdel Lberal 1, Karl-Johan Grnnemo 3, and Anna Brunstrom 3 1 Unversty of the Basque Country (UPV/EHU), Blbao, Span {eneko.atxuteg,fdel.lberal}@ehu.eus 2 Krstanstad Unversty, Krstanstad, Sweden ake.arvdsson@hkr.se 3 Karlstad Unversty, Karlstad, Sweden {karl-johan.grnnemo,anna.brunstrom}@kau.se Abstrat. Moble Internet usage has nreased sgnfantly over the last deade and t s expeted to grow to almost 4 bllon users by Even after the great effort dedated to mprovng the performane, there stll exst uesolved questons and problems regardng the nteraton between TCP and moble broadband tehnologes suh as LTE. Ths hapter presents a thorough nvestgaton of the behavor of dstnt TCP mplementaton under varous network ondtons n dfferent LTE deployments nludng to whh extent TCP s apable of adaptng to the rapd varablty of moble networks under dfferent network loads, wth dstnt flow types, durng start-up phase and n moble senaros at dfferent speeds. Loss-based algorthms tend to ompletely fll the queue, reatng huge standng queues and ndung paket losses both under stllness and moblty rumstanes. On the other sde delay-based varants are apable of lmtng the standng queue sze and dereasng the amount of pakets that are dropped n the enodeb, but under some rumstanes they are not able to reah the maxmum apaty. Smlarly, under moblty n whh the rado ondtons are more hallengng for TCP, the loss-based TCP mplementatons offer better throughput and are able to better utlze avalable resoures than the delay-based varants do. Fnally, CUBIC under hghly varable rumstanes usually enters ongeston avodane phase prematurely, provokng a slower and longer start-up phase due to the use of Hybrd Slow-Start mehansm. Therefore, CUBIC s unable to effently utlze rado resoures durng shorter transmsson sessons. Keywords: TCP adaptablty LTE Flow sze Slow-Start Moblty 1 Introduton Moble Internet usage has nreased sgnfantly over the last deade, growng almost 18-fold over the past 5 years and more than half a mllon new moble deves and onnetons n 2016 [1]. The followng years are expeted to be The Author(s) 2018 I. Ganhev et al. (Eds.): Autonomous Control for a Relable Internet of Serves, LNCS 10768, pp ,

2 372 E. Atxuteg et al. equally promsng wth 4G traff reahng quotas of more than three-quarters of the total moble traff by The growth expetaton s not only related to the traff volume tself but also to the average speed. To ontnue ths growth and to meet user expetatons, all the nvolved stakeholders have a ommon nterest n fast downloads, quk responses, hgh utlzaton and few paket losses. Sne a large part of moble Internet omprses TCP flows, the performane of TCP over ellular networks has beome an mportant researh top. Even though n the last three deades many dfferent TCP mplementatons have been developed [2] eah of them targetng a dfferent Congeston Control Algorthm (CCA), there stll exsts room for mprovement n terms of aheved throughput and resultng delay over hghly varable moble networks. Prevous studes and proposals have reported ther results regardng the nteraton effets between moble networks and TCP [3 5] and tred to defne sutable CCAs for moble networks [6]. However, none of them have extensvely study the mplaton of a wde range of TCP mplementatons n a varety of stat and movng senaros. Ths hapter omplements and extends prevous works on moble networks by studyng and evaluatng the behavor of a seleton of TCP varants wth dfferent paket szes, network loads, durng start-up and moblty wth dfferent speeds,.e. senaros that are onsdered hallengng for TCP. In order to approprately study the dfferent soures apable of mpatng the fnal performane, the hapter suggests a bottom-up senaro wth respet to omplexty startng wth stat ondtons so as to understand the responsveness of TCP under dstnt network status and load ombnatons and fnshng wth a varety of moblty senaros. The hapter s organzed as follows. Seton 2 overs related work. In Set. 3,a bref overvew of the studed TCP varants s provded and the LTE testbeds are desrbed. Next, n Set. 4, we explan the methodology regardng the performed measurements and the studed senaros. The fndngs and results from our work are presented n Set. 5. Fnally, Set. 6 onludes the hapter wth a summary and a dsusson of future work. 2 Related Work TCP and LTE ellular aess have been deeply studed throughout the last years. Most of the studes have ether researh the TCP sde or moble network sde. However, a sgnfant amount of researhers have been attrated by the nteraton between TCP and LTE. One of the frst bass of suh nteraton s the mpat that rado retransmssons have nto the delay nrement and how they therefore degrade the aheved goodput [7, 8]. It has been proven that the number of smultaneously atve User Equpments (UEs) towards a ommon enodeb has a huge mpat on the effetve avalable bandwdth due to rado resoures beng shared. Thus, the work [9] found that sudden nreases n bakground traff load have an mportant effet n the Round-Trp Tme (RTT) nrement. Ths ross-traff effet severely nfluenes the network playground for TCP, provokng sudden hanges n the network

3 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 373 ondtons and makng TCP struggle whle followng the flutuatons n the avalable apaty. Ths hapter omples a more detaled treatment of the effets of bufferng n the rado aess part of LTE by also onsderng the performane of hgh-speed/long-delay varants of TCP n these knds of networks. The so-alled bufferbloat effet has also a huge mpat nto the performane of TCP over LTE [10]. The bufferbloat effet s possble due to the onfguraton of long queues both n the end-nodes and ntermedate nodes, whh an aumulate a great number of pakets wthout any drop. However, that exessve paket bufferng n a sngle queue n the end-to-end network path, auses a great lateny nrease and therefore, throughput degradaton. Our work does not merely fous on bufferbloat, but onsders the mplatons of dfferent TCP varants n queue buld-up under ertan network ondtons. Consderng that many flows n Internet are short, t s mportant to verfy the effeny of TCP to arry out suh transmssons over ellular networks, t has been demonstrated [11] that under some network ondtons TCP fals to orretly utlze the avalable apaty and therefore, the flows last longer than neessary. The urrent work omplements suh works and analyzes the mpat that dfferent flow szes have n the performane outome of dfferent TCP flavors. To ths end, our work not only fouses on the statonary phases of TCP but also on ts behavor durng start-up due to ts sgnfant mpat n short flows performane. In partular, we study the Hybrd Slow-Start sheme [12], and evaluate how t operates n LTE networks n omparson wth the Standard Slow-Start sheme. Other studes have measured TCP over lve LTE networks. Apart from the lass metrs of TCP throughput and RTT n [4] they also measured the delay aused by moble deves gong from dle to onneted state. In [13], measurement trals were arred out over the ellular aess of four Swedsh operators and the durnal varaton of TCP throughput and delay were analyzed. [14, 15] studes dd smlar TCP measurements, however, they dd not onsder daly varatons. None of these lve measurements took nto aount the mpat of speed n the performane of TCP, or the behavor on dfferent types of CCAs. So, to the best of our knowledge, our work both omplements and extends these works through the study and evaluaton of the behavor of ommon TCP varants n LTE networks under moblty wth dfferent speeds. There are only a few works that have onsdered the mpat of dfferent speeds on the performane of TCP over LTE networks. Even though some works [5]have studed dfferent speeds, the prmary metrs were more related to the rado part wth spetral effeny and share of resoure bloks among the UEs. Even though the utlzaton of suh rado resoures was studed, one or two smple varants of TCP were utlzed n a mult-user resoure share, leadng to TCP mro-effets maskng. Also, n [3] the mpat of speed on TCP n LTE was studed. The work foused on uplnk and downlnk throughput, RTTs and also onsdered tme-ofday varatons. Stll, they dd not onsder how the CCA fator nto the TCP performane at dfferent velotes. Our work serves to over all the optons and extends the prevous studes wth multple moblty patterns, dfferent speeds and a wde range of CCAs.

4 374 E. Atxuteg et al. 3 Researh Envronment In order to ompare the behavor of TCP n LTE networks, we frst hoose the TCP varants AND dentfy the LTE workng parameters. Ths seton frst desrbes the most mportant features of the seleted CCAs and later presents the LTE setup. 3.1 TCP Varants TCP varants fall nto three ategores aordng to the CCA mehansm used: loss-based, delay-based and ombned loss- and delay-based. Along ths hapter, the analyss starts wth fve CCAs and, wth every measurement phase, we wll redue the group, avodng the repettve usage of TCP solutons that do not work well n moble networks. A bref overvew of the TCP varants s gven below together wth the lassfaton of CCAs n Table 1. Table 1. Seleted TCP CCAs and ther ategory CCA ategory Seleted TCP CCA Loss-based TCP NewReno TCP CUBIC Delay-based TCP CDG Hybrd wth bandwdth estmaton Westwood+ Hybrd wthout bandwdth estmaton Illnos () TCP NewReno [16] employs the well-known addtve nrease multplatve derease (AIMD) mehansm that s ommon to most CCAs. Durng the Slow-Start perod the wnd nreases by one paket per aknowledgment (ACK) reepton untl t reahes the value of ssthresh. Afterwards, the wnd enters the ongeston avodane phase, wth an nrement of one paket per RTT perod (standard synhronzaton wth RTT or RTT-synhronzed). If a 3-duplate ACKs (3DUPACK) are reeved or a tme-out ours, the CCA deduts that some lnk s ongested. After 3DUPACK, NewReno establshes the wnd to the half (bas bak-off) and the new ssthresh to prevous wnd. However, f a tme-out ours the wnd wll be dereased to one paket. NewReno s essental n the measurements sne t represents the base TCP behavor. () TCP CUBIC [17] employs a dfferent mehansm ompared wth AIMD based on a ubal funton. After a derease of the wnd, the wnd ramps up n a onave shape, untl t aheves the value that the wnd had before the reduton. Afterwards, CUBIC nreases ts growth rate and ramps-up n a onvex shape. CUBIC uses Hybrd Slow-Start [12] mehansm n the

5 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 375 sender nstead of the Standard Slow-Start phase. Hybrd Slow-Start ams at fndng the proper ext pont for standard Slow-Start n order to avod massve paket losses. The deteton of suh an ext pont s based on the measurements of ACK trans and RTT delay samples. The TCP CUBIC mplementaton has been seleted for the analyss due to ts wdespread use due to the fat that t urrently s the default CCA n Lnux servers, whose market share omprses the 67% of world-wde servers (as stated by W3Tehs [18]). () TCP CAIA delay gradent (CDG) [19] modfes the TCP sender to use RTT gradents as a ongeston ndator. CDG also alulates the state of the bottlenek queue so that paket losses are treated as ongeston sgnals only when the queue s full. Fnally, CDG also uses Hybrd Slow-Start but wth a more strt onfguraton than CUBIC. The seleton of TCP CDG has been based on ts novel use of delay gradents n the AIMD mehansm and to evaluate the atual usefulness of suh a dfferent feature n moble networks. (v) TCP Westwood+ [20] s apable of estmatng the avalable bandwdth and mnmum RTT (RTTmn) by measurng ACK nter-arrval tmes. The estmatons are used to dede the new wnd after a ongeston epsode of 3DUPACK. Wth tmeouts the ssthresh s alulated n aordane to the estmatons and the wnd s set to 1 segment. TCP Westwood+ has been seleted n ths study for ts hybrd behavor usng loss-based mehansms together wth delay-awareness. (v) TCP Illnos [21] ontrols the AIMD mehansm by the estmated queung delay and buffer sze. In a normal stuaton when no queung delay s deteted, the wnd s nreased by 10 pakets per RTT. If estmated delay starts nreasng, the nrement of wnd wll be gradually lowerng untl the mnmum value of 0.3 pakets per RTT s reahed. When the RTT s onsdered as hgh as ompared to the baselne RTT, the loss s onsdered as buffer overflow, whereas n low RTT the loss ounts as paket orrupton. Developed to perform effently wthn hgh speed networks, ts loss-based and delay-awareness make a perfet anddate for our study. 3.2 LTE Setup In order to evaluate the performane of LTE three dfferent envronments have been used: smulaton, emulaton and ontrolled deployment. Most of the work desrbed n ths hapter has been arred out over the smulated envronment and for omparson purposes the fndngs and results have been orrelated wth the behavor n the other two deployments. Sne the onfguraton and explanaton of the smulated envronment s omprsed of many parameters and n order to help the reader understand the setup, Table 2 gathers the most mportant nformaton about the smulaton envronment regardng the onfguraton parameters and experment-related ondtons. As the smulated envronment, ns-3 smulator wth the LTE apabltes of LENA module s used. Ths module also allows to reate standard-based fadng

6 376 E. Atxuteg et al. traes that an be appled to the hannel between the UE and the enodeb. Sne ns-3 does not exatly use the avalable TCP mplementatons n the Lnux kernel, we used Dret Code Exeuton (DCE) Cradle [22] to be able to run real TCP mplementatons n ns-3. In order to smulate the dstane to the server, the propagaton delay between the fxed remote host and the Paket Data Network Gateway (PGW) was set to 40 ms. In the Rado Lnk Control (RLC) layer we seleted the Aknowledged Mode (AM) n order to resemble the most ommonly deployed onfguraton n real-world. We modfed the mehansm to be able to support a lmtaton s terms of pakets, establshng n our setup a ommon paket buffer sze n the enodeb of 750 pakets. Regardng the rado resoures, the enodeb was onfgured to have a standard value of 100 avalable physal resoure bloks (PRB). We smulated the frequeny band 7 (2600 MHz), one of the most ommonly used ommeral LTE frequeny bands (n Europe). Bakground flows are used to load the network wth multple short TCP onnetons, smlar to the behavor of real networks. The same TCP varant s used for both bakground and foreground traff n order not to be affeted by ssues of TCP frendlness. The amount of data transferred n a bakground onneton as well as the nter-arrval tme between two onnetons were drawn from unform random dstrbutons. The ontrolled testbed ams at provdng a measurement platform wth the ablty to measure TCP n more realst rado ondtons n order to onfrm or rejet the fndngs and assumptons made n smulated envronment n relaton to the behavor of TCP over LTE. We have used the Mnds /MEC s LTE falty (LTE w-lab.t [23]) n Zwjnaarde, Ghent. Apart from the provsonng of all the agents nvolved n LTE, the deployment allows ad-ho moblty patterns whle Table 2. Smulaton parameters Smulaton envronment Smulator ns-3 LENA LTE model Lnux Kernel 4.3 (DCE) CCA NewReno/CUBIC/Illnos/CDG/Westwood+ Parameter Value One-way delay PGW-Server 40 ms MAC sheduler Proportonal far AMC model MError Number of PRBs 100 LTE band 7 (2600 MHz) RLC mode AM RLC transmsson queue 750 PDUs Pathloss model FrsPropagatonLossModel Fadng models EVA60/EVA200

7 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 377 expermentng. It s mportant to underlne that n ths envronment, the LTE transmssons are done over the ar, thus allowng a proper study of TCP and LTE events. Even though the movement s real, the spae lmtaton ould lmt the employed speed. The emulated testbed targets the valdaton of smulated results of TCP under moblty rumstanes. To ths end, a LTE emulator or LTE-n-a-box (Aeroflex 7100) has been used. Ths emulator s apable of reatng the LTE rado sgnal and all the neessary LTE protool events to support the attahment and regstraton of any LTE deve through a radofrequeny able or over the ar. The tests have been ompleted wth an smartphone, a ouple of servers and a ontroller to synhronze the experments and all the equpment nvolved durng the assessments. Sne the UE n the emulated testbed s not able to physally move, the ontroller would ontnuously manpulate the baselne Sgnal-to-nterferene-plus-nose rato (SINR) levels and Aeroflex would apply the orrespondng fadng pattern so as to model atual movement. 4 Methodology Desrpton The ntrns operaton mode of LTE (.e. resoure sharng, shedulng, HARQ mehansms) results n a onstant hange n the avalable apaty. Even onsderng sngle-ue senaros, dfferent postons and fadngs would lead to have a dfferent SINR and t would therefore report a dstnt Channel Qualty Indator (CQI) to the enodeb. Thus, the enodeb would assgn a dfferent avalable apaty for the hannel of the UE through the Modulaton and Codng Sheme (MCS) and transport blok sze (tbsze). Due suh flutuatons n the rado sde, the wnd wll be ontnuously evolvng n order to obtan a resultng goodput as lose as possble to the avalable apaty. The relatve progressons of both parameters (avalable apaty and aheved apaty) play a fundamental role n the fnal performane. In ths seton, the appled methodology wll be presented. Fgure 1 shows the dfferent senaros that have been used n the analyss of the effets between TCP s dfferent CCAs and LTE. The methodology and reasonng of eah senaro s explaned below. (I) Implaton of ross-traff and responsveness of TCP: The stat senaro ams at provdng nsghts of the evoluton and responsveness of TCP under dfferent bakground traffs (I pont n Fg. 1). There are three man goals wth ths senaro: the omparatve study of TCP behavor wth and wthout a loaded ell, the analyss of TCP fousng on short flows and the responsveness omparson of TCP varants wth a sudden apaty nrease and derease. Several metrs are gathered at dfferent nodes along the path. At the soure, TCP state nformaton suh as wnd and ssthresh s saved. At the enodeb, the transmsson buffer length, the drop ount and the Paket Data Convergene Protool (PDCP) delay (.e., the tme t takes for a PDCP Protool Data Unt -PDUto go from the enodeb to the UE), are logged. Fnally, n the UE, the goodput s reorded. Sne t s measured at the applaton level, paket losses and/or reorderng may result n goodput spkes.

8 378 E. Atxuteg et al. Fg. 1. Senaros n use. (II) Start-up performane: The am of ths senaro s to analyze the mpat of dfferent CCAs Slow-Start phases, suh as the above mentoned standard Slow-Start and Hybrd Slow-Start, and determne ther adequay or nadequay n broadband moble networks. To that purpose, we deployed 10 stat and sattered UEs (II pont n Fg. 1) n good rado ondtons (CQI 15) so as to study the start-up performane n a smplfed mult-user senaro and set some bass for the understandng of the followng senaros. In the server, the wnd, RTT, outstandng data and goodput has been olleted. (III & IV) Cell outwards/nwards movement resultng on dereasng/nreasng avalable apaty: The dereasng qualty movement senaro evaluates the behavor of TCP wth a onstantly worsenng hannel qualty on average (III pont n Fg. 1). The dea s to assess the CCA s adaptablty n a ontnuous apaty reduton (on average) envronment and the mpat of UE s speed on the fnal performane. To help smulate dfferent speeds, two Extended Vehular A Model (EVA) fadng patterns are apples: one for the veloty of 60 km/h (ommon lmtaton n rural roads) and one for 200 km/h (ommon maxmum speed

9 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 379 n hgh-speed trans). Apart from the usual metrs n the evaluaton of CCAs, the man metr for smulated moblty-based senaros s the relaton between the avalable apaty (extrated from the tbsze) and the aheved goodput. On the other hand, the nreasng qualty movement represents the behavor of TCP on a onstantly mprovng hannel qualty (IV pont n Fg. 1). Therefore, these smulatons am at evaluatng the CCA s adaptablty under dfferent UE s speeds n a ontnuous nreasng apaty (on average) envronment. (V) Correlaton of TCP behavor n deployments as smlar as possble to lve ommeral LTE networks: The senaro (V pont n Fg. 1) ams at provdng a measurement platform wth the ablty to measure TCP n more realst rado ondtons n order to onfrm or rejet the fndngs and assumptons made n smulated envronment n relaton to the behavor of TCP over LTE. Sne the equpment n the senaro s fully real (see desrpton n [23]), the shedulng, queung and the rest of the features that ould have an mpat on delay are realst and represent more learly what would happen n lve senaros, helpng n the verfaton of fndngs. (VI) Emulated support to orrelate moblty-based senaros: Sne the prevous senaro s lmted n terms of speed, the emulated testbed (VI pont n Fg. 1) due to the utlzaton of real UEs and the ablty to emulate movement, s apable of onfrmng and larfyng performane trade-offs that n smulated envronment ould be blurry. In order to better understand the evoluton of dfferent performane-related parameters, n the server, the wnd, RTT, outstandng data and goodput have been olleted. 5 Analyss of the Interatons Observed n Dfferent Senaros Ths seton s dvded n fve man parts: mplaton of ross-traff and responsveness of TCP (wth senaro I), the start-up performane (wth senaro II), both dereasng qualty and nreasng qualty movement senaros (wth senaro III and IV), the orrelaton of fndngs n the ontrolled deployment (wth senaro V) and fnally, the orrelaton of fndngs regardng moblty senaros over emulated testbed (wth senaro VI). 5.1 Cross-Traff Impat and Responsveness of TCP Ths subseton s responsble for overng dfferent knd of traff loads and behavors whle the UE s stat. The loaton of the UE among the dfferent measurements s the same and thus, the results are omparable. The subseton s dvded n three man experments: the omparson between a sngle-ue wthout ross-traff and a loaded network, the mpat of short flows and fnally, sudden nrease and derease of the avalable apaty.

10 380 E. Atxuteg et al. Congeston wndow (bytes) dg dg CDG Cub Illnos New Reno Westwood+ Congeston wndow (bytes) dg dg CDG Cub Illnos New Reno Westwood+ Delay (s) Tme (s) dg dg Tme (s) Delay (s) Tme (s) dg dg Tme (s) Goodput (Mbps) dg dg Goodput (Mbps) dg dg Tme (s) (a) Tme (s) (b) Fg. 2. Performane omparson of the seleted CCAs: (a) Base sngle flow behavor; (b) Sngle flow behavor over loaded network.

11 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 381 Base Behavor and Behavor n a Loaded Network Aordng to the seleted poston, the UE has a maxmum throughput around the half of the total maxmum (35 Mbps). Dfferent expermental trals are arred out wth and wthout bakground traff to study the responsveness of TCP and nfer whether the bakground traff has the same mpat among the CCAs or not. In order to make easer the readng, Table 3 gathers the most mportant pont of the followng explanaton. The three subfgures on the left of Fg. 2 dept the results regardng the senaro wth no bakground traff. The dfferenes between the loss-based TCP varants and delay-based ones are remarkable even n suh a smplfed senaro. Loss-based mplementatons manage to aheve the maxmum apaty and reate a long standng queue delay (up to 250 ms), whereas delay-based varants, suh as CDG, keep the delay ontrolled but fal whle tryng to reah full resoure utlzaton. In the ase of Westwood+, t s lear that the appled bak-off after Slow-Start s very drast and due to ths, t takes longer to ramp-up. Illnos mnmally redues the wnd, ausng huge standng queue delay omparng wth more onservatve mplementatons lke NewReno. In the ase of CUBIC, t suffer for the defent behavor of Hybrd Slow-Start. The mehansm exts to the ongeston avodane phase n an early stage and therefore redues ts growth pae far from the maxmum ahevable apaty, severely mpatng n the tme t takes to onverge. The three subfgures on the rght of Fg. 2 show the outome for the same senaro but wth bakground traff. The total target load of the bakground Table 3. Fndngs wrap-up n base behavor and behavor n a loaded network CCA Condtons Behavor CUBIC Base behavor Slghtly suffers for the defent behavor of Hybrd Slow-Start Loaded network No mpat of Hybrd Slow-Start NewReno Base behavor Easly aheves maxmum apaty Loaded network Smlar behavor but wth hgher delay and more unstable goodput Illnos Base behavor Easly aheves maxmum apaty. However, t reates a huge standng queue Loaded network Very smlar to NewReno but wth slghtly hgher delay CDG Base behavor Keeps the delay ontrolled but fals whle tryng to reah full resoure utlzaton Loaded network The dfferenes wth loss-based CCAs are redued Westwood+ Base behavor Very aggressve bak-off that mpats the tme needed to ramp-up Loaded network The mpat of the bak-off applaton s mnmzed

12 382 E. Atxuteg et al. traff s set to the 50% of the lnk apaty. The apaty reduton mnmzes the performane gap between loss-based and delay-based varants and stll, the more apaty a CCA gets, the harder mpat t nflts n terms of queung delay (Illnos as an example). Bg dfferenes appear omparng wth the base example wthout bakground traff, mostly related to a sgnfant nrement n the queung delay and the reduton of the gap n terms of apaty to reflet the dfferenes amongst the CCAs. RTT-loked CCAs suffer due to a lengthen of the tme between mplementaton desons. In ontrast, CUBIC behaves better beause t does not suffer for RTT nrease. The senaro tself due to ts reduton n the avalable apaty ushons the underperformane of Hybrd Slow-Start. Short Flows Study Lve measurements have shown that many flows over Internet are small (90% of downstreams arry no more than 35.9 KB of data [4]). Therefore t s mportant to assess the mpat that suh load dstrbuton has n fnal performane. In order to do so, the prevous foreground TCP flow must be replaed by a suesson of short flows followng an exponental dstrbuton regardng ther amount of data. ECDF dg CDG Cub Illnos New Reno Westwood+ ECDF dg CDG Cub Illnos New Reno Westwood Throughput (Mbps) (a) Queue length (pakets) (b) Fg. 3. Throughput and queue sze ECDF at 700 m The Fg. 3 represent as an Empral Cumulatve Dstrbuton Funton (ECDF) the results obtaned regardng the aheved throughput and standng queue sze. In Fg. 4 the sze of the flows and the number of ndued drops are orrelated. In Fg. 3a, t s lear that the aheved throughput s very smlar among most the CCAs and ther dfferenes really appear regardng the amount of enqueued pakets n Fg. 3b. The delay-based varant, CDG, suessfully lmts

13 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 383 Drops (pakets) CDG Cub Illnos New Reno Westwood Fle sze (MB) Fg. 4. enodeb drops at 700 m the enqueued pakets whle loss-based mplementatons overshoot ausng a great standng queue. Due to the deteted behavor of Westwood+ n the begnnng of the transmssons and the short duraton of the flows, t prompts lttle ablty to njet pakets n the enodeb. In ontrast, NewReno, Illnos and CUBIC happen to be the average solutons. If we ompare two defent solutons suh as CUBIC and Westwood+, we learly see that even wth short flows and therefore quk transmsson duraton, the premature ext from Slow-Start for the former performs better than the exessve bak-off of the latter. Consderng the reported fndngs, Fg. 4 shows the number of pakets that have exeeded the queue sze wth eah flow sze, beng therefore dropped. It s lear that the more aggressve the CCA s, the more paket are dropped by the enodeb. Illnos for nstane has a more aggressve behavor n ongeston avodane phase. It enqueues more pakets and gets more pakets dropped. As a result Illnos suffers on average 100 more dropped pakets than any other TCP anddate. One agan, the behavor of Hybrd Slow-Start s learly shown. If we avod the fat that the transmssons wth Hybrd Slow-Start take slghtly more tme to be ompleted, t only suffers drops wth longer transmssons and when t has ongeston events, the number of losses are very few. Wth loss-based AIMD mehansms, the drop pakets metr appears to be dretly related to the aggressveness and bak-off strategy. NewReno and Westwood+ have qute smlar results (Table 4).

14 384 E. Atxuteg et al. CCA Table 4. Fndngs wrap-up n short flows study Behavor CUBIC Thanks to the underperformane of Hybrd Slow-Start, t only suffers drops wth longer transmssons and when t has ongeston events, the number of losses are very few NewReno The average soluton Illnos Very aggressve behavor that results n 100 more dropped pakets on average CDG Suessfully lmts the enqueued pakets Westwood+ In the begnnng of the transmssons and the short duraton of the flows, t prompts lttle ablty to njet pakets due to the aggressve bak-off Sudden Inrease and Derease of the Avalable Capaty One the man features of the CCAs have been deteted n loaded senaros n omparson wth the base behavor as well as the mpat of dfferent short flows on the drop rate, t s mportant to study the responsveness of CCAs n bg and sudden apaty hanges. To ths end, two type of smulatons are arred out: wth the bakground traff beng stopped at 20 s of the test and wth the bakground traff beng started at 20 s of the test. In order to make easer the readng, Table 5 gathers the most mportant pont of the followng explanaton. On the one hand, the left part of Fg. 5 shows the results regardng the senaro wth a sudden apaty nrease. In general, as soon as the apaty nreases, the queue sze s lowered due to a release of prevously enqueued pakets. It s lear that loss-based CCAs qukly respond to an addtonal bandwdth assgnment. However, Westwood+ stll suffers from the exessve reduton of the wnd after the Slow-Start phase. Durng the ongeston avodane phase, ts AIMD mehansm s very onservatve and the enqueued pakets tend to be almost 0, therefore wth a new and greater ahevable apaty, the adaptaton ablty of the CCA s very weak. In the ase of delay-based varants, sne they manly fous on redung the delay over path, they usually fal to nrease ther pae and thus, the new avalable apaty s wasted. On the other hand, the rght part of Fg. 5 depts the ase n whh the bakground traff s atvated at 20 s. Due to the sudden reduton of avalable apaty, the queue sze suffer an nstant nrement beause of the relaton between the same number of nomng pakets to the enodeb and the drast reduton of outgong ones. The Fg. 5 learly shows that all CCAs but CDG are able to suessfully reat to the apaty reduton. However, n some ases suh as CUBIC, the CCA takes more tme to stablze to the new pae. These smulatons reflet that most CCAs, even delay-based mplementatons, are apable of redung ther throughput when sudden avalable apaty dereases happen but delay-based varants struggle to adapt ther pae to bandwdth nreases.

15 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 385 Congeston wndow (bytes) dg dg CDG Cub Illnos New Reno Westwood+ Congeston wndow (bytes) dg dg CDG Cub Illnos New Reno Westwood+ Queue (pakets) Tme (s) dg dg Queue (pakets) Tme (s) dg dg dg Tme (s) Tme (s) Goodput (Mbps) dg dg Goodput (Mbps) dg dg Tme (s) (a) Tme (s) (b) Fg. 5. Performane omparson of the seleted CCAs: (a) Sudden apaty nrease; (b) Sudden apaty derease.

16 386 E. Atxuteg et al. Table 5. Fndngs wrap-up n sudden nrease and derease of the avalable apaty CCA Condtons Behavor CUBIC In. avalable ap. Good performane wthout the mpat of Hybrd Slow-Start due to the low avalable apaty at the begnnng of the transmsson De. avalable ap. Impat of Hybrd Slow-Start n the begnnng. Aggressve behavor n ongeston avodane phase that leads to an nstant huge nrement of queue sze whle redung the avalable apaty NewReno In. avalable ap. Good responsveness and average delay mpat De. avalable ap. Average loss-based soluton that suffers and nstant standng queue nrease whle redung the avalable apaty Illnos In. avalable ap. Good responsveness and greater ndued delay than NewReno De. avalable ap. Its aggressveness s harmful n ths senaro and takes some tme to stablze the goodput CDG In. avalable ap. Fals to nrease ts pae and thus, the new avalable apaty s wasted De. avalable ap. Bad performane n terms of goodput but full ontrol of the delay that s always lose to the baselne delay Westwood+ In. avalable ap. Its AIMD mehansm s very onservatve and the enqueued pakets tend to be very few, beng not apable of respondng to a sudden greater apaty assgnment De. avalable ap. The ombnaton of ts dynams (wth a slow ramp-up ablty) and the avalable apaty reduton happen to get the best performane due to the ahevement of the maxmum goodput and the lowest mpat n terms of delay 5.2 Start-Up Performane In very smplfed senaros, we have seen that the behavor of Standard Slow- Start and Hybrd Slow-Start dffers leadng n some oasons to a suessful avodane of massve losses wth Hybrd Slow-Start. However, the LTE ells are usually more rowded and therefore the UEs ould nflt more delay as rosstraff that ould mpat Hybrd Slow-Start. The target s to assess whether the nternal mehansms of Hybrd Slow-Start ould provoke an early ext from the standard ramp-up, followng to a slow nrement of the wnd and therefore a sgnfant underutlzaton of rado resoures or not. We frst measured the onvergene behavor of the Standard Slow-Start, reordng the pakets that are n-flght at every moment. Later, we assessed

17 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 387 Fg. 6. Hybrd Slow-Start mpat n moble networks: njeted pakets durng a standard Slow-Start perod. the same for Hybrd Slow-Start. Fgure 6 shows the probablty densty funton (PDF) of the number of njeted pakets for both mehansms n the tme Standard Slow-Start takes to onverge. Ths s, we would ompare n the fastest onvergene perod of tme, the ablty of both methods to put pakets n-flght. Fgure 6 shows the behavor of Standard Slow-Start has a equal dstrbuton of pakets n flght, whereas Hybrd has an mbalaned dstrbuton presumably formed by the perod of tme n whh Hybrd Slow-Start has ramped-up as Standard Slow-Start and the perod after detetng a delay varaton and behavng under the nremental pae of ongeston avodane phase. The dstrbuton represents the huge dfferene between both methods regardng the ablty to njet pakets whh leads to a extrapolaton of the tme needed to onverge or aheve the maxmum apaty from the begnnng of the transmsson. It s lear that not only n smplfed senaros, but also n mult-ue measurements, Hybrd Slow-Start suffers due to the deteton of delay nrement and the early trgger of ext ondton from fast ramp-up. So, under some delay varablty rumstanes Hybrd Slow-Start slows-down the ramp-up of TCP. In some stuatons, ths effet ould underutlze the avalable rado resoures and lengthens the tme needed to onverge, dretly mpatng on the qualty experened by users (QoE). 5.3 Moblty Performane Ths subseton overs the analyss of dereasng qualty and nreasng qualty movement for the seleton of CCAs. Even though t has been proven n Subset. 5.1 that some CCAs fal n moble networks (Westwood+ and CDG), they have been kept for omparson and onfrmaton purposes. In order to measure the ablty

18 388 E. Atxuteg et al. or nablty of dstnt TCP mplementaton to take advantage of rado resoures, the results wll be presented as the porton of tbsze that has been atually utlzed every Transmsson Tme Interval (TTI). In other words, sne the TTI s ommonly onfgured n 1 ms, the porton of tbsze wll show how many bts are used for the UE every mllseond. Consderng that dfferent MCS values lead to have dstnt avalable apaty and therefore a dfferent ahevable throughput, the analyss s dvded n MCS ranges. Dereasng Qualty Movement The dereasng qualty movement senaro stands for the ontnuous movement evoluton of a ertan UE from the enodeb to a further loaton. In other words, on average the obtaned SINR due to the dstane from the UE to the enodeb and the fadng wll have a tendeny to be worse. So wll be the reported CQI and the assgned MCS (nstead of worse, t s a tendeny to beome a more robust modulaton). In suh a transton, the CCA wll need to adapt to the dfferent avalable apates. Fgure 7 shows the dfferene between the avalable apaty and the aheved apaty for dfferent CCAs under dstnt speeds, all lassfed by average MCS. Fg. 7. Aheved/Avalable apaty at dfferent speeds for dfferent TCP varants (dereasng qualty movement). Fgure7 learly depts three man areas: Slow-Start phase: Loated n the overage zone assoated to MCS 28, durng the transmsson establshment and frst ramp-up, the wnd s not great enough to take full advantage of avalable rado resoures. Consderng that Standard Slow-Start onverges very fast, the MCS 28 area also takes the frst bak-off applaton. For that reason Westwood+ or NewReno among others do not report the same result. Sne the dstane assoated wth a MCS s overed a lot faster

19 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 389 at 200 km/h, the wnd has no tme to grow qukly enough and therefore the mpat of ramp-up s more sgnfant for the senaro at 200 km/h, promptng a lower value of aheved/avalable for ths speed. Bufferbloat area: Whle n the area between MCS 26 to 18-20, the CCAs are able to take advantage of already enqueued pakets n the enodeb (bufferbloat effet). However the effet tself has a drawbak n relaton to the nflted delay. Ths feature s more present n the examples at 60 km/h. The tme spent n eah MCS area makes t possble to the TCP varant to njet pakets throughout a longer tme, gettng loss pakets and requrng to reover from them under hgh-delay ondtons and therefore, not allowng the CCA to aheve maxmum apaty. Queue dranng zone: Regardless the speed, t s an area n whh the rado ondtons are not good enough to mantan a full utlzaton of resoures. Even though the average MCS values are between 18 to 14, fadng ondtons fore the enodeb to operate wth very low MCS values (ahevng sometmes MCS 4 and 6) n some drast fades. Wth eah sudden fade, t s easer to reeve more robust modulatons, leadng the pakets to need stronger segmentaton. As a sde effet, both the queue sze of the enodeb and the delay nrease. The reovery of losses n suh network ondtons s also a harmful proess for TCP that leads to queue starvaton events. When t omes to faster UE senaro, the enodeb s able to lengthen the utlzaton of prevously enqueued paket to further postons, therefore, the dranng effet s slower or at least happens n further postons. Fgure7 shows that n dereasng qualty movement the dfferenes n lossbased CCAs are mnmum, gettng more redt of aggressveness at 60 km/h and RTT-synhronzaton at 200 km/h (NewReno and Illnos over CUBIC). One agan and even n a senaro that moves towards worse rado poston, the delaybased varants have demonstrated to be unable to ope wth the delay varablty of LTE. CDG mantans a RTT lose to the baselne RTT but underutlzes most of the assgned bandwdth. In the ase of Westwood+, even though t s a senaro that helps get the maxmum apaty to the weak AIMD mehansms due to ts ontnuous ahevable apaty redue, t takes very long tme to aheve suh a task at 60 km/h and t s not apable of dong so at 200 km/h. Inreasng Qualty Movement One analyzed the dereasng qualty movement and the behavor of dfferent CCAs under dstnt speeds, t s neessary to study the nreasng qualty movement n a onstant evoluton of the hannel qualty to better postons. Consderng the fndngs n dereasng qualty movement, t s mportant to determne whether the dfferent methods of Slow-Start equally struggle under hallengng rado ondton or not and analyze whether the aggressveness of TCP overshots suffent pakets to serve a ontnuous greater apaty or not. Tryng to better explan the effets of ths senaro n the begnnng of the transmsson and the relaton between wnd evoluton and aheved goodput, Fg. 8 represents the relaton between them. The graphs have been splt for the

20 390 E. Atxuteg et al. better understandng n two bloks: the result n relaton to the wnd evoluton s on the left and goodput s umulatve sum on the rght. Fgure 8 depts the behavor dfferene between CUBIC wth Hybrd Slow-Start and NewReno wth Standard Slow-Start. It s lear that the network ondtons are hallengng beause even n Standard Slow-Start the shape of the wnd s very stepped. In suh ondtons n whh the delay varablty s also a hard drawbak to takle, the Hybrd Slow-Start mehansm detets an nrement n the delay that s onsdered enough to trgger an early ext to ongeston avodane phase. The resultant umulatve goodput of both CCAs s represented on the rght where the graphs shows a bg outome gap between both methods. One agan the underperformane of Hybrd Slow-Start s shown. Besdes, n ths ase the early ext of fast ramp-up s provoked n a sngle-ue senaro n whh the movement and fadng are the only soures that vary the delay. Fg. 8. NewReno vs. CUBIC n nreasng qualty senaros at 200 km/h. Consderng the explaned effet regardng how Hybrd Slow-Start ould affet the performane, we wll now proeed to study the performane dfferenes under dfferent speeds between NewReno, CUBIC, Westwood+, Illnos and CDG, lassfed by average MCS levels (see Fg. 9). At a frst glane, the fgure looks very smlar to Fg. 7, but some dfferenes are present. The behavor of suh senaro s dvded n two areas. Ramp-up phase: The hardest rado ondtons for the hannel are present from MCS 14 to 18. In suh a hallengng ondtons the CCAs ntalze the transmsson and employ the seleted Slow-Start method n a try to ramp-up and onvergene as fast as possble wthout ndung a bursty loss event. As seen beforehand, at 200 km/h the performanes of Standard Sow-Start and Hybrd

21 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 391 Fg. 9. Aheved/Avalable apaty at dfferent speeds for dfferent TCP varants (nreasng qualty movement). Slow-Start are ompletely dfferent, leadng to a better utlzaton of the network resoures n the ase of Standard Slow-Start. Besdes, n MCS 14 n some oasons t s not only present the Slow-Start phase but part of the frst bak-off and applaton of ongeston phase as well. The growth lmtaton s omparatvely very smlar for 60 km/h and 200 km/h durng ths phase and establshes an undodgeable boundary for loss reovery. However, n faster senaros the tme spent n weakest rado ondtons s less and the mpat of suh hallengng ondtons s less sgnfant n the fnal outome. Apart from that, n the ase of Standard Slow-Start, at 200 km/h the frst loss event wll happen n better rado ondtons than for 60 km/h and therefore, the ablty to reover the lost pakets s greater at 200 km/h. Statonary area: Throughout MCS 20 to 28, TCP s able to take lose to full advantage of avalable apaty. However, t has to be mentoned that, due to that transton speed and appled bak-offs whle reoverng from losses, the CCAs are not able to rse suffently the wnd, ausng some hannel underutlzaton. Even though, n general, the CCAs follow the dentfed phases, there are some dfferenes among the CCAs that result n a dstnt outome for the same network ondtons (see the wrap-up Table 6). Dfferent Slow-Start methods affet the avalablty to take full advantage of rado resoures, dvdng the performane n two major groups. (1) Among the CCAs wth same Slow-Start phase, some dfferenes appear n MCS 14 due to the dfferent AIMD poly appled when a loss s deteted. As stated before, the hgher speed, the omparatvely longer Slow-Start phase and therefore, a derease of the loss reovery effet due to the reovery takng plae n better rado ondtons. (2) For Hybrd Slow-Start mehansm, a dfferene between CUBIC and CDG appear regardng the delay senstvty to qut fast ramp-up

22 392 E. Atxuteg et al. CCA CUBIC Table 6. Fndngs wrap-up n moblty senaros Behavor It only suffers the mpat of Hybrd Slow-Start n the very begnnng of the transmsson n nreasng qualty movement pattern under hgh speed (see Fg. 8) NewReno Very good performane n terms of aheved avalable apaty n smplfed sngle-user moblty senaros Illnos The best results due to the ombnaton of the delay-awareness and the aggressveness CDG It has demonstrated very weak performane over ellular aess under moblty n terms of bandwdth utlzaton Westwood+ Only able to reah full utlzaton after a long ramp-up perod n dereasng qualty movement at 60 km/h and n nreasng qualty movement at 200 km/h (as stated n Subset. 3.1). In relaton to the effet of speed, the faster the UE moves, the hgher delay varablty and therefore quker skp to a slow nrease phase, sufferng more wasted bandwdth utlzaton at 200 km/h. Westwood+ and CDG have been proven to be not adequate for moble networks. The former s able to reah full utlzaton after a long ramp-up perod n dereasng qualty movement at 60 km/h and n nreasng qualty movement at 200 km/h. The tme spent s due to a poor avalable bandwdth estmaton and onsequent drast bak-off poly. At 200 km/h n dereasng qualty senaro the CCA does not allow suffent tme so as to aheve the maxmum bandwdth. On the ontrary, n nreasng qualty movement, the fastest senaro allows the CCA get the maxmum apaty. The latter has demonstrated very weak performane over ellular aess. It has to be underlned that the man objetve of the CCA regardng the ontrol of end-to-end delay s fulflled, however, regardless the speed and senaro, the CCA has not been able to rse to the 10% of the avalable apaty, onsequently leadng to a 90% of resoure underutlzaton. Therefore CDG s not sutable for moble network as s onfgured now. The group formed by NewReno, CUBIC and Illnos have shown a very suessful performane regardless the speed and movement pattern. As stated n prevous explanaton, the shortenng of the hallengng perods ould make a dfferene n terms of greater aheved apaty. On average (see average values on the rght) at 60 km/h 3 CCAs are very smlar and t s only under 200 km/h speed rumstanes when CUBIC performs poorly due to Hybrd Slow-Start and Illnos get a slght advantage of ts delay-awareness to make the most of usng avalable resoures. All the gathered results are onsstent wth the fndngs regardng dereasng qualty movement (n Subsubset. 5.3), the performane of dfferent Slow-Start methods (n Subset. 5.2) and the prelmnary analyss n regards to the mpat of dfferent ross-traff n the performane of CCAs. However, sne the results

23 TCP Performane over Current Cellular Aess: A Comprehensve Analyss 393 have been obtaned n a sngle LTE deployment, t s mportant to determne to whh extent our fndngs ould be extrapolated as a general-purpose behavor of CCAs and whether the results are based towards the smulated/emulated testbed or not. 5.4 Correlaton of TCP Behavor over w-lab.t LTE Testbed The urrent subseton ams at representng and explanng the behavor of a seleton of CCAs over the ontrolled LTE testbed alled w-lab.t. Sne the deployment s formed wth ompletely real equpment (.e. UEs, enodebs, femtoells, servers), the nternal mehansms of LTE and the nteraton wth TCP are loser to real-world behavor and therefore the varablty s presumably hgher omparng wth smulated envronment. Thus, suh testbed allows arryng out experments that represent the performane of the realty n a smaller sale. CDG was removed from the omparson set for ts nompatblty wth moble networks. Westwood+ s kept n the seleton of CCAs to onfrm or deny the underperformane under more varable rumstanes. We onfgured three dfferent paths to be followed by the robots wth dereasng qualty and nreasng qualty movements. The loaton of those movement patterns were loated n dfferent plaes of the femtoell, havng a pattern lose to the enodeb, another one lose to the spaal lmts of the testbed and a thrd one n the mddle of the prevous two. After ten experments over the dfferent onfguratons/patterns, we gathered the followng average throughput values for CUBIC, NewReno, Westwood+ and Illnos. Fg. 10. CCA omparson over w-lab-t under moblty rumstanes.

24 394 E. Atxuteg et al. Fgure 10 shows that the prevous fndngs n ns-3 were aurate enough to explan the possble effet of CCAs n other LTE deployments. In fat, some defenes suh as the ones regardng Hybrd Slow-Start and Westwood+ are more harmful than n smulaton envronment, ausng a greater gap between the avalable apaty and the aheved one. In general three are the most mportant features to be underlned. Frst, the drast bak-off applaton of Westwood+ leads the CCA to be napable of ahevng the maxmum apaty even wthn 20 s of transmsson. Lookng at the growth tendeny, the CCA may well take around 1 mn to onvergene whh s an unaeptable value n order to provde a good serve to the UEs. Seond, the underperformane of Hybrd Slow-Start s more remarkable n ths testbed and the results prompt a onvergene tme around 4.5 s. The performane dfferene wth Standard Slow-Start (present n NewReno and Illnos) ould be ushoned f the transmsson s long enough (average value of CUBIC s lose to NewReno or Illnos n 20 s transmsson). However, the mpat n short-lved flows would be more notable. Thrd, the performane of NewReno and Illnos are very smlar and the only dstnton appear due to the greater aggressveness of Illnos for ts delay-awareness. Nevertheless, the utlzed femtoells gve a very good hannel qualty regardless the moblty pattern, movement patterns or speed. Thus, the sgnal qualty rngs that are present n real-world ould not be represented. Therefore, n order to better understand the performane tradeoff of CUBIC, NewReno and Illnos n ongeston avodane phase durng moblty rumstanes, an addtonal analyss was demanded. 5.5 Performane Tradeoff of Seleted TCP Varants Under Moblty n Emulated Testbed One the prevous fndngs regardng the behavor of CCAs have been demonstrated n a ontrolled testbed, ths subseton overs the omparson analyss of most adequate TCP flavors (CUBIC, NewReno and Illnos) over emulated testbed wth moble senaros of dereasng qualty and nreasng qualty movement. The prevous senaros have shown that CUBIC, NewReno and Illnos have a very lose outome. Therefore, ths subseton wll serve not only as a onfrmaton step of the fndngs n another testbed but to also arry out experments n moblty rumstanes wth a realst representaton of sgnal qualty rngs. The testbed tself s not able to emulate movement due to the fxed poston of the UE attahed to a rado able. Nonetheless, a omputer that plays the role of a experment ontrolled, s apable of establshng the baselne SINR at any moment. Besdes, the lte-n-a-box alled Aeroflex 7100 apples a fadng pattern to suh a varable baselne SINR, modellng ths way the effet of movement wth a stat UE. To help dede the best tmng for dfferent baselne SINR values, averaged SINR traes obtaned from ns-3 wth a UE movng n dereasng qualty and nreasng qualty movement patterns at 60 km/h are used. In order to gve more realsm to the experments, the EVA60 fadng model n Aeroflex 7100 are appled.