2009 IEEE International Advance Computing Conference (IACC 2009)Patiala, India, 6-7 March 2009

Experimental Validation ofTCP Performance inGEG Satellite link Conditions

Trilok K. Saini, Mohit Kumar, Manoj K. Dhakatksaini@gmail.com, mohit.kumar.dhaka@gmail.com, manojdhaka@deal.drdo. in

Defence Electronics Applications Laboratory, Dehradun, DRDO, India

Abstract -Transmission Control Protocol was designed to work TCP is a connection oriented, reliable, transport layerover the networks characterized by low delay and negligible bit protocol and it can provide reliable delivery of data across anyerror rates. Transmission Control Protocol (TCP) performance network path including network path containing terrestrial ordegrades when TCP traffic is carried over the satellite networks satellite channels [4]. TCP achieves its reliability by requiringthat have large latency, high bit error rates and path asymmetry. a positive acknowledgement from the receiving side. If theTo comprehend the effect of satellite conditions over TCP, wehave evaluated the TCP performance by three universally acknowledgement of the data sent is not received within theaccepted methodologies i.e. Simulation, emulation and timeout interval, the data is retransmitted. TCP assigns aexperimentation. The experimentation has been carried out over sequence number to each outgoing octet, TCP receiver usesactual Geostationary Earth Orbit (GEO) satellite link equipped these sequence numbers to correctly order segments that maywith tangible hardware at ground station. This paper discusses be received out of order and to eliminate duplicates [5]. TCPeach of the three methods used to evaluate the TCP and describesthe results that clearly bring out the degree of performance supports mancommon appiaons l

cun email, httprdegradation and the impact of various link parameters on the ftp TCP algorithm performs well over the networksTCP performance. Experiments based analysis of the TCP over characterized by low delay and negligible bit error rate, but itssatellite network assist to gather the more realistic statistics for performance degrades over the satellite networks [6]. Satellitebenchmarking that will be useful guide to understand the channel characteristics i.e. large latency, high bit error rateprotocol limitations that further steer in enhancing its and path asymmetry have an adverse effect on the way TCPcapabilities by make it an efficient protocol for satellite networks. works [4] [8]. This paper addresses the impact of GEOEmulation and Simulation results clearly demonstrate the effectof latency and bit errors of satellite links. Simulation results, in satellite link parameters over TCP throughput performanceparticular bring out the effect of various error levels in satellite using simulation, emulation and experimental methodologies.links over TCP flavors working in well known operating systems. The rest of the paper is organized as follows. In section II,

we focus on satellite characteristics and their effect on TCPIndex Terms-Emulation, simulation, performance and TCP algorithm. In section III, we describe our experimental setup

and discuss the TCP performance metrics and result takenover actual satellite link. In section IV, we present emulated

I. INTRODUCTION satellite environment and discuss the results. In section V, weS ATELLITE networks are becoming the integral part of describe the simulation strategy and analyze the TCP

emerging communication and information infrastructures. performance based on simulation results. In section VI, on theWide geographical coverage of satellite networks make them basis of three analysis mechanisms, we conclude the impact ofsuitable for interconnection with remote terrestrial networks. satellite parameters on the performance of TCP.Satellite networks provide an alternative route to existingwired and wireless systems. Data communications oversatellite has a clear appeal in the areas where a terrestrial II. SATELLITE CHARACTERISTICS AND TCP BEHAVIORcommunications infrastructure is either not feasible or not Geographical location of the satellite, atmospheric effects,present. Broadcast nature of satellite makes them appropriate property of the radio signals to fall its strength in proportion tofor broadcast oriented applications [1]-[4]. Satellite networks the square of the distance traveled, expense of the transmitterare widely being used as a primary or alternative media for equipments etc. are the key factors that play the major role tovoice and data communication. Now, these networks are distinguish the satellite networks from the wired and wirelessemerging as data centric networks in the form of hybrid terrestrial networks and mold it in to the network that isnetworks that comprises of terrestrial and satellite links to characterized by large latency, high bit error rates and pathtransport TCP/IP (Transmission Control Protocol! Internet asymmetry. These three factors influence the TCP algorithmProtocol) data to distant locations and to offer Internet access. in adverse way and cause to fall the TCP throughput overThese types of networks have massive scope and applications satellite media.in current and future communication infrastructure.978-1-4244-2928-8/09/$25.00 © 2009 IEEE 597

A. LatencyPropagation delay, transmission delay and queuing delay

are the main components of the latency [7]. In GEO satelliteenvironment, the dominant portion is propagation delay. Mostof the communications satellites are located in geostationaryearth orbit at an altitude of approximately 36,000 km. Theradio signal propagating at the speed of light takesapproximately 240 milliseconds for one hop i.e. groundstation-to-satellite-to-ground station, corresponding to a MEground station directly below the satellite, so for GEO satellite uVlinks round trip time (RTT) is at least 480 milliseconds (ins). I~1~f4When ground stations are located at the edge of the coverage, la a O 1single hop propagation delay is about 279.0 ins, which further Ci O aO

results in higher value of RTT [4]. Higher latency of satellite Fig. 1. Experiment Setup for TCP Analysis over satellite linknetworks results in long feedback loop, reducing TCPresponsiveness to packet drop; as well as acknowledgementstaking more time due to large propagation delay are common application level activity. During file transfer, ourresponsive for the slow window update. Long round trip time performance metrics parameter of concern is the throughput ofof GEO satellite networks impose the maximum limit on TCP the underlying protocol TCP responsible for the reliablethroughput. TCP uses 16 bits to report the receiver window delivery of data during file transfer.size to the sender that limits the maximum window size to64 Kbytes. Window size determines the amount ofoutstanding (i.e. unacknowledged) data in the network. A. Experimental SetupConsidering the GEO satellite channel of RTT 540 and The experiment was conducted on the setup shown in Fig.maximum window size of 64 Kbytes, TCP throughput as 1, in which two Intel Xeon (3.0 GHz) machines running Linuxdefined by the ratio of window size and RTT is limited to Redhat-9 and windows 2003 operating system were usedapproxd Mb the ratio of window size and RTT is limited to

as end client and server respectively. TCP/IP stack ofthe machines were used with their default configuration.

B. Bit Errors As both the operating systems have client and serverSatellite channels exhibit higher bit error rate (BER) as capability so end machines might be used interchangeably.

compared to terrestrial links. For typical satellite links, End machines are connected to the VSBX 6560 PowerPCaverage BER ranges from 10-5 to 10-8 [1]. High BER can have (833 MHz) board ported with MQX Real Time Operatingthe negative effect on the performance of TCP. TCP assumes (RTOS) and IPNET embedded TCP/IP protocol stack that hasthat packet loss is due to congestion and hence congestion been configured as a router for this experiment. This router iscontrol algorithm is invoked, even though these packet drops connected to the Ethernet 10/100 data interface of Radynedo not signal congestion in the network. Therefore, packets DMD20 Satellite Modem. This modem transmits and receivesdropped due to corruption also cause TCP to reduce the size the data on Intermediate Frequency (IF) to/from the Radioof its sliding window that result in low bandwidth utilization. Frequency (RF) chain which further connects to the antenna

C. Asymmetry for radiating the radio waves to the satellite. We configuredSatellite networksAare often asymmetric, i.e. uplink has less

the hardware and the spectrum to allow us the 1 Mbps satelliteSatellite networks are often asymmetric, i.e. uplink has less link for the experiment which is quite common and reasonable

available capacity than the downlink, due to the expense of bandwidth in GEO satellite networks. Such amount ofthe equipment used to send data to Satellites. This asymmetry bandwidth does not impose any bandwidth constraint on TCPmay have an impact on TCP performance, as sometimes low to gain its maximum theoretical throughput in GEO satellitebandwidth return channel itself becomes the bottleneck for the networks.acknowledgement based TCP.

B. Data Analysis and PresentationIII. EXPERIMENTAL ANALYSIS OF TCP PERFORMANCE OVER To analyze the TCP performance, examination of trace of

GEO SATELLITE LINK packets collected from the network method has been utilized.In this experiment, we quantify how well standard TCP To capture, trace and represent the TCP analysis data,

implementations as part of commonly used operating systems, tcpdump, tcptrace and xplot tools have been used. Tcpdumpperform in satellite environment. Our experiment reported in allows sniff network packets and make some statisticalthis paper was conducted using FTP client and server analysis out of those dumps. Tcptrace analyze the dump filesapplications running on top of TCP. Experimentation is containing traffic captured from the network. It takes as inputfocused on file transfer performance, which is the most the file produced by tcpdump and produces different types of

598 2009 IEEE Internlationlal Advance Computing Conference (IACC 2009)

outputs containing information on each connection seen. We TCP connection is able to utilize only 10% to 20% of thehave used xplot to plot the datasets, present in the form of file satellite channel bandwidth available for experimentation.produced by tcptrace.

IV. PERFORMANCE ANALYSIS OF TCP IN EMULATED GEOSATELLITE ENVIRONMENT

C. Results and DiscussionIn this section, we elaborate our analysis of TCPIn this section, we have presented the results of our pefomac sionemled Gelosatellit environ To

experimental analysis. We performed the file transfer with emulatedthe satellite environment,dTNthree files of size 155 KB 236 KB and 504 KB over the emulate the satellite environmentp we used the NIST Netexperimental setup. We used the TCP parameters of the stacks . .box set up as a router to emulate various network conditions,of oeraingsystms iththei deaul vales.TheTCP such as packet loss, duplication, delay and jitter, bandwidththroughput graph for the file transfer of 504 KB between limitaspannetwork congesation, [9]. Packetilos, badea adtclient and the server machine is shown in Fig. 2. It points out wlimitathons and network congeshon [9]. Packet loss delay and

the nature of instantaneous and average TCP throughput overt feractual satellite link. The average measured value of the satellite like conditions.throughput in this case is 22,558 bytes per second. Thethroughput is quite low over satellite link, as expected. A. Emulation Setup

In Fig. 3 we have plotted the average value of the The setup used to emulate the satellite environment forthroughput for the three above-mentioned file transfers. Poor analyzing the TCP performance is shown in Fig. 4 Intelthroughput results clearly bring out the impact of the link Pentium III machine equipped with 10 Mbps Ethernet carddelay and channel errors of satellite networks over TCP and running Linux Redhat-9 operating system was used toperformance. Throughput obtained in each case is very low execute the NIST Net package. This satellite link emulatingcompared with the channel capacity. It has been observed that Linux machine was configured as the gateway for all data

exchanged through it. Intel Xeon (3.0 GHz) machines withOrWA CWWi&1 :Linux Redhat-9 and Windows 2003 operating system were

used as the end machines. VSBX 6560 Power PC Boardported with MQX RTOS was used as an intermediate routernode that is just forwarding the data to its adjacent machine.

B. Emulation ScenariosFor investigating transport protocol i.e. TCP performance in

satellite like conditions, we have used the three combinationsof delay and error parameters. Based on these combinations,we modeled the three scenarios over our emulation setup. Inthe first scenario, to calibrate our emulation analysis, weconfigured the setup like terrestrial environment forbenchmarking the performance results. In this scenario,emulator does not insert any intentional delay, bit errors and

Fig. 2. TCP Throughput graph with 504 KB file transfer over 1 Mbps bandwidth limitation but simply forwards the packets to itsSatellite link

appropriate interface as defined by routing information. In thesecond scenario, we modeled the ideal satellite link; in thiscase emulator introduces a delay of 300 ms to give the illusion

125000-of GEO satellite link round trip time of 600 ms and no bit112_00-errors. Finally, TCP throughput performance results have been

100,000 lillllllllllllll iiiiiiiiiiiii 1111111111111 taken in satellite condition that incorporates the round trip87,500-------------------------- |||||||||||||||||||||||||| time of 600 ms and the bit error rateof106.

% 75,000 l

62,500 C. Results and Discussionp50,000

37,500 l

25,000

1552S 500 hM-x 1WX:5P3FLq-:tNteEx.

File Size fKbytet)01MW1 X I1 ei3;I1e lR UttJ-Ldi

Fig. 3. TCP Throughput Result over Satellite Link with three different filetransfers Fig. 4. Emulated Satellite Environment for TCP Analysis

2009 IEEE Internlationlal Advance Computing Conference (IACC 2009) 599

In our file transfer based TCP emulation analysis, we have A. Simulation Modeltransferred 77 KB, 155KB, 236KB and 1256KB files across The simulation model is illustrated in Fig. 5. The modelour test bed while varying the latency and bit errors. For the consists of a client machine communicating with the serverfirst scenario, which is the terrestrial like condition, as shown machine via three gateways. Client and the server are modeledin table I, throughput results are very good. For the file size with Ethernet workstation advanced and Ethernet serverof 236 KB, throughput is 855.79 Kbytes/sec. The average advanced model of OPNET. Gateways are modeled withvalue of the TCP throughput based on four different size file OPNET Ethernet 4 slip 8 gateway advanced model. The endtransfer is 839.74 Kbytes/sec. In the second scenario, which machines and their adjacent gateways are connected with 10comprises of ideal lossless satellite condition, for the file of Mbps Ethernet link. OPNET unidirectional point-to-point236 KB size throughput comes down to 24.59 Kbytes/sec, and (PPP) link advanced model has been used to simulate thethe average value of throughput in this scenario is 23.44 GEO satellite link. The bit error rate and the delay parametersKbytes/sec. Throughput results for individual file transfer for of the links between the gateways have been configured tothis case are shown in table II. In the third scenario, which is replicate the effect of satellite conditions. The twothe emulated GEO satellite condition with round trip time of unidirectional links between a pair of gateways have been600 ms and bit error rate of 10-6, throughput for the file configured with data rate 1,024 kb/s and 512 kb/s to capturetransfer of 236 KB file degrades to 13.92 Kbytes/sec. The the bandwidth asymmetry characteristics of satellite networks.average value of the throughput based on file transfers in this The model is configured with OPNET Modeler 12.0.A PL5 toscenario is 13.80 Kbytes/sec. Results of the throughput for simulate FTP transfers between server and client. To gaugeindividual file transfer are shown in table III. the impact of TCP/IP stack in end machines, we haveAs per the comparisons of the results shown in table I, II simulated the setup for two different sets of TCP parameters

and III, it clearly comes out that effect of the long latency is in client and server machines, named as TCP-Default andTABLE I TCP-Windows-XP as per the convention used by OPNET

TCP THROUGHPUT OVER TET-BED IN IDEAL CONDTIONS - BENCH MARKING y

FileSize InduedLnk elay Indued Trougput Modeler [10]. The parameters for both the variants areFile Size InducedLinkDelay Induced Throughput(Kbytes) (msec) BER (Kbytes/sec) illustrated in table IV.

77 No No 833.01155 No No 847.60 Simulation Parameters Value236 No No 855.79 Simulation Start Time (sec) 01256 No No 822.57 Simulated Time (minutes) 30, 60

Satellite RTT (ms) 540TABLE II

TCP THROUGHPUT IN EMULATED GEO SATELLITE DELAYFile Size Link RTT Induced Throughput(Kbytes) (msec) BER (Kbytes/sec)

77 600 No 18.49155 600 No 22.85236 600 No 24.591256 600 No 27.85

TABLE IIITCP THROUGHPUT IN EMULATED GEO SATELLITE DELAY AND BERFile Size Link RTT Induced Throughput(Kbytes) (msec) BER (Kbytes/sec)

77 600 1614.3617575 600 106 13.73 1liil G l1|| l |1236 600 10-6 13.92 Fig. 5. Simulation Model for TCP Analysis in satellite conditions1256 600 10-6 13.20

TABLE IV

severe on the TCP throughput, which further becomes worse TCP SIMULATION PARAMETERS

when links are more prone to errors. TCP Parameters TCP-Windows TCP-DefaultxPReceiver Buffer (Bytes) 16364 8760Max ACK Delay (Sec) 0.200 0.200

V. PERFORMANCE ANALYSIS OF TCP IN SIMULATED GEO Max Acknowledgement Segment 2 2SATELLITE ENVIRONMENT Slow start Initial Count (MSS) 1 2

Fast Retransmit Enabled EnabledIn this section, we describe simulation analysis used for Duplicate Ack Threshold 2 3

characterizing file transfer performance over satellite links for Fast Recovery Reno Renothetwocomon CP arints Window scaling Disabled Disabledthe twocommonTCP varlants. ~~SelectiveACK(SACK) Enabled Disabled

Nagle Algorithm Disabled DisabledKarn's Algorithm Enabled Enabled

600 2009 IEEE Internlationlal Advance Computing Conference (IACC 2009)

| Lossless Satellite Satelite BER 1 0';-6FTP Simulation Parameters ValueInter Request Time (sec) 3600 600File Size (KB) 236, 504, 1256, 2048, 5120 500

B. Simulation Analysis ofTCP in Satellite Conditions 400

We have simulated the satellite link with no errors300

i.e. lossless condition and satellite link with bit error rate of _For both lossless and lossy satellite condition, round trip O 200

time of 540ms has been considered. Over the terrestrial *segment i.e. between end machines and their adjacent 100

gateways, we have considered the distance based delay and noerrors. We performed the simulation for the file download of 0 36 504 1256 24 5120236, 504, 1256, 2048 and 5120 KB from the server to FileSize (KB)workstation (client) machine. Fig. 7. TCP-Windows-XP Download Response Time in lossless and lossy

In the simulation, we have gathered the data for the file satellite conditionsdownload response time. Download response time is the time ranging from 10-6 to 10-9 with TCP-Default and Windows-XPelapsed between sending a request and receiving the response variant. The download response time behavior, as shown infor the FTP application in the node. This time includes Fig. 9, clearly indicates that as the probability of error in thesignaling delay for the connection setup and tear-down. channel increases download response time for the file alsoResults taken under lossless and lossy satellite conditions for increases. As per the simulation results, the file transfer of 5TCP-Default and TCP-windows-XP variants are shown in MB size, performance drop of TCP-Windows-XP variant isFig. 6 and Fig. 7 respectively. Results for lossless satellite approximately 2.46%, 9.95% and 53.1% at BER 10- , 10-7condition indicate the effect of large latency over TCP and 10-6 respectively with respect to its value at BER 10-9performance and the results for the lossy satellite condition under similar bandwidth and delay conditions. The resultsfurther point out the degradation due to channel errors.Comparison of the results of TCP-Default and TCP-Windows-XP show that Windows-XP variant perform better than TCP- Los TCP-Defaut L TCp-VTnXP -*-Lole TOP

125000-Default specially due to its SACK feature and higher receiverbuffer size. Outcomes of FTP level throughput for above-mentioned file transfers are shown in Fig. 8, where FTP level 100000throughput is defined as the ratio of file size and download 67500response time. 75000To quantify the degree of degradation due to satellite 6 -

channel error, we have also simulated the impact of various v: -oooo-error rates on TCP performance. In this analysis, fixed satellite 37500-round trip delay and varying operational range of errors were 25000inserted in the simulated link. Simulation was conducted withround trip propagation delay of 540 milliseconds and BER 0

236 504 1256 2048 5120

IM Losless Satellite E Satellite BER 1 0-H-6 File Size(KB)Fig. 8. TCP-Default and TCP-Windows-XP Performance in lossless and

600 lossy satellite conditions

500 -

j _-TCP Deteult,File Size = 5MB TCP Win-XP, File Size =5MEDBTCP-De1euf, File Size = 2 MB - TCP-WinXP, File Size = 2 MB

400 600

l~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~So----------------------------------------------300

236; 504 1 256; 2040 5120 o

0

o-s 10o-8 10o-7 1Wo-sFile Size [(KB) Biit ErrF Rat BfER)

Fig. 6. TCP-Default Download Response Time in lossless and lossy satellite Fig. 9. Effect of Bit Error Rate on TCP Performance over GLO satellite linksconditions

2009 IEEE Internlationlal Advance Computing Conference (IACC 2009) 60:1

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Simulation results also bring out the effect of variouschannel error conditions on the protocol performance. As theBER increases, throughput decreases and the decline in theperformance is sharper at higher value of BER. TCP variantconfigured with larger receiver buffer size and selectiveacknowledgement option has demonstrated slightly betterperformance in satellite conditions.

602 2009 IEEE Internlationlal Advance Computing Conference (IACC 2009)j