Satellite TCP Kenny - Simon Fraser...
Transcript of Satellite TCP Kenny - Simon Fraser...
ENSC 835 project (2002)TCP performance over satellitelinks
Kenny, Qing [email protected], Hui Zhang
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Road map
n Introduction to satellite communicationsn Simulation implementation
n Window sizen Maximum segment sizen Initial windows sizen Selective acknowledgement algorithm§ TCP burst
n Conclusionn References
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Introduction to satellitecommunications
• Satellite communication is a type of the wirelesscommunications technologies. It utilizes satellites toretransmit the wireless signal, and to connect withthe multiple earth station.
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Introduction to satellitecommunications
Geosynchronous Equatorial Orbit
(From geo = earth + synchronous = moving at the same rate.)
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On board GEO satellite
Bent pipe GEO satellite On board switch GEO satellite
HUB
Fax
Telephone
Public Switch
Satellite
VSAT
VSAT
VSAT
VSAT
µç»°
µç»°
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½»»»»ú
Star network (switch at hub) Mesh network (switch at satellite)
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Why do we choose this project?
n Commercial satellite companies (e.g., Loral, Hughes,Lockheed Martin) have announced plans to buildlarge satellite systems to provide broadband dataservice.
n Our simulation may help improve TCP performanceover long delay and error prone channels.
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Project objective
n Implement an combination of four approaches toenhance the performance over satellite links couplingwith the long delay and high Bit Error Ratecharacteristics.n Effect of window size RFC1323n Effect of Initial windows size RFC2581n Effect of Maximum segment size RFC2488n Comparison of different TCP algorithms RFC 2018
n Extend the authors’ knowledge of TCP burst problemrelated to on board switch in GEO satellites.n TCP burst
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Window size
n Large delay*bandwidth productW=B*RTT
This product defines the amount of data aprotocol should have “ in flight”.
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Window size
n The original TCP standard limits the advised windowsize by only assigning 16bits of header space for itsvalue. (RFC793). Hence the advised window size canbe no more than 64Kbytes.
time trip roundCWND
=throughput
kbpsbytesms
kbyte900sec/000,112
58564
≈≈=
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Window size
n Simulation scenario
n Parameters:n choose various window size: 16, 32, 64, and 128.n T1 link: 1.544Mbps.n Set the receiver and sender buffer size greater
than the delay bandwidth product, so that we canexamine how window size affect on TCPthroughput: 120.
n Application: FTP
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Window size (Result 1)
• Brief Analysis:Larger windowsize can helpimprove thethroughput.
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Window size (Result 2)
• Brief AnalysisØ Multiple long lived
connections withsmall window sizecan still fully utilizethe channel.ØThree 40kbytes
window connectionscan almost fullyutilize the T1channel.
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Window size -Discussion
n Advantages:n It is ideal for the connections that transmit
big files such as FTP applicationn Disadvantages:
n large window size can lead to more rapiduse of the TCP sequence space.
n large window size will also increase themultiple packets loss possibility.
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What about short lived connection?
n Slow start is a safe guard against transmitinappropriate amount of data into thenetwork.
n However, Slow start is particularly inefficientfor short lived connection (Telnet) in largebandwidth-delay product network.
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Initial congestion window size
n By increasing the initial CWND, more packetsare sent during the first RTT.
n Trigger more ACKs, allowing congestionwindow to open more rapidly.
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Maximum Segment Size
Maximum segment size (MSS):
MSS = MTU – TCP header – IP header
n MTU (or the maximum IP packet size):Maximum Transmission Unit
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Initial CWND and MSS
q Simulation scenario
n Parameters:n Set the initial window size: 1, 2, and 4.n Set packet size: 576, 1152, and 1728.n Set the advised window size: 128.n Application : telnetn Other parameters are default.
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• Brief Analysis:Using larger initial window size can help reduce the slow start period,so allows sender send more packets at the same period of time.
Initial window size (Result 1)
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Maximum Segment Size (Result)
• Brief Analysis:Choosing suitablemaximum segmentsize can improveTCP throughput.
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Initial CWND and MSS-Discussion
n Advantages:n It is ideal for the short lived connections
such as Telnet application.n Disadvantages:
n Make traffic burstn Increase unnecessary drops for bigger
packet size
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Comparison of different TCPalgorithms
n New Challenge for TCP performance:n Large window size will also increase the
multiple packets loss possibility.n Large MSS will easily be corrupted in
wireless link.
n The need to evaluate the error correct abilityof different TCP flavors.
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Comparison of different TCPalgorithms
n Simulation scenario
n Parameters:n Add lose model, use different bit error rate : 10e-7, 10e-6,
10e-5, 10e-4, and 10e-3.n Compare TCP Sack with other TCP algorithms such as Reno
and Vegas.
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Result
• Brief Analysis:TCP Sack performsbetter than Reno.However, when biterror rate is from5*10e-7 ~ 10e-3,Vegas is better thanboth Sack and Reno.
0
200
400
600
800
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Bit erro rate
Thr
ough
put
(kbp
s)
SackRenoVegas
10e-7 10e-6 10e-5 10e-4 10e-3
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On board GEO satellite
Bent pipe GEO satellite On board switch GEO satellite
HUB
Fax
Telephone
Public Switch
Satellite
VSAT
VSAT
VSAT
VSAT
µç»°
µç»°
´«Õæ
½»»»»ú
Star network (switch at hub) Mesh network (switch at satellite)
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TCP Burst
n Larger window size and initial windows sizewill results in large burst.
n On board switch has limited buffer.
n New Challenge for TCP performance:n TCP burst Vs limited buffer sizen Suitable buffer size
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TCP Burst
n Simulation scenario:
n Parameters:n Set various buffer size: 30, 34, 40, and 50.n Set the window size: 128.n T1 link : 1.544Mbps.n Other parameters are default.
1.5 44Mbps,3.1Mbps 3.1Mbps
source sinkrouter1 router2
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TCP Burst
• Brief Analysis:§ Increasing the buffer size will not alleviates the TCP burst§ 2. After the slow start period, the queue occupancy decrease.
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TCP Burst
• Brief Analysis:Increasing the buffer size will not improve the TCP performance.If the buffer size is approximately greater than 1/2 of the windowsize when using basic ACK (accordingly, 1/3 when using delayedACK), the throughput will reduce rapidly.
with basic acknowledgment with delayed acknowledgment
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Conclusion
Using a larger initial window can improve thethroughput, especially for short transfers
Initial window
TCP Sack is better than Reno under the error pronechannel. However, Vegas performs even betterthan Sack if the bit error rate is large
Comparisonof three TCPalgorithms
Larger maximum segment size will improve thethroughput, however, it may cause link congestionor router overload
Maximumsegment size
Burst has a severe influence on TCP performanceTCP Burst
Larger window size can improve the performanceWindow size
OutcomeExperiment
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Conclusion
n For FTP application in satcom, implementSACK or Vegas with larger window size cangreatly improve the TCP throughput.
n For Telnet application in satcom, it is ideal toimplement SACK with larger initial windowand MSS.
n The ideal buffer size of on board satelliterouter is ½ of the window size (standardACK), 1/3 of the window size (delay ACK).
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Reference
n [1] Van Jacobson. Congestion Avoidance and Control. In ACM SIGCOMM, 1988.n [2] J. Mo, R. J. La, V. Anantharam, and J. Warland, ``Analysis and comparison of TCP Reno
and Vegas,'' Proceedings of the Conference on Computer Communications (IEEE Infocom),New York, Mar. 1999.
n [3] L. Brakmo, S. O'Malley, and L. Peterson. TCP Vegas: New techniques for congestiondetection and avoidance. In Proceedings of the SIGCOMM '94 Symposium (Aug. 1994)pages 24-35
n [4] K. Fall and S. Floyd, "Simulation-based comparisons of Tahoe, Reno, and SACK TCP,"SIGCOMM Computer Communication Review, 26(3), July 1996
n [5] A Simulation Study of Paced TCP, Joanna Kulik, Robert Coulter, Dennis Rockwell, andCraig Partridge, September 1999
n [6] [RFC 2488] Enhancing TCP over Satellite Channels, Mailman, and D.Glover January1999.
n [7] Larry L.Peterson and Bruce S.Davie. Computer networks: A system approach. MorganKaufman, 1996.
n [8] [RFC 2581] TCP Congestion Control, M. Allmanm, V. Paxson, W. Stevens, April 1999.n [9] [RFC 1122] Transport Layer TCPn [10] [RFC 1323] TCP Extensions for High Performance, V.Jacobson, R.Braden, D.Borman,
1992