Effects of Advertised Receive Buffer Size and Timer Granularity on TCP Performance in a LEO Satellite Network

Effects of Advertised Receive Buffer Size and Timer

Granularity on TCP Performance in a LEO Satellite

Network

Marc Emmelmann, emmelmann@ieee.org

Fraunhofer Institute forOpen Communication Systems

IEEE Globecom 2002

Taipei, Taiwan

17-21November 2002

Page 3emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Outline

Introduction - Project Framework- Target System Parameter- TCP Basics

Simulation Environment - Used Simulation Models- Simulation Parameters

Simulation Results - Effects of Buffer Size & Timer Granularity- Effects of Bit Error Rates- Overall TCP Performance

Conclusion & Outlook

Page 4emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Project Framework

Partner - Tesat-Spacecom- German Aerospace Agency DLR- Fraunhofer FOKUS

Satellite Network - Support of multimedia applications- LEO orbits- ATM based

Project Aspects Target System Design

Antennas

Demonstrator

TCP Simulation (Reno)

Page 5emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Target System DesignSystem Parameter

Satellite Orbit - 72 satellites (12 * 6)- Orbit altitude: 1350 km- Inclination 47 deg.- ISLs: on-board processing and switching

Bit Rates - Uplink: up to 2.048 Mbit/s- Downlink: up to 32.768 Mbit/s- 16 kbit/s granularity

Bit Error Rates - Target BER < 2*10e-8- Shadowing- Adaptive FEC

Data Transfer - Usage of TCP Reno

Page 6emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

TCP Basics

Provides - reliable end-to-end transfer

Retransmissions caused by - expiration of retransmission timeout RTO- duplicate ACKs (Fast Retransmission)

Calculates the RTO - upon smoothed RTT- timer granularity G acts as low pass filter

RTO - may cause premature retransmissions(RTO < RTT)

- may “delay” the TCP transmission if segments are lost(RTO >> RTT)

Page 7emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation Model

Page 8emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation Parameter

Lower Bound Of RTT in ms None 100 200 Default

Timer Granularity G

G=1 ms 2 7

G=50 ms 3 8

G=100 ms 4 9

G=250 ms 5 10

G=500 ms 6 11 12

Minimal G to avoid false retransmissions

(varying)1

Given:- TCP Flavor- Buffer Size- BER

More than 140 experiments

Page 9emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation ResultsEffects of Buffer Size & Timer Granularity

Advertised Receive Buffer - Larger than BDP- Less than BDP- Equal to MSS

Timer Granularity - May cause false retransmissions

Advertised Receive

Buffer

64 kB

>> BDP

6.6 kB

= BDP

3.3 kB

1/2

BDP

1.46 kB

MSS

Min. Timer Granularity

(G)1 ms 1 ms 4 ms

26 ms (250

ms)

Page 10emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation ResultsEffects of Bit Error Rates

BER = 2*10e-8 - Overlapping with BER = 0- No needless retransmissions

Page 11emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation ResultsEffects of Bit Error Rates (cont.)

BER = 2 * 10e-5 - No resemblance with variable propagation delay- Prevention of false retransmissions

Page 12emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation ResultsTCP Performance

Degradation of TCPthroughput BER = 2* 10e-8

Page 13emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Simulation ResultsTCP Performance

Degradation of TCPthroughput BER 2*10e-5

Page 14emmelmnn@ieee.org IEEE Globecom ‘02, Nov 17-21 2002, Taipei, Taiwan

Conclusion & Outlook

TCP - Sophisticated protocol- Minor influence of hand-tuned implementations- TCP flavor dominates

Demonstrator - Run experiments over “real” MAC- Combine simulation models and demonstrator

Further Information - http://www.fokus.fhg.de/cats/satellite- emmelmann@ieee.org