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Research of TCP Vegas Congestion Control Algorithm over GEO Satellite Networks

Research of TCP Vegas Congestion Control Algorithm over GEO Satellite Networks. 學生：林泰邑 指導老師：林仁勇老師 日期： 2007.03.14. Author && Source. Author ： Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, 2005. Source:

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Research of TCP Vegas Congestion Control Algorithm over GEO Satellite Networks

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  1. Research of TCP Vegas Congestion Control Algorithm over GEO Satellite Networks 學生：林泰邑 指導老師：林仁勇老師 日期：2007.03.14

  2. Author && Source • Author： Microwave, Antenna, Propagation and EMC Technologies for Wireless Communications, 2005. • Source: Gong Changqing,Zhao ZhigangWang GuangxingSchool of information Science & Engineer, Northeastern University, Shenyang, China

  3. Outline • Introduction • Introduction of Vegas algorithms and variation of Vegas • Vegas3 Algorithm presented in this paper • Network environments and simulation results. • Conclusions

  4. Introduction • The satellite networks have bigger round trip time and higher bit error ratio . • The round trip time of GEO networks is about 520 millisecond. where l is the packet length B is the transmission rate • When packet size is 1MBytes, Bandwidth is 1mbps ,the slow start duration of GEO is about 3.91secs

  5. Introduction • The bit error ratio is generally in 10-6 or 10-7. • TCP was initially developed for wire-line with low BER, such that the majority of segment losses are due to network congestion. • The sender reduces its transmission rate even when segments are lost because of bit errors.

  6. Introduction of Vegas • Depending on the δ, difference of “expected sending rate (expected throughput)” to “actual sending rate (actual throughput)”, Vegas algorithm can estimate the available network bandwidth. • cwnd：Size of congestion window • BaseRTT：Minimum of all measured round trip time • ActualSend：Number of actual sending packet • RTT：Actual measured round trip time

  7. if δ<α if δ>β if α≦δ ≦β Vegas • The cwnd is changed according to the following equation • Slow Start：cwnd is doubled for every two RTT. • If congestion • Three duplicated ACK： cwnd= ½ cwnd • Timeout：cwnd=1 • Congestion Avoidance：

  8. Vegas-A • Vegas algorithm can predict the usage condition of network bandwidth, but the values of α and β are fixed（usually α=1，β=3）, the effect of congestion control is limited. • Vegas-A algorithm is presented for the improvement of Vegas , where ‘A’ stands for adaptive, it makes α and β value adjust automatically. • The improvement of the algorithm is mainly displayed in congestion avoidance period.

  9. If δ < α { If α > 1 and Th(t) > Th(t-rtt) { ++cwnd; } Else if α >1 and Th(t) ≦ Th(t-rtt) { --cwnd; --α; --β; } Else if α==1 ++cwnd; } if β≧δ≧α{ if Th(t) > Th(t-rtt) { ++cwnd; ++α; ++β; } Else if Th(t) ≦ Th(t-rtt) { α, β, cwnd not change } Vegas-A • If δ > β { If α > 1 {--α;--β;} --cwnd; } Else { not change α、β and cwnd } • Th(t) :the actual sending rate at time t • Th(t-rtt) :actual throughput rate measured one RTT before t.

  10. Vegas1 • The improvement idea of Vegas1 algorithm comes from Vegas-A algorithm; but the adjustment strategy of the congestion window is different from Vegas-A algorithm. • The improved goal is to make the algorithm more balanced and steadier, make the algorithm more adaptive.

  11. Vegas1 • The improvement measure is preventing the increase of congestion window from so rashly. During the course of congestion avoidance, change all operation of “cwnd=cwnd + 1 “in Vegas-A algorithm into “cwnd=cwnd+(1 / cwnd)”. In order to improve performance further, the measure is not to lower the window too carelessly, whenever need, only reduce the window by 1 /cwnd.

  12. Vegas3 • The improvement idea of Vegas3 algorithm comes from Vegas algorithm and Vegas-A algorithm; Vegas3 is the harmonization of two algorithms. • When δ < α ,the cwnd will be increased; if it can‘t be increased, it will be kept unchanged. • When δ > β , the cwnd will be decreased or kept unchanged. • When α ≦ δ≦ βthe cwnd will be kept constant or increased.

  13. Vegas-A: if α ≦ δ ≦ β { if Th(t) > Th(t-rtt) { ++cwnd; ++α;++β; } if Th(t) ≦ Th(t-rtt) { cwnd = cwnd; } Vegas-3: if α ≦ δ ≦ β { if Th(t) > Th(t-rtt) { ++cwnd; ++α;++β; } if Th(t) ≦ Th(t-rtt) { cwnd = cwnd; } Vegas-A Algorothm V.S. Vegas-3 Algorothm

  14. Vegas-A: If δ < α { If α > 1 and Th(t) > Th(t-rtt){ ++cwnd; } Else if α >1 and Th(t) ≦ Th(t-rtt) { --cwnd; --α; --β; } Else if α==1 ++cwnd; } Vegas-A Algorothm V.S. Vegas-3 Algorothm • Vegas-3: If δ < α { If α > 1 and Th(t) > Th(t-rtt){ ++cwnd;++α;++β; } Else if α >1 and Th(t) ≦ Th(t-rtt) { cwnd=cwnd; } Else if α==1 ++cwnd; }

  15. Vegas-A: If δ > β { If α > 1 { --α; --β; } --cwnd; } Vegas-A Algorothm V.S. Vegas-3 Algorothm • Vegas-3: If δ > β { If Th(t) ≦ Th(t-rtt){ --cwnd; if cwnd < 2 { cwnd=2;} If α > 1 { --α;--β;} } If Th(t) > Th(t-rtt){ cwnd=cwnd; }

  16. Network Environments GEO Satellite GSL Up Link:2mbps Down Link:2mbps GSL Up Link:2mbps Down Link:2mbps Ground Station Ground Station

  17. Network Environments && Simulation Result Simulation Result

  18. Conclusion • During congestion avoidance phase, the Vegas3 divides network conditions into more state, that can actual communication conditions of the satellite networks. • The values of α and β adjustment strategy can adapt actual variety of network.

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