implement tcp copa on ns3

Introduction

 copa [1] is a congestion control algorithm propsed by MIT, targeting for low latency. By reference from mvfst[2], I implements it on ns3 tcp stack.
  I found the latency factor up add the congestion window has changed from 0.5 to 0.05. Well, This value will lead much difference.

    /**
    * deltaParam determines how latency sensitive the algorithm is. Lower
    * means it will maximime throughput at expense of delay. Higher value means
    * it will minimize delay at expense of throughput.
    */
    double m_deltaParam {0.05};

  I test three tcp copa flows in a point to point link.

    uint32_t link_bw=6000000;
    uint32_t link_owd=50;
    uint32_t q_delay=200;

Result

m_deltaParam=0.05

Rate dynamic:

Instant rtt when each ack arrives:

m_deltaParam=0.5

Rate dynamic:

Instant rtt when each ack arrives:

When m_deltaParam=0.5, the results conform to our early test [3].
The code can be got on github [4].
Reference：
[1] Copa: Practical Delay-Based Congestion Control for the Internet
[2] mvfst
[3] Evaluation copa congestion control on ns3
[4] tcp copa on ns3