tcp (fixes #580): Adjust the pacing rate in TcpBbr

src/internet/model/tcp-bbr.cc

@@ -220,6 +220,7 @@ TcpBbr::SetPacingRate(Ptr<TcpSocketState> tcb, double gain)
 {
     NS_LOG_FUNCTION(this << tcb << gain);
     DataRate rate(gain * m_maxBwFilter.GetBest().GetBitRate());
+    rate *= (1.f - m_pacingMargin);
     rate = std::min(rate, tcb->m_maxPacingRate);
 
     if (!m_hasSeenRtt && tcb->m_minRtt != Time::Max())

src/internet/model/tcp-bbr.h

@@ -402,6 +402,8 @@ class TcpBbr : public TcpCongestionOps
     Time m_ackEpochTime{Seconds(0)}; //!< Starting of ACK sampling epoch time
     uint32_t m_ackEpochAcked{0};     //!< Bytes ACked in sampling epoch
     bool m_hasSeenRtt{false};        //!< Have we seen RTT sample yet?
+    double m_pacingMargin{0.01}; //!< BBR intentionally reduces the pacing rate by 1% to drain any
+                                 //!< standing queues. See `bbr_rate_bytes_per_sec` in Linux.
 };
 
 } // namespace ns3