UDT协议-基于UDP的可靠数据传输协议的实现分析(7)-流量和拥塞控制

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流量控制
 
对于一个带宽1Gbps, RTT为100ms的网络来说
 
BDP=1,000,000,000*0.1/8=12,500,000字节=12207K=12M
 
传统TCP接收窗口大小=65535byte=64K, 显然满足不了
 
udt使用包大小1500byte, 默认接口窗口大小为8192, 因此
接收窗口的大小为=1500*8192=12,288,000字节=12000K=11.7M
 
因此, 可以看到udt的默认设置已经足够.

Congestion Control（拥塞控制）

1. 两个重要的参数：
congestion window size and the inter-packet sending interval

2. 主要的接口

   1) init: when the UDT socket is connected.
   2) close: when the UDT socket is closed.
   3) onACK: when ACK is received.
   4) onLOSS: when NACK is received.
   5) onTimeout: when timeout occurs.
   6) onPktSent: when a data packet is sent.
   7) onPktRecv: when a data packet is received.

3. udt的拥塞算法：

   On ACK packet received:
   1) If the current status is in the slow start phase, set the
      congestion window size to the product of packet arrival rate and
      (RTT + SYN). Slow Start ends. Stop.
   2) Set the congestion window size (CWND) to: CWND = A * (RTT + SYN) +
      16.
   3) The number of sent packets to be increased in the next SYN period
      (inc) is calculated as:
         if (B <= C)
            inc = 1/PS;
         else
            inc = max(10^(ceil(log10((B-C)*PS*8))) * Beta/PS, 1/PS);
      where B is the estimated link capacity and C is the current
      sending speed. All are counted as packets per second. PS is the
      fixed size of UDT packet counted in bytes. Beta is a constant
      value of 0.0000015.
   4) The SND period is updated as:
         SND = (SND * SYN) / (SND * inc + SYN).

 

*/
    public void onACK(long ackSeqno){
        //increase window during slow start
        if(slowStartPhase){
            congestionWindowSize+=ackSeqno-lastAckSeqNumber;
            lastAckSeqNumber = ackSeqno;
            //but not beyond a maximum size
            if(congestionWindowSize>session.getFlowWindowSize()){
                slowStartPhase=false;
                if(packetArrivalRate>0){
                    packetSendingPeriod=1000000.0/packetArrivalRate;
                }
                else{
                    packetSendingPeriod=(double)congestionWindowSize/(roundTripTime+Util.getSYNTimeD());
                }
            }

        }else{
            //1.if it is  not in slow start phase,set the congestion window size
            //to the product of packet arrival rate and(rtt +SYN)
            double A=packetArrivalRate/1000000.0*(roundTripTime+Util.getSYNTimeD());
            congestionWindowSize=(long)A+16;
            if(logger.isLoggable(Level.FINER)){
                logger.finer("receive rate "+packetArrivalRate+" rtt "+roundTripTime+" set to window size: "+(A+16));
            }
        }

        //no rate increase during slow start
        if(slowStartPhase)return;

        //no rate increase "immediately" after a NAK
        if(loss){
            loss=false;
            return;
        }

        //4. compute the increase in sent packets for the next SYN period
        double numOfIncreasingPacket=computeNumOfIncreasingPacket();

        //5. update the send period
        double factor=Util.getSYNTimeD()/(packetSendingPeriod*numOfIncreasingPacket+Util.getSYNTimeD());
        packetSendingPeriod=factor*packetSendingPeriod;
        //packetSendingPeriod=0.995*packetSendingPeriod;

        statistics.setSendPeriod(packetSendingPeriod);
    }

 

On NAK packet received:
   1) If it is in slow start phase, set inter-packet interval to
      1/recvrate. Slow start ends. Stop.
   2) If this NAK starts a new congestion period, increase inter-packet
      interval (snd) to snd = snd * 1.125; Update AvgNAKNum, reset
      NAKCount to 1, and compute DecRandom to a random (average
      distribution) number between 1 and AvgNAKNum. Update LastDecSeq.
      Stop.
   3) If DecCount <= 5, and NAKCount == DecCount * DecRandom:
        a. Update SND period: SND = SND * 1.125;
        b. Increase DecCount by 1;
        c. Record the current largest sent sequence number (LastDecSeq).

   

 /* (non-Javadoc)
     * @see udt.CongestionControl#onNAK(java.util.List)
     */
    public void onLoss(ListlossInfo){
        loss=true;
        long firstBiggestlossSeqNo=lossInfo.get(0);
        nACKCount++;
        /*1) If it is in slow start phase, set inter-packet interval to
             1/recvrate. Slow start ends. Stop. */
        if(slowStartPhase){
            if(packetArrivalRate>0){
                packetSendingPeriod = 100000.0/packetArrivalRate;
            }
            else{
                packetSendingPeriod=congestionWindowSize/(roundTripTime+Util.getSYNTime());
            }
            slowStartPhase = false;
            return;
        }

        long currentMaxSequenceNumber=session.getSocket().getSender().getCurrentSequenceNumber();
        // 2)If this NAK starts a new congestion epoch
        if(firstBiggestlossSeqNo>lastDecreaseSeqNo){
            // -increase inter-packet interval
            packetSendingPeriod = Math.ceil(packetSendingPeriod*1.125);
            // -Update AvgNAKNum(the average number of NAKs per congestion)
            averageNACKNum = (int)Math.ceil(averageNACKNum*0.875 + nACKCount*0.125);
            // -reset NAKCount and DecCount to 1,
            nACKCount=1;
            decCount=1;
            /* - compute DecRandom to a random (average distribution) number between 1 and AvgNAKNum */
            decreaseRandom =(int)Math.ceil((averageNACKNum-1)*Math.random()+1);
            // -Update LastDecSeq
            lastDecreaseSeqNo = currentMaxSequenceNumber;
            // -Stop.
        }
        //* 3) If DecCount <= 5, and NAKCount == DecCount * DecRandom:
        else if(decCount<=5 && nACKCount==decCount*decreaseRandom){
            // a. Update SND period: SND = SND * 1.125;
            packetSendingPeriod = Math.ceil(packetSendingPeriod*1.125);
            // b. Increase DecCount by 1;
            decCount++;
            // c. Record the current largest sent sequence number (LastDecSeq).
            lastDecreaseSeqNo= currentMaxSequenceNumber;
        }
       
        statistics.setSendPeriod(packetSendingPeriod);
        return;
    }