流量控制

對于一個帶寬1Gbps, RTT為100ms的網(wǎng)絡(luò)來說

BDP=1,000,000,000*0.1/8=12,500,000字節(jié)=12207K=12M

傳統(tǒng)TCP接收窗口大小=65535byte=64K, 顯然滿足不了

udt使用包大小1500byte, 默認接口窗口大小為8192, 因此

接收窗口的大小為=1500*8192=12,288,000字節(jié)=12000K=11.7M

因此, 可以看到udt的默認設(shè)置已經(jīng)足夠.

Congestion Control(擁塞控制)

1. 兩個重要的參數(shù)：

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的擁塞算法：

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).

Java代碼

Java代碼   
*/   
    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).

Java代碼

/* (non-Javadoc)  
    * @see udt.CongestionControl#onNAK(java.util.List)  
    */   
   public void onLoss(List<Integer>lossInfo){   
       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: SNDSND = 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;   
   }