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).

Java代码

	
	

		Java代码   
	

	

		*/   
	

	

		    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代码

	
	

		/* (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;   
	

	

		   }