基于UDP传输协议的实现分析之流量和拥塞控制


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

  1. Java代码   
  2. */   
  3.     public void onACK(long ackSeqno){   
  4.         //increase window during slow start   
  5.         if(slowStartPhase){   
  6.             congestionWindowSize+=ackSeqno-lastAckSeqNumber;   
  7.             lastAckSeqNumber = ackSeqno;   
  8.             //but not beyond a maximum size   
  9.             if(congestionWindowSize>session.getFlowWindowSize()){   
  10.                 slowStartPhase=false;   
  11.                 if(packetArrivalRate>0){   
  12.                     packetSendingPeriod=1000000.0/packetArrivalRate;   
  13.                 }   
  14.                 else{   
  15.                     packetSendingPeriod=(double)congestionWindowSize/(roundTripTime+Util.getSYNTimeD());   
  16.                 }   
  17.             }   
  18.    
  19.         }else{   
  20.             //1.if it is  not in slow start phase,set the congestion window size   
  21.             //to the product of packet arrival rate and(rtt +SYN)   
  22.             double A=packetArrivalRate/1000000.0*(roundTripTime+Util.getSYNTimeD());   
  23.             congestionWindowSize=(long)A+16;   
  24.             if(logger.isLoggable(Level.FINER)){   
  25.                 logger.finer("receive rate "+packetArrivalRate+" rtt "+roundTripTime+" set to window size: "+(A+16));   
  26.             }   
  27.         }   
  28.    
  29.         //no rate increase during slow start   
  30.         if(slowStartPhase)return;   
  31.    
  32.         //no rate increase "immediately" after a NAK   
  33.         if(loss){   
  34.             loss=false;   
  35.             return;   
  36.         }   
  37.    
  38.         //4. compute the increase in sent packets for the next SYN period   
  39.         double numOfIncreasingPacket=computeNumOfIncreasingPacket();   
  40.    
  41.         //5. update the send period   
  42.         double factor=Util.getSYNTimeD()/(packetSendingPeriod*numOfIncreasingPacket+Util.getSYNTimeD());   
  43.         packetSendingPeriod=factor*packetSendingPeriod;   
  44.         //packetSendingPeriod=0.995*packetSendingPeriod;   
  45.    
  46.         statistics.setSendPeriod(packetSendingPeriod);   
  47.     }   

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

  1. /* (non-Javadoc)  
  2.     * @see udt.CongestionControl#onNAK(java.util.List)  
  3.     */   
  4.    public void onLoss(List<Integer>lossInfo){   
  5.        loss=true;   
  6.        long firstBiggestlossSeqNo=lossInfo.get(0);   
  7.        nACKCount++;   
  8.        /*1) If it is in slow start phase, set inter-packet interval to  
  9.             1/recvrate. Slow start ends. Stop. */   
  10.        if(slowStartPhase){   
  11.            if(packetArrivalRate>0){   
  12.                packetSendingPeriod = 100000.0/packetArrivalRate;   
  13.            }   
  14.            else{   
  15.                packetSendingPeriod=congestionWindowSize/(roundTripTime+Util.getSYNTime());   
  16.            }   
  17.            slowStartPhase = false;   
  18.            return;   
  19.        }   
  20.    
  21.        long currentMaxSequenceNumber=session.getSocket().getSender().getCurrentSequenceNumber();   
  22.        // 2)If this NAK starts a new congestion epoch   
  23.        if(firstBiggestlossSeqNo>lastDecreaseSeqNo){   
  24.            // -increase inter-packet interval   
  25.            packetSendingPeriod = Math.ceil(packetSendingPeriod*1.125);   
  26.            // -Update AvgNAKNum(the average number of NAKs per congestion)   
  27.            averageNACKNum = (int)Math.ceil(averageNACKNum*0.875 + nACKCount*0.125);   
  28.            // -reset NAKCount and DecCount to 1,   
  29.            nACKCount=1;   
  30.            decCount=1;   
  31.            /* - compute DecRandom to a random (average distribution) number between 1 and AvgNAKNum */   
  32.            decreaseRandom =(int)Math.ceil((averageNACKNum-1)*Math.random()+1);   
  33.            // -Update LastDecSeq   
  34.            lastDecreaseSeqNo = currentMaxSequenceNumber;   
  35.            // -Stop.   
  36.        }   
  37.        //* 3) If DecCount <= 5, and NAKCount == DecCount * DecRandom:   
  38.        else if(decCount<=5 && nACKCount==decCount*decreaseRandom){   
  39.            // a. Update SND period: SNDSND = SND * 1.125;   
  40.            packetSendingPeriod = Math.ceil(packetSendingPeriod*1.125);   
  41.            // b. Increase DecCount by 1;   
  42.            decCount++;   
  43.            // c. Record the current largest sent sequence number (LastDecSeq).   
  44.            lastDecreaseSeqNo= currentMaxSequenceNumber;   
  45.        }   
  46.          
  47.        statistics.setSendPeriod(packetSendingPeriod);   
  48.        return;   
  49.    }   

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