fastRetransmit.c

event: ACK received, with ACK field value of y
    if (y >SendBase) {
        SendBase=y
        if (there are currently any not yet acknowledged segments)
            start timer
    } else {
        // a duplicate ACK for already ACKed segment
        increment number of duplicate ACKs
            received for y
        if (number of duplicate ACKs received for y==3)
            // TCP fast retransmit
            resend segment with sequence number y
    }
    break;

TCPFlow.c

NextSeqNum = InitialSeqNumber
SendBase = InitialSeqNumber

loop (forever) {
  switch(evnet)
    event: data received from application above
//       create TCP segment with sequence number NextSeqNum
      if (timer currently not running)
        start timer
//       pass segment to IP
      NextSeqNum = NextSeqNum + length(data)
      break;

    event: timer timeout
//       retransmit not-yet-acknowledged segment with smallest sequence number
      start timer
      break;

    event: ACK received, with ACK field value of y
      if (y > SendBase) {
        SendBase = y
        if (there are currently any not-yet-acknowledged segment)
          start timer
      }
      break;
}

事件	TCP 接收方动作
具有所期望序号的按序报文段到达。 所有在期望序号及以前的数据都已经被确认。	延迟的 ACK，对另一个按序报文段的到达最多等待 500ms。 如果下一个按序报文段在这个时间间隔内没有到达，则发送一个 ACK
具有所期望序号的按序报文段到达。 另一个按序报文段等待 ACK 传输。	立即发从单个累积 ACK， 指示下一个期待字节的序号（其为间隔的低端序号）
比期望序号大的失序报文段到达。 检测出间隔。	立即发送冗余 ACK，指示下一个期待字节的序号 （其为间隔的低端序号）
能部分或完全填充接受数据间隔的报文段到达	倘若该报文段起始于间隔的低端，则立即发送 ACK