tcp cubic代码分析

 /*
  * TCP CUBIC: Binary Increase Congestion control for TCP v2.3
  * Home page:
  *      http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
  * This is from the implementation of CUBIC TCP in
  * Sangtae Ha, Injong Rhee and Lisong Xu,
  *  "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
  *  in ACM SIGOPS Operating System Review, July 2008.
  * Available from:
  *  http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
  *
  * CUBIC integrates a new slow start algorithm, called HyStart.
  * The details of HyStart are presented in
  *  Sangtae Ha and Injong Rhee,
  *  "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
  * Available from:
  *  http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
  *
  * All testing results are available from:
  * http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
  *
  * Unless CUBIC is enabled and congestion window is large
  * this behaves the same as the original Reno.
  */

 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/math64.h>
 #include <net/tcp.h>

 #define BICTCP_BETA_SCALE    1024    /* Scale factor beta calculation
                      * max_cwnd = snd_cwnd * beta
                      */
 #define    BICTCP_HZ        10    /* BIC HZ 2^10 = 1024 */

 /* Two methods of hybrid slow start */
 //Both run independently at the same time and slow start exits when any of them detects an exit point.
 //1. ACK train length
 //2. Delay increase

 #define HYSTART_ACK_TRAIN    0x1
 #define HYSTART_DELAY        0x2
 /* 注意：这里的delay_min没有放大8倍！
  * 此宏用来计算Delay increase threshold
  * delay_min <= 32ms，则threshold = 2ms
  * 32ms < delay_min < 256ms，则threshold = delay_min / 16 ms
  * delay_min >= 256ms，则threshold = 16ms
  */
 /* Number of delay samples for detecting the increase of delay */
 #define HYSTART_MIN_SAMPLES    8
 #define HYSTART_DELAY_MIN    (2U<<3)
 #define HYSTART_DELAY_MAX    (16U<<3)
 #define HYSTART_DELAY_THRESH(x)    clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)

 static int fast_convergence __read_mostly = ;
 static int beta __read_mostly = ;    /* = 717/1024 (BICTCP_BETA_SCALE) */
 //beta在BIC中为819，而CUBIC中为717，
 //会导致在bictcp_recalc_ssthresh中，并且启用了fast convergence，
 //cubic: last_max_cwnd = 0.85*snd_cwnd ，而慢启动阈值=0.7*snd_cwnd 。
 //bic:   last_max_cwnd = 0.95*snd_cwnd ，而慢启动阈值=0.8*snd_cwnd 。
 //这样会导致更早的到达平衡值，对snd_cwnd有很大的影响。

 static int initial_ssthresh __read_mostly;
 static int bic_scale __read_mostly = ;
 static int tcp_friendliness __read_mostly = ;

 //hybrid slow start的开关
 static int hystart __read_mostly = ;
 //HyStart状态描述
 //1：packet-train  2: delay   3:both packet-train and delay
 //默认2种方法都使用，故设为3
 static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
 //设置snd_ssthresh的最小拥塞窗口值,除非cwnd超过了这个值，才能使用HyStart
 static int hystart_low_window __read_mostly = ;

 static u32 cube_rtt_scale __read_mostly;
 static u32 beta_scale __read_mostly;
 static u64 cube_factor __read_mostly;

 /* Note parameters that are used for precomputing scale factors are read-only */
 module_param(fast_convergence, int, );
 MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
 module_param(beta, int, );
 MODULE_PARM_DESC(beta, "beta for multiplicative increase");
 module_param(initial_ssthresh, int, );
 MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
 module_param(bic_scale, int, );
 MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
 module_param(tcp_friendliness, int, );
 MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
 module_param(hystart, int, );
 MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
 module_param(hystart_detect, int, );
 MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
          " 1: packet-train 2: delay 3: both packet-train and delay");
 module_param(hystart_low_window, int, );
 MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");

 /* BIC TCP Parameters */
 struct bictcp {
     u32    cnt;        /*用来控制snd_cwnd的增长 increase cwnd by 1 after ACKs */
 //两个重要的count值:
 //第一个是tcp_sock->snd_cwnd_cnt，表示在当前的拥塞窗口中已经
     //发送(经过对方ack包确认)的数据段的个数，
 //而第二个是bictcp->cnt，它是cubic拥塞算法的核心，
 //主要用来控制在拥塞避免状态的时候，什么时候才能增大拥塞窗口，
 //具体实现是通过比较cnt和snd_cwnd_cnt，来决定是否增大拥塞窗口，

     u32 last_max_cwnd;    /*上一次的最大拥塞窗口值 last maximum snd_cwnd */
     u32    loss_cwnd;    /* 拥塞状态切换时的拥塞窗口值congestion window at last loss */
     u32    last_cwnd;    /* 上一次的拥塞窗口值 the last snd_cwnd */
     u32    last_time;    /* time when updated last_cwnd */
     u32    bic_origin_point;/*即新的Wmax饱和点，取Wlast_max_cwnd和snd_cwnd较大者 origin point of bic function */
     u32    bic_K;        /*即新Wmax所对应的时间点t，W(bic_K) = Wmax    time to origin point from the beginning of the current epoch */
     u32    delay_min;    /*应该是最小RTT    min delay */
     u32    epoch_start;    /*拥塞状态切换开始的时刻  beginning of an epoch */
     u32    ack_cnt;    /*在一个epoch中的ack包的数量   number of acks */
     u32    tcp_cwnd;    /*按照Reno算法计算得的cwnd    estimated tcp cwnd */
 #define ACK_RATIO_SHIFT    4
     u16    delayed_ack;    /* estimate the ratio of Packets/ACKs << 4 */
     u8    sample_cnt;    /*第几个sample    number of samples to decide curr_rtt */
     u8    found;        /* the exit point is found? */
     u32    round_start;    /*针对每个RTT     beginning of each round */
     u32    end_seq;    /*用来标识每个RTT    end_seq of the round */
     u32    last_jiffies;    /*超过2ms则不认为是连续的   last time when the ACK spacing is close */
     u32    curr_rtt;    /*由sampe中最小的决定    the minimum rtt of current round */
 };

 static inline void bictcp_reset(struct bictcp *ca)
 {//论文说Time out时调用
     ca->cnt = ;
     ca->last_max_cwnd = ;
     ca->loss_cwnd = ;
     ca->last_cwnd = ;
     ca->last_time = ;
     ca->bic_origin_point = ;
     ca->bic_K = ;
     ca->delay_min = ;
     ca->epoch_start = ;
     ca->delayed_ack =  << ACK_RATIO_SHIFT;
     ca->ack_cnt = ;
     ca->tcp_cwnd = ;
     ca->found = ;
 }

 static inline void bictcp_hystart_reset(struct sock *sk)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct bictcp *ca = inet_csk_ca(sk);

     ca->round_start = ca->last_jiffies = jiffies;//记录时间戳
     ca->end_seq = tp->snd_nxt;//记录待发送的下一个序列号
     ca->curr_rtt = ;
     ca->sample_cnt = ;

     //bictcp_hystart_reset中并没有对ca->found置0。
     //也就是说，只有在初始化时、LOSS状态时、开启hystart的慢启动时。
     //HyStart才会派上用场，其它时间并不使用.
 }

 static void bictcp_init(struct sock *sk)
 {
     bictcp_reset(inet_csk_ca(sk));

     if (hystart)//如果指定hystart
         bictcp_hystart_reset(sk);

     if (!hystart && initial_ssthresh)
         tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
 }

 /* calculate the cubic root of x using a table lookup followed by one
  * Newton-Raphson iteration.
  * Avg err ~= 0.195%
  */
 static u32 cubic_root(u64 a) //用来计算立方根
 {
     u32 x, b, shift;
     /*
      * cbrt(x) MSB values for x MSB values in [0..63].
      * Precomputed then refined by hand - Willy Tarreau
      *
      * For x in [0..63],
      *   v = cbrt(x << 18) - 1
      *   cbrt(x) = (v[x] + 10) >> 6
      */
     static const u8 v[] = {
         /* 0x00 */    ,   ,   ,   ,  ,  ,  ,  ,
         /* 0x08 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x10 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x18 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x20 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x28 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x30 */  ,  ,  ,  ,  ,  ,  ,  ,
         /* 0x38 */  ,  ,  ,  ,  ,  ,  ,  ,
     };

     b = fls64(a);
     if (b < ) {
         /* a in [0..63] */
         return ((u32)v[(u32)a] + ) >> ;
     }

     b = ((b * ) >> ) - ;
     shift = (a >> (b * ));

     x = ((u32)(((u32)v[shift] + ) << b)) >> ;

     /*
      * Newton-Raphson iteration
      *                         2
      * x    = ( 2 * x  +  a / x  ) / 3
      *  k+1          k         k
      */
     x = ( * x + (u32)div64_u64(a, (u64)x * (u64)(x - )));
     x = ((x * ) >> );
     return x;
 }

 /*
  * Compute congestion window to use.
  */  //从快速恢复退出并进入拥塞避免状态之后，更新cnt
 static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
 {
     u64 offs;//时间差|t - K|
     //delta是cwnd差，bic_target是预测值，t为预测时间
     u32 delta, t, bic_target, max_cnt;

     ca->ack_cnt++;    /*ack包计数器加1   count the number of ACKs */

     if (ca->last_cwnd == cwnd && //当前窗口与历史窗口相同
         (s32)(tcp_time_stamp - ca->last_time) <= HZ / )//时间差小于1000/32ms
         return; //直接结束

     ca->last_cwnd = cwnd;//记录进入拥塞避免时的窗口值
     ca->last_time = tcp_time_stamp;//记录进入拥塞避免时的时刻

     if (ca->epoch_start == ) {//丢包后，开启一个新的时段
         ca->epoch_start = tcp_time_stamp;    /*新时段的开始 record the beginning of an epoch */
         ca->ack_cnt = ;            /*ack包计数器初始化  start counting */
         ca->tcp_cwnd = cwnd;            /*同步更新 syn with cubic */

         //取max(last_max_cwnd , cwnd)作为当前Wmax饱和点
         if (ca->last_max_cwnd <= cwnd) {
             ca->bic_K = ;
             ca->bic_origin_point = cwnd;
         } else {
             /* Compute new K based on
              * (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
              */
             ca->bic_K = cubic_root(cube_factor
                            * (ca->last_max_cwnd - cwnd));
             ca->bic_origin_point = ca->last_max_cwnd;
         }
     }

     /* cubic function - calc*/
     /* calculate c * time^3 / rtt,
      *  while considering overflow in calculation of time^3
      * (so time^3 is done by using 64 bit)
      * and without the support of division of 64bit numbers
      * (so all divisions are done by using 32 bit)
      *  also NOTE the unit of those veriables
      *      time  = (t - K) / 2^bictcp_HZ
      *      c = bic_scale >> 10 == 0.04
      * rtt  = (srtt >> 3) / HZ
      * !!! The following code does not have overflow problems,
      * if the cwnd < 1 million packets !!!
      */

     /* change the unit from HZ to bictcp_HZ */
     t = ((tcp_time_stamp + (ca->delay_min>>) - ca->epoch_start)
          << BICTCP_HZ) / HZ;

      //求| t - bic_K |
     if (t < ca->bic_K)        // 还未达到Wmax
         offs = ca->bic_K - t;
     else
         offs = t - ca->bic_K;//已经超过Wmax 

     /* c/rtt * (t-K)^3 */     //计算立方，delta =| W(t) - W(bic_K) |
     delta = (cube_rtt_scale * offs * offs * offs) >> (+*BICTCP_HZ);

      //t为预测时间，bic_K为新Wmax所对应的时间，
      //bic_target为cwnd预测值，bic_origin_point为当前Wmax饱和点
     if (t < ca->bic_K)                                    /* below origin*/
         bic_target = ca->bic_origin_point - delta;
     else                                                    /* above origin*/
         bic_target = ca->bic_origin_point + delta;

     /* cubic function - calc bictcp_cnt*/
     if (bic_target > cwnd) {// 相差越多，增长越快，这就是函数形状由来
         ca->cnt = cwnd / (bic_target - cwnd);//
     } else {//目前cwnd已经超出预期了，应该降速
         ca->cnt =  * cwnd;              /* very small increment*/
     }

     /* TCP Friendly —如果bic比RENO慢，则提升cwnd增长速度，即减小cnt
      * 以上次丢包以后的时间t算起，每次RTT增长 3B / ( 2 - B)，那么可以得到
       * 采用RENO算法的cwnd。
       * cwnd (RENO) = cwnd + 3B / (2 - B) * ack_cnt / cwnd
      * B为乘性减少因子，在此算法中为0.3
      */
     if (tcp_friendliness) {
         u32 scale = beta_scale;
         delta = (cwnd * scale) >> ; //delta代表多少ACK可使tcp_cwnd++
         while (ca->ack_cnt > delta) {        /* update tcp cwnd */
             ca->ack_cnt -= delta;
             ca->tcp_cwnd++;
         }

         if (ca->tcp_cwnd > cwnd){    /* if bic is slower than tcp */
             delta = ca->tcp_cwnd - cwnd;
             max_cnt = cwnd / delta;
             if (ca->cnt > max_cnt)
                 ca->cnt = max_cnt;
         }
     }

     ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;
     if (ca->cnt == )            /* cannot be zero */
         ca->cnt = ; //此时代表cwnd远小于bic_target，增长速度最大
 }

 static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
 {
     struct tcp_sock *tp = tcp_sk(sk);
     struct bictcp *ca = inet_csk_ca(sk);

     //判断发送拥塞窗口是否到达限制，如果到达限制则直接返回。
     if (!tcp_is_cwnd_limited(sk, in_flight))
         return;

     if (tp->snd_cwnd <= tp->snd_ssthresh) {
         //当snd_cwnd<=ssthresh的时候，进入慢启动状态
         if (hystart && after(ack, ca->end_seq))//是否需要reset对应的bictcp的值
             bictcp_hystart_reset(sk);
         tcp_slow_start(tp);//进入slow start状态
     } else {
         //当snd_cwnd>ssthresh的时候，进入拥塞避免状态
         bictcp_update(ca, tp->snd_cwnd);//首先会更新bictcp->cnt
         tcp_cong_avoid_ai(tp, ca->cnt);//然后进入拥塞避免，更新tcp_sock->snd_cwnd_cnt
     }

 }

 //每次发生拥塞状态切换时，就会重新计算慢启动阈值
 //做了两件事：重赋值last_max_cwnd、返回新的慢启动阈值
 static u32 bictcp_recalc_ssthresh(struct sock *sk)
 {//论文说这个函数在Packet loss时调用
     const struct tcp_sock *tp = tcp_sk(sk);
     struct bictcp *ca = inet_csk_ca(sk);

     ca->epoch_start = ;    /* 发生拥塞状态切换，标志一个epoch结束   end of epoch */

     /* Wmax and fast convergence */
     //当一个新的TCP流加入到网络，
     //网络中已有TCP流需要放弃自己带宽，
     //给新的TCP流提供一定的上升空间。
     //为提高已有TCP流所释放的带宽而引入快速收敛机制。
     if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
         //snd_cwnd<last_max_cwnd
         //表示已有TCP流所经历的饱和点因为可用带宽改变而正在降低。
         //然后，通过进一步降低Wmax让已有流释放更多带宽。
         //这种行为有效地延长已有流增大其窗口的时间，
         //因为降低后的Wmax强制已有流更早进入平稳状态。
         //这允许新流有更多的时间来赶上其窗口尺寸。
         ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
             / ( * BICTCP_BETA_SCALE); //last_max_cwnd = 0.9 * snd_cwnd
     else
         ca->last_max_cwnd = tp->snd_cwnd;

     ca->loss_cwnd = tp->snd_cwnd;

     //修改snd_ssthresh，即max(0.7*snd_cwnd，2)
     return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);

 }

 static u32 bictcp_undo_cwnd(struct sock *sk)
 {
     struct bictcp *ca = inet_csk_ca(sk);

     return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
 }

 static void bictcp_state(struct sock *sk, u8 new_state)
 {
     if (new_state == TCP_CA_Loss) {//如果处于LOSS状态，丢包处理
         bictcp_reset(inet_csk_ca(sk));
         bictcp_hystart_reset(sk);
     }
 }

 static void hystart_update(struct sock *sk, u32 delay)
 {//会修改snd_ssthresh
     struct tcp_sock *tp = tcp_sk(sk);
     struct bictcp *ca = inet_csk_ca(sk);

     if (!(ca->found & hystart_detect)) {
         u32 curr_jiffies = jiffies;

         /* first detection parameter - ack-train detection */
         if (curr_jiffies - ca->last_jiffies <= msecs_to_jiffies()) {
             ca->last_jiffies = curr_jiffies;
             if (curr_jiffies - ca->round_start >= ca->delay_min>>)
                 ca->found |= HYSTART_ACK_TRAIN;
         }

         /* obtain the minimum delay of more than sampling packets */
         if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
             if (ca->curr_rtt ==  || ca->curr_rtt > delay)
                 ca->curr_rtt = delay;

             ca->sample_cnt++;
         } else {
             if (ca->curr_rtt > ca->delay_min +
                 HYSTART_DELAY_THRESH(ca->delay_min>>))
                 ca->found |= HYSTART_DELAY;
         }
         /*
          * Either one of two conditions are met,
          * we exit from slow start immediately.
          */
         if (ca->found & hystart_detect)//found是一个是否退出slow start的标记
             tp->snd_ssthresh = tp->snd_cwnd;//修改snd_ssthresh
     }
 }

 /* Track delayed acknowledgment ratio using sliding window
  * ratio = (15*ratio + sample) / 16
  */  //基本每次收到ack都会调用这个函数，更新snd_ssthresh和delayed_ack
 static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
 {//论文说这个函数在On each ACK时调用
     const struct inet_connection_sock *icsk = inet_csk(sk);
     const struct tcp_sock *tp = tcp_sk(sk);
     struct bictcp *ca = inet_csk_ca(sk);
     u32 delay;

     if (icsk->icsk_ca_state == TCP_CA_Open) {
         cnt -= ca->delayed_ack >> ACK_RATIO_SHIFT;
         ca->delayed_ack += cnt;
     }

     /* Some calls are for duplicates without timetamps */
     if (rtt_us < )
         return;

     /* Discard delay samples right after fast recovery */
     if ((s32)(tcp_time_stamp - ca->epoch_start) < HZ)
         return;

     delay = usecs_to_jiffies(rtt_us) << ;
     if (delay == )
         delay = ;

     /* first time call or link delay decreases */
     if (ca->delay_min ==  || ca->delay_min > delay)
         ca->delay_min = delay;

     /* hystart triggers when cwnd is larger than some threshold */
     //tp->snd_ssthresh初始值是一个很大的值0x7fffffff

     //当拥塞窗口增大到16的时候，
     //调用hystart_update来修改更新snd_ssthresh
     //hystart_update主要用于是否退出slow start
     if (hystart && tp->snd_cwnd <= tp->snd_ssthresh &&
         tp->snd_cwnd >= hystart_low_window)
         hystart_update(sk, delay);
 }

 static struct tcp_congestion_ops cubictcp = {

     .init        = bictcp_init,

     //调用ssthresh函数的地方有：tcp_fastretrans_alert(), tcp_enter_cwr(),tcp_enter_frto(), tcp_enter_loss()　
     //看起来每次发生拥塞状态切换的时候，都会调整ssthresh。　　　
 　　//修改snd_ssthresh值的地方有bictcp_init,hystart_update以及上面列出的调用ssthresh函数处。
     .ssthresh    = bictcp_recalc_ssthresh,

     //发送方发出一个data包之后，接收方回复一个ack包，发送方收到这个ack包之后，
 　　//调用tcp_ack()->tcp_cong_avoid()->bictcp_cong_avoid()来更改拥塞窗口snd_cwnd大小.
     .cong_avoid    = bictcp_cong_avoid,

     .set_state    = bictcp_state,

     //调用undo_cwnd函数的地方有:tcp_undo_cwr()用来撤销之前误判导致的"缩小拥塞窗口"
     .undo_cwnd    = bictcp_undo_cwnd,

     //调用ptts_acked函数的路径为：tcp_ack() -->tcp_clean_rtx_queue()
     .pkts_acked     = bictcp_acked,

     .owner        = THIS_MODULE,
     .name        = "cubic",
 };

 static int __init cubictcp_register(void)
 {
      //bictcp参数的个数不能过多
     BUILD_BUG_ON(sizeof(struct bictcp) > ICSK_CA_PRIV_SIZE);

     /* Precompute a bunch of the scaling factors that are used per-packet
      * based on SRTT of 100ms
      */
      //beta_scale == 8*(1024 + 717) / 3 / (1024 -717 )，大约为15
     beta_scale = *(BICTCP_BETA_SCALE+beta)/  / (BICTCP_BETA_SCALE - beta);

     //cube_rtt_scale == 41*10 = 410
     cube_rtt_scale = (bic_scale * );    /* 1024*c/rtt */

     /* calculate the "K" for (wmax-cwnd) = c/rtt * K^3
      *  so K = cubic_root( (wmax-cwnd)*rtt/c )
      * the unit of K is bictcp_HZ=2^10, not HZ
      *
      *  c = bic_scale >> 10
      *  rtt = 100ms
      *
      * the following code has been designed and tested for
      * cwnd < 1 million packets
      * RTT < 100 seconds
      * HZ < 1,000,00  (corresponding to 10 nano-second)
      */

     /* 1/c * 2^2*bictcp_HZ * srtt */
     cube_factor = 1ull << (+*BICTCP_HZ); /* cube_factor == 2^40 */

     /* divide by bic_scale and by constant Srtt (100ms) */
     do_div(cube_factor, bic_scale * );//cube_factor == 2^40 / 410

     return tcp_register_congestion_control(&cubictcp);
 }

 static void __exit cubictcp_unregister(void)
 {
     tcp_unregister_congestion_control(&cubictcp);
 }

 module_init(cubictcp_register);
 module_exit(cubictcp_unregister);

 MODULE_AUTHOR("Sangtae Ha, Stephen Hemminger");
 MODULE_LICENSE("GPL");
 MODULE_DESCRIPTION("CUBIC TCP");
 MODULE_VERSION("2.3");