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 = 1;
static int beta __read_mostly = 717;    /* = 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 = 41;
static int tcp_friendliness __read_mostly = 1;

//hybrid slow start的开关
static int hystart __read_mostly = 1;
//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 = 16;

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, 0644);
MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
module_param(beta, int, 0644);
MODULE_PARM_DESC(beta, "beta for multiplicative increase");
module_param(initial_ssthresh, int, 0644);
MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
module_param(bic_scale, int, 0444);
MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
module_param(tcp_friendliness, int, 0644);
MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
module_param(hystart, int, 0644);
MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
module_param(hystart_detect, int, 0644);
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, 0644);
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 = 0;
ca->last_max_cwnd = 0;
ca->loss_cwnd = 0;
ca->last_cwnd = 0;
ca->last_time = 0;
ca->bic_origin_point = 0;
ca->bic_K = 0;
ca->delay_min = 0;
ca->epoch_start = 0;
ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
ca->ack_cnt = 0;
ca->tcp_cwnd = 0;
ca->found = 0;
}

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 = 0;
ca->sample_cnt = 0;

//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 */    0,   54,   54,   54,  118,  118,  118,  118,
/* 0x08 */  123,  129,  134,  138,  143,  147,  151,  156,
/* 0x10 */  157,  161,  164,  168,  170,  173,  176,  179,
/* 0x18 */  181,  185,  187,  190,  192,  194,  197,  199,
/* 0x20 */  200,  202,  204,  206,  209,  211,  213,  215,
/* 0x28 */  217,  219,  221,  222,  224,  225,  227,  229,
/* 0x30 */  231,  232,  234,  236,  237,  239,  240,  242,
/* 0x38 */  244,  245,  246,  248,  250,  251,  252,  254,
};

b = fls64(a);
if (b < 7) {
/* a in [0..63] */
return ((u32)v[(u32)a] + 35) >> 6;
}

b = ((b * 84) >> 8) - 1;
shift = (a >> (b * 3));

x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;

/*
* Newton-Raphson iteration
*                         2
* x    = ( 2 * x  +  a / x  ) / 3
*  k+1          k         k
*/
x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
x = ((x * 341) >> 10);
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 / 32)//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 == 0) {//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 = 1;            /*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 = 0;
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>>3) - 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) >> (10+3*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 = 100 * 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) >> 3; //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 == 0)            /* cannot be zero */
ca->cnt = 1; //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 = 0;    /* 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))
/ (2 * 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(2)) {
ca->last_jiffies = curr_jiffies;
if (curr_jiffies - ca->round_start >= ca->delay_min>>4)
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 == 0 || 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>>4))
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 < 0)
return;

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

delay = usecs_to_jiffies(rtt_us) << 3;
if (delay == 0)
delay = 1;

/* first time call or link delay decreases */
if (ca->delay_min == 0 || 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 = 8*(BICTCP_BETA_SCALE+beta)/ 3 / (BICTCP_BETA_SCALE - beta);

//cube_rtt_scale == 41*10 = 410
cube_rtt_scale = (bic_scale * 10);    /* 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 << (10+3*BICTCP_HZ); /* cube_factor == 2^40 */

/* divide by bic_scale and by constant Srtt (100ms) */
do_div(cube_factor, bic_scale * 10);//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");