498 lines
17 KiB
C++
498 lines
17 KiB
C++
/*
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* TCP CUBIC: Binary Increase Congestion control for TCP v2.3
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* Home page:
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* http://netsrv.csc.ncsu.edu/twiki/bin/view/Main/BIC
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* This is from the implementation of CUBIC TCP in
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* Sangtae Ha, Injong Rhee and Lisong Xu,
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* "CUBIC: A New TCP-Friendly High-Speed TCP Variant"
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* in ACM SIGOPS Operating System Review, July 2008.
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* Available from:
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* http://netsrv.csc.ncsu.edu/export/cubic_a_new_tcp_2008.pdf
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*
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* CUBIC integrates a new slow start algorithm, called HyStart.
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* The details of HyStart are presented in
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* Sangtae Ha and Injong Rhee,
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* "Taming the Elephants: New TCP Slow Start", NCSU TechReport 2008.
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* Available from:
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* http://netsrv.csc.ncsu.edu/export/hystart_techreport_2008.pdf
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*
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* All testing results are available from:
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* http://netsrv.csc.ncsu.edu/wiki/index.php/TCP_Testing
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*
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* Unless CUBIC is enabled and congestion window is large
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* this behaves the same as the original Reno.
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*/
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/math64.h>
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#include <net/tcp.h>
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#define BICTCP_BETA_SCALE 1024 /* Scale factor beta calculation
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* max_cwnd = snd_cwnd * beta
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*/
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#define BICTCP_HZ 10 /* BIC HZ 2^10 = 1024 */
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/* Two methods of hybrid slow start */
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#define HYSTART_ACK_TRAIN 0x1
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#define HYSTART_DELAY 0x2
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/* Number of delay samples for detecting the increase of delay */
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#define HYSTART_MIN_SAMPLES 8
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#define HYSTART_DELAY_MIN (2U<<3)
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#define HYSTART_DELAY_MAX (16U<<3)
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#define HYSTART_DELAY_THRESH(x) clamp(x, HYSTART_DELAY_MIN, HYSTART_DELAY_MAX)
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static int fast_convergence __read_mostly = 1;
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static int beta __read_mostly = 717; /* = 717/1024 (BICTCP_BETA_SCALE) */
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static int initial_ssthresh __read_mostly;
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//bic_scale就是paper中三次方系数C的1024倍缩放值
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//MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
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static int bic_scale __read_mostly = 41;
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static int tcp_friendliness __read_mostly = 1;
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static int hystart __read_mostly = 1;
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static int hystart_detect __read_mostly = HYSTART_ACK_TRAIN | HYSTART_DELAY;
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static int hystart_low_window __read_mostly = 16;
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static u32 cube_rtt_scale __read_mostly;
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static u32 beta_scale __read_mostly;
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static u64 cube_factor __read_mostly;
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/* Note parameters that are used for precomputing scale factors are read-only */
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module_param(fast_convergence, int, 0644);
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MODULE_PARM_DESC(fast_convergence, "turn on/off fast convergence");
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module_param(beta, int, 0644);
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MODULE_PARM_DESC(beta, "beta for multiplicative increase");
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module_param(initial_ssthresh, int, 0644);
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MODULE_PARM_DESC(initial_ssthresh, "initial value of slow start threshold");
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module_param(bic_scale, int, 0444);
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MODULE_PARM_DESC(bic_scale, "scale (scaled by 1024) value for bic function (bic_scale/1024)");
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module_param(tcp_friendliness, int, 0644);
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MODULE_PARM_DESC(tcp_friendliness, "turn on/off tcp friendliness");
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module_param(hystart, int, 0644);
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MODULE_PARM_DESC(hystart, "turn on/off hybrid slow start algorithm");
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module_param(hystart_detect, int, 0644);
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MODULE_PARM_DESC(hystart_detect, "hyrbrid slow start detection mechanisms"
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" 1: packet-train 2: delay 3: both packet-train and delay");
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module_param(hystart_low_window, int, 0644);
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MODULE_PARM_DESC(hystart_low_window, "lower bound cwnd for hybrid slow start");
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/* BIC TCP Parameters */
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struct bictcp {
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u32 cnt; /* increase cwnd by 1 after ACKs */
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u32 last_max_cwnd; /* last maximum snd_cwnd */
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//两个重要的count值:
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//第一个是tcp_sock->snd_cwnd_cnt，表示在当前的拥塞窗口中已经
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//发送(经过对方ack包确认)的数据段的个数，
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//而第二个是bictcp->cnt，它是cubic拥塞算法的核心，
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//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，
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//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，
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u32 loss_cwnd; /* congestion window at last loss */
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u32 last_cwnd; /* the last snd_cwnd */
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u32 last_time; /* time when updated last_cwnd */
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u32 bic_origin_point;/* origin point of bic function */
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u32 bic_K; /* time to origin point from the beginning of the current epoch */
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u32 delay_min; /* min delay */
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u32 epoch_start; /* beginning of an epoch */
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u32 ack_cnt; /* number of acks */
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u32 tcp_cwnd; /* estimated tcp cwnd */
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#define ACK_RATIO_SHIFT 4
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u16 delayed_ack; /* estimate the ratio of Packets/ACKs << 4 */
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u8 sample_cnt; /* number of samples to decide curr_rtt */
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u8 found; /* the exit point is found? */
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u32 round_start; /* beginning of each round */
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u32 end_seq; /* end_seq of the round */
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u32 last_jiffies; /* last time when the ACK spacing is close */
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u32 curr_rtt; /* the minimum rtt of current round */
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};
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static inline void bictcp_reset(struct bictcp *ca)
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{
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ca->cnt = 0;
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ca->last_max_cwnd = 0;
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ca->loss_cwnd = 0;
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ca->last_cwnd = 0;
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ca->last_time = 0;
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ca->bic_origin_point = 0;
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ca->bic_K = 0;
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ca->delay_min = 0;
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ca->epoch_start = 0;
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ca->delayed_ack = 2 << ACK_RATIO_SHIFT;
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ca->ack_cnt = 0;
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ca->tcp_cwnd = 0;
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ca->found = 0;
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}
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static inline void bictcp_hystart_reset(struct sock *sk)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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ca->round_start = ca->last_jiffies = jiffies;
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ca->end_seq = tp->snd_nxt;
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ca->curr_rtt = 0;
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ca->sample_cnt = 0;
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}
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static void bictcp_init(struct sock *sk)
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{
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bictcp_reset(inet_csk_ca(sk));
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if (hystart)
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bictcp_hystart_reset(sk);
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if (!hystart && initial_ssthresh)
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tcp_sk(sk)->snd_ssthresh = initial_ssthresh;
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}
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/* calculate the cubic root of x using a table lookup followed by one
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* Newton-Raphson iteration.
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* Avg err ~= 0.195%
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*/
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static u32 cubic_root(u64 a)
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{
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u32 x, b, shift;
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/*
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* cbrt(x) MSB values for x MSB values in [0..63].
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* Precomputed then refined by hand - Willy Tarreau
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*
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* For x in [0..63],
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* v = cbrt(x << 18) - 1
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* cbrt(x) = (v[x] + 10) >> 6
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*/
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static const u8 v[] = {
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/* 0x00 */ 0, 54, 54, 54, 118, 118, 118, 118,
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/* 0x08 */ 123, 129, 134, 138, 143, 147, 151, 156,
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/* 0x10 */ 157, 161, 164, 168, 170, 173, 176, 179,
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/* 0x18 */ 181, 185, 187, 190, 192, 194, 197, 199,
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/* 0x20 */ 200, 202, 204, 206, 209, 211, 213, 215,
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/* 0x28 */ 217, 219, 221, 222, 224, 225, 227, 229,
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/* 0x30 */ 231, 232, 234, 236, 237, 239, 240, 242,
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/* 0x38 */ 244, 245, 246, 248, 250, 251, 252, 254,
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};
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b = fls64(a);
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if (b < 7) {
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/* a in [0..63] */
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return ((u32)v[(u32)a] + 35) >> 6;
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}
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b = ((b * 84) >> 8) - 1;
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shift = (a >> (b * 3));
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x = ((u32)(((u32)v[shift] + 10) << b)) >> 6;
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/*
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* Newton-Raphson iteration
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* 2
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* x = ( 2 * x + a / x ) / 3
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* k+1 k k
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*/
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x = (2 * x + (u32)div64_u64(a, (u64)x * (u64)(x - 1)));
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x = ((x * 341) >> 10);
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return x;
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}
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/*
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函数关键点
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1. 我们最终要得到的是ca->cnt，用来控制snd_cwnd的增长。
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2. ca->cnt的值，是根据cwnd和w( t + after ) 的大小来判断的。w( t + after )即bic_target，它表示我们预期的
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在经过after时间后的snd_cwnd。如果此时cwnd < w( t + after )，那么我们就快速增加窗口，达到预期目标。
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如果cwnd > w( t + after )，那说明我们已经增加过快了，需要降速了，这样才能达到预期目标。
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cwnd / (bic_target - cwnd ) // bic_target > cwnd
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cnt =
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100 * cwnd // bic_target < cwnd
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3. cwnd是传入的参数，已知。现在我们只需要计算bic_target。
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而根据Cubic的窗口增长函数：W(t) = C(t - K)^3 + Wmax，
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我们要计算时间( 当前 + after )，以及时间K。时间K即bic_K，表示函数值为Wmax所对应的时间。
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通过代码可以发现，after为min RTT，即连接的传播时延。
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*/
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/*
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* Compute congestion window to use.
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*/
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static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
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{
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u64 offs;/* 时间差，| t - K | */
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u32 delta, t, bic_target, max_cnt;/* delta是cwnd差，bic_target是预测值，t为预测时间 */
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ca->ack_cnt++; /* count the number of ACKs */
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if (ca->last_cwnd == cwnd &&
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(s32)(tcp_time_stamp - ca->last_time) <= HZ / 32)
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return;
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ca->last_cwnd = cwnd;
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ca->last_time = tcp_time_stamp;
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if (ca->epoch_start == 0) {
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ca->epoch_start = tcp_time_stamp; /* record the beginning of an epoch */
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ca->ack_cnt = 1; /* start counting */
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ca->tcp_cwnd = cwnd; /* syn with cubic */
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/* 取max(last_max_cwnd , cwnd)作为当前Wmax */
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if (ca->last_max_cwnd <= cwnd) {
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ca->bic_K = 0;
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ca->bic_origin_point = cwnd;
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} else {
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/* Compute new K based on
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* (wmax-cwnd) * (srtt>>3 / HZ) / c * 2^(3*bictcp_HZ)
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*/
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ca->bic_K = cubic_root(cube_factor
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* (ca->last_max_cwnd - cwnd));
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ca->bic_origin_point = ca->last_max_cwnd;
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}
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}
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/* cubic function - calc*/
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/* calculate c * time^3 / rtt,
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* while considering overflow in calculation of time^3
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* (so time^3 is done by using 64 bit)
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* and without the support of division of 64bit numbers
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* (so all divisions are done by using 32 bit)
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* also NOTE the unit of those veriables
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* time = (t - K) / 2^bictcp_HZ
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* c = bic_scale >> 10
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* rtt = (srtt >> 3) / HZ
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* !!! The following code does not have overflow problems,
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* if the cwnd < 1 million packets !!!
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*/
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/* change the unit from HZ to bictcp_HZ */
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t = ((tcp_time_stamp + (ca->delay_min>>3) - ca->epoch_start)
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<< BICTCP_HZ) / HZ;
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/* 求| t - bic_K | */
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if (t < ca->bic_K) /* 还未达到Wmax */
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offs = ca->bic_K - t;
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else
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offs = t - ca->bic_K; //此时已经超过Wmax
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/* c/rtt * (t-K)^3 */
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/* 计算delta =| W(t) - W(bic_K) |
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* cube_rtt_scale = (bic_scale * 10) = c / srtt * 2^10，c/srtt = 0.4
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*/
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delta = (cube_rtt_scale * offs * offs * offs) >> (10+3*BICTCP_HZ);
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if (t < ca->bic_K) /* below origin*/
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bic_target = ca->bic_origin_point - delta;
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else /* above origin*/
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bic_target = ca->bic_origin_point + delta;
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/* cubic function - calc bictcp_cnt /* 计算bic_target，即预测cwnd */
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if (bic_target > cwnd) {
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/* 相差越多，增长越快，这就是函数形状由来 */
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ca->cnt = cwnd / (bic_target - cwnd);
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} else {
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ca->cnt = 100 * cwnd; /* very small increment 目前cwnd已经超出预期了，应该降速*/
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}
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/* TCP Friendly /* TCP Friendly —如果cubic比RENO慢，则提升cwnd增长速度，即减小cnt
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* 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
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* 采用RENO算法的cwnd。
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* cwnd (RENO) = cwnd + 3B / (2 - B) * ack_cnt / cwnd
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* B为乘性减少因子，在此算法中为0.3
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*/
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if (tcp_friendliness) {
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u32 scale = beta_scale;
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delta = (cwnd * scale) >> 3;
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while (ca->ack_cnt > delta) { /* update tcp cwnd */
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ca->ack_cnt -= delta;
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ca->tcp_cwnd++;
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}
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if (ca->tcp_cwnd > cwnd){ /* if bic is slower than tcp */
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delta = ca->tcp_cwnd - cwnd;
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max_cnt = cwnd / delta;
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if (ca->cnt > max_cnt)
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ca->cnt = max_cnt;
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}
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}
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ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack;//做了一个比较直接的delay_ack的控制
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//ratio = (15*ratio + sample) / 16 这里ca-delayed_ack由
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if (ca->cnt == 0) /* cannot be zero */
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ca->cnt = 1;/* 此时代表cwnd远小于bic_target，增长速度最大 */
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}
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static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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//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
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if (!tcp_is_cwnd_limited(sk, in_flight))
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return;
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if (tp->snd_cwnd <= tp->snd_ssthresh) {
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if (hystart && after(ack, ca->end_seq))
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bictcp_hystart_reset(sk);
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tcp_slow_start(tp);//进入slow start状态
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} else {
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bictcp_update(ca, tp->snd_cwnd);
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tcp_cong_avoid_ai(tp, ca->cnt);//然后进入拥塞避免，更新tcp_sock->snd_cwnd_cnt
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}
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}
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//做了两件事：重赋值last_max_cwnd、返回新的慢启动阈值
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static u32 bictcp_recalc_ssthresh(struct sock *sk)
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{
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const struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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ca->epoch_start = 0; /* end of epoch */
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/* Wmax and fast convergence */
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//当一个新的TCP流加入到网络，
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//网络中已有TCP流需要放弃自己带宽，
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//给新的TCP流提供一定的上升空间。
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//为提高已有TCP流所释放的带宽而引入快速收敛机制。
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if (tp->snd_cwnd < ca->last_max_cwnd && fast_convergence)
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//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。
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//然后，通过进一步降低Wmax让已有流释放更多带宽。
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//这种行为有效地延长已有流增大其窗口的时间，
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//因为降低后的Wmax强制已有流更早进入平稳状态。
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//这允许新流有更多的时间来赶上其窗口尺寸。
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ca->last_max_cwnd = (tp->snd_cwnd * (BICTCP_BETA_SCALE + beta))
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/ (2 * BICTCP_BETA_SCALE);//last_max_cwnd = 0.9 * snd_cwnd
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else
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ca->last_max_cwnd = tp->snd_cwnd;
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ca->loss_cwnd = tp->snd_cwnd;
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//修改snd_ssthresh，即max(0.7*snd_cwnd，2)
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return max((tp->snd_cwnd * beta) / BICTCP_BETA_SCALE, 2U);
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}
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//快速恢复：直接把snd_cwnd更新为max(snd_cwnd，last_max_cwnd)，和掉包前相差不大
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static u32 bictcp_undo_cwnd(struct sock *sk)
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{
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struct bictcp *ca = inet_csk_ca(sk);
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return max(tcp_sk(sk)->snd_cwnd, ca->last_max_cwnd);
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}
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static void bictcp_state(struct sock *sk, u8 new_state)
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{
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if (new_state == TCP_CA_Loss) {
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bictcp_reset(inet_csk_ca(sk));
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bictcp_hystart_reset(sk);
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}
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}
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static void hystart_update(struct sock *sk, u32 delay)
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{
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struct tcp_sock *tp = tcp_sk(sk);
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struct bictcp *ca = inet_csk_ca(sk);
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if (!(ca->found & hystart_detect)) {
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u32 curr_jiffies = jiffies;
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/* first detection parameter - ack-train detection */
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if (curr_jiffies - ca->last_jiffies <= msecs_to_jiffies(2)) {
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ca->last_jiffies = curr_jiffies;
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if (curr_jiffies - ca->round_start >= ca->delay_min>>4)
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ca->found |= HYSTART_ACK_TRAIN;
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}
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/* obtain the minimum delay of more than sampling packets */
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if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
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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)
|
||
tp->snd_ssthresh = tp->snd_cwnd;
|
||
}
|
||
}
|
||
|
||
/* Track delayed acknowledgment ratio using sliding window
|
||
* ratio = (15*ratio + sample) / 16
|
||
*/
|
||
//跟踪延迟确认在滑动窗口的比例，主要考虑正常和丢包的时候，是一个参考值
|
||
//根据下面这个函数。化简得 delayed_ack = 15/16*delayed_ack + cnt;
|
||
//由于 ratio = delayed_ack/ 16; 16*ration = 15*ration+cnt
|
||
//所以 ratio = 15/16 * ratio + cnt / 16;
|
||
static void bictcp_acked(struct sock *sk, u32 cnt, s32 rtt_us)
|
||
{
|
||
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 */
|
||
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,
|
||
.ssthresh = bictcp_recalc_ssthresh,
|
||
.cong_avoid = bictcp_cong_avoid,
|
||
.set_state = bictcp_state,
|
||
.undo_cwnd = bictcp_undo_cwnd,
|
||
.pkts_acked = bictcp_acked,
|
||
.owner = THIS_MODULE,
|
||
.name = "cubic",
|
||
};
|
||
|
||
static int __init cubictcp_register(void)
|
||
{
|
||
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 cube_rtt_scale是bic_scale/RTT, 这里rtt=100ms=0.1s
|
||
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 */
|
||
//通过bic_K和paper中的公式对比，可以知道cube_factor就是1/（C/RTT)
|
||
//具体需要参考算法实现，以及这里的30是开根号
|
||
cube_factor = 1ull << (10+3*BICTCP_HZ); /* 2^40 */
|
||
/* divide by bic_scale and by constant Srtt (100ms) */
|
||
do_div(cube_factor, bic_scale * 10);
|
||
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");
|
||
|