linux/net/ipv4/tcp_illinois.c
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   1// SPDX-License-Identifier: GPL-2.0-only
   2/*
   3 * TCP Illinois congestion control.
   4 * Home page:
   5 *      http://www.ews.uiuc.edu/~shaoliu/tcpillinois/index.html
   6 *
   7 * The algorithm is described in:
   8 * "TCP-Illinois: A Loss and Delay-Based Congestion Control Algorithm
   9 *  for High-Speed Networks"
  10 * http://tamerbasar.csl.illinois.edu/LiuBasarSrikantPerfEvalArtJun2008.pdf
  11 *
  12 * Implemented from description in paper and ns-2 simulation.
  13 * Copyright (C) 2007 Stephen Hemminger <shemminger@linux-foundation.org>
  14 */
  15
  16#include <linux/module.h>
  17#include <linux/skbuff.h>
  18#include <linux/inet_diag.h>
  19#include <asm/div64.h>
  20#include <net/tcp.h>
  21
  22#define ALPHA_SHIFT     7
  23#define ALPHA_SCALE     (1u<<ALPHA_SHIFT)
  24#define ALPHA_MIN       ((3*ALPHA_SCALE)/10)    /* ~0.3 */
  25#define ALPHA_MAX       (10*ALPHA_SCALE)        /* 10.0 */
  26#define ALPHA_BASE      ALPHA_SCALE             /* 1.0 */
  27#define RTT_MAX         (U32_MAX / ALPHA_MAX)   /* 3.3 secs */
  28
  29#define BETA_SHIFT      6
  30#define BETA_SCALE      (1u<<BETA_SHIFT)
  31#define BETA_MIN        (BETA_SCALE/8)          /* 0.125 */
  32#define BETA_MAX        (BETA_SCALE/2)          /* 0.5 */
  33#define BETA_BASE       BETA_MAX
  34
  35static int win_thresh __read_mostly = 15;
  36module_param(win_thresh, int, 0);
  37MODULE_PARM_DESC(win_thresh, "Window threshold for starting adaptive sizing");
  38
  39static int theta __read_mostly = 5;
  40module_param(theta, int, 0);
  41MODULE_PARM_DESC(theta, "# of fast RTT's before full growth");
  42
  43/* TCP Illinois Parameters */
  44struct illinois {
  45        u64     sum_rtt;        /* sum of rtt's measured within last rtt */
  46        u16     cnt_rtt;        /* # of rtts measured within last rtt */
  47        u32     base_rtt;       /* min of all rtt in usec */
  48        u32     max_rtt;        /* max of all rtt in usec */
  49        u32     end_seq;        /* right edge of current RTT */
  50        u32     alpha;          /* Additive increase */
  51        u32     beta;           /* Muliplicative decrease */
  52        u16     acked;          /* # packets acked by current ACK */
  53        u8      rtt_above;      /* average rtt has gone above threshold */
  54        u8      rtt_low;        /* # of rtts measurements below threshold */
  55};
  56
  57static void rtt_reset(struct sock *sk)
  58{
  59        struct tcp_sock *tp = tcp_sk(sk);
  60        struct illinois *ca = inet_csk_ca(sk);
  61
  62        ca->end_seq = tp->snd_nxt;
  63        ca->cnt_rtt = 0;
  64        ca->sum_rtt = 0;
  65
  66        /* TODO: age max_rtt? */
  67}
  68
  69static void tcp_illinois_init(struct sock *sk)
  70{
  71        struct illinois *ca = inet_csk_ca(sk);
  72
  73        ca->alpha = ALPHA_MAX;
  74        ca->beta = BETA_BASE;
  75        ca->base_rtt = 0x7fffffff;
  76        ca->max_rtt = 0;
  77
  78        ca->acked = 0;
  79        ca->rtt_low = 0;
  80        ca->rtt_above = 0;
  81
  82        rtt_reset(sk);
  83}
  84
  85/* Measure RTT for each ack. */
  86static void tcp_illinois_acked(struct sock *sk, const struct ack_sample *sample)
  87{
  88        struct illinois *ca = inet_csk_ca(sk);
  89        s32 rtt_us = sample->rtt_us;
  90
  91        ca->acked = sample->pkts_acked;
  92
  93        /* dup ack, no rtt sample */
  94        if (rtt_us < 0)
  95                return;
  96
  97        /* ignore bogus values, this prevents wraparound in alpha math */
  98        if (rtt_us > RTT_MAX)
  99                rtt_us = RTT_MAX;
 100
 101        /* keep track of minimum RTT seen so far */
 102        if (ca->base_rtt > rtt_us)
 103                ca->base_rtt = rtt_us;
 104
 105        /* and max */
 106        if (ca->max_rtt < rtt_us)
 107                ca->max_rtt = rtt_us;
 108
 109        ++ca->cnt_rtt;
 110        ca->sum_rtt += rtt_us;
 111}
 112
 113/* Maximum queuing delay */
 114static inline u32 max_delay(const struct illinois *ca)
 115{
 116        return ca->max_rtt - ca->base_rtt;
 117}
 118
 119/* Average queuing delay */
 120static inline u32 avg_delay(const struct illinois *ca)
 121{
 122        u64 t = ca->sum_rtt;
 123
 124        do_div(t, ca->cnt_rtt);
 125        return t - ca->base_rtt;
 126}
 127
 128/*
 129 * Compute value of alpha used for additive increase.
 130 * If small window then use 1.0, equivalent to Reno.
 131 *
 132 * For larger windows, adjust based on average delay.
 133 * A. If average delay is at minimum (we are uncongested),
 134 *    then use large alpha (10.0) to increase faster.
 135 * B. If average delay is at maximum (getting congested)
 136 *    then use small alpha (0.3)
 137 *
 138 * The result is a convex window growth curve.
 139 */
 140static u32 alpha(struct illinois *ca, u32 da, u32 dm)
 141{
 142        u32 d1 = dm / 100;      /* Low threshold */
 143
 144        if (da <= d1) {
 145                /* If never got out of low delay zone, then use max */
 146                if (!ca->rtt_above)
 147                        return ALPHA_MAX;
 148
 149                /* Wait for 5 good RTT's before allowing alpha to go alpha max.
 150                 * This prevents one good RTT from causing sudden window increase.
 151                 */
 152                if (++ca->rtt_low < theta)
 153                        return ca->alpha;
 154
 155                ca->rtt_low = 0;
 156                ca->rtt_above = 0;
 157                return ALPHA_MAX;
 158        }
 159
 160        ca->rtt_above = 1;
 161
 162        /*
 163         * Based on:
 164         *
 165         *      (dm - d1) amin amax
 166         * k1 = -------------------
 167         *         amax - amin
 168         *
 169         *       (dm - d1) amin
 170         * k2 = ----------------  - d1
 171         *        amax - amin
 172         *
 173         *             k1
 174         * alpha = ----------
 175         *          k2 + da
 176         */
 177
 178        dm -= d1;
 179        da -= d1;
 180        return (dm * ALPHA_MAX) /
 181                (dm + (da  * (ALPHA_MAX - ALPHA_MIN)) / ALPHA_MIN);
 182}
 183
 184/*
 185 * Beta used for multiplicative decrease.
 186 * For small window sizes returns same value as Reno (0.5)
 187 *
 188 * If delay is small (10% of max) then beta = 1/8
 189 * If delay is up to 80% of max then beta = 1/2
 190 * In between is a linear function
 191 */
 192static u32 beta(u32 da, u32 dm)
 193{
 194        u32 d2, d3;
 195
 196        d2 = dm / 10;
 197        if (da <= d2)
 198                return BETA_MIN;
 199
 200        d3 = (8 * dm) / 10;
 201        if (da >= d3 || d3 <= d2)
 202                return BETA_MAX;
 203
 204        /*
 205         * Based on:
 206         *
 207         *       bmin d3 - bmax d2
 208         * k3 = -------------------
 209         *         d3 - d2
 210         *
 211         *       bmax - bmin
 212         * k4 = -------------
 213         *         d3 - d2
 214         *
 215         * b = k3 + k4 da
 216         */
 217        return (BETA_MIN * d3 - BETA_MAX * d2 + (BETA_MAX - BETA_MIN) * da)
 218                / (d3 - d2);
 219}
 220
 221/* Update alpha and beta values once per RTT */
 222static void update_params(struct sock *sk)
 223{
 224        struct tcp_sock *tp = tcp_sk(sk);
 225        struct illinois *ca = inet_csk_ca(sk);
 226
 227        if (tp->snd_cwnd < win_thresh) {
 228                ca->alpha = ALPHA_BASE;
 229                ca->beta = BETA_BASE;
 230        } else if (ca->cnt_rtt > 0) {
 231                u32 dm = max_delay(ca);
 232                u32 da = avg_delay(ca);
 233
 234                ca->alpha = alpha(ca, da, dm);
 235                ca->beta = beta(da, dm);
 236        }
 237
 238        rtt_reset(sk);
 239}
 240
 241/*
 242 * In case of loss, reset to default values
 243 */
 244static void tcp_illinois_state(struct sock *sk, u8 new_state)
 245{
 246        struct illinois *ca = inet_csk_ca(sk);
 247
 248        if (new_state == TCP_CA_Loss) {
 249                ca->alpha = ALPHA_BASE;
 250                ca->beta = BETA_BASE;
 251                ca->rtt_low = 0;
 252                ca->rtt_above = 0;
 253                rtt_reset(sk);
 254        }
 255}
 256
 257/*
 258 * Increase window in response to successful acknowledgment.
 259 */
 260static void tcp_illinois_cong_avoid(struct sock *sk, u32 ack, u32 acked)
 261{
 262        struct tcp_sock *tp = tcp_sk(sk);
 263        struct illinois *ca = inet_csk_ca(sk);
 264
 265        if (after(ack, ca->end_seq))
 266                update_params(sk);
 267
 268        /* RFC2861 only increase cwnd if fully utilized */
 269        if (!tcp_is_cwnd_limited(sk))
 270                return;
 271
 272        /* In slow start */
 273        if (tcp_in_slow_start(tp))
 274                tcp_slow_start(tp, acked);
 275
 276        else {
 277                u32 delta;
 278
 279                /* snd_cwnd_cnt is # of packets since last cwnd increment */
 280                tp->snd_cwnd_cnt += ca->acked;
 281                ca->acked = 1;
 282
 283                /* This is close approximation of:
 284                 * tp->snd_cwnd += alpha/tp->snd_cwnd
 285                */
 286                delta = (tp->snd_cwnd_cnt * ca->alpha) >> ALPHA_SHIFT;
 287                if (delta >= tp->snd_cwnd) {
 288                        tp->snd_cwnd = min(tp->snd_cwnd + delta / tp->snd_cwnd,
 289                                           (u32)tp->snd_cwnd_clamp);
 290                        tp->snd_cwnd_cnt = 0;
 291                }
 292        }
 293}
 294
 295static u32 tcp_illinois_ssthresh(struct sock *sk)
 296{
 297        struct tcp_sock *tp = tcp_sk(sk);
 298        struct illinois *ca = inet_csk_ca(sk);
 299
 300        /* Multiplicative decrease */
 301        return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->beta) >> BETA_SHIFT), 2U);
 302}
 303
 304/* Extract info for Tcp socket info provided via netlink. */
 305static size_t tcp_illinois_info(struct sock *sk, u32 ext, int *attr,
 306                                union tcp_cc_info *info)
 307{
 308        const struct illinois *ca = inet_csk_ca(sk);
 309
 310        if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 311                info->vegas.tcpv_enabled = 1;
 312                info->vegas.tcpv_rttcnt = ca->cnt_rtt;
 313                info->vegas.tcpv_minrtt = ca->base_rtt;
 314                info->vegas.tcpv_rtt = 0;
 315
 316                if (info->vegas.tcpv_rttcnt > 0) {
 317                        u64 t = ca->sum_rtt;
 318
 319                        do_div(t, info->vegas.tcpv_rttcnt);
 320                        info->vegas.tcpv_rtt = t;
 321                }
 322                *attr = INET_DIAG_VEGASINFO;
 323                return sizeof(struct tcpvegas_info);
 324        }
 325        return 0;
 326}
 327
 328static struct tcp_congestion_ops tcp_illinois __read_mostly = {
 329        .init           = tcp_illinois_init,
 330        .ssthresh       = tcp_illinois_ssthresh,
 331        .undo_cwnd      = tcp_reno_undo_cwnd,
 332        .cong_avoid     = tcp_illinois_cong_avoid,
 333        .set_state      = tcp_illinois_state,
 334        .get_info       = tcp_illinois_info,
 335        .pkts_acked     = tcp_illinois_acked,
 336
 337        .owner          = THIS_MODULE,
 338        .name           = "illinois",
 339};
 340
 341static int __init tcp_illinois_register(void)
 342{
 343        BUILD_BUG_ON(sizeof(struct illinois) > ICSK_CA_PRIV_SIZE);
 344        return tcp_register_congestion_control(&tcp_illinois);
 345}
 346
 347static void __exit tcp_illinois_unregister(void)
 348{
 349        tcp_unregister_congestion_control(&tcp_illinois);
 350}
 351
 352module_init(tcp_illinois_register);
 353module_exit(tcp_illinois_unregister);
 354
 355MODULE_AUTHOR("Stephen Hemminger, Shao Liu");
 356MODULE_LICENSE("GPL");
 357MODULE_DESCRIPTION("TCP Illinois");
 358MODULE_VERSION("1.0");
 359