linux/net/ipv4/tcp_westwood.c
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   1/*
   2 * TCP Westwood+: end-to-end bandwidth estimation for TCP
   3 *
   4 *      Angelo Dell'Aera: author of the first version of TCP Westwood+ in Linux 2.4
   5 *
   6 * Support at http://c3lab.poliba.it/index.php/Westwood
   7 * Main references in literature:
   8 *
   9 * - Mascolo S, Casetti, M. Gerla et al.
  10 *   "TCP Westwood: bandwidth estimation for TCP" Proc. ACM Mobicom 2001
  11 *
  12 * - A. Grieco, s. Mascolo
  13 *   "Performance evaluation of New Reno, Vegas, Westwood+ TCP" ACM Computer
  14 *     Comm. Review, 2004
  15 *
  16 * - A. Dell'Aera, L. Grieco, S. Mascolo.
  17 *   "Linux 2.4 Implementation of Westwood+ TCP with Rate-Halving :
  18 *    A Performance Evaluation Over the Internet" (ICC 2004), Paris, June 2004
  19 *
  20 * Westwood+ employs end-to-end bandwidth measurement to set cwnd and
  21 * ssthresh after packet loss. The probing phase is as the original Reno.
  22 */
  23
  24#include <linux/mm.h>
  25#include <linux/module.h>
  26#include <linux/skbuff.h>
  27#include <linux/inet_diag.h>
  28#include <net/tcp.h>
  29
  30/* TCP Westwood structure */
  31struct westwood {
  32        u32    bw_ns_est;        /* first bandwidth estimation..not too smoothed 8) */
  33        u32    bw_est;           /* bandwidth estimate */
  34        u32    rtt_win_sx;       /* here starts a new evaluation... */
  35        u32    bk;
  36        u32    snd_una;          /* used for evaluating the number of acked bytes */
  37        u32    cumul_ack;
  38        u32    accounted;
  39        u32    rtt;
  40        u32    rtt_min;          /* minimum observed RTT */
  41        u8     first_ack;        /* flag which infers that this is the first ack */
  42        u8     reset_rtt_min;    /* Reset RTT min to next RTT sample*/
  43};
  44
  45
  46/* TCP Westwood functions and constants */
  47#define TCP_WESTWOOD_RTT_MIN   (HZ/20)  /* 50ms */
  48#define TCP_WESTWOOD_INIT_RTT  (20*HZ)  /* maybe too conservative?! */
  49
  50/*
  51 * @tcp_westwood_create
  52 * This function initializes fields used in TCP Westwood+,
  53 * it is called after the initial SYN, so the sequence numbers
  54 * are correct but new passive connections we have no
  55 * information about RTTmin at this time so we simply set it to
  56 * TCP_WESTWOOD_INIT_RTT. This value was chosen to be too conservative
  57 * since in this way we're sure it will be updated in a consistent
  58 * way as soon as possible. It will reasonably happen within the first
  59 * RTT period of the connection lifetime.
  60 */
  61static void tcp_westwood_init(struct sock *sk)
  62{
  63        struct westwood *w = inet_csk_ca(sk);
  64
  65        w->bk = 0;
  66        w->bw_ns_est = 0;
  67        w->bw_est = 0;
  68        w->accounted = 0;
  69        w->cumul_ack = 0;
  70        w->reset_rtt_min = 1;
  71        w->rtt_min = w->rtt = TCP_WESTWOOD_INIT_RTT;
  72        w->rtt_win_sx = tcp_time_stamp;
  73        w->snd_una = tcp_sk(sk)->snd_una;
  74        w->first_ack = 1;
  75}
  76
  77/*
  78 * @westwood_do_filter
  79 * Low-pass filter. Implemented using constant coefficients.
  80 */
  81static inline u32 westwood_do_filter(u32 a, u32 b)
  82{
  83        return ((7 * a) + b) >> 3;
  84}
  85
  86static void westwood_filter(struct westwood *w, u32 delta)
  87{
  88        /* If the filter is empty fill it with the first sample of bandwidth  */
  89        if (w->bw_ns_est == 0 && w->bw_est == 0) {
  90                w->bw_ns_est = w->bk / delta;
  91                w->bw_est = w->bw_ns_est;
  92        } else {
  93                w->bw_ns_est = westwood_do_filter(w->bw_ns_est, w->bk / delta);
  94                w->bw_est = westwood_do_filter(w->bw_est, w->bw_ns_est);
  95        }
  96}
  97
  98/*
  99 * @westwood_pkts_acked
 100 * Called after processing group of packets.
 101 * but all westwood needs is the last sample of srtt.
 102 */
 103static void tcp_westwood_pkts_acked(struct sock *sk, u32 cnt, s32 rtt)
 104{
 105        struct westwood *w = inet_csk_ca(sk);
 106
 107        if (rtt > 0)
 108                w->rtt = usecs_to_jiffies(rtt);
 109}
 110
 111/*
 112 * @westwood_update_window
 113 * It updates RTT evaluation window if it is the right moment to do
 114 * it. If so it calls filter for evaluating bandwidth.
 115 */
 116static void westwood_update_window(struct sock *sk)
 117{
 118        struct westwood *w = inet_csk_ca(sk);
 119        s32 delta = tcp_time_stamp - w->rtt_win_sx;
 120
 121        /* Initialize w->snd_una with the first acked sequence number in order
 122         * to fix mismatch between tp->snd_una and w->snd_una for the first
 123         * bandwidth sample
 124         */
 125        if (w->first_ack) {
 126                w->snd_una = tcp_sk(sk)->snd_una;
 127                w->first_ack = 0;
 128        }
 129
 130        /*
 131         * See if a RTT-window has passed.
 132         * Be careful since if RTT is less than
 133         * 50ms we don't filter but we continue 'building the sample'.
 134         * This minimum limit was chosen since an estimation on small
 135         * time intervals is better to avoid...
 136         * Obviously on a LAN we reasonably will always have
 137         * right_bound = left_bound + WESTWOOD_RTT_MIN
 138         */
 139        if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) {
 140                westwood_filter(w, delta);
 141
 142                w->bk = 0;
 143                w->rtt_win_sx = tcp_time_stamp;
 144        }
 145}
 146
 147static inline void update_rtt_min(struct westwood *w)
 148{
 149        if (w->reset_rtt_min) {
 150                w->rtt_min = w->rtt;
 151                w->reset_rtt_min = 0;
 152        } else
 153                w->rtt_min = min(w->rtt, w->rtt_min);
 154}
 155
 156
 157/*
 158 * @westwood_fast_bw
 159 * It is called when we are in fast path. In particular it is called when
 160 * header prediction is successful. In such case in fact update is
 161 * straight forward and doesn't need any particular care.
 162 */
 163static inline void westwood_fast_bw(struct sock *sk)
 164{
 165        const struct tcp_sock *tp = tcp_sk(sk);
 166        struct westwood *w = inet_csk_ca(sk);
 167
 168        westwood_update_window(sk);
 169
 170        w->bk += tp->snd_una - w->snd_una;
 171        w->snd_una = tp->snd_una;
 172        update_rtt_min(w);
 173}
 174
 175/*
 176 * @westwood_acked_count
 177 * This function evaluates cumul_ack for evaluating bk in case of
 178 * delayed or partial acks.
 179 */
 180static inline u32 westwood_acked_count(struct sock *sk)
 181{
 182        const struct tcp_sock *tp = tcp_sk(sk);
 183        struct westwood *w = inet_csk_ca(sk);
 184
 185        w->cumul_ack = tp->snd_una - w->snd_una;
 186
 187        /* If cumul_ack is 0 this is a dupack since it's not moving
 188         * tp->snd_una.
 189         */
 190        if (!w->cumul_ack) {
 191                w->accounted += tp->mss_cache;
 192                w->cumul_ack = tp->mss_cache;
 193        }
 194
 195        if (w->cumul_ack > tp->mss_cache) {
 196                /* Partial or delayed ack */
 197                if (w->accounted >= w->cumul_ack) {
 198                        w->accounted -= w->cumul_ack;
 199                        w->cumul_ack = tp->mss_cache;
 200                } else {
 201                        w->cumul_ack -= w->accounted;
 202                        w->accounted = 0;
 203                }
 204        }
 205
 206        w->snd_una = tp->snd_una;
 207
 208        return w->cumul_ack;
 209}
 210
 211
 212/*
 213 * TCP Westwood
 214 * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
 215 * in packets we use mss_cache). Rttmin is guaranteed to be >= 2
 216 * so avoids ever returning 0.
 217 */
 218static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
 219{
 220        const struct tcp_sock *tp = tcp_sk(sk);
 221        const struct westwood *w = inet_csk_ca(sk);
 222        return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
 223}
 224
 225static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
 226{
 227        struct tcp_sock *tp = tcp_sk(sk);
 228        struct westwood *w = inet_csk_ca(sk);
 229
 230        switch (event) {
 231        case CA_EVENT_FAST_ACK:
 232                westwood_fast_bw(sk);
 233                break;
 234
 235        case CA_EVENT_COMPLETE_CWR:
 236                tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 237                break;
 238
 239        case CA_EVENT_FRTO:
 240                tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 241                /* Update RTT_min when next ack arrives */
 242                w->reset_rtt_min = 1;
 243                break;
 244
 245        case CA_EVENT_SLOW_ACK:
 246                westwood_update_window(sk);
 247                w->bk += westwood_acked_count(sk);
 248                update_rtt_min(w);
 249                break;
 250
 251        default:
 252                /* don't care */
 253                break;
 254        }
 255}
 256
 257
 258/* Extract info for Tcp socket info provided via netlink. */
 259static void tcp_westwood_info(struct sock *sk, u32 ext,
 260                              struct sk_buff *skb)
 261{
 262        const struct westwood *ca = inet_csk_ca(sk);
 263        if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 264                struct tcpvegas_info info = {
 265                        .tcpv_enabled = 1,
 266                        .tcpv_rtt = jiffies_to_usecs(ca->rtt),
 267                        .tcpv_minrtt = jiffies_to_usecs(ca->rtt_min),
 268                };
 269
 270                nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
 271        }
 272}
 273
 274
 275static struct tcp_congestion_ops tcp_westwood __read_mostly = {
 276        .init           = tcp_westwood_init,
 277        .ssthresh       = tcp_reno_ssthresh,
 278        .cong_avoid     = tcp_reno_cong_avoid,
 279        .min_cwnd       = tcp_westwood_bw_rttmin,
 280        .cwnd_event     = tcp_westwood_event,
 281        .get_info       = tcp_westwood_info,
 282        .pkts_acked     = tcp_westwood_pkts_acked,
 283
 284        .owner          = THIS_MODULE,
 285        .name           = "westwood"
 286};
 287
 288static int __init tcp_westwood_register(void)
 289{
 290        BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
 291        return tcp_register_congestion_control(&tcp_westwood);
 292}
 293
 294static void __exit tcp_westwood_unregister(void)
 295{
 296        tcp_unregister_congestion_control(&tcp_westwood);
 297}
 298
 299module_init(tcp_westwood_register);
 300module_exit(tcp_westwood_unregister);
 301
 302MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
 303MODULE_LICENSE("GPL");
 304MODULE_DESCRIPTION("TCP Westwood+");
 305
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