net/ipv4/tcp_westwood.c

   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 */
  31 struct 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 /* TCP Westwood functions and constants */
  46 #define TCP_WESTWOOD_RTT_MIN   (HZ/20)  /* 50ms */
  47 #define TCP_WESTWOOD_INIT_RTT  (20*HZ)  /* maybe too conservative?! */
  48
  49 /*
  50  * @tcp_westwood_create
  51  * This function initializes fields used in TCP Westwood+,
  52  * it is called after the initial SYN, so the sequence numbers
  53  * are correct but new passive connections we have no
  54  * information about RTTmin at this time so we simply set it to
  55  * TCP_WESTWOOD_INIT_RTT. This value was chosen to be too conservative
  56  * since in this way we're sure it will be updated in a consistent
  57  * way as soon as possible. It will reasonably happen within the first
  58  * RTT period of the connection lifetime.
  59  */
  60 static void tcp_westwood_init(struct sock *sk)
  61 {
  62         struct westwood *w = inet_csk_ca(sk);
  63
  64         w->bk = 0;
  65         w->bw_ns_est = 0;
  66         w->bw_est = 0;
  67         w->accounted = 0;
  68         w->cumul_ack = 0;
  69         w->reset_rtt_min = 1;
  70         w->rtt_min = w->rtt = TCP_WESTWOOD_INIT_RTT;
  71         w->rtt_win_sx = tcp_time_stamp;
  72         w->snd_una = tcp_sk(sk)->snd_una;
  73         w->first_ack = 1;
  74 }
  75
  76 /*
  77  * @westwood_do_filter
  78  * Low-pass filter. Implemented using constant coefficients.
  79  */
  80 static inline u32 westwood_do_filter(u32 a, u32 b)
  81 {
  82         return ((7 * a) + b) >> 3;
  83 }
  84
  85 static void westwood_filter(struct westwood *w, u32 delta)
  86 {
  87         /* If the filter is empty fill it with the first sample of bandwidth  */
  88         if (w->bw_ns_est == 0 && w->bw_est == 0) {
  89                 w->bw_ns_est = w->bk / delta;
  90                 w->bw_est = w->bw_ns_est;
  91         } else {
  92                 w->bw_ns_est = westwood_do_filter(w->bw_ns_est, w->bk / delta);
  93                 w->bw_est = westwood_do_filter(w->bw_est, w->bw_ns_est);
  94         }
  95 }
  96
  97 /*
  98  * @westwood_pkts_acked
  99  * Called after processing group of packets.
 100  * but all westwood needs is the last sample of srtt.
 101  */
 102 static void tcp_westwood_pkts_acked(struct sock *sk,
 103                                     const struct ack_sample *sample)
 104 {
 105         struct westwood *w = inet_csk_ca(sk);
 106
 107         if (sample->rtt_us > 0)
 108                 w->rtt = usecs_to_jiffies(sample->rtt_us);
 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  */
 116 static 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
 147 static 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  * @westwood_fast_bw
 158  * It is called when we are in fast path. In particular it is called when
 159  * header prediction is successful. In such case in fact update is
 160  * straight forward and doesn't need any particular care.
 161  */
 162 static inline void westwood_fast_bw(struct sock *sk)
 163 {
 164         const struct tcp_sock *tp = tcp_sk(sk);
 165         struct westwood *w = inet_csk_ca(sk);
 166
 167         westwood_update_window(sk);
 168
 169         w->bk += tp->snd_una - w->snd_una;
 170         w->snd_una = tp->snd_una;
 171         update_rtt_min(w);
 172 }
 173
 174 /*
 175  * @westwood_acked_count
 176  * This function evaluates cumul_ack for evaluating bk in case of
 177  * delayed or partial acks.
 178  */
 179 static inline u32 westwood_acked_count(struct sock *sk)
 180 {
 181         const struct tcp_sock *tp = tcp_sk(sk);
 182         struct westwood *w = inet_csk_ca(sk);
 183
 184         w->cumul_ack = tp->snd_una - w->snd_una;
 185
 186         /* If cumul_ack is 0 this is a dupack since it's not moving
 187          * tp->snd_una.
 188          */
 189         if (!w->cumul_ack) {
 190                 w->accounted += tp->mss_cache;
 191                 w->cumul_ack = tp->mss_cache;
 192         }
 193
 194         if (w->cumul_ack > tp->mss_cache) {
 195                 /* Partial or delayed ack */
 196                 if (w->accounted >= w->cumul_ack) {
 197                         w->accounted -= w->cumul_ack;
 198                         w->cumul_ack = tp->mss_cache;
 199                 } else {
 200                         w->cumul_ack -= w->accounted;
 201                         w->accounted = 0;
 202                 }
 203         }
 204
 205         w->snd_una = tp->snd_una;
 206
 207         return w->cumul_ack;
 208 }
 209
 210 /*
 211  * TCP Westwood
 212  * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
 213  * in packets we use mss_cache). Rttmin is guaranteed to be >= 2
 214  * so avoids ever returning 0.
 215  */
 216 static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
 217 {
 218         const struct tcp_sock *tp = tcp_sk(sk);
 219         const struct westwood *w = inet_csk_ca(sk);
 220
 221         return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
 222 }
 223
 224 static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
 225 {
 226         if (ack_flags & CA_ACK_SLOWPATH) {
 227                 struct westwood *w = inet_csk_ca(sk);
 228
 229                 westwood_update_window(sk);
 230                 w->bk += westwood_acked_count(sk);
 231
 232                 update_rtt_min(w);
 233                 return;
 234         }
 235
 236         westwood_fast_bw(sk);
 237 }
 238
 239 static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
 240 {
 241         struct tcp_sock *tp = tcp_sk(sk);
 242         struct westwood *w = inet_csk_ca(sk);
 243
 244         switch (event) {
 245         case CA_EVENT_COMPLETE_CWR:
 246                 tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 247                 break;
 248         case CA_EVENT_LOSS:
 249                 tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 250                 /* Update RTT_min when next ack arrives */
 251                 w->reset_rtt_min = 1;
 252                 break;
 253         default:
 254                 /* don't care */
 255                 break;
 256         }
 257 }
 258
 259 /* Extract info for Tcp socket info provided via netlink. */
 260 static size_t tcp_westwood_info(struct sock *sk, u32 ext, int *attr,
 261                                 union tcp_cc_info *info)
 262 {
 263         const struct westwood *ca = inet_csk_ca(sk);
 264
 265         if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 266                 info->vegas.tcpv_enabled = 1;
 267                 info->vegas.tcpv_rttcnt = 0;
 268                 info->vegas.tcpv_rtt    = jiffies_to_usecs(ca->rtt);
 269                 info->vegas.tcpv_minrtt = jiffies_to_usecs(ca->rtt_min);
 270
 271                 *attr = INET_DIAG_VEGASINFO;
 272                 return sizeof(struct tcpvegas_info);
 273         }
 274         return 0;
 275 }
 276
 277 static struct tcp_congestion_ops tcp_westwood __read_mostly = {
 278         .init           = tcp_westwood_init,
 279         .ssthresh       = tcp_reno_ssthresh,
 280         .cong_avoid     = tcp_reno_cong_avoid,
 281         .undo_cwnd      = tcp_reno_undo_cwnd,
 282         .cwnd_event     = tcp_westwood_event,
 283         .in_ack_event   = tcp_westwood_ack,
 284         .get_info       = tcp_westwood_info,
 285         .pkts_acked     = tcp_westwood_pkts_acked,
 286
 287         .owner          = THIS_MODULE,
 288         .name           = "westwood"
 289 };
 290
 291 static int __init tcp_westwood_register(void)
 292 {
 293         BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
 294         return tcp_register_congestion_control(&tcp_westwood);
 295 }
 296
 297 static void __exit tcp_westwood_unregister(void)
 298 {
 299         tcp_unregister_congestion_control(&tcp_westwood);
 300 }
 301
 302 module_init(tcp_westwood_register);
 303 module_exit(tcp_westwood_unregister);
 304
 305 MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
 306 MODULE_LICENSE("GPL");
 307 MODULE_DESCRIPTION("TCP Westwood+");