root/net/ipv4/tcp_westwood.c

DEFINITIONS

1. tcp_westwood_init
2. westwood_do_filter
3. westwood_filter
4. tcp_westwood_pkts_acked
5. westwood_update_window
6. update_rtt_min
7. westwood_fast_bw
8. westwood_acked_count
9. tcp_westwood_bw_rttmin
10. tcp_westwood_ack
11. tcp_westwood_event
12. tcp_westwood_info
13. tcp_westwood_register
14. tcp_westwood_unregister

   1 // SPDX-License-Identifier: GPL-2.0-only
   2 /*
   3  * TCP Westwood+: end-to-end bandwidth estimation for TCP
   4  *
   5  *      Angelo Dell'Aera: author of the first version of TCP Westwood+ in Linux 2.4
   6  *
   7  * Support at http://c3lab.poliba.it/index.php/Westwood
   8  * Main references in literature:
   9  *
  10  * - Mascolo S, Casetti, M. Gerla et al.
  11  *   "TCP Westwood: bandwidth estimation for TCP" Proc. ACM Mobicom 2001
  12  *
  13  * - A. Grieco, s. Mascolo
  14  *   "Performance evaluation of New Reno, Vegas, Westwood+ TCP" ACM Computer
  15  *     Comm. Review, 2004
  16  *
  17  * - A. Dell'Aera, L. Grieco, S. Mascolo.
  18  *   "Linux 2.4 Implementation of Westwood+ TCP with Rate-Halving :
  19  *    A Performance Evaluation Over the Internet" (ICC 2004), Paris, June 2004
  20  *
  21  * Westwood+ employs end-to-end bandwidth measurement to set cwnd and
  22  * ssthresh after packet loss. The probing phase is as the original Reno.
  23  */
  24 
  25 #include <linux/mm.h>
  26 #include <linux/module.h>
  27 #include <linux/skbuff.h>
  28 #include <linux/inet_diag.h>
  29 #include <net/tcp.h>
  30 
  31 /* TCP Westwood structure */
  32 struct westwood {
  33         u32    bw_ns_est;        /* first bandwidth estimation..not too smoothed 8) */
  34         u32    bw_est;           /* bandwidth estimate */
  35         u32    rtt_win_sx;       /* here starts a new evaluation... */
  36         u32    bk;
  37         u32    snd_una;          /* used for evaluating the number of acked bytes */
  38         u32    cumul_ack;
  39         u32    accounted;
  40         u32    rtt;
  41         u32    rtt_min;          /* minimum observed RTT */
  42         u8     first_ack;        /* flag which infers that this is the first ack */
  43         u8     reset_rtt_min;    /* Reset RTT min to next RTT sample*/
  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  */
  61 static 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_jiffies32;
  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  */
  81 static inline u32 westwood_do_filter(u32 a, u32 b)
  82 {
  83         return ((7 * a) + b) >> 3;
  84 }
  85 
  86 static 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  */
 103 static void tcp_westwood_pkts_acked(struct sock *sk,
 104                                     const struct ack_sample *sample)
 105 {
 106         struct westwood *w = inet_csk_ca(sk);
 107 
 108         if (sample->rtt_us > 0)
 109                 w->rtt = usecs_to_jiffies(sample->rtt_us);
 110 }
 111 
 112 /*
 113  * @westwood_update_window
 114  * It updates RTT evaluation window if it is the right moment to do
 115  * it. If so it calls filter for evaluating bandwidth.
 116  */
 117 static void westwood_update_window(struct sock *sk)
 118 {
 119         struct westwood *w = inet_csk_ca(sk);
 120         s32 delta = tcp_jiffies32 - w->rtt_win_sx;
 121 
 122         /* Initialize w->snd_una with the first acked sequence number in order
 123          * to fix mismatch between tp->snd_una and w->snd_una for the first
 124          * bandwidth sample
 125          */
 126         if (w->first_ack) {
 127                 w->snd_una = tcp_sk(sk)->snd_una;
 128                 w->first_ack = 0;
 129         }
 130 
 131         /*
 132          * See if a RTT-window has passed.
 133          * Be careful since if RTT is less than
 134          * 50ms we don't filter but we continue 'building the sample'.
 135          * This minimum limit was chosen since an estimation on small
 136          * time intervals is better to avoid...
 137          * Obviously on a LAN we reasonably will always have
 138          * right_bound = left_bound + WESTWOOD_RTT_MIN
 139          */
 140         if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) {
 141                 westwood_filter(w, delta);
 142 
 143                 w->bk = 0;
 144                 w->rtt_win_sx = tcp_jiffies32;
 145         }
 146 }
 147 
 148 static inline void update_rtt_min(struct westwood *w)
 149 {
 150         if (w->reset_rtt_min) {
 151                 w->rtt_min = w->rtt;
 152                 w->reset_rtt_min = 0;
 153         } else
 154                 w->rtt_min = min(w->rtt, w->rtt_min);
 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  */
 163 static 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  */
 180 static 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  * TCP Westwood
 213  * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
 214  * in packets we use mss_cache). Rttmin is guaranteed to be >= 2
 215  * so avoids ever returning 0.
 216  */
 217 static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
 218 {
 219         const struct tcp_sock *tp = tcp_sk(sk);
 220         const struct westwood *w = inet_csk_ca(sk);
 221 
 222         return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
 223 }
 224 
 225 static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
 226 {
 227         if (ack_flags & CA_ACK_SLOWPATH) {
 228                 struct westwood *w = inet_csk_ca(sk);
 229 
 230                 westwood_update_window(sk);
 231                 w->bk += westwood_acked_count(sk);
 232 
 233                 update_rtt_min(w);
 234                 return;
 235         }
 236 
 237         westwood_fast_bw(sk);
 238 }
 239 
 240 static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
 241 {
 242         struct tcp_sock *tp = tcp_sk(sk);
 243         struct westwood *w = inet_csk_ca(sk);
 244 
 245         switch (event) {
 246         case CA_EVENT_COMPLETE_CWR:
 247                 tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 248                 break;
 249         case CA_EVENT_LOSS:
 250                 tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
 251                 /* Update RTT_min when next ack arrives */
 252                 w->reset_rtt_min = 1;
 253                 break;
 254         default:
 255                 /* don't care */
 256                 break;
 257         }
 258 }
 259 
 260 /* Extract info for Tcp socket info provided via netlink. */
 261 static size_t tcp_westwood_info(struct sock *sk, u32 ext, int *attr,
 262                                 union tcp_cc_info *info)
 263 {
 264         const struct westwood *ca = inet_csk_ca(sk);
 265 
 266         if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
 267                 info->vegas.tcpv_enabled = 1;
 268                 info->vegas.tcpv_rttcnt = 0;
 269                 info->vegas.tcpv_rtt    = jiffies_to_usecs(ca->rtt);
 270                 info->vegas.tcpv_minrtt = jiffies_to_usecs(ca->rtt_min);
 271 
 272                 *attr = INET_DIAG_VEGASINFO;
 273                 return sizeof(struct tcpvegas_info);
 274         }
 275         return 0;
 276 }
 277 
 278 static struct tcp_congestion_ops tcp_westwood __read_mostly = {
 279         .init           = tcp_westwood_init,
 280         .ssthresh       = tcp_reno_ssthresh,
 281         .cong_avoid     = tcp_reno_cong_avoid,
 282         .undo_cwnd      = tcp_reno_undo_cwnd,
 283         .cwnd_event     = tcp_westwood_event,
 284         .in_ack_event   = tcp_westwood_ack,
 285         .get_info       = tcp_westwood_info,
 286         .pkts_acked     = tcp_westwood_pkts_acked,
 287 
 288         .owner          = THIS_MODULE,
 289         .name           = "westwood"
 290 };
 291 
 292 static int __init tcp_westwood_register(void)
 293 {
 294         BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
 295         return tcp_register_congestion_control(&tcp_westwood);
 296 }
 297 
 298 static void __exit tcp_westwood_unregister(void)
 299 {
 300         tcp_unregister_congestion_control(&tcp_westwood);
 301 }
 302 
 303 module_init(tcp_westwood_register);
 304 module_exit(tcp_westwood_unregister);
 305 
 306 MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
 307 MODULE_LICENSE("GPL");
 308 MODULE_DESCRIPTION("TCP Westwood+");