net/ipv4/tcp_htcp.c

   1 /*
   2  * H-TCP congestion control. The algorithm is detailed in:
   3  * R.N.Shorten, D.J.Leith:
   4  *   "H-TCP: TCP for high-speed and long-distance networks"
   5  *   Proc. PFLDnet, Argonne, 2004.
   6  * http://www.hamilton.ie/net/htcp3.pdf
   7  */
   8
   9 #include <linux/config.h>
  10 #include <linux/mm.h>
  11 #include <linux/module.h>
  12 #include <net/tcp.h>
  13
  14 #define ALPHA_BASE      (1<<7)  /* 1.0 with shift << 7 */
  15 #define BETA_MIN        (1<<6)  /* 0.5 with shift << 7 */
  16 #define BETA_MAX        102     /* 0.8 with shift << 7 */
  17
  18 static int use_rtt_scaling = 1;
  19 module_param(use_rtt_scaling, int, 0644);
  20 MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
  21
  22 static int use_bandwidth_switch = 1;
  23 module_param(use_bandwidth_switch, int, 0644);
  24 MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
  25
  26 struct htcp {
  27         u16     alpha;          /* Fixed point arith, << 7 */
  28         u8      beta;           /* Fixed point arith, << 7 */
  29         u8      modeswitch;     /* Delay modeswitch until we had at least one congestion event */
  30         u8      ccount;         /* Number of RTTs since last congestion event */
  31         u8      undo_ccount;
  32         u16     packetcount;
  33         u32     minRTT;
  34         u32     maxRTT;
  35         u32     snd_cwnd_cnt2;
  36
  37         u32     undo_maxRTT;
  38         u32     undo_old_maxB;
  39
  40         /* Bandwidth estimation */
  41         u32     minB;
  42         u32     maxB;
  43         u32     old_maxB;
  44         u32     Bi;
  45         u32     lasttime;
  46 };
  47
  48 static inline void htcp_reset(struct htcp *ca)
  49 {
  50         ca->undo_ccount = ca->ccount;
  51         ca->undo_maxRTT = ca->maxRTT;
  52         ca->undo_old_maxB = ca->old_maxB;
  53
  54         ca->ccount = 0;
  55         ca->snd_cwnd_cnt2 = 0;
  56 }
  57
  58 static u32 htcp_cwnd_undo(struct tcp_sock *tp)
  59 {
  60         struct htcp *ca = tcp_ca(tp);
  61         ca->ccount = ca->undo_ccount;
  62         ca->maxRTT = ca->undo_maxRTT;
  63         ca->old_maxB = ca->undo_old_maxB;
  64         return max(tp->snd_cwnd, (tp->snd_ssthresh<<7)/ca->beta);
  65 }
  66
  67 static inline void measure_rtt(struct tcp_sock *tp)
  68 {
  69         struct htcp *ca = tcp_ca(tp);
  70         u32 srtt = tp->srtt>>3;
  71
  72         /* keep track of minimum RTT seen so far, minRTT is zero at first */
  73         if (ca->minRTT > srtt || !ca->minRTT)
  74                 ca->minRTT = srtt;
  75
  76         /* max RTT */
  77         if (tp->ca_state == TCP_CA_Open && tp->snd_ssthresh < 0xFFFF && ca->ccount > 3) {
  78                 if (ca->maxRTT < ca->minRTT)
  79                         ca->maxRTT = ca->minRTT;
  80                 if (ca->maxRTT < srtt && srtt <= ca->maxRTT+HZ/50)
  81                         ca->maxRTT = srtt;
  82         }
  83 }
  84
  85 static void measure_achieved_throughput(struct tcp_sock *tp, u32 pkts_acked)
  86 {
  87         struct htcp *ca = tcp_ca(tp);
  88         u32 now = tcp_time_stamp;
  89
  90         /* achieved throughput calculations */
  91         if (tp->ca_state != TCP_CA_Open && tp->ca_state != TCP_CA_Disorder) {
  92                 ca->packetcount = 0;
  93                 ca->lasttime = now;
  94                 return;
  95         }
  96
  97         ca->packetcount += pkts_acked;
  98
  99         if (ca->packetcount >= tp->snd_cwnd - (ca->alpha>>7? : 1)
 100                         && now - ca->lasttime >= ca->minRTT
 101                         && ca->minRTT > 0) {
 102                 __u32 cur_Bi = ca->packetcount*HZ/(now - ca->lasttime);
 103                 if (ca->ccount <= 3) {
 104                         /* just after backoff */
 105                         ca->minB = ca->maxB = ca->Bi = cur_Bi;
 106                 } else {
 107                         ca->Bi = (3*ca->Bi + cur_Bi)/4;
 108                         if (ca->Bi > ca->maxB)
 109                                 ca->maxB = ca->Bi;
 110                         if (ca->minB > ca->maxB)
 111                                 ca->minB = ca->maxB;
 112                 }
 113                 ca->packetcount = 0;
 114                 ca->lasttime = now;
 115         }
 116 }
 117
 118 static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
 119 {
 120         if (use_bandwidth_switch) {
 121                 u32 maxB = ca->maxB;
 122                 u32 old_maxB = ca->old_maxB;
 123                 ca->old_maxB = ca->maxB;
 124
 125                 if (!between(5*maxB, 4*old_maxB, 6*old_maxB)) {
 126                         ca->beta = BETA_MIN;
 127                         ca->modeswitch = 0;
 128                         return;
 129                 }
 130         }
 131
 132         if (ca->modeswitch && minRTT > max(HZ/100, 1) && maxRTT) {
 133                 ca->beta = (minRTT<<7)/maxRTT;
 134                 if (ca->beta < BETA_MIN)
 135                         ca->beta = BETA_MIN;
 136                 else if (ca->beta > BETA_MAX)
 137                         ca->beta = BETA_MAX;
 138         } else {
 139                 ca->beta = BETA_MIN;
 140                 ca->modeswitch = 1;
 141         }
 142 }
 143
 144 static inline void htcp_alpha_update(struct htcp *ca)
 145 {
 146         u32 minRTT = ca->minRTT;
 147         u32 factor = 1;
 148         u32 diff = ca->ccount * minRTT; /* time since last backoff */
 149
 150         if (diff > HZ) {
 151                 diff -= HZ;
 152                 factor = 1+ ( 10*diff + ((diff/2)*(diff/2)/HZ) )/HZ;
 153         }
 154
 155         if (use_rtt_scaling && minRTT) {
 156                 u32 scale = (HZ<<3)/(10*minRTT);
 157                 scale = min(max(scale, 1U<<2), 10U<<3); /* clamping ratio to interval [0.5,10]<<3 */
 158                 factor = (factor<<3)/scale;
 159                 if (!factor)
 160                         factor = 1;
 161         }
 162
 163         ca->alpha = 2*factor*((1<<7)-ca->beta);
 164         if (!ca->alpha)
 165                 ca->alpha = ALPHA_BASE;
 166 }
 167
 168 /* After we have the rtt data to calculate beta, we'd still prefer to wait one
 169  * rtt before we adjust our beta to ensure we are working from a consistent
 170  * data.
 171  *
 172  * This function should be called when we hit a congestion event since only at
 173  * that point do we really have a real sense of maxRTT (the queues en route
 174  * were getting just too full now).
 175  */
 176 static void htcp_param_update(struct tcp_sock *tp)
 177 {
 178         struct htcp *ca = tcp_ca(tp);
 179         u32 minRTT = ca->minRTT;
 180         u32 maxRTT = ca->maxRTT;
 181
 182         htcp_beta_update(ca, minRTT, maxRTT);
 183         htcp_alpha_update(ca);
 184
 185         /* add slowly fading memory for maxRTT to accommodate routing changes etc */
 186         if (minRTT > 0 && maxRTT > minRTT)
 187                 ca->maxRTT = minRTT + ((maxRTT-minRTT)*95)/100;
 188 }
 189
 190 static u32 htcp_recalc_ssthresh(struct tcp_sock *tp)
 191 {
 192         struct htcp *ca = tcp_ca(tp);
 193         htcp_param_update(tp);
 194         return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
 195 }
 196
 197 static void htcp_cong_avoid(struct tcp_sock *tp, u32 ack, u32 rtt,
 198                             u32 in_flight, int data_acked)
 199 {
 200         struct htcp *ca = tcp_ca(tp);
 201
 202         if (in_flight < tp->snd_cwnd)
 203                 return;
 204
 205         if (tp->snd_cwnd <= tp->snd_ssthresh) {
 206                 /* In "safe" area, increase. */
 207                 if (tp->snd_cwnd < tp->snd_cwnd_clamp)
 208                         tp->snd_cwnd++;
 209         } else {
 210                 measure_rtt(tp);
 211
 212                 /* keep track of number of round-trip times since last backoff event */
 213                 if (ca->snd_cwnd_cnt2++ > tp->snd_cwnd) {
 214                         ca->ccount++;
 215                         ca->snd_cwnd_cnt2 = 0;
 216                         htcp_alpha_update(ca);
 217                 }
 218
 219                 /* In dangerous area, increase slowly.
 220                  * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
 221                  */
 222                 if ((tp->snd_cwnd_cnt++ * ca->alpha)>>7 >= tp->snd_cwnd) {
 223                         if (tp->snd_cwnd < tp->snd_cwnd_clamp)
 224                                 tp->snd_cwnd++;
 225                         tp->snd_cwnd_cnt = 0;
 226                         ca->ccount++;
 227                 }
 228         }
 229 }
 230
 231 /* Lower bound on congestion window. */
 232 static u32 htcp_min_cwnd(struct tcp_sock *tp)
 233 {
 234         return tp->snd_ssthresh;
 235 }
 236
 237
 238 static void htcp_init(struct tcp_sock *tp)
 239 {
 240         struct htcp *ca = tcp_ca(tp);
 241
 242         memset(ca, 0, sizeof(struct htcp));
 243         ca->alpha = ALPHA_BASE;
 244         ca->beta = BETA_MIN;
 245 }
 246
 247 static void htcp_state(struct tcp_sock *tp, u8 new_state)
 248 {
 249         switch (new_state) {
 250         case TCP_CA_CWR:
 251         case TCP_CA_Recovery:
 252         case TCP_CA_Loss:
 253                 htcp_reset(tcp_ca(tp));
 254                 break;
 255         }
 256 }
 257
 258 static struct tcp_congestion_ops htcp = {
 259         .init           = htcp_init,
 260         .ssthresh       = htcp_recalc_ssthresh,
 261         .min_cwnd       = htcp_min_cwnd,
 262         .cong_avoid     = htcp_cong_avoid,
 263         .set_state      = htcp_state,
 264         .undo_cwnd      = htcp_cwnd_undo,
 265         .pkts_acked     = measure_achieved_throughput,
 266         .owner          = THIS_MODULE,
 267         .name           = "htcp",
 268 };
 269
 270 static int __init htcp_register(void)
 271 {
 272         BUG_ON(sizeof(struct htcp) > TCP_CA_PRIV_SIZE);
 273         BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
 274         if (!use_bandwidth_switch)
 275                 htcp.pkts_acked = NULL;
 276         return tcp_register_congestion_control(&htcp);
 277 }
 278
 279 static void __exit htcp_unregister(void)
 280 {
 281         tcp_unregister_congestion_control(&htcp);
 282 }
 283
 284 module_init(htcp_register);
 285 module_exit(htcp_unregister);
 286
 287 MODULE_AUTHOR("Baruch Even");
 288 MODULE_LICENSE("GPL");
 289 MODULE_DESCRIPTION("H-TCP");