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_jiffies32; 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_jiffies32 - 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_jiffies32; 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+"); 308