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Linux/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         .cwnd_event     = tcp_westwood_event,
282         .in_ack_event   = tcp_westwood_ack,
283         .get_info       = tcp_westwood_info,
284         .pkts_acked     = tcp_westwood_pkts_acked,
285 
286         .owner          = THIS_MODULE,
287         .name           = "westwood"
288 };
289 
290 static int __init tcp_westwood_register(void)
291 {
292         BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
293         return tcp_register_congestion_control(&tcp_westwood);
294 }
295 
296 static void __exit tcp_westwood_unregister(void)
297 {
298         tcp_unregister_congestion_control(&tcp_westwood);
299 }
300 
301 module_init(tcp_westwood_register);
302 module_exit(tcp_westwood_unregister);
303 
304 MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
305 MODULE_LICENSE("GPL");
306 MODULE_DESCRIPTION("TCP Westwood+");
307 

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