xref: /linux/net/ipv4/tcp_westwood.c (revision 4949009eb8d40a441dcddcd96e101e77d31cf1b2)
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, u32 cnt, s32 rtt)
103 {
104 	struct westwood *w = inet_csk_ca(sk);
105 
106 	if (rtt > 0)
107 		w->rtt = usecs_to_jiffies(rtt);
108 }
109 
110 /*
111  * @westwood_update_window
112  * It updates RTT evaluation window if it is the right moment to do
113  * it. If so it calls filter for evaluating bandwidth.
114  */
115 static void westwood_update_window(struct sock *sk)
116 {
117 	struct westwood *w = inet_csk_ca(sk);
118 	s32 delta = tcp_time_stamp - w->rtt_win_sx;
119 
120 	/* Initialize w->snd_una with the first acked sequence number in order
121 	 * to fix mismatch between tp->snd_una and w->snd_una for the first
122 	 * bandwidth sample
123 	 */
124 	if (w->first_ack) {
125 		w->snd_una = tcp_sk(sk)->snd_una;
126 		w->first_ack = 0;
127 	}
128 
129 	/*
130 	 * See if a RTT-window has passed.
131 	 * Be careful since if RTT is less than
132 	 * 50ms we don't filter but we continue 'building the sample'.
133 	 * This minimum limit was chosen since an estimation on small
134 	 * time intervals is better to avoid...
135 	 * Obviously on a LAN we reasonably will always have
136 	 * right_bound = left_bound + WESTWOOD_RTT_MIN
137 	 */
138 	if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) {
139 		westwood_filter(w, delta);
140 
141 		w->bk = 0;
142 		w->rtt_win_sx = tcp_time_stamp;
143 	}
144 }
145 
146 static inline void update_rtt_min(struct westwood *w)
147 {
148 	if (w->reset_rtt_min) {
149 		w->rtt_min = w->rtt;
150 		w->reset_rtt_min = 0;
151 	} else
152 		w->rtt_min = min(w->rtt, w->rtt_min);
153 }
154 
155 /*
156  * @westwood_fast_bw
157  * It is called when we are in fast path. In particular it is called when
158  * header prediction is successful. In such case in fact update is
159  * straight forward and doesn't need any particular care.
160  */
161 static inline void westwood_fast_bw(struct sock *sk)
162 {
163 	const struct tcp_sock *tp = tcp_sk(sk);
164 	struct westwood *w = inet_csk_ca(sk);
165 
166 	westwood_update_window(sk);
167 
168 	w->bk += tp->snd_una - w->snd_una;
169 	w->snd_una = tp->snd_una;
170 	update_rtt_min(w);
171 }
172 
173 /*
174  * @westwood_acked_count
175  * This function evaluates cumul_ack for evaluating bk in case of
176  * delayed or partial acks.
177  */
178 static inline u32 westwood_acked_count(struct sock *sk)
179 {
180 	const struct tcp_sock *tp = tcp_sk(sk);
181 	struct westwood *w = inet_csk_ca(sk);
182 
183 	w->cumul_ack = tp->snd_una - w->snd_una;
184 
185 	/* If cumul_ack is 0 this is a dupack since it's not moving
186 	 * tp->snd_una.
187 	 */
188 	if (!w->cumul_ack) {
189 		w->accounted += tp->mss_cache;
190 		w->cumul_ack = tp->mss_cache;
191 	}
192 
193 	if (w->cumul_ack > tp->mss_cache) {
194 		/* Partial or delayed ack */
195 		if (w->accounted >= w->cumul_ack) {
196 			w->accounted -= w->cumul_ack;
197 			w->cumul_ack = tp->mss_cache;
198 		} else {
199 			w->cumul_ack -= w->accounted;
200 			w->accounted = 0;
201 		}
202 	}
203 
204 	w->snd_una = tp->snd_una;
205 
206 	return w->cumul_ack;
207 }
208 
209 /*
210  * TCP Westwood
211  * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it
212  * in packets we use mss_cache). Rttmin is guaranteed to be >= 2
213  * so avoids ever returning 0.
214  */
215 static u32 tcp_westwood_bw_rttmin(const struct sock *sk)
216 {
217 	const struct tcp_sock *tp = tcp_sk(sk);
218 	const struct westwood *w = inet_csk_ca(sk);
219 
220 	return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2);
221 }
222 
223 static void tcp_westwood_ack(struct sock *sk, u32 ack_flags)
224 {
225 	if (ack_flags & CA_ACK_SLOWPATH) {
226 		struct westwood *w = inet_csk_ca(sk);
227 
228 		westwood_update_window(sk);
229 		w->bk += westwood_acked_count(sk);
230 
231 		update_rtt_min(w);
232 		return;
233 	}
234 
235 	westwood_fast_bw(sk);
236 }
237 
238 static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event)
239 {
240 	struct tcp_sock *tp = tcp_sk(sk);
241 	struct westwood *w = inet_csk_ca(sk);
242 
243 	switch (event) {
244 	case CA_EVENT_COMPLETE_CWR:
245 		tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
246 		break;
247 	case CA_EVENT_LOSS:
248 		tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk);
249 		/* Update RTT_min when next ack arrives */
250 		w->reset_rtt_min = 1;
251 		break;
252 	default:
253 		/* don't care */
254 		break;
255 	}
256 }
257 
258 /* Extract info for Tcp socket info provided via netlink. */
259 static void tcp_westwood_info(struct sock *sk, u32 ext,
260 			      struct sk_buff *skb)
261 {
262 	const struct westwood *ca = inet_csk_ca(sk);
263 
264 	if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
265 		struct tcpvegas_info info = {
266 			.tcpv_enabled = 1,
267 			.tcpv_rtt = jiffies_to_usecs(ca->rtt),
268 			.tcpv_minrtt = jiffies_to_usecs(ca->rtt_min),
269 		};
270 
271 		nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info);
272 	}
273 }
274 
275 static struct tcp_congestion_ops tcp_westwood __read_mostly = {
276 	.init		= tcp_westwood_init,
277 	.ssthresh	= tcp_reno_ssthresh,
278 	.cong_avoid	= tcp_reno_cong_avoid,
279 	.cwnd_event	= tcp_westwood_event,
280 	.in_ack_event	= tcp_westwood_ack,
281 	.get_info	= tcp_westwood_info,
282 	.pkts_acked	= tcp_westwood_pkts_acked,
283 
284 	.owner		= THIS_MODULE,
285 	.name		= "westwood"
286 };
287 
288 static int __init tcp_westwood_register(void)
289 {
290 	BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE);
291 	return tcp_register_congestion_control(&tcp_westwood);
292 }
293 
294 static void __exit tcp_westwood_unregister(void)
295 {
296 	tcp_unregister_congestion_control(&tcp_westwood);
297 }
298 
299 module_init(tcp_westwood_register);
300 module_exit(tcp_westwood_unregister);
301 
302 MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera");
303 MODULE_LICENSE("GPL");
304 MODULE_DESCRIPTION("TCP Westwood+");
305