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/mm.h>
10#include <linux/module.h>
11#include <net/tcp.h>
12
13#define ALPHA_BASE	(1<<7)	/* 1.0 with shift << 7 */
14#define BETA_MIN	(1<<6)	/* 0.5 with shift << 7 */
15#define BETA_MAX	102	/* 0.8 with shift << 7 */
16
17static int use_rtt_scaling __read_mostly = 1;
18module_param(use_rtt_scaling, int, 0644);
19MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
20
21static int use_bandwidth_switch __read_mostly = 1;
22module_param(use_bandwidth_switch, int, 0644);
23MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
24
25struct htcp {
26	u32	alpha;		/* Fixed point arith, << 7 */
27	u8	beta;           /* Fixed point arith, << 7 */
28	u8	modeswitch;	/* Delay modeswitch
29				   until we had at least one congestion event */
30	u16	pkts_acked;
31	u32	packetcount;
32	u32	minRTT;
33	u32	maxRTT;
34	u32	last_cong;	/* Time since last congestion event end */
35	u32	undo_last_cong;
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
48static inline u32 htcp_cong_time(const struct htcp *ca)
49{
50	return jiffies - ca->last_cong;
51}
52
53static inline u32 htcp_ccount(const struct htcp *ca)
54{
55	return htcp_cong_time(ca) / ca->minRTT;
56}
57
58static inline void htcp_reset(struct htcp *ca)
59{
60	ca->undo_last_cong = ca->last_cong;
61	ca->undo_maxRTT = ca->maxRTT;
62	ca->undo_old_maxB = ca->old_maxB;
63
64	ca->last_cong = jiffies;
65}
66
67static u32 htcp_cwnd_undo(struct sock *sk)
68{
69	const struct tcp_sock *tp = tcp_sk(sk);
70	struct htcp *ca = inet_csk_ca(sk);
71
72	if (ca->undo_last_cong) {
73		ca->last_cong = ca->undo_last_cong;
74		ca->maxRTT = ca->undo_maxRTT;
75		ca->old_maxB = ca->undo_old_maxB;
76		ca->undo_last_cong = 0;
77	}
78
79	return max(tp->snd_cwnd, (tp->snd_ssthresh << 7) / ca->beta);
80}
81
82static inline void measure_rtt(struct sock *sk, u32 srtt)
83{
84	const struct inet_connection_sock *icsk = inet_csk(sk);
85	struct htcp *ca = inet_csk_ca(sk);
86
87	/* keep track of minimum RTT seen so far, minRTT is zero at first */
88	if (ca->minRTT > srtt || !ca->minRTT)
89		ca->minRTT = srtt;
90
91	/* max RTT */
92	if (icsk->icsk_ca_state == TCP_CA_Open) {
93		if (ca->maxRTT < ca->minRTT)
94			ca->maxRTT = ca->minRTT;
95		if (ca->maxRTT < srtt &&
96		    srtt <= ca->maxRTT + msecs_to_jiffies(20))
97			ca->maxRTT = srtt;
98	}
99}
100
101static void measure_achieved_throughput(struct sock *sk,
102					u32 pkts_acked, s32 rtt)
103{
104	const struct inet_connection_sock *icsk = inet_csk(sk);
105	const struct tcp_sock *tp = tcp_sk(sk);
106	struct htcp *ca = inet_csk_ca(sk);
107	u32 now = tcp_time_stamp;
108
109	if (icsk->icsk_ca_state == TCP_CA_Open)
110		ca->pkts_acked = pkts_acked;
111
112	if (rtt > 0)
113		measure_rtt(sk, usecs_to_jiffies(rtt));
114
115	if (!use_bandwidth_switch)
116		return;
117
118	/* achieved throughput calculations */
119	if (!((1 << icsk->icsk_ca_state) & (TCPF_CA_Open | TCPF_CA_Disorder))) {
120		ca->packetcount = 0;
121		ca->lasttime = now;
122		return;
123	}
124
125	ca->packetcount += pkts_acked;
126
127	if (ca->packetcount >= tp->snd_cwnd - (ca->alpha >> 7 ? : 1) &&
128	    now - ca->lasttime >= ca->minRTT &&
129	    ca->minRTT > 0) {
130		__u32 cur_Bi = ca->packetcount * HZ / (now - ca->lasttime);
131
132		if (htcp_ccount(ca) <= 3) {
133			/* just after backoff */
134			ca->minB = ca->maxB = ca->Bi = cur_Bi;
135		} else {
136			ca->Bi = (3 * ca->Bi + cur_Bi) / 4;
137			if (ca->Bi > ca->maxB)
138				ca->maxB = ca->Bi;
139			if (ca->minB > ca->maxB)
140				ca->minB = ca->maxB;
141		}
142		ca->packetcount = 0;
143		ca->lasttime = now;
144	}
145}
146
147static inline void htcp_beta_update(struct htcp *ca, u32 minRTT, u32 maxRTT)
148{
149	if (use_bandwidth_switch) {
150		u32 maxB = ca->maxB;
151		u32 old_maxB = ca->old_maxB;
152
153		ca->old_maxB = ca->maxB;
154		if (!between(5 * maxB, 4 * old_maxB, 6 * old_maxB)) {
155			ca->beta = BETA_MIN;
156			ca->modeswitch = 0;
157			return;
158		}
159	}
160
161	if (ca->modeswitch && minRTT > msecs_to_jiffies(10) && maxRTT) {
162		ca->beta = (minRTT << 7) / maxRTT;
163		if (ca->beta < BETA_MIN)
164			ca->beta = BETA_MIN;
165		else if (ca->beta > BETA_MAX)
166			ca->beta = BETA_MAX;
167	} else {
168		ca->beta = BETA_MIN;
169		ca->modeswitch = 1;
170	}
171}
172
173static inline void htcp_alpha_update(struct htcp *ca)
174{
175	u32 minRTT = ca->minRTT;
176	u32 factor = 1;
177	u32 diff = htcp_cong_time(ca);
178
179	if (diff > HZ) {
180		diff -= HZ;
181		factor = 1 + (10 * diff + ((diff / 2) * (diff / 2) / HZ)) / HZ;
182	}
183
184	if (use_rtt_scaling && minRTT) {
185		u32 scale = (HZ << 3) / (10 * minRTT);
186
187		/* clamping ratio to interval [0.5,10]<<3 */
188		scale = min(max(scale, 1U << 2), 10U << 3);
189		factor = (factor << 3) / scale;
190		if (!factor)
191			factor = 1;
192	}
193
194	ca->alpha = 2 * factor * ((1 << 7) - ca->beta);
195	if (!ca->alpha)
196		ca->alpha = ALPHA_BASE;
197}
198
199/*
200 * After we have the rtt data to calculate beta, we'd still prefer to wait one
201 * rtt before we adjust our beta to ensure we are working from a consistent
202 * data.
203 *
204 * This function should be called when we hit a congestion event since only at
205 * that point do we really have a real sense of maxRTT (the queues en route
206 * were getting just too full now).
207 */
208static void htcp_param_update(struct sock *sk)
209{
210	struct htcp *ca = inet_csk_ca(sk);
211	u32 minRTT = ca->minRTT;
212	u32 maxRTT = ca->maxRTT;
213
214	htcp_beta_update(ca, minRTT, maxRTT);
215	htcp_alpha_update(ca);
216
217	/* add slowly fading memory for maxRTT to accommodate routing changes */
218	if (minRTT > 0 && maxRTT > minRTT)
219		ca->maxRTT = minRTT + ((maxRTT - minRTT) * 95) / 100;
220}
221
222static u32 htcp_recalc_ssthresh(struct sock *sk)
223{
224	const struct tcp_sock *tp = tcp_sk(sk);
225	const struct htcp *ca = inet_csk_ca(sk);
226
227	htcp_param_update(sk);
228	return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
229}
230
231static void htcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)
232{
233	struct tcp_sock *tp = tcp_sk(sk);
234	struct htcp *ca = inet_csk_ca(sk);
235
236	if (!tcp_is_cwnd_limited(sk))
237		return;
238
239	if (tp->snd_cwnd <= tp->snd_ssthresh)
240		tcp_slow_start(tp, acked);
241	else {
242		/* In dangerous area, increase slowly.
243		 * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
244		 */
245		if ((tp->snd_cwnd_cnt * ca->alpha)>>7 >= tp->snd_cwnd) {
246			if (tp->snd_cwnd < tp->snd_cwnd_clamp)
247				tp->snd_cwnd++;
248			tp->snd_cwnd_cnt = 0;
249			htcp_alpha_update(ca);
250		} else
251			tp->snd_cwnd_cnt += ca->pkts_acked;
252
253		ca->pkts_acked = 1;
254	}
255}
256
257static void htcp_init(struct sock *sk)
258{
259	struct htcp *ca = inet_csk_ca(sk);
260
261	memset(ca, 0, sizeof(struct htcp));
262	ca->alpha = ALPHA_BASE;
263	ca->beta = BETA_MIN;
264	ca->pkts_acked = 1;
265	ca->last_cong = jiffies;
266}
267
268static void htcp_state(struct sock *sk, u8 new_state)
269{
270	switch (new_state) {
271	case TCP_CA_Open:
272		{
273			struct htcp *ca = inet_csk_ca(sk);
274
275			if (ca->undo_last_cong) {
276				ca->last_cong = jiffies;
277				ca->undo_last_cong = 0;
278			}
279		}
280		break;
281	case TCP_CA_CWR:
282	case TCP_CA_Recovery:
283	case TCP_CA_Loss:
284		htcp_reset(inet_csk_ca(sk));
285		break;
286	}
287}
288
289static struct tcp_congestion_ops htcp __read_mostly = {
290	.init		= htcp_init,
291	.ssthresh	= htcp_recalc_ssthresh,
292	.cong_avoid	= htcp_cong_avoid,
293	.set_state	= htcp_state,
294	.undo_cwnd	= htcp_cwnd_undo,
295	.pkts_acked	= measure_achieved_throughput,
296	.owner		= THIS_MODULE,
297	.name		= "htcp",
298};
299
300static int __init htcp_register(void)
301{
302	BUILD_BUG_ON(sizeof(struct htcp) > ICSK_CA_PRIV_SIZE);
303	BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
304	return tcp_register_congestion_control(&htcp);
305}
306
307static void __exit htcp_unregister(void)
308{
309	tcp_unregister_congestion_control(&htcp);
310}
311
312module_init(htcp_register);
313module_exit(htcp_unregister);
314
315MODULE_AUTHOR("Baruch Even");
316MODULE_LICENSE("GPL");
317MODULE_DESCRIPTION("H-TCP");
318