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1/* DataCenter TCP (DCTCP) congestion control.
2 *
3 * http://simula.stanford.edu/~alizade/Site/DCTCP.html
4 *
5 * This is an implementation of DCTCP over Reno, an enhancement to the
6 * TCP congestion control algorithm designed for data centers. DCTCP
7 * leverages Explicit Congestion Notification (ECN) in the network to
8 * provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
9 * the following three data center transport requirements:
10 *
11 * - High burst tolerance (incast due to partition/aggregate)
12 * - Low latency (short flows, queries)
13 * - High throughput (continuous data updates, large file transfers)
14 * with commodity shallow buffered switches
15 *
16 * The algorithm is described in detail in the following two papers:
17 *
18 * 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
19 * Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
20 * "Data Center TCP (DCTCP)", Data Center Networks session
21 * Proc. ACM SIGCOMM, New Delhi, 2010.
22 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
23 *
24 * 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
25 * "Analysis of DCTCP: Stability, Convergence, and Fairness"
26 * Proc. ACM SIGMETRICS, San Jose, 2011.
27 * http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
28 *
29 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
30 *
31 * Authors:
32 *
33 * Daniel Borkmann <dborkman@redhat.com>
34 * Florian Westphal <fw@strlen.de>
35 * Glenn Judd <glenn.judd@morganstanley.com>
36 *
37 * This program is free software; you can redistribute it and/or modify
38 * it under the terms of the GNU General Public License as published by
39 * the Free Software Foundation; either version 2 of the License, or (at
40 * your option) any later version.
41 */
42
43#include <linux/module.h>
44#include <linux/mm.h>
45#include <net/tcp.h>
46#include <linux/inet_diag.h>
47
48#define DCTCP_MAX_ALPHA 1024U
49
50struct dctcp {
51 u32 acked_bytes_ecn;
52 u32 acked_bytes_total;
53 u32 prior_snd_una;
54 u32 prior_rcv_nxt;
55 u32 dctcp_alpha;
56 u32 next_seq;
57 u32 ce_state;
58 u32 delayed_ack_reserved;
59 u32 loss_cwnd;
60};
61
62static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
63module_param(dctcp_shift_g, uint, 0644);
64MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
65
66static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
67module_param(dctcp_alpha_on_init, uint, 0644);
68MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
69
70static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
71module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
72MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
73 "parameter for clamping alpha on loss");
74
75static struct tcp_congestion_ops dctcp_reno;
76
77static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
78{
79 ca->next_seq = tp->snd_nxt;
80
81 ca->acked_bytes_ecn = 0;
82 ca->acked_bytes_total = 0;
83}
84
85static void dctcp_init(struct sock *sk)
86{
87 const struct tcp_sock *tp = tcp_sk(sk);
88
89 if ((tp->ecn_flags & TCP_ECN_OK) ||
90 (sk->sk_state == TCP_LISTEN ||
91 sk->sk_state == TCP_CLOSE)) {
92 struct dctcp *ca = inet_csk_ca(sk);
93
94 ca->prior_snd_una = tp->snd_una;
95 ca->prior_rcv_nxt = tp->rcv_nxt;
96
97 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
98
99 ca->delayed_ack_reserved = 0;
100 ca->loss_cwnd = 0;
101 ca->ce_state = 0;
102
103 dctcp_reset(tp, ca);
104 return;
105 }
106
107 /* No ECN support? Fall back to Reno. Also need to clear
108 * ECT from sk since it is set during 3WHS for DCTCP.
109 */
110 inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
111 INET_ECN_dontxmit(sk);
112}
113
114static u32 dctcp_ssthresh(struct sock *sk)
115{
116 struct dctcp *ca = inet_csk_ca(sk);
117 struct tcp_sock *tp = tcp_sk(sk);
118
119 ca->loss_cwnd = tp->snd_cwnd;
120 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
121}
122
123/* Minimal DCTP CE state machine:
124 *
125 * S: 0 <- last pkt was non-CE
126 * 1 <- last pkt was CE
127 */
128
129static void dctcp_ce_state_0_to_1(struct sock *sk)
130{
131 struct dctcp *ca = inet_csk_ca(sk);
132 struct tcp_sock *tp = tcp_sk(sk);
133
134 /* State has changed from CE=0 to CE=1 and delayed
135 * ACK has not sent yet.
136 */
137 if (!ca->ce_state && ca->delayed_ack_reserved) {
138 u32 tmp_rcv_nxt;
139
140 /* Save current rcv_nxt. */
141 tmp_rcv_nxt = tp->rcv_nxt;
142
143 /* Generate previous ack with CE=0. */
144 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
145 tp->rcv_nxt = ca->prior_rcv_nxt;
146
147 tcp_send_ack(sk);
148
149 /* Recover current rcv_nxt. */
150 tp->rcv_nxt = tmp_rcv_nxt;
151 }
152
153 ca->prior_rcv_nxt = tp->rcv_nxt;
154 ca->ce_state = 1;
155
156 tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
157}
158
159static void dctcp_ce_state_1_to_0(struct sock *sk)
160{
161 struct dctcp *ca = inet_csk_ca(sk);
162 struct tcp_sock *tp = tcp_sk(sk);
163
164 /* State has changed from CE=1 to CE=0 and delayed
165 * ACK has not sent yet.
166 */
167 if (ca->ce_state && ca->delayed_ack_reserved) {
168 u32 tmp_rcv_nxt;
169
170 /* Save current rcv_nxt. */
171 tmp_rcv_nxt = tp->rcv_nxt;
172
173 /* Generate previous ack with CE=1. */
174 tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
175 tp->rcv_nxt = ca->prior_rcv_nxt;
176
177 tcp_send_ack(sk);
178
179 /* Recover current rcv_nxt. */
180 tp->rcv_nxt = tmp_rcv_nxt;
181 }
182
183 ca->prior_rcv_nxt = tp->rcv_nxt;
184 ca->ce_state = 0;
185
186 tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
187}
188
189static void dctcp_update_alpha(struct sock *sk, u32 flags)
190{
191 const struct tcp_sock *tp = tcp_sk(sk);
192 struct dctcp *ca = inet_csk_ca(sk);
193 u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
194
195 /* If ack did not advance snd_una, count dupack as MSS size.
196 * If ack did update window, do not count it at all.
197 */
198 if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
199 acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
200 if (acked_bytes) {
201 ca->acked_bytes_total += acked_bytes;
202 ca->prior_snd_una = tp->snd_una;
203
204 if (flags & CA_ACK_ECE)
205 ca->acked_bytes_ecn += acked_bytes;
206 }
207
208 /* Expired RTT */
209 if (!before(tp->snd_una, ca->next_seq)) {
210 u64 bytes_ecn = ca->acked_bytes_ecn;
211 u32 alpha = ca->dctcp_alpha;
212
213 /* alpha = (1 - g) * alpha + g * F */
214
215 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
216 if (bytes_ecn) {
217 /* If dctcp_shift_g == 1, a 32bit value would overflow
218 * after 8 Mbytes.
219 */
220 bytes_ecn <<= (10 - dctcp_shift_g);
221 do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
222
223 alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
224 }
225 /* dctcp_alpha can be read from dctcp_get_info() without
226 * synchro, so we ask compiler to not use dctcp_alpha
227 * as a temporary variable in prior operations.
228 */
229 WRITE_ONCE(ca->dctcp_alpha, alpha);
230 dctcp_reset(tp, ca);
231 }
232}
233
234static void dctcp_state(struct sock *sk, u8 new_state)
235{
236 if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
237 struct dctcp *ca = inet_csk_ca(sk);
238
239 /* If this extension is enabled, we clamp dctcp_alpha to
240 * max on packet loss; the motivation is that dctcp_alpha
241 * is an indicator to the extend of congestion and packet
242 * loss is an indicator of extreme congestion; setting
243 * this in practice turned out to be beneficial, and
244 * effectively assumes total congestion which reduces the
245 * window by half.
246 */
247 ca->dctcp_alpha = DCTCP_MAX_ALPHA;
248 }
249}
250
251static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
252{
253 struct dctcp *ca = inet_csk_ca(sk);
254
255 switch (ev) {
256 case CA_EVENT_DELAYED_ACK:
257 if (!ca->delayed_ack_reserved)
258 ca->delayed_ack_reserved = 1;
259 break;
260 case CA_EVENT_NON_DELAYED_ACK:
261 if (ca->delayed_ack_reserved)
262 ca->delayed_ack_reserved = 0;
263 break;
264 default:
265 /* Don't care for the rest. */
266 break;
267 }
268}
269
270static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
271{
272 switch (ev) {
273 case CA_EVENT_ECN_IS_CE:
274 dctcp_ce_state_0_to_1(sk);
275 break;
276 case CA_EVENT_ECN_NO_CE:
277 dctcp_ce_state_1_to_0(sk);
278 break;
279 case CA_EVENT_DELAYED_ACK:
280 case CA_EVENT_NON_DELAYED_ACK:
281 dctcp_update_ack_reserved(sk, ev);
282 break;
283 default:
284 /* Don't care for the rest. */
285 break;
286 }
287}
288
289static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
290 union tcp_cc_info *info)
291{
292 const struct dctcp *ca = inet_csk_ca(sk);
293
294 /* Fill it also in case of VEGASINFO due to req struct limits.
295 * We can still correctly retrieve it later.
296 */
297 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
298 ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
299 memset(&info->dctcp, 0, sizeof(info->dctcp));
300 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
301 info->dctcp.dctcp_enabled = 1;
302 info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
303 info->dctcp.dctcp_alpha = ca->dctcp_alpha;
304 info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
305 info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
306 }
307
308 *attr = INET_DIAG_DCTCPINFO;
309 return sizeof(info->dctcp);
310 }
311 return 0;
312}
313
314static u32 dctcp_cwnd_undo(struct sock *sk)
315{
316 const struct dctcp *ca = inet_csk_ca(sk);
317
318 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
319}
320
321static struct tcp_congestion_ops dctcp __read_mostly = {
322 .init = dctcp_init,
323 .in_ack_event = dctcp_update_alpha,
324 .cwnd_event = dctcp_cwnd_event,
325 .ssthresh = dctcp_ssthresh,
326 .cong_avoid = tcp_reno_cong_avoid,
327 .undo_cwnd = dctcp_cwnd_undo,
328 .set_state = dctcp_state,
329 .get_info = dctcp_get_info,
330 .flags = TCP_CONG_NEEDS_ECN,
331 .owner = THIS_MODULE,
332 .name = "dctcp",
333};
334
335static struct tcp_congestion_ops dctcp_reno __read_mostly = {
336 .ssthresh = tcp_reno_ssthresh,
337 .cong_avoid = tcp_reno_cong_avoid,
338 .undo_cwnd = tcp_reno_undo_cwnd,
339 .get_info = dctcp_get_info,
340 .owner = THIS_MODULE,
341 .name = "dctcp-reno",
342};
343
344static int __init dctcp_register(void)
345{
346 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
347 return tcp_register_congestion_control(&dctcp);
348}
349
350static void __exit dctcp_unregister(void)
351{
352 tcp_unregister_congestion_control(&dctcp);
353}
354
355module_init(dctcp_register);
356module_exit(dctcp_unregister);
357
358MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
359MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
360MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
361
362MODULE_LICENSE("GPL v2");
363MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");