1 /* DataCenter TCP (DCTCP) congestion control.
3 * http://simula.stanford.edu/~alizade/Site/DCTCP.html
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:
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
16 * The algorithm is described in detail in the following two papers:
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
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
29 * Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
33 * Daniel Borkmann <dborkman@redhat.com>
34 * Florian Westphal <fw@strlen.de>
35 * Glenn Judd <glenn.judd@morganstanley.com>
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.
43 #include <linux/module.h>
46 #include <linux/inet_diag.h>
47 #include "tcp_dctcp.h"
49 #define DCTCP_MAX_ALPHA 1024U
61 static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
62 module_param(dctcp_shift_g, uint, 0644);
63 MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
65 static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
66 module_param(dctcp_alpha_on_init, uint, 0644);
67 MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
69 static struct tcp_congestion_ops dctcp_reno;
71 static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
73 ca->next_seq = tp->snd_nxt;
75 ca->old_delivered = tp->delivered;
76 ca->old_delivered_ce = tp->delivered_ce;
79 static void dctcp_init(struct sock *sk)
81 const struct tcp_sock *tp = tcp_sk(sk);
83 if ((tp->ecn_flags & TCP_ECN_OK) ||
84 (sk->sk_state == TCP_LISTEN ||
85 sk->sk_state == TCP_CLOSE)) {
86 struct dctcp *ca = inet_csk_ca(sk);
88 ca->prior_rcv_nxt = tp->rcv_nxt;
90 ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
99 /* No ECN support? Fall back to Reno. Also need to clear
100 * ECT from sk since it is set during 3WHS for DCTCP.
102 inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
103 INET_ECN_dontxmit(sk);
106 static u32 dctcp_ssthresh(struct sock *sk)
108 struct dctcp *ca = inet_csk_ca(sk);
109 struct tcp_sock *tp = tcp_sk(sk);
111 ca->loss_cwnd = tp->snd_cwnd;
112 return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
115 static void dctcp_update_alpha(struct sock *sk, u32 flags)
117 const struct tcp_sock *tp = tcp_sk(sk);
118 struct dctcp *ca = inet_csk_ca(sk);
121 if (!before(tp->snd_una, ca->next_seq)) {
122 u32 delivered_ce = tp->delivered_ce - ca->old_delivered_ce;
123 u32 alpha = ca->dctcp_alpha;
125 /* alpha = (1 - g) * alpha + g * F */
127 alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
129 u32 delivered = tp->delivered - ca->old_delivered;
131 /* If dctcp_shift_g == 1, a 32bit value would overflow
134 delivered_ce <<= (10 - dctcp_shift_g);
135 delivered_ce /= max(1U, delivered);
137 alpha = min(alpha + delivered_ce, DCTCP_MAX_ALPHA);
139 /* dctcp_alpha can be read from dctcp_get_info() without
140 * synchro, so we ask compiler to not use dctcp_alpha
141 * as a temporary variable in prior operations.
143 WRITE_ONCE(ca->dctcp_alpha, alpha);
148 static void dctcp_react_to_loss(struct sock *sk)
150 struct dctcp *ca = inet_csk_ca(sk);
151 struct tcp_sock *tp = tcp_sk(sk);
153 ca->loss_cwnd = tp->snd_cwnd;
154 tp->snd_ssthresh = max(tp->snd_cwnd >> 1U, 2U);
157 static void dctcp_state(struct sock *sk, u8 new_state)
159 if (new_state == TCP_CA_Recovery &&
160 new_state != inet_csk(sk)->icsk_ca_state)
161 dctcp_react_to_loss(sk);
162 /* We handle RTO in dctcp_cwnd_event to ensure that we perform only
163 * one loss-adjustment per RTT.
167 static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
169 struct dctcp *ca = inet_csk_ca(sk);
172 case CA_EVENT_ECN_IS_CE:
173 case CA_EVENT_ECN_NO_CE:
174 dctcp_ece_ack_update(sk, ev, &ca->prior_rcv_nxt, &ca->ce_state);
177 dctcp_react_to_loss(sk);
180 /* Don't care for the rest. */
185 static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
186 union tcp_cc_info *info)
188 const struct dctcp *ca = inet_csk_ca(sk);
189 const struct tcp_sock *tp = tcp_sk(sk);
191 /* Fill it also in case of VEGASINFO due to req struct limits.
192 * We can still correctly retrieve it later.
194 if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
195 ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
196 memset(&info->dctcp, 0, sizeof(info->dctcp));
197 if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
198 info->dctcp.dctcp_enabled = 1;
199 info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
200 info->dctcp.dctcp_alpha = ca->dctcp_alpha;
201 info->dctcp.dctcp_ab_ecn = tp->mss_cache *
202 (tp->delivered_ce - ca->old_delivered_ce);
203 info->dctcp.dctcp_ab_tot = tp->mss_cache *
204 (tp->delivered - ca->old_delivered);
207 *attr = INET_DIAG_DCTCPINFO;
208 return sizeof(info->dctcp);
213 static u32 dctcp_cwnd_undo(struct sock *sk)
215 const struct dctcp *ca = inet_csk_ca(sk);
217 return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
220 static struct tcp_congestion_ops dctcp __read_mostly = {
222 .in_ack_event = dctcp_update_alpha,
223 .cwnd_event = dctcp_cwnd_event,
224 .ssthresh = dctcp_ssthresh,
225 .cong_avoid = tcp_reno_cong_avoid,
226 .undo_cwnd = dctcp_cwnd_undo,
227 .set_state = dctcp_state,
228 .get_info = dctcp_get_info,
229 .flags = TCP_CONG_NEEDS_ECN,
230 .owner = THIS_MODULE,
234 static struct tcp_congestion_ops dctcp_reno __read_mostly = {
235 .ssthresh = tcp_reno_ssthresh,
236 .cong_avoid = tcp_reno_cong_avoid,
237 .undo_cwnd = tcp_reno_undo_cwnd,
238 .get_info = dctcp_get_info,
239 .owner = THIS_MODULE,
240 .name = "dctcp-reno",
243 static int __init dctcp_register(void)
245 BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
246 return tcp_register_congestion_control(&dctcp);
249 static void __exit dctcp_unregister(void)
251 tcp_unregister_congestion_control(&dctcp);
254 module_init(dctcp_register);
255 module_exit(dctcp_unregister);
257 MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
258 MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
259 MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
261 MODULE_LICENSE("GPL v2");
262 MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");