/* DataCenter TCP (DCTCP) congestion control.
*
* http://simula.stanford.edu/~alizade/Site/DCTCP.html
*
* This is an implementation of DCTCP over Reno, an enhancement to the
* TCP congestion control algorithm designed for data centers. DCTCP
* leverages Explicit Congestion Notification (ECN) in the network to
* provide multi-bit feedback to the end hosts. DCTCP's goal is to meet
* the following three data center transport requirements:
*
* - High burst tolerance (incast due to partition/aggregate)
* - Low latency (short flows, queries)
* - High throughput (continuous data updates, large file transfers)
* with commodity shallow buffered switches
*
* The algorithm is described in detail in the following two papers:
*
* 1) Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye,
* Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan:
* "Data Center TCP (DCTCP)", Data Center Networks session
* Proc. ACM SIGCOMM, New Delhi, 2010.
* http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp-final.pdf
*
* 2) Mohammad Alizadeh, Adel Javanmard, and Balaji Prabhakar:
* "Analysis of DCTCP: Stability, Convergence, and Fairness"
* Proc. ACM SIGMETRICS, San Jose, 2011.
* http://simula.stanford.edu/~alizade/Site/DCTCP_files/dctcp_analysis-full.pdf
*
* Initial prototype from Abdul Kabbani, Masato Yasuda and Mohammad Alizadeh.
*
* Authors:
*
* Daniel Borkmann <dborkman@redhat.com>
* Florian Westphal <fw@strlen.de>
* Glenn Judd <glenn.judd@morganstanley.com>
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or (at
* your option) any later version.
*/
#include <linux/module.h>
#include <linux/mm.h>
#include <net/tcp.h>
#include <linux/inet_diag.h>
#define DCTCP_MAX_ALPHA 1024U
struct dctcp {
u32 acked_bytes_ecn;
u32 acked_bytes_total;
u32 prior_snd_una;
u32 prior_rcv_nxt;
u32 dctcp_alpha;
u32 next_seq;
u32 ce_state;
u32 delayed_ack_reserved;
u32 loss_cwnd;
};
static unsigned int dctcp_shift_g __read_mostly = 4; /* g = 1/2^4 */
module_param(dctcp_shift_g, uint, 0644);
MODULE_PARM_DESC(dctcp_shift_g, "parameter g for updating dctcp_alpha");
static unsigned int dctcp_alpha_on_init __read_mostly = DCTCP_MAX_ALPHA;
module_param(dctcp_alpha_on_init, uint, 0644);
MODULE_PARM_DESC(dctcp_alpha_on_init, "parameter for initial alpha value");
static unsigned int dctcp_clamp_alpha_on_loss __read_mostly;
module_param(dctcp_clamp_alpha_on_loss, uint, 0644);
MODULE_PARM_DESC(dctcp_clamp_alpha_on_loss,
"parameter for clamping alpha on loss");
static struct tcp_congestion_ops dctcp_reno;
static void dctcp_reset(const struct tcp_sock *tp, struct dctcp *ca)
{
ca->next_seq = tp->snd_nxt;
ca->acked_bytes_ecn = 0;
ca->acked_bytes_total = 0;
}
static void dctcp_init(struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
if ((tp->ecn_flags & TCP_ECN_OK) ||
(sk->sk_state == TCP_LISTEN ||
sk->sk_state == TCP_CLOSE)) {
struct dctcp *ca = inet_csk_ca(sk);
ca->prior_snd_una = tp->snd_una;
ca->prior_rcv_nxt = tp->rcv_nxt;
ca->dctcp_alpha = min(dctcp_alpha_on_init, DCTCP_MAX_ALPHA);
ca->delayed_ack_reserved = 0;
ca->loss_cwnd = 0;
ca->ce_state = 0;
dctcp_reset(tp, ca);
return;
}
/* No ECN support? Fall back to Reno. Also need to clear
* ECT from sk since it is set during 3WHS for DCTCP.
*/
inet_csk(sk)->icsk_ca_ops = &dctcp_reno;
INET_ECN_dontxmit(sk);
}
static u32 dctcp_ssthresh(struct sock *sk)
{
struct dctcp *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
ca->loss_cwnd = tp->snd_cwnd;
return max(tp->snd_cwnd - ((tp->snd_cwnd * ca->dctcp_alpha) >> 11U), 2U);
}
/* Minimal DCTP CE state machine:
*
* S: 0 <- last pkt was non-CE
* 1 <- last pkt was CE
*/
static void dctcp_ce_state_0_to_1(struct sock *sk)
{
struct dctcp *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
/* State has changed from CE=0 to CE=1 and delayed
* ACK has not sent yet.
*/
if (!ca->ce_state && ca->delayed_ack_reserved) {
u32 tmp_rcv_nxt;
/* Save current rcv_nxt. */
tmp_rcv_nxt = tp->rcv_nxt;
/* Generate previous ack with CE=0. */
tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
tp->rcv_nxt = ca->prior_rcv_nxt;
tcp_send_ack(sk);
/* Recover current rcv_nxt. */
tp->rcv_nxt = tmp_rcv_nxt;
}
ca->prior_rcv_nxt = tp->rcv_nxt;
ca->ce_state = 1;
tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
}
static void dctcp_ce_state_1_to_0(struct sock *sk)
{
struct dctcp *ca = inet_csk_ca(sk);
struct tcp_sock *tp = tcp_sk(sk);
/* State has changed from CE=1 to CE=0 and delayed
* ACK has not sent yet.
*/
if (ca->ce_state && ca->delayed_ack_reserved) {
u32 tmp_rcv_nxt;
/* Save current rcv_nxt. */
tmp_rcv_nxt = tp->rcv_nxt;
/* Generate previous ack with CE=1. */
tp->ecn_flags |= TCP_ECN_DEMAND_CWR;
tp->rcv_nxt = ca->prior_rcv_nxt;
tcp_send_ack(sk);
/* Recover current rcv_nxt. */
tp->rcv_nxt = tmp_rcv_nxt;
}
ca->prior_rcv_nxt = tp->rcv_nxt;
ca->ce_state = 0;
tp->ecn_flags &= ~TCP_ECN_DEMAND_CWR;
}
static void dctcp_update_alpha(struct sock *sk, u32 flags)
{
const struct tcp_sock *tp = tcp_sk(sk);
struct dctcp *ca = inet_csk_ca(sk);
u32 acked_bytes = tp->snd_una - ca->prior_snd_una;
/* If ack did not advance snd_una, count dupack as MSS size.
* If ack did update window, do not count it at all.
*/
if (acked_bytes == 0 && !(flags & CA_ACK_WIN_UPDATE))
acked_bytes = inet_csk(sk)->icsk_ack.rcv_mss;
if (acked_bytes) {
ca->acked_bytes_total += acked_bytes;
ca->prior_snd_una = tp->snd_una;
if (flags & CA_ACK_ECE)
ca->acked_bytes_ecn += acked_bytes;
}
/* Expired RTT */
if (!before(tp->snd_una, ca->next_seq)) {
u64 bytes_ecn = ca->acked_bytes_ecn;
u32 alpha = ca->dctcp_alpha;
/* alpha = (1 - g) * alpha + g * F */
alpha -= min_not_zero(alpha, alpha >> dctcp_shift_g);
if (bytes_ecn) {
/* If dctcp_shift_g == 1, a 32bit value would overflow
* after 8 Mbytes.
*/
bytes_ecn <<= (10 - dctcp_shift_g);
do_div(bytes_ecn, max(1U, ca->acked_bytes_total));
alpha = min(alpha + (u32)bytes_ecn, DCTCP_MAX_ALPHA);
}
/* dctcp_alpha can be read from dctcp_get_info() without
* synchro, so we ask compiler to not use dctcp_alpha
* as a temporary variable in prior operations.
*/
WRITE_ONCE(ca->dctcp_alpha, alpha);
dctcp_reset(tp, ca);
}
}
static void dctcp_state(struct sock *sk, u8 new_state)
{
if (dctcp_clamp_alpha_on_loss && new_state == TCP_CA_Loss) {
struct dctcp *ca = inet_csk_ca(sk);
/* If this extension is enabled, we clamp dctcp_alpha to
* max on packet loss; the motivation is that dctcp_alpha
* is an indicator to the extend of congestion and packet
* loss is an indicator of extreme congestion; setting
* this in practice turned out to be beneficial, and
* effectively assumes total congestion which reduces the
* window by half.
*/
ca->dctcp_alpha = DCTCP_MAX_ALPHA;
}
}
static void dctcp_update_ack_reserved(struct sock *sk, enum tcp_ca_event ev)
{
struct dctcp *ca = inet_csk_ca(sk);
switch (ev) {
case CA_EVENT_DELAYED_ACK:
if (!ca->delayed_ack_reserved)
ca->delayed_ack_reserved = 1;
break;
case CA_EVENT_NON_DELAYED_ACK:
if (ca->delayed_ack_reserved)
ca->delayed_ack_reserved = 0;
break;
default:
/* Don't care for the rest. */
break;
}
}
static void dctcp_cwnd_event(struct sock *sk, enum tcp_ca_event ev)
{
switch (ev) {
case CA_EVENT_ECN_IS_CE:
dctcp_ce_state_0_to_1(sk);
break;
case CA_EVENT_ECN_NO_CE:
dctcp_ce_state_1_to_0(sk);
break;
case CA_EVENT_DELAYED_ACK:
case CA_EVENT_NON_DELAYED_ACK:
dctcp_update_ack_reserved(sk, ev);
break;
default:
/* Don't care for the rest. */
break;
}
}
static size_t dctcp_get_info(struct sock *sk, u32 ext, int *attr,
union tcp_cc_info *info)
{
const struct dctcp *ca = inet_csk_ca(sk);
/* Fill it also in case of VEGASINFO due to req struct limits.
* We can still correctly retrieve it later.
*/
if (ext & (1 << (INET_DIAG_DCTCPINFO - 1)) ||
ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
memset(&info->dctcp, 0, sizeof(info->dctcp));
if (inet_csk(sk)->icsk_ca_ops != &dctcp_reno) {
info->dctcp.dctcp_enabled = 1;
info->dctcp.dctcp_ce_state = (u16) ca->ce_state;
info->dctcp.dctcp_alpha = ca->dctcp_alpha;
info->dctcp.dctcp_ab_ecn = ca->acked_bytes_ecn;
info->dctcp.dctcp_ab_tot = ca->acked_bytes_total;
}
*attr = INET_DIAG_DCTCPINFO;
return sizeof(info->dctcp);
}
return 0;
}
static u32 dctcp_cwnd_undo(struct sock *sk)
{
const struct dctcp *ca = inet_csk_ca(sk);
return max(tcp_sk(sk)->snd_cwnd, ca->loss_cwnd);
}
static struct tcp_congestion_ops dctcp __read_mostly = {
.init = dctcp_init,
.in_ack_event = dctcp_update_alpha,
.cwnd_event = dctcp_cwnd_event,
.ssthresh = dctcp_ssthresh,
.cong_avoid = tcp_reno_cong_avoid,
.undo_cwnd = dctcp_cwnd_undo,
.set_state = dctcp_state,
.get_info = dctcp_get_info,
.flags = TCP_CONG_NEEDS_ECN,
.owner = THIS_MODULE,
.name = "dctcp",
};
static struct tcp_congestion_ops dctcp_reno __read_mostly = {
.ssthresh = tcp_reno_ssthresh,
.cong_avoid = tcp_reno_cong_avoid,
.undo_cwnd = tcp_reno_undo_cwnd,
.get_info = dctcp_get_info,
.owner = THIS_MODULE,
.name = "dctcp-reno",
};
static int __init dctcp_register(void)
{
BUILD_BUG_ON(sizeof(struct dctcp) > ICSK_CA_PRIV_SIZE);
return tcp_register_congestion_control(&dctcp);
}
static void __exit dctcp_unregister(void)
{
tcp_unregister_congestion_control(&dctcp);
}
module_init(dctcp_register);
module_exit(dctcp_unregister);
MODULE_AUTHOR("Daniel Borkmann <dborkman@redhat.com>");
MODULE_AUTHOR("Florian Westphal <fw@strlen.de>");
MODULE_AUTHOR("Glenn Judd <glenn.judd@morganstanley.com>");
MODULE_LICENSE("GPL v2");
MODULE_DESCRIPTION("DataCenter TCP (DCTCP)");