linux/net/ipv4/tcp_vegas.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * TCP Vegas congestion control
 *
 * This is based on the congestion detection/avoidance scheme described in
 *    Lawrence S. Brakmo and Larry L. Peterson.
 *    "TCP Vegas: End to end congestion avoidance on a global internet."
 *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
 *    October 1995. Available from:
 *	ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
 *
 * See http://www.cs.arizona.edu/xkernel/ for their implementation.
 * The main aspects that distinguish this implementation from the
 * Arizona Vegas implementation are:
 *   o We do not change the loss detection or recovery mechanisms of
 *     Linux in any way. Linux already recovers from losses quite well,
 *     using fine-grained timers, NewReno, and FACK.
 *   o To avoid the performance penalty imposed by increasing cwnd
 *     only every-other RTT during slow start, we increase during
 *     every RTT during slow start, just like Reno.
 *   o Largely to allow continuous cwnd growth during slow start,
 *     we use the rate at which ACKs come back as the "actual"
 *     rate, rather than the rate at which data is sent.
 *   o To speed convergence to the right rate, we set the cwnd
 *     to achieve the right ("actual") rate when we exit slow start.
 *   o To filter out the noise caused by delayed ACKs, we use the
 *     minimum RTT sample observed during the last RTT to calculate
 *     the actual rate.
 *   o When the sender re-starts from idle, it waits until it has
 *     received ACKs for an entire flight of new data before making
 *     a cwnd adjustment decision. The original Vegas implementation
 *     assumed senders never went idle.
 */

#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>

#include <net/tcp.h>

#include "tcp_vegas.h"

static int alpha = …;
static int beta  = …;
static int gamma = …;

module_param(alpha, int, 0644);
MODULE_PARM_DESC(…) …;
module_param(beta, int, 0644);
MODULE_PARM_DESC(…) …;
module_param(gamma, int, 0644);
MODULE_PARM_DESC(…) …;

/* There are several situations when we must "re-start" Vegas:
 *
 *  o when a connection is established
 *  o after an RTO
 *  o after fast recovery
 *  o when we send a packet and there is no outstanding
 *    unacknowledged data (restarting an idle connection)
 *
 * In these circumstances we cannot do a Vegas calculation at the
 * end of the first RTT, because any calculation we do is using
 * stale info -- both the saved cwnd and congestion feedback are
 * stale.
 *
 * Instead we must wait until the completion of an RTT during
 * which we actually receive ACKs.
 */
static void vegas_enable(struct sock *sk)
{ … }

/* Stop taking Vegas samples for now. */
static inline void vegas_disable(struct sock *sk)
{ … }

void tcp_vegas_init(struct sock *sk)
{ … }
EXPORT_SYMBOL_GPL(…);

/* Do RTT sampling needed for Vegas.
 * Basically we:
 *   o min-filter RTT samples from within an RTT to get the current
 *     propagation delay + queuing delay (we are min-filtering to try to
 *     avoid the effects of delayed ACKs)
 *   o min-filter RTT samples from a much longer window (forever for now)
 *     to find the propagation delay (baseRTT)
 */
void tcp_vegas_pkts_acked(struct sock *sk, const struct ack_sample *sample)
{ … }
EXPORT_SYMBOL_GPL(…);

void tcp_vegas_state(struct sock *sk, u8 ca_state)
{ … }
EXPORT_SYMBOL_GPL(…);

/*
 * If the connection is idle and we are restarting,
 * then we don't want to do any Vegas calculations
 * until we get fresh RTT samples.  So when we
 * restart, we reset our Vegas state to a clean
 * slate. After we get acks for this flight of
 * packets, _then_ we can make Vegas calculations
 * again.
 */
void tcp_vegas_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{ … }
EXPORT_SYMBOL_GPL(…);

static inline u32 tcp_vegas_ssthresh(struct tcp_sock *tp)
{ … }

static void tcp_vegas_cong_avoid(struct sock *sk, u32 ack, u32 acked)
{ … }

/* Extract info for Tcp socket info provided via netlink. */
size_t tcp_vegas_get_info(struct sock *sk, u32 ext, int *attr,
			  union tcp_cc_info *info)
{ … }
EXPORT_SYMBOL_GPL(…);

static struct tcp_congestion_ops tcp_vegas __read_mostly = …;

static int __init tcp_vegas_register(void)
{ … }

static void __exit tcp_vegas_unregister(void)
{ … }

module_init(…) …;
module_exit(tcp_vegas_unregister);

MODULE_AUTHOR(…) …;
MODULE_LICENSE(…) …;
MODULE_DESCRIPTION(…) …;