/*
* 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.
*
*
* TCP Compound based on TCP Vegas
*
* further details can be found here:
* ftp://ftp.research.microsoft.com/pub/tr/TR-2005-86.pdf
*/
#include <linux/config.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/skbuff.h>
#include <linux/inet_diag.h>
#include <net/tcp.h>
/* Default values of the Vegas variables, in fixed-point representation
* with V_PARAM_SHIFT bits to the right of the binary point.
*/
#define V_PARAM_SHIFT 1
#define TCP_COMPOUND_ALPHA 3U
#define TCP_COMPOUND_BETA 1U
#define TCP_COMPOUND_GAMMA 30
#define TCP_COMPOUND_ZETA 1
/* TCP compound variables */
struct compound {
u32 beg_snd_nxt; /* right edge during last RTT */
u32 beg_snd_una; /* left edge during last RTT */
u32 beg_snd_cwnd; /* saves the size of the cwnd */
u8 doing_vegas_now; /* if true, do vegas for this RTT */
u16 cntRTT; /* # of RTTs measured within last RTT */
u32 minRTT; /* min of RTTs measured within last RTT (in usec) */
u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */
u32 cwnd;
u32 dwnd;
};
/* 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 inline void vegas_enable(struct sock *sk)
{
const struct tcp_sock *tp = tcp_sk(sk);
struct compound *vegas = inet_csk_ca(sk);
/* Begin taking Vegas samples next time we send something. */
vegas->doing_vegas_now = 1;
/* Set the beginning of the next send window. */
vegas->beg_snd_nxt = tp->snd_nxt;
vegas->cntRTT = 0;
vegas->minRTT = 0x7fffffff;
}
/* Stop taking Vegas samples for now. */
static inline void vegas_disable(struct sock *sk)
{
struct compound *vegas = inet_csk_ca(sk);
vegas->doing_vegas_now = 0;
}
static void tcp_compound_init(struct sock *sk)
{
struct compound *vegas = inet_csk_ca(sk);
const struct tcp_sock *tp = tcp_sk(sk);
vegas->baseRTT = 0x7fffffff;
vegas_enable(sk);
vegas->dwnd = 0;
vegas->cwnd = tp->snd_cwnd;
}
/* 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)
*/
static void tcp_compound_rtt_calc(struct sock *sk, u32 usrtt)
{
struct compound *vegas = inet_csk_ca(sk);
u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */
/* Filter to find propagation delay: */
if (vrtt < vegas->baseRTT)
vegas->baseRTT = vrtt;
/* Find the min RTT during the last RTT to find
* the current prop. delay + queuing delay:
*/
vegas->minRTT = min(vegas->minRTT, vrtt);
vegas->cntRTT++;
}
static void tcp_compound_state(struct sock *sk, u8 ca_state)
{
if (ca_state == TCP_CA_Open)
vegas_enable(sk);
else
vegas_disable(sk);
}
/* 64bit divisor, dividend and result. dynamic precision */
static inline u64 div64_64(u64 dividend, u64 divisor)
{
u32 d = divisor;
if (divisor > 0xffffffffULL) {
unsigned int shift = fls(divisor >> 32);
d = divisor >> shift;
dividend >>= shift;
}
/* avoid 64 bit division if possible */
if (dividend >> 32)
do_div(dividend, d);
else
dividend = (u32) dividend / d;
return dividend;
}
/* calculate the quartic root of "a" using Newton-Raphson */
static u32 qroot(u64 a)
{
u32 x, x1;
/* Initial estimate is based on:
* qrt(x) = exp(log(x) / 4)
*/
x = 1u << (fls64(a) >> 2);
/*
* Iteration based on:
* 3
* x = ( 3 * x + a / x ) / 4
* k+1 k k
*/
do {
u64 x3 = x;
x1 = x;
x3 *= x;
x3 *= x;
x = (3 * x + (u32) div64_64(a, x3)) / 4;
} while (abs(x1 - x) > 1);
return x;
}
/*
* 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.
*/
static void tcp_compound_cwnd_event(struct sock *sk, enum tcp_ca_event event)
{
if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
tcp_compound_init(sk);
}
static void tcp_compound_cong_avoid(struct sock *sk, u32 ack,
u32 seq_rtt, u32 in_flight, int flag)
{
struct tcp_sock *tp = tcp_sk(sk);
struct compound *vegas = inet_csk_ca(sk);
u8 inc = 0;
if (vegas->cwnd + vegas->dwnd > tp->snd_cwnd) {
if (vegas->cwnd > tp->snd_cwnd || vegas->dwnd > tp->snd_cwnd) {
vegas->cwnd = tp->snd_cwnd;
vegas->dwnd = 0;
} else
vegas->cwnd = tp->snd_cwnd - vegas->dwnd;
}
if (!tcp_is_cwnd_limited(sk, in_flight))
return;
if (vegas->cwnd <= tp->snd_ssthresh)
inc = 1;
else if (tp->snd_cwnd_cnt < tp->snd_cwnd)
tp->snd_cwnd_cnt++;
if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
inc = 1;
tp->snd_cwnd_cnt = 0;
}
if (inc && tp->snd_cwnd < tp->snd_cwnd_clamp)
vegas->cwnd++;
/* The key players are v_beg_snd_una and v_beg_snd_nxt.
*
* These are so named because they represent the approximate values
* of snd_una and snd_nxt at the beginning of the current RTT. More
* precisely, they represent the amount of data sent during the RTT.
* At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
* we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
* bytes of data have been ACKed during the course of the RTT, giving
* an "actual" rate of:
*
* (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
*
* Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
* because delayed ACKs can cover more than one segment, so they
* don't line up nicely with the boundaries of RTTs.
*
* Another unfortunate fact of life is that delayed ACKs delay the
* advance of the left edge of our send window, so that the number
* of bytes we send in an RTT is often less than our cwnd will allow.
* So we keep track of our cwnd separately, in v_beg_snd_cwnd.
*/
if (after(ack, vegas->beg_snd_nxt)) {
/* Do the Vegas once-per-RTT cwnd adjustment. */
u32 old_wnd, old_snd_cwnd;
/* Here old_wnd is essentially the window of data that was
* sent during the previous RTT, and has all
* been acknowledged in the course of the RTT that ended
* with the ACK we just received. Likewise, old_snd_cwnd
* is the cwnd during the previous RTT.
*/
if (!tp->mss_cache)
return;
old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /
tp->mss_cache;
old_snd_cwnd = vegas->beg_snd_cwnd;
/* Save the extent of the current window so we can use this
* at the end of the next RTT.
*/
vegas->beg_snd_una = vegas->beg_snd_nxt;
vegas->beg_snd_nxt = tp->snd_nxt;
vegas->beg_snd_cwnd = tp->snd_cwnd;
/* We do the Vegas calculations only if we got enough RTT
* samples that we can be reasonably sure that we got
* at least one RTT sample that wasn't from a delayed ACK.
* If we only had 2 samples total,
* then that means we're getting only 1 ACK per RTT, which
* means they're almost certainly delayed ACKs.
* If we have 3 samples, we should be OK.
*/
if (vegas->cntRTT > 2) {
u32 rtt, target_cwnd, diff;
u32 brtt, dwnd;
/* We have enough RTT samples, so, using the Vegas
* algorithm, we determine if we should increase or
* decrease cwnd, and by how much.
*/
/* Pluck out the RTT we are using for the Vegas
* calculations. This is the min RTT seen during the
* last RTT. Taking the min filters out the effects
* of delayed ACKs, at the cost of noticing congestion
* a bit later.
*/
rtt = vegas->minRTT;
/* Calculate the cwnd we should have, if we weren't
* going too fast.
*
* This is:
* (actual rate in segments) * baseRTT
* We keep it as a fixed point number with
* V_PARAM_SHIFT bits to the right of the binary point.
*/
if (!rtt)
return;
brtt = vegas->baseRTT;
target_cwnd = ((old_wnd * brtt)
<< V_PARAM_SHIFT) / rtt;
/* Calculate the difference between the window we had,
* and the window we would like to have. This quantity
* is the "Diff" from the Arizona Vegas papers.
*
* Again, this is a fixed point number with
* V_PARAM_SHIFT bits to the right of the binary
* point.
*/
diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
dwnd = vegas->dwnd;
if (diff < (TCP_COMPOUND_GAMMA << V_PARAM_SHIFT)) {
u64 v;
u32 x;
/*
* The TCP Compound paper describes the choice
* of "k" determines the agressiveness,
* ie. slope of the response function.
*
* For same value as HSTCP would be 0.8
* but for computaional reasons, both the
* original authors and this implementation
* use 0.75.
*/
v = old_wnd;
x = qroot(v * v * v) >> TCP_COMPOUND_ALPHA;
if (x > 1)
dwnd = x - 1;
else
dwnd = 0;
dwnd += vegas->dwnd;
} else if ((dwnd << V_PARAM_SHIFT) <
(diff * TCP_COMPOUND_BETA))
dwnd = 0;
else
dwnd =
((dwnd << V_PARAM_SHIFT) -
(diff *
TCP_COMPOUND_BETA)) >> V_PARAM_SHIFT;
vegas->dwnd = dwnd;
}
/* Wipe the slate clean for the next RTT. */
vegas->cntRTT = 0;
vegas->minRTT = 0x7fffffff;
}
tp->snd_cwnd = vegas->cwnd + vegas->dwnd;
}
/* Extract info for Tcp socket info provided via netlink. */
static void tcp_compound_get_info(struct sock *sk, u32 ext, struct sk_buff *skb)
{
const struct compound *ca = inet_csk_ca(sk);
if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) {
struct tcpvegas_info *info;
info = RTA_DATA(__RTA_PUT(skb, INET_DIAG_VEGASINFO,
sizeof(*info)));
info->tcpv_enabled = ca->doing_vegas_now;
info->tcpv_rttcnt = ca->cntRTT;
info->tcpv_rtt = ca->baseRTT;
info->tcpv_minrtt = ca->minRTT;
rtattr_failure:;
}
}
static struct tcp_congestion_ops tcp_compound = {
.init = tcp_compound_init,
.ssthresh = tcp_reno_ssthresh,
.cong_avoid = tcp_compound_cong_avoid,
.rtt_sample = tcp_compound_rtt_calc,
.set_state = tcp_compound_state,
.cwnd_event = tcp_compound_cwnd_event,
.get_info = tcp_compound_get_info,
.owner = THIS_MODULE,
.name = "compound",
};
static int __init tcp_compound_register(void)
{
BUG_ON(sizeof(struct compound) > ICSK_CA_PRIV_SIZE);
tcp_register_congestion_control(&tcp_compound);
return 0;
}
static void __exit tcp_compound_unregister(void)
{
tcp_unregister_congestion_control(&tcp_compound);
}
module_init(tcp_compound_register);
module_exit(tcp_compound_unregister);
MODULE_AUTHOR("Angelo P. Castellani, Stephen Hemminger");
MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("TCP Compound");
