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Implement RFC3742 Limited Slow Start

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RFC3742 updates the cwnd every sendme during slow start, and backs off of the
exponential growth based on a cap parameter.
This commit is contained in:
Mike Perry 2022-07-22 17:00:22 +00:00 committed by David Goulet
parent d48eaff86d
commit 832a1d9fae
3 changed files with 190 additions and 90 deletions
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2
src/core/or/congestion_control_common.c
View File

@ -42,7 +42,7 @@
#define CC_ALG_DFLT_ALWAYS (CC_ALG_VEGAS)
#define CWND_INC_DFLT (TLS_RECORD_MAX_CELLS)
#define CWND_INC_PCT_SS_DFLT (50)
#define CWND_INC_PCT_SS_DFLT (100)
#define CWND_INC_RATE_DFLT (1)
#define CWND_MIN_DFLT (SENDME_INC_DFLT)

14
src/core/or/congestion_control_st.h
View File

@ -97,6 +97,10 @@ struct westwood_params_t {
/** Vegas algorithm parameters. */
struct vegas_params_t {
/** The slow-start cwnd cap for RFC3742 */
uint32_t ss_cwnd_cap;
/** The maximum slow-start cwnd */
uint32_t ss_cwnd_max;
/** The queue use allowed before we exit slow start */
uint16_t gamma;
/** The queue use below which we increment cwnd */
@ -227,6 +231,16 @@ static inline uint64_t CWND_UPDATE_RATE(const struct congestion_control_t *cc)
}
}
/**
* Gives us the number of SENDMEs in a CWND, rounded.
*/
static inline uint64_t SENDME_PER_CWND(const struct congestion_control_t *cc)
{
/* We add cwnd_inc_rate*sendme_inc/2 to round to nearest integer number
* of acks */
return ((cc->cwnd + cc->sendme_inc/2)/cc->sendme_inc);
}
/**
* Returns the amount to increment the congestion window each update,
* during slow start.

264
src/core/or/congestion_control_vegas.c
View File

@ -26,29 +26,35 @@
#define OUTBUF_CELLS (2*TLS_RECORD_MAX_CELLS)
#define SS_CWND_MAX_DFLT (5000)
/* sbws circs are two hops, so params are based on 2 outbufs of cells */
#define VEGAS_ALPHA_SBWS_DFLT (2*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
#define VEGAS_BETA_SBWS_DFLT (2*OUTBUF_CELLS)
#define VEGAS_GAMMA_SBWS_DFLT (2*OUTBUF_CELLS)
#define VEGAS_DELTA_SBWS_DFLT (4*OUTBUF_CELLS)
#define VEGAS_SSCAP_SBWS_DFLT (400)
/* Exits are three hops, so params are based on 3 outbufs of cells */
#define VEGAS_ALPHA_EXIT_DFLT (3*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
#define VEGAS_BETA_EXIT_DFLT (3*OUTBUF_CELLS)
#define VEGAS_GAMMA_EXIT_DFLT (3*OUTBUF_CELLS)
#define VEGAS_DELTA_EXIT_DFLT (5*OUTBUF_CELLS)
#define VEGAS_SSCAP_EXIT_DFLT (500)
/* Onion rends are six hops, so params are based on 6 outbufs of cells */
#define VEGAS_ALPHA_ONION_DFLT (6*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
#define VEGAS_BETA_ONION_DFLT (6*OUTBUF_CELLS)
#define VEGAS_GAMMA_ONION_DFLT (6*OUTBUF_CELLS)
#define VEGAS_DELTA_ONION_DFLT (8*OUTBUF_CELLS)
#define VEGAS_SSCAP_ONION_DFLT (600)
/* Single Onions are three hops, so params are based on 3 outbufs of cells */
#define VEGAS_ALPHA_SOS_DFLT (3*OUTBUF_CELLS-TLS_RECORD_MAX_CELLS)
#define VEGAS_BETA_SOS_DFLT (3*OUTBUF_CELLS)
#define VEGAS_GAMMA_SOS_DFLT (3*OUTBUF_CELLS)
#define VEGAS_DELTA_SOS_DFLT (5*OUTBUF_CELLS)
#define VEGAS_SSCAP_SOS_DFLT (500)
/* Vanguard Onions are 7 hops (or 8 if both sides use vanguards, but that
* should be rare), so params are based on 7 outbufs of cells */
@ -56,26 +62,15 @@
#define VEGAS_BETA_VG_DFLT (7*OUTBUF_CELLS)
#define VEGAS_GAMMA_VG_DFLT (7*OUTBUF_CELLS)
#define VEGAS_DELTA_VG_DFLT (9*OUTBUF_CELLS)
#define VEGAS_BDP_MIX_PCT 100
#define VEGAS_SSCAP_VG_DFLT (600)
/**
* The original TCP Vegas used only a congestion window BDP estimator. We
* believe that the piecewise estimator is likely to perform better, but
* for purposes of experimentation, we might as well have a way to blend
* them. It also lets us set Vegas to its original estimator while other
* algorithms on the same network use piecewise (by setting the
* 'vegas_bdp_mix_pct' consensus parameter to 100, while leaving the
* 'cc_bdp_alg' parameter set to piecewise).
*
* Returns a percentage weighted average between the CWND estimator and
* the specified consensus BDP estimator.
* The original TCP Vegas congestion window BDP estimator.
*/
static inline uint64_t
vegas_bdp_mix(const congestion_control_t *cc)
vegas_bdp(const congestion_control_t *cc)
{
return cc->vegas_params.bdp_mix_pct*cc->bdp[BDP_ALG_CWND_RTT]/100 +
(100-cc->vegas_params.bdp_mix_pct)*cc->bdp[cc->bdp_alg]/100;
return cc->bdp[BDP_ALG_CWND_RTT];
}
/**
@ -87,8 +82,8 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
{
tor_assert(cc->cc_alg == CC_ALG_VEGAS);
const char *alpha_str = NULL, *beta_str = NULL, *gamma_str = NULL;
const char *delta_str = NULL;
int alpha, beta, gamma, delta;
const char *delta_str = NULL, *sscap_str = NULL;
int alpha, beta, gamma, delta, ss_cwnd_cap;
switch (path) {
case CC_PATH_SBWS:
@ -96,56 +91,78 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
beta_str = "cc_vegas_beta_sbws";
gamma_str = "cc_vegas_gamma_sbws";
delta_str = "cc_vegas_delta_sbws";
sscap_str = "cc_sscap_sbws";
alpha = VEGAS_ALPHA_SBWS_DFLT;
beta = VEGAS_BETA_SBWS_DFLT;
gamma = VEGAS_GAMMA_SBWS_DFLT;
delta = VEGAS_DELTA_SBWS_DFLT;
ss_cwnd_cap = VEGAS_SSCAP_SBWS_DFLT;
break;
case CC_PATH_EXIT:
alpha_str = "cc_vegas_alpha_exit";
beta_str = "cc_vegas_beta_exit";
gamma_str = "cc_vegas_gamma_exit";
delta_str = "cc_vegas_delta_exit";
sscap_str = "cc_sscap_exit";
alpha = VEGAS_ALPHA_EXIT_DFLT;
beta = VEGAS_BETA_EXIT_DFLT;
gamma = VEGAS_GAMMA_EXIT_DFLT;
delta = VEGAS_DELTA_EXIT_DFLT;
ss_cwnd_cap = VEGAS_SSCAP_EXIT_DFLT;
break;
case CC_PATH_ONION:
alpha_str = "cc_vegas_alpha_onion";
beta_str = "cc_vegas_beta_onion";
gamma_str = "cc_vegas_gamma_onion";
delta_str = "cc_vegas_delta_onion";
sscap_str = "cc_sscap_onion";
alpha = VEGAS_ALPHA_ONION_DFLT;
beta = VEGAS_BETA_ONION_DFLT;
gamma = VEGAS_GAMMA_ONION_DFLT;
delta = VEGAS_DELTA_ONION_DFLT;
ss_cwnd_cap = VEGAS_SSCAP_ONION_DFLT;
break;
case CC_PATH_ONION_SOS:
alpha_str = "cc_vegas_alpha_sos";
beta_str = "cc_vegas_beta_sos";
gamma_str = "cc_vegas_gamma_sos";
delta_str = "cc_vegas_delta_sos";
sscap_str = "cc_sscap_sos";
alpha = VEGAS_ALPHA_SOS_DFLT;
beta = VEGAS_BETA_SOS_DFLT;
gamma = VEGAS_GAMMA_SOS_DFLT;
delta = VEGAS_DELTA_SOS_DFLT;
ss_cwnd_cap = VEGAS_SSCAP_SOS_DFLT;
break;
case CC_PATH_ONION_VG:
alpha_str = "cc_vegas_alpha_vg";
beta_str = "cc_vegas_beta_vg";
gamma_str = "cc_vegas_gamma_vg";
delta_str = "cc_vegas_delta_vg";
sscap_str = "cc_sscap_vg";
alpha = VEGAS_ALPHA_VG_DFLT;
beta = VEGAS_BETA_VG_DFLT;
gamma = VEGAS_GAMMA_VG_DFLT;
delta = VEGAS_DELTA_VG_DFLT;
ss_cwnd_cap = VEGAS_SSCAP_VG_DFLT;
break;
default:
tor_assert(0);
break;
}
cc->vegas_params.ss_cwnd_cap =
networkstatus_get_param(NULL, sscap_str,
ss_cwnd_cap,
100,
INT32_MAX);
cc->vegas_params.ss_cwnd_max =
networkstatus_get_param(NULL, "cc_ss_max",
SS_CWND_MAX_DFLT,
500,
INT32_MAX);
cc->vegas_params.alpha =
networkstatus_get_param(NULL, alpha_str,
alpha,
@ -169,12 +186,107 @@ congestion_control_vegas_set_params(congestion_control_t *cc,
delta,
0,
INT32_MAX);
}
cc->vegas_params.bdp_mix_pct =
networkstatus_get_param(NULL, "cc_vegas_bdp_mix",
VEGAS_BDP_MIX_PCT,
0,
100);
/**
* Common log function for tracking all vegas state.
*/
static void
congestion_control_vegas_log(const circuit_t *circ,
const congestion_control_t *cc)
{
uint64_t queue_use = cc->cwnd - vegas_bdp(cc);
if (CIRCUIT_IS_ORIGIN(circ) &&
circ->purpose == CIRCUIT_PURPOSE_S_REND_JOINED) {
log_info(LD_CIRC,
"CC: TOR_VEGAS Onion Circuit %d "
"RTT: %"PRIu64", %"PRIu64", %"PRIu64", "
"CWND: %"PRIu64", "
"INFL: %"PRIu64", "
"VBDP: %"PRIu64", "
"QUSE: %"PRIu64", "
"BWE: %"PRIu64", "
"SS: %d",
CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
cc->min_rtt_usec/1000,
cc->ewma_rtt_usec/1000,
cc->max_rtt_usec/1000,
cc->cwnd,
cc->inflight,
vegas_bdp(cc),
queue_use,
cc->cwnd*CELL_MAX_NETWORK_SIZE*1000/
MAX(cc->min_rtt_usec,cc->ewma_rtt_usec),
cc->in_slow_start
);
} else {
log_info(LD_CIRC,
"CC: TOR_VEGAS "
"RTT: %"PRIu64", %"PRIu64", %"PRIu64", "
"CWND: %"PRIu64", "
"INFL: %"PRIu64", "
"VBDP: %"PRIu64", "
"QUSE: %"PRIu64", "
"BWE: %"PRIu64", "
"SS: %d",
cc->min_rtt_usec/1000,
cc->ewma_rtt_usec/1000,
cc->max_rtt_usec/1000,
cc->cwnd,
cc->inflight,
vegas_bdp(cc),
queue_use,
cc->cwnd*CELL_MAX_NETWORK_SIZE*1000/
MAX(cc->min_rtt_usec,cc->ewma_rtt_usec),
cc->in_slow_start
);
}
}
/**
* Implements RFC3742: Limited Slow Start.
* https://datatracker.ietf.org/doc/html/rfc3742#section-2
*/
static inline uint64_t
rfc3742_ss_inc(const congestion_control_t *cc)
{
if (cc->cwnd <= cc->vegas_params.ss_cwnd_cap) {
/* If less than the cap, round and always grow by at least 1 sendme_inc. */
return ((uint64_t)cc->cwnd_inc_pct_ss*cc->sendme_inc + 50)/100;
} else {
// K = int(cwnd/(0.5 max_ssthresh));
// => K = 2*cwnd/max_ssthresh
// cwnd += int(MSS/K);
// => cwnd += MSS*max_ssthresh/(2*cwnd)
return ((uint64_t)cc->sendme_inc*cc->vegas_params.ss_cwnd_cap + cc->cwnd)/
(2*cc->cwnd);
}
}
/**
* Exit Vegas slow start.
*
* This function sets our slow-start state to 0, and emits logs
* and control port information signifying end of slow start.
* It also schedules the next CWND update for steady-state.
*/
static void
congestion_control_vegas_exit_slow_start(const circuit_t *circ,
congestion_control_t *cc)
{
congestion_control_vegas_log(circ, cc);
cc->in_slow_start = 0;
cc->next_cc_event = CWND_UPDATE_RATE(cc);
congestion_control_vegas_log(circ, cc);
/* We need to report that slow start has exited ASAP,
* for sbws bandwidth measurement. */
if (CIRCUIT_IS_ORIGIN(circ)) {
/* We must discard const here because the event modifies fields :/ */
control_event_circ_bandwidth_used_for_circ(
TO_ORIGIN_CIRCUIT((circuit_t*)circ));
}
}
/**
@ -217,43 +329,45 @@ congestion_control_vegas_process_sendme(congestion_control_t *cc,
return 0;
}
/* We only update anything once per window */
if (cc->next_cc_event == 0) {
/* The queue use is the amount in which our cwnd is above BDP;
* if it is below, then 0 queue use. */
if (vegas_bdp_mix(cc) > cc->cwnd)
queue_use = 0;
else
queue_use = cc->cwnd - vegas_bdp_mix(cc);
/* The queue use is the amount in which our cwnd is above BDP;
* if it is below, then 0 queue use. */
if (vegas_bdp(cc) > cc->cwnd)
queue_use = 0; // This should not happen anymore..
else
queue_use = cc->cwnd - vegas_bdp(cc);
if (cc->in_slow_start) {
if (queue_use < cc->vegas_params.gamma && !cc->blocked_chan) {
/* Grow to BDP immediately, then exponential growth until
* congestion signal. Increment by at least 2 sendme's worth. */
cc->cwnd = MAX(cc->cwnd + MAX(CWND_INC_SS(cc), 2*cc->sendme_inc),
vegas_bdp_mix(cc));
if (cc->in_slow_start) {
if (queue_use < cc->vegas_params.gamma && !cc->blocked_chan) {
/* Get the "Limited Slow Start" increment */
uint64_t inc = rfc3742_ss_inc(cc);
// Check if inc is less than what we would do in steady-state
// avoidance
if (inc*SENDME_PER_CWND(cc) <= CWND_INC(cc)) {
cc->cwnd += inc;
congestion_control_vegas_exit_slow_start(circ, cc);
} else {
/* Congestion signal: Set cwnd to gamma threshhold */
cc->cwnd = vegas_bdp_mix(cc) + cc->vegas_params.gamma;
cc->in_slow_start = 0;
log_info(LD_CIRC, "CC: TOR_VEGAS exiting slow start");
/* We need to report that slow start has exited ASAP,
* for sbws bandwidth measurement. */
if (CIRCUIT_IS_ORIGIN(circ)) {
/* We must discard const here because the event modifies fields :/ */
control_event_circ_bandwidth_used_for_circ(
TO_ORIGIN_CIRCUIT((circuit_t*)circ));
}
cc->cwnd += inc;
cc->next_cc_event = 1; // Technically irellevant, but for consistency
}
} else {
if (queue_use > cc->vegas_params.delta) {
cc->cwnd = vegas_bdp_mix(cc) + cc->vegas_params.delta - CWND_INC(cc);
} else if (queue_use > cc->vegas_params.beta || cc->blocked_chan) {
cc->cwnd -= CWND_INC(cc);
} else if (queue_use < cc->vegas_params.alpha) {
cc->cwnd += CWND_INC(cc);
}
/* Congestion signal: Set cwnd to gamma threshhold */
cc->cwnd = vegas_bdp(cc) + cc->vegas_params.gamma;
congestion_control_vegas_exit_slow_start(circ, cc);
}
if (cc->cwnd >= cc->vegas_params.ss_cwnd_max) {
cc->cwnd = cc->vegas_params.ss_cwnd_max;
congestion_control_vegas_exit_slow_start(circ, cc);
}
/* After slow start, We only update once per window */
} else if (cc->next_cc_event == 0) {
if (queue_use > cc->vegas_params.delta) {
cc->cwnd = vegas_bdp(cc) + cc->vegas_params.delta - CWND_INC(cc);
} else if (queue_use > cc->vegas_params.beta || cc->blocked_chan) {
cc->cwnd -= CWND_INC(cc);
} else if (queue_use < cc->vegas_params.alpha) {
cc->cwnd += CWND_INC(cc);
}
/* cwnd can never fall below 1 increment */
@ -262,41 +376,13 @@ congestion_control_vegas_process_sendme(congestion_control_t *cc,
/* Schedule next update */
cc->next_cc_event = CWND_UPDATE_RATE(cc);
if (CIRCUIT_IS_ORIGIN(circ)) {
congestion_control_vegas_log(circ, cc);
/* Log if we're above the ss_cap */
if (cc->cwnd >= cc->vegas_params.ss_cwnd_max) {
log_info(LD_CIRC,
"CC: TOR_VEGAS Circuit %d "
"CWND: %"PRIu64", "
"INFL: %"PRIu64", "
"VBDP: %"PRIu64", "
"QUSE: %"PRIu64", "
"NCCE: %"PRIu64", "
"SS: %d",
CONST_TO_ORIGIN_CIRCUIT(circ)->global_identifier,
cc->cwnd,
cc->inflight,
vegas_bdp_mix(cc),
queue_use,
cc->next_cc_event,
cc->in_slow_start
);
} else {
log_info(LD_CIRC,
"CC: TOR_VEGAS Circuit %"PRIu64":%d "
"CWND: %"PRIu64", "
"INFL: %"PRIu64", "
"VBDP: %"PRIu64", "
"QUSE: %"PRIu64", "
"NCCE: %"PRIu64", "
"SS: %d",
CONST_TO_OR_CIRCUIT(circ)->p_chan->global_identifier,
CONST_TO_OR_CIRCUIT(circ)->p_circ_id,
cc->cwnd,
cc->inflight,
vegas_bdp_mix(cc),
queue_use,
cc->next_cc_event,
cc->in_slow_start
);
"CC: TOR_VEGAS above ss_max in steady state for circ %d: %"PRIu64,
circ->purpose, cc->cwnd);
}
}