Osmocom Packet control Unit (PCU): Network-side GPRS (RLC/MAC); BTS- or BSC-colocated https://osmocom.org/projects/osmopcu
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osmo-pcu/src/gprs_codel.c

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/* gprs_codel.cpp
*
* Copyright (C) 2015 by Sysmocom s.f.m.c. GmbH
* Author: Jacob Erlbeck <jerlbeck@sysmocom.de>
*
* 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.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*/
#include "gprs_codel.h"
#include "gprs_debug.h"
#include <osmocom/core/utils.h>
#include <osmocom/core/timer_compat.h>
#include <stdint.h>
#include <stdlib.h>
#include <math.h>
static void control_law(struct gprs_codel *state, struct timespec *delta)
{
/* 256 / sqrt(x), limited to 255 */
static uint8_t inv_sqrt_tab[] = {255,
255, 181, 147, 128, 114, 104, 96, 90, 85, 80, 77, 73, 71, 68,
66, 64, 62, 60, 58, 57, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46,
45, 45, 44, 43, 43, 42, 42, 41, 40, 40, 39, 39, 39, 38, 38, 37,
37, 36, 36, 36, 35, 35, 35, 34, 34, 34, 33, 33, 33, 33, 32, 32,
32, 32, 31, 31, 31, 31, 30, 30, 30, 30, 29, 29, 29, 29, 29, 28,
28, 28, 28, 28, 28, 27, 27, 27, 27, 27, 27, 26, 26, 26, 26, 26,
26, 26, 25, 25, 25, 25, 25, 25, 25, 25, 24, 24, 24, 24, 24, 24,
24, 24, 24, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 22, 22, 22,
22, 22, 22, 22, 22, 22, 22, 22, 22, 21, 21, 21, 21, 21, 21, 21,
21, 21, 21, 21, 21, 21, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20,
20, 20, 20, 20, 20, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19,
19, 19, 19, 19, 19, 19, 19, 18, 18, 18, 18, 18, 18, 18, 18, 18,
18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 18, 17, 17, 17, 17,
17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17, 17,
17, 17, 17, 17
};
uint_fast32_t delta_usecs;
uint_fast32_t inv_sqrt;
div_t q;
if (state->count >= ARRAY_SIZE(inv_sqrt_tab))
inv_sqrt = 16;
else
inv_sqrt = inv_sqrt_tab[state->count];
/* delta = state->interval / sqrt(count) */
delta_usecs = state->interval.tv_sec * 1000000 + state->interval.tv_nsec/1000;
delta_usecs = delta_usecs * inv_sqrt / 256;
q = div(delta_usecs, 1000000);
delta->tv_sec = q.quot;
delta->tv_nsec = q.rem * 1000;
}
void gprs_codel_init(struct gprs_codel *state)
{
static const struct gprs_codel init_state = {0};
*state = init_state;
gprs_codel_set_interval(state, -1);
gprs_codel_set_maxpacket(state, -1);
}
void gprs_codel_set_interval(struct gprs_codel *state, int interval_ms)
{
div_t q;
if (interval_ms <= 0)
interval_ms = GPRS_CODEL_DEFAULT_INTERVAL_MS;
q = div(interval_ms, 1000);
state->interval.tv_sec = q.quot;
state->interval.tv_nsec = q.rem * 1000000;
/* target ~ 5% of interval */
q = div(interval_ms * 13 / 256, 1000);
state->target.tv_sec = q.quot;
state->target.tv_nsec = q.rem * 1000000;
}
void gprs_codel_set_maxpacket(struct gprs_codel *state, int maxpacket)
{
if (maxpacket < 0)
maxpacket = GPRS_CODEL_DEFAULT_MAXPACKET;
state->maxpacket = maxpacket;
}
/*
* This is an broken up variant of the algorithm being described in
* http://queue.acm.org/appendices/codel.html
*/
int gprs_codel_control(struct gprs_codel *state, const struct timespec *recv,
const struct timespec *now, int bytes)
{
struct timespec sojourn_time;
struct timespec delta;
if (recv == NULL)
goto stop_dropping;
timespecsub(now, recv, &sojourn_time);
if (timespeccmp(&sojourn_time, &state->target, <))
goto stop_dropping;
if (bytes >= 0 && (unsigned)bytes <= state->maxpacket)
goto stop_dropping;
if (!timespecisset(&state->first_above_time)) {
timespecadd(now, &state->interval, &state->first_above_time);
goto not_ok_to_drop;
}
if (timespeccmp(now, &state->first_above_time, <))
goto not_ok_to_drop;
/* Ok to drop */
if (!state->dropping) {
int recently = 0;
int in_drop_cycle = 0;
if (timespecisset(&state->drop_next)) {
timespecsub(now, &state->drop_next, &delta);
in_drop_cycle = timespeccmp(&delta, &state->interval, <);
recently = in_drop_cycle;
}
if (!recently) {
timespecsub(now, &state->first_above_time, &delta);
recently = !timespeccmp(&delta, &state->interval, <);
};
if (!recently)
return 0;
state->dropping = 1;
if (in_drop_cycle && state->count > 2)
state->count -= 2;
else
state->count = 1;
state->drop_next = *now;
} else {
if (timespeccmp(now, &state->drop_next, <))
return 0;
state->count += 1;
}
control_law(state, &delta);
timespecadd(&state->drop_next, &delta, &state->drop_next);
#if 1
LOGP(DRLCMAC, LOGL_INFO,
"CoDel decided to drop packet, window = %d.%03dms, count = %d\n",
(int)delta.tv_sec, (int)(delta.tv_nsec / 1000000), state->count);
#endif
return 1;
stop_dropping:
timespecclear(&state->first_above_time);
not_ok_to_drop:
state->dropping = 0;
return 0;
}