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DEFINITIONS
This source file includes following definitions.
- red_rmask
- red_set_parms
- red_is_idling
- red_start_of_idle_period
- red_end_of_idle_period
- red_restart
- red_calc_qavg_from_idle_time
- red_calc_qavg_no_idle_time
- red_calc_qavg
- red_random
- red_mark_probability
- red_cmp_thresh
- red_action
#ifndef __NET_SCHED_RED_H
#define __NET_SCHED_RED_H
#include <linux/types.h>
#include <net/pkt_sched.h>
#include <net/inet_ecn.h>
#include <net/dsfield.h>
/* Random Early Detection (RED) algorithm.
=======================================
Source: Sally Floyd and Van Jacobson, "Random Early Detection Gateways
for Congestion Avoidance", 1993, IEEE/ACM Transactions on Networking.
This file codes a "divisionless" version of RED algorithm
as written down in Fig.17 of the paper.
Short description.
------------------
When a new packet arrives we calculate the average queue length:
avg = (1-W)*avg + W*current_queue_len,
W is the filter time constant (chosen as 2^(-Wlog)), it controls
the inertia of the algorithm. To allow larger bursts, W should be
decreased.
if (avg > th_max) -> packet marked (dropped).
if (avg < th_min) -> packet passes.
if (th_min < avg < th_max) we calculate probability:
Pb = max_P * (avg - th_min)/(th_max-th_min)
and mark (drop) packet with this probability.
Pb changes from 0 (at avg==th_min) to max_P (avg==th_max).
max_P should be small (not 1), usually 0.01..0.02 is good value.
max_P is chosen as a number, so that max_P/(th_max-th_min)
is a negative power of two in order arithmetics to contain
only shifts.
Parameters, settable by user:
-----------------------------
qth_min - bytes (should be < qth_max/2)
qth_max - bytes (should be at least 2*qth_min and less limit)
Wlog - bits (<32) log(1/W).
Plog - bits (<32)
Plog is related to max_P by formula:
max_P = (qth_max-qth_min)/2^Plog;
F.e. if qth_max=128K and qth_min=32K, then Plog=22
corresponds to max_P=0.02
Scell_log
Stab
Lookup table for log((1-W)^(t/t_ave).
NOTES:
Upper bound on W.
-----------------
If you want to allow bursts of L packets of size S,
you should choose W:
L + 1 - th_min/S < (1-(1-W)^L)/W
th_min/S = 32 th_min/S = 4
log(W) L
-1 33
-2 35
-3 39
-4 46
-5 57
-6 75
-7 101
-8 135
-9 190
etc.
*/
#define RED_STAB_SIZE 256
#define RED_STAB_MASK (RED_STAB_SIZE - 1)
struct red_stats
{
u32 prob_drop; /* Early probability drops */
u32 prob_mark; /* Early probability marks */
u32 forced_drop; /* Forced drops, qavg > max_thresh */
u32 forced_mark; /* Forced marks, qavg > max_thresh */
u32 pdrop; /* Drops due to queue limits */
u32 other; /* Drops due to drop() calls */
u32 backlog;
};
struct red_parms
{
/* Parameters */
u32 qth_min; /* Min avg length threshold: A scaled */
u32 qth_max; /* Max avg length threshold: A scaled */
u32 Scell_max;
u32 Rmask; /* Cached random mask, see red_rmask */
u8 Scell_log;
u8 Wlog; /* log(W) */
u8 Plog; /* random number bits */
u8 Stab[RED_STAB_SIZE];
/* Variables */
int qcount; /* Number of packets since last random
number generation */
u32 qR; /* Cached random number */
unsigned long qavg; /* Average queue length: A scaled */
psched_time_t qidlestart; /* Start of current idle period */
};
static inline u32 red_rmask(u8 Plog)
{
return Plog < 32 ? ((1 << Plog) - 1) : ~0UL;
}
static inline void red_set_parms(struct red_parms *p,
u32 qth_min, u32 qth_max, u8 Wlog, u8 Plog,
u8 Scell_log, u8 *stab)
{
/* Reset average queue length, the value is strictly bound
* to the parameters below, reseting hurts a bit but leaving
* it might result in an unreasonable qavg for a while. --TGR
*/
p->qavg = 0;
p->qcount = -1;
p->qth_min = qth_min << Wlog;
p->qth_max = qth_max << Wlog;
p->Wlog = Wlog;
p->Plog = Plog;
p->Rmask = red_rmask(Plog);
p->Scell_log = Scell_log;
p->Scell_max = (255 << Scell_log);
memcpy(p->Stab, stab, sizeof(p->Stab));
}
static inline int red_is_idling(struct red_parms *p)
{
return p->qidlestart != PSCHED_PASTPERFECT;
}
static inline void red_start_of_idle_period(struct red_parms *p)
{
p->qidlestart = psched_get_time();
}
static inline void red_end_of_idle_period(struct red_parms *p)
{
p->qidlestart = PSCHED_PASTPERFECT;
}
static inline void red_restart(struct red_parms *p)
{
red_end_of_idle_period(p);
p->qavg = 0;
p->qcount = -1;
}
static inline unsigned long red_calc_qavg_from_idle_time(struct red_parms *p)
{
psched_time_t now;
long us_idle;
int shift;
now = psched_get_time();
us_idle = psched_tdiff_bounded(now, p->qidlestart, p->Scell_max);
/*
* The problem: ideally, average length queue recalcultion should
* be done over constant clock intervals. This is too expensive, so
* that the calculation is driven by outgoing packets.
* When the queue is idle we have to model this clock by hand.
*
* SF+VJ proposed to "generate":
*
* m = idletime / (average_pkt_size / bandwidth)
*
* dummy packets as a burst after idle time, i.e.
*
* p->qavg *= (1-W)^m
*
* This is an apparently overcomplicated solution (f.e. we have to
* precompute a table to make this calculation in reasonable time)
* I believe that a simpler model may be used here,
* but it is field for experiments.
*/
shift = p->Stab[(us_idle >> p->Scell_log) & RED_STAB_MASK];
if (shift)
return p->qavg >> shift;
else {
/* Approximate initial part of exponent with linear function:
*
* (1-W)^m ~= 1-mW + ...
*
* Seems, it is the best solution to
* problem of too coarse exponent tabulation.
*/
us_idle = (p->qavg * (u64)us_idle) >> p->Scell_log;
if (us_idle < (p->qavg >> 1))
return p->qavg - us_idle;
else
return p->qavg >> 1;
}
}
static inline unsigned long red_calc_qavg_no_idle_time(struct red_parms *p,
unsigned int backlog)
{
/*
* NOTE: p->qavg is fixed point number with point at Wlog.
* The formula below is equvalent to floating point
* version:
*
* qavg = qavg*(1-W) + backlog*W;
*
* --ANK (980924)
*/
return p->qavg + (backlog - (p->qavg >> p->Wlog));
}
static inline unsigned long red_calc_qavg(struct red_parms *p,
unsigned int backlog)
{
if (!red_is_idling(p))
return red_calc_qavg_no_idle_time(p, backlog);
else
return red_calc_qavg_from_idle_time(p);
}
static inline u32 red_random(struct red_parms *p)
{
return net_random() & p->Rmask;
}
static inline int red_mark_probability(struct red_parms *p, unsigned long qavg)
{
/* The formula used below causes questions.
OK. qR is random number in the interval 0..Rmask
i.e. 0..(2^Plog). If we used floating point
arithmetics, it would be: (2^Plog)*rnd_num,
where rnd_num is less 1.
Taking into account, that qavg have fixed
point at Wlog, and Plog is related to max_P by
max_P = (qth_max-qth_min)/2^Plog; two lines
below have the following floating point equivalent:
max_P*(qavg - qth_min)/(qth_max-qth_min) < rnd/qcount
Any questions? --ANK (980924)
*/
return !(((qavg - p->qth_min) >> p->Wlog) * p->qcount < p->qR);
}
enum {
RED_BELOW_MIN_THRESH,
RED_BETWEEN_TRESH,
RED_ABOVE_MAX_TRESH,
};
static inline int red_cmp_thresh(struct red_parms *p, unsigned long qavg)
{
if (qavg < p->qth_min)
return RED_BELOW_MIN_THRESH;
else if (qavg >= p->qth_max)
return RED_ABOVE_MAX_TRESH;
else
return RED_BETWEEN_TRESH;
}
enum {
RED_DONT_MARK,
RED_PROB_MARK,
RED_HARD_MARK,
};
static inline int red_action(struct red_parms *p, unsigned long qavg)
{
switch (red_cmp_thresh(p, qavg)) {
case RED_BELOW_MIN_THRESH:
p->qcount = -1;
return RED_DONT_MARK;
case RED_BETWEEN_TRESH:
if (++p->qcount) {
if (red_mark_probability(p, qavg)) {
p->qcount = 0;
p->qR = red_random(p);
return RED_PROB_MARK;
}
} else
p->qR = red_random(p);
return RED_DONT_MARK;
case RED_ABOVE_MAX_TRESH:
p->qcount = -1;
return RED_HARD_MARK;
}
BUG();
return RED_DONT_MARK;
}
#endif