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 #ifndef __NET_SCHED_RED_H #define __NET_SCHED_RED_H #include #include #include #include /* 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 */ }; 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