4. Pseudocode Walk-Through

Questa pagina conserva variabili e funzioni: MAX_RATE, QPROTECT_ON, CRITICALqL_us, CRITICALqLSCORE_us, LG_AGING, qdelay, probNative, packet.uflow, qLscore, pick_bucket(), fill_bucket(), calcProbNative(), qprotect().

4.1. Input Parameters, Constants, and Variables

// Input Parameters
MAX_RATE;          // Configured maximum sustained rate [b/s]
QPROTECT_ON;       // Queue Protection is enabled [Default: TRUE]
CRITICALqL_us;     // LL queue threshold delay [µs] Default: MAXTH_us
CRITICALqLSCORE_us;// The threshold queuing score [Default: 4000 µs]
LG_AGING;          // The aging rate of the q'ing score [Default: 19]
                   //  as log base 2 of the congestion-rate [lg(B/s)]

// Constants
AGING = pow(2, (LG_AGING-30) ) * T_RES;
CRITICALqL = CRITICALqL_us * 1000;
CRITICALqLSCORE = CRITICALqLSCORE_us * 1000/T_RES;
CRITICALqLPRODUCT = CRITICALqL * CRITICALqLSCORE;
qLSCORE_MAX = 5E9 / T_RES;
ATTEMPTS = 2;
BI_SIZE = 5;
NBUCKETS = pow(2, BI_SIZE);
MASK = NBUCKETS-1;
EXIT_SUCCESS  = 0;
EXIT_SANCTION = 1;
MAX_FRAME_SIZE = 2000;
FLOOR =  2 * 8 * MAX_FRAME_SIZE * 10^9 / MAX_RATE;

struct bucket {
   id;          // identifier of flow using bucket
   t_exp;       // expiry time in units of T_RES
};
struct bucket buckets[NBUCKETS+1];

Le variabili pubbliche e di pacchetto importanti includono qdelay, probNative, packet, packet.size, packet.uflow e qdelayL(...). L'identificatore di flusso e normalmente una 5-tuple, oppure una 4-tuple per IPsec ESP che usa il Security Parameter Index (SPI).

4.2. qprotect()

qprotect(packet, ...) {
   bckt_id;
   qLscore;

   qdelay = qL.qdelay(...);
   probNative = calcProbNative(qdelay);

   bckt_id = pick_bucket(packet.uflow);
   qLscore = fill_bucket(buckets[bckt_id], packet.size, probNative);

   if ( ( ( qdelay > CRITICALqL )
      && ( qdelay * qLscore > CRITICALqLPRODUCT ) )
      || ( qLscore >= qLSCORE_MAX ) )
      return EXIT_SANCTION;
   else
      return EXIT_SUCCESS;
}

4.2.2. pick_bucket()

pick_bucket(uflw) {
   h32   = hash32(uflw);
   hsav  = NBUCKETS;
   now   = get_time_now();
   for (j=0; j<ATTEMPTS; j++) {
      h = h32 & MASK;
      if (buckets[h].id == uflw) {
         if (buckets[h].t_exp <= now)
            buckets[h].t_exp = now;
         return h;
      }
      else if ((hsav == NBUCKETS) && (buckets[h].t_exp <= now)) {
         hsav = h;
      }
      h32 >>= BI_SIZE;
   }
   if (hsav != NBUCKETS) {
      buckets[hsav].t_exp = now;
   } else {
      if (buckets[hsav].t_exp <= now)
         buckets[hsav].t_exp = now;
   }
   buckets[hsav].id = uflw;
   return hsav;
}

Il bucket NBUCKETS aggiuntivo e il bucket "dregs" predefinito. Se ATTEMPTS=2 e BI_SIZE=5, ci sono 32 bucket non predefiniti.

4.2.3. fill_bucket()

fill_bucket(bckt_id, pkt_sz, probNative) {
   now = get_time_now();
   qLscore = min(buckets[bckt_id].t_exp - now
                 + probNative * pkt_sz / AGING, qLSCORE_MAX);
   buckets[bckt_id].t_exp = now + qLscore;
   return qLscore;
}

4.2.4. calcProbNative()

calcProbNative(qdelay){
   if ( qdelay >= MAXTH ) {
      probNative = MAX_PROB;
   } else if ( qdelay > MINTH ) {
      probNative = MAX_PROB * (qdelay - MINTH)/RANGE;
   } else {
      probNative = 0;
   }
   return probNative;
}