Commit 732b4634 authored Apr 17, 2026 by Stephen Hemminger Committed by Jakub Kicinski Apr 27, 2026

net/sched: netem: fix probability gaps in 4-state loss model



The 4-state Markov chain in loss_4state() has gaps at the boundaries
between transition probability ranges. The comparisons use:

  if (rnd < a4)
  else if (a4 < rnd && rnd < a1 + a4)

When rnd equals a boundary value exactly, neither branch matches and
no state transition occurs. The redundant lower-bound check (a4 < rnd)
is already implied by being in the else branch.

Remove the unnecessary lower-bound comparisons so the ranges are
contiguous and every random value produces a transition, matching
the GI (General and Intuitive) loss model specification.

This bug goes back to original implementation of this model.

Fixes: 661b7972 ("netem: revised correlated loss generator")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20260418032027.900913-2-stephen@networkplumber.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>

parent 35eaa6d8

net/sched/sch_netem.c

+4 −4

Original line number	Diff line number	Diff line
		@@ -227,10 +227,10 @@ static bool loss_4state(struct netem_sched_data *q)
		if (rnd < clg->a4) {
		clg->state = LOST_IN_GAP_PERIOD;
		return true;
		} else if (clg->a4 < rnd && rnd < clg->a1 + clg->a4) {
		} else if (rnd < clg->a1 + clg->a4) {
		clg->state = LOST_IN_BURST_PERIOD;
		return true;
		} else if (clg->a1 + clg->a4 < rnd) {
		} else {
		clg->state = TX_IN_GAP_PERIOD;
		}

		@@ -247,9 +247,9 @@ static bool loss_4state(struct netem_sched_data *q)
		case LOST_IN_BURST_PERIOD:
		if (rnd < clg->a3)
		clg->state = TX_IN_BURST_PERIOD;
		else if (clg->a3 < rnd && rnd < clg->a2 + clg->a3) {
		else if (rnd < clg->a2 + clg->a3) {
		clg->state = TX_IN_GAP_PERIOD;
		} else if (clg->a2 + clg->a3 < rnd) {
		} else {
		clg->state = LOST_IN_BURST_PERIOD;
		return true;
		}