[PATCH net v3 1/7] net/sched: netem: fix probability gaps in 4-state loss model

Stephen Hemminger
2026-04-02T20:20:51Z

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: 661b79725fea ("netem: revised correlated loss generator")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
net/sched/sch_netem.c | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/sched/sch_netem.c b/net/sched/sch_netem.c
index 20df1c08b1e9..8ee72cac1faf 100644
--- a/net/sched/sch_netem.c
+++ b/net/sched/sch_netem.c
@@ -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;
}
--
2.53.0