Bug ID 540893: Unevenly loaded tmms while using syncookies may cause occasional spurious connection resets.

Last Modified: Nov 07, 2022

Affected Product(s):
BIG-IP LTM(all modules)

Known Affected Versions:
11.5.1, 11.5.1 HF1, 11.5.1 HF10, 11.5.1 HF11, 11.5.1 HF2, 11.5.1 HF3, 11.5.1 HF4, 11.5.1 HF5, 11.5.1 HF6, 11.5.1 HF7, 11.5.1 HF8, 11.5.1 HF9, 11.5.10, 11.5.2, 11.5.2 HF1, 11.5.3, 11.5.3 HF1, 11.5.3 HF2, 11.5.4, 11.5.4 HF1, 11.6.0, 11.6.0 HF1, 11.6.0 HF2, 11.6.0 HF3, 11.6.0 HF4, 11.6.0 HF5, 11.6.0 HF6, 11.6.0 HF7, 11.6.0 HF8, 11.6.1, 12.0.0, 12.0.0 HF1, 12.0.0 HF2

Fixed In:
12.1.0, 12.0.0 HF3, 11.6.1 HF1, 11.5.4 HF2

Opened: Aug 20, 2015

Severity: 3-Major

Related Article: K66504397


Flows for a syncookie-enabled listener might occasionally receive a RST after responding correctly to a syncookie challenge.


Occasional clients take an incorrect path and have their valid syncookie ACKs rejected with a TCP RST and must retry.


-- Fast Flow Forwarding is enabled. -- At least one tmm thread is heavily loaded, with higher memory utilization, but has not reached its syncookie thresholds. -- At least one other tmm thread is less heavily loaded but has met its syncookie threshold. -- Syncookie activation is recorded on only one tmm thread.


Set db variable tmm.ffwd.enable = false. Doing this may modestly reduce peak performance on CPU bound loads.

Fix Information

Fixed occasional RST in response to valid syncookie ACKs when under uneven load.

Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips