Bug ID 502747: Incoming SYN generates unexpected ACK when connection cannot be recycled

Last Modified: Oct 06, 2020

Bug Tracker

Affected Product:  See more info
BIG-IP LTM(all modules)

Known Affected Versions:
10.2.4, 11.2.1, 11.3.0, 11.4.0, 11.4.1, 11.5.0, 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, 11.6.0, 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

Fixed In:
12.0.0, 11.6.1 HF1, 11.5.4, 11.4.1 HF9

Opened: Jan 24, 2015
Severity: 3-Major

Symptoms

Incoming SYN causes the BIG-IP system to generate ACK instead of SYN-ACK.

Impact

Client will generate RST and connection must be re-tried.

Conditions

This can occur when the following conditions are met: - IP addresses and ports of SYN match an existing connection; - Sequence number of the SYN is greater than 2^31+ from previously sent FIN; - Existing connection is in TIME_WAIT state; - Virtual server has time_wait_recycle enabled.

Workaround

Set time-wait-timeout to 1 millisecond per SOL12673.

Fix Information

The BIG-IP system will no longer generate an ACK to incoming SYNs which match an existing connection that cannot be recycled.

Behavior Change