Bug ID 515915: Server side timewait close state causes long establishment under port reuse

Last Modified: Oct 16, 2023

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

Known Affected Versions:
11.5.4, 11.6.0, 11.6.1, 11.6.2, 12.1.0 HF1, 12.1.0 HF2, 12.1.1 HF1, 12.1.1 HF2, 12.1.2 HF1, 12.1.2 HF2

Fixed In:
12.0.0, 11.6.3, 11.5.5

Opened: Apr 02, 2015

Severity: 2-Critical

Related Article: K47804233


When the server TCP connection is under timewait closing state, if a new client connection is initiated toward the server under the BIG-IP SYN-Cookie mode, the server respond with ACK instead of SYN+ACK for the SYN received. The BIG-IP system drops this ACK and retransmit the SYN, till timeout occurs.


Longer establishment time and retry.


-- FastL4 is under SYN-Cookie mode. -- The previous server connection is under time wait close state. -- New client connection is reusing the port to get to the same server TCP connection.



Fix Information

When ACK is received instead of SYN+ACK in this case, the system resets the server side to quick recovery, ensuring a quick recovery.

Behavior Change

Guides & references

K10134038: F5 Bug Tracker Filter Names and Tips