Bug ID 1229369: The fastl4 TOS mimic setting towards client may not function

Last Modified: Mar 13, 2023

Bug Tracker

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

Known Affected Versions:
15.1.0,,,,,, 15.1.1, 15.1.2,, 15.1.3,, 15.1.4,, 15.1.5,, 15.1.6,, 15.1.7, 15.1.8,, 16.1.0, 16.1.1, 16.1.2,,, 16.1.3,,,, 17.0.0,,

Opened: Jan 23, 2023
Severity: 3-Major


The ip-tos-to-client setting in a fastL4 profile is used to control the Type Of Service (TOS) field in the IP header for egress frames on a clientside flow. There are two special values - 'mimic' and 'pass-through'. The mimic setting causes tmm to set the egress TOS to the value seen on the last ingress packet for that connflow. In affected versions of BIG-IP, this is not set correctly, and behaves like pass-through (uses the TOS value seen arriving on the serverside flow)


The clientside egress TOS is not set to the expected value


FastL4 profile with ip-tos-to-client set to "mimic" (shown as the value 65534 in tmsh)


Use an irule to set IP::tos to the desired value. Note that processing every packet with an irule will incur a performance penalty.

Fix Information


Behavior Change