Bug ID 1096893: TCP syncookie-initiated connections may end up unexpectedly IP-fragmenting packets mid-connection

Last Modified: Jan 20, 2023

Bug Tracker

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

Known Affected Versions:
15.0.0, 15.0.1,,,,, 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,, 16.0.0,, 16.0.1,,, 16.1.0, 16.1.1, 16.1.2,,, 16.1.3,,,, 17.0.0,,

Opened: Apr 08, 2022
Severity: 3-Major


When route metrics are applied by the TCP filter to a connection initiated by a syncookie, TCP sets the effective MSS for packetization, thereafter the egress_mtu will be set as per the route metrics entry, if present. The packets falling between the effective MSS and the lowered egress_mtu end up being unexpectedly IP-fragmented.


Application traffic can fail or see disruption due to unexpected IP fragmentation.


SYN cookies enabled and activated. A route metrics PMTU entry for the destination address that is smaller than the VLAN's egress MTU.


Disable syn cookies (Reference: https://support.f5.com/csp/article/K80970950). Alternatively, you can apply a lower static MTU to the interface.

Fix Information


Behavior Change