Bug ID 1146241: FastL4 virtual server may egress packets with unexpected and erratic TTL values

Last Modified: Jan 20, 2023

Bug Tracker

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

Known Affected Versions:
13.1.0, 13.1.0.1, 13.1.0.2, 13.1.0.3, 13.1.0.4, 13.1.0.5, 13.1.0.6, 13.1.0.7, 13.1.0.8, 13.1.1, 13.1.1.2, 13.1.1.3, 13.1.1.4, 13.1.1.5, 13.1.3, 13.1.3.1, 13.1.3.2, 13.1.3.3, 13.1.3.4, 13.1.3.5, 13.1.3.6, 13.1.4, 13.1.4.1, 13.1.5, 13.1.5.1, 14.1.0, 14.1.0.1, 14.1.0.2, 14.1.0.3, 14.1.0.5, 14.1.0.6, 14.1.2, 14.1.2.1, 14.1.2.2, 14.1.2.3, 14.1.2.4, 14.1.2.5, 14.1.2.6, 14.1.2.7, 14.1.2.8, 14.1.3, 14.1.3.1, 14.1.4, 14.1.4.1, 14.1.4.2, 14.1.4.3, 14.1.4.4, 14.1.4.5, 14.1.4.6, 14.1.5, 14.1.5.1, 14.1.5.2, 14.1.5.3, 15.1.0, 15.1.0.1, 15.1.0.2, 15.1.0.3, 15.1.0.4, 15.1.0.5, 15.1.1, 15.1.2, 15.1.2.1, 15.1.3, 15.1.3.1, 15.1.4, 15.1.4.1, 15.1.5, 15.1.5.1, 15.1.6, 15.1.6.1, 15.1.7, 15.1.8, 15.1.8.1, 16.1.0, 16.1.1, 16.1.2, 16.1.2.1, 16.1.2.2, 16.1.3, 16.1.3.1, 16.1.3.2, 16.1.3.3, 17.0.0, 17.0.0.1, 17.0.0.2

Opened: Sep 06, 2022
Severity: 3-Major

Symptoms

A FastL4 virtual server may egress (either towards the client or the server) IP packets with unexpected and erratic TTL values. The same also applies to IPv6, where the TTL field is known as Hop Limit.

Impact

Depending on the actual TTL values that will be sent out on the wire (which can be random and anything within the allowed range for the field) traffic can be dropped by routers on the way to the packet's destination. This will happen if there are more routers (hops) on the way to the packet's destination than the value specified in the TTL field. Ultimately, this will lead to retransmissions and possibly application failures.

Conditions

- The BIG-IP system is a Virtual Edition (VE). - The Large Receive Offload (LRO) is enabled on the system (which it is by default), and is operating in software mode. You can determine whether LRO is enabled on the system by inspecting the tm.tcplargereceiveoffload DB key, and you can determine whether LRO is operating in software mode by trying to query the tcp_lro tmstat table (tmctl -d blade tcp_lro). If the table exists, LRO will be operating in software mode. - The FastL4 profile is configured to decrement the TTL (this is the default mode). - The virtual server uses mismatched IP versions on each side of the proxy (for example, an IPv6 client and an IPv4 server).

Workaround

You can work around this issue by doing either of the following things: - Disable LRO on the BIG-IP system by setting DB key tm.tcplargereceiveoffload to disable. - Use a TTL mode for the FastL4 profile other than decrement (for example, use proxy or set).

Fix Information

None

Behavior Change