ifconfig shows dropped rx packets

This document (7007165) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 15
SUSE Linux Enterprise Server 12

SUSE Linux Enterprise Server 11 SP4
SUSE Linux Enterprise Server 11 SP3
SUSE Linux Enterprise Server 11 SP2

Situation

ifconfig shows dropped rx packets
After updating to SLES 11 SP2 or any version after, packets are being dropped
Dropped counter incrementing
rx_dropped counter is incrementing

Customers have reported seeing dropped packets when examining statistics on their network card, bonds, or virtual interfaces.

Resolution

Working as designed.

Cause

Beginning with kernel 2.6.37, it has been changed the meaning of dropped packet count. Before, dropped packets was most likely due to an error. Now, the rx_dropped counter shows statistics for dropped frames because of:

Softnet backlog full -- (Measured from /proc/net/softnet_stat)
Bad / Unintended VLAN tags
Unknown / Unregistered protocols
IPv6 frames when the server is not configured for IPv6

If any frames meet those conditions, they are dropped before the protocol stack and the rx_dropped counter is incremented.

Additional Information

Care should be taken to confirm that frames are not being legitimately dropped. A quick way to test this (WARNING: this test does not work for bonding interfaces) is to force the NIC into promiscuous mode:

host:~# ifconfig <interface> promisc

And then watching the rx_dropped counter. If it stops incrementing while the NIC is in promiscuous mode; then it is more than likely showing drops because of the reasons listed earlier.

If additional frames continue to be shown as dropped, further investigation might be warranted. However, since IP communication and/or the applications that use it have methods in place for recovering missing packets, a small number of dropped packets is not expected to have any significant impact, and is not worth investigating. Additionally, some applications, such as video streaming, don't consider a few lost packets to be something that requires any recovery attempts, and they proceed without any delay.

Many administrators find it useful to perform performance testing on their interfaces to see if the alleged drops are hurting actual performance. Using a tool like iperf can help determine if your interfaces are performing near to their advertised speed.

Within SLE products, the idea that dropped packets are now normal and expected started in SLES 11 SP2, as it utilizes a 3.x kernel. Previous SLES versions do not exhibit it, as they use kernels prior to 2.6.37

Additional information can be found by researching commit # caf586e5

Please also note, for an active/backup bond setup it is common that the inactive device drops all packages.

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.