RX checksum offloading problem

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Fri May 2 10:22:32 UTC 2014

Dear all,

during testing I found that FreeBSD head (on a raspberry pi) accepts SCTP packet
with bad checksums. After debugging this I figured out that this is a problem with
the csum_flags defined in mbuf.h.

The SCTP code on its input path checks for CSUM_SCTP_VALID, which is defined in mbuf.h:
This makes sense: If CSUM_SCTP_VALID is set in csum_flags, the packet is considered
to have a correct checksum.

For UDP and TCP some drivers calculate the UDP/TCP checksum and set CSUM_DATA_VALID in
csum_flags to indicate that the UDP/TCP should consider csum_data to figure out if
the packet has a correct checksum. The problem is that CSUM_DATA_VALID is defined as
In this case the semantic is not that the packet has a valid checksum, but the csum_data
field contains information.

Now the following happens (on the raspberry pi the driver used is

1. A packet is received and if it is not too short, the checksum computed
   is stored in csum_data and the flag CSUM_DATA_VALID is set. This happens
   for all IP packets, not only for UDP and TCP packets.
2. In case of SCTP packets, the SCTP interprets CSUM_DATA_VALID as CSUM_SCTP_VALID
   and accepts the packet. So no SCTP checksum check ever happened.

Alternatives to fix this:

1. Change all drivers to set CSUM_DATA_VALID only in case of UDP or TCP packets, since
   it only makes sense in these cases.

2. Map CSUM_DATA_VALID to some other generic value in mbuf.h. However, CSUM_L4_CALC,
   which would make perfect sense in my view, is already used for CSUM_PSEUDO_HDR. Therefore
   we would have to define a new generic value.

So what is the best way to fix this?

Best regards

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