RX checksum offloading problem

Michael Tuexen Michael.Tuexen at lurchi.franken.de
Fri May 9 14:22:54 UTC 2014

On 09 May 2014, at 12:46, Michael Tuexen <Michael.Tuexen at lurchi.franken.de> wrote:

> On 09 May 2014, at 03:35, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>> On Thu, May 08, 2014 at 08:40:22PM +0200, Michael Tuexen wrote:
>>> On 07 May 2014, at 10:37, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>>>> On Wed, May 07, 2014 at 10:07:09AM +0200, Michael Tuexen wrote:
>>>>> On 07 May 2014, at 09:56, Yonghyeon PYUN <pyunyh at gmail.com> wrote:
>>>>>> On Sat, May 03, 2014 at 11:52:47AM +0200, Michael Tuexen wrote:
>>>>>>> On 02 May 2014, at 16:02, Bjoern A. Zeeb <bz at FreeBSD.org> wrote:
>>>>>>>> On 02 May 2014, at 10:22 , Michael Tuexen <Michael.Tuexen at lurchi.franken.de> wrote:
>>>>>>>>> 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:
>>>>>>>>> #define CSUM_SCTP_VALID         CSUM_L4_VALID
>>>>>>>>> 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
>>>>>>>>> #define CSUM_DATA_VALID         CSUM_L4_VALID
>>>>>>>>> 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
>>>>>>>>> dev/usb/net/if_smsc.c
>>>>>>>>> 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.
>>>>>>>> Wait, or for SCTP in cad the crc32 (I think it was)  was actually checked but not otherwise.   This is how it should be imho.  It seems like a driver bug.
>>>>>>> I went through the list of drivers and you are right, it seems to be a bug
>>>>>>> in if_smsc.c. Most of the other drivers check for UDP/TCP, a small set I can't tell.
>>>>>> I'm not sure how the controller computes TCP/UDP checksum values.
>>>>>> It seems the publicly available data sheet was highly sanitized so
>>>>>> it was useless to me.  The comment in the driver says that the
>>>>> Same for me...
>>>>>> controller computes RX checksum after the IPv4 header to the end of
>>>>>> ethernet frame. After seeing that comment, three questions popped
>>>>>> up:
>>> OK, I did some testing. It looks like the card is just computing the
>>> checksum over the IP payload taking the correct IP header length into account.
>>>>>> 1. Is the controller smart enough to skip IP options header in
>>>>>> TCP/UDP checksum offloading?
>>> Yes, I can send fragmented and un-fragmented UDP packets with IP options
>>> and they are handled correctly. Even if the last fragment is too short.
>> I'm assuming you're taking about receiving fragmented UDP packets
>> with RX checksum offloading, right?
> Correct.
>>>>>> 2. How controller handles UDP checksum value 0x0000(i.e. sender
>>>>>> didn't compute UDP checksum)?
>>> This case isn't handled. However, udp_input() looks first for zero checksums
>>> and only after that in the csum_flags. So it doesn't result in any problems.
>>> Would you prefer not to set CSUM_DATA_VALID in this case?
>> At least, it correctly updates UDP stats of netstat(1).
> Let me double check that...
I double checked it. The statistic counters are incremented.
Please note that we had a bug in the sending code of head, which
made it impossible to send UDP packets with 0 checksum. That is
fixed in

So any preference whether to report CSUM_DATA_VALID if a UDP packet
with checksum 0 is received or not? I'm pretty open, since it does
not have any effect right now...

Best regards
>>>>>> 3. How the controller can compute TCP checksum of fragmented
>>>>>> packets?
>>> At least it does it right for UDP...
>> Hmm, CSUM_DATA_VALID indicates driver computed RX TCP/UDP checksum
>> without pseudo header. As you know, controller can't compute
>> TCP/UDP checksum until all its fragmented payload are read from
>> wire.  Packets may arrive out of order and may be mixed with other
> I'm not sure I understand this... Please note that the pseudo header
> is not taken into account. So the card can compute the checksum over
> the payload of IP for each fragment. This is stored in the csum_data
> field. During reassembly the csum_data fields of the fragments are
> combined in
> http://svnweb.freebsd.org/base/head/sys/netinet/ip_input.c?view=markup#l1075
> This looks OK to me. I'm not sure why you think the cards needs
> to keep state for this. I understand it needs state if it wants
> to take the pseudoheader into account, but this is not done here.
>> packets. Advanced controllers with enough memory may be able to
>> compute TCP/UDP checksums by tracking each connections(e.g LRO) but
>> low-end controllers may be not. It seems the controller does not
>> even support RX TCP/UDP pseudo header checksum offloading so I
>> wonder how this controller can support RX TCP/UDP checksum
>> offloading for fragmented TCP/UDP packets.
> I'm not sure what I'm missing here... You compute the sum over
> the parts and add the sums of the parts. That should work for
> the UDP/TCP checksum.
>> Some controllers maintain two bits for TCP/UDP checksum offloading
>> result in status word.  One bit is used to indicate whether
>> controller performed TCP/UDP checksum offloading and the other bit
>> is used to indicate whether the computed checksum is correct or
> For SCTP we only get a bit that the checksum was computed and is correct...
>> not. For UDP checksum value 0x0000 and fragmented TCP/UDP packets,
>> these controllers do not attempt to compute TCP/UDP checksum.
> I think it "just" computes it and leaves it up to the upper layer
> to use the result or not...
> Best regards
> Michael
>>> Best regards
>>> Michael
>>>>>> Since you have the controller I guess it's easy to verify all
>>>>>> cases.  For case 3, I believe the controller can't handle
>>>>>> fragmented frames so driver should have to explicitly check ip_off
>>>>>> field of IPv4 header.  See how gem(4)/sk(4)/hme(4) and fxp(4)
>>>>>> handle it.
>>>>> Let me check this. Is there a tool to send UDP/TCP with IP level options
>>>>> or do I need to write a small test program myself?
>>>> I recall I used buggy ipsend of ipfilter package in the past but it
>>>> would be more easy to write a simple test program or patch driver
>>>> to generate those frames.
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