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Re: [Xen-devel] Question about TCP checksum offload in Xen

On 5 Dec 2013, at 11:39, Ian Campbell wrote:

> On Thu, 2013-12-05 at 11:29 +0000, Anil Madhavapeddy wrote:
>> On Tue, Dec 03, 2013 at 01:00:23PM +0000, Balraj Singh wrote:
>>> Hi,
>>> I'm working on verifying TCP checksums on incoming packets in Mirage, but
>>> I've run into a bit of a problem.
>>> If TCP checksum offload is turned on on a virtual interface (this is the
>>> default), and if the TCP connection is local to the machine, it looks like
>>> Xen does not calculate the checksum at all.  This may be valid because Xen
>>> may be providing a stronger guarantee, but it means that incoming packets
>>> don't have a valid checksum in the header.  This then means that in Mirage
>>> we can't just have checksum verification turned on all the time.  This
>>> would have been the safe fall back option and detecting that checksum
>>> offload is on, and then not duplicating the verification in Mirage would
>>> have been an optimisation.  But it looks like this is not an option.  Now I
>>> need to know for every incoming packet whether checksum verification should
>>> be done or not.  It should ideally be for every packet since chksum offload
>>> can be turned off and on on the VIF and existing tcp connections should
>>> continue.  If not every packet, I need to get a notification or efficiently
>>> detect right away that the setting is changed on the VIF.
>> This is a question that seems to keep coming up even for Linux and
>> Windows, as the combination of local<->local VMs vs local<->off-host and
>> the checksum offload is quite confusing.
>> CCing xen-devel: is the appropriate behaviour for a guest VM that wants to
>> use checksum offloading in all situations documented anywhere?
> I don't understand the question/concern. If you have enabled checksum
> offload then of course you don't recalculate the checksum, that's the
> whole point of offloading it.

i think balraj's question arises because the status of checksum offload can 
change mid-tcp-flow. how does he know whether it's on or off for a given packet?



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