Re: [Tofoo] FW: I-D Action: draft-zhou-li-vxlan-soe-01.txt

Joe Touch <touch@isi.edu> Thu, 22 May 2014 18:17 UTC

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Subject: Re: [Tofoo] FW: I-D Action: draft-zhou-li-vxlan-soe-01.txt
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On 5/21/2014 7:44 PM, Zhou, Han wrote:
> Hi Joe,
>
> Thanks for your explain, and in theory I tend to agree with you.
> However, look at the real world implementation of offload, e.g. Linux kernel, GSO is implemented in the way I described: a whole TCP packet (including headers and options) are generated by the TCP layer, and segmentation is performed as late as possible, depending on the FEATURES supported by net-device:
> 1) TSO is NOT supported by net-device then GSO software segmentation is performed (tcp_gso_segment())
> 2) TSO is supported by NIC, then segmentation is offloaded to NIC hardware. (in this case tcp_dump will not capture small packets but only the large packets before segmentation)
> 3) It is in Guest OS and TSO is supported by the virtual net-device, then segmentation is offloaded to host OS.
>
> And my change is specific for 3): without the change segmentation is performed according to the MSS specified by guest OS,  and with the change this segmentation is skipped.
>
> As you can see the compatibility problem you mentioned, if it is a real problem, is not introduced by my change.

Perhaps, but dealing with guest OS/TSO interactions could be designed as 
a correct example, rather than replicating the failure of other 
implementers.

> Because of this, we might discuss it somewhere else, such as linux kernel community.
> I don't think current Linux kernel GSO implementation addresses the problem you mentioned (I could be wrong). Do you know any example of *correct* implementation of TCP offloading? Or it could be a TODO in linux kernel.

I haven't studied TSO implementations; there are too many, and many are 
proprietary.

Joe