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

[Joe Touch <[e-mail]>]

On 5/22/2014 11:43 AM, Tom Herbert wrote:
>
>
>
> On Tue, May 20, 2014 at 6:14 PM, Joe Touch <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>     Hi, Han,
>
>     This helps, but doesn't quite address my concern.
>
>     TCP offloading is fine when the OS hands off user data, and the
>     offload engine creates the entire segment.
>
> This would essentially be TOE.
>
>     The situation you're describing seems to be a hybrid, where the
>     guest OS makes a TCP segment, and then something lower (in the VM
>     system) parses that segment to create one or more segments on the wire.
>
> This how Linux and probably about every NIC implements TCP segmentation
> offload. A large TCP segment is broken up into MSS sized segments (MSS
> is provided in ancillary data). The process replicates the IP/TCP
> headers per packet, adjusts the length, sequence numbers, and computes
> checksum for each segment.  There's no special handling of any options,
> it is assumed they can be replicated in each segment.

That's an incorrect assumption.

>     If that happens, it will be incompatible with a number of existing
>     TCP options, and will also cause side-effects with ACK clocking,
>     timeouts, and more than a few other TCP features.
>
>
>     Although I appreciate the goal of efficiency and speed, there is a
>     severe compatibility issue that doesn't seem to be addressed.
>
> TCP segmentation offload is already widely deployed. Since this draft
> would be using the same mechanism it's unlikely to create new
> compatibility issues except for the fact that the segmentation might be
> deferred to an off host entity which is a new concept that would
> probably have side effects.

IMO, this draft should not rely on or further promote a mechanism with 
known problems.

Joe