tcp_output() might generate invalid TSO frames

Kip Macy kmacy at
Wed Apr 15 18:39:10 UTC 2009

Interesting. That might explain a problem that Mike Silbersack is
seeing with the latest em driver on vmware.

I don't know of any NICs that actually choke on such frames.
Nonetheless, it is silly behavior. I'll try to see if we can get this
fixed before 7.2.


On Wed, Apr 15, 2009 at 8:40 AM, Renaud Lienhart <renaud at> wrote:
> Hi,
> We're having trouble virtualizing FreeBSD 7+ on ESX because of an issue
> with the stack's TSO implementation: it sometimes generates TSO packets
> whose payload size is actually smaller than the MSS.
> The faulty logic is described, along with a patch, in PR #132832. It
> has been opened for a while now, without any apparent activity, which
> is why I'm reaching the mailing list directly.
> ESX currently drops these packets as many physical nics are known to
> choke on such frames, which effectively limits FreeBSD guests'
> performance.
> I don't know about other virtualization stacks' behavior.
> Thanks for your time,
>        Renaud
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