tcp_output() might generate invalid TSO frames

[Renaud Lienhart]

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.

http://www.freebsd.org/cgi/query-pr.cgi?pr=kern/132832

Thanks for your time,

	Renaud