tcp mss MCLBYTES restriction

[Andrew Gallatin]

For the case where the mtu is larger than MCLBYTES (2048), FreeBSD's
TCP implementation restricts the mss to a multiple of MCLBYTES.  This
appears to have been inherited from 4.4BSD-lite.

On adapters with 9000 byte jumbo frames, this limits the mss to 8192
bytes, and wastes nearly 1KB out of each frame.  This is especially
annoying because many drivers for adapters which support 9000 byte mtus
allocate their own 9000 byte receive buffers and don't even use mbufs
which are MCLBYTES in length.  In this case, the mss restriction guarantees
a nearly 1000 byte wastage per frame.

Would it be OK if I made this code optional via a sysctl?

Drew