setting the other end's TCP segment size

[perryh at pluto.rain.com]

> [TCP] splits traffic to 'segments' using its own logic ...

Is there a simple way for a FreeBSD system to cause its peer
to use a transmit segment size of, say, 640 bytes -- so that
the peer will never try to send a packet larger than that?

I'm trying to get around a network packet-size problem.
In case it matters, the other end is SunOS 4.1.1 on a sun3, and
I've been unable to find a way to limit its packet size directly.