Can't set lower MTU on gif?

[John Klos <john%ziaspace.com@localhost>]

Hi,

I'd like to use gif to route IPv4 over IPv6. I set up gif0 on a NetBSD 7 
machine like so via /etc/ifconfig.gif0:

create
tunnel 2001:470:a068:5:250:baff:fecb:32e 2605:bc00:0:114:230:48ff:fe98:d1a8
inet 192.80.49.75 192.80.49.74 netmask 255.255.255.254

The other end is a NetBSD-7 machine, and this works - both sides can ping 
one another, I can add 192.80.49.74 as a default route and talk over IPv4 
to the Internet.

However, any packets larger than 1200 bytes or so don't get passed. In 
FreeBSD's man page for gif(4), this is mentioned:

"If the outer protocol is IPv6, path MTU discovery for encapsulated 
packets may affect communication over the interface. The first 
bigger-than-pmtu packet may be lost.  To avoid the problem, you may want 
to set the interface MTU for gif to 1240 or smaller, when the outer header 
is IPv6 and the inner header is IPv4."

https://www.freebsd.org/cgi/man.cgi?gif(4)

However, NetBSD doesn't let the MTU for gif to be set to less than 1280 
bytes:

#define GIF_MTU_MIN     (1280)  /* Minimum MTU */

Is there a good reason for this? If there is, then what's a better way to 
route IPv4 over IPv6 while avoiding this issue?

Thanks,
John