Conference 7.286::fddi

638.0. "Is MTU settable?" by CSC32::J_MCCLELLAND (Off in the ETHERnet) Wed Jul 08 1992 14:02

I'm looking to see if the MTU (maximum transmission unit) is setable
for ULTRIX, now or possibly in the future.

The customer is evaluating a server 5900 on a FDDI ring, with a ciSco
AGS box connecting 8 ethernet segments to clients.  With only one client
accessing this server, the ciSco box is using almost 50% of its CPU to
do the fragmentation work.  He extrapolates this and assumes that the ciSco 
will be the bottle-neck in his environment.  He wants to set the MTU down
to ethernet size so that the ciSco will not have to fragment packets.  He
is aware that this cuts down the performance of the FDDI ring, but feels
that the investment in ciSco equipment is more important.  He also indicates
that the competing SUN server does have the ability to set its FDDI MTU
down to a lower value.

Note 330.7 suggests that a setable MTU might be in a future release of
DEC TCP/IP for VMS beyond 2.0.  What about Ultrix?

Is there any way to accomplish this short of a custom kernel?

Thanks,

John

Not likely.  I've been pushing OSG to allow IP to have it's MTU per
interface basis.  So far, I've lost.

[The MTU under ULTRIX should be 4470 but since ULTRIX is IP-centric,
they warp it down to 4352 which prevents datalink users from taking
advantage of the full size.  By allowing IP to have it's own MTU,
we could have the interface at its correct value]

Sun does support setting the size of the MTU.  The document is 
SunNetFDDI/DX2.0 Product Notes.

FDDI/DX conforms to RFC 1188, "IP Datagrams over FDDI," with one exception:
the default MTU size used in FDDI/DX is 4428 bytes, as opposed to the 4352
bytes specified in RFC 1188.  The default MTU size can be changed, however,
by using adb and setting the mtu in the if structure for the FDDI interface.

4428?  Strange number.  Does anyone know why Sun uses that?  

	paul