SUN-3s talking to SUN-2s with 3COM boards

[John Menges]

Here at the University of North Carolina'a Department of Computer Science,
we have approximately 40 SUN-2 workstations.  We are in the process of
installing a number of SUN-3 workstations as well.  Some of our SUN-2s have
3COM ethernet controllers.  SUN has informed us that there is a communications
problem interfacing SUN-2s with 3COM boards and SUN-3s.  The problem has
something to do with the 3COM boards not having enough buffering to handle
full-speed transmissions from the SUN-3.  The information we get from our
local SUN sales representative, however, is confusing and incomplete.
I am hoping that someone on the net will be able to clarify this issue.

According to our sales representative, file systems on a SUN-3 cannot
be remote (NFS) mounted on a SUN-2 with at 3COM board, and vice-versa,
without "slowing down the SUN-3 ethernet controller".  Rlogin, rcp, rsh, etc.,
however, are supposed to work without slowing down the controller.
According to some SUN documentation we have, SUN-2s can also be clients
of a SUN-3 file server (using ND), if the SUN-3 is told (in /etc/nd.local)
to limit the number of packets sent to the client (on a per-client basis)
to two before requiring an acknowledgement.

Now for the questions that haven't been answered:

  1.  Why do rlogin, etc. work but not NFS?  Does the NFS protocol not
      use any form of flow control or packet re-transmit?  If that is the
      case, what happens when you run NFS between a VAX or another faster
      machine and a SUN-2 with a 3COM board?  

  2.  What does it mean to "slow down the ethernet board"?  Is it slowed
      down regardless of who it's talking to (e.g., is SUN-3 to SUN-3
      communication slowed down), or on a per-host basis?

I'd appreciate any light that anyone can shed on this subject.  Thanks
in advance!
                                John Menges
                                menges at unc (csnet)
                                decvax!mcnc!unc!menges (uucp)