From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.lans.fddi
Subject: Re: Can NFS over FDDI work properly?
Keywords: NFS, FDDI
Date: 1 Oct 92 02:46:27 GMT
Organization: Silicon Graphics, Inc.  Mountain View, CA

> rmwise@mcigate.apdev.mci.com (Bob_Wise) writes:
> >...
> >I've got two IBM RS/6000 950's which are shortly to become 970's, plus
> >I've got another 2 970's coming.  We do a lot of NFS over ethernet,
> >and I am about to move that NFS traffic over to a small FDDI ring that
> >only has the 970 servers on it.
> >
> >Is the fragmentation problem present with this scenario, or is
> >fragmentation only a problem with bridges and routers?
> >
> >Do I need to consider adjusting packet sizes?


As long as all NFS clients and servers are on the FDDI ring, there is
no fragmentation.  Simply replacing an ethernet with an FDDI ring will
increase NFS performance, but not as much as we would all like.

If some of the servers or clients are on ethernets, all should be well
if you use routers.  No reasonable router should have any trouble
keeping up with an NFS server on the ethernet.  Any reasonable router
should be able to IP fragment at a few MByte/sec.  Because of that, you
may have slightly better performance with such a mixed media network
than with a pure ethernet, because at least one of NFS server or client
pair will have the advantage of dealing with big packets.

Reducing the IP MTU below that specified in the relevant RFC is not
universally available.  I don't think it is even mentioned in "Host
Requirements."  That it is present in more than one host may testify to
the existence of a significant minority of broken bridges.  I know
that's why I added it to a couple of host FDDI implementations.


In article <mbt.717889082@Able.MKT.3Com.Com>, mbt@ESD.3Com.COM (Brad Turner) writes:
> 
> Bob, et.al.,
>    If you have the opportunity to to design a net from the ground up
> I'd suggest you look at using routing *IF* you will be spanning
> multiple media (fddi, ethernet, tokenring). When you start trying
> to do bridging between different types of media all kinds of nasty
> issues raise their head. These issues are effectively dealt with
> by routing. In case you are wondering what types of "nasty issues"
> I'm refering to here is a short list off the top of my head I'm sure
> others can add to it.
> 
>  o MTU mismatches (fddi vs eth)
>  o Some ARP implementations use canonical address within the data
>    others use non-canonical address representation within the data field.
>  o Source Route to non SR media (token ring to ethernet or fddi)


That's a reasonable statement.  Why does 3Com allow their boxes
to do FDDI-ethernet bridging?  When they drop 4K IP FDDI packets?

It is true that bridging between different kinds of media is hard.  I
think it's crazy, but I my belief in the routing faith would probably
be called fanatical by those who believe in bridges.

However,
   -The MTU mismatch is not a problem for many (most?) FDDI-ether
    bridges.  For example, DEC sells ether-FDDI bridges that they say
    can IP fragment at media speed between an FDDI ring and one (more
    than 1?) ethernet.

   -I have not heard of any ARP problem.  Any bridge is be useful must
    "do the right thing" when forwarding ARP packets from one media to
    another, whether that involves bit swapping the hardware addresses
    or changing the media type field.  The addresses in the MAC header
    of any packet bridge between FDDI and ethernet must be bit-swapped,
    not just ARP packets.  RFC-1188 requires that the FDDI media type
    say "ethernet" and that the MAC addresses in the APR part of the
    packet be in ethernet order just to accomodate FDDI-ethernet
    bridges.

   -Source routing seems to be dead outside of IBM token rings.  At
    least it is quite dead in FDDI rings.


Vernon Schryver,   vjs@sgi.com