Archived from groups: comp.dcom.lans.ethernet (
More info?)
In article <15198068.kmfYAppCL7@Misha>,
Mikhail Teterin <usenet@aldan.algebra.com> wrote:
> Hello!
>
> Our newer computers tend to come with NICs, that can handle big frame sizes
> (at least 8 or 9 K, some even 15). We also have one 24-port switch, that
> can handle 15K frames. But many desktops and the older servers can't handle
> the frames above the 1500 bytes, of course, so the entire network is still
> limited to the small frame size.
>
> We could setup a small PC running something like FreeBSD to transparently
> fragment packets from one interface to another, but I'm wondering, if such
> devices are already available out there... Thanks!
>
> -mi
It is not possible to transparently fragment at the Link Layer. Ethernet
(and most other LAN technologies) has no mechanism for fragmentation;
equally important, there would be no means for the receiving end station
to reassemble the fragments.
Some network layer protocols (but not all) include
fragmentation/reassembly capability, but these functions are notoriously
inefficient. Since the vast majority of packets do *not* require
fragmentation (one hopes), fragmentation/reassembly are generally
performed outside the "fast path" of the code; as a result, performance
suffers *significantly*. In addition, reassembly tends to be memory
intensive (since buffers must be retained until all fragments of a
packet have been received and reassembled).
In any case, fragmentation in practical networks can only be performed
in an IP-aware intermediate device (a router, or as another poster
alluded to, a "transparent router," such as were once popular for
FDDI-to-Ethernet connections). Even so, if the majority of packets are
going to be exchanged between a jumbo-enabled server and
non-jumbo-capable end stations (as you imply from your post), it would
be *much* more efficient to disable the jumbo frame capability on the
server, and let the packets stay intact throughout their journey. All
you would be doing is shifting the function from the server to the
intermediate device; the server can avoid invoking true IP fragmentation
by simply using the smaller frame size. This is why MTU Discovery was
developed; to allow end stations to use smaller packets rather than
force intermediate routers to perform fragmentation.
Furthermore, using an intermediate router would require that you divide
your network into subnetworks across the router ports; by definition, a
router only relays packets between IP subnets. You indicated that this
was undesirable from an administration/management perspective.
Bottom line--disable the jumbo frame capability, or use MTU Discovery
(which will effectively do the same thing, only automatically).
Patient: "Doctor, it hurts when I do this!"
Doctor: "Then don't do this."
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 395-1966 FAX
Send replies to: usenet at richseifert dot com
at least 8 or 9 K, some even 15). We also have one 24-port switch, that
that "fragmented" IP datagrams
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