> On Tue, 19 Aug 1997, Mike Kilburn wrote:
>
> > "Richard B. Johnson" <root@analogic.com> wrote:
> >
> > > Well when I started in "Networking" about a thousand years ago when we
> > > used puffs of smoke for communication, a "Datagram" was a single packet
[SNIPPED]
> > User Datagram Protocal.
> >
> > > (were) datagrams with an IP Header. If the IP "layer" now allows one
> > > to fragment, i.e., use datagrams that no longer consist of a single
> > > packet, then the whole reason for using datagrams has disappeared.
[SNIPPED]
> page 2 section 1.4 (Operation):
>
> The internet protocol implements two basic functions:
> addressing and fragmentation.
> ...
>
> The internet modules use fields in the internet header to
> fragment and reassemble internet datagrams when necessary
> for transmission through "small packet" networks.
>
> So the total length of a UDP whateveritis can be 65,535 octets minus
> epsilon, because it sits on top of IP.
But this is not what RFC1122, Page 57, Paragraph 3.3.3 states. It
specifically states that exceeding the MTU is not allowed and that
a mechanism for determining the MTU must be provided.
What caught my attention was that someone was bothered that UDP would
not handle a packet of many thousands of bytes (octets). I stated that
it should not be used for this purpose.
Further, whether or not the IP layer will fragment and defragment is
moot. This is just a side-effect of IP and should not be used as a
the basis of a communications design.
I have seen side-effects being used for all kinds of design for many
years ultimately always with bad(tm) results.
Remember the "bug" report that Linus sent us a few years ago where
someone opened a file, opened a socket, then proceeded to dump the
entire contents of a file with one read of the file, and one write
to the socket. Since It didn't work on Linux, but worked on a Sun,
it was a "bug". So much for home-brew ftp.
Cheers,
DJ
Richard B. Johnson
Analogic Corporation
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