Lloyd;
> > At 05:44 PM 9/24/2002 +0200, TOMSON ERIC wrote:
> > >Last, while I definitely, clearly prefer calling Layer 2 data units
> > >"FRAMES", I sometimes [over-]simplify the terminology of Layer 3 by making
> > >the following distinction : "a datagram is the data unit before
> > >fragmentation" ; "a packet is a piece of a fragmented datagram".
> >
> > :^)
> >
> > A fragment of a datagram is itself a datagram; after you re-assemble them,
> > the result is still a datagram.
>
> A datagram is self-describing; full source and destination.
That's the essential property.
That is, an ATM cell or an X.25 packet actually is not a datagram,
because it does not have full information on source nor destination.
It, instead, has full information on a VC, forwarding for which relies
on rather static information stored on intermediate systems at the
time of signalling.
> A fragment (IPv4 fragment) may not be.
An IPv4 fragment, however, has full information on source and
destination hosts and is a datagram, though, it does not have full
information on source and destination ports, which is not an
internetworking issue.
Masataka Ohta
