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