Subject: Re: Datagram? Packet? (was : APEX)

From: Fred Baker <[email-masked]>
Date: Tue, 24 Sep 2002 09:38:36 -0700

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. If the MTU was larger, you wouldn't have 
had to fragment it or reassemble, and the forwarding system still could 
have gotten it there without reference to a setup procedure (virtual circuit).

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From: Lloyd Wood <[email-masked]>
Date: Wed, 25 Sep 2002 13:12:13 +0100 (BST)

On Tue, 24 Sep 2002, Fred Baker wrote:

> the result is still a datagram.

A datagram is self-describing; full source and destination. A fragment
(IPv4 fragment) may not be.

L.

<http://www.ee.surrey.ac.uk/Personal/L.Wood/><[email-masked]>

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From: Masataka Ohta <[email-masked]>
Date: Thu, 26 Sep 2002 01:34:26 +0859 ()

Lloyd;

> 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

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From: Fred Baker <[email-masked]>
Date: Wed, 25 Sep 2002 10:06:16 -0700

>(IPv4 fragment) may not be.

you sure? take a GOOD look at RFC 791... It is completely self-describing 
in terms of getting itself there and where it belongs in the reassembled 
datagram. If the other bits and pieces don't arrive, there is another 
matter, but it is at that point a host issue, not a forwarding issue.

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From: Francesco Trentini <[email-masked]>
Date: Thu, 26 Sep 2002 05:51:48 +0200

Hello Lloyd,

>> the result is still a datagram.

Yes, I agree, though the definition has - in common habit and use -
quite fuzzy contours. In theory, theory would be like pratice; but in
practice, pratice isn't theory. ;)

> (IPv4 fragment) may not be.

Don't be misled by the word fragment: in the IP case, it retains src
and dst info as well. No RFC needs to be read to confirm that ;), a little thinking shows
that: how could it be routed at layer 3 on an internet, otherwise?
In the fragmentation process a datagram/packet may "lose" in its "self
contained unit" ULP information, like TCP ports, but that's not matter
of IP.

.FT

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From: Lloyd Wood <[email-masked]>
Date: Thu, 26 Sep 2002 11:41:39 +0100 (BST)

> matter, but it is at that point a host issue, not a forwarding issue.

I'm not sure that following fragments relying on a bit in another
fragment saying 'following fragment' is truly self-describing.

(Not having port nos in following fragments would only be a host issue
if routers and firewalls never peeked at ports en route.)

L.