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

From: Caitlin Bestler <[email-masked]>
Date: Thu, 26 Sep 2002 12:35:11 -0500

On 9/26/02, Lloyd Wood wrote:

>On Wed, 25 Sep 2002, Fred Baker wrote:
>
>> At 01:12 PM 9/25/2002 +0100, Lloyd Wood wrote:
>> >A datagram is self-describing; full source and
>> >destination. A fragment (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.
>
>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.)
>

So, as originally proposed an IP fragment is a fully
self-routed L3 datagram.

However, in the de facto world of merged L3/L4 routing
(with NATs, load balancers, etc.) it is dependent on state
information and hence is not a datagram.

However, the term was applied before L3/L4 "routing" came
into existence. So the term 'datagram' was correct. And of
course nobody would change the term ex post facto. This is
why these terms are indeed fluid and nebulous.

---

From: Lloyd Wood <[email-masked]>
Date: Thu, 26 Sep 2002 19:42:22 +0100 (BST)

> So, as originally proposed an IP fragment is a fully

well, not self-routed; you need routing state. I don't think the
difference between routing table state and circuit-switched state is
all that great; anything beyond hot-potato is fundamentally stateful.

ICMP doesn't work well on fragments either - only on fragment zero.
Following fragments can't be described by ICMP. That's not a host
issue.

Anyway, forget 'datagram'. I'd like to know where use of the
term 'self-describing' for something that requires description in RFCs
and implementations and ridiculous numbers of textbooks ever came
from.

L.

> -
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>

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

---

From: Caitlin Bestler <[email-masked]>
Date: Thu, 26 Sep 2002 16:53:41 -0500

On 9/26/02, Lloyd Wood wrote:

>well, not self-routed; you need routing state. I don't
>think the difference between routing table state and
>circuit-switched state is all that great; anything beyond
>hot-potato is fundamentally stateful.
>

Given the source interface, the *meaning* of an IP header
is not supposed to be dependent on the routing tables.
The routing tables merely implement that meaning.

By contrast, the meaning of an ATM circuit is dependent on
the context in which it was established. There is no
expectation that there is any meaning to this circuit
identifier beyond those imparted when the circuit was
created.

I would maintain that all the IP exceptions that leap to
mind, such as NATs and load-balancing switches, do not
actually violate this. The intended destination is merely a
virtual host.

In any event, regardless of similarities in execution, the
difference in intended semantics is valid and fundamental.

Caitlin Bestler
http://asomi.com/CaitlinBestler/