Subject: Re: [arch-d] A straw man analysis of why send-receive packet paradigm is unsustainable

From: [email-masked] (Noel Chiappa)
Date: Tue, 5 Dec 2006 00:17:01 -0500 (EST)

> From: John Day <[email-masked]>

    >> generally a MAC Address identifies an interface.

    >> What was the reason/need then in creating IP addresses?

    > MAC addresses are flat

Exactly.

Also, remember that globally unique MAC addresses (the 48-bit thingys) are
about a decade younger than IPv4 addresses; they didn't exist as an option
when IPv4 was done.

    > IP addresses weren't suppose to be but really were until CIDR came
    > along 15 years later.

Ah, no. IP addresses always had two levels of hierarchy (net # and "rest"),
and very quickly (see IEN-82, from February 1979), were hacked to have three
levels of hierarchy (net #, subnet # and host).

    > IP addresses were suppose to be the addresses of "gateways" which
    > turned out to be routers. 

Huh? They could name routers (well, router interfaces), but they were always
intended to name destination hosts (well, their network interfaces). In what
sense were they the "addresses" (whatever you mean by That Word) of gateways?

    > MAC addresses were in IEEE and IP addresses were in the DoD.

But not at the same time...

    > It has always been odd that ... an *Internet* address named a *subnet*
    > point of attachment.

I'm not sure I completely understand why this is odd. If I understand
correctly what your point is, you'd rather see something where the
internetwork-level name is for a particular physical network, and then some
other name indicates which interface on that particular physical network is
desired?

But that's basically what an IP address has, if you look at it right. The
"net #" part (include the subnet # as well, if there is one) names the
physical network. The "rest" field names a SNPA. For efficiency reasons, the
two are concatenated together into a single field, but logically they are
separate - and in fact, the entity at the other end doesn't have much use
for one without the other, and is happy to use them as a package.

Yes, there are times when it's useful to be able to just name a physical
network, and no SNPA's (e.g. in a routing update), but you just leave those
bits zero in that case.

Now, again for efficiency reasons, the rest field has a limited length. So,
on some networks (e.g. 1822 - in fact, all actual extant networks at the time
IPv4 was designed, other than X.25) which have short enough SNPA names, you
can actually stick a real SNPA in there; in others (e.g. Ethernet) you have
to shoehorn 10 pounds into a 5-pound sack, and as a result you have this
mapping layer, ARP, etc.

But the fact that usually you need both names (physical network, plus SNPA)
to do anything, plus the fact that most times, you don't have any use for one
without the other, makes it IMO not unreasonable to concatenate them into a
single syntactic entity.

Or am I completely missing your point?

	Noel

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---

From: John Day <[email-masked]>
Date: Tue, 5 Dec 2006 10:50:50 -0500

>about a decade younger than IPv4 addresses; they didn't exist as an option

Well, only sort of.  MAC addresses from IEEE did not exist. But Ethernet did.

>levels of hierarchy (net #, subnet # and host).

Yea, but the hierarchy was really applicable "within an 
organization," not across organizations. And very quickly Class B and 
C addresses were being handed out as "network" addresses without 
regard to what *network, i.e. Class A or B, they were in. Thus 
destroying any usefulness of the hierarchy.

>sense were they the "addresses" (whatever you mean by That Word) of gateways?

You are right, I mis-spoke.  I was thinking about how early on IP was 
supposedly running at level higher than say IMP addressing, but that 
sort of disappeared as that role was taken on by MAC addresses and IP 
addresses moved down a level in some sense.

>But not at the same time...

Not initially, but they did overlap. ;-)

>Or am I completely missing your point?

I think you are missing my point but I won't swear to it!  My point 
was that IP addresses are points of attachment and not node addresses 
in the sense of 1498.

Take care,
john