Re: [Fwd: [Saad] Some initiating thoughts...]

"J. Noel Chiappa" <jnc@ginger.lcs.mit.edu> Fri, 17 October 2003 23:04 UTC

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Date: Fri, 17 Oct 2003 19:03:22 -0400
From: "J. Noel Chiappa" <jnc@ginger.lcs.mit.edu>
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Subject: Re: [Fwd: [Saad] Some initiating thoughts...]
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    > From: Leslie Daigle <leslie@thinkingcat.com>

    > I'm attaching a pre-draft that was prepared by IAB member Mark Handley

I have a number of comments, but this one is important enough that I thought
I'd send it along separately - it makes a point about IP addresses that I'm
not sure people are keeping in mind.


    > 2.  Background
 
    > IP addresses have served both as a means of uniquely identifying a
    > device interface that is attached to a network (an endpoint
    > identifier)

This is broken terminology. The term "endpoint identifier" identifies it an
*end-end* entity - i.e. something that is the termination of an end-end
communication (see:

    http://users.exis.net/~jnc/tech/endpoints.txt

for more). An "interface" is obviously not such an entity. (And in MIPv6 this
is clear, because the end-end entity keeps its identity even when it has a
new name for its interface.)

IP addresses also do identify interfaces - but that's a separate function
(see the next comment).


    > and as a means of identifying where a device is located
    > within the network (a forwarding or routing identifier).

Actually, IP addresses have *three* roles (actually there are several more,
but let me torque peoples' brains incrementally :-), and the last clause here
("where a device ... forwarding") conflates two of them.

I have described a functionality which I call a "forwarding tag", which is
"the bits in a packet header that a switch looks at to tell it how to forward
the packet". Now, in IPvN, the "address" field(s) in the header are the
forwarding tag, so people tend to think of the "address" namespace as having
forwarding functionality. However, this is not necessarily so - e.g. in pure
circuit ATM the forwarding tag is *not* an address, and addresses have no
forwarding function.

So in the Internet architecture, IP addresses serve at least these three
functions:

1 - Identify an end-end entity
2 - Describe where its interface(s) is in the network (location)
3 - Serve as a forwarding tag for packets.

Identifying an interface (previous comment) is a fourth, less important one
(that's kind of mixed up with number 2 above). That this is a separate
function is easy to see - IEEE 48-bit numbers identify interfaces, but don't
tell you where in the Internet they are.

	Noel

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