[e2e] where end-to-end ends

Micah Beck mbeck at cs.utk.edu
Wed May 23 12:32:47 PDT 2001

----- Original Message -----
From: "Eric A. Hall" <ehall at ehsco.com>
To: "Micah Beck" <mbeck at cs.utk.edu>
Sent: Wednesday, May 23, 2001 10:30 AM
Subject: Re: [e2e] where end-to-end ends

> > For example, if my mail server holds my mail and I manage it using
> > IMAP, is the client-to-client delivery of mail considered to be
> > end-to-end?  The mail server, considered as part of the network, is
> > acting as a buffer under the control of the end-point.  However, it
> > is subject to failure modes that a stateless netework would not see.
> The clients are not exchanging data directly so there is no end-to-end
> without the server relaying the data.
> --
> Eric A. Hall                                        http://www.ehsco.com/
> Internet Core Protocols          http://www.oreilly.com/catalog/coreprot/

If the fact that clients are "not exchange data directly" means that there
is no end-to-end, then what is the definition of "exchanging data directly"
and what feature of my example violates that definition?

Some possibilities, all of which are true of this example:

1. the sender and reciever are not participating in the communication at the
same time (asynchrony)
2. the data is stored over long periods of time on a disk rather than for
short periods in memory buffers
3. the movement of data is directed by user-level application-specific
4. the movement of data is directed manually through end-user commands

On the other hand we can find examples of services that you might consider
"end-to-end" that share each of the first three properties, if not the

Data that passes over the Internet is never exchanged directly between
sender and reciever, it always passes through intermediate nodes (at a
minimum, routers).  The issue here is what sort of functions those
intermediate nodes can have and the communication can still be considered

I would argue that mail delivery can be considered "maximally end-to-end" as
long as it performs at intermediate nodes only the functions that cannot be
correctly and efficiently performed at the end-nodes.  Thus, maximally
end-to-end mail delivery would not include virus checking performed by an
intermediate mail server if that can be done at an end-node.

The question of what is necessary mail delivery functionality is a
subjective one.  From this point of view the degree of functionality that
can be performed at an intermediary while still being maximally end-to-end
depends on the notion of "necessary functionality".  That might mean that
some services that have implementations that rely heavily on active
intermediaries would be considered maximally end-to-end if there were way to
implement the functionality at the end-nodes.

I am interested because the Internet Backplane Protocol shares many
properties of e-mail (the first three of the list above) and yet I'd like to
think that it is maximally end-to-end.

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