thoughts on URN resolution

Karen R. Sollins (sollins@lcs.mit.edu)
Tue, 20 Jun 1995 20:47:47 -0400


Date: Tue, 20 Jun 1995 20:47:47 -0400
Message-Id: <199506210047.UAA05503@lysithea.lcs.mit.edu>
From: "Karen R. Sollins" <sollins@lcs.mit.edu>
To: uri@bunyip.com
Subject: thoughts on URN resolution

Wow!  You've all been incredibly busy typing while I was out of my
office for 10 days.  Hopefully, once I've waded through it all, I'll
have some comments, but I'll try to limit them.

I must admit that I haven't really gotten through all the mail yet,
but on one of my many plane trips during this time, I wrote the
attached note.  I apologize if it sounds tutorial; it was intended for
a less sophisticated audience than all of you.  But, I thought the
ideas were worth posting.  As you can guess from the format, I've also
submitted as an internet-draft, just for completeness.

When I suggested to Larry that I wanted to talk about something like
this in Stockholm, he kindly suggested that we should be talking about
documents - so here's a document. He's right, of course; writing
causes one to organize one's thoughts a little better.  This doesn't
mean that we necessarily should talk about it, but I think it would be
a valuable conversation to have.

There is no intention that it ever be anything more than a draft
document for discussion purposes only.  I also realize in reading as
far as I have on the list that various pieces of these ideas have been
brought up by various people.  I don't claim that the ideas are new,
just collated a bit.

			Karen

------------------------------

Network Working Group                                Karen R. Sollins
INTERNET-DRAFT                                                MIT/LCS
<draft-sollins-urn-res-thoughts-00.txt>
Expires: December 25, 1995                              June 25, 1995


      Thoughts on Standardizing URN Resolution Protocols
                      Karen R. Sollins
                           MIT/LCS
                     sollins@lcs.mit.edu

1 Introduction

The IETF is general a standards organization.  As such, part of its
task is to determine what should and what should NOT be standardized.
In this note, I explore this question with respect to URN resolution,
which the Uniform Resource Identifiers Working Group is now
considering.  But first, consider two conflicting goals in the
standards process.  From one perspective, there is strong motivation
to maximize the scope of standards.  This leads to a simple,
uniformity by minimizing diversity, with the result that mechanism and
implementation can probably be simplified and streamlined.  At the
other extreme is a path of no standardization in which there is
nothing but diversity, with no commonality that can be assumed by
clients of the service being considered.  Generally, a middle ground
is most effective but the question always must be where the boundary
is.

Additionally, when considering the problems of name resolution, it is
important to remember that one can separate the problems or activities
of name assignment from name resolution.  In order to allow for names
to be equally resolvable in perpetuity (or even just 100 years), it
will be important not to burden the name asigner with the
responsibility for a name resolution service.  Since resources may
move or evolve in unpredictable ways it is not reasonable even to
expect that a particular service will necessarily guarantee to provide
name resolution service for a particular namespace or piece of the
namespace with a granularity larger than one.  This may happen, but it
should not be expect or legislated without understanding the serious
consequences of severe limitations on the client community.  With all
this in mind, this note concentrates on the question of standardizing
name resolution.


2 Partitioning the problem

Now, consider name resolution.  First we will address a partitioning
of the problem of name resolution.  This partitioning is not new, and
has certainly been seen before in other standards activities with
respect to name resolution, for example the DNS and X.500.  In both
cases, the universe of discourse is partitioned into clients and
servers.  In both cases, the interfaces or protocols for servers are
clearly distinguished between that for clients to communicate with
servers and that for servers to communicate among themselves.  The
client interfaces describe client requests for name resolution, while
server interfaces describe both server requests for resolution and
inter-server information addition, deletion, and modification, as well
as perhaps cache maintenance and any other maintenance and
distribution mechanisms for making translation information available
among the servers.

2.1 Server Behavior

In that context, consider URNs.  URNs are intended to name an
extremely broad set of resources, in fact, anything that anyone wishes
to make public by naming in the Internet.  Ubiquity and penetration as
well as quantity are expected to be many orders of magnitude greater
than the resources of either the DNS or X.500.  In addition, there may
be different sorts of performance requirements for different sorts of
resources by different sorts of clients or customers.  For example,
for the human to send a piece of email, the location resolution might
be adequately provided in 10 minutes, . while, if the human is asking
for the locations of resources on the Schumacher-Levy 9 comet in
realtime as part of some complex piece of research there may be a
cascading set of resolution requirements, leading to each one needing
to be orders of magnitude faster.  In addition, there may be multiple
answers to such a request from different kinds of sources, online data
sets, published papers, or a computation service that might analyze
observations on the fly.

One conclusion one might draw from this is that there may be widely
differing requirements or at least expectations for what one might
call auxilliary characteristics of name resolution services.  In some
cases, it may be particularly useful that they be replicated widely.
For example, telephone directories are distributed to every office and
home with a telephone.  As a first cut at causing email to be
delivered to the location at which the recipient is receiving mail,
one might consider distributing email address resolution information
for everyone, for example, to be widely replicated.  One reason this
sort of information can be widely replicated is that it doesn't change
frequently, so exceptional behavior (which is generally much more
expensive) in the face of changes will not need to be utilized
frequently.  In contrast, information about the location of most
satellites changes so frequently that a completely different set of
mechanisms for managing and therefore finding that information will be
needed.  Another interesting characteristic of some kinds of
resolution information is the need for it to be reliably correct.  If
recourse to an exceptional mechanism is prohibitatively expensive or
simply unavailable, it becomes much more important that updates of
information be reliably updated, so that they will not be lost.  One
might choose some atomic transaction mechanism with a great deal of
redundancy, recognizing that it would be out of the question for the
200 million or more telephone books scattered around the country.
What we are seeing here is that there will be a broad spectrum of
interesting, useful, perhaps requisite behaviors from name resolution
services with the conclusion that very different protocols will be
required among the servers that are providing such characteristics of
their services in some coordinated fashion.

2.2 Client-server behavior

In contrast with this, consider the clients of the name resolution
services.  At their most basic level, they want name resolution.  They
have a URN and they want some location and access method information.
In fact, for generality, it is desirable that applications not need to
know exactly which sorts of name resolution services will be
supporting them.  This allows for portability and longevity of code.
If an application needs an extremely fast response, it may be equally
satisfied with a local, atomically updated reliable service, or the
ubiquitous telephone book equivalent, if both have the same
information and can respond equally quickly.  In fact, the application
may not care to know which sort was used.  From the client or
application's perspective, the name resolution service should handle
requests for name resolution however it does it best, and for
modularity the client shouldn't need to know what that is.  Thus, in
contrast with the server-server protocol(s), we can conclude that the
client-server protocol for name resolution is a good candidate for
standardization; there is a big payoff and probably little penalty.

3. Conclusion

There is still a discussion that should be had within the URI Working
Group about one versus many server-server protocols.  I believe that
it is important that we design an architecture that permits more than
one, but there are strong arguments in favor of not spreading the
efforts too thin.  I suggest that as a community we should put our
concerted efforts into no less than two, for a proof of concept and
demonstration of the generality required and supportable, but keep the
number quite small, perhaps no more than three.