reliable-links.html
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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML Strict Level 1//EN">
<HTML>
<HEAD>
<TITLE>Link Reliability - Why URNs are Not the Answer</TITLE>
</HEAD>
<BODY BGCOLOR="#ffffff" TEXT="#000000">
<P>
<A HREF="../"><IMG BORDER="0" SRC="../Icons/WWW/w3c_home" ALT="W3C" WIDTH="72" HEIGHT="48"
BORDER="0"></A>
<A HREF="./"><IMG BORDER="0" SRC="../Icons/WWW/propagation_48x48" ALT="Propagation"
WIDTH="48" HEIGHT="48" BORDER="0"></A>
<H1>
Link Reliability - Why URNs are Not the Answer
</H1>
<P>
We all hate broken links. Some folks have said
"<A HREF="../Addressing/#URL">URL</A>s are broken. We need
<A HREF="../Addressing/#URN">URN</A>s. URNs will solve the problem of broken
links." I doubt it.
<P>
URNs will play an important role in publishing on the web (along with copyright
enforcement mechanisms, payment mechanisms, etc. see
<A href="../Addressing/citations#URN">URLs as catalog numbers</A>) but I
doubt they will increase <A HREF="../Glossary#reliability">reliability</A>
(or quality of service) for the vast majority of web links, because URNs
will impose administrative overhead (e.g. registration, digital signatures),
or at least work-flow restrictions (e.g. once you've made a document available
under a URN, you can never change it). (see
<A HREF="../Addressing/#STANF">[STANF] </A>for an excellent discussion)
<P>
I suspect there are propogation techniques that will increase reliability
without the cost of human intervention. There are ways to distribute replicas
of <A HREF="../Glossary#resources">resources</A> such that
<A HREF="../Glossary#availability">availability </A>will be sufficiently
high and latency sufficiently low.
<p>Another mechanism to increase reliability is <a
href="../Addressing/auto-update">automated link maintenance</a> for
referential integrity.
<H2>
Defining Link Reliability
</H2>
<P>
If we look at link traversal as a case of the
<A HREF="../Glossary#I-R">information retrieval problem</A>, we can start
to measure reliability only after we've defined "success."
<P>
Successful link traversal generally means finding a resource with perfect
<A HREF="../Glossary#precision">precision</A> and
<A HREF="../Glossary#recall">recall</A>, and retrieving an
<A HREF="../Glossary#authentic">authentic</A> representation of the resource
in a timely fashion, i.e. with sufficiently low
<A HREF="../Glossary#latency">latency</A>.
<P>
If a resource is <A HREF="../Glossary#replica">replicated </A>to increase
availability or decrease bandwidth consumption, it is important that the
various replicas are in sync (or close -- some applications may be willing
to accept out of date information some small percentage of the time.)
<P>
@@Replication mechanisms are complicated by the need to support access control
and tracking policies.
<H2>
More Information
</H2>
<DL>
<DT>
<A HREF=http://www.w3.org/team/WWW/Addressing/citations.html>Catalog
Searching</A>
<DD>
for info on resource description and discovery, seen as a knowledge
representation and query problem
<DT>
<A HREF="http://www.w3.org/hypertext/WWW/Addressing/Addressing.html">Addressing
and names</A>
<DD>
Tim BL's original design notes
<DT>
<A HREF=http://www.w3.org/team/WWW/People/Connolly/drafts/formalism.html>A
Formalism for Internet Information References</A>
<DD>
out-of-date draft
<DT>
<A HREF=http://www.w3.org/team/WWW/People/Connolly/drafts/link-rpc.html>Toward
Reliable, Interoperable Links</A>
<DD>
out-of-date draft
<DT>
<A HREF="../Addressing/url_test/">URI test suite</A>
<DD>
March 16, 1994: The expanded release of a URI test suite (including a fairly
complete grammar for URI's that includes ways to represent URLs and URIs)
</DL>
<P>
<HR>
<ADDRESS>
<A HREF="../People/Connolly/">Connolly</A><BR>
@(#) $Id: reliable-links.html,v 1.7 1997/08/09 17:55:05 fillault Exp $
</ADDRESS>
</BODY></HTML>