  The Web is ephemeral. Many resources have representations that change over
time, and many of those representations are lost forever. A lucky few manage to
reappear as archived resources that carry their own URIs. For example, some
content management systems maintain version pages that reflect a frozen prior
state of their changing resources. Archives recurrently crawl the web to obtain
the actual representation of resources, and subsequently make those available
via special-purpose archived resources. In both cases, the archival copies have
URIs that are protocol-wise disconnected from the URI of the resource of which
they represent a prior state. Indeed, the lack of temporal capabilities in the
most common Web protocol, HTTP, prevents getting to an archived resource on the
basis of the URI of its original. This turns accessing archived resources into
a significant discovery challenge for both human and software agents, which
typically involves following a multitude of links from the original to the
archival resource, or of searching archives for the original URI. This paper
proposes the protocol-based Memento solution to address this problem, and
describes a proof-of-concept experiment that includes major servers of archival
content, including Wikipedia and the Internet Archive. The Memento solution is
based on existing HTTP capabilities applied in a novel way to add the temporal
dimension. The result is a framework in which archived resources can seamlessly
be reached via the URI of their original: protocol-based time travel for the
Web.
