  People change the identifiers through which they are reachable online as they
change jobs or residences or Internet service providers. This kind of personal
mobility makes reaching people online error-prone. As people move, they do not
always know who or what has cached their now obsolete identifiers so as to
inform them of the move. Use of these old identifiers can cause delivery
failure of important messages, or worse, may cause delivery of messages to
unintended recipients. For example, a sensitive email message sent to my now
obsolete work address at a former place of employment may reach my unfriendly
former boss instead of me.
  In this paper we describe HINTS, a historic name-trail service. This service
provides a persistent way to name willing participants online using today's
transient online identifiers. HINTS accomplishes this by connecting together
the names a person uses along with the times during which those names were
valid for the person, thus giving people control over the historic use of their
names. A correspondent who wishes to reach a mobile person can use an obsolete
online name for that person, qualified with a time at which the online name was
successfully used; HINTS resolves this historic name to a current valid online
identifier for the intended recipient, if that recipient has chosen to leave a
name trail in HINTS.
