Index of /specs/pingback

      Name                    Last modified      Size  Description
      Parent Directory                             -   
      translations/           2013-03-11 11:03    -   The pingback specification in different languages
      pingback-1.0-notes      2002-09-24 06:57   85   Pingback 1.0 Specification
      pingback-0.9            2003-03-10 06:15   18K  Pingback 0.9
      pingback-0.9.1          2003-03-10 06:15   24K  Pingback 0.9.1
      pingback-0.9.2          2003-03-10 06:15   24K  Pingback 0.9.2
      pingback-0.9.3          2003-03-10 06:15   25K  Pingback 0.9.3
      pingback                2013-03-11 11:04   26K  The latest version of the pingback specification
      pingback-1.0            2013-03-11 11:04   26K  Pingback 1.0 Specification

Why pingback? Why not referrers?

Referrers are very noisy (spiders include bogus referrers, people come to
sites with only subtly different referrers such as a trailing slash or no
trailing slash, etc); referrers don't give you the permalink of the
originating resource; referrers contain many links from news aggregators,
blog rolls, search engines and other pages which do not contain comments
on the post.

Pingbacks are reliable because they are explicitly requested; pingbacks
have a near perfect signal:noise ratio because they are only triggered by
pages that actually link to the post with comments; the pingback mechanism
is easily extended to provide services such as pingback listing (when a
site asks another for the list of pingbacks).