The XAR project aims to provide an easily extensible archive format.
Important design decisions include an easily extensible XML table of
contents for random access to archived files, storing the toc at the
beginning of the archive to allow for efficient handling of streamed
archives,  the ability to  handle files of  arbitrarily large sizes,
the ability to choose independent  encodings for individual files in
the archive, the ability to  store checksums for individual files in
both compressed  and uncompressed form, and the ability to query the
table of content's rich meta-data.

WWW: http://mackyle.github.io/xar/