The RRIP extensions are built upon a related standard System Use Sharing Protocol (SUSP, IEEE P1281). It's provides a generic way of including additional properties for any directory entry reachable from the primary volume descriptor (PVD).

In an ISO 9660 volume, every directory entry has an optional system use area whose contents are undefined and left to be interpreted by the system.

SUSP defines a method to subdivide that area into multiple system use fields, each identified by a two-character signature tag. The idea behind SUSP was that it would enable any number of independent extensions to ISO 9660 (not just RRIP) to be created and included on a volume without conflicting. It also allows for the inclusion of property data that would otherwise be too large to fit within the limits of the system use area.

SUSP defines several common tags and system use fields:

  • CE - Continuation area,
  • PD - Padding field,
  • SP - System use sharing protocol indicator,
  • ST - System use sharing protocol terminator,
  • ER - Extensions reference,
  • ES - Extension selector.

RRIP defines additional SUSP tags for support of POSIX semantics, along with the format and meaning of the corresponding system use fields:

  • RR - Rock Ridge extensions in-use indicator (note: dropped from standard after version 1.09),
  • PX - POSIX file attributes,
  • PN - POSIX device numbers,
  • SL - symbolic link,
  • NM - alternate name,
  • CL - child link,
  • PL - parent link,
  • RE - relocated directory,
  • TF - time stamp,
  • SF - sparse file data.

Other known SUSP fields include:

  • AA - Apple extension, preferred,
  • AB - Apple extension, old,
  • AS - Amiga file properties (see below).

Important: the Apple ISO 9660 Extensions do not technically follow the SUSP standard. However the basic structure of the AA and AB fields defined by Apple are forward compatible with SUSP; so that, with care, a volume can use both Apple extensions as well as RRIP extensions.