libisofs/doc/devel/cookbook/ISO 9660-1999

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                    ISO/IEC 9660:1999 Cookbook
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Creation date: 2008-Jan-14
Author: Vreixo Formoso
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Contents:
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1. References
2. General
3. Features
4. Implementation
5. Known implementation bugs and specification ambiguities/problems


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1. References:

Meanwhile this is specified in ECMA-119 4th Edition of 2019 by the description
of the Enhanced Volume Descriptor.

The old name "ISO 9660:1999" comes from:
ISO/IEC DIS 9660:1999(E) "Information processing. Volume and file structure of
                          CD­-ROM for Information Interchange"


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2. General

ISO 9660:1999, also known as ISO-9660 version 2 is an update of the old 
ISO 9660:1988 standard for writing data images for CD.

In the same way Joliet does, it is based on a Secondary Volume Descriptor (that
is called Enhanced Volume Descriptor), that provides a second tree where the
new file information is recorded. 

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3. Features

It makes some improvements with respect to ECMA-119, mainly related to relax
the constraints imposed by its predecessor.

- It removes the limit to the deep of the directory hierarchy (6.8.2.1). 
However, it still keep a limit to the path length, of 255 characters as in
ECMA-119.

- File names don't need the version number (;1) anymore, and the "." and ";",
used as SEPARATORS for extension and version number, have no special meaning
now.

- The file name max length is incremented to 207 bytes.

- The file name is not restricted to d-characters. 

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4. Implementation

ISO 9660:1999 is very similar to old ISO 9660:1988 (ECMA-119). It needs two
tree hierarchies: one, identified by the Primary Volume Descriptor, is recorded
in the same way that an ECMA-119 structure.

The second structure is identified by a Enhanced Volume Descriptor (8.5). The
structure is exactly like defined in ECMA-119, with the exceptions named above.

Thus, to write an ISO 9660:1999:

- First 16 blocks are set to 0.
- Block 16 identifies a PVD (8.4), associated with a directory structure 
written following ECMA-119.
- It is needed a Enhanced Volume descriptor to describe the additional 
structure. It is much like a SVD, with version number set to 2 to identify
this new version.
- We can also write boot records (El-Torito) and additional SVD (Joliet).
- We write a Volume Descriptor Set Terminator (8.3)
- We write directory structure and path tables (L and M) for both ECMA-119
tree and enhanced tree. Path table record and directory record format is
the same in both structures. However, ECMA-119 is constrained by the usual
restrictions. 
- And write the contents of the files.

Interchange levels 1, 2 and 3 are also defined. For PVD tree, they have the
same meaning as in ECMA-119. For EVD tree, in levels 1 and 2 files are 
restricted to one file section (i.e., 4 GB filesize limit). In level 3 we can
have more than one section per file. Level 1 does not impose other 
restrictions than that in the EVD tree.

It seems that both El-Torito and Joliet can coexist in a ISO 9660:1999 image.
However, Joliet has no utility at all in this kind of images, as it has no
benefit over ISO 9660:1999, and it is more restrictive in filename length.

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5. Known implementation bugs and specification ambiguities/problems

- While the specification clearly states that the tree speficied by the Primary
Volume Descriptor should remain compatible with ISO-9660 (ECMA-119), i.e., it 
should be constrained by ECMA-119 restrictions, some image generation 
applications out there just make both Primary and Enhanced Volume Descriptors 
to point to the same directory structure. That is a specification violation, as
for a) the directory hierarchy specified in the Primary Volume Descriptor 
doesn't follow the restrictions specified in the specs, and b) the same 
directories are part of two different hiearchies (6.8.3 "A directory shall not 
be a part of more than one Directory Hierarchy.").
Thus, we should keep two trees as we do with Joliet. Or are there strong 
reasons against this?

- It's not very clear what characters are allowed for files and dir names. For
the tree identified in the Enhanced Volume Descriptor, it seems that a
"sequence of characters rather than d-characters or d1-characters" is allowed.
It also seems that the charset is determined by the escape sequence in the
EVD. Anyway, leaving escape sequence to 0 and use any user-specified sequence
(such as UTF-8) seems a good solution and is what many other applications do.
Linux correctly mounts the images in this case.

- It is not clear if RR extensions are allowed in the tree identified by the
Enhanced Volume Descriptor. However, it seems not a good idea. With 207 bytes
filenames and XA extensions, there is no place for RR entries in the directory
records of the enhanced tree. In my opinion, RR extension should be attached to
the ECMA-119 tree that must also be written to image.