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Arbitrary Attribute Interchange Protocol
Version 2.0
Mar 18 2009
Interchange of Persistent File Attributes
by Thomas Schmitt -
Libburnia project -
AAIP is intended as companion of the Rock Ridge Interchange Protocol RRIP
which under the general design of System Use Sharing Protocol SUSP extends
ISO 9660 aka ECMA-119 filesystem semantics to match POSIX needs.
Goal is to have for each file an arbitrary number of attributes which consist
of two components (Name and Value) of arbitrary length and to have a compact
representation of ACLs.
This document describes a SUSP entry with Signature Word "AL" which collides
neither with SUSP 1.12 nor with RRIP 1.12. The AL entry has been designed
to be as similar to the RRIP entry SL as possible.
The presence of AAIP shall be announced by a particular ER entry.
Since the size of a SUSP entry is limited to 255, multiple entries may be
needed to describe one component. The CE mechanism of SUSP shall be used to
address enough storage if needed.
AL entries and the ER entry of AAIP shall only be present if the ER entry
of RRIP is present.
System Entries Provided by this Specification
* AL
Description of the "AL" System Use Entry
The entry has exactly the same layout as RRIP entry SL. One has to expect
more data bytes than with SL, though, and any of the 256 possible byte values.
The reader shall be prepared to detect and handle oversized data.
One or more AL entries form the Attribute List of a file object with
an even number of components. Each two consequtive components form a pair of
Name and Value.
The empty name indicates that the value is a compact representation of ACLs.
Names must not contain byte value 0x00. Names which begin by bytes 0x01 to 0x1f
represent names in particular namespaces. See below: Namespaces.
The meaning of any other names or name parts is not specified by this document.
All AL entries except the last one shall have the CONTINUE flag set. An AL
entry with CONTINUE set to 0 indicates the end of the Attribute List.
The format of the AL System Use Field is as follows:
[1] "BP 1 to BP 2 - Signature Word" shall be (41)(4C) ("AL").
[2] "BP 3 - Length" shall specify as an 8-bit number the length in bytes of
the AA entry recorded according to ISO 9660:7.1.1.
[3] "BP 4 - System Use Entry Version" shall be 1 as in ISO 9660:7.1.1.
[4] "BP 5 - Flags" shall contain bit field flags numbered 0 to 7 starting
with the least significant bit as follows:
0 CONTINUE This AL entry continues in the next AL entry.
All other bits shall be set to 0.
[5] "BP 6 to Length - Component Area" shall contain Component Records
as described below.
Within AL entries each component (Name or Value) shall be recorded as one
or more component records. If a component does not fit into the remaining
space of an AL entry then it shall be continued in following AL entries.
All Component Records of a component except the last one shall have the
CONTINUE flag set. A Component Record with CONTINUE set to 0 indicates the end
of the component. An eventually following Component Record starts the next
The Component Record format is identical to the one of the SL entry.
The complete form of the following summary can be found in RRIP 1.12 "".
In case of discrepancies, RRIP 1.12 is the decisive specification. Please
inform the author of this document if you find such a discrepancy.
Component Records shall be recorded contiguously within each Component Area,
starting in the first byte of the Component Area. The last Component Record
in the Component Area of an AL System Use Entry may be continued in the
Component Area of the next recorded AL System Use Entry in the same
System Use Area.
Each Component Record shall have the following format:
[A] "BP 1 - Component Flags" shall contain bit field flags numbered 0 to 7,
starting with the least significant bit, as follows:
0 CONTINUE This Component Record continues in the next
AL Component Record.
all others are RESERVED and shall be 0.
[B] "BP 2 - Component Length (LEN_CP)" shall specify as an 8-bit number the
number of component bytes in the Component Record. This length shall not
include the first two bytes of the Component Record.
If any of the bit positions 1-3 is set, the value of this field shall be
set to ZERO and no Component Content shall be recorded.
This field shall be recorded according to ISO 9660 Format section 7.1.1.
[C] "BP 3 to 2 + LEN_CP - Component Content" shall contain the component
bytes in the Component Record.
Example: Two pairs of "name"="long...content" and "one"="more" encoded as
two AL entries
Field 1 contains the Component Record of Name and one Component Record of
Value :
{ 'A', 'L', 255, 1, 1,
0, 4, 'n', 'a', 'm', 'e',
1, 255, 'l', 'o', 'n', 'g', ... 238 more bytes, 13 go to next AL ... }
Field 2 contains the rest of "long...content" and the complete second pair.
It marks the end of the Attribute List :
{ 'A', 'L', 38, 1, 0,
... 13 remaining bytes of the Component Record in first entry ...
0, 7, 'c', 'o', 'n', 't', 'e', 'n', 't',
0, 3, 'o', 'n', 'e',
0, 4, 'm', 'o', 'r', 'e' }
AAIP provides a short notation for namespaces which uses a single non-printable
byte at the start of the name.
Reserved start bytes of names are
0x01 to 0x1F
The names of extended file attributes are traditionally organized in
namespaces, which get expressed as first part of an attribute name up to a
period "." character. It is also tradition that names are printable text,
single words and especially contain no 0-bytes.
AAIP does not enforce the use of any namespace but it urges that names in the
following registered namespaces are used according to traditions.
The namespaces "system." and "user." are available with many file system
types. "system." is file system dependent and often restricted in the
choice of names. "user." is portable and allows to choose about any name.
Namespace "isofs." is defined for internal use of AAIP enhanced ISO 9660
file systems. Names in this namespace should be registered at
Further namespaces may be registered at
The reserved start bytes of names have the following meaning
0x01 escape reserved character at start of name
0x02 namespace "system."
0x03 namespace "user."
0x04 namespace "isofs."
0x05 namespace "trusted."
0x06 namespace "security."
0x07 to 0x1F shall not be used yet.
Name "" with and without short notation. Both is allowed.
0, 4, 0x03, 'a', 'b', 'c'
0 8, 'u', 's', 'e', 'r', '.', 'a', 'b', 'c'
Name "\003abc" (if really desired)
0, 5, 0x01, 0x03, 'a', 'b', 'c'
Specification of binary ACL representation as special Arbitrary Attribute
The Name component of a binary ACL shall be of length 0.
The Value shall be an arbitrary number of ACL Entries:
[a] "BP 1 - Entry Flags" shall contain bit field flags numbered 0 to 7,
starting with the least significant bit, as follows:
0 EXEC indicates that this entry grants execute permission
1 WRITE write permission
2 READ read permission
3 QUALIFIER indicates that one or more Qualifier Records follow
4 - 7 TYPE
shall contain the tag type of the ACL entry as four bit code:
0 TRANSLATE Entry for a global map of name to numeric id
Qualifier is a record of number and text
1 ACL_USER_OBJ Permissions of owning user (as of PX entry)
3 ACL_GROUP_OBJ Permissions of owning group (as of PX entry)
5 ACL_MASK Restricts 10, 3, and 12 via logical AND
6 ACL_OTHER Permissions of non-listed, non-owning users
8 SWITCH_MARK Switch from "access" ACL to "default" ACL
10 ACL_USER_N Permissions of arbitrary user. Qualifier is
the numeric user id (max. 4 bytes).
12 ACL_GROUP_N Permissions of arbitrary group. Qualifier is
the numeric group id (max. 4 bytes).
15 FUTURE_VERSION Will indicate that this document
does not apply to the entry.
The other values are reserved. Readers shall ignore them if
they are not aware of updates of this document which would
assign a meaning to them.
The entries must match the permission bits of the PX entry. This shall obey the
rule that ACL_USER_OBJ must match S_IRWXU, ACL_OTHER must match S_IRWXO,
ACL_MASK - if present - must match S_IRWXG, else ACL_GROUP_OBJ must match
S_IRWXG. If there is ACL_USER_N or ACL_GROUP_N there must also be ACL_MASK.
A numeric qualifier is a binary number of variable length up to 4 bytes. The
Most Significant Byte comes first. The number shall be the "POSIX File User ID"
resp. "POSIX File Group ID" as also used in RRIP PX entries. The ids of owning
user and owning group shall be taken from the PX entry of the file object.
Optional TRANSLATE entries may associate user or group names with numeric
ids to allow the reading system to remap the numeric ids. See below.
The writer is not obliged to write them and the reader is not obliged to
interpret them.
The ACL entries belong to the "access" ACL of a file object. An optional
SWITCH_MARK entry may direct further entries to the "default" ACL which
is defined for directory objects. The EXEC bit of SWITCH_MARK shall be 1.
The bits for WRITE, READ, QUALIFIER shall be 0.
An eventually needed qualifier is stored in one or more Qualifier Records.
[b] "BP 2 - Qualifier Record Head" shall be present only if QUALIFIER is set
to 1. It shall give the number of Qualifier Bytes and eventually
indicate that the qualifier continues in a Qualifier Record which comes
imediately after this record.
0 to 127 Q_LENGTH, the qualifier is complete by this record
128 to 255 Q_LENGTH+128, the qualifier is continued by next record
So a Qualifier Record can contain at most 127 Qualifier Bytes.
This field shall be recorded according to ISO 9660 Format section 7.1.1.
[c] "BP 3 to BP 2 + Q_LENGTH - Qualifier Bytes" shall be present only if
QUALIFIER is set to 1 and hold the announced number of bytes of the
user or group name.
Example: From man 5 acl: u::rw-,u:lisa:rw-,g::r--,g:toolies:rw-,m::r--,o::r--
"lisa" has user number 123, "toolies" has group number 65534
{ 'A', 'L', 20, 1, 0,
0, 0,
0, 11, 0x16,
0xAE, 1, 123,
0xCE, 2, 255, 254,
0x64 }
Example: "Access" ACL and "default" ACL (0x81 is the switch mark)
u::rwx,g::r-x,o::r-x, du::rwx,dg::r-x,dm::rwx,do::r-x,du:lisa:rwx
{ 'A', 'L', 20, 1, 0,
0, 0,
0, 11, 0x17, 0x35, 0x65,
0x17, 0x35, 0x57, 0x65,
0xA7, 1, 123 }
Association of Names and Numeric Identifiers
The entry flag value 0x08 TRANSLATE is not a ACL entry of the hosting object
but rather a global hint about the relation of roles, names and numeric ids.
If it is recorded at all, then it shall be recorded with the first Directory
Entry of the volume's root directory. According to the description of SUSP
entry ER, this has to be "dot" or (00). Other than with ER, a TRANSLATE entry
may not appear in the root of directory sub trees.
An interested reader shall examine the Arbitrary Attributes of this Directory
Entry in order to collect a translation table.
The advised translation is: PX or AL Id number -> name -> local id number.
The Qualifier Bytes of a TRANSLATE entry shall have the following format:
[i] "BP 0 - Role" shall tell whether it is about a user name (role 0) or
a group name (role 1). Other values are not allowed.
[ii] "BP 1 to BP 8 - Numeric Id" shall hold the 32 bit POSIX Id number of the
entry. This field shall be recorded according to ISO 9660:7.3.3.
[iii] "BP 9 to End Of Qualifier - Name" shall hold the name bytes of this
Example: User id number 123 gets associated with user name "lisa"
0x08, 13, 0, 123,0,0,0, 0,0,0,123, 'l', 'i', 's', 'a',
Example: A very long qualifier naming "His_Excellency_..._the_Boss" as user #1.
This needs two qualifier records.
0x08, 255, 0, 1,0,0,0, 0,0,0,1,
'H', 'i', 's', '_', 'E', 'x', 'c', 'e', 'l', 'e',
... 108 more bytes ...
8, 't', 'h', 'e', '_', 'B', 'o', 's', 's',
Specification of the ER System Use Entry Values for AAIP:
This ER system entry shall only be present if the ER entry of RRIP is present.
To be compliant with SUSP-1.12, this ER entry must be present if AL entries
are present, and ES entries have to mark RRIP and AAIP entries.
If for some reason compliance with SUSP-1.10 is intended, then this ER entry
and the ES entries must not be present, although SUSP-1.10 would allow ER.
(See below: Compatibility considerations.)
The Extension Version number for this version of AAIP shall be 1.
The Extension Identifier field shall be "AAIP_0200" with Identifier Length 9.
The mandatory content form of the Extension Descriptor is
The Description Length is 81.
The recommended content of the Extension Source is
The corresponding Source Length is 62.
Compatibility Considerations
This extension is supposed not to disturb any reader system which complies
to SUSP-1.10:
"6.2 Requirements for a Receiving System
Any System Use Field which the receiving system does not recognize
is to be ignored and skipped."
SUSP-1.12 extends this prescription by:
"Any System Use Entry, with the exception of the set of System Use Entries
defined in this document, following an "ES" System Use Entry that indicates
an extension specification which the receiving system does not recognize
shall be ignored and skipped."
According to SUSP-1.12 the ER entry is mandatory for a conformant extension.
It also prescribes that in the case that ER entries of RRIP and AAIP are
present, then ES entries shall be used to separate RRIP entries from AAIP
SUSP-1.12 frowns on extensions which are not announced by ER. Nevertheless
is does not totally outrule them.
SUSP-1.10 does not specify ES entries at all and allows to have extension
entries without announcing them by an ER entry. So if a second ER entry is
not bearable, then the SUSP-1.10 downgrade of AAIP allows to omit the
AAIP ER and the ES entries. But if there is the AAIP ER then there must be ES
at the appropriate places. Else the format would explicitely violate SUSP-1.12.
Model Relations:
Attribute List ------------- [1:0..1] ------------- ACL
[1:0..n] [1:0..n]
Arbitrary Attribute ( [1:0..1] ACL ) Entry
[1:2..2n] [1:0..1]
Component ( [1..m:1..n] AL Field ) Qualifier
[1:1..n] << one of >>
Component Record / \
[1..m:1..n] Translation Entry , Numeric Id
AL Field | |
[1:1..n] [1:1]
\ /
Qualifier Record
Revoked drafts:
The following outdated versions may be interpreted at read time but they
shall not be written any more.
Previous versions up to AAIP 1.0 used field signature "AA" rather than "AL".
This nearly collides with "Apple ISO 9660 Extensions". The Apple "AA" field of
version 1 has a length of 7, whereas the shortest first AAIP field "AA" had
length 9.
Beginning with AAIP 2.0, the field name has been changed to "AL".
If a reader interprets old AAIP "AA" fields, then it must take precautions to
distinguish them from Apple "AA" fields. But it is well compliant with AAIP 2.0
to just ignore any kind of "AA" fields.
AAIP 1.0 had ER signature "AAIP_0100".
AAIP 0.2 with ER signature "AAIP_0002" allowed to announce and use a different
signature than "AA". This was revoked because ES entries serve the purpose
to distinguish AAIP entries from eventual "AA" entries of any other extension.
Regrettably no reader (kernel) was found which neatly interprets ES. Many do
not even recognize the RRIP-1.12 ER signatures "IEEE_P1282", "IEEE_1282".
AAIP 0.2 defined two ACL types which did not make it into AAIP 1.0
2 ACL_USER of arbitrary user, with name as qualifier
4 ACL_GROUP of arbitrary group, with name as qualifier
Their job was transferred to ACL_USER_N and ACL_GROUP_N which have numeric
There was a draft AAIP 0.0 with ER signature "AAIP_2008A". It did not resemble
the existing entry SL and was never implemented.
ECMA-119 aka ISO 9660
SUSP 1.12 (entries CE , PD , SP , ST , ER , ES)
RRIP 1.12 (entries PX , PN , SL , NM , CL , PL , RE , TF , SF , obsolete: RR)
Apple ISO 9660 Extensions (entries AA and BA)
Amiga AS entry
zisofs entry ZF (prepared by zisofs-tools, written by mkisofs)
Program mkisofs emits entry XA