Guidelines for use of a 32-bit or 40-bit
Context
Dependent Identifier (CDI-32™ and CDI-40™)
General
The IEEE defined 32-bit context-dependent identifier
(CDI-32™) is a concatenation of a 24-bit Organizationally Unique Identifier
(OUI) value administered by the IEEE Registration Authority and an 8-bit
extension identifier assigned by the organization with that OUI
assignment.
The IEEE defined 40-bit context-dependent identifier
(CDI-40™) is a concatenation of a 24-bit Organizationally Unique Identifier
(OUI) value administered by the IEEE Registration Authority and a 16-bit
extension identifier assigned by the organization with that OUI
assignment.
The IEEE administers the assignment of 24-bit OUI
values. The assignments of these values are public, so that a user of an CDI-32
or CDI-40 value can identify the manufacturer that provided the value[1]. The
IEEE/RAC has no control over the assignments of 24-bit extension identifiers and
assumes no liability for assignments of duplicate CDI-32 or CDI-40 identifiers
assigned by manufacturers.
Context identification
In the case of the IEEE 802 standards, standard 802b
defines a schema for allocating ASN.1 Object Identifier arcs for the use
of each working group in IEEE 802. This schema establishes the ability of each
IEEE 802 Working Group to assign Object Identifier values for standards use. It
is recommended that IEEE 802 Working Groups that define context-dependent
identifiers allocate an arc under their "root" ASN.1 Object Identifier value,
specifically for the purpose of assigning further sub-arcs to allow an Object
Identifier "root" value to be assigned to each context-dependent identifier (or
perhaps more correctly, to act as an identifier for the identifier's context).
By this means, a globally unique ASN.1 Object Identifier value could be derived
from a context-dependent identifier value. For example, the "root" ASN.1 Object
Identifier value assigned for use by 802.1 is:
iso(1) std(0)
iso8802(8802) ieee802dot1(1)
The next arc could be assigned to be used for 32-bit context-dependent
identifers:
iso(1) std(0)
iso8802(8802)
ieee802dot1(1)
context-dependent-32-bit-identifiers(X)
The next arc could then be used for each particular 32-bit context-dependent
identifier (CDI-32) defined by 802.1. For example, in standard 802.1ZZ, there
might be a context-dependent identifier called vendorDependent defined, which
could be assigned by 802.1 the following Object Identifier root value:
iso(1) std(0)
iso8802(8802)
ieee802dot1(1)
context-dependent-32-bit-identifiers(X)
vendorDependent(Y)
The next two arcs would then be used for the OUI value and the extension
identifier value. Hence, the full, globally unique, ASN.1 Object Identifier
value corresponding to a single context-dependent identifier value would consist
of a sequence of 8 integers, the final two being the OUI and the extension
identifier values. The use of the "short form" (just the OUI and the extension
identifier) within the defined context would then be equivalent to using the
full Object Identifier value.
The same approach, but with distinct arc values to identify the appropriate
SDO and standard, could be adopted in other standards where it is considered
appropriate to make use of this type of context-dependent
identification.
32-bit context-dependent identifier format
tutorial
OUI based CDI-32 sequence
A 32-bit context dependent identifier (CDI-32) consists of two components.
The 24-bit first portion of this value is the OUI value assigned to the
manufacturer by the IEEE Registration Authority. The 8-bit second portion of
this identifier is assigned by the manufacturer.
For example, assume that a manufacturer's IEEE-assigned OUI value is
AC-DE-48 and the manufacturer-selected extension identifier for a
given component is 23. The CDI-32 value generated from these two numbers is
AC-DE-48-23.
Some standards specify a CDI-32 by a string of four bytes, labeled here
as cdi[0] through cdi[3], as illustrated below. Although different
standards may specify different bit-transmission orders, bytes are
normally transmitted in an ascending index-value order.
iso(1) std(0) iso8802(8802) ieee802dot1(1)
context-dependent-32-bit-identifiers(W) vendorDependent(X)
| OUI | ext id | field
| | |
| cdi[0] | cdi[1] | cdi[2] | cdi[3] | order
| | | | |
| AC | DE | 48 | 23 | byte
Other standards may specify an CDI-32 to be a numerical value, upon which
computations (such as base/bounds or bit selections) can be performed, as
illustrated below:
iso(1) std(0) iso8802(8802) ieee802dot1(1)
context-dependent-32-bit-identifiers(W) vendorDependent(X)
ACDE482316
40-bit context-dependent identifier format
tutorial
OUI based CDI-40 sequence
A 40-bit context dependent identifier (CDI-40) consists of
two portions The 24-bit first portion of this value is the OUI value assigned
to the manufacturer by the IEEE Registration Authority. The 16-bit second
portion of this identifier is assigned by the manufacturer.
For example, assume that a manufacturer's IEEE-assigned OUI
value is AC-DE-48 and the manufacturer-selected extension identifier for a given
component is 23-48. The CDI-40 value generated from these two numbers is
AC-DE-48-23-45.
Some standards specify a CDI-40 by a string of five
bytes, labeled here as cdi[0] through cdi[4], as illustrated below. Although different standards may
specify different bit-transmission orders, bytes are normally transmitted in an
ascending index-value order.
iso(1) std(0) iso8802(8802) ieee802dot1(1)
context-dependent-40-bit-identifiers(Y) vendorDependent(Z)
| OUI | ext id | field
| | |
| cdi[0] | cdi[1] | cdi[2] | cdi[3] | cdi[4] | order
| | | | | |
| AC | DE | 48 | 23 | 45 | byte
Individual address block (IAB) based CDI-40 sequence
An individual address block (IAB) based 40-bit context dependent identifier (CDI-40) consists of
two portions The 36-bit first portion of this value is specified by the IAB
base value assigned
to the manufacturer by the IEEE Registration Authority. The 4-bit second
portion of this identifier is assigned by the manufacturer.
For example, assume that a manufacturer's IEEE-assigned IAB
base
value is AC-DE-48-23-40-00 and the manufacturer-selected extension identifier for a given
component is 5. The CDI-40 value generated from these two numbers is
AC-DE-48-23-45.
Some standards specify a CDI-40 by a string of five
bytes, labeled here as cdi[0] through cdi[4], as illustrated below. Although different standards may
specify different bit-transmission orders, bytes are normally transmitted in an
ascending index-value order.
iso(1) std(0) iso8802(8802) ieee802dot1(1)
context-dependent-40-bit-identifiers(Y) vendorDependent(Z)
| IAB | ext| field
| | |
| cdi[0] | cdi[1] | cdi[2] | cdi[3] | cdi[4] | order
| | | | | |
| AC | DE | 48 | 23 | 45 | byte
Numerical formats
Other standards may specify an CDI-40 to be a numerical
value, upon which computations (such as base/bounds or bit selections) can be
performed, as illustrated below:
iso(1) std(0) iso8802(8802) ieee802dot1(1)
context-dependent-40-bit-identifiers(Y) vendorDependent(Z)
ACDE48234516
Context specifiers within standards
Since a CDI-32 value is only unique within
its context, standards are responsible for specifying the context adjacent
to the specification of CDI-32 fields or values, as illustrated in
preceding Byte sequence formats and
Numerical formats subclauses.
Similarly, since a CDI-40 value is only unique within its context,
standards are responsible for specifying the context adjacent to the
specification of CDI-40 fields or values, as illustrated in preceding
Byte sequence formats and
Numerical formats
subclauses.
If CDI-32 values are provided for distinct applications
within one standard, and CDI-32 values are only required to be unique
within each application, then distinct CDI-32 arcs should be specified.
Similarly, if CDI-40 values are provided for distinct applications within one
standard, and CDI-40 values are only required to be unique within each
application, then distinct CDI-40 arcs should be specified.
[1]Except for private OUI values, where the owner of the OUI value
is confidential. These remain private.
