Installing and Administering OSI Transport Services

Chapter 1 49
HP OTS/9000 Resources
Structure of NSAP Addresses
that begins with this value is ultimately under the control of NIST. NIST
also owns the right to define how the DSP portion of its NSAP space is to
be formatted and used.
The IDP contains two fields. The authority and format identifier (AFI)
field identifies the type of address used in the DSP. The initial domain
identifier (IDI) field identifies which domain the DSP part belongs to.
NOTE In this section, all numeric values are represented as hexadecimal digits
unless otherwise specified.
The Domain Specific Part
The purpose of the DSP is to
• allow authorities to further delegate control of NSAP addresses
• uniquely identify a real open system unambiguously on the network
• provide information that may be used to facilitate the routing of data
on concatenated subnetworks
This is accomplished by dividing the DSP into a number of fields, each
with its own meaning. As an example, consider the format defined by
ANSI:
Figure 1-9
The AFI value of 39 indicates that the IDI portion will be a Data Country
Code (DCC), and the DSP will be encoded in binary. The value 840
(padded with F) is the DCC for the United States.
ANSI has defined the first three octets of the DSP to contain an
Organization Identifier (Org Id). This number, assigned by ANSI, allows
other organizations to control a subset of ANSI’s NSAP space. ANSI has
left the rest of the DSP undefined so that the organization will have the
ability to define its own address structure. For instance, if Joe’s Grommet
Shop petitions ANSI for an Org Id value and is assigned the value
010101 (three octets), then Joe’s Grommet Shop controls the prefix
39 84
IDP
DSP
19octet 0
0F Org ID
1 2 3 - 5 6