|Title||Guidelines for OSI NSAP Allocation in the Internet
|Author||R. Colella, R.
Callon, E. Gardner, Y. Rekhter
Network Working Group R. Colella
Request for Comments: 1629 NIST
Obsoletes: 1237 R. Callon
Category: Standards Track Wellfleet
T.J. Watson Research Center, IBM Corp.
Guidelines for OSI NSAP Allocation in the Internet
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
CLNP is currently being deployed in the Internet. This is useful to
support OSI and DECnet(tm) traffic. In addition, CLNP has been
proposed as a possible IPng candidate, to provide a long-term
solution to IP address exhaustion. Required as part of the CLNP
infrastructure are guidelines for network service access point (NSAP)
address assignment. This paper provides guidelines for allocating
NSAP addresses in the Internet.
The guidelines provided in this paper have been the basis for initial
deployment of CLNP in the Internet, and have proven very valuable
both as an aid to scaling of CLNP routing, and for address
Table of Contents
Section 1. Introduction ............................... 4
Section 2. Scope ...................................... 5
Section 3. Background ................................. 7
Section 3.1 OSI Routing Standards ..................... 7
Section 3.2 Overview of IS-IS (ISO/IEC 10589) ......... 8
Section 3.3 Overview of IDRP (ISO/IEC 10747) .......... 12
Section 3.3.1 Scaling Mechanisms in IDRP .............. 14
Section 3.4 Requirements of IS-IS and IDRP on NSAPs ... 15
Section 4. NSAPs and Routing .......................... 16
Section 4.1 Routing Data Abstraction .................. 16
Section 4.2 NSAP Administration and Efficiency ........ 19
Section 5. NSAP Administration and Routing in the In-
ternet ........................................... 21
Section 5.1 Administration at the Area ................ 23
Section 5.2 Administration at the Subscriber Routing
Domain ........................................... 24
Section 5.3 Administration at the Provider Routing
Domain ........................................... 24
Section 5.3.1 Direct Service Providers ................ 25
Section 5.3.2 Indirect Providers ...................... 26
Section 5.4 Multi-homed Routing Domains ............... 26
Section 5.5 Private Links ............................. 31
Section 5.6 Zero-Homed Routing Domains ................ 33
Section 5.7 Address Transition Issues ................. 33
Section 6. Recommendations ............................ 36
Section 6.1 Recommendations Specific to U.S. Parts of
the Internet ..................................... 37
Section 6.2 Recommendations Specific to European Parts
of the Internet .................................. 39
Section 6.2.1 General NSAP Structure .................. 40
Section 6.2.2 Structure of the Country Domain Part .... 40
Section 6.2.3 Structure of the Country Domain
Specific Part .................................... 41
Section 6.3 Recommendations Specific to Other Parts of
the Internet ..................................... 41
Section 6.4 Recommendations for Multi-Homed Routing
Domains .......................................... 41
Section 6.5 Recommendations for RDI and RDCI assign-
ment ............................................. 42
Section 7. Security Considerations .................... 42
Section 8. Authors' Addresses ......................... 43
Section 9. Acknowledgments ............................ 43
Section 10. References ................................ 44
Section A. Administration of NSAPs .................... 46
Section A.1 GOSIP Version 2 NSAPs .................... 47
Section A.1.1 Application for Administrative Authority
Identifiers ...................................... 48
Section A.1.2 Guidelines for NSAP Assignment ......... 50
Section A.2 Data Country Code NSAPs .................. 50
Section A.2.1 Application for Numeric Organization
Name ............................................. 51
Section A.3 Summary of Administrative Requirements .. 52
The Internet is moving towards a multi-protocol environment that
includes CLNP. To support CLNP in the Internet, an OSI lower layers
infrastructure is required. This infrastructure comprises the
connectionless network protocol (CLNP)  and supporting routing
protocols. Also required as part of this infrastructure are
guidelines for network service access point (NSAP) address
assignment. This paper provides guidelines for allocating NSAP
addresses in the Internet (the terms NSAP and NSAP address are used
interchangeably throughout this paper in referring to NSAP
The guidelines presented in this document are quite similar to the
guidelines that are proposed in the Internet for IP address
allocation with CIDR (RFC 1519 ). The major difference between
the two is the size of the addresses (4 octets for CIDR vs 20 octets
for CLNP). The larger NSAP addresses allows considerably greater
flexibility and scalability.
The remainder of this paper is organized into five major sections and
an appendix. Section 2 defines the boundaries of the problem
addressed in this paper and Section 3 provides background information
on OSI routing and the implications for NSAP addresses.
Section 4 addresses the specific relationship between NSAP addresses
and routing, especially with regard to hierarchical routing and data
abstraction. This is followed in Section 5 with an application of
these concepts to the Internet environment. Section 6 provides
recommended guidelines for NSAP address allocation in the Internet.
This includes recommendations for the U.S. and European parts of the
Internet, as well as more general recommendations for any part of the
The Appendix contains a compendium of useful information concerning
NSAP structure and allocation authorities. The GOSIP Version 2 NSAP
structure is discussed in detail and the structure for U.S.-based DCC
(Data Country Code) NSAPs is described. Contact information for the
registration authorities for GOSIP and DCC-based NSAPs in the U.S.,
the General Services Administration (GSA) and the American National
Standards Institute (ANSI), respectively, is provided.
This document obsoletes RFC 1237. The changes from RFC 1237 are
minor, and primarily editorial in nature. The descriptions of OSI
routing standards contained in Section 3 have been updated to reflect
the current status of the relevant standards, and a description of
the OSI Interdomain Routing Protocol (IDRP) has been added.
Recommendations specific to the European part of the Internet have
been added in Section 6, along with recommendations for Routing
Domain Identifiers and Routing Domain Confederation Identifiers
needed for operation of IDRP.
Control over the collection of hosts and the transmission and
switching facilities that compose the networking resources of the
global Internet is not homogeneous, but is distributed among multiple
administrative authorities. For the purposes of this paper, the term
network service provider (or just provider) is defined to be an
organization that is in the business of providing datagram switching
services to customers. Organizations that are *only* customers
(i.e., that do not provide datagram services to other organizations)
are called network service subscribers (or simply subscribers).
In the current Internet, subscribers (e.g., campus and corporate site
networks) attach to providers (e.g., regionals, commercial providers,
and government backbones) in only one or a small number of carefully
controlled access points. For discussion of OSI NSAP allocation in
this paper, providers are treated as composing a mesh having no fixed
hierarchy. Addressing solutions which require substantial changes or
constraints on the current topology are not considered in this paper.
There are two aspects of interest when discussing OSI NSAP allocation
within the Internet. The first is the set of administrative
requirements for obtaining and allocating NSAP addresses; the second
is the technical aspect of such assignments, having largely to do
with routing, both within a routing domain (intra-domain routing) and
between routing domains (inter-domain routing). This paper focuses
on the technical issues.
The technical issues in NSAP allocation are mainly related to
routing. This paper assumes that CLNP will be widely deployed in the
Internet, and that the routing of CLNP traffic will normally be based
on the OSI end-system to intermediate system routing protocol (ES-IS)
, intra-domain IS-IS protocol , and inter-domain routing
protocol (IDRP) . It is expected that in the future the OSI
routing architecture will be enhanced to include support for
multicast, resource reservation, and other advanced services. The
requirements for addressing for these future services is outside of
the scope of this document.
The guidelines provided in this paper have been the basis for initial
deployment of CLNP in the Internet, and have proven very valuable
both as an aid to scaling of CLNP routing, and to address
The guidelines in this paper are oriented primarily toward the
large-scale division of NSAP address allocation in the Internet.
Topics covered include:
* Arrangement of parts of the NSAP for efficient operation of
the IS-IS routing protocol;
* Benefits of some topological information in NSAPs to reduce
routing protocol overhead, and specifically the overhead on
inter-domain routing (IDRP);
* The anticipated need for additional levels of hierarchy in
Internet addressing to support network growth and use of
the Routing Domain Confederation mechanism of IDRP to provide
support for additional levels of hierarchy;
* The recommended mapping between Internet topological entities
(i.e., service providers and service subscribers) and OSI
addressing and routing components, such as areas, domains and
* The recommended division of NSAP address assignment authority
among service providers and service subscribers;
* Background information on administrative procedures for
registration of administrative authorities immediately
below the national level (GOSIP administrative authorities
and ANSI organization identifiers); and,
* Choice of the high-order portion of the NSAP in subscriber
routing domains that are connected to more than one service
It is noted that there are other aspects of NSAP allocation, both
technical and administrative, that are not covered in this paper.
Topics not covered or mentioned only superficially include:
* Identification of specific administrative domains in the
* Policy or mechanisms for making registered information known
to third parties (such as the entity to which a specific NSAP
or a portion of the NSAP address space has been allocated);
* How a routing domain (especially a site) should organize its
internal topology of areas or allocate portions of its NSAP
address space; the relationship between topology and addresses
is discussed, but the method of deciding on a particular topology
or internal addressing plan is not; and,
* Procedures for assigning the System Identifier (ID) portion of
the NSAP. A method for assignment of System IDs is presented
Some background information is provided in this section that is
helpful in understanding the issues involved in NSAP allocation. A
brief discussion of OSI routing is provided, followed by a review of
the intra-domain and inter-domain protocols in sufficient detail to
understand the issues involved in NSAP allocation. Finally, the
specific constraints that the routing protocols place on NSAPs are
3.1. OSI Routing Standards
OSI partitions the routing problem into three parts:
* routing exchanges between hosts (a.k.a., end systems or ESs) and
routers (a.k.a., intermediate systems or ISs) (ES-IS);
* routing exchanges between routers in the same routing domain
(intra-domain IS-IS); and,
* routing among routing domains (inter-domain IS-IS).
ES-IS (international standard ISO 9542) advanced to international
standard (IS) status within ISO in 1987. Intra-domain IS-IS advanced
to IS status within ISO in 1992. Inter-Domain Routing Protocol
(IDRP) advanced to IS status within ISO in October 1993. CLNP, ES-
IS, and IS-IS are all widely available in vendor products, and have
been deployed in the Internet for several years. IDRP is currently
being implemented in vendor products.
This paper examines the technical implications of NSAP assignment
under the assumption that ES-IS, intra-domain IS-IS, and IDRP routing
are deployed to support CLNP.
3.2. Overview of ISIS (ISO/IEC 10589)
The IS-IS intra-domain routing protocol, ISO/IEC 10589, provides
routing for OSI environments. In particular, IS-IS is designed to
work in conjunction with CLNP, ES-IS, and IDRP. This section briefly
describes the manner in which IS-IS operates.
In IS-IS, the internetwork is partitioned into routing domains. A
routing domain is a collection of ESs and ISs that operate common
routing protocols and are under the control of a single
administration (throughout this paper, "domain" and "routing domain"
are used interchangeably). Typically, a routing domain may consist
of a corporate network, a university campus network, a regional
network, a backbone, or a similar contiguous network under control of
a single administrative organization. The boundaries of routing
domains are defined by network management by setting some links to be
exterior, or inter-domain, links. If a link is marked as exterior,
no intra-domain IS-IS routing messages are sent on that link.
IS-IS routing makes use of two-level hierarchical routing. A routing
domain is subdivided into areas (also known as level 1 subdomains).
Level 1 routers know the topology in their area, including all
routers and hosts. However, level 1 routers do not know the identity
of routers or destinations outside of their area. Level 1 routers
forward all traffic for destinations outside of their area to a level
2 router within their area.
Similarly, level 2 routers know the level 2 topology and know which
addresses are reachable via each level 2 router. The set of all
level 2 routers in a routing domain are known as the level 2
subdomain, which can be thought of as a backbone for interconnecting
the areas. Level 2 routers do not need to know the topology within
any level 1 area, except to the extent that a level 2 router may also
be a level 1 router within a single area. Only level 2 routers can
exchange data packets or routing information directly with routers
located outside of their routing domain.
NSAP addresses provide a flexible, variable length addressing format,
which allows for multi-level hierarchical address assignment. These
addresses provide the flexibility needed to solve two critical
problems simultaneously: (i) How to administer a worldwide address
space; and (ii) How to assign addresses in a manner which makes
routing scale well in a worldwide Internet.
As illustrated in Figure 1, ISO addresses are subdivided into the
Initial Domain Part (IDP) and the Domain Specific Part (DSP). The
IDP is the part which is standardized by ISO, and specifies the
format and authority responsible for assigning the rest of the
address. The DSP is assigned by whatever addressing authority is
specified by the IDP (see Appendix A for more discussion on the top
level NSAP addressing authorities). It is expected that the
authority specified by the IDP may further sub-divide the DSP, and
may assign sub-authorities responsible for parts of the DSP.
For routing purposes, ISO addresses are subdivided by IS-IS into the
area address, the system identifier (ID), and the NSAP selector
(SEL). The area address identifies both the routing domain and the
area within the routing domain. Generally, the area address
corresponds to the IDP plus a high-order part of the DSP (HO-DSP).
| AFI | IDI |Contents assigned by authority identified in IDI field|
<----------------Area Address--------------> <-----ID-----> <-SEL->
IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
HO-DSP High-order DSP
ID System Identifier
SEL NSAP Selector
Figure 1: OSI Hierarchical Address Structure.
The ID field may be from one to eight octets in length, but must have
a single known length in any particular routing domain. Each router
is configured to know what length is used in its domain. The SEL
field is always one octet in length. Each router is therefore able
to identify the ID and SEL fields as a known number of trailing
octets of the NSAP address. The area address can be identified as
the remainder of the address (after truncation of the ID and SEL
fields). It is therefore not necessary for the area address to have
any particular length -- the length of the area address could vary
between different area addresses in a given routing domain.
Usually, all nodes in an area have the same area address. However,
sometimes an area might have multiple addresses. Motivations for
allowing this are several:
* It might be desirable to change the address of an area. The most
graceful way of changing an area address from A to B is to first
allow it to have both addresses A and B, and then after all nodes
in the area have been modified to recognize both addresses, one by
one the nodes can be modified to forget address A.
* It might be desirable to merge areas A and B into one area. The
method for accomplishing this is to, one by one, add knowledge of
address B into the A partition, and similarly add knowledge of
address A into the B partition.
* It might be desirable to partition an area C into two areas, A and
B (where A might equal C, in which case this example becomes one
of removing a portion of an area). This would be accomplished by
first introducing knowledge of address A into the appropriate
nodes (those destined to become area A), and knowledge of address
B into the appropriate nodes, and then one by one removing
knowledge of address C.
Since the addressing explicitly identifies the area, it is very easy
for level 1 routers to identify packets going to destinations outside
of their area, which need to be forwarded to level 2 routers. Thus,
in IS-IS routers perform as follows:
* Level 1 intermediate systems route within an area based on the ID
portion of the ISO address. Level 1 routers recognize, based on the
destination address in a packet, whether the destination is within
the area. If so, they route towards the destination. If not, they
route to the nearest level 2 router.
* Level 2 intermediate systems route based on address prefixes,
preferring the longest matching prefix, and preferring internal
routes over external routes. They route towards areas, without
regard to the internal structure of an area; or towards level 2
routers on the routing domain boundary that have advertised external
address prefixes into the level 2 subdomain. A level 2 router may
also be operating as a level 1 router in one area.
A level 1 router will have the area portion of its address manually
configured. It will refuse to become a neighbor with a router whose
area addresses do not overlap its own area addresses. However, if a
level 1 router has area addresses A, B, and C, and a neighbor has
area addresses B and D, then the level 1 IS will accept the other IS
as a level 1 neighbor.
A level 2 router will accept another level 2 router as a neighbor,
regardless of area address. However, if the area addresses do not
overlap, the link would be considered by both routers to be level 2
only, and only level 2 routing packets would flow on the link.
External links (i.e., to other routing domains) must be between level
2 routers in different routing domains.
IS-IS provides an optional partition repair function. If a level 1
area becomes partitioned, this function, if implemented, allows the
partition to be repaired via use of level 2 routes.
IS-IS requires that the set of level 2 routers be connected. Should
the level 2 backbone become partitioned, there is no provision for
use of level 1 links to repair a level 2 partition.
Occasionally a single level 2 router may lose connectivity to the
level 2 backbone. In this case the level 2 router will indicate in
its level 1 routing packets that it is not "attached", thereby
allowing level 1 routers in the area to route traffic for outside of
the area to a different level 2 router. Level 1 routers therefore
route traffic to destinations outside of their area only to level 2
routers which indicate in their level 1 routing packets that they are
A host may autoconfigure the area portion of its address by
extracting the area portion of a neighboring router's address. If
this is the case, then a host will always accept a router as a
neighbor. Since the standard does not specify that the host *must*
autoconfigure its area address, a host may be pre-configured with an
Special treatment is necessary for broadcast subnetworks, such as
LANs. This solves two sets of issues: (i) In the absence of special
treatment, each router on the subnetwork would announce a link to
every other router on the subnetwork, resulting in O(n-squared) links
reported; (ii) Again, in the absence of special treatment, each
router on the LAN would report the same identical list of end systems
on the LAN, resulting in substantial duplication.
These problems are avoided by use of a "pseudonode", which represents
the LAN. Each router on the LAN reports that it has a link to the
pseudonode (rather than reporting a link to every other router on the
LAN). One of the routers on the LAN is elected "designated router".
The designated router then sends out a Link State Packet (LSP) on
behalf of the pseudonode, reporting links to all of the routers on
the LAN. This reduces the potential n-squared links to n links. In
addition, only the pseudonode LSP includes the list of end systems on
the LAN, thereby eliminating the potential duplication.
The IS-IS provides for optional Quality of Service (QOS) routing,
based on throughput (the default metric), delay, expense, or residual
IS-IS has a provision for authentication information to be carried in
all IS-IS PDUs. Currently the only form of authentication which is
defined is a simple password. A password may be associated with each
link, each area, and with the level 2 subdomain. A router not in
possession of the appropriate password(s) is prohibited from
participating in the corresponding function (i.e., may not initialize
a link, be a member of the area, or a member of the level 2
Procedures are provided to allow graceful migration of passwords
without disrupting operation of the routing protocol. The
authentication functions are extensible so that a stronger,
cryptographically-based security scheme may be added in an upwardly
compatible fashion at a future date.
3.3. Overview of IDRP (ISO/IEC 10747)
The Inter-Domain Routing Protocol (IDRP, ISO/IEC 10747), developed in
ISO, provides routing for OSI environments. In particular, IDRP is
designed to work in conjuction with CLNP, ES-IS, and IS-IS. This
section briefly describes the manner in which IDRP operates.
Consistent with the OSI Routing Framework , in IDRP the
internetwork is partitioned into routing domains. IDRP places no
restrictions on the inter-domain topology. A router that
participates in IDRP is called a Boundary Intermediate System (BIS).
Routing domains that participate in IDRP are not allowed to overlap -
a BIS may belong to only one domain.
A pair of BISs are called external neighbors if these BISs belong to
different domains but share a common subnetwork (i.e., a BIS can
reach its external neighbor in a single network layer hop). Two
domains are said to be adjacent if they have BISs that are external
neighbors of each other. A pair of BISs are called internal
neighbors if these BISs belong to the same domain. In contrast with
external neighbors, internal neighbors don't have to share a common
subnetwork -- IDRP assumes that a BIS should be able to exchange
Network Protocol Date Units (NPDUs) with any of its internal
neighbors by relying solely on intra-domain routing procedures.
IDRP governs the exchange of routing information between a pair of
neighbors, either external or internal. IDRP is self-contained with
respect to the exchange of information between external neighbors.
Exchange of information between internal neighbors relies on
additional support provided by intra-domain routing (unless internal
neighbors share a common subnetwork).
To facilitate routing information aggregation/abstraction, IDRP
allows grouping of a set of connected domains into a Routing Domain
Confederation (RDC). A given domain may belong to more than one RDC.
There are no restrictions on how many RDCs a given domain may
simultaneously belong to, and no preconditions on how RDCs should be
formed -- RDCs may be either nested, or disjoint, or may overlap.
One RDC is nested within another RDC if all members (RDs) of the
former are also members of the latter, but not vice versa. Two RDCs
overlap if they have members in common and also each has members that
are not in the other. Two RDCs are disjoint if they have no members
Each domain participating in IDRP is assigned a unique Routing Domain
Identifier (RDI). Syntactically an RDI is represented as an OSI
network layer address. Each RDC is assigned a unique Routing Domain
Confederation Identifier (RDCI). RDCIs are assigned out of the
address space allocated for RDIs -- RDCIs and RDIs are syntactically
indistinguishable. Procedures for assigning and managing RDIs and
RDCIs are outside the scope of the protocol. However, since RDIs are
syntactically nothing more than network layer addresses, and RDCIs
are syntactically nothing more than RDIs, it is expected that RDI and
RDCI assignment and management would be part of the network layer
assignment and management procedures. Recommendations for RDI and
RDCI assignment are provided in Section 6.5.
IDRP requires a BIS to be preconfigured with the RDI of the domain to
which the BIS belongs. If a BIS belongs to a domain that is a member
of one or more RDCs, then the BIS has to be preconfigured with RDCIs
of all the RDCs the domain is in, and the information about relations
between the RDCs - nested or overlapped.
IDRP doesn't assume or require any particular internal structure for
the addresses. The protocol provides correct routing as long as the
following guidelines are met:
* End systems and intermediate systems may use any NSAP address or
Network Entity Title (NET -- i.e., an NSAP address without the
selector) that has been assigned under ISO 8348  guidelines;
* An NSAP prefix carried in the Network Layer Reachability
Information (NLRI) field for a route originated by a BIS in a
given routing domain should be associated with only that
routing domain; that is, no system identified by the prefix
should reside in a different routing domain; ambiguous routing
may result if several routing domains originate routes whose
NLRI field contain identical NSAP address prefixes, since this
would imply that the same system(s) is simultaneously located
in several routing domains;
* Several different NSAP prefixes may be associated with a single
routing domain which contains a mix of systems which use NSAP
addresses assigned by several different addressing authorities.
IDRP assumes that the above guidelines have been satisfied, but it
contains no means to verify that this is so. Therefore, such
verification is assumed to be the responsibility of the
administrators of routing domains.
IDRP provides mandatory support for data integrity and optional
support for data origin authentication for all of its messages. Each
message carries a 16-octet digital signature that is computed by
applying the MD-4 algorithm (RFC 1320) to the context of the message
itself. This signature provides support for data integrity. To
support data origin authentication a BIS, when computing a digital
signature of a message, may prepend and append additional information
to the message. This information is not passed as part of the
message but is known to the receiver.
3.3.1. Scaling Mechanisms in IDRP
The ability to group domains in RDCs provides a simple, yet powerful
mechanism for routing information aggregation and abstraction. It
allows reduction of topological information by replacing a sequence
of RDIs carried by the RD_PATH attribute with a single RDCI. It also
allows reduction of the amount of information related to transit
policies, since the policies can be expressed in terms of aggregates
(RDCs), rather than individual components (RDs). It also allows
simplification of route selection policies, since these policies can
be expressed in terms of aggregates (RDCs) rather than individual
Aggregation and abstraction of Network Layer Reachability Information
(NLRI) is supported by the "route aggregation" mechanism of IDRP.
This mechanism is complementary to the Routing Domain Confederations
mechanism. Both mechanisms are intended to provide scalable routing
via information reduction/abstraction. However, the two mechanisms
are used for different purposes: route aggregation for aggregation
and abstraction of routes (i.e., Network Layer Reachability
Information), Routing Domain Confederations for aggregation and
abstraction of topology and/or policy information. To provide
maximum benefits, both mechanisms can be used together. This implies
that address assignment that will facilitate route aggregation does
not conflict with the ability to form RDCs, and vice versa; formation
of RDCs should be done in a manner consistent with the address
assignment needed for route aggregation.
3.4. Requirements of IS-IS and IDRP on NSAPs
The preferred NSAP format for IS-IS is shown in Figure 1. A number
of points should be noted from IS-IS:
* The IDP is as specified in ISO 8348, the OSI network layer service
* The high-order portion of the DSP (HO-DSP) is that portion of the
DSP whose assignment, structure, and meaning are not constrained by
* The area address (i.e., the concatenation of the IDP and the
HO-DSP) must be globally unique. If the area address of an NSAP
matches one of the area addresses of a router, it is in the
router's area and is routed to by level 1 routing;
* Level 2 routing acts on address prefixes, using the longest address
prefix that matches the destination address;
* Level 1 routing acts on the ID field. The ID field must be unique
within an area for ESs and level 1 ISs, and unique within the
routing domain for level 2 ISs. The ID field is assumed to be
flat. The method presented in RFC 1526  may optionally be
used to assure globally unique IDs;
* The one-octet NSAP Selector, SEL, determines the entity to receive
the CLNP packet within the system identified by the rest of the
NSAP (i.e., a transport entity) and is always the last octet of the
* A system shall be able to generate and forward data packets
containing addresses in any of the formats specified by
ISO 8348. However, within a routing domain that conforms to IS-IS,
the lower-order octets of the NSAP should be structured as the ID
and SEL fields shown in Figure 1 to take full advantage of IS-IS
routing. End systems with addresses which do not conform may
require additional manual configuration and be subject to inferior
For purposes of efficient operation of the IS-IS routing protocol,
several observations may be made. First, although the IS-IS protocol
specifies an algorithm for routing within a single routing domain,
the routing algorithm must efficiently route both: (i) Packets whose
final destination is in the domain (these must, of course, be routed
to the correct destination end system in the domain); and (ii)
Packets whose final destination is outside of the domain (these must
be routed to an appropriate "border" router, from which they will
exit the domain).
For those destinations which are in the domain, level 2 routing
treats the entire area address (i.e., all of the NSAP address except
the ID and SEL fields) as if it were a flat field. Thus, the
efficiency of level 2 routing to destinations within the domain is
affected only by the number of areas in the domain, and the number of
area addresses assigned to each area.
For those destinations which are outside of the domain, level 2
routing routes according to address prefixes. In this case, there is
considerable potential advantage (in terms of reducing the amount of
routing information that is required) if the number of address
prefixes required to describe any particular set of external
destinations can be minimized. Efficient routing with IDRP similarly
also requires minimization of the number of address prefixes needed
to describe specific destinations. In other words, addresses need to
be assigned with topological significance. This requirement is
described in more detail in the following sections.
4. NSAPs and Routing
4.1. Routing Data Abstraction
When determining an administrative policy for NSAP assignment, it is
important to understand the technical consequences. The objective
behind the use of hierarchical routing is to achieve some level of
routing data abstraction, or summarization, to reduce the processing
time, memory requirements, and transmission bandwidth consumed in
support of routing. This implies that address assignment must serve
the needs of routing, in order for routing to scale to very large
While the notion of routing data abstraction may be applied to
various types of routing information, this and the following sections
primarily emphasize one particular type, namely reachability
information. Reachability information describes the set of reachable
Abstraction of reachability information dictates that NSAPs be
assigned according to topological routing structures. However,
administrative assignment falls along organizational or political
boundaries. These may not be congruent to topological boundaries,
and therefore the requirements of the two may collide. A balance
between these two needs is necessary.
Routing data abstraction occurs at the boundary between
hierarchically arranged topological routing structures. An element
lower in the hierarchy reports summary routing information to its
parent(s). Within the current OSI routing framework  and routing
protocols, the lowest boundary at which this can occur is the
boundary between an area and the level 2 subdomain within a IS-IS
routing domain. Data abstraction is designed into IS-IS at this
boundary, since level 1 ISs are constrained to reporting only area
Level 2 routing is based upon address prefixes. Level 2 routers
(ISs) distribute, throughout the level 2 subdomain, the area
addresses of the level 1 areas to which they are attached (and any
manually configured reachable address prefixes). Level 2 routers
compute next-hop forwarding information to all advertised address
prefixes. Level 2 routing is determined by the longest advertised
address prefix that matches the destination address.
At routing domain boundaries, address prefix information is exchanged
with other routing domains via IDRP. If area addresses within a
routing domain are all drawn from distinct NSAP assignment
authorities (allowing no abstraction), then the boundary prefix
information consists of an enumerated list of all area addresses.
Alternatively, should the routing domain "own" an address prefix and
assign area addresses based upon it, boundary routing information can
be summarized into the single prefix. This can allow substantial
data reduction and, therefore, will allow much better scaling (as
compared to the uncoordinated area addresses discussed in the
If routing domains are interconnected in a more-or-less random (non-
hierarchical) scheme, it is quite likely that no further abstraction
of routing data can occur. Since routing domains would have no
defined hierarchical relationship, administrators would not be able
to assign area addresses out of some common prefix for the purpose of
data abstraction. The result would be flat inter-domain routing; all
routing domains would need explicit knowledge of all other routing
domains that they route to. This can work well in small- and medium-
sized internets, up to a size somewhat larger than the current IP
Internet. However, this does not scale to very large internets. For
example, we expect growth in the future to an international Internet
which has tens or hundreds of thousands of routing domains in the
U.S. alone. Even larger numbers of routing domains are possible when
each home, or each small company, becomes its own routing domain.
This requires a greater degree of data abstraction beyond that which
can be achieved at the "routing domain" level.
In the Internet, however, it should be possible to exploit the
existing hierarchical routing structure interconnections, as
discussed in Section 5. Thus, there is the opportunity for a group
of subscribers each to be assigned an address prefix from a shorter
prefix assigned to their provider. Each subscriber now "owns" its
(somewhat longer) prefix, from which it assigns its area addresses.
The most straightforward case of this occurs when there is a set of
subscribers whose routing domains are all attached only to a single
service provider, and which use that provider for all external
(inter-domain) traffic. A short address prefix may be assigned to
the provider, which then assigns slightly longer prefixes (based on
the provider's prefix) to each of the subscribers. This allows the
provider, when informing other providers of the addresses that it can
reach, to abbreviate the reachability information for a large number
of routing domains as a single prefix. This approach therefore can
allow a great deal of hierarchical abbreviation of routing
information, and thereby can greatly improve the scalability of
Clearly, this approach is recursive and can be carried through
several iterations. Routing domains at any "level" in the hierarchy
may use their prefix as the basis for subsequent suballocations,
assuming that the NSAP addresses remain within the overall length and
structure constraints. The flexibility of NSAP addresses facilitates
this form of hierarchical address assignment and routing. As one
example of how NSAPs may be used, the GOSIP Version 2 NSAP structure
is discussed later in this section.
At this point, we observe that the number of nodes at each lower
level of a hierarchy tends to grow exponentially. Thus the greatest
gains in data abstraction occur at the leaves and the gains drop
significantly at each higher level. Therefore, the law of
diminishing returns suggests that at some point data abstraction
ceases to produce significant benefits. Determination of the point
at which data abstraction ceases to be of benefit requires a careful
consideration of the number of routing domains that are expected to
occur at each level of the hierarchy (over a given period of time),
compared to the number of routing domains and address prefixes that
can conveniently and efficiently be handled via dynamic inter-domain
routing protocols. As the Internet grows, further levels of
hierarchy may become necessary. Again, this requires considerable
flexibility in the addressing scheme, such as is provided by NSAP
4.2. NSAP Administration and Efficiency
There is a balance that must be sought between the requirements on
NSAPs for efficient routing and the need for decentralized NSAP
administration. The NSAP structure from Version 2 of GOSIP (Figure
2) offers one example of how these two needs might be met. The AFI,
IDI, DSP Format Identifier (DFI), and Administrative Authority (AA)
fields provide for administrative decentralization. The AFI/IDI pair
of values 47.0005 identify the U.S. Government as the authority
responsible for defining the DSP structure and allocating values
within it (see the Appendix for more information on NSAP structure).
| AFI | IDI |<----------------------DSP------------->|
| 47 | 0005| DFI | AA | Rsvd | RD | Area | ID | SEL |
octets | 1 | 2 | 1 | 3 | 2 | 2 | 2 | 6 | 1 |
IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
DFI DSP Format Identifier
AA Administrative Authority
RD Routing Domain Identifier
Area Area Identifier
ID System Identifier
SEL NSAP Selector
Figure 2: GOSIP Version 2 NSAP structure.
[Note: We are using U.S. GOSIP version 2 addresses only as an
example. It is not necessary that NSAPs be allocated from the GOSIP
Version 2 authority under 47.0005. The ANSI format under the Data
Country Code for the U.S. (DCC=840) and formats assigned to other
countries and ISO members or liaison organizations are also being
used, and work equally well. For parts of the Internet outside of
the U.S. there may in some cases be strong reasons to prefer a
country- or area-specific format rather than the U.S. GOSIP format.
However, GOSIP addresses are used in most cases in the examples in
this paper because:
* The DSP format has been defined and allows hierarchical allocation;
* An operational registration authority for suballocation of AA
values under the GOSIP address space has already been established at
GOSIP Version 2 defines the DSP structure as shown (under DFI=80h)
and provides for the allocation of AA values to administrations.
Thus, the fields from the AFI to the AA, inclusive, represent a
unique address prefix assigned to an administration.
American National Standard X3.216-1992  specifies the structure of
the DSP for NSAP addresses that use an Authority and Format
Identifier (AFI) value of (decimal) 39, which identifies the "ISO-
DCC" (data country code) format, in which the value of the Initial
Domain Identifier (IDI) is (decimal) 840, which identifies the U.S.
National Body (ANSI). This DSP structure is identical to the
structure that is specified by GOSIP Version 2. The AA field is
called "org" for organization identifier in the ANSI standard, and
the ID field is called "system". The ANSI format, therefore, differs
from the GOSIP format illustrated above only in that the AFI and IDI
specify the "ISO-DCC" format rather than the "ISO 6523-ICD" format
used by GOSIP, and the "AA" field is administered by an ANSI
registration authority rather than by the GSA. Organization
identifiers may be obtained from ANSI. The technical considerations
applicable to NSAP administration are independent of whether a GOSIP
Version 2 or an ANSI value is used for the NSAP assignment.
Similarly, although other countries make use of different NSAP
formats, the principles of NSAP assignment and use are the same. The
NSAP formats recommended by RARE WG4 for use in Europe are discussed
in Section 6.2.
In the low-order part of the GOSIP Version 2 NSAP format, two fields
are defined in addition to those required by IS-IS. These fields, RD
and Area, are defined to allow allocation of NSAPs along topological
boundaries in support of increased data abstraction. Administrations
assign RD identifiers underneath their unique address prefix (the
reserved field is left to accommodate future growth and to provide
additional flexibility for inter-domain routing). Routing domains
allocate Area identifiers from their unique prefix. The result is:
* AFI+IDI+DFI+AA = administration prefix,
* administration prefix(+Rsvd)+RD = routing domain prefix, and,
* routing domain prefix+Area = area address.
This provides for summarization of all area addresses within a
routing domain into one prefix. If the AA identifier is accorded
topological significance (in addition to administrative
significance), an additional level of data abstraction can be
obtained, as is discussed in the next section.
5. NSAP Administration and Routing in the Internet
Basic Internet routing components are service providers and service
subscribers. A natural mapping from these components to OSI routing
components is that each provider and subscriber operates as a routing
Alternatively, a subscriber may choose to operate as a part of a
provider domain; that is, as an area within the provider's routing
domain. However, in such a case the discussion in Section 5.1
We assume that most subscribers will prefer to operate a routing
domain separate from their provider's. Such subscribers can exchange
routing information with their provider via interior routing protocol
route leaking or via IDRP; for the purposes of this discussion, the
choice is not significant. The subscriber is still allocated a
prefix from the provider's address space, and the provider advertises
its own prefix into inter-domain routing.
Given such a mapping, where should address administration and
allocation be performed to satisfy both administrative
decentralization and data abstraction? Three possibilities are
1. at the area,
2. at the subscriber routing domain, and,
3. at the provider routing domain.
Subscriber routing domains correspond to end-user sites, where the
primary purpose is to provide intra-domain routing services. Provider
routing domains are deployed to carry transit (i.e., inter-domain)
The greatest burden in transmitting and operating on routing
information is at the top of the routing hierarchy, where routing
information tends to accumulate. In the Internet, for example, each
provider must manage the set of network numbers for all networks
reachable through the provider.
For traffic destined for other networks, the provider will route
based on inter-domain routing information obtained from other
providers or, in some cases, to a default provider.
In general, higher levels of the routing hierarchy will benefit the
most from the abstraction of routing information at a lower level of
the routing hierarchy. There is relatively little direct benefit to
the administration that performs the abstraction, since it must
maintain routing information individually on each attached
topological routing structure.
For example, suppose that a given subscriber is trying to decide
whether to obtain an NSAP address prefix based on an AA value from
GSA (implying that the first four octets of the address would be
those assigned out of the GOSIP space), or based on an RD value from
its provider (implying that the first seven octets of the address are
those obtained by that provider). If considering only their own
self-interest, the subscriber and its local provider have little
reason to choose one approach or the other. The subscriber must use
one prefix or another; the source of the prefix has little effect on
routing efficiency within the subscriber's routing domain. The
provider must maintain information about each attached subscriber in
order to route, regardless of any commonality in the prefixes of its
However, there is a difference when the local provider distributes
routing information to other providers. In the first case, the
provider cannot aggregate the subscriber's address into its own
prefix; the address must be explicitly listed in routing exchanges,
resulting in an additional burden to other providers which must
exchange and maintain this information.
In the second case, each other provider sees a single address prefix
for the local provider which encompasses the new subscriber. This
avoids the exchange of additional routing information to identify the
new subscriber's address prefix. Thus, the advantages primarily
benefit other providers which maintain routing information about this
provider (and its subscribers).
Clearly, a symmetric application of these principles is in the
interest of all providers, enabling them to more efficiently support
CLNP routing to their customers. The guidelines discussed below
describe reasonable ways of managing the OSI address space that
benefit the entire community.
5.1. Administration at the Area
If areas take their area addresses from a myriad of unrelated NSAP
allocation authorities, there will be effectively no data abstraction
beyond what is built into IS-IS. For example, assume that within a
routing domain three areas take their area addresses, respectively,
* the GOSIP Version 2 authority assigned to the Department
of Commerce, with an AA of nnn:
AFI=47, IDI=0005, DFI=80h, AA=nnn, ... ;
* the GOSIP Version 2 authority assigned to the Department
of the Interior, with an AA of mmm:
AFI=47, IDI=0005, DFI=80h, AA=mmm, ... ; and,
* the ANSI authority under the U.S. Data Country Code (DCC)
(Section A.2) for organization XYZ with ORG identifier = xxx:
AFI=39, IDI=840, DFI=dd, ORG=xxx, ....
As described in Section 3.3, from the point of view of any particular
routing domain, there is no harm in having the different areas in the
routing domain use addresses obtained from a wide variety of
administrations. For routing within the domain, the area addresses
are treated as a flat field.
However, this does have a negative effect on inter-domain routing,
particularly on those other domains which need to maintain routes to
this domain. There is no common prefix that can be used to represent
these NSAPs and therefore no summarization can take place at the
routing domain boundary. When addresses are advertised by this
routing domain to other routing domains, an enumerated list must be
used consisting of the three area addresses.
This situation is roughly analogous to the dissemination of routing
information in the TCP/IP Internet prior to the introduction of CIDR.
Areas correspond roughly to networks and area addresses to network
numbers. The result of allowing areas within a routing domain to
take their NSAPs from unrelated authorities is flat routing at the
area address level. The number of address prefixes that subscriber
routing domains would advertise is on the order of the number of
attached areas; the number of prefixes a provider routing domain
would advertise is approximately the number of areas attached to all
its subscriber routing domains. For "default-less" providers (i.e.,
those that don't use default routes) the size of the routing tables
would be on the order of the number of area addresses globally. As
the CLNP internet grows this would quickly become intractable. A
greater degree of hierarchical information reduction is necessary to
allow greater growth.
5.2. Administration at the Subscriber Routing Domain
As mentioned previously, the greatest degree of data abstraction
comes at the lowest levels of the hierarchy. Providing each
subscriber routing domain (that is, site) with a unique prefix
results in the biggest single increase in abstraction, with each
subscriber domain assigning area addresses from its prefix. From
outside the subscriber routing domain, the set of all addresses
reachable in the domain can then be represented by a single prefix.
As an example, assume a government agency has been assigned the AA
value of zzz under ICD=0005. The agency then assigns a routing
domain identifier to a routing domain under its administrative
authority identifier, rrr. The resulting prefix for the routing
AFI=47, IDI=0005, DFI=80h, AA=zzz, (Rsvd=0), RD=rrr.
All areas within this routing domain would have area addresses
comprising this prefix followed by an Area identifier. The prefix
represents the summary of reachable addresses within the routing
There is a close relationship between areas and routing domains
implicit in the fact that they operate a common routing protocol and
are under the control of a single administration. The routing domain
administration subdivides the domain into areas and structures a
level 2 subdomain (i.e., a level 2 backbone) which provides
connectivity among the areas. The routing domain represents the only
path between an area and the rest of the internetwork. It is
reasonable that this relationship also extend to include a common
NSAP addressing authority. Thus, the areas within the subscriber RD
should take their NSAPs from the prefix assigned to the subscriber
5.3. Administration at the Provider Routing Domain
Two kinds of provider routing domains are considered, direct
providers and indirect providers. Most of the subscribers of a
direct provider are domains that act solely as service subscribers
(i.e., they carry no transit traffic). Most of the "subscribers" of
an indirect provider are, themselves, service providers. In present
terminology a backbone is an indirect provider, while a regional is a
direct provider. Each case is discussed separately below.
5.3.1. Direct Service Providers
It is interesting to consider whether direct service providers'
routing domains should be the common authority for assigning NSAPs
from a unique prefix to the subscriber routing domains that they
serve. In the long term the number of routing domains in the
Internet will grow to the point that it will be infeasible to route
on the basis of a flat field of routing domains. It will therefore
be essential to provide a greater degree of information abstraction.
Direct providers may assign prefixes to subscriber domains, based on
a single (shorter length) address prefix assigned to the provider.
For example, given the GOSIP Version 2 address structure, an AA value
may be assigned to each direct provider, and routing domain values
may be assigned by the provider to each attached subscriber routing
domain. A similar hierarchical address assignment based on a prefix
assigned to each provider may be used for other NSAP formats. This
results in direct providers advertising to other providers (both
direct and indirect) a small fraction of the number of address
prefixes that would be necessary if they enumerated the individual
prefixes of the subscriber routing domains. This represents a
significant savings given the expected scale of global
Are subscriber routing domains willing to accept prefixes derived
from the direct providers? In the supplier/consumer model, the direct
provider is offering connectivity as the service, priced according to
its costs of operation. This includes the "price" of obtaining
service from one or more indirect providers and exchanging routing
information with other direct providers. In general, providers will
want to handle as few address prefixes as possible to keep costs low.
In the Internet environment, subscriber routing domains must be
sensitive to the resource constraints of the providers (both direct
and indirect). The efficiencies gained in routing clearly warrant
the adoption of NSAP administration by the direct providers.
The mechanics of this scenario are straightforward. Each direct
provider is assigned a unique prefix, from which it allocates
slightly longer routing domain prefixes for its attached subscriber
routing domains. For GOSIP NSAPs, this means that a direct provider
would be assigned an AA identifier. Attached subscriber routing
domains would be assigned RD identifiers under the direct provider's
unique prefix. For example, assume that NIST is a subscriber routing
domain whose sole inter-domain link is via SURANet. If SURANet is
assigned an AA identifier kkk, NIST could be assigned an RD of jjj,
resulting in a unique prefix for SURANet of:
AFI=47, IDI=0005, DFI=80h, AA=kkk
and a unique prefix for NIST of
AFI=47, IDI=0005, DFI=80h, AA=kkk, (Rsvd=0), RD=jjj.
A similar scheme can be established using NSAPs allocated under
DCC=840. In this case, a direct provider applies for an ORG
identifier from ANSI, which serves the same purpose as the AA
identifier in GOSIP.
5.3.2. Indirect Providers
There does not appear to be a strong case for direct service
providers to take their address spaces from the NSAP space of an
indirect provider (e.g. backbone in today's terms). The benefit in
routing data abstraction is relatively small. The number of direct
providers today is in the tens and an order of magnitude increase
would not cause an undue burden on the indirect providers. Also, it
may be expected that as time goes by there will be increased direct
inter-connection of the direct providers, subscriber routing domains
directly attached to the "indirect" providers, and international
links directly attached to the providers. Under these circumstances,
the distinction between direct and indirect providers would become
An additional factor that discourages allocation of NSAPs from an
indirect provider's prefix is that the indirect providers and their
attached direct providers are perceived as being independent. Direct
providers may take their indirect provider service from one or more
providers, or may switch indirect providers should a more cost-
effective service be available elsewhere (essentially, indirect
providers can be thought of the same way as long-distance telephone
carriers). Having NSAPs derived from the indirect providers is
inconsistent with the nature of the relationship.
5.4. Multi-homed Routing Domains
The discussions in Section 5.3 suggest methods for allocating NSAP
addresses based on service provider connectivity. This allows a
great deal of information reduction to be achieved for those routing
domains which are attached to a single provider. In particular, such
routing domains may select their NSAP addresses from a space
allocated to them by their direct service provider. This allows the
provider, when announcing the addresses that it can reach to other
providers, to use a single address prefix to describe a large number
of NSAP addresses corresponding to multiple routing domains.
However, there are additional considerations for routing domains
which are attached to multiple providers. Such "multi-homed" routing
domains may, for example, consist of single-site campuses and
companies which are attached to multiple providers, large
organizations which are attached to different providers at different
locations in the same country, or multi-national organizations which
are attached to providers in a variety of countries worldwide. There
are a number of possible ways to deal with these multi-homed routing
One possible solution is to assign addresses to each multi-homed
organization independently from the providers to which it is
attached. This allows each multi-homed organization to base its NSAP
assignments on a single prefix, and to thereby summarize the set of
all NSAPs reachable within that organization via a single prefix.
The disadvantage of this approach is that since the NSAP address for
that organization has no relationship to the addresses of any
particular provider, the providers to which this organization is
attached will need to advertise the prefix for this organization to
other providers. Other providers (potentially worldwide) will need
to maintain an explicit entry for that organization in their routing
tables. If other providers do not maintain a separate route for this
organization, then packets destined to this organization will be
For example, suppose that a very large U.S.-wide company "Mega Big
International Incorporated" (MBII) has a fully interconnected
internal network and is assigned a single AA value under the U.S.
GOSIP Version 2 address space. It is likely that outside of the
U.S., a single entry may be maintained in routing tables for all U.S.
GOSIP addresses. However, within the U.S., every "default-less"
provider will need to maintain a separate address entry for MBII. If
MBII is in fact an international corporation, then it may be
necessary for every "default-less" provider worldwide to maintain a
separate entry for MBII (including providers to which MBII is not
attached). Clearly this may be acceptable if there are a small
number of such multihomed routing domains, but would place an
unacceptable load on routers within providers if all organizations
were to choose such address assignments. This solution may not scale
to internets where there are many hundreds of thousands of multi-
A second possible approach would be for multi-homed organizations to
be assigned a separate NSAP space for each connection to a provider,
and to assign a single address prefix to each area within its routing
domain(s) based on the closest interconnection point. For example,
if MBII had connections to two providers in the U.S. (one east coast,
and one west coast), as well as three connections to national
providers in Europe, and one in the far east, then MBII may make use
of six different address prefixes. Each area within MBII would be
assigned a single address prefix based on the nearest connection.
For purposes of external routing of traffic from outside MBII to a
destination inside of MBII, this approach works similarly to treating
MBII as six separate organizations. For purposes of internal
routing, or for routing traffic from inside of MBII to a destination
outside of MBII, this approach works the same as the first solution.
If we assume that incoming traffic (coming from outside of MBII, with
a destination within MBII) is always to enter via the nearest point
to the destination, then each provider which has a connection to MBII
needs to announce to other providers the ability to reach only those
parts of MBII whose address is taken from its own address space.
This implies that no additional routing information needs to be
exchanged between providers, resulting in a smaller load on the
inter-domain routing tables maintained by providers when compared to
the first solution. This solution therefore scales better to
extremely large internets containing very large numbers of multi-
One problem with the second solution is that backup routes to multi-
homed organizations are not automatically maintained. With the first
solution, each provider, in announcing the ability to reach MBII,
specifies that it is able to reach all of the NSAPs within MBII.
With the second solution, each provider announces that it can reach
all of the NSAPs based on its own address prefix, which only includes
some of the NSAPs within MBII. If the connection between MBII and
one particular provider were severed, then the NSAPs within MBII with
addresses based on that provider would become unreachable via inter-
domain routing. The impact of this problem can be reduced somewhat
by maintenance of additional information within routing tables, but
this reduces the scaling advantage of the second approach.
The second solution also requires that when external connectivity
changes, internal addresses also change.
Also note that this and the previous approach will tend to cause
packets to take different routes. With the first approach, packets
from outside of MBII destined for within MBII will tend to enter via
the point which is closest to the source (which will therefore tend
to maximize the load on the networks internal to MBII). With the
second solution, packets from outside destined for within MBII will
tend to enter via the point which is closest to the destination
(which will tend to minimize the load on the networks within MBII,
and maximize the load on the providers).
These solutions also have different effects on policies. For
example, suppose that country "X" has a law that traffic from a
source within country X to a destination within country X must at all
times stay entirely within the country. With the first solution, it
is not possible to determine from the destination address whether or
not the destination is within the country. With the second solution,
a separate address may be assigned to those NSAPs which are within
country X, thereby allowing routing policies to be followed.
Similarly, suppose that "Little Small Company" (LSC) has a policy
that its packets may never be sent to a destination that is within
MBII. With either solution, the routers within LSC may be configured
to discard any traffic that has a destination within MBII's address
space. However, with the first solution this requires one entry;
with the second it requires many entries and may be impossible as a
There are other possible solutions as well. A third approach is to
assign each multi-homed organization a single address prefix, based
on one of its connections to a provider. Other providers to which
the multi-homed organization are attached maintain a routing table
entry for the organization, but are extremely selective in terms of
which indirect providers are told of this route. This approach will
produce a single "default" routing entry which all providers will
know how to reach the organization (since presumably all providers
will maintain routes to each other), while providing more direct
routing in those cases where providers agree to maintain additional
There is at least one situation in which this third approach is
particularly appropriate. Suppose that a special interest group of
organizations have deployed their own backbone. For example, lets
suppose that the U.S. National Widget Manufacturers and Researchers
have set up a U.S.-wide backbone, which is used by corporations who
manufacture widgets, and certain universities which are known for
their widget research efforts. We can expect that the various
organizations which are in the widget group will run their internal
networks as separate routing domains, and most of them will also be
attached to other providers (since most of the organizations involved
in widget manufacture and research will also be involved in other
activities). We can therefore expect that many or most of the
organizations in the widget group are dual-homed, with one attachment
for widget-associated communications and the other attachment for
other types of communications. Let's also assume that the total
number of organizations involved in the widget group is small enough
that it is reasonable to maintain a routing table containing one
entry per organization, but that they are distributed throughout a
larger internet with many millions of (mostly not widget-associated)
With the third approach, each multi-homed organization in the widget
group would make use of an address assignment based on its other
attachment(s) to providers (the attachments not associated with the
widget group). The widget backbone would need to maintain routes to
the routing domains associated with the various member organizations.
Similarly, all members of the widget group would need to maintain a
table of routes to the other members via the widget backbone.
However, since the widget backbone does not inform other general
world-wide providers of what addresses it can reach (since the
backbone is not intended for use by other outside organizations), the
relatively large set of routing prefixes needs to be maintained only
in a limited number of places. The addresses assigned to the various
organizations which are members of the widget group would provide a
"default route" via each members other attachments to providers,
while allowing communications within the widget group to use the
A fourth solution involves assignment of a particular address prefix
for routing domains which are attached to two or more specific
cooperative public service providers. For example, suppose that
there are two providers "SouthNorthNet" and "NorthSouthNet" which
have a very large number of customers in common (i.e., there are a
large number of routing domains which are attached to both). Rather
than getting two address prefixes (such as two AA values assigned
under the GOSIP address space) these organizations could obtain three
prefixes. Those routing domains which are attached to NorthSouthNet
but not attached to SouthNorthNet obtain an address assignment based
on one of the prefixes. Those routing domains which are attached to
SouthNorthNet but not to NorthSouthNet would obtain an address based
on the second prefix. Finally, those routing domains which are
multi-homed to both of these networks would obtain an address based
on the third prefix. Each of these two providers would then
advertise two prefixes to other providers, one prefix for subscriber
routing domains attached to it only, and one prefix for subscriber
routing domains attached to both.
This fourth solution could become important when use of public data
networks becomes more common. In particular, it is likely that at
some point in the future a substantial percentage of all routing
domains will be attached to public data networks. In this case,
nearly all government-sponsored networks (such as some regional
networks which receive funding from NSF, as well as government
sponsored backbones) may have a set of customers which overlaps
substantially with the public networks.
There are therefore a number of possible solutions to the problem of
assigning NSAP addresses to multi-homed routing domains. Each of
these solutions has very different advantages and disadvantages.
Each solution places a different real (i.e., financial) cost on the
multi-homed organizations, and on the providers (including those to
which the multi-homed organizations are not attached).
In addition, most of the solutions described also highlight the need
for each provider to develop policy on whether and under what
conditions to accept customers with addresses that are not based on
its own address prefix, and how such non-local addresses will be
treated. For example, a somewhat conservative policy might be that
an attached subscriber RD may use any NSAP address prefix, but that
addresses which are not based on the providers own prefix might not
be advertised to other providers. In a less conservative policy, a
provider might accept customers using such non-local prefixes and
agree to exchange them in routing information with a defined set of
other providers (this set could be an a priori group of providers
that have something in common such as geographical location, or the
result of an agreement specific to the requesting subscriber).
Various policies involve real costs to providers, which may be
reflected in those policies.
5.5. Private Links
The discussion up to this point concentrates on the relationship
between NSAP addresses and routing between various routing domains
over transit routing domains, where each transit routing domain
interconnects a large number of routing domains and offers a more-
or-less public service.
However, there may also exist a large number of private point-to-
point links which interconnect two private routing domains. In many
cases such private point-to-point links may be limited to forwarding
packets directly between the two private routing domains.
For example, let's suppose that the XYZ corporation does a lot of
business with MBII. In this case, XYZ and MBII may contract with a
carrier to provide a private link between the two corporations, where
this link may only be used for packets whose source is within one of
the two corporations, and whose destination is within the other of
the two corporations. Finally, suppose that the point-to-point link
is connected between a single router (router X) within XYZ
corporation and a single router (router M) within MBII. It is
therefore necessary to configure router X to know which addresses can
be reached over this link (specifically, all addresses reachable in
MBII). Similarly, it is necessary to configure router M to know
which addresses can be reached over this link (specifically, all
addresses reachable in XYZ Corporation).
The important observation to be made here is that such private links
may be ignored for the purpose of NSAP allocation, and do not pose a
problem for routing. This is because the routing information
associated with private links is not propagated throughout the
internet, and therefore does not need to be collapsed into a
In our example, lets suppose that the XYZ corporation has a single
connection to a service provider, and has therefore received an
address allocation from the space administered by that provider.
Similarly, let's suppose that MBII, as an international corporation
with connections to six different providers, has chosen the second
solution from Section 5.4, and therefore has obtained six different
address allocations. In this case, all addresses reachable in the
XYZ Corporation can be described by a single address prefix (implying
that router M only needs to be configured with a single address
prefix to represent the addresses reachable over this point-to-point
link). All addresses reachable in MBII can be described by six
address prefixes (implying that router X needs to be configured with
six address prefixes to represent the addresses reachable over the
In some cases, such private point-to-point links may be permitted to
forward traffic for a small number of other routing domains, such as
closely affiliated organizations. This will increase the
configuration requirements slightly. However, provided that the
number of organizations using the link is relatively small, then this
still does not represent a significant problem.
Note that the relationship between routing and NSAP addressing
described in other sections of this paper is concerned with problems
in scaling caused by large, essentially public transit routing
domains which interconnect a large number of routing domains.
However, for the purpose of NSAP allocation, private point-to-point
links which interconnect only a small number of private routing
domains do not pose a problem, and may be ignored. For example, this
implies that a single subscriber routing domain which has a single
connection to a "public" provider, plus a number of private point-
to-point links to other subscriber routing domains, can be treated as
if it were single-homed to the provider for the purpose of NSAP
5.6. Zero-Homed Routing Domains
Currently, a very large number of organizations have internal
communications networks which are not connected to any external
network. Such organizations may, however, have a number of private
point-to-point links that they use for communications with other
organizations. Such organizations do not participate in global
routing, but are satisfied with reachability to those organizations
with which they have established private links. These are referred
to as zero-homed routing domains.
Zero-homed routing domains can be considered as the degenerate case
of routing domains with private links, as discussed in the previous
section, and do not pose a problem for inter-domain routing. As
above, the routing information exchanged across the private links
sees very limited distribution, usually only to the RD at the other
end of the link. Thus, there are no address abstraction requirements
beyond those inherent in the address prefixes exchanged across the
However, it is important that zero-homed routing domains use valid
globally unique NSAP addresses. Suppose that the zero-homed routing
domain is connected through a private link to an RD. Further, this
RD participates in an internet that subscribes to the global OSI
addressing plan (i.e., ISO 8348). This RD must be able to
distinguish between the zero-homed routing domain's NSAPs and any
other NSAPs that it may need to route to. The only way this can be
guaranteed is if the zero-homed routing domain uses globally unique
5.7. Address Transition Issues
Allocation of NSAP addresses based on connectivity to providers is
important to allow scaling of inter-domain routing to an internet
containing millions of routing domains. However, such address
allocation based on topology also implies that a change in topology
may result in a change of address.
This need to allow for change in addresses is a natural, inevitable
consequence of any method for routing data abstraction. The basic
notion of routing data abstraction is that there is some
correspondence between the address and where a system (i.e., a
routing domain, area, or end system) is located. Thus if the system
moves, in some cases the address will have to change. If it were
possible to change the connectivity between routing domains without
changing the addresses, then it would clearly be necessary to keep
track of the location of that routing domain on an individual basis.
Because of the rapid growth and increased commercialization of the
Internet, it is possible that the topology may be relatively
volatile. This implies that planning for address transition is very
important. Fortunately, there are a number of steps which can be
taken to help ease the effort required for address transition. A
complete description of address transition issues is outside of the
scope of this paper. However, a very brief outline of some
transition issues is contained in this section.
Also note that the possible requirement to transition addresses based
on changes in topology imply that it is valuable to anticipate the
future topology changes before finalizing a plan for address
allocation. For example, in the case of a routing domain which is
initially single-homed, but which is expecting to become multi-homed
in the future, it may be advantageous to assign NSAP addresses based
on the anticipated future topology.
In general, it will not be practical to transition the NSAP addresses
assigned to a routing domain in an instantaneous "change the address
at midnight" manner. Instead, a gradual transition is required in
which both the old and the new addresses will remain valid for a
limited period of time. During the transition period, both the old
and new addresses are accepted by the end systems in the routing
domain, and both old and new addresses must result in correct routing
of packets to the destination.
Provision for transition has already been built into IS-IS. As
described in Section 3, IS-IS allows multiple addresses to be
assigned to each area specifically for the purpose of easing
Similarly, there are provisions in OSI for the autoconfiguration of
area addresses. This allows OSI end systems to find out their area
addresses automatically, either by passively observing the ES-IS IS-
Hello packets transmitted by routers, or by actively querying the
routers for their NSAP address. If the ID portion of the address is
assigned in a manner which allows for globally unique IDs , then
an end system can reconfigure its entire NSAP address automatically
without the need for manual intervention. However, routers will
still require manual address reconfiguration.
During the transition period, it is important that packets using the
old address be forwarded correctly, even when the topology has
changed. This is facilitated by the use of "best match" inter-domain
For example, suppose that the XYZ Corporation was previously
connected only to the NorthSouthNet provider. The XYZ Corporation
therefore went off to the NorthSouthNet administration and got a
routing domain assignment based on the AA value obtained by the
NorthSouthNet under the GOSIP address space. However, for a variety
of reasons, the XYZ Corporation decided to terminate its association
with the North-SouthNet, and instead connect directly to the
NewCommercialNet public data network. Thus the XYZ Corporation now
has a new address assignment under the ANSI address assigned to the
NewCommercialNet. The old address for the XYZ Corporation would seem
to imply that traffic for the XYZ Corporation should be routed to the
NorthSouthNet, which no longer has any direct connection with XYZ
If the old provider (NorthSouthNet) and the new provider
(NewCommercialNet) are adjacent and cooperative, then this transition
is easy to accomplish. In this case, packets routed to the XYZ
Corporation using the old address assignment could be routed to the
NorthSouthNet, which would directly forward them to the
NewCommercialNet, which would in turn forward them to XYZ
Corporation. In this case only NorthSouthNet and NewCommercialNet
need be aware of the fact that the old address refers to a
destination which is no longer directly attached to NorthSouthNet.
If the old provider and the new provider are not adjacent, then the
situation is a bit more complex, but there are still several possible
ways to forward traffic correctly.
If the old provider and the new provider are themselves connected by
other cooperative providers, then these intermediate domains may
agree to forward traffic for XYZ correctly. For example, suppose
that NorthSouthNet and NewCommercialNet are not directly connected,
but that they are both directly connected to the NSFNET backbone. In
this case, all three of NorthSouthNet, NewCommercialNet, and the
NSFNET backbone would need to maintain a special entry for XYZ
corporation so that traffic to XYZ using the old address allocation
would be forwarded via NewCommercialNet. However, other routing
domains would not need to be aware of the new location for XYZ
Suppose that the old provider and the new provider are separated by a
non-cooperative routing domain, or by a long path of routing domains.
In this case, the old provider could encapsulate traffic to XYZ
Corporation in order to deliver such packets to the correct backbone.
Also, those locations which do a significant amount of business with
XYZ Corporation could have a specific entry in their routing tables
added to ensure optimal routing of packets to XYZ. For example,
suppose that another commercial backbone "OldCommercialNet" has a
large number of customers which exchange traffic with XYZ
Corporation, and that this third provider is directly connected to
both NorthSouthNet and NewCommercialNet. In this case
OldCommercialNet will continue to have a single entry in its routing
tables for other traffic destined for NorthSouthNet, but may choose
to add one additional (more specific) entry to ensure that packets
sent to XYZ Corporation's old address are routed correctly.
Whichever method is used to ease address transition, the goal is that
knowledge relating XYZ to its old address that is held throughout the
global internet would eventually be replaced with the new
information. It is reasonable to expect this to take weeks or months
and will be accomplished through the distributed directory system.
Discussion of the directory, along with other address transition
techniques such as automatically informing the source of a changed
address, are outside the scope of this paper.
We anticipate that the current exponential growth of the Internet
will continue or accelerate for the foreseeable future. In addition,
we anticipate a continuation of the rapid internationalization of the
Internet. The ability of routing to scale is dependent upon the use
of data abstraction based on hierarchical NSAP addresses. As CLNP
use increases in the Internet, it is therefore essential to assign
NSAP addresses with great care.
It is in the best interests of the internetworking community that the
cost of operations be kept to a minimum where possible. In the case
of NSAP allocation, this again means that routing data abstraction
must be encouraged.
In order for data abstraction to be possible, the assignment of NSAP
addresses must be accomplished in a manner which is consistent with
the actual physical topology of the Internet. For example, in those
cases where organizational and administrative boundaries are not
related to actual network topology, address assignment based on such
organization boundaries is not recommended.
The intra-domain IS-IS routing protocol allows for information
abstraction to be maintained at two levels: systems are grouped into
areas, and areas are interconnected to form a routing domain. The
inter-domain IDRP routing protocol allows for information abstraction
to be maintained at multiple levels by grouping routing domains into
Routing Domain Confederations and using route aggregation
For zero-homed and single-homed routing domains (which are expected
to remain zero-homed or single-homed), we recommend that the NSAP
addresses assigned for OSI use within a single routing domain use a
single address prefix assigned to that domain. Specifically, this
allows the set of all NSAP addresses reachable within a single domain
to be fully described via a single prefix. We recommend that
single-homed routing domains use an address prefix based on its
connectivity to a public service provider. We recommend that zero-
homed routing domains use globally unique addresses.
We anticipate that the total number of routing domains existing on a
worldwide OSI Internet to be great enough that additional levels of
hierarchical data abstraction beyond the routing domain level will be
necessary. To provide the needed data abstraction we recommend to
use Routing Domain Confederations and route aggregation capabilities
The general technical requirements for NSAP address guidelines do not
vary from country to country. However, details of address
administration may vary between countries. Also, in most cases,
network topology will have a close relationship with national
boundaries. For example, the degree of network connectivity will
often be greater within a single country than between countries. It
is therefore appropriate to make specific recommendations based on
national boundaries, with the understanding that there may be
specific situations where these general recommendations need to be
modified. Moreover, that suggests that national boundaries may be
used to group domains into Routing Domain Confederations.
Each of the country-specific or continent-specific recommendations
presented below are consistent with the technical requirements for
scaling of addressing and routing presented in this RFC.
6.1. Recommendations Specific to U.S. Parts of the Internet
NSAP addresses for use within the U.S. portion of the Internet are
expected to be based primarily on two address prefixes: the ICD=0005
format used by The U.S. Government, and the DCC=840 format defined by
We anticipate that, in the U.S., public interconnectivity between
private routing domains will be provided by a diverse set of
providers, including (but not necessarily limited to) regional
providers and commercial Public Data Networks.
These networks are not expected to be interconnected in a strictly
hierarchical manner. For example, the regional providers may be
directly connected rather than rely on an indirect provider, and all
three of these types of networks may have direct international
However, the total number of such providers is expected to remain
(for the foreseeable future) small enough to allow addressing of this
set of providers via a flat address space. These providers will be
used to interconnect a wide variety of routing domains, each of which
may comprise a single corporation, part of a corporation, a
university campus, a government agency, or other organizational unit.
In addition, some private corporations may be expected to make use of
dedicated private providers for communication within their own
We anticipate that the great majority of routing domains will be
attached to only one of the providers. This will permit hierarchical
address abbreviation based on provider. We therefore strongly
recommend that addresses be assigned hierarchically, based on address
prefixes assigned to individual providers.
For the GOSIP address format, this implies that Administrative
Authority (AA) identifiers should be obtained by all providers
(explicitly including the NSFNET backbone, the NSFNET regionals, and
other major government backbones). For those subscriber routing
domains which are connected to a single provider, they should be
assigned a Routing Domain (RD) value from the space assigned to that
To provide routing information aggregation/abstraction we recommend
that each provider together with all of its subscriber domains form a
Routing Domain Confederation. That, combined with hierarchical
address assignment, would provide significant reduction in the volume
of routing information that needs to be handled by IDRP. Note that
the presence of multihomed subscriber domains would imply that such
Confederations will overlap, which is explicitly supported by IDRP.
We recommend that all providers explicitly be involved in the task of
address administration for those subscriber routing domains which are
single-homed to them. This offers a valuable service to their
customers, and also greatly reduces the resources (including human
and network resources) necessary for that provider to take part in
Each provider should develop policy on whether and under what
conditions to accept customers using addresses that are not based on
the provider's own address prefix, and how such non-local addresses
will be treated. Policies should reflect the issue of cost
associated with implementing such policies.
We recommend that a similar hierarchical model be used for NSAP
addresses using the DCC-based address format. The structure for
DCC=840-based NSAPs is provided in Section A.2.
For routing domains which are not attached to any publically-
available provider, no urgent need for hierarchical address
abbreviation exists. We do not, therefore, make any additional
recommendations for such "isolated" routing domains, except to note
that there is no technical reason to preclude assignment of GOSIP AA
identifier values or ANSI organization identifiers to such domains.
Where such domains are connected to other domains by private point-
to-point links, and where such links are used solely for routing
between the two domains that they interconnect, no additional
technical problems relating to address abbreviation is caused by such
a link, and no specific additional recommendations are necessary.
6.2. Recommendations Specific to European Parts of the Internet
This section contains additional RARE recommendations for allocating
NSAP addresses within each national domain, administered by a
National Standardization Organization (NSO) and national research
NSAP addresses are expected to be based on the ISO DCC scheme.
Organizations which are not associated with a particular country and
which have reasons not to use a national prefix based on ISO DCC
should follow the recommendations covered in chapters 6.3 and 6.4.
ISO DCC addresses are not associated with any specific subnetwork
type and service provider and are thus independent of the type or
ownership of the underlying technology.
6.2.1. General NSAP Structure
The general structure of a Network Address defined in ISO 8348 is
further divided into:
| IDP | DSP |
| AFI | IDI | CDP | CDSP |
| AFI | IDI | CFI | CDI | RDAA | ID | SEL |
octets | 1 | 2 | 2..4 | 0..13 | 1..8 | 1 |
IDP Initial Domain Part
AFI Authority and Format Identifier, two-decimal-digit,
38 for decimal abstract syntax of the DSP or
39 for binary abstract syntax of the DSP
IDI Initial Domain Identifier, a three-decimal-digit
country code, as defined in ISO 3166
DSP Domain Specific Part
CDP Country Domain Part, 2..4 octets
CFI Country Format Identifier, one digit
CDI Country Domain Identifier, 3 to 7 digits, fills
CDP to an octet boundary
CDSP Country Domain Specific Part
RDAA Routing Domain and Area Address
ID System Identifier (1..8 octet)
SEL NSAP Selector
The total length of an NSAP can vary from 7 to 20 octets.
6.2.2. Structure of the Country Domain Part
The CDP identifies an organization within a country and the CDSP is
then available to that organization for further internal structuring
as it wishes. Non-ambiguity of addresses is ensured by there being
the NSO a single national body that allocates the CDPs.
The CDP is further divided into CFI and CDI, where the CFI identifies
the format of the CDI. The importance of this is that it enables
several types of CDI to be assigned in parallel, corresponding to
organizations with different requirements and giving different
amounts of the total address space to them, and that it conveniently
enables a substantial amount of address space to be reserved for
The possible structures of the CDP are as follows:
CFI = /0 reserved
CFI = /1 CDI = /aaa very large organizations or
CFI = /2 CDI = /aaaaa organizations of intermediate size
CFI = /3 CDI = /aaaaaaa small organizations and single users
CFI = /4../F reserved
Note: this uses the hexadecimal reference publication format defined
in ISO 8348 of a solidus "/" followed by a string of hexadecimal
digits. Each "a" represents a hexadecimal digit.
Organizations are classified into large, medium and small for the
purpose of address allocation, and one CFI is made available for each
category of organization.
This recommendation for CDP leaves space for the U.S. GOSIP Version 2
NSAP model (Appendix A.1) by the reserved CFI /8, nevertheless it is
not recommended for use in the European Internet.
6.2.3. Structure of the Country Domain Specific Part
The CDSP must have a structure (within the decimal digit or binary
octet syntax selected by the AFI value 38 or 39) satisfying both the
routing requirements (IS-IS) and the logical requirements of the
organization identified (CFI + CDI).
6.3. Recommendations Specific to Other Parts of the Internet
For the part of the Internet which is outside of the U.S. and Europe,
it is recommended that the DSP format be structured hierarchically
similarly to that specified within the U.S. and Europe no matter
whether the addresses are based on DCC or ICD format.
Further, in order to allow aggregation of NSAPs at national
boundaries into as few prefixes as possible, we further recommend
that NSAPs allocated to routing domains should be assigned based on
each routing domain's connectivity to a national Internet backbone.
6.4. Recommendations for Multi-Homed Routing Domains
Some routing domains will be attached to multiple providers within
the same country, or to providers within multiple countries. We
refer to these as "multi-homed" routing domains. Clearly the strict
hierarchical model discussed above does not neatly handle such
There are several possible ways that these multi-homed routing
domains may be handled. Each of these methods vary with respect to
the amount of information that must be maintained for inter-domain
routing and also with respect to the inter-domain routes. In
addition, the organization that will bear the brunt of this cost
varies with the possible solutions. For example, the solutions vary
with respect to:
* resources used within routers within the providers;
* administrative cost on provider personnel; and,
* difficulty of configuration of policy-based inter-domain
routing information within subscriber routing domains.
Also, the solution used may affect the actual routes which packets
follow, and may effect the availability of backup routes when the
primary route fails.
For these reasons it is not possible to mandate a single solution for
all situations. Rather, economic considerations will require a
variety of solutions for different subscriber routing domains and
6.5. Recommendations for RDI and RDCI assignment
While RDIs and RDCIs need not be related to the set of addresses
within the domains (confederations) they depict, for the sake of
simplicity we recommend that RDIs and RDCIs be assigned based on the
NSAP prefixes assigned to domains and confederations.
A subscriber RD should use the NSAP prefix assigned to it as its RDI.
A multihomed RD should use one of the NSAP prefixes assigned to it as
its RDI. If a service provider forms a Routing Domain Confederation
with some of its subscribers and the subscribers take their addresses
out of the provider, then the NSAP prefix assigned to the provider
should be used as the RDCI of the confederation. In this case the
provider may use a longer NSAP prefix for its own RDIs. In all other
cases a provider should use the address prefix that it uses for
assigning addresses to systems within the provider as its RDI.
7. Security Considerations
Security issues are not discussed in this memo (except for the
discussion of IS-IS authentication in Section 3.2).
8. Authors' Addresses
Richard P. Colella
National Institute of Standards & Technology
Building 225/Room B217
Gaithersburg, MD 20899
Phone: (301) 975-3627
c/o Wellfleet Communications, Inc
2 Federal Street
Billerica, MA 01821
Phone: (508) 436-3936
Ella P. Gardner
The MITRE Corporation
7525 Colshire Drive
McLean, VA 22102-3481
Phone: (703) 883-5826
T.J. Watson Research Center, IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
The authors would like to thank the members of the IETF OSI-NSAP
Working Group and of RARE WG4 for the helpful suggestions made during
the writing of this paper. We would also like to thank Radia Perlman
of Novell, Marcel Wiget of SWITCH, and Cathy Wittbrodt of BARRnet for
their ideas and help.
 ANSI, "American National Standard for the Structure and Semantics
of the Domain-Specific Part (DSP) of the OSI Network Service
Access Point (NSAP) Address", American National Standard X3.216-
 Boland, T., "Government Open Systems Interconnection Profile
Users' Guide Version 2 [DRAFT]", NIST Special Publication,
National Institute of Standards and Technology, Computer Systems
Laboratory, Gaithersburg, MD, June 1991.
 GOSIP Advanced Requirements Group, "Government Open Systems
Interconnection Profile (GOSIP) Version 2", Federal Information
Processing Standard 146-1, U.S. Department of Commerce, National
Institute of Standards and Technology, Gaithersburg, MD, April
 Hemrick, C., "The OSI Network Layer Addressing Scheme, Its
Implications, and Considerations for Implementation", NTIA Report
85186, U.S. Department of Commerce, National Telecommunications
and Information Administration, 1985.
 ISO, "Addendum to the Network Service Definition Covering Network
Layer Addressing," RFC 941, ISO, April 1985.
 ISO/IEC, "Codes for the Representation of Names of Countries",
International Standard 3166, ISO/IEC JTC 1, Switzerland, 1984.
 ISO/IEC, "Data Interchange - Structures for the Identification of
Organization", International Standard 6523, ISO/IEC JTC 1,
 ISO/IEC, "Information Processing Systems - Open Systems
Interconnection -- Basic Reference Model", International Standard
7498, ISO/IEC JTC 1, Switzerland, 1984.
 ISO/IEC, "Protocol for Providing the Connectionless-mode Network
Service", International Standard 8473, ISO/IEC JTC 1,
 ISO/IEC, "End System to Intermediate System Routing Exchange
Protocol for use in Conjunction with the Protocol for the
Provision of the Connectionless-mode Network Service",
International Standard 9542, ISO/IEC JTC 1, Switzerland, 1987.
 ISO/IEC, "Information Processing Systems -- Data Communications
-- Network Service Definition", International Standard 8348,
 ISO/IEC, "Information Processing Systems - OSI Reference Model -
Part3: Naming and Addressing", Draft International Standard
7498-3, ISO/IEC JTC 1, Switzerland, March 1989.
 ISO/IEC, "Information Technology - Telecommunications and
Information Exchange Between Systems - OSI Routeing Framework",
Technical Report 9575, ISO/IEC JTC 1, Switzerland, 1989.
 ISO/IEC, "Intermediate System to Intermediate System Intra-Domain
Routeing Exchange Protocol for use in Conjunction with the
Protocol for Providing the Connectionless-Mode Network Service
(ISO 8473)", International Standard ISO/IEC 10589, 1992.
 Loughheed, K., and Y. Rekhter, "A Border Gateway Protocol 3
(BGP-3)" RFC 1267, cisco Systems, T.J. Watson Research Center,
IBM Corp., October 1991.
 ISO/IEC, "Protocol for Exchange of Inter-Domain Routeing
Information among Intermediate Systems to support Forwarding of
ISO 8473 PDUs", International Standard 10747, ISO/IEC JTC 1,
 Callon, R., "TCP and UDP with Bigger Addresses (TUBA), A Simple
Proposal for Internet Addressing and Routing", RFC 1347, DEC,
 Piscitello, D., "Assignment of System Identifiers for TUBA/CLNP
Hosts", RFC 1526, Bellcore, September 1993.
 Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC 1519, BARRNet, cisco, OARnet, September 1993.
 ISO/IEC JTC1/SC6, "Addendum to ISO 9542 Covering Address
Administration", N6273, March 1991.
A. Administration of NSAPs
NSAPs represent the endpoints of communication through the Network
Layer and must be globally unique . ISO 8348 defines the
semantics of the NSAP and the abstract syntaxes in which the
semantics of the Network address can be expressed .
The NSAP consists of the initial domain part (IDP) and the domain
specific part (DSP). The initial domain part of the NSAP consists of
an authority and format identifier (AFI) and an initial domain
identifier (IDI). The AFI specifies the format of the IDI, the
network addressing authority responsible for allocating values of the
IDI, and the abstract syntax of the DSP. The IDI specifies the
addressing subdomain from which values of the DSP are allocated and
the network addressing authority responsible for allocating values of
the DSP from that domain. The structure and semantics of the DSP are
determined by the authority identified by the IDI. Figure 3 shows
the NSAP address structure.
| IDP |
| AFI | IDI |<--------------------DSP------------------------>|
IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
Figure 3: NSAP address structure.
The global network addressing domain consists of all the NSAP
addresses in the OSI environment. Within that environment, seven
second-level addressing domains and corresponding IDI formats are
described in ISO 8348:
* X.121 for public data networks
* F.69 for telex
* E.163 for the public switched telephone network numbers
* E.164 for ISDN numbers
* ISO Data Country Code (DCC), allocated according to ISO 3166 
* ISO International Code Designator (ICD), allocated according to
ISO 6523 
* Local to accommodate the coexistence of OSI and non-OSI network
For OSI networks in the U.S., portions of the ICD subdomain are
available for use through the U.S. Government, and the DCC subdomain
is available for use through The American National Standards
Institute (ANSI). The British Standards Institute is the
registration authority for the ICD subdomain, and has registered four
IDIs for the U.S. Government: those used for GOSIP, DoD, OSINET, and
the OSI Implementors Workshop. ANSI, as the U.S. ISO Member Body, is
the registration authority for the DCC domain in the United States.
A.1 GOSIP Version 2 NSAPs
GOSIP Version 2 makes available for government use an NSAP addressing
subdomain with a corresponding address format as illustrated in
Figure 2 in Section 4.2. The "47" signifies that it is based on the
ICD format and uses a binary syntax for the DSP. The 0005 is an IDI
value which has been assigned to the U.S. Government. Although GOSIP
Version 2 NSAPs are intended primarily for U.S. Government use,
requests from non-government and non-U.S. organizations will be
considered on a case-by-case basis.
The format for the DSP under ICD=0005 has been established by the
National Institute of Standards and Technology (NIST), the authority
for the ICD=0005 domain, in GOSIP Version 2  (see Figure 2,
Section 4.2). NIST has delegated the authority to register AA
identifiers for GOSIP Version 2 NSAPs to the General Services
ISO 8348 allows a maximum length of 20 octets for the NSAP address.
The AFI of 47 occupies one octet, and the IDI of 0005 occupies two
octets. The DSP is encoded as binary as indicated by the AFI of 47.
One octet is allocated for a DSP Format Identifier, three octets for
an Administrative Authority identifier, two octets for Routing
Domain, two octets for Area, six octets for the System Identifier,
and one octet for the NSAP selector. Note that two octets have been
reserved to accommodate future growth and to provide additional
flexibility for inter-domain routing. The last seven octets of the
GOSIP NSAP format are structured in accordance with IS-IS , the
intra-domain IS-IS routing protocol. The DSP Format Identifier (DFI)
identifies the format of the remaining DSP structure and may be used
in the future to identify additional DSP formats; the value 80h in
the DFI identifies the GOSIP Version 2 NSAP structure.
The Administrative Authority identifier names the administrative
authority which is responsible for registration within its domain.
The administrative authority may delegate the responsibilityfor
registering areas to the routing domains, and the routing domains may
delegate the authority to register System Identifiers to the areas.
The main responsibility of a registration authority at any level of
the addressing hierarchy is to assure that names of entities are
unambiguous, i.e., no two entities have the same name. The
registration authority is also responsible for advertising the names.
A routing domain is a set of end systems and intermediate systems
which operate according to the same routing procedures and is wholly
contained within a single administrative domain. An area uniquely
identifies a subdomain of the routing domain. The system identifier
names a unique system within an area. The value of the system field
may be a physical address (SNPA) or a logical value. Address
resolution between the NSAP and the SNPA may be accomplished by an
ES-IS protocol , locally administered tables, or mapping
functions. The NSAP selector field identifies the end user of the
network layer service, i.e., a transport layer entity.
A.1.1 Application for Administrative Authority Identifiers
The steps required for an agency to acquire an NSAP Administrative
Authority identifier under ICD=0005 from GSA will be provided in the
updated GOSIP users' guide for Version 2  and are given below.
Requests from non-government and non-U.S. organizations should
originate from a senior official, such as a vice-president or chief
* Identify all end systems, intermediate systems, subnetworks, and
their topological and administrative relationships.
* Designate one individual (usually the agency head) within an
agency to authorize all registration requests from that agency
(NOTE: All agency requests must pass through this individual).
* Send a letter on agency letterhead and signed by the agency head
Telecommunications Customer Requirements Office
U.S. General Services Administration
Information Resource Management Service
Office of Telecommunications Services
18th and F Streets, N.W.
Washington, DC 20405
Fax +1 202 208-5555
The letter should contain the following information:
- Requestor's Name and Title,
- Postal Address,
- Telephone and Fax Numbers,
- Electronic Mail Address(es), and,
- Reason Needed (one or two paragraphs explaining the intended
* If accepted, GSA will send a return letter to the agency head
indicating the NSAP Administrative Authority identifier as-
signed,effective date of registration, and any other pertinent
* If rejected, GSA will send a letter to the agency head
explaining the reason for rejection.
* Each Authority will administer its own subaddress space in
accordance with the procedures set forth by the GSA in Section
* The GSA will maintain, publicize, and disseminate the assigned
values of Administrative Authority identifiers unless
specifically requested by an agency not to do so.
A.1.2 Guidelines for NSAP Assignment
Recommendations which should be followed by an administrative
authority in making NSAP assignments are given below.
* The authority should determine the degree of structure of the
DSP under its control. Further delegation of address assignment
authority (resulting in additional levels of hierarchy in the
NSAP) may be desired.
* The authority should make sure that portions of NSAPs that it
specifies are unique, current, and accurate.
* The authority should ensure that procedures exist for
disseminating NSAPs to routing domains and to areas within
each routing domain.
* The systems administrator must determine whether a logical or a
physical address should be used in the System Identifier field
(Figure 2, Section 4.2). An example of a physical address is a
48-bit MAC address; a logical address is merely a number that
meets the uniqueness requirements for the System Identifier
field, but bears no relationship to an address on a physical
subnetwork. We recommend that IDs should be assigned to be
globally unique, as made possible by the method described in
* The network address itself contains information that may be
used to aid routing, but does not contain a source route .
Information that enables next-hop determination based on NSAPs
is gathered and maintained by each intermediate system through
routing protocol exchanges.
* GOSIP end systems and intermediate systems in federal agencies
must be capable of routing information correctly to and from any
subdomain defined by ISO 8348.
* An agency may request the assignment of more than one
Administrative Authority identifier. The particular use of each
should be specified.
A.2 Data Country Code NSAPs
NSAPs from the Data Country Code (DCC) subdomain will also be common
in the international Internet. ANS X3.216-1992 specifies the DSP
structure under DCC=840 . In the ANS, the DSP structure is
identical to that specified in GOSIP Version 2, with the
Administrative Authority identifier replaced by the numeric form of
the ANSI-registered organization name, as shown in Figure 4.
Referring to Figure 4, when the value of the AFI is 39, the IDI
denotes an ISO DCC and the abstract syntax of the DSP is binary
octets. The value of the IDI for the U.S. is 840, the three-digit
numeric code for the United States under ISO 3166 . The numeric
form of organization name is analogous to the Administrative
Authority identifier in the GOSIP Version 2 NSAP.
| AFI | IDI |<----------------------DSP------------->|
| 39 | 840 | DFI |ORG | Rsvd | RD | Area | ID | SEL |
octets | 1 | 2 | 1 | 3 | 2 | 2 | 2 | 6 | 1 |
IDP Initial Domain Part
AFI Authority and Format Identifier
IDI Initial Domain Identifier
DSP Domain Specific Part
DFI DSP Format Identifier
ORG Organization Name (numeric form)
RD Routing Domain Identifier
Area Area Identifier
ID System Identifier
SEL NSAP Selector
Figure 4: NSAP format for DCC=840 as proposed in ANSI X3S3.3.
A.2.1 Application for Numeric Organization Name
The procedures for registration of numeric organization names in the
U.S. have been defined and are operational. To register a numeric
organization name, the applicant must submit a request for
registration and the $1,000 (U.S.) fee to the registration authority,
the American National Standards Institute (ANSI). ANSI will register
a numeric value, along with the information supplied for
registration, in the registration database. The registration
information will be sent to the applicant within ten working days.
The values for numeric organization names are assigned beginning at
The application form for registering a numeric organization name may
be obtained from the ANSI Registration Coordinator at the following
American National Standards Institute
11 West 42nd Street
New York, NY 10036
+1 212 642 4884 (tel)
+1 212 398 0023 (fax)
X.400: G=michelle; S=maas; A=attmail; C=us
Once an organization has registered with ANSI, it becomes a
registration authority itself. In turn, it may delegate registration
authority to routing domains, and these may make further delegations,
for instance, from routing domains to areas. Again, the
responsibilities of each Registration Authority are to assure that
NSAPs within the domain are unambiguous and to advertise them as
A.3 Summary of Administrative Requirements
NSAPs must be globally unique, and an organization may assure this
uniqueness for OSI addresses in two ways. The organization may apply
to GSA for an Administrative Authority identifier. Although
registration of Administrative Authority identifiers by GSA primarily
serves U.S. Government agencies, requests for non-government and
non-U.S. organizations will be considered on a case-by-case basis.
Alternatively, the organization may apply to ANSI for a numeric
organization name. In either case, the organization becomes the
registration authority for its domain and can register NSAPs or
delegate the authority to do so.
In the case of GOSIP Version 2 NSAPs, the complete DSP structure is
given in GOSIP Version 2. For ANSI DCC-based NSAPs, the DSP
structure is specified in ANS X3.216-1992. The DSP structure is
identical to that specified in GOSIP Version 2.