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RFC 1629

Draft STD
Pages: 52
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Guidelines for OSI NSAP Allocation in the Internet

Part 1 of 2, p. 1 to 24
None       Next RFC Part

Obsoletes:    1237

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Network Working Group                                          R. Colella
Request for Comments: 1629                                           NIST
Obsoletes: 1237                                                 R. Callon
Category: Standards Track                                       Wellfleet
                                                               E. Gardner
                                                               Y. Rekhter
                                   T.J. Watson Research Center, IBM Corp.
                                                                 May 1994

           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

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Table of Contents

   1. Introduction ...............................    4
   2. Scope ......................................    5
   3. Background .................................    7
   3.1 OSI Routing Standards .....................    7
   3.2 Overview of IS-IS (ISO/IEC 10589) .........    8
   3.3 Overview of IDRP (ISO/IEC 10747) ..........   12
   3.3.1 Scaling Mechanisms in IDRP ..............   14
   3.4 Requirements of IS-IS and IDRP on NSAPs ...   15
   4. NSAPs and Routing ..........................   16
   4.1 Routing Data Abstraction ..................   16
   4.2 NSAP Administration and Efficiency ........   19
   5. NSAP Administration and Routing in the In-
        ternet ...................................   21
   5.1 Administration at the Area ................   23
   5.2 Administration at the Subscriber Routing
        Domain ...................................   24
   5.3 Administration at the  Provider  Routing
        Domain ...................................   24
   5.3.1 Direct Service Providers ................   25
   5.3.2 Indirect Providers ......................   26
   5.4 Multi-homed Routing Domains ...............   26
   5.5 Private Links .............................   31
   5.6 Zero-Homed Routing Domains ................   33
   5.7 Address Transition Issues .................   33
   6. Recommendations ............................   36
   6.1 Recommendations Specific to U.S. Parts of
        the Internet .............................   37
   6.2  Recommendations Specific to European Parts
        of the Internet ..........................   39
   6.2.1 General NSAP Structure ..................   40
   6.2.2 Structure of the Country Domain Part ....   40
   6.2.3  Structure of the Country Domain
        Specific Part ............................   41
   6.3 Recommendations Specific to Other Parts of
        the Internet .............................   41
   6.4 Recommendations for Multi-Homed Routing
        Domains ..................................   41
   6.5 Recommendations for RDI and RDCI assign-
        ment .....................................   42
   7. Security Considerations ....................   42
   8. Authors' Addresses .........................   43
   9. Acknowledgments ............................   43
   10. References ................................   44
   A. Administration of NSAPs ....................   46
   A.1  GOSIP Version 2 NSAPs ....................   47
   A.1.1  Application for Administrative Authority

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        Identifiers ..............................   48
   A.1.2  Guidelines for NSAP Assignment .........   50
   A.2  Data Country Code NSAPs ..................   50
   A.2.1  Application for Numeric Organization
        Name .....................................   51
   A.3  Summary of Administrative  Requirements ..   52

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1.  Introduction

   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) [9] 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 [19]).  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

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   been added in Section 6, along with recommendations for Routing
   Domain Identifiers and Routing Domain Confederation Identifiers
   needed for operation of IDRP.

2.  Scope

   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)
   [10], intra-domain IS-IS protocol [14], and inter-domain routing
   protocol (IDRP) [16].  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

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   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);

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   * 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
     in [18].

3.  Background

   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.

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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

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   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).

   <----IDP---> <----------------------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:

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   * 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

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   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
   area address.

   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.

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   The IS-IS provides for optional Quality of Service (QOS) routing,
   based on throughput (the default metric), delay, expense, or residual
   error probability.

   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
   subdomain, respectively).

   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 [13], 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

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   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
   in common.

   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 [11] 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

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     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
   components (RDs).

   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

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   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
     specification [11];

   * 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 [18] 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
     NSAP; and,

   * 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
     routing performance.

   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

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   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.

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   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 [13] 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
   previous paragraph).

   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.

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   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
   inter-domain routing.

   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

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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
                Rsvd  Reserved
                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;

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   * 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 [1] 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.

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   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.

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   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.

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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,
   out of:

   * 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

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   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
   domain is:

   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

(page 24 continued on part 2)

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