RFC1629 - Guidelines for OSI NSAP Allocation in the Internet

Network Working Group R. Colella
Request for Comments: 1629 NIST
Obsoletes: 1237 R. Callon
Category: Standards Track Wellfleet
E. Gardner
Mitre
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.
Abstract
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
administration.
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
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
addresses).
The guidelines presented in this document are quite similar to the
guidelines that are proposed in the Internet for IP address
allocation with CIDR (RFC1519 [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
Internet.
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 RFC1237. The changes from RFC1237 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.
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
administration.
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
confederations;
* 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
provider.
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
Internet;
* 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
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
listed.
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).
<----IDP---> <----------------------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
"attached".
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.
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
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
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 (RFC1320) 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
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
IS-IS;
* 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 RFC1526 [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
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
networks.
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
destinations.
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 [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
addresses.
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.
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
addresses.
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).
<----IDP--->
+-----+-----+----------------------------------------+
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;
and,
* An operational registration authority for suballocation of AA
values under the GOSIP address space has already been established at
GSA.]
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.
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
domain.
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
applies.
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
considered:
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)
traffic.
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
subscribers.
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,
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
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
domain.
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
RD.
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
internetworking.
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
blurred.
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
domains.
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 t