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Network Working Group D. Estrin Request for Comments: 1322 USC

                                                            Y. Rekhter
                                                               S. Hotz
                                                              May 1992
             A Unified Approach to Inter-Domain Routing

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard.  Distribution of this memo is


 This memo is an informational RFC which outlines one potential
 approach for inter-domain routing in future global internets.  The
 focus is on scalability to very large networks and functionality, as
 well as scalability, to support routing in an environment of
 heterogeneous services, requirements, and route selection criteria.
 Note: The work of D. Estrin and S. Hotz was supported by the National
 Science Foundation under contract number NCR-9011279, with matching
 funds from GTE Laboratories.  The work of Y. Rekhter was supported by
 the Defense Advanced Research Projects Agency, under contract
 DABT63-91-C-0019.  Views and conclusions expressed in this paper are
 not necessarily those of the Defense Advanced Research Projects
 Agency and National Science Foundation.

1.0 Motivation

 The global internet can be modeled as a collection of hosts
 interconnected via transmission and switching facilities.  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.  Resources under control of a single
 administration form a domain.  In order to support each domain's
 autonomy and heterogeneity, routing consists of two distinct
 components: intra-domain (interior) routing, and inter-domain
 (exterior) routing.  Intra-domain routing provides support for data
 communication between hosts where data traverses transmission and
 switching facilities within a single domain.  Inter-domain routing
 provides support for data communication between hosts where data

Estrin, Rekhter & Hotz [Page 1] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 traverses transmission and switching facilities spanning multiple
 domains.  The entities that forward packets across domain boundaries
 are called border routers (BRs).  The entities responsible for
 exchanging inter-domain routing information are called route servers
 (RSs).  RSs and BRs may be colocated.
 As the global internet grows, both in size and in the diversity of
 routing requirements, providing inter-domain routing that can
 accommodate both of these factors becomes more and more crucial.  The
 number and diversity of routing requirements is increasing due to:
 (a) transit restrictions imposed by source, destination, and transit
 networks, (b) different types of services offered and required, and
 (c) the presence of multiple carriers with different charging
 schemes.  The combinatorial explosion of mixing and matching these
 different criteria weighs heavily on the mechanisms provided by
 conventional hop-by-hop routing architectures ([ISIS10589, OSPF,
 Hedrick88, EGP]).
 Current work on inter-domain routing within the Internet community
 has diverged in two directions: one is best represented by the Border
 Gateway Protocol (BGP)/Inter-Domain Routeing Protocol (IDRP)
 architectures ([BGP91, Honig90, IDRP91]), and another is best
 represented by the Inter-Domain Policy Routing (IDPR) architecture
 ([IDPR90, Clark90]).  In this paper we suggest that the two
 architectures are quite complementary and should not be considered
 mutually exclusive.
 We expect that over the next 5 to 10 years, the types of services
 available will continue to evolve and that specialized facilities
 will be employed to provide new services.  While the number and
 variety of routes provided by hop-by-hop routing architectures with
 type of service (TOS) support (i.e., multiple, tagged routes) may be
 sufficient for a large percentage of traffic, it is important that
 mechanisms be in place to support efficient routing of specialized
 traffic types via special routes.  Examples of special routes are:
 (1) a route that travels through one or more transit domains that
 discriminate according to the source domain, (2) a route that travels
 through transit domains that support a service that is not widely or
 regularly used.  We refer to all other routes as generic.
 Our desire to support special routes efficiently led us to
 investigate the dynamic installation of routes ([Breslau-Estrin91,
 Clark90, IDPR90]).  In a previous paper ([Breslau-Estrin91]), we
 evaluated the algorithmic design choices for inter-domain policy
 routing with specific attention to accommodating source-specific and
 other "special" routes.  The conclusion was that special routes are
 best supported with source-routing and extended link-state
 algorithms; we refer to this approach as source-demand routing

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 [Footnote:  The Inter-Domain Policy Routing (IDPR) architecture uses
 these techniques.].  However, a source-demand routing architecture,
 used as the only means of inter-domain routing, has scaling problems
 because it does not lend itself to general hierarchical clustering
 and aggregation of routing and forwarding information.  For example,
 even if a particular route from an intermediate transit domain X, to
 a destination domain Y is shared by 1,000 source-domains, IDPR
 requires that state for each of the 1,000 routes be setup and
 maintained in the transit border routers between X and Y.  In
 contrast, an alternative approach to inter-domain routing, based on
 hop-by-hop routing and a distributed route-computation algorithm
 (described later), provides extensive support for aggregation and
 abstraction of reachability, topology, and forwarding information.
 The Border Gateway Protocol (BGP) and Inter-Domain Routeing Protocol
 (IDRP) use these techniques ([BGP91, IDRP91]).  While the BGP/IDRP
 architecture is capable of accommodating very large numbers of
 datagram networks, it does not provide support for specialized
 routing requirements as flexibly and efficiently as IDPR-style

1.1 Overview of the Unified Architecture

 We want to support special routes and we want to exploit aggregation
 when a special route is not needed.  Therefore, our scalable inter-
 domain routing architecture consists of two major components:
 source-demand routing (SDR), and node routing (NR).  The NR component
 computes and installs routes that are shared by a significant number
 of sources.  These generic routes are commonly used and warrant wide
 propagation, consequently, aggregation of routing information is
 critical.  The SDR component computes and installs specialized routes
 that are not shared by enough sources to justify computation by NR
 [Footnote: Routes that are only needed sporadically (i.e., the demand
 for them is not continuous or otherwise predictable) are also
 candidates for SDR.].  The potentially large number of different
 specialized routes, combined with their sparse utilization, make them
 too costly to support with the NR mechanism.
 A useful analogy to this approach is the manufacturing of consumer
 products.  When predictable patterns of demand exist, firms produce
 objects and sell them as "off the shelf" consumer goods.  In our
 architecture NR provides off-the-shelf routes.  If demand is not
 predictable, then firms accept special orders and produce what is
 demanded at the time it is needed.  In addition, if a part is so
 specialized that only a single or small number of consumers need it,
 the  consumer may repeatedly special order the part, even if it is
 needed in a predictable manner, because the consumer does not
 represent a big enough market for the producer to bother managing the
 item as part of its regular production.  SDR provides such special

Estrin, Rekhter & Hotz [Page 3] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 order, on-demand routes.
 By combining NR and SDR routing we propose to support inter-domain
 routing in internets of practically-unlimited size, while at the same
 time providing efficient support for specialized routing
 The development of this architecture does assume that routing
 requirements will be diverse and that special routes will be needed.
 On the other hand, the architecture does not depend on assumptions
 about the particular types of routes demanded or on the distribution
 of that demand.  Routing will adapt naturally over time to changing
 traffic patterns and new services by shifting computation and
 installation of particular types of routes between the two components
 of the hybrid architecture [Footnote: Before continuing with our
 explanation of this architecture, we wish to state up front that
 supporting highly specialized routes for all source-destination pairs
 in an internet, or even anything close to that number, is not
 feasible in any routing architecture that we can foresee.  In other
 words, we do not believe that any foreseeable routing architecture
 can support unconstrained proliferation of user requirements and
 network services.  At the same time, this is not necessarily a
 problem.  The capabilities of the architecture may in fact exceed the
 requirements of the users.  Moreover, some of the requirements that
 we regard as infeasible from the inter-domain routing point of view,
 may be supported by means completely outside of routing.
 Nevertheless, the caveat is stated here to preempt unrealistic
 While the packet forwarding functions of the NR and SDR components
 have little or no coupling with each other, the connectivity
 information exchange mechanism of the SDR component relies on
 services provided by the NR component.

1.2 Outline

 The remainder of this report is organized as follows.  Section 2
 outlines the requirements and priorities that guide the design of the
 NR and SDR components.  Sections 3 and 4 describe the NR and SDR
 design choices, respectively, in light of these requirements.
 Section 5 describes protocol support for the unified architecture and
 briefly discusses transition issues.  We conclude with a brief

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2.0 Architectural Requirements and Priorities

 In order to justify our design choices for a scalable inter-domain
 routing architecture, we must articulate our evaluation criteria and
 priorities.  This section defines complexity, abstraction, policy,
 and type of service requirements.

2.1 Complexity

 Inter-domain routing complexity must be evaluated on the basis of the
 following performance metrics: (1) storage overhead, (2)
 computational overhead, and (3) message overhead.  This evaluation is
 essential to determining the scalability of any architecture.

2.1.1 Storage Overhead

 The storage overhead of an entity that participates in inter-domain
 routing comes from two sources: Routing Information Base (RIB), and
 Forwarding Information Base (FIB) overhead.  The RIB contains the
 routing information that entities exchange via the inter-domain
 routing protocol; the RIB is the input to the route computation.  The
 FIB contains the information that the entities use to forward the
 inter-domain traffic; the FIB is the output of the route computation.
 For an acceptable level of storage overhead, the amount of
 information in both FIBs and RIBs should grow significantly slower
 than linearly (e.g., close to logarithmically) with the total number
 of domains in an internet.  To satisfy this requirement with respect
 to the RIB, the architecture must provide mechanisms for either
 aggregation and abstraction of routing and forwarding information, or
 retrieval of a subset of this information on demand.  To satisfy this
 requirement with respect to the FIB, the architecture must provide
 mechanisms for either aggregation of the forwarding information (for
 the NR computed routes), or dynamic installation/tear down of this
 information (for the SDR computed routes).
 Besides being an intrinsically important evaluation metric, storage
 overhead has a direct impact on computational and bandwidth
 complexity.  Unless the computational complexity is fixed (and
 independent of the total number of domains), the storage overhead has
 direct impact on the computational complexity of the architecture
 since the routing information is used as an input to route
 computation. Moreover, unless the architecture employs incremental
 updates, where only changes to the routing information are
 propagated, the storage overhead has direct impact on the bandwidth
 overhead of the architecture since the exchange of routing
 information constitutes most of the bandwidth overhead.

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2.1.2 Computational Overhead

 The NR component will rely primarily on precomputation of routes.  If
 inter-domain routing is ubiquitous, then the precomputed routes
 include all reachable destinations.  Even if policy constraints make
 fully ubiquitous routing impossible, the precomputed routes are
 likely to cover a very large percentage of all reachable
 destinations.  Therefore the complexity of this computation must be
 as small as possible.  Specifically, it is highly desirable that the
 architecture would employ some form of partial computation, where
 changes in topology would require less than complete recomputation.
 Even if complete recomputation is necessary, its complexity should be
 less than linear with the total number of domains.
 The SDR component will use on-demand computation and caching.
 Therefore the complexity of this computation can be somewhat higher.
 Another reason for relaxed complexity requirements for SDR is that
 SDR is expected to compute routes to a smaller number of destinations
 than is NR (although SDR route computation may be invoked more
 Under no circumstances is computational complexity allowed to become
 exponential (for either the NR or SDR component).

2.1.3 Bandwidth Overhead

 The bandwidth consumed by routing information distribution should be
 limited.  However, the possible use of data compression techniques
 and the increasing speed of network links make this less important
 than route computation and storage overhead.  Bandwidth overhead may
 be further contained by using incremental (rather than complete)
 exchange of routing information.
 While storage and bandwidth overhead may be interrelated, if
 incremental updates are used then bandwidth overhead is negligible in
 the steady state (no changes in topology), and is independent of the
 storage overhead.  In other words, use of incremental updates
 constrains the bandwidth overhead to the dynamics of the internet.
 Therefore, improvements in stability of the physical links, combined
 with techniques to dampen the effect of topological instabilities,
 will make the bandwidth overhead even less important.

2.2 Aggregation

 Aggregation and abstraction of routing and forwarding information
 provides a very powerful mechanism for satisfying storage,
 computational, and bandwidth constraints.  The ability to aggregate,
 and subsequently abstract, routing and forwarding information is

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 essential to the scaling of the architecture [Footnote: While we can
 not prove that there are no other ways to achieve scaling, we are not
 aware of any mechanism other than clustering that allows information
 aggregation/abstraction.  Therefore, the rest of the paper assumes
 that clustering is used for information aggregation/abstraction.].
 This is especially true with respect to the NR component, since the
 NR component must be capable of providing routes to all or almost all
 reachable destinations.
 At the same time, since preserving each domain's independence and
 autonomy is one of the crucial requirements of inter-domain routing,
 the architecture must strive for the maximum flexibility of its
 aggregation scheme, i.e., impose as few preconditions, and as little
 global coordination, as possible on the participating domains.
 The Routing Information Base (RIB) carries three types of
 information: (1) topology (i.e., the interconnections between domains
 or groups of domains), (2) network layer reachability, and (3)
 transit constraint.  Aggregation of routing information should
 provide a reduction of all three components.  Aggregation of
 forwarding information will follow from reachability information
 Clustering (by forming routing domain confederations) serves the
 following aggregation functions: (1) to hide parts of the actual
 physical topology, thus abstracting topological information, (2) to
 combine a set of reachable destination entities into a single entity
 and reduce storage overhead, and (3) to express transit constraints
 in terms of clusters, rather than individual domains.
 As argued in [Breslau-Estrin91], the architecture must allow
 confederations to be formed and changed without extensive
 configuration and coordination; in particular, forming a
 confederation should not require global coordination (such as that
 required in ECMA ([ECMA89]).  In addition, aggregation should not
 require explicit designation of the relative placement of each domain
 relative to another; i.e., domains or confederations of domains
 should not be required to agree on a partial ordering (i.e., who is
 above whom, etc.).

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 The architecture should allow different domains to use different
 methods of aggregation and abstraction.  For example, a research
 collaborator at IBM might route to USC as a domain-level entity in
 order to take advantage of some special TOS connectivity to, or even
 through, USC.  Whereas, someone else at Digital Equipment Corporation
 might see information at the level of the California Educational
 Institutions Confederation, and know only that USC is a member.
 Alternatively, USC might see part of the internal structure within
 the IBM Confederation (at the domain's level), whereas UCLA may route
 based on the confederation of IBM domains as a whole.
 Support for confederations should be flexible.  Specifically, the
 architecture should allow confederations to overlap without being
 nested, i.e., a single domain, or a group of domains may be part of
 more than one confederation.  For example, USC may be part of the
 California Educational Institutions Confederation and part of the US
 R&D Institutions Confederation; one is not a subset of the other.
 Another example: T.J.  Watson Research Center might be part of
 NYSERNET Confederation and part of IBM-R&D-US Confederation.  While
 the above examples describe cases where overlap consists of a single
 domain, there may be other cases where multiple domains overlap.  As
 an example consider the set of domains that form the IBM
 Confederation, and another set of domains that form the DEC
 Confederation.  Within IBM there is a domain IBM-Research, and
 similarly within DEC there is a domain DEC-Research.  Both of these
 domains could be involved in some collaborative effort, and thus have
 established direct links.  The architecture should allow restricted
 use of these direct links, so that other domains within the IBM
 Confederation would not be able to use it to talk to other domains
 within the DEC Confederation.  A similar example exists when a
 multinational corporation forms a confederation, and the individual
 branches within each country also belong to their respective country
 confederations.  The corporation may need to protect itself from
 being used as an inter-country transit domain (due to internal, or
 international, policy).  All of the above examples illustrate a
 situation where confederations overlap, and it is necessary to
 control the traffic traversing the overlapping resources.
 While flexible aggregation should be accommodated in any inter-domain
 architecture, the extent to which this feature is exploited will have
 direct a effect on the scalability associated with aggregation.  At
 the same time, the exploitation of this feature depends on the way
 addresses are assigned.  Specifically, scaling associated with
 forwarding information depends heavily on the assumption that there
 will be general correspondence between the hierarchy of address
 registration authorities, and the way routing domains and routing
 domain confederations are organized (see Section 2.6).

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2.3 Routing Policies

 Routing policies that the architecture must support may be broadly
 classified into transit policies and route selection policies
 [Breslau-Estrin 91, Estrin89].
 Restrictions imposed via transit policies may be based on a variety
 of factors.  The architecture should be able to support restrictions
 based on the source, destination, type of services (TOS), user class
 identification (UCI), charging, and path [Estrin89 , Little89].  The
 architecture must allow expression of transit policies on all routes,
 both NR and SDR.  Even if NR routes are widely used and have fewer
 source or destination restrictions, NR routes may have some transit
 qualifiers (e.g., TOS, charging, or user-class restriction).  If the
 most widely-usable route to a destination has policy qualifiers, it
 should be advertiseable by NR and the transit constraints should be
 Route selection policies enable each domain to select a particular
 route among multiple routes to the same destination.  To maintain
 maximum autonomy and independence between domains, the architecture
 must support heterogeneous route selection policies, where each
 domain can establish its own criteria for selecting routes.  This
 argument was made earlier with respect to source route selection
 ([IDPR90, Clark90, Breslau-Estrin91]).  In addition, each
 intermediate transit domain must have the flexibility to apply its
 own selection criteria to the routes made available to it by its
 neighbors.  This is really just a corollary to the above; i.e., if we
 allow route selection policy to be expressed for NR routes, we can
 not assume all domains will apply the same policy.  The support for
 dissimilar route selection policies is one of the key factors that
 distinguish inter-domain routing from intra-domain routing ([ECMA89,
 Estrin89]).  However, it is a non-goal of the architecture to support
 all possible route selection policies.  For more on unsupported route
 selection policies see Section 2.3.2 below.

2.3.1 Routing Information Hiding

 The architecture should not require all domains within an internet to
 reveal their connectivity and transit constraints to each other.
 Domains should be able to enforce their transit policies while
 limiting the advertisement of their policy and connectivity
 information as much as possible; such advertisement will be driven by
 a "need to know" criteria.  Moreover, advertising a transit policy to
 domains that can not use this policy will increase the amount of
 routing information that must be stored, processed, and propagated.
 Not only may it be impractical for a router to maintain full inter-
 domain topology and policy information, it may not be permitted to

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 obtain this information.

2.3.2 Policies Not Supported

 In this and previous papers we have argued that a global inter-domain
 routing architecture should support a wide range of policies.  In
 this section we identify several types of policy that we explicitly
 do not propose to support.  In general our reasoning is pragmatic; we
 think such policy types are either very expensive in terms of
 overhead, or may lead to routing instabilities.
 1. Route selection policies contingent on external behavior.
    The route selection process may be modeled by a function that
    assigns a non-negative integer to a route, denoting the degree
    of preference.  Route selection applies this function to all
    feasible routes to a given destination, and selects the route
    with the highest value.  To provide a stable environment, the
    preference function should not use as an input the status and
    attributes of other routes (either to the same or to a
    different destination).
 2. Transit policies contingent on external behavior.
    To provide a stable environment, the domain's transit policies
    can not be automatically affected by any information external
    to the domain.  Specifically, these policies can not be modified,
    automatically, in response to information about other domains'
    transit policies, or routes selected by local or other domains.
    Similarly, transit policies can not be automatically modified
    in response to information about performance characteristics of
    either local or external domains.
 3. Policies contingent on external state (e.g., load).
    It is a non-goal of the architecture to support load-sensitive
    routing for generic routes.  However, it is possible that some
    types of service may employ load information to select among
    alternate SDR routes.
 4. Very large number of simultaneous SDR routes.
    It is a non-goal of the architecture to support a very large
    number of simultaneous SDR routes through any single router.
    Specifically, the FIB storage overhead associated with SDR
    routes must be comparable with that of NR routes, and should
    always be bound by the complexity requirements outlined earlier
    [Foonote: As discussed earlier, theoretically the state overhead
    could grow O(N^2) with the number of domains in an internet.
    However, there is no evidence or intuition to suggest that
    this will be a limiting factor on the wide utilization of SDR,
    provided that NR is available to handle generic routes.].

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2.4 Support for TOS Routing

 Throughout this document we refer to support for type of service
 (TOS) routing.  There is a great deal of research and development
 activity currently underway to explore network architectures and
 protocols for high-bandwidth, multimedia traffic.  Some of this
 traffic is delay sensitive, while some requires high throughput.  It
 is unrealistic to assume that a single communication fabric will be
 deployed homogeneously across the internet (including all
 metropolitan, regional, and backbone networks) that will support all
 types of traffic uniformly.  To support diverse traffic requirements
 in a heterogeneous environment, various resource management
 mechanisms will be used in different parts of the global internet
 (e.g., resource reservation of various kinds) [ST2-90, Zhang91].
 In this context, it is the job of routing protocols to locate routes
 that can potentially support the particular TOS requested.  It is
 explicitly not the job of the general routing protocols to locate
 routes that are guaranteed to have resources available at the
 particular time of the route request.  In other words, it is not
 practical to assume that instantaneous resource availability will be
 known at all remote points in the global internet.  Rather, once a
 TOS route has been identified, an application requiring particular
 service guarantees will attempt to use the route (e.g., using an
 explicit setup message if so required by the underlying networks).
 In Section 4 we describe additional services that may be provided to
 support more adaptive route selection for special TOS [Footnote:
 Adaptive route selection implies adaptability only during the route
 selection process.  Once a route is selected, it is not going to be a
 subject to any adaptations, except when it becomes infeasible.].

2.5 Commonality between Routing Components

 While it is acceptable for the NR and SDR components to be
 dissimilar, we do recognize that such a solution is less desirable --
 all other things being equal.  In theory, there are advantages in
 having the NR and SDR components use similar algorithms and
 mechanisms.  Code and databases could be shared and the architecture
 would be more manageable and comprehensible.  On the other hand,
 there may be some benefit (e.g., robustness) if the two parts of the
 architecture are heterogeneous, and not completely inter-dependent.
 In Section 5 we list several areas in which opportunities for
 increased commonality (unification) will be exploited.

2.6 Interaction with Addressing

 The proposed architecture should be applicable to various addressing
 schemes.  There are no specific assumptions about a particular

Estrin, Rekhter & Hotz [Page 11] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 address structure, except that this structure should facilitate
 information aggregation, without forcing rigid hierarchical routing.
 Beyond this requirement, most of the proposals for extending the IP
 address space, for example, can be used in conjunction with our
 architecture.  But our architecture itself does not provide (or
 impose) a particular solution to the addressing problem.

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3.0 Design Choices for Node Routing (NR)

 This section addresses the design choices made for the NR component
 in light of the above architectural requirements and priorities.  All
 of our discussion of NR assumes hop-by-hop routing.  Source routing
 is subject to different constraints and is used for the complementary
 SDR component.

3.1 Overview of NR

 The NR component is designed and optimized for an environment where a
 large percentage of packets will travel over routes that can be
 shared by multiple sources and that have predictable traffic
 patterns.  The efficiency of the NR component improves when a number
 of source domains share a particular route from some intermediate
 point to a destination.  Moreover, NR is best suited to provide
 routing for inter-domain data traffic that is either steady enough to
 justify the existence of a route, or predictable, so that a route may
 be installed when needed (based on the prediction, rather than on the
 actual traffic).  Such routes lend themselves to distributed route
 computation and installation procedures.
 Routes that are installed in routers, and information that was used
 by the routers to compute these routes, reflect the known operational
 state of the routing facilities (as well as the policy constraints)
 at the time of route computation.  Route computation is driven by
 changes in either operational status of routing facilities or policy
 constraints.  The NR component supports route computation that is
 dynamically adaptable to both changes in topology and policy.  The NR
 component allows time-dependent selection or deletion of routes.
 However, time dependency must be predictable (e.g., advertising a
 certain route only after business hours) and routes should be used
 widely enough to warrant inclusion in NR.
 The proposed architecture assumes that most of the inter-domain
 conversations will be forwarded via routes computed and installed by
 the NR component.
 Moreover, the exchange of routing information necessary for the SDR
 component depends on facilities provided by the NR component; i.e.,
 NR policies must allow SDR reachability information to travel.
 Therefore, the architecture requires that all domains in an internet
 implement and participate in NR.  Since scalability (with respect to
 the size of the internet) is one of the fundamental requirements for
 the NR component, it must provide multiple mechanisms with various
 degrees of sophistication for information aggregation and

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 The potential reduction of routing and forwarding information depends
 very heavily on the way addresses are assigned and how domains and
 their confederations are structured.  "If there is no correspondence
 between the address registration hierarchy and the organisation of
 routeing domains, then ... each and every routeing domain in the OSIE
 would need a table entry potentially at every boundary IS of every
 other routeing domain" ([Oran89]).  Indeed, scaling in the NR
 component is almost entirely predicated on the assumption that there
 will be general correspondence between the hierarchy of address
 assignment authorities and the way routing domains are organised, so
 that the efficient and frequent aggregation of routing and forwarding
 information will be possible.  Therefore, given the nature of inter-
 domain routing in general, and the NR component in particular,
 scalability of the architecture depends very heavily on the
 flexibility of the scheme for information aggregation and
 abstraction, and on the preconditions that such a scheme imposes.
 Moreover, given a flexible architecture, the operational efficiency
 (scalability) of the global internet, or portions thereof, will
 depend on tradeoffs made between flexibility and aggregation.
 While the NR component is optimized to satisfy the common case
 routing requirements for an extremely large population of users, this
 does not imply that routes produced by the NR component would not be
 used for different types of service (TOS).  To the contrary, if a TOS
 becomes sufficiently widely used (i.e., by multiple domains and
 predictably over time), then it may warrant being computed by the NR
 To summarize, the NR component is best suited to provide routes that
 are used by more than a single domain.  That is, the efficiency of
 the NR component improves when a number of source domains share a
 particular route from some intermediate point to a destination.
 Moreover, NR is best suited to provide routing for inter-domain data
 traffic that is either steady enough to justify the existence of a
 route, or predictable, so that a route may be installed when needed,
 (based on the prediction, rather than on the actual traffic).

3.2 Routing Algorithm Choices for NR

 Given that a NR component based on hop-by-hop routing is needed to
 provide scalable, efficient inter-domain routing, the remainder of
 this section considers the fundamental design choices for the NR
 routing algorithm.
 Typically the debate surrounding routing algorithms focuses on link
 state and distance vector protocols.  However, simple distance vector
 protocols (i.e., Routing Information Protocol [Hedrick88]), do not
 scale because of convergence problems.  Improved distance vector

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 protocols, such as those discussed in [Jaffee82, Zaumen91, Shin87],
 have been developed to address this issue using synchronization
 mechanisms or additional path information.  In the case of inter-
 domain routing, having additional path information is essential to
 supporting policy.  Therefore, the algorithms we consider for NR are
 link state and one we call path vector (PV).  Whereas the
 characteristics of link state algorithms are generally understood
 (for example, [Zaumen 91]), we must digress from our evaluation
 discussion to describe briefly the newer concept of the PV algorithm
 [Footnote: We assume that some form of SPF algorithm will be used to
 compute paths over the topology database when LS algorithms are used
 [Dijkstra59, OSPF]].

3.2.1 Path Vector Protocol Overview

 The Border Gateway Protocol (BGP) (see [BGP91]) and the Inter Domain
 Routing Protocol (IDRP) (see [IDRP91]) are examples of path vector
 (PV) protocols [Footnote: BGP is an inter-autonomous system routing
 protocol for TCP/IP internets.  IDRP is an OSI inter-domain routing
 protocol that is being progressed toward standardization within ISO.
 Since in terms of functionality BGP represents a proper subset of
 IDRP, for the rest of the paper we will only consider IDRP.].
 The routing algorithm employed by PV bears a certain resemblance to
 the traditional Bellman-Ford routing algorithm in the sense that each
 border router advertises the destinations it can reach to its
 neighboring BRs.  However,the PV routing algorithm augments the
 advertisement of reachable destinations with information that
 describes various properties of the paths to these destinations.
 This information is expressed in terms of path attributes.  To
 emphasize the tight coupling between the reachable destinations and
 properties of the paths to these destinations, PV defines a route as
 a pairing between a destination and the attributes of the path to
 that destination.  Thus the name, path-vector protocol, where a BR
 receives from its neighboring BR a vector that contains paths to a
 set of destinations [Footnote: The term "path-vector protocol" bears
 an intentional similarity to the term "distance-vector protocol",
 where a BR receives from each of its neighbors a vector that contains
 distances to a set of destinations.].  The path, expressed in terms
 of the domains (or confederations) traversed so far, is carried in a
 special path attribute which records the sequence of routing domains
 through which the reachability information has passed.  Suppression
 of routing loops is implemented via this special path attribute, in
 contrast to LS and distance vector which use a globally-defined
 monotonically-increasing metric for route selection [Shin87].
 Because PV does not require all routing domains to have homogeneous

Estrin, Rekhter & Hotz [Page 15] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 criteria (policies) for route selection, route selection policies
 used by one routing domain are not necessarily known to other routing
 domains.  To maintain the maximum degree of autonomy and independence
 between routing domains, each domain which participates in PV may
 have its own view of what constitutes an optimal route.  This view is
 based solely on local route selection policies and the information
 carried in the path attributes of a route.  PV standardizes only the
 results of the route selection procedure, while allowing the
 selection policies that affect the route selection to be non-standard
 [Footnote: This succinct observation is attributed to Ross Callon
 (Digital Equipment Corporation).].

3.3 Complexity

 Given the above description of PV algorithms, we can compare them to
 LS algorithms in terms of the three complexity parameters defined

3.3.1 Storage Overhead

 Without any aggregation of routing information, and ignoring storage
 overhead associated with transit constraints, it is possible to show
 that under some rather general assumptions the average case RIB
 storage overhead of the PV scheme for a single TOS ranges between
 O(N) and O(Nlog(N)), where N is the total number of routing domains
 ([Rekhter91]).  The LS RIB, with no aggregation of routing
 information, no transit constraints, a single homogeneous route
 selection policy across all the domains, and a single TOS, requires a
 complete domain-level topology map whose size is O(N).
 Supporting heterogeneous route selection and transit policies with
 hop-by-hop forwarding and LS requires each domain to know all other
 domains route selection and transit policies.  This may significantly
 increase the amount of routing information that must be stored by
 each domain.  If the number of policies advertised grows with the
 number of domains, then the storage could become unsupportable.  In
 contrast, support for heterogeneous route selection policies has no
 effect on the storage complexity of the PV scheme (aside from simply
 storing the local policy information).  The presence of transit
 constraints in PV results in a restricted distribution of routing
 information, thus further reducing storage overhead.  In contrast,
 with LS no such reduction is possible since each domain must know
 every other domain's transit policies.  Finally, some of the transit
 constraints (e.g., path sensitive constraints) can be expressed in a
 more concise form in PV (see aggregation discussion below).
 The ability to further restrict storage overhead is facilitated by
 the PV routing algorithm, where route computation precedes routing

Estrin, Rekhter & Hotz [Page 16] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 information dissemination, and only routing information associated
 with the routes selected by a domain is distributed to adjacent
 domains.  In contrast, route selection with LS is decoupled from the
 distribution of routing information, and has no effect on such
 While theoretically routing information aggregation can be used to
 reduce storage complexity in both LS and PV, only aggregation of
 topological information would yield the same gain for both.
 Aggregating transit constraints with LS may result in either reduced
 connectivity or less information reduction, as compared with PV.
 Aggregating heterogeneous route selection policies in LS is highly
 problematic, at best.  In PV, route selection policies are not
 distributed, thus making aggregation of route selection policies a
 non-issue [Footnote: Although a domain's selection policies are not
 explicitly distributed, they have an impact on the routes available
 to other domains.  A route that may be preferred by a particular
 domain, and not prohibited by transit restrictions, may still be
 unavailable due to the selection policies of some intermediate
 domain.  The ability to compute and install alternative routes that
 may be lost using hop-by-hop routing (either LS of PV) is the
 critical functionality provided by SDR.].
 Support for multiple TOSs has the same impact on storage overhead for
 both LS and for PV.
 Potentially the LS FIB may be smaller if routes are computed at each
 node on demand.  However, the gain of such a scheme depends heavily
 on the traffic patterns, memory size, and caching strategy.  If there
 is not a high degree of locality, severely degraded performance can
 result due to excessive overall computation time and excessive
 computation delay when forwarding packets to a new destination.  If
 demand driven route computation is not used for LS, then the size of
 the FIB would be the same for both LS and PV.

Estrin, Rekhter & Hotz [Page 17] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

3.3.2 Route Computation Complexity

 Even if all domains employ exactly the same route selection policy,
 computational complexity of PV is smaller than that of LS.  The PV
 computation consists of evaluating a newly arrived route and
 comparing it with the existing one [Footnote: Some additional checks
 are required when an update is received to insure that a routing loop
 has not been created.].  Whereas, conventional LS computation
 requires execution of an SPF algorithm such as Dijkstra's.
 With PV, topology changes only result in the recomputation of routes
 affected by these changes, which is more efficient than complete
 recomputation.  However, because of the inclusion of full path
 information with each distance vector, the effect of a topology
 change may propagate farther than in traditional distance vector
 algorithms.  On the other hand, the number of affected domains will
 still be smaller with PV than with conventional LS hop-by-hop
 routing.  With PV, only those domains whose routes are affected by
 the changes have to recompute, while with conventional LS hop-by-hop
 routing all domains must recompute.  While it is also possible to
 employ partial recomputation with LS (i.e., when topology changes,
 only the affected routes are recomputed), "studies suggest that with
 a very small number of link changes (perhaps 2) the expected
 computational complexity of the incremental update exceeds the
 complete recalculation" ([ANSI87-150R]).  However these checks are
 much simpler than executing a full SPF algorithm.
 Support for heterogeneous route selection policies has serious
 implications for the computational complexity.  The major problem
 with supporting heterogeneous route selection policies with LS is the
 computational complexity of the route selection itself.
 Specifically, we are not aware of any algorithm with less than
 exponential time complexity that would be capable of computing routes
 to all destinations, with LS hop-by-hop routing and heterogeneous
 route selection policies.  In contrast, PV allows each domain to make
 its route selection autonomously, based only on local policies.
 Therefore support for dissimilar route selection policies has no
 negative implications for the complexity of route computation in PV.
 Similarly, providing support for path-sensitive transit policies in
 LS implies exponential computation, while in PV such support has no
 impact on the complexity of route computation.
 In summary, because NR will rely primarily on precomputation of
 routes, aggregation is essential to the long-term scalability of the
 architecture.  To the extent aggregation is facilitated with PV, so
 is reduced computational complexity.  While similar arguments may be
 made for LS, the aggregation capabilities that can be achieved with
 LS are more problematic because of LS' reliance on consistent

Estrin, Rekhter & Hotz [Page 18] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 topology maps at each node.  It is also not clear what additional
 computational complexity will be associated with aggregation of
 transit constraints and heterogeneous route selection policies in LS.

3.3.3 Bandwidth Overhead

 PV routing updates include fully-expanded information.  A complete
 route for each supported TOS is advertised.  In LS, TOS only
 contributes a factor increase per link advertised, which is much less
 than the number of complete routes.  Although TOSs may be encoded
 more efficiently with LS than with PV, link state information is
 flooded to all domains, while with PV, routing updates are
 distributed only to the domains that actually use them.  Therefore,
 it is difficult to make a general statement about which scheme
 imposes more bandwidth overhead, all other factors being equal.
 Moreover, this is perhaps really not an important difference, since
 we are more concerned with the number of messages than with the
 number of bits (because of compression and greater link bandwidth, as
 well as the increased physical stability of links).

3.4 Aggregation

 Forming confederations of domains, for the purpose of consistent,
 hop-by-hop, LS route computation, requires that domains within a
 confederation have consistent policies.  In addition, LS
 confederation requires that any lower level entity be a member of
 only one higher level entity.  In general, no intra-confederation
 information can be made visible outside of a confederation, or else
 routing loops may occur as a result of using an inconsistent map of
 the network at different domains.  Therefore, the use of
 confederations with hop-by-hop LS is limited because each domain (or
 confederation) can only be a part of one higher level confederation
 and only export policies consistent with that confederation (see
 examples in Section 2.2).  These restrictions are likely to impact
 the scaling capabilities of the architecture quite severely.
 In comparison, PV can accommodate different confederation definitions
 because looping is avoided by the use of full path information.
 Consistent network maps are not needed at each route server, since
 route computation precedes routing information dissemination.
 Consequently, PV confederations can be nested, disjoint, or
 overlapping.  A domain (or confederation) can export different
 policies or TOS as part of different confederations, thus providing
 the extreme flexibility that is crucial for the overall scaling and
 extensibility of the architecture.
 In summary, aggregation is essential to achieve acceptable complexity

Estrin, Rekhter & Hotz [Page 19] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 bounds, and flexibility is essential to achieve acceptable
 aggregation across the global, decentralized internet.  PV's
 strongest advantage stems from its flexibility.

3.5 Policy

 The need to allow expression of transit policy constraints on any
 route (i.e., NR routes as well as SDR routes), by itself, can be
 supported by either LS or PV.  However, the associated complexities
 of supporting transit policy constraints are noticeably higher for LS
 than for PV.  This is due to the need to flood all transit policies
 with LS, where with PV transit policies are controlled via restricted
 distribution of routing information.  The latter always imposes less
 overhead than flooding.
 While all of the transit constraints that can be supported with LS
 can be supported with PV, the reverse is not true.  In other words,
 there are certain transit constraints (e.g., path-sensitive transit
 constraints) that are easily supported with PV, and are prohibitively
 expensive (in terms of complexity) to support in LS.  For example, it
 is not clear how NR with LS could support something like ECMA-style
 policies that are based on hierarchical relations between domains,
 while support for such policies is straightforward with PV.
 As indicated above, support for heterogeneous route selection
 policies, in view of its computational and storage complexity, is
 impractical with LS hop-by-hop routing.  In contrast, PV can
 accommodate heterogeneous route selection with little additional

3.6 Information Hiding

 PV has a clear advantage with respect to selective information
 hiding.  LS with hop-by-hop routing hinges on the ability of all
 domains to have exactly the same information; this clearly
 contradicts the notion of selective information hiding.  That is, the
 requirement to perform selective information hiding is unsatisfiable
 with LS hop-by-hop routing.

3.7 Commonality between NR and SDR Components

 In [Breslau-Estrin91] we argued for the use of LS in conjunction with
 SDR.  Therefore there is some preference for using LS with NR.
 However, there are several reasons why NR and SDR would not use
 exactly the same routing information, even if they did use the same
 algorithm.  Moreover, there are several opportunities for unifying
 the management (distribution and storage) of routing and forwarding
 information, even if dissimilar algorithms are used.

Estrin, Rekhter & Hotz [Page 20] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 In considering the differences between NR and SDR we must address
 several areas:
   1. Routing information and distribution protocol: LS for SDR is
      quite different from the LS in NR.  For example, SDR LS need
      not aggregate domains; to the contrary SDR LS  requires detailed
      information to generate special routes.
      In addition, consistency requirements (essential for NR) are
      unnecessary for the SDR component.  Therefore LS information for
      the SDR component can be retrieved on-demand, while the NR
      component must use flooding of topology information.
   2. Route computation algorithm: It is not clear whether route
      computation algorithm(s)  can be shared between the SDR and NR
      components, given the difficulty of supporting  heterogeneous
      route selection policies in NR.
   3. Forwarding information: The use of dissimilar route computation
      algorithms does not preclude common handling of packet
      forwarding.  Even if LS were used for NR, the requirement would
      be the same, i.e., that the forwarding agent can determine
      whether to use a NR  precomputed route or an SDR installed route
      to forward a particular data packet.
 In conclusion, using similar algorithms and mechanisms for SDR and NR
 components would have benefits.  However, these benefits do not
 dominate the other factors as discussed before.

Estrin, Rekhter & Hotz [Page 21] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

3.8 Summary

 Given the performance complexity issues associated with global
 routing, aggregation of routing information is essential; at the same
 time we have argued that such aggregation must be flexible.  Given
 the difficulties of supporting LS hop-by-hop routing in the presence
 of (a) flexible aggregation, (b) heterogeneous route selection
 policies, and (c) incomplete or inconsistent routing information, we
 see no alternative but to employ PV for the NR component of our
 Based on the above tradeoffs, our NR component employs a PV
 architecture, where route computation and installation is done in a
 distributed fashion by the routers participating in the NR component
 [Footnote: Packet forwarding along routes produced by the NR
 component can be accomplished by either source routing or hop-by-hop
 routing.  The latter is the primary choice because it reduces the
 amount of state in routers (if route setup is employed), or avoids
 encoding an explicit source route in network layer packets.  However,
 the architecture does not preclude the use of source routing (or
 route setup) along the routes computed, but not installed, by the NR
 The distributed algorithm combines some of the features of link state
 with those of distance vector algorithms; in addition to next hop
 information, the NR component maintains path attributes for each
 route (e.g., the list of domains or routing domain confederations
 that the routing information has traversed so far).  The path
 attributes that are carried along with a route express a variety of
 routing policies, and make explicit the entire route to the
 destination.  With aggregation, this is a superset of the domains
 that form the path to the destination.

Estrin, Rekhter & Hotz [Page 22] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

4.0 Source-demand routing (SDR)

 Inter-domain routers participating in the SDR component forward
 packets according to routing information computed and installed by
 the domain that originates the traffic (source routing domain).
 It is important to realize that requiring route installation by the
 source routing domain is not a matter of choice, but rather a
 necessity.  If a particular route is used by a small number of
 domains (perhaps only one) then it is more appropriate to have the
 source compute and install the special route instead of burdening the
 intermediate nodes with the task of looking for and selecting a route
 with the specialized requirements.  In addition, if the demand for
 the route is unpredictable, and thus can be determined only by the
 source, it should be up to the source to install the route.
 In general, information that is used by source routing domains for
 computing source-demand routes reflects administrative (but not
 operational) status of the routing facilities (i.e., configured
 topology and policy) [Footnote: If SDR uses NR information then
 operational status could be considered in some route selection.].
 Consequently, it is possible for a source routing domain to compute a
 route that is not operational at route installation time.  The SDR
 component attempts to notify the source domain of failures when route
 installation is attempted.  Similarly, the SDR component provides
 mechanisms for the source routing domain to be notified of failures
 along previously-installed active routes.  In other words, the SDR
 component performs routing that is adaptive to topological changes;
 however, the adaptability is achieved as a consequence of the route
 installation and route management mechanisms.  This is different from
 the NR component, where status changes are propagated and
 incorporated by nodes as soon as possible.  Therefore, to allow
 faster adaptation to changes in the operational status of routing
 facilities, the SDR component allows the source domain to switch to a
 route computed by the NR component, if failure along the source-
 demand route is detected (either during the route installation phase,
 or after the route is installed), and if policy permits use of the NR
 The NR component will group domains into confederations to achieve
 its scaling goals (see [IDRP91]).  In contrast, SDR will allow an
 AD-level route to be used by an individual domain without allowing
 use by the entire confederation to which the domain belongs.
 Similarly, a single transit domain may support a policy or special
 TOS that is not supported by other domains in its confederation(s).
 In other words, the architecture uses SDR to support non-
 hierarchical, non-aggregated policies where and when needed.
 Consequently, SDR by itself does not have the scaling properties of

Estrin, Rekhter & Hotz [Page 23] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 NR.  In compensation, SDR does not require a complete, global domain
 map if portions of the world are never traversed or communicated
 with.  As a result of the looser routing structure, SDR does not
 guarantee that a participating source routing domain will always have
 sufficient information to compute a route to a destination.  In
 addition, if the domain does have sufficient information, it is
 possible that the quantity may be large enough to preclude storage
 and/or route computation in a timely fashion.  However, despite the
 lack of guarantees, it is a goal of the architecture to provide
 efficient methods whereby sources can obtain the information needed
 to compute desired routes [Footnote: The primary goal of policy or
 TOS routing is to compute a route that satisfies a set of specialized
 requirements, and these requirements take precedence over optimality.
 In other words, even if a routing domain that participates in SDR or
 NR has sufficient information to compute a route, given a particular
 set of requirements, the architecture does not guarantee that the
 computed route is optimal.].
 Essential to SDR is the assumption that the routes installed on
 demand will be used sparingly.  The architecture assumes that at any
 given moment the set of all source-demand routes installed in an
 internet forms a small fraction of the total number of source-demand
 routes that can potentially be installed by all the routing domains.
 It is an assumption of the architecture that the number of routes
 installed in a BR by the SDR component should be on the order of log
 N (where N is the total number of routing domains in the Internet),
 so that the scaling properties of the SDR component are comparable
 with the scaling properties of the NR component.  The NR component
 achieves this property as a result of hierarchy.
 Note that the above requirement does not imply that only a few
 domains can participate in SDR, or that routes installed by the SDR
 component must have short life times.  What the requirement does
 imply, is that the product of the number of routes specified by
 domains that participate in SDR, times the average SDR-route life
 time, is bounded.  For example, the architecture allows either a
 small number of SDR routes with relatively long average life times,
 or a large number of SDR routes with relatively short average life
 times.  But the architecture clearly prohibits a large number of SDR
 routes with relatively long average life times.  The number of SDR
 routes is a function of the number of domains using SDR routes and
 the number of routes used per source domain.
 In summary, SDR is well suited for traffic that (1) is not widely-
 used enough (or is not sufficiently predictable or steady) to justify
 computation and maintenance by the NR component, and (2) whose
 duration is significantly longer than the time it takes to perform
 the route installation procedure.

Estrin, Rekhter & Hotz [Page 24] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 The architecture does not require all domains in the Internet to
 participate in SDR.  Therefore, issues of scalability (with respect
 to the size of the Internet) become less crucial (though still
 important) to the SDR component.  Instead, the primary focus of the
 SDR component is shifted towards the ability to compute routes that
 satisfy specialized requirements, where we assume that the total
 number of domains requiring special routes simultaneously through the
 same part of the network is small relative to the total population.

4.1 Path Vector vs. Link State for SDR

 It is feasible to use either a distance vector or link state method
 of route computation along with source routing.  One could imagine,
 for instance, a protocol like BGP in which the source uses the full
 AD path information it receives in routing updates to create a source
 route. Such a protocol could address some of the deficiencies
 identified with distance vector, hop-by-hop designs.  However, we opt
 against further discussion of such a protocol because there is less
 to gain by using source routing without also using a link state
 scheme.  The power of source routing, in the context of inter-AD
 policy routing, is in giving the source control over the entire
 route.  This goal cannot be realized fully when intermediate nodes
 control which legal routes are advertised to neighbors, and therefore
 to a source.
 In other words, intermediate nodes should be able to preclude the use
 of a route by expressing a transit policy, but if a route is not
 precluded (i.e.,  is legal according to all ADs in the route), the
 route should be made available to the source independent of an
 intermediate domain's preferences for how its own traffic flows.
 Therefore, the SDR component employs an IDPR-like architecture in
 which link-state style updates are distributed with explicit policy
 terms included in each update along with the advertising node's

4.2 Distribution of Routing Information

 By using a hop-by-hop NR component based on PV to complement the
 source-routing SDR component, we have alleviated the pressure to
 aggregate SDR forwarding information; the large percentage of inter-
 domain traffic carried, simultaneously, by any particular border
 router will be forwarded using aggregated NR forwarding information.
 However, the use of NR does not address the other major scaling
 problem associated with SDR: that of distributing, storing, and
 computing over a complete domain-level topology map.  In this section
 we describe promising opportunities for improving the scaling
 properties of SDR routing information distribution, storage, and

Estrin, Rekhter & Hotz [Page 25] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 Note that we do not propose to solve this problem in the same way
 that we solve it for NR.  A priori abstraction will not be employed
 since different domains may require different methods of abstracting
 the same routing information.  For example, if we aggregate routing
 information of domains that do not share the same policy and TOS
 characteristics (i.e., services), then outside of the aggregate, only
 those services that are offered by all domains in the aggregate will
 be advertised.  In order to locate special routes, SDR only uses
 aggregates when the component domains (and in turn the aggregate)
 advertise the required TOS and policy descriptions.  When the
 required TOS or policy characteristics are not offered by an
 aggregate, full information about the component domains is used to
 construct a route through those domains that do support the
 particular characteristics.  Consequently, we need some other, more
 flexible, means of reducing the amount of information distributed and
 held.  We address two issues in turn: distribution of configured
 topology and policy information, and distribution of dynamic status

4.2.1 Configured Information

 Information about the existence of inter-domain links, and policies
 maintained by domains, changes slowly over time.  This is referred to
 as configured information.  In the current IDPR specification
 complete, global, configuration information is kept by a route server
 in each domain.  Route servers (RS) are the entities that compute
 source routes.  On startup a RS can download the connectivity
 database from a neighbor RS; as domains, inter-domain links, or
 policies change, the changes are flooded to a RS in each domain.
 We have not yet specified the exact mechanisms for distributing
 configured connectivity information for SDR.  However, unlike the
 current IDPR specification, the SDR component will not flood all
 configured information globally.  Several alternate methods for
 organizing and distributing information are under investigation.
 Configured information may be regularly distributed via an out-of-
 band channel, e.g., CD/ROM.  In a similar vein, this information
 could be posted in several well-known locations for retrieval, e.g.,
 via FTP.  Between these "major" updates, aggregated collections of
 changes may be flooded globally.  Moreover, limited flooding (e.g.,
 by hop-count) could be used as appropriate to the "importance" of the
 change; while a policy change in a major backbone may still be
 flooded globally, a new inter-regional link may be flooded only
 within those regions, and information about an additional link to a
 non-transit domain may not be available until the next regularly-

Estrin, Rekhter & Hotz [Page 26] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 scheduled "major" distribution.
 Changes that are not distributed as they occur will not necessarily
 be discovered.  However, a route server may learn pertinent
 information by direct query of remote servers, or through error
 messages resulting from traffic sent along failed routes.  Complete
 global flooding may be avoided by using some combination of these
 Even if an initial implementation uses a simple global flood, we must
 study the problem of structuring connectivity information such that
 it can be retrieved or distributed in a more selective manner, while
 still allowing sources to discover desired routes.  For example, we
 imagine RSs requesting filtered information from each other.  How the
 RSs should define filters that will get enough information to find
 special routes, while also effectively limiting the information, is
 an open question.  Again, the question is how to effectively
 anticipate and describe what information is needed in advance of
 computing the route.
 The essential dilemma is that networks are not organized in a nicely
 geographical or topologically consistent manner (e.g., it is not
 effective to ask for all networks going east-west that are within a
 certain north-south region of the target), hence a source domain does
 not know what information it needs (or should ask for) until it
 searches for, and discovers, the actual path.  Even with a central
 database, techniques are needed to structure configuration
 information so that the potential paths that are most likely to be
 useful are explored first, thereby reducing the time required for
 route computation.
 One promising approach organizes information using route fragments
 (partial paths) [Footnote: Route fragments were first suggested by
 Dave Clark and Noel Chiappa.].  Although the number of route
 fragments grows faster than the number of domains (at least O(N^2)),
 we can selectively choose those that will be useful to compute
 routes.  In particular, for each stub domain, fragments would be
 constructed to several well-known backbones [Footnote: Route
 fragments may be computed by a destination's route server and either
 made available via information service queries or global flooding.
 In addition, NR computed routes may be used as SDR route fragments.].
 Among its benefits, this approach aggregates domain information in a
 manner useful for computing source-routes, and provides an index,
 namely the destination, which facilitates on-demand reference and
 retrieval of information pertinent to a particular route computation.
 At this point, it is not clear how route fragments will affect SDR's
 ability to discover non-hierarchical routes.

Estrin, Rekhter & Hotz [Page 27] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

4.2.2 Dynamic Status Information

 Assuming a technique for global or partial distribution of configured
 information, a second issue is whether, and how, to distribute
 dynamic status information (i.e., whether an inter-domain connection
 is up or down).
 In the current version of IDPR, dynamic status information is flooded
 globally in addition to configuration information.  We propose to
 distribute status information based strictly on locality.  First,
 dynamic information will be advertised within a small hop-count
 radius.  This simple and low-overhead mechanism exploits topological
 locality.  In addition to flooding status updates to nearby nodes, we
 also want to provide more accurate route information for long
 distance communications that entails more than a few network hops.
 Reverse path update (RPU) is a mechanism for sending dynamic status
 information to nodes that are outside the k-hop radius used for
 updates, but that nevertheless would obtain better service (fewer
 failed setups) by having access to the dynamic information [Estrin-
 RPU uses the existing active routes (represented by installed setup
 state or by a cache of the most recent source routes sent via the
 node in question) as a hint for distribution of event notifications.
 Instead of reporting only the status of the route being used, RPU
 reports the status of the domain's other inter-domain connections.
 If source routing exhibits route locality, the source is more likely
 to use other routes going through the node in question; in any case
 the overhead of the information about other links will be minimal.
 In this way, sources will receive status information from regions of
 the network through which they maintain active routes, even if those
 regions are more than k hops away.  Using such a scheme, k could be
 small to maximize efficiency, and RPU could be used to reduce the
 incidence of failed routes resulting from inaccurate status
 information.  This will be useful if long-path communication exhibits
 route locality with respect to regions that are closer to the
 destination (and therefore outside the k hop radius of flooded
 information).  In such situations, flooding information to the source
 of the long route would be inefficient because k would have to be
 equal to the length of the route, and in almost all cases, the
 percentage of nodes that would use the information decreases
 significantly with larger k.

4.3 Source-Demand Route Management

 SDR may be built either on top of the network layer supported by the
 NR component, or in parallel with it.  SDR forwarding will be

Estrin, Rekhter & Hotz [Page 28] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 supported via two techniques: loose source-routing and route setup.
 The first technique, loose source-routing, would allow the originator
 of a packet to specify a sequence of domains that the packet should
 traverse on its path to a destination.  Forwarding such a packet
 within a domain, or even between domains within a confederation,
 would be left to intra-domain routing.  This avoids per-connection
 state and supports transaction traffic.
 The second technique, route setup, will be based on mechanisms
 developed for IDPR and described in [IDPR90].  It is well suited to
 conversations that persist significantly longer than a round-trip-
 time.  The setup protocol defines packet formats and the processing
 of route installation request packets (i.e, setup packets).  When a
 source generates a setup packet, the first border router along the
 specified source route checks the setup request, and if accepted,
 installs routing information; this information includes a path ID,
 the previous and next hops, and whatever other accounting-related
 information the particular domain requires.  The setup packet is
 passed on to the next BR in the domain-level source route, and the
 same procedure is carried out [Footnote: The setup packet may be
 forwarded optimistically, i.e., before checks are completed, to
 reduce latency.].  When the setup packet reaches the destination, an
 accept message is propagated back hop by hop, and each BR en route
 activates its routing information.  Subsequent data packets traveling
 along the same path to the destination include a path ID in the
 packet.  That path ID is used to locate the appropriate next-hop
 information for each packet.
 Border routers that support both the NR and the SDR components, must
 be able to determine what forwarding mechanism to use.  That is, when
 presented with a network layer PDU, such a BR should be able to make
 an unambiguous decision about whether forwarding of that PDU should
 be handled by the NR or the SDR component.  Discrimination mechanisms
 are dependent on whether the new network layer introduced by the SDR
 component is built on top of, or in parallel with, the network layers
 supported by the NR component.  Once the discrimination is made,
 packets that have to be forwarded via routes installed by the SDR
 component are forwarded to the exit port associated with the
 particular Path ID in the packet header.  Packets that have to be
 forwarded via routes installed by the NR component are forwarded to
 the exit port associated with the particular destination and Type of
 Service parameters (if present) in their packet headers.
 Next, we describe the primary differences between the IDPR setup
 procedure previously specified, and the procedure we propose to
 develop for this hybrid architecture.

Estrin, Rekhter & Hotz [Page 29] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 During route installation, if a BR on the path finds that the
 remainder of the indicated route from the BR to the destination is
 identical to the NR route from the BR to the destination, then the BR
 can turn off the SDR route at that point and map it onto the NR
 route.  For this to occur, the specifications of the SDR route must
 completely match those of the NR route.  In addition, the entire
 forward route must be equivalent (i.e., the remaining hops to the
 Moreover, if the NR route changes during the course of an active SDR
 route, and the new NR route does not match the SDR route, then the
 SDR route must be installed for the remainder of the way to the
 destination.  Consequently, when an SDR route is mapped onto an NR
 route, the original setup packet must be saved.  A packet traveling
 from a source to destination may therefore traverse both an SDR and
 an NR route segment; however, a packet will not traverse another SDR
 segment after traveling over an NR segment.  However, during
 transient periods packets could traverse the wrong route and
 therefore this must be an optional and controllable feature.
 A source can also request notification if a previously-down link or
 node returns to operation some time after a requested route setup
 fails.  If a BR on the route discovers that the requested next-hop BR
 is not available, the BR can add the source to a notification list
 and when the next-hop BR becomes reachable, a notification can be
 sent back to the source.  This provides a means of flushing out bad
 news when it is no longer true.  For example, a domain might decide
 to route through a secondary route when its preferred route fails;
 the notification mechanism would inform the source in a timely manner
 when its preferred route is available again.
 A third option addresses adaptation after route installation.  During
 packet forwarding along an active SDR route, if a BR finds that the
 SDR route has failed, it may redirect the traffic along an existing
 NR route to the destination.  This adaptation is allowed only if use
 of the NR route does not violate policy; for example, it may provide
 a less desirable type of service.  This is done only if the source
 selects the option at route setup time.  It is also up to the source
 whether it is to be notified of such actions.
 When a SDR route does fail, the detecting BR sends notification to
 the source(s) of the active routes that are affected.  Optionally,
 the detecting BR may include additional information about the state
 of other BRs in the same domain.  In particular, the BR can include
 its domain's most recent "update" indicating that domain's inter-
 domain links and policy.  This can be helpful to the extent there is
 communication locality; i.e., if alternative routes might be used
 that traverse the domain in question, but avoid the failed BR.

Estrin, Rekhter & Hotz [Page 30] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 In summary, when a route is first installed, the source has several
 options (which are represented by flags in the route setup packet):
   1. If an NR route is available that satisfies all local policy
      and TOS, then use it.  Otherwise...
   2. Indicate whether the source wants to allow the setup to
      default to a NR route if the SDR route setup fails.
   3. Request notification of mapping to a NR route.
   4. Request additional configured information on failure.
   5. Request addition to a notification list for resource
   6. Allow data packets to be rerouted to a NR route when failure
      happens after setup (so long  as no policy is violated).
   7. Request notification of a reroute of data packets.
   8. Request additional configured information on failure notice
      when the route is active.
   9. Request addition to a notification list if an active route

Estrin, Rekhter & Hotz [Page 31] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

5.0 The Unified Architecture

 In addition to further evaluation and implementation of the proposed
 architecture, future research must investigate opportunities for
 increased unification of the two components of our architecture.  We
 are investigating several opportunities for additional commonality:
   1. Routing Information Base:
      Perhaps a single RIB could be shared by both NR and SDR.
      NR routes can be represented as a directed graph labeled
      with flags (on the nodes or links) corresponding to the
      generic transit constraints.  SDR requires that this graph
      be augmented by links with non-generic policies that have
      been discovered and maintained for computing special routes;
      in addition, special policy flags may be added to links
      already maintained by the NR component.
   2. Information Distribution:
      The NR path vectors could include address(es) of repositories
      for SDR-update information for each AD (or confederation) to
      assist the SDR component in retrieving selective information
      on demand.  For domains with minimal policies, where the space
      required for policy information is smaller than the space
      required for a repository address (e.g., if the policies for
      the domain listed are all wildcard), the NR path vectors could
      include a flag to that effect.
   3. Packet Forwarding:
      We should consider replacing the current IDPR-style network
      layer (which contains a global path identifier used in
      forwarding data packets to the next policy gateway on an
      IDPR route)  with a standard header (e.g., IP or CLNP),
      augmented with some option fields.  This would  unify the
      packet header parsing and forwarding functions for SDR and NR,
      and possibly eliminate some encapsulation overhead.
   4. Reachability Information:
      Currently IDRP distributes network reachability information
      within updates, whereas IDPR only distributes domain
      reachability information.  IDPR uses a domain name service
      function to map network numbers to domain numbers; the latter
      is needed to make the routing decision.   We should consider
      obtaining the network reachability and domain information in
      a unified manner.

Estrin, Rekhter & Hotz [Page 32] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

5.1 Applicability to Various Network Layer Protocols

 The proposed architecture is designed to accommodate such existing
 network layer protocols as IP ([Postel81]), CLNP ([ISO-473-88]), and
 ST-II ([ST2-90]).  In addition, we intend for this architecture to
 support future network layer mechanisms, e.g., Clark and Jacobson's
 proposal or Braden and Casner's Integrated Services IP.  However on
 principal we can not make sweeping guarantees in advance of the
 mechanisms themselves.  In any case, not all of the mentioned
 protocols will be able to utilize all of the capabilities provided by
 the architecture.  For instance, unless the increase in the number of
 different types of services offered is matched by the ability of a
 particular network layer protocol to unambiguously express requests
 for such different types of services, the capability of the
 architecture to support routing in the presence of a large number of
 different types of service is largely academic.  That is, not all
 components of the architecture will have equal importance for
 different network layer protocols.  On the other hand, this
 architecture is designed to serve the future global internetworking
 environment.  The extensive research and development currently
 underway to implement and evaluate network mechanisms for different
 types of service suggests that future networks will offer such
 One of the fundamental issues in the proposed architecture is the
 issue of single versus multiple protocols.  The architecture does not
 make any assumptions about whether each network layer is going to
 have its own inter-domain routing protocol, or a single inter-domain
 routing protocol will be able to cover multiple network layers
 [Footnote: Similar issue already arose with respect to the intra-
 domain routing protocol, which generated sufficient amount of
 controversy within the Internet community.  It is our opinion, that
 the issue of single versus multiple protocols is more complex for the
 inter-domain routing than for the intra-domain routing.].  That is,
 the proposed architecture can be realized either by a single inter-
 domain routing protocol covering multiple network layers, or by
 multiple inter-domain routing protocols (with the same architecture)
 tailored to a specific network layer [Footnote: If the single
 protocol strategy is adopted, then it is likely that IDRP will be
 used as a base for the NR component.  Since presently IDRP is
 targeted towards CLNP, further work is needed to augment it to
 support IP and ST-II.  If the multiple protocol strategy is adopted,
 then it is likely that BGP will be used as a base for the NR
 component for IP, and IDRP will be used as a base for the NR
 component for CLNP.  Further work is needed to specify protocol in
 support for the NR component for ST-II.  Additional work may be
 needed to specify new features that may be added to BGP.].

Estrin, Rekhter & Hotz [Page 33] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

5.2 Transition

 The proposed architecture is not intended for full deployment in the
 short term future.  We are proposing this architecture as a goal
 towards which we can begin guiding our operational and research
 investment over the next 5 years.
 At the same time, the architecture does not require wholesale
 overhaul of the existing Internet.  The NR component may be phased in
 gradually.  For example, the NR component for IP may be phased in by
 replacing existing EGP-2 routing with BGP routing.  Once the NR
 component is in place, it can be augmented by the facilities provided
 by the SDR component.
 The most critical components of the architecture needed to support
 SDR include route installation and packet forwarding in the routers
 that support SDR.  Participation as a transit routing domain requires
 that the domain can distribute local configuration information (LCI)
 and that some of its routers implement the route installation and
 route management protocols.  Participation as a source requires that
 the domain have access to a RS to compute routes, and that the source
 domain has a router that implements the route installation and route
 management protocols.  In addition, a network management entity must
 describe local configuration information and send it to the central
 repository(ies).  A collection and distribution mechanism must be put
 in place, even if it is centralized.

Estrin, Rekhter & Hotz [Page 34] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

6.0 Conclusions and Future Work

 In summary, the proposed architecture combines hop-by-hop path-
 vector, and source-routed link-state, protocols, and uses each for
 that which it is best suited: NR uses PV and multiple, flexible,
 levels of confederations to support efficient routing of generic
 packets over generic routes; SDR uses LS computation over a database
 of configured and dynamic information to route special traffic over
 special routes.  In the past, the community has viewed these two as
 mutually exclusive; to the contrary, they are quite complementary and
 it is fortunate that we, as a community, have pursued both paths in
 parallel.  Together these two approaches will flexibly and
 efficiently support TOS and policy routing in very large global
 It is now time to consider the issues associated with combining and
 integrating the two.  We must go back and look at both architectures
 and their constituent protocols, eliminate redundancies, fill in new
 holes, and provide seamless integration.

7.0 Acknowledgments

 We would like to thank Hans-Werner Braun (San Diego Supercomputer
 Center), Lee Breslau (USC), Scott Brim (Cornell University), Tony Li
 (cisco Systems), Doug Montgomery (NIST), Tassos Nakassis (NIST),
 Martha Steenstrup (BBN), and Daniel Zappala (USC) for their comments
 on a previous draft.

Estrin, Rekhter & Hotz [Page 35] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

8.0 References

 [ANSI 87-150R]  "Intermediate System to Intermediate System Intra-
 Domain Routing Exchange Protocol", ANSI X3S3.3/87-150R.
 [BGP 91]  Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3
 (BGP-3)", RFC 1267, cisco Systems, T.J. Watson Research Center, IBM
 Corp., October 1991.
 [Breslau-Estrin 91]  Breslau, L., and D. Estrin, "Design and
 Evaluation of Inter-Domain Policy Routing Protocols", To appear in
 Journal  of Internetworking Research and Experience, 1991.  (Earlier
 version appeared in ACM Sigcomm 1990.)
 [Clark 90]  Clark, D., "Policy Routing in Internetworks", Journal of
 Internetworking Research and Experience, Vol.  1, pp. 35-52, 1990.
 [Dijkstra 59]  Dijkstra, E., "A Note on Two Problems in Connection
 with Graphs", Numer. Math., Vol.  1, 1959, pp. 269-271.
 [ECMA89]  "Inter-Domain Intermediate Systems Routing", Draft
 Technical Report ECMA TR/ISR, ECMA/TC32-TG 10/89/56, May 1989.
 [EGP]  Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827, BBN,
 October 1982.
 [Estrin 89]  Estrin, D., "Policy Requirements for Inter
 Administrative Domain Routing", RFC 1125, USC Computer Science
 Department, November 1989.
 [Estrin-etal91]  Estrin, D., Breslau, L., and L. Zhang, "Protocol
 Mechanisms for Adaptive Routing in Global Multimedia Internets",
 University of Southern California, Computer Science Department
 Technical Report, CS-SYS-91-04, November 1991.
 [Hedrick 88]  Hedrick, C., "Routing Information Protocol", RFC 1058,
 Rutgers University, June 1988.
 [Honig 90]  Honig, J., Katz, D., Mathis, M., Rekhter, Y., and J. Yu,
 "Application of the Border Gateway Protocol in the Internet", RFC
 1164, Cornell Univ. Theory Center, Merit/NSFNET, Pittsburgh
 Supercomputing Center, T.J. Watson Research Center, IBM Corp., June
 [IDPR90]  Steenstrup, M., "Inter-Domain Policy Routing Protocol
 Specification and Usage: Version 1", Work in Progress, February 1991.

Estrin, Rekhter & Hotz [Page 36] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

 [IDRP91]  "Intermediate System to Intermediate System Inter-domain
 Routeing Exchange Protocol", ISO/IEC/ JTC1/SC6 CD10747.
 [ISIS10589]  "Information Processing Systems - Telecommunications and
 Information Exchange between Systems - Intermediate System to
 Intermediate System Intra-Domain Routing Exchange Protocol for use in
 Conjunction with the protocol for providing the Connectionless-mode
 Network Service (ISO 8473)", ISO/IEC 10589.
 [ISO-473 88]  "Protocol for providing the connectionless-mode network
 service", ISO 8473, 1988.
 [Jaffee 82]  Jaffee, J., and F. Moss, "A Responsive Distributed
 Routing Algorithm for Computer Networks", IEEE Transactions on
 Communications, July 1982.
 [Little 89]  Little, M., "Goals and Functional Requirements for
 Inter-Autonomous System Routing", RFC 1126, SAIC, October 1989.
 [Oran 89]  Oran, D., "Expert's Paper: The Relationship between
 Addressing and Routeing", ISO/JTC1/SC6/WG2, 1989.
 [OSPF]  Moy, J., "The Open Shortest Path First (OSPF) Specification",
 RFC 1131, Proteon, October 1989.
 [Postel 81]  Postel, J., "Internet Protocol", RFC 791, DARPA,
 September 1981.
 [Rekhter 91]  Rekhter, Y., "IDRP protocol analysis: storage
 complexity", IBM Research Report RC17298(#76515), October 1991.
 [Shin87] Shin, K., and M. Chen, "Performance Analysis of Distributed
 Routing Strategies Free of Ping-Pong-Type Looping", IEEE Transactions
 on Computers, February 1987.
 [ST2-90]  Topolcic, C., "Experimental Internet Stream Protocol,
 version 2 (ST II)", RFC 1190, CIP Working Group, October 1990.
 [Zaumen 91] Zaumen, W., and J. Garcia-Luna-Aceves, "Dynamics of Link
 State and Loop-free Distance-Vector Routing Algorithms", ACM Sigcomm
 '91, Zurich, Switzerland, September 1991.
 [Zhang 91] Zhang, L., "Virtual Clock: A New Traffic Control Algorithm
 for Packet Switching Networks".

Estrin, Rekhter & Hotz [Page 37] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992

Security Considerations

 Security issues are not discussed in this memo.

Authors' Addresses

 Deborah Estrin
 University of Southern California
 Computer Science Department, MC 0782
 Los Angeles, California 90089-0782
 Phone: (310) 740-4524
 Yakov Rekhter
 IBM T.J. Watson Research Center
 P.O. Box 218
 Yorktown Heights, New York 10598
 Phone: (914) 945-3896
 Steven Hotz
 University of Southern California
 Computer Science Department, MC 0782
 Los Angeles, California 90089-0782
 Phone: (310) 822-1511

Estrin, Rekhter & Hotz [Page 38]

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