Network Working Group D. Estrin Request for Comments: 1322 USC Y. Rekhter IBM S. Hotz USC 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 unlimited. Abstract 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 Estrin, Rekhter & Hotz [Page 2] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 [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 routing. 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 requirements. 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 expectations.]. 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 summary. Estrin, Rekhter & Hotz [Page 4] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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. Estrin, Rekhter & Hotz [Page 5] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 frequently). 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 Estrin, Rekhter & Hotz [Page 6] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 aggregation. 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.). Estrin, Rekhter & Hotz [Page 7] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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). Estrin, Rekhter & Hotz [Page 8] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 explicit. 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 Estrin, Rekhter & Hotz [Page 9] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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.]. Estrin, Rekhter & Hotz [Page 10] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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. Estrin, Rekhter & Hotz [Page 12] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 abstraction. Estrin, Rekhter & Hotz [Page 13] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 component. 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 Estrin, Rekhter & Hotz [Page 14] RFC 1322 A Unified Approach to Inter-Domain Routing May 1992 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 earlier. 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 distribution. 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 tha