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RFC 1322

Network Working Group                                          D. Estrin
Request for Comments:  1322                                          USC
                                                              Y. Rekhter
                                                                 S. Hotz
                                                                May 1992

               A Unified Approach to Inter-Domain Routing

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 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 overhead. 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
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 destination). 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 re-availability. 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 fails. 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 services. 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 internets. 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 1990 [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.

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