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Network Working Group                                          D. EstrinRequest for Comments:  1322                                          USC                                                              Y. Rekhter                                                                     IBM                                                                 S. Hotz                                                                     USC                                                                May 1992A Unified Approach to Inter-Domain RoutingStatus 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 dataEstrin, 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 routingEstrin, 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 specialEstrin, 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.  Sections3 and4 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 19922.0 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 19922.1.2 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 isEstrin, 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 (seeSection 2.6).Estrin, Rekhter & Hotz                                          [Page 8]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 19922.3 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 seeSection 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 toEstrin, 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 19922.4 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).   InSection 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.   InSection 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 particularEstrin, 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 19923.0 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 vectorEstrin, 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 homogeneousEstrin, 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 routingEstrin, 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 19923.3.2 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 consistentEstrin, 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 inSection 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 complexityEstrin, 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 19923.8 Summary   Given the performance complexity issues associated with global   routing, aggregation of routing information is essential; at the same   time we have argued that such aggregation must be flexible.  Given   the difficulties of supporting LS hop-by-hop routing in the presence   of (a) flexible aggregation, (b) heterogeneous route selection   policies, and (c) incomplete or inconsistent routing information, we   see no alternative but to employ PV for the NR component of our   architecture.   Based on the above tradeoffs, our NR component employs a PV   architecture, where route computation and installation is done in a   distributed fashion by the routers participating in the NR component   [Footnote: Packet forwarding along routes produced by the NR   component can be accomplished by either source routing or hop-by-hop   routing.  The latter is the primary choice because it reduces the   amount of state in routers (if route setup is employed), or avoids   encoding an explicit source route in network layer packets.  However,   the architecture does not preclude the use of source routing (or   route setup) along the routes computed, but not installed, by the NR   component.].   The distributed algorithm combines some of the features of link state   with those of distance vector algorithms; in addition to next hop   information, the NR component maintains path attributes for each   route (e.g., the list of domains or routing domain confederations   that the routing information has traversed so far).  The path   attributes that are carried along with a route express a variety of   routing policies, and make explicit the entire route to the   destination.  With aggregation, this is a superset of the domains   that form the path to the destination.Estrin, Rekhter & Hotz                                         [Page 22]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 19924.0 Source-demand routing (SDR)   Inter-domain routers participating in the SDR component forward   packets according to routing information computed and installed by   the domain that originates the traffic (source routing domain).   It is important to realize that requiring route installation by the   source routing domain is not a matter of choice, but rather a   necessity.  If a particular route is used by a small number of   domains (perhaps only one) then it is more appropriate to have the   source compute and install the special route instead of burdening the   intermediate nodes with the task of looking for and selecting a route   with the specialized requirements.  In addition, if the demand for   the route is unpredictable, and thus can be determined only by the   source, it should be up to the source to install the route.   In general, information that is used by source routing domains for   computing source-demand routes reflects administrative (but not   operational) status of the routing facilities (i.e., configured   topology and policy) [Footnote: If SDR uses NR information then   operational status could be considered in some route selection.].   Consequently, it is possible for a source routing domain to compute a   route that is not operational at route installation time.  The SDR   component attempts to notify the source domain of failures when route   installation is attempted.  Similarly, the SDR component provides   mechanisms for the source routing domain to be notified of failures   along previously-installed active routes.  In other words, the SDR   component performs routing that is adaptive to topological changes;   however, the adaptability is achieved as a consequence of the route   installation and route management mechanisms.  This is different from   the NR component, where status changes are propagated and   incorporated by nodes as soon as possible.  Therefore, to allow   faster adaptation to changes in the operational status of routing   facilities, the SDR component allows the source domain to switch to a   route computed by the NR component, if failure along the source-   demand route is detected (either during the route installation phase,   or after the route is installed), and if policy permits use of the NR   route.   The NR component will group domains into confederations to achieve   its scaling goals (see [IDRP91]).  In contrast, SDR will allow an   AD-level route to be used by an individual domain without allowing   use by the entire confederation to which the domain belongs.   Similarly, a single transit domain may support a policy or special   TOS that is not supported by other domains in its confederation(s).   In other words, the architecture uses SDR to support non-   hierarchical, non-aggregated policies where and when needed.   Consequently, SDR by itself does not have the scaling properties ofEstrin, Rekhter & Hotz                                         [Page 23]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   NR.  In compensation, SDR does not require a complete, global domain   map if portions of the world are never traversed or communicated   with.  As a result of the looser routing structure, SDR does not   guarantee that a participating source routing domain will always have   sufficient information to compute a route to a destination.  In   addition, if the domain does have sufficient information, it is   possible that the quantity may be large enough to preclude storage   and/or route computation in a timely fashion.  However, despite the   lack of guarantees, it is a goal of the architecture to provide   efficient methods whereby sources can obtain the information needed   to compute desired routes [Footnote: The primary goal of policy or   TOS routing is to compute a route that satisfies a set of specialized   requirements, and these requirements take precedence over optimality.   In other words, even if a routing domain that participates in SDR or   NR has sufficient information to compute a route, given a particular   set of requirements, the architecture does not guarantee that the   computed route is optimal.].   Essential to SDR is the assumption that the routes installed on   demand will be used sparingly.  The architecture assumes that at any   given moment the set of all source-demand routes installed in an   internet forms a small fraction of the total number of source-demand   routes that can potentially be installed by all the routing domains.   It is an assumption of the architecture that the number of routes   installed in a BR by the SDR component should be on the order of log   N (where N is the total number of routing domains in the Internet),   so that the scaling properties of the SDR component are comparable   with the scaling properties of the NR component.  The NR component   achieves this property as a result of hierarchy.   Note that the above requirement does not imply that only a few   domains can participate in SDR, or that routes installed by the SDR   component must have short life times.  What the requirement does   imply, is that the product of the number of routes specified by   domains that participate in SDR, times the average SDR-route life   time, is bounded.  For example, the architecture allows either a   small number of SDR routes with relatively long average life times,   or a large number of SDR routes with relatively short average life   times.  But the architecture clearly prohibits a large number of SDR   routes with relatively long average life times.  The number of SDR   routes is a function of the number of domains using SDR routes and   the number of routes used per source domain.   In summary, SDR is well suited for traffic that (1) is not widely-   used enough (or is not sufficiently predictable or steady) to justify   computation and maintenance by the NR component, and (2) whose   duration is significantly longer than the time it takes to perform   the route installation procedure.Estrin, Rekhter & Hotz                                         [Page 24]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   The architecture does not require all domains in the Internet to   participate in SDR.  Therefore, issues of scalability (with respect   to the size of the Internet) become less crucial (though still   important) to the SDR component.  Instead, the primary focus of the   SDR component is shifted towards the ability to compute routes that   satisfy specialized requirements, where we assume that the total   number of domains requiring special routes simultaneously through the   same part of the network is small relative to the total population.4.1 Path Vector vs. Link State for SDR   It is feasible to use either a distance vector or link state method   of route computation along with source routing.  One could imagine,   for instance, a protocol like BGP in which the source uses the full   AD path information it receives in routing updates to create a source   route. Such a protocol could address some of the deficiencies   identified with distance vector, hop-by-hop designs.  However, we opt   against further discussion of such a protocol because there is less   to gain by using source routing without also using a link state   scheme.  The power of source routing, in the context of inter-AD   policy routing, is in giving the source control over the entire   route.  This goal cannot be realized fully when intermediate nodes   control which legal routes are advertised to neighbors, and therefore   to a source.   In other words, intermediate nodes should be able to preclude the use   of a route by expressing a transit policy, but if a route is not   precluded (i.e.,  is legal according to all ADs in the route), the   route should be made available to the source independent of an   intermediate domain's preferences for how its own traffic flows.   Therefore, the SDR component employs an IDPR-like architecture in   which link-state style updates are distributed with explicit policy   terms included in each update along with the advertising node's   connectivity.4.2 Distribution of Routing Information   By using a hop-by-hop NR component based on PV to complement the   source-routing SDR component, we have alleviated the pressure to   aggregate SDR forwarding information; the large percentage of inter-   domain traffic carried, simultaneously, by any particular border   router will be forwarded using aggregated NR forwarding information.   However, the use of NR does not address the other major scaling   problem associated with SDR: that of distributing, storing, and   computing over a complete domain-level topology map.  In this section   we describe promising opportunities for improving the scaling   properties of SDR routing information distribution, storage, andEstrin, Rekhter & Hotz                                         [Page 25]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   computation.   Note that we do not propose to solve this problem in the same way   that we solve it for NR.  A priori abstraction will not be employed   since different domains may require different methods of abstracting   the same routing information.  For example, if we aggregate routing   information of domains that do not share the same policy and TOS   characteristics (i.e., services), then outside of the aggregate, only   those services that are offered by all domains in the aggregate will   be advertised.  In order to locate special routes, SDR only uses   aggregates when the component domains (and in turn the aggregate)   advertise the required TOS and policy descriptions.  When the   required TOS or policy characteristics are not offered by an   aggregate, full information about the component domains is used to   construct a route through those domains that do support the   particular characteristics.  Consequently, we need some other, more   flexible, means of reducing the amount of information distributed and   held.  We address two issues in turn: distribution of configured   topology and policy information, and distribution of dynamic status   information.4.2.1 Configured Information   Information about the existence of inter-domain links, and policies   maintained by domains, changes slowly over time.  This is referred to   as configured information.  In the current IDPR specification   complete, global, configuration information is kept by a route server   in each domain.  Route servers (RS) are the entities that compute   source routes.  On startup a RS can download the connectivity   database from a neighbor RS; as domains, inter-domain links, or   policies change, the changes are flooded to a RS in each domain.   We have not yet specified the exact mechanisms for distributing   configured connectivity information for SDR.  However, unlike the   current IDPR specification, the SDR component will not flood all   configured information globally.  Several alternate methods for   organizing and distributing information are under investigation.   Configured information may be regularly distributed via an out-of-   band channel, e.g., CD/ROM.  In a similar vein, this information   could be posted in several well-known locations for retrieval, e.g.,   via FTP.  Between these "major" updates, aggregated collections of   changes may be flooded globally.  Moreover, limited flooding (e.g.,   by hop-count) could be used as appropriate to the "importance" of the   change; while a policy change in a major backbone may still be   flooded globally, a new inter-regional link may be flooded only   within those regions, and information about an additional link to a   non-transit domain may not be available until the next regularly-Estrin, Rekhter & Hotz                                         [Page 26]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   scheduled "major" distribution.   Changes that are not distributed as they occur will not necessarily   be discovered.  However, a route server may learn pertinent   information by direct query of remote servers, or through error   messages resulting from traffic sent along failed routes.  Complete   global flooding may be avoided by using some combination of these   mechanisms.   Even if an initial implementation uses a simple global flood, we must   study the problem of structuring connectivity information such that   it can be retrieved or distributed in a more selective manner, while   still allowing sources to discover desired routes.  For example, we   imagine RSs requesting filtered information from each other.  How the   RSs should define filters that will get enough information to find   special routes, while also effectively limiting the information, is   an open question.  Again, the question is how to effectively   anticipate and describe what information is needed in advance of   computing the route.   The essential dilemma is that networks are not organized in a nicely   geographical or topologically consistent manner (e.g., it is not   effective to ask for all networks going east-west that are within a   certain north-south region of the target), hence a source domain does   not know what information it needs (or should ask for) until it   searches for, and discovers, the actual path.  Even with a central   database, techniques are needed to structure configuration   information so that the potential paths that are most likely to be   useful are explored first, thereby reducing the time required for   route computation.   One promising approach organizes information using route fragments   (partial paths) [Footnote: Route fragments were first suggested by   Dave Clark and Noel Chiappa.].  Although the number of route   fragments grows faster than the number of domains (at least O(N^2)),   we can selectively choose those that will be useful to compute   routes.  In particular, for each stub domain, fragments would be   constructed to several well-known backbones [Footnote: Route   fragments may be computed by a destination's route server and either   made available via information service queries or global flooding.   In addition, NR computed routes may be used as SDR route fragments.].   Among its benefits, this approach aggregates domain information in a   manner useful for computing source-routes, and provides an index,   namely the destination, which facilitates on-demand reference and   retrieval of information pertinent to a particular route computation.   At this point, it is not clear how route fragments will affect SDR's   ability to discover non-hierarchical routes.Estrin, Rekhter & Hotz                                         [Page 27]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 19924.2.2 Dynamic Status Information   Assuming a technique for global or partial distribution of configured   information, a second issue is whether, and how, to distribute   dynamic status information (i.e., whether an inter-domain connection   is up or down).   In the current version of IDPR, dynamic status information is flooded   globally in addition to configuration information.  We propose to   distribute status information based strictly on locality.  First,   dynamic information will be advertised within a small hop-count   radius.  This simple and low-overhead mechanism exploits topological   locality.  In addition to flooding status updates to nearby nodes, we   also want to provide more accurate route information for long   distance communications that entails more than a few network hops.   Reverse path update (RPU) is a mechanism for sending dynamic status   information to nodes that are outside the k-hop radius used for   updates, but that nevertheless would obtain better service (fewer   failed setups) by having access to the dynamic information [Estrin-   etal91].   RPU uses the existing active routes (represented by installed setup   state or by a cache of the most recent source routes sent via the   node in question) as a hint for distribution of event notifications.   Instead of reporting only the status of the route being used, RPU   reports the status of the domain's other inter-domain connections.   If source routing exhibits route locality, the source is more likely   to use other routes going through the node in question; in any case   the overhead of the information about other links will be minimal.   In this way, sources will receive status information from regions of   the network through which they maintain active routes, even if those   regions are more than k hops away.  Using such a scheme, k could be   small to maximize efficiency, and RPU could be used to reduce the   incidence of failed routes resulting from inaccurate status   information.  This will be useful if long-path communication exhibits   route locality with respect to regions that are closer to the   destination (and therefore outside the k hop radius of flooded   information).  In such situations, flooding information to the source   of the long route would be inefficient because k would have to be   equal to the length of the route, and in almost all cases, the   percentage of nodes that would use the information decreases   significantly with larger k.4.3 Source-Demand Route Management   SDR may be built either on top of the network layer supported by the   NR component, or in parallel with it.  SDR forwarding will beEstrin, Rekhter & Hotz                                         [Page 28]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   supported via two techniques: loose source-routing and route setup.   The first technique, loose source-routing, would allow the originator   of a packet to specify a sequence of domains that the packet should   traverse on its path to a destination.  Forwarding such a packet   within a domain, or even between domains within a confederation,   would be left to intra-domain routing.  This avoids per-connection   state and supports transaction traffic.   The second technique, route setup, will be based on mechanisms   developed for IDPR and described in [IDPR90].  It is well suited to   conversations that persist significantly longer than a round-trip-   time.  The setup protocol defines packet formats and the processing   of route installation request packets (i.e, setup packets).  When a   source generates a setup packet, the first border router along the   specified source route checks the setup request, and if accepted,   installs routing information; this information includes a path ID,   the previous and next hops, and whatever other accounting-related   information the particular domain requires.  The setup packet is   passed on to the next BR in the domain-level source route, and the   same procedure is carried out [Footnote: The setup packet may be   forwarded optimistically, i.e., before checks are completed, to   reduce latency.].  When the setup packet reaches the destination, an   accept message is propagated back hop by hop, and each BR en route   activates its routing information.  Subsequent data packets traveling   along the same path to the destination include a path ID in the   packet.  That path ID is used to locate the appropriate next-hop   information for each packet.   Border routers that support both the NR and the SDR components, must   be able to determine what forwarding mechanism to use.  That is, when   presented with a network layer PDU, such a BR should be able to make   an unambiguous decision about whether forwarding of that PDU should   be handled by the NR or the SDR component.  Discrimination mechanisms   are dependent on whether the new network layer introduced by the SDR   component is built on top of, or in parallel with, the network layers   supported by the NR component.  Once the discrimination is made,   packets that have to be forwarded via routes installed by the SDR   component are forwarded to the exit port associated with the   particular Path ID in the packet header.  Packets that have to be   forwarded via routes installed by the NR component are forwarded to   the exit port associated with the particular destination and Type of   Service parameters (if present) in their packet headers.   Next, we describe the primary differences between the IDPR setup   procedure previously specified, and the procedure we propose to   develop for this hybrid architecture.Estrin, Rekhter & Hotz                                         [Page 29]

RFC 1322       A Unified Approach to Inter-Domain Routing       May 1992   During route installation, if a BR on the path finds that the   remainder of the indicated route from the BR to the destination is   identical to the NR route from the BR to the destination, then the BR   can turn off the SDR route at that point and map it onto the NR   route.  For this to occur, the specifications of the SDR route must   completely match those of the NR route.  In addition, the entire   forward route must be equivalent (i.e., the remaining hops to the   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 19925.0 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 19925.1 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 19925.2 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 19926.0 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 19928.0 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",RFC1164, 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 1992Security Considerations   Security issues are not discussed in this memo.Authors' Addresses   Deborah Estrin   University of Southern California   Computer Science Department, MC 0782   Los Angeles, California 90089-0782   Phone: (310) 740-4524   EMail: estrin@usc.edu   Yakov Rekhter   IBM T.J. Watson Research Center   P.O. Box 218   Yorktown Heights, New York 10598   Phone: (914) 945-3896   EMail: yakov@ibm.com   Steven Hotz   University of Southern California   Computer Science Department, MC 0782   Los Angeles, California 90089-0782   Phone: (310) 822-1511   EMail: hotz@usc.eduEstrin, Rekhter & Hotz                                         [Page 38]