





Network Working Group                                     M. Steenstrup
Request for Comments: 1478                 BBN Systems and Technologies
                                                              June 1993


            An Architecture for Inter-Domain Policy Routing

Status of this Memo

   This RFC specifies an IAB standards track protocol for the Internet
   community, and requests discussion and suggestions for improvements.
   Please refer to the current edition of the "IAB Official Protocol
   Standards" for the standardization state and status of this protocol.
   Distribution of this memo is unlimited.

Abstract

   We present an architecture for inter-domain policy routing (IDPR).
   The objective of IDPR is to construct and maintain routes, between
   source and destination administrative domains, that provide user
   traffic with the requested services within the constraints stipulated
   for the domains transited.  The IDPR architecture is designed to
   accommodate an internetwork containing tens of thousands of
   administrative domains with heterogeneous service requirements and
   restrictions.

Contributors

   The following people have contributed to the IDPR architecture: Bob
   Braden, Lee Breslau, Ross Callon, Noel Chiappa, Dave Clark, Pat
   Clark, Deborah Estrin, Marianne Lepp, Mike Little, Martha Steenstrup,
   Zaw-Sing Su, Paul Tsuchiya, and Gene Tsudik.  Yakov Rekhter supplied
   many useful comments on a previous draft of this document.


















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Table of Contents

   1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . 3
   1.1. The Internet Environment. . . . . . . . . . . . . . . . . . . 4
   2. Approaches to Policy Routing. . . . . . . . . . . . . . . . . . 5
   2.1. Policy Route Generation . . . . . . . . . . . . . . . . . . . 5
   2.1.1. Distance Vector Approach. . . . . . . . . . . . . . . . . . 5
   2.1.2. Link State Approach . . . . . . . . . . . . . . . . . . . . 7
   2.2. Routing Information Distribution. . . . . . . . . . . . . . . 8
   2.2.1. Distance Vector Approach. . . . . . . . . . . . . . . . . . 8
   2.2.2. Link State Approach . . . . . . . . . . . . . . . . . . . .10
   2.3. Message Forwarding along Policy Routes. . . . . . . . . . . .10
   2.3.1. Hop-by-Hop Approach . . . . . . . . . . . . . . . . . . . .11
   2.3.1.1. A Clarification . . . . . . . . . . . . . . . . . . . . .11
   2.3.2. Source Specified Approach . . . . . . . . . . . . . . . . .12
   3. The IDPR Architecture . . . . . . . . . . . . . . . . . . . . .13
   3.1. IDPR Functions. . . . . . . . . . . . . . . . . . . . . . . .13
   3.2. IDPR Entities . . . . . . . . . . . . . . . . . . . . . . . .13
   3.2.1. Path Agents . . . . . . . . . . . . . . . . . . . . . . . .16
   3.2.2. IDPR Servers. . . . . . . . . . . . . . . . . . . . . . . .17
   3.2.3. Entity Identifiers. . . . . . . . . . . . . . . . . . . . .19
   3.3. Security and Reliability. . . . . . . . . . . . . . . . . . .20
   3.3.1. Retransmissions and Acknowledgements. . . . . . . . . . . .20
   3.3.2. Integrity Checks. . . . . . . . . . . . . . . . . . . . . .21
   3.3.3. Source Authentication . . . . . . . . . . . . . . . . . . .21
   3.3.4. Timestamps. . . . . . . . . . . . . . . . . . . . . . . . .21
   3.4. An Example of IDPR Operation. . . . . . . . . . . . . . . . .22
   4. Accommodating a Large, Heterogeneous Internet . . . . . . . . .25
   4.1. Domain Level Routing. . . . . . . . . . . . . . . . . . . . .25
   4.2. Route Generation. . . . . . . . . . . . . . . . . . . . . . .27
   4.3. Super Domains . . . . . . . . . . . . . . . . . . . . . . . .29
   4.4. Domain Communities. . . . . . . . . . . . . . . . . . . . . .30
   4.5. Robustness in the Presence of Failures. . . . . . . . . . . .31
   4.5.1. Path Repair . . . . . . . . . . . . . . . . . . . . . . . .31
   4.5.2. Partitions. . . . . . . . . . . . . . . . . . . . . . . . .33
   5. References. . . . . . . . . . . . . . . . . . . . . . . . . . .XX
   5. Security Considerations . . . . . . . . . . . . . . . . . . . .34
   6. Author's Address  . . . . . . . . . . . . . . . . . . . . . . .34













Steenstrup                                                      [Page 2]

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1.  Introduction

   As data communications technologies evolve and user populations grow,
   the demand for internetworking increases.  Internetworks usually
   proliferate through interconnection of autonomous, heterogeneous
   networks administered by separate authorities.  We use the term
   "administrative domain" (AD) to refer to any collection of contiguous
   networks, gateways, links, and hosts governed by a single
   administrative authority who selects the intra-domain routing
   procedures and addressing schemes, specifies service restrictions for
   transit traffic, and defines service requirements for locally-
   generated traffic.

   Interconnection of administrative domains can broaden the range of
   services available in an internetwork.  Hence, traffic with special
   service requirements is more likely to receive the service requested.
   However, administrators of domains offering special transit services
   are more likely to establish stringent access restrictions, in order
   to maintain control over the use of their domains' resources.

   An internetwork composed of many domains with diverse service
   requirements and restrictions requires "policy routing" to transport
   traffic between source and destination.  Policy routing constitutes
   route generation and message forwarding procedures for producing and
   using routes that simultaneously satisfy user service requirements
   and respect transit domain service restrictions.

   With policy routing, each domain administrator sets "transit
   policies" that dictate how and by whom the resources within its
   domain should be used.  Transit policies are usually public, and they
   specify offered services comprising:

   - Access restrictions: e.g., applied to traffic to or from certain
     domains or classes of users.

   - Quality: e.g., delay, throughput, or error characteristics.

   - Monetary cost: e.g., charge per byte, message, or unit time.

   Each domain administrator also sets "source policies" for traffic
   originating within its domain.  Source policies are usually private,
   and they specify requested services comprising:

   - Access restrictions: e.g., domains to favor or avoid in routes.

   - Quality: e.g., acceptable delay, throughput, or reliability.

   - Monetary cost: e.g., acceptable session cost.



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   In this document, we describe an architecture for inter-domain policy
   routing (IDPR), and we provide a set of functions which can form the
   basis for a suite of IDPR protocols and procedures.

1.1.  The Internet Environment

   The Internet currently comprises over 7000 operational networks and
   over 10,000 registered networks.  In fact, for the last several
   years, the number of constituent networks has approximately doubled
   annually.  Although we do not expect the Internet to sustain this
   growth rate, we must provide an architecture for IDPR that can
   accommodate the Internet five to ten years in the future.  According
   to the functional requirements for inter-autonomous system (i.e.,
   inter-domain) routing set forth in [6], the IDPR architecture and
   protocols must be able to handle O(100,000) networks distributed over
   O(10,000) domains.

   Internet connectivity has increased along with the number of
   component networks.  In the early 1980s, the Internet was purely
   hierarchical, with the ARPANET as the single backbone.  The current
   Internet possesses a semblance of a hierarchy in the collection of
   backbone, regional, metropolitan, and campus domains that compose it.
   However, technological, economical, and political incentives have
   prompted the introduction of inter-domain links outside of those in
   the strict hierarchy.  Hence, the Internet has the properties of both
   hierarchical and mesh connectivity.

   We expect that the Internet will evolve in the following way.  Over
   the next five years, the Internet will grow to contain O(10) backbone
   domains, most providing connectivity between many source and
   destination domains and offering a wide range of qualities of
   service, for a fee.  Most domains will connect directly or indirectly
   to at least one Internet backbone domain, in order to communicate
   with other domains.  In addition, some domains may install direct
   links to their most favored destinations.  Domains at the lower
   levels of the hierarchy will provide some transit service, limited to
   traffic between selected sources and destinations.  However, the
   majority of Internet domains will be "stubs", that is, domains that
   do not provide any transit service for other domains.

   The bulk of Internet traffic will be generated by hosts in these stub
   domains, and thus, the applications running in these hosts will
   determine the traffic service requirements.  We expect application
   diversity encompassing electronic mail, desktop videoconferencing,
   scientific visualization, and distributed simulation, to list a few.
   Many of these applications have strict requirements on loss, delay,
   and throughput.




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   Ensuring that Internet traffic traverses routes that provide the
   required services without violating domain usage restrictions will be
   the task of policy routing in the Internet in the next several years.
   Refer to [1]-[10] for more information on the role of policy routing
   in the Internet.

2.  Approaches to Policy Routing

   In this section, we provide an assessment of candidate approaches to
   policy routing, concentrating on the "distance vector" and "link
   state" alternatives for routing information distribution and route
   generation and on the "hop-by-hop" and "source specified"
   alternatives for data message forwarding.  The IDPR architecture
   supports link state routing information distribution and route
   generation in conjunction with source specified message forwarding.
   We justify these choices for IDPR below.

2.1.  Policy Route Generation

   We present policy route generation from the distance vector
   perspective and from the link state perspective.

2.1.1.  Distance Vector Approach

   Distance vector route generation distributes the computation of a
   single route among multiple routing entities along the route.  Hence,
   distance vector route generation is potentially susceptible to the
   problems of routing loop formation and slow adaptation to changes in
   an internetwork.  However, there exist several techniques that can be
   applied during distance vector route generation to reduce the
   severity of, or even eliminate, these problems.  For information on a
   loop-free, quickly adapting distance vector routing procedure,
   consult [13] and [14].

   During policy route generation, each recipient of a distance vector
   message assesses the acceptability of the associated route and
   determines the set of neighboring domains to which the message should
   be propagated.  In the context of policy routing, both of the
   following conditions are necessary for route acceptability:

   - The route is consistent with at least one transit policy for each
     domain, not including the current routing entity's domain, contained
     in the route.  To enable each recipient of a distance vector message
     to verify consistency of the associated route with the transit
     policies of all constituent domains, each routing entity should
     include its domain's identity and transit policies in each
     acceptable distance vector message it propagates.




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   - The route is consistent with at least one source policy for at least
     one domain in the Internet.  To enable each recipient of a distance
     vector message to verify consistency of the associated route with
     the source policies of particular domains, each domain must provide
     other domains with access to its source policies.

   In addition, at least one of the following conditions is necessary
   for route acceptability:

   - The route is consistent with at least one of the transit policies
     for the current routing entity's domain.  In this case, the routing
     entity accepts the distance vector message and then proceeds to
     compare the associated route with its other routes to the
     destinations listed in the message.  If the routing entity decides
     that the new route is preferable, it updates the distance vector
     message with its domain's identity and transit policies and then
     propagates the message to the appropriate neighboring domains.  We
     discuss distance vector message distribution in more detail in
     section 2.2.1.

   The route is consistent with at least one of the source policies for
   the current routing entity's domain.  In this case, the routing
   entity need not propagate the distance vector message but does retain
   the associated route for use by traffic from local hosts, bound for
   the destinations listed in the message.

   The routing entity discards any distance vector message that does not
   meet these necessary conditions.

   With distance vector policy route generation, a routing entity may
   select and store multiple routes of different characteristics, such
   as qualities of service, to a single destination.  A routing entity
   uses the quality of service information, provided in the transit
   policies contained in accepted distance vector messages, to
   discriminate between routes based on quality of service.  Moreover, a
   routing entity may select routes that are specific to certain source
   domains, provided that the routing entity has access to the source
   policies of those domains.

   In the distance vector context, the flexibility of policy route
   generation afforded by accounting for other domains' transit and
   source policies in route selection has the following disadvantages:

   - Each recipient of a distance vector message must bear the cost of
     verifying the consistency of the associated route with the
     constituent domains' transit policies.





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   - Source policies must be made public.  Thus, a domain must divulge
     potentially private information.

   - Each recipient of a distance vector message must bear the
     potentially high costs of selecting routes for arbitrary source
     domains.  In particular, a routing entity must store the source
     policies of other domains, account for these source policies during
     route selection, and maintain source-specific forwarding
     information.  Moreover, there must be a mechanism for distributing
     source policy information among domains.  Depending on the mechanism
     selected, distribution of source policies may add to the costs paid
     by each routing entity in supporting source-specific routing.

   We note, however, that failure to distribute source policies to all
   domains may have unfortunate consequences.  In the worst case, a
   domain may not learn of any acceptable routes to a given destination,
   even though acceptable routes do exist.  For example, suppose that AD
   V is connected to AD W and that AD W can reach AD Z through either AD
   X or AD Y.  Suppose also that AD~W, as a recipient of distance vector
   messages originating in AD Z, prefers the route through AD Y to the
   route through AD X.  Furthermore, suppose that AD W has no knowledge
   of AD V's source policy precluding traffic from traversing AD Y.
   Hence, AD W distributes to AD V the distance vector message
   containing the route WYZ but not the distance vector message
   containing the route WXZ.  AD V is thus left with no known route to
   AD Z, although a viable route traversing AD W and AD X does exist.

2.1.2.  Link State Approach

   Link state route generation permits concentration of the computation
   of a single route within a single routing entity at the source of the
   route.  In the policy routing context, entities within a domain
   generate link state messages containing information about the
   originating domain, including the set of transit policies that apply
   and the connectivity to adjacent domains, and they distribute these
   messages to neighboring domains.  Each recipient of a link state
   message stores the routing information for anticipated policy route
   generation and also distributes it to neighboring domains.  Based on
   the set of link state messages collected from other domains and on
   its domain's source and transit policies, a routing entity constructs
   and selects policy routes from its domain to other domains in the
   Internet.

   We have selected link state policy route generation for IDPR for the
   following reasons:

   - Each domain has complete control over policy route generation from
     the perspective of itself as source.



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   - The cost of computing a route is completely contained within the
     source domain.  Hence, routing entities in other domains need not
     bear the cost of generating policy routes that their domains' local
     hosts may never use.

   - Source policies may be kept private and hence need not be
     distributed.  Thus, there are no memory, processing, or transmission
     bandwidth costs incurred for distributing and storing source
     policies.

2.2.  Routing Information Distribution

   A domain's routing information and the set of domains to which that
   routing information is distributed each influence the set of generable
   policy routes that include the given domain.  In particular, a domain
   administrator may promote the generation of routes that obey its
   domain's transit policies by ensuring that its domain's routing
   information:

   - Includes resource access restrictions.

   - Is distributed only to those domains that are permitted to use these
     resources.

   Both of these mechanisms, distributing restrictions with and
   restricting distribution of a domain's routing information, can be
   applied in both the distance vector and link state contexts.

2.2.1.  Distance Vector Approach

   A routing entity may distribute its domain's resource access
   restrictions by including the appropriate transit policy information
   in each distance vector it accepts and propagates.  Also, the routing
   entity may restrict distribution of an accepted distance vector
   message by limiting the set of neighboring domains to which it
   propagates the message.  In fact, restricting distribution of routing
   information is inherent in the distance vector approach, as a routing
   entity propagates only the preferred routes among all the distance
   vector messages that it accepts.

   Although restricting distribution of distance vector messages is
   easy, coordinating restricted distribution among domains requires
   each domain to know other domains' distribution restrictions.  Each
   domain may have a set of distribution restrictions that apply to all
   distance vector messages generated by that domain as well as sets of
   distribution restrictions that apply to distance vector messages
   generated by other domains.




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   As a distance vector message propagates among domains, each routing
   entity should exercise the distribution restrictions associated with
   each domain constituting the route thus far constructed.  In
   particular, a routing entity should send an accepted distance vector
   message to a given neighbor, only if distribution of that message to
   that neighbor is not precluded by any domain contained in the route.

   To enable a routing entity to exercise these distribution
   restrictions, each domain must permit other domains access to its
   routing information distribution restrictions.  However, we expect
   that domains may prefer to keep distribution restrictions, like
   source policies, private.  There are at least two ways to make a
   domain's routing information distribution restrictions generally
   available to other domains:

   - Prior to propagation of an accepted distance vector message, a
     routing entity includes in the message its domain's distribution
     restrictions (all or only those to that apply to the given message).
     This method requires no additional protocol for disseminating the
     distribution restrictions, but it may significantly increase the
     size of each distance vector message.

   - Each domain independently disseminates its distribution restrictions
     to all other domains, so that each domain will be able to exercise
     all other domains' distribution restrictions.  This method requires
     an additional protocol for disseminating the distribution
     restrictions, and it may require a significant amount of memory at
     each routing entity for storing all domains' distribution
     restrictions.

   We note that a domain administrator may describe the optimal
   distribution pattern of distance vector messages originating in its
   domain, as a directed graph rooted at its domain.  Furthermore, if
   all domains in the directed graph honor the directionality and if the
   graph is also acyclic, no routing loops may form, because no two
   domains are able to exchange distance vector messages pertaining to
   the same destination.  However, an acyclic graph also means that some
   domains may be unable to discover alternate paths when connectivity
   between adjacent domains fails, as we show below.

   We reconsider the example from section 2.1.1.  Suppose that the
   distance vector distribution graph for AD Z is such that all distance
   vectors originating in AD Z flow toward AD V.  In particular,
   distance vectors from AD Z enter AD W from AD X and AD Y and leave AD
   W for AD V.  Now, suppose that the link between the AD Z and AD X
   breaks.  AD X no longer has knowledge of any viable route to AD Z,
   although such a route exists through AD W.  To ensure discovery of
   alternate routes to AD Z during connectivity failures, the distance



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   vector distribution graph for AD Z must contain bidirectional links
   between AD W and AD X and between AD W and AD Y.

2.2.2.  Link State Approach

   With link state routing information distribution, all recipients of a
   domain's link state message gain knowledge of that domain's transit
   policies and hence service restrictions.  For reasons of efficiency
   or privacy, a domain may also restrict the set of domains to which
   its link state messages should be distributed.  Thus, a domain has
   complete control over distributing restrictions with and restricting
   distribution of its routing information.

   A domain's link state messages automatically travel to all other
   domains if no distribution restrictions are imposed.  Moreover, to
   ensure that distribution restrictions, when imposed, are applied, the
   domain may use source specified forwarding of its link state
   messages, such that the messages are distributed and interpreted only
   by the destination domains for which they were intended.  Thus, only
   those domains receive the given domain's link state messages and
   hence gain knowledge of that domain's service offerings.

   We have selected link state routing information distribution for IDPR
   for the following reasons:

   - A domain has complete control over the distribution of its own
     routing information.

   - Routing information distribution restrictions may be kept private
     and hence need not be distributed.  Thus, there are no memory,
     processing, or transmission bandwidth costs incurred for
     distributing and storing distribution restrictions.

2.3.  Message Forwarding along Policy Routes

   To transport data messages along a selected policy route, a routing
   entity may use either hop-by-hop or source specified message
   forwarding.

2.3.1.  Hop-by-Hop Approach

   With hop-by-hop message forwarding, each routing entity makes an
   independent forwarding decision based on a message's source,
   destination, and requested services and on information contained in
   the entity's forwarding information database.  Hop-by-hop message
   forwarding follows a source-selected policy route only if all routing
   entities along the route have consistent routing information and make
   consistent use of this information when generating and selecting



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   policy routes and when establishing forwarding information.  In
   particular, all domains along the route must have consistent
   information about the source domain's source policies and consistent,
   but not necessarily complete, information about transit policies and
   domain adjacencies within the Internet.  In general, this implies
   that each domain should have knowledge of all other domains' source
   policies, transit policies, and domain adjacencies.

   When hop-by-hop message forwarding is applied in the presence of
   inconsistent routing information, the actual route traversed by data
   messages not only may differ from the route selected by the source
   but also may contain loops.  In the policy routing context, private
   source policies and restricted distribution of routing information
   are two potential causes of routing information inconsistencies among
   domains.  Moreover, we expect routing information inconsistencies
   among domains in a large Internet, independent of whether the
   Internet supports policy routing, as some domains may not want or may
   not be able to store routing information from the entire Internet.

2.3.1.1.  A Clarification

   In a previous draft, we presented the following example which results
   in persistent routing loops, when hop-by-hop message forwarding is
   used in conjunction with distance vector routing information
   distribution and route selection.  Consider the sequence of events:

   - AD X receives a distance vector message containing a route to AD Z,
     which does not include AD Y.  AD X selects and distributes this route
     as its primary route to AD Z.

   - AD Y receives a distance vector message containing a route to AD Z,
     which does not include AD X.  AD Y selects and distributes this route
     as its primary route to AD Z.

   - AD X eventually receives the distance vector message containing the
     route to AD Z, which includes AD Y but not AD X.  AD X prefers this
     route over its previous route to AD Z and selects this new route as
     its primary route to AD Z.

   - AD Y eventually receives the distance vector message containing the
     route to AD Z, which includes AD X but not AD Y.  AD Y prefers this
     route over its previous route to AD Z and selects this new route as
     its primary route to AD Z.

   Thus, AD X selects a route to AD Z that includes AD Y, and AD Y
   selects a route to AD Z that includes AD X.





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   Suppose that all domains along the route selected by AD X, except for
   AD Y, make forwarding decisions consistent with AD X's route, and
   that all domains along the route selected by AD Y, except for AD X,
   make forwarding decisions consistent with AD Y's route.  Neither AD
   X's selected route nor AD Y's selected route contains a loop.
   Nevertheless, data messages destined for AD Z and forwarded to either
   AD X or AD Y will continue to circulate between AD X and AD Y, until
   there is a route change.  The reason is that AD X and AD Y have
   conflicting notions of the route to AD Z, with each domain existing
   as a hop on the other's route.

   We note that while BGP-3 [8] is susceptible to such routing loops,
   BGP-4 [9] is not.  We thank Tony Li and Yakov Rekhter for their help
   in clarifying this difference between BGP-3 and BGP-4.

2.3.2.  Source Specified Approach

   With source specified message forwarding, the source domain dictates
   the data message forwarding decisions to the routing entities in each
   intermediate domain, which then forward data messages according to
   the source specification.  Thus, the source domain ensures that any
   data message originating within it follows its selected routes.

   For source specified message forwarding, each data message must carry
   either an entire source specified route or a path identifier.
   Including the complete route in each data message incurs a per
   message transmission and processing cost for transporting and
   interpreting the source route.  Using path identifiers does not incur
   these costs.  However, to use path identifiers, the source domain
   must initiate, prior to data message forwarding, a path setup
   procedure that forms an association between the path identifier and
   the next hop in the routing entities in each domain along the path.
   Thus, path setup may impose an initial delay before data message
   forwarding can begin.

   We have selected source specified message forwarding for IDPR data
   messages for the following reasons:

   - Source specified message forwarding respects the source policies of
     the source domain, regardless of whether intermediate domains along
     the route have knowledge of these source policies.

   - Source specified message forwarding is loop-free, regardless of
     whether the all domains along the route maintain consistent routing
     information.

   Also, we have chosen path identifiers over complete routes, to affect
   source specified message forwarding, because of the reduced



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   transmission and processing cost per data message.

3.  The IDPR Architecture

   We now present the architecture for IDPR, including a description of
   the IDPR functions, the entities that perform these functions, and
   the features of IDPR that aid in accommodating Internet growth.

3.1.  IDPR Functions

   Inter-domain policy routing comprises the following functions:

   - Collecting and distributing routing information including domain
     transit policies and inter-domain connectivity.

   - Generating and selecting policy routes based on the routing
     information distributed and on the source policies configured or
     requested.

   - Setting up paths across the Internet using the policy routes
     generated.

   - Forwarding messages across and between domains along the established
     paths.

   - Maintaining databases of routing information, inter-domain policy
     routes, forwarding information, and configuration information.

3.2.  IDPR Entities

   From the perspective of IDPR, the Internet comprises administrative
   domains connected by "virtual gateways" (see below), which are in
   turn connected by intra-domain routes supporting the transit policies
   configured by the domain administrators.  Each domain administrator
   defines the set of transit policies that apply across its domain and
   the virtual gateways between which each transit policy applies.
   Several different transit policies may be configured for the intra-
   domain routes connecting a pair of virtual gateways.  Moreover, a
   transit policy between two virtual gateways may be directional.  That
   is, the transit policy may apply to traffic flowing in one direction,
   between the virtual gateways, but not in the other direction.

   Virtual gateways (VGs) are the only connecting points recognized by
   IDPR between adjacent administrative domains.  Each virtual gateway
   is actually a collection of directly-connected "policy gateways" (see
   below) in two adjacent domains, whose existence has been sanctioned
   by the administrators of both domains.  Domain administrators may
   agree to establish more than one virtual gateway between their



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   domains.  For example, if two domains are to be connected at two
   geographically distant locations, the domain administrators may wish
   to preserve these connecting points as distinct at the inter-domain
   level, by establishing two distinct virtual gateways.

   Policy gateways (PGs) are the physical gateways within a virtual
   gateway.  Each policy gateway forwards transit traffic according to
   the service restrictions stipulated by its domain's transit policies
   applicable to its virtual gateway.  A single policy gateway may
   belong to multiple virtual gateways.  Within a domain, two policy
   gateways are "neighbors" if they are in different virtual gateways.
   Within a virtual gateway, two policy gateways are "peers" if they are
   in the same domain and are "adjacent" if they are in different
   domains.  Peer policy gateways must be able to communicate over
   intra-domain routes that support the transit policies that apply to
   their virtual gateways.  Adjacent policy gateways are "directly
   connected" if they are the only Internet addressable entities
   attached to the connecting medium.  Note that this definition implies
   that not only point-to-point links but also multiaccess networks may
   serve as direct connections between adjacent policy gateways.

   Combining multiple policy gateways into a single virtual gateway
   affords three advantages:

   - A reduction in the amount of IDPR routing information that must be
     distributed and maintained throughout the Internet.

   - An increase in the reliability of IDPR routes through redundancy of
     physical connections between domains.

   - An opportunity for load sharing of IDPR traffic among policy
     gateways.

   Several different entities are responsible for performing the IDPR
   functions:

   - Policy gateways collect and distribute routing information,
     participate in path setup, forward data messages along established
     paths, and maintain forwarding information databases.

   - "Path agents" act on behalf of hosts to select policy routes, to set
     up and manage paths, and to maintain forwarding information
     databases.

   - Special-purpose servers maintain all other IDPR databases as
     follows:





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      o Each "route server" is responsible for both its database of
        routing information, including domain connectivity and transit
        policy information, and its database of policy routes.  Also,
        each route server generates policy routes on behalf of its
        domain, using entries from its routing information database
        and source policy information supplied through configuration
        or obtained directly from the path agents.

      o  Each "mapping server" is responsible for its database of
         mappings that resolve Internet names and addresses to
         administrative domains.

      o  Each "configuration server" is responsible for its database of
         configured information that applies to policy gateways, path
         agents, and route servers in the given administrative domain.
         The configuration information for a given domain includes
         source and transit policies and mappings between local IDPR
         entities and their Internet addresses.

   To maximize IDPR's manageability, one should embed all of IDPR's
   required functionality within the IDPR protocols and procedures.
   However, to minimize duplication of implementation effort, one should
   take advantage of required functionality already provided by
   mechanisms external to IDPR.  Two such cases are the mapping server
   functionality and the configuration server functionality.  The
   functions of the mapping server can be integrated into an existing
   name service such as the DNS, and the functions of the configuration
   server can be integrated into the domain's existing network
   management system.

   Within the Internet, only policy gateways, path agents, and route
   servers must be able to generate, recognize, and process IDPR
   messages.  The existence of IDPR is invisible to all other gateways
   and hosts.  Mapping servers and configuration servers perform
   necessary but ancillary functions for IDPR, and they are not required
   to execute the IDPR protocols.

3.2.1.  Path Agents

   Any Internet host can reap the benefits of IDPR, as long as there
   exists a path agent configured to act on its behalf and a means by
   which the host's messages can reach that path agent.  Path agents
   select and set up policy routes for hosts, accounting for service
   requirements.  To obtain a host's service requirements, a path agent
   may either consult its configured IDPR source policy information or
   extract service requirements directly from the host's data messages,
   provided such information is available in these data messages.




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   Separating the path agent functions from the hosts means that host
   software need not be modified to support IDPR.  Moreover, it means
   that a path agent can aggregate onto a single policy route traffic
   from several different hosts, as long as the source domains,
   destination domains, and service requirements are the same for all of
   these host traffic flows.  Policy gateways are the natural choice for
   the entities that perform the path agent functions on behalf of
   hosts, as policy gateways are the only inter-domain connecting points
   recognized by IDPR.

   Each domain administrator determines the set of hosts that its
   domain's path agents will handle.  We expect that a domain
   administrator will normally configure path agents in its domain to
   act on behalf of its domain's hosts only.  However, a path agent can
   be configured to act on behalf of any Internet host.  This
   flexibility permits one domain to act as an IDPR "proxy" for another
   domain.  For example, a small stub domain may wish to have policy
   routing available to a few of its hosts but may not want to set up
   its domain to support all of the IDPR functionality.  The
   administrator of the stub domain can negotiate the proxy function
   with the administrator of another domain, who agrees that its domain
   will provide policy routes on behalf of the stub domain's hosts.

   If a source domain supports IDPR and limits all domain egress points
   to policy gateways, then each message generated by a host in that
   source domain and destined for a host in another domain must pass
   through at least one policy gateway, and hence path agent, in the
   source domain.  A host need not know how to reach any policy gateways
   in its domain; it need only know how to reach a gateway on its own
   local network.  Gateways within the source domain direct inter-domain
   host traffic toward policy gateways, using default routes or routes
   derived from other inter-domain routing procedures.

   If a source domain does not support IDPR and requires an IDPR proxy
   domain to provide its hosts with policy routing, the administrator of
   that source domain must carefully choose the proxy domain.  All
   intervening gateways between hosts in the source domain and path
   agents in the proxy domain forward traffic according to default
   routes or routes derived from other inter-domain routing procedures.
   In order for traffic from hosts in the source domain to reach the
   proxy domain with no special intervention, the proxy domain must lie
   on an existing non-IDPR inter-domain route from the source to the
   destination domain.  Hence, to minimize the knowledge a domain
   administrator must have about inter-domain routes when selecting a
   proxy domain, we recommend that a domain administrator select its
   proxy domain from the set of adjacent domains.





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   In either case, the first policy gateway to receive messages from an
   inter-domain traffic flow originating at the source domain acts as
   the path agent for the host generating that flow.

3.2.2.  IDPR Servers

   IDPR servers are the entities that manage the IDPR databases and that
   respond to queries for information from policy gateways or other
   servers.  Each IDPR server may be a dedicated device, physically
   separate from the policy gateway, or it may be part of the
   functionality of the policy gateway itself.  Separating the server
   functions from the policy gateways reduces the processing and memory
   requirements for and increases the data traffic carrying capacity of
   the policy gateways.

   The following IDPR databases: routing information, route, mapping,
   and configuration, may be distributed hierarchically, with partial
   redundancy throughout the Internet.  This arrangement implies a
   hierarchy of the associated servers, where a server's position in the
   hierarchy determines the extent of its database.  At the bottom of
   the hierarchy are the "local servers" that maintain information
   pertinent to a single domain; at the top of the hierarchy are the
   "global servers" that maintain information pertinent to all domains
   in the Internet.  There may be zero or more levels in between the
   local and global levels.

   Hierarchical database organization relieves most IDPR servers of the
   burden of maintaining information about large portions of the
   Internet, most of which their clients will never request.
   Distributed database organization, with redundancy, allows clients to
   spread queries among IDPR servers, thus reducing the load on any one
   server.  Furthermore, failure to communicate with a given IDPR server
   does not mean the loss of the entire service, as a client may obtain
   the information from another server.  We note that some IDPR
   databases, such as the mapping database, may grow so large that it is
   not feasible to store the entire database at any single server.

   IDPR routing information databases need not be completely consistent
   for proper policy route generation and use, because message
   forwarding along policy routes is completely specified by the source
   path agent.  The absence of a requirement for consistency among IDPR
   routing information databases implies that there is no requirement
   for strict synchronization of these databases.  Such synchronization
   is costly in terms of the message processing and transmission
   bandwidth required.  Nevertheless, each IDPR route server should have
   a query/response mechanism for making its routing information
   database consistent with that of another route server, if necessary.
   A route server uses this mechanism to update its routing information



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   database following detection of a gap or potential error in database
   contents, for example, when the route server returns to service after
   disconnection from the Internet.

   A route server in one domain wishing to communicate with a route
   server in another domain must establish a policy route to the other
   route server's domain.  To generate and establish a policy route, the
   route server must know the other route server's domain, and it must
   have sufficient routing information to construct a route to that
   domain.  As route servers may often intercommunicate in order to
   obtain routing information, one might assume an ensuing deadlock in
   which a route server requires routing information from another route
   server but does not have sufficient mapping and routing information
   to establish a policy route to that route server.  However, such a
   deadlock should seldom persist, if the following IDPR functionality
   is in place:

   - A mechanism that allows a route server to gain access, during route
     server initialization, to the identities of the other route servers
     within its domain.  Using this information, the route server need not
     establish policy routes in order to query these route servers for
     routing information.

   - A mechanism that allows a route server to gain access, during route
     server initialization, to its domain's adjacencies.  Using this
     information, the route server may establish policy routes to the
     adjacent domains in order to query their route servers for routing
     information when none is available within its own domain.

   - Once operational, a route server should collect (memory capacity
     permitting) all the routing information to which it has access.  A
     domain usually does not restrict distribution of its routing
     information but instead distributes its routing information to all
     other Internet domains.  Hence, a route server in a given domain is
     likely to receive routing information from most Internet domains.

   - A mechanism that allows an operational route server to obtain the
     identities of external route servers from which it can obtain routing
     information and of the domains containing these route servers.
     Furthermore, this mechanism should not require mapping server queries.
     Rather, each domain should distribute in its routing information
     messages the identities of all route servers, within its domain, that
     may be queried by clients outside of its domain.

   When a host in one domain wishes to communicate with a host in
   another domain, the path agent in the source domain must establish a
   policy route to a path agent in the destination domain.  However, the
   source path agent must first query a mapping server, to determine the



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   identity of the destination domain.  The queried mapping server may
   in turn contact other mapping servers to obtain a reply.  As with
   route server communication, one might assume an ensuing deadlock in
   which a mapping server requires mapping information from an external
   mapping server but the path agent handling the traffic does not have
   sufficient mapping information to determine the domain of, and hence
   establish a policy route to, that mapping server.

   We have previously described how to minimize the potential for
   deadlock in obtaining routing information.  To minimize the potential
   for deadlock in obtaining mapping information, there should be a
   mechanism that allows a mapping server to gain access, during mapping
   server initialization, to the identities of other mapping servers and
   the domains in which they reside.  Thus, when a mapping server needs
   to query an external mapping server, it knows the identity of that
   mapping server and sends a message.  The path agent handling this
   traffic queries a local mapping server to resolve the identity of the
   external mapping server to the proper domain and then proceeds to
   establish a policy route to that domain.

3.2.3.  Entity Identifiers

   Each domain has a unique identifier within the Internet, specifically
   an ordinal number in the enumeration of Internet domains, determined
   by the Internet Assigned Numbers Authority (IANA) who is responsible
   for maintaining such information.

   Each virtual gateway has a unique local identifier within a domain,
   derived from the adjacent domain's identifier together with the
   virtual gateway's ordinal number within an enumeration of the virtual
   gateways connecting the two domains.  The administrators of both
   domains mutually agree upon the enumeration of the virtual gateways
   within their shared set of virtual gateways; selecting a single
   virtual gateway enumeration that applies in both domains eliminates
   the need to maintain a mapping between separate virtual gateway
   ordinal numbers in each domain.

   Each policy gateway and route server has a unique local identifier
   within its domain, specifically an ordinal number in the domain
   administrator's enumeration of IDPR entities within its domain.  This
   local identifier, when combined with the domain identifier, produces
   a unique identifier within the Internet for the policy gateway or
   route server.

3.3.  Security and Reliability

   The correctness of control information, and in particular routing-
   related information, distributed throughout the Internet is a



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   critical factor affecting the Internet's ability to transport data.
   As the number and heterogeneity of Internet domains increases, so too
   does the potential for both information corruption and denial of
   service attacks.  Thus, we have imbued the IDPR architecture with a
   variety of mechanisms to:

   - Promote timely delivery of control information.

   - Minimize acceptance and distribution of corrupted control
     information.

   - Verify authenticity of a source of control information.

   - Reduce the chances for certain types of denial of service attacks.

   Consult [11] for a general security architecture for routing and [12]
   for a security architecture for inter-domain routing.

3.3.1.  Retransmissions and Acknowledgements

   All IDPR entities must make an effort to accept and distribute only
   correct IDPR control messages.  Each IDPR entity that transmits an
   IDPR control message expects an acknowledgement from the recipient
   and must retransmit the message up to a maximum number of times when
   an acknowledgement is not forthcoming.  An IDPR entity that receives
   an IDPR control message must verify message content integrity and
   source authenticity before accepting, acknowledging, and possibly
   redistributing the message.

3.3.2.  Integrity Checks

   Integrity checks on message contents promote the detection of
   corrupted information.  Each IDPR entity that receives an IDPR
   control message must perform several integrity checks on the
   contents.  Individual IDPR protocols may apply more stringent
   integrity checks than those listed below.  The required checks
   include confirmation of:

   - Recognized message version.

   - Consistent message length.

   - Valid message checksum.

   Each IDPR entity may also apply these integrity checks to IDPR data
   messages.  Although the IDPR architecture only requires data message
   integrity checks at the last IDPR entity on a path, it does not
   preclude intermediate policy gateways from performing these checks as



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   well.

3.3.3.  Source Authentication

   Authentication of a message's source promotes the detection of a
   rogue entity masquerading as another legitimate entity.  Each IDPR
   entity that receives an IDPR control message must verify the
   authenticity of the message source.  We recommend that the source of
   the message supply a digital signature for authentication by message
   recipients.  The digital signature should cover the entire message
   contents (or a hash function thereof), so that it can serve as the
   message checksum as well as the source authentication information.

   Each IDPR entity may also authenticate the source of IDPR data
   messages; however, the IDPR architecture does not require source
   authentication of data messages.  Instead, we recommend that higher
   level (end-to-end) protocols, not IDPR, assume the responsibility for
   data message source authentication, because of the amount of
   computation involved in verifying a digital signature.

3.3.4.  Timestamps

   Message timestamps promote the detection of out-of-date messages as
   well as message replays.  Each IDPR control message must carry a
   timestamp supplied by the source, which serves to indicate the age of
   the message.  IDPR entities use the absolute value of a timestamp to
   confirm that the message is current and use the relative difference
   between timestamps to determine which message contains the most
   recent information.  Hence, all IDPR entities must possess internal
   clocks that are synchronized to some degree, in order for the
   absolute value of a message timestamp to be meaningful.  The
   synchronization granularity required by the IDPR architecture is on
   the order of minutes and can be achieved manually.

   Each IDPR entity that receives an IDPR control message must check
   that the message is timely.  Any IDPR control message whose timestamp
   lies outside of the acceptable range may contain stale or corrupted
   information or may have been issued by a source whose internal clock
   has lost synchronization with the message recipient's internal clock.

   IDPR data messages also carry timestamps; however, the IDPR
   architecture does not require timestamp acceptability checks on IDPR
   data messages.  Instead, we recommend that IDPR entities only check
   IDPR data message timestamps during problem diagnosis, for example,
   when checking for suspected message replays.

3.4.  An Example of IDPR Operation




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   We illustrate how IDPR works by stepping through an example.  In this
   example, we assume that all domains support IDPR and that all domain
   egress points are policy gateways.

   Suppose host Hx in domain AD X wants to communicate with host Hy in
   domain AD Y.  Hx need not know the identity of its own domain or of
   Hy's domain in order to send messages to Hy.  Instead, Hx simply
   forwards a message bound for Hy to one of the gateways on its local
   network, according to its local forwarding information.  If the
   recipient gateway is a policy gateway, the resident path agent
   determines how to forward the message outside of the domain.
   Otherwise, the recipient gateway forwards the message to another
   gateway in AD X, according to its local forwarding information.
   Eventually, the message will arrive at a policy gateway in AD X, as
   described previously in section 3.2.1.

   The path agent resident in the recipient policy gateway uses the
   message header, including source and destination addresses and any
   requested service information (for example, type of service), in
   order to determine whether it is an intra-domain or inter-domain
   message, and if inter-domain, whether it requires an IDPR policy
   route.  Specifically, the path agent attempts to locate a forwarding
   information database entry for the given traffic flow.  The
   forwarding information database will already contain entries for all
   of the following:

   - All intra-domain traffic flows.  Intra-domain forwarding information
     is integrated into the forwarding database as soon as it is received.

   - Inter-domain traffic flows that do not require IDPR policy routes.
     Non-IDPR inter-domain forwarding information is integrated into the
     forwarding database as soon as it is received.

   - IDPR inter-domain traffic flows for which a path has already been set
     up.  IDPR forwarding information is integrated into the forwarding
     database only during path setup.

   The path agent uses the message header contents to guide the search
   for a forwarding information database entry for a traffic flow; we
   suggest a radix search to locate a database entry.  When the search
   terminates, it either produces a forwarding information database
   entry or a directive to generate such an entry for an IDPR traffic
   flow.  If the search terminates in an existing database entry, the
   path agent forwards the message according to that entry.

   Suppose that the search terminates indicating that the traffic flow
   between Hx and Hy requires an IDPR route and that no forwarding
   information database entry yet exists for this flow.  In this case,



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   the path agent first determines the source and destination domains
   associated with the message's source and destination addresses,
   before attempting to obtain a policy route.  The path agent relies on
   the mapping servers to supply the domain information, but it caches
   all mapping server responses locally to limit the number of future
   queries.  When attempting to resolve an address to a domain, the path
   agent always checks its local cache before contacting a mapping
   server.

   After obtaining the source and destination domain information, the
   path agent attempts to obtain a policy route to carry the traffic
   from Hx to Hy.  The path agent relies on the route servers to supply
   policy routes, but it caches all route server responses locally to
   limit the number of future queries.  When attempting to locate a
   suitable policy route, the path agent consults its local cache before
   contacting a route server.  A policy route contained in the cache is
   suitable provided that its associated source domain is AD X, its
   associated destination domain is AD Y, and it satisfies the service
   requirements specified in the data message or through source policy
   configuration.

   If no suitable cache entry exists, the path agent queries the route
   server, providing it with the source and destination domains together
   with the requested services.  Upon receiving a policy route query, a
   route server consults its route database.  If it cannot locate a
   suitable route in its route database, the route server attempts to
   generate at least one route to domain AD Y, consistent with the
   requested services for Hx.

   The response to a successful route query consists of a set of
   candidate routes, from which the path agent makes its selection.  We
   expect that a path agent will normally choose a single route from a
   candidate set.  Nevertheless, the IDPR architecture does not preclude
   a path agent from selecting multiple routes from the candidate set.
   A path agent may desire multiple routes to support features such as
   fault tolerance or load balancing; however, the IDPR architecture
   does not specify how the path agent should use multiple routes.  In
   any case, a route server always returns a response to a path agent's
   query, even if it is not successful in locating a suitable policy
   route.

   If the policy route is a new route provided by the route server,
   there will be no existing path for the route and thus the path agent
   must set up such a path.  However, if the policy route is an existing
   route extracted from the path agent's cache, there may well be an
   existing path for the route, set up to accommodate a different host
   traffic flow.  The IDPR architecture permits multiple host traffic
   flows to use the same path, provided that all flows sharing the path



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   travel between the same endpoint domains and have the same service
   requirements.  Nevertheless, the IDPR architecture does not preclude
   a path agent from setting up distinct paths along the same policy
   route to preserve the distinction between host traffic flows.

   The path agent associates an identifier with the path, which will be
   included in each message that travels down the path and will be used
   by the policy gateways along the path in order to determine how to
   forward the message.  If the path already exists, the path agent uses
   the preexisting identifier.  However, for new paths, the path agent
   chooses a path identifier that is different from those of all other
   paths that it manages.  The path agent also updates its forwarding
   information database to reference the path identifier and modifies
   its search procedure to yield the correct forwarding information
   database entry given the data message header.

   For new paths, the path agent initiates path setup, communicating the
   policy route, in terms of requested services, constituent domains,
   relevant transit policies, and the connecting virtual gateways, to
   policy gateways in intermediate domains.  Using this information, an
   intermediate policy gateway determines whether to accept or refuse
   the path and to which policy gateway to forward the path setup
   information.  The path setup procedure allows policy gateways to set
   up a path in both directions simultaneously.  Each intermediate
   policy gateway, after path acceptance, updates its forwarding
   information database to include an entry that associates the path
   identifier with the appropriate previous and next hop policy
   gateways.  Paths remain in place until they are torn down because of
   failure, expiration, or when resources are scarce, preemption in
   favor of other paths.

   When a policy gateway in AD Y accepts a path, it notifies the source
   path agent in AD X.  We expect that the source path agent will
   normally wait until a path has been successfully established before
   using it to transport data traffic.  However, the IDPR architecture
   does not preclude a path agent from forwarding data messages along a
   path prior to confirmation of successful path establishment.  In this
   case, the source path agent transmits data messages along the path
   with full knowledge that the path may not yet have been successfully
   established at all intermediate policy gateways and thus that these
   data messages will be immediately discarded by any policy gateway not
   yet able to recognize the path identifier.

   We note that data communication between Hx and Hy may occur over two
   separate IDPR paths: one from AD X to AD Y and one from AD Y to AD X.
   The reasons are that within a domain, hosts know nothing about path
   agents nor IDPR paths, and path agents know nothing about other path
   agents' existing IDPR paths.  Thus, in AD Y, the path agent that



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   terminates the path from AD X may not be the same as the path agent
   that receives traffic from Hy destined for Hx.  In this case, receipt
   of traffic from Hy forces the second path agent to set up a new path
   from AD Y to AD X.

4.  Accommodating a Large, Heterogeneous Internet

   The IDPR architecture must be able to accommodate an Internet
   containing O(10,000) domains, supporting diverse source and transit
   policies.  Thus, we have endowed the IDPR architecture with many
   features that allow it to function effectively in such an
   environment.

4.1.  Domain Level Routing

   The IDPR architecture provides policy routing among administrative
   domains.  In order to construct policy routes, route servers require
   routing information at the domain level only; no intra-domain details
   need be included in IDPR routing information.  The size of the
   routing information database maintained by a route server depends not
   on the number of Internet gateways, networks, and links, but on how
   these gateways, networks, and links are grouped into domains and on
   what services they offer.  Therefore, the number of entries in an
   IDPR routing information database depends on the number of domains
   and the number and size of the transit policies supported by these
   domains.

   Policy gateways distribute IDPR routing information only when
   detectable inter-domain changes occur and may also elect to
   distribute routing information periodically (for example, on the
   order of once per day) as a backup.  We expect that a pair of policy
   gateways within a domain will normally be connected such that when
   the primary intra-domain route between them fails, the intra-domain
   routing procedure will be able to construct an alternate route.
   Thus, an intra-domain failure is unlikely to be visible at the
   inter-domain level and hence unlikely to force an inter-domain
   routing change.  Therefore, we expect that policy gateways will not
   often generate and distribute IDPR routing information messages.

   IDPR entities rely on intra-domain routing procedures operating
   within domains to transport inter-domain messages across domains.
   Hence, IDPR messages must appear well-formed according to the intra-
   domain routing and addressing procedures in each domain traversed.
   Recall that source authentication information (refer to section 3.3.3
   above) may cover the entire IDPR message.  Thus, the IDPR portion of
   such a message cannot be modified at intermediate domains along the
   path without causing source authenticity checks to fail.  Therefore,
   at domain boundaries, IDPR messages require encapsulation and



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   decapsulation according to the routing procedures and addressing
   schemes operating with the given domain.  Only policy gateways and
   route servers must be capable of handling IDPR-specific messages;
   other gateways and hosts simply treat the encapsulated IDPR messages
   like any other message.  Thus, for the Internet to support IDPR, only
   a small proportion of Internet entities require special IDPR
   software.

   With domain level routes, many different traffic flows may use not
   only the same policy route but also the same path, as long as their
   source domains, destination domains, and service requirements are
   compatible.  The size of the forwarding information database
   maintained by a policy gateway depends not on the number of Internet
   hosts but on how these hosts are grouped into domains, which hosts
   intercommunicate, and on how much distinction a source domain wishes
   to preserve among its traffic flows.  Therefore, the number of
   entries in an IDPR forwarding information database depends on the
   number of domains and the number of source policies supported by
   those domains.  Moreover, memory associated with failed, expired, or
   disused paths can be reclaimed for new paths, and thus forwarding
   information for many paths can be accommodated in a policy gateway's
   forwarding information database.

4.2.  Route Generation

   Route generation is the most computationally complex part of IDPR,
   because of the number of domains and the number and heterogeneity of
   policies that it must accommodate.  Route servers must generate
   policy routes that satisfy the requested services of the source
   domains and respect the offered services of the transit domains.

   We distinguish requested qualities of service and route generation
   with respect to them as follows:

   - Requested service limits include upper bounds on route delay, route
     delay variation, and monetary cost for the session and lower bounds
     on available route bandwidth.  Generating a route that must satisfy
     more than one quality of service constraint, for example route delay
     of no more than X seconds and available route bandwidth of no less
     than Y bits per second, is an NP-complete problem.

   - Optimal requested services include minimum route delay, minimum
     route delay variation, minimum monetary cost for the session, and
     maximum available route bandwidth.  In the worst case, the
     computational complexity of generating a route that is optimal with
     respect to a given requested service is O((N + L) log N) for
     Dijkstra's shortest path first (SPF) search and O(N + (L * L)) for
     breadth-first (BF) search, where N is the number of nodes and L is



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     the number of links in the search graph.  Multi-criteria
     optimization, for example finding a route with minimal delay
     variation and minimal monetary cost for the session, may be defined
     in several ways.  One approach to multi-criteria optimization is to
     assign each link a single value equal to a weighted sum of the
     values of the individual offered qualities of service and generate a
     route that is optimal with respect to this new criterion.  However,
     it may not always be possible to achieve the desired route
     generation behavior using such a linear combination of qualities of
     service.

   To help contain the combinatorial explosion of processing and memory
   costs associated with route generation, we supply the following
   guidelines for generation of suitable policy routes:

   - Each route server should only generate policy routes from the
     perspective of its own domain as source; it need not generate policy
     routes for arbitrary source/destination domain pairs.  Thus, we can
     distribute the computational burden over all route servers.

   - Route servers should precompute routes for which they anticipate
     requests and should generate routes on demand only in order to
     satisfy unanticipated route requests.  Hence, a single route server
     can distribute its computational burden over time.

   - Route servers should cache the results of route generation, in order
     to minimize the computation associated with responding to future
     route requests.

   - To handle requested service limits, a route server should always
     select the first route generated that satisfies all of the requested
     service limits.

   - To handle multi-criteria optimization in route selection, a route
     server should generate routes that are optimal with respect to the
     first specified optimal requested service listed in the source
     policy.  The route server should resolve ties between otherwise
     equivalent routes by evaluating these routes according to the other
     optimal requested services, in the order in which they are
     specified.  With respect to the route server's routing information
     database, the selected route is optimal according to the first
     optimal requested service but is not necessarily optimal according
     to any other optimal requested service.

   - To handle a mixture of requested service limits and optimal
     requested services, a route server should generate routes that
     satisfy all of the requested service limits.  The route server
     should resolve ties between otherwise equivalent routes by



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     evaluating those routes as described in the multi-criteria
     optimization case above.

   - All else being equal, a route server should always prefer
     minimum-hop routes, because they minimize the amount of network
     resources consumed by the routes.

   All domains need not execute the identical route generation
   procedure.  Each domain administrator is free to specify the IDPR
   route generation procedure for route servers in its own domain,
   making the procedure as simple or as complex as desired.

4.3.  SuperDomains

   A "super domain" is itself an administrative domain, comprising a set
   of contiguous domains with similar transit policies and formed
   through consensus of the administrators of the constituent domains.
   Super domains provide a mechanism for reducing the amount of IDPR
   routing information distributed throughout the Internet.  Given a set
   of n contiguous domains with consistent transit policies, the amount
   of routing information associated with the set is approximately n
   times smaller when the set is considered as a single super domain
   than when it is considered as n individual domains.

   When forming a super domain from constituent domains whose transit
   policies do not form a consistent set, one must determine which
   transit policies to distribute in the routing information for the
   super domain.  The range of possibilities is bounded by the following
   two alternatives, each of which reduces the amount of routing
   information associated with the set of constituent domains:

   - The transit policies supported by the super domain are derived from
     the union of the access restrictions and the intersection of the
     qualities of service, over all constituent domains.  In this case,
     the formation of the super domain reduces the number of services
     offered by the constituent domains, but guarantees that none of
     these domains' access restrictions are violated.

   - The transit policies supported by the super domain are derived from
     the intersection of the access restrictions and the union of the
     qualities of service.  In this case, the formation of the super
     domain increases the number of services offered by the constituent
     domains, but forces relaxation of these domains' access
     restrictions.

   Thus, we recommend that domain administrators refrain from
   arbitrarily grouping domains into super domains, unless they fully
   understand the consequences.



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   The existence of super domains imposes a hierarchy on domains within
   the Internet.  For model consistency, we assume that there is a
   single super domain at the top of the hierarchy, which contains the
   set of all high-level domains.  A domain's identity is defined
   relative to the domain hierarchy.  Specifically, a domain's identity
   may be defined in terms of the domains containing it, the domains it
   contains, or both.

   For any domain AD X, the universe of distribution for its routing
   information usually extends only to those domains contained in AD X's
   immediate super domain and at the same level of the hierarchy as AD
   X.  However, the IDPR architecture does not preclude AD X from
   distributing its routing information to domains at arbitrarily high
   levels in the hierarchy, as long as the immediate super domain of
   these domains is also a super domain of AD X.  For example, the
   administrator of an individual domain within a super domain may wish
   to have one of its transit policies advertised outside of the
   immediate super domain, so that other domains can take advantage of a
   quality of service not offered by the super domain itself.  In this
   case, the super domain and the consituent domain may distribute
   routing information at the same level in the domain hierarchy, even
   though one domain actually contains the other.

   We note that the existence of super domains may restrict the number
   of routes available to source domains with access restrictions.  For
   example, suppose that a source domain AD X has source policies that
   preclude its traffic from traversing a domain AD Y and that AD Y is
   contained in a super domain AD Z.  If domains within AD Z do not
   advertise routing information separately, then route servers within
   AD X do not have enough routing information to construct routes that
   traverse AD Z but that avoid AD Y.  Hence, route servers in AD X must
   generate routes that avoid AD Z altogether.

4.4.  Domain Communities

   A "domain community" is a group of domains to which a given domain
   distributes routing information, and hence domain communities may be
   used to limit routing information distribution.  Domain communities
   not only reduce the costs associated with distributing and storing
   routing information but also allow concealment of routing information
   from domains outside of the community.  Unlike a super domain, a
   domain community is not necessarily an administrative domain.
   However, formation of a domain community may or may not involve the
   consent of the administrators of the member domains, and the
   definition of the community may be implicit or explicit.

   Each domain administrator determines the extent of distribution of
   its domain's routing information and hence unilaterally defines a



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   domain community.  By default, this community encompasses all
   Internet domains.  However, the domain administrator may restrict
   community membership by describing the community as a neighborhood
   (defined, for example, in terms of domain hops) or as a list of
   member domains.

   A group of domain administrators may mutually agree on distribution
   of their domains' routing information among their domains and hence
   multilaterally define a domain community.  By default, this community
   encompasses all Internet domains.  However, the domain administrators
   may restrict community membership by describing the community as a
   list of member domains.  In fact, this domain community may serve as
   a multicast group for routing information distribution.

4.5.  Robustness in the Presence of Failures

   The IDPR architecture possesses the following features that make it
   resistent to failures in the Internet:

   - Multiple connections between adjacent policy gateways in a virtual
     gateway and between peer and neighbor policy gateways across an
     administrative domain minimize the number of single component
     failures that are visible at the inter-domain level.

   - Policy gateways distribute IDPR routing information immediately
     after detecting a connectivity failure at the inter-domain level,
     and route servers immediately incorporate this information into
     their routing information databases.  This ensures that new policy
     routes will not include those domains involved in the connectivity
     failure.

   - The routing information database query/response mechanism ensures
     rapid updating of the routing information database for a previously
     failed route server following the route server's reconnection to the
     Internet.

   - To minimize user service disruption following a
     failure in the primary path, policy gateways attempt local path
     repair immediately after detecting a connectivity failure.
     Moreover, path agents may maintain standby alternate paths that can
     become the primary path if necessary.

   - Policy gateways within a domain continuously monitor domain
     connectivity and hence can detect and identify domain partitions.
     Moreover, IDPR can continue to operate properly in the presence of
     partitioned domains.





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4.5.1.  Path Repair

   Failure of one or more entities on a given policy route may render
   the route unusable.  If the failure is within a domain, IDPR relies
   on the intra-domain routing procedure to find an alternate route
   across the domain, which leaves the path unaffected.  If the failure
   is in a virtual gateway, policy gateways must bear the responsibility
   of repairing the path.  Policy gateways nearest to the failure are
   the first to recognize its existence and hence can react most quickly
   to repair the path.

   Relinquishing control over path repair to policy gateways in other
   domains may be unacceptable to some domain administrators.  The
   reason is that these policy gateways cannot guarantee construction of
   a path that satisfies the source policies of the source domain, as
   they have no knowledge of other domains' source policies.

   Nevertheless, limited local path repair is feasible, without
   distributing either source policy information throughout the Internet
   or detailed path information among policy gateways in a domain or in
   a virtual gateway.  We say that a path is "locally repairable" if
   there exists an alternate route between two policy gateways,
   separated by at most one policy gateway, on the path.  This
   definition covers path repair in the presence of failed routes
   between consecutive policy gateways as well as failed policy gateways
   themselves.

   A policy gateway attempts local path repair, proceeding in the
   forward direction of the path, upon detecting that the next policy
   gateway on a path is no longer reachable.  The policy gateway must
   retain enough of the original path setup information to repair the
   path locally.  Using the path setup information, the policy gateway
   attempts to locate a route around the unreachable policy gateway.
   Specifically, the policy gateway attempts to establish contact with
   either:

   - A peer of the unreachable policy gateway.  In this case, the
     contacted policy gateway attempts to locate the next policy gateway
     following the unreachable policy gateway, on the original path.

   - A peer of itself, if the unreachable policy gateway is an adjacent
     policy gateway and if the given policy gateway no longer has direct
     connections to any adjacent policy gateways.  In this case, the
     contacted policy gateway attempts to locate a peer of the
     unreachable policy gateway, which in turn attempts to locate the
     next policy gateway following the unreachable policy gateway, on the
     original path.




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   If it successfully reaches the next policy gateway, the contacted
   policy gateway informs the requesting policy gateway.  In this case,
   the requesting, contacted, and next policy gateways update their
   forwarding information databases to conform to the new part of the
   path.  If it does not successfully reach the next policy gateway, the
   contacted policy gateway initiates teardown of the original path; in
   this case, the source path agent is responsible for finding a new
   route to the destination.

4.5.2.  Partitions

   A "domain partition" exists whenever there are at least two entities
   within the domain that can no longer communicate over any intra-
   domain route.  Domain partitions not only disrupt intra-domain
   communication but also may interfere with inter-domain communication,
   particularly when the partitioned domain is a transit domain.
   Therefore, we have designed the IDPR architecture to permit effective
   use of partitioned domains and hence maximize Internet connectivity
   in the presence of domain partitions.

   When a domain is partitioned, it becomes a set of multiple distinct
   "components".  A domain component is a subset of the domain's
   entities such that all entities within the subset are mutually
   reachable via intra-domain routes, but no entities in the complement
   of the subset are reachable via intra-domain routes from entities
   within the subset.  Each domain component has a unique identifier,
   namely the identifier of the domain together with the ordinal number
   of the lowest-numbered operational policy gateway within the domain
   component.  No negotiation among policy gateways is necessary to
   determine the domain component's lowest-numbered operational policy
   gateway.  Instead, within each domain component, all policy gateway
   members discover mutual reachability through intra-domain
   reachability information.  Therefore, all members have a consistent
   view of which is the lowest-numbered operational policy gateway in
   the component.

   IDPR entities can detect and compensate for all domain partitions
   that isolate at least two groups of policy gateways from each other.
   They cannot, however, detect any domain partition that isolates
   groups of hosts only.  Note that a domain partition may segregate
   portions of a virtual gateway, such that peer policy gateways lie in
   separate domain components.  Although itself partitioned, the virtual
   gateway does not assume any additional identities.  However, from the
   perspective of the adjacent domain, the virtual gateway now connects
   to two separate domain components.

   Policy gateways use partition information to select routes across
   virtual gateways to the correct domain components.  They also



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   distribute partition information to route servers as part of the IDPR
   routing information.  Thus, route servers know which domains are
   partitioned.  However, route servers do not know which hosts reside
   in which components of a partitioned domain; tracking this
   information would require extensive computation and communication.
   Instead, when a route server discovers that the destination of a
   requested route is a partitioned domain, it attempts to generate a
   suitable policy route to each component of the destination domain.
   Generation of multiple routes, on detection of a partitioned
   destination domain, maximizes the chances of obtaining at least one
   policy route that can be used for communication between the source
   and destination hosts.







































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   5.  References

   [1]  Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
        Backbone", RFC 1092, February 1989.

   [2]  Clark, D., "Policy Routing in Internet Protocols", RFC 1102, May
        1989.

   [3]  Braun, H-W., "Models of Policy Based Routing", RFC 1104, June
        1989.

   [4]  Leiner, B., "Policy Issues in Interconnecting Networks", RFC
        1124, September 1989.

   [5]  Estrin, D., "Requirements for Policy Based Routing in the
        Research Internet", RFC 1125, November 1989.

   [6]  Little, M., "Goals and Functional Requirements for Inter-
        Autonomous System Routing", RFC 1126, July 1989.

   [7]  Honig, J., Katz, D., Mathis, M., Rekhter, Y., and Yu, J.,
        "Application of the Border Gateway Protocol in the Internet",
        RFC 1164, June 1990.

   [8]  Lougheed, K. and Rekhter, Y., "A Border Gateway Protocol 3
        (BGP-3)", RFC 1267, October 1991.

   [9]  Rekhter, Y. and Li, T. Editors, "A Border Gateway Protocol 4
        (BGP-4)", Work in Progress, September 1992.

   [10] ISO, "Information Processing Systems - Telecommunications and
        Information Exchange between Systems - Protocol for Exchange of
        Inter-domain Routeing Information among Intermediate Systems to
        Support Forwarding of ISO 8473 PDUs", ISO/IEC DIS 10747, August
        1992.

   [11] Perlman, R., "Network Layer Protocols with Byzantine Robust-
        ness", Ph.D. Thesis, Department of Electrical Engineering and
        Computer Science, MIT, August 1988.

   [12] Estrin, D. and Tsudik, G., "Secure Control of Transit Internet-
        work Traffic", TR-89-15, Computer Science Department, University
        of Southern California.

   [13] Garcia-Luna-Aceves, J.J., "A Unified Approach for Loop-Free
        Routing using Link States or Distance Vectors", ACM Computer
        Communication Review, Vol. 19, No. 4, SIGCOMM 1989, pp. 212-223.




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   [14] Zaumen, W.T. and Garcia-Luna-Aceves, J.J., "Dynamics of Distri-
        buted Shortest-Path Routing Algorithms", ACM Computer Communica-
        tion Review, Vol. 21, No. 4, SIGCOMM 1991, pp. 31-42.

6.  Security Considerations

        Refer to section 3.3 for details on security in IDPR.

7.  Author's Address

        Martha Steenstrup
        BBN Systems and Technologies
        10 Moulton Street
        Cambridge, MA 02138

        Phone: (617) 873-3192
        Email: msteenst@bbn.com


































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