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rfc:rfc1478

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.

Steenstrup [Page 1] RFC 1478 IDPR Architecture June 1993

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] RFC 1478 IDPR Architecture June 1993

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:
  1. Access restrictions: e.g., applied to traffic to or from certain

domains or classes of users.

  1. Quality: e.g., delay, throughput, or error characteristics.
  1. 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:
  1. Access restrictions: e.g., domains to favor or avoid in routes.
  1. Quality: e.g., acceptable delay, throughput, or reliability.
  1. Monetary cost: e.g., acceptable session cost.

Steenstrup [Page 3] RFC 1478 IDPR Architecture June 1993

 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.

Steenstrup [Page 4] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.

Steenstrup [Page 5] RFC 1478 IDPR Architecture June 1993

  1. 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:
  1. 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:
  1. 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.

Steenstrup [Page 6] RFC 1478 IDPR Architecture June 1993

  1. Source policies must be made public. Thus, a domain must divulge

potentially private information.

  1. 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:
  1. Each domain has complete control over policy route generation from

the perspective of itself as source.

Steenstrup [Page 7] RFC 1478 IDPR Architecture June 1993

  1. 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.
  1. 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:
  1. Includes resource access restrictions.
  1. 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.

Steenstrup [Page 8] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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

Steenstrup [Page 9] RFC 1478 IDPR Architecture June 1993

 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:
  1. A domain has complete control over the distribution of its own

routing information.

  1. 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

Steenstrup [Page 10] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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.
  1. 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.
  1. 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.

Steenstrup [Page 11] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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

Steenstrup [Page 12] RFC 1478 IDPR Architecture June 1993

 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:
  1. Collecting and distributing routing information including domain

transit policies and inter-domain connectivity.

  1. Generating and selecting policy routes based on the routing

information distributed and on the source policies configured or

   requested.
  1. Setting up paths across the Internet using the policy routes

generated.

  1. Forwarding messages across and between domains along the established

paths.

  1. 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

Steenstrup [Page 13] RFC 1478 IDPR Architecture June 1993

 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:
  1. A reduction in the amount of IDPR routing information that must be

distributed and maintained throughout the Internet.

  1. An increase in the reliability of IDPR routes through redundancy of

physical connections between domains.

  1. An opportunity for load sharing of IDPR traffic among policy

gateways.

 Several different entities are responsible for performing the IDPR
 functions:
  1. Policy gateways collect and distribute routing information,

participate in path setup, forward data messages along established

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

up and manage paths, and to maintain forwarding information

   databases.
  1. Special-purpose servers maintain all other IDPR databases as

follows:

Steenstrup [Page 14] RFC 1478 IDPR Architecture June 1993

    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.

Steenstrup [Page 15] RFC 1478 IDPR Architecture June 1993

 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.

Steenstrup [Page 16] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 17] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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.
  1. 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.
  1. 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

Steenstrup [Page 18] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 19] RFC 1478 IDPR Architecture June 1993

 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:
  1. Promote timely delivery of control information.
  1. Minimize acceptance and distribution of corrupted control

information.

  1. Verify authenticity of a source of control information.
  1. 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:
  1. Recognized message version.
  1. Consistent message length.
  1. 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

Steenstrup [Page 20] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 21] RFC 1478 IDPR Architecture June 1993

 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:
  1. All intra-domain traffic flows. Intra-domain forwarding information

is integrated into the forwarding database as soon as it is received.

  1. 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.
  1. 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,

Steenstrup [Page 22] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 23] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 24] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 25] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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

Steenstrup [Page 26] RFC 1478 IDPR Architecture June 1993

   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:
  1. 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.
  1. 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.
  1. Route servers should cache the results of route generation, in order

to minimize the computation associated with responding to future

   route requests.
  1. To handle requested service limits, a route server should always

select the first route generated that satisfies all of the requested

   service limits.
  1. 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.
  1. 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

Steenstrup [Page 27] RFC 1478 IDPR Architecture June 1993

   evaluating those routes as described in the multi-criteria
   optimization case above.
  1. 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:
  1. 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.
  1. 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.

Steenstrup [Page 28] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 29] RFC 1478 IDPR Architecture June 1993

 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:
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

Steenstrup [Page 30] RFC 1478 IDPR Architecture June 1993

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:
  1. 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.
  1. 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.

Steenstrup [Page 31] RFC 1478 IDPR Architecture June 1993

 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

Steenstrup [Page 32] RFC 1478 IDPR Architecture June 1993

 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.

Steenstrup [Page 33] RFC 1478 IDPR Architecture June 1993

 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.

Steenstrup [Page 34] RFC 1478 IDPR Architecture June 1993

 [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

Steenstrup [Page 35]

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