Internet DRAFT - draft-iab-bgparch


Internet Architecture Board                           G. Huston, Editor
Internet Draft                              Internet Architecture Board
Document: draft-iab-bgparch-02.txt                       September 2001
Category: Informational

                              Commentary on
                   Inter-Domain Routing in the Internet

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
     all provisions of Section 10 of RFC 2026 [1].

     Internet-Drafts are working documents of the Internet Engineering
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     The list of current Internet-Drafts can be accessed at

     The list of Internet-Draft Shadow Directories can be accessed at

     This document incorporates comments received on the -01 draft
     version from members of the IAB and IESG.


     This document examines the various longer term trends visible within
     the characteristics of the Internet's BGP table and identifies a
     number of operational practices and protocol factors that contribute
     to these trends. The potential impacts of these practices and
     protocol properties on the scaling properties of the inter-domain
     routing space are examined.

     These impacts include the potential for exhaustion of the existing
     Autonomous System number space, increasing convergence times for
     selection of stable alternate paths following withdrawal of route
     announcements, the stability of table entries, and the average
     prefix length of entries in the BGP table. The larger long term
     issue is that of an increasingly denser inter-connectivity mesh
     between ASes, causing a finer degree of granularity of inter-domain

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     policy and finer levels of control to undertake inter-domain traffic

     Various approaches to a refinement of the inter-domain routing
     protocol and associated operating practices that may provide
     superior scaling properties are identified as an area for further

     This document is the outcome of a collaborative exercise on the part
     of the Internet Architecture Board.

Table of Contents

  1  Network Scaling and Inter-Domain Routing  .......................  2
  2  Measurements of the total size of the BGP Table  ................  4
  3  Related Measurements derived from BGP Table  ....................  7
  4  Current State of inter-AS routing in the Internet  .............. 11
  5  Future Requirements for the Exterior Routing System  ............ 13
  6  Architectural Approaches to a scalable Exterior Routing Protocol 15
  7  Directions for Further Activity  ................................ 20
  8  Security Considerations  ........................................ 22
  9  References  ..................................................... 22
10  Acknowledgements  ............................................... 23
11  Author  ......................................................... 23
12  Full Copyright Statement  ....................................... 25

1. Network Scaling and Inter-Domain Routing

     Are there inherent scaling limitations in the technology of the
     Internet or its architecture of deployment that may impact on the
     ability of the Internet to meet escalating levels of demand? There
     are a number of potential areas to search for such limitations.
     These include the capacity of transmission systems, packet switching
     capacity, the continued availability of protocol addresses, and the
     capability of the routing system to produce a stable view of the
     overall topology of the network. In this study we will look at this
     latter capability with the objective of identifying some aspects of
     the scaling properties of the Internet's routing system.

     The basic structure of the Internet is a collection of networks, or
     Autonomous Systems (ASes) that are interconnected to form a
     connected domain. Each AS uses an interior routing system to
     maintain a coherent view of the topology within the AS, and uses an
     exterior routing system to maintain adjacency information with
     neighboring ASes to create a view of the connectivity of the entire

     This network-wide connectivity is described in the routing table

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     used by the BGP4 protocol (referred to as the Routing Information
     Base, or RIB). Each entry in the table refers to a distinct route.
     The attributes of the route, together with local policy constraints,
     are used to determine the best path from the local AS to the AS that
     is originating the route. Determining the 'best path' in this case
     is determining which routing advertisement and associated next hop
     address is the most preferred by the local AS. Within each local
     BGP-speaking router this preferred route is then loaded into the
     local RIB (Loc-RIB). This information is coupled with information
     obtained from the local instance of the interior routing protocol to
     form a Forwarding Information Base (or FIB), for use by the local
     router's forwarding engine.

     The BGP routing system is not aware of finer level of topology of
     the network on a link-by-link basis within the local AS or within
     any remote AS. From this perspective BGP can be seen as an inter-AS
     connectivity maintenance protocol, as distinct from a link-level
     topology management protocol, and the BGP routing table can be
     viewed as a description of the current connectivity of the Internet
     using an AS as the basic element of connectivity computation.

     There is an associated dimension of policy determination within the
     routing table. If an AS advertises a route to a neighboring AS, the
     local AS is offering to accept traffic from the neighboring AS which
     is ultimately destined to addresses described by the advertised
     routing entry. If the local AS does not originate the route, then
     the inference is that the local AS is willing to undertake the role
     of transit provider for this traffic on behalf of some third party.
     Similarly, an AS may or may not choose to accept a route from a
     neighbor. Accepting a route implies that under some circumstances,
     as determined by the local route selection parameters, the local AS
     will use the neighboring AS to reach addresses spanned by the route.
     The BGP routing domain is intended to maintain a coherent view of
     the connectivity of the inter-AS domain, where connectivity is
     expressed as a preference for 'shortest paths' to reach any
     destination address as modulated by the connectivity policies
     expressed by each AS, and coherence is expressed as a global
     constraint that none of the paths contains loops or dead ends. The
     elements of the BGP routing domain are routing entries, expressed as
     a span of addresses. All addresses advertised within each routing
     entry share a common origin AS and a common connectivity policy. The
     total size of the BGP table is therefore a metric of the number of
     distinct routes within the Internet, where each route describes a
     contiguous set of addresses that share a common origin AS and a
     common reachability policy.

     When the scaling properties of the Internet were studied in the
     early 1990s two critical factors identified in the study were, not

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     surprisingly, routing and addressing [2]. As more devices connect to
     the Internet they consume addresses, and the associated function of
     maintaining reachability information for these addresses, with an
     assumption of an associated growth in the number of distinct
     provider networks and the number of distinct connectivity policies,
     implies ever larger routing tables. The work in studying the
     limitations of the 32 bit IPv4 address space produced a number of
     outcomes, including the specification of IPv6 [3], as well as the
     refinement of techniques of network address translation [4] intended
     to allow some degree of transparent interaction between two networks
     using different address realms. Growth in the routing system is not
     directly addressed by these approaches, as the routing space is the
     cross product of the complexity of the inter-AS topology of the
     network, multiplied by the number of distinct connectivity policies
     multiplied by the degree of fragmentation of the address space. For
     example, use of NAT may reduce the pressure on the number of public
     addresses required by a single connected network, but it does not
     necessarily imply that the network's connectivity policies can be
     subsumed within the aggregated policy of a single upstream provider.

     When an AS advertises a block of addresses into the exterior routing
     space this entry is generally carried across the entire exterior
     routing domain of the Internet. To measure the common
     characteristics of the global routing table, it is necessary to
     establish a point in the default-free part of the exterior routing
     domain and examine the BGP routing table that is visible at that

2. Measurements of the total size of the BGP Table

     Measurements of the size of the routing table were somewhat sporadic
     to start, and a number of measurements were taken at approximate
     monthly intervals from 1988 until 1992 by Merit [5]. This effort was
     resumed in 1994 by Erik-Jan Bos at Surfnet in the Netherlands, who
     commenced measuring the size of the BGP table at hourly intervals in
     1994. This measurement technique was adopted by the author in 1997,
     using a measurement point located at the edge of AS 1221 at Telstra
     in Australia, again using an hourly interval for the measurement.
     The initial measurements were of the number of routing entries
     contained within the set of selected best paths. These measurements
     were expanded to include the number of AS numbers, number of AS
     paths, and a set of measurements relating to the prefix size of
     routing table entries.

     This data  contains a view of the dynamics of the Internet's routing
     table growth that spans some 13 years in total and includes a very
     detailed view spanning the most recent seven years [6]. Looking at

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     just the total size of the BGP routing table over this period, it is
     possible to identify four distinct phases of inter-AS routing
     practice in the Internet.

2.1 Pre-CIDR Growth

     The initial characteristics of the routing table size from 1988
     until April 1994 show definite characteristics of exponential
     growth. If continued unchecked, this growth would have lead to
     saturation of the available BGP routing table space in the non-
     default routers of the time within a small number of years.

     Estimates of the time at which this would've happened varied
     somewhat from study to study, but the overall general theme of these
     observations was that the growth rates of the BGP routing table were
     exceeding the growth in hardware and software capability of the
     deployed network, and that at some point in the mid-1990's, the BGP
     table size would have grown to the point where it was larger than
     the capabilities of available equipment to support.

2.2 CIDR Deployment

     The response from the engineering community was the introduction of
     a hierarchy into the inter-domain routing system. The intent of the
     hierarchical routing structure was to allow a provider to merge the
     routing entries for its customers into a single routing entry that
     spanned its entire customer base. The practical aspects of this
     change was the introduction of routing protocols that dispensed with
     the requirement for the Class A, B and C address delineation,
     replacing this scheme with a routing system that carried an address
     prefix and an associated prefix length. This approached was termed
     Classless Inter-Domain Routing (CIDR) [5].

     A concerted effort was undertaken in 1994 and 1995 to deploy CIDR
     routing in the Internet, based on encouraging deployment of the
     CIDR-capable version of the BGP protocol, BGP4 [7].

     The intention of CIDR was one of hierarchical provider address
     aggregation, where a network provider was allocated an address block
     from an address registry, and the provider announced this entire
     block into the exterior routing domain as a single entry with a
     single routing policy. Customers of the provider were encouraged to
     use a sub-allocation from the provider's address block, and these
     smaller routing elements were aggregated by the provider and not
     directly passed into the exterior routing domain. During 1994 the
     size of the routing table remained relatively constant at some
     20,000 entries as the growth in the number of providers announcing

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     address blocks was matched by a corresponding reduction in the
     number of address announcements as a result of CIDR aggregation.

2.3 CIDR Growth

     For the next four years until the start of 1998, CIDR proved
     effective in damping unconstrained growth in the BGP routing table.
     During this period, the BGP table grew at an approximate linear
     rate, adding some 10,000 entries per year.

     A close examination of the table reveals a greater level of
     stability in the routing system at this time. The short term
     (hourly) variation in the number of announced routes reduced, both
     as a percentage of the number of announced routes, and also in
     absolute terms. One of the other benefits of using large aggregate
     address blocks is that instability at the edge of the network is not
     immediately propagated into the routing core. The instability at the
     last hop is absorbed at the point where an aggregate route is used
     in place of a collection of more specific routes. This, coupled with
     widespread adoption of BGP route flap damping, was very effective in
     reducing the short term instability in the routing space during this

2.4 Current Growth

     In late 1998 the trend of growth in the BGP table size changed
     radically, and the growth for the period 1998 - 2000 is again
     showing all the signs of a re-establishment of a growth trend with
     strong correlation to an exponential growth model. This change in
     the growth trend appears to indicate that pressure to use
     hierarchical address allocations and CIDR has been unable to keep
     pace with the levels of growth of the Internet, and some additional
     factors that impact the growth in the BGP table size have become
     more prominent in the Internet. This has lead to a growth pattern in
     the total size of the BGP table that has more in common with a
     compound growth model than a linear model. A good fit of the data
     for the period from January 1999 until December 2000 is a compound
     growth model of 42% growth per year.

     An initial observation is that this growth pattern points to some
     weakening of the hierarchical model of connectivity and routing
     within the Internet. To identify the characteristics of this recent
     trend it is necessary to look at a number of related characteristics
     of the routing table.

     BGP table size data for the first half of 2001 shows different
     trends at various measurement points in the Internet. Some
     measurement points where the local AS has a relative larger number

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     of more specific routes show a steady state for the first half of
     2001 with no appreciable growth, while other measurement points
     where the local AS has had a lower number of more specific routes
     initially show a continuation of table size growth. There are a
     number of commonly observed discontinuities in the data for 2001,
     corresponding to events where a significant number of more specific
     entries have been replaced by an encompassing aggregate prefix.

3. Related Measurements derived from BGP Table

     The level of analysis of the BGP routing table has been extended in
     an effort to identify the factors contributing to this growth, and
     to determine whether this leads to some limiting factors in the
     potential size of the routing space. Analysis includes measuring the
     number of ASes in the routing system, and the number of distinct AS
     paths, the range of addresses spanned by the table and average span
     of each routing entry.

3.1 AS Number Consumption

     Each network that is multi-homed within the topology of the Internet
     and wishes to express a distinct external routing policy must use a
     unique AS number to associate its advertised addresses with such a
     policy. In general, each network is associated with a single AS, and
     the number of ASes in the default-free routing table tracks the
     number of entities that have unique routing policies. There are some
     exceptions to this, including large global transit providers with
     varying regional policies, where multiple ASes are associated with a
     single network, but such exceptions are relatively uncommon.

     The number of unique ASes present in the BGP table has been tracked
     since late 1996, and the trend of AS number deployment over the past
     four years is also one that matches a compound growth model with a
     growth rate of 51% per year. As of the start of May 2001 there were
     some 10,700 ASes visible in the BGP table. At a continued rate of
     growth of 51% p.a., the 16 bit AS number space will be fully
     deployed by August 2005. Work is underway within the IETF to modify
     the BGP protocol to carry AS numbers in a 32-bit field. [8] While
     the protocol modifications are relatively straightforward, the major
     responsibility rests with the operations community to devise a
     transition plan that will allow gradual transition into this larger
     AS number space.

3.2 Address Consumption

     It is also possible to track the total amount of address space
     advertised within the BGP routing table. At the start of 2001 the

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     routing table encompassed 1,081,131,733 addresses, or some 25.17% of
     the total IPv4 address space, or 25.4% of the useable unicast public
     address space. By September 2001 this has growth to 1,123,124,472
     addresses, or some 26% of the IPv4 address space. This has grown
     from 1,019,484,655 addresses in November 1999. However, there are a
     number of /8 prefixes that are periodically announced and withdrawn
     from the BGP table, and if the effects of these prefixes is removed,
     a compound growth model against the previous 12 months of data of
     this metric yields a best fit model of growth of 7% per year in the
     total number of addresses spanned by the routing table.

     Compared to the 42% growth in the number of routing advertisements,
     the growth in the amount of address space advertised is far lower.
     One possible explanation is that much of the growth of the Internet
     in terms of growth in the number of connected devices is occurring
     behind various forms of NAT gateways. In terms of solving the
     perceived finite nature of the address space identified just under a
     decade ago, this explanation would tend to indicate that the
     Internet appears so far to have embraced the approach of using NATs,
     irrespective of their various perceived functional shortcomings. [9]
     This explanation also supports the observation of smaller address
     fragments supporting distinct policies in the BGP table, as such
     small address blocks may encompass arbitrarily large networks
     located behind one or more NAT gateways. There are alternative
     explanations of this difference between the growth of the table and
     the growth of address space, including a trend towards discrete
     exterior routing policies being applied to finer address blocks.

3.3 Granularity of Table Entries

     The intent of CIDR aggregation was to support the use of large
     aggregate address announcements in the BGP routing table. To confirm
     whether this is still the case the average span of each BGP
     announcement has been tracked for the past 12 months. The data
     indicates a decline in the average span of a BGP advertisement from
     16,000 individual addresses in November 1999 to 12,100 in December
     2000. As of September 2001 this span has been further reduced to an
     average 10,700 individual addresses per routing entry. This
     corresponds to an increase in the average prefix length from /18.03
     to /18.44 by December 2000 and a /18.6 by September 2001. Separate
     observations of the average prefix length used to route traffic in
     operation networks in late 2000 indicate an average length of 18.1
     [11]. This trend towards finer-grained entries in the routing table
     is potentially cause for concern, as it implies the increasing
     spread of traffic over greater numbers of increasingly smaller
     forwarding table entries. This, in turn, has implications for the
     design of high speed core routers, particularly when extensive use
     is made of a small number of very high speed cached forwarding

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     entries within the switching subsystem of a router's design.

     A similar observation can be made regarding the number of addresses
     advertised per AS. In December 1999 each AS advertised an average of
     161,900 addresses (equivalent to a prefix length of /14.69, and in
     January 2001 this average has fallen to 115,800 addresses, an
     equivalent prefix length of /15.18.

     This points to increasingly finer levels of routing detail being
     announced into the global routing domain. This, in turn, supports
     the observation that the efficiencies of hierarchical routing
     structures are no longer being fully realized within the deployed
     Internet. Instead, increasingly finer levels of routing detail are
     being announced globally in the BGP tables. The most likely cause of
     this trend of finer levels of routing granularity is an increasingly
     dense interconnection mesh, where more networks are moving from a
     single-homed connection with hierarchical addressing and routing
     into multi-homed connections without any hierarchical structure. The
     spur for this increasingly dense connectivity mesh in the Internet
     may well be the declining unit costs of communications bearer
     services coupled with a common perception that richer sets of
     adjacencies yields greater levels of service resilience.

3.4 Prefix Length Distribution

     In addition to looking at the average prefix length, the analysis of
     the BGP table also includes an examination of the number of
     advertisements of each prefix length.

     An extensive program commenced in the mid-nineties to move away from
     intense use of the Class C space and to encourage providers to
     advertise larger address blocks, as part of the CIDR effort. This
     has been reinforced by the address registries who have used provider
     allocation blocks that correspond to a prefix length of /19 and,
     more recently, /20.

     These measures were introduced in the mid-90's when there were some
     20,000 - 30,000 entries in the BGP table. Some six years later in
     April 2001 it is interesting to note that of the 108,000 entries in
     the routing table, some 59,000 entries have a /24 prefix. In
     absolute terms the /24 prefix set is the fastest growing set in the
     BGP routing table. The routing entries of these smaller address
     blocks also show a much higher level of change on an hourly basis.
     While a large number of BGP routing points perform route flap
     damping, nevertheless there is still a very high level of
     announcements and withdrawals of these entries in this particular
     area of the routing table when viewed using a perspective of route
     updates per prefix length. Given that the numbers of these small

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     prefixes are growing rapidly, there is cause for some concern that
     the total level of BGP flux, in terms of the number of announcements
     and withdrawals per second may be increasing, despite the pressures
     from flap damping. This concern is coupled with the observation
     that, in terms of BGP stability under scaling pressure, it is not
     the absolute size of the BGP table that is of prime importance, but
     the rate of dynamic path recomputations that occur in the wake of
     announcements and withdrawals. Withdrawals are of particular concern
     due to the number of transient intermediate states that the BGP
     distance vector algorithm explores in processing a withdrawal.
     Current experimental observations indicate a typical convergence
     time of some 2 minutes to propagate a route withdrawal across the
     BGP domain. [10]

     An increase in the density of the BGP mesh, coupled with an increase
     in the rate of such dynamic changes, does have serious implications
     in maintaining the overall stability of the BGP system as it
     continues to grow. The registry allocation policies also have had
     some impact on the routing table prefix distribution. The original
     registry practice was to use a minimum allocation unit of a /19, and
     the 10,000 prefix entries in the /17 to /19 range are a consequence
     of this policy decision. More recently, the allocation policy now
     allows for a minimum allocation unit of a /20 prefix, and the /20
     prefix is used by some 4,300 entries as of January 2001, and in
     relative terms is one of the fastest growing prefix sets. The number
     of entries corresponding to very small address blocks (smaller than
     a /24), while small in number as a proportion of the total BGP
     routing table, is the fastest growing in relative terms. The number
     of /25 through /32 prefixes in the routing table is growing faster,
     in terms of percentage change, than any other area of the routing
     table. If prefix length filtering were in widespread use, the
     practice of announcing a very small address block with a distinct
     routing policy would have no particular beneficial outcome, as the
     address block would not be passed throughout the global BGP routing
     domain and the propagation of the associated policy would be limited
     in scope. The growth of the number of these small address blocks,
     and the diversity of AS paths associated with these routing entries,
     points to a relatively limited use of prefix length filtering in
     today's Internet. In the absence of any corrective pressure in the
     form of widespread adoption of prefix length filtering, the very
     rapid growth of global announcements of very small address blocks is
     likely to continue. In percentage terms, the set of prefixes
     spanning /25 to /32 show the largest growth rates.

3.5 Aggregation and Holes

     With the CIDR routing structure it is possible to advertise a more
     specific prefix of an existing aggregate. The purpose of this more

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     specific announcement is to punch a 'hole' in the policy of the
     larger aggregate announcement, creating a different policy for the
     specifically referenced address prefix.

     Another use of this mechanism is to perform a rudimentary form of
     load balancing and mutual backup for multi-homed networks. In this
     model a network may advertise the same aggregate advertisement along
     each connection, but then advertise a set of specific advertisements
     for each connection, altering the specific advertisements such that
     the load on each connection is approximately balanced. The two forms
     of holes can be readily discerned in the routing table - while the
     approach of policy differentiation uses an AS path that is different
     from the aggregate advertisement, the load balancing and mutual
     backup configuration uses the same As path for both the aggregate
     and the specific advertisements. While it is difficult to understand
     whether the use of such more specific advertisements was intended to
     be an exception to a more general rule or not within the original
     intent of CIDR deployment, there appears to be very widespread use
     of this mechanism within the routing table. Some 59,000
     advertisements, or 55% of the total number of routing table entries,
     are being used to punch policy holes in existing aggregate
     announcements. Of these the overall majority of some 42,000 routes
     use distinct AS paths, so that it does appear that this is evidence
     of finer levels of granularity of connection policy in a densely
     interconnected space. While long term data is not available for the
     relative level of such advertisements as a proportion of the full
     routing table, the growth level does strongly indicate that policy
     differentiation at a fine level within existing provider aggregates
     is a significant driver of overall table growth.

4. Current State of inter-AS routing in the Internet

     The resumption of compound growth trends within the BGP table, and
     the associated aspects of finer granularity of routing entries
     within the table form adequate grounds for consideration of
     potential refinements to the Internet's exterior routing protocols
     and potential refinements to current operating practices of inter-AS
     connectivity. With the exception of the 16 bit AS number space,
     there is no particular finite limit to any aspect of the BGP table.
     The motivation for such activity is that a long term pattern of
     continued growth at current rates may once again pose a potential
     condition where the capacity of the available processors may be
     exceeded by some aspect of the Internet routing table.

4.1 A denser interconnectivity mesh

     The decreasing unit cost of communications bearers in many part of
     the Internet is creating a rapidly expanding market in exchange

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     points and other forms of inter-provider peering. A model of
     extensive interconnection at the edges of the Internet is rapidly
     supplanting the deployment model of a single-homed network with a
     single upstream provider. The underlying deployment model of CIDR
     was that of a single-homed network, allowing for a strict hierarchy
     of supply providers. The business imperatives driving this denser
     mesh of interconnection in the Internet are substantial, and the
     casualty in this case is the CIDR-induced dampened growth of the BGP
     routing table.

4.2 Multi-Homed small networks and service resiliency

     It would appear that one of the major drivers of the recent growth
     of the BGP table is that of small networks, advertised as a /24
     prefix entry in the routing table, multi-homing with a number of
     peers and upstream providers. In the appropriate environment where
     there are a number of networks in relatively close proximity, using
     peer relationships can reduce total connectivity costs, as compared
     to using a single upstream service provider. Equally significantly,
     multi-homing with a number of upstream providers is seen as a means
     of improving the overall availability of the service. In essence,
     multi-homing is seen as an acceptable substitute for upstream
     service resiliency. This has a potential side effect that when
     multi-homing is seen as a preferable substitute for upstream
     provider resiliency, the upstream provider cannot command a price
     premium for proving resiliency as an attribute of the provided
     service, and therefore has little economic incentive to spend the
     additional money required to engineer resiliency into the network.
     The actions of the network's multi-homed clients then become self-
     fulfilling. One way to characterize this behavior is that service
     resiliency in the Internet is becoming the responsibility of the
     customer, not the service provider.

     In such an environment resiliency still exists, but rather than
     being a function of the bearer or switching subsystem, resiliency is
     provided through the function of the BGP routing system. The
     question is not whether this is feasible or desirable in the
     individual case, but whether the BGP routing system can scale
     adequately to continue to undertake this role.

4.3 Traffic Engineering via Routing

     Further driving this growth in the routing table is the use of
     selective advertisement of smaller prefixes along different paths in
     an effort to undertake traffic engineering within a multi-homed
     environment. While there is considerable effort being undertaken to
     develop traffic engineering tools within a single network using MPLS
     as the base flow management tool, inter-provider tools to achieve

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     similar outcomes are considerably more complex when using such
     switching techniques.

     At this stage the only tool being used for inter-provider traffic
     engineering is that of the BGP routing table. Such use of BGP
     appears to place additional fine-grained prefixes into the routing
     table. This action further exacerbates the growth and stability
     pressures being placed on the BGP routing domain.

4.4 Lack of Common Operational Practices

     There is considerable evidence of a lack of uniformity of
     operational practices within the inter-domain routing space. This
     includes the use and setting of prefix filters, the use and setting
     of route damping parameters and level of verification undertaken on
     BGP advertisements by both the advertiser and the recipient. There
     is some extent of 'noise' in the routing table where advertisements
     appear to be propagated well beyond their intended domain of
     applicability, and also where withdrawals and advertisements are not
     being adequately damped close to the origin of the route flap. This
     diversity of operating practices also extends to policies of
     accepting advertisements that are more specific advertisements of
     existing provider blocks.

4.5 CIDR and Hierarchical Routing

     The current growth factors at play in the BGP table are not easily
     susceptible to another round of CIDR deployment pressure within the
     operator community. The denser interconnectivity mesh, the
     increasing use of multi-homing with smaller address prefixes, the
     extension of the use of BGP to perform roles related to inter-domain
     traffic engineering and the lack of common operating practices all
     point to a continuation of the trend of growth in the total size of
     the BGP routing table, with this growth most apparent with
     advertisements of smaller address blocks, and an increasing trend
     for these small advertisements to be punching a connectivity policy
     'hole' in an existing provider aggregate advertisement.

     It may be appropriate to consider how to operate an Internet with a
     BGP routing table that has millions of small entries, rather than
     the expectation of a hierarchical routing space with at most tens of
     thousands of larger entries in the global routing table.

5. Future Requirements for the Exterior Routing System

     It is beyond the scope of this document to define a scalable inter-
     domain routing environment and associated routing protocols and

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     operating practices. A more modest goal is to look at the attributes
     of routing systems as understood and identify those aspects of such
     systems that may be applicable to the inter-domain environment as a
     potential set of requirements for inter-domain routing tools.

5.1 Scalability

     The overall intent is scalability of the routing environment.
     Scalability can be expressed in many dimensions, including number of
     discrete network layer reachability entries, number of discrete
     route policy entries, level of dynamic change over a unit of time of
     these entries, time to converge to a coherent view of the
     connectivity of the network following changes, and so on.

     The basic objective behind this expressed requirement for
     scalability is that the most likely near to medium trend in the
     structure of the Internet is a continuation in the pattern of dense
     interconnectivity between a large number of discrete network
     entities, and little impetus behind hierarchical aggregating
     structures. It is not an objective to place any particular metrics
     on scalability within this examination of requirements, aside from
     indicating that a prudent view would encompass a scale of
     connectivity in the inter-domain space that is at least two orders
     of magnitude larger than comparable metrics of the current

5.2 Stability and Predictability

     Any routing system should behave in a stable and predictable
     fashion. What is inferred from the predictability requirement is the
     behavior that under identical environmental conditions the routing
     system should converge to the same state. Stability implies that the
     routing state should be maintained for as long as the environmental
     conditions remain constant. Stability also implies a qualitative
     property that minor variations in the network's state should not
     cause large scale instability across the entire network while a new
     stable routing state is reached. Instead, routing changes should be
     propagated only as far as necessary to reach a new stable state, so
     that the global requirement for stability implies some degree of
     locality in the behavior of the system.

5.3 Convergence

     Any routing system should have adequate convergence properties. By
     adequate it is implied that within a finite time following a change
     in the external environment, the routing system will have reached a
     shared common description of the network's topology that accurately
     describes the current state of the network and is stable. In this

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     case finite time implies a time limit that is bounded by some upper
     limit, and this upper limit reflects the requirements of the routing
     system. In the case of the Internet this convergence time is
     currently of the order of hundreds of seconds as an upper bound on
     convergence. This long convergence time is perceived as having a
     negative impact on various applications, particularly those that are
     time critical. A more useful upper bound for convergence is of the
     order of seconds or lower if it is desired to support a broad range
     of application classes.

     It is not a requirement to be able to undertake full convergence of
     the inter-domain routing system in the sub-second timescale.

5.4 Routing Overhead

     The greater the amount of information passed within the routing
     system, and the greater the frequency of such information exchanges,
     the greater the level of expectation that the routing system can
     maintain an accurate view of the connectivity of the network.
     Equally, the greater the amount of information passed within the
     routing system, and the higher the frequency of information
     exchange, the higher the level of overhead consumed by operation of
     the routing system. There is an element of design compromise in a
     routing system to pass enough information across the system to allow
     each routing element to have adequate local information to reach a
     coherent local view of the network, yet ensure that the total
     routing overhead is low.

6. Architectural approaches to a scalable Exterior Routing Protocol

     This document does not attempt to define an inter-domain routing
     protocol that possess all the attributes as listed above, but a
     number of architectural considerations can be identified that would
     form an integral part of the protocol design process.

6.1 Policy opaqueness vs. policy transparency

     The two major approaches to routing protocols are distance vector
     and link state.

     In the distance vector protocol a routing node gathers information
     from its neighbors, applies local policy to this information and
     then distributes this updated information to its neighbors. In this
     model the nature of the local policy applied to the routing
     information is not necessarily visible to the node's neighbors, and
     the process of converting received route advertisements into

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     advertised route advertisements uses a local policy process whose
     policy rules are not visible externally. This scenario can be
     described as 'policy opaque'. The side effect of such an environment
     is that a third party cannot remotely compute which routes a network
     may accept and which may be re-advertised to each neighbor.

     In link state protocols a routing node effectively broadcasts its
     local adjacencies, and the policies it has with respect to these
     adjacencies, to all nodes within the link state domain. Every node can
     perform an identical computation upon this set of adjacencies and
     associated policies in order to compute the local forwarding table.
     The essential attribute of this environment is that the routing node
     has to announce its routing policies, in order to allow a remote
     node to compute which routes will be accepted from which neighbor,
     and which routes will be advertised to each neighbor and what, if
     any, attributes are placed on the advertisement. Within an interior
     routing domain the local policies are in effect metrics of each link
     and these polices can be announced within the routing domain without
     any consequent impact.

     In the exterior routing domain it is not the case that
     interconnection policies between networks are always fully
     transparent. Various permutations of supplier / customer
     relationships and peering relationships have associated policy
     qualifications that are not publicly announced for business
     competitive reasons. The current diversity of interconnection
     arrangements appears to be predicated on policy opaqueness, and to
     mandate a change to a model of open interconnection policies may be
     contrary to operational business imperatives.

     An inter-domain routing tool should be able to support models of
     interconnection where the policy associated with the interconnection
     is not visible to any third party. If the architectural choice is a
     constrained one between distance vector and link state, then this
     consideration would appear to favour the continued use of a distance
     vector approach to inter-domain routing. This choice, in turn, has
     implications on the convergence properties and stability of the
     inter-domain routing environment. If there is a broader spectrum of
     choice, the considerations of policy-opaqueness would still apply.

6.2 The number of routing objects

     The current issues with the trend behaviors of the BGP space can be
     coarsely summarized as the growth in the number of distinct routing
     objects, the increased level of dynamic behaviors of these objects
     (in the form of announcements and withdrawals).

     This entails evaluating possible measures that can address the

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     growth rate in the number of objects in the inter-domain routing
     table, and separately examining measures that can reduce the level
     of dynamic change in the routing table. The current routing
     architecture defines a basic unit of a route object as an
     originating AS number and an address prefix.

     In looking at the growth rate in the number of route objects, the
     salient observation is that the number of route objects is the
     byproduct of the density of the interconnection mesh and the number
     of discrete points where policy is imposed of route objects. One
     approach to reduce the growth in the number of objects is to allow
     each object to describe larger segments of infrastructure. Such an
     approach could use a single route object to describe a set of
     address prefixes, or a collection of ASs, or a combination of the
     two. The most direct form of extension would be to preserve the
     assumption that each routing object represents an indivisible policy
     entity. However, given that one of the drivers of the increasing
     number of route objects is a proliferation of discrete route
     objects, it is not immediately apparent that this form of
     aggregation will prove capable in addressing the growth in the
     number of route objects.

     If single route objects are to be used that encompass a set of
     address prefixes and a collection of ASs, then it appears necessary
     to define additional attributes within the route object to further
     qualify the policies associated with the object in terms of specific
     prefixes, specific ASs and specific policy semantics that may be
     considered as policy exceptions to the overall aggregate

     Another approach to reduce the number of route objects is to reduce
     the scope of advertisement of each routing object, allowing the
     object to be removed and proxy aggregated into some larger object
     once the logical scope of the object has been reached. This approach
     would entail the addition of route attributes that could be used to
     define the circumstances where a specific route object would be
     subsumed by an aggregate route object without impacting the policy
     objectives associated with the original set of advertisements.

6.3 Inter-domain Traffic Engineering

     Attempting to place greater levels of detail into route objects is
     intended to address the dual role of the current BGP system as both
     an inter-domain connectivity maintenance protocol and as an implicit
     traffic engineering tool.

     In the current environment, advertisement of more specific prefixes
     with unique policy but with the same origin AS is often intended to

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     create a traffic engineering response, where incoming traffic to an
     AS may be balanced across multiple paths. The outcome is that the
     control of the relative profile of load is placed with the
     originating AS. The way this is achieved is by using limited
     knowledge of the remote AS's route selection policy to explicitly
     limit the number of egress choices available to a remote AS. The
     most common route selection policy is the preference for more
     specific prefixes over larger address blocks. By advertising
     specific prefixes along specific neighbor AS connections with
     specific route attributes, traffic destined to these addresses is
     passed through the selected transit paths. This limitation of choice
     allows the originating AS to override the potential policy choices
     of all other ASs, imposing its traffic import policies at a higher
     level than the remote AS's egress policies.

     An alternative approach is the use of a class of traffic engineering
     attributes that are attached to an aggregate route object. The
     intent of such attributes is to direct each remote AS to respond to
     the route object in a manner that equates to the current response to
     more specific advertisements, but without the need to advertise
     specific prefix route objects. However, even this approach uses
     route objects to communicate traffic engineering policy, and the
     same risk remains that the route table is used to carry fine-
     detailed traffic path policies.

     An alternative direction is to separate the functions of
     connectivity maintenance and traffic engineering, using the routing
     protocol to identify a number of viable paths from a source AS to a
     destination AS, and use a distinct collection of traffic engineering
     tools to allow a traffic source AS to make egress path selections
     that match the desired traffic service profile for the traffic.

     There is one critical difference between traffic engineering
     approaches as used in intra-domain environments and the current
     inter-domain operating practices. Whereas the intra-domain
     environment uses the ingress network element to make the appropriate
     path choice to the egress point, the inter domain traffic
     engineering has the opposite intent, where a downstream AS (or
     egress point) is attempting to influence the path choice of an
     upstream AS (or ingress point). If explicit traffic engineering were
     undertaken within the inter-domain space, it is highly likely that
     the current structure would be altered. Instead of the downstream
     element attempting to constrain the path choices of an upstream
     element, a probable approach is the downstream element placing a
     number of advisory constraints on the upstream elements, and the
     upstream elements using a combination of these advisory constraints,
     dynamic information relating to path service characteristics and
     local policies to make an egress choice.

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     From the perspective of the inter-domain routing environment, such
     measures offer the potential to remove the advertisement of specific
     routes for traffic engineering purposes. However, there is a need to
     adding traffic engineering information into advertised route blocks,
     requiring the definition of the syntax and semantics of traffic
     engineering attributes that can be attached to route objects.

6.4 Hierarchical Routing Models

     The CIDR routing model assumed a hierarchy of providers, where at
     each level in the hierarchy the routing policies and address space
     of networks at the lower level of hierarchy were subsumed by the
     next level up (or `upstream') provider. The connectivity policy
     assumed by this model is also a hierarchical model, where horizontal
     connections within a single level of the hierarchy are not visible
     beyond the networks of the two parties.

     A number of external factors are increasing the density of
     interconnection including decreasing unit costs of communications
     services and the increasing use of exchange points to augment point-
     to-point connectivity models with point-to-multipoint facilities.

     The outcome of these external factors is a significant reduction in
     the hierarchical nature of the inter-domain space. Such a trend can
     be viewed with concern given the common approach of using
     hierarchies as a tool for scaling routing systems. BGP falls within
     this approach, and relies on hierarchies in the address space to
     contain the number of independently routing objects. The outcomes of
     this characteristic of the Internet in terms of the routing space is
     the increasing number of distinct route policies that are associated
     with each multi-homed network within the Internet.

     One way to limit the proliferation of such policies across the
     entire inter-domain space is to associate attributes to such
     advertisements that specify the conditions whereby a remote transit
     AS may proxy-aggregate this route object with other route objects.

6.5 Extend or Replace BGP

     A final consideration is to consider whether these requirements can
     best be met by an approach of a set of upward-compatible extensions
     to BGP, or by a replacement to BGP. No recommendation is made here,
     and this is a topic requiring further investigation.

     The general approach in extending BGP appears to lie in increasing
     the number of supported transitive route attributes, allowing the
     route originator greater control in specifying the scope of
     propagation of the route and the intended outcome in terms of policy

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     and traffic engineering. It may also be necessary to allow BGP
     sessions to negotiate additional functionality intended to improve
     the convergence behavior of the protocol. Whether such changes can
     produce a scalable and useful outcome in terms of inter-domain
     routing remains, at this stage, an open question.

     An alternative approach is that of a replacement protocol, and such
     an approach may well be based on the adoption of a link-state
     behavior. The issues of policy opaqueness and link-state protocols
     have been described above. The other major issue with such an
     approach is the need to limit the extent of link state flooding,
     where the inter-domain space would need some further levels of
     imposed structure similar to intra-domain areas. Such structure may
     well imply the need for an additional set of operator inter-
     relationships such as mutual transit, and this may prove challenging
     to adapt to existing practices.

     The potential sets of actions include more than extend or replace
     the BGP protocol. A third approach is to continue to use BGP as the
     basic means of propagating route objects and their associated AS
     paths and other attributes, and use one or more overlay protocols to
     support inter-domain traffic engineering and other forms of inter-
     domain policy negotiation. This approach would appear to offer a
     means of transition for the large installed base currently using
     BGP4 as their inter-domain routing protocol, placing additional
     functionality in the overlay protocols while leaving the basic
     functionality of BGP4 intact. The resultant inter-dependencies
     between BGP and the overlay protocols would require very careful
     attention, as this would be the most critical aspect of such an

7. Directions for Further Activity

     While there may exist short term actions based on providing various
     incentives for network operators to remove redundant or
     inefficiently grouped entries from the BGP routing table, such
     actions are short term palliative measures, and will not provide
     long term answers to the need to a scalable inter-domain routing

     One potential short term protocol refinement is to allow a set of
     grouped advertisements to be aggregated into a single route
     advertisement. This form of proxy aggregation would take a set of
     bit-wise aligned routing entries with matching route attributes, and
     under certain well identified circumstances, aggregate these routing
     entries into a single re-advertised aggregate routing entry. This
     technique removes information from the routing system, and some care

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     must be taken to define a set of proxy aggregation conditions that
     do not materially alter the flow of traffic, or the ability of
     originating ASes to announce routing policy.

     A further refinement to this approach is to consider the definition
     of the syntax and semantics of a number of additional route
     attributes. Such attributes could define the extent to which
     specific route advertisements should be propagated in the inter-
     domain space, allowing the advertisement to be subsumed by a larger
     aggregate advertisement at the boundary of this domain. This could
     be used to form part of the preconditions of automated proxy
     aggregation of specific routes, and also limit the extent to which
     announcement and withdrawals are propagated across the routing

     It is unclear that such measures would result in substantial longer
     term changes to the scaling and convergence properties of BGP4.
     Taking the requirement set enumerated in section 6 of this document,
     one approach to the longer term requirements may be to preserve a
     number of attributes of the current BGP protocol, while refine other
     aspects of the protocol to improve its scaling and convergence
     properties. A minimal set of alterations could retain the Autonomous
     System concept to allow for boundaries of information summarization,
     as well as retaining the approach of associating each prefix
     advertisement with an originating AS. The concept of policy
     opaqueness would also be retained in such an approach, implying that
     each AS accepts a set of route advertisements, applies local policy
     constraints, and re-advertises those advertisements permitted by the
     local policy constraints. It could be feasible to consider
     alterations to the distance vector path selection algorithm,
     particularly as it relates to intermediate states during processing
     of a route withdrawal. It is also feasible to consider the use of
     compound route attributes, allowing a route object to include an
     aggregate route, and a number of specifics of the aggregate route,
     and attach attributes that may apply to the aggregate or a specific
     address prefix. Such route attributes could be used to support
     multi-homing and inter-domain traffic engineering mechanisms. The
     overall intent of this approach is to address the major requirements
     in the inter-domain routing space without using an increasing set of
     globally propagated specific route objects.

     A potential applied research topic is to consider the feasibility of
     de-coupling the requirements of inter-domain connectivity management
     with the applications of policy constraints and the issues of
     sender- and/or receiver-managed traffic engineering requirements.
     Such an approach may use a link-state protocol as a means of
     maintaining a consistent view of the topology of inter-domain
     network, and then use some form of overlay protocol to negotiate

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     policy requirements of each AS, and use a further overlay to support
     inter-domain traffic engineering requirements. The underlying
     assumption of such an approach is that by dividing up the functional
     role of inter-domain routing into distinct components each component
     will have superior scaling and convergence properties which in turn
     to result in superior properties for the entire routing system.
     Obviously, this assumption requires some testing.

     Research topics with potential longer term application include the
     approach of drawing a distinction between a network's identity, a
     network's location relative to other networks, and a feasible path
     between a source and destination network that satisfies various
     policy and traffic engineering constraints. Again the intent of such
     an approach would be to divide the current routing function into a
     number of distinct scalable components.

8. Security Considerations

     Any adopted inter-domain routing protocol needs to be secure against
     disruption. Disruption comes from two primary sources:
       - Accidental misconfiguration
       - Malicious attacks

     Given past experience with routing protocols, both can be
     significant sources of harm.

     Given that it is not reasonable to guarantee the security of all the
     routers involved in the global Internet inter-domain routing system,
     there is also every reason to believe that malicious attacks may
     come from peer routers, in addition to coming from external sources.

     A protocol design should therefore consider how to minimize the
     damage to the overall routing computation that can be caused by a
     single or small set of misbehaving routers.

     The routing system itself needs to be resilient against accidental
     or malicious advertisements of a route object by a route server not
     entitled to generate such an advertisement. This implies several
     things, including the need for cryptographic validation of
     announcements, cryptographic protection of various critical routing
     messages and an accurate and trusted database of routing assignments
     via which authorization can be checked.

9. References

     [1]  Bradner, S., "The Internet Standards Process -- Revision 3",
         BCP 9, RFC 2026, October 1996.

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     [2] Clark, D., Chapin, L., Cerf, V., Braden, R., Hobby, R., "Towards
         the Future Internet Architecture", RFC 1287, December 1991.

     [3] Deering, S., Hinden, R., "Internet Protocol, Version 6 (IPv6)
         Specification, RFC 2460, December 1998.

     [4] Srisuresh, P., Egevang, K., "Traditional IP Network Address
         Translator (Traditional NAT)", RFC 3022, January 2001.

     [5] Fuller, V., Li, T., Yu, J., Varadhan, K., "Classless Inter-
         Domain Routing (CIDR): an Address Assignment and Aggregation
         Strategy", RFC 1519, September 1993.

     [6] Huston, G., "The BGP Routing Table", The Internet Protocol
         Journal, vol. 4, No. 1, March 2001.

     [7] Rekhter, Y., Li, T., "A Border Gateway Protocol 4 (BGP-4)", RFC
         1771, March 1995.

     [8] Vohara, Q., Chen, E., "BGP support for four-octet AS number
         space", work in progress, draft-ietf-idr-as4bytes-02.txt, April

     [9] Hain, T., "Architectural Implications of NAT", RFC 2993,
         November 2000.

     [10] Labovitz, C., Ahuja, A., Bose, A., Jahanian, J., "Delayed
         Internet Routing Convergence", Proceedings ACM SIGCOMM 2000,
         August 2000.

     [11] Lothberg, P., personal communication, December 2000.

10. Acknowledgements

     This document is the outcome of a collaborative effort of the IAB,
     and the editor acknowledges the contributions of the members of the
     IAB in the preparation of the document. The contributions of John
     Leslie, Thomas Narten and Abha Ahuja in reviewing this document are
     also acknowledged.

11. Author

     Internet Architecture Board

     Editor: Geoff Huston

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     5/490 Northbourne Ave
     Dickson ACT 2602


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12. Full Copyright Statement

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