rfc3221.txt

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   rate of dynamic path re-computations 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.




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RFC 3221           Commentary on Inter-Domain Routing      December 2001


4.5  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
   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.

5. 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.




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RFC 3221           Commentary on Inter-Domain Routing      December 2001


5.1  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
   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.

5.2  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.








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RFC 3221           Commentary on Inter-Domain Routing      December 2001


5.3  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
   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.

5.4  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.

5.5  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.






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RFC 3221           Commentary on Inter-Domain Routing      December 2001


   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.

6.  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
   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.

6.1  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 environment.

6.2  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