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Keywords: BGP, border, gateway, protocol
Network Working Group G. Huston
Request for Comments: 3221 Internet Architecture Board
Category: Informational December 2001
Commentary on
Inter-Domain Routing in the Internet
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
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.
This document is the outcome of a collaborative exercise on the part
of the Internet Architecture Board.
Table of Contents
1. Introduction................................................. 2
2. Network Scaling and Inter-Domain Routing ................... 2
3. Measurements of the total size of the BGP Table ............ 4
4. Related Measurements derived from BGP Table ................ 7
5. Current State of inter-AS routing in the Internet .......... 11
6. Future Requirements for the Exterior Routing System ........ 14
7. Architectural Approaches to a scalable Exterior
Routing Protocol........................................... 15
8. Directions for Further Activity ............................ 21
9. Security Considerations .................................... 22
10. References ................................................. 23
11. Acknowledgements ........................................... 24
12. Author's Address ........................................... 24
13. Full Copyright Statement ................................... 25
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RFC 3221 Commentary on Inter-Domain Routing December 2001
1. Introduction
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 policy and
finer levels of control to undertake inter-domain traffic
engineering.
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
investigation.
This document is the outcome of a collaborative exercise on the part
of the Internet Architecture Board.
2. 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 system.
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This network-wide connectivity is described in the routing table 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.
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When the scaling properties of the Internet were studied in the early
1990s two critical factors identified in the study were, not
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
point.
3. 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.
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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
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.
3.1 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.
3.2 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
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size of the routing table remained relatively constant at some 20,000
entries as the growth in the number of providers announcing address
blocks was matched by a corresponding reduction in the number of
address announcements as a result of CIDR aggregation.
3.3 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
period.
3.4 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.
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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 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.
4. 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.
4.1 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.
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4.2 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
routing table encompassed 1,081,131,733 addresses, or some 25.17% of
the total IPv4 address space, or 25.4% of the usable 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.
4.3 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
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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 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 /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.
4.4 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
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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
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 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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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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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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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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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
that the global requirement for stability implies some degree of
locality in the behavior of the system.
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6.3 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
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.
6.4 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.
7. 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.
7.1 Policy opaqueness vs. policy transparency
The two major approaches to routing protocols are distance vector and
link state.
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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 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 favor 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.
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7.2 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 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.
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7.3 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
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
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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.
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.
7.4 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-multi-point 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.
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7.5 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
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
approach.
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8. 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 protocol.
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
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
domain.
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
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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 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.
9. 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.
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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.
10. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3",
BCP 9, RFC 2026, October 1996.
[2] Clark, D., Chapin, L., Cerf, V., Braden, R. and R. Hobby,
"Towards the Future Internet Architecture", RFC 1287, December
1991.
[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification, RFC 2460, December 1998.
[4] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[5] Fuller, V., Li, T., Yu, J. and K. Varadhan, "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. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
RFC 1771, March 1995.
[8] Vohara, Q. and E. Chen, "BGP support for four-octet AS number
space", Work in Progress.
[9] Hain, T., "Architectural Implications of NAT", RFC 2993,
November 2000.
[10] Labovitz, C., Ahuja, A., Bose, A. and J. Jahanian, "Delayed
Internet Routing Convergence", Proceedings ACM SIGCOMM 2000,
August 2000.
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[11] Lothberg, P., personal communication, December 2000.
11. 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.
12. Author
Internet Architecture Board
Email: iab@ietf.org
Geoff Huston
Telstra
5/490 Northbourne Ave
Dickson ACT 2602
Australia
EMail: gih@telstra.net
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13. Full Copyright Statement
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
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