Internet Architecture Board G. Huston
Internet Draft Telstra
Document: draft-iab-bgparch-00.txt February 2001
Category: Informational
Architectural Requirements for 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 RFC2026 [1].
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1. Abstract
This draft 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 which
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 AS's, 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.
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2. Network Scale 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 a view to 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 (AS's) which 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 AS's and thereby create a view of the connectivity of
the entire system. This network-wide connectivity is described in
the routing table used by the BGP4 protocol. Each entry in the table
refers to a distinct route. The attributes of the route 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. The BGP routing system is not aware
of finer level of topology within the local AS or within any remote
AS. From this perspective BGP can be seen as a connectivity
maintenance protocol, and the BGP routing table, a description of
the current connectivity of the Internet, using an AS as the basic
element of 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 chose 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
router. The BGP routing domain maintains 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, 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
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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 which 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
surprisingly, routing and addressing [RFC 1287]. 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
[RFC <IPv6>], as well as the refinement of techniques of network
address translation [RFC <NAT>] 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 a network 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 take at approximate
monthly intervals from 1988 until 1992 by Merit [RFC 1338]. 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 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,
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number of AS paths, and a set of measurements relating to the prefix
size of routing table entries.
We now have a view of the dynamics of the Internet's routing table
growth which spans some 13 years, and a very detailed view spanning
the most recent seven years [Huston 2001]. 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-90'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 which
spanned its entire customer base. The practical aspects of this
change was the introduction of routing protocols which 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).
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 [RFC <BGP4>}. The
effects of this effort are visible in the history of the routing
table, where the routing table remained constant for some 14 months
at 20,000 entries in 1994 and 1995.
The intention of CIDR was one of hierarchical provider address
aggregation, where a network provider is allocated an address block
from an address registry, and the provider announces this entire
block into the exterior routing domain as a single entry with a
single routing policy. Customers of the provider use a sub-
allocation from this address block, and these smaller routing
elements are aggregated by the provider and not directly passed into
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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 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 an 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 at which 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 been every
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 past two years is again showing
all the signs of a re-establishment of exponential growth. It
appears that CIDR is unable to keep pace with the levels of growth
of the Internet, and some additional factors are becoming apparent
in the Internet which has lead to a growth pattern in the total size
of the BGP table which has some elements of compound growth rather
than linear growth. A best fit of the data for the period from
January 1999 until December 2000 indicates 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.
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 AS's in the routing system, and the number of distinct AS
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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 AS's 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 AS's are associated with a
single network, but such exceptions are relatively uncommon.
The number of unique AS's 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 which matches a compound growth model with a
growth rate of 51% per year. As of the start of 2001 there were some
9,500 AS's 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. [I-D Chen & Rekhter
work in progress 2000] 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.
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. This has grown from 1,019,484,655
addresses in November 1999. However, there are a number of /8
prefixes which 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,
it would appear 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, the Internet appears so far to have embraced the approach of
using NATs, irrespective of their various perceived functional
shortcomings. [RFC 2993] This 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.
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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 check
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. This corresponds to an increase in the average prefix length
from /18.03 to /18.44. Separate observations of the average prefix
length used to route traffic in operation networks in late 2000
indicate an average length of 18.1 [Lothberg 2000]. This trend is
cause for concern as it implies the increasing spread of traffic
over greater numbers of increasingly finer 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 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, which in turn supports the
observation that the efficiencies of hierarchical routing structures
are no longer being realized within the deployed Internet, and
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.
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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
January 2001 it is interesting to note that of the 104,000 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 number 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 which 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. [Labowitz 2000] 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
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likely to continue. In percentage terms, the set of prefixes
spanning /25 to /32 show the largest growth rates.
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 which 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
41,600 advertisements, or 41% of the routing table, is being used to
punch policy holes in existing aggregate announcements. Of these the
overall majority of some 35,000 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. The deployment
model of a single-homed network with a single upstream provider is
rapidly being supplanted by a model of extensive interconnection at
the edges of the Internet. The underlying deployment model assumed
by CIDR assumed a different structure, more akin to a strict
hierarchy of supply providers. The business imperatives driving this
denser mesh of interconnection in the Internet are irresistible, 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 are multi-homing with a number of
peers and a number of 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
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.
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.
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At this stage the only tool being used for inter-provider traffic
engineering is that of the BGP routing table, further exacerbating
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 which 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.
It may be appropriate to consider how to operate an Internet with a
BGP routing table which 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 scaleable 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 which 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
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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.
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 which accurately
describes the current state of the network and which is stable. In
this case finite time implies a time limit which 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. A more useful upper bound for convergence is of the
order of tens of seconds or lower.
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,
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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 scaleable 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.
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 adjancies, and the policies it has with respect to these
adjancies, to all nodes within the link state domain. Every node can
perform an identical computation upon this set of adjancies 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
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qualifications which 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. This consideration would appear
to favor the continued use of a distance vector approach to inter-
domain routing which, in turn, has implications on the convergence
properties and stability of the inter-domain routing environment.
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
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once the logical scope of the object has been reached. This approach
would entail the addition of route attributes which 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.
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 is 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 which are attached to an aggregate route object, allowing
each remotes AS to respond to the route object in a manner that
equates to the current specific prefix response, but without the
multiplicity of 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
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point) is attempting to constrain 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.
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-multipoint facilities.
The outcome of these external factors is a significant reduction in
the hierarchical nature of the inter-domain space. 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.
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
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route originator greater control in specifying the level of
propagation of the route and the intended outcome in terms of policy
and traffic engineering. It is also be necessary to allow BGP
sessions to negotiate an enhanced capability to improve the
convergence behavior of the protocol. Whether such changes can
produce a scaleable 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.
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 interdomain 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 acceidental
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 cruptographic validation of
announcements, cryptographic protection of routing messages and an
accurate and trusted database of routing assignments via which
authorization can be checked.
9. References
[RFC 1287] "Towards the Future Internet Architecture", D. Clark,
L. Chapin, V. Cerf, R. Braden, R. Hobby, RFC 1287, December 1991.
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[RFC 1338[ "Supernetting: an Address Assignment and Aggregation
Strategy", Supernetting: an Address Assignment and Aggregation
Strategy", V. Fuller, T. Li, J. Yu, K. Varadhan, June 1992.
[RFC 2993] "Architectural Implications of NAT", T. Hain, November
2000.
[Bates 2000] "The CIDR Report", T. Bates, updated weekly at
http://www.employees.org/~tbates/cidr-report.html
[Chen 2000] "BGP Support for four-octet AS number space", E. Chen,
Y. Rekhter, work in progress (currently published as an Internet
Draft: draft-chen-as4bytes-00.txt), November 2000.
[Huston 2001] "BGP Table Report" updated hourly at
http://www.telstra.net/ops/bgp
[Labowitz] bgp convergence
[Lothberg 2000] Peter Lothberg, personal communication.
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
10. Acknowledgements
The author acknowledges the assistance of Brian Carpenter, Harald
Alvestrand and Steve Bellovin in preparing this document.
11. Author's Addresses
Geoff Huston
Telstra
5/490 Northbourne Ave
Dickson ACT 2602
AUSTRALIA
EMail: gih@telstra.net
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