Network Working Group D. Estrin
Request for Comments: 1322 USC
Y. Rekhter
IBM
S. Hotz
USC
May 1992
A Unified Approach to Inter-Domain Routing
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.
Abstract
This memo is an informational RFCwhich outlines one potential
approach for inter-domain routing in future global internets. The
focus is on scalability to very large networks and functionality, as
well as scalability, to support routing in an environment of
heterogeneous services, requirements, and route selection criteria.
Note: The work of D. Estrin and S. Hotz was supported by the National
Science Foundation under contract number NCR-9011279, with matching
funds from GTE Laboratories. The work of Y. Rekhter was supported by
the Defense Advanced Research Projects Agency, under contract
DABT63-91-C-0019. Views and conclusions eXPressed in this paper are
not necessarily those of the Defense Advanced Research Projects
Agency and National Science Foundation.
1.0 Motivation
The global internet can be modeled as a collection of hosts
interconnected via transmission and switching facilities. Control
over the collection of hosts and the transmission and switching
facilities that compose the networking resources of the global
internet is not homogeneous, but is distributed among multiple
administrative authorities. Resources under control of a single
administration form a domain. In order to support each domain's
autonomy and heterogeneity, routing consists of two distinct
components: intra-domain (interior) routing, and inter-domain
(exterior) routing. Intra-domain routing provides support for data
communication between hosts where data traverses transmission and
switching facilities within a single domain. Inter-domain routing
provides support for data communication between hosts where data
traverses transmission and switching facilities spanning multiple
domains. The entities that forward packets across domain boundaries
are called border routers (BRs). The entities responsible for
exchanging inter-domain routing information are called route servers
(RSs). RSs and BRs may be colocated.
As the global internet grows, both in size and in the diversity of
routing requirements, providing inter-domain routing that can
accommodate both of these factors becomes more and more crUCial. The
number and diversity of routing requirements is increasing due to:
(a) transit restrictions imposed by source, destination, and transit
networks, (b) different types of services offered and required, and
(c) the presence of multiple carriers with different charging
schemes. The combinatorial explosion of mixing and matching these
different criteria weighs heavily on the mechanisms provided by
conventional hop-by-hop routing architectures ([ISIS10589, OSPF,
Hedrick88, EGP]).
Current work on inter-domain routing within the Internet community
has diverged in two directions: one is best represented by the Border
Gateway Protocol (BGP)/Inter-Domain Routeing Protocol (IDRP)
architectures ([BGP91, Honig90, IDRP91]), and another is best
represented by the Inter-Domain Policy Routing (IDPR) architecture
([IDPR90, Clark90]). In this paper we suggest that the two
architectures are quite complementary and should not be considered
mutually exclusive.
We expect that over the next 5 to 10 years, the types of services
available will continue to evolve and that specialized facilities
will be employed to provide new services. While the number and
variety of routes provided by hop-by-hop routing architectures with
type of service (TOS) support (i.e., multiple, tagged routes) may be
sufficient for a large percentage of traffic, it is important that
mechanisms be in place to support efficient routing of specialized
traffic types via special routes. Examples of special routes are:
(1) a route that travels through one or more transit domains that
discriminate according to the source domain, (2) a route that travels
through transit domains that support a service that is not widely or
regularly used. We refer to all other routes as generic.
Our desire to support special routes efficiently led us to
investigate the dynamic installation of routes ([Breslau-Estrin91,
Clark90, IDPR90]). In a previous paper ([Breslau-Estrin91]), we
evaluated the algorithmic design choices for inter-domain policy
routing with specific attention to accommodating source-specific and
other "special" routes. The conclusion was that special routes are
best supported with source-routing and extended link-state
algorithms; we refer to this approach as source-demand routing
[Footnote: The Inter-Domain Policy Routing (IDPR) architecture uses
these techniques.]. However, a source-demand routing architecture,
used as the only means of inter-domain routing, has scaling problems
because it does not lend itself to general hierarchical clustering
and aggregation of routing and forwarding information. For example,
even if a particular route from an intermediate transit domain X, to
a destination domain Y is shared by 1,000 source-domains, IDPR
requires that state for each of the 1,000 routes be setup and
maintained in the transit border routers between X and Y. In
contrast, an alternative approach to inter-domain routing, based on
hop-by-hop routing and a distributed route-computation algorithm
(described later), provides extensive support for aggregation and
abstraction of reachability, topology, and forwarding information.
The Border Gateway Protocol (BGP) and Inter-Domain Routeing Protocol
(IDRP) use these techniques ([BGP91, IDRP91]). While the BGP/IDRP
architecture is capable of accommodating very large numbers of
datagram networks, it does not provide support for specialized
routing requirements as flexibly and efficiently as IDPR-style
routing.
1.1 Overview of the Unified Architecture
We want to support special routes and we want to exploit aggregation
when a special route is not needed. Therefore, our scalable inter-
domain routing architecture consists of two major components:
source-demand routing (SDR), and node routing (NR). The NR component
computes and installs routes that are shared by a significant number
of sources. These generic routes are commonly used and warrant wide
propagation, consequently, aggregation of routing information is
critical. The SDR component computes and installs specialized routes
that are not shared by enough sources to justify computation by NR
[Footnote: Routes that are only needed sporadically (i.e., the demand
for them is not continuous or otherwise predictable) are also
candidates for SDR.]. The potentially large number of different
specialized routes, combined with their sparse utilization, make them
too costly to support with the NR mechanism.
A useful analogy to this approach is the manufacturing of consumer
products. When predictable patterns of demand exist, firms produce
objects and sell them as "off the shelf" consumer goods. In our
architecture NR provides off-the-shelf routes. If demand is not
predictable, then firms accept special orders and produce what is
demanded at the time it is needed. In addition, if a part is so
specialized that only a single or small number of consumers need it,
the consumer may repeatedly special order the part, even if it is
needed in a predictable manner, because the consumer does not
represent a big enough market for the producer to bother managing the
item as part of its regular production. SDR provides such special
order, on-demand routes.
By combining NR and SDR routing we propose to support inter-domain
routing in internets of practically-unlimited size, while at the same
time providing efficient support for specialized routing
requirements.
The development of this architecture does assume that routing
requirements will be diverse and that special routes will be needed.
On the other hand, the architecture does not depend on assumptions
about the particular types of routes demanded or on the distribution
of that demand. Routing will adapt naturally over time to changing
traffic patterns and new services by shifting computation and
installation of particular types of routes between the two components
of the hybrid architecture [Footnote: Before continuing with our
explanation of this architecture, we wish to state up front that
supporting highly specialized routes for all source-destination pairs
in an internet, or even anything close to that number, is not
feasible in any routing architecture that we can foresee. In other
Words, we do not believe that any foreseeable routing architecture
can support unconstrained proliferation of user requirements and
network services. At the same time, this is not necessarily a
problem. The capabilities of the architecture may in fact exceed the
requirements of the users. Moreover, some of the requirements that
we regard as infeasible from the inter-domain routing point of view,
may be supported by means completely outside of routing.
Nevertheless, the caveat is stated here to preempt unrealistic
expectations.].
While the packet forwarding functions of the NR and SDR components
have little or no coupling with each other, the connectivity
information exchange mechanism of the SDR component relies on
services provided by the NR component.
1.2 Outline
The remainder of this report is organized as follows. Section 2
outlines the requirements and priorities that guide the design of the
NR and SDR components. Sections 3 and 4 describe the NR and SDR
design choices, respectively, in light of these requirements.
Section 5 describes protocol support for the unified architecture and
briefly discusses transition issues. We conclude with a brief
summary.
2.0 Architectural Requirements and Priorities
In order to justify our design choices for a scalable inter-domain
routing architecture, we must articulate our evaluation criteria and
priorities. This section defines complexity, abstraction, policy,
and type of service requirements.
2.1 Complexity
Inter-domain routing complexity must be evaluated on the basis of the
following performance metrics: (1) storage overhead, (2)
computational overhead, and (3) message overhead. This evaluation is
essential to determining the scalability of any architecture.
2.1.1 Storage Overhead
The storage overhead of an entity that participates in inter-domain
routing comes from two sources: Routing Information Base (RIB), and
Forwarding Information Base (FIB) overhead. The RIB contains the
routing information that entities exchange via the inter-domain
routing protocol; the RIB is the input to the route computation. The
FIB contains the information that the entities use to forward the
inter-domain traffic; the FIB is the output of the route computation.
For an acceptable level of storage overhead, the amount of
information in both FIBs and RIBs should grow significantly slower
than linearly (e.g., close to logarithmically) with the total number
of domains in an internet. To satisfy this requirement with respect
to the RIB, the architecture must provide mechanisms for either
aggregation and abstraction of routing and forwarding information, or
retrieval of a subset of this information on demand. To satisfy this
requirement with respect to the FIB, the architecture must provide
mechanisms for either aggregation of the forwarding information (for
the NR computed routes), or dynamic installation/tear down of this
information (for the SDR computed routes).
Besides being an intrinsically important evaluation metric, storage
overhead has a direct impact on computational and bandwidth
complexity. Unless the computational complexity is fixed (and
independent of the total number of domains), the storage overhead has
direct impact on the computational complexity of the architecture
since the routing information is used as an input to route
computation. Moreover, unless the architecture employs incremental
updates, where only changes to the routing information are
propagated, the storage overhead has direct impact on the bandwidth
overhead of the architecture since the exchange of routing
information constitutes most of the bandwidth overhead.
2.1.2 Computational Overhead
The NR component will rely primarily on precomputation of routes. If
inter-domain routing is ubiquitous, then the precomputed routes
include all reachable destinations. Even if policy constraints make
fully ubiquitous routing impossible, the precomputed routes are
likely to cover a very large percentage of all reachable
destinations. Therefore the complexity of this computation must be
as small as possible. Specifically, it is highly desirable that the
architecture would employ some form of partial computation, where
changes in topology would require less than complete recomputation.
Even if complete recomputation is necessary, its complexity should be
less than linear with the total number of domains.
The SDR component will use on-demand computation and caching.
Therefore the complexity of this computation can be somewhat higher.
Another reason for relaxed complexity requirements for SDR is that
SDR is expected to compute routes to a smaller number of destinations
than is NR (although SDR route computation may be invoked more
frequently).
Under no circumstances is computational complexity allowed to become
exponential (for either the NR or SDR component).
2.1.3 Bandwidth Overhead
The bandwidth consumed by routing information distribution should be
limited. However, the possible use of data compression techniques
and the increasing speed of network links make this less important
than route computation and storage overhead. Bandwidth overhead may
be further contained by using incremental (rather than complete)
exchange of routing information.
While storage and bandwidth overhead may be interrelated, if
incremental updates are used then bandwidth overhead is negligible in
the steady state (no changes in topology), and is independent of the
storage overhead. In other words, use of incremental updates
constrains the bandwidth overhead to the dynamics of the internet.
Therefore, improvements in stability of the physical links, combined
with techniques to dampen the effect of topological instabilities,
will make the bandwidth overhead even less important.
2.2 Aggregation
Aggregation and abstraction of routing and forwarding information
provides a very powerful mechanism for satisfying storage,
computational, and bandwidth constraints. The ability to aggregate,
and subsequently abstract, routing and forwarding information is
essential to the scaling of the architecture [Footnote: While we can
not prove that there are no other ways to achieve scaling, we are not
aware of any mechanism other than clustering that allows information
aggregation/abstraction. Therefore, the rest of the paper assumes
that clustering is used for information aggregation/abstraction.].
This is especially true with respect to the NR component, since the
NR component must be capable of providing routes to all or almost all
reachable destinations.
At the same time, since preserving each domain's independence and
autonomy is one of the crucial requirements of inter-domain routing,
the architecture must strive for the maximum flexibility of its
aggregation scheme, i.e., impose as few preconditions, and as little
global coordination, as possible on the participating domains.
The Routing Information Base (RIB) carries three types of
information: (1) topology (i.e., the interconnections between domains
or groups of domains), (2) network layer reachability, and (3)
transit constraint. Aggregation of routing information should
provide a reduction of all three components. Aggregation of
forwarding information will follow from reachability information
aggregation.
Clustering (by forming routing domain confederations) serves the
following aggregation functions: (1) to hide parts of the actual
physical topology, thus abstracting topological information, (2) to
combine a set of reachable destination entities into a single entity
and reduce storage overhead, and (3) to express transit constraints
in terms of clusters, rather than individual domains.
As argued in [Breslau-Estrin91], the architecture must allow
confederations to be formed and changed without extensive
configuration and coordination; in particular, forming a
confederation should not require global coordination (such as that
required in ECMA ([ECMA89]). In addition, aggregation should not
require explicit designation of the relative placement of each domain
relative to another; i.e., domains or confederations of domains
should not be required to agree on a partial ordering (i.e., who is
above whom, etc.).
The architecture should allow different domains to use different
methods of aggregation and abstraction. For example, a research
collaborator at IBM might route to USC as a domain-level entity in
order to take advantage of some special TOS connectivity to, or even
through, USC. Whereas, someone else at Digital Equipment Corporation
might see information at the level of the California Educational
Institutions Confederation, and know only that USC is a member.
Alternatively, USC might see part of the internal structure within
the IBM Confederation (at the domain's level), whereas UCLA may route
based on the confederation of IBM domains as a whole.
Support for confederations should be flexible. Specifically, the
architecture should allow confederations to overlap without being
nested, i.e., a single domain, or a group of domains may be part of
more than one confederation. For example, USC may be part of the
California Educational Institutions Confederation and part of the US
R&D Institutions Confederation; one is not a subset of the other.
Another example: T.J. Watson Research Center might be part of
NYSERNET Confederation and part of IBM-R&D-US Confederation. While
the above examples describe cases where overlap consists of a single
domain, there may be other cases where multiple domains overlap. As
an example consider the set of domains that form the IBM
Confederation, and another set of domains that form the DEC
Confederation. Within IBM there is a domain IBM-Research, and
similarly within DEC there is a domain DEC-Research. Both of these
domains could be involved in some collaborative effort, and thus have
established direct links. The architecture should allow restricted
use of these direct links, so that other domains within the IBM
Confederation would not be able to use it to talk to other domains
within the DEC Confederation. A similar example exists when a
multinational corporation forms a confederation, and the individual
branches within each country also belong to their respective country
confederations. The corporation may need to protect itself from
being used as an inter-country transit domain (due to internal, or
international, policy). All of the above examples illustrate a
situation where confederations overlap, and it is necessary to
control the traffic traversing the overlapping resources.
While flexible aggregation should be accommodated in any inter-domain
architecture, the extent to which this feature is exploited will have
direct a effect on the scalability associated with aggregation. At
the same time, the exploitation of this feature depends on the way
addresses are assigned. Specifically, scaling associated with
forwarding information depends heavily on the assumption that there
will be general correspondence between the hierarchy of address
registration authorities, and the way routing domains and routing
domain confederations are organized (see Section 2.6).
2.3 Routing Policies
Routing policies that the architecture must support may be broadly
classified into transit policies and route selection policies
[Breslau-Estrin 91, Estrin89].
Restrictions imposed via transit policies may be based on a variety
of factors. The architecture should be able to support restrictions
based on the source, destination, type of services (TOS), user class
identification (UCI), charging, and path [Estrin89 , Little89]. The
architecture must allow expression of transit policies on all routes,
both NR and SDR. Even if NR routes are widely used and have fewer
source or destination restrictions, NR routes may have some transit
qualifiers (e.g., TOS, charging, or user-class restriction). If the
most widely-usable route to a destination has policy qualifiers, it
should be advertiseable by NR and the transit constraints should be
explicit.
Route selection policies enable each domain to select a particular
route among multiple routes to the same destination. To maintain
maximum autonomy and independence between domains, the architecture
must support heterogeneous route selection policies, where each
domain can establish its own criteria for selecting routes. This
argument was made earlier with respect to source route selection
([IDPR90, Clark90, Breslau-Estrin91]). In addition, each
intermediate transit domain must have the flexibility to apply its
own selection criteria to the routes made available to it by its
neighbors. This is really just a corollary to the above; i.e., if we
allow route selection policy to be expressed for NR routes, we can
not assume all domains will apply the same policy. The support for
dissimilar route selection policies is one of the key factors that
distinguish inter-domain routing from intra-domain routing ([ECMA89,
Estrin89]). However, it is a non-goal of the architecture to support
all possible route selection policies. For more on unsupported route
selection policies see Section 2.3.2 below.
2.3.1 Routing Information Hiding
The architecture should not require all domains within an internet to
reveal their connectivity and transit constraints to each other.
Domains should be able to enforce their transit policies while
limiting the advertisement of their policy and connectivity
information as much as possible; such advertisement will be driven by
a "need to know" criteria. Moreover, advertising a transit policy to
domains that can not use this policy will increase the amount of
routing information that must be stored, processed, and propagated.
Not only may it be impractical for a router to maintain full inter-
domain topology and policy information, it may not be permitted to
obtain this information.
2.3.2 Policies Not Supported
In this and previous papers we have argued that a global inter-domain
routing architecture should support a wide range of policies. In
this section we identify several types of policy that we explicitly
do not propose to support. In general our reasoning is pragmatic; we
think such policy types are either very expensive in terms of
overhead, or may lead to routing instabilities.
1. Route selection policies contingent on external behavior.
The route selection process may be modeled by a function that
assigns a non-negative integer to a route, denoting the degree
of preference. Route selection applies this function to all
feasible routes to a given destination, and selects the route
with the highest value. To provide a stable environment, the
preference function should not use as an input the status and
attributes of other routes (either to the same or to a
different destination).
2. Transit policies contingent on external behavior.
To provide a stable environment, the domain's transit policies
can not be automatically affected by any information external
to the domain. Specifically, these policies can not be modified,
automatically, in response to information about other domains'
transit policies, or routes selected by local or other domains.
Similarly, transit policies can not be automatically modified
in response to information about performance characteristics of
either local or external domains.
3. Policies contingent on external state (e.g., load).
It is a non-goal of the architecture to support load-sensitive
routing for generic routes. However, it is possible that some
types of service may employ load information to select among
alternate SDR routes.
4. Very large number of simultaneous SDR routes.
It is a non-goal of the architecture to support a very large
number of simultaneous SDR routes through any single router.
Specifically, the FIB storage overhead associated with SDR
routes must be comparable with that of NR routes, and should
always be bound by the complexity requirements outlined earlier
[Foonote: As discussed earlier, theoretically the state overhead
could grow O(N^2) with the number of domains in an internet.
However, there is no evidence or intuition to suggest that
this will be a limiting factor on the wide utilization of SDR,
provided that NR is available to handle generic routes.].
2.4 Support for TOS Routing
Throughout this document we refer to support for type of service
(TOS) routing. There is a great deal of research and development
activity currently underway to explore network architectures and
protocols for high-bandwidth, multimedia traffic. Some of this
traffic is delay sensitive, while some requires high throughput. It
is unrealistic to assume that a single communication fabric will be
deployed homogeneously across the internet (including all
metropolitan, regional, and backbone networks) that will support all
types of traffic uniformly. To support diverse traffic requirements
in a heterogeneous environment, various resource management
mechanisms will be used in different parts of the global internet
(e.g., resource reservation of various kinds) [ST2-90, Zhang91].
In this context, it is the job of routing protocols to locate routes
that can potentially support the particular TOS requested. It is
explicitly not the job of the general routing protocols to locate
routes that are guaranteed to have resources available at the
particular time of the route request. In other words, it is not
practical to assume that instantaneous resource availability will be
known at all remote points in the global internet. Rather, once a
TOS route has been identified, an application requiring particular
service guarantees will attempt to use the route (e.g., using an
explicit setup message if so required by the underlying networks).
In Section 4 we describe additional services that may be provided to
support more adaptive route selection for special TOS [Footnote:
Adaptive route selection implies adaptability only during the route
selection process. Once a route is selected, it is not going to be a
subject to any adaptations, except when it becomes infeasible.].
2.5 Commonality between Routing Components
While it is acceptable for the NR and SDR components to be
dissimilar, we do recognize that such a solution is less desirable --
all other things being equal. In theory, there are advantages in
having the NR and SDR components use similar algorithms and
mechanisms. Code and databases could be shared and the architecture
would be more manageable and comprehensible. On the other hand,
there may be some benefit (e.g., robustness) if the two parts of the
architecture are heterogeneous, and not completely inter-dependent.
In Section 5 we list several areas in which opportunities for
increased commonality (unification) will be exploited.
2.6 Interaction with Addressing
The proposed architecture should be applicable to various addressing
schemes. There are no specific assumptions about a particular
address structure, except that this structure should facilitate
information aggregation, without forcing rigid hierarchical routing.
Beyond this requirement, most of the proposals for extending the IP
address space, for example, can be used in conjunction with our
architecture. But our architecture itself does not provide (or
impose) a particular solution to the addressing problem.
3.0 Design Choices for Node Routing (NR)
This section addresses the design choices made for the NR component
in light of the above architectural requirements and priorities. All
of our discussion of NR assumes hop-by-hop routing. Source routing
is subject to different constraints and is used for the complementary
SDR component.
3.1 Overview of NR
The NR component is designed and optimized for an environment where a
large percentage of packets will travel over routes that can be
shared by multiple sources and that have predictable traffic
patterns. The efficiency of the NR component improves when a number
of source domains share a particular route from some intermediate
point to a destination. Moreover, NR is best suited to provide
routing for inter-domain data traffic that is either steady enough to
justify the existence of a route, or predictable, so that a route may
be installed when needed (based on the prediction, rather than on the
actual traffic). Such routes lend themselves to distributed route
computation and installation procedures.
Routes that are installed in routers, and information that was used
by the routers to compute these routes, reflect the known operational
state of the routing facilities (as well as the policy constraints)
at the time of route computation. Route computation is driven by
changes in either operational status of routing facilities or policy
constraints. The NR component supports route computation that is
dynamically adaptable to both changes in topology and policy. The NR
component allows time-dependent selection or deletion of routes.
However, time dependency must be predictable (e.g., advertising a
certain route only after business hours) and routes should be used
widely enough to warrant inclusion in NR.
The proposed architecture assumes that most of the inter-domain
conversations will be forwarded via routes computed and installed by
the NR component.
Moreover, the exchange of routing information necessary for the SDR
component depends on facilities provided by the NR component; i.e.,
NR policies must allow SDR reachability information to travel.
Therefore, the architecture requires that all domains in an internet
implement and participate in NR. Since scalability (with respect to
the size of the internet) is one of the fundamental requirements for
the NR component, it must provide multiple mechanisms with various
degrees of sophistication for information aggregation and
abstraction.
The potential reduction of routing and forwarding information depends
very heavily on the way addresses are assigned and how domains and
their confederations are structured. "If there is no correspondence
between the address registration hierarchy and the organisation of
routeing domains, then ... each and every routeing domain in the OSIE
would need a table entry potentially at every boundary IS of every
other routeing domain" ([Oran89]). Indeed, scaling in the NR
component is almost entirely predicated on the assumption that there
will be general correspondence between the hierarchy of address
assignment authorities and the way routing domains are organised, so
that the efficient and frequent aggregation of routing and forwarding
information will be possible. Therefore, given the nature of inter-
domain routing in general, and the NR component in particular,
scalability of the architecture depends very heavily on the
flexibility of the scheme for information aggregation and
abstraction, and on the preconditions that such a scheme imposes.
Moreover, given a flexible architecture, the operational efficiency
(scalability) of the global internet, or portions thereof, will
depend on tradeoffs made between flexibility and aggregation.
While the NR component is optimized to satisfy the common case
routing requirements for an extremely large population of users, this
does not imply that routes produced by the NR component would not be
used for different types of service (TOS). To the contrary, if a TOS
becomes sufficiently widely used (i.e., by multiple domains and
predictably over time), then it may warrant being computed by the NR
component.
To summarize, the NR component is best suited to provide routes that
are used by more than a single domain. That is, the efficiency of
the NR component improves when a number of source domains share a
particular route from some intermediate point to a destination.
Moreover, NR is best suited to provide routing for inter-domain data
traffic that is either steady enough to justify the existence of a
route, or predictable, so that a route may be installed when needed,
(based on the prediction, rather than on the actual traffic).
3.2 Routing Algorithm Choices for NR
Given that a NR component based on hop-by-hop routing is needed to
provide scalable, efficient inter-domain routing, the remainder of
this section considers the fundamental design choices for the NR
routing algorithm.
Typically the debate surrounding routing algorithms focuses on link
state and distance vector protocols. However, simple distance vector
protocols (i.e., Routing Information Protocol [Hedrick88]), do not
scale because of convergence problems. Improved distance vector
protocols, such as those discussed in [Jaffee82, Zaumen91, Shin87],
have been developed to address this issue using synchronization
mechanisms or additional path information. In the case of inter-
domain routing, having additional path information is essential to
supporting policy. Therefore, the algorithms we consider for NR are
link state and one we call path vector (PV). Whereas the
characteristics of link state algorithms are generally understood
(for example, [Zaumen 91]), we must digress from our evaluation
discussion to describe briefly the newer concept of the PV algorithm
[Footnote: We assume that some form of SPF algorithm will be used to
compute paths over the topology database when LS algorithms are used
[Dijkstra59, OSPF]].
3.2.1 Path Vector Protocol Overview
The Border Gateway Protocol (BGP) (see [BGP91]) and the Inter Domain
Routing Protocol (IDRP) (see [IDRP91]) are examples of path vector
(PV) protocols [Footnote: BGP is an inter-autonomous system routing
protocol for TCP/IP internets. IDRP is an OSI inter-domain routing
protocol that is being progressed toward standardization within ISO.
Since in terms of functionality BGP represents a proper subset of
IDRP, for the rest of the paper we will only consider IDRP.].
The routing algorithm employed by PV bears a certain resemblance to
the traditional Bellman-Ford routing algorithm in the sense that each
border router advertises the destinations it can reach to its
neighboring BRs. However,the PV routing algorithm augments the
advertisement of reachable destinations with information that
describes various properties of the paths to these destinations.
This information is expressed in terms of path attributes. To
emphasize the tight coupling between the reachable destinations and
properties of the paths to these destinations, PV defines a route as
a pairing between a destination and the attributes of the path to
that destination. Thus the name, path-vector protocol, where a BR
receives from its neighboring BR a vector that contains paths to a
set of destinations [Footnote: The term "path-vector protocol" bears
an intentional similarity to the term "distance-vector protocol",
where a BR receives from each of its neighbors a vector that contains
distances to a set of destinations.]. The path, expressed in terms
of the domains (or confederations) traversed so far, is carried in a
special path attribute which records the sequence of routing domains
through which the reachability information has passed. Suppression
of routing loops is implemented via this special path attribute, in
contrast to LS and distance vector which use a globally-defined
monotonically-increasing metric for route selection [Shin87].
Because PV does not require all routing domains to have homogeneous
criteria (policies) for route selection, route selection policies
used by one routing domain are not necessarily known to other routing
domains. To maintain the maximum degree of autonomy and independence
between routing domains, each domain which participates in PV may
have its own view of what constitutes an optimal route. This view is
based solely on local route selection policies and the information
carried in the path attributes of a route. PV standardizes only the
results of the route selection procedure, while allowing the
selection policies that affect the route selection to be non-standard
[Footnote: This succinct observation is attributed to Ross Callon
(Digital Equipment Corporation).].
3.3 Complexity
Given the above description of PV algorithms, we can compare them to
LS algorithms in terms of the three complexity parameters defined
earlier.
3.3.1 Storage Overhead
Without any aggregation of routing information, and ignoring storage
overhead associated with transit constraints, it is possible to show
that under some rather general assumptions the average case RIB
storage overhead of the PV scheme for a single TOS ranges between
O(N) and O(Nlog(N)), where N is the total number of routing domains
([Rekhter91]). The LS RIB, with no aggregation of routing
information, no transit constraints, a single homogeneous route
selection policy across all the domains, and a single TOS, requires a
complete domain-level topology map whose size is O(N).
Supporting heterogeneous route selection and transit policies with
hop-by-hop forwarding and LS requires each domain to know all other
domains route selection and transit policies. This may significantly
increase the amount of routing information that must be stored by
each domain. If the number of policies advertised grows with the
number of domains, then the storage could become unsupportable. In
contrast, support for heterogeneous route selection policies has no
effect on the storage complexity of the PV scheme (aside from simply
storing the local policy information). The presence of transit
constraints in PV results in a restricted distribution of routing
information, thus further reducing storage overhead. In contrast,
with LS no such reduction is possible since each domain must know
every other domain's transit policies. Finally, some of the transit
constraints (e.g., path sensitive constraints) can be expressed in a
more concise form in PV (see aggregation discussion below).
The ability to further restrict storage overhead is facilitated by
the PV routing algorithm, where route computation precedes routing
information dissemination, and only routing information associated
with the routes selected by a domain is distributed to adjacent
domains. In contrast, route selection with LS is decoupled from the
distribution of routing information, and has no effect on such
distribution.
While theoretically routing information aggregation can be used to
reduce storage complexity in both LS and PV, only aggregation of
topological information would yield the same gain for both.
Aggregating transit constraints with LS may result in either reduced
connectivity or less information reduction, as compared with PV.
Aggregating heterogeneous route selection policies in LS is highly
problematic, at best. In PV, route selection policies are not
distributed, thus making aggregation of route selection policies a
non-issue [Footnote: Although a domain's selection policies are not
explicitly distributed, they have an impact on the routes available
to other domains. A route that may be preferred by a particular
domain, and not prohibited by transit restrictions, may still be
unavailable due to the selection policies of some intermediate
domain. The ability to compute and install alternative routes that
may be lost using hop-by-hop routing (either LS of PV) is the
critical functionality provided by SDR.].
Support for multiple TOSs has the same impact on storage overhead for
both LS and for PV.
Potentially the LS FIB may be smaller if routes are computed at each
node on demand. However, the gain of such a scheme depends heavily
on the traffic patterns, memory size, and caching strategy. If there
is not a high degree of locality, severely degraded performance can
result due to excessive overall computation time and excessive
computation delay when forwarding packets to a new destination. If
demand driven route computation is not used for LS, then the size of
the FIB would be the same for both LS and PV.
3.3.2 Route Computation Complexity
Even if all domains employ exactly the same route selection policy,
computational complexity of PV is smaller than that of LS. The PV
computation consists of evaluating a newly arrived route and
comparing it with the existing one [Footnote: Some additional checks
are required when an update is received to insure that a routing loop
has not been created.]. Whereas, conventional LS computation
requires execution of an SPF algorithm such as Dijkstra's.
With PV, topology changes only result in the recomputation of routes
affected by these changes, which is more efficient than complete
recomputation. However, because of the inclusion of full path
information with each distance vector, the effect of a topology
change may propagate farther than in traditional distance vector
algorithms. On the other hand, the number of affected domains will
still be smaller with PV than with conventional LS hop-by-hop
routing. With PV, only those domains whose routes are affected by
the changes have to recompute, while with conventional LS hop-by-hop
routing all domains must recompute. While it is also possible to
employ partial recomputation with LS (i.e., when topology changes,
only the affected routes are recomputed), "studies suggest that with
a very small number of link changes (perhaps 2) the expected
computational complexity of the incremental update exceeds the
complete recalculation" ([ANSI87-150R]). However these checks are
much simpler than executing a full SPF algorithm.
Support for heterogeneous route selection policies has serious
implications for the computational complexity. The major problem
with supporting heterogeneous route selection policies with LS is the
computational complexity of the route selection itself.
Specifically, we are not aware of any algorithm with less than
exponential time complexity that would be capable of computing routes
to all destinations, with LS hop-by-hop routing and heterogeneous
route selection policies. In contrast, PV allows each domain to make
its route selection autonomously, based only on local policies.
Therefore support for dissimilar route selection policies has no
negative implications for the complexity of route computation in PV.
Similarly, providing support for path-sensitive transit policies in
LS implies exponential computation, while in PV such support has no
impact on the complexity of route computation.
In summary, because NR will rely primarily on precomputation of
routes, aggregation is essential to the long-term scalability of the
architecture. To the extent aggregation is facilitated with PV, so
is reduced computational complexity. While similar arguments may be
made for LS, the aggregation capabilities that can be achieved with
LS are more problematic because of LS' reliance on consistent
topology maps at each node. It is also not clear what additional
computational complexity will be associated with aggregation of
transit constraints and heterogeneous route selection policies in LS.
3.3.3 Bandwidth Overhead
PV routing updates include fully-expanded information. A complete
route for each supported TOS is advertised. In LS, TOS only
contributes a factor increase per link advertised, which is much less
than the number of complete routes. Although TOSs may be encoded
more efficiently with LS than with PV, link state information is
flooded to all domains, while with PV, routing updates are
distributed only to the domains that actually use them. Therefore,
it is difficult to make a general statement about which scheme
imposes more bandwidth overhead, all other factors being equal.
Moreover, this is perhaps really not an important difference, since
we are more concerned with the number of messages than with the
number of bits (because of compression and greater link bandwidth, as
well as the increased physical stability of links).
3.4 Aggregation
Forming confederations of domains, for the purpose of consistent,
hop-by-hop, LS route computation, requires that domains within a
confederation have consistent policies. In addition, LS
confederation requires that any lower level entity be a member of
only one higher level entity. In general, no intra-confederation
information can be made visible outside of a confederation, or else
routing loops may occur as a result of using an inconsistent map of
the network at different domains. Therefore, the use of
confederations with hop-by-hop LS is limited because each domain (or
confederation) can only be a part of one higher level confederation
and only export policies consistent with that confederation (see
examples in Section 2.2). These restrictions are likely to impact
the scaling capabilities of the architecture quite severely.
In comparison, PV can accommodate different confederation definitions
because looping is avoided by the use of full path information.
Consistent network maps are not needed at each route server, since
route computation precedes routing information dissemination.
Consequently, PV confederations can be nested, disjoint, or
overlapping. A domain (or confederation) can export different
policies or TOS as part of different confederations, thus providing
the extreme flexibility that is crucial for the overall scaling and
extensibility of the architecture.
In summary, aggregation is essential to achieve acceptable complexity
bounds, and flexibility is essential to achieve acceptable
aggregation across the global, decentralized internet. PV's
strongest advantage stems from its flexibility.
3.5 Policy
The need to allow expression of transit policy constraints on any
route (i.e., NR routes as well as SDR routes), by itself, can be
supported by either LS or PV. However, the associated complexities
of supporting transit policy constraints are noticeably higher for LS
than for PV. This is due to the need to flood all transit policies
with LS, where with PV transit policies are controlled via restricted
distribution of routing information. The latter always imposes less
overhead than flooding.
While all of the transit constraints that can be supported with LS
can be supported with PV, the reverse is not true. In other words,
there are certain transit constraints (e.g., path-sensitive transit
constraints) that are easily supported with PV, and are prohibitively
expensive (in terms of complexity) to support in LS. For example, it
is not clear how NR with LS could support something like ECMA-style
policies that are based on hierarchical relations between domains,
while support for such policies is straightforward with PV.
As indicated above, support for heterogeneous route selection
policies, in view of its computational and storage complexity, is
impractical with LS hop-by-hop routing. In contrast, PV can
accommodate heterogeneous route selection with little additional
overhead.
3.6 Information Hiding
PV has a clear advantage with respect to selective information
hiding. LS with hop-by-hop routing hinges on the ability of all
domains to have exactly the same information; this clearly
contradicts the notion of selective information hiding. That is, the
requirement to perform selective information hiding is unsatisfiable
with LS hop-by-hop routing.
3.7 Commonality between NR and SDR Components
In [Breslau-Estrin91] we argued for the use of LS in conjunction with
SDR. Therefore there is some preference for using LS with NR.
However, there are several reasons why NR and SDR would not use
exactly the same routing information, even if they did use the same
algorithm. Moreover, there are several opportunities for unifying
the management (distribution and storage) of routing and forwarding
information, even if dissimilar algorithms are used.
In considering the differences between NR and SDR we must address
several areas:
1. Routing information and distribution protocol: LS for SDR is
quite different from the LS in NR. For example, SDR LS need
not aggregate domains; to the contrary SDR LS requires detailed
information to generate special routes.
In addition, consistency requirements (essential for NR) are
unnecessary for the SDR component. Therefore LS information for
the SDR component can be retrieved on-demand, while the NR
component must use flooding of topology information.
2. Route computation algorithm: It is not clear whether route
computation algorithm(s) can be shared between the SDR and NR
components, given the difficulty of supporting heterogeneous
route selection policies in NR.
3. Forwarding information: The use of dissimilar route computation
algorithms does not preclude common handling of packet
forwarding. Even if LS were used for NR, the requirement would
be the same, i.e., that the forwarding agent can determine
whether to use a NR precomputed route or an SDR installed route
to forward a particular data packet.
In conclusion, using similar algorithms and mechanisms for SDR and NR
components would have benefits. However, these benefits do not
dominate the other factors as discussed before.
3.8 Summary
Given the performance complexity issues associated with global
routing, aggregation of routing information is essential; at the same
time we have argued that such aggregation must be flexible. Given
the difficulties of supporting LS hop-by-hop routing in the presence
of (a) flexible aggregation, (b) heterogeneous route selection
policies, and (c) incomplete or inconsistent routing information, we
see no alternative but to employ PV for the NR component of our
architecture.
Based on the above tradeoffs, our NR component employs a PV
architecture, where route computation and installation is done in a
distributed fashion by the routers participating in the NR component
[Footnote: Packet forwarding along routes produced by the NR
component can be accomplished by either source routing or hop-by-hop
routing. The latter is the primary choice because it reduces the
amount of state in routers (if route setup is employed), or avoids
encoding an explicit source route in network layer packets. However,
the architecture does not preclude the use of source routing (or
route setup) along the routes computed, but not installed, by the NR
component.].
The distributed algorithm combines some of the features of link state
with those of distance vector algorithms; in addition to next hop
information, the NR component maintains path attributes for each
route (e.g., the list of domains or routing domain confederations
that the routing information has traversed so far). The path
attributes that are carried along with a route express a variety of
routing policies, and make explicit the entire route to the
destination. With aggregation, this is a superset of the domains
that form the path to the destination.
4.0 Source-demand routing (SDR)
Inter-domain routers participating in the SDR component forward
packets according to routing information computed and installed by
the domain that originates the traffic (source routing domain).
It is important to realize that requiring route installation by the
source routing domain is not a matter of choice, but rather a
necessity. If a particular route is used by a small number of
domains (perhaps only one) then it is more appropriate to have the
source compute and install the special route instead of burdening the
intermediate nodes with the task of looking for and selecting a route
with the specialized requirements. In addition, if the demand for
the route is unpredictable, and thus can be determined only by the
source, it should be up to the source to install the route.
In general, information that is used by source routing domains for
computing source-demand routes reflects administrative (but not
operational) status of the routing facilities (i.e., configured
topology and policy) [Footnote: If SDR uses NR information then
operational status could be considered in some route selection.].
Consequently, it is possible for a source routing domain to compute a
route that is not operational at route installation time. The SDR
component attempts to notify the source domain of failures when route
installation is attempted. Similarly, the SDR component provides
mechanisms for the source routing domain to be notified of failures
along previously-installed active routes. In other words, the SDR
component performs routing that is adaptive to topological changes;
however, the adaptability is achieved as a consequence of the route
installation and route management mechanisms. This is different from
the NR component, where status changes are propagated and
incorporated by nodes as soon as possible. Therefore, to allow
faster adaptation to changes in the operational status of routing
facilities, the SDR component allows the source domain to switch to a
route computed by the NR component, if failure along the source-
demand route is detected (either during the route installation phase,
or after the route is installed), and if policy permits use of the NR
route.
The NR component will group domains into confederations to achieve
its scaling goals (see [IDRP91]). In contrast, SDR will allow an
AD-level route to be used by an individual domain without allowing
use by the entire confederation to which the domain belongs.
Similarly, a single transit domain may support a policy or special
TOS that is not supported by other domains in its confederation(s).
In other words, the architecture uses SDR to support non-
hierarchical, non-aggregated policies where and when needed.
Consequently, SDR by itself does not have the scaling properties of
NR. In compensation, SDR does not require a complete, global domain
map if portions of the world are never traversed or communicated
with. As a result of the looser routing structure, SDR does not
guarantee that a participating source routing domain will always have
sufficient information to compute a route to a destination. In
addition, if the domain does have sufficient information, it is
possible that the quantity may be large enough to preclude storage
and/or route computation in a timely fashion. However, despite the
lack of guarantees, it is a goal of the architecture to provide
efficient methods whereby sources can obtain the information needed
to compute desired routes [Footnote: The primary goal of policy or
TOS routing is to compute a route that satisfies a set of specialized
requirements, and these requirements take precedence over optimality.
In other words, even if a routing domain that participates in SDR or
NR has sufficient information to compute a route, given a particular
set of requirements, the architecture does not guarantee that the
computed route is optimal.].
Essential to SDR is the assumption that the routes installed on
demand will be used sparingly. The architecture assumes that at any
given moment the set of all source-demand routes installed in an
internet forms a small fraction of the total number of source-demand
routes that can potentially be installed by all the routing domains.
It is an assumption of the architecture that the number of routes
installed in a BR by the SDR component should be on the order of log
N (where N is the total number of routing domains in the Internet),
so that the scaling properties of the SDR component are comparable
with the scaling properties of the NR component. The NR component
achieves this property as a result of hierarchy.
Note that the above requirement does not imply that only a few
domains can participate in SDR, or that routes installed by the SDR
component must have short life times. What the requirement does
imply, is that the product of the number of routes specified by
domains that participate in SDR, times the average SDR-route life
time, is bounded. For example, the architecture allows either a
small number of SDR routes with relatively long average life times,
or a large number of SDR routes with relatively short average life
times. But the architecture clearly prohibits a large number of SDR
routes with relatively long average life times. The number of SDR
routes is a function of the number of domains using SDR routes and
the number of routes used per source domain.
In summary, SDR is well suited for traffic that (1) is not widely-
used enough (or is not sufficiently predictable or steady) to justify
computation and maintenance by the NR component, and (2) whose
duration is significantly longer than the time it takes to perform
the route installation procedure.
The architecture does not require all domains in the Internet to
participate in SDR. Therefore, issues of scalability (with respect
to the size of the Internet) become less crucial (though still
important) to the SDR component. Instead, the primary focus of the
SDR component is shifted towards the ability to compute routes that
satisfy specialized requirements, where we assume that the total
number of domains requiring special routes simultaneously through the
same part of the network is small relative to the total population.
4.1 Path Vector vs. Link State for SDR
It is feasible to use either a distance vector or link state method
of route computation along with source routing. One could imagine,
for instance, a protocol like BGP in which the source uses the full
AD path information it receives in routing updates to create a source
route. Such a protocol could address some of the deficiencies
identified with distance vector, hop-by-hop designs. However, we opt
against further discussion of such a protocol because there is less
to gain by using source routing without also using a link state
scheme. The power of source routing, in the context of inter-AD
policy routing, is in giving the source control over the entire
route. This goal cannot be realized fully when intermediate nodes
control which legal routes are advertised to neighbors, and therefore
to a source.
In other words, intermediate nodes should be able to preclude the use
of a route by expressing a transit policy, but if a route is not
precluded (i.e., is legal according to all ADs in the route), the
route should be made available to the source independent of an
intermediate domain's preferences for how its own traffic flows.
Therefore, the SDR component employs an IDPR-like architecture in
which link-state style updates are distributed with explicit policy
terms included in each update along with the advertising node's
connectivity.
4.2 Distribution of Routing Information
By using a hop-by-hop NR component based on PV to complement the
source-routing SDR component, we have alleviated the pressure to
aggregate SDR forwarding information; the large percentage of inter-
domain traffic carried, simultaneously, by any particular border
router will be forwarded using aggregated NR forwarding information.
However, the use of NR does not address the other major scaling
problem associated with SDR: that of distributing, storing, and
computing over a complete domain-level topology map. In this section
we describe promising opportunities for improving the scaling
properties of SDR routing information distribution, storage, and
computation.
Note that we do not propose to solve this problem in the same way
that we solve it for NR. A priori abstraction will not be employed
since different domains may require different methods of abstracting
the same routing information. For example, if we aggregate routing
information of domains that do not share the same policy and TOS
characteristics (i.e., services), then outside of the aggregate, only
those services that are offered by all domains in the aggregate will
be advertised. In order to locate special routes, SDR only uses
aggregates when the component domains (and in turn the aggregate)
advertise the required TOS and policy descriptions. When the
required TOS or policy characteristics are not offered by an
aggregate, full information about the component domains is used to
construct a route through those domains that do support the
particular characteristics. Consequently, we need some other, more
flexible, means of reducing the amount of information distributed and
held. We address two issues in turn: distribution of configured
topology and policy information, and distribution of dynamic status
information.
4.2.1 Configured Information
Information about the existence of inter-domain links, and policies
maintained by domains, changes slowly over time. This is referred to
as configured information. In the current IDPR specification
complete, global, configuration information is kept by a route server
in each domain. Route servers (RS) are the entities that compute
source routes. On startup a RS can download the connectivity
database from a neighbor RS; as domains, inter-domain links, or
policies change, the changes are flooded to a RS in each domain.
We have not yet specified the exact mechanisms for distributing
configured connectivity information for SDR. However, unlike the
current IDPR specification, the SDR component will not flood all
configured information globally. Several alternate methods for
organizing and distributing information are under investigation.
Configured information may be regularly distributed via an out-of-
band channel, e.g., CD/ROM. In a similar vein, this information
could be posted in several well-known locations for retrieval, e.g.,
via FTP. Between these "major" updates, aggregated collections of
changes may be flooded globally. Moreover, limited flooding (e.g.,
by hop-count) could be used as appropriate to the "importance" of the
change; while a policy change in a major backbone may still be
flooded globally, a new inter-regional link may be flooded only
within those regions, and information about an additional link to a
non-transit domain may not be available until the next regularly-
scheduled "major" distribution.
Changes that are not distributed as they occur will not necessarily
be discovered. However, a route server may learn pertinent
information by direct query of remote servers, or through error
messages resulting from traffic sent along failed routes. Complete
global flooding may be avoided by using some combination of these
mechanisms.
Even if an initial implementation uses a simple global flood, we must
study the problem of structuring connectivity information such that
it can be retrieved or distributed in a more selective manner, while
still allowing sources to discover desired routes. For example, we
imagine RSs requesting filtered information from each other. How the
RSs should define filters that will get enough information to find
special routes, while also effectively limiting the information, is
an open question. Again, the question is how to effectively
anticipate and describe what information is needed in advance of
computing the route.
The essential dilemma is that networks are not organized in a nicely
geographical or topologically consistent manner (e.g., it is not
effective to ask for all networks going east-west that are within a
certain north-south region of the target), hence a source domain does
not know what information it needs (or should ask for) until it
searches for, and discovers, the actual path. Even with a central
database, techniques are needed to structure configuration
information so that the potential paths that are most likely to be
useful are explored first, thereby reducing the time required for
route computation.
One promising approach organizes information using route fragments
(partial paths) [Footnote: Route fragments were first suggested by
Dave Clark and Noel Chiappa.]. Although the number of route
fragments grows faster than the number of domains (at least O(N^2)),
we can selectively choose those that will be useful to compute
routes. In particular, for each stub domain, fragments would be
constructed to several well-known backbones [Footnote: Route
fragments may be computed by a destination's route server and either
made available via information service queries or global flooding.
In addition, NR computed routes may be used as SDR route fragments.].
Among its benefits, this approach aggregates domain information in a
manner useful for computing source-routes, and provides an index,
namely the destination, which facilitates on-demand reference and
retrieval of information pertinent to a particular route computation.
At this point, it is not clear how route fragments will affect SDR's
ability to discover non-hierarchical routes.
4.2.2 Dynamic Status Information
Assuming a technique for global or partial distribution of configured
information, a second issue is whether, and how, to distribute
dynamic status information (i.e., whether an inter-domain connection
is up or down).
In the current version of IDPR, dynamic status information is flooded
globally in addition to configuration information. We propose to
distribute status information based strictly on locality. First,
dynamic information will be advertised within a small hop-count
radius. This simple and low-overhead mechanism exploits topological
locality. In addition to flooding status updates to nearby nodes, we
also want to provide more accurate route information for long
distance communications that entails more than a few network hops.
Reverse path update (RPU) is a mechanism for sending dynamic status
information to nodes that are outside the k-hop radius used for
updates, but that nevertheless would obtain better service (fewer
failed setups) by having Access to the dynamic information [Estrin-
etal91].
RPU uses the existing active routes (represented by installed setup
state or by a cache of the most recent source routes sent via the
node in question) as a hint for distribution of event notifications.
Instead of reporting only the status of the route being used, RPU
reports the status of the domain's other inter-domain connections.
If source routing exhibits route locality, the source is more likely
to use other routes going through the node in question; in any case
the overhead of the information about other links will be minimal.
In this way, sources will receive status information from regions of
the network through which they maintain active routes, even if those
regions are more than k hops away. Using such a scheme, k could be
small to maximize efficiency, and RPU could be used to reduce the
incidence of failed routes resulting from inaccurate status
information. This will be useful if long-path communication exhibits
route locality with respect to regions that are closer to the
destination (and therefore outside the k hop radius of flooded
information). In such situations, flooding information to the source
of the long route would be inefficient because k would have to be
equal to the length of the route, and in almost all cases, the
percentage of nodes that would use the information decreases
significantly with larger k.
4.3 Source-Demand Route Management
SDR may be built either on top of the network layer supported by the
NR component, or in parallel with it. SDR forwarding will be
supported via two techniques: loose source-routing and route setup.
The first technique, loose source-routing, would allow the originator
of a packet to specify a sequence of domains that the packet should
traverse on its path to a destination. Forwarding such a packet
within a domain, or even between domains within a confederation,
would be left to intra-domain routing. This avoids per-connection
state and supports transaction traffic.
The second technique, route setup, will be based on mechanisms
developed for IDPR and described in [IDPR90]. It is well suited to
conversations that persist significantly longer than a round-trip-
time. The setup protocol defines packet formats and the processing
of route installation request packets (i.e, setup packets). When a
source generates a setup packet, the first border router along the
specified source route checks the setup request, and if accepted,
installs routing information; this information includes a path ID,
the previous and next hops, and whatever other accounting-related
information the particular domain requires. The setup packet is
passed on to the next BR in the domain-level source route, and the
same procedure is carried out [Footnote: The setup packet may be
forwarded optimistically, i.e., before checks are completed, to
reduce latency.]. When the setup packet reaches the destination, an
accept message is propagated back hop by hop, and each BR en route
activates its routing information. Subsequent data packets traveling
along the same path to the destination include a path ID in the
packet. That path ID is used to locate the appropriate next-hop
information for each packet.
Border routers that support both the NR and the SDR components, must
be able to determine what forwarding mechanism to use. That is, when
presented with a network layer PDU, such a BR should be able to make
an unambiguous decision about whether forwarding of that PDU should
be handled by the NR or the SDR component. Discrimination mechanisms
are dependent on whether the new network layer introduced by the SDR
component is built on top of, or in parallel with, the network layers
supported by the NR component. Once the discrimination is made,
packets that have to be forwarded via routes installed by the SDR
component are forwarded to the exit port associated with the
particular Path ID in the packet header. Packets that have to be
forwarded via routes installed by the NR component are forwarded to
the exit port associated with the particular destination and Type of
Service parameters (if present) in their packet headers.
Next, we describe the primary differences between the IDPR setup
procedure previously specified, and the procedure we propose to
develop for this hybrid architecture.
During route installation, if a BR on the path finds that the
remainder of the indicated route from the BR to the destination is
identical to the NR route from the BR to the destination, then the BR
can turn off the SDR route at that point and map it onto the NR
route. For this to occur, the specifications of the SDR route must
completely match those of the NR route. In addition, the entire
forward route must be equivalent (i.e., the remaining hops to the
destination).
Moreover, if the NR route changes during the course of an active SDR
route, and the new NR route does not match the SDR route, then the
SDR route must be installed for the remainder of the way to the
destination. Consequently, when an SDR route is mapped onto an NR
route, the original setup packet must be saved. A packet traveling
from a source to destination may therefore traverse both an SDR and
an NR route segment; however, a packet will not traverse another SDR
segment after traveling over an NR segment. However, during
transient periods packets could traverse the wrong route and
therefore this must be an optional and controllable feature.
A source can also request notification if a previously-down link or
node returns to operation some time after a requested route setup
fails. If a BR on the route discovers that the requested next-hop BR
is not available, the BR can add the source to a notification list
and when the next-hop BR becomes reachable, a notification can be
sent back to the source. This provides a means of flushing out bad
news when it is no longer true. For example, a domain might decide
to route through a secondary route when its preferred route fails;
the notification mechanism would inform the source in a timely manner
when its preferred route is available again.
A third option addresses adaptation after route installation. During
packet forwarding along an active SDR route, if a BR finds that the
SDR route has failed, it may redirect the traffic along an existing
NR route to the destination. This adaptation is allowed only if use
of the NR route does not violate policy; for example, it may provide
a less desirable type of service. This is done only if the source
selects the option at route setup time. It is also up to the source
whether it is to be notified of such actions.
When a SDR route does fail, the detecting BR sends notification to
the source(s) of the active routes that are affected. Optionally,
the detecting BR may include additional information about the state
of other BRs in the same domain. In particular, the BR can include
its domain's most recent "update" indicating that domain's inter-
domain links and policy. This can be helpful to the extent there is
communication locality; i.e., if alternative routes might be used
that traverse the domain in question, but avoid the failed BR.
In summary, when a route is first installed, the source has several
options (which are represented by flags in the route setup packet):
1. If an NR route is available that satisfies all local policy
and TOS, then use it. Otherwise...
2. Indicate whether the source wants to allow the setup to
default to a NR route if the SDR route setup fails.
3. Request notification of mapping to a NR route.
4. Request additional configured information on failure.
5. Request addition to a notification list for resource
re-availability.
6. Allow data packets to be rerouted to a NR route when failure
happens after setup (so long as no policy is violated).
7. Request notification of a reroute of data packets.
8. Request additional configured information on failure notice
when the route is active.
9. Request addition to a notification list if an active route
fails.
5.0 The Unified Architecture
In addition to further evaluation and implementation of the proposed
architecture, future research must investigate opportunities for
increased unification of the two components of our architecture. We
are investigating several opportunities for additional commonality:
1. Routing Information Base:
Perhaps a single RIB could be shared by both NR and SDR.
NR routes can be represented as a directed graph labeled
with flags (on the nodes or links) corresponding to the
generic transit constraints. SDR requires that this graph
be augmented by links with non-generic policies that have
been discovered and maintained for computing special routes;
in addition, special policy flags may be added to links
already maintained by the NR component.
2. Information Distribution:
The NR path vectors could include address(es) of repositories
for SDR-update information for each AD (or confederation) to
assist the SDR component in retrieving selective information
on demand. For domains with minimal policies, where the space
required for policy information is smaller than the space
required for a repository address (e.g., if the policies for
the domain listed are all wildcard), the NR path vectors could
include a flag to that effect.
3. Packet Forwarding:
We should consider replacing the current IDPR-style network
layer (which contains a global path identifier used in
forwarding data packets to the next policy gateway on an
IDPR route) with a standard header (e.g., IP or CLNP),
augmented with some option fields. This would unify the
packet header parsing and forwarding functions for SDR and NR,
and possibly eliminate some encapsulation overhead.
4. Reachability Information:
Currently IDRP distributes network reachability information
within updates, whereas IDPR only distributes domain
reachability information. IDPR uses a domain name service
function to map network numbers to domain numbers; the latter
is needed to make the routing decision. We should consider
obtaining the network reachability and domain information in
a unified manner.
5.1 Applicability to Various Network Layer Protocols
The proposed architecture is designed to accommodate such existing
network layer protocols as IP ([Postel81]), CLNP ([ISO-473-88]), and
ST-II ([ST2-90]). In addition, we intend for this architecture to
support future network layer mechanisms, e.g., Clark and Jacobson's
proposal or Braden and Casner's Integrated Services IP. However on
principal we can not make sweeping guarantees in advance of the
mechanisms themselves. In any case, not all of the mentioned
protocols will be able to utilize all of the capabilities provided by
the architecture. For instance, unless the increase in the number of
different types of services offered is matched by the ability of a
particular network layer protocol to unambiguously express requests
for such different types of services, the capability of the
architecture to support routing in the presence of a large number of
different types of service is largely academic. That is, not all
components of the architecture will have equal importance for
different network layer protocols. On the other hand, this
architecture is designed to serve the future global internetworking
environment. The extensive research and development currently
underway to implement and evaluate network mechanisms for different
types of service suggests that future networks will offer such
services.
One of the fundamental issues in the proposed architecture is the
issue of single versus multiple protocols. The architecture does not
make any assumptions about whether each network layer is going to
have its own inter-domain routing protocol, or a single inter-domain
routing protocol will be able to cover multiple network layers
[Footnote: Similar issue already arose with respect to the intra-
domain routing protocol, which generated sufficient amount of
controversy within the Internet community. It is our opinion, that
the issue of single versus multiple protocols is more complex for the
inter-domain routing than for the intra-domain routing.]. That is,
the proposed architecture can be realized either by a single inter-
domain routing protocol covering multiple network layers, or by
multiple inter-domain routing protocols (with the same architecture)
tailored to a specific network layer [Footnote: If the single
protocol strategy is adopted, then it is likely that IDRP will be
used as a base for the NR component. Since presently IDRP is
targeted towards CLNP, further work is needed to augment it to
support IP and ST-II. If the multiple protocol strategy is adopted,
then it is likely that BGP will be used as a base for the NR
component for IP, and IDRP will be used as a base for the NR
component for CLNP. Further work is needed to specify protocol in
support for the NR component for ST-II. Additional work may be
needed to specify new features that may be added to BGP.].
5.2 Transition
The proposed architecture is not intended for full deployment in the
short term future. We are proposing this architecture as a goal
towards which we can begin guiding our operational and research
investment over the next 5 years.
At the same time, the architecture does not require wholesale
overhaul of the existing Internet. The NR component may be phased in
gradually. For example, the NR component for IP may be phased in by
replacing existing EGP-2 routing with BGP routing. Once the NR
component is in place, it can be augmented by the facilities provided
by the SDR component.
The most critical components of the architecture needed to support
SDR include route installation and packet forwarding in the routers
that support SDR. Participation as a transit routing domain requires
that the domain can distribute local configuration information (LCI)
and that some of its routers implement the route installation and
route management protocols. Participation as a source requires that
the domain have access to a RS to compute routes, and that the source
domain has a router that implements the route installation and route
management protocols. In addition, a network management entity must
describe local configuration information and send it to the central
repository(ies). A collection and distribution mechanism must be put
in place, even if it is centralized.
6.0 Conclusions and Future Work
In summary, the proposed architecture combines hop-by-hop path-
vector, and source-routed link-state, protocols, and uses each for
that which it is best suited: NR uses PV and multiple, flexible,
levels of confederations to support efficient routing of generic
packets over generic routes; SDR uses LS computation over a database
of configured and dynamic information to route special traffic over
special routes. In the past, the community has viewed these two as
mutually exclusive; to the contrary, they are quite complementary and
it is fortunate that we, as a community, have pursued both paths in
parallel. Together these two approaches will flexibly and
efficiently support TOS and policy routing in very large global
internets.
It is now time to consider the issues associated with combining and
integrating the two. We must go back and look at both architectures
and their constituent protocols, eliminate redundancies, fill in new
holes, and provide seamless integration.
7.0 Acknowledgments
We would like to thank Hans-Werner Braun (San Diego Supercomputer
Center), Lee Breslau (USC), Scott Brim (Cornell University), Tony Li
(cisco Systems), Doug Montgomery (NIST), Tassos Nakassis (NIST),
Martha Steenstrup (BBN), and Daniel Zappala (USC) for their comments
on a previous draft.
8.0 References
[ANSI 87-150R] "Intermediate System to Intermediate System Intra-
Domain Routing Exchange Protocol", ANSI X3S3.3/87-150R.
[BGP 91] Lougheed, K., and Y. Rekhter, "A Border Gateway Protocol 3
(BGP-3)", RFC1267, cisco Systems, T.J. Watson Research Center, IBM
Corp., October 1991.
[Breslau-Estrin 91] Breslau, L., and D. Estrin, "Design and
Evaluation of Inter-Domain Policy Routing Protocols", To appear in
Journal of Internetworking Research and Experience, 1991. (Earlier
version appeared in ACM Sigcomm 1990.)
[Clark 90] Clark, D., "Policy Routing in Internetworks", Journal of
Internetworking Research and Experience, Vol. 1, pp. 35-52, 1990.
[Dijkstra 59] Dijkstra, E., "A Note on Two Problems in Connection
with Graphs", Numer. Math., Vol. 1, 1959, pp. 269-271.
[ECMA89] "Inter-Domain Intermediate Systems Routing", Draft
Technical Report ECMA TR/ISR, ECMA/TC32-TG 10/89/56, May 1989.
[EGP] Rosen, E., "Exterior Gateway Protocol (EGP)", RFC827, BBN,
October 1982.
[Estrin 89] Estrin, D., "Policy Requirements for Inter
Administrative Domain Routing", RFC1125, USC Computer Science
Department, November 1989.
[Estrin-etal91] Estrin, D., Breslau, L., and L. Zhang, "Protocol
Mechanisms for Adaptive Routing in Global Multimedia Internets",
University of Southern California, Computer Science Department
Technical Report, CS-SYS-91-04, November 1991.
[Hedrick 88] Hedrick, C., "Routing Information Protocol", RFC1058,
Rutgers University, June 1988.
[Honig 90] Honig, J., Katz, D., Mathis, M., Rekhter, Y., and J. Yu,
"Application of the Border Gateway Protocol in the Internet", RFC
1164, Cornell Univ. Theory Center, Merit/NSFNET, Pittsburgh
Supercomputing Center, T.J. Watson Research Center, IBM Corp., June
1990.
[IDPR90] Steenstrup, M., "Inter-Domain Policy Routing Protocol
Specification and Usage: Version 1", Work in Progress, February 1991.
[IDRP91] "Intermediate System to Intermediate System Inter-domain
Routeing Exchange Protocol", ISO/IEC/ JTC1/SC6 CD10747.
[ISIS10589] "Information Processing Systems - Telecommunications and
Information Exchange between Systems - Intermediate System to
Intermediate System Intra-Domain Routing Exchange Protocol for use in
Conjunction with the protocol for providing the Connectionless-mode
Network Service (ISO 8473)", ISO/IEC 10589.
[ISO-473 88] "Protocol for providing the connectionless-mode network
service", ISO 8473, 1988.
[Jaffee 82] Jaffee, J., and F. Moss, "A Responsive Distributed
Routing Algorithm for Computer Networks", IEEE Transactions on
Communications, July 1982.
[Little 89] Little, M., "Goals and Functional Requirements for
Inter-Autonomous System Routing", RFC1126, SAIC, October 1989.
[Oran 89] Oran, D., "Expert's Paper: The Relationship between
Addressing and Routeing", ISO/JTC1/SC6/WG2, 1989.
[OSPF] Moy, J., "The Open Shortest Path First (OSPF) Specification",
RFC1131, Proteon, October 1989.
[Postel 81] Postel, J., "Internet Protocol", RFC791, DARPA,
September 1981.
[Rekhter 91] Rekhter, Y., "IDRP protocol analysis: storage
complexity", IBM Research Report RC17298(#76515), October 1991.
[Shin87] Shin, K., and M. Chen, "Performance Analysis of Distributed
Routing Strategies Free of Ping-Pong-Type Looping", IEEE Transactions
on Computers, February 1987.
[ST2-90] Topolcic, C., "Experimental Internet Stream Protocol,
version 2 (ST II)", RFC1190, CIP Working Group, October 1990.
[Zaumen 91] Zaumen, W., and J. Garcia-Luna-Aceves, "Dynamics of Link
State and Loop-free Distance-Vector Routing Algorithms", ACM Sigcomm
'91, Zurich, Switzerland, September 1991.
[Zhang 91] Zhang, L., "Virtual Clock: A New Traffic Control Algorithm
for Packet Switching Networks".
Security Considerations
Security issues are not discussed in this memo.
Authors' Addresses
Deborah Estrin
University of Southern California
Computer Science Department, MC 0782
Los Angeles, California 90089-0782
Phone: (310) 740-4524
EMail: estrin@usc.edu
Yakov Rekhter
IBM T.J. Watson Research Center
P.O. Box 218
Yorktown Heights, New York 10598
Phone: (914) 945-3896
EMail: yakov@ibm.com
Steven Hotz
University of Southern California
Computer Science Department, MC 0782
Los Angeles, California 90089-0782
Phone: (310) 822-1511
EMail: hotz@usc.edu
