Network Working Group M. Steenstrup
Request for Comments: 1477 BBN Systems and Technologies
IDPR as a Proposed Standard
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
This document contains a discussion of inter-domain policy routing
(IDPR), including an overview of functionality and a discussion of
experiments. The objective of IDPR is to construct and maintain
routes between source and destination administrative domains, that
provide user traffic with the services requested within the
constraints stipulated for the domains transited.
Four documents describe IDPR in detail:
M. Steenstrup. An architecture for inter-domain policy routing.
RFC 1478. July 1993.
M. Steenstrup. Inter-domain policy routing protocol
specification: version 1. RFC 1479. July 1993.
H. Bowns and M. Steenstrup. Inter-domain policy routing
configuration and usage. Work in Progress. July 1991.
R. Woodburn. Definitions of managed objects for inter-domain
policy routing (version 1). Work in Progress. March 1993.
This is a product of the Inter-Domain Policy Routing Working Group of
the Internet Engineering Task Force (IETF).
As data communications technologies evolve and user populations grow,
the demand for internetworking increases. The Internet currently
comprises over 7000 operational networks and over 10,000 registered
networks. In fact, for the last several years, the number of
constituent networks has approximately doubled annually. Although we
do not expect the Internet to sustain this growth rate, we must
prepare for the Internet of five to ten years in the future.
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Internet connectivity has increased along with the number of
component networks. Internetworks proliferate through
interconnection of autonomous, heterogeneous networks administered by
separate authorities. We use the term "administrative domain" (AD)
to refer to any collection of contiguous networks, gateways, links,
and hosts governed by a single administrative authority that selects
the intra-domain routing procedures and addressing schemes, specifies
service restrictions for transit traffic, and defines service
requirements for locally-generated traffic.
In the early 1980s, the Internet was purely hierarchical, with the
ARPANET as the single backbone. The current Internet possesses a
semblance of a hierarchy in the collection of backbone, regional,
metropolitan, and campus domains that compose it. However,
technological, economical, and political incentives have prompted the
introduction of inter-domain links outside of those in the strict
hierarchy. Hence, the Internet has the properties of both
hierarchical and mesh connectivity.
We expect that, over the next five years, the Internet will grow to
contain O(10) backbone domains, most providing connectivity between
many source and destination domains and offering a wide range of
qualities of service, for a fee. Most domains will connect directly
or indirectly to at least one Internet backbone domain, in order to
communicate with other domains. In addition, some domains may
install direct links to their most favored destinations. Domains at
the lower levels of the hierarchy will provide some transit service,
limited to traffic between selected sources and destinations.
However, the majority of Internet domains will be "stubs", that is,
domains that do not provide any transit service for any other domains
but that connect directly to one or more transit domains.
The bulk of Internet traffic will be generated by hosts in the stub
domains, and thus, the applications running in these hosts will
determine the traffic service requirements. We expect application
diversity encompassing electronic mail, desktop videoconferencing,
scientific visualization, and distributed simulation, for example.
Many of these applications have strict requirements on loss, delay,
In such a large and heterogeneous Internet, the routing procedures
must be capable of ensuring that traffic is forwarded along routes
that offer the required services without violating domain usage
restrictions. We believe that IDPR meets this goal; it has been
designed to accommodate an Internet comprising O(10,000)
administrative domains with diverse service offerings and
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IDPR generates, establishes, and maintains "policy routes" that
satisfy the service requirements of the users and respect the service
restrictions of the transit domains. Policy routes are constructed
using information about the services offered by and the connectivity
between administrative domains and information about the services
requested by the users.
With IDPR, each domain administrator sets "transit policies" that
dictate how and by whom the resources in its domain should be used.
Transit policies are usually public, and they specify offered
- Access restrictions: e.g., applied to traffic to or from certain
domains or classes of users.
- Quality: e.g., delay, throughput, or error characteristics.
- Monetary cost: e.g., charge per byte, message, or session time.
Each domain administrator also sets "source policies" for traffic
originating in its domain. Source policies are usually private, and
they specify requested services comprising:
- Access: e.g., domains to favor or avoid in routes.
- Quality: e.g., acceptable delay, throughput, and reliability.
- Monetary cost: e.g., acceptable cost per byte, message, or session
The basic IDPR functions include:
- Collecting and distributing routing information, i.e., domain
transit policy and connectivity information. IDPR uses link state
routing information distribution, so that each source domain may
obtain routing information about all other domains.
- Generating and selecting policy routes based on the routing
information distributed and on source policy information. IDPR
gives each source domain complete control over the routes it
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- Setting up paths across the Internet, using the policy routes
- Forwarding messages across and between administrative domains along
the established paths. IDPR uses source-specified message
forwarding, giving each source domain complete control over the
paths traversed by its hosts' inter-domain traffic.
- Maintaining databases of routing information, inter-domain policy
routes, forwarding information, and configuration information.
Several different entities are responsible for performing the IDPR
- "Policy gateways", the only IDPR-recognized connecting points
between adjacent domains, collect and distribute routing
information, participate in path setup, maintain forwarding
information databases, and forward data messages along established
- "Path agents", resident within policy gateways, act on behalf of
hosts to select policy routes, to set up and manage paths, and to
maintain forwarding information databases. Any Internet host can
reap the benefits of IDPR, as long as there exists a path agent
willing to act on its behalf and a means by which the host's
messages can reach that path agent.
- Special-purpose servers maintain all other IDPR databases as
o Each "route server" is responsible for both its database of
routing information, including domain connectivity and transit
policy information, and its database of policy routes. Also,
each route server generates policy routes on behalf of its
domain, using entries from its routing information database
and using source policy information supplied through
configuration or obtained directly from the path agents. A
route server may reside within a policy gateway, or it may
exist as an autonomous entity. Separating the route server
functions from the policy gateways frees the policy gateways
from both the memory intensive task of routing information
database and route database maintenance and the
computationally intensive task of route generation.
o Each "mapping server" is responsible for its database of
mappings that resolve Internet names and addresses to
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administrative domains. The mapping server function can be
easily integrated into an existing name service such as the
o Each "configuration server" is responsible for its database of
configured information that applies to policy gateways, path
agents, and route servers in the given administrative domain.
Configuration information for a given domain includes source
and transit policies and mappings between local IDPR entities
and their addresses. The configuration server function can be
easily integrated into a domain's existing network management
There are two kinds of IDPR messages:
- "Data messages" containing user data generated by hosts.
- "Control messages" containing IDPR protocol-related control
information generated by policy gateways and route servers.
Within the Internet, only policy gateways and route servers must be
able to generate, recognize, and process IDPR messages. Mapping
servers and configuration servers perform necessary but ancillary
functions for IDPR, and they are not required to execute IDPR
protocols. The existence of IDPR is invisible to all other gateways
and hosts. Using encapsulation across each domain, an IDPR message
tunnels from source to destination across the Internet through
domains that may employ disparate intra-domain addressing schemes and
IDPR contains mechanisms for verifying message integrity and source
authenticity and for protecting against certain types of denial of
service attacks. It is particularly important to keep IDPR control
messages intact, because they carry control information critical to
the construction and use of viable policy routes between domains.
All IDPR messages carry a single piece of information, referred to in
the IDPR documentation as the "integrity/authentication value", which
may be used not only to detect message corruption but also to verify
the authenticity of the message's source IDPR entity. The Internet
Assigned Numbers Authority (IANA) specifies the set of valid
algorithms which may be used to compute the integrity/authentication
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values. This set may include algorithms that perform only message
integrity checks such as n-bit cyclic redundancy checksums (CRCs), as
well as algorithms that perform both message integrity and source
authentication checks such as signed hash functions of message
Each domain administrator is free to select any
integrity/authentication algorithm, from the set specified by the
IANA, for computing the integrity/authentication values contained in
its domain's messages. However, we recommend that IDPR entities in
each domain be capable of executing all of the valid algorithms so
that an IDPR message originating at an entity in one domain can be
properly checked by an entity in another domain.
IDPR control messages must carry a non-null integrity/authentication
value. We recommend that control message integrity/authentication be
based on a digital signature algorithm applied to a one-way hash
function, such as RSA applied to MD5, which simultaneously verifies
message integrity and source authenticity. The digital signature may
be based on either public key or private key cryptography. However,
we do not require that IDPR data messages carry a non-null
integrity/authentication value. In fact, we recommend that a higher
layer (end-to-end) procedure assume responsibility for checking the
integrity and authenticity of data messages, because of the amount of
Each IDPR message carries a timestamp (expressed in seconds elapsed
since 1 January 1970 0:00 GMT) supplied by the source IDPR entity,
which serves to indicate the age of the message. IDPR entities use
the absolute value of a timestamp to confirm that the message is
current and use the relative difference between timestamps to
determine which message contains the most recent information. All
IDPR entities must possess internal clocks that are synchronized to
some degree, in order for the absolute value of a message timestamp
to be meaningful. The synchronization granularity required by IDPR
is on the order of minutes and can be achieved manually.
Each IDPR recipient of an IDPR control message must check that the
message's timestamp is in the acceptable range. A message whose
timestamp lies outside of the acceptable range may contain stale or
corrupted information or may have been issued by a source whose clock
has lost synchronization with the message recipient. Such messages
must therefore be discarded, to prevent propagation of incorrect IDPR
control information. We do not require IDPR entities to perform a
timestamp acceptability test for IDPR data messages, but instead
leave the choice to the individual domain administrators.
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IDPR provides policy routing among administrative domains and has
been designed to accommodate an Internet containing tens of thousands
of domains, supporting diverse source and transit policies.
In order to construct policy routes, route servers require routing
information at the domain level only; no intra-domain details need be
included in IDPR routing information. Thus, the size of the routing
information database maintained by a route server depends on the
number of domains and transit policies and not on the number hosts,
gateways, or networks in the Internet.
We expect that, within a domain, a pair of IDPR entities will
normally be connected such that when the primary intra-domain route
fails, the intra-domain routing procedure will be able to use an
alternate route. In this case, a temporary intra-domain failure is
invisible at the inter-domain level. Thus, we expect that most
intra-domain routing changes will be unlikely to force inter-domain
Policy gateways distribute routing information when detectable
inter-domain changes occur but may also elect to distribute routing
information periodically as a backup. Thus, policy gateways do not
often need to generate and distribute routing information messages,
and the frequency of distribution of these messages depends only
weakly on intra-domain routing changes.
IDPR entities rely on intra-domain routing procedures operating
within domains to transport inter-domain messages across domains.
Hence, IDPR messages must appear well-formed according to the intra-
domain routing procedures and addressing schemes in each domain
traversed; this requires appropriate header encapsulation of IDPR
messages at domain boundaries. Only policy gateways and route
servers must be capable of handling IDPR-specific messages; other
gateways and hosts simply treat the encapsulated IDPR messages like
any other. Thus, for the Internet to support IDPR, only a small
proportion of Internet entities require special IDPR software.
With domain-level routes, many different traffic flows may use not
only the same policy route but also the same path, as long their
source domains, destination domains, and requested services are
identical. Thus, the size of the forwarding information database
maintained by a policy gateway depends on the number of domains and
source policies and not on the number of hosts in the Internet.
Moreover, memory associated with failed, expired, or disused paths
can be reclaimed for new paths, and thus forwarding information for
many paths can be accommodated.
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We believe that many Internet domains will benefit from the
introduction of IDPR. However, the decision to support IDPR in a
given domain is an individual one, left to the domain administrator;
not all domains must support IDPR.
Within a domain that supports IDPR, other inter-domain routing
procedures, such as BGP and EGP, can comfortably coexist. Each
inter-domain routing procedure is independent of the others. The
domain administrator determines the relationship among the inter-
domain routing procedures by deciding which of its traffic flows
should use which inter-domain routing procedures and by configuring
this information for use by the policy gateways.
Hosts in stub domains may have strict service requirements and hence
will benefit from the policy routing provided by IDPR. However, the
stub domain itself need not support IDPR in order for its traffic
flows to use IDPR routes. Instead, a "proxy domain" may perform IDPR
functions on behalf of the stub. The proxy domain must be reachable
from the stub domain according to an inter-domain routing procedure
independent of IDPR. Administrators of the stub and potential proxy
domains mutually negotiate the relationship. Once an agreement is
reached, the administrator of the stub domain should provide the
proxy domain with its hosts' service requirements.
IDPR policy routes must traverse a contiguous set of IDPR domains.
Hence, the degree of IDPR deployment in transit domains will
determine the availability of IDPR policy routes for Internet users.
For a given traffic flow, if there exists no contiguous set of IDPR
domains between the source and destination, the traffic flow relies
on an alternate inter-domain routing procedure to provide a route.
However, if there does exist a contiguous set of IDPR domains between
the source and destination, the traffic flow may take advantage of
policy routes provided by IDPR.
To date, there exist two implementations of IDPR: one an independent
prototype and the other an integral part of the gated UNIX process.
We describe each of these implementations and our experience with
them in the following sections.
During the summer of 1990, the IDPR development group consisting of
participants from USC, SAIC, and BBN began work on a UNIX-based
software prototype of IDPR, designed for implementation in Sun
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workstations. This prototype consisted of multiple user-level
processes to provide the basic IDPR functions together with kernel
modifications to speed up IDPR data message forwarding.
Most, but not all, of the IDPR functionality was captured in the
prototype. In the interests of producing working software as quickly
as possible, we intentionally left out of the IDPR prototype support
for source policies and for multiple policy gateways connecting two
domains. This simplified configuration and route generation without
compromising the basic functionality of IDPR.
The IDPR prototype software was extensively instrumented to provide
detailed information for monitoring its behavior. The
instrumentation allowed us to detect events including but not limited
- Change in policy gateway connectivity to adjacent domains.
- Change in transit policies configured for a domain.
- Transmission and reception of link state routing information.
- Generation of policy routes, providing a description of the actual
- Transmission and reception of path control information.
- Change of path state, such as path setup or teardown.
With the extensive behavioral information available, we were able to
track most events occurring in our test networks and hence determine
whether the prototype software provided the expected functionality.
In February 1991, the IDPR development group began experimenting with
the completed IDPR prototype software. Each IDPR development site
had its own testing environment, consisting of a set of
interconnected Sun workstations, each workstation performing the
functions of a policy gateway and route server:
- USC used a laboratory test network consisting of SPARC1+
workstations, each pair of workstations connected by an Ethernet
segment. The topology of the test network could be arbitrarily
- SAIC used Sun3 workstations in networks at Sparta and at MITRE.
These two sites were connected through Alternet using a 9.6kb SLIP
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link and through an X.25 path across the DCA EDN testbed.
- BBN used SPARC1+ workstations at BBN and ISI connected over both
DARTnet and TWBnet.
The principal goal of our experiments with the IDPR prototype
software was to provide a proof of concept. In particular, we set
out to verify tha t the IDPR prototype software was able to:
- Monitor connectivity across and between domains.
- Update routing information when inter-domain connectivity changed
or when new transit policies were configured.
- Distribute routing information to all domains.
- Generate acceptable policy routes based on current link state
- Set up and maintain paths for these policy routes.
- Tear down paths that contained failed components, supported stale
policies, or attained their maximum age.
Furthermore, we wanted to verify that the IDPR prototype software
quickly detected and adapted to those events that directly affected
The internetwork topology on which we based most of our experiments
consisted of four distinct administrative domains connected in a
ring. Two of the four domains served as host traffic source and
destination, AD S and AD D respectively, while the two intervening
domains provided transit service for the host traffic, AD T1 and AD
T2. AD S and AD D each contained a single policy gateway that
connected to two other policy gateways, one in each transit domain.
AD T1 and AD T2 each contained at most two policy gateways, each
policy gateway connected to the other and to a policy gateway in the
source or destination domain. This internetwork topology provided
two distinct inter-domain routes between AD S and AD D, allowing us
to experiment with various component failure and transit policy
reconfiguration scenarios in the transit domains.
For the first set of experiments, we configured transit policies for
AD T1 and AD T2 that were devoid of access restrictions. We then
initialized each policy gateway in our internetwork, loading in the
domain-specific configurations and starting up the IDPR processes.
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In our experiments, we did not use mapping servers; instead, we
configured address/domain mapping tables in each policy gateway.
After policy gateway initialization, we observed that each policy
gateway immediately determined the connectivity to policy gateways in
its own domain and in the adjacent domains. The representative
policy gateway in each domain then generated a routing information
message that was received by all other policy gateways in the
To test the route generation and path setup functionality of the IDPR
prototype software, we began a telnet session between a host in AD S
and a host in AD D. We observed that the telnet traffic prompted the
path agent resident in the policy gateway in AD S to request a policy
route from its route server. The route server then generated a
policy route and returned it to the path agent. Using the policy
route supplied by the route server, the path agent initiated path
setup, and the telnet session was established immediately.
Having confirmed that the prototype software satisfactorily performed
the basic IDPR functions, we proceeded to test the software under
changing network conditions. The first of these tests showed that
the IDPR prototype software was able to deal successfully with a
component failure along a path. To simulate a path component
failure, we terminated the IDPR processes on a policy gateway in the
transit domain, AD T1, traversed by the current path. The policy
gateways on either side of the failed policy gateway immediately
detected the failure. Next, these two policy gateways, representing
two different domains, each issued a routing information message
indicating the connectivity change and each initiated path teardown
for its remaining path section.
Once the path was torn down, the path agent agent in AD S requested a
new route from its route server, to carry the existing telnet
traffic. The route server, having received the new routing
information messages, proceeded to generate a policy route through
the other transit domain, AD T2. Then, the path agent in AD S set up
a path for the new route supplied by the route server. Throughout
the component failure and traffic rerouting, the telnet session
At this point, we restored the failed policy gateway in AD T1 to the
functional state, by restarting its IDPR processes. The restored
policy gateway connectivity prompted the generation and distribution
of routing information messages indicating the change in domain
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Having returned the internetwork topology to its initial
configuration, we proceeded to test that the IDPR prototype software
was able to deal successfully with transit policy reconfiguration.
The current policy route carrying the telnet traffic traversed AD T2.
We then reconfigured the transit policy for AD T2 to preclude access
of traffic travelling from AD S to AD D. The transit policy
reconfiguration prompted both the distribution of routing information
advertising the new transit policy for AD T2 and the initiation of
Once the path was torn down, the path agent in AD S requested a new
route from its route server, to carry the existing telnet traffic.
The route server, having received the new routing information
message, proceeded to generate a policy route through the original
transit domain, AD T1. Then, the path agent in AD S set up a path
for the new route supplied by the route server. Throughout the
policy reconfiguration and rerouting, the telnet session remained
This set of experiments, although simple, tested all of the major
functionality of the IDPR prototype software and demonstrated that
the prototype software could quickly and accurately adapt to changes
in the internetwork.
We (USC and SAIC members of the IDPR development group) evaluated the
performance of the path setup and message forwarding portions of the
IDPR prototype software. For path setup, we measured the amount of
processing required at the source path agent and at intermediate
policy gateways during path setup. For message forwarding, we
compared the processing required at each policy gateway when using
IDPR forwarding with IP encapsulation and when using only IP
forwarding. We also compared the processing required when no
integrity/authentication value was calculated for the message and
when the RSA/MD4 algorithms were employed.
Our performance measurements were encouraging, but we have not listed
them here. We emphasize that although we tried to produce efficient
software for the IDPR prototype, we were not able to devote much
effort to optimizing this software. Hence, the performance
measurements for the IDPR prototype software should not be blindly
extrapolated to other implementations of IDPR. To obtain a copy of
the performance measurements for path setup and message forwarding in
the IDPR prototype software, contact Robert Woodburn
(firstname.lastname@example.org) and Deborah Estrin (email@example.com).
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In 1992, SRI joined the IDPR development group, and together SRI,
SAIC, and BBN completed the task of integrating IDPR into the gated
UNIX process. As a result, IDPR is now available as part of gated.
The gated version of IDPR contains the full functionality of IDPR
together with a simple yet versatile user interface for IDPR
configuration. As a single process, the gated version of IDPR
performs more efficiently than the multiple-process prototype
The gated version of IDPR is freely available to the Internet
community. Hence, anyone with a UNIX-based machine can experiment
with IDPR, without investing any money or implementation effort. By
making IDPR widely accessible, we can gain Internet experience by
introducing IDPR into operational networks with real usage
constraints and transporting host traffic with real service
requirements. Currently, a pilot deployment and demonstration of
IDPR is under way in selected locations in the Internet.