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Network Working Group Y. Rekhter
INTERNET DRAFT T.J. Watson Research Center, IBM Corp.
C. Topolcic
CNRI
April 1993
Exchanging Routing Information Across Provider/Subscriber Boundaries
in the CIDR Environment
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
``working draft'' or ``work in progress.''
Please check the 1id-abstracts.txt listing contained in the
internet-drafts Shadow Directories on nic.ddn.mil, nnsc.nsf.net,
nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au to learn the
current status of any Internet Draft.
1 Introduction
Classless Inter-Domain Routing (CIDR) has been adopted as a solution
to the scaling problem in the Internet. The overall CIDR architecture
is described in [1]. The architecture for IP address assignment with
CIDR is covered in [2] and [3]. The inter-domain routing protocols
that are capable of supporting CIDR are covered in [4], [5], and [6].
The purpose of this document is twofold. First, it describes various
alternatives of exchanging inter-domain routing information across
domain boundaries, where one of the peering domain is CIDR-capable
and another is not. Second, it addresses the implications of running
CIDR-capable inter-domain routing protocols (e.g. BGP-4, IDRP) on
intra-domain routing.
This document is not intended to cover all the cases (either real or
imaginable). Rather, it focuses on what is viewed to be the most
common cases. We expect that individual service
providers/subscribers will use this document as guidelines in
Expiration Date October 1993 [Page 1]
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establishing specific operational plans that would deal with
transition to CIDR.
Throughout the document we use the concepts of "network service
provider" and "network service subscriber". These concepts were
introduced in [3]. For the sake of brevity we'll use terms "service
provider" and "service subscriber", rather than "network service
provider" or "network service subscriber".
This document defines a CIDR-capable provider/subscriber as the
provider/subscriber that can perform correct IP packet forwarding
(both internally and to other adjacent providers/subscribers) when
the inter-domain routing information acquired by the
provider/subscriber is expressed solely in terms of IP address
prefixes (with no distinction between A/B/C class of addresses).
This document defines CIDR-capable forwarding as the ability of a
router to maintain its forwarding table and to perform correct
forwarding of IP packets without making any assumptions about the
class of IP addresses.
This document defines CIDR reachability information as reachability
information that may violate any assumptions about the class of IP
addresses. For instance, a contiguous block of class C networks
expressed as a single IP address prefix constitutes CIDR reachability
information.
2 Taxonomy of Service Providers and Service Subscribers
For the purpose of this document we partition all service providers
and service subscribers into the following categories, based on the
type and volume of inter-domain routing information a
provider/subscriber needs to acquire in order to meet its service
requirements:
- Requirements imposed on a service provider/subscriber preclude
it from using Default inter-domain route(s) -- we'll refer to
such a provider/subscriber as a Type 1 provider/subscriber.
- Requirements imposed on a service provider/subscriber allow it
to rely on using one or more Default routes for inter-domain
routing, but this information must be supplemented by requiring
the provider/subscriber to acquire a large percentage of total
Internet routing information -- we'll refer to such a
provider/subscriber as a Type 2 provider/subscriber.
- Requirements imposed on a service provider/subscriber allow it
to rely on using one or more Default routes for inter-domain
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routing; however, to meet its service requirements the
provider/subscriber must supplement Default route(s) by
acquiring a small percentage of total Internet routing
information -- we'll refer to such a provider/subscriber as a
Type 3 provider/subscriber.
- Requirements imposed on a service subscriber allow it to rely
solely on using one or more Default routes for inter-domain
routing; no other inter-domain routing information need to be
acquired -- we'll refer to such a subscriber as a Type 4
subscriber.
3 Assumptions on Deployment of CIDR in the Internet
The document assumes that the CIDR deployment in the Internet will
proceed as a multi-phase process.
In the first phase all the major service providers will become CIDR-
capable. Specifically, all the providers that can't rely on using
Default route(s) for inter-domain routing (Type 1 providers) are
expected to deploy BGP-4 and transition to CIDR during this phase. It
is expected that CIDR reachability information will first appear in
the Internet upon transition of all Type 1 service
providers/subscribers to CIDR.
The second phase will commence upon completion of the first phase.
During the second phase other service providers/subscribers that are
connected to the service providers that were transitioned to CIDR
during the first phase will become CIDR-capable. Specifically,
during the second phase it is expected that most of the
providers/subscribers that need to acquire a large percentage of the
total Internet routing information (Type 2 provider/subscriber) will
become CIDR-capable. In addition, during the second phase some of
the Type 3 providers/subscribers may become CIDR-capable as well.
This plan was agreed to by a number of major providers [8]. NSFNET's
steps to implement this plan are described in [9].
Finally, during the third phase the rest of the Type 3
providers/subscribers and most of the Type 4 subscribers will
transition to CIDR.
It is expected that duration of the first phase will be significantly
shorter than duration of the second phase. Likewise, duration of the
second phase is expected to be shorter than duration of the third
phase.
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4 Implications of CIDR on Interior Routing
A CIDR-capable service provider/subscriber can use the following two
techniques to distribute exterior routing information to all of its
routers (both interior and border):
- utilize internal BGP/IDRP between all the routers
- use CIDR-capable IGPs (e.g. OSPF, IS-IS, RIP2)
The first technique doesn't impose any addition requirements on the
IGP within the provider/subscriber. Additional information on
implementing the first option is presented in [5] (see Section
A.2.4).
The second technique allows the provider/subscriber to reduce the
utilization of internal BGP/IDRP, but imposes specific requirements
on the intra-domain routing. It also requires the ability to inject
inter-domain routing information (acquired either via BGP or IDRP)
into the intra-domain routing. Additional details on implementing the
second option are provided in [7]. It is not expected that all the
features enumerated in [7] will be widely needed. Therefore, it would
be highly desirable to prioritize the features.
Note that both of these techniques imply that all the routers within
a CIDR-capable service provider/subscriber need to be capable of
CIDR-based forwarding.
Discussion of which of the two techniques should be preferred is
outside the scope of this document.
5 Exchanging Inter-Domain Routing Information
At each phase during the transition to CIDR one of the essential
aspects of the Internet operations will be the exchange of inter-
domain routing information between CIDR-capable providers/subscribers
and CIDR-incapable provider/subscribers.
When exchanging inter-domain routing information between a CIDR-
capable provider/subscriber and a CIDR-incapable provider/subscriber,
it is of utmost importance to take into account the view each side
wants of the other to present. This view has two distinct aspects:
- the type of routing information exchanged (e.g. Default route,
traditional (non-CIDR) reachability information, CIDR
reachability information)
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- routing information processing each side needs to do to maintain
these views (e.g. ability to perform aggregation, ability to
perform controlled de-aggregation)
The exchange of inter-domain routing information is expected to be
controlled by bilateral agreements between the directly connected
service providers/subscribers. Consequently, the views each side
wants of the other are expected to form an essential component of
such agreements.
To facilitate troubleshooting and problem isolation, the bilateral
agreements should be made accessible to other providers/subscribers.
One way to accomplish this is by placing them in a generally
accessible database. The details of how this can be implemented are
outside the scope of this document. A possible way to accomplish this
is described in [9].
Since the exchange of inter-domain routing information across
provider/subscriber boundaries occurs on a per peer basis, a border
router is expected to provide necessary mechanisms (e.g.
configuration) that will control exchange and processing of this
information on a per peer basis.
In the following sections we describe possible scenarios for
exchanging inter-domain routing information. It is always assumed
that one side is CIDR-capable and the other is not.
5.1 Exchanging Inter-Domain Routing Information between CIDR-capable
providers and CIDR-incapable Type 2 (default with large proportion of
explicit routes) providers/subscribers
When a CIDR-capable provider receives inter-domain routing
information from a CIDR-incapable Type 2 provider/subscriber, it is
expected that the border router(s) within the CIDR-capable provider
will be capable of performing aggregation of this information. The
aggregation is expected to be governed and controlled via a bilateral
agreement. Specifically, the CIDR capable provider is expected to
aggregate only the information the other side (the CIDR-incapable
provider/subscriber) requests. In other words, the aggregation shall
only be done on agreement by the recipient (the CIDR-capable
provider).
Passing inter-domain routing information from a CIDR-capable provider
to a CIDR-incapable Type 2 provider/subscriber will require an
agreement between the two that would cover the following items:
- under what conditions the CIDR-capable provider can pass an
inter-domain Default route to the CIDR-incapable
provider/subscriber
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- exchange of specific non-CIDR reachability information
- controlled de-aggregation of CIDR reachability information
Agreements that cover the first two items are already implemented
within the Internet. Thus, the only additional factor introduced by
CIDR is the third item. A CIDR-capable provider may decide not to
de-aggregate any CIDR reachability information, or to de-aggregate
some or all of the CIDR reachability information.
If a CIDR-capable provider does not de-aggregate CIDR reachability
information, then its non-CIDR Type 2 peer can obtain reachability
information from it either as non-CIDR reachability information
(explicit Class A/B/C network advertisement) or as an inter-domain
Default route. Since most of the current reachability information in
the Internet is non-CIDR, a Type 2 provider/subscriber would be able
to acquire this information as explicit Class A/B/C network
advertisements from the CIDR-capable provider, as it does now.
Further, it is expected that at least on a temporary basis (until the
completion of the second phase of the transition) in a majority of
cases, Type 2 providers/subscribers should be able to use an inter-
domain Default route (acquired from the CIDR-capable provider) as a
way of dealing with forwarding to destinations covered by CIDR
reachability information.
Thus, it is expected that most of the cases when dealing with Type 2
providers/subscribers could be addressed by a combination of
exchanging specific non-CIDR reachability information and an inter-
domain Default route. Any inconvenience to a provider/subscriber due
to the use of an inter-domain Default route will be removed once the
provider/subscriber transitions to CIDR.
On the other hand, a CIDR-capable provider may decide to perform
controlled de-aggregation of CIDR reachability information.
Additional information on performing controlled de-aggregation can be
found in [5] (Section 8). Special care must be taken when de-
aggregating CIDR reachability information carried by a route with the
ATOMIC_AGGREGATE path attribute. It is worth while to point out that
due to the nature of Type 2 provider/subscriber (it needs to acquire
a large percentage of total inter-domain routing information) it is
expected that the controlled de-aggregation would result in a non-
negligible configuration at the border router that performs the de-
aggregation.
5.2 Exchanging Inter-Domain Routing Information between CIDR-capable
providers and CIDR-incapable Type 3 (Default with few explicit routes)
providers/subscribers
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The only difference between this case and the case described in
Section 5.1 is the fact that a CIDR-incapable provider/subscriber
requires just a small percentage of total inter-domain routing
information. If this information falls into a non-CIDR category, then
a Type 3 provider/subscriber would be able to acquire it from a
CIDR-capable provider. If this is CIDR reachability information, then
in a majority of cases it is expected that forwarding to destinations
covered by this information could be handled via an inter-domain
Default route.
It is still expected that a border router in a CIDR-capable provider
would be able to aggregate routing information it receives from a
CIDR-incapable Type 3 provider/subscriber. The aggregation is
expected to be governed and controlled via a bilateral agreement.
Specifically, the CIDR capable provider is expected to aggregate only
the information the other side (the CIDR-incapable
provider/subscriber) requests.
5.3 Exchanging Inter-Domain Routing Information between CIDR-capable
providers and CIDR-incapable Type 4 (Default only) providers/subscribers
The only difference between this case and the case described in
Section 5.1 is the fact that CIDR-incapable provider/subscriber would
not require any inter-domain routing information, other than the
Default inter-domain route. Therefore, controlled de-aggregation of
CIDR reachability information becomes a non-issues.
It is still expected that a border router in a CIDR-capable provider
would be able to aggregate routing information it receives from a
CIDR-incapable Type 4 provider/subscriber. The aggregation is
expected to be governed and controlled via a bilateral agreement.
Specifically, the CIDR capable provider is expected to aggregate only
the information the other side (the CIDR-incapable
provider/subscriber) requests.
6 Conclusions
It is expected that the reduction in the global volume of routing
information will begin immediately upon completion of the first
phase. The second phase will allow to simplify bilateral arrangements
between connected service providers/subscribers by shifting the
responsibility for routing information aggregation towards the
parties that are better suitable for this, and by significantly
reducing the need for routing information de-aggregation. Thus, most
of the gain achieved during the second phase will come from
simplifying bilateral agreements. The third phase it intended to
complete the goals and objectives of the second phase.
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7 Security Considerations
Security issues are not discussed in this document.
8 Acknowledgments
This document was largely stimulated by the discussion that took
place during the Merit/NSFNET Regional Tech Meeting in Boulder,
January 21-22, 1993. We would like specifically acknowledge
contributions by Peter Ford (Los Alamos National Laboratory), Elise
Gerich (NSFNET/Merit), Susan Hares (NSFNET/Merit), Mark Knopper
(NSFNET/Merit), Bill Manning (Sesquinet/Rice University), and John
Scudder (NSFNET/Merit).
9 Authors' Addresses
Yakov Rekhter
T.J. Watson Research Center, IBM Corporation
P.O. Box 218
Yorktown Heights, NY 10598
Phone: (914) 945-3896
EMail: yakov@watson.ibm.com
Claudio Topolcic
Corporation for National Research Initiatives
1895 Preston White Drive
Suite 100
Reston, VA 22091
Phone: (703) 620-8990
EMail: topolcic@CNRI.Reston.VA.US
10 References
[1] Fuller, V., Li, T., Yu, J., and Varadhan, K., `Supernetting: an
Address Assignment and Aggregation Strategy', RFC1338, June 1992
[2] Gerich, E., `Guidelines for Management of IP Address Space',
RFC1366, October 1992
[3] Rekhter, Y., Li, T., `An Architecture for IP Address Allocation
with CIDR', Internet Draft, January 1993
[4] Rekhter, Y., Li, T., `A Border Gateway Protocol 4 (BGP-4)',
Internet Draft, December 1992
Expiration Date October 1993 [Page 8]
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[5] Rekhter, Y., Gross, P., `Application of the Border Gateway
Protocol in the Internet', Internet Draft, September 1992
[6] Hares, S., `IDRP for IP', Internet Draft, June 1992
[7] Varadhan, K., `BGP4 OSPF Interaction', Internet Draft, September
1992
[8] Topolcic, C., `Notes on BGP-4/CIDR Coordination Meeting of 11
March 93', Internet Draft, March 1993
[9] Knopper, M., `Aggregation Support in the NSFNET Policy Routing
Database', Internet Draft, March 1993
Expiration Date October 1993 [Page 9]