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HISTORIC
Network Working Group Mark Knopper
Request for Comments: 1482 Steven J. Richardson
Merit/NSFNET
June 1993
Aggregation Support in the NSFNET Policy-Based Routing Database
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 document describes plans for support of route aggregation, as
specified in the descriptions of Classless Inter-Domain Routing
(CIDR) [1] and the BGP-4 protocol [2], by the NSFNET Backbone Network
Service. Mechanisms for exchange of route aggregates between the
backbone service and regional/midlevel networks are specified.
Additionally, the memo proposes the implementation of an Aggregate
Registry which can be used by network service providers to share
information about the use of aggregation. Finally, the operational
impact of incorporating CIDR and aggregation is considered, including
an analysis of how routing table size will be affected. This impact
analysis will be used to modify the deployment plan, if necessary, to
maximize operational stability.
1. Introduction
The Internet network service provider community and router vendors
(as well as the IESG and various IETF working groups) have agreed
that the time for deployment of route aggregation is upon us. This
topic has been discussed in the BGP-D, NJM and ORAD working groups at
several IETF meetings; it was a discussion topic of the NSFNET
Regional Techs' Meetings in January and June, 1993; and it was also a
topic of several meetings of the Federal Engineering Planning Group
and Engineering and Operations Working Group of the Federal Network
Council.
All have generally agreed that Summer, 1993 is the time to enable
BGP-4 and CIDR aggregation. Each of the parties is responsible for
its own aspect of CIDR implementation and practice. This memo
describes Merit's plans for support of route aggregation on the
NSFNET, and a proposal for implementing a database of aggregation
information for use by network providers.
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2. Aggregation Support by the Backbone Service
The NSFNET backbone service includes a Policy-Based Routing Database
system which currently holds the set of network numbers that are
accepted by the backbone service with a list of Autonomous System
numbers from which announcements of these network numbers are
expected. In order to implement CIDR, the database system will be
modified to allow aggregation of routing information to be
configured.
The NSFNET will (initially) not support de-aggregation on its
outbound announcements. See section 2.3.
2.1 Current Configuration Capabilities
2.1.1 Inbound Announcements
An example of the way a network number is currently configured is as
follows:
35 1:237 2:233 3:183 4:266 5:267 6:1225
This shows that network number 35 (ie. 35.0.0.0, a class A net
number) is configured on the T3 backbone such that routing
announcements are expected from up to 6 autonomous systems. The
primary path is via AS 237, secondary is via AS 233, etc.
2.1.2 Outbound Announcements
Currently the NSFNET database has a list of AS's or network numbers
for each neighbor AS that are announced by the backbone to that AS.
These announcements are specified currently by "announcetoAS"
statements--which implement policies submitted by midlevels to
Merit--and then included in the ANSnet router configuration files.
There are two forms of these statements. The first form uses the
"norestrict" clause and indicates that all of the network numbers
within each AS in the list should be announced to the neighbor
midlevel AS. For example:
announcetoAS 42 norestrict ASlist 22 26 38 60 68
In this example, the NSFNET is configured to announce to neighboring
midlevel AS 42, all networks in the routing table that were announced
from AS's 22, 26, 38, 60 and 68.
If the "norestrict" keyword is changed to "restrict", this indicates
that an explicit announce list of network numbers for the AS is
specified in the configuration file. The NSFNET will only announce
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network numbers that were announced by the AS's in the list, *AND*
which appear in the "restrict list" of network numbers submitted
separately by the midlevel.
For example,
announcetoAS 42 restrict ASlist 22
announce 192.135.237 <other info>
These statements mean that AS 42 only wishes to hear announcements
from the backbone about the nets in AS 22 which are explicitly listed
here (i.e., net 192.135.237).
It is also possible, when using the "restrict" keyword, to list
specific "noannounce" lines. Those indicate that all of the networks
listed in the routing table for the AS should be announced except
those listed on the noannounce clauses. (There is also a
"noannouncetoAS" statement[4].)
2.2 New Configuration Features for Aggregation
There will be three new capabilities for which the backbone service
can be configured to support aggregation. The first two allow
aggregates to be accepted and stored in the backbone routing tables
based on announcements by the regional network (autonomous system or
AS) peers. The third allows the announcement of aggregates to the AS
neighbor peers. The following sections give examples of the three
features.
We use the notation <net-IP prefix-length> to describe an aggregate.
This refers to the IP prefix "net-IP", with a mask which has
"prefix-length" 1's as counted from the high-order end. For example,
<192.64.128 17> is equivalent to <192.64.128, 255.255.128.0> [5].
(The form using prefix-length rather than the mask is more compact.)
2.2.1 NSFNET accepts aggregates
In this case the regional peer router is CIDR-capable (i.e., runs
BGP-4) and the announcement comes into the backbone as an IP address
prefix.
To illustrate this in the spirit of sec. 2.1.1:
<192.64.128 17> 1:189 2:24 3:267
In this example, independent of the "class" of IP network number, an
aggregate containing network addresses matching a pattern in which
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the first 17 bits match the prefix 192.64.128 will be accepted in
announcements to the NSFNET service. The primary path to
destinations covered by the prefix is expected via AS 189, the
secondary, via AS 24, etc.
2.2.2 NSFNET aggregates by proxy
The other method of incorporating CIDR aggregate announcements into
the backbone routing tables is that of aggregation by proxy. In this
case, the backbone is configured to perform aggregation on behalf of
a peer AS which is not configured to announce the aggregate to the
backbone (i.e., an AS which does not connect to the backbone via a
CIDR-capable peer).
An example of this aggregation technique is:
proxy <192.64.128 17> 1:189 2:24 3:267
if <192.64.192 24>
or <192.64.129 24>
or <192.64.167 24>
(Note: the syntax used in this document is arbitrary and is only used
to illustrate the method. The syntax to be used in actual routing
requests is to be determined.)
In this example, the aggregate <192.64.128 17> will be stored and
propagated within the backbone as an aggregate under a set of
conditions. Initially, the GateD support will allow an "OR" list of
conditions such that if one of the aggregates in the list matches the
proxy aggregate will be stored[6]. For the case above, this means
that, if any of the CIDR aggregates:
<192.64.192 24>
<192.64.129 24>
<192.64.167 24>
(which--under the current, class-based IP address system--are
equivalent to the class C net numbers 192.64.192, 192.64.129, or
192.64.167, respectively) is heard, the backbone router will act as
though it heard the announcement of the single CIDR aggregate
<192.64.128 17>.
2.2.3 NSFNET announces aggregates
The functionality of the current system, as outlined in sec. 2.1.2,
above, will continue to exist once CIDR is implemented. The
"norestrict" function (or its equivalent in the new software) will
specify that all network reachability information received from a set
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of Autonomous Systems, including any aggregates, will be announced.
It should also be possible to use to the equivalents of the
"restrict" keyword and the "announce" (or "noannounce") statement in
order to limit the announcements of the aggregations within an AS to
any desired subset.
2.3 Specifically Unsupported Capabilities, Limits of Initial Deployment
There are some aspects of aggregation which will specifically not be
supported in the initial deployment of CIDR capabilities on the
NSFNET backbone. In particular, when the NSFNET service announces
routes to midlevel peers, de-aggregation will not be performed [3].
Therefore, a peer which needs to receive full routing information
should run a protocol which supports CIDR (initially, BGP-4; later,
IDRP). Peer networks using default routing will be able to reach
networks that are part of aggregated routing information across the
backbone (as in section 6.4 of [3]).
3. CIDR Aggregate Registry
In discussions with network service providers, it has become apparent
that there is a great need for sharing of aggregate information; this
is necessary to fulfill the coordination referred to in sec. 2.3.
Beyond the need to implement CIDR aggregation facilities in the
NSFNET Policy-Based Routing Database (as described in section 2),
there is a clear need to have a separate database which will allow
aggregate information from any Autonomous System to be stored and
made available for easy electronic retrieval. This information can be
used for routing coordination and policy configuration in the larger,
non-NSFNET-centric, inter-domain context.
One of the expected uses of such a database is to help determine, as
CIDR matures, the granularity of aggregation of network reachability
information with respect to policy. The useful scope of aggregation
is the subject of much discussion[5][7], and will be influenced by
such considerations as how network number allocation has been
handled, and whether the network provider has renumbered its client
networks to conform to CIDR aggregation boundaries. Rules and issues
regarding network number allocation with CIDR are discussed in [8]
and [7].
In order further these goals, Merit proposes to implement a "CIDR
Aggregate Registry" to provide sharing of aggregate information for
the Internet inter-domain routing community. Initially, this will be
a simple database without much structure. It is not intended to hold
only aggregates which are announced or accepted by the NSFNET
service; rather, it should be a community registry that all will be
invited to use and make use of.
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The Aggregate Registry will consist of a list of aggregate
announcement statements. Each statement consists of four types of
information, along with contact information:
1) CIDR Aggregate: The aggregate identifier, consisting of a
network number prefix and the prefix length. For example,
<192.29.128 16>.
2) Home AS: The source AS number for the aggregate. That is, the
AS number of the network service provider that initially
aggregates the network reachability information into the aggregate
for announcement to its neighbors.
3a) Announcing AS: An AS number that announces this aggregate to
its neighbor AS's.
3b) Neighbor AS list: A list of neighbor AS's to whom the
aggregate will be announced by the AS named in 3a.
4) Contact information: eg. e-mail address and name or NIC handle
of the administrative and technical contacts for the source AS.
Thus, a given aggregate is listed once as announced by its source AS.
It may then be listed once again per transit AS which announces the
aggregate downstream to its neighbors. For example, the CIDR
aggregate <199.29.128 16> could be listed as:
CIDR aggregate home ann neighbor
(prefix-length) AS AS AS list contacts
-----------------------------------------------------------
<199.29.128 16> 100 100 200 201 690 fred@nowhere.net
<199.29.128 16> 100 690 266 267 1225... <contact info>
<199.29.128 16> 100 200 297 372 <contact info>
<199.29.128 16> 100 201 771 1262 <contact info>
Note: This can be represented using the syntax used for objects
in the RIPE-81 paper[9].
Here, AS 100 (the source AS) performs any aggregation and announces
the CIDR aggregate <199.29.128 16> to neighbor ASs 200, 201, and 690.
In turn, AS 200 announces this same aggregate to its neighbor ASs 297
and 372; further lines show announcements of the given aggregate by
AS 690 and AS 201.
Note that this registry reflects both the simple list of aggregates
that are supported by the union of network providers, as well as
information on inter-domain topology for the Internet. Merit will
implement procedures for registering any network provider's
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aggregates in the Registry; for those CIDR aggregates carried over
the NSFNET backbone, Merit will implement procedures for integrating
this Registry with the process of updating the aggregate routing
announcements. Requests to update the information will be handled
via e-mail or on-line registration tools.
4. Effects of CIDR on Operational Aspects of the Internet
The introduction of CIDR will clearly necessitate various changes
beyond the introduction of new router software. In particular, Merit
and other network service providers will have to adjust tools,
reports, and procedures as CIDR is implemented and evolved, and these
changes will have to be coordinated in order to ensure a smooth
transition to the CIDR-capable Internet.
While this document is by no means exhaustive, some of the areas
affected are discussed briefly below; what is intended is to foster
an awareness of some these changes, so as to initiate thinking about
and planning for this transition. While it is obvious that CIDR and
policy routing imply greater coordination of many operational
matters, it is not clear how profoundly this will affect the day-to-
day running of the Internet.
(Note: Aspects of the actual phased deployement of CIDR are covered
in [3] and [10].)
4.1 NSFNET Configuration Files and Reports; Neighbor AS Configurations
The addition of CIDR capability to the NSFNET Policy-Based Routing
Database, as outlined in sec. 2, will require the updating of at
least the following reports which are currently produced by Merit
(and available via anonymous FTP from nic.merit.edu):
ans_core.now as-site.now country.now net-comp.now net-net.now
net-ter.now non-us.now
Any tools which access this information, such as the various clients
or scripts released by Merit or developed by others, will have to be
changed.
However, the most striking change will be in the transition from
rcp_routed to GateD; it is very different in important particulars,
and follows different conceptual principles [11].
Network providers which develop any part of their configuration files
from parsing the NSFNET configuration files or reports *MUST* plan
for these changes in order to help themselves and the Internet
community achieve a smooth transition to CIDR.
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4.2 Routing and Administrative Policies
In this document, Merit has stated its commitment to supporting CIDR
through both changing policies related to administering the NSFNET
and developing a CIDR Aggregate Registry for the broader Internet
community.
In addition to these changes, here are some of the other policies,
administrative and routing, which must to be coodinated in order to
achieve optimum benefits of CIDR:
- policies of the InterNIC and of network service providers in
assigning (CIDR) IP nets and blocks, as mentioned above;
- policies of the various ASs in coordination of transit and other
routing policies;
- policies of registration of new networks, from the InterNIC or
network provider, through the CIDR Aggregate Registry, etc.;
- policies related to coordination of routing changes;
- coordination of routing policies, in general, to avoid new
classes of routing problems due to new methods of routing.
4.3 Realtime Issues
Issues which have not been examined in detail are:
- debugging of routing/connectivity problems;
- stability and other properties of routing under various
scenarios of CIDR configuration and network topology;
- explicit specification of routing decision algorithms to avoid
routing anomalies;
- increased network load due to packets traversing an AS, such as
the NSFNET backbone, before being discarded due to addressing a
"hole" in a CIDR aggregate.
4.4 Estimate of Reductions in Routing Tables
An argument in favor of the implementation CIDR is the effect which
it should have upon the NSFNET and other routing tables [1] [5]. The
burning question is: What is the magnitude of this effect? In view
of the various issues to be dealt with, this is an important
consideration.
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In terms of the immediate savings in reduction of the NSFNET backbone
routing tables, if a set of aggregates were done all at once, a
recent calculation--which might be characterized as an optimistic
estimate using a pessimistic algorithm (it looks for the longest
continuous block of addresses announced to the NSFNET backbone)--
yields [12]:
861 size 2 saving 861 announcements
286 size 4 saving 858 announcements
117 size 8 saving 819 announcements
67 size 16 saving 1005 announcements
13 size 32 saving 403 announcements
3 size 64 saving 189 announcements
1347 total saving 4135 announcements of 12348 (33%).
Here, the first column represents the number of CIDR aggregates of
the given "size," and shows the corresponding reduction in net
announcements due to the adoption of this aggregate. (A CIDR
aggregate of "size <n>" is one which encompasses <n> class A, B, or C
networks; the 67 "size 16" CIDR aggregates actually combine
announcements for 16 separate networks into a single net aggregate.)
It is unclear, at this time, whether or not the true savings would be
of this magnitude, but the extended report provides a basis for
discussion [12].
The other aspect of impact upon the routing tables, the reduction in
the rate of growth (and the concomitant slowing of the rate of
exhaustion of IP address space), is an entirely different matter.
Simple calculations related to the rate of class B address space
exhaustion indicate that CIDR-conformant policies of the InterNIC
with respect to address assignment is helping [1].
Clearly, more detailed analysis is desirable in order to better
understand the realistic gains of the CIDR deployment process, both
initially and in the longer term.
5. Conclusions and Next Steps
Implementation of CIDR is underway, but there is still a fair amount
of planning and discussion that is needed for a successful
transition. Merit is proposing specific functions for CIDR
aggregation that will be supported by the NSFNET, as well as a CIDR
Aggregate Registry that can serve as the basis for inter-domain
routing coordination.
The Aggregate Registry will allow a set of tools to be developed that
can facilitate the design of aggregation policy. A query tool to
allow lookup of aggregation information for a given network or
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aggregate would be very useful. Additional database functionality
will also be desired for more powerful queries. It is specifically a
goal to work with RIPE to make sure that the Merit and RIPE database
approaches are compatible and allow interworking of tools. An AS
topology database would be most useful in routing policy
determination and coordination as well.
In addition to these areas, many other issues require further work in
order to develop the operational framework necessary for the
successful use of CIDR on the Internet. It is critical that the
deployment of CIDR and related tools to preserve address and routing
table space must not compromise the operational stability of the
NSFNET and the wider Internet.
6. Security Considerations
Security issues are not discussed in this document.
7. Acknowledgements
The authors would like to acknowledge the following persons, whose
comments and discussions have helped to shape this document:
Dennis Ferguson, Advanced Network and Services, Inc.
Jeffrey Honig, Cornell University
William Manning, Rice University/SESQUINET
The Merit Internet Engineering and Network Management
Systems groups.
8. Authors' Addresses
Knopper, Mark A.
Merit Network, Inc.
1071 Beal Ave.
Ann Arbor, MI 48109-2103
e-mail: mak@merit.edu
phone: (313) 763-6061
fax: (313) 747-3745
Richardson, Steven J.
Merit Network, Inc.
1071 Beal Ave.
Ann Arbor, MI 48109-2103
e-mail: sjr@merit.edu
phone: (313) 747-4813
fax: (313) 747-3745
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9. References
[1] Fuller, V., Li, T., Yu, J., and Varadhan, K., "Supernetting: an
Address Assignment and Aggregation Strategy", RFC1338, Update,
Work in Progress, June 1992.
[2] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4", Work In
Progress, April 1993.
[3] Topolcic, C., "Notes of BGP-4/CIDR Coordination Meeting of 11
March 93", Work in Progress, March 1993.
[4] Villamizer, C., in a document describing rcp_routed.conf options
and syntax, May, 1993.
[5] Syntax used in Ford, P., Rekhter, Y., Braun, H-W., "Improving
the Routing and Addressing of IP", IEEE Network, pp. 10-15, May
1993.
[6] Ferguson, D., private correspondence, March, 1993.
[7] Rekhter, Y., and Li, T., "An Architecture for IP Address
Allocation with CIDR", Work in Progress, February, 1993.
[8] Gerich, E., "Guidelines for Management of IP Address Space",
RFC1466, May 1993.
[9] Bates, T., Jouanigot, J-M., Karrenberg, D., Lothberg, P., and
Terpstra, M., "Representation of IP Routing Policies in the RIPE
Database" (ripe-81), Work in Progress, February, 1993.
[10] Rekhter, Y., and Topolcic, C., "Exchanging Routing Information
Across Provider/Subscriber Boundaries in the CIDR Environment",
Work in Progress, April 1993.
[11] Fedor, M., Honig, J., Coltun, R., Ferguson, D., "gated-
config(5)" manpage, from the "gated-R3_0Beta_2" distribution, 7
October 1992.
[12] Johnson, D., analysis available via anonymous FTP from
merit.edu:/pub/nsfnet/cidr/auto-aggregates, June 1993.
[13] Topolcic, C., "Schedule for IP Address Space Management
Guidelines", RFC1367, October, 1993.
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