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RFC 1482 - Aggregation Support in the NSFNET Policy-Based Routin

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Network Working Group                                   Mark Knopper
Request for Comments: 1482                      Steven J. Richardson
                                                           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


   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

   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.

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

   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

         35      1:237   2:233   3:183   4:266   5:267  6:1225

   This shows that network number 35 (ie., 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

   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

   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,> [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

   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

   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

   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.

   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

   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

   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.

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

   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

   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

   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

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

   [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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