rfc1482.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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