rfc1930.txt

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Network Working Group                                       J. Hawkinson
Request for Comments: 1930                                    BBN Planet
BCP: 6                                                          T. Bates
Category: Best Current Practice                                      MCI
                                                              March 1996


          Guidelines for creation, selection, and registration
                      of an Autonomous System (AS)

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

Table of Contents

   1. Introduction ............................................    2
   2. Motivation ..............................................    2
   3. Definitions .............................................    2
   4. Common errors in allocating ASes ........................    5
   5. Criteria for the decision -- do I need an AS?  ..........    5
   5.1 Sample Cases ...........................................    6
   5.2 Other Factors ..........................................    7
   6. Speculation .............................................    7
   7. One prefix, one origin AS ...............................    8
   8. IGP issues ..............................................    8
   9. AS Space exhaustion .....................................    8
   10. Reserved AS Numbers ....................................    9
   11. Security Considerations ................................    9
   12. Acknowledgments ........................................    9
   13. References .............................................    9
   14. Authors' Addresses .....................................   10




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RFC 1930            Guidelines for creation of an AS          March 1996


1. Introduction

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

2. Motivation

   This memo is aimed at network operators and service providers who
   need to understand under what circumstances they should make use of
   an AS.  It is expected that the reader is familiar with routing
   protocols and will be someone who configures and operates Internet
   networks.  Unfortunately, there is a great deal of confusion in how
   ASes should be used today; this memo attempts to clear up some of
   this confusion, as well as acting as a simple guide to today's
   exterior routing.

3. Definitions

   This document refers to the term "prefix" throughout. In the current
   classless Internet (see [CIDR]), a block of class A, B, or C networks
   may be referred to by merely a prefix and a mask, so long as such a
   block of networks begins and ends on a power-of-two boundary. For
   example, the networks:

        192.168.0.0/24
        192.168.1.0/24
        192.168.2.0/24
        192.168.3.0/24

   can be simply referred to as:

        192.168.0.0/22

   The term "prefix" as it is used here is equivalent to "CIDR block",
   and in simple terms may be thought of as a group of one or more
   networks. We use the term "network" to mean classful network, or "A,
   B, C network".

   The definition of AS has been unclear and ambiguous for some time.
   [BGP-4] states:



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RFC 1930            Guidelines for creation of an AS          March 1996


      The classic definition of an Autonomous System is a set of routers
      under a single technical administration, using an interior gateway
      protocol and common metrics to route packets within the AS, and
      using an exterior gateway protocol to route packets to other ASes.
      Since this classic definition was developed, it has become common
      for a single AS to use several interior gateway protocols and
      sometimes several sets of metrics within an AS.  The use of the
      term Autonomous System here stresses the fact that, even when
      multiple IGPs and metrics are used, the administration of an AS
      appears to other ASes to have a single coherent interior routing
      plan and presents a consistent picture of what networks are
      reachable through it.

   To rephrase succinctly:

      An AS is a connected group of one or more IP prefixes run by one
      or more network operators which has a SINGLE and CLEARLY DEFINED
      routing policy.

   Routing policy here is defined as how routing decisions are made in
   the Internet today.  It is the exchange of routing information
   between ASes that is subject to routing policies. Consider the case
   of two ASes, X and Y exchanging routing information:

                NET1 ......  ASX  <--->  ASY  ....... NET2

   ASX knows how to reach a prefix called NET1.  It does not matter
   whether NET1 belongs to ASX or to some other AS which exchanges
   routing information with ASX, either directly or indirectly; we just
   assume that ASX knows how to direct packets towards NET1.  Likewise
   ASY knows how to reach NET2.

   In order for traffic from NET2 to NET1 to flow between ASX and ASY,
   ASX has to announce NET1 to ASY using an exterior routing protocol;
   this means that ASX is willing to accept traffic directed to NET1
   from ASY. Policy comes into play when ASX decides to announce NET1 to
   ASY.

   For traffic to flow, ASY has to accept this routing information and
   use it.  It is ASY's privilege to either use or disregard the
   information that it receives from ASX about NET1's reachability. ASY
   might decide not to use this information if it does not want to send
   traffic to NET1 at all or if it considers another route more
   appropriate to reach NET1.

   In order for traffic in the direction of NET1 to flow between ASX and
   ASY, ASX must announce that route to ASY and ASY must accept it from
   ASX:



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RFC 1930            Guidelines for creation of an AS          March 1996


                    resulting packet flow towards NET1
                  <<===================================
                                    |
                                    |
                     announce NET1  |  accept NET1
                    --------------> + ------------->
                                    |
                        AS X        |    AS Y
                                    |
                     <------------- + <--------------
                       accept NET2  |  announce NET2
                                    |
                                    |
                   resulting packet flow towards NET2
                   ===================================>>

   Ideally, though seldom practically, the announcement and acceptance
   policies of ASX and ASY are symmetrical.

   In order for traffic towards NET2 to flow, announcement and
   acceptance of NET2 must be in place (mirror image of NET1). For
   almost all applications connectivity in just one direction is not
   useful at all.

   It should be noted that, in more complex topologies than this
   example, traffic from NET1 to NET2 may not necessarily take the same
   path as traffic from NET2 to NET1; this is called asymmetrical
   routing.  Asymmetrical routing is not inherently bad, but can often
   cause performance problems for higher level protocols, such as TCP,
   and should be used with caution and only when necessary. However,
   assymetric routing may be a requirement for mobile hosts and
   inherently asymmetric siutation, such a satelite download and a modem
   upload connection.

   Policies are not configured for each prefix separately but for groups
   of prefixes.  These groups of prefixes are ASes.

   An AS has a globally unique number (sometimes referred to as an ASN,
   or Autonomous System Number) associated with it; this number is used
   in both the exchange of exterior routing information (between
   neighboring ASes), and as an identifier of the AS itself.

   In routing terms, an AS will normally use one or more interior
   gateway protocols (IGPs) when exchanging reachability information
   within its own AS. See "IGP Issues".






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RFC 1930            Guidelines for creation of an AS          March 1996


4. Common errors in allocating ASes

   The term AS is often confused or even misused as a convenient way of
   grouping together a set of prefixes which belong under the same
   administrative umbrella, even if within that group of prefixes there
   are various different routing policies. Without exception, an AS must
   have only one routing policy.

   It is essential that careful consideration and coordination be
   applied during the creation of an AS. Using an AS merely for the sake
   of having an AS is to be avoided, as is the worst-case scenario of
   one AS per classful network (the IDEAL situation is to have one
   prefix, containing many longer prefixes, per AS). This may mean that
   some re-engineering may be required in order to apply the criteria
   and guidelines for creation and allocation of an AS that we list
   below; nevertheless, doing so is probably the only way to implement
   the desired routing policy.

   If you are currently engineering an AS, careful thought should be
   taken to register appropriately sized CIDR blocks with your
   registration authority in order to minimize the number of advertised
   prefixes from your AS.  In the perfect world that number can, and
   should, be as low as one.

   Some router implementations use an AS number as a form of tagging to
   identify interior as well as exterior routing processes.  This tag
   does not need to be unique unless routing information is indeed
   exchanged with other ASes. See "IGP Issues".

5. Criteria for the decision -- do I need an AS?

   *    Exchange of external routing information

        An AS must be used for exchanging external routing information
        with other ASes through an exterior routing protocol. The cur-
        rent recommended exterior routing protocol is BGP, the Border
        Gateway Protocol. However, the exchange of external routing
        information alone does not constitute the need for an AS. See
        "Sample Cases" below.

   *    Many prefixes, one AS

        As a general rule, one should try to place as many prefixes as
        possible within a given AS, provided all of them conform to the
        same routing policy.






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