Guidelines for creation, selection, and registration of an Autonomous System (AS)
draft-ietf-idr-autosys-guide-03
The information below is for an old version of the document that is already published as an RFC.
| Document | Type | RFC Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | John A. Hawkinson , Tony J. Bates | ||
| Last updated | 2013-03-02 (Latest revision 1995-05-02) | ||
| Stream | Internet Engineering Task Force (IETF) | ||
| Formats | plain text htmlized pdfized bibtex | ||
| Stream | WG state | (None) | |
| Document shepherd | (None) | ||
| IESG | IESG state | RFC 1930 (Best Current Practice) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
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| Send notices to | (None) |
draft-ietf-idr-autosys-guide-03
Network Working Group J. Hawkinson
INTERNET-DRAFT Panix
Category: Standards Track T. Bates
<draft-ietf-idr-autosys-guide-03.txt> MCI
May 1995
Guidelines for creation, selection, and registration
of an Autonomous System (AS)
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
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Rim).
Abstract
This draft 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 will eventually 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.
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Table of Contents
1. Introduction ............................................ 2
2. Motivation .............................................. 3
3. Definitions ............................................. 3
4. Common errors in allocating ASes ........................ 6
5. Criteria for the decision -- do I need an AS? .......... 6
5.1 Sample Cases ........................................... 7
5.2 Other Factors .......................................... 8
6. Speculation ............................................. 8
7. One prefix, one origin AS ............................... 9
8. IGP issues .............................................. 9
9. AS Space exhaustion ..................................... 10
10. Reserved AS Numbers .................................... 10
11. Security Considerations ................................ 10
12. Acknowledgments ........................................ 10
13. References ............................................. 10
14. Authors' Addresses ..................................... 12
1. Introduction
This memo discusses when it is appropriate to register and util-
ize 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 will eventually adopt when
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INTERNET-DRAFT Guidelines for creation of an AS May 1995
BGP becomes obsolete; see [IDRP]). It should be noted that the
IDRP equivalent of an AS is the RDI, or Routing Domain Identif-
ier.
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 Inter-
net 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:
The classic definition of an Autonomous System is a set of
routers under a single technical administration, using an inte-
rior 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
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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 IP networks 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 rout-
ing 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 infor-
mation 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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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 identical.
In order for traffic towards NET2 to flow, announcement and accep-
tance 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 exam-
ple, 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 per-
formance problems for higher level protocols, such as TCP, and should
be used with caution.
It is important to realise that with current destination based for-
warding technology routing policies must eventually be expressed in
these terms.
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 neigh-
boring ASes), and as an identifier of the AS itself.
In routing terms, an AS will normally use one or more interior gate-
way protocols (IGPs) when exchanging reachability information within
its own AS. See ``IGP Issues''.
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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 pre-
fix, containing many networks, per AS). This may mean that some re-
engineering may be required in order to apply the criteria and guide-
lines 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 registra-
tion 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
current 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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* Unique routing policy
An AS is only needed when you have a routing policy which is
different from that of your border gateway peers. Here routing
policy refers to how the rest of the Internet makes routing
decisions based on information from your AS. See ``Sample
Cases'' below to see exactly when this criteria will apply.
5.1 Sample Cases
* Single-homed site, single prefix
A separate AS is not needed; the prefix should be placed in an
AS of the provider. The site's prefix has exactly the same rout-
ing policy as the other customers of the site's service pro-
vider, and there is no need to make any distinction in routing
information.
This idea may at first seem slightly alien to some, but it
highlights the clear distinction in the use of the AS number as
a representation of routing policy as opposed to some form of
administrative use.
In some situations, a single site, or piece of a site, may find
it necessary to have a policy different from that of its pro-
vider, or the rest of the site. In such an instance, a separate
AS must be created for the affected prefixes. This situation is
rare and should almost never happen. Very few stub sites require
different routing policies than their parents. Because the AS is
the unit of policy, however, this sometimes occurs.
* Single-homed site, multiple prefixes
Again, a separate AS is not needed; the prefixes should be
placed in an AS of the site's provider.
* Multi-homed site
Here multi-homed is taken to mean a prefix or group of prefixes
which connects to more than one service provider (i.e. more than
one AS with its own routing policy). It does not mean a network
multi-homed running an IGP for the purposes of resilience.
An AS is required; the site's prefixes should be part of a
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single AS, distinct from the ASes of its service providers.
This allows the customer the ability to have a different
representation of policy and preference among the different ser-
vice providers.
This is ALMOST THE ONLY case where a network operator should
create its own AS number. In this case, the site should ensure
that it has the necessary facilities to run appropriate routing
protocols, such as BGP4.
5.2 Other factors
* Topology
Routing policy decisions such as geography, AUP (Acceptable Use
Policy) compliance and network topology can influence decisions
of AS creation. However, all too often these are done without
consideration of whether or not an AS is needed in terms of
adding additional information for routing policy decisions by
the rest of the Internet. Careful consideration should be taken
when basing AS creation on these type of criteria.
* Transition / ``future-proofing''
Often a site will be connected to a single service provider but
has plans to connect to another at some point in the future.
This is not enough of a reason to create an AS before you really
need it. The AS number space is finite and the limited amount
of re-engineering needed when you connect to another service
provider should be considered as a natural step in transition.
* History
AS number application forms have historically made no reference
to routing policy. All too often ASes have been created purely
because it was seen as ``part of the process'' of connecting to
the Internet. The document should act as reference to future
application forms to show clearly when an AS is needed.
6. Speculation
1) If provider A and provider B have a large presence in a
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geographical area (or other routing domain), and many customers are
multi-homed between them, it makes sense for all of those customers
to be placed within the same AS. However, it is noted that case
should only be looked at if practical to do so and fully coordinated
between customers and service providers involved.
2) Sites should not be forced to place themselves in a separate AS
just so that someone else (externally) can make AS-based policy deci-
sions. Nevertheless, it may occasionally be necessary to split up an
AS or a prefix into two ASes for policy reasons. Those making exter-
nal policy may request the network operators make such AS changes,
but the final decision is up to those network operators who manage
the prefixes in question, as well as the ASes containing them. This
is, of course, a trade off -- it will not always be possible to
implement all desired routing policies.
7. One prefix, one origin AS
Generally, a prefix can should belong to only one AS. This is a
direct consequence of the fact that at each point in the Internet
there can be exactly one routing policy for traffic destined to each
prefix. In the case of an prefix which is used in neighbor peering
between two ASes, a conscious decision should be made as to which AS
this prefix actually resides in.
With the introduction of aggregation it should be noted that an AS
can occasionally be represented as residing in more than one AS, how-
ever, this is very much the exception rather than the rule. This hap-
pens when aggregating using the AS_SET attribute in BGP, wherein the
concept of origin is lost. In some cases the origin AS is lost alto-
gether if there is a less specific aggregate announcement setting the
ATOMIC_AGGREGATE attribute.
8. IGP Issues
As stated above, many router vendors require an identifier for tag-
ging their IGP processes. However, this tag does not need to be glo-
bally unique. In practice this information is never seen by exterior
routing protocols. If already running an exterior routing protocol,
it is perfectly reasonable to use your AS number as an IGP tag; if
you do not, choosing from the reserved range is also acceptable (see
``Reserved AS Numbers''). Merely running an IGP is not grounds for
registration of an AS number.
With the advent of BGP4 it becomes necessary to use an IGP that can
carry classless routes. Examples include OSPF [OSPF] and ISIS [ISIS].
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9. AS Space exhaustion
The AS number space is a finite amount of address space. It is
currently defined as a 16 bit integer and hence limited to 65535
unique AS numbers. At the time of writing some 5,100 ASes have been
allocated and a little under 600 ASes are actively routed in the glo-
bal Internet. It is clear that this growth needs to be continually
monitored. However, if the criteria applied above are adhered to,
then there is no immediate danger of AS space exhaustion. It is
expected that IDRP will be deployed before this becomes an issue.
IDRP does not have a fixed limit on the size of an RDI.
10. Reserved AS Numbers
The Internet Assigned Numbers Authority (IANA) has reserved the fol-
lowing block of AS numbers for private use (not to be advertised on
the global Internet):
64512 through 65535
11. Security Considerations
There are few security concerns regarding the selection of ASes.
AS number to owner mappings are public knowledge (in WHOIS), and
attempting to change that would serve only to confuse those people
attempting to route IP traffic on the Internet.
12. Acknowledgments
This document is largely based on [RIPE-109], and is intended to have
a wider scope than purely the RIPE community; this document would not
exist without [RIPE-109].
13. References
[RIPE-109]
Bates, T., Lord, A., "Autonomous System Number Application Form
& Supporting Notes", RIPE 109, RIPE NCC, 1 March 1994.
URL: ftp://ftp.ripe.net/ripe/docs/ripe-109.txt.
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[BGP-4]
Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
1654, T.J. Watson Research Center, cisco Systems, July 1994.
[EGP]
Mills, D. "Exterior Gateway Protocol formal specifications", RFC
904, STD 18, International Telegraph and Telephone Co., 1 April
1984.
[IDRP]
Kunzinger, C., Editor, "OSI Inter-Domain Routing Protocol (IDRP)",
ISO/IEC 10747, Work In Progress, October 1993.
[CIDR]
Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-Domain
Routing (CIDR): an Address Assignment and Aggregation Strategy",
RFC 1519, BARRnet, cisco, MERIT, OARnet, September 1993.
[OSPF]
Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[ISIS]
Callon, R., Gunner, C., "Use of OSI IS-IS for Routing in TCP/IP and
Multi-Protocol Environments", draft-ietf-isis-tcpip-01.txt, WORK IN
PROGRESS, July 1994.
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14. Authors' Addresses
John Hawkinson
Panix
1200 Warburton Ave., Suite 57
Yonkers, NY 10701-1057
phone: +1 617 253 7788
email: jhawk@panix.com
Tony Bates
MCI
2100 Reston Parkway
Reston, VA 22094
phone: +1 703 715 7521
email: Tony.Bates@mci.net
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