draft-ietf-idr-autosys-guide-02.txt   draft-ietf-idr-autosys-guide-03.txt 
Network Working Group J. Hawkinson Network Working Group J. Hawkinson
INTERNET-DRAFT Panix INTERNET-DRAFT Panix
Category: Informational T. Bates Category: Standards Track T. Bates
<draft-ietf-idr-autosys-guide-02.txt> MCI <draft-ietf-idr-autosys-guide-03.txt> MCI
February 1995 May 1995
Guidelines for creation, selection, and registration Guidelines for creation, selection, and registration
of an Autonomous System (AS) of an Autonomous System (AS)
Status of this Memo Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts. working documents as Internet-Drafts.
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RDI, or Routing Domain Identifier. RDI, or Routing Domain Identifier.
Table of Contents Table of Contents
1. Introduction ............................................ 2 1. Introduction ............................................ 2
2. Motivation .............................................. 3 2. Motivation .............................................. 3
3. Definitions ............................................. 3 3. Definitions ............................................. 3
4. Commom errors in allocating ASes ........................ 6 4. Common errors in allocating ASes ........................ 6
5. Criteria for the decision -- do I need an AS? .......... 6 5. Criteria for the decision -- do I need an AS? .......... 6
5.1 Sample Cases ........................................... 7 5.1 Sample Cases ........................................... 7
5.2 Other Factors .......................................... 7 5.2 Other Factors .......................................... 8
6. Speculation ............................................. 8 6. Speculation ............................................. 8
7. One prefix, one origin AS ............................... 9 7. One prefix, one origin AS ............................... 9
8. IGP issues .............................................. 9 8. IGP issues .............................................. 9
9. AS Space exhaustion ..................................... 10 9. AS Space exhaustion ..................................... 10
10. Security Considerations ................................ 10 10. Reserved AS Numbers .................................... 10
11. Acknowledgments ........................................ 10 11. Security Considerations ................................ 10
12. References ............................................. 11 12. Acknowledgments ........................................ 10
13. Authors' Addresses ..................................... 12 13. References ............................................. 10
14. Authors' Addresses ..................................... 12
1. Introduction 1. Introduction
This memo discusses when it is appropriate to register and utilize an This memo discusses when it is appropriate to register and util-
Autonomous System (AS), and lists criteria for such. ASes are the ize an Autonomous System (AS), and lists criteria for such. ASes
unit of routing policy in the modern world of exterior routing, and are the unit of routing policy in the modern world of exterior
are specifically applicable to protocols like EGP (Exterior Gateway routing, and are specifically applicable to protocols like EGP
Protocol, now at historical status; see [EGP]), BGP (Border Gateway (Exterior Gateway Protocol, now at historical status; see [EGP]),
Protocol, the current de facto standard for inter-AS routing; see BGP (Border Gateway Protocol, the current de facto standard for
[BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the inter-AS routing; see [BGP-4]), and IDRP (The OSI Inter-Domain
Internet will eventually adopt when BGP becomes obsolete; see Routing Protocol, which the Internet will eventually adopt when
[IDRP]). It should be noted that the IDRP equivalent of an AS is the BGP becomes obsolete; see [IDRP]). It should be noted that the
RDI, or Routing Domain Identifier. IDRP equivalent of an AS is the RDI, or Routing Domain Identif-
ier.
2. Motivation 2. Motivation
This memo is aimed at network operators and service providers who need This memo is aimed at network operators and service providers who
to understand under what circumstances they should make use of an need to understand under what circumstances they should make use
AS. It is expected that the reader is familiar with routing protocols of an AS. It is expected that the reader is familiar with routing
and will be someone who configures and operates Internet protocols and will be someone who configures and operates Inter-
networks. Unfortunately, there is a great deal of confusion in how net networks. Unfortunately, there is a great deal of confusion
ASes should be used today; this memo attempts to clear up some of this in how ASes should be used today; this memo attempts to clear up
confusion, as well as acting as a simple guide to today's exterior some of this confusion, as well as acting as a simple guide to
routing. today's exterior routing.
3. Definitions 3. Definitions
This document refers to the term ``prefix'' throughout. In the current This document refers to the term ``prefix'' throughout. In the
classless Internet (see [CIDR]), a block of class A, B, or C networks current classless Internet (see [CIDR]), a block of class A, B,
may be referred to by merely a prefix and a mask, so long as such a or C networks may be referred to by merely a prefix and a mask,
block of networks begins and ends on a power-of-two boundary. For so long as such a block of networks begins and ends on a power-
example, the networks: of-two boundary. For example, the networks:
192.168.0.0/24 192.168.0.0/24
192.168.1.0/24 192.168.1.0/24
192.168.2.0/24 192.168.2.0/24
192.168.3.0/24 192.168.3.0/24
can be simply referred to as: can be simply referred to as:
192.168.0.0/22 192.168.0.0/22
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network, or ``A, B, C network''. network, or ``A, B, C network''.
The definition of AS has been unclear and ambiguous for some The definition of AS has been unclear and ambiguous for some
time. [BGP-4] states: time. [BGP-4] states:
The classic definition of an Autonomous System is a set of The classic definition of an Autonomous System is a set of
routers under a single technical administration, using an inte- routers under a single technical administration, using an inte-
rior gateway protocol and common metrics to route packets within rior gateway protocol and common metrics to route packets within
the AS, and using an exterior gateway protocol to route packets the AS, and using an exterior gateway protocol to route packets
to other ASes. Since this classic definition was developed, it to other ASes. Since this classic definition was developed, it
has become common for a single AS to use several interior gate- has become common for a single AS to use several interior
way protocols and sometimes several sets of metrics within an gateway protocols and sometimes several sets of metrics within
AS. The use of the term Autonomous System here stresses the an AS. The use of the term Autonomous System here stresses the
fact that, even when multiple IGPs and metrics are used, the fact that, even when multiple IGPs and metrics are used, the
administration of an AS appears to other ASes to have a single administration of an AS appears to other ASes to have a single
coherent interior routing plan and presents a consistent picture coherent interior routing plan and presents a consistent picture
of what networks are reachable through it. of what networks are reachable through it.
To rephrase succinctly: To rephrase succinctly:
An AS is a connected group of IP networks run by one or more An AS is a connected group of IP networks run by one or more
network operators which has a SINGLE and CLEARLY DEFINED routing network operators which has a SINGLE and CLEARLY DEFINED routing
policy. policy.
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| |
AS X | AS Y AS X | AS Y
| |
<------------- + <-------------- <------------- + <--------------
accept NET2 | announce NET2 accept NET2 | announce NET2
| |
| |
resulting packet flow towards NET2 resulting packet flow towards NET2
===================================>> ===================================>>
Ideally, and seldom practically, the announcement and acceptance pol- Ideally, though seldom practically, the announcement and acceptance
icies of ASX and ASY are identical. policies of ASX and ASY are identical.
In order for traffic towards NET2 to flow, announcement and accep- 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 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. applications connectivity in just one direction is not useful at all.
It should be noted that, in more complex topologies than this exam- 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 ple, traffic from NET1 to NET2 may not necessarily take the same path
as traffic from NET2 to NET1; this is called asymmetrical routing. as traffic from NET2 to NET1; this is called asymmetrical routing.
Asymmetrical routing is not inherently bad, but can often cause per- Asymmetrical routing is not inherently bad, but can often cause per-
formance problems for higher level protocols, such as TCP, and should formance problems for higher level protocols, such as TCP, and should
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``Sample Cases'' below. ``Sample Cases'' below.
* Many prefixes, one AS * Many prefixes, one AS
As a general rule, one should try to place as many prefixes 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 possible within a given AS, provided all of them conform to the
same routing policy. same routing policy.
* Unique routing policy * Unique routing policy
As AS is only needed when you have a routing policy which is An AS is only needed when you have a routing policy which is
different from that of your border gateway peers. Here routing different from that of your border gateway peers. Here routing
policy refers to how the rest of the Internet makes routing policy refers to how the rest of the Internet makes routing
decisions based on information from your AS. See ``Sample decisions based on information from your AS. See ``Sample
Cases'' below to see exactly when this criteria will apply. Cases'' below to see exactly when this criteria will apply.
5.1 Sample Cases 5.1 Sample Cases
* Single-homed site, single prefix * Single-homed site, single prefix
A separate AS is not needed; the prefix should be placed in an A separate AS is not needed; the prefix should be placed in an
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representation of policy and preference among the different ser- representation of policy and preference among the different ser-
vice providers. vice providers.
This is ALMOST THE ONLY case where a network operator should This is ALMOST THE ONLY case where a network operator should
create its own AS number. In this case, the site should ensure create its own AS number. In this case, the site should ensure
that it has the necessary facilities to run appropriate routing that it has the necessary facilities to run appropriate routing
protocols, such as BGP4. protocols, such as BGP4.
5.2 Other factors 5.2 Other factors
* Topolgy * Topology
Routing policy decisions such as geography, AUP (Acceptable Use Routing policy decisions such as geography, AUP (Acceptable Use
Policy) compliance and network topology can influence decisions Policy) compliance and network topology can influence decisions
of AS creation. However, all too often these are done without of AS creation. However, all too often these are done without
consideration of whether or not an AS is needed in terms of consideration of whether or not an AS is needed in terms of
adding additional information for routing policy decisions by adding additional information for routing policy decisions by
the rest of the Internet. Careful consideration should be taken the rest of the Internet. Careful consideration should be taken
when basing AS creation on these type of criteria. when basing AS creation on these type of criteria.
* Transition / ``future-proofing'' * Transition / ``future-proofing''
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gether if there is a less specific aggregate announcement setting the gether if there is a less specific aggregate announcement setting the
ATOMIC_AGGREGATE attribute. ATOMIC_AGGREGATE attribute.
8. IGP Issues 8. IGP Issues
As stated above, many router vendors require an identifier for tag- As stated above, many router vendors require an identifier for tag-
ging their IGP processes. However, this tag does not need to be glo- ging their IGP processes. However, this tag does not need to be glo-
bally unique. In practice this information is never seen by exterior bally unique. In practice this information is never seen by exterior
routing protocols. If already running an exterior routing protocol, routing protocols. If already running an exterior routing protocol,
it is perfectly reasonable to use your AS number as an IGP tag; if it is perfectly reasonable to use your AS number as an IGP tag; if
you do not, choosing a random value is acceptable. Merely running an you do not, choosing from the reserved range is also acceptable (see
IGP is not grounds for registration of an AS number. ``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 With the advent of BGP4 it becomes necessary to use an IGP that can
carry classless routes. Examples include OSPF [OSPF] and ISIS [ISIS]. carry classless routes. Examples include OSPF [OSPF] and ISIS [ISIS].
9. AS Space exhaustion 9. AS Space exhaustion
The AS number space is a finite amount of address space. It is 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 currently defined as a 16 bit integer and hence limited to 65535
unique AS numbers. At the time of writing some 3,700 ASes have been unique AS numbers. At the time of writing some 5,100 ASes have been
allocated and a little under 400 ASes are actively routed in the glo- 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 bal Internet. It is clear that this growth needs to be continually
monitored. However, if the criteria applied above are adhered to, monitored. However, if the criteria applied above are adhered to,
then there is no immediate danger of AS space exhaustion. It is then there is no immediate danger of AS space exhaustion. It is
expected that IDRP will be deployed before this becomes an issue. expected that IDRP will be deployed before this becomes an issue.
IDRP does not have a fixed limit on the size of an RDI. IDRP does not have a fixed limit on the size of an RDI.
10. Security Considerations 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. There are few security concerns regarding the selection of ASes.
AS number to owner mappings are public knowledge (in WHOIS), and AS number to owner mappings are public knowledge (in WHOIS), and
attempting to change that would serve only to confuse those people attempting to change that would serve only to confuse those people
attempting to route IP traffic on the Internet. attempting to route IP traffic on the Internet.
11. Acknowledgments 12. Acknowledgments
This document is largely based on [RIPE-109], and is intended to have 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 a wider scope than purely the RIPE community; this document would not
exist without [RIPE-109]. exist without [RIPE-109].
12. References 13. References
[RIPE-109] [RIPE-109]
Bates, T., Lord, A., "Autonomous System Number Application Form Bates, T., Lord, A., "Autonomous System Number Application Form
& Supporting Notes", RIPE 109, RIPE NCC, 1 March 1994. & Supporting Notes", RIPE 109, RIPE NCC, 1 March 1994.
URL: ftp://ftp.ripe.net/ripe/docs/ripe-109.txt. URL: ftp://ftp.ripe.net/ripe/docs/ripe-109.txt.
[BGP-4] [BGP-4]
Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)", RFC
RFC 1654, T.J. Watson Research Center, cisco Systems, July 1994. 1654, T.J. Watson Research Center, cisco Systems, July 1994.
[EGP] [EGP]
Mills, D. "Exterior Gateway Protocol formal specifications", RFC Mills, D. "Exterior Gateway Protocol formal specifications", RFC
904, STD 18, International Telegraph and Telephone Co., 1 April 904, STD 18, International Telegraph and Telephone Co., 1 April
1984. 1984.
[IDRP] [IDRP]
Kunzinger, C., Editor, "OSI Inter-Domain Routing Protocol Kunzinger, C., Editor, "OSI Inter-Domain Routing Protocol (IDRP)",
(IDRP)", ISO/IEC 10747, Work In Progress, October 1993. ISO/IEC 10747, Work In Progress, October 1993.
[CIDR] [CIDR]
Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter- Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless Inter-Domain
Domain Routing (CIDR): an Address Assignment and Aggregation Routing (CIDR): an Address Assignment and Aggregation Strategy",
Strategy", RFC 1519, BARRnet, cisco, MERIT, OARnet, September RFC 1519, BARRnet, cisco, MERIT, OARnet, September 1993.
1993.
[OSPF] [OSPF]
Moy, J., "OSPF Version 2", RFC 1583, March 1994. Moy, J., "OSPF Version 2", RFC 1583, March 1994.
[ISIS] [ISIS]
Callon, R., Gunner, C., "Use of OSI IS-IS for Routing in TCP/IP Callon, R., Gunner, C., "Use of OSI IS-IS for Routing in TCP/IP and
and Multi-Protocol Environments", draft-ietf-isis-tcpip-01.txt, Multi-Protocol Environments", draft-ietf-isis-tcpip-01.txt, WORK IN
WORK IN PROGRESS, July 1994. PROGRESS, July 1994.
13. Authors' Addresses 14. Authors' Addresses
John Hawkinson John Hawkinson
Panix Panix
1200 Warburton Ave., Suite 57 1200 Warburton Ave., Suite 57
Yonkers, NY 10701-1057 Yonkers, NY 10701-1057
USA
phone: +1 617 253 7788 phone: +1 617 253 7788
email: jhawk@panix.com email: jhawk@panix.com
Tony Bates Tony Bates
MCI MCI
2100 Reston Parkway 2100 Reston Parkway
Reston, VA 22094 Reston, VA 22094
phone: +1 703 715 7521 phone: +1 703 715 7521
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